Chapter 2
REVIEW OF LITERATURE
Cancer
Cancer is a genetic disease, arising from an accumulation of mutations that promote
clonal selection of cells with increasingly aggressive behavior. The vast majorities of mutations
in cancer are somatic and are found only in an individual’s cancer cells. However, about 1% of
all cancers arise in individuals with an unmistakable hereditary cancer syndrome (Fearon, 1997).
Cancer is a progressive disease, occurring in a series of well-defined steps, typically arising as a
consequence of activating mutations (oncogenes) or deactivating mutations (tumor suppressor
genes) in proliferating cells (Lisa et al., 2001). Cancer is an English term, dissimilated from
Greek word 'Karakinos' (Sanskrit Karkita) for crab, the symbol for fourth zodiacal constellation
(the CANCER) or Cancer is a generic term that refers to a group of chronic diseases
characterized by the uncontrolled growth of abnormal cells within the body.. The word was
believed to be first used by Hippocrates, who attributed this disease to an excess of black bile.
Cancer was known in old ages, being described in early writings of Greeks and Romans.
Pathological evidences support the bone tumors in dinosaurs and other prehistoric animals but
the tumors in Egyptian mummies dating back to 5000 years represent the first known human
malignancies. Later, the Roman physician Celsus (28-50 BC) used the Latin term for crab,
cancer. Another Roman physician, Galen, (130-200 AD) used the Greek word for swelling,
oncos, to describe tumours -the root of the modern English word, oncology. Cancer has been
8
defined in terms of an autonomous growth that is unresponsive to normal growth factors and
antigrowth signals; in provisions of irreversibility with which the cancer cells progressively lose
the differentiated characteristics and functions of normal tissue of origin; on the basis of
morphologic and cytogenetic features; and on the basis of reversion to growth and antigenic
properties characteristics of fetal cells. Normally, cells divide and replicate to replace worn-out
cells or to repair some form of injury to tissues of the body. After a predictable period, normal
cells wear out and die. Cancer cells do not grow, divide and die in the same predictable fashion
as normal cells. Rather, they grow, divide and create more abnormal cells, which outlive normal
cells. The abnormal cells often spread to other body parts, invading other organs or systems.
Gastric cancer
Gastric cancer refers to cancer arising from any part of the stomach. Gastric cancer is
often either asymptomatic (producing no noticeable symptoms) or it may cause only nonspecific
symptoms (symptoms which are not specific to just gastric cancer, but also to other related or
unrelated disorders) in its early stages. By the time symptoms occur, the cancer has often reached
an advanced stage and may have also metastasized (spread to other, perhaps distant, parts of the
body), which is one of the main reasons for its relatively poor prognosis.
Gastric cancer can cause the following signs and symptoms:
Stage 1 (Early)

Indigestion or a burning sensation (heartburn)

Loss of appetite, especially for meat

Abdominal discomfort or irritation
9
Stage 2 (Middle)

Weakness and fatigue

Bloating of the stomach, usually after meals

Abdominal pain in the upper abdomen

Nausea and occasional vomiting

Diarrhea or constipation

Weight loss

Bleeding (vomiting blood or having blood in the stool)

Dysphagia; this feature suggests a tumor in the cardia.
Stage 3 (Late)
Pathology
The primary epithelial tumour of the stomach is the adenocarcinoma, and develops from
the stomach mucosa, usually maintaining glandular differentiation. Other less common tumours
of the stomach are the squamous cell carcinomas, and the adenosquamous carcinomas,
combining characteristics of both the adenocarcinoma and the squamous cell carcinoma to
approximately equal extent. Undifferentiated carcinoma lacks any differentiated features and
does not fit into any of the above categories. Gastric carcinomas can be classified according to
their localization in the stomach. The antral-pyloric region of the stomach is the most common
site of stomach cancer, and carcinomas of the body or corpus are located along the greater or
lesser curvature. Cancers of the cardia are often unable to be distinguished from cancers of the
gastroesophageal junction, and are believed to be a separate entity, probably originating from the
distal oesophagus. Early gastric cancers may feature protruded (Type I), elevated (Type IIa), flat
(Type IIb), depressed (Type IIc) or excavated (Type III) growth (Hamilton and Aaltonen,
10
2000), whereas advanced gastric carcinomas are classified into polypoid (Type I), fungating
(Type II), ulcerated (Type III) or infiltrative (Type IV) growth patterns (Borrmann, 1926;
Hamilton and Aalton, 2000). Type II or III advanced gastric cancers are commonly ulcerating,
and the risk of penetration of the submucosa is highest in early gastric cancers with a depressed
growth pattern (Type IIc), and in infiltrative advanced gastric carcinomas (Type IV). The
superficial spread of Type IV infiltrative (diffuse) tumours through the mucosa and submucosa
result in flat, plaque-like lesions, which may exhibit shallow ulcerations. Serosal, lymphatic, and
vascular invasion and lymph node metastases are most frequent in the diffusely growing tumours
(Mori et al., 1995; Carneiro, 1996).
Histological classification
Various systems have been applied to the classification of gastric carcinomas, including
the WHO (Laurén, 1965; Ming, 1977; Goseki and Koike, 1992; Hamilton and Aalton, 2000)
classifications. The clinical significance of these classifications is limited, with only the Lauren
and perhaps the Goseki classifications providing prognostic assessments (Alekseenko et al.,
2004). The TNM staging of the gastric carcinoma, according to the guidelines set out by the
International Union Against Cancer (UICC) (Wittekind and Sobin,
2002), is the most
important prognostic factor in clinical practice (Alekseenko et al., 2004). However, the Lauren
classification has been the most successful system, as it defines two distinct histological entities,
which clearly exhibit different clinical and epidemiological characteristics, even in advanced
gastric cancers (Satoh et al., 2007). In the Laurén classification(Laurèn, 1965), intestinal-type
carcinomas maintain the glandular phenotype, with well- to moderately-differentiated tumours
forming identifiable glands, often with poorly differentiated tumour cells at the invasive front.
Typically arising on a background of intestinal metaplasia, these tumours exhibit an intestinal,
11
gastric and gastrointestinal mucinous phenotype. Diffuse-type carcinomas form no or very few
glandular structures, instead usually infiltrating the gastric wall, appearing diffusely distributed
as small, round single cells or poorly cohesive cell clusters. They may resemble signet-ring cells,
and may contain small amounts of intestinal mucin. Additionally, mixed tumours exhibit both
intestinal and diffuse characteristics, and undifferentiated tumors are classified as indeterminate.
The natural history of gastric carcinoma, in particular the association with environmental factors,
incidence trends, and precursor lesions, is often evaluated with respect to the Laurén
classification.
Descriptive epidemiology
One of the notable features of the descriptive epidemiology concerning gastric cancer is
that it establishes some clear distinction between cancer localized to the gastric cardia and cancer
of the rest of the stomach, as discussed below.
International variations
Despite a major decline in the incidence and mortality over several decades, gastric
cancer is still the fourth most common cancer and the second to third most frequent cause of
cancer death in the world (Brenner et al., 2009; Herszenyi and Tulassay, 2010). There is
marked geographic variation in the incidence of gastric cancer. International Agency for
Research on Cancer data for 1996, demonstrate age-standardized incidence rates in males
ranging from 95.5/10.5 in Yamagata, Japan,to 7.5/10.5 in Whites in the United States. High-risk
areas include China and large parts of central and South America (Parkin and Ferlay, 1997).
Most of the geographic variation is accounted for by differences in the incidence of non cardia
cancer. Cancer localized to the cardia has a more uniform distribution. Gastric cardia cancer
accounts for only 4% of total gastric cancer cases in males in Osaka, Japan, compared to 39% in
12
white males in the United States (Parkin and Ferlay, 1997). On a histologic level, the incidence
of diffuse adenocarcinomas is reported to be similar in most populations, while the intestinal
type predominates in the high-risk geographic regions and is the type that has declined
significantly in incidence in many countries (Munoz and Cuello, 1968; Sipponen and Kekki,
1987). Ethnic groups who have migrated from high- to low-incidence countries have an overall
risk intermediate between that of their homeland and that of their new country. First generation
migrants tend to maintain their high-risk while subsequent generations have risk levels
approximating that of the host country (Haenszel, 1968). The prognosis is rather poor, with a
five-year survival below 30%. More than 90% of gastric cancers are adenocarcinomas, which are
malignant epithelial tumors, originating from glandular epithelium of the gastric mucosa. In the
Lauren classification, two major histological types of gastric adenocarcinoma can be
distinguished histopathologically: the diffuse and the intestinal type (Vauhkonen et al., 2006).
Intestinal metaplasia with goblet cells is considered to be a precursor lesion of the intestinal type
of gastric adenocarcinoma, which shows tubular differentiation (Vauhkonen et al., 2006). The
diffuse type gastric adenocarcinoma is characterized by non-cohesive single mucocellular cancer
cells (signet-ring cells) diffusely infiltrating the stroma (Vauhkonen et al., 2006). The diffuse
type gastric adenocarcinoma shows some predominance in the fundus and corpus of the stomach,
whereas the intestinal type gastric adenocarcinoma prevails in the antrum (Vauhkonen et al.,
2006). Furthermore, the remaining 10% of gastric malignancies are lymphomas or originate from
gastrointestinal stromal tissue (soft tissue tumors) (Verbeke et al., 2012).
Indian variations
Cancer is one of the leading causes of adult deaths worldwide. In India, the International
Agency for Research on Cancer estimated indirectly that about 635 000 people died from cancer
13
in 2008, representing about 8% of all estimated global cancer deaths and about 6% of all deaths
in India (Ferlay et al., 2010). 7137 of 122 429 study deaths were due to cancer, corresponding to
556 400 national cancer deaths in India in 2010. 395 400 (71%) cancer deaths occurred in people
aged 30–69 years (200 100 men and 195 300 women). At 30–69 years, the three most common
fatal cancers were oral (including lip and pharynx, 45 800 [22·9%]), stomach (25 200 [12·6%]),
and lung (including trachea and larynx, 22 900 [11·4%]) in men, and cervical (33 400 [17·1%]),
stomach (27 500 [14·1%]), and breast (19 900 [10·2%]) in women. Tobacco-related cancers
represented 42·0% (84 000) of male and 18·3% (35 700) of female cancer deaths and there were
twice as many deaths from oral cancers as lung cancers. Age-standardised cancer mortality rates
per 100 000 were similar in rural (men 95·6 [99% CI 89·6–101·7] and women 96·6 [90·7–
102·6]) and urban areas (men 102·4 [92·7–112·1] and women 91·2 [81·9–100·5]), but varied
greatly between the states, and were two times higher in the least educated than in the most
educated adults (men, illiterate 106·6 [97·4–115·7] vs most educated 45·7 [37·8–53·6]; women,
illiterate 106·7 [99·9–113·6] vs most educated 43·4 [30·7–56·1]). Cervical cancer was far less
common in Muslim than in Hindu women (study deaths 24, age-standardised mortality ratio 0·68
[0·64–0·71] vs 340, 1·06 [1·05–1·08] (Rajesh et al., 2012).
Kashmir variations
The valley of Kashmir is one of the divisions of Jammu and Kashmir State, situated in
the Himalayas. In Kashmir valley where incidences of almost all types of organ cancers have
shown a drastic increase in last couple of decades particularly gastric cancer. Gastric cancer is
the leading one with an average frequency of 19.2% followed by esophagus and lung as 16.5%
and 14.6%, respectively. In cancer types common to both sexes, the proportion in the men
exceeds that in the women particularly in lung and bladder cancer where around 80% were
14
males. Stomach (23%) and lung (21%) are the leading cancers in men while as esophageal
cancer tops (18.3%) in women followed by breast cancer (16.6%). Those tumors affecting the
stomach and extending to the esophageal lumen were considered gastric cancers. An interesting
finding was the presence of around 30% adenocarcinoma of GE junction among the total
frequency of stomach cancers. Around 21% of GE junction cancers were of sqamous cell
carcinoma of esophageal origin while as around 8% were recorded as adenocarcinoma of
esophageal origin. The age-standardized rates (ASR) of incidence of stomach cancer tops the list
with 10.2 cases/100,000/year followed by esophageal cancer (9.4/100,000) and lung cancer
accounts for third incident cancer (7.8 cases/100,000/year) (Arshad et al., 2012).
Age, sex and Race
The incidence of gastric cancer rises progressively with age, with most patients being
between the ages of 50 and 70 years at presentation. Cases in patients younger than 30 years are
very rare. Noncardia cancer is more common in males than females by a ratio of approximately
2:1. Gastric cardia cancer has a higher male-to-female ratio, of up to nearly 6:1 in U.S. Whites
(Parkin and Ferlay, 1997). There are significant variations in the overall incidence of gastric
cancer between different ethnic groups living in the same region (Parkin and Ferlay, 1997).
The ethnic distribution for cardia cancer is different, with a preponderance in Whites over Blacks
in the United States and non-Maoris over Maoris in New Zealand (Parkin and Ferlay, 1997).
Socioeconomic status
Low socioeconomic status has been consistently shown to be associated with an
increased risk of gastric cancer overall (Howson and Wynder, 1986). Remarkably, the increase
in incidence of cardia cancer has been predominantly in professional classes (Powell, 1992).
15
Aetiology and risk factors
Aetiological factors
Migrants from high- to low-incidence countries tend to maintain the high risk of the
population of origin. However, subsequent generations of migrants have risk levels
approximating those in the host country (Stewart BW 2003). It has been suggested that
approximately 66–75% of stomach cancer risk could be reduced with high intake of fruit and
vegetables and low consumption of salted foods (AICR 1997). The World Cancer Research Fund
(WCRF) and Association for International Cancer Research (AICR) panel of experts reached the
following conclusions:
 The evidence that diets high in vegetables and fruits, collectively and separately, decrease
the risk of stomach cancer is convincing. There is consistent evidence that raw
vegetables, allium vegetables and citrus fruits have a protective effect. A negative role
has been ascribed to salted nd pickled vegetables.
 There is also convincing evidence that the availability of refrigeration—which facilitates
year-round consumption of vegetables and fruits and may also reduce the need for salt as
a preservative—protects against stomach cancer.
 Diets high in salted foods and a high intake of salt, added in manufacture, in cooking, or
at the table, probably increase the risk of stomach cancer.
 Diets high in wholegrain cereals and green tea possibly decrease the risk of stomach
cancer.
 Although the data regarding wholegrain cereals and stomach cancer are somewhat
limited in number, they are almost entirely consistent.
16
In terms of the most important vitamins and compound contained in vegetables and
fruits, the WCRF and AICR report considered that Vitamin C probably, and carotenoids possibly
decrease the gastric cancer risk, while it is doubtful that
Vitamin E is involved. Alcohol
consumption possibly increases the risk of cancer of the gastric cardia (AICR 1997). Some
evidence has been provided by chemoprevention trials. An intervention study carried out in
volunteers(Correa P 2000) to assess the effect of Vitamin C and beta-carotene supplements
found a protective effect, that is an increase in the rate of regression of confirmed histological
diagnoses of intestinal metaplasia and/or atrophy (premalignant lesions). In another
chemoprevention trial, conducted in male smokers in Finland (Varis and Sipponen, 1998),
supplementation with alpha tocopherol and beta-carotene did not affect the risk of stomach
cancer and prevalence of premalignant lesions. The relationship between smoking and stomach
cancer has recently been recognised. The European Prospective Investigation Into Cancer and
Nutrition (EPIC) project (Gonzalez and Agudo, 2003), found a significant association between
cigarette smoking and gastric cancer risk: the hazard ratio (HR) was 1.45, 1.7 and 1.8 for ever
smokers, current male and current female smokers, respectively, and HR increased with intensity
and duration of cigarette smoking. An international population study performed by the
EUROGAST study group found that countries with a high incidence of stomach cancer have a
high prevalence of Helicobacter pylori infection (Group, 1993). However, the geographical
variation in gastric cancer risk in Europe cannot be explained by differences in the prevalence of
infection, since only a small fraction of those infected by Helicobacter develop stomach cancer.
In Japan, the country with the highest incidence of stomach cancer in the world, it has been
estimated that of the 60 million people infected by Helicobacter only 0.4% had stomach
cancer(Asaka M 1998). H. pylori was isolated in 1982, and was recognised as a human
17
carcinogen by IARC in 1994, but the specific mechanisms of action in the complex process of
stomach cancer are not known (Gonzalez, 2002). Moreover, H. pylori infection does not
increase the risk of cancer in the gastric cardia. Gastric ulcers, atrophic gastritis and autoimmune
gastritis with pernicious anaemia (Hsing and McLaughlin, 1993; Stewart, 2003), are
conditions which cause an excessive rate of cell proliferation in the gastric epithelium. Gastritis
is associated with increased production of oxidants and reactive nitrogen intermediates, including
nitritic oxide. Gastritis and atrophy alter gastric acid secretion, elevates gastric pH, changes the
gastric flora and allows anaerobic bacteria to colonize the stomach. The evidence for a role of
ionizing radiation in the aetiology of stomach cancer comes from the study of survivors of the
atomic bombings of Hiroshima and Nagasaki (Thompson, 1994). A linear dose–response effect
was observed, although the excess risk was small and the attributable risk was low (6.5%).
Studies of patients undergoing therapeutic radiation to the stomach for peptic ulcer disease (an
old treatment modality) and for testicular cancer, found a two- to four-fold increased risk in
patients exposed to radiation doses of 15–30 Gy (Griem and Boice, 1994). The risk of stomach
cancer is increased in first-degree relatives of patients with the disease by approximately two-to
three-fold(Lissowska J 1999). From the Scandinavian Twin Study, an increased risk of stomach
cancer in the twin of an affected person was found (Lichtenstein and Verkasald, 2000). Model
fitting to assess the contribution of hereditary and environmental factors found that inherited
genes contributed 28%, shared environmental factors 10%, and environmental factors 62%
(Vincenzo et al., 2005).
Dietary factors
Distinct variations in the incidence and mortality of gastric cancer, over time, between
and within countries, in differing socioeconomic groups, and in migrants and their offspring
18
suggest that diet may be etiologically important and its role has been extensively investigated
with inconclusive results.
Fruit and vegetables
The dominant dietary hypothesis is that fresh fruits and vegetables, or contained
micronutrients, are protective against gastric cancer. Numerous studies have shown, almost
uniformly, a protective association with fresh fruits and vegetables, independent of other dietary
factors. The association has been less pronounced in limited cohort studies (Kono, 1996).
Possible protective micronutrients include vitamins C (ascorbate) and E (alpha-tocopherol),
carotenoids (particularly beta carotene), and selenium (Kono, 1996). The evidence is strongest
for vitamin C, with an approximate halving of risk associated with high intake vs. low intake
demonstrated in case– control studies (Neugut et al., 1996). However, a 5-year intervention trial,
involving 30,000 40 to 69 year olds in China, did not show any change in risk of gastric cancer
in subjects receiving supplemental vitamin C (Blot et al., 1993).
Salt
The hypothesis that excess salt intake could be involved in the etiology of stomach cancer
was first presented in 1965. It was postulated that the continuous use of high doses of salt would
result in early atrophic gastritis, thereby increasing the later risk of stomach cancer (Joossens et
al., 1996). Since that time high salt consumption has been reasonably consistently associated
with an increased risk of gastric cancer in ecologic and analytical studies, although good
quantitative data are lacking (Joossens et al., 1996; Kono, 1996).
Nitrite and Nitrate
Many N -nitroso compounds have been shown to be carcinogenic in animal experiments.
Such compounds may be formed in the human stomach from dietary nitrite or nitrate. Hence, the
19
hypothesis that a diet high in nitrite or nitrate may predispose to gastric cancer. The major
sources of nitrate and nitrite are vegetables and preserved meats, respectively. Drinking water is
an additional source of nitrate, but usually contains negligible nitrite. In general, daily nitrate
intake is approximately 100 times that of nitrite. Small quantities of pre-formed N -nitroso
compounds may also be contained in some foods including cured meats (Gonzalez and Badosa,
1994; Kono, 1996). Case–control studies examining dietary intake of nitrate and the risk of
gastric cancer have consistently found a negative association. In such studies, vegetable intake
has been consistently related to a decreased risk of gastric cancer. Nitrate intake was probably an
index of vegetable intake and the negative association is not surprising in that context (Kono,
1996). Recent case– control studies have all reported a weak, statistically nonsignificant
increased risk of gastric cancer (relative risks from 1.12 to 1.28) for high vs. low nitrite intake
(Gonzalez and Badosa, 1994). Nitrite intake probably reflects consumption of preserved meats,
typically high-salt foods so isolating an effect of nitrite consumption is difficult. Further
evidence suggests that the interaction of the above dietary components is important. For
example, a diet high in nitrite does not appear to confer an increased risk if that diet is also high
in antioxidants from fruit and vegetables (Buiatti et al., 1990).
Other dietary factors
The collective literature on diet and gastric cancer provides data for a comprehensive
array of food groups, nutrients, micronutrients, and food-storage methods. Difficulties in
gathering and interpreting this evidence limit the conclusions that can be drawn. The advent of
widely available refrigeration, the consequent availability of fresh food and the decreased
consumption of preserved foods may have contributed to the decline in gastric cancer incidence
in the second half of this century. Dixon has pointed out how virulent strains of H. pylori release
20
reactive oxygen metabolites that could destroy neighboring glandular tissue leading to gastric
glandular atrophy, hastened by factors such as bile reflux or a high-salt intake but retarded by
antioxidants such as ascorbic acid, alpha tocopherol, beta carotene, and cysteine (Dixon, 1997).
Ionizing radiation
The best evidence concerning the role of ionizing radiation in the etiology of gastric
cancer comes from the study of survivors of the atomic bombings of Hiroshima and Nagasaki. In
a prospective incidence study of this cohort of approximately 80,000, Thompson et al. identified
more than 2,600 cases of gastric cancer (Thompson, 1994). A linear dose–response effect (P
>0.001) was observed between radiation dose and risk of gastric cancer, although the excess risk
was small (0.32 at one Sievert, 95% CI, 0.16 to 0.50) and the attributable risk was low (6.5%), in
the setting of a high background rate of gastric cancer in this Japanese population. Studies of
patients undergoing therapeutic radiation to the region of the stomach for peptic ulcer disease
(Griem and Boice, 1994) (a treatment modality used from the late 1930s to the mid-1960s) and
for testicular cancer (Moller et al., 1993; Van et al., 1993), also provide significant support for
this association. These studies suggest a two- to fourfold increased risk in patients exposed to
radiation doses of 15 to 30 Gy. Studies of occupational radiation exposure in radiographers
(Wang et al., 1990) and radiologists (Matanoski et al., 1975; Smith, 1981) have not
demonstrated increased risks, presumably due to the much lower radiation doses involved
compared to the atomic bomb survivors and the therapeutically irradiated. Differentiation of risk
based on the type of gastric cancer is not possible from the available evidence.
Pernicious anemia
An association between pernicious anemia and gastric cancer has long been recognized.
In the most recent and largest study of this topic, Hsing et al. observed a threefold increase in the
21
risk of gastric cancer in a cohort of 4,517 pernicious anemia patients, followed for up to 20 years
(Hsing and McLaughlin, 1993).
Smoking
The relationship between smoking and gastric cancer has been extensively examined yet
remains unclear; while most studies have reported a weak to moderate association, a few have
found none (Hansson et al., 1994; McLaughlin et al., 1995; Nomura et al., 1995; Ji et al.,
1996). In the positive studies the increased relative risks reported have generally been less than
twofold, and only a few studies have found a dose–response relationship (Hansson et al., 1994;
McLaughlin et al., 1995; Ji et al., 1996). A particular limitation of the available studies has
been a lack of control for confounding, particularly by H. pylori infection, which is positively
correlated with smoking, and by fruit and vegetable intake, which is inversely associated. Most
of the evidence does not allow any differentiation of risk by anatomic subsite or by histologic
type. One case– control study found a stronger association for gastric cardia cancer than for other
gastric cancer across multiple categories of smoking, whereas another did not (Ji et al., 1996).
Alcohol
A 1994 review of the experimental, descriptive, and analytical evidence relating to
alcohol and gastric cancer found little to support an association (Franceschi, 1994). After
examining over 50 mostly negative cohort and case–control studies, the authors concluded that
alcohol consumption was unlikely to be materially involved in the etiology of gastric cancer.
Subsequently, studies have not challenged that conclusion (Hansson et al., 1994; Ji et al.,
1996). Five case–control studies showed no association between alcohol consumption and cancer
of the gastric cardia (Unakami et al., 1989; Gray et al., 1992; Palli et al., 1992; Guo et al.,
1994; Ji et al., 1996) and one a doubling of risk in drinkers vs. nondrinkers (Kabat et al., 1993).
22
Epstein-Barr virus infection
Epstein-Barr virus has been isolated from gastric adenocarcinomas and poorly
differentiated carcinomas with lymphoid infiltrate by a number of investigators. Epstein-Barr
virus infection may contribute to the development of gastric carcinoma, but the data are limited.
Helicobacter pylori
The final sentence of Marshall’s, now classic, letter to the Lancet in June 1983
(Marshall, 1983) suggests that the, then unidentified, curved bacilli found in human gastric
epithelia ‘may have a part to play in poorly understood, gastritis associated diseases (i.e. peptic
ulcer and gastric cancer)‘. Exactly eleven years later, in June 1994, the International Agency for
Research on Cancer convened a Working Group which reviewed over 350 papers on the
relationship between Helicobacter pylori and gastric cancer and came to the conclusion that this
relationship was causal (IARC, 1994). This conclusion was reached after considering a wealth of
evidence concerning the epidemiology of H. pylori, specific studies of the association between
infection and the risk of cancer and mechanistic evidence about the pathogenicity of the
bacterium. It is extremely rare for such a body of evidence to accumulate in a short time period
and permit a definitive judgment about the carcinogenicity of an environmental exposure.
Marshall’s conjecture in 1983 has proven to be remarkably prophetic, so much so that H. pylori
eradication is now envisaged as a method of cancer prevention.
Asbestos
Several studies of workers with occupational exposure to asbestos have reported limited
evidence of an association (Frumkin, 1988; Andersen et al., 1993; Cocco et al., 1994).
However, methodologic problems cast doubt on the association. A case–control study of heavily
exposed asbestos miners and millers in Western Australia found no association between gastric
23
cancer mortality and intensity of exposure, duration of employment, or time since employment
began (De et al., 1989).
Other risk factors
The risk of gastric cancer is increased in first-degree relatives of patients with the disease
by approximately two- to threefold (La et al., 1992; Palli et al., 1994; Lissowska et al., 1999).
other suggested risk factors include blood group A and gastric polyps. However, the final word
on heredity in gastric cancer may well be the Scandinavian Twin Study of 44,788 pairs of twins
in the Swedish, Danish, and Finnish twin registries (Lichtenstein and Verkasald, 2000). This
found an increased risk of gastric cancer in the twin of an affected person. Model fitting to assess
the contribution of hereditary and environmental factors found that inherited genes contributed
28% (95% C.I. 0–51%), shared environmental factors 10% (95% CI, 0–34%), and environmental
factors 62% (95% CI, 0–76%). The statistical model used provided a perfect fit (P>1.0).
Chronic inflammation
Chronic inflammation plays a multifaceted role in carcinogenesis. Mounting evidence
from preclinical and clinical studies suggests that persistent inflammation functions as a driving
force in the journey to cancer. The possible mechanisms by which inflammation can contribute
to carcinogenesis include induction of genomic instability, alterations in epigenetic events and
subsequent inappropriate gene expression, enhanced proliferation of initiated cells, resistance to
apoptosis, aggressive tumor neovascularization, invasion through tumor-associated basement
membrane and metastasis, etc. Inflammation-induced reactive oxygen and nitrogen species cause
damage to important cellular components (e.g., DNA, proteins and lipids), which can directly or
indirectly contribute to malignant cell transformation. Overexpression, elevated secretion, or
abnormal activation of proinflammatory mediators, such as cytokines, chemokines,
24
cyclooxygenase-2, prostaglandins, inducible nitric oxide synthase, and nitric oxide, and a distinct
network of intracellular signaling molecules including upstream kinases and transcription factors
facilitate tumor promotion and progression. While inflammation promotes development of
cancer, components of the tumor microenvironment, such as tumor cells, stromal cells in
surrounding tissue and infiltrated inflammatory/immune cells generate an intratumoral
inflammatory state by aberrant expression or activation of some proinflammatory molecules.
Many of proinflammatory mediators, especially cytokines, chemokines and prostaglandins, turn
on the angiogenic switches mainly controlled by vascular endothelial growth factor, thereby
inducing inflammatory angiogenesis and tumor cell-stroma communication. This will end up
with tumor angiogenesis, metastasis and invasion. Moreover, cellular microRNAs are emerging
as a potential link between inflammation and cancer (Joydeb et al., 2012). Inflammation is
regarded as seventh hallmark of cancer (Francesco et al., 2009) .
Fig
1.
Fig. 1. Inflammation as the seventh hallmark of cancer. An integration to the six
hallmarks of cancer [modified from Hanahan and Weinberg (Hanahan, 2000) and Mantovani
(Mantovani, 2009) and Francesco (Francesco et al., 2009).
25
Chronic inflammation as a predisposing factor for malignant transformation
of cells
Chronic inflammation represents a major pathologic basis for the majority of human
malignancies. The role of inflammation in carcinogenesis has first been proposed by Rudolf
Virchow in 1863, when he noticed the presence of leukocytes in neoplastic tissues (Balkwill and
Mantovani, 2001). Since the Virchow’s early observation that linked inflammation and cancer,
accumulating data have supported that tumors can originate at the sites of infection or chronic
inflammation (Mueller, 2004). Approximately, 25% of all cancers are somehow associated with
chronic infection and inflammation (Jackson, 2006; Perwez, 2007). Although inflammation acts
as an adaptive host defense against infection or injury and is primarily a self-limiting process,
inadequate resolution of inflammatory responses often leads to various chronic ailments
including cancer (Jackson, 2006; Schottenfeld,2006). Multiple lines of evidence from
laboratory and population based studies suggest that organ-specific carcinogenesis is partly
associated with a persistent local inflammatory state (O’Byrne et al., 2001; Itzkowitz et al.,
2004; Nelson et al., 2004; Whitcomb, 2004). For instance, the development of carcinomas of
stomach, liver, gallbladder, prostate and pancreas has been attributed to Helicobacter pyloriinduced gastric inflammation, chronic hepatitis, cholecystitis, inflammatory atrophy of the
prostate and chronic pancreatitis, respectively (Philpott, 2004; Jackson, 2006; Matsuzaki et al.,
2007). Patients suffering from inflammatory bowel disorders, such as ulcerative colitis and
Crohn’s disease, have an increased risk of developing colorectal cancer (Seril et al., 2003;
Itzkowitz et al., 2004; Herszenyi and Tulassay, 2007), while the management of colitis with
anti-inflammatory drugs reduces this risk (Eaden et al., 2000).
26
Inflammation-associated carcinogenesis: roles of reactive oxygen and nitrogen
species
Sustained cellular injuries can cause inflammation, which may lead to carcinogenesis.
Various inflammatory and innate immune cells (e.g., mast cells, neutrophils, leukocytes,
macrophages, monocytes, eosinophils, dendritic cells, phagocytes, and natural killer cells) are
often recruited at the site of infection or inflammation. In response to proinflammatory stimuli,
activated inflammatory/immune cells generate reactive oxygen species (ROS) and reactive
nitrogen species (RNS), which can function as chemical effectors in inflammation-driven
carcinogenesis. Thus, one of the plausible mechanisms by which chronic inflammation can
initiate tumorigenesis is the generation of ROS and/or RNS in the inflamed tissue and subsequent
DNA damage leading to activation of oncogenes and/or inactivation of tumor suppressor genes.
Chronic exposure to ultraviolet (UV) B radiation is known to precipitate inflammatory tissue
damage and skin cancer (GM, 2005). Mutational changes in ras and p53 have been observed in
many types of human cancer (Rajalingam et al., 2007; Strano et al., 2007). The activation of
ras oncogene and loss-of-function of p53 tumor suppressor gene have been implicated in UVBinduced mouse skin carcinogenesis (Hattori et al., 1996). ROS-induced DNA damages
including DNA strand breaks, DNA base modifications, and DNA cross-links result in the
replication errors and the genomic instability and hence contribute to tumor initiation. Nitric
oxide (NO), another reactive species, plays a role in inflammation-associated carcinogenesis by
direct modification of DNA and inactivation of DNA repair enzymes. 8-Oxo-7,8-dihydro-20deoxyguanosine (8-oxo-dG), a major biochemical hallmark of oxidative and mutagenic DNA
27
damage (Hoki et al., 2007), has been found to be produced in association with H. pylori induced
gastric (Xu et al., 2004) and tumor necrosis factor-a (TNF-a)-induced pulmonary carcinogenesis
(Babbar et al., 2006). Peroxynitrite, a product formed by a reaction between NO radical and
superoxide anion, causes DNA damage by forming 8-nitroguanine (8-NG), which is another
potential biomarker of inflammation-associated cancers (Kawanishi, 2006). Thus, oxidative and
nitrosative DNA damage products, such as 8-oxo-dG and 8-NG, have been implicated in the
initiation of inflammation-driven carcinogenesis. ROS and RNS can induce lipid peroxidation to
generate other reactive species, such as manoldialdehyde and 4-hydroxynonenal (4-HNE), which
are capable of forming DNA-adducts. 4-HNE forms an adduct preferentially at the codon 249 of
the p53 gene. Elevated intracellular ROS (e.g., superoxide anion, H2O2, and hydroxyl radical)
and RNS (e.g., peroxynitrite, NO, and S-nitrosothiols) also cause alterations in cellular protein
functions, such as perturbation of DNA-protein cross-links and post-translational modification of
proteins involved in maintaining cellular homeostasis. For example, NO has been shown to
hyperphosphorylate and inactivate retinoblastoma protein resulting in increased proliferation of
human colon cancer cells. In colon tissues from patients with ulcerative colitis, a positive
correlation between the expression of iNOS and the phosphorylation of p53 at serine 15 residue,
as well as the activation of p53 transcriptional activity has been noted (Hofseth et al., 2003).
Nitrosative stress also plays a critical role in inflammation-associated carcinogenesis by
activating activator protein-1 (AP-1), a representative redox sensitive transcription factor, which
is involved in cell transformation and proliferation. Paradoxically, ROS and RNS can cause
apoptotic or necrotic cell death (Halliwell, 2007).
28
Major mediators linking inflammation and cancer
Chronic inflammation is implicated in all stages of carcinogenesis, i.e., initiation,
promotion and progression. In a persistently inflamed tissue, excessive generation of ROS can
cause genomic instability which leads to initiation of cancer (Philip et al., 2004; Hussain,
2007). A single initiated cell undergoes proliferation to produce a clone of mutated cells which
form premalignant mass, the event generally termed tumor promotion. Some of the preneoplastic
cells encounter additional mutations and become malignant. This process is referred to as tumor
progression. Proliferating tumor cells, their surrounding host stromal cells and tumor-infiltrating
inflammatory/ immune cells create a tumor microenvironment that reflects a persistent
inflammatory state (Balkwill and Mantovani, 2001; Ariztia et al., 2006). Within the tumor
microenvironment, various proinflammatory mediators participate in a complex inflammatory
signaling that facilitates extravasation of tumor cells through the stroma, thereby fostering tumor
progression (Balkwill and Mantovani, 2001; Ariztia et al., 2006)(Fig. 2).
Fig. 2. A journey to cancer: inflammation as the driving force. Inflammation is
implicated in multi-stage carcinogenesis. ROS/RNS or other reactive species derived from
inflammatory stress can attack DNA and cause mutations in oncogenes/tumor suppressor genes
or other genetic alterations. This will lead to initiation of carcinogenesis. Inflammation also
29
contributes to promotion and progression stages by stimulating the proliferation of initiated or
premalignant cells, enhancing angiogenesis and metastasis, rendering precancerous or neoplastic
cells resistant to apoptosis, etc., through epigenetic mechanisms (Joydeb et al., 2012).
Inflammation acts as a key regulator of tumor promotion and progression by several
mechanisms including acceleration of cell cycle progression and cell proliferation, evasion from
apoptotic cell death, and stimulation of tumor neovascularization (Philip et al., 2004). Among
the major molecular players involved in the inflammation-to-cancer axis, the notable members
are cytokines, chemokines, COX-2, prostaglandins, prostanoid receptors (EP 1–4), iNOS, NO,
and NF-kB. Table 1 represents the mechanisms by which the key inflammatory mediators
contribute to carcinogenesis.
Table 1 Key mediators linking inflammation and cancer (Joydeb et al.,2012).
30
Cytokines
Cytokines including interlukins, TNF-α, growth factors and differentiation factors are
secreted or membrane bound small protein molecules that regulate diverse physiological
processes, such as growth, development, differentiation, wound healing and immune response
(Lu and Huang, 2006; Miki et al., 2007). Cytokine signaling is initiated upon binding of
specific cytokines to cell-specific cognate receptors followed by activation of intracellular
kinases, such as Janus activated kinase (JAK), phosphatidylionositol-3-kinase (PI3/K)/ Akt, IKK,
and MAP kinases, with subsequent activation of transcription factors, predominantly STAT, NFkβ, and AP-1 (Jung et al., 2002; Yoshimura, 2006). The pleiotropic nature of cytokine
functions is evident from cross-regulation of one cytokine by other cytokines, differential
response of the same cytokine depending on the cell type, and synergistic or antagonistic effects
elicited by combined cytokine stimulation of cells (Szlosarek et al., 2006). Despite a complex
nature of their function, cytokines can broadly be classified as inflammatory (e.g., IL-1, IL-6, IL8, IL-17) and anti-inflammatory (e.g., IL-10) ones. Some cytokines have been reported to play a
role in inflammation associated carcinogenesis (Lin, 2007; Rigby et al., 2007). For example,
mice genetically modified to disrupt SOCS3 exhibit enhanced colonic crypt formation, crypt
proliferation, and the increased number and the size of colon tumors after challenge with dextran
sulfate sodium (DSS) or azoxymethane (AOM) plus DSS (Rigby et al., 2007). While persistent
local inflammation leads to cell transformation, a tumor cell further augments inflammatory
responses in its vicinity by secreting cytokines and chemokines, thereby creating a positive loop
between inflammation and cancer. Both cytokines and chemokines facilitate the communication
between tumor cells and tumor-associated host stromal tissue, thereby accelerating tumor
progression (Ben, 2003; Ariztia et al., 2006; Porta et al., 2007).
31
Chemokines
Chemokines are soluble chemotactic cytokines, which are classified as four major
groups, i.e., CXC, CC, XC and CX3C primarily based on the positions of conserved cysteine
residues (Balkwill and Mantovani, 2001; Lu and Huang, 2006; Allen and Handel, 2007). In
chronic inflammation, chemokines are usually produced by proinflammatory cytokines.
The central role of chemokines is to recruit leukocytes at the site of inflammation (Lu
and Huang, 2006). Most tumor cells can produce CXC and CC chemokines, which again differ
in selectivity for particular leukocytes. While lymphocytes represent a common target of both
CXC and CC, neutrophils are targeted only by CXC chemokines. CC chemokines can also act on
other leukocyte subtypes, such as monocytes and eosinophils as well as dendritic cells and
natural killer cells (Balkwill and Mantovani, 2001). Like cytokines, chemokines also act by
interacting with specific receptors expressed by both infiltrated leukocytes and tumor cells in an
autocrine or a paracrine fashion (Balkwill and Mantovani, 2001; Lu and Huang, 2006).
Several studies have reported the involvement of chemokines and chemokine receptors in cell
proliferation, migration, invasion and metastasis of different types of tumors. Overexpression of
CXCL-1/GROa, CXCL-2/GROb or CXCL-3/GROg promotes soft agar colony formation and
transformation of melanocytes in culture as well as tumorigenicity of transplanted melanoma
cells in nude mice.Treatment of cultured melanoma cells with anti-IL-8Rb antibody inhibited the
cell growth. Chemokine regulation of tumor angiogenesis results from a balance between
proangiogenic and angiostatic activities. Besides their role in chemoattraction of leukocytes,
chemokines direct the migration of tumor cells to the distal organs via circulation. The metastatic
potential of chemokines is attributed to their ability to induce the expression of matrix
metalloproteinases (MMPs), which facilitate tumor invasion. A stromal cell derived factor (SDF-
32
1)/CXCL-12 promoted the migration of colon adenocarcinoma (CT26) cells in culture and the
growth of implanted CT26 cells in BALB/c mice in vivo through angiogenesis-dependent
induction of tumor cell proliferation and inhibition of apoptotic cell death. Moreover, silencing
of endogenous CXCR4 gene expression by CXCR4-shRNA resulted in the inhibition of the
proliferation, adhesion, chemotaxis and invasion of mucoepidermoid carcinoma cells (Wen et
al., 2007).
COX-2 and prostaglandins
COX-2, an inducible form of cyclooxygenase, serves as an interface between
inflammation and cancer (Aggarwal et al., 2006; Surh, 2007). In response to various external
stimuli, such as proinflammatory cytokines, bacterial LPS, UV, ROS and phorbol ester, COX-2
is transiently elevated in certain tissues (Surh, 2007). Abnormally elevated COX-2 causes
promotion of cellular proliferation, suppression of apoptosis, enhancement of angiogenesis and
invasiveness, etc., which account for its oncogenic function (Fig. 3).
Fig. 3. Role of COX-2 and PGs in inflammation-induced carcinogenesis. Inflammatory signaling triggers
induction of COX-2 expression and subsequently production of an array of prostaglandins. While some
prostaglandins, especially PGE2, are implicated in carcinogenesis, others (e.g., PGI2) have cytoprotective effects.
Still another group of prostaglandins, including PGD2 and 15d-PGJ2, have dual effects on carcinogenesis. PGDH by
33
inactivating PGE2 can protect against carcinogenesis and is recognized as a tumor suppressor. EP and FP denote
PGE2 and PGF2a receptors, respectively (Joydeb et al., 2012).
Cytokine polymorphisms and gastrointestinal malignancy
Genetic polymorphisms have emerged in recent years as important determinants of
disease susceptibility and severity. This is particularly true for cytokine gene polymorphisms and
gastrointestinal malignancy. Perhaps the most compelling evidence for the role of inflammation
in GI malignancy comes from studies showing that proinflammatory cytokine gene
polymorphisms increase the risk of cancer and its precursors. An excellent example of this is the
role of these polymorphisms in the pathogenesis of H. pylori-induced gastric cancer. H. pylori
causes its damage by initiating chronic inflammation in the gastric mucosa. This inflammation is
mediated by an array of pro- and anti-inflammatory cytokines. Genetic polymorphisms directly
influence interindividual variation in the magnitude of cytokine response, and this clearly
contributes to an individual’s ultimate clinical outcome. In the case of H. pylori infection, we
speculated that the most relevant candidate genes would be ones whose products were involved
in handling the H. pylori attack (innate and adaptive immune responses) and ones that mediated
the resulting inflammation. H. pylori-induced gastritis is associated with three phenotypes that
correlate closely with clinical outcome. The first is an antrum-predominant/corpus-sparing
pattern associated with high acid secretion and increased risk of duodenal ulcer disease. Second
is mild mixed antrum/corpus gastritis with no major effect on acid secretion and, generally, no
serious clinical outcome. The last is a corpus-predominant or severe pangastritis pattern that is
associated with gastric atrophy, hypochlorhydria, and an increased risk of gastric cancer.
Inhibition of gastric acid pharmacologically can lead to a shift from an antrum-predominant
pattern to a corpus-predominant one with onset of gastric atrophy. Thus it was clear that an
34
endogenous agent that was upregulated in the presence of H. pylori, has a profound
proinflammatory effect, and was also an acid inhibitor would be the most relevant host genetic
factor to be studied. IL-1β fitted this profile perfectly, for not only is it one of the earliest and
most important proinflammatory cytokines in the context of H. pylori infection, it is also the
most powerful acid inhibitor known (El Omar, 2001). We have shown that proinflammatory IL1 gene cluster polymorphisms (IL-1β encoding IL-1β and IL-1RN encoding its naturally
occurring receptor antagonist) increase the risk of gastric cancer and its precursors in the
presence of H. pylori (El Omar et al., 2000). Individuals with the IL-1β-31C or -511T and IL1RN2/2 genotypes are at increased risk of developing hypochlorhydria and gastric atrophy in
response to H. pylori infection. This risk is extended to gastric cancer itself with a two- to
threefold increased risk of malignancy compared with subjects who have the less
proinflammatory genotypes (El-Omar et al., 2000; El-Omar et al., 2003). The association of
IL-1 gene cluster polymorphisms and gastric cancer has been confirmed in other reports
(Figueiredo et al., 2002). In addition to IL-1 gene cluster polymorphisms, proinflammatory
genotypes of TNF-α and IL-10 have also been identified as risk factors for gastric cancer, and, as
is the case with IL-1 gene cluster polymorphisms, this is restricted to noncardia adenocarcinomas
(El-Omar et al., 2003). We have shown that having an increasing number of proinflammatory
genotypes (IL-1β -511T, IL- 1RN2/2, TNF-α-308A, and IL-10 ATA/ATA) progressively
increases the risk of gastric cancer. Indeed, by the time three to four of these polymorphisms are
present, the risk of gastric cancer is increased 27-fold (El-Omar EM 2003). The fact that H.
pylori is a prerequisite for the association of these polymorphisms with malignancy demonstrates
that in this situation, inflammation is indeed driving carcinogenesis. It is likely that other
proinflammatory cytokine gene polymorphisms will be relevant to gastric cancer initiation and
35
progression. This exciting field has expanded greatly over the past few years, and the search is
now fully on for the full complement of risk genotypes that dictate an individual’s likelihood of
developing cancer. This approach has now been adopted for many other cancers as described
below. In Japanese patients with chronic HCV infection, the IL-1β - 511 T/T genotype has been
associated with an increased risk of progression to hepatocellular carcinoma (Tanaka et al.,
2003). Because the T/T proinflammatory genotype is related to greater IL-1β production, it is
feasible that risk of malignant transformation is higher. IL-1β leads to the production of PGE2
and hepatocyte growth factor and has angiogenic influence via inducible NO and COX-2
expression. Furthermore, the degree of HCV induced liver inflammation and fibrosis has been
correlated with hepatic expression of Th1 cytokines. At present, there is relatively little
information on the relationship between other gastrointestinal malignancies and cytokine
polymorphisms. Some studies have addressed the influence of polymorphisms on cancer
outcome. Barber et al. (Barber et al., 2000) found that possession of a genotype resulting in
increased IL-1β production was associated with shortened survival in pancreatic cancer. Park et
al. (Park et al., 1998) investigated TNF-A and –B polymorphisms in 136 colorectal cancer
patients and 325 healthy controls in an Asian population. Their results indicated that TNFB1/TNF-B1 genotypes showed an increased risk for colorectal cancer. De Jong and colleagues
(De Jong et al., 2002) recently performed pooled analyses on 30 polymorphisms in 20
lowpenetrance genes and identified an additional three studies investigating TNF-α
polymorphisms and colorectal cancer.Associations were detected for the a2, a5, and a13 TNF-α
alleles and colorectal cancer.
36
Mechanisms of inflammation associated tumor development in the gastro
intestinal (GI) tract
The mechanisms employed by ROS, COX-2, and cytokines to promote neoplasia are
discussed, these mechanisms include direct DNA damage, inhibition of apoptosis, subversion of
immunity, and stimulation of angiogenesis. In addition, chronic inflammation in the GI tract is
also known to affect proliferation, adhesion, and cellular transformation. Deregulation of cellular
proliferation is one of the hallmarks of cancer cells and is the outcome of interaction between a
variety of endogenous and exogenous factors that are active during the inflammatory process.
These include luminal contents, bacteria, inflammatory cytokines, and mediators such as the
matrix metalloproteinases. Direct mechanical irritation can also lead to epithelial proliferation,
and when this is combined with the effects of an additional inflammatory stimulus, such as a
bacterium for example, the resulting hyperproliferation can push the tissue further along the
pathway toward cancer. H. pylori infection, although initially enhancing apoptosis, ultimately
leads to a compensatory proliferation (Yanai et al., 2003). Pathways through which H. pylori
may influence apoptosis include those involving COX-2 and peroxisome proliferator-activated
receptor-γ(Gupta RA 2001). Proinflammatory cytokines, particularly TNF-α, are also able to
modulate apoptosis through altering the levels of the pro and anti apoptotic proteins Bcl-2 and
Bax. As well as affecting proliferation and apoptosis, the same mediators impact on cellular
adhesion and angiogenesis. Cancer cells responding to proinflammatory cytokines released from
macrophages may exploit the same mechanism used by leukocytes to migrate through the
vasculature. Upregulation of cell adhesion molecule expression is seen on exposure of colon
cancer cells to LPS, and COX-2 has also been shown to promote cell adhesion (Tsujii, 1995;
Sutton et al., 2000). Macrophages are important sources of VEGF, and studies have shown that
37
this can be augmented in tumors by the humoral antitumor immune response (Barbera et al.,
2002). Thus a Th2 environment promotes angiogenesis, and conversely, CMI/Th1 immune
responses tend to be inhibitory. Although infection and inflammation initially generate Th1
cytokines, a cycle involving COX-2-mediated upregulation of Th2 cytokines and subsequent
chronic downregulation of the Th1/CMI immune response can develop in neoplasia. This is
elegantly illustrated by Dalgleish and O’Byrne (Dalgleish, 2002) in their review of chronic
immune activation and inflammation as the cause of malignancy, from which Fig. 2.6a has been
modified.
Fig. 4. Mechanisms of inflammation-associated tumor development in the gastrointestinal tract:
angiogenesis. Chronic inflammatory stimuli such as Helicobacter pylori gastritis and inflammatory bowel disease
(IBD) initially generate a Th1 proinflammatory cytokine response. However, via cyclooxygenase (COX)-2
generation, Th2 cytokine production is upregulated and there is negative feedback inhibition of the Th1 response.
The presence of COX-2 and the change to a predominantly Th2 environment promote angiogenesis. COX-2
stimulates angiogenic growth factors, in particular VEGF, which is also stimulated by a number of other
inflammatory mediators including nitric oxide (NO) and certain cytokines. All may be acting through the
transcription factor hypoxia-inducible factor-1α (HIF-1α), which is regulated by NO, certain cytokines, and growth
factors and which, in turn, binds to the VEGF promoter region leading to its activation (Mairi et al., 2004).
The transition to a predominantly Th2 immune environment favors angiogenesis, and
COX-2 itself has proangiogenic activity. Hypoxia is a potent inducer of VEGF, and this is
38
mediated by the transcription factor hypoxia-inducible factor- 1α(HIF-1α). The VEGF gene
contains a number of HIF-1α-binding sites in its regulatory region, and HIF-1αis able to activate
the VEGF promoter. Liu and colleagues (Liu et al., 2002) demonstrated that PGE2 production
via COX-2-catalyzed pathways plays a critical role in HIF-1α regulation by hypoxia. They
showed that tumors treated with a COX-2 inhibitor were smaller, with increased apoptosis,
decreased microvessel density, and decreased tumor VEGF levels. ROS, NO, certain cytokines,
and growth factors are also regulators of HIF-1α expression, and this may explain their
proangiogenic activity. The significance of HIF-1αin inflammation has been highlighted by
Cramer et al.(Cramer et al., 2003), who revealed that it controls the redness and swelling of
injured tissues and the ability of leukocytes to enter inflamed areas. In the low oxygen
concentration of injured, inflamed, or neoplastic tissue, HIF-1α is required to generate ATP in
leukocytes and thus enable them to function. It also increases the production of NO, which acts
back to further increase HIF-1α activity. Thus acting through HIF-1α in a hypoxic environment,
various inflammatory mediators including growth factors, NO, cytokines, chemokines, COX-2,
and its products may “switch” on angiogenesis. They may do this via the generation of VEGF,
either directly or indirectly, and may also aid the process by activating other factors such as
proteases, which degrade the extracellular matrix. Therefore, it is not difficult to see that in
chronic GI inflammation, the development of hypoxic areas may increase the generation of
proangiogenic stimuli that tip the balance in favor of angiogenesis and further drive tissues
toward carcinogenesis. Finally, in addition to impacting on cellular proliferation, apoptosis,
adhesion, and angiogenesis, the stimuli and mediators of chronic inflammation can cause cellular
39
transformation. A number of viruses such as HBV, Epstein-Barr, and HPV are known to directly
bind to certain genes and affect protein activity, including transcriptional factors and oncogenes.
Animal models have also demonstrated that bacteria can lead to ultrastructural changes within
the colonic epithelia and subsequent hyperplasia (Luperchio, 2001). This may precede the
development of colonic adenomas. Fig.2.6b, provide an overview of how chronic inflammation
in the GI tract may ultimately lead to malignancy. This takes into account the various cellular
processes and pathways through which inflammation and its by products are thought to exert
their effects. There is ample epidemiological evidence to support links between chronic
inflammation and carcinogenesis of the GI tract, but increasingly, the basic molecular pathways
of this association are being uncovered. Inflammatory cells produce a wide range of mediators
including proinflammatory cytokines, chemokines, ROS, growth factors, and eicosanoids. COX2 may be a linchpin in orchestrating many of the mutagenic effects of these products, and this is
supported by studies showing the chemopreventative benefits of COX inhibitors. Cytokine gene
polymorphisms undoubtedly contribute to individual risks of malignancy, but their importance
lies in their contribution to the understanding of inflammation-mediated carcinogenesis. The fact
that chronic inflammation impacts crucial cellular processes such as proliferation, adhesion,
apoptosis, angiogenesis, and transformation highlights its pivotal role in the pathogenesis of GI
malignancy (Mairi et al., 2004).
40
Fig.5. Overview of mechanisms of inflammation-associated gastrointestinal
carcinogenesis. This diagram illustrates the complex interactions between inflammatory
mediators and the common cellular processes that could lead to cancer. When the factors are
considered in layers, the environment within the gastrointestinal tract forms the outermost layer.
The presence of a rapidly proliferating mucosa exposed to exogenous substances and a vast
quantity of LPS-rich bacteria provides an ideal environment for chronic inflammation to get
established. Moving inward, the next layer involves the release of mediators such as
proinflammatory cytokines, chemokines, growth factors, and NO, many of which share
transcriptional factors such as HIF-1α and NF-kB. These may act independently or in
combination, leading to the activation of COX-2. At the center are 5 cellular mechanisms that
could form a pathway to malignant transformation, either independently or more likely in
concert. COX-2 is known to impact on each of them and is placed in an inner layer because of
the growing evidence supporting its central role in gastrointestinal carcinogenesis. However, the
relationships among NO, cytokines, and activation of COX-2 and their impact on cellular
mechanisms are not purely linear. As described in the review, all of these factors are capable of
exerting effects independent of COX-2, and thus this layer may not always be a necessary
component in revealing the final core of pathways to cancer (Mairi et al., 2004).
41
Overall cancer statistics indicate that gastric cancer (GC) is the fourth most common
cancer worldwide, with 934,000 cases per year in 2002 (8.6% of total cases), and has the second
highest mortality after lung carcinoma (about 700,000 deaths annually) (Parkin et al., 2005).
Over the past 10 years GC has become significantly more common in Great Britain, the United
States, and other countries (Wayman et al., 2001). In Russia, GC is the second most widespread
malignancy, with approximately 48,300 cases per year (Aksel et al., 2001). Unfortunately, GC
prognosis remains poor. According to modern statistical data, the survival rate is around 25–30%
worldwide (mean lifetime, 17.2 months) after chemotherapy and radiotherapy (Parkin et al.,
2005). For unknown reasons, males are more predisposed to GC than females (Parkin et al.,
2005). Gastric carcinogenesis is a multifactorial process that includes environmental, host,
genetic, and bacterial factors. The main and foremost environmental etiological factors for GC
risk are nutrition pattern and individual dietary choices. High intakes of red or processed meat
(Larsson et al., 2006) and salty foods (Panel et al., 1997), excessive alcohol consumption (Liu
et al., 2008), and tobacco smoking (Tredaniel et al., 1997) may contribute to GC development.
Obesity has also been determined to be an etiological factor, but only for the cardiac subtype of
GC (Lagergren et al., 1999). A connection between Helicobacter pylori (HP) status and high
risk of GC has also been established (Group, 1993), and after numerous investigations this
bacterium was officially adopted as an etiological factor by the International Agency for
Research on Cancer (Hxcker et al., 2003). Modern cancer research is concerned with the
investigation of individual susceptibility of the human genome to various risk factors. During the
past decade many correlations between single nucleotide polymorphisms (SNPs) in DNA
structure and the risk of various diseases, including cancer, were reported (Tsigris et al., 2007).
42
Therefore, these polymorphisms can be applied as specific markers of predisposition for tumor
prevention.
Interleukins in gastric cancer development
Interleukins (ILs) belong to a diverse family of cytokines and represent small, specific
cell signaling protein molecules, which regulate the immune system of an organism. ILs are
synthesized predominantly by T cells, monocytes, macrophages, and endothelial cells. Their
functions are plural: facilitating communication between immune cells, controlling genes,
regulating transcription factors, and governing the inflammation, differentiation, proliferation,
and secretion of antibodies (Salazar et al., 2007). Thus, humoral regulation of the immune
response is greatly dependent on the functioning of interleukins. ILs implement cell coordination
by creating signals (Salazar et al., 2007), which provide autocrine or paracrine regulation. Each
IL has a specific ligand-dependent receptor (IL-R), which is expressed on the surface of the
target cell and directly participates in signaling. These receptors are membrane glycoproteins,
which consist of an external immunoglobulin-like domain, a transmembrane region, and a
cytoplasmic domain (McMahan et al., 1991). The impact of IL on a cell is a multistep process.
First, IL binds to the receptor on the cell surface and forms a single complex, which leads to
conformational changes in IL-R, bringing the JAKs (specific tyrosine kinases) close enough to
autophosphorylate themselves. Autophosphorylation of JAKs induces a conformational alteration
in its own structure, enabling it to further phosphorylate and activate transcription factors called
signal transducers and activators of transcription (STATs). Activated STATs dissociate from the
receptor and form dimers before translocating to the cell nucleus, where they regulate the
transcription of selected genes (Fig. 2.7). There are 35 ILs identified in the human body (Doan,).
Their coordinated work ensures the correct and effective functioning of immune system. Any
43
dysregulation between IL interactions or disruptions in the JAK/STAT pathway may lead to DNA
damage, excessive production of tumor-inducing factors, immune disorders, angiogenesis, and
dysplasia (Lederle et al., 2011). In addition, they often result in malignant transformation and
further formation of metastasis (El-Omar, 2006). Disruptions and dysregulations may be caused
by several factors, including activities of intracellular agents (Brauer et al., 2010), viruses
(Tacke et al., 2011), infections (Group, 1993), or smoking (Macha et al., 2011). Genetic
factors play an essential role in cytokine balance, including genetic polymorphisms. Thus, it
becomes clear that ILs may contribute to cancer development. In fact, many studies have proven
that the presence of a tumor is often connected with a high serum concentration of certain ILs.
An increased serum level of proinflammatory ILs was identified in patients with hepatocellular
carcinoma, oral squamous cell carcinoma, and prostate cancer (Al-Wabel et al., 1995; Tsai et
al., 1999; Michalaki et al., 2004). Anti-inflammatory ILs were also reported to contribute to the
development of cancer: high serum levels of IL-10 and IL-4 were indicated among melanoma,
gastric, pancreatic, and prostate cancer patients (Fortis et al., 1996; Takeshi et al., 2005).
Hence, a dramatic alteration of the IL level may reflect dysregulations in the production of ILs.
Individual genetic differences caused by SNPs may be closely related to these disruptions and
eventually play a role in carcinogenesis (Arseniy, 2011).
44
Fig. 6 Schematic plan illustrates how interleukins (ILs) mediate cell functioning. 1. IL binds with the
corresponding IL receptor (IL-R), inducing conformational change. 2. Activated JAK phosphorylates STAT with
mediation of the IL/IL-R complex. 3. STAT forms the dimer, which is able to enter the nucleus and bind to specific
DNA sequences in the promoters of genes that begin transcription of needful genes.
Several inflammatory interleukins have been linked with tumorigenesis, which suggests
that inflammation is associated with cancer development (Table 2). These interleukins include
IL-1, IL-6, IL-8,IL-10 and IL-18. Interleukins mediate different steps in the pathway leading to
tumorigenesis. Secretion of IL-1α promotes growth of cervical carcinoma (Woodworth et al.,
1995) and can also induce anchorage independence in embryo fibroblasts and tumor cell
revertants (Vanhamme et al., 1993). Autocrine production of interleukin IL-1β
promotes
growth and confers chemoresistance in pancreatic carcinoma cell lines (Arlt et al., 2002). High
levels of IL-1β have been identified as a key mediator of this activation in two of the
chemoresistant pancreatic cell lines. IL-1β secretion into the tumor milieu also induces several
45
angiogenic factors from tumor and stromal cells that promotes tumor growth through
hyperneovascularization in lung carcinoma growth in vivo (Saijo et al., 2002). IL-6 acts as a
paracrine growth factor for multiple myeloma, non-Hodgkin’s lymphoma, bladder cancer,
colorectal cancer, and renal cell carcinoma (RCC) (Klein et al., 1989; Okamoto et al., 1995;
Voorzanger et al., 1996; Angelo et al., 2002). Autocrine IL-6 production in RCC has been
linked with the involvement of p53. RCC cell lines containing mutant p53 produced higher
levels of IL-6 than those containing wild-type p53 (Angelo et al., 2002). Another important proinflammatory cytokine IL-8 has been reported to promote growth and metastasis of wide variety
of tumors. Expression of IL-8 by human melanoma cells and human ovarian cancer cells
correlates with their metastatic potential (Luca et al., 1997; Xu, 2000; Huang et al., 2002). IL-8
has been detected in astrocytomas, anaplastic astrocytomas, glioblastomas, and central nervous
system cervical carcinoma metastasis. Thus, IL-8 secretion could be a key factor involved in the
determination of the lymphoid infiltrates observed in brain tumors and the development of
cerebrospinal fluid pleocytosis in persons with meningoencephalitides (Van et al., 1992).
Polymorphisms in the IL-8 gene contributes to a high risk of gastric cardia adenocarcinoma
(GCC) and esophageal squamous cell carcinoma (ESCC) among the population of Linxian in
north-central China (Savage et al., 2004). IL-8 has been found to be transcriptional target of Ras
signaling. Ras-dependent IL-8 secretion was required for the initiation of tumor-associated
inflammation and neovascularization(Sparmann A 2004). Constitutive production of IL-18,
RANTES, and MIP-1b, has been linked to disease progression in large granular lymphocyte
(LGL) leukemia (Kothapalli et al., 2005).
46
Table 2 – Role of inflammatory interleukins and chemokines in tumorigenesis
IL-1β polymorphisms and gastric cancer
IL-1 plays a significant role in immune response signaling pathways (Dinarello, 1994).
The IL1 cluster consists of 3 related genes: IL1A, IL-1β, and IL1RA, which encode the signal
proteins IL-1α, IL-1β, and their receptor, IL-1RA, respectively. The relation of IL-1β and IL1RA
gene polymorphisms to GC development was observed in numerous studies (El-Omar et al.,
2000). Overexpression of the IL-1β gene is suggested to be promoted by H.pylori infection, as
well as chronic atrophic gastritis or duodenal ulcer (Blanchard et al.,1998). Overexpression of
IL-1β inhibits the secretion of gastric acid, which consequently results in the overexpression of
47
gastrin and may stimulate neoplastic growth (Rozengurt et al.,2001). However, overexpression
of the IL-1β gene may also be caused without H pylori exposure because of SNPs(Kuipers et al.,
1995). There are 3 diallelic polymorphisms at positions –511 (C-T, rs16944), –31 (T-C,
rs1143627), and-3954 (C-T, rs1143634) base pairs from the transcription start site, which are
involved in the regulation of IL-1β expression. Other SNPs of the IL-1β gene have been
investigated less intensively. Four comprehensive meta-analyses on the IL-1β gene were chosen
to assess the role of these SNPs in GC. Researchers have been studying the T allele of the -511
SNP for a long time. First, Camargo et al. (Camargo et al., 2006) demonstrated that –511 TT
carriers had increased GC risk in comparison with CC wild-type genotype carriers in 14 studies
(odds ratio [OR] -1.21). The effect was more evident for T allele carriers with noncardiac and
intestinal GC subtypes (OR -1.66 and 1.80, respectively) among Caucasians. Interestingly, there
were no correlations among Asian populations. An analysis of 39 case–control studies conducted
by Wang et al. (Wang et al., 2006) revealed similar results: the T allele of –511 was more
frequent among patients with GC (OR -1.26). Histopathologic stratification revealed that the
association was stronger for patients with intestinal subtype (OR -1.76) but not with diffuse type
(OR -1.16), which confirmed the results of Camargo et al. (Camargo et al., 2006). The authors
also indicated that there was no impact of ethnicity and H. Pylori status on cancer risk in this
study. The results of Xue et al. (Xue et al., 2010) also confirmed the findings of previous
authors: the T allele correlated with higher intestinal and noncardiac GC risk in 18 studies versus
CC genotype (OR -1.55 and 1.33, respectively). Interestingly, these significant results were also
obtained among Caucasians but not in Asian or Hispanic populations (OR -1.33). The fourth
meta-analysis, which included 28 studies, was performed by Kamangar et al. (Kamangar et al.,
2006). They reported an absence of correlations between CT/TT genotypes and the GC genotype
48
in comparison with the CC wild-type genotype (OR-1.07 and 1.16, respectively). Ethnic
stratification also indicated a lack of significant differences, as well as analysis of histopathology
or tumor site. Such controversial results may be explained by possible differences in study
design, inclusion criteria of investigations, errors during the statistical analysis, differences in
stratification, sample size, and chance. However, an overwhelming majority of studies
characterize the T allele of the IL-1β –511 polymorphism as frequent among Caucasian
individuals with noncardiac GC, preferably for the intestinal subtype of this cancer. Therefore, it
is feasible to suggest this SNP as a potential predictive marker for GC. Investigations of the IL1β –31 TATA-box polymorphism continue to be controversial even after several meta-analyses
have been carried out. Camargo et al. (Camargo et al., 2006) revealed a slight nonsignificant
connection between the C variant allele and GC risk compared with TT homozygotes (OR -1.04)
in 14 studies. Again, there was no association among Asian populations (OR -0.91) in
comparison with Hispanic or Caucasian populations (OR -0.91, 1.52, and 1.11, respectively).
Histologic stratification indicated a moderate increase of intestinal GC subtype among C allele
carriers in Caucasian populations (OR -1.61), but this statement was not true for the diffuse
subtype of GC. No associations were observed in the studies of Wang et al. (Wang et al., 2006),
Xue et al. (Xue et al., 2010), and Kamangar et al. (Kamangar et al., 2006) (OR -1.00, 0.97, and
0.99, respectively). Subgroup analysis did not indicate any correlations. Investigations on the IL1β –3954 SNP have indicated discordant results. Camargo et al. (Camargo et al.,2006) indicated
a slight non significant increase of GC risk among T mutant allele carriers compared with CC
individuals (OR-1.26), and the effect was more evident among Asian populations (OR -1.73).
49
However, the small number of studies and the deficiency of T and CT allele carriers in some
studies could distort the results. Wang et al.(Wang et al., 2006) also determined that the T allele
of the -3954 gene polymorphism contributes to cancer risk (OR -1.37) in comparison with the
CC genotype, although no analyses of H.pylori
status, tumor location, or subtype were
performed. Xue et al. (Xue et al., 2010) reported a lack of association between the -3954
polymorphism and GC. A small number of studies on the IL-1β -3954C/T polymorphism have
been conducted. A correlation of the T allele with GC was identified, which indicates a potential
role in gastric carcinogenesis. Recently, Lee et al. (Lee et al., 2004) discovered a new promoter
IL-1β –1473 G to C (rs1143623) polymorphism and reported an association between its G allele
and increased risk of the intestinal type of GC (OR -1.8 for the CG genotype and 2.1 for the GG
genotype) among a Korean population. The significance of this finding should be proven by
further functional and genetic association studies. It is important not to overlook the fact that the
above mentioned meta-analyses (Kamangar et al., 2006; Wang et al., 2006; Xue et al., 2010;
Camargo et al., 2006) are mostly composed of the same original studies, and therefore some
degree of overlap exists. First, meta-analyses devoted to the IL-1β –511C/T polymorphism are
composed of materials from 10 same case–control studies. Materials from 11 studies were
included in the above-mentioned meta-analyses on the IL-1β –31T/C SNP and data from 7
papers were involved in the meta-analyses on the IL-1β -3954C gene polymorphism. Such
overlapping of data creates sufficient difficulties in comparison of these meta-analyses among
each other, although the main trends and relations could be defined. The role of the T allele of
the IL-1β –511 polymorphism may be defined as cancer predictive. The impact of the –31 C
allele remains unclear based on contradictory results. Several epidemiologic studies have
reported that the T allele of the IL-1β –31 SNP is associated with vulnerable to persistent
50
H.pylori infection, which can be modified by smoking (Hamajima et al., 2001), but according
to case–control studies, this allele apparently does not play a role in the development of GC,
even taking into account the fact that some studies confirm this link.
The IL-1β gene encoding IL-1β has two diallelic polymorphisms in the promoter region
at positions -511 and -31, representing C/T and T/C transitions, respectively, in near total linkage
disequilibrium (Hamajima et al., 2001; Machado et al., 2001; Machado et al., 2003). The less
common alleles of these loci (IL-1β -511T and IL-1β -31C) have been found to be associated
with Gastric carcinoma (Bidwell et al., 1999; El Omar et al., 2000; Machado et al., 2003). The
capacity to produce different cytokines varies among different individuals and may be
genetically determined. Such interindividual differences can be attributed to several molecular
mechanisms, including single nucleotide polymorphisms (SNPs) in the functional regions of
cytokine or cytokine receptor genes. These SNPs may affect the overall expression and secretion
of cytokines and may account for some of the heterogeneity of infectious diseases. These two
SNPs were associated with an increased risk of gastric cancer in Scottish and Polish subjects (
El-Omar and Bream 2000) and were subsequently confirmed by studies in other ethnic groups
from the USA (El-Omar et al., 2003) and Portugal (Machado et al., 2003). However, several
other studies failed to demonstrate the correlation (Kato et al., 2001; Zeng et al., 2003). The
diverse ethnic background with different sample size, different environmental exposures
infection in the population and genetic heterogeneity in the pathogenesis of gastric cancer may
account for the variability in different studies. Therefore, whether these polymorphisms of IL-1β
gene were related to the risk of gastric cancer remains inconclusive and needs to be replicated in
ethnically diverse populations.
51
IL-8 polymorphisms and gastric cancer
Interleukin-8 (IL-8), a member of the CXC chemokine family, is a chemoattractant of
neutrophils and lymphocytes. A wide variety of normal and tumour cells could express IL-8, and
the principal role of IL-8 is to initiate and amplify acute inflammatory reactions. Additionally,
growing evidence has shown that the important roles IL-8 may play in the pathogenesis of
cancer, including angiogenesis, tumour growth, and metastasis. The interleukin-8 (IL-8) gene,
located on chromosome 4q13– q21 in humans, is composed of four exons, three introns, and a
proximal promoter region. IL-8enhances cell proliferation and migration and acts like a
chemoattractant for neutrophils and leukocytes, as well as a proangiogenic factor and mediator of
chronic inflammatory processes (Brat et al., 2005). The angiogenic and tumorigenic properties
of IL-8 were proven in experiments with mice long ago (Kitadai et al., 1999), and recent studies
revealed its involvement in adhesion, migration, and invasion in human GC cells in vitro (Ju et
al., 2010). The mucosal levels of IL-8 were reported to be elevated among GC patients in
comparison with healthy individuals, and the prognosis for patients with a high expression of IL8 was significantly poorer compared with that of patients who had a moderate level of this
protein (Kido et al., 2001). IL-8 may stimulate the expression of Reg protein in stomach cells,
which intensifies the proliferation of gastric mucosal cells and may indirectly promote GC
initiation (Yoshino et al., 2005). Moreover, IL-8 was reported to be involved in lung cancer
etiopathogenesis as well as in the initiation and progression of multiple myeloma(Xie JY 2010;
Vetvicka V 2011). Undoubtedly, these observations indicate a carcinogenic role of IL-8 and
underscore the significance of studying its tumorigenic properties. Fifteen functional
polymorphisms were identified in the IL-8 gene locus, and several may alter gene expression
(Hull et al., 2004). These “altering” SNPs were investigated in several case–control studies: IL-
52
8–251A/T (rs4073), IL-8-396T/G (rs2227307), and IL-8-781C/T (rs2227306). First, the A allele
of the IL-8–251A/T polymorphism was associated with elevated IL-8 synthesis and also
contributed to the development of breast and bladder carcinomas (Hull et al., 2004; Lu et al.,
2007; Ahirwar et al., 2010; Snoussi et al., 2010). This allele correlated with increased cancer
risk compared with the TT genotype, according to the study of Lu et al. (Lu et al., 2007) (OR 1.21). Kang et al. (Kang et al., 2009) also observed that the AA genotype of H.pylori. positive
individuals is associated with a higher GC risk (OR - 2.0) in comparison with H pylori.positive
controls. Unfortunately, no data regarding ethnic impact, histopathology, or tumor site were
available. These findings forward a hypothesis that the presence of a high producing allele
dramatically increases the production of the IL-8protein, which may lead to malignant
transformation resulting from the angiogenic and iatrogenic properties of this cytokine.
However, the analysis of Sugimoto et al. (Sugimoto et al., 2010) indicated an absence of
association between A allele carriers and noncardiac GC risk (OR -0.99), whereas peptic ulcer
risk was significantly higher among Western compared with Asian non-GC patients (OR-1.49).
Most studies in the meta-analysis have reported positive associations for Asians and negative
associations for Caucasians. The sample size was approximately equal to that in the study of Lu
et al. (Lu et al., 2007), and discrepancies between these investigations are difficult to explain.
Possible reasons may be differences in ascertainment, study design, and populations, as well as
features of the analysis. The relationship between other polymorphisms of the IL-8 gene (-396
and -781) and GC risk has not been established by the combined analysis of Sugimoto et al.
(Sugimoto et al., 2010), although Savage et al. (Savage et al., 2004) demonstrated a correlation
of the IL-8-251/-396/-781 AGT/ AGC haplotype with a 4-fold increased risk of cardiac GC (OR
- 4.14). Taking into account the fact that the AA genotype of –251 was associated with a 2-fold
53
greater risk of GC (OR - 1.96), the contribution of -396 and -781 polymorphisms is obvious.
Sugimoto et al., (Sugimoto et al., 2010) demonstrates the efficiency of studying the cumulative
effect of haplotype, which may indicate a more sensitive result or reflect a lesser or greater share
of each polymorphism in its total impact. According to the above, the role of IL-8 gene
polymorphisms in GC remains obscure because of contradictory findings. However, the -251
gene polymorphism seems to play a major role in GC development and requires further
investigation.
Alterations in various genetic factors are important in increasing gastric cancer risk (Jung
et al., 2011).IL-8 a chemoattractant of neutrophils and lymphocytes, a wide variety of normal
and tumour cells could express IL-8, and role of IL-8 is to initiate and amplify acute
inflammatory reactions. Growing evidence has shown that the important roles IL-8 may play in
the pathogenesis of cancer, including angiogenesis, tumour growth, and metastasis (Lin et al.,
2010). Number of molecular epidemiological studies have been done to evaluate the association
between IL-8 –251 A/T polymorphism and tumour risk in diverse populations. The tumour types
included gastric cancer, (Taguchi et al., 2005) breast cancer(Kamali et al., 2007) colorectal
cancer, (Cacev et al., 2008) and lung cancer, (Campaet al., 2005). Taguchi and colleagues
reported that the IL-8–251AA genotype of IL-8 was associated with a significantly increased risk
of gastric cancer in a Japanese population (Taguchi et al., 2005); nevertheless, Savage and
colleagues did not find any significant association between –251A/T polymorphism of IL-8 and
gastric cancer in a case–control study based on a Polish population.(Savage et al., 2006). Many
studies have demonstrated the relationship between IL-8 and the risk of GC. IL-8 expression is
also strongly correlated with neovascularization in the tissues from GC patients. The -251 A/T
polymorphism in the IL-8 promoter region has been associated with increased expression of IL-8.
54
Many researchers reported that the -251 A genotype is associated with the risk of GC as well as
antral atrophy and metaplasia compared with the T genotype. Furthermore, the association
between the -251 A genotype and the risk of GC varied according to histological type and tumor
location. Meta analysis done by Liu et al showed
IL-8 -251 A/T polymorphism was not
associated with the risk of GC and the association may be varied when stratified for histological
type, tumor location and ethnicity/country (Liu et al., 2010).
IL-10 polymorphisms and gastric cancer
IL-10 generally functions as an immunosuppressor and anti-inflammatory mediator and is
also known as cytokine synthesis inhibitory factor for its ability to decrease the synthesis of
proinflammatory cytokines by activated T cells and NK cells. IL-10 also blocks the antigenpresenting abilities of macrophages and stimulates the proliferation of B cells, T cells, and mast
cells (Rousset et al., 1992). Many recent in vitro and in vivo investigations observed an impact
of IL-10 on autoimmune diseases and progression of malignancies (Asadullah et al., 2003).
Throughout the past 2 decades intensive genetic association studies have demonstrated
controversial results on polymorphisms in the IL-10 gene, but maintain that they promote cancer
appearance and development one way or another. There are 3 functional promoter SNPs in the
IL-10 locus at –1082 (A to G, rs1800896), –819 (C to T, rs1800871), and –592 (A to C,
rs1800872) pairs from the transcriptional start site. Most studies on these polymorphisms have
investigated haplotypes, but not distinct polymorphisms, which is important. The GCC haplotype
for IL-10 –1082/-819/-592 polymorphisms is associated with higher cytokine production
compared with the ATA haplotype. Alteration of the IL-10 protein level may be directly linked
to the fact that polymorphisms of the IL-10 gene are associated with susceptibility to prostate,
cervical, bladder, and breast carcinoma (Ahirwar et al., 2009; Gerger et al., 2010; Liu et al.,
55
2010; Matsumoto et al., 2010). Furthermore, a high serum level of IL-10 was reported to
contribute to chronic hepatitis C and several autoimmune diseases (Edwards et al., 1999). Wu et
al.(Wu et al., 2003) revealed that the high-producing GCC haplotype of IL-10 -1082/-819/-592
genetic polymorphisms was more frequent among GC patients in comparison with healthy
controls in a Thai population (OR - 2.67). The effect was more evident for cardiac GC (OR 3.21) compared with a noncardiac tumor location (OR- 1.96). A high GC risk was also reported
for smokers and H pylori.-infected carriers of the GCC haplotype, whereas for both smokers and
H pylori.-positive patients the risk was extremely high (OR -3.05, 2.32, and 6.00, respectively).
Sugimoto et al. (Sugimoto et al., 2007) also identified the GCC combination as a risk haplotype
for GC in comparison with the ATA haplotype among H pylori.-positive Japanese individuals
(OR- 2.805). In contrast with previous papers, El-Omar et al. (El-Omar et al., 2003)
demonstrated the hypoactive ATA haplotype to be associated with noncardiac GC (OR -2.5).
Carriers of the low-producing haplotype apparently have a stronger inflammatory response to H
pylori. infection, which may result in the reduction of acid production, atrophy of the stomach,
and progressive growth of neoplastic cells. However, it remains unclear how the high-producing
GCC haplotype correlates with increased cancer risk. Perhaps this correlation may be explained
by the immunosuppressive properties of IL-10 or by the inhibitory effect on other cytokines
involved in antitumor defense. Finally, separate gene polymorphism investigations, among
which are the IL-10 –1082A/G, IL-10 –592A/C, and IL-10 –819C/T polymorphisms,
demonstrated the absence of significant differences in genotype distribution between patients
with GC and healthy controls (Zhou et al., 2008; Chen et al., 2010; Zhuang et al., 2010). In
addition, Kang et al. (Kang et al., 2009) reported that CC carriers of the IL10–592A/C
polymorphism have a decreased risk of intestinal GC (OR -0.4). These findings revealed the
56
advantage of haplotype analysis, which allows to define plausible and consistent results. The
investigations on IL-10 gene polymorphisms allow to suggest that they impact GC (Arseniy,
2011).
Though several genetic changes may occur in gastric cancer, but polymorphisms in the
IL-1, IL-8 and IL-10 genes are the most common and most specific genetic abnormalities in
gastric cancer; this motivated us to study the aberrations of these genetic events in the
etiopathogenesis of gastric cancer.
57