[CANCER RESEARCH 44,1472-1477,
April 1984]
Effect of Dietary Unsaturated and Saturated Fats on Azoxymethaneinduced Colon Carcinogenesis in Rats1
Michitomo Sakaguchi,2 Yoshifumi Hiramatsu, Hideho Takada, Manabu Yamamura, Koshiro Hioki, Kunihiko
Saito, and Masakatsu Yamamoto
Departments of Surgery [M. S., Y. H., H. T., M. Y., K. H., M. Y.¡and Medical Chemistry [K. S.¡,Kansai Medical University, 1 Fumizono, Moriguchi, Osaka, 570, Japan
ABSTRACT
The effects of dietary unsaturated and saturated fats on
chemically induced colon carcinogenesis were examined in male
Donryu rats. The rats were fed two types of semipurified diets
consisting of 5% linoleic acid or 4.7% stearic acid plus 0.3%
essential fatty acid as dietary fats. The rats were treated with
azoxymethane (7.4 mg/kg body weight) s.c. once a week for 11
weeks and sacrificed 15 weeks after the last injection of the
carcinogen. The rats fed unsaturated fat diet demonstrated a
significantly higher incidence of colon tumors [100%], more
tumors per rat [2.68 ± 1.60 (S.D.)], and greater malignant
differentiation histologically than did those fed saturated fat diet
[76%, 1.79 ±1.59, respectively].
Lipid analysis of colon tumors and colon mucosa showed that
unsaturated fat diet altered the phosphatide fatty acyl composi
tion of colon mucosa markedly and increased the content of
arachidonic acid in the neutral lipid of colon tumors. The altered
lipid composition of the mucosa may increase the sensitivity of
the colon to the carcinogen, and the excess of arachidonic acid
or its metabolites may be the primary agents of cocarcinogenesis
of colon tumors. These findings suggest that dietary unsaturated
fats have potent cocarcinogenic effects on colon carcinogenesis.
INTRODUCTION
Epidemiological studies suggest that ingestion of excessive
amounts of fat is an important factor in colon carcinogenesis in
humans (11, 36). In animal studies also, Nigro et al. (32) showed
that a high-beef-fat diet enhanced the carcinogenic effect of
azoxymethane in rats. Broitman ef a/. (6) reported a higher
number of 1,2-dimethylhydrazine-induced
colon tumors in rats
fed a safflower oil diet (neh in polyunsaturated fatty acids) than
in those fed the same concentration of a coconut oil diet (rich in
saturated fatty acids). Although the mechanism by which highfat diets (or polyunsaturated fatty acids) enhance colon carcino
genesis is not known, it has been speculated that high-fat diets
increase bile acid and neutral sterol excretions (20, 30, 35, 37),
alter the intestinal microflora (17, 34, 45), and suppress the
immune system (1). Furthermore, high-fat diets rich in unsatu
rated fatty acids affect phospholipid metabolism in cell mem
branes in mammary carcinogenesis (7,19) and in transplantable
hepatoma (47). In addition, dietary vitamin E (10) and selenium
(22. 43), which suppress the peroxidation of polyunsaturated
fatty acids in cell membranes, inhibit colon carcinogenesis. How
ever, these studies used natural oils that contained other com
ponents at various concentrations. Therefore, it remains uncer
tain whether unsaturated fat is a major factor in colon carcino
genesis.
In this study, we used purified linoleic acid (ds*)3 and stearic
acid (Ci8:o) with a semipurified, fat-free diet and examined the
effects of dietary fats on colon carcinogenesis and the alteration
of lipid metabolism in colon tumors.
MATERIALS AND METHODS
Chemicals. Linoleic acid ethyl ester was kindly provided by Ono
Pharmaceutical Co. (Osaka, Japan). Stearic acid ethyl ester was pur
chased from Nakarai Chemical, Inc. (Kyoto, Japan). Vitamin E was kindly
provided by Eisai Co. (Tokyo, Japan) as DL-a-tocopheryl nicotinate.
Azoxymethane was purchased from Ash Stevens Co. (Detroit, Ml) and
dissolved in 0.9% NaCI solution. SilicAR CC-7 Special was purchased
from Mallinckrodt Chemical Works (St. Louis, MO).
Animal Diets. The basal semipurified, fat-free, and vitamin E-free diets
were made at Oriental Yeast Co. (Tokyo, Japan) in cooperation with
Eisai Co. Two types of 5% fat diets were prepared using a mixing
apparatus in our laboratory and were kept at 4°in a dark depot. The
rats were fed 1 or 2 days after preparation to reduce the possibility of
peroxidation of polyunsaturated fatty acids. Unsaturated fat diet was
made by adding 5% (v/v) linoleic acid ethyl ester and 40 ID vitamin E/kg
diet to the basal diet. Saturated fat diet was made by adding 4.7% (v/v)
stearic acid ethyl ester and 0.3% (v/v) linoleic acid ethyl ester, as an
essential fatty acid, and 40 ID vitamin E/kg diet to the basal diet as
shown in Table 1.
Animals and Treatment Groups. Weanling male Donryu rats weighing
140 to 160 g were purchased from Kitayama Labs, Inc. (Kyoto, Japan).
After a 1-week period of acclimatization, the 7-week-old rats were divided
into 4 experimental groups according to the type of semipurified diet
with or without azoxymethane (7.4 mg/kg body weight). Group 1 was
40 rats fed unsaturated-fat diet. Group 2 contained 40 rats fed saturatedfat diet. In both of these groups, the carcinogen was injected s.c. into
the inguinal region once a week for 11 weeks. The 2 untreated groups
consisted of Group 3, 10 rats fed unsaturated-fat diet (control of Group
1), and Group 4,10 rats fed saturated-fat diet (control of Group 2).
Fifteen weeks after the last injection, the azoxymethane-treated
rats
were killed (25 rats/group). At autopsy, colon tumors were removed,
fixed, sectioned, and examined for histological types. The remaining rats
were killed for lipid analysis of colon tumors and host livers at 20 weeks
after the last injection. The additional 5 weeks were necessary, because
colon tumors were too small for lipid analysis at 15 weeks. The control
groups were also killed at the same time for lipid analysis of control colon
mucosa and control livers.
Lipid Analysis. Total lipids were extracted from tissue samples by
the method of Bligh and Dyer (4) and separated into neutral lipid and
phospholipid fractions by silicic column chromatography, with SilicAR
CC-7 Special used as an absorbent. The efficiency of separation was
examined by thin-layer chromatography. Lipid phosphorus was mea
sured by the method of King (24). Fatty acid methyl esters of each
' Supported by Grant-in-Aid 57570471 for Scientific Research of the Ministry of
Education, Science and Culture of Japan.
2 To whom requests for reprints should be addressed.
3 The fatty acids are designated by chain length and number of double bonds.
Received August 26, 1983; accepted January 9,1984.
For example, C,e ; indicates a fatty acid with 18 carbon atoms and 2 double bonds.
1472
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RESEARCH
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VOL. 44
Dietary Fats and Colon Carcinogenesis
Table 1
Composition per 100 g of semipurified unsatorated- and saturated-fat diet
and also higher numbers of tumors per rat in comparison with
those fed the saturated-fat diet. Grossly, the colon tumors were
Basal
dietCom
polypoid or sessile. The number of tumors in the proximal half
starchVitamin-tree
g25
of the colon in rats fed an unsaturated-fat diet was slightly higher
casein«-Wheat
g10g8g5g692
than in rats fed a saturated-fat diet. No significant difference in
starchPurified
fibrinSucrosePotassiumPhosphorusCalciumSodiumMagnesiaIronZincManganeseCopperIodineVitamin
the average diameter of the tumors was seen between the 2
mg597mg411
mg270mg86mg41
groups. Histologically, rats fed an unsaturated-fat diet showed a
higher number of moderately and poorly differentiated adenocarcinoma but a lower number of well-differentiated adenocarcino-
mas, than did those fed saturated fat diet, significantly (Table 3).
The invasion of tumors into the colon wall was not significantly
different between the 2 groups. The spreading of colon tumors
into regional lymph nodes and into liver occurred in 3 rats fed an
mg7.7mg1000IU200
unsaturated-fat diet but not in those fed a saturated-fat diet. The
incidence and the average number of small intestinal tumors
IU2.4mg8.0mg1.6mg0.001
were 20% and 0.52 ±1.29 (S.D.) in rats fed an unsaturated-fat
diet, respectively, and 12% and 0.40 ± 1.19 in those fed a
saturated fat diet, respectively. These differences are not signif
mg60.0mg10.4mg0.04
icant.
Phospholipid Analysis. Phospholipid contents of protein in
mg0.4
colon tumors and control colon mucosa of the 4 groups were
mg10.0mg10.0mg12.0mg12.0mg4000.0
not different significantly, whereas those in host livers were lower
than those in control livers of both groups fed unsaturated-fat (p
< 0.01) and saturated-fat (p < 0.05) diets (Table 4).
mg0.4mg1.3mg0.08
AVitamin
D2Vitamin
B,Vitamin
B..Vitamin
B,Vitamin
B,2Vitamin
CVitamin
KBiotinFolie
acidCalcium
pantothenatepacidNiacinInositolCholine
Ammobenzoic
chloride39
Addition
Vitamin E
Lmoleic acid
Stearicacid
mg
Fatty Acid Composition of Phospholipid. The fatty acid
compositions of phospholipids in colon tumors, control colon
mucosa, host livers, and control livers in each diet group are
shown in Table 5. In control groups, colon mucosa and livers in
rats fed saturated-fat diet contained less C16:o,CÃ-e*,and C^*
but more Ci6:i, Ci8:0, and Ci8:i compared to these fatty acids in
rats fed unsaturated-fat diets. In those fed saturated-fat diet, the
4IU
5986
0.3g*
4.7g6
'' Unsaturated-fat diet.
b Saturated-fat diet.
fraction were prepared by the method of Rogozinski et al. (39). The
details of these procedures were described in our previous papers (25,
33). Fatty acid methyl esters were analyzed by gas-liquid chromatography (Shimadzu GC-7AG) using a flame ionization detector. The column
was a packed glass tube with 10% Silar 10C on Gas-Chrom Q (100 to
120 mesh), 2 m x 3 mm. The column temperature was kept at 183°for
8 min, then increased to 215° at a rate of 5°/min, and maintained
thereafter. The injection temperature was 270°. Peak areas were cal
culated automatically
with a Shimadzu C-RIA Chromatopac
*
; 300
computer.
Tissue protein was measured by the method of Lowry ef al. (27).
Statistical Analysis. The significance of differences in tumor incidence
was assessed with Fisher's exact test, histopathological differences with
x2 tests, and the other data with Student's f test (3).
Chart 1. Body weight of rats fed unsaturated-fat diet with azoxymethane (•)
or
without azoxymethane (O) and of those fed saturated-fat diet with azoxymethane
(A) or without azoxymethane (A).
RESULTS
General Observations. Some rats in the 4 groups died during
the experimental period. These rats were fully examined, and no
abnormalities were found other than pneumonia. All rats were
weighed weekly. The mean weight gain in all 4 groups was
almost the same (Chart 1). The liver, spleen, and kidneys were
weighed at autopsy, and there were no significant differences
among the 4 groups and no histological abnormalities other than
the spreading of colon tumor cells into the liver in 3 rats treated
with azoxymethane and fed an unsaturated-fat diet.
Tumor Induction. The groups of rats without carcinogen had
no colon tumors. Table 2 summarizes the incidence of colon
tumors in 2 groups treated with azoxymethane. Rats fed the
unsaturated-fat diet had a significantly higher tumor incidence
APRIL
1984
Table 2
Colon tumor yields in rats fed unsaturated-fat diet or saturated-fat diet and treated
azoxymethaneGroupUnsaturated-fat
with
Of
no.
rats with
of co
of
lon tumor
of colon
colon tu
examined
rats2525No. mors25a incidence100 tumors67
tumors/
rats2.68
examined
±1.60*' c
diet
Saturated-fat
76Total
44Colon 1.79 ±1.59
19%
dietNo.
8 Significantly greater than saturated-fat diet group (p < 0.02) by Fisher's exact
test.
8 Mean ±S.D.
c Significantly greater than saturated-fat diet group (p < 0.05) by Student's f
test.
1473
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M. Sakaguchi et al.
fatty acid compositions of colon tumors and control colon mu
cosa were almost identical, but in rats fed unsaturated-fat diet,
saturated, monoenoic, and dienoic fatty acid levels increased,
and polyenoic fatty acid levels decreased. In addition, the fatty
acid composition of phospholipids in host and control livers was
affected by the fatty acid composition of dietary fats with no
remarkable changes being seen between host and control livers
in each diet group.
Fatty Acid Composition of Neutral Lipid. The fatty acid
compositions of neutral lipids in colon tumors, control colon
mucosa, host livers, and control livers in each diet group are
shown in Table 6. The fatty acid composition of neutral lipids,
TabteS
Histology of colon tumors from rats fed unsaturated-fat diet or saturated-fat diet
and treated with azoxymethane
WeM-differentiated adenocar-
Moderately differentiated ade-
Poorly differen
tiated adeno-
Group
cinoma
nocarcmoma
carcinoma
Unsaturated-fat diet
32* (47.8)"
18C(26.9)
17 (25.4)
Saturated-fat diet
33(75.0)
3(6.8)
8(18.2)
* Significantly fewer than saturated-fat diet group (p < 0.01) by x2 analysis.
0 Numbers in parentheses,percentageof tumors/total number of tumors in each
diet'' group.
Significantlygreater than saturated-fat diet group (p < 0.01).
Table 4
Contents of phospholipids in colon tumors, control colon mucosa, host livers, and
control livers in each diet group
mg phosphorus/g protein
Unsaturated-fat
dietColon
.8 ±0.7a
tumors
mucosaSaturated-fat
Control colon
2.4
0.71.8
±
diet
Coton tumors
mucosaUnsaturated-fat
Control colon
±0.0
±0.76.0
1.7
diet
Host livers
liversSaturated-fat
Control
diet
Host livers
Control livers1
* Mean ±S.D. of 3 or more samples.
" Significantly different from controls (p < 0.01).
0 Significantly different from controls (p < 0.05).
compared to phospholipids, was affected by the fatty acid com
position of dietary fats more directly; but in rats fed a saturatedfat diet, the contents of Ci8:i were at least 3-fold higher than
was Cis;o. In rats fed saturated-fat diet, the fatty acid composition
of colon tumors and control colon mucosa were almost identical;
but in rats fed unsaturated-fat diet, the content of C,8 2decreased
and the content of C^* increased markedly.
Variation of Calculated Characteristics of Fatty Acid Com
position. Chart 2 shows the ratio of the sum of the contents of
all polyunsaturated fatty acid with 4 or more double bonds to
the sum of Ci&i, Cien, and Ci8:2 among the 4 experimental
groups. The ratios in colon tumors in rats fed unsaturated- and
saturated-fat diets were 0.59 and 1.00, respectively, and those
in control colon mucosa in each diet group were 1.86 and 1.49,
respectively. In rats fed unsaturated-fat diet, the ratio in colon
tumors was markedly lower than that of control colon mucosa.
The ratios in host livers in rats fed unsaturated- and saturatedfat diets were 1.65 and 2.01, respectively, and the ratios in
control livers in each diet group were 1.96 and 2.01, respectively.
The ratio in host livers in rats fed unsaturated-fat diet was slightly
low compared to the other groups, which were almost identical.
Chart 3 shows the contents of C;»«
in both the phospholipids
and the neutral lipids of colon tumors and in control colon
mucosa. In rats fed an unsaturated-fat diet, colon tumors con
tained high neutral C204 but low phosphatide C-0 4 compared to
control colon mucosa; while in those fed a saturated-fat diet,
colon tumors exhibited levels almost identical to those in control
colon mucosa. The ratio of the contents of phosphatide CM* to
neutral CM* ¡ncolon tumors of rats fed unsaturated-fat diet was
0.54, whereas the ratios in the other groups were at least 1.04.
DISCUSSION
In this study, we prepared semipurified diets so as to include
the same concentration of unsaturated and saturated fatty acid
at low levels (5%), since Reddy ef al. (38) reported that at low
levels of dietary fat (5%) polyunsaturated fat was more effective
in dimethylhydrazine-induced colon carcinogenesis than was sat
urated fat; no differences were found when the level of fat was
20%. Sugano ef a/. (44) found that the concentrations of biliary
cholesterol and bile acids and the fecal excretion of neutral and
acidic steroids were not altered by fat types. Therefore, factors
±0.5"
±1.86.8
7.3
±0.4C
7.5 ±0.2
Tables
Fatty acid composition of phospholipids in colon tumors, control colon mucosa, host livers, and control livers in each diet group
mol%
tumorsFatty
diet24.5
acidCÃ-e«Cl8:1
Colon
mucosaUnsaturated-fat
colon
diet20.1
diet19.5
diet17.6
±0.5**
±0.5"
±2.0
±0.2
5.7 ±0.2"-"
7.5 ±0.7e
9.1 ±0.3
10.4 ±1.1
14.5±0.2"-c
16.5 ±0.8"
18.2 ±1.0
20.0 ±0.6
Clifl
12.2 ±1.1"
16.5 ±0.8"
9.8 ±1.3"
13.4 ±0.3
Cl8:1
11.5±0.9d-e
4.9 ±0.46
8.3 ±1.5"
2.9 ±0.2
ClB.2CmC*»
2.4 ±0.2"-"
1.5
±0.1°
1.2 ±0.1
1.0 ±0.2
9.1 ±0.2o-"
13.2 ±1.4
15.6 ±1.2
13.5 ±1.1
3.8±0.2b'tf
1.0 ±0.2*
1.9 + 0.2
2.3 ±0.4
Cza*
0.4±0.00'"Saturated-fat
0.8 ±0.1Control 0.8 ±0.1Saturated-fat
1.1 ±1.0Host
Co*Unsaturated-fat
* Mean ±S.D. of 3 or more samples.
" Significantlydifferent from controls (p < 0.01).
' Significantly different from saturated-fat diet (p < 0.05).
" Significantlydifferent from saturated-fat diet (p < 0.01).
8 Significantly different from controls (p < 0.05).
1474
liversUnsaturated-fat
liversUnsaturated-fat
diet17.0
±0.8°
1.1 ±0.2
20.1 ±2.2"
6.4 ±0.8°
14.2 ±2.7°
0.7 ±0.1"
diet14.1
diet18.6
±0.8*
1.3 ±0.3"
28.1 ±1.3
10.0 ±1.2
5.6 ±2.0
4.2 ±0.4e
28.2 ±2.1
27.6 ±2.5
4.8 ±1.3
3.1 ±0.9
2.9 ±0.6Saturated-fat
3.3 ±0.6Control
diet12.2
±0.6d
1.4 ±0.2"
19.4 ±0.6"
7.3 ±0.8o
10.7 ±0.2"
0.6 ±0.2"
30.0 ±0.9C
±0.4
2.3 ±0.1
28.3 ±0.3
10.9 ±0.8
3.5 ±0.5
7.1 ±1.5
26.3 ±2.0
5.7 ±0.9
3.8 ±0.8
2.5 ±0.3Saturated-fat
3.0 ±0.5
CANCER
RESEARCH
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VOL. 44
Dietary Fats and Colon Carcinogenesis
Table6
Fatty acid composition of neutral llpids in colon tumors, control colon mucosa, host livers, and control livers in each diet group
mol%
tumorsFatty
mucosaUnsaturated-fat
colon
Colon
acidCl«:O diet1
5.4 ±0.7a'"
2.3 ±0.4"
Ctat
12.2 ±0.5d
ClMc,.:,C,t,
21.5±0.5i>'e
15.2±2.1c'd
diet17.9
diet16.6
diet18.5
±0.7
6.1 ±0.7e
7.5 ±0.4C
1941*1.0°
30.4 ±0.7e
1.4 + 0.2*
12.5 ±2.4
37.0 ±8.5
5.6 ±0.6"
ratio
| colon tumors
I
| host livers
iiversIUnsaturated
roÃ-coioimJCOSl^6»¿¿6ilinrttroi
2.0-1.5
1.0
0.5Y¿¿¿IoonIt
fat
diet
Saturated
fat diet
Unsaturated
fat diet
Saturated
fat diet
Chart 2. Ratio of the sum of the contents of all polyunsaturated fatty acids with
4 or more double bonds divided by the sum of Ci*i, C,8i, and Ciea in the
phospholipid contents of colon tumors, control colon mucosa, host livers, and
control livers in each diet group.
90
30-
20
10
1IIÃŽ&&tumor
Unsaturated
mucosa
fat diet
tumor
Saturated
mucosa
fat diet
Chart 3. Contents of arachidonic acid in the phospholipids and the neutral lipids
in colon tumors and control colon mucosa in each diet group.
other than bile acids are involved in the promoting effects of
unsaturated-fat diets on colon carcinogenesis.
The level of vitamin E in these diets was maintained at a
constant value, because vitamin E-enriched diets reduced the
incidence of dimethylhydrazine-induced
colon tumors in mice,
probably by the radical scavenging ability of this substance (10).
We added vitamin E to each diet at 40 ID/kg diet as a-tocopheryl
nicotinate based on dietary studies showing that adequate vita
APRIL 1984
diet19.9±1.4C
±0.7
8.0 ±0.4
11.3 ±0.6
37.5 ±0.9
4.3 ±0.5
2.7 ±0.4
9.8 ±1.2
10.5 ±0.6
0.6 ±0.3
0.5 ±0.3
0.3 ±0.2Saturated-fat
0.3 ±0.2Host
±1.8
4.8 ±1.8*
2.4 ±0.4
2.9 ±O.r
Coa
16.9 ±0.6"
13.0 ±5.9
Caw
1.1 ±0.0°
0.8 ±0.5
Co*
1.0±0.2"'eSaturated-fat
0.3 ±0.2Control
Co»Unsaturated-fat
" Mean ±S.D. of 3 or more samples.
" Significantlydifferent from controls (p < 0.05).
'•'
Significantlydifferent from saturated-fat diet (p < 0.01).
" Significantlydifferent from controls (p < 0.01).
" Significantly different from saturated-fat diet (p < 0.05).
[
liversUnsaturated-fat
liversUnsaturated-fat
diet23.9
±0.4*
diet21
.6 ±0.1"
4.3 ±0.5
9.2 ±0.8*
41.8 ±3.7*
5.1 ±0.9
7.1 ±1.3°
16.9 ±1.8°
28.2 ±3.4C
0.6 ±0.4C
diet20.6
±0.5
6.1 ±1.3
12.2 ±1.0
34.3 ±1.8
5.6 ±0.9
4.8 ±0.6
2.4 ±0.8
3.4 ±0.7
8.1 ±1.3"
14.4 ±1.9
12.9 ±0.5
0.7 ±0.1
0.5 ±0.3
0.5 ±0.1
0.5 ±0.2
0.2 ±0.2eSaturated-fat
0.9 ±0.3Control 0.5 ±0.1Saturated-fat
0.7 ±0.0
2.4 ±0.2e'"
5.3 ±0.7°
14.2 ±0.0°
36.5±2.1U-C
0.9 ±0.3e
13.710.2o
min E status can be achieved with diets providing an 0.8 ratio of
ID of vitamin E to mg of polyunsaturated fatty acids (16).
For rats fed saturated fatty acid, it was necessary to add 0.3%
(v/v) linotele acid as an essential fatty acid because dietary linoleic
acid, fed in excess of 1% of total calories, maintained good
growth without deficiency of essential fatty acid in rats (29).
Linoleic acid is an essential fatty acid since it cannot be synthe
sized de novo and must be supplied preformed in the diet.
Linoleic acid is the main source of arachidonic acid; the precursor
of prostaglandins is arachidonic acid in cellular phospholipids.
The absorbed dietary fats are converted to neutral lipid which
is utilized to provide energy, and some of them, after being
converted to phospholipid, exhibit important membrane-associ
ated functions in cell membranes. The qualitative change in
dietary fats alters the fatty acid composition of phospholipids of
cell membranes and both the structure and functions of cell
membranes (46). The fluidity and permeability of biological mem
branes is mainly determined by their fatty acid composition of
phospholipids (9,12,15, 23). In our study, rats fed unsaturatedfat diet contained more Ci6:o, CÃ-e«,
and Cao* but less Cien,
Ci8:o, and dai than did rats fed saturated-fat diet in the fatty
acid composition of phospholipids in control colon mucosa and
control livers. These results suggest that the fluidity and perme
ability of cell membranes are increased structurally and function
ally in rats fed unsaturated-fat diet. Recent studies have revealed
that specific changes in the lipid composition of cell membranes,
as a function of nutritional factors, promote tumor growth in the
intact animal tissue (13). Other investigators have reported that
the alteration of properties of liquid-crystalline phases at aqueous
interfaces in cell membranes interferes with the control systems
that maintain the normal behavior of the initiated cell, thus
causing neoplastia proliferation (21). Therefore, in our study, the
sensitivity or permeability of cell membranes to carcinogen might
have been enhanced in rats fed an unsaturated-fat diet.
Numerous investigations on lipid metabolism in a variety of
tumors have demonstrated increases in the ratio of cholesterol
to phospholipid (8), the contents of ether-linked glycerolipid (42)
and monoenoic fatty acid levels, but a decrease in polyenoic
fatty acid levels (2, 40, 41). Recently, Hartz ef al. (18), investi
gating phosphatide fatty acyl compositions in rat transplantable
tumors, observed that the ratio of the sum of the content of
polyunsaturated fatty acids with 4 or more double bonds to the
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M. Sakaguchi et al.
sum of the content of C16.i, Cien, and Ci8:2corresponded directly
with tumor transfer time or inversely with tumor growth for Morris
hepatoma. In our study, this Hartz ratio, both in colon and in
liver, decreased in rats fed unsaturated fatty acid but not in those
fed saturated fatty acid. Therefore, the Hartz ratio may also
reflect the malignancy of colon tumors. This ratio is mainly
affected by the content of arachidonic acid, which is a major
component of polyunsaturated fatty acids with 4 or more double
bonds. If the content of arachidonic acid decreases, this ratio
decreases, enhancing tumor growth. In fact, the content of
arachidonic acid was lower in rats fed unsaturated fatty acid
than in those fed saturated fatty acid in this study. In contrast to
phospholipid, the content of arachidonic acid in the neutral lipid
of colon tumors was high in rats fed unsaturated fatty acid.
Levin ef al. (26) showed that phorbol diesters (tumor pro
moters) stimulate deacylation of the cellular phospholipids of
canine kidney cells radioactively labeled with [3H]arachidonic acid
into prostaglandins. Bresnick ef al. (5) found that phorbol diesters
increase the incorporation of arachidonic acid into mouse skin
and significantly activate epidermal cell membrane phospholipase
A2 without affecting intracellular acid phospholipase. The phos
pholipase A2 inhibitors, in particular, appear to be among the
most potent inhibitors of skin tumor promotion known, and at
least some of the metabolic products from arachidonic acid are
essential for tumor growth (14). Narisawa ef al. (31) showed that
indomethacin, an inhibitor of prostaglandin synthesis, reduces
the development of methylnitrosourea-induced colon tumors in
rats. Moveover, prostaglandins act as cocarcinogens and might
have important implications in mouse cutaneous carcinogenesis
(28). Therefore, it is possible that tumor formation is enhanced
by an excess of arachidonic acid or by some of the metabolic
products from arachidonic acid.
In summary, unsaturated-fat diets may increase the permea
bility and fluidity of cell membranes through the alteration of
phosphatide fatty acyl composition, and the sensitivity or perme
ability of cell membranes to carcinogen may be enhanced. In
addition, the excess of arachidonic acid or its metabolites may
be the primary agents of cocarcinogenesis of colon tumors.
Further studies are needed to clarify the role of arachidonic acid
in tumor formation and tumor proliferation.
ACKNOWLEDGMENTS
The authors wish to thank Dr. M. Ogura, Department
University, for his help in the histológica! evaluation.
of Pathology of this
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Effect of Dietary Unsaturated and Saturated Fats on
Azoxymethane-induced Colon Carcinogenesis in Rats
Michitomo Sakaguchi, Yoshifumi Hiramatsu, Hideho Takada, et al.
Cancer Res 1984;44:1472-1477.
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