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The Journal of Clinical Endocrinology & Metabolism 89(8):3629 –3643
Copyright © 2004 by The Endocrine Society
doi: 10.1210/jc.2004-0405
HOT TOPIC
Islet Amyloid: A Critical Entity in the Pathogenesis of
Type 2 Diabetes
REBECCA L. HULL, GUNILLA T. WESTERMARK, PER WESTERMARK,
AND
STEVEN E. KAHN
Division of Metabolism, Endocrinology and Nutrition, Department of Medicine, Veterans Affairs Puget Sound Health Care
System and University of Washington (R.L.H., S.E.K.), Seattle, Washington 98108; Division of Cell Biology, Linköping
University (G.T.W.), Linköping, Sweden; and Department of Genetics and Pathology, Rudbeck Laboratory, Uppsala
University (P.W.), Uppsala, Sweden
Islet amyloid deposition is a pathogenic feature of type 2 diabetes, and these deposits contain the unique amyloidogenic
peptide islet amyloid polypeptide. Autopsy studies in humans
have demonstrated that islet amyloid is associated with loss
of ␤-cell mass, but a direct role for amyloid in the pathogenesis
of type 2 diabetes cannot be inferred from such studies. Animal studies in both spontaneous and transgenic models of
islet amyloid formation have shown that amyloid forms in
islets before fasting hyperglycemia and therefore does not
arise merely as a result of the diabetic state. Furthermore, the
extent of amyloid deposition is associated with both loss of
␤-cell mass and impairment in insulin secretion and glucose
metabolism, suggesting a causative role for islet amyloid in
the islet lesion of type 2 diabetes. These animal studies have
also shown that ␤-cell dysfunction seems to be an important
prerequisite for islet amyloid formation, with increased secretory demand from obesity and/or insulin resistance acting
to further increase islet amyloid deposition. Recent in vitro
studies suggest that the cytotoxic species responsible for islet
amyloid-induced ␤-cell death are formed during the very early
stages of islet amyloid formation, when islet amyloid polypeptide aggregation commences. Interventions to prevent islet
amyloid formation are emerging, with peptide and small molecule inhibitors being developed. These agents could thus
lead to a preservation of ␤-cell mass and amelioration of the
islet lesion in type 2 diabetes. (J Clin Endocrinol Metab 89:
3629 –3643, 2004)
I
General features of amyloid deposits
SLET AMYLOID IS a pathological hallmark of the pancreatic islet present in a substantial proportion of individuals from all ethnic groups with type 2 diabetes (1–5) (Fig.
1). Islet amyloid deposits were first described in diabetes
more than a century ago (6, 7), but due to the extreme insolubility of these deposits, further study and analysis of the
nature of islet amyloid were hampered by the inability to
extract and characterize its constituents. However, insight
into the mechanism(s) underlying the formation of islet amyloid and its contribution to the pathogenesis of type 2 diabetes has made great strides since we (8, 9) and others (10)
in 1986 and 1987 successfully extracted and determined the
amino acid sequence of the unique constituent peptide of
islet amyloid. This peptide is islet amyloid polypeptide
(IAPP) and is also known as amylin. This review will highlight 1) factors that may underlie islet amyloid deposition, 2)
recent evidence for a causative role of islet amyloid in the
pathogenesis of type 2 diabetes, and 3) new approaches
aimed at preventing the formation of islet amyloid, the goal
being to ameliorate the loss of islet ␤-cell mass and function
that characterizes type 2 diabetes.
Abbreviations: GAG, Glycosaminoglycan; IAPP, islet amyloid
polypeptide; SAP, serum amyloid P component.
JCEM is published monthly by The Endocrine Society (http://www.
endo-society.org), the foremost professional society serving the endocrine community.
Amyloid deposits occur in association with several distinct
diseases and can be classified in two general forms, systemic
and localized. Amyloid deposits comprise fibrils formed
from one of more than 20 different amyloidogenic precursor
proteins, and it is these unique fibrillogenic proteins that
differentiate the various forms of amyloid that develop in a
variety of tissues and in association with numerous diseases
(11, 12).
Systemic amyloidoses, as the name suggests, involve deposition of a circulating amyloidogenic precursor protein in
several different organs, including the heart, liver, and kidney. The most common examples of systemic amyloidoses
are the following. Primary systemic amyloidosis, resulting
from the deposition of a monoclonal Ig light chain, occurs in
almost all disorders of B lymphocyte lineage (13). Secondary
or AA amyloidosis is associated with diseases of chronic
inflammation, such as tuberculosis and rheumatoid arthritis,
and occurs through accumulation of an N-terminal fragment
of the acute phase protein serum amyloid A (14). Finally, the
ATTR amyloidoses, senile systemic amyloidosis and familial
transthyretin-associated amyloidosis, arise due to the deposition of wild-type or one of more than 50 mutated forms
of transthyretin, the most common being the point mutation
Val30Met (15, 16).
Localized amyloidoses comprise a number of diseases
characterized by the deposition of amyloid in a target organ.
Usually, the production of the amyloidogenic peptide occurs
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Hull et al. • Hot Topic
FIG. 1. A, Islet amyloid deposition in an islet from an individual with type 2 diabetes stained with Congo Red and viewed under partially
cross-polarized light. Islet amyloid is visible as areas of pink/brown staining and apple green birefringence. B, A pancreatic islet from an individual
with type 2 diabetes showing islet amyloid deposition by thioflavin S staining (green) and residual ␤-cells with insulin immunostaining (red). C, No
amyloid is present in a pancreatic islet from a nondiabetic individual; insulin immunostaining shows ␤-cells (red). D, Islet amyloid formation visualized
by thioflavin S staining (green) in a human IAPP transgenic mouse fed a high-fat diet for 1 yr; insulin immunostaining shows ␤-cells (red). E, Islet
from a nontransgenic mouse also fed a high-fat diet for 1 yr, showing insulin immunostaining (red), but no islet amyloid.
proximal to the site of amyloid formation. Common examples include type 2 diabetes, where IAPP is deposited in the
pancreatic islets (6, 8, 10, 17); Alzheimer’s disease, which is
characterized by cerebrovascular and cortical accumulation
of A␤ (18 –20); and medullary thyroid carcinoma, which is
associated with the deposition of (pro)calcitonin as amyloid
fibrils (21). Despite thorough and careful characterization of
the nature of amyloid deposits in systemic and localized
amyloidoses, the mechanism(s) underlying the formation of
amyloid fibrils from normally soluble precursors remains
essentially unknown.
Several structural features define all amyloid fibrils, regardless of their constituent protein. Electron microscopy has
shown amyloid fibrils to be unbranching structures, 5–10 nm
in diameter and of indeterminate length (22). Within these
fibrils, the amyloidogenic protein assumes a predominantly
␤-sheet structure, giving rise to extensive hydrogen bonding
along the length of the fibril and generating the characteristic
cross-␤ x-ray diffraction pattern that has been observed for
numerous types of amyloid fibril (23–25). This characteristic
and ordered structure permits specific binding of amyloid
fibrils by histological stains, including thioflavin S and thio-
flavin T, which can be visualized by fluorescence microscopy
(26 –29), and Congo Red, which, when viewed under crosspolarized light, appears as apple green birefringence (27, 30)
(Fig. 1). This invariant structure shared by all amyloid fibrils
may provide a common mechanism by which amyloid contributes to disease pathogenesis through cell toxicity and
death. In fact, it has been recently demonstrated that small,
cytotoxic aggregates from a number of amyloidogenic precursor proteins assume a common structural conformation
regardless of amino acid sequence, thus providing a potential
therapeutic target for general antiamyloid therapies (31).
Rather than classical amyloid fibrils, these cytotoxic aggregates are soluble and appear to represent a very early intermediate in the pathway of amyloid fibril formation (32). This
is consistent with a long-standing model for the progression
of amyloid formation proposed based on electron microscopy studies of amyloid fibrils (33). Soluble aggregates of
amyloidogenic peptides are likely precursors of subprotofibrils or represent subprotofibrils themselves. These assemble
to form protofibrils, then fibrils, and ultimately organized
deposits (Fig. 2). Thus, amyloid deposition represents a continuum from early cytotoxic aggregates to mature collections
Hull et al. • Hot Topic
J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643 3631
FIG. 2. Model for islet amyloid fibril formation and cytotoxicity. A, In normal individuals, IAPP is present in its native conformation.
Amyloidogenic amino acid regions 8 –20, 20 –29, and 30 –37 are shown (䊐, f, and , respectively). B, An alteration in the folding and or trafficking
mechanisms within ␤-cell leads to the misfolding of IAPP, which forms an intramolecular ␤-sheet according to the model of Jaikaran and Clark
(108). C and D, Assembly of these misfolded molecules leads first to the formation of soluble aggregates that are cytotoxic (31, 32, 34) (C) and
then to protofibrils (D); both species are visible by electron microscopy. Once protofibrils are formed, a more rapid phase of islet amyloid formation
ensues, first leading to amyloid fibril formation visible in vitro (E) and extracellularly in vivo (Am in F) by electron microscopy and finally to
classical light microscopy-visible amyloid (G). Panels C–E reprinted with permission from Porat et al.: Biochemistry 42:10971–10977, 2003 (32).
Copyright © (2003) American Chemical Society.
of fibrils, with the early aggregates conveying the cytotoxicity associated with amyloid (31, 32), and classical light
microscopy-visible amyloid deposits representing the end
stage of the process and being less cytotoxic (34).
Islet amyloid polypeptide synthesis, secretion, and function
In the case of islet amyloid, IAPP is the unique amyloidogenic precursor peptide. IAPP is a normal product of the
pancreatic islet ␤-cell and is stored along with insulin in
secretory granules (35–38). Like insulin, IAPP is derived from
a larger propeptide precursor, preproIAPP, that in humans
is an 89-amino acid peptide that contains an amino-terminal
signal sequence, consistent with this being a secreted peptide
(39 – 41). Once the signal peptide is removed, the 67-amino
acid propeptide proIAPP is enzymatically cleaved to the
mature 37-amino acid peptide IAPP by the prohormone convertases PC1/3 and PC2 (42– 45), which are also responsible
for proteolytic conversion of proinsulin to insulin (46, 47).
Additional posttranslational modifications include formation of a disulfide bridge between cysteine residues at positions 2 and 7 and amidation of the C-terminal tyrosine (48).
The release of IAPP from the ␤-cell occurs in response to
nutrient stimuli, such that its secretion closely mirrors that of
insulin (49 –53). In the fasting state, IAPP levels are 10 –15%
those of insulin (54 –56). The plasma clearance rates of IAPP
and insulin differ, with IAPP being cleared more slowly than
insulin and at a comparable rate to C peptide. This is consistent with the fact that IAPP, like C peptide is cleared
through the kidney (57). This difference in clearance rates of
IAPP and insulin contributes to the fasting IAPP/insulin
ratio being higher than that observed shortly after the release
of these peptides from the ␤-cell. Thus, IAPP plasma levels
immediately after acute stimulation of ␤-cell peptide release
with glucose or nonglucose secretagogues are closer to 1%
that of insulin (55, 58 – 60). Because in the stimulated state the
impact of differences in peptide clearance on the plasma level
is diminished, the lower molar IAPP to insulin ratio under
these conditions is probably a better reflection of the proportions of these two peptides that exists within the ␤-cell
secretory granule in humans. This lower ratio is, in fact, in
keeping with in vitro data for IAPP and insulin content in
human islets (61).
In humans, the factors known to affect insulin secretion
also appear to be important when considering IAPP secretion. For example, the importance of insulin sensitivity as a
modulator of insulin secretion has been well established (62).
Similarly, insulin sensitivity has been shown to be an important modulator of IAPP release by the ␤-cell (58, 60). Thus,
IAPP levels are elevated in conditions associated with insulin
resistance, such as obesity (53, 55, 63) and pregnancy (64).
Conversely, in disease states associated with reduced ␤-cell
peptide release, namely impaired glucose tolerance and both
type 1 and type 2 diabetes, IAPP release has been shown to
be diminished in response to both oral and iv stimulation,
paralleling the reduction in insulin release (50, 51, 53, 59,
65– 67). Furthermore, first degree relatives of individuals
with type 2 diabetes and older individuals, both groups at
increased risk of developing type 2 diabetes, manifest both
decreased IAPP and insulin responses when presented with
an iv glucose challenge (58, 60). In individuals with impaired
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J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643
glucose tolerance as well as in those with type 2 diabetes,
IAPP release in response to an oral glucose load is decreased.
This abnormality is most evident during the first 30 min after
glucose administration, when clearance rates have less impact on plasma IAPP levels (53, 59, 65).
As described above, the synthesis and secretion of IAPP by
␤-cells have been well studied. In contrast, the physiological
function of the peptide remains largely unclear. In keeping
with the role of IAPP as an islet hormone, some of the first
functions ascribed to the peptide were related to glucose
metabolism: suppression of insulin-mediated glucose uptake
in skeletal muscle (68, 69) and inhibition of glucose-stimulated insulin secretion (70 –72). In addition, IAPP suppresses
glucagon release from isolated islets (73). In support of these
findings, male IAPP knockout mice show increased insulin
secretion and enhanced glucose clearance compared with
mice with normal IAPP levels (74). However, many in vitro
studies examining these issues required supraphysiological
doses of IAPP for their effects to be observed, and the findings were not replicated in all studies. Some further insight
comes from studies in humans using the nonamyloidogenic
human IAPP analog, pramlintide. Injection of pramlintide at
near-physiological and supraphysiological levels reduces
gastric emptying (75), thereby decreasing postprandial hyperglycemia in individuals with type 1 diabetes (76, 77) who
are, by nature of their disease process, IAPP deficient. This
effect may be mediated by vagal inhibition (78).
A role for IAPP in the regulation of food intake and body
weight has also been suggested by a number of groups.
Central or peripheral administration of IAPP in rats is associated with reduced food intake (79 – 82); the specificity of
this action of IAPP is demonstrated by increases in food
intake and body weight when an IAPP antagonist is administered intracerebroventricularly (83). Consistent with this
finding is the localization in rodents of IAPP-binding sites in
the nucleus accumbens, area postrema, nucleus of the solitary tract, and various hypothalamic regions that are well
known to mediate food intake and body weight (84 – 86).
Other effects described for IAPP include regulation of renal
filtration (87, 88) with IAPP-binding sites being localized to
the kidney (89), calcium homeostasis (90, 91), and vasodilatation (92, 93). Typically, the characterization of specific binding site aids in the identification of a receptor and the elucidation of the physiological role of a substrate. For IAPP, the
closest evidence to date for the existence of an IAPP receptor
comes from several studies showing that IAPP exerts physiological effects through either the calcitonin receptor, modified by receptor activity-modifying protein 1 or 3 (94 –96), or
calcitonin gene-related peptide receptor 1 (97–99).
Although a definite physiological function(s) has yet to be
clearly ascribed to IAPP, its fundamental role in the formation of islet amyloid in the pancreas of individuals with type
2 diabetes has been clearly defined. In this manner, this islet
peptide plays a role in the pathogenesis of the islet ␤-cell
dysfunction observed in type 2 diabetes.
Factors affecting amyloidogenicity of IAPP
Despite the fact that the amino acid sequence of IAPP is
greater than 80% homologous across mammalian species,
Hull et al. • Hot Topic
only humans, nonhuman primates, and cats express a form
of IAPP capable of forming amyloid fibrils, whereas rodent
IAPP is nonamyloidogenic. Almost all of the species-specific
differences in IAPP amino acid sequence occur between residues 20 and 29; the residues in this central portion of human
IAPP are critical for the formation of antiparallel ␤-pleated
sheets and thus amyloid fibrils (41, 100). Three proline residues occur within this region of rodent IAPP (rat and mouse
IAPP are identical at the amino acid level). Proline is a ␤-sheet
breaker; thus, the presence of three prolines in this critical
region of IAPP is thought to preclude the formation of
␤-sheets necessary for amyloid fibril formation and accounts
for the lack of amyloidogenicity of rodent IAPP (100). This
has been confirmed by substituting amino acids in this critical 20 –29 region, the result being alterations in the amyloidogenicity of human IAPP (100 –102). A mutation in this
region of IAPP (S20G) has been described in a cohort of
individuals with early-onset type 2 diabetes in Japan (103).
Interestingly, in vitro this mutated IAPP is more aggressively
amyloidogenic and cytotoxic than wild-type IAPP (104, 105).
Two other potentially amyloidogenic regions of IAPP have
also been described: the carboxyl-terminal 30 –37 region (106)
and amino acids 8 –20 (107). The 30 –37 amino acid sequence
is identical for human and rodent IAPP, whereas one amino
acid substitution exists between human and rodent IAPP in
the 8 –20 amino acid region: residue 18 is histidine in human
IAPP, but is arginine in monkey, cat, and rodent IAPP. This
histidine for arginine substitution does not influence the
amyloidogenicity of the 8 –20 IAPP sequence (107). Thus, the
propensity of these regions of IAPP to form amyloid fibrils
is present in both amyloidogenic human IAPP and the normally nonamyloidogenic rodent IAPP. In humans, the amyloidogenic regions of IAPP 8 –20 and 30 –37 are linked by the
aggressively amyloidogenic 20 –29 amino acid region, thus
allowing the formation of an intramolecular ␤-sheet, as proposed in the model described by Jaikaran and Clark (108)
(Fig. 2B). However, in rodents, the amyloidogenic 8 –20 and
30 –37 regions are linked by the 20 –29 sequence, which contains several proline residues. Thus, an intramolecular
␤-sheet conformation cannot form in rodent IAPP, and this
may be the reason for the lack of islet amyloid formation in
rats and mice (108). Taken together, these data show that an
amyloidogenic amino acid sequence is an absolute prerequisite for islet amyloid formation to occur. How, then, is it
that the vast majority of humans do not deposit islet amyloid
in the absence of diabetes?
Because IAPP must assume a ␤-sheet conformation to form
amyloid fibrils, it seems logical that a structural change must
occur in the peptide during amyloidogenesis. The first evidence for such an alteration was the demonstration that
immunoreactivity of IAPP to a monoclonal antiserum occurred only when the IAPP was in its native conformation
within the ␤-cell, but not when it existed in an altered conformation in islet amyloid deposits (109). These data have
recently been supported by the finding that IAPP, in common
with other amyloidogenic peptides, forms soluble oligomers
with a structure distinct from monomeric IAPP and common
to other amyloidogenic peptides (31) (Fig. 2C). These soluble
oligomers represent an early intermediate in the amyloid
fibril formation pathway and constitute the cytotoxic form of
Hull et al. • Hot Topic
amyloidogenic peptides that may therefore be responsible
for amyloid fibril-induced cell death (31, 32).
Ordinarily, IAPP does not form amyloid fibrils, suggesting
that mechanisms must exist within the ␤-cell that maintain
IAPP in a monomeric form. Typically, the pH and calcium
concentration within the ␤-cell secretory granule are tightly
controlled to allow the correct trafficking and maturation of
insulin and IAPP to occur. Alterations in either or both secretory granule pH and calcium concentration have been
shown to alter IAPP fibril formation, suggesting that the
normal granule environment keeps IAPP in a soluble, nonfibrillar form (110 –112). Furthermore, transgenic mice that
express amyloidogenic human IAPP in their islet ␤-cells, but
lack endogenous mouse IAPP, develop more severe islet
amyloid deposits than mice that express both forms of IAPP,
suggesting the endogenous mouse IAPP may protect against
amyloid fibril formation (113). In addition, in vitro studies
have suggested that the normal molar ratio of IAPP to insulin, proinsulin, and C peptide protects against IAPP fibril
formation (111, 112, 114). Insulin and IAPP can form a stable
complex that prevents IAPP from assuming the ␤-sheet
structure necessary for islet amyloid formation to occur (115,
116). Conversely, changes in any of these granule components, which may possibly occur with the ␤-cell dysfunction
seen in type 2 diabetes, may result in the fibrillogenesis of
IAPP (110 –112).
Amyloid formation can also be reduced by degradation
or clearance of the amyloidogenic peptide (117). Insulindegrading enzyme or insulysin is a cellular enzyme that was
named as such because it degrades insulin (118, 119) and has
recently been implicated in the breakdown of other amyloidogenic peptides (117, 120), thus providing a mechanism for
the normal clearance of these fibrillogenic precursors (121).
IAPP has also been shown to be a substrate for insulindegrading enzyme (122), suggesting that normal intracellular clearance of IAPP may occur by the action of enzymes
such as insulin-degrading enzyme. Inhibition of insulindegrading enzyme with bacitracin, a protease inhibitor
known to block insulin degradation, resulted in increased
amyloid fibril formation and cytotoxicity in islet ␤-cell lines
treated with exogenous human IAPP (123). This finding provides evidence that impaired IAPP clearance may be a factor
in the formation of islet amyloid.
Because islet amyloid formation in humans is so closely
associated with type 2 diabetes, it seems likely that the altered islet milieu may provide conditions that promote the
amyloidogenesis of IAPP. Type 2 diabetes is associated with
chronically elevated glucose and free fatty acids, both of
which have been demonstrated to enhance amyloid fibril
formation, albeit by different processes. One result of chronic
hyperglycemia in combination with oxidative stress is the
formation of advanced glycation end products. Evidence
exists for the glycation of IAPP in type 2 diabetes (109), and
advanced glycation end product-modified IAPP is more aggressively amyloidogenic than the unmodified peptide (124).
As discussed below in more detail, human IAPP transgenic
mice, a model of islet amyloid deposition, develop progressively more amyloid when fed diets containing increasing
amounts of dietary fat (125). In addition, culture of human
IAPP transgenic mouse islets in the presence of free fatty
J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643 3633
acids leads to acceleration of the formation of IAPP-immunoreactive fibrils, and incubation of human IAPP with free
fatty acids in vitro markedly enhances fibril formation (126).
The disproportionate release of proinsulin relative to mature insulin from the islet is a well described feature of type
2 diabetes (127–130) that is already evident in nondiabetic
individuals who are at high risk of developing the disease
(131, 132). Because insulin and IAPP are both derived from
their propeptides by the actions of the enzymes PC1/3 and
PC2 (42– 44), it is possible that inefficient processing of
proIAPP to IAPP could occur in type 2 diabetes, as has been
demonstrated to occur with proinsulin to insulin conversion
in type 2 diabetes (110, 130). Because proIAPP itself is amyloidogenic (Fig. 3A), it is possible that increased levels of
proIAPP in diabetes may promote islet amyloid formation
(133, 134); the presence of proIAPP or an incompletely processed intermediate in islet amyloid deposits from type 2
diabetes has been demonstrated using antisera raised against
the N-terminal peptide of proIAPP (135) (Fig. 3B).
These data therefore suggest that inefficient proteolytic
conversion of proIAPP to IAPP may occur in type 2 diabetes,
and an increase in proIAPP levels may contribute to islet
amyloid deposition.
The site of the initiation of amyloid formation is still unclear. Autopsy studies of human pancreas have indicated
that deposition of islet amyloid is always an extracellular
event. However, studies in human islets transplanted into
nude mice (136, 137) and in islets of human IAPP transgenic
mice (135) have indicated that the early stages of islet amyloid formation may take place intracellularly. These intracellular aggregates are immunoreactive for IAPP propeptides (135), underlining the possibility that insufficiently
processed proIAPP may be important in early intracellular
amyloid formation. It is possible that these intracellular aggregates of (pro)IAPP may act as a nidus to which mature
IAPP then associates, leading to more rapid and extracellular
amyloid deposition.
Taken together, these data allow us to propose a working
model for islet amyloid formation, as outlined in Fig.2. In a
normal, functioning ␤-cell, IAPP is synthesized, processed,
and secreted from the ␤-cell along with insulin and does not
accumulate as amyloid fibrils (Fig. 2A). However, when
␤-cell dysfunction is present, as occurs in and before the
onset of type 2 diabetes (138), protein folding and/or trafficking in the ␤-cell are likely to be impaired. Misfolding of
proIAPP in the endoplasmic reticulum and/or reduced processing of proIAPP in the secretory granule are possible
manifestations. The resulting misfolded and/or unprocessed
(pro)IAPP present in secretory granules would be released
from the cell along with insulin and, outside the cell, would
be exposed to an altered chemical environment (increased
pH and decreased calcium concentration) as well as other
molecules (such as heparan sulfate proteoglycans, as discussed below) that could elicit a further structural change in
the peptide and initiate fibril formation (Fig. 2, B–D). In
addition, the presence of misfolded (pro)IAPP in secretory
granules may cause the granule contents to be targeted to the
lysosome for degradation, because the lysosomal system is
responsible for the removal of excess or misfolded peptides,
such as IAPP and insulin. Misfolded (pro)IAPP appears to be
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J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643
Hull et al. • Hot Topic
FIG. 3. A, Amyloidogenicity of human proIAPP demonstrated by the presence of amyloid-like fibrils in a preparation of synthetic human
proIAPP stained with phosphotungstic acid and viewed by electron microscopy. B, Immunoreactivity for proIAPP in islet amyloid in a subject
with type 2 diabetes. Panel B reprinted from Diabetes Res Clin Pract, Vol. 7, Westmark et al., Islet amyloid polypeptide (IAPP) and pro-IAPP
immunoreactivity in human islets of Langerhans, pp 219 –226, Copyright © 1989, with permission from Elsevier.
resistant to this normal degradative process, as demonstrated by the presence of proIAPP and IAPP aggregates in
␤-cell lysosomes in pancreas sections from humans with type
2 diabetes and human IAPP transgenic mice (139). Thus, it is
possible that fibril formation could also commence intracellularly due to aggregation of (pro)IAPP in the lysosome, with
the nascent fibrils being released into the extracellular space
upon cell death (Fig. 2, B–D). Once formed, these fibrils,
which originate either within or outside the ␤-cell, provide
the “seed” required to facilitate a second stage of rapid amyloid fibril accumulation. This second stage has been well
documented in vitro (Fig. 2E) (124, 140) and would, in the
process of further fibril formation, induce cytotoxicity from
the exterior of the ␤-cell via disruption of the plasma membrane (32, 34, 141–143) and result eventually in the formation
of classical amyloid deposits visible in vivo (Fig. 2, F and G).
Other components of islet amyloid deposits
Amyloid deposits contain, in addition to their unique fibrillogenic peptide, other components that include apolipoprotein E, the heparan sulfate proteoglycan perlecan, and
serum amyloid P component (SAP) (4, 144 –149). Histochemical studies have shown the presence of these components in
the islet amyloid deposits found in humans with type 2
diabetes (4, 145, 146, 149) and in human IAPP transgenic mice
(150). These findings are similar to those in cerebrovascular
amyloid plaques in Alzheimer’s disease (144, 147, 148) and
in amyloid deposits associated with other diseases (144, 148).
Although these other proteins are clearly constituents of
amyloid deposits, their role as causative factors in amyloi-
dogenesis appear to vary. Transgenic mice lacking SAP show
reduced amyloid deposition in a model of AA amyloidosis,
suggesting that SAP may mediate the extent of amyloid
deposition. However, it is not absolutely required for AA
amyloid formation, because amyloid deposition was not entirely prevented in the absence of SAP (151). Apolipoprotein
E is a good example of a component whose importance in
amyloid deposition differs with the type of amyloid studied.
An association between apolipoprotein E and Alzheimer’s
disease is well established. The ⑀4 genotype has been shown
to be associated with an earlier onset of Alzheimer’s disease
and increased cortical amyloid deposition (152). In keeping
with these observations, cross-breeding of a transgenic
mouse model of A␤-derived amyloid in the brain with apolipoprotein E knockout mice resulted in a marked reduction
or absence of amyloid deposition in offspring lacking one or
both apolipoprotein E alleles, respectively (153, 154). In sharp
contrast, no relationship between the apolipoprotein E genotype and the prevalence or age of onset of human type 2
diabetes has been demonstrated (155, 156). Furthermore, in
our human IAPP transgenic mice that were cross-bred so that
they lacked apolipoprotein E, there was no decrease in islet
amyloid deposition among mice that completely lacked apolipoprotein E and their littermates with either one or both
copies of the apolipoprotein E allele (150). These findings
suggest that apolipoprotein E is not required for islet amyloid deposition, which contrasts with A␤ amyloid, where the
presence of apolipoprotein E appears to be critical (153, 154).
Thus, although there are potentially important similarities
between the different forms of localized amyloid, the role of
Hull et al. • Hot Topic
apolipoprotein E in the pathogenesis of islet amyloid in type
2 diabetes highlights the fact that differences may exist, in
this case with A␤ amyloid, as occurs in Alzheimer’s disease.
Interestingly, although apolipoprotein E immunoreactivity
is clearly present in islet amyloid deposits, it is not detected
in normal islets at either the protein or mRNA level (150),
suggesting that rather than being synthesized locally in the
islet, circulating apolipoprotein E may be trapped in islet
amyloid during the formation of these deposits. This is also
thought to be similar to the situation with SAP, which is
synthesized exclusively in the liver (157).
In contrast to apolipoprotein E and SAP, another class of
amyloid constituents, the heparan sulfate proteoglycans are
present in islet amyloid deposits (4, 146), and we have recently shown that they are synthesized and secreted by islet
␤-cells (158). These ␤-cell-derived heparan sulfate proteoglycans are capable of binding amyloidogenic human, but
not nonamyloidogenic rodent IAPP (158), an observation
that has also been made for the heparan sulfate proteoglycan
perlecan derived from other cellular sources (159, 160). This
direct interaction with human IAPP is thought to occur
through the highly sulfated and negatively charged glycosaminoglycan (GAG) side-chains of proteoglycans (160, 161),
which can interact with basic amino acid motifs within the
amyloidogenic peptide. Upon proteoglycan binding, an acceleration of and an increase in total IAPP amyloid fibril
formation are observed, providing compelling evidence for
a role for heparan sulfate proteoglycans in islet amyloid
formation (159, 160). As proIAPP also contains a linear glycosaminoglycan binding sequence and is capable of binding
heparan sulfate GAGs specifically and with high affinity, an
interaction between proIAPP and heparan sulfate proteoglycans could also play a role in islet amyloid formation
(162). Studies using serum amyloid A, the precursor for AA
amyloid, provide a possible mechanism by which proteoglycans may stimulate amyloid fibril formation. Interaction
between proteoglycans and serum amyloid A is associated
with an increase in the ␤-sheet structure of the amyloidogenic peptide (163), a prerequisite for fibrillogenesis.
Evidence for a role for islet amyloid in decreased ␤-cell
mass and function
The association between amyloid fibril formation and disease pathogenesis is thought to be common for all amyloidoses and to occur by induction of cell death. The cytotoxic
properties of amyloidogenic peptides, including IAPP, are
well documented (164 –167). An amyloidogenic form of IAPP
is required for the cytotoxic effect to occur, with human
IAPP, but not the nonamyloidogenic rat IAPP, resulting in
increased cell death when incubated with isolated islets or
islet cells (34, 141, 167, 168). The primary form of IAPPinduced cell death is apoptosis; morphological changes in
cells after human IAPP treatment are well documented and
are consistent with this mechanism (34, 167, 169). In addition,
IAPP administration is associated with the up-regulation of
proapoptotic genes, c-fos, fosB, c-jun, and junB (170), and
increased expression of apoptotic markers, such as p53 and
p21 (171).
The most potent cytotoxic effect of human IAPP occurs
J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643 3635
soon after peptide reconstitution, when IAPP is in small
oligomeric aggregates (34, 142, 143). This is consistent with
the observation that several amyloidogenic peptides, including IAPP, form soluble oligomers that share a common structural configuration and are cytotoxic (31). These
IAPP oligomers produced in vitro probably represent the
precursors of amyloid deposition and may represent precursors of amyloid fibrils that form the first of a two-stage
process that is hypothesized to constitute amyloid formation in vivo (124, 140). This first stage is a nucleation or
seeding process that is thought to progress relatively
slowly. Once a critical mass of fibrils has formed, a second,
exponential phase of fibril deposition ensues, leading to
the formation of classical, light microscopy-visible amyloid deposits (Fig. 2G). These mature amyloid deposits are
less cytotoxic than the small aggregates (34, 142, 143) and
probably represent a space-filling lesion that progressively replaces ␤-cell mass (172) and possibly acts as a
diffusion barrier, thereby compounding the loss of ␤-cells
that probably occurred by apoptosis during the early
stages of amyloid development.
Consistent with the increased cell death associated with
small aggregates of human IAPP, prefibrillar assemblies of
human IAPP have been shown to be the most active molecular form of the peptide in binding and disruption of lipid
bilayers in vitro (32). Disruption of the plasma membrane
leading to the formation of ion-permeable pores is a possible
mechanism by which IAPP and other amyloidogenic peptides trigger cell death (34, 141, 142, 173). Rather than being
a specific cell surface receptor-based mechanism, this is a
rather nonspecific, generalized form of cell death that occurs
due to the destabilization of the intracellular ionic environment and the generation of reactive oxygen species (166).
Amyloid fibril-induced toxicity is thought to be mediated at
least in part by increased cellular free radical production,
reflected by increases in levels of NADPH oxidase, glutathione reductase, and other proteins associated with redox
control after treatment with human IAPP and other amyloidogenic peptides (170, 174 –177). This finding is of particular
relevance to the ␤-cell, which is poorly equipped to defend
against damage by reactive oxygen species due to low levels
of antioxidant enzymes, such as superoxide dismutase and
catalase (178).
In addition to the effects of IAPP aggregation to induce
␤-cell death, IAPP aggregation and/or islet amyloid may
also reduce ␤-cell replication. We have recently shown that
islet amyloid formation in transplanted human IAPP transgenic islets is associated with a decline in ␤-cell replication
in the transplanted graft (179). The cause(s) of this maladaptation is unknown, although a recent in vitro study demonstrated that actively replicating cells are at increased risk
of human IAPP-induced cytotoxicity (180). These data suggest that under conditions where ␤-cell replication should be
increased to compensate for human IAPP-induced ␤-cell
loss, the newly replicating ␤-cells may be preferentially targeted by the same IAPP-induced cytotoxic process, leading
to both an increase in ␤-cell death and a decrease in ␤-cell
replication.
3636
J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643
Role of islet amyloid in the pathogenesis of type 2 diabetes
Although it is clear that islet amyloid deposits are present
in almost all individuals with type 2 diabetes, the role of this
lesion on the pathogenesis of type 2 diabetes remains somewhat controversial. Studies in humans have necessarily been
limited to autopsy series, because a method for visualizing
islet amyloid in vivo does not yet exist. These studies have
shown that the postmortem islet amyloid load is increased in
individuals who received insulin treatment for diabetes compared with those who were treated with oral antidiabetic
agents alone, presumably reflecting a milder form of diabetes
in those taking oral agents (3). Decreased ␤-cell number also
occurs in those individuals with amyloid deposition (2, 5, 17),
suggesting that amyloid deposition is associated with decreased ␤-cell mass. Despite their limitations, these studies
provide a basis to hypothesize a causal link between islet
amyloid formation and the pathogenesis of type 2 diabetes.
Several spontaneous or genetically manipulated animal
models of islet amyloid formation have been developed and
have extended these findings in humans.
The closest model to islet amyloid formation in humans is
the nonhuman primate. Macaques produce an amyloidogenic form of IAPP and spontaneously develop islet amyloid
and subsequently diabetes as they age. An elegant longitudinal study by Howard (181) evaluated islet amyloid formation over 4 –10 yr in Macaca nigra monkeys using serial
pancreatic biopsies. Animals with mild islet amyloid deposition displayed only a small decrement in both glucose
clearance and the incremental insulin response after an iv
glucose load. Only in those animals with severe islet amyloid
deposition (⬎50% of islets involved) were more marked
changes in insulin release and elevated fasting plasma glucose levels observed. In a similar cross-sectional study performed in Macaca mulatta monkeys, which also spontaneously develop islet amyloid, animals were grouped
according to body weight and glucose control. Islet amyloid
deposits were already present in these animals in the absence
of elevated fasting glucose. Hyperglycemic and overtly diabetic animals developed more extensive islet amyloid deposits, and these were associated with the loss of islet ␤-cells
(182). Domestic cats are one of the few other species that also
develop diabetes associated with the deposition of IAPP as
islet amyloid. Similar to the observations in nonhuman primates, islet amyloid deposition in cats precedes the increase
in fasting glucose, occurring in animals with impaired glucose tolerance (183, 184). Together, these data provide strong
evidence that islet amyloid is a lesion that probably occurs
early in the pathogenesis of type 2 diabetes, before the onset
of fasting hyperglycemia. Furthermore, its formation is progressive and is associated with a worsening of ␤-cell function
and glucose homeostasis and a loss of islet ␤-cell mass.
Because the use of these aforementioned animal models is
expensive, several lines of transgenic mice with targeted
expression of human IAPP in their islet ␤-cells have been
developed as small animal models of islet amyloid formation
(185–188). Importantly, these mice produce, process, and
store human IAPP normally in secretory granules before
releasing it along with insulin in a regulated manner (189).
In the presence of a normal metabolic environment, these
Hull et al. • Hot Topic
mice did not develop islet amyloid deposits, although it has
been reported by one group that small deposits of fibrillar
material were present within ␤-cell secretory granules (187).
This lack of amyloid formation in the presence of a normal
metabolic environment further supports the concept that the
synthesis and secretion of an amyloidogenic form of IAPP
alone are not sufficient for islet amyloid to form and underscores the requirement for an altered metabolic environment
for islet amyloid deposition. What, then, are the factors that
may have accounted for the lack of islet amyloid development in these human IAPP transgenic mice (185–188)?
Firstly, the relatively short duration of the early human IAPP
transgenic mouse studies may have been important. Islet
amyloid typically forms over many, many years in humans
and larger animals, in parallel with the gradual development
and progressive nature of ␤-cell mass loss and dysfunction
in type 2 diabetes. Islet amyloid is present in older individuals who do not have diabetes, but show elevated postprandial glucose levels (1, 190). Aging is associated with obesity,
insulin resistance, and reduced ␤-cell function (191–193),
which predispose individuals to type 2 diabetes. Thus, islet
amyloid formation may play a role in the impaired glucose
metabolism of aging. Secondly, the initial human IAPP transgenic mouse studies were performed in the face of a relatively normal metabolic environment. It is well recognized
that the majority of young or middle-aged, nondiabetic humans do not develop islet amyloid deposits even in the
presence of obesity and insulin resistance. ␤-Cell dysfunction
is a critical prerequisite for the development of the hyperglycemia that characterizes type 2 diabetes, with the abnormalities in ␤-cell dysfunction comprising both reduced insulin release and inefficient proinsulin processing being
demonstrable in subjects at high risk of developing the disease and in those who subsequently develop hyperglycemia
(60, 131, 194). Furthermore, as mentioned above, islet amyloid is observed in older individuals who do not have overt
diabetes, but display abnormal glucose metabolism (1, 190)
and as a population have reduced ␤-cell function (193). These
observations strongly suggest that the presence of impaired
␤-cell function is required for amyloid fibril formation to
occur in humans and that the same is likely to be the case in
human IAPP transgenic mice.
In support of the hypothesis that underlying ␤-cell dysfunction is needed for islet amyloidogenesis, studies in human IAPP transgenic mice that have resulted in islet amyloid
formation have used interventions that result in ␤-cell dysfunction with or without insulin resistance. Increased dietary
fat intake has been shown to be associated with an increased
prevalence of type 2 diabetes in different populations (195,
196) and has been demonstrated to result in impaired ␤-cell
function in animals (197, 198). The administration of a high
fat diet to our human IAPP transgenic mice for 1 yr was
associated with the development of islet amyloid in more
than 80% of male human IAPP transgenic mice, but in only
11% of female mice (199). This effect of dietary fat was dose
dependent (125) (Fig. 4) and appears in this model to be in
part sex steroid dependent, because the combination of oophorectomy and increased dietary fat intake was associated
with the development of amyloid in 64% of female mice
(200). A strong correlation exists between the magnitude of
Hull et al. • Hot Topic
FIG. 4. Prevalence (percentage of islets containing amyloid; A) and
severity (percentage of islet area occupied by amyloid; B) of islet
amyloid formation in human IAPP transgenic mice fed for 12 months
with diets containing 15% (low fat; n ⫽ 17; 䡺), 30% (medium fat; n ⫽
16; o), or 45% kilocalories from fat (high fat; n ⫽ 15; f). Islet amyloid
prevalence increased in a dose-dependent manner with increased
dietary fat (P ⬍ 0.05). The severity of islet amyloid deposition was
significantly higher in mice receiving the high-fat diet (P ⫽ 0.05).
Copyright © 2003 American Diabetes Association. Adapted from Diabetes, Vol. 52, 2003:372–379 (Ref. 125). Reprinted with permission
from the American Diabetes Association.
amyloid deposition and the degree of ␤-cell loss (Fig. 5A),
with impaired insulin secretion and increased fasting plasma
glucose being associated with islet amyloid deposition (125,
172).
Other approaches have been equally successful in promoting islet amyloid formation, focusing on increasing the
secretory demand on a dysfunctional ␤-cell. GH and dexamethasone produce both insulin resistance (201, 202) and
impaired ␤-cell function (202, 203), and administration of this
combination of agents to human IAPP transgenic mice was
associated with electron microscopy-visible amyloid fibril
formation in islets (204). Genetic murine models of obesity,
insulin resistance, and ␤-cell dysfunction that ultimately develop diabetes (205, 206), namely ob/ob (207) and Agouti
viable yellow (Avy/a) (208) mice, have also been intercrossed
with human IAPP transgenic mice. In keeping with the findings in the model involving increased dietary fat intake described above, islet amyloid was observed in 83% of male
homozygous human IAPP⫻ob/ob double transgenic mice,
with increased amyloid deposition being positively corre-
J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643 3637
lated with fasting plasma glucose (207). Human IAPP transgenic mice carrying the Avy/a mutation were also found to
develop classical light microscopy-visible islet amyloid deposits, which were associated at about 1 yr of age with
reduced plasma insulin levels and fasting hyperglycemia
(208). These alterations in glucose homeostasis observed in
these double genetically modified mice appears to have resulted, as with the dietary fat fed model, from a loss of islet
mass as a result of amyloid deposition. In contrast, subtotal
pancreatectomy, an intervention that increases ␤-cell secretory demand, but does not induce ␤-cell dysfunction, did not
result in islet amyloid formation in human IAPP transgenic
mice, even 7 months after pancreatectomy (209).
A critical question that remains is whether islet amyloid
deposition occurs in the same manner in human IAPP transgenic mice as it does in humans. Although the administration
of GH and dexamethasone to human IAPP transgenic mice
resulted in amyloid deposits that were associated with loss
of islet mass, but were visible only by electron microscopy,
this morphology is in contrast to the classical light microscopy-visible amyloid deposits observed in human type 2
diabetes (204). However, in the high fat-fed human IAPP
transgenic mice (125, 199), human IAPP⫻ob/ob mice (207),
and human IAPP⫻Avy/a transgenic mice (208), islet amyloid
deposits are histologically indistinguishable from those seen
in human type 2 diabetes (Fig. 1). They are visible by light
microscopy and staining with the histological dyes thioflavin
S and Congo Red (Fig. 1) and occur extracellularly in close
proximity to islet ␤-cells (4, 199, 207, 208). The high fat-fed
model also demonstrates that islet amyloid formation in
human IAPP transgenic mice occurs in the same progressive
manner and with the same pattern of distribution as that in
human type 2 diabetes (unpublished observation). As shown
in Fig. 5B, deposition of islet amyloid in transgenic mice
occurs diffusely throughout the pancreas, involving all islets
before the magnitude of amyloid deposition within each islet
increases exponentially (172).
Taken together, then, these data suggest that these different human IAPP transgenic mouse colonies are indeed good
models of islet amyloid formation as occurs in human type
2 diabetes. A large body of evidence has been generated from
these transgenic mice that confirms and extends the findings
from humans and spontaneous animal models of islet amyloid formation. As clearly observed in humans, the expression of an amyloidogenic form of IAPP is not, by itself,
sufficient to induce islet amyloid formation. An intervention
that has the dual effect of increasing ␤-cell secretory demand
by inducing obesity and/or insulin resistance and impairing
␤-cell function appears in these human IAPP transgenic mice
to be key in promoting islet amyloid deposition, reproducing
that seen in human type 2 diabetes. As observed in nonhuman primates and cats, islet amyloid formation can occur
without marked elevations in fasting plasma glucose, but
increased islet amyloid formation is associated with reduced
insulin secretion, islet ␤-cell mass loss, and ultimately increased fasting plasma glucose levels. Thus, not only have
these transgenic mouse models provided valuable information regarding the development of islet amyloid, but they
also provide valuable models to test interventions aimed at
preventing islet amyloid formation and thus the deleterious
3638
J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643
Hull et al. • Hot Topic
FIG. 5. A, Relationship between the proportion of insulin-immunoreactive area per islet and the severity of islet amyloid (percentage of islet
area occupied by amyloid) in 240 human IAPP transgenic mouse islets. Increasing islet amyloid severity is strongly associated with a decline
in the insulin-positive area within islets (r ⫽ ⫺0.59; P ⬍ 0.0001). B, Relationship between islet amyloid prevalence (percentage of islets
containing amyloid) and severity (percentage of islet area occupied by amyloid) in 12 human IAPP transgenic mice. Islet amyloid prevalence
and severity are related in a nonlinear manner (r ⫽ 0.93; P ⬍ 0.001), such that the vast majority of islets contain amyloid before the severity
of amyloid deposition increases. Copyright © 2001 American Diabetes Association. Adapted from Diabetes, Vol. 50, 2001:2514 –2520 (Ref. 172).
Reprinted with permission from the American Diabetes Association.
effect of islet amyloid on ␤-cell mass and function observed
in type 2 diabetes.
Approaches to inhibiting islet amyloid formation
The increasing body of evidence linking aggregation and
cytotoxicity of IAPP and islet amyloid formation to a decline
in ␤-cell mass and function underscores the importance of
developing methods to decrease or prevent islet amyloid
formation.
One approach has been to reduce the secretion of IAPP
from the islet ␤-cell to reduce the precursor pool of amyloidogenic IAPP available for fibril formation. In a small study
of partially pancreatectomized cats made diabetic with injections of dexamethasone and GH, islet amyloid was observed in all four cats when the hyperglycemia was treated
with the ␤-cell secretagogue glipizide, whereas only one of
four cats treated with insulin developed islet amyloid. This
suggests that the increased secretory demand with glipizide
can induce amyloid formation, whereas decreasing ␤-cell
secretory demand with insulin may reduce amyloid formation (210). Intercrossing human IAPP transgenic mice with
mice heterozygous for a knockout mutation in the glucokinase gene generated animals with the potential to deposit
amyloid, but with reduced ability to secrete IAPP. The double-transgenic offspring (human IAPP transgenic and glucokinase deficient) showed a marked reduction in islet amyloid deposition compared with mice expressing human IAPP
that had a normal complement of glucokinase (211). Another
approach to decreasing IAPP release and thus the potential
for amyloid formation has been the use of antidiabetic agents
that act by reducing ␤-cell secretory demand. Metformin,
which acts primarily by suppressing hepatic glucose production (212), reduces plasma IAPP levels in obese individuals with newly diagnosed type 2 diabetes (213), whereas
thiazolidinediones, which are administered to improve insulin sensitivity in the peripheral tissues, also reduce ␤-cell
peptide release (214). We have recently successfully used
these pharmacological approaches to reduce ␤-cell peptide
secretion and decrease amyloid formation in human IAPP
transgenic mice. Treatment with metformin or the thiazolidinedione rosiglitazone for 1 yr markedly reduced islet
amyloid formation in these mice, decreasing both the number of islets that contained amyloid and the proportion of
islet area that was replaced by amyloid. The effect of these
interventions to reduce islet amyloid formation was greater
than their effect to reduce ␤-cell secretory demand alone,
suggesting that they have an additional, as yet unidentified
effect to reduce islet amyloid deposition (215). Interestingly,
however, these approaches did not prevent the development
of amyloid formation, strongly supporting the concept that
other ␤-cell functional changes induced by the high fat diet
were sufficient to allow amyloid formation to occur. It is
important to recognize that despite the marked reduction in
islet amyloid formation with these interventions that reduce
IAPP secretion, the presence of amyloid deposits still makes
it possible that the cytotoxic effects of the small aggregates
of IAPP associated with amyloid fibril formation may be
occurring. Therefore, it appears that the development of approaches to prevent the loss of ␤-cells due to amyloid formation should also focus on the very early stages in the
generation of amyloid fibrils, rather than being solely aimed
at the dissolution of mature islet amyloid deposits.
The development of inhibitors of islet amyloid fibril formation is in relatively early stages, although many findings
from inhibitor studies targeted at other forms of amyloid,
such as A␤ fibril formation in Alzheimer’s disease, may be
applicable to diabetes. Inhibitors based on small molecules
such as Congo Red, which bind all amyloid fibrils, have been
shown to have effects on IAPP. Although Congo Red does
not affect the fibrillogenesis of human IAPP (142), it has been
shown to inhibit the cytotoxic effect of both human IAPP and
A␤ (142, 216). Alternatively, the antibiotic rifampicin or its
Hull et al. • Hot Topic
J Clin Endocrinol Metab, August 2004, 89(8):3629 –3643 3639
analogs, through their free radical scavenging ability, have
been shown to be effective in inhibiting the cytotoxic effects
of human IAPP (142, 217), providing more evidence for an
association between human IAPP cytotoxicity, oxidative
stress, and islet amyloid formation. Small molecule analogs
of glucosamine, a basic subunit of the GAG chains of heparan
sulfate proteoglycans, have also been shown to be effective
in blocking GAG chain elongation and thereby inhibiting
amyloid formation in vivo in a model of AA amyloidosis
(218 –220).
A promising target for the development of islet amyloid
inhibitors is the 20 –29 amino acid region of human IAPP that
confers the amyloidogenicity of this molecule. Synthetic peptides that correspond to the amino acid sequences within
IAPP, but have been modified by methylation, have been
shown to inhibit IAPP fibrillogenesis and to reduce the cytotoxic effects of IAPP fibrils (221). An alternative approach
has been to synthesize short peptides based on the 20 –29
amyloidogenic region of human IAPP that are not intrinsically amyloidogenic (222). These peptides are effective in
inhibiting both the fibrillogenesis and cytotoxicity of human
IAPP in islet ␤-cell lines (222). However, it is unclear whether
these peptide inhibitors are able to enter the cell. Thus, if the
initial amyloid formation is intracellular, such an approach
may be less efficient.
All of these approaches hold promise for potentially reducing the burden and effects of islet amyloid on the ␤-cell.
If apoptosis and ␤-cell mass loss as a result of islet amyloid
formation are important contributors to the loss of ␤-cell
function observed in type 2 diabetes and clearly precede the
diagnosis, as we believe, then the use of agents that slow or
prevent these effects will be appealing. What will surely be
the challenge will be to determine which individuals would
benefit from their use and whether these approaches should
be initiated at an early stage to prevent the progressive decline of ␤-cell function that characterizes the development
and progression of hyperglycemia (194, 223, 224).
Conclusions
A great deal of progress has been made recently that has
identified IAPP and islet amyloid as potentially important
contributors to the pathogenesis of the ␤-cell dysfunction of
type 2 diabetes. With the establishment of good small animal
models of islet amyloid formation and the ongoing development of inhibitors to prevent its formation, it should soon
be possible to test the hypothesis that early intervention
aimed at preventing the cytotoxic effects of small aggregates
of IAPP and the aggregation of amyloid fibrils to form the
larger amyloid deposits will slow or even prevent the loss of
␤-cell dysfunction and the development of the hyperglycemia of type 2 diabetes observed in humans.
Acknowledgments
Received March 1, 2004. Accepted April 12, 2004.
Address all correspondence and requests for reprints to: Dr. Steven
E. Kahn, Veterans Affairs Puget Sound Health Care System (151), 1660
South Columbian Way, Seattle, Washington 98108. E-mail: skahn@
u.washington.edu.
This work was supported by the Medical Research Service of the
Department of Veterans Affairs; Grants DK-02654, DK-17047, and DK-
50703; the American Diabetes Association; the Juvenile Diabetes Research Foundation; the Swedish Research Council (Projects 5941, 9237,
and 14040); the Swedish Diabetes Association; the Nordic Insulin Fund;
the Novo Nordisk Research Foundation; and the Family Ernfors Fund.
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