Expression Of Islet Amyloid Polypeptide Research Articles

Autophagy protects against human islet amyloid polypeptide-associated apoptosis.

Aims/Introduction: Islets in type 2 diabetes are characterized by deposition of islet amyloid polypeptide (IAPP) as well as β-cell dysfunction. The unique amyloidogenic character of human (h)IAPP is associated with cytotoxicity. Autophagy is a ubiquitous system of cellular recycling that contributes to cell survival. Thus, we examined whether autophagy could ameliorate hIAPP-associated cytotoxicity. First, we used a COS-1 cell model, lacking endogenous IAPP that might affect cytotoxicity related to exogenous hIAPP. Next, we used the mouse β-cell line, MIN-6 cells. Both cells were transfected with hIAPP or rat (r)IAPP expression constructs, or transfected with bicistronic vectors expressing green fluorescent protein (GFP) and either hIAPP or rIAPP for flow cytometry analysis. Cell viability and apoptosis markers were studied in relation to chemical or genetic modulation of autophagy. The viability of cells expressing hIAPP was significantly decreased as compared with those expressing rIAPP and the cleavage of pro-caspase-3 was elevated in hIAPP-transfected cells. The formation of autophagosomes and the conversion of microtubule-associated protein light chain 3B I to II were elevated in hIAPP-expressing cells. The viability of hIAPP-expressing cells was increased after treatment with rapamycin, an inducer of autophagy, and decreased after treatment with 3-methyladenine, an inhibitor of autophagy. In MIN-6 cells, annexin positive cells were increased by 3-methyladenine and decreased by rapamycin using flow cytometry. Knocking down of the autophagy protein 5 gene decreased hIAPP-transfected cell viability. Autophagy is co-localized with hIAPP expression and it plays a protective role in hIAPP-associated apoptosis. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2010.00065.x, 2010).

Human-IAPP disrupts the autophagy/lysosomal pathway in pancreatic β-cells: protective role of p62-positive cytoplasmic inclusions

In type II diabetes (T2DM), there is a deficit in β-cells, increased β-cell apoptosis and formation of intracellular membrane-permeant oligomers of islet amyloid polypeptide (IAPP). Human-IAPP (h-IAPP) is an amyloidogenic protein co-expressed with insulin by β-cells. IAPP expression is increased with obesity, the major risk factor for T2DM. In this study we report that increased expression of human-IAPP led to impaired autophagy, due at least in part to the disruption of lysosome-dependent degradation. This action of IAPP to alter lysosomal clearance in vivo depends on its propensity to form toxic oligomers and is independent of the confounding effect of hyperglycemia. We report that the scaffold protein p62 that delivers polyubiquitinated proteins to autophagy may have a protective role against human-IAPP-induced apoptosis, apparently by sequestrating protein targets for degradation. Finally, we found that inhibition of lysosomal degradation increases vulnerability of β-cells to h-IAPP-induced toxicity and, conversely, stimulation of autophagy protects β-cells from h-IAPP-induced apoptosis. Collectively, these data imply an important role for the p62/autophagy/lysosomal degradation system in protection against toxic oligomer-induced apoptosis.

Pathologic Changes and Glucose Homeostasis According to Expression of Human Islet Amyloid Polypeptide in Type 2 Diabetic Patients

The amount of amyloid detectable in islets varies, and is not always correlated with degree of beta-cell loss. It has been hypothesized that islet amyloid polypeptide (IAPP) aggregation causes beta-cell dysfunction. In this study, we investigated the relationship between IAPP expression and glucose homeostasis in pancreatectomized patients. Human pancreatic head tissues were collected from 46 pancreatic tumor patients. We divided the diabetic cases into two groups, patients with higher IAPP-expressing islets (DM-H) and patients with lower IAPP-expressing islets (DM-L), and compared both groups to patients with normal glucose tolerance (NGT). Interestingly, oral glucose tolerance test analyses showed that DM-L patients had significantly higher glucose levels and lower C-peptide levels than DM-H patients. Furthermore, the DM-H group showed a relative beta-cell volume similar to that of the NGT group, as well as a significantly higher relative beta-cell volume than the DM-L group. The DM-L group was significantly higher than the DM-H group, not only in the rates of replication and apoptosis, but also in the nuclear C/EBP homologous protein and the ratio of oligomer to IAPP. Thus, IAPP expression may not be an indicator of cell death induction. IAPP, including oligomer, may be an important determinant in the fate of islet beta-cells. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Small Molecule Inhibitors of Islet Amyloid Polypeptide Fibrillogenesis and Cytotoxicity

Protein fiber formation is associated with diseases ranging from Alzheimer's to type II diabetes. The fiber formation is a complex reaction, which includes a number of conformational and oligomeric intermediate states. In recent years, it has become clear that it is these states, and not the end product (i.e. fibers) of amyloid formation that are the toxic agents associated with disease. These insights indicate that small molecule screens must to be directed at assembly mechanism in order to maximize the prospects for success. Islet amyloid polypeptide (IAPP) is a 37 residue peptide hormone co-secreted with insulin by the ß-cells of the pancreas. In patients with type II diabetes, this protein aggregates as amyloid in a process that is correlated with ß-cell dysfunction and the loss of ß-cell mass. In in vitro studies, the addition of soluble IAPP has been shown to be toxic to many ß- and non-ß-cell lines. IAPP fibrillogenesis, as is the case for many other amylodiogenic proteins, can be catalyzed by lipid bilayers. Paradoxically, while amyloid fibers are ß-sheet rich, membrane-stabilized states are α-helical. We have identified a small molecule alpha helix mimetic, IS5, which inhibits bilayer catalysis of fibrillogenesis, and rescues IAPP-induced toxicity in cell culture. Importantly, IAPP:IS5 interactions localize to the putative α-helical region of IAPP, revealing that α-helical states are on pathway to fiber formation. Normally, IAPP is not amyloidogenic as its cosecreted partner, insulin, prevents self-assembly. Here, we show that IS5 inhibition is synergistic with insulin. IS5 therefore represents a new approach to amyloid inhibition as the target is an assembly intermediate that may additionally restore functional IAPP expression.

Expression of islet amyloid polypeptide in duodenum of rabbits during the postnatal development period

Expression of islet amyloid polypeptide in duodenum of rabbits during the postnatal development period

Reduced Ghrelin, Islet Amyloid Polypeptide, and Peptide YY Expression in the Stomach of Gastrin-Cholecystokinin Knockout Mice

The antral hormone gastrin and its intestinal relative, cholecystokinin (CCK), are pivotal in the regulation of gastric functions. Other gastric hormones like ghrelin, peptide YY (PYY), and islet amyloid polypeptide (IAPP), however, also contribute to the regulation of acid secretion, motility, and feeding. Because gastrin and CCK are crucial for gastric homeostasis, we examined how loss of gastrin alone and gastrin plus CCK affected the expression of ghrelin, IAPP, and PYY and ghrelin secretion. The expression of ghrelin, IAPP, and PYY and the CCK-A receptor genes were examined in both gastrin and gastrin-CCK double-knockout (KO) mice using immunocytochemistry and quantitative RT-PCR. Ghrelin concentrations in plasma were measured using RIA. Gastrin and CCK were infused in gastrin-CCK KO mice using osmotic minipumps. The number of ghrelin cells and ghrelin gene expression were unaffected, albeit the ghrelin cells were located closer to the base of the glands in both KO mouse strains when freely fed. However, lack of both gastrin and CCK attenuated fasting-induced ghrelin expression and secretion. Fundic ghrelin cells expressed the CCK-A receptor, and ghrelin expression increased after CCK infusion. Furthermore, gastric IAPP and PYY expression as well as the number of IAPP- and PYY-containing cells were reduced in both gastrin and gastrin-CCK KO mice. Gastrin infusion increased gastric IAPP but not PYY expression. In conclusion, lack of gastrin plus CCK but not gastrin alone reduced ghrelin secretion in response to fasting through both direct and indirect mechanisms. Both gastrin and combined gastrin-CCK deficiency reduced the gastric IAPP and PYY expression.

Postnatally disturbed pancreatic islet cell distribution in human islet amyloid polypeptide transgenic mice

Objective: Islet amyloid polypeptide (IAPP)/amylin is produced by the pancreatic islet β-cells, which also produce insulin. To study potential functions of IAPP, we have generated transgenic mice overexpressing human IAPP (hIAPP) in the β-cells. These mice show a diabetic phenotype when challenged with an oral glucose load. In this study, we examined the islet cytoarchitecture in the hIAPP mice by examining islet cell distribution in the neonatal period, as well as 1, 3 and 6 months after birth. Results: Neonatal transgenic mice exhibited normal islet cell distribution with β-cells constituting the central islet portion, whereas glucagon and somatostatin-producing cells constituted the peripheral zone. In contrast, in hIAPP transgenic mice at the age of 1 month, the glucagon-immunoreactive (IR) cells were dispersed throughout the islets. Furthermore, at the age of 3 and 6 months, the islet organisation was similarly severely disturbed as at 1 month. Expression of both endogenous mouse IAPP and transgenic hIAPP was clearly higher in 6-month-old mice as compared to newborns, as revealed by mRNA in situ hybridisation. Conclusions: Mice transgenic for hIAPP have islets with disrupted islet cytoarchitecture in the postnatal period, particularly affecting the distribution of glucagon-IR cells. This islet cellular phenotype of hIAPP transgenic mice is similar to that of other mouse models of experimental diabetes and might contribute to the impaired glucose homeostasis.

Estrogen can prevent or reverse obesity and diabetes in mice expressing human islet amyloid polypeptide.

Type 2 diabetes is characterized by loss of beta-cell mass and concomitant deposition of amyloid derived from islet amyloid polypeptide (IAPP). Previously we have shown that expression of human IAPP (huIAPP) in islets of transgenic mice results in either a rapid onset of hyperglycemia in mice homozygous for the huIAPP transgene on a lean background (FVB/N) or a gradual hyperglycemia in mice hemizygous for the huIAPP transgene on an obese background (A(vy)/A). In both strains, only the males routinely develop diabetes. To investigate this sexual dimorphism, we treated young prediabetic A(vy)/A mice transgenic for huIAPP (huIAPP-A(vy)) with 17beta-estradiol (E2). The treatment completely blocked the progression to hyperglycemia but also prevented the associated weight gain in these mice. Immunohistochemistry of pancreatic sections demonstrated normal islet morphology with no apparent deposition of islet amyloid. E2 treatment of 1-year-old huIAPP-A(vy) diabetic males rapidly reverses obesity and hyperglycemia. To determine the effects of E2 in a nonobese model, we also treated prediabetic, ad libitum-fed and pair-fed Lean-huIAPP transgenic males. E2 completely blocked the progression to hyperglycemia with no significant effect on body weight. Pancreatic insulin content and plasma insulin concentration of Lean-huIAPP transgenic mice increased in a dose-dependent manner. We demonstrated the presence of estrogen receptor (ER)-alpha mRNA in mouse and human islets. By also confirming the presence of ER-alpha protein in islets, we discovered a novel 58-kDa ER-alpha isoform in mice and a 52-kDa isoform in humans, in the absence of the classic 67-kDa protein found in most tissues of both species. The demonstrated presence of ER-alpha in mouse and human islets is consistent with a direct effect on islet function. We conclude that exogenous E2 administered to male mice may block human IAPP-mediated beta-cell loss both by direct action on beta-cells and by decreasing insulin demand through inhibition of weight gain or increasing insulin action.

Islet amyloid polypeptide in the islets of Langerhans: friend or foe?

Islet amyloid polypeptide (IAPP), or amylin, was originally discovered as the constituent peptide in amyloid occurring in human insulinomas and in pancreatic islets in human subjects with Type II (non-insulin-dependent) diabetes mellitus. Its normal expression in beta cells and its co-secretion with insulin in response to nutrient stimuli, suggest a metabolic function for the peptide. Specifically, IAPP has most frequently been shown to inhibit insulin secretion, implying that IAPP has a role in the regulation of islet hormone homeostasis. The physiological significance of IAPP in islets has been difficult to assess; very high IAPP concentrations are required to alter insulin secretion. Moreover, until recently, IAPP receptors have not been characterised at the molecular level, thus leaving the actual target cells for IAPP unidentified. Furthermore, in experimental diabetes in rodents, the ratio of IAPP expression to that of insulin invariably is increased. In view of the pleiotropic effects attributed to IAPP, such regulation could be both adverse and beneficial in diabetes. Metabolic characterisation of mice carrying a null mutation in the IAPP gene or which overexpress IAPP in beta cells have recently confirmed that IAPP is a physiological inhibitor of insulin secretion. Based on experiments in which IAPP-deficient mice develop a more severe form of alloxan-induced diabetes, we argue that the action of IAPP in the islets normally is beneficial for beta-cell function and survival; thus, the established up regulation of IAPP expression compared with that of insulin in experimental rodent diabetes could serve to protect islets under metabolically challenging circumstances.

Glucose Regulates Islet Amyloid Polypeptide Gene Transcription in a PDX1- and Calcium-dependent Manner

Islet amyloid polypeptide (IAPP) and insulin are expressed in the beta-cells of the islets of Langerhans. They are co-secreted in response to changes in glucose concentration, and their mRNA levels are also regulated by glucose. The promoters of both genes share similar cis-acting sequence elements, and both bind the homeodomain transcription factor PDX1, which plays an important role in the regulation of the insulin promoter and insulin mRNA levels by glucose. Here we examine the role of PDX1 in the regulation of the human IAPP promoter by glucose. The experiments were facilitated by the availability of a human beta-cell line (NES2Y) that lacks PDX1. NES2Y cells also lack operational K(ATP) channels, resulting in a loss of control of calcium signaling. We have previously used these cells to show that glucose regulation of the insulin gene is dependent on PDX1, but not calcium. In the mouse beta-cell line Min6, glucose (16 mm) stimulated a 3.5-4-fold increase in the activity of a -222 to +450 IAPP promoter construct compared with values observed in 0.5 mm glucose. In NES2Y cells, glucose failed to stimulate transcriptional activation of the IAPP promoter. Overexpression of PDX1 in NES2Y cells failed to reinstate glucose-responsive control of the IAPP promoter. Glucose effects on the IAPP promoter were observed only in the presence of PDX1 when normal calcium signaling was restored by overexpression of the two K(ATP) channel subunits SUR1 and Kir6.2. The importance of calcium was further emphasized by an experiment in which glucose-stimulated IAPP promoter activity was inhibited by the calcium channel blocker verapamil (50 microm). Verapamil was further shown to inhibit the stimulatory effect of glucose on IAPP mRNA levels. These results demonstrate that like the insulin promoter, glucose regulation of the IAPP promoter is dependent on the activity of PDX1, but unlike the insulin promoter, it additionally requires the activity of another, as yet uncharacterized factor(s), the activity of which is calcium-dependent.

Expression of Islet Amyloid Polypeptide. Localization and regulation in the pancreatic islets, gastrointestinal tract and sensory nervous system

Islet amyloid polypeptide (IAPP; also named ”amylin”) is a member of the calcitonin superfamily of peptides, displaying the closest structural relationship to the neuropeptide calcitonin gene-related peptide (CGRP). It is expressed in islet beta-cells and forms islet amyloid in type 2 diabetic patients. IAPP may restrain release and peripheral actions of insulin. Thus, IAPP has been implicated in the pathogenesis of type 2 diabetes. By examinations of tissue-specific expression of IAPP and its regulation under normal and experimental conditions, we aimed to illuminate a possible role of IAPP. IAPP was expressed in islet beta-cells in nearly all species examined; in some species IAPP occurred also in delta-cells. IAPP expression was of early onset during embryonic islet development. In vivo, we found a parallel regulation of IAPP and insulin expression by administration of glucose. These findings agree with a role for IAPP in glucose homeostasis by autocrine/paracrine regulation of islet function as well as by endocrine secretion. Following dexamethasone treatment of rats, a model for type 2 diabetes, we found a non-parallel expression of IAPP and insulin; in relative terms, IAPP was over-expressed compared with insulin. In rodents given streptozotocin or alloxan, conferring beta-cell depletion reminiscent of type 1 diabetes, IAPP expression was consistently less impaired than that of insulin. These studies show that IAPP and insulin gene expression are differentially regulated. Apparently, beta-cell stress, regardless of ethiology, results in over-expression of IAPP. IAPP was also found to be expressed in the gastrointestinal tract in a region- and species-dependent manner; when present, IAPP expression was most abundant proximally, occurring in somatostatin and gastrin cells. Also gastrointestinal IAPP expression was of early embryonic onset. In vitro, IAPP relaxed pre-contracted gut musculature. In the sensory nervous system IAPP was expressed in CGRP neurons and displayed a distribution typical of sensory neuropeptides. Upon sciatic axotomy, IAPP and CGRP expression were co-ordinately downregulated. In contrast, following adjuvant-induced rat paw inflammation, IAPP expression was upregulated in innervating neurons, indicating that IAPP may play a role in neurogenic inflammation. In conclusion, our studies have expanded the view of IAPP as an exclusive islet hormone to become a ”classical” neurohormonal peptide, with several endocrine and neuronal expression sites. The studies on the regulation of islet IAPP and insulin expression in response to glucose and in experimental diabetes may form the basis for an understanding of the role of IAPP in diabetes. Provided that the amyloidogenic mechanisms and metabolic effects of IAPP are elucidated, an over-expression of IAPP may prove to be relevant for diabetes development.

The new proof of neuro-endocrine-immune network-expression of islet amyloid polypeptide in plasma cells in gastric mucosa of peptic ulcer patients.

New proof of neuro-endocrine-immune network expression of islet amyloid polypeptide in plasma cells in gastric mucosa of peptic ulcer patients

Islet amyloid polypeptide/amylin messenger RNA and protein expression in human insulinomas in relation to amyloid formation.

Islet amyloid polypeptide (IAPP), also named amylin, is the predominant protein component of amyloid deposits in human islet beta cell tumours of the pancreas (insulinomas). IAPP is co-produced with insulin by islet beta cells. We investigated IAPP expression in relation to insulin expression and to amyloid formation in eleven insulinomas. RNA and protein extracts were prepared from the same pieces of tumour tissue, and from specimens of two normal human pancreata. IAPP and insulin mRNA and peptide content were quantified using Northern blot analysis and radioimmunoassay (RIA) respectively. Molecular forms of IAPP immunoreactivity were analysed by reversed-phase high-performance liquid chromatography (HPLC). The presence of islet hormones and of amyloid was assessed by (immuno)histochemical staining of paraffin sections. Plasma levels of IAPP and insulin prior to tumour resection were determined by RIA. IAPP and insulin mRNA and peptide content varied widely between the tumour specimens, and there was considerable intratumour heterogeneity of peptide content. HPLC analysis indicated correct proteolytic processing of the IAPP precursor protein. Amyloid deposits were detected only in the three tumours with the highest IAPP content. In contrast to insulin, plasma levels of IAPP were not elevated in the insulinoma patients. The spectrum of hormone production by insulinomas cannot be inferred from only a few tissue sections due to intratumour heterogeneity. Expression of the IAPP and insulin genes is not coupled in insulinomas, which produce properly processed mature IAPP. In addition to IAPP overproduction, additional factors such as intracellular accumulation of IAPP are involved in amyloidogenesis in insulinomas.

Expression of non-classical islet hormone-like peptides during the embryonic development of the pancreas.

Understanding of islet embryogenesis may prove to be key in the design of future therapies for diabetes directed at re-initiating islet growth, with the goal to replace and/or replenish the impaired beta-cell mass in the disease. In this context, studies of islet neurohormonal peptides, known to play a role in the local regulation of islet function, and their expression during islet embryogenesis are important. Here we review our studies on the embryonic islet expression of islet amyloid polypeptide (IAPP) and the PP-fold peptides pancreatic polypeptide (PP), peptide YY (PYY) and neuropeptide Y (NPY). IAPP, which is constitutively expressed in beta- and delta-cells in the adult rat, was found to occur in the assumed pluripotent islet progenitor cell, together with PYY, glucagon, and to a lesser extent with insulin. As development proceeds, the insulin/IAPP phenotype is segregated from that of PYY/glucagon; with the formation of islet-like structures, insulin/IAPP-expressing cells primarily occupy their central portions, while PYY/glucagon-expressing cells are found in their periphery. At the time of formation of islet-like structures, expression of NPY is induced in the insulin/IAPP-containing cells. Whereas NPY-expression ceases at birth, PYY is constitutively expressed in non-beta-cells in the mature rat. Expression of PP is induced just prior to birth in a separate population of islet cells, occasionally co-expressed with PYY. Although a clear role for these peptides during embryogenesis has not been identified, they conceivably could play a role in the control of insulin secretion, islet growth and islet blood flow.

Beta-cell-specific inactivation of the mouse Ipf1/Pdx1 gene results in loss of the beta-cell phenotype and maturity onset diabetes.

To study the late beta-cell-specific function of the homeodomain protein IPF1/PDX1 we have generated mice in which the Ipf1/Pdx1 gene has been disrupted specifically in beta cells. These mice develop diabetes with age, and we show that IPF1/PDX1 is required for maintaining the beta cell identity by positively regulating insulin and islet amyloid polypeptide expression and by repressing glucagon expression. We also provide evidence that IPF1/PDX1 regulates the expression of Glut2 in a dosage-dependent manner suggesting that lowered IPF1/PDX1 activity may contribute to the development of type II diabetes by causing impaired expression of both Glut2 and insulin.

Islet amyloid polypeptide and calcitonin gene-related peptide expression are upregulated in lumbar dorsal root ganglia after unilateral adjuvant-induced inflammation in the rat paw

After unilateral adjuvant-induced inflammation, expression of neuropeptides believed to be involved in the inflammatory response, e.g. substance P and calcitonin gene-related peptide (CGRP), is upregulated in innervating sensory neurons. Islet amyloid polypeptide (IAPP) is structurally related to CGRP and constitutively expressed in sensory CGRP-containing neurons; the role of IAPP in sensory neurons is unknown. To examine whether IAPP could play a role in inflammation, IAPP expression in L 5 dorsal root ganglion (DRG) and its distribution in the dorsal horn were investigated after unilateral adjuvant-induced inflammation in the rat paw and compared with CGRP, using in situ hybridization and immunocytochemistry. At 12 h and day 3, but not day 21, the percentage of nerve cell profiles expressing IAPP and CGRP mRNA was greater in the ipsilateral L 5 DRG; these changes paralleled the occurrence of edema around the tarsotibial joint and a slight limp. IAPP expression in individual nerve cell profiles was higher in the ipsilateral L 5 DRG at 12 h, but not at days 3 and 21; the corresponding CGRP mRNA level was higher at days 3 and 21. At day 3, the higher expression of IAPP and CGRP on the ipsilateral side was accompanied by increased numbers of immunoreactive DRG neurons and fibers in the spinal cord dorsal horn. Largely, expression of IAPP and CGRP seems to be co-ordinately regulated by localized inflammation, although the rapid, but transient, upregulation in DRG neurons of IAPP mRNA expression and the slower, but sustained, upregulation of CGRP mRNA expression may indicate dissociated regulation of the peptides. Thus, IAPP could play a role in the initial phase of localized inflammation.

Islet amyloid polypeptide and calcitonin gene-related peptide expression are down-regulated in dorsal root ganglia upon sciatic nerve transection

Islet amyloid polypeptide (IAPP) is structurally related to calcitonin gene-related peptide (CGRP) and has been implicated in glucose homeostasis and diabetes pathogenesis because it is expressed in insulin cells and forms amyloid in pancreatic islets from type II diabetic patients. IAPP is also constitutively co-expressed with CGRP in rat sensory neurons. Whether expression of IAPP is altered by nerve injury with or without regeneration was investigated in adult rats subjected to unilateral sciatic axotomy; IAPP and CGRP expression were determined by quantitative in situ hybridization and immunocytochemistry at days 3, 10 and 30 after axotomy. In ipsilateral L4–L5 dorsal root ganglia (DRG), the percentages of nerve cell profiles labelled for IAPP and CGRP mRNA were reduced at all time points studied. IAPP and CGRP mRNA expression were lower in nerve cell profiles in ipsilateral DRGs compared to the contralateral side after axotomy alone whereas epineurial nerve suture maintained or restored IAPP and CGRP expression. The numbers of IAPP- and CGRP-immunoreactive DRG nerve cell profiles and dorsal horn fibers were reduced on the ipsilateral side at all time points. Thus, IAPP and CGRP expression are down-regulated upon axotomy. Nerve repair maintains or restores IAPP and CGRP expression in individual neurons but does not prevent the loss of CGRP/IAPP phenotype of some of these neurons in response to axotomy.

Differential effect of insulin treatment on islet amyloid polypeptide (amylin) and insulin gene expression in streptozotocin-induced diabetes in rats.

Islet amyloid polypeptide (IAPP) is a beta cell hormone, which forms islet amyloid in non-insulin-dependent diabetes mellitus and may oppose insulin action and release. Therefore, the previously observed relative overexpression of IAPP compared with insulin in streptozotocin-treated rats could be unfavourable if it occurs in diabetes. Using quantitative in situ hybridization, we examined whether insulin treatment affected IAPP and insulin gene expression and their ratio at day 8 and 20 after induction of streptozotocin diabetes (plasma glucose approximately 30 mM). Total islet IAPP mRNA levels were less reduced than those of insulin at both time points. Differential regulation of the two hormones was further reflected by mean IAPP mRNA levels in the remaining islet cells being unaffected by streptozotocin treatment, whereas those of insulin were reduced. At both time points, insulin treatment decreased total islet levels of IAPP and insulin mRNA even more, IAPP mRNA levels being less reduced. Mean insulin mRNA levels, but not IAPP mRNA levels, in the remaining islet cells were reduced by insulin treatment. Between time points, total islet mRNA levels were higher at day 20, while mean mRNA levels in the remaining islet cells were unchanged, suggesting that regeneration of beta cells had occurred. In contrast, insulin gene expression was the same at both time points in insulin-treated rats, suggesting that insulin impaired insulin expression. To summarize, we found that IAPP and insulin were differentially expressed in experimental diabetes and that insulin treatment inhibited insulin, but not IAPP, gene expression. It is therefore unlikely that insulin will protect against amyloid formation and metabolic perturbations which may arise as a consequence of IAPP overexpression.

Islet amyloid polypeptide in the gut and pancreas: localization, ontogeny and gut motility effects.

The occurrence of islet amyloid polypeptide (IAPP) in the gut and pancreas of several species and during ontogeny of the rat, was studied using immunocytochemistry. Effects of IAPP on rat ileal smooth muscle were assessed in vitro. Islets of most, but not all, species examined, displayed IAPP in insulin cells and, in some species, also in somatostatin- and peptide YY (PYY)-containing cells. In the gut, expression of IAPP varied among species; when present, IAPP was most abundant in the proximal part and co-localized with somatostatin, PYY, gastrin/cholecystokinin, enteroglucagon or serotonin. IAPP was first demonstrated at embryonic day 12 and 16 in islet and gastrointestinal endocrine cells, respectively. IAPP relaxed gut muscle and reduced electrical field stimulation-evoked contractions, presumably by inhibiting acetylcholine release. Thus, IAPP expression in islets is consistent with an important role for IAPP in fuel metabolism; the gastrointestinal expression and motor effects of IAPP suggest that IAPP may modulate gastrointestinal function.

Islet amyloid polypeptide (amylin) does not seem to be directly involved in the development of gestational diabetes mellitus.

In order to define the islet amyloid polypeptide (IAPP) levels in gestational diabetes mellitus (GDM) and their interrelationship with the insulin levels, we studied (1) the placental RNA from 10 women (5 with GDM and 5 normals) for IAPP expression by Northern blotting and (2) 10 women with GDM during a 100-gram oral glucose tolerance test and compared these with 11 normal women matched for obesity and age. Plasma levels of glucose, IAPP, insulin, and C peptide were determined. No IAPP expression was detected in any of the placentae after a long exposure. We could not demonstrate any differences in plasma IAPP levels (basal or stimulated) between the two groups of pregnant women. However, in women with GDM we found a lower IAPP/insulin ratio (p < 0.05) and a lower maximal IAPP/maximal insulin response ratio during the oral glucose tolerance test (p < 0.05) than in normal women. Therefore, IAPP does not appear to be directly involved in the development of GDM. The peripheral levels of IAPP relative to insulin are lower in GDM, a finding similar to that described in type 2 diabetes mellitus (non-insulin-dependent diabetes mellitus). This observation further confirms that GDM resembles the early stages of type 2 diabetes mellitus.