Two novel immortal pancreatic beta-cell lines expressing and secreting human islet amyloid polypeptide do not spontaneously develop islet amyloid

Fructose-1,6-bisphosphatase overexpression in pancreatic beta-cells results in reduced insulin secretion: a new mechanism for fat-induced impairment of beta-cell function

OBJECTIVE

Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme that is upregulated in islets or pancreatic beta-cell lines exposed to high fat. However, whether specific beta-cell upregulation of FBPase can impair insulin secretory function is not known. The objective of this study therefore is to determine whether a specific increase in islet beta-cell FBPase can result in reduced glucose-mediated insulin secretion.

RESEARCH DESIGN AND METHODS

To test this hypothesis, we have generated three transgenic mouse lines overexpressing the human FBPase (huFBPase) gene specifically in pancreatic islet beta-cells. In addition, to investigate the biochemical mechanism by which elevated FBPase affects insulin secretion, we made two pancreatic beta-cell lines (MIN6) stably overexpressing huFBPase.

RESULTS

FBPase transgenic mice showed reduced insulin secretion in response to an intravenous glucose bolus. Compared with the untransfected parental MIN6, FBPase-overexpressing cells showed a decreased cell proliferation rate and significantly depressed glucose-induced insulin secretion. These defects were associated with a decrease in the rate of glucose utilization, resulting in reduced cellular ATP levels.

CONCLUSIONS

Taken together, these results suggest that upregulation of FBPase in pancreatic islet beta-cells, as occurs in states of lipid oversupply and type 2 diabetes, contributes to insulin secretory dysfunction.

Recent insights in islet amyloid polypeptide-induced membrane disruption and its role in beta-cell death in type 2 diabetes mellitus

The presence of fibrillar protein deposits (amyloid) of human islet amyloid polypeptide (hIAPP) in the pancreatic islets of Langerhans is thought to be related to death of the insulin-producing islet β-cells in type 2 diabetes mellitus (DM2). The mechanism of hIAPP-induced β-cell death is not understood. However, there is growing evidence that hIAPP-induced disruption of β-cell membranes is the cause of hIAPP cytotoxicity. Amyloid cytotoxicity by membrane damage has not only been suggested for hIAPP, but also for peptides and proteins related to other misfolding diseases, like Alzheimer's disease, Parkinson's disease, and prion diseases. Here we review the interaction of hIAPP with membranes, and discuss recent progress in the field, with a focus on hIAPP structure and on the proposed mechanisms of hIAPP-induced membrane damage in relation to β-cell death in DM2.

Prevention and promotion effects of apolipoprotein E4 on amylin aggregation

The misfolding of islet amyloid polypeptide (IAPP, amylin) results in the formation of islet amyloid, which is one of the most common pathological features of type 2 diabetes (T2D). Amylin, a 37-amino-acid peptide co-secreted with insulin and apolipoprotein E (ApoE) from the beta-cells of pancreatic islets, is thought to be responsible for the reduced mass of insulin-producing beta-cells. However, neither the relationship between amylin and ApoE nor the biological consequence of amylin misfolding is known. Here we have characterized the interaction between ApoE4 and amylin in vitro. We found that ApoE4 can strongly bind to amylin, and insulin can hardly inhibit amylin-ApoE binding. We further found that amylin fibrillization can be prevented by low concentration of ApoE4 and promoted by high concentration of ApoE4. Taken together, we propose that under physiological conditions ApoE4 efficiently binds and sequesters amylin, preventing its aggregation, and in T2D the enhanced ApoE4-amylin binding leads to the critical accumulation of amylin, facilitating islet amyloid formation.

Biological importance of the peptides of the calcitonin family as revealed by disruption and transfer of corresponding genes

The hormone calcitonin (CT) of thyroid C-cell origin, the neuropeptides alpha- and beta-calcitonin gene-related peptide (CGRP), the widely expressed hormone and tissue factor adrenomedullin (AM), and amylin (AMY) that is co-produced with insulin in pancreatic beta-cells, are structurally related peptides. They have in common six or seven amino acid ring structures, linked by disulfide bridges between cysteine residues, and amidated carboxyl termini that are both required for biological activity. The actions of the peptides in vivo have traditionally been studied after intravenous and intracerebroventricular administration. As a result, CT lowers serum calcium and reduces pain perception. alpha- and beta CGRP and AM are highly potent vasodilatory peptides. AMY inhibits food intake through its action in the area postrema of the brain. Physiological actions of the peptides summarized in the present review have been defined through gene knockout and overexpression strategies.

Elevated SNAP-25 is associated with fatty acid-induced impairment of mouse islet function

The role of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in insulin secretion following chronic exposure to non-esterified fatty acids (NEFAs) has not been extensively investigated. Here, we show that synaptosome-associated protein of 25 kDa (SNAP-25) levels were predominantly elevated in the soluble fraction of mouse islets exposed to palmitate. This coincided with an impairment of insulin secretion to glucose and non-glucose secretagogues, consistent with a defect at a distal regulatory step in exocytosis. Removal of palmitate from the media restored both SNAP-25 protein levels and insulin secretion to control levels. We conclude that increased expression of SNAP-25 is associated with NEFA-induced impairment of insulin secretion in mouse islets.

Extended life span is associated with insulin resistance in a transgenic mouse model of insulinoma secreting human islet amyloid polypeptide

Pancreatic amyloid is found in patients with insulinomas and type 2 diabetes. To study mechanisms of islet amyloidogenesis, we produced transgenic mice expressing the unique component of human islet amyloid, human islet amyloid polypeptide (hIAPP). These mice develop islet amyloid after 12 mo of high-fat feeding. To determine whether we could accelerate the rate of islet amyloid formation, we crossbred our hIAPP transgenic animals with RIP-Tag mice that develop islet tumors and die at 12 wk of age from hypoglycemia. At 12 wk of age, this new line of hIAPPxRIP-Tag mice was heavier (29.7 +/- 1.0 vs. 25.0 +/- 1.3 g, P < 0.05) and had increased plasma glucose levels (4.6 +/- 0.4 vs. 2.9 +/- 0.6 mmol/l, P < 0.05) compared with littermate RIP-Tag mice. However, the hIAPPxRIP-Tag mice did not display islet amyloid or amyloid fibrils despite high circulating hIAPP levels (24.6 +/- 7.0 pmol/l). Interestingly, hIAPPxRIP-Tag mice had a longer life span than RIP-Tag mice (121 +/- 8 vs. 102 +/- 5 days, P < 0.05). This increase in life span in hIAPPxRIP-Tag was positively correlated with body weight (r = 0.48, P < 0.05) and was associated with decreased insulin sensitivity compared with RIP-Tag mice. hIAPPxRIP-Tag mice did not develop amyloid during their 4-mo life span, suggesting that increased hIAPP secretion is insufficient for islet amyloid formation within such a short time. However, hIAPPxRIP-Tag mice did have an increase in life span that was associated with insulin resistance, suggesting that hIAPP has extrapancreatic effects, possibly on peripheral glucose metabolism.

Amylin-induced cytotoxicity is associated with activation of caspase-3 and MAP kinases

Nanomolar concentrations of human amylin promote death of RINm5F cells in a time- and concentrationdependent manner. Morphological changes of chromatin integrity suggest that cells are predominantly undergoing apoptosis. Human amylin induces significant activation of caspase-3 and strong and sustained phosphorylation of stress-activated protein kinases, c-Jun N-terminal kinase (JNK) and p38, that precedes cell death. Extracellular signal-regulated kinase (ERK) activation was not concomitant with JNK and/or p38 activation. Activation of caspase-3 and mitogen-activated protein kinases (MAPKs) was detected by Western blot analysis. Addition of the MEK1 inhibitor PD 98059 had no effect on amylin-induced apoptosis, suggesting that ERK activation does not play a role in this apoptotic scenario. A correlative inhibition of JNK activation by the immunosuppressive drug FK506, as well as a selective inhibition of p38 MAPK activation by SB 203580, significantly suppressed procaspase-3 processing and the extent of amylin-induced cell death. Moreover, simultaneous pretreatment with both FK506 and SB 203580, or with the caspase-3 inhibitor Ac-DEVD-CHO alone, almost completely abolished procaspase-3 processing and cell death. Thus, our results suggest that amylin-induced apoptosis proceeds through sustained activation of JNK and p38 MAPK followed by caspase-3 activation.

The hexosamine biosynthesis pathway regulates insulin secretion via protein glycosylation in mouse islets

The hexosamine biosynthesis pathway plays a role in the modification of cellular proteins via the provision of substrate for addition of O-linked N-acetylglucosamine (GlcNAc). The relative importance of the GlcNAc modification of proteins to insulin secretion from pancreatic beta-cells has not been investigated and so remains unclear. In the present study, we show that inhibition of the hexosamine biosynthesis pathway decreases insulin secretion from mouse islets in response to a number of secretagogues, including glucose. This impairment in beta-cell function could not be attributed to reduced islet insulin content, altered ATP levels, or cell death and was restored with the addition of N-acetylglucosamine, a substrate that enters the pathway below the point of inhibition. Western blot analysis revealed that decreased islet protein glycosylation paralleled the decrease in insulin secretion following inhibition of the pathway. In conclusion, the data suggest a role for the hexosamine biosynthesis pathway in regulating the secretion of insulin by altering protein glycosylation. This finding may have implications for the development of type 2 diabetes, as chronic increase in flux through the hexosamine biosynthesis pathway may lead to the deterioration of beta-cell function via abnormal protein glycosylation.

Effects of free fatty acids on insulin secretion in obesity

The prevalence of obesity in Western society has reached epidemic proportions and its aetiological role in the development of type 2 diabetes has made finding an effective treatment for the condition of crucial importance. Of the many consequences of obesity, derangements in glucose metabolism present one of the greatest problems to health. While the role of obesity in causing insulin resistance has received much attention, the effect of obesity on beta-cell failure and the consequent development of type 2 diabetes requires re-emphasis. In this review, the current understanding of the effects of elevated free-fatty acids on beta-cell function will be examined, including a discussion of potential mechanisms. In particular, dysregulation of biochemical pathways and alterations in key enzymes, proteins and hormones will be considered as grounds for the progression to a diabetic phenotype.

Islet amyloid develops diffusely throughout the pancreas before becoming severe and replacing endocrine cells

Islet amyloid occurs in >90% of type 2 diabetic patients and may play a role in the pathogenesis of this disease. To determine whether islet amyloid occurs diffusely throughout the pancreas, whether it affects islets equally, and whether it decreases islet endocrine cells, we characterized islet amyloidosis by computerized fluorescence microscopy in transgenic mice that develop typical islet amyloid. These mice produce the unique amyloidogenic component of human islet amyloid, human islet amyloid polypeptide (hIAPP). The prevalence of amyloid (number of islets containing amyloid/total number of islets x 100) and the severity of amyloid (Sigmaamyloid area/Sigmaislet area x 100) were found to be uniform throughout the pancreas. Furthermore, a high prevalence of amyloid was observed in islets when the severity of amyloid was only 1.5% of the islet area, suggesting a diffuse distribution of amyloid from the very early stages of islet amyloidosis. In 12 hIAPP transgenic mice with an amyloid severity of 9.6 +/- 3.4%, the proportion of islets composed of beta- and delta-cells was reduced in the transgenic mice compared with 6 nontransgenic mice that do not develop amyloid (beta-cells: 62.9 +/- 3.1% vs. 75.5 +/- 0.9%, P = 0.02; delta-cells: 2.8 +/- 0.5% vs. 4.4 +/- 0.4%, P = 0.05), whereas the proportion of islets composed of alpha-cells did not significantly differ between the two groups of mice. In the individual islets in these transgenic mice, amyloid severity was inversely correlated with beta-cell, (r = -0.59, P < 0.0001), alpha-cell (r = -0.32, P < 0.0001), and delta-cell (r = -0.25, P < 0.0001) areas. In conclusion, islet amyloidosis occurs uniformly throughout the pancreas, affecting all islets before becoming severe. A reduction in islet endocrine mass starts at this early stage of islet amyloid development and progresses as amyloid mass increases.

Agouti signaling protein stimulates islet amyloid polypeptide (amylin) secretion in pancreatic beta-cells

Ectopic overexpression of the murine agouti gene results in yellow coat color, obesity, hyperinsulinemia, and type II diabetes. We have shown the human homologue of agouti (agouti signaling protein; ASP) to regulate human adipocyte metabolism and lipid storage via a Ca(2+)-dependent mechanism. We have also demonstrated agouti expression in human pancreas, and that ASP stimulates insulin release via a similar Ca(2+)-dependent mechanism. Plasma amylin is also elevated in agouti mutant mice. Amylin is cosecreted with insulin from beta-cells, and overexpression of human amylin in beta-cells in yellow agouti mutant mice resulted in accelerated pancreatic amyloid deposition, severely impaired beta-cell function, and a diabetic phenotype. We report here that ASP stimulates amylin release in both the HIT-T15 beta-cell line and human pancreatic islets in the presence of a wide range of glucose concentrations (0-16.7 mmol/L), similar to its effect on insulin release; this effect was blocked by 30 mumol/L nitrendipine, confirming a Ca(2+)-dependent mechanism. Accordingly, ASP stimulation of amylin release may serve as a compensatory system to regulate blood glucose in yellow agouti mutants.

The prohormone convertase enzyme 2 (PC2) is essential for processing pro-islet amyloid polypeptide at the NH2-terminal cleavage site

Impaired processing of pro-islet amyloid polypeptide (proIAPP), the precursor of the beta-cell peptide islet amyloid polypeptide (IAPP) (amylin), has been implicated in islet amyloid formation in type 2 diabetes. The prohormone convertase enzymes PC3 (also known as PC1) and PC2 are localized to beta-cell secretory granules with proIAPP and proinsulin and are responsible for proinsulin processing. To determine whether PC2 might be essential for proIAPP processing, we performed Western blot analysis of freshly isolated islets from normal mice and mice lacking active PC2. As expected, the primary species of IAPP immunoreactivity in islets from wild-type mice was fully processed (4-kDa) IAPP, with only small amounts of the 8-kDa precursor (unprocessed proIAPP) present. Islets from heterozygous PC2 null mice were identical to wild-type animals, suggesting that half the normal complement of PC2 is sufficient for normal proIAPP processing. By contrast, in islets from homozygous PC2 null mice, the predominant IAPP-immunoreactive form was of intermediate size (approximately 6 kDa), with no detectable mature IAPP and slightly elevated amounts of the 8-kDa precursor form present. Thus, in the absence of PC2, proIAPP processing appears to be blocked at the level of a proIAPP conversion intermediate. Immunofluorescence of pancreas sections and immunoblotting using antisera raised to the NH2- and COOH-terminal flanking regions of mouse proIAPP demonstrated that the 6-kDa intermediate form was an NH2-terminally extended proIAPP conversion intermediate (processed only at the COOH-terminus). These data indicate that PC2 is essential for processing of proIAPP at the NH2-terminal cleavage site in vivo and that PC3 is likely only capable of processing proIAPP at the COOH-terminal cleavage site.