Beta cell response to nutrient overload involves phospholipid remodelling and lipid peroxidation.

AIMS/HYPOTHESIS: Membrane phospholipids are the major intracellular source for fatty acid-derived mediators, which regulate myriad cell functions. We showed previously that high glucose levels triggered the hydrolysis of polyunsaturated fatty acids from beta cell phospholipids. These fatty acids were subjected to free radical-catalysed peroxidation to generate the bioactive aldehyde 4-hydroxy-2E-nonenal (4-HNE). The latter activated the nuclear peroxisome proliferator-activated receptor-δ (PPARδ), which in turn augmented glucose-stimulated insulin secretion. The present study aimed at investigating the combined effects of glucose and fatty acid overload on phospholipid turnover and the subsequent generation of lipid mediators, which affect insulin secretion and beta cell viability. METHODS: INS-1E cells were incubated with increasing glucose concentrations (5-25 mmol/l) without or with palmitic acid (PA; 50-500 µmol/l) and taken for fatty acid-based lipidomic analysis and functional assays. Rat isolated islets of Langerhans were used similarly. RESULTS: PA was incorporated into membrane phospholipids in a concentration- and time-dependent manner; incorporation was highest at 25 mmol/l glucose. This was coupled to a rapid exchange with saturated, mono-unsaturated and polyunsaturated fatty acids. Importantly, released arachidonic acid and linoleic acid were subjected to peroxidation, resulting in the generation of 4-HNE, which further augmented insulin secretion by activating PPARδ in beta cells. However, this adaptive increase in insulin secretion was abolished at high glucose and PA levels, which induced endoplasmic reticulum stress, apoptosis and cell death. CONCLUSIONS/INTERPRETATION: These findings highlight a key role for phospholipid remodelling and fatty acid peroxidation in mediating adaptive and cytotoxic interactions induced by nutrient overload in beta cells.

Foam cell-derived 4-hydroxynonenal induces endothelial cell senescence in a TXNIP-dependent manner.

Vascular endothelial cell (VEC) senescence is considered an early event in the development of atherosclerotic lesions. Stressful stimuli, in particular oxidative stress, have been linked to premature senescence in the vasculature. Foam cells are a major source of reactive oxygen species and may play a role in the induction of VEC senescence; hence, we investigated their involvement in the induction of VEC senescence in a co-culture transwell system. Primary bovine aortic endothelial cells, exposed to the secretome of THP-1 monocyte-derived foam cells, were analysed for the induction of senescence. Senescence associated ß-galactosidase activity and the expression of p16 and p21 were increased, whereas phosphorylated retinoblastoma protein was reduced. This senescent phenotype was mediated by 4-hydroxnonenal (4-HNE), a lipid peroxidation product secreted from foam cells; scavenging of 4-HNE in the co-culture medium blunted this effect. Furthermore, both foam cells and 4-HNE increased the expression of the pro-oxidant thioredoxin-interacting protein (TXNIP). Molecular manipulation of TXNIP expression confirmed its involvement in foam cell-induced senescence. Previous studies showed that peroxisome proliferator-activated receptor (PPAR)δ was activated by 4-hydroalkenals, such as 4-HNE. Pharmacological interventions supported the involvement of the 4-HNE-PPARδ axis in the induction of TXNIP and VEC senescence. The association of TXNIP with VEC senescence was further supported by immunofluorescent staining of human carotid plaques in which the expression of both TXNIP and p21 was augmented in endothelial cells. Collectively, these findings suggest that foam cell-released 4-HNE activates PPARδ in VEC, leading to increased TXNIP expression and consequently to senescence.

Metabolic and genomic dissection of diabetes in the Cohen rat.

We investigated the metabolic and genetic basis of diabetes in the Cohen Diabetic rat, a model of diet-induced diabetes, as a means to identify the molecular mechanisms involved. By altering individual components in the diabetogenic diet, we established that the dietary susceptibility that leads to the development of diabetes in this model is directly related to the high casein and low copper content in chow. The development of diabetes is accompanied by depletion of the acini from the exocrine pancreas and replacement with fat cells, while the appearance of the islets of Langerhans remains intact. With reversion back from diabetogenic to regular diet, the diabetic phenotype disappears but the histological changes in the exocrine pancreas prevail. Using positional cloning, we detected a major quantitative trait locus (QTL) on rat chromosome 4 with a chromosomal span of 4.9 cM, and two additional loci on chromosomes 7 and X. A screen for genes within that QTL in the rat and in the syntenic regions in mouse and man revealed only 23 candidate genes. Notable among these genes is Ica1, which has been causally associated with diabetes and bovine casein. We conclude that the development of diabetes in our model is dependent upon high casein and low copper in diet, that it is accompanied by histomorphological changes in the exocrine but not endocrine pancreas, that it is reversible, and that it is associated with a major QTL on chromosome 4 in which we detected Ica1, a high priority candidate gene.

Glucose-induced beta cell production of IL-1beta contributes to glucotoxicity in human pancreatic islets.

In type 2 diabetes, chronic hyperglycemia is suggested to be detrimental to pancreatic beta cells, causing impaired insulin secretion. IL-1beta is a proinflammatory cytokine acting during the autoimmune process of type 1 diabetes. IL-1beta inhibits beta cell function and promotes Fas-triggered apoptosis in part by activating the transcription factor NF-kappaB. Recently, we have shown that increased glucose concentrations also induce Fas expression and beta cell apoptosis in human islets. The aim of the present study was to test the hypothesis that IL-1beta may mediate the deleterious effects of high glucose on human beta cells. In vitro exposure of islets from nondiabetic organ donors to high glucose levels resulted in increased production and release of IL-1beta, followed by NF-kappaB activation, Fas upregulation, DNA fragmentation, and impaired beta cell function. The IL-1 receptor antagonist protected cultured human islets from these deleterious effects. beta cells themselves were identified as the islet cellular source of glucose-induced IL-1beta. In vivo, IL-1beta-producing beta cells were observed in pancreatic sections of type 2 diabetic patients but not in nondiabetic control subjects. Similarly, IL-1beta was induced in beta cells of the gerbil Psammomys obesus during development of diabetes. Treatment of the animals with phlorizin normalized plasma glucose and prevented beta cell expression of IL-1beta. These findings implicate an inflammatory process in the pathogenesis of glucotoxicity in type 2 diabetes and identify the IL-1beta/NF-kappaB pathway as a target to preserve beta cell mass and function in this condition.

Calreticulin destabilizes glucose transporter-1 mRNA in vascular endothelial and smooth muscle cells under high-glucose conditions.

Substrate autoregulation of glucose transporter-1 (GLUT-1) mRNA and protein expression provides vascular endothelial and smooth muscle cells a sensitive mechanism to adapt their rate of glucose transport in response to changing glycemic conditions. Hyperglycemia-induced downregulation of glucose transport is particularly important in protecting these cells against an excessive influx of glucose and consequently increased intracellular protein glycation and generation of free radicals; both are detrimental in the development of vascular disease in diabetes. We aimed to investigate the molecular mechanism of high glucose-induced downregulation of GLUT-1 mRNA expression in primary bovine aortic vascular endothelial (VEC) and smooth muscle (VSMC) cell cultures. Using RNA mobility shift, UV cross-linking, and in vitro degradation assays, followed by mass-spectrometric analysis, we identified calreticulin as a specific destabilizing trans-acting factor that binds to a 10-nucleotide cis-acting element (CAE(2181-2190)) in the 3'-untranslated region of GLUT-1 mRNA. Pure calreticulin accelerated the rate of GLUT-1 mRNA-probe degradation in vitro, whereas overexpression of calreticulin in vascular cells decreased significantly the total cell content of GLUT-1 mRNA and protein. The expression of calreticulin was augmented in vascular cells exposed to high glucose in comparison with low-glucose conditions. Similarly, increased expression of calreticulin was observed in aortae of diabetic Psammomys obesus in comparison with normoglycemic controls. These data suggest that CAE(2181-2190)-calreticulin complex, which is formed in VSMC and VEC exposed to hyperglycemic conditions, renders GLUT-1 mRNA susceptible to degradation. This interaction underlies the process of downregulation of glucose transport in vascular cells under high-glucose conditions.

Regulation of insulin secretion and proinsulin biosynthesis by succinate.

Succinate stimulates insulin secretion and proinsulin biosynthesis. We studied the effects of reduced nicotinamide adenine dinucleotide phosphate (NADPH)-modulating pathways on glucose- and succinate-stimulated insulin secretion and proinsulin biosynthesis in the rat and the insulin-resistant Psammomys obesus. Disruption of the anaplerotic pyruvate/malate shuttle by phenylacetic acid inhibited glucose- and succinate-stimulated insulin secretion and succinate-stimulated proinsulin biosynthesis in both species. In contrast, phenylacetic acid failed to inhibit glucose-stimulated proinsulin biosynthesis in P. obesus islets. Inhibition of the NADPH-consuming enzyme neuronal nitric oxide synthase (nNOS) with l-N(G)-nitro-l-arginine methyl ester or with N(G)-monomethyl-l-arginine(G) doubled succinate-stimulated insulin secretion in rat islets, suggesting that succinate- and nNOS-derived signals interact to regulate insulin secretion. In contrast, nNOS inhibition had no effect on succinate-stimulated proinsulin biosynthesis in both species. In P. obesus islets, insulin secretion was not stimulated by succinate in the absence of glucose, whereas proinsulin biosynthesis was increased 5-fold. Conversely, under stimulating glucose levels, succinate doubled insulin secretion, indicating glucose-dependence. Pyruvate ester and inhibition of nNOS partially mimicked the permissive effect of glucose on succinate-stimulated insulin secretion, suggesting that anaplerosis-derived signals render the beta-cells responsive to succinate. We conclude that beta-cell anaplerosis via pyruvate carboxylase is important for glucose- and succinate-stimulated insulin secretion and for succinate-stimulated proinsulin biosynthesis. In P. obesus, pyruvate/malate shuttle dependent and independent pathways that regulate proinsulin biosynthesis coexist; the latter can maintain fuel stimulated biosynthetic activity when the succinate-dependent pathway is inhibited. nNOS signaling is a negative regulator of insulin secretion, but not of proinsulin biosynthesis.

Dynamic changes in {beta}-cell mass and pancreatic insulin during the evolution of nutrition-dependent diabetes in psammomys obesus: impact of glycemic control.

Recent studies ascribe a major role to pancreatic beta-cell loss in type 2 diabetes. We investigated the dynamics of beta-cell mass during diabetes evolution in Psammomys obesus, a model for nutrition-dependent type 2 diabetes, focusing on the very early and the advanced stages of the disease. P. obesus fed a high-calorie diet for 26 days developed severe hyperglycemia, beta-cell degranulation, and markedly reduced pancreatic insulin content. Reducing calories for 7 days induced normoglycemia in 90% of the animals, restoring beta-cell granulation and insulin content. To dissociate effects of diet from blood glucose reduction, diabetic animals received phlorizin for 2 days, which normalized glycemia and increased the pancreatic insulin reserve to 50% of control, despite a calorie-rich diet. During diabetes progression, beta-cell mass decreased initially but recovered spontaneously to control levels, despite persistent hyperglycemia. Strikingly, however, beta-cell mass did not correlate with degree of hyperglycemia or pancreatic insulin content. We conclude that reduced insulin reserve is the main cause of diabetes progression, whereas irreversible beta-cell mass reduction is a late event in P. obesus. The rapid recovery of the pancreas by phlorizin-induced normoglycemia implies a causal relationship between hyperglycemia and islet dysfunction. Similar mechanisms could be operative during the evolution of type 2 diabetes in humans.

Psammomys obesus, a model for environment-gene interactions in type 2 diabetes.

Type 2 diabetes is characterized by insulin resistance and progressive beta-cell failure. Deficient insulin secretion, with increased proportions of insulin precursor molecules, is a common feature of type 2 diabetes; this could result from inappropriate beta-cell function and/or reduced beta-cell mass. Most studies using tissues from diabetic patients are retrospective, providing only limited information on the relative contribution of beta-cell dysfunction versus decreased beta-cell mass to the "beta-cell failure" of type 2 diabetes. The gerbil Psammomys obesus is a good model to address questions related to the role of insulin resistance and beta-cell failure in nutritionally induced diabetes. Upon a change from its natural low-calorie diet to the calorie-rich laboratory food, P. obesus develops moderate obesity associated with postprandial hyperglycemia. Continued dietary load, superimposed on its innate insulin resistance, results in depletion of pancreatic insulin stores, with increased proportions of insulin precursor molecules in the pancreas and the blood. Inadequate response of the preproinsulin gene to the increased insulin needs is an important cause of diabetes progression. Changes in beta-cell mass do not correlate with pancreatic insulin stores and are unlikely to play a role in disease initiation and progression. The major culprit is the inappropriate insulin production with depletion of insulin stores as a consequence. Similar mechanisms could operate during the evolution of type 2 diabetes in humans.

Beta-cell protein kinases and the dynamics of the insulin response to glucose.

A full biphasic insulin response is the most sensitive index for well-coupled beta-cell signal transduction. While first-phase insulin response is extremely sensitive to potentiating and inhibiting modulations, full expression of second-phase response requires near maximally activated beta-cell fuel metabolism. In the isolated rat pancreas, accelerated calcium entry or activation of protein kinase (PK)-A or PKC result in no insulin response in the absence of fuel metabolism. At submaximal levels of beta-cell fuel secretagogue, arginine (which promotes calcium entry) or glucagon (which activates PKA) produces a small first-phase insulin response but minimal or no second-phase response; carbachol (which activates PKC and promotes calcium entry) generates biphasic insulin response in the presence of minimal fuel (3.3 mmol/l glucose). Glucagon produces full biphasic response in the presence of 10.0 mmol/l glucose, whereas arginine requires near-maximal stimulatory glucose (16.7 mmol) to produce full biphasic insulin response. Thus, PKA and PKC signal pathways potentiate primary signals generated by fuel secretagogues to induce full biphasic insulin response, while calcium recruitment alone is insufficient to potentiate primary signals generated at low levels of fuel secretagogue. We suggest that three families of PKs (calmodulin-dependent PK [CaMK], PKA, and PKC) function as distal amplifiers for stimulus-secretion coupling signals originating from fuel metabolism, as well as from incretins acting through membrane receptors, adenylate cyclase, and phospholipase C. Several isoenzymes of PKA and PKC are present in pancreatic beta-cells, but the specific function of most is still undefined. Each PK isoenzyme is activated and subsequently phosphorylates its specific effector protein by binding to a highly specific anchoring protein. Some diabetes-related beta-cell derangements may be linked to abnormal function of one or more PK isoenzymes. Identification and characterization of the specific function of the individual PK isoenzymes may provide the tool to improve the insulin response of the diabetic patient.

Increased glucose sensitivity of stimulus-secretion coupling in islets from Psammomys obesus after diet induction of diabetes.

When fed a high-energy (HE) diet, diabetes-prone (DP) Psammomys obesus develop type 2 diabetes with altered glucose-stimulated insulin secretion (GSIS). Beta-cell stimulus-secretion coupling was investigated in islets isolated from DP P. obesus fed a low-energy (LE) diet (DP-LE) and after 5 days on a HE diet (DP-HE). DP-LE islets cultured overnight in 5 mmol/l glucose displayed glucose dose-dependent increases in NAD(P)H, mitochondrial membrane potential, ATP/(ATP + ADP) ratio, cytosolic calcium concentration ([Ca(2+)](c)), and insulin secretion. In comparison, DP-HE islets cultured overnight in 10 mmol/l glucose were 80% degranulated and displayed an increased sensitivity to glucose at the level of glucose metabolism, [Ca(2+)](c), and insulin secretion. These changes in DP-HE islets were only marginally reversed after culture in 5 mmol/l glucose and were not reproduced in DP-LE islets cultured overnight in 10 mmol/l glucose, except for the 75% degranulation. Diabetes-resistant P. obesus remain normoglycemic on HE diet. Their beta-cell stimulus-secretion coupling was similar to that of DP-LE islets, irrespective of the type of diet. Thus, islets from diabetic P. obesus display an increased sensitivity to glucose at the level of glucose metabolism and a profound beta-cell degranulation, both of which may affect their in vivo GSIS.

Insulin does not mediate glucose stimulation of proinsulin biosynthesis.

It has recently been suggested that insulin augments its own production by a physiologically important feed-forward autocrine loop. We studied the kinetics of glucose-regulated proinsulin gene expression and proinsulin biosynthesis in normal rat islets with emphasis on the potential role of insulin as a mediator of the glucose effect. There was a time-dependent increase in steady-state proinsulin mRNA in islets cultured at 16.7 mmol/l compared with 3.3 mmol/l glucose; no early (1-3 h) increase in proinsulin gene expression was observed. In contrast, there was a threefold increase in proinsulin biosynthesis within 1 h of glucose stimulation that was not affected by inhibition of glucose-stimulated proinsulin gene transcription with actinomycin D. In addition, inhibition of glucose-stimulated insulin secretion with diazoxide had no effect on glucose-stimulated proinsulin mRNA or biosynthesis. Furthermore, addition of different concentrations of insulin to islets cultured in low glucose failed to affect proinsulin biosynthesis. Taken together, our data suggest that the early glucose-dependent increase in proinsulin biosynthesis is mainly regulated at the translational level, rather than by changes in proinsulin gene expression. Moreover, we could not demonstrate any effect of insulin on islet proinsulin mRNA level or rate of proinsulin biosynthesis. Thus, if insulin has any effect on the proinsulin biosynthetic apparatus, it is a minor one. We conclude that the secreted insulin is not an important mediator of insulin production in response to glucose.

Characterization of the action of S 21403 (mitiglinide) on insulin secretion and biosynthesis in normal and diabetic beta-cells.

S 21403 (mitiglinide) is a new drug for type 2 diabetes mellitus (T2DM). Its action on insulin release and biosynthesis was investigated in several experimental systems utilizing pancreas from normal and T2DM animals. At high concentrations (10 microM), S 21403, like classical sulphonylurea, induced insulin release in the absence of glucose. In contrast, at therapeutic (0.1-1.0 microM) concentrations, S 21403 amplified insulin secretion glucose dose-dependently and with similar magnitude in normal and diabetic GK rat islets. In perfused GK rat pancreas, S 21403 induced normal kinetics of insulin secretion including first-phase response. The effect of S 21403 was strongly modulated by physiological factors. Thus, 0.1 microM adrenaline inhibited S 21403-induced insulin release. There was marked synergism between S 21403 and arginine in GK rat islets, combination of the two normalizing insulin secretion. In primary islet cultures from normal rats or prediabetic Psammomys obesus, prolonged exposure to S 21403 did not induce further depletion of insulin stores under normal or 'glucotoxic' conditions. Proinsulin biosynthesis was not affected by 2-h exposure of rat or prediabetic P. obesus islets to 1 microM S 21403. Yet, 24-h exposure of rat islets to S 21403 resulted in 30% increase in proinsulin biosynthesis at 8.3 mM glucose. Amplification by S 21403 of glucose-induced insulin secretion in diabetic GK beta-cells with restoration of first-phase response, a strong synergistic interaction with arginine and marked inhibition by adrenaline, make it a prime candidate for successful oral antidiabetic agent.

Glucose-regulated proinsulin gene expression is required for adequate insulin production during chronic glucose exposure.

Psammomys obesus, an animal model of type 2 diabetes, shows rapid and marked depletion of pancreatic insulin content as hyperglycemia develops when fed a high-calorie diet. P. obesus islets do not increase proinsulin gene expression when exposed to high glucose, which may be related to absence of the conserved form of the transcription factor insulin promoter factor 1/pancreatic-duodenal homeobox 1. The present study assesses the importance of regulation of proinsulin gene expression by glucose for insulin production. Islets of diabetes-prone P. obesus and diabetes-resistant Wistar rats, cultured at various glucose concentrations for up to 24 h, were analyzed for proinsulin mRNA by quantitative RT-PCR, proinsulin biosynthesis by leucine incorporation into proinsulin, and insulin content and secretion by RIA. No increase in proinsulin mRNA was observed in P. obesus islets during 24-h exposure to increasing concentrations of glucose. In contrast, rat islets exposed to high glucose responded with a 2- to 3-fold stimulation of proinsulin mRNA. The failure of P. obesus islets to increase proinsulin mRNA was accompanied by a reduced proinsulin biosynthetic response: after 24 h, maximal proinsulin biosynthesis was blunted, associated with depletion of islet insulin content. Inhibition of glucose-stimulated proinsulin gene transcription in rat islets by actinomycin D did not affect the early proinsulin biosynthetic response, which, however, was reduced to the level of P. obesus islets after 24 h in culture. We conclude that stimulation of proinsulin gene transcription by glucose is necessary for maintaining proinsulin biosynthesis and hence conserving pancreatic insulin stores, under conditions of sustained secretory drive, but not for short-term regulation of proinsulin biosynthesis Our findings support the hypothesis that inadequate regulation of proinsulin gene expression by glucose contributes to the failure of P. obesus to cope with the increased demand for insulin associated with caloric excess, leading to depletion of insulin stores and diabetes.

Glucotoxicity and beta-cell failure in type 2 diabetes mellitus.

Type 2 diabetes mellitus is increasing worldwide with a trend of declining age of onset. It is characterized by insulin resistance and a progressive loss of beta-cell function. The ability to secrete adequate amounts of insulin is determined by the functional integrity of beta-cells and their overall mass. Glucose, the main regulator of insulin secretion and production, exerts negative effects on beta-cell function when present in excessive amounts over a prolonged period. The multiple metabolic aberrations induced by chronic hyperglycemia in the beta-cell include increased sensitivity to glucose, increased basal insulin release, reduced response to stimulus to secrete insulin, and a gradual depletion of insulin stores. Inadequate insulin production during chronic hyperglycemia results from decreased insulin gene transcription due to hyperglycemia-induced changes in the activity of beta-cell specific transcription factors. Hyperglycemia may negatively affect beta-cell mass by inducing apoptosis without a compensatory increase in beta-cell proliferation and neogenesis. The detrimental effect of excessive glucose concentrations is referred to as 'glucotoxicity'. The present review discusses the role of glucotoxicity in beta-cell dysfunction in type 2 diabetes mellitus.

Improvement of ER stress-induced diabetes by stimulating autophagy.

Pancreatic ß-cell dysfunction is central in diabetes. The diabetic milieu may impair proinsulin folding, leading to ß-cell endoplasmic reticulum (ER) stress and apoptosis, and thus a worsening of the diabetes. Autophagy is crucial for the well-being of the ß-cell; however, the impact of stimulating autophagy on ß-cell adaptation to ER stress is unknown. We studied the crosstalk between ER stress and autophagy in a rodent model of diabetes, called Akita, in which proinsulin gene mutation leads to protein misfolding and ß-cell demise. We found that proinsulin misfolding stimulates autophagy and, in symmetry, inhibition of autophagy induces ß-cell stress and apoptosis. Under conditions of excessive proinsulin misfolding, stimulation of autophagy by inhibiting MTORC1 alleviates stress and prevents apoptosis. Moreover, treatment of diabetic Akita mice with the MTORC1 inhibitor rapamycin improves diabetes and prevents ß-cell apoptosis. Thus, autophagy is a central adaptive mechanism in ß-cell stress. Stimulation of autophagy may become a novel therapeutic strategy in diabetes.

Signaling properties of 4-hydroxyalkenals formed by lipid peroxidation in diabetes.

Peroxidation of polyunsaturated fatty acids is intensified in cells subjected to oxidative stress and results in the generation of various bioactive compounds, of which 4-hydroxyalkenals are prominent. During the progression of type 2 diabetes mellitus, the ensuing hyperglycemia promotes the generation of reactive oxygen species (ROS) that contribute to the development of diabetic complications. It has been suggested that ROS-induced lipid peroxidation and the resulting 4-hydroxyalkenals markedly contribute to the development and progression of these pathologies. Recent findings, however, also suggest that noncytotoxic levels of 4-hydroxyalkenals play important signaling functions in the early phase of diabetes and act as hormetic factors to induce adaptive and protective responses in cells, enabling them to function in the hyperglycemic milieu. Our studies and others' have proposed such regulatory functions for 4-hydroxynonenal and 4-hydroxydodecadienal in insulin secreting ß-cells and vascular endothelial cells, respectively. This review presents and discusses the mechanisms regulating the generation of 4-hydroxyalkenals under high glucose conditions and the molecular interactions underlying the reciprocal transition from hormetic to cytotoxic agents.

Stimulation of autophagy improves endoplasmic reticulum stress-induced diabetes.

Accumulation of misfolded proinsulin in the ß-cell leads to dysfunction induced by endoplasmic reticulum (ER) stress, with diabetes as a consequence. Autophagy helps cellular adaptation to stress via clearance of misfolded proteins and damaged organelles. We studied the effects of proinsulin misfolding on autophagy and the impact of stimulating autophagy on diabetes progression in Akita mice, which carry a mutation in proinsulin, leading to its severe misfolding. Treatment of female diabetic Akita mice with rapamycin improved diabetes, increased pancreatic insulin content, and prevented ß-cell apoptosis. In vitro, autophagic flux was increased in Akita ß-cells. Treatment with rapamycin further stimulated autophagy, evidenced by increased autophagosome formation and enhancement of autophagosome-lysosome fusion. This was associated with attenuation of cellular stress and apoptosis. The mammalian target of rapamycin (mTOR) kinase inhibitor Torin1 mimicked the rapamycin effects on autophagy and stress, indicating that the beneficial effects of rapamycin are indeed mediated via inhibition of mTOR. Finally, inhibition of autophagy exacerbated stress and abolished the anti-ER stress effects of rapamycin. In conclusion, rapamycin reduces ER stress induced by accumulation of misfolded proinsulin, thereby improving diabetes and preventing ß-cell apoptosis. The beneficial effects of rapamycin in this context strictly depend on autophagy; therefore, stimulating autophagy may become a therapeutic approach for diabetes.

Diet-induced diabetes in the sand rat (Psammomys obesus).

Insulin deficiency is the underlying cause of hyperglycemia in type 2 diabetes. The gerbil Psammomys obesus (P. obesus) is a naturally insulin resistant rodent with tendency to develop diet-induced hyperglycemia associated with obesity. P. obesus does not exhibit hyperglycemia in its natural desert habitat, feeding on low caloric vegetation. However, when fed regular laboratory chow containing higher caloric density, the animals develop moderate obesity and hyperglycemia. Diabetes development and progression is very fast in P. obesus. The animals reach the irreversible hypoinsulinemic stage of the disease, in which a marked reduction of ß-cell mass is apparent, within 4-6 weeks of high caloric diet. The present review describes the P. obesus of the Hebrew University colony, with emphasis on its use for the study of ß-cell dysfunction in type 2 diabetes.

ß-Cell failure in type 2 diabetes.

Type 2 diabetic patients are insulin resistant as a result of obesity and a sedentary lifestyle. Nevertheless, it has been known for the past five decades that insulin response to nutrients is markedly diminished in type 2 diabetes. There is now a consensus that impaired glucose regulation cannot develop without insulin deficiency. First-phase insulin response to glucose is lost very early in the development of type 2 diabetes. Several prospective studies have shown that impaired insulin response to glucose is a predictor of future impaired glucose tolerance (IGT) and type 2 diabetes. Recently discovered type 2 diabetes-risk gene variants influence ß-cell function, and might represent the molecular basis for the low insulin secretion that predicts future type 2 diabetes. We believe type 2 diabetes develops on the basis of normal but 'weak'ß-cells unable to cope with excessive functional demands imposed by overnutrition and insulin resistance. Several laboratories have shown a reduction in ß-cell mass in type 2 diabetes and IGT, whereas others have found modest reductions and most importantly, a large overlap between ß-cell masses of diabetic and normoglycemic subjects. Therefore, at least initially, the ß-cell dysfunction of type 2 diabetes seems more functional than structural. However, type 2 diabetes is a progressive disorder, and animal models of diabetes show ß-cell apoptosis with prolonged hyperglycemia/hyperlipemia (glucolipotoxicity). ß-Cells exposed in vitro to glucolipotoxic conditions show endoplasmic reticulum (ER) and oxidative stress. ER stress mechanisms might participate in the adaptation of ß-cells to hyperglycemia, unless excessive. ß-Cells are not deficient in anti-oxidant defense, thioredoxin playing a major role. Its inhibitor, thioredoxin-interacting protein (TXNIP), might be important in leading to ß-cell apoptosis and type 2 diabetes. These topics are intensively investigated and might lead to novel therapeutic approaches. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2010.00094.x, 2011).