Differential effects of p27 in regulation of beta-cell mass during development, neonatal period, and adult life

beta-Cell cycle progression and proliferation are critical to maintain beta-cell mass in adult mice. Of the cell cycle inhibitors, p27Kip1 is thought to be the primary modulator of the proliferative status in most cell types. p27 plays a role in beta-cell adaptation in genetic models of insulin resistance. To study the role of p27 in beta-cells during physiological conditions and at different stages of beta-cell differentiation, we studied mice deficient of or overexpressing p27. Experiments in p27-deficient mice showed improved glucose tolerance and hyperinsulinemia. These changes were associated with increased islet mass and proliferation. The experiments overexpressing p27 in beta-cells were performed using a doxycycline-inducible model. Interestingly, overexpression of p27 for 16 weeks in beta-cells from adult mice had no effect on glucose tolerance, beta-cell mass, or proliferation. In contrast, induction of p27 expression during beta-cell development or early neonatal period resulted in severe glucose intolerance and reduced beta-cell mass by decreased proliferation. These changes were reversible upon discontinuation of doxycycline. These experiments suggest that p27 is a critical molecule for beta-cell proliferation during beta-cell development and early postnatal life but not for maintenance of adult mass.     

p27 Regulates the transition of beta-cells from quiescence to proliferation

Diabetes results from an inadequate mass of functional beta-cells. Such inadequacy could result from loss of beta-cells due to an immune assault or the inability to compensate for insulin resistance. Thus, mechanisms that regulate the number of beta-cells will be key to understanding both the pathogenesis of diabetes and for developing therapies. In this study, we show that cell cycle regulator p27 plays a crucial role in establishing the number of beta-cells formed before birth. We show that p27 accumulates in terminally differentiated beta-cells during embryogenesis. Disabling p27 allows newly differentiated beta-cells that are normally quiescent during embryogenesis to reenter the cell cycle and proliferate. As a consequence, excess beta-cells are generated in the p27(-/-) mice, doubling their beta-cell mass at birth. The early postnatal expansion of beta-cell mass was unaffected in p27(-/-) mice, indicating that the main function of p27 is to maintain the quiescent state of newly differentiated beta-cells generated during embryogenesis. The expanded beta-cell mass was accompanied by increased insulin secretion; however, the p27(-/-) mice were glucose intolerant, as these mice were insulin insensitive. To assess the role of p27 to affect regeneration of beta-cells in models of diabetes, p27(-/-) mice were injected with streptozotocin (STZ). In contrast to control mice that displayed elevated blood glucose levels, p27(-/-) mice showed decreased susceptibility to develop STZ-induced diabetes. Furthermore, beta-cells retained the ability to reenter the cell cycle at a far greater frequency in p27(-/-) mice after developing STZ-induced diabetes compared with wild-type littermates. These data indicate that p27 is a key regulator in establishing beta-cell mass and an important target for facilitating beta-cell regeneration in therapies for diabetes.     

Heterogeneity in pancreatic beta-cell population.

All pancreatic beta-cells are identified by specific morphological characteristics. Similarity in microscopic features is not necessarily associated with identity in functional properties. In vitro studies on isolated rat beta-cells have indicated intercellular differences in the threshold for glucose-induced shifts in metabolic redox state. The cellular heterogeneity in glucose sensitivity results in a dose-dependent recruitment of glucose-exposed beta-cells into biosynthetic and secretory activities. The molecular basis of this diversity is not known. Indirect evidence supports the concept that the in situ pancreatic beta-cell population is also composed of functionally diverse subpopulations. The heterogeneity in glucose responsiveness is expected to create subpopulations of beta-cells with either constant, fluctuating, or occasional glucose-dependent functions; whether any subpopulation is preferentially responsive to other regulatory factors and/or committed to other activities is unknown. Morphological markers may help identify beta-cell subpopulations in situ and quantify their size in conditions known to affect total beta-cell mass or function. The concept of a functionally heterogeneous beta-cell population influences views on the role of pancreatic beta-cells in health and disease.     

Compensatory growth of pancreatic beta-cells in adult rats after short-term glucose infusion

The extent to which adult pancreatic beta-cells can respond in vivo to a sustained glucose stimulus by increasing their mass through either hyperplasia or hypertrophy has remained unanswered. Therefore, we studied the in vivo effect of short-term (96-h) hyperglycemia on the growth of beta-cells by infusing adult rats with 35 or 50% glucose or 0.45% saline. After 96 h of glucose infusion, the beta-cell mass, quantified by point-counting morphometrics of immunoperoxidase-stained paraffin sections, showed a 50% increase (9.57 +/- 0.87 mg, n = 5, 50% glucose infused; 9.50 +/- 1.23, n = 7, 35% glucose infused; 6.15 +/- 0.55, n = 6, 0.45% saline infused). This growth was selective for beta-cells; the non-beta-cell mass was unchanged. The mitotic index, measured by accumulated mitotic frequency after a 4-h colchicine treatment, increased fivefold in glucose-infused animals compared to saline-infused animals. This enhanced replication of beta-cells provides evidence for increase in cell number or hyperplasia. In addition, hypertrophy of the beta-cell was also quantified. Mean cell volume, determined from the mean cell cross-sectional area measured planimetrically from low-magnification electron micrographs, increased to 150% of control values after 96 h of 50% glucose infusion. Seven days after the 96-h infusion, in reversal experiments, the beta-cell mass had not returned to saline-infused levels. In addition, the non-beta-cell mass of glucose-infused animals had increased. The mitotic index of the beta-cell of glucose-infused rats was, however, significantly lower than that of the saline controls, but the mean cell volume of the beta-cells remained elevated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta-cell adaptation and decompensation during the progression of diabetes

Inadequate beta-cell function is an essential component of all forms of diabetes. The most obvious problem is a failure to maintain sufficient beta-cell mass and function to cope with whatever insulin resistance is present. The most striking functional defect is a loss of acute glucose-induced insulin secretion (GIIS). This review discusses the ways in which beta-cells successfully adapt to increased demand and then decompensate as diabetes develops. Successful adaptation is achieved through increased beta-cell mass and increased insulin secretion. The hypothesis is explored that beta-cells exposed to the diabetic milieu lose their differentiation, which leads to loss of specialized functions such as GIIS. This concept has been strengthened by the finding of dedifferentiation of beta-cells in a rat model of partial pancreatectomy that includes a reduction of insulin gene expression, which may further contribute to decreased insulin production. Another finding was increased expression of c-Myc, which probably contributes to an increase in the expression of lactate dehydrogenase and the development of beta-cell hypertrophy. Arguments are developed that the beta-cell changes found in diabetes are better correlated with increased glucose levels than with non-esterified fatty acid levels, thus supporting the importance of glucose toxicity.     

Tissue-specific deletion of the retinoblastoma protein in the pancreatic beta-cell has limited effects on beta-cell replication, mass, and function

Animal studies show that G(1/S) regulatory molecules (D-cyclins, cdk-4, p18, p21, p27) are critical for normal regulation of beta-cell proliferation, mass, and function. The retinoblastoma protein, pRb, is positioned at the very end of a cascade of these regulatory proteins and is considered the final checkpoint molecule that maintains beta-cell cycle arrest. Logically, removal of pRb from the beta-cell should result in unrestrained beta-cell replication, increased beta-cell mass, and insulin-mediated hypoglycemia. Because global loss of both pRb alleles is embryonic lethal, this hypothesis has not been tested in beta-cells. We developed two types of conditional knockout (CKO) mice in which both alleles of the pRb gene were inactivated specifically in beta-cells. Surprisingly, although the pRb gene was efficiently recombined in beta-cells of both CKO models, changes in beta-cell mass, beta-cell replication rates, insulin concentrations, and blood glucose levels were limited or absent. Other pRb family members, p107 and p130, were not substantially upregulated. In contrast to dogma, the pRb protein is not essential to maintain cell cycle arrest in the pancreatic beta-cell. This may reflect fundamental inaccuracies in models of beta-cell cycle control or complementation for pRb by undefined proteins.     

Sialylated form of the neural cell adhesion molecule (NCAM): a new tool for the identification and sorting of beta-cell subpopulations with different functional activity

To clarify the relationship between variations in beta-cell mass and pancreatic function, we investigated the possibility to analyze, quantify, and sort beta-cell subpopulations with different functional maturity. To this aim, we tested the reliability of the sialylated form of neural cell adhesion molecule (NCAM) (PSA-NCAM) as a marker of beta-cell functional activity. Islet cells isolated from adult rats were analyzed for their PSA-NCAM abundance using an anti-PSA-NCAM antibody. We found that PSA-NCAM is expressed only in beta-cells. The PSA-NCAM labeling was also studied with a fluorescence-activated cell sorter. We showed that the beta-cell population is heterogeneous for PSA-NCAM labeling. To directly determine the relationship between PSA-NCAM labeling and beta-cell activity, in vitro insulin secretion studies were performed on sorted beta-cell subpopulations using a perifusion technique. Two beta-cell subpopulations were analyzed: one that was highly labeled for PSA-NCAM and another that was poorly labeled. Insulin secretion from high PSA-NCAM-labeled beta-cells was significantly higher than that in low PSA-NCAM-labeled beta-cells. This differential expression in the beta-cell population was well correlated with differences in glucose responsiveness. PSA-NCAM seems thus suitable for use as a tool to identify beta-cell subpopulations according to their glucose responsiveness.     

Glibenclamide treatment recruits beta-cell subpopulation into elevated and sustained basal insulin synthetic activity

Use of sulfonylureas in diabetes treatment is based on their insulin-releasing effect on pancreatic beta-cells. Prolonged action is known to degranulate beta-cells, but functional consequences have not been examined at the cellular level. This study investigates influences of in vivo (48-h) and in vitro (24-h) glibenclamide treatment on the functional state of the beta-cell population. Both conditions decreased cellular insulin content by >50% and caused an elevated basal insulin biosynthetic activity that was maintained for at least 24 h after drug removal. Glibenclamide stimulation of basal insulin synthesis was not achieved after a 2-h exposure; it required a calcium-dependent translational activity and involved an increase in the percent activated beta-cells (50% after glibenclamide pretreatment vs. 8% in control cells). The glibenclamide-activated beta-cell subpopulation corresponded to the degranulated beta-cell subpopulation that was isolated by fluorescence-activated cell sorter on the basis of lower cellular sideward scatter. Glibenclamide pretreatment did not alter cellular rates of glucose oxidation but sensitized beta-cells to glucose-induced changes in metabolic redox and insulin synthesis and release. In conclusion, chronic exposure to glibenclamide results in degranulation of a subpopulation of beta-cells, which maintain an elevated protein and insulin synthetic activity irrespective of the presence of the drug and of glucose. Our study demonstrates that the in situ beta-cell population also exhibits a functional heterogeneity that can vary with drug treatment. Glibenclamide induces degranulated beta-cells with a sustained elevated basal activity that might increase the risk for hypoglycemic episodes.     

Decreased insulin secretion in type 2 diabetes: a problem of cellular mass or function?

Type 2 diabetes is characterized by diminished or inappropriate secretion of insulin, which could be a defect of either islet cell function or beta-cell mass. Quantitation of islet cell populations in postmortem pancreas demonstrates little change of beta-cell mass in type 2 diabetes. Reduction of islet cell mass (up to 30%) is associated largely with islet amyloid deposition, and the degree of amyloidosis is independent of the duration of the disease. Insulin secretory capacity is dependent on both function and mass of cells. beta-Cell secretion is heterogeneous; increasing glucose concentrations result in recruitment of beta-cells into the secretory pool, indicating a large reserve of secretory capacity that can be recruited in insulin resistant conditions. The Starling curve of islet function describes the relationship of insulin secretion to increasing levels of insulin resistance and hyperglycemia in type 2 diabetes. Longitudinal studies in Macaca mulatta monkeys show that insulin resistance is accompanied by increased islet mass and onset of diabetes is associated with deposition of amyloid and reduction of beta-cells. Increasing the function of unresponsive beta-cells rather than the mass of cells may be a more effective therapeutic target for type 2 diabetes.     

Overexpression of cyclin D1 in pancreatic beta-cells in vivo results in islet hyperplasia without hypoglycemia

Cyclin D1 can stimulate proliferation by driving cells from the G1 into the S-phase of the mammalian cell cycle. Previous animal studies have implicated the G1-S transition as a key regulatory checkpoint governing the proliferation of pancreatic islet cells. We expressed cyclin D1 in the beta-cells of mice and islet hyperplasia developed in a time-dependent manner. The hyperplastic beta-cells exhibited higher rates of proliferation. However, blood glucose levels in fasting as well as nonfasting conditions remained normal. Furthermore, glucose tolerance tests demonstrated nearly normal responses, and diabetes did not develop in any of the animals. No islet cell tumors were observed, even among animals >2 years of age. Under our experimental conditions, the proliferative stimulus provided by cyclin D1 is not tumorigenic, does not result in diabetes, and does not result in hypoglycemia. Cyclin D1 may thus be considered a potential candidate to augment the beta-cell population ex vivo as a prelude to islet transplantation for diabetes.     

Effects of autoimmunity and immune therapy on beta-cell turnover in type 1 diabetes

beta-Cell mass can expand in response to demand: during pregnancy, in the setting of insulin resistance, or after pancreatectomy. It is not known whether similar beta-cell hyperplasia occurs following immune therapy of autoimmune diabetes, but the clinical remission soon after diagnosis and the results of recent immune therapy studies suggest that beta-cell recovery is possible. We studied changes in beta-cell replication, mass, and apoptosis in NOD mice during progression to overt diabetes and following immune therapy with anti-CD3 monoclonal antibodies (mAbs) or immune regulatory T-cells (Tregs). beta-Cell replication increases in pre-diabetic mice, after adoptive transfer of diabetes with increasing islet inflammation but before an increase in blood glucose concentration or a significant decrease in beta-cell mass. The pathogenic cells are responsible for increasing beta-cell replication because replication was reduced during diabetes remission induced by anti-CD3 mAb or Tregs. beta-Cell replication stimulated by the initial inflammatory infiltrate results in increased production of new beta-cells after immune therapy and increased beta-cell area, but the majority of this increased beta-cell area represents regranulated beta-cells rather than newly produced cells. We conclude that beta-cell replication is closely linked to the islet inflammatory process. A significant proportion of degranulated beta-cells remain, at the time of diagnosis of diabetes, that can recover after metabolic correction of hyperglycemia. Correction of the beta-cell loss in type 1 diabetes will, therefore, require strategies that target both the immunologic and cellular mechanisms that destroy and maintain beta-cell mass.     

Overexpression of parathyroid hormone-related protein inhibits pancreatic beta-cell death in vivo and in vitro

Pancreatic beta-cell survival is critical in the setting of diabetes as well as in islet transplantation. Transgenic mice overexpressing parathyroid hormone-related protein (PTHrP) targeted to beta-cells using the rat insulin II promoter (RIP) display hyperinsulinemia, hypoglycemia, and islet hyperplasia, without a concomitant increase in beta-cell proliferation rate or enlargement of individual beta-cell size. Thus, the mechanism for increased beta-cell mass is unknown. In this study, we demonstrated that beta-cells of transgenic mice are resistant to the cytotoxic effects of streptozotocin (STZ) in vivo, as documented by a sixfold reduction in the rate of STZ-induced beta-cell death in RIP-PTHrP mice relative to their normal siblings. The reduced cell death in transgenic mice is due neither to their increased islet mass nor to a decrease in their sensing of STZ, but rather results from PTHrP-induced resistance to beta-cell death. This is also demonstrated in vitro by markedly reduced cell death rates observed in beta-cells of transgenic mice compared with normal mice when cultured in the absence of serum and glucose or in the presence of STZ. Finally, we demonstrated that NH(2)-terminal PTHrP inhibits beta-cell death. These findings support the concept that PTHrP overexpression increases islet mass in transgenic mice through inhibition of beta-cell death.     

Tolbutamide as mimic of glucose on beta-cell electrical activity. ATP-sensitive K+ channels as common pathway for both stimuli

It is accepted for insulin-secreting cells in culture that the closure of ATP-sensitive K+ channels causes the glucose-dependent depolarization of pancreatic beta-cells seen at subthreshold levels (less than 100 mg/dl) of glucose. The question remains for the more thoroughly studied beta-cells in freshly dissected intact islets, however, whether closure of these channels is responsible for subthreshold glucose-dependent depolarization and suprathreshold glucose-dependent regulation of membrane electrical activity. To answer this, we took advantage of the ability of tolbutamide, an orally active antidiabetic agent, to specifically inhibit ATP-sensitive K+ channels in pancreatic beta-cells to determine whether these channels are active at sub- and suprathreshold levels of glucose and whether channel closure by tolbutamide reproduces the electrophysiological effects of glucose stimulation. We recorded membrane electrical activity from freshly dissected adult mouse pancreatic islets exposed to various levels of glucose and tolbutamide. As previously found by others, tolbutamide depolarizes islet cells in the absence of glucose, but we have found that, although the depolarization can trigger Ca2+ action potentials (spikes), a glucose-dependent permissive factor may be required for the normal bursting pattern of spiking. More significantly, we found that, unlike other beta-cell stimuli, tolbutamide specifically mimics the effects of glucose stimulation on the pattern of suprathreshold electrical activity. The effects were seen with levels of tolbutamide that correspond to those required to inhibit ATP-sensitive K+ channels. These data suggest that ATP-sensitive K+ channels are active at sub- and suprathreshold levels of glucose and may be the sole pathway by which either glucose or tolbutamide depolarizes beta-cells and controls beta-cell electrical activity.     

Role of pancreatic beta-cells in the process of beta-cell death

Studies on the pathogenesis of type 1 diabetes have mainly focused on the role of the immune system in the destruction of pancreatic beta-cells. Lack of data on the cellular and molecular events at the beta-cell level is caused by the inaccessibility of these cells during development of the disease. Indirect information has been collected from isolated rodent and human islet cell preparations that were exposed to cytotoxic conditions. This article reviews in vitro experiments that investigated the role of beta-cells in the process of beta-cell death. beta-Cells rapidly die in necrosis because of toxic levels of oxidizing radicals or of nitric oxide; they progressively become apoptotic after prolonged culture at low glucose or with proinflammatory cytokines. Their susceptibility to necrosis or apoptosis varies with their functional state and thus with the environmental conditions. A change in cellular phenotype can alter its recognition of potentially cytotoxic agents and its defense mechanisms against cell death. These observations support the view that beta-cells are not necessarily passive victims of a cytotoxic process but can actively participate in a process of beta-cell death. Their role will be influenced by neighboring non-beta-cells, which can make the islet internal milieu more protective or toxic for the beta-cells. We consider duct cells as potentially important contributors to this local process.     

Glucose induces opposite intracellular Ca2+ concentration oscillatory patterns in identified alpha- and beta-cells within intact human islets of Langerhans

Homeostasis of blood glucose is mainly regulated by the coordinated secretion of glucagon and insulin from alpha- and beta-cells within the islets of Langerhans. The release of both hormones is Ca(2+) dependent. In the current study, we used confocal microscopy and immunocytochemistry to unequivocally characterize the glucose-induced Ca(2+) signals in alpha- and beta-cells within intact human islets. Extracellular glucose stimulation induced an opposite response in these two cell types. Although the intracellular Ca(2+) concentration ([Ca(2+)](i)) in beta-cells remained stable at low glucose concentrations, alpha-cells exhibited an oscillatory [Ca(2+)](i) response. Conversely, the elevation of extracellular glucose elicited an oscillatory [Ca(2+)](i) pattern in beta-cells but inhibited low-glucose-induced [Ca(2+)](i) signals in alpha-cells. These Ca(2+) signals were synchronic among beta-cells grouped in clusters within the islet, although they were not coordinated among the whole beta-cell population. The response of alpha-cells was totally asynchronic. Therefore, both the alpha- and beta-cell populations within human islets did not work as a syncitium in response to glucose. A deeper knowledge of alpha- and beta-cell behavior within intact human islets is important to better understand the physiology of the human endocrine pancreas and may be useful to select high-quality islets for transplantation.     

Beta-cell glucose toxicity, lipotoxicity, and chronic oxidative stress in type 2 diabetes

The relentless decline in beta-cell function frequently observed in type 2 diabetic patients, despite optimal drug management, has variously been attributed to glucose toxicity and lipotoxicity. The former theory posits hyperglycemia, an outcome of the disease, as a secondary force that further damages beta-cells. The latter theory suggests that the often-associated defect of hyperlipidemia is a primary cause of beta-cell dysfunction. We review evidence that patients with type 2 diabetes continually undergo oxidative stress, that elevated glucose concentrations increase levels of reactive oxygen species in beta-cells, that islets have intrinsically low antioxidant enzyme defenses, that antioxidant drugs and overexpression of antioxidant enzymes protect beta-cells from glucose toxicity, and that lipotoxicity, to the extent it can be attributable to hyperlipidemia, occurs only in the context of preexisting hyperglycemia, whereas glucose toxicity can occur in the absence of hyperlipidemia.     

Increased intracellular calcium is required for spreading of rat islet beta-cells on extracellular matrix

Rat islet beta-cells spread in response to glucose when attached on the matrix produced by a rat bladder carcinoma cell line (804G). Furthermore, in a mixed population of cells, it has been observed previously that spread cells secrete more insulin acutely in response to glucose, compared with cells that remain rounded. These results suggest bi-directional signaling between the islet beta-cell and the extracellular matrix. In the present study, the role of increased intracellular free Ca2+ concentration [Ca2+]i as an intracellular step linking glucose stimulation and beta-cell spreading (inside-out signaling) was investigated. Purified rat beta-cells were attached to this matrix and incubated under various conditions known to affect [Ca2+]i. The effect of glucose on beta-cell spreading was mimicked by 25 mmol/l KCl (which induces calcium influx) and inhibited by diazoxide (which impairs depolarization and calcium entry) and by the L-type Ca2+ channel blocker SR-7037. When a 24-h incubation at 16.7 glucose was followed by 24 h at 2.8 mmol/l, beta-cells that had first spread regained a round phenotype. In the presence of thapsigargin, spreading progressed throughout the experiment, suggesting that capture of calcium by the endoplasmic reticulum is involved in the reversibility of spreading previously induced by glucose. Spreading was still observed in degranulated beta-cells and in botulinum neurotoxin E-expressing beta-cells when exocytosis was prevented. In summary, the results indicate that increased [Ca2+]i is required for the glucose-induced spreading of beta-cells on 804G matrix and that it is not a consequence of exocytotic processes that follow elevation of [Ca2+]i.     

Targeted inactivation of hepatocyte growth factor receptor c-met in beta-cells leads to defective insulin secretion and GLUT-2 downregulation without alteration of beta-cell mass

Overexpression of hepatocyte growth factor (HGF) in the beta-cell of transgenic mice enhances beta-cell proliferation, survival, and function. In the current studies, we have used conditional ablation of the c-met gene to uncover the physiological role of HGF in beta-cell growth and function. Mice in which c-met is inactivated in the beta-cell (MetCKO mice) display normal body weight, blood glucose, and plasma insulin compared with control littermates. In contrast, MetCKO mice displayed significantly diminished glucose tolerance and reduced plasma insulin after a glucose challenge in vivo. This impaired glucose tolerance in MetCKO mice was not caused by insulin resistance because sensitivity to exogenous insulin was similar in both groups. Importantly, in vitro glucose-stimulated insulin secretion in MetCKO islets was decreased by approximately 50% at high glucose concentrations compared with control islets. Furthermore, whereas insulin and glucokinase expression in MetCKO islets were normal, GLUT-2 expression was decreased by approximately 50%. These changes in beta-cell function in MetCKO mice were not accompanied by changes in total beta-cell mass, islet morphology, islet cell composition, and beta-cell proliferation. Interestingly, however, MetCKO mice display an increased number of small islets, mainly single and doublet beta-cells. We conclude that HGF/c-met signaling in the beta-cell is not essential for beta-cell growth, but it is essential for normal glucose-dependent insulin secretion.     

Glucose induces beta-cell apoptosis via upregulation of the Fas receptor in human islets

In autoimmune type 1 diabetes, Fas-to-Fas-ligand (FasL) interaction may represent one of the essential pro-apoptotic pathways leading to a loss of pancreatic beta-cells. In the advanced stages of type 2 diabetes, a decline in beta-cell mass is also observed, but its mechanism is not known. Human islets normally express FasL but not the Fas receptor. We observed upregulation of Fas in beta-cells of type 2 diabetic patients relative to nondiabetic control subjects. In vitro exposure of islets from nondiabetic organ donors to high glucose levels induced Fas expression, caspase-8 and -3 activation, and beta-cell apoptosis. The effect of glucose was blocked by an antagonistic anti-Fas antibody, indicating that glucose-induced apoptosis is due to interaction between the constitutively expressed FasL and the upregulated Fas. These results support a new role for glucose in regulating Fas expression in human beta-cells. Upregulation of the Fas receptor by elevated glucose levels may contribute to beta-cell destruction by the constitutively expressed FasL independent of an autoimmune reaction, thus providing a link between type 1 and type 2 diabetes.