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

Glucotoxicity and lipotoxicity contribute to the impaired β-cell function observed in type 2 diabetes. Here we examine the effect of saturated and unsaturated fatty acids at different glucose concentrations on β-cell proliferation and apoptosis. Adult rat pancreatic islets were cultured onto plates coated with extracellular matrix derived from bovine corneal endothelial cells. Exposure of islets to saturated fatty acid (0.5 mmol/l palmitic acid) in medium containing 5.5, 11.1, or 33.3 mmol/l glucose for 4 days resulted in a five- to ninefold increase of β-cell DNA fragmentation. In contrast, monounsaturated palmitoleic acid alone (0.5 mmol/l) or in combination with palmitic acid (0.25 or 0.5 mmol/l each) did not affect DNA fragmentation. Increasing concentrations of glucose promoted β-cell proliferation that was dramatically reduced by palmitic acid. Palmitoleic acid enhanced the proliferation activity in medium containing 5.5 mmol/l glucose but had no additional effect at higher glucose concentrations (11.1 and 33.3 mmol/l). The cell-permeable ceramide analog C2-ceramide mimicked both the palmitic acid—induced β-cell apoptosis and decrease in proliferation. Moreover, the ceramide synthetase inhibitor fumonisin B1 blocked the deleterious effects of palmitic acid on β-cell viability. Additionally, palmitic acid but not palmitoleic acid decreased the expression of the mitochondrial adenine nucleotide translocator and induced release of cytochrome c from the mitochondria into the cytosol. Finally, palmitoleic acid improved β-cell—secretory function that was reduced by palmitic acid. Taken together, these results suggest that the lipotoxic effect of the saturated palmitic acid involves an increased apoptosis rate coupled with reduced proliferation capacity of β-cells and impaired insulin secretion. The deleterious effect of palmitate on β-cell turnover is mediated via formation of ceramide and activation of the apoptotic mitochondrial pathway. In contrast, the monounsaturated palmitoleic acid does not affect β-cell apoptosis, yet it promotes β-cell proliferation at low glucose concentrations, counteracting the negative effects of palmitic acid as well as improving β-cell function.

Glucotoxicity and lipotoxicity contribute to the impaired β-cell function observed in type 2 diabetes. Here we examine the effect of saturated and monounsaturated fatty acids at different glucose concentrations on human β-cell turnover and secretory function. Exposure of cultured human islets to saturated fatty acid and/or to an elevated glucose concentration for 4 days increased β-cell DNA fragmentation and decreased β-cell proliferation. In contrast, the monounsaturated palmitoleic acid or oleic acid did not affect DNA fragmentation and induced β-cell proliferation. Moreover, each monounsaturated fatty acid prevented the deleterious effects of both palmitic acid and high glucose concentration. The cell-permeable ceramide analogue C2-ceramide mimicked both the palmitic acid-induced β-cell apoptosis and decrease in proliferation. Furthermore, the ceramide synthetase inhibitor fumonisin B1 blocked the deleterious effects of palmitic acid on β-cell turnover. In addition, palmitic acid decreased Bcl-2 expression and induced release of cytochrome c from the mitochondria into the cytosol, which was prevented by fumonisin B1 and by oleic acid. Finally, each monounsaturated fatty acid improved β-cell secretory function that was reduced by palmitic acid and by high glucose. Thus, in human islets, the saturated palmitic acid and elevated glucose concentration induce β-cell apoptosis, decrease β-cell proliferation, and impair β-cell function, which can be prevented by monounsaturated fatty acids. The deleterious effect of palmitic acid is mediated via formation of ceramide and activation of the apoptotic mitochondrial pathway, whereas Bcl-2 may contribute to the protective effect of monounsaturated fatty acids.

A decrease in the number of functional insulin-producing β-cells contributes to the pathophysiology of type 2 diabetes. Opinions diverge regarding the relative contribution of a decrease in β-cell mass versus an intrinsic defect in the secretory machinery. Here we review the evidence that glucose, dyslipidemia, cytokines, leptin, autoimmunity, and some sulfonylureas may contribute to the maladaptation of β-cells. With respect to these causal factors, we focus on Fas, the ATP-sensitive K⁺ channel, insulin receptor substrate 2, oxidative stress, nuclear factor-κB, endoplasmic reticulum stress, and mitochondrial dysfunction as their respective mechanisms of action. Interestingly, most of these factors are involved in inflammatory processes in addition to playing a role in both the regulation of β-cell secretory function and cell turnover. Thus, the mechanisms regulating β-cell proliferation, apoptosis, and function are inseparable processes.

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 β-cells. In the advanced stages of type 2 diabetes, a decline in β-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 β-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 β-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 β-cells. Upregulation of the Fas receptor by elevated glucose levels may contribute to β-cell destruction by the constitutively expressed FasL independent of an autoimmune reaction, thus providing a link between type 1 and type 2 diabetes.