TOSO promotes ß-cell proliferation and protects from apoptosis.

Decreased ß-cell mass reflects a shift from quiescence/proliferation into apoptosis, it plays a crucial role in the pathophysiology of diabetes. A major attempt to restore ß-cell mass and normoglycemia is to improve ß-cell survival. Here we show that switching off the Fas pathway using Fas apoptotic inhibitory protein (Faim/TOSO), which regulates apoptosis upstream of caspase 8, blocked ß-cell apoptosis and increased proliferation in human islets. TOSO was clearly expressed in pancreatic ß-cells and down-regulated in T2DM. TOSO expression correlated with ß-cell turnover; at conditions of improved survival, TOSO was induced. In contrast, TOSO downregulation induced ß-cell apoptosis. Although TOSO overexpression resulted in a 3-fold induction of proliferation, proliferating ß-cells showed a very limited capacity to undergo multiple rounds of replication. Our data suggest that TOSO is an important regulator of ß-cell turnover and switches ß-cell apoptosis into proliferation.

Purinergic P2X7 receptors regulate secretion of interleukin-1 receptor antagonist and beta cell function and survival.

AIMS/HYPOTHESIS: In obesity, beta cells activate compensatory mechanisms to adapt to the higher insulin demand. Interleukin-1 receptor antagonist (IL-1Ra) prevents obesity-induced hyperglycaemia and is a potent target for the treatment of diabetes, but the mechanisms of its secretion and regulation in obesity are unknown. In the present study, we hypothesise the regulation of IL-1Ra secretion by purinergic P2X(7) receptors in islets. METHODS: Production and regulation of P2X(7) were studied in pancreatic sections from lean and obese diabetic patients, non-diabetic controls and in isolated islets. IL-1Ra, IL-1beta and insulin secretion, glucose tolerance and beta cell mass were studied in P2x7 (also known as P2Rx7)-knockout mice. RESULTS: P2X(7) levels were elevated in beta cells of obese patients, but downregulated in patients with type 2 diabetes mellitus. Elevated glucose and non-esterified fatty acids rapidly activated P2X(7) and IL-1Ra secretion in human islets, and this was inhibited by P2X(7) blockade. In line with our results in vitro, P2x7-knockout mice had a lower capacity to secrete IL-1Ra. They exhibited severe and rapid hyperglycaemia, glucose intolerance and impaired beta cell function in response to a high-fat/high-sucrose diet, were unable to compensate by increasing their beta cell mass in response to the diet and showed increased beta cell apoptosis. CONCLUSIONS/INTERPRETATION: Our study shows a tight correlation of P2X(7) activation, IL-1Ra secretion and regulation of beta cell mass and function. The increase in P2X(7) production is one mechanism that may explain how beta cells compensate by adapting to the higher insulin demand. Disturbances within that system may result in the progression of diabetes.

Identification of ALOX5 as a gene regulating adiposity and pancreatic function.

AIMS/HYPOTHESIS: We previously used an integrative genetics approach to demonstrate that 5-lipoxygenase (5-LO) deficiency in mice (Alox5 (-/-)) protects against atherosclerosis despite increasing lipid levels and fat mass. In the present study, we sought to further examine the role of 5-LO in adiposity and pancreatic function. METHODS: Alox5 (-/-) and wild-type (WT) mice were characterised with respect to adiposity and glucose/insulin metabolism using in vivo and in vitro approaches. The role of ALOX5 in pancreatic function in human islets was assessed through short interfering RNA (siRNA) knockdown experiments. RESULTS: Beginning at 12 weeks of age, Alox5 (-/-) mice had significantly increased fat mass, plasma leptin levels and fasting glucose levels, but lower fasting insulin levels (p<0.05). Although Alox5 (-/-) mice did not exhibit insulin resistance, they had impaired insulin secretion in response to a bolus glucose injection. Histological analyses revealed that Alox5 (-/-) mice had increased islet area, beta cell nuclear size, and numbers of beta cells/mm(2) islet (p<0.05), indicative of both hyperplasia and hypertrophy. Basal and stimulated insulin secretion in isolated Alox5 (-/-) islets were significantly lower than in WT islets (p<0.05) and accompanied by a three- to fivefold decrease in the expression of the genes encoding insulin and pancreatic duodenal homeobox 1 (Pdx1). Direct perturbation of ALOX5 in isolated human islets with siRNA decreased insulin and PDX1 gene expression by 50% and insulin secretion by threefold (p<0.05). CONCLUSIONS/INTERPRETATION: These results provide strong evidence for pleiotropic metabolic effects of 5-LO on adiposity and pancreatic function and may have important implications for therapeutic strategies targeting this pathway for the treatment of cardiovascular disease.