OBJECTIVE-Type 2 diabetes is characterized by impaired insulin secretion in response to increased metabolic demand. This defect in ␤-cell compensation seems to result from the interplay between environmental factors and genetic predisposition. Genome-wide association studies reveal that common variants in transcription factor 7-like 2 (TCF7L2) are associated with increased risk of type 2 diabetes. The aim of the present study was to establish whether TCF7L2 plays a role in ␤-cell function and/or survival. RESEARCH DESIGN AND METHODS-To investigate the effects of TCFL7L2 depletion, isolated islets were exposed to TCF7L2 small interfering RNA (siRNA) versus scrambled siRNA, and ␤-cell survival and function were examined. For TCF7L2 overexpression, islets were cultured in glucose concentrations of 5.5-33.3 mmol/l and the cytokine mix interleukin-1␤/␥-interferon with or without overexpression of TCF7L2. Subsequently, glucose-stimulated insulin secretion (GSIS), ␤-cell apoptosis [by transferase-mediated dUTP nick-end labeling assay and Western blotting for poly(ADP-ribose) polymerase and Caspase-3 cleavage], and ␤-cell proliferation (by Ki67 immunostaining) were analyzed. RESULTS-DepletingTCF7L2 by siRNA resulted in a 5.1-fold increase in ␤-cell apoptosis, 2.2-fold decrease in ␤-cell proliferation (P Ͻ 0.001), and 2.6-fold decrease in GSIS (P Ͻ 0.01) in human islets. Similarly, loss of TCF7L2 resulted in impaired ␤-cell function in mouse islets. In contrast, overexpression of TCF7L2 protected islets from glucose and cytokine-induced apoptosis and impaired function. CONCLUSIONS-TCF7L2is required for maintaining GSIS and ␤-cell survival. Changes in the level of active TCF7L2 in ␤-cells from carriers of at-risk allele may be the reason for defective insulin secretion and progression of type 2 diabetes. Diabetes 57: [645][646][647][648][649][650][651][652][653] 2008

In type 1 and type 2 diabetes (T1/T2DM), beta cell destruction by apoptosis results in decreased beta cell mass and progression of the disease. In this study, we found that the interferon gamma-inducible protein 10 plays an important role in triggering beta cell destruction. Islets isolated from patients with T2DM secreted CXCL10 and contained 33.5-fold more CXCL10 mRNA than islets from control patients. Pancreatic sections from obese nondiabetic individuals and patients with T2DM and T1DM expressed CXCL10 in beta cells. Treatment of human islets with CXCL10 decreased beta cell viability, impaired insulin secretion, and decreased insulin mRNA. CXCL10 induced sustained activation of Akt, JNK, and cleavage of p21-activated protein kinase 2 (PAK-2), switching Akt signals from proliferation to apoptosis. These effects were not mediated by the commonly known CXCL10 receptor CXCR3 but through TLR4. Our data suggest CXCL10 as a binding partner for TLR4 and as a signal toward beta cell failure in diabetes.

Subclinical inflammation is a recently discovered phenomenon in type 2 diabetes. Elevated cytokines impair beta-cell function and survival. A recent clinical trial shows that blocking IL-1beta signaling by IL-1 receptor antagonist (IL-1Ra) improves beta-cell secretory function in patients with type 2 diabetes. In the present study, we provide further mechanisms of the protective role of IL-1Ra on the beta-cell. IL-1Ra prevented diabetes in vivo in C57BL/6J mice fed a high-fat/high-sucrose diet (HFD) for 12 wk; it improved glucose tolerance and insulin secretion. High-fat diet treatment increased serum levels of free fatty acids and of the adipokines resistin and leptin, which were reduced by IL-1Ra treatment. In addition, IL-1Ra counteracted adiponectin levels, which were decreased by high-fat feeding. Studies on isolated islets revealed that IL-1Ra specifically acted on the beta-cell. IL-1Ra protected islets from HFD treated animals from beta-cell apoptosis, induced beta-cell proliferation, and improved glucose-stimulated insulin secretion. Insulin mRNA was reduced in islets from mice fed a HFD but normalized in the IL-1Ra group. Our results show that IL-1Ra improves beta-cell survival and function, and support the potential role for IL-1Ra in the treatment of diabetes.

Type 2 diabetes is characterized by a deficit in ␤-cell mass, and its incidence increases with age. Here, we analyzed ␤-cell turnover in islets from 2-to 3-compared with 7-to 8-month-old rats and in human islets from 53 organ donors with ages ranging from 17 to 74 years. In cultured islets from 2-to 3-month-old rats, the age at which rats are usually investigated, increasing glucose from 5.5 to 11.1 mmol/l decreased ␤-cell apoptosis, which was augmented when glucose was further increased to 33.3 mmol/l. In parallel, ␤-cell proliferation was increased by both 11.1 and 33.3 mmol/l glucose compared with 5.5 mmol/l. In contrast, in islets from 7-to 8-month-old rats and from adult humans, increasing glucose concentrations from 5.5 to 33.3 mmol/l induced a linear increase in ␤-cell death and a decrease in proliferation. Additionally, in cultivated human islets, age correlated positively with the sensitivity to glucose-induced ␤-cell apoptosis and negatively to baseline proliferation. In rat islets, constitutive expression of Fas ligand and glucose-induced Fas receptor expression were observed only in 7-to 8-month-old but not in 2-to 3-month-old islets, whereas no age-dependent changes in the Fas/Fas ligand system could be detected in human islets. However, pancreatic duodenal homeobox (PDX)-1 expression decreased with age in pancreatic tissue sections of rats and humans. Furthermore, older rat islets were more sensitive to the high-glucose-mediated decrease in PDX-1 expression than younger islets. Therefore, differences in glucose sensitivity between human and 2-to 3-month-old rat islets may be due to both differences in age and in the genetic background. These data provide a possible explanation for the increased incidence of type 2 diabetes at an older age and support the use of islets from older rats as a more appropriate model to study glucose-induced ␤-cell apoptosis. Diabetes 55: [2455][2456][2457][2458][2459][2460][2461][2462] 2006

Aims/hypothesisIn 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 P2X7 receptors in islets.MethodsProduction and regulation of P2X7 were studied in pancreatic sections from lean and obese diabetic patients, non-diabetic controls and in isolated islets. IL-1Ra, IL-1β and insulin secretion, glucose tolerance and beta cell mass were studied in P2x7 (also known as P2Rx7)-knockout mice.ResultsP2X7 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 P2X7 and IL-1Ra secretion in human islets, and this was inhibited by P2X7 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/interpretationOur study shows a tight correlation of P2X7 activation, IL-1Ra secretion and regulation of beta cell mass and function. The increase in P2X7 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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00125-009-1349-0) contains supplementary material, which is available to authorised users.