Deletion of the mitochondrial flavoprotein apoptosis inducing factor (AIF) induces beta-cell apoptosis and impairs beta-cell mass.

BACKGROUND: Apoptosis is a hallmark of beta-cell death in both type 1 and type 2 diabetes mellitus. Understanding how apoptosis contributes to beta-cell turnover may lead to strategies to prevent progression of diabetes. A key mediator of apoptosis, mitochondrial function, and cell survival is apoptosis inducing factor (AIF). In the present study, we investigated the role of AIF on beta-cell mass and survival using the Harlequin (Hq) mutant mice, which are hypomorphic for AIF. METHODOLOGY/PRINCIPAL FINDINGS: Immunohistochemical evaluation of pancreata from Hq mutant mice displayed much smaller islets compared to wild-type mice (WT). Analysis of beta-cell mass in these mice revealed a greater than 4-fold reduction in beta-cell mass together with an 8-fold increase in beta-cell apoptosis. Analysis of cell cycle dynamics, using BrdU pulse as a marker for cells in S-phase, did not detect significant differences in the frequency of beta-cells in S-phase. In contrast, double staining for phosphorylated Histone H3 and insulin showed a 3-fold increase in beta-cells in the G2 phase in Hq mutant mice, but no differences in M-phase compared to WT mice. This suggests that the beta-cells from Hq mutant mice are arrested in the G2 phase and are unlikely to complete the cell cycle. beta-cells from Hq mutant mice display increased sensitivity to hydrogen peroxide-induced apoptosis, which was confirmed in human islets in which AIF was depleted by siRNA. AIF deficiency had no effect on glucose stimulated insulin secretion, but the impaired effect of hydrogen peroxide on beta-cell function was potentiated. CONCLUSIONS/SIGNIFICANCE: Our results indicate that AIF is essential for maintaining beta-cell mass and for oxidative stress response. A decrease in the oxidative phosphorylation capacity may counteract the development of diabetes, despite its deleterious effects on beta-cell survival.

CXCL10 impairs beta cell function and viability in diabetes through TLR4 signaling.

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.

The antiinflammatory cytokine interleukin-1 receptor antagonist protects from high-fat diet-induced hyperglycemia.

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.

Glucose and leptin induce apoptosis in human beta-cells and impair glucose-stimulated insulin secretion through activation of c-Jun N-terminal kinases.

c-Jun N-terminal kinases (SAPK/JNKs) are activated by inflammatory cytokines, and JNK signaling is involved in insulin resistance and beta-cell secretory function and survival. Chronic high glucose concentrations and leptin induce interleukin-1beta (IL-1beta) secretion from pancreatic islets, an event that is possibly causal in promoting beta-cell dysfunction and death. The present study provides evidence that chronically elevated concentrations of leptin and glucose induce beta-cell apoptosis through activation of the JNK pathway in human islets and in insulinoma (INS 832/13) cells. JNK inhibition by the dominant inhibitor JNK-binding domain of IB1/JIP-1 (JNKi) reduced JNK activity and apoptosis induced by leptin and glucose. Exposure of human islets to leptin and high glucose concentrations leads to a decrease of glucose-induced insulin secretion, which was partly restored by JNKi. We detected an interplay between the JNK cascade and the caspase 1/IL-1beta-converting enzyme in human islets. The caspase 1 gene, which contains a potential activating protein-1 binding site, was up-regulated in pancreatic sections and in isolated islets from type 2 diabetic patients. Similarly, cultured human islets exposed to high glucose- and leptin-induced caspase 1 and JNK inhibition prevented this up-regulation. Therefore, JNK inhibition may protect beta-cells from the deleterious effects of high glucose and leptin in diabetes.

Transcription factor 7-like 2 regulates beta-cell survival and function in human pancreatic islets.

OBJECTIVE: Type 2 diabetes is characterized by impaired insulin secretion in response to increased metabolic demand. This defect in beta-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 beta-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 beta-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 beta/gamma-interferon with or without overexpression of TCF7L2. Subsequently, glucose-stimulated insulin secretion (GSIS), beta-cell apoptosis [by transferase-mediated dUTP nick-end labeling assay and Western blotting for poly(ADP-ribose) polymerase and Caspase-3 cleavage], and beta-cell proliferation (by Ki67 immunostaining) were analyzed. RESULTS: Depleting TCF7L2 by siRNA resulted in a 5.1-fold increase in beta-cell apoptosis, 2.2-fold decrease in beta-cell proliferation (P < 0.001), and 2.6-fold decrease in GSIS (P < 0.01) in human islets. Similarly, loss of TCF7L2 resulted in impaired beta-cell function in mouse islets. In contrast, overexpression of TCF7L2 protected islets from glucose and cytokine-induced apoptosis and impaired function. CONCLUSIONS: TCF7L2 is required for maintaining GSIS and beta-cell survival. Changes in the level of active TCF7L2 in beta-cells from carriers of at-risk allele may be the reason for defective insulin secretion and progression of type 2 diabetes.

Evidence that N-acetylcysteine inhibits TNF-alpha-induced cerebrovascular endothelin-1 upregulation via inhibition of mitogen- and stress-activated protein kinase.

N-acetylcysteine (NAC) is neuroprotective in animal models of acute brain injury such as caused by bacterial meningitis. However, the mechanism(s) by which NAC exerts neuroprotection is unclear. Gene expression of endothelin-1 (ET-1), which contributes to cerebral blood flow decline in acute brain injury, is partially regulated by reactive oxygen species, and thus a potential target of NAC. We therefore examined the effect of NAC on tumor necrosis factor (TNF)-alpha-induced ET-1 production in cerebrovascular endothelial cells. NAC dose dependently inhibited TNF-alpha-induced preproET-1 mRNA upregulation and ET-1 protein secretion, while upregulation of inducible nitric oxide synthase (iNOS) was unaffected. Intriguingly, NAC had no effect on the initial activation (i.e., IkappaB degradation, nuclear p65 translocation, and Ser536 phosphorylation) of NF-kappaB by TNF-alpha. However, transient inhibition of NF-kappaB DNA binding suggested that NAC may inhibit ET-1 upregulation by inhibiting (a) parallel pathway(s) necessary for full transcriptional activation of NF-kappaB-mediated ET-1 gene expression. Similar to NAC, the MEK1/2 inhibitor U0126, the p38 inhibitor SB203580, and the protein kinase inhibitor H-89 selectively inhibited ET-1 upregulation without affecting nuclear p65 translocation, suggesting that NAC inhibits ET-1 upregulation via inhibition of mitogen- and stress-activated protein kinase (MSK). Supporting this notion, cotreatment with NAC inhibited the TNF-alpha-induced rise in MSK1 and MSK2 kinase activity, while siRNA knock-down experiments showed that MSK2 is the predominant isoform involved in TNF-alpha-induced ET-1 upregulation.