MCL-1 Is a Key Antiapoptotic Protein in Human and Rodent Pancreatic ß-Cells.

Induction of endoplasmic reticulum stress and activation of the intrinsic apoptotic pathway is widely believed to contribute to ß-cell death in type 1 diabetes (T1D). MCL-1 is an antiapoptotic member of the BCL-2 protein family, whose depletion causes apoptosis in rodent ß-cells in vitro. Importantly, decreased MCL-1 expression was observed in islets from patients with T1D. We report here that MCL-1 downregulation is associated with cytokine-mediated killing of human ß-cells, a process partially prevented by MCL-1 overexpression. By generating a ß-cell-specific Mcl-1 knockout mouse strain (ßMcl-1KO), we observed that, surprisingly, MCL-1 ablation does not affect islet development and function. ß-Cells from ßMcl-1KO mice were, however, more susceptible to cytokine-induced apoptosis. Moreover, ßMcl-1KO mice displayed higher hyperglycemia and lower pancreatic insulin content after multiple low-dose streptozotocin treatment. We found that the kinase GSK3ß, the E3 ligases MULE and ßTrCP, and the deubiquitinase USP9x regulate cytokine-mediated MCL-1 protein turnover in rodent ß-cells. Our results identify MCL-1 as a critical prosurvival protein for preventing ß-cell death and clarify the mechanisms behind its downregulation by proinflammatory cytokines. Development of strategies to prevent MCL-1 loss in the early stages of T1D may enhance ß-cell survival and thereby delay or prevent disease progression.

A20 Inhibits ß-Cell Apoptosis by Multiple Mechanisms and Predicts Residual ß-Cell Function in Type 1 Diabetes.

Activation of the transcription factor nuclear factor kappa B (NFkB) contributes to ß-cell death in type 1 diabetes (T1D). Genome-wide association studies have identified the gene TNF-induced protein 3 (TNFAIP3), encoding for the zinc finger protein A20, as a susceptibility locus for T1D. A20 restricts NF-κB signaling and has strong antiapoptotic activities in ß-cells. Although the role of A20 on NF-κB inhibition is well characterized, its other antiapoptotic functions are largely unknown. By studying INS-1E cells and rat dispersed islet cells knocked down or overexpressing A20 and islets isolated from the ß-cell-specific A20 knockout mice, we presently demonstrate that A20 has broader effects in ß-cells that are not restricted to inhibition of NF-κB. These involves, suppression of the proapoptotic mitogen-activated protein kinase c-Jun N-terminal kinase (JNK), activation of survival signaling via v-akt murine thymoma viral oncogene homolog (Akt) and consequently inhibition of the intrinsic apoptotic pathway. Finally, in a cohort of T1D children, we observed that the risk allele of the rs2327832 single nucleotide polymorphism of TNFAIP3 predicted lower C-peptide and higher hemoglobin A1c (HbA1c) levels 12 months after disease onset, indicating reduced residual ß-cell function and impaired glycemic control. In conclusion, our results indicate a critical role for A20 in the regulation of ß-cell survival and unveil novel mechanisms by which A20 controls ß-cell fate. Moreover, we identify the single nucleotide polymorphism rs2327832 of TNFAIP3 as a possible prognostic marker for diabetes outcome in children with T1D.

Heterozygous inactivation of plasma membrane Ca(2+)-ATPase in mice increases glucose-induced insulin release and beta cell proliferation, mass and viability.

AIMS/HYPOTHESIS: Calcium plays an important role in the process of glucose-induced insulin release in pancreatic beta cells. These cells are equipped with a double system responsible for Ca(2+) extrusion--the Na/Ca exchanger (NCX) and the plasma membrane Ca(2+)-ATPase (PMCA). We have shown that heterozygous inactivation of NCX1 in mice increased glucose-induced insulin release and stimulated beta cell proliferation and mass. In the present study, we examined the effects of heterozygous inactivation of the PMCA on beta cell function. METHODS: Biological and morphological methods (Ca(2+) imaging, Ca(2+) uptake, glucose metabolism, insulin release and immunohistochemistry) were used to assess beta cell function and proliferation in Pmca2 (also known as Atp2b2) heterozygous mice and control littermates ex vivo. Blood glucose and insulin levels were also measured to assess glucose metabolism in vivo. RESULTS: Pmca (isoform 2) heterozygous inactivation increased intracellular Ca(2+) stores and glucose-induced insulin release. Moreover, increased beta cell proliferation, mass, viability and islet size were observed in Pmca2 heterozygous mice. However, no differences in beta cell glucose metabolism, proinsulin immunostaining and insulin content were observed. CONCLUSIONS/INTERPRETATION: The present data indicates that inhibition of Ca(2+) extrusion from the beta cell and its subsequent intracellular accumulation stimulates beta cell function, proliferation and mass. This is in agreement with our previous results observed in mice displaying heterozygous inactivation of NCX, and indicates that inhibition of Ca(2+) extrusion mechanisms by small molecules in beta cells may represent a new approach in the treatment of type 1 and type 2 diabetes.