Abstract

Objective – Pro-inflammatory cytokines are cytotoxic to β-cells and have been implicated in the pathogenesis of type 1 diabetes and islet graft failure. The importance of the intrinsic mitochondrial apoptotic pathway in cytokine-induced β-cell death is unclear. Here, cytokine activation of the intrinsic apoptotic pathway and the role of the two pro-apoptotic Bcl-2 proteins, Bad and Bax were examined in β-cells.

Research Design and Methods – Human islets, rat islets, and INS-1 cells were exposed to a combination of pro-inflammatory cytokines: interleukin (IL)-1β, interferon (IFN)γ and/or tumor necrosis factor (TNF)α. Activation of Bad was determined by Ser136 dephosphorylation; mitochondrial stress by changes in mitochondrial metabolic activity and cytochrome c release; and downstream apoptotic signalling by activation of caspase-9 and -3, and DNA fragmentation. The inhibitors FK506 and V5 were used to investigate the role of Bad and Bax activation, respectively.

Results – We found that pro-inflammatory cytokines induced calcineurin-dependent dephosphorylation of Bad Ser136, mitochondrial stress, cytochrome c release, activation of caspase-9 and -3, and DNA fragmentation. Inhibition of Bad Ser136 dephosphorylation or Bax was found to inhibit cytokine-induced intrinsic pro-apoptotic signalling.

Conclusions – Our findings demonstrate that the intrinsic mitochondrial apoptotic pathway contributes significantly to cytokine-induced β-cells death and suggest a functional role of calcineurin-mediated Bad Ser136 dephosphorylation and Bax activity in cytokine-induced apoptosis.