Proinflammatory Cytokines Activate the Intrinsic Apoptotic Pathway in β-Cells

Abstract

OBJECTIVE Proinflammatory 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 proapoptotic Bcl-2 proteins, Bad and Bax, were examined in β-cells. RESEARCH DESIGN AND METHODS Human and rat islets and INS-1 cells were exposed to a combination of proinflammatory cytokines (interleukin-1β, interferon-γ, and/or tumor necrosis factor-α). Activation of Bad was determined by Ser136 dephosphorylation, mitochondrial stress by changes in mitochondrial metabolic activity and cytochrome c release, downstream apoptotic signaling 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 proinflammatory 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 proapoptotic signaling. CONCLUSIONS Our findings demonstrate that the intrinsic mitochondrial apoptotic pathway contributes significantly to cytokine-induced β-cell death and suggest a functional role of calcineurin-mediated Bad Ser136 dephosphorylation and Bax activity in cytokine-induced apoptosis.