UBC Theses and Dissertations
The role of mitochondrial (intrinsic) apoptotic pathway in hIAPP mediated β-cell death Wong, Helen Yunshan
Type 2 diabetes (T2D) is a complex and heterogeneous metabolic disorder characterized by insulin deficiency due to progressive loss of pancreatic β-cell function/mass and insulin resistance in peripheral tissues. Islet amyloid is a common pathologic characteristic of the pancreas in patients with T2D which also forms in cultured and transplanted human islets. Intra- and extra-cellular amyloid formation due to aggregation of the β-cell peptide human islet amyloid polypeptides (hIAPP) contributes to progressive β-cell dysfunction and death in T2D and islet grafts. Our research group previously showed that blocking the amyloid-induced Fas-mediated apoptotic pathway markedly reduces, but does not completely prevent, amyloid toxicity. Since hIAPP aggregates also form intracellularly, we proposed to investigate the potential role of mitochondrial-dependent (intrinsic) apoptotic pathway in amyloid-induced β-cell toxicity. In the present study, we examined if intracellular hIAPP aggregates can initiate the mitochondrial apoptotic pathway and tested if blocking this pathway protects β-cells from toxic hIAPP aggregates. INS-1 transformed rat β-cells and dispersed human islet cells were transduced with an adenovirus that codes for the expression of hIAPP (Ad-prohIAPP; to form intracellular hIAPP aggregates) or non-fibrillogenic rIAPP (Ad-prorIAPP; as control) and cultured with or without the inhibitor of Bax or caspase-9 (to block the intrinsic apoptotic pathway). Adenoviral mediated hIAPP expression and formation of intracellular hIAPP oligomers in both transformed INS-1 β-cells and primary human islet β-cells increased cytochrome c release leading to caspase-9 activation and β-cell apoptosis, both of which were prevented by Bax or caspase-9 inhibitor. We next used mouse islets isolated from CytcKA/KA and BaxBak DKO mice transduced with Ad-prohIAPP, as two models of ex vivo hIAPP aggregation with blocked intrinsic apoptotic pathway. Blocking the pro-apoptotic function of cytochrome c (CytcKA/KA) or activation of Bax and Bak (BaxBak DKO) in islets, prevented activation of the mitochondrial apoptotic pathway mediated by intracellular hIAPP aggregates and enhanced β-cell survival as compared to wild-type islets despite comparable hIAPP aggregation. These data suggest that: 1) intracellular hIAPP aggregates trigger the mitochondrial apoptotic pathway in islet β-cells; 2) blocking the pro-apoptotic function of cytochrome c or deletion of Bax and Bak protects islet β-cells from hIAPP aggregates.
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