Cytokines activate caspase-3 in insulinoma cells of diabetes-prone NOD mice directly and via upregulation of Fas

Abstract

In type 1 diabetes, autoimmune inflammation of pancreatic islets of Langerhans (‘insulitis’) results in destruction of insulin-producing β cells. Cytokines released from islet-infiltrating mononuclear cells are known to be cytotoxic both directly and by upregulating Fas for FasL-induced apoptosis. To investigate the role of caspase-3, a major effector of apoptosis in β-cell death, we asked whether cytokine- and/or FasL-induced apoptosis was associated with increased activity of caspase-3 in NIT-1 insulinoma cells and islets of autoimmune diabetes-prone NOD mice. Measurement of caspase-3 activity using a fluorogenic cleavage assay was validated in NOD mouse thymocytes undergoing dexamethasone (Dex)-induced apoptosis. For cytokine-induced apoptosis, NIT-1 cells or islets were exposed to IL-1β and IFN-γ for 24 h. Caspase-3-like activity was increased 2.1 ± 0.7 and 2.4 ± 0.9-fold in lysates of cytokine-treated NIT-1 cells and NOD mouse islets, respectively. However, NIT-1 cells exhibited 2.1% (4.7 pg active caspase-3/μg protein) and islets 0.8% (1.9 pg active caspase-3/μg protein) of the active caspase-3 content observed in Dex-treated thymocytes (225.1 pg active caspase-3/μg protein). After 24 h cytokine-exposure, the percentage of Fas-positive NIT-1 cells increased from 1.4 ± 1.1 to 29.7 ± 11.6%. Addition of FasL for a further 3 h increased caspase-3-like activity an additional 1.8-fold in cytokine-treated NIT-1 cells. In summary, exposure of NOD mouse insulinoma cells or islets to IL-1β and IFN-γ for 24 h induced caspase-3-like activity that, in the case of insulinoma cells at least, can be further enhanced by interaction of cytokine-induced Fas receptor with FasL. Compared to thymocytes, insulinoma cells and islets from NOD mice were characterised by low basal and cytokine-induced caspase-3 activity.