Role of MAPK in apolipoprotein CIII-induced apoptosis in INS-1E cells

Background: Individuals with type 2 diabetes mellitus (T2DM) have elevated levels of circulating apolipoprotein CIII (apoCIII). ApoCIII plays an important role for plasma triglyceride levels and elevated levels of the apolipoprotein have been connected with dyslipidemia in T2DM subjects. In addition, apoCIII has been linked to enhanced beta-cell apoptosis. The present study was undertaken to investigate apoptotic mechanisms induced by the apolipoprotein. Results: ApoCIII (10 mu g/ml) enhanced apoptosis 2-fold in insulin-producing INS-1E cells after 24 hours exposure to the apolipoprotein. At this time point phosphorylation of mitogen activated protein kinase (MAPK) p38 had doubled but ERK1/2 and JNK were not activated. Instead, ERK1/2 showed rapid and transient phosphorylation (2-fold after 0.5 hour). No JNK phosphorylation was observed. In support of a role of activation of not only p38 but also ERK1/2 in apoCIII-induced apoptosis, inclusion of p38 inhibitor SB203580 (10 mu M) or ERK1/2 inhibitor PD98059 (100 mu M) normalized apoptosis. Whereas influx of Ca(2+) was linked to apoCIII- induced ERK1/2 activation, pro-apoptotic protein CHOP/GADD of the unfolded protein response (UPR) was not affected by apoCIII. Conclusion: It is suggested that elevated circulating apoCIII levels may contribute to beta-cell apoptosis via activation of p38 and ERK1/2 in individuals with T2DM. Therapies aiming at normalizing levels of apoCIII could be beneficial not only for the function of the beta-cell but also for cardiovascular protection.