Insulin aggravates free fatty acid-induced hepatocyte cell death by activation of CD95 death receptor pathways in liver

Introduction: Free fatty acids (FFA) induce lipoapoptosis in hepatocytes in a JNK-dependent but death receptor-independent way.¹ Insulin has been reported to induce EGFR activation in liver.² Since JNK-dependent and EGFR-mediated CD95-Tyr-P is a crucial step in CD95 activation,³ the aim of the study was to elucidate whether addition of insulin may aggravate FFA-induced cell death by activation of CD95. Methods: Studies were performed in hepatocytes isolated from rats and the CD95-k.o.-mouse. Results: Caprylate (C8; 200µM) as well as palmitate (C16 200µM) induced within 18h apoptosis in hepatocytes as measured by TUNEL staining. Even if activating JNK-phosphorylation occurred, no activation of CD95 became detectable with respect to CD95/EGFR association, CD95-Tyr-P and DISC formation. Insulin (100 nM) induced an increase in BrdU incorporation which was almost totally abolished when incubated together with C8- or C16-FFA. Moreover, incubation of C8- or C16-FFA together with insulin markedly increased the number of apoptotic cells compared to the FFA alone. IP-studies revealed that insulin-ind. EGFR-activation couples to CD95 signaling pathway by the FFA-ind. JNK activation and subsequent EGFR/CD95 association. Therefore, only upon FFA/insulin-coadministration CD95-Tyr-P and DISC formation occurred. To determine whether insulin-ind. increase in FFA-mediated apoptosis is due to CD95-activation, experiments in hepatocytes isolated from CD95-k.o.-mice were performed. Again C8 (50µM)- as well as C16 (50µM)-FFA induced apoptotic cell death within 24h. Addition of insulin (100nM) roughly doubled FFA-induced amount of apoptotic cells in wild type mice, whereas insulin was ineffective in CD95-k.o.-mice indicating an involvement of CD95. Conclusion: These data suggest that FFA-induced lipoapoptosis can be aggravated by coadministration of insulin in a CD95-dependent way, since FFA-induced JNK activation couples insulin-induced EGFR activation to CD95 death receptor pathway.

Literatur: 1) Malhi et al., J Biol Chem. 2006;281:12093-101. 2) Reinehr et al., J Biol Chem. 2010;285:25904-12. 3) Reinehr et al., FASEB J. 2003;17:731-3.

CD95 - Hyperinsulinemie - NASH - free fatty acids