Activation of an alternative death receptor-induced signaling pathway in human hepatocytes under caspase arrest

Caspases are thought to be essential in execution of death receptor-induced apoptosis. However, recent findings suggest the existence of alternative pathways independent of caspases. We provide further evidence for such signaling in hepatocytes. Results: Death receptor-induced activation of caspases and apoptosis in primary murine hepatocytes was completely blocked in presence of 1.5 μM N-benzyloxycarbonyl-Val-Ala-Asp-(O-methyl)fluoromethylketone (zVAD-fmk). Whereas the same concentration of the inhibitor was sufficient to block TNF receptor 1-, CD95- or TRAIL receptor 1/-2-induced activation of caspases in primary human hepatocytes or HepG2 cells, complete prevention apoptotic cell death needed almost 100 μM zVAD-fmk. Under caspase-inhibitory but non-protective conditions, i.e. at 1.5 μM zVAD-fmk, various serine protease inhibitors prevented apoptosis-like cell death. Neither sole arrest of caspases nor inhibition of serine proteases alone protected human hepatocytes. Conclusion: Human but not murine hepatocytes bear the potential to activate a permissive, serine protease inhibitor-sensitive alternative death signaling pathway under caspase-inhibitory conditions.