Overexpression of Bcl-2 enhances LIGHT- and interferon-gamma-mediated apoptosis in Hep3BT2 cells

LIGHT is a member of the tumor necrosis factor superfamily and is the Ligan d for LT-betaR, HVEM, and decoy receptor 3. LIGHT has a cytotoxic effect, w hich is further enhanced by the presence of interferon-gamma (IFN-gamma). A lthough LIGHT/IFN-gamma can activate caspase activity, neither benzyloxycar bonyl-Asp-Glu-Val-Asp-fluoromethylketone nor benzyloxycarbonyl-Val-Ala-Asp- fluoromethylketone can completely inhibit LIGHT/IFN-gamma -mediated apoptos is. Moreover, overexpression of Bcl-2 further enhances LIGHT/IFN-gamma -med iated apoptosis. It appears that LIGHT and IFN-gamma act synergistically to activate caspase-3, with the resultant cleavage of Bcl-2, removal of the B H4 domain, leading to conversion of Bcl-2 from an antiapoptotic to a proapo ptotic form in p53-deficient hepatocellular carcinoma Hep3BT2 cells. Thus, LIGHT seems to be able to override the protective effect of Bcl-2 and induc e cell death. Although benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone and benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone can prevent the cleav age of Bcl-2 by LIGHT/IFN-gamma they only partially inhibit apoptosis in He p3BT2 cells that are overexpressing Bcl-2. In contrast, both LIGHT/IFN-gamm a -mediated apoptosis and Bcl-2 cleavage are inhibited by free radical scav engers, indicating that free radicals may play an essential role in LIGHT/I FN-gamma -mediated apoptosis at a step upstream of caspase-3 activation. Th ese results suggest that LIGHT signaling may diverge into multiple, separat e processes.