BCL-2 PROTECTS CELLS FROM CYTOKINE-INDUCED NITRIC-OXIDE-DEPENDENT APOPTOSIS

Cytokine-mediated cell death in tumor cells can be achieved through en dogenous nitric oxide (NO) from within tumor cells or exogenous NO fro m either activated macrophages or endothelial cells. The purpose of th is study was to determine the role of Bcl-2 in NO-mediated apoptosis. The incubation of murine L929 and NIH3T3 cells with interleukin-1 alph a (IL-1 alpha) and interferon gamma (IFN gamma) induced high endogenou s NO production only in the L929 cells that also underwent apoptosis. NIH3T3 cells were not resistant to NO-mediated apoptosis. In fact, the incubation of L929 and NIH3T3 cells with exogenous NO derived from NO donors, sodium nitroprusside, or S-nitroso-N-acetyl-DL-penicillamine (SNAP) induced death? characterized by typical apoptotic morphology an d DNA fragmentation, in both cell types, but to a higher degree in NIH 3T3 cells than in the L929 cells. We then measured the effect of Bcl-2 expression on exogenous NO-induced apoptosis. At both the and protein levels, L929 fibroblasts expressed levels of endogenous mouse Bcl-2 t han did NIH3T3 cells. At the same time, L929 cells were much more resi stant to exogenous NO-induced cell death than were NIH3T3 cells. The i nverse correlation between mouse Bcl-2 expression and sensitivity to e xogenous NO-mediated cell death was also found in the murine K-1735 me lanoma C-23 and X-21 clonal populations. Transfection of both NIH3T3 c ells and L929 cells with the human bcl-2 gene led to resistance to bot h exogenous and endogenous NO-mediated apoptosis. These data demonstra te that NO-mediated apoptosis can be suppressed by expression of Bcl-2 , suggesting that abnormal expression of Bcl-2 may influence the effic acy of tumor immunotherapy.