We previously reported synergistic induction of apoptosis by IFN-gamma plus either cyclosporin A (CsA) or tacrolimus (FK506) in gastric carcinoma cells. In this study, we aimed to elucidate the mechanism for this synergistic induction of apoptosis. IFN-gamma plus CsA synergistically induced caspase-3 mediated apoptosis in gastric carcinoma cells. Although IFN-gamma induced activation of signal transducer and activator of transcription1 (STAT1) and expression of interferon regulatory factor-1 (IRF-1) mRNA, IFN-gamma alone was not able to induce caspase-3 activation and apoptosis. When gastric carcinoma cells were treated with cyclohexamide, a protein synthesis inhibitor, following IFN-gamma pretreatment, caspase-3 was activated, and apoptosis was markedly induced. These findings suggest the existence of IFN-gamma-induced anti-apoptotic pathway and we evaluated the effect of IFN-gamma and CsA on calcium-sensitive nuclear factor-kappaB (NF-kappaB) activation. IFN-gamma increased intracellular calcium ion concentration ([Ca2+](i)) consisting of a spike and a sustained phase. and the latter was completely abrogated by CsA. Activation of NF-kappaB occurred in response to IFN-gamma, and which was markedly inhibited by either CsA or FK506. NF-kappaB decoy also enhanced the cytotoxic effect of IFN-gamma. These results suggest that IFN-gamma may simultaneously induce the STAT1-mediated apoptotic pathway and the anti-apoptotic pathway through calcium-activated NF-kappaB and that inhibition of the latter by CsA may result in dominance of the apoptosis-inducing pathway. (C) 2003 Elsevier Science (USA). All rights reserved.