Fas-mediated apoptosis represents one major mechanism by which
tumor cells can be eliminated by activated cytotoxic immune lymphocytes. Previously, we have reported that
interferon-gamma (IFN-gamma) sensitizes human ovarian
carcinoma cell lines to Fas-mediated apoptosis. Furthermore, IFN-gamma, together with many other proinflammatory
cytokines (
TNF-alpha, IL-1beta, LPS, etc.), can stimulate the induction of
inducible nitric oxide synthase (iNOS) and the generation of
nitric oxide (NO). In this study, we examined whether
nitric oxide is a mediator of IFN-gamma-induced sensitization of human ovarian
carcinoma cell lines (A2780 and
AD10) to Fas-mediated apoptosis and whether NO regulates the expression of the
Fas receptor. Treatment of quiescent A2780 and
AD10 ovarian
carcinoma cells with IFN-gamma alone induced the expression of iNOS
mRNA as examined by RT-PCR. There was accumulation of
nitrite in the culture medium of IFN-gamma-treated cells, suggesting the generation of NOx. Like IFN-gamma, the use of exogenous sources of NO (
S-nitroso-N-acetylpenicillamine (SNAP)) mimicked the sensitization of both cell lines to anti-Fas cytotoxic antibody (CH11) by IFN-gamma. Endogenously produced NO, by IFN-gamma pretreatment or exogenous nitrodonors, resulted in the upregulation of
Fas receptor mRNA and
protein expression. Blocking iNOS activity by
NG-monomethyl-l-arginine (l-NMA) significantly reduced the sensitization, Fas
mRNA, and
protein expression observed with IFN-gamma pretreatment of the
tumor cells. These findings demonstrate that sensitization of human ovarian
carcinoma cell lines to Fas-mediated apoptosis by IFN-gamma can be due, in part, to the induction of iNOS and the subsequent upregulation of Fas gene expression by reactive
nitrogen intermediates. Thus, the sensitivity of
tumor cells to Fas-L-mediated cytotoxic immune lymphocytes can be regulated by the induction of NO or intermediates.