SENSITIZATION OF HUMAN OVARIAN TUMOR-CELLS BY SUBTOXIC CDDP TO ANTI-FAS ANTIBODY-MEDIATED CYTOTOXICITY AND APOPTOSIS

Treatment of drug-sensitive or resistant tumor cells with combination of anti-Fas antibody and drugs (e.g., CDDP) results in augmented cytot oxicity and synergy, This study examined a possible underlying mechani sm of synergy achieved by anti-Fas and CDDP, Three human ovarian tumor cell lines were selected for the studies, namely, the CDDP-sensitive A2780 and CDDP-resistant variants AD10 and C30 tumor cells, All three lines express Fas but are resistant to cytotoxicity by anti-Fas antibo dy, Treatment of tumor cells with monoclonal mouse antihuman Fas (IgM) antibody and CDDP resulted in significant augmentation of cytotoxicit y and synergy in all three lines, The magnitude of synergy was a funct ion of the concentrations of both the anti-Fas antibody and the CDDP u sed. Pretreatment of tumor cells first with CDDP, but not with anti-Fa s and then treatment with anti-Fas, resulted in synergy, suggesting th at CDDP sensitizes the cells to anti-Fas-mediated cytotoxicity. This w as corroborated by inhibiting synergy by the addition of neutralizing anti-Fas (IgG) antibody. Sensitization of tumor cells by CDDP resulted in upregulation of surface Fas expression which was dependent on de n ovo protein synthesis. Findings similar to those obtained in cytotoxic ity were also obtained in apoptosis as determined by DNA hypoploidy an d DNA fragmentation, The effect of CDDP-mediated sensitization to anti -Fas and cytotoxicity was compared to CDDP-mediated toxicity. The addi tion of the antioxidant butylated hydroxyanisole (BHA) inhibited CDDP- mediated cytotoxicity in both AD10 and C30 bur not in A2780 ovarian tu mor cells, However, BHA did not inhibit upregulation of Fas expression by CDDP in all three lines and further BHA did not inhibit the synerg y achieved with combination of subtoxic concentrations of CDDP and ant i-Fas (IgM) antibody, These findings revealed that CDDP exerts its cyt otoxic effect and its sensitization to Fas cytotoxicity by different m echanisms, Since cytotoxic T lymphocytes (CTL) express Fas ligand and kill Fas(+) target cells, a significant potentiation of tumor cell kil ling will be achieved following sensitization of tumor cells to Fas si gnaling by subtoxic concentrations of CDDP. These findings suggest a n ew approach for augmenting CTL-mediated immune interventions in the th erapy of resistant ovarian tumor cells. (C) 1996 Academic Press, Inc.