Tamoxifen modulation of cisplatin sensitivity in human malignant melanoma cells.

Tamoxifen (TAM) markedly increases the response rate of malignant melanoma to treatment with cisplatin (DDP), carmustine, and dacarbazine, and we have previously reported that there is a highly synergistic interaction between TAM and DDP with respect to the cytotoxic effect against the human melanoma cell line T-289 (E. F. Mc Clay et al., Cancer Res., 52: 6790-6796, 1992). The mechanism underlying synergy was investigated by examining the effect of selection for either DDP or TAM resistance on the magnitude of the synergy quantitated by median effect analysis. The combination index at 50% cell kill was 0.26 +/- 0.02 (SD) for parental T-289 cells (indicating marked synergy), 0.54 +/- 0.14 for cells selected for low-level DDP resistance (indicating moderate synergy), and 1.39 +/- 0.20 for cells selected for low-level TAM resistance (indicating antagonism). Thus, factors that regulate DDP sensitivity have a moderate effect on reducing the DDP/TAM synergy, but determinants of TAM sensitivity have a major effect. The known biochemical effects of TAM include antagonism of estrogen at the estrogen receptor (ER) and inhibition of calmodulin and protein kinase C activity. T-289 cells contained undetectable amounts of ER by the dextran-coated charcoal assay and expressed only trace amounts of ER mRNA, and another more avid ER antagonist, droloxifene, failed to interact synergistically with DDP. N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), a potent calmodulin antagonist, failed to demonstrate synergy with DDP, and activation of protein kinase C, instead of interacting antagonistically with DDP, yielded synergy. TAM did not alter the cell cycle phase perturbation produced by exposure to DDP alone. We conclude that the synergy between TAM and DDP is not mediated by the effects of TAM on the ER, calmodulin, protein kinase C, or cell cycle regulation. However, the factors that determine cellular sensitivity to TAM also determine whether TAM interacts synergistically with DDP.