Synergy between Tamoxifen and Cisplatin in Human Melanoma Cells Is Dependent on the Presence of Antiestrogen-binding Sites

Abstract We have demonstrated previously that cisplatin (DDP) and tamoxifen (TAM) act synergistically to kill human melanoma T-289 cells, and that the observed synergy is lost in the 3-fold TAM-resistant subline, 289/TAM 6 . We have identified the intracellular antiestrogen-binding sites (AEBSs), defined by their ability to bind antiestrogens while having no affinity for estrogen, as a possible mediator of this synergy. We report here that [ 3 H]TAM binds to AEBSs, as defined by the ability of N,N -diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine-HCl, an AEBS-specific ligand, to compete with [ 3 H]TAM binding. Furthermore, we have characterized the number of binding sites and their affinity for [ 3 H]TAM by Scatchard analysis in whole-cell lysates, microsomal fractions, and nuclear fractions of both cell lines by competing [ 3 H]TAM binding with increasing concentrations of unlabeled TAM. These data demonstrate that the loss of a high-affinity AEBS from the nuclear fraction of the 289/TAM 6 cell line correlates with the loss of synergy between DDP and TAM in these cells. This implicates AEBSs as a critical component of the mechanism that mediates the synergistic interaction of DDP and TAM in human melanoma cells.