The effects of terbium on the cellular accumulation of cisplatin in MDA-MB-231 human breast tumor cells

Purpose: Cisplatin (DDP) is an effective antitumor agent limited in its efficacy by the development of tumor cell resistance. The defective accumulation of DDP has been shown to be a prominent feature in many DDP-resistant cell lines. In an effort to circumvent this problem, we examined the cellular accumulation of DDP in the presence of terbium (Tb3+). We also examined the effects of verapamil on the cellular accumulation of DDP in order to delineate the specific interaction of Tb3+ and DDP. All experiments were performed on DDP-sensitive or DDP-resistant MDA-MB-231 human breast tumor cells. Methods: The cellular accumulation of DDP and verapamil were determined by electrothermal atomic absorption spectro-photometry. Time-resolved luminescence spectroscopy was used to obtain equilibrium binding constants for the Tb3+/MDA cell complexes. Results: We found that 100 mu M Tb3+ increased DDP accumulation in the parent MDA cell line, 5.7-fold resistant MDA/A13 and 10-fold resistant MDA/CH cells by 56.2 +/- 7.4, 71.9 +/- 9.4 and 50.8 +/- 9.4%, respectively (P < 0.0001 for all MDA cell types). In contrast, 20 mu M verapamil had no significant effect on DDP accumulation in the MDA cell lines. In addition, a positive correlation between the membrane binding of Tb3+ and the cellular accumulation of DDP was found to exist in the parent cell line and sublines (r = 0.9). Conclusions: In agreement with earlier studies, the plasma membrane of MDA cell lines contain a specific Tb3+-binding protein. Our findings suggest that the Tb3+-binding protein may be intimately associated with the accumulation of DDP in breast tumor cells.