Ethyldeshydroxy-sparsomycin (EdSm) Is a rlbosomal protein synthesis Inhibitor which synergistlcally enhances the antitumor activity of cisplatin against L1210 leukemia in vivo. Because cellular glutathione (GSH) and glutathione Stransferases (GST) are reported to Interfere with the antitumor activity of cisplatin, we analyzed the effect of EdSm and cisplatin on GSH and GST activity in selected tumor cells. For this purpose we used three murlne leukemia tumors with different sensitivities towards EdSm and cisplatin: L1210-WT, sensitive to both drugs, L1210-Sm, resistant to EdSm, and L1210-CDDP, resistant to cisplatin. No significant differences were detectable between these three cell lines regarding the population doubling time, the cell size, and the cellular level of protein and glutathione. Neither of the resistant L1210 subclones showed P-glycoproteln expression. Drug exposure, however, changed the Intracellular dynamics. Exposure to EdSm strongly decreased the amount of cellular protein, decreased the overall GST activity and led to GSH depletion, whereas exposure to cisplatin Induced a rise In the amount of protein, In GSH, and in the total GST activity. These effects are dose-dependent and correlate well with the sensitivity of the tumor cells for EdSm or cisplatin. In addition, exposure to EdSm lowered the Vmn of GST in L1210-WT and L1210-Sm; however, in L1210-CDDP both the Vm.x and the Km were increased. That this was not a direct effect of EdSm on GST was shown In a cell-free system, where EdSm did not influence the GST activity nor could it act as a substrate for GST. Our results suggest that the synerglstlc combination of EdSm and cisplatin might be explained by EdSm switching off the cellular detoxification mechanism for cisplatin, I.e. by inhibition of de novo synthesis and subsequent depletion of GSH and GST.
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