THE EFFECT OF ETHYLDESHYDROXY-SPARSOMYCIN AND CISPLATIN ON THE INTRACELLULAR GLUTATHIONE LEVEL AND GLUTATHIONE-S-TRANSFERASE ACTIVITY

Ethyldeshydroxy-sparsomycin (EdSm) is a ribosomal protein synthesis in hibitor which synergistically enhances the antitumor activity of cispl atin against L1210 leukemia in vivo. Because cellular glutathione (GSH ) and glutathione S-transferases (GST) are reported to interfere with the antitumor activity of cisplatin, we analyzed the effect of EdSm an d cisplatin on GSH and GST activity in selected tumor cells. For this purpose we used three murine leukemia tumors with different sensitivit ies towards EdSm and cisplatin: L1210-WT, sensitive to both drugs, L12 10-Sm, resistant to EdSm, and L1210-CDDP, resistant to cisplatin. No s ignificant differences were detectable between these three cell lines regarding the population doubling time, the cell size, and the cellula r level of protein and glutathione. Neither of the resistant L1210 sub clones showed P-glycoprotein expression. Drug exposure, however, chang ed the intracellular dynamics. Exposure to EdSm strongly decreased the amount of cellular protein, decreased the overall GST activity and le d to GSH depletion, whereas exposure to cisplatin induced a rise in th e amount of protein, in GSH, and in the total GST activity. These effe cts are dose-dependent and correlate well with the sensitivity of the tumor cells for EdSm or cisplatin. In addition, exposure to EdSm lower ed the V-max of GST in L1210-WT and L1210-Sm; however, in L1210-CDDP b oth the V-max and the K-m were increased. That this was not a direct e ffect of EdSm on GST was shown in a cell-free system, where EdSm did n ot influence the GST activity nor could it act as a substrate for GST. Our results suggest that the synergistic combination of EdSm and cisp latin might be explained by EdSm switching off the cellular detoxifica tion mechanism for cisplatin, i.e. by inhibition of de novo synthesis and subsequent depletion of GSH and GST.