Modulation of cisplatin cytotoxicity and cisplatin-induced DNA cross-linksin HepG2 cells by regulation of glutathione-related mechanisms

Glutathione (GSH), glutathione S-transferase (GST), and glutathione conjuga te export pump (GS-X pump) have been shown to participate collectively in t he detoxification of many anticancer drugs, including cisplatin. Identifica tion and regulation of the rate-limiting step in the overall system for cis platin detoxification is of crucial importance for sensitization of human t umor cells to cisplatin. In this study, the GSH content, GST activity, and GS-X pump activity were regulated separately to examine effects of the regu lation on cisplatin cytotoxicity and cisplatin-induced DNA interstrand cros s-links (ICL) in HepG2 cells. Seventy-percent depletion of GSH by buthionin e sulfoximine (BSO) and 50% increase of GSH by monoethyl GSH ester (GSHe) p otentiated and decreased cisplatin cytotoxicity, respectively. This was ref lected by a significant decrease and increase of their respective IC50 valu es by 62 and 107%. Cisplatin-induced ICL was also potentiated by depletion of GSH by BSO and decreased by enrichment of GSH by GSHe, as shown by a 125 % increase and a 34% decrease of cross-linked DNA compared with control sam ples exposed to cisplatin alone (p = 0.008 and 0.03, respectively). On the other hand, inhibition of GST and GS-X pump by ethacrynic acid, quercetin, tannic acid, and indomethacin at concentrations that inhibited activities o f GST and GS-X pump by more than 50% had no significant effects on cisplati n cytotoxicity and cisplatin-induced DNA ICL in these cells. The results sh owed that of the parameters measured, intracellular GSH seems to be the rat e-limiting factor, and its regulation would provide a more promising strate gy for sensitization of human liver tumor cells to cisplatin.