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Vol. 59, Issue 4, 837-843, April 2001
Department of Experimental Surgery, Singapore General Hospital,
Singapore (K.Z., M.C., E.B.Y., P.M); and Department of Biochemistry,
National University of Singapore, Singapore (K.P.W)
Glutathione (GSH), glutathione S-transferase
(GST), and glutathione conjugate export pump (GS-X pump) have been
shown to participate collectively in the detoxification of many
anticancer drugs, including cisplatin. Identification and regulation of
the rate-limiting step in the overall system for cisplatin
detoxification is of crucial importance for sensitization of human
tumor cells to cisplatin. In this study, the GSH content, GST activity,
and GS-X pump activity were regulated separately to examine effects of
the regulation on cisplatin cytotoxicity and cisplatin-induced DNA
interstrand cross-links (ICL) in HepG2 cells. Seventy-percent
depletion of GSH by buthionine sulfoximine (BSO) and 50% increase of
GSH by monoethyl GSH ester (GSHe) potentiated and decreased cisplatin cytotoxicity, respectively. This was reflected by a significant decrease and increase of their respective IC50 values 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 samples 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 of GST and GS-X pump by
more than 50% had no significant effects on cisplatin cytotoxicity and
cisplatin-induced DNA ICL in these cells. The results showed that of
the parameters measured, intracellular GSH seems to be the
rate-limiting factor, and its regulation would provide a more promising
strategy for sensitization of human liver tumor cells to cisplatin.
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