The Role of Glutathione and Glutathione S-Transferases in Acquired Drug Resistance

Abstract

A selected population from a wild type (WS) Walker 256 rat mammary carcinoma has been demonstrated to have a 20-fold resistance to nitrogen mustards. However, these resistant cells (WR) have no collateral resistance to nitrosoureas. A direct correlation between the carbamoylating activity and cytotoxicity has been found in this cell line. A one-hour incubation with N,N’-bis-(trans-4-hydroxycyclohexyl)-N’,-nitrosourea (BCyNU) (which decomposes to produce carbamoylating isocyanate species, but no alkylating species) at a concentration of 5 x 10^-3M resulted in a 50% inhibition of glutathione reductase (GR) activity in WS and 100% in WR. Similar nitrogen mustard concentrations had no effect on GR activity. The basal levels of this enzyme showed that the GR activity in the WR cells was approximately half of the WS (3.98 vs 8.67 nmoles NADPH oxidised/mg protein/min). Glutathione S-transferases have been shown to protect cells against alkylating species. Basal activity in WR is twice that of WS (28.3 vs 14.1 nmoles/mg/min with CDNB as substrate). ID₅₀ concentrations of alkylating and carbamoylating agents did not deplete transferase activity. The reduced efficiency of GR in WR cells may account for their relative sensitivity to carbamoylation. The presence of double minutes, a marker for gene amplification, and the increased glutathione S-transferase activity in the WR may be commensurate with an increased capacity of the resistant cells to deal with the electrophilic alkylating species and express resistance.