Protection by transfected glutathione S-transferase isozymes against carcinogen-induced alkylation of cellular macromolecules in human MCF-7 cells.

Increased expression of glutathione S-transferase (GST) isozymes has been correlated with development of resistance both to cytotoxic anticancer agents and to genotoxic carcinogens. While most anticancer agents are poor GST substrates, the model alkylating carcinogen 4-nitroquinoline-1-oxide (NQO) is a good substrate for human pi class GST (hGSTP1-1) and murine GST mu-1 (mGSTM1-1), but not human GST alpha-2 (hGSTA2-2). We investigated whether expression of these GST isozymes in stably transfected clonal cell lines could protect against the genotoxic and cytotoxic effects of NQO. Compared to parental MCF-7 or pSV2neotransfected control cell lines, covalent labeling of total cellular macromolecules by [3H]NQO (0.1-1.0 mM) was reduced by 70% and 92% in hGSTP1-1- and mGSTM1-1-transfected cell lines, respectively, but was not affected in the hGSTA2-2 expressing line. The observed protection was closely correlated with the relative specific activity of each cell line for conjugation of NQO by the transfected GST isozymes and this protection was reversible by pretreatment of cells with the GST inhibitor ethacrynic acid. Similar results were obtained when covalent labeling of total cellular nucleic acid or DNA was measured. However, clonogenic survival assays indicated that the sensitivity of these cell lines to the cytotoxic effects of NQO was similar for the control and GST-transfected MCF-7 cell lines. Thus, while expression of hGSTP1-1 and mGSTM1-1 (but not hGSTA2-2) was highly protective against alkylation of cellular macromolecules by NQO, this protection was not effective against cytotoxicity induced by NQO as measured by clonogenic assay. These results indicate that expression of GST isozymes can protect differentially against the acute genotoxic and potentially mutagenic effects, as compared to the cytotoxic effects, of electrophiles that are detoxified by glutathione conjugation.