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 b een correlated with development of resistance both to cytotoxic antica ncer agents and to genotoxic carcinogens. While most anticancer agents are poor GST substrates, the model alkylating carcinogen 4-nitroquino line-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 pSV2neo-transf ected control cell lines, covalent labeling of total cellular macromol ecules by [H-3]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 affec ted in the hGSTA2-2 expressing line. The observed protection was close ly correlated with the relative specific activity of each cell line fo r conjugation of NQO by the transfected GsT isozymes and this protecti on was reversible by pretreatment of cells with the GST inhibitor etha crynic acid. Similar results were obtained when covalent labeling of t otal cellular nucleic acid or DNA was measured. However, clonogenic su rvival assays indicated that the sensitivity of these cell lines to th e cytotoxic effects of NQO was similar for the control and GST-transfe cted MCF-7 cell lines. Thus, while expression of hGSTP1-1 and mGSTM1-1 (but not hGSTA2-2) was highly protective against alkylation of cellul ar macromolecules by NQO, this protection was not effective against cy totoxicity induced by NQO as measured by clonogenic assay. These resul ts indicate that expression of GST isozymes can protect differentially against the acute genotoxic and potentially mutagenic effects, as com pared to the cytotoxic effects, of electrophiles that are detoxified b y glutathione conjugation.