Expression of human μ or α class glutathione S-transferases in stably transfected human MCF-7 breast cancer cells: Effect on cellular sensitivity to cytotoxic agents

Increased expression of certain glutathione S-transferase (GST) isoenzymes has frequently been associated with the development of resistance to alkylating agents and other classes of antineoplastic drugs in drug-selected cell lines. The question arises whether this phenomenon is causal or is a stress-induced response associated with drug resistance in these cell lines. We have constructed mammalian expression vectors containing the human GSTμ and GSTα2 (Ha2) cDNAs and stably transfected them into the human breast cancer cell line MCF-7. Whereas the parental and pSV2neo-transfected cell lines display low GST activity, three individual transfected clones were identified in each group that expressed either GSTμ or GSTα2. The range of GST activities was similar to those observed in cells selected for anticancer drug resistance. The GSTμ specific activities were 56, 150, and 340 mlU/mg, compared with 10 mlU/mg of endogenous GSTμ in control lines. Specific activities in GSTα2-transfected clones were 17, 28, and 52 mlU/mg, compared with no detectable α class GST in control lines. These clonal lines and the parental and pSV2neo-transfected control lines were tested for sensitivity to antineoplastic agents and other cytotoxic compounds. The clones with the highest activity in each group were 17-fold (GSTα2) to 2.1-fold (GSTμ) resistant to the toxic effects of ethacrynic acid, a known substrate for GSTs. However, the GST-transfected cell lines were not resistant to doxorubicin, L-phenylalanine mustard, bis(2-chloroethyl)-1-nitrosourea, cisplatin, chlorambucil, or the GST substrates 1-chloro-2,4-dinitrobenzene or tert-butyl hydroperoxide. Thus, although L-phenylalanine mustard, bis(2-chloroethyl)-1-nitrosourea, chlorambucil, tert-butyl hydroperoxide, and 1-chloro-2,4-dinitrobenzene are known to be metabolized by glutathione-dependent GST-catalyzed reactions, there was no protection against any of these agents in MCF-7 cell lines overexpressing GST μ or GSTα2. We conclude that, at the levels of GST obtained in this transfection model system, overexpression of GSTμ or GSTα2 is not by itself sufficient to confer resistance to these anticancer agents. These studies do not exclude the possibility that GST may be a marker of drug resistance or that other gene products not expressed in MCF-7 cells might cooperate with GST to confer drug resistance.