Differential increases in glutathione S-transferase activities in a range of multidrug-resistant human tumor cell lines

Modified glutathione S-transferase (GST) activity was detected in MCF-7 cell lines selected in vitro for modest levels of resistance (2.6- to 13.7-fold) not only to Adriamycin but also to Novantrone, vincristine (VCR), and etoposide (VP-16). Western blot analysis revealed a qualitative increase in the expression of the pi class GST isozyme and some apparent down regulation of the mu class in these resistant lines relative to the parental line. Overexpression of GST pi protein was also observed after in vitro exposure of MCF-7 cells to fractionated x-irradiation, which resulted in a subline that exhibited 5-fold resistance to VCR and 3-fold resistance to VP-16. Northern blot analysis provided qualitative evidence that these changes in GST pi and mu appeared to be at the transcriptional level in these VCR- and VP-16-resistant MCF-7 sublines. In contrast, similarly selected VCR-resistant, VP-16-resistant, and x-ray-pretreated-resistant sublines, derived from the SuSa cell line established from a human testicular teratoma, did not exhibit significant alterations in GST isozyme profiles compared with their parental cell lines. All parental and resistant SuSa lines that were studied expressed the GST pi isozyme constitutively. Total glutathione content and glutathione peroxidase activities were not modified in parallel with the GST alterations in these human tumor drug resistant sublines. These data suggest that the increased cellular GST activity was associated with selection of resistance to many of the drugs associated with the multidrug resistant phenotype in MCF-7 sublines but that this was not the case for the other human tumor cell line (SuSa) tested.