OVEREXPRESSION OF GLUTATHIONE-S-TRANSFERASE AND ELEVATION OF THIOL POOLS IN A MULTIDRUG-RESISTANT HUMAN COLON CANCER CELL-LINE

A human colon cancer line with acquired multidrug resistance (MDR) was assayed for the intracellular GSH level and the activity of GSH-S-tra nsferase (GST), which catalyzes the conjugation reaction of electrophi lic drugs with GSH. The GSH level and GST activity (as measured with 1 -chloro-2,4-dinitrobenzene) were elevated in the resistant cells by 1. 7-fold and 2-fold, respectively. This elevated catalytic activity of t he resistant cells was reflected in a 2-fold increase in GST-pi mRNA, which was not the result of gene amplification. In addition, buthionin e sulfoximine, a specific inhibitor of GSH synthesis, significantly in creased Adriamycin sensitivity in both the MDR and the parental cells, affecting the former more than the latter. The effects seen with buth ionine sulfoximine were not seen with puromycin and actinomycin D.A dr amatic overexpression of mdr1, a P-glycoprotein gene responsible for t he MDR phenotype, was also observed in the MDR cells. In contrast, non e of these products (i.e., mdr P-glycoprotein, GSH level, total GST ac tivity, GST-pi gene copy, and GST-pi mRNA level) was elevated in HeLa cells resistant to cisplatin and some alkylating agents, supporting th e notion that the acquisition of cisplatin resistance differs from the mechanism of MDR. These results indicate that the intrinsic GSH level and GST-pi activity affect anthracycline resistance per se and not MD R in the human colon cancer cells.