Reduced topoisomerase II and elevated α class glutathione S-transferase expression in a multidrug resistant CHO cell line highly cross-resistant to mitomycin C

Abstract We have isolated a multidrug-resistant derivative of Chinese hamster ovary CHO-K1 cells by exposure to progressively increasing concentrations of Adriamycin®. This cell line, designated CHO-Adr r , was 27-fold more resistant than the parental line to Adriamycin and showed similar degrees of cross-resistance to several other topoisomerase II (topo II) inhibitors, including mitoxantrone, daunomycin and etoposide. CHO-Adr r cells showed a lower (4-fold) level of cross-resistance to vincristine and colchicine, drugs associated with the multidrug-resistant phenotype. While CHO-Adr r cells showed no enhanced resistance to several mono- and bi-functional alkylating agents or to UV and ionizing radiation, they were greater than 80-fold resistant to mitomycin C (MMC). There was a 5-fold decreased level of daunomycin accumulation in CHO-Adr r cells compared to CHO-K1 cells and this was associated with increased drug efflux. The resistant cells had amplified multidrug resistance gene ( mdr ) sequences and overexpressed ( mdr ) mRNA. Verapamil was able to completely reverse Adriamycin resistance but reversal of MMC resistance was only partial, with residual 23-fold resistance. CHO-Adr r cells expressed a 4-fold reduced level of topo II protein but overexpressed an α class (basic) glutathione S -transferase (GST). Analysis of cell hybrids showed that while the level of resistance to Adriamycin dropped by a factor of 3 in CHO-K1/CHO-Adr r hybrids compared to CHO-Adr r /CHO-Adr r hybrids, resistance to MMC dropped 10-fold. Thus, CHO-Adr r cells appear to exhibit simultaneously several different drug resistance mechanisms including MDR and GST overexpression, and topo II reduction.