REDUCED TOPOISOMERASE-II AND ELEVATED ALPHA CLASS GLUTATHIONE-S-TRANSFERASE EXPRESSION IN A MULTIDRUG RESISTANT CHO CELL-LINE HIGHLY CROSS-RESISTANT TO MITOMYCIN-C

We have isolated a multidrug-resistant derivative of Chinese hamster ovary CHO-K1 cells by exposure to progressively increasing concentrations of Adriamycin(R). 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 that 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 alpha-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.