Characterization of auromomycin-resistant hamster cell mutants that display a multidrug resistance phenotype

We have selected and characterized Chinese hamster ovary (CHO) cells resistant to auromomycin (AUR), an antitumor antibiotic composed of a protein moiety and a nonpeptide chromophore. AUR is cytotoxic as a consequence of DNA strand-scission activity associated with the chromophore. Initial single-step selections for clones resistant to AUR detected a subpopulation of phenotypically resistant colonies, but nearly all such clones failed to display heritable resistance. One isolate that did show somewhat increased resistance was selected further and yielded a clone designated AURR-R1 that exhibits stable 10-fold increased resistance to AUR. The R1 line is also resistant to the AUR chromophore and cross-resistant to the closely related agent neocarzinostatin (NCS) and to the NCS chromophore. For AUR-treated whole cells, resistance to AUR cytotoxicity was inversely correlated with DNA damage as measured by filter elution; by contrast, isolated nuclei from sensitive and resistant cells displayed similar levels of AUR-induced DNA damage. The R1 cell line was found to be cross-resistant to colchicine, Adriamycin, Daunomycin, and vinblastine. The resistance phenotype is expressed with incomplete dominance in cell hybrids and appears similar to the "classic" multidrug resistance of CHO cells selected with other agents. Indeed, we found the multidrug-resistant CHO line CCHR-C5 to be about 5-fold cross-resistant to AUR and to NCS. We ascertained that AUR-resistant (AURR) isolates express elevated levels of the molecular weight 170,000 P-glycoprotein often associated with multidrug resistance and also contain amplified DNA sequences that contain the gene for P-glycoprotein. When multiple-step enrichment selections were carried out as an alternative approach for isolating AURR mutants, each of nine clonal isolates showed phenotypes resembling the AURR-R1 line. Thus, our findings imply that increased cellular resistance to AUR may frequently be associated with P-glycoprotein-mediated multidrug resistance.