Isolation and characterization of Saccharomyces cerevisiae mutants with enhanced resistance to the anticancer drug bleomycin

Bleomycin is an antitumor agent believed to act by damaging DNA. It is currently used for treating testicular carcinomas, but other types of cancers such as ovarian and colon are resistant to the drug from the outset. The mechanism involved in allowing cells to confer resistant to bleomycin is not known. We exploited the power of yeast genetics to isolate for the first time several bleomycin-resistant mutants derived from a strain deleted for the IMP2 gene encoding a transcriptional co-activator. imp2Delta mutants are known to be hypersensitive to bleomycin, monovalent and divalent cations, and high pH. The suppressors of imp2Delta showed extreme resistance to bleomycin and also either fully or partially rescued the phenotypes associated with the imp2Delta mutant, suggesting that bleomycin resistance is linked to other phenotypes. Using fluorescently labeled bleomycin, we demonstrated that two bleomycin-resistant variants, MAY1 and MAY2, were compromised for uptake of the drug, as compared with the parent. In contrast, the imp2Delta mutant showed a substantial increase in the uptake of fluorescently labeled bleomycin. We further showed that strains MAY1 and MAY2 contain a reduced amount of a plasma membrane protein, which binds to Co-57-labeled bleomycin and is believed to mediate drug entry into the cell. We propose that the bleomycin-resistant mutants are likely defective in a process responsible for transporting the drug into the cell.