Cisplatin resistance and regulation of DNA repair in cAMP-dependent protein kinase mutants

Drug resistance in cancer poses a major problem to the success of chemotherapy. Increased resistance to the DNA-damaging chemotherapeutic drug cisplatin may be associated with a variety of factors including decreased drug accumulation, increased intracellular levels of thiols, and increased DNA repair. We have found that mutants of the Chinese hamster ovary (CHO) and the mouse adrenocortical carcinoma Y1 cells harboring a defective regulatory subunit (RI) of the cAMP-dependent protein kinase (PKA) exhibited increased resistance to cisplatin. These mutants are cross-resistant to other DNA-damaging chemotherapeutic agents, including bleomycin and melphalan. In addition, wild-type CHO cells transfected with and overexpressing the yeast phosphodiesterase gene or a dominant mutant Rl alpha subunit gene also displayed similar increased resistance to cisplatin. However, mutants with altered catalytic (C) subunits showed a sensitivity to cisplatin similar to the wild-type cells. Further analysis by gel shift assay using cisplatin-damaged DNA as probes and nuclear extracts derived from the Rl subunit mutants showed increased binding of nuclear factor(s) to the damaged DNA. In addition, a host cell reactivation assay of DNA repair, using a cisplatin-damaged reporter plasmid, detected enhanced capacity for repair of DNA lesions in the PKA mutants. These results suggest that DNA repair may be increased in the PKA mutants. We speculate that functional inactivation of PKA may result in increased DNA repair and the acquisition of resistance to DNA-damaging anticancer drugs in cancer.