Increased poly(ADP-ribose) formation in cisplatin-resistant rat ovarian tumor cells

Abstract

Poly(ADP-ribose) polymerase (PARP) is well known for its involvement in DNA repair, although the underlying mechanisms remain unclear. Poly(ADP-ribose) is synthesized on nuclear proteins in response to DNA damage and consequently implicated in the toxicity of various xenobiotics, including anticancer agents. The metabolism of poly(ADP-ribose) in cisplatin (DDP)-sensitive (O342) and -resistant (O-342/DDP) rat ovarian tumor cells was investigated to explore its possible roles in DDP resistance. The poly(ADP-ribose) synthesis assayed as [3H]-NAD incorporation was higher by up to two-fold in the resistant O-342/DDP cells, when compared with that of its DDP-sensitive subline O-342. Furthermore, this difference still existed even in the presence of saturating concentrations of a double-stranded octameric deoxynucleotide that stimulates the enzyme directly, indicating a higher maximal poly(ADP-ribosyl)ation capacity of the resistant cells. In addition, acute treatment of O-342 cells with DDP also stimulated the polymer synthesis by up to 1.6-fold, which was totally suppressed by inclusion of 2.5 mM 3AB in the post-exposure incubation. Western blot analysis, however, failed to reveal higher levels of the enzyme proteins in the resistant cells. A higher level of endogenous DNA single strand breaks was also detected in both intact and permeabilized cells of O-342/DDP line. Taken together, these results demonstrate that the DDP resistance phenotype in these rat ovarian tumor cells is accompanied by a higher cellular poly(ADP-ribosyl)ation capacity, which may be linked with DDP resistance by enhancing the repair of DDP-inflicted DNA damage.