Increased glutathione level is not involved in enhanced bleomycin sensitivity in cisplatin-resistant 2780CP cells

The cytotoxic activity of bleomycin (BLM) was evaluated in cisplatin (CDDP)-sensitive (A2780) and -resistant (2780CP) human ovarian cancer cells, and the mechanism of increased antitumor activity of BLM in the 2780CP cells was investigated. Compared with the A2780 cells, the 2780CP cells exhibited a 4.5-fold increase in resistance to CDDP, but were 4.0-fold more sensitive to BLM. The cellular glutathione (GSH) levels in the 2780CP cells were significantly higher than those in the A2780 cells, however, GSH depletion in the 2780CP cells below the levels in the A2780 cells by using buthionine-[S,R]-sulfoximine (BSO) did not affect the sensitivity to BLM. BLM decreased 5-bromo-2'-deoxyuridine (BrdU) incorporation after 24-h exposure by 27.5%-90% compared to that of the untreated control at BLM doses of 25-500 ng/ml in the 2780CP cells, but only by 1.5% -45.8% in the A2780 cells. Furthermore, in the 2780CP cells, the percentage of S-phase cells markedly decreased, with an increase in G2/M-phase cells as determined by flow cytometry after exposure to BLM. The enhanced cytotoxity of BLM in CDDP-resistant 2780CP cells could be attributed to BLM-induced G2/M accumulation and significantly inhibited DNA synthesis, not to increased cellular GSH levels.