Mechanisms of enhancement of cytotoxicity in etoposide and ionising radiation-treated cells by the protein kinase inhibitor wortmannin.

We have investigated the effects of the protein kinase inhibitor wortmannin (WM) on the cytotoxic mechanisms of etoposide and ionising radiation (IR) in the Chinese hamster ovary K1 (CHO-K1) cell line, and its radiation-sensitive derivative, xrs-6, which is defective in DNA-dependent protein kinase (DNA-PK) function. WM potentiated the cytotoxicity of etoposide and IR in CHO-K1 cells approximately 1.6 and 3-fold, respectively, and this potentiation was abolished in xrs-6 cells, which were themselves more sensitive to etoposide and IR alone. WM partially inhibited the repair of etoposide-induced DNA double-strand breaks. Etoposide treatment caused a biphasic inhibition of DNA synthesis in both cell lines, and this was abrogated by co-incubation with WM. These data suggest that WM inhibits in intact cells both DNA-PK and either or both the ataxia telangiectasia (AT) and AT-related gene products ATM and ATR.