BCNU IS A CASPASE-MEDIATED INHIBITOR OF DRUG-INDUCED APOPTOSIS

BCNU [1,3-bis(2-chloroethyl)-1-nitrosourea], a bifunctional (alkylatin g/carbamoylating) anticancer agent, in noncytotoxic doses (12-50 mu M) inhibited drug-induced apoptosis in HT58 human lymphoma cells exposed to etoposide (ETO; 50 mu M) as well as in mouse thymocytes exposed to dexamethasone (5 mu g/ml) in vitro in 4-h cultures, The cytoplasmic e xtracts of ETO-treated HT58 cells cleaved both purified poly(ADP-ribos e)polymerase and Ac-Asp-Glu-Val-Asp-7-amino-4-methylcoumarin fluorogen ic caspase substrate, indicating the presence of active caspases, and these effects were inhibited by BCNU concentration dependently, The ca rbamoylating decomposite, 2-chloroethyl-isocyanate (6-25 mu M), also d ecreased ETO-induced apoptosis in HT58 cells in vitro and their caspas e 3-like activity ex vivo, whereas N-(2-chloroethyl)-N-nitrosocarbamoy l-valinamide, an alkylating and mainly intramolecularly carbamoylating nitrosourea derivative (400 mu M), did not influence these phenomena. Furthermore, the activity of recombinant caspase 3 was also strongly inhibited by BCNU and 2-chloroethyl-isocyanate. These results indicate that BCNU, via its carbamoylating capacity, can inactivate cysteine p rotease(s) essential for ETO-induced apoptosis, This apoptosis-modulat ing property of BCNU, in turn, may influence the efficacy of chemother apeutic protocols in the treatment of cancer.