[Enhanced apoptosis-inducing effect of etoposide on leukemic cell lines M-07e and TF-1 by the proteasome inhibitor Z-LLL-CHO].

Recent researches indicate that ubiquitin-protea some pathway plays an important role in apoptosis regulation. Proteasome inhibitors induce apoptosis in many kinds of neoplastic cells, thus provide a great opportunity for exploring synergy of proteasome inhibitors and other apoptosis-inducing agents. In this study, the effect of the proteasome inhibitor Z-LLL-CHO combined with etoposide (VP16) on leukemic cell lines M-07e and TF-1 was investigated by MTT assay, trypan blue exclusion, flow cytometry and Western blot. The results showed that the combination of Z-LLL-CHO and VP16 was much more effective than either agents alone in promoting cytotoxicity in both cell lines evaluated. Accumulation of cells in S + G2/M phase of the cell cycle was observed in the cells treated with VP16 and Z-LLL-CHO alone, while apparent increase of sub-G0/G1 fraction was detected in cells treated with combination of the agents. The cleavage of Bcl-2 into a shortened 22 kD fragment was detected in M-07e cells exposed to either agents alone, and the fraction of 22 kD fragment was increased in the cells treated with combination of the agents. In conclusion, the combination of Z-LLL-CHO and VP16 enhanced their individual cytotoxic effect by inducing apoptosis, in which increase of S + G2/M fraction in cell cycle as well as the enhanced cleavage of Bcl-2 are the possible mechanism of the additive effect on leukemic cells by Z-LLL-CHO and VP16.