Dual effects of cycloheximide on U937 apoptosis induced by its combination with VP-16

In this study, cycloheximide (CHX) and VP-16 alone and in combination (C&V) have been used to strongly trigger apoptosis in U937 cells. The presence of CHX markedly prevented VP-16-induced apoptosis, suggesting that in this process de novo protein synthesis is required. But interestingly, C&V had shown more similarities with CHX but not VP-16 alone, including the effects on cell cycle distribution and induction of apoptosis, which occurred more quickly and was steadily enhanced by increasing concentrations of CHX or by N-alpha-tosyl-L-lysyl-chloromethyl ketone (TLCK), a serine protease inhibitor. These results indicate that CHX, not VP-16, is indeed the dominant inducer of U937 apoptosis, when they are coadministered. In particular, VP-16 even promoted CHX-induced apoptosis, but did not alter its selection of cell types. In T-cells resistant to CHX (Molt-4), we have detected no apoptotic response to their combination. These findings may well explain why the inhibitory effects of CHX on apoptosis induced by the same stimuli are usually different according to the cell type used, and also suggest that CHX may have the potential to lower side effects and drug resistance of cancer therapy.