INHIBITION OF ETOPOSIDE-INDUCED APOPTOSIS WITH PEPTIDE ALDEHYDE INHIBITORS OF PROTEASOME

Recent investigations have indicated the involvement of proteasome in programmed cell death. The present studies show that although peptide aldehyde inhibitors of proteasome are by themselves weak inducers of a poptosis, they inhibit the apoptotic effect of the anticancer drug eto poside in rat thymocytes, Acetyl-Leu-Leu-norvalinal (LLnV-al) and othe r related peptide aldehydes inhibited the increase in caspase activity and DNA fragmentation that followed treatment with etoposide and thei r effect was related to their potency as proteasome inhibitors. To inh ibit etoposide-induced apoptosis, LLnV-al must be present within 3 h o f treatment with etoposide, in the same way as the inhibitor of protei n synthesis cycloheximide must be. Etoposide caused a rapid accumulati on of p53 protein that was not inhibited by LLnV-al, which was also a strong inducer of p53. Peptide aldehydes were also weak activators of caspase activity, suggesting that the same mechanism, i.e. the blockin g of proteasome function, both triggers apoptosis and inhibits the eff ect of etoposide. These results are consistent with a model in which p roteasome is selectively involved in the pathway used by etoposide to induce cell suicide.