Failure to activate interleukin 1beta-converting enzyme-like proteases and to cleave retinoblastoma protein in drug-resistant cells.

We previously found that retinoblastoma (RB) is cleaved at the initiation of apoptotic execution. Here we report that when an HL-60 cell line resistant to cytosine arabinoside (Ara-C) was exposed to this anticancer drug, neither RB cleavage nor apoptosis was detected. Consistent with that, processing of interleukin 1beta-converting enzyme (ICE) and CPP32 (an ICE-like protease) was also prevented in these cells. In contrast, treatment of the HL-60-Ara-C-resistant cells with etoposide induced all of these apoptotic events. Furthermore, the etoposide-induced RB cleavage was inhibited by a specific tetrapeptide ICE-like inhibitor. Our results demonstrate that activation of the RB cleavage enzyme, an ICE-like protease, is required for overcoming drug resistance.