The proteasome regulates caspase-dependent and caspase-independent protease cascades during apoptosis of MO7e hematopoietic progenitor cells

Withdrawal of trophic support from growth factor-dependent MO7e human myelo id progenitor cells induces apoptosis characterized by DNA fragmentation an d degradation of the catalytic subunit of DNA-dependent protein kinase (DNA -PKcs). Inhibitors of caspase (ICE) protease family members did not inhibit apoptosis or DNA fragmentation induced by factor withdrawal, but blocked d egradation of DNA-PKcs. Thus, caspase activity accounts for only a componen t of the apoptotic program in MO7e hematopoietic cells. The protease inhibi tor TPCK, but not other protease inhibitors, blocked DNA fragmentation, but not degradation of DNA-PKcs during apoptosis of MO7e cells. Thus, caspase- independent and caspase-dependent protease cascades mediate distinct featur es of MO7e cell apoptosis. The proteasome inhibitors calpain inhibitor I an d lactacystin promoted DNA fragmentation, degradation of DNA-PKcs and apopt osis of MO7e cells. The ability of lactacystin to promote DNA fragmentation was abrogated by TPCK, but not by caspase inhibitors, whereas the ability of lactacystin to promote degradation of DNA-PKcs was blocked by caspase in hibitors, but not by TPCK, Thus, caspase-dependent and caspase-independent protease cascades are downstream of and regulated by the proteasome, which plays a central role in regulating the multiple protease cascades that indu ce apoptosis. (C) 1998 Academic Press.