A FUNCTIONAL-ROLE FOR DEATH PROTEASES IN S-MYC-MEDIATED AND C-MYC-MEDIATED APOPTOSIS

Upon activation, cell surface death receptors, Fas/APO-1/CD95 and tumo r necrosis factor receptor-1 (TNFR-1), are attached to cytosolic adapt or proteins, which in turn recruit caspase-8 (MACH/FLICE/Mch5) to acti vate the interleukin-1 beta-converting enzyme (ICE)/CED-3 family prote ase (caspase) cascade. However, it remains unknown whether these apopt otic proteases are generally involved in apoptosis triggered by other stimuli such as Myc and p53. In this study, we provide lines of eviden ce that a death protease cascade consisting of caspases and serine pro teases plays an essential role in Myc-mediated apoptosis. When Rat-1 f ibroblasts stably expressing either s-Myc or c-Myc were induced to und ergo apoptosis by serum deprivation, a caspase-3 (CPP32)-like protease activity that cleaves a specific peptide substrate, Ac-DEVD-MCA, appe ared in the cell lysates. Induction of s-Myc- and c-Myc-mediated apopt otic cell death was effectively prevented by caspase inhibitors such a s Z-Asp-CH2-DCB and Ac-DEVD-CHO. Furthermore, exposing the cells to a serine protease inhibitor, 4-(2-aminoethyl)benzenesulfonyl fluoride (A EBSF), also significantly inhibited s-Myc- and c-Myc-mediated apoptosi s and the appearance of the caspase-3-like protease activity in vivo, However, AEBSF did not directly inhibit caspase-3-like protease activi ty in the apoptotic cell lysates in vitro. Together, these results ind icate that caspase-3-like proteases play a critical role in both s-Myc - and c-Myc-mediated apoptosis and that caspase-3-like proteases funct ion downstream of the AEBSF-sensitive step in the signaling pathway of Myc-mediated apoptosis.