Myc potentiates apoptosis by stimulating Bax activity at the mitochondria

The ability of the c-Myc oncoprotein to potentiate apoptosis has been well documented; however, the mechanism of action remains ill defined. We have p reviously identified spatially distinct apoptotic pathways within the same cell that are differentially inhibited by Bcl-2 targeted to either the mito chondria (Bcl-acta) or the endoplasmic reticulum (Bcl-cb5). We show here th at in Rat1 cells expressing an exogenous c-myc allele, distinct apoptotic p athways can be inhibited by Bcl-2 or Bcl-acta yet be distinguished by their sensitivity to Bcl-cb5 as either susceptible (serum withdrawal, taxol, and ceramide) or refractory (etoposide and doxorubicin). Myc expression and ap optosis were universally associated with Bcl-acta and not Bcl-cb5, suggesti ng that Myc acts downstream at a point common to these distinct apoptotic s ignaling cascades. Analysis of Rat1 c-myc null cells shows these same death stimuli induce apoptosis with characteristic features of nuclear condensat ion, membrane blebbing, poly (ADP-ribose) polymerase cleavage, and DNA frag mentation; however, this Myc-independent apoptosis is not inhibited by Bcl- 2. During apoptosis, Bax translocation to the mitochondria occurs in the pr esence or absence of Myc expression. Moreover, Bax mRNA and protein express ion remain unchanged in the presence or absence of Myc. However, in the abs ence of Myc, Bax is not activated and cytochrome c is not released into the cytoplasm. Reintroduction of Myc into the c-myc null cells restores Bax ac tivation, cytochrome c release, and inhibition of apoptosis by Bcl-2. These results demonstrate a role for Myc in the regulation of Bax activation dur ing apoptosis. Moreover, apoptosis that can be triggered in the absence of Myc provides evidence that signaling pathways exist which circumvent Bax ac tivation and cytochrome c release to trigger caspase activation. Thus, Myc increases the cellular competence to die by enhancing disparate apoptotic s ignals at a common mitochondrial amplification step involving Bax activatio n and cytochrome c release.