p14ARF induces apoptosis via an entirely caspase-3-dependent mitochondrial amplification loop.

The p14^ARF tumor suppressor triggers cell death or cell cycle arrest upon oncogenic stress. In MCF-7 breast carcinoma cells, expression of the tumor suppressor gene p14^ARF fails to trigger apoptosis but induces an arrest in the G1 and, to a lesser extent, in the G2 phase in the cell division cycle. Here, inhibition of cell cycle arrest resulted in apoptosis induction in caspase-3 proficient MCF-7 cells upon expression of p14^ARF. This occurred in the absence of S-phase progression or mitotic entry. In contrast, syngeneic, caspase-3-deficient MCF-7 cells remained entirely resistant to p14^ARF-induced apoptosis. Thus, cell cycle checkpoint abrogation overcomes resistance to p14^ARF-induced cell death and promotes cell death via a caspase-3-dependent pathway. Cell death coincided with dissipation of the mitochondrial membrane potential, release of cytochrome c, and was inhibitable by pan-caspase inhibitors and the caspase-3/7 inhibitor zDEVD-fmk. Of note, mitochondrial events of apoptosis execution depended entirely on caspase-3 proficiency indicating that caspase-3 either acts 'up-stream' of the mitochondria in a 'non-canonical' pathway or mediates a mitochondrial feedback loop to amplify the apoptotic caspase signal in p14^ARF-induced stress signaling. [ABSTRACT FROM AUTHOR]