Activation of the CPP32 apoptotic protease by distinct signaling pathways with differential sensitivity to Bcl-x(L)

In the absence of growth factors, many types of mam malian cells undergo apoptosis. We and others have shown recently that growth factors promote cell survival by activating phosphatidylinositol 3-kinase (PI 3-kinase) in several cell types, In the present study, we have compared downstream elements of the apoptotic pathways induced by PI 3-kinase inhibitors and other stimuli. In U937 cells, both PI 3-kinase inhibitors (wortmannin and LY294002) and etoposide activated the CPP32 apoptotic protease by cleavage to active p17 subunits, In contrast, treatment with tumor necrosis factor alpha (TNF alpha) resulted in the accumulation of a distinct active CPP32 subunit, p20. Furthermore, overexpression of Bcl-x(L) blocked DNA fragmentation, CPP32 activation and cleavage of poly(ADP ribose) polymerase in U937 cells treated with both PI 3-kinase inhibitors and etoposide, but not in cells treated with TNF alpha. Distinct patterns of CPP32 activation and differential sensitivities to Bcl-x(L) thus distinguish the cell death pathways activated by PI 3-kinase inhibition and DNA damage from that activated by TNF alpha.