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 under go apoptosis. We and others have shown recently that growth factors pr omote 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 i nhibitors and other stimuli. In U937 cells, both PI 3-kinase inhibitor s (wortmannin and LY294002) and etoposide activated the CPP32 apoptoti c protease by cleavage to active p17 subunits, In contrast, treatment with tumor necrosis factor alpha (TNF alpha) resulted in the accumulat ion of a distinct active CPP32 subunit, p20. Furthermore, overexpressi on of Bcl-x(L) blocked DNA fragmentation, CPP32 activation and cleavag e of poly(ADP ribose) polymerase in U937 cells treated with both PI 3- kinase inhibitors and etoposide, but not in cells treated with TNF alp ha. Distinct patterns of CPP32 activation and differential sensitiviti es to Bcl-x(L) thus distinguish the cell death pathways activated by P I 3-kinase inhibition and DNA damage from that activated by TNF alpha.