Insulin Regulates Cleavage of Procaspase-9 via Binding of X Chromosome-Linked Inhibitor of Apoptosis Protein in HT-29 Cells

Insulin significantly reduced tumor necrosis factor (TNF)-–induced cleavage of procaspase-8,-9, and-3 and poly(ADP-ribose) polymerase when observed for up to 24 hours in a dose-dependent manner. Signaling pathways responsible for the inhibitory effects of insulin were investigated by using protein kinase inhibitors. Both phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase kinase pathways mediate the ability of insulin to decrease the TNF-–induced cleavage of procaspase-8. In contrast, only the PI3K inhibitor reversed the effect of insulin on the TNF-–induced cleavage of procaspase-9. Moreover, insulin decreased the apoptotic level induced by TNF-, whereas the PI3K inhibitor en-hanced it. The protein level of Apaf-1, an activator of procaspase-9, remained constant with the application of agents affecting the cleavage of procaspase-9. In examining another regulator of cleaved caspase-9, X chromosome-linked inhibitor of apoptosis protein (XIAP), we observed that TNF- treatment induced fragmentation of XIAP, which was also enhanced by the PI3K inhibitor. In addition, XIAP was coimmunopre-cipitated with procaspase-9. The treatment with TNF- reduced the level of XIAP precipitated with procaspase-9, whereas insulin reversed this effect. Moreover, PI3K and Akt inhibitors, but not mammalian target of rapamycin inhibitor, inhibited the effect of insulin on the coprecipitation of procaspase-9 and XIAP. Our data suggest that insulin decreases the TNF-–induced cleavage of procaspase-9 and subsequent apoptosis by regulating XIAP via the PI3K/Akt pathway