Up-regulation of IAPs by PI-3K: a cell survival signal-mediated anti-apoptotic mechanism

Under normal cell physiology, a balance between cell survival and apoptosis is crucial for homeostasis. Many studies have demonstrated that apoptosis is modulated by cell survival stimuli. Active Akt, a common mediator of cell survival signals, has been shown to inhibit apoptosis by attenuating activity of pro-apoptotic factors Bad and caspase-9. However, the anti-apoptotic mechanisms mediated by various cell survival signals are poorly understood. Human prostate cancer LNCaP cells, known to contain constitutively activated Akt as a result of a frame-shift mutation in PTEN, an inhibitor of PI-3K/Akt pathway, were observed to be completely resistant to TRAIL-induced apoptosis. In agreement with the known action of Akt, blockade of the PI-3K/Akt pathway rendered LNCaP cells highly susceptible to TRAIL. Importantly, active PI-3K/Akt prevented processing/activation of caspase-3, a phenomenon associated with the function of inhibitor of apoptosis proteins (IAPs). In fact, inhibition of PI-3K activity using Wortmannin significantly decreased the protein levels of IAPs, concomitantly promoting processing/activation of caspase-3 and TRAIL-induced apoptosis. My data indicate that in addition to blocking Bad and caspase-9 through Akt, PI-3K also inhibits caspase-3 through up-regulating IAPs, thereby attenuates apoptosis.