Regulation of vascular smooth muscle cell apoptosis - Modulation of bad bya phosphatidylinositol 3-kinase-dependent pathway

Our objective was to define the signaling mechanisms by which mitogens such as insulin-like growth factor-I (IGF-I) regulate vascular smooth muscle ce ll (VSMC) apoptosis. We confirmed that IGF-I inhibits serum withdrawal-indu ced apoptosis of cultured VSMCs in a dose-dependent and time-dependent fash ion. To test the hypothesis that the phosphatidylinositol (PI) 3-kinase sig naling pathway regulates VSMC survival, we examined the relationship betwee n PI 3-kinase activity and cell fate. PI 3-kinase was elevated in viable VS MCs maintained in serum-containing medium, declined significantly in respon se to serum withdrawal, and increased in response to IGF-I-induced survival . Moreover, blockade of PI 3-kinase with 2 structurally dissimilar inhibito rs (wortmannin or LY294002) abolished the capacity of IGF-I to maintain VSM C viability. Similarly, transient transfection of a dominant-negative Delta p85 PI 3-kinase mutant construct abrogated the capacity of IGF-I to preven t VSMC death. Thus, PI 3-kinase is a critical antiapoptotic signal in VSMCs . To define the distal element of the antiapoptotic cascade, we tested the hypothesis that IGF-I inhibits the influence of the proapoptotic gene Bad. Indeed, IGF-I stimulates increased expression of the inactive, phosphorylat ed form of Bad by a PI 3-kinase-dependent pathway. Moreover, the proapoptot ic effect of Bad was attenuated by the stimulation of IGF-I. Thus, growth f actors appear to prevent VSMC death by activating signal transduction pathw ays linked to apoptotic regulatory genes.