SUPPRESSION OF TRANSFORMING GROWTH FACTOR-BETA-INDUCED APOPTOSIS THROUGH A PHOSPHATIDYLINOSITOL 3-KINASE AKT-DEPENDENT PATHWAY

Insulin and insulin receptor substrate 1 (IRS-1) are capable of protec ting liver cells from apoptosis induced by transforming growth factor- beta (TGF-beta), The Ras/mitogen-activated protein kinase (MAP kinase) and the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathways are both activated upon insulin stimulation and can protect against apopto sis under certain circumstances. We investigated which of these pathwa ys is responsible for the protective effect of insulin on TGF-beta-ind uced apoptosis. An activated Pas, although elicited a strong mitogenic effect, could not protect Hep3B cells from TGF-beta-induced apoptosis , Furthermore, PD98059, a selective inhibitor of MEK, did not suppress the antiapoptotic effect of insulin. In contrast, the PI 3-kinase inh ibitor, LY294002, efficiently blocked the effect of insulin. Protectio n against TGF-beta-induced apoptosis conferred by PI 3-kinase was furt her verified by stable transfection of an activated PI 3-kinase, Downs tream targets of PI 3-kinase involved in this protection was further i nvestigated. An activated Akt mimicked the antiapoptotic effect of ins ulin, whereas a dominant-negative Akt inhibited such effect. However, rapamycin, the p70(S6) kinase inhibitor, had no effect on the protecti vity of insulin against TGF-beta-induced apoptosis, suggesting that th e antiapoptotic target of PI 3-kinase/Akt pathway is independent or li es upstream of the p70S6 kinase, The mechanism by which PI 3-kinase/Ak t pathway interferes with the apoptotic signaling of TGF-beta was expl ored. Activation of PI 3-kinase did not lead to a suppression of Smad hetero-oligomerization or nuclear translocation but blocked TGF-beta-i nduced caspase-3-like activity. In summary, the PI 3-kinase/Akt pathwa y, but not the Ras/MAP kinase pathway, protects against TGF-beta-induc ed apoptosis by inhibiting a step downstream of Smad but upstream of c aspase-3.