Phosphatidylinositol 3-kinase/Akt signaling controls endothelial cell sensitivity to Fas-mediated apoptosis via regulation of FLICE-inhibitory protein (FLIP)

Fas is constitutively expressed on endothelial cells, but in contrast to sm ooth muscle and other cell types, endothelial cells are highly resistant to Fas-mediated apoptosis. In this study, we examined the role of the serine/ threonine kinase Akt/PKB in controlling the sensitivity of endothelial cell s to Fas-mediated apoptosis. Serum deprivation inhibited expression of the caspase-8 inhibitor FLICE-inhibitory protein (FLIP), which functions downst ream from Fas. FLIP expression levels were restored when serum-depleted cel ls were treated with vascular endothelial growth factor. Treatment with the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitors wortmannin or LY294 002 or infection of the adenoviral construct expressing dominant-negative A kt (Adeno-dnAkt) also inhibited the expression of FLIP in endothelial cells , whereas the MEK inhibitor PD98059 had no effect. Conversely, adenovirus-m ediated transfection of a constitutively-active Akt gene abolished the wort mannin- and LY294002-mediated downregulation of FLIP. Suppression of PI 3-k inase signaling sensitized endothelial cells to Fas-mediated apoptosis. Und er conditions of suppressed PI 3-kinase signaling, restoration of FLIP expr ession reversed the induced sensitivity of endothelial cells to Fas-mediate d apoptosis. These data suggest that inhibition of Fas-mediated apoptosis, via promotion of FLIP expression, is a mechanism through which Akt signalin g can promote endothelial cell survival.