Cytochrome P450-derived epoxyeicosatrienoic acids (EETs) stimulate endothelial cell proliferation and angiogenesis. In this study, we investigated the involvement of the forkhead box, class O (FOXO) family of transcription factors and their downstream target p27Kip1 in EET-induced endothelial cell proliferation. Incubation of human umbilical vein endothelial cells with 11,12-EET induced a time-and dose-dependent decrease in p27Kip1 protein expression, whereas p21 Cip1 was not significantly affected. This effect on p27Kip1 protein was associated with decreased mRNA levels as well as p27Kip1 promoter activity. 11,12-EET also stimulated the time-dependent phosphorylation of Akt and of the forkhead factors FOXO1 and FOXO3a, effects prevented by the phosphatidylinositol 3-kinase inhibitor LY 294002. Transfection of endothelial cells with either a dominantnegative or an "Akt-resistant"/constitutively active FOXO3a mutant reversed the 11,12-EET-induced downregulation of p27Kip1 , whereas transfection of a constitutive active Akt decreased p27Kip1 expression independently of the presence or absence of 11,12-EET. To determine whether these effects are involved in EETinduced proliferation, endothelial cells were transfected with the 11,12-EET-generating epoxygenase CYP2C9. Transfection of CYP2C9 elicited endothelial cell proliferation and this effect was inhibited in cells co-transfected with CYP2C9 and either a dominant-negative Akt or constitutively active FOXO3a. Reducing FOXO expression using RNA interference, on the other hand, attenuated p27Kip1 expression and stimulated endothelial cell proliferation. These results indicate that EET-induced endothelial cell proliferation is associated with the phosphatidylinositol 3-kinase/Akt-dependent phosphorylation and inactivation of FOXO factors and the subsequent decrease in expression of the cyclin-dependent kinase inhibitor p27Kip1 .The epoxyeicosatrienoic acids (EETs) 1 are biologically active eicosanoids generated by cytochrome P450 epoxygenases (1, 2). Epoxygenases of the CYP2B, -2C, and -2J subfamilies are expressed in vascular endothelial cells and metabolize arachidonic acid into a series of regio-and stereo-selective EETs (5,8,11,and 14, that are potent vasodilators and have been identified as endothelium-derived hyperpolarizing factors (3-5).Recent reports have shown that EETs exert multiple effects on the vascular wall that are not directly related to changes in membrane potential or vascular tone. Indeed, EETs seem to act as second messengers in numerous signaling pathways (6, 7) and are involved in the regulation of inflammation, migration, apoptosis, hypoxia-reoxygenation injury, and platelet aggregation (8 -13). Moreover, we and others have recently been able to demonstrate that EETs stimulate endothelial cell proliferation and elicit an angiogenic response (14 -16).Progression through the mammalian cell cycle requires the activation of cyclin-dependent kinases (CDKs) through association with regulatory subunits (cyclins) that phosphorylate the retinoblastoma ...