Gas6 Is Required for Thrombin-Induced Expression of VCAM-1 through Foxo-1 in Endothelial Cells

Venous thromboembolism (VTE) is a leading cause of morbidity and mortality. Gas6^-/- mice are protected against VTE suggesting an important role for Gas6 in the pathophysiology of this disorder. VCAM-1 is a key adhesion molecule, expressed on the endothelium, implicated in the pathophysiology of venous thrombosis. We previously demonstrated that Gas6 induced the phosphorylation of Forkhead box O1 (FoxO-1) through the activation of the PI3K/Akt signaling pathway. Thus, we hypothesize that Gas6 promotes thrombin-induced VCAM-1 expression through the regulation of FoxO-1 in endothelial cells. Thrombin induces VCAM-1 mRNA and protein expression in WT but not in Gas6^-/- endothelial cells. Silencing FoxO-1, using siRNA, enhances VCAM-1 expression both in WT and in Gas6^-/- endothelial cells. Thrombin induces FoxO-1 phosphorylation and nuclear exclusion in WT but not in Gas6^-/- endothelial cells. Inhibition of FoxO-1 phosphorylation by overexpression of a phosphorylation resistant FoxO-1 mutant (Tm-FoxO-1) decreases VCAM-1 expression, both in WT and Gas6^-/- endothelial cells. Inhibition of Akt by LY294002 prevents FoxO-1 phosphorylation and VCAM-1 expression. Finally, thrombin-induced VCAM-1 expression in WT endothelial cells was associated with an increase in bone marrow mononuclear cell (BM-MC) adhesion. BM-MC adhesion was further increased when endothelial cells were transfected with a FoxO-1 siRNA and decreased when transfected with the Tm-FoxO-1 plasmid. Thus, we demonstrate that Gas6 regulates VCAM-1 expression through phosphorylation and nuclear exclusion of FoxO-1, thereby promoting BM-MC adhesion. These data suggest that the Gas6/FoxO-1 signaling axis plays an important role in VCAM-1 expression during thrombosis.