Tumor necrosis factor-α induces adhesion molecule expression through the sphingosine kinase pathway

The signaling pathways that couple tumor necrosis factor-alpha (TNF alpha) receptors to functional, especially inflammatory, responses have remained elusive. We report here that TNF alpha induces endothelial cell activation, as measured by the expression of adhesion protein E-selectin and vascular adhesion molecule-1, through the sphingosine kinase (SKase) signaling pathway. Treatment of human umbilical vein endothelial cells with TNF alpha resulted in a rapid SKase activation and sphingosine l-phosphate (S1P) generation. SLP, but not ceramide or sphingosine, was a potent dose-dependent stimulator of adhesion protein expression. S1P was able to mimic the effect of TNF alpha on endothelial cells leading to extracellular signal-regulated kinases and NF-KB activation, whereas ceramide or sphingosine was not. Furthermore, N,N-dimethylsphingosine, an inhibitor of SKase, profoundly inhibited TNF alpha-induced extracellular signal-regulated kinases and NF-KB activation and adhesion protein expression. Thus we demonstrate that the SKase pathway through the generation of SLP is critically involved in mediating TNF alpha-induced endothelial cell activation.