Activation of the sphingosine kinase-signaling pathway by high glucose mediates the proinflammatory phenotype of endothelial cells

Vascular endothelial cells are key targets for hyperglycemic damage that facilitates vascular inflammation and the vasculopathy associated with diabetes mellitus. However, the mechanisms underlying this damage remain undefined. We now demonstrate that hyperglycemia induces activation of sphingosine kinase (SphK), which represents a novel signaling pathway that mediates endothelial damage under ambient high glucose conditions. SphK activity was significantly increased in aorta and heart of streptozotocin-induced diabetic rats. Interestingly, this increase in SphK activity was prevented by insulin treatment, which achieved euglycemia in the diabetic animals. Hyperglycemia-induced increase in SphK activity was also evident in endothelial cells that received long-term exposure to high glucose (22 mmol/L). Studies using a small interfering RNA strategy demonstrated that endogenous SphK1, but not SphK2, is the major isoenzyme that was activated by high glucose. In addition, an increase in SphK1 phosphorylation was detected in a protein kinase C- and extracellular signal-regulated kinase 1/2-dependent manner, which accounts for the high glucose-induced increases in SphK activity. Importantly, inhibition of SphK1 by either a chemical inhibitor (N', N'-dimethylsphingosine) or expression of a dominant-negative mutant of SphK1 (SphKG82D), or SphK1-specific small interfering RNA, strongly protected endothelial cells against high glucose-induced damage, as characterized by an attenuation in the expression of proinflammatory adhesion molecules, adhesion of leukocytes to endothelial cells, and nuclear factor kappa B activation. Thus, interventions that target the SphK-signaling pathway may have the potential to prevent vascular lesions under hyperglycemic conditions.