The role of sphingosine kinase in insulin signaling and its actions in hepatocytes

Abstract

Sphingosine kinase (SphK) catalyzes the phosphorylation of sphingosine to sphingosine-1-phosphate (S1P), is a key enzyme controlling sphingolipids metabolism. Current study was aimed to investigate potential roles of SphK in regulation of hepatic insulin signaling and its metabolic actions. This thesis has provided evidence demonstrating the importance of SphK. The key findings are: (1) Insulin induces SphK activation in hepatocytes. (2) Insulin-stimulated Akt and Erk1/2 phosphorylation was significantly attenuated by SphK inhibitor (SKI II). (3) Abrogated insulin-induced SphK activation by overexpressing of dominant negative SphK (SphKG82D) resulted in dramatic attenuation of insulin signaling, whereas overexpressing wild-type SphK (SphKwt) caused significant potentiation of insulin signaling. (4) siRNA-mediated knockdown of SphK2 led to significant attenuation of insulin signaling (5) Down-regulating S1P receptor 3 (S1P3) by siRNA strategy profoundly inhibited insulin and S1P-mediated Akt phosphorylation, suggesting the effect of SphK is specifically mediated by extracellular action of S1P on receptor of S1P3. (6) Insulin-mediated suppression of glucose production and gluconeogenic gene expression were significantly attenuated by SphK2 inhibition. Furthermore, we demonstrated that insulin induces SphK2 phosphorylation at Thr-578, which may also necessary for nuclear translocation of the enzyme. In summary, this project has identified a novel regulatory role of SphK2 in insulin signaling and glucose production in hepatocytes.