Abstract
Sphingosine 1-phosphate (S1P) is a lysophospholipid that exerts numerous biological activities, including promotion of angiogenic differentiation of vascular endothelial cells, both as a receptor ligand and as an intracellular second messenger. Because molecular mechanisms that underlie differentiation processes of pre-adipocytes to adipocytes (adipogenesis) remain incompletely understood, we explored the roles of sphingosine kinase-1 (SK-1), an enzyme that phosphorylates and converts sphingosine to S1P. RT-PCR assays revealed that the expression levels of SK-1 mRNA, but not those of SK-2, are upregulated during adipogenesis of mouse 3T3-L1 cells (4.0 folds at day 4 vs. pre-adipocytes, p < 0.005), concomitant with elevated intracellular S1P content (4.1 folds at day 5, HPLC analysis, p < 0.01). Conversely, cells transfected with siRNA specific to SK-1, which exhibit markedly reduced S1P content (61% vs. control), undergo lower degrees of adipogenesis, characterized by decreases in lipid droplet accumulation (Oil Red O staining, 51% vs. control) and in adipocyte marker gene expression (PPARγ and aP2, RT-PCR). In vivo relevance of SK-1 in adipogenesis was explored in 13-weeks old ob/ob mice with obese phenotype, which exhibits markedly upregulated levels of SK-1 mRNA in both subcutaneous and epididymal adipose tissues (RT-PCR, p < 0.001 vs. wild type). These results suggest that the increased expression of SK-1, an S1P-producing enzyme, may promote adipogenesis, potentially providing a novel point of control in cardiovascular disorders associated with obesity. [J Physiol Sci. 2007;57 Suppl:S74]
