Sphingosine kinase (SphK) is a conserved lipid kinase that catalyzes the formation of sphingosine 1-phosphate (S1P), an important lipid mediator, which regulates fundamental biological processes. Here, we provide evidence that SphK is required for the achievement of cell growth arrest as well as myogenic differentiation of C2C12 myoblasts. Indeed, SphK activity, SphK1 protein content and S1P formation were found to be enhanced in myoblasts that became confluent as well as in differentiating cells. Enforced expression of SphK1 reduced the myoblast proliferation rate, enhanced the expression of myogenic differentiation markers and anticipated the onset of differentiated muscle phenotype. Conversely, down-regulation of SphK1 by specific silencing by RNA interference or overexpression of the catalytically inactive SphK1, significantly increased cell growth and delayed the beginning of myogenesis; noticeably, exogenous addition of S1P rescued the biological processes. Importantly, stimulation of myogenesis in SphK1-overexpressing myoblasts was abrogated by treatment with short interfering RNA specific for S1P(2) receptor. This is the first report of the role of endogenous SphK1 in myoblast growth arrest and stimulation of myogenesis through the formation of S1P that acts as morphogenic factor via the engagement of S1P(2).
(c) 2007 Wiley-Liss, Inc.