Sphingosine 1-phosphate induces cell contraction via calcium-independent/Rho-dependent pathways in undifferentiated skeletal muscle cells

We have previously shown that sphingosine 1-phosphate (SIP) can induce intracellular Ca2+ mobilization and cell contraction in C2C12 myoblasts and that the two phenomena are temporally unrelated. Although Ca2+-independent mechanisms of cell contraction have been the focus of numerous studies on Ca2+ sensitization of smooth muscle, comparatively less studies have focused on the role that these mechanisms play in the regulation of skeletal muscle contractility. Phosphorylation and activation of myosin by Rho-dependent kinase mediate most of Ca2+-independent contractile responses. In the present study, we examined the potential role of Rho/Rho-kinase cascade activation in SI P-induced C2C12 cell contraction. First, we showed that depletion of Ca2+, by pre-treatment with BAPTA, did not affect SI P-induced myoblastic contractility, whereas it abolished S1P-induced Ca2+ transients. These results correlated with the absence of troponin C and with the immature cytoskeletal organization of these cells. Experimental evidence demonstrating the involvement of Rho pathway in SIP-stimulated myoblast contraction included: the activation/translocation of RhoA to the membrane in response to agonist-stimulation in cells depleted of Ca2+ and the inhibition of dynamic changes of the actin cytoskeleton in cells where Rho functions had been inhibited either by overexpression of RhoGDI, a physiological inhibitor of GDP dissociation from Rho proteins, or by pretreatment with Y-27632, a specific Rho kinase inhibitor. Contribution of protein kinase C in this cytoskeletal rearrangement was also evaluated. However, the pretreatment with Go6976 or rottlerin, specific inhibitors of PKCalpha and PKCdelta, respectively, failed to inhibit the agonist-induced myoblastic contraction. Single particle tracking of G-actin fluorescent probe was performed to statistically evaluate actin cytoskeletal dynamics in response to S1P. Stimulation with S1P was also able to increase the phosphorylation level of myosin light chain II. In conclusion, our results strongly suggest that Ca2+-independent/Rho- Rho kinase-dependent pathways may exert an important role in SI P-induced myoblastic cell contraction. (C) 2003 Wiley-Liss, Inc.