Transforming Growth Factor-β1 Induces Transdifferentiation of Myoblasts into Myofibroblasts via Up-Regulation of Sphingosine Kinase-1/S1P3 Axis

The pleiotropic cytokine transforming growth factor (TGF)-beta 1 is a key player in the onset of skeletal muscle fibrosis, which hampers tissue repair. However, the molecular mechanisms implicated in TGF beta 1-dependent transdifferentiation of myoblasts into myofibroblasts are presently unknown. Here, we show that TGF beta 1 up-regulates sphingosine kinase (SK)-1 in C2C12 myoblasts in a Smad-dependent manner, and concomitantly modifies the expression of sphingosine 1-phosphate (S1P) receptors (S1PRs). Notably, pharmacological or short interfering RNA-mediated inhibition of SK1 prevented the induction of fibrotic markers by TGF beta 1. Moreover, inhibition of S1P(3), which became the highest expressed S1PR after TGF beta 1 challenge, strongly attenuated the profibrotic response to TGF beta 1. Furthermore, downstream of S1P(3), Rho/Rho kinase signaling was found critically implicated in the profibrotic action of TGF beta 1. Importantly, we demonstrate that SK/S1P axis, known to play a key role in myogenesis via S1P(2), consequently to TGF beta 1-dependent S1PR pattern remodeling, becomes responsible for transmitting a profibrotic, antidifferentiating action. This study provides new compelling information on the mechanism by which TGF beta 1 gives rise to fibrosis in skeletal muscle, opening new perspectives for its pharmacological treatment. Moreover, it highlights the pleiotropic role of SK/S1P axis in skeletal myoblasts that, depending on the expressed S1PR pattern, seems capable of eliciting multiple, even contrasting biological responses.