Intracellular sphingosine 1-phosphate contributes to collagen expression of hepatic myofibroblasts in human liver fibrosis independent of its receptors.

Sphingosine 1-phosphate (S1P) is involved in multiple pathological processes, including fibrogenesis. S1P participates in mouse liver fibrogenesis via a paracrine manner. Herein, we investigated the involvement of S1P in human liver fibrosis. Human fibrotic samples were obtained from livers of patients undergoing liver transplantation. Expression of sphingosine kinase (SphK1), collagen (Col) α1(I), Col α1(III), α-smooth muscle actin, and p-Smad2/3 was characterized by immunofluorescence, real-time RT-PCR, high-content analysis, or Western blot analysis in the fibrotic liver, human bone marrow-derived mesenchymal stem cells, and human hepatogenic profibrotic cells. The effect of SphK1 was assessed using siSphK1 or SphK-specific inhibitor. SphK1, which was expressed in human fibrotic liver myofibroblasts, could be detected in human bone marrow-derived mesenchymal stem cells or human hepatogenic profibrotic cells activated by transforming growth factor β1 (TGF-β1). TGF-β1 evoked the activation of SphK1, increased intracellular S1P, and up-regulated expression of SphK1, Col α1(I), and Col α1(III) in a TGF-β receptor-dependent manner. TGF-β1 induced expression of Col α1(I) and Col α1(III) via SphK1, which was mediated by intracellular S1P, independent of S1P receptors. TGF-β1 evoked nuclear translocation of p-Smad2 and p-Smad3 in TGF-β receptor-dependent, but SphK1-independent, manner. In conclusion, intracellular S1P plays a crucial role in the TGF-β1-induced expression of Col α1(I) and Col α1(III), which is required for human fibrosis development. S1P exerts its effects in S1P receptor-independent manner.