Sphingosine 1-phosphate regulates regeneration and fibrosis after liver injury via sphingosine 1-phosphate receptor 2

Sphingosine 1-phosphate (S1P), a bioactive lipid mediator, stimulates proliferation and contractility in hepatic stellate cells, the principal matrix-producing cells in the liver, and inhibits proliferation via S1P receptor 2 (S1P₂) in hepatocytes in rats in vitro. A potential role of S1P and S1P₂ in liver regeneration and fibrosis was examined in S1P₂-deficient mice. Nuclear 5-bromo-2′-deoxy-uridine labeling, proliferating cell nuclear antigen (PCNA) staining in hepatocytes, and the ratio of liver weight to body weight were enhanced at 48 h in S1P₂-deficient mice after a single carbon tetrachloride (CCl₄) injection. After dimethylnitrosamine (DMN) administration with a lethal dose, PCNA staining in hepatocytes was enhanced at 48 h and survival rate was higher in S1P₂-deficient mice. Serum aminotransferase level was unaltered in those mice compared with wild-type mice in both CCl₄-and DMN-induced liver injury, suggesting that S1P₂ inactivation accelerated regeneration not as a response to enhanced liver damage. After chronic CCl₄ administration, fibrosis was less apparent, with reduced expression of smooth-muscle α-actin-positive cells in the livers of S1P₂-deficient mice, suggesting that S1P2 inactivation ameliorated CCl₄-induced fibrosis due to the decreased accumulation of hepatic stellate cells. Thus, S1P plays a significant role in regeneration and fibrosis after liver injury via S1P₂.