Abstract
Background:
Current studies demonstrate that sphingosine kinase 1 (SphK1)/sphingosine 1-phosphate (S1P) signaling is involved in liver fibrosis. Here, we aimed to investigate the molecular mechanism and define the cell type involved in mediating the SphK1-dependent effect on liver fibrosis.
Methods
The expression and activity of SphK1 were examined in both normal and diseased human and murine liver tissues. Hepatic fibrosis was induced in SphK1-/- and wild-type controls with carbon tetrachloride (CCl4) and bile duct ligation (BDL) treatment. The functional contributions of SphK1 in liver fibrosis and activation of HSCs and Kupffer cells (KC) were investigated. Additionally, mice were lethally irradiated and subjected by bone marrow transplantation (BMT) from donor mice. CCl4-induced mice were also treated with SphK1 inhibitor to test the therapeutical effect of SphK1 on liver fibrosis.
Results:
The levels of expression and activity of SphK1 were low in normal liver of humans and mice but was significantly increased in fibrotic liver. Liver transaminases and histology revealed reduced injury in SphK1-/- compared with WT mice after CCl4 or BDL treatment. Collagen deposition and α-smooth muscle actin (α-SMA) expression, as well as inflammation, was reduced in SphK1-/- mice compared with wild-type mice. SphK1 was co-expressed with a range of HSC/KC markers including desmin, α-SMA and F4/80 in murine sections. Secretion of CCL2 in SphK1-/- mice decreased compared with WT mice by Chemokine array. The level of microRNA-19b-3p in SphK1-/- mice increased compared with WT mice and microRNA-19b-3p downregulated CCR2 (CCL2 receptor) in HSCs. SphK1 knockout also reduced the migration of HSCs and KC, respectively. Moreover, SphK1 knockout in KC reduced the secretion of CCL2; SphK1 knockout in HSCs inhibited the activation of HSCs through the CCL2-CCR2 axis. BMT experiments excluded bone marrow-derived cells from having a major impact on the SphK1-dependent effect on fibrogenesis. Finally, the protective effect of 5C on the CCl4-induced liver injury and fibrosis was further verified in mice.
Conclusion:
SphK1 promotes hepatic fibrosis through inducing intrahepatic HSCs and KC to produce CCL2 and collagen. Mechanistically, SphK1 deficiency attenuated HSC activation by down-regulation of CCR2 through upregulation of miR-19b-3p.
