Antiproliferative property of sphingosine 1-phosphate in rat hepatocytes involves activation of Rho via Edg-5

Background & Aims: Sphingosine 1-phosphate (SIP), a ligand for G protein-coupled endothelial differentiation gene-1 (Edg-1), Edg-3, Edg-5, Edg-6, and Edg-8, elicits a variety of responses by cells. Prominent among these is cell proliferation. SIP is abundantly stored in platelets and released upon their activation, suggesting that SIP plays a pathophysiologic role in vivo. Because the major part of injected SIP was distributed into the liver in mice, we wondered whether the liver would be one of its targets. The effects of SIP on hepatocytes, the major constituent cells in the liver, were examined. Methods & Results: Northern blot analysis revealed the expression of Edg-1 and Edg-5 messenger RNA (mRNA) in cultured rat hepatocytes, in which SIP decreased DNA synthesis induced by hepatocyte growth factor (HGF) or epidermal growth factor (EGF) without affecting total protein synthesis. This inhibitory effect was attenuated by inactivation of small GTPase Rho with C3 exotoxin but not by inactivation of G_i with pertussis toxin. Moreover, in the presence of JTE-013, a newly developed and specific binding antagonist for Edg-5, the inhibitory effect was also cancelled. Finally, the administration of SIP after 70% partial hepatectomy in rats reduced the peak of DNA synthesis in hepatocytes with increased Rho activity. Furthermore, Edg-5 but not Edg-1 mRNA expression was enhanced in hepatocytes 24-72 hours after partial hepatectomy, which coincides with decreasing hepatocyte proliferation. Conclusions: SIP has an antiproliferative property in rat hepatocytes by activating Rho via Edg-5. Our results raise the possibility that SIP is a negative regulator in liver regeneration.