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(Circulation Research. 2008;102:669.)
© 2008 American Heart Association, Inc.

Molecular Medicine

Vascular Endothelium As a Contributor of Plasma Sphingosine 1-Phosphate

Krishnan Venkataraman^*, Yong-Moon Lee^*, Jason Michaud, Shobha Thangada, Youxi Ai, Herbert L. Bonkovsky, Nehal S. Parikh, Cheryl Habrukowich, Timothy Hla

From the Center for Vascular Biology (K.V., J.M., S.T., Y.A., C.H., T.H.), Department of Cell Biology, Departments of Medicine and Molecular, Microbial and Structural Biology, University of Connecticut Health Center, Farmington; Liver-Biliary-Pancreatic Center (H.L.B.), University of Connecticut Health Center, Farmington; College of Pharmacy and CBITRC (Y.-M.L.), Chungbuk National University, Chongju, Korea; and Division of Hematology & Oncology (N.S.P.), Connecticut Children’s Medical Center, Hartford.

Correspondence to Timothy Hla, University of Connecticut Health Center, Center for Vascular Biology, MC3501, 263 Farmington Ave, Farmington, CT 06030. E-mail hla{at}nso2.uchc.edu

Sphingosine 1-phosphate (S1P), an abundant lipid mediator in plasma, regulates vascular and immune cells by activating S1P receptors. In this report, we investigated the mechanisms by which high plasma S1P levels are maintained in mice. We found that plasma S1P turns over rapidly with a half-life of 15 minutes, suggesting the existence of a high-capacity biosynthetic source(s). Transplantation of bone marrow from wild-type to Sphk1^–/–Sphk2^+/– mice restored plasma S1P levels, suggesting that hematopoietic cells are capable of secreting S1P into plasma. However, plasma S1P levels were not appreciably altered in mice that were thrombocytopenic, anemic, or leukopenic. Surprisingly, reconstitution of Sphk1^–/–Sphk2^+/– bone marrow cells into wild-type hosts failed to reduce plasma S1P, suggesting the existence of an additional, nonhematopoietic source for plasma S1P. Adenoviral expression of Sphk1 in the liver of Sphk1^–/– mice restored plasma S1P levels. In vitro, vascular endothelial cells, but not hepatocytes, secreted S1P in a constitutive manner. Interestingly, laminar shear stress downregulated the expression of S1P lyase (Sgpl) and S1P phosphatase-1 (Sgpp1) while concomitantly stimulating S1P release from endothelial cells in vitro. Modulation of expression of endothelial S1P lyase with small interfering RNA and adenoviral expression altered S1P secretion, suggesting an important role played by this enzyme. These data suggest that the vascular endothelium, in addition to the hematopoietic system, is a major contributor of plasma S1P.

Key Words: sphingosine 1-phosphate (S1P) • sphingosine kinase (Sphk) • S1P lyase (Sgpl) • plasma S1P gradient • Shear stress

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