title: Extracellular export of sphingosine kinase-1a contributes to the vascular S1P gradient.
abstract: Sphingosine 1-phosphate (S1P), produced by Sphks (sphingosine kinases), is a multifunctional lipid mediator that regulates immune cell trafficking and vascular development. Mammals maintain a large concentration gradient of S1P between vascular and extravascular compartments. Mechanisms by which S1P is released from cells and concentrated in the plasma are poorly understood. We recently demonstrated [Ancellin, Colmont, Su, Li, Mittereder, Chae, Stefansson, Liau and Hla (2002) J. Biol. Chem. 277, 6667-6675] that Sphk1 activity is constitutively secreted by vascular endothelial cells. In the present study, we show that among the five Sphk isoforms expressed in endothelial cells, the Sphk-1a isoform is selectively secreted in HEK-293 cells (human embryonic kidney cells) and human umbilical-vein endothelial cells. In sharp contrast, Sphk2 is not secreted. The exported Sphk-1a isoform is enzymatically active and produced sufficient S1P to induce S1P receptor internalization. Wild-type mouse plasma contains significant Sphk activity (179 pmol x min(-1) x g(-1)). In contrast, Sphk1-/- mouse plasma has undetectable Sphk activity and approx. 65% reduction in S1P levels. Moreover, human plasma contains enzymatically active Sphk1 (46 pmol x min(-1) x g(-1)). These results suggest that export of Sphk-1a occurs under physiological conditions and may contribute to the establishment of the vascular S1P gradient.
authors.affiliation: Center for Vascular Biology, Department of Cell Biology, University of Connecticut Health Center, Farmington, CT 06030-3501, USA.
authors.forename: Krishnan
authors.initials: K
authors.lastname: Venkataraman
authors.forename: Shobha
authors.initials: S
authors.lastname: Thangada
authors.forename: Jason
authors.initials: J
authors.lastname: Michaud
authors.forename: Myat Lin
authors.initials: ML
authors.lastname: Oo
authors.forename: Youxi
authors.initials: Y
authors.lastname: Ai
authors.forename: Yong-Moon
authors.initials: YM
authors.lastname: Lee
authors.forename: Mingtao
authors.initials: M
authors.lastname: Wu
authors.forename: Nehal S
authors.initials: NS
authors.lastname: Parikh
authors.forename: Faraz
authors.initials: F
authors.lastname: Khan
authors.forename: Richard L
authors.initials: RL
authors.lastname: Proia
authors.forename: Timothy
authors.initials: T
authors.lastname: Hla
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D017077:Culture Media, Conditioned; D004730:Endothelium, Vascular / Q000166:cytology; D005110:Extracellular Space / Q000378:metabolism; D006801:Humans; D042541:Intracellular Space / Q000378:metabolism; D007527:Isoenzymes / Q000097:blood / Q000378:metabolism; D008246:Lysophospholipids / Q000096:biosynthesis* / Q000097:blood; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000097:blood / Q000378:metabolism*; D021381:Protein Transport; D049349:Receptors, Lysosphingolipid / Q000378:metabolism; D013110:Sphingosine / Q000031:analogs & derivatives* / Q000096:biosynthesis / Q000097:blood; D000099157:Sphingosine Kinase
chemical_list: D017077:Culture Media, Conditioned; D007527:Isoenzymes; D008246:Lysophospholipids; D017853:Phosphotransferases (Alcohol Group Acceptor); D049349:Receptors, Lysosphingolipid; D013110:Sphingosine; D000099157:Sphingosine Kinase; C060506:sphingosine 1-phosphate