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Journal of Lipid Research, Vol. 46, 2458-2467, November 2005
Copyright © 2005 by American Society for Biochemistry and Molecular Biology

Mechanisms of sphingosine and sphingosine 1-phosphate generation in human platelets

Motohiro Tani^*, Takamitsu Sano^*, Makoto Ito and Yasuyuki Igarashi^1,*

^* Department of Biomembrane and Biofunctional Chemistry, Graduate School of Pharmaceutical Sciences, Hokkaido University, Kita 12-jo, Nishi 6-choume, Kita-ku, Sapporo 060-0812, Japan
Department of Bioscience and Biotechnology, Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan

Published, JLR Papers in Press, August 1, 2005. DOI 10.1194/jlr.M500268-JLR200

¹ To whom correspondence should be addressed. e-mail: yigarash{at}pharm.hokudai.ac.jp

The bioactive molecule sphingosine 1-phosphate (S1P) is abundantly stored in platelets and can be released extracellularly. However, although they have high sphingosine (Sph) kinase activity, platelets lack the de novo sphingolipid biosynthesis necessary to provide the substrates. Here, we reveal a generation pathway for Sph, the precursor of S1P, in human platelets. Platelets incorporated extracellular ³H-labeled Sph much faster than human megakaryoblastic cells and rapidly converted it to S1P. Furthermore, Sph formed from plasma sphingomyelin (SM) by bacterial sphingomyelinase (SMase) and neutral ceramidase (CDase) was rapidly incorporated into platelets and converted to S1P, suggesting that platelets use extracellular Sph as a source of S1P. Platelets abundantly express SM, possibly supplied from plasma lipoproteins, at the cell surface. Treating platelets with bacterial SMase resulted in Sph generation at the cell surface, conceivably by the action of membrane-bound neutral CDase. Simultaneously, a time-dependent increase in S1P levels was observed. Finally, we demonstrated that secretory acid SMase also induces S1P increases in platelets.

In conclusion, our results suggest that in platelets, Sph is supplied from at least two sources: generation in the plasma followed by incorporation, and generation at the outer leaflet of the plasma membrane, initiated by cell surface SM degradation.

Abbreviations: CDase, ceramidase; Cer, ceramide; FB₁, fumonisin B₁; NBD, 4-nitrobenzo-2-oxa-1,3-diazole; SM, sphingomyelin; SMase, sphingomyelinase; Sph, sphingosine; S1P, sphingosine 1-phosphate

Supplementary key words ceramide • sphingomyelinase • neutral ceramidase

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