Possible involvement of sphingomyelin in the regulation of the plasma sphingosine 1-phosphate level in human subjects

Objectives: Sphingosine 1-phosphate (SIP) is a bioactive sphingolipid mediator. Although the plasma SIP concentration is reportedly determined by cellular components, including erythrocytes, platelets, and vascular endothelial cells, the possible involvement of other factors, such as serum sphingomyelin (SM) and autotaxin (ATX), remains to be elucidated. Design and methods: We measured SIP using high-performance liquid chromatography (HPLC), SM and lysophosphatidic acid (LPA) using enzymatic assays, ATX antigen using a two-site enzyme immunoassay, and ATX activity using a lysophospholipase D activity assay. To fractionate the lipoproteins, plasma samples were separated using fast protein liquid chromatography (FPLC) utilizing a Superose 6 column. Results: The plasma SIP level was positively correlated with the levels of SM and lysophosphatidylcholine, but not with the level of phosphatidylcholine. Although SM was present in the very low-density lipoprotein (VLDL) fraction, neither the plasma SIP level nor the SM level was affected by feeding. The plasma SIP level was negatively correlated with the ATX activity. Although the incubation of 100 1.unol/L of sphingosylphosphorylcholine (SPC) with the serum resulted in a significant increase in the S1P level because of the presence of ATX, the physiological concentration of SPC did not mimic this effect. Conclusion: The plasma SIP level was affected by the serum SM level, while the possibility of ATX involvement in the increase in the plasma SW level was considered to be remote at least in healthy human subjects. (C) 2015 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.