Mechanism of physiological function of sphingosine-1-phosphate: extracellular action and demonstration of alleged receptor
Sphingosine-1-phosphate (Sph-1-P), the first product of sphingosine (Sph) catabolism, causes changes of cell physiology when exogenously added to culture medium. Cellular response varies depending on type of cell. For example: (i) 10-20 $\mu$M exogenous Sph-1-P causes shape change and aggregation of platelets. (ii) 10-100 nM exogenous Sph-1-P inhibits motility of B16 mouse melanoma and various other tumor cells. (iii) 5-10 $\mu$M exogenous Sph-1-P stimulates proliferation of Swiss 3T3 fibroblasts.Platelets lack Sph-1-P lyase activity, possess persistently active Sph kinase, and therefore abundantly store Sph-1-P. Exogenous Sph-1-P promptly activates platelets, even though the quantity of Sph-1-P transported into cells is negligible as compared to the high level of stored intracellular Sph-1-P. This suggests that the initial action of Sph-1-P is based on binding of this agonist at a cell surface receptor. This concept was applied to study the initial effects of exogenous Sph-1-P causing two processes: platelet activation and melanoma cell motility inhibition.Contact of cell surface with immobilized Sph-1-P whose $\omega$-carbon is covalently linked to porous glass beads resulted in activation of platelets, and motility inhibition of melanoma cells. Binding assays with radiolabeled Sph-1-P revealed the presence of specific Sph-1-P binding sites on platelets and melanoma cells. These results suggest that Sph-1-P acts extracellularly on these cells through surface binding sites.Sph-1-P inhibits cell motility at very low concentrations (10-100 nM) which may be physiologically relevant. Internally stored Sph-1-P was released from platelets and other cells upon stimulation, and concentration of Sph-1-P in human plasma was high enough to control cell motility. Thus, Sph-1-P may act as an intercellular (cell-to-cell) modulator or messenger in regulation of cell motility.
- Pathobiology