Abstract

Sphingosine 1-phosphate (S1P) has been shown to exert a variety of biological responses through extracellular specific receptors or intracellular mechanisms. In the present study, we characterized a signaling pathway of S1P-induced cAMP accumulation in human coronary artery smooth muscle cells (CASMCs). S1P induced biphasic cAMP accumulation composed of a short-term and transient response (a peak at 2.5 min) and a late and sustained response (∼4-6 h). The late phase of cAMP accumulation was parallel to the increment of cyclooxygenase-2 protein expression and was inhibited by N-[2-(cyclohexyloxyl)-4-nitrophenyl]-methane sulfonamide (NS398), a cyclooxygenase-2-specific inhibitor. We were surprised to find that the cyclooxygenase-2 inhibitor also inhibited short-term cAMP accumulation even when cyclooxygenase-2 protein expression was not yet increased. More interestingly, the short-term cAMP accumulation was also completely inhibited by pertussis toxin, an inhibitor of G_i/o proteins. JTE-013, a specific antagonist for S1P₂ receptors, inhibited the S1P-induced cAMP accumulation. Furthermore, small interfering RNAs targeted for S1P₂ receptors significantly inhibited the S1P-induced cAMP accumulation. The cAMP response was also inhibited by specific inhibitors for phospholipase C, extracellular signal-regulated kinase pathways, and cytosolic phospholipase A₂. S1P actually activated these enzyme activities and stimulated prostaglandin I₂ (PGI₂) synthesis. Finally, exogenously applied arachidonic acid and PGI₂ induced cAMP accumulation to a similar extent as S1P. In conclusion, S1P induced cAMP accumulation through S1P receptors, including S1P₂ receptor and G_i/o protein-mediated stimulation of intracellular signaling pathways involving cyclooxygenase-2-dependent PGI₂ synthesis.