Sphingosine-1-phosphate (S1P) displays sustained S1P sub(1) receptor agonism and signaling through S1P lyase-dependent receptor recycling

The sphingosine-1-phosphate (S1P) type 1 receptor (S1P sub(1)R) is a novel therapeutic target in lymphocyte-mediated autoimmune diseases. S1P sub(1) receptor desensitization caused by synthetic S1P sub(1) receptor agonists prevents T-lymphocyte egress from secondary lymphoid organs into the circulation. The selective S1P sub(1) receptor agonist ponesimod, which is in development for the treatment of autoimmune diseases, efficiently reduces peripheral lymphocyte counts and displays efficacy in animal models of autoimmune disease. Using ponesimod and the natural ligand S1P, we investigated the molecular mechanisms leading to different signaling, desensitization and trafficking behavior of S1P sub(1) receptors. In recombinant S1P sub(1) receptor-expressing cells, ponesimod and S1P triggered G alpha i protein-mediated signaling and beta -arrestin recruitment with comparable potency and efficiency, but only ponesimod efficiently induced intracellular receptor accumulation. In human umbilical vein endothelial cells (HUVEC), ponesimod and S1P triggered translocation of the endogenous S1P sub(1) receptor to the Golgi compartment. However, only ponesimod treatment caused efficient surface receptor depletion, receptor accumulation in the Golgi and degradation. Impedance measurements in HUVEC showed that ponesimod induced only short-lived G alpha i protein-mediated signaling followed by resistance to further stimulation, whereas S1P induced sustained G alpha i protein-mediated signaling without desensitization. Inhibition of S1P lyase activity in HUVEC rendered S1P an efficient S1P sub(1) receptor internalizing compound and abrogated S1P-mediated sustained signaling. This suggests that S1P lyase - by facilitating S1P1 receptor recycling - is essential for S1P-mediated sustained signaling, and that synthetic agonists are functional antagonists because they are not S1P lyase substrates.