The
sphingosine-1-phosphate (S1P) type 1 receptor (S1P1R) is a novel therapeutic target in lymphocyte-mediated
autoimmune diseases.
S1P1 receptor desensitization caused by synthetic
S1P1 receptor agonists prevents T-lymphocyte egress from secondary lymphoid organs into the circulation. The selective
S1P1 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 S1P1 receptors. In recombinant
S1P1 receptor-expressing cells,
ponesimod and S1P triggered Gαi
protein-mediated signaling and β-
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
S1P1 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 Gαi
protein-mediated signaling followed by resistance to further stimulation, whereas S1P induced sustained Gαi
protein-mediated signaling without desensitization. Inhibition of S1P
lyase activity in HUVEC rendered S1P an efficient
S1P1 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.