Sphingosine 1-phosphate subtype 1 (S1P(1)) receptor agonists alter lymphocyte trafficking and endothelial barrier integrity in vivo. Among these is the potent, non-selective agonist, FTY720-P, whose mechanism of action has been suggested to correlate with S1P(1) down-regulation. Discovery of the in vivo active S1P(1)-selective agonist, SEW2871, has broadened our understanding of minimal requirements for S1P(1) function while highlighting differences regarding agonist effect on S1P(1) fate, because SEW2871 does not degrade S1P(1). To further understand the mechanism of agonist-induced S1P(1) down-regulation, we compared signaling and fate of human S1P(1)-green fluorescent protein (GFP) in stable 293 cells, using AFD-R, a chiral analog of FTY720-P, SEW2871, and S1P. Although all agonists acutely internalized S1P(1) to late endosomal vesicles and activated GTP gamma S-35 binding and PERK to similar maxima, only AFD-R led to significant S1P(1) down-regulation, as shown by GFP immunoprecipitation studies. Down-regulation was time- and concentration-dependent, was partially blocked by proteasomal inhibition and reversed by chloroquine and an antagonist to S1P(1). All agonists induced a receptor-associated increase in ubiquitination, with AFD-R inducing 3-fold more accumulation than S1P and being 3-4 logs more potent than SEW2871. The formation of AFD-R-receptor ubiquitin complex was inhibited by antagonist and chloroquine and was enhanced by proteasomal inhibition. Identification of proteins by PAGE liquid chromatography-tandem mass spectrometry in cells treated with AFD-R confirmed the co-migration of ubiquitin peptides with those of S1P(1) and GFP, relative to vehicle alone. These data suggest that the hierarchy of ubiquitin recruitment to SIP1 (AFD-R > S1P > SEW2871) correlates with the efficiency of lysosomal receptor degradation and reflects intrinsic differences between agonists.
