Agonism with the omega-3 fatty acids a-linolenic acid and docosahexaenoic acid mediates phosphorylation of both the short and long isoforms of the human GPR120 receptor

The newly discovered G protein-coupled receptor GPR120 has recently been shown to stimulate secretion of the gut hormones glucagon-like peptide-1 and cholecystokinin upon binding of free fatty acids, thrusting it to the forefront of drug discovery efforts for treatment of type 2 diabetes as well as satiety and obesity. Although sequences for two alternative splice variants of the human GPR120 receptor have been reported, there have been no studies which directly compare the signaling of these isoforms. We have
identified an additional 16 amino acid gap containing four phospho-labile serine/ threonine residues which is localized to the third intracellular loop of the GPR120-long (GPR120-L) isoform. Based on this finding, we hypothesized that the agonist-stimulated phosphorylation profiles of this isoform would be distinct from that of the short isoform (GPR120-S). Using a clonal HEK293 cell model, we examined agonist-mediated phosphorylation of GPR120-S and GPR120-L with the omega-3 fatty acids a-linolenic acid (ALA) and docosahexaenoic acid (DHA). Our results show rapid phosphorylation of both isoforms following agonism by either ALA or DHA. Moreover, we show no significant difference in the degree or rate of phosphorylation of both isoforms upon agonism with either ALA or DHA, suggesting that the additional gap in the longer variant is not phosphorylated. Importantly, our results demonstrate that the shorter variant exhibits significantly more pronounced basal phosphorylation in the absence of agonist, suggesting that the additional gap in the long variant may contribute to masking of constitutive phosphorylation sites. These are the first results which demonstrate specific phosphorylation of GPR120 isoforms upon agonism by free fatty acids and the first which distinguish the phosphorylation profiles of the two GPR120 isoforms. (Authors Abstract)

GPR120 is a recently discovered G protein-coupled receptor (GPCR) that  has become a  target for treatment of type 2 diabetes due to its ability to stimulate the release of the insulin secretagogue glucagon-like peptide-1 from secretory L-cells of the intestinal lumen. Stimulation of GPR120 has also been shown to elicit secretion of cholecystokinin in vivo as well as in mouse intestinal enteroendocrine cells and thus may have a role in regulating secretion of intestinal peptides. The molecular aspects involved in GPR120 signaling are unknown. In this study, the authors examined the sequence differences between the two human GPR120 isoforms and report an additional 16 amino acid gap encoded for in GPR120-L (long). This additional gap consists of four phospholabile serine/threonine residues, which the authors suggest indicate that that GPR120-S (short) and GPR120-L could have distinct agonist-stimulated phosphorylation profiles. A human embryonic kidney clonal cell model was used to assess whether agonism of cloned human GPR120-S and GPR120-L with the alpha-linolenic acid (ALA) and docosahexaenoic acid (DHA) would facilitate receptor phosphorylation. This is the first report that ALA and DHA agonist stimulation facilitates GPR120 phosphorylation in a rapid and transient manner. In analysis of the differences between the two human GPR120 isoforms,  four phospho-labile sites were identified. These sites may contribute to greater agonist-induced phosphorylation in the long isoform compared to the short. Stimulation with ALA seemed to have a minimally greater effect on phosphorylation of the long isoform which was statistically equivalent to phosphorylation of the short isoform. With results for DHA, it was demonstrated that there is no significant difference in the intensity or rate at which GPR120-S and GPR120-L are phosphorylated, at least upon stimulation with these two agonists. The results suggest differential maximal efficacies between the two agonists for inducing phosphorylation at the two GPR120 isoforms, e.g., DHA seems to be more efficacious at phosphorylating the short receptor while ALA more so at the long. These effects could be due to differences in lipophilicity or structurally dependent as DHA, a 22-carbon fatty acid with six unconjugated double bonds could interact in a distinct manner with aromatic residues that make up the receptor ligand binding pocket than could ALA, which is comprised of 18 carbons with three unconjugated double bonds. The number of double bonds is likely critical in binding to the receptor not only due to pi-electron interactions within the ligand binding domain, but due to the ability to constrain bond rotation thereby arranging the fatty acids in variable ��kinked� arrangements. The importance of chain length and degree of saturation in promoting receptor agonism
and phosphorylation will be elucidated by the authors with future studies using a broader range of fatty acids which comprise diverse lengths, double bonds, and degrees of conjugation. Further ongoing studies are being conducted on the mechanisms of GPR120 phosphorylation which are aimed to further identify differences in efficacy between fatty acid agonists. (Editors Comments)