The molecular basis of ligand interaction at free fatty acid receptor 4 (FFA4/GPR120)

The long-chain fatty acid receptor FFA4(previously GPR120) is receiving substantial interest as a novel target for the treatment of metabolic and inflammatory disease. The current study examines for the first time the detailed mode of binding of both a long-chain fatty acid and synthetic agonist ligands at FFA4 by integrating molecular modeling, receptor mutagenesis, and ligand structure-activity relationship approaches in an iterative format. In doing so, residues required for binding of fatty acid and synthetic agonists to FFA4 have been identified. This has allowed for the refinement of a well-validated model of the mode of ligand-FFA4 interaction which will be invaluable in the identification of novel ligands and future development of this receptor as a therapeutic target. The model reliably predicted the effects of substituent variations on agonist potency, and was also able to predict the qualitative effect of binding site mutations in the majority of cases

Concomitant action of structural elements and receptor phosphorylation determines arrestin-3 interaction with the free fatty acid receptor FFA4.

In addition to being nutrients, free fatty acids act as signaling molecules by activating a family of G protein-coupled receptors. Among these is FFA4, previously called GPR120, which responds to medium and long chain fatty acids, including health-promoting ω-3 fatty acids, which have been implicated in the regulation of metabolic and inflammatory responses. Here we show, using mass spectrometry, mutagenesis, and phosphospecific antibodies, that agonist-regulated phosphorylation of the human FFA4 receptor occurred primarily at five residues (Thr(347), Thr(349), Ser(350), Ser(357), and Ser(360)) in the C-terminal tail. Mutation of these residues reduced both the efficacy and potency of ligand-mediated arrestin-3 recruitment as well as affecting recruitment kinetics. Combined mutagenesis of all five of these residues was insufficient to fully abrogate interaction with arrestin-3, but further mutagenesis of negatively charged residues revealed additional structural components for the interaction with arrestin-3 within the C-terminal tail of the receptor. These elements consist of the acidic residues Glu(341), Asp(348), and Asp(355) located close to the phosphorylation sites. Receptor phosphorylation thus operates in concert with structural elements within the C-terminal tail of FFA4 to allow for the recruitment of arrestin-3. Importantly, these mechanisms of arrestin-3 recruitment operate independently from Gq/11 coupling, thereby offering the possibility that ligands showing stimulus bias could be developed that exploit these differential coupling mechanisms. Furthermore, this provides a strategy for the design of biased receptors to probe physiologically relevant signaling

Complex Pharmacology of Free Fatty Acid Receptors

G protein-coupled receptors (GPCRs) are historically the most successful family of drug targets. In recent times it has become clear that the pharmacology of these receptors is far more complex than previously imagined. Understanding of the pharmacological regulation of GPCRs now extends beyond simple competitive agonism or antagonism by ligands interacting with the orthosteric binding site of the receptor to incorporate concepts of allosteric agonism, allosteric modulation, signaling bias, constitutive activity, and inverse agonism. Herein, we consider how evolving concepts of GPCR pharmacology have shaped understanding of the complex pharmacology of receptors that recognize and are activated by nonesterified or “free” fatty acids (FFAs). The FFA family of receptors is a recently deorphanized set of GPCRs, the members of which are now receiving substantial interest as novel targets for the treatment of metabolic and inflammatory diseases. Further understanding of the complex pharmacology of these receptors will be critical to unlocking their ultimate therapeutic potential

The GPR120 agonist TUG‐891 promotes metabolic health by stimulating mitochondrial respiration in brown fat

Brown adipose tissue (BAT) activation stimulates energy expenditure in human adults, which makes it an attractive target to combat obesity and related disorders. Recent studies demonstrated a role for G protein‐coupled receptor 120 (GPR120) in BAT thermogenesis. Here, we investigated the therapeutic potential of GPR120 agonism and addressed GPR120‐mediated signaling in BAT. We found that activation of GPR120 by the selective agonist TUG‐891 acutely increases fat oxidation and reduces body weight and fat mass in C57Bl/6J mice. These effects coincided with decreased brown adipocyte lipid content and increased nutrient uptake by BAT, confirming increased BAT activity. Consistent with these observations, GPR120 deficiency reduced expression of genes involved in nutrient handling in BAT. Stimulation of brown adipocytes in vitro with TUG‐891 acutely induced O2 consumption, through GPR120‐dependent and GPR120‐independent mechanisms. TUG‐891 not only stimulated GPR120 signaling resulting in intracellular calcium release, mitochondrial depolarization, and mitochondrial fission, but also activated UCP1. Collectively, these data suggest that activation of brown adipocytes with the GPR120 agonist TUG‐891 is a promising strategy to increase lipid combustion and reduce obesity

Discovery of a Potent and Selective GPR120 Agonist

GPR120 is a receptor of unsaturated long-chain fatty acids reported to mediate GLP-1 secretion, insulin sensitization, anti-inflammatory, and anti-obesity effects and is therefore emerging as a new potential target for treatment of type 2 diabetes and metabolic diseases. Further investigation is however hindered by the lack of suitable receptor modulators. Screening of FFA1 ligands provided a lead with moderate activity on GPR120 and moderate selectivity over FFA1. Optimization led to the discovery of the first potent and selective GPR120 agonist