The HETE Is on FFAR1 and Pancreatic Islet Cells

A review of non-prostanoid, eicosanoid receptors: expression, characterization, regulation, and mechanism of action

Eicosanoid signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain and to cell survival itself. Disruption of normal eicosanoid signaling is implicated in numerous disease states. Eicosanoid signaling is facilitated by G-protein-coupled, eicosanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of non-prostanoid, eicosanoid receptors.

Autocrine negative feedback regulation of lipolysis through sensing of NEFAs by FFAR4/GPR120 in WAT

OBJECTIVES

Long-chain fatty acids (LCFAs) released from adipocytes inhibit lipolysis through an unclear mechanism. We hypothesized that the LCFA receptor, FFAR4 (GPR120), which is highly expressed in adipocytes, may be involved in this feedback regulation.

METHODS AND RESULTS

Liquid chromatography mass spectrometry (LC-MS) analysis of conditioned media from isoproterenol-stimulated primary cultures of murine and human adipocytes demonstrated that most of the released non-esterified free fatty acids (NEFAs) are known agonists for FFAR4. In agreement with this, conditioned medium from isoproterenol-treated adipocytes stimulated signaling strongly in FFAR4 transfected COS-7 cells as opposed to non-transfected control cells. In transfected 3T3-L1 cells, FFAR4 agonism stimulated Gi- and Go-mini G protein binding more strongly than Gq, effects which were blocked by the selective FFAR4 antagonist AH7614. In primary cultures of murine white adipocytes, the synthetic, selective FFAR4 agonist CpdA inhibited isoproterenol-induced intracellular cAMP accumulation in a manner similar to the antilipolytic control agent nicotinic acid acting through another receptor, HCAR2. In vivo, oral gavage with the synthetic, specific FFAR4 agonist CpdB decreased the level of circulating NEFAs in fasting lean mice to a similar degree as nicotinic acid. In agreement with the identified anti-lipolytic effect of FFAR4, plasma NEFAs and glycerol were increased in FFAR4-deficient mice as compared to littermate controls despite having elevated insulin levels, and cAMP accumulation in primary adipocyte cultures was augmented by treatment with the FFAR4 antagonist conceivably by blocking the stimulatory tone of endogenous NEFAs on FFAR4.

CONCLUSIONS

In white adipocytes, FFAR4 functions as an NEFA-activated, autocrine, negative feedback regulator of lipolysis by decreasing cAMP though Gi-mediated signaling.

20-HETE Enzymes and Receptors in the Neurovascular Unit: Implications in Cerebrovascular Disease

20-HETE is a potent vasoconstrictor that is implicated in the regulation of blood pressure, cerebral blood flow and neuronal death following ischemia. Numerous human genetic studies have shown that inactivating variants in the cytochrome P450 enzymes that produce 20-HETE are associated with hypertension, stroke and cerebrovascular disease. However, little is known about the expression and cellular distribution of the cytochrome P450A enzymes (CYP4A) that produce 20-HETE or the newly discovered 20-HETE receptor (GPR75) in the brain. The present study examined the cell types and regions in the rat forebrain that express CYP4A and GPR75. Brain tissue slices from Sprague Dawley (SD), Dahl Salt-Sensitive (SS) and CYP4A1 transgenic rat strains, as well as cultured human cerebral pericytes and cerebral vascular smooth muscle cells, were analyzed by fluorescent immunostaining. Tissue homogenates from these strains and cultured cells were examined by Western blot. In the cerebral vasculature, CYP4A and GPR75 were expressed in endothelial cells, vascular smooth muscle cells and the glial limiting membrane of pial arteries and penetrating arterioles but not in the endothelium of capillaries. CYP4A, but not GPR75, was expressed in astrocytes. CYP4A and GPR75 were both expressed in a subpopulation of pericytes on capillaries. The diameters of capillaries were significantly decreased at the sites of first and second-order pericytes that expressed CYP4A. Capillary diameters were unaffected at the sites of other pericytes that did not express CYP4A. These findings implicate 20-HETE as a paracrine mediator in various components of the neurovascular unit and are consistent with 20-HETE's emerging role in the regulation of cerebral blood flow, blood-brain barrier integrity, the pathogenesis of stroke and the vascular contributions to cognitive impairment and dementia. Moreover, this study highlights GPR75 as a potential therapeutic target for the treatment of these devastating conditions.

Structural basis for GPCR signaling by small polar versus large lipid metabolites-discovery of non-metabolite ligands

Key metabolites act through specific G protein-coupled receptors (GPCRs) as extracellular signals of fuel availability and metabolic stress. Here, we focus on the succinate receptor SUCNR1/GPR91 and the long chain fatty acid receptor FFAR1/GPR40, for which 3D structural information is available. Like other small polar acidic metabolites, succinate is excreted from the cell by transporter proteins to bind to an extracellular, solvent-exposed pocket in SUCNR1. Non-metabolite pharmacological tool compounds are currently being designed based on the structure of the SUCNR1 binding pocket. In FFAR1, differently signaling lipid mimetics bind in two distinct membrane-exposed sites corresponding to each of the lipid bilayer leaflets. Conceivably endogenous lipid ligands gain access to these sites by way of the membrane and probably occupy both sites under physiological circumstances. Design of polar agonists for a dynamic, solvent-exposed pocket in FFAR1 underlines the possibility of structure-based approaches for development of novel tool compounds even in lipid sensing metabolite GPCRs.

Connecting pancreatic islet lipid metabolism with insulin secretion and the development of type 2 diabetes

Obesity is the major contributing factor for the increased prevalence of type 2 diabetes (T2D) in recent years. Sustained positive influx of lipids is considered to be a precipitating factor for beta cell dysfunction and serves as a connection between obesity and T2D. Importantly, fatty acids (FA), a key building block of lipids, are a double-edged sword for beta cells. FA acutely increase glucose-stimulated insulin secretion through cell-surface receptor and intracellular pathways. However, chronic exposure to FA, combined with elevated glucose, impair the viability and function of beta cells in vitro and in animal models of obesity (glucolipotoxicity), providing an experimental basis for the propensity of beta cell demise under obesity in humans. To better understand the two-sided relationship between lipids and beta cells, we present a current view of acute and chronic handling of lipids by beta cells and implications for beta cell function and health. We also discuss an emerging role for lipid droplets (LD) in the dynamic regulation of lipid metabolism in beta cells and insulin secretion, along with a potential role for LD under nutritional stress in beta cells, and incorporate recent advancement in the field of lipid droplet biology.

Molecular dynamics-guided discovery of an ago-allosteric modulator for GPR40/FFAR1

Structures of G protein-coupled receptors (GPCRs) in complex with ligands mainly provide frozen pictures with little information about the actual molecular mechanism of action of the ligand in the normally highly dynamic receptor. Through computer-based molecular dynamics simulations of a receptor for long-chain fatty acids, free fatty acid receptor 1 (FFAR1), we discover that an unoccupied, solvent-exposed pocket closes on removal of the lipid-like agonist; that is, during a major conformational change of the receptor. Importantly, a compound designed to prevent closure of this previously unrecognized, dynamic pocket was identified through structure-based virtual screening and shown to function as an allosteric agonist for the receptor. The study demonstrates that molecular dynamics simulations can be used in drug discovery to identify different modes of stabilizing specific receptor states.

The long-chain fatty acid receptor FFAR1/GPR40 binds agonists in both an interhelical site between the extracellular segments of transmembrane helix (TM)-III and TM-IV and a lipid-exposed groove between the intracellular segments of these helices. Molecular dynamics simulations of FFAR1 with agonist removed demonstrated a major rearrangement of the polar and charged anchor point residues for the carboxylic acid moiety of the agonist in the interhelical site, which was associated with closure of a neighboring, solvent-exposed pocket between the extracellular poles of TM-I, TM-II, and TM-VII. A synthetic compound designed to bind in this pocket, and thereby prevent its closure, was identified through structure-based virtual screening and shown to function both as an agonist and as an allosteric modulator of receptor activation. This discovery of an allosteric agonist for a previously unexploited, dynamic pocket in FFAR1 demonstrates both the power of including molecular dynamics in the drug discovery process and that this specific, clinically proven, but difficult, antidiabetes target can be addressed by chemotypes different from existing ligands.