Predicting FFAR4 agonists using structure-based machine learning approach based on molecular fingerprints

Free Fatty Acid Receptor 4 (FFAR4), a G-protein-coupled receptor, is responsible for triggering intracellular signaling pathways that regulate various physiological processes. FFAR4 agonists are associated with enhancing insulin release and mitigating the atherogenic, obesogenic, pro-carcinogenic, and pro-diabetogenic effects, normally associated with the free fatty acids bound to FFAR4. In this research, molecular structure-based machine-learning techniques were employed to evaluate compounds as potential agonists for FFAR4. Molecular structures were encoded into bit arrays, serving as molecular fingerprints, which were subsequently analyzed using the Bayesian network algorithm to identify patterns for screening the data. The shortlisted hits obtained via machine learning protocols were further validated by Molecular Docking and via ADME and Toxicity predictions. The shortlisted compounds were then subjected to MD Simulations of the membrane-bound FFAR4-ligand complexes for 100 ns each. Molecular analyses, encompassing binding interactions, RMSD, RMSF, RoG, PCA, and FEL, were conducted to scrutinize the protein-ligand complexes at the inter-atomic level. The analyses revealed significant interactions of the shortlisted compounds with the crucial residues of FFAR4 previously documented. FFAR4 as part of the complexes demonstrated consistent RMSDs, ranging from 3.57 to 3.64, with minimal residue fluctuations 5.27 to 6.03 nm, suggesting stable complexes. The gyration values fluctuated between 22.8 to 23.5 nm, indicating structural compactness and orderliness across the studied systems. Additionally, distinct conformational motions were observed in each complex, with energy contours shifting to broader energy basins throughout the simulation, suggesting thermodynamically stable protein-ligand complexes. The two compounds CHEMBL2012662 and CHEMBL64616 are presented as potential FFAR4 agonists, based on these insights and in-depth analyses. Collectively, these findings advance our comprehension of FFAR4's functions and mechanisms, highlighting these compounds as potential FFAR4 agonists worthy of further exploration as innovative treatments for metabolic and immune-related conditions.

Positive interplay between FFAR4/GPR120, DPP-IV inhibition and GLP-1 in beta cell proliferation and glucose homeostasis in obese high fat fed mice

G-protein coupled receptor-120 (GPR120; FFAR4) is a free fatty acid receptor, widely researched for its glucoregulatory and insulin release activities. This study aimed to investigate the metabolic advantage of FFAR4/GPR120activation using combination therapy. C57BL/6 mice, fed a High Fat Diet (HFD) for 120 days to induce obesity-diabetes, were subsequently treated with a single daily oral dose of FFAR4/GPR120 agonist Compound A (CpdA) (0.1μmol/kg) alone or in combination with sitagliptin (50mg/kg) for 21 days. After 21-days, glucose homeostasis, islet morphology, plasma hormones and lipids, tissue genes (qPCR) and protein expression (immunocytochemistry) were assessed. Oral administration of CpdA improved glucose tolerance (34% p<0.001) and increased circulating insulin (38% p<0.001). Addition of CpdA with the dipeptidyl peptidase-IV (DPP-IV) inhibitor, sitagliptin, further improved insulin release (44%) compared to sitagliptin alone and reduced fat mass (p<0.05). CpdA alone (50%) and in combination with sitagliptin (89%) induced marked reductions in LDL-cholesterol, with greater effects in combination (p<0.05). All treatment regimens restored pancreatic islet and beta-cell area and mass, complemented with significantly elevated beta-cell proliferation rates. A marked increase in circulating GLP-1 (53%) was observed, with further increases in combination (38%). With treatment, mice presented with increased Gcg (proglucagon) gene expression in the jejunum (130% increase) and ileum (120% increase), indicative of GLP-1 synthesis and secretion. These data highlight the therapeutic promise of FFAR4/GPR120 activation and the potential for combined benefit with incretin enhancing DPP-IV inhibitors in the regulation of beta cell proliferation and diabetes.

FFAR4 activation inhibits lung adenocarcinoma via blocking respiratory chain complex assembly associated mitochondrial metabolism

Despite notable advancements in the investigation and management of lung adenocarcinoma (LUAD), the mortality rate for individuals afflicted with LUAD remains elevated, and attaining an accurate prognosis is challenging. LUAD exhibits intricate genetic and environmental components, and it is plausible that free fatty acid receptors (FFARs) may bridge the genetic and dietary aspects. The objective of this study is to ascertain whether a correlation exists between FFAR4, which functions as the primary receptor for dietary fatty acids, and various characteristics of LUAD, while also delving into the potential underlying mechanism. The findings of this study indicate a decrease in FFAR4 expression in LUAD, with a positive correlation (P < 0.01) between FFAR4 levels and overall patient survival (OS). Receiver operating characteristic (ROC) curve analysis demonstrated a significant diagnostic value [area under the curve (AUC) of 0.933] associated with FFAR4 expression. Functional investigations revealed that the FFAR4-specific agonist (TUG891) effectively suppressed cell proliferation and induced cell cycle arrest. Furthermore, FFAR4 activation resulted in significant metabolic shifts, including a decrease in oxygen consumption rate (OCR) and an increase in extracellular acidification rate (ECAR) in A549 cells. In detail, the activation of FFAR4 has been observed to impact the assembly process of the mitochondrial respiratory chain complex and the malate-aspartate shuttle process, resulting in a decrease in the transition of NAD+ to NADH and the inhibition of LUAD. These discoveries reveal a previously unrecognized function of FFAR4 in the negative regulation of mitochondrial metabolism and the inhibition of LUAD, indicating its potential as a promising therapeutic target for the treatment and diagnosis of LUAD.

Double knockout of FFAR4 and FGF21 aggravates metabolic disorders in mice

Several investigations have examined the involvement of free fatty acid receptor 4 (FFAR4) in metabolic disorders, but its action remains controversial. To investigate whether endogenous fibroblast growth factor 21 (FGF21)-mediated signaling controls the metabolic status in FFAR4-deficient mice, we generated FFAR4/FGF21 double knockout (DKO) mice. We also evaluated the role of FGF21 on glucose and lipid metabolism in FFAR4 KO mice fed a high-fat diet. Levels of FGF21 were significantly increased in FFAR4-deficient mice and double deletion of FGF21 and FFAR4 led to severe metabolic disorders. Additionally, FFAR4/FGF21 DKO mice displayed metabolic abnormalities that may be caused by decreased energy expenditure. Collectively, this study characterized the effects of endogenous FGF21, which acts as a master feedback regulator in the absence of FFAR4.

GPR120/FFAR4 stimulation attenuates airway remodeling and suppresses IL-4- and IL-13-induced airway epithelial injury via inhibition of STAT6 and Akt

BACKGROUND

Airway remodeling is associated with severity and treatment insensitivity in asthma. This study aimed to investigate the effects of G protein-coupled receptor 120 (GPR120) stimulation on alleviating allergic inflammation and remodeling of airway epithelium.

RESEARCH DESIGN AND METHODS

Ovalbumin (OVA)-challenged BALB/c mice and type-2-cytokine (IL-4 and IL-13)-exposed 16HBE human bronchial epithelial cells were treated with GSK137647A, a selective GPR120 agonist. Markers of allergic inflammation and airway remodeling were determined.

RESULTS

GSK137647A attenuated inflammation and mucus secretion in airway epithelium of OVA-challenged mice. Stimulation of GPR120 in 16HBE suppressed expression of asthma-associated cytokines and cytokine-induced expression of pathogenic mucin-MUC5AC. These effects were abolished by co-treatment with AH7614, a GPR120 antagonist. Moreover, GPR120 stimulation in 16HBE cells reduced expression of fibrotic markers including fibronectin protein and ACTA2 mRNA and inhibited epithelial barrier leakage induced by type-2 inflammation via rescuing expression of zonula occludens-1 protein. Furthermore, GPR120 stimulation prevented the cytokine-induced airway epithelial remodeling via suppression of STAT6 and Akt phosphorylation.

CONCLUSIONS

Our findings suggest that GPR120 activation alleviates allergic inflammation and remodeling of airway epithelium partly through inhibition of STAT6 and Akt. GPR120 may represent a novel therapeutic target for diseases associated with remodeling of airway epithelium, including asthma.

A high-throughput structural dynamics approach for identification of potential agonists of FFAR4 for type 2 diabetes mellitus therapy

Diabetes mellitus is a metabolic disorder that persists as a global threat to the world. A G-protein coupled receptor (GPCR), free fatty acid receptor 4 (FFAR4), has emerged as a potential target for type 2 diabetes mellitus (T2DM) and obesity-related disorders. The current study has investigated the FFAR4, deploying 3-dimensional structure modeling, molecular docking, machine learning, and high-throughput virtual screening methods to unravel the receptor's crucial and non-crucial binding site residues. We screened four lakh compounds and shortlisted them based on binding energy, stereochemical considerations, non-bonded interactions, and pharmacokinetic profiling. Out of the screened compounds, four compounds were selected for ligand-bound simulations. The molecular dynamic simulations were carried out for 1µs for native FFAR4 and 500 ns each for complexes of FFAR4 with compound 1, compound 2, compound 3, and compound 4. Our findings showed that in addition to reported binding site residues ARG99, ARG183, and VAL98 in known agonists like TUG-891, the amino acids ARG22, ARG24, THR23, TRP305, and GLU43 were also critical binding site residues. These amino acids impart stability to the FFAR4 complexes and contribute to the stronger binding affinity of the compounds. The study also indicated that aromatic residues like PHE211 are crucial for recognizing the active site's pi-pi and C-C double bonds. Since FFAR4 is a membrane protein, the simulation studies give an insight into the mechanisms of the crucial protein-lipid and lipid-water interactions. The analysis of the molecular dynamics trajectories showed all four compounds as potential hit molecules that can be developed further into potential agonists for T2DM therapy. Amongst the four compounds, compound 4 showed relatively better binding affinity, stronger non-bonded interactions, and a stable complex.Communicated by Ramaswamy H. Sarma.

Akkermansia muciniphila supplementation improves glucose tolerance in intestinal Ffar4 knockout mice during the daily light to dark transition

IMPORTANCE

Alterations in the intestinal environment are associated with various diseases, and FFAR4 is abundantly enriched in the intestine, where it has been shown to have the ability to regulate intestinal hormone secretion and intestinal microbiota; here, we confirmed previous reports. Meanwhile, we found that intestinal FFAR4 regulates glucagon-like peptide 1 secretion by decreasing Akkermansia muciniphila abundance and show that such change is associated with the level of glucose utilization at ZT12 in mice. Intestinal FFAR4 deficiency leads to severely impaired glucose tolerance at the ZT12 moment in mice, and Akkermansia muciniphila supplementation ameliorates the abnormal glucose utilization at the ZT12 moment caused by FFAR4 deficiency, which is very similar to the dawn phenomenon in diabetic patients. Collectively, our data suggest that intestinal Ffar4 deteriorates glucose tolerance at the daily light to dark transition by affecting Akkermansia muciniphila.

GPCR-mediated effects of fatty acids and bile acids on glucose homeostasis

Fatty acids and glucose are key biomolecules that share several commonalities including serving as energy substrates and as signaling molecules. Fatty acids can be synthesized endogenously from intermediates of glucose catabolism via de-novo lipogenesis. Bile acids are synthesized endogenously in the liver from the biologically important lipid molecule, cholesterol. Evidence abounds that fatty acids and bile acids play direct and indirect roles in systemic glucose homeostasis. The tight control of plasma glucose levels during postprandial and fasted states is principally mediated by two pancreatic hormones, insulin and glucagon. Here, we summarize experimental studies on the endocrine effects of fatty acids and bile acids, with emphasis on their ability to regulate the release of key hormones that regulate glucose metabolism. We categorize the heterogenous family of fatty acids into short chain fatty acids (SCFAs), unsaturated, and saturated fatty acids, and highlight that along with bile acids, these biomolecules regulate glucose homeostasis by serving as endogenous ligands for specific G-protein coupled receptors (GPCRs). Activation of these GPCRs affects the release of incretin hormones by enteroendocrine cells and/or the secretion of insulin, glucagon, and somatostatin by pancreatic islets, all of which regulate systemic glucose homeostasis. We deduce that signaling induced by fatty acids and bile acids is necessary to maintain euglycemia to prevent metabolic diseases such as type-2 diabetes and related metabolic disorders.

Lipid-Sensing Receptor FFAR4 Modulates Pulmonary Epithelial Homeostasis following Immunogenic Exposures Independently of the FFAR4 Ligand Docosahexaenoic Acid (DHA)

The role of pulmonary free fatty acid receptor 4 (FFAR4) is not fully elucidated and we aimed to clarify the impact of FFAR4 on the pulmonary immune response and return to homeostasis. We employed a known high-risk human pulmonary immunogenic exposure to extracts of dust from swine confinement facilities (DE). WT and Ffar4-null mice were repetitively exposed to DE via intranasal instillation and supplemented with docosahexaenoic acid (DHA) by oral gavage. We sought to understand if previous findings of DHA-mediated attenuation of the DE-induced inflammatory response are FFAR4-dependent. We identified that DHA mediates anti-inflammatory effects independent of FFAR4 expression, and that DE-exposed mice lacking FFAR4 had reduced immune cells in the airways, epithelial dysplasia, and impaired pulmonary barrier integrity. Analysis of transcripts using an immunology gene expression panel revealed a role for FFAR4 in lungs related to innate immune initiation of inflammation, cytoprotection, and immune cell migration. Ultimately, the presence of FFAR4 in the lung may regulate cell survival and repair following immune injury, suggestive of potential therapeutic directions for pulmonary disease.

Fatty Acid Transport and Signaling: Mechanisms and Physiological Implications

Long-chain fatty acids (FAs) are components of plasma membranes and an efficient fuel source and also serve as metabolic regulators through FA signaling mediated by membrane FA receptors. Impaired tissue FA uptake has been linked to major complications of obesity, including insulin resistance, cardiovascular disease, and type 2 diabetes. Fatty acid interactions with a membrane receptor and the initiation of signaling can modify pathways related to nutrient uptake and processing, cell proliferation or differentiation, and secretion of bioactive factors. Here, we review the major membrane receptors involved in FA uptake and FA signaling. We focus on two types of membrane receptors for long-chain FAs: CD36 and the G protein-coupled FA receptors FFAR1 and FFAR4. We describe key signaling pathways and metabolic outcomes for CD36, FFAR1, and FFAR4 and highlight the parallels that provide insight into FA regulation of cell function.

Icosabutate: targeting metabolic and inflammatory pathways for the treatment of NASH

INTRODUCTION

Via pleiotropic targeting of membrane and nuclear fatty acid receptors regulating key metabolic and inflammatory pathways in the liver, long-chain omega-3 fatty acids could offer a unique therapeutic approach for the treatment of metabolic-inflammatory diseases such as NASH. However, they lack efficacy for the treatment of NASH, likely due to unfavorable distribution, metabolism, and susceptibility to peroxidation.

AREAS COVERED

Structurally engineered fatty acids (SEFAs), as exemplified by icosabutate, circumvent the inherent limitations of unmodified long-chain fatty acids, and demonstrate markedly enhanced pharmacodynamic effects without sacrificing safety and tolerability. We cover icosabutate's structural modifications, their rationale and the fatty acid receptor and pathway targeting profile. We also provide an overview of the clinical data to date, including interim data from a Phase 2b trial in NASH subjects.

EXPERT OPINION

Ideally, candidate drugs for NASH and associated liver fibrosis should be pleiotropic in mechanism and work upstream on multiple drivers of NASH, including lipotoxic lipid species, oxidative stress, and key modulators of inflammation, liver cell injury, and fibrosis. Icosabutate has demonstrated the ability to target these pathways in preclinical NASH models with interim data from the ICONA trial supporting, at least noninvasively, the clinical translation of highly promising pre-clinical data.

The secretion of total and acyl ghrelin from the mouse gastric mucosa: Role of nutrients and the lipid chemosensors FFAR4 and CD36

AIMS

This study investigated the nutrient-mediated modulation of total ghrelin (TG) and acyl ghrelin (AG) secretion from the mouse gastric mucosa, and the role of long-chain fatty acid chemosensors, FFAR4 and CD36, in lipid-mediated modulation of TG and AG release.

METHODS

Ex-vivo experiments were conducted using mouse gastric mucosa to examine the effects of nutrients (D-glucose, L-phenylalanine, peptone (mixture of oligopeptides & single amino acids), D-mannitol, α-linolenic acid and fat emulsion (intralipid)) on TG and AG secretion. Additionally, inhibition of FFAR4 and CD36 on α-linolenic acid and intralipid-mediated regulation of TG and AG secretion was assessed.

RESULTS

TG and AG secretion were unaffected by glucose and D-mannitol. Peptone stimulated the release of TG and AG. In contrast, L-phenylalanine reduced AG secretion only. Intralipid reduced TG secretion and stimulated AG secretion, and α-linolenic acid reduced AG release, without affecting TG mobilisation. Modulation of ghrelin secretion by lipids occurred in an FFAR4 and CD36-independent manner.

CONCLUSION

Ghrelin secretion is modulated in a nutrient-specific manner by proteins and lipids, with TG and AG displaying independent responses to the same stimuli. In addition, FFAR4 and CD36 do not participate in modulation of TG and AG secretion by α-linolenic acid and intralipid.

The molecular mechanism of phytosphingosine binding to FFAR4/GPR120 differs from that of other fatty acids

Free fatty acid receptor 4 (FFAR4)/GPR120 comprises a receptor for medium‐ and long‐chain fatty acids. We previously identified phytosphingosine (PHS) as a novel ligand of FFAR4. Although many natural FFAR4 ligands have carboxyl groups, PHS does not, thus suggesting that binding to FFAR4 is driven by a completely different mechanism than other natural ligands such as α‐linolenic acid (ALA). To test this hypothesis, we performed docking simulation analysis using a FFAR4 homology model based on a protein model derived from the crystal structure of activated turkey beta‐1 adrenoceptor. The docking simulation revealed that the probable hydrogen bonds to FFAR4 differ between various ligands. In particular, binding was predicted between R264 of the FFAR4 and the oxygen of the carboxylate group in ALA, as well as between E249 of the FFAR4 and the oxygen of the hydroxy group at the C4‐position in PHS. Alanine substitution at E249 (E249A) dramatically reduced PHS‐induced FFAR4 activation but demonstrated a weaker effect on ALA‐induced FFAR4 activation. Kinetic analysis and K_m values clearly demonstrated that the E249A substitution resulted in reduced affinity for PHS but not for ALA. Additionally, we observed that sphingosine, lacking a hydroxyl group at C4‐position, could not activate FFAR4. Our data show that E249 of the FFAR4 receptor is crucial for binding to the hydroxy group at the C4‐position in PHS, and this is a completely different molecular mechanism of binding from ALA. Because GPR120 agonists have attracted attention as treatments for type 2 diabetes, our findings may provide new insights into their development.

The Anti-Atherosclerotic Action of FFAR4 Agonist TUG-891 in ApoE-Knockout Mice Is Associated with Increased Macrophage Polarization towards M2 Phenotype

Background: Over the past few years, a better understanding of the biology of G-protein coupled receptors (GPRs) has led to the identification of several receptors as novel targets for free fatty acids (FFAs). FFAR4 has received special attention in the context of chronic inflammatory diseases, including atherosclerosis, obesity and NAFLD, through to its anti-inflammatory effect. Methods: The present study investigates the influence of prolonged treatment with TUG-891-FFAR4 agonist on the development of atherosclerosis plaque in apoE-knockout mice, using morphometric and molecular methods. Results: TUG-891 administration has led to the reduction of atherosclerotic plaque size and necrotic cores in an apoE-knockout mice model. TUG-891-treated mice were administered subcutaneously at a dose of 20 mg/kg three times a week for 4 months. The FFAR4 agonist reduced the content of pro-inflammatory M1-like macrophages content in atherosclerotic plaques, as evidenced by immunohistochemical phenotyping and molecular methods. In atherosclerotic plaque, the population of smooth muscle cells increased as evidenced by α-SMA staining. We observed changes in G-CSF and eotaxin markers in the plasma of mice; changes in the levels of these markers in the blood may be related to macrophage differentiation. Importantly, we observed a significant increase in M2-like macrophage cells in atherosclerotic plaque and peritoneum. Conclusions: Prolonged administration of TUG-891 resulted in significant amelioration of atherogenesis, providing evidence that the strategy based on macrophage phenotype switching toward an M2-like activation state via stimulation of FFAR4 receptor holds promise for a new approach in the prevention or treatment of atherosclerosis.

FFAR4: A New Player in Cardiometabolic Disease?

Free fatty acids (FFAs) are implicated in the pathogenesis of metabolic diseases that includes obesity, type 2 diabetes mellitus, and cardiovascular disease (CVD). FFAs serve as ligands for free fatty acid receptors (FFARs) that belong to the family of rhodopsin-like G protein-coupled receptors (GPCRs) and are expressed throughout the body to maintain energy homeostasis under changing nutritional conditions. Free fatty acid receptor 4 (FFAR4), also known as G protein-coupled receptor 120, is a long-chain fatty acid receptor highly expressed in adipocytes, endothelial cells, and macrophages. Activation of FFAR4 helps maintain metabolic homeostasis by regulating adipogenesis, insulin sensitivity, and inflammation. Furthermore, dysfunction of FFAR4 is associated with insulin resistance, obesity, and eccentric remodeling in both humans and mice, making FFAR4 an attractive therapeutic target for treating or preventing metabolic diseases. While much of the previous literature on FFAR4 has focused on its role in obesity and diabetes, recent studies have demonstrated that FFAR4 may also play an important role in the development of atherosclerosis and CVD. Most notably, FFAR4 activation reduces monocyte-endothelial cell interaction, enhances cholesterol efflux from macrophages, reduces lesion size in atherogenic mouse models, and stimulates oxylipin production in myocytes that functions in a feed-forward cardioprotective mechanism. This review will focus on the role of FFAR4 in metabolic diseases and highlights an underappreciated role of FFAR4 in the development of atherosclerosis and CVD.

Regulation of Free Fatty Acid Receptor 4 on Inflammatory Gene Induced by LPS in Large Yellow Croaker (Larimichthys crocea)

Free fatty acid receptor 4 (FFAR4) plays a key role in regulating the inflammatory response in mammals. The present study aimed to investigate the function of large yellow croaker FFAR4 on inflammation. In the present study, ffar4 was widely expressed in 10 tissues of large yellow croaker including gill, head kidney and spleen. Further studies showed that treatment of head kidney macrophages with agonists (TUG891 or GSK137647A) or overexpression of ffar4 reduced the mRNA expression of pro-inflammatory genes induced by LPS, and increased the expression of pparγ. Treatment of macrophages with antagonist AH7614 increased the mRNA expression of pro-inflammatory genes induced by LPS, and decreased the mRNA expression of pparγ. In order to verify the immunomodulatory effect of PPARγ, PPARγ was overexpressed in macrophages which significantly reduced the mRNA expression of pro-inflammatory genes il6, il1β, il8, tnfα and cox2. Moreover, results of dual-luciferase assays showed that PPARγ downregulated the transcriptional activity of il6 and il1β promoters. In conclusion, FFAR4 showed anti-inflammatory effects on LPS-induced inflammation in large yellow croaker.

Anti-Atherosclerotic Potential of Free Fatty Acid Receptor 4 (FFAR4)

Fatty acids (FAs) are considered not only as a basic nutrient, but are also recognized as signaling molecules acting on various types of receptors. The receptors activated by FAs include the family of rhodopsin-like receptors: GPR40 (FFAR1), GPR41 (FFAR3), GPR43 (FFAR2), GPR120 (FFAR4), and several other, less characterized G-protein coupled receptors (GPR84, GPR109A, GPR170, GPR31, GPR132, GPR119, and Olfr78). The ubiquitously distributed FFAR4 can be activated by saturated and unsaturated medium- and long-chain fatty acids (MCFAs and LCFAs), as well as by several synthetic agonists (e.g., TUG-891). The stimulation of FFAR4 using selective synthetic agonists proved to be promising strategy of reduction of inflammatory reactions in various tissues. In this paper, we summarize the evidence showing the mechanisms of the potential beneficial effects of FFAR4 stimulation in atherosclerosis. Based partly on our own results, we also suggest that an important mechanism of such activity may be the modulatory influence of FFAR4 on the phenotype of macrophage involved in atherogenesis.

Free fatty acid receptor 4 inhibitory signaling in delta cells regulates islet hormone secretion in mice

OBJECTIVE

Maintenance of glucose homeostasis requires the precise regulation of hormone secretion from the endocrine pancreas. Free fatty acid receptor 4 (FFAR4/GPR120) is a G protein-coupled receptor whose activation in islets of Langerhans promotes insulin and glucagon secretion and inhibits somatostatin secretion. However, the contribution of individual islet cell types (α, β, and δ cells) to the insulinotropic and glucagonotropic effects of GPR120 remains unclear. As gpr120 mRNA is enriched in somatostatin-secreting δ cells, we hypothesized that GPR120 activation stimulates insulin and glucagon secretion via inhibition of somatostatin release.

METHODS

Glucose tolerance tests were performed in mice after administration of selective GPR120 agonist Compound A. Insulin, glucagon, and somatostatin secretion were measured in static incubations of isolated mouse islets in response to endogenous (ω-3 polyunsaturated fatty acids) and/or pharmacological (Compound A and AZ-13581837) GPR120 agonists. The effect of Compound A on hormone secretion was tested further in islets isolated from mice with global or somatostatin cell-specific knock-out of gpr120. Gpr120 expression was assessed in pancreatic sections by RNA in situ hybridization. Cyclic AMP (cAMP) and calcium dynamics in response to pharmacological GPR120 agonists were measured specifically in α, β, and δ cells in intact islets using cAMPER and GCaMP6 reporter mice, respectively.

RESULTS

Acute exposure to Compound A increased glucose tolerance, circulating insulin, and glucagon levels in vivo. Endogenous and/or pharmacological GPR120 agonists reduced somatostatin secretion in isolated islets and concomitantly demonstrated dose-dependent potentiation of glucose-stimulated insulin secretion and arginine-stimulated glucagon secretion. Gpr120 was enriched in δ cells. Pharmacological GPR120 agonists reduced cAMP and calcium levels in δ cells but increased these signals in α and β cells. Compound A-mediated inhibition of somatostatin secretion was insensitive to pertussis toxin. The effect of Compound A on hormone secretion was completely absent in islets from mice with either global or somatostatin cell-specific deletion of gpr120 and partially reduced upon blockade of somatostatin receptor signaling by cyclosomatostatin.

CONCLUSIONS

Inhibitory GPR120 signaling in δ cells contributes to both insulin and glucagon secretion in part by mitigating somatostatin release.

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.

The dietary fatty acids α-linolenic acid (ALA) and linoleic acid (LA) selectively inhibit microglial nitric oxide production

Alzheimer's disease (AD) is a neurodegenerative disorder without a known cure or effective treatment. Research has identified several modifiable risk factors and suggested preventative measures to reduce the risk of developing AD, including alterations in diet. Polyunsaturated fatty acids (PUFAs) have been shown to regulate inflammatory responses in the central nervous system (CNS), the main site of inflammation in AD. In the CNS, microglia are immune cells responsible for the maintenance of homeostasis. However, in AD, microglia can become adversely activated, causing them to release increased levels of cytotoxins and inflammatory mediators, including nitric oxide (NO) and monocyte-chemoattractant protein (MCP)-1. We assessed the effects of two PUFAs, α-linolenic acid (ALA) and linoleic acid (LA), on select microglial immune functions, since the effects of these dietary fatty acids on neuroimmune responses are not well characterized. In BV-2 mouse microglia activated with lipopolysaccharide (LPS), exposure to LA reduced NO secretion and inducible nitric oxide synthase (iNOS) levels, whereas exposure to ALA reduced NO without a corresponding reduction of iNOS. Neither ALA nor LA altered MCP-1 levels or cytotoxins released by THP-1 human microglia-like cells stimulated with a combination of LPS and interferon (IFN)-γ. Specific receptor antagonists were used to demonstrate that the inhibitory effect of LA on NO secretion did not depend on the free fatty acid receptor (FFAR) 1 or FFAR4. Furthermore, gas chromatography with a flame ionization detector (GC-FID) revealed that exposure to LA or ALA did not alter the fatty acid composition of BV-2 microglia. Our data indicate that regulation of select microglial immune functions by ALA and LA could be one of the mechanisms underlying the observed link between certain dietary patterns and AD, such as reduced risk of cognitive decline and dementia associated with the Mediterranean diet.

The Role of GPR120 Receptor in Essential Fatty Acids Metabolism in Schizophrenia

A growing body of evidence confirms abnormal fatty acid (FAs) metabolism in the pathophysiology of schizophrenia. Omega-3 polyunsaturated fatty acids (PUFAs) are endogenous ligands of the G protein-coupled receptors, which have anti-inflammatory properties and are a therapeutic target in many diseases. No clinical studies are concerned with the role of the GPR120 signaling pathway in schizophrenia. The aim of the study was to determine the differences in PUFA nutritional status and metabolism between patients with schizophrenia (SZ group) and healthy individuals (HC group). The study included 80 participants (40 in the SZ group, 40 in the HC group). There were no differences in serum GPR120 and PUFA concentrations and PUFA intake between the examined groups. In the HC group, there was a relationship between FAs in serum and GPR120 concentration (p < 0.05): α-linolenic acid (ALA) (R = −0.46), docosahexaenoic acid (DHA) (R = −0.54), omega-3 PUFAs (R = −0.41), arachidonic acid (AA) (R = −0.44). In the SZ group, FA serum concentration was not related to GPR120 (p > 0.05). In the HC group, ALA and DHA serum concentrations were independently associated with GPR120 (p < 0.05) in the model adjusted for eicosapentaenoic acid (EPA) and accounted for 38.59% of GPR120 variability (p < 0.05). Our results indicate different metabolisms of FAs in schizophrenia. It is possible that the diminished anti-inflammatory response could be a component connecting GPR120 insensitivity with schizophrenia.

Omega-3 Fatty Acids Activate Ciliary FFAR4 to Control Adipogenesis

Adult mesenchymal stem cells, including preadipocytes, possess a cellular sensory organelle called the primary cilium. Ciliated preadipocytes abundantly populate perivascular compartments in fat and are activated by a high-fat diet. Here, we sought to understand whether preadipocytes use their cilia to sense and respond to external cues to remodel white adipose tissue. Abolishing preadipocyte cilia in mice severely impairs white adipose tissue expansion. We discover that TULP3-dependent ciliary localization of the omega-3 fatty acid receptor FFAR4/GPR120 promotes adipogenesis. FFAR4 agonists and ω-3 fatty acids, but not saturated fatty acids, trigger mitosis and adipogenesis by rapidly activating cAMP production inside cilia. Ciliary cAMP activates EPAC signaling, CTCF-dependent chromatin remodeling, and transcriptional activation of PPARγ and CEBPα to initiate adipogenesis. We propose that dietary ω-3 fatty acids selectively drive expansion of adipocyte numbers to produce new fat cells and store saturated fatty acids, enabling homeostasis of healthy fat tissue.

Role of phospholipase D in migration and invasion induced by linoleic acid in breast cancer cells

Linoleic acid (LA) is an essential and omega-6 polyunsaturated fatty acid that mediates a variety of biological processes, including migration and invasion in breast cancer cells. Phospholipase D (PLD) catalyses the hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. Increases of expression and activity of PLD are reported in several human cancers, including gastric, colorectal, renal, stomach, lung and breast. In this article, we demonstrate that LA induces an increase of PLD activity in MDA-MB-231 breast cancer cells. Particularly, PLD1 and/or PLD2 mediate migration and invasion induced by LA. Moreover, LA induces increases in number and size of spheroids via PLD activity. FFAR1 also mediates migration and invasion, whereas PLD activation induced by LA requires the activities of FFAR1, FFAR4 and EGFR in MDA-MB-231 cells. In summary, PLD plays a pivotal role in migration and invasion induced by LA in MDA-MB-231 breast cancer cells.

Aberrant fatty acid profile and FFAR4 signaling confer endocrine resistance in breast cancer

BACKGROUND

Evidence suggests that fatty acid receptor FFAR4 plays a tumor-promoting role in adipose tissue-adjacent malignancies, but its clinical relevance remains unexplored. Here, we investigated the clinical significance and underlying mechanisms of FFAR4 in hormone receptor-positive breast cancer (HRPBC).

METHODS

FFAR4 expression was assessed by immunohistochemistry in an exploration cohort of 307 breast cancer cases collected from two independent institutes. Two public breast cancer microarray datasets served as validation cohorts. Gas chromatography-mass spectrometry was employed to identify FFAR4 ligands in normal and cancerous breast tissues. Survival analyses were performed in all cohorts and designated molecular subgroups. Mechanistic studies were performed in vitro in hormone receptor-positive breast cancer cell lines MCF-7 and T-47D.

RESULTS

Aberrant FFAR4 expression and endogenous FFAR4 ligands were identified in breast cancer tissues, five FFAR4 ligands showed significantly elevated proportions in cancerous versus normal tissues. In the exploration cohort, FFAR4 was demonstrated as an independent prognostic factor for recurrences (HR: 2.183, 95% CI: 1.360-3.504, P = 0.001) and breast cancer-specific deaths (HR: 2.102, 95% CI: 1.173-3.766, P = 0.013) in HRPBC cases. In contrast, FFAR4 expression was not associated with prognosis in hormone receptor-negative cases. In the validation cohorts, FFAR4 mRNA levels were also observed to be associated with disease recurrence in estrogen receptor-positive cases, but not so in estrogen receptor-negative cases. FFAR4 activation by endogenous ligands and a synthetic ligand TUG891 significantly dampened tamoxifen's efficacy on HRPBC cells, whereas FFAR4 knockdown or antagonist AH7614 abrogated this effect. Furthermore, FFAR4-induced tamoxifen resistance was dependent on ERK and AKT pathways in HRPBC.

CONCLUSIONS

Our results establish a novel role of FFAR4 and its ligands in the complicated interactions between tissue lipid profile and cancer biology. FFAR4 signaling confers tamoxifen resistance in HRPBC cell line and FFAR4 expression can serve as a prognostic biomarker for tamoxifen-treated HRPBC patients. FFAR4 may serve as a potential target for anti-breast cancer therapies, especially in endocrine resistant cases.

GPR120 protects lipotoxicity-induced pancreatic β-cell dysfunction through regulation of PDX1 expression and inhibition of islet inflammation

G-protein coupled receptor 120 (GPR120) has been shown to act as an omega-3 unsaturated fatty acid sensor and is involved in insulin secretion. However, the underlying mechanism in pancreatic β cells remains unclear. To explore the potential link between GPR120 and β-cell function, its agonists docosahexaenoic acid (DHA) and GSK137647A were used in palmitic acid (PA)-induced pancreatic β-cell dysfunction, coupled with GPR120 knockdown (KD) in MIN6 cells and GPR120 knockout (KO) mice to identify the underlying signaling pathways. In vitro and ex vivo treatments of MIN6 cells and islets isolated from wild-type (WT) mice with DHA and GSK137647A restored pancreatic duodenal homeobox-1 (PDX1) expression levels and β-cell function via inhibiting PA-induced elevation of proinflammatory chemokines and activation of nuclear factor κB, c-Jun amino (N)-terminal kinases1/2 and p38MAPK signaling pathways. On the contrary, these GPR120 agonism-mediated protective effects were abolished in GPR120 KD cells and islets isolated from GPR120 KO mice. Furthermore, GPR120 KO mice displayed glucose intolerance and insulin resistance relative to WT littermates, and β-cell functional related genes were decreased while inflammation was exacerbated in islets with increased macrophages in pancreas from GPR120 KO mice. DHA and GSK137647A supplementation ameliorated glucose tolerance and insulin sensitivity, as well as improved Pdx1 expression and islet inflammation in diet-induced obese WT mice, but not in GPR120 KO mice. These findings indicate that GPR120 activation is protective against lipotoxicity-induced pancreatic β-cell dysfunction, via the mediation of PDX1 expression and inhibition of islet inflammation, and that GPR120 activation may serve as a preventative and therapeutic target for obesity and diabetes.

Discovery of N-arylpyrroles as agonists of GPR120 for the treatment of type II diabetes

The discovery of a novel series of N-arylpyrroles as agonists of GPR120 (FFAR4) is discussed. One lead compound is a potent GPR120 agonist, has good selectivity for related receptor GPR40 (FFAR1), has acceptable PK properties, and is active in 2 models of Type 2 Diabetes in mice.

GPR120: Mechanism of action, role and potential for medical applications

G protein-coupled receptors (GPCRs) constitute a family of transmembrane proteins that mediate many cellular processes. GPR120/FFAR4, a receptor from this family that is activated by fatty acids, has received considerable attention recently. This paper presents a literature review concerning the role of GPR120 and its mechanism of action in animal and human studies as well as the potential use of GPR120 for the treatment of chronic diseases. Two electronic databases - Medline and Google Scholar - were searched for available studies addressing the review topic that were written in English and published from 2000 to June 2017. The following key terms were used in the search: GPR120, FFA4, GPR120 agonist, PUFAs, EPA, DHA, adipocyte, obesity, hyperlipidemia, inflammation, cancer, diabetes, insulin resistance, taste, atherogenesis, hepatis, central nervous system. In humans, GPR120 expression is expressed in macrophages, eosinophils, and adipose tissue, in cells of the tongue, liver, lungs, small and large intestine, gastric mucosa, and pancreas, in the central nervous system and placental microvilli. Medium- and long-chain fatty acids act as ligands for the receptor. Through the internalization of beta-arrestin-2 complex and the inhibition of NF-κB, GPR120 mediates the activation of the cell's anti-inflammatory mechanisms. The receptor is also involved in the maturation of adipocytes, the modulation of insulin signalling pathways, the regulation of glucose metabolism, and the secretion of intestinal hormones. GPR120 is a promising target for the treatment of numerous diseases, whose pathophysiology is associated with low-grade inflammation. As a result of intensive searches, a likely group of synthetic agonists of the receptor was determined with potential therapeutic applications in conditions such as obesity, impaired carbohydrate metabolism, inflammatory bowel diseases, cancer, mental disorders.

Docosahexaenoic acid mechanisms of action on the bovine oocyte-cumulus complex

Background

Supplementation of bovine oocyte-cumulus complexes during in vitro maturation (IVM) with 1 μM of docosahexaenoic acid (DHA), C22:6 n-3 polyunsaturated fatty acid, was reported to improve in vitro embryo development. The objective of this paper was to decipher the mechanisms of DHA action.

Results

Transcriptomic analysis of 1 μM DHA-treated and control cumulus cells after 4 h IVM showed no significant difference in gene expression. MALDI-TOF mass spectrometry analysis of lipid profiles in DHA-treated and control oocytes and cumulus cells after IVM showed variations of only 3 out of 700 molecular species in oocytes and 7 out of 698 species in cumulus cells (p < 0.01).

We showed expression of free fatty acid receptor FFAR4 in both oocytes and cumulus cells, this receptor is known to be activated by binding to DHA. FFAR4 protein was localized close to the cellular membrane by immunofluorescence. Functional studies demonstrated that supplementation with FFAR4 agonist TUG-891 (1 μM or 5 μM) during IVM led to an increased blastocyst rate (39.5% ± 4.1%, 41.3% ± 4.1%), similar to DHA 1 μM treatment (39.2% ± 4.1%) as compared to control (25.2% ± 3.6%).

FFAR4 activation via TUG-891 led to beneficial effect on oocyte developmental competence and might explain in part similar effects of DHA.

Conclusions

In conclusion, we suggested that low dose of DHA (1 μM) during IVM might activate regulatory mechanisms without evident effect on gene expression and lipid content in oocyte-cumulus complexes, likely through signaling pathways which need to be elucidated in further studies.

Electronic supplementary material

The online version of this article (10.1186/s13048-017-0370-z) contains supplementary material, which is available to authorized users.

Linoleic acid induces migration and invasion through FFAR4- and PI3K-/Akt-dependent pathway in MDA-MB-231 breast cancer cells. Med Oncol. 2017;34:111. [PMID: 28456993 DOI: 10.1007/s12032-017-0969-3]

An increased risk of developing breast cancer has been associated with high levels of dietary fat intake. Linoleic acid (LA) is an essential fatty acid and the major ω-6 polyunsaturated fatty acid in occidental diets, which is able to induce inappropriate inflammatory responses that contribute to several chronic diseases including cancer. In breast cancer cells, LA induces migration. However, the signal transduction pathways that mediate migration and whether LA induces invasion in MDA-MB-231 breast cancer cells have not been studied in detail. We demonstrate here that LA induces Akt2 activation, invasion, an increase in NFκB-DNA binding activity, miR34a upregulation and miR9 downregulation in MDA-MB-231 cells. Moreover, Akt2 activation requires EGFR and PI3K activity, whereas migration and invasion are dependent on FFAR4, EGFR and PI3K/Akt activity. Our findings demonstrate, for the first time, that LA induces migration and invasion through an EGFR-/PI3K-/Akt-dependent pathway in MDA-MB-231 breast cancer cells.

Fatty acid 16:4(n-3) stimulates a GPR120-induced signaling cascade in splenic macrophages to promote chemotherapy resistance

Although chemotherapy is designed to eradicate tumor cells, it also has significant effects on normal tissues. The platinum-induced fatty acid 16:4(n-3) (hexadeca-4,7,10,13-tetraenoic acid) induces systemic resistance to a broad range of DNA-damaging chemotherapeutics. We show that 16:4(n-3) exerts its effect by activating splenic F4/80⁺/CD11b^low macrophages, which results in production of chemoprotective lysophosphatidylcholines (LPCs). Pharmacologic studies, together with analysis of expression patterns, identified GPR120 on F4/80⁺/CD11b^low macrophages as the relevant receptor for 16:4(n-3). Studies that used splenocytes from GPR120-deficient mice have confirmed this conclusion. Activation of the 16:4(n-3)-GPR120 axis led to enhanced cPLA₂ activity in these splenic macrophages and secretion of the resistance-inducing lipid mediator, lysophosphatidylcholine(24:1). These studies identify a novel and unexpected function for GPR120 and suggest that antagonists of this receptor might be effective agents to limit development of chemotherapy resistance.—Houthuijzen, J. M., Oosterom, I., Hudson, B. D., Hirasawa, A., Daenen, L. G. M., McLean, C. M., Hansen, S. V. F., van Jaarsveld, M. T. M., Peeper, D. S., Jafari Sadatmand, S., Roodhart, J. M. L., van de Lest, C. H. A., Ulven, T., Ishihara, K., Milligan, G., Voest, E. E. Fatty acid 16:4(n-3) stimulates a GPR120-induced signaling cascade in splenic macrophages to promote chemotherapy resistance.

Discovery of Chromane Propionic Acid Analogues as Selective Agonists of GPR120 with in Vivo Activity in Rodents

GPR120 (FFAR4) is a fatty acid sensing G protein coupled receptor (GPCR) that has been identified as a target for possible treatment of type 2 diabetes. A selective activator of GPR120 containing a chromane scaffold has been designed, synthesized, and evaluated in vivo. Results of these efforts suggest that chromane propionic acid 18 is a suitable tool molecule for further animal studies. Compound 18 is selective over the closely related target GPR40 (FFAR1), has a clean off-target profile, demonstrates suitable pharmacokinetic properties, and has been evaluated in wild-type/knockout GPR120 mouse oGTT studies.

Design, Synthesis, and Evaluation of Novel and Selective G-protein Coupled Receptor 120 (GPR120) Spirocyclic Agonists

Type 2 diabetes mellitus (T2DM) is an ever increasing worldwide epidemic, and the identification of safe and effective insulin sensitizers, absent of weight gain, has been a long-standing goal of diabetes research. G-protein coupled receptor 120 (GPR120) has recently emerged as a potential therapeutic target for treating T2DM. Natural occurring, and more recently, synthetic agonists have been associated with insulin sensitizing, anti-inflammatory, and fat metabolism effects. Herein we describe the design, synthesis, and evaluation of a novel spirocyclic GPR120 agonist series, which culminated in the discovery of potent and selective agonist 14. Furthermore, compound 14 was evaluated in vivo and demonstrated acute glucose lowering in an oral glucose tolerance test (oGTT), as well as improvements in homeostatic measurement assessment of insulin resistance (HOMA-IR; a surrogate marker for insulin sensitization) and an increase in glucose infusion rate (GIR) during a hyperinsulinemic euglycemic clamp in diet-induced obese (DIO) mice.

Discovery of benzofuran propanoic acid GPR120 agonists: From uHTS hit to mechanism-based pharmacodynamic effects

The transformation of an aryloxybutanoic acid ultra high-throughput screening (uHTS) hit into a potent and selective series of G-protein coupled receptor 120 (GPR120) agonists is reported. uHTS hit 1 demonstrated an excellent rodent pharmacokinetic profile and selectivity over the related fatty acid receptor GPR40, but only modest GPR120 potency. Optimization of the "left-hand" aryl group led to compound 6, which demonstrated a GPR120 mechanism-based pharmacodynamic effect in a mouse oral glucose tolerance test (oGTT). Further optimization gave rise to the benzofuran propanoic acid series (exemplified by compound 37), which demonstrated acute mechanism-based pharmacodynamic effects. The combination of in vivo efficacy and attractive rodent pharmacodynamic profiles suggests compounds generated from this series may afford attractive candidates for the treatment of Type 2 diabetes.

Central Agonism of GPR120 Acutely Inhibits Food Intake and Food Reward and Chronically Suppresses Anxiety-Like Behavior in Mice

BACKGROUND

GPR120 (FFAR4) is a G-protein coupled receptor implicated in the development of obesity and the antiinflammatory and insulin-sensitizing effects of omega-3 (ω-3) polyunsaturated fatty acids. Increasing central ω-3 polyunsaturated fatty acid levels has been shown to have both anorectic and anxiolytic actions. Despite the strong clinical interest in GPR120, its role in the brain is largely unknown, and thus we sought to determine the impact of central GPR120 pharmacological activation on energy balance, food reward, and anxiety-like behavior.

METHODS

Male C57Bl/6 mice with intracerebroventricular cannulae received a single injection (0.1 or 1 µM) or continuous 2-week infusion (1 µM/d; mini-pump) of a GPR120 agonist or vehicle. Free-feeding intake, operant lever-pressing for palatable food, energy expenditure (indirect calorimetry), and body weight were measured. GPR120 mRNA expression was measured in pertinent brain areas. Anxiety-like behavior was assessed in the elevated-plus maze and open field test.

RESULTS

GPR120 agonist injections substantially reduced chow intake during 4 hours postinjection, suppressed the rewarding effects of high-fat/-sugar food, and blunted approach-avoidance behavior in the open field. Conversely, prolonged central GPR120 agonist infusions reduced anxiety-like behavior in the elevated-plus maze and open field, yet failed to affect free-feeding intake, energy expenditure, and body weight on a high-fat diet.

CONCLUSION

Acute reductions in food intake and food reward suggest that GPR120 could mediate the effects of central ω-3 polyunsaturated fatty acids to inhibit appetite. The anxiolytic effect elicited by GPR120 agonist infusions favors the testing of compounds that can enter the brain to activate GPR120 for the mitigation of anxiety.

Functional and genetic epidemiological characterisation of the FFAR4 (GPR120) p.R270H variant in the Danish population

BACKGROUND

p.R270H (rs116454156) in the long chain fatty acid 7TM receptor FFAR4 (GPR120) which results in impaired Gαq (Gq) coupled signalling, has been associated with obesity. We aimed to extend the functional in vitro analyses of p.R270H and to investigate the association with obesity and glucose-related traits in the Danish population.

METHODS

Surface expression, Gq and Gi coupled signalling as well as β-arrestin recruitment were examined in vitro. p.R270H was genotyped using the exome chip array in 11 479 Danish adult individuals. Of these 4391 were obese and 4415 were normal weight. Association with quantitative metabolic traits comprised 8720 non-diabetic individuals.

RESULTS

p.R270H showed reduced surface expression of FFAR4. Ligand-independent activity was eliminated and strongly impaired through the Gq and Gi signalling pathways, respectively. The ligand-induced maximal signalling efficacy of p.R270H was reduced only through the Gq pathway. The p.R270H variant did not affect β-arrestin recruitment. p.R270H was not associated with increased risk of obesity nor increased fasting plasma glucose levels in the Danish study populations. Nor was it associated with these two traits in the European Network for Genetic and Genomic Epidemiology consortium data (N=34 901-71 175; p>0.70). It was also not associated with waist-hip ratio, glucose metabolism during an oral glucose tolerance test, lipid levels or with markers of adiposity (leptin, adiponectin), inflammation (high-sensitive C reactive protein; hs-CRP) and liver function (alanine aminotransferase) in the Danish population (p>0.05).

CONCLUSIONS

We demonstrate that p.R270H of FFAR4 impairs Gq and Gi signalling of FFAR4 in vitro; however, this impaired signalling for p.R270H does not translate into associations with human metabolic phenotypes in the investigated populations.

A novel anti-inflammatory role of GPR120 in intestinal epithelial cells

GPR120 (free fatty acid receptor-4) is a G protein-coupled receptor for medium- and long-chain unsaturated fatty acids, including ω-3 fatty acids. Recent studies have shown GPR120 to play cardinal roles in metabolic disorders via modulation of gut hormone secretion and insulin sensitivity and to exert anti-inflammatory effects in macrophages and adipose tissues. However, information on anti-inflammatory role of GPR120 at the level of intestinal epithelium is very limited. Current studies demonstrated differential levels of GPR120 mRNA and protein along the length of the human, mouse, and rat intestine and delineated distinct anti-inflammatory responses following GPR120 activation in model human intestinal epithelial Caco-2 cells, but not in model mouse intestinal epithelial endocrine cell line STC-1. In Caco-2 cells, GPR120 was internalized, bound to β-arrestin-2, and attenuated NF-κB activation in response to 30-min exposure to the agonists GW9508, TUG-891, or docosahexaenoic acid. These effects were abrogated in response to small interfering RNA silencing of β-arrestin-2. Treatment of STC-1 cells with these agonists did not induce receptor internalization and had no effects on NF-κB activation, although treatment with the agonists GW9508 or TUG-891 for 6 h augmented the synthesis and secretion of the gut hormone glucagon-like peptide-1 in this cell line. Our studies for the first time demonstrated a GPR120-mediated novel anti-inflammatory pathway in specific intestinal epithelial cell types that could be of therapeutic relevance to intestinal inflammatory disorders.

Novel identification of the free fatty acid receptor FFAR1 that promotes contraction in airway smooth muscle

Obesity is one of the major risk factors for asthma. Previous studies have demonstrated that free fatty acid levels are elevated in the plasma of obese individuals. Medium- and long-chain free fatty acids act as endogenous ligands for the free fatty acid receptors FFAR1/GPR40 and FFAR4/GPR120, which couple to Gq proteins. We investigated whether FFAR1 and FFAR4 are expressed on airway smooth muscle and whether they activate Gq-coupled signaling and modulate airway smooth muscle tone. We detected the protein expression of FFAR1 and FFAR4 in freshly dissected native human and guinea pig airway smooth muscle and cultured human airway smooth muscle (HASM) cells by immunoblotting and immunohistochemistry. The long-chain free fatty acids (oleic acid and linoleic acid) and GW9508 (FFAR1/FFAR4 dual agonist) dose-dependently stimulated transient intracellular Ca(2+) concentration ([Ca(2+)]i) increases and inositol phosphate synthesis in HASM cells. Downregulation of FFAR1 or FFAR4 in HASM cells by small interfering RNA led to a significant inhibition of the long-chain free fatty acids-induced transient [Ca(2+)]i increases. Oleic acid, linoleic acid, or GW9508 stimulated stress fiber formation in HASM cells, potentiated acetylcholine-contracted guinea pig tracheal rings, and attenuated the relaxant effect of isoproterenol after an acetylcholine-induced contraction. In contrast, TUG-891 (FFAR4 agonist) did not induce the stress fiber formation or potentiate acetylcholine-induced contraction. These results suggest that FFAR1 is the functionally dominant free fatty acid receptor in both human and guinea pig airway smooth muscle. The free fatty acid sensors expressed on airway smooth muscle could be an important modulator of airway smooth muscle tone.

Activity of dietary fatty acids on FFA1 and FFA4 and characterisation of pinolenic acid as a dual FFA1/FFA4 agonist with potential effect against metabolic diseases

Various foods are associated with effects against metabolic diseases such as insulin resistance and type 2 diabetes; however, their mechanisms of action are mostly unclear. Fatty acids may contribute by acting as precursors of signalling molecules or by direct activity on receptors. The medium- and long-chain NEFA receptor FFA1 (free fatty acid receptor 1, previously known as GPR40) has been linked to enhancement of glucose-stimulated insulin secretion, whereas FFA4 (free fatty acid receptor 4, previously known as GPR120) has been associated with insulin-sensitising and anti-inflammatory effects, and both receptors are reported to protect pancreatic islets and promote secretion of appetite and glucose-regulating hormones. Hypothesising that FFA1 and FFA4 mediate therapeutic effects of dietary components, we screened a broad selection of NEFA on FFA1 and FFA4 and characterised active compounds in concentration-response curves. Of the screened compounds, pinolenic acid, a constituent of pine nut oil, was identified as a relatively potent and efficacious dual FFA1/FFA4 agonist, and its suitability for further studies was confirmed by additional in vitro characterisation. Pine nut oil and free and esterified pure pinolenic acid were tested in an acute glucose tolerance test in mice. Pine nut oil showed a moderately but significantly improved glucose tolerance compared with maize oil. Pure pinolenic acid or ethyl ester gave robust and highly significant improvements of glucose tolerance. In conclusion, the present results indicate that pinolenic acid is a comparatively potent and efficacious dual FFA1/FFA4 agonist that exerts antidiabetic effects in an acute mouse model. The compound thus deserves attention as a potential active dietary ingredient to prevent or counteract metabolic diseases.

Free fatty acid receptors as therapeutic targets for the treatment of diabetes

Nutrition regulates energy balance; however, dysfunction of energy balance can cause metabolic disorders, such as obesity and diabetes. Fatty acids are an essential energy source and signaling molecules that regulate various cellular processes and physiological functions. Recently, several orphan G protein-coupled receptors were identified as free fatty acid receptors (FFARs). GPR40/FFAR1 and GPR120/FFAR4 are activated by medium- and/or long-chain fatty acids, whereas GPR41/FFAR3 and GPR43/FFAR2 are activated by short-chain fatty acids. FFARs are regarded as targets for novel drugs to treat metabolic disorders, such as obesity and type 2 diabetes, because recent studies have showed that these receptors are involved in the energy metabolism in various tissues, including adipose, intestinal, and immune tissue. In this review, we summarize physiological roles of the FFARs, provide a comprehensive overview of energy regulation by FFARs, and discuss new prospects for treatment of metabolic disorders.

Lipid droplet formation in response to oleic acid in Huh-7 cells is mediated by the fatty acid receptor FFAR4

It is unclear how changes in lipid droplet size and number are regulated - for example, it is not known whether this involves a signalling pathway or is directed by cellular lipid uptake. Here, we show that oleic acid stimulates lipid droplet formation by activating the long-chain fatty acid receptor FFAR4, which signals through a pertussis-toxin-sensitive G-protein signalling pathway involving phosphoinositide 3-kinase (PI3-kinase), AKT (also known as protein kinase B) and phospholipase D (PLD) activities. This initial lipid droplet formation is not dependent upon exogenous lipid, whereas the subsequent more sustained increase in the number of lipid droplets is dependent upon lipid uptake. These two mechanisms of lipid droplet formation point to distinct potential intervention points.

Expression and localization of the omega-3 fatty acid receptor GPR120 in human term placenta

Fatty acids can function as signaling molecules, acting through receptors in the cytosol or on the cell surface. G-Protein Receptor (GPR)120 is a membrane-bound receptor mediating anti-inflammatory and insulin-sensitizing effects of the omega-3 fatty acid docohexaenoic acid (DHA). GPR120 dysfunction is associated with obesity in humans. Cellular localization of GPR120 and the influence of maternal obesity on GPR120 protein expression in the placenta are unknown. Herein we demonstrate that GPR120 is predominantly expressed in the microvillous membrane (MVM) of human placenta and that the expression level of this receptor in MVM is not altered by maternal body mass index (BMI).

Omega-3 Fatty Acids and FFAR4

The beneficial roles of omega-3 fatty acids (ω3-FAs) on obesity, type 2 diabetes, and other metabolic diseases are well known. Most of these effects can be explained by their anti-inflammatory effects triggered through their receptor, free fatty acid receptor 4 (FFAR4) activation. Although the whole mechanism of action is not fully described yet, it has been shown that stimulation of ω3-FA to FFAR4 is followed by receptor phosphorylation. This makes FFAR4 to be capable of interacting with β-arrestin-2, which in turn, results in association of β-arrestin-2 with TAB1. This stealing of an important partaker of the inflammatory cascade leads to interruption of the pathway, resulting in reduced inflammation. Besides this regulation of the anti-inflammatory response, FFAR4 signaling also has been shown to regulate glucose homeostasis, adiposity, gastrointestinal peptide secretion, and taste preference. In this review, we summarize the current knowledge about the interaction of ω3-FAs with FFAR4 and the consequent opportunities for the application of ω3-FAs and possible FFAR4 targets.

Regulation of Energy Homeostasis via GPR120

Free fatty acids (FFAs) are fundamental units of key nutrients. FFAs exert various biological functions, depending on the chain length and degree of desaturation. Recent studies have shown that several FFAs act as ligands of G-protein-coupled receptors (GPCRs), activate intracellular signaling and exert physiological functions via these GPCRs. GPR120 (also known as free fatty acid receptor 4) is activated by unsaturated medium- to long-chain FFAs and has a critical role in various physiological homeostasis mechanisms such as incretin hormone secretion, food preference, anti-inflammation, and adipogenesis. Recent studies showed that a lipid sensor GPR120 has a key role in sensing dietary fat in white adipose tissue and regulates the whole body energy homeostasis in both humans and rodents. Genetic study in human identified the loss-of-functional mutation of GPR120 associated with obesity and insulin resistance. In addition, dysfunction of GPR120 has been linked as a novel risk factor for diet-induced obesity. This review aims to provide evidence from the recent development in physiological function of GPR120 and discusses its functional roles in the regulation of energy homeostasis and its potential as drug targets.