Dietary Fatty Acids and Their Potential for Controlling Metabolic Diseases Through Activation of FFA4/GPR120

Protective Effects of Omega-3 Fatty Acids in Cancer-Related Complications

Omega-3 polyunsaturated fatty acids (PUFAs) are considered immunonutrients and are commonly used in the nutritional therapy of cancer patients due to their ample biological effects. Omega-3 PUFAs play essential roles in cell signaling and in the cell structure and fluidity of membranes. They participate in the resolution of inflammation and have anti-inflammatory and antinociceptive effects. Additionally, they can act as agonists of G protein-coupled receptors, namely, GPR40/FFA1 and GPR120/FFA4. Cancer patients undergo complications, such as anorexia-cachexia syndrome, pain, depression, and paraneoplastic syndromes. Interestingly, the 2017 European Society for Clinical Nutrition and Metabolism (ESPEN) guidelines for cancer patients only discuss the use of omega-3 PUFAs for cancer-cachexia treatment, leaving aside other cancer-related complications that could potentially be managed by omega-3 PUFA supplementation. This critical review aimed to discuss the effects and the possible underlying mechanisms of omega-3 PUFA supplementation in cancer-related complications. Data compilation in this critical review indicates that further investigation is still required to assess the factual benefits of omega-3 PUFA supplementation in cancer-associated illnesses. Nevertheless, preclinical evidence reveals that omega-3 PUFAs and their metabolites might modulate pivotal pathways underlying complications secondary to cancer, indicating that this is a promising field of knowledge to be explored.

Fatty acid receptor GPR120 promotes breast cancer chemoresistance by upregulating ABC transporters expression and fatty acid synthesis

BACKGROUND

Chemoresistance is the major cause of neoadjuvant treatment failure in breast cancer patients. Despite recent progress, the mechanism underlying chemoresistance remains to be further defined.

METHODS

Expression of G protein-coupled receptor 120 (GPR120) was analyzed by immunohistochemistry in the biopsies of primary breast cancer who subsequently underwent preoperative neoadjuvant chemotherapy. In vitro and in vivo loss- and gain-of -function studies were performed to reveal the effects and related mechanism of GPR120 signaling pathway in the chemoresistance of breast cancer cells.

FINDINGS

We identified that GPR120, a receptor for long-chain fatty acids, was important for the acquisition of chemoresistance in breast cancer cells. We showed that GPR120 expression was positively associated with clinical response to neoadjuvant chemotherapy in patients. In breast cancer cells, GPR120 enhanced the de novo synthesis of fatty acids that served as GPR120 ligands to activate GPR120 signaling via a feedback mechanism. Upregulated GPR120 signaling rendered cells resistant to epirubicin-induced cell death by upregulating ABC transporters expression and thus decreasing the intracellular accumulation of epirubicin. Akt/NF-κB pathway was responsible for the GPR120-mediated expression of ABC transporters leading to modulation of the concentration of chemotherapeutic drugs in cells. The functional importance of GPR120 in chemoresistance was further validated using epirubicin-treated tumor xenografts, in which we showed that blockade of GPR120 signaling with AH7614 or GPR120-siRNA significantly compromised chemoresistance.

INTERPRETATION

Our results highlight that GPR120 might be a promising therapeutic target for breast cancer chemoresistance. FUND: National Natural Science Foundation of China, Ministry of Science and Technology of China, Program of Science and Technology Commission of Shanghai Municipality.

Adipose tissue dysfunction is associated with low levels of the novel Palmitic Acid Hydroxystearic Acids

Adipose tissue dysfunction is considered an important contributor to systemic insulin resistance and Type 2 diabetes (T2D). Recently, a novel family of endogenous lipids, palmitic acid hydroxy stearic acids (PAHSAs), was discovered. These have anti-diabetic and anti-inflammatory effects in mice and are reduced in serum and adipose tissue of insulin resistant humans. In the present study, we investigate if adipose tissue dysfunction is associated with reduced PAHSA levels in human subjects and if PAHSAs influence adipocyte differentiation. Our results show that low expression of adipocyte GLUT4 and adipocyte hypertrophy, markers of adipose tissue dysfunction, are associated with reduced expression of key enzymes for de novo lipogenesis and adipose tissue levels of PAHSAs in human subjects. We also show that GLUT4 is not only a marker of adipose tissue dysfunction, but may be causally related to the observed impairments. PAHSAs may also act locally in the adipose tissue to improve adipogenesis through a mechanism bypassing direct activation of peroxisome proliferator-activated receptor (PPARγ). The discovery of PAHSAs and our current results provide novel insights into positive effects of lipid species in adipose tissue and mechanisms by which dysfunctional adipose tissue is associated with insulin resistance and risk of developing T2D.

IL-33 stimulates expression of the GPR84 (EX33) fatty acid receptor gene and of cytokine and chemokine genes in human adipocytes

Expression of GPCR fatty acid sensor/receptor genes in adipocytes is modulated by inflammatory mediators, particularly IL-1β. In this study we examined whether the IL-1 gene superfamily member, IL-33, also regulates expression of the fatty acid receptor genes in adipocytes. Human fat cells, differentiated from preadipocytes, were incubated with IL-33 at three different dose levels for 3 or 24 h and mRNA measured by qPCR. Treatment with IL-33 induced a dose-dependent increase in GPR84 mRNA at 3 h, the level with the highest dose being 13.7-fold greater than in controls. Stimulation of GPR84 expression was transitory; the mRNA level was not elevated at 24 h. In contrast to GPR84, IL-33 had no effect on GPR120 expression. IL-33 markedly stimulated expression of the IL1B, CCL2, IL6, CXCL2 and CSF3 genes, but there was no effect on ADIPOQ expression. The largest effect was on CSF3, the mRNA level of which increased 183-fold over controls at 3 h with the highest dose of IL-33; there was a parallel increase in the secretion of G-CSF protein into the medium. It is concluded that in human adipocytes IL-33, which is synthesised in adipose tissue, has a strong stimulatory effect on the expression of cytokine and chemokine genes, particularly CSF3, and on the expression of GPR84, a pro-inflammatory fatty acid receptor.

IL-1β and TNFα inhibit GPR120 (FFAR4) and stimulate GPR84 (EX33) and GPR41 (FFAR3) fatty acid receptor expression in human adipocytes: implications for the anti-inflammatory action of n-3 fatty acids

Regulation of the expression of GPCR fatty acid receptor genes has been examined in human adipocytes differentiated in culture. TNFα and IL-1β induced a marked reduction in GPR120 expression, mRNA level falling 17-fold at 24 h in adipocytes incubated with TNFα. In contrast, GPR84 mRNA was dramatically increased by these cytokines (>500-fold for IL-1β at 4 h); GPR41 expression was also stimulated. Rosiglitazone did not affect GPR84 expression, but GPR120 and GPR41 expression increased. Dexamethasone, insulin, linoleic and docosahexaenoic acids (DHA), and TUG891 (GPR120 agonist) had little effect on GPR120 and GPR84 expression. TUG891 did not attenuate the pro-inflammatory actions of TNFα and IL-1β. DHA slightly countered the actions of IL-1β on CCL2, IL6 and ADIPOQ expression, though not on secretion of these adipokines. GPR120 and GP84 gene expression in human adipocytes is highly sensitive to pro-inflammatory mediators; the inflammation-induced inhibition of GPR120 expression may compromise the anti-inflammatory action of GPR120 agonists.

Diabetes Mellitus and Obesity as Risk Factors for Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is among the deadliest types of cancer. The worldwide estimates of its incidence and mortality in the general population are eight cases per 100,000 person-years and seven deaths per 100,000 person-years, and they are significantly higher in the United States than in the rest of the world. The incidence of this disease in the United States is more than 50,000 new cases in 2017. Indeed, total deaths due to PDAC are projected to increase dramatically to become the second leading cause of cancer-related deaths before 2030. Considering the failure to date to efficiently treat existing PDAC, increased effort should be undertaken to prevent this disease. A better understanding of the risk factors leading to PDAC development is of utmost importance to identify and formulate preventive strategies. Large epidemiologic and cohort studies have identified risk factors for the development of PDAC, including obesity and type 2 diabetes mellitus. This review highlights the current knowledge of obesity and type 2 diabetes as risk factors for PDAC development and progression, their interplay and underlying mechanisms, and the relation to diet. Research gaps and opportunities to address this deadly disease are also outlined.

Fish oil as a potential activator of brown and beige fat thermogenesis

Numerous studies have shown that feeding rodents n-3 polyunsaturated fatty acids attenuates adiposity. Moreover, meta-analyses of human dietary intervention studies indicate that fish oil (eicosapentaenoic and docosahexaenoic acid) supplementation might reduce waist circumference. A recent line of research suggests that browning of white adipose depots and activation of uncoupled respiration in brown fat contributes to these effects. This mini-review summarizes the observations in rodents, highlights several mechanisms that might explain these observations and discusses the translational potential. Given the available in vivo evidence and the ability of human adipocytes to aquire a beige phenotype in response to eicosapentaenoic acid incubation, future studies should test the hypothesis that fish oil activates thermogenic brown and beige adipose tissue in humans.

Palmitic and Oleic Acid: The Yin and Yang of Fatty Acids in Type 2 Diabetes Mellitus

Increased plasma non-esterified fatty acids (NEFAs) link obesity with insulin resistance and type 2 diabetes mellitus (T2DM). However, in contrast to the saturated FA (SFA) palmitic acid, the monounsaturated FA (MUFA) oleic acid elicits beneficial effects on insulin sensitivity, and the dietary palmitic acid:oleic acid ratio impacts diabetes risk in humans. Here we review recent mechanistic insights into the beneficial effects of oleic acid compared with palmitic acid on insulin resistance and T2DM, including its anti-inflammatory actions, and its capacity to inhibit endoplasmic reticulum (ER) stress, prevent attenuation of the insulin signaling pathway, and improve β cell survival. Understanding the molecular mechanisms of the antidiabetic effects of oleic acid may contribute to understanding the benefits of this FA in the prevention or delay of T2DM.

The role of free-fatty acid receptor-4 (FFA4) in human cancers and cancer cell lines

A dietary influence on cancer progression has been evident for many decades, and dietary fatty acids, particularly long chain mono- and polyunsaturated fatty acids, have been shown to play significant roles in influencing growth of a variety of human cancers. The discovery of the family of cell-surface free-fatty acid receptors, which include the long-chain fatty acid receptors FFA1 and FFA4, suggest that many of the effects of dietary fats could be receptor-mediated. FFA4 is ubiquitously expressed and has recently been shown to modulate a variety of important anti-inflammatory and metabolic processes. Since FFA4 is currently an attractive drug target for treatment of metabolic disorders such as diabetes and obesity, understanding its role in cancer progression is critical towards the drug discovery process. In this research update, the current body of knowledge on the role of this receptor in regulating cancer cell proliferation, migration, and invasion, as well as in vivo tumorigenesis is reviewed.

20-HETE promotes glucose-stimulated insulin secretion in an autocrine manner through FFAR1

The long-chain fatty acid receptor FFAR1 is highly expressed in pancreatic β-cells. Synthetic FFAR1 agonists can be used as antidiabetic drugs to promote glucose-stimulated insulin secretion (GSIS). However, the physiological role of FFAR1 in β-cells remains poorly understood. Here we show that 20-HETE activates FFAR1 and promotes GSIS via FFAR1 with higher potency and efficacy than dietary fatty acids such as palmitic, linoleic, and α-linolenic acid. Murine and human β-cells produce 20-HETE, and the ω-hydroxylase-mediated formation and release of 20-HETE is strongly stimulated by glucose. Pharmacological inhibition of 20-HETE formation and blockade of FFAR1 in islets inhibits GSIS. In islets from type-2 diabetic humans and mice, glucose-stimulated 20-HETE formation and 20-HETE-dependent stimulation of GSIS are strongly reduced. We show that 20-HETE is an FFAR1 agonist, which functions as an autocrine positive feed-forward regulator of GSIS, and that a reduced glucose-induced 20-HETE formation contributes to inefficient GSIS in type-2 diabetes.

FFAR1 receptor is highly expressed in beta cells and its activation has been suggested as therapy against type-2 diabetes. Here, Tunaru et al. show that 20-hydroxyeicosatetraenoic acid, produced within the islets upon glucose stimulation, acts in an autocrine manner to stimulate insulin secretion via FFAR1 activation.

Omega-3 PUFA ameliorates hyperhomocysteinemia-induced hepatic steatosis in mice by inhibiting hepatic ceramide synthesis

Hyperhomocysteinemia (HHcy) is a key risk factor in hepatic steatosis. In this study, we applied a metabolomic approach to investigate the changes in the metabolite profile due to HHcy-induced hepatic steatosis and the effects of omega-3 PUFA (polyunsaturated fatty acid) supplementation in mice. HHcy was induced in mice by giving DL-Hcy (1.8 g/L) in drinking water for 6 weeks, then the mice were sacrificed, and the metabolic profiles of the liver and plasma were analyzed through UPLC-ESI-QTOFMS-based lipidomics. Hepatic triglycerides and cholesterol were further assayed. The expression of ceramide metabolism-related genes was measured by quantitative PCR. Compared with control mice, HHcy mice exhibited hepatic steatosis with a notable increase in ceramide-related metabolites and subsequent upregulation of ceramide synthesis genes such as Sptlc3, Degs2, Cer4 and Smpd4. Omega-3 PUFA was simultaneously administered in HHcy mice through chow diet containing 3.3% omega-3 PUFA supplement for 6 weeks, which significantly ameliorated Hcy-induced hepatic steatosis. The decrease in hepatic lipid accumulation was mainly due to reduced hepatic levels of ceramides, which was partly the result of the lower expression of ceramide synthesis genes, Sptlc3 and Degs2. Similar beneficial effects of DHA were observed in Hcy-stimulated primary hepatocytes in vitro. In summary, Hcy-induced ceramide elevation in hepatocytes might contribute to the development of hepatic steatosis. Furthermore, downregulation of ceramide levels through omega-3 PUFA supplementation ameliorates hepatic lipid accumulation. Thus, ceramide is a potential therapeutic target for the treatment of hepatic steatosis.

Perinatal deficiency in dietary omega-3 fatty acids potentiates sucrose reward and diet-induced obesity in mice

Insufficient dietary intake of essential omega-3 polyunsaturated fatty acids (N-3), especially during critical stages of development, is well-associated with negative neurological and metabolic consequences. The increased availability and intake of foods rich in saturated fat coincides with reduced N-3 consumption, yet how N-3 dietary deficiency during perinatal development modulates motivation for palatable food and interacts with a high-fat diet to affect body weight and emotional states is not clear. Pregnant C57Bl6 mice and pups were subjected to diets either deficient or adequate (control) in N-3 until postnatal day 21. Adult male N-3 deficient or control offspring were tested in a progressive ratio operant task for sucrose motivated behavior or given prolonged access to a saturated high-fat diet or chow followed by measures of energy balance and anxiety-like behavior in the elevated-plus maze and open field test. Brain fatty acid profiles were measured via gas chromatography mass spectrometry. Perinatal dietary N-3 deficiency lowered brain N-3 levels, augmented the rewarding effects of sucrose, heightened diet-induced weight gain and fat mass accumulation and diminished spontaneous physical activity. Finally, perinatal N-3 deficiency increased anxiety-like behaviour independent of diet in the open field but not in the elevated-plus maze test. Insufficient dietary N-3 during critical periods of developmental can amplify the obesogenic effects of saturated fat intake, enhance motivated behaviour for palatable foods and may elicit negative emotional states that can perpetuate overeating and obesity.

Tannerella forsythia and coating color on the tongue dorsum, and fatty food liking associate with fat accumulation and insulin resistance in adult catch-up fat

BACKGROUND/OBJECTIVES

We aimed to determine the alteration of Tannerella forsythia and coating color on the dorsal tongue, and fatty food liking in catch-up fat in adult (CUFA), as well as the probable associations between fat accumulation, insulin resistance (IR) and these changes.

SUBJECTS/METHODS

T. forsythia on the tongue dorsum, fatty food liking, fat accumulation and insulin sensitivity were investigated in CUFA humans and rats, and tongue-coating color was observed in CUFA individuals. We further determined the changes of fatty food liking, fat accumulation and IR in T. forsythia-infected rodents by oral lavage.

RESULTS

Increases in fat accumulation, IR, percentage of subjects with yellow tongue coating and that with T. forsythia detected were observed in CUFA individuals. Additionally, the fat ranking scores were significantly lower and the hedonic ratings of low-fat options of sampled food were lower, while the ratings of high-fat options were remarkably higher in CUFA subjects. Additionally, T. forsythia level elevated in CUFA rats, and fatty food liking, fat accumulation and IR increased in CUFA and T. forsythia-infected animals, with the increases in T. forsythia infection and fatty food liking preceding the occurrence of fat accumulation and IR.

CONCLUSIONS

T. forsythia and yellow coating on the dorsal tongue and fatty food liking associate fat accumulation and IR in CUFA. Moreover, we tentatively put forward that T. forsythia, which is very important in yellow tongue-coating microbiota, and its consequent increases in fatty food liking, might be crucial in the development of fat accumulation and IR in CUFA.

GPR120: a critical role in adipogenesis, inflammation, and energy metabolism in adipose tissue

It is well known that adipose tissue has a critical role in the development of obesity and metabolic diseases and that adipose tissue acts as an endocrine organ to regulate lipid and glucose metabolism. Accumulating in the adipose tissue, fatty acids serve as a primary source of essential nutrients and act on intracellular and cell surface receptors to regulate biological events. G protein-coupled receptor 120 (GPR120) represents a promising target for the treatment of obesity-related metabolic disorders for its involvement in the regulation of adipogenesis, inflammation, glucose uptake, and insulin resistance. In this review, we summarize recent studies and advances regarding the systemic role of GPR120 in adipose tissue, including both white and brown adipocytes. We offer a new perspective by comparing the different roles in a variety of homeostatic processes from adipogenic development to adipocyte metabolism, and we also discuss the effects of natural and synthetic agonists that may be potential agents for the treatment of metabolic diseases.

FFA4/GPR120: Pharmacology and Therapeutic Opportunities

Free Fatty Acid receptor 4 (FFA4), also known as GPR120, is a G-protein-coupled receptor (GPCR) responsive to long-chain fatty acids that is attracting considerable attention as a potential novel therapeutic target for the treatment of type 2 diabetes mellitus (T2DM). Although no clinical studies have yet been initiated to assess efficacy in this indication, a significant number of primary publications and patents have highlighted the ability of agonists with potency at FFA4 to improve glucose disposition and enhance insulin sensitivity in animal models. However, the distribution pattern of the receptor suggests that targeting FFA4 may also be useful in other conditions, ranging from cancer to lung function. Here, we discuss and contextualise the basis for these ideas and the results to support these conclusions.

Substantial focus on the therapeutic potential of FFA4/GPR120 is currently directed towards type 2 diabetes.

Progress in the identification and characterisation of FFA4/GPR120 agonist ligands is apparent in both the primary scientific and patent literatures.

In models of glucose handling, FFA4/GPR120 agonists appear highly effective.

Recent indications provide support for consideration of FFA4/GPR120 ligands in areas of cancer treatment.

High levels of expression of FFA4/GPR120 in the lung suggest utility in analysis of the potential therapeutic roles of FFA4/GPR120 ligands in both acute and chronic airway inflammatory conditions.

Stearic acid suppresses mammary gland development by inhibiting PI3K/Akt signaling pathway through GPR120 in pubertal mice

It has been demonstrated that dietary high fat diet negatively affects the pubertal mammary gland development. The aim of the present study was to investigate the effects of stearic acid (SA), an 18-carbon chain saturated fatty acid, on mammary gland development in pubertal mice and to explore the underlying mechanism. Our results demonstrated that dietary supplementation of 2% SA suppressed mammary duct development, with significant reduction of terminal end bud (TEB) number and ductal branch. In accord, the expression of proliferative marker Cyclin D1 was markedly decreased by dietary SA. Furthermore, dietary SA led to increase of G protein-coupled receptor 120 (GPR120) expression and inhibition of PI3K/Akt signaling pathway in mammary gland of pubertal mice. In good agreement with the in vivo findings, the in vitro results showed that 40 μM SA significantly suppressed proliferation of mouse mammary epithelial cell HC11 by regulating mRNA and/or protein expression of proliferative markers such as Cyclin D1/3, p21, and PCNA. Meanwhile, SA activated GPR120 and inhibited PI3K/Akt signaling pathway in a GPR120-dependent manner. In addition, SA-induced inhibition of PI3K/Akt signaling pathway, suppression of HC11 proliferation, and alteration of proliferative markers expression were abolished by knockdown of GPR120 with siRNA. Collectively, these findings showed that SA suppressed mammary gland development of pubertal mice, which was coincident with the SA-inhibited HC11 proliferation, and was associated with inhibition of PI3K/Akt signaling pathway through activation of GPR120. These data provided new insights into the regulation of mammary gland development by dietary fatty acids.

Nrg4 promotes fuel oxidation and a healthy adipokine profile to ameliorate diet-induced metabolic disorders

Objective

Brown and white adipose tissue exerts pleiotropic effects on systemic energy metabolism in part by releasing endocrine factors. Neuregulin 4 (Nrg4) was recently identified as a brown fat-enriched secreted factor that ameliorates diet-induced metabolic disorders, including insulin resistance and hepatic steatosis. However, the physiological mechanisms through which Nrg4 regulates energy balance and glucose and lipid metabolism remain incompletely understood. The aims of the current study were: i) to investigate the regulation of adipose Nrg4 expression during obesity and the physiological signals involved, ii) to elucidate the mechanisms underlying Nrg4 regulation of energy balance and glucose and lipid metabolism, and iii) to explore whether Nrg4 regulates adipose tissue secretome gene expression and adipokine secretion.

Methods

We examined the correlation of adipose Nrg4 expression with obesity in a cohort of diet-induced obese mice and investigated the upstream signals that regulate Nrg4 expression. We performed metabolic cage and hyperinsulinemic-euglycemic clamp studies in Nrg4 transgenic mice to dissect the metabolic pathways regulated by Nrg4. We investigated how Nrg4 regulates hepatic lipid metabolism in the fasting state and explored the effects of Nrg4 on adipose tissue gene expression, particularly those encoding secreted factors.

Results

Adipose Nrg4 expression is inversely correlated with adiposity and regulated by pro-inflammatory and anti-inflammatory signaling. Transgenic expression of Nrg4 increases energy expenditure and augments whole body glucose metabolism. Nrg4 protects mice from diet-induced hepatic steatosis in part through activation of hepatic fatty acid oxidation and ketogenesis. Finally, Nrg4 promotes a healthy adipokine profile during obesity.

Conclusions

Nrg4 exerts pleiotropic beneficial effects on energy balance and glucose and lipid metabolism to ameliorate obesity-associated metabolic disorders. Biologic therapeutics based on Nrg4 may improve both type 2 diabetes and non-alcoholic fatty liver disease (NAFLD) in patients.

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Nrg4 is a target of pro-inflammatory and anti-inflammatory signaling in adipocytes.

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Transgenic expression of Nrg4 increased energy expenditure and glucose metabolism.

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Nrg4 stimulates hepatic fatty acid oxidation and ketogenesis during starvation.

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Nrg4 promotes a beneficial adipokine profile during obesity.

Succinct synthesis of saturated hydroxy fatty acids and in vitro evaluation of all hydroxylauric acids on FFA1, FFA4 and GPR84

Saturated hydroxy fatty acids make up a class of underexplored lipids with potentially interesting biological activities. We report a succinct and general synthetic route to saturated hydroxy fatty acids hydroxylated at position 6 or higher, and exemplify this with the synthesis of hydroxylauric acids. All regioisomers of hydroxylauric acids were tested on free fatty acid receptors FFA1, FFA4 and GPR84. The results show that the introduction of a hydroxy group and its position have a high impact on receptor activity.

Probe-Dependent Negative Allosteric Modulators of the Long-Chain Free Fatty Acid Receptor FFA4

High-affinity and selective antagonists that are able to block the actions of both endogenous and synthetic agonists of G protein–coupled receptors are integral to analysis of receptor function and to support suggestions of therapeutic potential. Although there is great interest in the potential of free fatty acid receptor 4 (FFA4) as a novel therapeutic target for the treatment of type II diabetes, the broad distribution pattern of this receptor suggests it may play a range of roles beyond glucose homeostasis in different cells and tissues. To date, a single molecule, 4-methyl-N-9H-xanthen-9-yl-benzenesulfonamide (AH-7614), has been described as an FFA4 antagonist; however, its mechanism of antagonism remains unknown. We synthesized AH-7614 and a chemical derivative and demonstrated these to be negative allosteric modulators (NAMs) of FFA4. Although these NAMs did inhibit FFA4 signaling induced by a range of endogenous and synthetic agonists, clear agonist probe dependence in the nature of allosteric modulation was apparent. Although AH-7614 did not antagonize the second long-chain free fatty acid receptor, free fatty acid receptor 1, the simple chemical structure of AH-7614 containing features found in many anticancer drugs suggests that a novel close chemical analog of AH-7614 devoid of FFA4 activity, 4-methyl-N-(9H-xanthen-9-yl)benzamide (TUG-1387), will also provide a useful control compound for future studies assessing FFA4 function. Using TUG-1387 alongside AH-7614, we show that endogenous activation of FFA4 expressed by murine C3H10T1/2 mesenchymal stem cells is required for induced differentiation of these cells toward a more mature, adipocyte-like phenotype.

Fatty acid and mineral receptors as drug targets for gastrointestinal disorders

Nutrient-sensing receptors, including fatty acid receptors (FFA1–FFA4), Ca2+-sensing receptors and Zn2+-sensing receptors, are involved in several biological processes. These receptors are abundantly expressed in the GI tract, where they have been shown to play crucial roles in regulating GI function. This review provides an overview of the GI functions of fatty acid and mineral receptors, including the regulation of gastric and enteroendocrine functions, GI motility, ion transport and cell growth. Recently, several lines of evidence have implicated these receptors as promising therapeutic targets for the treatment of GI disorders, for example, inflammatory bowel disease, colorectal cancer, metabolic syndrome and diarrheal diseases. A future perspective on drug discovery research targeting these receptors is discussed.

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.

Ginsenoside Rb2 enhances the anti-inflammatory effect of ω-3 fatty acid in LPS-stimulated RAW264.7 macrophages by upregulating GPR120 expression

Recent studies confirm that chronic low-grade inflammation is closely associated with metabolic syndromes, and anti-inflammatory therapy is a potential approach for treating cardiovascular diseases and type 2 diabetes. Accumulating evidence suggests that GPR120 activation is a feasible solution to ameliorating chronic inflammation and improving glucose metabolism. In this study we investigated whether ginsenoside Rb2 (Rb2), which exhibited regulatory activities in glucose and lipid metabolism, affected GPR120 expression in lipopolysaccharide (LPS)-activated mouse macrophage RAW264.7 cells, and examined the contribution of GPR120 activation to reducing the LPS-induced inflammatory response. LPS (100 ng/mL) activated the macrophages, resulting in dramatic increases in TNF-α, IL-6, IL-1β and NO production. Treatment with a ω-3 fatty acid α-linolenic acid (ALA, 50 μmol/L) produced moderate reduction in LPS-stimulated inflammatory cytokines and NO production (TNF-α and IL-6 were decreased by 46% and 42%, respectively). Pre-incubation with Rb2 (1 or 10 μmol/L) for 12 h before ALA treatment dramatically amplified the inhibitory effects of ALA (TNF-α and IL-6 were decreased by 74% and 86%, respectively). Compared to the treatment with ALA alone, pre-incubation with Rb2 resulted in a more prominent reduction in LPS-stimulated expression of iNOS and COX-2 and LPS-stimulated IKK/NF-κB phosphorylation and MAPK pathway activation. Rb2 (0.1-100 μmol/L) dose- and time-dependently increased both mRNA and protein expression of GPR120 in RAW264.7 cells, but treatment with Rb2 alone did not exert anti-inflammatory effect in LPS-activated RAW264.7 cells. In RAW264.7 cells transfected with GPR120 shRNA, the ameliorating effects of Rb2 on LPS-induced inflammation were abolished. In conclusion, Rb2 exerts anti-inflammatory effect in LPS-stimulated mouse macrophage RAW264.7 cells in vitro by increasing GPR120 expression and subsequently enhancing ω-3 fatty acid-induced GPR120 activation.

Effects of arachidonic acid on FFA4 receptor: Signaling, phosphorylation and internalization

Arachidonic acid increased intracellular calcium, in cells expressing green fluorescent protein-tagged human FFA4 receptors, with an EC₅₀ of ~40µM. This action was not blocked by cyclooxygenase or lipoxigenase inhibitors but it was inhibited by AH7614, a FFA4 antagonist. Arachidonic acid induced ERK activation accompanied by EGF receptor transactivation. However, EGF transactivation was not the major mechanism through which the fatty acid induced ERK phosphorylation, as evidenced by the inability of AG1478 to block it. Arachidonic acid increased FFA4 receptor phosphorylation that reached its maximum within 15min with an EC₅₀ of ~30µM; inhibitors of protein kinase C partially diminish this effect and AH7614 blocked it. Arachidonic acid induced rapid and sustained Akt/PKB phosphorylation and FFA4 - β-arrestin interaction. Confocal microscopy evidenced that FFA4 receptor activation and phosphorylation were associated to internalization. In conclusion, arachidonic acid is a bona fide FFA4 receptor agonist.

Polymorphic Variation in FFA Receptors: Functions and Consequences

Overfeeding of fat can cause various metabolic disorders including obesity and type 2 diabetes (T2D). Diet provided free fatty acids (FFAs) are not only essential nutrients, but they are also recognized as signaling molecules, which stimulate various important biological functions. Recently, several G protein-coupled receptors (GPCRs), including FFA1-4, have been identified as receptors of FFAs by various physiological and pharmacological studies. FFAs exert physiological functions through these FFA receptors (FFARs) depending on carbon chain length and degree of unsaturation. Functional analyses have revealed that several important metabolic processes, such as peptide hormone secretion, cell maturation and nerve activities, are regulated by FFARs and thereby FFARs contribute to the energy homeostasis through these physiological functions. Hence, FFARs are expected to be promising pharmacological targets for metabolic disorders since imbalances in energy homeostasis lead to metabolic disorders. In human, it is established that different responses of individuals to endogenous ligands and chemical drugs may be due to differences in the ability of such ligands to activate nucleotide polymorphic variants of receptors. However, the clear links between genetic variations that are involved in metabolic disorders and polymorphisms receptors have been relatively difficult to assess. In this review, I summarize current literature describing physiological functions of FFARs and genetic variations of those receptors to discuss the potential of FFARs as drug targets for metabolic disorders.

Pharmacological Tool Compounds for the Free Fatty Acid Receptor 4 (FFA4/GPR120)

The free fatty acid receptor 4 (FFA4), also known as GPR120, is a G protein-coupled receptor that is activated by long-chain fatty acids and that has been associated with regulation of appetite, release of insulin controlling hormones, insulin sensitization, anti-inflammatory and potentially anti-obesity activity, and is progressively appearing as an attractive potential target for the treatment of metabolic dysfunctions such as obesity, type 2 diabetes and inflammatory disorders. Ongoing investigations of the pharmacological functions of FFA4 and validation of its potential as a therapeutic target depend critically on the appropriateness and quality of the available pharmacological probes or tool compounds. After a brief summary of the pharmacological functions of FFA4 and some general considerations on desirable properties for these pharmacological tool compounds, the individual compounds that have been or are currently being used as tools for probing the function of FFA4 in various in vitro and in vivo settings will be discussed and evaluated.

FFAR4 (GPR120) Signaling Is Not Required for Anti-Inflammatory and Insulin-Sensitizing Effects of Omega-3 Fatty Acids

Free fatty acid receptor-4 (FFAR4), also known as GPR120, has been reported to mediate the beneficial effects of omega-3 polyunsaturated fatty acids (ω3-PUFAs) by inducing an anti-inflammatory immune response. Thus, activation of FFAR4 has been reported to ameliorate chronic low-grade inflammation and insulin resistance accompanying obesity. However, conflicting reports on the role of FFAR4 in mediating the effects of ω3-PUFAs are emerging, suggesting that FFAR4 may not be the sole effector. Hence analyses of the importance of this receptor in relation to other signaling pathways and prominent effects of ω3-PUFAs remain to be elucidated. In the present study, we used Ffar4 knockouts (KO) and heterozygous (HET) mice fed either low fat, low sucrose reference diet; high fat, high sucrose ω3-PUFA; or high fat, high sucrose ω6-PUFA diet for 36 weeks. We demonstrate that both KO and HET mice fed ω3-PUFAs were protected against obesity, hepatic triacylglycerol accumulation, and whole-body insulin resistance. Moreover, ω3-PUFA fed mice had increased circulating protein levels of the anti-inflammatory adipokine, adiponectin, decreased fasting insulin levels, and decreased mRNA expression of several proinflammatory molecules within visceral adipose tissue. In conclusion, we find that FFAR4 signaling is not required for the reported anti-inflammatory and insulin-sensitizing effects mediated by ω3-PUFAs.

The lipid sensor GPR120 promotes brown fat activation and FGF21 release from adipocytes

The thermogenic activity of brown adipose tissue (BAT) and browning of white adipose tissue are important components of energy expenditure. Here we show that GPR120, a receptor for polyunsaturated fatty acids, promotes brown fat activation. Using RNA-seq to analyse mouse BAT transcriptome, we find that the gene encoding GPR120 is induced by thermogenic activation. We further show that GPR120 activation induces BAT activity and promotes the browning of white fat in mice, whereas GRP120-null mice show impaired cold-induced browning. Omega-3 polyunsaturated fatty acids induce brown and beige adipocyte differentiation and thermogenic activation, and these effects require GPR120. GPR120 activation induces the release of fibroblast growth factor-21 (FGF21) by brown and beige adipocytes, and increases blood FGF21 levels. The effects of GPR120 activation on BAT activation and browning are impaired in FGF21-null mice and cells. Thus, the lipid sensor GPR120 activates brown fat via a mechanism that involves induction of FGF21.

GPR120 is a G-protein-coupled receptor that binds polyunsaturated fatty acids. Here, the authors show that GPR120 is upregulated in brown fat in cold-exposed mice, and mediates thermogenic activation of brown fat via a mechanism that, at least in part, depends on the release of the adipokine FGF21.

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.

GPR120 promotes adipogenesis through intracellular calcium and extracellular signal-regulated kinase 1/2 signal pathway

Numerous researches have demonstrated that GPR120 (also called FFAR4) exerts novel functions in insulin resistance and adipogenesis. However, the molecular mechanism of GPR120-mediated adipogenic differentiation is still unclear. This study was aimed to interpret the relevant function mechanism of GPR120 in the differentiation of 3T3-L1 adipocytes. The results showed that GPR120 expression was dramatically increased along with the adipogenic differentiation of 3T3-L1 adipocytes and the adipogenic ability was significantly inhibited in shGPR120-transfected cells. TUG-891, a selective agonist of GPR120, promoted the intracellular triglyceride accumulation in a dose-dependent manner and did not enhance adipogenesis in shGPR120-transfected cells. Markedly, TUG-891 increased the activation of PPARγ in a GPR120-dependent pathway as assessed by luciferase reporter assay. Furthermore, in the adipogenic differentiation process of 3T3-L1 adipocytes, TUG-891 increased the [Ca(2+)]i and phosphorylation level of ERK1/2. Pretreatment with inhibitors of either ERK1/2 (U0126) or [Ca(2+)]i (BAPTA-AM) notably attenuated the GPR120-mediated adipogenesis. These results show that GPR120 promotes adipogenesis by increasing PPARγ expression via [Ca(2+)]i and ERK1/2 signal pathway in 3T3-L1 adipocytes.

For Better or Worse: FFAR1 and FFAR4 Signaling in Cancer and Diabetes

Increased energy intake can lead to obesity, which increases the risk for the development of diabetes and cancer. Free fatty acids regulate numerous cellular processes, like insulin secretion, inflammation, proliferation, and cell migration. Dysregulation of these cellular functions by increased lipid intake plays a significant role in the development of diseases like diabetes and cancer. Free fatty acid receptors 1 and 4 (FFAR1 and FFAR4) are two free fatty acid receptors under increasing investigation for their roles in diabetes and more recently also cancer. Both receptors bind medium- to long-chain, saturated and omega-3 unsaturated fatty acids. Increasing evidence shows that enhanced FFAR1 and FFAR4 signaling reduces diabetes symptoms but enhances tumor growth and migration of various cancer types like melanoma and prostate cancer. This review gives an overview of the role of FFAR1 and FFAR4 in diabetes and cancer and discusses their potential to function as targets for treatment.

Eicosapentaenoic Acid Potentiates Brown Thermogenesis through FFAR4-dependent Up-regulation of miR-30b and miR-378

Emerging evidence suggests that n-3 polyunsaturated fatty acids (PUFA) promote brown adipose tissue thermogenesis. However, the underlying mechanisms remain elusive. Here, we hypothesize that n-3 PUFA promotes brown adipogenesis by modulating miRNAs. To test this hypothesis, murine brown preadipocytes were induced to differentiate the fatty acids of palmitic, oleate, or eicosapentaenoic acid (EPA). The increases of brown-specific signature genes and oxygen consumption rate by EPA were concurrent with up-regulation of miR-30b and 378 but not by oleate or palmitic acid. Next, we hypothesize that free fatty acid receptor 4 (Ffar4), a functional receptor for n-3 PUFA, modulates miR-30b and 378. Treatment of Ffar4 agonist (GW9508) recapitulated the thermogenic activation of EPA by increasing oxygen consumption rate, brown-specific marker genes, and miR-30b and 378, which were abrogated in Ffar4-silenced cells. Intriguingly, addition of the miR-30b mimic was unable to restore EPA-induced Ucp1 expression in Ffar4-depleted cells, implicating that Ffar4 signaling activity is required for up-regulating the brown adipogenic program. Moreover, blockage of miR-30b or 378 by locked nucleic acid inhibitors significantly attenuated Ffar4 as well as brown-specific signature gene expression, suggesting the signaling interplay between Ffar4 and miR-30b/378. The association between miR-30b/378 and brown thermogenesis was also confirmed in fish oil-fed C57/BL6 mice. Interestingly, the Ffar4 agonism-mediated signaling axis of Ffar4-miR-30b/378-Ucp1 was linked with an elevation of cAMP in brown adipocytes, similar to cold-exposed or fish oil-fed brown fat. Taken together, our work identifies a novel function of Ffar4 in modulating brown adipogenesis partly through a mechanism involving cAMP activation and up-regulation of miR-30b and miR-378.

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.

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.