N-oleoyldopamine enhances glucose homeostasis through the activation of GPR119

Effect of N-oleoyl dopamine on myofibroblast trans-differentiation of retinal pigment epithelial cells

Retinal pigment epithelial (RPE) cells contribute to several clinical conditions resulting in retinal fibrotic scars. Myofibroblast trans-differentiation of RPE cells is a critical step in the process of retinal fibrosis. In this study, we investigated the effects of N-oleoyl dopamine (OLDA), a newer endocannabinoid with a structure distinct from classic endocannabinoids, on TGF-β2-induced myofibroblast trans-differentiation of porcine RPE cells. Using an in vitro collagen matrix contraction assay, OLDA was found to inhibit TGF-β2 induced contraction of collagen matrices by porcine RPE cells. This effect was concentration-dependent, with significant inhibition of contraction observed at 3 μM and 10 μM. OLDA did not affect the proliferation of porcine RPE cells. Immunocytochemistry showed that at 3 μM, OLDA decreased incorporation of α-SMA in the stress fibers of TGF-β2-treated RPE cells. In addition, western blot analysis showed that 3 μM OLDA significantly downregulated TGF-β2-induced α-SMA protein expression. Taken together these results demonstrate that OLDA inhibits TGF-β induced myofibroblast trans-differentiation of RPE cells. It has been established that classic endocannabinoid such as anandamide, by activating the CB1 cannabinoid receptor, promote fibrosis in multiple organ systems. In contrast, this study demonstrates that OLDA, an endocannabinoid with a chemical structure distinct from classic endocannabinoids, inhibits myofibroblast trans-differentiation, an important step in fibrosis. Unlike classic endocannabinoids, OLDA has weak affinity for the CB1 receptor. Instead, OLDA acts on non-classic cannabinoid receptors such as GPR119, GPR6, and TRPV1. Therefore, our study indicates that the newer endocannabinoid OLDA and its non-classic cannabinoid receptors could potentially be novel therapeutic targets for treating ocular diseases involving retinal fibrosis and fibrotic pathologies in other organ systems.

Activation of TRPV1-Expressing Renal Sensory Nerves of Rats with N-Oleoyldopamine Attenuates High-Fat-Diet-Induced Impairment of Renal Function

Enhanced renal sympathetic nerve activity (RSNA) contributes to obesity-induced renal disease, while the role of afferent renal nerve activity (ARNA) is not fully understood. The present study tested the hypothesis that activating the transient receptor potential vanilloid 1 (TRPV1) channel in afferent renal nerves suppresses RSNA and prevents renal dysfunction and hypertension in obese rats. N-oleoyldopamine (OLDA, 1 ng/kg, daily) was administrated intrathecally (T8-L3) via an indwelled catheter to chronically activate, TRPV1-positive afferent renal nerves in rats fed a chow diet or high-fat diet (HFD) for 8 weeks. HFD intake significantly increased the body weight, impaired glucose and insulin tolerance, decreased creatinine clearance, and elevated systolic blood pressure in rats compared with the levels of the chow-fed rats (all p < 0.05). An intrathecal OLDA treatment for 8 weeks did not affect the fasting glucose level, glucose tolerance, and insulin tolerance in rats fed either chow or HFD. As expected, the chronic OLDA treatment significantly increased the levels of plasma calcitonin gene-related peptide and substance P and ARNA in the HFD-fed rats (all p < 0.05). Interestingly, the OLDA treatment decreased the urinary norepinephrine level and RSNA in rats fed HFD (both p < 0.05). Importantly, the OLDA treatment attenuated HFD-induced decreases in creatinine clearance and urinary Na+ excretion and increases in the plasma urea level, urinary albumin level, and systolic blood pressure at the end of an 8-week treatment (all p < 0.05). Taken together, the intrathecal administration of OLDA ameliorates the enhancement of RSNA, renal dysfunction, and hypertension in obese rats. These findings shed light on the roles of TRPV1-positive renal afferent nerves in obesity-related renal dysfunction and hypertension.

Activation and signaling mechanism revealed by GPR119-Gs complex structures

Agonists selectively targeting cannabinoid receptor-like G-protein-coupled receptor (GPCR) GPR119 hold promise for treating metabolic disorders while avoiding unwanted side effects. Here we present the cryo-electron microscopy (cryo-EM) structures of the human GPR119-G_s signaling complexes bound to AR231453 and MBX-2982, two representative agonists reported for GPR119. The structures reveal a one-amino acid shift of the conserved proline residue of TM5 that forms an outward bulge, opening up a hydrophobic cavity between TM4 and TM5 at the middle of the membrane for its endogenous ligands-monounsaturated lipid metabolites. In addition, we observed a salt bridge between ICL1 of GPR119 and Gβ_s. Disruption of the salt bridge eliminates the cAMP production of GPR119, indicating an important role of Gβ_s in GPR119-mediated signaling. Our structures, together with mutagenesis studies, illustrate the conserved binding mode of the chemically different agonists, and provide insights into the conformational changes in receptor activation and G protein coupling.

N-Oleoyl dopamine induces IL-10 via central nervous system TRPV1 and improves endotoxemia and sepsis outcomes

Background

The transient receptor potential vanilloid 1 (TRPV1) participates in thermosensation and inflammatory pain, but its immunomodulatory mechanisms remain enigmatic. N-Oleoyl dopamine (OLDA), an endovanilloid and endocannabinoid, is a TRPV1 agonist that is produced in the central nervous system and the peripheral nervous system. We studied the anti-inflammatory effects and TRPV1-dependent mechanisms of OLDA in models of inflammation and sepsis.

Methods

Mice were challenged intratracheally or intravenously with LPS, or intratracheally with S. aureus to induce pneumonia and sepsis, and then were treated intravenously with OLDA. Endpoints included plasma cytokines, leukocyte activation marker expression, mouse sepsis scores, lung histopathology, and bacterial counts. The role of TRPV1 in the effects of OLDA was determined using Trpv1^−/− mice, and mice with TRPV1 knockdown pan-neuronally, in peripheral nervous system neurons, or in myeloid cells. Circulating monocytes/macrophages were depleted using clodronate to determine their role in the anti-inflammatory effects of OLDA in endotoxemic mice. Levels of exogenous OLDA, and of endovanilloids and endocannabinoids, at baseline and in endotoxemic mice, were determined by LC–MS/MS.

Results

OLDA administration caused an early anti-inflammatory response in endotoxemic and septic mice with high serum levels of IL-10 and decreased levels of pro-inflammatory cytokines. OLDA also reduced lung injury and improved mouse sepsis scores. Blood and lung bacterial counts were comparable between OLDA- and carrier-treated mice with S. aureus pneumonia. OLDA’s effects were reversed in mice with pan-neuronal TRPV1 knockdown, but not with TRPV1 knockdown in peripheral nervous system neurons or myeloid cells. Depletion of monocytes/macrophages reversed the IL-10 upregulation by OLDA in endotoxemic mice. Brain and blood levels of endovanilloids and endocannabinoids were increased in endotoxemic mice.

Conclusions

OLDA has strong anti-inflammatory actions in mice with endotoxemia or S. aureus pneumonia. Prior studies focused on the role of peripheral nervous system TRPV1 in modulating inflammation and pneumonia. Our results suggest that TRPV1-expressing central nervous system neurons also regulate inflammatory responses to endotoxemia and infection. Our study reveals a neuro-immune reflex that during acute inflammation is engaged proximally by OLDA acting on neuronal TRPV1, and through a multicellular network that requires circulating monocytes/macrophages, leads to the systemic production of IL-10.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02485-z.

Metabolic Consequences of Gestational Cannabinoid Exposure

Up to 20% of pregnant women ages 18-24 consume cannabis during pregnancy. Moreover, clinical studies indicate that cannabis consumption during pregnancy leads to fetal growth restriction (FGR), which is associated with an increased risk of obesity, type II diabetes (T2D), and cardiovascular disease in the offspring. This is of great concern considering that the concentration of Δ9- tetrahydrocannabinol (Δ9-THC), a major psychoactive component of cannabis, has doubled over the last decade and can readily cross the placenta and enter fetal circulation, with the potential to negatively impact fetal development via the endocannabinoid (eCB) system. Cannabis exposure in utero could also lead to FGR via placental insufficiency. In this review, we aim to examine current pre-clinical and clinical findings on the direct effects of exposure to cannabis and its constituents on fetal development as well as indirect effects, namely placental insufficiency, on postnatal metabolic diseases.

Endocannabinoid signaling pathways: beyond CB1R and CB2R

The search for cannabinoid receptors other than CB1R and CB2R has been ongoing for over a decade. A number of orphan receptors have been proposed as potential cannabinoid receptors primarily based on phylogenic arguments and reactivity towards known endocannabinoids and phytocannabinoids. Seven putative cannabinoid receptors are described and discussed, and evidence for and against their inclusion in this category are presented.

Targeting the GPR119/incretin axis: a promising new therapy for metabolic-associated fatty liver disease

In the past decade, G protein-coupled receptors have emerged as drug targets, and their physiological and pathological effects have been extensively studied. Among these receptors, GPR119 is expressed in multiple organs, including the liver. It can be activated by a variety of endogenous and exogenous ligands. After GPR119 is activated, the cell secretes a variety of incretins, including glucagon-like peptide-1 and glucagon-like peptide-2, which may attenuate the metabolic dysfunction associated with fatty liver disease, including improving glucose and lipid metabolism, inhibiting inflammation, reducing appetite, and regulating the intestinal microbial system. GPR119 has been a potential therapeutic target for diabetes mellitus type 2 for many years, but its role in metabolic dysfunction associated fatty liver disease deserves further attention. In this review, we discuss relevant research and current progress in the physiology and pharmacology of the GPR119/incretin axis and speculate on the potential therapeutic role of this axis in metabolic dysfunction associated with fatty liver disease, which provides guidance for transforming experimental research into clinical applications.

Gut-inhabiting Clostridia build human GPCR ligands by conjugating neurotransmitters with diet- and human-derived fatty acids

Human physiology is regulated by endogenous signalling compounds, including fatty acid amides (FAAs), chemical mimics of which are made by bacteria. The molecules produced by human-associated microbes are difficult to identify because they may only be made in a local niche or they require a substrate sourced from the host, diet or other microbes. We identified a set of uncharacterized gene clusters in metagenomics data from the human gut microbiome. These clusters were discovered to make FAAs by fusing exogenous fatty acids with amines. Using an in vitro assay, we tested their ability to incorporate 25 fatty acids and 53 amines known to be present in the human gut, from which the production of six FAAs was deduced (oleoyl dopamine, oleoyl tyramine, lauroyl tryptamine, oleoyl aminovaleric acid, α-linolenoyl phenylethylamine and caproyl tryptamine). These molecules were screened against panels of human G-protein-coupled receptors to deduce their putative human targets. Lauroyl tryptamine is found to be an antagonist to the immunomodulatory receptor EBI2 against its native oxysterol ligand (0.98 μM half-maximal inhibitory concentration), is produced in culture by Eubacterium rectale and is present in human faecal samples. FAAs produced by Clostridia may serve as a mechanism to modulate their host by mimicking human signalling molecules.

GPR119 agonists: Novel therapeutic agents for type 2 diabetes mellitus

Diabetes mellitus type 2 (T2D) is a group of genetically heterogeneous metabolic disorders whose frequency has gradually risen worldwide. Diabetes mellitus Type 2 (T2D) has started to achieve a pandemic level, and it is estimated that within the next decade, cases of diabetes might get double due to increase in aging population. Diabetes is rightly called the 'silent killer' because it has emerged to be one of the major causes, leading to renal failure, loss of vision; besides cardiac arrest in India. Thus, a clinical requirement for the oral drug molecules monitoring glucose homeostasis appears to be unmet. GPR119 agonist, a family of G-protein coupled receptors, usually noticed in β-cells of pancreatic as well as intestinal L cells, drew considerable interest for type 2 diabetes mellitus (T2D). GPR119 monitors physiological mechanisms that enhance homeostasis of glucose, such as glucose-like peptide-1, gastrointestinal incretin hormone levels, pancreatic beta cell-dependent insulin secretion and glucose-dependent insulinotropic peptide (GIP). In this manuscript, we have reviewed the work done in the last five years (2015-2020) which gives an approach to design, synthesize, evaluate and study the structural activity relationship of novel GPR119 agonist-based lead compounds. Our article would help the researchers and guide their endeavours in the direction of strategy and development of innovative, effective GPR119 agonist-based compounds for the management of diabetes mellitus type 2.

GPR119 agonists for the treatment of type 2 diabetes: an updated patent review (2014-present)

Introduction: Type 2 diabetes is a rapid-growing complex chronic metabolic disease characterized by hyperglycemia due to lessened insulin secretion, insulin resistance and hepatic glucose overproduction. GPR119 is a class A of G protein-coupled receptor, expressed on certain enteroendocrine L and K cells in the small intestine and by β-cells within the islets of Langerhans of the pancreas. Activation of GPR119 stimulates the secretion of glucagon-like peptide-1 (GLP-1) in the intestinal tract and glucose-dependent release of insulin in pancreatic β-cells.Area covered: This review summarized the reported patents on GPR119 agonists from 2014 to present. The authors described the structural features of these novel synthetic molecules and compared their biological activities (including in vitro and in vivo) as potent GPR119 agonists for the treatment of diabetes.Expert opinion: GPR119 agonists remain the advantage of stimulating both insulin and incretin release in a glucose-dependent manner over other hypoglycemic agents, although some GPR119 agonist clinical candidates have been discontinued in Phase І or Phase II. GPR119 agonists will succeed to be developed as anti-diabetic drugs after accumulated scaffolds of agonists are discovered and the crystallographic structure of GPR119 is elucidated. The synergic effect of GPR119 agonist and DPP-4 inhibitor will also elicit a benefit for the new therapeutic of diabetes.

Discovery of a novel series of indolinylpyrimidine-based GPR119 agonists: Elimination of ether-a-go-go-related gene liability using a hydrogen bond acceptor-focused approach

We previously identified a novel series of indolinylpyrimidine derivatives exemplified by 2 in Figure 1, which is an indoline based derivative, as potent GPR119 agonists. Despite the attractive potency of 2, this compound inhibited the human ether-a-go-go-related gene (hERG) K⁺ channel. We elucidated crucial roles of the methylsulfonyl group of 2 in its interaction with the hERG channel and the GPR119 receptor, presumably as a hydrogen bond acceptor (HBA). To remove the undesirable hERG inhibitory activity, a strategy was implemented to arrange an HBA on a less conformationally flexible framework at the indoline 5-position instead of the methylsulfonyl group. This successfully led to the discovery of a piperidinone ring as a desirable motif at the indoline 5-position, which could minimize hERG liability as shown by 24b. Further optimization focused on the reduction of lipophilicity in terms of more favorable drug-like properties. Consequently, the introduction of a hydroxy group at the 3-position of the piperidinone ring effectively reduced lipophilicity without compromising GPR119 potency, resulting in the identification of (3S)-3-hydroxy-1-{1-[6-({1-[3-(propan-2-yl)-1,2,4-oxadiazol-5-yl]piperidin-4-yl}oxy)pyrimidin-4-yl]- 2,3-dihydro-1H-indol-5-yl}piperidin-2-one ((S)-29) as a novel, potent, and orally bioavailable GPR119 agonist with a well-balanced profile. The pharmacological effects of this compound were also confirmed after single and chronic oral administration in diabetic animal models.

Nutrient-Induced Cellular Mechanisms of Gut Hormone Secretion

The gastrointestinal tract can assess the nutrient composition of ingested food. The nutrient-sensing mechanisms in specialised epithelial cells lining the gastrointestinal tract, the enteroendocrine cells, trigger the release of gut hormones that provide important local and central feedback signals to regulate nutrient utilisation and feeding behaviour. The evidence for nutrient-stimulated secretion of two of the most studied gut hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), along with the known cellular mechanisms in enteroendocrine cells recruited by nutrients, will be the focus of this review. The mechanisms involved range from electrogenic transporters, ion channel modulation and nutrient-activated G-protein coupled receptors that converge on the release machinery controlling hormone secretion. Elucidation of these mechanisms will provide much needed insight into postprandial physiology and identify tractable dietary approaches to potentially manage nutrition and satiety by altering the secreted gut hormone profile.

Targeting lipid GPCRs to treat type 2 diabetes mellitus - progress and challenges

Therapeutic approaches to the treatment of type 2 diabetes mellitus that are designed to increase insulin secretion either directly target β-cells or indirectly target gastrointestinal enteroendocrine cells (EECs), which release hormones that modulate insulin secretion (for example, incretins). Given that β-cells and EECs both express a large array of G protein-coupled receptors (GPCRs) that modulate insulin secretion, considerable research and development efforts have been undertaken to design therapeutic drugs targeting these GPCRs. Among them are GPCRs specific for free fatty acid ligands (lipid GPCRs), including free fatty acid receptor 1 (FFA1, otherwise known as GPR40), FFA2 (GPR43), FFA3 (GPR41) and FFA4 (GPR120), as well as the lipid metabolite binding glucose-dependent insulinotropic receptor (GPR119). These lipid GPCRs have demonstrated important roles in the control of islet and gut hormone secretion. Advances in lipid GPCR pharmacology have led to the identification of a number of synthetic agonists that exert beneficial effects on glucose homeostasis in preclinical studies. Yet, translation of these promising results to the clinic has so far been disappointing. In this Review, we present the physiological roles, pharmacology and clinical studies of these lipid receptors and discuss the challenges associated with their clinical development for the treatment of type 2 diabetes mellitus.

GPCRs get fatty: the role of G protein-coupled receptor signaling in the development and progression of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD), characterized by the abnormal deposition of lipids within the liver not due to alcohol consumption, is a growing epidemic affecting over 30% of the United States population. Both simple fatty liver and its more severe counterpart, nonalcoholic steatohepatitis, represent one of the most common forms of liver disease. Recently, several G protein-coupled receptors have emerged as targets for therapeutic intervention for these disorders. These include those with known hepatic function as well as those involved in global metabolic regulation. In this review, we highlight these emerging therapeutic targets, focusing on several common themes including their activation by microbial metabolites, stimulatory effect on insulin and incretin secretion, and contribution to glucose tolerance. The overlap in ligands, localization, and downstream effects of activation indicate the interdependent nature of these receptors and highlight the importance of this signaling family in the development and prevention of NAFLD.

GPR119 and GPR55 as Receptors for Fatty Acid Ethanolamides, Oleoylethanolamide and Palmitoylethanolamide

Oleoylethanolamide and palmitoylethanolamide are members of the fatty acid ethanolamide family, also known as acylethanolamides. Their physiological effects, including glucose homeostasis, anti-inflammation, anti-anaphylactic, analgesia, and hypophagia, have been reported. They have affinity for different receptor proteins, including nuclear receptors such as PPARα, channels such as TRPV1, and membrane receptors such as GPR119 and GPR55. In the present review, the pathophysiological functions of fatty acid ethanolamides have been discussed from the perspective of receptor pharmacology and drug discovery.

Druggable Lipid GPCRs: Past, Present, and Prospects

G protein-coupled receptors (GPCRs) have seven transmembrane spanning domains and comprise the largest superfamily with ~800 receptors in humans. GPCRs are attractive targets for drug discovery because they transduce intracellular signaling in response to endogenous ligands via heterotrimeric G proteins or arrestins, resulting in a wide variety of physiological and pathophysiological responses. The endogenous ligands for GPCRs are highly chemically diverse and include ions, biogenic amines, nucleotides, peptides, and lipids. In this review, we follow the KonMari method to better understand druggable lipid GPCRs. First, we have a comprehensive tidying up of lipid GPCRs including receptors for prostanoids, leukotrienes, specialized pro-resolving mediators (SPMs), lysophospholipids, sphingosine 1-phosphate (S1P), cannabinoids, platelet-activating factor (PAF), free fatty acids (FFAs), and sterols. This tidying up consolidates 46 lipid GPCRs and declutters several perplexing lipid GPCRs. Then, we further tidy up the lipid GPCR-directed drugs from the literature and databases, which identified 24 clinical drugs targeting 16 unique lipid GPCRs available in the market and 44 drugs under evaluation in more than 100 clinical trials as of 2019. Finally, we introduce drug designs for GPCRs that spark joy, such as positive or negative allosteric modulators (PAM or NAM), biased agonism, functional antagonism like fingolimod, and monoclonal antibodies (MAbs). These strategic drug designs may increase the efficacy and specificity of drugs and reduce side effects. Technological advances will help to discover more endogenous lipid ligands from the vast number of remaining orphan GPCRs and will also lead to the development novel lipid GPCR drugs to treat various diseases.

Small-molecule GLP-1 secretagogs: challenges and recent advances

Glucagon-like peptide-1 (GLP-1) is a potent anti-hyperglycemic hormone that is an alternative treatment choice for patients with type 2 diabetes mellitus (T2DM). The glucose-dependent mechanism of GLP-1 is particularly important because it does not stimulate insulin secretion and cause hypoglycemia when plasma glucose concentrations are in the normal fasting range. Although several peptide drugs of GLP-1 analogs are clinically available, research on the small molecules that stimulate GLP-1 secretion is still struggling. In this review, we summarize recent updates in the discovery of small-molecule GLP-1 secretagogs targeting the G-protein-coupled receptor (GPCR) family. We also discuss the challenges and strategies for the study and describe the latest developments.

An Update of Palmitoylethanolamide and Luteolin Effects in Preclinical and Clinical Studies of Neuroinflammatory Events

The inflammation process represents of a dynamic series of phenomena that manifest themselves with an intense vascular reaction. Neuroinflammation is a reply from the central nervous system (CNS) and the peripheral nervous system (PNS) to a changed homeostasis. There are two cell systems that mediate this process: the glia of the CNS and the lymphocites, monocytes, and macrophages of the hematopoietic system. In both the peripheral and central nervous systems, neuroinflammation plays an important role in the pathogenesis of neurodegenerative diseases, such as Parkinson’s and Alzheimer’s diseases, and in neuropsychiatric illnesses, such as depression and autism spectrum disorders. The resolution of neuroinflammation is a process that allows for inflamed tissues to return to homeostasis. In this process the important players are represented by lipid mediators. Among the naturally occurring lipid signaling molecules, a prominent role is played by the N-acylethanolamines, namely N-arachidonoylethanolamine and its congener N-palmitoylethanolamine, which is also named palmitoylethanolamide or PEA. PEA possesses a powerful neuroprotective and anti-inflammatory power but has no antioxidant effects per se. For this reason, its co-ultramicronization with the flavonoid luteolin is more efficacious than either molecule alone. Inhibiting or modulating the enzymatic breakdown of PEA represents a complementary therapeutic approach to treating neuroinflammation. The aim of this review is to discuss the role of ultramicronized PEA and co-ultramicronized PEA with luteolin in several neurological diseases using preclinical and clinical approaches.

TRPV1 Mediates Glucose-induced Insulin Secretion Through Releasing Neuropeptides

BACKGROUND/AIM

Transient receptor potential vanilloid 1 (TRPV1)-expressing sensory nerves innervate the pancreatic islets. Sensory neuropeptides, including calcitonin gene-related peptide (CGRP) and substance P (SP), participate in insulin secretion. This study aimed to investigate the role of TRPV1 in glucose-induced insulin secretion.

MATERIALS AND METHODS

TRPV1^-/- and wild-type (WT) mice were fed a normal diet for 24 weeks. Glucose tolerance and insulin secretion were measured at the end of the experiments.

RESULTS

TRPV1^-/- mice had greater impairments in glucose tolerance and higher decrease in glucose-induced insulin secretion than WT mice. Capsaicin (a TRPV1 agonist) increased insulin secretion in WT, but not in TRPV1^-/- mice. Glucose-induced insulin secretion was blunted in TRPV1^-/- mice, and was attenuated by AMG9810 (a TRPV1 inhibitor), CGRP_8-37 (a CGRP receptor antagonist), or RP67580 (a NK-1 receptor antagonist) in WT mice. Glucose-induced SP and CGRP release from WT pancreas was higher than that from TRPV1^-/- pancreas.

CONCLUSION

TRPV1 mediates glucose-induced insulin secretion likely through CGRP and SP release.

Identification of Novel, Structurally Diverse, Small Molecule Modulators of GPR119

GPR119 drug discovery efforts in the pharmaceutical industry for the treatment of type 2 diabetes mellitus (T2DM) and obesity, were initiated based on its restricted distribution in pancreas and GI tract, and its possible role in glucose homeostasis. While a number of lead series have emerged, the pharmacological endpoints they provide have not been clear. In particular, many lead series have demonstrated loss of efficacy and significant toxic side effects. Thus, we sought to identify novel, potent, positive modulators of GPR119. In this study, we have successfully developed and optimized a high-throughput screening strategy to identify GPR119 modulators using a live cell assay format that utilizes a cyclic nucleotide-gated channel as a biosensor for cAMP production. Our high-throughput screening (HTS) approach is unique to that of previous HTS approaches targeting this receptor, as changes in cAMP were measured both in the presence and absence of an EC₁₀ of the endogenous ligand, oleoylethanolamide, enabling detection of both agonists and potential allosteric modulators in a single assay. From these efforts, we have identified positive modulators of GPR119 with similar as well as unique scaffolds compared to existing compounds and similar as well as unique signaling properties. Our compounds will not only serve as novel molecular probes to better understand GPR119 pleiotropic signaling and the underlying physiological consequences of receptor activation, but are also well-suited for translation as potential therapeutic agents.

Free Fatty Acid Receptors in Health and Disease

Fatty acids are metabolized and synthesized as energy substrates during biological responses. Long- and medium-chain fatty acids derived mainly from dietary triglycerides, and short-chain fatty acids (SCFAs) produced by gut microbial fermentation of the otherwise indigestible dietary fiber, constitute the major sources of free fatty acids (FFAs) in the metabolic network. Recently, increasing evidence indicates that FFAs serve not only as energy sources but also as natural ligands for a group of orphan G protein-coupled receptors (GPCRs) termed free fatty acid receptors (FFARs), essentially intertwining metabolism and immunity in multiple ways, such as via inflammation regulation and secretion of peptide hormones. To date, several FFARs that are activated by the FFAs of various chain lengths have been identified and characterized. In particular, FFAR1 (GPR40) and FFAR4 (GPR120) are activated by long-chain saturated and unsaturated fatty acids, while FFAR3 (GPR41) and FFAR2 (GPR43) are activated by SCFAs, mainly acetate, butyrate, and propionate. In this review, we discuss the recent reports on the key physiological functions of the FFAR-mediated signaling transduction pathways in the regulation of metabolism and immune responses. We also attempt to reveal future research opportunities for developing therapeutics for metabolic and immune disorders.

Targeting GPCRs Activated by Fatty Acid-Derived Lipids in Type 2 Diabetes

G protein-coupled receptors (GPCRs) are the most intensively studied drug targets, because of their diversity, cell-specific expression, and druggable sites accessible at the cell surface. Preclinical and clinical studies suggest that targeting GPCRs activated by fatty acid-derived lipids may have potential to improve glucose homeostasis and reduce complications in patients with type 2 diabetes (T2D). Despite the discontinued development of fasiglifam (TAK-875), the first FFA1 agonist to reach late-stage clinical trials, lipid-sensing receptors remain a viable target, albeit with a need for further characterization of their binding mode, intracellular signaling, and toxicity. Herein, we analyze general discovery trends, various signaling pathways, as well as possible challenges following activation of GPCRs that have been validated clinically to control blood glucose levels.

The metabolic role of vagal afferent innervation

The regulation of energy and glucose balance contributes to whole-body metabolic homeostasis, and such metabolic regulation is disrupted in obesity and diabetes. Metabolic homeostasis is orchestrated partly in response to nutrient and vagal-dependent gut-initiated functions. Specifically, the sensory and motor fibres of the vagus nerve transmit intestinal signals to the central nervous system and exert biological and physiological responses. In the past decade, the understanding of the regulation of vagal afferent signals and of the associated metabolic effect on whole-body energy and glucose balance has progressed. This Review highlights the contributions made to the understanding of the vagal afferent system and examines the integrative role of the vagal afferent in gastrointestinal regulation of appetite and glucose homeostasis. Investigating the integrative and metabolic role of vagal afferent signalling represents a potential strategy to discover novel therapeutic targets to restore energy and glucose balance in diabetes and obesity.

Effect of N-acyl-dopamines on beta cell differentiation and wound healing in diabetic mice

N-acyl-dopamines are endolipids with neuroprotective, antiinflammatory and immunomodulatory properties. Previously, we showed the ability of these compounds to induce HIF-1α stabilization. Hypoxia and HIF-1α play an important role in the most relevant stages of diabetic pathogenesis. This work analyzes the possible role of these molecules on beta cell differentiation, insulin production and diabetic foot ulcer. Hypoxia response pathway has been characterized in beta-cell differentiation in rat pancreatic acinar cell line and human islet-derived precursor cells. Protein and mRNA expression of key proteins in this process have been analyzed, as well as those involved in beta cells reprogramming. The effect of N-acyl-dopamines on hypoxia response pathway, beta cells reprogramming and insulin production have been studied in both cell types, as well as its role in angiogenesis models in vitro and wound closure in type 2 diabetic mice. Our results show how the hypoxia response pathway is altered during beta cells differentiation, accompanied by an induction of the transcription factor HIF-1α. We demonstrate how some N-acyl-dopamines induce beta cell differentiation and insulin production in two different cell models. In parallel, these endolipids promote angiogenesis in vitro and wound closure in type 2 diabetic mice. These results provide a biological mechanism through which some endolipids could induce beta cell differentiation. We demonstrate how N-acyl-dopamines can modulate insulin production and, in parallel, reverse HIF-1α inhibition in a wound healing model in diabetic mice. Therefore, the potential use of the pharmacological modulation of N-acyl-dopamines may have implications for diabetes prevention and treatment strategies.

Modulation of the Oxidative Stress and Lipid Peroxidation by Endocannabinoids and Their Lipid Analogues

Growing evidence supports the pivotal role played by oxidative stress in tissue injury development, thus resulting in several pathologies including cardiovascular, renal, neuropsychiatric, and neurodegenerative disorders, all characterized by an altered oxidative status. Reactive oxygen and nitrogen species and lipid peroxidation-derived reactive aldehydes including acrolein, malondialdehyde, and 4-hydroxy-2-nonenal, among others, are the main responsible for cellular and tissue damages occurring in redox-dependent processes. In this scenario, a link between the endocannabinoid system (ECS) and redox homeostasis impairment appears to be crucial. Anandamide and 2-arachidonoylglycerol, the best characterized endocannabinoids, are able to modulate the activity of several antioxidant enzymes through targeting the cannabinoid receptors type 1 and 2 as well as additional receptors such as the transient receptor potential vanilloid 1, the peroxisome proliferator-activated receptor alpha, and the orphan G protein-coupled receptors 18 and 55. Moreover, the endocannabinoids lipid analogues N-acylethanolamines showed to protect cell damage and death from reactive aldehydes-induced oxidative stress by restoring the intracellular oxidants-antioxidants balance. In this review, we will provide a better understanding of the main mechanisms triggered by the cross-talk between the oxidative stress and the ECS, focusing also on the enzymatic and non-enzymatic antioxidants as scavengers of reactive aldehydes and their toxic bioactive adducts.

Bioactive Oleic Derivatives of Dopamine: A Review of the Therapeutic Potential

Lipid derivatives of dopamine are a novel class of compounds raising a research interest due to the potential of their being a vehicle for dopamine delivery to the brain. The aim of the present paper is to review the main features of the two most prominent bioactive members of this family, namely, N-oleoyl-dopamine (OLDA) and 3'-O-methyl-N-oleoyl-dopamine (OMe-OLDA), with emphasis on the possible therapeutic properties.

Bidirectional GPR119 Agonism Requires Peptide YY and Glucose for Activity in Mouse and Human Colon Mucosa

The lipid sensor G protein-coupled receptor 119 (GPR119) is highly expressed by enteroendocrine L-cells and pancreatic β-cells that release the hormones, peptide YY (PYY) and glucagonlike peptide 1, and insulin, respectively. Endogenous oleoylethanolamide (OEA) and the dietary metabolite, 2-monoacylglycerol (2-OG), can each activate GPR119. Here, we compared mucosal responses with selective, synthetic GPR119 agonists (AR440006 and AR231453) and the lipids, OEA, 2-OG, and N-oleoyldopamine (OLDA), monitoring epithelial ion transport as a readout for L-cell activity in native mouse and human gastrointestinal (GI) mucosae. We also assessed GPR119 modulation of colonic motility in wild-type (WT), GPR119-deficient (GPR119-/-), and PYY-deficient (PYY-/-) mice. The water-soluble GPR119 agonist, AR440006 (that cannot traverse epithelial tight junctions), elicited responses, when added apically or basolaterally in mouse and human colonic mucosae. In both species, GPR119 responses were PYY, Y1 receptor mediated, and glucose dependent. AR440006 efficacy matched the GI distribution of L-cells in WT tissues but was absent from GPR119-/- tissue. OEA and 2-OG responses were significantly reduced in the GPR119-/- colon, but OLDA responses were unchanged. Alternative L-cell activation via free fatty acid receptors 1, 3, and 4 and the G protein-coupled bile acid receptor TGR5 or by the melanocortin 4 receptor, was unchanged in GPR119-/- tissues. The GPR119 agonist slowed transit in WT but not the PYY-/- colon in vitro. AR440006 (intraperitoneally) slowed WT colonic and upper-GI transit significantly in vivo. These data indicate that luminal or blood-borne GPR119 agonism can stimulate L-cell PYY release with paracrine consequences and slower motility. We suggest that this glucose-dependent L-cell response to a gut-restricted GPR119 stimulus has potential therapeutic advantage in modulating insulinotropic signaling with reduced risk of hypoglycemia.

Structure and biological activity of endogenous and synthetic agonists of GPR119

A G-protein-coupled receptor, GPR119, is a promising pharmacological target for a new class of hypoglycaemic drugs with an original mechanism of action, namely, increase in the glucose-dependent incretin and insulin secretion. In 2005, the first ligands were found and in the subsequent years, a large number of GPR119 agonists were synthesized in laboratories in various countries; the safest and most promising agonists have entered phase I and II clinical trials as agents for the treatment of type 2 diabetes mellitus and obesity. The review describes the major endogenous GPR119 agonists and the main trends in the design and modification of synthetic structures for increasing the hypoglycaemic activity. The data on synthetic agonists are arranged according to the type of the central core of the molecules.

The bibliography includes 104 references.

Therapeutic application of GPR119 ligands in metabolic disorders

GPR119 belongs to the G protein-coupled receptor family and exhibits dual modes of action upon ligand-dependent activation: pancreatic secretion of insulin in a glucose-dependent manner and intestinal secretion of incretins. Hence, GPR119 has emerged as a promising target for treating type 2 diabetes mellitus without causing hypoglycaemia. However, despite continuous efforts by many major pharmaceutical companies, no synthetic GPR119 ligand has been approved as a new class of anti-diabetic agents thus far, nor has any passed beyond phase II clinical studies. Herein, we summarize recent advances in research concerning the physiological/pharmacological effects of GPR119 and its synthetic ligands on the regulation of energy metabolism, and we speculate on future applications of GPR119 ligands for the treatment of metabolic diseases, focusing on non-alcoholic fatty liver disease.

Peripheral modulation of the endocannabinoid system in metabolic disease

Dysfunction of the endocannabinoid system (ECS) has been identified in metabolic disease. Cannabinoid receptor 1 (CB₁) is abundantly expressed in the brain but also expressed in the periphery. Cannabinoid receptor 2 (CB₂) is more abundant in the periphery, including the immune cells. In obesity, global antagonism of overexpressed CB₁ reduces bodyweight but leads to centrally mediated adverse psychological outcomes. Emerging research in isolated cultured cells or tissues has demonstrated that targeting the endocannabinoid system in the periphery alleviates the pathologies associated with metabolic disease. Further, peripheral specific cannabinoid ligands can reverse aspects of the metabolic phenotype. This Keynote review will focus on current research on the functionality of peripheral modulation of the ECS for the treatment of obesity.

[The role of pharmacology to produce firuglipel (DS-8500a), an orally available GPR119 agonist for type 2 diabetes mellitus]

GPR119 (G-protein coupled receptor 119) has been shown to be highly expressed in the lower small intestinal and colorectal L-cells and pancreatic β-cells, and mediates intracellular cAMP concentration, glucagon like peptide (GLP-1) secretion, and glucose stimulated insulin secretion (GSIS). As the next generation for the treatment of type 2 diabetes mellitus (T2DM), GPR119 agonist has been intensively studied by pharmaceutical companies and a lot of patents have been applied by them. In such highly competitive condition, biological differentiation and to find an advantage among GPR119 agonists were necessary to proceed the candidate compound in further clinical investigation. Firuglipel (DS-8500a) is an orally available GPR119 agonist synthesized in DAIICHI SANKYO CO., LTD (DS). It was originated from DS-chemical library and optimized in the aspect of bioavailability and safety. Firuglipel had a higher intrinsic activity (IA) of the production of intracellular cAMP in human GPR119 expressing CHO-K1 cells than those of other GPR119 agonists studied. The level of IA in each GPR119 agonist was correlated with the existence of agonist conformer. In parallel with the study for the differentiation from other GPR119 agonists, we compared firuglipel with dipeptidyl peptide-4 (DPP-4) inhibitor in NONcNZO10/LtJ mice and evaluated their combination in streptozotocin (STZ) treated C57BL/6J mice to clarify future positioning among anti-diabetics therapy. These pharmacological studies illustrated here can draw out a clinical value of compound and expected to lead the production of first-in-class agent in pharmaceutical companies.

Fatty Acid-Induced Lipotoxicity in Pancreatic Beta-Cells During Development of Type 2 Diabetes

Type 2 diabetes is caused by chronic insulin resistance and progressive decline in beta-cell function. Optimal beta-cell function and mass is essential for glucose homeostasis and beta-cell impairment leads to the development of diabetes. Elevated levels of circulating fatty acids (FAs) and disturbances in lipid metabolism regulation are associated with obesity, and they are major factors influencing the increase in the incidence of type 2 diabetes. Chronic free FA (FFA) treatment induces insulin resistance and beta-cell dysfunction; therefore, reduction of elevated plasma FFA levels might be an important therapeutic target in obesity and type 2 diabetes. Lipid signals via receptors, and intracellular mechanisms are involved in FFA-induced apoptosis. In this paper, we discuss lipid actions in beta cells, including effects on metabolic pathways and stress responses, to help further understand the molecular mechanisms of lipotoxicity-induced type 2 diabetes.

Peripheral tissue levels and molecular species compositions of N-acyl-phosphatidylethanolamine and its metabolites in mice lacking N-acyl-phosphatidylethanolamine-specific phospholipase D

N-acylethanolamines (NAEs), a class of lipid mediators, are produced from N-acyl-phosphatidylethanolamine (NAPE) by several pathways, including the direct release by NAPE-specific phospholipase D (NAPE-PLD) or the multistep pathway via sn-glycero-3-phospho-N-acylethanolamine (Gp-NAE). Using liquid chromatography-tandem mass spectrometry, we compared peripheral tissue levels of NAPE, Gp-NAE and NAE in NAPE-PLD-deficient (NAPE-PLD-/-) and wild type (WT) mice. NAPE-PLD was suggested to play a major role in the NAPE degradation in heart, kidney, and liver, but not in jejunum, because the NAPE levels except jejunum were significantly higher in NAPE-PLD-/- mice than in WT mice. The deletion of NAPE-PLD failed to alter the NAE levels of these tissues, suggesting its limited role in the NAE production. The enzyme assays with tissue homogenates confirmed the presence of NAPE-PLD-independent pathways in these peripheral tissues. Gp-NAE species having an acyl moiety with 22 carbons and 6 double bonds was enriched in these peripheral tissues. As for sn-2 acyl species of NAPE, 18:2-acyl-containing NAPE species were predominant over 18:1-containing species in heart, liver, and jejunum. Our results show that both molecular species composition of NAPE, NAE and Gp-NAE and their dependencies on Napepld are different among the peripheral tissues, suggesting that each tissue has distinct metabolic pathways and these NAE-containing lipids play tissue-specific roles.

Cannabinoid Receptor-Related Orphan G Protein-Coupled Receptors

Of the druggable group of G protein-coupled receptors in the human genome, a number remain which have yet to be paired with an endogenous ligand-orphan GPCRs. Among these 100 or so entities, 3 have been linked to the cannabinoid system. GPR18, GPR55, and GPR119 exhibit limited sequence homology with the established CB₁ and CB₂ cannabinoid receptors. However, the pharmacology of these orphan receptors displays overlap with CB₁ and CB₂ receptors, particularly for GPR18 and GPR55. The linking of GPR119 to the cannabinoid receptors is less convincing and emanates from structural similarities of endogenous ligands active at these GPCRs, but which do not cross-react. This review describes the evidence for describing these orphan GPCRs as cannabinoid receptor-like receptors.

YH18421, a novel GPR119 agonist exerts sustained glucose lowering and weight loss in diabetic mouse model

G-protein-coupled receptor 119 (GPR119) represents a promising target for the treatment of type 2 diabetes as it can increase both GLP-1 secretion from intestinal L cells and glucose-stimulated insulin secretion (GSIS) from pancreatic β cells. Due to this dual mechanism of action, the development of small molecule GPR119 agonists has received much interest for the treatment of type 2 diabetes. Here, we identified a novel small-molecule GPR119 agonist, YH18421 and evaluated its therapeutic potential. YH18421 specifically activated human GPR119 with high potency and potentiated GLP-1 secretion and GSIS in vitro cell based systems. In normal mice, single oral administration of YH18421 improved glucose tolerance. Combined treatment of YH18421 and the DPP-4 inhibitor augmented both plasma active GLP-1 levels and glycemic control. In diet induced obese (DIO) mice model, glucose lowering effect of YH18421 was maintained after 4 weeks of repeat dosing and YH18421 acted additively with DPP-IV inhibitor. We also observed that YH18421 inhibited weight gain during 4 weeks of administration in DIO mice. These data demonstrate that YH18421 is capable of delivering sustained glucose control and preventing weight gain and combination with the DPP-IV inhibitor maybe an effective strategy for the treatment of type 2 diabetes.

Synthesis and biological evaluation of pyrimidine derivatives with diverse azabicyclic ether/amine as novel GPR119 agonist

A class of novel pyrimidine derivatives bearing diverse conformationally restricted azabicyclic ether/amine were designed, synthesized and evaluated for their GPR119 agonist activities against type 2 diabetes. Most compounds exhibited superior hEC₅₀ values to endogenous lipid oleoylethanolamide (OEA). Analogs with 2-fluoro substitution in the aryl ring showed more potent GPR119 activation than those without fluorine. Especially compound 27m synthesized from endo-azabicyclic alcohol was observed to have the best EC₅₀ value (1.2nM) and quite good agonistic activity (112.2% max) as a full agonist.

Cannabinoid CB1 and CB2 Receptor Signaling and Bias

An agonist that acts through a single receptor can activate numerous signaling pathways. Recent studies have suggested that different ligands can differentially activate these pathways by stabilizing a limited range of receptor conformations, which in turn preferentially drive different downstream signaling cascades. This concept, termed "biased signaling" represents an exciting therapeutic opportunity to target specific pathways that elicit only desired effects, while avoiding undesired effects mediated by different signaling cascades. The cannabinoid receptors CB1 and CB2 each activate multiple pathways, and evidence is emerging for bias within these pathways. This review will summarize the current evidence for biased signaling through cannabinoid receptor subtypes CB1 and CB2.

Synthesis and biological evaluation of 1, 2, 4-oxadiazole derivatives as novel GPR119 agonists

A series of 1, 2, 4-oxadiazole derivatives have been designed and synthesized, and 25 compounds were evaluated their abilities by the assay of cAMP concentration in GPR119-transfected HEK293T cells. All compounds showed acceptable agonistic effects on GPR119. Among these compounds, 4p exhibited the best agonistic effects with the EC₅₀ of 20.6 nm, which was comparable to that of positive control GPR119 agonist GSK1292263. The agonistic activity of these 1,2,4-oxadiazole derivatives led to the establishment of a structure-activity relationship.

Oral 2-oleyl glyceryl ether improves glucose tolerance in mice through the GPR119 receptor

The intestinal G protein-coupled receptor GPR119 is a novel metabolic target involving glucagon-like peptide-1 (GLP-1)-derived insulin-regulated glucose homeostasis. Endogenous and diet-derived lipids, including N-acylethanolamines and 2-monoacylglycerols (2-MAG) activate GPR119. The purpose of this work is to evaluate whether 2-oleoyl glycerol (2-OG) improves glucose tolerance through GPR119, using wild type (WT) and GPR 119 knock out (KO) mice. We here show that GPR119 is essential for 2-OG-mediated release of GLP-1 and CCK from GLUTag cells, since a GPR119 specific antagonist completely abolished the hormone release. Similarly, in isolated primary colonic crypt cultures from WT mice, GPR119 was required for 2-OG-stimulated GLP-1 release while there was no response in crypts from KO mice. In vivo, gavage with 2-oleyl glyceryl ether ((2-OG ether), a stable 2-OG analog with a potency of 5.3 µM for GPR119 with respect to cAMP formation as compared to 2.3 µM for 2-OG), significantly (P < 0.05) improved glucose clearance in WT littermates, but not in GPR119 KO mice. Finally, deletion of GPR119 in mice resulted in lower glucagon levels, whereas the levels of insulin and GIP were unchanged. In the present study we show that 2-OG stimulates GLP-1 secretion through GPR119 activation in vitro, and that fat-derived 2-MAGs are potent candidates for mediating fat-induced GLP-1 release through GPR119 in vivo. © 2016 BioFactors, 42(6):665-673, 2016.

A role of CB1R in inducing θ-rhythm coordination between the gustatory and gastrointestinal insula

Anandamide (AEA) and N-oleoylethanolamine (OEA) are produced in the intestine and brain during fasting and satiety, respectively. Subsequently, AEA facilitates food intake via activation of cannabinoid type-1 receptors (CB1Rs) while OEA decreases food intake via activation of peroxisome proliferator-activated receptor-α (PPARα) and/or G-protein-coupled receptor 119 (GPR119). Neuronal activity in the gastrointestinal region of the autonomic insula (GI-Au-I) that rostrally adjoins the gustatory insula (Gu-I) increases during fasting, enhancing appetite while umami and sweet taste sensations in Gu-I enhances appetite in GI-Au-I, strongly suggesting the presence of a neural interaction between the Gu-I and GI-Au-I which changes depending on the concentrations of AEA and OEA. However, this possibility has never been investigated. In rat slice preparations, we demonstrate with voltage-sensitive dye imaging that activation of CB1Rs by AEA induces θ-rhythm oscillatory synchronization in the Gu-I which propagates into the GI-Au-I but stops at its caudal end, displaying an oscillatory coordination. The AEA-induced oscillation was abolished by a CB1R antagonist or OEA through activation of GPR119. Our results demonstrate that the neural coordination between the Gu-I and GI-Au-I is generated or suppressed by the opposing activities between CB1R and GPR119. This mechanism may be involved in the feeding behavior based on taste recognition.

Broad impact of deleting endogenous cannabinoid hydrolyzing enzymes and the CB1 cannabinoid receptor on the endogenous cannabinoid-related lipidome in eight regions of the mouse brain

BACKGROUND AND PURPOSE

The enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) hydrolyze endogenous cannabinoids (eCBs), N-arachidonoyl ethanolamine (AEA) and 2-arachidonoyl glycerol (2-AG), respectively. These enzymes also metabolize eCB analogs such as lipoamines and 2-acyl glycerols, most of which are not ligands at CB1. To test the hypothesis that deleting eCB hydrolyzing enzymes and CB1 shifts lipid metabolism more broadly and impacts more families of eCB structural analogs, targeted lipidomics analyses were performed on FAAH KO, MAGL KO, and CB1 KO mice and compared to WT controls in 8 brain regions.

EXPERIMENTAL APPROACH

Methanolic extracts of discrete brain regions (brainstem, cerebellum, cortex, hippocampus, hypothalamus, midbrain, striatum and thalamus) were partially purified on C-18 solid-phase extraction columns. Over 70 lipids per sample were then analyzed with HPLC/MS/MS.

KEY RESULTS

AEA and 2-AG were unaffected throughout the brain in CB1 KO mice; however, there was an increase in the arachidonic acid (AA) metabolite, PGE2 in the majority of brain areas. By contrast, PGE2 and AA levels were significantly reduced throughout the brain in the MAGL KO corresponding to significant increases in 2-AG. No changes in AA or PGE2 were seen throughout in the FAAH KO brain, despite significant increases in AEA, suggesting AA liberated by FAAH does not contribute to steady state levels of AA or PGE2. Changes in the lipidome were not confined to the AA derivatives and showed regional variation in each of the eCB KO models.

CONCLUSIONS AND IMPLICATIONS

AEA and 2-AG hydrolyzing enzymes and the CB1 receptor link the eCB system to broader lipid signaling networks in contrasting ways, potentially altering neurotransmission and behavior independently of cannabinoid receptor signaling.

Nutritional Signaling via Free Fatty Acid Receptors

Excess energy is stored primarily as triglycerides, which are mobilized when demand for energy arises. Dysfunction of energy balance by excess food intake leads to metabolic diseases, such as obesity and diabetes. Free fatty acids (FFAs) provided by dietary fat are not only important nutrients, but also contribute key physiological functions via FFA receptor (FFAR)-mediated signaling molecules, which depend on FFAs' carbon chain length and the ligand specificity of the receptors. Functional analyses have revealed that FFARs are critical for metabolic functions, such as peptide hormone secretion and inflammation, and contribute to energy homeostasis. In particular, recent studies have shown that the administration of selective agonists of G protein-coupled receptor (GPR) 40 and GPR120 improved glucose metabolism and systemic metabolic disorders. Furthermore, the anti-inflammation and energy metabolism effects of short chain FAs have been linked to the activation of GPR41 and GPR43. In this review, we summarize recent progress in research on FFAs and their physiological roles in the regulation of energy metabolism.

GPR119: a promising target for nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is associated with metabolic syndrome and has the unique characteristic of excess lipid accumulation in liver. G-protein-coupled receptor 119 (GPR119) is a promising target for type 2 diabetes. However, the role of GPR119 activation in hepatic steatosis and its precise mechanism has not been investigated. In primary cultured hepatocytes from wild-type and GPR119 knockout (KO) mice, expression of lipogenic enzymes was elevated in GPR119 KO hepatocytes. Treatment of hepatocytes and HepG2 cells with GPR119 agonists in phase 2 clinical trials (MBX-2982 [MBX] and GSK1292263) inhibited protein expression of both nuclear and total sterol regulatory element binding protein (SREBP)-1, a key lipogenesis transcription factor. Oral administration of MBX in mice fed a high-fat diet potently inhibited hepatic lipid accumulation and expression levels of SREBP-1 and lipogenesis-related genes, whereas the hepatic antilipogenesis effects of MBX were abolished in GPR119 KO mice. MBX activated AMPK and increased Ser-372 phosphorylation of SREBP-1c, an inhibitory form of SREBP-1c. Moreover, inhibition of AMPK recovered MBX-induced down-regulation of SREBP-1. These findings demonstrate for the first time that the GPR119 ligand alleviates hepatic steatosis by inhibiting SREBP-1-mediated lipogenesis in hepatocytes.

G Protein-Coupled Receptor 119 (GPR119) Agonists for the Treatment of Diabetes: Recent Progress and Prevailing Challenges

In this Perspective, recent advances and challenges in the development of GPR119 agonists as new oral antidiabetic drugs will be discussed. Such agonists are expected to exhibit a low risk to induce hypoglycemia as well as to have a beneficial impact on body weight. Many pharmaceutical companies have been active in the search for GPR119 agonists, making it a highly competitive area in the industrial environment. Several GPR119 agonists have been entered into clinical studies, but many have failed either in phase I or II and none has progressed beyond phase II. Herein we describe the strategies chosen by the different medicinal chemistry teams in academia and the pharmaceutical industry to improve potency, physicochemical properties, pharmacokinetics, and the safety profile of GPR119 agonists in the discovery phase in order to improve the odds for successful development.

Mutation-Guided Unbiased Modeling of the Fat Sensor GPR119 for High-Yield Agonist Screening

Recent benchmark studies have demonstrated the difficulties in obtaining accurate predictions of ligand binding conformations to comparative models of G-protein-coupled receptors. We have developed a data-driven optimization protocol, which integrates mutational data and structural information from multiple X-ray receptor structures in combination with a fully flexible ligand docking protocol to determine the binding conformation of AR231453, a small-molecule agonist, in the GPR119 receptor. Resulting models converge to one conformation that explains the majority of data from mutation studies and is consistent with the structure-activity relationship for a large number of AR231453 analogs. Another key property of the refined models is their success in separating active ligands from decoys in a large-scale virtual screening. These results demonstrate that mutation-guided receptor modeling can provide predictions of practical value for describing receptor-ligand interactions and drug discovery.