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

Past and present of beta arrestins: A new perspective on insulin secretion and effect

BACKGROUND

Beta arrestins had been known as intracellular adaptors that uncouple and inactivate the G protein-coupled receptors that they interact with. Their roles as signal initiators for some receptors have recently been recognized.

SCOPE OF REVIEW

In this review, we focused on their role in mediating metabolic modulation primarily in relation to insulin signaling. Commenced by the upstream receptor, they seem to act like intracellular hubs that divert the metabolic profile of the cell. The amount of metabolic substrates in circulation and their usage/deposition by tissues are controlled by the contribution of all systems in the organism. This control is enabled by the release of hormones such as insulin, glucagon and glucagon-like peptide-1. Intriguingly, some ligands -either agonists or antagonists-of different classes of receptors have preferential properties mediated by β arrestins. This is not surprizing considering that substrate supply and usage should parallel physiological function such as hormone release or muscle contraction.

MAJOR CONCLUSIONS

Available data indicate that β arrestins conduct the regulatory role in insulin secretion and action. They may be good candidates to target when the upstream signal demands the function that may compromise the cell. An example is carvedilol that is protective by preventing the stimulatory effects of excessive catecholamines, stimulates mitochondrial function and has preferential clinical outcomes in metabolic disorders.

Middle-Down Mass Spectrometry Reveals Activity-Modifying Phosphorylation Barcode in a Class C G Protein-Coupled Receptor

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors in humans. They mediate nearly all aspects of human physiology and thus are of high therapeutic interest. GPCR signaling is regulated in space and time by receptor phosphorylation. It is believed that different phosphorylation states are possible for a single receptor, and each encodes for unique signaling outcomes. Methods to determine the phosphorylation status of GPCRs are critical for understanding receptor physiology and signaling properties of GPCR ligands and therapeutics. However, common proteomic techniques have provided limited quantitative information regarding total receptor phosphorylation stoichiometry, relative abundances of isomeric modification states, and temporal dynamics of these parameters. Here, we report a novel middle-down proteomic strategy and parallel reaction monitoring (PRM) to quantify the phosphorylation states of the C-terminal tail of metabotropic glutamate receptor 2 (mGluR2). By this approach, we found that mGluR2 is subject to both basal and agonist-induced phosphorylation at up to four simultaneous sites with varying probability. Using a PRM tandem mass spectrometry methodology, we localized the positions and quantified the relative abundance of phosphorylations following treatment with an agonist. Our analysis showed that phosphorylation within specific regions of the C-terminal tail of mGluR2 is sensitive to receptor activation, and subsequent site-directed mutagenesis of these sites identified key regions which tune receptor sensitivity. This study demonstrates that middle-down purification followed by label-free quantification is a powerful, quantitative, and accessible tool for characterizing phosphorylation states of GPCRs and other challenging proteins.

Oncogenic signaling of the free-fatty acid receptors FFA1 and FFA4 in human breast carcinoma cells

Globally, breast cancer is the most frequent type of cancer in women, and most breast cancer-associated deaths are due to metastasis and recurrence of the disease. Dietary habits, specifically dietary fat intake is a crucial risk factor involved in breast cancer development and progression. Decades of research has revealed that free-fatty acids (FFA) modulate carcinogenic processes through fatty acid metabolism and lipid peroxidation. The ground-breaking discovery of free-fatty acid receptors, which are members of the G-protein coupled receptor (GPCR) superfamily, has led to the realization that FFA can also act via these receptors to modulate carcinogenic effects. The long-chain free-fatty acid receptors FFA1 (previously termed GPR40) and FFA4 (previously termed GPR120) are activated by mono- and polyunsaturated fatty acids including ω-3, 6, and 9 fatty acids. Initial enthusiasm towards the study of these receptors focused on their insulin secretagogue and sensitization effects, and the downstream associated metabolic regulation. However, recent studies have demonstrated that abnormal expression and/or aberrant FFA1/FFA4 signaling are evident in human breast carcinomas, suggesting that FFA receptors could be a promising target in the treatment of breast cancer. The current review discusses the diverse roles of FFA1 and FFA4 in the regulation of cell proliferation, migration, invasion, and chemotherapy resistance in human breast carcinoma cells and tissue.

How Arrestins and GRKs Regulate the Function of Long Chain Fatty Acid Receptors

FFA1 and FFA4, two G protein-coupled receptors that are activated by long chain fatty acids, play crucial roles in mediating many biological functions in the body. As a result, these fatty acid receptors have gained considerable attention due to their potential to be targeted for the treatment of type-2 diabetes. However, the relative contribution of canonical G protein-mediated signalling versus the effects of agonist-induced phosphorylation and interactions with β-arrestins have yet to be fully defined. Recently, several reports have highlighted the ability of β-arrestins and GRKs to interact with and modulate different functions of both FFA1 and FFA4, suggesting that it is indeed important to consider these interactions when studying the roles of FFA1 and FFA4 in both normal physiology and in different disease settings. Here, we discuss what is currently known and show the importance of understanding fully how β-arrestins and GRKs regulate the function of long chain fatty acid receptors.

Accelerated Multiphosphorylated Peptide Synthesis

Preparing phosphorylated peptides with multiple adjacent phosphorylations is synthetically difficult, leads to β-elimination, results in low yields, and is extremely slow. We combined synthetic chemical methodologies with computational studies and engineering approaches to develop a strategy that takes advantage of fast stirring, high temperature, and a very low concentration of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to produce multiphosphorylated peptides at an extremely rapid time and high purity.

Cell Trafficking and Function of G Protein-coupled Receptors

The G protein-coupled receptors (GPCRs) are plasma membrane proteins that function as sensors of changes in the internal and external milieux and play essential roles in health and disease. They are targets of hormones, neurotransmitters, local hormones (autacoids), and a large proportion of the drugs currently used as therapeutics and for "recreational" purposes. Understanding how these receptors signal and are regulated is fundamental for progress in areas such as physiology and pharmacology. This review will focus on what is currently known about their structure, the molecular events that trigger their signaling, and their trafficking to endosomal compartments. GPCR phosphorylation and its role in desensitization (signaling switching) are also discussed. It should be mentioned that the volume of information available is enormous given the large number and variety of GPCRs. However, knowledge is fragmentary even for the most studied receptors, such as the adrenergic receptors. Therefore, we attempt to present a panoramic view of the field, conscious of the risks and limitations (such as oversimplifications and incorrect generalizations). We hope this will provoke further research in the area. It is currently accepted that GPCR internalization plays a role signaling events. Therefore, the processes that allow them to internalize and recycle back to the plasma membrane are briefly reviewed. The functions of cytoskeletal elements (mainly actin filaments and microtubules), the molecular motors implicated in receptor trafficking (myosin, kinesin, and dynein), and the GTPases involved in GPCR internalization (dynamin) and endosomal sorting (Rab proteins), are discussed. The critical role phosphoinositide metabolism plays in regulating these events is also depicted.

Selective phosphorylation of threonine residues defines GPR84-arrestin interactions of biased ligands

GPR84 is an immune cell-expressed, proinflammatory receptor currently being assessed as a therapeutic target in conditions including fibrosis and inflammatory bowel disease. Although it was previously shown that the orthosteric GPR84 activators 2-HTP and 6-OAU promoted its interactions with arrestin-3, a G protein-biased agonist DL-175 did not. Here, we show that replacement of all 21 serine and threonine residues within i-loop 3 of GPR84, but not the two serines in the C-terminal tail, eliminated the incorporation of [32P] and greatly reduced receptor-arrestin-3 interactions promoted by 2-HTP. GPR84 was phosphorylated constitutively on residues Ser221 and Ser224, while various other amino acids are phosphorylated in response to 2-HTP. Consistent with this, an antiserum able to identify pSer221/pSer224 recognized GPR84 from cells treated with and without activators, whereas an antiserum able to identify pThr263/pThr264 only recognized GPR84 after exposure to 2-HTP and not DL-175. Two distinct GPR84 antagonists as well as inhibition of G protein-coupled receptor kinase 2/3 prevented phosphorylation of pThr263/pThr264, but neither strategy affected constitutive phosphorylation of Ser221/Ser224. Furthermore, mutation of residues Thr263 and Thr264 to alanine generated a variant of GPR84 also limited in 2-HTP-induced interactions with arrestin-2 and -3. By contrast, this mutant was unaffected in its capacity to reduce cAMP levels. Taken together, these results define a key pair of threonine residues, regulated only by subsets of GPR84 small molecule activators and by GRK2/3 that define effective interactions with arrestins and provide novel tools to monitor the phosphorylation and functional status of GPR84.

Agonist-induced phosphorylation of orthologues of the orphan receptor GPR35 functions as an activation sensor

G protein-coupled receptor 35 (GPR35) is poorly characterized but nevertheless has been revealed to have diverse roles in areas including lower gut inflammation and pain. The development of novel reagents and tools will greatly enhance analysis of GPR35 functions in health and disease. Here, we used mass spectrometry, mutagenesis, and [³²P] orthophosphate labeling to identify that all five hydroxy-amino acids in the C-terminal tail of human GPR35a became phosphorylated in response to agonist occupancy of the receptor and that, apart from Ser²⁹⁴, each of these contributed to interactions with arretin-3, which inhibits further G protein-coupled receptor signaling. We found that Ser³⁰³ was key to such interactions; the serine corresponding to human GPR35a residue 303 also played a dominant role in arrestin-3 interactions for both mouse and rat GPR35. We also demonstrated that fully phospho-site–deficient mutants of human GPR35a and mouse GPR35 failed to interact effectively with arrestin-3, and the human phospho-deficient variant was not internalized from the surface of cells in response to agonist treatment. Even in cells stably expressing species orthologues of GPR35, a substantial proportion of the expressed protein(s) was determined to be immature. Finally, phospho-site–specific antisera targeting the region encompassing Ser³⁰³ in human (Ser³⁰¹ in mouse) GPR35a identified only the mature forms of GPR35 and provided effective sensors of the activation status of the receptors both in immunoblotting and immunocytochemical studies. Such antisera may be useful tools to evaluate target engagement in drug discovery and target validation programs.

Roles of receptor phosphorylation and Rab proteins in G protein-coupled receptor function and trafficking

The G Protein-Coupled Receptors form the most abundant family of membrane proteins and are crucial physiological players in the homeostatic equilibrium, which we define as health. They also participate in the pathogenesis of many diseases and are frequent targets of therapeutic intervention. Considering their importance, it is not surprising that different mechanisms regulate their function, including desensitization, resensitization, internalization, recycling to the plasma membrane, and degradation. These processes are modulated in a highly coordinated and specific way by protein kinases and phosphatases, ubiquitin ligases, protein adaptors, interaction with multifunctional complexes, molecular motors, phospholipid metabolism, and membrane distribution. This review describes significant advances in the study of the regulation of these receptors by phosphorylation and endosomal traffic (where signaling can take place); we revisited the bar code hypothesis and include two additional observations: a) that different phosphorylation patterns seem to be associated with internalization and endosome sorting for recycling or degradation, and b) that, surprisingly, phosphorylation of some G protein-coupled receptors appears to be required for proper receptor insertion into the plasma membrane. Significance Statement G protein-coupled receptor phosphorylation is an early event in desensitization/ signaling switching, endosomal traffic, and internalization. These events seem crucial for receptor responsiveness, cellular localization, and fate (recycling/ degradation) with important pharmacological/ therapeutic implications. Phosphorylation sites vary depending on the cells in which they are expressed and on the stimulus that leads to such covalent modification. Surprisingly, evidence suggests that phosphorylation also seems to be required for proper insertion into the plasma membrane for some receptors.

Structure based prediction of a novel GPR120 antagonist based on pharmacophore screening and molecular dynamics simulations

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Hypothesis of the important residues in conserving the GPR120S “ionic-lock”.

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Computational model targeting W277 and N313 for virtual screening of GPR120S ligands.

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Cpd 9 emerged as a potential GPR120S antagonist and anti-cancer treatment.

The G-protein coupled receptor, GPR120, has ubiquitous expression and multifaceted roles in modulating metabolic and anti-inflammatory processes. Recent implications of its role in cancer progression have presented GPR120 as an attractive oncogenic drug target. GPR120 gene knockdown in breast cancer studies revealed a role of GPR120-induced chemoresistance in epirubicin and cisplatin-induced DNA damage in tumour cells. Higher expression and activation levels of GPR120 is also reported to promote tumour angiogenesis and cell migration in colorectal cancer. Some agonists targeting GPR120 have been reported, such as TUG891 and Compound39, but to date development of small-molecule inhibitors of GPR120 is limited.

Herein, following homology modelling of the receptor a pharmacophore hypothesis was derived from 300 ns all-atomic molecular dynamics (MD) simulations on apo, TUG891-bound and Compound39-bound GPR120S (short isoform) receptor models embedded in a water solvated lipid bilayer system. We performed comparative MD analysis on protein–ligand interactions between the two agonist and apo simulations on the stability of the “ionic lock” – a Class A GPCRs characteristic of receptor activation and inactivation. The detailed analysis predicted that ligand interactions with W277 and N313 are critical to conserve the “ionic-lock” conformation (R136 of Helix 3) and prevent GPR120S receptor activation. The results led to generation of a W277 and N313 focused pharmacophore hypothesis and the screening of the ZINC15 database using ZINCPharmer through the structure-based pharmacophore.

100 ns all-atomic molecular dynamics (MD) simulations were performed on 9 small molecules identified and Cpd 9, (2-hydroxy-N-{4-[(6-hydroxy-2-methylpyrimidin-4-yl) amino] phenyl} benzamide) was predicted to be a small-molecule GPR120S antagonist. The conformational results from the collective all-atomic MD analysis provided structural information for further identification and optimisation of novel druggable inhibitors of GPR120S using this rational design approach, which could have future potential for anti-cancer drug development studies.

Delineating the interactions between the cannabinoid CB2 receptor and its regulatory effectors; β-arrestins and G protein-coupled receptor kinases

BACKGROUND AND PURPOSE

The cannabinoid CB₂ receptor (CB₂ ) is a promising therapeutic target for modulating inflammation. However, little is known surrounding the mechanisms underpinning CB₂ desensitisation and regulation, particularly the role of G protein-coupled receptor kinases (GRKs). Here, we evaluated the role of six GRK isoforms in β-arrestin recruitment to CB₂ . Mutagenesis of several distal C-terminal aspartic acid residues was also performed in an attempt to delineate additional structural elements involved in the regulation of CB₂ .

EXPERIMENTAL APPROACH

In CB₂ -expressing HEK 293 cells, β-arrestin translocation was measured using real-time BRET assays. G protein dissociation BRET assays were performed to assess the activation and desensitisation of CB₂ in the presence of β-arrestin 2.

KEY RESULTS

Overexpression of GRK isoforms 1-6 failed to considerably improve translocation of either β-arrestin 1 or β-arrestin 2 to CB₂ . Consistent with this, inhibition of endogenous GRK2/3 did not substantially reduce β-arrestin 2 translocation. Mutagenesis of C-terminal aspartic acid residues resulted in attenuation of β-arrestin 2 translocation, which translated to a reduction in desensitisation of G protein activation.

CONCLUSION AND IMPLICATIONS

Our findings suggest that CB₂ does not adhere to the classical GPCR regulatory paradigm, entailing GRK- and β-arrestin-mediated desensitisation. Instead, C-terminal aspartic acid residues may act as phospho-mimics to induce β-arrestin activation. This study provides novel insights into the regulatory mechanisms of CB₂ , which may aid in our understanding of drug tolerance and dependence.

Naturally occurring mutations in G protein-coupled receptors associated with obesity and type 2 diabetes mellitus

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors involved in the regulation of almost all known physiological processes. Dysfunctions of GPCR-mediated signaling have been shown to cause various diseases. The prevalence of obesity and type 2 diabetes mellitus (T2DM), two strongly associated disorders, is increasing worldwide, with tremendous economical and health burden. New safer and more efficacious drugs are required for successful weight reduction and T2DM treatment. Multiple GPCRs are involved in the regulation of energy and glucose homeostasis. Mutations in these GPCRs contribute to the development and progression of obesity and T2DM. Therefore, these receptors can be therapeutic targets for obesity and T2DM. Indeed some of these receptors, such as melanocortin-4 receptor and glucagon-like peptide 1 receptor, have provided important new drugs for treating obesity and T2DM. This review will focus on the naturally occurring mutations of several GPCRs associated with obesity and T2DM, especially incorporating recent large genomic data and insights from structure-function studies, providing leads for future investigations.

Pathophysiological regulation of lung function by the free fatty acid receptor FFA4

Increased prevalence of inflammatory airway diseases including asthma and chronic obstructive pulmonary disease (COPD) together with inadequate disease control by current frontline treatments means that there is a need to define therapeutic targets for these conditions. Here, we investigate a member of the G protein-coupled receptor family, FFA4, that responds to free circulating fatty acids including dietary omega-3 fatty acids found in fish oils. We show that FFA4, although usually associated with metabolic responses linked with food intake, is expressed in the lung where it is coupled to G_q/11 signaling. Activation of FFA4 by drug-like agonists produced relaxation of murine airway smooth muscle mediated at least in part by the release of the prostaglandin E₂ (PGE₂) that subsequently acts on EP₂ prostanoid receptors. In normal mice, activation of FFA4 resulted in a decrease in lung resistance. In acute and chronic ozone models of pollution-mediated inflammation and house dust mite and cigarette smoke-induced inflammatory disease, FFA4 agonists acted to reduce airway resistance, a response that was absent in mice lacking expression of FFA4. The expression profile of FFA4 in human lung was similar to that observed in mice, and the response to FFA4/FFA1 agonists similarly mediated human airway smooth muscle relaxation ex vivo. Our study provides evidence that pharmacological targeting of lung FFA4, and possibly combined activation of FFA4 and FFA1, has in vivo efficacy and might have therapeutic value in the treatment of bronchoconstriction associated with inflammatory airway diseases such as asthma and COPD.

Effect of docosahexaenoic acid, phorbol myristate acetate, and insulin on the interaction of the FFA4 (short isoform) receptor with Rab proteins

Human embryonic kidney (HEK) 293 cells were co-transfected with plasmids for the expression of mCherry fluorescent protein-tagged FFA4 receptors and the enhanced green fluorescent protein-tagged Rab proteins involved in retrograde transport and recycling, to study their possible interaction through Förster Resonance Energy Transfer (FRET), under the action of agents that induce FFA4 receptor phosphorylation and internalization through different processes, i.e., the agonist, docosahexaenoic acid, the protein kinase C activator phorbol myristate acetate, and insulin. Data indicate that FFA4 receptor internalization varied depending on the agent that induced the process. Agonist activation (docosahexaenoic acid) induced an association with early endosomes (as suggested by interaction with Rab5) and rapid recycling to the plasma membrane (as indicated by receptor interaction with Rab4). More prolonged agonist stimulation also appears to allow the FFA4 receptors to interact with late endosomes (interaction with Rab9), slow recycling (interaction with Rab 11), and target to degradation (Rab7). Phorbol myristate acetate, triggered a rapid association with early endosomes (Rab5), slow recycling to the plasma membrane (Rab11), and some receptor degradation (Rab7). Insulin-induced FFA4 receptor internalization appears to be associated with interaction with early endosomes (Rab5) and late endosomes (Rab9) and fast and slow recycling to the plasma membrane (Rab4, Rab11). Additionally, we observed that agonist- and PMA-induced FFA4 internalization was markedly reduced by paroxetine, which suggests a possible role of G protein-coupled receptor kinase 2.

Carboxy-Terminal Phosphoregulation of the Long Splice Isoform of Free-Fatty Acid Receptor-4 Mediates β-Arrestin Recruitment and Signaling to ERK1/2

Free-fatty acid receptor-4 (FFA4), previously termed GPR120, is a G protein-coupled receptor (GPCR) for medium and long-chained fatty acids, agonism of which can regulate a myriad of metabolic, sensory, inflammatory, and proliferatory signals. Two alternative splice isoforms of FFA4 exist that differ by the presence of an additional 16 amino acids in the longer (FFA4-L) transcript, which has been suggested to be an intrinsically β-arrestin-biased GPCR. Although the shorter isoform (FFA4-S) has been studied more extensively, very little is known about mechanisms of regulation or signaling of the longer isoform. Because β-arrestin recruitment is dependent on receptor phosphorylation, in the current study, we used the endogenous agonist docosahexaenoic acid (DHA) to examine the mechanisms of FFA4-L phosphorylation, as well as DHA-dependent β-arrestin recruitment and DHA-dependent extracellular-signal regulated kinase-1/2 (ERK1/2) signaling in human embryonic kidney 293 cells. Our results reveal differences in basal phosphorylation of the two FFA4 isoforms, and we show that DHA-mediated phosphorylation of FFA4-L is primarily regulated by G protein-coupled receptor kinase 6, whereas protein kinase-C can also contribute to agonist-induced and heterologous phosphorylation. Moreover, our data demonstrate that FFA4-L phosphorylation occurs on the distal C terminus and is directly responsible for recruitment and interactions with β-arrestin-2. Finally, using CRISPR/Cas9 genome-edited cells, our data reveal that unlike FFA4-S, the longer isoform is unable to facilitate phosphorylation of ERK1/2 in cells that are devoid of β-arrestin-1/2. Together, these results are the first to demonstrate phosphoregulation of FFA4-L as well as the effects of loss of phosphorylation sites on β-arrestin recruitment and ERK1/2 activation. SIGNIFICANCE STATEMENT: Free-fatty acid receptor-4 (FFA4) is a cell-surface G protein-coupled receptor for medium and long-chained fatty acids that can be expressed as distinct short (FFA4-S) or long (FFA4-L) isoforms. Although much is known about FFA4-S, the longer isoform remains virtually unstudied. Here, we reveal the mechanisms of docosahexaenoic acid-induced phosphorylation of FFA4-L and subsequent β-arrestin-2 recruitment and extracellular-signal regulated kinase-1/2 activity.

Sites phosphorylated in human α1B-adrenoceptors in response to noradrenaline and phorbol myristate acetate

Phosphorylation of the human α_1B-adrenergic receptor (fused with the green fluorescent protein) was studied employing the inducible Flp-ln HEK293 T-Rex system for expression. Serine/alanine substitutions were performed in five sites corresponding to those previously identified as phosphorylation targets in the hamster ortholog. Desensitization was decreased in these mutants but receptor phosphorylation was still clearly detected. The protein phosphorylation of the wild-type receptor (fused to the green fluorescent protein) was studied, using mass spectrometry, under baseline and stimulated conditions (noradrenaline or phorbol myristate acetate). Basal phosphorylation was detected at sites located at the intracellular loop 3 and carboxyl terminus, and the number of sites detected increased under agonist activation and stimulation of protein kinase C. The phosphorylation patterns differed under the distinct conditions. Three of the phosphorylation sites detected in this work corresponded to those observed in the hamster receptor. The phosphorylation sites detected included the following: a) at the intracellular loop 3: serines 246, 248, 257, 267, and 277; and threonines 252, 264, and 268, and b) at the carboxyl terminus: serines 396, 400, 402, 406, 423, 425, 427, 455, and 470, and threonines 387, 392, 420, and 475. Our data indicate that complex phosphorylation patterns exist and suggest the possibility that such differences could be relevant in receptor function and subcellular localization.

Receptor tyrosine kinase activation induces free fatty acid 4 receptor phosphorylation, β-arrestin interaction, and internalization

FFA4 (Free Fatty Acid receptor 4, previously known as GPR120) is a G protein-coupled receptor that acts as a sensor of long-chain fatty acids, modulates metabolism, and whose dysfunction participates in endocrine disturbances. FFA4 is known to be phosphorylated and internalized in response to agonists and protein kinase C activation. In this paper report the modulation of this fatty acid receptor by activation of receptor tyrosine kinases. Cell-activation with growth factors (insulin, epidermal growth factor, insulin-like growth factor-I, and platelet-derived growth factor) increases FFA4 phosphorylation in a time- and concentration-dependent fashion. This effect was blocked by inhibitors of protein kinase C and phosphoinositide 3-kinase, suggesting the involvement of these kinases in it. FFA4 phosphorylation did not alter agonist-induced FFA4 calcium signaling, but was associated with decreased ERK 1/2 phosphorylation. In addition, insulin, insulin-like growth factor-I, epidermal growth factor, and to a lesser extent, platelet-derived growth factor, induce receptor internalization. This action of insulin, insulin-like growth factor I, and epidermal growth factor was blocked by inhibitors of protein kinase C and phosphoinositide 3-kinase. Additionally, cell treatment with these growth factors induced FFA4-β-arrestin coimmunoprecipitation. Our results evidenced cross-talk between receptor tyrosine kinases and FFA4 and suggest roles of protein kinase C and phosphoinositide 3-kinase in such a functional interaction.

Updates in the function and regulation of α1 -adrenoceptors

α₁ -Adrenoceptors are seven transmembrane domain GPCRs involved in numerous physiological functions controlled by the endogenous catecholamines, noradrenaline and adrenaline, and targeted by drugs useful in therapeutics. Three separate genes, whose products are named α_1A -, α_1B -, and α_1D - adrenoceptors, encode these receptors. Although the existence of multiple α₁ -adrenoceptors has been acknowledged for almost 25 years, the specific functions regulated by each subtype are still largely unknown. Despite the limited comprehension, the identification of a single class of subtype-selective ligands for the α_1A - adrenoceptors, the so-called α-blockers for prostate dysfunction, has led to major improvement in therapeutics, demonstrating the need for continued efforts in the field. This review article surveys the tissue distribution of the three α₁ -adrenoceptor subtypes in the cardiovascular system, genitourinary system, and CNS, highlighting the functions already identified as mediated by the predominant activation of specific subtypes. In addition, this review covers the recent advances in the understanding of the molecular mechanisms involved in the regulation of each of the α₁ -adrenoceptor subtypes by phosphorylation and interaction with proteins involved in their desensitization and internalization. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

S1P1 receptor phosphorylation, internalization, and interaction with Rab proteins: effects of sphingosine 1-phosphate, FTY720-P, phorbol esters, and paroxetine

Sphingosine 1-phosphate (S1P) and FTY720-phosphate (FTYp) increased intracellular calcium in cells expressing S1P₁ mCherry-tagged receptors; the synthetic agonist was considerably less potent. Activation of protein kinase C by phorbol myristate acetate (PMA) blocked these effects. The three agents induced receptor phosphorylation and internalization, with the action of FTYp being more intense. S1P₁ receptor–Rab protein (GFP-tagged) interaction was studied using FRET. The three agents were able to induce S1P₁ receptor–Rab5 interaction, although with different time courses. S1P₁ receptor–Rab9 interaction was mainly increased by the phorbol ester, whereas S1P₁ receptor–Rab7 interaction was only increased by FTYp and after a 30-min incubation. These actions were not observed using dominant negative (GDP-bound) Rab protein mutants. The data suggested that the three agents induce interaction with early endosomes, but that the natural agonist induced rapid receptor recycling, whereas activation of protein kinase C favored interaction with late endosome and slow recycling and FTYp triggered receptor interaction with vesicles associated with proteasomal/lysosomal degradation. The ability of bisindolylmaleimide I and paroxetine to block some of these actions suggested the activation of protein kinase C was associated mainly with the action of PMA, whereas G protein-coupled receptor kinase (GRK) 2 (GRK2) was involved in the action of the three agents.

Distinct phosphorylation sites/clusters in the carboxyl terminus regulate α1D-adrenergic receptor subcellular localization and signaling

The human α_1D-adrenergic receptor is a seven transmembrane-domain protein that mediates many of the physiological actions of adrenaline and noradrenaline and participates in the development of hypertension and benign prostatic hyperplasia. We recently reported that different phosphorylation patterns control α_1D-adrenergic receptor desensitization. However, to our knowledge, there is no data regarding the role(s) of this receptor's specific phosphorylation residues in its subcellular localization and signaling. In order to address this issue, we mutated the identified phosphorylated residues located on the third intracellular loop and carboxyl tail. In this way, we experimentally confirmed α_1D-AR phosphorylation sites and identified, in the carboxyl tail, two groups of residues in close proximity to each other, as well as two individual residues in the proximal (T442) and distal (S543) regions. Our results indicate that phosphorylation of the distal cluster (T507, S515, S516 and S518) favors α_1D-AR localization at the plasma membrane, i. e., substitution of these residues for non-phosphorylatable amino acids results in the intracellular localization of the receptors, whereas phospho-mimetic substitution allows plasma membrane localization. Moreover, we found that T442 phosphorylation is necessary for agonist- and phorbol ester-induced receptor colocalization with β-arrestins. Additionally, we observed that substitution of intracellular loop 3 phosphorylation sites for non-phosphorylatable amino acids resulted in sustained ERK1/2 activation; additional mutations in the phosphorylated residues in the carboxyl tail did not alter this pattern. In contrast, mobilization of intracellular calcium and receptor internalization appear to be controlled by the phosphorylation of both third-intracellular-loop and carboxyl terminus-domain residues. In summary, our data indicate that a) both the phosphorylation sites present in the third intracellular loop and in the carboxyl terminus participate in triggering calcium signaling and in turning-off α_1D-AR-induced ERK activation; b) phosphorylation of the distal cluster appears to play a role in receptor's plasma membrane localization; and c) T442 appears to play a critical role in receptor phosphorylation and receptor-β-arrestin colocalization.

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.

FFA4 (GPR120) as a fatty acid sensor involved in appetite control, insulin sensitivity and inflammation regulation

Unsaturated long-chain fatty acids have been suggested to be beneficial in the context of cardiovascular disorders based in epidemiologic studies conducted in Greenland and Mediterranean. DHA and EPA are omega-3 polyunsaturated fatty acids that are plentiful in fish oil, and oleic acid is an omega-9 monounsaturated fatty acid, rich in olive oil. Dietary intake of these unsaturated long-chain fatty acids have been associated with insulin sensitivity and weight loss, which contrasts with the impairment of insulin sensitivity and weight gain associated with high intakes of saturated long-chain fatty acids. The recent discovery that free fatty acid receptor 4 (FFA4, also known as GPR120) acts as a sensor for unsaturated long-chain fatty acids started to unveil the molecular mechanisms underlying the beneficial functions played by these unsaturated long-chain fatty acids in various physiological processes, which include the secretions of gastrointestinal peptide hormones and glucose homeostasis. In this review, the physiological roles and therapeutic significance of FFA4 in appetite control, insulin sensitization, and inflammation reduction are discussed in relation to obesity and type 2 diabetes from pharmacological viewpoints.

Free-fatty acid receptor-4 (FFA4) modulates ROS generation and COX-2 expression via the C-terminal β-arrestin phosphosensor in Raw 264.7 macrophages

Agonism of the G protein-coupled free-fatty acid receptor-4 (FFA4) has been shown to promote numerous anti-inflammatory effects in macrophages that arise due to interaction with β-arrestin partner proteins. Humans express functionally distinct short and long FFA4 splice variants, such that FFA4-S signals through Gαq/11 and β-arrestin, while FFA4-L is intrinsically biased solely towards β-arrestin signaling. Recently, we and others have shown that phosphorylation of the FFA4 C-terminal tail is responsible for β-arrestin interactability and signaling. Given the significance of β-arrestin in the anti-inflammatory function of FFA4, the goal of this study was to examine the role of the C-terminal β-arrestin phosphosensor in FFA4 signaling induced by PMA and LPS in murine Raw 264.7 macrophages. Our data reveal for the first time that both FFA4 isoforms modulate PMA-induced ROS generation, and that abolishment of the FFA4-S, but not FFA4-L C-terminal phosphosensor, is detrimental to this effect. Furthermore, we show that while both isoforms reduce PMA-induced expression of COX-2, removal of the FFA4-S phosphosensor significantly decreases this response, suggesting that these effects of FFA4-S are β-arrestin mediated. On the contrary, FFA4-S, as well as the truncated C-terminal congener lacking the β-arrestin phosphosensor were both able to reduce LPS-induced NF-κB activity and ERK1/2 phosphorylation. However, FFA4-L and its corresponding mutant were incapable of modulating either, suggesting that these responses are mediated by G protein coupling. Taken together, our data reveal important structure-function and signaling differences between the two FFA4 isoforms, and for the first time link FFA4 to modulation of ROS in macrophages.

Free Fatty Acid Receptors and Cancer: From Nutrition to Pharmacology

The effects of fatty acids on cancer cells have been studied for decades. The roles of dietary long-chain n-3 polyunsaturated fatty acids, and of microbiome-generated short-chain butyric acid, have been of particular interest over the years. However, the roles of free fatty acid receptors (FFARs) in mediating effects of fatty acids in tumor cells have only recently been examined. In reviewing the literature, the data obtained to date indicate that the long-chain FFARs (FFA1 and FFA4) play different roles than the short-chain FFARs (FFA2 and FFA3). Moreover, FFA1 and FFA4 can in some cases mediate opposing actions in the same cell type. Another conclusion is that different types of cancer cells respond differently to FFAR activation. Currently, the best-studied models are prostate, breast, and colon cancer. FFA1 and FFA4 agonists can inhibit proliferation and migration of prostate and breast cancer cells, but enhance growth of colon cancer cells. In contrast, FFA2 activation can in some cases inhibit proliferation of colon cancer cells. Although the available data are sometimes contradictory, there are several examples in which FFAR agonists inhibit proliferation of cancer cells. This is a unique response to GPCR activation that will benefit from a mechanistic explanation as the field progresses. The development of more selective FFAR agonists and antagonists, combined with gene knockout approaches, will be important for unraveling FFAR-mediated inhibitory effects. These inhibitory actions, mediated by druggable GPCRs, hold promise for cancer prevention and/or therapy.

Noradrenaline, oxymetazoline and phorbol myristate acetate induce distinct functional actions and phosphorylation patterns of α1A-adrenergic receptors

In LNCaP cells that stably express α_1A-adrenergic receptors, oxymetazoline increased intracellular calcium and receptor phosphorylation, however, this agonist was a weak partial agonist, as compared to noradrenaline, for calcium signaling. Interestingly, oxymetazoline-induced receptor internalization and desensitization displayed greater effects than those induced by noradrenaline. Phorbol myristate acetate induced modest receptor internalization and minimal desensitization. α_1A-Adrenergic receptor interaction with β-arrestins (colocalization/coimmunoprecipitation) was induced by noradrenaline and oxymetazoline and, to a lesser extent, by phorbol myristate acetate. Oxymetazoline was more potent and effective than noradrenaline in inducing ERK 1/2 phosphorylation. Mass spectrometric analysis of immunopurified α_1A-adrenergic receptors from cells treated with adrenergic agonists and the phorbol ester clearly showed that phosphorylated residues were present both at the third intracellular loop and at the carboxyl tail. Distinct phosphorylation patterns were observed under the different conditions. The phosphorylated residues were: a) Baseline and all treatments: T233; b) noradrenaline: S220, S227, S229, S246, S250, S389; c) oxymetazoline: S227, S246, S381, T384, S389; and d) phorbol myristate acetate: S246, S250, S258, S351, S352, S401, S402, S407, T411, S413, T451. Our novel data, describing the α_1A-AR phosphorylation sites, suggest that the observed different phosphorylation patterns may participate in defining adrenoceptor localization and action, under the different conditions examined.

Barcoding of GPCR trafficking and signaling through the various trafficking roadmaps by compartmentalized signaling networks

Proper signaling by G protein coupled receptors (GPCR) is dependent on the specific repertoire of transducing, enzymatic and regulatory kinases and phosphatases that shape its signaling output. Activation and signaling of the GPCR through its cognate G protein is impacted by G protein-coupled receptor kinase (GRK)-imprinted "barcodes" that recruit β-arrestins to regulate subsequent desensitization, biased signaling and endocytosis of the GPCR. The outcome of agonist-internalized GPCR in endosomes is also regulated by sequence motifs or "barcodes" within the GPCR that mediate its recycling to the plasma membrane or retention and eventual degradation as well as its subsequent signaling in endosomes. Given the vast number of diverse sequences in GPCR, several trafficking mechanisms for endosomal GPCR have been described. The majority of recycling GPCR, are sorted out of endosomes in a "sequence-dependent pathway" anchored around a type-1 PDZ-binding module found in their C-tails. For a subset of these GPCR, a second "barcode" imprinted onto specific GPCR serine/threonine residues by compartmentalized kinase networks was required for their efficient recycling through the "sequence-dependent pathway". Mutating the serine/threonine residues involved, produced dramatic effects on GPCR trafficking, indicating that they played a major role in setting the trafficking itinerary of these GPCR. While endosomal SNX27, retromer/WASH complexes and actin were required for efficient sorting and budding of all these GPCR, additional proteins were required for GPCR sorting via the second "barcode". Here we will review recent developments in GPCR trafficking in general and the human β1-adrenergic receptor in particular across the various trafficking roadmaps. In addition, we will discuss the role of GPCR trafficking in regulating endosomal GPCR signaling, which promote biochemical and physiological effects that are distinct from those generated by the GPCR signal transduction pathway in membranes.

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.

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.

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.

Protein Kinase C Activation Promotes α1B-Adrenoceptor Internalization and Late Endosome Trafficking through Rab9 Interaction. Role in Heterologous Desensitization

Upon agonist stimulation, α_1B-adrenergic receptors couple to G_q proteins, calcium signaling and protein kinase C activation; subsequently, the receptors are phosphorylated, desensitized, and internalized. Internalization seems to involve scaffolding proteins, such as β-arrestin and clathrin. However, the fine mechanisms that participate remain unsolved. The roles of protein kinase C and the small GTPase, Rab9, in α_1B-AR vesicular traffic were investigated by studying α_1B-adrenergic receptor-Rab protein interactions, using Förster resonance energy transfer (FRET), confocal microscopy, and intracellular calcium quantitation. In human embryonic kidney 293 cells overexpressing Discosoma spp. red fluorescent protein (DsRed)-tagged α_1B-ARs and enhanced green fluorescent protein--tagged Rab proteins, pharmacological protein kinase C activation mimicked α_1B-AR traffic elicited by nonrelated agents, such as sphingosine 1-phosphate (i.e., transient α_1B-AR-Rab5 FRET signal followed by a sustained α_1B-AR-Rab9 interaction), suggesting brief receptor localization in early endosomes and transfer to late endosomes. This latter interaction was abrogated by blocking protein kinase C activity, resulting in receptor retention at the plasma membrane. Similar effects were observed when a dominant-negative Rab9 mutant (Rab9-GDP) was employed. When α_1B-adrenergic receptors that had been mutated at protein kinase C phosphorylation sites (S396A, S402A) were used, phorbol ester-induced desensitization of the calcium response was markedly decreased; however, interaction with Rab9 was only partially decreased and internalization was observed in response to phorbol esters and sphingosine 1-phosphate. Finally, Rab9-GDP expression did not affect adrenergic-mediated calcium response but abolished receptor traffic and altered desensitization. Data suggest that protein kinase C modulates α_1B-adrenergic receptor transfer to late endosomes and that Rab9 regulates this process and participates in G protein-mediated signaling turn-off.

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.

Novel Structural Approaches to Study GPCR Regulation

BACKGROUND

Upon natural agonist or pharmacological stimulation, G protein-coupled receptors (GPCRs) are subjected to posttranslational modifications, such as phosphorylation and ubiquitination. These posttranslational modifications allow protein-protein interactions that turn off and/or switch receptor signaling as well as trigger receptor internalization, recycling or degradation, among other responses. Characterization of these processes is essential to unravel the function and regulation of GPCR.

METHODS

In silico analysis and methods such as mass spectrometry have emerged as novel powerful tools. Both approaches have allowed proteomic studies to detect not only GPCR posttranslational modifications and receptor association with other signaling macromolecules but also to assess receptor conformational dynamics after ligand (agonist/antagonist) association.

RESULTS

this review aims to provide insights into some of these methodologies and to highlight how their use is enhancing our comprehension of GPCR function. We present an overview using data from different laboratories (including our own), particularly focusing on free fatty acid receptor 4 (FFA4) (previously known as GPR120) and α1A- and α1D-adrenergic receptors. From our perspective, these studies contribute to the understanding of GPCR regulation and will help to design better therapeutic agents.

Targeted Elimination of G Proteins and Arrestins Defines Their Specific Contributions to Both Intensity and Duration of G Protein-coupled Receptor Signaling

G protein-coupled receptors (GPCRs) can initiate intracellular signaling cascades by coupling to an array of heterotrimeric G proteins and arrestin adaptor proteins. Understanding the contribution of each of these coupling options to GPCR signaling has been hampered by a paucity of tools to selectively perturb receptor function. Here we employ CRISPR/Cas9 genome editing to eliminate selected G proteins (Gα_q and Gα₁₁) or arrestin2 and arrestin3 from HEK293 cells together with the elimination of receptor phosphorylation sites to define the relative contribution of G proteins, arrestins, and receptor phosphorylation to the signaling outcomes of the free fatty acid receptor 4 (FFA4). A lack of FFA4-mediated elevation of intracellular Ca²⁺ in Gα_q/Gα₁₁-null cells and agonist-mediated receptor internalization in arrestin2/3-null cells confirmed previously reported canonical signaling features of this receptor, thereby validating the genome-edited HEK293 cells. FFA4-mediated ERK1/2 activation was totally dependent on G_q/₁₁ but intriguingly was substantially enhanced for FFA4 receptors lacking sites of regulated phosphorylation. This was not due to a simple lack of desensitization of G_q/₁₁ signaling because the G_q/₁₁-dependent calcium response was desensitized by both receptor phosphorylation and arrestin-dependent mechanisms, whereas a substantially enhanced ERK1/2 response was only observed for receptors lacking phosphorylation sites and not in arrestin2/3-null cells. In conclusion, we validate CRISPR/Cas9 engineered HEK293 cells lacking G_q/₁₁ or arrestin2/3 as systems for GPCR signaling research and employ these cells to reveal a previously unappreciated interplay of signaling pathways where receptor phosphorylation can impact on ERK1/2 signaling through a mechanism that is likely independent of arrestins.

Identification of key phosphorylation sites in PTH1R that determine arrestin3 binding and fine-tune receptor signaling

The parathyroid hormone receptor 1 (PTH1R) is a member of family B of G-protein-coupled receptors (GPCRs), predominantly expressed in bone and kidney where it modulates extracellular Ca²⁺ homeostasis and bone turnover. It is well established that phosphorylation of GPCRs constitutes a key event in regulating receptor function by promoting arrestin recruitment and coupling to G-protein-independent signaling pathways. Mapping phosphorylation sites on PTH1R would provide insights into how phosphorylation at specific sites regulates cell signaling responses and also open the possibility of developing therapeutic agents that could target specific receptor functions. Here, we have used mass spectrometry to identify nine sites of phosphorylation in the C-terminal tail of PTH1R. Mutational analysis revealed identified two clusters of serine and threonine residues (Ser489–Ser495 and Ser501–Thr506) specifically responsible for the majority of PTH(1–34)-induced receptor phosphorylation. Mutation of these residues to alanine did not affect negatively on the ability of the receptor to couple to G-proteins or activate extracellular-signal-regulated kinase 1/2. Using fluorescence resonance energy transfer and bioluminescence resonance energy transfer to monitor PTH(1–34)-induced interaction of PTH1R with arrestin3, we show that the first cluster Ser489–Ser495 and the second cluster Ser501–Thr506 operated in concert to mediate both the efficacy and potency of ligand-induced arrestin3 recruitment. We further demonstrate that Ser503 and Thr504 in the second cluster are responsible for 70% of arrestin3 recruitment and are key determinants for interaction of arrestin with the receptor. Our data are consistent with the hypothesis that the pattern of C-terminal tail phosphorylation on PTH1R may determine the signaling outcome following receptor activation.

Complex Pharmacology of Free Fatty Acid Receptors

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

The Pharmacochaperone Activity of Quinine on Bitter Taste Receptors

Bitter taste is one of the five basic taste sensations which is mediated by 25 bitter taste receptors (T2Rs) in humans. The mechanism of bitter taste signal transduction is not yet elucidated. The cellular processes underlying T2R desensitization including receptor internalization, trafficking and degradation are yet to be studied. Here, using a combination of molecular and pharmacological techniques we show that T2R4 is not internalized upon agonist treatment. Pretreatment with bitter agonist quinine led to a reduction in subsequent quinine-mediated calcium responses to 35 ± 5% compared to the control untreated cells. Interestingly, treatment with different bitter agonists did not cause internalization of T2R4. Instead, quinine treatment led to a 2-fold increase in T2R4 cell surface expression which was sensitive to Brefeldin A, suggesting a novel pharmacochaperone activity of quinine. This phenomenon of chaperone activity of quinine was also observed for T2R7, T2R10, T2R39 and T2R46. Our results suggest that the observed action of quinine for these T2Rs is independent of its agonist activity. This study provides novel insights into the pharmacochaperone activity of quinine and possible mechanism of T2R desensitization, which is of fundamental importance in understanding the mechanism of bitter taste signal transduction.

Free-fatty acid receptor-4 (GPR120): Cellular and molecular function and its role in metabolic disorders

Over the last decade, a subfamily of G protein-coupled receptors that are agonized by endogenous and dietary free-fatty acids (FFA) has been discovered. These free-fatty acid receptors include FFA2 and FFA3, which are agonized by short-chained FFA, as well as FFA1 and FFA4, which are agonized by medium-to-long chained FFA. Ligands for FFA1 and FFA4 comprise the family of long chain polyunsaturated omega-3 fatty acids including α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), suggesting that many of the long-known beneficial effects of these fats may be receptor mediated. In this regard, FFA4 has gathered considerable interest due to its role in ameliorating inflammation, promoting insulin sensitization, and regulating energy metabolism in response to FFA ligands. The goal of this review is to summarize the body of evidence in regard to FFA4 signal transduction, its mechanisms of regulation, and its functional role in a variety of tissues. In addition, recent endeavors toward discovery of small molecules that modulate FFA4 activity are also presented.

Using Biosensors to Study Free Fatty Acid Receptor Pharmacology and Function

The free fatty acid (FFA) family of G protein coupled receptors (GPCRs) has generated significant interest for exploiting its members as potential drug targets. However, unravelling the complex pharmacology of this family of receptors has proven challenging. In recent years the use of biosensor technologies capable of assessing biological functions in living cells, and in real time, has greatly enhanced our ability to study GPCR pharmacology and function. These include genetically encoded sensors that change the intensity or wavelength of light emitted from a bioluminescent or fluorescent protein in response to a stimulus, as well as non-genetically encoded sensors able to measure more global cellular changes, such as mass redistribution within a cell. This chapter will examine how these sensors can be used to study GPCRs, and in particular how they are helping uncover the pharmacology of the FFA family of receptors.

The experimental power of FR900359 to study Gq-regulated biological processes

Despite the discovery of heterotrimeric αβγ G proteins ∼25 years ago, their selective perturbation by cell-permeable inhibitors remains a fundamental challenge. Here we report that the plant-derived depsipeptide FR900359 (FR) is ideally suited to this task. Using a multifaceted approach we systematically characterize FR as a selective inhibitor of Gq/11/14 over all other mammalian Gα isoforms and elaborate its molecular mechanism of action. We also use FR to investigate whether inhibition of Gq proteins is an effective post-receptor strategy to target oncogenic signalling, using melanoma as a model system. FR suppresses many of the hallmark features that are central to the malignancy of melanoma cells, thereby providing new opportunities for therapeutic intervention. Just as pertussis toxin is used extensively to probe and inhibit the signalling of Gi/o proteins, we anticipate that FR will at least be its equivalent for investigating the biological relevance of Gq.

Phosphorylation and Internalization of Lysophosphatidic Acid Receptors LPA1, LPA2, and LPA3

Results

The lysophosphatidic acid receptors LPA₁, LPA₂, and LPA₃ were individually expressed in C9 cells and their signaling and regulation were studied. Agonist-activation increases intracellular calcium concentration in a concentration-dependent fashion. Phorbol myristate acetate markedly inhibited LPA₁- and LPA₃-mediated effect, whereas that mediated by LPA₂ was only partially diminished; the actions of the phorbol ester were inhibited by bisindolylmaleimide I and by overnight incubation with the protein kinase C activator, which leads to down regulation of this protein kinase. Homologous desensitization was also observed for the three LPA receptors studied, with that of LPA₂ receptors being consistently of lesser magnitude; neither inhibition nor down-regulation of protein kinase C exerted any effect on homologous desensitization. Activation of LPA_1–3 receptors induced ERK 1/2 phosphorylation; this effect was markedly attenuated by inhibition of epidermal growth factor receptor tyrosine kinase activity, suggesting growth factor receptor transactivation in this effect. Lysophosphatidic acid and phorbol myristate acetate were able to induce LPA_1–3 phosphorylation, in time- and concentration-dependent fashions. It was also clearly observed that agonists and protein kinase C activation induced internalization of these receptors. Phosphorylation of the LPA₂ subtype required larger concentrations of these agents and its internalization was less intense than that of the other subtypes.

Conclusion

Our data show that these three LPA receptors are phosphoproteins whose phosphorylation state is modulated by agonist-stimulation and protein kinase C-activation and that differences in regulation and cellular localization exist, among the subtypes.

Phospho-selective mechanisms of arrestin conformations and functions revealed by unnatural amino acid incorporation and (19)F-NMR

Specific arrestin conformations are coupled to distinct downstream effectors, which underlie the functions of many G-protein-coupled receptors (GPCRs). Here, using unnatural amino acid incorporation and fluorine-19 nuclear magnetic resonance (¹⁹F-NMR) spectroscopy, we demonstrate that distinct receptor phospho-barcodes are translated to specific β-arrestin-1 conformations and direct selective signalling. With its phosphate-binding concave surface, β-arrestin-1 ‘reads' the message in the receptor phospho-C-tails and distinct phospho-interaction patterns are revealed by ¹⁹F-NMR. Whereas all functional phosphopeptides interact with a common phosphate binding site and induce the movements of finger and middle loops, different phospho-interaction patterns induce distinct structural states of β-arrestin-1 that are coupled to distinct arrestin functions. Only clathrin recognizes and stabilizes GRK2-specific β-arrestin-1 conformations. The identified receptor-phospho-selective mechanism for arrestin conformation and the spacing of the multiple phosphate-binding sites in the arrestin enable arrestin to recognize plethora phosphorylation states of numerous GPCRs, contributing to the functional diversity of receptors.

G-protein-coupled receptors (GPCRs) signal via G proteins or arrestin-mediated pathways; the plasticity of arrestin proteins is thought to underlie their function. Here, the authors use NMR to examine how β-arrestin-1 recognizes different GPCR phospho-barcodes, and how this triggers structural rearrangements to fulfill selective functions.

Determination of GPCR Phosphorylation Status: Establishing a Phosphorylation Barcode

G protein-coupled receptors (GPCRs) are rapidly phosphorylated following agonist occupation in a process that mediates receptor uncoupling from its cognate G protein, a process referred to as desensitization. In addition, this process provides a mechanism by which receptors can engage with arrestin adaptor molecules and couple to downstream signaling pathways. The importance of this regulatory process has been highlighted recently by the understanding that ligands can direct receptor signaling along one pathway in preference to another, the phenomenon of signaling bias that is partly mediated by the phosphorylation status or phosphorylation barcode of the receptor. Methods to determine the phosphorylation status of a GPCR in vitro and in vivo are necessary to understand not only the physiological mechanisms involved in GPCR signaling, but also to fully examine the signaling properties of GPCR ligands. This unit describes detailed methods for determining the overall phosphorylation pattern on a receptor (the phosphorylation barcode), as well as mass spectrometry approaches that can define the precise sites that become phosphorylated. These techniques, coupled with the generation and characterization of receptor phosphorylation-specific antibodies, provide a full palate of techniques necessary to determine the phosphorylation status of any given GPCR subtype.