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.

Multiple AMPK activators inhibit l-carnitine uptake in C2C12 skeletal muscle myotubes

Mutations in the gene that encodes the principal l-carnitine transporter, OCTN2, can lead to a reduced intracellular l-carnitine pool and the disease Primary Carnitine Deficiency. l-Carnitine supplementation is used therapeutically to increase intracellular l-carnitine. As AMPK and insulin regulate fat metabolism and substrate uptake, we hypothesized that AMPK-activating compounds and insulin would increase l-carnitine uptake in C2C12 myotubes. The cells express all three OCTN transporters at the mRNA level, and immunohistochemistry confirmed expression at the protein level. Contrary to our hypothesis, despite significant activation of PKB and 2DG uptake, insulin did not increase l-carnitine uptake at 100 nM. However, l-carnitine uptake was modestly increased at a dose of 150 nM insulin. A range of AMPK activators that increase intracellular calcium content [caffeine (10 mM, 5 mM, 1 mM, 0.5 mM), A23187 (10 μM)], inhibit mitochondrial function [sodium azide (75 μM), rotenone (1 μM), berberine (100 μM), DNP (500 μM)], or directly activate AMPK [AICAR (250 μM)] were assessed for their ability to regulate l-carnitine uptake. All compounds tested significantly inhibited l-carnitine uptake. Inhibition by caffeine was not dantrolene (10 μM) sensitive despite dantrolene inhibiting caffeine-mediated calcium release. Saturation curve analysis suggested that caffeine did not competitively inhibit l-carnitine transport. To assess the potential role of AMPK in this process, we assessed the ability of the AMPK inhibitor Compound C (10 μM) to rescue the effect of caffeine. Compound C offered a partial rescue of l-carnitine uptake with 0.5 mM caffeine, suggesting that AMPK may play a role in the inhibitory effects of caffeine. However, caffeine likely inhibits l-carnitine uptake by alternative mechanisms independently of calcium release. PKA activation or direct interference with transporter function may play a role.

Characterizing pharmacological ligands to study the long-chain fatty acid receptors GPR40/FFA1 and GPR120/FFA4

The free fatty acid receptors (FFA) 1 (previously designated GPR40) and FFA4 (previously GPR120) are two GPCRs activated by saturated and unsaturated longer-chain free fatty acids. With expression patterns and functions anticipated to directly or indirectly promote insulin secretion, provide homeostatic control of blood glucose and improve tissue insulin sensitivity, both receptors are being studied as potential therapeutic targets for the control of type 2 diabetes. Furthermore, genetic and systems biology studies in both humans and mouse models link FFA4 receptors to diabetes and obesity. Although activated by the same group of free fatty acids, FFA1 and FFA4 receptors are not closely related and, while the basis of recognition of fatty acids by FFA1 receptors is similar to that of the short-chain fatty acid receptors FFA2 and FFA3, the amino acid residues involved in endogenous ligand recognition by FFA4 receptors are more akin to those of the sphingosine 1 phosphate receptor S1P1 . Screening and subsequent medicinal chemistry programmes have developed a number of FFA1 receptor selective agonists that are effective in promoting insulin secretion in a glucose concentration-dependent manner, and in lowering blood glucose levels. However, the recent termination of Phase III clinical trials employing TAK-875/fasiglifam has caused a setback and raises important questions over the exact nature and mechanistic causes of the problems. Progress in the identification and development of highly FFA4 receptor-selective pharmacological tools has been less rapid and several issues remain to be clarified to fully validate this receptor as a therapeutic target. Despite this, the ongoing development of a range of novel ligands offers great opportunities to further unravel the contributions of these receptors.

Treatment of type 2 diabetes by free Fatty Acid receptor agonists

Dietary free fatty acids (FFAs), such as ω-3 fatty acids, regulate metabolic and anti-inflammatory processes, with many of these effects attributed to FFAs interacting with a family of G protein-coupled receptors. Selective synthetic ligands for free fatty acid receptors (FFA1-4) have consequently been developed as potential treatments for type 2 diabetes (T2D). In particular, clinical studies show that Fasiglifam, an agonist of the long-chain FFA receptor, FFA1, improved glycemic control and reduced HbA1c levels in T2D patients, with a reduced risk of hypoglycemia. However, this ligand was removed from clinical trials due to potential liver toxicity and determining if this is a target or a ligand-specific feature is now of major importance. Pre-clinical studies also show that FFA4 agonism increases insulin sensitivity, induces weight loss, and reduces inflammation and the metabolic and anti-inflammatory effects of short chain fatty acids (SCFAs) are linked with FFA2 and FFA3 activation. In this review, we therefore show that FFA receptor agonism is a potential clinical target for T2D treatment and discuss ongoing drug development programs within industry and academia aimed at improving the safety and effectiveness of these potential treatments.

AMPK modulates glucose-sensing in insulin-secreting cells by altered phosphotransfer to KATP channels

Glucose-sensing (GS) behaviour in pancreatic β-cells is dependent on ATP-sensitive K(+) channel (KATP) activity, which is controlled by the relative levels of the KATP ligands ATP and ADP, responsible for closing and opening KATP, respectively. However, the mechanism by which β-cells transfer energy status from mitochondria to KATP, and hence to altered electrical excitability and insulin secretion, is presently unclear. Recent work has demonstrated a critical role for AMP-activated protein kinase (AMPK) in GS behaviour of cells. Electrophysiological recordings, coupled with measurements of gene and protein expression were made from rat insulinoma cells to investigate whether AMPK activity regulates this energy transfer process. Using the whole-cell recording configuration with sufficient intracellular ATP to keep KATP closed, raised AMPK activity induced GS electrical behaviour. This effect was prevented by the AMPK inhibitor, compound C and required a phosphotransfer process. Indeed, high levels of intracellular phosphocreatine or the presence of the adenylate kinase (AK) inhibitor AP5A blocked this action of AMPK. Using conditions that maximised AMPK-induced KATP opening, there was a significant increase in AK1, AK2 and UCP2 mRNA expression. Thus we propose that KATP opening in response to lowered glucose concentration requires AMPK activity, perhaps in concert with increased AK and UCP2 to enable mitochondrial-derived ADP signals to be transferred to plasma membrane KATP by phosphotransfer cascades.

Anorexigenic and orexigenic hormone modulation of mammalian target of rapamycin complex 1 activity and the regulation of hypothalamic agouti-related protein mRNA expression

Activation of mammalian target of rapamycin 1 (mTORC1) by nutrients, insulin and leptin leads to appetite suppression (anorexia). Contrastingly, increased AMP-activated protein kinase (AMPK) activity by ghrelin promotes appetite (orexia). However, the interplay between these mechanisms remains poorly defined. The relationship between the anorexigenic hormones, insulin and leptin, and the orexigenic hormone, ghrelin, on mTORC1 signalling was examined using S6 kinase phosphorylation as a marker for changes in mTORC1 activity in mouse hypothalamic GT1-7 cells. Additionally, the contribution of AMPK and mTORC1 signalling in relation to insulin-, leptin- and ghrelin-driven alterations to mouse hypothalamic agouti-related protein (AgRP) mRNA levels was examined. Insulin and leptin increase mTORC1 activity in a phosphoinositide-3-kinase (PI3K)- and protein kinase B (PKB)-dependent manner, compared to vehicle controls, whereas increasing AMPK activity inhibits mTORC1 activity and blocks the actions of the anorexigenic hormones. Ghrelin mediates an AMPK-dependent decrease in mTORC1 activity and increases hypothalamic AgRP mRNA levels, the latter effect being prevented by insulin in an mTORC1-dependent manner. In conclusion, mTORC1 acts as an integration node in hypothalamic neurons for hormone-derived PI3K and AMPK signalling and mediates at least part of the assimilated output of anorexigenic and orexigenic hormone actions in the hypothalamus.

Leptin-dependent phosphorylation of PTEN mediates actin restructuring and activation of ATP-sensitive K+ channels

Leptin activates multiple signaling pathways in cells, including the phosphatidylinositol 3-kinase pathway, indicating a degree of cross-talk with insulin signaling. The exact mechanisms by which leptin alters this signaling pathway and how it relates to functional outputs are unclear at present. A previous study has established that leptin inhibits the activity of the phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10), an important tumor suppressor and modifier of phosphoinositide signaling. In this study we demonstrate that leptin phosphorylates multiple sites on the C-terminal tail of PTEN in hypothalamic and pancreatic beta-cells, an action not replicated by insulin. Inhibitors of the protein kinases CK2 and glycogen synthase kinase 3 (GSK3) block leptin-mediated PTEN phosphorylation. PTEN phosphorylation mutants reveal the critical role these sites play in transmission of the leptin signal to F-actin depolymerization. CK2 and GSK3 inhibitors also prevent leptin-mediated F-actin depolymerization and consequent ATP-sensitive K(+) channel opening. GSK3 kinase activity is inhibited by insulin but not leptin in hypothalamic cells. Both hormones increase N-terminal GSK3 serine phosphorylation, but in hypothalamic cells this action of leptin is transient. Leptin, not insulin, increases GSK3 tyrosine phosphorylation in both cell types. These results demonstrate a significant role for PTEN in leptin signal transmission and identify GSK3 as a potential important signaling node contributing to divergent outputs for these hormones.

Sphingosine‐1‐phosphate and the immunosuppressant, FTY720‐phosphate, regulate detrusor muscle tone

Overactive bladder syndrome (OBS) results from disturbances of bladder function. Bladder smooth muscle (detrusor) exhibits spontaneous rhythmic activity (tone) independent of neurogenic control, which is enhanced in patients with OBS. We have now uncovered a prominent role for the bioactive sphingolipid metabolite, sphingosine-1-phosphate (S1P), in regulating rabbit detrusor smooth muscle tone and contraction. S1P-induced contraction of detrusor muscle was dependent on stretch and intracellular calcium. Although detrusor expresses the S1P receptors S1P1 and S1P2, only S1P2 appeared to be involved in S1P-induced contraction, since SEW2871 (S1P1 agonist) and dihydro-S1P (potent agonist for all S1P receptors except S1P2) were poor contractile agents. In agreement, the S1P2 antagonist JTE013 inhibited S1P-induced contraction. The fast, transient muscle contraction (phasic) mediated by S1P was dependent on phospholipase C (PLC) whereas the slower, sustained contraction (tonic) was not. Surprisingly, the immunosuppressant FTY720-phosphate, an agonist for all S1P receptors except S1P2, had distinct contractile properties and also induced slow, sustained contraction. Thus, FTY720-phosphate and/or S1P may regulate calcium channels in an S1P receptor-independent manner. Collectively, our results demonstrate that S1P may regulate detrusor smooth muscle tone and suggest that dysregulation of complex S1P signaling might contribute to OBS.

The S1P2 receptor negatively regulates platelet-derived growth factor-induced motility and proliferation

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid metabolite, is the ligand for five specific G protein-coupled receptors, named S1P(1) to S1P(5). In this study, we found that cross-communication between platelet-derived growth factor receptor and S1P(2) serves as a negative damper of PDGF functions. Deletion of the S1P(2) receptor dramatically increased migration of mouse embryonic fibroblasts toward S1P, serum, and PDGF but not fibronectin. This enhanced migration was dependent on expression of S1P(1) and sphingosine kinase 1 (SphK1), the enzyme that produces S1P, as revealed by downregulation of their expression with antisense RNA and small interfering RNA, respectively. Although S1P(2) deletion had no significant effect on tyrosine phosphorylation of the PDGF receptors or activation of extracellular signal-regulated kinase 1/2 or Akt induced by PDGF, it reduced sustained PDGF-dependent p38 phosphorylation and markedly enhanced Rac activation. Surprisingly, S1P(2)-null cells not only exhibited enhanced proliferation but also markedly increased SphK1 expression and activity. Conversely, reintroduction of S1P(2) reduced DNA synthesis and expression of SphK1. Thus, S1P(2) serves as a negative regulator of PDGF-induced migration and proliferation as well as SphK1 expression. Our results suggest that a complex interplay between PDGFR and S1P receptors determines their functions.

Expression of SphK1 impairs degranulation and motility of RBL-2H3 mast cells by desensitizing S1P receptors

Mast cells play a central role in inflammatory and immediate-type allergic reactions by secreting a variety of biologically active substances, including sphingosine-1 phosphate (S1P). Sphingosine kinase 1 (SphK1) and formation of S1P, which leads to transactivation of S1P receptors and their downstream signaling pathways, regulates mast-cell functions initiated by cross-linking of the high-affinity immunoglobulin E (IgE) receptor FcepsilonRI. Surprisingly, overexpression of SphK1 in rat basophilic leukemia (RBL)-2H3 mast cells impaired degranulation as well as migration toward antigen. These effects were reversed by serum withdrawal, yet the increased formation and secretion of S1P were the same as in the presence of serum. Nonetheless, serum increased localization of SphK1 at the plasma membrane. This restricted formation of S1P induced internalization and desensitization of S1P receptors on the surface of mast cells as determined by confocal immunofluorescence microscopy, aberrant S1P receptor signaling, and lack of S1P receptor coupling to G proteins. Serum starvation, which significantly reduced membrane-associated SphK1 activity, restored S1P receptor functions. Our results have important implications for mast-cell migration and degranulation as well as for the biologic functions of the S1P receptors on cells that are circulating in the bloodstream.

The role of sphingosine-1-phosphate in smooth muscle contraction

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that is known to mediate diverse cellular responses including cell growth, survival, and migration. Most of these effects have been attributed to its binding to a specific subfamily of G protein-coupled receptors (GPCR), namely S1P(1-5). Recent studies have suggested that S1P also plays a prominent role in the contraction of various types of smooth muscle. This review provides a brief overview of its role in this process and also highlights how S1P-dependent signaling serves as an important regulator of smooth muscle contraction.

Differential transactivation of sphingosine-1-phosphate receptors modulates NGF-induced neurite extension

The process of neurite extension after activation of the TrkA tyrosine kinase receptor by nerve growth factor (NGF) involves complex signaling pathways. Stimulation of sphingosine kinase 1 (SphK1), the enzyme that phosphorylates sphingosine to form sphingosine-1-phosphate (S1P), is part of the functional TrkA signaling repertoire. In this paper, we report that in PC12 cells and dorsal root ganglion neurons, NGF translocates SphK1 to the plasma membrane and differentially activates the S1P receptors S1P₁ and S1P₂ in a SphK1-dependent manner, as determined with specific inhibitors and small interfering RNA targeted to SphK1. NGF-induced neurite extension was suppressed by down-regulation of S1P₁ expression with antisense RNA. Conversely, when overexpressed in PC12 cells, transactivation of S1P₁ by NGF markedly enhanced neurite extension and stimulation of the small GTPase Rac, important for the cytoskeletal changes required for neurite extension. Concomitantly, differentiation down-regulated expression of S1P₂ whose activation would stimulate Rho and inhibit neurite extension. Thus, differential transactivation of S1P receptors by NGF regulates antagonistic signaling pathways that modulate neurite extension.

Sphingosine-1-phosphate stimulates contraction of human airway smooth muscle cells

The bioactive sphingolipid sphingosine-1-phosphate (S1P) that is increased in airways of asthmatic subjects markedly induced contraction of human airway smooth muscle (HASM) cells embedded in collagen matrices in a Gi-independent manner. Dihydro-S1P, which binds to S1P receptors, also stimulated contractility. S1P induced formation of stress fibers, contraction of individual HASM cells, and stimulated myosin light chain phosphorylation, which was inhibited by the Rho-associated kinase inhibitor Y-27632. S1P-stimulated HASM cell contractility was independent of the ERK1/2 and PKC signaling pathways, important regulators of airway smooth muscle contraction. However, removal of extracellular calcium completely blocked S1P-mediated contraction and Y-27632 reduced it. S1P also induced calcium mobilization that was not desensitized by repeated additions. Pretreatment with thapsigargin to deplete InsP3-sensitive calcium stores partially blocked increases in [Ca2+]i induced by S1P, yet did not inhibit S1P-stimulated contraction. In sharp contrast, the L-type calcium channel blocker verapamil markedly decreased S1P-induced HASM cell contraction, supporting a role for calcium influx from extracellular sources. Collectively, our results suggest that S1P may regulate HASM contractility, important in the pathobiology of asthma.

Subtype-specific regulation of receptor internalization and recycling by the carboxyl-terminal domains of the human A1 and rat A3 adenosine receptors: consequences for agonist-stimulated translocation of arrestin3

In this study, we have characterized the differential effects on inhibitory adenosine receptor (AR) trafficking of disrupting predicted sites for palmitoylation and phosphorylation within each receptor's carboxyl terminus. While a Cys(302,305)Ala-mutated rat A(3)AR mutant internalizes significantly faster than the wild-type (WT) receptor in response to agonist exposure, analogous mutation of the human A(1)AR (Cys(309)Ala) had no effect on receptor internalization. Moreover, unlike the WT A(3)AR, the entire pool of internalized mutant A(3)AR is able to recycle back to the plasma membrane following agonist removal. These properties do not reflect utilization of an alternative trafficking pathway, as internalized WT and mutant A(3)ARs both accumulate into transferrin receptor-positive endosomal compartments. However, receptor accumulation into endosomes is dependent upon prior G-protein-coupled receptor kinase (GRK)-mediated phosphorylation of the receptor's carboxyl terminus, as replacement of the carboxyl-terminal domain of the human A(1)AR with the 14 GRK-phosphorylated amino acids of the rat A(3)AR confers rapid agonist-mediated endosomal accumulation of the resulting chimeric A(1)CT3AR. Sensitivity to GRK-mediated phosphorylation also dictates the distinct redistribution of arrestin3 observed upon agonist exposure. Thus, while the nonphosphorylated A(1)AR redistributes arrestin3 from the cytoplasm to punctate clusters at the plasma membrane, GRK-phosphorylated WT and Cys(302,305)Ala-mutated A(3)ARs, as well as the A(1)CT3AR chimera, each induce the redistribution of arrestin3 into punctate accumulations both at the plasma membrane and within the cytoplasm. Neither the human A(1)AR nor the rat A(3)AR colocalized with arrestin3 under basal or agonist-stimulated conditions. Together, these results demonstrate that inhibitory AR-mediated changes in arrestin3 distribution are subtype-specific, with specificity correlating with the sensitivity of the receptor's carboxyl-terminal domain to GRK phosphorylation. In the case of the rat A(3)AR, sensitivity to GRK-mediated internalization appears to be regulated in part by the integrity of putative palmitate attachment sites upstream of its GRK phosphoacceptor sites.

Dual regulation of EDG1/S1P(1) receptor phosphorylation and internalization by protein kinase C and G-protein-coupled receptor kinase 2

Here we demonstrate that phosphorylation of the sphingosine 1-phosphate (SSP) receptor "endothelial differentiation gene 1" (EDG1 or S1P(1)) receptor is increased in response to either SSP or phorbol 12-myristate 13-acetate (PMA) exposure but not lysophosphatidic acid. Phosphoamino acid analysis demonstrated that SSP stimulated the accumulation of phosphoserine and phosphothreonine but not phosphotyrosine. An inhibitor of PMA-stimulated EDG1 phosphorylation failed to block SSP-stimulated phosphorylation. Additionally, removal of 12 amino acids from the carboxyl terminus of EDG1 specifically reduced SSP- but not PMA-stimulated phosphorylation, suggesting that SSP and PMA increase EDG1 phosphorylation via distinct mechanisms. In vitro assays revealed that G-protein-coupled receptor kinase 2 may be at least partially responsible for SSP-stimulated EDG1 phosphorylation observed in intact cells. In addition, phosphorylation by PMA and SSP were associated with a loss of EDG1 from the cell surface by distinct mechanisms. Removal of 12 residues from the carboxyl terminus of EDG1 completely inhibited SSP-mediated internalization, suggesting that this domain dictates susceptibility to receptor internalization while retaining sensitivity to SSP-stimulated phosphorylation. Thus, we conclude that (a) EDG1 phosphorylation and internalization are controlled via independent mechanisms by agonist occupation of the receptor and protein kinase C activation, and (b) although determinants within the receptor's carboxyl-terminal tail conferring EDG1 sensitivity to agonist-mediated internalization and G-protein-coupled receptor kinase phosphorylation exhibit a degree of overlap, the two phenomena are separable.

Subtype-specific kinetics of inhibitory adenosine receptor internalization are determined by sensitivity to phosphorylation by G protein-coupled receptor kinases

Despite coupling to the same class of inhibitory G proteins and binding the same physiological ligand, the human A(1) and rat A(3) adenosine receptors (ARs) desensitize at different rates in response to sustained agonist exposure. This is due to the ability of the A(3)AR, but not the A(1)AR, to serve as a substrate for rapid phosphorylation and desensitization by members of the G protein-coupled receptor kinase (GRK) family. The aim of this study was to investigate whether these differences were also manifested in their abilities to undergo agonist-dependent receptor internalization. For the first time, we report that A(3)ARs internalize profoundly in response to short-term exposure to agonist but not activators of second messenger-regulated kinases. The A(3)AR-selective antagonist MRS1523 blocked both A(3)AR phosphorylation and internalization. Moreover, in contrast to the A(1)AR, which internalized quite slowly (t(1/2) = 90 min), A(3)ARs internalized rapidly (t(1/2) = 10 min) over a time frame that followed the onset of receptor phosphorylation. A nonphosphorylated A(3)AR mutant failed to internalize over a 60-min time course, suggesting that receptor phosphorylation was essential for rapid A(3)AR internalization to occur. In addition, fusion onto the A(1)AR of the A(3)AR C-terminal domain containing the sites for phosphorylation by GRKs conferred rapid agonist-induced internalization kinetics (t(1/2) = 10 min) on the resulting chimeric AR. In conclusion, these data suggest that GRK-stimulated phosphorylation of threonine residues within the C-terminal domain of the A(3)AR is obligatory to observe rapid agonist-mediated internalization of the receptor.