Elucidation of signaling and functional activities of an orphan GPCR, GPR81

GPR81 is an orphan G protein-coupled receptor (GPCR) that has a high degree of homology to the nicotinic acid receptor GPR109A. GPR81 expression is highly enriched and specific in adipocytes. However, the function and signaling properties of GPR81 are unknown because of the lack of natural or synthetic ligands. Using chimeric G proteins that convert Gi-coupled receptors to Gq-mediated inositol phosphate (IP) accumulation, we show that GPR81 can constitutively increase IP accumulation in HEK293 cells and suggest that GPR81 couples to the Gi signaling pathway. We also constructed a chimeric receptor that expresses the extracellular domains of cysteinyl leukotriene 2 receptor (CysLT2R) and the intracellular domains of GPR81. We show that the CysLT2R ligand, leukotriene D4 (LTD4), is able to activate this chimeric receptor through activation of the Gi pathway. In addition, LTD4 is able to inhibit lipolysis in adipocytes expressing this chimeric receptor. These results suggest that GPR81 couples to the Gi signaling pathway and that activation of the receptor may regulate adipocyte function and metabolism. Hence, targeting GPR81 may lead to the development of a novel and effective therapy for dyslipidemia and a better side effect profile than nicotinic acid. GPR81 is an orphan G protein-coupled receptor (GPCR) that has a high degree of homology to the nicotinic acid receptor GPR109A. GPR81 expression is highly enriched and specific in adipocytes. However, the function and signaling properties of GPR81 are unknown because of the lack of natural or synthetic ligands. Using chimeric G proteins that convert Gi-coupled receptors to Gq-mediated inositol phosphate (IP) accumulation, we show that GPR81 can constitutively increase IP accumulation in HEK293 cells and suggest that GPR81 couples to the Gi signaling pathway. We also constructed a chimeric receptor that expresses the extracellular domains of cysteinyl leukotriene 2 receptor (CysLT2R) and the intracellular domains of GPR81. We show that the CysLT2R ligand, leukotriene D4 (LTD4), is able to activate this chimeric receptor through activation of the Gi pathway. In addition, LTD4 is able to inhibit lipolysis in adipocytes expressing this chimeric receptor. These results suggest that GPR81 couples to the Gi signaling pathway and that activation of the receptor may regulate adipocyte function and metabolism. Hence, targeting GPR81 may lead to the development of a novel and effective therapy for dyslipidemia and a better side effect profile than nicotinic acid. cysteinyl leukotriene 2 receptor G protein-coupled receptor inositol phosphate leukotriene D4 pertussis toxin Nicotinic acid has been used for the treatment of dyslipidemia for >50 years. The drug improves multiple cardiovascular risk factors, including increase of HDL and reduction of VLDL, LDL, lipoprotein a, and triglycerides (TGs), that overall result in a reduction in mortality (1.Carlson L.A. Nicotinic acid: the broad-spectrum lipid drug. A 50th anniversary review.J. Intern. Med. 2005; 258: 94-114Crossref PubMed Scopus (503) Google Scholar). The receptor for nicotinic acid, termed GPR109A (HM74A in humans and PUMA-G in mice), identified in recent years couples to G proteins of the Gi family and is expressed mainly in adipocytes and immune cells (2.Wise A. Foord S.M. Fraser N.J. Barnes A.A. Elshourbagy N. Eilert M. Ignar D.M. Murdock P.R. Steplewski K. Green A. et al.Molecular identification of high and low affinity receptors for nicotinic acid.J. Biol. Chem. 2003; 278: 9869-9874Abstract Full Text Full Text PDF PubMed Scopus (460) Google Scholar, 3.Tunaru S. Kero J. Schaub A. Wufka C. Blaukat A. Pfeffer K. Offermanns S. PUMA-G and HM74 are receptors for nicotinic acid and mediate its anti-lipolytic effect.Nat. Med. 2003; 9: 352-355Crossref PubMed Scopus (657) Google Scholar, 4.Soga T. Kamohara M. Takasaki J. Matsumoto S. Saito T. Ohishi T. Hiyama H. Matsuo A. Matsushime H. Furuichi K. Molecular identification of nicotinic acid receptor.Biochem. Biophys. Res. Commun. 2003; 303: 364-369Crossref PubMed Scopus (285) Google Scholar). Although the precise mechanism of action for nicotinic acid is unknown, GPR109A-mediated inhibition of lipolysis in adipocytes, which results in a reduction in plasma FFA levels, is postulated to play an important role in the improvement of plasma lipid parameters (5.Pike N.B. Wise A. Identification of a nicotinic acid receptor: is this the molecular target for the oldest lipid-lowering drug?.Curr. Opin. Investig. Drugs. 2004; 5: 271-275PubMed Google Scholar, 6.Offermanns S. The nicotinic acid receptor GPR109A (HM74A or PUMA-G) as a new therapeutic target.Trends Pharmacol. Sci. 2006; 27: 384-390Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar). Changes in plasma FFA levels may also lead to changes in the peripheral tissue responsiveness to insulin as well. There is an observed correlation between increased plasma FFA levels and type 2 diabetes as well as an increased risk of type 2 diabetes in individuals with high plasma FFA levels. Furthermore, increased plasma FFA levels can lead directly to the inhibition of insulin-dependent glucose disposal, increased hepatic gluconeogenesis, and impaired pancreatic islet functions (7.Wang M. Fotsch C. Small-molecule compounds that modulate lipolysis in adipose tissue: targeting strategies and molecular classes.Chem. Biol. 2006; 13: 1019-1027Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar). Indeed, it has been observed that short-term treatment with nicotinic acid and its analogs improves insulin action (6.Offermanns S. The nicotinic acid receptor GPR109A (HM74A or PUMA-G) as a new therapeutic target.Trends Pharmacol. Sci. 2006; 27: 384-390Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar), while sustained reduction in plasma FFAs by treatment with the nicotinic acid analog acipimox improves the clinical course of type 2 diabetes (8.Worm D. Henriksen J.E. Vaag A. Thye-Ronn P. Melander A. Beck-Nielsen H. Pronounced blood glucose-lowering effect of the antilipolytic drug acipimox in noninsulin-dependent diabetes mellitus patients during a 3-day intensified treatment period.J. Clin. Endocrinol. Metab. 1994; 78: 717-721Crossref PubMed Scopus (0) Google Scholar). Therefore, targeting fat tissue and regulating adipocyte lipolysis may be a general strategy to regulate lipid and glucose homeostasis. Despite its great efficacy in improving plasma lipid parameters, nicotinic acid treatment results in a number of undesirable side effects, including flushing in the face and upper body and gastrointestinal upset. Recently, it was reported that in rodents the activation of GPR109A in the epidermal Langerhans cells via the release of prostaglandins mediates the flushing side effect of nicotinic acid (9.Pike N.B. Flushing out the role of GPR109A (HM74A) in the clinical efficacy of nicotinic acid.J. Clin. Invest. 2005; 115: 3400-3403Crossref PubMed Scopus (73) Google Scholar, 10.Benyo Z. Gille A. Kero J. Csiky M. Suchankova M.C. Nusing R.M. Moers A. Pfeffer K. Offermanns S. GPR109A (PUMA-G/HM74A) mediates nicotinic acid-induced flushing.J. Clin. Invest. 2005; 115: 3634-3640Crossref PubMed Scopus (286) Google Scholar). A number of different approaches have been taken to alleviate the side effects, including the use of an extended-release formulation (1.Carlson L.A. Nicotinic acid: the broad-spectrum lipid drug. A 50th anniversary review.J. Intern. Med. 2005; 258: 94-114Crossref PubMed Scopus (503) Google Scholar), receptor-selective agonists/modulators that only activate the antilipolytic pathway while avoiding the activation of the immune-related flush-inducing pathway (11.Richman J.G. Kanemitsu-Parks M. Gaidarov I. Cameron J.S. Griffin P. Zheng H. Guerra N.C. Cham L. Maciejewski-Lenoir D. Behan D.P. et al.Nicotinic acid receptor agonists differentially activate downstream effectors.J. Biol. Chem. 2007; 282: 18028-18036Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar), and combination treatment of nicotinic acid with prostaglandin D2 receptor 1 antagonists (12.Cheng K. Wu T.J. Wu K.K. Sturino C. Metters K. Gottesdiener K. Wright S.D. Wang Z. O'Neill G. Lai E. et al.Antagonism of the prostaglandin D2 receptor 1 suppresses nicotinic acid-induced vasodilation in mice and humans.Proc. Natl. Acad. Sci. USA. 2006; 103: 6682-6687Crossref PubMed Scopus (264) Google Scholar). Another alternative strategy is to investigate other adipocyte Gi-coupled G protein-coupled receptors (GPCRs) that are either not expressed in immune cells or whose activation does not induce flushing. GPR81 is an orphan GPCR that belongs to the same subfamily of receptors as GPR109A and GPR109B (13.Lee D.K. Nguyen T. Lynch K.R. Cheng R. Vanti W.B. Arkhitko O. Lewis T. Evans J.F. George S.R. O'Dowd B.F. Discovery and mapping of ten novel G protein-coupled receptor genes.Gene. 2001; 275: 83-91Crossref PubMed Scopus (165) Google Scholar). GPR81 shares ∼51% sequence identity with GPR109A and is colocalized in humans on the same chromosome, 12q24.31, as GPR109A and GPR109B (6.Offermanns S. The nicotinic acid receptor GPR109A (HM74A or PUMA-G) as a new therapeutic target.Trends Pharmacol. Sci. 2006; 27: 384-390Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar). GPR81 expression was found to be highest in adipose tissue among a number of human tissues tested (2.Wise A. Foord S.M. Fraser N.J. Barnes A.A. Elshourbagy N. Eilert M. Ignar D.M. Murdock P.R. Steplewski K. Green A. et al.Molecular identification of high and low affinity receptors for nicotinic acid.J. Biol. Chem. 2003; 278: 9869-9874Abstract Full Text Full Text PDF PubMed Scopus (460) Google Scholar). Because no ligands have been described for GPR81, the signaling pathway for receptor activation as well as the functions of the receptor are largely unknown. Here, using chimeric G proteins as well as chimeric cysteinyl leukotriene 2 receptor (CysLT2R) and GPR81 receptors, we demonstrate that GPR81 couples to Gi pathways for receptor activation. Furthermore, we show that activation of our chimeric receptor by leukotriene D4 (LTD4) results in the inhibition of lipolysis in adipocytes. These results suggest that activation of adipocyte-specific GPR81 has the potential to promote a similar positive clinical outcome as nicotinic acid treatment, but without the skin-mediated flushing side effect. GPR81 expression in tissues and differentiated adipocytes was analyzed using quantitative PCR. Mouse RNA was from Ambion FirstChoice Total RNA: Mouse Normal Tissue Survey Panel (catalog No. AM7800). Mouse 3T3-L1 preadipocytes were cultured and differentiated into adipocytes as described previously (14.Lee G. Elwood F. McNally J. Weiszmann J. Lindstrom M. Amaral K. Nakamura M. Miao S. Cao P. Learned R.M. et al.T0070907, a selective ligand for peroxisome proliferator-activated receptor gamma, functions as an antagonist of biochemical and cellular activities.J. Biol. Chem. 2002; 277: 19649-19657Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar). The primers used for GPR81 were 5′-TCTTCCTGCCCCTGACAATC-3′, 5′-CCGTCTCAGGCTCCAAACA-3′, and 5′-6FAM-TCTTGTTCTGCTCGGTCAACG-BHQ1-3′; the primers for CysLT2R/GPR81 were 5′-CAACCATCCATCTCCGTATCAG-3′, 5′-TTGTGCAGTTCCTGCTGTTGT-3′, and 5′-6FAM-AATGGAACCAAATGGCACCTTCAGCAAT-BHQ1-3′; the primers for GPR109A were from ABI (catalog No. Mm02620500 s1); and the primers for GAPDH were from ABI (catalog No. 4352932E). Quantitative PCR was performed on Stratagene Mx3000P quantitative PCR machines with the Stratagene Brilliant QRT-PCR Master Mix Kit, 1-Step (catalog No. 600551) using 100 ng RNA/well and normalized with mouse GAPDH (ABI). HEK293 cells were obtained from the American Type Culture Collection. Yttrium silicate scintillation proximity assay beads were obtained from Amersham. Tritiated inositol (50 to 80 Ci/mmol) was obtained from Amersham. Chimeric G proteins, Gα15, Gα16, aequorin, and GPR81, were cloned by PCR using the published sequence data (15.Coward P. Chan S.D. Wada H.G. Humphries G.M. Conklin B.R. Chimeric G proteins allow a high-throughput signaling assay of Gi-coupled receptors.Anal. Biochem. 1999; 270: 242-248Crossref PubMed Scopus (205) Google Scholar) and inserted into the plasmid pcDNA3.1 or pZeo (Gα16). HEK293 cells were dispensed onto a poly-d-lysine tissue culture-treated 96-well plate at a density of 25,000 cells/well. The next day, the cells (∼80–90% confluent) were transfected with 100 ng of GPR81 and 20 ng of G protein (5:1 ratio) or control vector containing the aequorin gene using Lipofectamine2000 according to the manufacturer's instructions. Six hours after transfection, the medium was replaced with inositol-free DMEM/10% dialyzed FCS supplemented with 1 μCi/ml tritiated inositol. After incubation overnight, the medium was replaced with HBSS/0.01% BSA containing 10 mM LiCl and incubated at 37°C for 1 h. The medium was aspirated, and the cells were fixed with ice-cold 20 mM formic acid. After incubation at 4°C for 5 h, the lysates were added to yttrium silicate scintillation proximity assay beads, allowed to settle overnight, and read on a Beckman TopCount scintillation counter. The extracellular, transmembrane, and intracellular domains for CysLT2R and GPR81 were using the The chimeric receptor of the extracellular and domains from CysLT2R and intracellular and the from GPR81. The CysLT2R/GPR81 was by using according to the sequence The chimeric receptor was using PCR and primers and and into vector cells were transfected with CysLT2R/GPR81 and using Lipofectamine2000 GPR81, and were used as A of pertussis toxin was added as to the medium to the cells for h. after transfection, the cells were in 20 of containing F. The cells were for 2 in the with from intracellular the of LTD4 was using the CysLT2R/GPR81 chimeric receptor was cloned into the and of vector S.R. A. A is the of gene expression for adipocyte in Natl. Acad. Sci. USA. PubMed Scopus Google Scholar). The containing the and the CysLT2R/GPR81 chimeric receptor from the vector was and into the of were into the of mice were identified by PCR using primers and which the highest of chimeric receptor was for described in this were by the and of were from fat of CysLT2R/GPR81 mice or mice by No. similar to previously described S. Kero J. Schaub A. Wufka C. Blaukat A. Pfeffer K. Offermanns S. PUMA-G and HM74 are receptors for nicotinic acid and mediate its anti-lipolytic effect.Nat. Med. 2003; 9: 352-355Crossref PubMed Scopus (657) Google Scholar, M. of fat I. of on glucose and Biol. Chem. Full Text PDF PubMed Google Scholar). were with No. with BSA and 1 The adipocytes from mice were onto plate and incubated at 37°C with in the or of Nicotinic acid was added as a positive were from the of the for assay using No. that GPR81 expression is to adipocytes in humans (2.Wise A. Foord S.M. Fraser N.J. Barnes A.A. Elshourbagy N. Eilert M. Ignar D.M. Murdock P.R. Steplewski K. Green A. et al.Molecular identification of high and low affinity receptors for nicotinic acid.J. Biol. Chem. 2003; 278: 9869-9874Abstract Full Text Full Text PDF PubMed Scopus (460) Google Scholar). the potential role of GPR81 and to this expression we analyzed GPR81 expression in a mouse tissue using quantitative results show similar to the for human tissue (2.Wise A. Foord S.M. Fraser N.J. Barnes A.A. Elshourbagy N. Eilert M. Ignar D.M. Murdock P.R. Steplewski K. Green A. et al.Molecular identification of high and low affinity receptors for nicotinic acid.J. Biol. Chem. 2003; 278: 9869-9874Abstract Full Text Full Text PDF PubMed Scopus (460) Google Scholar), GPR81 was expressed mainly in the adipose tissues and that low levels were in other tissues The expression also that GPR81 expression was during the 3T3-L1 in Because GPR81 and GPR109A sequence GPR109A expression was also analyzed from the same tissue as a similar to the for human tissues (2.Wise A. Foord S.M. Fraser N.J. Barnes A.A. Elshourbagy N. Eilert M. Ignar D.M. Murdock P.R. Steplewski K. Green A. et al.Molecular identification of high and low affinity receptors for nicotinic acid.J. Biol. Chem. 2003; 278: 9869-9874Abstract Full Text Full Text PDF PubMed Scopus (460) Google Scholar), GPR109A expression was largely to adipose tissue and GPR109A expression was also during the 3T3-L1 in than for GPR81 of GPR81 in mouse Quantitative PCR of mouse tissue with mouse and mouse taken at different during the mouse 3T3-L1 adipocyte were also the of data from a and results were normalized with mouse Chimeric G proteins are for by Gi-coupled receptors to a Gq-mediated signaling pathway (15.Coward P. Chan S.D. Wada H.G. Humphries G.M. Conklin B.R. Chimeric G proteins allow a high-throughput signaling assay of Gi-coupled receptors.Anal. Biochem. 1999; 270: 242-248Crossref PubMed Scopus (205) Google Scholar, B.R. Z. D. of receptor of to that of Gi PubMed Scopus Google Scholar). for the of Gi-coupled receptor in an inositol phosphate (IP) accumulation The IP assay is than L.A. Zheng proximity assay of inositol in high-throughput of receptor Biochem. 2003; PubMed Scopus Google Scholar). The development of chimeric G proteins by Conklin and B.R. Z. D. of receptor of to that of Gi PubMed Scopus Google Scholar) has a GPCR and on orphan GPCR in in 2 are the results of GPR81 into HEK293 cells with or without chimeric G proteins and of IP accumulation in the of 10 mM LiCl to inhibit inositol In the of chimeric G proteins, GPR81 no IP However, in the of and Gα15, was observed with the vector the highest of which was than the of the next highest of this was not different from that of and also to a The of was that of the other chimeric G is to the of to Gi-coupled receptors in HEK293 the human of Gα15, Gα16, no S. G and G a of receptors to Biol. Chem. 270: Full Text Full Text PDF PubMed Scopus Google Scholar, S. Chan J.S. of and receptors to G of PubMed Scopus Google Scholar). These results suggest that GPR81 couples to the Gi signaling pathway in HEK293 are also with data that the nicotinic acid which is to the GPR81, is also to the Gi signaling to inhibit accumulation (6.Offermanns S. The nicotinic acid receptor GPR109A (HM74A or PUMA-G) as a new therapeutic target.Trends Pharmacol. Sci. 2006; 27: 384-390Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar). Gi-coupled receptors have been to to chimeric G proteins with a of as GPR81 J. D. P. H. A. R. J. H. and S. for orphan and Chem. In of GPR81 in an inositol phosphate (IP) accumulation HEK293 cells were transfected with GPR81 or in combination with chimeric G proteins, Gα15, or vector containing aequorin as a hours after transfection, the cells were incubated with containing 10 mM LiCl to IP accumulation and are of are as of from are and expressed as Because GPR81 is an orphan it is to the receptor signaling and function without to natural or synthetic that GPR81 couples to the Gi pathway and to the potential of the receptor activation on adipocyte we a chimeric receptor between cysteinyl leukotriene CysLT2R A. G. and receptors in and other and Res. 2007; 27: PubMed Scopus Google Scholar, C. J. Evans J.F. S. T. T. of for leukotriene and 2003; PubMed Scopus Google Scholar), and GPR81. The chimeric receptor was to the intracellular and the of GPR81, with the that the receptor signaling that of GPR81. The chimeric receptor also extracellular and domains from CysLT2R to allow the of the chimeric receptor to CysLT2R ligand, LTD4 strategy has well for other T. Wang H. S. G protein signaling from to the 2001; PubMed Scopus Google Scholar, Wang The receptor properties of a G Pharmacol. 2002; PubMed Scopus Google Scholar, H. Wang of and expression of receptor in mouse Pharmacol. 2004; PubMed Scopus Google Scholar, D. S. Wang receptor with chimeric Pharmacol. 2004; PubMed Scopus Google Scholar, D. E. Wang receptors as for of protein and in receptor 2006; PubMed Scopus Google Scholar, J. G. H. of signaling properties of receptor 1 by using the chimeric receptor Natl. Acad. Sci. USA. 2004; PubMed Scopus Google and signaling of cysteinyl leukotriene 2 receptor chimeric receptor. acid sequence of GPR81, and chimeric CysLT2R/GPR81 receptors by the cells were transfected with containing receptors, chimeric G and was and as from pertussis and signaling of cysteinyl leukotriene 2 receptor chimeric receptor. acid sequence of GPR81, and chimeric CysLT2R/GPR81 receptors by the cells were transfected with containing receptors, chimeric G and was and as from pertussis We next the signaling pathways that are by the CysLT2R/GPR81 chimeric receptor in to LTD4 activation was observed through the and pathways not However, CysLT2R/GPR81 was with into intracellular in to LTD4 treatment, and this signaling was The GPR81 receptor not to LTD4 treatment in this assay a control for the transfected into to LTD4 treatment as described and this was as the CysLT2R to the signaling pathway A. G. and receptors in and other and Res. 2007; 27: PubMed Scopus Google Scholar, C. J. Evans J.F. S. T. T. of for leukotriene and 2003; PubMed Scopus Google Scholar). These results demonstrate that the chimeric receptor through Gi Because the intracellular and the of the chimeric receptor are from GPR81, results that GPR81 is able to through Gi proteins for receptor activation. the potential of GPR81 activation on adipocyte mice CysLT2R/GPR81 of the control of a were with no Furthermore, was no in body or plasma acid levels in not The expression levels of GPR81 and CysLT2R/GPR81 chimeric receptors in the and were in chimeric receptor expression was only in the and not in the The effect of on GPR81 expression was also and a increase was observed the chimeric receptor lipolysis in adipocytes, release was after LTD4 treatment in adipocytes from either or CysLT2R/GPR81 Nicotinic acid was used as a positive control in in nicotinic acid treatment in the inhibition of lipolysis in and adipocytes to a similar in a In LTD4 treatment only lipolysis in adipocytes from CysLT2R/GPR81 but not in cells from The of antilipolytic was similar between LTD4 and nicotinic acid These results suggest that the activation of GPR81 receptor in adipocytes may also lead to the inhibition of D4 (LTD4) lipolysis in adipocytes expressing CysLT2R/GPR81 chimeric receptor. levels of CysLT2R/GPR81 chimeric receptor and GPR81 in the fat of or mice were by quantitative PCR and normalized to mouse are of data from different from adipocytes from or CysLT2R/GPR81 mice were with or nicotinic acid. release was in the medium using are of are as of D4 (LTD4) lipolysis in adipocytes expressing CysLT2R/GPR81 chimeric receptor. levels of CysLT2R/GPR81 chimeric receptor and GPR81 in the fat of or mice were by quantitative PCR and normalized to mouse are of data from different from adipocytes from or CysLT2R/GPR81 mice were with or nicotinic acid. release was in the medium using are of are as of D4 (LTD4) lipolysis in adipocytes expressing CysLT2R/GPR81 chimeric receptor. levels of CysLT2R/GPR81 chimeric receptor and GPR81 in the fat of or mice were by quantitative PCR and normalized to mouse are of data from different from adipocytes from or CysLT2R/GPR81 mice were with or nicotinic acid. release was in the medium using are of are as of is with insulin and which are important cardiovascular risk The identification of and effective for cardiovascular has been the of Nicotinic acid the effective therapy for increased HDL while other cardiovascular risk factors, including VLDL, LDL, lipoprotein a, and However, the therapeutic of nicotinic acid is by its side effect of which is by the expression of GPR109A in immune cells (6.Offermanns S. The nicotinic acid receptor GPR109A (HM74A or PUMA-G) as a new therapeutic target.Trends Pharmacol. Sci. 2006; 27: 384-390Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar). is for other novel effective with side effect In this we the potential signaling pathways and functions of orphan GPR81. from observed in the IP accumulation assay using chimeric G proteins that convert Gi signaling to as well as Gi pathway activation from a chimeric receptor between CysLT2R and GPR81, that GPR81 couples to the Gi family of These with the of antilipolytic of Gi suggest that GPR81 may regulate this in adipocytes as well. Indeed, the activation of adipocytes from expressing the CysLT2R/GPR81 chimeric receptor by LTD4 results in the inhibition of lipolysis in Although our of the lipid-lowering profile of nicotinic acid is from it has been that the mechanism of action the inhibition of adipocyte lipolysis via activation of the Gi pathway. that GPR81 couples to the Gi signaling pathway and that its expression is to adipocytes than we that the activation of GPR81 has potential in dyslipidemia without the flushing side effect observed with nicotinic acid Identification of GPR81 ligands better of its functions in adipocyte and its potential in The the and the for