- Email: [email protected]
GPR55 and the regulation of glucose homeostasis
Accepted Manuscript Title: GPR55 and the regulation of glucose homeostasis Authors: Eva Tudur´ı, Miguel L´opez, Carlos Di´eguez, Angel Nadal, Ruben Nogueiras PII: DOI: Reference:
S1357-2725(17)30088-2 http://dx.doi.org/doi:10.1016/j.biocel.2017.04.010 BC 5114
To appear in:
The International Journal of Biochemistry & Cell Biology
Received date: Revised date: Accepted date:
27-2-2017 25-4-2017 26-4-2017
Please cite this article as: Tudur´ı, Eva., L´opez, Miguel., Di´eguez, Carlos., Nadal, Angel., & Nogueiras, Ruben., GPR55 and the regulation of glucose homeostasis.International Journal of Biochemistry and Cell Biology http://dx.doi.org/10.1016/j.biocel.2017.04.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
GPR55 and the regulation of glucose homeostasis
Eva Tudurí1,2, Miguel López1,2,3, Carlos Diéguez1,2,3, Angel Nadal4, Ruben Nogueiras1,2,3
1
Instituto de Investigaciones Sanitarias (IDIS), CIMUS, University of Santiago de
Compostela, Santiago de Compostela 15782, Spain. 2
CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), Santiago de
Compostela 15706, Spain. 3
Department of Physiology, CIMUS, University of Santiago de Compostela,
Santiago de Compostela 15782, Spain. 4
Instituto de Bioingeniería and CIBER de Diabetes y Enfermedades Metabólicas
Asociadas (CIBERDEM), Universidad Miguel Hernández, Elche, Spain
Running title: GPR55 and glucose Address correspondence to:
Eva Tudurí, Instituto de Investigaciones Sanitarias (IDIS), Centro de Investigaciones Médicas de la Universidad de Santiago (CIMUS), University of Santiago de Compostela; Avda de Barcelona s/n, 15782 Santiago de Compostela (A Coruña), Spain. Email: [email protected] Phone number: +34 881815436
Ruben Nogueiras, Department of Physiology, Centro de Investigaciones Médicas de la Universidad de Santiago (CIMUS), University of Santiago de Compostela, and CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn); Avda de Barcelona s/n, 15782 Santiago de Compostela (A Coruña), Spain. Email: [email protected] Phone number: +34 881815437 1
Abstract Pathophysiological conditions such as obesity and type 2 diabetes (T2D) are reportedly associated to over-activation of the endocannabinoid system (ECS). Therefore, modulation of the ECS offers potential therapeutic benefits on those diseases. GPR55, the receptor for L-α-lysophosphatidylinositol (LPI) that has also affinity for various cannabinoid ligands, is distributed at the central and peripheral level and it is involved in several physiological processes. This review summarizes the localization and role of GPR55 in tissues that are crucial for the regulation of glucose metabolism, and provides an update on its contribution in obesity and insulin resistance. The therapeutic potential of targeting the GPR55 receptor is also discussed.
Keywords: endocannabinoids, insulin, islets, obesity
1.- Introduction Obesity and type 2 diabetes (T2D) are two major chronic non-communicable diseases that are increasing worldwide at an alarming rate; the majority of people with T2D are overweight or obese, and T2D associated with obesity is expected to have doubled by 2030 (1). Obesity is characterized by an excess of body weight in the form of fat, whereas the main feature of T2D is hyperglycemia, caused by defects in insulin secretion from pancreatic β-cells and/or resistance to this hormone. Both obesity and alterations in insulin release are closely related to high blood glucose and free fatty acid levels, and are major risks for cardiovascular disease and other health complications. 2
Growing evidence points to the endocannabinoid system (ECS) as an important regulator of glucose metabolism. The ECS comprises endogenous cannabinoids, metabolizing enzymes that regulate their production and degradation, and two classical cannabinoid receptors: CB1 and CB2. The ECS components can be found in many tissues and cells at the central and peripheral level, and over-activity of the ECS has been observed in humans and animal models with obesity or T2D (2). It was therefore reasonable to hypothesize that the attenuation of the activity of this system would have therapeutic benefit in treating disorders that might have a component of excess appetitive drive or over-activity of the endocannabinoid system, such as obesity. For instance, Rimonabant, a CB1 blocker, was approved for obese and overweight patients with BMI>27 kg/m2 and associated risk factors such as dyslipidemia or type 2 diabetes. However, the presence of undesirable side effects, such as a higher tendency to commit suicide, forced its withdrawal from the human use (3, 4).
One of the more recently identified constituents of the ECS is the GPR55 receptor. GPR55 belongs to the G protein-coupled receptor (GPCR) family, and was firstly cloned by Sawzdargo et al. in 1999 (5). Although several agonists and antagonists of the classical receptors CB1 and CB2 exert actions through GPR55, L-αlysophosphatidylinositol (LPI) is considered its putative endogenous ligand; and binding of LPI to GPR55 induces rapid phosphorylation of ERK and increases intracellular Ca2+ (6). During the last years, numerous publications have described the participation of GPR55 signaling in several physiological and pathophysiological processes including cancer, nociception or inflammation. The present review focuses
3
primarily on the role of the GPR55 receptor in the metabolism of glucose, and synthesizes the evidence found from in vitro and in vivo studies.
2.- GPR55 structure The human GPR55 gene resides on chromosome 2q37 and encodes for the GPR55 receptor, a 319 amino acid protein (5). This receptor shows higher homology with human chemokine receptors (CCR4, 23%), purinoceptors (P2Y5, 29%) and purinoceptor-like orphan receptors (GPR23, 30%; GPR35, 27%) (5) than with the classical cannabinoid CB1 and CB2 receptors (13% and 14%, respectively) (7). When comparing between species, it was revealed that human GPR55 gene sequence shares 75% and 78% homology with rat and mouse, respectively (8). An important difference respect to CB1 and CB2 is that GPR55 lacks the classical cannabinoid binding pocket. Instead, a GPR55 model in its active conformation showed a binding pocket with many hydrophilic residues (contrary to CB1 and CB2 binding pockets, which are highly hydrophobic), and that accommodates ligands that have inverted-L or T shapes and display long and thin profiles (9).
3.- GPR55 expression, activation and turnover GPR55 expression GPR55 mRNA is widely distributed in the CNS. In rodents, Gpr55 expression has been detected in hypothalamus (8), hippocampus (5, 8, 10), striatum (8, 11), forebrain (10), frontal cortex (8, 10) and cerebellum (8, 10), whereas within the human CNS, the
4
GPR55 receptor is expressed in nucleus accumbens (12), hypothalamus (12), striatum (12), caudate nucleus (5, 12) and putamen (5, 12).
GPR55 is also present in key tissues that regulate glucose homeostasis such as liver (13), skeletal muscle (14), white adipose tissue (WAT) (13), gastrointestinal tract (8, 12, 15, 16) and pancreas (17-19). The outcomes stated from rodent studies show differences in the expression of Gpr55 in the gut, being this receptor highly expressed in jejunum and ileum compared to the levels detected in colon or stomach (8). Within the islets of Langerhans, where GPR55 protein levels were reportedly greater in rat than in mouse islets (18), immunohistological studies revealed the presence of GPR55 in the majority of rat β-cells but not in α-cells (18), and in 95% and 16% of mouse insulin-secreting and glucagon-releasing cells, respectively (19). Interestingly, important differences were found between human and rodents; despite the low percentage of mouse α-cells expressing GPR55, 84% of human glucagon-positive cells co-stained for GPR55 (19). GPR55 co-localization with somatostatin was practically absent in rat, mouse or human islets (17-19).
Finally, GPR55 has also been detected in human placenta, lungs and spleen, among others (20).
GPR55 activation Although initially identified as a cannabinoid receptor (8), GPR55 is not only activated by the endogenous cannabinoids anandamide (AEA), 2-arachidonoyglycerol (2-AG) and virodhamine, the phytocannabinoid delta-9-tetrahidrocannabinol (Δ9-THC), the endogenous fatty acid amide that enhances AEA activity N-palmitoyl-ethanolamine
5
(PEA), and several synthetic cannabinoid ligands (O-1602, AM251, HU-210, and CP55940 (8)), but also by the non-cannabinoid phospholipid LPI (6, 21, 22), which is considered the endogenous ligand with the highest affinity for GPR55. The GPR55 receptor belongs to the rhodopsin-like (Class A) family and couples to Gαq, Gα12 and Gα13 proteins (8, 21, 22). Reportedly, GPR55 activation triggers activation of Rhoassociated protein kinase and phospholipase C, phosphorylation of the extracellular signal-regulated kinase (ERK) and increased intracellular Ca2+ (6, 21, 22). Unfortunately, the current pharmacology for GPR55 (which includes potential ligands and signaling) is undetermined, presumably by biased signaling (23).
GPR55 internalization and trafficking Several studies carried out in HEK293 (21, 24), U2OS (24) and MCF-7 (25) have demonstrated that in the absence of an agonist, GPR55 is mainly located on the cell surface and its internalization markedly occurs upon stimulation with the ligand. The GPCR associated sorting protein-1 (GASP-1) has been recently reported to play a crucial role in GPR55 sorting. Experiments performed in GPR55-expressing HEK293 cells exposed to rimonabant or LPI showed that although GASP-1 is not responsible for GPR55 internalization, it targets GRP55 to the lysosomes/degradative pathway (26).
4.- Biological function GPR55 in obesity and insulin resistance Given its wide expression in some key areas of the CNS such as the hypothalamus, important in the control of both energy balance and glucose metabolism, as well as in peripheral metabolic tissues, it could be easily speculated that GPR55 modulates energy balance as other cannabinoid receptors do. Indeed, genetically modified mice with
6
global deletion of GPR55 (GPR55-/- mice) manifest obesity, attributable to reduced voluntary and spontaneous physical activity since no alterations in energy intake were observed (27). In addition, GPR55-/- mice displayed insulin resistance, seemingly due to increased fat mass and liver steatosis (27). Hence, proper GPR55 signaling seems to be necessary for the maintenance of energy balance and insulin sensitivity. In fact, a recent report showed that streptozotocin (STZ)-induced diabetic mice that followed a chronic treatment with a potent GPR55 agonist, namely abnormal cannabidiol (Abn-CBD), displayed improved insulin sensitivity by the end of the study (28). Abn-CBD decreased plasma glucose and increased circulating insulin, compared with saline-treated diabetic controls (28). In line with this, Moreno-Navarrete et al. reported reduced GPR55 protein levels in WAT from two rodent models characterized by displaying obesity and insulin resistance: rats fed a high fat diet (HFD) and mice lacking leptin (ob/ob mice) (13). At the cellular level, the GPR55 agonist O-1602 increased Ca2+ levels and lipid accumulation in 3T3-L1 adipocytes (29).
The role of GPR55 in human obesity seems to be quite different to what has been observed in rodents. Firstly, clinical data have shown high LPI circulating levels in obese females (13). Secondly, in vitro studies showed increased GPR55 protein expression in human adipose explants from obese patients compared to those obtained from lean subjects (13). Interestingly, the obese individuals that also had T2D displayed the highest levels of adipose GPR55. Furthermore, in human adipose tissue explants LPI potentiated the expression of lipogenic genes (13). Altogether, this suggests that the GPR55 receptor plays a potential role not only in human obesity, but also in T2D.
7
Finally, no differences in protein levels of GPR55 have been detected in human livers from glucose tolerant and intolerant patients (13), and GPR55 has also been found in rat skeletal muscle (14); however, it is still unknown whether hepatic and/or muscle GPR55 are directly involved in the regulation of glucose metabolism.
GPR55 in the islets of Langerhans The direct stimulation of islet β-cells with GPR55 agonists increased intracellular Ca2+ concentration and cAMP levels (17, 18, 30) (figure 1). Consequently, insulin release is potentiated as observed in human (19), rat (18) and mouse (18, 19) islets, as well as in the glucose-responsive rat cell line BRIN-BD11 (17), under stimulation with GPR55 ligands. Accordingly, the exposure of BRIN-BD11 cells to a GPR55 antagonist inhibited insulin secretion (17). It is important to note that whereas O-1602 did not produce changes in GSIS in islets isolated from GPR55-/- mice (18, 19), other GPR55 agonists such as LPI and CBD did show increased insulin secretion at high glucose concentrations in ex vivo experiments performed with GPR55-/- islets (19). This suggests that the insulinotropic actions of those compounds are partially independent of GPR55.
Some in vivo studies performed in different animal models also confirmed the participation of GPR55 signaling in the modulation of glucose metabolism. For instance, wild type animals that received a single intraperitoneal injection of GPR55 agonists displayed improved glucose tolerance and enhanced insulin secretion in response to a glucose load (17, 18). Furthermore, a recent publication showed that diabetic STZ-treated mice followed a 28 day treatment with Abn-CBD displayed reduced blood glucose and plasma glucagon, and increased β-cell proliferation,
8
pancreatic insulin content, plasma insulin levels and enhanced glucose tolerance; independently of changes in body weight (28). A separate study that employed mice globally lacking GPR55 described unchanged glucose tolerance without changes in GSIS (27). Overall, multiple reports have demonstrated that the major effect of GPR55 upon activation of β-cell is to potentiate insulin release, which subsequently improves glucose handling.
It has been recently revealed that human α-cells also express GPR55 (19); though, its participation in glucagon release remains yet to be unraveled. Given that GPR55 has a potential role in human obesity and T2D (13), and considering the large number of diabetic patients that display hyperglucagonemia, it would be of interest to decipher whether GPR55 actually modulates glucagon secretion and, if so, under which physiological conditions.
Possible medical applications Although further research is needed, current human data points to GPR55 as a regulator of lipid and glucose metabolism and therefore, it may be used as a potential therapeutic target. Importantly, there are some hurdles that need to be firstly solved. The complex structure of the GPR55 binding pocket complicates the synthesis of robust, selective GPR55 agonists and antagonists. Hence, many GPR55 ligands already employed in research have mediated effects in a GPR55-independent manner. So far, this has not only brought contradictory observations and possible misinterpretations, but also impedes the gain of knowledge about the specific physiological role of GPR55. In addition, and given its wide distribution throughout the body, tissue-specific approaches
9
should be designed for the achievement of particular outcomes and also to prevent undesired off-target adverse effects. This is particularly important since, for instance, inhibition of GPR55 in WAT would potentially contribute to fight obesity, whereas activation of GPR55 in the endocrine pancreas would benefit T2D management. Furthermore, compounds targeting GPR55 will likely brain areas related to mood and/or behavior, and therefore, it might be possible the appearance of adverse psychiatric events as happened with CB1 blockers.
In addition to the relevance of tissue-specificity, another importat aspect to be considered is the difference between species regarding GPR55 expression and signaling, making very difficult the translation of animal data to human research. And finally, the expanding literature on the expression of GPR55 in several types of human tumors as well as its role in metastasis (31-33) is also indeed a challenging handicap, even though the precise role of GPR55 in cancer is not clear yet. Nevertheless, despite presenting some obstacles to be overcome, these possible new strategies deserve further exploration as they might have a significant clinical impact in preventing and managing T2D.
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REFERENCES 1. Chen L, Magliano DJ, Zimmet PZ. The worldwide epidemiology of type 2 diabetes mellitus--present and future perspectives. Nat Rev Endocrinol. 2011;8(4):228-36. 2. Matias I, Gonthier MP, Orlando P, Martiadis V, De Petrocellis L, Cervino C, et al. Regulation, function, and dysregulation of endocannabinoids in models of adipose and betapancreatic cells and in obesity and hyperglycemia. J Clin Endocrinol Metab. 2006;91(8):317180. Epub 2006/05/11. 3. European Medicines Agency E. Assessment Report for Acomplia (Rimonabant). 2009;http://www ema europa eu/docs/en_GB/document_library/EPAR_-_Assessment_Report__Variation/human/000666/WC500021280 pdf (accessed 23 Feb 2017). 4. Bermudez-Silva FJ, Viveros MP, McPartland JM, Rodriguez de Fonseca F. The endocannabinoid system, eating behavior and energy homeostasis: the end or a new beginning? Pharmacology, biochemistry, and behavior. 2010;95(4):375-82. Epub 2010/03/30. 5. Sawzdargo M, Nguyen T, Lee DK, Lynch KR, Cheng R, Heng HH, et al. Identification and cloning of three novel human G protein-coupled receptor genes GPR52, PsiGPR53 and GPR55: GPR55 is extensively expressed in human brain. Brain Res Mol Brain Res. 1999;64(2):193-8. Epub 1999/02/05. 6. Oka S, Nakajima K, Yamashita A, Kishimoto S, Sugiura T. Identification of GPR55 as a lysophosphatidylinositol receptor. Biochem Biophys Res Commun. 2007;362(4):928-34. Epub 2007/09/04. 7. Elbegdorj O, Westkaemper RB, Zhang Y. A homology modeling study toward the understanding of three-dimensional structure and putative pharmacological profile of the Gprotein coupled receptor GPR55. J Mol Graph Model. 2013;39:50-60. Epub 2012/12/12. 8. Ryberg E, Larsson N, Sjogren S, Hjorth S, Hermansson NO, Leonova J, et al. The orphan receptor GPR55 is a novel cannabinoid receptor. British journal of pharmacology. 2007;152(7):1092-101. Epub 2007/09/19. 9. Kotsikorou E, Madrigal KE, Hurst DP, Sharir H, Lynch DL, Heynen-Genel S, et al. Identification of the GPR55 agonist binding site using a novel set of high-potency GPR55 selective ligands. Biochemistry. 2011;50(25):5633-47. Epub 2011/05/04. 10. Wu CS, Chen H, Sun H, Zhu J, Jew CP, Wager-Miller J, et al. GPR55, a G-Protein Coupled Receptor for Lysophosphatidylinositol, Plays a Role in Motor Coordination. PLoS One. 2013;8(4):e60314. Epub 2013/04/09. 11. Sylantyev S, Jensen TP, Ross RA, Rusakov DA. Cannabinoid- and lysophosphatidylinositol-sensitive receptor GPR55 boosts neurotransmitter release at central synapses. Proc Natl Acad Sci U S A. 2013;110(13):5193-8. Epub 2013/03/09. 12. Henstridge CM, Balenga NA, Kargl J, Andradas C, Brown AJ, Irving A, et al. Minireview: recent developments in the physiology and pathology of the lysophosphatidylinositol-sensitive receptor GPR55. Mol Endocrinol. 2011;25(11):1835-48. Epub 2011/10/04. 13. Moreno-Navarrete JM, Catalan V, Whyte L, Diaz-Arteaga A, Vazquez-Martinez R, Rotellar F, et al. The L-alpha-lysophosphatidylinositol/GPR55 system and its potential role in human obesity. Diabetes. 2012;61(2):281-91. Epub 2011/12/20. 14. Simcocks AC, O'Keefe L, Jenkin KA, Mathai ML, Hryciw DH, McAinch AJ. A potential role for GPR55 in the regulation of energy homeostasis. Drug Discov Today. 2014;19(8):114551. 15. Li K, Fichna J, Schicho R, Saur D, Bashashati M, Mackie K, et al. A role for O-1602 and G protein-coupled receptor GPR55 in the control of colonic motility in mice. Neuropharmacology. 2013;71C:255-63. Epub 2013/04/23. 16. Lin XH, Yuece B, Li YY, Feng YJ, Feng JY, Yu LY, et al. A novel CB receptor GPR55 and its ligands are involved in regulation of gut movement in rodents. Neurogastroenterol Motil. 2011;23(9):862-e342. Epub 2011/07/06. 17. McKillop AM, Moran BM, Abdel-Wahab YH, Flatt PR. Evaluation of the insulin releasing and antihyperglycaemic activities of GPR55 lipid agonists using clonal beta-cells, isolated pancreatic islets and mice. British journal of pharmacology. 2013;170(5):978-90. Epub 2013/09/03. 18. Romero-Zerbo SY, Rafacho A, Diaz-Arteaga A, Suarez J, Quesada I, Imbernon M, et al. A role for the putative cannabinoid receptor GPR55 in the islets of Langerhans. J Endocrinol. 2011;211(2):177-85. Epub 2011/09/03. 19. Liu B, Song S, Ruz-Maldonado I, Pingitore A, Huang GC, Baker D, et al. GPR55dependent stimulation of insulin secretion from isolated mouse and human islets of Langerhans. Diabetes Obes Metab. 2016;18(12):1263-73. Epub 2016/08/27. 11
20. Kremshofer J, Siwetz M, Berghold VM, Lang I, Huppertz B, Gauster M. A role for GPR55 in human placental venous endothelial cells. Histochem Cell Biol. 2015;144(1):49-58. 21. Henstridge CM, Balenga NA, Ford LA, Ross RA, Waldhoer M, Irving AJ. The GPR55 ligand L-alpha-lysophosphatidylinositol promotes RhoA-dependent Ca2+ signaling and NFAT activation. FASEB J. 2009;23(1):183-93. Epub 2008/09/02. 22. Lauckner JE, Jensen JB, Chen HY, Lu HC, Hille B, Mackie K. GPR55 is a cannabinoid receptor that increases intracellular calcium and inhibits M current. Proc Natl Acad Sci U S A. 2008;105(7):2699-704. Epub 2008/02/12. 23. Zeng Y, Irvine R, Hiley C. Biased Signalling Might be the Answer to the Inconsistent Pharmacology of GPR55. The FASEB Journal. 2015;29(1 Supplement). 24. Kapur A, Zhao P, Sharir H, Bai Y, Caron MG, Barak LS, et al. Atypical responsiveness of the orphan receptor GPR55 to cannabinoid ligands. J Biol Chem. 2009;284(43):29817-27. Epub 2009/09/03. 25. Ford LA, Roelofs AJ, Anavi-Goffer S, Mowat L, Simpson DG, Irving AJ, et al. A role for L-alpha-lysophosphatidylinositol and GPR55 in the modulation of migration, orientation and polarization of human breast cancer cells. British journal of pharmacology. 2010;160(3):762-71. Epub 2010/07/02. 26. Kargl J, Balenga NA, Platzer W, Martini L, Whistler JL, Waldhoer M. The GPCRassociated sorting protein 1 regulates ligand-induced down-regulation of GPR55. British journal of pharmacology. 2012;165(8):2611-9. Epub 2011/07/02. 27. Meadows A, Lee JH, Wu CS, Wei Q, Pradhan G, Yafi M, et al. Deletion of G-proteincoupled receptor 55 promotes obesity by reducing physical activity. Int J Obes (Lond). 2016;40(3):417-24. 28. McKillop AM, Moran BM, Abdel-Wahab YH, Gormley NM, Flatt PR. Metabolic effects of orally administered small-molecule agonists of GPR55 and GPR119 in multiple low-dose streptozotocin-induced diabetic and incretin-receptor-knockout mice. Diabetologia. 2016;59(12):2674-85. Epub 2016/09/30. 29. Diaz-Arteaga A, Vazquez MJ, Vazquez-Martinez R, Pulido MR, Suarez J, Velasquez DA, et al. The atypical cannabinoid O-1602 stimulates food intake and adiposity in rats. Diabetes Obes Metab. 2012;14(3):234-43. 30. Metz SA. Lysophosphatidylinositol, but not lysophosphatidic acid, stimulates insulin release. A possible role for phospholipase A2 but not de novo synthesis of lysophospholipid in pancreatic islet function. Biochem Biophys Res Commun. 1986;138(2):720-7. 31. Andradas C, Blasco-Benito S, Castillo-Lluva S, Dillenburg-Pilla P, Diez-Alarcia R, Juanes-Garcia A, et al. Activation of the orphan receptor GPR55 by lysophosphatidylinositol promotes metastasis in triple-negative breast cancer. Oncotarget. 2016;7(30):47565-75. Epub 2016/06/25. 32. Kargl J, Andersen L, Hasenohrl C, Feuersinger D, Stancic A, Fauland A, et al. GPR55 promotes migration and adhesion of colon cancer cells indicating a role in metastasis. British journal of pharmacology. 2016;173(1):142-54. Epub 2015/10/06. 33. Falasca M, Ferro R. Role of the lysophosphatidylinositol/GPR55 axis in cancer. Advances in biological regulation. 2016;60:88-93. Epub 2015/11/22.
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Figure 1. Schematic representation of the effects of different GPR55 agonists on intracellular pathways and insulin secretion in a glucose-stimulated β-cell. Some ligands exert their effect in a GPR55-independent manner via other receptor/s (referred as a ?). LPI: L-α-lysophosphatidylinositol; CBD: cannabidiol.
13
S1357-2725(17)30088-2 http://dx.doi.org/doi:10.1016/j.biocel.2017.04.010 BC 5114
To appear in:
The International Journal of Biochemistry & Cell Biology
Received date: Revised date: Accepted date:
27-2-2017 25-4-2017 26-4-2017
Please cite this article as: Tudur´ı, Eva., L´opez, Miguel., Di´eguez, Carlos., Nadal, Angel., & Nogueiras, Ruben., GPR55 and the regulation of glucose homeostasis.International Journal of Biochemistry and Cell Biology http://dx.doi.org/10.1016/j.biocel.2017.04.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
GPR55 and the regulation of glucose homeostasis
Eva Tudurí1,2, Miguel López1,2,3, Carlos Diéguez1,2,3, Angel Nadal4, Ruben Nogueiras1,2,3
1
Instituto de Investigaciones Sanitarias (IDIS), CIMUS, University of Santiago de
Compostela, Santiago de Compostela 15782, Spain. 2
CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), Santiago de
Compostela 15706, Spain. 3
Department of Physiology, CIMUS, University of Santiago de Compostela,
Santiago de Compostela 15782, Spain. 4
Instituto de Bioingeniería and CIBER de Diabetes y Enfermedades Metabólicas
Asociadas (CIBERDEM), Universidad Miguel Hernández, Elche, Spain
Running title: GPR55 and glucose Address correspondence to:
Eva Tudurí, Instituto de Investigaciones Sanitarias (IDIS), Centro de Investigaciones Médicas de la Universidad de Santiago (CIMUS), University of Santiago de Compostela; Avda de Barcelona s/n, 15782 Santiago de Compostela (A Coruña), Spain. Email: [email protected] Phone number: +34 881815436
Ruben Nogueiras, Department of Physiology, Centro de Investigaciones Médicas de la Universidad de Santiago (CIMUS), University of Santiago de Compostela, and CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn); Avda de Barcelona s/n, 15782 Santiago de Compostela (A Coruña), Spain. Email: [email protected] Phone number: +34 881815437 1
Abstract Pathophysiological conditions such as obesity and type 2 diabetes (T2D) are reportedly associated to over-activation of the endocannabinoid system (ECS). Therefore, modulation of the ECS offers potential therapeutic benefits on those diseases. GPR55, the receptor for L-α-lysophosphatidylinositol (LPI) that has also affinity for various cannabinoid ligands, is distributed at the central and peripheral level and it is involved in several physiological processes. This review summarizes the localization and role of GPR55 in tissues that are crucial for the regulation of glucose metabolism, and provides an update on its contribution in obesity and insulin resistance. The therapeutic potential of targeting the GPR55 receptor is also discussed.
Keywords: endocannabinoids, insulin, islets, obesity
1.- Introduction Obesity and type 2 diabetes (T2D) are two major chronic non-communicable diseases that are increasing worldwide at an alarming rate; the majority of people with T2D are overweight or obese, and T2D associated with obesity is expected to have doubled by 2030 (1). Obesity is characterized by an excess of body weight in the form of fat, whereas the main feature of T2D is hyperglycemia, caused by defects in insulin secretion from pancreatic β-cells and/or resistance to this hormone. Both obesity and alterations in insulin release are closely related to high blood glucose and free fatty acid levels, and are major risks for cardiovascular disease and other health complications. 2
Growing evidence points to the endocannabinoid system (ECS) as an important regulator of glucose metabolism. The ECS comprises endogenous cannabinoids, metabolizing enzymes that regulate their production and degradation, and two classical cannabinoid receptors: CB1 and CB2. The ECS components can be found in many tissues and cells at the central and peripheral level, and over-activity of the ECS has been observed in humans and animal models with obesity or T2D (2). It was therefore reasonable to hypothesize that the attenuation of the activity of this system would have therapeutic benefit in treating disorders that might have a component of excess appetitive drive or over-activity of the endocannabinoid system, such as obesity. For instance, Rimonabant, a CB1 blocker, was approved for obese and overweight patients with BMI>27 kg/m2 and associated risk factors such as dyslipidemia or type 2 diabetes. However, the presence of undesirable side effects, such as a higher tendency to commit suicide, forced its withdrawal from the human use (3, 4).
One of the more recently identified constituents of the ECS is the GPR55 receptor. GPR55 belongs to the G protein-coupled receptor (GPCR) family, and was firstly cloned by Sawzdargo et al. in 1999 (5). Although several agonists and antagonists of the classical receptors CB1 and CB2 exert actions through GPR55, L-αlysophosphatidylinositol (LPI) is considered its putative endogenous ligand; and binding of LPI to GPR55 induces rapid phosphorylation of ERK and increases intracellular Ca2+ (6). During the last years, numerous publications have described the participation of GPR55 signaling in several physiological and pathophysiological processes including cancer, nociception or inflammation. The present review focuses
3
primarily on the role of the GPR55 receptor in the metabolism of glucose, and synthesizes the evidence found from in vitro and in vivo studies.
2.- GPR55 structure The human GPR55 gene resides on chromosome 2q37 and encodes for the GPR55 receptor, a 319 amino acid protein (5). This receptor shows higher homology with human chemokine receptors (CCR4, 23%), purinoceptors (P2Y5, 29%) and purinoceptor-like orphan receptors (GPR23, 30%; GPR35, 27%) (5) than with the classical cannabinoid CB1 and CB2 receptors (13% and 14%, respectively) (7). When comparing between species, it was revealed that human GPR55 gene sequence shares 75% and 78% homology with rat and mouse, respectively (8). An important difference respect to CB1 and CB2 is that GPR55 lacks the classical cannabinoid binding pocket. Instead, a GPR55 model in its active conformation showed a binding pocket with many hydrophilic residues (contrary to CB1 and CB2 binding pockets, which are highly hydrophobic), and that accommodates ligands that have inverted-L or T shapes and display long and thin profiles (9).
3.- GPR55 expression, activation and turnover GPR55 expression GPR55 mRNA is widely distributed in the CNS. In rodents, Gpr55 expression has been detected in hypothalamus (8), hippocampus (5, 8, 10), striatum (8, 11), forebrain (10), frontal cortex (8, 10) and cerebellum (8, 10), whereas within the human CNS, the
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GPR55 receptor is expressed in nucleus accumbens (12), hypothalamus (12), striatum (12), caudate nucleus (5, 12) and putamen (5, 12).
GPR55 is also present in key tissues that regulate glucose homeostasis such as liver (13), skeletal muscle (14), white adipose tissue (WAT) (13), gastrointestinal tract (8, 12, 15, 16) and pancreas (17-19). The outcomes stated from rodent studies show differences in the expression of Gpr55 in the gut, being this receptor highly expressed in jejunum and ileum compared to the levels detected in colon or stomach (8). Within the islets of Langerhans, where GPR55 protein levels were reportedly greater in rat than in mouse islets (18), immunohistological studies revealed the presence of GPR55 in the majority of rat β-cells but not in α-cells (18), and in 95% and 16% of mouse insulin-secreting and glucagon-releasing cells, respectively (19). Interestingly, important differences were found between human and rodents; despite the low percentage of mouse α-cells expressing GPR55, 84% of human glucagon-positive cells co-stained for GPR55 (19). GPR55 co-localization with somatostatin was practically absent in rat, mouse or human islets (17-19).
Finally, GPR55 has also been detected in human placenta, lungs and spleen, among others (20).
GPR55 activation Although initially identified as a cannabinoid receptor (8), GPR55 is not only activated by the endogenous cannabinoids anandamide (AEA), 2-arachidonoyglycerol (2-AG) and virodhamine, the phytocannabinoid delta-9-tetrahidrocannabinol (Δ9-THC), the endogenous fatty acid amide that enhances AEA activity N-palmitoyl-ethanolamine
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(PEA), and several synthetic cannabinoid ligands (O-1602, AM251, HU-210, and CP55940 (8)), but also by the non-cannabinoid phospholipid LPI (6, 21, 22), which is considered the endogenous ligand with the highest affinity for GPR55. The GPR55 receptor belongs to the rhodopsin-like (Class A) family and couples to Gαq, Gα12 and Gα13 proteins (8, 21, 22). Reportedly, GPR55 activation triggers activation of Rhoassociated protein kinase and phospholipase C, phosphorylation of the extracellular signal-regulated kinase (ERK) and increased intracellular Ca2+ (6, 21, 22). Unfortunately, the current pharmacology for GPR55 (which includes potential ligands and signaling) is undetermined, presumably by biased signaling (23).
GPR55 internalization and trafficking Several studies carried out in HEK293 (21, 24), U2OS (24) and MCF-7 (25) have demonstrated that in the absence of an agonist, GPR55 is mainly located on the cell surface and its internalization markedly occurs upon stimulation with the ligand. The GPCR associated sorting protein-1 (GASP-1) has been recently reported to play a crucial role in GPR55 sorting. Experiments performed in GPR55-expressing HEK293 cells exposed to rimonabant or LPI showed that although GASP-1 is not responsible for GPR55 internalization, it targets GRP55 to the lysosomes/degradative pathway (26).
4.- Biological function GPR55 in obesity and insulin resistance Given its wide expression in some key areas of the CNS such as the hypothalamus, important in the control of both energy balance and glucose metabolism, as well as in peripheral metabolic tissues, it could be easily speculated that GPR55 modulates energy balance as other cannabinoid receptors do. Indeed, genetically modified mice with
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global deletion of GPR55 (GPR55-/- mice) manifest obesity, attributable to reduced voluntary and spontaneous physical activity since no alterations in energy intake were observed (27). In addition, GPR55-/- mice displayed insulin resistance, seemingly due to increased fat mass and liver steatosis (27). Hence, proper GPR55 signaling seems to be necessary for the maintenance of energy balance and insulin sensitivity. In fact, a recent report showed that streptozotocin (STZ)-induced diabetic mice that followed a chronic treatment with a potent GPR55 agonist, namely abnormal cannabidiol (Abn-CBD), displayed improved insulin sensitivity by the end of the study (28). Abn-CBD decreased plasma glucose and increased circulating insulin, compared with saline-treated diabetic controls (28). In line with this, Moreno-Navarrete et al. reported reduced GPR55 protein levels in WAT from two rodent models characterized by displaying obesity and insulin resistance: rats fed a high fat diet (HFD) and mice lacking leptin (ob/ob mice) (13). At the cellular level, the GPR55 agonist O-1602 increased Ca2+ levels and lipid accumulation in 3T3-L1 adipocytes (29).
The role of GPR55 in human obesity seems to be quite different to what has been observed in rodents. Firstly, clinical data have shown high LPI circulating levels in obese females (13). Secondly, in vitro studies showed increased GPR55 protein expression in human adipose explants from obese patients compared to those obtained from lean subjects (13). Interestingly, the obese individuals that also had T2D displayed the highest levels of adipose GPR55. Furthermore, in human adipose tissue explants LPI potentiated the expression of lipogenic genes (13). Altogether, this suggests that the GPR55 receptor plays a potential role not only in human obesity, but also in T2D.
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Finally, no differences in protein levels of GPR55 have been detected in human livers from glucose tolerant and intolerant patients (13), and GPR55 has also been found in rat skeletal muscle (14); however, it is still unknown whether hepatic and/or muscle GPR55 are directly involved in the regulation of glucose metabolism.
GPR55 in the islets of Langerhans The direct stimulation of islet β-cells with GPR55 agonists increased intracellular Ca2+ concentration and cAMP levels (17, 18, 30) (figure 1). Consequently, insulin release is potentiated as observed in human (19), rat (18) and mouse (18, 19) islets, as well as in the glucose-responsive rat cell line BRIN-BD11 (17), under stimulation with GPR55 ligands. Accordingly, the exposure of BRIN-BD11 cells to a GPR55 antagonist inhibited insulin secretion (17). It is important to note that whereas O-1602 did not produce changes in GSIS in islets isolated from GPR55-/- mice (18, 19), other GPR55 agonists such as LPI and CBD did show increased insulin secretion at high glucose concentrations in ex vivo experiments performed with GPR55-/- islets (19). This suggests that the insulinotropic actions of those compounds are partially independent of GPR55.
Some in vivo studies performed in different animal models also confirmed the participation of GPR55 signaling in the modulation of glucose metabolism. For instance, wild type animals that received a single intraperitoneal injection of GPR55 agonists displayed improved glucose tolerance and enhanced insulin secretion in response to a glucose load (17, 18). Furthermore, a recent publication showed that diabetic STZ-treated mice followed a 28 day treatment with Abn-CBD displayed reduced blood glucose and plasma glucagon, and increased β-cell proliferation,
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pancreatic insulin content, plasma insulin levels and enhanced glucose tolerance; independently of changes in body weight (28). A separate study that employed mice globally lacking GPR55 described unchanged glucose tolerance without changes in GSIS (27). Overall, multiple reports have demonstrated that the major effect of GPR55 upon activation of β-cell is to potentiate insulin release, which subsequently improves glucose handling.
It has been recently revealed that human α-cells also express GPR55 (19); though, its participation in glucagon release remains yet to be unraveled. Given that GPR55 has a potential role in human obesity and T2D (13), and considering the large number of diabetic patients that display hyperglucagonemia, it would be of interest to decipher whether GPR55 actually modulates glucagon secretion and, if so, under which physiological conditions.
Possible medical applications Although further research is needed, current human data points to GPR55 as a regulator of lipid and glucose metabolism and therefore, it may be used as a potential therapeutic target. Importantly, there are some hurdles that need to be firstly solved. The complex structure of the GPR55 binding pocket complicates the synthesis of robust, selective GPR55 agonists and antagonists. Hence, many GPR55 ligands already employed in research have mediated effects in a GPR55-independent manner. So far, this has not only brought contradictory observations and possible misinterpretations, but also impedes the gain of knowledge about the specific physiological role of GPR55. In addition, and given its wide distribution throughout the body, tissue-specific approaches
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should be designed for the achievement of particular outcomes and also to prevent undesired off-target adverse effects. This is particularly important since, for instance, inhibition of GPR55 in WAT would potentially contribute to fight obesity, whereas activation of GPR55 in the endocrine pancreas would benefit T2D management. Furthermore, compounds targeting GPR55 will likely brain areas related to mood and/or behavior, and therefore, it might be possible the appearance of adverse psychiatric events as happened with CB1 blockers.
In addition to the relevance of tissue-specificity, another importat aspect to be considered is the difference between species regarding GPR55 expression and signaling, making very difficult the translation of animal data to human research. And finally, the expanding literature on the expression of GPR55 in several types of human tumors as well as its role in metastasis (31-33) is also indeed a challenging handicap, even though the precise role of GPR55 in cancer is not clear yet. Nevertheless, despite presenting some obstacles to be overcome, these possible new strategies deserve further exploration as they might have a significant clinical impact in preventing and managing T2D.
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Figure 1. Schematic representation of the effects of different GPR55 agonists on intracellular pathways and insulin secretion in a glucose-stimulated β-cell. Some ligands exert their effect in a GPR55-independent manner via other receptor/s (referred as a ?). LPI: L-α-lysophosphatidylinositol; CBD: cannabidiol.
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