Cannabinoids Receptors in Liver: Diet and Physiopathology
Valeska Castillo1, Cynthia Barrera1, Rodrigo Troncoso2, Ana Maria Ronco1 and Miguel Llanos1*
1Laboratorio de Nutritióny Regulatión Metabólica, Instituto de Nutritión Y Tecnologia de !os AHmentos, Universidad de Chile, Chile
2Laboratorio de Enfermedades Crónicas Asociadas a la Nutritión, Instituto de Nutritión Y Tecnologia de los Alimentos, Universidad de Chile, Chile
Submission: March 9, 2017; Published: September 14, 2017
*Corresponding author: Miguel N Llanos S, Laboratorio de Nutrition y Regulation Metabolica, Instituto de Nutrition Y Tecnologia de los Alimentos, Universidad de Chile, Macul # 5540, Casilla 138-11, Santiago, Chile, Tel: 56-2-2978-1507; Fax: 56-2-2221-4030; Email: mllanos@inta.uchile.cl
How to cite this article: Valeska C, Cynthia B, Rodrigo T, Ana Marķa R, Miguel Llanos. Cannabinoids Receptors in Liver. Diet and Physiopathology. Curre Res Diabetes & Obes J. 2017; 4(1): 555628. DOI:10.19080/CRDOJ.2017.04.555628
Abstract
The Endocannabinoid system (SEC), located in the central nervous system and several peripheral tissues, is an important modulator of many metabolic functions. This system, is composed by cannabinoid receptors type 1 and 2 (CB1R; CB2R), their endogenous ligands, known as Endocannabinoids, and the enzymes involved in their synthesis and degradation. It has been suggested that a hyperactivated SEC originates metabolic disruptions in several tissues, resulting in typical manifestations of the metabolic syndrome. Liver steatosis due to consumption of a high fat diet is a pathophysiological condition associated to a perturbed SEC. In this condition, it has been shown an increased expression of CB1R and/or higher endocannabinoid levels in hepatic cells, which may exert an autocrine/paracrine stimulation of CB1R and CB2R. Activation of CB1R stimulate the expression of several hepatocyte lipogenic factors, leading to increased de novo fatty acids synthesis and as a consequence, abnormal accumulation of triglycerides. In addition, CB1R activity is necessary for development of insulin and leptin resistance. A role for CB2R in hepatic function is still controversial, because on one side, its stimulation has an interesting protective effect on liver injury, while on the other, may worsen the development of hepatic steatosis and insulin resistance in experimental models of diet-induced obesity. In this minireview, we discuss a suggested sequential interaction of CB1R and CB2R, linking development of steatosis and insulin resistance, associated to a mechanism resulting in a deteriorated function of phosphorylated proteins involved in insulin signaling, due to tyrosine nitration of those proteins.
Keywords: Endocannabinoids; Cannabinoids receptors; Insulin resistance; Steatosis; Hepatic lipogenesis; Protein nitration
Introduction
The endocannabinoid system: general and historical features
Cannabis Sativa has been used for its psychoactive and medicinal properties for millennia. However, knowledge into its mechanism of action has only emerged the last decades. It was first demonstrated that brain had specific binding sites for Δ9-tetrahidrocannabinol (Δ9-THC; the psychoactive compound of cannabis) named type 1-cannabinoid receptors (CB1R) and then cloned [1,2]. Discovery of CB1R, stimulated research to find its endogenous ligand (s). Arachidonoylethanolamide (Anandamide, AEA) a compound discovered and isolated from porcine brain was able to specifically bind CB1R and had a Δ9-THC-like behavior in selected bioassays [3]. This important finding led to the endocannabinoids (ECs) research era, with extensive research activity during the last 25 years. Subsequently, type 2 cannabinoid receptors (CB2R) were then cloned [4,5], and other lipid molecule, 2-arachidonoylglycerol (2-AG), was proposed to be its physiological agonist [6]. At present, some others molecules are considered ECs such as Homo-y-linoleylethanolamide and docosatetraenoylethanolamide, but most of research in this area has been carried out with AEA or its synthetic agonists, and 2-AG. At present, AEA is considered the ligand for CB1R and less selective for CB2R, while 2-AG binds with almost same affinity to CB1R and CB2R [7]. Anandamide and 2-AG derive from membrane phospholipids and are biosynthesized through different pathways [8,9]. Enzymes able to hydrolyze AEA (fatty acid amido hydrolase, FAAH) and 2-AG (monoacylglicerol lipase, MAGL) are becoming very important by their ability to modulate availability of endocannabinoids to exert their endocrine/paracrine actions [10]. Type 1 and 2 cannabinoid/endocannabinoid receptors are G-protein coupled receptors sensitive to pertussis toxin (Gi/o). Classical signaling mechanisms involve modulation of cAMP levels, intracellular free calcium concentration by internal ion mobilization or calcium channels gating regulation, and nitric oxide production [11-15]. Type 1 CBR were first described in brain; subsequently, they have been reported to be present in several peripheral tissues such as pancreas, adipose tissue, gastro-intestinal tract, muscle, heart, thyroid, liver etc., where they are involved in several metabolic actions [16]. The type 2 CBR mainly present in immune and hematopoietic cells have been described more recently in brain (as heteromers together with CB1R), pancreas and liver [14,17-19]. All the molecules previously described constitute the Endocannabinoid System (ECS).
Our review will be focused now in the liver ECS, which has been recently recognized to have pleiotropic functions under physiological and pathological conditions [20,21]. It is well documented now that up regulation of CB1R and elevated AEA levels in liver play an important role in the development of fatty liver associated with ethanol intake, high-fat diet, and obesity [22,23]. Although CB2R up regulation also appears to contribute to fatty liver [24], this finding needs more research to clarify whether CB2R up regulation is a cause or consequence of hepatic steatosis. In our opinion CB2R up regulation is a consequence of a primary CB1R overactivity leading to steatosis as a first signal of liver dysfunction. Thus, exacerbated steatosis should be the alarm signal to up-regulate CB2R, a fact that may be important for hepatoprotection, as previously suggested by Lotersztajn et al. [20]. However, consequences derived from long lasting CB2R over expression and activity deserve further research to have a clear picture of its effects on liver physiology and pathophysiology.
Discussion
CB1R and CB2R in liver, from steatosis to insulin resistance?
Last ten years, studies have been focused to examine the liver and its molecular machinery as a target for metabolic actions of the endocannabinoid system. Hepatocytes express CB1R; these activated receptors stimulate expression of the lipogenesis transcription factor named Steroid Regulatory Element Binding Protein 1c (SREBP-1c) and its targets enzymes Acetyl Coenzyme A carboxilase-1 (ACC1) and Fatty Acid Synthase (FAS) leading to de novo fatty acid synthesis [22]. There is no liver lipogenic response in CB1R-/- mice and SR141716A-treated (antagonist/inverse agonist of CB1R) native mice show decreased hepatic de novo fatty acids synthesis [22]. In addition, a high fat diet contribute to elevate hepatic AEA levels, increase CB1R expression, and as a result, a CB1R-mediated increase in de novo fatty acids synthesis and triglycerides accumulation in hepatocytes [22]. It is important to mention that elevated hepatic AEA levels and subsequent CB1R overactivation is also a consequence of depressed AEA degradation due to decreased FAAH activity and a sustained synthesis rate. Interestingly, we have demonstrated that nociceptive stress during lactation leads to a decreased protein amount and activity of FAAH in adult mice liver concomitant to accumulation of triglycerides [25,26]. Results reported in a liver- specific CB1R knockout mouse (LCB1R-/-), clearly demonstrated that hepatic CB1R are required for development of diet induced hepatic steatosis, dyslipidemia, and insulin and leptin resistance in mice [27]. Furthermore, high fat diet (HFD)-induced elevation of plasma insulin and leptin concentrations with simultaneous hyperglycemia found in native mice, were greatly attenuated in LCB1R-/- mice. More interestingly, deletion of hepatic CB1R led to dissociate obesity from insulin and leptin resistance due to a HFD [27]. The question how hepatic CB1R activation could help to develop a permissive effect in insulin resistance development?, has been an interesting issue to face with different scientific approaches. Previous reports have demonstrated that activation of hepatic CB1R suppresses insulin-induced phosphorylation of Akt-2, increased expression of serine/threonine phosphatases Phlpp1 [28] and specific ceramides species, which are involved in HFD-associated hepatic insulin resistance. More recently, the involvement of CB1R in insulin resistance has been associated to the increased activity of the forkhead box O1 (FoxO1) [29].
The other crucial endocannabinoid/cannabinoid receptor with important implications in liver physiology is the CB2R. It has been reported that CB2R, which are normally undetectable in the liver, are strongly induced by steatosis and non-alcoholic fatty liver disease [30]. In these conditions, they are mainly found in hepatocytes and cholangiocytes, while in cirrhosis they are found in hepatocytes, cholangiocytes, stellate cells and myofibroblasts. Type 2 CBR are also important in Kupffer cells during embryogenesis [23]. In cirrhotic rats, the CB2R agonist JWH-133 improves regenerative response to acute liver injury and decreases fibrosis [31]. Type 2 CBR activity may also have beneficial effects on liver injury due to CCl4-induced hepatitis promoting liver regeneration via a mechanism on hepatocytes originating from myofibroblasts [32,33]. All these antecedents indicate that activation of CB2R by its agonist(s) could play an important role in a paracrine mechanism leading to liver regeneration in cases of liver injury. In this sense, the finding that CB2R are induced by steatosis with a primary function to protect the liver is important but may have future physiological consequences for this tissue. Molecular mechanisms associated to CB2R-mediated liver physiology/pathophysiology may constitute an important issue to develop future research in this area.
Conclusions and perspectives
Even if a slight level of hepatic steatosis becomes chronic, a chronic overactivity of CB2R may be displayed and together with protective effects, a late consequence on liver physiology may arise, such as a state of hepatic insulin resistance. In this regard, it has been reported that CB2R potentiates insulin resistance associated to obesity in a murine obesity model [24]. Thus, in addition to CB2R effects on fat inflammation, presence of CB2R in other insulin sensitive tissues such as skeletal muscle and liver, may also contribute to systemic insulin resistance. In this case, treatments with CB2R antagonists may become a therapeutic contribution to manage obesity and its long term associated metabolic perturbations. In rats, however, the selective CB2R agonist JWH-133 recuperate glucose tolerance, while the compound AM630, a CB2R-antagonist, had opposite effects [34]. Further studies in other animal models, are needed to solve this discrepancy.
Inducible nitric oxide synthase (iNOS) has been identified as the key molecule able to mediate beneficial effects of CB2R in liver [32]. Thus, CB2R knockout mice (CB2R-/-) show decreased induction of hepatic iNOS when challenged with CCl4, and iNOS- /- mice have increased hepatocyte apoptosis when exposed to CCl4 [35]. Interplay of CB2R and nitric oxides synthases has been also demonstrated in remote neurodegeneration due to oxidative and nitrative stress [36]. Although iNOS activity was shown to be beneficial to liver function, a previous report has demonstrated that liver insulin resistance is associated to an increased induction of the hepatic iNOS [37]. This effect was obtained after lipid infusion to wild type mice. Conversely, iNOS-/- mice were protected from hepatic and peripheral insulin resistance when challenged with the lipid infusion. Hepatic insulin resistance was due to tyrosine nitration of insulin signaling molecules instead of optimum extent of tyrosine phosphorylation. Thus, lipid infusion induced tyrosine nitration of insulin receptor p subunit (IRP), insulin receptor substrate (IRS-1, IRS-2) and Akt in wild type mice but not in iNOS-/- mice. Tyrosine nitration of proteins is due to peroxynitrite (ONOO-) action, a compound formed by the quick reaction of nitric oxide (NO) with superoxide radicals (O2-). Although the role of ONOO- in protein tyrosine nitration is still debated, is not less true that it is recognized as the most efficient mechanism for nitrating proteins under biological conditions [38]. It is important to remember that NO overproduction due to induction of iNOS occurring in one type of hepatic cell may exert paracrine actions in other type of hepatic cells, included hepatocytes, due to NO ability to diffuse throughout the whole tissue.
Being increased induction of hepatic iNOS a crucial player in hepatic insulin resistance, an elegant study of Shinozaki et al. [39] has generated a liver-specific iNOS transgenic mice (L-iNOS-Tg), showing that its increased expression is sufficient to cause hepatic insulin resistance. In this case, insulin-stimulated phosphorylation of signaling proteins such as IRS-1 and Akt was diminished in liver but not in skeletal muscle. In this way, it was demonstrated that selective expression of iNOS in liver plays a key role in inducing insulin resistance. Interestingly, L-iNOS-Tg mice also showed mild elevated levels of circulating glucose and insulin, together with peripheral insulin resistance. Under these circumstances, we propose a sequential link involving CB1R-mediated triglycerides accumulation, leading to over expression and activity of CB2R, then, a long lasting induction of iNOS expression and activity and as a result elevated levels of NO available to form ONOO-. Hepatic insulin resistance should be a consequence of abnormally levels of protein phosphorylation and function, through a mechanism involving tyrosine nitration of proteins involved in insulin signaling. This topic is matter of present investigation in our laboratory.
Acknowledgements
This work received financial support from Fondo Nacional de Ciencia y Tecnologia de Chile (FONDECYT) Grant N° 1130106 to MLl.
Conflict of Interest
The authors declare no conflicts of interests
References
- Devane WA, Dysarz FA, Johnson MR, Melvin LS, Howlett AC (1988) Determination and characterization of a cannabinoid receptor in rat brain. Mol Pharmacol 34(5): 605-613.
- Matsuda LA, Lolait SJ, Brownstein MJ, Young AC, Bonner TI (1990) Structure of a cannabinoid receptor and functional expression of the cloned cDNA. Nature 346(6284): 561-564.
- Devane WA, Hanus L, Breuer A, Pertwee RG, Stevenson LA, et al. (1992) Isolation and structure of a brain constituent that binds to the cannabinoid receptor. Science 258(5090): 1946-1949.
- Matsuda LA (1997) Molecular aspects of cannabinoids receptors. Crit Rev Neurobiol 11(2-3): 143-166.
- Munro S, Thomas KL, Abu Shaar M (1993) Molecular characterization of a peripheral receptor for cannabinoids. Nature 365(6441): 61-65.
- Sugiura T, Kondo S, Kishimoto S, Miyashita T, Nakane S, et al. (2000) Evidence that 2-Arachidonoylglycerol but not N-palmitoylethanolamine or anandamide is the physiological ligand for the cannabinoid CB2 receptor. J Biol Chem 275(1): 605-612.
- Pertwee RG, Howlett AC, Abood ME, Alexander SP, Di Marzo V, et al. (2010) International union of basic and clinical pharmacology. LXXIX. Cannabinoid receptor and their ligands: beyond CBa and CB2. Pharmacol Rev 62(4): 588-631.
- Liu J, Wang L, Harvey White J, Osei Hyiaman D, Razdan R, et al. (2006) A biosynthetic pathway for anandamide. Proc Natl Acad Sci USA 103(36): 13345-13350.
- Simon GM, Cravatt BF (2008) Anandamide biosynthesis catalysed by the phosphodiesterase GDE1 and detection of glycerophospho-N-acyl ethanolamine precursors in mouse brain. J Biol Chem 283(14): 93419349.
- Maccarrone M, Dainese E, Oddi S (2010) Intracellular trafficking of anandamide: new concepts for signaling. Trends Biochem Sci 35(11): 601-608.
- Maccarrone M, Bari M, Lorenzon T, Bisogno T, Di Marzo V, et al. (2000) Anandamide uptake by human endothelial cells and its regulation by nitric oxide. J Biol Chem 275(18): 13484-13492.
- Habayeb OM, Bell SC, Konje JC (2002) Endogenous cannabinoids: metabolism and their role in reproduction. Life Sci 70(17): 1963-1977.
- Stefano GB, Esch T, Cadet P, Zhu W, Mantione K, et al. (2003) Endocannabinoids as autoregulatory signaling molecules: coupling to nitric oxide and a possible association with the relaxation response. Med Sci Monit 9(4): RA63- RA75.
- Juan Pico P, Fuentes E, Bermudez Silva FJ, Javier Di'az Molina F, Ripoll C, et al. (2006) Cannabinoids receptors regulate Ca(2+) signals and insulin secretion in pancreatic beta cell. Cell Calcium 39(2): 155-162.
- Ronco AM, Llanos M, Tamayo D, Hirsch S (2007) Anandamide inhibits endothelin-1 production by human cultured endothelial cells: a new vascular action of this endocannabinoid. Pharmacology 79(1): 12-16.
- Matias I, Cristino L, Di Marzo V (2008) Endocannabinoids: some like it fat (and sweet too). J Neuroendocrinol 20(Suppl 1): 100-109.
- Julien B, Grenard P, Teixeira Clerc F, Van Nhieu JT, Li L, et al. (2005) Antifibrogenic role of the cannabinoid receptor CB2 in the liver. Gastroenterology 128(3): 742-755.
- Batkai S, Osei Hyiaman D, Pan H, El Assal O, Rajesh M, et al. (2007) Cannabinoid-2 receptor mediates protection against hepatic ischemia/ reperfusion injury. FASEB J 21(8): 1788-1800.
- Callen L, Moreno E, Barroso Chinea P, Moreno Delgado D, Cortes A, et al. (2012) Cannabinoid receptors CB1 and CB2 form functional heteromers in the brain. J Biol Chem 287(25): 20851-20865.
- Lotersztajn S, Teixeira Clerc F, Julien B, Deveaux V, Ichigotani Y, et al.(2008) CB2 receptors as new therapeutic targets for liver disease. Br J Pharmacol 153(2): 286-289.
- Mallat A, Teixeira Clerc F, Deveaux V, Manin S, Lotersztajn S (2011) The endocannabinoid system as a key mediator during liver diseases: new insights and therapeutic openings. Br J Pharmacol 163(7): 1432-1440.
- Osei Hyiaman D, DePetrillo M, Pacher P, Liu J, Radaeva S, et al. (2005) Endocannabinoid activation at hepatic CB1 receptors stimulates fatty acid synthesis and contributes to diet-induced obesity. J Clin Invest 115(5): 1298-1305.
- Tam J, Liu J, Mukhopadhyay B, Cinar R, Godlewski G, et al. (2011) Endocannabinoids in Liver Disease. Hepatology 53(1): 346-355.
- Deveaux V, Cadoudal T, Ichigotani Y, Teixeira Clerc F, Louvet A, et al. (2009) Cannabinoid CB2 receptor potentiates obesity-associated inflammation, insulin resistance and hepatic steatosis. PLos One 4(6): e5844.
- Valeska AC, Carina AV, Paula P, Ana MR, Miguel NL (2015) Stress during Lactation Affects Fatty Acid Amide Hydrolase Protein Expression in Adipose Tissue and Liver of Adult Mice. Endocrinol Metab Synd 4: 1.
- Valenzuela C, Castillo VA, Aguirre CA, Ronco AM, Llanos MN (2011). The endocannabinoid CB1 receptor antagonist SR141716A reverses adult male mice overweight and metabolic alterations induced by nociceptive stress. Obesity (Silver Spring) 19(1): 29-35.
- Osei Hyiaman D, Liu J, Zhou L, Godlewski G, Harvey White J, et al. (2008) Hepatic CB1 receptor is required for development of diet- induced steatosis, dyslipidemia, and insulin and leptin resistance in mice. J Clin Invest 118(9): 3160-3169.
- Liu J, Zhou L, Xiong K, Godlewski G, Mukhopadhyay B, et al. (2012) Hepatic cannabinoid receptor-1 mediates diet-induced insulin resistance via inhibition of insulin signaling and clearance in mice. Gastroenterology 142(5): 1218-1228.
- Chen CC, Lee TY, Kwok CF, Hsu YP, Shih KC, et al. (2016) Cannabinoid receptor type 1 mediates high-fat diet-induced insulin resistance by increasing forkhead box O1 activity in a mouse model of obesity. Int J Mol Med 2016 37(3): 743-754.
- Mendez Sanchez N, Zamora Valdes D, Pichardo Bahena R, Barredo Prieto B, Ponciano Rodriguez G, et al. (2007) Endocannabinoid receptor CB2 in nonalcoholic fatty liver disease. Liver Int 27(2): 215219.
- Munoz Luque J, Ros J, Fernandez Varo G, Tugues S, Morales Ruiz M, et al. (2008) Regression of fibrosis after chronic stimulation of cannabinoid CB2 receptor in cirrhotic rats. J Pharmacol Exp Ther 324(2): 475-483.
- Teixeira Clerc F, Belot MP, Manin S, Deveaux V, Cadoudal T, et al. (2010) Beneficial paracrine effects of cannabinoid receptor 2 on liver injury and regeneration. Hepatology 52(3): 1046-1059.
- Huang SS, Chen DZ, Wu H, Chen RC, Du SJ, et al. (2016) Cannabinoid receptors are involved in the protective effect of a novel curcumin derivative C66 against CCl4-induced liver fibrosis. Eur J Pharmacol 779: 22-30.
- Bermudez Silva FJ, Sanchez Vera I, Suarez J, Serrano A, Fuentes E, et al. (2007) Role of cannabinoid CB2 receptors in glucose homeostasis in rats. Eur J Pharmacol 565(1-3): 207-211.
- Aram G, Potter JJ, Liu X, Torbenson MS, Mezey E (2008) Lack of inducible nitric oxide synthase leads to increased hepatic apoptosis and decreased fibrosis in mice after chronic carbon tetrachloride administration. Hepatology 47(6): 2051-2058.
- Pacher P, Mackie K (2012) Interplay of cannabinoid 2 (CB2) receptors with nitric oxide synthases, oxidative and nitrative stress, and cell death during remote neurodegeneration. J Mol Med (Berl) 90(4): 347351.
- Charbonneau A, Marette A (2010) Inducible nitric oxide synthase induction underlies lipid-induced hepatic insulin resistance in mice. Potential role of tyrosine nitration of insulin signaling proteins. Diabetes 59(4): 861-871.
- Monteiro HP, Arai RJ, Travassos LR (2008) Protein tyrosine phosphorylation and protein tyrosine nitration in redox signaling. Antioxid Redox Signal 10(5): 843-889.
- Shinozaki S, Choi CS, Shimizu N, Yamada M, Kim M, et al. (2011) Liver- specific inducible nitric-oxide synthase expression is sufficient to cause hepatic insulin resistance and mild hyperglycemia in mice. J Biol Chem 286(40): 34959-34975.