The role of adipocyte insulin resistance in the pathogenesis of obesity-related elevations in endocannabinoids

Resistance to diet-induced adiposity in cannabinoid receptor-1 deficient mice is not due to impaired adipocyte function

Background

Overactivity and/or dysregulation of the endocannabinoid system (ECS) contribute to development of obesity. In vitro studies indicate a regulatory role for the cannabinoid receptor 1 (CB₁) in adipocyte function and CB₁-receptor deficient (CB₁^-/-) mice are resistant to high fat diet-induced obesity. Whether this phenotype of CB₁^-/- mice is related to altered fat metabolism in adipose tissue is unknown.

Methods

We evaluated adipose tissue differentiation/proliferation markers and quantified lipogenic and lipolytic activities in fat tissues of CB₁^-/- and CB₁^+/+ mice fed a high-fat (HF) or a high-fat/fish oil (HF/FO) diet as compared to animals receiving a low-fat chow diet. Comparison between HF diet and HF/FO diet allowed to investigate the influence of dietary fat quality on adipose tissue biology in relation to CB₁ functioning.

Results

The adiposity-resistant phenotype of the CB₁^-/- mice was characterized by reduced fat mass and adipocyte size in HF and HF/FO-fed CB₁^-/- mice in parallel to a significant increase in energy expenditure as compared to CB₁^+/+ mice. The expression levels of adipocyte differentiation and proliferation markers were however maintained in these animals. Consistent with unaltered lipogenic gene expression, the fatty acid synthesis rates in adipose tissues from CB₁^-/- and CB₁^+/+ mice were unchanged. Whole-body and adipose-specific lipoprotein lipase (LPL) activities were also not altered in CB₁^-/- mice.

Conclusions

These findings indicate that protection against diet-induced adiposity in CB₁-deficient mice is not related to changes in adipocyte function per se, but rather results from increased energy dissipation by oxidative and non-oxidative pathways.

Altered endocannabinoid signalling after a high-fat diet in Apoe(-/-) mice: relevance to adipose tissue inflammation, hepatic steatosis and insulin resistance

AIMS/HYPOTHESIS

Apolipoprotein E (ApoE) deficiency is associated with reduced fat accumulation in white adipose tissue (WAT) and high liver triacylglycerol content. Elevated levels of endocannabinoids and cannabinoid receptor type 1 (CB(1)) receptors in the liver and in epididymal vs subcutaneous WAT are associated with fatty liver, visceral adipose tissue, inflammatory markers and insulin resistance.

METHODS

We investigated, in Apoe (-/-) and wild-type (WT) mice, the effect of a high-fat diet (HFD) on: (1) subcutaneous and epididymal WAT accumulation, liver triacylglycerols, phospholipid-esterified fatty acids, inflammatory markers in WAT and liver, and insulin resistance; and (2) endocannabinoid levels, and the gene expression levels of the Cb ( 1 ) receptor and endocannabinoid metabolic enzymes in liver and WAT.

RESULTS

After a 16 week HFD, Apoe (-/-) mice exhibited lower body weight, WAT accumulation and fasting leptin, glucose and insulin levels, and higher hepatic steatosis, than WT mice. Glucose clearance and insulin-mediated glucose disposal following the HFD were slower in WT than Apoe (-/-) mice, which exhibited higher levels of mRNA encoding inflammatory markers (tumour necrosis factor-α [TNF-α], monocyte chemoattractant protein-1 [MCP-1], cluster of differentiation 68 [CD68] and EGF-like module-containing mucin-like hormone receptor-like 1 [EMR1]) in the liver, but lower levels in epididymal WAT. HFD-induced elevation of endocannabinoid levels in the liver or epididymal WAT was higher or lower, respectively, in Apoe (-/-) mice, whereas HFD-induced decrease of subcutaneous WAT endocannabinoid and CB(1) receptor levels was significantly less marked. Alterations in endocannabinoid levels reflected changes in endocannabinoid catabolic enzymes in WAT, or the availability of phospholipid precursors in the liver.

CONCLUSIONS/INTERPRETATION

Liver and adipose tissue endocannabinoid tone following an HFD is altered on Apoe deletion and strongly associated with inflammation, insulin resistance and hepatic steatosis, or lack thereof.

Protective role of cannabinoid receptor type 2 in a mouse model of diabetic nephropathy

OBJECTIVE

The cannabinoid receptor type 2 (CB2) has protective effects in chronic degenerative diseases. Our aim was to assess the potential relevance of the CB2 receptor in both human and experimental diabetic nephropathy (DN).

RESEARCH DESIGN AND METHODS

CB2 expression was studied in kidney biopsies from patients with advanced DN, in early experimental diabetes, and in cultured podocytes. Levels of endocannabinoids and related enzymes were measured in the renal cortex from diabetic mice. To assess the functional role of CB2, streptozotocin-induced diabetic mice were treated for 14 weeks with AM1241, a selective CB2 agonist. In these animals, we studied albuminuria, renal function, expression of podocyte proteins (nephrin and zonula occludens-1), and markers of both fibrosis (fibronectin and transforming growth factor-β1) and inflammation (monocyte chemoattractant protein-1 [MCP-1], CC chemokine receptor 2 [CCR2], and monocyte markers). CB2 signaling was assessed in cultured podocytes.

RESULTS

Podocytes express the CB2 receptor both in vitro and in vivo. CB2 was downregulated in kidney biopsies from patients with advanced DN, and renal levels of the CB2 ligand 2-arachidonoylglycerol were reduced in diabetic mice, suggesting impaired CB2 regulation. In experimental diabetes, AM1241 ameliorated albuminuria, podocyte protein downregulation, and glomerular monocyte infiltration, without affecting early markers of fibrosis. In addition, AM1241 reduced CCR2 expression in both renal cortex and cultured podocytes, suggesting that CB2 activation may interfere with the deleterious effects of MCP-1 signaling.

CONCLUSIONS

The CB2 receptor is expressed by podocytes, and in experimental diabetes, CB2 activation ameliorates both albuminuria and podocyte protein loss, suggesting a protective effect of signaling through CB2 in DN.

The activity of the endocannabinoid metabolising enzyme fatty acid amide hydrolase in subcutaneous adipocytes correlates with BMI in metabolically healthy humans

Background

The endocannabinoid system (ECS) is a ubiquitously expressed signalling system, with involvement in lipid metabolism and obesity. There are reported changes in obesity of blood concentrations of the endocannabinoids anandamide (AEA) and 2-arachidonoylglcyerol (2-AG), and of adipose tissue expression levels of the two key catabolic enzymes of the ECS, fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL). Surprisingly, however, the activities of these enzymes have not been assayed in conditions of increasing adiposity. The aim of the current study was to investigate whether FAAH and MGL activities in human subcutaneous adipocytes are affected by body mass index (BMI), or other markers of adiposity and metabolism.

Methods

Subcutaneous abdominal mature adipocytes, fasting blood samples and anthropometric measurements were obtained from 28 metabolically healthy subjects representing a range of BMIs. FAAH and MGL activities were assayed in mature adipocytes using radiolabelled substrates. Serum glucose, insulin and adipokines were determined using ELISAs.

Results

MGL activity showed no relationship with BMI or other adiposity indices, metabolic markers (fasting serum insulin or glucose) or serum adipokine levels (adiponectin, leptin or resistin). In contrast, FAAH activity in subcutaneous adipocytes correlated positively with BMI and waist circumference, but not with skinfold thickness, metabolic markers or serum adipokine levels.

Conclusions

In this study, novel evidence is provided that FAAH activity in subcutaneous mature adipocytes increases with BMI, whereas MGL activity does not. These findings support the hypothesis that some components of the ECS are upregulated with increasing adiposity in humans, and that AEA and 2-AG may be regulated differently.

Altered gut microbiota and endocannabinoid system tone in obese and diabetic leptin-resistant mice: impact on apelin regulation in adipose tissue

Growing evidence supports the role of gut microbiota in the development of obesity, type 2 diabetes, and low-grade inflammation. The endocrine activity of adipose tissue has been found to contribute to the regulation of glucose homeostasis and low-grade inflammation. Among the key hormones produced by this tissue, apelin has been shown to regulate glucose homeostasis. Recently, it has been proposed that gut microbiota participate in adipose tissue metabolism via the endocannabinoid system (eCB) and gut microbiota-derived compounds, namely lipopolysaccharide (LPS). We have investigated gut microbiota composition in obese and diabetic leptin-resistant mice (db/db) by combining pyrosequencing and phylogenetic microarray analysis of 16S ribosomal RNA gene sequences. We observed a significant higher abundance of Firmicutes, Proteobacteria, and Fibrobacteres phyla in db/db mice compared to lean mice. The abundance of 10 genera was significantly affected by the genotype. We identified the roles of the eCB and LPS in the regulation of apelinergic system tone (apelin and APJ mRNA expression) in genetic obese and diabetic mice. By using in vivo and in vitro models, we have demonstrated that both the eCB and low-grade inflammation differentially regulate apelin and APJ mRNA expression in adipose tissue. Finally, deep-gut microbiota profiling revealed that the gut microbial community of type 2 diabetic mice is significantly different from that of their lean counterparts. This indicates specific relationships between the gut microbiota and the regulation of the apelinergic system. However, the exact roles of specific bacteria in shaping the phenotype of db/db mice remain to be determined.

Effect of the cannabinoid receptor-1 antagonist rimonabant on inflammation in mice with diet-induced obesity

We studied whether cannabinoid receptor (CB1) blockade with rimonabant has an anti-inflammatory effect in obese mice, and whether this effect depends on weight loss and/or diet consumption. High-fat diet (HFD)-induced obese mice were treated orally with rimonabant (HFD-R) or vehicle (HFD-V) for 4 weeks. Paired-feeding was conducted in two additional groups of obese mice to achieve either the same body weight (HFD-BW) or the same HFD intake (HFD DI) as HFD-R. All these groups of mice were maintained on HFD throughout, with mice on normal diet (ND) throughout as lean controls. Rimonabant treatment of obese mice induced marked diet-intake reduction and weight loss during the first week, which was followed by maintenance of low body weight but not diet-intake reduction. Lower HFD intake was required to reach the same degree of weight loss in HFD-BW. HFD-DI had similar weight loss initially, but then started to gain weight, reaching a higher body weight than HFD-R. Despite the same degree of weight loss, HFD-R had less fat mass and lower adipogenic gene expression than HFD-BW. Compared to HFD-V or HFD-DI, HFD-R had reduced inflammation in adipose tissue (AT) and/or liver indicated primarily by lower monocyte chemoattractant protein-1 (MCP-1) levels. However, MCP-1 levels were not significantly different between HFD-R and HFD-BW. In vitro incubation of rimonabant with AT explants did not change MCP-1 levels. Thus, rimonabant induced weight loss in obese mice by diet-intake-dependent and -independent fashions. Rimonabant decreased inflammation in obese mice, possibly through a primary effect on weight reduction.

The gut microbiome as therapeutic target

Obesity, type-2 diabetes and low-grade inflammation are becoming worldwide epidemics. In this regard, the literature provides a novel concept that we call "MicrObesity" (Microbes and Obesity), which is devoted to deciphering the specific role of dysbiosis and its impact on host metabolism and energy storage. In the present review, we discuss novel findings that may partly explain how the microbial community participates in the development of the fat mass development, insulin resistance and low-grade inflammation that characterise obesity. In recent years, numerous mechanisms have been proposed and several proteins identified. Amongst the key players involved in the control of fat mass development, Fasting induced adipose factor, AMP-activated protein kinase, G-protein coupled receptor 41 and G-protein coupled receptor 43 have been linked to gut microbiota. In addition, the discovery that low-grade inflammation might be directly linked to the gut microbiota through metabolic endotoxaemia (elevated plasma lipopolysaccharide levels) has led to the identification of novel mechanisms involved in the control of the gut barrier. Amongst these, the impacts of glucagon-like peptide-2, the endocannabinoid system and specific bacteria (e.g., Bifidobacterium spp.) have been investigated. Moreover, the advent of probiotic and prebiotic treatments appears to be a promising "pharmaco-nutritional" approach to reversing the host metabolic alterations linked to the dysbiosis observed in obesity. Although novel powerful molecular system biology approaches have offered great insight into this "small world within", more studies are needed to unravel how specific changes in the gut microbial community might affect or counteract the development of obesity and related disorders.

Pharmacological and non-pharmacological interventions to influence adipose tissue function

Obesity is associated with metabolic derangements such as insulin resistance, inflammation and hypercoagulobility which can all be understood as consequences of adipose tissue dysfunction. The potential role for adipose tissue derived cytokines and adipokines in the development of vascular disease and diabetes may produce a clinical need to influence adipose tissue function. Various pharmacological and non-pharmacological interventions affect plasma cytokine and adipokine levels. The effects of these interventions depend on weight loss per se, changes in fat distribution without weight loss and/or direct effects on adipose tissue inflammation.

Weight loss, as a result of diet, pharmacology and surgery, positively influences plasma adipokines and systemic inflammation. Several classes of drugs influence systemic inflammation directly through their anti-inflammatory actions. PPAR-γ agonism positively influences adipose tissue inflammation in several classes of intervention such as the thiazolidinediones and perhaps salicylates, CB1-antagonists and angiotensin II receptor blockers. Furthermore, within drug classes there are differential effects of individual pharmacologic agents on adipose tissue function.

It can be concluded that several commonly used pharmacological and non-pharmacological interventions have unintended influences on adipose tissue function. Improving adipose tissue function may contribute to reducing the risk of vascular diseases and the development of type 2 diabetes.

Effects of Chronic Antagonism of Endocannabinoid-1 Receptors on Glucose Tolerance and Insulin Action in Skeletal Muscles of Lean and Obese Zucker Rats

BACKGROUND/AIMS

Antagonism of the endocannabinoid receptor-1 (CB1R) directly improves whole-body metabolic parameters of insulin resistance. The present investigation determined the effects of chronic CB1R antagonism on whole-body and skeletal-muscle insulin action in insulin-sensitive lean and insulin-resistant obese Zucker rats.

METHODS

Animals were either fed ad libitum or in pairs, or treated with SR141716 (10 mg/kg i.p. for 14 days).

RESULTS

Food intake was significantly reduced (p < 0.05) after initial SR141716 treatment and remained decreased in both lean and obese animals until day 13. Fasting plasma glucose decreased (24%) and insulin increased (43%) in lean SR141716-treated (24%) rats compared to lean ad libitum-fed controls, but not in the corresponding obese groups. Fasting plasma free fatty acids were reduced by CB1R antagonism in lean (21%) and obese (42%) animals. Whole-body insulin sensitivity was increased (36%) in obese SR141716-treated rats compared to obese ad libitum-fed controls, which was associated with reduced insulin secretion during an oral glucose tolerance test. Insulin-stimulated glucose transport activity in the soleus was greatest in the respective SR141716-treated lean and obese groups compared to the corresponding ad libitum- and pair-fed controls. Chronic SR141716 treatment did not induce alterations in signaling factors associated with the regulation of glucose transport [protein kinase B (Akt), glycogen synthase kinase-3β, 5'-AMP-dependent protein kinase, or p38 mitogen-activated protein kinase] in the soleus.

CONCLUSIONS

These results indicate that, while the chronic treatment with CB1R antagonism markedly diminished food intake in lean and obese Zucker rats, there are also significant metabolic improvements in whole-body and skeletal-muscle insulin action mediated by CB1R antagonism through mechanisms independent of reduced caloric intake.

Cannabinoids and endocannabinoids in metabolic disorders with focus on diabetes

The cannabinoid receptors for Δ(9)-THC, and particularly, the CB(1) receptor, as well as its endogenous ligands, the endocannabinoids anandamide and 2-arachidonoylglycerol, are deeply involved in all aspects of the control of energy balance in mammals. While initially it was believed that this endocannabinoid signaling system would only facilitate energy intake, we now know that perhaps even more important functions of endocannabinoids and CB(1) receptors in this context are to enhance energy storage into the adipose tissue and reduce energy expenditure by influencing both lipid and glucose metabolism. Although normally well controlled by hormones and neuropeptides, both central and peripheral aspects of endocannabinoid regulation of energy balance can become dysregulated and contribute to obesity, dyslipidemia, and type 2 diabetes, thus raising the possibility that CB(1) antagonists might be used for the treatment of these metabolic disorders. On the other hand, evidence is emerging that some nonpsychotropic plant cannabinoids, such as cannabidiol, can be employed to retard β-cell damage in type 1 diabetes. These novel aspects of endocannabinoid research are reviewed in this chapter, with emphasis on the biological effects of plant cannabinoids and endocannabinoid receptor antagonists in diabetes.

The endocannabinoid system links gut microbiota to adipogenesis

We investigated several models of gut microbiota modulation: selective (prebiotics, probiotics, high-fat), drastic (antibiotics, germ-free mice) and mice bearing specific mutations of a key gene involved in the toll-like receptors (TLR) bacteria-host interaction (Myd88^−/−). Here we report that gut microbiota modulates the intestinal endocannabinoid (eCB) system-tone, which in turn regulates gut permeability and plasma lipopolysaccharide (LPS) levels.

The activation of the intestinal endocannabinoid system increases gut permeability which in turn enhances plasma LPS levels and inflammation in physiological and pathological conditions such as obesity and type 2 diabetes.

The investigation of adipocyte differentiation and lipogenesis (both markers of adipogenesis) indicate that gut microbiota controls adipose tissue physiology through LPS-eCB system regulatory loops and may play a critical role in the adipose tissue plasticity during obesity.

In vivo, ex vivo and in vitro studies indicate that LPS acts as a master switch on adipose tissue metabolism, by blocking the cannabinoid-driven adipogenesis.

Obesity and type II diabetes have reached epidemic proportions and are associated with a massive expansion of the adipose tissue. Recent data have shown that these metabolic disorders are characterised by low-grade inflammation of unknown molecular origin (Hotamisligil and Erbay, 2008; Shoelson and Goldfine, 2009); therefore, it is of the utmost importance to identify the link between inflammation and adipose tissue metabolism and plasticity. Among the latest important discoveries published in the field, two new concepts have driven this study. First, emerging data have shown that gut microbiota is involved in the control of energy homeostasis (Ley et al, 2005; Turnbaugh et al, 2006; Claus et al, 2008) Obesity is characterised by the massive expansion of adipose tissues and is associated with inflammation (Weisberg et al, 2003). It is possible that both this expansion and the associated inflammation are controlled by microbiota and lipopolysaccharide (LPS) (Cani et al, 2007a, 2008), a cell wall component of Gram-negative bacteria that is among the most potent inducers of inflammation (Cani et al, 2007a, 2007b, 2008; Cani and Delzenne, 2009). Second, obesity is also characterised by greater endocannabinoid (eCB) system tone (increased eCB plasma levels, altered expression of the cannabinoid receptor 1 (CB₁ mRNA) and increased eCB levels in the adipose tissue) (Engeli et al, 2005; Bluher et al, 2006; Matias et al, 2006; Cote et al, 2007; D'Eon et al, 2008; Starowicz et al, 2008; Di Marzo et al, 2009; Izzo et al, 2009).

Several studies have suggested a close relationship between LPS, gut microbiota and the eCB system. Indeed, LPS controls the synthesis of eCB in macrophages, whereas macrophage infiltration in the adipose tissue occurring during obesity is an important factor in the development of the metabolic disorders (Weisberg et al, 2003). We have shown that macrophage infiltration is not only dependent on the activation of the receptor CD14 by LPS, but is also dependent on the gut microbiota composition and the gut barrier function (gut permeability) (Cani et al, 2007a, 2008). Moreover, LPS controls the synthesis of eCBs both in vivo (Hoareau et al, 2009) and in vitro (Di Marzo et al, 1999; Maccarrone et al, 2001) through mechanisms dependent of the LPS receptor signalling pathway (Liu et al, 2003). Thus, obesity is nowadays associated with changes in gut microbiota and a higher endocannabinoid system tone, both having a function in the disease's pathophysiology.

Given that the convergent molecular mechanisms that may affect these different supersystem activities and adiposity remain to be elucidated, we tested the hypothesis that the gut microbiota and the eCB system control gut permeability and adipogenesis, by a LPS-dependent mechanism, under both physiological and obesity-related conditions.

First, we found that high-fat diet-induced obese and diabetic animals exhibit threefold higher colonic CB₁ mRNA, whereas no modification was observed in the small intestinal segment (jejunum). Moreover, selective modulation of gut microbiota using prebiotics (i.e. non-digestible compounds fermented by specific bacteria in the gut) (Gibson and Roberfroid, 1995) reduces by about one half this effect. Similarly, in genetically obese mice (ob/ob), prebiotic treatment decreases colonic CB₁ mRNA and colonic eCB concentrations (AEA) (Figure 2A). In addition, we have observed a modulation of FAAH and MGL mRNA (Figure 2A). Furthermore, we have found that antibiotic treatment decreasing the number of gut bacteria content was associated with a strong reduction of the CB₁ receptor levels in the colon of healthy mice.

Second, we show that the endocannabinoid system controls gut barrier function (in vivo and in vitro) and endotoxaemia. More precisely, we designed two in vivo experiments in obese and lean mice (Figure 2). In a first experiment, we blocked the CB₁ receptor in obese mice with a specific and selective antagonist (SR141716A) and found that the blockade of the CB₁ receptor reduces plasma LPS levels by a mechanism linked to the improvement of the gut barrier function (Figure 2C) as shown by the lower alteration of tight junctions proteins (zonula occludens-1 (ZO-1) and occludin) distribution and localisation, and independently of food intake behaviour (Figures 2D and 3). In a second set of experiments performed in lean wild-type mice, we mimicked the increased eCB system tone observed during obesity by chronic (4-week) infusion of a cannabinoid receptor agonist (HU-210) through mini-pumps implanted subcutaneously. We found that cannabinoid agonist administration significantly increased plasma LPS levels. Furthermore, increased plasma fluorescein isothiocyanate-dextran levels were observed after oral gavage (Figure 2F and G). These sets of in vivo experiments strongly suggest that an overactive eCB system increases gut permeability. Finally, in a cellular model of intestinal epithelial barrier (Caco-2 cells monolayer), we found that CB₁ receptor antagonist normalised LPS and the cannabinoid receptors agonist HU-210-induced epithelial barrier alterations.

Third, we provide evidence that adipogenesis is under the control of the gut microbiota, through the modulation of the gut and adipose tissue endocannabinoid systems in both physiological and pathological conditions. We found that the higher eCB system tone (found in obesity or mimicked by eCB agonist) participates to the regulation of adipogenesis by directly acting on the adipose tissue, but also indirectly by increasing plasma LPS levels, which consequently impair adipogenesis and promote inflammatory states. Here, we found that both the specific modulation of the gut microbiota and the blockade of the CB₁ receptor decrease plasma LPS levels and is associated with higher adipocyte differentiation and lipogenesis rate. One possible explanation for these surprising data could be as follows: plasma LPS levels might be under the control of CB₁ in the intestine (gut barrier function); therefore, under particular pathophysiological conditions in vivo (e.g. obesity/type II diabetes), this could lead to higher circulating LPS levels. Furthermore, CB₁ receptor blockade might paradoxically increase adipogenesis because of the ability of CB₁ antagonist to reduce gut permeability and counteract the LPS-induced inhibitory effect on adipocyte differentiation and lipogenesis (i.e. a disinhibition mechanism). In summary, given that these treatments reduce gut permeability and, hence, plasma LPS levels and inflammatory tone, we hypothesised that LPS could act as a regulator in this process. This hypothesis was further supported in vitro and in vivo by the observation that cannabinoid-induced adipocyte differentiation and lipogenesis were directly altered (i.e. reduced) in the presence of physiological levels of LPS. In summary, because these treatments reduce gut permeability, hence, plasma LPS and inflammatory tone, we hypothesised that LPS acts as a regulator in this process. Altogether, our data provide the evidence that the consequences of obesity and gut microbiota dysregulation on gut permeability and metabolic endotoxaemia are clearly mediated by the eCB system, those observed on adiposity are likely the result of two systems interactions: LPS-dependent pathways activities and eCB system tone dysregulation (Figure 9).

Our results indicate that the endocannabinoid system tone and the plasma LPS levels have a critical function in the regulation of the adipose tissue plasticity. As obesity is commonly characterised by increased eCB system tone, higher plasma LPS levels, altered gut microbiota and impaired adipose tissue metabolism, it is likely that the increased eCB system tone found in obesity is caused by a failure or a vicious cycle within the pathways controlling the eCB system.

These findings show that two novel therapeutic targets in the treatment of obesity, the gut microbiota and the endocannabinoid system, are closely interconnected. They also provide evidence for the presence of a new integrative physiological axis between gut and adipose tissue regulated by LPS and endocannabinoids. Finally, we propose that the increased endotoxaemia and endocannabinoid system tone found in obesity might explain the altered adipose tissue metabolism.

Obesity is characterised by altered gut microbiota, low-grade inflammation and increased endocannabinoid (eCB) system tone; however, a clear connection between gut microbiota and eCB signalling has yet to be confirmed. Here, we report that gut microbiota modulate the intestinal eCB system tone, which in turn regulates gut permeability and plasma lipopolysaccharide (LPS) levels. The impact of the increased plasma LPS levels and eCB system tone found in obesity on adipose tissue metabolism (e.g. differentiation and lipogenesis) remains unknown. By interfering with the eCB system using CB₁ agonist and antagonist in lean and obese mouse models, we found that the eCB system controls gut permeability and adipogenesis. We also show that LPS acts as a master switch to control adipose tissue metabolism both in vivo and ex vivo by blocking cannabinoid-driven adipogenesis. These data indicate that gut microbiota determine adipose tissue physiology through LPS-eCB system regulatory loops and may have critical functions in adipose tissue plasticity during obesity.

Modulation of adipocyte biology by δ(9)-tetrahydrocannabinol

It is recognized that the endocannabinoid system (ECS) plays a crucial role in the modulation of food intake and other aspects of energy metabolism. In this study, we aimed to investigate the effects of Δ(9)-tetrahydrocannabinol (THC) on adipocyte biology. 3T3-L1 cells were used to evaluate proliferation by sulforhodamine B (SRB) staining and methyl-(3)H-thymidine incorporation after 48 or 72 h of treatment with THC (1-500 nmol/l). Cells were differentiated in the presence or absence of the cannabinoid, and adipogenesis was determined by measuring lipid accumulation and peroxisome proliferator-activated receptor γ (PPARγ) transcription through reverse transcriptase-PCR (RT-PCR). Lipolysis was quantified under basal conditions or after isoproterenol (IP, 100 nmol/l) or insulin (INS, 100 nmol/l) treatment. Transforming growth factor β (TGFβ), diacylglycerol lipase α, and N-acylphosphatidylethanolamine-specific phospholipase D (NAPE-PLD) transcriptions were determined by RT-PCR in preadipocytes and adipocytes and adiponectin only in adipocytes. THC treatment increased culture protein content and reduced methyl-(3)H-thymidine incorporation. Cells treated with THC underwent adipogenesis shown by the expression of PPARγ and had increased lipid accumulation. Basal and IP-stimulated lipolyses were inhibited by THC and there was no effect on lipolysis of INS-treated adipocytes. The effects on methyl-(3)H-thymidine incorporation and lipolysis seem to be mediated through CB1- and CB2-dependent pathways. THC decreased NAPE-PLD in preadipocytes and increased adiponectin and TGFβ transcription in adipocytes. These results show that the ECS interferes with adipocyte biology and may contribute to adipose tissue (AT) remodeling. Although these observations point toward increased AT deposition, the stimulation of adiponectin production and inhibition of lipolysis may be in favor of improved INS sensitivity under cannabinoid influence.

The endocannabinoid system: its roles in energy balance and potential as a target for obesity treatment

Obesity and cardiometabolic risk continue to be major public health concerns. A better understanding of the physiopathological mechanisms leading to obesity may help to identify novel therapeutic targets. The endocannabinoid system discovered in the early 1990s is believed to influence body weight regulation and cardiometabolic risk factors. This article aims to review the literature on the endocannabinoid system including the biological roles of its major components, namely, the cannabinoid receptors, their endogenous ligands the endocannabinoids and the ligand-metabolising enzymes. The review also discusses evidence that the endocannabinoid system constitutes a new physiological pathway occurring in the central nervous system and peripheral tissues that has a key role in the control of food intake and energy expenditure, insulin sensitivity, as well as glucose and lipid metabolism. Based on the important finding that there is a close association between obesity and the hyperactivity of the endocannabinoid system, interest in blocking stimulation of this pathway to aid weight loss and reduce cardiometabolic risk factor development has become an important area of research. Among the pharmacological strategies proposed, the antagonism of the cannabinoid receptors has been particularly investigated and several clinical trials have been conducted. One challenging pharmacological task will be to target the endocannabinoid system in a more selective, and hence, safe way. As the management of obesity also requires lifestyle modifications in terms of healthy eating and physical activity, the targeting of the endocannabinoid system may represent a novel approach for a multifactorial therapeutic strategy.

Differential alterations of the concentrations of endocannabinoids and related lipids in the subcutaneous adipose tissue of obese diabetic patients

BACKGROUND

The endocannabinoids, anandamide and 2-AG, are produced by adipocytes, where they stimulate lipogenesis via cannabinoid CB1 receptors and are under the negative control of leptin and insulin. Endocannabinoid levels are elevated in the blood of obese individuals and nonobese type 2 diabetes patients. To date, no study has evaluated endocannabinoid levels in subcutaneous adipose tissue (SAT) of subjects with both obesity and type 2 diabetes (OBT2D), characterised by similar adiposity and whole body insulin resistance and lower plasma leptin levels as compared to non-diabetic obese subjects (OB).

DESIGN AND METHODS

The levels of anandamide and 2-AG, and of the anandamide-related PPARalpha ligands, oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), in the SAT obtained by abdominal needle biopsy in 10 OBT2D, 11 OB, and 8 non-diabetic normal-weight (NW) subjects, were measured by liquid chromatography-mass spectrometry. All subjects underwent a hyperinsulinaemic euglycaemic clamp.

RESULTS

As compared to NW, anandamide, OEA and PEA levels in the SAT were 2-4.4-fold elevated (p < 0.05), and 2-AG levels 2.3-fold reduced (p < .05), in OBT2D but not in OB subjects. Anandamide, OEA and PEA correlated positively (p < .05) with SAT leptin mRNA and free fatty acid during hyperinsulinaemic clamp, and negatively with SAT LPL activity and plasma HDL-cholesterol, which were all specifically altered in OBT2D subjects.

CONCLUSIONS

The observed alterations emphasize, for the first time in humans, the potential different role and regulation of adipose tissue anandamide (and its congeners) and 2-AG in obesity and type 2 diabetes.

CB1 antagonism exerts specific molecular effects on visceral and subcutaneous fat and reverses liver steatosis in diet-induced obese mice

OBJECTIVE

The beneficial effects of the inactivation of endocannabinoid system (ECS) by administration of antagonists of the cannabinoid receptor (CB) 1 on several pathological features associated with obesity is well demonstrated, but the relative contribution of central versus peripheral mechanisms is unclear. We examined the impact of CB1 antagonism on liver and adipose tissue lipid metabolism in a mouse model of diet-induced obesity.

RESEARCH DESIGN AND METHODS

Mice were fed either with a standard diet or a high-sucrose high-fat (HSHF) diet for 19 weeks and then treated with the CB1-specific antagonist SR141716 (10 mg x kg(-1) x day(-1)) for 6 weeks.

RESULTS

Treatment with SR141716 reduced fat mass, insulin levels, and liver triglycerides primarily increased by HSHF feeding. Serum adiponectin levels were restored after being reduced in HSHF mice. Gene expression of scavenger receptor class B type I and hepatic lipase was induced by CB1 blockade and associated with an increase in HDL-cholesteryl ether uptake. Concomitantly, the expression of CB1, which was strongly increased in the liver and adipose tissue of HSHF mice, was totally normalized by the treatment. Interestingly, in visceral but not subcutaneous fat, genes involved in transport, synthesis, oxidation, and release of fatty acids were upregulated by HSHF feeding, while this effect was counteracted by CB1 antagonism.

CONCLUSIONS

A reduction in the CB1-mediated ECS activity in visceral fat is associated with a normalization of adipocyte metabolism, which may be a determining factor in the reversion of liver steatosis induced by treatment with SR141716.

Role of lipid-derived mediators in skeletal muscle insulin resistance

Imbalance between nutritional intake and energy expenditure has been described to culminate in obesity, which predisposes to insulin resistance and type 2 diabetes mellitus. In such states of energy oversupply, excess amounts of lipids are available in tissues and circulation. Over the past years, an increasingly important role in development of skeletal muscle (SkM) insulin resistance has been attributed to lipids and impaired fatty acid metabolism. In this review, we reflect the current state of knowledge about the effects of various lipid-derived mediators on SkM insulin sensitivity. Furthermore, potential mechanisms underlying the biogenesis of intramyocellular ectopic lipid stores are discussed. Previously, a pivotal role was attributed to mitochondrial dysfunction. However, results of recent studies have suggested an important role for exercise deficiency, accompanied by decreased expression levels of peroxisome proliferator-activated receptor-γ coactivator-1α and subsequent, incomplete β-oxidation. Additionally, we summarize the implications of increased levels of lipid-derived endocannabinoids (ECs) for metabolic control in peripheral tissue and highlight the benefits of targeting the EC system.

Metabolic responses to long-term pharmacological inhibition of CB1-receptor activity in mice in relation to dietary fat composition

BACKGROUND AND OBJECTIVES

The antiobesity effects of suppressed endocannabinoid signaling may rely, at least in part, on changes in lipid fluxes. As fatty acids exert specific effects depending on their level of saturation, we hypothesized that the dietary fatty acid composition would influence the outcome of treatment with a CB(1)-receptor antagonist (rimonabant).

METHODS

Mice were treated with rimonabant (10 mg kg(-1) body weight per day) or vehicle while equicalorically fed either a low-fat diet (LF), a high-fat (HF) diet or an HF diet in which 10% of the saturated fatty acids (SFAs) were replaced by poly-unsaturated fatty acids (PUFA) from fish oil (FO). Food intake and body weight were registered daily. Indirect calorimetry was performed and feces were collected. After 3 weeks, mice were killed for blood and tissue collection.

RESULTS

Relative to the LF diet, the HF diet caused anticipated metabolic derangements, which were partly reversed by the HF/FO diet. The HF/FO diet, however, was most obesity-promoting despite inhibiting lipogenesis as indicated by low gene expression levels of lipogenic enzymes. On all three diets, rimonabant treatment improved metabolic derangements and led to significantly lower body weight gain than their respective controls. This latter effect appeared largest in the HF/FO group, but occurred without major changes in nutrient absorption and energy expenditure.

CONCLUSION

The effects of chronic rimonabant treatment on body weight gain occurred irrespective of diet-induced changes in lipogenic activity, food intake and daily energy expenditure, and were, in fact, most pronounced in HF/FO mice. The effects of dietary PUFA replacement in an HF diet on expansion of adipose tissue might allow the favorable effects of dietary PUFA on dyslipidemia and hepatic steatosis. In light of other disadvantageous effects of weight gain, this might be a risky trade-off.

Regulation and possible role of endocannabinoids and related mediators in hypercholesterolemic mice with atherosclerosis

In this study we analysed the possible modulation of endocannabinoids and related molecules during atherosclerosis development in mice. Wild-type and apolipoprotein E knockout (ApoE(-/-)) mice were fed either normal chow or high-cholesterol diet for 8-12 weeks, and tissue endocannabinoid levels were measured by liquid chromatography-mass spectrometry. We found increased levels of 2-AG in aortas and visceral adipose tissue (VAT) of ApoE(-/-) mice fed on high-cholesterol diet for 12 weeks as compared to ApoE(-/-) mice fed on normal chow or wild-type mice fed on cholesterol. No significant difference in 2-AG levels was observed after 8 weeks of diet, and no changes in anandamide levels were found in any group. The levels of the anandamide-related mediators with anti-inflammatory or anti-lipogenic properties, palmitoylethanolamide (PEA) and oleoylethanolamide (OEA), decreased or increased only in VAT or in both tissues, respectively. Endocannabinoid- and OEA/PEA-degrading enzymes were expressed by macrophages within atherosclerotic lesions. In vitro, 2-AG and OEA-induced monocyte migration at 0.3-1microM, which corresponds to the levels observed in aortas. PEA 1microM also induced monocyte migration but counteracted the effect of 2-AG, whereas OEA enhanced it. Enhanced 2-AG levels in advanced atherosclerotic lesions may trigger the inflammatory process by recruiting more inflammatory cells and inducing extracellular matrix degradation via CB(2) receptors, and this possibility was supported in vitro but not in vivo by experiments with the CB(2) antagonist, SR144528.

The emerging role of the endocannabinoid system in cardiovascular disease

Endocannabinoids are endogenous bioactive lipid mediators present both in the brain and various peripheral tissues, which exert their biological effects via interaction with specific G-protein-coupled cannabinoid receptors, the CB(1) and CB(2). Pathological overactivation of the endocannabinoid system (ECS) in various forms of shock and heart failure may contribute to the underlying pathology and cardiodepressive state by the activation of the cardiovascular CB(1) receptors. Furthermore, tonic activation of CB(1) receptors by endocannabinoids has also been implicated in the development of various cardiovascular risk factors in obesity/metabolic syndrome and diabetes, such as plasma lipid alterations, abdominal obesity, hepatic steatosis, inflammation, and insulin and leptin resistance. In contrast, activation of CB(2) receptors in immune cells exerts various immunomodulatory effects, and the CB(2) receptors in endothelial and inflammatory cells appear to limit the endothelial inflammatory response, chemotaxis, and inflammatory cell adhesion and activation in atherosclerosis and reperfusion injury. Here, we will overview the cardiovascular actions of endocannabinoids and the growing body of evidence implicating the dysregulation of the ECS in a variety of cardiovascular diseases. We will also discuss the therapeutic potential of the modulation of the ECS by selective agonists/antagonists in various cardiovascular disorders associated with inflammation and tissue injury, ranging from myocardial infarction and heart failure to atherosclerosis and cardiometabolic disorders.

Cannabinoid receptor antagonists: pharmacological opportunities, clinical experience, and translational prognosis

The endogenous cannabinoid (CB) (endocannabinoid) signaling system is involved in a variety of (patho)physiological processes, primarily by virtue of natural, arachidonic acid-derived lipids (endocannabinoids) that activate G protein-coupled CB1 and CB2 receptors. A hyperactive endocannabinoid system appears to contribute to the etiology of several disease states that constitute significant global threats to human health. Consequently, mounting interest surrounds the design and profiling of receptor-targeted CB antagonists as pharmacotherapeutics that attenuate endocannabinoid transmission for salutary gain. Experimental and clinical evidence supports the therapeutic potential of CB1 receptor antagonists to treat overweight/obesity, obesity-related cardiometabolic disorders, and substance abuse. Laboratory data suggest that CB2 receptor antagonists might be effective immunomodulatory and, perhaps, anti-inflammatory drugs. One CB1 receptor antagonist/inverse agonist, rimonabant, has emerged as the first-in-class drug approved outside the United States for weight control. Select follow-on agents (taranabant, otenabant, surinabant, rosonabant, SLV-319, AVE1625, V24343) have also been studied in the clinic. However, rimonabant's market withdrawal in the European Union and suspension of rimonabant's, taranabant's, and otenabant's ongoing development programs have highlighted some adverse clinical side effects (especially nausea and psychiatric disturbances) of CB1 receptor antagonists/inverse agonists. Novel CB1 receptor ligands that are peripherally directed and/or exhibit neutral antagonism (the latter not affecting constitutive CB1 receptor signaling) may optimize the benefits of CB1 receptor antagonists while minimizing any risk. Indeed, CB1 receptor-neutral antagonists appear from preclinical data to offer efficacy comparable to or better than that of prototype CB1 receptor antagonists/inverse agonists, with less propensity to induce nausea. Continued pharmacological profiling, as the prelude to first-in-man testing of CB1 receptor antagonists with unique modes of targeting/pharmacological action, represents an exciting translational frontier in the critical path to CB receptor blockers as medicines.

Peripheral endocannabinoid dysregulation in obesity: relation to intestinal motility and energy processing induced by food deprivation and re-feeding

BACKGROUND AND PURPOSE

Endocannabinoids in tissues controlling energy homeostasis are altered in obesity, thus contributing to metabolic disorders. Here we evaluate endocannabinoid dysregulation in the small intestine of mice with diet-induced obesity (DIO) and in peripheral tissues of Zucker and lean rats following food deprivation and re-feeding.

EXPERIMENTAL APPROACH

Intestinal transit, evaluated using rhodamine-B-labelled dextran, and small intestinal endocannabinoid levels, measured by liquid chromatography mass spectrometry, were measured in mice fed normal or high-fat diets (HFDs). Endocannabinoid levels were measured also in various tissues of lean and Zucker rats fed ad libitum or following overnight food deprivation with and without subsequent re-feeding.

KEY RESULTS

After 8 weeks of HFD, baseline intestinal transit was increased in DIO mice and enhanced by cannabinoid CB(1) receptor antagonism less efficaciously than in lean mice. Small intestinal anandamide and 2-arachidonoylglycerol levels were reduced and increased respectively. In Zucker rats, endocannabinoids levels were higher in the pancreas, liver and duodenum, and lower in the subcutaneous adipose tissue. Food deprivation increased endocannabinoid levels in the duodenum and liver of both rat strains, in the pancreas of lean rats and in adipose tissues of Zucker rats.

CONCLUSIONS AND IMPLICATIONS

Reduced anandamide levels might account for increased intestinal motility in DIO mice. Regulation of endocannabinoid levels in rat peripheral tissues, induced by food deprivation and re-feeding, might participate in food intake and energy processing and was altered in Zucker rats. These data, together with previous observations, provide further evidence for dysregulation of peripheral endocannabinoids in obesity.

The dysregulation of the endocannabinoid system in diabesity-a tricky problem

Endocannabinoids (ECs) are small lipid mediators that play a critical role in energy metabolism. Human studies have shown that the EC tone in peripheral tissues positively correlates with increased adiposity. Furthermore, pharmacological inhibition of EC signaling results in weight loss in humans. However, the mechanisms that cause the dysregulation of the EC system in obesity are not well-understood. Since the clinical utility of currently available EC blockers is severely limited due to their side effects like depression and suicidal ideation that are caused by central effects, it is important to delineate the role of central and peripheral effects of EC signaling in regulating glucose and lipid metabolism.

The role of the endocannabinoid system in lipogenesis and fatty acid metabolism

Endocannabinoids (ECs) regulate energy balance by modulating hypothalamic circuits controlling food intake and energy expenditure. However, convincing evidence has accumulated indicating that the EC system is present also in peripheral tissues, in particular in adipose tissue. Fat cells produce and are targets of ECs. Glucose uptake and lipoprotein lipase (LPL) activity, lipogenesis and adipogenesis are stimulated by ECs through cannabinoid 1 (CB1) receptors. Moreover, CB1 activation leads to a decreased mitochondrial biogenesis and function through inhibition of endothelial nitric oxide synthase (eNOS). All these effects are blocked by the CB1 antagonist rimonabant, suggesting that the weight-reducing effect of CB1 blockade is due not only to the transient suppression of food intake and reduction of lipogenesis but also to an increased mitochondrial biogenesis and oxidative metabolism which counteracts the inhibitory effects of ECs, levels of which are increased in fat tissues of obese rodents and humans. This review focuses on the role of ECs in adipose tissue metabolism, adipokine production, and interactions between ECs and peroxisome proliferator-activated receptors (PPARs) during adipogenesis.

Use of cannabinoid CB1 receptor antagonists for the treatment of metabolic disorders

Abdominal obesity is associated with numerous metabolic abnormalities, including insulin resistance, impaired glucose tolerance/type-2 diabetes, and atherogenic dyslipidaemia with low high-density lipoprotein (HDL) cholesterol, high triglycerides, and increased small dense low-density lipoprotein (LDL) cholesterol. A proportion of these metabolic disorders may be attributed to increased endocannabinoid activity. The selective cannabinoid 1 (CB1) receptor antagonist rimonabant has been shown to reduce body weight, waist circumference, insulin resistance, triglycerides, dense LDL, C-reactive protein (CRP), and blood pressure, and to increase HDL and adiponectin concentrations in both non-diabetic and diabetic overweight/obese patients. Besides an improvement in glucose tolerance in non-diabetic subjects, a reduction of 0.5-0.7% in haemoglobin A1C (HbA(1c)) levels was consistently observed in various groups of patients with type-2 diabetes. Almost half the metabolic changes could not be explained by weight loss, supporting direct peripheral effects of rimonabant. Ongoing studies should demonstrate whether improved metabolic disorders with CB1 receptor antagonists (rimonabant, taranabant, etc.) would translate into fewer cardiovascular complications among high-risk individuals.

The role of the endocannabinoid system in the regulation of energy expenditure

Endocannabinoids, a lipid-derived signaling system, regulate appetite and motivation to eat via effects in the hypothalamus and nucleus accumbens. Not all the effects of endocannabinoids on fat mass can be explained by the regulation of food intake alone. Endocannabinoids and their receptors are located in areas of the central nervous system and multiple peripheral tissues involved in the regulation of intermediary metabolism and energy expenditure. In addition to regulating food intake by both central and peripherally mediated effects, endocannabinoids modify glucose and lipid metabolism so as to promote energy storage via lipogenesis and reduce energy expenditure. The endocannabinoid system appears to be overactive in obesity and may serve to maintain fat mass and underlies some of the metabolic consequences of obesity. Inhibition of the cannabinoid type-1 receptor ameliorates the effects of endocannabinoids on food intake and energy metabolism; lipogenesis is inhibited, lipolysis, fatty acid oxidation and glucose uptake increase.

Peripheral, but not central, CB1 antagonism provides food intake-independent metabolic benefits in diet-induced obese rats

OBJECTIVE

Blockade of the CB1 receptor is one of the promising strategies for the treatment of obesity. Although antagonists suppress food intake and reduce body weight, the role of central versus peripheral CB1 activation on weight loss and related metabolic parameters remains to be elucidated. We therefore specifically assessed and compared the respective potential relevance of central nervous system (CNS) versus peripheral CB1 receptors in the regulation of energy homeostasis and lipid and glucose metabolism in diet-induced obese (DIO) rats.

RESEARCH DESIGN AND METHODS

Both lean and DIO rats were used for our experiments. The expression of key enzymes involved in lipid metabolism was measured by real-time PCR, and euglycemic-hyperinsulinemic clamps were used for insulin sensitivity and glucose metabolism studies.

RESULTS

Specific CNS-CB1 blockade decreased body weight and food intake but, independent of those effects, had no beneficial influence on peripheral lipid and glucose metabolism. Peripheral treatment with CB1 antagonist (Rimonabant) also reduced food intake and body weight but, in addition, independently triggered lipid mobilization pathways in white adipose tissue and cellular glucose uptake. Insulin sensitivity and skeletal muscle glucose uptake were enhanced, while hepatic glucose production was decreased during peripheral infusion of the CB1 antagonist. However, these effects depended on the antagonist-elicited reduction of food intake.

CONCLUSIONS

Several relevant metabolic processes appear to independently benefit from peripheral blockade of CB1, while CNS-CB1 blockade alone predominantly affects food intake and body weight.

The endocannabinoid system in obesity and type 2 diabetes

Endocannabinoids (ECs) are defined as endogenous agonists of cannabinoid receptors type 1 and 2 (CB1 and CB2). ECs, EC anabolic and catabolic enzymes and cannabinoid receptors constitute the EC signalling system. This system participates in the control of lipid and glucose metabolism at several levels, with the possible endpoint of the accumulation of energy as fat. Following unbalanced energy intake, however, the EC system becomes dysregulated, and in most cases overactive, in several organs participating in energy homeostasis, particularly, in intra-abdominal adipose tissue. This dysregulation might contribute to excessive visceral fat accumulation and reduced adiponectin release from this tissue, and to the onset of several cardiometabolic risk factors that are associated with obesity and type 2 diabetes. This phenomenon might form the basis of the mechanism of action of CB1 antagonists/inverse agonists, recently developed by several pharmaceutical companies as adjuvants to lifestyle modification for weight reduction, glycaemic control and dyslipidaemia in obese and type 2 diabetes patients. It also helps to explain why some of the beneficial actions of these new therapeutics appear to be partly independent from weight loss.