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Introduction/Overview

Adipose tissue is an endocrine organ; it produces and secretes hormones (such as leptin and adiponectin) that are physiologically important in the regulation of energy balance and metabolism.¹ The role of the endocannabinoid system (ECS) in adipose tissue metabolism involves the modulation of the activity of the fat-storage enzyme lipoprotein lipase as well as the regulation of adiponectin production.^2-4 Cannabinoid receptor type 1 (CB₁ receptors) and fatty acid amide hydrolase, which is the enzyme that metabolizes some endocannabinoids, are expressed in human adipocytes.⁵ Moreover, obesity is associated with increased CB₁ receptor expression in rodent adipocytes and increased endocannabinoid levels in both rodent and human visceral adipose tissue.^{6, 7}

Energy Balance and Metabolic Regulation

Peripheral Metabolic Regulation: Role of the ECS

Compared with lean rats, there was a three- to fourfold increase in CB₁ receptor mRNA expression in the adipose tissue of genetically obese rats.¹ A similar increase in CB₁ receptor mRNA expression was observed in differentiated adipocytes compared with undifferentiated adipocytes.¹ In differentiated and hypertrophic adipocytes, significantly higher levels of the endocannabinoid 2-arachidonoyl glycerol (2-AG) were found compared with undifferentiated adipocytes, possibly as a result of elevated expression of the 2-AG biosynthesizing enzyme, diacylglycerol lipase-α, and loss of negative regulation by peroxisome proliferator-activated receptor γ (PPAR-γ), a marker of adipocyte differentiation (Figure 1A).⁷ Accordingly, the levels of 2-AG but not anandamide were significantly elevated in the epididymal fat of mice with diet-induced obesity (Figure 1B).⁷ These findings are consistent with a role for CB₁ receptors and 2-AG in the regulation of fat storage. Thus, adipose tissue may be a key peripheral target for the ECS.¹

CB₁ receptor blockade is associated with an inhibition of ECS-induced lipogenesis in adipose tissue. Cota et al showed that stimulation of primary adipocytes (derived from epididymal fat pads of male mice) with the CB₁ receptor-agonist WIN-55,212 increased heparin-releasable lipoprotein lipase activity, and that this effect was blocked by the pre-incubation with the CB₁ receptor-selective antagonist SR141716A.³ Matias et al showed that stimulation of mouse 3T3F44 adipocytes with the CB₁ receptor-agonist HU-210 stimulated adipocyte differentiation and lipogenesis.⁷ In addition, these two effects were blocked by co-incubation with SR141716A (Figure 2). Taken together, these findings indicate a CB₁ receptor-mediated effect on lipogenesis and strongly support adipose tissue as an important target of the ECS.¹³

Dysregulation of the ECS within adipose tissue may lead to reduced levels of adiponectin, an adipocyte-derived protein that has both insulin-sensitizing and anti-inflammatory actions.^8-10 Matias et al showed that pharmacologic stimulation of CB₁ receptors was associated with significantly reduced expression of adiponectin mRNA in adipocytes.⁷ In contrast, CB₁ receptor blockade with SR141716 significantly increased the expression of adiponectin protein.⁷ Data support the hypothesis that a loss of insulin sensitivity could result as a consequence of a CB₁ receptor-mediated decrease in the level of adiponectin. Thus, stimulation of the ECS in adipose tissue may contribute to some of the metabolic complications associated with obesity.⁷

Data from other investigators indicate that CB₁ receptor blockade, while reducing body weight may also reverse the morphological changes in white adipose tissue produced by diet-induced obesity. White adipocytes of mice fed a high-fat diet were large and heterogeneous.¹¹ In contrast, the fat cells of SR141716-treated animals on the high-fat diet were significantly smaller in diameter (by 67%) than fat cells of pair-fed control animals and slightly smaller (around 10%) than those of mice fed a standard chow (Figure 3).¹¹

Implications

That the ECS is overactive in obesity was conclusively shown when Matias et al.⁷ demonstrated that visceral, but not subcutaneous, adipose tissue from male subjects contained significantly higher levels of the endocannabinoid 2-AG than the visceral fat from nonobese male subjects (Figure 2).⁷ These authors also, found a nonsignificant trend toward decrease in CB₁ receptor expression in the visceral adipose tissue of obese subjects.⁷ Subcutaneous fat from obese subjects exhibited significantly lower levels of 2-AG than visceral fat. Thus, higher levels of endocannabinoids in the adipose tissue of obese people may depend on the specific fat depot. Overall, experimental data indicate that the ECS is activated in obesity. As a result, the role of the peripheral ECS in energy balance and metabolic regulation is now being studied with great interest.

Dyslipidemia

Role of the ECS

In the setting of obesity, lipoprotein lipase activity in adipose tissue is increased compared with that in skeletal muscle.¹² One of the consequences of this shift may be the shunting of dietary fat into adipose tissue for storage, and increased flux of fatty acids to the liver. Furthermore, CB₁ receptor stimulation in mouse 3T3 adipocytes accelerated adipocyte differentiation, as assessed by PPAR-γ expression, and lipogenesis (Figure 3).⁷

Adiponectin, which has a number of important regulatory roles in glucose and lipid metabolism, is the most abundant protein secreted by adipocytes.¹³ Adiponectin increases fatty-acid oxidation in skeletal muscle, and may protect against the excess accumulation of triglycerides in the tissues of obese mice. Stimulation of the ECS decreases adiponectin secretion from adipocytes,⁷ while CB₁ receptor blockade by SR141617 is associated with an increase in serum adiponectin levels.¹

Implications

Recent clinical trials have demonstrated a relationship between adiponectin and lipid levels. Yamamoto et al demonstrated that serum adiponectin concentration is positively correlated with high-density lipoprotein cholesterol levels and is negatively correlated with total cholesterol, low-density lipoprotein cholesterol, and triglycerides.¹⁴ Thus, one mechanism whereby CB₁ receptor blockade leads to an improved lipid profile may be through increased adiponectin levels.

Glucose Homeostasis

Glucose Metabolism: Role of the ECS

In diet-induced obese mice, where endocannabinoid (2-AG) levels in the epididymal fat are significantly increased,⁷ oral treatment with the CB₁ receptor antagonist SR141716 enhanced the expression of critical regulators of glucose metabolism.¹¹ Global gene expression analyses demonstrated that SR141716 treatment enhanced the expression of 4 of the 8 glycolytic enzymes found in adipose tissue.⁵ Thus, CB₁ receptor stimulation is associated with changes in the expression of genes involved in glucose metabolism.

Adiponectin, a hormone that is derived primarily from adipocytes,^{15, 16} inhibits both the expression of hepatic gluconeogenic enzymes and the rate of endogenous glucose production.¹⁷ Studies in mice have shown that central administration (intracerebroventricular) of adiponectin leads to reductions in body weight and improvements in glucose metabolism¹⁸ and adiponectin levels predict glucose tolerance in a manner that is partly independent of the contribution of visceral adiposity.¹⁹ Additional studies are needed to determine if changes in adiponectin are related to ECS effects on glucose metabolism.

Implications

Preclinical data indicate that CB₁ receptor blockade may produce a pattern of gene-expression changes in adipose tissue that is associated with increased glucose uptake and metabolism.¹¹ Additional studies are needed to determine if 1) CB₁ receptor blockade changes the expression of these genes and their protein products in human adipose tissue and 2) if these changes are associated with clinically significant effects on glucose metabolism.

Glycemic Control and Type 2 Diabetes Mellitus: Role of the ECS

There appears to be a strong link between the ECS and the adipocyte-derived hormone adiponectin. First, CB₁ receptor stimulation may decrease adiponectin expression in adipocytes.⁷ Second, CB₁ receptor blockade with SR141716 increased adiponectin levels in both diet-induced mice¹³ and genetically obese rats.¹ Importantly, these changes in adiponectin levels were associated with favorable changes in serum insulin and glucose levels.^{1, 13} It is unclear if the increased levels of adiponectin were independent of weight loss in the animals treated with SR141716. However, treatment of mouse adipocyte cells in vitro with SR141716 was associated with significantly increased levels of adiponectin mRNA compared with control cells.¹

Visceral adiposity, which is present in many persons with obesity as well as type 2 diabetes and other insulin-resistant states, is associated with increased release of FFAs into the circulation.⁸ High levels of circulating FFAs may impair insulin sensitivity by modifying signaling events downstream from the insulin receptor.²⁰

In diet-induced obese mice, where endocannabinoid (2-AG) levels in the epididymal fat are significantly increased⁷, oral treatment with the CB₁ receptor antagonist SR141716 increased gene expression of the insulin-sensitive glucose transporter GLUT4 in adipose tissue.¹¹ Increased expression of GLUT4 can lead to increased insulin-stimulated glucose uptake by adipocytes. Thus, CB₁ receptor blockade may be involved in regulating insulin sensitivity in adipose tissue.

Implications

Results from clinical studies indicate that adiponectin levels may be an important link between the ECS and the regulation of glucose metabolism.

Figures

Figure 1. A role for CB₁ receptors and 2-AG in the regulation of fat storage. A, In differentiated and hypertrophic adipocytes, significantly higher levels of the endocannabinoid 2-AG were found as compared with undifferentiated adipocytes. Levels of endocannabinoids are shown during adipocyte differentiation induced with insulin alone (0.9 µm) or in a mixture with 3-isobutyl-1-methylxanthine (0.5 mM) and dexamethasone (1.0 mM). Endocannabinoids levels were measured by isotope-dilution liquid chromatography-mass spectrometry. Data are means ± SE of n= 3-6 separate experiments. *, **, ***, P <0.05, 0.01, 0.005 versus control, respectively, as assessed by ANOVA followed by the Bonferroni’s test. B, The levels of 2-AG but not anandamide were significantly elevated in the epididymal fat of mice with diet-induced obesity. P <0.05 versus controls, as assessed by ANOVA followed by Bonferroni’s test. DIO, diet-induced obesity. From Matias et al.⁷

Figure 2. Endocannabinoid levels in the visceral adipose tissue of normal weight (normoweight) and overweight/obese subjects and in the subcutaneous fat of obese subjects. **, P ≤0.01 versus visceral fat from normal weight volunteers; ##, P < 0.01 versus visceral fat from subjects as assessed by the Kuskal-Wallis nonparametric test. From Matias et al.⁷

Figure 3. Stimulation of mouse 3T3F44 adipocyte differentiation and lipogenesis with the CB₁/CB₂ receptor-agonist HU-210. A, Effect of chronic treatment with HU-210 (100 nM), with or without co-incubation with SR141716 on lipid droplet formation in differentiated adipocytes, as revealed by Oil Red-O staining and microscopic observation (day 8). B, Effect on PPAR-γ and adiponectin expression, in differentiated and mature adipocytes, respectively, of chronic treatment with HU-210, SR141716, and HU-210 + SR141716. Expression was measured by real-time RT-PCR and is expressed as fold enhancement over control (vehicle). Error bars are not shown and they were always less than or equal to 10%. SD values for cycle threshold were always less than 1%. ** P <0.01, *** 0.005 versus vehicle, respectively; ##, P <0.01 versus HU-210.. From Matias et al.⁷

Figure 3. CB₁ receptor blockade attenuates histological changes in adipose tissue of obese mice fed a high-fat diet. White adipocytes from lumbar fat tissue of mice fed a high-fat diet were large and heterogeneous. In contrast, the fat cells of SR141716-treated animals on the high-fat diet were significantly smaller in diameter (by 67%) than fat cells of pair-fed control animals and slightly smaller (around 10%) than those of mice fed a standard chow. HFD, high-fat diet; STD, standard diet. Magnification x400. From Jbilo.¹¹

References

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