Endogenous cannabinoids and their CB1 receptors play an important role in the control of body weight, and the CB1 receptor blocking drug rimonabant has been recently released in Europe for the treatment of obesity/metabolic syndrome. In a previous study reported last year we provided evidence that the liver is a major target of the metabolic effects of endocannabinoids in diet-induced obesity in mice. This study has established that endocannabinoids stimulate hepatic lipogenesis via CB1 receptors, and an increase in this effect in response to high fat diet plays an essential role in the development of diet-induced obesity. The resistance of CB1 receptor deficient (CB1-/-) mice to diet-induced obesity despite similar caloric intake further suggested that these mice must also have increased energy expenditure. A major mechanism of energy expenditure is fatty acid beta-oxidation, for which the rate limiting enzyme is carnitine palmitoyl transferase 1 (CPT1). An upstream regulator of CPT1 activity is AMP kinase, activation of which inhibits lipogenesis and activates fatty acid beta-oxidation. In ongoing experiments we found that treatment of mice with CB1 receptor agonists decreases hepatic CPT1 activity and AMP kinase activities, whereas pharmacologic blockade or genetic ablation of CB1 results in increased hepatic CPT1 and AMP kinase activities. The most important known activator of fatty acid beta-oxidation is the adipocyte-derived hormone, adiponectin, whose plasma levels were reported to increase in response to rimonabant treatment. Therefore, we have started experiments using adiponectin receptor-2 knockout mice to test whether the effects of cannabinoids on CPT1 and AMP kinase are mediated directly via CB1 receptors in hepatocytes, or indirectly through changes in adiponectin secretion by adipocytes.? Additional experiments have been aimed to determine the relative importance of CB1 receptors located in the liver vs in the CNS in the hepatic lipogenic effect of cannabinoids. Hepatic lipogenesis was quantified in rats by the incorporation of tritium into hepatic fatty acids following intrahepatic injection of tritiated water. The stimulation of hepatic lipogenesis in response to CB1 agonist treatment was reduced by about 50% in animals previously subjected to hepatic vagal denervation. These preliminary results suggest that both central and peripheral CB1 receptors contribute to the activation of hepatic lipogenesis.? ? In a separate study just published we analyzed the biosynthesis of the endocannabinoid anandamide (arachidonoyl ethanolamide) in a mouse macrophage cell line (RAW264.7). In these cells, bacterial endotoxin (LPS) causes a dramatic increase in anandamide levels, which has been implicated in the hypotension of septic shock an advanced liver cirrhosis. Anandamide can be generated from its membrane precursor, N-arachidonoyl phosphatidyl ethanolamine (NAPE) through cleavage by a phospholipase D (NAPE-PLD). In this study we reported on a novel biosynthetic pathway for anandamide that involves the phospholipase C (PLC)-catalyzed cleavage of NAPE to generate phospho-anandamide, which is subsequently dephosphorylated by phosphatases, including PTPN22, previosuly described as a protein tyrosine phosphatase. LPS-induced anandamide synthesis in macrophages is mediated exclusively through the PLC/phosphatase pathway, which is upregulated by LPS, whereas NAPE-PLD is down-regulated by LPS and functions as a salvage pathway of anandamide synthesis when the PLC/phosphatase pathway is compromised. Both PTPN22 and endocannabinoids have been implicated in autoimmune diseases, suggesting that the PLC/phosphatase pathway may be a therapeutic target.? ? Endocannabinoids and CB1 receptors are essential components of the mesolimbic dopaminergic reward pathway, and CB1 receptor blockade was found to disrupt drug-seeking behavior, including voluntary alcohol drinking in rodent models. Based on such findings in animal studies, including our own, we had earlier started a phase I/II clinical trial to assess the safety of rimonabant treatment in young, heavy drinking subjects and its efficacy to reduce their desire to drink. This is a double-blind, placebo controlled study involving forty heavy drinking subjects between the ages of 21 and 45 years who take rimonabant (20 mg/day) or placebo for two weeks, followed by an in-hospital laboratory drinking paradigm where their desire to drink as well as their physiological, psychological and hormonal response to exposure to alcohol and drinking is evaluated. This study has just been completed and analysis of the results will start soon.
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