The endocannabinoid system (ECS), which includes endogenous cannabinoids, their receptors, and the enzymes involved in their biosynthesis and degradation, plays an important role in the control of body weight and energy homeostasis. In several large clinical trials, the CB1 receptor (CB1R) blocking drug rimonabant had been found effective in reducing body weight and improving cardiometabolic abnormalities in patients with the metabolic syndrome, but was recently withdrawn from the market due to neuropsychiatric side effects, including depression, anxiety and suicidal ideation. We had earlier provided evidence that the liver is a major target of the metabolic effects of endocannabinoids in mice with diet-induced obesity (DIO)(JCI 115:1298, 2005). In a subsequent study using mice with liver-specific knockout of CB1R we have demonstrated that activation of hepatic CB1R contributes to steatosis, insulin and leptin resistance as well as dyslipidemias, but not to increased adiposity (JCI 118:3160-69, 2008). Functional CB1R are also present in adipocytes, skeletal muscle and pancreatic beta cells, raising the possibility that peripheral CB1 receptors may be targeted for the treatment of fatty liver, impaired glucose homeostasis, and dyslipidemia in order the minimize the neuropsychiatric side effects of non-selective CB1 blockade during the treatment of obesity-associated conditions. In a collaborative study with Alex Makriyannis (Northeastern Univ., Boston), we have developed a peripherally restricted CB1R antagonist. AM6545 is a novel, orally bioavailable CB1R neutral antagonist with high CB1R binding affinity similar to that of the brain-penetrant CB1R antagonist rimonabant. AM6545 has markedly reduced brain penetrance, with a brain to plasma ratio of 0.03 compared to 0.76 for rimonabant. Unlike rimonabant, AM6545 fails to inhibit central CB1R-mediated increases in locomotor activity or cataleptic behavior, and does not induce anxiety-related behaviors in mice. On the other hand, AM6545 has effects similar to rimonabant on hormonal/metabolic parameters in mice with diet-induced obesity (DIO). Treatment of DIO mice with 10 mg Kg-1 day-1 AM6545 for 28 days significantly reduced body weight without an overall effect on caloric intake, by shifting energy expenditure from carbohydrate to fat oxidation. Elevated serum leptin, glucose and insulin levels were significantly reduced or normalized, while adiponectin levels increased. AM6545 treatment ameliorated glucose tolerance and insulin sensitivity. Finally, AM6545 treatment fully reversed the diet-induced hepatic steatosis and hepatocellular damage, as well as CB1R-mediated de novo hepatic lipogenesis. Thus, selective blockade of peripheral CB1R is sufficient to improve the metabolic abnormalities associated with obesity, which suggests that non brain-penetrant CB1R antagonists may have value in the treatment of these disorders in humans. These findings have been published in the Journal of Clinical Investigation. Our earlier findings using hepatocyte-specific CB1R knockout mice indicated that hepatic CB1R are necessary for diet-induced steatosis, insulin and leptin resistance and dyslipidemias to develop in mice. To address the question whether activation of hepatic CB1R is sufficient for these effect, we have developed a rescue model, transgenic mice that express CB1R only in hepatocytes. This was achieved by transfecting fertilized egges of CB1R knockout mice with a construct of the mouse CB1R driven by the mouse albumin promoter. We have established several lines with variable degrees of CB1R expression in hepatocytes and verified the absence of CB1R in all other tissues. When fed a high fat diet, these mice remained lean, but developed glucose intolerance and insulin resistance. Also, glucose intolerance and insulin resistance can be acutely induced by the ip injection of the endocannabinoid anandamide in wild-type and liver CB1Rtransgenic mice, but not in CB1R-/- or hepatocyte-specific CB1R-/- (LCB1R-/-) mice. CB1-/- mice with transgenic overexpression of CB1 restricted to hepatocytes are hyperinsulinemic due to reduced insulin clearance, yet have increased hepatic glucose production due to increased glycogenolysis, indicating hepatic insulin resistance. CB1 activation induces the endoplasmic reticulum (ER) stress response in liver. In human and murine isolated hepatocytes, CB1 activation causes ER stress-dependent suppression of insulin-induced phosphorylation of akt-2 through stimulation of the ser/thre phosphatase Phlpp1. In human liver, CB1 expression is upregulated in non-alcoholic fatty liver disease. In conclusion, endocannabinoids contribute to diet-induced insulin resistance via hepatic CB1-mediated inhibition of insulin signaling and clearance. Thes findings have been written up and submitted for publication. 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 have completed 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 was a double-blind, placebo controlled study involving forty heavy drinking subjects (consuming between 20 and 50 drinks per week) between the ages of 21 and 45 years who took 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 was evaluated. Results from both the outpatient call-ins and the alcohol self-administration paradigm failed to show a statistically significant difference between those receiving rimonabant or placebo. This suggests that the daily administration of 20 mg rimonabant for two weeks has no effect on alcohol consumption in heavy social drinkers. A manuscript summarizing these results has been published in Psychopharmacology. Chronic alcoholism can result in fatty liver that can progress to steatohepatitis, cirrhosis and liver cancer. Mice fed alcohol develop fatty liver through endocannabinoid activation of hepatic CB1 cannabinoid receptors (CB1R), which increases lipogenesis and decreases fatty-acid oxidation. Chronic alcohol feeding also upregulates CB1R in hepatocytes in vivo, which could be replicated in vitro by co-culturing control mouse hepatocytes with stellate cells (HSC) isolated from ethanol-fed mice, implicating HSC-derived mediator(s) in the regulation of hepatic CB1R. HSC being a rich source of retinoic acid (RA), we tested whether RA and its receptors may regulate CB1R expression in primary mouse hepatocytes. Incubation of hepatocytes with RA or RA receptor (RAR) agonists increased CB1R mRNA and protein, the most efficacious being the RAR-agonist CD437 and the pan-RAR agonist TTNPB. The endocannabinoid 2-AG also increased hepatic CB1R expression, which was mediated indirectly via RA, as it was absent in hepatocytes from mice lacking retinaldehyde dehydrogenase 1, the enzyme catalyzing the generation of RA from retinaldehyde. Liganded RAR regulates gene transcription by binding to RAR response elements in the promoters of target genes. The binding of RAR to the CB1R gene 5 upstream domain in hepatocytes treated with RAR agonists or 2-AG was confirmed by chromatin immunoprecipitation and electrophoretic mobility shift and antibody supershift assays. Finally, TTNPB-induced CB1R expression was attenuated by siRNA knock-down of RAR in hepatocytes. These findings indicate that RAR regulates CB1R expression and is thus involved in the control of hepatic lipid metabolism (published in JBC)
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