Endocannabinoids (ECs) acting via CB1 receptors (CB1R) play an important role in the control of body weight and energy homeostasis. In clinical trials, the CB1R blocking drug rimonabant had been found effective in reducing body weight and improving cardiometabolic abnormalities in patients with the metabolic syndrome, but was withdrawn from the market in 2008 due to neuropsychiatric side effects. Several studies, including our own, indicate that CB1R in peripheral tissues contribute to the metabolic benefit of CB1R blockade (JCI 115:1298, 2005;JCI 118:3160-9, 2008), raising the possibility that peripheral CB1R may be selectively targeted in the treatment of the metabolic syndrome in order to minimize neuropsychiatric side effects. This concept was supported by our recent studies, using a peripherally restricted neutral CB1 antagonist(AM6545;JCI 120:2953-66, 2010) or the peripheral CB1 inverse agonists JD5037 (Cell Metabolism 16:167-79, 2012) in mice with high-fat diet-induced obesity (DIO). DIO mice replicate the components of the human metabolic syndrome, including insulin resistance, but do not become overtly diabetic due to compensatory hyperplasia of islet beta cells, the source of insulin. We therefore tested the effect of peripheral CB1 blockade in a rat model of type-2 diabetes (T2DM). T2DM progresses from compensated insulin resistance to beta cell failure resulting in uncompensated hyperglycemia, a process replicated in the Zucker diabetic fatty (ZDF) rat. The Nlrp3 inflammasome has been implicated in obesity-induced insulin resistance and beta cell failure, and ECs are known to promote insulin resistance via peripheral CB1 receptors, and can also cause beta cell loss. In a study in ZDF rats, we demonstrated that beta cell loss in the adult animals is not associated with CB1 signaling in beta cells, but rather in M1 macrophages infiltrating into pancreatic islets, and that this leads to activation of the Nlrp3-ASC inflammasome in the macrophages. These effects are replicated in vitro by incubating wild-type human or rodent macrophages, but not macrophages from CB1-/- or Nlrp3-/- mice, with the endocannabinoid anandamide. Peripheral CB1 blockade with JD5037, clodronate-induced in vivo depletion of macrophages, or macrophage-specific siRNA-mediated knockdown of CB1 receptors reverses or prevents these changes and restores normoglycemia and glucose-stimulated insulin secretion. These findings implicate ECs and inflammasome activation in beta cell failure, and identify macrophage-expressed CB1 receptors as a therapeutic target in T2DM. These findings have been published in Nature Medicine. Obesity is associated with increased activity of two lipid signaling systems - ECs and ceramides with both being implicated in insulin resistance. CB1 antagonists reverse obesity and insulin resistance, but have psychiatric side effects. We analyzed the role of ceramide in CB1R-mediated insulin resistance in DIO mice, using JD5037, a peripherally restricted CB1R inverse agonist. Chronic JD5037 treatment of DIO mice reduced body weight and steatosis, and improved glucose tolerance and insulin sensitivity. Peripheral CB1R blockade also attenuated the diet-induced increase in C14:0, C16:0, C18:0 and C20:0 ceramide species with either C16 or C18 sphingosine-base in the liver. Decreased ceramide levels reflected their reduced de novo synthesis, due to inhibition of the activity of serine-palmitoyl transferase (SPT) and the expression of its SPTLC3 catalytic subunit, as well as reduced ceramide synthase (CerS) activity related to reduced expression of CerS1 and CerS6. JD5037 treatment also increased ceramide degradation due to increased expression of ceramidases. In primary cultured mouse hepatocytes and HepG2 cells, the EC anandamide increased ceramide synthesis in an eIF2-dependent manner, and inhibited insulin-induced akt phosphorylation by increased serine phosphorylation of IRRS1 and increased expression of the serine/threonine phosphatase Phlpp1. These effects were abrogated by JD5037 or the SPT inhibitor myriocin. Chronic treatment of DIO mice with myriocin or JD5037 similarly reversed hepatic insulin resistance, as verified using a euglycemic/hyperinsulinemic clamp. These findings indicate that ECs induce CB1R-mediated, ER stress-dependent synthesis of specific ceramide subspecies in the liver, which plays a key role in obesity-related hepatic insulin resistance. These results have been published in Hepatology. High-fat diet (HFD)-induced obesity and insulin resistance are associated with increased activity of the EC/CB1 receptor system that promotes the hepatic expression of lipogenic genes, including stearoyl CoA desaturase-1 (SCD1). Mice deficient in either CB1 or SCD1 remain lean and insulin sensitive on HFD, suggesting a functional link between the two systems. The HFD-induced increase in the hepatic levels of the anandamide (AEA) has been attributed to reduced activity of the AEA-degrading enzyme, fatty acid amide hydrolase (FAAH). We have demonstrated that HFD-induced increased hepatic AEA levels and decreased FAAH activity are absent in SCD1-/- mice, and the monounsaturated fatty acid (MUFA) products of SCD1, palmitoleic and oleic acid, inhibit FAAH activity in vitro at low micromolar concentrations. HFD markedly increases hepatic SCD1 activity in wild-type mice as well as in CB1R-/- mice with transgenic re-expression of CB1R in hepatocytes (htgCB1R-/- mice), but not in global CB1R-/- mice. Treatment of HFD-fed mice with the SCD1 inhibitor A939572 prevents the diet-induced reduction of hepatic FAAH activity, normalizes hepatic AEA levels, and improves insulin sensitivity. SCD1-/- mice on HFD remain insulin sensitive, but develop glucose intolerance and insulin resistance in response to chronic treatment with the FAAH inhibitor URB597. HFD rich in MUFA or feeding mice pure oleic acid fail to inhibit hepatic FAAH activity. We conclude that MUFA generated via SCD1 activity, but not diet-derived MUFA, function as endogenous FAAH inhibitors mediating the HFD-induced increase in hepatic AEA, which then activates hepatic CB1R to induce insulin resistance. This study is under review by PNAS.

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National Institute on Alcohol Abuse and Alcoholism
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