Obesity-induced accumulation of ectopic fat in the liver is thought to contribute to the development of insulin resistance, and increased activity of hepatic cannabinoid-1 receptors (CB1R) has been shown to promote both processes. However, lipid accumulation in liver can be experimentally dissociated from insulin resistance under certain conditions, suggesting the involvement of additional mechanisms. Obesity is also associated with proinflammatory changes which, in turn, can promote insulin resistance. Kupffer cells (KCs), the livers resident macrophages, are the major source of proinflammatory cytokines in the liver, such as TNF-, which has been shown to inhibit insulin signaling in multiple cell types, including hepatocytes. We sought to identify the role of CB1R in KCs in obesity-induced hepatic insulin resistance. For this, we used intravenously administered -D-glucan-encapsulated siRNA to knock-down CB1R gene expression selectively in KCs. We demonstrated that a robust knock-down of the expression of Cnr1 limited to KCs, the gene encoding CB1R, results in improved glucose tolerance and insulin sensitivity in diet-induced obese mice, without affecting hepatic lipid content or body weight. Moreover, Cnr1 knock-down in KCs was associated with a shift from pro-inflammatory M1 to anti-inflammatory M2 cytokine profile and improved insulin signaling as reflected by increased insulin-induced Akt phosphorylation. These findings suggest that CB1R expressed in KCs play a critical role in obesity-related hepatic insulin resistance via a pro-inflammatory mechanism. This study has been published in Molecular Metabolism. In another study, we examined the role of CB1R expressed on glomerular podocytes in type I diabetic nephropathy (DN). Glomerular podocytes are specialized epithelial cells with an essential role in maintaining the integrity of the glomerular filtration apparatus, and podocyte injury is a pathogenic feature of DN. Podocytes express components of the endocannabinoid system, including CB1R, activation of which mediates podocyte injury caused by both hyperglycemia and increased renin angiotensin system activity. Using podocyte-specific CB1R knockout (pCB1Rko) mice developed in our laboratory, we found pCB1Rko mice are partially protected against streptozotocin (STZ)-induced type-1 DN. Although STZ-treated pCB1Rko mice are as hyperglycemic as their wild-type littermates, they display less albuminuria, higher GFR and less podocyte loss than STZ-treated wild-type mice. Unexpectedly, pCB1Rko mice also have milder tubular dysfunction, fibrosis and reduction of cortical microcirculation than wild-type controls. Thus, activation of CB1R in podocytes contributes to both glomerular and tubular dysfunction in type-1 DN, which underlies the therapeutic potential of peripheral CB1R blockade. This study was published in Diabetes Obesity and Metabolism. In a third study, we analyzed the molecular mechanisms involved in the catabolic effects of peripheral CB1R blockade in the liver in diet-induced obesity/metabolic syndrome. Endocannabinoids promote energy conservation in obesity, whereas CB1R blockade reverses body weight gain and insulin resistance and increases energy expenditure. Exposure of primary mouse hepatocytes and HepG2 cells to the CB1R agonist ACEA inhibited the expression of Sirt1 and Rictor, a component of mTORC2, and suppressed insulin-induced Akt phosphorylation at ser473. These effects were reversed by peripheral CB1R antagonist JD5037 in control hepatocytes but not in cells deficient in Sirt1 and/or Rictor, indicating that these two proteins are required for the CB1R-mediated inhibition of insulin signaling. Feeding C57BL/6J mice a high-fat diet (HFD) inhibited hepatic Sirt1/mTORC2/Akt signaling, and the inhibition was reversed by rimonabant or JD5037 in wild-type but not liver-specific Sirt1-/- (Sirt1-LKO) mice, to levels observed in hepatocyte-specific CB1R-/- (LCB1R-/-) mice. A similar attenuation of hyperglycemia and hyperinsulinemia in obese wild-type but not Sirt1-LKO mice could be attributed to insufficient reversal of HFD-induced mitochondrial ROS generation in peripheral tissues in the latter. In contrast, JD5037 treatment was equally effective in HFD-fed wild-type and Sirt1-LKO mice in reducing hepatic steatosis, increasing fatty acid -oxidation, activating AMPK and normalizing circulating FGF21 levels and the expression of FGF21 receptor Fgfr1 and co-receptor -Klotho (Klb) in adipose tissue and brain, resulting in a similar increase in total energy expenditure in the two strains. We conclude that peripheral CB1R blockade in obese mice improves glycemic control via hepatic Sirt1/mTORC2/Akt pathway, whereas it increases fatty acid oxidation via AMPK and FGF21 signaling. This study has been written up and submitted for publication.
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