Introduction

Endocannabinoids increase food intake through their interaction with the leptin-regulated central neural appetitive circuitry (1, 2), and pharmacological blockade of CB1 receptors has shown promise in the treatment of visceral obesity and the metabolic syndrome (3–5). Mice exposed to high-fat diets develop obesity, steatosis, and insulin and leptin resistance as well as plasma lipid changes similar to those associated with the metabolic syndrome (6). Mice deficient in CB1 receptors have a lean phenotype (7) and are resistant to these diet-induced changes, even though their total caloric intake is not different from that of wild-type mice fed the same diet (8, 9). Similarly, chronic treatment of obese mice or rats with a CB1 antagonist induces sustained weight loss and protects rodents from this detrimental metabolic phenotype, even though it causes only a transient reduction in food intake (10–13). These findings indicate that endocannabinoids can directly affect peripheral energy metabolism by mechanisms unrelated to their effect on appetite. We recently reported that activation of CB1 receptors in wild-type mice stimulates de novo lipogenesis in the liver through induction of the lipogenic transcription factor SREBP1c and its target enzymes acetyl-CoA carboxylase–1 (ACC1) and fatty acid synthase (FAS) (9). Furthermore, a high-fat diet increased the hepatic levels of the endocannabinoid anandamide and CB1 receptors as well as the basal rate of de novo lipogenesis, and the latter could be attenuated by CB1 blockade (9). Although these findings strongly implicate hepatic CB1 receptors in the development of diet-induced obesity and related metabolic changes, they do not exclude the alternative possibility that endocannabinoids act on CB1 receptors in the CNS to influence peripheral energy metabolism indirectly or produce their effects via CB1 receptors at extrahepatic sites, such as the adipose tissue (14). For example, centrally acting leptin was previously shown to affect hepatic glucose fluxes (15) and hepatic lipid metabolism (16), possibly through neural input to the liver. Moreover, communication between the liver and adipose tissue may occur through neural pathways involving hepatic vagal afferents (17). In order to more directly test the role of hepatic CB1 receptors in diet-induced metabolic changes, we generated a mouse model with a hepatocyte-selective deletion of CB1 receptors and compared the metabolic effects of a high-fat diet as well as the effects of cannabinoid agonist and antagonist treatment with those observed in wild-type mice and in mice with global knockout of CB1 receptors.
The resistance of CB1 receptor–deficient mice to diet-induced obesity and steatosis despite their similar caloric intake indicates enhanced energy expenditure and fat elimination, but the mechanism by which CB1 receptor deficiency or blockade increases energy expenditure is not yet clear. Therefore, we also investigated the effects of pharmacological activation, inhibition, or genetic ablation of CB1 receptors on the gene expression and enzymatic activity of carnitine palmitoyltransferase–1 (CPT1), the rate-limiting enzyme in fatty acid β-oxidation. Overall, our results indicated that endocannabinoids acting at hepatic CB1 receptors mediated diet-induced steatosis through increasing de novo lipogenesis and inhibiting fatty acid oxidation in the liver. Endocannabinoids also played an essential role in diet-induced changes in plasma lipid profile and insulin and leptin resistance, but did not contribute to the parallel increase in fat deposition in adipose tissue and the resulting diet-induced obesity.

Results
High-fat diet feeding upregulates CB1 receptors in hepatocytes. We previously demonstrated increased expression of CB1 receptors in whole liver tissue of mice fed a high-fat diet for 3 weeks (9). To further define the cellular localization of CB1 receptors regulated by the diet, we used Western blotting to analyze CB1 receptor protein levels in isolated pure fractions of hepatocytes from wild-type mice fed regular chow or high-fat diet for 3 weeks. As shown in Figure 1A, there was a marked increase in the level of CB1 receptor protein in cells from the latter group compared with controls.

J. Clin. Invest. doi:10.1172/JCI34827.