The role of CB1 receptors has been investigated in the development of fatty liver induced by ethanol, high-fat diet, and obesity.
- Role of CB1 receptors in the development of alcoholic fatty liver:
Chronic ethanol consumption is known to induce fatty liver, which is an initial stage of alcoholic liver disease characterized by fatty liver, steatohepatitis, fibrosis, and cirrhosis. Jeong et al.
) investigated the role of CB1 receptors in the development of ethanol-induced fatty liver using a mouse model of alcoholic liver injury. Ethanol exposure of male mice in a low-fat diet induced fatty liver as assessed by histological evaluation and hepatic triglyceride levels. In this study, ethanol was administered in Lieber–DeCarli low-fat liquid diet comprised of 27% ethanol calories and 12% fat calories. In contrast to ethanol-exposed mice, pair-fed control mice did not develop fatty liver. Ethanol-induced fatty liver was associated with increased hepatic expression of the gene encoding for CB1 receptors. In these mice, ethanol increased hepatic levels of 2-AG (but not anandamide), which was selectively produced by hepatic stellate cells. Treatment of mice with rimonabant (SR141716), a CB1 receptor antagonist, prevented the development of fatty liver by ethanol. Consistent with rimonabant findings, mice with global or hepatocyte-specific CB1 receptor knockout were resistant to ethanol-induced steatosis. These findings suggest that ethanol induces fatty liver via hepatocyte CB1 receptor activation.
In order to understand the underlying mechanisms of CB1 receptor-mediated induction of fatty liver, these researchers measured the expression/level of a lipogenic transcription factor–SREBP-1c, fatty acid synthase (FAS), and carnitine palmitoyltransferase (CPT-1), which is the rate-limiting enzyme for fatty acid oxidation. Ethanol exposure increased the nuclear level of SREBP-1c in the liver of wild-type mice but not in the liver of global or hepatocyte-specific CB1 receptor knockout mice. Consistent with this, ethanol increased FAS protein levels in the liver of wild-type mice, but this effect was absent or blunted in both global and hepatocyte-specific CB1 receptor knockout mice. On the other hand, ethanol exposure decreased both hepatic CPT-1 protein level and CPT-1 activity in wild-type mice, but these decreases were not seen in hepatocyte-specific CB1 receptor knockout mice. In global CB1 receptor knockout mice, although ethanol did not decrease CPT-1 activity, it did decrease CPT-1 protein levels compared to pair-fed control. Coculture of hepatic stellate cells from ethanol-fed mice with hepatocytes from pair-fed mice upregulated gene expression of CB1 receptor, SREBP-1c, and FAS in hepatocytes. In contrast, in hepatocytes from hepatocyte-specific CB1 receptor knockout mice cocultured with hepatic stellate cells from ethanol-fed mice, CB1 mRNA was absent, and the induction of SREBP-1c and FAS expression was blunted. Taken together, these results suggest that ethanol-induced increase in 2-AG from stellate cells activates hepatocyte CB1 receptors in a paracrine manner, which leads to steatosis via increasing fatty acid synthesis and decreasing fatty acid oxidation.
- Role of CB1 receptors in the development of high-fat diet-induced fatty liver:
Consumption of a high-fat diet is associated with obesity and development of hepatic steatosis. Osei-Hyiaman et al.
) used this approach to investigate the role of CB1 receptors in the development of fatty liver. Feeding of high-fat diet (60% of total calories) to wild-type mice for 14 weeks made these animals obese, and they developed fatty liver as diagnosed by histological examination and triglyceride accumulation. In contrast, global CB1 receptor knockout mice on high-fat diet remained lean, and they did not develop fatty liver despite consuming caloric intake similar to that of wild-type mice on the high-fat diet. In addition, high-fat diet upregulated hepatic expression of CB1 receptors in wild-type mice and increased hepatic levels of anandamide, but not 2-AG, in both wild-type and CB1 receptor knockout mice. Furthermore, in wild-type mice, high-fat diet increased basal rates of fatty acid synthesis, which was blocked by the CB1 antagonist rimonabant. Taken together, these results suggest that high-fat diet-induced steatosis resulting from increased fatty acid synthesis is mediated via anandamide-induced CB1 receptor activation. Mechanistic studies by these investigators further revealed that activation of CB1 receptors by CB1 agonist (HU-210) in wild-type mice, maintained on regular chow diet, increased hepatic gene expression of the lipogenic transcription factor SREBP-1c and its target enzymes, acetyl-CoA carboxylase-1 (ACC-1) and FAS. These effects were blocked or prevented by CB1 antagonist. Consistently, treatment with CB1 agonist also increased de novo
fatty acid synthesis in the liver or in isolated CB1 receptor-expressing hepatocytes, and this effect was also blocked or prevented by CB1 antagonist, further corroborating the role of CB1 receptor activation in fatty acid synthesis.
In order to discern the specific role of liver CB1 receptors in the development of steatosis, researchers have used liver-specific CB1 knockout mice along with global knockout mice (17
). In this study, global CB1 receptor knockout mice were totally resistant to high-fat-induced fatty liver and obesity. In liver-specific CB1 knockout mice, high-fat diet induced obesity and increased hepatic triglyceride levels to some extent, but this increase was significantly less (about 75% over pair-fed control) than that observed in wild-type mice (about 200% over pair-fed control) maintained on high-fat diet. In this study, high-fat diet was also shown to increase CB1 expression in hepatocytes in wild-type mice (17
). Furthermore, CB1 receptor agonist (HU-210) increased hepatic de novo
lipogenesis in wild-type chow-fed mice, but not in global CB1 knockout or liver-specific CB1 knockout mice. These findings support the contention that high-fat diet induces fatty liver primarily via activation of hepatic CB1 receptors.
In addition to hepatic lipogenesis, CB1 receptor activation also appears to regulate hepatic fatty acid oxidation by modulating the activity of hepatic CPT-1, the rate-limiting enzyme in fatty acid β-oxidation. High-fat diet, compared to chow-fed control, significantly reduced the activity of hepatic CPT-1 in wild-type mice, but not in global or liver-specific CB1 receptor knockout mice. In addition, rimonabant treatment of high-fat diet-fed mice increased protein levels of CPT1A, the hepatic isoform of CPT1. Furthermore, in wild-type chow-fed mice, CB1 receptor agonist (HU210) significantly decreased hepatic CPT-1 activity, whereas rimonabant significantly increased CPT-1 activity and prevented the inhibitory effect of subsequently administered HU-210. Taken together, these results suggest that CB1 activation decreases fatty acid oxidation via decreasing CPT-1 activity, which may contribute, in part, to high-fat diet-induced steatosis.
- Role of CB1 receptors in obesity-associated fatty liver:
Obesity is a chronic metabolic disorder characterized by increased body weight and development of adipose tissue with excessive fat storage, and it is invariably associated with fatty liver. The Zucker fatty rats (fa/fa) are genetically obese with defective leptin receptors. These rats have elevated plasma leptin levels and are resistant to exogenous leptin administration. In addition, they have severe hepatic steatosis characterized by accumulation of fat within hepatocytes. Gary-Bobo et al.
) investigated the role of CB1 receptors in the development of fatty liver in Zucker rats by administering rimonabant (SR141716), a CB1 receptor antagonist. In this study, oral treatment of obese (fa/fa) rats with rimonabant (30 mg/kg) daily for 8 weeks abolished hepatic steatosis and attenuated hepatomegaly. Liver slices from the obese (fa/fa) rats treated with rimonabant were found to be histologically comparable to those from lean rats. In contrast, in pair-fed rats, which consumed the same amount of food as that consumed by rimonabant-treated rats, steatosis and hepatomegaly were not significantly reduced. This suggests that rimonabant, and not reduced food intake, was responsible for reducing steatosis. Taken together, these results indicate that in the obese rats, development of fatty liver is mediated via activation of CB1 receptors.