We read with great interest the article by Teixera-Clerc et al.,1 regarding the hepatoprotective properties displayed by cannabinoid receptor 2 (CB2) agonists in a mouse model of carbon tetrachloride (CCl4)-induced liver injury. Acute hepatitis induced by CCl4 was accelerated in CB2 knockout mice, resulting in increased serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels compared to wild-type (WT) animals. Conversely, the ALT and AST levels were reduced in CCl4-treated WT mice following the treatment with the CB2 agonist JWH-133.
In apparent disagreement with these results, the same group previously demonstrated that, in the liver of obese mice which were fed a high-fat diet, CB2 stimulation potentiated insulin resistance and nonalcoholic fatty liver disease (NAFLD), whereas CB2 inactivation reduced liver inflammation.2 In obese children, NAFLD ranges from simple liver steatosis to steatosis associated with inflammation and increased liver enzymes, which may eventually progress to liver cirrhosis.3
To highlight the role of CB2 in NAFLD progression, we took advantage of a functionally relevant CB2 polymorphism. We studied the rs35761398 variant of the CNR2 (cannabinoid receptor 2; GeneID 1269) gene, encoding for the CB2 receptor, which causes the substitution of a glutamine with a positively charged arginine at codon 63, in the first intracellular signaling loop. Receptor carrying arginine shows reduced function when activated by an endogenous cannabinoid.4
The variant was screened by direct sequencing in 438 Italian obese children with liver steatosis at ultrasound imaging in order to evaluate the association of the CB2 receptor Gln63Arg (Q63R) functional variant with serum AST and ALT levels. Results from this analysis (Table 1) demonstrate that the less functional R63 variant is statistically associated with higher ALT and AST levels independent of sex, age and pubertal stage. The variant is not associated with Body Mass Index, insulin resistance (assessed by Homeostasis Model Assessment of Insulin Resistance score) and size of abdominal fat (assessed by waist-to-hip ratio).
|N (%)||27 (7.5)||222 (50.7)||189 (41.8)|
|Females (%)||14 (53)||115 (52)||102 (54)|
|Prepubertal (%)||13 (48.1)||111 (50)||93 (49.2)|
|Age (years)||10.7 ± 1.9||11.3 ± 2.6||11.4 ± 3.2||0.29|
|BMI SDS||2.9 ± 0.4||3 ± 0.6||3.1 ± 0.7||0.043|
|W/Hr||0.65 ± 0.03||0.64 ± 0.05||0.65 ± 0.07||0.16|
|HOMA-IR||5.7 ± 3.4||6.5 ± 4.8||6.6 ± 4.3||0.78|
|ALT (U/L)||24.4 ± 8.6||32.3 ± 16.4||37.2 ± 21.4||0.0001|
|AST (U/L)||22.5 ± 6.5||24.5 ± 7.0||27 ± 13.2||0.002|
To be homozygous for the R63 allele results in a significant risk factor for liver damage, with an odds ratio for ALT >40 of 2.7 (95% confidence interval = 1.65-3.92, P = 0.001).
These data, obtained using an approach (i.e., association study in humans) completely different from that of previously published studies (i.e., analysis in murine models), showing that the partially inactive R63 CB2 variant is associated with liver damage progression in obese children with liver steatosis, corroborate the idea of a hepatoprotective role for CB2 receptors.