Activation of the peripheral endocannabinoid system in human obesity

Abstract

Obesity is the main risk factor for the development of type 2 diabetes. Activation of the central endocannabinoid system increases food intake and promotes weight gain. Blockade of the cannabinoid type 1 (CB-1) receptor reduces body weight in animals by central and peripheral actions; the role of the peripheral endocannabinoid system in human obesity is now being extensively investigated. We measured circulating endocannabinoid concentrations and studied the expression of CB-1 and the main degrading enzyme, fatty acid amide hydrolase (FAAH), in adipose tissue of lean (n = 20) and obese (n = 20) women and after a 5% weight loss in a second group of women (n = 17). Circulating levels of anandamide and 1/2-arachidonoylglycerol were increased by 35 and 52% in obese compared with lean women (P < 0.05). Adipose tissue mRNA levels were reduced by -34% for CB-1 and -59% for FAAH in obese subjects (P < 0.05). A strong negative correlation was found between FAAH expression in adipose tissue and circulating endocannabinoids. Circulating endocannabinoids and CB-1 or FAAH expression were not affected by 5% weight loss. The expression of CB-1 and FAAH was increased in mature human adipocytes compared with in preadipocytes and was found in several human tissues. Our findings support the presence of a peripheral endocannabinoid system that is upregulated in human obesity.

FIG. 1

Expression of CB-1 and FAAH genes in human tissues. Real-time RT-PCR of a multiple tissue human RNA panel, normalized by 18S rRNA expression. Data are given in arbitrary units (AUs) relative to CB-1 or FAAH gene expression in adipose tissue.

FIG. 2

Expression of CB-1 and FAAH by human adipose cells. A: CB-1 and FAAH mRNA was detected by real-time PCR in isolated human adipocytes. B: Confocal microscopy revealed localization of the CB-1 receptor in the adipocyte membrane. The negative control (left; only second antibody added) confirms the specificity of fluorescence signals seen in the right confocal image. C: Increased expression of CB-1 and FAAH genes in isolated mature human adipocytes (AC) compared with isolated human preadipocytes (PAC). CB-1 gene expression data were confirmed by Western blotting (insert). Data are means ± SE from seven pairs and are given as arbitrary units (AUs), normalized by 18S rRNA expression. Group comparison by Student’s t test. *P < 0.05.

FIG. 3

Influence of obesity and weight loss on the peripheral endocannabinoid system. Subcutaneous adipose tissue biopsies and blood samples were obtained from 20 lean and 20 obese postmenopausal women in a cross-sectional study (A) and from 17 obese postmenopausal women before and after a 5% body weight loss by a dietary protocol (B). Circulating levels of AEA and AGs (1/2-AG) were increased in the obese women by 35 and 52%, respectively. In contrast, adipose tissue CB-1 mRNA was decreased by −34% and FAAH mRNA by −59% in the obese subjects. Neither circulating levels of AEA and 1/2-AG nor adipose tissue, CB-1, or FAAH mRNA were influenced by the weight loss protocol. Gene expression is given in arbitrary units (AUs), normalized by GAPDH expression. Group comparison was performed with Student’s t test for independent samples (cross-sectional study) or the t test for paired samples (weight loss study). *P < 0.05 vs. lean.

FIG. 4

Relation of FAAH expression in adipose tissue and circulating endocannabinoids. Both circulating AEA and Ags (1/2-AG) were negatively correlated with the expression of the FAAH gene in 40 human adipose tissue samples. Gene expression is given in arbitrary units (AUs), normalized by GAPDH expression.