Factors secreted from white adipose tissue (WAT) play a significant role in the regulation of food intake. As there is also considerable evidence to support a role for the endocannabinoid system in the control of appetite, the presence of endocannabinoid system components has been examined in WAT. However, there are mixed reports on the expression of cannabinoid-1 (CB₁) receptors in rodent adipose tissue. Some studies indicate expression (Cota et al., 2003; Bensaid et al., 2003), whilst others have failed to identify CB₁ mRNA (Nieri et al., 2003). On close examination of the primers used to detect CB₁ mRNA in these studies, it is evident that they may not be of an optimal design and this may be a confounding factor in the interpretation of the data. The aim of this study was to undertake a systematic analysis of CB₁ expression in rodents and to extend this investigation to human adipose tissue.

We have employed RT-PCR to identify the expression of the CB₁ gene in tissues obtained from freely-fed adult CD-1 mice (35 – 40 g), in a human adipocyte cell line (SGBS cells) and in subcutaneous and omental fat from 2 human subjects undergoing gastroplasty. Ethical permission was obtained through the Sefton Ethics Committee; the subjects gave informed consent for the removal of fat samples.

A band consistent with CB₁ mRNA was readily detectable in mouse brain, kidney and epididymal WAT (Figure 1), and sequencing confirmed the identity of the 598-bp product. Other major WAT depots (perirenal, omental, mesenteric, subcutaneous) and brown adipose tissue (BAT) were also examined. CB₁ was expressed in each, apart from omental WAT and BAT. As mature adipocytes constitute at most 50% of the total cell content of WAT, epididymal fat was also digested with collagenase and the adipocyte and S-V fraction separated. RT-PCR indicated that CB₁ mRNA was present in both fractions, but the signal appeared more intense in the mature adipocytes. RT-PCR was also used to investigate expression of the CB₁ gene in human adipose tissue. CB₁ mRNA was present in both human subcutaneous and omental fat depots, and was additionally detected in human SGBS adipocytes differentiated in cell culture.

Figure 1: CB₁ gene expression in mouse adipose and non-adipose tissues (epi, epididymal WAT; -ve, no template control).

The present study demonstrates that the CB₁ gene is expressed in the major WAT depots of mice, and in the mature adipocytes. The gene is also expressed in human WAT, indicating that it is not simply a factor characteristic of rodents. Further studies are required to establish the physiological role of CB₁ receptors in adipose tissue.

Bensaid M, et al., (2003) Mol. Pharmacol. 63, 908-914.
Cota D, et al., (2003) J.Clin.Invest. 112, 423-431.
Nieri P, et al., (2003) Naunyn SchmiedebergsArch Pharmacol. 368, 352-359.