Lack of beta 3-adrenergic receptor mRNA expression in adipose and other metabolic tissues in the adult human.
The beta 3-adrenergic receptor (beta 3AR) has been purported to play important roles in a number of metabolic functions, suggesting that beta 3AR agonists might be useful as antidiabetic and antiobesity therapeutic agents. However, these assertions are based entirely on extensive metabolic studies with such agonists in rodents. To clarify the role that the beta 3AR might have in humans, we sought to define the tissue distribution of the beta 3AR in adult human tissue by the use of a highly specific and sensitive approach. Northern blots of selected tissues failed to reveal any beta 3AR mRNA, suggesting little or no expression. To detect minute amounts of transcripts, we developed a reverse transcriptase-polymerase chain reaction (RT-PCR) method that uses primers to amplify a region of the beta 3AR that has little homology with the closely related beta 1- and beta 2-AR genes, and we verified the specificity of this approach using plasmids containing the cloned human beta 1-, beta 2-, and beta 3AR genes. RT-PCR performed on as little as 20 ng of total RNA from 3T3-F442A cells, which expressed beta 3AR at very low levels (approximately 20 fmol/mg of protein), provided an easily detectable signal by ethidium bromide staining and Southern blotting of electrophoresed products. RT-PCR was performed on RNA obtained from 23 different human tissues, using primers for the beta 3AR, the beta 2AR, and beta-actin, which acted as a control. Whereas beta-actin and the beta 2AR were detected in virtually all tissues, RT-PCR using beta 3AR primers gave products from 13 tissues, including skeletal muscle, lung, adipose tissue, kidney, small intestine, pancreas, spleen, and adrenal gland. An end-labeled 50-nucleotide probe identical to an internal region of the expected beta 3AR product hybridized under low stringency conditions to seven of these products. However, sequencing of these products, which were somewhat smaller in molecular size than expected, did not reveal beta 3AR DNA sequence. Given the specificity and sensitivity of our approach, we conclude that the beta 3AR is not expressed to any significant degree in the adult human tissues studied, including adipose tissue and other metabolic sites.