PCR-select subtraction for characterization of messages differentially expressed in brown compared with white adipose tissue

To understand the energy metabolism occurring in brown adipose tissue (BAT), we subtracted the messages expressed in white adipose tissue (WAT) from those in BAT. Thereby we succeeded in identifying 37 cDNA clones as being significantly expressed in BAT but not in WAT. Of these, 24 clones were found to code for mitochondrial proteins. Since BAT is well known to have a higher mitochondrial content than WAT, these results would seem to reflect simply the differences in mitochondrial content between BAT and WAT. To examine this possibility, we next measured the amount of mitochondrial DNA (mtDNA) in various rat tissues. As a result, the mtDNA copy number per cell was found to be markedly different among the tissues analyzed, and the highest value of about 5.3 * 10^4 copies per cell was observed with the rat brain. BAT showed a value similar to that of brain, but this value was only about 3.5-fold higher than that for WAT. Since observed differences in mitochondrial content between BAT and WAT was smaller than those observed with transcript levels of proteins, we conclude that the observed differences in the transcript levels of certain proteins between BAT and WAT reflect the functional differences between BAT and WAT, and do not reflect the differences in mitochondrial content between BAT and WAT.