Is all fat created equal?: A comparison of absolute copy number for leptin expression from fish to mammals

We cloned complete cDNAs for Bowhead whale, and compared the absolute copy number of mRNA for the whales to other mammals and fish. In mammals the hormone leptin is most robustly expressed by adipose tissues. Of its many functions, the role of leptin in fat regulation has been the most studied. Most expression studies examine relative copy number among tissues and/or treatments within the same species. Relative expression does not allow direct comparison among species; a comparison made possible using absolute quantification. Adult bowhead whales ( Balaena mysticetus ) have the largest stores of adipose tissue (blubber) of any mammal. Current leptin dogma predicts that such large fat reserves would result in very high leptin production and would signal a reduction in food intake. In both male and female adults, the most internal layer of blubber (closest to underlying muscle tissue) exhibited the highest leptin expression (males with 19,953 and females with 21,827 copies/50ng total RNA) with decreasing expression levels in intermediate and superficial blubber layers. Juvenile (<2 years) whales express leptin maximally at ~4000 copies/50ng total RNA, similar to mouse and fish (carp and zebrafish). Absolute quantification of leptin mRNA suggests that great whales express leptin at 4–5x values found in other vertebrates. This research was funded by NIH 1R15DK079282‐01A1 to RLL and a COF to HCB.