Chapter Twenty-Two Thermogenic Characterization of Ghrelin Receptor Null Mice
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Ghrelin is the only known circulating orexigenic hormone that increases food intake and promotes adiposity, and these physiological functions of ghrelin are mediated through its receptor growth hormone secretagogue receptor (GHS-R). Ghrelin/GHS-R signaling plays a crucial role in energy homeostasis. Old GHS-R null mice exhibit a healthy phenotype-lean and insulin sensitive. Interestingly, the GHS-R null mice have increased energy expenditure, yet exhibit no difference in food intake or locomotor activity compared to wild-type mice. We have found that GHS-R is expressed in brown adipose tissue (BAT) of old mice. Ablation of GHS-R attenuates age-associated decline in thermogenesis, exhibiting a higher core body temperature. Indeed, the BAT of old GHS-R null mice reveals enhanced thermogenic capacity, which is consistent with the gene expression profile of increases in glucose/lipid uptake, lipogenesis, and lipolysis in BAT. The data collectively suggest that ghrelin/GHS-R signaling has important roles in thermogenesis. The recent discovery that BAT also regulates energy homeostasis in adult humans makes the BAT a new antiobesity target. Understanding the roles and molecular mechanisms of ghrelin/GHS-R in thermogenesis is of great significance. GHS-R antagonists might be a novel means of combating obesity by shifting adiposity balance from obesogenesis to thermogenesis.
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