Thyroid hormone exerts negative feedback on hypothalamic type 4 melanocortin receptor expression

The type 4 melanocortin receptor MC4R, a key relay in leptin signaling, links central energy control to peripheral reserve status. MC4R activation in different brain areas reduces food intake and increases energy expenditure. Mice lacking Mc4r are obese. Mc4r is expressed by hypothalamic paraventricular Thyrotropin-releasing hormone (TRH) neurons and increases energy usage through activation of Trh and production of the thyroid hormone tri-iodothyronine (T-3). These facts led us to test the hypothesis that energy homeostasis should require negative feedback by T-3 on Mc4r expression. Quantitative PCR and in situ hybridization showed hyperthyroidism reduces Mc4r mRNA levels in the paraventricular nucleus. Comparative in silico analysis of Mc4r regulatory regions revealed two evolutionarily conserved potential negative thyroid hormone-response elements (nTREs). In vivo ChIP assays on mouse hypothalamus demonstrated association of thyroid hormone receptors (TRs) with a region spanning one nTRE. Further, in vivo gene reporter assays revealed dose-dependent T-3 repression of transcription from the Mc4r promoter in mouse hypothalamus, in parallel with T-3-dependent Trh repression. Mutagenesis of the nTREs in the Mc4r promoter demonstrated direct regulation by T-3, consolidating the ChIP results. In vivo shRNA knockdown, TR overexpression approaches and use of mutant mice lacking specific TRs showed that both TR alpha and TR beta contribute to Mc4r regulation. T-3 repression of Mc4r transcription ensures that the energy-saving effects of T-3 feedback on Trh are not overridden by MC4R activation of Trh. Thus parallel repression by T-3 on hypothalamic Mc4r and Trh contributes to energy homeostasis.