Département Régulations, Centre National de la Recherche Scientifique Unité Mixte de Recherche 7221, Evolution des Régulations Endocriniennes, Muséum National d'Histoire Naturelle, Paris, France.
Proc Natl Acad Sci U S A. 2010 Mar 2;107(9):4471-6. doi: 10.1073/pnas.0905190107. Epub 2010 Feb 16.
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 over-expression approaches and use of mutant mice lacking specific TRs showed that both TRalpha and TRbeta 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.
4 型黑色素皮质受体（MC4R）是瘦素信号的关键中继器，将中枢能量控制与外周储备状态联系起来。不同脑区的 MC4R 激活可减少食物摄入并增加能量消耗。缺乏 Mc4r 的小鼠肥胖。Mc4r 由下丘脑室旁核促甲状腺素释放激素（TRH）神经元表达，并通过激活 Trh 和产生甲状腺激素三碘甲状腺原氨酸（T(3)）来增加能量消耗。这些事实使我们提出了一个假设，即能量平衡应该需要 T(3)对 Mc4r 表达的负反馈。定量 PCR 和原位杂交显示，甲状腺功能亢进症降低了室旁核中的 Mc4r mRNA 水平。对 Mc4r 调节区域的比较计算机分析显示了两个进化上保守的潜在负甲状腺激素反应元件（nTRE）。体内 ChIP 测定表明，甲状腺激素受体（TR）与跨越一个 nTRE 的区域结合。此外，体内基因报告基因测定显示，T(3)依赖性转录抑制作用与 T(3)依赖性 Trh 抑制作用平行，T(3)在体内抑制 Mc4r 启动子的转录。Mc4r 启动子中 nTRE 的突变显示出 T(3)的直接调节作用，巩固了 ChIP 结果。体内 shRNA 敲低、TR 过表达方法和使用缺乏特定 TR 的突变小鼠表明，TRalpha 和 TRbeta 都有助于 Mc4r 的调节。T(3)对 Mc4r 转录的抑制确保了 T(3)对 Trh 的能量节约作用不会被 MC4R 对 Trh 的激活所掩盖。因此，T(3)对下丘脑 Mc4r 和 Trh 的平行抑制作用有助于能量平衡。
