Fozzatti Laura, Lu Changxue, Kim Dong-Wook, Cheng Sheue-yann
Laboratory of Molecular Biology, National Cancer Institute, 37 Convent Drive, Bethesda, Maryland 20892-4264, USA.
Mol Endocrinol. 2011 Jun;25(6):908-21. doi: 10.1210/me.2010-0474. Epub 2011 Apr 7.
Studies using mice deficient in thyroid hormone receptors (TR) indicate that the two TR isoforms, TRα1 and TRβ1, in addition to mediating overlapping biological activities of the thyroid hormone, T3, also mediate distinct functions. Mice harboring an identical dominant negative mutation (denoted PV) at the C terminus of TRα1 (Thra1(PV) mice) or β1 (Thrb(PV) mice) also exhibit distinct phenotypes. These knockin mutant mice provide an opportunity to understand the molecular basis of isoform-dependent functions in vivo. Here we tested the hypothesis that the distinct functions of TR mutant isoforms are directed by a subset of nuclear regulatory proteins. Tandem-affinity chromatography of HeLa nuclear extracts showed that distinct 33 nuclear proteins including nuclear receptor corepressor (NCoR1) and six other proteins preferentially associated with TRα1PV or TRβ1PV, respectively. These results indicate that recruitment of nuclear regulatory proteins by TR mutants is subtype dependent. The involvement of NCoR1 in mediating the distinct liver phenotype of Thra1(PV) and Thrb(PV) mice was further explored. NCoR1 preferentially interacted with TRα1PV rather than with TRβ1PV. NCoR1 was recruited more avidly to the thyroid hormone response element-bound TRα1PV than to TRβ1PV in the promoter of the CCAAT/enhancer-binding protein α gene to repress its expression in the liver of Thra1(PV) mice, but not in Thrb(PV) mice. This preferential recruitment of NCoR1 by mutant isoforms could contribute, at least in part, to the distinct liver lipid phenotype of these mutant mice. The present study highlights a novel mechanism by which TR isoforms direct their selective functions via preferential recruitment of a subset of nuclear coregulatory proteins.
使用甲状腺激素受体（TR）缺陷小鼠的研究表明，两种TR亚型，即TRα1和TRβ1，除了介导甲状腺激素T3的重叠生物学活性外，还介导不同的功能。在TRα1（Thra1（PV）小鼠）或β1（Thrb（PV）小鼠）的C末端携带相同显性负突变（称为PV）的小鼠也表现出不同的表型。这些敲入突变小鼠为了解体内亚型依赖性功能的分子基础提供了一个机会。在这里，我们测试了一个假设，即TR突变亚型的不同功能是由一部分核调节蛋白指导的。对HeLa细胞核提取物进行串联亲和层析显示，包括核受体共抑制因子（NCoR1）和其他六种蛋白质在内的33种不同的核蛋白分别优先与TRα1PV或TRβ1PV结合。这些结果表明，TR突变体对核调节蛋白的招募是亚型依赖性的。进一步探讨了NCoR1在介导Thra1（PV）和Thrb（PV）小鼠不同肝脏表型中的作用。NCoR1优先与TRα1PV相互作用，而不是与TRβ1PV相互作用。在CCAAT/增强子结合蛋白α基因启动子中，NCoR1被更强烈地招募到与甲状腺激素反应元件结合的TRα1PV上，而不是TRβ1PV上，以抑制其在Thra1（PV）小鼠肝脏中的表达，但在Thrb（PV）小鼠中则不然。突变亚型对NCoR1的这种优先招募可能至少部分导致了这些突变小鼠不同的肝脏脂质表型。本研究突出了一种新机制，即TR亚型通过优先招募一部分核共调节蛋白来指导其选择性功能。
