Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Center for Cancer Research, 37 Convent Drive, Room 5128, Bethesda, Marryland 20892-4264, USA.
J Mol Endocrinol. 2011 Jun 9;46(3):233-44. doi: 10.1530/JME-10-0163. Print 2011 Jun.
We previously showed that two thyroid hormone receptor (TR) isoforms--TRα1 and TRβ1--differentially regulate thyroid hormone (triiodothyroxine, T(3))-stimulated adipogenesis in vivo. This study aims to understand the role of the nuclear receptor corepressor, NCoR1, in TR isoform-dependent adipogenesis. We found that T(3)-stimulated adipogenesis of 3T3-L1 cells was accompanied by progressive loss of NCoR1 protein levels. In 3T3-L1 cells stably expressing a mutated TRα1, PV (L1-α1PV cells), the T(3)-stimulated adipogenesis was more strongly inhibited than that in 3T3-L1 cells stably expressing an identical mutation in TRβ1 (L1-β1PV cells). The stronger inhibition of adipogenesis in L1-α1PV cells was associated with a higher NCoR1 protein level. These results indicate that the degree of loss of NCoR1 correlates with the extent of adipogenesis. siRNA knockdown of NCoR1 promoted adipogenesis of control 3T3-L1 cells and reversed the inhibited adipogenesis of L1-α1PV and L1-β1PV cells, indicating that NCoR1 plays an essential role in TR isoform-dependent adipogenesis. An ubiquitin ligase, mSiah2, that targets NCoR1 for proteasome degradation was upregulated on day 1 before the onset of progressive loss of NCoR1. NCoR1 was found to associate with mSiah2 and with TR, TRα1PV, or TRβ1PV, but a stronger interaction of NCoR1 with TRα1PV than with TRβ1PV was detected. Furthermore, TRα1PV-NCoR1 complex was more avidly recruited than TRβ1PV-NCoR1 to the promoter of the C/ebpα gene, leading to more inhibition in its expression. These results indicate that differential interaction of NCoR1 with TR isoforms accounted for the TR isoform-dependent regulation of adipogenesis and that aberrant interaction of NCoR1 with TR could underlie the pathogenesis of lipid disorders in hypothyroidism.
我们之前的研究表明，两种甲状腺激素受体（TR）亚型——TRα1 和 TRβ1——在体内以不同的方式调节甲状腺激素（三碘甲状腺原氨酸，T3）刺激的脂肪生成。本研究旨在探讨核受体共抑制因子 NCoR1 在 TR 亚型依赖性脂肪生成中的作用。我们发现，3T3-L1 细胞中 T3 刺激的脂肪生成伴随着 NCoR1 蛋白水平的逐渐降低。在稳定表达突变型 TRα1 的 3T3-L1 细胞（L1-α1PV 细胞）中，T3 刺激的脂肪生成受到的抑制比在稳定表达 TRβ1 相同突变的 3T3-L1 细胞（L1-β1PV 细胞）中更强。L1-α1PV 细胞中脂肪生成的抑制更强，与 NCoR1 蛋白水平更高有关。这些结果表明，NCoR1 的丢失程度与脂肪生成的程度相关。用 siRNA 敲低 NCoR1 可促进对照 3T3-L1 细胞的脂肪生成，并逆转 L1-α1PV 和 L1-β1PV 细胞中受抑制的脂肪生成，表明 NCoR1 在 TR 亚型依赖性脂肪生成中发挥重要作用。一种靶向 NCoR1 进行蛋白酶体降解的泛素连接酶 mSiah2 在 NCoR1 逐渐丢失之前的第一天就被上调。发现 NCoR1 与 mSiah2 以及 TR、TRα1PV 或 TRβ1PV 相关，但与 TRα1PV 的相互作用比与 TRβ1PV 的相互作用更强。此外，与 TRβ1PV-NCoR1 复合物相比，TRα1PV-NCoR1 复合物更易募集到 C/ebpα 基因的启动子上，导致其表达受到更强烈的抑制。这些结果表明，NCoR1 与 TR 亚型的差异相互作用解释了 TR 亚型依赖性脂肪生成的调节，而 NCoR1 与 TR 的异常相互作用可能是甲状腺功能减退症中脂质紊乱发病机制的基础。
