Meier-Heusler S C, Zhu X, Juge-Aubry C, Pernin A, Burger A G, Cheng S Y, Meier C A
Thyroid Unit, Division of Endocrinology, University Hospital Geneva, Switzerland.
Mol Cell Endocrinol. 1995 Jan;107(1):55-66. doi: 10.1016/0303-7207(94)03422-p.
Thyroid hormone action is not only determined by hormone availability, but also by target organ sensitivity. A dominant negative interaction is known to occur between thyroid hormone receptors (TRs) and the non-ligand binding splicing variant c-erbA alpha 2 as well as mutant TR beta 1 from kindreds with resistance to thyroid hormone. We compared the inhibitory effect of naturally occurring mutant hTR beta 1, artificially created hTR alpha 1 mutants, c-erbA alpha 2 and the human peroxisome proliferator-activated receptor (hPPAR) on three prototypic T3-response elements (TREs), TRE-PAL, DR + 4 and TRE-LAP. The inhibitory effect of mutant hTR alpha 1 and beta 1 occurred only on TRE-LAP and to a minor degree on DR + 4 when equimolar ratios of mutant/wildtype receptor were present. In contrast, the c-erbA alpha 2 splicing variant and the hPPAR inhibited TR action on all three TREs. Gel mobility shift experiments in the presence of T3 showed increased binding of mutant hTR alpha 1 and beta 1 only to TRE-LAP compared to the binding of wildtype hTRs, thereby explaining their TRE-selective dominant negative potency. Contrarily, equal amounts of c-erbA alpha 2 or hPPAR protein did not bind to either of the three response elements even in the presence of RXR. Since the TR:RXR heterodimers were only partially displaced from DNA in the presence of excess amounts of c-erbA alpha 2, it is likely that the TRE-unspecific dominant negative action of c-erbA alpha 2 is due in part to competition for DNA-binding and for TR-auxiliary proteins. In contrast, equimolar amounts of hPPAR completely inhibited the DNA-binding of hTR beta 1:RXR heterodimers, but not of TR:TR homodimers, suggesting that hPPAR has a higher RXR-binding affinity and is therefore a potent competitor for intranuclear RXR. Since thyroid hormones and peroxisome proliferators regulate in part a similar subset of target genes involved in fatty acid metabolism, these results suggest the possibility of cross-talk among the thyroid hormone and peroxisome proliferator signalling pathways. In summary, the results suggest that thyroid hormone action can be modulated by at least three different mechanisms: (i) increased binding of mutant hTRs to specific TREs; (ii) efficient competition for limiting amounts of RXR through the preferential formation of hPPAR:RXR, rather than TR:RXR heterodimers; and (iii) competition for binding to DNA and to auxiliary proteins other than RXR in the case of c-erbA alpha 2.
甲状腺激素的作用不仅取决于激素的可利用性，还取决于靶器官的敏感性。已知甲状腺激素受体（TRs）与非配体结合剪接变体c-erbAα2以及来自甲状腺激素抵抗家系的突变型TRβ1之间会发生显性负性相互作用。我们比较了天然存在的突变型hTRβ1、人工构建的hTRα1突变体、c-erbAα2和人类过氧化物酶体增殖物激活受体（hPPAR）对三种典型的T3反应元件（TREs），即TRE-PAL、DR + 4和TRE-LAP的抑制作用。当存在等摩尔比的突变体/野生型受体时，突变型hTRα1和β1的抑制作用仅发生在TRE-LAP上，对DR + 4的抑制作用较小。相比之下，c-erbAα2剪接变体和hPPAR抑制TR对所有三种TREs的作用。在T3存在的情况下进行的凝胶迁移率变动实验表明，与野生型hTRs的结合相比，突变型hTRα1和β1仅与TRE-LAP的结合增加，从而解释了它们对TRE具有选择性的显性负性作用。相反，即使在存在RXR的情况下，等量的c-erbAα2或hPPAR蛋白也不与三种反应元件中的任何一种结合。由于在存在过量c-erbAα2的情况下，TR:RXR异二聚体仅部分从DNA上被置换，因此c-erbAα2对TRE非特异性的显性负性作用可能部分归因于对DNA结合和TR辅助蛋白的竞争。相比之下，等摩尔量的hPPAR完全抑制hTRβ1:RXR异二聚体与DNA的结合，但不抑制TR:TR同二聚体与DNA的结合，这表明hPPAR具有更高的RXR结合亲和力，因此是核内RXR的有效竞争者。由于甲状腺激素和过氧化物酶体增殖物部分调节参与脂肪酸代谢的相似子集的靶基因，这些结果提示甲状腺激素和过氧化物酶体增殖物信号通路之间可能存在相互作用。总之，结果表明甲状腺激素的作用可以通过至少三种不同机制进行调节：（i）突变型hTRs与特定TREs的结合增加；（ii）通过优先形成hPPAR:RXR而非TR:RXR异二聚体，对有限量的RXR进行有效竞争；（iii）在c-erbAα2的情况下，对与DNA和除RXR之外的辅助蛋白的结合进行竞争。
