Nishiyama K, Andoh S, Kitahara A, Natsume H, Mikami T, Genma R, Nakamura H
Department of Internal Medicine, Hamamatsu University School of Medicine, Japan.
Mol Cell Endocrinol. 1998 Mar 16;138(1-2):95-104. doi: 10.1016/s0303-7207(98)00014-8.
Although different expression patterns of thyroid hormone receptor (TR) alpha1 and beta1 have been reported, no essential distinction has been established in their functions. Unlike the TR beta gene, a mutation in the TR alpha1 gene has never been found in patients with resistance to thyroid hormone (RTH). Previously we found a mutant TR beta with an 11-carboxyl (C)-terminal amino acid truncation (betaF451X) in a girl with severe RTH. BetaF451X is a natural mutant with disruption of the transactivation domain, tau4, and it had very strong dominant negative activities. Based on the fact that the 46 amino acid sequence in the extreme C-terminal region is identical in TR alpha1 and TR beta, except for a C-terminal three amino acid extension of TR alpha1, we constructed a mutant TR alpha1 (alphaF397X) with the identical C-terminal truncation to betaF451X, to study functional differences between TR alpha1 and beta1. Both betaF451X and alphaF397X had negligible T3 binding and transcriptional activities even with 1 microM T3. The dominant negative activities of the mutant TRs were remarkable and T3 response element (TRE)-dependent. Co-expression of betaF451X decreased the CAT activity of either wild-type TR alpha1 or beta1 at 100 nM T3 by approximately 90% on the TRE-pal2 and 70% on DR4. AlphaF397X inhibited the transcriptional activities of both wild-type TR alpha1 and beta1 by approximately 50% on TRE-pal2 and by 60% on DR4. The dominant negative potency of betaF451X was significantly stronger than that of alphaF397X on the TRE-pal2, -DR4 and chicken lysozyme silencer F2, but similar on TRE-myosin heavy chain alpha and malic enzyme. No partiality for the TR subtypes was found in the dominant negative effects of betaF451X and alphaF397X. Co-expression with RXR enhanced the dominant negative effects of alphaF397X, but not of betaF451X. The results indicate that there are different dominant negative properties between alphaF397X and betaF451X, which are TRE-dependent, despite their identical C-terminal truncation. Deletion in the tau4 domain might affect the receptor structures of TR alpha1 and beta1 differently.
尽管已有报道称甲状腺激素受体（TR）α1和β1存在不同的表达模式，但尚未明确它们在功能上的本质区别。与TRβ基因不同，在甲状腺激素抵抗（RTH）患者中从未发现TRα1基因发生突变。此前我们在一名严重RTH女童中发现了一种突变型TRβ，其羧基（C）末端有11个氨基酸截短（βF451X）。βF451X是一种天然突变体，其反式激活结构域tau4被破坏，具有很强的显性负性活性。基于TRα1和TRβ极端C末端区域的46个氨基酸序列相同，除了TRα1的C末端有三个氨基酸延伸这一事实，我们构建了一种与βF451X具有相同C末端截短的突变型TRα1（αF397X），以研究TRα1和β1之间的功能差异。即使在1μM T3存在的情况下，βF451X和αF397X的T3结合和转录活性也可忽略不计。突变型TR的显性负性活性显著且依赖于T3反应元件（TRE）。在100 nM T3时，βF451X与野生型TRα1或β1共表达，在TRE-pal2上使CAT活性降低约90%，在DR4上降低70%。αF397X在TRE-pal2上使野生型TRα1和β1的转录活性降低约50%，在DR4上降低60%。在TRE-pal2、-DR4和鸡溶菌酶沉默子F2上，βF451X的显性负性效力明显强于αF397X，但在TRE-肌球蛋白重链α和苹果酸酶上两者相似。在βF451X和αF397X的显性负性作用中未发现对TR亚型的偏好。与RXR共表达增强了αF397X的显性负性作用，但未增强βF451X的显性负性作用。结果表明，尽管αF397X和βF451X具有相同的C末端截短，但它们之间存在不同的显性负性特性，且这些特性依赖于TRE。tau4结构域的缺失可能对TRα1和β1的受体结构产生不同影响。
