Design of thyroid hormone receptor antagonists from first principles

It is desirable to obtain TR antagonists for treatment of hyperthyroidism and other conditions. We have designed TR antagonists from first principles based on TR crystal structures. Since agonist ligands are buried in the fold of the TR ligand binding domain (LBD), we reasoned that ligands that resemble agonists with large extensions should bind the LBD, but would prevent its folding into an active conformation. In particular, we predicted that extensions at the 5′ aryl position of ligand should reposition helix (H) 12, which forms part of the co-activator binding surface, and thereby inhibit TR activity. We have found that some synthetic ligands with 5′ aryl ring extensions behave as antagonists (DIBRT, NH-3), or partial antagonists (GC-14, NH-4). Moreover, one compound (NH-3) represents the first potent TR antagonist with nanomolar affinity that also inhibits TR action in an animal model. However, the properties of the ligands also reveal unexpected aspects of TR behavior. While nuclear receptor antagonists generally promote binding of co-repressors, NH-3 blocks co-activator binding and also prevents co-repressor binding. More surprisingly, many compounds with extensions behave as full or partial agonists. We present hypotheses to explain both behaviors in terms of dynamic equilibrium of H12 position.     

Resistance to thyroid hormone mediated by defective thyroid hormone receptor alpha

Background

Thyroid hormone acts via receptor subtypes (TRα1, TRβ1, TRβ2) with differing tissue distributions, encoded by distinct genes (THRA, THRB). THRB mutations cause a disorder with central (hypothalamic–pituitary) resistance to thyroid hormone action with markedly elevated thyroid hormone and normal TSH levels.

Scope of review

This review describes the clinical features, genetic and molecular pathogenesis of a homologous human disorder mediated by defective THRA. Clinical features include growth retardation, skeletal dysplasia and constipation associated with low-normal T4 and high-normal T3 levels and a low T4/T3 ratio, together with subnormal reverse T3 levels. Heterozygous TRa1 mutations in affected individuals generate defective mutant receptors which inhibit wild-type receptor action in a dominant negative manner.

Major conclusions

Mutations in human TRα1 mediate RTH with features of hypothyroidism in particular tissues (e.g. skeleton, gastrointestinal tract), but are not associated with a markedly dysregulated pituitary–thyroid axis.

General significance

Human THRA mutations could be more common but may have eluded discovery due to the absence of overt thyroid dysfunction. Nevertheless, in the appropriate clinical context, a thyroid biochemical signature (low T4/T3 ratio, subnormal reverse T3 levels), may enable future identification of cases.This article is part of a Special Issue entitled Thyroid hormone signalling.     

A role of unliganded thyroid hormone receptor in postembryonic development in Xenopus laevis

A fascinating feature of thyroid hormone (T3) receptors (TR) is that they constitutively bind to promoter regions of T3-response genes, providing dual functions. In the presence of T3, TR activates T3-inducible genes, while unliganded TR represses these same genes. Although this dual function model is well demonstrated at the molecular level, few studies have addressed the presence or the role of unliganded TR-induced repression in physiological settings. Here, we analyze the role of unliganded TR in Xenopus laevis development. The total dependence of amphibian metamorphosis upon T3 provides us a valuable opportunity for studying TR function in vivo. First, we designed a dominant negative form of TR-binding corepressor N-CoR (dnN-CoR) consisting of its receptor interacting domain. We confirmed its dominant negative activity by showing that dnN-CoR competes away the binding of endogenous N-CoR to unliganded TR and relieves unliganded TR-induced gene repression in frog oocytes. Next, we overexpressed dnN-CoR in tadpoles through transgenesis and analyzed its effect on gene expression and development. Quantitative RT-PCR revealed significant derepression of T3-response genes in transgenic animals. In addition, transgenic tadpoles developed faster than wild type siblings, with an acceleration of as much as 7 days out of the 30-day experiment. These data thus provide in vivo evidence for the presence and a role of unliganded TR-induced gene repression in physiological settings and strongly support our earlier model that unliganded TR represses T3-response genes in premetamorphic tadpoles to regulate the progress of development.     

Severe impairment of cerebellum development in mice expressing a dominant-negative mutation inactivating thyroid hormone receptor alpha1 isoform

Thyroid hormone deficiency is known to deeply affect cerebellum post-natal development. We present here a detailed analysis of the phenotype of a recently generated mouse model, expressing a dominant-negative TRα1 mutation. Although hormonal level is not affected, the cerebellum of these mice displays profound alterations in neuronal and glial differentiation, which are reminiscent of congenital hypothyroidism, indicating a predominant function of this receptor isoform in normal cerebellum development. Some of the observed effects might result from the cell autonomous action of the mutation, while others are more likely to result from a reduction in neurotrophic factor production.     

Cloning of partial putative gonadotropin hormone receptor sequence from fish

A search for the presence of mariner-like elements in the Labeo rohita genome by polymerase chain reaction led to the amplification of a partial DNA sequence coding for a putative transmembrane domain of gonadotropin hormone receptor. The amplified DNA sequence shows a high degree of homology to the available turkey and human luteinizing and follicle stimulating hormone receptor coding sequences. This is the first report on cloning such sequences of piscine origin.     

Sequential up-regulation of thyroid hormone β receptor,ornithine transcarbamylase,and carbamyl phosphate synthetase mRNAs in the liver of Rana catesbeiana tadpoles during spontaneous and thyroid hormone-induced metamorphosis

During both spontaneous and thyroid hormone (TH)-induced metamorphosis, the Rana catesbeiana tadpole undergoes postembryonic developmental changes in its liver which are necessary for its transition from an ammonotelic larva to a ureotelic adult. Although this transition ultimately results from marked increases in the activities and/or de novo synthesis of the urea cycle enzymes, the precise molecular means by which TH exerts this tissue-specific response are presently unknown. Recent reports, using RNA from whole Xenopus laevis tadpole homogenates and indirect means of measuring TH receptor (TR) mRNAs, suggest a correlation between the up-regulation of TRβ-mRNAs and the general morphological changes occurring during amphibian metamorphosis. To assess whether or not this same relationship exists in a TH-responsive tissue, such as liver, we isolated and characterized a cDNA clone containing the complete nucleotide sequence for a R. catesbeiana urea cycle enzyme, ornithine transcarbamylase (OTC), as well as a genomic clone containing a portion of the hormone-binding domain of a R. catesbeiana TRβ gene. Through use of these homologous sequences and a heterologous cDNA fragment encoding rat carbamyl phosphate synthetase (CPS), we directly determined the relative levels of the TRβ, OTC, and CPS mRNAs in liver from spontaneous and TH-induced tadpoles. Our results establish that TH affects an up-regulation of mRNAs for its own receptor prior to up-regulating CPS and OTC mRNAs. Moreover, results with cultured tadpole liver demonstrate that TH, in the absence of any other hormonal influence, can affect an up-regulation of both the TRβ and OTC mRNAs. © 1992 Wiley-Liss, Inc.     

甲状腺癌患者血清促甲状腺激素和甲状腺激素表达水平及临床意义

目的:探讨甲状腺癌患者血清促甲状腺激素和甲状腺激素表达水平及临床意义。方法:应用电化学发光方法检测甲状腺癌组、甲状腺良性病变组和正常对照组血清促甲状腺激素(TSH)和甲状腺激素(TT3、FT3、TT4、FT4)水平。结果:①血清TSH在三组中比较有统计学意义(P〈0.001),甲状腺癌组血清TSH水平(3.56±0.93ulU/ml)明显高于甲状腺良性病变组(2.82±0.70ulU/ml)和正常对照组(2.04±0.56ulU/ml);TSH与肿瘤病理分期和肿瘤大小呈正相关(P<0.05)。②血清FT3、FT4水平在三组中有统计学意义(均P〈0.001),甲状腺癌组FT3、FT4水平处于较低水平,二者均明显低于甲状腺良性病变组和正常对照组(P<0.001);FT3与肿瘤病理分期和淋巴结转移呈负相关(P<0.05)。③TT3和TT4水平在三组之间比较均无统计学意义(P>0.05)。结论:高水平TSH可增加甲癌复发的危险性。低甲状腺激素水平在甲状腺癌形成中可能起到一定的作用,因此可以将其作为预测甲癌复发的重要指标之一。     

Understanding the role of thyroid hormone in Sertoli cell development: a mechanistic hypothesis

More than a decade of research has shown that Sertoli cell proliferation is regulated by thyroid hormone. Neonatal hypothyroidism lengthens the period of Sertoli cell proliferation, leading to increases in Sertoli cell number, testis weight, and daily sperm production (DSP) when euthyroidism is re-established. In contrast, the neonatal Sertoli cell proliferative period is shortened under hyperthyroid conditions, but the mechanism by which thyroid hormone is able to negatively regulate Sertoli cell proliferation has been unclear. Recent progress in the understanding of the cell cycle has provided the opportunity to dissect the molecular targets responsible for thyroid-hormone-mediated effects on Sertoli cell proliferation. In this review, we discuss recent results indicating a critical role for the cyclin-dependent kinase inhibitors (CDKI) p27^Kip1 and p21^Cip1 in establishing Sertoli cell number, testis weight, and DSP, and the ability of thyroid hormone to modulate these CDKIs. Based on these recent results, we propose a working hypothesis for the way in which thyroid hormone regulates the withdrawal of the cell cycle by controlling CDKI degradation. Finally, although Sertoli cells have been shown to have two biologically active thyroid hormone receptor (TR) isoforms, TRα1 and TRβ1, experiments with transgenic mice lacking TRα or TRβ illustrate that only one TR mediates thyroid hormone effects in neonatal Sertoli cells. Although significant gaps in our knowledge still remain, advances have been made toward appreciation of the molecular sequence of events that occur when thyroid hormone stimulates Sertoli cell maturation. We gratefully acknowledge the support of this work by the NIH, USDA, the University of Illinois, the Lalor Foundation, and the Thanis A. Field Endowment at the University of Illinois. D.R. Holsberger was supported by postdoctoral fellowships from the Lalor Foundation and Reproductive Biology Research Training Program (NIH grant T32 HD07028), University of Illinois at Urbana–Champaign.