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.     

Compensatory role of thyroid hormone receptor (TR) alpha 1 in resistance to thyroid hormone: study in mice with a targeted mutation in the TR beta gene and deficient in TR alpha 1

Resistance to thyroid hormone (RTH) is caused by mutations of the thyroid hormone receptor beta (TR beta) gene. Almost all RTH patients are heterozygous with an autosomal dominant pattern of inheritance. That most are clinically euthyroid suggests a compensatory role of the TR alpha1 isoform in maintaining the normal functions of thyroid hormone (T3) in these patients. To understand the role of TR alpha1 in the manifestation of RTH, we compared the phenotypes of mice with a targeted dominantly negative mutant TR beta (TR betaPV) with or without TR alpha1. TR betaPV mice faithfully recapitulate RTH in humans in that these mice demonstrate abnormalities in the pituitary-thyroid axis and impairment in growth. Here we show that the dysregulation of the pituitary-thyroid axis was worsened by the lack of TR alpha1 in TR betaPV mice, and severe impairment of postnatal growth was manifested in TR betaPV mice deficient in TR alpha1. Furthermore, abnormal expression patterns of T3-target genes in TR betaPV mice were altered by the lack of TR alpha1. These results demonstrate that the lack of TR alpha1 exacerbates the manifestation of RTH in TR betaPV mice. Therefore, TR alpha1 could play a compensatory role in mediating the functions of T3 in heterozygous patients with RTH. This compensatory role may be especially crucial for postnatal growth.     

Thyroxine promotes association of mitogen-activated protein kinase and nuclear thyroid hormone receptor (TR) and causes serine phosphorylation of TR

Activated nongenomically by l-thyroxine (T(4)), mitogen-activated protein kinase (MAPK) complexed in 10-20 min with endogenous nuclear thyroid hormone receptor (TRbeta1 or TR) in nuclear fractions of 293T cells, resulting in serine phosphorylation of TR. Treatment of cells with the MAPK kinase inhibitor, PD 98059, prevented both T(4)-induced nuclear MAPK-TR co-immunoprecipitation and serine phosphorylation of TR. T(4) treatment caused dissociation of TR and SMRT (silencing mediator of retinoid and thyroid hormone receptor), an effect also inhibited by PD 98059 and presumptively a result of association of nuclear MAPK with TR. Transfection into CV-1 cells of TR gene constructs in which one or both zinc fingers in the TR DNA-binding domain were replaced with those from the glucocorticoid receptor localized the site of TR phosphorylation by T(4)-activated MAPK to a serine in the second zinc finger of the TR DNA-binding domain. In an in vitro cell- and hormone-free system, purified activated MAPK phosphorylated recombinant human TRbeta1 (). Thus, T(4) activates MAPK and causes MAPK-mediated serine phosphorylation of TRbeta1 and dissociation of TR and the co-repressor SMRT.     

Ligand-independent orphan receptor TR2 activation by phosphorylation at the DNA-binding domain

In a previous report we demonstrated protein kinase C (PKC)-mediated phosphorylation of the ligand-binding domain (LBD) of orphan nuclear receptor TR2. In this report, we provide the evidence of PKC-mediated phosphorylation of the DNA-binding domain (DBD) of TR2. Two PKC target sites were predicted within the DBD, at Ser-170 and Ser-185, but only Ser-185 was confirmed by MS. Phosphorylation of DBD facilitated DNA binding of the TR2 receptor and its recruiting of coactivator p300/CBP-associated factor (P/CAF). Ser-185 was required for DNA binding, whereas both Ser-170 and Ser-185 were necessary for receptor interaction with P/CAF. The P/CAF-interacting domain of TR2 was located in its DBD. A double mutant (Ser-170 and Ser-185) of TR2 significantly lowered the activation of its target gene RARbeta2. This study provides the first evidence for ligand-independent activation of TR2 orphan receptor through PTM at the DBD, which enhanced its DNA-binding ability and interaction with coactivator P/CAF.     

Regulation of neonatal Sertoli cell development by thyroid hormone receptor alpha1

Neonatal hypothyroidism increases adult Sertoli cell populations by extending Sertoli cell proliferation. Conversely, hyperthyroidism induces premature cessation of Sertoli cell proliferation and stimulates maturational events like seminiferous tubule canalization. Thyroid hormone receptors alpha1 and beta1, which are commonly referred to as TRalpha1 and TRbeta1, respectively, are expressed in neonatal Sertoli cells. We determined the relative roles of TRalpha1 and TRbeta1 in the thyroid hormone effect on testicular development and Sertoli cell proliferation using Thra knockout (TRalphaKO), Thrb knockout (TRbetaKO), and wild-type (WT) mice. Triiodothyronine (T3) treatment from birth until Postnatal Day 10 reduced Sertoli cell proliferation to minimal levels in WT and TRbetaKO mice versus that in their untreated controls, whereas T3 had a diminished effect on TRalphaKO Sertoli cell proliferation. Seminiferous tubule patency and luminal diameter were increased in T3-treated WT and TRbetaKO testes. In contrast, T3 had no effect on these parameters in TRalphaKO mice. In untreated adult TRalphaKO mice, Sertoli cell number, testis weight, and daily sperm production were increased or trended toward an increase, but the increase in magnitude was smaller than that seen in WT mice following neonatal hypothyroidism. Conversely, in TRbetaKO mice, Sertoli cell number, testis weight, and daily sperm production were similar to those in untreated WT mice. In addition, Sertoli cell number and testis weight in adult WT and TRbetaKO mice showed comparable increases following hypothyroidism. Our results show that TRalphaKO mice have testicular effects similar to those seen in WT mice following neonatal hypothyroidism and that TRbetaKO mice, but not TRalphaKO mice, have normal Sertoli cell responsiveness to T3. Thus, effects of exogenous manipulation of T3 on neonatal Sertoli cell development are predominately mediated through TRalpha1.     

Negative regulation of growth hormone receptor/JAK2 signaling by signal regulatory protein alpha

Signal regulatory proteins (SIRPs) are receptor-like transmembrane proteins, the majority of which contain a cytoplasmic proline-rich region and four cytoplasmic tyrosines that, when phosphorylated, bind SH2 domain-containing protein tyrosine phosphatases (SHP). We demonstrated previously that growth hormone (GH) induces tyrosyl phosphorylation of SIRPalpha and association of SIRPalpha with SHP-2. The GH-activated tyrosine kinase JAK2 associates with and tyrosyl-phosphorylates SIRPalpha1. Here we show that JAK2-SIRPalpha1 association does not require phosphotyrosines in SIRPalpha1 or JAK2 or the proline-rich region of SIRPalpha1. However, when the C-terminal 30 amino acids of SIRPalpha1 containing the proline-rich region and tyrosine 495 are deleted, tyrosyl phosphorylation of SIRPalpha1 by JAK2 and association of SHP-2 with SIRPalpha1 are reduced. GH-dependent tyrosyl phosphorylation of JAK2 is reduced when wild-type SIRPalpha1 compared with SIRPalpha1 lacking the four cytoplasmic tyrosines (SIRP 4YF) is expressed in cells, suggesting that SIRPalpha1 negatively regulates GHR/JAK2 signaling. Consistent with reduced JAK2 activity, overexpression of wild-type SIRPalpha1 but not SIRP 4YF reduces GH-induced phosphorylation of ERKs 1 and 2, STAT3, and STAT5B. These results suggest that SIRPalpha1 is a negative regulator of GH signaling and that the ability of SIRPalpha1 mutants to negatively regulate GHR-JAK2 signaling correlates with their ability to bind SHP-2.