Environmental chemicals as thyroid hormone analogues: new studies indicate that thyroid hormone receptors are targets of industrial chemicals?

Thyroid hormone (TH) is essential for normal brain development, but the specific actions of TH differ across developmental time and brain region. These actions of TH are mediated largely by a combination of thyroid hormone receptor (TR) isoforms that exhibit specific temporal and spatial patterns of expression during animal and human brain development. In addition, TR action is influenced by different co-factors, proteins that directly link the TR protein to functional changes in gene expression. Several recent studies now show that TRs may be unintended targets of chemicals manufactured for industrial purposes, and to which humans and wildlife are routinely exposed. Polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and bisphenol-A (BPA), and specific halogenated derivatives and metabolites of these compounds, have been shown to bind to TRs and perhaps have selective effects on TR functions. A number of common chemicals including polybrominated biphenyls (PBBs) and phthalates may also exert such effects. Considering the importance of TH in brain development, it will be important to pursue the possibilities that these chemicals - or interactions among chemical classes - are affecting children's health by influencing TH signaling in the developing brain.     

Environmental chemicals impacting the thyroid: targets and consequences.

Thyroid hormone (TH) is essential for normal brain development, but the specific actions of TH differ across developmental time and brain region. These actions of TH are mediated largely by a combination of thyroid hormone receptor (TR) isoforms that exhibit specific temporal and spatial patterns of expression during animal and human brain development. In addition, TR action is influenced by different cofactors, proteins that directly link the TR protein to functional changes in gene expression. Considering the importance of TH signaling in development, it is important to consider environmental chemicals that may interfere with this signaling. Recent research indicates that environmental chemicals can interfere with thyroid function and with TH signaling. The key issues are to understand the mechanism by which these chemicals act and the dose at which they act, and whether adaptive responses intrinsic to the thyroid system can ameliorate potential adverse consequences (i.e., compensate). In addition, several recent studies show that TRs may be unintended targets of chemicals manufactured for industrial purposes to which humans and wildlife are routinely exposed. Polychlorinated biphenyls, polybrominated diphenyl ethers, bisphenol-A, and specific halogenated derivatives and metabolites of these compounds have been shown to bind to TRs and perhaps have selective effects on TR functions. A number of common chemicals, including polybrominated biphenyls and phthalates, may also exert such effects. When we consider the importance of TH in brain development, it will be important to pursue the possibilities that these chemicals-or interactions among chemical classes-are affecting children's health by influencing TH signaling in the developing brain.     

An unliganded thyroid hormone receptor causes severe neurological dysfunction

Congenital hypothyroidism and the thyroid hormone (T(3)) resistance syndrome are associated with severe central nervous system (CNS) dysfunction. Because thyroid hormones are thought to act principally by binding to their nuclear receptors (TRs), it is unexplained why TR knock-out animals are reported to have normal CNS structure and function. To investigate this discrepancy further, a T(3) binding mutation was introduced into the mouse TR-beta locus by homologous recombination. Because of this T(3) binding defect, the mutant TR constitutively interacts with corepressor proteins and mimics the hypothyroid state, regardless of the circulating thyroid hormone concentrations. Severe abnormalities in cerebellar development and function and abnormal hippocampal gene expression and learning were found. These findings demonstrate the specific and deleterious action of unliganded TR in the brain and suggest the importance of corepressors bound to TR in the pathogenesis of hypothyroidism.     

Thyroid hormone receptors in brain development and function

Thyroid hormones are important during development of the mammalian brain, acting on migration and differentiation of neural cells, synaptogenesis, and myelination. The actions of thyroid hormones are mediated through nuclear thyroid hormone receptors (TRs) and regulation of gene expression. The purpose of this article is to review the role of TRs in brain maturation. In developing humans maternal and fetal thyroid glands provide thyroid hormones to the fetal brain, but the timing of receptor ontogeny agrees with clinical data on the importance of the maternal thyroid gland before midgestation. Several TR isoforms, which are encoded by the THRA and THRB genes, are expressed in the brain, with the most common being TRalpha1. Deletion of TRalpha1 in rodents is not, however, equivalent to hormone deprivation and, paradoxically, even prevents the effects of hypothyroidism. Unliganded receptor activity is, therefore, probably an important factor in causing the harmful effects of hypothyroidism. Accordingly, expression of a mutant receptor with impaired triiodothyronine (T(3)) binding and dominant negative activity affected cerebellar development and motor performance. TRs are also involved in adult brain function. TRalpha1 deletion, or expression of a dominant negative mutant receptor, induces consistent behavioral changes in adult mice, leading to severe anxiety and morphological changes in the hippocampus.     

mu-crystallin, a NADPH-dependent T(3)-binding protein in cytosol.

Thyroid hormone action is initiated through nuclear thyroid hormone receptors (TRs). Before the discovery of these nuclear receptors, possible major binding sites for thyroid hormones were thought to be cytosolic owing to high thyroid hormone-binding activity in crude cytosolic fractions. Several cytosolic thyroid hormone-binding proteins have been identified, including reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent 3,5,3'-triiodo-L-thyronine (T(3))-binding protein, also known as mu-crystallin, which was initially cloned as the ortholog of bacterial ornithine cyclodeaminase. The expression of mu-crystallin is developmentally regulated and cell-type specific. Recently, patients with nonsyndromic deafness were reported to have point mutations in the mu-crystallin gene. Cytosolic thyroid hormone-binding proteins, especially mu-crystallin, have roles in adaptation to environmental alterations by thyroid hormone, which might have a role in hearing function.     

Quantitative analysis of DNA binding affinity and dimerization properties of wild-type and mutant thyroid hormone receptor beta1.

Thyroid hormone (triiodothyronine [T3]) actions are mediated through binding of thyroid hormone receptors (TRs) to specific DNA sequences (thyroid hormone response elements [TREs]) as monomers, homodimers, and heterodimers with thyroid hormone receptor auxiliary proteins (TRAPs). We quantitatively characterized dimerization of wild-type (WT) and mutant TRbetas by coimmunoprecipitation, and binding to DNA by electrophoretic gel mobility shift assays (EMSA). Binding affinities of TR retinoid X receptor-alpha (RXRalpha) heterodimers to DNA were determined by competing with excess nonradiolabeled TREs in EMSA. TRs in vitro synthesized in reticulocyte lysates (RL), and human RXRalpha expressed in a Sf9 cell-baculovirus system (BAC), were coincubated with 32P-labeled rat malic enzyme (ME), palindromic (PAL), or chicken lysozyme F2 (F2) TREs. The mutant TRbetas tested were R316H and G345R, which have nondetectable T3 binding and have previously been reported to show weak and potent dominant negative effect, respectively. Scatchard analysis showed no significant differences in Kas between WT and mutant TR-RXRalpha heterodimers binding to DNA. We measured affinity of heterodimerization between TRs and RXRalpha in solution in the absence of DNA, and by coimmunoprecipitation using anti-TRbeta1WT specific antibodies. 35S-labeled RL-RXRalpha was incubated with BAC-WT or TRbeta or R316H in the absence or presence of increasing amounts of nonlabeled BAC-RXRalpha. Displacement curves were obtained by counting radioactivity of precipitated 35S-RXRalpha, that was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography. Kds of WT and TRbeta R316H heterodimerizing with RXRalpha were approximately the same. Binding affinity of TR homodimers for F2-TRE was studied because this TRE binds homodimers strongly. Scatchard analysis clearly showed that DNA binding affinity of BAC-WT homodimers did not differ with or without 100 nM T3, but maximal binding capacity (MBC) was reduced three-fold to fourfold in the presence of 100 nM T3. In contrast, BACTRbeta-R316H homodimers showed a fivefold reduction in DNA binding affinity for F2, both in the presence and absence of T3, and approximately the same MBC as WT in the absence of T3. Mutant RL-G345R homodimers showed approximately the same Ka as RL-WT homodimers for binding to F2 and the same MBC in the presence and absence of T3. These results indicate that (1) T3 reduced TRbeta homodimerization in solution but does not effect DNA binding of formed homodimers; (2) T3 does not influence DNA binding affinity of TR/RxR heterodimers; and (3) TRbeta mutant R316H homodimers have reduced DNA binding affinity but homodimerization and heterodimerization in solution does not differ from WT TRbeta.