Thyroid hormone alters the DNA binding properties of chicken thyroid hormone receptors alpha and beta.

The effects of thyroid hormone agonists on thyroid hormone receptor (TR)/DNA complex formation was investigated to elucidate the mechanism by which TRs transactivate genes in response to ligand. The data, obtained from gel shift experiments, indicate that thyroid hormones alter the conformation of TRs bound to DNA, irrespective of if the element is occupied by monomeric TR, homodimeric TR/TR, or heterodimeric complexes with the retinoid receptors RAR or RXR. Furthermore, triiodo-thyronine (T3) prevents 2 TR molecules from binding to oligonucleotides containing direct repeats or inverted palindromes of the consensus AGGTCA motif, an effect that was not detected with palindromic elements. Heterodimers bound to direct repeats were less affected: RXR/TR were fully and RAR/TR complexes partially resistant to thyroid hormone. The data suggest that a ligand-induced conformational change in TR prevents double TR occupancy of a response element containing 2 direct repeats of the consensus binding motif, possibly by steric hindrance, whereas such an event does not prevent TR/RXR heterodimers from binding to DNA. Finally, our data show that a monomeric, liganded TR bound preferentially to the second half site in a AGGTCActcaAGGTCA element, and therefore indicate that nucleotides adjacent to the consensus half site contribute to binding specificity.     

DNA binding of thyroid hormone receptors.

Thyroid hormone receptors, isolated from rat liver nuclei, bind to purified DNA. By contrast, free triiodothyronine and plasma proteins which bind thyroid hormone do not associate with DNA. Thus, the nuclear localization of thyroid hormone in target tissues may be explained by the association of its receptors with DNA.     

The binding of thyroid hormone receptors to DNA

The behaviour of tri-iodothyronine (T3)- and thyroxine (T4)-receptor complexes when bound to native DNA-cellulose is reported. Equal and large proportions of both T3- and T4-receptor complexes bind to DNA but although T3-receptor complexes are 99% recoverable by 0.5 M NaCl buffer elution, only 60-70% of the T4-receptor complexes are regained. The balance appears as free T4, apparently released as the T4-receptor complexes bind to the DNA whilst the corresponding receptor remains bound. This effect is independent of T4-receptor complex/DNA ratio up to ca. 4 fmol/micrograms DNA, of the presence of an equal amount of unoccupied receptor and of an eight-fold concentration range of both T4-receptor complex and DNA at a fixed ratio, in the cellulose matrix. Pre-formed receptor-DNA material, likewise, only accepts some 60% of the expected quantity of T4 whereas the capacity for T3 appears to be similar to that of free receptors.     

A nuclear factor that enhances binding of thyroid hormone receptors to thyroid hormone response elements

Recent studies from this laboratory have demonstrated the presence of thyroid hormone response elements (TREs) in the 5'-flanking region of the rat alpha and TSH beta subunit genes. Using an avidin-biotin complex DNA binding assay, we have shown that these TREs bind the thyroid hormone (T3) receptor present in nuclear extracts of GH3 cells, as well as the in vitro synthesized Hc-erbA beta, which has been identified as a member of the family of T3 receptors. The binding of Hc-erbA beta to the alpha subunit TRE can be enhanced 3-4-fold by including GH3 nuclear extract in the binding assay. Binding to the TRE present in the TSH beta gene or the rat growth hormone gene was similarly enhanced, although to a lesser degree. The enhanced binding activity is trypsin-sensitive and heat labile, and is not reproduced by the addition of histones, bovine serum albumin, or cytosol instead of nuclear extract. Gel exclusion chromatography suggests a molecular size of approximately 65,000 Da. This protein, which is present in several different cell types, is also able to complement binding of the rat erbA alpha-1 and the pituitary-specific erbA beta-2 forms of the receptor. These data suggest that the binding of the T3 receptor to a TRE is augmented by another nuclear protein, which may be involved in the mechanism of action of thyroid hormone.     

DNA binding of the Bordetella pertussis H1 homolog alters in vitro DNA flexibility.

BpH1, the Bordetella pertussis H1 homolog, interacts with chromosomal DNA. With DNase I protection assays, we demonstrate in this study that BpH1 binds DNA in a nonspecific manner and that it may cover DNA fragments from end to end. Although the binding was shown to be nonspecific, preferential binding sites and sites resistant to BpH1 binding were identified within and upstream of the pertussis toxin promoter sequence. In the presence of DNA ligase, BpH1 favored the formation of multimeric DNA fragments of various sizes and prevented ring closures, suggesting a diminished flexibility of the DNA fragments and thus indicating that BpH1 acts as a macromolecular crowding agent.     

An assay for thyroid hormone binding to chromatin receptors

I have measured the interaction of T₃ with highly soluble, expanded, rat liver chromatin using a new assay for the study of hormone binding to nucleoprotein. Bound hormone and free hormone were rapidly and quantitatively separated by the adsorption of the hormone-nucleoprotein complex onto hydroxylapatite. This procedure satisfies several criteria for a successful binding assay: (1) The binding capacity is stable throughout the time required to reach equilibrium, (2) the ratio of specific to nonspecific binding (signal/noise) is at least 20:1, (3) large numbers of samples can be handled easily, (4) the amount of bound hormone is directly proportional to the quantity of chromatin employed, (5) the hormone and its analogs display a range of affinities for the binding site, and (6) the binding occurs to a limited number of sites, over a free hormone concentration range which is similar to the hormone concentrations found in vivo.     

Thyroid hormone receptors form distinct nuclear protein-dependent and independent complexes with a thyroid hormone response element

We have examined the binding of nuclear proteins and recombinant thyroid hormone receptors (TRs) to the palindromic thyroid hormone responsive element AGGTCATGACCT (TREp) using a gel electrophoretic mobility shift assay. Four specific protein-DNA complexes were detected after incubation of nuclear extracts (NE) from T3-responsive pituitary (GH3) cells with a TREp-containing DNA fragment. This was compared with the TREp binding of reticulocyte lysate-synthesized TRs. TR alpha 1 and TR beta 2 each formed a single major TR:TREp complex which comigrated with the least retarded complex formed by GH3 NE, while TR beta 1 formed multiple complexes suggesting that it can bind to TREp as an oligomer. Interestingly, coincubation of 35S-TR alpha 1, GH3 NE, and unlabeled TREp resulted in not only the 35S-TR:TREp complex, but in two additional more greatly retarded complexes containing 35S-TR alpha 1 and comigrating with those formed by GH3 extract alone. Incubation of each of the TRs with NE from COS-7 cells, which do not possess sufficient endogenous TRs to mediate T3-responses, resulted in formation of a new, more greatly shifted complex. A similar, heat labile activity which altered mobility of the TR:TRE complex was also present in NE from T3-unresponsive JEG-3 cells. At high concentration of NE, all of the TR bound to TREp was more greatly retarded than in the absence of NE. Truncation of TR alpha 1 at amino acid 210 prevented additional complex formation in the presence of NE without affecting DNA binding, suggesting that the carboxyl-terminus of the TRs is essential for interaction with nuclear proteins.(ABSTRACT TRUNCATED AT 250 WORDS)     

The thyroid hormone receptors: molecular basis of thyroid hormone resistance.

Major progress has been achieved in the mechanism of action of thyroid hormones thanks to the identification of the T3 receptor as the product of the proto-oncogene c-erbA. Recognition of subsets of receptors with and without T3-binding properties and of the interaction of different receptors with each other leads to new insights in cell regulation and development. In thyroid hormone resistance, distinct mutations in the T3-binding domain of thyroid hormone receptor (TR)beta have been identified in unrelated families. No correlation between the type of mutation and tissue resistance has been established. Mutant TRs bind to thyroid hormone response elements (TREs) on both negative or positive T3-controlled genes. Subjects with heterozygous TR beta gene deletion are not affected, supporting the hypothesis that mutant TRs act through a dominant negative effect. In generalized thyroid hormone resistance, mutated TR beta may interfere through competition for TREs and/or formation of inactive dimers. Finally, deficiency in T3 receptor auxiliary protein or other accessory proteins or competition between mutant and normal TRs for these factors is not excluded.     

The structural basis for enhanced stability and reduced DNA binding seen in engineered second-generation Cro monomers and dimers

It was previously shown that the Cro repressor from phage lambda, which is a dimer, can be converted into a stable monomer by a five-amino acid insertion. Phe58 is the key residue involved in this transition, switching from interactions which stabilize the dimer to those which stabilize the monomer. Structural studies, however, suggested that Phe58 did not penetrate into the core of the monomer as well as it did into the native dimer. This was strongly supported by the finding that certain core-repacking mutations, including in particular, Phe58-->Trp, increased the stability of the monomer. Unexpectedly, the same substitution also increased the stability of the native dimer. At the same time it decreased the affinity of the dimer for operator DNA. Here we describe the crystal structures of the Cro F58W mutant, both as the monomer and as the dimer. The F58W monomer crystallized in a form different from that of the original monomer. In contrast to that structure, which resembled the DNA-bound form of Cro, the F58W monomer is closer in structure to wild-type (i.e. non-bound) Cro. The F58W dimer also crystallizes in a form different from the native dimer but has a remarkably similar overall structure which tends to confirm the large changes in conformation of Cro on binding DNA. Introduction of Trp58 perturbs the position occupied by the side-chain of Arg38, a DNA-contact residue, providing a structural explanation for the reduction in DNA-binding affinity.The improved thermal stability is seen to be due to the enhanced solvent transfer free energy of Trp58 relative to Phe58, supplemented in the dimer structure, although not the monomer, by a reduction in volume of internal cavities.     

Low-resolution structures of thyroid hormone receptor dimers and tetramers in solution

High-resolution X-ray structures of thyroid hormone (TH) receptor (TR) DNA and ligand binding domains (DBD and LBD) have yielded significant insights into TR action. Nevertheless, the TR DBD and LBD act in concert to mediate TH effects upon gene expression, and TRs form multiple oligomers; however, structures of full-length TRs or DBD-LBD constructs that would clarify these influences are not available. Here, we report low-resolution X-ray structures of the TRbeta DBD-LBD construct in solution which define the shape of dimers and tetramers and likely positions of the DBDs and LBDs. The holo TRbeta DBD-LBD construct forms a homodimer with LBD-DBD pairs in close contact and DBDs protruding from the base in the same direction. The DBDs are connected to the LBDs by crossed extended D domains. The apo hTRbeta DBD-LBD construct forms tetramers that resemble bulged cylinders with pairs of LBD dimers in a head-to-head arrangement with DBD pairs packed tightly against the LBD core. Overall, there are similarities with our previous low-resolution structures of retinoid X receptors, but TRs exhibit two unique features. First, TR DBDs are closely juxtaposed in the dimer and tetramer forms. Second, TR DBDs are closely packed against LBDs in the tetramer, but not the dimer. These findings suggest that TRs may be able to engage in hitherto unknown interdomain interactions and that the D domain must rearrange in different oligomeric forms. Finally, the data corroborate our suggestion that apo TRs form tetramers in solution which dissociate into dimers upon hormone binding.     

In vitro mycotoxin binding to bovine uterine steroid hormone receptors

The mycotoxins, aflatoxin B(1), aflatoxin M(1), aflatoxicol and zearalenone were tested for binding to bovine endometrial estrogen and progestin receptors. Radioinert estradiol-17beta, estrone, testosterone, and cholesterol were evaluated for binding to the estrogen receptor. Zearalenone and aflatoxicol but not aflatoxins B(1) and M(1) competed with estradiol-17beta for the estrogen receptor. The order of binding affinities for the estrogen receptor were zearalenone > estradiol-17beta > estrone > aflatoxicol. The affinity of zearalenone for the estrogen receptor was 2-3 times that of estradiol-17beta. Progesterone, cortisol, radioinert R 5020, and cholesterol were evaluated for binding to the progestin receptor. None of the tested compounds except R 5020 and progesterone competed for the progestin receptor. The significance of aflatoxicol binding to the estrogen receptor is unclear. It is proposed that aflatoxicol binding to the receptor may alter gene expression in target tissues or act at the level of the hypothalamus to inhibit gonadotropin secretion and ovulation. These effects could explain reports of reduced fertility in domestic animals following ingestion of aflatoxin contaminated feedstuffs. It is also suggested that the mechanism of adverse effects on fertility of chronic aflatoxin ingestion in cattle and other livestock should be more thoroughly investigated.     

Hormone selectivity in thyroid hormone receptors

Separate genes encode thyroid hormone receptor subtypes TRalpha (NR1A1) and TRbeta (NR1A2). Products from each of these contribute to hormone action, but the subtypes differ in tissue distribution and physiological response. Compounds that discriminate between these subtypes in vivo may be useful in treating important medical problems such as obesity and hypercholesterolemia. We previously determined the crystal structure of the rat (r) TRalpha ligand-binding domain (LBD). In the present study, we determined the crystal structure of the rTRalpha LBD in a complex with an additional ligand, Triac (3,5, 3'-triiodothyroacetic acid), and two crystal structures of the human (h) TRbeta receptor LBD in a complex with either Triac or a TRbeta-selective compound, GC-1 [3,5-dimethyl-4-(4'-hydroy-3'-isopropylbenzyl)-phenoxy acetic acid]. The rTRalpha and hTRbeta LBDs show close structural similarity. However, the hTRbeta structures extend into the DNA-binding domain and allow definition of a structural "hinge" region of only three amino acids. The two TR subtypes differ in the loop between helices 1 and 3, which could affect both ligand recognition and the effects of ligand in binding coactivators and corepressors. The two subtypes also differ in a single amino acid residue in the hormone-binding pocket, Asn (TRbeta) for Ser (TRalpha). Studies here with TRs in which the subtype-specific residue is exchanged suggest that most of the selectivity in binding derives from this amino acid difference. The flexibility of the polar region in the TRbeta receptor, combined with differential recognition of the chemical group at the 1-carbon position, seems to stabilize the complex with GC-1 and contribute to its beta-selectivity. These results suggest a strategy for development of subtype-specific compounds involving modifications of the ligand at the 1-position.     

The nature of thyroid hormone receptors. Intracellular transport and compartmentalization of chromatin receptors and thyroid hormones

It is well established that in vivo administered labelled TBPA penetrates into liver, brain and lung cells, is translocated from cytosol into the nucleus and is accepted by chromatin without being affected by modifications touching upon the antigenic determinants of this protein. Electron microscopic autoradiography demonstrated that 125I-TBPA translocated from cytosol into the nucleus is localized on the border between hetero- and euchromatin. The data obtained may serve as an additional proof of the universal structure of intracellular thyroid hormone receptors and suggest that TBPA participate in manifestation of genetic effects of thyroid hormones.