Separation of DNA binding domain from hormone and core histone binding domains by trypsin digestion of rat liver nuclear thyroid hormone receptor

Kinetics of Micronucleus Induction by125I-labelled Thyroid Hormone in Hormone-responsive Cells

Two cell lines, CHO and GC, different in their tissue origin, were investigated with the aim of discovering the correlation between the level of 125I-T3 binding and chromosomal damage induced by 125I decay. Incubation of cells with 125I-T3 has been performed in two exposure schedules: continuous incubation for one to six cell cycles and a pulse-chase schedule involving exposure for one cell cycle. The cellular uptake of 125I-T3, its compartmentization and kinetics were different in the two cell lines. GC cells contained about 7 times more 125I-T3 than CHO cells when incubated with the same external 125I activity concentration (74 kBq of 125I-T3 ml-1 medium). Approximately 70% of the cellular 125I-T3 was found in nuclei of GC cells and only 5% in the nuclei of CHO cells. During the long-term incubation of GC cells with 74 kBq of 125I-T3 ml-1 medium, the 125I activity concentration in cells and their nuclei initially decreased by a half, and thereafter reached a plateau after the third doubling time. In CHO cells and nuclei a very slow linear increase of 125I activity was observed. In GC cells, micronucleus frequency was found to be correlated with nuclear 125I activity. One cell cycle pulse labelling with 74 kBq of 125I-T3 ml-1 medium caused a significant enhancement of micronucleus frequency above the control level during six doubling times, with a maximum at the first post-labelling doubling time. In GC cells continuously incubated with 74 kBq of 125I-T3 ml-1 medium, the micronucleus frequency increased with the incubation time. A model of T3 receptor-dependent dose delivery to nuclei of GC cells continuously incubated with 125I-T3 is proposed. The frequency of micronuclei in the CHO cell line continuously incubated with 125I-T3 did not differ significantly from the control, whereas in the pulse-chase schedule the mean frequency of micronucleated binuclear cells was lower during 4 post-labelling doubling times (significantly at the first and second post-labelling doubling time and insignificantly at the later doubling times) than in the control. Incubation of GC cells with various activity concentrations in medium for four cell cycles resulted in a linear increase of 125I activity in cells and nuclei; however, with a saturation in the region of highest 125I-T3 concentrations used. The frequency of binuclear cells bearing micronuclei was linearly dependent on the nuclear 125I-T3 concentration.

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.

DNA binding affinity of hTRbeta1 mutants as heterodimers with traps from different tissues

Patients with generalized resistance to thyroid hormone (GRTH) show various organ-specific features, for example mental retardation, growth abnormalities, liver damage, delayed bone age or cardiac disorders. Could this reflect aberrant mutant thyroid hormone receptor beta1 (TRbeta1) heterodimerization with specific TR auxiliary proteins (TRAPs) from different tissues, altering the mutant's ability to transactivate tissue-specific genes? To answer this question, we examined the heterodimerization of TRbeta1 mutants and TRAPs of several rat tissues (cerebrum, cerebellum, liver, heart, lung, spleen, and kidney), and in vitro translated RXRalpha, beta and gamma by electrophoretic gel mobility shift assay (EMSA). Mutant TRbeta1 proteins, synthesized in reticulocyte lysate, were incubated with 32P rat malic enzyme (rME) thyroid hormone response elements (TRE) and nuclear extracts of rat tissues. The TRbeta1 mutants used were Mf (G345R), and GH (R316H). Both have non-detectable T3 binding affinity. GH has weak dominant negative effect and Mf has strong dominant negative effect. Two major bands were observed in EMSA. Cerebrum, cerebellum, lung and liver extracts formed a slower migrating band than a TR homodimer, while kidney extracts formed a faster migrating band, and heart and spleen extracts had both bands. There were no qualitative differences in heterodimerization between TRbeta1wt, and TRbeta1 mutants, when using tissue extracts and DNA in excess ratio to TR. We found that RXRalpha, beta, and gamma were differentially expressed in each rat tissue and formed heterodimer complexes with wild type (WT) TRbeta1. Scatchard analysis of affinity and capacity of the binding of TR-TRAP heterodimers to response elements was performed by competing with 2.5-, 5-, 10-, 25-, and 250-fold excess non-radiolabeled rME-TRE. When using kidney extract, the DNA binding affinity of heterodimers was significantly decreased both in wild type and mutant TRs, suggesting that the DNA binding affinity of the faster migrating band was lower than that of the slower migrating band. Mutant GH, which causes 'pituitary RTH' and shows weak dominant negative effect, tended to form heterodimers with lower DNA binding affinity than TRbeta1wt with all extracts. Mutant Mf, which has strong dominant negative effect, tended to show higher DNA binding affinity than TRbeta1WT. When the data were pooled for all tissues, GH and Mf were found to form heterodimers with significantly lower, or higher, affinity for TREs than TRbeta1wt. These results indicate that: 1) differences of DNA binding affinity of mutant TR-TRAP heterodimers to response elements in DNA play a part in its reduced or strong dominant negative effect; and 2) differences in formation of heterodimers with TRAPs present in tissues do not appear to explain the apparent tissue-specific and mutant-specific variations seen in RTH.

Identification of functional positive and negative thyroid hormone-responsive elements in the rat apolipoprotein AI promoter

Transcription of the antiatherogenic protein apolipoprotein AI is regulated by the thyroid hormone, L-triiodothyronine. Transient transfection and electrophoretic mobility shift assays were used to identify the cis-acting elements involved. In transient transfection assays, hormone bound to either thyroid hormone receptor alpha or beta exerts a positive effect through a thyroid hormone response element, site A (-208 to -193). In the absence of site A, liganded receptor alpha or beta have a negative effect on promoter activity. This negative effect is mediated by a 40 bp fragment spanning nucleotides -46 to -7. Closer examination of this region of the gene shows there to be a negative thyroid hormone response element at position -25 to -20 which is fused to the 3' end of the TATA element. Electrophoretic mobility shift assays show that bacterially expressed chicken or rat thyroid hormone receptor alpha 1 binds to site A, either as a homodimer or as a heterodimer with the human 9-cis-retinoic acid receptor alpha. In contrast, the negative thyroid hormone responsive element binds chicken thyroid hormone receptor alpha exclusively as a monomer. Site-directed mutagenesis of the negative thyroid hormone response element abolished the inhibitory effects of the hormone and increased basal promoter activity by up to 40-fold. These data suggest that functional positive and negative thyroid hormone response elements coexist within the rat apolipoprotein AI promoter and both elements contribute to the control of apolipoprotein AI gene expression.

Purification of putative thyroid hormone receptor from the ovarian nuclei of fresh water perch, Anabas testudineus

Perch ovarian putative T3 (3,5,3'-triiodo-L-thyroxine) receptor was purified to 580-fold by extracting the ovarian nuclear preparation with 0.4 M KCl, gel filtration on Sephadex G-25, DEAE-Sephacel chromatography and FPLC Superose 6 chromatography. To monitor the T3 binding protein at each purification step, aliquots from each peak protein fractions were incubated with (125)I-T3 (0.16 pmol to 3.2 nmol) in the absence or presence of 500-fold excess of unlabelled T3. Maximum binding capacity (Bmax) obtained from the Scatchard plot analysis was estimated to determine the extent of purification at each step. Purified putative T3 receptors showed a single band in polyacrylamide gel electrophoresis (PAGE) indicating homogeneity of the putative receptor protein. The molecular weight of the putative T3 receptor protein, as determined on a FPLC Superose 6 column, was 50 kD. Treatment of putative T3 receptor protein with β-mercaptoethanol followed by SDS-PAGE resulted in two subunits of 26 and 31 kD. Purification increased the specific activity of the receptor, but did not alter its affinity. Analogue specificity of the purified receptor corresponded to that of the crude nuclear preparation. Triiodothyroacetic acid (Triac) and T3 equally competed in inhibiting radiolabelled T3 binding while thyroxine (T4) was a poor competitor. T3 receptor antiserum crossreacted with the receptor protein. (125)I-labelled receptor protein binding with its antiserum was inhibited by increasing logarithmic concentrations of unlabelled receptor. In contrast to earlier reports on hepatic T3 receptor, which is a monomer, present investigation demonstrated T3 binding protein in the perch ovary to be a heterodimer held together by disulphide bond.

Fatty acyl-CoA binding activity of the nuclear thyroid hormone receptor

Long-chain fatty acids and their acyl-CoA esters are potent inhibitors of nuclear thyroid hormone (T3) receptor in vitro. In the present study, we obtained evidence for acyl-CoA binding activity in the nuclear extract from rat liver. The activity sedimented at a position (3.5 S) identical with that of the T3 receptor, and the two activities sedimented together. Similarly, they coeluted on DEAE-Sephadex. After partial purification of the receptor, it was again inhibited strongly by acyl-CoAs. Heat stability and a partial trypsin digestion of the receptor both suggested that the action site of oleoyl-CoA overlapped the T3-binding domain of the receptor. In addition, thyroid hormone receptor beta 1, synthesized in vitro, bound oleoyl-CoA specifically and its T3-binding activity was inhibited. The dissociation constant for oleoyl-CoA binding to the partially purified receptor was 1.2 x 10(-7) M. This value as well as its molecular size distinguished the nuclear binding sites from the cytoplasmic fatty acid/acyl-CoA binding proteins. Oleoyl-CoA had no effect on the glucocorticoid receptor, another member of the nuclear hormone-receptor superfamily. From these results, we propose that thyroid hormone receptor is a specific acyl-CoA binding protein of the cell nucleus.

High level expression of functional full length human thyroid hormone receptor beta 1 in insect cells using a recombinant baculovirus

We have cloned the human thyroid hormone receptor beta 1 (hThR beta) from the human breast cancer cell line T47D using the PCR technique. A recombinant baculovirus transfer vector pVL1392/hThR beta was constructed and the full length receptor was expressed in the insect cell line Spodoptera frugiperda (Sf9). Approx. 10-15 x 10(6) receptors are expressed/cell which implies a production level of 2.5-4.0 mg hThR beta/l of cell culture. The expressed hThR beta displayed a single class of binding sites for T3 with high affinity. Western blot analysis using a polyclonal antibody indicated that the molecular weight of the baculovirus expressed receptor is approx. 50 kDa. Crude nuclear extract of hThR beta labeled with [125I]T3 sedimented as a 4 S peak on a glycerol gradient. No receptor could be detected in the cytoplasm indicating its proper translocation to the nuclear compartment. An oligonucleotide containing a palindromic thyroid hormone response element is specifically recognized and retarded in a gel-mobility-shift assay in the presence of nuclear extract of Sf9 cells expressing hThR beta. These data suggest that hThR beta expressed in Sf9 cells is functional and displays characteristics virtually indistinguishable from those of the thyroid hormone receptor (ThR) extracted from mammalian cells. Furthermore, the data indicate that the baculovirus expression system is adequate for large-scale production of receptor for detailed structural and functional studies.

Cellular binding proteins of thyroid hormones

Cellular binding proteins of thyroid hormones are present in the cell nucleus, cytosol, cell membrane, and mitochondria. While nuclear binding is proven to mediate hormone action, the exact roles of the other binding sites remain to be established. Nuclear receptor associates with DNA, core histone, and nuclear matrix and preferentially distributes in transcriptionally active chromatin due to interaction with H1 histone. Of particular importance is the binding of nuclear receptor to specific DNA sequences of target genes, termed thyroid-responsive elements. The binding is stabilized by non-receptor nuclear protein. Upon binding thyroid hormone, nuclear receptor is activated through alterations in the steric configuration, leading to changes in the rate of transcription of the target genes. Multiple nuclear receptor forms exist with likely distinct functional roles. Cytosolic thyroid hormone binding proteins are also heterogeneous. One form is under the control of cell metabolism (NADP and NADPH) and it may have a role in transport of the hormone to mitochondria and nucleus. Membrane-linked thyroid hormone binding proteins may have dual functional roles: one is to mediate hormone action and the other is to support active uptake of hormones by cells. Mitochondrial function may be regulated by thyroid hormone through mitochondrial binding sites in cooperation with nuclear receptor-mediated pathway. Further studies are required to elucidate the exact functional roles of non nuclear thyroid hormone binding proteins.

Differences in nuclear thyroid hormone receptors among species

Hepatic nuclear thyroid hormone receptors from rat, dog, chicken, and rainbow trout were compared. Receptor affinities for 3,5,3'-triiodo-L-thyronine (T3) were similar in preparations from rat, dog, and chicken, using isolated nuclei and nuclear extracts. Rainbow trout nuclear receptor showed a lower affinity for T3. Almost half of the receptors were released into the medium with rat and chicken nuclei, and 79.7 +/- 1.1% of the receptors were released with rainbow trout nuclei, when isolated nuclei were incubated with T3 at 22 degrees for 2 hr. The affinity constant of rat liver receptor for calf thymus DNA-cellulose at 0.17 M KCl, pH 7.4, was 3.98 +/- 1.47 x 10(5) M-1, when determined using DNA-cellulose columns. The number of salt bridges involved in DNA binding of the rat receptor was 5.73 +/- 0.38. When receptor-DNA interactions were compared among species, significant differences were found, but the receptors from dog and rainbow trout liver were similar. Sephacryl S-200 column chromatography showed that chicken receptor had a Stokes radius significantly smaller than that of rat receptor. Partial proteolysis of T3-receptor complex using trypsin alpha-chymotrypsin, elastase, and papain produced distinct T3-binding fragments in different species. Our data provide evidence that nuclear thyroid hormone receptors from different species have significant structural dissimilarities.

Thyroid hormone nuclear receptors and their role in the metabolic action of the hormone

Thyroid hormone nuclear receptor molecules have been characterized as proteins of approximately 49,000 molecular weight existing in cells attached to chromatin and with 4000-8000 copies per nucleus. They bind T3 with Ka of 0.2 X 10(10) l/mol and show microheterogeneity on isoelectric focusing. Hormone responsiveness varies with receptor content in the nucleus and occupancy of receptor by T3. Recent investigations have shown that the receptors are part of the v-erbA related super family of nuclear hormone receptors. At least two types of T3 receptors (TR) exist, one coded by a gene on chromosome 3 (TR beta) and a second coded on chromosome 17 (hTR alpha). Receptors are low in the fetus and, in the adult, are dramatically reduced by starvation, illness and glucagon. Receptors function through binding of T3 or other hormone analogs to a domain in the carboxyl portion of the protein, and binding of the receptor-T3 complex through 'DNA-fingers' to specific response elements as enhancers and located in the 5'-flanking DNA of thyroid hormone responsive genes. Extensive studies on regulation of rat growth hormone have suggested binding of receptor or associated factors to several positions in the 5'-flanking DNA, and recent studies suggest that a crucial area may be a 15 bp segment between bases -179 and -164. Abnormal receptors are believed to be responsible for the syndrome of generalized resistance to thyroid hormone action, but it is yet unclear as to which form (or forms) of the receptor is abnormal in this syndrome.

Antibody against nuclear thyroid hormone receptors

Rat liver nuclear thyroid hormone receptor was purified to 700-1600 pmol T3 binding capacity/mg protein by sequentially using hydroxylapatite column, ammonium sulfate precipitation, Sephadex G-150 gel filtration, DNA-cellulose column, DEAE-Sephadex A-50 column, and heparin-Sepharose column. Serum from a mouse immunized using this purified receptor preparation caused a shift of [125I]T3-receptor peak on glycerol density gradient sedimentation from 3.4 S to approximately 7 S. [125I]T3-receptor complex was immunoprecipitated using this serum and goat anti-mouse IgG. The serum showed reduced ability to immunoprecipitate the globular T3 binding fragment with Stokes radius of 22 A produced by trypsin digestion, a receptor fragment which has core histone and hormone binding but not DNA binding activity. These data indicate the production of anti-nuclear thyroid hormone receptor antibody which mainly recognized epitopes unrelated to hormone and core histone binding domain.