Two separate NCoR (nuclear receptor corepressor) interaction domains mediate corepressor action on thyroid hormone response elements

The nuclear corepressor (NCoR) binds to the thyroid hormone receptor (TR) in the absence of ligand. NCoR-TR interactions are mediated by two interaction domains in the C-terminal portion of NCoR. Binding of NCoR to TR results in ligand-independent repression on positive thyroid hormone response elements. The interactions between NCoR interaction domains and TR on DNA response elements, however, have not been well characterized. We have found that both interaction domains are capable of binding TR on thyroid hormone response elements. In addition, the NCoR interaction domains interact much more strongly with the TR than those present in the silencing mediator of retinoic acid and TRs (SMRT). Furthermore, deletion of either NCoR interaction domain does not significantly impair ligand-independent effects on positive or negative thyroid hormone response elements. Finally, both NCoR interaction domains appear to preferentially bind TR homodimer over TR-retinoid X receptor heterodimer in electrophoretic mobility shift assays. These data suggest that either NCoR interaction domain is capable of mediating the ligand-independent effects of TR on positive and negative thyroid hormone response elements.     

Thyroid hormone negatively regulates the transcriptional activity of the beta-amyloid precursor protein gene

The expression of the beta-amyloid precursor protein (APP), which plays a key role in the development of Alzheimer's disease, is regulated by a variety of cellular mediators in a cell-dependent manner. In the present study, we present evidence that thyroid hormones negatively regulate the expression of the APP gene in neuroblastoma cells. Transient transfection studies using plasmids that contain progressive deletions of the 5' region of the gene demonstrate that triiodothyronine (T3), the more active form of the thyroid hormones, represses APP promoter activity by a mechanism that requires binding of the nuclear T3 receptor (TR) to a specific sequence located in the first exon. The unliganded receptor increases promoter activity, and T3 reverses that activity to basal levels. The repressive effect of T3 does not exhibit TR isoform specificity, and it is equally mediated by TRalpha and TRbeta. Gel mobility shift assays using in vitro synthesized nuclear receptors and nuclear extracts led to the identification of a negative thyroid hormone response element, at nucleotide position +80/+96, that preferentially binds heterodimers of TR with the retinoid X receptor. Insertion of sequences containing this element confers negative regulation by T3 to a heterologous TK promoter, thus indicating the functionality of the element.     

Negative feedback control of the retinoid-retinoic acid/retinoid X receptor pathway by the human TR4 orphan receptor, a member of the steroid receptor superfamily

Amino acid sequence analysis indicates that the human TR4 orphan receptor (TR4) is a member of the estrogen/thyroid receptor subfamily of the steroid/thyroid receptor superfamily and recognizes the AGGTCA direct repeat (DR) of the hormone response element. Here we demonstrate using the electrophoretic mobility shift assay that TR4 binds specifically to DR with a spacing of 1 and 5 base pairs (DR1 and DR5), which are the response elements for retinoic acid receptor (RAR) and retinoid X receptor (RXR), respectively. A reporter gene assay using chloramphenicol acetyltransferase demonstrated that TR4 repressed RA-induced transactivation in a TR4 dose-dependent manner. Inhibition of the retinoid signal pathway also occurs through natural response elements found in CRBPII and RARbeta genes. Our data suggest that the mechanism of repression may not involve the formation of functionally inactive heterodimers between TR4 and RAR or RXR. Instead, we show that TR4 may compete for hormone response elements with RAR and RXR due to its higher binding affinity. Furthermore, treatment of F9 murine teratocarcinoma (F9) cells with 10(-6) M all-trans-retinoic acid increased TR4 mRNA levels, and this change was accompanied by an increased amount of endogenous TR4 protein that can bind to RXRE in electrophoretic mobility shift assay. Our data therefore strongly suggest that the retinoid signal pathway can be regulated by TR4 in a negative feedback control mechanism, which may restrict retinoic acid signaling to certain elements in a cell-specific fashion.     

Cell type-dependent modulation of the dominant negative action of human mutant thyroid hormone beta 1 receptors

BACKGROUND: Mutations in the ligand-binding domain of the thyroid hormone receptor beta (TR beta) gene cause the syndrome of resistance to thyroid hormone (RTH). The clinical phenotype results from the antagonism of the normal TR alpha and the non-mutated TR beta alleles by the TR beta 1 mutants, via a dominant negative effect. There is, however, marked heterogeneity of organ resistance within and among kindreds with RTH. This study examines the potential role of cell type in modulating the dominant negative potency of human TR beta 1 (h-TR beta 1) mutants. MATERIALS AND METHODS: Transient transfections were performed in HeLa and NIH3T3 cells, using a wild type (WT) and three naturally occurring mutant h-TR beta 1 constructs, and three natural thyroid hormone response elements (TREs). Immunocytochemistry was performed to detect levels of TR beta 1 expression in these two cell types. In order to determine how TR beta 1 interacts with other cellular partners, gel-shift analyses using HeLa and NIH3T3 nuclear extracts were performed. RESULTS: Transfection studies using WT h-TR beta 1 in HeLa and NIH3T3 cells, showed that the 3,3',5-triiodothyronine (T3)-induced transactivation of the different TREs varied between cell types. Unlike the non-T3-binding h-TR beta 1 mutant, PV, mutants ED and OK displayed the expected T3-induced dose responsiveness in these two cell types. For each TRE examined, the magnitude of the dominant negative effect varied between the cell types. The levels of receptor expression in HeLa and NIH3T3 cells were identical, as determined by immunocytochemistry. Gel-shift analyses showed differences in the formation of hetero- and homodimers depending on both the cell type and TRE motif. CONCLUSIONS: The cell type in which a mutant receptor operates affects the relative amounts of hetero- and homodimers. Together with the nature of the mutation and the TRE-motif, this could modulate the dominant negative action of mutant receptors in different tissues, which, in turn, could contribute to the variable phenotypic characteristics of RTH.     

Lack of coactivator interaction can be a mechanism for dominant negative activity by mutant thyroid hormone receptors

We studied the interactions of two natural thyroid hormone receptor (TR) mutants from patients with resistance to thyroid hormone (RTH) and an artificial TR mutant with a nuclear receptor corepressor, N-CoR, and a steroid receptor coactivator, SRC-1. In electrophoretic mobility shift assays, wild-type TRbeta-1 interacted with N-CoR in the absence of ligand, whereas T3 caused dissociation of the TRbeta-1/N-CoR complex and formation of TRbeta-1/SRC-1 complex. In contrast, a natural mutant (G345R) with poor T3-binding affinity formed TRbeta-1/N-CoR complex, both in the absence and presence of T3, but could not form TRbeta-1/SRC-1 complex. Another TR mutant, which bound T3 with normal affinity and containing a mutation in the AF-2 region (E457D), had normal interactions with N-CoR but could not bind SRC-1. Both these mutants had strong dominant negative activity on wild-type TR transactivation. Studies with a TR mutant that had slightly decreased T3-binding affinity (R320H) showed a T3-dependent decrease in binding to N-CoR and increase in binding to SRC-1 that reflected its decreased ligand binding affinity. Additionally, when N-CoR and SRC-1 were added to these receptors at various T3 concentrations in electrophoretic mobility shift assays, TR/N-CoR and TR/SRC-1 complexes, but not intermediate complexes were observed, suggesting that N-CoR release is necessary before SRC-1 binding to TR. Our data provide new insight on the molecular mechanisms of dominant negative activity in RTH and suggest that the inability of mutant TRs to interact with coactivators such as SRC-1, which results from reduced T3-binding affinity, is a determinant of dominant negative activity.     

TOR: a new orphan receptor expressed in the thymus that can modulate retinoid and thyroid hormone signals

Vitamin A and other fat-soluble hormones and vitamins have important roles as modulators of essential biological processes such as homeostasis, development, differentiation, and oncogenesis and also as regulators of the immune system. The active form of vitamin A, retinoic acid, as well as vitamin D3 and thyroid hormones exert their actions by binding to specific nuclear receptors that represent one subfamily of the steroid/thyroid hormone receptor superfamily. To identify new members of the retinoid/thyroid hormone receptor subfamily that could play a role in the immune system, a screening of a T cell cDNA library was performed using a retinoid X receptor probe. A clone was isolated encoding a novel nuclear receptor expressed mainly in the thymus and T cell lines. This new receptor, TOR (thymus orphan receptor), is most closely related in both its DNA-binding domain and ligand-binding domain, 90% and 53%, respectively, to ROR alpha/RZR alpha and clusters with these two receptors and RZR beta in a phylogenetic tree, when both the DNA-binding domain and the ligand-binding domain sequences of nuclear receptors are compared. Thus, TOR is part of a subgroup of receptors, one of which has recently been reported to be activated by melatonin. TOR binds specifically to a direct repeat of the half-site sequence 5'-AGGTCA-3' with a four- or five-nucleotide spacer, DNA sequences that also serve as binding sites for thyroid hormone (TR), and retinoic acid receptors (RAR). In transient transfection experiments TOR does not activate a reporter gene carrying these sequences in the absence or the presence of any known nuclear receptor ligands. TOR, however, is able to repress TR and RAR activity on DR-4-TREs or DR-5-RAREs, respectively. Therefore, our data suggest that TOR, similar to COUP-TF, can negatively regulate retinoic acid and thyroid hormone signals. However, the response elements recognized by TOR and COUP-TF differ as do the expression patterns of these receptors. Thus, one important role of TOR could be to modulate retinoid and thyroid hormone signals in the thymus.     

Endocytosis of functional epidermal growth factor receptor-green fluorescent protein chimera

A chimera of the epidermal growth factor receptor (EGFR) and green fluorescent protein (GFP) has been engineered by fusing GFP to the carboxyl terminus of EGFR. Data are provided to demonstrate that the GFP moiety does not affect the expected functioning of EGFR. EGFR-GFP becomes phosphorylated at tyrosine residues in response to EGF and is capable of phosphorylating endogenous substrates and initiating signaling cascades. EGF-dependent association of the chimeric receptor with the clathrin adaptor protein AP-2, involved in endocytosis, and with Shc adaptor protein, which binds in close proximity to the fusion point, is not affected by the GFP moiety. Receptor down-regulation and internalization occur at rates similar to those in cells expressing wild-type EGFR. Western blot analysis reveals that lysosomal degradation of EGFR-GFP proceeds from the extracellular domain and that GFP is not preferentially cleaved. Time-dependent co-localization of EGFR-GFP and Texas Red-conjugated EGF in living cells using digital deconvolution microscopy demonstrates the trafficking of ligand-receptor complexes through the early and multivesicular endosomes followed by segregation of the ligand and receptor at the late stages of endocytosis. Time-lapse optical analysis of the early stages of endocytosis reveals localization of EGFR-GFP in the tubular-vesicular endosomal compartments. Rapid dynamics of membrane movement and fusion within these compartments were observed. This approach and the fidelity of the biochemical properties of the EGFR-GFP demonstrate that real-time visualization of trafficking and protein interactions of tyrosine kinase receptors in the presence or absence of the ligand are feasible.     

Lymphocyte apoptosis induced by Fas ligand- expressing ovarian carcinoma cells. Implications for altered expression of T cell receptor in tumor-associated lymphocytes

We have recently reported that tumor-associated lymphocytes obtained from ascitic fluids of women with ovarian carcinoma (OvCA) demonstrate a marked decrease in expression of cytoplasmic CD3-zeta and surface CD3-epsilon chains, which is associated with altered function of T cell receptor (TcR). We now demonstrate that OvCAs in situ and in culture express functional Fas ligand (FasL), capable of triggering an intrinsic cell death program in Fas-expressing T cells. The possibility of a relationship between cell death and altered expression of TcR was examined. The data indicate that alterations in expression of CD3-zeta and CD3-epsilon chains in T cells coincubated with OvCA are related to tumor-induced apoptosis, as the addition of pan-caspase inhibitors, DEVD-cho or YVAD-cho, prevents both the in vitro induction of T cell death by OvCA cells and the changes in the level of expression of CD3-zeta and CD3-epsilon chains. In the presence of Fas-Fc fusion protein, but not Fc-control protein, the loss in expression of CD3-zeta and CD3-epsilon chains induced in T cells by FasL+ OvCA cells was prevented. These results suggest that the loss in expression of CD3-zeta and CD3-epsilon chains in T lymphocytes interacting with OvCA cells is associated with apoptosis mediated by FasL-expressing tumor cells.