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F Saatcioglu

Publications and source records attributed to F Saatcioglu.

27 records · Page 2Linked to original sources
PubMed1996

A shift in the ligand responsiveness of thyroid hormone receptor alpha induced by heterodimerization with retinoid X receptor alpha.

Thyroid hormone (T3) receptors (T3Rs) are ligand-modulated transcription factors that bind to thyroid hormone response elements (T3REs) and mediate either positive or negative transcriptional regulation of target genes. In addition, in response to ligand binding, T3Rs can interfere with AP-1 activity and thereby inhibit transcription of AP-1-responsive genes. T3Rs were recently shown to form heterodimers with retinoid X receptors (RXRs), leading to increased binding to T3REs in vitro and potentiation of transcriptional responses in vivo. Here we demonstrate that T3R alpha forms stable heterodimers with RXR alpha in living cells. Most important, we describe a new role for RXR alpha in modulating ligand-dependent T3R alpha activity: heterodimerization with RXR alpha greatly increases transcriptional interference with AP-1 activity, augments T3-dependent transcriptional activation, and potentiates the reversal of ligand-independent activation by T3R alpha. In all three cases, the responses occur at substantially lower T3 concentrations when elicited by T3R alpha plus RXR alpha than by T3R alpha alone. In vitro, the binding of T3 decreases the DNA-binding activity of T3R alpha homodimers but does not affect DNA binding by T3R alpha:RXR alpha heterodimers. We provide evidence that increased activities of T3R alpha at lower T3 concentrations are not due to changes in its T3 binding properties. Instead, the altered response could be mediated by either RXR alpha-induced conformational changes, increased stability of heterodimers over homodimers, especially following T3 binding, or both.

Amino Acid Sequence↗
PubMed1995

A unique thyroid hormone response element in the human immunodeficiency virus type 1 long terminal repeat that overlaps the Sp1 binding sites.

Long terminal repeat (LTR) of human immunodeficiency virus (HIV) type 1 is activated by thyroid hormone (T3) receptor alpha (T3R alpha) in the absence of ligand. Addition of T3 reverses this effect. This activity is mediated by a high affinity T3 response element (T3RE) within the HIV-1 LTR, termed the HIV-T3RE (bases -74 to -50), which coincides with the Sp1 element as demonstrated by mobility shift, DNaseI footprinting, and methylation interference analyses. HIV-T3RE mediates ligand-independent activation of transcription by T3R alpha when linked to a heterologous promoter. In addition, the viral transactivator Tat synergizes with T3R alpha to activate the HIV-1 LTR in the absence of T3, which is relieved in its presence. These findings have implications for the possible control of HIV-1 LTR activity by T3.

Base Sequence↗
PubMed1994

Negative transcriptional regulation by nuclear receptors.

Steroid and thyroid hormones, and vitamins A and D bind to nuclear receptors, which act as ligand-modulated transcription factors. In many cases, ligand-activated nuclear receptor binds to positively acting hormone response elements (p-HREs) to induce gene transcription. However, ligand-activated receptors also repress transcription of specific genes and several mechanisms that account for negative regulation have recently emerged. One major form of negative regulation is based on transcriptional interference between nuclear receptors and other transcription factors, such as AP-1. In this case, the liganded receptor prevents AP-1 or other positively acting transcription factors from fruitful interaction with the transcription initiation complex. A second form of negative regulation is based on binding of nuclear receptors to specialized negative HREs (n-HREs). Binding of unliganded receptor to such an element results in constitutive activation, which is terminated by the binding of ligand. While transcriptional interference with AP-1 has been described for many members of the nuclear receptor family, negative regulation through n-HREs so far has been shown only for one of the thyroid hormone receptors. However, this type of negative regulation is likely to be widespread.

Animals↗
PubMed1993

A novel cis element mediating ligand-independent activation by c-ErbA: implications for hormonal regulation.

A novel type of hormone-responsive element (HRE) is described. Unlike classical HREs, this element, RSV-T3RE (found in Rous sarcoma virus-long terminal repeat), mediates strong activation by the c-ErbA alpha thyroid hormone (T3) receptor in the absence of T3, and addition of T3 reverses this response. Whereas both c-ErbA alpha and v-ErbA are potent ligand-independent activators through the RSV-T3RE, c-ErbA beta is not. The RSV-T3RE is recognized and activated by either c-ErbA alpha homodimers or c-ErbA alpha/retinoid X receptor (RXR) heterodimers. Ligand-independent activation by c-ErbA alpha depends on a unique N-terminal activation domain, while the C-terminal activation domain is not absolutely required. Ligand-dependent activation, on the other hand, requires the C-terminal but not the N-terminal activation domain. Upon binding to the RSV-T3RE, c-ErbA alpha assumes a different conformation than when bound to a classical T3RE. c-ErbA alpha is therefore capable of selective deployment of activation domains, dictated both by the HRE with which it interacts and by T3 binding.

Alkylation↗
PubMed1993

Various modes of gene regulation by nuclear receptors for steroid and thyroid hormones.

AP-1 is a transcriptional activator composed of homo- and heterodimers of Jun and Fos proteins. It is involved in activation of genes, such as collagenase, stromelysin, IL-2 and TGF beta 1, by tumour promoters, growth factors and cytokines. AP-1 activity is also elevated in response to transforming oncogenes and is required for cell proliferation. AP-1 activity is subject to complex regulation both transcriptionally and post-transcriptionally. Transcriptional control of jun and fos gene expression determines the amount and composition of the AP-1 complex. The jun and fos genes are regulated both positively and negatively and are highly inducible in response to extracellular stimuli. Post translational control is also important. Both cJun and cFos are subject to regulated phosphorylation. In the case of cJun, phosphorylation of sites near the DNA-binding domain inhibits DNA-binding, while dephosphorylation reverses this inhibition. Phosphorylation of cJun on sites within the N-terminal activation domain increases its ability to activate transcription. The protein kinase phosphorylating these sites is stimulated by cytokines and growth factors. Another mechanism modulating AP-1 activity is transcriptional interference by members of the nuclear receptor family and is relevant for the pathophysiology of rheumatoid arthritis (RA). In RA, chronic inflammation leads to increased AP-1 activity in T cells,macrophages and synoviocytes as a response to secretion of cytokines such as IL-1 and TNF alpha. While the IL-2 gene plays a major role in T cell activation, another AP-1 target gene encodes an enzyme, collagenase, responsible for destruction of bone and tendon.(ABSTRACT TRUNCATED AT 250 WORDS)

DNA↗
PubMed1993

A conserved C-terminal sequence that is deleted in v-ErbA is essential for the biological activities of c-ErbA (the thyroid hormone receptor).

The thyroid hormone (T3) receptor type alpha, the c-ErbA alpha proto-oncoprotein, stimulates transcription of T3-dependent promoters, interferes with AP-1 activity, and induces erythroid differentiation in a ligand-dependent manner. The v-ErbA oncoprotein does not bind hormone and has lost all of these activities. Using c-ErbA/v-ErbA chimeras, we found that a deletion of 9 amino acids, conserved among many members of the nuclear receptor superfamily, which are located at the extreme carboxy terminus of c-ErbA alpha is responsible for loss of both transactivation and transcriptional interference activities. Single, double, and triple amino acid substitutions within this region completely abolished T3-dependent transcriptional activation, interference with AP-1 activity, and decreased T3 binding by c-ErbA alpha. However, the lower T3 binding by these mutants does not fully account for the loss of transactivation and transcriptional interference, since a c-ErbA/v-ErbA chimera which was similarly reduced in T3 binding activity has retained both of these functions. Deletion of homologous residues in the retinoic acid receptor alpha (RAR alpha) resulted in a similar loss of transactivation and transcriptional interference activities. The ability of c-ErbA alpha to induce differentiation of transformed erythroblasts is also impaired by all of the mutations introduced into the conserved carboxy-terminal sequence. We conclude that this 9-amino-acid conserved region is essential for normal biological function of c-ErbA alpha and RAR alpha and possibly other T3 and RA receptors.

Amino Acid Sequence↗
PubMed1990

Aryl hydrocarbon (Ah) receptor DNA-binding activity. Sequence specificity and Zn2+ requirement.

The aryl hydrocarbon (Ah) receptor, also called the xenobiotic or TCDD receptor, mediates transcriptional activation of the cytochrome P-450c (CYP1A1) gene by interacting with Ah or xenobiotic response elements. This paper presents evidence that a metal ion, probably Zn2+, is an essential cofactor for the Ah receptor. This paper also maps in detail the interactions between the Ah response element XRE1 and the Ah receptor from the rat hepatocyte-derived cell line LCS7. Interactions were mapped by three methods, 1) methylation interference footprinting, 2) mobility shift competition experiments, using a series of synthetic oligonucleotides with systematic alterations in the Ah response element core sequence, and 3) orthophenanthroline/Cu+ footprinting. These findings suggest the following consensus sequence for DNA recognition by the Ah receptor: CNA/TNA/TCACGCA/TA/T. The chelators 1,10-phenanthroline and oxalic acid inhibited the sequence-specific DNA-binding activity of the AH receptor in a concentration dependent manner, suggesting that the DNA-binding activity of the receptor requires divalent metal ions. Inhibition was due to metal-chelation, since: 1) inhibition was almost completely prevented by the presence of Zn2+, or other divalent metal ions having high affinity for the chelators used, while metal ions with low affinity did not protect; 2) the DNA-binding activity of the receptor could be restored by dialysis to remove 1,10-phenanthroline, but only in the presence of Zn2+, while dialysis in the absence of metal ions reversed inhibition by the nonchelating isomer 4,7-phenanthroline. The involvement of a divalent cation in receptor function, possibly bound via sulfhydryls, was also suggested by the finding that Cd2+ and Co2+ inhibited DNA-binding activity. Once bound to the XRE1 DNA sequence, the receptor could not be inhibited by 1,10-phenanthroline, suggesting that the essential metal ion must become inaccessible to chelation when the receptor binds DNA. The Zn2+ requirement of the Ah receptor is similar to that of the estrogen and the glucocorticoid receptors and is consistent with the hypothesis that the Ah receptor is a member of the steroid and thyroid hormone receptor superfamily.

Animals↗
PubMed1990

Multiple DNA-binding factors interact with overlapping specificities at the aryl hydrocarbon response element of the cytochrome P450IA1 gene.

Three nuclear factors, the Ah receptor, XF1, and XF2, bind sequence specifically to the Ah response elements or xenobiotic response elements (XREs) of the cytochrome P450IA1 (P450c) gene. The interactions of these factors with the Ah response element XRE1 were compared by three independent methods, methylation interference footprinting, orthophenanthroline-Cu+ footprinting, and mobility shift competition experiments, using a series of synthetic oligonucleotides with systematic alterations in the XRE core sequence. These studies established the following (i) all three factors interact sequence specifically with the core sequence of XRE1; (ii) the pattern of contacts made with this sequence by the Ah receptor are different from those made by XF1 and XF2; and (iii) although XF1 and XF2 can be distinguished by the mobility shift assay, the sequence specificities of their interactions with XRE1 are indistinguishable. Further characterization revealed the following additional differences among these three factors: (i) XF1 and XF2 could be extracted from nuclei under conditions quite different from those required for extraction of the Ah receptor; (ii) XF1 and XF2 were present in the nuclei of untreated cells and did not respond to polycyclic compounds, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and beta-napthoflavone, while nuclear Ah receptor was undetectable in untreated cells and rapidly increased in response to TCDD; (iii) inhibition of protein synthesis did not affect the TCDD-induced appearance of the Ah receptor but substantially decreased the constitutive activities of XF1 and XF2, suggesting that the Ah receptor must be present in untreated cells in an inactive form that can be rapidly activated by polycyclic compounds, while the constitutive expression of XF1 and XF2 depends on the continued synthesis of a relatively unstable protein; (iv) the receptor-deficient and nuclear translocation-defective mutants of the hepatoma cell line Hepa1, which are known to lack nuclear Ah receptor, expressed normal levels of XF1 and XF2, suggesting that the former factor is genetically distinct from the latter two; and (v) a divalent metal ion, probably Zn2+, is known to be an essential cofactor for the Ah receptor but was not required for the DNA-binding activities of XF1 and XF2. Together, these findings indicate that the Ah receptor is distinct from XF1 and XF2, while the latter two activities may be related. Because the DNA-binding domains of these three factors overlap substantially, their binding to XREs is probably mutually exclusive, which suggests that the interplay of these factors at Ah response elements may be important to the regulation of CYP1A1 gene transcription. The results of preliminary transfection experiments with constructs harboring XREs upstream of the chloramphenicol acetyltransferase gene driven by a minimal simian virus 40 promoter are presented that are consistent with this hypothesis.

Base Sequence↗