A c-erb-A binding site in rat growth hormone gene mediates trans-activation by thyroid hormone

Thyroid hormone regulates expression of a transfected human alpha-myosin heavy-chain fusion gene in fetal rat heart cells

The rat alpha-myosin heavy-chain (alpha-MHC) gene is regulated by 3,5,3'-triiodo-L-thyronine (T3) in ventricular myocardium and is constitutively expressed in atrial tissue. Less is known about regulation of the human gene, but conservation of sequences in the 5'-flanking region between the rat and human alpha-MHC genes suggests that the human gene may be regulated similarly. Accordingly, T3-responsiveness and tissue-specific expression of human and rat alpha-MHC/chloramphenicol acetyltransferase fusion constructs have been compared in rat fetal heart cells, L6E9 myoblasts and myotubes, 3T3 fibroblasts, and HeLa cells. Transient transfection assays revealed a complex series of cis-regulatory elements in the 5'-flanking sequences in the human genes, including a basal promoter element with canonical TATAA and CAAT sequences, two positive regulatory element(s), and two negative regulatory elements, which markedly diminished both constitutive and T3-inducible activity. Interestingly, the human gene seemed to contain a proximal thyroid-hormone response element(s) not found in the rat gene. In L6E9 myoblasts and myotubes, the human constructs were constitutively expressed but not T3-regulated; none of the constructs were active in 3T3 or HeLa cells. We propose that interactions among the thyroid hormone responsive elements and other cis-acting elements in the human alpha-MHC 5'-flanking sequences may be sufficient to explain the characteristic features of expression of this gene in cardiac tissues.

1,25-dihydroxyvitamin D-responsive element and glucocorticoid repression in the osteocalcin gene

The active hormonal form of vitamin D3, 1,25-dihydroxyvitamin D3[1,25(OH), which regulates cellular replication and function in many tissues and has a role in bone and calcium homeostasis, acts through a hormone receptor homologous with other steroid and thyroid hormone receptors. A 1,25(OH)2D3-responsive element (VDRE), which is within the promoter for osteocalcin [a bone protein induced by 1,25(OH)2D3] is unresponsive to other steroid hormones, can function in a heterologous promoter, and contains a doubly palindromic DNA sequence (TTGGTGACTCACCGGGTGAAC; -513 to -493 bp), with nucleotide sequence homology to other hormone responsive elements. The potent glucocorticoid repression of 1,25(OH)2D3 induction and of basal activity of this promoter acts through a region between -196 and +34 bp, distinct from the VDRE.

Negative regulation of the thyroid-stimulating hormone alpha gene by thyroid hormone: receptor interaction adjacent to the TATA box

Thyroid-stimulating hormone alpha and beta subunit genes are negatively regulated by thyroid hormone at the transcriptional level. Transient gene expression studies were used to demonstrate that the erbA beta form of the thyroid hormone receptor mediates negative regulation of the alpha-subunit promoter in a hormone-dependent manner. In JEG-3 choriocarcinoma cells, which are deficient in thyroid hormone receptors, coexpression of erbA beta with alpha CAT reporter genes markedly suppressed alpha CAT expression after treatment with thyroid hormone, whereas a reporter gene containing a known positive thyroid response element was induced. Thus, a single form of thyroid hormone receptor mediates both positive and negative responses to thyroid hormone in this system. Transient expression analyses of alpha gene 5' flanking sequence deletion mutants localized the negative thyroid response element to the proximal region of the promoter between -100 and +4 base pairs. The location of the negative thyroid response element in the alpha gene is therefore distinct from that of previously identified regulatory elements including the tissue-specific upstream regulatory elements, the cAMP response elements, and the glucocorticoid response elements. Overlapping segments of the alpha promoter were examined for potential thyroid hormone receptor binding sites by using gel shift assays and biotinylated DNA-binding studies. A specific thyroid hormone receptor binding site was identified between -22 and -7 base pairs, which is immediately downstream from the TATA box. This region of the alpha promoter interacts with erbA beta receptor synthesized in vitro as well as with endogenous nuclear thyroid hormone receptors, and it competes for receptor binding to a known positive thyroid response element. These studies suggest a mechanism for negative regulation in which the thyroid hormone receptor interacts with the alpha gene promoter immediately downstream of the TATA box to inhibit transcription.

Positive and negative regulation of gene transcription by a retinoic acid-thyroid hormone receptor heterodimer

We present evidence that the human thyroid hormone receptor forms a heterodimer with the human retinoic acid receptor. This interaction results in a cooperative increase in binding of the alpha retinoic acid receptor to a subset of thyroid hormone response elements. Mutations within the DNA binding domain or near the C-terminus abolish either receptor's ability to interact cooperatively on these elements. The thyroid hormone-retinoic acid receptor heterodimer exhibits novel transcriptional properties in that coexpression of both receptors at low levels in Green monkey kidney (CV1) cells results in a positive transcriptional effect on promoters containing a palindromic thyroid hormone response element, but has a surprisingly negative effect on a thyroid hormone response element derived from the alpha myosin heavy chain gene. These results suggest that by forming heterodimers, more elab-orate control of transcription can be achieved by creating receptor combinations with differing activities.

Enhancement of c-erbA proto-oncogene expression by glucocorticoid hormones in S49.1 lymphoma cells

The modifications of the mRNA levels of the c-myc and c-erbA proto-oncogenes during the dexamethasone-induced decrease of S49.1 cell proliferation have been studied. The levels of c-myc mRNA decreased significantly between 3 and 18 h after dexamethasone (1 microM) treatment. In contrast, a significant increase in the levels of a 2.6 kb c-erbA mRNA was observed between 6 and 18 h after hormone treatment. Cycloheximide treatment of S49.1 cells increased the levels of c-erbA RNA and overcome the enhancing effect of dexamethasone on the expression of this proto-oncogene, suggesting that ongoing protein synthesis is necessary to elicit this hormone effect. The associated decrease of cell proliferation and changes in c-myc and c-erbA mRNA levels after dexamethasone treatment suggest that such oncogenes might be involved in the dexamethasone-mediated control of lymphoid cell growth.

H-2RIIBP, a member of the nuclear hormone receptor superfamily that binds to both the regulatory element of major histocompatibility class I genes and the estrogen response element

Transcription of major histocompatibility complex (MHC) class I genes is regulated by the conserved MHC class I regulatory element (CRE). The CRE has two factor-binding sites, region I and region II, both of which elicit enhancer function. By screening a mouse lambda gt 11 library with the CRE as a probe, we isolated a cDNA clone that encodes a protein capable of binding to region II of the CRE. This protein, H-2RIIBP (H-2 region II binding protein), bound to the native region II sequence, but not to other MHC cis-acting sequences or to mutant region II sequences, similar to the naturally occurring region II factor in mouse cells. The deduced amino acid sequence of H-2RIIBP revealed two putative zinc fingers homologous to the DNA-binding domain of steroid/thyroid hormone receptors. Although sequence similarity in other regions was minimal, H-2RIIBP has apparent modular domains characteristic of the nuclear hormone receptors. Further analyses showed that both H-2RIIBP and the natural region II factor bind to the estrogen response element (ERE) of the vitellogenin A2 gene. The ERE is composed of a palindrome, and half of this palindrome resembles the region II binding site of the MHC CRE. These results indicate that H-2RIIBP (i) is a member of the superfamily of nuclear hormone receptors and (ii) may regulate not only MHC class I genes but also genes containing the ERE and related sequences. Sequences homologous to the H-2RIIBP gene are widely conserved in the animal kingdom. H-2RIIBP mRNA is expressed in many mouse tissues, in agreement with the distribution of the natural region II factor.

Generalized resistance to thyroid hormone associated with a mutation in the ligand-binding domain of the human thyroid hormone receptor beta

The syndrome of generalized resistance to thyroid hormone is characterized by elevated circulating levels of thyroid hormone in the presence of an overall eumetabolic state and failure to respond normally to triiodothyronine. We have evaluated a family with inherited generalized resistance to thyroid hormone for abnormalities in the thyroid hormone nuclear receptors. A single guanine----cytosine replacement in the codon for amino acid 340 resulted in a glycine----arginine substitution in the hormone-binding domain of one of two alleles of the patient's thyroid hormone nuclear receptor beta gene. In vitro translation products of this mutant human thyroid hormone nuclear receptor beta gene did not bind triiodothyronine. Thus, generalized resistance to thyroid hormone can result from expression of an abnormal thyroid hormone nuclear receptor molecule.

Two chicken erythrocyte band 3 mRNAs are generated by alternative transcriptional initiation and differential RNA splicing

The erythrocyte anion transport protein (band 3) mediates two distinct cellular functions: it provides plasma membrane attachment sites for the erythroid cytoskeletal network, and it also functions as the anion transporter between the erythrocyte cytoplasm and extracellular milieu. We previously showed that two chicken band 3 polypeptides are encoded by two different mRNAs with different translation initiation sites. Here we show that these two band 3 mRNAs are transcribed from two separate promoters within a single gene. In addition, the two pre-mRNAs are differentially spliced, leading to fusion with coding exons used in common in the two mRNAs. The chicken erythrocyte band 3 gene is therefore the first example of a gene that has two promoters within a single locus which function equally efficiently in one cell type at the same developmental stage.

Glutathione-S-transferases are major cytosolic thyroid hormone binding proteins

Thyroid hormone binding proteins of rat liver cytosol were characterized. Glutathione-S-transferases were identified among major cytosolic proteins adsorbed by thyroxine affinity matrices. The Ya and Yb subunits of the glutathione-S-transferases were also principal proteins of cytosol covalently labeled with 3,3',5-triiodo-L-thyronine (T3) or 3,3',5,5'-tetraiodo-L-thyronine (T4) by photoaffinity methods. T3 and T4, but not L-thyronine or iodinated tyrosines, were bound with high affinity to purified glutathione-S-transferases and were potent inhibitors of their enzymatic activities. These results suggest that glutathione-S-transferases have the potential to function in the intracellular binding and transport of thyroid hormones. The proteins provide a means for regulating the action and metabolism of thyroid hormones by acting as high capacity binding components.

Regulation of the transfected human glycoprotein hormone alpha-subunit gene by dexamethasone and thyroid hormone

A chimeric plasmid, (-1,500)h alpha CAT, containing approximately 1,500 bp of 5'-flanking DNA of the human glycoprotein hormone alpha-subunit gene directing the expression of the bacterial chloramphenicol acetyl transferase (CAT) gene, was transfected transiently into rat pituitary-derived GH3 cells. (-1,500)h alpha CAT expression was stimulated 5- to 20-fold by dexamethasone and 3- to 5-fold by 8-bromo-cAMP (8-Br-cAMP), and was inhibited by 50% by L-triiodothyronine (T3). Thus, suppression by T3 in this system was similar to that seen in pituitary thyrotropes. Induction of (-1,500)h alpha CAT expression by dexamethasone was antagonized by T3 but was unaffected by 8-Br-cAMP. However, T3 augmented the stimulation of (-1,500)h alpha CAT activity by 8-Br-cAMP. Deletants containing less than 346 bp of 5'-flanking alpha DNA showed a stepwise decrease in induction by dexamethasone, suggesting that multiple sequence elements located in this region are required for full induction of h alpha CAT activity. Deletion analysis also indicated that a thyroid hormone response element is located between 207 and 172 bp of the alpha gene transcriptional start site. Our finding of induction of alpha expression by dexamethasone in pituitary cells contrasts with the inhibition of alpha gene activity by glucocorticoids which has previously been shown in placental cells. Therefore, these data indicate that cell-type-specific factors play an important role in the modulation of alpha gene transcription.

Differential expression of alpha and beta thyroid hormone receptor genes in rat brain and pituitary

Multiple thyroid hormone receptor cDNAs have previously been identified in rat and are classified into alpha and beta subtypes. Alternative splicing of the alpha gene gives rise to the functional receptor, rTR alpha 1, and the non-thyroid hormone-binding isotype, rTR alpha 2. Recent evidence suggests the beta gene encodes two functional receptors, rTR beta 1, and the pituitary-specific receptor, rTR beta 2. By using synthetic DNA probes common to rTR beta transcripts and specific for rTR alpha 1 and rTR alpha 2 mRNAs, we mapped the expression of these transcripts in adult rat brain and pituitary by hybridization histochemistry. We also localized mRNAs encoding the putative nuclear receptor REV-ErbA alpha, a portion of which is derived from the opposite strand of the rTR alpha gene. rTR alpha 1 and rTR alpha 2 transcripts were widely distributed in a similar, if not identical, pattern. Highest levels of rTR alpha 1 and rTR alpha 2 transcripts were found in the olfactory bulb, hippocampus, and granular layer of the cerebellar cortex. REV-ErbA alpha and rTR beta mRNAs were found in more restricted patterns of expression distinct from those of rTR alpha 1 and rTR alpha 2. REV-ErbA alpha mRNA was highest in the neocortex. High levels of rTR beta transcripts in the anterior pituitary and the parvocellular part of the paraventricular hypothalamic nucleus suggest rTR beta gene products may mediate thyroid hormone feedback regulation of thyroid-stimulating hormone and thyrotropin-releasing hormone. Our results identify nuclei and structures in the mammalian central nervous system in which regulation of gene expression by specific thyroid hormone receptor subtypes may occur.

Dual regulatory role for thyroid-hormone receptors allows control of retinoic-acid receptor activity

Both thyroid hormone (T3) and retinoic acid signal essential steps in development, differentiation and morphogenesis. Specific nuclear receptors for these ligands have recently been cloned. Previously we have noted a close homology between the DNA-binding domains of the epsilon-retinoic acid receptor (RAR-epsilon, also designated RAR-beta), the thyroid hormone receptors and the oestrogen receptor. We have now found that RAR-epsilon is very efficient at inducing transcription from two distinct thyroid-hormone responsive elements (TREs). Transcription induced by ligand-activated RAR-epsilon from a TRE can, however, be repressed by thyroid-hormone receptor in the absence of its ligand. Conversely, in the presence of its ligand, thyroid-hormone receptor will activate transcription from a TRE irrespective of the presence of unbound RAR. The use of hybrid receptors has shown that the DNA-binding domain of RAR is the essential target for inhibition by thyroid-hormone receptors. These data, together with in vitro DNA-binding studies, suggest that thyroid-hormone receptors may have dual regulatory roles: in the presence of hormone they function as TRE-specific transcriptional activators; in the absence of hormone, however, they can function as TRE-specific repressors.

Identification of cis-acting sequences responsible for phorbol ester induction of human serum amyloid A gene expression via a nuclear factor kappaB-like transcription factor

We have analyzed the 5'-flanking region of one of the genes coding for the human acute-phase protein, serum amyloid A (SAA). We found that SAA mRNA could be increased fivefold in transfected cells by treatment with phorbol 12-myristate 13-acetate (PMA). To analyze this observation further, we placed a 265-base-pair 5' SAA fragment upstream of the reporter chloramphenicol acetyltransferase (CAT) gene and transfected this construct into HeLa cells. PMA treatment of these transient transfectants resulted in increased CAT expression. Nuclear proteins from PMA-treated HeLa cells bound to this DNA fragment, and methylation interference analysis showed that the binding was specific to the sequence GGGACTTTCC (between -82 and -91), a sequence previously described by R. Sen and D. Baltimore (Cell 46:705-716, 1986) as the binding site for the nuclear factor NF kappa B. In a cotransfection competition experiment, we could abolish PMA-induced CAT activity by using cloned human immunodeficiency virus long-terminal-repeat DNA containing the NF kappa B-binding sequence. The same long-terminal-repeat DNA containing mutant NF kappa B-binding sequences (G. Nabel and D. Baltimore, Nature [London] 326:711-713, 1987) did not affect CAT expression, which suggested that binding by an NF kappa B-like factor is required for increased SAA transcription.

Repression of transcription mediated at a thyroid hormone response element by the v-erb-A oncogene product

Several recent observations, such as the identification of the cellular homologue of the v-erb-A oncogene as a thyroid-hormone receptor, have strongly implicated nuclear oncogenes in transcriptional control mechanisms. The v-erb-A oncogene blocks the differentiation of erythroid cells, and changes the growth requirements of fibroblasts and erythroblasts. Mutations in v-erb-A protein have led to the loss of its affinity for thyroid hormones but do not affect its DNA-binding ability, a property required for biological activity. We report here the identification of a novel thyroid-hormone response element (TRE) in the long terminal repeat of Moloney murine leukaemia virus that binds the c-erb-A-alpha protein. The v-erb-A protein abolishes the responsiveness of this TRE to thyroid hormone, although it has a lower affinity than the normal receptor for the TRE. The data indicate that overexpressed v-erb-A protein negatively interferes with normal transcriptional-control mechanisms, and that amino-acid substitutions have altered its DNA-binding properties.

Thyroid hormone response elements in pituitary genes

Thyroid hormones regulate the transcription of a number of genes in the anterior pituitary gland. A thyroid hormone response element in the regulatory region of the rat growth hormone gene has previously been shown to mediate the effects of thyroid hormones on growth hormone gene transcription. The 5' flanking regions of the thyrotrophin alpha and beta subunit and prolactin genes have now been examined for the presence of sequences similar to this response element. Southern blotting reveals that no such sequences are present in the regions of the thyrotrophin subunit and prolactin genes examined, suggesting that the thyroid response elements of these genes differ from that of the rat growth hormone gene.

A pituitary POU domain protein, Pit-1, activates both growth hormone and prolactin promoters transcriptionally

The anterior pituitary gland provides a model for investigating the molecular basis for the appearance of phenotypically distinct cell types within an organ, a central question in development. The rat prolactin and growth hormone genes are expressed selectively in distinct cell types (lactotrophs and somatotrophs, respectively) of the anterior pituitary gland, reflecting differential mechanisms of gene activation or restriction, as a result of the interactions of multiple factors binding to these genes. We find that when the pituitary-specific 33-kD transcription factor Pit-1, expressed normally in both lactotrophs and somatotrophs, is expressed in either the heterologous HeLa cell line or in bacteria, it binds to and activates transcription from both growth hormone and prolactin promoters in vitro at levels even 10-fold lower than those normally present in pituitary cells. This suggests that a single factor, Pit-1, may be capable of activating the expression of two genes that define different anterior pituitary cell phenotypes. Because a putative lactotroph cell line (235-1) that does not express the growth hormone gene, but only the prolactin gene, appears to contain high levels of functional Pit-1, a mechanism selectively preventing growth hormone gene expression may, in part, account for the lactotroph phenotype.

Protein encoded by v-erbA functions as a thyroid-hormone receptor antagonist

The thyroid-hormone receptor can, in the absence of its ligand, suppress activity of a responsive promoter. Addition of thyroid hormone, however, results in the stimulation of expression. The oncogenic derivative of the thyroid-hormone receptor, v-erbA, acts as a constitutive repressor and, when coexpressed with the receptor, blocks activation by thyroid hormone. Thus, v-erbA may be the first example of a dominant negative oncogene.

Protein disulphide isomerase, a multifunctional endoplasmic reticulum protein

Protein disulphide isomerase (E.C. 5.3.4.1) has been purified, cloned, and sequenced from a variety of vertebrate tissues. The enzyme and its isoforms have been assigned a role in four functional activities: (1) hydroxylation of proline residues in procollagen; (2) disulphide bond oxidation, isomerization, and reduction; (3) the major non-nuclear binding protein of the thyroid hormone 3,3',5-triiodo-L-thyronine; and (4) a component of oligosaccharide transferase. The concentration of the enzyme has been shown to be positively correlated with an endoplasmic reticulum network which is active in secreting disulphide-bonded polypeptides. The enzyme is directed into the endoplasmic reticulum by virtue of a 19 residue N-terminal signal peptide; a four amino acid C-terminal KDEL sequence prevents the enzyme from being secreted. Careful inspection of the sequence data of the isoforms from human tissues reveals a 97% similarity; whereas, analyses of the data from chick tissues reveals only a 80% level of similarity. Chromosomal localizations using human cDNA probes against different human isoforms have assigned the gene(s) to opposite ends of the long arm of chromosome 17. The compiled data suggest the presence of a family of related polypeptides, all of which reside within the lumen of the endoplasmic reticulum.

Sequence elements in the human osteocalcin gene confer basal activation and inducible response to hormonal vitamin D3

Osteoblast-specific expression of the bone protein osteocalcin is controlled at the transcriptional level by the steroid hormone 1 alpha,25-dihydroxyvitamin D3. As this protein may represent a marker for bone activity in human disease, we examined the regulation of its expression at the molecular level by evaluating human osteocalcin gene promoter function. We describe regions within the promoter that contribute to basal expression of the gene in osteoblast-like cells in culture. Further, we define a 21-base-pair DNA element with the sequence 5'-GTGACTCACCGGGTGAACGGG-3', which acts in cis to mediate 1 alpha,25-dihydroxyvitamin D3 inducibility of the osteocalcin gene. This response element bears sequence similarity with other short DNA segments, particularly those for estrogen and thyroid hormone, which act together with their respective trans-acting receptors to modulate gene transcription.

Identification of cis-acting sequences responsible for phorbol ester induction of human serum amyloid A gene expression via a nuclear factor kappaB-like transcription factor

We have analyzed the 5'-flanking region of one of the genes coding for the human acute-phase protein, serum amyloid A (SAA). We found that SAA mRNA could be increased fivefold in transfected cells by treatment with phorbol 12-myristate 13-acetate (PMA). To analyze this observation further, we placed a 265-base-pair 5' SAA fragment upstream of the reporter chloramphenicol acetyltransferase (CAT) gene and transfected this construct into HeLa cells. PMA treatment of these transient transfectants resulted in increased CAT expression. Nuclear proteins from PMA-treated HeLa cells bound to this DNA fragment, and methylation interference analysis showed that the binding was specific to the sequence GGGACTTTCC (between -82 and -91), a sequence previously described by R. Sen and D. Baltimore (Cell 46:705-716, 1986) as the binding site for the nuclear factor NF kappa B. In a cotransfection competition experiment, we could abolish PMA-induced CAT activity by using cloned human immunodeficiency virus long-terminal-repeat DNA containing the NF kappa B-binding sequence. The same long-terminal-repeat DNA containing mutant NF kappa B-binding sequences (G. Nabel and D. Baltimore, Nature [London] 326:711-713, 1987) did not affect CAT expression, which suggested that binding by an NF kappa B-like factor is required for increased SAA transcription.

Trans-activation by thyroid hormone receptors: functional parallels with steroid hormone receptors

The effects of thyroid hormones are mediated through nuclear receptor proteins that modulate the transcription of specific genes in target cells. We previously isolated cDNAs encoding two different mammalian thyroid hormone receptors, one from human placenta (hTR beta) and the other from rat brain (rTR alpha), and showed that their in vitro translation products bind thyroid hormones with the characteritistic affinities of the native thyroid hormone receptor. We now demonstrate that both of the cloned receptors activate transcription from a thyroid hormone-responsive promoter in a hormone-dependent manner, with rTR alpha eliciting a greater response than hTR beta. The putative functional domains of the thyroid hormone receptors were examined by creating chimeric thyroid hormone/glucocorticoid receptors, producing receptors with hybrid functional properties. These experiments support the proposal that the thyroid hormone receptors are composed of interchangeable functional domains, and indicate that the mechanism of hormone-inducible gene regulation has been conserved in steroid and thyroid hormone receptors.

Structure of human angiotensinogen gene

A cDNA clone encoding human angiotensinogen was isolated from a cDNA library prepared from human liver mRNA and used to isolate the angiotensinogen gene. The complete exon sequence of this gene together with extensive intron and flanking sequences are reported. The human angiotensinogen gene contains five exons interrupted by four intervening sequences. We compared the intron-exon structure of this human gene with that of the rat gene or the genes coding for proteins such as alpha 1-antitrypsin and antithrombin III, whose primary amino acid sequences show similarities. The human angiotensinogen gene shows identical organization with the alpha 1-antitrypsin gene, but is different from the antithrombin III gene. The 5'-flanking sequence (-500 to -1 bp) of the human angiotensinogen gene was examined for hormone regulatory elements (HRE), which may be implicated in the interaction with the hormone receptor complexes.

Inhibition of thyroid hormone action by a non-hormone binding c-erbA protein generated by alternative mRNA splicing

Thyroid hormone (T3) binds to a nuclear receptor protein which regulates gene expression by binding to specific DNA sequences near hormone-responsive genes. Proteins encoded by two cellular proto-oncogenes, c-erbA alpha and beta, bind T3 and can act as functional T3 receptors. In rats, alternative splicing of the alpha-gene transcript generates at least two distinct protein products, termed r-erbA alpha 1 and r-erbA alpha 2. Although these proteins bind to the same DNA sequence, r-erbA alpha 2 does not bind T3. We show here that expression of r-erbA alpha 2 inhibits the T3-dependent inductive effect of either r-erbA beta or r-erbA alpha 1 on expression of a T3-responsive test gene. Alternative splicing of the erbA alpha transcript thus generates products with opposing biological activities, suggesting a novel mechanism for the modulation of hormonal responsiveness.

Liver cell specific gene transcription in vitro: the promoter elements HP1 and TATA box are necessary and sufficient to generate a liver-specific promoter

The hepatocyte-specific promoter element HP1, which is present in several genes specifically expressed in the liver, is active in an in vitro transcription system. The liver-specificity is retained in the in vitro system, as the activity is found in extracts of rat liver or hepatoma cells but is absent in an L-cell extract. Mutational analysis identifies HP1 as a 13 bp element: Two point mutations abolish the function of HP1. This inactivation is correlated with a reduced binding affinity of the transcription factors recognizing HP1. Two other mutants, which reduce the activity of HP1, bind the transcription factors with an affinity identical to the wildtype sequence. This suggests that the binding of the transcription factors is not sufficient for activation of HP1 dependent transcription. The function of HP1 depends on the presence of a TATA box within a distance of some 70 bp. Various TATA sequences are functional and no stereospecific alignment between HP1 and the TATA box is required.

Thyroid hormone action and the erbA oncogene family

In this review, we discuss the biological action and biochemical function of the v-erbA oncogene product, and the role of c-erbA proto-oncogene products as thyroid hormone receptors, as related to the molecular structure and function of the nuclear hormone receptors at large.

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.

Analysis of cis- and trans-acting elements in the hormone-sensitive human somatotropin gene promoter

The expression of the human growth hormone (hGH) gene is regulated by several transcription factors. Basal level transcription factors include TATA box-binding proteins, Sp1, USF and CTF/NF-1. The hGH gene is expressed only in pituitary somatotrophs, and the pituitary-specific GHF-1/Pit-1 protein is a potent transcriptional stimulator. Glucocorticoid and thyroid hormones, insulin, and GHRH which acts via cAMP, also control hGH gene transcription via trans-acting factors some of which are the hormone receptors themselves. Three transcription initiation sites were detected when hGH gene promoter activity was studied in a cell-free system. This system enabled us to delineate the respective role of some transcription factors and to propose a model that accounts for the basal, pituitary-specific, and hormonal control of hGH gene expression.

c-erbA encodes multiple proteins in chicken erythroid cells

To identify and characterize the proteins encoded by the erbA proto-oncogene, we expressed the C-terminal region of v-erbA in a bacterial trpE expression vector system and used the fusion protein to prepare antiserum. The anti-trp-erbA serum recognized the P75gag-erbA protein encoded by avian erythroblastosis virus and specifically precipitated six highly related proteins ranging in size from 27 to 46 kilodaltons from chicken embryonic erythroid cells. In vitro translation of a chicken erbA cDNA produced essentially the same pattern of proteins. Partial proteolytic maps and antigenicity and kinetic analyses of the in vivo and in vitro proteins indicated that they are related and that the multiple bands are likely to arise from internal initiations within c-erbA to generate a nested set of proteins. All of the c-erbA proteins are predominantly associated with chicken erythroblast nuclei. However, Nonidet P-40 treatment resulted in extraction of the three smaller proteins, whereas the larger proteins were retained. During differentiation of erythroid cells in chicken embryos, we found maximal levels of c-erbA protein synthesis at days 7 to 8 of embryogenesis. By contrast, c-erbA mRNA levels remained essentially constant from days 5 to 12. Together, our results indicate that posttranscriptional or translational mechanisms are involved in regulation of c-erbA expression and in the complexity of its protein products.

Genetic dissection of functional domains within the avian erythroblastosis virus v-erbA oncogene

The avian erythroblastosis virus v-erbA locus potentiates the oncogenic transformation of erythroid and fibroblast cells and is derived from a host cell gene encoding a thyroid hormone receptor. We report here the use of site-directed mutagenesis to identify and characterize functional domains within the v-erbA protein. Genetic lesions introduced into a putative hinge region or at the extreme C-terminus of the v-erbA coding domain had no significant effect on the biological activity of this polypeptide. In contrast, mutations introduced within the cysteine-lysine-arginine-rich center of the v-erbA coding region, a DNA-binding domain in the thyroid and steroid hormone receptors, abolished or severely compromised the ability of the viral protein to function. Our results suggest that the mechanism of action of the v-erbA protein in establishing the neoplastic phenotype is closely related to its ability to interact with DNA, presumably thereby altering expression of host target genes by either mimicking or interfering with the action of the normal c-erbA gene product.

Organization and nucleotide sequence of the mouse alpha-subunit gene of the pituitary glycoprotein hormones

We have isolated and determined the nucleotide sequence of the mouse alpha-subunit gene of the pituitary glycoprotein hormones. No detectable rearrangements were observed from the cloned gene when compared with total genomic DNA. The gene is approximately 13.5 kb in size and consists of 4 exons and 3 introns. The 5' untranslated region is encoded by exon 1 and 14 bp of exon 2. A large first intron of about 10 kb interrupts the 5' untranslated region. The coding region is present in exons 2, 3, and 4, while the 3' untranslated region is contained entirely within exon 4. A number of mouse B1 and B2 repeats are present in the 5'-flanking region, the first and third introns, as well as in the 3'-flanking region. In contrast to the mouse beta-subunit gene of thyrotropin, primer extension analysis revealed that the alpha-subunit gene has a single transcriptional start site and that the primary transcript does not exhibit alternative exon splicing. The transcriptional start site is identical in both mouse pituitaries and in TtT97 thyrotropic tumors and is responsive to thyroidal status. The 5'-flanking region contains a sequence that is homologous to a single copy of the duplicated 18-bp cyclic AMP-responsive element of the human alpha-subunit gene. Sequences homologous to putative thyroid-responsive and estrogen responsive elements are present in the 5'-flanking region and may be important in the multihormonal regulation of the alpha-subunit gene in throtrophs and gonadotrophs, respectively.

Retinoic acid and thyroid hormone induce gene expression through a common responsive element

Studies of steroid receptors have led to the identification of a superfamily of ligand-inducible regulatory proteins that includes receptors for thyroid hormones and retinoic acid. This family of receptors regulates gene expression through binding to short cis-acting sequences referred to as hormone-response elements. Identification of a functional retinoic acid responsive element is crucial to our understanding of the mechanisms by which retinoic acid receptors activate gene expression and regulate cell differentiation. One impediment to such a study is the absence of any identified gene whose transcription is directly dependent on the receptor-hormone complex. Because the DNA-binding domains of the retinoic acid and thyroid hormone receptors are highly related (62% identical in their amino acid sequences), we have investigated the possibility that the retinoic acid receptor could activate gene expression through a thyroid hormone response element. We now report that a human retinoic acid receptor expressed from cloned complementary DNA or the endogenous retinoic acid receptor present in F9 teratocarcinoma cells can activate gene expression from promoters fused to a natural or synthetic thyroid hormone response element. The product translated in vitro from the human retinoic acid receptor cDNA can bind to a thyroid hormone response element with high affinity. The unexpected implication of these findings is that retinoic acid and thyroid hormones, acting through their respective receptors, could control overlapping gene networks involved in the regulation of vertebrate morphogenesis and homeostasis.

A tissue-specific transcription factor containing a homeodomain specifies a pituitary phenotype

Multiple related cis-active elements required for cell-specific activation of the rat prolactin gene appear to bind a pituitary-specific positive transcription factor(s), referred to as Pit-1. DNA complementary to Pit-1 mRNA, cloned on the basis of specific binding to AT-rich cell-specific elements in the rat prolactin and growth hormone genes, encodes a 33 kd protein with significant similarity at its carboxyl terminus to the homeodomains encoded by Drosophila developmental genes. Pit-1 mRNA is expressed exclusively in the anterior pituitary gland in both somatotroph and lactotroph cell types, which produce growth hormone and prolactin, respectively. Pit-1 expression in heterologous cells (HeLa) selectively activates prolactin and growth hormone fusion gene expression, suggesting that Pit-1 is sufficient to confer a characteristic pituitary phenotype. The structure of Pit-1 and its recognition elements suggests that metazoan tissue phenotype is controlled by a family of transcription factors that bind to related cis-active elements and contain several highly conserved domains.

Cyclic AMP and phorbol ester-stimulated transcription mediated by similar DNA elements that bind distinct proteins

cAMP and phorbol esters mediate cellular metabolism by the activation of distinct signal transduction pathways consisting of a cascade of sequential protein phosphorylations. An important consequence of the activation of these pathways is the stimulation of gene transcription by way of interactions of specific proteins with DNA control elements. The 8-base-pair (bp) DNA consensus sequence TGACGTCA [cAMP response element (cAMP-RE)] has been shown to confer cAMP responsivity on transcription from various promoters, and the closely related 7-bp consensus sequence TGA-(C or G)TCA [phorbol 12-myristate 13-acetate response element (PMA-RE)] lends transcriptional responsiveness to phorbol esters. In the JEG-3 placental cell line we find that several variants of the cAMP-REs fused to a gonadotropin alpha promoter chloramphenicol acetyltransferase reporter gene mediate responsiveness to cAMP but not to phorbol esters. The PMA-RE is responsive to phorbol esters but also imparts submaximal sensitivity to cAMP in the JEG-3 cells and in the Hep G2 hepatoma cell line. The transcriptional activities of cAMP-RE and PMA-RE are markedly influenced by the composition of the neighboring bases, but different sequences are permissive for the activity of the cAMP-RE versus the PMA-RE. The two signaling agents together display a supraadditive effect on reporter genes containing active PMA-REs but not cAMP-REs. Gel-mobility-shift and UV cross-linking analyses show that distinct proteins bind to the two control elements. One protein of 38 kDa binds to the cAMP-RE and several proteins of 48-84 kDa bind to the PMA-RE.