c-erbA encodes multiple proteins in chicken erythroid cells

Genomic and Non-Genomic Mechanisms of Action of Thyroid Hormones and Their Catabolite 3,5-Diiodo-L-Thyronine in Mammals

Since the realization that the cellular homologs of a gene found in the retrovirus that contributes to erythroblastosis in birds (v-erbA), i.e. the proto-oncogene c-erbA encodes the nuclear receptors for thyroid hormones (THs), most of the interest for THs focalized on their ability to control gene transcription. It was found, indeed, that, by regulating gene expression in many tissues, these hormones could mediate critical events both in development and in adult organisms. Among their effects, much attention was given to their ability to increase energy expenditure, and they were early proposed as anti-obesity drugs. However, their clinical use has been strongly challenged by the concomitant onset of toxic effects, especially on the heart. Notably, it has been clearly demonstrated that, besides their direct action on transcription (genomic effects), THs also have non-genomic effects, mediated by cell membrane and/or mitochondrial binding sites, and sometimes triggered by their endogenous catabolites. Among these latter molecules, 3,5-diiodo-L-thyronine (3,5-T2) has been attracting increasing interest because some of its metabolic effects are similar to those induced by T3, but it seems to be safer. The main target of 3,5-T2 appears to be the mitochondria, and it has been hypothesized that, by acting mainly on mitochondrial function and oxidative stress, 3,5-T2 might prevent and revert tissue damages and hepatic steatosis induced by a hyper-lipid diet, while concomitantly reducing the circulating levels of low density lipoproteins (LDL) and triglycerides. Besides a summary concerning general metabolism of THs, as well as their genomic and non-genomic effects, herein we will discuss resistance to THs and the possible mechanisms of action of 3,5-T2, also in relation to its possible clinical use as a drug.

A truncated PPAR gamma 2 localizes to mitochondria and regulates mitochondrial respiration in brown adipocytes

Peroxisome proliferator-activated receptor gamma (PPARγ) is a key regulator of brown adipocyte differentiation and thermogenesis. The PPARγ gene produces two isoforms, PPARγ1 and PPARγ2. PPARγ2 is identical to PPARγ1 except for additional 30 amino acids present in the N-terminus of PPARγ2. Here we report that the C-terminally truncated form of PPARγ2 is predominantly present in the mitochondrial matrix of brown adipocytes and that it binds to the D-loop region of mitochondrial DNA (mtDNA), which contains the promoter for mitochondrial electron transport chain (ETC) genes. Expression of mitochondrially targeted MLS-PPARγ2 in brown adipocytes increases mtDNA-encoded ETC gene expression concomitant with enhanced mitochondrial respiration. These results suggest that direct regulation of mitochondrially encoded ETC gene expression by mitochondrial PPARγ2, in part, underlies the isoform-specific role for PPARγ2 in brown adipocytes.

Thyroid Hormone Action: The p43 Mitochondrial Pathway

The possibility that several pathways are involved in the multiplicity of thyroid hormone physiological influences led to searches for the occurrence of T3 extra nuclear receptors. The existence of a direct T3 mitochondrial pathway is now well established. The demonstration that TRα1 mRNA encodes not only a nuclear thyroid hormone receptor but also two proteins imported into mitochondria with molecular masses of 43 and 28 kDa has provided new clues to understand the pleiotropic influence of iodinated hormones.The use of a T3 photo affinity label derivative (T3-PAL) allowed detecting two mitochondrial T3 binding proteins. In association with western blots using antibodies raised against the T3 nuclear receptor TRα1, mitochondrial T3 receptors were identified as truncated TRα1 forms. Import and in organello transcription experiments performed in isolated mitochondria led to the conclusion that p43 is a transcription factor of the mitochondrial genome, inducing changes in the mitochondrial/nuclear crosstalk. In vitro experiments indicated that this T3 mitochondrial pathway affects cell differentiation, apoptosis, and transformation. Generation of transgenic mice demonstrated the involvement of this mitochondrial pathway in the determination of muscle phenotype, glucose metabolism, and thermogenesis.

Mitochondrial T3 receptor and targets

The demonstration that TRα1 mRNA encodes a nuclear thyroid hormone receptor and two proteins imported into mitochondria with molecular masses of 43 and 28 kDa has brought new clues to better understand the pleiotropic influence of iodinated hormones. If p28 activity remains unknown, p43 binds to T3 responsive elements occurring in the organelle genome, and, in the T3 presence, stimulates mitochondrial transcription and the subsequent synthesis of mitochondrial encoded proteins. This influence increases mitochondrial activity and through changes in the mitochondrial/nuclear cross talk affects important nuclear target genes regulating cell proliferation and differentiation, oncogenesis, or apoptosis. In addition, this pathway influences muscle metabolic and contractile phenotype, as well as glycaemia regulation. Interestingly, according to the process considered, p43 exerts opposite or cooperative effects with the well-known T3 pathway, thus allowing a fine tuning of the physiological influence of this hormone.

Fatty acid metabolism and thyroid hormones

The importance of thyroid hormone signaling in the acute regulation of metabolic activity has been recognized for decades. Slowly, the underlying mechanisms responsible for this activity are being elucidated. A prominent characteristic of thyroid signaling is rapid increases in oxygen consumption and ATP production. This discovery implicated a non-genomic regulation of mitochondrial metabolism by thyroid hormones. Another important clue came from the discovery that thyroid hormones stimulated fatty acid oxidation (FAO) in a variety of tissues in a receptor-dependent, but transcriptional-independent manner. Recently, key linkages between thyroid hormone signaling and specific mitochondrial-targeted pathways have been discovered. This review focuses on the molecular mechanisms by which mitochondrial FAO can be increased through thyroid hormone signaling. The roles of both the full-length and shortened mitochondrial isoforms of thyroid hormone receptor will be discussed. Additionally, the impact of thyroid hormone signaling on dyslipidemias such as obesity, type II diabetes, and fatty liver disease will be considered.

The thyroid hormone receptor alpha1 protein is expressed in embryonic postmitotic neurons and persists in most adult neurons

Thyroid hormone is essential for brain development where it acts mainly through the thyroid hormone receptor α1 (TRα1) isoform. However, the potential for the hormone to act in adult neurons has remained undefined due to difficulties in reliably determining the expression pattern of TR proteins in vivo. We therefore created a mouse strain that expresses TRα1 and green fluorescent protein as a chimeric protein from the Thra locus, allowing examination of TRα1 expression during fetal and postnatal development and in the adult. Furthermore, the use of antibodies against other markers enabled identification of TRα1 expression in subtypes of neurons and during specific stages of their maturation. TRα1 expression was first detected in postmitotic cells of the cortical plate in the embryonic telencephalon and preceded the expression of the mature neuronal protein NeuN. In the cerebellum, TRα1 expression was absent in proliferating cells of the external granular layer, but switched on as the cells migrated towards the internal granular layer. In addition, TRα1 was expressed transiently in developing Purkinje cells, but not in mature cells. Glial expression was found in tanycytes in the hypothalamus and in the cerebellum. In the adult brain, TRα1 expression was detected in essentially all neurons. Our data demonstrate that thyroid hormone, unexpectedly, has the capacity to play an important role in virtually all developing and adult neurons. Because the role of TRα1 in most neuronal cell types in vivo is largely unknown, our findings suggest that novel functions for thyroid hormone remain to be identified in the brain.

Involvement of thyroid hormones in chicken embryonic brain development

Thyroid hormones (THs) play an important role in vertebrate brain development by stimulating and coordinating cell proliferation, migration and differentiation. Several TH-responsive genes involved in these processes have been identified, but the information is mainly derived from studies of late brain development, while relatively little is known about the more early stages, prior to the onset of embryonic TH secretion. We have chosen the chick embryo to investigate the role of THs in both late and early brain development. T(4) and T(3) are found in chicken brain from the earliest stages tested (day 4). Indirect clues for the involvement of T(3) in brain development are found in the ontogenetic expression profiles of proteins regulating its bioavailability and action, including TH transporters, deiodinases and TH-receptors. All of them are expressed in whole embryos tested on day 2 of incubation and in developing brain tested from day 4 onwards. Their distribution patterns vary over time and according to the brain area and cell type studied. Hypothyroidism induced during the second half of incubation disturbs cell migration in the cerebellum, providing more direct evidence for the requirement for THs during the later stages of brain development. Direct morphological proof for the requirement for THs during the first half of incubation is still missing, but microarray analysis of telencephalon shows a clearly divergent gene expression profile in hypothyroid embryos. In vivo knockdown of TH transporters and deiodinases in chick embryos cultured ex ovo provides an excellent tool to study the role of THs in early brain development in more detail.

Expression of nuclear and mitochondrial thyroid hormone receptors in postnatal rat tongue muscle

In this quantitative study, a competitive RT-PCR analysis was used to measure the level of the thyroid hormone receptors (TRs) in rat tongue muscle during the development of male Wistar rats aged 0, 5, 10, 15 and 21 postnatal days. There were differences between the expression of TR-alpha1 mRNA and the mRNAs for TR-beta1 and TR-beta2 in rat tongue muscle. Using Western blot analysis, a difference in expression between TR-alpha1 protein (c-ErbAalpha1 protein) and 43-kD c-ErbAalpha1 protein (T(3)-binding 43-kD mitochondrial protein) was detected during the development of the rat tongue muscle. Immunohistochemical examination using electron microscopy showed that TR-alpha1 was found in the mitochondria and nuclei in contrast to TR-beta1 detected in rat tongue muscle. In mitochondrial fractions from rat tongue muscle, the expression of 43-kD c-ErbAalpha1 protein was increased dramatically at 15 and 21 days, and a similar tendency was seen in cytochrome c proteins using Western blot analysis. We presume that the 43-kD c-ErbAalpha1 protein plays a role in regulating mitochondrial RNA synthesis during the postnatal development of rat tongue. The mRNA and protein myosin heavy chain isoforms of muscle also had a different expression during development. The slow myosin isoform protein was not found from day 10 in contrast to fast myosin isoforms. It is likely that the expression of TR-alpha1 mRNA from the rat tongue muscle may be related to a specific phase in muscle phenotype during the development.

Multiple Messenger Ribonucleic Acid Variants Regulate Cell-Specific Expression of Human Thyroid Hormone Receptor β1

Thyroid hormones are essential for development, growth, and metabolism and act via T3 receptors (TR) alpha and beta. The THRA and THRB genes have discrete physiological roles but their mRNAs are expressed widely in overlapping patterns. There is poor correlation between TR mRNA and protein, indicating that expression may be regulated by posttranscriptional mechanisms. Differences in the relative levels of expressed TRalpha and beta proteins have been suggested to modulate tissue T3 responsiveness. We determined the structure of the human THRB gene, cloned seven alternately spliced 5'-untranslated region (5'-UTR) TRbeta1 mRNAs, and identified five polyadenylation position elements in the 3'-UTR. At least six TRbeta1 mRNAs between 1.35 and 7.5 kb in length were expressed in discrete temporospatial patterns in fetal and adult human tissues. The 5'-UTRs contained up to seven upstream short open reading frames, which did not influence the structure of the TRbeta1 protein. In transfection studies, 5'-UTRs exerted cell-specific effects on mRNA expression but consistently reduced protein expression. Furthermore, each 5'-UTR strongly inhibited translation in vitro. Thus, developmental and tissue-specific expression of human thyroid hormone receptor beta1 5'-UTR mRNAs may regulate T3-responsiveness in target tissues by modulating TRbeta protein translation and thereby controlling the ratio of expressed TRalpha and -beta proteins.

New molecular aspects of regulation of mitochondrial activity by fenofibrate and fasting

Fenofibrate and fasting are known to regulate several genes involved in lipid metabolism in a similar way. In this study measuring several mitochondrial enzyme activities, we demonstrate that, in contrast to citrate synthase and complex II, cytochrome c oxidase (COX) is a specific target of these two treatments. In mouse liver organelles, Western blot experiments indicated that mitochondrial levels of p43, a mitochondrial T3 receptor, and mitochondrial peroxisome proliferator activated receptor (mt-PPAR), previously described as a dimeric partner of p43 in the organelle, are increased by both fenofibrate and fasting. In addition, in PPAR alpha-deficient mice, this influence was abolished for mt-PPAR but not for p43, whereas the increase in COX activity was not altered. These data indicate that: (1) PPAR alpha is involved in specific regulation of mt-PPAR expression by both treatments; (2) fenofibrate and fasting regulate the mitochondrial levels of p43 and thus affect the efficiency of the direct T3 mitochondrial pathway.

Differential expression and regulation of estrogen receptors (ERs) in rat pituitary and cell lines: estrogen decreases ERalpha protein and estrogen responsiveness

Estrogen (E) regulates the synthesis and secretion of several pituitary hormones during the reproductive cycle in a cell- and promoter-specific manner. One mechanism underlying cell specificity is the differential expression of estrogen receptor (ER) isoforms. We used in vivo and in vitro rodent pituitary cell models to examine the expression and regulation of ERalpha, ERbeta, and the pituitary-specific ERalpha isoform, truncated estrogen receptor product-1 (TERP-1). In cycling female rat pituitaries, ERbeta messenger RNA (mRNA) levels fell 40% on the morning of proestrus and were suppressed by E or dihydrotestosterone in ovariectomized females. In lactotrope and gonadotrope cell lines (GH3, RC4B, LbetaT2), progesterone (P) or P plus E also suppressed ERbeta. TERP-1 mRNA increased 3-fold at proestrus and in response to E treatment in vivo and in cell lines. ERalpha mRNA levels were not regulated significantly by any treatment in vivo or in cell lines. However, E suppressed ERalpha protein levels in vivo and in cell lines, and reduction of ERalpha protein levels by E or the antiestrogen ICI182,780 reduced E-stimulated transcriptional activation of the PRL promoter in GH3 cells. TERP-1 and ERbeta protein levels were low to undetectable in cell lines, but E stimulated TERP-1. Because E treatment decreases ERbeta mRNA and ERalpha protein and increases levels of TERP-1 (which can suppress ERalpha/beta activity), the dynamic steroid-induced changes in ER expression in the rat pituitary during the midcycle gondaotropin/PRL surge may provide a means for ovarian steroids to limit positive feedback.

The adenovirus E1A protein is a potent coactivator for thyroid hormone receptors

The thyroid hormone receptors interact with several different cofactors when activating transciption. In this study, we show that the adenovirus E1A oncoprotein functions as a strong coactivator for the thyroid hormone receptor (TR), and that TR and E1A synergistically activate transcription via direct (DR4) or palindromic (IRO) hormone-responsive sites. Cotransfection experiments using different isoforms of the chicken TR and E1A show synergistic, ligand-enhanced transactivation. This transactivation is accomplished through a direct, ligand-independent interaction between TR and E1A. The interaction domains in TR are localized to the DNA-binding domain and to the carboxy-terminal part of the ligand-binding domain. In E1A, the regions of interactions are localized to the conserved regions 1 and 3. Both of these domains in E1A are required for a 40-fold enhancement of TR-mediated activation in transfection experiments. Taken together, we show that E1A strongly enhances transcriptional activation, which suggests that it serves as a bridging factor between the receptor and other components of the transcription machinery.

Cis-acting elements in the 5'-flanking DNA of the malic enzyme gene regulate tissue-specific T3-responsiveness in chick embryo fibroblasts

Triiodothyronine (T3) stimulates transcription of the malic enzyme gene in chick embryo hepatocytes (CEH), but not in chick embryo fibroblasts (CEF), even though the two cell types contain similar nuclear T3 binding activities (F. B. Hillgartner, W. Chen, and A. G. Goodridge, J. Biol. Chem. 267, 12299-12306, 1992). Based on Western blot analyses and gel electrophoretic mobility-shift assays, differences in mass of thyroid hormone receptor (TR)alpha or binding of TRalpha to T3 response element (T3RE) are not responsible for tissue-specific T3 responsiveness. Using transfection assays, we show that the primary T3RE in RCAS-TRalpha-CEF, cells that constitutively over-express TRalpha, is located downstream of the T3REs that are primarily responsible for T3 responsiveness in CEH and is only weakly functional in CEH. T3RE 2, the major T3RE of the malic enzyme gene in CEH is active in CEF when the construct does not contain additional malic enzyme DNA, but not in constructs containing DNA from -3858 to -3541 bp. Responsiveness conferred by T3RE 2 is inhibited in CEF and RCAS-TRalpha-CEF by three or more cis-acting elements downstream from T3RE 2. One element each was localized to fragments from -3622 to -3595 and -3561 to -3541 bp. The inhibitory effect of these elements was not observed in CEH and, although they cannot explain all of the difference in responsiveness in the two cell types, may contribute to the tissue-specific T3 responsiveness of the malic enzyme gene.

Thyroid hormone receptor gene knockouts

The thyroid hormone receptor genes, TRalpha and TRbeta, differ in developmental expression and tissue distribution. TRbeta knockout mice have goiter, elevated thyroid hormone and TSH levels, and a functional auditory defect. In contrast, mice with TRalpha 1/alpha2 inactivation have thyroid hypoplasia, low serum thyroid hormone levels, growth arrest and delayed small intestine maturation. Mice with selective TRalpha1 inactivation have apparent normal growth and development, but have bradycardia and reduced body temperature. The dramatic differences between these mice with TRbeta and TRalpha gene inactivations indicate the differential function of these genes. The influence of these gene inactivations on thyroid-stimulating hormone regulation is central to the resulting phenotypes.

Chicken thyroid hormone receptor alpha requires the N-terminal amino acids for exclusive nuclear localization

The subcellular localization of natural and engineered forms of the chicken thyroid hormone receptor (cTR alpha) is dependent on amino acids encoded in the N-terminal region. The full length receptor protein, cTR alpha-p46, was found to localize exclusively to the nucleus, whereas the N-terminally shorter variant, cTR alpha-p40, localizes to both the nucleus and the cytoplasm. The exclusive nuclear localization of cTR alpha-p46 is dependent on the presence of the first 11 N-terminal amino acids, but independent of the phosphorylation of the serine at position 12. Our data identify a novel role for an N-terminal domain of the full length thyroid hormone receptor.

Thyroid hormone resistance syndrome manifests as an aberrant interaction between mutant T3 receptors and transcriptional corepressors

Nuclear hormone receptors are hormone-regulated transcription factors that play critical roles in chordate development and homeostasis. Aberrant nuclear hormone receptors have been implicated as causal agents in a number of endocrine and neoplastic diseases. The syndrome of Resistance to Thyroid Hormone (RTH) is a human genetic disease characterized by an impaired physiological response to thyroid hormone. RTH is associated with diverse mutations in the thyroid hormone receptor beta-gene. The resulting mutant receptors function as dominant negatives, interfering with the actions of normal thyroid hormone receptors coexpressed in the same cells. We report here that RTH receptors interact aberrantly with a newly recognized family of transcriptional corepressors variously denoted as nuclear receptor corepressor (N-CoR), retinoid X receptor interacting protein-13 (RIP-13), silencing mediator for retinoid and thyroid hormone receptors (SMRT), and thyroid hormone receptor-associating cofactor (TRAC). All RTH receptors tested exhibit an impaired ability to dissociate from corepressors in the presence of thyroid hormone. Two of the RTH mutations uncouple corepressor dissociation from hormone binding; two additional RTH mutants exhibit an unusually strong interaction with corepressor under all hormone conditions tested. Finally, artificial mutants that abolish corepressor binding abrogate the dominant negative activity of RTH mutants. We suggest that an altered corepressor interaction is likely to play a critical role in the dominant negative potency of RTH mutants and may contribute to the variable phenotype in this disorder.

Starvation-induced decrease in the maximal binding capacity for triiodothyronine of the thyroid hormone receptor is due to a decrease in the receptor protein

Biological responses to thyroid hormones are mediated by the nuclear thyroid hormone receptor (TR). Alterations in the maximal triiodothyronine (T3)-binding capacity (Cmax) of TR measured using a ligand binding assay have been reported under some pathophysiological conditions. Northern blot analysis has indicated that TR mRNA concentrations do not necessarily correlate with Cmax levels. For example, although the decrease in Cmax in rat liver induced by prolonged fasting is well established, TR mRNA concentrations have been reported to be constant. In the present study, we examined starvation-induced changes in TR by Western blot with anti-TR(alpha 1 + beta)antiserum and by Scatchard plot analysis. Starvation of rats for 72 hours decreased Cmax in the liver to 72.5% of control levels. The 47- and 55-kd TR proteins detected in hepatic nuclear extract by Western blotting also decreased to 64% and 66% of control values, respectively. The starvation-induced changes in Cmax and TR protein levels paralleled the change in total hepatic nuclear protein concentration. These results suggest that the decrease in T3-binding activity of the TR is due to a reduction of the TR protein itself.

A 43-kDa protein related to c-Erb A alpha 1 is located in the mitochondrial matrix of rat liver

In order to characterize Sterling's triiodothyronine (T3) mitochondrial receptor using photoaffinity labeling, we observed two specific T3-binding proteins in the inner membrane (28 kDa) and in the matrix (43 kDa) of rat liver mitochondria. Western blots and immunoprecipitation using antibodies raised against the T3-binding domain of the T3 nuclear receptor c-Erb A alpha 1 indicated that at least the 43-kDa protein was c-Erb A alpha 1-related. In addition, gel mobility shift assays demonstrated the occurrence of a c-Erb A alpha 1-related mitochondrial protein that specifically binds to a natural or a palindromic thyroid-responsive element. Moreover, this protein specifically binds to a direct repeat 2 sequence located in the D-loop of the mitochondrial genome. Furthermore, electron microscopy studies allowed the direct observation of a c-Erb A-related protein in mitochondria. Lastly, the relative amounts of the 43-kDa protein related to c-Erb A alpha 1 were in good correlation with the known mitochondrial mass in three typical tissues. Interestingly, expression of a truncated form of the c-Erb A alpha 1 nuclear receptor in CV1 cells was associated with a mitochondrial localization and a stimulation of mitochondrial activity. These results supply evidence of the localization of a member of the nuclear receptor superfamily in the mitochondrial matrix involved in the regulation of mitochondrial activity that could act as a mitochondrial T3-dependent transcription factor.

Retinoid and thyroid hormone receptors: ligand-regulated transcription factors as proto-oncogenes

The retroviral v-erb A locus is derived from a cellular gene, c-erb A, encoding a thyroid hormone receptor. The v-erb A and c-erb A proteins are, in turn, members of a larger family of structurally and functionally interrelated polypeptides that includes the steroid, retinoic acid, and vitamin D3 receptors. These nuclear hormone receptors act by binding to specific sites on the cell genome and, in response to cognate hormone, modulating the transcription of adjacent 'target' genes. The expression, properties, and mechanisms of action of the thyroid hormone receptors (c-erb A proteins) and the closely related retinoic acid receptors are discussed.

Isolation and characterization of a cDNA encoding a chicken beta thyroid hormone receptor

We have isolated and characterized a cDNA encoding a chicken beta homolog of c-erbA, or thyroid hormone receptor (TR). Chicken liver cDNA libraries were screened with a rat TR beta-1 cDNA probe, and several cDNA inserts were isolated and characterized. The sequence of one cDNA predicts a 369-amino-acid open reading frame (ORF), with a protein sequence that possesses 96% identity with that of rat TR beta-1, but only 88% identity with chicken TR alpha. These data indicate that the cDNA likely encodes a beta form of TR that has the expected putative DNA and T3 binding domains. The chicken TR beta (chTR beta) in vitro translated protein binds T3 with high affinity, and binds both the thyroid hormone response element (TRE) from the rat growth hormone gene and the Xenopus vitellogenin A2 gene estrogen response element (ERE), similarly to that of the rat TR beta-1. Northern blot analysis revealed the expression of a 7.0-kb RNA in several tissues including cerebellum, pituitary, kidney, and liver. This chicken liver TR beta cDNA sequence varies in both the 5' and 3' untranslated regions from the chicken kidney TR beta cDNA sequence recently reported (Forrest et al., 1990). The 5' untranslated cDNA sequence divergence occurs near a potential splice site junction of the human TR beta gene, suggesting that this chicken liver cDNA may represent an alternatively spliced RNA product of the chicken TR beta gene.

Developmental and thyroidal regulation of the nuclear T3 receptors/c-erb A oncogene products in the Ob 17 preadipocyte cell line

In a thyroid hormone-sensitive mouse preadipocyte cell line (Ob 17), the concentration of nuclear T3 receptors increases during differentiation in an insulin-independent manner and decreases by 50-60 p.cent after medium supplementation with physiological concentrations of T3. The down-regulation of T3 receptors implies both quantitative and qualitative changes. The preadipocyte T3 receptors were previously reported to be heterogeneous in gel filtration and in their reactivity towards rabbit antibodies raised against large erb A alpha peptides. This report shows that the receptor heterogeneity is not modified during cell development, whereas T3 mainly depletes the receptor species that are both the most retarded in gel filtration and preferentially recognized by c-erb A alpha-specific antisera. The c-erb A alpha-related T3 receptors which predominate in preadipocytes, are thus probably mainly involved in receptor depletion by T3. A similar T3 receptor half-life of 12-13 h was estimated after cycloheximide addition to cells at confluence or later in the differentiation phase with or without T3. This suggested that development, or T3, might have mainly affected the production of T3 receptors. In Northern hybridization studies, using alpha- or beta-type c-erb A cDNA probes containing the entire coding sequence, only alpha-type mRNAs were detected with a predominant band of 2.8 kbases and two fainter bands of about 5.5 and 6.0 kbases. The alpha-type mRNA abundance relative to beta-actin significantly increased during differentiation and decreased after T3 addition.

v-erbA oncogene activation entails the loss of hormone-dependent regulator activity of c-erbA

The v-erbA oncogene, one of the two oncogenes of the avian erythroblastosis virus, efficiently blocks erythroid differentiation and suppresses erythrocyte-specific gene transcription. Here we show that the overexpressed thyroid hormone receptor c-erbA effectively modulates erythroid differentiation and erythrocyte-specific gene expression in a T3-dependent fashion, when introduced into erythroid cells via a retrovirus. In contrast, the endogenous thyroid hormone receptor does not detectably affect erythroid differentiation. The analysis of a series of chimeric v-/c-erbA proteins suggests that the v-erbA oncoprotein has lost one type of thyroid hormone receptor function (regulating erythrocyte gene transcription in response to T3), but constitutively displays another function: it represses transcription in the absence of T3. The region responsible for the loss of hormone-dependent regulator activity of v-erbA has been mapped to the very C-terminus of c-erbA, encompassing a cluster of highly conserved amino acid residues with the potential to form an amphipathic alpha-helix.

A subpopulation of the avian erythroblastosis virus v-erbA protein, a member of the nuclear hormone receptor family, is glycosylated

The v-erbA oncogene of avian erythroblastosis virus is derived from a cellular gene for a thyroid hormone (T4/T3 thyronine) receptor and encodes a DNA-binding protein found principally in the nucleus of the infected cell. I report here that a subpopulation of the v-erbA protein is glycosylated. The v-erbA protein, therefore, is another member of the newly recognized family of eucaryotic transcription factors and related polypeptides which are glycoproteins.

Expression of the v-erbA product, an altered nuclear hormone receptor, is sufficient to transform erythrocytic cells in vitro

We investigated the effect of the v-erbA oncogene product, an altered thyroid hormone receptor, in chicken erythrocyte progenitor cells. Bone marrow cells were infected with a retrovirus vector (XJ12) carrying the v-erbA gene in association with the neoR gene. XJ12-infected erythrocyte progenitor cells gave rise to G418-resistant clones. Some were composed of blast cells identified as transformed CFU-Es blocked in their differentiation. These cells could be grown in culture for at least 25 generations and required anemic chicken serum as a source of erythropoietic growth factors. XJ12 can infect erythrocyte progenitor cells in vivo but is not sufficient to induce erythroleukemia. These data suggest that the activation of a nuclear hormone receptor might represent one step toward the development of neoplasms.