Involvement of a C/EBP-like protein in the acquisition of responsiveness to glucocorticoid hormones during chick neural retina development

The glucocorticoid receptor represses the positive autoregulation of the trout estrogen receptor gene by preventing the enhancer effect of a C/EBPbeta-like protein

Stress and cortisol are known to have negative effects on vitellogenesis in oviparous species. This provides a physiological context in which to explore in more detail the molecular mechanisms involved in transcriptional interferences between two steroids receptors, the estradiol receptor (ER) and the glucocorticoid receptor (GR). We have previously shown that the cortisol inhibitory effect on rainbow trout (rt) vitellogenesis is the result of a repression of the estradiol-induced ER-positive autoregulation by activated GR. In the present study, we demonstrate that the GR repression involves a proximal region of the rtER promoter that is unable to bind GR. This inhibition is counteracted in part by the orphan receptor COUP-TF1 that has been previously shown to cooperate with ERs on the same promoter. A detailed analysis allowed us to identify a C/EBPbeta-like protein that is implicated in both the maximal stimulatory effect of estradiol and the GR repression. Indeed, GR, through its DNA-binding domain, suppresses the binding of C/EBPbeta on the rtER promoter by protein-protein interactions and thereby prevents the enhancer effect of this transcription factor.

Structure/activity elements of the multifunctional protein, GMEB-1. Characterization of domains relevant for the modulation of glucocorticoid receptor transactivation properties

GMEB-1 was initially described as a component of a 550-kDa heteromeric DNA binding complex that is involved in the modulation of two properties of glucocorticoid receptor (GR) transactivation, the dose-response curve of agonists and the partial agonist activity of antagonists. Subsequently, GMEB-1 was also found to bind to hsp27, to associate with the coactivator TIF2 in yeast cells, and to participate in Parvovirus replication. To understand these multiple activities of GMEB-1 at a molecular level, we have now determined which regions are associated with the various activities associated with the modulation of GR transactivation properties. These activities include, homooligomerization, heterooligomerization, DNA binding, binding to GR and the transcriptional cofactor CBP, and GR modulation. Complex activities such as DNA binding and GR modulation, are found to require the physical combination of those domains that would be predicted from the involved biochemical processes. We have previously documented that GMEB-1 possesses both GR modulatory and intrinsic transactivation activity. However, the domains for these two activities of GMEB-1 are found not to overlap. This separation of activities provides a structural basis for our prior biological observations that the modulation of the dose-response curve and partial agonist activity of GR complexes is independent of the total levels of gene activation by the same GR complexes.

The role of C/EBP in nutrient and hormonal regulation of gene expression

C/EBPs are a family of transcription factors that play important roles in energy metabolism. Although initially thought to be constitutive regulators of transcription, an increasing amount of evidence indicates that their transactivating capacity within the cell can be modulated by nutrients and hormones. There are several mechanisms whereby this occurs. First, hormones/nutrients are known to directly alter the expression of C/EBPs. Second, hormones/nutrients may cause an alteration in the phosphorylation state of C/EBPs, which can affect their DNA-binding activity or transactivating capacity. Third, C/EBPs can function as accessory factors on gene promoters within a hormone response unit, interacting with other transcription factors to enhance the degree of responsiveness to specific hormones. Given their role in regulating genes involved in a wide variety of metabolic events, advancing our understanding of the molecular mechanism of action of C/EBPs will undoubtedly further our appreciation for the role these transcription factors play in both health and disease.

Roles of the NFI/CTF gene family in transcription and development

The Nuclear Factor I (NFI) family of site-specific DNA-binding proteins (also known as CTF or CAAT box transcription factor) functions both in viral DNA replication and in the regulation of gene expression. The classes of genes whose expression is modulated by NFI include those that are ubiquitously expressed, as well as those that are hormonally, nutritionally, and developmentally regulated. The NFI family is composed of four members in vertebrates (NFI-A, NFI-B, NFI-C and NFI-X), and the four NFI genes are expressed in unique, but overlapping, patterns during mouse embryogenesis and in the adult. Transcripts of each NFI gene are differentially spliced, yielding as many as nine distinct proteins from a single gene. Products of the four NFI genes differ in their abilities to either activate or repress transcription, likely through fundamentally different mechanisms. Here, we will review the properties of the NFI genes and proteins and their known functions in gene expression and development.

Lactogenic hormones and tenascin-C regulate C/EBPalpha and beta in mammary epithelial cells

Mammary epithelial cell differentiation depends on lactogenic hormones, growth factors, and cell-cell and cell-substrate interactions, all of which modulate transcription factors essential for milk protein gene expression. The CCAAT/enhancer binding protein (C/EBP) family and the signal transducer and activator of transcription 5 (Stat5) have been implicated in mammary epithelial cell growth and differentiation. We have investigated the effects of extracellular matrix components and lactogenic hormones on C/EBP and Stat5 activity. In the mammary gland, tenascin is expressed mainly during embryogenesis and carcinogenesis and in cell culture tenascin downregulates beta-casein gene expression. In HC11 mammary cells, we found that tenascin, but not laminin or fibronectin, specifically downregulated C/EBPalpha levels but had no effect on Stat5 amount or DNA binding activity. Furthermore, we found that the lactogenic hormones, glucocorticoids, prolactin, and insulin, had no effect on C/EBPalpha and C/EBPbeta protein levels but downregulated the DNA binding activity of the transcriptional repressor C/EBPbetaLIP. Thus, C/EBPalpha and beta are regulated by tenascin and lactogenic hormones in mammary epithelial cells.

Glucocorticoid control of glial gene expression

The glucocorticoid signaling pathway is responsive to a considerable number of internal and external signals and can therefore establish diverse patterns of gene expression. A glial-specific pattern, for example, is shown by the glucocorticoid-inducible gene glutamine synthetase. The enzyme is expressed at a particularly high level in glial cells, where it catalyzes the recycling of the neurotransmitter glutamate, and at a low level in most other cells, for housekeeping duties. Glial specificity of glutamine synthetase induction is achieved by the use of positive and negative regulatory elements, a glucocorticoid response element and a neural restrictive silencer element. Though not glial specific by themselves, these elements may establish a glial-specific pattern of expression through their mutual activity and their combined effect. The inductive activity of glucocorticoids is markedly repressed by the c-Jun protein, which is expressed at relatively high levels in proliferating glial cells. The signaling pathway of c-Jun is activated by the disruption of glia-neuron cell contacts, by transformation with v-src, and in proliferating retinal cells of early embryonic ages. The c-Jun protein inhibits the transcriptional activity of the glucocorticoid receptor and thus represses glutamine synthetase expression. This repressive mechanism might also affect the ability of glial cells to cope with glutamate neurotoxicity in injured tissues.

A silencer element in the regulatory region of glutamine synthetase controls cell type-specific repression of gene induction by glucocorticoids

Glutamine synthetase is a key enzyme in the recycling of the neurotransmitter glutamate. Expression of this enzyme is regulated by glucocorticoids, which induce a high level of glutamine synthetase in neural but not in various non-neural tissues. This is despite the fact that non-neural cells express functional glucocorticoid receptor molecules capable of inducing other target genes. Sequencing and functional analysis of the upstream region of the glutamine synthetase gene identified, 5' to the glucocorticoid response element (GRE), a 21-base pair glutamine synthetase silencer element (GSSE), which showed considerable homology with the neural restrictive silencer element NRSE. The GSSE was able to markedly repress the induction of gene transcription by glucocorticoids in non-neural cells and in embryonic neural retina. The repressive activity of the GSSE could be conferred on a heterologous GRE promoter and was orientation- and position-independent with respect to the transcriptional start site, but appeared to depend on a location proximal to the GRE. Gel-shift assays revealed that non-neural cells and cells of early embryonic retina contain a high level of GSSE binding activity and that this level declines progressively with age. Our results suggest that the GSSE might be involved in the restriction of glutamine synthetase induction by glucocorticoids to differentiated neural tissues.

Hypoglycemia and impaired hepatic glucose production in mice with a deletion of the C/EBPbeta gene

The transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta) is enriched in liver and adipose tissue and controls the expression of a wide variety of genes coding for important metabolic pathways, including gluconeogenesis and lipid synthesis. To investigate the role of C/EBPbeta on glucose homeostasis, we studied mice with a targeted deletion of the gene for C/EBPbeta-/- mice. Adult C/EBPbeta-/- mice have hypoglycemia after an 18-hour fast, accompanied by lower hepatic glucose production (40% of that of wild-type mice), with no change in plasma insulin and a lower concentration of plasma free fatty acids (FFA). Glucagon infusion during a pancreatic clamp acutely stimulated hepatic glucose production by 38% in wild-type animals, with no change detected in C/EBPbeta-/- mice. Unexpectedly, both the basal and glucagon-stimulated hepatic cyclic adenosine monophosphate (cAMP) levels were lower in C/EBPbeta-/- mice, indicating an essential role for C/EBPbeta in controlling proximal signal transduction. Fasting hypoglycemia was associated with normal levels of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) gene expression, however net liver glycogenolysis was impaired in C/EBPbeta-/- mice. FFA release from isolated adipose tissue in response to epinephrine was 68% lower in C/EBPbeta-/- mice than in control animals; however, N6,O2'-dibutyryladenosine (Bt2) cAMP stimulated a twofold increase in FFA release in C/EBPbeta-/- compared with no further increase in wild-type mice. Because a deletion in the gene for C/EBPbeta reduces blood glucose and circulating FFA, it could be an important therapeutic target for the treatment of non-insulin-dependent diabetes and possibly obesity, based on designing antagonists that decrease C/EBPbeta activity.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Identification of a functional glucocorticoid response element in the CYP3A1/IGC2 gene

The rat CYP3A subfamily of cytochrome P450 consists of steroid- and drug-metabolizing enzymes inducible by pregnenolone 16alpha-carbonitrile and by supra-physiological doses of dexamethasone. The induction of CYP3A by dexamethasone has been proposed to be mediated by a mechanism distinct from the glucocorticoid receptor mediated response. However, a synergistic induction of CYP3A has been observed with physiological doses of glucocorticoids and other CYP3A inducers. We have identified the presence of a glucocorticoid-responsive element in the CYP3A1/IGC2 gene that mediates the induction with physiological doses of glucocorticoids. A 219-bp dexamethasone responsive fragment of the CYP3A1/IGC2 gene localized at -2100/-1882 bp upstream of the transcription initiation site was identified in transfection experiments with HepG2 cells. Maximum induction was achieved with 50-100 nM dexamethasone. DNase I footprinting analysis revealed two glucocorticoid receptor-protected sequences in the 5' flank of the CYP3A1/IGC2 gene. Point mutations in footprint I (-1982/-1960-bp) completely abolished binding and transcription activation whereas a mutation in footprint II (-2001/-1986-bp) only decreased the binding and had no effect on transcription activation. These results led to the conclusion that the glucocorticoid response element present in footprint I mediated the dexamethasone response in transfection experiments with HepG2 cells. Pregnenolone 16alpha-carbonitrile failed to induce any transcriptional effect mediated by this response element in the HepG2 cells.

Differential stimulation by CCAAT/enhancer-binding protein alpha isoforms of the estrogen-activated promoter of the very-low-density apolipoprotein II gene

The transcription factors CCAAT/enhancer-binding proteins alpha and beta (C/EBP alpha and C/EBP beta) are highly expressed in liver and are believed to function in maintaining the differentiated state of the hepatocytes. C/EBP alpha appears to be a critical regulator of genes involved in metabolic processes. We are interested in the roles of C/EBP in the expression of the very-low-density apolipoprotein II (apoVLDL II) gene. This gene encodes an avian yolk protein, is induced by estrogens and is only expressed in liver. To examine the role of C/EBP in apoVLDL II expression, footprinting and electromobility-shift analysis were performed. For three of the protein-binding sites in the apoVLDL II promoter region, C/EBP alpha and C/EBP beta were identified as the major DNA-binding activities. For one of the C/EBP genes, C/EBP alpha, the effect of the gene products on apoVLDL II transcription was examined. From transfection experiments we conclude that maximal estrogen-dependent activity of the apoVLDL II promoter requires the dual action of the estrogen receptor and C/EBP. The level of activity is different depending on the nature of the C/EBP alpha translational isoform transfected, the full-length C/EBP alpha polypeptide being the most active isoform and the N-terminally truncated isoform being moderately active. The present results suggest a role of C/EBP alpha translational isoform ratio in the modulation of expression of C/EBP target genes, such as those involved in metabolic processes.

Organ-specific activity of the 5' regulatory region of the glutamine synthetase gene in developing mice

Glutamine synthetase (GS) converts ammonia and glutamate into glutamine. We assessed the activity of the 5' regulatory region of the GS gene in developing transgenic mice carrying the chloramphenicol acetyltransferase (CAT) gene under the control of 3150 bp of the upstream sequence of the rat GS gene to obtain insight into the spatiotemporal regulation of its pattern of expression. To determine the organ-specific activity of the 5' regulatory region CAT and GS mRNA expression were compared by ribonuclease-protection and semi-quantitative in situ hybridization analyses. Three patterns were observed: the 5' region is active and involved in the regulation of GS expression throughout development (pericentral hepatocytes, intestines and epididymis); the 5' region shows no activity at any of the ages investigated (periportal hepatocytes and white adipose tissue); and the activity of the 5' region becomes repressed during development (stomach, muscle, brown adipose tissue, kidney, lung and testis). In the second group, an additional element must be responsible for the activation of GS expression. The last group included organs in which the 5' regulatory region is active, but not in the cells that express GS. In these organs, the activity of the 5' regulatory region must be repressed by other regulatory regions of the GS gene that are missing from the transgenic construct. These findings indicate that in addition to the 5' regulatory region, at least two unidentified elements are involved in the spatiotemporal pattern of expression of GS.

Glucocorticoid receptor lacking the tau 1 transactivation domain is a gene-specific regulator of the wild-type glucocorticoid-receptor activity

The glucocorticoid receptor (GR) contains a major transactivation function (tau 1), located in the N-terminal domain. tau 1 contributes to about 80% of the ligand-inducible transcriptional activity of GR. In this study, we show that GR devoid of tau 1 (symbol: see text] GR) can inhibit activation of gene expression by wild-type GR but this does not occur for all target genes. Activation of the mouse mammary tumor virus promoter by wild-type GR in transiently transfected chinese hamster ovary (CHO) cells lacking endogenous GR was repressed by cotransfecting [symbol: see text] GR. This effect was proportional to the amount of transfected [symbol: see text] GR and was not due to squelching. A moderate expression level of stably transfected [symbol: see text] GR mutant was also shown to repress the transcriptional activity of endogenous GR present in rat skeletal myoblast L8 cells. Glucocorticoid mediated down regulation of endogenous GR gene expression can be blocked by the [symbol: see text] GR mutant in stably transfected L8 cells. In contrast, no inhibition was observed on glucocorticoid induction of the endogenous glutamine synthetase gene in L8 cells. However, glucocorticoid induction of a reporter gene driven by the chicken glutamine synthetase promoter was inhibited by [symbol: see text] GR in L8 cells. Stable expression of wild-type GR in CHO cells rendered the cells glucocorticoid responsive with regard to glutamine synthetase induction but coexpression of [symbol: see text] GR did not repress induction of the endogenous glutamine synthetase gene expression by wild-type GR. Expression of [symbol: see text] GR alone in CHO cells did not render the glutamine synthetase gene glucocorticoid responsive, indicating that [symbol: see text] GR has no transcriptional activity on the glutamine synthetase gene. We conclude from these results that the structure of glucocorticoid-response elements within target genes may be very critical for the ability of the mutant receptor to exhibit a dominant negative effect.

Characterization of an NF-1/CTF family member as a functional activator of the mouse mammary tumor virus long terminal repeat 5' enhancer

The long terminal repeat of the mouse mammary tumor virus restricts virus expression primarily to the mammary epithelium. The extreme 5' end of the long terminal repeat contains an enhancer that has been associated with tissue-specific expression of the virus. A total of six functional cis-acting elements have been identified in the enhancer. Although proteins binding to these elements have been reported, only one has been identified; this factor, mp5, is identical or closely related to the transcription factor AP-2 (Mellentin-Michelotti, J., John, S., Pennie, W. D., Williams, T., and Hager, G. L. (1994) J. Biol. Chem. 269, 31983-31990). The other factors are hitherto unidentified and poorly described. We report here the characterization of another of the six elements, previously referred to as the F3 site (Mink, S., Hartig, E., Jennewein, P., Doppler, W., and Cato, A. C. (1992) Mol. Cell Biol. 12, 4906-4918). We show that the F3 binding activity and AP-2 act synergistically to enhance mouse mammary tumor virus-directed transcription, but only in the presence of glucocorticoid hormone. The F3 element has an NF-1-like half-site, but the activity recognizing this element has binding characteristics distinct from the NF-1/CTF family as well as the rest of the CCAAT-binding proteins. We conclude that the F3 activity represents a new member of the NF-1/CTF family.

Transcriptional regulation by steroid hormones

Steroid hormones influence the transcription of a large number of genes by virtue of their interaction with intracellular receptors, which are modular proteins composed of a ligand binding domain, a DNA binding domain, and several transactivation functions distributed along the molecule. The DNA binding domain is organized around two zinc ions and allows the receptors to bind as homodimers to palindromic DNA sequences, the hormones responsive elements (HRE), is such a way that each homodimer contacts one half of the palindrome. Since the two halves are separated by three base pairs, the two homodimers contact the same face of the double helix. Before hormone binding, the receptors are part of a complex with multiple chaperones which maintain the receptor in its steroid binding conformation. Following hormone binding, the complex dissociates and the receptors bind to HREs in chromatin. Regulation of gene expression by hormones involves an interaction of the DNA-bound receptors with other sequence-specific transcription factors and with the general transcription factors, which is partly mediated by co-activators and co-repressors. The specific array of cis regulatory elements in a particular promoter/enhancer region, as well as the organization of the DNA sequences in nucleosomes, specifies the network of receptor interactions. Depending on the nature of these interactions, the final outcome can be induction or repression of transcription. The various levels at which these interactions are modulated are discussed using as an example the promoter of the Mouse Mammary Tumor Virus and its organization in chromatin.

Interaction of steroid hormone receptors with transcription factors involves chromatin remodelling

The mechanism by which steroid hormones modulate promoter utilization is not clear. Evidence from transfection studies and cell-free assays points to an interaction of the hormone receptors with general transcription factors, as well as with sequence-specific transcription factors. Moreover co-activators or transcription intermediary factors, have been identified which could mediate some of the transcriptional effects of the hormone-receptor complex. However, in addition to this interaction of receptors with proteins directly involved in transcription, a participation of chromatin structure in gene regulation by steroid hormones is becoming increasingly evident. In the case of the MMTV promoter, the nucleosomal organization seems to be responsible for transcriptional repression prior to hormonal stimulation. This effect is due to occlusion by a nucleosome positioned on the MMTV promoter sequences in such a way that essential transcription factors cannot access their recognition sites. Following hormone induction, a remodelling of the nucleosome structure takes place which enables a whole complement of sequence specific transcription factors to assemble on the promoter. Since a complete occupancy of binding sites does not take place when the promoter is present as naked DNA, the nucleosomal organization appears to be required for the proper synergism between transcription factors following hormonal induction. According to this model, the positioning of a nucleosome sets the stage for constitutive repression and hormone induction of the MMTV promoter.

Characterization of an EcR/USP heterodimer target site that mediates ecdysone responsiveness of the Drosophila Lsp-2 gene

The Larval serum protein-2 gene (Lsp-2) of Drosophila melanogaster is uniquely expressed in the fat body tissue from the beginning of the third instar to the end of adult life. Accumulation of the larval Lsp-2 transcript is enhanced by 20-hydroxyecdysone. To study the molecular basis for ecdysone regulated Lsp-2 activity, deletion mutants of the Lsp-2 5'-flanking region were constructed by fusion to either the Escherichia coli chloramphenicol acetyltransferase (CAT) gene or to an hsp70-lacZ hybrid gene encoding beta-galactosidase. Constructs transfected into Drosophila S2/M3 cells were shown to confer transient ecdysone inducibility on the reporter genes. A single functional ecdysone response element (EcRE) was localized at position -75 relative to the Lsp-2 transcription initiation site. In gel mobility shift assays using fat body nuclear extracts or nuclear receptors synthesized in vitro, a 27-bp sequence harboring the EcRE bound both the Drosophila ecdysone receptor and the Drosophila retinoid-X homologue, Ultraspiracle, in a cooperative manner. Competition experiments indicate that the affinity of the Lsp-2 EcRE for the ecdysone receptor complex is comparable to that of the canonical EcRE of the hsp27 gene and is at least 4-fold greater than that of Fbp1, another fat body-specific Drosophila gene. Our results suggest that structural features of this EcRE determine its ability to induce ecdysone responsiveness at a lower ligand concentration and may form the basis for differential hormone responsiveness within the fat body.

Analysis of a vitellogenin gene of the mosquito, Aedes aegypti and comparisons to vitellogenins from other organisms

A genomic clone of the Aedes aegypti vitellogenin A1 gene was sequenced including 2015 bp of 5' untranscribed sequence, 6369 bp of open reading frame interrupted by two introns, and a short 3' untranslated region. Primer extension was used to identify the transcription initiation site. The amino termini of the large and small subunits were located by N-terminal sequencing of vitellin purified from eggs. The length of the signal sequence and the position of the cleavage site between the two subunits were also determined. Three sequential imperfect repeats were found near the beginning of the small subunit. The sequence of the coding region appears to be polymorphic. Comparison of the signal sequences of seven insect vitellogenin genes revealed several conserved leucines, and a conserved position of an intron. However, the signal sequences are not conserved between these genes and the yolk protein genes of Cyclorraphid Dipteran insects. The cleavage sites between the small and large subunits in the vitellogenins of the mosquito, A. aegypti, sawfly, Athalia rosae, boll weevil, Anthonomus grandis, and silkworm, Bombyx mori are flanked by sequences rich in serine. Pairwise dot matrix analysis at the protein level showed that the mosquito, boll weevil and silkworm vitellogenins are significantly related with approx. 50% similarity. One region of the three insect vitellogenin genes, near the N-terminal of the large subunit, showed the highest levels of similarity, from 57.5 to 64.4%. The position of cysteines in insect vitellogenins is conserved, particularly in the C-terminus of the large subunit. Dot matrix comparison of the mosquito vitellogenin with that of Xenopus laevis and Caenorhabditis elegans showed much lower, but still significant degrees of relationship. Pairwise comparisons of the mosquito vitellogenin and the Drosophila melanogaster yolk proteins did not show significant similarities. Potential regulatory regions in the mosquito VgA1 gene were identified by comparison to regulatory elements known from other organisms, especially D. melanogaster, which could provide useful information for further functional analysis.

Selectivity of cell cycle regulation of glucocorticoid receptor function

The restricted expression of some genes to distinct stages of the cell cycle is often brought about through alterations in the activity and/or abundance of specific transcription factors. Many cells have been shown to be unresponsive to glucocorticoid hormone action during the G2 phase of the mammalian cell cycle, suggesting that some activities of the glucocorticoid receptor (GR), a ligand-activated transcription factor, are subjected to cell cycle control. We show here that GR insensitivity in G2 is selective, affecting receptor-mediated transactivation from a simple glucocorticoid response element, but not repression from a composite glucocorticoid response element. Since glucocorticoid-dependent down-regulation of GR protein levels is also unaffected in G2, distinct activities of the receptor that participate in this homologous down-regulation must be operating as effectively in G2-synchronized cells as in asynchronous cells. Finally, the phosphorylation state of the GR is altered in G2-synchronized cells reflecting, in part, both site-specific phosphorylation and dephosphorylation events. These results suggest that, while GR may be a target for cell cycle regulated kinases and phosphatases, the resulting changes in receptor phosphorylation have an impact only on selected GR functions.

Gene regulation and differentiation in vertebrate ocular tissues

Molecular biological techniques have contributed greatly to the study of vertebrate ocular tissues. The specification of ocular tissues has been shown to be closely related to the expression of transcription factors encoded by genes such as Pax6 and microphthalmia. Lens-specific expression of the delta 1-crystallin gene is controlled by factors, such as delta EF1, binding to its enhancer sequences. Retinal activity of the glucocorticoid hormone receptor is regulated by its binding with another transcription factor. Degeneration of photoreceptors in a retinal disease, retinitis pigmentosa, can be caused by the introduction of a mutated opsin gene into mice. In addition, the process of transdifferentiation in ocular tissues has been described at the level of gene expression.

Glucocorticoids regulate the transcription of Na(+)-K(+)-ATPase genes in the infant rat kidney

Glucocorticoids modulate the maturation of Na(+)-K(+)-ATPase mRNA in a tissue- and age-dependent manner. In this study, we report the effect of glucocorticoids on Na(+)-K(+)-ATPase gene transcription in the infant rat kidney. Ten-day-old rats were treated with one intraperitoneal injection of betamethasone. In glucocorticoid-treated rats, there was a significant increase in renal cortical alpha 1- and beta 1-mRNAs (3.08 +/- 0.34- and 4.06 +/- 0.10-fold). Pretreatment with cycloheximide, an inhibitor of protein synthesis, did not abolish the increase in alpha 1- and beta 1-mRNA after glucocorticoids. The alpha 1- and beta 1-gene transcription rates were significantly increased in nuclei isolated from kidneys of glucocorticoid-treated rat (2.16 +/- 0.05- and 3.12 +/- 0.50-fold). Interaction between nuclear proteins and Na(+)-K(+)-ATPase alpha 1-promoter was studied by gel retardation assay. Nuclear protein from glucocorticoid-treated rats retarded a fragment of alpha 1-promoter that includes a half-consensus glucocorticoid response element (GRE) at position -750 bp but did not retard a fragment including a half-consensus GRE at position -481. Retardation of alpha 1-promoter was inhibited by incubation with molar excess of GRE or with a monoclonal antibody against glucocorticoid receptor. We conclude that in the infant kidney, glucocorticoids directly stimulate the transcription of alpha 1- and beta 1-Na(+)-K(+)-ATPase subunits. It is likely that the binding of glucocorticoid receptor to alpha 1-Na(+)-K(+)-ATPase promoter requires the presence of an auxiliary factor.

Expression of the glucocorticoid receptor gene is regulated during early embryogenesis of Xenopus laevis

To study the possible role of the glucocorticoid receptor (GR) in early embryogenesis, we isolated a Xenopus glucocorticoid receptor cDNA from an embryonic stage 17 cDNA library. Overexpression of this Xenopus GR in COS cells confers the ability to transactivate a GRE-tk CAT promoter construct in a ligand dependent manner. Expression of the Xenopus GR gene at the RNA level was analyzed by Northern blot hybridization. Transcripts of 4 and 6 kb are present in oocytes. The 4 kb mRNA is abundant and is degraded together with the 6 kb mRNA during cleavage stages of early development. Between stages 17 and 24, GR messengers are extremely rare. From stage 32 onwards, both GR transcripts start to be expressed again at intermediate levels. These results provide the first evidence that expression of the GR gene is regulated during early embryonic development.

Role of glucocorticoids in activation of hepatic PEPCK gene transcription during exercise

The objective of these studies was to determine the molecular basis for the activation of phosphoenolpyruvate carboxykinase (PEPCK) gene transcription during prolonged submaximal exercise. Mice were fed a high-carbohydrate diet for 1 wk and exercised continuously by swimming for up to 120 min. The level of hepatic PEPCK mRNA increased progressively during exercise, reaching 510% above control, whereas transcription of the PEPCK gene increased 1,000%, before decreasing to control levels within 60 min of recovery. In transgenic mice carrying a chimeric gene consisting of the PEPCK promoter linked to a reporter gene for bovine growth hormone (bGH), PEPCK(-460)-bGH, the level of hepatic bGH mRNA increased by 490% in response to exercise, similar to the increase in the expression of the native PEPCK gene. However, in transgenic mice with a deletion of the glucocorticoid regulatory unit, PEPCK(-355)-bGH, bGH mRNA did not increase above control values. In transgenic mice with a block mutation in adenosine 3',5'-cyclic monophosphate (cAMP) regulatory regions -90/-82 and -250/-234, PEPCK cAMP response element 1 (CRE-1)/P3(1)-bGH, exercise increased bGH mRNA 260% above controls. Adrenalectomy (Adx) had no effect on PEPCK mRNA levels in nonexercised mice, whereas in adrenalectomized (Adx)-exercised mice, PEPCK mRNA increased only 80% above basal, and, in Adx mice injected with dexamethasone, PEPCK mRNA increased with exercise 570% above controls. Exercise was also associated with a large increase in transcription of the gene for the transcription factor CCAAT/enhancer-binding protein beta (C/EBP-beta) and a smaller rise in transcription of c-jun gene, both of which returned to control levels during recovery.(ABSTRACT TRUNCATED AT 250 WORDS)