Steroid-selective initiation of chromatin remodeling and transcriptional activation of the mouse mammary tumor virus promoter is controlled by the site of promoter integration

Endocrine-Disrupting Activities of Flavones on Steroid Receptors: Structural Requirements and Synthesis of Novel Flavone with Improved Estrogenic Activity

Background/Objectives: Flavonoids are common ubiquitous components of plants and are consumed by humans and livestock in their diets. Many different activities have been proposed for a variety of flavonoids that play a role in the benefits of a plant-rich diet. On the downside, excessive exposure to some flavonoids comes with a risk of endocrine disruption. Our objective was to define the structural elements of flavones and selected other flavonoids required for endocrine-disrupting activities on each of four steroid receptors, estrogen, androgen, progesterone, and glucocorticoid receptors. Methods: This work presents a systematic screen for the hormone agonist or antagonist activity of a selected panel of flavonoids on estrogen, androgen, progesterone, and glucocorticoid receptors. The screen is focused on the positional requirements of hydroxyl substituents on the flavone backbone. Results: Each receptor exhibited a distinct pattern for structural requirements of the flavones to impact receptor signaling. The most active flavones exhibited antagonist activity on androgen and progesterone receptors with an IC50 of 0.5 and 2 µM, respectively. Flavones only exhibited weak antagonism on glucocorticoid receptors. When active, flavones acted as estrogen receptor agonists. The findings were utilized to design and synthesize a novel flavone, 3-fluoro, 6,4'-dihydroxyflavone 14, that displays increased potency as an estrogen agonist (EC50~30 nM). Modeling of the binding of this novel flavone predicts increased preference for ERα versus ERβ relative to the estrogenic phytoestrogen, genistein. Conclusions: The structural requirements for flavones to act as estrogen agonists and antagonists of other steroid receptors are defined. The synthesis of a novel flavone offers potential for topical applications where systemic estrogen activity is undesired. However, the results highlight the potential for endocrine disruption when certain flavones are consumed in quantity as supplements.

Minireview: The versatile roles of lysine deacetylases in steroid receptor signaling

Lysine deacetylases have been known to regulate nuclear receptor function for many years. In the unliganded state, nuclear receptors that form heterodimers with retinoid X receptors, such as the retinoic acid and thyroid hormone receptors, associate with deacetylases to repress target genes. In the case of steroid receptors, binding of an antagonist ligand was initially reported to induce association of deacetylases to prevent activation of target genes. Since then, deacetylases have been shown to have diverse functions in steroid receptor signaling, from regulating interactions with molecular chaperones to facilitating their ability to activate transcription. The purpose of this review is to summarize recent studies on the role of deacetylases in steroid receptor signaling, which show deacetylases to be highly versatile regulators of steroid receptor function.

The WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

HDAC activity is required for efficient core promoter function at the mouse mammary tumor virus promoter

Histone deacetylases (HDACs) have been shown to be required for basal or inducible transcription at a variety of genes by poorly understood mechanisms. We demonstrated previously that HDAC inhibition rapidly repressed transcription from the mouse mammary tumor virus (MMTV) promoter by a mechanism that does not require the binding of upstream transcription factors. In the current study, we find that HDACs work through the core promoter sequences of MMTV as well as those of several cellular genes to facilitate transcriptional initiation through deacetylation of nonhistone proteins.

Upregulation of human with-no-lysine kinase-4 gene expression by GATA-1 acetylation

With-no-lysine kinase-4 (WNK4), a member of the serine-threonine protein kinase family, acts as a multifunctional regulator of diverse ion transporters. Therefore, it is interesting to investigate the mechanisms that control its expression. We have previously demonstrated that glucocorticoid downregulates human WNK4 (hWNK4) expression through the negative glucocorticoid responsive element. Here, using real-time PCR and Western blot assays, we show that trichostatin A (TSA), a histone deacetylase inhibitor, upregulated hWNK4 mRNA and protein expression in human embryo kidney 293 cells. Analysis of the transcriptional activity of a series of the truncated hWNK4 promoters by luciferase assay indicated that the region -484 to -337 of the hWNK4 promoter was sensitive to TSA, and a GATA-1 binding motif was identified at position -426 using TRANSFAC-TESS program. Moreover, using electrophoresis mobility shift assay and chromatin immunoprecipitation assay, the GATA-1 binding affinity to the hWNK4 promoter was shown to increase with TSA under in vitro and in vivo conditions. Immunoprecipitation and Western blot analyses showed that the levels of acetylated GATA-1 were increased with TSA, in agreement with changes in its DNA-binding affinity. These findings indicate that TSA induces hWNK4 expression, at least in part, by increasing GATA-1 acetylation, and thereby its binding to the GATA-1 responsive element, within the hWNK4 promoter.

A miniaturized glucocorticoid receptor translocation assay using enzymatic fragment complementation evaluated with qHTS

Nuclear translocation is an important step in glucocorticoid receptor (GR) signaling and assays that measure this process allow the identification of nuclear receptor ligands independent of subsequent functional effects. To facilitate the identification of GR-translocation agonists, an enzyme fragment complementation (EFC) cell-based assay was scaled to a 1536-well plate format to evaluate 9,920 compounds using a quantitative high throughput screening (qHTS) strategy where compounds are assayed at multiple concentrations. In contrast to conventional assays of nuclear translocation the qHTS assay described here was enabled on a standard luminescence microplate reader precluding the requirement for imaging methods. The assay uses beta-galactosidase alpha complementation to indirectly detect GR-translocation in CHO-K1 cells. 1536-well assay miniaturization included the elimination of a media aspiration step, and the optimized assay displayed a Z' of 0.55. qHTS yielded EC(50) values for all 9,920 compounds and allowed us to retrospectively examine the dataset as a single concentration-based screen to estimate the number of false positives and negatives at typical activity thresholds. For example, at a 9 microM screening concentration, the assay showed an accuracy that is comparable to typical cell-based assays as judged by the occurrence of false positives that we determined to be 1.3% or 0.3%, for a 3sigma or 6sigma threshold, respectively. This corresponds to a confirmation rate of approximately 30% or approximately 50%, respectively. The assay was consistent with glucocorticoid pharmacology as scaffolds with close similarity to dexamethasone were identified as active, while, for example, steroids that act as ligands to other nuclear receptors such as the estrogen receptor were found to be inactive.

The contribution of different androgen receptor domains to receptor dimerization and signaling

The androgen receptor (AR) is a ligand-activated transcription factor of the nuclear receptor superfamily that plays a critical role in male physiology and pathology. Activated by binding of the native androgens testosterone and 5alpha-dihydrotestosterone, the AR regulates transcription of genes involved in the development and maintenance of male phenotype and male reproductive function as well as other tissues such as bone and muscle. Deregulation of AR signaling can cause a diverse range of clinical conditions, including the X-linked androgen insensitivity syndrome, a form of motor neuron disease known as Kennedy's disease, and male infertility. In addition, there is now compelling evidence that the AR is involved in all stages of prostate tumorigenesis including initiation, progression, and treatment resistance. To better understand the role of AR signaling in the pathogenesis of these conditions, it is important to have a comprehensive understanding of the key determinants of AR structure and function. Binding of androgens to the AR induces receptor dimerization, facilitating DNA binding and the recruitment of cofactors and transcriptional machinery to regulate expression of target genes. Various models of dimerization have been described for the AR, the most well characterized interaction being DNA-binding domain- mediated dimerization, which is essential for the AR to bind DNA and regulate transcription. Additional AR interactions with potential to contribute to receptor dimerization include the intermolecular interaction between the AR amino terminal domain and ligand-binding domain known as the N-terminal/C-terminal interaction, and ligand-binding domain dimerization. In this review, we discuss each form of dimerization utilized by the AR to achieve transcriptional competence and highlight that dimerization through multiple domains is necessary for optimal AR signaling.

A shifting paradigm: histone deacetylases and transcriptional activation

Transcriptional repression and silencing have been strongly associated with hypoacetylation of histones. Accordingly, histone deacetylases, which remove acetyl groups from histones, have been shown to participate in mechanisms of transcriptional repression. Therefore, current models of the role of acetylation in transcriptional regulation focus on the acetylation status of histones and designate histone acetyltransferases, which add acetyl groups to histones, as transcriptional coactivators and histone deacetylases as corepressors. In recent years, an accumulation of studies have shown that these enzymes also target non-histone proteins and that histone deacetylases have clear roles as coactivators at a variety of genes, some of which are key regulators of cell growth and survival. This review summarizes the evidence for histone deacetylases as coactivators and provides models of coactivation mechanisms, some of which integrate roles of acetylated histones and non-histone proteins in transcription.

Probasin promoter assembles into a strongly positioned nucleosome that permits androgen receptor binding

The promoter of the murine probasin (PB) gene exhibits strong androgen receptor (AR)-specific and tissue-specific regulation and is considered a promising candidate for gene therapy treatment of advanced prostate cancer. To characterize the determinants of chromatin specificity of the PB promoter with the AR we initially investigated the in vitro interactions of recombinant AR DNA binding domain (AR-DBD) with reconstituted nucleosomes incorporating the proximal PB promoter (nucleotides -268 to -76). We demonstrate that a DNA fragment of this promoter region exhibits strong nucleosome positioning. The phased DNA sequence protected by the histone octamer includes four androgen receptor response elements (AREs) which are arranged as two sets of class I and class II sites spaced approximately 90bp apart. Class I AREs form classical contacts with the AR, whereas class II AREs contain atypical binding sequences and have been shown to stabilize AR binding to adjacent class I sites, resulting in synergistic transcriptional activation and increased hormone sensitivity. We used DNase 1 footprinting and electrophoretic mobility shift assays (EMSA) to show that the AR-DBD binds to its cognate sequences independently of their nucleosomal organization. In addition, we show that the ability of the AR-DBD to interact with the nucleosomal PB promoter is not affected by histone acetylation. Thus the AR-DBD is able to bind to its cognate sequences within the PB promoter in a way that is indifferent to the presence or absence of histones and nucleosomal structure.

Effects of acetylation, polymerase phosphorylation, and DNA unwinding in glucocorticoid receptor transactivation

Varying the concentration of selected factors alters the induction properties of steroid receptors by changing the position of the dose-response curve (or the value for half-maximal induction=EC(50)) and the amount of partial agonist activity of antisteroids. We now describe a rudimentary mathematical model that predicts a simple Michaelis-Menten curve for the multi-step process of steroid-regulated gene induction. This model suggests that steps far downstream from receptor binding to steroid can influence the EC(50) of agonist-complexes and partial agonist activity of antagonist-complexes. We therefore asked whether inhibitors of three possible downstream steps can reverse the effects of increased concentrations of two factors: glucocorticoid receptors (GRs) and Ubc9. The downstream steps (with inhibitors in parentheses) are protein deacetylation (TSA and VPA), DNA unwinding (CPT), and CTD phosphorylation of RNA polymerase II (DRB and H8). None of the inhibitors mimic or prevent the effects of added GRs. However, inhibitors of DNA unwinding and CTD phosphorylation do reverse the effects of Ubc9 with high GR concentrations. These results support our earlier conclusion that different rate-limiting steps operate at low and high GR concentrations versus high GR with Ubc9. The present data also suggest that downstream steps can modulate the EC(50) of GR-mediated induction, thus both supporting the utility of our mathematical model and widening the field of biochemical processes that can modify the EC(50).

Trichostatin A reduces hormone-induced transcription of theMMTVpromoter and has pleiotropic effects on its chromatin structure

The deacetylase inhibitor trichostatin A (TSA) has long been used to study the relationship between gene transcription and the acetylation status of chromatin. We have used Xenopus laevis oocytes to study the effects of TSA on glucocorticoid receptor (GR)-dependent transcription and we have related these effects to changes in the chromatin structure of a reporter mouse mammary tumor virus (MMTV) promoter. We show that TSA induces a low level of constitutive transcription. This correlates with a change of acetylation pattern and a more open chromatin structure over the MMTV chromatin, and with specific acetylation and remodeling events in the promoter region. Specifically, a repositioning of initially randomly positioned nucleosomes along the distal MMTV long terminal repeat is seen. This nucleosome rearrangement is similar to the translational nucleosome positioning that occurs upon hormone activation. We also note a reduced hormone response in the presence of TSA. TSA effects have for a long time been associated with transcriptional activation and chromatin opening through inhibition of the deacetylation of histones. However, our results and those of others show that TSA-induced changes in expression and chromatin structure can be quite different in different promoter contexts and, thus, the effects of TSA are more complex than previously believed.

The interplay between the glucocorticoid receptor and nuclear factor-kappaB or activator protein-1: molecular mechanisms for gene repression

The inflammatory response is a highly regulated physiological process that is critically important for homeostasis. A precise physiological control of inflammation allows a timely reaction to invading pathogens or to other insults without causing overreaction liable to damage the host. The cellular signaling pathways identified as important regulators of inflammation are the signal transduction cascades mediated by the nuclear factor-kappaB and the activator protein-1, which can both be modulated by glucocorticoids. Their use in the clinic includes treatment of rheumatoid arthritis, asthma, allograft rejection, and allergic skin diseases. Although glucocorticoids have been widely used since the late 1940s, the molecular mechanisms responsible for their antiinflammatory activity are still under investigation. The various molecular pathways proposed so far are discussed in more detail.

Inhibition of MMTV transcription by HDAC inhibitors occurs independent of changes in chromatin remodeling and increased histone acetylation

Increased histone acetylation has been associated with activated gene transcription and decreased acetylation with repression. However, there is a growing number of genes known, which are downregulated by histone deacetylase (HDAC) inhibitors through unknown mechanisms. This study examines the mechanism by which the mouse mammary tumor virus (MMTV) promoter is repressed by the HDAC inhibitor, trichostatin A (TSA). We find that this repression is transcriptional in nature and that it occurs in the presence and absence of glucocorticoids. TSA decreases MMTV transcription at a rapid rate, reaching maximum in 30-60 min. In contrast with previous reports, the repression does not correlate with an inhibition of glucocorticoid-induced nuclease hypersensitivity or NF1-binding at the MMTV promoter. Surprisingly, TSA does not induce sizable increases in histone acetylation at the MMTV promoter nor does it inhibit histone deacetylation, which accompanies deactivation of the glucocorticoid-activated MMTV promoter. Repression of MMTV transcription by TSA does not depend on the chromatin organization of the promoter because a transiently transfected MMTV promoter construct with a disorganized nucleoprotein structure was also repressed by TSA treatment. Mutational analysis of the MMTV promoter indicates that repression by TSA is mediated through the TATA box region. These results suggest a novel mechanism that involves acetylation of nonhistone proteins necessary for basal transcription.

CBP recruitment and histone acetylation in differential gene induction by glucocorticoids and progestins

We have analyzed histone acetylation at the steroid-responsive mouse mammary tumor virus (MMTV) promoter in five separate cell lines that express functional glucocorticoid and/or progesterone receptors. Chromatin immunoprecipitation assays reveal that glucocorticoid and progesterone receptors bind the MMTV promoter after hormone addition but that receptor binding is not associated with an increase in acetylation of histone H3 or H4. We have, however, found one exception to this rule. Previously we described a cell line [T47D(C&L)] that displayed a remarkable differential induction of MMTV by glucocorticoids and progestins. At one chromosomal locus (MMTV-luciferase), MMTV is preferentially induced by glucocorticoids, whereas at another locus within the same cell (MMTV-CAT), MMTV is activated by both glucocorticoids and progestins. Here we show that the glucocorticoid-mediated induction of MMTV-luciferase is accompanied by increased recruitment of CBP to the promoter and increased histone H3 and H4 acetylation, whereas the hormonal induction of MMTV-CAT in the same cell exhibits a more modest CBP recruitment without any increase in histone acetylation. These studies suggest that increased histone acetylation may serve a potentiating function for MMTV promoter activation at certain loci. However, increased histone acetylation is not requisite for steroid-mediated induction of transcription at all genes.

Chromatin exposes intrinsic differences in the transcriptional activities of estrogen receptors alpha and beta

The biological actions of estrogens are mediated via two distinct intranuclear estrogen receptor (ER) proteins, ERalpha and ERbeta. We have used an in vitro chromatin assembly and transcription system to compare the transcriptional activities of the two ERs in the context of chromatin, the physiological template for transcription by RNA polymerase II. We find that under conditions where many biochemical activities of the receptors are similar (e.g. ligand binding, chromatin binding, chromatin remodeling and co-activator recruitment), liganded ERalpha is a much more potent transcriptional activator than ERbeta with chromatin templates, but not with naked DNA. This difference is attributable to the N-terminal A/B region of ERalpha, which contains a transferable activation function that facilitates transcription specifically with chromatin templates. Interestingly, chromatin selectively restricts ligand-dependent transcriptional activation by ERbeta under some conditions (e.g. with a closed chromatin architecture), while allowing it under other conditions (e.g. with an open chromatin architecture). Collectively, our results define an important role for chromatin in determining signaling outcomes mediated by distinct subtypes of signal-transducing transcriptional activator proteins.

Attenuation of glucocorticoid signaling through targeted degradation of p300 via the 26S proteasome pathway

The effects of acetylation on gene expression are complex, with changes in chromatin accessibility intermingled with direct effects on transcriptional regulators. For the nuclear receptors, both positive and negative effects of acetylation on specific gene transcription have been observed. We report that p300 and steroid receptor coactivator 1 interact transiently with the glucocorticoid receptor and that the acetyltransferase activity of p300 makes an important contribution to glucocorticoid receptor-mediated transcription. Treatment of cells with the deacetylase inhibitor, sodium butyrate, inhibited steroid-induced transcription and altered the transient association of glucocorticoid receptor with p300 and steroid receptor coactivator 1. Additionally, sustained sodium butyrate treatment induced the degradation of p300 through the 26S proteasome pathway. Treatment with the proteasome inhibitor MG132 restored both the level of p300 protein and the transcriptional response to steroid over 20 h of treatment. These results reveal new levels for the regulatory control of gene expression by acetylation and suggest feedback control on p300 activity.

Transactivation specificity of glucocorticoid versus progesterone receptors. Role of functionally different interactions of transcription factors with amino- and carboxyl-terminal receptor domains

A major unanswered question of glucocorticoid and progesterone action is how different whole cell responses arise when both of the cognate receptors can bind to, and activate, the same hormone response elements. We have documented previously that the EC(50) of agonist complexes, and the partial agonist activity of antagonist complexes, of both glucocorticoid receptors (GRs) and progesterone receptors (PRs) are modulated by increased amounts of homologous receptor and of coregulators. We now ask whether these components can differentially alter GR and PR transcriptional properties. To remove possible cell-specific differences, we have examined both receptors in the common environment of a line of mouse mammary adenocarcinoma (1470.2) cells. In order to segregate the responses that might be due to unequal nucleosome reorganization by the two receptors from those reflecting interactions with other components, we chose a transiently transfected reporter containing a simple glucocorticoid response element (i.e. GREtkLUC). No significant differences are found with elevated levels of either receptor. However, major, qualitative differences are seen with the corepressors SMRT and NCoR, which afford opposite responses with GR and PR. Studies with chimeric GR/PR receptors indicate that no one segment of PR or GR is responsible for these properties and that the composite response likely involves interactions with both the amino and carboxyl termini of receptors. Collectively, the data suggest that GR and PR induction of responsive genes in a given cell can be differentially controlled, in part, by unequal interactions of multiple receptor domains with assorted nuclear cofactors.

Androgen-receptor-specific DNA binding to an element in the first exon of the human secretory component gene

Androgens and glucocorticoids are steroid hormones, which exert their effects in vivo by binding and activating their cognate receptors. These intracellular receptors are transcription factors that can bind specific DNA sequences, called hormone response elements, located near the target genes. Although the androgen receptor (AR) and the glucocorticoid receptor (GR) bind the same consensus DNA sequence, androgen-specific responses can be achieved by non-conventional androgen response elements (AREs). Here we determine the specificity mechanism of such a selective element recently identified in the first exon of the human gene for secretory component (sc ARE). This sc ARE consists of two receptor-binding hexamers separated by three nucleotides. The DNA-binding domains of the AR and GR both bind the sc ARE, but, although the AR fragment dimerizes on the element, the GR fragment does not. Comparing the affinities of the DNA-binding domains for mutant forms of the sc ARE revealed that dimeric GR binding is actively excluded by the left hexamer and more precisely by the presence of a G residue at position -3, relative to the central spacer nucleotide. Inserting a G at this position changed a non-selective element into an androgen-selective one. We postulate that the AR recognizes the sc ARE as a direct repeat of two 5'-TGTTCT-3'-like core sequences instead of the classical inverted repeat. Direct repeat binding is not possible for the GR, thus explaining the selectivity of the sc ARE. This alternative dimerization by the AR on the sc ARE is also indicated by the DNA-binding characteristics of receptor fragments in which the dimerization interfaces were swapped. In addition, the flanking and spacer sequences seem to affect the functionality of the sc ARE.

Separable features of the ligand-binding domain determine the differential subcellular localization and ligand-binding specificity of glucocorticoid receptor and progesterone receptor

Glucocorticoid receptor (GR) and progesterone receptor (PR) are closely related members of the steroid receptor family of transcription factors. The two receptors share a similar domain structure, substantial sequence identity, DNA binding specificity, and the ability to induce many of the same genes. Despite these similarities, the unliganded GR is localized predominantly in the cytoplasm, while unliganded PR is found predominantly in the nucleus. By expressing green fluorescent protein (GFP)-tagged receptors and assessing subcellular localization in living cells by confocal microscopy, we have investigated the structural basis for the differential localization of GR and PR. By constructing a series of GFP-tagged receptor chimeras between GR and PR, we have shown that multiple features in the N-terminal half of the ligand-binding domain (LBD) are the critical determinants that mandate the differential localization of GR and PR. Replacement of residues encompassing helices 1-5 of GR with those of PR yields a receptor that is nuclear. However, this domain is unable to mediate nuclear import by itself when removed from the context of the receptor. The chimeric receptors also indicate that regions encompassing helices 6 and 7 are key determinants of the ligand binding potential and the transactivation potential of receptors. Thus, the determinants specifying localization of hormone-free receptors are separable from those governing ligand binding character.

Trichostatin A inhibits beta-casein expression in mammary epithelial cells

Many aspects of cellular behavior are defined by the content of information provided by association of the extracellular matrix (ECM) and with cell membrane receptors. When cultured in the presence of laminin-containing ECM and prolactin (Prl), normal mammary epithelial cells express the milk protein beta-casein. We have previously found that the minimal ECM- and Prl-responsive enhancer element BCE-1 was only active when stably integrated into chromatin, and that trichostatin A (TSA), a reagent that leads to alterations in chromatin structure, was able to activate the integrated enhancer element. We now show that endogenous beta-casein gene, which is controlled by a genetic assembly that is highly similar to that of BCE-1 and which is also activated by incubation in ECM and Prl, is instead inhibited by TSA. We provide evidence that the differing response of beta-casein and BCE-1 to TSA is neither due to an unusual effect of TSA on mammary epithelial cells, nor to secondary consequences from the expression of a separate gene, nor to a particular property of the BCE-1 construct. As a component of this investigation, we also showed that ECM mediated rapid histone deacetylation in mammary epithelial cells. These results are discussed in combination with previous work showing that TSA mediates the differentiation of many types of cancer cells but inhibits differentiation of some nonmalignant cell types.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.

Opposing effects of corepressor and coactivators in determining the dose-response curve of agonists, and residual agonist activity of antagonists, for glucocorticoid receptor-regulated gene expression

A distinguishing, but unexplained, characteristic of steroid hormone action is the dose-response curve for the regulation of gene expression. We have previously reported that the dose-response curve for glucocorticoid induction of a transfected reporter gene in CV-1 and HeLa cells is repositioned in the presence of increasing concentrations of glucocorticoid receptors (GRs). This behavior is now shown to be independent of the reporter, promoter, or enhancer, consistent with our proposal that a transacting factor(s) was being titrated by added receptors. Candidate factors have been identified by the observation that changes in glucocorticoid induction parameters in CV-1 cells could be reproduced by varying the cellular levels of coactivators [transcriptional intermediary factor 2 (TIF2), steroid receptor coactivator 1 (SRC-1), and amplified in breast cancer 1 (AIB1)], comodulator [CREB-binding protein (CBP)], or corepressor [silencing mediator for retinoid and thyroid-hormone receptors (SMRT)] without concomitant increases in GR. Significantly, the effects of TIF2 and SMRT were mutually antagonistic. Similarly, additional SMRT could reverse the action of increased levels of GRs in HeLa cells, thus indicating that the effects of cofactors on transcription may be general for GR in a variety of cells. These data further indicate that GRs are yet an additional target of the corepressor SMRT. At the same time, these cofactors were found to be capable of regulating the level of residual agonist activity displayed by antiglucocorticoids. Finally, these observations suggest that a novel role for cofactors is to participate in processes that determine the dose-response curve, and partial agonist activity, of GR-steroid complexes. This new activity of cofactors is disconnected from their ability to increase or decrease GR transactivation. An equilibrium model is proposed in which the ratio of coactivator-corepressor bound to either receptor-agonist or -antagonist complexes regulates the final transcriptional properties.

Components of vectors for gene transfer and expression in mammalian cells

Progress in diverse scientific fields has been realized partly by the continued refinement of mammalian gene expression vectors. A growing understanding of biological processes now allows the design of vector components to meet specific objectives. Thus, gene expression in a tissue-selective or ubiquitous manner may be accomplished by selecting appropriate promoter/enhancer elements; stabilization of labile mRNAs may be effected through removal of 3' untranslated regions or fusion to heterologous stabilizing sequences; protein targeting to selected tissues or different organelles is carried out using specific signal sequences; fusion moieties effect the detection, enhanced yield, surface expression, prolongation of half-life, and facile purification of recombinant proteins; and careful tailoring of the codon content of heterologous genes enhances protein production from poorly translated transcripts. The use of viral as well as nonviral genetic elements in vectors allows the stable replication of episomal elements without the need for chromosomal integration. The development of baculovirus vectors for both transient and stable gene expression in mammalian cells has expanded the utility of such vectors for a broad range of cell types. Internal ribosome entry sites are now widely used in many applications that require coexpression of different genes. Progress in gene targeting techniques is likely to transform gene expression and amplification in mammalian cells into a considerably less labor-intensive operation. Future progress in the elucidation of eukaryotic protein degradation pathways holds promise for developing methods to minimize proteolysis of specific recombinant proteins in mammalian cells and tissues.

Long-distance chromatin mechanisms controlling tissue-specific gene locus activation

Several different types of regulatory mechanisms contribute to the tissue- and development-specific regulation of a gene. It is now well established that, in addition to promoters, upstream cis-regulatory elements, which bind a variety of trans-acting factors, are essential for correct gene activation. In the last few years, however, it has become evident that the chromatin structure of eukaryotic genes is an important additional regulatory layer that is essential for correct gene expression during development. Chromatin is essentially a repressive environment for transcription factors; hence, much effort in recent years has been devoted to the elucidation of how these repressive forces are overcome during the process of gene locus activation. A particular interesting question in this context is: what are the molecular mechanisms by which extensive regions of chromatin, in many cases far outside the coding region, are reorganized during development? In this review, I summarize data from recent investigations that have uncovered a surprising variety of factors involved in this process.

Comparison of chromatin remodeling and transcriptional activation of the mouse mammary tumor virus promoter by the androgen and glucocorticoid receptor

We examined the interaction between the androgen (AR) and glucocorticoid receptor (GR) at the transcriptional level using mouse fibroblast cell lines harboring an integrated mouse mammary tumor virus (MMTV) promoter. We found that the AR, after induction with dihydrotestosterone (DHT), caused a progressive increase in MMTV-CAT reporter activity over 72 h which was correlated to an increase in chromatin remodeling of the MMTV promoter in the vicinity of the hormone response element (HRE). In contrast, stimulation of the GR by the synthetic glucocorticoid dexamethasone (Dex) caused a transient increase in MMTV transcriptional activity which returned to basal levels after 72 h. These changes were correlated to a transient increase in chromatin remodeling in the region of the HRE. Neither cotreatment nor pretreatment with Dex affected the DHT response. In fact, there was a more than additive effect of the two hormones on transcription at early time points. This suggests that the inability of GR to remodel chromatin, after 24 h of hormone treatment, is most likely related to changes in the GR itself and not the chromatin remodeling process. Consistent with this, nuclear GR levels dropped by greater than 50% after Dex treatment whereas the AR was induced fourfold after 24 h of DHT treatment. We conclude that a promoter with an ordered chromatin structure can still respond to androgens even after its glucocorticoid responsiveness is lost. This may be one mechanism cells utilize to establish target gene specificity for nuclear receptors that recognize identical DNA sequences.