The glucocorticoid receptor hormone binding domain mediates transcriptional activation in vitro in the absence of ligand

Auxin co-receptor IAA17/AXR3 controls cell elongation in Arabidopsis thaliana root solely by modulation of nuclear auxin pathway

The nuclear TIR1/AFB-Aux/IAA auxin pathway plays a crucial role in regulating plant growth and development. Specifically, the IAA17/AXR3 protein participates in Arabidopsis thaliana root development, response to auxin and gravitropism. However, the mechanism by which AXR3 regulates cell elongation is not fully understood. We combined genetical and cell biological tools with transcriptomics and determination of auxin levels and employed live cell imaging and image analysis to address how the auxin response pathways influence the dynamics of root growth. We revealed that manipulations of the TIR1/AFB-Aux/IAA pathway rapidly modulate root cell elongation. While inducible overexpression of the AXR3-1 transcriptional inhibitor accelerated growth, overexpression of the dominant activator form of ARF5/MONOPTEROS inhibited growth. In parallel, AXR3-1 expression caused loss of auxin sensitivity, leading to transcriptional reprogramming, phytohormone signaling imbalance and increased levels of auxin. Furthermore, we demonstrated that AXR3-1 specifically perturbs nuclear auxin signaling, while the rapid auxin response remains functional. Our results shed light on the interplay between the nuclear and cytoplasmic auxin pathways in roots, revealing their partial independence but also the dominant role of the nuclear auxin pathway during the gravitropic response of Arabidopsis thaliana roots.

Anti-Inflammatory Effects of Synthetic Peptides Based on Glucocorticoid-Induced Leucine Zipper (GILZ) Protein for the Treatment of Inflammatory Bowel Diseases (IBDs)

Glucocorticoids (GCs) are commonly used to treat autoimmune and inflammatory diseases, but their clinical effects and long-term use can lead to serious side effects. New drugs that can replace GCs are needed. Glucocorticoid-induced leucine zipper (GILZ) is induced by GCs and mediates many of their anti-inflammatory effects, such as inhibiting the pro-inflammatory molecule NF-κB. The GILZ C-terminal domain (PER region) is responsible for GILZ/p65NF-κB interaction and consequent inhibition of its transcriptional activity. A set of five short peptides spanning different parts of the PER region of GILZ protein was designed, and their anti-inflammatory activity was tested, both in vitro and in vivo. We tested the biological activity of GILZ peptides in human lymphocytic and monocytic cell lines to evaluate their inhibitory effect on the NF-κB-dependent expression of pro-inflammatory cytokines. Among the tested peptides, the peptide named PEP-1 demonstrated the highest efficacy in inhibiting cell activation in vitro. Subsequently, PEP-1 was further evaluated in two in vivo experimental colitis models (chemically induced by DNBS administration and spontaneous colitis induced in IL-10 knock-out (KO) mice (to assess its effectiveness in counteracting inflammation. Results show that PEP-1 reduced disease severity in both colitis models associated with reduced NF-κB pro-inflammatory activity in colon lamina propria lymphocytes. This study explored GILZ-based 'small peptides' potential efficacy in decreasing lymphocyte activation and inflammation associated with experimental inflammatory bowel diseases (IBDs). Small peptides have several advantages over the entire protein, including higher selectivity, better stability, and bioavailability profile, and are easy to synthesize and cost-effective. Thus, identifying active GILZ peptides could represent a new class of drugs for treating IBD patients.

The Cellular Coactivator HCF-1 Is Required for Glucocorticoid Receptor-Mediated Transcription of Bovine Herpesvirus 1 Immediate Early Genes

Following productive infection, bovine herpesvirus 1 (BoHV-1) establishes latency in sensory neurons. As in other alphaherpesviruses, expression of BoHV-1 immediate early (IE) genes is regulated by an enhancer complex containing the viral IE activator VP16, the cellular transcription factor Oct-1, and transcriptional coactivator HCF-1, which is assembled on an IE enhancer core element (TAATGARAT). Expression of the IE transcription unit that encodes the viral IE activators bICP0 and bICP4 may also be induced by the activated glucocorticoid receptor (GR) via two glucocorticoid response elements (GREs) located upstream of the enhancer core. Strikingly, lytic infection and reactivation from latency are consistently enhanced by glucocorticoid treatment in vivo As the coactivator HCF-1 is essential for IE gene expression of alphaherpesviruses and recruited by multiple transcription factors, we tested whether HCF-1 is required for glucocorticoid-induced IE gene expression. Depletion of HCF-1 reduced GR-mediated activation of the IE promoter in mouse neuroblastoma cells (Neuro-2A). More importantly, HCF-1-mediated GR activation of the promoter was dependent on the presence of GRE sites but independent of the TAATGARAT enhancer core element. HCF-1 was also recruited to the GRE region of a promoter lacking the enhancer core, consistent with a direct role of the coactivator in mediating GR-induced transcription. Similarly, during productive lytic infection, HCF-1 and GR occupied the IE region containing the GREs. These studies indicate HCF-1 is critical for GR activation of the viral IE genes and suggests that glucocorticoid induction of viral reactivation proceeds via an HCF-1-GR mechanism in the absence of the viral IE activator VP16.IMPORTANCE BoHV-1 transcription is rapidly activated during stress-induced reactivation from latency. The immediate early transcription unit 1 (IEtu1) promoter is regulated by the GR via two GREs. The IEtu1 promoter regulates expression of two viral transcriptional regulatory proteins, infected cell proteins 0 and 4 (bICP0 and bICP4), and thus must be stimulated during reactivation. This study demonstrates that activation of the IEtu1 promoter by the synthetic corticosteroid dexamethasone requires HCF-1. Interestingly, the GRE sites, but not the IE enhancer core element (TAATGARAT), were required for HCF-1-mediated GR promoter activation. The GR and HCF-1 were recruited to the IEtu1 promoter in transfected and infected cells. Collectively, these studies indicate that HCF-1 is critical for GR activation of the viral IE genes and suggest that an HCF-1-GR complex can stimulate the IEtu1 promoter in the absence of the viral IE activator VP16.

Bovine herpesvirus 1 productive infection and immediate early transcription unit 1 promoter are stimulated by the synthetic corticosteroid dexamethasone

The primary site for life-long latency of bovine herpesvirus 1 (BHV-1) is sensory neurons. The synthetic corticosteroid dexamethasone consistently induces reactivation from latency; however the mechanism by which corticosteroids mediate reactivation is unclear. In this study, we demonstrate for the first time that dexamethasone stimulates productive infection, in part, because the BHV-1 genome contains more than 100 potential glucocorticoid receptor (GR) response elements (GREs). Immediate early transcription unit 1 (IEtu1) promoter activity, but not IEtu2 or VP16 promoter activity, was stimulated by dexamethasone. Two near perfect consensus GREs located within the IEtu1 promoter were necessary for dexamethasone-mediated stimulation. Electrophoretic mobility shift assays and chromatin immunoprecipitation studies demonstrated that the GR interacts with IEtu1 promoter sequences containing the GREs. Although we hypothesize that DEX-mediated stimulation of IEtu1 promoter activity is important during productive infection and perhaps reactivation from latency, stress likely has pleiotropic effects on virus-infected cells.

Helix 8 of the ligand binding domain of the glucocorticoid receptor (GR) is essential for ligand binding

Membrane association of estrogen receptors (ER) depends on cysteine palmitoylation and two leucines in the ligand binding domain (LBD), conserved in most steroid receptors. The role of this region, corresponding to helix 8 of the glucocorticoid receptor (GR) LBD, on membrane association of GR was studied in 4B cells, expressing endogenous GR, and Cos-7 cells transfected EGFP-GR constructs. 4B cells preloaded with radiolabeled palmitic acid showed no radioactivity incorporation into immunoprecipitated GR. Moreover, mutation C683A (corresponding to ER palmitoylation site) did not affect corticosterone-induced membrane association of GR. Mutations L687-690A, L682A, E680G and K685G prevented membrane and also nuclear localization through reduced ligand binding. L687-690A mutation decreased association of GR with heat shock protein 90 and transcriptional activity, without overt effects on receptor protein stability. The data demonstrate that palmitoylation does not mediate membrane association of GR, but that the region 680-690 (helix 8) is critical for ligand binding and receptor function.

Biochemical analyses of nuclear receptor-dependent transcription with chromatin templates

Chromatin, the physiological template for transcription, plays important roles in gene regulation by nuclear receptors (NRs). It can (1) restrict the binding of NRs or the transcriptional machinery to their genomic targets, (2) serve as a target of regulatory posttranslational modifications by NR coregulator proteins with histone-directed enzymatic activities, and (3) function as a binding scaffold for a variety of transcription-related proteins. The advent of in vitro or "cell-free" systems that accurately recapitulate ligand-dependent transcription by NRs with chromatin templates has allowed detailed analyses of these processes. Biochemical studies have advanced our understanding of the mechanisms of gene regulation, including the role of ligands, coregulators, and nucleosome remodeling. In addition, they have provided new insights about the dynamics of NR-mediated transcription. This chapter reviews the current methodologies for assembling, transcribing, and analyzing chromatin in vitro, as well as the new information that has been gained from these studies.

Glucocorticoid receptor domain requirements for chromatin remodeling and transcriptional activation of the mouse mammary tumor virus promoter in different nucleoprotein contexts

The glucocorticoid receptor (GR) contains several activation domains, tau1 (AF-1), tau2, and AF-2, which were initially defined using transiently transfected reporter constructs. Using domain mutations in the context of full-length GR, this study defines those domains required for activation of the mouse mammary tumor virus (MMTV) promoter in two distinct nucleoprotein configurations. A transiently transfected MMTV template with a disorganized, accessible chromatin structure was largely dependent on the AF-2 domain for activation. In contrast, activation of an MMTV template in organized, replicated chromatin requires both domains but has a relatively larger dependence on the tau1 domain. Domain requirements for GR-induced chromatin remodeling of the latter template were also investigated. Mutation of the AF-2 helix 12 domain partially inhibits the induction of nuclease hypersensitivity, but the inhibition was relieved in the absence of tau1, suggesting the occurrence of an important interaction between the two domains. Further mutational analysis indicates that GR-induced chromatin remodeling requires the ligand-binding domain in the region of helix 3. Our study shows that the GR activation surfaces required for transcriptional modulation of a target promoter were determined in part by its chromatin structure. Within a particular cellular environment the GR appears to possess a significant degree of versatility in the mechanism by which it activates a target promoter.

Differential transcription of the orphan receptor RORbeta in nuclear extracts derived from Neuro2A and HeLa cells

An important model system for studying the process leading to productive transcription is provided by the superfamily of nuclear receptors, which are for the most part ligand-controlled transcription factors. Over the past years several 'orphan' nuclear receptors have been isolated for which no ligand has yet been identified. Very little is known about how these 'orphan' receptors regulate transcription. In this study we have analysed the biochemical and transcriptional properties of the neuronally expressed orphan nuclear receptor RORbeta (NR1F2) and compared them with the retinoic acid receptor heterodimer RXRalpha-RARalpha (NR2B1-NR1B1) and Gal-VP16 in vitro. Although RORbeta binds to its DNA-binding sites with comparatively low affinity, it efficiently directs transcription in nuclear extracts derived from a neuronal cell line, Neuro2A, but not in nuclear extracts from non-neuronal HeLa cells. In contrast, RXRalpha-RARalpha and the acidic transcription factor Gal-VP16 support transcription in Neuro2A and HeLa nuclear extracts equally efficiently. These observations point to a different (co)factor requirement for transactivation by members of the NR1 subfamily of nuclear receptors.

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.

Differential binding and transcriptional behaviour of two highly related orphan receptors, ROR alpha(4) and ROR beta(1)

Nuclear receptors are ligand-inducible transcription factors that can be classified into two major groups according to their DNA-binding properties. Members of the first group bind to DNA as dimers, either homo- or heterodimers; members of the second group are also able to bind as monomers. While the first group has been extensively studied biochemically, very little is known about nuclear receptors that bind and act as monomers. In this study, we compared the binding and transcriptional behaviour of ROR alpha (NR1F1) and ROR beta (NR1F2), two representatives of the subgroup of monomer-binding receptors. We show that although they are highly related in their amino acid structures, they display remarkably different binding behaviours. Furthermore, we provide evidence that ROR beta can efficiently activate transcription in vitro as a monomer.

Androgen specificity of a response unit upstream of the human secretory component gene is mediated by differential receptor binding to an essential androgen response element

The expression of secretory component (SC), the epithelial receptor for poly-immunoglobulins, is regulated in a highly tissue-specific manner. In several tissues, e.g. lacrimal gland and prostate, SC synthesis is enhanced by androgens at the transcriptional level. In this study, we describe the presence of an androgen response unit, located 3.3 kb upstream of the sc transcription initiation site and containing several 5'-TGTTCT-3'-like motifs. Although each of these elements is implicated in the enhancer function, one element, the ARE1.2 motif, is found to be the main interaction site for the androgen receptor as demonstrated in in vitro binding assays as well as in transient transfection assays. A high-affinity binding site for nuclear factor I, adjacent to this ARE, is also involved in the correct functioning of the sc upstream enhancer. The ARE1.2 motif consists of an imperfect direct repeat of two core binding elements with a three-nucleotide spacer and therefore constitutes a nonconventional ARE. We demonstrate that this element displays selectivity for the androgen receptor as opposed to glucocorticoid receptor both in in vitro binding assays and in transfection experiments. Mutational analysis suggests that the direct nature of the half-site repeat is responsible for this selectivity. We have thus determined a complex and androgen-specific response unit in the far upstream region of the human SC gene, which we believe to be involved in its androgen responsiveness in epithelial cells of different organs such as prostate and lacrimal gland. We were also able to demonstrate that the primary sequence of a single nonconventional ARE motif within the enhancer is responsible for its androgen specificity.

Ligand-dependent activation of transcription in vitro by retinoic acid receptor alpha/retinoid X receptor alpha heterodimers that mimics transactivation by retinoids in vivo

All-trans and 9-cis retinoic acids (RA) signals are transduced by retinoic acid receptor/retinoid X receptor (RAR/RXR) heterodimers that act as functional units controlling the transcription of RA-responsive genes. With the aim of elucidating the underlying molecular mechanisms, we have developed an in vitro transcription system using a chromatin template made up of a minimal promoter and a direct repeat with 5-spacing-based RA response element. RARalpha and RXRalpha were expressed in and purified from baculovirus-infected Sf9 cells, and transcription was carried out by using naked DNA or chromatin templates. Transcription from naked templates was not affected by the presence of RA and/or RAR/RXR heterodimers. In contrast, very little transcription occurred from chromatin templates in the absence of RA or RAR/RXR heterodimers whereas their addition resulted in a dosage-dependent stimulation of transcription that never exceeded that occurring on naked DNA templates. Most importantly, the addition of synthetic agonistic or antagonistic retinoids to the chromatin transcription system mimicked their stimulatory or inhibitory action in vivo, and activation by a RXR-specific retinoid was subordinated to the binding of an agonist ligand to the RAR partner. Moreover, the addition of the p300 coactivator generated a synergistic enhancement of transcription. Thus, the dissection of this transcription system ultimately should lead to the elucidation of the molecular mechanisms by which RAR/RXR heterodimers control transcription in a ligand-dependent manner.

p300 and estrogen receptor cooperatively activate transcription via differential enhancement of initiation and reinitiation

Estrogen- and antiestrogen-regulated, AF-2-dependent transcriptional activation by purified full-length human estrogen receptor (ER) was carried out with chromatin templates in vitro. With this system, the ability of purified human p300 to function as a transcriptional coactivator was examined. In the absence of ligand-activated ER, p300 was found to have little effect (less than twofold increase) on transcription, whereas, in contrast, p300 was observed to act synergistically with ligand-activated ER to enhance transcription. When transcription was limited to a single round, p300 and ER were found to enhance the efficiency of transcription initiation in a cooperative manner. On the other hand, when transcription reinitiation was allowed to occur, ER, but not p300, was able to increase the number of rounds of transcription. These results suggest a two-stroke mechanism for transcriptional activation by ligand-activated ER and p300. In the first stroke, ER and p300 function cooperatively to increase the efficiency of productive transcription initiation. In the second stroke, ER promotes the reassembly of the transcription preinitiation complex. Therefore, ER exhibits distinct, dual functions in transcription initiation and reinitiation.

The glucocorticoid receptor is associated with the RNA-binding nuclear matrix protein hnRNP U

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor that is able to modulate gene activity by binding to its response element, interacting with other transcription factors, and contacting several accessory proteins such as coactivators. Here we show that GRIP120, one of the factors we have identified to interact with the glucocorticoid receptor, is identical to the heterogeneous nuclear ribonucleoprotein U (hnRNP U), a nuclear matrix protein binding to RNA as well as to scaffold attachment regions. GR.hnRNP U complexes were identified by blotting and coimmunoprecipitation. The subnuclear distribution of GR and hnRNP U was characterized by indirect immunofluorescent labeling and confocal laser microscopy demonstrating a colocalization of both proteins. Using a nuclear transport-deficient deletion of hnRNP U, nuclear translocation was seen to be dependent on GR and dexamethasone. Transient transfections were used to identify possible interaction domains. Overexpressed hnRNP U interfered with glucocorticoid induction, and the COOH-terminal domains of both proteins were sufficient in mediating the transcriptional interference. A possible functional role for this GR binding-protein in addition to its binding to the nuclear matrix, to RNA, and to scaffold attachment regions is discussed.

Retinoid X receptor:vitamin D3 receptor heterodimers promote stable preinitiation complex formation and direct 1,25-dihydroxyvitamin D3-dependent cell-free transcription

The numerous members of the steroid/nuclear hormone receptor superfamily act as direct transducers of circulating signals, such as steroids, thyroid hormone, and vitamin or lipid metabolites, and modulate the transcription of specific target genes, primarily as dimeric complexes. The receptors for 9-cis retinoic acid and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], RXR and VDR, respectively, as members of this superfamily, form a heterodimeric complex and bind cooperatively to vitamin D responsive elements (VDREs) to activate or repress the transcription of a multitude of genes which regulate a variety of physiological functions. To directly investigate RXR- and VDR-mediated transactivation, we developed a cell-free transcription system for 1,25(OH)2D3 signaling by utilizing crude nuclear extracts and a G-free cassette-based assay. Transcriptional enhancement in vitro was dependent on purified, exogenous RXR and VDR and was responsive to physiological concentrations of 1,25(OH)2D3. We found that RXR and VDR transactivated selectively from VDRE-linked templates exclusively as a heterodimeric complex, since neither receptor alone enhanced transcription in vitro. By the addition of low concentrations of the anionic detergent Sarkosyl to limit cell-free transcription to a single round and the use of agarose gel mobility shift experiments to assay factor complex assembly, we observed that 1,25(OH)2D3 enhanced RXR:VDR-mediated stabilization or assembly of preinitiation complexes to effect transcriptional enhancement from VDRE-linked promoter-containing DNA.

Mechanism of gene expression by the glucocorticoid receptor: role of protein-protein interactions

The glucocorticoid receptor belongs to an important class of transcription factors that alter the expression of target genes in response to a specific hormone signal. The glucocorticoid receptor can function at least at three levels: (1) recruitment of the general transcription machinery; (2) modulation of transcription factor action, independent of DNA binding, through direct protein-protein interactions; and (3) modulation of chromatin structure to allow the assembly of other gene regulatory proteins and/or the general transcription machinery on the DNA. This review will focus on the multifaceted nature of protein-protein interactions involving the glucocorticoid receptor and basal transcription factors, coactivators and other transcription factors, occurring at these different levels of regulation.

Induction of cell-free, in vitro transcription by recombinant androgen receptor peptides

An in vitro, cell-free transcription system, based on prostate-derived transcriptional machinery and very powerful androgen response elements (AREs), has been developed. Multiple (p(ARR3)LovTATA) AREs from the androgen-regulated probasin gene were linked to G-free cassettes and used in nuclear extracts prepared from prostate carcinoma cell lines (PC3 and LNCaP cells) to test specific induction of transcription by full-length AR and by glutathione-S-transferase (GST)-fusion peptides in which the androgen receptor (AR) DNA-binding domain alone (AR524-649), or together with the ligand-binding domain (AR524-902), or a portion of the NH2-terminal domain (AR232-649) were incorporated. In the presence of AR, nuclear extracts from PC3 cells had greater activity in supporting transcription than those from LNCaP cells; and lower background activity than those from HeLa cells. All of the AR forms correctly initiated in vitro transcription of ARE-templates in an androgen-independent manner. The amount of specific, inducible transcript was dependent on the concentration of AR peptide present. AR524-902 was the most potent transactivator tested, with the maximal level of specific transcript over 900-fold higher than the minimal level. At all concentrations this peptide was three to four times more active than either AR524-649 or AR232-649. In conclusion, we have developed a very specific and sensitive cell-free transcription system for delineating trans-activational regions of the AR.

Differential binding of mutant glucocorticoid receptors to the glucocorticoid response element of the tyrosine aminotransferase gene

Glucocorticoid receptors (GCRs) in sublines of the mouse P1798 lymphosarcoma that are sensitive (S) or resistant (R) to glucocorticoid-induced cell lysis were examined for their ability to bind to a single glucocorticoid responsive element (GRE). Mobility shift assays detected two specific complexes that were identical in both S and R cellular extracts. Antibodies against the GCR N-terminus supershifted complexes, suggesting that the 97 kDa wild-type GCR (WT-GCR) in S cells, and the variant, 97 kDa non-steroid-binding GCR (NSB-GCR) in R cells were components of both complexes. Sephacryl S300 gel filtration column fractions containing the WT-GCR and NSB-GCR formed complexes with the GRE, while fractions containing a second GCR variant in R cells, the 45 kDa steroid-binding truncated GCR (TR-GCR), did not. Southwestern blotting detected a GRE-binding, 97 kDa protein band in both S and R extracts. A 45 kDa band was not detected. UV crosslinking of protein to DNA revealed protein in the range of 92-120 kDa crosslinked to the GRE in both S and R extracts. No crosslinking was detected at 45 kDa. Strong interaction of the NSB-GCR with GREs and lack of binding of the TR-GCR to single GREs illustrate a complex receptor system in the P1798 lymphosarcoma.

A fraction enriched in a novel glucocorticoid receptor-interacting protein stimulates receptor-dependent transcription in vitro

Glucocorticoids influence numerous cell functions by regulating gene activity. The glucocorticoid receptor (GR) is a ligand-activated transcription factor and, like any other transcription factor, does not modulate gene activity just by binding to DNA. Interaction with other proteins is probably required to enhance the establishment of a functional transcription initiation complex. To identify such proteins, we analyzed the in vitro interaction of the glucocorticoid receptor bound to a double glucocorticoid response element with nuclear proteins and describe here three interacting proteins with different molecular weights. One of them, which we named GRIP 170 (GR-interacting protein), was purified and microsequenced, and it turned out to be an unknown protein. When tested in a cell-free transcription assay, the fraction highly enriched for GRIP 170 does not influence basal promoter activity but does enhance GR induction.

Identification and localization of steroid-binding and nonsteroid-binding forms of the glucocorticoid receptor in the mouse P1798 lymphosarcoma

Glucocorticoid receptors (GCRs) were characterized in sublines of the mouse P1798 lymphosarcoma that are sensitive (S) or resistant (R) to glucocorticoid-mediated apoptosis. Previous work had identified two steroid-binding GCRs in S and R cells: a 97 kDa wild-type GCR in S cells (WT-GCR), and a 45 kDa truncated GCR in R cells (TR-GCR). A third GCR, a 97 kDa nonsteroid-binding GCR (NSB-GCR), was also identified in R cells. Using subcellular fractionation and Western blotting, we now show that in contrast to the WT-GCR which is localized in both the cytoplasm and nucleus of S cells, the NSB-GCR is localized predominantly in R cell nuclei. Moreover, gel filtration chromatography revealed that treatment with 400 mM NaCl and heat did not significantly alter the Stokes radius of the NSB-GCR suggesting that this receptor is not present in a heterooligomeric complex with other proteins. The TR-GCR was localized predominantly in the soluble cytoplasmic fraction but also in the crude membrane fractions of R cell nuclei, suggesting that this receptor is tightly associated with nuclear structures. It was not detected in the soluble nuclear fraction. Unexpectedly, a 45 kDa nonsteroid-binding immunoreactive protein was detected in crude membrane fractions of S cells. These studies describe a complex GCR system in the P1798 lymphosarcoma that necessitates a further consideration of glucocorticoid signaling in S and R cells.

Retinoid-dependent in vitro transcription mediated by the RXR/RAR heterodimer

The effects of retinoids on gene regulation are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Here, we provide the first biochemical evidence that, in vitro, ligand governs the transcriptional activity of RXR alpha/RAR alpha by inducing conformational changes in the ligand-binding domains. Using limited proteolytic digestion we show that binding of the cognate ligand causes a conformational change in the carboxy-terminal part of the receptor. We also show that recombinant RXR alpha/RAR alpha is partially active in the absence of exogenously added ligand. Trans-activation depends critically on the ligand-dependent transcriptional activation function AF-2 of RAR alpha. Full activation by recombinant RXR alpha/RAR alpha, however, requires the addition of either all-trans RA, 9-cis RA, or other RAR-specific agonists, whereas an RAR alpha-specific antagonist abolishes trans-activation. Intriguingly, the ligand-dependent AF-2 of RXR does not contribute to the level of transcription from the RAR beta 2 promoter in vitro even when the cognate ligand (9-cis RA) is bound. Thus, the major role of RXR in trans-activation of the RAR beta 2 promoter is to serve as an auxiliary factor required for the binding of RAR which, in turn, is directly responsible for transcriptional activity.

Human androgen receptor expressed in HeLa cells activates transcription in vitro

The androgen receptor (AR) is a ligand-responsive transcription factor, belonging to the class of steroid receptors. AR mutations have been associated with various X-linked diseases, characterized by complete or partial resistance to androgens. To further analyse the molecular mechanism of action of the AR, we have produced the human AR in HeLa cells with a Vaccinia virus expression system. Binding studies on infected HeLa cells demonstrate that the recombinant AR interacts specifically and with high affinity with natural and synthetic androgens. In a gel retardation assay the partially purified AR specifically recognizes an androgen response element of the rat prostatic binding protein gene. Moreover, the recombinant AR activates transcription in vitro from a synthetic promoter construct containing glucocorticoid response elements (GRE).

Affinity purification of histidine-tagged proteins

Expression of recombinant proteins is a standard technique in molecular biology and a wide variety of prokaryotic as well as eukaryotic expression systems are currently in use. A limiting step is often the purification of the expressed recombinant protein, particularly if mammalian expression systems that yield low expression levels are employed. Here, we discuss the advantages and restrictions of tagging recombinant proteins with histidines and purifying them by Ni(2+)-NTA chromatography.