Steroid hormone receptors: many actors in search of a plot

Expression of the estrogen-related receptor 1 (ERR-1) orphan receptor during mouse development

We studied the expression of the estrogen-related receptor 1 (ERR-1) during mouse embryonic development. ERR-1 is expressed at very early stages in ES cells and at E8.5 in the mesodermal cells of the visceral yolk sac. ERR-1 continues to be expressed later in mesodermal tissues and particularly in heart and in skeletal muscles. This expression persists during all the embryonic development and in adult stage. ERR-1 transcripts level increases during muscle differentiation. Accordingly, we show that ERR-1 expression increases during the myoblast to myotube transition in differentiating C2 myoblastic cells. ERR-1 has also been detected in the nervous system during embryonic development. At E10.5, a high level of ERR-1 transcripts can be observed in differentiated cells of the intermediate zone of the spinal cord which also suggests a role of ERR-1 in the differentiation of the nervous system. The same is observed in the telencephalon vesicules at E13.5. Later, at E15.5 and E17.5, expression persists in the spinal cord but decreases dramatically in the central nervous system. Moreover, ERR-1 expression increases during skin formation and is detected in the stratum spinosum which contains differentiated Malpighian cells. Finally, we also observed ERR-1 in endodermal derivatives such as the epithelium of intestine and urogenital system. The DNA target of ERR-1 has been identified to be the SF-1/FTZ-F1 responsive element (SFRE) and we show in this paper that SF-1/FTZ-F1 and ERR-1 bind to and activate transcription independently through the SFRE element. Our study suggests that ERR-1 may be implicated in numerous physiological or developmental functions, particularly in the muscle, the central and peripheral nervous system and the epidermis. Interestingly, in these various systems ERR-1 expression is correlated with post-mitotic cells stage, suggesting that ERR-1 may play a role in the differentiation process.

Novel progesterone target genes identified by an improved differential display technique suggest that progestin-induced growth inhibition of breast cancer cells coincides with enhancement of differentiation

Progesterone is an important regulator of normal and malignant breast epithelial cells. In addition to stimulating development of normal mammary epithelium, it can be used to treat hormone-dependent breast tumors. However, the mechanism of growth inhibition by progestins is poorly understood, and only a limited number of progesterone target genes are known so far. We therefore decided to clone such target genes by means of differential display polymerase chain reaction. In this paper, we describe an improved differential display strategy that eliminates false positives, along with the identification of nine positive (TSC-22, CD-9, Na+/K+-ATPase alpha1, desmoplakin, CD-59, FKBP51, and three unknown genes) and one negative progesterone target genes (annexin-VI) from the mammary carcinoma cell line T47D, which is growth-inhibited by progestins. None of these genes have been reported before to be progesterone targets. Regulation of desmoplakin, CD-9, CD-59, Na+/K+-ATPase alpha1, and annexin-VI by the progestin suggests that progesterone induces T47D cells to differentiate. Three of these genes were repressed by estradiol and up-regulated by the progestin. Estradiol treatment of T47D cells also leads to formation of lamellipodia and delocalization of two cell adhesion proteins, E-cadherin and alpha-catenin. All these effects were reversed by the progestin. These data suggest that estradiol dedifferentiates T47D cells, while progestins have the opposite effect. This may be linked to the capacity of progestins to inhibit tumor growth.

Modulation of hormone-dependent transcriptional activity of the glucocorticoid receptor by the tumor suppressor p53

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor which regulates growth, development and metabolic functions. To test the hypothesis that the pleiotropic effect of the GR could be mediated by other transcription factors/oncogenes, the present study assessed its interaction with the tumor suppressor p53. p53 is a transcription factor which is involved in cell cycle regulation and apoptosis. We found that the wild-type p53 physically interacted with the GR and repressed the glucocorticoid-dependent transcriptional activity. In contrast, mutant p53 had no or a lesser effect depending on the type of p53 mutant. These findings raised the possibility that p53 may play an important role in modulating the activities of glucocorticoids in cells.

Interaction of androgen receptors with androgen response element in intact cells. Roles of amino- and carboxyl-terminal regions and the ligand

Promoter interference assay was employed to examine in intact cells the roles of the functional domains of androgen receptor (AR) and the ligand for specific DNA interactions using a cytomegalovirus-(androgen response element)-chloramphenicol acetyltransferase reporter (pCMV-ARE2-CAT). Native rat and human ARs interfered with pCMV-ARE2-CAT expression in a hormone-dependent fashion. Low steroid-independent interference seemed to occur because of the ligand binding domain (LBD), which was transcriptionally inhibitory also in a heterologous context. AR devoid of LBD (rARDelta641-902) decreased pCMV-ARE2-CAT activity by 50%. The rARDelta46-408 mutant devoid of the NH2-terminal transcription activation region exhibited ligand-dependent promoter interference of a similar magnitude. Ligand and DNA binding-deficient mutants (hARM807R and rARC562G, respectively) did not influence pCMV-ARE2-CAT expression, although hARM807R binds to ARE in vitro. Non-steroidal anti-androgens casodex and hydroxyflutamide antagonized agonist-dependent promoter interference, whereas cyproterone acetate, RU 56187, RU 57073, and RU 59063 were partial agonists/antagonists. Collectively, interaction of ARs with ARE in intact cells does not require the presence of the COOH-terminal or NH2-terminal domain and/or their interaction. In the context of native AR, however, the androgen-induced conformational change in LBD is mandatory for generation of a transcriptionally competent receptor that binds to DNA in intact cells.

Transcription of the vasoactive intestinal peptide gene in response to glucocorticoids: differential regulation of alternative transcripts is modulated by a labile protein in rat anterior pituitary

Expression of the vasoactive intestinal peptide (VIP) gene is controlled by glucocorticoids in a tissue- and endocrine status-specific manner. We have investigated the molecular mechanisms that determine glucocorticoid regulation of VIP gene expression in the rat pituitary. In initial experiments, using explant cultures of rat pituitary glands, we have demonstrated that treatment with the glucocorticoid agonist dexamethasone leads to a marked increase in VIP mRNA levels. This effect was found to be selective for the larger of two alternatively polyadenylated VIP transcripts, and in addition, protein synthesis inhibitors markedly enhanced the magnitude of this response indicating that a labile pituitary protein acts to attenuate the transcript-selective response to glucocorticoids. Nuclear run-on analysis of transcription demonstrated that the effects of dexamethasone in vitro are mediated largely, if not completely, at the level of transcription. In order to investigate the role of VIP promoter sequence in the glucocorticoid response, we then demonstrated that the activity of rat VIP gene promoter/reporter constructs in GH3 pituitary cells are up-regulated by dexamethasone. This up-regulation is virtually abolished following removal of promoter sequence between -162 and -89 of the start of transcription. Using an in vitro electrophoretic mobility shift assay, we have also demonstrated that this region of the promoter binds recombinant glucocorticoid receptor protein. The results of our study therefore indicate a direct mechanism of action for the modulation of VIP gene expression by glucocorticoids, and furthermore provide evidence of a mechanism that permits selective glucocorticoid regulation of alternative VIP transcripts.

Crosslinking of progesterone receptor to DNA using tuneable nanosecond, picosecond and femtosecond UV laser pulses

UV laser crosslinking is a potentially powerful tool to investigate transient DNA-protein interactions and binding kinetics in intact cells. As the processes underlying UV laser crosslinking are not fully understood, we have performed a study of the influence of laser pulses with different physical parameters on crosslinking of the progesterone receptor to an oligonucleotide containing a hormone-responsive element. We also studied the influence of the various parameters on the amount of laser-irradiated DNA that can be correctly primer extended as an operational measurement of DNA integrity. A strong influence of pulse intensity and pulse length on the crosslink yield was found, likely due to a change in the 'two photon' processes responsible for crosslinking. The highest efficiency of protein crosslinking to DNA was achieved with femtosecond pulses and should be sufficient to enable use of this technique for in vivo studies.

Effects of natural and synthetic polyamines on the conformation of an oligodeoxyribonucleotide with the estrogen response element

We studied the effects of natural and synthetic polyamines on the conformation of an oligodeoxyribonucleotide (ODN1) harboring the estrogen response element (ERE) by circular dichroism (CD) spectroscopy and polyacrylamide gel electrophoresis. Putrescine and spermidine had no marked effect on the CD spectrum of ODN1. In contrast, spermine provoked and stabilized two characteristic changes in the CD spectrum. The first change was indicated by an increase in the intensity of the CD band at 280 nm at 0.5 mM spermine in Tris-HCl buffer containing 50 mM NaCl. This change appears to be related to changes in base tilt and conformational alterations similar to A-DNA. At 1-2 mM spermine, the CD spectrum was characterized by a loss of positive bands at 220 and 270 nm. This change might have contributions from polyamine-induced condensation/aggregation of DNA. Spectral measurements were also conducted in Tris-HCl buffer containing 150 mM NaCl to minimize contributions from condensation and aggregation of ODN1. Under these conditions, CD spectral changes were retained by (ODN1), although the magnitude of the change was diminished. In contrast, a control oligdeoxyribonucleotide (ODN2) having similar base composition did not show any significant change in the CD spectrum in the presence of 150 mM NaCl and 2 mM spermine. The changes in the CD spectrum of ODN1 were highly sensitive to polyamine structure, as evidenced by experiments using spermine analogs with altered number of -CH2- groups separating the amino and imino groups. Electrophoretic mobility shift analysis further showed ODN1 stabilization by spermine and its analogs. These data demonstrate the ability of an ODN containing ERE to undergo conformational transitions in the presence of polyamines and suggest a possible mechanism for polyamine-mediated alterations in the interaction of estrogen receptor with ERE.

A signature motif in transcriptional co-activators mediates binding to nuclear receptors

The binding of lipophilic hormones, retinoids and vitamins to members of the nuclear-receptor superfamily modifies the DNA-binding and transcriptional properties of these receptors, resulting in the activation or repression of target genes. Ligand binding induces conformational changes in nuclear receptors and promotes their association with a diverse group of nuclear proteins, including SRC-1/p160, TIF-2/GRIP-1 and CBP/p300 which function as co-activators of transcription, and RIP-140, TIF-1 and TRIP-1/SUG-1 whose functions are unclear. Here we report that a short sequence motif LXXLL (where L is leucine and X is any amino acid) present in RIP-140, SRC-1 and CBP is necessary and sufficient to mediate the binding of these proteins to liganded nuclear receptors. We show that the ability of SRC-1 to bind the oestrogen receptor and enhance its transcriptional activity is dependent upon the integrity of the LXXLL motifs and on key hydrophobic residues in a conserved helix (helix 12) of the oestrogen receptor that are required for its ligand-induced activation function. We propose that the LXXLL motif is a signature sequence that facilitates the interaction of different proteins with nuclear receptors, and is thus a defining feature of a new family of nuclear proteins.

The transcriptional co-activator p/CIP binds CBP and mediates nuclear-receptor function

The functionally conserved proteins CBP and p300 act in conjunction with other factors to activate transcription of DNA. A new factor, p/CIP, has been discovered that is present in the cell as a complex with CBP and is required for transcriptional activity of nuclear receptors and other CBP/p300-dependent transcription factors. The highly related nuclear-receptor co-activator protein NCoA-1 is also specifically required for ligand-dependent activation of genes by nuclear receptors. p/CIP, NCoA-1 and CBP all contain related leucine-rich charged helical interaction motifs that are required for receptor-specific mechanisms of gene activation, and allow the selective inhibition of distinct signal-transduction pathways.

Identification of two estrogen receptor transcripts with novel 5' exons isolated from a MCF7 cDNA library

Two novel transcripts of human estrogen receptor (ER) have been identified that differ in the 5' untranslated sequence. It has previously been determined that an alternate ER transcript is generated from transcription initiated upstream of the main ER cap site (P1), and utilizes a splice acceptor site at +163. Here we report the isolation of 21 ER clones from a MCF7 cDNA library. Eleven of these clones correspond to transcripts that initiate at the P1 cap site, whereas the remaining 10 clones are derived from two previously unidentified ER transcripts (designated E and H) that both utilize the +163 splice acceptor site. A panel of breast and endometrial carcinoma cell lines were screened by reverse transcriptase-polymerase chain reaction (RT-PCR) for expression of the E and H transcripts. It was found that all ER-positive cell lines expressed both of the novel transcripts. In addition, 10 primary human breast cancers were analyzed, of which six expressed the E transcript and five abundantly expressed the H transcript. These data indicate that expression of ER in human breast cancers can be dependent upon an alternate promoter at least 20 kb upstream of the primary cap site for ER.

Human TAF(II)135 potentiates transcriptional activation by the AF-2s of the retinoic acid, vitamin D3, and thyroid hormone receptors in mammalian cells

We report for the first time the cloning of a complete cDNA encoding the human TFIID subunit hTAF(II)135 (hTAF(II)130). Full-length hTAF(II)135 comprises 1083 amino acids and contains two conserved domains present also in dTAF(II)110 and hTAF(II)105. We show that expression of hTAF(II)135 in mammalian cells strongly and selectively potentiates transcriptional stimulation by the activation function-2 (AF-2) of the retinoic acid, thyroid hormone, and vitamin D3 receptors (RAR, TR, and VDR), but does not affect the AF-2s of the estrogen (ER) or retinoid X (RXR) receptors. The coactivator activity requires an hTAF(II)135 region that is located between the conserved domains but is itself not conserved in dTAF(II)110 and hTAF(II)105. Expression of hTAF(II)135 also stimulates RAR AF-2 activity when a promoter with a low-affinity TATA element (TGTA) is used, indicating that hTAF(II)135 overexpression compensates for the low-affinity of TBP for this promoter and may facilitate the recruitment of TFIID by the RAR AF-2.

Repression of vasopressin gene expression by glucocorticoids in transgenic mice: evidence of a direct mechanism mediated by proximal 5' flanking sequence

Glucocorticoids are known to exert multiple effects upon neuronal systems and neuronal gene expression but the molecular mechanisms through which these effects are mediated are largely undefined. In this study, a transgenic mouse model that expresses a bovine vasopressin transgene was used to investigate the mechanisms by which this neuropeptide gene is repressed by glucocorticoids. Using both northern analysis and a reverse transcriptase polymerase chain reaction assay, depletion of glucocorticoids with the 11,beta-hydroxylase inhibitor metyrapone was shown to result in a dexamethasone-reversed increase in ectopic adrenal transgene messenger RNA levels. This result shows that sequences within the confines of the 3.5 kb transgene are sufficient to mediate repression by glucocorticoids, and indicates the involvement of a type II glucocorticoid receptor mechanism which is independent of cellular context. Evidence for the involvement of cis-acting repressive elements in the proximal 5' flanking sequence was obtained in further studies in which bovine transgene constructs were shown to be negatively regulated by dexamethasone in 293 cells. The further demonstration that recombinant glucocorticoid receptor binds to a vasopressin promoter fragment in an in vitro electrophoretic mobility shift assay provided additional evidence of a direct mechanism of repression. Both in vitro studies were consistent with the presence of a glucocorticoid regulatory element within the region -300 to 155 of the transcription start site. The use of an in vivo transgenic system combined with in vitro analyses of gene promoter fragments enabled the characterization of the molecular mechanisms which effect physiological changes in vasopressin gene expression, and provided evidence of a direct mechanism of repression mediated by sequences within the vasopressin gene promoter.

Analysis of the functional role of steroid receptor coactivator-1 in ligand-induced transactivation by thyroid hormone receptor

The nuclear hormone receptors belonging to the steroid/thyroid/retinoid receptor superfamily are ligand-inducible transcription factors. These receptors modulate transcription of specific cellular genes, either positively or negatively, by interacting with specific hormone response elements located near the target promoters. Recent studies indicated that the hormone- occupied, DNA-bound receptor acts in concert with a cellular coregulatory factor, termed coactivator, and the basal transcription machinery to mediate gene activation. Consistent with this scenario, a number of nuclear proteins with potential coactivator function have been isolated. In the present study, we demonstrate that steroid receptor coactivator-1 (SRC-1), a recently isolated candidate coactivator, functions as a positive regulator of the thyroid hormone receptor (TR)-mediated transactivation pathway. In transient transfection experiments, coexpression of SRC-1 significantly enhanced ligand-dependent transactivation of a thyroid hormone response element (TRE)-linked promoter by human TRbeta. Our studies revealed that deletion of six amino acids (451-456) in the extreme COOH-terminal region of TRbeta resulted in a receptor that retained the ability to bind T3 but failed to be stimulated by SRC-1. These six amino acids are part of an amphipathic helix that is highly conserved among nuclear hormone receptors and contains the core domain of the ligand-dependent transactivation function, AF-2. In agreement with this observation, in vitro protein binding studies showed that SRC-1 interacted with a ligand binding domain peptide (145-456) of TRbeta in a T3-dependent manner, whereas it failed to interact with a mutant ligand binding domain lacking the amino acids (451-456). We demonstrated that a synthetic peptide containing the COOH-terminal amino acids (437-456) of TRbeta efficiently blocked the ligand-induced binding of SRC-1 to the receptor. These results suggest that the conserved amphipathic helix that constitutes the AF-2 core domain of TRbeta is critical for interaction with SRC-1 and thereby plays a central role in coactivator-mediated transactivation. We further observed that a heterodimer of TRbeta and retinoid X receptor-alpha (RXR alpha), either in solution or bound to a DR+4 TRE, recruited SRC-1 in a T3-dependent manner. The AF-2 of TR was clearly involved in this process because a TR-RXR heterodimer containing a mutant TRbeta (1-450) with impaired AF-2 failed to bind to SRC-1. Surprisingly, the RXR-specific ligand 9-cis-retinoic acid induced binding of SRC-1 to the RXR component of the TRE-bound heterodimer. This novel finding suggests that RXR, as a heterodimeric partner of TR, has the potential to play an active role in transcriptional regulation. Our results raise the interesting possibility that a RXR-specific ligand may modulate T3-mediated signaling by inducing additional interactions between TRE-bound TR-RXR heterodimer and the coactivator.

Determinants of specificity of transactivation by the mineralocorticoid or glucocorticoid receptor

Glucocorticoids and mineralocorticoids have distinct in vivo roles despite close structural homology and similarities in vitro. Known mechanisms of specificity focus on factors extrinsic to the receptor; interactions that directly regulate the receptor to confer specificity are less well understood, particularly for the mineralocorticoid receptor (MR). To examine relative MR vs. glucocorticoid receptor (GR) function in a more physiological context, we compared transactivation by GR and MR in the standard experimental fibroblast CV-1 cell line, the renal epithelial LLC-PK1 line, and neuronal medullary raphe RN33B cells. Maximal transactivational activity mediated by MR, relative to that mediated by GR, is enhanced in both of these cell lines and is primarily conferred by an N-terminal-mediated enhancement of the MR response. In addition, the ligand concentration required for maximal transcriptional activity of the GR varies significantly between cell lines. This is independent of binding affinity or 11beta-hydroxysteroid dehydrogenase-mediated inactivation and may contribute to in vivo tissue-specific differences in responses to the GR. Although ligand binding affinity is clearly conferred by the LBD, receptor-specific variations between cell lines in transcriptional sensitivity to ligand appear, rather, to be associated with the N-terminus. These studies demonstrate that the specificity of the MR vs. the GR response may be mediated via unique cellular factors, as well as suggesting a novel means of expanding the cellular response to cortisol.

Patterning the expression of a tissue-specific transcription factor in embryogenesis: HNF1 alpha gene activation during Xenopus development

Tissue-specific transcription factors play an essential role in establishing cell identity during development. We review our knowledge of the molecular events involved in the activation of the gene encoding the tissue-specific transcription factor HNF1 alpha (LFB1). The available data suggest that the maternal factors OZ-1, HNF4 alpha and HNF4 beta act as initial activators of the HNF1 alpha promoter. We present evidence suggesting that the mesoderm-inducing factor activin A plays a critical role by acting through the HNF4 binding site of the HNF1 alpha promoter. The activity of this embryonic morphogen seems to form a gradient opposing the distribution of the maternal HNF4 proteins that are concentrated at the animal pole of the egg. After zygotic gene transcription the HNF1 alpha-related transcription factor HNF1 beta accumulates faster than HNF1 alpha itself and thus is likely to contribute to the activation of the HNF1 alpha transcription via the HNF1 binding site. The cofactor of the HNF1 proteins (DCoH) is present throughout development and thus cannot limit the activation potential of HNF1 alpha in early development. Our results provide a detailed description of setting up the expression pattern of a tissue-specific transcription factor during embryogenesis.

New steroidal anti-inflammatory antedrugs: methyl 3,20-dioxo-9 alpha-fluoro-11 beta,17 alpha,21-trihydroxy-1,4-pregnadiene-16 alpha-carboxylate and methyl 21-acetyloxy-3,20-dioxo-11 beta, 17 alpha-dihydroxy-9 alpha-fluoro-1,4-pregnadiene-16 alpha-carboxylate

Focused efforts have been made to increase local-to-systemic activity ratios of potent anti-inflammatory steroids for local and/or topical applications. The approach taken in the present investigation is based upon the concept of "antedrug," defined as a locally active compound that exerts its action at the application site but rapidly undergoes a predictable biotransformation to an inactive metabolite that is readily excreted upon entry into the systemic circulation. In continuing efforts to synthesize potent, anti-inflammatory steroids without systemic glucocorticoid activities, 9 alpha-fluoro-methyl 11 beta, 17 alpha, 21-trihydroxy-3,20-dioxo-pregna-1,4-diene-16 alpha-carboxylate (FP16CM) and its 21-acetate derivative (FP16CMAc) have been synthesized and screened. Novel antedrugs were evaluated for antiinflammatory activity in the acute croton oil-induced ear edema bioassay, adverse systemic effects in the 5-day croton oil model, receptor binding, and concomitant L-tyrosine-2-oxoglutarate aminotransferase (EC 2.6.1.5) (TAT) enzyme induction in HTC cells in culture. Following a single topical application in the croton oil-induced ear edema bioassay, treatment with all compounds resulted in dose-dependent inhibition of edema. From these dose-response profiles, the following ID50 values (nmol resulting in a 50% reduction of edema) were calculated: 817, 540, 266, and 67 for hydrocortisone (HC), prednisolone (P), FP16CM, and FP16CMAc, respectively. Calculated relative potencies, setting HC = 1.0, were P, 1.5; FP16CM, 3.1, and FP16CMAc, 12.2. Results of the 5-day rat croton oil ear edema bioassay indicated that, in contrast to the parent compound P, the novel steroidal antedrugs did not significantly alter body weight gain, thymus weights, or plasma corticosterone levels. Relative binding potencies for cytosolic HTC glucocorticoid receptors were 1.0, 20.1, 5.4, and 2.5 for HC, P, FP16CM, and FP16CMAc, respectively. As predicted by the antedrug concept, FP16CM and FP16CMAc were very weak agonists for induction of TAT in HTC cells. Collectively, results of these investigations suggest that modification of P, which included addition of the 9-fluoro and 16-methoxycarbonyl group alone or in conjunction with a 21-acetoxy moiety, increase topical anti-inflammatory activity without significant adverse systemic effects. These new antedrugs may be useful as anti-inflammatory steroids for local applications.

Differential interaction of nuclear receptors with the putative human transcriptional coactivator hTIF1

Hormonal regulation of gene activity is mediated by nuclear receptors acting as ligand-activated transcription factors. Intermediary factors interacting with their activation functions are required to mediate transcriptional stimulation. In search of such receptor interacting proteins, we have screened a human cDNA expression library and isolated a human protein that interacts in vitro with transcriptionally active estrogen receptors (ER). Sequence analysis reveals that this protein is the human homolog of mouse TIF1 (transcription intermediary factor 1) shown to enhance nuclear receptor ligand-dependent activation function 2 (AF2) in yeast. We have characterized the nuclear receptor binding site on hTIF1 and shown that a region of 26 residues is sufficient for hormone-dependent binding to the estrogen receptor. As shown by point mutagenesis, the AF2 activation domain of ER is required for the binding of hTIF1 but not sufficient, since a short region encompassing the conserved amphipathic alpha-helix corresponding to this domain fails to precipitate hTIF1. We also demonstrate that hTIF1 association with DNA-bound ER requires the presence of estradiol. Finally, we show that the interaction of hTIF1 with receptors is selective since strong in vitro hormone-dependent binding is only observed with some members of the nuclear receptor superfamily.

How hormone receptor-DNA binding affects nucleosomal DNA: the role of symmetry

Molecular dynamics simulations have been employed to determine the optimal conformation of an estrogen receptor DNA binding domain dimer bound to a consensus response element, ds(AGGTCACAGTGACCT), and to a nonconsensus response element, ds(AGAACACAGTGACCT). The structures simulated were derived from a crystallographic structure and solvated by a sphere (45-A radius) of explicit water and counterions. Long-range electrostatic interactions were accounted for during 100-ps simulations by means of a fast multipole expansion algorithm combined with a multiple time-step scheme in the molecular dynamics package NAMD. The simulations demonstrate that the dimer induces a bent and underwound (10.7 bp/turn) conformation in the DNA. The bending reflects the dyad symmetry of the receptor dimer and can be described as an S-shaped curve in the helical axis of DNA when projected onto a plane. A similar bent and underwound conformation is observed for nucleosomal DNA near the nucleosome's dyad axis that reflects the symmetry of the histone octamer. We propose that when a receptor dimer binds to a nucleosome, the most favorable dimer-DNA and histone-DNA interactions are achieved if the respective symmetry axes are aligned. Such positioning of a receptor dimer over the dyad of nucleosome B in the mouse mammary tumor virus promoter is in agreement with experiment.

Myocyte-specific enhancer factor 2 and thyroid hormone receptor associate and synergistically activate the alpha-cardiac myosin heavy-chain gene

The muscle-specific regulatory region of the alpha-cardiac myosin heavy-chain (MHC) gene contains the thyroid hormone response element (TRE) and two A/T-rich DNA sequences, designated A/T1 and A/T2, the putative myocyte-specific enhancer factor 2 (MEF2) binding sites. We investigated the roles of the TRE and MEF2 binding sites and the potential interaction between thyroid hormone receptor (TR) and MEF2 proteins regulating the alpha-MHC promoter. Deletion mutation analysis indicated that both the A/T2 motif and TRE were required for muscle-specific expression of the alpha-MHC gene. The alpha-MHC enhancer containing both the A/T2 motif and TRE was synergistically activated by coexpression of MEF2 and TR in nonmuscle cells, whereas neither factor by itself activated the alpha-MHC reporters. The reporter construct containing the A/T2 sequence and the TRE linked to a heterologous promoter also showed synergistic activation by coexpression of MEF2 and TR in nonmuscle cells. Moreover, protein binding assays demonstrated that MEF2 and TR specifically bound to one another in vitro and in vivo. The MADS domain of MEF2 and the DNA-binding domain of TR were necessary and sufficient to mediate their physical interaction. Our results suggest that the members of the MADS family (MEF2) and steroid receptor superfamily (TR) interact with one another to synergistically activate the alpha-cardiac MHC gene expression.

A steroid-triggered switch in E74 transcription factor isoforms regulates the timing of secondary-response gene expression

The steroid hormone 20-hydroxyecdysone (referred to here as ecdysone) directs Drosophila metamorphosis by activating a series of genetic regulatory hierarchies. ETS domain transcription factors encoded by the ecdysone-inducible E74 early gene, E74A and E74B, act at the top of these hierarchies to coordinate the induction of target genes. We have ectopically expressed these E74 isoforms to understand their regulatory functions during the onset of metamorphosis. We show that E74 can regulate its own transcription, most likely through binding sites within its gene. Ectopic expression of E74B can partially repress the E78B and DHR3 orphan receptor genes, suggesting a role for E74 in the appropriate timing of early-late gene expression. Furthermore, E74A is both necessary and sufficient for E78B induction, implicating E74A as a key regulator of E78B expression. We also show, consistent with our studies of E74 loss-of-function mutations, that E74B is a potent repressor of late gene transcription and E74A is sufficient to prematurely induce the L71-1 late gene. However, ectopic expression of both Broad-Complex and E74A activators in an E74B mutant background is not sufficient to prematurely induce all late genes, indicating that other factors contribute to this regulatory circuit. These observations demonstrate that the steroid-triggered switch in E74 transcription factor isoforms plays a central role in the proper timing of secondary-response gene expression.

Retinoid X and retinoic acid receptors interact with transcription factor II-B by distinct mechanisms

Nuclear hormone receptors are believed to modulate target gene expression by interacting with the general transcriptional machinery of the cell. We demonstrate here that two otherwise closely related members of the nuclear hormone receptor family, retinoid acid receptors (RARs) and retinoid X receptors (RXRs), exhibit significant differences in their interactions with the transcriptional machinery. RARs display a strong constitutive interaction with transcription factor II-B (TFIIB) that requires the TFIIB C-terminus, whereas RXR exhibits a weaker, hormone-stimulated interaction with the TFIIB that maps outside of the TFIIB C-terminus. Use of a dominant-negative mutant of TFIIB suggests that the TFIIB interaction is essential for full transcriptional activation by RXR.

Plasticity of tetramer formation by retinoid X receptors. An alternative paradigm for DNA recognition

Retinoid X receptors (RXRs) are transcription factors that traditionally have been thought to bind DNA as protein dimers. Recently, however, it has been recognized that RXRs can also bind to DNA as protein tetramers. Receptor tetramers form cooperatively on response elements containing suitably reiterated half-sites, and play an important role in determining the specificity of DNA recognition by different nuclear receptors. We report here that RXR tetramers exhibit significant functional plasticity, and form on response elements possessing diverse half-site orientations and spacings. This ability of RXRs to form tetramers and related oligomers appears to contribute to the synergistic transcriptional activation observed when multiple, spatially separated response elements are introduced into a single promoter. Oligomerization may therefore be a common paradigm for DNA recognition and combinatorial regulation by several different classes of transcription factors.

Nuclear receptor coactivators

Retinoic acid, steroid and thyroid hormones regulate complex programs of gene expression by binding to intracellular receptors that are members of the nuclear receptor superfamily of ligand-dependent transcription factors. Recent studies have led to the identification and cloning of genes encoding coactivator molecules that appear to play important roles in mediating ligand-dependent transcription by members of this family. The identification of these coactivator molecules suggests a point of entry into the general transcriptional machinery that is common to several other classes of regulated transcription factors.

Nucleosome-mediated synergism between transcription factors on the mouse mammary tumor virus promoter

In unstimulated mammalian cells and in Saccharomyces cerevisiae, the mouse mammary tumor virus (MMTV) promoter is silent and organized into positioned nucleosomes, one of which encompasses the binding sites for glucocorticoid receptor (GR) and nuclear factor I (NFI). Glucocorticoid induction in vivo involves a functional synergism between GR and NFI and simultaneous occupancy of the promoter sites for both proteins that cannot be reproduced on naked DNA. The role of chromatin in the process of induction was investigated by manipulating the nucleosome density in yeast strains carrying a regulated histone H4 gene. Following depletion of nucleosomes, independent transactivation by NFI or by GR, as well as binding of the individual proteins to the MMTV promoter, were enhanced, in agreement with a repressive function of nucleosomes. In contrast, NFI-dependent hormone induction of the promoter and the simultaneous binding of receptor and NFI were compromised by nucleosome depletion. This effect could be partly mediated by a cryptic binding site for the receptor that is functional only in the nucleosomal context. Thus, positioned nucleosomes do not only account for constitutive repression of the MMTV promoter, but also participate in induction by mediating cooperative binding and functional synergism between GR and NFI.

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

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

Ligand-dependent interaction between the estrogen receptor and the human homologues of SWI2/SNF2

The human SNF2alpha (or hbrm) and SNF2beta (or BRG1) proteins have previously been shown to enhance transcriptional activation by nuclear receptors (NRs) in cultured human cells, and to be present in SWI/SNF complexes which are thought to be involved in control of transcription by facilitating remodelling of chromatin templates. Using the yeast two-hybrid system, we now demonstrate that the N-terminal regions of hSNF2alpha and hSNF2beta, preceding the DNA-dependent ATPase domain, specifically interact with the region of the estrogen receptor (ER) which includes the ligand binding domain and the ligand-dependent activation function AF-2. These interactions are increased by estrogen, but not by the ER AF-2 antagonist hydroxytamoxifen. Furthermore, mutants of ER that lack AF-2 activity are unable to interact with hSNF2alpha and -beta. These results suggest that the human homologues of the yeast SWI2/SNF2 protein may participate in the enhancement of transcription by the ER in vivo through interactions with the AF-2 activating domain, thus leading to ligand-dependent remodelling of chromatin templates.

Inducibility of E4BP4 suggests a novel mechanism of negative gene regulation by glucocorticoids

The major biological effects of glucocorticoids are thought to be initiated through changes in the expression of cellular genes. However, the mechanisms involved in the suppression of some gene products by glucocorticoids are not well defined. ID13 mouse fibroblast cells were treated with the synthetic glucocorticoid dexamethasone (Dex) or with vehicle. Plus/ minus (+Dex/-Dex) cDNAs were used to screen an ID13 cDNA library. Using this method, we found that the recently identified human protein adenovirus E4 promoter binding protein (E4BP4) mRNA was induced by Dex in the ID13 cell line. E4BP4 is a transcriptional repressor and binds to a specific DNA element. Further investigation indentified E4BP4-like DNA elements located in the promoters of glucocorticoid repressed genes Cox-2, iNOS, and cPLA2. These findings suggest that E4BP4 may play a role in the glucocorticoid repression of these and other genes.

Ligand-independent activation of the oestrogen receptor by mutation of a conserved tyrosine

The oestrogen receptor is a member of the nuclear receptor family of transcription factors which, on binding the steroid hormone 17beta-oestradiol, interacts with co-activator proteins and stimulates gene expression. Replacement of a single tyrosine in the hormone-binding domain generated activated forms of the receptor which stimulated transcription in the absence of hormone. This increased activation is related to a decrease in hydrophobicity and a reduction in size of the side chain of the amino acid with which the tyrosine is replaced. Ligand-independent, in common with ligand-dependent transcriptional activation, requires an amphipathic alpha-helix at the C-terminus of the ligand-binding domain which is essential for the interaction of the receptor with a number of potential co-activator proteins. In contrast to the wild-type protein, constitutively active receptors were able to bind both the receptor-interacting protein RIP-140 and the steroid receptor co-activator SRC-1 in a ligand-independent manner, although in the case of SRC-1 this was only evident when the receptors were prebound to DNA. We propose, therefore, that this tyrosine is required to maintain the receptor in a transcriptionally inactive state in the absence of hormone. Modification of this residue may generate a conformational change in the ligand-binding domain of the receptor to form an interacting surface which allows the recruitment of co-activators independent of hormone binding. This suggests that this tyrosine may be a target for a different signalling pathway which forms an alternative mechanism of activating oestrogen receptor-mediated transcription.

SUG1, a putative transcriptional mediator and subunit of the PA700 proteasome regulatory complex, is a DNA helicase

Yeast SUG1 was originally characterized as a transcriptional mediator for the GAL4 transactivator. A similar role in vertebrates was suggested by the ligand-enhanced interaction between mammalian homologues of yeast SUG1 and the ligand-dependent activating domain (AF-2) of nuclear receptors. SUG1 was also shown to be a component of the PA700 regulatory complex of the 26 S proteasome and a member of a large family of putative ATPases. However, no catalytic function has yet been attributed to SUG1. We show here that SUG1 is a 3'-5' DNA helicase whose activity is dependent on an intact ATP binding domain. The sedimentation heterogeneity of mammalian SUG1 suggests that it may be associated with distinct protein complexes and therefore play multiple roles.

Cloning, chromosomal localization, and functional analysis of the murine estrogen receptor beta

Estrogen receptor beta (ER beta) is a novel steroid receptor that is expressed in rat prostate and ovary. We have cloned the mouse homolog of ER beta and mapped the gene, designated Estrb, to the central region of chromosome 12. The cDNA encodes a protein of 485 amino acids that shares, respectively, 97% and 60% identity with the DNA- and ligand-binding domains of mouse (m) ER alpha. Mouse ER beta bind to an inverted repeat spaced by three nucleotides in a gel mobility shift assay and transactivates promoters containing synthetic or natural estrogen response elements in an estradiol (E2)-dependent manner. Scatchard analysis indicates that mER beta has slightly lower affinity for E2 [dissociation constant (Kd) = 0.5 nM] when compared with mER alpha (Kd = 0.2 nM). Antiestrogens, including 4-hydroxytamoxifen (OHT), ICI 182,780, and a novel compound, EM-800, inhibit E2-dependent transactivation efficiently. However, while OHT displays partial agonistic activity with ER alpha on a basal promoter linked to estrogen response elements in Cos-1 cells, this effect is not observed with mER beta. Cotransfection of mER beta and H-RasV12 causes enhanced activation in the presence of E2. Mutagenesis of a serine residue (position 60), located within a mitogen-activated protein kinase consensus phosphorylation site abolishes the stimulatory effect of Ras, suggesting that the activity of mER beta is also regulated by the mitogen-activated protein kinase pathway. Surprisingly, the coactivator SRC-1 up-regulates mER beta transactivation both in the absence and presence of E2, and in vitro interaction between SRC-1 and the ER beta ligand-binding domain is enhanced by E2. Moreover, the ligand-independent stimulatory effect of SRC-1 on ER beta transcriptional activity is abolished by ICI 182,780, but not by OHT. Our results demonstrate that while ER beta shares many of the functional characteristics of ER alpha, the molecular mechanisms regulating the transcriptional activity of mER beta may be distinct from those of ER alpha.

Identification of a novel transferable cis element in the promoter of an estrogen-responsive gene that modulates sensitivity to hormone and antihormone

The estrogen receptor (ER) is a ligand-regulated transcription factor that acts at the promoters of estrogen-regulated genes to modulate their expression. In the present study, we examined three estrogen-regulated promoters, namely the rat progesterone receptor gene distal (PRD) and proximal (PRP) promoters and the human pS2 gene promoter, and observed marked differences in their sensitivity to stimulation by estrogen and repression of estrogen-stimulated transcription by antiestrogen (AE)-occupied ER. ER-containing MCF-7 human breast cancer cells were transfected with reporter gene constructs containing estrogen response elements upstream of the three gene promoters. In this system, PRP and PRD showed similar dose-response curves for stimulation by estradiol whereas pS2 was activated by even lower concentrations of estradiol. By contrast, PRD was much less sensitive to repression of estrogen-stimulated activity by all AEs studied, relative to the PRP and the pS2 promoters. Using deletion and mutational analysis, we have identified a transferable cis element at -131 to -94 bp in PRD that is involved in modulating the sensitivity of this promoter to both estrogens and AEs. The element reduced the magnitude of estrogen-stimulated activity, enhanced the ability of AEs to repress estrogen-stimulated activity, and elicited similiar effects when transferred to the promoter of another estrogen-responsive gene. Thus, removal of this region from PRD further accentuated the insensitivity of this promoter to AE while enhancing its sensitivity (both EC50 and fold induction) to estrogen. Gel mobility shift assays showed that proteins from nuclear extracts of MCF-7 cells interact with this element and that the binding of these proteins is inversely correlated with the transcriptional effectiveness of the ER. The findings demonstrate that a specific cis element from the promoter of an estrogen-responsive gene can alter the transcriptional activity of hormone and antihormone-occupied receptor bound at its response element near the promoter. Such ligand response modulatory elements, and changes in the levels and activity of factors that bind to such elements, may underlie the different sensitivities of steroid hormone-regulated genes to both hormones and antihormones.

Dimeric binding of the mouse germ cell nuclear factor

The mouse germ cell nuclear factor (GCNF), a member of the nuclear receptor superfamily, is highly expressed during spermatogenesis, oogenesis, and during neuronal embryonic differentiation. The in vitro translated receptor binds autonomously to the direct repeat of the sequence 5'-AGGTCA-3'. To gain insights into the determinants necessary for DNA binding, I have generated truncated GCNF molecules by introducing carboxy-terminal deletions into expression constructs. An electrophoretic mobility-shift assay with these polypeptides shows that amino acids in addition to the core DNA-binding domain are important for specific binding. To address the question of whether the protein binds as monomer, homodimer, or heterodimer, I used different approaches. Analysis of the full-length protein was possible with GCNF polypeptides that contain epitopes of six consecutive histidines. Using a monoclonal antibody directed against these epitopes, I demonstrate that two GCNF molecules bind to a direct repeat. Dimerization between wild-type and truncated GCNF is shown by an electrophoretic mobility-shift analysis with a mixture of the proteins. In addition, I show that there is no in vitro interaction of GCNF with the retinoid X receptor, a promiscous partner of many nuclear receptors. The data suggest that GCNF may excert its in vivo function independently of other nuclear receptors.

Three amino acid substitutions selectively disrupt the activation but not the repression function of the glucocorticoid receptor N terminus

A 210-amino acid region, termed enh2, near the N terminus of the rat glucocorticoid receptor, is necessary for both transcriptional activation and repression. The mechanism(s) of transcriptional regulation conferred by this region, however, are poorly understood. We screened in Saccharomyces cerevisiae a library of random mutants in the enh2 region of a constitutive glucocorticoid receptor derivative and isolated a series of multiply substituted receptors that are specifically defective in transcriptional activation. Although many substitutions in this area were tolerated, three amino acid substitutions (E219K, F220L, and W234R) within a 16-amino acid region were sufficient to disrupt the enh2 transcriptional activation function both in yeast and in mammalian cells. Although this region is rich in acidic residues, the conserved tryptophan at position 234 appears to be a more critical feature for enh2 activity; hydrophobic but not charged residues were tolerated at this position. Notably, the mutants uncoupled the activation and repression functions of enh2, as the activation defective isolates remained competent for repression of AP-1 at the composite response element plfG.

CDK-independent activation of estrogen receptor by cyclin D1

Both cyclin D1 and estrogens have an essential role in regulating proliferation of breast epithelial cells. We show here a novel role for cyclin D1 in growth regulation of estrogen-responsive tissues by potentiating transcription of estrogen receptor-regulated genes. Cyclin D1 mediates this activation independent of complex formation to a CDK partner. Cyclin D1 activates estrogen receptor-mediated transcription in the absence of estrogen and enhances transcription in its presence. The activation of estrogen receptor by cyclin D1 is not inhibited by anti-estrogens. A direct physical binding of cyclin D1 to the hormone binding domain of the estrogen receptor results in an increased binding of the receptor to estrogen response element sequences, and upregulates estrogen receptor-mediated transcription. These results highlight a novel role for cyclin D1 as a CDK-independent activator of the estrogen receptor.

Studies on the mechanism of glucocorticoid-mediated repression from a negative glucocorticoid response element from the bovine prolactin gene

Several models for repression of transcription by glucocorticoid hormone, some of which involve so-called negative glucocorticoid response elements (nGRE), have been suggested. In the cases where nGREs are required, the glucocorticoid receptor (GR) is thought to bind to the nGRE and interfere with transcriptional activation by positively acting transactivating factors. We have studied an nGRE from the bovine prolactin gene promoter (PRL3), which increases basal expression from a heterologous promoter in rat pituitary cells (GH3) and is repressed by glucocorticoids. Two proteins in addition to the GR were identified in pituitary cells to bind specifically to the PRL3 nGRE, one of which was the pituitary-specific transcription factor Pit-1/GHF-1. A mutation in the PRL3 nGRE, which destroyed Pit-1/GHF-1 binding, totally abolished the increased basal expression as well as glucocorticoid repression in transfected GH3 cells. A mutation in the binding site for the second protein, termed XTF, partially impaired basal activity but totally abrogated glucocorticoid repression. The same mutation had no effect on GR binding to the PRL3 nGRE. Mixing experiments with whole-cell extracts containing overexpressed GR from COS cells decreased the binding of both Pit-1/GHF-1 and XTF to the PRL3 element. However, Pit-1/GHF-1 displacement from the PRL3 element by the GR required XTF binding. Furthermore, GR binding to the PRL3 nGRE was required for glucocorticoid repression to occur, because a mutation of the GR binding site abolished the glucocorticoid effect. Moreover, the PRL3 nGRE was found to contain only half a palindromic GRE, allowing only one GR moiety to contact the DNA. These data demonstrate that the PRL3 nGRE is composite in nature and that the ability of the GR to repress transactivation by displacement requires an intermediary factor, XTF.

Estrogen receptor-beta mRNA expression in rat ovary: down-regulation by gonadotropins

We have examined the expression and regulation of the two estrogen receptor (ER alpha and ER beta) genes in the rat ovary, using Northern blotting, RT-PCR, and in situ hybridization histochemistry. Northern blotting results show that the ovary expresses both ER alpha and ER beta genes as single (approximately 6.5-kb) and multiple (ranging from approximately 1.0-kb to approximately 10.0-kb) transcripts, respectively. ER alpha mRNA is expressed at a level lower than ER beta mRNA in immature rat ovaries. This relationship appears unchanged between sexually mature adult rats and immature rats. In sexually mature adult rats undergoing endogenous hormonal changes, whole ovarian content of ER beta mRNA, as determined by RT-PCR, remained more or less constant with the exception of the evening of proestrus when ER beta mRNA levels were decreased. Examination of ER beta mRNA expression at the cellular level, by in situ hybridization, showed that ER beta mRNA is expressed preferentially in granulosa cells of small, growing, and preovulatory follicles, although weak expression of ER beta mRNA was observed in a subset of corpora lutea, and that the decrease in ER beta mRNA during proestrous evening is attributable, at least in part, to down-regulation of ER beta mRNA in the preovulatory follicles. This type of expression and regulation was not typical for ER alpha mRNA in the ovary. Although whole ovarian content of ER alpha mRNA was clearly detected by RT-PCR, no apparent modulation of ER alpha mRNA levels was observed during the estrous cycle. Examination of ER alpha mRNA expression at the cellular level, by in situ hybridization, showed that ER alpha mRNA is expressed at a low level throughout the ovary with no particular cellular localization. To further examine the potential role of the preovulatory pituitary gonadotropins in regulating ER beta mRNA expression in the ovary, we used immature rats treated with gonadotropins. In rats undergoing exogenous hormonal challenges, whole ovarian content of ER beta mRNA, as determined by RT-PCR, remained more or less unchanged after an injection of PMSG. In contrast, a subsequent injection of human CG (hCG) resulted in a substantial decrease in whole ovarian content of ER beta mRNA. In situ hybridization for ER beta mRNA shows that small, growing, and preovulatory follicles express ER beta mRNA in the granulosa cells. The preovulatory follicles contain ER beta mRNA at a level lower than that observed for small and growing follicles. In addition, there is an abrupt decrease in ER beta mRNA expression in the preovulatory follicles after hCG injection. The inhibitory effect of hCG on ER beta mRNA expression was also observed in cultured granulosa cells. Moreover, agents stimulating LH/CG receptor-associated intracellular signaling pathways (forskolin and a phorbol ester) readily mimicked the effect of hCG in down-regulating ER beta mRNA in cultured granulosa cells. Taken together, our results demonstrate that 1) the ovary expresses both ER alpha and ER beta genes, although ER beta is the predominant form of estrogen receptor in the ovary, 2) ER beta mRNA is localized predominantly to the granulosa cells of small, growing, and preovulatory follicles, and 3) the preovulatory LH surge down-regulates ER beta mRNA. These results clearly implicate the physiological importance of ER beta in female reproductive functions.

The glucocorticoid receptor is a key regulator of the decision between self-renewal and differentiation in erythroid progenitors

During development and in regenerating tissues such as the bone marrow, progenitor cells constantly need to make decisions between proliferation and differentiation. We have used a model system, normal erythroid progenitors of the chicken, to determine the molecular players involved in making this decision. The molecules identified comprised receptor tyrosine kinases (c-Kit and c-ErbB) and members of the nuclear hormone receptor superfamily (thyroid hormone receptor and estrogen receptor). Here we identify the glucocorticoid receptor (GR) as a key regulator of erythroid progenitor self-renewal (i.e. continuous proliferation in the absence of differentiation). In media lacking a GR ligand or containing a GR antagonist, erythroid progenitors failed to self-renew, even if c-Kit, c-ErbB and the estrogen receptor were activated simultaneously. To induce self-renewal, the GR required the continuous presence of an activated receptor tyrosine kinase and had to cooperate with the estrogen receptor for full activity. Mutant analysis showed that DNA binding and a functional AF-2 transactivation domain are required for proliferation stimulation and differentiation arrest. c-myb was identified as a potential target gene of the GR in erythroblasts. It could be demonstrated that delta c-Myb, an activated c-Myb protein, can functionally replace the GR.

Transcriptional regulation by competition between ELP isoforms and nuclear receptors

ELP is a transcription factor belonging to the nuclear receptor superfamily. The consensus binding sequence for ELP contains a half site of the nuclear receptor recognition element. We demonstrated previously that ELP1, the repressor type isoform of ELP, competes for binding with the retinoic acid receptor and represses retinoic acid-induced transactivation. In this study, competitive repression by ELP1 was investigated for several other nuclear receptors. As in the case of the retinoic acid receptor, binding of vitamin D receptor, thyroid hormone receptor, and estrogen receptor could be competed by ELP1, resulting in repression of their ligand-dependent transactivation. Interestingly, the activator-type ELP isoforms were capable of repressing retinoic acid-induced transactivation through binding to the retinoic acid receptor binding element. These data suggest that competition for target DNA binding is a general mechanism of transcriptional repression by ELP isoforms.

The testis and the adrenal are (transcriptionally) the same

We have been studying the transcriptional regulation of the rat P450c17 gene in both adrenocortical and Leydig cells, to assess which DNA sequences are required for its basal and hormonally stimulated transcription. Comparing the transcriptional regulation in both of these cell types enables us to demonstrate whether specific nuclear factors required for transcriptional regulation of the rat P450c17 gene are tissue-specifically expressed, and whether the same cis-acting DNA elements in the gene are required for transcriptional regulation in both of these two different steroidogenic tissues. Using such an approach, we previously demonstrated that the transcriptional regulation of the rat P450scc gene uses different cis-acting DNA sequences in steroidogenic versus neural tissues, and requires the expression of tissue-specific nuclear factors that are unique to neural tissue. However, in studying the transcriptional regulation of the rat P450c17 gene in cultured mouse adrenocortical Y-1 and mouse Leydig MA-10 cells, we have shown that identical DNA sequences necessary for basal and cAMP-stimulated transcriptional regulation in these two cell types, and that identical nuclear factors from Y-1 and from MA-10 cells bind to these sequences. We have identified four transcriptionally active regions within 500 bp of the transcription initiation start site that are important for basal and/or cAMP-stimulated transcriptional regulation of this gene in Y-1 and MA-10 cells. This paper will discuss two of these regions in greater detail. By studying the regulation of the rat P450c17 gene, we have identified two new members of the orphan nuclear receptor gene family and have discovered new alternative mechanisms by which orphan nuclear receptors activate gene transcription in both mouse adrenocortical Y-1 and Leydig MA-10 cells.

Inhibition of mineralocorticoid activity by the beta-isoform of the human glucocorticoid receptor

Mineralocorticoids and glucocorticoids are important regulators of electrolyte homeostasis and arterial blood pressure. Their effects are mediated by the mineralocorticoid (MR) and the glucocorticoid receptor (GR), respectively. The present study was designed to determine how the two isoforms of the human GR, the "classic" GR alpha and the non-hormone-binding GR beta, interfere with the transcriptional effects of the hormone-activated human MR. COS-7 monkey kidney cells were transfected with different mineralocorticoid-responsive reporter plasmids and a vector expressing the human MR protein. Different amounts of either control, GR alpha, or GR beta plasmid were co-transfected, and luciferase activity was measured after stimulation with aldosterone and/or dexamethasone. MR-mediated stimulation of transcription was enhanced by co-transfection of the GR alpha expression vector. In contrast, MR-mediated stimulation of transcription was strongly inhibited by co-transfection of equal amounts of the GR beta expression vector. Reverse transcription-polymerase chain reaction (RT-PCR) showed expression of both GR isoforms as well as of MR in the human kidney. These data indicate that the two isoforms of the human GR exert opposite effects on mineralocorticoid activity. We conclude that the ratio between GR alpha and GR beta can define the sensitivity of mineralocorticoid target tissues to aldosterone. Imbalances of this ratio may participate in clinical syndromes of impaired or augmented mineralocorticoid sensitivity, such as certain cases of pseudohypoaldosteronism or, possibly, primary arterial hypertension.

Weak estrogenic activity from continuous-release pellets

In the process of evaluating a proprietary compound for weak estrogenic activity, two different types of dosing regimens were used, repeated daily dosing (three times/d) and continuous-release pellets. In studies using the proprietary compound, rats that were dosed via intraperitoneal injection showed no estrogenic responses, while those receiving the test compound via continuous-release pellets displayed several estrogenic responses. Because of the conflicting results, the control pellets were evaluated for estrogenic activity in the same battery of tests using the same number of pellets. In studies using only the control pellets, several estrogenic responses were observed including increased uterine weight, uterine stromal cell proliferation, estrous conversion, uterine progesterone receptor content, and decreased uterine estrogen receptor content. Animals receiving no pellet implant showed no estrogenic responses. In addition, a methylene chloride/DMSO extract of the control pellets promoted expression of a reporter gene controlled by the estrogen receptor and demonstrated competition with 17 beta-estradiol for binding to the human estrogen receptor. It is concluded that component(s) of the control pellets possess weak estrogenic activity.

Expression of EcR and USP in Escherichia coli: purification and functional studies

The functional ecdysteroid receptor complex consists of a nuclear receptor heterodimer of ecdysteroid receptor (EcR) and ultraspiracle (USP). EcR and USP of both Chironomus tentans and Drosophila melanogaster were expressed in Escherichia coli as fusion proteins with glutathione S-transferase (GST). Cell lysis and protein solubilization with the anionic detergent sarkosyl yielded preparations of EcR and USP with properties similar to those of the endogenous receptors in various respects. The heterodimer of the expressed proteins specifically bound the labeled ecdysteroid (Ec) [3H]ponasterone A. Furthermore, it preferentially recognized the palindromic ecdysone response element (EcRE) PALI. Interestingly, binding to the PAL1 element was also observed for EcR homodimers. USP homodimers, in turn, preferentially bound to the direct repeat element DR1. When incubated with native polytene chromosomes of Chironomus, EcR/USP specifically accumulated at the early Ec-inducible puff site IV-2B.