Molecular mechanisms of action of steroid/thyroid receptor superfamily members

Structure and chromosomal locations of mouse steroid receptor coactivator gene family

The newly recognized steroid receptor coactivators (SRC-1, SRC-2, and SRC-3) belong to a homologous gene family and are important transcriptional mediators for nuclear receptors. Through fluorescence in situ hybridization, we have mapped the mouse SRC-1, SRC-2, and SRC-3 genes to chromosomal locations 12A2-A3, 1A3-A5, and 2H2-H4, respectively. By screening a mouse genomic DNA library, performing long-range polymerase chain reaction and sequencing, we have cloned and characterized the mouse SRC-3 gene. The SRC-3 gene contains 19 exons and spans more than 38 kilobases (kb). Intron sizes are variable. Intron 1 (13.5 kb) and intron 15 (4.6 kb) contribute to almost half the total length of the gene. Among 20 exons identified, exon 10 is the largest (869 bp) and encodes the receptor interaction domain. The start and stop codons for translation are in exon 2 and 20, respectively. The relationship between SRC-3 gene structure and its functional protein domains suggests that many functional domains or subdomains are encoded by individual exons. The correlation between gene structure and alternative splice variants is also discussed. In summary, we have defined the structure of mouse SRC-3 gene and found that the genes in the SRC family are located in different mouse chromosomes. This information is important for developing valuable animal models harboring multiple disruptions of the SRC gene family to study their biological functions.

Reporter gene technology: the future looks bright

Reporter gene technology is widely used to monitor the cellular events associated with signal transduction and gene expression. Based upon the splicing of transcriptional control elements to a variety of reporter genes (with easily measurable phenotypes), it "reports" the effects of a cascade of signalling events on gene expression inside cells. The principal advantage of these assays is their high sensitivity, reliability, convenience, and adaptability to large-scale measurements. This review summarises the current status of reporter gene technology including its role in monitoring gene transfer and expression and its development as a biological screen. With the advances in this technology and in detection methods, it is likely that luciferase and green fluorescent protein will become increasingly popular for the non-invasive monitoring of gene expression in living tissues and cells. Such techniques will be important in defining the molecular events associated with gene transcription, which has implications for our understanding of the molecular basis of disease and will influence our approach to gene therapy and drug development.

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.

Melatonin regulates glucocorticoid receptor: an answer to its antiapoptotic action in thymus

We have previouslyreported that low doses of melatonin inhibit apoptosis in both dexamethasone-treated cultured thymocytes (standard model for the study of apoptosis) and the intact thymus. Here we elucidate the mechanism by which this agent protects thymocytes from cell death induced by glucocorticoids. Our results demonstrate an effect of melatonin on the mRNA for antioxidant enzymes in thymocytes, also showing an unexpected regulation by dexamethasone of these mRNA. Both an effect of melatonin on the general machinery of apoptosis and a possible regulation of the expression of the cell death related genes bcl-2 and p53 are shown not to be involved. We found melatonin to down-regulate the mRNA for the glucocorticoid receptor in thymocytes (glucocorticoids up-regulate their own receptor). The decrease by melatonin of mRNA levels for this receptor in IM-9 cells (where glucocorticoids down-regulate it) demonstrates that melatonin actually down-regulates glucocorticoid receptor. These findings allow us to propose the effects of melatonin on this receptor as the likely mediator of its thymocyte protection against dexamethasone-induced cell death. This effect of melatonin, given the oxidant properties of glucocorticoids, adds another mechanism to explain its antioxidant effects.

The androgen receptor amino-terminal domain plays a key role in p160 coactivator-stimulated gene transcription

Steroid receptors are conditional transcription factors that, upon binding to their response elements, regulate the expression of target genes via direct protein interactions with transcriptional coactivators. We have analyzed the functional interactions between the androgen receptor (AR) and 160-kDa nuclear receptor coactivators. Upon overexpression in mammalian cells, these coactivators enhance the transcriptional activity of both the amino-terminal domain (NTD) and the ligand-binding domain (LBD) of the AR. The coactivator activity for the LBD is strictly ligand-controlled and depends on the nature of the DNA-binding domain to which it is fused. We demonstrate that the NTD physically interacts with coactivators and with the LBD and that this interaction, like the functional interaction between the LBD and p160 coactivators, relies on the activation function 2 (AF2) core domain. The mutation of a highly conserved lysine residue in the predicted helix 3 of the LBD (K720A), however, blunts the functional interaction with coactivators but not with the NTD. Moreover, this mutation does not affect the transcriptional activity of the full-size AR. A mutation in the NTD of activation function AF1a (I182A/L183A), which dramatically impairs the activity of the AR, has no effect on the intrinsic transcriptional activity of the NTD but interferes with the cooperation between the NTD and the LBD. Finally, p160 proteins in which the three LXXLL motifs are mutated retain most of their coactivator activity for the full-size AR, although they are no longer functional for the isolated LBD. Together, these data suggest that in the native AR the efficient recruitment of coactivators requires a functional association of the NTD with the LBD and that the binding of coactivators occurs primarily through the NTD.

A truncated human peroxisome proliferator-activated receptor alpha splice variant with dominant negative activity

The peroxisome proliferator-activated receptor alpha (PPARalpha) plays a key role in lipid and lipoprotein metabolism. However, important inter- and intraspecies differences exist in the response to PPARalpha activators. This incited us to screen for PPARalpha variants with different signaling functions. In the present study, using a RT-PCR approach a variant human PPARalpha mRNA species was identified, which lacks the entire exon 6 due to alternative splicing. This deletion leads to the introduction of a premature stop codon, resulting in the formation of a truncated PPARalpha protein (PPARalphatr) lacking part of the hinge region and the entire ligand-binding domain. RNase protection analysis demonstrated that PPARalphatr mRNA is expressed in several human tissues and cells, representing between 20-50% of total PPARalpha mRNA. By contrast, PPARalphatr mRNA could not be detected in rodent tissues. Western blot analysis using PPARalpha-specific antibodies demonstrated the presence of an immunoreactive protein migrating at the size of in vitro produced PPARalphatr protein both in human hepatoma HepG2 cells and in human hepatocytes. Both in the presence or absence of 9-cis-retinoic acid receptor, PPARalphatr did not bind to DNA in gel shift assays. Immunocytochemical analysis of transfected CV-1 cells indicated that, whereas transfected PPARalphawt was mainly nuclear localized, the majority of PPARalphatr resided in the cytoplasm, with presence in the nucleus depending on cell culture conditions. Whereas a chimeric PPARalphatr protein containing a nuclear localization signal cloned at its N-terminal localized into the nucleus and exhibited strong negative activity on PPARalphawt transactivation function, PPARalphatr interfered with PPARalphatr transactivation function only under culture conditions inducing its nuclear localization. Cotransfection of the coactivator CREB-binding protein relieved the transcriptional repression of PPARalphawt by PPARalphatr, suggesting that the dominant negative effect of PPARalphatr might occur through competition for essential coactivators. In addition, PPARalphatr interfered with transcriptional activity of other nuclear receptors such as PPARgamma, hepatic nuclear factor-4, and glucocorticoid receptor-alpha, which share CREB-binding protein/p300 as a coactivator. Thus, we have identified a human PPARalpha splice variant that may negatively interfere with PPARalphawt function. Factors regulating either the ratio of PPARalphawt vs. PPARalphatr mRNA or the nuclear entry of PPARalphatr protein should therefore lead to altered signaling via the PPARalpha and, possibly also, other nuclear receptor pathways.

Effects of glucocorticoids on the trabecular meshwork: towards a better understanding of glaucoma

Glucocorticoid effects on the human trabecular meshwork can be used as a model system in which to study glaucomatous damage to the trabecular meshwork. One of the most important risk factors for glaucoma is an elevated intraocular pressure. The administration of glucocorticoids also can cause elevated intraocular pressure in some individuals. In addition, there is suggestive evidence linking glucocorticoids with the development of glaucoma. Glucocorticoids cause multiple effects on the human trabecular meshwork including changes in extracellular matrix metabolism, organisation of the cytoskeleton, and changes in gene expression and cell function. New discoveries on the molecular mechanisms of glucocorticoid receptor action provide new opportunities to study the possible role of this receptor in the development of glaucoma. For example, alternate spliced forms of the glucocorticoid receptor, glucocorticoid receptor response element half-sites, numerous modulatory factors, and direct effects of nuclear transcription factors have been recently described. Other recent information has shown that the new glaucoma gene (GLC1A/myocilin) is induced in the human trabecular meshwork by glucocorticoids. Although the exact function of myocilin is currently unknown, it offers the opportunity to dissect the molecular pathways regulating aqueous humor outflow. Future challenges include determining (1) which glucocorticoid effects in the human trabecular meshwork are responsible for elevated intraocular pressure; and (2) the significance of these findings to the development of glaucoma.

Multiple signal input and output domains of the 160-kilodalton nuclear receptor coactivator proteins

Members of the 160-kDa nuclear receptor coactivator family (p160 coactivators) bind to the conserved AF-2 activation function found in the hormone binding domains of nuclear receptors (NR) and are potent transcriptional coactivators for NRs. Here we report that the C-terminal region of p160 coactivators glucocorticoid receptor interacting protein 1 (GRIP1), steroid receptor coactivator 1 (SRC-1a), and SRC-1e binds the N-terminal AF-1 activation function of the androgen receptor (AR), and p160 coactivators can thereby enhance transcriptional activation by AR. While they all interact efficiently with AR AF-1, these same coactivators have vastly different binding strengths with and coactivator effects on AR AF-2. p160 activation domain AD1, which binds secondary coactivators CREB binding protein (CBP) and p300, was previously implicated as the principal domain for transmitting the activating signal to the transcription machinery. We identified a new highly conserved motif in the AD1 region which is important for CBP/p300 binding. Deletion of AD1 only partially reduced p160 coactivator function, due to signaling through AD2, another activation domain located at the C-terminal end of p160 coactivators. C-terminal coactivator fragments lacking AD1 but containing AD2 and the AR AF-1 binding site served as efficient coactivators for full-length AR and AR AF-1. The two signal input domains (one that binds NR AF-2 domains and one that binds AF-1 domains of some but not all NRs) and the two signal output domains (AD1 and AD2) of p160 coactivators played different relative roles for two different NRs: AR and thyroid hormone receptor.

Steroid receptor coactivator-1 (SRC-1) enhances ligand-dependent and receptor-dependent cell-free transcription of chromatin

Progesterone receptor (PR) functions as a transcription factor that modulates the transcription of target genes in response to progesterone and other signals. The transcriptional activity of PR requires the involvement of coactivators such as steroid receptor coactivator-1 (SRC-1). To dissect the role of SRC-1 in PR transactivation, we established an in vitro transcription system with chromatin templates, in which PR induced transcription in a ligand-dependent and PRE-dependent manner. In the presence of ligand, purified PR bound to chromatin templates, resulting in chromatin remodeling. With this system, the ability of purified SRC-1 to act as a coactivator of PR was examined. SRC-1 potentiated transcription by ligand-activated PR, whereas it had no effect on transcription in the absence of ligands. As SRC-1 possesses intrinsic histone acetyltransferase activity, we tested the role of acetylation in PR-mediated transcription by using a histone deacetylase inhibitor, trichostatin A (TSA). We found that addition of TSA strongly enhanced PR-dependent transcription on chromatin but not on naked DNA template, and the effects of SRC-1 and TSA on PR transactivation were partially redundant. In addition, SRC-1 was able to potentiate PR transactivation with nonchromatin templates. Thus, our results substantiate a two-step mechanism whereby recruitment of coactivator SRC-1 by the ligand-activated PR in vivo leads to (i) chromatin remodeling through histone acetylation and (ii) recruitment/stabilization of the preinitiation complex.

Basic transcription element-binding protein (BTEB) is a thyroid hormone-regulated gene in the developing central nervous system. Evidence for a role in neurite outgrowth

Thyroid hormone (3,5,3'-triiodothyronine; T(3)) is essential for normal development of the vertebrate brain, influencing diverse processes such as neuronal migration, myelin formation, axonal maturation, and dendritic outgrowth. We have identified basic transcription element-binding protein (BTEB), a small GC box-binding protein, as a T(3)-regulated gene in developing rat brain. BTEB mRNA levels in cerebral cortex exhibit developmental regulation and thyroid hormone dependence. T(3) regulation of BTEB mRNA is neural cell-specific, being up-regulated in primary cultures of embryonic neurons (E16) and in neonatal astrocytes (P2), but not in neonatal oligodendrocytes (P2). T(3) rapidly up-regulated BTEB mRNA in neuro-2a cells engineered to express thyroid hormone receptor (TR) beta1 but not in cells expressing TRalpha1, suggesting that the regulation of this gene is specific to the TRbeta1 isoform. Several lines of evidence support a transcriptional action of T(3) on BTEB gene expression. Overexpression of BTEB in Neuro-2a cells dramatically increased the number and length of neurites in a dose-dependent manner suggesting a role for this transcription factor in neuronal process formation. However, other T(3)-dependent changes were not altered; i.e. overexpression of BTEB had no effect on the rate of cell proliferation nor on the expression of acetylcholinesterase activity.

Hormone-independent transcriptional activation and coactivator binding by novel orphan nuclear receptor ERR3

Orphan nuclear receptors share sequence homology with members of the nuclear receptor superfamily, but ligands are unknown or unnecessary. A novel orphan receptor, estrogen receptor-related protein 3 (ERR3), was identified by yeast two-hybrid screening, using the transcriptional coactivator glucocorticoid receptor interacting protein 1 (GRIP1) as bait. The putative full-length mouse ERR3 contains 458 amino acids and is closely related to two known orphan receptors ERR1 and ERR2. All the ERR family members share an almost identical DNA-binding domain, which has 68% amino acid identity with that of estrogen receptor. ERR3 bound specifically to an estrogen response element and activated reporter genes controlled by estrogen response elements, both in yeast and in mammalian cells, in the absence of any added ligand. A conserved AF-2 activation domain located in the hormone-binding domain of ERR3 was primarily responsible for transcriptional activation. The ERR3 AF-2 domain bound GRIP1 in a ligand-independent manner both in vitro and in vivo, through the LXXLL motifs of GRIP1, and GRIP1 functioned as a transcriptional coactivator for ERR3 in both yeast and mammalian cells. Expression of ERR3 in adult mouse was restricted; highest expression was observed in heart, kidney, and brain. In the mouse embryo no expression was observed at day 7, and highest expression occurred around the 11-15 day stages. Although ERR3 is much more closely related to ERR2 than to ERR1, the expression pattern for ERR3 was similar to that of ERR1 and distinct from that for ERR2, suggesting a unique role for ERR3 in development.

Homologous androgen receptor up-regulation in osteoblastic cells may be associated with enhanced functional androgen responsiveness

Although androgens have myriad effects on the skeleton, the regulation of androgen action in bone is not well understood. Androgen receptors (ARs) are known to play an important role in mediating androgen action. We have examined the effects of androgens and other sex steroids on AR levels in osteoblastic cells in vitro using two clonal human cell lines, SaOS-2 and U-2 OS. AR protein levels were quantitated both by specific androgen binding studies and Western analyses, and AR messenger RNA was measured with RNase protection assays. Potential changes in AR functionality was assessed by reporter assays. Treatment of osteoblastic cells with the nonaromatizable androgen 5alpha-dihydrotestosterone (DHT) increased specific androgen binding 2-to 4-fold. Similar increases in AR protein levels were documented by Western analysis in both cell lines. The androgen-mediated increase in receptor levels was time and dose dependent as well as androgen specific. Steady-state AR messenger RNA levels were also increased by DHT. When AR concentrations in osteoblastic cells were elevated with exogenous receptor, there was an enhancement of DHT responsiveness, measured by increased trans-activation of an androgen-responsive promoter. Thus, androgen exposure increased androgen receptor protein levels and specific androgen binding in osteoblastic cells. Androgen action as measured by androgen-mediated transcriptional activation is enhanced in the presence of elevated AR levels. Consequently, these studies have revealed an additional means by which androgens may modulate skeletal metabolism.

Identification of mouse TRAP100: a transcriptional coregulatory factor for thyroid hormone and vitamin D receptors

Nuclear hormone receptors (NRs) regulate transcription in part by recruiting distinct transcriptional coregulatory complexes to target gene promoters. The thyroid hormone receptor (TR) was recently purified from thyroid hormone-cultured HeLa cells in association with a complex of novel nuclear proteins termed TRAPs (thyroid hormone receptor-associated proteins) ranging in size from 20 to 240 kDa. The TRAP complex markedly enhances TR-mediated transcription in vitro, suggesting a coactivator role for one or more of the TRAP components. Here we present the mouse cDNA for the 100-kDa component of the TRAP complex (mTRAP100). The mTRAP100 protein contains seven LxxLL motifs thought to be potential binding surfaces for liganded NRs, yet surprisingly fails to interact with TR and other NRs in vitro. By contrast, mTRAP100 coprecipitates in vivo with another component of the TRAP complex (TRAP220), which directly contacts TR and the vitamin D receptor in a ligand-dependent manner. Our findings thus suggest that TRAP100 is targeted to NRs in association with TRAP complexes specifically containing TRAP220. Transient overexpression of mTRAP100 in mammalian cells further enhances ligand-dependent transcription by both TR and the vitamin D receptor, revealing a functional role for mTRAP100 in NR-mediated transactivation. The presence of an intrinsic mTRAP100 transactivation function is suggested by the ability of mTRAP100 to activate transcription constitutively when tethered to the GAL4 DNA-binding domain. Collectively, these findings suggest that TRAP100, in concert with other TRAPs, plays an important functional role in mediating transactivation by specific NRs.

Inhibition of estrogen receptor function promotes porcine coronary artery smooth muscle cell proliferation

Estradiol-17beta (E2) can inhibit vascular smooth muscle cell (VSMC) proliferation probably through its ability to activate its nuclear estrogen receptors (ER). Activation or inhibition of the ER by cognate permissive or non-permissive ligands, respectively, would indicate whether ER action is critical for this vascular protective effect. We investigated a previously characterized population of cultured porcine coronary artery SMCs for ER expression and for the response of these cells to estrogens and antiestrogens. Reverse transcription-polymerase chain reaction and Western blot analyses demonstrated ER mRNA and protein, respectively, in these cells. While the culture conditions required may have prevented the demonstration of physiological effects of E2, the antiestrogens, ICI 182,780 and 4-hydroxytamoxifen, stimulated VSMC proliferation. The data suggest that, by interrupting ER function, antiestrogens significantly increased the VSMC mitotic rate. This model may be used to identify ER-regulated genes that function to control the growth of these coronary artery SMCs.

Regulation of transcription by a protein methyltransferase

The p160 family of coactivators, SRC-1, GRIP1/TIF2, and p/CIP, mediate transcriptional activation by nuclear hormone receptors. Coactivator-associated arginine methyltransferase 1 (CARM1), a previously unidentified protein that binds to the carboxyl-terminal region of p160 coactivators, enhanced transcriptional activation by nuclear receptors, but only when GRIP1 or SRC-1a was coexpressed. Thus, CARM1 functions as a secondary coactivator through its association with p160 coactivators. CARM1 can methylate histone H3 in vitro, and a mutation in the putative S-adenosylmethionine binding domain of CARM1 substantially reduced both methyltransferase and coactivator activities. Thus, coactivator-mediated methylation of proteins in the transcription machinery may contribute to transcriptional regulation.

Cloning of bovine estrogen receptor beta (ERbeta): expression of novel deleted isoforms in reproductive tissues

cDNAs coding for bovine estrogen receptor beta (ERbeta) isoforms were cloned from bovine granulosa cells using a combination of several RT-PCR strategies. The cloned full-length receptor contains an open reading frame of 474 amino acids encoding a protein with high homology to the ERbeta sequences from other species. A second isoform nearly totally lacking the ligand binding domain was cloned that is expressed to relatively high levels in reproductive tissues. Expression of both ERbeta isoforms is down-regulated in corpus luteum and endometrium during the luteal phase of the female cycle. In addition, in granulosa cells several ERbeta isoforms carrying major internal deletions were detected by RT-PCR and cloned. Transient transfection studies expressing the two major bovine ERbeta isoforms together with an ERE reporter construct show estrogen-dependent transactivation by the full-length isoform, whereas the isoform lacking the ligand binding domain did not show any transactivation.

Tip60 is a nuclear hormone receptor coactivator

The androgen receptor (AR) is a member of the nuclear hormone receptor superfamily. Recent work in this field has been focused upon defining the mechanisms of transcriptional control exacted by members of this superfamily. Using a COOH-terminal region of the human AR in a yeast two-hybrid screen, we have identified Tip60 as an AR-interacting protein. In this report, we show that Tip60, which was originally identified as a coactivator for the human immunodeficiency virus TAT protein, can enhance AR-mediated transactivation in a ligand-dependent manner in LNCaP and COS-1 cell lines. In addition, our experiments show that Tip60 can also enhance transactivation through the estrogen receptor and progesterone receptor in a ligand-dependent manner; thus identifying Tip60 as a nuclear hormone receptor coactivator. Our studies also demonstrate that Tip60 co-immunoprecipitates with the full-length AR in vitro and that, in our system, Tip60 enhances transactivation to levels observed with the coactivators steroid receptor coactivator 1, p300, and CREB-binding protein. The importance of such proteins in enhancing nuclear hormone receptor-mediated transcriptional activation is widely accepted, and this work suggests that Tip60 may have an equally important role to play.

Conformational dynamics and molecular recognition: backbone dynamics of the estrogen receptor DNA-binding domain

We examined the internal mobility of the estrogen receptor DNA-binding domain (ER DBD) using NMR15N relaxation measurements and compared it to that of the glucocorticoid receptor DNA-binding domain (GR DBD). The studied protein fragments consist of residues Arg183-His267 of the human ER and residues Lys438-Gln520 of the rat GR. The15N longitudinal (R1) and transverse (R2) relaxation rates and steady state {1H}-15N nuclear Overhauser enhancements (NOEs) were measured at 30 degrees C at1H NMR frequencies of 500 and 600 MHz. The NOE versus sequence profile and calculated order parameters for ER DBD backbone motions indicate enhanced internal dynamics on pico- to nanosecond time-scales in two regions of the core DBD. These are the extended strand which links the DNA recognition helix to the second zinc domain and the larger loop region of the second zinc domain. The mobility of the corresponding regions of the GR DBD, in particular that of the second zinc domain, is more limited. In addition, we find large differences between the ER and GR DBDs in the extent of conformational exchange mobility on micro- to millisecond time-scales. Based on measurements of R2as a function of the15N refocusing (CPMG) delay and quantitative (Lipari-Szabo-type) analysis, we conclude that conformational exchange occurs in the loop of the first zinc domain and throughout most of the second zinc domain of the ER DBD. The conformational exchange dynamics in GR DBD is less extensive and localized to two sites in the second zinc domain. The different dynamical features seen in the two proteins is consistent with previous studies of the free state structures in which the second zinc domain in the ER DBD was concluded to be disordered whereas the corresponding region of the GR DBD adopts a stable fold. Moreover, the regions of the ER DBD that undergo conformational dynamics on the micro- to millisecond time-scales in the free state are involved in intermolecular protein-DNA and protein-protein interactions in the dimeric bound state. Based on the present data and the previously published dynamical and DNA binding properties of a GR DBD triple mutant which recognize an ER binding site on DNA, we argue that the free state dynamical properties of the nuclear receptor DBDs is an important element in molecular recognition upon DNA binding.

Expression of intracellular progesterone receptors in rat brain during different reproductive states, and involvement in maternal behavior

Progesterone is one of a complex of hormones which influences the occurrence of maternal behavior in rats. The present study provides information on progesterone's mechanism and possible neural site(s) of action with respect to maternal responsiveness. Progesterone can exert cellular effects by acting on membrane receptors or by acting on intracellular receptors. In the first experiment we show that RU 486 can antagonize progesterone's inhibitory effect on maternal behavior. Since RU 486 acts as an antagonist to progesterone's action at its intracellular receptor, these results support the involvement of that receptor in maternal behavior control. The second experiment employs immunocytochemical techniques to detect the number of cells in various forebrain regions which contain intracellular progesterone receptors during different reproductive states. The number of cells which contained progesterone receptors was higher toward the end of pregnancy (progesterone is presumably exerting its effects on maternal behavior at this time) when compared to either early pregnancy or lactation in the following forebrain regions: anteroventral periventricular nucleus of the preoptic area; medial preoptic area; ventral part of the bed nucleus of stria terminalis; ventrolateral division of the ventromedial nucleus; arcuate nucleus; anterior paraventricular nucleus of the hypothalamus; and medial amygdala. The possible involvement of these regions as a site or sites where progesterone might exert its effects on maternal behavior is discussed.

Differential regulation of direct repeat 3 vitamin D3 and direct repeat 4 thyroid hormone signaling pathways by the human TR4 orphan receptor

In situ hybridization analysis demonstrated that abundant testicular orphan receptor (TR4) transcripts were detected in kidney, intestine, and bone, which are vitamin D3 target organs. Cell transfection studies also demonstrated that the expression of the vitamin D3 target gene, 25-hydroxyvitamin D3 24-hydroxylase, can be repressed by TR4 through high affinity binding (Kd = 1.32 nM) to the direct repeat 3 vitamin D3 receptor response element (DR3VDRE). This TR4-mediated repression of DR3VDRE is in contrast to our earlier report that TR4 could induce thyroid hormone target genes containing a direct repeat 4 thyroid hormone response element (DR4T3RE). Electrophoretic mobility shift assay using several TR4 monoclonal antibodies when combined with either TR4-DR3VDRE or TR4-DR4T3RE showed a distinct supershifted pattern, and proteolytic analysis further demonstrated distinct digested peptides with either TR4-DR3VDRE or TR4-DR4T3RE. These results may therefore suggest that TR4 can adapt to different conformations once bound to DR3VDRE or DR4T3RE. The consequence of these different conformations of TR4-DR3VDRE and TR4-DR4T3RE may allow each of them to recruit different coregulators. The differential repression of TR4-mediated DR3VDRE and DR4T3RE transactivation by the receptor interacting protein 140, a TR4 coregulator, further strengthens our hypothesis that the specificity of gene regulation by TR4 can be modulated by protein-DNA and protein-protein interactions.

Nuclear receptor coregulators: cellular and molecular biology

Nuclear receptor coregulators are coactivators or corepressors that are required by nuclear receptors for efficient transcripitonal regulation. In this context, we define coactivators, broadly, as molecules that interact with nuclear receptors and enhance their transactivation. Analogously, we refer to nuclear receptor corepressors as factors that interact with nuclear receptors and lower the transcription rate at their target genes. Most coregulators are, by definition, rate limiting for nuclear receptor activation and repression, but do not significantly alter basal transcription. Recent data have indicated multiple modes of action of coregulators, including direct interactions with basal transcription factors and covalent modification of histones and other proteins. Reflecting this functional diversity, many coregulators exist in distinct steady state precomplexes, which are thought to associate in promoter-specific configurations. In addition, these factors may function as molecular gates to enable integration of diverse signal transduction pathways at nuclear receptor-regulated promoters. This review will summarize selected aspects of our current knowledge of the cellular and molecular biology of nuclear receptor coregulators.

Distribution of estrogen receptor-beta messenger ribonucleic acid in the male sheep hypothalamus

As a first step in determining possible influences of the newly discovered estrogen receptor (ER)-beta on reproduction, we have localized mRNA for ER-beta within the male sheep hypothalamus using in situ hybridization and a rat ER-beta cRNA probe. Highest amounts of hybridization signal were observed in the preoptic area (POA), bed nucleus of the stria terminalis, paraventricular nucleus, and supraoptic nucleus. Relatively moderate amounts of hybridization signal were observed in the retrochiasmatic area (RCH), anterior hypothalamic area, dorsomedial hypothalamus, and lateral hypothalamus. Only a low level of hybridization signal was observed in the ventromedial hypothalamus, suprachiasmatic nucleus, and arcuate nucleus. The presence of ER-beta mRNA in several areas of the male sheep hypothalamus suggests multiple functions for this receptor. The distribution of ER-beta in the ovine hypothalamus was similar to that described for the rat, suggesting a high degree of functional conservation across species. A role for ER-beta in influencing reproduction is suggested by its presence in the POA and RCH, regions of the hypothalamus that control reproduction.

Stage- and region-specific expression of estrogen receptor alpha isoforms during ontogeny of the pituitary gland

The expression time course of estrogen receptor alpha (ER alpha) was analyzed by RT-PCR in fetal and newborn rat pituitaries. In addition to the classical ER alpha messenger RNA (mRNA), three shorter transcripts were detected and subsequently cloned. Sequence analysis showed that they corresponded to ER alpha mRNAs lacking exon 3 (which encodes a zinc finger in the DNA-binding domain), exon 4 (which encodes the nuclear localization signal and part of the steroid-binding domain), or both exons 3 and 4. As analyzed by RT-PCR and ribonuclease protection assay, the respective expression levels of the different transcripts varied dramatically during pituitary development; short forms appeared 4 days before full-length ER alpha mRNA. On Western blots from rat pituitaries of different ages, an ER alpha-specific antiserum labeled four protein bands of the expected molecular weights, revealing that all four ER alpha mRNAs are translated in vivo. Immunocytochemistry, using the same antiserum, showed the ER alpha to be present first in the cytosol of intermediate lobe cells (around embryonic day 16). Only 5 days later, nuclear staining became detectable in the anterior lobe. We argue that the observed cytosolic staining will be essentially due to short ER alpha isoforms, which are indeed more abundantly expressed in the intermediate lobe. These data suggest that during pituitary development, the activity of the ER alpha might be specifically regulated by differential splicing of its primary transcript, resulting in a differential subcellular localization of the isoforms.

Hormone-dependent interaction between the amino- and carboxyl-terminal domains of progesterone receptor in vitro and in vivo

Full transcriptional activation by steroid hormone receptors requires functional synergy between two transcriptional activation domains (AF) located in the amino (AF-1) and carboxyl (AF-2) terminal regions. One possible mechanism for achieving this functional synergy is a physical intramolecular association between amino (N-) and carboxyl (C-) domains of the receptor. Human progesterone receptor (PR) is expressed in two forms that have distinct functional activities: full-length PR-B and the amino-terminally truncated PR-A. PR-B is generally a stronger activator than PR-A, whereas under certain conditions PR-A can act as a repressor in trans of other steroid receptors. We have analyzed whether separately expressed N- (PR-A and PR-B) and C-domains [hinge plus ligand-binding domain (hLBD)] of PR can functionally interact within cells by mammalian two-hybrid assay and whether this involves direct protein contact as determined in vitro with purified expressed domains of PR. A hormone agonist-dependent interaction between N-domains and the hLBD was observed functionally by mammalian two-hybrid assay and by direct protein-protein interaction assay in vitro. With both experimental approaches, N-C domain interactions were not induced by the progestin antagonist RU486. However, in the presence of the progestin agonist R5020, the N-domain of PR-B interacted more efficiently with the hLBD than the N-domain of PR-A. Coexpression of steroid receptor coactivator-1 (SRC-1) and the CREB binding protein (CBP), enhanced functional interaction between N- and C-domains by mammalian two-hybrid assay. However, addition of SRC-1 and CBP in vitro had no influence on direct interaction between purified N- and C-domains. These results suggest that the interaction between N- and C-domains of PR is direct and requires a hormone agonist-induced conformational change in the LBD that is not allowed by antagonists. Additionally, coactivators are not required for physical association between the N- and C-domains but are capable of enhancing a functionally productive interaction. In addition, the more efficient interaction of the hLBD with the N-domain of PR-B, compared with that of PR-A, suggests that distinct interactions between N- and C-terminal regions contribute to functional differences between PR-A and PR-B.

Insect pheromones--an overview of biosynthesis and endocrine regulation

This overview describes, compares, and attempts to unify major themes related to the biosynthetic pathways and endocrine regulation of insect pheromone production. Rather than developing and dedicating an entirely unique set of enzymes for pheromone biosynthesis, insects appear to have evolved to add one or a few tissue-specific auxiliary or modified enzymes that transform the products of "normal" metabolism to pheromone compounds of high stereochemical and quantitative specificity. This general understanding is derived from research on model species from one exopterygote insect order (Blattodea) and three endopterygote insect orders (Coleoptera, Diptera, and Lepidoptera). For instance, the ketone hydrocarbon contact sex pheromone of the female German cockroach, Blattella germanica, derives its origins from fatty acid biosynthesis, arising from elongation of a methyl-branched fatty acyl-CoA moiety followed by decarboxylation, hydroxylation, and oxidation. Coleopteran sex and aggregation pheromones also arise from modifications of fatty acid biosynthesis or other biosynthetic pathways, such as the isoprenoid pathway (e.g. Cucujidae, Curculionidae, and Scolytidae), or from simple transformations of amino acids or other highly elaborated host precursors (e.g. Scarabaeidae and Scolytidae). Like the sex pheromone of B. germanica, female-produced dipteran (e.g. Drosophilidae and Muscidae) sex pheromone components originate from elongation of fatty acyl-CoA moieties followed by loss of the carbonyl carbon and the formation of the corresponding hydrocarbon. Female-produced lepidopteran sex pheromones are also derived from fatty acids, but many moths utilize a species-specific combination of desaturation and chain-shortening reactions followed by reductive modification of the carbonyl carbon. Carbon skeletons derived from amino acids can also be used as chain initiating units and elongated to lepidopteran pheromones by this pathway (e.g. Arctiidae and Noctuidae). Insects utilize at least three hormonal messengers to regulate pheromone biosynthesis. Blattodean and coleopteran pheromone production is induced by juvenile hormone III (JH III). In the female common house fly, Musca domestica, and possibly other species of Diptera, it appears that during hydrocarbon sex pheromone biosynthesis, ovarian-produced ecdysteroids regulate synthesis by affecting the activities of one or more fatty acyl-CoA elongation enzyme(s) (elongases). Lepidopteran sex pheromone biosynthesis is often mediated by a 33 or 34 amino acid pheromone biosynthesis activating neuropeptide (PBAN) through alteration of enzyme activities at one or more steps prior to or during fatty acid synthesis or during modification of the carbonyl group. Although a molecular level understanding of the regulation of insect pheromone biosynthesis is in its infancy, in the male California fivespined ips, Ips paraconfusus (Coleoptera: Scolytidae), JH III acts at the transcriptional level by increasing the abundance of mRNA for 3-hydroxy-3-methylglutaryl-CoA reductase, a key enzyme in de novo isoprenoid aggregation pheromone biosynthesis.

Estrogen receptor null mice: what have we learned and where will they lead us?

All scientific investigations begin with distinct objectives: first is the hypothesis upon which studies are undertaken to disprove, and second is the overall aim of obtaining further information, from which future and more precise hypotheses may be drawn. Studies focusing on the generation and use of gene-targeted animal models also apply these goals and may be loosely categorized into sequential phases that become apparent as the use of the model progresses. Initial studies of knockout models often focus on the plausibility of the model based on prior knowledge and whether the generation of an animal lacking the particular gene will prove lethal or not. Upon the successful generation of a knockout, confirmatory studies are undertaken to corroborate previously established hypotheses of the function of the disrupted gene product. As these studies continue, observations of unpredicted phenotypes or, more likely, the lack of a phenotype that was expected based on models put forth from past investigations are noted. Often the surprising phenotype is due to the loss of a gene product that is downstream from the functions of the disrupted gene, whereas the lack of an expected phenotype may be due to compensatory roles filled by alternate mechanisms. As the descriptive studies of the knockout continue, use of the model is often shifted to the role as a unique research reagent, to be used in studies that 1) were not previously possible in a wild-type model; 2) aimed at finding related proteins or pathways whose existence or functions were previously masked; or 3) the subsequent effects of the gene disruption on related physiological and biochemical systems. The alpha ERKO mice continue to satisfy the confirmatory role of a knockout quite well. As summarized in Table 4, the phenotypes observed in the alpha ERKO due to estrogen insensitivity have definitively illustrated several roles that were previously believed to be dependent on functional ER alpha, including 1) the proliferative and differentiative actions critical to the function of the adult female reproductive tract and mammary gland; 2) as an obligatory component in growth factor signaling in the uterus and mammary gland; 3) as the principal steroid involved in negative regulation of gonadotropin gene transcription and LH levels in the hypothalamic-pituitary axis; 4) as a positive regulator of PR expression in several tissues; 5) in the positive regulation of PRL synthesis and secretion from the pituitary; 6) as a promotional factor in oncogene-induced mammary neoplasia; and 7) as a crucial component in the differentiation and activation of several behaviors in both the female and male. The list of unpredictable phenotypes in the alpha ERKO must begin with the observation that generation of an animal lacking a functional ER alpha gene was successful and produced animals of both sexes that exhibit a life span comparable to wild-type. The successful generation of beta ERKO mice suggests that this receptor is also not essential to survival and was most likely not a compensatory factor in the survival of the alpha ERKO. In support of this is our recent successful generation of double knockout, or alpha beta ERKO mice of both sexes. The precise defects in certain components of male reproduction, including the production of abnormal sperm and the loss of intromission and ejaculatory responses that were observed in the alpha ERKO, were quite surprising. In turn, certain estrogen pathways in the alpha ERKO female appear intact or unaffected, such as the ability of the uterus to successfully exhibit a progesterone-induced decidualization response, and the possible maintenance of an LH surge system in the hypothalamus. [ABSTRACT TRUNCATED]

Androgen effects on the solubility and conformational change of the androgen receptor in baculovirus expression system

To purify the androgen receptor (AR) efficiently from baculovirus expression system, we fused 6 histidine residues with the N-terminal domain of AR as a tag to specifically bind to Ni+2-affinity column. Our data indicated that adding androgen can increase the binding capacity of his-tag AR to the Ni+2-affinity column, and this increased binding capacity of AR could be due to the exposure of histidine residues of N-terminal domain induced by androgen. The androgen-enhanced binding to Ni+2-column also correlated with the increasing solubility of AR. Electrophoretic mobility shift assay further indicated that only purified AR could interact with androgen response element. Together, our data suggest that the binding of androgen to the hormone binding domain of AR may result in the conformational change of the N-terminal domain of AR and increase the hydrophilic property of AR.

The structure of the nuclear hormone receptors

The functions of the group of proteins known as nuclear receptors will be understood fully only when their working three-dimensional structures are known. These ligand-activated transcription factors belong to the steroid-thyroid-retinoid receptor superfamily, which include the receptors for steroids, thyroid hormone, vitamins A- and D-derived hormones, and certain fatty acids. The majority of family members are homologous proteins for which no ligand has been identified (the orphan receptors). Molecular cloning and structure/function analyses have revealed that the members of the superfamily have a common functional domain structure. This includes a variable N-terminal domain, often important for transactivation of transcription; a well conserved DNA-binding domain, crucial for recognition of specific DNA sequences and protein:protein interactions; and at the C-terminal end, a ligand-binding domain, important for hormone binding, protein: protein interactions, and additional transactivation activity. Although the structure of some independently expressed single domains of a few of these receptors have been solved, no holoreceptor structure or structure of any two domains together is yet available. Thus, the three-dimensional structure of the DNA-binding domains of the glucocorticoid, estrogen, retinoic acid-beta, and retinoid X receptors, and of the ligand-binding domains of the thyroid, retinoic acid-gamma, retinoid X, estrogen, progesterone, and peroxisome proliferator activated-gamma receptors have been solved. The secondary structure of the glucocorticoid receptor N-terminal domain, in particular the taul transcription activation region, has also been studied. The structural studies available not only provide a beginning stereochemical knowledge of these receptors, but also a basis for understanding some of the topological details of the interaction of the receptor complexes with coactivators, corepressors, and other components of the transcriptional machinery. In this review, we summarize and discuss the current information on structures of the steroid-thyroid-retinoid receptors.

Polyglutamine-expanded androgen receptors form aggregates that sequester heat shock proteins, proteasome components and SRC-1, and are suppressed by the HDJ-2 chaperone

Spinal bulbar muscular atrophy is a neurodegenerative disorder caused by a polyglutamine expansion in the androgen receptor (AR). We show in transiently transfected HeLa cells that an AR containing 48 glutamines (ARQ48) accumulates in a hormone-dependent manner in both cytoplasmic and nuclear aggregates. Electron microscopy reveals both types of aggregates to have a similar ultrastructure. ARQ48 aggregates sequester mitochondria and steroid receptor coactivator 1 and stain positively for NEDD8, Hsp70, Hsp90 and HDJ-2/HSDJ. Co-expression of HDJ-2/HSDJ significantly represses aggregate formation. ARQ48 aggregates also label with antibodies recognizing the PA700 proteasome caps but not 20S core particles. These results suggest that ARQ48 accumulates due to protein misfolding and a breakdown in proteolytic processing. Furthermore, the homeostatic disturbances associated with aggregate formation may affect normal cell function.

Altered expression of exon 6 deleted progesterone receptor variant mRNA between normal human breast and breast tumour tissues

The progesterone receptor (PR) is an important prognostic marker in breast cancer as well as a marker of responsiveness to endocrine therapies. The presence of several exon-deleted PR variant mRNAs in both normal and neoplastic breast samples has recently been reported. Amongst them, a variant mRNA deleted in exon 6 (D6-PR mRNA) that if translated would encode a truncated PR-like protein missing the hormone binding domain and one of the transactivating domains of the wild-type PR protein. In order to determine whether changes in D6-PR variant expression could occur during tumorigenesis, its expression was investigated by reverse transcription and polymerase chain reaction in ten normal reduction mammoplasty samples, nine breast tumours with high PR levels (> 100 fmol mg(-1) protein) and eight breast tumours with low PR levels (< 15 fmol mg(-1) protein), as determined by ligand binding assay. The relative expression of D6-PR to wild-type PR mRNA was lower (P< 0.01 ) in normal than in all tumour breast samples. Moreover, a trend to lower (P < 0.1) relative D6-PR expression was observed in high PR tumours, compared to low PR tumours. These data suggest that increased expression of D6-PR occurs during tumorigenesis.

Structure and hormone responsiveness of the gene encoding the alpha-subunit of the rat amiloride-sensitive epithelial sodium channel

The rat amiloride-sensitive epithelial sodium channel (rENaC) is the rate-limiting step for vectorial transport of Na+ across tight epithelia. The complex is composed of three subunits, alpha, beta, and gamma. Expression of the subunits has been shown to be tissue-specific and developmentally and hormonally regulated. To study mechanisms involved in transcriptional regulation of alpharENaC, we determined the genomic organization of the alpharENaC gene. By 5' rapid amplification of cDNA ends and primer extension, two transcriptional start sites were detected 453 base pairs (bp) apart, resulting in alternative 5' untranslated region (UTR) lengths of 515 or 62 bp. The longer 5' UTR is more prevalent in fetal lung than in adult lung or kidney. The 5' untranslated and coding regions are contained within 12 exons, with the translation start site located within the first exon. Sequence analysis of approximately 1,500 bp of 5' flanking DNA identified putative binding sites for transcription factors PEA3, SP1, AP-1, nuclear factor-kappaB, and thyroid and glucocorticoid receptors. alpharENaC promoter-reporter gene constructs produced low levels of reporter gene activity in transiently transfected cells, which could be increased by dexamethasone (DEX) treatment. Tri-iodothyronine treatment alone had no effect but potentiated stimulation by DEX.

Gene expression in the epididymis

The epididymis is a tubular organ exhibiting vectorial functions of sperm concentration, maturation, transport, and storage. The molecular basis for these functions is poorly understood. However, it has become increasingly clear that regional differences along the length of the duct play a role in epididymal physiology and that region-specific gene expression is involved in the formation of these differences. Although not an overtly segmented organ, the epididymis consists of a series of highly coiled "zones," separated by connective tissue septulae and distinct by cell morphology and their pattern of gene expression. Thus, it constitutes an interesting mammalian model to study how pattern formation is achieved by differential gene activity. A large number of epididymis-expressed genes have been cloned and analyzed at the molecular level, most of them have been characterized by a distinct temporal and spatial expression pattern within the organ. Only recently have theories been developed about how and when during ontogenesis this pattern formation takes place and what its significance might be. This review summarizes the current knowledge on regionalized gene expression in the epididymis and presents hypotheses concerning its ontogenetic origin and regulation in the adult.

The orphan nuclear receptor COUP-TFII is required for angiogenesis and heart development

The embryonic expression of COUP-TFII, an orphan nuclear receptor, suggests that it may participate in mesenchymal-epithelial interactions required for organogenesis. Targeted deletion of the COUP-TFII gene results in embryonic lethality with defects in angiogenesis and heart development. COUP-TFII mutants are defective in remodeling the primitive capillary plexus into large and small microcapillaries. In the COUP-TFII mutant heart, the atria and sinus venosus fail to develop past the primitive tube stage. Reciprocal interactions between the endothelium and the mesenchyme in the vascular system and heart are essential for normal development of these systems. In fact, the expression of Angiopoietin-1, a proangiogenic soluble factor thought to mediate the mesenchymal-endothelial interactions during heart development and vascular remodeling, is down-regulated in COUP-TFII mutants. This down-regulation suggests that COUP-TFII may be required for bidirectional signaling between the endothelial and mesenchymal compartments essential for proper angiogenesis and heart development.

Tbx5 is essential for heart development

Mutations in the Tbx5 transcription factor cause heart septal defects found in human Holt-Oram Syndrome. The complete extent to which Tbx5 functions in heart development, however, has not been established. Here we show that, in Xenopus embryos, Tbx5 is expressed in the early heart field, posterior to the cardiac homeobox transcription factor, Nkx2.5. During morphogenesis, Tbx5 is expressed throughout the heart tube except the anterior portion, the bulbus cordis. When Tbx5 activity is antagonized with a hormone-inducible, dominant negative version of the protein, the heart fails to develop. These results suggest that, in addition to its function in heart septation, Tbx5 has a more global role in cardiac specification and heart development in vertebrate embryos.

Protein kinase A linked phosphorylation mediates triiodothyronine induced actin gene expression in developing brain

In the developing rat cerebra, triiodothyronine (T3) stimulates actin mRNA by acting predominantly at the level of transcription whereas tubulin mRNA is enhanced primarily by post-transcriptional regulation. We report here that in primary cultures of rat cerebra, the T3-induced actin gene expression is mediated by phosphorylation events. Inhibition of protein kinase A (PKA), but not of protein kinase C (PKC) or tyrosine kinase, totally blocked the induction of actin mRNA by T3. Under identical conditions, induction of tubulin mRNA by T3 was virtually unaffected by all the inhibitors. Activators of PKA, but not of PKC, potentiated the T3-induced actin gene expression, both at mRNA and protein level, by about 2-fold. In the absence of T3, neither the inhibitor nor the activator of PKA had any significant effect on this induction. The involvement of PKA in mediating the induction of actin mRNA by T3 was confirmed by transfecting primary cultures of rat cerebra with an expression vector encoding the protein kinase A inhibitor which totally abolished the induction. T3 is shown to enhance the phosphorylation of the thyroid hormone receptor, TRalpha, by about 2-fold but the level of phosphorylation of TRbeta remained virtually unaffected.

A steroid receptor coactivator, SRA, functions as an RNA and is present in an SRC-1 complex

Nuclear receptors play critical roles in the regulation of eukaryotic gene expression. We report the isolation and functional characterization of a novel transcriptional coactivator, termed steroid receptor RNA activator (SRA). SRA is selective for steroid hormone receptors and mediates transactivation via their amino-terminal activation function. We provide functional and mechanistic evidence that SRA acts as an RNA transcript; transfected SRA, unlike other steroid receptor coregulators, functions in the presence of cycloheximide, and SRA mutants containing multiple translational stop signals retain their ability to activate steroid receptor-dependent gene expression. Biochemical fractionation shows that SRA exists in distinct ribonucleoprotein complexes, one of which contains the nuclear receptor coactivator steroid receptor coactivator 1. We suggest that SRA may act to confer functional specificity upon multiprotein complexes recruited by liganded receptors during transcriptional activation.

A corepressor and chicken ovalbumin upstream promoter transcriptional factor proteins modulate peroxisome proliferator-activated receptor-gamma2/retinoid X receptor alpha-activated transcription from the murine lipoprotein lipase promoter

Complex physiological stimuli differentially regulate the tissue-specific transcription of the lipoprotein lipase (LPL) gene. A conserved DNA recognition element (-171 to -149 bp) within the promoter functions as a transcriptional enhancer when bound by the peroxisome proliferator-activated receptor-gamma2 (PPARgamma2)/retinoid X receptor alpha (RXRalpha) heterodimer, but serves as a transcriptional silencer in the presence of unidentified double and single stranded DNA-binding proteins. To address this apparent paradox, the current study examined the effect of two classes of candidate comodulatory proteins, COUP-TF (chicken ovalbumin upstream promoter transcriptional factor) and the corepressor SMRT (silencing mediator of retinoic acid receptor and thyroid receptor). The expression of COUP-TF was detected by Western and Northern blots in a preadipocyte 3T3-L1 cell model during periods corresponding to increased LPL transcription. Cotransfection of COUP-TF expression constructs in the renal epithelial 293T cell line significantly increased transcription from the LPL promoter in synergy with PPARgamma2/RXRalpha heterodimers. The COUP-TFII (ARP-1) protein specifically bound the LPL PPAR recognition element inelectromobility shift assays and interacted directly with the ligand-binding domain of PPARgamma in pull-down experiments. In contrast, cotransfection of SMRT repressed PPARgamma2/ RXRalpha-mediated LPL transcription in the absence or presence of COUP-TFII (ARP-1). The interaction between PPARgamma2 and SMRT localized to the receptor-interactive domain 2 (amino acids 1260-1495) of the SMRT protein based on cotransfection and pull-down assays. These in vitro data indicate that COUP-TF proteins and SMRT modulate PPARgamma-mediated LPL transcription in the 293T cell line.

Induction of TR4 orphan receptor by retinoic acid in human HaCaT keratinocytes

Human TR4 orphan receptor (TR4) can modulate the transcriptional activity of the reporter gene containing an AGGTCA direct repeat-hormone response element. Here we studied the potential role of TR4 in human HaCaT keratinocytes. Using a chloramphenicol acetyl-transferase reporter gene assay, it was shown that TR4 can suppress retinoic acid-induced transactivation by 47.3% in human HaCaT keratinocytes. Electrophoretic mobility shift assay indicated that this suppression may be due to TR4 binding with higher affinity to the retinoic acid response element than retinoid receptors. Western blot analysis further suggested that retinoic acid can increase the expression of TR4 protein in human HaCaT keratinocytes, indicating that TR4 acts as a negative feedback modulator for retinoic acid action. Interestingly, TR4 expression is increased in normal human keratinocytes when substituting a low calcium medium with a high calcium medium. Together, our data suggested, for the first time, that an orphan receptor, such as TR4, may play an important part in retinoid-mediated signaling pathways in human keratinocytes, providing a new insight into keratinocyte biology.

Suppression of gene expression by tethering KRAB domain to promoter of ER target genes

Estrogens play an important role in the development and progression of breast cancer. Although estrogen antagonist treatment often results in the arrest or remission of breast cancer growth, most breast cancers recur and become resistant to estrogen ablative therapy. The molecular mechanisms underlying these actions remain largely undefined. It is hypothesized that tumor cells of an advanced stage may develop compensatory pathways to stimulate the expression of estrogen receptor (ER) target genes or downstream events, independent of estrogen action. In this study, we developed a chimeric repressor to turn off ER target genes with the aim of directly investigating the role of ER target genes in tumor progression. The chimeric repressor contains the ER DNA-binding domain that recognizes estrogen response elements (EREs), a Krupple-associated box (KRAB) repressor domain which silences target genes when tethered to their promoter regions and a truncated progesterone ligand-binding domain which responds only to the exogenous synthetic ligand, RU486. The ability of the chimeric repressor to block ER mediated transcription was assessed in transient transfection assays. ER-induced reporter activity was inhibited by the repressor in a dose-dependent manner, with the maximum effect of more than 80% reduction. The inhibitory activity of the chimeric repressor was tightly under the control of RU486. Effective suppression by the repressor on the natural promoter of ER target gene, complement factor 3 (C3), was also observed. The inhibitory activity was specific to ER, since the repressor has no effect on other nuclear receptor systems tested. Furthermore, the repressor could inhibit the 4-hydroxy-tamoxifen (4OH-T)-induced ER activity. Taken together, our results demonstrate that the inducible repressor we have designed could specifically inhibit ER target gene expression in response to an exogenous synthetic ligand. This repressor will provide a useful tool to study the role of ER target genes in breast cancer progression and it may be potentially useful for gene therapy of breast cancer.

Upregulation of mineralocorticoid- and glucocorticoid-receptor gene expression by Sp-I

The mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) belong to the steroid/thyroid hormone superfamily of ligand-induced transcription factors. Both activate the human Na/K ATPase alpha1 and beta1 genes transcriptionally. To assess the role of the transcription factor Sp1 and the nuclear factor I (NF-I), in MR- and GR-mediated gene expression using the human Na/K ATPase beta1 full-length promoter, we have examined the functions of Sp-I and NF-I functions in two different cell lines, COS-1 and T-84. By transient transfections we have shown that Sp-I significantly enhances MR and GR expression, whereas NF-I had negligible effect. We propose that the transcriptional enhancement could be through a direct interaction physically between MR or GR with Sp1 that allows other factors to bind the responsive element resulting in synergistic upregulation of transcription.

Nuclear receptor coactivators: multiple enzymes, multiple complexes, multiple functions

Nuclear receptors are ligand-inducible transcription factors which mediate the physiological effects of steroid, thyroid and retinoid hormones. By regulating the assembly of a transcriptional preinitiation complex at the promoter of target genes, they enhance the expression of these genes in response to hormone. Recent evidence suggests that nuclear receptors act in part by recruiting multiple coregulator proteins which may have specific functions during transcriptional initiation. Liganded receptors recruit members of the SRC family, a group of structurally and functionally related transcriptional coactivators. Receptors also interact with the transcriptional cointegrators p300 and CBP, which are proposed to integrate diverse afferent signals at hormone-regulated promoters. p300/CBP and members of the SRC coactivator family have intrinsic histone acetyltransferase activity which is believed to disrupt the nucleosomal structure at these promoters. Other nuclear receptor coactivators include a member of the SWI/SNF complex, BRG-1, which couples ATP hydrolysis to chromatin remodelling, and the E3 ubiquitin-protein ligases E6-AP and RPF-1. Finally, nuclear receptor coactivators appear to be organized into preformed subcomplexes, an arrangement that may facilitate their efficient assembly into diverse higher order configurations.

G-protein signalling pathways and oestrogen: a role of balanced maintenance in osteoblasts

Oestrogen (E2) is an important regulator of bone cell function and alterations in oestrogen levels may cause abnormal bone metabolism in vivo. In this study we examined the long term effects of 17beta-oestradiol (17beta-E2) on G-proteins and the secondary signalling pathways of phospholipase C (PLC), cyclic adenosine monophosphate (cAMP), and 1,4,5-inositol triphosphate (IP3). Cells from neonatal mouse calvariae were cultured in phenol red-free RPMI 1640 medium supplemented with charcoal stripped foetal calf serum for 192 h with either oestrogen (10(-8) M), or oestrogen withdrawal after 48 h. Cultures were stimulated for the final 48 h with IL-6 (10(-10) M), or left unstimulated. Western blot analysis was undertaken on osteoblast membrane preparations obtained by 10 mM Tris-HCl, 0.1 mM EDTA pH 7.8 and centrifugation at 40,000 x g for 2 h. For cAMP study, cells were stimulated with IL-6 for either 15 min or 30 min. Intracellular cAMP was extracted from cells and measured by ELISA methodology. For the IP3 assay, cells were stimulated with IL-6 for 20 s and IP3 levels measured using radioimmunoassay. The blots revealed increased levels of Gialpha-, and Gqalpha-proteins with oestrogen withdrawal and IL-6 stimulation. This was in comparison to cells which were unstimulated, or stimulated with IL-6 with continuous 17beta-E2, or IL-6 alone. Gsalpha expression decreased with oestrogen withdrawal compared to the control. Limited amounts of Gialpha-, Gsalpha-, and Gqalpha-proteins were identified with continuous 17beta-E2. The levels of PLC isoforms PLCbeta1-2 were not affected by the differing oestrogen conditions. The cAMP production induced by IL-6 stimulation for 30 min and withdrawal of 17beta-E2 was lower and significantly different compared to the control study (P<0.05). Also IL-6 activation with continuous oestradiol increased cAMP levels and was significantly different from the control cells (P<0.01). However, 17beta-E2 had no effect on the formation of intracellular IP3, although IL-6 significantly lowered IP3 levels in all the groups compared to the control (P<0.01). These results suggest that oestrogen modulates the signal transduction pathways of G-protein molecules, and the secondary pathways of cAMP in mouse osteoblast-like cells.

Stable transfection of PC12 cells with estrogen receptor (ERalpha): protective effects of estrogen on cell survival after serum deprivation

Potential protective effects of the gonadal steroid estrogen on neurons are of particular interest in aging, neurodegenerative disease, and other traumatic conditions. In this study, we examined the hypothesis that estrogen, acting through the estrogen receptor (ERalpha), can enhance neuronal cell survival in the face of serious apoptotic challenge. PC12 cells were transfected with full-length rat ERalpha cDNA and a number of stable transfectants that expressed ER mRNA and protein (PCER cells) at levels comparable to those present in uterus or the MCF7 breast cancer cell line were obtained. A control line of cells transfected with vector DNA alone (PCCON cells) was used for comparisons. The apoptotic challenge used in the experiments was serum-free media, as it is well established that undifferentiated PC12 cells rapidly undergo cell death via apoptosis under those conditions. Estrogen treatment of PCER cells markedly increased the viability of these cells relative to PCCON cells in serum-free media, as assessed by trypan blue staining and TUNEL staining. We also examined the mitotic effects of estrogen treatment. While estrogen significantly stimulated bromodeoxy uridine (BrdU) incorporation into PCER cells in low-serum, but otherwise steroid-free media, no BrdU incorporation occurred in serum-free media. Mitotic effects of estrogen in low-serum steroid-free media were completely abolished by treatment with the estrogen receptor antagonist ICI 182,780. From this we conclude that the effects of estrogen on PCER cells in serum-free media can be attributed to increased cell survival, rather than proliferation.

COUP-TF upregulates NGFI-A gene expression through an Sp1 binding site

The formation of various tissues requires close communication between two groups of cells, epithelial and mesenchymal cells. COUP-TFs are transcription factors which have been shown to have functions in embryonic development. COUP-TFI is expressed mainly in the nervous system, and its targeted deletion leads to defects in the central and peripheral nervous systems. COUP-TFII is highly expressed in the mesenchymal component of the developing organs. A null mutation of COUP-TFII results in the malformation of the heart and blood vessels. From their expression pattern, we proposed that COUP-TFs regulate paracrine signals important for mesenchymal cell-epithelial cell interactions. In order to identify genes regulated by COUP-TF in this process, a rat urogenital mesenchymal cell line was stably transfected with a COUP-TFI expression vector. We found that NGFI-A, a gene with important functions in brain, organ, and vasculature development, has elevated mRNA and protein levels upon overexpression of COUP-TFI in these cells. A study of the promoter region of this gene identified a COUP-TF-responsive element between positions -64 and -46. Surprisingly, this region includes binding sites for members of the Sp1 family of transcription factors but no COUP-TF binding site. Mutations that abolish the Sp1 binding activity also impair the transactivation of the NGFI-A promoter by COUP-TF. Two regions of the COUP-TF molecule are shown to be important for NGFI-A activation: the DNA binding domain and the extreme C terminus of the putative ligand binding domain. The C-terminal region is likely to be important for interaction with coactivators. In fact, the coactivators p300 and steroid receptor activator 1 can enhance the transactivation of the NGFI-A promoter induced by COUP-TFI. Finally, we demonstrated that COUP-TF can directly interact with Sp1. Taken together, these results suggest that NGFI-A is a target gene for COUP-TFs and that the Sp1 family of transcription factors mediates its regulation by COUP-TFs.

Cloning and characterization of human prostate coactivator ARA54, a novel protein that associates with the androgen receptor

Androgen receptor (AR) is a member of the steroid receptor superfamily that may require coactivators for proper or maximal transactivation. Using a yeast two-hybrid screening followed by mammalian cell analyses, we identified a novel ligand-dependent AR-associated protein, ARA54, which consists of 474 amino acids with a molecular mass of 54 kDa. We demonstrated that ARA54 might function as a preferential coactivator for AR-mediated transactivation in human prostate cancer DU145 cells. Interestingly, our data also showed that ARA54 could significantly enhance the transcriptional activity of LNCaP mutant AR (ARt877a) but not wild type AR or another mutant AR (ARe708k) in the presence of 10 nM 17beta-estradiol or 1 microM hydroxyflutamide. These results imply that both ARA54 and the positions of the AR mutation (877 versus 708) might contribute to the specificity of AR-mediated transactivation. Our findings further demonstrated that the C-terminal domain of ARA54 can serve as a dominant negative inhibitor and exogenous full-length ARA54 can reverse this squelching effect on AR transcriptional activity. Co-expression of ARA54 with other AR coactivators, such as ARA70 or SRC-1, showed additive stimulation of AR-mediated transactivation, which indicates that these cofactors may function individually as AR coactivators to induce AR target gene expression. Through our findings, we have identified and characterized a novel AR coactivator, ARA54, which may play an important role in the AR signaling pathway in human prostate.