The steroid hormone estrogen plays a critical role in female development and homeostasis. Estrogen mediates its effects through binding and activation of specific estrogen receptors alpha (ERα) and beta (ERβ), members of the steroid/nuclear receptor family of ligand-induced transcription factors. Due to their intimate roles in genomic and nongenomic signaling pathways, these hormones and their receptors have been also implicated in the pathologies of a variety of cancers and metabolic disorders, and have been the target of large therapeutic development efforts. The binding of estrogen to its respective receptors initiates a cascade of events that include receptor dimerization, nuclear localization, DNA binding and recruitment of co-regulatory protein complexes. In this manuscript, we investigate the potential for manipulating steroid receptor gene expression activity through the development of bivalent steroid hormones that are predicted to facilitate hormone receptor dimerization events. Data are presented for the development and testing of novel estrogen dimers, linked through their C-17 moiety, that can activate estrogen receptor alpha (ERα)-mediated transcription events with efficacy and potency equal to or greater than that of ERα’s cognate ligand, 17β-estradiol. These bivalent estrogen structures open the door to the development of a variety of steroid therapeutics that could dramatically impact future drug development in this area.
ERα; steroid hormone; Girard reagent; dimerization
Steroid receptors define a family of ligand-activated transcription factors. Recent work has demonstrated that the receptors regulate distinct but overlapping gene networks, yet the mechanisms by which they do so remain unclear. We previously determined the microscopic binding energetics for progesterone receptor (PR) isoform assembly at promoters containing multiple response elements. We found that the two isoforms (PR-A and PR-B) share nearly identical dimerization and intrinsic DNA binding free energies, but maintain large differences in cooperative free energy. Moreover, cooperativity can be modulated by monovalent ion binding and promoter layout, suggesting that differences in cooperativity might control isoform-specific promoter occupancy and thus receptor function. To determine whether cooperative binding energetics are common to other members of the steroid receptor family, we dissected the thermodynamics of estrogen receptor-α (ER-α):promoter interactions. We find that the ER-α intrinsic DNA binding free energy is identical to that of the PR isoforms. This was expected, noting that receptor DNA binding domains are highly conserved. Unexpectedly, ER-α generates negligible cooperativity – orders of magnitude less than predicted based on our studies of the PR isoforms. However, analysis of the cooperativity term suggests that it reflects a balance between highly favorable cooperative stabilization and unfavorable promoter bending. Moreover, ER-α cooperative free energy is compensated for by a large increase in dimerization free energy. Collectively, the results demonstrate that steroid receptors differentially partition not only cooperative energetics, but also dimerization energetics. We speculate that this ability serves as a framework for regulating receptor-specific promoter occupancy and thus receptor-specific gene regulation.
estrogen receptor; thermodynamics; protein-DNA interactions; quantitative footprinting; analytical ultracentrifugation
Prolactin and glucocorticoid hormone are signals which regulate the transcription of milk protein genes in mammary epithelial cells. We have investigated the molecular mechanisms by which these hormones cooperate in the induction of transcription. Both hormones activate latent transcription factors in the cytoplasm of mammary epithelial cells. Prolactin exerts its effect through binding to the extracellular domain of the prolactin receptor and through receptor dimerization. This leads to the activation of a protein tyrosine kinase (Jak2), which is noncovalently associated with the cytoplasmic domain of the prolactin receptor. Jak2 phosphorylates the signal transducer and transcription activator (Stat5) which causes its dimerization and nuclear translocation where Stat5 specifically binds to sequence elements in the promoter regions of milk protein genes. In comparison, the glucocorticoid receptor is activated by a lipophilic steroid ligand in the cytoplasm which causes allosteric changes in the molecule, dimerization, and nuclear localization. It has been demonstrated that Stat5 and the glucocorticoid receptor form a molecular complex which cooperates in the induction of transcription of the beta-casein gene. We have defined the DNA sequence requirements for this cooperative mechanism and have delimited the functional domains in Stat5 and the glucocorticoid receptor that are necessary for the functional interaction. We find that the Stat5 response element (Stat5RE) within the beta-casein gene promoter is sufficient to elicit the cooperative action of Stat5 and the glucocorticoid receptor on transcription. Activation of Stat5 through phosphorylation of tyrosine 694 is an absolute prerequisite for transcription. Deletion of the transactivation domain of Stat5 results in a molecule which cannot mediate transactivation by itself but can still cooperate with the glucocorticoid receptor. Mutated variants of the glucocorticoid receptor with a nonfunctional DNA binding domain or a DNA binding domain contributed by the estrogen receptor are still able to cooperate with Stat5 in transcriptional induction. Deletion of the ligand binding domain of the glucocorticoid receptor does not impede cooperation with Stat5, whereas deletion of the AF-1 transactivation domain does prevent cooperation. Our results indicate that the glucocorticoid receptor acts as a ligand-dependent coactivator of Stat5 independently of its DNA binding function.
Gonadal steroid hormones act throughout the body to elicit changes in gene expression that result in profound effects on reproductive physiology and behavior. Steroid hormones exert many of these effects by binding to their respective intracellular receptors, which are members of a nuclear receptor superfamily of transcriptional activators. A variety of in vitro studies indicate that nuclear receptor coactivators are required for efficient transcriptional activity of steroid receptors. Many of these coactivators are found in a variety of steroid hormone-responsive reproductive tissues, including the reproductive tract, mammary gland, and brain. While many nuclear receptor coactivators have been investigated in vitro, we are only now beginning to understand their function in reproductive physiology and behavior. In this review, we discuss the general mechanisms of action of nuclear receptor coactivators in steroid-dependent gene transcription. We then review some recent and exciting findings on the function of nuclear receptor coactivators in steroid-dependent brain development and reproductive physiology and behavior.
behavior; hypothalamus; neuroendocrinology; steroid hormone receptors
The steroid hormones, including estradiol (E) and progesterone, act in the brain to regulate female reproductive behavior and physiology. These hormones mediate many of their biological effects by binding to their respective intracellular receptors. The receptors for estrogens (ER) and progestins (PR) interact with nuclear receptor coactivators to initiate transcription of steroid-responsive genes. Work from our laboratory and others reveals that nuclear receptor coactivators, including steroid receptor coactivator-1 (SRC-1) and SRC-2, function in brain to modulate ER-mediated induction of the PR gene and hormone-dependent behaviors. In order for steroid receptors and coactivators to function together, both must be expressed in the same cells.
Triple-label immunofluorescence was used to determine if E-induced PR cells also express SRC-1 or SRC-2 in reproductively relevant brain regions of the female mouse.
The majority of E-induced PR cells in the medial preoptic area (61%), ventromedial nucleus of the hypothalamus (63%) and arcuate nucleus (76%) coexpressed both SRC-1 and SRC-2. A smaller proportion of PR cells expressed either SRC-1 or SRC-2, while a few PR cells expressed neither coactivator. In addition, compared to control animals, 17β-estradiol benzoate (EB) treatment increased SRC-1 levels in the arcuate nucleus, but not the medial preoptic area or the ventromedial nucleus of the hypothalamus. EB did not alter SRC-2 expression in any of the three brain regions analyzed.
Taken together, the present findings identify a population of cells in which steroid receptors and nuclear receptor coactivators may interact to modulate steroid sensitivity in brain and regulate hormone-dependent behaviors in female mice. Given that cell culture studies reveal that SRC-1 and SRC-2 can mediate distinct steroid-signaling pathways, the present findings suggest that steroids can produce a variety of complex responses in these specialized brain cells.
Estrogen receptor; Hypothalamus; Progestin receptor; Sexual behavior; Steroid receptor coactivator; Steroid receptor coactivator-1
Epigenetic changes in the nervous system are emerging as a critical component of enduring effects induced by early life experience, hormonal exposure, trauma and injury or learning and memory. Sex differences in the brain are largely determined by steroid hormone exposure during a perinatal sensitive period that alters subsequent hormonal and non-hormonal responses throughout the life span. Steroid receptors are members of a nuclear receptor transcription factor superfamily and recruit multiple proteins that possess enzymatic activity relevant to epigenetic changes such as acetylation and methylation. Thus steroid hormones are uniquely poised to exert epigenetic effects on the developing nervous system to dictate adult sex differences in brain and behavior. Sex differences in the methylation pattern in the promoter of estrogen and progesterone receptor genes are evident in newborns and persist in adults but with a different pattern. Changes in response to injury and in methyl binding proteins and steroid receptor coregulatory proteins are also reported. Many steroid-induced epigenetic changes are opportunistic and restricted to a single lifespan but new evidence suggests endocrine disrupting compounds can exert multigenerational effects. Similarly, maternal diet also induces transgenerational effects but the impact is sex specific. The study of epigenetics of sex differences is in its earliest stages, with needed advances in understanding of the hormonal regulation of enzymes controlling acetylation and methylation, coregulatory proteins, transient versus stable DNA methylation patterns and sex differences across the epigenome in order to fully understand sex differences in brain and behavior.
We previously reported that the chromatin high-mobility group protein 1 (HMG-1) enhances the sequence-specific DNA binding activity of progesterone receptor (PR) in vitro, thus providing the first evidence that HMG-1 may have a coregulatory role in steroid receptor-mediated gene transcription. Here we show that HMG-1 and the highly related HMG-2 stimulate DNA binding by other steroid receptors, including estrogen, androgen, and glucocorticoid receptors, but have no effect on DNA binding by several nonsteroid nuclear receptors, including retinoid acid receptor (RAR), retinoic X receptor (RXR), and vitamin D receptor (VDR). As highly purified recombinant full-length proteins, all steroid receptors tested exhibited weak binding affinity for their optimal palindromic hormone response elements (HREs), and the addition of purified HMG-1 or -2 substantially increased their affinity for HREs. Purified RAR, RXR, and VDR also exhibited little to no detectable binding to their cognate direct repeat HREs but, in contrast to results with steroid receptors, the addition of HMG-1 or HMG-2 had no stimulatory effect. Instead, the addition of purified RXR enhanced RAR and VDR DNA binding through a heterodimerization mechanism and HMG-1 or HMG-2 had no further effect on DNA binding by RXR-RAR or RXR-VDR heterodimers. HMG-1 and HMG-2 (HMG-1/-2) themselves do not bind to progesterone response elements, but in the presence of PR they were detected as part of an HMG-PR-DNA ternary complex. HMG-1/-2 can also interact transiently in vitro with PR in the absence of DNA; however, no direct protein interaction was detected with VDR. These results, taken together with the fact that PR can bend its target DNA and that HMG-1/-2 are non-sequence-specific DNA binding proteins that recognize DNA structure, suggest that HMG-1/-2 are recruited to the PR-DNA complex by the combined effect of transient protein interaction and DNA bending. In transient-transfection assays, coexpression of HMG-1 or HMG-2 increased PR-mediated transcription in mammalian cells by as much as 7- to 10-fold without altering the basal promoter activity of target reporter genes. This increase in PR-mediated gene activation by coexpression of HMG-1/-2 was observed in different cell types and with different target promoters, suggesting a generality to the functional interaction between HMG-1/-2 and PR in vivo. Cotransfection of HMG-1 also increased reporter gene activation mediated by other steroid receptors, including glucocorticoid and androgen receptors, but it had a minimal influence on VDR-dependent transcription in vivo. These results support the conclusion that HMG-1/-2 are coregulatory proteins that increase the DNA binding and transcriptional activity of the steroid hormone class of receptors but that do not functionally interact with certain nonsteroid classes of nuclear receptors.
Estradiol and other steroid hormones modulate the nervous system and behavior on both acute and long-term time scales. Though estradiol was originally characterized as a regulator of gene expression through the action of nuclear estrogen receptors (ER) that directly bind DNA to regulate gene expression, research over the past thirty years has firmly established that estradiol can initiate signaling pathways via extra-nuclear ERs associated with the cellular membrane, producing changes in neurons through stimulation of various intracellular signaling pathways. Several studies have determined that the classical ERs, ERα and ERβ, mediate some of these fast-acting signaling pathways through activation of G proteins. Since ERα and ERβ are not G protein-coupled receptors, the mechanisms by which ERs can stimulate signal transduction pathways are a focus of recent research. Here we discuss recent studies illustrating one mechanism by which ERα and ERβ initiate these pathways: through direct association with metabotropic glutamate receptors (mGluRs). Estradiol binding to these membrane-localized estrogen receptors results in mGluR signaling independent of glutamate. ERs are organized with mGluRs into functional signaling microdomains via caveolin proteins. The pairing of ERs to specific mGluRs via caveolins is region specific, with ERs being linked to different mGluRs in hippocampal, striatal, and other neurons. It is becoming clear that ER signaling through mGluRs is one important mechanism by which estrogens can modulate neuron and glial physiology, ultimately impacting various aspects of nervous system function.
Estradiol; metabotropic glutamate receptors; rapid; estrogen; caveolin; hippocampus; striatum
Sirtuins, which are class III NAD-dependent histone deacetylases (HDACs) that regulate a number of physiological processes, play important roles in the regulation of metabolism, aging, oncogenesis and cancer progression. More recently, a role for the sirtuins in the regulation of steroid hormone receptor signaling is emerging. In this mini-review, we will summarize current research into the regulation of estrogen, androgen, progesterone, mineralocorticoid and glucocorticoid signaling by sirtuins in cancer. Sirtuins can regulate steroid hormone signaling through a variety of molecular mechanisms, including acting as co-regulatory transcription factors, deacetylating histones in the promoters of genes with nuclear receptor binding sites, directly deacetylating steroid hormone nuclear receptors, and regulating pathways which modify steroid hormone receptors through phosphorylation. Furthermore, disruption of sirtuin activity may be an important step in the development of steroid hormone-refractory cancers.
Sirtuin; steroid hormone receptor activity/regulation; SIRT1; endocrine cancer
Steroid hormones largely exert their actions by activating nuclear receptors, which as transcription factors, powerfully influence fundamental processes of neural development. Often, steroid receptor action demonstrates remarkable specificity under different developmental, anatomical or hormonal conditions. Yet, the mechanisms underlying such specificity are poorly understood. The present study examined the anatomically-specific regulation of progestin receptor (PR) expression by estrogen receptor (ER) activation in the ventromedial nucleus (VMN) of the hypothalamus and the medial preoptic nucleus (MPN) of the neonatal female rat brain, using the selective estrogen receptor modulators (SERMs), tamoxifen and ICI 182, 780 (ICI), in the presence or absence of estradiol benzoate (EB) treatment. The results demonstrate that PR immunoreactivity (PRir) in the neonatal female MPN was significantly increased by EB and this increase was abolished by either tamoxifen or ICI treatment. In contrast, within the VMN of the same animals, EB had no effect on PRir and the SERMs only modestly decreased PRir. Interestingly, ICI acted as a true antagonist regardless of EB treatment, whereas tamoxifen acted as an ER agonist in the absence of EB in the MPN, but not the VMN, representing one of the first in vivo demonstrations of tissue-specific and estradiol-independent effects of tamoxifen on ER activation. The present results indicate that PR expression is highly dependent on oestradiol and its receptor in the MPN, but is independent of both oestradiol and ER activation within the neonatal VMN. These findings demonstrate anatomically-specific actions of estradiol and its receptor to induce PR in two brain regions controlling different aspects of female reproductive behaviors in adulthood.
estradiol; estrogen receptor; progestin receptor; SERMs; tamoxifen; development; preoptic; hypothalamus; ventromedial nucleus; VMN; VMH
The mechanism of signal transduction by the estrogen receptor (ER) is complex and not fully understood. In addition to the ER, a number of accessory proteins are apparently required to efficiently transduce the steroid hormone signal. In the absence of estradiol, the ER, like other steroid receptors, is complexed with Hsp90 and other molecular chaperone components, including an immunophilin, and p23. This Hsp90-based chaperone complex is thought to repress the ER’s transcriptional regulatory activities while maintaining the receptor in a conformation that is competent for high-affinity steroid binding. However, a role for p23 in ER signal transduction has not been demonstrated. Using a mutant ER (G400V) with decreased hormone binding capacity as a substrate in a dosage suppression screen in yeast cells (Saccharomyces cerevisiae), we identified the yeast homologue of the human p23 protein (yhp23) as a positive regulator of ER function. Overexpression of yhp23 in yeast cells increases ER transcriptional activation by increasing estradiol binding in vivo. Importantly, the magnitude of the effect of yhp23 on ER transcriptional activation is inversely proportional to the concentration of both ER and estradiol in the cell. Under conditions of high ER expression, ER transcriptional activity is largely independent of yhp23, whereas at low levels of ER expression, ER transcriptional activation is primarily dependent on yhp23. The same relationship holds for estradiol levels. We further demonstrate that yhp23 colocalizes with the ER in vivo. Using a yhp23-green fluorescent protein fusion protein, we observed a redistribution of yhp23 from the cytoplasm to the nucleus upon coexpression with ER. This nuclear localization of yhp23 was reversed by the addition of estradiol, a finding consistent with yhp23’s proposed role as part of the aporeceptor complex. Expression of human p23 in yeast partially complements the loss of yhp23 function with respect to ER signaling. Finally, ectopic expression of human p23 in MCF-7 breast cancer cells increases both hormone-dependent and hormone-independent transcriptional activation by the ER. Together, these results strongly suggest that p23 plays an important role in ER signal transduction.
Steroid hormone receptors represent a major target in drug discovery. As ligand inducible transcription factors, their activity can be modulated by small lipophilic molecules. Here we describe two panels of potent and selective luciferase reporter cell lines based on cells with low endogenous steroid receptor activity (U2OS). The panels contain reporter cell lines for estrogen receptors α and β, androgen, glucocorticoid, mineralocorticoid, and progesterone receptors. In the first panel, the activation of either synthetic, steroid response elements containing promoter or viral promoter is mediated by full-length steroid receptors. The second panel is based on the expression of the chimeric receptor, which was created by the replacement of the N-terminal part of the molecule by Gal4 DBD and that binds to multiple UAS sites in the reporter promoter. Both panels were extensively characterized by profiling 28 ligands in dose response manner in agonist and antagonist mode. We have analyzed and compared the responses to tested ligands from both panels and concluded that in general both systems generated similar qualitative response in terms of potency, efficacy, partial agonism/antagonism, mixed agonistic/antagonistic profiles and the rank of potencies was well conserved between both panels. However, we have also identified some artifacts introduced by the Gal4/LBD reporter assays in contrast to their full-length receptor reporter counterparts. Keeping in mind the advantages and drawbacks of each reporter format, these cell lines represent powerful and selective tools for profiling large compound libraries (HTS) and for detailed study of mechanisms by which compounds exert their biological effects.
U2OS; cell-based luciferase reporter assay; ligand binding domain; Gal4; nuclear hormone receptor; steroid receptor; HTS; drug discovery.
RNA synthesis was stimulated directly in a cell-free expression system by crude preparations of recombinant mouse estrogen receptor (ER). Receptor-stimulated transcription required the presence of estrogen response elements (EREs) in the test template and could be specifically inhibited by addition of competitor oligonucleotides containing EREs. Moreover, polyclonal antibodies directed against the DNA-binding region of ER inhibited ER-dependent transcription. In our cell-free expression system, hormone-free ER induced transcription in a hormone-independent manner. Evidence is presented suggesting that ER acts by facilitating the formation of a stable preinitiation complex at the target gene promoter and thus augments the initiation of transcription by RNA polymerase II. These observations lend support to our current understanding of the mechanism of steroid receptor-regulated gene expression and suggest strong conservation of function among members of the steroid receptor superfamily.
The effect of steroid hormones on multiplication of the human polyomavirus BK (BKV) was studied. Physiological concentrations of the synthetic glucocorticoid dexamethasone, progesterone R5020, or estrogen 17 beta-estradiol enhanced the permissivity of the host cell for BKV, resulting in an up to 11-fold (dexamethasone), 5-fold (progesterone), or 3-fold (17 beta-estradiol) higher virus yield. The increase in virus yield in dexamethasone-stimulated cells correlated with enhanced steady-state levels of viral transcripts. The late leader sequence of the BKV control region contains a hormone response unit composed of a nonconsensus glucocorticoid and/or progesterone response element (GRE/PRE) and a fully consensus estrogen response element (ERE). DNA-protein binding studies showed that the glucocorticoid receptor and the progesterone receptor bound to this BKV GRE/PRE-like sequence, while the estrogen receptor could bind to the BKV ERE motif. By transient transfection assays, we were able to show that these sequences can mediate steroid hormone-induced gene expression. However, no cooperative transactivation effect between the BKV GRE/PRE-like motif and BKV ERE motif was observed. This BKV hormone response unit may play an important role in vivo by enhancing a productive BKV infection, and perhaps also by reactivating a latent infection, during physiological or pathological conditions accompanied by increased steroid hormone levels.
The pregnane X receptor (PXR, NR1I2) is a ligand activated transcription factor that belongs to the nuclear hormone receptor (NR) superfamily. PXR is highly expressed in the liver and intestine, but low levels of expression have also been found in many other tissues. PXR plays an integral role in xenobiotic and endobiotic metabolism by regulating the expression of drug metabolizing enzymes and transporters, as well as genes implicated in the metabolism of endobiotics. PXR exerts its transcriptional regulation by binding to its DNA response elements as a heterodimer with the retinoid X receptor (RXR) and recruitment of a host of coactivators. The biological and physiological implications of PXR activation are broad, ranging from drug metabolism and drug-drug interactions to the homeostasis of numerous endobiotics, such as glucose, lipids, steroids, bile acids, bilirubin, retinoic acid, and bone minerals. The purpose of this article is to provide an overview on the transcriptional circuits and metabolic relevance controlled by PXR.
Nuclear receptor; gene regulation; xenobiotic receptor; xenobiotics; endobiotics
The ovarian hormones, estradiol (E) and progesterone (P) facilitate the expression of sexual behavior in female rats. E and P mediate many of these behavioral effects by binding to their respective intracellular receptors in specific brain regions. Nuclear receptor coactivators, including Steroid Receptor Coactivator-1 (SRC-1) and CREB Binding Protein (CBP), dramatically enhance ligand-dependent steroid receptor transcriptional activity in vitro. Previously, our lab has shown that SRC-1 and CBP modulate estrogen receptor (ER)-mediated induction of progestin receptor (PR) gene expression in the ventromedial nucleus of the hypothalamus (VMN) and hormone-dependent sexual receptivity in female rats. Female sexual behaviors can be activated by high doses of E alone in ovariectomized rats, and thus are believed to be ER-dependent. However, the full repertoire of female sexual behavior, in particular, proceptive behaviors such as hopping, darting and ear wiggling, are considered to be PR-dependent. In the present experiments, the function of SRC-1 and CBP in distinct ER- (Exp. 1) and PR- (Exp. 2) dependent aspects of female sexual behavior was investigated. In Exp. 1, infusion of antisense oligodeoxynucleotides to SRC-1 and CBP mRNA into the VMN decreased lordosis intensity in rats treated with E alone, suggesting that these coactivators modulate ER-mediated female sexual behavior. In Exp. 2, antisense to SRC-1 and CBP mRNA around the time of P administration reduced PR-dependent ear wiggling and hopping and darting. Taken together, these data suggest that SRC-1 and CBP modulate ER and PR action in brain and influence distinct aspects of hormone-dependent sexual behaviors. These findings support our previous studies and provide further evidence that SRC-1 and CBP function together to regulate ovarian hormone action in behaviorally-relevant brain regions.
Steroid hormone; SRC-1; CBP; Hypothalamus; Reproductive behavior; Lordosis; Proceptive behavior; Nuclear receptor coactivator
Estradiol is the most potent and ubiquitous member of a class of steroid hormones called estrogens. Fetuses and newborns are exposed to estradiol derived from their mother, their own gonads, and synthesized locally in their brains. Receptors for estradiol are nuclear transcription factors that regulate gene expression but also have actions at the membrane, including activation of signal transduction pathways. The developing brain expresses high levels of receptors for estradiol. The actions of estradiol on developing brain are generally permanent and range from establishment of sex differences to pervasive trophic and neuroprotective effects. Cellular end points mediated by estradiol include the following: 1) apoptosis, with estradiol preventing it in some regions but promoting it in others; 2) synaptogenesis, again estradiol promotes in some regions and inhibits in others; and 3) morphometry of neurons and astrocytes. Estradiol also impacts cellular physiology by modulating calcium handling, immediate-early-gene expression, and kinase activity. The specific mechanisms of estradiol action permanently impacting the brain are regionally specific and often involve neuronal/glial cross-talk. The introduction of endocrine disrupting compounds into the environment that mimic or alter the actions of estradiol has generated considerable concern, and the developing brain is a particularly sensitive target. Prostaglandins, glutamate, GABA, granulin, and focal adhesion kinase are among the signaling molecules co-opted by estradiol to differentiate male from female brains, but much remains to be learned. Only by understanding completely the mechanisms and impact of estradiol action on the developing brain can we also understand when these processes go awry.
Steroid hormone receptors act to regulate specific gene transcription primarily as steroid-specific dimers bound to palindromic DNA response elements. DNA-dependent dimerization contacts mediated between the receptor DNA binding domains stabilize DNA binding. Additionally, some steroid receptors dimerize prior to their arrival on DNA through interactions mediated through the receptor ligand binding domain. In this report, we describe the steroid-induced homomeric interaction of the rat glucocorticoid receptor (GR) in solution in vivo. Our results demonstrate that GR interacts in solution at least as a dimer, and we have delimited this interaction to a novel interface within the hinge region of GR that appears to be both necessary and sufficient for direct binding. Strikingly, we also demonstrate an interaction between GR and the mineralocorticoid receptor in solution in vivo that is dependent on the ligand binding domain of GR alone and is separable from homodimerization of the glucocorticoid receptor. These results indicate that functional interactions between the glucocorticoid and mineralocorticoid receptors in activating specific gene transcription are probably more complex than has been previously appreciated.
The Ser/Thr kinase family, RSK, has been implicated in numerous types of hormone-dependent and -independent cancers. However, there has been little consideration of RSKs as downstream mediators of steroid hormone non-genomic effects or of their ability to facilitate steroid receptor-mediated gene expression. Steroid hormone signaling can directly stimulate the MEK/ERK/RSK pathway to regulate cellular proliferation and survival in transformed cells. To date, multiple mechanisms of RSK and steroid hormone receptor-mediated proliferation/survival have been elucidated. For example, RSK enhances proliferation of breast and prostate cancer cells via its ability to control the levels of the estrogen receptor co-activator, cyclin D1. While in lung and other tumors RSK may control apoptosis via estrogen-mediated regulation of mitochondrial integrity. Thus the RSKs could be important anti-cancer therapeutic targets in many different transformed tissues. The recent discovery of RSK-specific inhibitors will advance our current understanding of RSK in transformation and drive these studies into animal and clinical models. In this review we explore the mechanisms associated with RSK in tumorigenesis and their relationship to steroid hormone signaling.
MAPK pathway; p90 ribosomal S6 kinase; estrogen receptor; androgen receptor; tumorigenesis
Steroid hormone receptors are ligand-dependent transcriptional activators that exert their effects by binding as dimers to cis-acting DNA sequences termed hormone response elements. When human progesterone receptor (PR), expressed as a full-length protein in a baculovirus system, was purified to homogeneity, it retained its ability to bind hormonal ligand and to dimerize but exhibited a dramatic loss in DNA binding activity for specific progesterone response elements (PREs). Addition of nuclear extracts from several cellular sources restored DNA binding activity, suggesting that PR requires a ubiquitous accessory protein for efficient interaction with specific DNA sequences. Here we have demonstrated that the high-mobility-group chromatin protein HMG-1, as a highly purified protein, dramatically enhanced binding of purified PR to PREs in gel mobility shift assays. This effect appeared to be highly selective for HMG-1, since a number of other nonspecific proteins failed to enhance PRE binding. Moreover, HMG-1 was effective when added in stoichiometric amounts with receptor, and it was capable of enhancing the DNA binding of both the A and B amino-terminal variants of PR. The presence of HMG-1 measurably increased the binding affinity of purified PR by 10-fold when a synthetic palindromic PRE was the target DNA. The increase in binding affinity for a partial palindromic PRE present in natural target genes was greater than 10-fold. Coimmunoprecipitation assays using anti-PR or anti-HMG-1 antibodies demonstrated that both PR and HMG-1 are present in the enhanced complex with PRE. HMG-1 protein has two conserved DNA binding domains (A and B), which recognize DNA structure rather than specific sequences. The A- or B-box domain expressed and purified from Escherichia coli independently stimulated the binding of PR to PRE, and the B box was able to functionally substitute for HMG-1 in enhancing PR binding. DNA ligase-mediated ring closure assays demonstrated that both the A and B binding domains mediate DNA flexure. It was also demonstrated in competition binding studies that the intact HMG-1 protein binds to tightly curved covalently closed or relaxed DNA sequences in preference to the same sequence in linear form. The finding that enhanced PRE binding was intrinsic to the HMG-1 box, combined with the demonstration that HMG-1 or its DNA binding boxes can flex DNA, suggests that HMG-1 facilitates the binding of PR by inducing a structural change in the target DNA.
Nuclear hormone receptors are hormone-regulated transcription factors that bind to specific sites on DNA and modulate the expression of adjacent target genes. Many nuclear hormone receptors display bimodal transcriptional properties; thyroid hormone receptors, for example, typically repress target gene expression in the absence of hormone, but activate target gene expression in the presence of hormone. The ability to repress is closely linked to the ability of the apo-receptor to physically bind to auxiliary corepressor proteins denoted SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) and N-CoR (nuclear receptor corepressor), which, in turn, help mediate the actual molecular events involved in transcriptional silencing. We report here that repression by thyroid hormone receptors can be regulated not only by cognate hormone, but also by certain tyrosine kinase signal transduction pathways, such as that represented by the epidermal growth factor-receptor. Activation of tyrosine kinase signaling leads to inhibition of T3R-mediated repression with relatively little effect on activation. These effects appear to be mediated by a kinase-initiated disruption of the ability of T3R to interact with SMRT corepressor. Intriguingly, tyrosine kinase signaling similarly disrupted the interactions of SMRT with v-Erb A, with retinoic acid receptors, and with PLZF, a nonreceptor transcriptional repressor. We conclude that tyrosine kinase signaling exerts potentially important regulatory effects on transcriptional silencing mediated by a variety of transcription factors that operate through the SMRT corepressor complex.
The numerous members of the steroid/nuclear hormone receptor superfamily act as direct transducers of circulating signals, such as steroids, thyroid hormone, and vitamin or lipid metabolites, and modulate the transcription of specific target genes, primarily as dimeric complexes. The receptors for 9-cis retinoic acid and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], RXR and VDR, respectively, as members of this superfamily, form a heterodimeric complex and bind cooperatively to vitamin D responsive elements (VDREs) to activate or repress the transcription of a multitude of genes which regulate a variety of physiological functions. To directly investigate RXR- and VDR-mediated transactivation, we developed a cell-free transcription system for 1,25(OH)2D3 signaling by utilizing crude nuclear extracts and a G-free cassette-based assay. Transcriptional enhancement in vitro was dependent on purified, exogenous RXR and VDR and was responsive to physiological concentrations of 1,25(OH)2D3. We found that RXR and VDR transactivated selectively from VDRE-linked templates exclusively as a heterodimeric complex, since neither receptor alone enhanced transcription in vitro. By the addition of low concentrations of the anionic detergent Sarkosyl to limit cell-free transcription to a single round and the use of agarose gel mobility shift experiments to assay factor complex assembly, we observed that 1,25(OH)2D3 enhanced RXR:VDR-mediated stabilization or assembly of preinitiation complexes to effect transcriptional enhancement from VDRE-linked promoter-containing DNA.
Estrogen receptors (ERs) are hormone-regulated transcription factors that regulate key aspects of reproduction and development. ERs are unusual in that they do not typically repress transcription in the absence of hormone but instead possess otherwise cryptic repressive functions that are revealed upon binding to certain hormone antagonists. The roles of corepressors in the control of these aspects of ER function are complex and incompletely understood. We report here that ERs recruit SMRT through an unusual mode of interaction involving multiple contact surfaces. Two surfaces of SMRT, located at the N- and C-terminal domains, contribute to the recruitment of the corepressor to ERs in vitro and are crucial for the corepressor modulation of ER transcriptional activity in cells. These corepressor surfaces contact the DNA binding domain of the receptor, rather than the hormone binding domain previously elucidated for other corepressor/nuclear receptor interactions, and are modulated by the ER's recognition of cognate DNA binding sites. Several additional nuclear receptors, and at least one other corepressor, N-CoR, share aspects of this novel mode of corepressor recruitment. Our results highlight a molecular mechanism that helps explain several previously paradoxical aspects of ER-mediated transcriptional antagonism, which may have a broader significance for an understanding of target gene repression by other nuclear receptors.
Although the primary signal for the activation of steroid hormone receptors is binding of hormone, there is increasing evidence that the activities of cell signaling pathways and the phosphorylation status of these transcription factors and their coregulators determine the overall response to the hormone. In some cases, enhanced cell signaling is sufficient to cause activation of receptors in medium depleted of steroids. Steroid receptors are targets for multiple kinases. Many of the phosphorylation sites contain Ser/Thr-Pro motifs implicating proline-directed kinases such as the cyclin-dependent kinases and the mitogen-activated kinases (MAPK) in receptor phosphorylation. Although some sites are constitutively phosphorylated, others are phosphorylated in response to hormone. Still others are only phosphorylated in response to specific cell signaling pathways. Phosphorylation of specific sites has been implicated not only in overall transcriptional activity, but also in nuclear localization, protein stability, and DNA binding. The studies of the roles of phosphorylation in coregulator function are more limited, but it is now well established that many of them are highly phosphorylated and that phosphorylation regulates their function. There is good evidence that some of the phosphorylation sites in the receptors and coregulators are targets of multiple signaling pathways. Individual sites have been associated both with functions that enhance the activity of the receptor, as well as with functions that inhibit activity. Thus, the specific combinations of phosphorylations of the steroid receptor combined with the expression levels and phosphorylation status of coregulators will determine the genes regulated and the biological response.
Thyroid hormone (T3) mediates diverse physiological functions including growth, differentiation, and energy homeostasis through the thyroid hormone receptors (TR). The TR bind DNA at specific recognition sequences in the promoter regions of their target genes known as the thyroid hormone response elements (TREs). Gene expression at TREs regulated by TRs is mediated by coregulator recruitment to the DNA bound receptor. This TR-coregulator interaction controls transcription of target genes by multiple mechanisms including covalent histone modifications and chromatin remodeling. Our previous studies identified a β-aminoketone as a potent inhibitor of the TR-coactivator interaction. We describe here the activity of one of these inhibitors in modulating effects of T3 signaling in comparison to an established ligand-competitive inhibitor of TR, NH-3. The β-aminoketone was found to reverse thyroid hormone induced gene expression by inhibiting coactivator recruitment at target gene promoters, thereby regulating downstream effects of thyroid hormone. While mimicking the downstream effects of a ligand competitive inhibitor at the molecular level, the β-aminoketone affects only a subset of the thyroid responsive signaling network. Thus antagonists directed to the coregulator binding site have distinct pharmacological properties relative to ligand based antagonists and may provide complementary activity in vivo.