Steroid hormone receptors: many actors in search of a plot

The structural basis of estrogen receptor/coactivator recognition and the antagonism of this interaction by tamoxifen

Ligand-dependent activation of transcription by nuclear receptors (NRs) is mediated by interactions with coactivators. Receptor agonists promote coactivator binding, and antagonists block coactivator binding. Here we report the crystal structure of the human estrogen receptor alpha (hER alpha) ligand-binding domain (LBD) bound to both the agonist diethylstilbestrol (DES) and a peptide derived from the NR box II region of the coactivator GRIP1 and the crystal structure of the hER alpha LBD bound to the selective antagonist 4-hydroxytamoxifen (OHT). In the DES-LBD-peptide complex, the peptide binds as a short alpha helix to a hydrophobic groove on the surface of the LBD. In the OHT-LBD complex, helix 12 occludes the coactivator recognition groove by mimicking the interactions of the NR box peptide with the LBD. These structures reveal the two distinct mechanisms by which structural features of OHT promote this "autoinhibitory" helix 12 conformation.

Mutations in the AF-2/hormone-binding domain of the chimeric activator GAL4.estrogen receptor.VP16 inhibit hormone-dependent transcriptional activation and chromatin remodeling in yeast

GAL4.estrogen receptor.VP16 (GAL4.ER.VP16), which contains the GAL4 DNA-binding domain, the human ER hormone binding (AF-2) domain, and the VP16 activation domain, functions as a hormone-dependent transcriptional activator in yeast (Louvion, J.-F., Havaux-Copf, B., and Picard, D. (1993) Gene (Amst.) 131, 129-134). Previously, we showed that this activator can remodel chromatin in yeast in a hormone-dependent manner. In this work, we show that a weakened VP16 activation domain in GAL4.ER.VP16 still allows hormone-dependent chromatin remodeling, but mutations in the AF-2 domain that abolish activity in the native ER also eliminate the ability of GAL4.ER.VP16 to activate transcription and to remodel chromatin. These findings suggest that an important role of the AF-2 domain in the native ER is to mask the activation potential of the AF-1 activation domain in the unliganded state; upon ligand activation, a conformational change releases AF-2-mediated repression and transcriptional activation ensues. We also show that the AF-2 domain, although inactive at simple promoters on its own in yeast, can enhance transcription by the MCM1 activator in hormone-dependent manner, consistent with its having a role in activation as well as repression in the native ER.

Regionalized metabolic activity establishes boundaries of retinoic acid signalling

The competence of a cell to respond to the signalling molecule retinoic acid (RA) is thought to depend largely on its repertoire of cognate zinc finger nuclear receptors. XCYP26 is an RA hydroxylase that is expressed differentially during early Xenopus development. In Xenopus embryos, XCYP26 can rescue developmental defects induced by application of exogenous RA, suggesting that the enzymatic modifications introduced inhibit RA signalling activities in vivo. Alterations in the expression pattern of a number of different molecular markers for neural development induced upon ectopic expression of XCYP26 reflect a primary function of RA signalling in hindbrain development. Progressive inactivation of RA signalling results in a stepwise anteriorization of the molecular identity of individual rhombomeres. The expression pattern of XCYP26 during gastrulation appears to define areas within the prospective neural plate that develop in response to different concentrations of RA. Taken together, these observations appear to reflect an important regulatory function of XCYP26 for RA signalling; XCYP26-mediated modification of RA modulates its signalling activity and helps to establish boundaries of differentially responsive and non-responsive territories.

Selective inhibition of human type 1 11beta-hydroxysteroid dehydrogenase by synthetic steroids and xenobiotics

Functional analyses were performed with microsomal human 11beta-hydroxysteroid dehydrogenase type 1 overexpressed in the yeast Pichia pastoris. Cell extracts or microsomes from transformed strains displayed dehydrogenase and reductase activities, which were up to 10 times higher than in human liver microsomes, while for whole cells cortisone reduction but no dehydrogenase activity was observed. The synthetic glucocorticoids prednisolone and prednisone were efficiently metabolized by subcellular fractions, whereas no activity was observed with dexamethasone, budesonide and deflazacort. Inhibitors found to be effective towards the recombinant 11beta-hydroxysteroid dehydrogenase include synthetic steroids and xenobiotic compounds, revealing selective inhibition of the reaction direction, useful for development of specific inhibitors.

A tale of dwarfs and drugs: brassinosteroids to the rescue

Brassinosteroids (BRs), a specific class of low abundance plant steroids, are capable of eliciting strong growth responses and a variety of physiological changes through exogenous application to plants. Recently, BRs gained general acceptance as important regulators (hormones) of plant growth and development through genetic and molecular identification and characterization of genes involved in BR biosynthesis or response in Arabidopsis, pea and tomato. This major advance in the molecular genetics of BR biosynthesis and mode of action disclosed another case of amazing functional conservation of signalling molecules utilized in plants and animals, and provided a set of extremely valuable tools for the functional analysis of BRs.

RORgamma t, a novel isoform of an orphan receptor, negatively regulates Fas ligand expression and IL-2 production in T cells

We have identified RORgamma t, a novel, thymus-specific isoform of the orphan nuclear receptor RORgamma that is expressed predominantly in CD4+ CD8+ double-positive thymocytes. Ectopic expression of RORgamma t protects T cell hybridomas from activation-induced cell death by inhibiting the upregulation of Fas ligand. Following hybridoma stimulation, RORgamma t also inhibits IL-2 production but does not affect the induction of Nur-77 and Egr-3 nor the upregulation of CD69. Both the ligand-binding and DNA-binding domains of RORgamma t are required for this effect. We propose that the role of RORgamma t expression in immature thymocytes is to inhibit Fas ligand expression and cytokine secretion following engagement of their TCR during positive or negative selection.

Correlating notch signaling with thymocyte maturation

The Notch receptor and its ligands are involved in many developmental processes. They are highly expressed in the thymus and have been implicated in the CD4 versus CD8 lineage decision. We identified the constitutively active intracellular fragment of murine Notch-1 as capable of rendering thymomas resistant to glucocorticoid-induced apoptosis. This effect was confirmed in other T cell lines and in CD4+ CD8+ DP thymocytes. Activation of the Notch signaling pathway also upregulated a number of other markers that, like steroid resistance, correlate with DP maturation into both the CD4 and CD8 lineages. These results suggest that Notch signaling is critically involved in the maturation of DP thymocytes into both CD4+ and CD8+ SP thymocytes.

Estrogen activates raf-1 kinase and induces expression of Egr-1 in MCF-7 breast cancer cells

We have investigated whether the raf-1 kinase, a downstream mediator of both receptor tyrosine kinase and protein kinase C signalling, is activated by estrogen (E2) in an estrogen receptor positive human breast cancer cell line. Autophosphorylation of raf-1 kinase was studied after treatment of MCF-7 cells with E2. E2-deprived cells contained low levels of raf-1 kinase activity. Treatment of cells for 1 min with E2 resulted in raf-1 autophosphorylation which was maximal within 5 min. Western blot analysis showed that raf-1 undergoes an electrophoretic mobility shift following E2 treatment. Egr-1 is a zinc finger-containing transcription factor which is expressed in association with raf-1 activation. Untreated MCF-7 cells expressed low levels of Egr-1 while E2 treatment resulted in an induction of egr-1 mRNA expression. These kinetics followed closely behind the E2 induction of c-myc mRNA. Egr-1 protein was similarly low in E2-deprived MCF-7 cells and was transiently increased following E2 treatment. Several studies have suggested that kinase activity may play a role in estrogen receptor (ER) activation. While activated v-raf failed to augment ER activation of transcription in transient transfection assays, a dominant negative mutant of raf-1 inhibited E2-induced transcription by 50% primarily as a result of increased baseline levels of E2 independent transcription. The results show that E2 can induce raf-1 kinase activity in MCF-7 breast cancer cells associated with the expression of an early growth response gene and modulation of ER signalling.

Phytoestrogens and diseases of the prostate gland

Both benign hyperplasia (BPH) and cancer of the prostate are manifest in men beyond the age of 50. Approximately 50% of men greater than 50 years of age will suffer from the symptoms associated with BPH, especially from bladder outlet obstruction. With the ever-increasing proportion of the population over 65 years of age worldwide, BPH is becoming an important medical problem as the world moves into the next millennium. Cancer of the prostate is the second most commonly diagnosed cancer after skin cancer in the male population of the United States, and the second most common cause of death from cancer after that of the lung. Overall, around the world the incidence of carcinoma of the prostate is increasing annually by 2-3%. Both race and geographical location have a profound influence of the prevalence of prostate cancer worldwide. Black men in the USA have the highest incidence, while the incidence is much lower in Asian men from China, Japan and Thailand. Although the prostate gland is androgen-dependent, it is now recognized that the biological actions of endocrine-related factors, such as androgens, oestrogens, glucocorticoids and certain dietary and environmental factors, are mediated within the gland by various growth regulatory factors. The growth regulatory factors such as epidermal growth factor (EGF), keratinocyte growth factors (KGF), fibroblast growth factors (FGFs) and insulin-like growth factors II and I are mitogenic and directly stimulate cell proliferation under the modulating influence of steroid hormones. Steroids are therefore essential but not directly responsible for cell proliferation. Certain plant compounds such as isoflavonoids, flavonoids and lignans have been proposed as cancer protective compounds in populations with low incidences of prostate diseases. In particular, soya contains the isoflavone genistein, a compound with many properties which could influence both endocrine and growth factor signalling pathways.

A winged-helix family member is involved in a steroid hormone-triggered regulatory circuit

A common theme emerging in eukaryotic gene regulation is that maximal gene induction requires several transcription factors acting in concert to regulate the activation of critical genes. Increasingly, nuclear receptors play key roles in orchestrating this regulation, often by integrating additional signaling pathways, through complex regulatory elements known as hormone response units. The ovalbumin gene contains one such unit, known as the steroid-dependent regulatory element. The binding of the chicken ovalbumin induced regulatory protein-I (Chirp-I) to this element occurs only in response to treatment with estrogen and glucocorticoid. Evidence presented herein demonstrates that Chirp-I has many features in common with the winged-helix (W-H) family of transcription factors. The binding sites for Chirp-I and for the W-H proteins have similar sequence recognition requirements. Northern blots establish that members of the W-H family are expressed in oviduct. Most convincing, the Chirp-I complex interacts with two different antibodies specific to W-H family members. The culmination of this work supports the hypothesis that Chirp-I is a member of the W-H family, and it lends credence to the idea that W-H proteins are essential components of some steroid hormone regulatory circuits.

Molecular mechanism of a cross-talk between estrogen and growth-factor signaling pathways

The actions of estrogen (E2) are considered to be mediated through its nuclear E2 receptor (ER). In cancer development, growth factors are shown to act synergistically with E2. Recently, we found that the mitogen-activated protein kinase, activated by growth factors, phosphorylates human ERalpha and this phosphorylation potentiates the transactivation function of human ERalpha demonstrating a novel cross-talk between E2 and growth factor-signaling pathways. In this review, the molecular mechanism of this cross-talk is discussed.

Glucocorticoid signalling--multiple variations of a common theme

Recent advances in molecular genetics have brought us closer to answering the question as to which mechanisms are used by steroid hormone receptors to control transcription. While binding to specific response elements in the promoter and enhancer regions of many genes has for a long time been considered as the major mode of action, a growing number of alternative transcriptional control mechanisms have been identified in the last couple of years. With the recent finding that DNA binding of the glucocorticoid receptor is not essential for survival, mechanisms depending on cross-talk with other transcription factors through protein-protein interactions have gained attention. In this mini-review, we will discuss the roles of DNA binding-dependent and -independent transcriptional modes of action in development and physiology as exemplified by the analysis of glucocorticoid receptor function.

Crystal structure of the ligand binding domain of the human nuclear receptor PPARgamma

The peroxisome proliferator-activated receptors (PPAR) are members of the nuclear receptor supergene family and are considered as key sensors of both lipid and glucose homeostasis. The role of the PPARgamma isoform in glucose metabolism is illustrated by the fact that anti-diabetic thiazolidinediones have been shown to be bona fide PPARgamma ligands. Here we report the crystal structure of apo-PPARgamma ligand binding domain (LBD) determined to 2.9-A resolution. Although the structure of apo-PPARgamma-LBD retains the overall fold described previously for other nuclear receptor LBDs, three distinct structural differences are evident. 1) The core AF-2 activation domain of apo-PPARgamma LBD is folded back toward the predicted ligand binding pocket similar to that observed in the holo-forms of other nuclear receptors. 2) The proposed ligand binding pocket of apo-PPARgamma-LBD is larger and more accessible to the surface in contrast to other LBDs. 3) The region of the LBD called the omega-loop is extended in PPARgamma and contains additional structural elements. Taken together, the apo-PPARgamma-LBD structure is in several aspects different from previously described LBDs. Given the central role of PPARgamma as a mediator in glucose regulation, the structure should be an important tool in the development of improved anti-diabetic agents.

Hepatocyte nuclear factor 1alpha is an accessory factor required for activation of glucose-6-phosphatase gene transcription by glucocorticoids

Deficiency of glucose-6-phosphatase (G6Pase), a key enzyme in glucose homeostasis, causes glycogen storage disease type 1a (GSD-1a), also know as von Gierke disease. Expression of the G6Pase gene is regulated by multiple hormones, including glucocorticoids. The synthetic glucocorticoid dexamethasone increased G6Pase mRNA abundance and gene transcription in H4-IIE hepatoma cells. Transient transfection assays demonstrated that the G6Pase promoter was active in H4-IIE cells only in the presence of dexamethasone. The minimal G6Pase promoter was contained within nucleotides -234/+3, which has two putative glucocorticoid response elements (GREs) at nucleotides -178/-164 (site 1) and -154/-140 (site 2). Electromobility shift and transient transfection assays showed that only GRE site 1 was required for glucocorticoid-activated transcription from the G6Pase promoter. Deletion analysis demonstrated that the DNA elements absolutely essential for glucocorticoid-stimulated transcription from the G6Pase promoter were contained within nucleotides -234/-212, encompassing binding motifs for hepatocyte nuclear factors (HNFs) 1 (-226/-212) and 4 (-231/-220). Electromobility shift and cotransfection assays showed that HNF1alpha bound to its cognate site and mediated transcription activation of the G6Pase gene by glucocorticoids.

Activation of vitellogenin II gene expression by steroid hormones in the old Japanese quail

Alterations in the basal transcription rates of eukaryotic genes are believed to involve the binding of trans-acting factor(s) with specific DNA sequences in the promoter. We show here two interrelated events for the VTGII gene of the old, non-egg laying Japanese quail: alterations in the structure of the chromatin encompassing the gene, and binding of trans-acting factors to the promoter of the gene. Estradiol/progesterone alone or together cause alterations in the conformation of the chromatin of the promoter region of the gene. This may allow free access of nuclear protein(s) to the cis-acting elements, ERE, PRE and NF1, in the promoter of the gene and cause activation of transcription.

Glucocorticoid receptor, C/EBP, HNF3, and protein kinase A coordinately activate the glucocorticoid response unit of the carbamoylphosphate synthetase I gene

A single far-upstream enhancer is sufficient to confer hepatocyte-specific, glucocorticoid- and cyclic AMP-inducible periportal expression to the carbamoylphosphate synthetase I (CPS) gene. To identify the mechanism of hormone-dependent activation, the composition and function of the enhancer have been analyzed. DNase I protection and gel mobility shift assays revealed the presence of a cyclic AMP response element, a glucocorticoid response element (GRE), and several sites for the liver-enriched transcription factor families HNF3 and C/EBP. The in vivo relevance of the transcription factors interacting with the enhancer in the regulation of CPS expression in the liver was assessed by the analysis of knockout mice. A strong reduction of CPS mRNA levels was observed in glucocorticoid receptor- and C/EBPalpha-deficient mice, whereas the CPS mRNA was normally expressed in C/EBPbeta knockout mice and in HNF3alpha and -gamma double-knockout mice. (The role of HNFbeta could not be assessed, because the corresponding knockout mice die at embryonic day 10). In hepatoma cells, most of the activity of the enhancer is contained within a 103-bp fragment, which depends for its activity on the simultaneous occupation of the GRE, HNF3, and C/EBP sites, thus meeting the requirement of a glucocorticoid response unit. In fibroblast-like CHO cells, on the other hand, the GRE in the CPS enhancer does not cooperate with the C/EBP and HNF3 elements in transactivation of the CPS promoter. In both hepatoma and CHO cells, stimulation of expression by cyclic AMP depends mainly on the integrity of the glucocorticoid pathway, demonstrating cross talk between this pathway and the cyclic AMP (protein kinase A) pathway.

Different classes of coactivators recognize distinct but overlapping binding sites on the estrogen receptor ligand binding domain

We have analyzed interaction of coactivators with the wild-type estrogen receptor alpha (ER), HEG0, and a mutant, L536P-HEG0, which is constitutively active in several transiently transfected cells and a HeLa line that stably propagates an estrogen-sensitive reporter gene. Different classes of coactivators do not recognize the ER ligand binding domain (LBD) in the same manner. Steroid receptor coactivator-1 (SRC-1), amplified in breast cancer-1 (AIB-1), transcriptional intermediary factor-1 (TIF-1), transcriptional intermediary factor-2 (TIF-2), and receptor interacting protein 140 (RIP140) interacted with HEG0 and L536P-HEG0 in the presence of estradiol, but generally not in the presence of anti-estrogens. However, ICI164,384 stimulated some interaction of RIP140 with LBDs. SRC-1, AIB-1, and RIP140 interacted constitutively with the L536P ER, whereas TIF-1 and TIF-2 interacted only weakly in the absence of hormone. Reciprocal competition for binding to the ER LBD was observed between different classes of coactivators. Moreover, coexpression of RIP140 blocked enhanced transactivation by HEG0 observed in the presence of TIF-2, suggesting that RIP140 may play a negative role in ER signaling. We conclude that constitutive activity of L536P-HEG0 is manifested to similar degrees in different cell types and likely arises from constitutive coactivator binding; different classes of coactivators recognize distinct but overlapping binding sites on the ER LBD. Finally, the observation that L536P-HEG0 interacted constitutively with AIB-1, a coactivator that has been implicated in ER signaling in breast and ovarian cancer, suggests that similar mutations in the ER may contribute to hormone-independent proliferation of breast and ovarian cells.

SXR, a novel steroid and xenobiotic-sensing nuclear receptor

An important requirement for physiologic homeostasis is the detoxification and removal of endogenous hormones and xenobiotic compounds with biological activity. Much of the detoxification is performed by cytochrome P-450 enzymes, many of which have broad substrate specificity and are inducible by hundreds of different compounds, including steroids. The ingestion of dietary steroids and lipids induces the same enzymes; therefore, they would appear to be integrated into a coordinated metabolic pathway. Instead of possessing hundreds of receptors, one for each inducing compound, we propose the existence of a few broad specificity, low-affinity sensing receptors that would monitor aggregate levels of inducers to trigger production of metabolizing enzymes. In support of this model, we have isolated a novel nuclear receptor, termed the steroid and xenobiotic receptor (SXR), which activates transcription in response to a diversity of natural and synthetic compounds. SXR forms a heterodimer with RXR that can bind to and induce transcription from response elements present in steroid-inducible cytochrome P-450 genes and is expressed in tissues in which these catabolic enzymes are expressed. These results strongly support the steroid sensor hypothesis and suggest that broad specificity sensing receptors may represent a novel branch of the nuclear receptor superfamily.

c-Fos dimerization with c-Jun represses c-Jun enhancement of androgen receptor transactivation

The transcriptional activity of the human androgen receptor (hAR), like other nuclear receptors, is dependent on accessory factors. One such factor is c-Jun, which has been shown to have a selective function of mediating androgen receptor-dependent transactivation. This c-Jun activity is inhibited by c-Fos, another protooncoprotein that can dimerize with c-Jun to form the transcription factor AP-1. Here we show that c-jun mediates hAR-induced transactivation from the promoter of the androgen-regulated gene, human kallikrein-2 (hKLK2), and c-Fos blocks this activity. Using c-Fos truncation mutants and measuring hKLK2-dependent transcription, we have determined that the bZIP region of c-Fos is required and sufficient for inhibiting c-Jun enhancement of hAR transactivation. Further truncation analysis of the bZIP shows that the c-Fos dimerization function, mediated through the leucine zipper, is essential for the negative activity, whereas DNA binding, mediated through the basic region, is dispensable. These results suggest that heterodimerization by c-Fos with c-Jun blocks c-Jun's ability to enhance hAR-induced transactivation.

Inhibition of estrogen receptor action by the orphan receptor SHP (short heterodimer partner)

SHP (short heterodimer partner) is an unusual orphan receptor that lacks a conventional DNA-binding domain. Previous results have shown that it interacts with several other nuclear hormone receptors, including the retinoid and thyroid hormone receptors, and inhibits their ligand-dependent transcriptional activation. Here we show that SHP also interacts with estrogen receptors and inhibits their function. In mammalian and yeast two-hybrid systems as well as glutathione-S-transferase pull-down assays, SHP interacts specifically with estrogen receptor-alpha (ERalpha) in an agonist-dependent manner. The same assay systems using various deletion mutants of SHP map the interaction domain with ERalpha to the same SHP sequences required for interaction with the nonsteroid hormone receptors such as retinoid X receptor and thyroid hormone receptor. In transient cotransfection assays, SHP inhibits estradiol -dependent activation by ERalpha by about 5-fold. In contrast, SHP interacts with ERbeta independent of ligand and reduces its ability to activate transcription by only 50%. These data suggest that SHP functions to regulate estrogen signaling through a direct interaction with ERalpha.

p300/CREB-binding protein enhances the prolactin-mediated transcriptional induction through direct interaction with the transactivation domain of Stat5, but does not participate in the Stat5-mediated suppression of the glucocorticoid response

Stat5 was discovered as a PRL-induced member of the Stat (signal transducer and activator of transcription) family. Its induction by many other cytokines and interleukins suggests that Stat5 plays a crucial role not only in mammary epithelial, but also in hematopoietic cells. Cell type- and promoter-specific functions of Stat5 are most likely modulated by the interaction with other transcription factors. We recently showed cross-talk between Stat5 and the glucocorticoid receptor. The activated glucocorticoid receptor forms a complex with Stat5 and enhances Stat5-mediated transcriptional induction. Conversely, Stat5 diminishes the induction of glucocorticoid-responsive genes. Here, we investigated the role of p300/CBP(CREB-binding protein), a transcriptional coactivator of several groups of transcription factors, in Stat5-mediated transactivation and in the cross-talk between Stat5 and the glucocorticoid receptor. p300/ CBP acts as a coactivator of Stat5. Its ectopic expression enhances PRL-induced Stat5-mediated transcriptional activation. Consistent with this observation, we find that the adenovirus E1A protein, which binds to p300/CBP, suppresses Stat5-induced transcriptional activation. This inhibition requires the Stat5 transactivation domain and the p300/CBP binding site of E1A. Coimmunoprecipitation and mammalian two-hybrid assays demonstrate a direct interaction between the carboxyl-terminal transactivation domain of Stat5 and p300/CBP. p300/CBP also positively interacts with the glucocorticoid receptor and enhances glucocorticoid receptor-dependent transcriptional activation of the mouse mammary tumor virus-long terminal repeat promoter. Overexpression of p300/CBP does not counteract the Stat5-mediated inhibition of glucocorticoid receptor-dependent transactivation, i.e. the repression of the glucocorticoid response by Stat5 is not a consequence of competition for limiting amounts of p300/CBP.

Transcriptional silencing is defined by isoform- and heterodimer-specific interactions between nuclear hormone receptors and corepressors

Nuclear hormone receptors are ligand-regulated transcription factors that play critical roles in metazoan homeostasis, development, and reproduction. Many nuclear hormone receptors exhibit bimodal transcriptional properties and can either repress or activate the expression of a given target gene. Repression appears to require a physical interaction between a receptor and a corepressor complex containing the SMRT/TRAC or N-CoR/RIP13 polypeptides. We wished to better elucidate the rules governing the association of receptors with corepressors. We report here that different receptors interact with different domains in the SMRT and N-CoR corepressors and that these divergent interactions may therefore contribute to distinct repression phenotypes. Intriguingly, different isoforms of a single nuclear hormone receptor class also differ markedly in their interactions with corepressors, indicative of their nonidentical actions in cellular regulation. Finally, we present evidence that combinatorial interactions between different receptors can, through the formation of heterodimeric receptors, result in novel receptor-corepressor interactions not observed for homomeric receptors.

The LXRs: a new class of oxysterol receptors

The liver X receptors (LXRs) are a family of transcription factors that were first identified as orphan members of the nuclear receptor superfamily. The identification of a specific class of oxidized derivatives of cholesterol as ligands for the LXRs has been crucial to helping understand the function of these receptors in vivo and first suggested their role in the regulation of lipid metabolism. Confirmation of this role has come from the recent analysis of LXR-deficient mice, which has demonstrated the essential function of one of these receptors in the liver as a major sensor of dietary cholesterol.

The estrogen receptor beta subtype: a novel mediator of estrogen action in neuroendocrine systems

The recent discovery that an additional estrogen receptor (ERbeta) subtype is present in many rat, mouse, and human tissues has advanced our understanding of the mechanisms underlying estrogen signalling. Ligand-binding experiments have shown specific binding of 17beta-estradiol by ERbeta with an affinity similar to that of ERalpha. The rat tissue distribution and/or the relative level of ERalpha and ERbeta expression seems to be quite different, i.e., moderate to high expression in uterus, testis, pituitary, ovary, kidney, epididymis, and adrenal for ERalpha and prostate, ovary, lung, bladder, brain, bone, uterus, and testis for ERbeta. Within the same organ it often appears that the ER subtypes are expressed in different cell types, supporting the hypothesis that the ER's may have different biological functions. The cell type-specific expression of ERalpha and ERbeta in rat prostate, testis, uterus, ovary, and brain and the distribution of ERbeta mRNA in the ERalpha knock-out mouse brain are discussed. The discovery of ERbeta suggests the existence of two previously unrecognized pathways of estrogen signalling; via the ERbeta subtype in tissues exclusively expressing this subtype and via the formation of heterodimers in tissues expressing both ER subtypes. The existence of two ER subtypes, their differential expression pattern, and different actions on certain response elements could provide explanations for the striking species-, cell-, and promoter-specific actions of estrogens and antiestrogens. The challenge for the future is to unravel the detailed physiological role of each subtype and to use this knowledge to develop the next generation of ER-targeted drugs with improved therapeutic profiles in the treatment or prevention of osteoporosis, cardiovascular system disorders, Alzheimer's disease, breast cancer, and disorders of the urogenital tract.

Coactivator TIF1beta interacts with transcription factor C/EBPbeta and glucocorticoid receptor to induce alpha1-acid glycoprotein gene expression

The transcription of the alpha1-acid glycoprotein gene is induced by inflammatory cytokines and glucocorticoids. C/EBPbeta is a major transcription factor involved in the induction of the agp gene by some cytokines. In this report, we have identified a novel transcriptional intermediary factor, TIF1beta, which could enhance the transcription of the agp gene by the glucocorticoid receptor (GR) and C/EBPbeta. TIF1beta belongs to a subgroup of RING (really interesting new gene) finger proteins that contain a RING finger preceding two B box-type fingers and a putative coiled-coil domain (RBCC domain). Immunoprecipitation experiments showed that the interaction between GR and TIF1beta is ligand independent. The overexpression of the TIF1beta gene enhances GR-regulated expression in a ligand- and glucocorticoid-responsive element (GRE)-dependent manner. TIF1beta can also augment C/EBPbeta-mediated activity on wild-type and GRE-mutated agp genes, but this augmentation is diminished when all three C/EBPbeta-binding elements are mutated. Functional and biochemical characterizations indicated that the bZIP domain of C/EBPbeta and the RBCC domain, plant homeodomain finger, and bromodomain of TIF1beta are crucial for the interactions of these proteins. Taken together, these results suggest that TIF1beta serves as a converging mediator of signal transduction pathways of glucocorticoids and some inflammatory cytokines.

Genetic dissection of glucocorticoid receptor function in mice

Upon hormone binding, the activated glucocorticoid receptor (GR) functions as a transcription factor via different modes of action to control gene expression. Recent gene-targeting studies in mice provide new insight into the role of GR in vivo and are helping decipher the molecular mechanisms underlying its actions.

Energetic methods to study bifunctional biotin operon repressor

Application of a broad range of approaches and techniques to analysis of the functional energetics of the biotin regulatory system has enabled dissection of each of the steps in the assembly of this transcriptional repression complex. Although the molecular details of the interactions are not yet completely understood, the studies described in this article have laid a solid foundation for future studies of the system. The application of kinetic and equilibrium methods to studies of binding of the allosteric effector has allow determination of the kinetic parameters governing the interaction of the protein and ligand. The kinetic parameters have, furthermore, been utilized to calculate the equilibrium parameters associated with the binding. The great advantage of using kinetic methods to study the binding process is the additional information provide about the mechanism of allosteric activation of the protein. Based on the initial observation of a kinetic time course that is consistent with the occurrence of a structural change concomitant with effector binding, additional measurements have been performed that have allowed formulation of a testable hypothesis concerning the nature and location of one locus: the structural change in the three-dimensional structure of BirA. Studies of assembly of the protein indicate the bio-5-AMP is an allosteric activator of dimerization of the protein. The dimerization is, however, weak. These results have been critical in analyzing site-specific DNA binding measurements. Application of the DNase I footprinting technique has allowed formulation of a model for association of holoBirA with bioO. Results of studies of binding of the protein to mutant operator templates, although not yielding the anticipated results, provide further insight into the mechanism of association of the protein and DNA. Two models for binding, the validity of which can be tested via the application of kinetic techniques, have been derived from these measurements. The results of quantitative studies of the biotin regulatory system can be interpreted in the context of the biological function of the system. The biotin holoenzyme ligases are a class of enzymes found across the evolutionary spectrum. Only a subset of these enzymes, including BirA, also function as transcriptional repressors. The tight binding of the allosteric effector may be understood in light of the bifunctional nature of the BirA-bio-5'-AMP complex. It is possible that the unusually high thermodynamic and kinetic stability of the complex ensures that the most probable state of the protein in vivo is the adenylate-bound form. This complex, not the unliganded protein, is active in both enzymatic transfer of biotin and site-specific DNA binding. This ensures that on depletion of the intracellular pool of apoBCCP, BirA-bio-5'-AMP accumulates and binds to bioO to repress transcription of the biotin biosynthesis operon. The intracellular demand for and synthesis of biotin are, consequently, tightly coupled in the system. The dimerization that accompanies adenylate binding to BirA appears to be significant for site-specific binding of the protein to bioO. Functionally, the simultaneous binding of the two monomers to the two operator half-sites, regardless of the kinetic mechanism by which it occurs, ensures coordinate regulation of transcription initiation from both biotin operon promoters. The multifaceted approach utilized in studies of the biotin regulatory system can serve as a model for studies of any complex transcriptional regulatory system. It is critical in elucidating the functional energetics of any of these systems that the assembly first be dissected into the constituent interactions and that each of these interactions be studied in isolation. This is not only critical for understanding the physicochemical properties of each individual contributing interaction, but is also a necessary precursor to studies of thermodynamic linkage in the system. (AB

Two wavelength femtosecond laser induced DNA-protein crosslinking

Nucleic acid-protein interactions are essential for storage, reproduction and expression of genetic information. Biochemical methods, such as dimethyl sulfate genomic footprinting, have been developed to study stable protein-DNA interactions in vivo and chemical crosslinking has been used for less stable interactions, but the chemical agents are slow, damage cells and perturb native equilibria. To avoid these perturbations, UV laser crosslinking offers an alternative, although the energies required for significant crosslinking cause extensive DNA damage. We find that a combination of femtosecond laser pulses at two different wavelengths, in the UV and the visible range, increases the crosslinking efficiency while minimizing DNA damage. This technique also allowed us to directly measure the singlet S1lifetime of native DNA (tauS1 = 3.2 +/- 0.2 ps), which is mainly determined by the lifetime of thymine [tauS1 = 2.8 +/- 0.4 ps for (dT)16], the photochemically most reactive base. Our results suggest that two wavelength femtosecond laser pulses are well suited for the identification of transcription factors interacting with defined sequences and for studying the kinetics of protein-nucleic acid interactions in intact cells.

Using yeast to study glucocorticoid receptor phosphorylation

The glucocorticoid receptor (GR) is a phosphoprotein and a member of the steroid/thyroid receptor superfamily of ligand dependent transcription factors. When the glucocorticoid receptor is expressed in yeast (Saccharomyces cerevisiae), it is competent for signal transduction and transcriptional regulation. We have studied the glucocorticoid receptor phosphorylation in yeast and demonstrated that the receptor is phosphorylated in both the absence and presence of hormone, on serine and threonine residues. This phosphorylation occurs within 15 min upon addition of radioactivity in both hormone treated and untreated cells. As reported for mammalian cells, additional phosphorylation occurs upon hormone binding and this phosphorylation is dependent on the type of the ligand. We have followed the hormone dependent receptor phosphorylation by electrophoretic mobility shift assay, and have shown that this mobility change is sensitive to phosphatase treatment. In addition, the appearance of hormone dependent phosphoisoforms of the receptor depends on the potency of the agonist used. Using this method we show that the residues contributing to the hormone dependent mobility shift are localized in one of the transcriptional activation domains, between amino acids 130-247. We altered the phosphorylation sites within this domain that correspond to the amino acids phosphorylated in mouse hormone treated cells. Using phosphopeptide maps we show that hormone changes the peptide pattern of metabolically labelled receptor, and we identify peptides which are phosphorylated in hormone dependent manner. Then we determine that phosphorylation of residues S224 and S232 is increased in the presence of hormone, whereas phosphorylation of residues T171 and S246 is constitutive. Finally, we show that in both yeast and mammalian cells the same residues on the glucocorticoid receptor are phosphorylated. Our results suggest that yeast cells would be a suitable system to study glucocorticoid receptor phosphorylation. The genetic manipulability of yeast cells, together with conservation of the phosphorylation of GR in yeast and mammalian cells and identification of hormone dependent phosphorylation, would facilitate the isolation of molecules involved in the glucocorticoid receptor phosphorylation pathway and further our understanding of this process.

Identification of a novel RING finger protein as a coregulator in steroid receptor-mediated gene transcription

Using the DNA-binding domain of androgen receptor (AR) as a bait in a yeast two-hybrid screening, we have identified a small nuclear RING finger protein, termed SNURF, that interacts with AR in a hormone-dependent fashion in both yeast and mammalian cells. Physical interaction between AR and SNURF was demonstrated by coimmunoprecipitation from cell extracts and by protein-protein affinity chromatography. Rat SNURF is a highly hydrophilic protein consisting of 194 amino acid residues and comprising a consensus C3HC4 zinc finger (RING) structure in the C-terminal region and a bipartite nuclear localization signal near the N terminus. Immunohistochemical experiments indicated that SNURF is a nuclear protein. SNURF mRNA is expressed in a variety of human and rat tissues. Overexpression of SNURF in cultured mammalian cells enhanced not only androgen, glucocorticoid, and progesterone receptor-dependent transactivation but also basal transcription from steroid-regulated promoters. Mutation of two of the potential Zn2+ coordinating cysteines to serines in the RING finger completely abolished the ability of SNURF to enhance basal transcription, whereas its ability to activate steroid receptor-dependent transcription was maintained, suggesting that there are separate domains in SNURF that mediate interactions with different regulatory factors. SNURF is capable of interacting in vitro with the TATA-binding protein, and the RING finger domain is needed for this interaction. Collectively, we have identified and characterized a ubiquitously expressed RING finger protein, SNURF, that may function as a bridging factor and regulate steroid receptor-dependent transcription by a mechanism different from those of previously identified coactivator or integrator proteins.

Activation of androgen receptor function by a novel nuclear protein kinase

Androgen receptor (AR) belongs to the nuclear receptor superfamily and mediates the biological actions of male sex steroids. In this work, we have characterized a novel 130-kDa Ser/Thr protein kinase ANPK that interacts with the zinc finger region of AR in vivo and in vitro. The catalytic kinase domain of ANPK shares considerable sequence similarity with the minibrain gene product, a protein kinase suggested to contribute to learning defects associated with Down syndrome. However, the rest of ANPK sequence, including the AR-interacting interface, exhibits no apparent homology with other proteins. ANPK is a nuclear protein that is widely expressed in mammalian tissues. Its overexpression enhances AR-dependent transcription in various cell lines. In addition to the zinc finger region, ligand-binding domain and activation function AF1 of AR are needed, as the activity of AR mutants devoid of these domains was not influenced by ANPK. The receptor protein does not appear to be a substrate for ANPK in vitro, and overexpression of ANPK does not increase the extent of AR phosphorylation in vivo. In view of this, it is likely that ANPK-mediated activation of AR function is exerted through modification of AR-associated proteins, such as coregulatory factors, and/or through stabilization of the receptor protein against degradation.

Signaling by tyrosine kinases negatively regulates the interaction between transcription factors and SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) corepressor

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.

Age-related changes in human oestrogen receptor alpha function and levels in osteoblasts

Oestrogen receptors (ERs) are present in human osteoblasts and mediate anti-resorptive effects on bone. Human osteoblast-like cells derived from different aged healthy female donors not on hormone replacement therapy were utilized under well-defined conditions in vitro to investigate ER function and levels. Treatment with 0.1 nM oestradiol-17beta of cell strains derived from eight young women (less than 50 years of age) increased hydroxyproline levels significantly [an average (2.2+/-0.1 S.E.M.)-fold increase], whereas cells derived from nine older women (more than 50 years of age) were not significantly affected. Similarly, cell strains, derived from younger women, transfected with a consensus oestrogen-responsive element linked to chloramphenicol acetyltransferase exhibited a greater response to oestrogen than strains derived from older women. When basal ERalpha levels were measured by enzyme immunoassay and normalized on a per cell basis, osteoblast-like strains derived from younger women (n=24) had a mean value of 2.54+/-0.16 fmol of ERalpha per 10(6) cells. In contrast, strains derived from older women (n=20) had a mean value of 5.44+/-0.48 fmol of ERalpha per 10(6) cells. An age-related increase in ERalpha number was also observed in human skin-derived fibroblasts and directly in dermal biopsies from women not on hormone replacement therapy. The results demonstrate ligand concentration-dependent ERalpha induction and indicate a loss of receptor regulation and diminution of ligand-receptor signal transduction with increasing donor age.

Cellular components interact with adenovirus type 5 minimal DNA packaging domains

Adenovirus type 5 DNA packaging is initiated from the left end of the viral genome and depends on the presence of a cis-acting packaging domain located between nucleotides 194 and 380. Multiple redundant packaging elements (termed A repeats I through VII [AI through AVII]) are contained within this domain and display differential abilities to support DNA packaging in vivo. The functionally most important repeats, AI, AII, AV, and AVI, follow a bipartite consensus motif exhibiting AT-rich and CG-rich core sequences. Results from previous mutational analyses defined a fragment containing AV, AVI, and AVII as a minimal packaging domain in vivo, which supports a functional independence of the respective cis-acting sequences. Here we describe multimeric versions of individual packaging elements as minimal packaging domains that can confer viability and packaging activity to viruses carrying gross truncations within their left end. These mutant viruses directly rate the functional role that different packaging elements play relative to each other. The A repeats are likely to be binding sites for limiting, trans-acting packaging factors of cellular and/or viral origin. We report here the characterization of two cellular binding activities interacting with all of the minimal packaging domains in vitro, an unknown binding activity termed P-complex, and the transcription factor chicken ovalbumin upstream promoter transcription factor. The binding of both activities is dependent on the integrity of the AT-rich, but not the CG-rich, consensus half site. In the case of P-complex, binding affinity for different minimal packaging domains in vitro correlates well with their abilities to support DNA packaging in vivo. Interestingly, P-complex interacts not only with packaging elements but also with the left terminus of the viral genome, the core origin of replication. Our data implicate cellular factors as components of the viral packaging machinery. The dual binding specificity of P-complex for packaging and replication sequences may further suggest a direct involvement of left-end replication sequences in viral DNA encapsidation.

Positive and negative discrimination of estrogen receptor agonists and antagonists using site-specific DNA recombinase fusion proteins

Activation of the estrogen receptor (ER) by hormone involves at least two steps. First, hormone binding initially relieves repression, a property imposed on ER in cis by its ligand-binding domain (EBD). Subsequently, the derepressed ER binds specific genomic sites and regulates transcription. In addition to the natural hormone, ER binds a broad range of ligands that evoke a spectrum of responses ranging from full ER activation by agonists to partial activation and inhibition by partial or complete antagonists. How these different ligands evoke different ER responses remains unclear. To address this issue, we have developed a nontranscriptional assay for ER ligand responsiveness based on Flp recombinase/human EBD protein chimeras. These fusion proteins transduce the transient event of ligand binding into a permanent DNA change in a human cell line system. A fusion protein including ER D, E, and F domains was activated by all the ER ligands tested, demonstrating that both agonists and antagonists serve to relieve initial repression, and that differences between them lie downstream in the activation pathway. Mutant variants of the Flp-ER protein that distinguish between agonists and antagonists, and a mutant EBD that selectively lost the ability to respond to 17beta,-estradiol but not to other ligands, were also identified. Thus, agonists and antagonists can be functionally distinguished in a nontranscriptional assay.

Heterodimerization is mainly responsible for the dominant negative activity of amino-terminally truncated rat androgen receptor forms

Rat androgen receptor (rAR) mutants devoid of the amino-terminal transactivation domain are able to behave as dominant negative regulators of wild-type rAR. To address the underlying mechanisms of the trans-dominant negative action, we have examined the roles of the DNA-binding domain (DBD) and the ligand-binding domain (LBD) in this process. Transactivation experiments in CV-1 cells complemented by electrophoretic mobility shift assays revealed that the dominant negative receptor forms repress the function of wild-type rAR mainly through heterodimer formation, rather than through competition for binding to cognate DNA elements. Heterodimerization of receptor forms containing LBDs may take place even in the absence of specific DNA binding.

Requirement for cooperative interaction of interleukin-6 responsive element type 2 and glucocorticoid responsive element in the synergistic activation of mouse metallothionein-I gene by interleukin-6 and glucocorticoid

Metallothionein (MT)-inducing activity of interleukin (IL)-6 depends on the presence of glucocorticoid in hepatic cells. The synergistic action of IL-6 and glucocorticoid was observed in the transcriptional activation of the mouse MT (mMT)-I gene. We found that a 281-bp promoter was sufficient for IL-6 and glucocorticoid stimulation. Our inspection of this region revealed the putative type 1 and 2 IL-6 responsive elements (REs). Functional analyses of these regions were performed using luciferase reporter constructs, and it was observed that the type 2 IL-6RE exerted the major response to the IL-6 signal. The transcriptional factor binding to type 1 IL-6RE, nuclear factor-IL-6, hardly contributed to the activation of the mMT-I promoter by IL-6 and glucocorticoid. A glucocorticoid responsive element (GRE) was also required for the synergistic activation by IL-6 and glucocorticoid. Interestingly, this synergism was not observed when the type 2 IL-6RE and the GRE were kept apart. Therefore, the synergistic activation of the mMT-I gene by IL-6 and glucocorticoid may require not only that signal transducers and activators 3 (Stat3) and the glucocorticoid receptor (GR) bind to their respective responsive elements, but also that Stat3 and the GR physically interact with one another.

Restoration of the glucocorticoid receptor function by the phosphodiester compound of vitamins C and E, EPC-K1 (L-ascorbic acid 2-[3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-2H-1-benzopyran-6-yl hydrogen phosphate] potassium salt), via a redox-dependent mechanism

We examined the effect of the novel antioxidant EPC-K1 (L-ascorbic acid 2-[3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-2H -1-benzopyran-6-yl hydrogen phosphate] potassium salt) on glucocorticoid receptor function. We used cloned CHOpMTGR cells in which human glucocorticoid receptor cDNA was stably transfected and the glucocorticoid receptor was expressed at high levels. We recently suggested that glucocorticoid-mediated gene expression is modulated via the cellular redox state [Makino et al., J Clin Invest 98: 2469-2477, 1996]. In the present study, this issue was clearly evidenced by the finding that cellular treatment with H2O2 decreased the ligand binding and transcriptional activity of the glucocorticoid receptor, and we showed that these inhibitory effects of H2O2 were effectively titrated by the addition of EPC-K1. Moreover, DNA-binding activity of the bacterially expressed DNA-binding domain of the glucocorticoid receptor was repressed by the thiol-oxidizing reagent diamide; EPC-K1 also counteracted this repressive effect of diamide. Thus, the redox state was indicated to influence glucocorticoid receptor function at various steps, and EPC-K1 may be useful in restoring the cellular glucocorticoid-responsiveness in oxidative conditions.

Effect of chronic antidepressant treatment on transcription factor binding activity in rat hippocampus and frontal cortex

1. The effect of chronic antidepressant administration on CRE-, SP1- and GRE-binding activity was studied in rat hippocampus and frontal cortex. 2. Fluoxetine and desipramine (3 and 10 mg/kg/day respectively) were given to rats for 21 consecutive days. The animals were killed 3 hr after the last injection and nuclear extracts were prepared to perform the DNA-protein reaction with consensus CRE, SP1 and GRE oligonucleotides. 3. Gel-shift assays showed that CRE-binding activity was increased in both frontal cortex and hippocampus by chronic fluoxetine treatment. Desipramine, however, only enhanced this activity in the frontal cortex. 4. Chronic fluoxetine decreased SP1-binding activity in the two selected brain regions. Again, desipramine only produced a significant reduction in the frontal cortex. 5. GRE-binding in the hippocampus was only enhanced by desipramine. Since chronic desipramine, and not fluoxetine, is able to increase hippocampal glucocorticoid receptor (GR) expression, interactions of GR with CREB and SP1 may determine the lack of effect of desipramine on binding activity of the two latter transcription factors in this brain region. 6. Overall, the results show a differential and region-specific effect of chronic, and not acute, antidepressant treatment on the DNA-binding activities studied and are consonant with the possible role of changes in gene expression in the mechanism of antidepressant action.

Interaction of vault particles with estrogen receptor in the MCF-7 breast cancer cell

A 104-kD protein was coimmunoprecipitated with the estrogen receptor from the flowtrough of a phosphocellulose chromatography of MCF-7 cell nuclear extract. mAbs to this protein identified several cDNA clones coding for the human 104-kD major vault protein. Vaults are large ribonucleoprotein particles of unknown function present in all eukaryotic cells. They have a complex morphology, including several small molecules of RNA, but a single protein species, the major vault protein, accounts for >70% of their mass. Their shape is reminiscent of the nucleopore central plug, but no proteins of known function have been described to interact with them. Western blot analysis of vaults purified on sucrose gradient showed the presence of estrogen receptor co-migrating with the vault peak. The AER317 antibody to estrogen receptor coimmunoprecipitated the major vault protein and the vault RNA also in the 20,000 g supernatant fraction. Reconstitution experiments of estrogen receptor fragments with the major vault protein mapped the site of the interaction between amino acids 241 and 280 of human estrogen receptor, where the nuclear localization signal sequences are located. Estradiol treatment of cells increased the amount of major vault protein present in the nuclear extract and coimmunoprecipitated with estrogen receptor, whereas the anti-estrogen ICI182,780 had no effect. The hormone-dependent interaction of vaults with estrogen receptor was reproducible in vitro and was prevented by sodium molybdate. Antibodies to progesterone and glucocorticoid receptors were able to coimmunoprecipitate the major vault protein. The association of nuclear receptors with vaults could be related to their intracellular traffic.

Recruitment of octamer transcription factors to DNA by glucocorticoid receptor

Glucocorticoid receptor (GR) and octamer transcription factors 1 and 2 (Oct-1/2) interact synergistically to activate the transcription of mouse mammary tumor virus and many cellular genes. Synergism correlates with cooperative DNA binding of the two factors in vitro. To examine the molecular basis for these cooperative interactions, we have studied the consequences of protein-protein binding between GR and Oct-1/2. We have determined that GR binds in solution to the octamer factor POU domain. Binding is mediated through an interface in the GR DNA binding domain that includes amino acids C500 and L501. In transfected mammalian cells, a transcriptionally inert wild-type but not an L501P GR peptide potentiated transcriptional activation by Oct-2 100-fold above the level that could be attained in the cell by expressing Oct-2 alone. Transcriptional activation correlated closely with a striking increase in the occupancy of octamer motifs adjacent to glucocorticoid response elements (GREs) on transiently transfected DNAs. Intriguingly, GR-Oct-1/2 binding was interrupted by the binding of GR to a GRE. We propose a model for transcriptional cooperativity in which GR-Oct-1/2 binding promotes an increase in the local concentration of octamer factors over glucocorticoid-responsive regulatory regions. These results reveal transcriptional cooperativity through a direct protein interaction between two sequence-specific transcription factors that is mediated in a way that is expected to restrict transcriptional effects to regulatory regions with DNA binding sites for both factors.

Divergent pathways regulate ligand-independent activation of ER alpha in SK-N-BE neuroblastoma and COS-1 renal carcinoma cells

The alpha-estrogen receptor (ER alpha) transcriptional activity can be regulated either by binding to the cognate ligand or by intracellular signaling pathways responsive to a variety of factors acting through cell membrane receptors. Studies carried out in HeLa and COS-1 cells demonstrated that the cross-coupling between estrogen and growth factor receptors is mediated by p21ras and requires phosphorylation of a specific serine residue (Ser 118 in the human ER alpha and Ser 122 in mouse ER alpha) located in the ER alpha N-terminal activation function 1 (AF-1). Likewise, in the SK-N-BE neuroblastoma cell line p21ras is involved in the cross-coupling between insulin and ER alpha receptors. However, in this cell line Ser 122 is not necessary for insulin-dependent activation of unliganded ER alpha. In addition, after insulin activation, the electrophoretic mobility associated to serine hyperphosphorylation of ER alpha in SK-N-BE and in COS-1 cells is different. Our study rules out the possibility of tyrosine phosphorylation in unliganded ER alpha activation by means of transactivation studies of ER alpha tyrosine mutants and analysis of Tyr phosphorylation immunoreactivity. The two cofactors for steroid receptors RIP 140 and SRC-1 do not seem to be specifically involved in the insulin-induced ER alpha transactivation. The present study demonstrates the possibility of an alternative, cell-specific pathway of cross-coupling between intracellular and membrane receptors, which might be of importance for the understanding of the physiological significance of this mode of activation in the nervous system.

The nuclear receptor ligand-binding domain: structure and function

In the past few years our understanding of nuclear receptor action has dramatically improved as a result of the elucidation of the crystal structures of the empty (apo) ligand-binding domains of the nuclear receptor and of complexes formed by the nuclear receptor's ligand-binding domain bound to agonists and antagonists. Furthermore, the concomitant identification and functional analysis of co-regulators (transcriptional intermediary factors [TIFs], comprising co-activators and co-repressors) previously predicted from squelching studies, have deepened this understanding. Recent data have provided the structural basis for the specific recognition of ligands and the molecular mechanisms of agonism and antagonism, enabling us to gain a comprehensive view of the early steps of nuclear receptor action.

BRASSINOSTEROIDS: Essential Regulators of Plant Growth and Development

Brassinosteroids (BRs) are growth-promoting natural products found at low levels in pollen, seeds, and young vegetative tissues throughout the plant kingdom. Detailed studies of BR biosynthesis and metabolism, coupled with the recent identification of BR-insensitive and BR-deficient mutants, has greatly expanded our view of steroids as signals controlling plant growth and development. This review examines the microchemical and molecular genetic analyses that have provided convincing evidence for an essential role of BRs in diverse developmental programs, including cell expansion, vascular differentiation, etiolation, and reproductive development. Recent advances relevant to the molecular mechanisms of BR-regulated gene expression and BR signal transduction are also discussed.

Crystallographic comparison of the estrogen and progesterone receptor's ligand binding domains

The 2.8-A crystal structure of the complex formed by estradiol and the human estrogen receptor-alpha ligand binding domain (hERalphaLBD) is described and compared with the recently reported structure of the progesterone complex of the human progesterone receptor ligand binding domain, as well as with similar structures of steroid/nuclear receptor LBDs solved elsewhere. The hormone-bound hERalphaLBD forms a distinctly different and probably more physiologically important dimer interface than its progesterone counterpart. A comparison of the specificity determinants of hormone binding reveals a common structural theme of mutually supported van der Waals and hydrogen-bonded interactions involving highly conserved residues. The previously suggested mechanism by which the estrogen receptor distinguishes estradiol's unique 3-hydroxy group from the 3-keto function of most other steroids is now described in atomic detail. Mapping of mutagenesis results points to a coactivator-binding surface that includes the region around the "signature sequence" as well as helix 12, where the ligand-dependent conformation of the activation function 2 core is similar in all previously solved steroid/nuclear receptor LBDs. A peculiar crystal packing event displaces helix 12 in the hERalphaLBD reported here, suggesting a higher degree of dynamic variability than expected for this critical substructure.