Steroid hormone receptors: many actors in search of a plot

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

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

Drug discovery using receptors that modulate gene expression

Cytokines and non-peptidyl small molecules, such as steroid hormones, exert many of their effects on cells through rapid regulation of gene expression. This is achieved by the activation of different families of latent transcription factors, which bind to specific sequences in the promoters of regulated genes. High throughput assay systems have been developed based on a detailed molecular understanding of these transcriptional regulation processes, and are being used as screens for both agonists and antagonists of specific cytokines and hormones. The opportunities for the discovery of novel and selective compounds using these systems is discussed.

Thioredoxin: a redox-regulating cellular cofactor for glucocorticoid hormone action. Cross talk between endocrine control of stress response and cellular antioxidant defense system

Adaptation to stress evokes a variety of biological responses, including activation of the hypothalamic-pituitary-adrenal (HPA) axis and synthesis of a panel of stress-response proteins at cellular levels: for example, expression of thioredoxin (TRX) is significantly induced under oxidative conditions. Glucocorticoids, as a peripheral effector of the HPA axis, exert their actions via interaction with a ligand-inducible transcription factor glucocorticoid receptor (GR). However, how these stress responses coordinately regulate cellular metabolism is still unknown. In this study, we demonstrated that either antisense TRX expression or cellular treatment with H2O2 negatively modulates GR function and decreases glucocorticoid-inducible gene expression. Impaired cellular response to glucocorticoids is rescued by overexpression of TRX, most possibly through the functional replenishment of the GR. Moreover, not only the ligand binding domain but the DNA binding domain of the GR is also suggested to be a direct target of TRX. Together, we here present evidence showing that cellular glucocorticoid responsiveness is coordinately modulated by redox state and TRX level and propose that cross talk between neuroendocrine control of stress responses and cellular antioxidant systems may be essential for mammalian adaptation processes.

Thyroid parafollicular cells. An accessible model for the study of serotonergic neurons

Serotonergic neurons play key roles in modulating a wide variety of behavioral and homeostatic processes. However, there is a paucity of good model systems to study these neurons at a molecular level. In this review we will present evidence that cell lines derived from an unexpected source, thyroid parafollicular cells (PF) (also called C cells), fit the criteria for use as models for the study of serotonergic neurons. A strength of PF cell lines over other cell lines is that the parental PF cells have serotonergic properties and a neuronal potential that is consistent with their neural crest origin. Furthermore, PF cells and PF cell lines are capable of expressing the fundamental properties of serotonergic neurons, including: (1) serotonin (5-HT) biosynthesis by tryptophan hydroxylase (TPH), (2) vesicular 5-HT storage and regulated release, (3) expression of a 5-HT autoreceptor, and (4) expression of the 5-HT transporter. In this review, we will focus primarily on the serotonergic and neuronal properties of the rat CA77 PF cell line and the parental rat PF cells. The applicability of CA77 cells for molecular analyses will be described. First, their use for studies on the glucocorticoid regulation of the TPH gene will be discussed. Second, control of the calcitonin/calcitonin gene-related peptide (CT/CGRP) gene will be discussed, with particular emphasis on the application of serotonergic drugs in treating migraine headaches. These examples highlight the versatility of thyroid PF cell lines as a system for studying the control of both serotonin biosynthesis and physiological actions.

Cloning of a cDNA encoding the murine orphan receptor RZR/ROR gamma and characterization of its response element

In this study, we describe the cloning of the mouse homologue of the orphan receptor, RZR/ROR gamma, a member of the nuclear receptor superfamily, from a mouse muscle cDNA library. The amino acid sequence of mouse ROR gamma (mROR gamma) is highly homologous to that of human ROR gamma, with an overall identity of 88%. Northern blot analysis using RNA from different tissues showed that mROR gamma was found to be highly expressed in skeletal muscle, liver and kidney. Analysis of the ROR gamma-response element using in vitro synthesized ROR gamma revealed that it binds as a monomer to response elements composed of a single core motif GGTCA preceded by a 6 bp AT-rich sequence. The ROR gamma-binding specificity was further defined by mutational analysis of the consensus RORE. ROR gamma was able to activate RORE-dependent transcription of the CAT reporter gene in mouse fibroblast D1 cells. ROR alpha 1 and ROR gamma inhibit the transactivation induced by GAL4(DBD)-ROR gamma in fibroblast D1 cells suggesting that these receptors compete for binding to the same coactivators.

A p56(lck) ligand serves as a coactivator of an orphan nuclear hormone receptor

Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII), an orphan member of the nuclear hormone receptor superfamily, acts as a transcriptional repressor by antagonizing the functions of other nuclear hormone receptors and by actively silencing transcription. However, in certain contexts, COUP-TFII stimulates transcription directly. A cellular factor, isolated by interaction cloning, bound COUP-TFII in vitro and allowed COUP-TFII to function as a transcriptional activator in mammalian cells. This factor is identical to a recently described ligand of the tyrosine kinase signaling molecule p56(lck), suggesting that it mediates cross-talk between mitogenic and nuclear hormone receptor signal transduction pathways.

Up-regulation of hormone response of human papillomavirus type 16 expression and increased DNA-protein binding by consensus mutations of viral glucocorticoid response elements

Human papillomaviruses (HPVs) and steroid hormones are linked to the development of cervical cancer. Studies from our laboratory and others showed that the steroid glucocorticoid and progesterone hormones activated the expression of HPV type 16. This activation was attributed to the specific interaction of the glucocorticoid receptor (GR) with the three glucocorticoid response elements (GREs) in the HPV16 regulatory region. In the present study, we first examined the glucocorticoid response mediated through the GREs, using GRE consensus (GREc) mutations and expression assays from a heterologous basal promoter. Both single and triple HPV16 GREc constructs increased expression in the presence of the dexamethasone glucocorticoid in HeLa cervical carcinoma cells and primary baby rat kidney epithelial cells, in comparison with the triple wild-type GREs. Further, the hormone increased significantly the expression of the viral E6-E7 oncogene mRNA from intact HPV in primary human ectocervical cells in in situ hybridization assays. Three in vitro assays of DNA-protein interaction with oligonucleotides and HeLa cell extracts showed a higher binding of protein to two of the HPV16 GREcs than to the wild-type GREs. This applied especially to the GRE containing an overlapping NF1 half site, that also had a greater differential induction by dexamethasone of expression in vivo. The NF1 site was mutated in the GREc that also was bound by unique, lower-mobility complexes in electrophoretic mobility shift assays. UV-crosslinking assays confirmed the increased binding and showed binding by a 96-kDa protein, probably the GR. Our results show an important role of glucocorticoids in HPV16 expression. The direct action through the HPV16 GREs is suggested to be mediated by the hormone-activated GR in association with other factors.

Inhibition of rabbit collagenase (matrix metalloproteinase-1; MMP-1) transcription by retinoid receptors: evidence for binding of RARs/RXRs to the -77 AP-1 site through interactions with c-Jun

Treatment of synovial fibroblasts with retinoic acid (RA) decreases their expression of collagenase (matrix metalloproteinase-1 or MMP-1), an enzyme that degrades interstitial collagens and contributes to the pathology of rheumatoid arthritis. This inhibition results, at least in part, from RA-induced decreases in the mRNA for the transactivators Fos and Jun (with concominant increases in RAR mRNA) and by sequestration of Fos/Jun by RARs/RXRs. Previously, we provided evidence that retinoid receptors are also present in complexes that bind to fragments of rabbit MMP-1 promoter DNA containing an AP-1 site at -77 (Pan et al., 1995, J. Cell. Biochem., 57:575-589). However, it was unclear whether RARs and retinoid X receptors (RXRs) were binding directly to the DNA or indirectly through another protein. We now use a sensitive MMP-1 promoter/luciferase reporter construct to confirm the transcriptional role of the AP-1 site at -77. In addition, with electrophoretic mobility shift analyses (EMSAs), antibody "supershifts" and DNAase 1 footprinting, we examine the interaction of retinoid receptors and AP-1 protein on the MMP-1 promoter. We demonstrate that RARs, RXRs, and c-Jun form a complex at the AP-1 site in which c-Jun binds directly to the DNA and apparently tethers the retinoid receptors to the complex. We conclude that retinoid receptors/AP-1 protein interactions at the DNA may provide an additional means of controlling collagenase gene transcription by retinoids.

The extreme C terminus of progesterone receptor contains a transcriptional repressor domain that functions through a putative corepressor

Binding of a hormone agonist to a steroid receptor leads to the dissociation of heat shock proteins, dimerization, specific DNA binding, and target gene activation. Although the progesterone antagonist RU486 can induce most of these events, it fails to activate human progesterone receptor (hPR)-dependent transcription. We have previously demonstrated that a conformational change is a key event leading to receptor activation. The major conformational distinction between hormone- and antihormone-bound receptors occurs within the C-terminal portion of the molecule. Furthermore, hPR mutants lacking the C terminus become transcriptionally active in the presence of RU486. These results suggest that the C terminus contains a repressor domain that inhibits the transcriptional activity of the RU486-bound hPR. In this study, we have defined a 12 amino acid (12AA) region in the C terminus of hPR that is necessary and sufficient for the repressor function when fused to the C-terminal truncated hPR or to the GAL4 DNA-binding domain. Mutations in the 12AA domain (aa 917-928) generate an hPR that is active in the presence of RU486. Furthermore, overexpression of the 12AA peptide activates the RU486-bound wild-type hPR without affecting progesterone-dependent activation. These results suggest that association of the 12AA repressor region with a corepressor might inactivate hPR activity when it is bound to RU486. We propose that binding of a hormone agonist to the receptor changes its conformation in the ligand-binding domain so that association with coactivator is promoted and activation of target gene occurs.

Functional interactions between Stat5 and the glucocorticoid receptor

Signal transduction pathways enable extracellular signals to activate latent transcription factors in the cytoplasm of cells. Dimerization, nuclear localization and binding to specific DNA sequences result in the induction of gene transcription by these proteins. These events are necessary for the functioning of the JAK/STAT pathway and of the glucocorticoid-receptor pathway. In the former, the protein Stat5, which is a member of a family of signal transducers and activators of transcription, is activated by cytokines, hormones and growth factors. These polypeptide ligands bind at the outside of the cell to specific transmembrane receptors and activate intracellular Janus protein tyrosine kinases (JAKs) to tyrosine-phosphorylate STAT proteins; interaction with the SH2 domain of the dimerization partner then confers the ability to bind to DNA at the STAT-response element and induce transcription. In the glucocorticoid-receptor pathway, the receptor interacts with its steroid hormone ligand in the cytoplasm, undergoes an allosteric change that enables the hormone receptor complex to bind to specific DNA-response elements (glucocorticoid response elements, or GRE) and modulate transcription. Although these pathways appear to be unrelated, we show here that the glucocorticoid receptor can act as a transcriptional co-activator for Stat5 and enhance Stat5-dependent transcription. Stat5 forms a complex with the glucocorticoid receptor which binds to DNA independently of the GRE. This complex formation between Stat5 and the glucocorticoid receptor diminishes the glucocorticoid response of a GRE-containing promoter.

p300 is a component of an estrogen receptor coactivator complex

The estrogen receptor (ER) is a ligand-dependent transcription factor that regulates expression of target genes in response to estrogen in concert with other cellular signaling pathways. This suggests that the mechanism by which ER transmits an activating signal to the general transcription machinery may include factors that integrate these diverse signals. We have previously characterized the estrogen receptor-associated protein, ERAP160, as a factor that complexes with ER in an agonist-dependent manner. We have now found that the transcriptional coactivator p300 associates with agonist bound ER and augments ligand-dependent activation by ER. Our studies show that an ER coactivator complex involves a direct hormone-dependent interaction between ER and ERAP160, resulting in the recruitment of p300. In addition, antibodies directed against the cloned steroid receptor coactivator 1 (SRC1) recognize ERAP160. The known role of p300 in multiple signal transduction pathways, including those involving the second messenger cAMP, suggests p300 functions as a point of integration between ER and these other pathways.

Retinoid receptors in transcriptional regulation

Our understanding of the mechanism of action of retinoids has been greatly expanded by a series of recent findings. First, the three-dimensional structure of the ligand-binding domain of two retinoid receptors has been solved and suggests that ligand binding induces marked allosteric changes. Second, several co-factors interacting with the receptors have been cloned, some of which are capable of regulating the function of receptors. Third, the advent of synthetic retinoids helped define the activities of the receptors. Fourth, the study of the in vivo receptor-DNA interactions has revealed a previously unrecognized role of the ligand in regulating the stability of receptor-DNA complexes. These advances have revealed complex molecular interactions operating at multiple levels, opening new avenues of research for addressing their mechanisms.

Transcription factors as drug targets in cancer

Within multicellular organisms, cells are continually signalling to each other to keep in tune with their environment. The ultimate targets for the majority of these signal pathways are upstream transcription factors, whose activity is thereby modulated, resulting in a new pattern of gene expression suitably coupled to the needs of the cell. It has been estimated that up to 10% of human genes may encode transcription factors, thus emphasising how fundamental the control of gene expression is to the processes of cellular division and differentiation during normal development. As a corollary of this, transcriptional regulation can also profoundly affect the course of growth-related diseases such as cancer. Of course it has been realised for some time that the normal counterparts of many oncogenes are transcription factors whose proper role is in the control of normal cell growth. More recent work has begun to identify several other transcription factors which may play a role in cancer, and strategies are now being developed which are designed to use our growing knowledge of transcriptional control mechanisms in the development of novel cancer therapies.

Glucocorticoid-stimulated CCAAT/enhancer-binding protein alpha expression is required for steroid-induced G1 cell cycle arrest of minimal-deviation rat hepatoma cells

By genetic correlation with the growth-suppressible phenotype and direct functional tests, we demonstrate that the glucocorticoid-stimulated expression of the CCAAT/enhancer-binding protein alpha (C/EBP alpha) transcription factor is required for the steroid-mediated G1 cell cycle arrest of minimal-deviation rat hepatoma cells. Comparison of C/EBP alpha transcript and active protein levels induced by the synthetic glucocorticoid dexamethasone in glucocorticoid growth-suppressible (BDS1), nonsuppressible receptor-positive (EDR1) and nonsuppressible receptor-deficient (EDR3) hepatoma cell proliferative variants revealed that the stimulation of C/EBP alpha expression is a rapid, glucocorticoid receptor-mediated response associated with the G1 cell cycle arrest. Consistent with the role of C/EBP alpha as a critical intermediate in the growth suppression response, maximal induction of transcription factor mRNA occurred within 2 h of dexamethasone treatment whereas maximal inhibition of [3H] thymidine incorporation was observed 24 h after steroid treatment. As a direct functional approach, ablation of C/EBP alpha protein expression and DNA-binding activity by transfection of an antisense C/EBP alpha expression vector blocked the dexamethasone-induced G1 cell cycle arrest of hepatoma cells but did not alter general glucocorticoid responsiveness. Transforming growth factor beta induced a G1 cell cycle arrest in C/EBP alpha antisense transfected cells, demonstrating the specific involvement of C/EBP alpha in the glucocorticoid growth suppression response. Constitutive expression of a conditionally activated form of C/EBP alpha caused a G1 cell cycle arrest of BDS1 hepatoma cells in the absence of glucocorticoids. In contrast, overexpression of C/EBP beta or C/EBP delta had no effect on hepatoma cell growth. Taken together, these results demonstrate that the steroid-induced expression of C/EBP alpha is necessary to mediate the glucocorticoid G1 cell cycle arrest of rat hepatoma cells and implicates a role for this transcription factor in the growth control of liver-derived epithelial tumor cells.

Mutual transcriptional interference between RelA and androgen receptor

Cross-modulation between androgen receptor (AR) and NF-kappaB/Rel proteins was studied using various androgen- and NF-kappaB-regulated reporter genes under transient transfection conditions. In COS-1 cells, elevated expression of RelA (p65) repressed AR-mediated transactivation in a dose-dependent manner, whereas NFkappaB1 (p50), another major member of the NF-kappaB family, did not influence transactivation. The repression of AR appeared to involve the N-terminal region of the protein between residue 297 and the DNA-binding domain. RelA-mediated transrepression could not be overcome by increasing the amount of AR. Transcriptional interference between RelA and AR was mutual in that cotransfected AR was able to attenuate transactivation by RelA in a dose- and steroid-dependent fashion. An excess of RelA was able to rescue the repression to some extent. Immunological analyses of RelA and AR protein levels indicated that transrepression was not due to reciprocal decrease in their amounts. Neither did AR increase the concentration of IkappaBalpha, which can sequester and inactivate RelA. Electrophoretic mobility shift assays using extracts from cotransfected cells and purified recombinant proteins showed that AR and RelA did not significantly influence each other's DNA binding activity. Nevertheless, protein-protein interaction experiments demonstrated a weak association between AR and RelA. Collectively, these data suggest that the mutual repression in intact cells is due to formation of AR-RelA complexes that are held together by another partner or to competition for a coactivator required for transcription.

Cross-talk between the T cell antigen receptor and the glucocorticoid receptor regulates thymocyte development

The fate of an immature thymocyte, life or death, is largely determined by the ligand-specificity of its T cell antigen receptor (TCR). The default pathway for thymocytes bearing TCRs with subthreshold avidity for self-antigens is death (death by neglect). Thymocytes bearing TCRs with high avidity for self also undergo apoptosis (negative selection). Those thymocytes with intermediate avidities, or that perhaps recognize self-peptides that have partial agonist or antagonist properties, survive and differentiate into mature immunocompetent T cells (positive selection). How TCR avidity is interpreted as a "rescue" signal or a death signal is unknown. Based upon a T cell hybridoma model, our laboratory proposed that glucocorticoids, which themselves are potent inducers of thymocyte apoptosis, antagonize TCR-mediated thymocyte deletion and allow positive selection to occur. In fact, epithelial cells in the thymus proved to be a source of steroid production, and interference with steroid synthesis in fetal thymic organ culture resulted in a greatly enhanced sensitivity of thymocytes to TCR-mediated apoptosis. Transgenic mice with reduced glucocorticoid receptor (GR) levels were produced by tissue-specific expression of GR antisense. Thymocytes in these mice had high levels of spontaneous apoptosis, and were exquisitely sensitive to deletion induced by cross-linking the TCR. Moreover, there was a very large (> or = 90%) loss of CD4+CD8+ thymocytes, signifying a block at the CD4-CD8- to CD4+CD8+ transition, perhaps due to apoptosis of cells upon engagement of the pre-TCR in the absence of an antagonizing glucocorticoid stimulus. The molecular mechanism of the antagonism is currently being investigated. These data indicate that there is cross-talk in thymocytes between the TCR and glucocorticoid signaling pathways resulting in apoptosis, and that locally produced steroids, in a paracrine fashion, participate in setting the TCR avidity thresholds that determine whether developing thymocytes survive or die, and therefore help to mold the antigen-specific T cell repertoire.

Glucocorticoid and estrogen receptors have elevated activity in human endometrial and ovarian tumors as compared to the adjacent normal tissues and recognize sequence elements of the H-ras proto-oncogene

We examined the level of receptor binding in H-ras elements, using nuclear extracts derived from human endometrial and ovarian lesions and from adjacent normal tissue in gel retardation assays. We found increased binding of the glucocorticoid receptor (GR) to the H-ras GR element in more than 90% of endometrial tumors and in all ovarian tumors tested, as compared to the corresponding adjacent normal tissue. Additionally, we found elevated binding of the estrogen receptor (ER) in H-ras ER element in all pairs of ovarian tumor/normal tissue tested, whereas in ER-negative control breast tumor/normal tissue pairs, no differences in ER DNA-binding levels were observed. These results suggest that steroid hormone receptor binding could directly activate the H-ras oncogenic potency in human endometrial and ovarian lesions, providing additional evidence for the role of H-ras expression in hormonally responsive human cancers.

Induction of phosphoglycerate kinase 1 gene expression by hypoxia. Roles of Arnt and HIF1alpha

To identify new dimerization partners for the aromatic hydrocarbon receptor nuclear translocator (Arnt), we used its N-terminal region (amino acids 1-470) as a target in a two-hybrid screening procedure, and we cloned the murine form of hypoxia-inducible factor 1alpha (HIF1alpha). Sequence comparisons reveal substantial identity between mouse and human HIF1alpha. Hypoxia induces a 10-fold accumulation of phosphoglycerate kinase 1 mRNA in wild type mouse hepatoma (Hepa 1c1c7) cells; the induction mechanism is Arnt dependent because induction does not occur in Arnt-defective cells. Furthermore, induction of phosphoglycerate kinase 1 mRNA requires Arnt's N-terminal region, which mediates DNA binding and heterodimerization; in contrast, induction does not require Arnt's C-terminal region, which mediates transactivation. We also show that a GAL4-HIF1alpha fusion protein transactivates a GAL4-dependent gene in the absence of Arnt, that HIF1alpha's transactivation capability is inducible by hypoxia, and that both hypoxia responsiveness and transactivation capability reside within the C-terminal 83 amino acids of HIF1alpha. Our findings generate new insights into the mechanism by which Arnt and HIF1alpha induce transcription in response to hypoxia.

ER beta: identification and characterization of a novel human estrogen receptor

A novel estrogen receptor (hereinafter referred to as ER beta) was cloned using degenerate PCR primers. A comparison of the amino acid sequence of ER beta with the "classical' ER (ER alpha) shows a high degree of conservation of the DNA-binding domain (96%), and of the ligand-binding domain (58%). In contrast, the A/B domain, the hinge region and the F-domain are not conserved. Northern blot analysis revealed that ER beta is expressed in human thymus, spleen, ovary and testis. Transient transfections of an ER beta expression construct together with an ERE-based reporter construct in CHO cells clearly demonstrated transactivation of ER beta by 17 beta-estradiol. In addition, the ER alpha antagonist ICI-164384 is a potent antagonist for ER beta as well. Interestingly, the level of transactivation by 17 beta-estradiol is higher for ER alpha than for ER beta, which may reflect suboptimal conditions for ER beta at the level of the ligand, responsive element or cellular context.

Multiple interactions between hTAFII55 and other TFIID subunits. Requirements for the formation of stable ternary complexes between hTAFII55 and the TATA-binding protein

We have cloned and characterized the human TATA-binding protein (TBP)-associated factor hTAFII55. hTAFII55, which has no known Drosophila counterpart, is present in both of the previously described TFIIDalpha and TFIIDbeta subpopulations. We describe the interactions of hTAFII55 with other subunits of the transcription factor TFIID. By cotransfection in COS cells, we show that hTAFII55 interacts with hTAFII250, hTAFII100, hTAFII28, hTAFII20, and hTAFII18, but not with hTAFII30 or TBP. Analysis of the binding of hTAFII55 and TBP to hTAFII28 deletion mutants indicates that distinct regions of hTAFII28 are required for these interactions. Although hTAFII55 does not interact by itself with TBP, stable ternary complexes containing hTAFII55 and TBP can be formed in the presence of hTAFII250, hTAFII100, or hTAFII28. These results not only show that hTAFII100 and hTAFII28 interact with TBP, but also that they can nucleate the formation of partial TFIID complexes.

The mosquito ultraspiracle homologue, a partner of ecdysteroid receptor heterodimer: cloning and characterization of isoforms expressed during vitellogenesis

We report the cloning and characterization of two isoforms of the Ultraspiracle homologue (AaUSP) from the mosquito, Aedes aegypti. The 2.33-kb AaUSPa cDNA has an open reading frame (ORF) of 484 amino acids encoding a polypeptide of 54 kDa, whereas the 2.14-kb AaUSPb ORF of 459 amino acids encodes a 51.3 kDa polypeptide. The AaUSPa and AaUSPb proteins differ only in the N-terminal portion of the variable A/B domain. The AaUSP DNA-binding domain shares 92% and 97% identities with the respective domains of the Drosophila (DmUSP) and Bombyx (BmUSP) Ultraspiracles. However, the AaUSP ligand-binding domain is only 57% and 52% identical to those of DmUSP and BmUSP, respectively. In spite of the relatively low level of sequence conservation, electrophoretic mobility shift assay (EMSA) and hormone-binding assay clearly demonstrated that the products of the AaUSPa and AaUSPb cDNAs are functional heterodimeric partners of the mosquito ecdysteroid receptor. In vitellogenic tissues, each of the two AaUSP isoforms is expressed differently: the AaUSPa is predominant in the fat body and the AaUSPb in the ovary. The kinetics of ovarian AaUSP mRNA coincide with those of the ecdysteroid receptor, being elevated during the previtellogenic period and shortly after the onset of vitellogenesis. In contrast, the level of the AaUSP in the fat body remains relatively constant throughout most of the vitellogenic cycle.

Biological and biochemical inhibitors of the NF-kappa B/Rel proteins and cytokine synthesis

The NF-kappa B/Rel family of transcription factors participates in the activation of a diverse range of genes involved in inflammation, immune response, lymphoid differentiation, growth control and development. The present review provides a brief overview of NF-kappa B/Rel activation and a detailed analysis of important biological and biochemical inhibitors of the NF-kappa B/Rel pathway. Given the pleiotropic role of NF-kappa B in controlling cytokines and other immunoregulatory genes, the inhibition of NF-kappa B activation by steroid hormones, antioxidants, protease inhibitors and other compounds may provide a pharmacological basis for interfering with pathological inflammatory conditions, cancer and AIDS.

The peroxisome proliferator activated receptors (PPARS) and their effects on lipid metabolism and adipocyte differentiation

The three types of peroxisome proliferator activated receptor (PPAR), alpha, beta (or delta), and gamma, each with a specific tissue distribution, compose a subfamily of the nuclear hormone receptor gene family. Although peroxisome proliferators, including fibrates and fatty acids, activate the transcriptional activity of these receptors, only prostaglandin J2 derivatives have been identified as natural ligands of the PPAR gamma subtype, which also binds thiazolidinedione antidiabetic agents with high affinity. Activated PPARs heterodimerize with RXR and alter the transcription of target genes after binding to specific response elements or PPREs, consisting of a direct repeat of the nuclear receptor hexameric DNA core recognition motif spaced by one nucleotide. The different PPARs can be considered key messengers responsible for the translation of nutritional, pharmacological and metabolic stimuli into changes in the expression of genes, more specifically those genes involved in lipid metabolism. PPAR alpha is involved in stimulating beta-oxidation of fatty acids. In rodents, a PPAR alpha-mediated change in the expression of genes involved in fatty acid metabolism lies at the basis of the phenomenon of peroxisome proliferation, a pleiotropic cellular response, mainly limited to liver and kidney and which can lead to hepatocarcinogenesis. In addition to their role in peroxisome proliferation in rodents, PPAR is also involved in the control of HDL cholesterol levels by fibrates and fatty acids in rodents and humans. This effect is, at least partially, based on a PPAR-mediated transcriptional regulation of the major HDL apolipoproteins, apo A-I and apo A-II. The hypotriglyceridemic action of fibrates and fatty acids also involves PPARs and can be summarized as follows: (1) an increased lipolysis and clearance of remnant particles, due to changes in LPL and apo C-III levels, (2) a stimulation of cellular fatty acid uptake and their conversion to acyl-CoA derivatives by the induction of FAT, FATP and ACS activity, (3) an induction of fatty acid beta-oxidation pathways, (4) a reduction in fatty acid and triglyceride synthesis, and finally (5) a decrease in VLDL production. Hence, both enhanced catabolism of triglyceride-rich particles as well as reduced secretion of VLDL particles are mechanisms that contribute to the hypolipidemic effect of fibrates and FFAs. Whereas for PPAR beta no function so far has been identified, PPAR gamma triggers adipocyte differentiation by inducing the expression of several genes critical for adipogenesis.

Multiple steps in the regulation of transcription-factor level and activity

This review focuses on the regulation of transcription factors, many of which are DNA-binding proteins that recognize cis-regulatory elements of target genes and are the most direct regulators of gene transcription. Transcription factors serve as integration centres of the different signal-transduction pathways affecting a given gene. It is obvious that the regulation of these regulators themselves is of crucial importance for differential gene expression during development and in terminally differentiated cells. Transcription factors can be regulated at two, principally different, levels, namely concentration and activity, each of which can be modulated in a variety of ways. The concentrations of transcription factors, as of intracellular proteins in general, may be regulated at any of the steps leading from DNA to protein, including transcription, RNA processing, mRNA degradation and translation. The activity of a transcription factor is often regulated by (de) phosphorylation, which may affect different functions, e.g. nuclear localization DNA binding and trans-activation. Ligand binding is another mode of transcription-factor activation. It is typical for the large super-family of nuclear hormone receptors. Heterodimerization between transcription factors adds another dimension to the regulatory diversity and signal integration. Finally, non-DNA-binding (accessory) factors may mediate a diverse range of functions, e.g. serving as a bridge between the transcription factor and the basal transcription machinery, stabilizing the DNA-binding complex or changing the specificity of the target sequence recognition. The present review presents an overview of different modes of transcription-factor regulation, each illustrated by typical examples.

The alpha1-fetoprotein locus is activated by a nuclear receptor of the Drosophila FTZ-F1 family

The alpha1-fetoprotein (AFP) gene is located between the albumin and alpha-albumin genes and is activated by transcription factor FTF (fetoprotein transcription factor), presumed to transduce early developmental signals to the albumin gene cluster. We have identified FTF as an orphan nuclear receptor of the Drosophila FTZ-F1 family. FTF recognizes the DNA sequence 5'-TCAAGGTCA-3', the canonical recognition motif for FTZ-F1 receptors. cDNA sequence homologies indicate that rat FTF is the ortholog of mouse LRH-1 and Xenopus xFF1rA. Rodent FTF is encoded by a single-copy gene, related to the gene encoding steroidogenic factor 1 (SF-1). The 5.2-kb FTF transcript is translated from several in-frame initiator codons into FTF isoforms (54 to 64 kDa) which appear to bind DNA as monomers, with no need for a specific ligand, similar KdS (approximately equal 3 x 10(-10) M), and similar transcriptional effects. FTF activates the AFP promoter without the use of an amino-terminal activation domain; carboxy-terminus-truncated FTF exerts strong dominant negative effects. In the AFP promoter, FTF recruits an accessory trans-activator which imparts glucocorticoid reactivity upon the AFP gene. FTF binding sites are found in the promoters of other liver-expressed genes, some encoding liver transcription factors; FTF, liver alpha1-antitrypsin promoter factor LFB2, and HNF-3beta promoter factor UF2-H3beta are probably the same factor. FTF is also abundantly expressed in the pancreas and may exert differentiation functions in endodermal sublineages, similar to SF-1 in steroidogenic tissues. HepG2 hepatoma cells seem to express a mutated form of FTF.

Estrogen effects in the heart

Gender specific differences in cardiovascular disease are largely mediated by sex hormones. The use of estrogens significantly reduces the overall incidence of heart disease in postmenopausal women. Beneficial effects of estrogens on plasma lipoprotein levels are clearly established. However, these do not explain the magnitude of risk reduction seen in clinical studies. Thus additional and currently unknown functions of estrogens must be operative. Elucidation of the exact estrogen action in the heart will have important implications in the treatment of cardiovascular disease. It will probably enhance the therapeutic repertoire in treating heart disease, the most common cause of death in industrialized countries. We will review the current understanding of the function of estrogens in the heart and discuss potential strategies on how to apply these data to clinical practice.

The androgen receptor in prostate cancer

The androgen receptor is a member of the family of nuclear receptors. In its activated form as an androgen receptor ligand complex (the ligand can either be testosterone or 5a-dihydrotestosterone), the androgen receptor is able to regulate a specific expression of target genes. The androgen receptor is expressed at high levels in male reproductive tissues. Mutations in the androgen receptor gene are the molecular cause of the androgen insensitivity syndrome, which is characterized by an aberrant male or an apparently female phenotype. Expansion of a CAG-repeat, encoding a polymorphic glutamine stretch is the cause of a rare motor neuron disease (Kennedy's disease). Hormonal therapy is the treatment of choice for metastatic prostate cancer. Hormone refractory prostate tumors in general still express androgen receptor. In a proportion of the late stage prostate tumors, somatic mutations in the androgen receptor gene have been described. Mutations can result in diminished ligand specificity of the androgen receptor. Furthermore, it has been hypothesized that ligand independent mechanisms can also be involved in androgen receptor activation.

Nuclear hormone receptors: ligand-activated regulators of transcription and diverse cell responses

Signal transduction via nuclear hormone receptors is unusual in that the hormone ligand forms an integral part of the protein complex involved in DNA binding and transcriptional activation. New structural and biochemical results have begun to unravel how these receptors produce different effects in different cells, and the structural changes involved in transcriptional activation.

A CBP integrator complex mediates transcriptional activation and AP-1 inhibition by nuclear receptors

Nuclear receptors regulate gene expression by direct activation of target genes and inhibition of AP-1. Here we report that, unexpectedly, activation by nuclear receptors requires the actions of CREB-binding protein (CBP) and that inhibition of AP-1 activity is the apparent result of competition for limiting amounts of CBP/p300 in cells. Utilizing distinct domains, CBP directly interacts with the ligand-binding domain of multiple nuclear receptors and with the p160 nuclear receptor coactivators, which upon cloning have proven to be variants of the SRC-1 protein. Because CBP represents a common factor, required in addition to distinct coactivators for function of nuclear receptors, CREB, and AP-1, we suggest that CBP/p300 serves as an integrator of multiple signal transduction pathways within the nucleus.

Interaction of the Ubc9 human homologue with c-Jun and with the glucocorticoid receptor

Glucocorticoid hormones convert the glucocorticoid receptor (GR) from an inactive cytosolic complex to a nuclear form that regulates transcription. Binding of GR to palindromic DNA-recognition sites (hormone response elements) leads to activated target gene transcription. GR also exerts negative actions on transcription, e.g., by interfering with the function of several other transcription factors such as AP-1, NK-kappa B, CREB, and Oct-1. Physical interactions of GR with AP-1 subunits are readily detectable but do not seem sufficient since nonrepressing GR mutants still interact in vitro, so that specific conformational changes and/or interactions with additional partner proteins may be required for negative action. In an attempt to find such partner proteins, we defined regions of c-Jun and GR essential for mutual interference and used in those a yeast two-hybrid screen for interacting proteins. Repeatedly we isolated overlapping cDNA sequences of one protein interaction with both c-Jun and GR. This protein does not interact with c-Fos or a non-repressing GR mutant and expressed in mammalian cells does not substantially affect AP-1 or GR activity. Interestingly, however, the protein rescues yeast cells from the toxic effects of the GR fragment used for screening. The protein represents the human homologue of the yeast E2 ubiquitin-conjugating enzyme, Ubc9; its specific interactions with both GR and c-Jun, but not mutant GR, suggest that it may exert physiologic regulatory functions.