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

Nuclear Hormone Receptor Coregulator GRIP1 Suppresses, whereas SRC1A and p/CIP Coactivate, by Domain-specific Binding of MyoD

p160 coregulators were initially identified as nuclear hormone receptor coactivators. In this study, functional data demonstrate that members of the three p160 families can have opposing roles in regulating gene expression by the same transcription factor. Both SRC1A and p/CIP function as coactivators for MyoD-mediated transcription whereas GRIP1 acts negatively as a (co)repressor. SRC1A and p/CIP predominantly interact with distinct sites on the NH2-terminal activation domain of MyoD. GRIP1 binds to both these regions but it alone, and neither SRC1A nor p/CIP, also interacts with specific sites on MyoD that are critical for the binding of the essential MyoD coactivator, p300. This suggests that competition by GRIP1 for SRC1A, p/CIP, and p300 binding sites on a transcription factor may regulate the activity of the factor.

1,25-Dihydroxyvitamin D3 stimulates cyclic vitamin D receptor/retinoid X receptor DNA-binding, co-activator recruitment, and histone acetylation in intact osteoblasts

1,25(OH)2D3 induces gene expression through the VDR. We used chromatin immunoprecipitation techniques to explore this 1,25(OH)2D3-induced process on the 25-hydroxyvitamin D3-24-hydroxylase (Cyp24) and Opn gene promoters in intact osteoblasts. Our studies show that 1,25(OH)2D3-induced transactivation is a dynamic process that involves promoter-specific localization of VDR and RXR, recruitment of histone acetyltransferase complexes, and in the case of the Cyp24 gene, modification of histone 4.

INTRODUCTION

The vitamin D receptor (VDR) binds as a retinoid X receptor (RXR) heterodimer to target DNA sequences and facilitates the recruitment of protein complexes that are essential for transcriptional modulation. These complexes include an acetyltransferase component that contains members of the p160 family and p300/CBP as well as human mediator that contains D receptor interacting protein (DRIP205). The objective of this study was to investigate the kinetics of VDR/RXR binding to 25-hydroxyvitamin D3-24-hydroxylase (Cyp24) and osteopontin (Opn) target gene promoters and to explore the recruitment and subsequent activities of co-activator complexes on these target genes in intact cells.

MATERIALS AND METHODS

Mouse osteoblastic MC3T3-E1 cells and mouse primary calvarial osteoblasts (MOBs) were cultured in alphaMEM medium supplemented with 10% FBS. Confluent cells were treated with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] or the vitamin D antagonist ZK159222, and the ability of these compounds to induce localization of VDR and RXR to specific regions of Cyp24 and Opn target genes was examined using chromatin immunoprecipitation techniques. The ability of both compounds to induce the recruitment of co-activator proteins such as p160 family members, CBP and DRIP205, and to increase the level of histone acetylation on the two gene promoters in MC3T3-E1 cells was also examined.

RESULTS

1,25(OH)2D3 induces rapid association of the VDR and RXR with both the Cyp24 and the Opn gene promoters in both MC3T3-E1 osteoblasts and MOBs, interactions that are both rapid and cyclic in nature. 1,25(OH)2D3 treatment also induces rapid recruitment of co-regulators such as SRC-1, -2, and -3, CBP, and p300 to both promoters, recruitment that leads to acetylation of histone 4 on Cyp24 but not the Opn. DRIP205 is also recruited to the two promoters in response to hormonal stimulation, an appearance that correlates directly with entry of RNA pol II. Studies with the vitamin D antagonist ZK159222 suggest a complex mode of action of this compound in blocking 1,25(OH)2D3-induced transcription. Our studies indicate that 1,25(OH)2D3-induced transactivation in intact osteoblasts is a dynamic process that involves promoter-specific localization of VDR and RXR as well as the recruitment of a number of co-regulators essential to 1,25(OH)2D3-induced transcription.

CONCLUSIONS

We conclude that co-regulators essential for the transcriptional activity of the steroid receptor gene family are indeed critical for the actions of 1,25(OH)2D3. Selective use of co-regulators by target genes, however, may provide a mechanism for the unique and perhaps gene-selective responses observed with synthetic analogs such as ZK159222.

Coordinate transcriptional regulation of bile acid homeostasis and drug metabolism

Drugs and bile acids are taken up into hepatocytes by specialized transport proteins localized at the basolateral membrane, e.g., organic anion transporting polypeptides . Following intracellular metabolism by cytochrome P450 (CYP) enzymes, drug metabolites are excreted into bile or urine via ATP-dependent multidrug resistance proteins (MDR1 and MRPs). Bile acids are excreted mainly via the bile salt export pump (BSEP, ABCB11). The genes coding for drug and bile acid transporters and CYP enzymes are regulated by a complex network of transcriptional cascades, notably by the ligand-activated nuclear receptors FXR, PXR, and CAR and by the ligand-independent nuclear receptor HNF-4alpha. The bile acid synthesizing enzymes CYP7A1, CYP8B1, and CYP27A1 are subject to negative feedback regulation by bile acids, which is partly mediated through the transcriptional repressor SHP. The role of transcriptional cofactors, such as SRC-1 and PGC-1, in mediating the gene-specific effects of individual nuclear receptors is becoming increasingly evident.

Identification and characterization of the protein-associated splicing factor as a negative co-regulator of the progesterone receptor

Progesterone is essential in all species for the maintenance of pregnancy, and its withdrawal is required to activate the myometrium and to initiate labor. However, unlike most other species, progesterone levels do not fall at term in humans, raising the paradox as to how labor can occur under the continued influence of progesterone. We hypothesized that an endogenous (myometrial) repressor of the progesterone receptor (PR) could induce a functional withdrawal of progesterone and hence lead to the initiation of labor. We used the human PR as bait in a protein pull-down assay and identified polypyrimidine tract-binding protein-associated splicing factor (PSF) as a PR-interacting protein. PSF functions as a potent inhibitor of PR (but not estrogen receptor) transcriptional activity in mammalian cells. It acts through two novel mechanisms, inducing degradation of the PR through the proteasomal pathway and also interfering with binding of PR to its DNA response element. Importantly, in vivo studies in rats demonstrated a dramatic increase in myometrial PSF expression at term that was temporally associated with reduced levels of the myometrial PR. Accordingly, we propose that PSF acts as a PR corepressor and contributes to the functional withdrawal of progesterone and the initiation of human labor.

Heterodimers of retinoic acid receptors and thyroid hormone receptors display unique combinatorial regulatory properties

Nuclear receptors are ligand-regulated transcription factors that regulate key aspects of metazoan development, differentiation, and homeostasis. Nuclear receptors recognize target genes by binding to specific DNA recognition sequences, denoted hormone response elements (HREs). Many nuclear receptors can recognize HREs as either homodimers or heterodimers. Retinoid X receptors (RXRs), in particular, serve as important heterodimer partners for many other nuclear receptors, including thyroid hormone receptors (TRs), and RXR/TR heterodimers have been proposed to be the primary mediators of target gene regulation by T3 hormone. Here, we report that the retinoic acid receptors (RARs), a distinct class of nuclear receptors, are also efficient heterodimer partners for TRs. These RAR/TR heterodimers form with similar affinities as RXR/TR heterodimers on an assortment of consensus and natural HREs, and preferentially assemble with the RAR partner 5' of the TR moiety. The corepressor and coactivator recruitment properties of these RAR/TR heterodimers and their transcriptional activities in vivo are distinct from those observed with the corresponding RXR heterodimers. Our studies indicate that RXRs are not unique in their ability to partner with TRs, and that RARs can also serve as robust heterodimer partners and combinatorial regulators of T3-modulated gene expression.

A nuclear isoform of the focal adhesion LIM-domain protein Trip6 integrates activating and repressing signals at AP-1- and NF-kappaB-regulated promoters

Glucocorticoid receptor (GR)-mediated transrepression of the transcription factors AP-1 and NF-kappaB, responsible for most of the anti-inflammatory effects of glucocorticoids, is initiated by the tethering of GR to the promoters of target genes. We report that this tethering is mediated by a nuclear isoform of the focal adhesion LIM domain protein Trip6. Trip6 functions as a coactivator for both AP-1 and NF-kappaB. As shown by chromatin immunoprecipitation, Trip6 is recruited to the promoters of target genes together with AP-1 or NF-kappaB. In the presence of glucocorticoids, GR joins the Trip6 complex. Reducing the level of Trip6 by RNA interference or abolishing its interaction with GR by dominant-negative mutation eliminates transrepression. We propose that GR tethering to the target promoter through Trip6 forms the basis of transrepression, and that Trip6 exerts its nuclear functions by acting as a molecular platform, enabling target promoters to integrate activating or repressing signals.

Retinoic acid receptor-alpha is stabilized in a repressive state by its C-terminal, isotype-specific F domain

Retinoic acid receptors (RARs) are hormone-regulated transcription factors that play multiple roles in vertebrate development and differentiation. Three isotypes of RARs, alpha, beta, and gamma, are encoded by distinct genetic loci and possess distinct transcriptional properties. Typically, RARalpha represses target gene transcription in the absence of hormone, whereas RARbeta and gamma fail to repress under these conditions. This inability of RARbeta and RARgamma to repress transcription is due to intramolecular interactions between helix 3 and helix 12 of the hormone binding domains of these isotypes that inhibit corepressor binding while favoring coactivator binding. We report here that the converse ability of RARalpha to repress requires the integrity of the receptor F domain, a domain that maps C-terminal to helix 12, varies in sequence among different nuclear receptors, and is of poorly understood function. The F domain appears to help stabilize helix 12 of RARalpha in a more open position that enhances corepressor binding and inhibits coactivator binding in the absence of hormone. Intriguingly, the RARalpha F domain is isotype autonomous in its function. We speculate that the RARalpha F domain may dock elsewhere on the receptor surface, and this intramolecular interaction may maintain RARalpha helix 12 in an open, repression-competent conformation.

Interaction of nuclear receptor ligands with the Vitamin D signaling pathway in prostate cancer

A number of hormonal ligands and/or the nuclear receptors that mediate their actions have been targeted for prostate cancer therapy. Androgens, the ligands for the androgen receptor (AR), are critical for the growth of prostate cancer. Inhibition of androgen production has been the mainstay of treatment for advanced prostate cancer for decades. Other more recently tested targets include retinoid receptors (RAR and RXR), glucocorticoid receptors (GR), estrogen receptors (ER) and peroxisome proliferator-activated receptors (PPAR). Calcitriol, acting through the Vitamin D receptor (VDR), has many tumor suppressive activities in the prostate, including inhibition of proliferation, induction of apoptosis and/or differentiation, and reduction of cellular invasion. Because of these properties, calcitriol and its less hypercalcemic analogs are being evaluated as agents to prevent or treat prostate cancer. Androgens, retinoids, glucocorticoids, estrogens and agonists of PPAR directly or indirectly impact Vitamin D signaling pathways, and vice versa. In order to design the most effective strategies to use calcitriol to prevent or treat prostate cancer, the interactions of other nuclear receptors and their ligands with the Vitamin D signaling pathway need to be considered.

Human ADA3 binds to estrogen receptor (ER) and functions as a coactivator for ER-mediated transactivation

We have recently identified the hADA3 protein, the human homologue of yeast transcriptional coactivator yADA3, as a novel HPV16 E6 target. Using ectopic expression approaches, we further demonstrated that hADA3 directly binds to the 9-cis retinoic acid receptors alpha and beta, and functions as a coactivator for retinoid receptor-mediated transcriptional activation. Here, we examined the role of endogenous hADA3 as a coactivator for estrogen receptor (ER), an important member of the nuclear hormone receptor superfamily. We show that ADA3 directly interacts with ER alpha and ER beta. Using the chromatin immunoprecipitation assay, we also show that hADA3 is a component of the activator complexes bound to the native ER response element within the promoter of the estrogen-responsive gene pS2. Furthermore, using an ER response element-luciferase reporter, we show that overexpression of ADA3 enhances the ER alpha- and ER beta-mediated sequence-specific transactivation. Reverse transcription-PCR analysis showed an ADA3-mediated increase in estrogen-induced expression of the endogenous pS2 gene. More importantly, using RNA interference against hADA3, we demonstrate that inhibition of endogenous hADA3 inhibited ER-mediated transactivation and the estrogen-induced increase in the expression of pS2, cathepsin D, and progesterone receptor, three widely known ER-responsive genes. The HPV E6 protein, by targeting hADA3 for degradation, inhibited the ER alpha-mediated transactivation and the protein expression of ER target genes. Thus, our results demonstrate that ADA3 directly binds to human estrogen receptor and enhances the transcription of ER-responsive genes, suggesting a broader role of mammalian hADA3 as a coactivator of nuclear hormone receptors and the potential role of these pathways in HPV oncogenesis.

Acetylation of nuclear receptors in cellular growth and apoptosis

Post-translational modification of chromatin histones governs a key mechanism of transcriptional regulation. Histone acetylation, together with methylation, phosphorylation, ubiquitylation, sumoylation, glycosylation, and ADP ribosylation, modulate the activity of many genes by modifying both core histones and non-histone transcription factors. Epigenetic protein modification plays an important role in multiple cellular processes including DNA repair, protein stability, nuclear translocation, protein-protein interactions, and in regulation of cellular proliferation, differentiation and apoptosis. Histone acetyltransferases modify histones, coactivators, nuclear transport proteins, structural proteins, cell cycle components and transcription factors including p53 and nuclear receptors. The estrogen, PPARgamma and androgen receptor are members of the nuclear receptor (NR) superfamily. The androgen receptor (AR) and estrogen receptor alpha (ERalpha) are directly acetylated by histone acetyltransferases at a motif that is conserved between species and other NR. Point mutations at the lysine residue within the acetylation motif of the AR and ERalpha have been identified in prostate cancer as well as in breast cancer tissue. Acetylation of the NR governs ligand sensitivity and hormone antagonist responses. The AR is acetylated by p300, P/CAF and TIP60 and acetylation of the AR regulates co-regulator recruitment and growth properties of the receptors in cultured cells and in vivo. AR acetylation mimic mutants convey reduced apoptosis and enhanced growth properties correlating with altered promoter specificity for cell-cycle target genes. Cell-cycle control proteins, including cyclins, in turn alter the access of transcription factors and nuclear receptors to the promoters of target genes.

High tumor incidence and activation of the PI3K/AKT pathway in transgenic mice define AIB1 as an oncogene

The gene encoding AIB1, an estrogen receptor coactivator, is amplified in a subset of human breast cancers. Here we show that overexpression of AIB1 in transgenic mice (AIB1-tg) leads to mammary hypertrophy, hyperplasia, abnormal postweaning involution, and the development of malignant mammary tumors. Tumors are also increased in other organs, including the pituitary and uterus. AIB1 overexpression increases mammary IGF-I mRNA and serum IGF-I protein levels. In addition, IGF-I receptor and downstream signaling molecules are activated in primary mammary epithelial cells and mammary tumor cells derived from AIB1-tg mice. Knockdown of AIB1 expression in cultured AIB1-tg mammary tumor cells leads to reduced IGF-I mRNA levels and increased apoptosis, suggesting that an autocrine IGF-I loop underlies the mechanism of AIB1-induced oncogenesis.

Familial/sporadic glucocorticoid resistance: clinical phenotype and molecular mechanisms

Glucocorticoids regulate a variety of biologic processes and exert profound influences on many physiologic functions. Their actions are mediated by the glucocorticoid receptor (GR), which belongs to the nuclear receptor family of ligand-dependent transcription factors. Alterations in tissue sensitivity to glucocorticoids may manifest as states of resistance or hypersensitivity. Glucocorticoid resistance is a rare, familial or sporadic, condition characterized by generalized, partial target-tissue resistance to glucocorticoids. Compensatory elevations in circulating adrenocorticotropic hormone (ACTH) concentrations lead to increased production of adrenal steroids with mineralocorticoid and/or androgenic activity and their corresponding clinical manifestations, as well as increased urinary free-cortisol excretion in the absence of symptomatology suggestive of hypercortisolism. The molecular basis of the condition has been ascribed to mutations in the GR gene, which impair normal glucocorticoid signal transduction, altering tissue sensitivity to glucocorticoids. The present review focuses on the mechanisms of GR action and the clinical manifestations and molecular mechanisms of familial/sporadic glucocorticoid resistance.

The LATS2/KPM Tumor Suppressor Is a Negative Regulator of the Androgen Receptor

The androgen receptor (AR) is a member of the steroid receptor superfamily that plays critical roles in the development and maintenance of the male reproductive system and in prostate cancer. Actions of AR are controlled by interaction with several classes of coregulators. In this study, we have identified LATS2/KPM as a novel AR-interacting protein. Human LATS1 and LATS2 are tumor suppressors that are homologs of Drosophila warts/lats. The interaction surface of LATS2 is mapped to the central region of the protein, whereas the AR ligand binding domain is sufficient for this interaction. LATS2 functions as a modulator of AR by inhibiting androgen-regulated gene expression. The mechanism of LATS2-mediated repression of AR activity appears to involve the inhibition of AR NH2- and COOH-terminal interaction. Chromatin immunoprecipitation assays in human prostate carcinoma cells reveal that LATS2 and AR are present in the protein complex that binds at the promoter and enhancer regions of prostate-specific antigen, and overexpression of LATS2 results in a reduction in androgen-induced expression of endogenous prostate-specific antigen mRNA. Immunohistochemistry shows that LATS2 and AR are localized within the prostate epithelium and that LATS2 expression is lower in human prostate tumor samples than in normal prostate. The results suggest that LATS2 may play a role in AR-mediated transcription and contribute to the development of prostate cancer.

Cell type-specific bidirectional regulation of the glucocorticoid-induced leucine zipper (GILZ) gene by estrogen

Estrogen has numerous beneficial physiological actions; however, by acting as a mitogen, it plays a significant role in the induction and maintenance of breast cancer. Although the positive effects of estrogen on gene expression are well described, negative gene regulation is not. Using microarray analysis, we identified 27 genes that were up-regulated and 20 that were down-regulated by estrogen in MCF-7 human breast cancer cells. One gene encoding GILZ (glucocorticoid-induced leucine zipper protein), a putative apoptosis-regulating transcription factor, is rapidly down-regulated by estrogen in these cells. Estrogen antagonists block the down-regulation. The region of the GILZ promoter between nucleotides -104 and -69 mediates both basal activity and estrogen-dependent down-regulation in MCF-7 cells. This region contains a functional Oct-1 binding site and a cyclic AMP response element binding protein (CREB) binding site. The same DNA region mediates up-regulation by estrogen in HeLa and HEK293 cells, indicating that cell-specific factors are involved in estrogen regulation of this gene. The estrogen receptor (ER) is present in GILZ promoter protein complexes, but it does not bind directly to the promoter itself, as the DNA-binding domain of the estrogen receptor is not required for down-regulation. Elimination of the CREB binding site blocks both basal activity and estrogen regulation. Our results suggest that ER action at the CRE may mediate estrogen-dependent, cell-specific regulation of this gene.

The Fanconi anemia core complex associates with chromatin during S phase

Fanconi anemia (FA) is an autosomal recessive disease marked by bone marrow failure, birth defects, and cancer. The FA proteins FANCA, FANCC, FANCE, FANCF, FANCG, and FANCL participate in a core complex. We previously have shown that several members of this complex bind to chromatin until mitosis and that this binding increases after DNA damage. The purpose of the present study was to determine the dynamics of complex movement between cytoplasm and nuclear compartments. Fluorescent-tagged versions of FANCA, FANCC, and FANCG colocalize in cytoplasm and nucleus, chiefly in chromatin. At the G1-S border, the FA core complex exists as foci on chromatin, progressively diffusing and migrating to the nuclear periphery and becoming completely excluded from condensed chromosomes by mitosis. Chromatin fiber analysis shows FA proteins diffusely staining along chromatin fibers during G1-S and S phase. Treatment with the DNA cross-linker mitomycin C results in a diffusion of foci and increased binding of complex proteins to chromatin, as well as diffuse and increased complex binding to chromatin fibers. These data are consistent with the idea that the FA proteins function at the level of chromatin during S phase to regulate and maintain genomic stability.

Human Tissue Kallikreins: Physiologic Roles and Applications in Cancer

Tissue kallikreins are members of the S1 family (clan SA) of trypsin-like serine proteases and are present in at least six mammalian orders. In humans, tissue kallikreins (hK) are encoded by 15 structurally similar, steroid hormone–regulated genes (KLK) that colocalize to chromosome 19q13.4, representing the largest cluster of contiguous protease genes in the entire genome. hKs are widely expressed in diverse tissues and implicated in a range of normal physiologic functions from the regulation of blood pressure and electrolyte balance to tissue remodeling, prohormone processing, neural plasticity, and skin desquamation. Several lines of evidence suggest that hKs may be involved in cascade reactions and that cross-talk may exist with proteases of other catalytic classes. The proteolytic activity of hKs is regulated in several ways including zymogen activation, endogenous inhibitors, such as serpins, and via internal (auto)cleavage leading to inactivation. Dysregulated hK expression is associated with multiple diseases, primarily cancer. As a consequence, many kallikreins, in addition to hK3/PSA, have been identified as promising diagnostic and/or prognostic biomarkers for several cancer types, including ovarian, breast, and prostate. Recent data also suggest that hKs may be causally involved in carcinogenesis, particularly in tumor metastasis and invasion, and, thus, may represent attractive drug targets to consider for therapeutic intervention.

Estrogens and the hair follicle. Ostrogene und der Haarfollikel

While it is undisputed that estrogens (1 beta-estradiol, E2) are mainly involved in skin physiology and operate as potent hair growth modulators, our knowledge about the estrogen target cells in skin and exact signaling pathways is still very limited. The current review provides an overview of estrogen effects on hair follicle cycling, cutaneous expression of estrogen receptors, and potential functions of estrogens in hair biology. We discuss potential target genes of estrogen receptor-mediated signaling in the skin, explore the interplay of estrogens with other hormones, growth factors and enzymes, and define major open questions in this intriguing and far too long neglected area of hair research.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

TIP30 interacts with an estrogen receptor alpha-interacting coactivator CIA and regulates c-myc transcription

Deregulation of c-myc expression is implicated in the pathogenesis of many neoplasias. Estrogen receptor alpha (ERalpha) can increase the rate of c-myc transcription through the recruitment of a variety of cofactors to the promoter, yet the precise roles of these cofactors in transcription and tumorigenesis are largely unknown. We show here that a putative tumor suppressor TIP30, also called CC3 or Htatip2, interacts with an ERalpha-interacting coactivator CIA. Using chromatin immunoprecipitation assays, we demonstrate that TIP30 and CIA are distinct cofactors that are dynamically associated with the promoter and downstream regions of the c-myc gene in response to estrogen. Both TIP30 and CIA are recruited to the c-myc gene promoter by liganded ERalpha in the second transcription cycle. TIP30 overexpression represses ERalpha-mediated c-myc transcription, whereas TIP30 deficiency enhances c-myc transcription in both the absence and presence of estrogen. Ectopic CIA cooperates with TIP30 to repress ERalpha-mediated c-myc transcription. Moreover, virgin TIP30 knockout mice exhibit increased c-myc expression in mammary glands. Together, these results reveal an important role for TIP30 in the regulation of ERalpha-mediated c-myc transcription and suggest a mechanism for tumorigenesis promoted by TIP30 deficiency.

Partner Molecules of Accessory Protein Vpr of the Human Immunodeficiency Virus Type 1

Vpr (Viral protein-R) of the Human Immunodeficiency Virus type-1 is a 14-kDa virion-associated protein, conserved in HIV-1, -2 and the Simian Immunodeficiency Virus (SIV). Vpr is incorporated into the virion, travels to the nucleus, and has multiple activities including promoter activation, cell cycle arrest at the G2/M transition and apoptosis induction. Through these activities, Vpr is thought to influence not only viral replication but also numerous host cell functions. These functions may be categorized in three groups depending on the domains of Vpr that support them: (1) functions mediated by the amino terminal portion of Vpr, like virion packaging; (2) functions mediated by the carboxyl terminal portion such as cell cycle arrest; and (3) functions that depend on central alpha-helical structures such as transcriptional activation, apoptosis and subcellular shuttling. Association of these activities to specific regions of the Vpr molecule appears to correlate to the host/viral molecules that interact with corresponding portion of Vpr. They include Gag, host transcription factors/coactivators such as SP1, the glucocorticoid receptor, p300/CREB-binding protein and TFIIB, apoptotic adenine nucleotide translocator, cyclophilin A and 14-3-3 proteins. The properties of Vpr molecule has made it difficult to assess its function and determine the true cellular interactors. Further studies on Vpr function are needed to fully assess the function of this important early regulatory molecule of HIV and other lentiviruses.

Urban Renewal in the Nucleus: Is Protein Turnover by Proteasomes Absolutely Required for Nuclear Receptor-Regulated Transcription?

The importance of the ubiquitin proteasome pathway in higher eukaryotes has been well established in cell cycle regulation, signal transduction, and cell differentiation, but has only recently been linked to nuclear hormone receptor-regulated gene transcription. Characterization of a number of ubiquitin proteasome pathway enzymes as coactivators and observations that several nuclear receptors are ubiquitinated and degraded in the course of their nuclear activities provide evidence that ubiquitin proteasome-mediated protein degradation plays an integral role in eukaryotic transcription. In addition to receptors, studies have revealed that coactivators are ubiquitinated and degraded via the proteasome. The notion that the ubiquitin proteasome pathway is involved in gene transcription is further strengthened by the fact that ubiquitin proteasome pathway enzymes are recruited to the promoters of target genes and that proteasome-dependent degradation of nuclear receptors is required for efficient transcriptional activity. These findings suggest that protein degradation is coupled with nuclear receptor coactivation activity. It is possible that the ubiquitin proteasome pathway modulates transcription by promoting remodeling and turnover of the nuclear receptor-transcription complex. In this review, we discus the possible role of the ubiquitin proteasome pathway in nuclear hormone receptor-regulated gene transcription.

Steroid receptor control of reproductive behavior

Steroid hormone receptors in the brain were thought to be only activated by steroid hormones. Once steroid binds to the receptor, it would act on DNA to regulate gene transcription. Recent data indicate that steroid receptor action is more complex. Steroid receptor activity in the brain is under the control of co-regulatory proteins, such as coactivators. It is the expression of these additional proteins that modulate the activity of steroid receptors. Furthermore, steroid receptors are not only activated by steroid, but can also be activated by neurotransmitters in the absence of steroid. For example, progestin receptors in rodent brain are sensitive to progesterone and to social cues in the environment. This review discusses these emerging mechanisms for steroid receptor control in developing and adult brain.

Molecular cloning and developmental expression patterns of thyroid hormone receptors and T3 target genes in the turbot (Scophtalmus maximus) during post-embryonic development

Thyroid hormones (TH) are pleiotropic factors important for many developmental and physiological functions in vertebrates and particularly in amphibian metamorphosis. Their effects are mediated by two specific receptors (TRalpha and TRbeta), which are ligand-dependent transcription factors, members of the nuclear hormone receptor superfamily. Besides their pivotal role in amphibian metamorphosis, TH are also critical for fish metamorphosis. As this later role of TH is less studied, we analyzed their action in the turbot (Scophtalmus maximus), a metamorphosing flat fish. We describe the isolation of sequences for the turbot orthologs of a number of Xenopus genes, which are induced during amphibian metamorphosis. Developmental expression of these genes during turbot metamorphosis was studied by several methods and the expression patterns of these genes compared with those in Xenopus and flounder. We find that the period between the onset and the end of eye migration (day 22 to day 30 post-hatching) most likely corresponds to the metamorphic climax with either high TRalpha or high TH levels. Our results show that in contrast to amphibians, it is TRalpha and not TRbeta mRNA that is up-regulated during metamorphosis. Our results highlight the notion that TH regulates, through a rise of TR expression, a genetic cascade during turbot metamorphosis. The fact that TH regulates metamorphosis in amphibian and teleost fishes suggests that TH-regulated metamorphosis is a post-embryonic process conserved in most vertebrates.

Effects of environmental estrogenic chemicals on AP1 mediated transcription with estrogen receptors alpha and beta

There has been much discussion concerning endocrine disrupting chemicals suspected of exerting adverse effects in both wildlife and humans. Since the majority of these compounds are estrogenic, a large number of in vitro tests for estrogenic characteristics have been developed for screening purpose. One reliable and widely used method is the reporter gene assay employing estrogen receptors (ERs) and a reporter gene with a cis-acting estrogen responsive element (ERE). Other elements such as AP1 also mediate estrogenic signals and the manner of response could be quite different from that of ERE. Since this has yet to be explored, the ER mediated AP1 activity in response to a series of environmental estrogens was investigated in comparison with ERE findings. All the compounds exhibited estrogenic properties with ERE-luc and their AP1 responses were quite similar. These was one exception, however, p,p'-DDT (1,1,1,-trichloro-2,2-bis(p-chlorophenyl)ethane) did not exert any AP1-luc activity, while it appeared to be estrogenic at 10(-7) to 10(-5)M with the ERE action. None of the compounds demonstrated ER beta:AP1 activity. These data suggest that significant differences can occur in responses through the two estrogen pathways depending on environmental chemicals.

Vitamin D Receptor–DNA Interactions

The vitamin D receptor (VDR) is a member of the steroid and nuclear hormone receptor superfamily of eukaryotic transcription factors and binds target DNA, or response elements, as a homodimer or heterodimer with the 9-cis retinoid X receptor (RXR). In this chapter, we survey the current understanding of VDR-DNA interactions, emphasizing recent structural insights. We highlight the stereochemical interactions that dictate DNA binding and hexameric half-site sequence affinity as well as the protein-protein interactions that account for preferential binding to a direct repeat of half-sites with three base pairs of spacer DNA (DR3). In addition, we review alternative response element arrangements other than those with DR3. Finally, the chapter discusses the VDR DNA binding domain (DBD) and suggests that it violates classical canons because it does not heterodimerize with the RXR DBD. This unique behavior of VDR is considered in light of recent results demonstrating the formation of VDR DBD-DNA and DR3 DBD-DNA complexes with RXR using a mutant VDR protomer.

Coactivators in Assay Design for Nuclear Hormone Receptor Drug Discovery

Nuclear hormone receptors (NHRs) represent one of the most important drug targets in terms of therapeutic applications. The NHR superfamily consists of a family of DNA binding transcription factors whose function can be controlled by small molecules (steroids or organic acids). Therefore, NHRs are suitable protein targets for the therapies of human diseases. Some of the current marketed drugs, including several anticancer and antidiabetic drugs, are known to target NHRs. Examples include the anticancer and retinoid X receptor-targeting Targretin and the antidiabetic and peroxisome proliferative-activated receptor-gamma-targeting thiaozolidinediones. More NHR-targeting drugs are expected in the coming years. Identification of specific NHR modulators, as well as identification of ligands for orphan NHRs, will lead to new therapies for many human diseases. Many pharmaceutical companies are investing in NHR-targeted drugs, which are estimated to be 10-15% of the US dollars 400 billion global pharmaceutical market. This minireview discusses various aspects of NHR drug discovery, with a focus on the application of NHR coactivators in assay design for NHR ligand identification.

Molecular action of the estrogen receptor and hormone dependency in breast cancer

The measurement of estrogen receptor (ER)alpha in breast cancer tissues is important to discriminate between the hormone dependent and independent tumors. Recently, a second ER, referred to as ERbeta, has been identified. The DNA binding domain of ERbeta is 96% conserved compared with ERalpha, and the ligand binding domain shows 53% conserved residues, suggesting that both receptors can bind estrogen responsive elements on target genes, and that they may also bind similar ligand. While both receptors bind to 17beta-estradiol with equal affinity, other compounds bind with varying affinities to the two receptors. Since the function of ERbeta in breast cancer progression is not well understood, further characterization of the function of ERbeta and its isoforms in breast cancer is warranted. Various kinds of cofactors, such as steroid receptor coactivator-1 (SRC-1), transcription intermediary factor 2 (TIF2), and amplified in breast cancer 1 (AIB1), have also been reported. These coactivators interact with nuclear receptors in a ligand-dependent manner and enhance transcriptional activation by the receptor via histone acetylation/methylation and recruitment of additional coactivator, such as CREB binding protein (CBP)/p300. Thus, action of estrogen is not as simple as thought previously, and is likely influenced by ERbeta, its variants and interaction with cofactors. Improved understanding of the ER mechanism may follow from the discovery of these proteins, although their precise roles remain to be determined.

Making sense of cross-talk between steroid hormone receptors and intracellular signaling pathways: who will have the last word?

In classical models of nuclear steroid hormone receptor function, ligand binds receptor, heat shock proteins dissociate, and receptor dimers enter or are withheld in the nucleus and interact with coregulatory molecules to mediate changes in gene expression. The footnotes, "receptors become phosphorylated" and "dynamic nucleo-cytoplasmic shuttling occurs" describe well-accepted, but less well-understood aspects of receptor action. Recently, the idea that several protein kinases are activated in response to steroid hormone binding to cognate cytoplasmic or membrane-associated receptors has become fashionable. However, the precise role of steroid hormone receptor phosphorylation and our understanding of which cytoplasmic kinases are activated and their functional significance remain elusive. This review provides an overview of the primary ways in which steroid hormone receptor and growth factor cross-talk occurs, using the human progesterone receptor (PR) as a model. The functional consequences of PR phosphorylation by protein kinases classically activated in response to peptide growth factors and novel extranuclear or nongenomic functions of PR as potential independent initiators of signal transduction pathways are discussed. Intracellular protein kinases are emerging as key mediators of steroid hormone receptor action. Cross-talk between steroid receptor- and growth factor-initiated signaling events may explain how gene subsets are coordinately regulated by mitogenic stimuli in hormonally responsive normal tissues, and is suspected to play a role in their cancer biology.

Purification and identification of a novel complex which is involved in androgen receptor-dependent transcription

The androgen receptor (AR) binds to and activates transcription of target genes in response to androgens. In an attempt to isolate cofactors capable of influencing AR transcriptional activity, we used an immunoprecipitation method and identified a 44-kDa protein, designated p44, as a new AR-interacting protein. p44 interacts with AR in the nucleus and with an androgen-regulated homeobox protein (NKX3.1) in the cytoplasm of LNCaP cells. Transient-transfection assays revealed that p44 enhances AR-, glucocorticoid receptor-, and progesterone receptor-dependent transcription but not estrogen receptor- or thyroid hormone receptor-dependent transcription. p44 was recruited onto the promoter of the prostate-specific antigen gene in the presence of the androgen in LNCaP cells. p44 exists as a multiprotein complex in the nuclei of HeLa cells. This complex, but not p44 alone, enhances AR-driven transcription in vitro in a cell-free transcriptional system and contains the protein arginine methyltransferase 5, which acts synergistically with p44 to enhance AR-driven gene expression in a methyltransferase-independent manner. Our data suggest a novel mechanism by which the protein arginine methyltransferase is involved in the control of AR-driven transcription. p44 expression is dramatically enhanced in prostate cancer tissue compared with adjacent benign prostate tissue.

Protein inhibitor of activated signal transducer and activator of transcription 1 interacts with the N-terminal domain of mineralocorticoid receptor and represses its transcriptional activity: implication of small ubiquitin-related modifier 1 modification

Molecular mechanisms underlying mineralocorticoid receptor (MR)-mediated gene expression are not fully understood but seem to largely depend upon interactions with specific coregulators. To identify novel human MR (hMR) molecular partners, yeast two-hybrid screenings performed using the N-terminal domain as bait, allowed us to isolate protein inhibitor of activated signal transducer and activator of transcription (PIAS)1 and PIASxbeta, described as SUMO (small ubiquitin-related modifier) E3-ligases. Specific interaction between PIAS1 and hMR was confirmed by glutathione-S-transferase pull-down experiments and N-terminal subdomains responsible for physical contacts were delineated. Transient transfections demonstrated that PIAS1 is a corepressor of aldosterone-activated MR transactivation but has no significant effect on human glucocorticoid receptor transactivation. The agonist or antagonist nature of the bound ligand also determines PIAS1 corepressive action. We provided evidence that PIAS1 conjugated SUMO-1 to hMR both in vitro and in vivo. Deciphering the unique sumoylation pattern of hMR, which possesses five consensus SUMO-1 binding sites, by combinatorial lysine substitutions, revealed a major impact of sumoylation on hMR properties. Using a murine mammary tumor virus promoter, PIAS1 action was independent of sumoylation whereas with glucocorticoid response element promoter, PIAS1 corepressive action depended on hMR sumoylation status. Taken together, our results identify a novel function for PIAS1 which interacts with the N-terminal domain of hMR and represses its ligand-dependent transcriptional activity, at least in part, through SUMO modifications.

SNF2-related CBP activator protein (SRCAP) functions as a coactivator of steroid receptor-mediated transcription through synergistic interactions with CARM-1 and GRIP-1

SRCAP (SNF2-related CBP activator protein) is a 350-kDa protein that shares homology with the SNF2 family of proteins whose members function in various aspects of transcriptional regulation. In various cell types, SRCAP is found in distinct multiprotein complexes that include proteins found in SWI/SNF chromatin remodeling complexes. SRCAP was identified by its ability to bind to CBP and was found to potentiate the ability of CBP to activate transcription. Studies in our laboratory have demonstrated that SRCAP functions as a coactivator for CREB-mediated transcription of a number of promoters, including that of the phosphoenolpyruvate carboxykinase gene. Our current studies demonstrate that SRCAP enhances phosphoenolpyruvate carboxykinase promoter transcription induced by glucocorticoids. SRCAP also enhances glucocorticoid receptor-mediated transcription of a simple promoter containing only two glucocorticoid response elements, indicating that SRCAP functions as a glucocorticoid receptor coactivator. In similar studies, SRCAP was also found to serve as a coactivator for the androgen receptor. SRCAP exhibits synergistic activation with nuclear receptor coactivators and functionally interacts in vivo with glucocorticoid receptor-interacting protein-1 and coactivator-associated arginine methyltransferase-1. We propose that SRCAP, by virtue of its ability to interact with CBP, functions as a coactivator to regulate transcription initiated by several signaling pathways.

Subcellular localization and mechanisms of nucleocytoplasmic trafficking of steroid receptor coactivator-1

Steroid hormone receptors are ligand-stimulated transcription factors that modulate gene transcription by recruiting coregulators to gene promoters. Subcellular localization and dynamic movements of transcription factors have been shown to be one of the major means of regulating their transcriptional activity. In the present report we describe the subcellular localization and the dynamics of intracellular trafficking of steroid receptor coactivator 1 (SRC-1). After its synthesis in the cytoplasm, SRC-1 is imported into the nucleus, where it activates transcription and is subsequently exported back to the cytoplasm. In both the nucleus and cytoplasm, SRC-1 is localized in speckles. The characterization of SRC-1 nuclear localization sequence reveals that it is a classic bipartite signal localized in the N-terminal region of the protein, between amino acids 18 and 36. This sequence is highly conserved within the other members of the p160 family. Additionally, SRC-1 nuclear export is inhibited by leptomycin B. The region involved in its nuclear export is localized between amino acids 990 and 1038. It is an unusually large domain differing from the classic leucine-rich NES sequences. Thus SRC-1 nuclear export involves either an alternate type of NES or is dependent on the interaction of SRC-1 with a protein, which is exported through the crm1/exportin pathway. Overall, the intracellular trafficking of SRC-1 might be a mechanism to regulate the termination of hormone action, the interaction with other signaling pathways in the cytoplasm and its degradation.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Growth inhibitory retinoid effects after recruitment of retinoid X receptor beta to the retinoic acid receptor beta promoter

Nuclear retinoid receptors mediate retinoid effects through tissue-specific, ligand-receptor interactions and subsequent transcriptional regulation of secondary target genes. Retinoic acid receptor beta (RARbeta) is itself a retinoid target gene with a retinoic acid response element (betaRARE) in the 5' untranslated region of the RARbeta2 gene. Altered transcriptional regulation of RARbeta may play a role in human carcinogenesis and the retinoid-responsiveness of malignant cells. Here we used retinoid X receptor-specific antibodies in electrophoretic mobility shift assays to show that the retinoid X receptor beta (RXRbeta) protein was recruited to the betaRARE, after retinoid treatment of retinoid-sensitive neuroblastoma (NB), lung and breast cancer cell lines, but not retinoid-resistant lung and breast cancer cell lines. RXRbeta selectively enhanced retinoid-induced transcriptional activation of the betaRARE. Stable overexpression of RXRalpha and RXRbeta in NB cells resulted in marked growth inhibition and cell death, which increased after retinoid treatment. However, only proteins from the RXRbeta transfectants exhibited specific RXRbeta binding to the betaRARE in vitro and in vivo, enhanced histone acetylation and increased endogenous RARbeta expression. These data indicate that recruitment of RXRbeta to the betaRARE, and consequent induction of endogenous RARbeta expression, is an important component in the retinoid anticancer signal. RXRalpha may also participate in the retinoid signal, but through mechanisms that do not involve RARbeta.

Structural insights into regulation of nuclear receptors by ligands

Nuclear receptors are DNA-binding transcription factors, the transcriptional function of many of which depends on the binding of ligands, a feature that distinguishes nuclear receptors from other transcription factors. This review will summarize recent advances in our knowledge of the interaction between selected nuclear receptors and their cognate ligands.

The roles of protein-protein interactions and protein methylation in transcriptional activation by nuclear receptors and their coactivators

Hormone-activated nuclear receptors (NR) bind to the promoters of their target genes and recruit coactivator proteins to help activate transcription. The p160 coactivators bind directly to activated NRs and recruit secondary coactivators CBP/p300 with protein acetyltransferase activity and CARM1 with protein methyltransferase activity. To further investigate the components of the p160 coactivator complex and their mechanisms of action, we have used two guiding assumptions. First, the coactivators constitute a signal transduction pathway that convey the signal from DNA-bound NRs to the transcription machinery. Second, each coactivator has signal input and signal output domains that facilitate signal transduction. These assumptions were used to address the mechanism by which CARM1 and the N-terminal region of p160 coactivators transmit activating signals to the transcription machinery. The p160-binding activity of CARM1 is in the same centrally located structural domain as the methyltransferase activity; the p160-binding domain anchors CARM1 to the target gene promoter and thereby serves as its signal input domain. CARM1 has two signal output mechanisms: the protein methyltransferase activity, which methylates histones and other proteins in the transcription initiation complex; and a strong autonomous activation function in the C-terminal region. We identified a protein, CCCP, which binds to the C-terminal region of CARM1 and cooperates synergistically with CARM1 to enhance NR function. We also defined the N-terminal region of p160 coactivators as another signal output domain, which binds a novel coactivator called coiled-coil coactivator (CoCoA). CoCoA acts synergistically with p160 coactivators to enhance NR function.

Domains of estrogen receptor alpha (ERalpha) required for ERalpha/Sp1-mediated activation of GC-rich promoters by estrogens and antiestrogens in breast cancer cells

Estrogen receptor alpha (ERalpha)/Sp1 activation of GC-rich gene promoters in breast cancer cells is dependent, in part, on activation function 1 (AF1) of ERalpha, and this study investigates contributions of the DNA binding domain (C) and AF2 (DEF) regions of ERalpha on activation of ERalpha/Sp1. 17Beta-estradiol (E2) and the antiestrogens 4-hydroxytamoxifen and ICI 182,780 induced reporter gene activity in MCF-7 and MDA-MB-231 cells cotransfected with human or mouse ERalpha (hERalpha or MOR), but not ERbeta and GC-rich constructs containing three tandem Sp1 binding sites (pSp13) or other E2-responsive GC-rich promoters. Estrogen and antiestrogen activation of hERalpha/Sp1 was dependent on overlapping and different regions of the C, D, E, and F domains of ERalpha. Antiestrogen-induced activation of hERalpha/Sp1 was lost using hERalpha mutants deleted in zinc finger 1 [amino acids (aa) 185-205], zinc finger 2 (aa 218-245), and the hinge/helix 1 (aa 265-330) domains. In contrast with antiestrogens, E2-dependent activation of hERalpha/Sp1 required the C-terminal F domain (aa 579-595), which contains a beta-strand structural motif. Moreover, in peptide competition experiments overexpression of a C-terminal (aa 575-595) F domain peptide specifically blocked E2-dependent activation of hERalpha/Sp1, suggesting that F domain interactions with nuclear cofactors are required for ERalpha/Sp1 action.

Variation in the FABP2 promoter alters transcriptional activity and is associated with body composition and plasma lipid levels

The fatty acid-binding proteins (FABPs) are cytoplasmic proteins involved in intracellular fatty acid transport and metabolism. FABP2, the intestinal-type FABP, is expressed exclusively in enterocytes in the small intestine. In previous studies of an Ala54Thr substitution in FABP2, the Thr-allele showed association with increased lipid oxidation, elevated plasma lipids, and impaired insulin sensitivity. We screened roughly 1 kb 5' of the FABP2 initiation codon and identified three insertion/deletion polymorphisms and four single nucleotide polymorphisms (SNPs). Three of the SNPs were in complete linkage disequilibrium with the three insertion/deletion polymorphisms, defining exactly two haplotypes (FABP2p-ID). We tested the hypothesis that this variation alters gene expression by transfecting Caco-2 cells with pGL3-Basic constructs containing opposite FABP2p-ID haplotypes. Luciferase assays showed a statistically significant two-fold increase in gene expression of the pGL3-insertion construct over the pGL3-deletion construct (P<0.001; n=5). We also tested for association between three FABP2 variants and measurements of body composition, plasma lipids, and insulin sensitivity in non-diabetic control subjects from the San Luis Valley Diabetes Study (n=714). The only informative variant, FABP2p-ID, was statistically significantly associated with body mass index (P=0.042) and marginally associated with fat mass (P=0.084), cholesterol (P=0.066), and HOMA IR (a derived measure of insulin resistance; P=0.062) in the entire cohort. Similar associations were seen only in non-Hispanics when the analysis was stratified by ethnicity. Within the non-Hispanic subgroup, the effects of FABP2p-ID on plasma lipids were sex-specific. These results suggest that genetic variation in the 5' region of FABP2 affects transcriptional activity, presumably leading to alterations in body composition and lipid processing.

A role for cofactor-cofactor and cofactor-histone interactions in targeting p300, SWI/SNF and Mediator for transcription

Transcriptional activation from chromatin by nuclear receptors (NRs) requires multiple cofactors including CBP/p300, SWI/SNF and Mediator. How NRs recruit these multiple cofactors is not clear. Here we show that activation by androgen receptor and thyroid hormone receptor is associated with the promoter targeting of SRC family members, p300, SWI/SNF and the Mediator complex. We show that recruitment of SWI/SNF leads to chromatin remodeling with altered DNA topology, and that both SWI/SNF and p300 histone acetylase activity are required for hormone-dependent activation. Importantly, we show that both the SWI/SNF and Mediator complexes can be targeted to chromatin by p300, which itself is recruited through interaction with SRC coactivators. Furthermore, histone acetylation by CBP/p300 facilitates the recruitment of SWI/SNF and Mediator. Thus, our data indicate that multiple cofactors required for activation are not all recruited through their direct interactions with NRs and underscore a role of cofactor-cofactor interaction and histone modification in coordinating the recruitment of multiple cofactors.

Smad4 as a transcription corepressor for estrogen receptor alpha

Antiestrogen compounds exhibit a variety of different effects in different tissues and are widely used for the treatment of osteoporosis, breast cancer, and other diseases. Upon examining the molecular mechanisms, we found that Smad4, a common signal transducer in the bone morphogenetic protein (BMP)/transforming growth factor-beta (TGF-beta) signaling pathway, functions as a transcription corepressor for human estrogen receptor alpha (ERalpha). Endogenous ERalpha was co-immunoprecipitated with Smad4, and the interaction was induced by antiestrogen ligands such as tamoxifen, raloxifene, and droloxifen, which was confirmed in chromatin immunoprecipitation assays. Smad4 and ERalpha form a complex when ERalpha binds to the estrogen-responsive element within the estrogen target gene promoter. Importantly, the expression of Smad4 inhibits both antiestrogen-induced luciferase activity and estrogen downstream target gene transcription in breast cancer cells. Mapping of the interaction domains indicates that the activation function 1 (AF1) domain of ERalpha is essential for its interaction with Smad4, while the MH1 domain and linker region of Smad4 are essential for the interaction. Our findings represent a novel mechanism that TGF-beta may regulate cell fate through Smad4-mediated cross-talk with estrogen.

Sumoylation of the progesterone receptor and of the steroid receptor coactivator SRC-1

SUMO-1 (small ubiquitin-like modifier) conjugation regulates the subcellular localization, stability, and activity of a variety of proteins. We show here that SUMO-1 overexpression markedly enhances progesterone receptor (PR)-mediated gene transcription. PR undergoes a sumoylation at lysine 388 located in its N-terminal domain. However, sumoylation of the receptor is not responsible for enhanced transcription because substitution of its target lysine did not abolish the effect of SUMO-1 and even converted the receptor into a slightly more active transactivator. Furthermore estrogen receptor alpha (ERalpha)-driven transcription is also enhanced by SUMO-1 overexpression contrasting with the absence of sumoylation of this receptor. We thus analyzed SUMO-1 conjugation to the steroid receptor coactivator SRC-1. We showed that this protein contains two major sites of conjugation at Lys-732 and Lys-774. Sumoylation was shown to increase PR-SRC-1 interaction and to prolong SRC-1 retention in the nucleus. It did not prevent SRC-1 ubiquitinylation and did not exert a clear effect on the stability of the protein. Overexpression of SUMO-1 enhanced PR-mediated gene transcription even in the presence of non-sumoylated mutants of SRC-1. This observation suggests that among the many protein partners involved in steroid hormone-mediated gene regulation several are probably targets of SUMO-1 modification.

Isotype-restricted corepressor recruitment: a constitutively closed helix 12 conformation in retinoic acid receptors beta and gamma interferes with corepressor recruitment and prevents transcriptional repression

Retinoic acid receptors (RARs) are ligand-regulated transcription factors that play multiple roles in vertebrate development and differentiation. RARs as a class are capable of both repressing and activating target gene expression. Transcriptional repression is mediated through the recruitment of corepressor proteins such as SMRT. Notably, vertebrates encode three major forms of RARs, alpha, beta, and gamma, and these distinct RAR isotypes differ in the ability to recruit a corepressor. RAR alpha strongly interacts with SMRT and can repress target gene transcription, whereas RAR beta and -gamma interact with SMRT only weakly and fail to repress. We report here the use of a genetic suppressor approach, based on a yeast two-hybrid interaction assay using Saccharomyces cerevisiae, for the isolation of RAR beta mutants that have gained the RAR alpha-like corepressor phenotype, i.e., a strong interaction with SMRT and the ability to repress gene expression in vertebrate cells. Analysis of these gain-of-function mutants indicates that the different corepressor interaction properties of RAR alpha, -beta and -gamma are determined by a gating mechanism through which amino acid differences in the helix 3 region of these receptors influence the position of the receptor C-terminal helix 12 domain. As a consequence, the RAR beta and RAR gamma receptors appear to adopt a constitutively closed helix 12 conformation in the absence of hormone that may approximate the conformation of RAR alpha when bound to hormone agonist. This closed helix 12 conformation in RAR beta and RAR gamma blocks corepressor binding, prevents repression, and permits significant levels of target gene activation even in the absence of hormone. We refer to this phenomenon as a "gate-latch" model of corepressor regulation.

New insights into the mechanisms of vitamin D action

The biologically active metabolite of vitamin D, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is a secosteroid whose genomic mechanism of action is similar to that of other steroid hormones and is mediated by stereospecific interaction of 1,25(OH)(2)D(3) with the vitamin D receptor (VDR) which heterodimerizes with the retinoid X receptor (RXR). After interaction with the vitamin D response element (VDRE) in the promoter of target genes, transcription proceeds through the interaction of VDR with coactivators and with the transcription machinery. The identification of the steps involved in this process has been a major focus of recent research in the field. However, the functional significance of target proteins as well as the functional significance of proteins involved in the transport and metabolism of vitamin D is also of major importance. Within the past few years much new information has been obtained from studies using knockout and transgenic mice. New insight has been obtained using this technology related to the physiological significance of the vitamin D binding protein (DBP), used to transport vitamin D metabolites, as well as the physiological significance of target proteins including 25-hydroxyvitamin D(3) 24-hydroxylase (24(OH)ase), 25-hydroxyvitamin D(3)-1 alpha-hydroxylase (1 alpha-(OH)ase), VDR, and osteopontin. The crystal structure of the DBP and the ligand binding domain of the VDR have recently been reported, explaining, in part, the unique properties of these proteins. In addition novel 1,25(OH)(2)D(3) target genes have been identified including the epithelial calcium channel, present in the proximal intestine and in the distal nephron. Thus in recent years a number of exciting discoveries have been made that have enhanced our understanding of mechanisms involved in the pleiotropic actions of 1,25(OH)(2)D(3).

Cyclic, proteasome-mediated turnover of unliganded and liganded ERalpha on responsive promoters is an integral feature of estrogen signaling

We present an integrated model of hERalpha-mediated transcription where both unliganded and liganded receptors cycle on estrogen-responsive promoters. Using ChIP, FRAP, and biochemical analysis we evaluate hERalpha at several points in these cycles, establishing the ubiquitination status and subnuclear distribution of hERalpha, its mobility, the kinetics of transcriptional activation, and the cyclic recruitment of E3 ligases and the 19S regulatory component of the proteasome. These experiments, together with an evaluation of the inhibition of transcription and proteasome action, demonstrate that proteasome-mediated degradation and hERalpha-mediated transactivation are inherently linked and act to continuously turn over hERalpha on responsive promoters. Cyclic turnover of hERalpha permits continuous responses to changes in the concentration of estradiol.

Regulation of androgen receptor activity by the nuclear receptor corepressor SMRT

Androgen receptor (AR) is a hormone-regulated transcription factor that mediates a wide array of biological processes including sexual differentiation, spermatogenesis, and prostate cancer progression. The transcriptional activity of AR and other members of the nuclear receptor superfamily are modulated by coregulatory proteins. In this study, we have investigated the regulation of AR transcriptional activity by the silencing mediator for retinoid and thyroid hormone receptors (SMRT). We found that AR possesses an intrinsic transcriptional repression activity, and AR interacts directly with SMRT. One interacting surface on AR is mapped to the ligand-binding domain, and the presence of a DNA binding/hinge region enhances this interaction. The binding surface on SMRT is mapped to the C-terminal ID2 region, and mutation in the ID2 corepressor motif inhibits the interaction. Overexpression of SMRT inhibits dihydrotestosterone-dependent transactivation by AR and further suppresses the antiandrogen flutamide-mediated inhibition of AR activity. We provide evidence to suggest that the mechanisms of SMRT-mediated inhibition of AR activity involves inhibition of AR N/C interaction and competition with the p160 coactivator. Our data establish a significant role of SMRT in modulating AR transcriptional activity.

Homology modelling of the nuclear receptors: human oestrogen receptorbeta (hERbeta), the human pregnane-X-receptor (PXR), the Ah receptor (AhR) and the constitutive androstane receptor (CAR) ligand binding domains from the human oestrogen receptor alpha (hERalpha) crystal structure, and the human peroxisome proliferator activated receptor alpha (PPARalpha) ligand binding domain from the human PPARgamma crystal structure

We have generated by homology the three-dimensional structures of the ligand binding domain (LBD) of several interrelated human steroid hormone receptors (SHRs). These are the oestrogen receptor beta (hERbeta), the pregnane-X-receptor (PXR), the Ah receptor (AhR) and the constitutive androstane receptor (CAR). They were produced by homology modelling from the human oestrogen receptor alpha (hERalpha) crystallographic coordinates [Nature 389 (1997) 753] as a template together with the amino acid sequences for hERbeta [FEBS Lett. 392 (1996) 49], PXR [J. Clin. Invest. 102 (1998) 1016], AhR [Proc. Natl. Acad. Sci. U.S.A. 89 (1992) 815] and CAR [Nature 395 (1998) 612; Mol. Cell. Biol. 14 (1994) 1544], respectively. The selective endogenous ligand, in each case, was docked interactively within the putative ligand binding site using the position of oestradiol in hERalpha as a guide, and the total energy was calculated. In each receptor model a number of different ligands known to fit closely within the ligand binding site were interactively docked and binding interactions noted. Specific binding interactions included combinations of hydrogen bonding and hydrophobic contacts with key amino acid sidechains, which varied depending on the nature of the ligand and receptor concerned. We also produced the human peroxisome proliferator activated receptor alpha (PPARalpha) by homology modelling using the human PPARgamma (hPPARgamma) LBD crystallographic coordinates summarised in [Toxicol. In Vitro 12 (1998) 619] as a template together with the amino acid sequence for hPPARalpha [Toxicol. In Vitro 12 (1998) 619; Nature 395 (1998) 137]. The models will provide a useful tool in unravelling the complexity in the physiologic response to xenobiotics by examining the ligand binding interactions and differences between the steroid hormone receptors activation or inactivation by their ligands.