Study of nuclear receptor-induced transcription complex assembly and histone modification by chromatin immunoprecipitation assays

The injectable-only contraceptive medroxyprogesterone acetate, unlike norethisterone acetate and progesterone, regulates inflammatory genes in endocervical cells via the glucocorticoid receptor

Clinical studies suggest that the injectable contraceptive medroxyprogesterone acetate (MPA) increases susceptibility to infections such as HIV-1, unlike the injectable contraceptive norethisterone enanthate (NET-EN). We investigated the differential effects, molecular mechanism of action and steroid receptor involvement in gene expression by MPA as compared to NET and progesterone (P4) in the End1/E6E7 cell line model for the endocervical epithelium, a key point of entry for pathogens in the female genital mucosa. MPA, unlike NET-acetate (NET-A) and P4, increases mRNA expression of the anti-inflammatory GILZ and IκBα genes. Similarly, MPA unlike NET-A, decreases mRNA expression of the pro-inflammatory IL-6, IL-8 and RANTES genes, and IL-6 and IL-8 protein levels. The predominant steroid receptor expressed in the End1/E6E7 and primary endocervical epithelial cells is the glucocorticoid receptor (GR), and GR knockdown experiments show that the anti-inflammatory effects of MPA are mediated by the GR. Chromatin-immunoprecipitation results suggest that MPA, unlike NET-A and P4, represses pro-inflammatory cytokine gene expression in cervical epithelial cells via a mechanism involving recruitment of the GR to cytokine gene promoters, like the GR agonist dexamethasone. This is at least in part consistent with direct effects on transcription, without a requirement for new protein synthesis. Dose response analysis shows that MPA has a potency of ∼24 nM for transactivation of the anti-inflammatory GILZ gene and ∼4–20 nM for repression of the pro-inflammatory genes, suggesting that these effects are likely to be relevant at injectable contraceptive doses of MPA. These findings suggest that in the context of the genital mucosa, these GR-mediated glucocorticoid-like effects of MPA in cervical epithelial cells are likely to play a critical role in discriminating between the effects on inflammation caused by different progestins and P4 and hence susceptibility to genital infections, given the predominant expression of the GR in primary endocervical epithelial cells.

Grail as a molecular determinant for the functions of the tumor suppressor p53 in tumorigenesis

The transcription factor p53 is a multifunctional tumor suppressor that arrests the cell cycle in response to stress and modulates the DNA repair process or induces apoptosis. The cellular level and activity of p53 are tightly controlled to maintain proper functioning. This study identified a novel p53-binding glycoprotein, gene related to anergy in lymphocytes (Grail), which formed a negative feedback loop (similar to that of Mdm2). Grail physically and functionally interacted with the N-terminus of p53 to target its degradation and modulate its transactivation activity. Grail also senses and regulates cellular p53 levels, modulates a panel of p53-targeted promoters, and has a role in p53-induced apoptosis in cultured cells. Overexpression of Grail inhibited p53-induced apoptosis by increasing p53 degradation. However, cells not expressing Grail failed to undergo p53-dependent apoptosis, resulting in p21-dependent G1 arrest. Thus, Grail may provide a novel regulatory route for controlling p53 activity under stress conditions.

Foxc1 controls the growth of the murine frontal bone rudiment by direct regulation of a Bmp response threshold of Msx2

The mammalian skull vault consists of several intricately patterned bones that grow in close coordination. The growth of these bones depends on the precise regulation of the migration and differentiation of osteogenic cells from undifferentiated precursor cells located above the eye. Here, we demonstrate a role for Foxc1 in modulating the influence of Bmp signaling on the expression of Msx2 and the specification of these cells. Inactivation of Foxc1 results in a dramatic reduction in skull vault growth and causes an expansion of Msx2 expression and Bmp signaling into the area occupied by undifferentiated precursor cells. Foxc1 interacts directly with a Bmp responsive element in an enhancer upstream of Msx2, and acts to reduce the occupancy of P-Smad1/5/8. We propose that Foxc1 sets a threshold for the Bmp-dependent activation of Msx2, thus controlling the differentiation of osteogenic precursor cells and the rate and pattern of calvarial bone development.

Differential nuclear localisation and promoter occupancy play a role in glucocorticoid receptor ligand-specific transcriptional responses

The glucocorticoid receptor (GR) is a ligand-activated transcription factor for which a number of endogenous and synthetic ligands exist. A key question in steroid receptor biology is how different ligands elicit different maximal transcriptional responses via the same receptor and on the same promoter. This question was addressed quantitatively for the GR, using a panel of agonists, partial agonists and antagonists, on the endogenous GILZ gene in two different human cell lines. It was found that the extent of GR nuclear localization correlated with the efficacy for GILZ transactivation by the GR in U2OS cells. However, in A549 cells there was no significant correlation, with all ligands resulting in similar levels of GR nuclear localization, despite different levels of transcriptional activation of the GILZ gene. Chromatin immunoprecipitation analysis on the other hand, revealed ligand-specific differences in GILZ promoter occupancy in the A549 cells, which correlated with the transcriptional efficacy of the subset of ligands investigated. This suggests that ligand-specific differences in promoter occupancy by activated GR play a major role in discrimination between agonist, partial agonist and antagonist responses on the endogenous GILZ gene in A549 cells, while differences in nuclear localisation of liganded GR play a role in determining the transcriptional outcome in U2OS cells. These cell line-specific differences were not dependent on the amount of GR present, since transient overexpression of GR in U2OS did not alter the relative ligand-selective nuclear localisation. Our results show that there is a relationship between ligand-specific transactivation efficacy, extent of nuclear translocation and recruitment of GR to the promoter. However, the relative contribution of nuclear translocation and GR promoter recruitment to ligand-specific transactivation efficacy is cell-specific.

Glucocorticoid-independent repression of tumor necrosis factor (TNF) alpha-stimulated interleukin (IL)-6 expression by the glucocorticoid receptor: a potential mechanism for protection against an excessive inflammatory response

TNFα signaling and cytokine levels play a crucial role in cervical immunity and the host response to infections. We investigated the role of liganded and unliganded glucocorticoid receptor (GR) in IL-6 and IL-8 gene regulation in response to TNFα in the End1/E6E7 immortalized human endocervical epithelial cell line. In the absence of glucocorticoids, both decreasing GR protein levels by an siRNA strategy and results with the GR antagonist RU486 suggest a role for the unliganded GR in reduction of TNFα-induced IL-6 and IL-8 mRNA levels in End1/E6E7 cells. Moreover, GR-dependent repression of endogenous IL-6 mRNA as well as a minimal IL-6 promoter-reporter gene is also demonstrated in COS-1 cells in the absence of GR ligand, suggesting a transcriptional mechanism that is not cell-specific. TNFα induced recruitment of both the unliganded GR and GR-interacting protein type 1 (GRIP-1) to the IL-6 promoter. This, together with GRIP-1 overexpression studies, suggests a function for GRIP-1 as a GR co-repressor in this context. TNFα was shown to induce phosphorylation of the unliganded human GR at Ser-226 but not Ser-211, unlike dexamethasone, which induced hyperphosphorylation at both serine residues. Ser-226 is shown to be required for the ligand-independent GR-mediated repression of IL-6 in response to TNFα. Taken together, these results support a model whereby the unliganded GR attenuates TNFα-stimulated IL-6 transcription by a mechanism involving selective phosphorylation and recruitment of the unliganded GR and GRIP-1 to the IL-6 promoter. These findings suggest the presence of a novel autoregulatory mechanism that may prevent overproduction of IL-6 in the endocervix, possibly protecting against negative effects of excessive inflammation.

Genomic and nongenomic cross talk between the gonadotropin-releasing hormone receptor and glucocorticoid receptor signaling pathways

The GnRH receptor (GnRHR), a member of the G protein-coupled receptor family, is a central regulator of reproductive function in all vertebrates. The peptide hormone GnRH exerts its effects via binding to the GnRHR in pituitary gonadotropes. We investigated the mechanisms of regulation of transcription of the mGnRHR gene in the mouse pituitary gonadotrope L beta T2 cell line by GnRH and dexamethasone (dex). Reporter assays with transfected mGnRHR promoter show that both dex and GnRH increase transcription of the mGnRHR gene via an activating protein-1 (AP-1) site. Real-time PCR confirmed this on the endogenous mGnRHR gene, and small interfering RNA experiments revealed a requirement for the glucocorticoid receptor (GR) for both the dex and GnRH response. Chromatin immunoprecipitation (ChIP) and immunofluorescence assays provide evidence that both GnRH and dex up-regulate the GnRHR gene via nuclear translocation and interaction of the GR with the AP-1 region on the mGnRHR promoter. We show that GnRH activates the unliganded GR by rapid phosphorylation of the GR at Ser-234 in a GnRHR-dependent fashion to transactivate a GRE reporter gene in L beta T2 and COS-1 cells. Using kinase inhibitors, we established a direct link between GnRH-induced protein kinase C and MAPK activation, leading to unliganded GR phosphorylation at Ser-234 and transactivation of the glucocorticoid response element. Furthermore, we show that GnRH and dex synergistically activate the endogenous GnRHR promoter in L beta T2 cells, via a mechanism involving steroid receptor coactivator-1 recruitment to the GnRHR AP-1 region. Our results suggest a novel mechanism of rapid nongenomic cross talk between the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes via GnRHR-dependent phosphorylation and activation of the unliganded GR in response to GnRH.

The orphan receptor ERRalpha interferes with steroid signaling

The estrogen receptor-related receptor alpha (ERRalpha) is an orphan member of the nuclear receptor superfamily that has been shown to interfere with the estrogen-signaling pathway. In this report, we demonstrate that ERRalpha also cross-talks with signaling driven by other steroid hormones. Treatment of human prostatic cells with a specific ERRalpha inverse agonist reduces the expression of several androgen-responsive genes, in a manner that does not involve perturbation of androgen receptor expression or activity. Furthermore, ERRalpha activates the expression of androgen response elements (ARE)-containing promoters, such as that of the prostate cancer marker PSA, in an ARE-dependent manner. In addition, promoters containing a steroid response element can be activated by all members of the ERR orphan receptor subfamily, and this, even in the presence of antisteroid compounds.

The initiation and pattern of spread of histone H4 acetylation parallel the order of transcriptional activation of genes in the aflatoxin cluster

The 27 genes involved in aflatoxin biosynthesis are clustered within a 70 kb region in the Aspergillus parasiticus genome. Using chromatin immunoprecipitation, we demonstrated a positive correlation between the initiation and spread of histone H4 acetylation in aflatoxin promoters and the onset of accumulation of aflatoxin proteins and aflatoxin. Histone H4 acetylation in the pksA (encodes an 'early' biosynthetic pathway enzyme) promoter peaked at 30 h, prior to the increased acetylation in the omtA and ordA (encode 'late' enzymes) promoters detected at 40 h. The specific order in which pksA, ver-1 (encodes a 'middle' enzyme) and omtA transcripts accumulated in cells paralleled the pattern of spread of histone H4 acetylation. Binding of AflR, a positive regulator of aflatoxin biosynthesis, to the ordA promoter showed a positive correlation with the spread of histone H4 acetylation. The data suggest that the order of genes within the aflatoxin cluster determines the timing and order of transcriptional activation, and that the site of initiation and spread of histone H4 acetylation mediate this process. Our data indicate that the aflatoxin and adjacent sugar utilization clusters are part of a larger 'regulatory unit'.

Glutathione depletion down-regulates tumor necrosis factor alpha-induced NF-kappaB activity via IkappaB kinase-dependent and -independent mechanisms

Reduced glutathione (GSH) plays a crucial role in hepatocyte function, and GSH depletion by diethyl maleate was shown previously to inhibit expression of NF-kappaB target genes induced by tumor necrosis factor alpha (TNFalpha) and sensitize primary cultured mouse hepatocytes to TNF-mediated apoptotic killing. Here we demonstrate in the same system that GSH depletion down-regulates TNF-induced NF-kappaB transactivation via two mechanisms, depending on the extent of the depletion. With moderate GSH depletion (approximately 50%), the down-regulation is IkappaB kinase (IKK)-independent and likely acts on NF-kappaB transcriptional activity because TNF-induced IKK activation, IkappaBalpha phosphorylation and degradation, NF-kappaB nuclear translocation, NF-kappaB DNA binding in vitro, and NF-kappaB subunit RelA(p65) recruitment to kappaB sites of target gene promoters all appear unaltered. On the other hand, with profound GSH depletion (approximately 80%), the down-regulation also is IKK-dependent, and a timeline is established linking the inhibition of polyubiquitination of receptor-interacting protein 1 in TNF receptor 1 complex to partial blockage of IKK activation, IkappaBalpha phosphorylation and degradation, and NF-kappaB nuclear translocation. Of note, pretreatment with antioxidant trolox protects against the inhibitory effect of profound GSH depletion on IKK activation and NF-kappaB nuclear translocation but fails to restore expression of NF-kappaB target genes, revealing both IKK-dependent and -independent inhibition. These findings provide new insights into the complex effects of oxidative stress and redox perturbations on the NF-kappaB pathway.

Ikaros is expressed in developing striatal neurons and involved in enkephalinergic differentiation

The Ikaros (Ik) gene encodes alternatively spliced zinc-finger proteins originally identified in developing hematopoietic organs and acts as master regulator of lymphoid development. During our search for transcription factors that control the developmental expression of the enkephalin (ENK) gene we found that Ik-1 and Ik-2 isoforms are specifically expressed in the embryonic striatum and bind the Ik-like cis-regulatory DNA element present on the ENK gene. Ik proteins are expressed by both proliferating (BrdU+/nestin+) and by post-mitotic differentiating (MAP2+) cells in the developing striatum between embryonic day 12 and post-natal day 2 and mRNAs encoding for the Ik and ENK genes are co-expressed by a subset of differentiating striatal neurons. Blocking the DNA binding of Ik proteins in differentiating embryonic striatal neuronal cultures resulted in decreased ENK expression and mutant animals lacking the DNA-binding domain of Ik had a deficit in the number of ENK but not in dynorphin or substance P mRNA+ cells. Animals lacking the protein interaction domain of Ik showed no deficit. These results demonstrate that Ik-1 and Ik-2 proteins through their DNA binding act as positive regulators of ENK gene expression in the developing striatum and participate in regulating enkephalinergic differentiation.

Importin alpha1 is involved in the nuclear localization of Zac1 and the induction of p21WAF1/CIP1 by Zac1

Zac1, a novel seven-zinc-finger transcription factor, preferentially binds GC-rich DNA elements and has intrinsic transactivation activity. To date, the NLS (nuclear localization signal) of Zac1 has not been empirically determined. We generated a series of EGFP (enhanced green fluorescence protein)-tagged deletion mutants of Zac1 and examined their subcellular localization, from which we defined two NLSs within the DNA-binding (or zinc-finger) domain. Fusion proteins consisting of the two EGFP-tagged zinc-finger clusters (zinc finger motifs 1-3 and 4-7) were located exclusively in the nucleus, demonstrating that each of the zinc-finger clusters is sufficient for nuclear localization. Physical interactions between these two zinc-finger clusters and importin alpha1 were demonstrated using an in vitro glutathione S-transferase pull-down assay. Finally, our results indicate that the association of Zac1 with importin alpha1 is also involved in regulating the transactivation activity of Zac1 on the p21WAF1/CIP1 gene and protein expression.

Two functional modes of a nuclear receptor-recruited arginine methyltransferase in transcriptional activation

Nuclear receptors, like other transcriptional activators, switch on gene transcription by recruiting a complex network of coregulatory proteins. Here, we have identified the arginine methyltransferase PRMT1 as a coactivator for HNF4, an orphan nuclear receptor that regulates the expression of genes involved in diverse metabolic pathways. Remarkably, PRMT1, whose methylation activity on histone H4 strongly correlates with induction of HNF4 target genes in differentiating enterocytes, regulates HNF4 activity through a bipartite mechanism. First, PRMT1 binds and methylates the HNF4 DNA-binding domain (DBD), thereby enhancing the affinity of HNF4 for its binding site. Second, PRMT1 is recruited to the HNF4 ligand-binding domain (LBD) through a mechanism that involves the p160 family of coactivators and methylates histone H4 at arginine 3. This, together with recruitment of the histone acetyltransferase p300, leads to nucleosomal alterations and subsequent RNA polymerase II preinitiation complex formation.

STAMP, a novel predicted factor assisting TIF2 actions in glucocorticoid receptor-mediated induction and repression

The coactivator TIF2 was predicted to interact with an unknown factor to modify both the relative inhibition in glucocorticoid receptor (GR)-mediated gene repression and several parameters of agonists and antisteroids in GR-regulated induction. Here, we describe the isolation and characterization of the predicted factor as a new 1,277-amino-acid endogenous protein (STAMP). STAMP associates with coactivators (TIF2 and SRC-1) and is selective for a subset of the steroid/nuclear receptors including GRs. Transfected STAMP increases the effects of TIF2 in GR-mediated repression and induction. Conversely, the levels of both induction and repression of endogenous genes are reduced when STAMP small interfering RNAs are used to lower the level of endogenous STAMP. Endogenous STAMP colocalizes with GR in intact cells and is recruited to the promoters of endogenous GR-induced and -repressed genes. We suggest that STAMP is an important new, downstream component of GR action in both gene activation and gene repression.

Histone H3 lysine 9 methyltransferase G9a is a transcriptional coactivator for nuclear receptors

Methylation of Lys-9 of histone H3 has been associated with repression of transcription. G9a is a histone H3 Lys-9 methyltransferase localized in euchromatin and acts as a corepressor for specific transcription factors. Here we demonstrate that G9a also functions as a coactivator for nuclear receptors, cooperating synergistically with nuclear receptor coactivators glucocorticoid receptor interacting protein 1, coactivator-associated arginine methyltransferase 1 (CARM1), and p300 in transient transfection assays. This synergy depends strongly on the arginine-specific protein methyltransferase activity of CARM1 but does not absolutely require the enzymatic activity of G9a and is specific to CARM1 and G9a among various protein methyltransferases. Reduction of endogenous G9a diminished hormonal activation of an endogenous target gene by the androgen receptor, and G9a associated with regulatory regions of this same gene. G9a fused to Gal4 DNA binding domain can repress transcription in a lysine methyltransferase-dependent manner; however, the histone modifications associated with transcriptional activation can inhibit the methyltransferase activity of G9a. These findings suggest a link between histone arginine and lysine methylation and a mechanism for controlling whether G9a functions as a corepressor or coactivator.

GAC63, a GRIP1-dependent nuclear receptor coactivator

Nuclear receptors (NRs) regulate target gene transcription through the recruitment of multiple coactivator complexes to the promoter regions of target genes. One important coactivator complex includes a p160 coactivator (GRIP1, SRC-1, or ACTR) and its downstream coactivators (e.g., p300, CARM1, CoCoA, and Fli-I), which contribute to transcriptional activation by protein acetylation, protein methylation, and protein-protein interactions. In this study, we identified a novel NR coactivator, GAC63, which binds to the N-terminal region of p160 coactivators as well as the ligand binding domains of some NRs. GAC63 enhanced transcriptional activation by NRs in a hormone-dependent and GRIP1-dependent manner in transient transfection assays and cooperated synergistically and selectively with other NR coactivators, including GRIP1 and CARM1, to enhance estrogen receptor function. Endogenous GAC63 was recruited to the estrogen-responsive pS2 gene promoter of MCF-7 cells in response to the hormone. Reduction of the endogenous GAC63 level by small interfering RNA inhibited transcriptional activation by the hormone-activated estrogen receptor. Thus, GAC63 is a physiologically relevant part of the p160 coactivator signaling pathway that mediates transcriptional activation by NRs.

A phylogenetically conserved cis-regulatory module in the Msx2 promoter is sufficient for BMP-dependent transcription in murine and Drosophila embryos

To understand the actions of morphogens, it is crucial to determine how they elicit different transcriptional responses in different cell types. Here, we identify a BMP-responsive enhancer of Msx2, an immediate early target of bone morphogenetic protein (BMP) signaling. We show that the BMP-responsive region of Msx2 consists of a core element, required generally for BMP-dependent expression, and ancillary elements that mediate signaling in diverse developmental settings. Analysis of the core element identified two classes of functional sites: GCCG sequences related to the consensus binding site of Mad/Smad-related BMP signal transducers; and a single TTAATT sequence, matching the consensus site for Antennapedia superclass homeodomain proteins. Chromatin immunoprecipitation and mutagenesis experiments indicate that the GCCG sites are direct targets of BMP restricted Smads. Intriguingly, however, these sites are not sufficient for BMP responsiveness in mouse embryos; the TTAATT sequence is also required. DNA sequence comparisons reveal this element is highly conserved in Msx2 promoters from mammalian orders but is not detectable in other vertebrates or non-vertebrates. Despite this lack of conservation outside mammals, the Msx2 BMP-responsive element serves as an accurate readout of Dpp signaling in a distantly related bilaterian - Drosophila. Strikingly, in Drosophila embryos, as in mice, both TTAATT and GCCG sequences are required for Dpp responsiveness, showing that a common cis-regulatory apparatus can mediate the transcriptional activation of BMP-regulated genes in widely divergent bilaterians.

Dnmt1 deficiency leads to enhanced microsatellite instability in mouse embryonic stem cells

DNA hypomethylation is frequently seen in cancer and imparts genomic instability in mouse models and some tissue culture systems. However, the effects of genomic DNA hypomethylation on mutation rates are still elusive. We have developed a model system to analyze the effects of DNA methyltransferase 1 (Dnmt1) deficiency on DNA mismatch repair (MMR) in mouse embryonic stem (ES) cells. We generated sibling ES cell clones with and without functional Dnmt1 expression, containing a stable reporter gene that allowed us to measure the slippage rate at a mononucleotide repeat. We found that Dnmt1 deficiency led to a 7-fold increase in the microsatellite slippage rate. Interestingly, the region flanking the mononucleotide repeat was unmethylated regardless of Dnmt1 status, suggesting that it is not the local levels of DNA methylation that direct the increase in microsatellite instability (MSI). The enhanced MSI was associated with higher levels of histone H3 acetylation and lower MeCP2 binding at regions near the assayed microsatellite, suggesting that Dnmt1 loss may decrease MMR efficiency by modifying chromatin structure.

Developmentally essential protein flightless I is a nuclear receptor coactivator with actin binding activity

Hormone-activated nuclear receptors (NR) activate transcription by recruiting multiple coactivator complexes to the promoters of target genes. One important coactivator complex includes a p160 coactivator (e.g., GRIP1, SRC-1, or ACTR) that binds directly to activated NR, the histone acetyltransferase p300 or CBP, and the arginine-specific histone methyltransferase CARM1. We previously demonstrated that the coactivator function of CARM1 depends both on the methyltransferase activity and on additional unknown proteins that bind to CARM1. In this study a yeast two-hybrid screen for proteins that bind CARM1 identified the protein Flightless I (Fli-I), which has essential roles in Drosophila and mouse development. Fli-I bound to CARM1, GRIP1, and NRs and cooperated synergistically with CARM1 and GRIP1 to enhance NR function. Fli-I bound poorly to and did not cooperate with PRMT1, a CARM1-related protein arginine methyltransferase that also functions as an NR coactivator. The synergy between GRIP1, CARM1, and Fli-I required the methyltransferase activity of CARM1. The C-terminal AD1 (binding site for p300/CBP) and AD2 (binding site for CARM1) activation domains of GRIP1 contributed to the synergy but were less stringently required than the N-terminal region of GRIP1, which is the binding site for Fli-I. Endogenous Fli-I was recruited to the estrogen-regulated pS2 gene promoter of MCF-7 cells in response to the hormone, and reduction of endogenous Fli-I levels by small interfering RNA reduced hormone-stimulated gene expression by the endogenous estrogen receptor. A fragment of Fli-I that is related to the actin binding protein gelsolin enhanced estrogen receptor activity, and mutations that reduced actin binding also reduced the coactivator function of this Fli-I fragment. These data suggest that Fli-I may facilitate interaction of the p160 coactivator complex with other coactivators or coactivator complexes containing actin or actin-like proteins.

CoCoA, a Nuclear Receptor Coactivator which Acts through an N-Terminal Activation Domain of p160 Coactivators

The p160 coactivators bind to and potentiate transcriptional activation by nuclear receptors by recruiting secondary coactivators such as the histone acetyltransferases p300 and CBP and the protein methyltransferase CARM1. The function of the highly conserved N-terminal basic-helix-loop-helix/Per-Arnt-Sim (bHLH-PAS) domain of p160 coactivators is unknown. This region is required for coactivator synergy among p160, p300, and CARM1 coactivators. We identified a coactivator, coiled-coil coactivator (CoCoA), which binds to this domain and thereby enhances transcriptional activation by the estrogen receptor and other nuclear receptors. Endogenous CoCoA was found simultaneously with p160 coactivators on the promoter of an endogenous estrogen-responsive gene. Reduction of endogenous cellular CoCoA levels inhibited the estrogen-stimulated expression of transiently transfected and endogenous genes. Moreover, CoCoA cooperated synergistically with GRIP1, CARM1, and p300 to enhance ER-mediated transcription. Thus, the N-terminal region of p160 coactivators contains an additional activation domain which contributes to coactivator function by recruitment of CoCoA.

Synergistic effects of coactivators GRIP1 and beta-catenin on gene activation: cross-talk between androgen receptor and Wnt signaling pathways

The p160 coactivators, such as GRIP1, bind nuclear receptors and help to mediate transcriptional activation. beta-Catenin binds to and serves as a coactivator for the nuclear receptor, androgen receptor (AR), and the Lymphoid Enhancer Factor/T Cell Factor family member, Lef1. Here we report that GRIP1 and beta-catenin can bind strongly to each other through the AD2 domain of GRIP1. Furthermore, GRIP1 and beta-catenin can synergistically enhance the activity of both AR and Lef1, and both coactivators are recruited specifically to AR-driven and Lef1-driven promoters. However, the mechanism of beta-catenin-GRIP1 coactivator function and synergy is different with AR and Lef1. While beta-catenin can bind directly to both AR and Lef1, GRIP1 can only bind directly to AR; the ability of GRIP1 to associate with and function as a coactivator for Lef1 is entirely dependent on the presence of beta-catenin. Thus, whereas GRIP1 coactivator function involves direct binding to nuclear receptors and most other classes of DNA-binding transcriptional activator proteins, the coactivator function of GRIP1 with Lef1 follows a novel paradigm where GRIP1 is recruited indirectly to Lef1 through their mutual association with beta-catenin. The beta-catenin-GRIP1 interaction represents another potential point of cross-talk between the AR and Wnt signaling pathways.