Role of CBP/P300 in nuclear receptor signalling

GA-binding protein and p300 are essential components of a retinoic acid-induced enhanceosome in myeloid cells

Expression of CD18, the beta chain of the leukocyte integrins, is transcriptionally regulated by retinoic acid (RA) in myeloid cells. Full RA responsiveness of the CD18 gene requires its proximal promoter, which lacks a retinoic acid response element (RARE). Rather, RA responsiveness of the CD18 proximal promoter requires ets sites that are bound by GA-binding protein (GABP). The transcriptional coactivator, p300, further increases CD18 RA responsiveness. We demonstrate that GABPalpha, the ets DNA-binding subunit of GABP, physically interacts with p300 in myeloid cells. This interaction involves the GABPalpha pointed domain (PNT) and identifies p300 as the first known interaction partner of GABPalpha PNT. Expression of the PNT domain, alone, disrupts the GABPalpha-p300 interaction and decreases the RA responsiveness of the CD18 proximal promoter. Chromatin immunoprecipitation and chromosome conformation capture demonstrate that, in the presence of RA, GABPalpha and p300 at the proximal promoter recruit retinoic acid receptor/retinoid X receptor from a distal RARE to form an enhanceosome. A dominant negative p300 construct disrupts enhanceosome formation and reduces the RA responsiveness of CD18. Thus, proteins on the CD18 proximal promoter recruit the distal RARE in the presence of RA. This is the first description of an RA-induced enhanceosome and demonstrates that GABP and p300 are essential components of CD18 RA responsiveness in myeloid cells.

Cancer-preventive anti-oxidants that attenuate free radical generation by inflammatory cells

Active inflammatory leukocytes are a major endogenous source of reactive oxygen and nitrogen oxide species (RONS). We have recently established novel bioassay systems, in which either phorbol ester-stimulated, differentiated HL-60 human leukemia cells or lipopolysaccharide (LPS)-stimulated RAW264.7 murine macrophages were co-cultured with AS52 Chinese hamster ovary cells. Extensive screening of extracts from Asian vegetables and fruits led to the identification of 1′-acetoxychavicol acetate (ACA), auraptene, nobiletin, and zerumbone, all of which were found to be highly anti-mutagenic in the above co-culture systems. Pretreatment of RAW264.7 macrophages with LPS led to the activation of mitogen-activated protein kinases (MAPKs) and Akt, together with the degradation of IκB-α protein, and the resultant activation of the AP-1, NF-κB, and CREB transcription factors. ACA abrogated ERK1/2 and JNK1/2, but not p38 activation, as well as the activation and transcriptional activation of NF-κB and CREB, whereas nobiletin allowed phosphorylation of these MAPKs, while it suppressed AP-1, NF-κB, and CREB activation. Interestingly, zerumbone did not have any effects on the latter transcription factors, although it did attenuate iNOS mRNA expression. In addition, auraptene suppressed iNOS protein production, but not mRNA expression, implying that it targets the translation step. Our model systems may be useful for identifying potentially anti-carcinogenic inhibitors of RONS generation.

Regulation of ERK1 gene expression by coactivator proteins

RARs (retinoic acid receptors) mediate the effect of their ligand RA (retinoic acid) on gene expression. We previously showed that RA inhibited cellular proliferation in part by decreasing expression of the mitogen activated protein kinase ERK1 (extracellular signal regulated kinase 1). However, the mechanism by which RA regulates ERK1 expression is largely uncharacterized. The present study characterizes coactivator-mediated regulation of RA target gene expression by analysing ERK1 promoter activation. CBP (CREB-binding protein) and PCAF (p300/CBP associated factor) are transcriptional coactivators that interact with nuclear hormone receptors such as RARs. CBP and PCAF differentially regulated ERK1 expression in stable clones. CBP clones expressed higher ERK1 protein levels, proliferated faster in culture and were resistant to RA-mediated growth inhibition. PCAF clones expressed lower levels of ERK1 protein and cells grew more slowly than controls. CBP and PCAF regulation of the ERK1 promoter was dependent on two Sp1 (specificity protein 1) sites located between -86 and -115 bp. Immunoprecipitation and yeast two-hybrid analysis revealed that PCAF interacted with Sp1 via CBP. A putative p53 binding site at -360 bp functioned as a major repressor of ERK1 promoter activity even in the absence of exogenous p53 expression. CBP and PCAF occupancy of the proximal ERK1 promoter was dramatically decreased by RA treatment. PCAF mediated inhibition of ERK1 expression was due to decreased stability of the kinase mRNA. We conclude that CBP and PCAF coactivators mediate ERK1 gene expression at both the transcriptional and post-transcriptional level.

Amphibian metamorphosis as a model for the developmental actions of thyroid hormone

Thyroid hormone (TH) elicits multiple physiological actions in vertebrates from fish to man. These actions can be divided into two broad categories: those where the hormone regulates developmental processes and those that involve actions in the adult organism. Amphibian metamorphosis is a most dramatic example of extensive morphological, biochemical and cellular changes occurring during post-embryonic development, which is obligatorily initiated and sustained by TH. It is, therefore, an ideal model system to understand the action of the hormone. Each tissue of the frog tadpole responds differently to TH, ranging from altered gene expression, morphogenesis, tissue re-structuring and extensive cell death, according to a developmental programme set in place before the thyroid gland begins to secrete the hormone. The key element determining the response to the hormone is the nuclear thyroid hormone receptor (TR). As in most vertebrates, there are two thyroid hormone receptors, TRalpha and TRbeta, which repress transcription in the absence of the ligand and whose concentration in the tissues is directly modulated by the hormone itself. In Xenopus, biochemical and in situ techniques have shown that the amount of TRbeta mRNA and protein are elevated 50-100 times during TH-induced metamorphic climax. This phenomenon of "autoinduction" of receptor is also seen with developmental or inductive processes regulated by other hormones acting through nuclear receptors. It is possible that receptor upregulation may be a pre-requisite for hormonal response. Recent molecular and cell biological studies have suggested that TRs function as multimeric complexes with other nuclear or chromatin proteins, such as co-repressors and co-activators, to regulate the structure of the chromatin, and thereby determine the transcription of the receptor-specified target gene. There is evidence that this may also be so for thyroid hormone regulated transcription during amphibian metamorphosis.

Ligand-specific allosteric regulation of coactivator functions of androgen receptor in prostate cancer cells

The androgen receptor not only mediates prostate development but also serves as a key regulator of primary prostatic cancer growth. Although initially responsive to selective androgen receptor modulators (SARMs), which cause recruitment of the nuclear receptor-corepressor (N-CoR) complex, resistance invariably occurs, perhaps in response to inflammatory signals. Here we report that dismissal of nuclear receptor-corepressor complexes by specific signals or androgen receptor overexpression results in recruitment of many of the cohorts of coactivator complexes that permits SARMs and natural ligands to function as agonists. SARM-bound androgen receptors appear to exhibit failure to recruit specific components of the coactivators generally bound by liganded nuclear receptors, including cAMP response element-binding protein (CBP)/p300 or coactivator-associated arginine methyltransferase 1 (CARM1) to the SARM-bound androgen receptor, although still causing transcriptional activation of androgen receptor target genes. SARM-bound androgen receptors use distinct LXXLL (L, leucine; X, any amino acid) helices in the p160 nuclear receptor interaction domains that may impose selective allosteric effects, providing a component of the molecular basis of differential responses to different classes of ligands by androgen receptor.

Critical roles of the p160 transcriptional coactivators p/CIP and SRC-1 in energy balance

Several transcriptional coactivators have been implicated in modulating the transcriptional activities of nuclear hormone receptors in vitro. Potential roles of these cofactors in important physiological processes such as energy homeostasis remain unknown. We report here that a developmental arrest in interscapular brown fat and defective adaptive thermogenesis occur in mice lacking both the p160 family transcriptional coactivators SRC-1 and p/CIP due to a failure in induction of selective PPARgamma target genes involved in adipogenesis and mitochondrial uncoupling. In the absence of p/CIP and SRC-1, mice eat more food on both regular chow and a high-fat diet because of decreased blood leptin levels. However, the p/CIP(-/-)/SRC-1(-/-) mice are lean and resistant to high-fat-diet-induced obesity. They exhibit increased basal metabolic rates and heightened levels of physical activity. Therefore, p/CIP and SRC-1 play critical roles in energy balance by controlling both energy intake and energy expenditure.

Role of peroxisome proliferator-activated receptor gamma in glucose-induced insulin secretion

Peroxisome proliferator-activated receptor (PPAR) isoforms (alpha and gamma) are known to be expressed in pancreatic islets as well as in insulin-producing cell lines. Ligands of PPAR have been shown to enhance glucose-induced insulin secretion in rat pancreatic islets. However, their effect on insulin secretion is still unclear. To understand the molecular mechanism by which PPARgamma exerts its effect on glucose-induced insulin secretion, we examined the endogenous activity of PPAR isoforms, and studied the PPARgamma function and its target gene expression in INS-1 cells. We found that: (1) endogenous PPARg was activated in a ligand-dependent manner in INS-1 cells; (2) overexpression of PPARgamma in the absence of PPARgamma ligands enhanced glucose-induced insulin secretion, which indicates that the increased glucose-induced insulin secretion is a PPARgamma-mediated event; (3) the addition of both PPARgamma and retinoid X receptor (RXR) ligands showed a synergistic effect on the augmentation of reporter activity, suggesting that the hetero-dimerization of PPARgamma and RXR is required for the regulation of the target genes; (4) PPARs upregulated both the glucose transporter 2 (GLUT2) and Cb1-associated protein (CAP) genes in INS-1 cells. Our findings suggest an important mechanistic pathway in which PPARgamma enhances glucose-induced insulin secretion by activating the expression of GLUT2 and CAP genes in a ligand-dependent manner.

Molecular and developmental analyses of thyroid hormone receptor function in Xenopus laevis, the African clawed frog

The current review focuses on the molecular mechanisms and developmental roles of thyroid hormone receptors (TRs) in gene regulation and metamorphosis in Xenopus laevis and discusses implications for TR function in vertebrate development and diversity. Questions addressed are: (1) what are the molecular mechanisms of gene regulation by TR, (2) what are the developmental roles of TR in mediating the thyroid hormone (TH) signal, (3) what are the roles of the different TR isoforms, and (4) how do changes in these molecular and developmental mechanisms affect evolution? Even though detailed knowledge of molecular mechanisms of TR-mediated gene regulation is available from in vitro studies, relatively little is known about how TR functions in development in vivo. Studies on TR function during frog metamorphosis are leading the way toward bridging the gap between in vitro and in vivo studies. In particular, a dual function model for the role of TR in metamorphosis has been proposed and investigated. In this model, TRs repress genes allowing tadpole growth in the absence of TH during premetamorphosis and activate genes important for metamorphosis when TH is present. Despite the lack of metamorphosis in most other vertebrates, TR has important functions in development across vertebrates. The underlying molecular mechanisms of TR in gene regulation are conserved through evolution, so other mechanisms involving TH-target genes and TH tissue-sensitivity and dependence underlie differences in role of TR across vertebrates. Continued analysis of molecular and developmental roles of TR in X. laevis will provide the basis for understanding how TR functions in gene regulation in vivo across vertebrates and how TR is involved in the generation of evolutionary diversity.

The APC/C and CBP/p300 cooperate to regulate transcription and cell-cycle progression

The anaphase-promoting complex/cyclosome (APC/C) is a multicomponent E3 ubiquitin ligase that, by targeting protein substrates for 26S proteasome-mediated degradation through ubiquitination, coordinates the temporal progression of eukaryotic cells through mitosis and the subsequent G1 phase of the cell cycle. Other functions of the APC/C are, however, less well defined. Here we show that two APC/C components, APC5 and APC7, interact directly with the coactivators CBP and p300 through protein-protein interaction domains that are evolutionarily conserved in adenovirus E1A. This interaction stimulates intrinsic CBP/p300 acetyltransferase activity and potentiates CBP/p300-dependent transcription. We also show that APC5 and APC7 suppress E1A-mediated transformation in a CBP/p300-dependent manner, indicating that these components of the APC/C may be targeted during cellular transformation. Furthermore, we establish that CBP is required in APC/C function; specifically, gene ablation of CBP by RNA-mediated interference markedly reduces the E3 ubiquitin ligase activity of the APC/C and the progression of cells through mitosis. Taken together, our results define discrete roles for the APC/C-CBP/p300 complexes in growth regulation.

New role for nuclear hormone receptors and coactivators in regulation of BRCA1-mediated DNA repair in breast cancer cell lines

INTRODUCTION

The breast cancer susceptibility gene BRCA1 is involved in the repair of double-strand breaks induced by ionizing radiation and chemotherapy drugs. BRCA1 interacts with coactivators such as p300 and CREB-binding protein (CBP) to activate target gene transcription. Estrogen and retinoic acid receptors (ER and RAR) also require coactivator proteins for their ligand-dependent functions. Few studies have suggested a role for nuclear hormone receptors in DNA repair.

METHODS

DNA damage and repair activity were quantified with the use of single-cell gel electrophoresis and plasmid end-joining assays. Cell cycle progression and apoptosis were determined by bromodeoxyuridine and TdT-mediated dUTP nick end labelling assays. Stable transfection was accomplished with the lipofection procedure. Protein interaction and expression were determined by immunoprecipitation and western blotting.

RESULTS

17beta-estradiol (E2) and all-trans retinoic acid (RA) had opposing effects on DNA damage and breast cancer cell survival after double-strand break damage. Treatment with E2, but not with RA, resulted in complex formation between ERalpha, CBP, and BRCA1 in ER-positive cell lines. Mutant BRCA1 reduced the expression and activity of DNA damage repair proteins but did not block nuclear hormone-dependent effects. Mutant BRCA1 failed to form complexes with ERalpha and CBP, which correlated with its ability to exert E2-independent effects on DNA repair. Mutant BRCA1 inhibited cell cycle progression and produced increased survival in cells with double-strand breaks. Ectopic ERalpha expression reproduced the E2-mediated effects on DNA damage, repair, and survival.

CONCLUSION

The present study proposes a new mechanism by which ER and RAR regulate BRCA1-mediated DNA repair by means of CBP.

Steroid hormone receptor signaling in tumorigenesis

Excessive activation of the hormone signaling pathways is implicated in several disorders of the target tissues, with cancer being one of the most serious fallouts. Steroid hormone receptors are key proteins through which steroid hormones convey their signals to the cells. Deregulated activity of the hormone receptors due to their altered activation; stability or sub-cellular localization is heavily implicated in the onset and progress of cancers. The role played by estrogen and its receptors in breast cancer remains the most thoroughly investigated steroid-dependent cancer system till date. Choosing it as an example, we have summarized the molecular mechanisms underlying the action of the estrogen receptors (ERs) in manifesting the effects of the estrogens in the cells. A special emphasis is placed on the molecular mechanism of their functionality, role of the coactivator proteins, and the reasons for the deregulated signaling. The therapeutic approaches resulting from the mechanistic study of the ER action and their efficacies are also discussed.

Zingiberaceous and citrus constituents, 1'-acetoxychavicol acetate, zerumbone, auraptene, and nobiletin, suppress lipopolysaccharide-induced cyclooxygenase-2 expression in RAW264.7 murine macrophages through different modes of action

In the present study, we explored the suppressive activities of 1'-acetoxychavicol acetate (ACA), auraptene, nobiletin, and zerumbone toward LPS-induced cyclooxygenase (COX)-2 mRNA expression in mouse macrophages and the underlying molecular mechanisms. Pretreatment of RAW264.7 cells with LPS led to the activation of mitogen-activated protein kinase (MAPK)s [p38, extracellular signal-regulated kinase (ERK)1/2, c-Jun NH2-terminal kinase (JNK)1/2] and Akt, together with degradation of the inhibitor of nuclear factor-kappaB (IkappaB)-alpha protein and nuclear translocation of nuclear factor (NF)-kappaB p65, and the resultant activation of activator protein (AP)-1, NF-kappaB, and cAMP-responsive element-binding protein (CREB) transcription factors. ACA abrogated ERK1/2 and JNK1/2, but not p38 MAPK, as well as the activation of those transcription factors. Although it allowed LPS-triggered phosphorylation of those MAPKs and NF-kappaB nuclear translocation, nobiletin suppressed the activation of AP-1, NF-kappaB, and CREB. Zerumbone had no effect on those transcription factors, though it attenuated COX-2 mRNA expression, suggesting that it disrupts the stabilization of COX-2 mRNA. Conversely, zerumbone significantly accelerated spontaneous COX-2 mRNA decay, the potency of which was comparable with that of SB203580, an inhibitor of p38 MAPK, whose activation has key roles in the proinflammatory mRNA stabilization processes. Because SB203580 but not zerumbone suppressed LPS-induced p38 MAPK activation, the molecular targets of zerumbone may be MAPK-activated protein kinase-2 or located downstream. However, auraptene suppressed the expression of COX-2 protein but not mRNA, implying that it targets translation. We propose that these phytochemicals are promising chemopreventive agents for inflammation-associated carcinogenesis. Their use in combination may enhance their efficacy because of their different modes of action.

Nuclear receptor coactivators: the key to unlock chromatin

The biological effects of hormones, ranging from organogenesis, metabolism, and proliferation, are transduced through nuclear receptors (NRs). Over the last decade, NRs have been used as a model to study transcriptional control. The conformation of activated NRs is favorable for the recruitment of coactivators, which promote transcriptional activation by directly communicating with chromatin. This review will focus on the function of different classes of coactivators and associated complexes, and on progress in our understanding of gene activation by NRs through chromatin remodeling.

T:G mismatch-specific thymine-DNA glycosylase (TDG) as a coregulator of transcription interacts with SRC1 family members through a novel tyrosine repeat motif

Gene activation involves protein complexes with diverse enzymatic activities, some of which are involved in chromatin modification. We have shown previously that the base excision repair enzyme thymine DNA glycosylase (TDG) acts as a potent coactivator for estrogen receptor-alpha. To further understand how TDG acts in this context, we studied its interaction with known coactivators of nuclear receptors. We find that TDG interacts in vitro and in vivo with the p160 coactivator SRC1, with the interaction being mediated by a previously undescribed motif encoding four equally spaced tyrosine residues in TDG, each tyrosine being separated by three amino acids. This is found to interact with two motifs in SRC1 also containing tyrosine residues separated by three amino acids. Site-directed mutagenesis shows that the tyrosines encoded in these motifs are critical for the interaction. The related p160 protein TIF2 does not interact with TDG and has the altered sequence, F-X-X-X-Y, at the equivalent positions relative to SRC1. Substitution of the phenylalanines to tyrosines is sufficient to bring about interaction of TIF2 with TDG. These findings highlight a new protein-protein interaction motif based on Y-X-X-X-Y and provide new insight into the interaction of diverse proteins in coactivator complexes.

Cyclin D3 interacts with vitamin D receptor and regulates its transcription activity

D-type cyclins are essential for the progression through the G1 phase of the cell cycle. Besides serving as cell cycle regulators, D-type cyclins were recently reported to have transcription regulation functions. Here, we report that cyclin D3 is a new interacting partner of vitamin D receptor (VDR), a member of the superfamily of nuclear receptors for steroid hormones, thyroid hormone, and the fat-soluble vitamins A and D. The interaction was confirmed with methods of yeast two-hybrid system, in vitro binding analysis and in vivo co-immunoprecipitation. Cyclin D3 interacted with VDR in a ligand-independent manner, but treatment of the ligand, 1,25-dihydroxyvitamin D3, strengthened the interaction. Confocal microscopy analysis showed that ligand-activated VDR led to an accumulation of cyclin D3 in the nuclear region. Cyclin D3 up-regulated transcriptional activity of VDR and this effect was counteracted by overexpression of CDK4 and CDK6. These findings provide us a new clue to understand the transcription regulation functions of D-type cyclins.

Differential recruitment of p160 coactivators by glucocorticoid receptor between Schwann cells and astrocytes

In the nervous system, glucocorticoids can exert beneficial or noxious effects, depending on their concentration and the duration of hormonal stimulation. They exert their effects on neuronal and glial cells by means of their cognate receptor, the glucocorticoid receptor (GR), which recruits the p160 coactivator family members SRC-1 (steroid receptor coactivator 1), SRC-2, and SRC-3 after hormone binding. In this study, we investigated the molecular pathways used by the GR in cultured glial cells of the central and the peripheral nervous systems, astrocytes and Schwann cells (MSC80 cells), respectively. We performed functional studies based on transient transfection of a minimal glucocorticoid-sensitive reporter gene into the glial cells to test the influence of overexpression or selective inhibition by short interfering RNA of the three p160 coactivator family members on GR transactivation. We demonstrate that, depending on the glial cell type, GR differentially recruits p160 family members: in Schwann cells, GR recruited SRC-1a, SRC-1e, or SRC-3, whereas in astrocytes, SRC-1e and SRC-2, and to a lesser extent SRC-3, were active toward GR signaling. The C-terminal nuclear receptor-interacting domain of SRC-1a participates in its exclusion from the GR transcriptional complex in astrocytes. Immunolocalization experiments revealed a cell-specific intracellular distribution of the p160s, which was dependent on the duration of the hormonal induction. For example, within astrocytes, SRC-1 and SRC-2 were mainly nuclear, whereas SRC-3 unexpectedly localized to the lumen of the Golgi apparatus. In contrast, in Schwann cells, SRC-1 showed a nucleocytoplasmic shuttling depending on hormonal stimulation, whereas SRC-2 remained strictly nuclear and SRC-3 remained predominantly cytoplasmic. Altogether, these results highlight the cell specificity and the time dependence of p160s recruitment by the activated GR in glial cells, revealing the complexity of GR-p160 assembly in the nervous system.

Novel interaction between nuclear coactivator CBP and the protein inhibitor of activated Stat1 (PIAS1)

cAMP response element binding protein (CREB)-binding protein (CBP) is a multifunctional transcriptional coactivator that plays important roles in gene regulation. CBP is expressed in murine embryonic orofacial tissue, where it is developmentally regulated. To identify nuclear factors associated with CBP in developing orofacial tissue, a yeast two-hybrid screen of a cDNA library derived from embryonic orofacial tissue from gestational days 11-13 mouse embryos was conducted. Using the carboxy terminal region of CBP as bait, the protein inhibitor of activated Stat1 (PIAS1) was identified as a novel CBP binding protein. The association of PIAS1 with CBP was confirmed in vitro by glutathione S-transferase (GST) pull-down assays and in vivo by coimmunoprecipitation. Reporter assays demonstrated that PIAS1 inhibited CBP-mediated transcriptional activation in the presence or absence of transforming growth factor-beta (TGF-beta). These results identify PIAS1 as a novel binding partner for CBP and inhibitor of CBP-mediated transcription, suggesting that PIAS1 might play a role in regulating cell proliferation, migration, and differentiation during embryonic development.

Enhanced coactivator binding and transcriptional activation of mutant vitamin D receptors from patients with hereditary 1,25-dihydroxyvitamin D-resistant rickets by phosphorylation and vitamin D analogs

In this study, we report that the function of certain mutant VDRs from patients with hereditary HVDRR can at least be partially restored by phosphorylation and hexafluoro 1,25(OH)2D3 analogs. Our study provides new insights into mechanisms involved in enhancement of mutant VDR function.

INTRODUCTION

1,25-Dihydroxyvitamin D-resistant rickets (HVDRR) is a rare genetic disorder caused by inactivating mutations in the vitamin D receptor (VDR). In this study, we examined VDR from patients with HVDRR having mutations in the ligand-binding domain (F251C, I268T, H305Q, E420K). We examined methods of restoring transcriptional activity of these mutants and the mechanisms involved.

MATERIALS AND METHODS

Reporter gene transcriptional assays were used to examine the activation of mutant VDRs. Western-blot analysis, glutathione S-transferase (GST) pull-down assays, and chromatin immunoprecipitation (ChIP) assays were also used in this study.

RESULTS

Using mutant VDRs, H305Q, F251C, I268T, and 10(-8) M 1,25(OH)2D3, only 10-30% of the activity of wildtype (WT) VDR in activating 24(OH)ase transcription was observed. The transcriptional response of mutant VDR mutants was significantly enhanced 2- to 3-fold by co-treatment of VDR mutant transfected COS-7 cells with 1,25(OH)2D3 and okadaic acid (OA; inhibitor of phosphatase; 50 nM). The H305Q mutant was the most responsive (90% of the response exhibited by WT VDR was restored). The E420K mutant was unresponsive to 1,25(OH)2D3 in the presence or absence of OA. The increased transcriptional response correlated with an increase in the interaction between DRIP205 and the mutant VDR. We further provide evidence that OA induces the phosphorylation of CREB-binding protein (CBP), indicating for the first time a correlation between phosphorylation of CBP and enhanced VDR function. Hexafluoro 1,25(OH)2D3 analogs (RO-26-2198 and RO-4383561) also resulted in at least a partial restoration of the transcriptional responsiveness of mutant VDRs I268T, F251C, and H305Q. Our data indicate that the enhanced potency of the hexafluoro analogs may be caused by increased DRIP205 and glucocorticoid receptor interacting protein 1 (GRIP-1) binding to VDRs and enhanced association of VDRs with DNA, as suggested by results of ChIP assays.

CONCLUSION

Our study provides new insights into the mechanisms involved in the enhancement of VDR function by both phosphorylation and hexafluoro analogs and forms a basis for future study of vitamin D analogs or specifically designed kinase activity mediators as potential therapy for the treatment of selected patients with HVDRR.

13-cis retinoic acid inhibits development and progression of chronic allograft nephropathy

Chronic allograft nephropathy is characterized by chronic inflammation and fibrosis. Because retinoids exhibit anti-proliferative, anti-inflammatory, and anti-fibrotic functions, the effects of low and high doses of 13-cis-retinoic acid (13cRA) were studied in a chronic Fisher344-->Lewis transplantation model. In 13cRA animals, independent of dose (2 or 20 mg/kg body weight/day) and start (0 or 14 days after transplantation) of 13cRA administration, serum creatinine was significantly lower and chronic rejection damage was dramatically reduced, including subendothelial fibrosis of preglomerular vessels and chronic tubulointerstitial damage. The number of infiltrating mononuclear cells and their proliferative activity were significantly diminished. The mRNA expression of chemokines (MCP-1/CCL2, MIP-1alpha/CCL3, IP-10/CXCL10, RANTES/CCL5) and proteins associated with fibrosis (plasminogen activator inhibitor-1, transforming growth factor-beta1, and collagens I and III) were strikingly lower in treated allografts. In vitro, activated peritoneal macrophages of 13cRA-treated rats showed a pronounced decrease in protein secretion of inflammatory cytokines (eg, tumor necrosis factor-alpha, interleukin-6). The suppression of the proinflammatory chemokine RANTES/CCL5 x 13cRA in fibroblasts could be mapped to a promoter module comprising IRF-1 and nuclear factor-kappaB binding elements, but direct binding of retinoid receptors to promoter elements could be excluded. In summary, 13cRA acted as a potent immunosuppressive and anti-fibrotic agent able to prevent and inhibit progression of chronic allograft nephropathy.

Histone acetyltransferase activity of p300 enhances the activation of IL-18 promoter

Interleukin-18 (IL-18), an important regulator of innate and acquired immune responses expressed from a variety of cell types, is a pleiotropic cytokine in the development of T helper type 1 (Thl) cells. The p300/CBP (CREB-binding protein) coactivator proteins are important histone acetyltransferases (HATs) that regulate the transcription of many genes. Whether p300/CBP play a role in the IL-18 expression has not been investigated previously. In this study, we analyzed the roles of p300 in the regulation of mouse IL-18 by using RT-PCR and a series of co-transfection studies. We showed that p300 had a stimulating effect on the endogenous IL-18 mRNA synthesis and on the activity of IL-18 p1 promoter. The results also showed that IL-18 p1 promoter activity was enhanced by p300 in a dose-dependent manner. Moreover, the p300-mediated activation function can be suppressed by the adenovirus E1A protein, which inhibits the HAT function of p300. Also, a mutation in p300 HAT region abolished the effect of p300 on IL-18 activation. These data further indicate that the acetylase activity of p300 was indispensable to its function. Furthermore, we found that p300 was able to enhance the effect of the transcription factor c-Fos on activation of the IL-18 promoter. Data presented in this paper implicate important roles of p300 in the transcriptional regulation of IL-18.

Proteomic analysis of steady-state nuclear hormone receptor coactivator complexes

We report our initial efforts in the analysis of endogenous nuclear receptor coactivator complexes as a research bridging strand of the Nuclear Receptor Signaling Atlas (NURSA) (www.NURSA.org). A proteomic approach is used to systematically isolate a variety of coactivator complexes using HeLa cells as a model cell line and to identify the coactivator-associated proteins with mass spectrometry. We have isolated and identified seven coactivator complexes including the p160 steroid receptor coactivator family, cAMP response element binding protein-binding protein, p300, coactivator of activating protein-1 and estrogen receptors, and E6 papillomavirus-associated protein. The newly identified coactivator-associated proteins provide unbiased clues and links for understanding of the endogenous hormone receptor coregulator network and its regulation. We hope that the electronic availability of these data to the general scientific community will facilitate generation and testing of new hypotheses to further our understanding of nuclear receptor signaling and coactivator functions.

A genetic screen identifies putative targets and binding partners of CREB-binding protein in the developing Drosophila eye

Drosophila CREB-binding protein (dCBP) is a very large multidomain protein, which belongs to the CBP/p300 family of proteins that were first identified by their ability to bind the CREB transcription factor and the adenoviral protein E1. Since then CBP has been shown to bind to >100 additional proteins and functions in a multitude of different developmental contexts. Among other activities, CBP is known to influence development by remodeling chromatin, by serving as a transcriptional coactivator, and by interacting with terminal members of several signaling transduction cascades. Reductions in CBP activity are the underlying cause of Rubinstein-Taybi syndrome, which is, in part, characterized by several eye defects, including strabismus, cataracts, juvenile glaucoma, and coloboma of the eyelid, iris, and lens. Development of the Drosophila melanogaster compound eye is also inhibited in flies that are mutant for CBP. However, the vast array of putative protein interactions and the wide-ranging roles played by CBP within a single tissue such as the retina can often complicate the analysis of CBP loss-of-function mutants. Through a series of genetic screens we have identified several genes that could either serve as downstream transcriptional targets or encode for potential CBP-binding partners and whose association with eye development has hitherto been unknown. The identification of these new components may provide new insight into the roles that CBP plays in retinal development. Of particular interest is the identification that the CREB transcription factor appears to function with CBP at multiple stages of retinal development.

Transcriptional activities of retinoic acid receptors

Vitamin A derivatives plays a crucial role in embryonic development, as demonstrated by the teratogenic effect of either an excess or a deficiency in vitamin A. Retinoid effects extend however beyond embryonic development, and tissue homeostasis, lipid metabolism, cellular differentiation and proliferation are in part controlled through the retinoid signaling pathway. Retinoids are also therapeutically effective in the treatment of skin diseases (acne, psoriasis and photoaging) and of some cancers. Most of these effects are the consequences of retinoic acid receptors activation, which triggers transcriptional events leading either to transcriptional activation or repression of retinoid-controlled genes. Synthetic molecules are able to mimic part of the biological effects of the natural retinoic acid receptors, all-trans retinoic acid. Therefore, retinoic acid receptors are considered as highly valuable therapeutic targets and limiting unwanted secondary effects due to retinoid treatment requires a molecular knowledge of retinoic acid receptors biology. In this review, we will examine experimental evidence which provide a molecular basis for the pleiotropic effects of retinoids, and emphasize the crucial roles of coregulators of retinoic acid receptors, providing a conceptual framework to identify novel therapeutic targets.

Nuclear hormone receptor coregulator: role in hormone action, metabolism, growth, and development

Nuclear hormone receptor coregulator (NRC) (also referred to as activating signal cointegrator-2, thyroid hormone receptor-binding protein, peroxisome proliferator activating receptor-interacting protein, and 250-kDa receptor associated protein) belongs to a growing class of nuclear cofactors widely known as coregulators or coactivators that are necessary for transcriptional activation of target genes. The NRC gene is also amplified and overexpressed in breast, colon, and lung cancers. NRC is a 2063-amino acid protein that harbors a potent N-terminal activation domain (AD1) and a second more centrally located activation domain (AD2) that is rich in Glu and Pro. Near AD2 is a receptor-interacting domain containing an LxxLL motif (LxxLL-1), which interacts with a wide variety of ligand-bound nuclear hormone receptors with high affinity. A second LxxLL motif (LxxLL-2) located in the C-terminal region of NRC is more restricted in its nuclear hormone receptor specificity. The intrinsic activation potential of NRC is regulated by a C-terminal serine, threonine, leucine-regulatory domain. The potential role of NRC as a cointegrator is suggested by its ability to enhance transcriptional activation of a wide variety of transcription factors and from its in vivo association with a number of known transcriptional regulators including CBP/p300. Recent studies in mice indicate that deletion of both NRC alleles leads to embryonic lethality resulting from general growth retardation coupled with developmental defects in the heart, liver, brain, and placenta. NRC(-/-) mouse embryo fibroblasts spontaneously undergo apoptosis, indicating the importance of NRC as a prosurvival and antiapoptotic gene. Studies with 129S6 NRC(+/-) mice indicate that NRC is a pleiotropic regulator that is involved in growth, development, reproduction, metabolism, and wound healing.

Teaching resources. Nuclear receptors

This teaching resource provides lecture notes and slides for a class covering nuclear receptors and is part of the course "Cell signaling systems: a course for graduate students." The lecture begins with a discussion of the structure of nuclear hormone receptors and then proceeds to describe mechanisms of transcriptional regulation, modulation of nuclear receptor activity by phosphorylation, and the role of nuclear receptors in cancer.

Identification of novel CBP interacting proteins in embryonic orofacial tissue

cAMP response element-binding protein (CREB)-binding protein (CBP) plays an important role as a general co-integrator of multiple signaling pathways and interacts with a large number of transcription factors and co-factors, through its numerous protein-binding domains. To identify nuclear factors associated with CBP in developing orofacial tissue, a yeast two-hybrid screen of a cDNA library derived from orofacial tissue from gestational day 11 to 13 mouse embryos was conducted. Using the carboxy terminus (amino acid residues 1676-2441) of CBP as bait, several novel proteins that bind CBP were identified, including an Msx-interacting-zinc finger protein, CDC42 interaction protein 4/thyroid hormone receptor interactor 10, SH3-domain GRB2-like 1, CCR4-NOT transcription complex subunit 3, adaptor protein complex AP-1 beta1 subunit, eukaryotic translation initiation factor 2B subunit 1 (alpha), and cyclin G-associated kinase. Results of the yeast two-hybrid screen were confirmed by glutathione S-transferase pull-down assays. The identification of these proteins as novel CBP-binding partners allows exploration of new mechanisms by which CBP regulates and integrates diverse cell signaling pathways.

p21WAF1/CIP1 selectively controls the transcriptional activity of estrogen receptor alpha

Estrogen receptors (ER) are ligand-dependent transcription factors that regulate growth, differentiation, and maintenance of cellular functions in a wide variety of tissues. We report here that p21WAF1/CIP1, a cyclin-dependent kinase (Cdk) inhibitor, cooperates with CBP to regulate the ERalpha-mediated transcription of endogenous target genes in a promoter-specific manner. The estrogen-induced expression of the progesterone receptor and WISP-2 mRNA transcripts in MCF-7 cells was enhanced by p21WAF1/CIP1, whereas that of the cyclin D1 mRNA was reduced and the pS2 mRNA was not affected. Chromatin immunoprecipitation assays revealed that p21WAF1/CIP1 was recruited simultaneously with ERalpha and CBP to the endogenous progesterone receptor gene promoter in an estrogen-dependent manner. Experiments in which the p21WAF1/CIP1 protein was knocked down by RNA interference showed that the induction of the expression of the gene encoding the progesterone receptor required p21WAF1/CIP1, in contrast with that of the cyclin D1 and pS2 genes. p21WAF1/CIP1 induced not only cell cycle arrest in breast cancer cells but also milk fat globule protein and lipid droplets, indicators of the differentiated phenotype, as well as cell flattening and increase of the volume of the cytoplasm. These results indicate that p21WAF1/CIP1, in addition to its Cdk-regulatory role, behaves as a transcriptional coactivator in a gene-specific manner implicated in cell differentiation.

Potentiation of estrogen receptor transcriptional activity by breast cancer amplified sequence 2

Breast cancer amplified sequence 2 (BCAS2) was initially identified as a gene that was overexpressed and amplified in some breast cancer cell lines. It was later found to be a component of the spliceosome. Here, we identified BCAS2 as an estrogen receptor (ER) alpha interacting protein by yeast two-hybrid screening. In addition to ER alpha, BCAS2 also interacted with ER beta, TR beta, PR, and PPAR gamma in a ligand-independent way. Transient transfection assays revealed that overexpression of BCAS2 enhanced while inhibition of BCAS2 expression attenuated the estrogen receptor-mediated transcription. BCAS2 potentiated the activation function-2 (AF-2) activity of ER alpha but had no effect on the AF-1 activity. This study suggested that BCAS2 might play an important role in breast cancer development by increasing the estrogen receptor's function.

B56 regulatory subunit of protein phosphatase 2A mediates valproic acid-induced p300 degradation

Transcriptional coactivator p300 is required for embryonic development and cell proliferation. Valproic acid, a histone deacetylase inhibitor, is widely used in the therapy of epilepsy and bipolar disorder. However, it has intrinsic teratogenic activity through unidentified mechanisms. We report that valproic acid stimulates proteasome-dependent p300 degradation through augmentation of gene expression of the B56gamma regulatory subunits of protein phosphatase 2A. The B56gamma3 regulatory and catalytic subunits of protein phosphatase 2A interact with p300. Overexpression of the B56gamma3 subunit leads to proteasome-mediated p300 degradation and represses p300-dependent transcriptional activation, which requires the B56gamma3 interaction domain of p300. Conversely, silencing of the B56gamma subunit expression by RNA interference increases the stability and transcriptional activity of the coactivator. Our study establishes the functional interaction between protein phosphatase 2A and p300 activity and provides direct evidence for signal-dependent control of p300 function.

CREB binding protein functions during successive stages of eye development in Drosophila

During the development of the compound eye of Drosophila several signaling pathways exert both positive and inhibitory influences upon an array of nuclear transcription factors to produce a near-perfect lattice of unit eyes or ommatidia. Individual cells within the eye are exposed to many extracellular signals, express multiple surface receptors, and make use of a large complement of cell-subtype-specific DNA-binding transcription factors. Despite this enormous complexity, each cell will make the correct developmental choice and adopt the appropriate cell fate. How this process is managed remains a poorly understood paradigm. Members of the CREB binding protein (CBP)/p300 family have been shown to influence development by (1) acting as bridging molecules between the basal transcriptional machinery and specific DNA-binding transcription factors, (2) physically interacting with terminal members of signaling cascades, (3) acting as transcriptional coactivators of downstream target genes, and (4) playing a key role in chromatin remodeling. In a screen for new genes involved in eye development we have identified the Drosophila homolog of CBP as a key player in both eye specification and cell fate determination. We have used a variety of approaches to define the role of CBP in eye development on a cell-by-cell basis.

Effects of FSH and 17beta-estradiol on the transactivation of estrogen-regulated promoters and cell proliferation in L cells

In the present study, we analyzed human follicle-stimulating hormone (FSH)-induced cell proliferation and transactivation of estrogen-sensitive reporter genes-in L cells stably expressing the human FSH receptor [L-(hFSHR(+)) cells]. In order to dissect the signaling pathways involved in this process, L-(hFSHR(+)) cells were transiently transfected with either the 3X-ERE-TAT-Luc or the ERE-VitA2-TK-CAT reporter genes and treated with FSH or PKA activators (cholera toxin, forskolin and 8-Br-cAMP) in the presence or absence of various kinase inhibitors. We found that FSH and all PKA activators, specifically induced transactivation of both reporter genes. Transactivation of estrogen-sensitive genes by FSH or PKA activators were blocked (approximately 90%) by H89 (PKA inhibitor) and LY294002 but not by Wortmannin (PI3-K inhibitors), 4-OH-tamoxifen, ICI182,780 or SB203580 (p38 MAPK inhibitor); PD98059 (ERK1/2 inhibitor) partially (approximately 30%) blocked the FSH-mediated effect. The combination of FSH and estradiol resulted in a synergistic effect on transactivation as well as on cell proliferation, and this enhancement was attenuated by antiestrogens. We additionally analyzed the participation of the coactivators SRC-1 and cAMP response element binding protein (CREB)-binding protein (CBP) in FSH-evoked estrogen receptor (ER)-dependent transactivation; we found that CBP but not SRC-1 potentiated FSH-induced transcriptional activation of both ER-sensitive reporters, being this effect stronger on the ERE-VitA2-TK-CAT than on the 3X-ERE-TAT-Luc reporter. Thus, in L-(hFSHR(+)) cells FSH induces transcriptional activation of estrogen-sensitive genes through an A-kinase-triggered signaling pathway, using also to a lesser extent the ERK1/2 and p38 pathways. PI3-K is not apparently involved in this FSH-mediated process since LY294002, but not Wortmannin, specifically binds ERs and completely blocks estrogen action. Presumably, CBP cooperates with the ER on genes that contain estrogen responsive elements through mechanisms involving the participation of other proteins and/or basal transcription factors (e.g. CREB), which in turn mediate the transcriptional response of estrogen-sensitive reporter genes to FSH stimulation.

Biochemical and NMR Mapping of the Interface between CREB-binding Protein and Ligand Binding Domains of Nuclear Receptor

CBP, cAMP-response element-binding protein (CREB)-binding protein, plays an important role as a general cointegrator of various signaling pathways and interacts with a large number of transcription factors. Interactions of CBP with ligand binding domains (LBDs) of nuclear receptors are mediated by LXXLL motifs, as are those of p160 proteins, although the number, distribution, and precise sequences of the motifs differ. We used a large N-terminal fragment of murine CBP to map by biochemical methods and NMR spectroscopy the interaction domain of CBP with the LBDs of several nuclear receptors. We show that distinct zones of that fragment are involved in the interactions: a 20-residue segment containing the LXXLL motif (residues 61-80) is implicated in the interaction with all three domains tested (peroxisome proliferator-activated receptor gamma-LBD, retinoid X receptor alpha-LBD, and estrogen-related receptor gamma-LBD), whereas a second N-terminal well conserved block of around 25 residues centered on a consensus L(40)PDEL(44) motif constitutes a secondary motif of interaction with peroxisome proliferator-activated receptor gamma-LBD. Sequence analysis reveals that both zones are well conserved in all vertebrate p300/CBP proteins, suggesting their functional importance. Interactions of p300/CBP coactivators with the LBDs of nuclear receptors are not limited to the canonical LXXLL motifs, involving both a longer contiguous segment around the motif and, for certain domains, an additional zone.

Histone acetyltransferase activity of p300 enhances the activation of IL-12 p40 promoter

Interleukin-12 (IL-12) is a heterodimeric cytokine produced by macrophages in response to intracellular pathogens and provides an obligatory signal for the differentiation of T-helper-1 cells. p300 is an important histone acetyltransferase (HAT) and has been implicated in the regulation of gene expression. Whether p300 plays a role in the IL-12 expression has not been investigated before. In this study, we analyzed the roles of p300 in the regulation of human IL-12 p40. By using RT-PCR and a series of co-transfection studies, we found that p300 had a stimulating effect on the expression of endogenous IL-12 p40 mRNA and on the activity of IL-12 p40 promoter. We also showed that the HAT activity of p300 was essential to its function in regulating IL-12 promoter, since the mutant p300 with the HAT domain deleted did not have such a stimulation effect. In addition, the adenovirus E1A protein suppressed the p300 function, whereas the mutant E1A lacking the p300 interaction domain did not. Furthermore, p300 was able to reinforce the c-Rel-mediated activation of IL-12 p40. Results presented in this paper implicate that p300 is involved in the transcriptional regulation of IL-12 p40, and IL-12 p40 is one of the target genes of p300.

Coactivators in Gene Regulation by STAT5

Signal transducer and activator of transcription 5 (STAT5) is a member of the STAT family of transcription factors that relay the effect of diverse cytokines, hormones, and growth factors by regulating the transcription of distinct target genes. This function is emphasized by its crucial role in the development of the mammary gland and the hematopoietic system. Cytokine receptor-associated Janus kinases (JAKs) induce dimerization, nuclear translocation, and DNA binding through tyrosine phosphorylation of STAT5. STAT5 regulates the expression of cytokine target genes by binding to gamma interferon-activated sequence (GAS) motifs. Transcriptional activation requires the contact of STAT5 to coactivators and components of the transcription machinery. Another important point in transcriptional activation is the cooperation with other transcription factors that bind in close vicinity to the target gene promoters and enhancers. Their concerted action can result in an enhanced binding to the promoters or in cooperative recruitment of coactivators. In addition, cross-talk with other signaling pathways as well as secondary modifications of STAT5 have been described to affect transactivation function.

Peroxisome proliferator-activated receptor-gamma calls for activation in moderation: lessons from genetics and pharmacology

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a prototypical member of the nuclear receptor superfamily and integrates the control of energy, lipid, and glucose homeostasis. PPARgamma can bind a variety of small lipophilic compounds derived from metabolism and nutrition. These ligands, in turn, determine cofactor recruitment to PPARgamma, regulating the transcription of genes in a variety of metabolic pathways. PPARgamma is the main target of the thiazolidinedione class of insulin-sensitizing drugs, which are currently a mainstay of therapy for type 2 diabetes. However, this therapy has a number of side effects. Here, we review the clinical consequences of PPARgamma polymorphisms in humans, as well as several studies in mice using general or tissue-specific knockout techniques. We also discuss the recent pharmacological literature describing a variety of new PPARgamma partial agonists and antagonists, as well as pan-PPAR agonists. The results of these studies have added to the understanding of PPARgamma function, allowing us to hypothesize a general mechanism of PPARgamma action and speculate on future trends in the use of PPARgamma as a target in the treatment of type II diabetes.

A role for p21 (WAF1) in the cAMP-dependent differentiation of F9 teratocarcinoma cells into parietal endoderm

Combined treatment of teratocarcinoma F9 cells with retinoic acid and dibutyryl-cAMP induces the differentiation into cells with a phenotype resembling parietal endoderm. We show that the levels of cyclin-dependent kinase inhibitor p21/WAF1/Cip1 (p21) protein and mRNA are dramatically elevated at the end of this differentiation, concomitantly with the appearance of p21 in the immunoprecipitated CDK2-cyclin E complex. The induction of differentiation markers could not be achieved by expression of ectopic p21 alone and still required treatment with differentiation agents. Clones of F9 cells transfected with sense or antisense p21 cDNA constructs revealed, upon differentiation, upregulated levels of mRNA for thrombomodulin, a parietal endoderm-specific marker, or increased fraction of cells in sub-G1 phase of the cell cycle, respectively. Consistent with this observation, whereas p21 was strictly nuclear in undifferentiated cells, a large proportion of differentiated cells had p21 localized also in the cytoplasm, a site associated with the antiapoptotic function of p21. Furthermore, p21 activated the thrombomodulin promoter in transient reporter assays and the p21 mutant defective in binding to cyclin E was equally efficient in activation. The promoter activity in differentiated cells was reduced by cotransfection of p21-specific siRNA or antisense cDNA. Coexpression of p21 increased the activity of the GAL-p300(1-1303) fusion protein on the GAL sites-containing TM promoter. This implies that p21 might act through a derepression of the p300 N-terminal-residing repression domain, thereby enhancing the p300 coactivator function. As differentiation of F9 cells into parietal endoderm-like cells requires the cAMP signaling, the results together suggest that the cyclin-dependent kinase inhibitor p21 may promote specifically this pathway in F9 cells.

Chromatin-dependent E1A activity modulates NF-kappaB RelA-mediated repression of glucocorticoid receptor-dependent transcription

The role of chromatin-dependent regulatory mechanisms in the repression of glucocorticoid-dependent transcription from the murine mammary tumor virus (MMTV) promoter by p65 and E1A was investigated by using chromatin and transiently transfected reporters. The p65 RelA subunit of NF-kappaB represses MMTV expression on either transient or integrated reporters. In contrast, the viral oncoprotein E1A represses a transient but not an integrated MMTV. E1A repression is attenuated by chromatin, suggesting p65 but not E1A manipulates chromatin appropriately to inhibit the GR. Coexpression of p65 and E1A additively represses the transient MMTV but restores the transcriptional activation of the chromatin MMTV in response to glucocorticoids. This indicates that E1A has a dominant chromatin-dependent activity that attenuates repression by p65. E1A, p65, and GR bind the MMTV promoter, and chromatin remodeling enhances binding on both repressed and activated promoters. In addition, p65 requires Brg for repression of the integrated MMTV. This suggests that neither p65 repression nor E1A attenuation of repression results from an inhibition of remodeling that prevents transcription factor binding. Furthermore, p300/CBP is also required for both repression and attenuation by p65 and E1A. E1A and p65 mutants that do not bind p300/CBP are inactive, indicative of a requirement for p300/CBP-dependent complex formation for both repression and attenuation with chromatin. These data suggest that both the p65-dependent repression and the E1A-mediated attenuation of repression require the Brg1-dependent chromatin remodeling function and p300/CBP-dependent complex formation at a promoter assembled within chromatin.

The Src kinase pathway promotes tamoxifen agonist action in Ishikawa endometrial cells through phosphorylation-dependent stabilization of estrogen receptor (alpha) promoter interaction and elevated steroid receptor coactivator 1 activity

Tamoxifen is the most widely used selective estrogen receptor modulator for breast cancer in clinical use today. However, tamoxifen agonist action in endometrium remains a major hurdle for tamoxifen therapy. Activation of the nonreceptor tyrosine kinase src promotes tamoxifen agonist action, although the mechanisms remain unclear. To examine these mechanisms, the effect of src kinase on estrogen and tamoxifen signaling in tamoxifen-resistant Ishikawa endometrial adenocarcinoma cells was assessed. A novel connection was identified between src kinase and serine 167 phosphorylation in estrogen receptor (ER)-alpha via activation of AKT kinase. Serine 167 phosphorylation stabilized ER interaction with endogenous ER-dependent promoters. Src kinase exhibited the additional function of potentiating the transcriptional activity of Gal-steroid receptor coactivator 1 (SRC-1) and Gal-cAMP response element binding protein-binding protein in endometrial cancer cells while having no effect on Gal-p300-associated factor and Gal fusions of the other p160 coactivators glucocorticoid-interacting protein 1 (transcriptional intermediary factor 2/nuclear coactivator-2/SRC-2) and amplified in breast cancer 1 (receptor-associated coactivator 3/activator of transcription of nuclear receptor/SRC-3). Src effects on ER phosphorylation and SRC-1 activity both contributed to tamoxifen agonist action on ER-dependent gene expression in Ishikawa cells. Taken together, these data demonstrate that src kinase potentiates tamoxifen agonist action through serine 167-dependent stabilization of ER promoter interaction and through elevation of SRC-1 and cAMP response element binding protein-binding protein coactivation of ER.

Coregulators and chromatin remodeling in transcriptional control

Despite many years of investigation by numerous investigators, transcriptional regulatory control remains an intensely investigated and continuously evolving field of research. Transcriptional regulation is dependent not only on transcription factor activation and chromatin remodeling, but also on a host of transcription factor coregulators-coactivators and corepressors. In addition to transcription factor activation and chromatin changes, there is an expanding array of additional modifications that titrate transcriptional regulation for the specific conditions of a particular cell type, organ system, and developmental stage, and such events are likely to be greatly influenced by upstream signaling cascades. Here, we will briefly review the highlights and perspectives of chromatin remodeling and transcription controls as affected by cofactor availability, cellular energy state, relative ratios of reducing equivalents, and upstream signaling. We also present the C-terminal binding protein (CtBP) as a novel nuclear receptor (NR) coregulator, which exemplifies the integration of a number of transcriptional regulatory controls.

Selective coactivator interactions in gene activation by SREBP-1a and -1c

Requisite levels of intracellular cholesterol and fatty acids are maintained in part by the sterol regulatory element binding proteins (SREBPs). Three major SREBP isoforms exist; SREBP-1a and SREBP-1c are expressed from overlapping mRNAs, whereas SREBP-2 is encoded by a separate gene. The active forms of SREBP-1a and SREBP-1c differ only at their extreme N termini; SREBP-1c lacks 28 aa present in SREBP-1a and instead contains 4 unique aa of its own. While the SREBP-1a and -1c isoforms differentially activate transcription, the molecular basis of this difference is unknown. Here we define the differences between these proteins that confer the enhanced activity of SREBP-1a and demonstrate that this enhancement is a direct result of its avid binding to the coactivator CREB binding protein (CBP) and the mammalian mediator complex. While previous work determined that the C/H1 zinc finger and KIX domains of CBP bind to SREBP-1a, we provide evidence that the interaction with C/H1 is important for gene activation. We further show that the association between the activation domain of SREBP-1 and mediator is through aa 500 to 824 of DRIP150. Finally, we demonstrate the recruitment of mediator to an SREBP-responsive promoter in a sterol-dependent manner.

CBP and p300: HATs for different occasions

The transcriptional coactivators CREB binding protein (CBP) and p300 are key regulators of RNA polymerase II-mediated transcription. Genetic alterations in the genes encoding these regulatory proteins and their functional inactivation have been linked to human disease. Findings in patients, knockout mice and cell-based studies indicate that the ability of these multidomain proteins to acetylate histones and other proteins is critical for many biological processes. Furthermore, despite their high degree of homology, accumulating evidence indicates that CBP and p300 are not completely redundant but also have unique roles in vivo. Recent studies suggest that these functional differences could be due to differential association with other proteins or differences in substrate specificity between these acetyltransferases. Inactivation of the acetyltransferase function of either CBP or p300 in various experimental systems will no doubt teach us more about the specific biological roles of these proteins. Given the wide range of human diseases in which CBP and/or p300 have been implicated, understanding the mechanisms that regulate their activity in vivo could help to develop novel approaches for the development of therapeutic strategies.

A histone methyltransferase is required for maximal response to female sex hormones

RIZ1 is an estrogen receptor (ER) coactivator but is also a histone lysine methyltransferase that methylates lysine 9 of histone H3, an activity known to repress transcription. We show here that target organs of mice deficient in RIZ1 exhibit decreased response to female sex hormones. RIZ1 interacted with SRC1 and p300, suggesting that the coactivator function of RIZ1 may be mediated by its interaction with other transcriptional coactivators. In the presence of estrogen, RIZ1 binding to estrogen target genes became less direct and followed the binding of ER to DNA and RIZ1 methyltransferase activity on H3-Lys 9 was inhibited, indicating derepression may play a role in estrogen induction of gene transcription. Reducing RIZ1 level correlated with decreased induction of pS2 gene by estrogen in MCF7 cells. The data suggest that a histone methyltransferase is required for optimal estrogen response in female reproductive tissues and that estrogen-bound ER may turn a transcriptional repressor into a coactivator.

Nuclear structure-associated TIF2 recruits glucocorticoid receptor and its target DNA

Assembly of multi-protein complexes on promoter and enhancer elements is a prerequisite for onset of gene transcription. At the beginning of this process, transcription factors are thought to act as nucleating centers for complex formation through the binding of their target DNA sequences, and thereafter recruit coactivators. Here, we investigated this process of assembly by determining the distribution of the glucocorticoid receptor (GR) and its coactivator, TIF2. Both endogenously and ectopically expressed TIF2 were shown to form foci in the nucleus, and GR could be recruited to the TIF2 foci upon GR agonist but not antagonist treatment. Moreover, we show that the coactivators, p300 and PCAF, are also recruited to the TIF2 foci. The TIF2 foci could recruit GR carrying a microinjected GR responsive element. We propose that TIF2 provides a nuclear compartment that allows the assembly of multi-protein complexes required for GR-mediated gene activation.

Multiple-color fluorescence imaging of chromosomes and microtubules in living cells

Microscopic observation of fluorescently-stained intracellular molecules within a living cell provides a straightforward approach to understanding their temporal and spatial relationships. However, exposure to the excitation light used to visualize these fluorescently-stained molecules can be toxic to the cells. Here we describe several important considerations in microscope instrumentation and experimental conditions for avoiding the toxicity associated with observing living fluorescently-stained cells. Using a computer-controlled fluorescence microscope system designed for live observation, we recorded time-lapse, multi-color images of chromosomes and microtubules in living human and fission yeast cells. In HeLa cells, a human cell line, microtubules were stained with rhodamine-conjugated tubulin, and chromosomes were stained with a DNA-specific fluorescent dye, Hoechst33342, or with rhodamine-conjugated histone. In fission yeast cells, microtubules were stained with alpha-tubulin fused with the jellyfish green fluorescent protein (GFP), and chromosomes were stained with Hoechst33342.

Subfertility, Uterine Hypoplasia, and Partial Progesterone Resistance in Mice Lacking the Krüppel-like Factor 9/Basic Transcription Element-binding Protein-1 (Bteb1) Gene

Progesterone receptor (PR), a ligand-activated transcription factor, is a key regulator of cellular proliferation and differentiation in reproductive tissues. The transcriptional activity of PR is influenced by co-regulatory proteins typically expressed in a tissue- and cell-specific fashion. We previously demonstrated that basic transcription element-binding protein-1 (BTEB1), a member of the Sp/Krüppel-like family of transcription factors, functionally interacts with the two PR isoforms, PR-A and PR-B, to mediate progestin sensitivity of target genes in endometrial epithelial cells in vitro. Here we report that ablation of the Bteb1 gene in female mice results in uterine hypoplasia, reduced litter size, and increased incidence of neonatal deaths in offspring. The reduced litter size is solely a maternal genotype effect and results from fewer numbers of implantation sites, rather than defects in ovulation. In the early pregnant uterus, Bteb1 expression in stromal cells temporally coincides with PR-A isoform-dependent decidual formation at the time of implantation. Expression of two implantation-specific genes, Hoxa10 and cyclin D3, was decreased in uteri of early pregnant Bteb1-null mutants, whereas that of Bteb3, a related family member, was increased, the latter possibly compensating for the loss of Bteb1. Progesterone responsiveness of several uterine genes was altered with Bteb1-null mutation. These results identify Bteb1 as a functionally relevant PR-interacting protein and suggest its selective modulation of cellular processes that are regulated by PR-A in the uterine stroma.

Runx2 control of organization, assembly and activity of the regulatory machinery for skeletal gene expression

We present an overview of Runx involvement in regulatory mechanisms that are requisite for fidelity of bone cell growth and differentiation, as well as for skeletal homeostasis and the structural and functional integrity of skeletal tissue. Runx-mediated control is addressed from the perspective of support for biological parameters of skeletal gene expression. We review recent findings that are consistent with an active role for Runx proteins as scaffolds for integration, organization and combinatorial assembly of nucleic acids and regulatory factors within the three-dimensional context of nuclear architecture.

Pancreas duodenum homeobox-1 transcriptional activation requires interactions with p300

The homeodomain transcription factor, pancreas duodenum homeobox (PDX)-1, is essential for pancreas development, insulin production, and glucose homeostasis. Mutations in pdx-1(ipf-1) are associated both with maturity-onset diabetes of the young and type 2 diabetes. PDX-1 interacts with multiple transcription factors and coregulators, including the coactivator p300, to activate the transcription of the insulin gene and other target genes within pancreatic beta-cells. In characterizing the protein-protein interactions of PDX-1 and p300, we identified mutations in PDX-1 that disrupt its function and are associated with increased or decreased interactions with p300. Several mutant PDX-1 proteins that are associated with heritable forms of diabetes in humans, in particular the mutant P63fsdelC, exhibited increased binding to a carboxy-terminal segment of p300 in the setting of decreased DNA-binding activities, suggesting that sequestration of p300 by mutant PDX-1 proteins may be an additional mechanism by which insulin gene expression is reduced in heterozygous carriers of pdx-1(ipf-1) mutations. The introduction of the point mutations S66A/Y68A in the highly conserved amino-terminal PDX-1 transactivation domain reduced the ability of PDX-1 to interact with p300, substantially diminished the transcriptional activation of PDX-1, and reduced the synergistic activation of glucose-responsive insulin promoter enhancer sequences by PDX-1, E12, and E47. We propose that interactions of PDX-1 with p300 are required for the transcriptional activation of PDX-1 target genes. Impairment of interactions between PDX-1 and p300 in pancreatic beta-cells may limit insulin production and lead to the development of diabetes.

A two-gene expression ratio predicts clinical outcome in breast cancer patients treated with tamoxifen

Tamoxifen significantly reduces tumor recurrence in certain patients with early-stage estrogen receptor-positive breast cancer, but markers predictive of treatment failure have not been identified. Here, we generated gene expression profiles of hormone receptor-positive primary breast cancers in a set of 60 patients treated with adjuvant tamoxifen monotherapy. An expression signature predictive of disease-free survival was reduced to a two-gene ratio, HOXB13 versus IL17BR, which outperformed existing biomarkers. Ectopic expression of HOXB13 in MCF10A breast epithelial cells enhances motility and invasion in vitro, and its expression is increased in both preinvasive and invasive primary breast cancer. The HOXB13:IL17BR expression ratio may be useful for identifying patients appropriate for alternative therapeutic regimens in early-stage breast cancer.

The nuclear hormone receptor coactivator NRC is a pleiotropic modulator affecting growth, development, apoptosis, reproduction, and wound repair

Nuclear hormone receptor coregulator (NRC) is a 2,063-amino-acid coregulator of nuclear hormone receptors and other transcription factors (e.g., c-Fos, c-Jun, and NF-kappaB). We and others have generated C57BL/6-129S6 hybrid (C57/129) NRC(+/-) mice that appear outwardly normal and grow and reproduce. In contrast, homozygous deletion of the NRC gene is embryonic lethal. NRC(-/-) embryos are always smaller than NRC(+/+) embryos, and NRC(-/-) embryos die between 8.5 and 12.5 days postcoitus (dpc), suggesting that NRC has a pleotrophic effect on growth. To study this, we derived mouse embryonic fibroblasts (MEFs) from 12.5-dpc embryos, which revealed that NRC(-/-) MEFs exhibit a high rate of apoptosis. Furthermore, a small interfering RNA that targets mouse NRC leads to enhanced apoptosis of wild-type MEFs. The finding that C57/129 NRC(+/-) mice exhibit no apparent phenotype prompted us to develop 129S6 NRC(+/-) mice, since the phenotype(s) of certain gene deletions may be strain dependent. In contrast with C57/129 NRC(+/-) females, 20% of 129S6 NRC(+/-) females are infertile while 80% are hypofertile. The 129S6 NRC(+/-) males produce offspring when crossed with wild-type 129S6 females, although fertility is reduced. The 129S6 NRC(+/-) mice tend to be stunted in their growth compared with their wild-type littermates and exhibit increased postnatal mortality. Lastly, both C57/129 NRC(+/-) and 129S6 NRC(+/-) mice exhibit a spontaneous wound healing defect, indicating that NRC plays an important role in that process. Our findings reveal that NRC is a coregulator that controls many cellular and physiologic processes ranging from growth and development to reproduction and wound repair.