CREB binding protein acts synergistically with steroid receptor coactivator-1 to enhance steroid receptor-dependent transcription

LKB1 catalytic activity contributes to estrogen receptor alpha signaling

The tumor suppressor serine-threonine kinase LKB1 is mutated in Peutz-Jeghers syndrome (PJS) and in epithelial cancers, including hormone-sensitive organs such as breast, ovaries, testes, and prostate. Clinical studies in breast cancer patients show low LKB1 expression is related to poor prognosis, whereas in PJS, the risk of breast cancer is similar to the risk from germline mutations in breast cancer (BRCA) 1/BRCA2. In this study, we investigate the role of LKB1 in estrogen receptor alpha (ERalpha) signaling. We demonstrate for the first time that LKB1 binds to ERalpha in the cell nucleus in which it is recruited to the promoter of ERalpha-responsive genes. Furthermore, LKB1 catalytic activity enhances ERalpha transactivation compared with LKB1 catalytically deficient mutants. The significance of our discovery is that we demonstrate for the first time a novel functional link between LKB1 and ERalpha. Our discovery places LKB1 in a coactivator role for ERalpha signaling, broadening the scientific scope of this tumor suppressor kinase and laying the groundwork for the use of LKB1 as a target for the development of new therapies against breast cancer.

Nuclear receptor coactivators: essential players for steroid hormone action in the brain and in behaviour

Steroid hormones act both in the brain and throughout the body to influence behaviour and physiology. Many of these effects of steroid hormones are elicited by transcriptional events mediated by their respective receptors. A variety of cell culture studies reveal that nuclear receptor coactivators are critical for modulating steroid receptor-dependent transcription. Thus, in addition to the availability of the hormone and the expression of its receptor, nuclear receptor coactivators are essential for steroid-dependent transactivation of genes. This review discusses the mounting evidence indicating that nuclear receptor coactivators are critical for modulating steroid hormone action in the brain and in the regulation of behaviour.

Isolation of novel coregulatory protein networks associated with DNA-bound estrogen receptor alpha

Background

DNA-bound transcription factors recruit an array of coregulatory proteins that influence gene expression. We previously demonstrated that DNA functions as an allosteric modulator of estrogen receptor α (ERα) conformation, alters the recruitment of regulatory proteins, and influences estrogen-responsive gene expression and reasoned that it would be useful to develop a method of isolating proteins associated with the DNA-bound ERα using full-length receptor and endogenously-expressed nuclear proteins.

Results

We have developed a novel approach to isolate large complexes of proteins associated with the DNA-bound ERα. Purified ERα and HeLa nuclear extracts were combined with oligos containing ERα binding sites and fractionated on agarose gels. The protein-DNA complexes were isolated and mass spectrometry analysis was used to identify proteins associated with the DNA-bound receptor. Rather than simply identifying individual proteins that interact with ERα, we identified interconnected networks of proteins with a variety of enzymatic and catalytic activities that interact not only with ERα, but also with each other. Characterization of a number of these proteins has demonstrated that, in addition to their previously identified functions, they also influence ERα activity and expression of estrogen-responsive genes.

Conclusion

The agarose gel fractionation method we have developed would be useful in identifying proteins that interact with DNA-bound transcription factors and should be easily adapted for use with a variety of cultured cell lines, DNA sequences, and transcription factors.

Molecular and cellular mechanisms for differentiation and regeneration of the uterine endometrium

The human endometrium undergoes cyclical changes including proliferation, differentiation, tissue breakdown, and shedding (menstruation) throughout a woman's reproductive life. The postovulatory rise in ovarian progesterone induces profound remodeling and differentiation of the estradiol-primed endometrium. This change, termed decidualization, is crucial for embryo implantation and maintenance of the pregnancy. To date, activation and crosstalk of cAMP- and progesterone-mediated signaling pathways have emerged as key cellular events to drive integrated changes at both the transcriptome and the proteome levels. This results in the induction and maintenance of the decidual phenotype and function. Our recent series of studies highlights the critical role of SRC kinase activation (v-src sarcoma viral oncogene homolog) and STAT5 (signal transducer and activator of transcription 5) phosphorylation in decidualization. After separation of the functional layer of the differentiated endometrium that follows progesterone withdrawal, i.e., menstruation, the basal layer of the endometrium, under the influence of estradiol, regrows and initiates a unique form of angiogenesis and regenerates a new functional layer. The molecular and cellular mechanisms for this process remain elusive, mainly because of difficulties in reproducing menstrual tissue breakdown, shedding, and subsequent tissue regeneration in vitro. We have recently developed a "humanized" mouse model in which a functional human endometrium is reconstituted. It may be used as an in vivo experimental tool for the study of endometrial angiogenesis and regeneration. This model may also be used to identify and test new therapeutic strategies for endometriosis, endometrial cancer, implantation failure, and infertility related to endometrial dysfunction.

Estrogenic regulation of gene and protein expression within the amygdala of female mice

Estrogens exert important actions on fear and anxiety in both humans and non-humans. Currently, the mechanisms underlying estrogenic modulation of fear are not known. However, evidence suggests that estrogens may exert their influence on fear and anxiety within the amygdala. The purpose of the present study was to examine the genomic effects of estrogens within the amygdala of female mice using high-density oligonucleotide microarrays. We examined the effects of estrogens on gene expression at 2 and 24 h after an acute subcutaneous injection. Data from the microarrays revealed that 2 h following an acute injection of estradiol, 44 genes were significantly up- or downregulated, and at 24 h, 13 transcripts were significantly up- or downregulated. One interesting estrogen-regulated gene, (CaMKIIalpha), was downregulated ninefold 2 h following an acute estradiol injection but was not altered 24 h after injection. We further examined estrogen regulation of CaMKIIalpha, as well as CaMKIIbeta and CaMKIV within the amygdala using quantitative PCR and western blot analysis. The data indicate that estrogen decreases CaMKIIalpha and CaMKIV but not CaMKIIbeta gene expression within the amygdala. However, CaMKII protein levels were not different, and CaMKIV protein levels increased 2 h post-EB treatment. These results indicate that estrogen regulates CaMK gene expression and protein levels within the amygdala.

PRMT2, a member of the protein arginine methyltransferase family, is a coactivator of the androgen receptor

The basal transcriptional activity of nuclear receptors (NRs) is regulated by interactions with additional comodulator proteins (coactivator/corepressor). Here, we describe a new androgen receptor (AR)-associated coactivator, PRMT2, which belongs to the arginine methyltransferase protein family. To search for AR-interacting proteins a fragment of the AR was used in a library screen exploiting the yeast two-hybrid technique and identifying the C-terminal region of PRMT2. We demonstrated that PRMT2 acts as a strong coactivator of the AR, had modest or none influence on transcriptional activation mediated by other NRs. Interestingly, PRMT2 interaction with the estrogen receptor (ER) was strongly dependent on the cellular background, thus, suggesting the involvement of additional, differentially expressed coregulators. We also demonstrated synergistic interaction of PRMT2 with other known nuclear receptor coactivators, such as GRIP1/TIF-2. Potentiation of AR-mediated transactivation by PRMT2 alone and in synergism with GRIP1 was prevented by a competitive inhibitor of methyltransferase activity. The PRMT2 expression profile overlaps with the distribution of AR, with strongest PRMT2 abundance in androgen target tissues. Immunofluorescence experiments showed that the intracellular localization of PRMT2 depends on the presence of the cognate receptor ligand. Under androgen-free conditions, both AR and PRMT2 are confined to the cytoplasm, whereas in the presence of androgens both proteins colocalize and translocate into the nucleus. Treatment with the AR antagonist hydroxyflutamide results in nuclear translocation of the AR, but not the coactivator PRMT2. Thus, it appears that the ligand-dependent AR conformation is essential for the recruitment and nuclear translocation of PMRT2 which acts as AR-coactivator, presumably by arginine methylation.

The prognostic significance of steroid receptor co-regulators in breast cancer: co-repressor NCOR2/SMRT is an independent indicator of poor outcome

BACKGROUND

Advances in understanding the molecular basis of breast cancer has necessitated a definition of improved indicators of prognosis that are central to the underlying cancer biology and that reflect the heterogeneous nature of the disease. This study investigates the pattern of expression of the steroid receptor co-regulators NCOA1/SRC1, NCOA3/RAC3, NCOR2/SMRT, and CBP/p300 in breast cancer. The aims were to identify whether their expression was related to patient outcome, their relationships to known prognostic factors and to provide a basis for further research into the mechanistic significance of such associations.

METHODS

The protein levels of steroid receptor co-regulators were assessed by immunohistochemistry in a large well-characterised series of breast carcinomas prepared as tissue microarrays. Relationships between these targets, other clinicopathological variables and patients' outcome were examined.

RESULTS

NCOR2/SMRT was an independent prognostic indicator of overall patient survival (OS) and disease free interval (DFI) and was significantly correlated with distant metastases and local recurrence whereas tumours expressing NCOA1/SRC1 had a significantly longer OS and DFI. There were also significant correlations between co-regulator expression of NCOA1/SRC1, CBP/p300 and NCOA3/RAC3, which were associated with lower tumour grade. NCOA1/SRC1 was also correlated with smaller tumour size. Furthermore, the co-activators had a significant association with steroid receptors, particularly ERalpha.

CONCLUSIONS

NCOR2/SMRT is associated with poor patient outcome, independent of other prognostic factors. In contrast, steroid receptor co-activator expression is generally associated with a good prognosis. Further investigations are needed to establish the mechanisms of these links between the steroid receptor co-regulator system and patient outcome.

The evolution of progesterone receptor ligands

Progesterone is one of the first nuclear receptor hormones to be described functionally and subsequently approached as a drug target. Because progesterone (1) affects both menstruation and gestation via the progesterone receptor (PR), research aimed at modulating its activity is usually surrounded by controversy. However, ligands for PR were developed into drugs, and their evolution can be crudely divided into three periods: (1) drug-like steroids that mimic the gestational properties of progesterone; (2) drug-like steroids with different properties from progesterone and expanded therapeutic applications; and (3) non-steroidal PR ligands with improved selectivity and modulator properties and further expanded therapeutic applications. Although the latter have yet to see widespread clinical applications, their development is founded on a half century of research, and they represent the future for this drug target.

Cells in behaviourally relevant brain regions coexpress nuclear receptor coactivators and ovarian steroid receptors

Oestradiol and progesterone act in the brain to elicit profound effects on behaviour and physiology. One physiological function of oestradiol is the induction of progesterone receptor (PR) expression in a variety of behaviourally relevant brain regions, including the ventromedial nucleus of the hypothalamus (VMN), the medial preoptic nucleus of the preoptic area (MPOA), the arcuate nucleus (ARC) and the medial central grey (MCG). Ligand-dependent transcriptional activity of steroid receptors, including oestrogen receptors (ER) and Pr, is dramatically influenced by nuclear receptor coactivators. In previous studies, we have found that two of these nuclear receptor coactivators, steroid receptor coactivator-1 (SRC-1) and CREB-binding protein (CBP), are important in ER-mediated induction of PR in the VMN and in steroid-dependent behaviours. For nuclear receptor coactivators to function in hormone-dependent transcription in the brain and regulate behaviour, both receptor and coactivator must be expressed in the same cell. In the present study, we used a dual-label immunohistochemical technique to investigate if individual cells in behaviourally relevant brain regions coexpress nuclear receptor coactivators and steroid receptors. Confocal analysis revealed that in oestrogen-primed rats, most of the E-induced PR cells in the VMN (89.6%), MPOA (63%), ARC (82.6%), and many in the MCG (39%), also express SRC-1. In addition, the majority of the cells containing E-induced PR in the VMN (78.3%), MPOA (83.1%), ARC (83.6%), and MCG (60%) also express CBP. These results, taken together with the findings that virtually all oestradiol-induced PR containing cells in the brain express ER, suggest that these neurones represent sites of functional interaction of nuclear receptor coactivators with ovarian steroid receptors in the brain. The present findings provide neuroanatomical evidence that nuclear receptor coactivators are integral in mediating steroid hormone action in behaviourally relevant brain regions.

Stromal-epithelial cell interactions and androgen receptor-coregulator recruitment is altered in the tissue microenvironment of prostate cancer

Tissue recombination experiments show that prostate mesenchyma directs prostate epithelial cell growth and development in an androgen-dependent manner, and that functional differentiation of prostate epithelium requires androgen-driven processes in both epithelia and stroma. The androgen induction of target genes in primary cultures of prostate stromal and epithelial cells was determined using an adenoviral expression system, which employed the MMTV-enhancer driven luciferase reporter as an androgen receptor (AR)-mediated transcription assay. These studies indicate that both cell types contain functional AR. Androgen induction of luciferase reporter activity is 3-fold in stromal cells compared with 10-fold in epithelial cells. AR-mediated transcription activity in stroma cells was enhanced by coculture with epithelial cells or epithelial cell-conditioned media. The elevated AR-mediated transcription activity in stromal cells that were exposed to epithelial factors correlated with increased recruitment of coactivators to the AR transcriptional complex. Epithelial cells facilitated interactions of AR with SRC-1 in an androgen-dependent manner. However, AR-mediated transcriptional activity in stromal cells isolated from prostate cancer was reduced compared with stromal cells isolated from benign prostate and continued to be reduced when cocultured with tumor-derived prostate epithelial cells. The occupancy of AR and coregulators on target genes showed that androgen-bound AR in prostate cancer stromal cells was associated with the corepressor silencing mediator for retinoid and thyroid hormone receptor. Thus, the ability of epithelial cells to modulate coregulator recruitment to the AR transcriptional complex on androgen-responsive genes seems altered in the stromal microenvironment of prostate cancer.

TReP-132 is a novel progesterone receptor coactivator required for the inhibition of breast cancer cell growth and enhancement of differentiation by progesterone

The sex steroid progesterone is essential for the proliferation and differentiation of the mammary gland epithelium during pregnancy. In relation to this, in vitro studies using breast carcinoma T47D cells have demonstrated a biphasic progesterone response, consisting of an initial proliferative burst followed by a sustained growth arrest. However, the transcriptional factors acting with the progesterone receptor (PR) to mediate the progesterone effects on mammary cell growth and differentiation remain to be determined. Recently, it has been demonstrated that the transcriptional regulating protein of 132 kDa (TReP-132), initially identified as a regulator of steroidogenesis, is also a cell growth suppressor. Similar to progesterone-bound PR, TReP-132 acts by inducing the gene expression of the G1 cyclin-dependent kinase inhibitors p21WAF1/Cip1 (p21) and p27Kip1 (p27). The putative interaction between TReP-132 and progesterone pathways in mammary cells was therefore analyzed in the present study. Our results show that TReP-132 interacts in vitro and in T47D cells with progesterone-activated PR. TReP-132 synergizes with progesterone-bound PR to trans activate the p21 and p27 gene promoters at proximal Sp1-binding sites. Moreover, TReP-132 overexpression and knockdown, respectively, increased or prevented the induction of p21 and p27 gene expression by progesterone. As a consequence, TReP-132 knockdown also resulted in the loss of the inhibitory effects of progesterone on pRB phosphorylation, G1/S cell cycle progression, and cell proliferation. Furthermore, the knockdown of TReP-132 expression also prevented the induction of both early and terminal markers of breast cell differentiation which had been previously identified as progesterone target genes. As well, the progesterone-induced accumulation of lipid vacuoles was inhibited in the TReP-132-depleted cells. Finally, TReP-132 gene expression levels increased following progesterone treatment, indicating the existence of a positive auto-regulatory loop between PR and TReP-132. Taken together, these data identify TReP-132 as a coactivator of PR mediating the growth-inhibitory and differentiation effects of progesterone on breast cancer cells.

What clinicians need to know about antioestrogen resistance in breast cancer therapy

Tamoxifen is the drug most used for early breast cancer treatment in oestrogen receptor (ER) positive patients. Unfortunately, despite high ER tumour levels in a tumour, resistance to endocrine therapy, either de novo or acquired after prolonged treatment, can occur. In this review, we will try to summarise the postulated mechanisms of hormonal-resistance, namely, the role of co-regulators and the crosstalk between the HER-2, IGF-IR, Cox-2 and ER pathways. Other predictive markers of tamoxifen-resistance/response, such as cyclin E and UPA/PAI-1, are also discussed.

Nuclear receptor coactivators function in estrogen receptor- and progestin receptor-dependent aspects of sexual behavior in female rats

The ovarian hormones, estradiol (E) and progesterone (P) facilitate the expression of sexual behavior in female rats. E and P mediate many of these behavioral effects by binding to their respective intracellular receptors in specific brain regions. Nuclear receptor coactivators, including Steroid Receptor Coactivator-1 (SRC-1) and CREB Binding Protein (CBP), dramatically enhance ligand-dependent steroid receptor transcriptional activity in vitro. Previously, our lab has shown that SRC-1 and CBP modulate estrogen receptor (ER)-mediated induction of progestin receptor (PR) gene expression in the ventromedial nucleus of the hypothalamus (VMN) and hormone-dependent sexual receptivity in female rats. Female sexual behaviors can be activated by high doses of E alone in ovariectomized rats, and thus are believed to be ER-dependent. However, the full repertoire of female sexual behavior, in particular, proceptive behaviors such as hopping, darting and ear wiggling, are considered to be PR-dependent. In the present experiments, the function of SRC-1 and CBP in distinct ER- (Exp. 1) and PR- (Exp. 2) dependent aspects of female sexual behavior was investigated. In Exp. 1, infusion of antisense oligodeoxynucleotides to SRC-1 and CBP mRNA into the VMN decreased lordosis intensity in rats treated with E alone, suggesting that these coactivators modulate ER-mediated female sexual behavior. In Exp. 2, antisense to SRC-1 and CBP mRNA around the time of P administration reduced PR-dependent ear wiggling and hopping and darting. Taken together, these data suggest that SRC-1 and CBP modulate ER and PR action in brain and influence distinct aspects of hormone-dependent sexual behaviors. These findings support our previous studies and provide further evidence that SRC-1 and CBP function together to regulate ovarian hormone action in behaviorally-relevant brain regions.

The pure estrogen receptor antagonist ICI 182,780 promotes a novel interaction of estrogen receptor-alpha with the 3',5'-cyclic adenosine monophosphate response element-binding protein-binding protein/p300 coactivators

Estrogen receptor-alpha (ERalpha) is a member of the nuclear receptor superfamily of ligand-activated transcription factors. Abundant evidence demonstrates that ERalpha agonists promote, whereas antagonists inhibit, receptor binding to coactivators. In this report we demonstrate that binding of the ICI 182,780 (ICI) pure antiestrogen to ERalpha promotes its interaction with the cAMP response element-binding protein-binding protein (CBP)/p300 but not the p160 family of coactivators, demonstrating the specificity of this interaction. Amino acid mutations within the coactivator binding surface of the ERalpha ligand-binding domain revealed that CBP binds to this region of the ICI-liganded receptor. The carboxy-terminal cysteine-histidine rich domain 3 of CBP, rather than its amino-terminal nuclear interacting domain, shown previously to mediate agonist-dependent interactions of CBP with nuclear receptors, is required for binding to ICI-liganded ERalpha. Chromatin immunoprecipitation assays revealed that ICI but not the partial agonist/antagonist 4-hydroxytamoxifen is able to recruit CBP to the pS2 promoter, and this distinguishes ICI from this class of antiestrogens. Chromatin immunoprecipitation assays for pS2 and cytochrome P450 1B1 promoter regions revealed that ICI-dependent recruitment of CBP, but not receptor, to ERalpha targets is gene specific. ICI treatment did not recruit the steroid receptor coactivator 1 to the pS2 promoter, and it failed to induce the expression of this gene. Taken together, these data indicate that recruitment of the CBP coactivator/cointegrator without steroid receptor coactivator 1 to ERalpha is insufficient to promote transcription of ERalpha target genes.

Role of estrogen receptor alpha transcriptional coregulators in tamoxifen resistance in breast cancer

Tamoxifen is the endocrine agent most commonly used at all stages of breast cancer. Estrogen receptor (ER) alpha, which belongs to the superfamily of nuclear receptors, has been used to identify breast cancer patients who are likely to respond to tamoxifen, but resistance nonetheless occurs in 30-50% of treated ER alpha-positive breast cancer patients. The antiproliferative activity of tamoxifen, relying primarily on its ability to compete with estrogen for the ER alpha ligand binding site in breast tumor tissue, hypotheses forwarded to explain treatment failure include: (1) the existence of a second estrogen receptor (ER beta), (2) an imbalance in estrogen biosynthesis and catabolism, (3) altered bioavailability of tamoxifen, (4) altered cellular trafficking of ER alpha, (5) non genomic effects of ER alpha, directly interacting with several signal transduction pathways, and (6) transcriptional dysregulation of ER alpha target genes, which may involve both genomic (ERE alteration) and non genomic alterations. A first non genomic alteration involves the regulation of ER alpha activity by its phosphorylation mediated by growth factors-kinases signaling pathways. A second non genomic alteration, which is the purpose of this review, involves regulatory factors (coregulators) known as coactivators and corepressors, which activate (or repress) the transcription of ER alpha-responsive genes. The regulation process involves both chromatin remodeling and ER alpha interaction with the transcriptional machinery. Thus, dysregulated expression (coactivator overexpression or corepressor underexpression) and/or mutation of these coregulators is thought to impair the action of tamoxifen. Many altered pathways may account for tamoxifen resistance which may be best studied by multigene approaches.

New and improved glucocorticoid receptor ligands

The therapeutic and prophylactic use of glucocorticoids is widespread due to their powerful anti-inflammatory, antiproliferative and immunomodulatory activity. However, long-term use of these drugs can result in severe dose-limiting side effects. One of the most critical and debilitating side effects is osteoporosis, which leads to increased risk of fractures. Glucocorticoids damage bone through several different mechanisms. The search for novel glucocorticoids that have reduced side effects in bone and other tissues is being driven by the identification of new mechanisms of action of the glucocorticoid receptor. This may facilitate the detection of new, safer therapies with efficacies equivalent to currently prescribed steroids.

BARX2 and estrogen receptor-alpha (ESR1) coordinately regulate the production of alternatively spliced ESR1 isoforms and control breast cancer cell growth and invasion

The estrogen receptor-alpha gene (ESR1) was previously identified as a direct target of the homeobox transcription factor BARX2 in MCF7 cells. Here, we show that BARX2 and ESR1 proteins bind to different ESR1 gene promoters and regulate the expression of alternatively spliced mRNAs that encode 66 and 46 kDa ESR1 protein isoforms. BARX2 increases the expression of both ESR1 isoforms; however, it has a greater effect on the 46 kDa isoform, leading to an increased ratio between the 46 and 66 kDa proteins. BARX2 also influences estrogen-dependent processes such as anchorage-independent growth and modulates the expression of the estrogen-responsive genes SOX5, RBM15, Dynein and Mortalin. In addition, BARX2 expression promotes cellular invasion and increases the expression of active matrix metalloproteinase-9 (MMP9). BARX2 also increases the expression of the tissue inhibitor of metalloproteinase (TIMP) genes, TIMP1 and TIMP3, in cooperation with estrogen signaling. Overall, these data indicate that BARX2 and ESR1 may coordinately regulate cell growth, survival and invasion pathways that are critical to breast cancer progression.

Switching of NR5A proteins associated with the inhibin alpha-subunit gene promoter after activation of the gene in granulosa cells

The inhibin alpha-subunit gene is transcriptionally activated by FSH in ovarian granulosa cells during follicular growth. We have investigated the roles of the NR5A family nuclear receptors steroidogenic factor 1 (SF-1) and liver receptor homolog 1 (LRH-1) in transcriptional activation of the inhibin alpha-subunit gene. Transfection assays using an inhibin alpha-subunit promoter reporter in GRMO2 granulosa cells show that LRH-1 and SF-1 act similarly to increase promoter activity, and that the activity of both transcription factors is augmented by the coactivators cAMP response element-binding protein-binding protein and steroid receptor coactivator 1. However, chromatin immunoprecipitation experiments illustrate differential dynamic association of LRH-1 and SF-1 with the alpha-subunit inhibin promoter in both primary cells and the GRMO2 granulosa cell line such that hormonal stimulation of transcription results in an apparent replacement of SF-1 with LRH-1. Transcriptional stimulation of the inhibin alpha-subunit gene is dependent on MAPK kinase activity, as is the dynamic association/disassociation of SF-1 and LRH-1 with the promoter. Inhibition of the phosphatidylinositol 3-kinase signaling pathway influences promoter occupancy and transcriptional activation by SF-1 but not LRH-1, suggesting a possible mechanistic basis for the distinct functions of these NR5A proteins in inhibin alpha-subunit gene regulation.

Nuclear receptors and their coregulators in kidney

Nuclear receptors are transcription factors that are essential in embryonic development, maintenance of differentiated cellular phenotypes, metabolism, and apoptosis. Dysfunction of nuclear receptor signaling leads to a wide spectra of proliferative, reproductive, and metabolic diseases, including cancers, infertility, obesity, and diabetes. In addition, many proteins have been identified as coregulators which can be recruited by DNA-binding nuclear receptors to affect transcriptional regulation. The cellular level of coregulators is crucial for nuclear receptor-mediated transcription and many coregulators have been shown to be targets for diverse intracellular signaling pathways and posttranslational modifications. This review provides a general overview of the roles and mechanism of action of nuclear receptors and their coregulators. Since progression of renal diseases is almost always associated with inflammatory processes and/or involve metabolic disorders of lipid and glucose, cell proliferation, hypertrophy, apoptosis, and hypertension, the importance of nuclear receptors and their coregulators in these contexts will be addressed.

Interaction of nuclear receptors with the Wnt/beta-catenin/Tcf signaling axis: Wnt you like to know?

The cross-regulation of Wnt/beta-catenin/Tcf ligands, kinases, and transcription factors with members of the nuclear receptor (NR) family has emerged as a clinically and developmentally important area of endocrine cell biology. Interactions between these signaling pathways result in a diverse array of cellular effects including altered cellular adhesion, tissue morphogenesis, and oncogenesis. Analyses of NR interactions with canonical Wnt signaling reveal two broad themes: Wnt/beta-catenin modulation of NRs (theme I), and ligand-dependent NR inhibition of the Wnt/beta-catenin/Tcf cascade (theme II). Beta-catenin, a promiscuous Wnt signaling member, has been studied intensively in relation to the androgen receptor (AR). Beta-catenin acts as a coactivator of AR transcription and is also involved in co-trafficking, increasing cell proliferation, and prostate pathogenesis. T cell factor, a transcriptional mediator of beta-catenin and AR, engages in a dynamic reciprocity of nuclear beta-catenin, p300/CREB binding protein, and transcriptional initiation factor 2/GC receptor-interaction protein, thereby facilitating hormone-dependent coactivation and transrepression. Beta-catenin responds in an equally dynamic manner with other NRs, including the retinoic acid (RA) receptor (RAR), vitamin D receptor (VDR), glucocorticoid receptor (GR), progesterone receptor, thyroid receptor (TR), estrogen receptor (ER), and peroxisome proliferator-activated receptor (PPAR). The NR ligands, vitamin D(3), trans/cis RA, glucocorticoids, and thiazolidines, induce dramatic changes in the physiology of cells harboring high Wnt/beta-catenin/Tcf activity. Wnt signaling regulates, directly or indirectly, developmental processes such as ductal branching and adipogenesis, two processes dependent on NR function. Beta-catenin has been intensively studied in colorectal cancer; however, it is now evident that beta-catenin may be important in cancers of the breast, prostate, and thyroid. This review will focus on the cross-regulation of AR and Wnt/beta-catenin/Tcf but will also consider the dynamic manner in which RAR/RXR, GR, TR, VDR, ER, and PPAR modulate canonical Wnt signaling. Although many commonalities exist by which NRs interact with the Wnt/beta-catenin signaling pathway, striking cell line and tissue-specific differences require deciphering and application to endocrine pathology.

Activation of extracellular signal-regulated kinase signaling by epidermal growth factor mediates c-Jun activation and p300 recruitment in keratin 16 gene expression

In studies of gene regulation of keratin 16, we reported previously that simian virus 40 promoter factor 1 shows a functional cooperation with c-Jun and coactivators p300/CBP in driving the transcriptional regulation of epidermal growth factor (EGF)-induced keratin 16 gene expression. In the present study, we found that the stimulated expression of keratin 16 by EGF was mediated mainly through the mitogen-activated protein kinase kinase-extracellular signal-regulated kinases 1 and 2 (ERK1/2) signaling pathway. Ser63 and Ser73 on the c-Jun NH(2)-terminal transactivation domain could be phosphorylated in cells treated with EGF; nevertheless, we found that the c-Jun COOH terminus played a pivotal role in EGF-induced expression of keratin 16. The activation of keratin 16 by EGF treatment could not be enhanced by the overexpression of myc-c-JunK3R, in which three putative acetylation lysine residues on the c-Jun COOH terminus were all mutated into arginines, suggesting that c-Jun acetylation on the COOH terminus might partially play a functional role in this system. In addition, by using a chromatin immunoprecipitation assay and a DNA affinity precipitation assay, EGF treatment up-regulated the p300 recruitment through ERK signaling to the promoter region in regulating keratin 16 transcriptional activity. Furthermore, the enhancement of acetyl-histone H3 to the keratin 16 chromatin promoter induced by EGF was also mediated via ERK activation. In conclusion, these results strongly suggest that both c-Jun induction and p300 recruitment to gene promoter, mediated through ERK activation, played an essential role in regulating keratin 16 gene expression by EGF. p300 mediated and regulated EGF-induced keratin 16 gene expression, at least in part, through multiple mechanisms, including a selective acetylation of c-Jun and histone H3.

Calmodulin-Dependent Kinase IV Stimulates Vitamin D Receptor-Mediated Transcription

1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] promotes intestinal absorption of calcium primarily by binding to the vitamin D receptor (VDR) and regulating gene expression. 1,25-(OH)2D3 also exerts rapid actions at the cell membrane that include increasing intracellular calcium levels and activating protein kinase cascades. To explore potential cross talk between calcium signaling elicited by the nongenomic actions of 1,25-(OH)2D3 and the genomic pathway mediated by VDR, we examined the effects of activated Ca2+/calmodulin-dependent kinases (CaMKs) on 1,25-(OH)2D3/VDR-mediated transcription. Expression of a constitutively active form of CaMKIV dramatically stimulated 1,25-(OH)2D3-activated reporter gene expression in COS-7, HeLa, and ROS17/2.8 cell lines. Metabolic labeling studies indicated that CaMKIV increased VDR phosphorylation levels. In addition, CaMKIV increased the independent transcription activity of the VDR coactivator SRC (steroid receptor coactivator) 1, and promoted ligand-dependent interaction between VDR and SRC coactivator proteins in mammalian two-hybrid studies. The functional consequences of this multifaceted mechanism of CaMKIV action were revealed by reporter gene studies, which showed that CaMKIV and select SRC coactivators synergistically enhanced VDR-mediated transcription. These studies support a model in which CaMKIV signaling stimulates VDR-mediated transcription by increasing phosphorylation levels of VDR and enhancing autonomous SRC activity, resulting in higher 1,25-(OH)2D3-dependent interaction between VDR and SRC coactivators.

The 1,25(OH)2D3-Regulated Transcription Factor MN1 Stimulates Vitamin D Receptor-Mediated Transcription and Inhibits Osteoblastic Cell Proliferation

The vitamin D endocrine system is essential for maintaining mineral ion homeostasis and preserving bone density. The most bioactive form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] elicits its effects by binding to the vitamin D receptor (VDR) and regulating the transcription of target genes. In osteoblasts, the bone-forming cells of the skeleton, 1,25-(OH)2D3 regulates cell proliferation, differentiation, and mineralization of the extracellular matrix. Despite these well-characterized biological functions, relatively few 1,25-(OH)2D3 target genes have been described in osteoblasts. In this study, we characterize the regulation and function of MN1, a novel 1,25-(OH)2D3-induced gene in osteoblastic cells. MN1 is a nuclear protein first identified as a gene disrupted in some meningiomas and leukemias. Our studies demonstrate that MN1 preferentially stimulates VDR-mediated transcription through its ligand-binding domain and synergizes with the steroid receptor coactivator family of coactivators. Furthermore, forced expression of MN1 in osteoblastic cells results in a profound decrease in cell proliferation by slowing S-phase entry, suggesting that MN1 is an antiproliferative factor that may mediate 1,25-(OH)2D3-dependent inhibition of cell growth. Collectively, these data indicate that MN1 is a 1,25-(OH)2D3-induced VDR coactivator that also may have critical roles in modulating osteoblast proliferation.

The breast cancer susceptibility gene BRCA1 regulates progesterone receptor signaling in mammary epithelial cells

The progesterone receptor (PR) plays roles in normal mammary development and breast cancer formation, where it may exert both stimulatory and inhibitory actions. Previously, the breast cancer susceptibility gene product BRCA1 was found to interact with and inhibit the transcriptional activity of estrogen receptor-alpha. In this study, we found that exogenous wild-type BRCA1 inhibited the activity of the PR in transient transfection assays utilizing a mouse mammary tumor virus-Luc reporter. Wild-type BRCA1 inhibited the activity of endogenous PR in human breast cancer cells (T47D and MCF-7) and inhibited the activity of exogenous PR-A, PR-B, and [PR-A plus PR-B] isoforms. On the other hand, knockdown of endogenous BRCA1 using small interfering RNA enhanced the progesterone-stimulated activity of the PR by about 4-fold. We documented an in vivo association of the endogenous BRCA1 with PR isoforms A and B and a direct in vitro interaction between BRCA1 and PR, which was partially mapped. Whereas down-regulation of the coactivator p300 contributes to the BRCA1-mediated repression of estrogen receptor-alpha, this mechanism does not contribute to inhibition of PR activity, because exogenous p300 did not rescue the BRCA1 repression of PR activity. The BRCA1-PR interaction has functional consequences. Thus, we showed that BRCA1 inhibits the expression of various endogenous progesterone-responsive genes and inhibits progesterone-stimulated proliferation of T47D cells. Finally, exogenous progesterone caused an exaggerated proliferative response in the mammary glands of mice harboring a mammary-targeted conditional deletion of the full-length isoform of Brca1. These findings suggest that BRCA1 regulates the activity of progesterone, a major hormone of pregnancy that may also participate in mammary carcinogenesis.

Recruitment of histone deacetylase 4 to the N-terminal region of estrogen receptor alpha

Transcriptional activation of estrogen receptor alpha (ERalpha) is regulated by the ligand-dependent activation function 2 and the constitutively active N-terminal activation function 1. To identify ERalpha N-terminal-specific coregulators, we screened a breast cDNA library by T7 phage display and isolated histone deacetylase 4 (HDAC4). HDAC4 interacts with the ERalpha N terminus both in vitro and in vivo. Presence of the ERalpha DNA binding domain and hinge region reduce HDAC4 recruitment whereas full-length ERalpha enhances recruitment. HDAC4 interaction is selective for the ERalpha and not ERbeta N terminus and occurs in the nucleus. We demonstrate in vivo that HDAC4 is recruited by the N terminus to the promoter of an endogenous estrogen responsive gene. HDAC4 suppresses transcriptional activation of ERalpha by estrogen and selective ER modulators (SERMs) such as tamoxifen in a cell type-dependent manner. Consistently, silencing of HDAC4 promotes the agonist effect of SERMs (tamoxifen and raloxifene) in a cell type-specific manner. These findings indicate a role for HDAC4 in regulating ERalpha activity as a novel N-terminal coregulator and uncover a mechanism by which certain cell types regulate SERM behavior.

Vitamin D

The vitamin D endocrine system plays an essential role in calcium homeostasis and bone metabolism, but research during the past two decades has revealed a diverse range of biological actions that include induction of cell differentiation, inhibition of cell growth, immunomodulation, and control of other hormonal systems. Vitamin D itself is a prohormone that is metabolically converted to the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. This vitamin D hormone activates its cellular receptor (vitamin D receptor or VDR), which alters the transcription rates of target genes responsible for the biological responses. This review focuses on several recent developments that extend our understanding of the complexities of vitamin D metabolism and actions: the final step in the activation of vitamin D, conversion of 25-hydroxyvitamin D to 1,25(OH)(2)D in renal proximal tubules, is now known to involve facilitated uptake and intracellular delivery of the precursor to 1alpha-hydroxylase. Emerging evidence using mice lacking the VDR and/or 1alpha-hydroxylase indicates both 1,25(OH)(2)D(3)-dependent and -independent actions of the VDR as well as VDR-dependent and -independent actions of 1,25(OH)(2)D(3). Thus the vitamin D system may involve more than a single receptor and ligand. The presence of 1alpha-hydroxylase in many target cells indicates autocrine/paracrine functions for 1,25(OH)(2)D(3) in the control of cell proliferation and differentiation. This local production of 1,25(OH)(2)D(3) is dependent on circulating precursor levels, providing a potential explanation for the association of vitamin D deficiency with various cancers and autoimmune diseases.

Mechanisms underlying "functional" progesterone withdrawal at parturition

Progesterone is a major factor maintaining uterine quiescence throughout pregnancy. In most species, peripheral progesterone levels decline before initiation of labor, and treatments that inhibit progesterone synthesis or action cause termination of pregnancy and/or premature deliveries. These findings suggest that progesterone withdrawal is required for activation of myometrial contractions. However, in humans, circulating progesterone levels remain elevated until birth, which leads to the notion that a "functional" progesterone withdrawal occurs before parturition. The apparent loss of progesterone sensitivity at term could be a consequence of several different mechanisms including: (1) the catabolism of progesterone in the uterus into inactive compounds; (2) alterations in progesterone receptor (PR) isoform ratios; (3) changes in cofactor protein levels affecting PR transactivation; and (4) inflammation-induced trans-repression of PR by nuclear factor kappaB. All of these mechanisms are potentially capable of decreasing uterine progesterone responsiveness at term, thus enabling the expression of pathways that originally were blocked by progesterone in early pregnancy. However, the specific uterine genes whose transcription is directly controlled by PR, and thus affected by "functional" progesterone withdrawal, remain to be identified.

The coactivator p300 directly acetylates the forkhead transcription factor Foxo1 and stimulates Foxo1-induced transcription

The FOXO (Forkhead box class O) subgroup of forkhead transcription factors controls the expression of many genes involved in fundamental cellular processes. Until recently, studies conducted on posttranslational modifications of Forkhead proteins were restricted to their phosphorylation. In this report, we show that the coactivator p300 directly acetylates lysines in the carboxyl-terminal region of Foxo1 in vivo and in vitro, and potently stimulates Foxo1-induced transcription of IGF-binding protein-1 in transient transfection experiments. The intrinsic acetyltransferase activity of p300 is required for both activities. Our results suggest that acetylation of Foxo1 by p300 is responsible, at least in part, for its increased transactivation potency, although acetylation of histones cannot be excluded. Insulin, the major negative regulator of Foxo1-stimulated transcription, potently enhances p300 acetylation of Foxo1. Three consensus protein kinase B/Akt phosphorylation sites whose phosphorylation is stimulated by insulin are required for insulin-induced acetylation of Foxo1. In contrast to its importance in regulating the transcriptional activity of Foxo1 in the absence of insulin, acetylation plays only a minor role compared with phosphorylation in insulin inhibition of Foxo1 transcriptional activity.

Transcriptional regulation by steroid receptor coactivator phosphorylation

The basic mechanisms underlying ligand-dependent transcriptional activation by nuclear receptors (NRs) require the sequential recruitment of various coactivators. Increasing numbers of coactivators have been identified in recent years, and both biochemical and genetic studies demonstrate that these coactivators are differentially used by transcription factors, including NRs, in a cell/tissue type- and promoter-specific manner. However, the molecular basis underlying this specificity remains largely unknown. Recently, NRs and coregulators were shown to be targets of posttranslational modifications activated by diverse cellular signaling pathways. It is argued that posttranslational modifications of these proteins provide the basis for a combinatorial code required for specific gene activation by NRs and coactivators, and that this code also enables coactivators to efficiently stimulate the activity of other classes of transcription factors. In this review, we will focus on coactivators and discuss the recent progress in understanding the role of phosphorylation of the steroid receptor coactivator family and the potential ramifications of this posttranslational modification for regulation of gene expression.

The search for safer glucocorticoid receptor ligands

Steroidal glucocorticoids are commonly used due to their powerful antiinflammatory activity. However, despite their excellent efficacy, severe side effects frequently limit the use of these drugs. The search for novel glucocorticoids with reduced side effects has been intensified by the discovery of new molecular details regarding the function of the glucocorticoid receptor. These new insights may pave the way for novel, safer therapies that retain the efficacy of currently prescribed steroids.

Regulation of endogenous gene expression in human non-small cell lung cancer cells by estrogen receptor ligands

Estrogen receptor (ER) agonists and antagonists elicit distinct responses in non-small cell lung cancer (NSCLC) cells. To determine how such responses are generated, the expression of ERalpha, ERbeta, and ER coregulators in human lung fibroblasts and human NSCLC cell lines was evaluated by immunoblot. Ligand-dependent estrogenic responses in NSCLC cells are probably generated via ERbeta and the p160 coactivator GRIP1/TIF2, because expression of these proteins was detected, but not full-length ERalpha or the p160 coactivator SRC-1. ERbeta and GRIP1/TIF2 are shown to interact in vitro in a ligand-dependent manner and thus may form functional transcription complexes in NSCLC cells. Furthermore, the capacity of ER ligands to regulate gene expression in NSCLC cells was explored using gene miniarrays. Expression profiles were examined after treatment with ER agonist 17-beta-estradiol (E2), the pure ER antagonist ICI 182,780 (fulvestrant, Faslodex), or epidermal growth factor, which served as a positive control for an alternative growth stimulus. E-cadherin and inhibitor of differentiation 2 were differentially regulated by E2 versus ICI 182,780 in 201T and 273T NSCLC cell lines. Epidermal growth factor also stimulated proliferation of these cells but had no effect on expression of E-cadherin and inhibitor of differentiation 2, suggesting they are specific targets of ER signaling. These data show that NSCLC cells respond to estrogens/antiestrogens by altering endogenous gene expression and support a model in which ICI 182,780 reduces proliferation of NSCLC cells via its ability to disrupt ER signaling. ICI 182,780 may therefore have therapeutic benefit in NSCLC.

Endogenous coactivator ARA70 interacts with estrogen receptor alpha (ERalpha) and modulates the functional ERalpha/androgen receptor interplay in MCF-7 cells

Overexpression of androgen receptor (AR) decreases estrogen receptor alpha (ERalpha) transactivation, which plays a basic role in hormone-dependent breast cancer. This transcriptional interference can be due to shared coactivators. Here we demonstrated that in MCF-7 cells ARA70, an AR-specific coactivator, interacted with endogenous ERalpha, increasing its transcriptional activity, and it was recruited to the pS2 gene promoter. Moreover, a dominant negative ARA70 down-regulated ERalpha transcriptional activity as well as pS2 mRNA. ARA70 overexpression reversed the AR down-regulatory effect on ERalpha signaling. However, in the presence of a progressive increase of transfected AR, ARA70 switched into enhancing the inhibitory effect of AR on ERalpha signaling. These opposite effects of ARA70 were further evidenced by coimmunoprecipitation assay in MCF-7wt, MCF-7-overexpressing AR, and HeLa cells, exogenously expressing an excess of ERalpha with respect to AR or an excess of AR with respect to ERalpha. Thus, ARA70 is a coactivator for ERalpha and may represent a functional link between ERalpha/AR modulating their cross-talk in models of estrogen signaling in MCF-7 and HeLa cells.

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.

Differential role of progesterone receptor isoforms in the transcriptional regulation of human gonadotropin-releasing hormone I (GnRH I) receptor, GnRH I, and GnRH II

Hypothalamic GnRH is a decapeptide that plays a pivotal role in mammalian reproduction by stimulating the synthesis and secretion of gonadotropins via binding to the GnRH receptor on the pituitary gonadotropins. It is hypothesized that sex steroids may regulate GnRH I (a classical form of GnRH), GnRH II (a second form of GnRH), and GnRH I receptor (GnRHRI) at the transcriptional level in target tissues. Thus, in the present study a role for progesterone (P4) in the regulation of GnRH I, GnRH II, and GnRHRI was investigated using a human neuronal medulloblastoma cell line (TE671) as an in vitro model. The cells were transfected with human GnRHRI promoter-luciferase constructs, and promoter activities were analyzed after P4 treatment by luciferase and beta-galactosidase assay. The mRNA levels of GnRH I and GnRH II were analyzed by RT-PCR. Treatment of TE671 cells with P4 resulted in a decrease in GnRHRI promoter activity compared with the control level in a dose- and time-dependent manner. Cotreatment of these cells with RU486, an antagonist of P4, reversed P4-induced inhibition of GnRHRI promoter activity, suggesting that the P4 effect is mediated by P4 receptor (PR). In the cells transfected with a full-length of PR A- or PR B-expressing vector, overexpression of PR A increased the sensitivity toward P4 in an inhibition of GnRHRI promoter, whereas PR B increased transcriptional activity of GnRHRI promoter in the presence of P4. However, PR B itself did not act as a transcriptional activator of GnRHRI promoter. Because TE671 cells have been recently demonstrated to express and synthesize two forms of GnRHs, we also investigated the regulation of GnRH mRNAs by P4. In the present study, P4 increased GnRH I mRNA levels in a time- and dose-dependent manner. This stimulatory effect of P4 in the regulation of GnRH I mRNAs was significantly attenuated by RU486, whereas no significant difference in the expression level of GnRH II was observed with P4 or RU496. Interestingly, although the expression level of PR B was low compared with that of PR A, P4 action on the GnRH I gene was mediated by PR B. In conclusion, these results indicate that P4 is a potent regulator of GnRHRI at the transcriptional level as well as GnRH I mRNA. This distinct effect of P4 on the GnRH system may be derived from different pathways through PR A or PR B.

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.

Identification of a novel functional androgen response element withinhPar1promoter: implications to prostate cancer progression

Human protease-activated receptor-1 (hPar1) plays a role in malignant and physiological invasion processes. We have identified a functional androgen response element (ARE) located in the hPar1 promoter upstream of the transcription start site at -1791 to -1777. Dihydrotestosterone treatment of the prostate cancer cell line LNCaP increased endogenous hPar1 mRNA levels, consistent with the threefold increase in promoter activity of hPar1-luciferase reporter construct. Specific binding of the hPar1-derived ARE to LNCaP nuclear extracts was demonstrated by electrophoretic mobility shift assay. This binding was abrogated by antiandrogen receptor (anti-AR) antibodies or excess cold oligonucleotide but not by a mutated oligonucleotide. Moreover, using chromatin immunoprecipitation assays, we confirm the in vivo interaction between the AR and ARE domain of the hPar1 promoter. In parallel, we show that hormone ablation therapy markedly reduces the otherwise high hPar1 expression levels in prostate cancer biopsy specimens. We suggest that the hPar1 gene is regulated transcriptionally by androgens, representing one of several target genes effectively reduced during hormone ablation therapy. A major limitation of hormonal deprivation is that it causes only a temporary remission, and the cancer eventually reappears in a more malignant, androgen-independent form. hPar1 is also overexpressed in CL1 cells, an aggressively metastasizing, hormone-independent subclone of LNCaP, and in PC3 prostate adenocarcinoma lacking AR in a mechanism yet to be fully elucidated. These data may imply that hPar1 expression correlates with prostate cancer progression in androgen-dependent and -independent phases and therefore, provides an instrumental, therapeutic target for treatment in prostate cancer.

A Novel Estrogen Receptor α-Associated Protein Alters Receptor-Deoxyribonucleic Acid Interactions and Represses Receptor-Mediated Transcription

Estrogen receptor α (ERα) serves as a ligand-activated transcription factor, turning on transcription of estrogen-responsive genes in target cells. Numerous regulatory proteins interact with the receptor to influence ERα-mediated transactivation. In this study, we have identified pp32, which interacts with the DNA binding domain of ERα when the receptor is free, but not when it is bound to an estrogen response element. Coimmunoprecipitation experiments demonstrate that endogenously expressed pp32 and ERα from MCF-7 breast cancer cells interact. Although pp32 substantially enhances the association of the receptor with estrogen response element-containing DNA, overexpression of pp32 in MCF-7 cells decreases transcription of an estrogen-responsive reporter plasmid. pp32 Represses p300-mediated acetylation of ERα and histones in vitro and inhibits acetylation of ERα in vivo. pp32 Also binds to other nuclear receptors and inhibits thyroid hormone receptor β-mediated transcription. Taken together, our studies provide evidence that pp32 plays a role in regulating transcription of estrogen-responsive genes by modulating acetylation of histones and ERα and also influences transcription of other hormone-responsive genes as well.

Triphasic pattern in the ex vivo response of human proliferative phase endometrium to oestrogens

The aim of this study was to evaluate the ex vivo oestrogen responsiveness of human proliferative phase endometrium using short-term explant cultures. The effects of oestrogen (17beta-E2) on proliferation and the expression of oestrogen-responsive genes known to be involved in regulating endometrial function were evaluated. Three distinct response patterns could be distinguished: (1) the menstrual (M) phase pattern (cycle days 2-5), which is characterised by a complete lack in the proliferative response to 17beta-E2, while an increased expression of AR (2.6-fold, P<0.01), PR (2.7-fold, P<0.01) and COX-2 (3.5-fold, P<0.01) at the mRNA level was observed and a similar upregulation was also found for AR, PR and COX-2 at the protein level; (2) the early proliferative (EP) phase pattern (cycle days 6-10) with 17beta-E2 enhanced proliferation in the stroma (1.7-fold, P<0.05), whereas the expression of AR, PR and COX-2 were not affected at the mRNA and protein levels and ER-alpha mRNA and protein levels were significantly reduced by 17beta-E2; (3) the late proliferative (LP) phase pattern (cycle days 11-14), which is characterised by a moderate stimulation of proliferation (1.4-fold, P<0.05) and PR mRNA expression (1.7-fold, P<0.01) by 17beta-E2. In conclusion, three distinct response patterns to 17beta-E2 could be identified with respect to proliferation and the expression of known oestrogen-responsive genes in human proliferative phase endometrium explant cultures.

Tumor necrosis factor-alpha suppresses the expression of steroid receptor coactivator-1 and -2: a possible mechanism contributing to changes in steroid hormone responsiveness

Inflammation is associated in some tissues with diminished responsiveness to steroid hormone action. We hypothesized that proinflammatory cytokines alter steroid hormone sensitivity, in part, by reducing levels of key nuclear receptor coactivators. Treatment of cultured human uterine smooth muscle cells (UtSMC) with TNF-alpha significantly reduced mRNA for the coactivators, SRC-1 (42%, P<0.01) and 2 (47%, P<0.03), and diminished the respective protein levels, but did not significantly alter the mRNAs encoding SRC-3, CBP and the corepressors, NCoR and SMRT; or progesterone receptor protein levels. To assess TNF-alpha effects on steroid hormone-mediated transcriptional activity, UtSMC were transfected with progesterone receptor B (PR-B) and a model PRE2-luciferase reporter construct. Transfected UtSMC were treated with progesterone alone or in the presence of TNF-alpha, and assayed for luciferase activity. TNF-alpha (10 ng/ml) diminished progesterone-stimulated PR-B-mediated transactivation by approximately 60% (P<0.02). The TNF-alpha-dependent decrease in PRE-luciferase activity was fully prevented by cotransfection with SRC-2, and partially prevented with exogenous SRC-1. In conclusion, TNF-alpha impairs progesterone-stimulated PR-B-mediated transactivation, and these effects appear to be due, in part, to reduced expression of SRC-1 and -2, which is a novel mechanism by which inflammation can functionally block steroid hormone action.

Tamoxifen resistance by a conformational arrest of the estrogen receptor alpha after PKA activation in breast cancer

Using a novel approach that detects changes in the conformation of ERalpha, we studied the efficacy of anti-estrogens to inactivate ERalpha under different experimental conditions. We show that phosphorylation of serine-305 in the hinge region of ERalpha by protein kinase A (PKA) induced resistance to tamoxifen. Tamoxifen bound but then failed to induce the inactive conformation, invoking ERalpha-dependent transactivation instead. PKA activity thus induces a switch from antagonistic to agonistic effects of tamoxifen on ERalpha. In clinical samples, we found that downregulation of a negative regulator of PKA, PKA-RIalpha, was associated with tamoxifen resistance prior to treatment. Activation of PKA by downregulation of PKA-RIalpha converts tamoxifen from an ERalpha inhibitor into a growth stimulator, without any effect on ICI 182780 (Fulvestrant).

Functional domain and motif analyses of androgen receptor coregulator ARA70 and its differential expression in prostate cancer

Androgen receptor (AR)-associated coregulator 70 (ARA70) was the first identified AR coregulator. However, its molecular mechanism and biological relevance to prostate cancer remain unclear. Here we show that ARA70 interacts with and promotes AR activity via the consensus FXXLF motif within the ARA70-N2 domain (amino acids 176-401). However, it does not promote AR activity via the classic LXXLL motif located at amino acids 92-96, although this classic LXXLL motif is important for ARA70 to interact with other receptors, such as PPARgamma. The molecular mechanisms by which ARA70 enhances AR transactivation involve the increase of AR expression, protein stability, and nuclear translocation. Furthermore, ARA70 protein is more frequently detected in prostate cancer specimens (91.74%) than in benign tissues (64.64%, p < 0.0001). ARA70 expression is also increased in high-grade prostate cancer tissues as well as the hormone-refractory LNCaP xenografts and prostate cancer cell lines. Because ARA70 can promote the antiandrogen hydroxyflutamide (HF)-enhanced AR transactivation, the increased ARA70 expression in hormone-refractory prostate tumors may confer the development of HF withdrawal syndrome, commonly diagnosed in patients with the later stages of prostate cancer. Because ARA70-N2 containing the AR-interacting FXXLF motif without coactivation function can suppress HF-enhanced AR transactivation in the hormone-refractory LNCaP cells, using the ARA70-N2 inhibitory peptide at the hormone refractory stage to battle the HF withdrawal syndrome may become an alternative strategy to treat prostate cancer.

Controlling transgene expression to study Xenopus laevis metamorphosis

Sperm-mediated transgenesis of Xenopus laevis is the first application of genetic methodology to an amphibian. However, some transgenes are lethal when they are expressed constitutively. To study the influence of these genes on amphibian metamorphosis and to generate F1 progeny from mature transgenic adults, these transgenes must be placed under the control of an inducible system so that they can be activated at specific times in development. We show that two well known binary inducible gene expression systems supplement transgenesis for the study of X. laevis metamorphosis, one system controlled by the progesterone analogue RU-486 and the other controlled by the tetracycline derivative doxycycline. By inducing a dominant negative form of the thyroid hormone receptor under the control of doxycycline specifically in the nervous system we have delimited the developmental periods within which thyroid hormone controls innervation of the developing limb from the spinal cord.

Role of p160 coactivator complex in the activation of liver X receptor

OBJECTIVE

Liver X receptor (LXR) is a member of a nuclear receptor family regulating the expression of several key proteins involved in lipid metabolism and inflammation. In contrast to several other nuclear receptors, very little is known about the coactivators needed for the agonist-mediated transactivation by LXR. In this study, we have investigated the role of p160 coactivator complex in the regulation of ATP-binding transporter A1 (ABCA1), a clinically important gene transcriptionally upregulated by LXR/RXR (retinoid X receptor) heterodimer.

METHODS AND RESULTS

Overexpression of LXRalpha, SRC-1, and p300, either alone or in combination, increased the luciferase activity driven by the wild-type ABCA1 promoter. The same coactivators bound to the ABCA1 promoter on oxysterol induction in chromatin immunoprecipitation assays. To the contrary, CARM-1 and P/CAF had no effect on ABCA1 transactivation, nor do they bind the promoter. When the DR-4 element was mutated from the ABCA1 promoter, only p300 was able to activate ABCA1 transcription in a ligand-independent manner.

CONCLUSIONS

The p160 coactivator complex members SRC-1 and p300, but not CARM-1 and P/CAF, coactivate LXR-mediated transcription of ABCA1 gene. In addition, p300 activates ABCA1 transcription independently of DR-4 element and LXR/RXR.

Interaction of estrogen receptor alpha with 3-methyladenine DNA glycosylase modulates transcription and DNA repair

Estrogen receptor alpha (ERalpha) interacts with basal transcription factors, coregulatory proteins, and chromatin modifiers to initiate transcription of the target genes. We have identified a novel interaction between ERalpha and the DNA repair protein 3-methyladenine DNA glycosylase (MPG) thereby providing a functional link between gene expression and DNA repair. Interestingly, the ERalpha-MPG interaction was enhanced by the presence of estrogen response element (ERE)-containing DNA. In vitro pull-down assays indicated that the interaction of ERalpha with MPG was direct and occurred through the DNA- and ligand-binding domains and the hinge region of the receptor. More importantly, endogenously expressed ERalpha and MPG from MCF-7 cells coimmunoprecipitated with ERalpha- and MPG-specific antibodies. The ERalpha-MPG interaction had functional consequences on the activities of both proteins. ERalpha increased MPG acetylation, stabilized the binding of MPG with hypoxanthine-containing oligos, and enhanced MPG-catalyzed removal of hypoxanthine from DNA. In turn, MPG dramatically stabilized the interaction of ERalpha with ERE-containing oligos, decreased p300-mediated acetylation of the receptor, and reduced transcription of simple and complex ERE-containing reporter plasmids in a dose-dependent manner. Our studies suggest that recruitment of MPG to ERE-containing genes influences transcription and plays a role in maintaining integrity of the genome by recruiting DNA repair proteins to actively transcribing DNA.

Antiestrogen resistance in breast cancer and the role of estrogen receptor signaling

Antiestrogens include agents such as tamoxifen, toremifene, raloxifene, and fulvestrant. Currently, tamoxifen is the only drug approved for use in breast cancer chemoprevention, and it remains the treatment of choice for most women with hormone receptor positive, invasive breast carcinoma. While antiestrogens have been available since the early 1970s, we still do not fully understand their mechanisms of action and resistance. Essentially, two forms of antiestrogen resistance occur: de novo resistance and acquired resistance. Absence of estrogen receptor (ER) expression is the most common de novo resistance mechanism, whereas a complete loss of ER expression is not common in acquired resistance. Antiestrogen unresponsiveness appears to be the major acquired resistance phenotype, with a switch to an antiestrogen-stimulated growth being a minor phenotype. Since antiestrogens compete with estrogens for binding to ER, clinical response to antiestrogens may be affected by exogenous estrogenic exposures. Such exposures include estrogenic hormone replacement therapies and dietary and environmental exposures that directly or indirectly increase a tumor's estrogenic environment. Whether antiestrogen resistance can be conferred by a switch from predominantly ERalpha to ERbeta expression remains unanswered, but predicting response to antiestrogen therapy requires only measurement of ERalpha expression. The role of altered receptor coactivator or corepressor expression in antiestrogen resistance also is unclear, and understanding their roles may be confounded by their ubiquitous expression and functional redundancy. We have proposed a gene network approach to exploring the mechanistic aspects of antiestrogen resistance. Using transcriptome and proteome analyses, we have begun to identify candidate genes that comprise one component of a larger, putative gene network. These candidate genes include NFkappaB, interferon regulatory factor-1, nucleophosmin, and the X-box binding protein-1. The network also may involve signaling through ras and MAPK, implicating crosstalk with growth factors and cytokines. Ultimately, signaling affects the expression/function of the proliferation and/or apoptotic machineries.

NCoA-1/SRC-1 is an essential coactivator of STAT5 that binds to the FDL motif in the alpha-helical region of the STAT5 transactivation domain

Signal transducer and activator of transcription 5 (STAT5) is a transcription factor that activates prolactin (PRL)-dependent gene expression in the mammary gland. For the activation of its target genes, STAT5 recruits coactivators like p300 and the CREB-binding protein (CBP). In this study we analyzed the function of p300/CBP-associated members of the p160/SRC/NCoA-family in STAT5-mediated transactivation of beta-casein expression. We found that only one of them, NCoA-1, acts as a coactivator for both STAT5a and STAT5b. The two coactivators p300/CBP and NCoA-1 cooperatively enhance STAT5a-mediated transactivation. For NCoA-1-dependent coactivation of STAT5, both the activation domain 1 and the amino-terminal bHLH/PAS domain are required. The amino-terminal region mediates the interaction with STAT5a in cells. A motif of three amino acids in an alpha-helical region of the STAT5a-transactivation domain is essential for the binding of NCoA-1 and for the transcriptional activity of STAT5a. Moreover we observed that NCoA-1 is involved in the synergistic action of the glucocorticoid receptor and STAT5a on the beta-casein promoter. These findings support a model in which STAT5, in concert with the glucocorticoid receptor, recruits a multifunctional coactivator complex to initiate the PRL-dependent transcription.

Induction of disease-associated keratin 16 gene expression by epidermal growth factor is regulated through cooperation of transcription factors Sp1 and c-Jun

Overexpression of keratin 16 has been observed in keratinocytes in those skin diseases characterized by hyperproliferation such as psoriasis. Therefore, keratin 16 is usually referred to as a disease-associated keratin. In the present study, we found that epidermal growth factor (EGF) increased the expression of keratin 16 mRNA and protein synthesis in a time-dependent manner in HaCaT cells. Reporter assays revealed that the EGF response region was in the range of -162 to -114 bp. Disruption of the Sp1 site (-127 to -122 bp) and the AP1 site (-148 to -142 bp) of the keratin 16 promoter by site-directed mutagenesis significantly inhibited keratin 16 promoter activity induced by EGF. Furthermore, keratin 16 gene expression induced by Ras activation was also regulated in the same manner as the EGF response. By using the DNA affinity precipitation assay in HaCaT and SL2 cells, Sp1 directly interacted with the Sp1 site of the promoter, and c-Jun and c-Fos precipitated with the Sp1 oligonucleotide was attributable to the interaction between the Sp1 and AP1 proteins. Moreover, cotransfection assays revealed that Sp1 acted synergistically with c-Jun to activate keratin 16. The coactivators p300/CBP could collaborate with Sp1 and c-Jun in the activation of keratin 16 promoter, and EGF-induced promoter activation was blocked by the viral oncoprotein E1A. Taken together, these results suggest that Sp1 and AP1 sites in the essential promoter region are critical for EGF response, and Sp1 showed a functional cooperation with c-Jun and coactivators p300/CBP in driving the transcriptional regulation of EGF-induced keratin 16 gene expression.