Ligand and DNA-dependent phosphorylation of human progesterone receptor in vitro

Sex steroid hormones and DNA repair regulation: Implications on cancer treatment responses

The role of sex steroid hormones (SSHs) has been shown to modulate cancer cytotoxic treatment sensitivity. Dysregulation of DNA repair associated with genomic instability, abnormal cell survival and not only promotes cancer progression but also resistance to cancer treatment. The three major SSHs, androgen, estrogen, and progesterone, have been shown to interact with several essential DNA repair components. The presence of androgens directly regulates key molecules in DNA double-strand break (DSB) repair. Estrogen can promote cell proliferation and DNA repair, allowing cancer cells to tolerate chemotherapy and radiotherapy. Information on the role of progesterone in DNA repair is limited: progesterone interaction with some DNA repair components has been identified, but the biological significance is still unknown. Here, we review the roles of how each SSH affects DNA repair regulation and modulates response to genotoxic therapies and discuss future research that can be beneficial when combining SSHs with cancer therapy. We also provide preliminary analysis from publicly available databases defining the link between progesterone/PR and DDRs & DNA repair regulation that plausibly contribute to chemotherapy response and breast cancer patient survival.

Targeted and systemic insights into the crosstalk between DNA-dependent protein kinase catalytic subunit and receptors of estrogen, progesterone and epidermal growth factor in the context of cancer

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) has emerged as a regulator of carcinogenesis. Increased expression of DNA-PKcs correlates with metastatic cancers. Here we review recently reported crosstalk of DNA-PKcs with estrogen (ER), progesterone (PR) and epidermal growth factor (EGFR) receptors. The reports show an extensive network of functional and direct interactions. Targeted studies focused on specific molecular mechanisms, and a systems biology network analysis shows unbiasedly engagement of various cellular functions. Feedforward regulation between expression and activities of DNA-PKcs and ER, DNA-PKcs-dependent phosphorylation of PR and an impact on PR-dependent transcription, and DNA-PKcs-promoted EGFR-dependent aggressiveness and metastases are examples of the results of targeted studies. Systems biology approach extracted many more genes and proteins engaged by DNA-PKcs in interaction with ER, PR, and EGFR. Examples are such regulators and predictors of breast tumorigenesis as BRCA1, TP53, and 18 genes of the MammaPrint signature. Reviewed here data suggest that the diagnostic value of DNA-PKcs in the context of ER, PR and EGFR signaling is defined by a network signature rather than by single markers. This review summarizes mechanisms of DNA-PKcs interaction with ER, PR, and EGFR, highlights tumor suppressors and oncogenes engaged by DNA-PKcs, and emphasizes the importance of diagnostic network-based signatures.

DNA-PK, Nuclear mTOR, and the Androgen Pathway in Prostate Cancer

Androgen and its receptor (AR) are major drivers of prostate cancer (PCa), a leading cause of mortality in aging men. Thus, understanding the numerous mechanisms by which AR can promote the growth and proliferation of PCa cells and enable their escape from hormone-dependent therapies, eventually leading to metastasis and death of the patient, is essential to discover alternative therapeutic approaches. Recently, two structurally related members of the phosphatidylinositol 3-kinase-like protein kinase (PIKK) family, DNA-dependent protein kinase (DNA-PK) and mammalian target of rapamycin (mTOR), were shown to have a direct role in modulating AR activity on chromatin of PCa cells. In this review, the common features of DNA-PK and mTOR and the similarities in their noncanonical roles as transcription coregulators of the AR are highlighted. An outlook on how these findings could be translated into new approaches to manage and treat PCa is provided.

Differential Regulation of Progesterone Receptor-Mediated Transcription by CDK2 and DNA-PK

Progesterone receptor (PR) function is altered by cell signaling, but the mechanisms of kinase-specific regulation are not well defined. To examine the role of cell signaling in the regulation of PR transcriptional activity, we have utilized a previously developed mammalian-based estrogen-response element promoter array cell model and automated cell imaging and analysis platform to visualize and quantify effects of specific kinases on different mechanistic steps of PR-mediated target gene activation. For these studies, we generated stable estrogen-response element array cell lines expressing inducible chimeric PR that contains a swap of the estrogen receptor-α DNA-binding domain for the DNA-binding domain of PR. We have focused on 2 kinases important for steroid receptor activity: cyclin-dependent kinase 2 and DNA-dependent protein kinase. Treatment with either a Cdk1/2 inhibitor (NU6102) or a DNA-dependent protein kinase inhibitor (NU7441) decreased hormone-mediated chromatin decondensation and transcriptional activity. Further, we observed a quantitative reduction in the hormone-mediated recruitment of select coregulator proteins with NU6102 that is not observed with NU7441. In parallel, we determined the effect of kinase inhibition on hormone-mediated induction of primary and mature transcripts of endogenous genes in T47D breast cancer cells. Treatment with NU6102 was much more effective than NU7441, in inhibiting induction of PR target genes that exhibit a rapid increase in primary transcript expression in response to hormone. Taken together, these results indicate that the 2 kinases regulate PR transcriptional activity by distinct mechanisms.

Expression and regulation of scavenger receptor class B type 1 in the rat ovary and uterus during the estrous cycle

Scavenger receptor class B type 1 (SR-B1) preferentially mediates the selective uptake of high density lipoprotein-cholesterol ester and the delivery of cholesterol for steroidogenesis. Although multiple analyses have investigated the function of SR-B1 in the liver, adrenal and ovary, its expression in rat ovary and uterus during the estrous cycle is lacking. In the present study, real-time PCR, western blot and immunohistochemistry (IHC) were used to investigate SR-B1 expression in the rat ovary and uterus during the estrous cycle. The results demonstrated that ovarian SR-B1 expression was in a stage-dependent manner, continuously increased from proestrus and kept elevated during metoestrus, while uterine SR-B1 expression decreased from proestrus to diestrus. To determine whether ovarian and uterine SR-B1 expression were affected by sex steroid hormones, immature rats were treated with 17 β-estradiol (E2), progesterone (P4), or their antagonists from postnatal days 24-26. Results showed that the levels of SR-B1 mRNA and protein were significantly up-regulated by E2 in both the ovary and uterus. IHC results showed that SR-B1 was primarily localized in the oocytes, theca internal cells (T-I) of follicles, interstitial cells (IC) as well as corpus luteum (CL), but not granulosa cells (GC) in the ovary during the estrous cycle. Uterine SR-B1 was highly expressed in the endometrial luminal epithelial cells (LEC) and glandular epithelial cells (GEC) as well as in the circular muscle (CM) cells, and weak staining in stromal cells (SC) through estrous cycle. Taken together, SR-B1 expression in the ovary and uterus across the estrous cycle demonstrate that SR-B1 may be involved in uterine function, follicular development as well as luteal function.

Expression and regulation of Enpp2 in rat uterus during the estrous cycle

Ectonucleotide pyrophosphatase/phosphodiestrase 2 (Enpp2) isolated from the supernatant of human melanoma cells is a lysophospholipase D that transforms lysophosphatidylcholine into lysophospatidic acid. Although multiple analyses have investigated the function of Enpp2 in the hypothalamus, its role in the uterus during the estrous cycle is not well understood. In the present study, rat uterine Enpp2 was analyzed by RT-PCR, Western blotting, and immunohistochemistry. Quantitative PCR analysis demonstrated that uterine Enpp2 mRNA was decreased during estrus compared to proestrus and diestrus. To determine whether uterine Enpp2 expression is affected by sex steroid hormones, immature rats were treated with 17β-estradiol (E2), progesterone, or both on postnatal days 14 to 16. Interestingly, the expression of Enpp2 mRNA and protein were down-regulated by E2 in the uterus during estrus but not during proestrus or diestrus, suggesting that Enpp2 may play a role in uterine function during estrus. Enpp2 is primarily localized in the stromal cells of the endometrium during proestrus and estrus. During diestrus, Enpp2 was highly expressed in the epithelial cells of the endometrium. Taken together, these results suggest that uterine Enpp2 may be regulated by E2 and plays a role in reproductive functions during female rat development.

Activation of the DNA-dependent protein kinase stimulates nuclear export of the androgen receptor in vitro

The androgen receptor undergoes nuclear import in response to ligand, but the mechanism by which it undergoes nuclear export is poorly understood. We developed a permeabilized cell assay to characterize nuclear export of the androgen receptor in LNCaP prostate cancer cells. We found that nuclear export of endogenous androgen receptor can be stimulated by short double-stranded DNA oligonucleotides. This androgen receptor export pathway is dependent on ATP hydrolysis and is enhanced by phosphatase inhibition with okadaic acid. Fluorescence recovery after photobleaching in permeabilized cells, under the conditions that stimulate androgen receptor export, suggested that double-stranded DNA-dependent export does not simply reflect the relief of a nuclear retention mechanism. A radiolabeled androgen was used to show that the androgen receptor remains ligand-bound during translocation through the nuclear pore complex. A specific inhibitor to the DNA-dependent protein kinase, NU7026, inhibits androgen receptor export and phosphorylation. In living cells, NU7026 treatment increases androgen-dependent transcription from endogenous genes that are regulated by androgen receptor. We suggest that DNA-dependent protein kinase phosphorylation of the androgen receptor, or an interacting component, helps target the androgen receptor for export from the nucleus.

Effects of mifepristone on proliferation and apoptosis of human endometrium in new users of medroxyprogesterone acetate

BACKGROUND

Mifepristone has been demonstrated to decrease breakthrough bleeding (BTB) in users of progestin-only contraceptives.

METHODS

Endometrial biopsies were collected from 50 normal cycling women who were new users of depot medroxyprogesterone acetate (DMPA) randomized to receive either mifepristone or placebo before, during and after treatment. Proliferation, apoptosis and sex steroid receptors were evaluated by either immunohistochemistry or TUNEL assay.

RESULTS

Administration of mifepristone to DMPA-exposed endometrium for 1 week significantly increased endometrial expression of Ki-67 (MKI67), estrogen receptor (ER)alpha and progesterone receptors A and B (PRAB) and decreased the number of TUNEL-positive and caspase-3 (CASP3)-active cells in the endometrial stroma. However, after 10 weeks of mifepristone treatment, no significant difference in proliferation, apoptosis and the expression of ERalpha or PRAB could be detected between the endometrium treated with DMPA alone and endometrium treated with mifepristone and DMPA.

CONCLUSIONS

Administration of mifepristone to DMPA users significantly increases endometrial proliferation and decreases endometrial stromal apoptosis in the short term. Prolonged exposure to mifepristone does not counteract the inhibitory effects of progestin therapy on endometrial proliferation. Estrogen and progesterone receptors may play an important role in these effects.

Compartment-specific phosphorylation of rat thyroid hormone receptor alpha1 regulates nuclear localization and retention

The thyroid hormone receptor alpha1 (TRalpha1) is a transcription factor, which can activate or repress gene expression in response to thyroid hormone. In addition, some of its actions, including DNA binding and transcriptional activation, are thought to be regulated by phosphorylation. Results presented here, using Xenopus oocyte microinjection assays, demonstrate that a phosphorylated form of rat TRalpha1 is present in the nucleus, whereas unphosphorylated TRalpha1 remains cytoplasmic. Changes in the phosphorylation state of TRalpha1 occur rapidly and point to the possibility that phosphorylation occurs in the nucleus. Furthermore, increasing the overall phosphorylation state of the cell leads to enhanced nuclear retention of TRalpha1, suggesting that compartment-specific phosphorylation regulates nuclear localization of TRalpha1. Enhanced nuclear retention of TRalpha1 is not dependent on phosphorylation of serine 12, a well-characterized casein kinase II site, nor is phosphorylation of this site necessary for import of TRalpha1 into the Xenopus oocyte nucleus. Similarly, mutational analysis in mammalian cells shows that nuclear localization and partitioning of TRalpha1 to the nuclear matrix are independent of serine 12 phosphorylation. Taken together, these studies suggest that phosphorylation of one or more sites in TRalpha1, excluding serine 12, enhances nuclear retention and/or inhibits nuclear export but is not directly involved in nuclear import.

New human breast cancer cells to study progesterone receptor isoform ratio effects and ligand-independent gene regulation

All known progesterone target cells coexpress two functionally different progesterone receptor (PR) isoforms: 120-kDa B-receptors (PR-B) and N-terminally truncated, 94-kDa A-receptors (PR-A). Their ratio varies in normal and malignant tissues. In human breast cancer cells, homodimers of progesterone-occupied PR-A or PR-B regulate different gene subsets. To study PR homo- and heterodimers, we constructed breast cancer cell lines in which isoform expression is controlled by an inducible system. PR-negative cells or cells that stably express one or the other isoform were used to construct five sets of cells: (i) PR-negative control cells (Y iNull), (ii) inducible PR-A cells (Y iA), (iii) inducible PR-B cells (Y iB), (iv) stable PR-B plus inducible PR-A cells (B iA), and (v) stable PR-A plus inducible PR-B cells (A iB). Expression levels of each isoform and/or the PR-A/PR-B ratios could be tightly controlled by the dose of inducer as demonstrated by immunoblotting and transcription studies. Induced PRs underwent normal progestin-dependent phosphorylation and down-regulation and regulated exogenous promoters as well as endogenous gene expression. Transcription of exogenous promoters was dependent on the PR-A/PR-B ratio, whereas transcription of endogenous genes was more complex. Finally, we have described several genes that are regulated by induced PR-A even in the absence of ligand.

Transcriptional hyperactivity of human progesterone receptors is coupled to their ligand-dependent down-regulation by mitogen-activated protein kinase-dependent phosphorylation of serine 294

Breast cancers often exhibit elevated expression of tyrosine kinase growth factor receptors; these pathways influence breast cancer cell growth in part by targeting steroid hormone receptors, including progesterone receptors (PR). To mimic activation of molecules downstream of growth factor-initiated signaling pathways, we overexpressed mitogen-activated protein kinase (MAPK; also known as extracellular signal-regulated kinase) kinase kinase 1 (MEKK1) in T47D human breast cancer cells expressing the B isoform of PR. MEKK1 is a strong activator of p42 and p44 MAPKs. MEKK1 expression increased progestin-mediated transcription 8- to 10-fold above normal PR-driven transcription levels. This was dependent on the presence of a progesterone response element and functional PR. PR protein levels were unchanged by MEKK1 alone but were extensively down-regulated by MEKK1 plus the progestin R5020. MEKK1 expression resulted in phosphorylation of PR on Ser294, a MAPK consensus site known to mediate ligand-dependent PR degradation. MEK inhibitors blocked phosphorylation of Ser294 and attenuated PR transcriptional hyperactivity in response to MEKK1 plus R5020; stabilization of PR by inhibition of the 26S proteasome produced similar results. T47D cells stably expressing mutant S294A PR, in which serine 294 is replaced by alanine, fail to undergo ligand-dependent down-regulation and are resistant to MEKK1-plus-R5020-induced transcriptional synergy but respond to progestins alone. Similarly, c-myc protein levels are synergistically increased by epidermal growth factor and R5020 in cells expressing wild-type PR, but not S294A PR. Thus, highly stable mutant PR are functional in response to progestins but are incapable of cross talk with MAPK-driven pathways. These studies demonstrate a paradoxical coupling between steroid receptor down-regulation and transcriptional hyperactivity. They also suggest a link between phosphorylation of PR by MAPKs in response to peptide growth factor signaling and steroid hormone control of breast cancer cell growth.

Temperature dependence of estrogen binding: importance of a subzone in the ligand binding domain of a novel piscine estrogen receptor

The full length estrogen receptor from Oreochromis aureus (OaER) was cloned and expressed in vitro and in vivo as a functional transcription factor. Amino acid residues involved in the thermal stability of the receptor are located at/near subzones beta1 and beta3, which are highly conserved in other non-piscine species but not in OaER. Hormone binding studies, however, indicate that OaER is thermally stable but exhibited a approximately 3-fold reduced affinity for estrogen at elevated temperatures. Transfection of OaER into various cell lines cultured at different temperatures displayed a significant estrogen dose-response shift compared with that of chicken ER (cER). At 37 degrees C, OaER requires approximately 80-fold more estrogen to achieve half-maximal stimulation of CAT. Lowering of the incubation temperature from 37 degrees C to 25 degrees C or 20 degrees C resulted in a 4-fold increase in its affinity for estrogen. The thermally deficient transactivation of OaER at temperatures above 25 degrees C was fully prevented by high levels of estrogen. Thus, compared to cER, the OaER exhibits reduced affinity for estrogen at elevated temperature as reflected in its deficient transactivation capability. Amino acid replacements of OaER beta3 subzones with corresponding amino acids from cER could partially rescue this temperature sensitivity. The three-dimensional structure of the OaER ligand binding domain (LBD) was modelled based on conformational similarity and sequence homology with human RXRalpha apo, RARgamma holo and ERalpha LBDs. Unliganded and 17beta-estradiol-liganded OaER LBD retained the overall folding pattern of the nuclear receptor LBDs. The residues at/near the subzone beta3 of the LBD constitute the central core of OaER structure. Thus, amino acid alteration at this region potentially alters the structure and consequently its temperature-dependent ligand binding properties.

Agonist and antagonist-induced qualitative and quantitative alterations of progesterone receptor from breast cancer cells

T47D cells, cultured in medium containing serum stripped of endogenous steroids, proliferate in response to treatment with the progesterone receptor (PR) agonist, R5020 or the PR agonist/antagonist, RU486, whereas the full PR antagonist, ZK98299 has no proliferative effects. Under estrogenized conditions, all of the PR ligands tested inhibit cell growth [23]. In order to determine whether the levels or phosphorylation state of PR are reflected in the growth patterns of T47D cells, we monitored the effects of these PR ligands on the immunoblotted PR band intensities, the relative intensities of PR-A and PR-B, and their phosphorylation states that are reflected in their altered mobility during SDS-PAGE. Under conditions where the PR ligands inhibit cell proliferation, each ligand had distinctively different qualitative and quantitative effects on PR. Short term treatment of the cells with R5020 or RU486 induced a characteristic phosphorylation-dependent upshift of both PR-A and PR-B. The phosphorylated PR was stable for up to 4 days after treatment of the cells with RU486, but was down regulated between 6-24 h after treatment with R5020. No replenishment of PR in cells treated with R5020 was detected. ZK98299, at concentrations tested, had no qualitative or quantitative effects on PR. Culturing cells for 8 days in medium containing steroid-depleted serum caused a significant reduction in the PR band intensity without causing a change in the ratio of PR-A and PR-B or their phosphorylation states. This decrease in the PR band intensity was reversed by maintaining the cells in 1 nM estrogen, but was potentiated by RU486 or ZK98299. These observations support the view that decreased PR levels may play a role in the stimulatory effects of R5020 and RU486 when cells are cultured under non-estrogenized conditions.

Characterization of ligand binding, DNA binding and phosphorylation of progesterone receptor by two novel progesterone receptor antagonist ligands

In order to gain a better understanding of the distinctive mechanisms of the various types of antiprogestins, we have characterized in vitro ligand binding, specific DNA binding and phosphorylation of progesterone receptor (PR) from T47D cells after treatment of cells with progestins (progesterone, R5020) and antiprogestins (RU486, ZK98299, Org 31806 and Org 31710). Treatment of the cells with R5020 or PR antagonists, with the exception of ZK98299, resulted in a quantitative upshift of PR-A and PR-B indicative of ligand/DNA-induced phosphorylation of PR. Treatment of cells with RU486, Org 31710 or Org 31806, but not R5020 or ZK98299 resulted in detectable PR-progesterone response element complexes (PR-PREc) as assessed by gel mobility shift assay. Although treatment of cells with ZK98299, a type I PR antagonist, did not induce phosphorylation, the antiprogestins, Org 31806 and Org 31710, in a manner identical to RU486, did. Our data suggest that Org 31806 and Org 31710 affect properties of PR from T47D cells that are similar to RU486.

A subpopulation of estrogen receptors are modified by O-linked N-acetylglucosamine

Estrogen receptors (ER) are ligand-inducible transcription factors regulated by Ser(Thr)-O-phosphorylation. Many transcription factors and eukaryotic RNA polymerase II itself are also dynamically modified by Ser(Thr)-O-linked N-acetylglucosamine moieties (O-GlcNAc). Here we report that subpopulations of murine, bovine, and human estrogen receptors are modified by O-GlcNAc. O-GlcNAc moieties were detected on insect cell-expressed, mouse ER (mER) by probing with bovine milk galactosyltransferase, followed by structural analysis. Wheat germ agglutinin-Sepharose affinity chromatography also readily detected terminal GlcNAc residues on subpopulations of ER purified from calf uterus, from human breast cancer cells (MCF-7), or from mER produced by in vitro translation. These data suggest that greater than 10% of these populations of estrogen receptors bear O-GlcNAc. Site mapping of insect cell expressed mER localized one major site of O-GlcNAc addition to Thr-575, within a PEST region of the carboxyl-terminal F domain. Based upon their relative resistance to both hexosaminidase and to in vitro galactosylation, O-GlcNAc moieties appear to be largely buried on native mER. This dynamic saccharide modification, like phosphorylation, may play a role in modulating the dimerization, stability, or transactivation functions of estrogen receptors.

Androgen receptor phosphorylation

Phosphorylation of transcription factors plays an important role in regulation of gene expression. DNA-binding, transactivation activity, and subcellular trafficking of specific transcription factors have been shown to be regulated by phosphorylation/dephosphorylation. Steroid hormone receptors are phospho-proteins, and mutations in phosphorylation sites significantly affect the transactivation capacity of these ligand-dependent transcription factors. At present, it is unknown which amino acid residues of the human androgen receptor are phosphorylated and whether phosphorylation of particular sites is a prerequisite for proper androgen receptor function. The aim of our future research is to map all phosphorylation sites in the human androgen receptor, and to analyze their importance by mutational analysis in vitro and in vivo using a number of functional assays.

Role of phosphorylation on DNA binding and transcriptional functions of human progesterone receptors

To study the function of human progesterone receptor (hPR) phosphorylation, we have tested four sets of serine to alanine substitution mutants: 10 serine clusters, located in regions common to both hPR isoforms (the M-series mutants) were mutated in A-receptors and B-receptors; 6 serine clusters located in the B-upstream segment (BUS; the B-series mutants) were mutated individually and collectively and cloned into B-receptors and into BUS-DBD-NLS, a constitutive transactivator, in which the AF3 function of BUS is fused to the DNA binding domain (DBD) and nuclear localization signal (NLS) of hPR. Transcription by most of the M-series mutants resembles that of wild-type A- or B-receptors. Mutation of 3 sites, Ser190 at the N terminus of A-receptors, a cluster of serines just upstream of the DBD, or Ser676 in the hinge region, inhibits transcription by 20-50% depending on cell or promoter context. These sites lie outside the AF1 activation function. M-series mutants are substrates for a hormone-dependent phosphorylation step, and they all bind well to DNA. Progressive mutation of the B-series clusters leads to the gradual dephosphorylation of BUS, but only the 6-site mutant, involving 10 serine residues, is completely dephosphorylated. These data suggest that in BUS alternate serines are phosphorylated or dephosphorylated at any time. However, even when BUS is completely dephosphorylated, both BUS-DBD-NLS and full-length B-receptors remain strong transactivators. Mutant B-receptors also do not acquire the dominant negative properties of A-receptors, and they retain the ability to activate transcription in synergy with 8-Br-cAMP and antiprogestins. We conclude that phosphorylation has subtle effects on the complex transcriptional repertoire that distinguishes the two hPR isoforms and does not influence transactivation mediated by AF1 or AF3, but subserves other functions.

TFIIB-directed transcriptional activation by the orphan nuclear receptor hepatocyte nuclear factor 4

The orphan nuclear receptor hepatocyte nuclear factor 4 (HNF-4) is required for development and maintenance of the liver phenotype. HNF-4 activates several hepatocyte-specific genes, including the gene encoding apolipoprotein AI (apoAI), the major protein component of plasma high-density lipoprotein. The apoAI gene is activated by HNF-4 through a nuclear receptor binding element (site A) located in its liver-specific enhancer. To decipher the mechanism whereby HNF-4 enhances apoAI gene transcription, we have reconstituted its activity in a cell-free system. Functional HNF-4 was purified to homogeneity from a bacterial expression system. In in vitro transcription assays employing nuclear extract from HeLa cells, which do not contain HNF-4, recombinant HNF-4 stimulated transcription from basal promoters linked to site A. Activation by HNF-4 did not exhibit a ligand requirement, but phosphorylation of HNF-4 in the in vitro transcription system was observed. The activation function of HNF-4 was localized to a domain displaying strong homology to the conserved AF-2 region of nuclear receptors. Dissection of the transcription cycle revealed that HNF-4 activated transcription by facilitating assembly of a preinitiation complex intermediate consisting of TBP, the TATA box-binding protein component of TFIID and TFIID, via direct physical interactions with TFIIB. However, recruitment of TFIIB by HNF-4 was not sufficient for activation, since HNF-4 deletion derivatives lacking AF-2 bound TFIIB. On the basis of these results, HNF-4 appears to activate transcription at two distinct levels. The first step involves AF-2-independent recruitment of TFIIB to the promoter complex; the second step is AF-2 dependent and entails entry of preinitiation complex components acting downstream of TFIIB.

Negative interaction between the RelA(p65) subunit of NF-kappaB and the progesterone receptor

Interactions between transcription factors are an important means of regulating gene transcription. The present study describes the mutual repression of two transcription factors, the RelA(p65) subunit of NF-kappaB and the progesterone receptor (PR). This trans-repression is shown to occur independent of PR isoform, reporter construct, or cell type used. Together with the demonstration of an interaction between PR and RelA in vitro, these findings suggest that the mutual repression is due to a direct interaction between these proteins. Furthermore, activation of NF-kappaB by tumor necrosis factor-alpha also results in repression of PR, while PR is able to repress tumor necrosis factor-alpha-induced NF-kappaB activity. Since NF-KB-regulating cytokine receptors are expressed in progesterone target tissues, like breast and endometrium, the mutual repression of PR and RelA could play an important role in a wide variety of physiological processes in these tissues, including maintenance of pregnancy, immunosuppression, and tumorigenesis.

Recent advances in the molecular biology of vitamin D action

Following the cloning and deletion analysis of the vitamin D receptor, most recent advances have been in the isolation and characterization of the DNA response elements found in the promoter region of target genes of vitamin D. Vitamin D, like the thyroid and retinoid hormones, binds to repeat sequences, but the repeats are separated by three nonspecified bases. The action of the VDR requires the presence of the RXR proteins and evidently other proteins that are involved in regulating transcriptions. A possible role of phosphorylation of the ligand binding domain of the VDR in transcription has also appeared. Very likely, the molecular events involved in vitamin D stimulation or suppression of a target gene will include its interaction with a number of transcription factors, both in the regulation of transcription and in the actual machinery involved in the transcription process through polymerase II. Although likely, it is not entirely clear whether the genomic action of vitamin D can account for all of its biological activities. Nongenomic actions of the vitamin D hormone have been reported, but convincing evidence that this is of biological importance in vivo is lacking. Advances in our understanding of the vitamin D mechanism of action can clearly be expected from physical studies of cloned and expressed vitamin D receptor and its subdomains, elucidation of the transcription factors in vitamin D-modulated transcription of target genes, elucidation of the role of phosphorylation in the transcription process, and the identification of important genes that are regulated in the specific target tissues responsive to vitamin D. This will definitely remain as a very active field of investigation well into the future.

Retinoid-dependent in vitro transcription mediated by the RXR/RAR heterodimer

The effects of retinoids on gene regulation are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Here, we provide the first biochemical evidence that, in vitro, ligand governs the transcriptional activity of RXR alpha/RAR alpha by inducing conformational changes in the ligand-binding domains. Using limited proteolytic digestion we show that binding of the cognate ligand causes a conformational change in the carboxy-terminal part of the receptor. We also show that recombinant RXR alpha/RAR alpha is partially active in the absence of exogenously added ligand. Trans-activation depends critically on the ligand-dependent transcriptional activation function AF-2 of RAR alpha. Full activation by recombinant RXR alpha/RAR alpha, however, requires the addition of either all-trans RA, 9-cis RA, or other RAR-specific agonists, whereas an RAR alpha-specific antagonist abolishes trans-activation. Intriguingly, the ligand-dependent AF-2 of RXR does not contribute to the level of transcription from the RAR beta 2 promoter in vitro even when the cognate ligand (9-cis RA) is bound. Thus, the major role of RXR in trans-activation of the RAR beta 2 promoter is to serve as an auxiliary factor required for the binding of RAR which, in turn, is directly responsible for transcriptional activity.

Changes in the abundance of androgen receptor isotypes: effects of ligand treatment, glutamine-stretch variation, and mutation of putative phosphorylation sites

The SDS-polyacrylamide gel electrophoresis (SDS-PAGE) migration pattern of wild-type and mutated human androgen receptors (ARs) expressed in COS-1 cells was analyzed. In the absence of hormone, the wild-type AR migrated as a closely spaced 110-112 kDa doublet. Alkaline phosphatase treatment resulted in a single 110 kDa band showing that the 112 kDa upshift reflects receptors phosphorylation. Deletion of the N-terminal amino acids 46-101 or 100-142 resulted in mutant ARs migrating as single protein bands. Three consensus phosphorylation sites in this region were substituted, and the resulting mutated proteins were analyzed. Two Ser-Pro-directed kinase consensus sites at positions Ser-80 and Ser-93 were both necessary for the AR 112 kDa upshift. Substitution of the putative casein kinase II Ser-118 site had no effect on the AR migration pattern. Surprisingly, deletion of the glutamine repeat, located directly N-terminal of the Ser-Pro sites, resulted also in an AR single form. Lengthening of the glutamine repeat caused an increase in the spacing between the two isotypes of the doublet, showing that the number of glutamine residues determines the extent of the upshift. Hormone treatment induced an extra isotype with an apparent molecular mass of 114 kDa, resulting in a 110-112-114 kDa AR triplet. The hormone-induced upshift was dependent on the Ser-80 consensus phosphorylation site. Mutations in the DNA binding domain caused a different distribution of receptor protein over the three AR isotypes.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation sites in ligand-induced and ligand-independent activation of the progesterone receptor

Steroid hormone receptors are phosphoproteins that undergo hyperphosphorylation upon binding of hormone. The mechanism and the role of this reaction remain poorly understood. Two-dimensional analysis of ligand-free progesterone receptor (PR) tryptic digests showed the existence of seven main phosphopeptides. Incubation of the cells with the progestin R5020 led to a global increase in the levels of PR phosphorylation. However, the same phosphopeptides were seen, and their levels of labeling relative to each other were unchanged. A similar result was observed after incubation of cells with the antiprogestin RU486. The antiprogestin ZK98299 demonstrated only half of the activity of RU486 in terms of receptor hyperphosphorylation, but the same phosphopeptides, proportionally labeled to the same extent, were observed by chromatography electrophoresis. Ligand-induced DNA binding did not play a role in receptor hyperphosphorylation since the mutant delta 547-592, which is devoid of the first zinc finger region, exhibited the same phosphopeptides, labeled to the same extent, as did wild-type receptor after incubation of cells with hormone. These results suggest that the same kinase(s) act in vivo on ligand-free and on agonist or antagonist-bound progesterone receptor. Binding of different ligands produces different conformational changes in the ligand binding domain of the receptor which enhance, to varying extents, affinity of the receptor for the kinase(s). The DNA binding region also plays a role in the interaction with the kinase(s), although binding to DNA per se is not necessary for the hyperphosphorylation of the receptor to take place.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation enhances the target gene sequence-dependent dimerization of thyroid hormone receptor with retinoid X receptor

To understand the molecular basis of the phosphorylation-enhanced transcriptional activity of human thyroid hormone nuclear receptor subtype beta 1 (hTR beta 1), we studied the effect of phosphorylation on the interaction of hTR beta 1 with the retinoid X receptor beta (RXR beta), we studied the effect of phosphorylation on the interaction of hTR beta 1 with the retinoid X receptor beta (RXR beta). In vitro, the extent of hTR beta 1.RXR beta heterodimer bound to various thyroid hormone response elements (TREs) was compared before and after phosphorylation of hTR beta 1. Without phosphorylation, hTR beta 1.RXR beta heterodimer was barely detectable under the experimental conditions. After phosphorylation of hTR beta 1, heterodimer bound to (i) the chicken lysozyme gene TRE, (ii) a TRE consisting of direct repeats of half-site binding motifs separated by four gaps, and (iii) a palindromic TRE was enhanced by approximately 10-, 7-, and 6-fold, respectively. The effect of phosphorylation on hTR beta 1.RXR beta heterodimerization was reversible. Dephosphorylation of the phosphorylated hTR beta 1 by alkaline phosphatase led to loss of the ability of hTR beta 1 to form a heterodimer with RXR beta in either the absence or the presence of DNA. These results indicate that the heterodimerization is enhanced by phosphorylation. To evaluate the effect of phosphorylation on the interaction of hTR beta 1 with RXR beta in vivo, we cotransfected hTR beta 1, RXR beta and TRE-chloramphenicol acetyltransferase (CAT) expression plasmids into CV-1 cells. CAT activity was assessed in the presence or absence of okadaic acid. Okadaic acid is a potent inhibitor of phosphatases 1 and 2A and increases the in vivo phosphorylation of hTR beta 1 by approximately 10-fold. Using the CAT reporter gene under control of the TRE from the malic enzyme gene, we found that RXR beta increased the okadaic acid-enhanced hTR beta 1-mediated CAT activity by 2- to 3-fold in the presence of 3,3',5-triiodo-L-thyronine. However, 9-cis-retinoic acid did not enhance the effect of okadaic acid. Our results indicate that phosphorylation is essential for the interaction of hTR beta 1 with RXR beta. Thus, phosphorylation plays a pivotal role in the gene-regulating activity of hTR beta 1.

Developmental changes in the expression and compartmentalization of the glucocorticoid receptor in embryonic retina

Inducibility by glucocorticoids of the glutamine synthetase gene in chicken embryo retina and the transcriptional activity of the glucocorticoid receptor (GR) greatly increase between embryonic days 6 and 10 (E6, E10), although the level of GR does not markedly change during that time. This apparent discrepancy was investigated by examining the pattern of GR expression in undifferentiated E6 retina and in E10 retina, which consists mostly of differentiated cells. Two GR isoforms, 90 and 95 kDa, were found to be expressed at both of these ages at a similar total level but in different proportions: in E6 retina the level of the 90-kDa isoform was higher, whereas in E10 retina the 95-kDa receptor was higher. However, following treatment of the retinas with cortisol, the 95-kDa isoform became the predominant receptor at both ages. Immunohistochemical analysis revealed that the cellular localization of GR markedly changed in the course of development: in the undifferentiated E6 retina GR was expressed in virtually all cells, whereas in the more differentiated E10 and E12 retina, GR was detected only in Müller glia cells. The latter represent approximately 20% of the cells in this tissue and are the only cells in which glucocorticoid hormone induces the glutamine synthetase gene. We suggest that the compartmentalization of GR in Müller glia is a major aspect of the mechanism that modulates receptor activity during retina development and results in the temporal increase in the inducibility of glutamine synthetase and its specific localization in Müller glia cells.

Chicken progesterone receptor expressed in Saccharomyces cerevisiae is correctly phosphorylated at all four Ser-Pro phosphorylation sites

This study describes the phosphorylation of chicken progesterone receptor (cPR) produced in yeast, Saccharomyces cerevisiae, and examines the dependence of specific phosphorylations on hormone and DNA binding. The chicken progesterone receptor is expressed in vivo as two forms, cPRB and a smaller form, cPRA. Characterization of the phosphorylation sites in the cPRB form expressed in yeast shows that progesterone receptor is phosphorylated on the three serines (Ser211, Ser260, and Ser530) reported previously in chicken oviduct. An additional site which was phosphorylated in response to hormone was also detected and was subsequently identified as Ser367. Although cPRB and cPRA are phosphorylated identically in chicken oviduct, cPRA expressed in yeast is phosphorylated on Ser211, Ser260, and Ser367, but phosphorylation of Ser530 is almost undetectable. In contrast, cPRB expressed in yeast is phosphorylated on all four sites. No phosphorylations were found in or near the region required for hormone binding, indicating that phosphorylation is not required for hormone binding. In order to determine whether any of the phosphorylations were DNA-dependent, phosphorylation was also studied using cPRA containing a partial deletion of the DNA binding domain. Two of the sites, Ser211 and Ser367, showed reduced phosphorylation in this mutant, suggesting a possible requirement for DNA binding activity for the phosphorylation of these sites. To our knowledge, this is one of the first demonstrations that a eucaryotic protein expressed in yeast is correctly phosphorylated.

Modulation of the ligand-independent activation of the human estrogen receptor by hormone and antihormone

It has been previously demonstrated that several members of the steroid receptor superfamily may be activated by the neurotransmitter dopamine in the apparent absence of cognate ligand. We have examined wild-type and mutant human estrogen receptors (ERs, [Gly400]ER and [Val400]ER, respectively) for their abilities to activate ER-dependent transcription of a transgene in a ligand-independent manner. In cells expressing the wild-type ER, dopamine was nearly as effective as 17 beta-estradiol at inducing the chloramphenicol acetyltransferase activity of the reporter gene in a dose-dependent manner; simultaneous addition of suboptimal concentrations of 17 beta-estradiol and dopamine stimulated transcription more than either compound alone. Dopamine alone was unable to induce gene expression in cells expressing [Val400]ER mutant receptors, but concomitant treatment with 17 beta-estradiol produced a synergistic increase in transcription, suggesting that the ligand may alter the mutant receptor's conformation such that it can be activated subsequently by a dopaminergic signaling mechanism. In the presence of the antiestrogen ICI 164,384, dopamine-stimulated gene expression was undetectable in cells expressing either form of ER. However, simultaneous treatment of cells expressing wild-type ER with trans-4-hydroxytamoxifen and dopamine resulted in transgene expression that was additive in nature compared to either compound alone; similar treatment of cells expressing [Val400]ER produced a synergistic increase. Our results suggest that ligand and ligand-independent activation of the ER initiate from distinct pathways and that the latter may occur in a variety of target tissues subject to modulation by receptor ligands.

The glucocorticoid receptor hormone binding domain mediates transcriptional activation in vitro in the absence of ligand

We show that recombinant rat glucocorticoid receptor (vvGR) expressed using vaccinia virus is indistinguishable from authentic GR with respect to DNA and hormone binding. In the absence of hormone, vvGR is mainly found in the cytoplasm in a complex with heat shock protein 90. Upon incubation with ligand, vvGR is released from this complex and translocated to the nucleus. Thus, the ligand binding domain displays the known biochemical properties. However, in vitro, transcription from a synthetic promoter and from the mouse mammary tumor virus (MMTV) promoter is enhanced by recombinant GR in a ligand independent manner. Both transactivation domains contribute to the transcriptional activity, additively on a synthetic promoter and cooperatively on the MMTV promoter. We thus provide the first evidence that in vitro the hormone binding domain has a transcriptional activity even in the absence of ligand.

The progesterone antagonist RU486 acquires agonist activity upon stimulation of cAMP signaling pathways

The protein kinase A stimulator cAMP can potentiate the ability of progestins to induce the transactivation function of the human progesterone receptor (hPR). We questioned in the present study whether cAMP could functionally cooperate with the progestin antagonist RU486. In T47D human breast cancer cells, RU486 behaves as a pure antagonist with respect to induction of the progesterone-responsive mouse mammary tumor virus chloramphenicol acetyltransferase (MMTV-CAT) reporter gene. It fails to stimulate MMTV-CAT expression and completely inhibits induction by the synthetic progestin R5020. However, when RU486 is combined with 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP), MMTV-CAT is induced to levels approaching that stimulated by R5020 alone. Also, RU486 in the presence of 8-Br-cAMP is only partially effective in antagonizing R5020 action. The agonist activity exhibited under these conditions appears to be due to RU486 acting through hPR as evidenced by the fact that 8-Br-cAMP alone has no effect on MMTV-CAT, whereas induction by the combination of 8-Br-cAMP and RU486 is dose responsive to RU486 in a saturable manner and can be inhibited by the type I antiprogestin (prevents hPR-DNA binding) ZK98299, which does not exhibit positive functional cooperation with cAMP. Acquisition of agonist activity in the presence of 8-Br-cAMP also extends to the type II antiprogestin (permits hPR-DNA binding) ZK112993. Since RU486 is also a type II antagonist, these results suggest that detection of functional synergism between cAMP and antiprogestins may require binding of the hPR-antagonist complex to DNA. We propose that cross-talk between second messenger and steroid receptor signal transduction pathways may be one mechanism for resistance to steroid antagonists that frequently develops in breast cancer.

Two nuclear signalling pathways for vitamin D

The dihydroxylated form of vitamin D3 (1,25-dihydroxy-D3)mediates a biological response by binding to intracellular receptors which belong to the steroid receptor superfamily. These receptors act as ligand-dependent transcription factors that bind to specific DNA sequences (reviewed in refs 6-9). We have identified two classes of vitamin D response elements that are activated either by the vitamin D receptor (VDR) alone or by heterodimers of VDR and the retinoid-X receptor-alpha (RXR-alpha). The motif GGGTGA arranged as a direct repeat with a spacing of six nucleotides or as a palindrome without spacing, or as an inverted palindrome with a 12-nucleotide spacing, confers vitamin D inducibility mediated by VDR alone. A second class of response elements, composed of directly repeated pairs of motifs (GGTCCA, AGGTCA, or GGGTGA) spaced by three nucleotides, is synergistically activated by RXR and VDR, but only in the presence of both ligands. Thus, the RXR ligand and the nature of the response element determine whether a nuclear receptor is co-regulated by RXR.

Mechanisms of transcriptional activation by steroid hormone receptors

Steroid hormones are involved in cell growth, development, and differentiation. The hormonal signal is mediated by nuclear receptors which represent a specific class of transcription factors. During the last few years, the cloning of all the major steroid hormone receptors increased our insight into how the hormonal signal converts the receptor into a transcriptional activator. Good progress has been made towards understanding the mechanism of steroid hormone action. In this review we will discuss the role of heat shock proteins in the process of transcriptional activation, the mechanistic differences between the hormone (agonist) and the antihormone (antagonist), the resulting functional consequences, and a possible mode by which transcriptional activation is mediated.

Developmental control of glucocorticoid receptor transcriptional activity in embryonic retina

In chicken embryo retina, competence for induction of the glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming); EC 6.3.1.2] gene by glucocorticoid hormones increases progressively with development; this competence is minimal in 6-day retina (E6) and high by day 10 (E10). Because the level of glucocorticoid receptors (GRs) in the retina does not increase during that time, we investigated whether the transcriptional activity of GR increased between days 6 and 10 of development. The glucocorticoid-inducible chloramphenicol acetyltransferase (CAT) constructs 2GRE-37TK and p delta G46TCO, which contain glucocorticoid-responsive elements attached to a TATA box and to the thymidine kinase promoter, respectively, were transfected into E6 and E10 retinas, and their inducibility was examined. CAT expression could be induced in the transfected E10 retina but was not induced in the transfected E6 retina. However, induction was obtained also in E6 retina after cotransfection with a GR expression vector. Noninducible CAT constructs (pRSV-CAT, pSV2CAT, and pBLCAT2) were expressed at both ages at similar levels. The CAT construct pGS2.1CAT, which is controlled by the upstream sequence of the chicken glutamine synthetase gene, could be induced in E10 retina but was not induced in E6 retina; however, cotransfection with the GR expression vector resulted in induction of pGS2.1CAT also in E6 retina. We interpret these results as showing that the transcriptional activity of GR in embryonic retina is developmentally controlled and suggest that its increase is causally implicated in the development of competence for glutamine synthetase induction.

Modulators of cellular protein phosphorylation alter the trans-activation function of human progesterone receptor and the biological activity of progesterone antagonists

Addition of progesterone to breast cancer cells in vivo increases phosphorylation of human progesterone receptor (PR), suggesting that phosphorylation has a regulatory role in producing the activated form of receptor. Kinetic analysis indicates that hormone-dependent phosphorylation is sequential and that early stages of phosphorylation(s) are closely associated with enhancement of PR-DNA binding while later stages are associated with a trans-activation function. Various agents that stimulate cellular protein phosphorylation (8-Br cAMP, okadaic acid, TPA) functionally synergize with progesterone to enhance progesterone-dependent PR trans-activation in intact cells. These results suggest that protein phosphorylation does have a role in modulating the trans-activation function of PR in vivo. They also demonstrate cross-talk between second messenger signal transduction pathways and nuclear steroid receptors. Whether the phosphorylated target that provides the link between these two signal transduction pathways is PR itself or another protein involved in PR-mediated gene transcription is not known. Positive cooperative interactions were also observed between cAMP signaling pathways and the progesterone antagonist RU486, that resulted in RU486 exerting substantial agonist activities. This ability of cross-talk between second messenger and steroid receptor signal transduction pathways to override the antagonistic effects of RU486 suggests a novel mechanism to explain the problem of resistance to clinically important steroid antagonists.

Hypoxic induction of the human erythropoietin gene: cooperation between the promoter and enhancer, each of which contains steroid receptor response elements

Transcription of the human erythropoietin (Epo) gene is stimulated by exposure to hypoxia and/or cobalt in whole animals and in Hep3B cells. We have systematically investigated the promoter and 3' enhancer elements necessary for this induction by transient transfection of Hep3B cells. We define a promoter region of 53 bp and an enhancer region of 43 bp that confer hypoxia and cobalt inducibility. Each element gives rise to a 6- to 10-fold induction alone. In combination they produce a 50-fold induction after stimulation, similar to the 50- to 100-fold induction of the endogenous Epo gene. Two areas of DNA sequence homology are present in these regions. We demonstrate specific DNA-protein interactions in the enhancer and the ability of the promoter element to compete with these interactions in electrophoretic mobility shift assays. DNase I footprinting and methylation interference data further refine the cis-acting element in the 43-bp enhancer to a short region containing a direct repeat of a steroid/thyroid hormone receptor response element half-site separated by a 2-bp gap. Two half-site consensus sequences are also present in the 53-bp promoter. Site-specific mutation of the half-site sequences in the enhancer destroys the functional activity of the enhancer.

Hypoxic induction of the human erythropoietin gene: cooperation between the promoter and enhancer, each of which contains steroid receptor response elements

Transcription of the human erythropoietin (Epo) gene is stimulated by exposure to hypoxia and/or cobalt in whole animals and in Hep3B cells. We have systematically investigated the promoter and 3' enhancer elements necessary for this induction by transient transfection of Hep3B cells. We define a promoter region of 53 bp and an enhancer region of 43 bp that confer hypoxia and cobalt inducibility. Each element gives rise to a 6- to 10-fold induction alone. In combination they produce a 50-fold induction after stimulation, similar to the 50- to 100-fold induction of the endogenous Epo gene. Two areas of DNA sequence homology are present in these regions. We demonstrate specific DNA-protein interactions in the enhancer and the ability of the promoter element to compete with these interactions in electrophoretic mobility shift assays. DNase I footprinting and methylation interference data further refine the cis-acting element in the 43-bp enhancer to a short region containing a direct repeat of a steroid/thyroid hormone receptor response element half-site separated by a 2-bp gap. Two half-site consensus sequences are also present in the 53-bp promoter. Site-specific mutation of the half-site sequences in the enhancer destroys the functional activity of the enhancer.