Steroid hormone receptors: many actors in search of a plot

The glucocorticoid receptor is required for stress erythropoiesis

The glucocorticoid receptor (GR) coordinates a multitude of physiological responses in vivo. In vitro, glucocorticoids are required for sustained proliferation of erythroid progenitors (ebls). Here, we analyze the impact of the GR on erythropoiesis in vivo, using GR-deficient mice or mice expressing a GR defective for transactivation. In vitro, sustained proliferation of primary ebls requires an intact GR. In vivo, the GR is required for rapid expansion of ebls under stress situations like erythrolysis or hypoxia. A particular, GR-sensitive progenitor could be identified as being responsible for the stress response. Thus, GR-mediated regulation of ebl proliferation is essential for stress erythropoiesis in vivo.

Rapid signalling by androgen receptor in prostate cancer cells

Androgens are important growth regulators in prostate cancer. Their known mode of action in target cells requires binding to a cytoplasmic androgen receptor followed by a nuclear translocation event and modulation of the expression of specific genes. Here, we report another mode of action of this receptor. Treatment of androgen responsive prostate cancer cells with dihydrotestosterone leads to a rapid and reversible activation of mitogen-activated protein kinases MAPKs (also called extracellular signal-regulated kinases or Erks). Transient transfection assays demonstrated that the androgen receptor-mediated activation of MAP kinase results in enhanced activity of the transcription factor Elk-1. This action of the androgen receptor differs from its known transcriptional activity since it is rapid and insensitive to androgen antagonists such as hydroxyflutamide or casodex. Biochemical studies as well as analyses with dominant negative mutants showed the involvement of kinases such as MAPK/Erk kinase, phosphatidyl-inositol 3-kinase and protein kinase C in the androgen receptor-mediated activation of MAP kinase. These results demonstrate a novel regulatory action of the androgen receptor and prove that in addition to its known transcriptional effects, it also uses non-conventional means to modulate several cellular signalling processes.

Identification of a DNA binding protein cooperating with estrogen receptor as RIZ (retinoblastoma interacting zinc finger protein)

Double-stranded DNA fragments were selected from a random pool by repeated cycles of estrogen receptor-specific immunoprecipitation in the presence of a nuclear extract and PCR amplification (cyclic amplification and selection of target, CAST, for multiple elements). Fragments were cloned and sequence analysis indicated the 5-nucleotide word TTGGC was the most recurrent sequence unrelated to the known estrogen responsive element. Screening a HeLa cell expression library with a probe designed with multiple repeats of this sequence resulted in the identification of a 1700-aa protein showing a complete homology with the product of the human retinoblastoma-interacting zinc-finger gene RIZ. In transfection experiments, RIZ protein was able to bestow estrogen inducibility to a promoter containing an incomplete estrogen responsive element and a TTGGC motif. RIZ protein present in MCF-7 cell nuclear extract retarded the TTGGC-containing probe in an EMSA. Estrogen receptor was co-immunoprecipitated from MCF-7 cell extract by antibodies to RIZ protein and vice versa, thus indicating an existing interaction between these two proteins.

p57Kip2, a glucocorticoid-induced inhibitor of cell cycle progression in HeLa cells

Glucocorticoids exert antiproliferative effects on a number of cell types, including the HeLa cervical carcinoma cell line. However, the mechanism responsible for the antiproliferative effect is poorly understood. In this report we have investigated the role of the recently identified cyclin-dependent kinase inhibitor (CDI) p57Kip2 in the antiproliferative effect conferred by glucocorticoids. When HeLa cells were treated with the synthetic glucocorticoid dexamethasone (DEX), the doubling time of exponentially growing cells increased 2-fold. Within 11 h of DEX treatment, this was accompanied by an accumulation of cells in the G1 phase of the cell cycle with a corresponding decreased proportion of cells in the S phase and decreased CDK2 activity. DEX treatment of the HeLa cells dramatically induced the protein and mRNA expression of the CDI p57Kip2. This induction was seen within 4 h of DEX treatment, preceding a major DEX-induced accumulation of cells in the G1 phase. DEX-induced mRNA expression of p57Kip2 did not require de novo protein synthesis, and the transcription of the p57Kip2 gene was increased as determined by a run-on transcription assay. Furthermore, DEX induction of p57Kip2 was not a consequence of the cell cycle arrest, since other growth inhibition signals did not result in strong p57Kip2 induction. Overexpression of p57Kip2 using HeLa cells stably transfected with a tetracycline-inducible vector showed that p57Kip2 is sufficient to reconstitute an antiproliferative effect similar to that seen in DEX-treated cells. Selective p57Kip2 expression by the tetracycline analog doxycycline to levels comparable to those observed on DEX induction resulted in a 1.7-fold increase in the doubling time and a shift of HeLa cells to the G1 phase as well as a decrease in CDK2 activity. Taken together, these results suggest that glucocorticoid treatment directly induces transcription of the p57Kip2 gene and that the p57Kip2 protein is involved in the glucocorticoid-induced antiproliferative effect.

Dissociative glucocorticoid activity of medroxyprogesterone acetate in normal human lymphocytes

The immunosuppressive effects of glucocorticoids (GC) have led to their wide application in the treatment of inflammatory and autoimmune states. However, long term GC treatment is associated with severe side-effects. The development of agents displaying a more favorable ratio of wanted and unwanted GC effects, is, therefore, a major goal of pharmacological and clinical research. In this study, the progesterone receptor agonist medroxyprogesterone acetate (MPA), which also binds to the glucocorticoid receptor (GR), was tested with regard to its immunosuppressive properties. Using a recently established electroporation protocol, we show that MPA (but not progesterone) can suppress a human interleukin-2 (IL-2) promoter-luciferase construct to the same extent as the synthetic GC dexamethasone in normal human lymphocytes. MPA also markedly suppressed IL-2 (as well as IL-1 and IL-6) release, as assessed by specific enzyme-linked immunosorbent assays. In contrast, a highly dexamethasone-inducible glucocorticoid response element-driven promoter construct was only marginally stimulated by MPA in both normal human lymphocytes and HeLa cells. RT-PCR and Western blot analysis of normal human lymphocytes revealed that they do not express progesterone receptor messenger ribonucleic acid and protein, respectively. In contrast, the GR protein was clearly detectable in all samples and was shown to mediate the effects of MPA in transfected Jurkat T lymphoma cells. Our data indicate that 1) MPA can transrepress the human IL-2 gene in normal human lymphocytes in the absence of significant trans-activation; and 2) this effect is mediated by GR. Because of its dissociative GC activity, MPA is a highly promising substance for the treatment of inflammatory/autoimmune states.

Differential uterine expression of estrogen and progesterone receptors correlates with uterine preparation for implantation and decidualization in the mouse

The present investigation examined the spatiotemporal expression of estrogen receptors (ER-alpha and ER-beta) and progesterone receptor (PR) in the periimplantation mouse uterus (days 1-8). ER-alpha messenger RNA (mRNA) was detected at much higher levels in the periimplantation uterus compared with that of ER-beta mRNA, the levels of which were very low in all uterine cells during this period. Results of in situ hybridization demonstrated expression of ER-alpha mRNA primarily in the luminal and glandular epithelia on days 1 and 2 of pregnancy. On days 3 and 4, the accumulation was localized primarily in stromal cells in addition to its presence in the epithelium. Following implantation on day 5, the accumulation of this mRNA was more condensed in the luminal and glandular epithelia, but declined in the subluminal epithelial stroma at the sites of implanting embryos. On days 6-8, the accumulation of ER-alpha mRNA was primarily localized in the secondary decidual zone (SDZ) with more intense localization in the subepithelial cells at the mesometrial pole. In contrast, signals were very low to undetectable in the primary decidual zone (PDZ), and no signals were detected in implanting embryos. The undifferentiated stroma underneath the myometrium also showed positive signals. The immunolocalization of ER-alpha protein correlated with the mRNA localization. Western blot analysis showed down-regulation of ER-alpha in day 8 decidual cell extracts consistent with the down-regulation of ER-alpha mRNA in decidual cells immediately surrounding the embryo on this day. The expression pattern of PR was also dynamic in the periimplantation uterus. On day 1, the accumulation of PR mRNA was very low to undetectable, whereas only a modest level of accumulation in the epithelium was noted on day 2. On days 3 and 4, the accumulation of this mRNA was detected in both the epithelium and stroma. In contrast, the expression was restricted only to the stroma with increased signals at the sites of implantation on day 5. On days 6-8, PR mRNA accumulation increased dramatically throughout the deciduum. The localization of immunoreactive PR correlated with the mRNA distribution in the periimplantation uterus. Taken together, the results demonstrate that the expression of ER-alpha, ER-beta, and PR is differentially regulated in the periimplantation mouse uterus. This compartmentalized expression of ER and PR provides information regarding the sites of coordinated effects ofestrogen and progesterone in the preparation of the uterus for implantation and decidualization during early pregnancy.

Two distinct subpopulations of ecdysone receptor complex in the female mosquito during vitellogenesis

The native functional ecdysone receptor complex, a heterodimer of the ecdysone receptor (EcR) and ultraspiracle (USP) proteins, was identified in the fat body of adult female mosquitoes, Aedes aegypti, through electrophoretic mobility shift assays (EMSA) using previously characterized Drosophila ecdysone response elements (EcREs). The use of different salt concentrations during preparation of nuclear extracts enabled us to characterize two distinct subpopulations of the receptor complex, one of which was high salt-sensitive and responsive to exogenous 20-hydroxyecdysone (20E), and the other of which was high salt-resistant and refractory to exogenous 20E. Salt-sensitivity correlated with ligand responsiveness. Developmental EMSA analyses demonstrated that previtellogenic fat body nuclei and nuclei from the termination phase of vitellogenesis with low 20E titer contained solely high-salt-sensitive, ligand responsive complexes, which could be recovered in nuclear extracts (NEs) only by low salt tissue homogenization, suggesting these complexes were unliganded. In contrast, the fat body nuclei from stages of active vitellogenesis with high 20E titer contained almost exclusively high salt-resistant, ligand refractory complexes, implying these complexes were liganded; the nuclei from the intermediate stages, early and late phases of vitellogenesis, contained a mixture of the two subpopulations. The developmental profile of fully activated, ligand refractory receptor complexes closely correlated with that of yolk protein expression, suggesting an intimate involvement of the ecdysone receptor complex in both the induction and maintenance of high level expression of yolk protein genes.

Estrogen receptor cofactors expression in breast and endometrial human cancer cells

In order to approach the molecular basis of the tissue-specific agonistic activity of antioestrogens, we have compared, at the mRNA level, the expression of various transcriptional cofactors (activators or repressors) of estrogen receptors in different breast (MCF7, ZR75-1, T47D, MDA-MB231) and endometrial (Ishikawa, RL-95-2 and HEC1A) human cancer cell lines. We showed that for SRC-1, CBP, TIF1alpha, RIP140, N-CoR, and SMRT, no significant differences in the expression levels were observed between breast and endometrial cells. For TIF1alpha mRNA, both isoforms were also detected at similar levels in all the cells tested. By contrast, over-expression of AIB1 mRNA was observed in MCF7 cells, but not in other breast or endometrial cells, irrespective of their ER-status. We then used protein-protein interaction assay (far-Western blot) to confirm the increased expression of at least one of the p160 proteins in MCF7 cells. Finally, we demonstrated that RIP140 mRNA is directly induced by estrogens in ER-positive MCF7 breast cancer cell lines but not in Ishikawa endometrial cells. Together these results indicate that some differences exist between breast and endometrial cancer cell lines at the level of estrogen receptor transcription cofactor expression.

Up-regulation of multidrug resistance-associated protein 2 (MRP2) expression in rat hepatocytes by dexamethasone

Regulation of multidrug resistance-associated protein (MRP2) expression in response to dexamethasone (DEX) was analyzed using mainly primary rat hepatocytes. Enhanced levels of MRP2 mRNAs associated with increased amounts of a 190 kDa MRP2 were found in cultured DEX-treated hepatocytes; similarly, administration of DEX to rats (100 mg/kg, i.p.) led to a marked increase of hepatic amounts of MRP2 mRNAs. Maximal induction of MRP2 expression in DEX-treated primary hepatocytes was reached with 10(-5) M DEX, a concentration higher than that (10(-7) M) required for maximal up-regulation of tyrosine aminotransferase (TAT), a typical glucocorticoid receptor-regulated enzyme. In addition, the anti-glucocorticoid compound RU486 failed to inhibit MRP2 induction caused by DEX whereas it fully blocked that of TAT. These findings therefore demonstrate that DEX is a potent inducer of MRP2 expression in rat hepatocytes through a mechanism that seems not to involve the classical glucocorticoid receptor pathway.

Underdeveloped uterus and reduced estrogen responsiveness in mice with disruption of the estrogen-responsive finger protein gene, which is a direct target of estrogen receptor alpha

The biological roles of estrogen-responsive finger protein (efp) in vivo were evaluated in mice carrying a loss-of-function mutation in efp by gene-targeted mutagenesis. Although efp homozygous mice were viable and fertile in both sexes, the uterus that expressed abundant estrogen receptor alpha exhibited significant underdevelopment. When the ovariectomized homozygotes were subjected to 17beta-estradiol treatment, they showed remarkably attenuated responses to estrogen, as exemplified by decreased interstitial water imbibition and retarded endometrial cell increase, at least, attributable to the lower ratio of G1 to S-phase progression in epithelial cells. These results suggest that efp is essential for the normal estrogen-induced cell proliferation and uterine swelling as one of the direct targets of estrogen receptor alpha.

Glucocorticoid receptor inhibits transforming growth factor-beta signaling by directly targeting the transcriptional activation function of Smad3

The transforming growth factor-beta (TGF-beta) family of cytokines and glucocorticoids regulate diverse biological processes through modulating the expression of target genes. Here we report that glucocorticoid receptor (GR) represses TGF-beta transcriptional activation of the type-1 plasminogen activator inhibitor (PAI-1) gene in a ligand-dependent manner. Similarly, GR represses TGF-beta activation of the TGF-beta responsive sequence containing Smad3/4-binding sites. Using mammalian two-hybrid assays, we demonstrate that GR inhibits transcriptional activation by both Smad3 and Smad4 C-terminal activation domains. Finally, we show that GR interacts with Smad3 both in vitro and in vivo. These results suggest a molecular basis for the cross-regulation between glucocorticoid and TGF-beta signaling pathways.

Developmental effects of ectopic expression of the glucocorticoid receptor DNA binding domain are alleviated by an amino acid substitution that interferes with homeodomain binding

Steroid hormone receptors are distinguished from other members of the nuclear hormone receptor family through their association with heat shock proteins and immunophilins in the absence of ligands. Heat shock protein association represses steroid receptor DNA binding and protein-protein interactions with other transcription factors and facilitates hormone binding. In this study, we investigated the hormone-dependent interaction between the DNA binding domain (DBD) of the glucocorticoid receptor (GR) and the POU domains of octamer transcription factors 1 and 2 (Oct-1 and Oct-2, respectively). Our results indicate that the GR DBD binds directly, not only to the homeodomains of Oct-1 and Oct-2 but also to the homeodomains of several other homeodomain proteins. As these results suggest that the determinants for binding to the GR DBD are conserved within the homeodomain, we examined whether the ectopic expression of GR DBD peptides affected early embryonic development. The expression of GR DBD peptides in one-cell-stage zebra fish embryos severely affected their development, beginning with a delay in the epibolic movement during the blastula stage and followed by defects in convergence-extension movements during gastrulation, as revealed by the abnormal patterns of expression of several dorsal gene markers. In contrast, embryos injected with mRNA encoding a GR peptide with a point mutation that disrupted homeodomain binding or with mRNA encoding the DBD of the closely related mineralocorticoid receptor, which does not bind octamer factors, developed normally. Moreover, coinjection of mRNA encoding the homeodomain of Oct-2 completely rescued embryos from the effects of the GR DBD. These results highlight the potential of DNA-independent effects of GR in a whole-animal model and suggest that at least some of these effects may result from direct interactions with homeodomain proteins.

Distribution of 1,25-dihydroxyvitamin D3 receptor immunoreactivity in the rat olfactory system

1. The rat olfactory system contains numerous target sites for 1,25-dihydroxyvitamin D3, as determined by receptor protein (VDR) immunocytochemistry and in situ hybridization. 2. Nuclear and cytoplasmic VDR immunoreactivity as well as the corresponding hybridization signal was observed in neurons in the olfactory epithelium, the olfactory bulb, and throughout the limbic system in locations also known to be glucocorticoid targets. 3. The widespread distribution of VDR indicates the distinct functional importance of 1,25-dihydroxyvitamin D3 for olfactory perception.

Effects of glucocorticoids and mineralocorticoids on proliferation and maturation of human peripheral blood stem cells

It has been shown that hematopoietic progenitors can be expanded ex vivo in the presence of various cytokine combinations. Glucocorticoids (GC) are involved in the self-renewal of erythroid progenitors in chicken. To see whether GC have a similar effect on hematopoiesis in humans, CD34(+) peripheral blood stem cells were cultured in serum free medium in the presence of a GC, triamcinolone acetonide. However, our results demonstrate an inhibition of both erythroid and granulocyte-macrophage (GM) proliferation and a modification of erythroid colony morphology. Furthermore, RU38486 (Mifepristone), a potent GC antagonist, was unable to reverse the inhibitory effect of triamcinolone acetonide. We also identified and characterized another steroid subfamily, the mineralocorticoid (MC) subfamily, in human PB CD34(+) cells. The MC, aldosterone, significantly enhanced GM colony formation and diminished the erythroid colony number. Neither of effects were inhibited by ZK91587, an antagonist specific to the MC receptor (MCR). In contrast, ZK91587 reversed the stimulatory effect of deoxycorticosterone on GM colony formation. Cytoplasmic staining for MCR was observed in CD34(+) cells incubated with a polyclonal antiserum raised against human MCR. To our knowledge, this is the first demonstration of the presence of MCR in human PB CD34(+) cells.

Estrogen induction of the cyclin D1 promoter: involvement of a cAMP response-like element

Estrogens induce cell proliferation in target tissues by stimulating progression through the G(1) phase of the cell cycle. Induction of cyclin D1 expression is a critical feature of the mitogenic action of estrogen. We have determined a region between -96 and -29 in the cyclin D1 promoter that confers regulation by estrogens in the human mammary carcinoma cells MCF-7. This region encompasses a unique known transcription factor binding site with a sequence of a potential cAMP response element (CRE-D1). The induction is strictly hormone dependent and requires the DNA binding domain as well as both AF-1 and AF-2 domains of the estrogen receptor (ER) alpha. Destruction of the CRE-D1 motif caused complete loss of estrogen responsiveness. Both c-Jun and ATF-2 transactivated the cyclin D1 promoter in transient transfection experiments, and a clear additional increase was detected when ER was cotransfected with either c-Jun or with c-Jun and ATF-2 but not with ATF-2 alone. Furthermore, the expression of a dominant negative variant of c-Jun, TAM67, completely abolished the induction of the cyclin D1 promoter both in the absence and presence of ER. We show that ATF-2 homodimers and ATF-2/c-Jun heterodimers, but not c-Jun homodimers, were able to bind the CRE of the cyclin D1 promoter. To interpret these results, we propose a mechanism in which ATF-2/c-Jun heterodimers bind to the CRE-D1 element and mediate the activation of cyclin D1 promoter by the ER. This mechanism represents a pathway by which estrogens control the proliferation of target cells.

Selective binding of steroid hormone receptors to octamer transcription factors determines transcriptional synergism at the mouse mammary tumor virus promoter

Transcriptional synergism between glucocorticoid receptor (GR) and octamer transcription factors 1 and 2 (Oct-1 and Oct-2) in the induction of mouse mammary tumor virus (MMTV) transcription has been proposed to be mediated through directed recruitment of the octamer factors to their binding sites in the viral long terminal repeat. This recruitment correlates with direct binding between the GR DNA binding domain and the POU domain of the octamer factors. In present study, in vitro experiments identified several nuclear hormone receptors to have the potential to bind to the POU domains of Oct-1 and Oct-2 through their DNA binding domains, suggesting that POU domain binding may be a property shared by many nuclear hormone receptors. However, physiologically relevant binding to the POU domain appeared to be a property restricted to only a few nuclear receptors as only GR, progesterone receptor (PR), and androgen receptor (AR), were found to interact physically and functionally with Oct-1 and Oct-2 in transfected cells. Thus GR, PR, and AR efficiently promoted the recruitment of Oct-2 to adjacent octamer motifs in the cell, whereas mineralocorticoid receptor (MR), estrogen receptor alpha, and retinoid X receptor failed to facilitate octamer factor DNA binding. For MMTV, although GR and MR both induced transcription efficiently, mutation of the promoter proximal octamer motifs strongly decreased GR-induced transcription without affecting the total level of reporter gene activity in response to MR. These results suggest that the configuration of the hormone response element within the MMTV long terminal repeat may promote a dependence for the glucocorticoid response upon the recruitment of octamer transcription factors to their response elements within the viral promoter.

Glia as mediators of steroid hormone action on the nervous system: An overview

This special issue on steroids and glia represents the intersection of two emerging themes in the neurosciences: (a) Glia actively modulate and participate in brain function throughout life, and (b) glia are sensitive to steroid hormones. This overview begins by reviewing some of the basic principles of steroid hormone action on the brain and introducing the various glia that inhabit the peripheral and central nervous system. A prominent theme among the articles that follow is that glia may be direct targets for steroid hormones since they possess steroid receptors and the promoter region of glial-specific genes such as glutamine synthetase contain hormone-responsive elements. The articles in this special issue discuss evidence that glia may mediate steroid action on the nervous system in the context of (a) steroid metabolism, which may control the hormonal microenvironment of neurons both in the normal and injured brain; (b) brain development including sexual differentiation; (c) synaptic plasticity which may underlie the cyclic release of luteinizing hormone releasing hormone in the female rodent brain; (d) neural repair and aging; and (e) brain immune function. Another theme among these articles is that glia influence neurons via specific secreted and cell-surface molecules, and that steroids affect this mode of communication by altering the level of glial production of these signaling molecules and/or the sensitivity of neurons to such signals.

A nuclear action of the eukaryotic cochaperone RAP46 in downregulation of glucocorticoid receptor activity

RAP46 is a eukaryotic cochaperone that associates with several proteins, including the heat shock protein hsp70/hsc70 and the glucocorticoid receptor (GR). Here we show a downregulation of GR-mediated transactivation by RAP46 via a mechanism independent of a cytoplasmic action of this cochaperone. We demonstrate a specific cytoplasmic-nuclear recruitment of RAP46 by the liganded GR that results in inhibition of the transactivation function of the receptor. A repeated sequence motif [EEX(4)](8) at the NH(2) terminus of RAP46 or BAG-1L, a larger isoform of RAP46, is responsible for this downregulation of GR activity. BAG-1, a shorter isoform with only a duplication of the [EEX(4)] sequence, does not inhibit GR activity. The [EEX(4)](8) motif, when linked to an otherwise unrelated protein, abrogated the inhibitory action of endogenous RAP46 on GR-mediated transactivation. The nuclear effects of RAP46 and BAG-1L are specific since GR-mediated inhibition of AP-1 activity was not affected. These studies identify the [EEX(4)](8) sequence as a signature motif for inhibition of GR-mediated transactivation and demonstrate a specific nuclear action of a eukaryotic cochaperone in the regulation of GR activity.

The human fetal retinal pigment epithelium: A target tissue for thyroid hormones

Thyroid hormone (T(3)) has previously been shown to regulate visual function in experimental animals and humans. To determine if T(3) exerts direct effects on retinal function, cultured human fetal retinal pigment epithelial (RPE) cells were tested for the presence of thyroid hormone receptors (TRs) and T(3) responses. Using TR-isoform-specific reverse-transcriptase polymerase chain reaction techniques, mRNA was detected for alpha1, alpha2 and beta1 TR isoforms. Immunohistochemistry using a polyclonal antibody that simultaneously recognizes alpha1, alpha2 and beta1 TRs showed nuclear staining of the fetal RPE. Specific binding of (125)I-T(3) to RPE cell nuclear extracts was detected, and Scatchard analysis revealed a K(d) of 110 pM. To determine if RPE cells can respond to T(3), hyaluronic acid (HA) levels in cell culture media were measured after 2, 4 or 6 days of growth in medium containing 10(-7) M T(3). T(3) inhibited accumulation of HA in the cell culture medium of RPE cells. This effect was not evident at 2 days, but at 4 days there was 42.8% less HA in cell culture medium of RPE cells grown in 10(-7) M T(3) (p < 0.01, t test). The effect persisted through 6 days, when there was 46.3% less HA in cell culture medium of RPE cells grown in 10(-7) M T(3) (p < 0.001, t test). The data indicate that human fetal RPE cells are a direct target for thyroid hormones.

Expression of mutant thyroid hormone nuclear receptors in human hepatocellular carcinoma cells

To understand the expression and role of thyroid hormone nuclear receptors (TRs) in hepatocarcinogenesis, we characterized the TRs in 16 human hepatocellular carcinoma (HCC) specimens. The full-length cDNAs for the two TR subtypes, alpha1 and beta1, were cloned from several tumors by reverse transcription-polymerase chain reaction. Southern blot analysis indicated that, in addition to the full-length cDNA, truncated TRalpha1 and TRbeta1 cDNAs were present in nine tumors (53%). In addition, point mutations detected by the mismatch RNase cleavage assay in TRalpha1 and TRbeta1 were found in 65% and 76% of the tumors, respectively. The mutations were confirmed by DNA sequencing. Interestingly, most of the TRalpha1 mutations were in amino acid codons 209-228 and 245-256, two hot-spots in HCC patients. However, no hot-spot was detected in TRbeta1. The expression of TRalpha1 and TRbeta1 proteins was determined in the tissue extracts by western blotting. TRbeta1 protein was expressed or elevated in 10 tumors but not in normal livers, whereas the expression of TRalpha1 was variable among tumors. The mutant TR proteins were translated in vitro, and their hormone- and DNA-binding activities were evaluated. Abnormal binding to the thyroid hormone response elements was observed. The proteins' DNA binding activity was either partially impaired or completely lost. The high prevalence of TR mutations found in the tumors of patients with hepatocellular carcinoma suggests that mutant TRs could play an important role in liver carcinogenesis.

Development of selective aryl hydrocarbon receptor modulators for treatment of breast cancer

The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix DNA-binding protein that forms a transcriptionally-active heterodimer with the AhR nuclear translocator (Arnt) protein. The nuclear AhR complex is a ligand-induced transcription factor and the environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a high affinity ligand for the AhR. TCDD induces a diverse spectrum of tissue-, sex- and species-specific biochemical and toxic responses in Ah-responsive cells/tissues including the inhibition of 17beta-oestradiol (E2)-induced gene expression in the rodent uterus and mammary and in human breast cancer cell lines. TCDD also inhibits spontaneous and carcinogen-induced mammary tumour formation and growth in rodent models. Research in this laboratory has utilised the AhR as a target for developing anticancer drugs for treatment of breast cancer and two different structural classes of selective AhR modulators (SAhRMs) have been developed. Alternate-substituted (1,3,6,8- and 2,4,6,8-) alkyl polychlorinated dibenzofurans (PCDFs) and substituted diindolylmethanes (DIMs) bind the AhR and induce a pattern of AhR-oestrogen receptor (ER) inhibitory cross-talk similar to that observed for TCDD including inhibition of mammary tumour growth at doses < 1.0 mg/kg/day. In contrast, effective doses of these compounds do not induce hepatic CYP1A1-dependent activity or other AhR-mediated toxic responses induced by TCDD. These results indicate that SAhRMs may be an important new class of drugs for clinical treatment of breast cancer via AhR-ER inhibitory cross-talk.

Overlapping and divergent actions of estrogen and the neurotrophins on cell fate and p53-dependent signal transduction in conditionally immortalized cerebral cortical neuroblasts

The developing cerebral cortex undergoes overlapping periods of neurogenesis, suicide, and differentiation to generate the mature cortical plate. The following experiments examined the role of the gonadal hormone estrogen in comparison to the neurotrophins, in the regulation of p53-dependent cortical cell fate. To synchronize choices between neurogenesis, apoptosis, and neural differentiation, embryonic rat cerebral cortical neuroblasts were conditionally immortalized with the SV40 large T antigen containing the tsA58/U19 temperature-sensitive mutations. At the nonpermissive temperature, cessation of large T antigen expression was accompanied by induction of p53, as well as the p53-dependent proteins, wild-type p53-activated fragment-1/Cdk (cyclin-dependent kinase)-interacting protein-1 (p21/Waf1), Bcl (B-cell lymphoma)-associated protein (Bax), and murine double minute 2 (MDM2), that lead to cell cycle-arrest, suicide, and p53 inhibition, respectively. Simultaneously, neuroblasts exit cell cycle and die apoptotically or differentiate primarily into astrocytes and immature postmitotic neuroblasts. At the nonpermissive temperature, estrogen specifically induced an antagonist-independent increase in phosphorylated p53 expression, while increasing p21/Waf1 and decreasing Bax. Coincidentally, estrogen rapidly increased and then decreased MDM2 relative to controls, suggesting temporal modulation of p53 function. Both estrogen and neurotrophins prevented DNA fragmentation, a marker for apoptosis. However, estrogen also induced a transient increase in released lactate dehydrogenase, suggesting that estrogen simultaneously induced rapid cell death in a subpopulation of cells. In contrast to the neurotrophins, estrogen also increased cell proliferation. Both estrogen and the neurotrophins supported neuronal differentiation. However, in contrast to the neurotrophins, estrogen only supported the expression of a subset of oligodendrocytic markers. These results suggest that estrogen and the neurotrophins support overlapping and distinct aspects of differentiation in the developing cerebral cortex.

Multiple signal input and output domains of the 160-kilodalton nuclear receptor coactivator proteins

Members of the 160-kDa nuclear receptor coactivator family (p160 coactivators) bind to the conserved AF-2 activation function found in the hormone binding domains of nuclear receptors (NR) and are potent transcriptional coactivators for NRs. Here we report that the C-terminal region of p160 coactivators glucocorticoid receptor interacting protein 1 (GRIP1), steroid receptor coactivator 1 (SRC-1a), and SRC-1e binds the N-terminal AF-1 activation function of the androgen receptor (AR), and p160 coactivators can thereby enhance transcriptional activation by AR. While they all interact efficiently with AR AF-1, these same coactivators have vastly different binding strengths with and coactivator effects on AR AF-2. p160 activation domain AD1, which binds secondary coactivators CREB binding protein (CBP) and p300, was previously implicated as the principal domain for transmitting the activating signal to the transcription machinery. We identified a new highly conserved motif in the AD1 region which is important for CBP/p300 binding. Deletion of AD1 only partially reduced p160 coactivator function, due to signaling through AD2, another activation domain located at the C-terminal end of p160 coactivators. C-terminal coactivator fragments lacking AD1 but containing AD2 and the AR AF-1 binding site served as efficient coactivators for full-length AR and AR AF-1. The two signal input domains (one that binds NR AF-2 domains and one that binds AF-1 domains of some but not all NRs) and the two signal output domains (AD1 and AD2) of p160 coactivators played different relative roles for two different NRs: AR and thyroid hormone receptor.

Differential expression of tyrosine hydroxylase in catecholaminergic neurons of neonatal wild-type and Nurr1-deficient mice

The orphan nuclear receptor Nurr1 is a transcription factor that belongs to the steroid/thyroid hormone receptor superfamily and is expressed in many regions of the brain. To determine the physiological role of Nurr1, we previously generated mice with a null mutation in the Nurr1 gene. Nurr1-null mice appear to develop normally but die within 12 h after birth. Subsequent analysis revealed the absence of neurotransmitter dopamine and tyrosine hydroxylase immunoreactivity in the central dopaminergic area of newborn pups. Herein, using in situ hybridization histochemistry, we show that Nurr1 is expressed only in subset of catecholamine producing neurons (A2 partly, A8-A10 and A11 catecholaminergic cell groups), and is excluded from the norepinephrine producing neurons (A1, A2, A5-A6 catecholaminergic cell groups). Nurr1 was not expressed in the dopamine synthesizing cell groups (A12-A16 catecholaminergic cell groups) of the diencephalon and the olfactory bulb. As previously shown and confirmed in this study, tyrosine hydroxylase immunoreactivity was absent in the substantia nigra and ventral tegmental area of Nurr1-deficient mice. However, the loss of Nurr1 expression in A2 and A11 dopaminergic neurons did not affect their tyrosine hydroxylase immunoreactivity. This study begins to dissect cues necessary for understanding the complex regulation of the catecholaminergic biosynthetic pathway with regard to local, chemical and developmental changes in the brain.

The glucocorticoid receptor is essential for maintaining basal and dexamethasone-induced repression of the murine corticosteroid-binding globulin gene

We have investigated hepatic expression and glucocorticoid regulation of the corticosteroid-binding globulin (CBG) gene in mice lacking a functional glucocorticoid receptor (GR). GR-/- mice show impaired negative feedback in the hypothalamic-pituitary-adrenal axis, resulting in elevated circulating levels of ACTH and corticosterone. This is seen in the neonatal period and continues into adulthood where ACTH and corticosterone levels are increased up to 4-5 fold. Despite high elevation of corticosterone we find no change in mean arterial blood pressure in GR-/- mice and no change in the renal activity of the glucocorticoid-metabolising enzymes 11beta-hydroxysteroid dehydrogenase type-1 (HSD1) and type-2 (HSD2). We do find markedly increased hepatic expression of CBG with a 50% increase in plasma CBG levels. Increased expression of CBG was detected in adult GR-/- mice and also at birth with a greater than 10-fold increase in CBG hepatic mRNA in day-18.5 embryonic GR-/- mice. Adult GR-/- mice were also resistant to dexamethasone-induced repression of CBG expression in the liver. These results indicate that in mice, GR is essential for maintaining the basal level of CBG gene expression in the liver, and is also required for dexamethasone-induced repression of the CBG gene in the adult.

Steroid receptor coactivator-1 (SRC-1) enhances ligand-dependent and receptor-dependent cell-free transcription of chromatin

Progesterone receptor (PR) functions as a transcription factor that modulates the transcription of target genes in response to progesterone and other signals. The transcriptional activity of PR requires the involvement of coactivators such as steroid receptor coactivator-1 (SRC-1). To dissect the role of SRC-1 in PR transactivation, we established an in vitro transcription system with chromatin templates, in which PR induced transcription in a ligand-dependent and PRE-dependent manner. In the presence of ligand, purified PR bound to chromatin templates, resulting in chromatin remodeling. With this system, the ability of purified SRC-1 to act as a coactivator of PR was examined. SRC-1 potentiated transcription by ligand-activated PR, whereas it had no effect on transcription in the absence of ligands. As SRC-1 possesses intrinsic histone acetyltransferase activity, we tested the role of acetylation in PR-mediated transcription by using a histone deacetylase inhibitor, trichostatin A (TSA). We found that addition of TSA strongly enhanced PR-dependent transcription on chromatin but not on naked DNA template, and the effects of SRC-1 and TSA on PR transactivation were partially redundant. In addition, SRC-1 was able to potentiate PR transactivation with nonchromatin templates. Thus, our results substantiate a two-step mechanism whereby recruitment of coactivator SRC-1 by the ligand-activated PR in vivo leads to (i) chromatin remodeling through histone acetylation and (ii) recruitment/stabilization of the preinitiation complex.

Tumor susceptibility gene 101 protein represses androgen receptor transactivation and interacts with p300

BACKGROUND

Functional inactivation of the tsg101 gene in mouse fibroblasts leads to cell transformation and the ability to form metastatic tumors in nude mice. Abnormal TSG101 transcripts with highly-specific deletions in the protein-coding region have been identified in human tumor samples and cancer cell lines, including prostate and breast carcinomas, and have been attributed to alternative splicing of TSG101 mRNA. The function of the TSG101 protein is not known, although its predicted sequence has suggested that it may function as a transcription factor.

METHODS

Human TSG101 N-terminal (encoding amino acids 10-240) and C-terminal (encoding amino acids 230-391) fragments were cloned and used in both transient transfection and protein binding experiments. The transient transfections were carried in CV-1 cells. Protein-protein interactions were determined by both glutathione-S-transferase fusion protein binding and co-immunoprecipitation.

RESULTS

The N-terminal region of TSG101, when fused to the GAL4 DNA binding domain, can activate transcription; whereas the C-terminal region mediates transcriptional repression. Full-length TSG101 or its separated regions repressed ligand-dependent transcriptional activation by nuclear receptors, including androgen receptor and estrogen receptor, which play central roles in prostate carcinoma and breast carcinoma, respectively. In addition, a direct association between TSG101 and the transcriptional co-factor p300 was demonstrated in vitro and in vivo.

CONCLUSIONS

These results indicate that TSG101 can function as a transcription modulator to affect nuclear receptor-mediated transcriptional activation, which raises the possibility that the tumor suppression by TSG101 observed previously may be mediated at least in part by its effects on nuclear receptor function.

Analysis of estrogen response element binding by genetically selected steroid receptor DNA binding domain mutants exhibiting altered specificity and enhanced affinity

To analyze the role of amino acids in the steroid receptor DNA binding domain (DBD) recognition helix in binding of the receptor to the estrogen response element (ERE), we adapted the powerful P22 challenge phage selection system for use with a vertebrate protein. We used the progesterone receptor DNA binding domain and selected for mutants that gained the ability to bind to the ERE. We used a mutagenesis protocol based on degenerate oligonucleotides to create a large and diverse pool of mutants in which 10 nonconsensus amino acids in the DNA recognition helix of the progesterone receptor DNA binding domain were randomly mutated. After a single cycle of modified P22 challenge phage selection, 37 mutant proteins were identified, all of which lost the ability to bind to the progesterone response element. In gel mobility shift assays, approximately 70% of the genetically selected mutants bound to the consensus ERE with a >4-fold higher affinity than the naturally occurring estrogen receptor DBD. In the P-box region of the DNA recognition helix, the selected mutants contained the amino acids found in the wild-type estrogen receptor DBD, as well as other amino acid combinations seen in naturally occurring steroid/nuclear receptors that bind the aGGTCA half-site. We also obtained high affinity DBDs with Trp(585) as the first amino acid of the P-box, although this is not found in the known steroid/nuclear receptors. In the linker region between the two zinc fingers, G597R was by far the most common mutation. In transient transfections in mammalian cells using promoter interference assays, the mutants displayed enhanced affinity for the ERE. When linked to an activation domain, the transfected mutants activated transcription from ERE-containing reporter genes. We conclude that the P-box amino acids can display considerable variation and that the little studied linker amino acids play an important role in determining affinity for the ERE. This work also demonstrates that the P22 challenge phage genetic selection system, modified for use with a mammalian protein, provides a novel, single cycle selection for steroid/nuclear receptor DBDs with altered specificity and greatly enhanced affinity for their response elements.

Hormone-independent transcriptional activation and coactivator binding by novel orphan nuclear receptor ERR3

Orphan nuclear receptors share sequence homology with members of the nuclear receptor superfamily, but ligands are unknown or unnecessary. A novel orphan receptor, estrogen receptor-related protein 3 (ERR3), was identified by yeast two-hybrid screening, using the transcriptional coactivator glucocorticoid receptor interacting protein 1 (GRIP1) as bait. The putative full-length mouse ERR3 contains 458 amino acids and is closely related to two known orphan receptors ERR1 and ERR2. All the ERR family members share an almost identical DNA-binding domain, which has 68% amino acid identity with that of estrogen receptor. ERR3 bound specifically to an estrogen response element and activated reporter genes controlled by estrogen response elements, both in yeast and in mammalian cells, in the absence of any added ligand. A conserved AF-2 activation domain located in the hormone-binding domain of ERR3 was primarily responsible for transcriptional activation. The ERR3 AF-2 domain bound GRIP1 in a ligand-independent manner both in vitro and in vivo, through the LXXLL motifs of GRIP1, and GRIP1 functioned as a transcriptional coactivator for ERR3 in both yeast and mammalian cells. Expression of ERR3 in adult mouse was restricted; highest expression was observed in heart, kidney, and brain. In the mouse embryo no expression was observed at day 7, and highest expression occurred around the 11-15 day stages. Although ERR3 is much more closely related to ERR2 than to ERR1, the expression pattern for ERR3 was similar to that of ERR1 and distinct from that for ERR2, suggesting a unique role for ERR3 in development.

Antiglucocorticoid activity of hepatocyte nuclear factor-6

Glucocorticoids exert their effects on gene transcription through ubiquitous receptors that bind to regulatory sequences present in many genes. These glucocorticoid receptors are present in all cell types, yet glucocorticoid action is controlled in a tissue-specific way. One mechanism for this control relies on tissue-specific transcriptional activators that bind in the vicinity of the glucocorticoid receptor and are required for receptor action. We now describe a gene-specific and tissue-specific inhibitory mechanism through which glucocorticoid action is repressed by a tissue-restricted transcription factor, hepatocyte nuclear factor-6 (HNF-6). HNF-6 inhibits the glucocorticoid-induced stimulation of two genes coding for enzymes of liver glucose metabolism, namely 6-phosphofructo-2-kinase and phosphoenolpyruvate carboxykinase. Binding of HNF-6 to DNA is required for inhibition of glucocorticoid receptor activity. In vitro and in vivo experiments suggest that this inhibition is mediated by a direct HNF-6/glucocorticoid receptor interaction involving the amino-terminal domain of HNF-6 and the DNA-binding domain of the receptor. Thus, in addition to its known property of stimulating transcription of liver-expressed genes, HNF-6 can antagonize glucocorticoid-stimulated gene transcription.

The promoter of the rat 3-hydroxy-3-methylglutaryl coenzyme A reductase gene contains a tissue-specific estrogen-responsive region

The isoprenoid metabolic pathway is mainly regulated at the level of conversion of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) to mevalonate, catalyzed by HMG CoA reductase. As estrogens are known to influence cholesterol metabolism, we have explored the potential regulation of the HMG CoA reductase gene promoter by estrogens. The promoter contains an estrogen-responsive element-like sequence at position -93 (termed Red-ERE), which differs from the ERE consensus by one mismatch in each half of the palindrome. A Red-ERE oligonucleotide specifically bound estrogen receptor in vitro and conferred receptor-dependent estrogen responsiveness to a heterologous promoter in all cell lines tested. However, expression of a reporter driven by the rat HMG CoA reductase promoter was induced by estrogen treatment after transient transfection into the breast cancer cell line MCF-7 cells but not in hepatic cell lines expressing estrogen receptor. Estrogen induction in MCF-7 cells was dependent on the Red-ERE and was strongly inhibited by the antiestrogen ICI 164,384. A functional cAMP-responsive element is located immediately upstream of the Red-ERE, but cAMP and estrogens inhibit each other in terms of transactivation of the promoter. Similarly, induction by estrogens was inhibited by micromolar concentrations of cholesterol, likely acting via changes in occupancy of the sterol-responsive element located 70 bp upstream of the Red-ERE. Thus, within its natural context, Red-ERE is able to mediate hormonal regulation of the HMG CoA reductase gene in tissues that respond to estrogens with enhanced cell proliferation, while it is not operative in liver cells. We postulate that this tissue-specific regulation of HMG CoA reductase by estrogens could partially explain the protective effect of estrogens against heart disease.

Human TAF(II)55 interacts with the vitamin D(3) and thyroid hormone receptors and with derivatives of the retinoid X receptor that have altered transactivation properties

We have identified novel interactions between the human (h)TATA-binding protein-associated factor TAF(II)55 and the ligand-binding domains (LBDs) of the nuclear receptors for vitamin D(3) (VDR) and thyroid hormone (TRalpha). Following expression in Cos cells, hTAF(II)55 interacts with the VDR and TRalpha LBDs in a ligand-independent manner whereas no interactions with the retinoid X receptors (RXRs) or with other receptors were observed. Deletion mapping indicates that hTAF(II)55 interacts with a 40-amino-acid region spanning alpha-helices H3 to H5 of the VDR and TRalpha LBDs but not with the equivalent highly related region of RXRgamma. TAF(II)55 also interacts with chimeric receptors in which the H3-to-H5 region of RXRgamma has been replaced with that of the VDR or TRalpha. Furthermore, replacement of two single amino acids of the RXRgamma LBD with their VDR counterparts allows the RXRgamma LBD to interact with hTAF(II)55 while the corresponding double substitution allows a much stronger interaction. In transfection experiments, the single mutated RXRgamma LBDs activate transcription to fivefold higher levels than wild-type RXRgamma while the double mutation activates transcription to a level comparable to that observed with the VDR. There is therefore a correlation between the ability of the modified RXRs to interact with hTAF(II)55 and transactivation. These results strongly suggest that the TAF(II)55 interactions with the modified RXR LBDs modulate transcriptional activation.

Oestrogen receptors - an overview

The oestrogen receptor (ER) is a ligand-activated transcription factor that mediates the effects of the steroid hormone 17beta-oestradiol in both males and females. Since the isolation and cloning of ER, the prevailing opinion has been that only one such receptor exists. The finding of a second subtype of ER (ERbeta) has caused considerable excitement and has forced endocrinologists to re-evaluate many aspects of the actions of oestrogens. In this article, we will try to summarize the current knowledge about the two oestrogen receptor subtypes, with the emphasis on oestrogen receptor beta (ERbeta), and to comment on the observations in mice lacking either receptor or the hormone itself.

Purification and identification of p68 RNA helicase acting as a transcriptional coactivator specific for the activation function 1 of human estrogen receptor alpha

The estrogen receptor (ER) regulates the expression of target genes in a ligand-dependent manner. The ligand-dependent activation function AF-2 of the ER is located in the ligand binding domain (LBD), while the N-terminal A/B domain (AF-1) functions in a ligand-independent manner when isolated from the LBD. AF-1 and AF-2 exhibit cell type and promoter context specificity. Furthermore, the AF-1 activity of the human ERalpha (hERalpha) is enhanced through phosphorylation of the Ser(118) residue by mitogen-activated protein kinase (MAPK). From MCF-7 cells, we purified and cloned a 68-kDa protein (p68) which interacted with the A/B domain but not with the LBD of hERalpha. Phosphorylation of hERalpha Ser(118) potentiated the interaction with p68. We demonstrate that p68 enhanced the activity of AF-1 but not AF-2 and the estrogen-induced as well as the anti-estrogen-induced transcriptional activity of the full-length ERalpha in a cell-type-specific manner. However, it did not potentiate AF-1 or AF-2 of ERbeta, androgen receptor, retinoic acid receptor alpha, or mineralocorticoid receptor. We also show that the RNA helicase activity previously ascribed to p68 is dispensable for the ERalpha AF-1 coactivator activity and that p68 binds to CBP in vitro. Furthermore, the interaction region for p68 in the ERalpha A/B domain was essential for the full activity of hERalpha AF-1. Taken together, these findings show that p68 acts as a coactivator specific for the ERalpha AF-1 and strongly suggest that the interaction between p68 and the hERalpha A/B domain is regulated by MAPK-induced phosphorylation of Ser(118).

The gene expression of the amiloride-sensitive epithelial sodium channel alpha-subunit is regulated by antagonistic effects between glucocorticoid hormone and ras pathways in salivary epithelial cells

The functional expression of the amiloride-sensitive epithelial sodium channel (ENaC) in select epithelia is critical for maintaining electrolyte and fluid homeostasis. Although ENaC activity is strictly dependent upon its alpha-subunit expression, little is known about the molecular mechanisms by which cells modulate alpha-ENaC gene expression. Previously, we have shown that salivary alpha-ENaC expression is transcriptionally repressed by the activation of Raf/extracellular signal-regulated protein kinase pathway. Here, this work further investigates the molecular mechanism(s) by which alpha-ENaC expression is regulated in salivary epithelial Pa-4 cells. A region located between -1.5 and -1.0 kilobase pairs of the alpha-ENaC 5'-flanking region is demonstrated to be indispensable for the maximal and Ras-repressible reporter expression. Deletional analyses using heterologous promoter constructs reveal that a DNA sequence between -1355 and -1269 base pairs functions as an enhancer conferring the high level of expression on reporter constructs, and this induction effect is inhibited by Ras pathway activation. Mutational analyses indicate that full induction and Ras-mediated repression require a glucocorticoid response element (GRE) located between -1323 and -1309 base pairs. The identified alpha-ENaC GRE encompassing sequence (-1334/-1306) is sufficient to confer glucocorticoid receptor/dexamethasone-dependent and Ras-repressible expression on both heterologous and homologous promoters. This report demon- strates for the first time that the cross-talk between glucocorticoid receptor and Ras/extracellular signal-regulated protein kinase signaling pathways results in an antagonistic effect at the transcriptional level to modulate alpha-ENaC expression through the identified GRE. In summary, this study presents a mechanism by which alpha-ENaC expression is regulated in salivary epithelial cells.

Molecular mechanism for cooperation between Sp1 and steroidogenic factor-1 (SF-1) to regulate bovine CYP11A gene expression

Bovine cholesterol side-chain cleavage cytochrome P450 (P450scc; product of the CYP11A gene) gene expression is regulated by gonadotropins via cAMP in the ovary, and by ACTH via cAMP in adrenal cortical cells. Previously, we characterized response elements located at -57/-32 and at -111/-101 bp in the 5'-flanking region of the bovine CYP11A gene required for cAMP-stimulated transcription in both mouse Y-1 adrenal tumor cells and bovine ovarian cells in primary culture, which bind SF-1 (or Ad4-BP) and Sp1, respectively. The role of these transcription factors in CYP11A transcription was further confirmed by deletion and mutation analyses. In addition, results obtained employing a double mutation of the Sp1- and SF-1-binding sites and a mammalian two-hybrid system indicate that Sp1 and SF-1 function cooperatively in the transactivation of the bovine CYP11A promoter in both bovine luteal cells and Y-1 cells. Here we report that SF-1 and Sp1 are able to associate with one another in vitro and in vivo. The NH2-terminal region of SF-1, especially the DNA-binding domain, is the binding site for Sp1. In addition, as CBP is a common coactivator required for the transcriptional activity of numerous transcription factors including nuclear receptors, we investigated whether CBP functions as a cofactor for the regulation of bovine CYP11A promoter activity. We show here that CBP enhanced the PKA-induced CYP11A promoter activity, while a double mutation of both Sp1 and SF-1 sites within the CYP11A promoter region abolished CBP-induced activity. Furthermore, CBP stimulated Sp1-dependent transactivation, and a CBP/Sp1 complex in vivo was demonstrated by a co-immunoprecipitation assay. Also, CBP potentiated the transcriptional activity of GAL4-SF-1 in the presence of PKA. Thus, the cooperation between SF-1 and Sp1, required for the regulation of bovine CYP11A gene expression, is mediated by a direct protein-protein interaction and/or the common coactivator CBP.

The Glucocorticoid Receptor Cooperates With the Erythropoietin Receptor and c-Kit to Enhance and Sustain Proliferation of Erythroid Progenitors In Vitro

Although erythropoietin (Epo) is essential for the production of mature red blood cells, the cooperation with other factors is required for a proper balance between progenitor proliferation and differentiation. In avian erythroid progenitors, steroid hormones cooperate with tyrosine kinase receptors to induce renewal of erythroid progenitors. We examined the role of corticosteroids in the in vitro expansion of primary human erythroid cells in liquid cultures and colony assays. Dexamethasone (Dex), a synthetic glucocorticoid hormone, cooperated with Epo and stem cell factor to induce erythroid progenitors to undergo 15 to 22 cell divisions, corresponding to a 105- to 106-fold amplification of erythroid cells. Dex acted directly on erythroid progenitors and maintained the colony-forming capacity of the progenitor cells expanded in liquid cultures. The hormone delayed terminal differentiation into erythrocytes, which was assayed by morphology, hemoglobin accumulation, and the expression of genes characteristic for immature cells. Sustained proliferation of erythroid progenitors could be induced equally well from purified erythroid burst-forming units (BFU-E), from CD34+ blast cells, and from bone marrow depleted from CD34+ cells.

The Glucocorticoid Receptor Cooperates With the Erythropoietin Receptor and c-Kit to Enhance and Sustain Proliferation of Erythroid Progenitors In Vitro

Although erythropoietin (Epo) is essential for the production of mature red blood cells, the cooperation with other factors is required for a proper balance between progenitor proliferation and differentiation. In avian erythroid progenitors, steroid hormones cooperate with tyrosine kinase receptors to induce renewal of erythroid progenitors. We examined the role of corticosteroids in the in vitro expansion of primary human erythroid cells in liquid cultures and colony assays. Dexamethasone (Dex), a synthetic glucocorticoid hormone, cooperated with Epo and stem cell factor to induce erythroid progenitors to undergo 15 to 22 cell divisions, corresponding to a 105- to 106-fold amplification of erythroid cells. Dex acted directly on erythroid progenitors and maintained the colony-forming capacity of the progenitor cells expanded in liquid cultures. The hormone delayed terminal differentiation into erythrocytes, which was assayed by morphology, hemoglobin accumulation, and the expression of genes characteristic for immature cells. Sustained proliferation of erythroid progenitors could be induced equally well from purified erythroid burst-forming units (BFU-E), from CD34+ blast cells, and from bone marrow depleted from CD34+ cells.

p27(Kip1) induction and inhibition of proliferation by the intracellular Ah receptor in developing thymus and hepatoma cells

The Ah receptor (AhR), a bHLH/PAS transcription factor, mediates dioxin toxicity in the immune system, skin, testis and liver. Toxic phenomena are associated with altered cell proliferation or differentiation, but signaling pathways of AhR in cell cycle regulation are poorly understood. Here we show that AhR induces the p27(Kip1) cyclin/cdk inhibitor by altering Kip1 transcription in a direct mode without the need for ongoing protein synthesis or cell proliferation. This is the first example of Kip1 being a direct transcriptional target of a toxic agent that affects cell proliferation. Kip1 causes dioxin-induced suppression of 5L hepatoma cell proliferation because Kip1 antisense-expressing cells are resistant to dioxins. Kip1 is also induced by dioxins in cultures of fetal thymus glands concomitant with inhibition of proliferation and severe reduction of thymocyte recovery. Kip1 expression is likely to mediate these effects as thymic glands of Kip1-deficient mice (Kip1(Delta51)) are largely, though not completely, resistant.

Transcription activating and repressing functions of the androgen receptor are differentially influenced by mutations in the deoxyribonucleic acid-binding domain

Despite the wide spectrum of androgen receptor (AR) mutants described in androgen insensitivity syndromes (AIS), their influence on transactivating and, in particular, transrepressing functions of AR are poorly defined. Rat AR mutants with substitutions in the DNA-binding domain, corresponding to several mutations in AIS patients, were examined for these activities. AR variants (G551V and C562G) with mutations in the first zinc finger (ZF) exhibited reduced DNA binding activity and attenuated transactivation. An R590Q substitution in the second ZF diminished transcriptional activity only from a promoter with a single androgen response element, whereas activation at multiple androgen response element sites was unaffected, despite the poor DNA-binding affinity of R590Q. Another substitution in the second ZF, A579T, yielded similar findings. In comparison to wild-type AR, G551V, and C562G variants had markedly reduced ability to repress an NF-kappaB/RelA-activated promoter but R590Q behaved like the native receptor. AP1 function was repressed not only by wild-type AR but also by the transactivating mutants A579T and R590Q as well as by the transcriptionally inactive mutants G551V and C562G. Furthermore, a Lys-to-Ala substitution in codon 563 of the first ZF switched AR into a ligand-dependent activator at AP1 sites but maintained the ability to repress NF-kappaB/RelA function. Taken together, DNA-binding domain mutations in AIS patients influence transcriptional activating and repressing functions of AR in a selective fashion, which probably contributes to the complexity in the presentation of the AIS phenotype.

Regulation of transcription by a protein methyltransferase

The p160 family of coactivators, SRC-1, GRIP1/TIF2, and p/CIP, mediate transcriptional activation by nuclear hormone receptors. Coactivator-associated arginine methyltransferase 1 (CARM1), a previously unidentified protein that binds to the carboxyl-terminal region of p160 coactivators, enhanced transcriptional activation by nuclear receptors, but only when GRIP1 or SRC-1a was coexpressed. Thus, CARM1 functions as a secondary coactivator through its association with p160 coactivators. CARM1 can methylate histone H3 in vitro, and a mutation in the putative S-adenosylmethionine binding domain of CARM1 substantially reduced both methyltransferase and coactivator activities. Thus, coactivator-mediated methylation of proteins in the transcription machinery may contribute to transcriptional regulation.

Antagonistic action of novel 1alpha,25-dihydroxyvitamin D3-26, 23-lactone analogs on differentiation of human leukemia cells (HL-60) induced by 1alpha,25-dihydroxyvitamin D3

We examined the effects of two novel 1alpha,25-dihydroxyvitamin D3-26,23-lactone (1alpha,25-lactone) analogues on human promyelocytic leukemia cell (HL-60) differentiation using the evaluation system of the vitamin D nuclear receptor (VDR)/vitamin D-responsive element (DRE)-mediated genomic action stimulated by 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) and its analogues. We found that the 1alpha,25-lactone analogues (23S)-25-dehydro-1alpha-hydroxyvitamin-D3-26,23-lactone (TEI-9647), and (23R)-25-dehydro-1alpha-hydroxyvitamin-D3-26,23-lactone (TEI-9648) bound much more strongly to the VDR than the natural (23S, 25R)-1alpha,25(OH)2D3-26,23-lactone, but did not induce cell differentiation even at high concentrations (10(-6) M). Intriguingly, the differentiation of HL-60 cells induced by 1alpha,25(OH)2D3 was inhibited by either TEI-9647 or TEI-9648 but not by the natural lactone. In contrast, retinoic acid or 12-O-tetradecanoylphorbol-13-acetate-induced HL-60 cell differentiation was not blocked by TEI-9647 or TEI-9648. In separate studies, TEI-9647 (10(-7) M) was found to be an effective antagonist of both 1alpha,25(OH)2D3 (10(-8) M) mediated induction of p21(WAF1, CIP1) in HL-60 cells and activation of the luciferase reporter assay in COS-7 cells transfected with cDNA containing the DRE of the rat 25(OH)D3-24-hydroxylase gene and cDNA of the human VDR. Collectively the results strongly suggest that our novel 1alpha,25-lactone analogues, TEI-9647 and TEI-9648, are specific antagonists of 1alpha, 25(OH)2D3 action, specifically VDR/DRE-mediated genomic action. As such, they represent the first examples of antagonists, which act on the nuclear VDR.

Catecholamines participate in the induction of ornithine decarboxylase gene expression in normal and hyperplastic mouse kidney

In the quinazoline antifolate (CB 3717)-induced hyperplastic kidney model, a remarkable increase of ornithine decarboxylase (ODC) activity was paralleled by a smaller, but highly significant augmentation of the ODC transcript level. Catecholamine depletion, evoked by reserpine, strongly impaired antifolate-induced ODC expression; the enzyme activity was almost completely abolished while the mRNA level decreased by 60%. Moreover, under conditions of a depleted catecholamine pool, kidney enlargement was significantly reduced confirming our earlier reports on the indispensability of ODC induction for renal hyperplasia (M. Manteuffel-Cymborowska et al. , Biochim. Biophys. Acta, 1182 (1993) 133-141[1]). In normal mouse kidney catecholamines appeared to be inducers of ODC expression. Use of selective agonists of catecholamine receptors demonstrated the importance of dopamine D2 receptors, and to a lower extent beta adrenoreceptors, in the catecholamine mediation of induction of ODC activity and of ODC mRNA levels. These increases were not abolished by an antiandrogen, casodex, suggesting that catecholamine control of ODC expression is an androgen receptor-independent process. The results obtained point to the critical role of renal catecholamines; these biogenic amines are not only involved in the regulation of ODC expression in normal kidney but are also required for the induction of ODC in hyperplastic kidney evoked by antifolate and, as shown recently (M. Manteuffel-Cymborowska et al., Biochim. Biophys. Acta, 1356 (1997) 292-298[2]), in testosterone-induced hypertrophic kidney.

Identification of nuclear receptor corepressor as a peroxisome proliferator-activated receptor alpha interacting protein

Nuclear receptor corepressor (NCoR) was demonstrated to interact strongly with peroxisome proliferator-activated receptor alpha (PPARalpha), and PPARalpha ligands suppressed this interaction. In contrast to the interaction of PPARalpha with the coactivator protein, p300, association of the receptor with NCoR did not require any part of the PPARalpha ligand binding domain. NCoR was found to suppress PPARalpha-dependent transcriptional activation in the context of a PPARalpha.retinoid X receptor alpha (RXRalpha) heterodimeric complex bound to a peroxisome proliferator-responsive element in human embryonic kidney 293 cells. This repression was reversed agonists of either receptor demonstrating a functional interaction between NCoR and PPARalpha.RXRalpha heterodimeric complexes in mammalian cells. NCoR appears to influence PPARalpha signaling pathways and, therefore, may modulate tissue responsiveness to peroxisome proliferators.

Phosphorylation/dephosphorylation of androgen receptor as a determinant of androgen agonistic or antagonistic activity

Protein phosphorylation/dephosphorylation is an important posttranslational modification that plays a critical role in signal transduction. The androgen receptor (AR) is under such control. We demonstrate that androgen receptor phosphorylation determines whether or not AR ligands perform as agonists or antagonists in LNCaP cells. Androgen receptor ligands (such as dihydrotestosterone and beta-estradiol) stimulate receptor expression and phosphorylation and, as a result, they act as agonists or partial agonists. In contrast, agents such as bicalutamide and estramustine inhibit the receptor phosphorylation and act as antagonists. This model is supported by gene expression and transactivation assays. Significant increases in levels of both mRNA and protein of prostate-specific antigen (PSA), a natural AR target gene, occur following the treatment of LNCaP cells with DHT, beta-estradiol, or hydroxyflutamide. In contrast, exposure of LNCaP cells to bicalutamide or estramustine results in a sharp decrease of PSA expression. Agonistic or antagonistic effect of these compounds on PSA expression parallels the level of phosphorylated, but not dephosphorylated androgen receptors. These agonistic or antagonistic effects are also observed in HeLa cells transfected with wild-type AR expression plasmid (pAR0) and AR-driven luciferase expression plasmid GRE-tk-LUC in the presence of different groups of AR blockers. Our data indicate that the functional status of androgen receptors is strongly correlated with the phosphorylation status of the receptors, and that the phosphorylated androgen receptor is the form of the receptor transcriptionally active in regulation. Thus the androgen receptor phosphorylation/dephosphorylation may serve as a new molecular target for screening androgen antagonists for the treatment of prostate cancer.

P/CAF associates with cyclin D1 and potentiates its activation of the estrogen receptor

Cyclin D1 is overexpressed in a significant percentage of human breast cancers, particularly in those that also express the estrogen receptor (ER). We and others have demonstrated previously that experimentally overexpressed cyclin D1 can associate with the ER and stimulate its transcriptional functions in the absence of estrogen. This effect is separable from the established function of cyclin D1 as a regulator of cyclin-dependent kinases. Here, we demonstrate that cyclin D1 can also interact with the histone acetyltransferase, p300/CREB-binding protein-associated protein (P/CAF), thereby facilitating an association between P/CAF and the ER. Ectopic expression of P/CAF potentiates cyclin D1-stimulated ER activity in a dose-dependent manner. This effect is largely dependent on the acetyltransferase activity of P/CAF. These results suggest that cyclin D1 may trigger the activation of the ER through the recruitment of P/CAF, by providing histone acetyltransferase activity and, potentially, links to additional P/CAF-associated transcriptional coactivators.

Thioredoxin in the endocrine response to stress

Adaptation to stress evokes a variety of biological responses, including activation of the hypothalamic--pituitary--adrenal (HPA) axis and synthesis of a panel of stress-response proteins at cellular levels: for example, expression of thioredoxin (TRX) is significantly induced under oxidative conditions. Glucocorticoids, as a peripheral effector of the HPA axis, exert their action via interaction with a ligand-inducible transcription factor glucocorticoid receptor (GR). However, how these stress responses coordinately regulate cellular metabolism is still unknown. We demonstrate that either antisense TRX expression or cellular treatment with H2O2 negatively modulates GR function and decreases glucocorticoid-inducible gene expression. Impaired cellular response to glucocorticoids is rescued by overexpression of TRX, most probably through the functional replenishment of the GR. Moreover, not only the ligand binding domain but the DNA binding domain of the GR is also suggested to be a direct target of TRX. Together, we propose that cellular glucocorticoid responsiveness is coordinately modulated by redox state and TRX level, suggesting that cross-talk between neuro-endocrine control of stress responses and cellular antioxidant systems may be essential for mammalian adaptation processes.

A possible mechanism of TPA-mediated downregulation of neurotrophin-3 gene expression in rat cultured vascular smooth muscle cells

We have previously reported that in cultured rat vascular smooth muscle cells (VSMCs), neurotrophin-3 (NT-3) gene expression was suppressed by TPA (12-O-tetradecanoyl phorbol-13-acetate), which induces an AP-1 transcription factor. In the present study, to clarify the mechanism for TPA-mediated downregulation of NT-3 gene expression, effects of cycloheximide and dexamethasone (Dex) on the TPA-mediated downregulation were examined in VSMCs. Pretreatment with cycloheximide, an inhibitor of protein synthesis, or simultaneous treatment with Dex, an inhibitor of AP-1, suppressed the TPA-mediated downregulation of NT-3 gene expression. Furthermore, co-transfection of c-fos and c-jun expression vectors into VSMCs resulted in decrease in the NT-3 gene expression. The present findings suggest that TPA-induced AP-1 de novo synthesis causes the downregulation of NT-3 gene expression in VSMCs.