Molecular mechanisms of action of steroid/thyroid receptor superfamily members

Steroid receptor induction of gene transcription: a two-step model

Coactivators, such as steroid receptor coactivator 1 (SRC-1A) and CREB (cAMP response element binding protein)-binding protein (CBP), are required for efficient steroid receptor transactivation. Using an in vitro transcription assay, we found that progesterone receptor (PR)-driven transcription is inhibited by a dominant negative PR ligand-binding domain-interacting region of SRC-1A, indicating that SRC-1A is required for actual transcriptional processes. In addition, these coactivators also possess intrinsic histone acetyltransferase (HAT) activity and bind to each other and another HAT, p300/CBP-associated factor. Here we show that the human PR also interacts with p300/CBP-associated factor in vitro. Recruitment of multiple HATs to target promoters suggests an important role for chromatin remodeling in transcriptional activation of genes by steroid receptors. In transient transfection assays, we found that addition of a histone deacetylase inhibitor, trichostatin A, strongly potentiated PR-driven transcription. In contrast, directing histone deacetylase-1 (HD1) to a promoter using the GAL4 DNA binding domain inhibited transcription. Furthermore, PR transactivation was repressed by recruiting HD1 into the PR-DNA complex by fusing HD1 to a PR ligand-binding domain-interacting portion of SRC-1. Collectively, these results suggest that targeted histone acetylation by recruited HAT cofactors and histone deacetylation are important factors affecting PR transactivation. Recruitment of coactivators and HATs by the liganded PR in vivo may result in (i) remodeling of transcriptionally repressed chromatin to facilitate assembly and (ii) enhanced stabilization of the preinitiation complex by the activation functions of coactivators and the liganded PR itself.

Analysis of chromatin structure of rat alpha1-acid glycoprotein gene; changes in DNase I hypersensitive sites after thyroid hormone, glucocorticoid hormone and turpentine oil treatment

Transcription of the ratalpha1-acid glycoprotein (AGP) gene is activated by glucocorticoid, thyroid hormone (T3) and cytokines. Following these treatments, the chromatin structure of this gene was analyzed by means of digestion with DNase I or micrococcal nuclease. Four DNase I hypersensitive sites were observed in the 5'-upstream region of the rat AGP gene of liver cells. They were designated HS1, HS2, HS3 and HS4 (3'-->5'). After T3treatment the sensitivity of HS1 and HS2 increased and after dexamethasone (Dex) treatment that of all four sites did so. Three new sites appeared after turpentine oil treatment, while the sensitivities of HS3 and HS4 increased. We conclude that transcriptional activation of the gene by T3and Dex have very similar mechanisms, but that at the inflammation stage they become slightly different. The increase in sensitivity at HS1 and HS2 after T3treatment in vivo was successfully reproduced in a cell-free system by in vitro treatment with T3. HS1, HS2 and HS3 were also sensitive for micrococcal nuclease.

Identification of novel inducible genes in airway epithelium

DNA differential display analysis (DD-PCR) was utilized to identify genes that are expressed in airway epithelium and are relevant to airway inflammation; cytokine-mediated induction of gene expression and inhibition of that induction by glucocorticoids were the criteria for selection. The IB3-1 cell line was cultured in the presence of tumor necrosis factor-alpha (TNF-alpha), dexamethasone, or dimethyl sulfoxide (DMSO) as a control, and analyzed via DD-PCR and Northern blot analyses. With this approach, two TNF-alpha-inducible and dexamethasone (DEX)-sensitive expressed sequence tags (EST8 and EST19) were identified. In IB3-1 cells, TNF-alpha increased messenger RNA (mRNA) expression of EST8 (34%, P < or = 0.005) and EST19 (41%, P < or = 0.01), whereas dexamethasone reduced this expression to resting levels. This pattern of mRNA expression was also observed in normal human bronchial epithelial cells (EST8: 21%, P < or = 0.009; EST19: 11%, P < or = 0.02) and in the basophil leukemia cell line KU812 (EST8: 34%, P < or = 0.01). Through basic local alignment search tool (BLAST) analysis, it was determined that these ESTs exhibited significant homology with the monomeric G protein rhoC (EST8: 100% homology, P = 1.6 x 10(-100)) and the UFO tyrosine kinase receptor (EST19: 86% homology, 5.3 x 10(-28).

An N-terminally truncated third progesterone receptor protein, PR(C), forms heterodimers with PR(B) but interferes in PR(B)-DNA binding

Multiple progesterone receptor (PR) isoforms may explain in part the complex and diverse biological actions of progestins. Recent studies demonstrate that the human 116 kDa B-receptor (PR[B]) and the 94 kDa A-receptor (PR[A]) can have very different transcriptional functions that are cell- and promoter-specific. Additionally, we have shown the existence of a smaller N-terminally truncated 60 kDa progestin-specific binding protein, called the C-receptor (PR[C]), that has unique transcriptional potentiating properties. In the presence of the other two PR isoforms, PR(C) enhances the transcriptional activities of the larger PR proteins. In order to determine the mechanism of action for the transcriptional promoting abilities of PR(C), the structural and functional properties of PR(C) were analysed and compared to those of PR(A) and PR(B). PR(C) consistently displayed a dissociation constant (Kd) approximately 5 times higher than that for PR(B) and PR(A), suggesting that the N-terminal truncation of PR(C) results in a conformation different from the two larger PR isoforms, that affects the hormone-binding region and its interaction with hormone. Despite this change, PR(C) is still capable of forming heterodimers with the larger PR(B) in solution, as determined by co-immunoprecipitation studies, but PR(C) interferes in tight PR(B) binding to DNA in gel-shift assays. Surprisingly, progestin and antiprogestin autoregulation of PR(C) protein levels parallel those for PR(B) and PR(A).

The novel estrogen receptor-beta subtype: potential role in the cell- and promoter-specific actions of estrogens and anti-estrogens

The recent discovery that an additional estrogen receptor (ER) subtype is present in various rat, mouse and human tissues has advanced our understanding of the mechanisms underlying estrogen signalling. The discovery of a second ER subtype (ERbeta) suggests the existence of two previously unrecognised pathways of estrogen signalling: via the ERbeta subtype in tissues exclusively expressing this subtype and via the formation of heterodimers in tissues expressing both ER subtypes. Various models have been suggested as explanations for the striking cell- and promoter-specific effects of estrogens and anti-estrogens, all on the basis of the assumption that only a single ER gene exists. This minireview describes several of these models and focuses on the potential role which the novel ERbeta subtype might have in this regard.

An exported peptide functions intracellularly to contribute to cell density signaling in B. subtilis

Competence development and sporulation in B. subtilis are partly controlled by peptides that accumulate in culture medium as cells grow to high density. We constructed two genes that encode mature forms of two different signaling molecules, the PhrA peptide that stimulates sporulation, and CSF, the competence- and sporulation-stimulating factor. Both pentapeptides are normally produced by secretion and processing of precursor molecules. The mature pentapeptides were functional when expressed inside the cell, indicating that they normally need to be imported to function. Furthermore, at physiological concentrations (10 nM), CSF was transported into the cell by the oligopeptide permease encoded by spo0K (opp). CSF was shown to have at least three different targets corresponding to its three activities: stimulating competence gene expression at low concentrations, and inhibiting competence gene expression and stimulating sporulation at high concentrations.

The transcriptional co-activator p/CIP binds CBP and mediates nuclear-receptor function

The functionally conserved proteins CBP and p300 act in conjunction with other factors to activate transcription of DNA. A new factor, p/CIP, has been discovered that is present in the cell as a complex with CBP and is required for transcriptional activity of nuclear receptors and other CBP/p300-dependent transcription factors. The highly related nuclear-receptor co-activator protein NCoA-1 is also specifically required for ligand-dependent activation of genes by nuclear receptors. p/CIP, NCoA-1 and CBP all contain related leucine-rich charged helical interaction motifs that are required for receptor-specific mechanisms of gene activation, and allow the selective inhibition of distinct signal-transduction pathways.

Cloning and characterization of the vitamin D receptor from Xenopus laevis

The Vitamin D receptor (VDR), a member of the nuclear receptor superfamily, mediates the effects of 1,25-dihydroxyvitamin D3 on mineral ion homeostasis. Although the mammalian and avian VDRs have been extensively studied, little is known about the VDR in lower vertebrate species. To address this, we have isolated the Xenopus laevis VDR (xVDR) complementary DNA. Overall, the xVDR shares 79%, 73%, 73%, and 75% identity at the amino acid level with the chicken, mouse, rat, and human VDRs, respectively. The amino acid residues and subdomains important for DNA binding, hormone binding, dimerization, and transactivation are mostly conserved among all VDR species. The xVDR polypeptide can heterodimerize with the mouse retinoid X receptor alpha, bind to the rat osteocalcin vitamin D response element (VDRE), and induce vitamin D-dependent transactivation in transfected mammalian cells. Northern analysis reveals two xVDR messenger RNA species of 2.2 kb and 1.8 kb in stage 60 Xenopus tissues. In the adult, xVDR expression is detected in many tissues including kidney, intestine, skin, and bone. During Xenopus development, xVDR messenger RNA first appears at developmental stage 13 (pre-neurulation), increasing to maximum at stages 57-61 (metamorphosis). Our data demonstrate that, in Xenopus, VDR expression is developmentally regulated and that the vitamin D endocrine system is highly conserved during evolution.

Human TAF(II)135 potentiates transcriptional activation by the AF-2s of the retinoic acid, vitamin D3, and thyroid hormone receptors in mammalian cells

We report for the first time the cloning of a complete cDNA encoding the human TFIID subunit hTAF(II)135 (hTAF(II)130). Full-length hTAF(II)135 comprises 1083 amino acids and contains two conserved domains present also in dTAF(II)110 and hTAF(II)105. We show that expression of hTAF(II)135 in mammalian cells strongly and selectively potentiates transcriptional stimulation by the activation function-2 (AF-2) of the retinoic acid, thyroid hormone, and vitamin D3 receptors (RAR, TR, and VDR), but does not affect the AF-2s of the estrogen (ER) or retinoid X (RXR) receptors. The coactivator activity requires an hTAF(II)135 region that is located between the conserved domains but is itself not conserved in dTAF(II)110 and hTAF(II)105. Expression of hTAF(II)135 also stimulates RAR AF-2 activity when a promoter with a low-affinity TATA element (TGTA) is used, indicating that hTAF(II)135 overexpression compensates for the low-affinity of TBP for this promoter and may facilitate the recruitment of TFIID by the RAR AF-2.

In vivo protein-DNA interactions on a glucocorticoid response unit of a liver-specific gene: hormone-induced transcription factor binding to constitutively open chromatin

Transcription from the liver promoter of a 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) gene depends on the presence of glucocorticoids that act via a glucocorticoid response unit (GRU) located in the first intron. The promoter and the GRU are in a constitutively open chromatin configuration. To determine how glucocorticoids would affect factor binding to the GRU in absence of chromatin remodeling, we have used a combination of in vitro DNA-binding assays and in vivo genomic footprinting in rat hepatocytes and hepatoma cells. We found that, in the absence of glucocorticoids, the GRU binds nuclear factor-I (NF-I). Glucocorticoid treatment modified factor binding to the NF-I site and induced the binding of hepatocyte nuclear factor-3 (HNF-3). Transfection assays showed that HNF-3 cooperates with the glucocorticoid receptor in stimulating transcription. In contrast with the lack of effect of glucocorticoids on factor binding to constitutively open GRUs of other genes, HNF-3 binding to the open PFK-2 GRU was hormone-dependent. Therefore, the PFK-2 GRU behaves as a novel type of GRU.

Calreticulin inhibits glucocorticoid- but not cAMP-sensitive expression of tyrosine aminotransferase gene in cultured McA-RH7777 hepatocytes

Calreticulin is a ubiquitously expressed Ca2+ binding protein of the endoplasmic reticulum which inhibits DNA binding and transcriptional activation by steroid hormone receptors. In this study the effects of calreticulin on tyrosine aminotransferase (TAT) gene expression in cultured McA-RH7777 hepatocytes was investigated. McA-RH7777 cells were stably transfected with calreticulin expression vector to generate cells overexpressing the protein. The transcriptional activity of the TAT gene, which is glucocorticoid-sensitive and cAMP-dependent, was investigated in the mock transfected McA-RH7777 and in cells overexpressing calreticulin (designated McA-11 and McA-17). In the presence of dexamethasone or the cAMP analog (CTP-cAMP) expression of the TAT gene was induced in mock transfected McA-RH7777 cells by approximately 4.5 and 5 fold, respectively. In McA-11 and McA-17 cells, overexpressing calreticulin, glucocorticoid-sensitive expression of the TAT gene was significantly inhibited, however, the CTP-cAMP-dependent expression of the TAT gene was not affected. The ability of calreticulin to inhibit glucocorticoid-sensitive TAT gene expression but not the cAMP-dependent expression of the gene suggests that the protein affects specifically the action of transcription pathways involving steroid receptors or transcription factors containing KxFF(K/R)R-like motifs. Calreticulin may play an important role in the regulation of glucocorticoid-sensitive pathway of expression of the hepatocytes specific genes during development.

Analysis of the functional role of steroid receptor coactivator-1 in ligand-induced transactivation by thyroid hormone receptor

The nuclear hormone receptors belonging to the steroid/thyroid/retinoid receptor superfamily are ligand-inducible transcription factors. These receptors modulate transcription of specific cellular genes, either positively or negatively, by interacting with specific hormone response elements located near the target promoters. Recent studies indicated that the hormone- occupied, DNA-bound receptor acts in concert with a cellular coregulatory factor, termed coactivator, and the basal transcription machinery to mediate gene activation. Consistent with this scenario, a number of nuclear proteins with potential coactivator function have been isolated. In the present study, we demonstrate that steroid receptor coactivator-1 (SRC-1), a recently isolated candidate coactivator, functions as a positive regulator of the thyroid hormone receptor (TR)-mediated transactivation pathway. In transient transfection experiments, coexpression of SRC-1 significantly enhanced ligand-dependent transactivation of a thyroid hormone response element (TRE)-linked promoter by human TRbeta. Our studies revealed that deletion of six amino acids (451-456) in the extreme COOH-terminal region of TRbeta resulted in a receptor that retained the ability to bind T3 but failed to be stimulated by SRC-1. These six amino acids are part of an amphipathic helix that is highly conserved among nuclear hormone receptors and contains the core domain of the ligand-dependent transactivation function, AF-2. In agreement with this observation, in vitro protein binding studies showed that SRC-1 interacted with a ligand binding domain peptide (145-456) of TRbeta in a T3-dependent manner, whereas it failed to interact with a mutant ligand binding domain lacking the amino acids (451-456). We demonstrated that a synthetic peptide containing the COOH-terminal amino acids (437-456) of TRbeta efficiently blocked the ligand-induced binding of SRC-1 to the receptor. These results suggest that the conserved amphipathic helix that constitutes the AF-2 core domain of TRbeta is critical for interaction with SRC-1 and thereby plays a central role in coactivator-mediated transactivation. We further observed that a heterodimer of TRbeta and retinoid X receptor-alpha (RXR alpha), either in solution or bound to a DR+4 TRE, recruited SRC-1 in a T3-dependent manner. The AF-2 of TR was clearly involved in this process because a TR-RXR heterodimer containing a mutant TRbeta (1-450) with impaired AF-2 failed to bind to SRC-1. Surprisingly, the RXR-specific ligand 9-cis-retinoic acid induced binding of SRC-1 to the RXR component of the TRE-bound heterodimer. This novel finding suggests that RXR, as a heterodimeric partner of TR, has the potential to play an active role in transcriptional regulation. Our results raise the interesting possibility that a RXR-specific ligand may modulate T3-mediated signaling by inducing additional interactions between TRE-bound TR-RXR heterodimer and the coactivator.

Coactivator and corepressor regulation of the agonist/antagonist activity of the mixed antiestrogen, 4-hydroxytamoxifen

Mixed antiestrogens, such as 4-hydroxytamoxifen (4HT), act as either partial agonists or antagonists of estrogen receptor (ER) function in a tissue-, cell-, and promoter-specific manner, suggesting that intracellular factors modulate their ability to regulate transcription. To determine whether coactivators and corepressors have the capacity to modulate the relative agonist/antagonist activity of 4HT, ER-dependent gene expression was measured in the absence or presence of expression vectors for SRC-1 (steroid receptor coactivator-1) or SMRT (silencing mediator of retinoic acid and thyroid hormone receptors). In Hep G2 cells in which 4HT is an agonist, exogenous SRC-1 enhanced estradiol (E2)- and 4HT-stimulated transcription in a dose-dependent manner, while SMRT overexpression strongly reduced basal and 4HT-stimulated gene expression with no effect on E2 activity. These observations were not cell- or promoter-specific inasmuch as similar results were obtained in HeLa cells under conditions in which 4HT is an antagonist. A protein-protein interaction assay indicated that the full-length ER binds to SMRT in vitro. To assess whether relative coactivator and corepressor expression within a given cell could modulate the balance of 4HT agonist/antagonist activity, SRC-1 and SMRT were coexpressed. SMRT overexpression blocked SRC-1 coactivation of 4HT-stimulated gene expression and preferentially inhibited 4HT agonist activity whether or not exogenous SRC-1 was present. The cumulative data in this model system indicate that the relative expression of coactivators and corepressors can modulate 4HT regulation of ER transcriptional activity and suggest they could contribute to the tissue-specific ability of mixed antiestrogens to activate or inhibit ER-mediated gene expression.

Gene silencing by chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) is mediated by transcriptional corepressors, nuclear receptor-corepressor (N-CoR) and silencing mediator for retinoic acid receptor and thyroid hormone receptor (SMRT)

Chicken ovalbumin upstream promoter-transcription factors (COUP-TFs) are orphan receptors that belong to the steroid/thyroid hormone receptor (TR) superfamily and can repress the transcriptional activity of several target genes; however, the precise mechanism of this repression is unknown. Transfection of a Gal4 DNA-binding domain fused to the putative ligand-binding domain of COUP-TFI (Gal4-COUP-TFI) significantly represses the basal transcriptional activity of a reporter gene containing Gal4-binding sites. Cotransfection of COUP-TFI can relieve the Gal4-COUP-TFI repression in a dose-dependent manner. In contrast, COUP-TFI delta35, which lacks the repressor domain (the C-terminal 35 amino acids), fails to relieve this repression. This finding suggests that the repressor domain of COUP-TFI may squelch a limiting amount of corepressor in HeLa cells. In addition, increasing concentrations of TRbeta also can relieve the COUP-TFI repression in a hormone-sensitive manner. Similarly, overexpression of increasing concentration of COUP-TFI, but not COUP-TFI delta35, can squelch the silencing activity of the unliganded TRbeta. Collectively, these results indicate that COUP-TFI and TRbeta share a common corepressor(s) for their silencing activity. To determine which corepressor is involved in the COUP-TF-silencing activity, we used a yeast two-hybrid and in vitro GST pull-down assays to demonstrate that COUP-TFI can interact with the fragment of N-CoR (nuclear receptor-corepressor) encoding amino acids 921-2453 and the fragments of SMRT (silencing mediator for retinoic acid receptor and TR) encoding amino acids 29-564 and 565-1289, respectively. Interestingly, the fragment of SMRT encoding amino acids 1192-1495, which strongly interacts with TRbeta, interacts very weakly with COUP-TFI. Furthermore, overexpression of N-CoR or SMRT potentiates the silencing activity of COUP-TFI and can relieve the COUP-TFI-mediated squelching of Gal4-COUP-TFI activity. Therefore, our studies indicate that N-CoR and SMRT act as corepressors for the COUP-TFI silencing activity.

Oestrogen receptor variants and mutations in human breast cancer

Several oestrogen receptor variant and mutated mRNA species have been identified in human breast samples and cell lines. Over-expression and altered expression of some of these mRNAs have been correlated with breast tumourigenesis and progression. The following review focuses on the current knowledge available in the scientific literature with respect to the type and characteristics of oestrogen receptor variants and mutations that have been identified as occurring naturally in human breast tissues and cell lines.

Utilization of recombinant adenovirus and dominant negative mutants to characterize hepatocyte nuclear factor 4-regulated apolipoprotein AI and CIII expression

Using recombinant adenoviral vectors and a dominant negative mutant of HNF-4, we have examined the contribution of hepatocyte nuclear factor 4 (HNF-4) to endogenous apolipoprotein AI and CIII mRNA expression. Overexpression of HNF-4 leads to a 7.4-fold increase in apolipoprotein CIII expression, while infection with the dominant negative mutant of HNF-4 reduces the level of apolipoprotein CIII mRNA by 80%, demonstrating that endogenous HNF-4 is necessary for apolipoprotein CIII expression. Experiments using the hepatoma cell lines, HepG2 and Hep3B, indicate that HNF-4 is also involved in the regulation of apolipoprotein AI expression in these lines. However, the effect of HNF-4 on apolipoprotein AI expression is much more dramatic in cell lines derived from intestinal epithelium. Infection of the intestinal-derived cell line IEC-6 with the HNF-4 adenovirus resulted in a greater than 20-fold increase in the level of apolipoprotein AI mRNA. These results indicate that HNF-4 does regulate apolipoprotein AI and CIII mRNA expression and suggest that HNF-4 is critical for intestinal apolipoprotein AI expression.

Involvement of the N-terminal region of the human mineralocorticoid receptor hormone-binding domain in agonist and antagonist binding as revealed by a new monoclonal antibody

To gain a better understanding of the mechanism of binding to the human mineralocorticoid receptor (hMR), we developed a new monoclonal antibody (mAb) raised against the hormone-binding domain (HBD). For this purpose, mice were immunized with a fusion protein including the sequence Thr729-Lys984 of hMR. After ELISA screening, mAb 18C7 was selected for its specificity towards the HBD. This antibody recognized both the denatured and native MR forms, as well as the hetero-oligomeric MR form and the transformed MR state. By using several HBD subfragments, the mAb 18C7 epitope was located in the N-terminal region of the HBD from Thr729 to Leu765. We then studied the effect of the antibody on aldosterone and progesterone binding to the hMR. When 18C7 was incubated with liganded MR, it was able to partly displace (20%) the hormone from its binding site. When 18C7 was incubated with MR before aldosterone or progesterone, the antibody inhibited 75-80% of the binding. The effect of 18C7 on the binding was similar with both hormones. A sucrose gradient analysis indicated the simultaneous presence of two kinds of receptor complexes: the steroid-MR complex and the antibody-MR complex. After its associated proteins, especially the heat-shock protein hsp90, had been cross-linked with the hMR by dimethylpimelimidate, 18C7 was still able to react with the receptor. Our results indicated that the epitope recognized by 18C7 was directly implicated in hormone binding. The lack of steroid binding of HBD mutants with the Thr729-Leu765 sequence deleted [Jalaguier, Mesnier, Léger and Auzou (1996) J. Steroid Biochem. Mol. Biol. 57, 43-50] supports this hypothesis. Because of the similar behaviours of aldosterone and progesterone, we conclude that the N-terminal Thr729-Leu765 region of the HBD is similarly involved in the binding of both hormones.

Subnuclear trafficking of glucocorticoid receptors in vitro: chromatin recycling and nuclear export

We have used digitonin-permeabilized cells to examine in vitro nuclear export of glucocorticoid receptors (GRs). In situ biochemical extractions in this system revealed a distinct subnuclear compartment, which collects GRs that have been released from chromatin and serves as a nuclear export staging area. Unliganded nuclear GRs within this compartment are not restricted in their subnuclear trafficking as they have the capacity to recycle to chromatin upon rebinding hormone. Thus, GRs that release from chromatin do not require transit through the cytoplasm to regain functionality. In addition, chromatin-released receptors export from nuclei of permeabilized cells in an ATP- and cytosol-independent process that is stimulated by sodium molybdate, other group VI-A transition metal oxyanions, and some tyrosine phosphatase inhibitors. The stimulation of in vitro nuclear export by these compounds is not unique to GR, but is restricted to other proteins such as the 70- and 90-kD heat shock proteins, hsp70 and hsp90, respectively, and heterogeneous nuclear RNP (hnRNP) A1. Under analogous conditions, the 56-kD heat shock protein, hsp56, and hnRNP C do not export from nuclei of permeabilized cells. If tyrosine kinase inhibitors genistein and tyrphostin AG126 are included to prevent increased tyrosine phosphorylation, in vitro nuclear export of GR is inhibited. Thus, our results are consistent with the involvement of a phosphotyrosine system in the general regulation of nuclear protein export, even for proteins such as GR and hnRNP A1 that use distinct nuclear export pathways.

Identification of direct repeat 4 as a positive regulatory element for the human TR4 orphan receptor. A modulator for the thyroid hormone target genes

While the TR4 orphan receptor (TR4) is able to repress the expression of its target genes via its interaction with the direct repeat 1-hormone response element (DR1-HRE) and DR2-HRE, we now report that TR4 can also induce the transcriptional activity of the reporter gene containing a DR4-HRE via chloramphenicol acetyltransferase assay. Electrophoretic mobility shift assay and Scatchard analysis reveal a strong binding affinity (dissociation constant = 2 nM) between TR4 and DR4-HRE. The induction mediated by TR4 was detected not only in the synthetic DR4-HRE but also in some genes, such as rat alpha-myosin heavy-chain and S14 genes, containing the DR4 or DR4-like motif, which have been suggested to be the response elements for a thyroid hormone receptor. Our data also demonstrate this TR4-mediated gene induction is TR4 dose- and DR4 sequence-dependent. Together, our data suggest that DR4-HRE can be a positive regulatory element for TR4, which may be able to induce the transcriptional activity of the genes containing such positive HREs.

Differential interaction of nuclear receptors with the putative human transcriptional coactivator hTIF1

Hormonal regulation of gene activity is mediated by nuclear receptors acting as ligand-activated transcription factors. Intermediary factors interacting with their activation functions are required to mediate transcriptional stimulation. In search of such receptor interacting proteins, we have screened a human cDNA expression library and isolated a human protein that interacts in vitro with transcriptionally active estrogen receptors (ER). Sequence analysis reveals that this protein is the human homolog of mouse TIF1 (transcription intermediary factor 1) shown to enhance nuclear receptor ligand-dependent activation function 2 (AF2) in yeast. We have characterized the nuclear receptor binding site on hTIF1 and shown that a region of 26 residues is sufficient for hormone-dependent binding to the estrogen receptor. As shown by point mutagenesis, the AF2 activation domain of ER is required for the binding of hTIF1 but not sufficient, since a short region encompassing the conserved amphipathic alpha-helix corresponding to this domain fails to precipitate hTIF1. We also demonstrate that hTIF1 association with DNA-bound ER requires the presence of estradiol. Finally, we show that the interaction of hTIF1 with receptors is selective since strong in vitro hormone-dependent binding is only observed with some members of the nuclear receptor superfamily.

The DNA binding and activation domains of Gal4p are sufficient for conveying its regulatory signals

The transcriptional activation function of the Saccharomyces cerevisiae activator Gal4p is known to rely on a DNA binding activity at its amino terminus and an activation domain at its carboxy terminus. Although both domains are required for activation, truncated forms of Gal4p containing only these domains activate poorly in vivo. Also, mutations in an internal conserved region of Gal4p inactivate the protein, suggesting that this internal region has some function critical to the activity of Gal4p. We have addressed the question of what is the minimal form of Gal4 protein that can perform all of its known functions. A form with an internal deletion of the internal conserved domain of Gal4p is transcriptionally inactive, allowing selection for suppressors. All suppressors isolated were intragenic alterations that had further amino acid deletions (miniGAL4s). Characterization of the most active miniGal4 proteins demonstrated that they possess all of the known functions of full-length Gal4p, including glucose repression, galactose induction, response to deletions of gal11 or gal6, and interactions with other proteins such as Ga180p, Sug1p, and TATA binding protein. Analysis of the transcriptional activities, protein levels, and DNA binding abilities of these miniGal4ps and a series of defined internal mutants compared to those of the full-length Gal4p indicates that the DNA binding and activation domains are necessary and sufficient qualitatively for all of these known functions of Gal4p. Our observations imply that the internal region of Gal4 protein may serve as a spacer to augment transcription and/or may be involved in intramolecular or Gal4p-Gal4p interactions.

GRIP1, a transcriptional coactivator for the AF-2 transactivation domain of steroid, thyroid, retinoid, and vitamin D receptors

After binding to enhancer elements, transcription factors require transcriptional coactivator proteins to mediate their stimulation of transcription initiation. A search for possible coactivators for steroid hormone receptors resulted in identification of glucocorticoid receptor interacting protein 1 (GRIP1). The complete coding sequence for GRIP1, isolated from a mouse brain cDNA library, contains an open reading frame of 1,462 codons. GRIP1 is the probable ortholog of the subsequently identified human protein transcription intermediary factor 2 (TIF2) and is also partially homologous to steroid receptor coactivator 1 (SRC-1). The full-length GRIP1 interacted with the hormone binding domains (HBDs) of all five steroid receptors in a hormone-dependent manner and also with HBDs of class II nuclear receptors, including thyroid receptor alpha, vitamin D receptor, retinoic acid receptor alpha, and retinoid X receptor alpha. In contrast to agonists, glucocorticoid antagonists did not promote interaction between the glucocorticoid receptor and GRIP1. In yeast cells, GRIP1 dramatically enhanced the transcriptional activation function of proteins containing the HBDs of any of the above-named receptors fused to the GAL4 DNA binding domain and thus served as a transcriptional coactivator for them. This finding contrasts with previous reports of TIF2 and SRC-1, which in mammalian cells enhanced the transactivation activities of only a subset of the steroid and nuclear receptors that they physically interacted with. GRIP1 also enhanced the hormone-dependent transactivation activity of intact glucocorticoid receptor, estrogen receptor, and mineralocorticoid receptor. Experiments with glucocorticoid receptor truncation and point mutants indicated that GRIP1 interacted with and enhanced the activity of the C-terminal AF-2 but not the N-terminal AF-1 transactivation domain of the glucocorticoid receptor. These results demonstrate directly that AF-1 and AF-2 domains accomplish their transactivation activities through different mechanisms: AF-2 requires GRIP1 as a coactivator, but AF-1 does not.

William L. McGuire Memorial Lecture. Antiestrogens: mechanisms of action and resistance in breast cancer

Antiestrogens have proven to be highly effective in the treatment of hormone-responsive breast cancer. However, resistance to antiestrogen therapy often develops. In addition, although tamoxifen-like antiestrogens are largely inhibitory and function as estrogen antagonists in breast cancer cells, they also have some estrogen-like activity in other cells of the body. Thus, recent efforts are being directed toward the development of even more tissue-selective antiestrogens, i.e. compounds that are antiestrogenic on breast and uterus while maintaining the beneficial estrogen-like actions on bone and the cardiovascular system. Efforts are also being directed toward understanding ligand structure-estrogen receptor (ER) activity relationships and characterizing the molecular changes that underlie alterations in parallel signal transduction pathways that impact on the ER. Recent findings show that antiestrogens, which are known to exert most of their effects through the ER of breast cancer cells, contact a different set of amino acids in the hormone binding domain of the ER than those contacted by estrogen, and evoke a different receptor conformation that results in reduced or no transcriptional activity on most genes. Resistance to antiestrogen therapy may develop due to changes at the level of the ER itself, and at pre- and post-receptor points in the estrogen receptor-response pathway. Resistance could arise in at least four ways: (1) ER loss or mutation; (2) Post-receptor alterations including changes in cAMP and phosphorylation pathways, or changes in coregulator and transcription factor interactions that affect the transcriptional activity of the ER; (3) Changes in growth factor production/sensitivity or paracrine cell-cell interactions; or (4) Pharmacological changes in the antiestrogen itself, including altered uptake and retention or metabolism of the antiestrogen. Model cell systems have been developed to study changes that accompany and define the antiestrogen resistant versus sensitive breast cancer phenotype. This information should lead to the development of antiestrogens with optimized tissue selectivity and agents to which resistance may develop more slowly. In addition, antiestrogens which work through somewhat different mechanisms of interaction with the ER should prove useful in treatment of some breast cancers that become resistant to a different category of antiestrogens.

Coordination of larval and prepupal gene expression by the DHR3 orphan receptor during Drosophila metamorphosis

The DHR3 orphan receptor gene is induced directly by the steroid hormone ecdysone at the onset of Drosophila metamorphosis. DHR3 expression peaks in early prepupae, as the early puff genes are repressed and betaFTZ-F1 is induced. Here we provide evidence that DHR3 directly contributes to both of these regulatory responses. DHR3 protein is bound to many ecdysone-induced puffs in the polytene chromosomes, including the early puffs that encode the BR-C and E74 regulatory genes, as well as the E75, E78 and betaFTZ-F1 orphan receptor loci. Three DHR3 binding sites were identified downstream from the start site of betaFTZ-F1 transcription, further indicating that this gene is a direct target of DHR3 regulation. Ectopic expression of DHR3 revealed that the polytene chromosome binding pattern is of functional significance. DHR3 is sufficient to repress BR-C, E74A, E75A and E78B transcription as well as induce betaFTZ-F1. DHR3 thus appears to function as a switch that defines the larval-prepupal transition by arresting the early regulatory response to ecdysone at puparium formation and facilitating the induction of the betaFTZ-F1 competence factor in mid-prepupae. This study also provides evidence for direct cross-regulation among orphan members of the nuclear receptor superfamily and further implicates these genes as critical transducers of the hormonal signal during the onset of Drosophila metamorphosis.

A steroid-triggered switch in E74 transcription factor isoforms regulates the timing of secondary-response gene expression

The steroid hormone 20-hydroxyecdysone (referred to here as ecdysone) directs Drosophila metamorphosis by activating a series of genetic regulatory hierarchies. ETS domain transcription factors encoded by the ecdysone-inducible E74 early gene, E74A and E74B, act at the top of these hierarchies to coordinate the induction of target genes. We have ectopically expressed these E74 isoforms to understand their regulatory functions during the onset of metamorphosis. We show that E74 can regulate its own transcription, most likely through binding sites within its gene. Ectopic expression of E74B can partially repress the E78B and DHR3 orphan receptor genes, suggesting a role for E74 in the appropriate timing of early-late gene expression. Furthermore, E74A is both necessary and sufficient for E78B induction, implicating E74A as a key regulator of E78B expression. We also show, consistent with our studies of E74 loss-of-function mutations, that E74B is a potent repressor of late gene transcription and E74A is sufficient to prematurely induce the L71-1 late gene. However, ectopic expression of both Broad-Complex and E74A activators in an E74B mutant background is not sufficient to prematurely induce all late genes, indicating that other factors contribute to this regulatory circuit. These observations demonstrate that the steroid-triggered switch in E74 transcription factor isoforms plays a central role in the proper timing of secondary-response gene expression.

Differential phosphorylation of chicken progesterone receptor in hormone-dependent and ligand-independent activation

Many steroid receptors, including chicken progesterone receptor, have been shown to be activated in the absence of their cognate ligands by modulators of kinases and phosphatases. To investigate the molecular mechanism of ligand-independent activation, chicken progesterone receptor mutants in which either one or all four of the previously identified phosphorylation sites have been changed to nonphosphorylatable alanine were analyzed for their ability to be activated by progesterone, 8-bromoadenosine 3':5'-cyclic monophosphate, or a dopamine agonist, SKF82958. Our current study shows that the receptor is differently phosphorylated in ligand-dependent and ligand-independent activation. The transcriptional activity of the receptor in response to 8-bromoadenosine 3':5'-cyclic monophosphate is affected by mutation of either Ser211 or Ser260. In addition, our data demonstrated that none of the four sites is absolutely required for the activation of the receptor by either 8-bromoadenosine 3':5'-cyclic monophosphate or the dopamine agonist. Treatment with 8-bromoadenosine 3':5'-cyclic monophosphate did not increase the overall level of receptor phosphorylation or cause phosphorylation of the receptor at alternate sites. These data raise the possibility that ligand-independent activation of the chicken progesterone receptor may be mediated through changes in the phosphorylation of coregulators or other protein factors interacting with the receptors.

Conservation of function between mammalian and plant steroid 5alpha-reductases

Arabidopsis det2 mutants are small dark-green dwarfs displaying pleiotropic defects in light-regulated development during multiple stages of the plant life cycle. The DET2 gene encodes a protein that shares approximately 40% sequence identity with mammalian steroid 5alpha-reductases and is implicated in the synthesis of a class of plant steroids, the brassinosteroids. Here we show that the DET2 protein, when expressed in human embryonic kidney 293 cells, catalyzes the 5alpha-reduction of several animal steroid substrates and has similar kinetic properties to the mammalian steroid 5alpha-reductase enzymes. Moreover, human steroid 5alpha-reductases expressed in det2 mutant plants can substitute for DET2 in brassinosteroid biosynthesis. These data indicate that DET2 is an ortholog of the mammalian steroid 5alpha-reductases and provide further evidence that brassinosteroids play an essential role in light-regulated plant development. The structural and functional conservation between DET2 and human steroid 5alpha-reductases raise interesting issues concerning the evolutionary origin of the steroid hormone signaling system.

The Caenorhabditis elegans orphan nuclear hormone receptor gene nhr-2 functions in early embryonic development

We have identified a Caenorhabditis elegans gene, nhr-2, that is a member of the nuclear hormone receptor superfamily of transcription factors and defines a new subclass of the superfamily. nhr-2 messenger RNA is expressed in the maternal germline and during the first half of embryogenesis. Zygotic expression of nhr-2 begins by the 16-cell stage, making it one of the earliest genes known to be transcribed in the embryo. Immunolocalization detects NHR-2 protein in embryonic nuclei as early as the 2-cell stage. The protein is present in every nucleus until the 16- to 20-cell stage. Subsequently, expression continues in many, but not all, cell lineages, becoming progressively restricted to the anterior and dorsal regions of the embryo and disappearing during the initial stages of morphogenesis. Disruption of nhr-2 function with antisense RNA results in embryonic and early larval arrest, indicating that the gene has an essential function in embryonic development. nhr-2 does not correspond to known mutations mapped to the same genetic interval, and will provide an entry point for further study of a heretofore uncharacterized zygotic gene regulatory pathway.

Characterization of rainbow trout (Oncorhynchus mykiss) growth hormone 1 gene and the promoter region of growth hormone 2 gene

Studies by Agellon et al. (Mol. Reprod. Dev. 1, 11-17) showed the presence of two growth hormone (rtGH1 and rtGH2) mRNA species in pituitary glands of adult rainbow trout (Oncorhynchus mykiss). In this study, we have detected rtGH1 and rtGH2 mRNAs in pituitary glands of rainbow trout from fry to 2 years of age. The level of rtGH1 mRNA is notably higher than that of rtGH2 mRNA in 10-day-old fry and 2-year-old females. These results suggest differential expression of rtGH1 and rtGH2 genes in different sexes and developmental stages. As a step toward elucidating the mechanism of differential expression of both GH genes, DNA fragments encoding rtGH1 gene and the promoter/regulatory region of rtGH2 gene were isolated and characterized. Rainbow trout GH genes span approximately 4.5 kb and are composed of six exons and five introns. The 5'-flanking region of both genes contain consensus sequences for TATA boxes and several Pit-1 binding sequences. Consensus sequences related to the cAMP response element, thyroid hormone response element, retinoic acid response element, estrogen response element (ERE), and glucocorticoid response element are present not only in the 5'-flanking region, but also in introns and exons in rtGH1 gene. These hormone response elements, except ERE, are also present in rtGH2 gene.

5'-flanking sequences in thyroid hormone response element half-sites determine the requirement of retinoid X receptor for receptor-mediated gene expression

Thyroid hormone receptors are ligand-inducible transcription factors that can potentially interact with thyroid hormone response elements as homodimers or heterodimers with the retinoid X receptor. It has generally been felt, however, that the heterodimer is responsible for induction of gene expression. We have demonstrated previously that the optimal thyroid hormone receptor binding sequence is not the consensus hexamer half-site AGGTCA but is an octamer, TAAGGTCA. Based upon these findings, we hypothesize that thyroid hormone response elements composed of optimal half-sites (TAAGGTCA) will bind thyroid hormone receptors readily and activate gene expression independently of the retinoid X receptor. In contrast, response elements composed of suboptimal half-sites (e.g. GCAGGTCA) will require the retinoid X receptor to facilitate thyroid hormone receptor-mediated gene expression. To test this hypothesis, we have reconstituted thyroid hormone receptor-mediated gene expression in yeast. Our studies confirm the hypothesis that the retinoid X receptor is required for gene expression from response elements composed of suboptimal half-sites, whereas thyroid hormone receptors are sufficient to activate gene expression maximally from response elements containing optimal half-sites. Furthermore, coexpression of steroid receptor coactivator-1 is required for ligand-dependent gene activation from single response elements. Surprisingly, however, coexpression of the retinoid X receptor decreases the steroid receptor coactivator-1-dependent thyroid hormone induction. Overall these data demonstrate that the architecture of the thyroid hormone response element dictates the nuclear receptor requirements for gene activation. The studies suggest that different coactivators may be required for gene activation depending upon the response element architecture and the nature of the bound thyroid hormone receptor complex (homo- versus heterodimer).

Characterization of a negative response DNA element in the upstream region of the cellular retinoic acid-binding protein-I gene of the mouse

A negative, regulatory DNA element from the mouse cellular retinoic acid-binding protein I gene promoter was identified. This DNA element, located approximately 1 kilobase upstream from the transcription initiation site of this gene, contained a pair of direct repeats (DRs) separated by 4 base pairs (DR4, TGACCTTTGGGGACCT). By examining a series of reporters deleted or mutated within this DR4 region, it was concluded that the core sequence of this DR4, including both repeats and the spacer, was required for suppressive activity in the mouse embryonal carcinoma cell line P19. From gel retardation experiments, it was concluded that both repeated sequences were essential for specific protein binding, but the spacer sequence was not as critical. Specific residues required for protein binding to this DR4 were identified. In P19 cells, retinoic acid induced the binding of nuclear factors to DR4 and suppressed the activities of the reporters containing this DR4. Co-expression of retinoic acid receptor beta or thyroid hormone receptor beta1 (T3Rbeta1) significantly inhibited the expression of this reporter in P19 cells. Gel retardation with in vitro-synthesized nuclear receptors demonstrated specific binding of this DR4 by T3Rbeta1 monomers, homodimers, or heterodimers of T3Rbeta1/retinoid receptor X beta. A biological function of DR4 in crabp-I gene regulation in P19 cells was suggested.

Neurotransmitters activate the human estrogen receptor in a neuroblastoma cell line

The human neuroblastoma cell line SK-N-SH has been used as a model system to study the interactions of the human estrogen receptor (hER) with neurotransmitters. We have successfully transfected these cells using an adenoviral delivery system and have reconstituted ligand-dependent responses to estradiol and ligand-independent responses to a series of dopamine D1 receptor agonists. The full agonist for the D1 receptor, SKF 82958, shows a robust activation of hER, comparable to that induced by estradiol. This activation is blocked by the protein kinase A inhibitor H-89, is mimicked by forskolin, and is therefore thought to be mediated in part through the cAMP/protein kinase A pathway. We have examined deletion mutants of hER for activation by SKF 82958 and find that both its transactivation domains, AF-1 and AF-2, must cooperate to impart the full response to the agonist. Significantly, an agonist of the muscarinic acetylcholine receptor, carbachol, though not active by itself, synergistically activates hER in conjunction with suboptimal doses of SKF 82958. This is the first reported instance of two neurotransmitters synergizing to activate a member of the nuclear receptor superfamily, and might predict a role for multiple neural inputs modulating the effects of these receptors in the central nervous system.

Wide variation in androgen receptor dysfunction in complete androgen insensitivity syndrome

Androgen insensitivity syndrome (AIS) is a disorder of male sexual differentiation caused by mutations in the androgen receptor (AR) gene. The partial form (PAIS), associated with varying degrees of receptor dysfunction, presents with a range of undervirilization phenotypes. The complete form (CAIS) is characterized by normal female external appearance at birth. In these cases the receptor is often absent or inactive. However, cases have been described where the mutant receptor concerned has considerable residual activity in in vitro assays. Here we describe the effects of five mutations, Gly750Asp, Leu762Phe, Ala765Thr, Asp864Asn and Leu907Phe, identified in complete androgen insensitivity patients. In vitro assays of mutant androgen receptors expressed in a mammalian cell line showed that the Gly750Asp, Leu762Phe and Ala765Thr mutations cause almost complete loss of androgen-binding activity, suggesting that these residues are critical for ligand binding. However, receptors with Asp864Asn and Leu907Phe, although defective, were capable of considerable binding and transactivation activity. Given that some mutations identified in PAIS patients have a more severe effect on androgen receptor function than two CAIS mutations described here, these results provide further evidence that other factors, including genetic background, can have a significant impact on the phenotype associated with a particular AR mutation.

Estrogen receptor is not primarily responsible for altered responsiveness of ovalbumin mRNA induction in the oviduct from genetically selected high- and low-albumen chicken lines

The role of estrogen receptor on ovalbumin mRNA induction by steroid hormones was investigated in primary cultures of oviduct cells from estrogen-stimulated immature chicks of genetically selected high- and low-albumen egg laying lines (H- and L-lines). In experiment 1, the extent of ovalbumin mRNA induction and changes in estrogen and progesterone receptors were compared between the oviduct cells from H- and L-lines with or without steroid hormones in the culture medium. In experiment 2, the effect of estrogen receptor gene transfection on the induction of ovalbumin mRNA was studied in the oviduct cells from the L-line chicks. The results showed a close correlation of the changes in ovalbumin mRNA with the numbers of nuclear and total estrogen receptors in the oviduct cells but not with the numbers of nuclear and total progesterone receptors. Estrogen receptor gene transfection induced ovalbumin mRNA to a moderate extent in the absence of the steroid hormones. To our surprise, however, estrogen receptor gene transfection apparently suppressed the ovalbumin mRNA responsiveness to estrogen to a considerable extent. It was concluded, therefore, that the extent of estrogen receptor expression might not be primarily responsible for the differences in responsiveness to steroid hormones of oviduct cells from genetically selected H- and L-line chickens.

Functional characterization and purification of an intracellular vitamin D-binding protein in vitamin D-resistant new world primate cells. Amino acid sequence homology with proteins in the hsp-70 family

Most genera of New World primates exhibit resistance to vitamin D. These monkeys harbor high circulating concentrations of the prohormone 25-hydroxyvitamin D and the active vitamin D hormone 1, 25-dihydroxyvitamin D. Previous work from this laboratory indicated that resistance is associated with the overexpression of a 60-65-kDa intracellular protein that binds vitamin D metabolites competitively. In the current studies 25-[3H]hydroxyvitamin D3 (25-OHD3) was used as a competitive ligand to investigate the ability of a number of small lipid molecules to interact with this intracellular vitamin D-binding protein (IDBP) in post-nuclear extracts of a prototypical lymphoblast cell line from the common marmoset, a vitamin D-resistant New World primate. Only those vitamin D metabolites with a hydroxyl moiety in the C-25 position were bound by IDBP. Disruption of the C-25 hydroxyl obviated binding, whereas more proximal alterations in the vitamin D side chain did not. Modifications in the A-ring of 25-hydroxylated vitamin D metabolites, most specifically hydroxylation of C-1, diminished but did not abolish ligand binding. Of more than two dozen other small lipid molecules examined, only the C-19 17-hydroxysteroids, 17beta-estradiol and testosterone, and the C-21 steroid progesterone were found to be capable of binding specifically to IDBP. Using a combination of physical and serial chromatographic techniques, we enriched IDBP 25-OHD3 binding activity 17,588-fold in extracts of B95-8 cells. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis of this purified fraction demonstrated a predominant 65-kDa molecular species with a pI approximately 4.5. Seven different peptide fragments were isolated from the 65-kDa protein, each possessing sequence similarity to the hsp-70 family of proteins. Ligand binding analyses confirmed that human inducibly expressed hsp-70-bound 25-OHD3 with approximately similar affinity ( approximately 10(-7) M) as did purified IDBP. In summary, these results suggest a novel action for the hsp-70 family of proteins as intracellular vitamin D- and gonadal steroid hormone-binding molecules.

Ligand-dependent interaction between the estrogen receptor and the human homologues of SWI2/SNF2

The human SNF2alpha (or hbrm) and SNF2beta (or BRG1) proteins have previously been shown to enhance transcriptional activation by nuclear receptors (NRs) in cultured human cells, and to be present in SWI/SNF complexes which are thought to be involved in control of transcription by facilitating remodelling of chromatin templates. Using the yeast two-hybrid system, we now demonstrate that the N-terminal regions of hSNF2alpha and hSNF2beta, preceding the DNA-dependent ATPase domain, specifically interact with the region of the estrogen receptor (ER) which includes the ligand binding domain and the ligand-dependent activation function AF-2. These interactions are increased by estrogen, but not by the ER AF-2 antagonist hydroxytamoxifen. Furthermore, mutants of ER that lack AF-2 activity are unable to interact with hSNF2alpha and -beta. These results suggest that the human homologues of the yeast SWI2/SNF2 protein may participate in the enhancement of transcription by the ER in vivo through interactions with the AF-2 activating domain, thus leading to ligand-dependent remodelling of chromatin templates.

Cloning and structural organization of the gene encoding the murine nuclear receptor transcription factor, NURR1

NURR1 is an immediate early gene product and a member of the nuclear receptor superfamily of transcription factors. Using the NURR1 cDNA as a probe, we isolated the genomic DNA encoding NURR1 from a mouse 129SvEv genomic library. The NURR1 gene is approximately 6.2 kb long and is organized into 7 exons separated by 6 introns. Structural analysis of the NURR1 reveals that this gene shares a similar structure with that of the nuclear receptor NUR77/NGF1-B. As in NUR77, the promoter region of NURR1 lacks an identifiable TATA box, but is GC-rich. The proximal promoter region also contains an ATF/CREB consensus binding site that may participate in cAMP-mediated induction of this immediate early gene product. Isolation and structural characterization of the NURR1 gene provides information for further developmental and transcriptional regulation studies of this gene.

Interaction between estradiol and growth factors in the regulation of specific gene expression in MCF-7 human breast cancer cells

The response of two endogenous, estrogen-induced genes, LIV-1 and pS2, to growth factor stimulation of MCF-7 cells was examined. Epidermal growth factor (EGF), transforming growth factor alpha (TGF alpha) and insulin-like growth factor-1 (IGF-1) were each able to induce an increase in the two mRNAs in the absence of estradiol, and their effects were additive to that of an optimally inducing concentration (10(-8) M) of the hormone. Induction by EGF and TGF alpha, but not by IGF-1, were also additive to induction by a saturating concentration (2 microg/ml) of insulin. TGFbeta, an antimitogenic growth factor for MCF-7 cells, did not induce LIV-1 or pS2 mRNA but inhibited induction by estradiol. Increases in mRNA were shown to reflect increases in specific gene transcription. Induction by growth factors, but not by estradiol, was dependent upon protein synthesis. Induction by both growth factors and estradiol was inhibited by the pure antiestrogen, ICI 164384 (ICI), and by the mixed agonist/antagonist, tamoxifen. Despite differences in patterns of expression in vivo and in vitro, both LIV-1 and pS2 appeared to be responsive to growth factors via a mechanism distinct from that of estradiol but requiring the estrogen receptor.

Identification of a novel transferable cis element in the promoter of an estrogen-responsive gene that modulates sensitivity to hormone and antihormone

The estrogen receptor (ER) is a ligand-regulated transcription factor that acts at the promoters of estrogen-regulated genes to modulate their expression. In the present study, we examined three estrogen-regulated promoters, namely the rat progesterone receptor gene distal (PRD) and proximal (PRP) promoters and the human pS2 gene promoter, and observed marked differences in their sensitivity to stimulation by estrogen and repression of estrogen-stimulated transcription by antiestrogen (AE)-occupied ER. ER-containing MCF-7 human breast cancer cells were transfected with reporter gene constructs containing estrogen response elements upstream of the three gene promoters. In this system, PRP and PRD showed similar dose-response curves for stimulation by estradiol whereas pS2 was activated by even lower concentrations of estradiol. By contrast, PRD was much less sensitive to repression of estrogen-stimulated activity by all AEs studied, relative to the PRP and the pS2 promoters. Using deletion and mutational analysis, we have identified a transferable cis element at -131 to -94 bp in PRD that is involved in modulating the sensitivity of this promoter to both estrogens and AEs. The element reduced the magnitude of estrogen-stimulated activity, enhanced the ability of AEs to repress estrogen-stimulated activity, and elicited similiar effects when transferred to the promoter of another estrogen-responsive gene. Thus, removal of this region from PRD further accentuated the insensitivity of this promoter to AE while enhancing its sensitivity (both EC50 and fold induction) to estrogen. Gel mobility shift assays showed that proteins from nuclear extracts of MCF-7 cells interact with this element and that the binding of these proteins is inversely correlated with the transcriptional effectiveness of the ER. The findings demonstrate that a specific cis element from the promoter of an estrogen-responsive gene can alter the transcriptional activity of hormone and antihormone-occupied receptor bound at its response element near the promoter. Such ligand response modulatory elements, and changes in the levels and activity of factors that bind to such elements, may underlie the different sensitivities of steroid hormone-regulated genes to both hormones and antihormones.

Use of type I and type IV hypersensitivity responses to define the immunopharmacological profile of drugs

Administration of antigen suspended in incomplete Freund's adjuvant supplemented with either heat-killed Mycobacterium tuberculosis (complete Freund's adjuvant, CFA) or Bordetella pertussis toxin sensitizes animals so that subsequent antigen challenge leads to delayed-type (DTH) or immediate type hypersensitivity (ITH) responses, named type IV and type I, respectively. Appropriate timing of administration of drugs with respect to immunization or antigen challenge allowed to detect predominantly immunosuppressive, antiinflammatory or antianaphylactic activities. Among the reference drugs tested, only cyclosporin A (CsA) and dexamethasone (Dex) markedly inhibited DTH reaction, due to their immunosuppressive and antiinflammatory activities, respectively, whereas leflunomide and indomethacin resulted less potent. On the other hand, only dexchlorpheniramine, a histamine-receptor antagonist, afforded significant protection against anaphylactic shock, a form of ITH. Two new chemical entities were studied according to this protocol: ITF 1697, a chemically stabilized C-reactive protein-derived tetrapeptide, and ITF 2018, a leflunomide analogue. Data obtained with these new compounds showed that ITF 1697 has antianaphylactic activity, while ITF 2018 is endowed, mainly, with antiinflammatory activity. These results show that, through appropriate timing of administration, established in vivo models of immunologically mediated disease states allow an accurate profiling of the effects of pharmacologically active molecules and the detection of unsuspected activities for new drugs.

Epithelial and stromal estrogen and progesterone receptor expression in endometrial cancer: true color computer-assisted image analysis versus subjective scoring

Immunohistochemical methods provide the opportunity to evaluate the staining reaction in different histologic compartments of malignant tumors. Eighty-four formalin-fixed, paraffin-embedded and immunohistochemically stained endometrial cancer specimens were assessed for estrogen receptor (ER) and progesterone receptor (PR) expression by conventional light microscopy, a light microscopy scoring system, and by true color computer-assisted image analysis. Measurements of mean optical density (MOD) in the epithelium as well as in the stroma of the tumor were performed. A negative correlation was established between the MOD of ER and PR staining in the nuclei of the epithelium and the MOD of PR staining in the stroma of the tumor vs histological stage (Spearman correlation coefficient -0.32/P < 0.004, -0.23/P < 0.03, -0.26/P < 0.02, respectively) and depth of myometrial invasion (Spearman correlation coefficient -0.34/P < 0.002, -0.24/P < 0.02, -0.25/P < 0.02, respectively). The staining pattern in endometrial cancer was heterogeneous and %PA (percentage of positive stained area) and MOD were therefore linked by multiplication in order to correct for this potentially confounding phenomenon. A negative correlation with histological stage (Spearman correlation coefficient -0.29/P < 0.007) and depth of myometrial invasion (Spearman correlation coefficient -0.34/P < 0.001) was found for PR staining in the stroma of the tumor. MOD and subjective ranking of staining intensity showed a high correlation as well as %PA and subjective estimation of stained surface. ER and PR content in the epithelium and also PR content in the stroma of endometrial cancer appear to be of value in the assessment of steroid hormone receptor status. This may be indicative of stromal-epithelial interaction.

Analysis of ligand dependence and hormone response element synergy in transcription by estrogen receptor

In this work we examined two questions: (1) Is the low, but readily detectable, ability of estrogen receptor (ER) to activate transcription in the absence of added 17beta-estradiol caused by traces of estrogen in the growth medium, or by a weak ligand-independent ability of ER to activate transcription? (2) Does the ER exhibit synergistic activation of transcription on reporter genes containing multiple estrogen response elements (EREs)? To study these questions we developed a powerful new reporter gene, containing four EREs, which achieves inductions of up to 330-fold in the presence of liganded ER. We provided several types of evidence indicating that under standard cell culture conditions unliganded ER is unable to activate transcription. We demonstrated that when cells are grown in serum-free medium, estrogenic compounds may be in the base tissue culture medium. We demonstrated a strong cell and ER-dependence in transcriptional synergy, and suggest that cooperative binding of ER to multiple EREs can be responsible for transcriptional synergy in vivo.

Cooperativity and dimerization of recombinant human estrogen receptor hormone-binding domain

The estrogen receptor dimerizes and exhibits cooperative ligand binding as part of its normal functioning. Interaction of the estrogen receptor with its ligands is mediated by a C-terminal hormone-binding domain (HBD), and residues within the HBD are thought to contribute to dimerization. To examine dimer interactions in the isolated HBD, a human estrogen receptor HBD fragment was expressed in high yield as a cleavable fusion protein in Escherichia coli. The isolated HBD peptide exhibited affinity for estradiol, ligand discrimination, and cooperative estradiol binding (Hill coefficient approximately 1.6) similar to the full-length protein. Circular dichroism spectroscopy suggests that the HBD contains significant amounts of alpha-helix ( approximately 60%) and some beta-strand ( approximately 7%) and that ligand binding induces little change in secondary structure. HBD dimer dissociation, measured using size exclusion chromatography, exhibited a half-life of approximately 1.2 h, which ligand binding increased approximately 3-fold (estradiol) to approximately 4-fold (4-hydroxytamoxifen). These results suggest that the isolated estrogen receptor HBD dimerizes and undergoes conformational changes associated with cooperative ligand binding in a manner comparable to the full-length protein, and that one effect of ligand binding is to alter the receptor dimer dissociation kinetics.

Different residues of the human estrogen receptor are involved in the recognition of structurally diverse estrogens and antiestrogens

We have previously examined, by alanine scanning mutagenesis, amino acids 515-535 of the estrogen receptor (ER) ligand binding domain to determine which of these residues are important in estradiol binding. Mutation at four sites that potentially lie along one face of an alpha-helix, Gly521, His524, Leu525, and Met528, all significantly impaired estradiol binding by the ER (Ekena, K., Weis, K. E., Katzenellenbogen, J. A., and Katzenellenbogen, B. S. (1996) J. Biol. Chem. 271, 20053-20059). In this report, we compare the pattern of residues that are important in the recognition of several structurally diverse estrogen agonists and antagonists (the synthetic nonsteroidal agonist hexestrol, an agonist derived from the mold metabolite zearalenone, P1496, and the partial agonist-antagonist trans-hydroxytamoxifen) with those that are predicted to contact estradiol in the receptor-ligand complex. Although there are some similarities in the pattern of residue recognition among all four ligands, each ligand showed distinct differences as well. Interestingly, alanine substitution at only one residue, the leucine at position 525, was found to inhibit binding of all the ligands tested. Another residue, His524, was found to be important in the recognition of three different agonists but not trans-hydroxytamoxifen (the only ligand lacking a second hydroxyl group). The recognition of estradiol and another agonist, P1496, was impaired by the G521A mutation, whereas ligand-induced activity by the two compounds that lack B- and C-rings, hexestrol and trans-hydroxytamoxifen, was unaffected. Our findings demonstrate that these ligands fit into the ER ligand binding pocket differently and that each contacts a distinct set of amino acids. The smaller ligands (estradiol and hexestrol) have a narrower footprint of interacting residues than the larger ligands (P1496 and trans-hydroxytamoxifen). This pattern of interaction is most consistent with the amino acids within this region being in contact with the portion of these ligands that corresponds to the D-ring end of estradiol. The interplay between the shape of an ER ligand and the residues that support its binding to ER may potentially underlie the selective actions of different ER ligands in various cell and promoter contexts.

Vitamin D and gonadal steroid-resistant New World primate cells express an intracellular protein which competes with the estrogen receptor for binding to the estrogen response element

New World primates (NWP) exhibit a form of compensated resistance to vitamin D and other steroid hormones, including 17beta-estradiol. One postulated cause of resistance is that NWP cells overexpress one or more proteins which block hormone action by competing with hormone for its cognate hormone response element. Here we report that both nuclear and postnuclear extracts from NWP, but not Old World primate, cells contained a protein(s) capable of binding directly to the estrogen response element (ERE). This ERE binding protein(s) (ERE-BP) was dissociated from the ERE by excess of either unlabeled ERE or excess of the ERE half-site motif AGGTCAcag. DNA affinity chromatography using concatamers of the latter resulted in > 20,000-fold purification of the ERE-BP. The intensity of the ERE-BP-ERE complex in electromobility shift assay was indirectly related to the amount of wild-type Old World primate estrogen receptor (ER) but not affected when potential ligands, including 17beta-estradiol (up to 100 nM), or anti-ER antibody was added to the binding reaction. We conclude that vitamin D-resistant and gonadal steroid-resistant NWP cells contain a protein(s) that may "silence" ER action by interacting directly with the ERE and interfering with ER binding.

Three amino acid substitutions selectively disrupt the activation but not the repression function of the glucocorticoid receptor N terminus

A 210-amino acid region, termed enh2, near the N terminus of the rat glucocorticoid receptor, is necessary for both transcriptional activation and repression. The mechanism(s) of transcriptional regulation conferred by this region, however, are poorly understood. We screened in Saccharomyces cerevisiae a library of random mutants in the enh2 region of a constitutive glucocorticoid receptor derivative and isolated a series of multiply substituted receptors that are specifically defective in transcriptional activation. Although many substitutions in this area were tolerated, three amino acid substitutions (E219K, F220L, and W234R) within a 16-amino acid region were sufficient to disrupt the enh2 transcriptional activation function both in yeast and in mammalian cells. Although this region is rich in acidic residues, the conserved tryptophan at position 234 appears to be a more critical feature for enh2 activity; hydrophobic but not charged residues were tolerated at this position. Notably, the mutants uncoupled the activation and repression functions of enh2, as the activation defective isolates remained competent for repression of AP-1 at the composite response element plfG.

Thyroid hormone-responsive genes in developing cerebellum include a novel synaptotagmin and a hairless homolog

Proper development of the mammalian CNS requires sufficient thyroid hormone; thyroid hormone deficiency during a brief perinatal period produces severe neurological defects in humans and experimental animals. Thyroid hormone exerts its effects through nuclear receptors, which modulate the transcription of downstream genes in response to hormone binding. Surprisingly, few genes that are regulated by thyroid hormone receptors in the CNS have been described. Here, I report the isolation and characterization of genes that are expressed in response to thyroid hormone in developing rat brain. One such gene (Srg1) encodes a novel protein related to synaptotagmin, a protein involved in regulating neurotransmitter release; another (hr) encodes a putative zinc finger protein related to the product of a recently identified mouse gene, hairless. Both Srg1 and hr are induced rapidly (<4 hr), suggesting that they are regulated directly by thyroid hormone. The temporal and spatial expression of both Srg1 and hr is characteristic of genes important to nervous system development. Srg1 and hr are likely part of a cascade of gene activation induced by thyroid hormone that is critical for CNS organization and development.