Steroid receptor-mediated inhibition of rat prolactin gene expression does not require the receptor DNA-binding domain

HNF4α antagonists discovered by a high-throughput screen for modulators of the human insulin promoter

Hepatocyte nuclear factor (HNF)4α is a central regulator of gene expression in cell types that play a critical role in metabolic homeostasis, including hepatocytes, enterocytes, and pancreatic β cells. Although fatty acids were found to occupy the HNF4α ligand-binding pocket and were proposed to act as ligands, there is controversy about both the nature of HNF4α ligands as well as the physiological role of the binding. Here, we report the discovery of potent synthetic HNF4α antagonists through a high-throughput screen for effectors of the human insulin promoter. These molecules bound to HNF4α with high affinity and modulated the expression of known HNF4α target genes. Notably, they were found to be selectively cytotoxic to cancer cell lines in vitro and in vivo, although in vivo potency was limited by suboptimal pharmacokinetic properties. The discovery of bioactive modulators for HNF4α raises the possibility that diseases involving HNF4α, such as diabetes and cancer, might be amenable to pharmacologic intervention by modulation of HNF4α activity.

Clinicopathological Features of Growth Hormone-Producing Pituitary Adenomas in 242 Acromegaly Patients: Classification according to Hormone Production and Cytokeratin Distribution

The aim of this study was to clarify the relationship between the histological features of GH-producing adenomas surgically resected at the Toranomon Hospital and the clinical features of the patients. Histological examinations, including immunohistochemistry for anterior pituitary hormones and cytokeratin (CK), were performed on 242 consecutively excised GH-producing pituitary adenomas. Immunohistochemistry showed 45% of the adenomas to be monohormonal and 55% to be plurihormonal, producing GH-PRL (77%), GH-TSH (13%), and GH-PRL-TSH (10%). One-fourth of the monohormonal GH adenomas had a dot-like pattern of CK immunoreactivity in the majority of the tumor cells (>80%); they were significantly more common in female or younger patients and usually tended to be larger and more invasive than monohormonal GH adenomas with perinuclear CK. Interestingly, CK-immunonegative adenomas were found in only 5% of the patients; they also showed a tendency to be larger, suggesting that they are a distinct type of GH adenoma with clinically aggressive features. Serum hormone levels correlated well with tumor size only in GH-producing adenomas with a perinuclear pattern of CK immunoreactivity. Each histological subtype of adenoma, classified according to the pattern of CK immunoreactivity, was associated with distinct clinical characteristics. This information is useful for understanding the pathophysiology of acromegaly-causing GH-producing adenomas.

Minireview: Conversing with chromatin: the language of nuclear receptors

Nuclear receptors are transcription factors that are activated by physiological stimuli to bind DNA in the context of chromatin and regulate complex biological pathways. Major advances in nuclear receptor biology have been aided by genome scale examinations of receptor interactions with chromatin. In this review, we summarize the roles of the chromatin landscape in regulating nuclear receptor function. Chromatin acts as a central integrator in the nuclear receptor-signaling axis, operating in distinct temporal modalities. Chromatin effects nuclear receptor action by specifying its genomic localization and interactions with regulatory elements. On receptor binding, changes in chromatin operate as an effector of receptor signaling to modulate transcriptional events. Chromatin is therefore an integral component of the pathways that guide nuclear receptor action in cell-type-specific and cell state-dependent manners.

A Retrospective on Nuclear Receptor Regulation of Inflammation: Lessons from GR and PPARs

Members of the nuclear receptor superfamily have vital roles in regulating immunity and inflammation. The founding member, glucocorticoid receptor (GR), is the prototype to demonstrate immunomodulation via transrepression of the AP-1 and NF-κB signaling pathways. Peroxisome proliferator-activated receptors (PPARs) have emerged as key regulators of inflammation. This review examines the history and current advances in nuclear receptor regulation of inflammation by the crosstalk with AP-1 and NF-κB signaling, focusing on the roles of GR and PPARs. A better understanding of the molecular mechanism by which nuclear receptors inhibit proinflammatory signaling pathways will enable novel therapies to treat chronic inflammation.

Dynamic regulation of glucocorticoid signalling in health and disease

Activation of the glucocorticoid receptor (GR) by endogenous and synthetic glucocorticoids regulates hundreds of genes to control regulatory networks in development, metabolism, cognition and inflammation. Elucidation of the mechanisms that regulate glucocorticoid action has highlighted the dynamic nature of hormone signalling and provides novel insights into genomic glucocorticoid actions. The major factors that regulate GR function include chromatin structure, epigenetics, genetic variation and the pattern of glucocorticoid hormone secretion. We review our current understanding of the mechanisms that contribute to GR signalling and how these contribute to glucocorticoid sensitivity, resistance and side effects.

Transcription factor AP1 potentiates chromatin accessibility and glucocorticoid receptor binding

Ligand-dependent transcription by the nuclear receptor glucocorticoid receptor (GR) is mediated by interactions with coregulators. The role of these interactions in determining selective binding of GR to regulatory elements remains unclear. Recent findings indicate that a large fraction of genomic GR binding coincides with chromatin that is accessible prior to hormone treatment, suggesting that receptor binding is dictated by proteins that maintain chromatin in an open state. Combining DNaseI accessibility and chromatin immunoprecipitation with high-throughput sequencing, we identify the activator protein 1 (AP1) as a major partner for productive GR-chromatin interactions. AP1 is critical for GR-regulated transcription and recruitment to co-occupied regulatory elements, illustrating an extensive AP1-GR interaction network. Importantly, the maintenance of baseline chromatin accessibility facilitates GR recruitment and is dependent on AP1 binding. We propose a model in which the basal occupancy of transcription factors acts to prime chromatin and direct inducible transcription factors to select regions in the genome.

Negative regulation by nuclear receptors: a plethora of mechanisms

Nuclear receptors are arguably the best understood transcriptional regulators. We know a great deal about the mechanisms through which they activate transcription in response to ligand binding and about the mechanisms through which they repress transcription in the absence of ligand. However, endocrine regulation often requires that ligand-bound receptors repress transcription of a subset of genes. An understanding of the mechanism for ligand-induced repression and how this differs from activation has proven elusive. A number of recent studies have directly or indirectly addressed this problem. Yet it seems the more evidence that accumulates, the more complex the mystery becomes.

Pathology, pathogenesis and therapy of growth hormone (GH)-producing pituitary adenomas: technical advances in histochemistry and their contribution

Growth hormone (GH)-producing adenomas (GHomas) are one of the most frequently-occurring pituitary adenomas. Differentiation of hormone-producing cells in the pituitary gland is regulated by transcription factors and co-factors. The transcription factors include Pit-1, Prop-1, NeuroD1, Tpit, GATA-2, SF-1. Aberrant expression of transcription factors such as Pit-1 results in translineage expression of GH in adrenocorticotropic hormone-producing adenomas (ACTHomas). This situation has been substantiated by GFP-Pit-1 transfection expression in the AtT20 cell line. Experimentally, GHomas have been induced in GH-releasing hormone (GHRH) or Prop-1 transgenic animals. Immunohistochemical detection of somatostatin receptor (SSTR2a) has recently emphasized their role in the response of GHomas to somatostatin analogue therapy. In this review, the advances in technology and their contribution to cell biology and medical practice are discussed.

Corticosterone impairs the mRNA expression and activity of 3beta- and 17beta-hydroxysteroid dehydrogenases in adult rat Leydig cells

Clinical and experimental studies, including our own observations, have shown the adverse effects of excess glucocorticoids on testicular steroid hormone production. The present study was designed to gain insight into the molecular mechanisms by which excess corticosterone impairs Leydig cell steroidogenesis. To achieve this, adult rats were administered with corticosterone-21-acetate (2 mg/100 g body weight) twice daily for 15 days. After the treatment period, rats were killed by decapitation. The testes were removed, decapsulated aseptically and used for the isolation of Leydig cells. Purified Leydig cells were used for assessing the activity of 3beta- and 17beta-hydroxysteroid dehydrogenases (HSDs) and total RNA isolation. For in vitro studies, purified Leydig cells (7.5 x 10(6) cells) of control rats were plated in culture flasks and exposed to different concentrations (50, 100, 200, 400, and 800 nmol/L) of corticosterone for 24 h. At the end of incubation, total RNA was isolated from cultured Leydig cells, and the mRNA of 3beta- and 17beta-HSDs was quantified by RT-PCR. A significant reduction in the activities and levels of 3beta-HSD type-I and 17beta-HSD type-III mRNAs in Leydig cells were observed. In vitro studies demonstrated a dose-dependent significant impairment in both the activity and mRNA expression of these enzymes. These results suggest that corticosterone might have a direct effect on the transcription of the genes of 3beta- and 17beta-HSD. It is inferred from the present in vivo and in vitro studies that one of the molecular mechanisms by which excess corticosterone decreases the steroidogenic potency of Leydig cells is by suppressing the mRNA expression of 3beta-HSD type-I and 17beta-HSD type-III enzymes.

Direct, androgen receptor-mediated regulation of the FKBP5 gene via a distal enhancer element

Androgen signaling via the androgen receptor (AR) transcription factor is crucial to normal prostate homeostasis and prostate tumorigenesis. Current models of AR function are predominantly based on studies of prostate-specific antigen regulation in androgen-responsive cell lines. To expand on these in vitro paradigms, we used the mouse prostate to elucidate the mechanisms through which AR regulates another direct target, FKBP5, in vivo. FKBP5 encodes an immunophilin that has been previously implicated in glucocorticoid and progestin signaling pathways and that likely influences prostate physiology in the presence of androgens. In this work, we show that androgens directly regulate FKBP5 via an interaction between the AR and a distal enhancer located 65 kb downstream of the transcription start site in the fifth intron of the FKBP5 gene. We have found that AR selectively recruits cAMP response element-binding protein to this enhancer. These interactions, in turn, result in chromatin remodeling that affects the enhancer proper but not the FKBP5 locus as a whole. Furthermore, in contrast to prostate-specific antigen-regulatory mechanisms, we show that transactivation of the FKBP5 gene does not rely on a single looping complex to mediate communication between the distal enhancer and proximal promoter. Rather, the distal enhancer complex and basal transcription apparatus communicate indirectly with one another, implicating a regulatory mechanism that has not been previously appreciated for AR target genes.

Acquired prolactin deficiency (APD) after treatment for Cushing's disease is a reliable marker of irreversible severe GHD but does not reflect disease status

OBJECTIVE

We have previously reported that acquired prolactin deficiency (APD) is a marker for severe hypopituitarism and observed a high prevalence of APD in patients treated for Cushing's disease. Recovery of GH secretion is recognized to occur in a proportion of patients treated for Cushing's disease after the effects of glucocorticoid excess on GH secretion have subsided. The aim of this study was to investigate further the association between APD, treated Cushing's disease and, in particular, to determine whether recovery of GH secretion may occur in these patients.

METHODS

Fifty-seven patients (42 female), in remission after treatment for Cushing's disease, were studied. The cohort comprised 13 patients with, and 44 without APD. APD was defined as a serum prolactin persistently below the detection limit of the assay. Severe GH deficiency was defined as a peak GH response of less than 9 mU/l during a GH stimulation test. Age and gender did not significantly differ between subgroups.

RESULTS

Of the 13 patients with APD, a macroadenoma was present in one patient, a microadenoma was present in 10, no lesion was detected in one, and in one patient (treated with an yttrium implant) the size of the tumour was unknown. Of the 28 patients who did not have APD, who were treated with primary surgery a microadenoma was present in 23 and a macroadenoma was present in five. Detailed pituitary imaging was not available in 16 patients who did not have APD, who were treated with primary external XRT. Deficiencies of GH, TSH, LH/FSH (P < 0.0001) and ADH (P = 0.006) status, by conventional testing, were present more frequently in the APD subgroup. In contrast, the prevalence of ACTH deficiency after treatment was not different between the APD and non-APD groups. However, the requirement for additional therapy, targeting the pituitary or adrenal gland, after primary treatment, in those patients not rendered ACTH-deficient, was significantly greater in the APD compared with the non-APD groups (P = 0.003). After pituitary surgery, a significant correlation between peak GH response and interval since remission of Cushing's syndrome was found in the subgroup without APD (r = 0.4, P = 0.04). Four patients who did not have APD, who had documented severe GHD in the immediate postoperative period displayed normalization of GH secretion, when re-tested after a mean interval of 27.2 months. In contrast, no patient with APD after pituitary surgery demonstrated a detectable GH response after up to 132 months of follow-up. No patient with APD showed recovery of prolactin secretion by the time of the most recent measurement (mean 57 months). All 10 patients who developed APD immediately after pituitary surgery had undergone a radical procedure (either a subtotal or total hypophysectomy). In contrast, of 28 patients with Cushing's disease who did not develop APD, only four underwent radical surgery (P < 0.0001). Seven of 14 patients (50%) who underwent a radical operation and two of 20 treated by selective adenomectomy (10%) required additional treatment to achieve control of Cushing's syndrome (P = 0.04).

CONCLUSION

In the presence of APD, patients with Cushing's disease do not experience recovery of GH secretion after treatment, even after the effects of glucocorticoid excess subside. In the absence of APD, GH status may normalize after successful surgical treatment. Although a marker for severe hypopituitarism, APD does not indicate success of treatment of Cushing's disease and may be associated with detectable ACTH secretion from residual corticotroph adenoma activity. APD after pituitary surgery for Cushing's disease occurs only after a radical operation. When a selective adenomectomy is not possible, control of Cushing's disease by operation is less frequent and when achieved, is more often at the cost of hypopituitarism. The optimal management of such patients requires further study.

Acquired prolactin deficiency indicates severe hypopituitarism in patients with disease of the hypothalamic-pituitary axis

OBJECTIVE

Prolactin deficiency has been the subject of many scientific studies, but there is a paucity of information regarding prolactin deficiency in humans. In this report, adults with disease of the hypothalamic-pituitary axis (HPA) were studied to determine the prevalence of severe acquired prolactin deficiency (APD) and the pathophysiological characteristics associated with it.

PATIENTS AND METHODS

APD was defined as a serum prolactin level persistently below the detection limit of the assay, i.e. less than 50 mU/l (normal range: male 85-444, female 85-530). Patients with a diagnosis of acromegaly, prolactinoma or with congenital or drug induced prolactin deficiency were excluded. Three hundred and sixty-nine patients (190 women, age range 17-79 years) with disease of the HPA, meeting the specified criteria were identified.

RESULTS

Twenty-two (13 women, age range 29-76 years), showed evidence of APD. Thirteen of the 22 patients with APD had been treated for Cushing's disease. In all, 62 patients treated for Cushing's disease were identified, resulting in a prevalence of APD in treated Cushing's disease of 20.97%. Excluding treated Cushing's disease, the prevalence of APD in the remainder of the cohort was 2.93%. Nineteen patients with APD (86.4%) and 183 without APD (52.7%) underwent surgery in the region of the HPA (P = 0.0042). In contrast, nine patients with APD (40.9%) and 283 without APD (80.4%) had received radiotherapy, with fields which included the HPA (P < 0.001). No patient with isolated APD was identified. All patients with APD had evidence of severe GH deficiency (GHD) with a peak GH response to provocative stimuli of < 1.6 mU/l and a median IGF-I standard deviation score (SDS) of -4.85 (quartiles -9.56 to -2.80). Of the 13 patients with APD and Cushing's disease, all were gonadotrophin and TSH-deficient, six were adrenocorticotropic hormone (ACTH)-deficient and six (46.1%) had cranial diabetes insipidus (CDI). Of the remaining nine patients with APD, total anterior pituitary hormone failure was present in all and CDI was present in two (22.2%).

CONCLUSIONS

The presence of APD indicates severe hypopituitarism in adults with HPA disease. It is universally associated with severe GHD. It is more common after surgery to the HP region. It has a low overall prevalence except in patients surgically treated for Cushing's disease.

Cross-talk between glucocorticoid and retinoic acid signals involving glucocorticoid receptor interaction with the homoeodomain protein Pbx1

Glucocorticoid (GC) signalling influences the response of the cell to a number of other signals via a mechanism referred to as 'cross-talk'. This cross-talk may act at several levels, including an interaction between the transcription factors involved in the signalling pathways. In the present paper, we demonstrate a novel functional interaction between GC and all- trans -retinoic acid (RA) signalling. We show that, in P19 embryonal carcinoma cells, GCs potentiate RA-induced expression of the murine Hoxb -1 gene through an autoregulatory element, b1-ARE, recognized by the Pbx1 and HOXB1 homoeodomain proteins. The synergistic effect of GC did not involve GC receptor (GR) binding to the b1-ARE, and the GC-GR complex alone was unable to activate transcription via the element. Furthermore, the ability of the GR to transactivate was not required, excluding expression of a GC-induced protein as the mechanism for the GC/RA synergy. Additional transfection experiments showed that the Pbx1/HOXB1 heterodimer was the target for the GC effect. Furthermore, functional dissection of the GR demonstrated that the DNA-binding domain (DBD) of the GR was required for the synergy. A physical interaction between the GR and Pbx1 proteins was demonstrated in vivo by co-immunoprecipitation experiments. These results are compatible with a model in which the GC/RA synergy is mediated by a direct interaction between the GR and Pbx1. On the basis of the ubiquitous expression of both GR and Pbx1, a number of genes regulated by Pbx are likely to be important targets for GC-mediated 'cross-talk'.

The regulation of HIV-1 gene expression: the emerging role of chromatin

Host and viral factors that regulate the expression of the human immunodeficiency virus type 1 (HIV-1) 5' long terminal repeat (LTR) promoter have been studied since the recognition that HIV is the cause of the acquired immunodeficiency syndrome (AIDS). However, complex modifications of nucleosomes within chromatin has been recently recognized as an important mechanism of gene regulation. Nucleosome remodelling can alter the accessibility of DNA to specific activators or repressors, general transcription factors, and RNA polymerase. Emerging data now suggests that dynamic regulation of chromatin structure in the vicinity of the LTR promoter adds an additional level of complexity to the regulation of HIV expression. A better understanding of the role of chromatin in the regulation of HIV expression could lead to much-needed therapies against proviral genomes that are being actively transcribed, and those that are quiescent and persistent.

Activities of a non-classical estrogen, Z-bis-dehydrodoisynolic acid, with ERalpha and ERbeta

(+/-)-Z-bis-Dehydrodoisynolic acid [(+/-)-Z-BDDA] is highly estrogenic in vivo, yet binds to estrogen receptor (ER) poorly. This paradox has raised the possibility of alternative ERs and/or molecular mechanisms. To address the possibility of high activities of Z-BDDA with ERbeta, we determined the activities of (+)-Z-BDDA and (-)-Z-BDDA, in cell culture and in vitro, comparing ERbeta to ERalpha. Transfectional analysis in Hela cells showed (-)-Z-BDDA is an agonist for gene activation with both ERalpha (EC(50) congruent with 0.3nM) and ERbeta (EC(50) congruent with 5nM), while little to no activity was observed with (+)-Z-BDDA. Similarly, in gene repression assays, (-)-Z-BDDA was active (EC(50) congruent with 0.2nM), but again minimal activity was exhibited by (+)-Z-BDDA. Binding to ERalpha and ERbeta in vitro used both competition and a direct binding assay. For ERalpha, the relative affinity of (-)-Z-BDDA was approximately 6% by competition and 1.7% by direct binding versus 17beta-estradiol (E2; 100%), while (+)-Z-BDDA also demonstrated binding, but with relative affinities of only 0.08% by competition and 0.3% by the direct assay. For ERbeta, the affinity of (-)-Z-BDDA was approximately 7% by competition and 1.5% by the direct assay relative to E2 (100%), while (+)-Z-BDDA had lower affinity, approximately 0.2% that of E2 by both assays. The paradox of potent in vivo activity but lower activity in receptor binding and in cell culture reporter gene assays, previously seen with ERalpha is now also associated with ERbeta. The failure of ERbeta to explain the activity-binding paradox indicates the need for additional in vivo metabolic and pharmacokinetic studies and continued consideration of alternative mechanisms.

Transcriptional regulation of the mouse fatty acid amide hydrolase gene

Fatty acid amide hydrolase (FAAH) is a membrane-bound enzyme that inactivates a family of fatty acid amide molecules which are implicated in physiological processes such as pain and sleep. We cloned a 1.9 kb fragment of the 5'-untranslated region of the mouse FAAH gene into the pGL3 basic luciferase reporter vector and showed that this sequence has promoter activity in vitro. By primer extension analysis, we have determined the transcription start site to be 200 bases upstream of the ATG initiation codon and found that a TATA motif was absent. A number of putative response elements, including those for estrogen and glucocorticoids, were identified in this sequence. We have demonstrated that the estrogen and glucocorticoid receptors down-regulate transcriptional activity independent of their ligand. These data should help in understanding the mechanisms of FAAH gene transcription.

17beta-Estradiol inhibits cytokine induction of the human E-selectin promoter

Estradiol has been shown to decrease levels of the cell adhesion molecule E-selectin in cultured cells and in women on hormone replacement therapy. We set out to determine if the mechanism of estradiol action on E-selectin is at the level of its promoter. It was found that estradiol repressed the cytokine-stimulated induction of luciferase activity driven by the human E-selectin promoter in a reporter plasmid (hE-sel-LUC) in co-transfected human hepatoma cells (Hep G2) and human umbilical cord endothelial cells (ECV-304). Repression by estradiol was dependent on the presence of transfected estrogen receptor (ER) alpha or beta expression vectors. The ER antagonist ICI-182,780 blocked the repression by estradiol, confirming the receptor-dependence of the effect. The intact DNA-binding domain of ERalpha was required for estradiol repression of the cytokine-induced stimulation of the promoter in each cell line as demonstrated by the inability of an ER construct with two point mutations in the DNA-binding domain to inhibit reporter activity. Mutation of the NFK-B site at -94 to -85 within the E-selectin promoter led to less stimulation of hE-sel-LUC by interleukin one beta (IL-1beta). Estradiol did not inhibit this IL-1beta stimulated luciferase activity, indicating that the NFK-B site is necessary for ER-mediated inhibition of this promoter. Mutation of the AP-1 site at -500 to -494 within the E-selectin promoter had no effect on the ability of IL-1beta to stimulate its transcription, and estradiol repressed this activation in an ER-dependent manner with identical efficacy and potency in comparison with the wild-type promoter. Therefore, the E-selectin promoter is down-regulated by estradiol working through either ERalpha or ERbeta and requires the NFK-B site at -94 to -85 within the promoter.

Estradiol decreases IGF-1 and IGF-1 receptor expression in rat aortic smooth muscle cells. Mechanisms for its atheroprotective effects

Insulin-like growth factor (IGF-1) is a potent mitogen for vascular smooth muscle cells. Both IGF-1 and its receptor have been shown to be highly expressed in atherosclerotic lesions. Here we investigated whether part of the vasculoprotective properties of E(2) may be mediated by its negative regulation of the IGF-1 system. HeLa cells, which do not contain endogenous estrogen receptors (ER), were transiently transfected with IGF-1R promoter constructs with or without a plasmid encoding human ERalpha or ERbeta and treated with 100 nm 17beta-estradiol (E(2)) for 24 h. E(2) treatment decreased basal luciferase activity by 51%, and this effect was dependent on co-expression of ERalpha, whereas no repression was observed with ERbeta. A mutation within the DNA binding domain of the ERalpha abolished the repressor function of the ER receptor. Similarly, E(2) decreased IGF-1R transcription by 21% in rat aortic smooth muscle cells (RASMC), which express endogenous ER. This effect was specific for E(2), because it was inhibited by an antiestrogen and because progesterone did not have any effect on IGF-1R expression in HeLa or RASMC transfected with progesterone receptor. Accordingly, E(2) decreased IGF-1R and IGF-1 mRNA in RASMC by 47% and 33%. Western blot analysis and radioligand binding studies showed that E(2) also dose-dependently decreased IGF-1R protein expression in RASMC by 40% and 30%, respectively, and that IGF-1 protein was reduced by 43%. Repression of IGF-1R promoter activity by a combination of ERalpha and E(2) did not appear to be mediated via direct binding of ER to the IGF-1R promoter but rather by inhibition of SP1 binding to the IGF-1R promoter. Thus, E(2) down-regulates IGF-1R and IGF-1 expression in vascular smooth muscle cells. This may have important implications for the understanding of the beneficial effects of estrogen in the cardiovascular system.

Cloning and sequencing of the gilthead sea bream estrogen receptor cDNA

We report here the complete nucleotide sequence of a cDNA clone containing the full-coding sequence of the Sparus aurata estrogen receptor (ER) isolated from an expression library prepared from gilthead sea bream liver poly A+ RNA. The library was screened using a single strand rainbow trout ER cDNA probe, corresponding to the C-D domain. The cDNA sequence containing an insert of 2369 nucleotides was found to encode a protein of 579 amino acids. The 5'- and 3'-untranslated regions of the message are 186 and 392 nucleotides long, respectively. The gilthead sea bream ER shows the higher homology with the ER of another perciform, Chrysophrys major (93%), moderate to high homology with Oreocromis aureus (78%) medaka (77%) and rainbow trout (70.7%) ERs and lower homology with japanese eel (45%), amphibian (47%), avian (48.5%) and mammalian (47-47.5%) ERs. The sequence homologies and phylogenetic analysis of the various ERs suggest that gilthead sea bream ER should be considered as a ER alpha-like.

Influence of a cap site element on tissue-restricted expression of the glycoprotein hormone alpha-subunit gene

Little is known of the transcriptional regulators important for expression of the glycoprotein hormone alpha-subunit (GPHalpha) gene in nonendocrine tumors, which secrete free alpha-subunit at an incidence of 25-80%. Consequently, attempts were made to define cis-regulatory elements and their cognate trans-acting factors that modulate promoter activity in epithelial cell types that do not normally express the glycoprotein hormones. DNA-mediated transient expression of promoter-reporter constructs was used to identify a novel negative regulatory element located at the GPHalpha gene transcription start site. Mutagenesis of this element produced a 2- to 10-fold increase in promoter activity, depending on the particular mutation and the transfected tumor cell line. Electrophoretic mobility shift analysis detected a protein that binds specifically to a DNA motif encompassing the cap site. It was present at different levels in a variety of cell types. Significantly, the degree to which activity of the wild-type promoter was suppressed relative to that of the mutant promoter was proportional to the level of cap site binding protein in the collection of cell lines examined. These results indicate that a negative regulatory element centered at the GPHalpha gene cap site and its cognate DNA-binding protein make a significant contribution to the production of alpha-subunit in a variety of tumor tissues. A detailed understanding of this cis/trans pair may further suggest a mechanism to explain, at least in part, how this gene becomes activated in nonendocrine tumors.

The AD1 and AD2 transactivation domains of E2A are essential for the antiapoptotic activity of the chimeric oncoprotein E2A-HLF

The chimeric oncoprotein E2A-HLF, generated by the t(17;19) chromosomal translocation in pro-B-cell acute lymphoblastic leukemia, incorporates the transactivation domains of E2A and the basic leucine zipper (bZIP) DNA-binding and protein dimerization domain of HLF (hepatic leukemic factor). The ability of E2A-HLF to prolong the survival of interleukin-3 (IL-3)-dependent murine pro-B cells after IL-3 withdrawal suggests that it disrupts signaling pathways normally responsible for cell suicide, allowing the cells to accumulate as transformed lymphoblasts. To determine the structural motifs that contribute to this antiapoptotic effect, we constructed a panel of E2A-HLF mutants and programmed their expression in IL-3-dependent murine pro-B cells (FL5.12 line), using a zinc-inducible vector. Neither the E12 nor the E47 product of the E2A gene nor the wild-type HLF protein was able to protect the cells from apoptosis induced by IL-3 deprivation. Surprisingly, different combinations of disabling mutations within the HLF bZIP domain had little effect on the antiapoptotic property of the chimeric protein, so long as the amino-terminal portion of E2A remained intact. In the context of a bZIP domain defective in DNA binding, mutants retaining either of the two transactivation domains of E2A were able to extend cell survival after growth factor deprivation. Thus, the block of apoptosis imposed by E2A-HLF in pro-B lymphocytes depends critically on the transactivating regions of E2A. Since neither DNA binding nor protein dimerization through the bZIP domain of HLF is required for this effect, we propose mechanisms whereby protein-protein interactions with the amino-terminal region of E2A allow the chimera to act as a transcriptional cofactor to alter the expression of genes regulating the apoptotic machinery in pro-B cells.

Glucocorticoids repress transcription from a negative glucocorticoid response element recognized by two homeodomain-containing proteins, Pbx and Oct-1

Several studies have established that the prolactin (PRL) gene is expressed not only in lactotrophs and somatotrophs of the anterior pituitary but, albeit to a lesser extent, in non-pituitary cells like human thymocytes, decidualized endometrium, mammary glands during lactation, and some human non-pituitary cell lines. Despite the requirement in the pituitary for the pituitary-specific transcription factor Pit-1/GHF-1 for PRL expression, the expression in non-pituitary cells occurs in the absence of Pit-1/GHF-1 and can be repressed by glucocorticoids. This prompted us to investigate the transcription factors in non-pituitary cells which are involved in controlling expression and glucocorticoid repression of a previously characterized negative glucocorticoid response element from the bovine prolactin gene (PRL3 nGRE). Here we have demonstrated that non-pituitary cells (COS-7 and mouse hepatoma Hepa1c1c7 cells) conferred increased expression via the PRL3 nGRE mainly because of the binding of the ubiquitously expressed POU-homeodomain-containing octamer transcription factor-1 (Oct-1) to an AT-rich sequence present in the PRL3 sequence. However, full transcriptional activity required the binding of a second ubiquitously expressed homeodomain-containing protein, Pbx, previously shown to bind cooperatively with several homeotic selector proteins. The Pbx binding site in the PRL3 nGRE, located just upstream of the Oct-1 binding site, showed a strong sequence similarity with known Pbx binding sites and bound Pbx with an affinity similar to that of other established Pbx target sequences. Interestingly, both Oct-1 and Pbx binding to the PRL3 nGRE were found to be required for glucocorticoid repression. Addition of in vitro translated glucocorticoid receptor DNA binding domain to the nuclear extract prevented Oct-1 and Pbx from binding to the PRL element. The involvement of the homeobox protein Pbx in glucocorticoid repression via an nGRE identifies a new role for this protein.

Differential expression of Nur77 family members in human T-lymphotropic virus type 1-infected cells: transactivation of the TR3/nur77 gene by Tax protein

We analyzed the differential expression and regulation of three members of the Nur77 transcription factor family by the human T-lymphotropic virus type 1 (HTLV-1) Tax protein. We have demonstrated that in both HTLV-1-infected cells and Tax-expressing JPX-9 cells, TR3/nur77 is highly expressed, whereas neither NOR-1 nor NOT expression is detectable. Transient transfection analysis further confirmed the Tax transactivation of the TR3/nur77 promoter but not the NOR-1 promoter in different cell types. Furthermore, expression of a luciferase reporter gene driven by the NGFI-B (rat homolog of TR3/Nur77) response element (NBRE) provided evidence that Tax-mediated transactivation resulted in the induction of a functional protein. Cotransfection assays with the TR3/nur77 promoter sequence or the NBRE binding motif together with a series of Tax mutants have shown that Tax-induced TR3/nur77 expression is mediated by CREB/ATF-related transcription factors.

A negative vitamin D response DNA element in the human parathyroid hormone-related peptide gene binds to vitamin D receptor along with Ku antigen to mediate negative gene regulation by vitamin D

We found that the human parathyroid hormone-related peptide (hPTHrP) gene contained a DNA element (nVDREhPTHrP) homologous to a negative vitamin D response element in the human parathyroid hormone gene. It bound to vitamin D receptor (VDR) but not retinoic acid Xalpha receptor (RXRalpha) in the human T cell line MT2 cells. VDR binding to this element was confirmed by the Southwestern assay combined with immunodepletion using anti-VDR monoclonal antibody, and this binding activity was repressed by 1,25-dihydroxyvitamin D3. Such a repression was reversed by acid phosphatase treatment, suggesting that 1,25-dihydroxyvitamin D3 phosphorylates VDR to weaken its binding activity to nVDREhPTHrP. In electrophoretic mobility shift assay, we found anti-Ku antigen antibody specifically supershifted the MT2 nuclear proteinnVDREhPTHrP complex. The nVDREhPTHrP-bearing reporter plasmid produced vitamin D-dependent inhibition of the reporter activity in MT2 cells, which was markedly masked by the introduction of the Ku antigen expression vector in the antisense orientation. On the other hand, such a procedure did not perturb the vitamin D response element-mediated gene stimulation by vitamin D. These results indicate that nVDREhPTHrP interacts with Ku antigen in addition to VDR to mediate gene suppression by vitamin D.

The loss of estrogen and progesterone receptor gene expression in human breast cancer

Hormone responsiveness is a critical determinant of breast cancer progression and management, and the response to endocrine therapy is highly correlated with the estrogen receptor (ER)3 and progesterone receptor (PR) status of tumor cells. Thus, key areas of study in breast cancer are those mechanisms that regulate ER and PR expression in normal and malignant breast tissues. One-third of all breast cancers lack ER and PR; these conditions are associated with less differentiated tumors and poorer clinical outcome. In addition, approximately one-half of ER-positive tumors lack PR protein and patients with this phenotype are less likely to respond to hormonal therapies than those whose tumors express both receptors. Since PR is induced by ER; its presence is a marker of a functional ER. In this review, we will discuss possible mechanisms for loss of ER and PR gene expression, especially structural changes within each gene including deletions, polymorphisms or methylation. Improved understanding of the pathways that lead to loss of ER and/or PR proteins should allow the development of better predictive indicators as well as novel therapeutic approaches to target these hormone-independent cancers.

Positive and negative regulation of the rat vasopressin gene promoter

To study the transcriptional regulation of the vasopressin gene in vitro, 3 kb of the 5' regulatory region of the rat vasopressin gene was isolated and subcloned, along with a series of various deletion mutants, into vectors containing the luciferase reporter gene. After transfecting these genes transiently into the human choriocarcinoma cell line JEG-3 along with a glucocorticoid receptor (GR) expression vector, transcriptional activity was quantitated using the luciferase assay. Forskolin, 8-bromo-cAMP, and protein kinase A catalytic subunit expression all markedly increased transcription from the 3-kb promoter. Analyses with deletion mutants of the promoter showed that two cAMP-responsive element (CRE)-like sequences (-227 to -220 bp and -123 to -116 bp) contribute to this positive regulation. Expression of KCREB, a dominant negative mutant of the cAMP-responsive element binding protein (CREB), suggested the involvement of CREB. Transfection of the activator protein 2 (AP2) DNA consensus sequence partially blocked transcription. Dexamethasone suppressed forskolin-stimulated expression. The negative effect of glucocorticoid was GR dependent and may be mediated by a mechanism not involving GR binding to DNA because it was independent of the putative glucocorticoid-responsive element previously reported in the vasopressin promoter (-622 to -608 bp) and was preserved in the shorter promoter constructs in which no glucocorticoid-responsive element-like sequence was found. Our data suggest that several trans-acting factors including CREB, AP2, and GR are likely to be involved in vasopressin gene transcription and that the positive and negative regulation of vasopressin gene transcription is complex.

The glucocorticoid receptor inhibits the human prolactin gene expression by interference with Pit-1 activity

Glucocorticoids have been shown to inhibit the activity of the human prolactin (hPRL) promoter. Using transient expression experiments in rat pituitary cells, we located the sequence conferring glucocorticoid inhibition to a region which contains Pit-1 binding sites, responsible for pituitary-specific expression, but does not seem to contain a glucocorticoid receptor (GR) binding site. Co-transfection experiments in non-pituitary cell lines, using expression vectors for Pit-1 and different mutants of the human GR show that inhibition of the hPRL gene is seen only in the presence of Pit-1 and GR, and that the DNA binding function of the receptor is not required. Immunoprecipitation studies show that either anti-GR or anti-Pit-1 antibodies are able to co-precipitate GR and Pit-1, suggesting an interaction between these factors. We conclude that the activated GR functionally interferes with the pituitary specific factor Pit-1, thereby leading to the observed transcriptional repression.

Androgen receptor-mediated antagonism of estrogen-dependent low density lipoprotein receptor transcription in cultured hepatocytes

Postmenopausal women receiving hormone replacement therapy have a lower risk of coronary heart disease than women who do not receive hormone treatment. Multiple mechanisms are likely to underlie estrogen's cardioprotective action, including lowering of plasma low density lipoprotein (LDL) cholesterol. Using an in vitro system exhibiting normal regulation of LDL receptor (LDLR) gene transcription, we show that 17beta-estradiol activates the LDLR promoter in transiently transfected HepG2 cells. LDLR activation by estrogen in HepG2 cells is dependent on the presence of exogenous estrogen receptor, and the estrogen-responsive region of the LDLR promoter colocalizes with the sterol response element previously identified. The estrogen response is concentration dependent, saturable, and sensitive to antagonism by estrogen receptor antagonists. Further, we show that compounds with androgen receptor agonist activity attenuate the estrogen-induced up-regulation of LDLR in our model system. Progestins with androgen receptor agonist activity, such as medroxyprogesterone acetate, also suppress estrogen's effects on LDLR expression through their androgenic properties. Characterization of the interplay between these hormone receptors on the LDLR in vitro system may allow a better understanding of the actions of sex steroids on LDLR gene expression and their roles in cardiovascular disease.

Transcriptional regulation of estrogen-responsive genes by non-steroidal estrogens: doisynolic and allenolic acids

Estrogen receptor (ER), a member of the nuclear receptor superfamily, exerts prominent physiological roles in both humans and other species by acting directly as a transcription factor, altering nuclear gene expression. One peculiarity of estrogenic regulation is that it is affected by a wide variety of non-steroidal compounds in addition to the natural hormone, estradiol. Doisynolic and allenolic acid compounds are non-steroidal compounds that act as potent estrogens in animal studies, yet bind to ER extremely poorly in competitive binding assays, raising the possibility of alternative molecular mechanisms for the observed estrogenic effects. In this work we demonstrate that (+/-)-Z-bisdehydrodoisynolic acid, (+/-)-Z-bisdehydrodoisynolic acid 3-methyl ether, and (-) allenolic acid can interact directly with ER. These compounds all serve as ligands for ER in mechanism-specific tissue culture-based reporter gene assays for both positive and negative gene regulation. We have also used a novel assay based on electromobility shift by ER for directly determining relative binding affinities for ER. In addition, we show cell-type-specific activity differences for (+/-)-Z-bisdehydrodoisynolic acid 3-methyl ether, supporting clinical observations indicating a higher potency of this compound in female animals than in humans.

Cells transformed by a v-Myb-estrogen receptor fusion differentiate into multinucleated giant cells

In order to make conditional alleles of the v-myb oncogene, we constructed and tested avian retroviruses which produce a number of different fusion proteins between v-Myb and the human estrogen receptor (ER). We found that the portion of the ER used in making these fusions profoundly affected their transcriptional activation. However, all the fusions tested were only weakly transforming in embryonic yolk sac assays and there was no direct correlation between the level of transcriptional activation and strength of oncogenic transformation. Nevertheless, transformation by a v-Myb-ER fusion was estrogen dependent, and upon withdrawal of the hormone, monocytic-lineage cells differentiated into multinucleated giant cells. Surprisingly, the withdrawal of estrogen caused a dramatic increase in the stability of the fusion protein, although it remained unable to promote cell growth or block differentiation.

Studies on the mechanism of glucocorticoid-mediated repression from a negative glucocorticoid response element from the bovine prolactin gene

Several models for repression of transcription by glucocorticoid hormone, some of which involve so-called negative glucocorticoid response elements (nGRE), have been suggested. In the cases where nGREs are required, the glucocorticoid receptor (GR) is thought to bind to the nGRE and interfere with transcriptional activation by positively acting transactivating factors. We have studied an nGRE from the bovine prolactin gene promoter (PRL3), which increases basal expression from a heterologous promoter in rat pituitary cells (GH3) and is repressed by glucocorticoids. Two proteins in addition to the GR were identified in pituitary cells to bind specifically to the PRL3 nGRE, one of which was the pituitary-specific transcription factor Pit-1/GHF-1. A mutation in the PRL3 nGRE, which destroyed Pit-1/GHF-1 binding, totally abolished the increased basal expression as well as glucocorticoid repression in transfected GH3 cells. A mutation in the binding site for the second protein, termed XTF, partially impaired basal activity but totally abrogated glucocorticoid repression. The same mutation had no effect on GR binding to the PRL3 nGRE. Mixing experiments with whole-cell extracts containing overexpressed GR from COS cells decreased the binding of both Pit-1/GHF-1 and XTF to the PRL3 element. However, Pit-1/GHF-1 displacement from the PRL3 element by the GR required XTF binding. Furthermore, GR binding to the PRL3 nGRE was required for glucocorticoid repression to occur, because a mutation of the GR binding site abolished the glucocorticoid effect. Moreover, the PRL3 nGRE was found to contain only half a palindromic GRE, allowing only one GR moiety to contact the DNA. These data demonstrate that the PRL3 nGRE is composite in nature and that the ability of the GR to repress transactivation by displacement requires an intermediary factor, XTF.

An NF1-like protein functions as a repressor of the von Willebrand factor promoter

The expression of the von Willebrand factor (vWf) gene is restricted to endothelial cells and megakaryocytes. We have previously reported the identification of a region of the vWf gene that regulates its cell-type-specific expression in cell culture. This region (spanning nucleotides -487 to +247) consists of a core promoter (spanning nucleotides -90 to +22), a positive regulatory region (spanning nucleotides +155 to +247), and a negative regulatory region spanning nucleotides -312 to -487. To identify the trans-acting factor(s) that interacts with the negative regulatory region, we carried out gel mobility and DNase1 footprint analyses of sequences -312 to -487. These analyses demonstrated that an NF1-like protein interacts with DNA sequences spanning -440 to -470 nucleotides in the negative regulatory region of the vWf promoter. Base substitution mutations of the NF1 binding site abolished the NF1-DNA interaction. Furthermore, mutation of the NF1 binding site in the promoter fragment (-487 to +155) that contained the core and the negative regulatory region resulted in activation of the mutant promoter in both endothelial and nonendothelial cells. The wild type promoter fragment (-487 to +155) was not activated in either cell type. These results demonstrate that an NF1-like protein functions as a repressor of vWf promoter activity. In contrast, the mutation of the same NF1 binding site, but now in the context of the larger 734-base pair endothelial cell-specific promoter fragment (-487 to +247), did not result in promoter activation in nonendothelial cells. The data indicate that there are additional repressor elements within the vWf promoter region suppressing its activity specifically, in nonendothelial cells, and suggest that there is a secondary repressor element(s) that is located in the terminal region of the first exon of this gene.

Sex steroid hormones and macrophage function

Macrophages are versatile cells whose activities are programmed by environmental signals. In this review, we discuss the potential impact of sex steroid hormones on macrophage activation and production of various effector molecules. The evidence accumulated to date supports the postulate that estrogens, progesterone, androgens and testosterone profoundly influence host defense by controlling the ability of macrophages to participate in immune responses.

Selective repression of transcriptional activators by Pbx1 does not require the homeodomain

PBX1 is a homeobox-containing gene identified as the chromosome 1 participant of the t(1;19) chromosomal translocation of childhood pre-B-cell acute lymphoblastic leukemia. This translocation produces a fusion gene encoding the chimeric oncoprotein E2A-Pbx1, which can induce both acute myeloid and T-lymphoid leukemia in mice. The binding of Pbx1 to DNA is weak; however, both Pbx1 and E2A-Pbx1 exhibit tight binding to specific DNA motifs in conjunction with certain other homeodomain proteins, and E2A-Pbx1 activates transcription through these motifs, whereas Pbx1 does not. In this report, we investigate potential transcriptional functions of Pbx1, using transient expression assays. While no segments of Pbx1 activated transcription, an internal domain of Pbx1 repressed transcription induced by the activation domain of Sp1, but not by the activation domains of VP16 or p53. This Pbx1 domain, which lies upstream of the homeodomain and is highly conserved among Pbx proteins, is thus predicted to bind a specific transcription factor. Surprisingly, the repression activity of Pbx1 did not require homeodomain-dependent DNA binding. Thus, Pbx1 may be able to alter gene transcription by both DNA-binding-dependent and DNA-binding-independent mechanisms.

Transcription factor repression and activation of the human acetylcholinesterase gene

Acetylcholinesterase in man is encoded by a single gene, ACHE, located on chromosome 7q22. In this study, the transcription start sites and major DNA promoter elements controlling the expression of this gene have been characterized by structural and functional studies. Immediately upstream of the first untranslated exon of the gene are GC-rich sequences containing consensus binding sites for several transcription factors, including Sp1, EGR-1 and AP2. In vitro transcription studies and RNase protection analyses of mRNA isolated from human NT2/D1 teratocarcinoma cells reveal that two closely spaced transcription cap sites are located at a consensus initiator (Inr) element similar to that found in the terminal transferase gene. Transient transfection of mutant genes shows that removal of three bases of this initiator sequence reduces promoter activity by 98% in NT2/D1 cells. In vitro transcription studies and transient transfection of a series of 5' deletion mutants of the ACHE promoter linked to a luciferase reporter show an Sp1 site at -71 to be essential for promoter activity. Purified Sp1 protein protects this site from DNase cleavage during in vitro footprinting experiments. A conserved AP2 consensus binding site, located between the GC box elements and the Inr, is protected by recombinant AP2 protein in DNase footprinting experiments, induces a mobility shift with AP2 protein and AP2-containing cell extracts, and fosters inhibition of transcription by AP2 as measured by transient transfection in mouse and human cell lines and in in vitro transcription reactions. These results indicate that AP2 functions as a repressor of human ACHE and mouse Ache transcription.

Retinoid receptors and keratinocytes

In 1987, a tremendous boost in our understanding of the action of dietary vitamin A occurred with the discovery and characterization of nuclear receptors for retinoic acid, the active form of the vitamin, in the laboratories of P. Chambon and R. Evans. They have shown that the nuclear receptors are ligand-activated transcription factors capable of specific gene regulation. Since that discovery, it has been determined that there are at least six retinoic acid receptors belonging to two families, RARs and RXRs, that they are differentially expressed in various mammalian tissues, and that they act as homo- and heterodimers interacting with other ligand-activated nuclear receptors. The domain structure of the receptors has been described, and their DNA-binding, ligand-binding, dimerization, and transcriptional activation regions characterized. Among the most important retinoid-regulated genes are the homeobox proteins, regulatory transcription factors which are responsible for body axis formation, patterning, limb formation, and other crucial processes during development. Retinoic acid and its receptors also regulate many differentiation markers which are particularly important in stratified epithelia, such as skin and oral epithelia. Our increased understanding led to improved therapy of a large number of skin disorders, ranging from acne to wrinkles and including epidermal and oral carcinomas.

Pit-1 binding to specific DNA sites as a monomer or dimer determines gene-specific use of a tyrosine-dependent synergy domain

Transcriptional activation of the prolactin and growth hormone genes, occurring in a cell-specific fashion, requires short-range synergistic interactions between the pituitary-specific POU domain factor Pit-1 and other transcription factors, particularly nuclear receptors. Unexpectedly, we find that these events involve the gene-specific use of alternative Pit-1 synergy domains. Synergistic activation of the prolactin gene by Pit-1 and the estrogen receptor requires a Pit-1 amino-terminal 25-amino-acid domain that is not required for analogous synergistic activation of the growth hormone promoter. The action of this Pit-1 synergy domain is dependent on the presence of two of three tyrosine residues spaced by 6 amino acids and can be replaced by a comparable tyrosine-dependent trans-activation domain of an unrelated transcription factor (hLEF). The gene-specific utilization of this tyrosine-dependent synergy domain is conferred by specific Pit-1 DNA-binding sites that determine whether Pit-1 binds as a monomer or a dimer. Thus, the critical DNA site in the prolactin enhancer, where this domain is required, binds Pit-1 as a monomer, whereas the Pit-1 sites in the growth hormone gene, which do not utilize this synergy domain, bind Pit-1 as a dimer. The finding that the sequence of specific DNA sites dictates alternative Pit-1 synergy domain utilization based on monomeric or dimeric binding suggests an additional regulatory strategy for differential target gene activation in distinct cell types.

Transcriptional regulation of hippocampal 5-HT1a receptors by corticosteroid hormones

5-HT1a receptors in the hippocampus play a critical role in modulating limbic system output. The activity and level of 5-HT1a receptors are modulated by glucocorticoid levels. The present study was undertaken to test the hypothesis that glucocorticoids attenuate the transcriptional activity of the 5-HT1a receptor gene. Using in situ hybridization and RNase protection assays, we observed a substantial increase in 5-HT1a mRNA expression after adrenalectomy in the same hippocampal regions in which 5-HT1a binding sites are increased. This increase in 5-HT1a mRNA expression occurs as early as 1 h after adrenalectomy and precedes the increase in receptor binding sites. Further in situ hybridization analysis showed that 5-HT1a mRNA is increased within individual hippocampal cells after adrenalectomy. Administration of dexamethasone completely prevents the adrenalectomy-induced elevation in hippocampal 5-HT1a receptor mRNA. Nuclear run-on assays showed that the rate of transcription of 5-HT1a mRNA after adrenalectomy increased 70% above the rate from control preparations and could be reduced to basal levels by the administration of dexamethasone. Adrenalectomy did not cause an increase in functional coupling of 5-HT1a receptors to adenylyl cyclase or phospholipase C. These results suggest that transcription of hippocampal 5-HT1a receptor mRNA is under negative regulation by corticosteroid hormones.

A silencer element for the lipoprotein lipase gene promoter and cognate double- and single-stranded DNA-binding proteins

Transfection experiments with constructs containing various 5'-deleted fragments of the human lipoprotein lipase (LPL) promoter and the chloramphenicol acetyltransferase reporter gene revealed an LPL silencer element (LSE) in the region of nucleotides -225 to -81 of the LPL gene that functioned in Chinese hamster ovary (CHO) and HeLa cells. Gel retardation competition analysis showed the presence of a nuclear factor(s) capable of binding to the sequence of nucleotides -169 to -152 of LSE (LSE-6) in a single-stranded (opposite-strand) and double-stranded specific fashion, the binding affinity being almost the same in the two binding forms. Site-directed mutagenesis indicated that almost the entire sequence of LSE-6 was necessary to form the complexes and also critical for silencing activity in CHO cells. The amounts of this binding factor(s) in CHO and HeLa cells were closely associated with transcriptional silencing activity. Photochemical cross-linking experiments indicated that the single- and double-stranded elements recognized the same binding factor(s) with molecular masses of 54 to 63 kDa and 109 to 124 kDa. The 109- to 124-kDa DNA binding factor(s) was found to be a doublet of that of the 54- to 63-kDa factor by isoelectric focusing or by increasing the time of exposure to UV irradiation. When inserted upstream of another gene such as that of the simian virus 40 enhancer/promoter of pSV2CAT, the sequence of nucleotides -190 to -143 (LSE-1) also suppressed transcription of the reporter gene in CHO cells. These results strongly suggest that the LSE plays a role in regulation of LPL gene expression by suppressing its transcription.

Analysis of regulatory regions in the COL1A1 gene responsible for 1,25-dihydroxyvitamin D3-mediated transcriptional repression in osteoblastic cells

The synthesis of type I collagen in bone cells is inhibited by the calcium-regulating hormone 1,25-dihydroxyvitamin D3. Earlier work from our laboratories has indicated that vitamin D regulation is at the level of transcription, based on results from both nuclear run-off assays and functional promoter analysis of a hybrid gene consisting of a 3.6 kb COL1A1 promoter fragment fused to the chloramphenicol acetyltransferase reporter gene. In the present study, we investigated the molecular basis for vitamin D-mediated transcriptional repression of the COL1A1 gene and report the identification of a region within the COL1A1 upstream promoter (the HindIII-Pstl restriction fragment between nucleotides -2295 and -1670) which is necessary for 1,25-dihydroxyvitamin D3 responsiveness in osteoblastic cells. This hormone-mediated inhibitory effect on the marker gene parallels the inhibition of the endogenous collagen gene. A 41 bp fragment from this region (between nucleotides -2256 and -2216) contains a sequence which is very similar to vitamin D-responsive elements identified in the osteocalcin gene. Extracts from cultured cells which express a high level of vitamin D receptor contain a hormone:receptor complex that binds specifically to this 41 bp fragment, as demonstrated by bandshift analysis. However, deletion of this vitamin D receptor binding region from either a -3.5 kb or a -2.3 kb promoter fragment did not abolish vitamin D responsiveness. These results indicate that a vitamin D response element similar to that described for other vitamin D responsive genes (osteocalcin and osteopontin) does not alone mediate the repression of COL1A1 by 1,25-dihydroxyvitamin D3.

How do breast cancers become hormone resistant?

We propose that the molecular heterogeneity of estrogen receptors (ER) in breast tumor cells characterized by the presence of mutant receptor forms, generates the cellular heterogeneity evident when progesterone receptor (PR) or DNA ploidy are analyzed in cell subpopulations. Furthermore, it is likely that cellular heterogeneity leads to the lack of uniformity in response to tamoxifen that we have described. We find that heterogeneity of PR distribution and DNA ploidy reflects the existence of mixed subpopulations of breast cancer cells that are substantially remodeled under the influence of tamoxifen. It appears likely that rather than being "resistant", different subsets of cells can be inhibited or stimulated by tamoxifen and their suppression or outgrowth alters the phenotype of the tumor. PR heterogeneity in solid tumors of patients may predict for such a mixed, and potentially dangerous, response to antiestrogen treatment. Similarly, the molecular heterogeneity resulting from the presence of two normal PR isotypes can lead to inappropriate responses to progesterone antagonists in certain genes or cell types. These agonist-like responses are due to cooperative interactions between the receptors and other transcription factors. As we learn more about the heterogeneity of PR, ER and other proteins in tumors, we may be able to recognize such lethal cell subpopulations, or combinations of regulatory factors. Specifically, with respect to tamoxifen, our data suggest that its use as a chemopreventant in women at high risk of developing breast cancer [Kiang, J. Natn. Cancer Inst. 83, 1991, 462-463] should be viewed with caution, since in the presence of tamoxifen subpopulations of cells may arise that are stimulated, rather than inhibited, by the drug.