Positive and negative effects of nuclear receptors on transcription activation by AP-1 of the human choline acetyltransferase proximal promoter

Kisspeptin Neurons and Estrogen-Estrogen Receptor α Signaling: Unraveling the Mystery of Steroid Feedback System Regulating Mammalian Reproduction

Estrogen produced by ovarian follicles plays a key role in the central mechanisms controlling reproduction via regulation of gonadotropin-releasing hormone (GnRH) release by its negative and positive feedback actions in female mammals. It has been well accepted that estrogen receptor α (ERα) mediates both estrogen feedback actions, but precise targets had remained as a mystery for decades. Ever since the discovery of kisspeptin neurons as afferent ERα-expressing neurons to govern GnRH neurons, the mechanisms mediating estrogen feedback are gradually being unraveled. The present article overviews the role of kisspeptin neurons in the arcuate nucleus (ARC), which are considered to drive pulsatile GnRH/gonadotropin release and folliculogenesis, in mediating the estrogen negative feedback action, and the role of kisspeptin neurons located in the anteroventral periventricular nucleus-periventricular nucleus (AVPV-PeN), which are thought to drive GnRH/luteinizing hormone (LH) surge and consequent ovulation, in mediating the estrogen positive feedback action. This implication has been confirmed by the studies showing that estrogen-bound ERα down- and up-regulates kisspeptin gene (Kiss1) expression in the ARC and AVPV-PeN kisspeptin neurons, respectively. The article also provides the molecular and epigenetic mechanisms regulating Kiss1 expression in kisspeptin neurons by estrogen. Further, afferent ERα-expressing neurons that may regulate kisspeptin release are discussed.

Sexual hormones and diabetes: The impact of estradiol in pancreatic β cell

Diabetes is one of the most prevalent metabolic diseases and its incidence is increasing throughout the world. Data from World Health Organization (WHO) point-out that diabetes is a major cause of blindness, kidney failure, heart attacks, stroke and lower limb amputation and estimated 1.6 million deaths were directly caused by it in 2016. Population studies show that the incidence of this disease increases in women after menopause, when the production of estrogen is decreasing in them. Knowing the impact that estrogenic signaling has on insulin-secreting β cells is key to prevention and design of new therapeutic targets. This chapter explores the role of estrogen and their receptors in the regulation of insulin secretion and biosynthesis, proliferation, regeneration and survival in pancreatic β cells. In addition, delves into the genetic animal models developed and its application for the specific study of the different estrogen signaling pathways. Finally, discusses the impact of menopause and hormone replacement therapy on pancreatic β cell function.

Estrogen signaling: An emanating therapeutic target for breast cancer treatment

Breast cancer, a most common malignancy in women, was known to be associated with steroid hormone estrogen. The discovery of estrogen receptor (ER) gave us not only a powerful predictive and prognostic marker, but also an efficient target for the treatment of hormone-dependent breast cancer with various estrogen ligands. ER consists of two subtypes i.e. ERα and ERβ, that are mostly G-protein-coupled receptors and activated by estrogen, specially 17β-estradiol. The activation is followed by translocation into the nucleus and binding with DNA to modulate activities of different genes. ERs can manage synthesis of RNA through genomic actions without directly binding to DNA. Receptors are tethered by protein-protein interactions to a transcription factor complex to communicate with DNA. Estrogens also exhibit nongenomic actions, a characteristic feature of steroid hormones, which are so rapid to be considered by the activation of RNA and translation. These are habitually related to stimulation of different protein kinase cascades. Majority of post-menopausal breast cancer is estrogen dependent, mostly potent biological estrogen (E2) for continuous growth and proliferation. Estrogen helps in regulating the differentiation and proliferation of normal breast epithelial cells. In this review we have investigated the important role of ER in development and progression of breast cancer, which is complicated by receptor's interaction with co-regulatory proteins, cross-talk with other signal transduction pathways and development of treatment strategies viz. selective estrogen receptor modulators (SERMs), selective estrogen receptor down regulators (SERDs), aromatase and sulphatase inhibitors.

Bone Remodeling in Post-menopausal Osteoporosis

Bone mass in the skeleton is dependent on the coordinated activities of bone-forming osteoblasts and bone-resorbing osteoclasts in discrete bone multi-cellular units. Remodeling of bone in these units is important not only for maintaining bone mass, but also to repair microdamage, to prevent accumulation of too much old bone, and for mineral homeostasis. The activities of osteoblasts and osteoclasts are controlled by a variety of hormones and cytokines, as well as by mechanical loading. Most importantly, sex hormones are very crucial for keeping bone mass in balance, and the lack of either estrogen or testosterone leads to decreased bone mass and increased risk for osteoporosis. The prevalence of osteoporotic fractures is increasing dramatically in the Western part of the world and is a major health problem in many countries. In the present review, the cellular and molecular mechanisms controlling bone remodeling and the influence of sex hormones on these processes are summarized. In a separate paper in this issue, the pathogenesis of post-menopausal osteoporosis will be compared with that of inflammation-induced bone remodeling, including the evidence for and against the hypothesis that concomitant post-menopausal osteoporotic disease influences the progression of periodontal disease.

Chromatin remodeling regulates catalase expression during cancer cells adaptation to chronic oxidative stress

Regulation of ROS metabolism plays a major role in cellular adaptation to oxidative stress in cancer cells, but the molecular mechanism that regulates catalase, a key antioxidant enzyme responsible for conversion of hydrogen peroxide to water and oxygen, remains to be elucidated. Therefore, we investigated the transcriptional regulatory mechanism controlling catalase expression in three human mammary cell lines: the normal mammary epithelial 250MK primary cells, the breast adenocarcinoma MCF-7 cells and an experimental model of MCF-7 cells resistant against oxidative stress resulting from chronic exposure to H₂O₂ (Resox), in which catalase was overexpressed. Here we identify a novel promoter region responsible for the regulation of catalase expression at -1518/-1226 locus and the key molecules that interact with this promoter and affect catalase transcription. We show that the AP-1 family member JunB and retinoic acid receptor alpha (RARα) mediate catalase transcriptional activation and repression, respectively, by controlling chromatin remodeling through a histone deacetylases-dependent mechanism. This regulatory mechanism plays an important role in redox adaptation to chronic exposure to H₂O₂ in breast cancer cells. Our study suggests that cancer adaptation to oxidative stress may be regulated by transcriptional factors through chromatin remodeling, and reveals a potential new mechanism to target cancer cells.

Interplay between AP-1 and estrogen receptor α in regulating gene expression and proliferation networks in breast cancer cells

Estrogen receptor α (ERα) is a ligand-dependent transcription factor that plays an important role in breast cancer. Estrogen-dependent gene regulation by ERα can be mediated by interaction with other DNA-binding proteins, such as activator protein-1 (AP-1). The nature of such interactions in mediating the estrogen response in breast cancer cells remains unclear. Here we show that knockdown of c-Fos, a component of the transcription factor AP-1, attenuates the expression of 37% of all estrogen-regulated genes, suggesting that c-Fos is a fundamental factor for ERα-mediated transcription. Additionally, knockdown of c-Fos affected the expression of a number of genes that were not regulated by estrogen. Pathway analysis reveals that silencing of c-Fos downregulates an E2F1-dependent proproliferative gene network. Thus, modulation of the E2F1 pathway by c-Fos represents a novel mechanism by which c-Fos enhances breast cancer cell proliferation. Furthermore, we show that c-Fos and ERα can cooperate in regulating E2F1 gene expression by binding to regulatory elements in the E2F1 promoter. To start to dissect the molecular details of the cross talk between AP-1 and estrogen signaling, we identify a novel ERα/AP-1 target, PKIB (cAMP-dependent protein kinase inhibitor-β), which is overexpressed in ERα-positive breast cancer tissues. Knockdown of PKIB results in robust growth suppression of breast cancer cells. Collectively, our findings support c-Fos as a critical factor that governs estrogen-dependent gene expression and breast cancer proliferation programs.

Regulation of SVEP1 gene expression by 17β-estradiol and TNFα in pre-osteoblastic and mammary adenocarcinoma cells

Breast cancer is one of several tumors, including prostate, thyroid and kidney, which display a remarkable predilection for metastasis to bone. The preference to metastasize to bone by tumor cells relies on specific interactions among tumor cells, bone marrow microenvironment and bone cells. Osteomimicry is postulated to enable the survival of tumor cells in the bone tissue. Using gene profiling array and RT-PCR we demonstrated the message expression of few bone matrix proteins in mammary adenocarcinoma cells as well as that of cell adhesion molecules (CAMs). A CAM molecule, named SVEP1, was previously shown to be expressed in osteoblastic cells both in vivo and in vitro mediating cell adhesion in the bone-marrow niches. Both estradiol (17βE(2)) and TNFα regulate the expression of adhesion molecules and act in bone-cancer-crosstalk. We focused on differential regulation of SVEP1 gene comparing pre-osteoblastic MBA-15 and mammary adenocarcinoma DA3 cells. 17βE(2) and TNFα activated SVEP1 promoter, increased its message and protein levels in both cell types. Using chromatin immunoprecipitation assay, we quantified SVEP1 promoter occupancy by transcription factors; TFIIB, ERα, NF-κB, Sp1 and their binding was also regulated by both factors. By comparing pre-osteoblastic with mammary adenocarcinoma cells, the study expands our understanding of SVEP1 gene expression regulation and it sheds light on its involvement in bone-cancer-microenvironment interactions.

Genome-wide mapping of estrogen receptor-beta-binding regions reveals extensive cross-talk with transcription factor activator protein-1

Estrogen signaling can occur through a nonclassical pathway involving the interaction of estrogen receptors (ER) with other transcription factors such as activator protein-1 (AP-1) and SP-1. However, there is little mechanistic understanding about this pathway, with conflicting results from in vitro investigations. In this study, we applied the ChIP-on-chip approach to identify ERbeta-binding sites on a genome-wide scale, identifying 1,457 high-confidence binding sites in ERbeta-overexpressing MCF7 breast cancer cells. Genes containing ERbeta-binding sites can be regulated by E2. Notably, approximately 60% of the genomic regions bound by ERbeta contained AP-1-like binding regions and estrogen response element-like sites, suggesting a functional association between AP-1 and ERbeta signaling. Chromatin immunoprecipitation (ChIP) analysis confirmed the association of AP-1, which is composed of the oncogenic transcription factors c-Fos and c-Jun, to ERbeta-bound DNA regions. Using a re-ChIP assay, we showed co-occupancy of ERbeta and AP-1 on chromatin. Short interfering RNA-mediated knockdown of c-Fos or c-Jun expression decreased ERbeta recruitment to chromatin, consistent with the role of AP-1 in mediating estrogen signaling in breast cancer cells. Additionally, ERalpha and ERbeta recruitment to AP-1/ERbeta target regions exhibited gene-dependent differences in response to antiestrogens. Together, our results broaden insights into ERbeta DNA-binding at the genomic level by revealing crosstalk with the AP-1 transcription factor.

Estrogen-induced and TAFII30-mediated gene repression by direct recruitment of the estrogen receptor and co-repressors to the core promoter and its reversal by tamoxifen

Estradiol (E2) acts through the estrogen receptor (ER) to downregulate many genes, and tamoxifen (Tam) largely reverses this repression but the underlying mechanisms are unclear. Repression of the folate receptor (FR)-alpha P4 core promoter by ER is enhanced by E2 and reversed by Tam. This effect was unaffected by inhibition of new protein synthesis and required the E/F and the DNA-binding domains of ER without direct binding of ER to DNA. The repression by E2/ER was not specific for either Sp1 or TATA elements but was loosely selective for the initiator and flanking sequence. Insertion of a response element or a relatively strong Sp1 cluster to recruit ER upstream of the core promoters caused a switch to activation by E2/ER that was inhibited by Tam. In nuclear extracts, association of ER with a biotinylated core promoter fragment was promoted by E2 but Tam blocked this effect. Repression/de-repression of the P4 promoter and endogenous FR-alpha expression by E2/Tam required SMRT and/or NCoR. ER associated with the chromosomal P4 promoter and SMRT and NCoR associated with it in an ER-dependent manner; these associations were favored by E2 but disrupted by Tam, in the short term, without changes in ER expression. TAFII30 was required for optimal P4 promoter activity and for the repressive association of ER. E2 may thus maintain a low transcriptional status of genes by favoring direct TAFII30-dependent association of ER with the core promoter in a co-repressor complex containing SMRT and/or NCoR; this repression is overridden in target genes containing an upstream element that strongly recruits ER. In addition to suppressing the activation of classical E2 target genes, Tam may upregulate genes by passively dissociating the ER co-repressor complex.

Mechanisms of estrogen receptor signaling: convergence of genomic and nongenomic actions on target genes

Estrogen receptors (ERs) act by regulating transcriptional processes. The classical mechanism of ER action involves estrogen binding to receptors in the nucleus, after which the receptors dimerize and bind to specific response elements known as estrogen response elements (EREs) located in the promoters of target genes. However, ERs can also regulate gene expression without directly binding to DNA. This occurs through protein-protein interactions with other DNA-binding transcription factors in the nucleus. In addition, membrane-associated ERs mediate nongenomic actions of estrogens, which can lead both to altered functions of proteins in the cytoplasm and to regulation of gene expression. The latter two mechanisms of ER action enable a broader range of genes to be regulated than the range that can be regulated by the classical mechanism of ER action alone. This review surveys our knowledge about the molecular mechanism by which ERs regulate the expression of genes that do not contain EREs, and it gives examples of the ways in which the genomic and nongenomic actions of ERs on target genes converge. Genomic and nongenomic actions of ERs that do not depend on EREs influence the physiology of many target tissues, and thus, increasing our understanding of the molecular mechanisms behind these actions is highly relevant for the development of novel drugs that target specific receptor actions.

An activator protein 1-like motif mediates 17beta-estradiol repression of gonadotropin-releasing hormone receptor promoter via an estrogen receptor alpha-dependent mechanism in ovarian and breast cancer cells

Although it is recognized that estrogen is one of the most important regulators of GnRH receptor (GnRHR) gene expression, the mechanism underlying the regulation at the transcriptional level is unknown. In the present study, we demonstrated that 17beta-estradiol (E2) repressed human GnRHR promoter via an activator protein 1-like motif and estrogen receptor-alpha, of which the DNA-binding domain and the ligand-binding domain were indispensable for the repression. Interestingly, the same cis-acting motif was also found to be important for both the basal activity and phorbol 12-myristate 13-acetate responsiveness of the GnRHR promoter. EMSAs indicated that multiple transcription factors including c-Jun and c-Fos bound to the activator protein 1-like site and that their DNA binding activity was not significantly affected by E2 treatment. In addition, we demonstrated that the E2 repression could be antagonized by phorbol 12-myristate 13-acetate, which stimulated c-Jun phosphorylation on serine 63, a process that is a prerequisite for recruitment of the transcriptional coactivator cAMP response element binding protein (CREB)-binding protein (CBP). Concomitantly, we found that overexpression of CBP could reverse the suppression in a dose-dependent manner. Taken together, our data indicate that E2-activated estrogen receptor-alpha represses human GnRHR gene transcription via an indirect mechanism involving CBP and possibly other transcriptional regulators.

An estrogen receptor (ER)alpha deoxyribonucleic acid-binding domain knock-in mutation provides evidence for nonclassical ER pathway signaling in vivo

We created a nonclassical estrogen receptor (ER) knock-in mouse model by introducing a mutation that selectively eliminates classical ER signaling through estrogen response elements, while preserving the nonclassical ER pathway. Heterozygous nonclassical ER knock-in (NERKI) females are infertile. Their ovaries contain no corpora lutea, reflecting a defect in ovulation, and the stromal cells contain lipid droplets, suggesting altered steroidogenesis. The uteri are enlarged with evidence of cystic endometrial hyperplasia, and the mammary glands are hypoplastic. These phenotypic features indicate differential ER effects on growth and development in various estrogen-responsive tissues. These findings suggest that nonclassical ER signaling pathways play an important physiological role in the development and function of the reproductive system.

Functional analysis of the rat bile salt export pump gene promoter

The 5' flanking region of the bile salt export pump (Bsep) gene was systematically analysed to provide the basis for understanding the mechanisms which regulate Bsep transcription. In addition substrates and drugs were investigated for their ability to alter Bsep promoter activity. Bsep promoter function was restricted to hepatocyte derived HepG2 cells. The 5' deletional analysis revealed a biphasic shape of reporter gene activities, indicating a suppressive element between nucleotides -800 and -512. Two consensus sites for the farnesoid X receptor (FXR) were located at nucleotides -473 and -64. The latter was characterized as functionally active in bile acid-mediated feed-back regulation of Bsep transcription. Bsep promoter activity was reduced by rifampin and beta-estradiol. The anti-estrogen tamoxifen stimulated promoter activity. Dexamethasone, hydrocortisone and phenobarbital had no effect on Bsep promoter activity. In conclusion, the data suggest that transcriptional regulation of the Bsep gene can be modulated by a number of endogenous compounds and xenobiotics. FXR was a major regulatory factor, mediating bile acid feed-back stimulation of Bsep transcription.

Changes in proinflammatory cytokine activity after menopause

There is now a large body of evidence suggesting that the decline in ovarian function with menopause is associated with spontaneous increases in proinflammatory cytokines. The cytokines that have obtained the most attention are IL-1, IL-6, and TNF-alpha. The exact mechanisms by which estrogen interferes with cytokine activity are still incompletely known but may potentially include interactions of the ER with other transcription factors, modulation of nitric oxide activity, antioxidative effects, plasma membrane actions, and changes in immune cell function. Experimental and clinical studies strongly support a link between the increased state of proinflammatory cytokine activity and postmenopausal bone loss. Preliminary evidence suggests that these changes also might be relevant to vascular homeostasis and the development of atherosclerosis. Better knowledge of the mechanisms and the time course of these interactions may open new avenues for the prevention and treatment of some of the most prevalent and important disorders in postmenopausal women.

A novel untranslated 'exon H' of the human choline acetyltransferase gene in placenta

To investigate the existence of 5'-region(s) of human choline acetyltransferase (hChAT) mRNA in placenta we analyzed the presence or absence of ChAT 5'-untranslated regions (UTR) in human neuronal and non-neuronal cells. Total RNA from human spinal cord, placenta, cultured choriocarcinoma JEG-3 and neuroblastoma CHP126 and MC-IXC cells was reverse transcribed and used for polymerase chain reaction amplification (RT-PCR). We used a sense primer located in the 5'-flanking region, in the previously defined intronic sequence and an anti-sense primer located in the common coding exon 2 of the hChAT gene. An amplified product of 567 bp in size was obtained only in human placenta and in JEG-3 cells whereas it was absent in spinal cord, CHP126 and MC-IXC cells. It was designated 'H-type' of ChAT mRNA. Whereas CHP126 produced the R- and N-type of ChAT mRNAs, no transcript of the N-and R-type was detected in JEG-3 and human placenta. In addition, CHP126 and JEG-3 cells and placenta showed the expression of the M-type of ChAT mRNA. The identity of the amplified 567 bp product (H-type) was confirmed by Southern hybridization and sequencing. The nucleotide sequence of the amplified fragment in placenta revealed the existence of a previously unknown type of ChAT mRNA produced by alternative splicing. Using primer extension we further determined the transcription initiation site of the H-type hChAT mRNA in placenta. These results demonstrate the expression of a novel ChAT mRNA isoform in human placenta in addition to the M-type. These data may be possibly explained by the presence of a placenta specific promoter in the ChAT gene, which might be the proximal promoter P1.

Estrogen suppression of EGFR expression in breast cancer cells: a possible mechanism to modulate growth

Epidermal growth factor receptor (EGFR) is a transmembrane receptor whose overexpression in breast cancer predicts for poor prognosis and is inversely correlated with expression of estrogen receptor (ER). This study was designed to investigate whether estrogen plays an active role in suppression of EGFR expression in estrogen-responsive breast cancer cell lines expressing low levels of EGFR. Upon withdrawal of estrogen, EGFR mRNA and protein increased 3-6 fold in MCF-7, T47D, and BT474 ER+ breast cancer cells. This was reversible upon addition of estradiol back to the culture media, but only after prolonged treatment. Nuclear run-on assays and studies with the transcription inhibitor actinomycin D demonstrated that regulation is at the transcriptional level. These results indicate that in the presence of estrogen, ER+ breast cancer cells possess active mechanisms to suppress EGFR expression. Up-regulation of EGFR in response to estrogen depletion and growth inhibition could represent an attempt to rescue cell growth by utilizing an alternative pathway. Indeed, we found that estrogen-depleted breast cancer cells are more sensitive to the mitogenic effects of EGF and TGF-alpha, and simultaneous blockade of both estrogen and EGFR signaling pathways induced cell death. J. Cell. Biochem. Suppl. 36: 232-246, 2001.

Estrogen receptor binding to DNA is not required for its activity through the nonclassical AP1 pathway

In the classical signaling pathway, the estrogen receptor (ER) binds directly to estrogen response elements (EREs) to regulate gene transcription. To test the hypothesis that the nonclassical pathway involves ER interactions with other proteins rather than direct binding to DNA, mutations were introduced into the DNA binding domain (DBD) of the mouse ERalpha. The effects of these DBD mutations were examined in DNA binding assays using reporter constructs containing either EREs (classical) or AP1 (nonclassical) response elements. Using the AP1 reporter, there was a reversal of ER action relative to that seen with the ERE reporter. Estradiol induced suppression, and the antiestrogen ICI 182,780 stimulated transcription of the AP1 reporter. DBD mutations in the proximal (P-box) of the first zinc finger of the ER (E207A/G208A and E207G/G208S) eliminated ERE binding. These mutants were inactive using the ERE reporter but retained partial or full activity with the AP1 reporter. The DBD mutant ERs interacted with Jun when tested in mammalian cell two-hybrid assays. Two mutations (K366D and I362R) in the ER ligand binding domain known to alter coactivator interactions impaired transcriptional responses using either the ERE or AP1 reporters. We concluded that ER action through the AP1 response element involves interactions with other promoter-bound proteins instead of, or in addition to, direct binding to DNA. Interactions with coactivators were required for both pathways. These data supported a model in which ER-mediated transcriptional activation or repression is dependent on the ligand and the nature of the response element in the target gene.

Par-4 induces cholinergic hypoactivity by suppressing ChAT protein synthesis and inhibiting NGF-inducibility of ChAT activity

Profound reductions in choline acetyl-transferase (ChAT) activity are reliable markers for cholinergic hypoactivity associated with cognitive function deficit in Alzheimer's disease (AD). Par-4 (prostate apoptosis response-4) is a novel mediator of neuronal apoptosis associated with the pathogenesis of AD. Par-4 contains a leucine zipper domain (Leu.zip) that presumably mediates protein-protein interactions critical for its functions in apoptosis. Par-4 activity can be effectively blocked by overexpression of Leu. zip because it exerts a dominant negative action possibly by competitively blocking the interaction of Par-4 with other proteins. Whether Par-4 participates in regulation of cholinergic signaling has not been determined. We report that overexpression of Par-4 results in apoptotic and non-apoptotic reductions in ChAT activity in transfected PC12 cells following exposure to a toxic concentration (50 microM) of aggregated amyloid beta peptide 1-42 (Abeta 1-42) and a non-toxic concentration (1 microM) of soluble Abeta 1-42, respectively. Non-apoptotic reduction in ChAT activity induced by Par-4 can be completely blocked by co-overexpression of Leu.zip, indicating that enhanced Par-4 activity is a necessary event for cholinergic hypoactivity in PC12 cells. Further studies found that Par-4 induces non-apoptotic reduction in ChAT activity by: (1) reducing ChAT protein levels following exposure to non-toxic concentration of Abeta, and (2) blocking the cellular capability to increase ChAT activity following exposure to nerve growth factor (NGF). The role of Par-4 in inducing cholinergic hypoactivity may have significant implications in the understanding and the treatment of memory impairment in AD.

Estrogen suppresses transcription of lipoprotein lipase gene. Existence of a unique estrogen response element on the lipoprotein lipase promoter

Estrogen exerts a variety of effects not only on female reproductive organs but also on nonreproductive organs, including adipose tissue. Estrogen inhibits obesity triggered by ovariectomy in rodents. We studied the mechanism underlying this estrogen-dependent inhibition of obesity. Estrogen markedly decreased the amounts of fat accumulation and lipoprotein lipase (LPL) mRNA as well as triglyceride accumulation in genetically manipulated 3T3-L1 adipocytes stably expressing the estrogen receptor (ER). A pLPL(1980)-CAT construct, along with an ER expression vector, was introduced into differentiated 3T3-L1 cells, and CAT activities were determined. ER, mostly ligand-dependently, inhibited the basal LPL promoter activity by 7-fold. We searched the LPL promoter for an estrogen-responsive suppressive element by employing a set of 5'-deletion mutants of the pLPL-CAT reporter. Although there was no classical estrogen response element, it was demonstrated that an AP-1-like TGAATTC sequence located at (-1856/-1850) was responsible for the suppression of the LPL gene transcription by estrogen. An electrophoretic mobility shift assay probed with the TGAATTC sequence demonstrated formation of a specific DNA-nuclear protein complex. Interestingly, this complex was not affected by the addition of any antibodies against ER, c-Jun, c-Fos, JunB, or JunD. Because this TGAATTC element responded to phorbol ester and overexpression of CREB-binding protein abrogated the suppressive effect of estrogen on the LPL promoter, we conclude that a unique protein that is related to the AP-1 transcription factor families may be involved in the complex that binds to the TGAATTC element.

Estrogen decreases TNF gene expression by blocking JNK activity and the resulting production of c-Jun and JunD

Central to the bone-sparing effect of estrogen (E(2)) is its ability to block the monocytic production of the osteoclastogenic cytokine TNF-alpha (TNF). However, the mechanism by which E(2) downregulates TNF production is presently unknown. Transient transfection studies in HeLa cells, an E(2) receptor-negative line, suggest that E(2) inhibits TNF gene expression through an effect mediated by estrogen receptor beta (ERbeta). We also report that in RAW 264.7 cells, an E(2) receptor-positive murine monocytic line, E(2) downregulates cytokine-induced TNF gene expression by decreasing the activity of the Jun NH(2)-terminal kinase (JNK). The resulting diminished phosphorylation of c-Jun and JunD at their NH(2)-termini decreases the ability of these nuclear proteins to autostimulate the expression of the c-Jun and JunD genes, thus leading to lower production of c-Jun and JunD. The consequent decrease in the nuclear levels of c-Jun and JunD leads to diminished binding of c-Jun/c-Fos and JunD/c-Fos heterodimers to the AP-1 consensus sequence in the TNF promoter and, thus, to decreased transactivation of the TNF gene.

NGF-induction of the expression of ChAT mRNA in PC12 cells and primary cultures of embryonic rat basal forebrain

The objective of this study was to examine the role of nerve growth factor (NGF) in regulation of expression of the cholinergic phenotype. NGF was administered to PC12 cells or primary cultures of embryonic (E17) rat basal forebrain for 2 days, then steady-state levels of choline acetyltransferase (ChAT) mRNA was monitored. Expression of ChAT mRNA isoforms was investigated using reverse transcription-polymerase chain reaction (RT-PCR) to amplify different upstream regions of the ChAT transcripts, and Southern blot analysis was used to verify identity of the PCR products. An NGF-induced increase of 1.8- and 1.5-fold in steady-state level of the ChAT transcript containing the M-exon (M-ChAT) was observed in PC12 cells and embryonic rat basal forebrain neurons, respectively. Also, a 2-fold increase in ChAT protein as determined by western blot analysis was associated with an NGF-mediated increase of 1.7-fold in ChAT activity in rat basal forebrain neurons within the same cultures following 4 days of NGF treatment.

Molecular mechanisms regulating NGF-mediated enhancement of cholinergic neuronal phenotype: c-fos trans-activation of the choline acetyltransferase gene

Nerve growth factor (NGF) enhances expression of the cholinergic phenotype observed as increased choline acetyltransferase (ChAT) activity, immunoreactivity, and mRNA. In the present study, treatment of cultured rat embryonic basal forebrain neurons with anti-c-fos, prior to administering NGF, blocked NGF-mediated increases in ChAT activity by 67%; basal ChAT activity was not affected by the antisense oligonucleotide treatment. Reverse transcription-polymerase chain reaction (RT-PCR) revealed that anti-c-fos treatment resulted in not only blockade but enhancement of steady-state ChAT mRNA at different time points. These data suggest that c-fos is an important component in NGF-mediated changes in the cholinergic phenotype and support the hypothesis that c-fos plays a role in the regulation of transcription of the ChAT gene. Elucidation of mechanisms underlying this regulation may aid drug development in neurodegenerative disease.

Synergistic transcriptional activation of the mouse urokinase plasminogen activator (uPA) gene and of its enhancer activator protein 1 (AP1) site by cAMP and retinoic acid

We have investigated the mechanism whereby all-trans retinoic acid (tRA) potentiates the 8-bromo-cAMP (8-BrcAMP)-dependent transcription of the urokinase plasminogen activator (uPA) gene in SC115 mouse mammary carcinoma cells. Photoaffinity labelling experiments showed that tRA did not alter the cellular content of cAMP-dependent protein kinase regulatory subunits I and II. In agreement with this, nuclear run-on analysis in the presence of the translational inhibitor puromycin demonstrated that the effect of 8-BrcAMP and its potentiation by tRA were independent of protein synthesis. A transiently transfected 6.6 kb uPA 5'-flanking region-chloramphenicol acetyltransferase (CAT) fusion gene mimicked the response of the endogenous uPA gene. Thus 1 mM 8-BrcAMP induced a 100-200% increase in CAT content, 100 nM tRA had no effect and 100 nM tRA+1 mM 8-BrcAMP induced a 300-500% increase in cells co-transfected with tRA receptor and/or 9-cis-RA receptor. Analysis of 5'-deleted constructs showed that the tRA effect required at least two cis regions: -2657 to -2186, encompassing the 100 bp uPA enhancer, and -709 to -324, which exhibited silencing activity. Neither region contained a tRA-response element-like motif. Because tRA receptor and 9-cis-RA receptor interact with activator protein 1 (AP1), we tested whether tRA regulated the uPA enhancer AP1 site in the presence of 8-BrcAMP. We found that a dimer of this site fused to a minimal uPA-CAT fusion gene was responsive to 1 mM 8-BrcAMP (100% CAT increase), not responsive to 100 nM tRA, and synergistically responsive to 100 nM tRA+1 mM 8-BrcAMP (240% CAT increase) in cells co-transfected with Fos and Jun. Synergistic activation of the same construct and of the 6.6 kb uPA-CAT fusion gene was also obtained using tRA and 100 nM PMA. We conclude that multiple cis elements, probably including the uPA enhancer AP1 site, mediate the tRA potentiation of uPA transcription.

Potential role of NF-kB and RXR beta like proteins in interferon induced HLA class I and beta globin gene transcription in K562 erythroleukaemia cells

Positive effects of biological response modifiers on cancer cells are usually measured using markers for increased immunogenicity as well as those for increased differentiation of the cells. An increase in levels of HLA class I antigens and the adult (beta) globin molecules are two such markers that may be used to assess the effect of modulators like interferons on the K562 erythroleukaemia cell line. Although interferon mediated up regulation of gene expression is thought to be primarily regulated by binding of proteins to the Interferon responsive cis elements in the promoters of IFN responsive genes, recent evidence has shown the induction of other transcriptional activators in response to IFN treatment. We present evidence for one such instance wherein up regulation of HLA class I and beta globin gene transcription are accompanied by induction of binding activities similar to RXR beta and kB proteins in K562 cells.

Estrogens and glucocorticoids induce the expression of c-erbB2/NEU receptor in Ishikawa human endometrial cells

We investigated the effects of estrogens and other steroid hormones on c-erbB2 gene expression in Ishikawa human endometrial adenocarcinoma cells. We have found that the c-erbB2/NEU transcripts are present in the Ishikawa endometrial cell line as well as in human endometrial adenocarcinoma cells. Both cell types express the 4.6 and 2.3 kb c-erbB2 mRNAs. Estradiol significantly increased in a time- and dose-dependent manner the content of c-erbB2 mRNA and the concentration of NEU protein in Ishikawa cell extracts, while progesterone was devoid of any activity. The effect of estradiol was partially reversed by the antiestrogen 4-hydroxytamoxifen, which, however, given alone exhibited agonist effects. Glucocorticoid dexamethasone augmented in a time- and dose-dependent fashion the content of c-erbB2 mRNA and the concentration of NEU protein in Ishikawa cell extracts. The antiglucocorticoid RU 486 acted as a glucocorticoid agonist increasing c-erbB2 gene activation. To our knowledge, this is the first report documenting the induction by steroid hormones of c-erbB2 gene expression in neoplastic human endometrial cells. Our data support the hypothesis that the oncogenic effect of estrogens on human endometrial cells may be partially mediated by its effect on the expression of the c-erbB2 proto-oncogene. The finding that glucocorticoids may induce endometrial c-erbB2 gene expression suggests that they may participate in the emergence of uterine neoplasias.

Transcriptional activation of human choline acetyltransferase by AP2- and NGF-induced factors

ChAT (choline acetyltransferase) is the enzyme responsible for acetylcholine synthesis and is specifically expressed in cholinergic neurons. To further characterize the transcriptional regulation of the hCHAT (human ChAT) gene by NGF, we examined the effects upon ChAT promoter activity of a family of transcription factors which are activated by NGF and several extracellular stimuli and encoded by immediate-early genes. These include NGFI-A (Egr1, zif268), NGFI-C (Egr2), Krox-20 and NGFI-B (Nurr77). Two fragments of the hChAT gene were used for functional analysis carrying 944 bp (P1) and 4000 bp (P1 + P2) of the 5' flanking region in front of the chloramphenicol acetyltransferase (CAT) reporter gene. They were transiently co-transfected with NGFI-A, NGFI-C, Krox-20 and NGFI-B expression vectors in NG108-15, SN6 and COS-1 cells. CAT activity after transfection of the p4000 ChAT-CAT reporter into both neuronal cell lines (NG108-15 and SN6 cells) was increased up to 5-fold in the presence of co-transfected NGFI-A and up to 5- and 12-fold after co-transfection of NGFI-C expression vector in NG108-15 and SN6 cells, respectively. In NG108-15 cells, dbcAMP excerted a strong enhancing activity on the transactivation properties of NGFI-C while this was not observed when cells were transfected with NGFI-A. These trans-activation effects were specific for neuronal cells. When NG108-15 cells were treated with dbcAMP in the presence of H89, a specific PKA inhibitor, the increase of transcriptional activity of NGFI-C was abolished, indicating that a signalling transduction mechanism through PKA plays a role in NGFI-C-induced trans-activation. Electrophoretic mobility-shift assays showed that the sequence GCCCGGGGAG (NGFRE) located 1205 bp upstream of the first coding ATG (E1) can bind NGFI-A but not NGFI-C. Several possibilities explaining the observed results are discussed. Finally, transfections of ChAT-CAT reporters including the P1 + P2 region or a minimal ChAT enhancer present in the P2 region in front of a heterologous promoter indicated the presence of a regulatory element which conferred AP2-dependent trans-activation with homologous as well as with heterologous promoter constructs.

Modulation of cholinergic locus expression by glucocorticoids and retinoic acid is cell-type specific

Modulation of mRNA expression of choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (VAChT) by the glucocorticoid dexamethasone and by retinoic acid was examined in two neuronal cell lines: basal forebrain-derived SN56 and pheochromocytoma PC12. Dexamethasone up-regulated ChAT and VAChT in SN56 cells, while it had inhibitory effects on these genes in PC12 cells. Retinoic acid stimulated the cholinergic markers in both cell types, but in SN56 cells its effect was partially additive with that of dexamethasone, whereas it was much smaller and abrogated by dexamethasone in PC12 cells. Acetylcholine content correlated with these mRNA changes. The presence of a glucocorticoid response element consensus sequence in the VAChT/ChAT gene locus suggests direct transcriptional regulation by glucocorticoids.

Causal role for jun protein in the stimulation of choline acetyltransferase by insulin in embryonic chick retina

Previous work showed that the availability of insulin to the embryonic chick retina at a critical developmental stage stimulated the activity of the acetylcholine synthetic enzyme, choline acetyltransferase (ChAT) (R. E. Hausman et al., 1991, Dev. Brain Res. 59, 31-37). Here we show that a 2- to 5-min exposure to insulin results in a greater than 24 hr elevation in ChAT protein. Immediately following exposure to insulin there is a transient increase in the level of jun protein followed by an increase in ChAT. The stimulation of ChAT protein is not the result of an overall stimulation of protein synthesis as other proteins are not affected. Exposure of the cells to antisense oligonucleotide to jun, but not to sense oligonucleotide, reduces the increase in both jun and ChAT. These and previous results suggest that insulin is necessary for the characteristic increase in ChAT protein during retina development and that this increase requires the transient synthesis of jun.

Glucocorticoid upregulation of glutamate dehydrogenase gene expression in vitro in astrocytes

Glutamate, the major excitatory neurotransmitter, is preferentially catabolized in astrocytes by glutamate dehydrogenase (GDH). Treatment of an astrocytic cell line with hydrocortisone (10(-5) M) resulted in increased expression of GDH mRNA. Transfection of the cells with truncated parts of the GDH promoter showed that genomic responsive elements activated by hydrocortisone are localized in the -557/+1 region of the promoter. This control of GDH expression by glucocorticoids may be involved in their protective effect against glutamate excitotoxicity.

A previously unidentified choline acetyltransferase transcript in the human foetal brain

To identify choline acetyltransferase (ChAT) transcript(s) in human foetal brain we used the reverse polymerase chain reaction including different couples of oligonucleotide primers. The analysis indicated the presence of a new choline acetyltransferase transcript containing at its 5' end an untranslated exon (E1A) of 57 nucleotides in length extending from Nt 3771 to Nt 3828. It differs in the 5'-non coding region from the M-N-R type mRNA previously described in mouse and rat.