Multiple mechanisms for Pitx-1 transactivation of a luteinizing hormone beta subunit gene

Ovaprim, a commercial spawning inducer, stimulates gonadotropin subunit gene transcriptional activity: A study correlated with plasma steroid profile, ovulation and fertilization in the catfish Heteropneustes fossilis

The commercial fish spawning inducer Ovaprim (OVP) containing a salmon gonadotropin-releasing hormone analogue and domperidone (a dopamine receptor-2 antagonist) has been widely used as an effective spawning inducer in artificial breeding of fishes. It induces a preovulatory LH surge resulting in final oocyte maturation (FOM) and ovulation through a mechanism involving a steroidogenic shift to secrete a maturation-inducing steroid (MIS). In the present study, a 0.5μL/g body weight dose of OVP each injected at 0h and 24h intraperitoneally into gravid female catfish, Heteropneustes fossilis resulted in periovulatory changes in gonadotropin (GtH) subunit gene expression and steroid hormone levels. The OVP injections induced ovulation time-dependently from 6h onwards with 100% ovulation recorded from 24h to 48h. The fertilization rate was high from 6h to 18h and declined from 24h onwards. The OVP treatment up regulated the expression of GtH subunit genes differentially. The expression of glycoprotein-α (GPα) and luteinizing hormone (LHβ) peaked at 6h and 12h, and declined at 18h and 24h after the first injection. The second OVP injection at 24h elicited only a transient increase in the GPα expression at 6h and a sustained increase in the LHβ expression from 6h to 18h after the second injection, but both transcripts decreased subsequently. The follicle-stimulating hormone (FSHβ) expression responded to the OVP treatment from 12h onwards and maintained a constant level from 18h to 36h after the first injection; the second dose had little effect. Plasma steroids were differentially altered: the levels of estradiol-17β decreased while that of the MIS 17,20β-dihydroxy-4-pregnen-3-one; 17,20β-DP increased, causing the steroidogenic shift preceding FOM and ovulation. The present results indicate that LHβ expression coincides with the ovulation response and the late induction and maintenance of the FSH expression may be related to post-ovulatory events in the ovary.

Transcriptional enhancers: Transcription, function and flexibility

Active transcriptional enhancers are often transcribed to eRNAs, whose changing levels mirror those of the target gene mRNA. We discuss some of the reported functions of these eRNAs and their likely diversity to allow utilization of distinct cis regulatory regions to enhance transcription in diverse developmental and cellular contexts.

RNA transcribed from a distal enhancer is required for activating the chromatin at the promoter of the gonadotropin α-subunit gene

Since the discovery that many transcriptional enhancers are transcribed into long noncoding RNAs termed "enhancer RNAs" (eRNAs), their putative role in enhancer function has been debated. Very recent evidence has indicted that some eRNAs play a role in initiating or activating transcription, possibly by helping recruit and/or stabilize binding of the general transcription machinery to the proximal promoter of their target genes. The distal enhancer of the gonadotropin hormone α-subunit gene, chorionic gonadotropin alpha (Cga), is responsible for Cga cell-specific expression in gonadotropes and thyrotropes, and we show here that it encodes two bidirectional nonpolyadenylated RNAs whose levels are increased somewhat by exposure to gonadotropin-releasing hormone but are not necessarily linked to Cga transcriptional activity. Knockdown of the more distal eRNA led to a drop in Cga mRNA levels, initially without effect on the forward eRNA levels. With time, however, the repression on the Cga increased, and the forward eRNA levels were suppressed also. We demonstrate that the interaction of the enhancer with the promoter is lost after eRNA knockdown. Dramatic changes also were seen in the chromatin, with an increase in total histone H3 occupancy throughout this region and a virtual loss of histone H3 Lys 4 trimethylation at the promoter following the eRNA knockdown. Moreover, histone H3 Lys 27 (H3K27) acetylation, which was found at both enhancer and promoter in wild-type cells, appeared to have been replaced by H3K27 trimethylation at the enhancer. Thus, the Cga eRNA mediates the physical interaction between these genomic regions and determines the chromatin structure of the proximal promoter to allow gene expression.

Rgs6 is required for adult maintenance of dopaminergic neurons in the ventral substantia nigra

Parkinson disease (PD) is characterized by the preferential, but poorly understood, vulnerability to degeneration of midbrain dopaminergic (mDA) neurons in the ventral substantia nigra compacta (vSNc). These sensitive mDA neurons express Pitx3, a transcription factor that is critical for their survival during development. We used this dependence to identify, by flow cytometry and expression profiling, the negative regulator of G-protein signaling Rgs6 for its restricted expression in these neurons. In contrast to Pitx3^−/− mDA neurons that die during fetal (vSNc) or post-natal (VTA) period, the vSNc mDA neurons of Rgs6^−/− mutant mice begin to exhibit unilateral signs of degeneration at around 6 months of age, and by one year cell loss is observed in a fraction of mice. Unilateral cell loss is accompanied by contralateral degenerating neurons that exhibit smaller cell size, altered morphology and reduced dendritic network. The degenerating neurons have low levels of tyrosine hydroxylase (TH) and decreased nuclear Pitx3; accordingly, expression of many Pitx3 target gene products is altered, including Vmat2, Bdnf, Aldh1a1 (Adh2) and Fgf10. These low TH neurons also express markers of increased dopamine signaling, namely increased DAT and phospho-Erk1/2 expression. The late onset degeneration may reflect the protective action of Rgs6 against excessive DA signaling throughout life. Rgs6-dependent protection is thus critical for adult survival and maintenance of the vSNc mDA neurons that are most affected in PD.

The locomotor deficits associated with Parkinson disease result from the death of a specific subset of dopamine neurons in the ventral part of the midbrain. The reason for the greater sensitivity to degeneration of those, relative to other, neurons is not clear. Prior work showed that the Pitx3 transcription factor is specifically expressed in these neurons where it has a survival role during development. The present work identified a cell signaling component, Rgs6, that is also restricted to the sensitive neurons in the midbrain and that exerts a protective function, particularly late in life. While the loss of Rgs6 function may predispose or contribute to Parkinson disease, its stimulation may provide a novel therapeutic avenue to treat Parkinson disease.

Effects of estrogens and estrogenic disrupting compounds on fish mineralized tissues

Estrogens play well-recognized roles in reproduction across vertebrates, but also intervene in a wide range of other physiological processes, including mineral homeostasis. Classical actions are triggered when estrogens bind and activate intracellular estrogen receptors (ERs), regulating the transcription of responsive genes, but rapid non-genomic actions initiated by binding to plasma membrane receptors were recently described. A wide range of structurally diverse compounds from natural and anthropogenic sources have been shown to interact with and disrupt the normal functions of the estrogen system, and fish are particularly vulnerable to endocrine disruption, as these compounds are frequently discharged or run-off into waterways. The effect of estrogen disruptors in fish has mainly been assessed in relation to reproductive endpoints, and relatively little attention has been given to other disruptive actions. This review will overview the actions of estrogens in fish, including ER isoforms, their expression, structure and mechanisms of action. The estrogen functions will be considered in relation to mineral homeostasis and actions on mineralized tissues. The impact of estrogenic endocrine disrupting compounds on fish mineralized tissues will be reviewed, and the potential adverse outcomes of exposure to such compounds will be discussed. Current lacunae in knowledge are highlighted along with future research priorities.

Embryonic development of gonadotrope cells and gonadotropic hormones--lessons from model fish

Pituitary gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are key regulators of vertebrate reproduction. The differential regulation of these hormones, however, is poorly understood and little is known about gonadotrope embryonic development. The different cell types in the vertebrate pituitary develop from common progenitor cells just after gastrulation. Proper development and merging of the anterior and posterior pituitary is dependent upon carefully regulated cell-to-cell interactions, and a suite of signaling pathways with precisely organized temporal and spatial expression patterns, which include transcription factors and their co-activators and repressors. Among the pituitary endocrine cell types, the gonadotropes are the last to develop and become functional. Although much progress has been made during the last decade regarding details of gonadotrope development, the coordinated program for their maturation is not well described. FSH and LH form an integral part of the hypothalamo-pituitary-gonad axis, the main regulator of gonad development and reproduction. Besides regulating gonad development, pre- and early post-natal activity in this axis is thought to be essential for proper development, especially of the central nervous system in mammals. As a means to investigate early functions of FSH and LH in more detail, we have developed a stable transgenic line of medaka with the LH beta subunit gene (lhb) promoter driving green fluorescent protein (Gfp) expression to characterize development of lhb-expressing gonadotropes. The lhb gene is maternally expressed early during embryogenesis. lhb-Expressing cells are initially localized outside the primordial pituitary in the developing gut tube as early as 32 hpf. At hatching, lhb-Gfp is clearly detected in the gut epithelium and in the anterior digestive tract. lhb-Gfp expression later consolidates in the developing pituitary by 2 weeks post-fertilization. This review discusses status of knowledge regarding pituitary morphology and development, with emphasis on gonadotrope cells and gonadotropins during early development, comparing main model species like mouse, zebrafish and medaka, including possible developmental functions of the observed extra pituitary expression of lhb in medaka.

A novel model for development, organization, and function of gonadotropes in fish pituitary

The gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are key regulators of the reproductive axis in vertebrates. Despite the high popularity of zebrafish as a model organism for studying reproductive functions, to date no transgenic zebrafish with labeled gonadotropes have been introduced. Using gonadotropin regulatory elements from tilapia, we generated two transgenic zebrafish lines with labeled gonadotropes. The tilapia and zebrafish regulatory sequences were highly divergent but several conserved elements allowed the tilapia promoters to correctly drive the transgenes in zebrafish pituitaries. FSH cells reacted to stimulation with gonadotropin releasing hormone by proliferating and showing increased transgene fluorescence, whereas estrogen exposure caused a decrease in cell number and transgene fluorescence. Transgene fluorescence reflected the expression pattern of the endogenous fshb gene. Ontogenetic expression of the transgenes followed typical patterns, with FSH cells appearing early in development, and LH cells appearing later and increasing dramatically in number with the onset of puberty. Our transgenic lines provide a powerful tool for investigating the development, anatomy, and function of the reproductive axis in lower vertebrates.

Calcineurin mediates the gonadotropin-releasing hormone effect on expression of both subunits of the follicle-stimulating hormone through distinct mechanisms

Gonadotropin-releasing hormone (GnRH) regulates the expression of all three gonadotropin genes, encoding the common α subunit (αGSU) and hormone-specific β subunits, through the activation of several signal transduction pathways. We have shown that GnRH also upregulates calcineurin, and we hypothesized that calcineurin mediates the effects of GnRH on the transcription of the αGSU and follicle-stimulating hormone β (FSHβ) genes through two of its targets: nuclear factor of activated T cells (NFAT) and CREB-regulated transcription coactivator (TORC). We show that calcineurin is essential for GnRH-induced expression of both genes but that NFAT and TORC1 play quite distinct roles in activating each gene. GnRH induces calcineurin-dependent nuclear import of NFAT3, which activates the αGSU promoter, while TORC1 also mediates GnRH activation of this promoter, but not through CREB. GnRH initially stimulates the degradation of TORC1 but protects the N terminus of the newly synthesized protein to enhance its activity. Calcineurin induces Nur77 expression, likely via NFAT3, and Nur77 interacts synergistically with TORC1 and CREB to increase FSHβ promoter activity. Although TORC plays a role in the basal activity of the FSHβ promoter, it does not interact with phosphorylated CREB and probably does not play a major role in direct GnRH signaling to this gene. TORC may be part of an alternatively regulated pathway, possibly involving cross talk with other stimulatory hormones.

The estrogen-regulated transcription factor PITX1 coordinates gene-specific regulation by estrogen receptor-alpha in breast cancer cells

The estrogen receptor α (ERα) is a master regulator of gene expression and works along with cooperating transcription factors in mediating the actions of the hormone estradiol (E2) in ER-positive tissues and breast tumors. Here, we report that expression of paired-like homeodomain transcription factor (PITX1), a tumor suppressor and member of the homeobox family of transcription factors, is robustly up-regulated by E2 in several ERα-positive breast cancer cell lines via ERα-dependent interaction between the proximal promoter and an enhancer region 5' upstream of the PITX1 gene. Overexpression of PITX1 selectively inhibited the transcriptional activity of ERα and ERβ, while enhancing the activities of the glucocorticoid receptor and progesterone receptor. Reduction of PITX1 by small interfering RNA enhanced ERα-dependent transcriptional regulation of a subset of ERα target genes. The consensus PITX1 binding motif was found to be present in 28% of genome-wide ERα binding sites and was in close proximity to estrogen response elements in a subset of ERα binding sites, and E2 treatment enhanced PITX1 as well as ERα recruitment to these binding sites. These studies identify PITX1 as a new ERα transcriptional target that acts as a repressor to coordinate and fine tune target-specific, ERα-mediated transcriptional activity in human breast cancer cells.

Perspectives on fish gonadotropins and their receptors

Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve fibers from the preoptic region to the pituitary. The various cell types in the teleost pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH) similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist of a common alpha subunit non-covalently associated with a hormone-specific beta subunit. In recent years the availability of molecular cloning techniques allowed the isolation of the genes coding for the GtH subunits in 56 fish species representing at least 14 teleost orders. Advanced molecular engineering provides the technology to produce recombinant GtHs from isolated cDNAs. Various expression systems have been used for the production of recombinant proteins. Recombinant fish GtHs were produced for carp, seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA, PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal steroids and peptides exert their effects on the gonadotropins either directly or via the hypothalamus. All these are discussed in detail in this review. In mammals, the biological activities of FSH and LH are directed to different gonadal target cells through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR), respectively, and the interaction between each gonadotropin-receptor couple is highly selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific as a result of promiscuous hormone-receptor interactions, while FSHR expression in Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.

Pin1 facilitates the phosphorylation-dependent ubiquitination of SF-1 to regulate gonadotropin beta-subunit gene transcription

Pin1 is a peptidyl-prolyl cis-trans isomerase which catalyzes the isomerization of phosphorylated Ser/Thr-Pro peptide bonds. Pin1 knockout mice have marked abnormalities in their reproductive development and function. However, the molecular mechanisms underlying their reproductive defects are poorly understood. Herein, we demonstrate that Pin1 is required for both basal and GnRH-induced gonadotropin beta-subunit gene transcription, through interactions with the transcription factors SF-1, Pitx1, and Egr-1. Pin1 activates transcription of the gonadotropin beta-subunit genes synergistically with these transcription factors, either by modulating their stability or by increasing their protein-protein interactions. Notably, we provide evidence that Pin1 is required for the Ser203 phosphorylation-dependent ubiquitination of SF-1, which facilitates SF-1-Pitx1 interactions and therefore results in an enhancement of SF-1 transcriptional activity. Furthermore, we demonstrate that in gonadotrope cells, sufficient levels of activated Pin1 are maintained through transcriptional and posttranslational regulation by GnRH-induced signaling cascades. Our results suggest that Pin1 functions as a novel player in GnRH-induced signal pathways and is involved in gonadotropin beta-subunit gene transcription by modulating the activity of various specific transcription factors.

The molecular regulation of Chinook salmon gonadotropin beta-subunit gene transcription

Expression of the gonadotropin beta-subunit genes is tightly regulated both cell-specifically and by the regulatory hormones to achieve the appropriate gonadotropic hormone levels required for reproductive development and function. Although the cDNA sequences of these genes are highly conserved across species, their promoter sequences are not and few functional studies have been carried out to understand the molecular mechanisms through which their expression is regulated. We and others have carried out several studies on the LHbeta gene promoter of Chinook salmon (Oncorhynchus tschawytscha), and also isolated the FSHbeta gene from the same species. We present here a review of these studies and also novel data pertaining to both genes, in an attempt to collate the current understanding of the molecular regulation of the gonadotropin beta-subunit genes in these fish.

Pitx3 potentiates Nurr1 in dopamine neuron terminal differentiation through release of SMRT-mediated repression

In recent years, the meso-diencephalic dopaminergic (mdDA) neurons have been extensively studied for their association with Parkinson's disease. Thus far, specification of the dopaminergic phenotype of mdDA neurons is largely attributed to the orphan nuclear receptor Nurr1. In this study, we provide evidence for extensive interplay between Nurr1 and the homeobox transcription factor Pitx3 in vivo. Both Nurr1 and Pitx3 interact with the co-repressor PSF and occupy the promoters of Nurr1 target genes in concert. Moreover, in vivo expression analysis reveals that Nurr1 alone is not sufficient to drive the dopaminergic phenotype in mdDA neurons but requires Pitx3 for full activation of target gene expression. In the absence of Pitx3, Nurr1 is kept in a repressed state through interaction with the co-repressor SMRT. Highly resembling the effect of ligand activation of nuclear receptors, recruitment of Pitx3 modulates the Nurr1 transcriptional complex by decreasing the interaction with SMRT, which acts through HDACs to keep promoters in a repressed deacetylated state. Indeed, interference with HDAC-mediated repression in Pitx3(-/-) embryos efficiently reactivates the expression of Nurr1 target genes, bypassing the necessity for Pitx3. These data position Pitx3 as an essential potentiator of Nurr1 in specifying the dopaminergic phenotype, providing novel insights into mechanisms underlying development of mdDA neurons in vivo, and the programming of stem cells as a future cell replacement therapy for Parkinson's disease.

Paired-like homeodomain transcription factors 1 and 2 regulate follicle-stimulating hormone beta-subunit transcription through a conserved cis-element

Paired-like homeodomain transcription factors (PITX) regulate the activity of pituitary hormone-encoding genes. Here, we examined mechanisms through which the family of PITX proteins control murine FSH beta-subunit (Fshb) transcription. We observed that endogenous PITX1 and PITX2 isoforms from murine LbetaT2 gonadotrope cells could bind a highly conserved proximal cis-element. Transfection of PITX1 or PITX2C in heterologous cells stimulated both murine and human Fshb/FSHB promoter-reporter activities, and in both cases, mutation of the critical cis-element abrogated these effects. In homologous LbetaT2 cells, the same mutation decreased basal reporter activity and greatly reduced activin A-stimulated transcription from murine and human promoter-reporters. Transfecting dominant-negative forms of PITX1 or PITX2C or knocking down PITX1 or -2 expression by RNA interference in LbetaT2 cells inhibited murine Fshb transcription, confirming roles for endogenous PITX proteins. Both PITX1 and PITX2C interacted with Smad3 (an effector of the activin signaling cascade in these cells) in coprecipitation experiments, and the PITX binding site mutation greatly inhibited Smad2/3/4-stimulated Fshb transcription. In summary, both PITX1 and PITX2C regulate murine and human Fshb/FSHB transcription through a conserved cis-element in the proximal promoter. Furthermore, the data indicate both common and distinct mechanisms of PITX1 and PITX2C action.

A proteomic comparison of immature and mature mouse gonadotrophs reveals novel differentially expressed nuclear proteins that regulate gonadotropin gene transcription and RNA splicing

The alphaT3-1 and LbetaT2 gonadotroph cell lines contain all the known factors required for expression of gonadotropin genes, yet only the LbetaT2 cells express the beta subunits. We hypothesized that comparison of their nuclear proteomes would reveal novel proteins and/or modifications that regulate expression of these genes. We identified nine proteins with different expression profiles in the two cell lines, of which several were chosen for further functional studies. Of those found at higher levels in alphaT3-1 nuclei, 1110005A23RIK was found associated with the Fshb gene promoter and repressed its expression. Transgelin 3 overexpression reduced transcript levels of Fshb, and its knockdown elevated Lhb and Cga transcript levels, indicating an ongoing repressive effect on these more highly expressed genes, possibly through altering levels of phosphorylated mitogen-activated protein kinase. Heterogeneous nuclear ribonucleoprotein A2/B1 repressed splicing of the Fshb primary transcript, which it binds in the first intron. Proteins at higher levels in LbetaT2 nuclei included prohibitin, the overexpression of which reduced promoter activity of all three gonadotropin subunits, and appeared to mediate the differential effect of GnRH on proliferation of the two cell lines; its knockdown also altered cell morphology. Two other splicing factors were also found at higher levels in LbetaT2 nuclei: the knockdown of PRPF19 or EIF4A3 decreased splicing of Lhb, or of both beta subunit transcripts, respectively. The levels of Eif4a3 mRNA were increased by activin, and both factors increased Fshb splicing. This study has revealed a number of novel factors that alter gonadotropin expression and gonadotroph function, and likely mediate or moderate effects of the regulatory hormones.

Distinct mechanisms involving diverse histone deacetylases repress expression of the two gonadotropin beta-subunit genes in immature gonadotropes, and their actions are overcome by gonadotropin-releasing hormone

The gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are produced in the embryonic pituitary in response to delivery of the hypothalamic gonadotropin releasing hormone (GnRH). GnRH has a pivotal role in reestablishing gonadotropin levels at puberty in primates, and for many species with extended reproductive cycles, these are reinitiated in response to central nervous system-induced GnRH release. Thus, a clear role is evident for GnRH in overcoming repression of these genes. Although the mechanisms through which GnRH actively stimulates LH and FSH beta-subunit (FSHbeta) gene transcription have been described in some detail, there is currently no information on how GnRH overcomes repression in order to terminate reproductively inactive stages. We show here that GnRH overcomes histone deacetylase (HDAC)-mediated repression of the gonadotropin beta-subunit genes in immature gonadotropes. The repressive factors associated with each of these genes comprise distinct sets of HDACs and corepressors which allow for differentially regulated derepression of these two genes, produced in the same cell by the same regulatory hormone. We find that GnRH activation of calcium/calmodulin-dependent protein kinase I (CaMKI) plays a crucial role in the derepression of the FSHbeta gene involving phosphorylation of several class IIa HDACs associated with both the FSHbeta and Nur77 genes, and we propose a model for the mechanisms involved. In contrast, derepression of the LH beta-subunit gene is not CaMK dependent. This demonstration of HDAC-mediated repression of these genes could explain the temporal shut-down of reproductive function at certain periods of the life cycle, which can easily be reversed by the actions of the hypothalamic regulatory hormone.

Proline-rich tyrosine kinase 2 mediates gonadotropin-releasing hormone signaling to a specific extracellularly regulated kinase-sensitive transcriptional locus in the luteinizing hormone beta-subunit gene

G protein-coupled receptor regulation of gene transcription primarily occurs through the phosphorylation of transcription factors by MAPKs. This requires transduction of an activating signal via scaffold proteins that can ultimately determine the outcome by binding signaling kinases and adapter proteins with effects on the target transcription factor and locus of activation. By investigating these mechanisms, we have elucidated how pituitary gonadotrope cells decode an input GnRH signal into coherent transcriptional output from the LH beta-subunit gene promoter. We show that GnRH activates c-Src and multiple members of the MAPK family, c-Jun NH2-terminal kinase 1/2, p38MAPK, and ERK1/2. Using dominant-negative point mutations and chemical inhibitors, we identified that calcium-dependent proline-rich tyrosine kinase 2 specifically acts as a scaffold for a focal adhesion/cytoskeleton-dependent complex comprised of c-Src, Grb2, and mSos that translocates an ERK-activating signal to the nucleus. The locus of action of ERK was specifically mapped to early growth response-1 (Egr-1) DNA binding sites within the LH beta-subunit gene proximal promoter, which was also activated by p38MAPK, but not c-Jun NH2-terminal kinase 1/2. Egr-1 was confirmed as the transcription factor target of ERK and p38MAPK by blockade of protein expression, transcriptional activity, and DNA binding. We have identified a novel GnRH-activated proline-rich tyrosine kinase 2-dependent ERK-mediated signal transduction pathway that specifically regulates Egr-1 activation of the LH beta-subunit proximal gene promoter, and thus provide insight into the molecular mechanisms required for differential regulation of gonadotropin gene expression.

Gonadotropin-releasing hormone activation of c-jun, but not early growth response factor-1, stimulates transcription of a luteinizing hormone beta-subunit gene

Transcription of mammalian LH beta-subunit genes (LHbeta) is regulated by GnRH through activation of early growth response factor-1 (Egr-1), which interacts synergistically with steroidogenic factor-1 (Sf-1) and pituitary homeobox-1 (Pitx1) at the promoter; Egr-1 is thought to comprise the major mediator of this effect. However, the proximal promoters of LHbeta genes in lower vertebrates lack an Egr-1 response element yet are responsive to GnRH; we demonstrate here that the promoter of the Chinook salmon LHbeta (csLHbeta) gene is also unresponsive to Egr-1. The homologous LHbeta promoters in other fish contain a conserved estrogen response element-like sequence, which we recently demonstrated is not required for estrogen receptor (ER) alpha association with the csLHbeta gene. Here we show that the estrogen response element-like element is required for the GnRH effect and for a response to c-jun overexpression. Using plasmid immunoprecipitation, we show that after GnRH exposure, c-jun associates with the intact csLHbeta gene promoter through this element. We further show that the effect of c-jun requires its DNA-binding domain and that c-jun interacts with Sf-1 and ERalpha and exerts synergistic effects on promoter activity with Sf-1, ERalpha, and Pitx1. Finally, we demonstrate the role of c-jun in mediating the GnRH effect on this gene through knockdown of c-jun expression or use of a dominant negative. We conclude that c-jun mediation of the GnRH effect on the LHbeta gene may be common in lower vertebrates and may have preceded an evolutionary divergence in the cis-regulatory elements that led to its function being replaced in mammals by Egr-1.

Transcription of gonadotropin beta subunit genes involves cross-talk between the transcription factors and co-regulators that mediate actions of the regulatory hormones

The gonadotropins LH and FSH have distinct temporal patterns of expression as a result of differential regulation by hormones such as GnRH, steroids and activin. This specific regulation is due to diverse sets of transcription factors that are recruited to the promoters of these genes, and recruit specific co-activator complexes which function to stabilize interactions with the general transcription factors and RNA polymerase II, and also to induce covalent modifications of the histone tails at these gene loci. As these molecular mechanisms are elucidated, the nature of nuclear cross-talk between the various hormonally induced pathways is becoming evident, revealing both negative and positive effects of interacting transcription factors and co-regulators. This paper will review current knowledge on the transcriptional regulation of gonadotropin beta subunit gene expression in the chromatin setting, and will present new data pertaining to nuclear cross-talk between the various endocrine-induced pathways regulating gonadotropin gene transcription.

Application of bioaffinity mass spectrometry for analysis of ligands

Bioaffinity mass spectrometry is a novel technology for analysis of binding proteins and their ligands. In this review, we introduce the concepts and principles of bioaffinity surface-enhanced laser desorption/ionization-time of flight mass spectrometry (SELDI-TOF MS). Various preactivated chip types and several approaches for binding of ligands or their binders to the chips are discussed. We also provide specific examples for the use of this technology for screening antibodies, analyzing ligands, glycoconjugates, protein-protein inter-actions, and DNA (RNA) binding proteins. In pursuit of developing new tests or studies of mechanism of drug action in therapeutic drug monitoring practice, this technology may provide a more rapid approach for ligand-binder studies.

Cross talk in hormonally regulated gene transcription through induction of estrogen receptor ubiquitylation

Estrogen tightly regulates the levels of circulating gonadotropins, but a direct effect of estrogen receptor alpha (ERalpha) on the mammalian LHbeta gene has remained poorly defined. We demonstrate here that ERalpha can associate with the LHbeta promoter through interactions with Sf-1 and Pitx1 without requiring an estrogen response element (ERE). We show that gonadotropin-releasing hormone (GnRH) promotes ERalpha ubiquitylation and also degradation while stimulating expression of ubc4. GnRH also increases the association and lengthens the cycling time of ERalpha on the LHbeta promoter. The ERalpha association and transactivation of the LHbeta gene, as well as ERalpha degradation, are increased following ubc4 overexpression, while the effects of GnRH are abated following ubc4 knockdown. Our results indicate that ERalpha ubiquitylation and subsequent transactivation of the LHbeta gene can be induced by increasing the levels of the E2 enzyme as a result of signaling by an extracellular hormone, thus providing a new form of cross talk in hormonally stimulated regulation of gene expression.

Molecular regulation of gonadotropin secretion by gonadotropin-releasing hormone in salmonid fishes

Gonadotropin-releasing hormone (GnRH) plays a central role in the control of reproductive function in vertebrates. In salmonids, salmon GnRH (sGnRH) secreted by preoptic GnRH neurons regulates gonadal maturation through stimulation of synthesis and release of pituitary gonadotropins (GTHs). In addition, several lines of our evidence indicate that sGnRH is involved in spawning behavior, and serves to integrate the gonadal maturation with the reproductive behavior. A growing number of studies show that the effects of GnRH are mediated by multiple subtypes of GnRH receptors, successive multiple signaling pathways, and finally multiple transcription factors which act cooperatively to stimulate transcription of GTH subunit genes. This complex regulatory system of the action of GnRH may serve as a molecular basis of divergent physiological strategies of reproductive success in various vertebrate species. In this article, recent data on the molecular mechanisms of action of GnRH are reviewed with special reference to the regulation of synthesis and release of GTHs in the pituitary of salmonids to elucidate the multifunctional action of GnRH.

Isolation of dopamine D(2) receptor (D (2)R) promoters in Mugil cephalus

This paper reports the isolation of two putative D(2)R promoters from grey mullet, one 5'flanking and the other an intronic sequence immediately upstream of the first coding exon. Promoter activity of the intronic sequence was confirmed in vitro through functional analysis using luciferase as reporter gene. The functional characteristics of the region flanking the 5'UTR is currently under investigation.

Essential role of the homeodomain for pituitary homeobox 1 activation of mouse gonadotropin-releasing hormone receptor gene expression through interactions with c-Jun and DNA

The gonadotropin-releasing hormone receptor (GnRHR) is expressed primarily in the gonadotropes of the anterior pituitary. Pituitary homeobox 1 (Pitx-1) has been shown to activate pituitary-specific gene expression by direct DNA binding and/or protein-protein interaction with other transcription factors. We hypothesized that Pitx-1 might also dictate tissue-specific expression of the mouse GnRHR (mGnRHR) gene in a similar manner. Pitx-1 activated the mGnRHR gene promoter, and transactivation was localized to sequences between -308 and -264. Pitx-1 bound to this region only with low affinity. This region includes an activating protein 1 (AP-1) site, which was previously shown to be important for mGnRHR gene expression. Further characterization indicated that an intact AP-1 site was required for full Pitx-1 responsiveness. Furthermore, Pitx-1 and AP-1 were synergistic in the activation of the mGnRHR gene promoter. A Pitx-1 homeodomain (HD) point mutation, which eliminated DNA binding ability, caused only a partial reduction of transactivation, whereas deletion of the HD completely prevented transactivation. Pitx-1 interacted directly with c-Jun, and the HD was sufficient for this interaction. While the point mutation in the Pitx-1 HD did not affect interaction with c-Jun, deletion of the HD eliminated the interaction. Taken together, our studies indicate that Pitx-1 can direct transactivation of the mGnRHR gene, in part by DNA binding and in part by an action of Pitx-1 as a cofactor for AP-1, augmenting AP-1 activity through a novel protein-protein interaction between c-Jun and the HD of Pitx-1.

Synergistic effects of salmon gonadotropin-releasing hormone and estradiol-17beta on gonadotropin subunit gene expression and release in masu salmon pituitary cells in vitro

Effects of salmon gonadotropin-releasing hormone (sGnRH) and estradiol-17beta (E2) on gene expression and release of gonadotropins (GTHs) were examined in masu salmon (Oncorhynchus masou) using primary pituitary cell cultures at three reproductive stages, initiation of sexual maturation in May, pre-spawning in July, and spawning in September. Amounts of GTH subunit mRNAs were determined by real-time polymerase chain reaction, and levels of GTH released in the medium were determined by RIA. In control cells, the amounts of three GTH subunit mRNAs (alpha2, FSHbeta, and LHbeta) peaked in July prior to spawning. FSH release spontaneously increased with gonadal maturation and peaked in September, whereas LH release remained low until July and extensively increased in September. Addition of E2 to the culture extensively increased the amounts of LHbeta mRNA in May and July in both sexes. It also increased the alpha2 mRNA in July in the females. In contrast, sGnRH alone did not have any significant effects on the amounts of three GTH subunit mRNAs at all stages, except for the elevation of alpha2 and FSHbeta mRNAs in July in the females. Nevertheless, synergistic effects by sGnRH and E2 were evident for all three GTH subunit mRNAs. In May, sGnRH in combination with E2 synergistically increased the amounts of LHbeta mRNA in the males and alpha2 mRNA in the females. However, in July the combination suppressed the amounts of alpha2 and FSHbeta mRNAs in the females. sGnRH alone stimulated LH release at all stages in both sexes, and the release was synergistically enhanced by E2. Synergistic stimulation of FSH release was also observed in May and July in both sexes. These results indicate that a functional interaction of sGnRH with E2 is differently involved in synthesis and release of GTH. The synergistic interaction modulates GTH synthesis differentially, depending on subunit, stage, and gender, whereas it potentiates the activity of GnRH to release GTH in any situation.

Isolation and characterization of the follicle-stimulating hormone beta subunit gene and 5' flanking region of the Chinook salmon

In this study we have isolated the follicle-stimulating hormone beta subunit gene from the Chinook salmon (csFSHbeta). This gene encodes for a protein that is highly similar to those isolated from other salmonids and shares all of the structural constraints seen in mammalian gonadotropins, including twelve conserved cysteines and a putative N-linked glycosylation site. The organization of the gene follows the conserved pattern regarding the numbers and positions of the introns, although the csFSHbeta gene contains a particularly large 6.2-kb first intron due to the inclusion of several transposon-like elements. Isolation of 1.2 kb of the 5' flanking region of the csFSHbeta gene and subsequent analysis in silico have revealed a number of putative elements which appear highly conserved in teleost FSHbeta gene promoters and are thus likely involved in basal and hormone-induced transcriptional regulation. The functionality of this 1.2-kb fragment in driving expression of a reporter gene and its response to GnRH was shown in gonadotropes, while the overexpression of AP-1 factors, Sf-1, estrogen receptor or Smad1 revealed that the promoter is responsive to these transcription factors. Our current study has opened the way for future analysis to verify the role of these factors in mediating hormonally induced transcription of this gene.

Regulation of fish gonadotropins

Neurohormones similar to those of mammals are carried in fish by hypothalamic nerve fibers to regulate directly follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH and the expression of the glycoprotein hormone alpha (GPalpha), FSHbeta, and LHbeta, as well as their secretion. Its signal transduction leading to LH release is similar to that in mammals although the involvement of cyclic AMP-protein kinase A (cAMP-PKA) cannot be ruled out. Dopamine (DA) acting through DA D2 type receptors may inhibit LH release, but not that of FSH, at sites distal to activation of protein kinase C (PKC) and PKA. GnRH increases the steady-state levels of GPalpha, LHbeta, and FSHbeta mRNAs. Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and neuropeptide Y (NPY) potentiate GnRH effect on gonadotropic cells, and also act directly on the pituitary cells. Whereas PACAP increases all three subunit mRNAs, NPY has no effect on that of FSHbeta. The effect of these peptides on the expression of the gonadotropin subunit genes is transduced differentially; GnRH regulates GPalpha and LHbeta via PKC-ERK and PKA-ERK cascades, while affecting the FSHbeta transcript through a PKA-dependent but ERK-independent cascade. The signals of both NPY and PACAP are transduced via PKC and PKA, each converging at the ERK level. NPY regulates only GPalpha- and LHbeta-subunit genes whereas PACAP regulates the FSHbeta subunit as well. Like those of the mammalian counterparts, the coho salmon LHbeta gene promoter is driven by a strong proximal tripartite element to which three different transcription factors bind. These include Sf-1 and Pitx-1 as in mammals, but the function of the Egr-1 appears to have been replaced by the estrogen receptor (ER). The GnRH responsive region in tilapia FSHbeta 5' flanking region spans the canonical AP1 and CRE motifs implicating both elements in conferring GnRH responsiveness. Generally, high levels of gonadal steroids are associated with high LHbeta transcript levels whereas those of FSHbeta are reduced when pituitary cells are exposed to high steroid levels. Gonadal or hypophyseal activin also participate in the regulation of FSHbeta and LHbeta mRNA levels. However, gonadal effects are dependent on the gender and stage of maturity of the fish.

Rapid discovery and identification of a tissue-specific tumor biomarker from 39 human cancer cell lines using the SELDI ProteinChip platform

Useful biomarkers are needed for early detection of cancers. To demonstrate the potential diagnostic usefulness of a new proteomic technology, we performed Expression Difference Mapping analysis on 39 cancer cell lines from 9 different tissues using ProteinChip technology. A protein biomarker candidate of 12kDa was found in colon cancer cells. We then optimized the purification conditions for this biomarker by utilizing Retentate Chromatography mass spectrometry (RC-MS). The optimized purification conditions developed "on-chip" were directly transferred to conventional chromatography to purify the biomarker, which was identified as prothymosin-alpha by ProteinChip time-of-flight mass spectrometry (TOF MS) and ProteinChip-Tandem MS systems. The relative expression level of prothymosin-alpha between colon cancer cells and normal colon mucosal cells was evaluated on the same ProteinChip platform. Prothymosin-alpha expression in colon cancer cells was clearly higher than in normal colon cells. These results indicate that prothymosin-alpha could be a potential biomarker for colon cancer, and that the ProteinChip platform could perform the whole process of biomarker discovery from screening to evaluation of the identified marker.

Structure of the GnRH receptor-stimulated signaling network: insights from genomics

The GnRH receptor influences gene expression in the gonadotrope through activating signaling cascades that modulate transcription factor expression and activity. A longstanding question in neuroendocrinology is how instructions received at the membrane in the form of the pattern of receptor stimulation are processed into specific biosynthetic changes at each gonadotropin promoter. Signal transduction from the membrane to preformed transcription factors relies on recognition of altered conformations. Signal transduction through the layers of the gene network also requires the biosynthesis of new transcription factors. The signal processing of this system depends on its molecular connectivity map and its feedback and feed-forward loops. Review of signal transduction, gene control, and genomic studies provide evidence of key loops that cross between cellular and nuclear compartments. Genomic studies suggest that the signal transduction and gene network form a continuum. We propose that information transfer in the gonadotrope depends on robust signaling modules that serve to integrate events at different time scales across cytoplasmic and nuclear compartments.

PITX2 isoform-specific regulation of atrial natriuretic factor expression: synergism and repression with Nkx2.5

PITX2 and Nkx2.5 are two of the earliest known transcriptional markers of vertebrate heart development. Pitx2-/- mice present with severe cardiac malformations and embryonic lethality, demonstrating a role for PITX2 in heart development. However, little is known about the downstream targets of PITX2 in cardiogenesis. We report here that the atrial natriuretic factor (ANF) promoter is a target of PITX2. PITX2A, PITX2B, and PITX2C isoforms differentially activate the ANF promoter. However, only PITX2C can synergistically activate the ANF promoter in the presence of Nkx2.5. We further demonstrate that the procollagen lysyl hydroxylase (PLOD1) promoter is regulated by Nkx2.5. Mechanistically, PITX2C and Nkx2.5 synergistically regulate ANF and PLOD1 expression through binding to their respective DNA elements. Surprisingly, PITX2A activation of the ANF and PLOD1 promoters is repressed by co-transfection of Nkx2.5 in the C3H10T1/2 embryonic fibroblast cell line. Pitx2a and Pitx2c are endogenously expressed in C3H10T1/2 cells, and these cells express factors that differentially regulate PITX2 isoform activities. We provide a new mechanism for the regulation of heart development by PITX2 isoforms through the regulation of ANF and PLOD1 gene expression and Nkx2.5 transcriptional activity.