FTZ-F1, a steroid hormone receptor-like protein implicated in the activation of fushi tarazu

A zebrafish ftz-F1 (Fushi tarazu factor 1) homologue requires multiple subdomains in the D and E regions for its transcriptional activity

A zebrafish Ftz-F1 homologue, zFF1A (zebrafish Ff1a or Nr5a2, a member of nuclear receptor superfamily) and its C-terminally truncated variant (zFF1B) were previously identified. Due to lack of the identity box (I-box) and activation function 2 (AF-2) domain, zFF1B lacks transactivation function and fails to synergize with estrogen receptor (ER) in regulating promoters. It was speculated that the I-box might be involved in the zFF1A/ER interaction. In the present study, the function of the I-box was examined. In the absence of the I-box or with an altered heptad 9, the AF-2 of zFF1A was not functional, either in the presence or absence of ER. The GST pull-down assay showed that zFF1A and its mutants exerted similar physical contacts with ER-LBD, suggesting that the "dimerization" domain (I-box) is essential for the transcriptional activity of zFF1A. Moreover, nuclear receptor coactivator selectively activated zFF1 with the I-box but exerted no effect on zFF1B, indicating that the I-box is able to interact with the coactivators. By deletion study and analysis of the identified domains in GAL4-DNA binding domain, other regions of zFF1A critical for its AF were also delineated. Consistent with the mutation analysis, AF-2 was active only in the presence of the I-box. We also identified a novel AF domain (AF-3) located in the hinge region (amino acids 155-267), although the activity of AF-3 was inhibited by its flanking region. We suggest that the D and E regions of zFF1A possess both positive and negative transactivation functions, and interdomain "cross-talk" may confer the full transcriptional activity of the protein.

Two isoforms of FTZ-F1 messenger RNA: molecular cloning and their expression in the frog testis

FTZ-F1, a member of the orphan nuclear receptors, is a transcriptional factor regulating the expression of the fushi tarazu gene in Drosophila (Lavorgna et al., 1991. Science 252, 848-851). Previously, we cloned a frog homologue of FTZ-F1 (rrFTZ-F1alpha; GenBank Accession No. AB035498). In this study, we isolated the rrFTZ-F1beta cDNA encoding a protein of 469 amino acids. Then, expressions of two types (alpha and beta) of rrFTZ-F1 mRNAs were examined during development of embryos and gonads in the frog Rana rugosa. They were expressed in the embryo at stage 12. Expressions of both the alpha and beta mRNAs became stronger in the testis of frogs at stage XXV and were most prominent in that of frogs 2months after metamorphosis. In the former testis, spermatogonia were the only germ cells in the seminiferous tubules, whilst postmeiotic cells were observed in the latter testis. Expression of the typealpha mRNA was more prominent. In addition, we cloned the regions with either exon I or II of the rrFTZ-F1 gene. Genomic structure analysis revealed that rrFTZ-F1beta is a partial exon I-truncated variant of rrFTZ-F1alpha. The results suggest that rrFTZ-F1alpha and -beta are expressed from the same gene by alternative splicing and that they may play an important role(s) in differentiation of premeiotic germ cells in the testis of the frog R. rugosa.

nhr-25, the Caenorhabditis elegans ortholog of ftz-f1, is required for epidermal and somatic gonad development

We have analyzed the expression and function of the Caenorhabditis elegans gene nhr-25, a member of the widely conserved FTZ-F1 family of nuclear receptors. The gene encodes two protein isoforms, only one of which has a DNA binding domain. nhr-25 is transcribed during embryonic and larval development. A nhr-25::GFP fusion gene is expressed in the epidermis, the developing somatic gonad, and a subset of other epithelial cells. RNA-mediated interference indicates a requirement for nhr-25 function during development: disruption of nhr-25 function leads to embryonic arrest due to failure of the epidermally mediated process of embryo elongation. Animals that survive to hatching arrest as misshapen larvae that occasionally exhibit defects in shedding molted cuticle. In addition, somatic gonad development is defective in these larvae. These results further establish the importance of FTZ-F1 nuclear receptors in molting and developmental control across evolutionarily distant phyla.

Encroaching genomics: adapting large-scale science to small academic laboratories

The process of conducting biological research is undergoing a profound metamorphosis due to the technological innovations and torrent of information resulting from the execution of multiple species genome projects. The further tasks of mapping polymorphisms and characterizing genome-wide protein-protein interaction (the characterization of the proteome) will continue to garner resources, talent, and public attention. Although some elements of these whole genome size projects can only be addressed by large research groups, consortia, or industry, the impact of these projects has already begun to transform the process of research in many small laboratories. Although the impact of this transformation is generally positive, laboratories engaged in types of research destined to be dominated by the efforts of a genomic consortium may be negatively impacted if they cannot rapidly adjust strategies in the face of new large-scale competition. The focus of this report is to outline a series of strategies that have been productively utilized by a number of small academic laboratories that have attempted to integrate such genomic resources into research plans with the goal of developing novel physiological insights.

Expression of steroidogenic factor 1 in the testis requires an E box and CCAAT box in its promoter proximal region

Steroidogenic factor 1 (SF-1), also known as adrenal 4-binding protein, is a member of the nuclear hormone receptor family that regulates transcription of genes encoding hormones and steroidogenic enzymes important to the function of the hypothalamic-pituitary-gonadal axis. The mammalian Ftz-F1 gene encodes SF-1 and is required for development of adrenal glands and gonads. To better understand the mechanisms regulating this gene in the gonads, we have examined its expression in the testis and characterized the promoter region for SF-1 in two testicular cell types. SF-1 promoter activity was examined in primary cultures of Sertoli cells and cell lines representative of Sertoli and Leydig cells. Deletion mutagenesis of the promoter identified several regions: both 5' and 3' to the transcriptional start sites that are important for transcriptional activity. Two elements, an E box and a CCAAT box, were found to be important for SF-1 transcription in the testis. An oligodeoxynucleotide containing both of these elements bound three specific protein complexes. The binding of one complex required only sequences within the E box and cross-reacted with antibodies against the basic helix-loop-helix ZIP proteins USF1 and USF2. A second specific complex required sequences within both the E box and CCAAT box for efficient binding, while a third complex predominantly interacted with sequences within the CCAAT motif. The presence of multiple protein complexes binding these sites suggests that regulation through these elements may involve interactions with different factors that depend on the state of the cell and its environment.

The timing of drosophila salivary gland apoptosis displays an l(2)gl-dose response

During Drosophila metamorphosis, larval tissues, such as the salivary glands, are histolysed whereas imaginal tissues differentiate into adult structures forming at eclosion a fly-shaped adult. Inactivation of the lethal(2)giant larvae (l(2)gl) gene encoding the cytoskeletal associated p127 protein, causes malignant transformation of brain neuroblasts and imaginal disc cells with developmental arrest at the larval-pupal transition phase. At this stage, p127 is expressed in wild-type salivary glands which become fully histolysed 12 - 13 h after pupariation. By contrast to wild-type, administration of 20-hydroxyecdsone to l(2)gl-deficient salivary glands is unable to induce histolysis, although it releases stored glue granules and gives rise to a nearly normal pupariation chromosome puffing, indicating that p127 is required for salivary gland apoptosis. To unravel the l(2)gl function in this tissue we used transgenic lines expressing reduced ( approximately 0.1) or increased levels of p127 (3.0). Here we show that the timing of salivary gland histolysis displays an l(2)gl-dose response. Reduced p127 expression delays histolysis whereas overexpression accelerates this process without affecting the duration of third larval instar, prepupal and pupal development. Similar l(2)gl-dependence is noticed in the timing of expression of the cell death genes reaper, head involution defective and grim, supporting the idea that p127 plays a critical role in the implementation of ecdysone-triggered apoptosis. These experiments show also that the timing of salivary gland apoptosis can be manipulated without affecting normal development and provide ways to investigate the nature of the components specifically involved in the apoptotic pathway of the salivary glands.

Microarray analysis of Drosophila development during metamorphosis

Metamorphosis is an integrated set of developmental processes controlled by a transcriptional hierarchy that coordinates the action of hundreds of genes. In order to identify and analyze the expression of these genes, high-density DNA microarrays containing several thousand Drosophila melanogaster gene sequences were constructed. Many differentially expressed genes can be assigned to developmental pathways known to be active during metamorphosis, whereas others can be assigned to pathways not previously associated with metamorphosis. Additionally, many genes of unknown function were identified that may be involved in the control and execution of metamorphosis. The utility of this genome-based approach is demonstrated for studying a set of complex biological processes in a multicellular organism.

Characterization and DNA-binding properties of GRF, a novel monomeric binding orphan receptor related to GCNF and betaFTZ-F1

A PCR approach has been used to isolate, from Bombyx mori, a cDNA encoding a novel orphan receptor (GRF) that is most closely related to Bombyx betaFTZ-F1 and to the vertebrate germ cell nuclear factor. The major GRF mRNA is detected in most tissues as an 8-kb transcript whose amount follows the circulating ecdysteroid concentration with a delay. The expression pattern of GRF is similar to that of the Bombyx homologue of the Drosophila early-late gene DHR3, and precedes that of betaFTZ-F1 in all stages and tissues examined. The GRF protein is thus likely to be required in many tissues, but in a temporally restricted manner suggesting that GRF has a well-defined function in the ecdysteroid-induced transcription cascade. The GRF protein binds in vitro to a single oestrogen receptor half-site AGGTCA preceded by a 5'-TCA extension, and is therefore a potential co-regulator of the orphan receptors betaFTZ-F1 and DHR39.

Cloning and characterization of new orphan nuclear receptors and their developmental profiles during Tenebrio metamorphosis

Five PCR fragments corresponding to a part of the DNA-binding domain of different hormone nuclear receptors were isolated from Tenebrio molitor mRNAs. The sequence identity of three of them with known Drosophila nuclear receptors strongly suggests that they are the Tenebrio orthologs of seven-up, DHR3 and beta-FTZ-F1, and thus named Tmsvp, TmHR3 and TmFTZ-F1. The full-length sequences of the other two were established. TmHR78 is either a new receptor of the DHR78 family or the same gene which has evolved rapidly, particularly in the E domain. TmGRF belongs to the GCNF1 family and its in vitro translated product binds to the extended half site TCAAGGTCA with high affinity. The periods of expression of the corresponding transcripts in epidermal cells during Tenebrio metamorphosis were analyzed as a function of 20-hydroxyecdysone titers measured in the hemolymph of the animals taken for RNA extraction. Comparison of the expression profiles of these nuclear receptors with those observed during Drosophila metamorphosis revealed similar temporal correlations as a function of ecdysteroid variations, which further supported the sequence identity data for TmSVP, TmHR3, TmFTZ-F1 and TmHR78.

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

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

Steroidogenic factor-1: its role in endocrine organ development and differentiation

The cloning of the first steroid hormone receptor over a decade ago provided vital insight into the mechanisms by which steroid hormones activate gene transcription. When bound by hormone, these receptors function as ligand-dependent transcription factors by binding to unique response elements in the promoter of specific target genes. Over 60 receptors have now been characterized in this superfamily of steroid receptors. Many receptors known as orphan receptors have been cloned by homology and have no known ligands but appear to be mediators of endocrine function in the adult and in many cases are essential developmental regulators in endocrine organogenesis. One such receptor is steroidogenic factor-1 (SF-1). While initially cloned as a transcriptional regulator of the various steroidogenic enzyme genes in the adrenal and gonad, it has become clear through genetic ablation experiments in mice that SF-1 is an essential factor in adrenal and gonadal development and for the proper functioning of the hypothalamic-pituitary-gonadal axis. In addition, these studies have revealed that SF-1 is necessary for the formation of the ventromedial nucleus of the hypothalamus. While we have learned much since the initial cloning of SF-1, the mechanisms by which SF-1 regulates these various developmental programs remain elusive. This article focuses on the characterization of SF-1 and its emerging role in endocrine homeostasis. Specific attention is placed on the mechanisms of action of this unique member of the nuclear receptor superfamily.

CPF: an orphan nuclear receptor that regulates liver-specific expression of the human cholesterol 7alpha-hydroxylase gene

Cholesterol 7alpha-hydroxylase is the first and rate-limiting enzyme in a pathway through which cholesterol is metabolized to bile acids. The gene encoding cholesterol 7alpha-hydroxylase, CYP7A, is expressed exclusively in the liver. Overexpression of CYP7A in hamsters results in a reduction of serum cholesterol levels, suggesting that the enzyme plays a central role in cholesterol homeostasis. Here, we report the identification of a hepatic-specific transcription factor that binds to the promoter of the human CYP7A gene. We designate this factor CPF, for CYP7A promoter binding factor. Mutation of the CPF binding site within the CYP7A promoter abolished hepatic-specific expression of the gene in transient transfection assays. A cDNA encoding CPF was cloned and identified as a human homolog of the Drosophila orphan nuclear receptor fushi tarazu F1 (Ftz-F1). Cotransfection of a CPF expression plasmid and a CYP7A reporter gene resulted in specific induction of CYP7A-directed transcription. These observations suggest that CPF is a key regulator of human CYP7A gene expression in the liver.

A double interaction screen identifies positive and negative ftz gene regulators and ftz-interacting proteins

Regulatory genes directing embryonic development are expressed in complex patterns. The Drosophila homeobox gene fushi tarazu (ftz) is expressed in a striped pattern that is controlled by several discrete and large cis- regulatory elements. One key cis-element is the ftz proximal enhancer which is required for stripe establishment and which mediates autoregulation by direct binding of Ftz protein. To identify the trans-acting factors that regulate ftz expression and autoregulation, we developed a modified yeast two hybrid screen, the Double Interaction Screen (DIS). The DIS was designed to isolate both DNA binding transcriptional regulators that interact with the proximal enhancer and proteins that interact with Ftz itself when it is bound to the enhancer. The screen identified two candidate Ftz protein cofactors as well as activators and repressors of ftz transcription that bind directly to the enhancer. One of these (Tramtrack (Ttk)) was previously shown to bind to at least five sites in the proximal enhancer; genetic studies suggested that Ttk acts as a repressor of ftz in the embryo. Here we show that, in yeast cells, Ttk protein strongly activates transcription, suggesting that yeast may be missing a necessary co-repressor which is present in Drosophila embryos. Further, we have characterized the activity of a second candidate ftz repressor isolated in the screen - the product of the pair-rule gene sloppy paired - a member of the forkhead family. We show that Slp1 is a DNA binding protein. We have identified a high affinity binding site for Slp1 in the ftz proximal enhancer. Slp1 represses transcription via this binding site in yeast cells, consistent with its role as a direct repressor of ftz stripes in interstripe regions during late stages of embryogenesis. The DIS should be a generally useful method to identify DNA binding transcriptional regulators and protein partners of previously characterized DNA binding proteins.

Steroidogenic factor 1 (SF-1) is essential for endocrine development and function

Steroidogenic factor 1 (SF-1), an orphan nuclear receptor, initially was isolated as a key regulator of the tissue-specific expression of the cytochrome P450 steroid hydroxylases. Thereafter, analyses of sites of SF-1 expression during mouse embryological development hinted at considerably expanded roles for SF-1, roles that were strikingly confirmed through the analyses of SF-1 knockout mice. These SF-1 knockout mice exhibited adrenal and gonadal agenesis, associated with male-to-female sex reversal of their internal and external genitalia and death from adrenocortical insufficiency. These findings showed unequivocally that SF-1 is essential for the embryonic survival of the primary steroidogenic organs. SF-1 knockout mice also had impaired pituitary expression of gonadotropins and agenesis of the ventromedial hypothalamic nucleus (VMH), establishing that SF-1 regulates reproductive function at all three levels of the hypothalamic-pituitary gonadal axis. This article reviews the experiments that have defined these essential roles of SF-1 in endocrine development and highlights important areas for future studies.

Human Ad4BP/SF-1 and its related nuclear receptor

Ad4BP (or SF-1) is an essential transcriptional factor for steroidogenesis as well as for the development of the reproductive axis. We elucidated the structure of the human Ad4BP gene. The spliced variants of Ad4BP gene, ELP1 and ELP2 in mice, are unlikely to be present in humans since the analysis of the human gene revealed an in frame stop codon, 36-bp before the first ATG of Ad4BP. The promoter sequence of human Ad4BP, upstream of non-coding exon 1 was highly conserved, and E-box was also found to be essential for the transcription of human Ad4BP gene. During the process of the human Ad4BP gene cloning, we happened to obtain an Ad4BP-related gene, FTZ-F1beta which also belongs to the nuclear receptor family. We revealed cDNA structures of rat FTZ-F1beta, and found that rat has at least two types of FTZ-F1beta isoforms, which differ only by 21 amino acids length in the A/B domain. The tissue distributions of FTZ-F1beta in rat examined by RT-PCR, was found to be abundant in liver, pancreas, and gastrointestinal tracts. These results suggest that the physiological significance of FTZ-F1beta is different from that of Ad4BP.

The Drosophila beta FTZ-F1 orphan nuclear receptor provides competence for stage-specific responses to the steroid hormone ecdysone

The acquisition of competence is a key mechanism for refining global signals to distinct spatial and temporal responses. The molecular basis of competence, however, remains poorly understood. Here, we show that the beta FTZ-F1 orphan nuclear receptor functions as a competence factor for stage-specific responses to the steroid hormone ecdysone during Drosophila metamorphosis. beta FTZ-F1 mutants pupariate normally in response to the late larval pulse of ecdysone but display defects in stage-specific responses to the subsequent ecdysone pulse in prepupae. The ecdysone-triggered genetic hierarchy that directs these developmental responses is severely attenuated in beta FTZ-F1 mutants, although ecdysone receptor expression is unaffected. This study define beta FTZ-F1 as an essential competence factor for stage-specific responses to a steroid signal and implicates interplay among nuclear receptors as a mechanism for achieving hormonal competence.

The Nuclear Receptor Superfamily Has Undergone Extensive Proliferation and Diversification in Nematodes

The nuclear receptor (NR) superfamily is the most abundant class of transcriptional regulators encoded in the Caenorhabditis elegans genome, with >200 predicted genes revealed by the screens and analysis of genomic sequence reported here. This is the largest number of NR genes yet described from a single species, although our analysis of available genomic sequence from the related nematode Caenorhabditis briggsae indicates that it also has a large number. Existing data demonstrate expression for 25% of theC. elegans NR sequences. Sequence conservation and statistical arguments suggest that the majority represent functional genes. An analysis of these genes based on the DNA-binding domain motif revealed that several NR classes conserved in both vertebrates and insects are also represented among the nematode genes, consistent with the existence of ancient NR classes shared among most, and perhaps all, metazoans. Most of the nematode NR sequences, however, are distinct from those currently known in other phyla, and reveal a previously unobserved diversity within the NR superfamily. In C. elegans, extensive proliferation and diversification of NR sequences have occurred on chromosome V, accounting for > 50% of the predicted NR genes.

[The sequence data described in this paper have been submitted to the GenBank data library under accession nos.AF083222–AF083225 and AF083251–AF083234.]

DNA-binding properties of the ecdysteroid receptor-complex (EcR/USP) of the epithelial cell line from Chironomus tentans

DNA-binding features of EcR and USP were investigated using a 0.4 M NaCl extract of the epithelial cell line of Chironomus tentans by means of electrophoretic mobility shift assays (EMSAs). It is shown that the DNA-binding is enhanced by hormone administration and that in the hormone dependent shift, both EcR and USP, are present. Furthermore, we demonstrate that under these conditions, EcR/USP form a unique complex on inverted repeat elements (PAL1 and hsp27-EcRE), while on direct repeat elements (DR1-5), a second complex with higher mobility is formed. In this second complex, neither EcR nor USP are present. Thus, an additional difference between PAL1 and DR-elements is the competition of other factors for DR-elements, modulating its function as an EcRE. A competition EMSA, using PAL1 as radiolabeled probe, reveals the following order of binding strength: PAL1>DR4/5>DR1>DR2/3/hsp27. Surprisingly, using DR1 as radiolabeled probe, shows a different order of binding strength: DR1>DR2>DR3/4/5/PAL1>hsp27. This indicates that the complexes formed on PAL1 are not identical to the ones formed on DR1 and that both are not easily convertible. Furthermore, the affinity of the EcR/USP complex may be altered under various conditions or by interaction with cofactors. Upon hormone administration, DNA binding of the receptor complex is enhanced, but the difference to hormone-free binding reactions decreases in course of time, indicating an additional hormone independent activation. Arch.

Molecular cloning and expression of the SF-1/Ad4BP gene in the frog, Rana rugosa

SF-1/Ad4BP is a transcriptional factor that was originally found to be a mammalian homologue of the Drosophila Ftz-F1 (fushi tarazu factor 1) (), and transcribed from a gene designated the Ftz-F1 gene (). Ftz-F1 gene-deficient mice lack adrenal glands and gonads. Besides mammals, however, the SF-1/Ad4BP cDNA has only been isolated to date in fish and birds. To understand its role(s) for adrenal and gonadal development in vertebrates, cloning of this gene in animals other than mammals is required. In this study, we succeeded to isolate frog (Rana rugosa) SF-1/Ad4BP cDNA from a testis lambdagt10 cDNA library. It encoded a protein of 468 amino acids, and its open reading frame (ORF) shared 70% similarity with that of chicken OR2.1 (a SF-1/Ad4BP homologue) and 62% with bovine SF-1/Ad4BP. SF-1/Ad4BP mRNA was expressed in the testes, brains, adrenals/kidneys and spleens, but not ovaries, of adult frogs. In addition, we also cloned the 5'-untranslated region (4.6kb) of the SF-1/Ad4BP gene with exons I and II. Genomic structure analysis revealed that frog SF-1/Ad4BP was also transcribed from the same gene as that of mammals. However, many Ftz-F1-related proteins have been reported so far. The Ftz-F1 gene does not encode all of those Ftz-F1-related proteins. Thus, the name of Ftz-F1 is not adequate for the gene coding SF-1/Ad4BP. Here, we propose the use of SF-1/Ad4BP instead of Ftz-F1 for the gene that encodes SF-1/Ad4BP in vertebrates.

Cloning and characterization of a novel human hepatocyte transcription factor, hB1F, which binds and activates enhancer II of hepatitis B virus

Enhancer II (ENII) of hepatitis B virus (HBV) is one of the essential cis-elements for the transcriptional regulation of HBV gene expression. Its function is highly liver-specific, suggesting that liver-enriched transcriptional factors play critical roles in regulating the activity of ENII. In this report, a novel hepatocyte transcription factor, which binds specifically to the B1 region (AACGACCGACCTTGAG) within the major functional unit (B unit) of ENII, has been cloned from a human liver cDNA library by yeast one-hybrid screening, and demonstrated to trans-activate ENII via the B1 region. We named this factor hB1F, for human B1-binding factor. Amino acid analysis revealed this factor structurally belongs to nuclear receptor superfamily. Based on the sequence similarities, hB1F is characterized to be a novel human homolog of the orphan receptor fushi tarazu factor I (FTZ-F1). Using reverse transcription-polymerase chain reaction, a splicing isoform of hB1F (hB1F-2) was identified, which has an extra 46 amino acid residues in the A/B region. Examination of the tissue distribution has revealed an abundant 5.2-kilobase transcript of hB1F is present specifically in human pancreas and liver. Interestingly, an additional transcript of 3.8 kilobases was found to be present in hepatoma cells HepG2. Fluorescent in situ hybridization has mapped the gene locus of hB1F to the region q31-32.1 of human chromosome 1. Altogether, this study provides the first report that a novel human homolog of FTZ-F1 binds and regulates ENII of HBV. The potential roles of this FTZ-F1 homolog in tissue-specific gene regulation, in embryonic development, as well as in liver carcinogenesis are discussed.

The roles of steroidogenic factor 1 in endocrine development and function

The orphan nuclear receptor steroidogenic factor 1 (SF-1) was initially isolated as a key regulator of the cytochrome P450 steroid hydroxylases in adrenocortical and gonadal cells. Subsequent analyses of SF-1 knockout mice have expanded considerably our understanding of the roles that SF-1 plays in endocrine development. These SF-1 knockout mice lacked adrenal glands and gonads, with consequent male-to-female sex reversal of their internal and external genitalia. Thus, SF-1 is essential for the embryonic survival of the primary steroidogenic organs. They further exhibited impaired gonadotrope function and agenesis of the ventromedial hypothalamic nucleus, establishing that SF-1 contributes to reproductive function at all three levels of the hypothalamic-pituitary-gonadal axis. This report reviews experiments that have defined these critical roles of SF-1 in endocrine development, and highlights areas of ongoing investigation.

A binding site for multiple transcriptional activators in the fushi tarazu proximal enhancer is essential for gene expression in vivo

The Drosophila homeobox gene fushi tarazu (ftz) is expressed in a highly dynamic striped pattern in early embryos. A key regulatory element that controls the ftz pattern is the ftz proximal enhancer, which mediates positive autoregulation via multiple binding sites for the Ftz protein. In addition, the enhancer is necessary for stripe establishment prior to the onset of autoregulation. We previously identified nine binding sites for multiple Drosophila nuclear proteins in a core 323-bp region of the enhancer. Three of these nine sites interact with the same cohort of nuclear proteins in vitro. We showed previously that the nuclear receptor Ftz-F1 interacts with this repeated module. Here we purified additional proteins interacting with this module from Drosophila nuclear extracts. Peptide sequences of the zinc finger protein Ttk and the transcription factor Adf-1 were obtained. While Ttk is thought to be a repressor of ftz stripes, we have shown that both Adf-1 and Ftz-F1 activate transcription in a binding site-dependent fashion. These two proteins are expressed ubiquitously at the time ftz is expressed in stripes, suggesting that either may activate striped expression alone or in combination with the Ftz protein. The roles of the nine nuclear factor binding sites were tested in vivo, by site-directed mutagenesis of individual and multiple sites. The three Ftz-F1-Adf-1-Ttk binding sites were found to be functionally redundant and essential for stripe expression in transgenic embryos. Thus, a biochemical analysis identified cis-acting regulatory modules that are required for gene expression in vivo. The finding of repeated binding sites for multiple nuclear proteins underscores the high degree of redundancy built into embryonic gene regulatory networks.

The DHR78 nuclear receptor is required for ecdysteroid signaling during the onset of Drosophila metamorphosis

Pulses of ecdysteroids direct Drosophila through its life cycle by activating stage- and tissue-specific genetic regulatory hierarchies. Here we show that an orphan nuclear receptor, DHR78, functions at the top of the ecdysteroid regulatory hierarchies. Null mutations in DHR78 lead to lethality during the third larval instar with defects in ecdysteroid-triggered developmental responses. Consistent with these phenotypes, DHR78 mutants fail to activate the mid-third instar regulatory hierarchy that prepares the animal for metamorphosis. DHR78 protein is bound to many ecdysteroid-regulated puff loci, suggesting that DHR78 directly regulates puff gene expression. In addition, ectopic expression of DHR78 has no effects on development, indicating that its activity is regulated post-translationally. We propose that DHR78 is a ligand-activated receptor that plays a central role in directing the onset of Drosophila metamorphosis.

The basic helix-loop-helix, leucine zipper transcription factor, USF (upstream stimulatory factor), is a key regulator of SF-1 (steroidogenic factor-1) gene expression in pituitary gonadotrope and steroidogenic cells

Tissue-specific expression of the mammalian FTZ-F1 gene is essential for adrenal and gonadal development and sexual differentiation. The FTZ-F1 gene encodes an orphan nuclear receptor, termed SF-1 (steroidogenic factor-1) or Ad4BP, which is a primary transcriptional regulator of several hormone and steroidogenic enzyme genes that are critical for normal physiological function of the hypothalamic-pituitary-gonadal axis in reproduction. The objective of the current study is to understand the molecular mechanisms underlying transcriptional regulation of SF-1 gene expression in the pituitary. We have studied a series of deletion and point mutations in the SF-1 promoter region for transcriptional activity in alphaT3-1 and L/betaT2 (pituitary gonadotrope), CV-1, JEG-3, and Y1 (adrenocortical) cell lines. Our results indicate that maximal expression of the SF-1 promoter in all cell types requires an E box element at -82/-77. This E box sequence (CACGTG) is identical to the binding element for USF (upstream stimulatory factor), a member of the helix-loop-helix family of transcription factors. Studies of the SF-1 gene E box element using gel mobility shift and antibody supershift assays indicate that USF may be a key transcriptional regulator of SF-1 gene expression.

cDNA cloning of a new member of the FTZ-F1 subfamily from a rainbow trout

We describe here cDNA cloning of an orphan nuclear receptor family member, tFZR1, which has a FTZ-F1 box. The amino acid sequences of the zinc finger domain and the FTZ-F1 box has 92.8% and 100% identity, respectively, with those of zebrafish FTZ-F1. On the other hand, the overall homology between tFZR1 and zebrafish FTZ-F1 is low (33.0%). The results indicate that tFZR1 is a new member of fushitarazu factor 1 (FTZ-F1) subfamily.

Steroidogenic factor-1 contains a carboxy-terminal transcriptional activation domain that interacts with steroid receptor coactivator-1

The orphan nuclear receptor, steroidogenic factor-1 (SF-1), plays an important role in the development of the adrenal gland and in sexual differentiation. SF-1 regulates the transcription of variety of genes, including several steroidogenic enzymes, Müllerian inhibiting substance, and gonadotropin genes. In this report, we sought to identify domains in SF-1 that are required for transactivation and to determine whether SF-1 interacts with a subset of known coactivators. Natural variants of the FTZ-F1 locus include embryonal long terminal repeat-binding protein (ELP)-1, ELP-2, and SF-1, which share the DNA-binding domain. Analyses of the transcriptional activity of these variants revealed that the activity of ELP-2 and SF-1 was much greater than ELP-1, which contains a distinct carboxy terminus. Further studies were performed using GAL4-SF-1 fusion proteins that were constructed by replacement of the zinc finger region and FTZ-F1 box of SF-1 with the DNA-binding domain of GAL4. Elimination of the putative AF-2 domain at the carboxy terminus of GAL4-SF-1 proteins resulted in a complete loss of transactivation. Several lines of evidence demonstrated that SF-1 interacts with steroid receptor coactivator-1 (SRC-1). Full-length SRC-1 enhanced GAL4-SF-1-mediated transactivation, whereas a dominant negative form of SRC-1, consisting of its interaction domain alone, inhibited the activity of GAL4-SF-1. In mammalian two-hybrid assays, fusion of the VP16 activation domain to the interaction domain of SRC-1 confirmed the interaction between SRC-1 and GAL4-SF-1 and demonstrated that the AF-2 domain is required for interaction with SRC-1. Furthermore, SRC-1, together with the cAMP responsive element binding protein (CBP) or a closely related factor, p300, synergistically enhanced transcriptional activity of GAL4-SF-1. We conclude that the carboxy-terminal AF-2 region of SF-1 functions as an activation domain and that SRC-1 and CBP/p300 are components of the coactivator complex with SF-1.

Molecular cloning of chicken FTZ-F1-related orphan receptors

FTZ-F1 is a member of the orphan nuclear receptors, which belongs to the steroid hormone receptor superfamily, and plays a role in the blastoderm and nervous system development in Drosophila. Recently, several FTZ-F1 family genes have been cloned in several species. SF-1/Ad4BPs have been identified as master regulators controlling steroidogenic P-450 genes in mammals and are considered to be the mammalian homologues of FTZ-F1. Moreover, SF-1/Ad4BP plays a critical role in the sexual differentiation of gonads in mammals. In vertebrates, except for mammals, the functional homologue of SF-1/Ad4BP has not been identified before. Herein, we cloned two chicken cDNAs (OR2.0 and OR2.1), which encode putative FTZ-F1 family receptors, by reverse transcriptase-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). OR2.1 consists of 3255 bp, is expressed in the adrenal glands and gonads, and is considered to be the chicken counterpart of mammalian SF-1/Ad4BP. However, OR2.0 consists of 2945 bp, is expressed in the livers and the adrenal glands, and is considered to be the chicken counterpart of mouse LRH-1, which is a member of the FTZ-F1 family in mammals.

NMR spectroscopic studies of the DNA-binding domain of the monomer-binding nuclear orphan receptor, human estrogen related receptor-2. The carboxyl-terminal extension to the zinc-finger region is unstructured in the free form of the protein

Unlike steroid and retinoid receptors, which associate with DNA as dimers, human estrogen related receptor-2 (hERR2) belongs to a growing subclass of nuclear hormone receptors that bind DNA with high affinity as monomers. A carboxyl-terminal extension (CTE) to the zinc-finger domain has been implicated to be responsible for determining the stoichiometry of binding by a nuclear receptor to its response element. To better understand the mechanism by which DNA specificity is achieved, the solution structure of the DNA-binding domain of hERR2 (residues 96-194) consisting of the two putative zinc fingers and the requisite 26-amino acid CTE was analyzed by multidimensional heteronuclear magnetic resonance spectroscopy. The highly conserved zinc-finger region (residues 103-168) has a fold similar to those reported for steroid and retinoid receptors, with two helices that originate from the carboxyl-terminal ends of the two zinc fingers and that pack together orthogonally, forming a hydrophobic core. The CTE element of hERR2 is unstructured and highly flexible, exhibiting nearly random coil chemical shifts, extreme sensitivity of the backbone amide protons to solvent presaturation, and reduced heteronuclear (1H-15N) nuclear Overhauser effect values. This is in contrast to the dimer-binding retinoid X and thyroid hormone receptors, where, in each case, a helix has been observed within the CTE. The implications of this property of the hERR2 CTE are discussed.

Multiprotein bridging factor 1 (MBF1) is an evolutionarily conserved transcriptional coactivator that connects a regulatory factor and TATA element-binding protein

Multiprotein bridging factor 1 (MBF1) is a transcriptional cofactor that bridges between the TATA box-binding protein (TBP) and the Drosophila melanogaster nuclear hormone receptor FTZ-F1 or its silkworm counterpart BmFTZ-F1. A cDNA clone encoding MBF1 was isolated from the silkworm Bombyx mori whose sequence predicts a basic protein consisting of 146 amino acids. Bacterially expressed recombinant MBF1 is functional in interactions with TBP and a positive cofactor MBF2. The recombinant MBF1 also makes a direct contact with FTZ-F1 through the C-terminal region of the FTZ-F1 DNA-binding domain and stimulates the FTZ-F1 binding to its recognition site. The central region of MBF1 (residues 35-113) is essential for the binding of FTZ-F1, MBF2, and TBP. When the recombinant MBF1 was added to a HeLa cell nuclear extract in the presence of MBF2 and FTZ622 bearing the FTZ-F1 DNA-binding domain, it supported selective transcriptional activation of the fushi tarazu gene as natural MBF1 did. Mutations disrupting the binding of FTZ622 to DNA or MBF1, or a MBF2 mutation disrupting the binding to MBF1, all abolished the selective activation of transcription. These results suggest that tethering of the positive cofactor MBF2 to a FTZ-F1-binding site through FTZ-F1 and MBF1 is essential for the binding site-dependent activation of transcription. A homology search in the databases revealed that the deduced amino acid sequence of MBF1 is conserved across species from yeast to human.

Teleost FTZ-F1 homolog and its splicing variant determine the expression of the salmon gonadotropin IIbeta subunit gene

Steroidogenic factor 1, a member of the fushi tarazu factor 1 (FTZ-F1) subfamily of nuclear receptors, is a key regulator in mammalian reproduction. From an embryonic complementary DNA library, the zebrafish homolog of FTZ-F1 (zFF1A) and an alternatively spliced variant (zFF1B) were isolated. zFF1B represented a C-terminally truncated version of zFF1A. Whole mount in situ hybridization and reverse transcriptase-PCR analysis revealed that both zFF1A and B transcripts were present in the developing pituitaries, adult fish brain, gonads, and liver, albeit zFF1B messenger RNA was absent in testis. Comparison of the primary sequences of zFF1 with those of other FTZ-F1 subfamily members showed a close structural relationship between the mouse liver receptor homolog, which activated the alpha1-fetoprotein gene in rodent liver. However, similar to mouse steroidogenic factor 1, zFF1A regulated chinook salmon gonadotropin IIbeta subunit gene expression. On the contrary, zFF1B, which could bind a consensus gonadotrope-specific element with an affinity similar to that of zFF1A, lacked both the trans-activation function and synergistic interaction with the estrogen receptor. Furthermore, cotransfection studies in HeLa cells showed that zFF1B was a strong competitor for the action of zFF1A on the chinook salmon gonadotropin IIbeta subunit gene promoter. Our investigation suggests that 1) zFF1 represents an ancestor protein of the vertebrate FTZ-F1 homologs; 2) the antagonistic relationship between zFF1A and -B may dictate the expression of the FTZ-F1 target genes in a variety of tissues, including the pituitary; and 3) the naturally occurring zFF1B provides evidence that the C-terminal portion of zFF1A (80 amino acid residues) contains a major trans-activation function and a protein-protein interface.

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

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

Synergistic activation of the human type II 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase promoter by the transcription factor steroidogenic factor-1/adrenal 4-binding protein and phorbol ester

Steroidogenic factor-1/adrenal 4-binding protein (SF-1/Ad4BP) is an orphan nuclear receptor/transcription factor known to regulate the P450 steroid hydroxylases; however, mechanisms that regulate the activity of SF-1/Ad4BP are not well defined. In addition, little is known about the mechanisms that regulate the human steroidogenic enzyme, type II 3beta-hydroxysteroid dehydrogenase (3beta-HSD II), the major gonadal and adrenal isoform. Regulation of the 3beta-HSD II promoter was examined using human adrenal cortical (H295R; steroidogenic) and cervical (HeLa; non-steroidogenic) carcinoma cells. H295R cells were transfected with a series of 5' deletions of 1251 base pairs (bp) of the 3beta-HSD II 5'-flanking region fused to a chloramphenicol acetyltransferase (CAT) reporter gene followed by treatment with or without phorbol ester (phorbol 12-myristate 13-acetate; PMA). CAT assay data indicated that the region from -101 to -52 bp of the promoter was required for PMA-induced expression. A putative SF-1/Ad4BP regulatory element, TCAAGGTAA, was identified by sequence homology at -64 to -56 bp of the promoter. Cotransfection of HeLa cells with the -101 3beta-HSD-CAT construct and an expression vector for SF-1/Ad4BP increased CAT activity 49-fold. Subsequent treatment with PMA induced an unexpected synergistic increase in transcriptional activity 540-fold over basal. Mutation of the putative response element (TCAAGGTAA to TCAATTTAA) abolished SF-1-induced CAT activity and the synergistic response to PMA. Gel mobility shift assays confirmed that SF-1/Ad4BP interacts with the putative element and transcripts for SF-1/Ad4BP were detected in H295R cells by Northern analysis. These data are the first to demonstrate 1) regulation of a non-cytochrome P450 steroidogenic enzyme promoter by SF-1/Ad4BP, 2) a powerful synergistic effect of PMA on SF-1/Ad4BP-induced transcription, and 3) the importance of the SF-1/Ad4BP regulatory element in the regulation of the 3beta-HSD II promoter.

Ftz-F1 is a cofactor in Ftz activation of the Drosophila engrailed gene

The fushi tarazu pair-rule gene is required for the formation of alternating parasegmental boundaries in the Drosophila embryo. fushi tarazu encodes a homeodomain protein necessary for transcription of the engrailed gene in even-numbered parasegments. Here we report that, within an engrailed enhancer, adjacent and conserved binding sites for the Fushi tarazu protein and a cofactor are each necessary, and together sufficient, for transcriptional activation. Footprinting shows that the cofactor site can be bound specifically by Ftz-F1, a member of the nuclear receptor superfamily. Ftz-F1 and the Fushi tarazu homeodomain bind the sites with 4- to 8-fold cooperativity, suggesting that direct contact between the two proteins may contribute to target recognition. Even parasegmental reporter expression is dependent on Fushi tarazu and maternal Ftz-F1, suggesting that these two proteins are indeed the factors that act upon the two sites in embryos. The two adjacent binding sites are also required for continued activity of the engrailed enhancer after Fushi tarazu protein is no longer detectable, including the period when engrailed, and the enhancer, become dependent upon wingless. We also report the existence of a separate negative regulatory element that apparently responds to odd-skipped.

The nuclear receptor homologue Ftz-F1 and the homeodomain protein Ftz are mutually dependent cofactors

Nuclear hormone receptors and homeodomain proteins are two classes of transcription factor that regulate major developmental processes. Both depend on interactions with other proteins for specificity and activity. The Drosophila gene fushi tarazu (ftz), which encodes a homeodomain protein (Ftz), is required zygotically for the formation of alternate segments in the developing embryo. Here we show that the orphan nuclear receptor alphaFtz-F1 (ref. 3), which is deposited in the egg during oogenesis, is an obligatory cofactor for Ftz. The two proteins interact specifically and directly, both in vitro and in vivo, through a conserved domain in the Ftz polypeptide. This interaction suggests that other nuclear receptor/homeodomain protein interactions maybe important and common in developing organisms.

The nuclear hormone receptor Ftz-F1 is a cofactor for the Drosophila homeodomain protein Ftz

Homeobox genes specify cell fate and positional identity in embryos throughout the animal kingdom. Paradoxically, although each has a specific function in vivo, the in vitro DNA-binding specificities of homeodomain proteins are overlapping and relatively weak. A current model is that homeodomain proteins interact with cofactors that increase specificity in vivo. Here we use a native binding site for the homeodomain protein Fushi tarazu (Ftz) to isolate Ftz-F1, a protein of the nuclear hormone-receptor superfamily and a new Ftz cofactor. Ftz and Ftz-F1 are present in a complex in Drosophila embryos. Ftz-F1 facilitates the binding of Ftz to DNA, allowing interactions with weak-affinity sites at concentrations of Ftz that alone bind only high-affinity sites. Embryos lacking Ftz-F1 display ftz-like pair-rule cuticular defects. This phenotype is a result of abnormal ftz function because it is expressed but fails to activate downstream target genes. Cooperative interaction between homeodomain proteins and cofactors of different classes may serve as a general mechanism to increase HOX protein specificity and to broaden the range of target sites they regulate.

Characterization of the promoter of SF-1, an orphan nuclear receptor required for adrenal and gonadal development

Steroidogenic factor 1 (SF-1) is a transcription factor shown to be critical for regulation of adrenal and gonadal development and function. To dissect the mechanisms that direct expression of this regulator, we have studied the promoter of the SF-1 gene and have identified cis-acting elements that recognize a basic-helix-loop-helix transcription factor; the CAAT binding factor; and Sp1. We demonstrate in Y1 adrenocortical cells that a 90-bp proximal promoter fragment is sufficient to direct steroidogenic-specific expression and that all three elements are required for activity of the SF-1 promoter. Functional analysis of the binding sites on a heterologous TATA box-containing promoter demonstrates that the CAAT box and Sp1 site are not essential for promoter activity when a TATA box is present, whereas the E box is absolutely required for gene expression and is most likely the steroidogenic cell-specific element. We also demonstrate that SF-1 itself does not significantly affect the transcription of its own gene, and thus conclude that the E box, CAAT box, and Sp1 site of the proximal promoter direct expression of the SF-1 gene.

Steroidogenic factor 1 acts at all levels of the reproductive axis

The conversion of cholesterol into steroid hormones occurs through the sequential actions of the cytochrome P450 steroid hydroxylases. Attempts to understand the mechanisms responsible for the temporal and spatial expression patterns of these enzymes led to the identification of a shared regulator, termed steroidogenic factor 1 (SF-1). SF-1 coordinately regulates the steroid hydroxylase genes and thus functions as a global mediator of steroidogenesis. Of greater significance, recent studies using a knockout mouse model have further implicated SF-1 in a variety of processes ranging from development of the steroidogenic organs to the normal function of gonadotropes and the development of the ventromedial hypothalamic nucleus. A fundamental aspect of elucidating the role of SF-1 at all levels of the reproductive axis is to identify its cell-specific target genes. The recent purification and cloning of the steroidogenic acute regulatory (StAR) protein has provided an intriguing new candidate through which SF-1 acts to mediate its effects on reproductive competence. These studies yield novel insights into the processes of steroidogenesis, endocrine development, and reproductive function.

Orphan nuclear receptor ROR alpha gene: isoform-specific spatiotemporal expression during postnatal development of brain

We analyzed expression of mouse orphan nuclear receptor ROR alpha during postnatal development of rodent brain. Using a riboprobe corresponding to the 3'-end of mROR alpha cDNA a peak of ROR alpha expression was observed at postnatal 16 day (P16) in the Purkinje cells of cerebellum, neurons of the thalamus and the olfactory bulb. The hippocampus was also shown to express ROR alpha with an earlier peak at P7. Expression in cell types other than the Purkinje cells appeared transient. On the other hand, when a probe to the 5'-end of mROR alpha cDNA was used, we observed patterns of ROR alpha expression that are different from those observed with the 3'-probe. No specific transcripts of ROR alpha were detected with the 5'-probe in the Purkinje cells until P16. Additionally, the relative level of the hybridization signals with the 5'-probe and the 3'-probe were different among the various brain regions. Together with the previous findings that ROR alpha comprises at least four isoforms which differ from one another in their N-terminal regions, these observations suggest that the spatiotemporal expression of ROR alpha is under isoform-specific regulation. The timing of its expression suggests that ROR alpha may be involved in regulation of postnatal maturation of specific classes of neurons.

Regulation of the EDG84A gene by FTZ-F1 during metamorphosis in Drosophila melanogaster

The transcription factor FTZ-F1 is a member of the nuclear hormone receptor superfamily and is transiently expressed during the mid- and late prepupal periods in Drosophila melanogaster. A putative pupal cuticle gene, EDG84A, is expressed slightly following FTZ-F1 expression during the prepupal period and carries a strong FTZ-F1 binding site between bases 100 and 92 upstream of its transcription start site. In this study, EDG84A mRNA was found to be prematurely expressed upon heat induction of FTZ-F1 in prepupae carrying the heat shock promoter-FTZ-F1 cDNA fusion gene construct. Transgenic fly lines having the 0.8-kb region of the EDG84A promoter fused to lacZ expressed the reporter gene in a tissue- and stage-specific manner. Base substitutions in the FTZ-F1 binding site within the 0.8-kb promoter abolished expression of lacZ. These results strongly suggest that the EDG84A gene is a direct target of FTZ-F1. Deletion studies of the cis-regulatory region of the EDG84A gene revealed that space-specific expression in imaginal disc-derived epidermis is controlled by the region between bp -408 and -104 from the transcription start site. The region between bp -408 and -194 is necessary to repress expression in a posterior part of the body, while the region between bp -193 and -104 carries a positive element for activation in an anterior part of the body. These results suggest that FTZ-F1 governs expression of the EDG84A gene in conjunction with putative tissue-specific regulators.

The roles of the nuclear receptor steroidogenic factor 1 in endocrine differentiation and development

The orphan nuclear receptor steroidogenic factor 1 (SF-1) has emerged as a critical determinant of adrenal and gonadal differentiation, development, and function. SF-1 was initially isolated as a positive regulator of the cytochrome P450 steroid hydroxylases in the adrenal glands and gonads; developmental analyses subsequently showed that SF-1 was also expressed in the diencephalon and anterior pituitary, suggesting additional roles in endocrine function. Analyses of knockout mice deficient in SF-1 revealed multiple abnormalities, including adrenal and gonadal agenesis, male to female sex reversal of the internal genitalia, impaired gonadotrope function, and absence of the ventromedial hypothalamic nucleus. Taken together, these results implicate SF-1 as a global regulator within the hypothalamic-pituitary-gonadal axis and the adrenal cortex.

SF-1: a key regulator of development and function in the mammalian reproductive system

The orphan nuclear receptor steroidogenic factor 1 (SF-1) was isolated as a transcription factor expressed specifically in the mouse primary steroidogenic tissues. SF-1 expression occurs at the earliest stages of adrenal and gonadal development and the expression pattern is sexually dimorphic in gonads during sexual differentiation. The two hormones required for male differentiation, testosterone and Müllerian-inhibiting substance, are regulated by SF-1. Analyses of knockout mice lacking SF-1 by gene targeting disruption demonstrated that the SF-1-disrupted mice lack adrenal glands and gonads, supporting the suggestion that SF-1 is an essential regulator of the endocrine development and differentiation. Additionally, SF-1 is expressed in the pituitary gonadotropes and the ventrolateral hypothalamic nucleus, which are higher levels of the reproductive regulatory axis, of both adults and embryos. These tissues are also affected in SF-1 knockout mice, indicating that SF-1 plays extended roles at all levels of the reproductive axis, by regulating more genes involved in reproductive function and development.

The alpha1-fetoprotein locus is activated by a nuclear receptor of the Drosophila FTZ-F1 family

The alpha1-fetoprotein (AFP) gene is located between the albumin and alpha-albumin genes and is activated by transcription factor FTF (fetoprotein transcription factor), presumed to transduce early developmental signals to the albumin gene cluster. We have identified FTF as an orphan nuclear receptor of the Drosophila FTZ-F1 family. FTF recognizes the DNA sequence 5'-TCAAGGTCA-3', the canonical recognition motif for FTZ-F1 receptors. cDNA sequence homologies indicate that rat FTF is the ortholog of mouse LRH-1 and Xenopus xFF1rA. Rodent FTF is encoded by a single-copy gene, related to the gene encoding steroidogenic factor 1 (SF-1). The 5.2-kb FTF transcript is translated from several in-frame initiator codons into FTF isoforms (54 to 64 kDa) which appear to bind DNA as monomers, with no need for a specific ligand, similar KdS (approximately equal 3 x 10(-10) M), and similar transcriptional effects. FTF activates the AFP promoter without the use of an amino-terminal activation domain; carboxy-terminus-truncated FTF exerts strong dominant negative effects. In the AFP promoter, FTF recruits an accessory trans-activator which imparts glucocorticoid reactivity upon the AFP gene. FTF binding sites are found in the promoters of other liver-expressed genes, some encoding liver transcription factors; FTF, liver alpha1-antitrypsin promoter factor LFB2, and HNF-3beta promoter factor UF2-H3beta are probably the same factor. FTF is also abundantly expressed in the pancreas and may exert differentiation functions in endodermal sublineages, similar to SF-1 in steroidogenic tissues. HepG2 hepatoma cells seem to express a mutated form of FTF.

Activation of transcription in Drosophila embryos is a gradual process mediated by the nucleocytoplasmic ratio

We have observed that zygotic transcription does not initiate at a single point in Drosophila embryos. Rather, a gene initiates transcription in a few nuclei of a fraction of embryos. During succeeding cycles, the frequency of transcribing embryos, and of nuclei transcribing in those embryos, gradually increases. For the fushi tarazu (ftz) gene, the timing of this process is regulated by the concentration of the maternally loaded, repressing transcription factor tramtrack (ttk). Altering the dose of Ttk protein in embryos shifts the activation of ftz transcription either forward or backward during development but does not effect Krüppel (Kr) activation. We have observed that the transcription of several genes, including ftz, is triggered in embryos at a critical nuclear density; therefore, we suggest that titration of transcription factors like ttk by the nucleocytoplasmic ratio triggers zygotic transcription in Drosophila.