The serine/threonine transmembrane receptor ALK2 mediates Müllerian inhibiting substance signaling

Müllerian inhibiting substance (MIS or anti-Müllerian hormone) is a member of the transforming growth factor-beta family and plays a pivotal role in proper male sexual differentiation. Members of this family signal by the assembly of two related serine/threonine kinase receptors, referred to as type I or type II receptors, and downstream cytoplasmic Smad effector proteins. Although the MIS type II receptor (MISRII) has been identified, the identity of the type I receptor is unclear. Here we report that MIS activates a bone morphogenetic protein-like signaling pathway, which is solely dependent on the presence of the MISRII and bioactive MIS ligand. Among the multiple type I candidates tested, only ALK2 resulted in significant enhancement of the MIS signaling response. Furthermore, dominant-negative and antisense strategies showed that ALK2 is essential for MIS-induced signaling in two independent assays, the cellular Tlx-2 reporter gene assay and the Müllerian duct regression organ culture assay. In contrast, ALK6, the other candidate MIS type I receptor, was not required. Expression analyses revealed that ALK2 is present in all MIS target tissues including the mesenchyme surrounding the epithelial Müllerian duct. Collectively, we conclude that MIS employs a bone morphogenetic protein-like signaling pathway and uses ALK2 as its type I receptor. The use of this ubiquitously expressed type I receptor underscores the role of the MIS ligand and the MIS type II receptor in establishing the specificity of the MIS signaling cascade.

Autocrine and paracrine Müllerian inhibiting substance hormone signaling in reproduction

Members of the transforming growth factor beta (TGFbeta) superfamily are polypeptide growth factors that exhibit diverse effects on normal cell growth, adhesion, mesenchymal-epithelial interactions, cell differentiation, and programmed cell death. This chapter will discuss the work of ourselves and others on one member of this large superfamily, Müllerian inhibiting substance (MIS, or anti-Müllerian hormone, AMH) and its role in reproductive tract development and the adult gonad. Using recombinant MIS protein, it is possible to begin unraveling the molecular mechanism of duct involution in the embryo. Our recent results suggest that MIS triggers cell death by altering mesenchymal-epithelial interactions. In addition to the developmental effects of MIS in secondary sexual differentiation, expression studies of the MIS ligand and the MIS type II receptor (MISIIR) suggest a potential regulatory role for MIS in adult germ cell maturation and gonadal function. Recent data from others suggest that MIS may act in a paracrine manner to block differentiation of interstitial cells of the adult gonad by repressing all or some steps of steroidogenesis. Our studies are highly suggestive of direct repression of steroidogenic enzyme gene expression by activation of the MIS signaling pathway. Thus, for the first time, an opportunity to define fully target genes and components of the MIS signaling pathway may be possible.

Haploinsufficiency of steroidogenic factor-1 in mice disrupts adrenal development leading to an impaired stress response

Adrenal steroids are essential for homeostasis and survival during severe physiological stress. Analysis of a patient heterozygous for the steroidogenic factor-1 (SF-1) gene suggested that reduced expression of this nuclear receptor leads to adrenal failure. We therefore examined SF-1 heterozygous (+/-) mice as a potential model for delineating mechanisms underlying this disease. Here we show that SF-1 +/- mice exhibit adrenal insufficiency resulting from profound defects in adrenal development and organization. However, compensatory mechanisms, such as cellular hypertrophy and increased expression of the rate-limiting steroidogenic protein StAR, help to maintain adrenal function at near normal capacity under basal conditions. In contrast, adrenal deficits in SF-1 heterozygotes are revealed under stressful conditions, demonstrating that normal gene dosage of SF-1 is required for mounting an adequate stress response. Our findings predict that natural variations leading to reduced SF-1 function may underlie some forms of subclinical adrenal insufficiency, which become life threatening during traumatic stress.

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.

Phosphorylation of the nuclear receptor SF-1 modulates cofactor recruitment: integration of hormone signaling in reproduction and stress

Steroidogenic factor 1 (SF-1) is an orphan nuclear receptor that serves as an essential regulator of many hormone-induced genes in the vertebrate endocrine system. The apparent absence of a SF-1 ligand prompted speculation that this receptor is regulated by alternative mechanisms involving signal transduction pathways. Here we show that maximal SF-1-mediated transcription and interaction with general nuclear receptor cofactors depends on phosphorylation of a single serine residue (Ser-203) located in a major activation domain (AF-1) of the protein. Moreover, phosphorylation-dependent SF-1 activation is likely mediated by the mitogen-activated protein kinase (MAPK) signaling pathway. We propose that this single modification of SF-1 and the subsequent recruitment of nuclear receptor cofactors couple extracellular signals to steroid and peptide hormone synthesis, thereby maintaining dynamic homeostatic responses in stress and reproduction.

Paracrine-mediated apoptosis in reproductive tract development

In mammalian development, the signaling pathways that couple extracellular death signals with the apoptotic machinery are still poorly understood. We chose to examine Müllerian duct regression in the developing reproductive tract as a possible model of apoptosis during morphogenesis. The TGFbeta-like hormone, Müllerian inhibiting substance (MIS), initiates regression of the Müllerian duct or female reproductive tract anlagen; this event is essential for proper male sexual differentiation and occurs between embryonic days (E) 14 and 17 in the rat. Here, we show that apoptosis occurs during Müllerian duct regression in male embryos beginning at E15. Female Müllerian ducts exposed to MIS also exhibited prominent apoptosis within 13 h, which was blocked by a caspase inhibitor. In both males and females the MIS type-II receptor is expressed exclusively in the mesenchymal cell layer surrounding the duct, whereas apoptotic cells localize to the epithelium. In addition, tissue recombination experiments provide evidence that MIS does not act directly on the epithelium to induce apoptosis. Based on these data, we suggest that MIS triggers cell death by altering mesenchymal-epithelial interactions.

Neuronal expression of the 5HT3 serotonin receptor gene requires nuclear factor 1 complexes

The 5HT3 receptor (5HT3R) is a serotonin-gated ion channel whose expression is restricted to a subset of cells within the central and peripheral nervous systems. In vitro analysis shows that a small proximal region of the TATA-less 5HT3R promoter is sufficient to direct neuronal-specific reporter gene expression. Three potential regulatory elements conserved between the mouse and human genes were identified within this proximal promoter, two of which are known sites for the ubiquitously expressed factors Sp1 and nuclear factor 1 (NF1). Surprisingly, mutation of the NF1 binding site abolished all reporter activity in cell transfection studies, suggesting that this element is essential for neuronal-specific transcriptional activity of the 5HT3R. Furthermore, a complex of neuronal proteins that includes a member(s) of the NF1 family binds to this site, as shown by gel mobility super shift and DNaseI footprinting analyses. Although NF1 has been proposed to mediate basal transcription of many ubiquitously expressed genes, our data suggest that a member of the NF1 transcription factor family participates in neuronal-specific gene expression by promoting interactions with other regulatory factors found in sensory ganglia.

Wilms' tumor 1 and Dax-1 modulate the orphan nuclear receptor SF-1 in sex-specific gene expression

Products of steroidogenic factor 1 (SF-1) and Wilms' tumor 1 (WT1) genes are essential for mammalian gonadogenesis prior to sexual differentiation. In males, SF-1 participates in sexual development by regulating expression of the polypeptide hormone Müllerian inhibiting substance (MIS). Here, we show that WT1 -KTS isoforms associate and synergize with SF-1 to promote MIS expression. In contrast, WT1 missense mutations, associated with male pseudohermaphroditism in Denys-Drash syndrome, fail to synergize with SF-1. Additionally, the X-linked, candidate dosage-sensitive sex-reversal gene, Dax-1, antagonizes synergy between SF-1 and WT1, most likely through a direct interaction with SF-1. We propose that WT1 and Dax-1 functionally oppose each other in testis development by modulating SF-1-mediated transactivation.

The nuclear receptor SF-1 mediates sexually dimorphic expression of Mullerian Inhibiting Substance, in vivo

Mullerian Inhibiting Substance (MIS) functions to promote regression of the Mullerian duct during male development. Maintaining the sexually dimorphic pattern of MIS expression is essential for proper mammalian reproductive tract development. Here, we show that the intricate spatial and temporal pattern of MIS expression is directed by a remarkably small proximal promoter of only 180 base pairs in length. Expression of the MIS-human growth hormone transgene (MIS/GH) is restricted to Sertoli cells in embryonic testis and to granulosa cells of postnatal ovary, consistent with the known MIS expression pattern. The proximal MIS promoter is therefore sufficient to direct the initiation and the maintenance of MIS gene expression in both sexes. Moreover, in vivo MIS promoter activity requires an intact binding site for the orphan nuclear receptor SF-1. Taken together, these data strongly suggest that SF-1 directly activates MIS in embryonic and postnatal gonads. Consistent with the proposed role of SF-1 in mammalian sex-determination, our study provides physiological evidence that a SF-1 binding site is essential for gene activation of an embryonic testis-specific marker.

Bioactivation of Müllerian inhibiting substance during gonadal development by a kex2/subtilisin-like endoprotease

During male gonadal development Müllerian duct regression is mediated by the actions of the hormone Müllerian inhibiting substance (MIS), a member of the transforming growth factor beta superfamily. MIS is considered to be unique among members of this superfamily because bioactivation of MIS via proteolytic processing is hypothesized to occur at its target organ, the Müllerian duct. We find instead that the majority of MIS is processed and secreted from the embryonic testes as a complex in which the mature region remains noncovalently associated with the prodomain. In addition, we have identified two candidate endoproteases that are expressed in the testes and that may be capable of processing MIS in vivo. These kex2/subtilisin-like enzymes, PC5 and furin, are members of the proprotein convertase family that have been implicated in hormone bioactivation via proteolytic processing after dibasic amino acid cleavage recognition sites. Coexpression of PC5 and MIS in transfected mammalian cells results in efficient processing and bioactivation of MIS. Our results suggest that MIS is a natural substrate for PC5, thereby supporting a role for prohormone convertases in the activation of transforming growth factor beta-related hormones during development.

Molecular biology of pituitary development and disease

The major endocrine cell types of the anterior and intermediate pituitary arise in sequential order during development. Our laboratory seeks to understand the molecular basis for different lineages among these cell types. Previous data from our group and others have shown that the POU-domain factor, Pit-1, and the orphan nuclear receptor, SF-1, are critical in the specification and maintenance of these cell types. The analysis of naturally occurring mutations revealed that Pit-1 is needed for development of three cell types, the thyrotropes (thyroid-stimulating hormone), somatotropes (growth hormone), and lactotropes (prolactin). Recently, a genetically engineered mouse mutant demonstrated that SF-1 is required for the maintenance of the gonadotrope (luteinizing hormone/follicle-stimulating hormone) cellular phenotype. To date, a similar factor for the corticotrope and melanotrope lineages expressing propiomelanocortin (POMC) has not been identified. Surprisingly, the serotonin (5-HT) neurotransmitter receptor 5-HT3 was found to be expressed in the anterior and intermediate lobes of the developing rodent pituitary. We are using this new marker to examine the molecular basis of the POMC lineages.

The nuclear receptor steroidogenic factor 1 acts at multiple levels of the reproductive axis

Steroidogenic factor 1 (SF-1), an orphan nuclear receptor, regulates the enzymes that produce sex steroids, and disruption of the Ftz-F1 gene encoding SF-1 precludes adrenal and gonadal development. We now study the role of SF-1 at other levels of the hypothalamic/pituitary/gonadal axis. In Ftz-F1-disrupted mice, immunohistochemical analyses with antibodies against pituitary trophic hormones showed a selective loss of gonadotrope-specific markers, supporting the role of SF-1 in gonadotrope function. In situ hybridization analyses confirmed these results; pituitaries from Ftz-F1-disrupted mice lacked transcripts for three gonadotrope-specific markers (LH beta, FSH beta, and the receptor for gonadotropin-releasing hormone), whereas they exhibited decreased but detectable expression of the alpha-subunit of glycoprotein hormones. SF-1 transcripts in the developing mouse pituitary, which first became detectable at embryonic day 13.5-14.5, preceded the appearance of FSH beta and LH beta transcripts. In adult rat pituitary cells, SF-1 transcripts colocalized with immunoreactivity for the gonadotrope-specific LH. Finally, SF-1 interacted with a previously defined promoter element in the glycoprotein hormone alpha-subunit gene, providing a possible mechanism for the impaired gonadotropin expression in Ftz-F1-disrupted mice. These studies establish novel roles of this orphan nuclear receptor in reproductive function.

Nuclear receptor steroidogenic factor 1 regulates the müllerian inhibiting substance gene: a link to the sex determination cascade

Normal male sex differentiation requires that Sertoli cells in the embryonic testes produce müllerian inhibiting substance (MIS), a TGF beta-like hormone that causes müllerian duct regression. In primary Sertoli cells, the orphan nuclear receptor, steroidogenic factor 1 (SF-1), regulates the MIS gene by binding to a conserved upstream regulatory element. In heterologous (HeLa) cells, MIS gene activation by SF-1 requires removal of the SF-1 ligand-binding domain, implicating a Sertoli cell-specific ligand or cofactor. Finally, the sexually dimorphic expression of SF-1 during development coincides with MIS expression and müllerian duct regression. We propose that SF-1 regulates MIS in vivo and participates directly in the process of mammalian sex determination.

Developmental expression of mouse steroidogenic factor-1, an essential regulator of the steroid hydroxylases

As an initial step toward understanding its role in steroidogenesis, we studied the developmental profile of steroidogenic factor-1 (SF-1), a nuclear receptor that regulates the steroid hydroxylases. SF-1 transcripts first appear on embryonic day 9 (E9) in the urogenital ridge, the probable source of steroidogenic cells of both adrenals and gonads. By E11, after the adrenals and gonads are clearly separate, SF-1 transcripts are detected throughout the adrenal primordium. Thereafter, adrenal expression of SF-1 localizes to the cortex. Consistent with its proposed role in regulating cholesterol side-chain cleavage enzyme (SCC), SF-1 is expressed before SCC. During the sexually undifferentiated stage of gonadal development (E9-E12), all embryos express SF-1 in the genital ridge. As testicular cords form in males, SF-1 transcripts are diffusely expressed throughout the testis, whereas SCC mRNA is limited to the interstitium. These differences between SF-1 and SCC reflect SF-1 expression by Sertoli cells, as shown by Northern blotting and in situ hybridization. In contrast to its persistent expression in the embryonic testis, SF-1 transcripts disappear from the ovary between E13.5-E16.5, reappearing only during late gestation (E18.5). Thus, expression of SF-1 in the embryonic gonad is sexually dimorphic. Coupled with the demonstration of SF-1 mRNA in Sertoli cells, these data suggest that SF-1 plays a role in gonadal development distinct from regulating the steroidogenic enzymes. Additionally, SF-1 is expressed in the embryonic forebrain, implying a role in neural development.

Mutation of the POU-specific domain of Pit-1 and hypopituitarism without pituitary hypoplasia

A point mutation in the POU-specific portion of the human gene that encodes the tissue-specific POU-domain transcription factor, Pit-1, results in hypopituitarism, with deficiencies of growth hormone, prolactin, and thyroid-stimulating hormone. In two unrelated Dutch families, a mutation in Pit-1 that altered an alanine in the first putative alpha helix of the POU-specific domain to proline was observed. This mutation generated a protein capable of binding to DNA response elements but unable to effectively activate its known target genes, growth hormone and prolactin. The phenotype of the affected individuals suggests that the mutant Pit-1 protein is competent to initiate other programs of gene activation required for normal proliferation of somatotrope, lactotrope, and thyrotrope cell types. Thus, a mutation in the POU-specific domain of Pit-1 has a selective effect on a subset of Pit-1 target genes.

Structural analysis of the transmembrane domain of the epidermal growth factor receptor

The ligand-binding domain of the epidermal growth factor (EGF) receptor is separated from the cytoplasmic protein tyrosine kinase domain by a predicted single transmembrane segment. Antipeptide antibodies prepared against the outer portion of the predicted transmembrane segment confirmed this area was exposed only when cells were treated with permeabilizing agents. To investigate structural requirements for signal transduction by the transmembrane domain, three types of mutant EGF receptor were prepared. The first type was designed to shorten the transmembrane domain, the second to place proline substitutions within this domain, and the third to make amino acid substitutions analogous to those present in the transforming c-erbB2/neu oncoprotein. Mutant human receptors were expressed in null recipient mouse B82L and Chinese hamster ovary cells. All receptors bound EGF and exhibited EGF-stimulated protein tyrosine kinase activity in vivo as assayed using a 125I-labeled monoclonal anti-phosphotyrosine antibody. EGF stimulated growth of cells expressing each mutant receptor with similar dose-response characteristics. In contrast to other growth factor receptors, the transmembrane domain of the EGF receptor is tolerant to a variety of changes which neither mimic EGF action by constitutive activation nor interfere with ligand-induced signal transduction.

A family of POU-domain and Pit-1 tissue-specific transcription factors in pituitary and neuroendocrine development

The anterior pituitary gland provides a model for investigating the molecular basis for the appearance of phenotypically distinct cell types, within an organ, a central question in development. The rat prolactin and growth hormone genes are selectively expressed in distinct cell types (lactotrophs and somatotrophs) of the anterior pituitary gland, which reflect differential mechanisms of gene activation or restriction because of interactions of multiple factors binding to these genes. We find that the pituitary-specific 33,000 dalton transcription factor, Pit-1, normally expressed in somatotrophs, lactotrophs, and thyrotrophs, can bind to and activate both growth hormone and prolactin promoters in vitro at levels even tenfold lower than those normally present in pituitary cells. In the case of the prolactin gene, high levels of expression in transgenic animals required two cis-active regions; a distal enhancer (-1.8 to -1.5 kb) and a proximal region (-422 to +33 bp). Each of these regions alone can direct low levels of fusion gene expression to prolactin-producing cell types in transgenic mice, but a synergistic interaction between these regions is necessary for high levels of expression. The initial appearance of the prolactin transgene expression closely follows the appearance of high levels of Pit-1, but later increases in expression coincident with appearance of mature lactotrophs suggest the operation of additional, critical positive factor(s). Unexpectedly, transgenes containing the distal enhancer removed from its normal context are expressed in both the prolactin-producing lactotrophs and the TSH-producing thyrotrophs, thereby suggesting that sequences flanking this enhancer are necessary to restrict expression to the correct cell type within the pituitary. These data indicate that distinct processes of gene activation and restriction are necessary for the fidelity of cell-type specific expression within an organ. Consistent with this model, we find that lactotroph cell lines that cannot express the growth hormone gene contain high levels of functional Pit-1. We suggest a large, highly related POU-domain gene family, potentially exceeding 100 members, has been conserved and expanded in evolution to meet the increasing requirements for more intricate patterns of cell phenotypes. The POU-domain subgroup of the homeodomain gene family, in concert with other homeodomain proteins and with other classes of transcription factors, is likely to contribute to the establishment of the mammalian neuroendocrine system.

Autoregulation of pit-1 gene expression mediated by two cis-active promoter elements

The pit-1 gene is a member of a large family of genes that encode proteins which are involved in development and which contain a highly homologous region, referred to as the POU domain. Pit-1, a pituitary-specific transcription factor, can activate the transcription of the growth hormone and prolactin promoters. It is expressed in mature thyrotroph, somatotroph and lactotroph cell types of the anterior pituitary which arise sequentially during development; somatotrophs and lactotrophs, which secrete growth hormone and prolactin, respectively, are the last to arise. Intriguingly, during ontogeny, pit-1 transcripts are observed in the rat neural tube and neural plate (embryonic day 10-11) and disappear thereafter (day 13), only to reappear exclusively in the anterior lobe of the pituitary gland (day 15) just before activation of prolactin and growth hormone. This biphasic pattern suggests a complex mechanism of initial activation of pit-1 gene expression. Transcription and transfection analyses in vitro using wild-type and mutated promoters indicate that Pit-1 can positively autoregulate the expression of the pit-1 promoter as a consequence of binding to two Pit-1-binding elements. Mutation of the 5' Pit-1-binding site abolished positive autoregulation, whereas mutation of the element located immediately 3' of the cap site markedly increased expression of the pit-1 promoter. These data are consistent with a positive, attenuated autoregulatory loop that seems to function in maintaining pit-1 gene expression.

Mutational removal of the Thr669 and Ser671 phosphorylation sites alters substrate specificity and ligand-induced internalization of the epidermal growth factor receptor

The epidermal growth factor (EGF) receptor contains multiple sites of phosphorylation on serine, threonine, and tyrosine residues. Because the biological responsiveness of the EGF receptor is regulated by phosphorylation at several of these sites, we studied the functional consequences of removal of the Thr669 and Ser671 phosphorylation sites using site-directed mutagenesis. The mutant EGF receptor expressed in mouse B82 cells displayed normal EGF binding and in vivo autophosphorylation and was fully active in biological signal transduction as measured by EGF-stimulated gene transcription. However, the EGF-dependent phosphorylation of an 85-kDa cellular substrate by the mutant receptor was impaired relative to the wild type receptor, indicating that the mutated region may specifically interact with this substrate. Endocytosis of the mutant receptor was also impaired as measured by both receptor down-regulation and ligand internalization studies. This was due to impaired uptake of the mutant receptor by the saturable, high affinity endocytic system. Several aspects of mutant receptor function were regulated normally by TPA, indicating a lack of interaction between the mutated phosphorylation sites and the nearby protein kinase C phosphorylation site Thr654. These results suggest that phosphorylation of the EGF receptor at Thr669 and Ser671 mediates interaction of the receptor with a specific tyrosine kinase substrate and is required for efficient ligand-induced receptor internalization.

The POU-specific domain of Pit-1 is essential for sequence-specific, high affinity DNA binding and DNA-dependent Pit-1-Pit-1 interactions

Pit-1 is a member of a family of transcription factors sharing two regions of homology: a highly conserved POU-specific (POUS) domain and a more divergent homeodomain (POUHD). Analysis of mutant Pit-1 proteins suggests that, while the POUHD is required and sufficient for low affinity DNA binding, the POUS domain is necessary for high affinity binding and accurate recognition of natural Pit-1 response elements. Pit-1 is monomeric in solution but associates as a dimer on its DNA response element, exhibiting DNA-dependent protein-protein interactions requiring the POUS domain. Analysis of alpha-helical domains and conserved structures in Pit-1 suggests that POU domain proteins interact with their DNA recognition sites differently than classic homeodomain proteins, with both the POUHD and the POUS domain contacting DNA. Transcriptional activity of Pit-1 on enhancer elements is conferred primarily by a Ser- and Thr-rich N-terminal region unrelated to other known transcription-activating motifs.

Pituitary cell phenotypes involve cell-specific Pit-1 mRNA translation and synergistic interactions with other classes of transcription factors

Development of the anterior pituitary gland involves proliferation and differentiation of ectodermal cells in Rathke's pouch to generate five distinct cell types that are defined by the trophic hormones they produce. A detailed ontogenetic analysis of specific gene expression has revealed novel aspects of organogenesis in this model system. The expression of transcripts encoding the alpha-subunit common to three pituitary glycoprotein hormones in the single layer of somatic ectoderm on embryonic day 11 established that primordial pituitary cell commitment occurs prior to formation of a definitive Rathke's pouch. Activation of Pit-1 gene expression occurs as an organ-specific event, with Pit-1 transcripts initially detected in anterior pituitary cells on embryonic day 15. Levels of Pit-1 protein closely parallel those of Pit-1 transcripts without a significant lag. Unexpectedly, Pit-1 transcripts remain highly expressed in all five cell types of the mature pituitary gland, but the Pit-1 protein is detected in only three cell types--lactotrophs, somatotrophs, and thyrotrophs and not in gonadotrophs or corticotrophs. The presence of Pit-1 protein in thyrotrophs suggests that combinatorial actions of specific activating and restricting factors act to confine prolactin and growth hormone gene expression to lactotrophs and somatotrophs, respectively. A linkage between the initial appearance of Pit-1 protein and the surprising coactivation of prolactin and growth hormone gene expression is consistent with the model that Pit-1 is responsible for the initial transcriptional activation of both genes. The estrogen receptor, which has been reported to be activated in a stereotypic fashion subsequent to the appearance of Pit-1, appears to be capable, in part, of mediating the progressive increase in prolactin gene expression characteristic of the mature lactotroph phenotype. This is a consequence of synergistic transcriptional effects with Pit-1, on the basis of binding of the estrogen receptor to a response element in the prolactin gene distal enhancer. These data imply that both transcriptional and post-transcriptional regulation of Pit-1 gene expression and combinatorial actions with other classes of transcription factors activated in distinct temporal patterns, are required for the mature physiological patterns of gene expression that define distinct cell types within the anterior pituitary gland.

Expression of a large family of POU-domain regulatory genes in mammalian brain development

A novel region referred to as the POU-domain is present in two tissue-specific transcription factors, Pit-1 and Oct-2, that activate expression of genes specifying pituitary and lymphocyte phenotypes. We report the identification of multiple new members of a large family of POU-domain genes expressed in adult brain, and document that all the known mammalian POU-domain genes, including Pit-1 and Oct-2, are expressed widely in the developing nervous system.

A pituitary POU domain protein, Pit-1, activates both growth hormone and prolactin promoters transcriptionally

The anterior pituitary gland provides a model for investigating the molecular basis for the appearance of phenotypically distinct cell types within an organ, a central question in development. The rat prolactin and growth hormone genes are expressed selectively in distinct cell types (lactotrophs and somatotrophs, respectively) of the anterior pituitary gland, reflecting differential mechanisms of gene activation or restriction, as a result of the interactions of multiple factors binding to these genes. We find that when the pituitary-specific 33-kD transcription factor Pit-1, expressed normally in both lactotrophs and somatotrophs, is expressed in either the heterologous HeLa cell line or in bacteria, it binds to and activates transcription from both growth hormone and prolactin promoters in vitro at levels even 10-fold lower than those normally present in pituitary cells. This suggests that a single factor, Pit-1, may be capable of activating the expression of two genes that define different anterior pituitary cell phenotypes. Because a putative lactotroph cell line (235-1) that does not express the growth hormone gene, but only the prolactin gene, appears to contain high levels of functional Pit-1, a mechanism selectively preventing growth hormone gene expression may, in part, account for the lactotroph phenotype.

A tissue-specific transcription factor containing a homeodomain specifies a pituitary phenotype

Multiple related cis-active elements required for cell-specific activation of the rat prolactin gene appear to bind a pituitary-specific positive transcription factor(s), referred to as Pit-1. DNA complementary to Pit-1 mRNA, cloned on the basis of specific binding to AT-rich cell-specific elements in the rat prolactin and growth hormone genes, encodes a 33 kd protein with significant similarity at its carboxyl terminus to the homeodomains encoded by Drosophila developmental genes. Pit-1 mRNA is expressed exclusively in the anterior pituitary gland in both somatotroph and lactotroph cell types, which produce growth hormone and prolactin, respectively. Pit-1 expression in heterologous cells (HeLa) selectively activates prolactin and growth hormone fusion gene expression, suggesting that Pit-1 is sufficient to confer a characteristic pituitary phenotype. The structure of Pit-1 and its recognition elements suggests that metazoan tissue phenotype is controlled by a family of transcription factors that bind to related cis-active elements and contain several highly conserved domains.

Characterization of two atypical promoters and alternate mRNA processing in the mouse Thy-1.2 glycoprotein gene

The promoter and 5' flanking region of the mouse Thy-1.2 glycoprotein gene were characterized by DNA sequencing, primer extension analysis, and deletion analysis. Transcriptional initiation sites were identified which corresponded to two separate exons upstream of the portion of the gene encoding the Thy-1.2 glycoprotein. We demonstrated that the mouse Thy-1.2 gene was transcribed from two atypical promoters separated by 260 base pairs in the genomic sequence. These promoters contained neither TATAAG nor GGPyCCAATCT homologous sequences but defined a conserved nonamer CTCCCTGCT at -48 from each initiation site. Two Thy-1.2 mRNA species of 1,835 and 1,939 nucleotides, differing in the 5' untranslated region of the mRNA, were thus transcribed from the single Thy-1.2 gene by mRNA splicing to the same downstream exon. Recombinant genomes in which the bacterial chloramphenicol acetyltransferase gene was expressed from either of the two Thy-1.2 promoters demonstrated that each promoter functioned independently and did not direct cell-specific expression in lymphoid cells. The 5' flanking region of the Thy-1.2 gene upstream of -68 could be eliminated without altering cell-type-specific expression. This suggests that regulatory elements responsible for tissue and developmental stage-specific expression of the Thy-1.2 gene are not present in the 5' flanking DNA but may reside downstream of the promoters.

DNA fragmentation and cytotoxicity from increased cellular deoxyuridylate

Previous results from this laboratory have shown that thymidylate deprivation results in dramatic elevation of intracellular dUTP and incorporation of dUMP into DNA. The goal of the present studies was to determine whether the latter changes may play a part in the associated cytotoxicity ("thymineless death"), which is ordinarily assumed to be a direct result of reduced intracellular dTTP. The approach used here was to increase intracellular dUTP without allowing dTTP to diminish and observe the effects on cell viability. dUMP pools were expanded by exposure of cells to deoxyuridine [in cell growth medium containing hypoxanthine, methotrexate, and thymidine (HAT medium)], resulting in accumulation of dUTP to levels that approached those of dTTP, which were at, or higher than, the levels in untreated cells. In conjunction with this the cells became nonviable, and newly synthesized DNA was fragmented, both of which occur with thymidylate depletion and, we assume, result from the active process of excision repair at the many uracil-containing sites in DNA. The results indicate that, although the relative importance of low dTTP remains unknown, elevated dUTP can account for the cytotoxicity caused by thymidine starvation. Most of the "dTTP" measured by the DNA polymerase assay in cells treated with methotrexate (MTX) (plus purine supplement) was, in fact, dUTP, which may explain some previous observations of only modest depression of dTTP in cells treated with MTX or similarly acting drugs.

The mouse Thy-1.2 glycoprotein gene: complete sequence and identification of an unusual promoter

Recombinant bacteriophage and cosmid clones containing the gene for the mouse Thy-1.2 glycoprotein were isolated and characterized. The complete sequence of the gene was determined, including a previously unidentified exon located 2.1 kb upstream of the portion of the gene encoding the Thy-1.2 glycoprotein. The transcriptional initiation site was located by S1 nuclease protection mapping in both T lymphocytes and neural cells and was found to be located immediately upstream of this exon. S1 nuclease protection mapping was also used to define the 3' end of the Thy-1.2 transcription unit, and no evidence for alternate mRNA processing was found. Thus, the mouse Thy-1.2 gene is 5447 base pairs in length, including promoter sequences, rather than 2094 as previously described. The mouse and rat Thy-1 genes are highly homologous in both introns and exons. However, the mouse Thy-1 cDNA and rat Thy-1 cDNA differ significantly in sequence in the 5' untranslated region. This suggests that the transcriptional initiation site of the mouse and rat genes may be located at different positions within the genomic sequence and may be related to the differing tissue distribution of Thy-1 in the two species.

The mouse Thy-1.2 glycoprotein gene: complete sequence and identification of an unusual promoter

Recombinant bacteriophage and cosmid clones containing the gene for the mouse Thy-1.2 glycoprotein were isolated and characterized. The complete sequence of the gene was determined, including a previously unidentified exon located 2.1 kb upstream of the portion of the gene encoding the Thy-1.2 glycoprotein. The transcriptional initiation site was located by S1 nuclease protection mapping in both T lymphocytes and neural cells and was found to be located immediately upstream of this exon. S1 nuclease protection mapping was also used to define the 3' end of the Thy-1.2 transcription unit, and no evidence for alternate mRNA processing was found. Thus, the mouse Thy-1.2 gene is 5447 base pairs in length, including promoter sequences, rather than 2094 as previously described. The mouse and rat Thy-1 genes are highly homologous in both introns and exons. However, the mouse Thy-1 cDNA and rat Thy-1 cDNA differ significantly in sequence in the 5' untranslated region. This suggests that the transcriptional initiation site of the mouse and rat genes may be located at different positions within the genomic sequence and may be related to the differing tissue distribution of Thy-1 in the two species.

Expression of the Thy-1 glycoprotein gene by DNA-mediated gene transfer

We isolated a gene encoding the Thy-1.2 glycoprotein from a recombinant library constructed from BALB/c mouse DNA. To evaluate the expression of this cloned gene in different genomic environments, we introduced it into cell lines derived from fibroblast, lymphoid, and neuronal tissues by DNA-mediated gene transfer. When integrated into the genome of mouse L cells, cell-surface Thy-1 can be detected with anti-Thy-1 monoclonal antibodies. These L-cell lines contain between two and four copies of the cloned Thy-1 gene stably integrated in the host genome. After subcloning into a plasmid vector containing the bacterial Eco-gpt gene as a selectable marker, the Thy-1 gene was introduced into the Thy-1-deficient mouse lymphoma AKR1 (Thy-1-d), and the rat neuronal cell line, B50. The resulting transformants also contain two to four copies of the cloned Thy-1 gene but express up to 50-fold more cell-surface Thy-1.2 than the L cell transformants. The expression of vastly differing amounts of cell-surface Thy-1 from similar numbers of genes suggests that the gene encoding this differentiation antigen is under tissue-specific regulation.

2'-deoxyribosyl analogues of UDP-N-acetylglucosamine in cells treated with methotrexate or 5-fluorodeoxyuridine

dUDP-GlcNAc, the 2'-deoxyribosyl analogue of UDP-GlcNAc, has been identified in human lymphoid cells treated with the dihydrofolate reductase inhibitor, methotrexate. It was shown previously that elevation of dUTP accompanies the gross expansion in intracellular deoxyuridylate pools that results from the methotrexate-induced block in thymidylate synthetase activity (1). dUDP-GlcNAc presumably is formed from dUTP acting in place of UTP in the normal pathway for formation of UDP-GlcNAc. Neither dUTP nor dUDP-GlcNAc has been detected in untreated cells. Inhibition of thymidylate synthetase by treatment of cells with 5-fluorodeoxyuridine (5-FdUrd) also causes the appearance of dUDP-GlcNAc, and, in addition, 5-FdUDP-GlcNAc, synthesized from 5-FdUTP. The metabolic effects, if any, of these analogues are not known. Synthesis of the analogues may help to limit accumulation of dUTP and 5-FdUTP under circumstances in which the deoxyuridine triphosphatase mechanism is insufficient.

Nucleotide levels and incorporation of 5-fluorouracil and uracil into DNA of cells treated with 5-fluorodeoxyuridine

Intracellular pools of 5-fluoro-2'-deoxyuridine (FdUrd) and dUrd nucleotides were measured in cultured human lymphoblasts treated with FdUrd. At 1 microM FdUrd, intracellular 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) was approximately 400 pmoles/10(6) cells, and FdUTP was approximately 0.1 pmole/10(6) cells. Intracellular dUMP and dUTP were elevated to values of approximately 1000 pmoles/10(6) cells and approximately 0.1 pmol/10(6) cells, respectively. With decrease in dTTP levels, utilization of FdUTP and dUTP as substrates for DNA synthesis became significant. FdUMP and dUMP, approximately 5 pmoles of each per micromole of DNA nucleotide, were found in DNA of cells treated with FdUrd (1 microM). The active removal of FUra and Ura from DNA of FdUrd-treated cells by the normal repair mechanism may lead to fragmentation of DNA and contribute to the cytotoxic effect of FdUrd.

Mechanism for exclusion of 5-fluorouracil from DNA

5-Fluorodeoxyuridine 5'-triphosphate is hydrolyzed by the enzyme deoxyuridine triphosphate diphosphohydrolase (EC 3.6.1.23). Uracil-DNA glycosylase removes 5-fluorouracil (FUra) from FUra-containing DNA similar to its removal of uracil from uracil-containing DNA. The absence of FUra in DNA following exposure of cells to FUra can be explained by the activities of these two enzymes.

Characterization of sialytransferase in noncancerous and neoplastic human liver tissue

When compared to uninvolved adjacent tissue, metastatic tumors in human liver appear to have significantly reduced sialytransferase activity. No significant kinetic differences (Michaelis constants, thermostability, and pH optima) between noncancerous and cancerous tissue sialytransferase were found. Mixing experiments between cancerous and noncancerous tissues indicated that inhibitors of sialytransferase activity were present in cancerous tissue. Subsequent experiments demonstrated increased levels of bound sialic acid in the tumor tissues. Inasmuch as futuin, a sialoglycoprotein, inhibits sialyltransferase activity, the increased levels of bound sialic acid in tumor tissue may be responsible for the reduced enzyme activity in these tissues.

Characterization of the association of specific proteins with messenger ribonucleic acid

Isolation of messenger ribonucleic acid (mRNA)-protein particles from cytosol or dissociated polyribosomes yielded complexes in which several proteins were consistently associated with mRNA. Some of the mRNA-associated proteins appeared to have a high affinity for mRNA since they remained complexed to mRNA during centrifugation in CsCl gradients. Quantitation of RNA and protein in polyribosomal mRNPs suggested that each molecule of mRNA bound a molecule of each of the two major proteins of 78 000 and 52 000 apparent molecular weights and/or one or more of several minor proteins found in mRNPs. Of the several mRNP proteins, only the protein of 78 000 apparent molecular weight appeared to form a stable complex with the polyriboadenylic acid [poly(A)]-tract of mRNA, suggesting that the remaining mRNA-associated proteins bind to other regions which may be common to many or all mRNAs. Binding of [3H]poly(A)-rich RNA to mRNP proteins was effectively inhibited by unlabeled poly(A)-rich RNA or the homopolymer polyriboguanylic acid [poly(G)], but not by poly(A) or other natural or synthetic mRNAs. The properties of non-poly(A)-dependent binding of mRNA by mRNP protein were similar to those of mRNA binding by the guanosine triphosphate dependent Met-tRNAfMet-binding protein.

Isolation of messenger-like ribonucleoproteins

Subribosomal and polyribosomal messenger ribonucleoproteins (mRNPs) were isolated from Ehrlich ascites tumor cells by a method involving sedimentation of polyribosomal and subribosomal particles, dissociation with EDTA, and rate-zonal sedimentation. The fractions containing mRNA protein particles were applied to glass fiber filters and extensively washed with buffer containing 0.5 M KCl. The eluted material was demonstrated to be an RNA-protein complex containing poly(A)-rich RNA, heterogeneous in size, and free of 18S or 28S rRNA. mRNA function for the RNA was suggested by its ability to direct protein synthesis in a cell-free protein-synthesizing system derived from wheat germ embryos. Analysis of the proteins associated with subribosomal and polyribosomal mRNPs by iodination and sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed at least seven similar proteins. The apparent molecular weights of the three most prominent proteins were 78,000, 52,000, and 34,000. Analysis of reticulocyte polyribosomal mRNPs revealed an increased prominence of the 78,000 and 52,000 molecular weight proteins relative to the other protein bands.