Holly A. Ingraham

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.

Anti-Müllerian Hormone Inhibits Initiation of Primordial Follicle Growth in the Mouse Ovary

Recruitment of primordial follicles is essential for female fer- tility; however, the exact mechanisms regulating this process are largely unknown. Earlier studies using anti-Mullerian hormone (AMH)-deficient mice suggested that AMH is in- volved in the regulation of primordial follicle recruitment. We tested this hypothesis in a neonatal ovary culture system, in which ovaries from 2-d-old C57Bl/6J mice were cultured for 2 or 4di n theabsence or presence of AMH. Ovaries from 2-d-old mice contain multiple primordial follicles, some naked oo- cytes, and no follicles at later stages of development. We ob- served that in the cultured ovaries, either nontreated or AMH- treated, follicular development progressed to the same extent as in in vivo ovaries of comparable age, confirming the validity of our culture system. However, in the presence of AMH, cul- tured ovaries contained 40% fewer growing follicles compared with control ovaries. A similar reduction was found after 4 d of culture. Consistent with these findings, we noted lower inhibin -subunit expression in AMH-treated ovaries com- pared with untreated ovaries. In contrast, expression of AMH ligand type II receptor and the expression of oocyte markers growth and differentiation factor 9 and zona pellucida protein 3 were not influenced by AMH. Based on the results, we suggest that AMH inhibits initia- tion of primordial follicle growth and therefore functions as an inhibitory growth factor in the ovary during these early stages of folliculogenesis. (Endocrinology 143: 1076 -1084, 2002)

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.

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 tissuespecific 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 α helix of the POU-specific domain to proline was observed. This mutation generated a protein capable of binding to DNA response elments 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.

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.

Structural analyses reveal phosphatidyl inositols as ligands for the NR5 orphan receptors SF-1 and LRH-1

Vertebrate members of the nuclear receptor NR5A subfamily, which includes steroidogenic factor 1 (SF-1) and liver receptor homolog 1 (LRH-1), regulate crucial aspects of development, endocrine homeostasis, and metabolism. Mouse LRH-1 is believed to be a ligand-independent transcription factor with a large and empty hydrophobic pocket. Here we present structural and biochemical data for three other NR5A members-mouse and human SF-1 and human LRH-1-which reveal that these receptors bind phosphatidyl inositol second messengers and that ligand binding is required for maximal activity. Evolutionary analysis of structure-function relationships across the SF-1/LRH-1 subfamily indicates that ligand binding is the ancestral state of NR5A receptors and was uniquely diminished or altered in the rodent LRH-1 lineage. We propose that phospholipids regulate gene expression by directly binding to NR5A nuclear receptors.

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.

Wnt4 overexpression disrupts normal testicular vasculature and inhibits testosterone synthesis by repressing steroidogenic factor 1/β-catenin synergy

Genetic studies in mice suggest that Wnt4 signaling antagonizes expression of male hormones and effectively blocks male development in the female embryo. We recently identified an XY intersex patient carrying a chromosomal duplication of the WNT4 locus and proposed that this patients feminization arises from an increased dosage of WNT4. To test this hypothesis, a transgenic mouse was generated with a large genomic P1 containing the human WNT4. Although a complete male to female intersex phenotype was not observed in WNT4 transgenic male mice, a dramatic reduction in steroidogenic acute regulatory protein was detected consistent with the marked reduction in serum and testicular androgen levels. Furthermore, a mild reduction of germ cells and a disorganized vascular system were observed in testes of WNT4 transgenic males. Consistent with these in vivo data, Wnt4 repressed steroidogenesis in adrenocortical and Leydig cell lines, as evidenced by reduced progesterone secretion and 3β-hydroxysteroid dehydrogenase activity. In vitro studies showed that Wnt4 antagonizes the functional synergy observed between the major effector of the Wnt signaling pathway, β-catenin and steroidogenic factor 1, and chromatin immunoprecipitation showed that Wnt4 attenuates recruitment of β-catenin to the steroidogenic acute regulatory protein promoter. Our findings suggest a model in which Wnt4 acts as an anti-male factor by disrupting recruitment of β-catenin at or near steroidogenic factor 1 binding sites present in multiple steroidogenic genes.

International Union of Pharmacology. LXVI. Orphan nuclear receptors.

Half of the members of the nuclear receptors superfamily are so-called “orphan” receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

Structural Basis for Ligand-Independent Activation of the Orphan Nuclear Receptor LRH-1

The orphan nuclear receptors SF-1 and LRH-1 are constitutively active, but it remains uncertain whether their activation is hormone dependent. We report the crystal structure of the LRH-1 ligand binding domain to 2.4 A resolution and find the receptor to be a monomer that adopts an active conformation with a large but empty hydrophobic pocket. Adding bulky side chains into this pocket resulted in full or greater activity suggesting that, while LRH-1 could accommodate potential ligands, these are dispensable for basal activity. Constitutive LRH-1 activity appears to be conferred by a distinct structural element consisting of an extended helix 2 that provides an additional layer to the canonical LBD fold. Mutating the conserved arginine in helix 2 reduced LRH-1 receptor activity and coregulator recruitment, consistent with the partial loss-of-function phenotype exhibited by an analogous SF-1 human mutant. These findings illustrate an alternative structural strategy for nuclear receptor stabilization in the absence of ligand binding.