J L Jameson

Nonclassical estrogen receptor α signaling mediates negative feedback in the female mouse reproductive axis

Ovarian estrogen exerts both positive and negative feedback control over luteinizing hormone (LH) secretion during the ovulatory cycle. Estrogen receptor (ER) α but not ERβ knockout mice lack estrogen feedback. Thus, estrogen feedback appears to be primarily mediated by ERα. However, it is now recognized that, in addition to binding to estrogen response elements (EREs) in DNA to alter target gene transcription, ERα signals through ERE-independent or nonclassical pathways, and the relative contributions of these pathways in conveying estrogen feedback remain unknown. Previously we created a knockin mouse model expressing a mutant form of ERα (AA) with ablated ERE-dependent but intact ERE-independent activity. Breeding this allele onto the ERα-null (−/−) background, we examine the ability of ERE-independent ERα signaling pathways to convey estrogen feedback regulation of the female hypothalamic–pituitary axis in vivo. ERα−/AA exhibited 69.9% lower serum LH levels compared with ERα−/− mice. Additionally, like wild type, ERα−/AA mice exhibited elevated LH after ovariectomy (OVX). Furthermore, the post-OVX rise in serum LH was significantly suppressed by estrogen treatment in OVX ERα−/AA mice. However, unlike wild type, both ERα−/AA and ERα−/− mice failed to exhibit estrous cyclicity, spontaneous ovulation, or an afternoon LH surge response to estrogen. These results indicate that ERE-independent ERα signaling is sufficient to convey a major portion of estrogens negative feedback actions, whereas positive feedback and spontaneous ovulatory cyclicity require ERE-dependent ERα signaling.

Aromatase (Cyp19) expression is up-regulated by targeted disruption of Dax1

DAX-1 [dosage-sensitive sex reversal, adrenal hypoplasia congenita (AHC) critical region on the X chromosome, gene 1] is an orphan nuclear receptor that represses transcription by steroidogenic factor-1 (SF-1), a factor that regulates expression of multiple steroidogenic enzymes and other genes involved in reproduction. Mutations in the human DAX1 gene (also known as AHC) cause the X-linked syndrome AHC, a disorder that is associated with hypogonadotropic hypogonadism also. Characterization of Dax1-deficient male mice revealed primary testicular defects that included Leydig cell hyperplasia (LCH) and progressive degeneration of the germinal epithelium, leading to infertility. In this study, we investigated the effect of Dax1 disruption on the expression profile of various steroidogenic enzyme genes in Leydig cells isolated from Dax1-deficient male mice. Expression of the aromatase (Cyp19) gene, which encodes the enzyme that converts testosterone to estradiol, was increased significantly in the Leydig cells isolated from mutant mice, whereas the expression of other proteins (e.g., StAR and Cyp11a) was not altered. In in vitro transfection studies, DAX-1 repressed the SF-1-mediated transactivation of the Cyp19 promoter but did not inhibit the StAR or Cyp11a promoters. Elevated Cyp19 expression was accompanied by increased intratesticular levels of estradiol. Administration of tamoxifen, a selective estrogen-receptor modulator, restored fertility to the Dax1-deficient male mice and partially corrected LCH, suggesting that estrogen excess contributes to LCH and infertility. Based on these in vivo and in vitro analyses, aromatase seems to be a physiologic target of Dax-1 in Leydig cells, and increased Cyp19 expression may account, in part, for the infertility and LCH in Dax1-deficient mice.

Mechanisms that mediate negative regulation of the thyroid-stimulating hormone alpha gene by the thyroid hormone receptor.

A group of transcriptional cofactors for nuclear hormone receptors, referred to as corepressors (CoRs) and coactivators (CoAs), has been shown to induce transcriptional silencing and hormone-induced activation, respectively, of genes that contain positive hormone response elements. Transcriptional silencing by CoRs involves the recruitment of histone deacetylases (HDACs), whereas ligand-dependent activation is associated with the recruitment of CoAs, which possess or recruit histone acetyltransferases (HATs). In a reciprocal manner, negatively regulated genes are stimulated by nuclear receptors in the absence of ligand and are repressed in response to ligand binding to receptors. We show here that negative regulation of the thyroid-stimulating hormone α (TSHα) promoter by the thyroid hormone receptor (TR) involves a novel mechanism in which the recruitment of CoRs by TR is associated with transcriptional stimulation and histone acetylation. Expression of excess HDAC reverses the stimulation mediated by the TR·CoR complex, consistent with a pivotal role for acetylation in this event. Addition of the ligand, 3,5,3′-triiodothyronine (T3), induces transcriptional repression of the TSHα promoter and is associated with the loss of histone acetylation. T3-dependent repression is blocked by phosphorylation of cAMP response element binding protein, or by inhibition of HDAC, indicating that receptor action is subverted by maneuvers that stimulate histone acetylation of the target gene. We propose that negative regulation of a subset of genes by TR involves the active exchange of CoRs and CoAs with intrinsic promoter regulatory elements that normally strongly induce histone acetylation and transcriptional activation.

A naturally occurring steroidogenic factor-1 mutation exhibits differential binding and activation of target genes.

Steroidogenic factor-1 (SF-1) is an orphan nuclear receptor that binds DNA as a monomer and regulates the transcription of multiple target genes. A mutation in the proximal (P)-box of the first zinc finger of SF-1 (G35E) has been reported to cause complete XY sex reversal and adrenal insufficiency. Because this P-box region dictates DNA binding specificity, we investigated the effect of this mutation on DNA binding and regulation of target genes. Binding of the P-box mutant was markedly impaired for most native SF-1 response elements. However, mutant SF-1 bound to a subset of response elements containing a CCA AGGTCA motif. Mutagenesis studies of response elements revealed that the first nucleotide position in the 5′-flanking sequence triplet and the central part of the half-site dictate DNA binding specificity by the mutant SF-1. Further, introduction of a mutation into the SF-1 A-box, which has been proposed to bind to the 5′-flanking sequence triplet, eliminated binding by mutant SF-1 to all response elements tested. These data support the idea that the A-box stabilizes monomeric binding by nuclear receptors. This action may be particularly important when P-box binding affinity is compromised either by mutations in SF-1 or by sequence alterations in its binding site.

Estrogen Receptor α Signaling Pathways Differentially Regulate Gonadotropin Subunit Gene Expression and Serum Follicle-Stimulating Hormone in the Female Mouse

Estrogen, acting via estrogen receptor (ER)alpha, regulates serum gonadotropin levels and pituitary gonadotropin subunit expression. However, the cellular pathways mediating this regulation are unknown. ERalpha signals through classical estrogen response element (ERE)-dependent genomic as well as nonclassical ERE-independent genomic and nongenomic pathways. Using targeted mutagenesis in mice to disrupt ERalpha DNA binding activity, we previously demonstrated that ERE-independent signaling is sufficient to suppress serum LH levels. In this study, we examined the relative roles of ERE-dependent and -independent estrogen signaling in estrogen regulation of LH, FSH, prolactin, and activin/inhibin subunit gene expression, pituitary LH and FSH protein content, and serum FSH levels. ERE-independent signaling was not sufficient for estrogen to induce pituitary prolactin mRNA or suppress pituitary LHbeta mRNA, LH content, or serum FSH in estrogen-treated ovariectomized mice. However, ERE-independent signaling was sufficient to reduce pituitary glycoprotein hormone alpha-subunit, FSHbeta, and activin-betaB mRNA expression. Together with previous serum LH results, these findings suggest ERE-independent ERalpha signaling suppresses serum LH via reduced secretion, not synthesis. Additionally, ERE-dependent and ERE-independent ERalpha pathways may distinctly regulate steps involved in the synthesis and secretion of FSH.

Variable presentation of X-linked adrenal hypoplasia congenita.

Abstract We present a family with X-linked adrenal hypoplasia congenita (AHC) due to a truncation mutation in the DAX1 gene. The three patient reports demonstrate variable clinical and biochemical features at presentation. They presented with adrenal crises at 3 years, 4 weeks, and 3 weeks. Mineralocorticoid deficiency preceded glucocorticoid deficiency in patient 3 and an early ultrasound indicated normal sized adrenal tissue. Genetic analysis showed that potential female carriers were unaffected.

Advances in the molecular genetics of hypogonadotropic hypogonadism

Mutations in several genes have been shown to cause hypogonadotropic hypogonadism (HHG) in humans. This condition may result from abnormalities in hypothalamic gonadotropin-releasing hormone (GnRH) secretion, impaired pituitary gonadotropin release, or both. Here, we consider mutations in KAL in X-linked Kallmann syndrome; DAX1 in X-linked adrenal hypoplasia congenita; the related orphan nuclear receptor, steroidogenic factor-1; leptin and prohormone convertase-1, which may influence GnRH release and processing; the GnRH receptor; the pituitary transcription factors, HESX-1, LHX3 and PROP-1; and the gonadotropins, follicle stimulating hormone (FSH) and luteinizing hormone (LH). Identifying naturally occurring mutations in these genes provides important information about the role of these factors in the development and function of the hypothalamic-pituitary gonadal axis in humans. Different approaches to treatment and counseling may be needed, depending on the condition. Furthermore, the pathophysiological basis of HHG in the majority of individuals remains unclear, despite recent advances. Other candidate genes may be involved in these patients.

X-linked adrenal hypoplasia congenita and DAX-1

X-linked adrenal hypoplasia congenita (AHC) (OMIM, 300200) is a potentially life-threatening condition that results from mutations in the orphan nuclear receptor, DAX-1. More than 50 different mutations in the DAX1 gene have been reported. DAX1 is expressed in the adrenal gland and at multiple levels throughout the hypotharamic-pituitary gonadal axis. In keeping with this expression pat tern, boys with X-linked AHC present with primary adrenal failure in early infancy or childhood and hypogonadotropic hypogonadism (HHG) be comes apparent at the expected time of puberty. This form of HHG reflects combined hypothalamic and pituitary abnormalities, and the gonadotropin response to gonadotropin-releasing hormone is of ten poor. Impaired spermatogenesis is also observed in Ahch (Dax1) knockout mice. The limited data available to date suggest that DAX-1 may affect Sertoli cell function in humans as well. In this review, we discuss the clinical presentation of patients with X-linked AHC, Several recently reported cases are described that broaden the phenotypic spectrum of this condition and include presentation with adrenal failure in adulthood, The molecular actions of DAX-1 and its pathophysiological role in AHC/HHG are also considered, Finally, future directions for the detection and potential treatment of this condition are proposed.

Human disorders caused by nuclear receptor gene mutations

The identification of naturally occurring nuclear receptor mutations highlights the critical role that many of these transcription factors play in human endocrine development and function. Inactivating mutations in the ligand-dependent nuclear receptors (TRβ, VDR, ERα, GR, MR, AR) are well characterized in patients with conditions such as androgen insensitivity syndrome (AIS) and vitamin D resistance. On the other hand, mutations in TRβ act in a dominant negative manner to cause hormone resistance. Inactivating mutations in orphan nuclear receptors have also been identified (PPARγ2, HNF4α, PNR, NURR1, SF1, DAX1, SHP) and reveal important developmental and metabolic functions for this group of receptors with previously elusive physiologic roles. In addition to loss of function mutations, receptor activation can result from mutations that confer constitutive activity or altered ligand responsiveness to the receptor (MR, AR), or from genetic duplication (DAX1) or the expression of fusion proteins (RARA, PPARγ1). Together, these naturally occurring mutations provide fascinating insight into key structural and functional receptor domains to reveal the diverse role nuclear receptors play in human biology.