Effects of puerarin on estrogen-regulated gene expression in gonadotropin-releasing hormone pulse generator of ovariectomized rats

Effects of puerarin on the hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator function is investigated, for the first time, in ovariectomized rats at the level of mRNA expression of estrogen-responsive genes, e.g., estrogen receptor (ER), GnRH and its receptor (GnRHR). Rats were treated orally for 90 days either with a soy-free diet containing two different doses of puerarin (low dose of 600 mg/kg and high dose of 3000 mg/kg) or estradiol benzoate (E2B) at either low dose (4.3 mg/kg) or high dose (17.3 mg/kg). Levels of mRNA expression in the medial preoptic area/anterior hypothalamus (MPOA/AH), mediobasal hypothalamus/median eminence (MBH/ME) and adenohypophysis were measured by quantitative TaqMan® real-time RT-PCR. Plasma levels of luteinizing hormone (LH) and prolactin (PRL) were measured by radioimmunoassay. In the MPOA/AH, both puerarin and E2B decreased ERα mRNA levels without any significant changes in ERβ and GnRH mRNA levels. Both puerarin and E2B did not significantly alter the expression levels of ERα, ERβ and GnRHR in the MBH/ME. E2B exerted significant effects on the down-regulation of adenohypophyseal GnRHR mRNA transcripts and serum LH levels. Puerarin did not cause significant changes in pituitary GnRHR mRNA transcripts and serum LH and PRL levels. This is the first study to demonstrate that in ovariectomized rat models of ovarian hormone deprivation, puerarin acted as a weak estrogen-active compound in the hypothalamic GnRH pulse generator through the downregulation of MPOA/AH ERα mRNA expression.

Antiandrogen Treatment Ameliorates Reproductive and Metabolic Phenotypes in the Letrozole-Induced Mouse Model of PCOS

Polycystic ovary syndrome (PCOS), the most common endocrinopathy in women of reproductive age, is characterized by hyperandrogenism, anovulation, and polycystic ovaries. Although its etiology is unknown, excess androgens are thought to be a critical factor driving the pathology of PCOS. We previously demonstrated that continuous exposure to the aromatase inhibitor letrozole (LET) in mice produces many hallmarks of PCOS, including elevated testosterone (T) and luteinizing hormone, anovulation, and obesity. In the current study, we sought to determine whether androgen receptor (AR) actions are responsible for any of the phenotypes observed in LET mice. C57BL/6 female mice were subcutaneously implanted with LET or placebo control and subsequently treated with the nonsteroidal AR antagonist flutamide or vehicle control. Flutamide treatment in LET females reversed elevated T levels and restored ovarian expression of Cyp17a1 (critical for androgen synthesis) to normal levels. Pituitary expression of Lhb was decreased in LET females that received flutamide treatment, with no changes in expression of Fshb or Gnrhr. Flutamide treatment also restored estrous cycling and reduced the number of ovarian cyst-like follicles in LET females. Furthermore, body weight and adipocyte size were decreased in flutamide-treated LET females. Altogether, our findings provide strong evidence that AR signaling is responsible for many key reproductive and metabolic PCOS phenotypes and further establish the LET mouse model as an important tool for the study of androgen excess.

GnRH-mediated DAN production regulates the transcription of the GnRH receptor in gonadotrope cells

The primary function of gonadotropin-releasing hormone (GnRH) is the regulation of pituitary gonadotropin hormone gene transcription, biosynthesis and release. These effects are mediated through intracellular mobilization of Ca2+ and activation of PKC isoforms and MAP kinases. We show here that DAN (differential screening-selected gene aberrative in neuroblastoma) which is a secreted bone morphogenic protein (BMP) antagonist belonging to the TGFbeta protein superfamily, is controlled by GnRH in murine gonadotrope cells. Acute GnRH stimulation induced a rapid, 27-fold, elevation of DAN mRNA, accompanied by an approximate 3-fold increase in the amount of mature DAN glycoprotein in the cell cytoplasm and in DAN secretion into the culture medium. Incubation of L beta T2 cells in DAN-containing medium altered the levels of a number of cellular proteins. Two of these were identified as the steroidogenic acute regulatory protein (StAR) and the actin-related protein 2/3 complex subunits 2 (p34-ARC) which are primarily involved in steroidogenesis and cytoskeleton remodelling, respectively. DAN caused an approximate 2-fold specific elevation in the cytoplasmic levels of both these proteins in L beta T2 cells. We further tested the effects of DAN on classical GnRH effects viz. gonadotropin and GnRH receptor gene expression. Co-transfection of L beta T2 cells with DAN and gonadotropin subunit promoter luciferase reporter genes had no effect on GnRH stimulation of alpha GSU and LH beta or on the additive GnRH and activin induction of FSH beta subunit transcription. However, co-transfection of DAN markedly inhibited the synergistic activation of GnRH and activin on GnRH receptor gene expression thus implicating DAN as a novel autocrine/paracrine factor that modulates GnRH function in pituitary gonadotropes.

Gonadotropin-releasing hormone (GnRH) positively regulates corticotropin-releasing hormone-binding protein expression via multiple intracellular signaling pathways and a multipartite GnRH response element in alphaT3-1 cells

CRH-binding protein (CRH-BP) binds CRH with high affinity and inhibits CRH-mediated ACTH release from anterior pituitary corticotrope-like cells in vitro. In female mouse pituitary, CRH-BP is localized not only in corticotropes, but is also expressed in gonadotropes and lactotropes. To investigate the functional significance of gonadotrope CRH-BP, we examined the molecular mechanisms underlying GnRH-regulated CRH-BP expression in alphaT3-1 gonadotrope-like cells. CRH-BP is endogenously expressed in alphaT3-1 cells, and quantitative real-time RT-PCR and ribonuclease protection assays demonstrate that GnRH induces a 3.7-fold increase in CRH-BP mRNA levels. GnRH also induces intracellular CRH-BP (2.0-fold) and secreted CRH-BP (5.3-fold) levels, as measured by [125I]CRH:CRH-BP chemical cross-linking. Transient transfection assays using CRH-BP promoter-luciferase constructs indicate that GnRH regulation involves protein kinase C-, ERK- and calcium-dependent signaling pathways and is mediated via a multipartite GnRH response element that includes activator protein 1 and cAMP response element (CRE) sites. The CRE site significantly contributes to GnRH responsiveness, independent of protein kinase A, representing a unique form of multipartite GnRH regulation in alphaT3-1 cells. Furthermore, EMSAs indicate that alphaT3-1 nuclear proteins specifically bind at activator protein 1 and CRE sites. These data demonstrate novel regulation of pituitary CRH-BP, highlighting the importance of the pituitary gonadotrope as a potential interface between the stress and reproductive axes.

Oct-1 and nuclear factor Y bind to the SURG-1 element to direct basal and gonadotropin-releasing hormone (GnRH)-stimulated mouse GnRH receptor gene transcription

The cis-regulatory element localized to position -292/-285 of the mouse GnRH receptor (mGnRHR) gene promoter, designated Sequence Underlying Responsiveness to GnRH 1 (SURG-1), has been shown previously to contribute to stimulation of mGnRHR gene expression by GnRH. We have identified three specific protein-DNA complexes on the SURG-1 element by EMSA using nuclear extracts from the gonadotrope-derived alphaT3-1 and LbetaT2 cell lines. Serial mutagenesis and supershift assays identified nuclear factor Y (NF-Y) binding to -288/-284 and Oct-1 binding to a TAAT sequence at -290/-287. Binding of these two transcription factors was confirmed in vivo by chromatin immunoprecipitation assay and increased in response to GnRH stimulation. To define the functional significance of these sequences in the regulation of mGnRHR gene transcription, transient transfection assays were performed in alphaT3-1 cells using a 1.2-kb mGnRHR (-1164/+62) gene promoter-luciferase reporter construct with selective mutations of the Oct-1, NF-Y, and/or the previously characterized activating protein 1 (AP-1) binding site (-274/-268). Individual mutations in the Oct-1, NF-Y, and AP-1 sites decreased both basal expression and stimulation by GnRH agonist, and the combined mutation of the Oct-1 and AP-1 binding sites further reduced basal transcriptional activity and abolished GnRH stimulation. Overexpression of NF-YA increased GnRHR promoter activity, whereas expression of a dominant negative NF-YA mutant decreased activity, further supporting a role of NF-Y in regulation of mGnRHR gene transcription. In addition, knockdown of Oct-1 by small interfering RNA confirmed that Oct-1 is important for mGnRHR gene expression. In conclusion, NF-Y and Oct-1 bind to the SURG-1 element to direct basal and GnRH-stimulated expression of the mGnRHR gene.