Lisa M. Halvorson

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors are expressed in the hypothalamus, the gonadotrope cells of the anterior pituitary gland, and the gonads, forming an autocrine–paracrine system in these tissues. Within the pituitary, PACAP functions either alone or synergistically with gonadotropin-releasing hormone (GnRH) to stimulate gonadotropin gene expression and secretion. Our goal was to define the hormonal regulation of pituitary PACAP and PACAP receptor (PAC1) gene expression by dihydrotestosterone (DHT), estradiol, and progesterone alone or in conjunction with GnRH. Treatment of adult male rat pituitary cell cultures with DHT or progesterone augmented GnRH-mediated increase in PACAP messenger RNA (mRNA) levels, but neither had an effect when present alone. Conversely, estradiol treatment blunted PACAP gene expression but did not alter GnRH effects on PACAP expression. Expression of PACAP receptor mRNA was decreased by GnRH treatment, minimally increased by DHT treatment, but not altered by the addition of estradiol or progesterone. DHT and GnRH together blunted PACAP receptor gene expression. Taken together, these results suggest that the activity of the intrapituitary PACAP-PAC1 system is regulated via the complex interaction of gonadal steroids and hypothalamic GnRH.

Pituitary adenylate cyclase-activating polypeptide 1 (PACAP or ADCYAP1) regulates gonadotropin biosynthesis and secretion, both alone and in conjunction with GNRH. Initially identified as a hypothalamic-releasing factor, ADCYAP1 subsequently has been identified in pituitary gonadotropes, suggesting it may act as an autocrine–paracrine factor in this tissue. GNRH has been shown to increase pituitary Adcyap1 gene expression through the interaction of CREB and jun/fos with CRE/AP1 cis-elements in the proximal promoter. In these studies, we were interested in identifying additional transcription factors and cognate cis-elements which regulate Adcyap1 gene promoter activity and chose to focus on the GATA family of transcription factors known to be critical for both pituitary cell differentiation and gonadotropin subunit expression. By transient transfection and electrophoretic mobility shift assay analysis, we demonstrate that GATA2 and GATA4 stimulate Adcyap1 promoter activity via a GATA cis-element located at position −191 in the rat Adcyap1 gene promoter. Furthermore, we show that addition of GATA2 or GATA4 significantly augments GNRH-mediated stimulation of Adcyap1 gene promoter activity in the gonadotrope LβT2 cell line. Conversely, blunting GATA expression with specific siRNA inhibits the ability of GNRH to stimulate ADCYAP1 mRNA levels in these cells. These data demonstrate a complex interaction between GNRH and GATA on ADCYAP1 expression, providing important new insights into the regulation of gonadotrope function.

Pituitary adenylate cyclase–activating polypeptide (PACAP), an ancient molecule highly preserved across species, has been classified as a member of the secretin/glucagon/vasoactive intestinal peptide/growth hormone-releasing hormone polypeptide family. PACAP was first identified as a hypothalamic-releasing factor; nevertheless, it has subsequently been determined to have widespread distribution and function, including expression in the pituitary, gonads, placenta, central and peripheral nervous systems, intestinal tract, and adrenal gland. Consistent with its widespread distribution, PACAP has been found to exert pleiotropic effects. Although first described over 20 years ago, only relatively recently has substantial attention turned to evaluating PACAP’s role in the reproductive system. This review will focus on our current understanding of the expression pattern and function of PACAP in the hypothalamic–pituitary–gonadal axis.

Gonadotropin expression is precisely regulated within the hypothalamic-pituitary-gonadal axis through the complex interaction of neuropeptides, gonadal steroids. and both gonadal- and pituitary-derived peptides. In the anterior pituitary gland, the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) modulates gonadotropin biosynthesis and secretion, acting both alone and in conjunction with GnRH. Steroid hormone feedback also influences gonadotropin expression via both direct and indirect mechanisms. Evidence from nonpituitary tissues suggests that PACAP may be a target for gonadal steroid regulation. In the present study, we show that androgen markedly stimulates rat (r) PACAP promoter-reporter activity in the LβT2 mature mouse gonadotrope cell line. 5′-Serial deletion analysis of reporter constructs identifies 2 regions of androgen responsiveness located at (−915 to −818) and (−308 to −242) of the rPACAP promoter. Androgen receptor (AR) binds directly to DNA cis-elements in each of these regions in vitro. Site-directed mutagenesis of 3 conserved hormone response element half-sites straddling the (−308 to −242) region dramatically blunts androgen-dependent PACAP promoter activity and prevents AR binding at the mutated promoter element. Chromatin immunoprecipitation demonstrates that endogenous AR binds the homologous region on mouse chromatin in LβT2 cells in both the presence and absence of androgen. These data demonstrate that androgen stimulates PACAP gene expression in the pituitary gonadotrope via direct binding of AR to a specific cluster of evolutionarily conserved hormone response elements in the proximal rPACAP gene promoter. Thus, androgen regulation of pituitary PACAP expression may provide an additional layer of control over gonadotropin expression within the hypothalamic-pituitary-gonadal axis.

The GATA family of transcription factors are critical determinants of cell differentiation as well as regulation of adult gene expression throughout the reproductive axis. Within the anterior pituitary gland, GATA factors have been shown to increase glycoprotein α-subunit gene promoter activity; however, nothing has been known about the impact of these factors on expression of the gonadotropin β-subunits. In this study, we demonstrate expression of both GATA2 and GATA4 in primary mouse gonadotropes and the gonadotrope cell line, LβT2. Based on the transient transfection in fibroblast cells, GATA factors increase LH β-subunit gene (LHβ) promoter activity alone and in synergy with the orphan nuclear receptors steroidogenic factor-1 (SF-1) and liver receptor homologue-1 (LRH-1). The GATA response was localized to a DNA regulatory region at position −101 in the ratLHβgene promoter which overlaps with a previously described cis-element for pituitary homeobox-1 (Pitx1) and is flanked by two SF-1/LRH-1 regulatory sites. As determined by gel shift, GATA and Pitx1 can compete for binding to this element. Furthermore, mutation analysis revealed a requirement for both the GATA/Pitx1 and the SF-1/LRH-1 cis-elements in order to achieve synergy. These studies identify a novel role for GATA transcription factors in the pituitary and reveal additional molecular mechanisms by which precise modulation ofLHβgene expression can be achieved.

Recent studies have demonstrated a clear role for pituitary adenylate cyclase-activating polypeptide (PACAP) in the regulation of gonadotropin biosynthesis and secretion, both alone and in conjunction with GnRH. First defined as a hypothalamic releasing factor, PACAP subsequently has been identified in the gonadotrope subpopulation of the anterior pituitary gland, suggesting that PACAP may act as an autocrine-paracrine factor in this tissue. In initial studies, we determined that GnRH markedly stimulated endogenous PACAP mRNA levels and promoter-reporter activity in the mature gonadotrope cell line, LβT2. GnRH-stimulated rat PACAP promoter activity was blunted with deletion from position −915 to −402 and eliminated with further truncation to position −77 relative to the transcriptional start site. Site-directed mutagenesis demonstrated a functional requirement for a cAMP response element (CRE)-like site at position −205 and an activating protein-1 (AP-1)-like site at position −275, both of which bound CRE binding protein and AP-1 family members on EMSA. Treatment with pharmacological activators or inhibitors of second messenger signaling pathways implicated the protein kinase A, protein kinase C, and MAPK pathways in the GnRH response. In support of these in vitro data, we demonstrate that JunB binds to the rat PACAP gene promoter by chromatin immunoprecipitation assay and that small interfering RNA knockdown of JunB, cFos, and CRE binding protein factors blunts PACAP expression. In summary, these results further elucidate the complex functional interactions between PACAP and GnRH in the anterior pituitary. Specifically, these studies demonstrate that GnRH-stimulated PACAP gene expression is mediated via multiple signaling pathways acting on CRE/AP-1 sites in the proximal gene promoter. Because both PACAP and GnRH regulate gonadotropin biosynthesis and secretion, these results provide important insight into the critical fine tuning of gonadotrope function and, thereby, the maintenance of normal reproductive function.

Although reproductive aging in women is classically attributed to loss of ovarian follicles, recent data have suggested that the entire hypothalamic-

Luteinizing hormone (LH) plays a central role in the reproductive axis, stimulating both gonadal steroid biosynthesis and the development of mature gametes. Over the past decade, significant progress has been made in characterizing the transcription factors and associated DNA-regulatory sites which mediate expression of the LH β-subunit gene (LHβ). One of these factors, pituitary homeobox 1 (Pitx1), has been shown to stimulate LHβ gene promoter activity, both alone and in synergy with the orphan nuclear receptor, steroidogenic factor-1 (SF-1), and the early growth response gene 1 (Egr-1). Prior reports have attributed the Pitx1 response to a cis-element located at position -101 in the rat LHβ gene promoter. While investigating the role of Pitx1 in regulating rat LHβ gene expression, we observed a small, but significant, residual Pitx1 response despite mutation or deletion of this site. In the studies presented here, we identify the presence of a second functional Pitx1 region spanning positions −73 to −52 in the rat LHβ gene promoter. Based on electrophoretic mobility shift assay, Pitx1 binds to both the initially described 5′Pitx1 site as well as this putative 3′Pitx1 region. In transient transfection analysis, mutation of the LHβ-3′Pitx1 site significantly blunted Pitx1 responsiveness, with elimination of the Pitx1 response in a construct containing mutations in both Pitx1 cis-elements. We also analyzed the importance of each of these Pitx1 sites for providing functional synergy with SF-1 and with Egr-1. We observed a markedly decreased synergistic response with mutation of the 5′Pitx1 site with further loss following mutation of the 3′Pitx1 site. In contrast, functional interaction between Pitx1 and Egr-1 persisted with mutation of both Pitx1 regions. We conclude that Pitx1 stimulates the rat LHβ gene promoter via two Pitx1 DNA-regulatory regions. These results further our understanding of the molecular mechanisms that regulate expression of this critical reproductive gene promoter.