Abstract
Estrogen acts to prime the pituitary prior to the GnRH-induced LH surge by undiscovered mechanisms. This study aimed to identify the key components that mediate estrogen action in priming the pituitary. RNA extracted from the pituitaries of metestrous (low estrogen) and proestrus (high estrogen) stage mice, as well as from ovariectomized wild-type and estrogen receptor α (ERα) knockout mice treated with 17β-estradiol (E2) or vehicle, was used for gene expression microarray. Microarray data were then aggregated, built into a functional electronic database, and used for further characterization of E2/ERα-regulated genes. These data were used to compile a list of genes representing diverse biological pathways that are regulated by E2 via an ERα-mediated pathway in the pituitary. This approach substantiates ERα regulation of membrane potential regulators and intracellular vesicle transporters, among others, but not the basic components of secretory machinery. Subsequent characterization of six selected genes (Cacna1a, Cacna1g, Cited1, Abep1, Opn3, and Kcne2) confirmed not only ERα dependency for their pituitary expression but also the significance of their expression in regulating GnRH-induced LH secretion. In conclusion, findings from this study suggest that estrogen primes the pituitary via ERα by equipping pituitary cells with critical cellular components that potentiate LH release on subsequent GnRH stimulations.
Introduction
The ovarian steroid estradiol (E2) plays a critical physiological role in inducing the LH surge by acting on both the hypothalamus and the pituitary (Clarke 2002, Christian et al. 2005). While much focus has been placed on the role of estrogen in the hypothalamic GnRH surge, less work has been done concerning the actions of estrogen on the pituitary. Estrogen has been shown to be involved in priming or sensitizing the pituitary gonadotroph to GnRH stimulus (Clarke & Cummins 1984, Clarke 1995a) by increasing expression of GnRH receptor (GnRH-R) in gonadotrophs (Liu & Yen 1983, Leung & Peng 1996, Strauss & Barbieri 2009), mobilizing secretory granules to the periphery of the cell (Thomas & Clarke 1997, Thomas et al. 1998), and recruiting the number of gonadotrophs to the pool of those that are capable of responding to GnRH stimulation (Smith et al. 1984). It has also been shown that estrogen downstream pathways include cytoskeleton rearrangement (Powers 1986, Sapino et al. 1986, DePasquale 1999), regulation of ion channels (Clarke 2002), and energy metabolism (Simpson et al. 2005, Jones et al. 2006). The molecular mechanisms by which pituitary priming occurs remain largely unknown, but these functions may play a part. Both estrogen receptor (ER) subtypes ERα and ERβ are expressed throughout the pituitary (Mitchner et al. 1998). However, diverse lines of evidence indicate that ERα is the predominant mediator of estrogen action in the pituitary. Agonists for ERα, but not ERβ, were capable of inducing increased LH secretion in estrogen-primed GnRH-stimulated rat pituitaries in vitro (Sanchez-Criado et al. 2004, 2005). ERα activation was shown to be primarily responsible for the reorganization of the disrupted organelle morphology seen in the gonadotroph after ovariectomy (Sanchez-Criado et al. 2006). In addition to ERα knockout (ERαKO) female mice exhibiting complete infertility and lack of ovulation (Dupont et al. 2000, Hewitt & Korach 2003), we recently reported that targeted deletion of ERα in the gonadotroph caused infertility in female mice (Gieske et al. 2008).
In the gonadotroph, binding of GnRH to GnRH-R, a G-protein-coupled receptor, activates intracellular signaling pathways causing membrane depolarization and a rapid change in intracellular Ca2+ concentration, which subsequently elicits multiple intracellular events that facilitate LH secretion (Ghosh et al. 1996, Shacham et al. 2001). The well-timed and rapid nature of LH secretion on GnRH stimulation at the time of surge suggests the need for synchronization of the cellular secretory machinery. Considering the evidence that estrogen priming of the pituitary is required for the induction of the LH surge (Liu & Yen 1983, Strauss & Barbieri 2009), we hypothesize that estrogen via ERα primes the pituitary by equipping gonadotrophs and other pituitary cells with key regulators of the LH secretion machinery.
This study aimed to identify components that play critical roles in estrogen priming of the pituitary. For this purpose, genes that are differentially regulated in the pituitary under various estrogen and ERα backgrounds were identified, and the expression patterns and functional roles of six selected genes were characterized. Future study on the functional roles of the identified genes will begin to reveal the molecular mechanism of pituitary estrogen priming for the GnRH-induced LH surge.
Materials and Methods
Animals and treatments
Animal handling procedures were carried out in accordance with the University of Kentucky Animal Care and Use Committee. Mice were maintained with food and water made available ad libitum in a 14 h light:10 h darkness cycle at 24 °C. All mice used in this study were of C57BL/6 genetic background. ERαKO mice were produced as described previously (Gieske et al. 2008). For microarray analysis, wild-type (WT) mice were divided into two subgroups at the age of 7 weeks after birth. The first group of WT mice was ovariectomized (OVX), kept for 3 weeks, and injected (s.c.) with 10 μg E2 per mouse or 100 μl sesame oil (vehicle (veh)) at 0900 h for two consecutive days. On the second day, the mice were killed at 1500 h by carbon dioxide inhalation, and the pituitaries were harvested and frozen on dry ice. The second group of WT mice was kept for a week, and their estrous cycling patterns were determined by daily vaginal smear for next 2 weeks using a standard procedure (Becker et al. 2005). During the second week of vaginal smear, mice were killed on proestrus or metestrus at 1500 h and pituitaries were collected. The pituitaries of ERαKO mice were collected at 1500 h at the age of 10 weeks. ERαKO mice did not cycle but displayed a consistent pattern of diestrus. For the histological analyses, cardiac perfusion was performed on 10-week-old mice using 4% neutralized buffered paraformaldehyde. After postfixation with the same fixative, tissues were stored in 20% sucrose and then frozen in OCT compound (Tissue-Tek, Sakura Finetek, Torrance, CA, USA). For primary pituitary culture, 10-week-old cycling WT female mice were used.
Reagents
Antibodies raised in rabbit for adipocyte enhancer binding protein 1 (AEBP1; ARP31592_P050, AVIVA Systems Biology, San Diego, CA, USA), Cav2.1 (encoded by Cacna1a; ACC-001, Alomone Labs, Jerusalem, Israel), Cav3.1 (encoded by Cacna1g; ACC-021, Alomone Labs), potassium voltage-gated channel, Isk-related subfamily, gene 2 (KCNE2; APC-054, Alomone Labs), Cbp/p300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 1 (Cited1; XAV-8490, ProSci, Inc., CA, USA), and encephalopsin, opsin 3 (Opn3; NLS2134, Novus Biologicals, Inc., CO, USA) were purchased from indicated company. Polyclonal antiserum for mouse pituitary LH was purchased from the National Hormone and Pituitary Program (Harbor–UCLA Medical Center, Torrance, CA, USA). GnRH and E2 were purchased from Sigma. Molecular reagents were purchased from Invitrogen. Cell culture reagents including DMEM, gentamicin, BSA, HEPES, trypsin, trypsin inhibitor, and DNase I were purchased from Sigma. Other reagents including ITS, fungizone, and fetal bovine serum were purchased from Gibco-BRL. ω-Agatoxin TK (selective blocker of Cav2.1 channel), r-Kurtoxin (selective blocker of Cav3.1), and E-4031 (selective blocker of HERG K+ channel) were purchased from Alomone Labs.
Gene expression microarray
Gene expression microarray was performed with total RNA (5 μg/group) at the University of Kentucky DNA Microarray Core Facility using the Affymetrix Mouse 430 2.0 oligonucleotide array set (Affymetrix, Santa Clara, CA, USA). Briefly, the total RNA was extracted from the pituitaries of mice in six groups: naturally cycling WT mice in either metestrus (group 1) or proestrus (group 2), OVX WT mice treated with vehicle (group 3) or E2 (group 4), and OVX ERαKO mice treated with vehicle (group 5) or E2 (group 6). Total RNA was extracted using Trizol reagent (Invitrogen Life Technologies, Inc.) and purified using an RNeasy Kit (Qiagen, Inc.). The integrity of RNA was checked by visualizing 28S and 18S rRNA bands on a 1.5% agarose gel. For each group, total RNA extracted from at least three different mice were pooled together for microarray. Completely different sets of mice were used for generating triplicate samples. Subsequently, microarray was performed for n=2 samples. RNA was labeled and hybridized according to the standard Affymetrix procedures. Data were prestatistically filtered as reported previously (Kadish et al. 2009). Briefly, the MAS5 algorithm was used to generate quality control metrics, produce presence/absence calls, and calculate signal intensity values. Results were transferred to Excel spreadsheets, filtered to remove genes rated absent (>2 presence calls across the study), and statistically analyzed (see section below) using the Multi-Experiment Viewer (Saeed et al. 2003).
RT-PCR
The gene expression patterns of selected genes were confirmed by real-time RT-PCR analysis using the total RNA (1 μg/group) used for microarray. The primers used were as follows: AEBP1, forward (5′-AGA CAC ACC CTT CCC AAA TG-3′) and reverse (5′-GTG GGC ATC TCA GTC TCC TC-3′); Cav2.1, forward (5′-AGG CAC CCT TTT GAT GGA G-3′) and reverse (5′-GCG GAT GTA GAA ACG CAT TC-3′; Xu et al. 2007); Cav3.1, forward (5′-TGC TGT GGA AAT GGT GGT GA-3′) and reverse (5′-AGC ATC CCA GCA ATG ACG AT-3′; Nordskog et al. 2006); KCNE2, forward (5′-GCA TGT TCT CGT TCA TCG TG-3′) and reverse (5′-CCT TGG AGT CTT CCA GAT GC-3′); Cited1, forward (5′-CAT CCT TCA ACC TGC ATC CT-3′) and reverse (5′-ACC AGC AGG AGG AGA GAC AG-3′; Howlin et al. 2006); Opn3, forward (5′-TCT TCA TGA ACA GAA AGT TTC G-3′) and reverse (5′-CCT GTC CCC ATC TTT CTG TGA C-3′; Henkel et al. 2006); and L7 ribosomal protein, forward (5′-TCA ATG GAG TAA GCC CAA AG-3′) and reverse (5′-CAA GAG ACC GAG CAA TCA AG-3′; Jeong et al. 2005). The L7 ribosomal protein gene was used as internal control. Real-time RT-PCR was performed using SyberGreen Master Mix (Ambion, Austin, TX, USA), and all of the triplicate samples that were originally prepared were used for each gene. Ct values used were each automatically generated by PCR machine software (Bio-Rad iQ5, version 2.0). The relative mRNA amount (RMA) was calculated by the following equation: 
Immunohistological analysis
For all immunohistological analyses, tissues were fixed and processed as described previously (Kim et al. 2005). Metestrus WT, proestrus WT, and diestrus ERαKO pituitary sections were mounted on the same slide for procedural control purposes. Tissue sections were incubated with 5% normal serum for 30 min at room temperature followed by incubation with specific antibody overnight at 4 °C. Immunopositive signals were then visualized by ABC method (Vector, Burlingame, CA, USA) and hematoxylin counterstaining was used on all slides except that for Cited1 (as this is a nuclear protein). Densitometric analysis was performed using the analySIS TS, OLYMPUS Soft Imaging Solutions Software (Münster, Germany). Relative signal intensities were calculated and proestrus and ERαKO sections were standardized to metestrus sections, which were given a relative intensity of 1 (n=4 for each gene). For double-immunofluorescent detection, sections were blocked by 10% normal serum (5% normal goat+5% normal horse serum) and then anti-LHβ (1:500) and specific antibody against each of the selected gene products (1:50 for Cav2.1, Cav3.1, Kcne2, and AEBP1; 1:100 for Opn3; and 1:300 for Cited1) were co-treated overnight at 4 °C. The Alexa Fluor 488-conjugated anti-rabbit IgG and the Alexa Fluor 594-conjugated anti-guinea pig IgG (both at 1:500) were incubated to detect each selected gene and LH signal respectively. After washing with distilled water, sections were mounted with ProLong Gold antifade reagent with DAPI (Molecular Probes, Eugene, Oregon, USA). Photographs were taken using a fluorescent microscope (Olympus) and a digital camera (DP70, Olympus). The proportions of double-positive signals were calculated as described previously (Kim et al. 2007).
Cell cultures and treatment for LH assay
Anterior pituitary lobes were dissected from whole pituitaries of 10-week-old female C57BL/6 mice after carbon dioxide inhalation. Pituitaries were pooled, cells were isolated, and then maintained as described previously (Kim et al. 2007). For assessment of the effect of specific channel blockers on LH secretion, primary pituitary cells were counted and plated (1×105 cells/well) in 96-well plates coated with poly-l-lysine. After 2 days of culture, incubation media were exchanged for medium supplemented with 10% charcoal-treated fetal bovine serum and cultured for an additional 2 days. The cells were then treated with either 0.00001% ethanol or 1 nM E2 in 0.00001% ethanol for 48 h. Cells were then treated with ω-Agatoxin TK (50 and 200 nM), r-Kurtoxin (50 and 200 nM), and E-4031 (100 and 1000 nM) for 30 min followed by GnRH (10 nM) challenge in the presence of the blockers for an additional 2 h. Media were snap-frozen and stored at −80 °C until assay. RIA of LH concentration was performed using a mouse LH sandwich assay by the University of Virginia Center for Research in Reproduction Ligand Assay and Analysis Core (NICHD (SCCPRR) Grant U54-HD28934, University of Virginia, VA, USA). Channel blocker concentrations were chosen based on the Alomone company assay for E-4031, and cited literature for ω-Agatoxin (Barral et al. 2001) and r-Kurtoxin (Chuang et al. 1998), and slight variations on these concentrations.
Statistical analysis
For densitometry and RIA data analyses, data were analyzed using one-way ANOVA and the Student–Newman–Keuls method or t-test. Microarray data were analyzed using one-way ANOVA to test each comparison group individually (metestrus versus proestrus, WT OVX veh versus E2, and ERαKO OVX veh versus E2). The probe sets were then standardized to maintain variability shifted about zero. Those probe sets that were found significant with ANOVA (P<0.05) were compared in a post hoc all-pairwise strategy with Fisher's protected least significant difference (pLSD). Stringency in the pLSD analysis was tuned until 90% of genes found significant by ANOVA yielded at least one significant contrast between treatments. Real-time PCR data were compared using t-test. In consideration of small sample size (n=2) for each microarray group, the presented s.e.m. should be given less weight than others of samples sizes larger than n=3.
Results
Identification of estrogen/ERα-regulated genes in the pituitary
To identify the genes that are under the regulation of estrogen via ERα, microarray analyses were performed using pituitaries collected from six different experimental groups. Pituitaries were collected from naturally cycling WT mice on the afternoons (1500 h) of metestrus (low circulating estrogen levels; group 1) and proestrus (high circulating estrogen levels; group 2). Pituitaries were also collected from OVX WT mice following treatment with vehicle (group 3) or E2 (group 4), as well as OVX ERαKO mice following treatment with vehicle (group 5) or E2 (group 6). A gene was determined to be E2/ERα regulated if its pattern of expression met all of the following criteria: 1) significantly higher or lower expression on proestrus compared with metestrus, 2) significantly higher or lower expression in E2-treated group compared with veh-treated group in the OVX WT mice, and 3) no significant difference in expression between E2-treated and veh-treated OVX ERαKO mice. Statistical analysis of the raw microarray data, as described above, revealed 64 E2-inducible/ERα-dependent genes (Table 1) and 17 E2-repressible/ERα-dependent genes (Table 2).
Estradiol (E2)/estrogen receptor α-regulated genes (upregulation)
| Gene | Gene title | Pro/Met | WT E2/veh | KO E2/veh |
|---|---|---|---|---|
| Abcb6 | ATP-binding cassette, sub-family B (MDR/TAP), member 6 | 1.61 | 2.30 | 0.92 |
| Accn1 | Amiloride-sensitive cation channel 1, neuronal (degenerin) | 2.14 | 5.73 | 0.85 |
| Adam12 | A disintegrin and metallopeptidase domain 12 (meltrin alpha) | 3.62 | 13.81 | 1.56 |
| Aebp1* | AE binding protein 1 | 3.60 | 9.41 | 1.21 |
| Ammecr1 | Alport syndrome, mental retardation, midface hypoplasia and elliptocytosis chromosomal region gene 1 homolog (human) | 1.83 | 2.89 | 1.48 |
| Amn | Amnionless | 30.38 | 25.42 | 0.60 |
| Ank1 | Ankyrin 1, erythroid | 1.64 | 7.14 | 0.84 |
| Arhgap24 | Rho GTPase activating protein 24 | 1.66 | 2.56 | 0.96 |
| Asahl | N-acylsphingosine amidohydrolase (acid ceramidase)-like | 1.47 | 1.61 | 0.96 |
| Asns | Asparagine synthetase | 1.19 | 1.50 | 1.00 |
| Bcan | Brevican | 1.46 | 1.72 | 1.03 |
| Cacna1a** | Calcium channel, voltage-dependent, P/Q type, alpha 1A subunit | 2.01 | 3.60 | 0.95 |
| Cacna1g* | Calcium channel, voltage-dependent, T type, alpha 1G subunit | 1.89 | 7.74 | 0.66 |
| Ccdc123 | Coiled-coil domain containing 123 | 1.70 | 2.52 | 1.26 |
| Cckar | Cholecystokinin A receptor | 5.82 | 126.33 | 0.75 |
| Cgref1 | Cell growth regulator with EF hand domain 1 | 2.09 | 1.49 | 0.85 |
| Cited1** | Cbp/p300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 1 | 3.13 | 15.94 | 1.40 |
| Dbh | Dopamine beta hydroxylase | 1.62 | 3.03 | 1.22 |
| Fbxo31 | F-box protein 31 | 1.57 | 2.14 | 0.95 |
| Fbxw7 | F-box and WD-40 domain protein 7, archipelago homolog (Drosophila) | 1.50 | 1.66 | 0.78 |
| G6pd2 | Glucose-6-phosphate dehydrogenase 2 | 1.55 | 3.87 | 1.19 |
| G6pdx | Glucose-6-phosphate dehydrogenase X-linked | 1.55 | 2.93 | 0.95 |
| Gadd45g | Growth arrest and DNA-damage-inducible 45 gamma | 2.56 | 4.52 | 0.95 |
| Gcs1 | Glucosidase 1 | 1.41 | 2.27 | 1.10 |
| Ggtla1 | Gamma-glutamyltransferase-like activity 1 | 3.81 | 3.42 | 1.13 |
| Hif1a | Hypoxia inducible factor 1, alpha subunit | 1.23 | 1.74 | 1.02 |
| Hnt | Neurotrimin | 1.34 | 2.40 | 0.98 |
| Hspb8 | Heat-shock protein 8 | 2.14 | 2.05 | 0.96 |
| Immt | Inner membrane protein, mitochondrial | 1.20 | 1.18 | 1.09 |
| Impdh1 | Inosine 5′-phosphate dehydrogenase 1 | 1.65 | 1.93 | 0.83 |
| Impg1 | Interphotoreceptor matrix proteoglycan 1 | 2.13 | 25.50 | 1.71 |
| Isg20l1 | Interferon stimulated exonuclease gene 20-like 1 | 1.26 | 1.28 | 0.99 |
| Jtv1 | JTV1 gene | 1.35 | 1.56 | 0.94 |
| Kcne2** | Potassium voltage-gated channel, Isk-related subfamily, gene 2 | 3.97 | 44.10 | 0.56 |
| Lamb3 | Laminin, beta 3 | 2.34 | 5.35 | 1.15 |
| Lancl3 | LanC lantibiotic synthetase component C-like 3 (bacterial) | 1.80 | 2.08 | 0.80 |
| Mars | Methionine-tRNA synthetase | 1.37 | 1.67 | 1.07 |
| Mcm2 | Minichromosome maintenance deficient 2 mitotin (Saccharomyces cerevisiae) | 1.26 | 2.15 | 0.96 |
| Mesdc2 | Mesoderm development candidate 2 | 1.28 | 1.46 | 1.01 |
| Mfge8 | Milk fat globule-EGF factor 8 protein | 1.36 | 1.81 | 1.04 |
| Myc | Myelocytomatosis oncogene | 2.60 | 6.75 | 1.11 |
| Nhlh2 | Nescient helix loop helix 2 | 3.10 | 3.92 | 1.06 |
| Nol5a | Nucleolar protein 5A | 1.87 | 2.60 | 1.12 |
| Nomo1 | Nodal modulator 1 | 1.31 | 1.41 | 1.03 |
| Nudt19 | Nudix (nucleoside diphosphate linked moiety X)-type motif 19 | 1.81 | 2.84 | 0.88 |
| Opn3* | Opsin (encephalopsin) | 2.01 | 29.49 | 0.89 |
| Oxtr | Oxytocin receptor | 3.03 | 7.17 | 1.13 |
| Pcsk6 | Proprotein convertase subtilisin/kexin type 6 | 1.81 | 3.49 | 0.99 |
| Peo1 | Progressive external ophthalmoplegia 1 (human) | 1.52 | 1.57 | 1.04 |
| Perp | PERP, TP53 apoptosis effector | 1.96 | 2.88 | 0.91 |
| Pgm2 | Phosphoglucomutase 2 | 1.38 | 1.56 | 1.05 |
| Phyhipl | Phytanoyl-CoA hydroxylase interacting protein-like | 1.24 | 1.82 | 1.18 |
| Plod1 | Procollagen–lysine, 2-oxoglutarate 5-dioxygenase 1 | 1.54 | 2.33 | 1.10 |
| Prmt3 | Protein arginine N-methyltransferase 3 | 1.41 | 1.43 | 0.98 |
| Ptpn5 | Protein tyrosine phosphatase, non-receptor type 5 | 4.04 | 25.08 | 0.30 |
| Rnd2 | Rho family GTPase 2 | 1.45 | 2.19 | 0.94 |
| Rybp | RING1 and YY1 binding protein | 1.30 | 1.90 | 1.10 |
| Scnn1g | Sodium channel, nonvoltage-gated 1 gamma | 2.75 | 3.16 | 0.55 |
| Slc10a3 | Solute carrier family 10 (sodium/bile acid cotransporter family), member 3 | 1.26 | 1.55 | 1.18 |
| Srebf1 | Sterol regulatory element binding factor 1 | 1.32 | 1.54 | 0.98 |
| Stat5a | Signal transducer and activator of transcription 5A | 2.47 | 5.19 | 1.10 |
| Tbl3 | Transducin (beta)-like 3 | 1.50 | 1.62 | 1.24 |
| Tcam1 | Testicular cell adhesion molecule 1 | 2.72 | 4.09 | 0.17 |
| Tmem86a | Transmembrane protein 86A | 1.81 | 3.01 | 0.88 |
| Zfp804a | Zinc finger protein 804A | 3.07 | 6.92 | 1.20 |
Asterisks (*,**) indicate genes chosen for further study (see text).
Estradiol (E2)/estrogen receptor α-regulated genes (downregulation)
| Gene | Gene title | Pro/Met | WT E2/veh | KO E2/veh |
|---|---|---|---|---|
| Caps2 | Calcyphosphine 2 | 0.54 | 0.31 | 0.99 |
| Chrna6 | Cholinergic receptor, nicotinic, alpha polypeptide 6 | 0.36 | 0.18 | 1.00 |
| Ctsf | Cathepsin F | 0.78 | 0.80 | 1.10 |
| Cyp39a1 | Cytochrome P450, family 39, subfamily a, polypeptide 1 | 0.67 | 0.74 | 0.91 |
| Fbp2 | Fructose bisphosphatase 2 | 0.50 | 0.46 | 0.97 |
| Glra2 | Glycine receptor, alpha 2 subunit | 0.63 | 0.27 | 0.92 |
| Gpm6a | Glycoprotein m6a | 0.71 | 0.58 | 0.90 |
| Hmgcll1 | 3-Hydroxymethyl-3-methylglutaryl-coenzyme A lyase-like 1 | 0.77 | 0.72 | 0.97 |
| Oasl2 | 2′,5′-Oligoadenylate synthetase-like 2 | 0.65 | 0.62 | 1.17 |
| Ociad2 | OCIA domain containing 2 | 0.76 | 0.66 | 0.92 |
| Rod1 | ROD1 regulator of differentiation 1 (Schizosaccharomyces pombe) | 0.82 | 0.72 | 1.03 |
| St8sia2 | ST8 alpha-N-acetyl-neuraminide alpha-2,8-sialyltransferase 2 | 0.60 | 0.56 | 1.04 |
| Tacstd1 | Tumor-associated calcium signal transducer 1 | 0.82 | 0.74 | 1.00 |
| Tmem51 | Transmembrane protein 51 | 0.63 | 0.43 | 0.88 |
| Tmhs | Tetraspan transmembrane protein, hair cell stereocilia | 0.75 | 0.60 | 0.97 |
| Trdmt1 | TRNA aspartic acid methyltransferase 1 | 0.85 | 0.84 | 1.01 |
| Vangl1 | Vang-like 1 (van gogh, Drosophila) | 0.70 | 0.66 | 0.95 |
Further characterization of microarray-identified ERα-inducible genes
The validity of the microarray and statistical analyses was evaluated using six genes. From the final list, we chose three genes (Table 1): Cacna1g (encodes protein Cav3.1; voltage-dependent calcium channel, T type, alpha 1G subunit), Aebp1, and Opn3. These three genes (marked with asterisk in Table 1) were selected on the basis of increased gene expression with high-estrogen environments found significant by the previously stated statistical analyses. In addition, selection of these genes was supported by the known function of their encoded proteins in other tissues. Cacna1g was chosen on the basis of its well-established regulation of ion transport, which has been thought to be an important regulatory mechanism in release of hormones and neurotransmitters (Douglas & Rubin 1961, Stojilkovic et al. 2005, Qiu et al. 2006, Stojilkovic 2006), and has previously been reported to be under estrogenic regulation in the pituitary (Bosch et al. 2009). Aebp1 has been shown to be estrogen/ERα-regulated in other tissues (Zhang et al. 2005). The Aebp1-encoded protein is also an augmenter of MAPK function, which is significant considering that MAPK family downstream regulators are known to be activated by GnRH (Kim et al. 2001, Navratil et al. 2010). Opn3 was chosen because of its known role in exocytosis in other cell types (Henkel et al. 2006).
Interestingly, we found a large number of genes that, while significantly different within treatment groups by ANOVA (metestrus versus proestrus, veh versus E2), were not found to be significant by our post hoc analysis among the groups. We reasoned that the basal level of estrogen in metestrus in the cycling mice would be enough to induce or maintain the expression of some estrogen-inducible genes at relatively higher levels, which might reduce the gene expression differential between metestrus and proestrus. Therefore, the fold changes shown in the naturally cycling pituitaries, while significant themselves by ANOVA between metestrus and proestrus, were relatively low compared with the higher fold changes seen in the OVX group, thus eliminating them from the list of genes that were found significant with post hoc analysis. Among the list of the genes in this category, we selected three (marked with double asterisk in Table 1) for further characterization: Cited1, Cacna1a (encodes protein Cav2.1; voltage-dependent calcium channel, P/Q type, alpha 1A subunit), and Kcne2. Cited1 was chosen because of its well-established role as a mediator of ER action (Yahata et al. 2001). Cacna1a and Kcne2, as with Cacna1g, were chosen because of their known roles as ion transporters.
The expression levels of the six genes from the microarray analysis are shown in Fig. 1a. Real-time RT-PCR assay using RNA extracted from metestrus, proestrus, OVX veh, and OVX E2 pituitaries (Fig. 1b) shows gene expression patterns closely resembling the microarray data for all six genes. For the scope of this study, only E2/ERα-inducible genes were chosen for further evaluation. Additional characterization of the significant E2/ERα-repressible genes (Table 2) will be intriguing topics for future investigation.
mRNA expression profiles of estrogen/ERα-regulated genes. Six of the E2-upregulated/ERα-dependent genes were chosen for further confirmation and study. Gene expression profiles of the microarray data are shown for each gene (a). Gene expression profiles were compared by ANOVA within treatment groups for WT metestrus (met) and proestrus (pro) mice, as well as OVX WT and OVX ERαKO, each treated with vehicle or E2. Data presented as mean±s.e.m. (n=2; *P≤0.05). RNA expression was confirmed by real-time RT-PCR analysis for six selected genes (b). The y-axis represents the range of signal intensity detected by DNA microarray. Total RNA were extracted from WT Pro and Met, WT OVX with E2- or veh-treated pituitaries. Data presented were mean±s.e.m. (n=3; *P<0.05; **P<0.01; ***P<0.001).
Citation: Journal of Endocrinology 210, 3; 10.1530/JOE-11-0098
Localization of ERα-regulated genes in the gonadotroph
In order to examine whether protein expression of these genes correlated with their mRNA expression pattern, immunohistochemistry was performed on pituitary sections from metestrus and proestrus females, and ERαKO females constitutively displaying diestrus vaginal cytology (Fig. 2). All six proteins showed a significant increase in positive staining in proestrus sections compared with their respective metestrus and ERαKO sections, as determined by densitometric analysis (Fig. 2, right panels). All proteins are located to cytoplasmic compartments, with the exception of Cited1, which is mostly nuclear. To investigate whether these proteins are localized in gonadotrophs, further analysis was performed using double immunofluorescence on proestrus pituitaries with LHβ antibody and antibody against each specific protein (Fig. 3). Each of the six proteins was found to be localized in the LHβ containing cells. Interestingly, the proteins were not homogenously expressed in all gonadotrophs, but rather in a subset of LHβ-positive cells. Likewise, protein expression of the six genes was detected in cell types other than gonadotroph. Quantification of the percent co-localization indicated that 85% of LHβ-positive gonadotrophs also show Cited1 expression, approximately half stain positive for Cav2.1, Kcne2, AEBP1, and Opn3, and about 34% show positive signals for Cav3.1 (Fig. 3).
Immunohistochemical analysis of six selected genes using pituitaries from wild-type metestrus, proestrus, and ERαKO mice. Tissue sections from WT metestrus, WT proestrus, and ERαKO (diestrus) pituitaries were sectioned and incubated with antibodies against each specified protein and immunopositive signals were detected by ABC method. Scale bar is 25 μm. The right panel shows relative density of each protein on metestrus (Met), proestrus (Pro), and ERαKO pituitaries, which was calculated by dividing mean density of proestrus and ERαKO by mean value of metestrus. Data presented were mean±s.e.m. (n=4). All the relative density values from proestrus of each protein are statistically significant compared with those from metestrus and ERαKO (*P<0.05).
Citation: Journal of Endocrinology 210, 3; 10.1530/JOE-11-0098
Localization of estrogen/ERα-regulated genes in the gonadotroph. Detection of the six selected genes in gonadotrophs in proestrus pituitary was achieved using immunofluorescence with antibodies against each gene and LHβ. Green color represents each selected gene as detected by each specific antibody and red color represents LHβ immunopositive signals. Yellow color, further indicated by white arrows, represents double-positive signaling. Numbers in parentheses indicate the proportion of gonadotrophs that stain double positive for the indicated gene (mean %±s.e.m., n=4). Scale bar=200 μm.
Citation: Journal of Endocrinology 210, 3; 10.1530/JOE-11-0098
Functional validation of ERα-regulated genes in the primary pituitary cell culture
Validity of LH secretion being influenced by the expression of these ERα-regulated genes in pituitary cells was further tested using the three channel proteins (Cav2.1, Cav3.1, and Kcne2) as subjects of functional confirmation. Cells dissected from anterior pituitary were pre-treated with E2 for 48 h, and the efficacy of GnRH-induced LH release was measured in the presence or absence of specific blockers of the chosen channel proteins. ω-Agatoxin TK was used to selectively block Cav2.1 (P/Q type) channels (Teramoto et al. 1993), r-Kurtoxin to block Cav3.1 (T-type) channels (Chuang et al. 1998), and E-4031 to selectively block HERG K+ channels, with which KCNE2 is associated (Spector et al. 1996). Treatment with E2 alone or with the channel blockers, without GnRH challenge, did not yield a change in basal secretion of LH (non-GnRH-induced secretion; Fig. 4). On GnRH stimulation, E2-treated cells produced significantly larger amount of LH (18%) compared with the control group (Fig. 5). Channel blockers at concentrations of 200 nM ω-Agatoxin, 50 nm r-Kurtoxin, and 1000 nm E-4031 showed complete negation of the priming effect of E2 on GnRH-stimulated LH secretion, whereas other concentrations of the blockers showed no significant difference from E2+GnRH-treated cells.
The effect of three ion channel blockers on LH secretion without GnRH challenge. Primary pituitary cells were incubated with estrogen (1 nM) or vehicle and in the presence of ion channel blockers ω-Agatoxin TK (50 or 200 nM), r-Kurtoxin (50 or 200 nM), and E-4031 (100 or 1000 nM) to determine whether channel blockers had an effect on basal LH secretion (non-GnRH-induced secretion) into the media. Data were represented by mean±s.e.m. (n=4).
Citation: Journal of Endocrinology 210, 3; 10.1530/JOE-11-0098
The effect of three ion channel blockers on LH secretion with GnRH challenge. Primary pituitary cells were incubated with or without estrogen (1 nM) and in the presence of ion channel blockers ω-Agatoxin TK (50 or 200 nM), r-Kurtoxin (50 or 200 nM), and E-4031 (100 or 100 nM) to determine whether channel blockers had an effect on LH secretion (GnRH-induced secretion) into the media. Data were represented by mean±s.e.m. (n=4). Statistics were calculated for treatments with the three channel blockers compared with E2+GnRH by ANOVA and Student–Newman–Keuls method (*P<0.05; **P<0.01).
Citation: Journal of Endocrinology 210, 3; 10.1530/JOE-11-0098
Discussion
The aim of this study was to examine genome-wide pituitary gene expression profiles in order to decipher the molecular networks involved in the process of estrogen priming in the pituitary prior to the LH surge. Preceding the surge period, it is well known that the gonadotrophs exhibit increased sensitivity to GnRH and maintain capacity to release a comparable amount of LH on each GnRH stimulus for an extended period (Gallo 1981, van Dieten & de Koning 1995, Hoeger et al. 1999). There is evidence that estrogen plays a role in the increased gonadotroph responsiveness to GnRH (Tilbrook et al. 1995, Clarke 2002). In regard to the mechanism, E2-induced increase in GnRH-R expression has been postulated as a key event (Liu & Yen 1983, Leung & Peng 1996, Strauss & Barbieri 2009). Estrogen facilitates redistribution of secretory granules, positioning them to be readily secreted on GnRH stimulation (Thomas & Clarke 1997, Thomas et al. 1998). ERα has been implicated as the major mediator of this E2 action in the pituitary. The ERα agonist PPT elicits increased LH secretion from rat pituitaries in response to consecutive GnRH challenges in vitro, comparable to that induced by E2 (Sanchez-Criado et al. 2004). Others and ourselves recently generated mouse models that lack functional ERα in the gonadotrophs (ERαflox/flox αGSUCre mouse; Gieske et al. 2008, Singh et al. 2009). Based on the infertility and irregular estrous cycles observed in these mice, it was hypothesized that gonadotroph ERα is necessary for the positive feedback action of estrogen. Interestingly, the absence of gonadotroph ERα did not affect the mRNA expression of LHβ, αGSU, or FSHβ (Gieske et al. 2008). Therefore, estrogen/ERα may rather regulate other cellular processes enhancing the responsiveness of proestrous gonadotrophs to GnRH, presumably by regulating a cohort of ERα downstream genes (Hoeger et al. 1999, Turgeon & Waring 2001).
In this study, we identified estrogen-responsive/ERα-dependent genes in the pituitary (Tables 1 and 2) and suggest that those genes may play a role in the sensitization of the pituitary to GnRH stimuli during the surge period. The localization of these proteins in the proestrus gonadotrophs (Fig. 3), in combination with their increased expression at proestrus compared with metestrus (Fig. 2), implicates putative functional roles of those proteins in gonadotroph function and LH secretion. However, our data also show expression of the selected genes in pituitary cell types other than gonadotroph, suggesting that E2/ERα regulation of these genes in other cell types may play indirect roles on gonadotroph function and LH secretion, or that their expression may influence other pituitary functions. The anterior pituitary is a complex organ with a great degree of heterogeneity in the physiology of the cell types that reside there. The intricacy of paracrine interactions between these cell types is only beginning to be teased apart. A growing volume of evidence indicates that pituitary cells form extensive networks for long-distance communication and coordination (Fauquier et al. 2002, Bonnefont et al. 2005). Estrogen facilitates this organization and interaction between cells prior to and during the LH surge, even after GnRH levels have fallen (Lyles et al. 2010). An apparent connectivity exists between gonadotrophs and lactotroph cells, exhibited by the presence of adherent and gap junctions (Horvath et al. 1977, Morand et al. 1996). Prolactin (PRL), the hormone product of lactotroph cells, acts on gonadotrophs cells to modulate the secretion of LH (Cheung 1983, Hodson et al. 2010a,b). Lactotrophs are also estrogen sensitive, and gonadotrophs express PRL receptors (Henderson et al. 2008). This evidence further signifies that, in addition to their function in the gonadotroph cells, the genes presented in this study may further modify the LH surge through their actions in lactotroph physiology.
While diverse molecular events would be necessary to increase the sensitivity to GnRH and the secretion of LH from gonadotrophs, the regulation of membrane potential via ion conductivity has been expected to be a key regulatory mechanism (Stojilkovic et al. 2005, Qiu et al. 2006, Stojilkovic 2006). In addition, the regulation of ion transport through plasma membrane or intracellular organelles is critical for exocytosis. The GnRH-R activates the PKA pathway resulting in release of calcium from intracellular stores (Hamid et al. 2008). Mobilization of Ca2+ is mediated through PKC pathways, including Gq/G11 and phospholipase Cβ (PLCβ) activation. Activation of Gq/G11 and PLCβ lead to the production of inositol 1,4,5-trisphosphate and diacylglycerol second messengers, which results in calcium ion (Ca2+) mobilization and gonadotropin release (Naor 1990, Clarke 1995b, Shacham et al. 2001). The biphasic LH secretion is initially dependent on intracellular calcium, while the subsequent plateau phase relies on extracellular calcium influx (Ortmann et al. 1994, 1995). These events may also be pertinent to the secretion of PRL from neighboring lactotrophs and thus the regulation of PRL on LH secretion. In this study, we identified three channel components as ERα-dependent estrogen-inducible genes in the proestrus pituitary. Cacna1a encodes a subunit for Cav2.1, a P/Q type Ca2+ channel, which is known to mediate neurotransmitter release via Ca2+-dependent excitation secretion coupling at many central synapses and at the peripheral neuromuscular junction (Uchitel et al. 1992). In the pituitary, GnRH triggers action potentials. In this process, the induction of Cav2.1 via estrogen may contribute to subsequent Ca2+-mediated LH secretion. In order to maintain the pulsatility of LH secretion during the surge, a quick recovery of membrane potential through repolarization is needed, which can be manifested by increasing K+ currents or by increasing other types of Ca2+ channels such as transient (T)-type calcium channel (Costantin & Charles 2001). Thus, the identification of Cacna1g, a component for the T-type Ca2+ channel Cav3.1, and Kcne2, a component of K+ current, as ERα-dependent estrogen-inducible genes in the pituitary is quite relevant. In particular, the recent finding of Ca2+-activated K+ channels as potential mediators of estrogen action in priming pituitary gonadotroph in preparation for the LH surge (Waring & Turgeon 2009) substantiates the identification of these channel proteins as estrogen/ERα-inducible genes in the pituitary. Recent studies have also alluded to the role of estrogen in the regulation of T-type calcium channel subunits in the pituitary via an ERα predominant pathway (Bosch et al. 2009). To see whether these three ion channels could increase the responsiveness of estrogen-primed gonadotrophs to GnRH, individual channel function was blocked after estrogen priming and GnRH stimulus in vitro. All three channel blockers significantly impaired E2 enhancement of GnRH-stimulated LH release (Fig. 5). Interestingly, in the case of r-Kurtoxin, treatment with the lower concentration of the Cav3.1-associated channel blocker provided the reduction in the priming effect, while the higher concentration did not show this effect. However, it is often seen in studies that with increasing concentrations of the administered molecule, the observed effects plateau off and sometimes reverse at higher concentrations as the binding sites or receptors become saturated and oversaturated (Trotta et al. 1980, Hyvelin et al. 2000). Although more detailed experiments will be required to verify their supposed electro-physiological roles in regulation of LH secretion by GnRH, these initial findings lay the groundwork for the involvement of all three channels in regulating LH secretion.
In addition, the known functions of AEBP1, Opn3, and Cited1 could be directly and/or indirectly related to the role of estrogen in priming the pituitary for GnRH stimulus. AEBP1, verified here as an estrogen/ERα-dependent gene in the proestrus pituitary (Fig. 2), and as previously shown in the white adipose tissue (Zhang et al. 2005), maintains the activation of MAPK by protecting it from the effects of a MAPK-specific phosphatase (Kim et al. 2001, Navratil et al. 2010). The signal pathways activated by GnRH-R include major members of the MAPK family (Yang et al. 2005, Dobkin-Bekman et al. 2006). Opn3 has been shown to be involved in light sensation and circadian rhythms, with its implicated function being light reception for light-induced exocytosis in the PC12 cells and primary embryonic telencephalon cells (Henkel et al. 2006). It is well established that the LH surge is closely related to photoperiod in experimental animals (Legan & Karsch 1975, Legan et al. 1975), suggesting a possibly similar role for this gene in the regulation of LH secretion. Cited1 has been shown to function as a transcriptional coregulator of estrogen in a manner dependent on ER ligand binding and its interaction with CBP/p300 transcriptional coactivator (Shioda et al. 1996, 1998, Yahata et al. 2000, 2001, Nair et al. 2001). While its role in the pituitary has yet to be studied in depth, the evidence presented in this study suggests that Cited1 may be a coregulator of ERα in this tissue as well. Based on the known functions of these three verified estrogen-inducible/ERα-dependent genes, at least two molecular events would be suggested as mechanisms of estrogen priming in the proestrus pituitary: the reinforcement of the signal transduction pathways from GnRH-R and the regulation of circadian homeostasis related to light–dark cycle. These events might be eventually responsible for increasing sensitivity, capacity, and synchronicity of the proestrous pituitary for the surge level of LH secretion. To this end, estrogen may regulate various genes, but the regulation is expected to have a temporal and spatial specificity (Coser et al. 2003), which could be manifested by a pituitary-specific ERα coregulator such as Cited1.
While we show here for the first time genome-wide information consisting of the genes regulated by E2/ERα in the pituitary and suggest a few molecular mechanisms of estrogen priming of the pituitary for the LH surge, there is much work to be done to elucidate the estrogen priming mechanism. It must be taken into account that estrogen has been shown to cause cellular responses through both rapid, non-genomic action (involving the activation of growth factor receptors and G-protein-coupled receptors, initiating multiple downstream pathways) and the ‘classical’ genomic responses (involving ER acting as a ligand-activated transcription factor). Most hormones, including estrogen, are capable of simultaneously activating both of these mechanisms (Prossnitz & Maggiolini 2009). Therefore, further studies are necessary to elucidate the precise mechanisms of the regulation of these genes by estrogen. In addition, as demonstrated by cholecystokinin-type A receptor, recently reported as a mediator of estrogen priming (Kim et al. 2007), the identified genes in this study have various functions. To further verify these mechanisms, it will be necessary to use various approaches including pituitary-specific gene knockout animal models for each candidate.
While not elaborated in this report, our microarray data showed that the expression of most of the genes that constitute the basic components of secretory machinery (e.g. microtubules, t-SNARE complex) are not influenced by E2/ERα (data not shown). Whereas, we found that some intracellular transport molecules, such as ankyrin, are under E2/ERα regulation in the mouse pituitary (Table 1). Interestingly, the microarray analysis found that mRNA expression of GnRH-R, a well-established E2-regulated gene in the pituitary (Leung & Peng 1996), was significantly higher in the OVX E2 pituitaries compared with the OVX veh. However, no such significant increase was shown between metestrous and proestrous mouse pituitaries. This finding indicates that while some genes are under E2/ERα regulation, their expressions are also subject to regulation by other factors, which may render tighter temporal and spatial regulations of the genes under changing physiological conditions.
In conclusion, findings from this study suggest that preovulatory estrogen priming of pituitary is achieved at least in part by regulating the expression of critical components that potentiate gonadotrophs, in addition to other pituitary cells, to be fully responsive to GnRH stimulation for LH surge stimulation.
Declaration of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding
This work was supported by National Institutes of Health grants: 1IR01HD052694 (C K) and P20 RR15592 (C K) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0006200).
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(H J Kim, M C Gieske, and K L Trudgen contributed equally to this work)
