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The Journal of Clinical Endocrinology & Metabolism Vol. 83, No. 7 2545-2553
Copyright © 1998 by The Endocrine Society


Original Studies

Expression of Functional Prolactin Receptors in Nonpregnant Human Endometrium: Janus Kinase-2, Signal Transducer and Activator of Transcription-1 (STAT1), and STAT5 Proteins Are Phosphorylated after Stimulation with Prolactin

H. N. Jabbour, H. O. D. Critchley and S. C. Boddy

Medical Research Council Reproductive Biology Unit (H.N.J., S.C.B.) and the Department of Obstetrics and Gynecology (H.O.D.C.), Center for Reproductive Biology, Edinburgh, United Kingdom EH3 9EW

Address all correspondence and requests for reprints to: Dr. H. N. Jabbour, Medical Research Council Reproductive Biology Unit, Center for Reproductive Biology, 37 Chalmers Street, Edinburgh, United Kingdom EH3 9EW. E-mail: h.jabbour{at}ed-rbu.mrc.ac.uk


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
PRL is synthesized by decidualized endometrial stromal cells from the midsecretory phase in a nonconception cycle and throughout pregnancy. The exact role of PRL in the human endometrium remains to be elucidated; however, the pattern of expression supports a role for PRL during implantation and placentation. This study investigated the site and pattern of expression of PRL receptors in the nonpregnant human endometrium. In situ hybridization and immunohistochemistry localized expression of the receptor in the glandular epithelium and a subset of stromal cells of the endometrium. As judged by the intensity of staining, expression of the receptor was dramatically up-regulated during the secretory phase. Expression of the PRL receptor gene in the endometrium from the secretory phase of the menstrual cycle was confirmed by ribonuclease protection assay using 50 µg total ribonucleic acid. Phosphorylation of Janus kinase-2 (JAK2), STAT1 (signal transducer and activator of transcription-1), and STAT5 proteins in response to PRL was investigated to establish the signaling pathway of PRL in the human endometrium. Endometrial tissue was collected during the secretory phase of the menstrual cycle and incubated in the presence of 100 ng/mL human PRL for 0, 5, 10, and 20 min. JAK2 phosphorylation was induced by PRL at 5 min, whereas STAT1 and STAT5 phosphorylation was apparent 20 min after stimulation with PRL. Immunohistochemistry localized the JAK/STAT proteins in the glandular epithelial cells and a subset of stromal cells, as was observed for the PRL receptor. Secretory phase stromal and glandular cells cultured separately and in the presence or absence of 100 ng/mL PRL confirmed the PRL-induced phosphorylation of JAK2/STAT proteins, at least in the glandular compartment. These studies demonstrate an up-regulation of expression of functional PRL receptors during the secretory phase of the menstrual cycle. Further, decidual PRL through a paracrine mechanism may influence glandular epithelial function/secretions and direct gene transcription through the JAK/STAT pathway. The target genes activated by PRL in the glandular epithelium of the nonpregnant human endometrium remain to be elucidated.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
THE UTERUS is one of the first extrapituitary sites that has been described to synthesize and secrete the hormone PRL (1). In the nonpregnant uterus, PRL synthesis is detected between the midsecretory phase and menses and coincides with the first histological signs of decidualization. If pregnancy occurs, the decidual cells differentiate, and decidual PRL synthesis increases after implantation, reaches a peak at 20–25 weeks of pregnancy, and declines towards term (2, 3). Decidual PRL is indistinguishable from pituitary PRL structurally, chemically, immunologically, and biologically (4). However, decidual and pituitary PRL arise from the utilization of alternative promoter regions located approximately 6 kb apart (5). This indicates that in vivo pituitary and decidual PRL are transcribed from mutually exclusive transcriptional regulatory regions and that the molecular mechanisms regulating cell-specific expression of the human PRL gene are divergent in the pituitary and uterus (6). In the pituitary, PRL expression is regulated by a multitude of neuronal factors (dopamine, TRH, epidermal growth factor, estrogen, and calcium) via the transcription factor Pit-1 (7). However, Pit-1 is not expressed in the uterus and is not capable of inducing PRL transcription in decidual cells (6, 8). In the uterus, the process of decidualization and subsequent synthesis and secretion of PRL are controlled most effectively by progesterone (9). Evidence suggests, however, that although progesterone is important for inducing and maintaining decidualization, it does not induce PRL gene expression directly. Progesterone is suggested to influence gene expression of the decidual PRL gene through induction or posttranscriptional regulation of trans-activators (6). This can be inferred from the absence of colocalization of at least genomic progesterone receptors in PRL-secreting cells (10) and the inability of activated progesterone receptors to induce the transcription of the decidual PRL promoter (6).

The precise role and mechanism of action of PRL in pregnant and nonpregnant endometrium have not been clarified. The temporal pattern of expression of both PRL and its receptor in the human endometrium suggest a pivotal role for the hormone during pregnancy. PRL receptors have been localized to the decidua, chorionic cytotrophoblast, placental trophoblast, and amniotic epithelium (11). Moreover, PRL receptor expression has been localized by immunohistochemistry to the stromal and glandular compartments of the endometrium during the menstrual cycle (12). This observation outlined a possible role for PRL in preparation of the endometrium for implantation of the trophoblast. A critical role for PRL in the process of implantation and successful establishment of pregnancy was ascertained recently after "knockout" of the PRL receptor gene. Female mice with a homozygous null mutation of the PRL receptor are sterile, and their uteri are refractory to implantation (13). PRL may influence the process of implantation through modification of the immune environment of the endometrium at the time of implantation (14, 15) and/or by regulation of the expression of a factor(s) within the glandular secretions that may control trophoblast proliferation and/or invasion of the endometrium.

The following study was designed to investigate further the pattern of expression of the PRL receptor gene in nonpregnant human endometrium. In addition, PRL intracellular signaling in nonpregnant endometrium was assessed by investigating the phosphorylation of Janus kinase (JAK) and signal transducer and activator of transcription (STAT) proteins after short term culture of human endometrium with PRL. Tyrosine phosphorylation of the JAK/STAT proteins after interaction of PRL with its receptor is part of the signaling pathway for transcription of PRL-responsive genes that are involved in mediating both proliferative and differentiative effects of PRL (15, 16).


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Tissue collection and processing

Normal endometrial tissue was collected with a pipelle suction curette (Pipelle, Laboratoire CCD, Paris, France) from fertile women with regular menstrual cycles (25–35 days) who were undergoing minor gynecological procedures. Subjects had not been exposed to exogenous hormones for at least 6 months before inclusion in the study. Written informed consent was received before tissue collection, and ethical approval was received from the Lothian Research ethics committee. Tissue collected for in situ hybridization (proliferative phase endometrium, n = 3; secretory phase endometrium, n = 3) was snap-frozen in isopentane precooled with dry ice and subsequently stored at -70 C. Tissue collected for immunohistochemistry (proliferative phase endometrium, n = 4; secretory phase endometrium, n = 4) was fixed by immersion in 10% neutral buffered saline overnight at 4 C before routine paraffin embedding. Tissue collected for the ribonuclease (RNase) protection assay (RPA; secretory phase endometrium, n = 5) was snap-frozen in dry ice and stored at -70 C. Tissue used for in vitro culture (secretory phase endometrium, n = 7) was promptly immersed in RPMI 1640 medium (Sigma Chemical Co., Dorset, UK) containing 2 mmol/L L-glutamine, 100 U/mL penicillin, and 100 µg/mL streptomycin and transported to the culture facility.

In situ hybridization

Cryostat tissue sections (5 µm) were thaw-mounted onto 3-aminopropyltriethoxy silane (TESPA)-coated and baked slides (2% TESPA in acetone; Sigma) and fixed for 5 min in 4% (wt/vol) paraformaldehyde made up in 0.1 mol/L phosphate-buffered saline. The slides were acetylated and prehybridized for 2 h at 55 C in prehybridization buffer consisting of 50% deionized formamide, 5 x SSPE [single x SSPE contains 0.15 mol/L NaCl, 10 mmol/L NaH2PO4·H2O, and 1 mmol/L ethylenediamine tetraacetate (EDTA)], 5 x Denhardt’s solution, 200 µg/mL yeast transfer ribonucleic acid (RNA), 200 µg/mL denatured salmon sperm DNA, and 1% SDS. Hybridization was then performed overnight in hybridization buffer (prehybridization buffer plus 4% dextran sulfate and 10 mmol/L dithiothreitol) containing 1 x 106 cpm [{alpha}-33P]UTP-labeled complementary RNA (cRNA). Excess probe was removed by washing in 4 x SSC (single x SSC contains 0.15 mol/L NaCl and 15 mmol/L sodium citrate, pH 7) at room temperature before the sections were treated with RNase A (20 µg/mL). The sections were subsequently incubated with 4 x SSC, 2 x SSC, and 0.1 x SSC for 30 min each at room temperature. Tissues were dehydrated progressively in alcohol (50%, 85%, and 95%) containing 0.3 mol/L ammonium acetate and air-dried before being dipped in NTB-2 emulsion (Eastman Kodak, Cambridge, UK). After incubation in a humidified box overnight, tissues were placed in a sealed dark box at 4 C for 3 weeks, developed with D19 developer, and fixed with Unifix (Kodak) at 14 C in the dark room.

Labeled sense and antisense cRNA were synthesized by incubation of linearized template [1 µg; pGEM containing a 645 bp complementary DNA (cDNA) fragment previously generated by PCR (12) between bp 154 and 798 of the human PRL receptor sequence (16)] with 50 µCi [{alpha}-33P]UTP (2000 Ci/mmol; Amersham, Aylesbury, UK) in the presence of T7 or SP6 RNA polymerase for 30 min at 37 C according to the manufacturer’s recommendations (Promega, Southhampton, UK).

Histology and immunocytochemistry

Five-micron paraffin wax-embedded sections were cut and mounted on slides coated with 2% TESPA in acetone. Slides were then dried overnight at 50 C before dewaxing in Histoclear (National Diagnostics, Hull, UK). Tissues were rehydrated in graded ethanol and washed in water followed by TBS (0.05 mol/L Tris-HCl, pH 7.4, and 0.85% NaCl). Sections were treated with 10% hydrogen peroxide in methanol for 30 min and then blocked for 30 min with normal swine serum (NSS) diluted 1:5 in TBS and 5% BSA. The primary antibodies used were PRL receptor raised in rabbit against a peptide sequence from the extracellular domain of the rat PRL receptor (1:50 dilution; donated by Dr. P. M. Ingleton, University of Sheffield, Sheffield, UK) (17), rabbit anti-human PRL (1:100 dilution; Dako, High Wycombe, Buckinghamshire, UK), rabbit anti-mouse JAK2 (1:200 dilution; the JAK2 antibody is raised against a peptide sequence corresponding to amino acids 758–776 and is non cross-reactive with JAK1, JAK3, and Tyk2; supplied by Autogenbioclear, Wiltshire, Calne, UK), rabbit antihuman STAT1 (1:50 dilution; the STAT1 antibody is raised against a peptide sequence corresponding to amino acids 688–710 mapping within a carboxyl-terminal sequence common to STAT1ß p84 and STAT1{alpha} p91 and is noncross-reactive with STAT2 p113, STAT3, STAT4, STAT5, and STAT6; supplied by Autogenbioclear), and mouse antihuman STAT5 (1:50 dilution; the STAT5 antibody is raised against a peptide sequence corresponding to amino acids 759–775 mapping at the carboxyl-terminus of STAT5b and is noncross-reactive with STAT1ß p84, STAT2p113, STAT3, STAT4, and STAT6; supplied by Autogenbioclear). The polyclonal antibody was diluted in NSS-TBS and 5% BSA (see above) and incubated on the sections overnight at 4 C under plastic coverslips. Control sections were incubated with nonimmune rabbit or mouse serum. After removal of coverslips, sections were washed twice in TBS (5 min each), incubated for 30 min with biotinylated swine antirabbit or goat antimouse IgG (Dako) diluted 1:500 in NSS-TBS, then washed again twice in TBS (5 min each) and incubated with peroxidase-antiperoxidase conjugated to avidin-biotin complex (Dako) for 30 min at room temperature. Color reaction was developed by incubation in a mixture of 0.05% 3,3'-diaminobenzidine (Sigma) in 10 mL 0.05 mol/L Tris-HCl buffer (pH 7.4) and 0.033% hydrogen peroxide.

RNA extraction and RNase protection assay

RNA was extracted from secretory phase endometrium using Tri-Reagent as recommended by the manufacturer (Sigma). RNA yields were estimated by spectrophotometry at 260 nm. For the RPA, an antisense cRNA was prepared from SphI-linearized pGEM plasmid containing the 645-bp cDNA fragment of the human PRL receptor described above. The RPA was conducted using the Ambion RPA II kit (AMS Biotechnology Europe, Oxfordshire, UK) as instructed by the manufacturer. Briefly, the linearized plasmid was incubated with SP6 RNA polymerase for 30 min in the presence of [{alpha}-32P]UTP (800 Ci/mmol; Amersham) mixed with loading dye (95% formamide, 0.025% bromophenol blue, 0.025% xylene cyanol, 30% glycerol, 0.5 mmol/L EDTA, and 0.025% SDS), heated at 95 C for 3 min, and separated on a denaturing 5% (wt/vol) acrylamide gel for 3 h at 50 watts. The gel was then covered in Saran wrap and exposed for 2 min to XAR-5 film (Kodak) to determine the location of the full-length radiolabeled cRNA. The appropriate portion of the gel was cut out, placed in an Eppendorf tube together with 350 µL elution buffer [containing 0.5 mol/L ammonium acetate, 1 mmol/L EDTA, and 0.2% (wt/vol) SDS], and incubated overnight at 37 C. The activity of 2 x 1 µL aliquots was determined by liquid scintillation spectroscopy. The average of the samples was determined, and the volume of radiolabeled probe required to give 2 x 105 cpm was calculated. Total RNA (50 µg) from secretory phase endometrium (n = 5) and yeast (n = 2; used as reaction controls in the presence or absence of RNase digestion to establish the specificity of the hybridization reaction and the size of the unprotected RNA fragment) was mixed with the radiolabeled probe and hybridization buffer, heated to 90 C for 4 min, and incubated overnight at 45 C. The integrity of the RNA and the relative amount of total RNA in each reaction were determined by including radiolabeled cRNA prepared from an 18S ribosomal standard cDNA in each reaction. The next day, single stranded RNA were digested using 250 U/mL RNase A and 10,000 U/mL RNase T1 at 37 C for 30 min. The protected RNA was precipitated and resuspended in gel loading buffer (95% formamide, 0.025% xylene cyanol, 0.025% bromophenol blue, 0.5 mmol/L EDTA, and 0.025% SDS), heated at 95 C for 3 min, and separated on a 5% acrylamide gel under denaturing conditions. The gel was dried under vacuum and initially exposed to a phosphorescent screen (Molecular Dynamic, UK) followed by exposure to an autoradiographic film (XAR-5, Kodak, Chesham).

In vitro culture and Western blotting

The tissue was washed in PBS (prewarmed to 37 C) twice and subsequently minced thoroughly with fine scissors. Two sets of experiments were conducted. Exp A was designed to investigate the time course of phosphorylation of JAK/STAT proteins in the endometrium, and Exp B was designed to investigate the differential phosphorylation of JAK/STAT proteins in the stromal and glandular compartments of endometrium. For Exp A, each sample (n = 3 samples of secretory phase endometrium) was separated into five equal aliquots. One aliquot was snap-frozen in dry ice at the beginning of the experiment and used as a control. The other four aliquots were cultured in 2 mL RPMI 1640 medium (containing 2 mmol/L L-glutamine, 100 U/mL penicillin, and 100 µg/mL streptomycin) and 100 ng/mL human PRL (hPRL-SIAFP-B2, donated by NIDDK, NIH). The tissue was incubated in a 37 C incubator with 5% CO2 for 5, 10, 15, and 20 min after the addition of human PRL. Subsequently, the tissue was snap-frozen in dry ice and stored at -20 C.

For Exp B, each sample (n = 4 samples of secretory phase endometrium; two samples for JAK2 phosphorylation and two samples for STAT1 and STAT5 phosphorylation) was digested with 1 mg/mL collagenase and 0.1 mg/mL deoxyribonuclease for approximately 45 min at 37 C. The digested tissue was filtered through 200- and 40-µm pore size filters to separate the stromal and glandular compartments of the endometrium. Cells from the stromal and glandular compartments were divided into two aliquots and incubated in RPMI 1640 medium (containing 2 mmol/L L-glutamine, 100 U/mL penicillin, and 100 µg/mL streptomycin) with 0 or 100 ng/mL human PRL. The cells were cultured for either 5 min (for JAK2 phosphorylation) or 20 min (for STAT1 and STAT5 phosphorylation). Subsequently, the cells were snap-frozen in dry ice and stored at -20 C.

After culture, the cells were lysed in 500 µL lysis buffer [60 mmol/L Tris-HCl (pH 6.8), 1 mmol/L sodium vanadate, 10% glycerol, and 2% SDS] containing protease inhibitors (44 µg/mL aprotinin and 1 mmol/L phenylmethylsulfonylfluoride). The insoluble material was discarded by centrifugation, and 100 µg lysate protein were incubated with mouse monoclonal antiphosphotyrosine antibody (5 µg/mL; Affiniti, Exeter, UK) overnight at 4 C. The precipitated phosphorylated proteins were separated by incubation with Dynabeads M-450 rat antimouse IgG2b (Dynal, Wirral, UK). The complexes were extensively washed in PBS and boiled for 5 min in sample buffer [125 mmol/L Tris-HCl (pH 6.8), 4% SDS, 2.5% dithiothreitol, 20% glycerol, and 0.05% bromophenol blue], and the beads were separated from the precipitated proteins by a Dynal MPC magnet. The precipitated proteins were loaded on a 7.5% polyacrylamide gel, transferred to a polyvinylidene difluoride membrane (Millipore, Watford, UK), and subjected to immunoblot analysis with JAK2 (1:200), STAT1 (1:100), and STAT5 (1:500) antibodies for 2 h at room temperature. Subsequently, the membranes were incubated with an antirabbit or antimouse IgG conjugated to horseradish peroxidase (1:2000) for 1 h at room temperature before being washed three times for 15 min each time in washing buffer [50 mmol/L Tris-HCl, 150 mmol/L NaCl, and 0.05% (vol/vol) Tween-20], and labeled bands were revealed by chemiluminescence (ECL kit, Amersham). In Exp A, each blot (total of three) was probed with the JAK2, STAT1, and STAT5 antibodies. Probing of the blots with STAT6 antibody (1:500; the STAT6 antibody was raised against a peptide sequence corresponding to amino acids 828–847; supplied by Autogenbioclear) was conducted as a negative control to confirm the specificity of phosphorylation of the STAT1/STAT5 antibodies in response to stimulation with PRL.

In Exp B, each blot (total of two) prepared from tissue incubated with or without PRL for 20 min was probed with STAT1 and STAT5 antibodies (the blots incubated with or without PRL for 5 min were probed with JAK2 only). For reprobing, the blots were stripped by incubation in stripping buffer [62.5 mmol/L Tris (pH 6.8), 3% SDS, and 50 mmol/L dithiothreitol] twice at 55 C for 30 min each time followed by three washes in TBS-Tween [50 mmol/L Tris-HCl, 150 mmol/L NaCl, and 0.05% (vol/vol) Tween-20].

Successful separation of the stromal from glandular compartments in Exp B was confirmed by Western blotting of proteins extracted form the two compartments with a rabbit antihuman PRL antibody (1:75 dilution; Dako) as a stromal cell marker and a mouse monoclonal cytokeratin 18 antibody (1:100 dilution; Autogenbioclear) as a glandular epithelial cell marker. PRL immunoreactivity was evident only in the stromal compartment, whereas cytokeratin immunoreactivity was evident only in the glandular compartment (Fig. 1Go).



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Figure 1. Western blot analysis of proteins extracted form stromal and glandular cells using PRL (stromal cell marker) and cytokeratin (glandular epithelial cell marker) antibodies. The stromal and glandular compartments were separated by enzyme digestion and filtration. PRL and cytokeratin immunoreactivities were observed in the stromal and glandular compartments, respectively, demonstrating successful separation of the two compartments.

 

    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The temporal pattern and localization of the site of expression of PRL and the PRL receptor genes were identified by in situ hybridization and/or immunohistochemistry. PRL expression in the human endometrium was negligible during the proliferative phase (Fig. 2AGo) and was up-regulated during the secretory phase (Fig. 2BGo) of the menstrual cycle, and the site of expression was localized to the stromal compartment of the endometrium. In situ hybridization confirmed expression of the PRL receptor gene in the human endometrium. Negligible signal was detected in samples collected during the proliferative phase (data not shown). In secretory phase endometrium, PRL receptor expression was apparent and localized to the stromal and glandular compartments (Fig. 2Go, D and E). A similar pattern of expression was detected by immunohistochemistry using a PRL receptor antibody. Expression of the receptor was detected during the secretory phase of the menstrual cycle and was localized to endometrial glands and a subset of stromal cells (Fig. 2Go, G and H). In addition, positive staining was observed in a population of hematopoietic cells within the endometrium (inset of Fig. 2HGo). PRL receptor expression in the human endometrium during the secretory phase was confirmed further by RPA (Fig. 3Go). All five samples collected between days 19–21 of the menstrual cycle demonstrated expression of the PRL receptor.



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Figure 2. Expression of PRL and PRL receptor in the human endometrium. Minimal PRL immunoreactivity is observed in the proliferative phase (A), whereas intense PRL staining is apparent in the stromal compartment during the midsecretory phase (B). C is a secretory endometrium incubated with nonimmune sera (negative control). PRL receptor expression was investigated by in situ hybridization (D–F) and immunocytochemistry (G–I). D and E demonstrate expression of the PRL receptor gene by in situ hybridization during the secretory phase (dark- and lightfields, respectively). F is a section from a secretory endometrium incubated with the sense probe (negative control). Minimal PRL receptor immunoreactivity was observed in the proliferative phase (G), whereas intense staining was apparent in the stromal and glandular compartments during the midsecretory phase (H). Within the stromal compartment, PRL receptor immunoreactivity was also apparent in cells of hematopoietic origin (inset of H). I is a section incubated with nonimmune sera (negative control). Scale bar = 100 µm.

 


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Figure 3. RPA conducted using 50 µg total RNA isolated from endometrium collected during the secretory phase (between days 19 and 21) and a 645-bp homologous PRL receptor cRNA probe generated from the extracellular domain. Reaction controls were yeast RNA with (Y+) and without (Y-) RNase digestion. The integrity of RNA and the relative amount of total RNA in each reaction were determined using a ribosomal 18S cRNA probe.

 
The intracellular signaling pathway was examined by investigating JAK/STAT protein phosphorylation after stimulation of endometrial samples with human PRL (Exp A). Rapid phosphorylation of JAK2 was detected after stimulation of secretory endometrium with PRL. Reactive bands of approximately 125 kDa corresponding to JAK2 were detected in samples after stimulation with PRL for 5 min (Fig. 4AGo). Similarly, STAT1 and STAT5, but not STAT6 (data not shown), phosphorylation was observed after stimulation with PRL. Reactive bands of approximately 84/91 kDa corresponding to the two variant forms of STAT1 (Fig. 4BGo) and of 94 kDa corresponding to STAT5 (Fig. 4CGo) were detected within 20 min after stimulation with PRL.



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Figure 4. Tyrosine phosphorylation of JAK2, STAT1, and STAT5 by PRL in human endometrium tissue collected during the secretory phase of the menstrual cycle. Human endometrium was incubated with 100 ng/mL human PRL for 0, 5, 10, 15, or 20 min. Proteins were immunoprecipitated with antiphosphotyrosine antibody and subsequently immunoblotted with JAK2, STAT1, and STAT5 antibodies. Phosphorylation of JAK2 was apparent within 5 min after stimulation with PRL (A), whereas STAT1 (B) and STAT5 (C) proteins were phosphorylated by 20 min after stimulation with PRL.

 
Immunocytochemistry localized the site of expression of the JAK2, STAT1, and STAT5 proteins to the endometrial glands and a subset of stromal cells. JAK2 (Fig. 5Go, A and B) and STAT5 (Fig. 5Go, G and H) displayed a similar temporal pattern of expression. JAK2 and STAT5 immunoreactivity was observed during the secretory phase and was negligible during the proliferative phase of the menstrual cycle. However, immunoreactivity with the STAT1 antibody was detected during both the proliferative and secretory phases of the menstrual cycle (Fig. 5Go, D and E).



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Figure 5. Immunolocalization of JAK2, STAT1, and STAT5 proteins in human endometrium collected during the proliferative and secretory phases of the menstrual cycle. Minimal immunoreactivity is observed for JAK2 (A) and STAT5 (G) during the proliferative phase. During the secretory phase, both JAK2 (B) and STAT5 (H) immunoreactivities were observed predominantly in the glandular compartment. STAT1 immunoreactivity was detected during the proliferative (D) and secretory (E) phases of the menstrual cycle. C, F, and I are sections incubated with nonimmune sera for JAK2, STAT1, and STAT5, respectively (negative controls). Scale bar = 100 µm.

 
To determine whether PRL receptors expressed in the different compartments of the endometrium are functioning through the JAK/STAT pathway, endometrial glands were separated from the stromal tissue, and the differential phosphorylation of JAK/STAT proteins was assessed after stimulation with PRL (Exp B). In Exp B, the periods of stimulation with PRL were 5 and 20 min for JAK2 and STAT1/STAT5 phosphorylation studies, respectively; these were the optimal times established for the phosphorylation of JAK2 and STAT1/STAT5 proteins after stimulation with PRL in Exp A. Phosphorylation of JAK2 in response to PRL was detected in the stroma and endometrial glands (Fig. 6AGo). By contrast, phosphorylation of the STAT1 and STAT5 proteins was more apparent in the endometrial glands than in the stromal tissue after stimulation with PRL (Fig. 6Go, B and C).



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Figure 6. Tyrosine phosphorylation of JAK2, STAT1, and STAT5 proteins by PRL in the glandular (G) and stromal (S) compartments of human endometrium tissue collected during the secretory phase of the menstrual cycle. Human endometrium was separated into the glandular and stromal compartments and incubated with (+P) or without (-P) 100 ng/mL human PRL for 5 (JAK2 phosphorylation) or 20 (STAT1 and STAT5 phosphorylation) min. Proteins were immunoprecipitated with antiphosphotyrosine antibody and subsequently immunoblotted with JAK2, STAT1, and STAT5 antibodies. Phosphorylation of JAK2 was induced in glandular and stromal compartments after stimulation with PRL (A), whereas PRL-induced phosphorylation of STAT1 (B) and STAT5 (C) was more apparent in the glandular compartment.

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
This study confirms the expression of the PRL receptor gene in nonpregnant human endometrium, as demonstrated by in situ hybridization, immunohistochemisty, and RPA. The data demonstrate that PRL receptor expression is negligible during the proliferative phase and is up-regulated in the secretory phase. Expression of the receptor was localized to cells within the stromal and glandular compartments of the endometrium. Using the same tissue biopsies, PRL expression was detected in the stromal compartment of the endometrium during the midsecretory phase. Negligible level of PRL expression was detected during the proliferative or early secretory phases of the menstrual cycle. Taken together, these data suggest a similar temporal pattern of expression of the PRL and PRL receptor genes. It is well established that the expression of PRL in the endometrium is promoted by progesterone and is closely linked to the process of decidualization (18, 19, 20). However, little is known about the regulation of PRL receptor expression in the endometrium. The temporal pattern of expression of the PRL receptor gene suggests that it may also be under regulation by progesterone. However, this is unlikely to be a direct modulatory effect, at least in the glandular compartment of the endometrium, because progesterone receptors do not localize to the glandular epithelial cells, which in the current study have been shown to be a major site of expression of the PRL receptor (21). Whether progesterone has a direct influence on PRL receptor expression in the stromal compartment of the endometrium remains to be established. A positive effect of progesterone on PRL receptor expression has been demonstrated both in human breast cancer cell lines (22) and in primary cultures of mammary epithelial cells (23). It is also likely that PRL receptor expression in the endometrium is regulated by PRL; this idea is supported by the close temporal pattern of expression of the two genes observed by immunohistochemistry. PRL has been reported previously to influence the expression of its own receptor. In the mammary gland, PRL receptor expression increases during lactation (24, 25) and has been demonstrated to be up-regulated by PRL (26, 27).

PRL, through its membrane-bound receptor, is known to regulate the growth and differentiation of a wide variety of cells. The pattern of expression of the PRL receptor gene in glandular epithelial cells suggests that endometrial PRL is mediating a differentiative, rather than a mitogenic, effect in the glandular compartment. This is inferred from the absence of expression of the receptor during the proliferative phase of the menstrual cycle, the period during which the glandular epithelial cells are undergoing rapid mitogenesis. The abundant expression of the receptor during the secretory phase implies that PRL may be regulating the secretory function of the endometrial glands. Whether PRL-modulated factors within the endometrial glands influence trophoblast function and subsequent establishment of pregnancy remains to be established. Expression of the PRL receptor has also been demonstrated in cells of hematopoietic origin within the stromal compartment of the endometrium. In recent years, a role for PRL in immunomodulation has been emphasized (28, 29). Hypophysectomy or bromocriptine treatment suppresses the growth of the thymus (30), T cell proliferation (31), and production of cytokines (32). In addition, PRL and PRL receptor expression have been described in a number of hematopoietic cells in the peripheral circulation, such as lymphocytes and mononuclear cells (14, 33), suggesting that PRL acts in an autocrine/paracrine fashion to influence immune cell proliferation and function. The demonstration of expression of PRL receptors in hematopoietic cells within the endometrium is not surprising because these cells are likely to be recruited from the peripheral circulation (34). However, these cells may undergo in situ proliferation and differentiation. Endometrial hematopoietic cells increase in number during the secretory phase of the menstrual cycle (35) and have different phenotypic characteristics from those of hematopoietic cells within the peripheral circulation (36, 37). Hematopoietic cell proliferation and differentiation during the secretory phase of the menstrual cycle and early pregnancy may play a vital role in the control of implantation and establishment of vasculature and blood supply at the fetoplacental unit (15). The local signal(s) responsible for these uterine immune cell modifications may be orchestrated by PRL, which is expressed at increasing concentrations by the pseudodecidualized and decidualized endometrium during the secretory phase of the menstrual cycle and early pregnancy.

PRL has been demonstrated previously to activate JAK2 and different STAT proteins, including STAT1 and STAT5 (reviewed in Ref. 38). The activated STAT proteins are translocated into the nucleus and subsequently up-regulate the transcription of target promoters, such as ß-casein (39), ß-lactoglobulin (40), whey acidic protein (41), {alpha}2-macroglobulin (42), and Interferon Regulatory Factor-1 (43), which mediate differentiative or mitogenic effects of PRL. The data presented in this study provide evidence that the PRL receptors expressed in the human endometrium may use the JAK/STAT signaling pathway in vivo. In vivo phosphorylation of JAK/STAT proteins after stimulation with PRL has been reported recently in the rat ovaries (44) and mammary gland (45). The immunohistochemistry and Western blotting data have demonstrated the colocalization of the PRL receptors with the JAK and STAT proteins and the activation/phosphorylation of JAK/STAT proteins in response to stimulation with PRL at least in the glandular compartment. The temporal pattern of phosphorylation of the JAK/STAT proteins confirms that upon ligand binding, JAK2 is rapidly phosphorylated and is followed by phosphorylation of the different STAT proteins. A similar pathway may be operating in stromal cells, although this was not as apparent in the present study. This may be a reflection of the lower level of expression of PRL receptors in the stromal tissue compared with the endometrial glands. The target genes for PRL function in the endometrial glands remain to be established, although it is evident that the putative PRL-inducible genes are regulated in part by the STAT1 and STAT5 proteins. The function of the different STAT proteins in signal transduction may vary with the different target genes investigated. Comparative studies have demonstrated that STAT1 and STAT5 proteins can act as both positive and negative regulators of gene transcription (46). For example, STAT5 is a positive regulator of PRL-induced transcription of the ß-casein promoter (46) and the {alpha}2-macroglobulin promoters (47), whereas PRL-induced transcription of the Interferon Regulatory Factor-1 promoter is inhibited by STAT5 and promoted by STAT1 (46).

In conclusion, this study has demonstrated expression of functional PRL receptors in the nonpregnant human endometrium during the secretory phase of the menstrual cycle. PRL induces gene transcription in the endometrium through the JAK/STAT pathway. The temporal pattern and site of expression of the receptor in the endometrium suggest that PRL may mediate an array of functions that are linked to the described mitogenic and differentiative effects of the hormone on target tissue.


    Acknowledgments
 
The authors acknowledge Prof. A. S. McNeilly, Dr. L.-y. Yu-Lee, Dr. R. W. Kelly, and Ms. T. A. Drudy for advice and assistance.

Received November 6, 1997.

Revised March 16, 1998.

Accepted April 16, 1998.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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