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Regulation of Human Glycoprotein Hormone -Subunit Gene Transcription in LßT2 Gonadotropes by Protein Kinase C and Extracellular Signal-Regulated Kinase 1/2¹

^a Department of Endocrinology, St. Bartholomew's and the Royal London School of Medicine and Dentistry, West Smithfield, London EC1A 7BE, United Kingdom

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Transcriptional activation of the human glycoprotein hormone -subunit (GSU) promoter in response to GnRH and phorbol-12-myristate-13-acetate (PMA) has been well characterized in T3-1 gonadotropes but not investigated in the more differentiated LßT2 clonal gonadotrope. We have evaluated GSU transcription in the more mature LßT2 cell line, using deletion and heterologous constructs of the GSU promoter linked to a luciferase reporter gene. Basal GSU-promoter activity was significantly less in LßT2 cells than in T3-1 cells, but stimulation of transfected cells with GnRH and PMA resulted in similar increases in GSU-promoter activity. Deletional analysis of the human GSU promoter in LßT2 cells indicated that sequences between -398 and -244 and between -244 and -195 base pairs (bp) were involved in regulating basal GSU-promoter transcription, whereas the region between -244 and -195 bp regulated PMA-stimulated promoter activity. Deletion of this promoter region containing a steroidogenic factor-1 (SF-1) binding site abolished basal and PMA-stimulated transcription. Site-directed mutagenesis of the SF-1 binding site resulted in a significant attenuation of basal and PMA-stimulated GSU transcription. Pretreatment of LßT2 cells with a mitogen-activated protein kinase kinase-specific inhibitor, U0126, abolished the PMA-stimulated increase in MAPK activity and significantly reduced basal and PMA-stimulated promoter activity. Electrophoretic mobility shift assays for SF-1 and GATA revealed that PMA failed to affect SF-1 binding but enhanced GATA binding to a consensus GATA oligonucleotide, an effect that was blocked with U0126 pretreatment, suggesting that GATA may mediate ERK activation of GSU transcription. Our data suggests that, in the mature LßT2 gonadotrope cell line, two regions of the human GSU promoter regulate basal transcription and that SF-1 is involved in mediating basal and PMA-stimulated promoter activity. Furthermore, PKC-stimulated transcription partially relies on ERK acting on elements downstream of -244 bp of the human GSU promoter.

gene regulation, neuroendocrinology, pituitary, pituitary hormones, signal transduction

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The glycoprotein hormone -subunit (GSU) gene is expressed in distinct populations of cells during early development of the pituitary gland from Rathke's pouch [1]. In the mature gland, GSU expression is confined to gonadotropes and thyrotropes, where it heterodimerizes with the ß-subunits of LH, FSH, and thyroid-stimulating hormone (TSH) [2]. Investigation of regulated GSU expression has been enhanced by the availability of the mouse gonadotrope-derived T3-1 cell line [3], an early gonadotrope precursor cell representative of Embryonic Day e13.5. These cells express many gonadotrope-specific markers (GSU, GnRH receptor, steroidogenic factor-1 [SF-1]) but do not express intact gonadotropins (i.e., no LH or FSH ß-subunits). More recently, a gonadotrope cell line that expresses the ß-subunit of LH [4] and, under certain conditions, FSH in addition to GnRH-R, SF-1, and GSU has been developed. The LßT2 cell line is more representative of gonadotropes at Day e17.5 [5], and as such may act as a valuable tool when investigating gene expression in mature gonadotropes. The mechanisms involved in regulation of the human GSU promoter have not been explored in LßT2 cells.

In rat pituitary cells and T3-1 cells, GSU expression has been shown to be regulated by GnRH, acting via its specific G-protein coupled receptor. GnRH activates several signal transduction pathways in gonadotropes, including four of the mitogen-activated protein kinase (MAPK) cascades [6, 7], in particular, the extracellular signal-regulated kinase (ERK1/2). However, its effects on GSU transcription appear to be predominantly mediated by its ability to increase extracellular calcium (Ca²⁺) influx and activate the protein kinase C (PKC) pathway [8], although the ERK1/2 pathway has also been implicated in the regulation of basal and GnRH-stimulated GSU transcription in T3-1 cells. However, discrepancies between species have been noticed in that the GnRH effect on the rodent GSU promoter appears to be ERK mediated [9] whereas the role for ERK in mediating the GnRH effect on the human promoter is much less pronounced [10].

The human GSU promoter has several regions that are involved in basal and regulated expression in the pituitary; the pituitary glycoprotein hormone basal element (PGBE), which includes the -basal elements (BE1 and 2), the gonadotrope-specific element (GSE); a consensus GATA site, and tandem cAMP response elements (CREs) [11–13]. The PGBE has been identified as a major regulatory region of basal GSU transcription in T3-1 cells, and it is unclear which transcription factors elicit these effects [11], although LIM homeodomain proteins are thought to bind to sites within this region [9]. Consensus Ets sites, which reside between -411 and -384 in the rat GSU, bind Ets proteins and are ERK sensitive, forming a putative GnRH-response element (RE) [14]. However, the corresponding region in the human promoter is not completely conserved, and no definitive GnRH-RE has been identified within the human promoter [15]. It is more likely that the GnRH effect relies on several interacting elements within the context of the human GSU. Deletion studies have shown that regions upstream of -195 base pairs (bp) are required to mediate GnRH responsiveness in gonadotropes [16, 17], suggesting that the CRE, which is downstream of -195 bp, is not directly involved in mediating these effects. In contrast, the CREs are the major regulatory regions involved in regulating GSU transcription in placental tissues [18].

An SF-1 binding site, the GSE, is found between -218 and -200 bp of the promoter, and both the GSE and SF-1 have been implicated in basal GSU transcription [12]. It has been suggested that SF-1 may be involved in mediating the transcriptional response to cAMP [17], although this is not the case in extrapituitary tissues that also express the GSU. There is also evidence to suggest that SF-1 is phosphorylated by activators of cAMP/protein kinase A (PKA) or ERK signaling pathways in other tissues [19–21], potentially allowing SF-1 to be regulated by stimulators of a number of different signaling pathways in gonadotropes. Therefore, the human GSU promoter appears to be regulated by several distinct response elements in a tissue-specific manner.

In this study, we have compared basal GSU-promoter activity in LßT2 and T3-1 cells and their response to activators of PKC/ERK (GnRH and phorbol ester) and investigated the intracellular signaling pathways important for stimulating GSU transcription in LßT2 cells. Furthermore, we have identified regions of the GSU promoter that are responsible for the regulation of basal and stimulated GSU transcription in the more mature, differentiated LßT2 gonadotrope.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Materials

All chemicals were purchased from Sigma (Poole, UK) or BDH-Merck (Poole, UK) unless otherwise stated. The long-acting GnRH synthetic analogue (des-Gly¹⁰, [D-Ala⁶] GnRH ethylamide) was obtained from Sigma. Phorbol-12-myristate-13-acetate (PMA) (CN Biosciences, Nottingham, UK) was obtained in lyophilized form and stored at -20°C as stock solutions of 1 mM in sterile water. All stimulants were diluted directly into culture medium (see below) before each experiment. The specific mitogen-activated protein kinase kinase (MEK) inhibitor U0126 was obtained from Promega (Southampton, UK) and used at 1 µM, a concentration previously reported to produce half-maximal ERK activity [22]. All the above components were prepared as stock solutions in sterile water and applied to the cells in culture media. Nifedipine and GF109203X were purchased from CN Biosciences, prepared as stock solutions in Me₂SO, and applied to the cells in culture media. Cells were exposed to less than 0.01% Me₂SO, and these concentrations of vehicle had no effect on responses of T3-1 or LßT2 cells. All concentrations of inhibitors were previously optimized [23].

Cell Culture

T3-1 and LßT2 cells were grown in monolayer culture in Dulbecco modified Eagle medium (DMEM) supplemented with high glucose (4500 mg/L) containing 10% (v/v) fetal calf serum, penicillin (100 IU/ml), streptomycin (100 mg/L), and fungizone (125 mg/L) (Life Technologies, Paisley, UK) (hereafter referred to as culture medium). Cells were passaged twice weekly and incubated at 37°C in a humidified 5% (v/v) CO₂/95% (v/v) air incubator. Cells were plated in six-well plates at a density of 1 x 10⁶ cells/well for transient transfection experiments, 2 x 10⁶ cells/well for ERK activity assays, and 5 x 10⁶ cells/T25 for Western blotting.

Plasmids and Transient Transfection Studies

The reporter construct -846LUC contains 846 bp of the 5' flanking sequence and 44 bp of exon 1 of the human GSU gene, linked to the luciferase (LUC) reporter gene in the plasmid pA3LUC [24]. Deletions of the 846-bp 5' flanking sequences linked to a LUC reporter gene and termed -517, -244, and -195LUC have been previously characterized and described [8]. The internal control plasmid BosßGal contains the promoter of the human elongation factor 1 gene-driving expression of ß-galactosidase [25]. The control plasmid TKLUC contains the promoter of the herpes simplex virus thymidine kinase (TK) gene linked to the LUC reporter gene in the plasmid pA3LUC. For analysis of specific GSU-promoter elements, constructs containing only upstream GSU-promoter sequences from -517 to -195 bp, fused to the heterologous TK promoter and termed -517, -398, -298, and -195TKLUC, were used. The promoterless LUC expression vector pA3LUC was used as a control plasmid for basal luciferase expression, and the expression vector BosßGal was used as an internal control to normalize transfection efficiencies. The plasmids containing the Elk-1 activation domain fused to the Gal-4 DNA-binding domain (Gal-4-Elk-1) and the Gal-4 promoter luciferase (Gal-4-LUC) were obtained from Professor A.F. Russo (University of Iowa, Iowa City, IA) and have been described previously [26]. The SF-1 wild-type expression vector in the expression vector pCIneo has been described previously [27]. All constructs were verified for orientation and correct sequence by restriction endonuclease digests and the dideoxy-DNA sequencing method. Large-scale preparation and purification of plasmids were performed by alkaline lysis and resin purification (Qiagen Ltd., Dorking, Surrey, UK); 1 x 10⁶ cells/well were transfected by the calcium phosphate method as described previously [28] without glycerol shock. Cells were transfected for 4 h with 10 µg of -846LUC, -517LUC, -244LUC, -195LUC, pA3LUC, -517TKLUC, -398TKLUC, -298TKLUC, -195TKLUC, TKLUC, Gal4-Elk-1, or Gal4-LUC and cotransfected with 5 µg of BosßGal. The cells were stimulated for 8 h in culture medium without (control) or with 100 nM GnRH or 100 nM PMA. The cells were harvested and cellular extracts were assayed for luciferase and ß-galactosidase activity as described previously [17]. Luciferase data from separate experiments were pooled by normalizing the data to the level of ß-galactosidase activity. Each treatment group contained triplicate cultures and experiments were repeated at least twice.

Site-Directed Mutagenesis

The -517LUC construct was subcloned in to the HindIII site in pBluescript, and site-directed mutagenesis using the QuikChange kit (Stratagene, Cambridge, UK) was conducted to introduce a 2-bp mutation within the GSE region. Mutagenesis was performed according to manufacturer's instructions and using the following primers: forward, 5'-CTCTCTTTT-CATGGGCTGATTGTCGTCACCATCACCTG-3'; reverse, 5'-CAGG-TGATGGTGACGACAATCAGCCCATGAAAAGAGAG-3', with the mutation underlined. Following sequence analysis, the mutated -517LUC (named -517MUT) was cloned back into the HindIII site of pA3LUC.

Nuclear Protein Extraction and Western Blotting

Nuclear protein extracts were prepared from LßT2 and T3-1 cells using a modification of a method described previously [29]. Briefly, 5 x 10⁶ cells were cultured in T25 flasks overnight in serum-free DMEM before replacement of media with 0 or 100 nM PMA for 30 min. The media was removed and the cells washed with PBS and scraped into 2 ml ice-cold phosphate-buffered saline (PBS). Following centrifugation (5 min, 1500 x g, 4°C), the cells were resuspended in 400 µl of ice-cold buffer A (10 mM HEPES, pH 7.9, 10 mM KCl, 0.1 mM EDTA, 0.1 mM EGTA, 1 mM DTT, and 0.5 mM PMSF) and transferred to a cold microfuge tube. Having left the cells to swell on ice for 15 min, 25 µl of a 10% solution of Nonidet P-40 (made in buffer A) was added to each sample, followed by vortexing for 10 sec. Following microcentrifugation (10 000 x g, 30 sec), the pellets were resuspended in 300 µl of buffer B (20 mM HEPES, pH 7.9, 0.4 M NaCl, 1 mM EDTA, 1 mM EGTA, 1 mM DTT, 1 mM PMSF) before vigorous rocking on a shaking platform at 4°C for 15 min. The nuclear extract was centrifuged (10 000 x g, 5 min) and stored at -70°C prior to protein determination (by Bradford assay). Normalized concentrations of nuclear extract (typically between 10 and 15 µg/well) were loaded onto a 10% SDS-polyacrylamide (SDS-PAGE) stacking gel and electrophoresed at 200 V. Briefly, samples were boiled in an equal volume of 2x sample buffer to cell lysate, electrophoresed, and transferred to Hybond-ECL membranes (Amersham-Pharmacia, Bucks, UK). The membrane was blocked with 5% nonfat milk and incubated overnight at 4°C with agitation, using 1 µg/ml of rabbit anti-SF-1 (Upstate Biotechnology, Lake Placid, NY) or rabbit anti-DAX-1 antisera (Santa Cruz, CA). The membrane was then washed three times with PBS containing 0.05% Tween and subsequently incubated with goat anti-rabbit IgG coupled to horseradish peroxidase (DAKO, Glostrup, Denmark) for 2 h at room temperature, again with agitation. The membrane was washed as before and bound antibody detected using enhanced chemiluminescence (ECL; Amersham-Pharmacia, Amersham, UK).

Electrophoretic Mobility Shift Assay

Electrophoretic mobility shift assay (EMSA) was performed using 1–3 µg nuclear extract per reaction. Probes were created by filling in the 5' AGCT overhangs of the annealed GSE or GATA oligonucleotides with Klenow polymerase using a mixture of dATP, dGTP, dTTP, and ³²P-dCTP (ICN, Hampshire, UK) (GSE: forward, 5'-AGCTGCTGACCTTGTCGTCAC-3'; reverse, 5'-AGCTGTGACGACAAGGTCAGC-3'; GSE-MUT: forward, 5'-AGCTGCTGATTTTGTCGTCAC-3'; reverse, 5'-AGCTGTGACGACGGGGTCAGC-3'; GATA: forward, 5'-AGCTAGATAAGAT-3'; reverse, 5'-AGCTATCTTATCT-3'). Three microliters of nuclear extracts were incubated at room temperature for 5 min in a 20-µl volume of 20 mM Tris (pH 8.0), 60 mM KCl, 2 mM MgCl₂, 1.2 mM DTT, 12% glycerol, 2.5 µg [poly(dI.dC)·poly(dI.dC)] (Amersham-Pharmacia). In some instances, excess cold GSE or GATA oligo (x50) or 1 µl of anti-SF-1 was added to the GSE reaction to complete specific binding, and these samples were incubated on ice for 60 min. The reactions were then incubated for 15 min at 30°C in the presence of 1 ng of probe. Complexes were electrophoresed on a 5% native acrylamide gel, dried, and visualized by autoradiography.

Measurement of ERK Activity

The p44/42 MAPK assay kit (New England Biolabs, Hitchin, UK) was used to detect ERK activity. After stimulation for the indicated time, the cells were washed briefly with ice-cold PBS before treatment with lysis buffer (20 mM Tris, pH 7.5, 150 mM NaCl, 1 mM EDTA, 1 mM EGTA, 1% [v/v] Triton-X-100, 2.5 mM sodium pyrophosphate, 1 mM ß-glycerolphosphate, 1 mM Na₃VO₄, 1 µg/ml [w/v] leupeptin, and 1 mM PMSF, added directly before use) and were incubated on ice for 5 min. The cells were scraped, transferred to microcentrifuge tubes, and immunoprecipitated with an immobilized phospho-p44/42 MAP kinase (Thr202/Tyr204) monoclonal antibody. The immunoprecipitated protein was used in a kinase assay with 200 µM ATP and 2 µg Elk-1 fusion protein before treatment with SDS sample buffer. The samples were subjected to SDS-PAGE separation, protein transfer to nitrocellulose membrane, and Western blotting (see above) using a phospho-Elk-1 antibody (1:1000) as the primary antibody and a combination of HRP-conjugated anti-rabbit secondary antibody (1:2000) with an HRP-conjugated anti-biotin antibody (1:1000) to detect the biotinylated protein markers. The proteins were visualized by enhanced chemiluminescence using the supplied LumiGLO and peroxidase reagents.

Data Presentation and Statistical Analysis

All graphical data were prepared using GraphPad Prism 3.0 (GraphPad, San Diego, CA) and analyzed using preprogrammed analysis equations within Prism. Transfection data are presented as normalized data pooled from multiple experiments (each in triplicate and performed at least twice). Where appropriate, an ANOVA was performed on data followed by a Student t-test or Tukey multiple comparisons test, accepting P < 0.05 as significant.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Basal Comparison of Human -Subunit-Promoter Activity in LßT2 and T3-1 Cells

In order to compare the transcriptional activity of the human GSU in LßT2 and T3-1 cells, a -517-bp GSU-promoter luciferase construct (-517LUC) was transiently transfected into both cell lines. Luciferase activity was determined from cell extracts 8 h posttransfection. GSU-promoter activity was significantly greater than promoterless control (pA3LUC) in both LßT2 and T3-1 cells (see Fig. 1A), but promoter activity was significantly greater in T3-1 cells than in LßT2 cells (by 14.1 ± 3.7-fold compared with LßT2 cells, P < 0.01). Part of this difference was accounted for by differences in transfection efficiency in that the expression of ß-galactosidase in T3-1 cells was 2.2-fold greater than in LßT2 cells (Fig. 1A), suggesting a better transfection efficiency in T3-1 cells. However, this does not fully account for the discrepancy between the relative GSU-promoter activity in the two cell lines.

View larger version (22K): [in this window] [in a new window]   FIG. 1. Comparison of basal GSU-promoter activity and SF-1/DAX-1 expression in T3-1 and LßT2 cells. A) T3-1 (open bars) and LßT2 (filled bars) cells were transfected with 10 µg of the -517LUC human GSU-promoter-luciferase construct or with 10 µg of pA3LUC promoterless control and 5 µg of the control plasmid BosßGal. Cells were incubated subsequently with culture medium for 8 h before harvesting the cells and assaying the cellular extracts for luciferase and ß-galactosidase activity. Each bar represents the mean ± SEM of at least three independent experiments, each performed in triplicate. **P < 0.01. B) Western blotting for SF-1 and DAX-1 was performed on nuclear proteins from T3-1 and LßT2 cells. Each blot is representative of at least three separate blots

Several transcription factors have previously been implicated in the regulation of GSU expression in T3-1 cells, including the orphan nuclear receptor SF-1 [12] and the GATA family of proteins [13]. However, there are at least six members of the GATA family, and it is still unclear which members are expressed in LßT2 and T3-1 cells. Therefore, SF-1 is a good candidate to explore in the regulation of GSU transcription because it is the only protein that has been shown to bind to its consensus site in the promoter, the GSE [12]. Furthermore, other studies have shown that increasing SF-1 concentrations can enhance transactivation of SF-1 target genes [30]. To establish whether the levels of SF-1 and its putative inhibitor DAX-1 were different in T3-1 and LßT2 cells, Western blotting was performed on the same concentrations of T3-1 and LßT2 nuclear extracts. There was no discernible difference in the levels of DAX-1 protein, but less SF-1 protein was detected in LßT2 cells compared with T3-1 cells (Fig. 1B), suggesting a difference in the ratio of DAX-1 to SF-1 protein in these two cell lines.

Comparison of GnRH and PMA-Stimulated -Subunit-Promoter Activity in LßT2 and T3-1 Cells

The comparative responsiveness of the human GSU promoter in T3-1 and LßT2 cells to GnRH and PMA (a PKC activator) was determined by transiently transfecting these cells with 10 µg of the -517LUC plasmid. Cells were incubated for 8 h posttransfection in the absence or presence of 100 nM GnRH or PMA, and results are expressed as mean fold increase in LUC expression relative to untreated control cells (Fig. 2A). The concentrations of hormones/drugs used were those shown previously to achieve maximal stimulatory effects on GSU transcription in T3-1 cells [8, 17]. As expected, basal GSU-promoter activity was significantly stimulated in T3-1 cells in response to GnRH (7.4 ± 2.3-fold, P < 0.05) and PMA (6.4 ± 1.6-fold, P < 0.05). LßT2 cells treated with identical stimulants also responded to GnRH and PMA, although the GnRH response was less than the PMA response (4.2 ± 0.7-fold and 8.7 ± 0.9-fold, P < 0.01 and P < 0.001, respectively) in this series of experiments. Luciferase activity from the promoterless control vector pA3LUC was unchanged by any of these treatments (data not shown).

View larger version (28K): [in this window] [in a new window]   FIG. 2. Comparison of GnRH and PMA-stimulated GSU-promoter activity in T3-1 and LßT2 cells and the role of Ca2+ entry and PKC activation. A) T3-1 (open bars) and LßT2 (filled bars) cells were transfected with 10 µg of the -517LUC human GSU-promoter-luciferase construct and 5 µg of the control plasmid BosßGal. Cells were incubated subsequently with culture medium containing 0 (control) or 100 nM GnRH or PMA for 8 h before harvesting the cells and assaying the cellular extracts for luciferase activity. Levels of luciferase activity are internally standardized according to levels of ß-galactosidase activity. The data were normalized to the basal expression of -517LUC in each cell line. Each bar represents the mean ± SEM of at least three independent experiments each performed in triplicate. ***P < 0.001; **P < 0.01; *P < 0.05, significantly different from control. B) LßT2 cells were transfected with 10 µg -517LUC and pretreated with culture medium alone or containing 1 µM nifedipine or GF109203X for 30 min before incubation with culture medium alone (Fig. 5B) or with 100 nM PMA (open bars) or GnRH (shaded bars) (B) in the presence and absence of nifedipine or GF109203X for 8 h before harvesting the cells and assaying the cellular extracts for luciferase activity. Levels of luciferase activity are internally standardized according to levels of ß-galactosidase activity. The data are normalized as a percentage of the maximum control response to PMA or GnRH (set at 100%) compared with the relevant basal values (dotted line) in the absence or presence of nifedipine or GF109203X. Each bar represents the mean ± SEM of at least three independent experiments, each performed in triplicate. *P < 0.05; ***P < 0.001, significantly different from the relevant control maximum

Previous studies have revealed that activation of the GnRH receptor in gonadotrope cells leads to the recruitment of several different signaling cascades. We have previously revealed a role for both Ca²⁺ entry and PKC activity in the GnRH regulation of GSU transcription in T3-1 cells [8]. However, the factors involved in mediating Ca²⁺-stimulated GSU transcription are poorly understood in comparison with the PKC pathway. We therefore sought to compare the relative contribution of both of these pathways in mediating GnRH and PMA-stimulated GSU transcription in LßT2 cells using specific pharmacologic inhibitors of Ca²⁺ entry (nifedipine) and PKC activation (GF109203X). Following transfection with -517LUC, LßT2 cells were pretreated for 30 min in the absence or presence of 1 µM nifedipine or 1 µM GF109203X before stimulation with either 100 nM GnRH or PMA in the continued absence or presence of inhibitor for a further 8 h (Fig. 2B). Basal GSU-promoter activity was not affected by GF109203X but was modestly affected by nifidipine (data not shown). However, normalizing the data to account for these differences in basal revealed that both nifedipine and GF109203X significantly attenuated GnRH-stimulated GSU-promoter activity in LßT2 cells, to 55.1 ± 3.5% and 46.5 ± 2.1% of control responses (P < 0.05 and P < 0.001, respectively). However, PMA-stimulated GSU-promoter activity was only significantly attenuated by GF109203X pretreatment (to 27.1 ± 3.0% of control responses, P < 0.001).

Deletional Analysis of GSU-Promoter Sequences Required for Basal and PKC-Mediated Responsiveness in LßT2 Cells

In order to study PKC-mediated GSU-promoter activity in LßT2 cells in isolation from additional effects mediated by calcium entry, subsequent experiments were conducted using PMA as a stimulus rather than GnRH. To establish which regions of the human GSU promoter are responsible for basal and PMA-stimulated expression in LßT2 cells, a series of deletions of the -846-bp promoter were made at the 5' termini at -517, -244, and -195. Posttransfection, cells were treated for 8 h with either no addition or 100 nM PMA. There was no significant difference in basal promoter activity between the -846 and -517 constructs (Fig. 3A). Stepwise reductions in basal LUC activity from -517 to -244 and from -244 to -195 were seen, suggesting that there are two regions (between -517 and -244 and between -244 and -195) that contribute to basal expression of GSU in LßT2 gonadotropes (Fig. 3A), similar to data described previously in T3-1 cells [17]. Following PMA stimulation, the activity of -846, -517, and -244LUC constructs were enhanced to a similar extent (by 4.8 ± 0.9-, 4.2 ± 0.7-, and 3.7 ± 0.4-fold, P < 0.001, P < 0.01, and P < 0.05, respectively) (Fig. 3B). However, these effects were lost upon deletion to -195 bp (-195 vs. pA3LUC; not significant). PMA had no effect on the expression of pA3LUC (data not shown).

View larger version (15K): [in this window] [in a new window]   FIG. 3. Delineation of GSU-promoter sequences responsive to PMA in LßT2 cells. LßT2 cells were transfected with a series of 5'-deletion LUC fusion constructs or vector alone (pA3LUC) and subsequently incubated with either culture medium alone (Fig. 4A) or 100 nM PMA (Fig. 4B) for 8 h before harvesting the cells and assaying the cellular extracts for luciferase activity. Levels of luciferase activity are internally standardized according to levels of ß-galactosidase activity. The data are normalized to the basal expression of -846LUC (set at 100%, [A]) or as fold increase over the relevant basal value (B). Each bar represents the mean ± SEM of at least three independent experiments, each performed in triplicate. ***P < 0.001; **P < 0.01; *P < 0.05, significantly different from promoterless control, or as indicated. PGBE, Pituitary glycoprotein hormone basal element; BE, -basal element; GSE, gonadotrope-specific element; CRE, cyclic AMP response element

Identification of Regions in the Human GSU Promoter That Confer Basal and PMA Responsiveness to the TK Promoter in LßT2 Cells

The results of the 5'-deletion analysis suggested that at least two regions (between -517 and -244 and between -244 and -195) were responsible for mediating basal and PMA-stimulated transcription of the human GSU promoter in LßT2 cells. To determine whether these regions could function as basal and PMA-responsive elements when linked to a heterologous promoter, a series of deletions between -517 and -198 of the human GSU promoter were subcloned upstream of a minimal TK promoter in a LUC reporter vector. The basal and PMA-stimulated transcriptional activity of these constructs in LßT2 cells was assessed following transient transfection. The -517, -398, and -298 TKLUC constructs showed a similar basal expression pattern in LßT2 cells (Fig. 4A) to that seen with the native promoter constructs. These same constructs also responded to PMA stimulation to a similar magnitude as that observed when using the native constructs (by 12.9 ± 3.6-, 10.9 ± 4.2-, and 10.7 ± 3.2-fold, P < 0.01, P < 0.05, and P < 0.01, respectively) (Fig. 4A). No significant LUC activity (either basal or PMA stimulated) was seen when the -195TKLUC construct was used, supporting earlier observations with the native constructs and previous studies in T3-1 cells [17]. Using these overlapping native and heterologous -promoter constructs, these results strongly suggest that basal and PMA-responsive regions of the human GSU promoter in LßT2 cells reside predominantly between -398 and -244 (basal) and between -244 and -195 bp (basal and stimulated) upstream of the start site in the human GSU promoter.

View larger version (15K): [in this window] [in a new window]   FIG. 4. PMA responsiveness of TKLUC constructs in LßT2 cells. LßT2 cells were transfected with -517, -398, -298, or -195TKLUC constructs or the vector (TKLUC) and treated with culture medium alone (Fig. 5A) or with 100 nM PMA (Fig. 5B) for 8 h before harvesting the cells and assaying the cellular extracts for luciferase activity. Levels of luciferase activity are internally standardized according to levels of ß-galactosidase activity. The data are normalized to the basal expression of -517TKLUC (set at 100%, [A]) or as fold increase over the relevant basal value (B). Each bar represents the mean ± SEM of at least three independent experiments, each performed in triplicate. ***P < 0.001; **P < 0.01; *P < 0.05, significantly different from promoterless control, or as indicated

Role of GSE in Mediating Basal and PMA-Stimulated GSU-Promoter Activity in LßT2 Cells

The findings of the deletional studies revealed a potential role for a transcription factor that binds the GSU promoter between the region -244 and -195. The orphan nuclear receptor SF-1 binds the region between -218 and -200 and as such is an ideal candidate to investigate for its role in GSU transcription. Initially, we established that LßT2 nuclear extracts bind SF-1 to the human GSU consensus GSE sequence by competing binding with excess cold GSE probe (50x) or preincubation with SF-1 antibody (Fig. 5A). By mutating the central CC to TT within the GSE, SF-1 binding was completely abolished (Fig. 5A). We next introduced this same mutation within the context of the -517LUC promoter by site-directed mutagenesis (to make -517MUT) and compared the wild-type and GSE mutated promoters in transient transfection studies. Basal, GnRH, and PMA-stimulated luciferase activity was compared 8 h posttransfection. Basal GSU-promoter activity was significantly reduced in the -517MUT construct to 16.1 ± 1.7% of wild-type activity (P < 0.001) (Fig. 5B). Although both GnRH and PMA still significantly stimulated the -517MUT promoter (by 2.7 ± 0.3-fold and 2.2 ± 0.6-fold, P < 0.01 and P < 0.05, respectively), the magnitude of these fold increases were significantly reduced when compared with the wild-type promoter (GnRH: 9.7 ± 0.8-fold, P < 0.001; PMA: 5.8 ± 0.9-fold, P < 0.001). These data strongly support a role for SF-1 and the GSE in mediating basal and GnRH/PMA-stimulated transcription of the human GSU promoter in LßT2 cells.

View larger version (38K): [in this window] [in a new window]   FIG. 5. Role of the GSE in mediating basal and PMA-stimulated GSU transcription in LßT2 cells. A) EMSA analysis of LßT2 nuclear extracts (3 µg) binding to consensus human GSU GSE oligonucleotide (WT GSE) in the absence and presence of excess cold oligonucleotide (50x, competitor) or 1 µl anti-SF-1 (aSF-1) or with a mutated GSE oligonucleotide (MUT GSE) as the probe. B) LßT2 cells were transfected with 10 µg of either -517LUC or -517MUT and 5 µg of the control plasmid BosßGal. Cells were incubated subsequently with culture medium containing 0 (control) or 100 nM GnRH or PMA for 8 h before harvesting the cells and assaying the cellular extracts for luciferase activity. Levels of luciferase activity are internally standardized according to levels of ß-galactosidase activity. The data were normalized to the basal expression of -517LUC, with fold increases over basal indicated above the relevant bars. Each bar represents the mean ± SEM of at least three independent experiments, each performed in triplicate. ***P < 0.001; **P < 0.01; *P < 0.05, significantly different from the relevant basal (LUC or MUT), or as indicated

PMA Stimulation of the Human GSU Promoter in LßT2 Cells Involves ERK Activation

Previous studies in T3-1 [10] and LßT2 gonadotropes [31] have suggested that both GnRH and PMA activate ERK and that this factor may play a role in mediating their effects on these cells. In order to investigate the potential role of ERK in mediating the PMA effect on the GSU promoter, we first examined the ability of the specific MEK inhibitor U0126 to block the PMA effect on ERK activation. To do this, an ERK immunocomplex assay was performed on LßT2 cells treated for 5 min without or with 100 nM PMA following a 30-min pretreatment without or with 1 µM U0126. Following immunoprecipitation of phospho-ERK and immunocomplex assay of these samples with the transcription factor Elk-1 in the presence of ATP, immunoblotting revealed that PMA markedly stimulated Elk-1 phosphorylation after 5 min, an effect that was obliterated in the presence of U0126 (Fig. 6A).

View larger version (22K): [in this window] [in a new window]   FIG. 6. Involvement of the ERK1/2 signaling pathway in PMA-stimulated GSU-promoter activity in LßT2 cells. A) LßT2 cells were incubated in serum-free media for 30 min in the presence or absence of 1 µM U0126 before incubation with culture medium alone (basal) or with 100 nM PMA in the presence and absence of U0126 for 5 min. Total proteins were extracted and phosphorylated ERK was immunoprecipitated and applied to an immunocomplex assay in the presence of Elk-1 and ATP before Western blotting for phosphorylated Elk-1. B) LßT2 cells were transfected with 10 µg -517LUC or -244LUC and pretreated with culture medium alone or containing 1 µM U0126 for 30 min before incubation with culture medium alone (basal) (B) in the presence and absence of U0126 for 8 h before harvesting the cells and assaying the cellular extracts for luciferase activity. The data are normalized to the control activity of either -517 or -244 (set to 1). C) Following transfection (see B), LßT2 cells were stimulated with 0 or 100 nM PMA in the continued absence or presence of U0126 for 8 h. The data are normalized to the relevant basal activity (-517 or -244, with or without U0126) and are expressed as a percentage of the control maximum response to PMA for each construct (set to 100%), with fold increases over basal indicated above the relevant bars. Each bar represents the mean ± SEM of at least three independent experiments, each performed in triplicate. *P < 0.05; ***P < 0.001, significantly different as indicated. D) LßT2 cells were cotransfected with 5 µg of the Gal4-LUC reporter gene, 5 µg of an Gal4-Elk-1 expression vector, and 5 µg of the control plasmid BosßGal and pretreated with culture medium alone or containing 1 µM U0126 for 30 min before incubation with culture medium alone (basal) or with 100 nM PMA in the presence and absence of U0126 for 8 h before harvesting the cells and assaying the cellular extracts for luciferase activity. The data were normalized to the basal activation of Gal4-LUC by Gal4-Elk-1 in control and pretreated cells and expressed as a percentage of the control maximum PMA response (set to 100%), with fold increases over basal indicated above the relevant bars. Each bar represents the mean ± SEM of at least two independent experiments, each performed in triplicate. ***P < 0.001, significantly different from basal

Having established that PMA-stimulated ERK activation could be inhibited by U0126, we used this inhibitor in transient transfection studies. LßT2 cells were transiently transfected with the native -517LUC or -244LUC construct and then pretreated without (control) or with 1 µM U0126 for 30 min prior to stimulation without (basal) or with 100 nM PMA for 8 h. Basal promoter activity of the -517 and -244 constructs was inhibited to 70.3 ± 5.8% and 59.3 ± 7.3% of control values (P < 0.05 and P < 0.001, respectively) (Fig. 6B). As expected, both promoter constructs responded to PMA treatment, but these responses were significantly inhibited in cells pretreated in the presence of U0126 (to 44.3 ± 8.8% and 65.5 ± 11.2%, P < 0.001 and P < 0.05 for -517 and -244, respectively) (Fig. 6C). Similar effects were seen in parallel experiments using a heterologous promoter (-517TKLUC) (data not shown). To confirm that the MEK inhibitor was affecting ERK1/2-mediated transcription, LßT2 cells were transiently transfected with a Gal4-Elk-1 expression vector and Gal4-LUC reporter construct before preincubation with 0 or 1 µM U0126 for 30 min prior to stimulation (Fig. 6D). Both basal and PMA-stimulated luciferase activity were significantly inhibited by U0126 pretreatment to 52.2 ± 8.0% (basal, not shown) and 51.4 ± 13.6% (PMA stimulated) of control values, respectively (P < 0.001). GnRH stimulation of this construct was also attenuated by U0126 pretreatment in LßT2 cells (data not shown). These data taken together strongly imply a role for ERK activation in partially mediating PMA-stimulated transcription of the human GSU in LßT2 gonadotropes and that one or more regions below -244 are likely to be responsible for this response.

Effect of PMA on Transcription Factor Binding to GSU-Promoter Consensus Oligonucleotides

The results of the previous experiments indicated the importance of SF-1 and the GSE in PMA-stimulated GSU transcription. To establish whether PMA treatment can alter the binding of SF-1 protein from T3-1 and LßT2 cells to the consensus GSE oligonucleotide, EMSAs were performed with nuclear extracts from untreated, PMA-stimulated (100 nM for 30 min) or U0126 pretreated (10 µM for 30 min) followed by PMA stimulation (Fig. 7A). SF-1 binding to the consensus GSE was not significantly altered by PMA treatment nor by inactivation of the MEK/ERK pathway with U0126 (Fig. 7A) in either cell type. For comparison, we also performed EMSAs with consensus GATA oligonucleotides (which resides downstream of the GSE in the human GSU promoter at -155 to -151 bp) because this protein has also been implicated in regulating GSU transcriptional activation [13]. Interestingly, PMA treatment enhanced GATA complex formation from both T3-1 and LßT2 nuclear extracts, an effect that was blocked following U0126 pretreatment (Fig. 7B), suggesting that binding of as yet unidentified GATA proteins to this region of the GSU promoter may play a role in mediating PKC transcriptional activation. To confirm the specificity of this complex, the GATA EMSA was repeated in the presence of excess cold GATA oligo, which completely abolished the complex formation (Fig. 7C).

View larger version (52K): [in this window] [in a new window]   FIG. 7. Binding of T3-1 and LßT2 nuclear extracts to consensus SF-1 and GATA binding site oligonucleotides. T3-1 and LßT2 cells were serum starved overnight before pretreatment with 0 or 10 µM U0126 for 30 min before stimulation with 0 or 100 nM PMA for 30 min and subsequent preparation of nuclear proteins. Electrophoretic mobility shift assays were performed using 32P-labeled GSE (for SF-1, A) or GATA (B) oligonucleotides and identical concentrations of T3-1 and LßT2 nuclear extracts were loaded. The autoradiograph is representative of three separate assays. To confirm the specificity of the GATA complex, excess cold GATA oligonucleotide (50x) was included in the incubation (C)

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The common pituitary glycoprotein hormone -subunit is one of the first genes expressed in the developing pituitary gland, and during pituitary development, its expression becomes confined to gonadotropes and thyrotropes [1]. The regulation of GSU transcription in these two anterior pituitary cell types has been shown to involve both common and distinct pathways [32]. A number of gonadotrope cell lines have been established that have been immortalized at different stages of gonadotrope development [5]. These are reported as having different patterns of gene expression and thus potentially different transcriptional regulatory mechanisms [30, 33]. The T3-1 cell line has been used extensively to study GSU gene regulation, whereas no data is yet available for the LßT2 cell line, which is more representative of the mature gonadotrope. Using transient transfection of the cell lines with a human GSU-promoter-luciferase construct, we have demonstrated that GSU transcription occurs at a reduced rate in LßT2 cells compared with T3-1 cells but remains sensitive to GnRH and PKC activation. The regulatory regions of the human GSU promoter involved in mediating basal and PKC-stimulated activity in LßT2 cells appear to be similar to those reported previously in T3-1 cells [8, 17], but in addition, we have been able to show in more detail the signaling pathways and transcription factors mediating these effects.

Comparison of the basal GSU-promoter-luciferase activity revealed that promoter activity in LßT2 cells was considerably reduced compared with T3-1 cells. Relative levels of stimulatory or inhibitory transcription factors might explain the difference in basal GSU transcription in these two cell lines, as suggested previously [30], and Western blotting for SF-1 and DAX-1 revealed that there was less SF-1 protein expression in LßT2 cells than in T3-1 cells but comparable levels of DAX-1 protein. These findings suggest an increase in the DAX-1:SF-1 protein ratio in LßT2 cells, which potentially could manifest itself as increased DAX-1 inhibition of SF-1-mediated GSU transcription. As SF-1 has been implicated in the basal regulation of GSU transcription in T3-1 cells [12], it is possible that the reduced protein expression of SF-1 in LßT2 cells might partially explain the differences in basal GSU transcription.

Our previous studies examining the effects of GnRH on GSU transcription found that Ca²⁺ entry and PKC activation are important steps in mediating the GnRH stimulation [8]. PKC activation and its consequent coupling to the MAPK pathways in gonadotrope cell lines have been explored extensively, yet the mechanisms behind the role of Ca²⁺ in regulating GSU transcription has remained poorly understood. To clarify the contribution of these two pathways in regulating GSU transcription in LßT2 cells, we used pharmacologic inhibitors of Ca²⁺ entry and PKC activation. The GnRH effect on GSU-promoter activity in LßT2 cells was sensitive to inhibition of both Ca²⁺ entry and PKC activation, but the PMA effect appeared solely to be mediated via PKC activation. We therefore chose to use PMA as a stimulus in the subsequent transfection experiments to avoid the complication of dissecting poorly identified factors involved in Ca²⁺-stimulated GSU transcription.

A reduction in basal activity of the human LUC promoter was observed following progressive 5' truncations from -517 bp, as has been shown previously in T3-1 cells [11, 12]. Two regions were found to contribute to basal GSU transcription in LßT2 cells, one between -398 and -244 bp and the other between -244 and -195 bp. There are several proposed transcription factor response elements that reside upstream of -244 bp, such as the PGBE and the BE [11]. It would appear that these upstream elements are probably involved in regulating basal GSU transcription in LßT2 cells, as is the case in T3-1 cells [11, 12]. The second region that we identified as being involved in basal transcription (between -244 and -195 bp) has been shown to contain a putative SF-1 binding site, the GSE. Previous studies using site-directed mutagenesis have revealed that the GSE can regulate basal GSU transcription in T3-1 cells [11, 12]. In our studies, the contribution of the region containing the GSE on basal transcription in LßT2 cells is modest by comparison with the upstream region, yet the -244 bp construct was still significantly active, implying a role for a transcription factor(s) that binds the GSU promoter between -244 and -195 bp in mediating basal GSU transcription, as has previously been shown in T3-1 cells [12, 17].

Although reductions in basal transcription were seen when using the deletion constructs, a different pattern of activation was observed following PMA stimulation. There was no change in the four- to fivefold stimulation of GSU transcription by PMA until deletion of the sequence between -244 and -195 bp, suggesting that the more upstream elements are not involved in mediating this activation. Experiments using heterologous constructs supported these findings and are in agreement with studies investigating similar deletion constructs in T3-1 cells [8, 17]. GnRH and PMA stimulation of GSU transcription is mediated partially by common signal transduction pathways in T3-1 and rat pituitary cells [6, 32]. However, in primary cultures of dispersed rat pituitary cells, a GnRH-responsive region in the human GSU was found to be upstream of the GSE, between -346 and -244 bp [34]. A definitive site for this GnRH-response element has not been identified, and it is possible that the heterogeneous nature of the primary cell population might contribute to this observation. Somatotropes can express GSU under certain conditions, and subpopulations of these cells in rat pituitary cultures have been shown to respond to GnRH stimulation, resulting in increased fos expression [35]. Therefore, part of the GnRH effect on GSU transcription in transfected rat pituitary cells may be occurring in cell types other than gonadotropes.

The combined findings of deletion studies using overlapping homologous and heterologous highlighted the region between -244 and -195 as being required for PMA-stimulated GSU transcription in LßT2 cells. An SF-1 binding site, the GSE, resides within this part of the proximal promoter and has previously been implicated in regulating basal GSU transcription in T3-1 cells [11, 12]. We therefore examined the role of the GSE and of SF-1 in the regulation of basal and PMA/GnRH-stimulated GSU transcription in LßT2 cells. Using site-directed mutagenesis to introduce a CC to TT mutation within the GSE region of a -517LUC construct, we were able to confirm the importance of the GSE in regulating basal and PKC-stimulated GSU transcription in LßT2 cells. This mutation obliterated SF-1 binding in nuclear extracts from LßT2 cells. Basal GSU-promoter activity was significantly reduced in the presence of the mutated GSE, and the fold PMA effect was also significantly reduced by at least 50%. These data demonstrate for the first time a role for SF-1 and the GSE, not only in basal GSU transcription but also in mediating the effects of PMA on the activity of the human GSU promoter in LßT2 cells.

Several previous studies have revealed a possible role for the ERK component of the MAPK pathway in mediating GSU transcription in response to PKC activation [9, 10, 14, 36]. A recent study has shown that GnRH activates ERK and JNK in LßT2 cells, although ERK was not implicated in the GnRH regulation of LHß transcription [31]. Our studies revealed that the PMA effect on GSU transcription was inhibited between 40–50% in LßT2 cells (-244 and -517 constructs) following U0126 treatment with the specific MEK inhibitor, implying that ERK activation is a partial requirement for the PMA transcriptional effect. Similar results were seen when using the heterologous -517TKLUC construct (data not shown), and U0126 pretreatment also significantly attenuated PMA activation of a Gal4-Elk-1 expression vector, confirming that the MEK inhibitor was blocking ERK-stimulated transcription. Thus, ERK activation plays a role in mediating the transcriptional effects of PKC activation on the human GSU promoter in LßT2 cells, as has been previously suggested in T3-1 cells [7, 9, 10].

The results of the deletion studies and the transfections using the MEK inhibitor revealed that the region below -244 bp of the human GSU promoter contains an element(s) that regulates PMA-stimulated GSU transcription in LßT2 cells. This region has previously been implicated for PACAP/cAMP and GnRH/calcium-stimulated GSU transcription in T3-1 cells [8, 17]. An SF-1 binding site (the GSE) resides within the -244 to -195 region of the promoter. Neither PMA treatment nor blockade of the MEK/ERK pathway with U0126 pretreatment altered SF-1 binding to the GSE in EMSAs. This supports data showing that mutation of the ERK phosphorylation site in SF-1 (Ser203) fails to alter its binding activity [20] in transfected JEG-3 cells. However, a recent study suggests that SF-1 binding to consensus SF-1 sites in the StAR promoter is upregulated in adrenal Y1 cells by ERK activation [21]. Thus, ERK regulation of SF-1-DNA binding appears to be cell-type specific.

Although the GATA site, which is downstream of the GSE in the human GSU promoter, was still intact in the -195-bp constructs that were used, no transcriptional activation was seen when using this construct in LßT2 cells. However, this site has been implicated in regulating basal GSU transcription in T3-1 cells in the context of a -220-bp human GSU promoter [13], which still contains the GSE. Therefore, as a comparison with SF-1 binding the GSE, we examined whether GATA binding in LßT2 and T3-1 cells was affected by PMA treatment. In contrast with the SF-1 binding, the formation of GATA complexes following PMA treatment was enhanced in both LßT2 and T3-1 nuclear extracts, an effect that was blocked by U0126 pretreatment. Thus, GATA binding, but not SF-1 binding, appears to be regulated by PMA activation of ERK in both T3-1 and LßT2 cells. This response element may therefore be the ERK-sensitive site in the regulation of GSU transcription in these two cell lines and suggests that GATA may interact with other proximally residing factors (such as SF-1). Thus, our data suggests that cooperation between SF-1 and GATA sites (as has been reported for other promoters) [37] may be required for the regulation of stimulated GSU transcription.

In summary, the basal transcription rate of the human GSU gene promoter in LßT2 cells is less than in T3-1 gonadotropes but involves common components that reside between -398 and -244 and between -244 and -195 bp. The regions of the promoter that are important for PKC-mediated transcription in LßT2 cells appear to be downstream of -244 bp and include consensus SF-1 and GATA binding sites. Our data suggest that SF-1 and its binding site, the GSE, are involved in regulating basal and PMA/GnRH-stimulated GSU transcription in LßT2 cells. ERK-stimulated GSU transcription appears to be regulated via the -244 to -195 region of the promoter and might involve the activation and cooperation of SF-1 and GATA proteins, although the precise nature of the interaction of these transcription factors in mediating these responses in LßT2 cells remains to be established.

	ACKNOWLEDGMENTS

 
We thank Prof. P.L. Mellon (University of California San Diego, La Jolla, CA) for the generous gift of T3-1 and LßT2 cell lines, Prof. K.L. Parker (University of Texas, Dallas, TX) for the wild-type SF-1 expression vector, and Prof. A.F. Russo (University of Iowa, Iowa City, IA) for the Gal4-Elk-1 and Gal4-LUC plasmids.

	FOOTNOTES

 
First decision: 24 October 2001.

¹ Supported by Wellcome Trust grant RG8E8 to J.M.B.

² Correspondence: Rob Fowkes, Molecular Endocrinology Lab, 1.4, 1st Floor Dominion House, 59 Bartholomew Close, West Smithfield, London EC1A 7BE, U.K. FAX: 44 0 207 601 8468; r.c.fowkes{at}qmul.ac.uk

Accepted: March 28, 2002.

Received: September 7, 2001.

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