TY - JOUR
AU - Andò,, Sebastiano
AB - Abstract Leydig cell tumors (LCTs) of the testis are steroid-secreting tumors associated with various steroid biosynthetic abnormalities and endocrine dysfunctions. Despite their overall rarity, LCTs are still of substantial interest owing to the paucity of information regarding their exact nature and malignant potential. In the present study, we disclose the ability of androgens to inhibit Leydig tumor cell proliferation by opposing to self-sufficient in situ estrogen production. In rat Leydig tumor cells, R2C, androgen treatment significantly decreases the expression and the enzymatic activity of cytocrome P450 aromatase, responsible for the local conversion of androgens into estrogens. This inhibitory effect relies on androgen receptor (AR) activation and involves negative regulation of the CYP19 gene transcriptional activity through the nuclear orphan receptor DAX-1 (dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1). Ligand-activated AR up-regulates the expression of DAX-1 and promotes its increased recruitment within the steroidogenic factor-1 site-containing region of the aromatase proximal promoter II in association with the nuclear receptor corepressor. The biological relevance in LCTs of the newly highlighted functional interplay between AR, DAX-1, and aromatase is underlined by our in vivo observations, revealing a marked down-regulation of AR and DAX-1 expression and a strong increase in aromatase levels in testes tissues from old Fischer rats with spontaneously developed Leydig cell neoplasia, compared with normal testes tissues from younger animals. In elucidating a mechanism by which androgens modulate the growth of Leydig tumor cells, our finding support the hypothesis that maintaining the adequate balance between androgen and estrogens may represent the key for blocking estrogen-secreting Leydigioma development, opening new prospects for therapeutic intervention. Testicular Leydig cell tumors (LCTs) are the most common neoplasms of male gonadal interstitium (1) and account for about 3% of all testicular cancers. Most frequently diagnosed in 5- to 10-year-old prepubertal boys and 30–60 aged adult men (2, 3), LCTs are commonly benign, but about 10% of them reveal a malignant phenotype in adult patients (4). Metastatic LCTs are resistant to irradiation and to most of the chemotherapic agents (5, 6), rendering it necessary to individuate as many new therapeutic target as possible, in the perspective of a multifaceted treatment. Because LCTs are steroid-secreting tumors, they are often associated with endocrine disturbance (7–10). It is now apparent that the adequate balance in the androgen/estrogen ratio is necessary for normal testicular development and function (11) and may potentially represent a clinical central factor in LCTs growth and progression. Androgens, which are mainly produced in testicular Leydig cells, drive the expression of the male phenotype, including male sexual differentiation, development of secondary sex characteristics, and manteinance of spermatogenesis (12). Androgen action is mediated by the androgen receptor (AR) that acts as a ligand-inducible transcription factor (13). In testicular cell-specific AR knockout mice, the lack of the receptor in Leydig cell compartment mainly affects steroidogenic functions (12). Mutations in the AR gene are responsible for the onset of varying levels of androgen insensitivity syndrome (14, 15) that, as result of the hormonal imbalance, is associated to a higher risk of developing testicular tumors (16). Immuno-histochemical findings in the testes of androgen insensitivity syndrome patients suggest the high expression of the cytochrome P450 aromatase as one of the molecular changes responsible for the increased risk of LTCs (17). The P450 aromatase enzyme is crucially involved in the maintenance of the androgen/estrogen essential balance, because it governs estrogen biosynthesis within the testis by catalyzing the irreversible conversion of C19 androgenic substrates, testosterone (T) and androstenedione, into the C18 estrogens, estradiol (E2) and estrone (18). Several observations suggest that estrogens can elicit proliferative effects in human and rodents tumor Leydig cells through an autocrine mechanism (19). Findings in transgenic mice show that increased E2 to T ratio, including excessive estrogen exposure, disturbs Leydig cell function and might cause hyperplasia, hypertrophy, and development of Leydig cell adenomas (20–22). In humans, elevated P450 aromatase expression, with consequent high plasma E2 levels, has been described in patients with testicular LCTs, further substantiating the role played by P450 aromatase on the pathogenesis of leydigiomas (23–26). All these experimental and clinical observations strongly support that abnormalities of the male gonadal functions may be associated with the decreased ratio of androgen/estrogen levels. So far, it has been demonstrated that androgens are able to suppress the expression of a number of steroidogenic enzyme genes eventually resulting in decreased testicular steroidogenesis (27, 28). However, information regarding the precise function of AR in Leydig tumor cells is still lacking. Here, we investigated the effect of androgens on the expression of the P450 aromatase in rat Leydig tumor cells R2C, a well-documented experimental model for leydigioma (19, 29–31). We demonstrated the existence of a novel mechanism, through which androgens are able to down-regulate the in situ estrogen production and Leydig tumor cell proliferation by inhibiting the expression of the P450 aromatase gene. This occurs through a functional cross talk between the AR, the orphan nuclear receptor DAX-1 (dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1) and the P450 aromatase. Materials and Methods Reagents and antibodies Mibolerone (Mb) was from PerkinElmer; bicalutammide (Bic) (Casodex) was from Astra-Zeneca; hydroxyflutamide (OH-Fl) and 4′,6-diamidino-2-phenylindole (DAPI) were from Sigma-Aldrich; BSA and AR (C-19), DAX-1 (K-17), nuclear receptor corepressor (N-CoR) (H-303), β-actin (AC-15), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (FL-335), cyclin D1(M-20), p21 (H-164), and Ki-67 antigen (Ki-67) (M-19) antibodies were from Santa Cruz Biotechnology, Inc; cytochrome P450 aromatase antibody was from Serotec. Cell cultures Authenticated rat Leydig tumor cells R2C were acquired from ATCC (LGC Standards), stored according to supplier’s instructions and used within 4 months after frozen aliquots resuscitations. Cells were cultured in Ham’s F-10 (Sigma) medium supplemented with 15% horse serum, 2.5% fetal bovine serum, and antibiotics (Invitrogen, S.R.L.). Before each experiment, cells were synchronized in phenol red-free (PRF) serum-free media for 24 hours. All the experiments were performed in PRF media containing 2.5% charcoal-treated horse serum (PRF-CT). Animal studies Male Fischer 344 rats were a gift of Sigma-Tau. 6-month-old Fischer rats (n = 3) and 24-month-old Fischer rats (n = 4) were used for in vivo studies. Twenty-four-month-old animals presented spontaneously developed LCTs (Fischer rats tumor testes [FRTT]), which were absent in younger animals (Fischer rats normal testes [FRNT]). Testes of all animals were surgically removed by qualified, specialized animal care staff in accordance with the recommendation of the Guidelines for the Care and Use of Laboratory Animals (NIH) and with the Animal Care Committee of University of Calabria. Cell viability and proliferation assays Cell viability was evaluated by 3-[4,5-dimethylthiaoly]-2,5-diphenyltetrazolium bromide (MTT) and cell counting assays, as previously described (32). Briefly, for MTT (Sigma) assay, a total of 5 × 104 cells were seeded onto 24-well plates and treated for 6 days with increasing concentrations of Mb. The MTT assay was performed as the following: 100-mL MTT stock solution in PBS (2 mg/mL) was added into each well and incubated at 37°C for 2 hours followed by media removal and solubilization in 500-mL dimethyl sulfoxide. After shaking the plates for 15 minutes, the absorbance in each well, including the blanks, was measured at 570 nm in Beckman Coulter. For cell counting, R2C cells were seeded on 6-well plates (2 × 105cells/well) in 2.5% PRF-CT. After 24 hours, cells were exposed for 6 days to 10−8M Mb or vehicle treated. The effects of Mb on cell proliferation were measured 0, 3, and 6 days after initial exposure to treatments by counting R2C cells using a Burker’s chamber, with cell viability determined by trypan blue dye exclusion. Anchorage-independent soft agar growth assays R2C cells (104/well) were plated in 4 mL of Ham’s F-10 with 0.5% agarose and 5% charcoal-stripped fetal bovine serum, in 0.7% agarose base in 6-well plates. After 2 days, media containing vehicle or treatments were added to the top layer and replaced every 2 days. Anchorage-independent growth was assessed as previously described (31). Immunocytochemical staining Immunocytochemical staining of R2C was performed as previously described (33). Cells were fixed for 30 minutes in freshly prepared para-formaldehyde (2%) and incubated for further 30 minutes with 10% normal rabbit serum to block the nonspecific binding sites. Immunocytochemical staining was performed using an affinity purified goat anti-Ki-67 antibody (Santa Cruz Biotechnology, Inc). The cells were then incubated with the secondary antibody biotinylated rabbit-antigoat IgG (Vector Laboratories) for 1 hour at room temperature followed by incubation with avidin/biotin/horseradish peroxidase complex (Vector Laboratories). The peroxidase reaction was developed using Stable 3,3′-diaminobenzidine (Sigma Chemical) for 3 minutes. RNA extraction, RT-PCR, and real-time PCR Cells were maintained overnight in serum-free medium and then treated for 24 or 48 hours in 2.5% PRF-CT medium. RNA extraction and reverse transcription were performed as previously described (33) with minor modifications. For RT-PCR, primers were DAX-1 (forward), 5′-CAGGCCATGGCGTTCCTGTA-3′ and (reverse), 5′-TCCTGCCGCCTGGTGGTGAG-3′; and CYP19 (forward), 5′-CAGCTATACTGAAGGAAATCC-3′ and (reverse), 5′-AATCGTTTCAAAAGTGTAACCA-3′. Quantitative PCR was performed using SYBR Green universal PCR master mix (Bio-Rad Laboratories). Primers used were CYP19 (forward), 5′-GAGAAACTGGAAGACTGTATGGATTTT-3′ and (reverse), 5′-ACTGATTCACGTTCTCCTTTGTCA-3′; and DAX-1 (forward), 5′-CTGGGTGGGGAGGGA CTGC-3′ (reverse), 5′-CCTGGCGCGGGTGGTTCTC-3′. PCRs were performed in the iCycler iQ Detection System (Bio-Rad), using 0.1μM each primer in a total volume of 30 μL of reaction mixture following the manufacturer’s recommendations. Each sample was normalized on the basis of its 18S ribosomal RNA content. The 18S quantification was performed using a TaqMan Ribosomal RNA Reagent kit (Applied Biosystems) following the method provided in the TaqMan Ribosomal RNA Control Reagent kit. The relative gene expression levels were normalized to a calibrator that was chosen to be the basal, untreated sample. Final results were expressed as n-fold differences in gene expression relative to 18S ribosomal RNA and calibrator, calculated following the ΔΔThreshold cycle (Ct) method, as published previously (19). Immunoblotting analysis R2C cells or total tissue of FRNT and FRTT were lysed for protein extraction (31). Total and cytosolic/nuclear extracts were prepared and subjected to SDS-PAGE as described (34). The intensity of bands representing relevant proteins was measured by Scion Image laser densitometry scanning program. Immunofluorescence Immunofluorescence analysis was performed as previously described, with minor modifications (35). Briefly, cells were fixed with 4% paraformaldehyde, permeabilized with 0.2% Triton X-100, followed by blocking with BSA (3%, 30 min), and incubated with anti-P450 aromatase or anti-DAX-1 antibodies (4°C, overnight) and with fluorescein-conjugated secondary antibody (30 min, room temperature). IgG primary antibody was used as negative control (NC). DAPI (Sigma) staining was used for nuclei detection. Protein cellular localization was observed under a fluorescence microscope (Olympus BX51 fluorescence microscope; Olympus Italia S.R.L.). Cells were photographed at ×100 magnification using ViewFinder Software, through an Olympus camera system dp50 and the optical densities of stained proteins were analyzed by ImageJ software (NIH). P450 aromatase activity assay The P450 aromatase activity in subconfluent R2C cells culture medium was measured by the tritiated water release assay using 0.5μM [1β3H]androst-4-ene-3,17-dione as substrate (36). The incubations were performed at 37°C for 2 hours under an air/CO2 (5%) atmosphere. The results, obtained as picomoles per hours and normalized to milligrams of protein (pmol/h·mg of protein), were expressed as fold change over control. Radioimmunoassay R2C cells were seeded on 6-well plates (2 × 105cells/well) in 2.5% PRF-CT. After 24 hours, cells were exposed for 72 hours to vehicle or 10−8M Mb and/or 10−6M OH-Fl. The E2 content of medium recovered from each well was determined against standards prepared in low-serum medium using a RIA kit (DSL 43100; Diagnostic System Laboratories). Plasmids, transfections, and luciferase reporter assays The plasmids containing different segments of the rat P450 aromatase promoter II (PII) sequence ligated to a luciferase reporter gene were kindly provided by Dr M.J. McPhaul and were −1037/+94 (p-1037), −688/+94 (p-688), and −688/+94 mut (5′cAMP response element (CRE)m, 3′CREm, XCREm, and steroidogenic factor-1 (SF-1)m) containing mutations of 5′CRE, 3′CRE, XCRE and SF-1, respectively (29). Firefly luciferase reporter plasmid XETL is a construct containing an estrogen-responsive element from the Xenopus vitellogenin promoter (37). Cells were transfected using FuGENE HD Transfection Reagent (Promega) according to manufacturer’s instructions. Renilla reniformis luciferase expression vector pRL-Tk (Promega) was used for transfection efficiency. Empty pGL2-basic vector was used as a control to measure basal activity. Luciferase activity was measured using Dual Luciferase Assay System (Promega). Chromatin immunoprecipitation (ChIP) ChIP assay was performed as described (38) using anti-DAX-1 and anti-N-CoR antibodies. Normal rabbit or mouse IgGs were used as NCs. A 3-μL volume of each sample and DNA input were used for PCR using the primers flanking SF-1 sequence in the rat P450 aromatase promoter region (forward), 5′-ATGCACGTCACTCTACCCACTCAA-3′ and (reverse), 5′-TAGCACGCAAAGCAGTAGTTTGGC-3′. The amplification products were analyzed in a 2% agarose gel and visualized by ethidium bromide staining. Statistical analysis All data were expressed as the mean ± SD of 3 independent experiments. Statistical significances were analyzed using Student’s t test or one-way ANOVA, where appropriate. *, P < .05 was considered as statistically significant. Results Androgens reduce cell proliferation rate in estrogen-dependent R2C Leydig tumor cells To investigate whether androgens might play a role in the modulation of estrogen-dependent Leydig tumor cell growth, we used the rat Leydig tumor cell line R2C, a well-documented experimental model for Leydigioma (19, 29–31). In the present study, experiments were carried out using the synthetic AR agonist Mb to minimize the metabolic conversion of androgen to estrogenic compounds by cultured cells. The effect of increasing concentrations of Mb on R2C Leydig tumor cell viability was evaluated by MTT assay. As shown in Figure 1A, Mb treatment significantly decreased R2C cell viability. The inhibitory effect of 10−8M Mb was further confirmed by trypan blue exclusion test, showing that it began 2 days after androgen administration and persisted thereafter. The inhibitory effect exerted by 10−8M Mb was abrogated by the addition of the AR antagonist OH-Fl (Figure 1B). In addition, Mb treatment was also able to decrease the number of colonies present in soft agar (Figure 1C). Reduction of cell proliferation after Mb administration was further proved by evaluating the expression levels of the proliferation marker Ki-67 (Figure 1D), the cyclin-dependent kinase inhibitor p21, which is also a well-known androgen-dependent gene (39), and cyclin D1 (Figure 1E), which represents a key factor in the E2-induced proliferative effects in estrogen-responsive tissues (40). According to the reduced cell number, Mb-treated R2C cells exhibited a decreased expression of Ki-67 and cyclin D1 but increased levels of p21. Figure 1 Open in new tabDownload slide Mb treatment reduces proliferation in R2C cells. A, MTT assays in R2C cells treated with vehicle (−) or different Mb concentrations (M) as indicated for 6 days. B, Cells were treated with vehicle or 10−8M Mb and/or 10−6M OH-Fl. Drug effects on cell proliferation were measured at day 0, 3, and 6 after initial exposure to treatments by cell counting using a Burker’s Chamber, with cell viability determined by trypan blue dye exclusion. C, R2C cells were seeded (1 × 104/well) in 0.5% agarose and treated as described above. Cells were allowed to grow for 14 days and then the number of colonies more than 50 μm were quantified and the results graphed. The results represent the mean ± SD of 3 different experiments each performed with triplicate samples. *, P < .05 vs vehicle; Δ, P < .05 vs Mb treated. D, Immunocytochemical staining of proliferation marker Ki-67 in R2C cells treated with vehicle (C) or 10−8M Mb for 72 hours. No immunoreactivity was detected when R2C cells were incubated without the primary antibody (NC). Scale bar, 12.5 μm. E, Immunoblotting of p21 and cyclin D1 in R2C cells treated with vehicle (C) or 10−8M Mb for 72 hours. GAPDH, loading control. Data from D and E are representative of 3 separate experiments. Figure 1 Open in new tabDownload slide Mb treatment reduces proliferation in R2C cells. A, MTT assays in R2C cells treated with vehicle (−) or different Mb concentrations (M) as indicated for 6 days. B, Cells were treated with vehicle or 10−8M Mb and/or 10−6M OH-Fl. Drug effects on cell proliferation were measured at day 0, 3, and 6 after initial exposure to treatments by cell counting using a Burker’s Chamber, with cell viability determined by trypan blue dye exclusion. C, R2C cells were seeded (1 × 104/well) in 0.5% agarose and treated as described above. Cells were allowed to grow for 14 days and then the number of colonies more than 50 μm were quantified and the results graphed. The results represent the mean ± SD of 3 different experiments each performed with triplicate samples. *, P < .05 vs vehicle; Δ, P < .05 vs Mb treated. D, Immunocytochemical staining of proliferation marker Ki-67 in R2C cells treated with vehicle (C) or 10−8M Mb for 72 hours. No immunoreactivity was detected when R2C cells were incubated without the primary antibody (NC). Scale bar, 12.5 μm. E, Immunoblotting of p21 and cyclin D1 in R2C cells treated with vehicle (C) or 10−8M Mb for 72 hours. GAPDH, loading control. Data from D and E are representative of 3 separate experiments. Similar results on cell proliferation (eg, R2C cell number and Ki-67 expression) were obtained by treating R2C cell with the AR natural ligand dihydrotestosterone (DHT) (Supplemental Figure 1, A and B). Androgen administration decreases P450 aromatase expression and activity in R2C cells Local estrogen production by highly expressed P450 aromatase represents a major feature involved in the positive control of R2C cell proliferation (19, 23). Thus, we explored the possibility that androgen treatment might impact on P450 aromatase gene expression in R2C cells. Mb administration was able to reduce the cellular content of the enzyme at both mRNA and protein levels, as detected by real-time PCR (Figure 2A) and immunoblotting (Figure 2B) assays, respectively. Similar findings were obtained after DHT administration (Supplemental Figure 1, C and D). These results were further confirmed by immunofluorescence analysis detecting a reduced P450 aromatase immunoreactivity in the cytoplasm of R2C cells as well as in the perinuclear region, when cells were treated with Mb. No reaction was noticed in the nuclei and in the cells processed without primary antibody (NC) (Figure 2C). Notably, the reduction of P450 aromatase protein levels upon Mb treatment was also reflected by a change in its enzymatic activity, as measured by the tritiated water release assay (Figure 2D) and a reduction in E2 production (E2 production in vehicle-treated samples was 3138 ± 140 pg/mL) (Figure 2E). The inhibitory effect exerted by androgen treatment on P450 aromatase expression and activity was abrogated by the contemporary addition of AR antagonists, OH-Fl or Bic, indicating that it was mediated by AR activation (Figure 2). Having demonstrated the ability of Mb to down-regulate aromatase expression and activity in R2C cells, we investigated whether it was able to affect E2/estrogen receptor (ER) signaling in R2C cells. To this aim, we performed a transient transfection experiment using the XETL plasmid, which carries firefly luciferase sequences under control of an estrogen-response element upstream of the thymidine kinase promoter. As shown in Figure 2F, we observed that R2C cell exposure to Mb significantly reduced XETL activation. As expected, addition of OH-Fl completely reversed this effect. Figure 2 Open in new tabDownload slide Effects of Mb on P450 aromatase expression and activity in R2C cells. A, P450 aromatase mRNA content evaluated by real-time RT-PCR in cells treated for 24 hours with vehicle (−), or 10−8M Mb and/or 10−6M OH-Fl or 10−6M Bic as indicated. Each sample was normalized to its 18S rRNA content. The values represent the mean ± SD of 3 different experiments each performed in triplicate. B, Total protein extracts from R2C cells treated with vehicle (−), 10−8M Mb and/or 10−6M OH-Fl or 10−6M Bic as indicated for 48 hours were used for immunoblotting analysis of P450 aromatase expression. GAPDH, loading control. Histograms represent the mean ± SD of 3 separate experiments in which band intensities were evaluated in terms of optical density arbitrary units and expressed as fold change over basal, which was assumed to be 1. C, Immunofluorescence of P450 aromatase (a–d) in cells treated for 48 hours with vehicle (−), 10−8M Mb and/or 10−6M OH-Fl. DAPI staining was used to visualize the cell nucleus (e–h). Scale bar, 15 μm. NC (i and l). Histograms represent the mean ± SD of 4 separate experiments, in which stained P450 aromatase protein was evaluated in terms of optical density arbitrary units by ImageJ software (NIH). P450 aromatase activity (D) and E2 production (E) in cells treated for 72 hours with vehicle (−), 10−8M Mb and/or 10−6M OH-Fl. F, R2C cells were transiently cotransfected with XETL (0.5 μg/well) and treated for 48 hours with vehicle (−), 10−8M Mb, and/or 10−6M OH-Fl. Activation of reporter gene expression XETL in vehicle-treated cells is arbitrarily set at 100%. Data represent the mean ± SD of 3 different experiments each performed in triplicate. *, P < .05 vs vehicle; Δ, P < .05 vs Mb treated. Figure 2 Open in new tabDownload slide Effects of Mb on P450 aromatase expression and activity in R2C cells. A, P450 aromatase mRNA content evaluated by real-time RT-PCR in cells treated for 24 hours with vehicle (−), or 10−8M Mb and/or 10−6M OH-Fl or 10−6M Bic as indicated. Each sample was normalized to its 18S rRNA content. The values represent the mean ± SD of 3 different experiments each performed in triplicate. B, Total protein extracts from R2C cells treated with vehicle (−), 10−8M Mb and/or 10−6M OH-Fl or 10−6M Bic as indicated for 48 hours were used for immunoblotting analysis of P450 aromatase expression. GAPDH, loading control. Histograms represent the mean ± SD of 3 separate experiments in which band intensities were evaluated in terms of optical density arbitrary units and expressed as fold change over basal, which was assumed to be 1. C, Immunofluorescence of P450 aromatase (a–d) in cells treated for 48 hours with vehicle (−), 10−8M Mb and/or 10−6M OH-Fl. DAPI staining was used to visualize the cell nucleus (e–h). Scale bar, 15 μm. NC (i and l). Histograms represent the mean ± SD of 4 separate experiments, in which stained P450 aromatase protein was evaluated in terms of optical density arbitrary units by ImageJ software (NIH). P450 aromatase activity (D) and E2 production (E) in cells treated for 72 hours with vehicle (−), 10−8M Mb and/or 10−6M OH-Fl. F, R2C cells were transiently cotransfected with XETL (0.5 μg/well) and treated for 48 hours with vehicle (−), 10−8M Mb, and/or 10−6M OH-Fl. Activation of reporter gene expression XETL in vehicle-treated cells is arbitrarily set at 100%. Data represent the mean ± SD of 3 different experiments each performed in triplicate. *, P < .05 vs vehicle; Δ, P < .05 vs Mb treated. Ligand-activated AR down-regulates the transcriptional activity of P450 aromatase PII-proximal promoter Next, we investigated whether the down-regulatory effect of Mb on P450 aromatase expression could be due to a negative influence on P450 aromatase gene transcriptional activity. In human fetal and adult testes, R2C and H540 rat Leydig tumor cells and in purified preparation of rat Leydig, Sertoli, and germ cells, P450 aromatase expression is regulated by a promoter proximal to the translational starting site named PII (41–43). Thus, R2C cells were transiently transfected with a luciferase reporter plasmid containing the PII P450 aromatase promoter (PII-Arom) sequence (−1037/+94). As shown in Figure 3A, a significant reduction in PII-Arom activity could be observed upon Mb administration. This effect was no longer detectable after the addition of the AR antagonist OH-Fl, further confirming the involvement of AR activation. In addition, no inhibitory effects were observed in Mb-treated cells transfected with the empty-luciferase reporter plasmid (data not shown). PII-regulated P450 aromatase expression is driven by specific response elements: a nuclear receptor half-site binding SF-1/liver receptor homolog-1 (LRH-1) (44) and CRE-like sequences binding CRE binding protein/activating transcription factors protein family members (29). Therefore, to characterize PII-Arom regions that are functionally important for transcriptional regulation by androgens, transient transfection experiments were performed by using different sized or mutated PII-Arom fragments fused to the luciferase reporter gene (29), as schematically exemplified in Figure 3B, left panel. Figure 3 Open in new tabDownload slide Effects of Mb on P450 aromatase promoter activity in R2C cells. A, R2C cells were transiently transfected with a luciferase reporter plasmid containing the PII-Arom (−1037/+94) and treated for 24 hours with vehicle (−), 10−8M Mb, and/or 10−6M OH-Fl. Activation of reporter plasmids in vehicle treated samples is arbitrarily set at 1. B, Schematic representation of the PII P450 aromatase proximal promoter constructs used in this study (left panel). All of the promoter constructs contain the same 3′ boundary (+94). The 5′ boundaries of the promoter fragments varied from −1037 to −688. Three putative CRE motifs (5′-CRE at −335; 3′-CRE at −231; XCRE at −169) are indicated as circles. The SF-1 motif at −90 is indicated as a square. A mutated 5′CRE, 3′CRE, XCRE, and SF-1-binding site is present, respectively, in 5′CREm, 3′CREm, XCREm, and SF-1m constructs. PII P450 aromatase transcriptional activity in R2C cells, transfected with the above described promoter constructs and treated with vehicle (C) or 10−8M Mb for 24 hours, is shown (right panel). Activation of reporter plasmids in vehicle treated samples is arbitrarily set at 1. Results represent the mean ± SD of 3 different experiments each performed in triplicate. *, P < .05 vs vehicle; Δ, P < .05 vs Mb treated. Figure 3 Open in new tabDownload slide Effects of Mb on P450 aromatase promoter activity in R2C cells. A, R2C cells were transiently transfected with a luciferase reporter plasmid containing the PII-Arom (−1037/+94) and treated for 24 hours with vehicle (−), 10−8M Mb, and/or 10−6M OH-Fl. Activation of reporter plasmids in vehicle treated samples is arbitrarily set at 1. B, Schematic representation of the PII P450 aromatase proximal promoter constructs used in this study (left panel). All of the promoter constructs contain the same 3′ boundary (+94). The 5′ boundaries of the promoter fragments varied from −1037 to −688. Three putative CRE motifs (5′-CRE at −335; 3′-CRE at −231; XCRE at −169) are indicated as circles. The SF-1 motif at −90 is indicated as a square. A mutated 5′CRE, 3′CRE, XCRE, and SF-1-binding site is present, respectively, in 5′CREm, 3′CREm, XCREm, and SF-1m constructs. PII P450 aromatase transcriptional activity in R2C cells, transfected with the above described promoter constructs and treated with vehicle (C) or 10−8M Mb for 24 hours, is shown (right panel). Activation of reporter plasmids in vehicle treated samples is arbitrarily set at 1. Results represent the mean ± SD of 3 different experiments each performed in triplicate. *, P < .05 vs vehicle; Δ, P < .05 vs Mb treated. Similarly to what observed for the PII-Arom (−1037/+94), the transcriptional activity of the PII-Arom construct (−688/+94), which still includes the 5′CRE, 3′CRE, XCRE, and SF-1 binding sites, was decreased in response to Mb stimulation, thus confirming the importance of these sites for PII-driven regulation of P450 aromatase expression (Figure 3B, right panel). Mutations of the 5′CRE (5′CRE m), the 3′CRE (3′CRE m), or the XCRE (XCREm) did not influence the response to androgens, because PII-Arom transcriptional activity was still reduced after Mb administration, mirroring wild-type PII-Arom activity. On the contrary, we evidenced the loss of the inhibitory effect exerted by Mb in the presence of the construct bearing a SF-1/LRH-1 mutated binding site (SF-1m), demonstrating that P450 aromatase regulation by androgens requires the integrity of the SF-1 motif. Mb induces DAX-1 expression and its recruitment within the P450 aromatase gene promoter in R2C cells One of the major repressors of SF-1-mediated transactivation of target genes in steroidogenic tissues is the nuclear orphan receptor DAX-1 (28, 45–47). Moreover, previous findings demonstrated that P450 aromatase is a physiologic target gene of the nuclear orphan receptor DAX-1 that, acting as an adaptor molecule, represses P450 aromatase transactivation through a variety of mechanisms (48–50). Thus, we investigated, in our experimental system, whether DAX-1 may represent a molecular link between androgens and P450 aromatase. First, we evaluated, in R2C cells, the effects of Mb administration on DAX-1 levels. By using quantitative real-time PCR, we performed a time-course study to evaluate DAX-1 mRNA levels in R2C cells. As shown in Figure 4A, a Mb-dependent induction of DAX-1 mRNA levels could be already observed after a 12-hour treatment and persisted thereafter. The enhanced DAX-1 mRNA levels were concomitant with a decreased P450 aromatase mRNA content (Figure 4B). Induction of DAX-1 expression by Mb (Figure 4C) or DHT (Supplemental Figure 1, E and F) was confirmed by Western blot analysis, also revealing that DAX-1 protein localizes prevalently into the nucleus wherein it was further enhanced upon Mb administration. Immunofluorescence analysis confirmed these observations (Figure 4D). DAX-1 regulation involves AR activation, because addition of the AR antagonists OH-Fl reduced the Mb-dependent up-regulation of DAX-1 (Figure 4C). Figure 4 Open in new tabDownload slide Mb up-regulates DAX-1 expression in R2C cells. A, Time-course analysis of DAX-1 mRNA content evaluated by real-time RT-PCR in cells treated with vehicle (−), or 10−8M Mb, as indicated. Each sample was normalized to its 18S rRNA content. Values represent the mean ± SD of 3 different experiments each performed in triplicate. B, RT-PCR for DAX-1 and P450 aromatase mRNA expression in R2C cells treated with vehicle (−) or 10−8M Mb for 48 hours. 18S, internal standard. C, Immunoblotting for DAX-1 expression in cytosolic or nuclear extracts from cells treated for 48 hours with vehicle (−), 10−8M Mb, and/or 10−6M OH-Fl. GAPDH and Lamin B were assessed as control of protein loading and purity of lysate fractions. Histograms represent the mean ± SD of 3 separate experiments, in which band intensities were evaluated in terms of optical density arbitrary units. D, Immunofluorescence of DAX-1 (a and b) in cells treated for 48 hours with vehicle (C) or 10−8M Mb. DAPI staining was used to visualize the cell nucleus (d and e). Scale bar, 15 μm. NC (c and f). Histograms represent the mean ± SD of 4 separate experiments, in which stained proteins were evaluated in terms of optical density arbitrary units by ImageJ software (NIH). E, Sheared chromatin from R2C cells treated with vehicle (−) or 10−8M Mb for 2 hours was precipitated using anti-DAX-1 or anti-N-CoR antibodies. The 5′-flanking sequence of the PII P450 aromatase proximal promoter containing the SF-1 site was detected by PCR with specific primers listed in Materials and Methods. Inputs DNA were amplified as loading controls. IgG, control samples. *, P < .05 vs vehicle; Δ, P < .05 vs Mb treated. Figure 4 Open in new tabDownload slide Mb up-regulates DAX-1 expression in R2C cells. A, Time-course analysis of DAX-1 mRNA content evaluated by real-time RT-PCR in cells treated with vehicle (−), or 10−8M Mb, as indicated. Each sample was normalized to its 18S rRNA content. Values represent the mean ± SD of 3 different experiments each performed in triplicate. B, RT-PCR for DAX-1 and P450 aromatase mRNA expression in R2C cells treated with vehicle (−) or 10−8M Mb for 48 hours. 18S, internal standard. C, Immunoblotting for DAX-1 expression in cytosolic or nuclear extracts from cells treated for 48 hours with vehicle (−), 10−8M Mb, and/or 10−6M OH-Fl. GAPDH and Lamin B were assessed as control of protein loading and purity of lysate fractions. Histograms represent the mean ± SD of 3 separate experiments, in which band intensities were evaluated in terms of optical density arbitrary units. D, Immunofluorescence of DAX-1 (a and b) in cells treated for 48 hours with vehicle (C) or 10−8M Mb. DAPI staining was used to visualize the cell nucleus (d and e). Scale bar, 15 μm. NC (c and f). Histograms represent the mean ± SD of 4 separate experiments, in which stained proteins were evaluated in terms of optical density arbitrary units by ImageJ software (NIH). E, Sheared chromatin from R2C cells treated with vehicle (−) or 10−8M Mb for 2 hours was precipitated using anti-DAX-1 or anti-N-CoR antibodies. The 5′-flanking sequence of the PII P450 aromatase proximal promoter containing the SF-1 site was detected by PCR with specific primers listed in Materials and Methods. Inputs DNA were amplified as loading controls. IgG, control samples. *, P < .05 vs vehicle; Δ, P < .05 vs Mb treated. To provide evidence of the participation of DAX-1 in the androgen-dependent modulation of P450 aromatase expression, we assessed, by ChIP assay, the ability of DAX-1 to associate to the SF-1/LRH-1-containing region of the PII P450 aromatase proximal promoter upon androgen treatment. According with previous findings, both DAX-1 and N-CoR were recruited at the SF-1/LRH-1 motif-containing region of the PII P450 aromatase gene promoter (Figure 4E). Interestingly, DAX-1 occupancy of the SF-1/LRH-1 site-containing sequence of the PII promoter was induced in a ligand-dependent manner, because DAX-1 recruitment was enhanced by Mb administration. Concomitantly, recruitment of the steroid receptor corepressor N-CoR within the same promoter region was also significantly increased by Mb treatment (Figure 4E). These results demonstrate, for the first time, how DAX-1 enhancement by androgens may negatively affect the modulation of the P450 aromatase gene in Leydig tumor cells. AR/DAX-1/P450 aromatase expression pattern in male Fischer rats As a final step of the current study, we evaluated the expression pattern of DAX-1, P450 aromatase, and AR in testis tissues from younger (FRNT) and older (FRTT) Fischer rats. Aged animals presented spontaneous LCTs, a phenomenon not observed in younger animals (51, 52), allowing us to use them as a good in vivo model useful to corroborate our in vitro observations. As shown in Figure 5, DAX-1 protein was expressed in FRNT but scarcely detectable in FRTT. A similar expression pattern was displayed by AR. On the contrary, a strongly increased P450 aromatase immunoreactivity could be observed in FRTT compared with FRNT. The AR/DAX-1/P450 aromatase protein expression pattern in normal (FRTT) and tumoral (FRNT) testis tissues was mirrored by the corresponding mRNA content, as detected by RT-PCR analysis (data not shown). Thus, our in vivo results indicate that the occurrence of Leydigioma is associated to a significant decrease in AR and DAX-1 levels and to a remarkable enhancement of P450 aromatase expression, confirming results obtained in R2C cell line. Figure 5 Open in new tabDownload slide DAX-1, P450 aromatase, and AR expression in Fischer rat testes. Immunoblotting of DAX-1, P450 aromatase, and AR in tissues from normal (FRNT) and tumor (FRTT) Fischer rat testes. β-Actin, loading control. Immunoblots show a single representative result. Histograms represent the mean ± SD of 3 separate experiments, in which band intensities were evaluated in terms of optical density arbitrary units. *, P < .05 vs FRNT. Figure 5 Open in new tabDownload slide DAX-1, P450 aromatase, and AR expression in Fischer rat testes. Immunoblotting of DAX-1, P450 aromatase, and AR in tissues from normal (FRNT) and tumor (FRTT) Fischer rat testes. β-Actin, loading control. Immunoblots show a single representative result. Histograms represent the mean ± SD of 3 separate experiments, in which band intensities were evaluated in terms of optical density arbitrary units. *, P < .05 vs FRNT. Discussion In the present study, we highlighted, in Leydig tumor cells, the existence of a novel functional interplay between the AR, the orphan nuclear receptor DAX-1, and the P450 aromatase enzyme that could play a role in leydigiomas’ development and progression. Testicular sex steroid synthesis is an elaborated dynamic process that, in addition to the negative feedback throughout the hypothalamic-pituitary-gonadal axis, is also finely regulated by various autocrine/paracrine factors (53, 54). In this scenario, the maintenance of a delicate balance between androgens and estrogens appears to have a fundamental role for male gonadal integrity and function (55). This balance is governed by cytochrome P450 aromatase, encoded by the CYP19 gene, which catalyzes the local conversion of androgens to estrogens and whose expression has been detected in Leydig cells and some population of germ cells (56). Interestingly, the expression of AR at the same site of P450 aromatase and ER in the testis (23, 57–59) suggests a possible cross-regulation between the 2 steroid-induced signaling pathways. It is now generally accepted that locally secreted estrogens may act as autocrine factors exerting proliferative effects on human and rodents Leydig cells (10, 19). Increased E2 to T ratio disrupts Leydig cell function and might cause hyperplasia, hypertrophy, and Leydig cell adenomas (20, 21). Indeed, in mice, chronic estrogen treatment induces LCTs (60), eventually regressing after estrogen withdrawal with cellular alterations suggestive of apoptosis (61). On the other hand, in male occult LCTs patients, gynecomastia, the more frequent clinical sign observed, is accompanied by increased E2 and decreased T levels, strongly suggesting the presence of an estrogen-secreting tumor (7, 8, 10, 62). However, less is known regarding the role of AR in the modulation of the complex testis paracrine/autocrine signaling especially concerning the molecular mechanism for androgen/AR regulation of Sertoli cell, Leydig cell, and peritubular myoid cell proliferation and/or differentiation (12). Here, we demonstrate that prolonged administration of the synthetic AR agonist Mb is able to reduce proliferation rate in a well-documented in vitro model for Leydig cell neoplasms, such as rat Leydig tumor cells R2C (63). Our study indicates that androgen-dependent reduction of cell growth is consequent to the decrease of the self-sufficient in situ estrogen production, which represents a major feature of R2C cells (19, 23). Indeed, in this cell type, androgen treatment negatively impacts on P450 aromatase by down-regulating its expression at both mRNA and protein levels and determining a decrease of its enzymatic activity. It has been shown that androgens are able to affect the expression of steroidogenic enzymes via an AR-mediated (64) or a local feedback mechanism (65, 66). Our results indicate that the inhibitory effects induced by androgen administration on P450 aromatase expression involve AR activation, because they disappear in the presence of the AR antagonist OH-Fl or bicalutamide. These data confirm previous in vitro and in vivo findings that AR signaling in Leydig cells displays autocrine regulation and that lacking functional AR in Leydig cells has a major influence on Leydig cell steroidogenic function (12, 27). The observed AR-mediated inhibition of P450 aromatase cellular levels involves regulation of CYP19 gene transcriptional activity. Distinctive tissue-specific CYP19 promoters are employed to direct the expression of P450 aromatase mRNA (67). The main regulator of normal testicular cell aromatization is the CYP19 proximal PII, located immediately upstream of the transcriptional initiation site. PII is also been reported as the main active promoter in LCTs and in R2C cells (24, 25, 31, 42, 43). Different response elements have been identified in PII: 3 motifs resembling CRE and an SF-1 binding site (29, 68), but no androgen-response element has been defined. By functional studies, using constructs containing wild-type or different 5′-deleted regions of rat PII-Arom, we demonstrated the ability of androgens to decrease PII transcriptional activity. This inhibitory effect was abrogated when a promoter fusion containing a mutated SF-1 element was employed, suggesting that the integrity of the SF-1 sequence is a prerequisite for the down-regulatory effects of the AR ligands on P450 aromatase promoter activity. The above described observations are consistent with recent studies showing that AR inhibits the transactivation of SF-1 in Leydig cells (27) as in gonadotrope-derived cells (69). The importance of the androgen-induced inhibition of SF-1-dependent P450 aromatase gene transactivation in Leydig tumor cells is highlighted by a recent case report of a 29-year-old male diagnosed with an estrogen-producing LCT, severe gynecomastia, and elevated plasma E2 levels. In fact, in this report, authors demonstrate elevated estrogen synthesis in LCT to be consequent to enhanced PII-driven CYP19 transcription, most likely caused by elevated levels of the transcription factor SF-1 (26). Thus, our findings are suggestive of the possibility that androgen action on testicular estrogen production might be accomplished by cross talk between androgen signaling and major transcription factors responsible for P450 aromatase gene expression. In the aim of getting insights into this issue, we investigated the involvement of the nuclear orphan receptor DAX-1 that has key roles in the development and the maintenance of reproductive function, because it is a crucial regulator of steroidogenesis in mammals. Indeed, DAX-1 has a restricted expression pattern to tissues directly involved in steroid hormone production (70). Within these tissues, DAX-1 colocalizes with SF-1 (45) and acts as a global antisteroidogenic factor, likely by suppressing SF-1-mediated transcription through a variety of mechanisms (28, 46, 47, 49, 50, 71). For instance, DAX-1 is a repressor of several steroidogenic enzymes (28, 70, 72), blocking steroid production at multiple levels and decreasing the formation of steroids that are precursors of estrogens. In Y-1 adrenocortical cells, DAX-1 blocks the rate-limiting step in steroid biosynthesis by repressing steroidogenic acute regulatory protein expression (46) as well as by inhibiting both P450 cholesterol side-chain cleavage (CYP11A) and 3-β-hydroxysteroid dehydrogenase cellular levels (50). Moreover, it has also been shown that in insulin-treated MA-10 Leydig cells, triggering of the insulin receptor signaling pathway decreases cAMP-induced steroidogenic acute regulatory protein, P450 cholesterol side-chain cleavage, and 3-β-hydroxysteroid dehydrogenase via induction of DAX-1, without any change in serum LH or FSH levels (73). Worthily the aromatase coding gene, CYP19 represents a physiological target for DAX-1 in Leydig cells. A very elegant study examining the consequences of DAX-1 deficiency on Leydig steroidogenesis in vivo demonstrated that there was no alteration in the expression of the 5 steroidogenic genes required for T biosynthesis, whereas P450 aromatase mRNA, protein, and enzymatic activity was increased significantly in DAX-1-deficient Leydig cells. Enhanced P450 aromatase expression was accompanied by a 40-fold increase in intratesticular E2 (48). Consistently, in a patient with adrenal insufficiency and hypogonadotropic hypogonadisms, deletion of the exon 2 of DAX-1 gene, dramatically affecting its function (74), has been associated with disturbed biological function of testis, especially hyperplasia of Leydig cells (75). Our findings indicate that, in rat Leydig tumor cells R2C, DAX-1 expression is regulated by androgens. In our experimental system, androgen-dependent up-regulation of nuclear DAX-1 is associated to a significant increase of its recruitment within the SF-1 site-containing region of the PII P450 aromatase proximal promoter. According to previous observations (49), DAX-1 occupancy of the PII promoter is concomitant with the enhanced recruitment of the steroid receptor corepressor N-CoR. Therefore, the mechanism we described might contribute to explain the negative influence of androgens on rat Leydig tumor cell proliferation and well correlates with observations in endometrial carcinoma, where loss or decreased DAX-1 expression results in increased intratumoral steroid production and enhanced estrogen-dependent proliferation of cancer cells (76). Thus, in Leydig tumor cells, the induction of DAX-1 expression and the consequent modulation of P450 aromatase expression and activity exerted by androgens may represent an in situ defensive mechanism for modulating unopposed estrogenic effects. Thus, the maintenance of a proper ratio between androgen and estrogen testicular levels appears to be fundamental. As shown in the present study, long-term administration (3 and 6 d) of low doses of androgens (in the nanomolar range) decreases R2C cell number by binding the AR and reducing the effect of locally produced E2. On the contrary, high doses (in the micromolar range) of aromatizable and nonaromatizable anabolic androgenic steroids induce, after a 24-hour treatment, proliferation of R2C cells as a consequence of the binding to the ER (32). Although reinforcing the concept that the finely tuned balance between androgens and estrogens is critical in the development of proliferative diseases, these observations make it appealing to speculate whether such a mechanism can also exist in other endocrine-related cancers. In fact, the aberrant expression of P450 aromatase is believed to contribute to the development and progression of breast cancer (18, 36, 77–79). To this regard, we recently demonstrated that ligand-activated AR negatively regulates in situ estrogen production by activating DAX-1 gene transcription in estrogen-related MCF-7 breast cancer cells, providing new clues for the inhibitory role exerted by androgens on estrogen-dependent cancer cell proliferation (79). More surprisingly, emerging evidences indicate the prostate as a target for locally produced estrogens (80, 81). Specifically, P450 aromatase is expressed in nonmalignant prostatic stroma but not in normal epithelial cells. In contrast, the onset and/or progression of malignancy is accompanied by the appearance and increase of P450 aromatase expression and activity within the prostate epithelium together with potential alteration of P450 aromatase promoter usage, similarly to that occurred in breast cancer (82, 83). Besides, prostate epithelium phenotypic changes are also associated to dysregulation of DAX-1, whose expression is considerably reduced in benign prostate hyperplasia compared with normal prostate tissue (84, 85), thus leaving the issue opened for future challenges. Direct confirmation of the biological relevance of our findings comes from results obtained in normal (FRNT) and tumoral (FRTT) testes tissues from younger and older, respectively, Fischer rats, which are characterized by exceptionally high incidence of spontaneous Leydig cell neoplasm associated with aging (51, 52). It appears extremely interesting to observe that tumor development is concomitant with a marked down-regulation of AR and DAX-1 expression and a strong increase in P450 aromatase levels, as it emerges comparing young normal rat testes with the older ones showing the presence of the neoplasia. Specifically, FRNTs are characterized by high expression levels of DAX-1 and very low levels of P450 aromatase enzyme. On the contrary, Leydig tumor development in FRTT is associated with an opposite expression pattern showing a barely detectable DAX-1 content and a strongly increased P450 aromatase expression. Importantly, DAX-1 expression pattern is mirrored by AR expression. Collectively, our study, for the first time, identifies the existence of a functional AR/DAX-1/P450 aromatase interplay involved in the inhibition of the estrogen-dependent testicular cancer cell proliferation. In elucidating a mechanism by which androgens modulate the growth of Leydig tumor cells, these findings reinforce the hypothesis that restoring the correct balance between androgen and estrogen levels may open new opportunities for therapeutic intervention in this type of endocrine-related cancer. Acknowledgments We thank Dr E.R. Simpson, Dr C.D. Clyne, and Dr M.J. McPhaul for generously providing P450 aromatase promoter plasmids. This work was supported by the Progetti di Ricerca di Interesse Nazionale-Ministero Istruzione Università e Ricerca Grant 20085Y7XT5, and the Associazione Italiana Ricerca sul Cancro Grant 11595, and Fondazione “Lilli Funaro.” Disclosure Summary: The authors have nothing to disclose. 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TI - Androgens Inhibit Aromatase Expression Through DAX-1: Insights Into the Molecular Link Between Hormone Balance and Leydig Cancer Development
JF - Endocrinology
DO - 10.1210/en.2014-1654
DA - 2015-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/androgens-inhibit-aromatase-expression-through-dax-1-insights-into-the-wNstarIZT9
SP - 1251
VL - 156
IS - 4
DP - DeepDyve
ER -