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Molecular and Cellular Biology, July 2007, p. 4807-4814, Vol. 27, No. 13
0270-7306/07/$08.00+0     doi:10.1128/MCB.02039-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

The Pituitary Function of Androgen Receptor Constitutes a Glucocorticoid Production Circuit{triangledown}

Junko Miyamoto,1,{dagger} Takahiro Matsumoto,1,2,{dagger} Hiroko Shiina,1 Kazuki Inoue,1 Ichiro Takada,1 Saya Ito,1 Johbu Itoh,3 Takeo Minematsu,4 Takashi Sato,1 Toshihiko Yanase,5 Hajime Nawata,5 Yoshiyuki R. Osamura,4 and Shigeaki Kato1,2*

Institute of Molecular and Cellular Biosciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan,1 ERATO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchisi, Saitama 332-0012, Japan,2 Teaching and Research Support Center,3 Department of Pathology, Tokai University School of Medicine, Boseidai, Isehara, Kanagawa 259-1193, Japan,4 Department of Medicine and Bioregulatory Science, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, Japan5

Received 1 November 2006/ Returned for modification 18 December 2006/ Accepted 16 April 2007


   ABSTRACT
Top
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Androgen receptor (AR) mediates diverse androgen actions, particularly reproductive processes in males and females. AR-mediated androgen signaling is considered to also control metabolic processes; however, the molecular basis remains elusive. In the present study, we explored the molecular mechanism of late-onset obesity in male AR null mutant (ARKO) mice. We determined that the obesity was caused by a hypercorticoid state. The negative feedback system regulating glucocorticoid production was impaired in ARKO mice. Male and female ARKO mice exhibited hypertrophic adrenal glands and glucocorticoid overproduction, presumably due to high levels of adrenal corticotropic hormone. The pituitary glands of the ARKO males had increased expression of proopiomelanocortin and decreased expression of the glucocorticoid receptor (GR). There were no overt structural abnormalities and no alteration in the distribution of cell types in the pituitaries of male ARKO mice. Additionally, there was normal production of the other hormones within the glucocorticoid feedback system in both the pituitary and hypothalamus. In a cell line derived from pituitary glands, GR expression was under the positive control of the activated AR. Thus, this study suggests that the activated AR supports the negative feedback regulation of glucocorticoid production via up-regulation of GR expression in the pituitary gland.


   INTRODUCTION
Top
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Sex steroid hormones exert a wide variety of biological actions. They are also involved in pathological events, such as the development of hormone-dependent cancers in reproductive organs (5, 37). In vertebrates, sex hormones play a pivotal role in male reproductive function and metabolic control. Most sex steroid actions are mediated through transcriptional control of target genes by nuclear receptors (NRs). NRs form a gene superfamily and act as transcriptional factors (9, 20). Sex hormone receptors have been shown to transactivate particular sets of target genes in a hormone-dependent manner through direct DNA binding to specific elements in target gene promoters. Hormone receptors activated by hormone binding recruit a number of coregulator-coregulator complexes for transactivation (28). These complexes then affect transcription through chromatin remodeling (12, 17, 22) and histone modification (1, 7). Hormone binding to the receptors may also transrepress target genes. The mechanisms of hormone-dependent transrepression of steroid receptors likely involve protein-protein interactions and are thus more diverse than that of transactivation (8, 10, 13, 21).

The molecular mechanisms behind the regulation of gene transcription by hormones and their NRs are complicated. Gene disruption studies have clarified the role of various NRs in steroid hormone action. By combining a Cre-loxP system with a canonical gene disruption approach, we succeeded in disrupting the androgen receptor (AR) on the X chromosome in mice in a manner that did not result in male infertility (14). Male AR null mutant (ARKO) mice exhibit abnormalities typical of testicular feminization mutants, including female external genitalia with atrophic testis and impaired sex behavior (29). Growth of the male ARKO mice is partially retarded, with impaired bone growth coupled with high bone turnover (16). The male mice also develop late-onset obesity (30). In contrast, no clear phenotypic abnormalities are present in female ARKO mice. However, normal folliculogenesis does require the AR, which suggests that androgen/AR signaling is also physiologically important in females (32).

To study how and why obesity develops in ARKO males, we began by examining the adrenal glands, which were hypertrophic in both males and females. In the present study, we explored the molecular basis of this observation. Dissection of the gland revealed that the layers of the zona fasciculata were thicker and coupled to the remaining layers of the X-zone (fetal zone). The hypertrophy resulted from a hypercorticoid state. Adrenal corticotropic hormone (ACTH) overproduction was driven by impaired negative feedback through the hypothalamus-pituitary-adrenal (HPA) axis. No clear alteration in the numbers of hormone-producing cells in the pituitary glands and hypothalamus was detected, but there were increased proopiomelanocortin (POMC) and decreased glucocorticoid receptor (GR) expression levels of transcripts in the ARKO pituitary glands. Androgen-induced GR gene activation was further confirmed in a pituitary gland-derived cell line (AtT-20 cells). These findings suggest that androgen/AR signaling in the pituitary gland supports the normal feedback system of glucocorticoid production through the HPA axis.


   MATERIALS AND METHODS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals. ARKO mice were generated by targeted disruption of the AR gene by means of a Cre-loxP system (19) and maintained as described previously (16, 29, 30, 32). Experiments were performed with 2- to 25-week-old male mice. All mice protocols were approved by the Animal Care and Use Committee of the University of Tokyo (31, 40).

Cell culture. Adherent AtT-20 cells, a murine corticotropic tumor cell line, were cultured in a 5% CO2 atmosphere at 37°C with Dulbecco's modified Eagle's medium-Ham's F12 at 1:1 containing 15% fetal calf serum (FCS) and penicillin-streptomycin. 3T3-L1 cells, a murine preadipocyte cell line, were cultured with Dulbecco's modified Eagle's medium containing 10% FCS. FCS in the culture media was replaced with charcoal-treated FCS for 1 week prior to the administration of 5{alpha}-dihydrotestosterone (DHT). For Northern and Western blot analyses, the cultured AtT-20 cells were subcultured in six-well plates. After incubation for 24 h, DHT (10–7 M) was added to the medium.

Histology and immunohistochemistry. Adrenal glands and pituitary glands were fixed by immersion with 4% paraformaldehyde for 24 h at 4°C. They were then embedded in paraffin, sliced into 4-µm sections by standard methods, and mounted onto silane-coated slides. After perfusion by 0.9% saline followed by 4% paraformaldehyde, brains were postfixed in the same fixative for 2 h at 4°C and soaked in phosphate-buffered saline containing 20% sucrose. Frontal sections were cut at 30-µm thickness using a cryostat. Serial sections were divided into four groups and used for single-labeling immunohistochemistry for the GR, corticotropin-releasing hormone (CRH), {alpha}-melanocyte-stimulating hormone ({alpha}-MSH), or thionin to allow determination of the areas to be measured.

Immunostaining was carried out using antibodies as described below (34). The primary antibodies included rabbit polyclonal anti-human AR (N-20; Santa Cruz Biotechnology, Santa Cruz, CA), rabbit polyclonal anti-GR (M-20; Santa Cruz Biotechnology, Santa Cruz, CA), mouse anti-human ACTH (DAKO, Carpinteria, CA), mouse anti-human luteinizing hormone ß (Immunotech, Marseille, France), mouse anti-human follicle-stimulating hormone ß (DAKO, Carpinteria, CA), mouse anti-human thyroid-stimulating hormone ß (Advanced Immunochemical Inc., CA), rabbit anti-rat glycoprotein hormone (kindly supplied by A. F. Parlow, the National Institute of Diabetes and Digestive and Kidney Diseases [NIDDK], Bethesda, MD), rabbit anti-human growth hormone (DAKO, Carpinteria, CA), and rabbit anti-rat prolactin (kindly supplied by A. F. Parlow, NIDDK).

After treatment with 0.5% H2O2 (30 min) and 5% normal serum (1 h), the sections were incubated for 24 h at 4°C with specific primary antibodies. The sections were then incubated with secondary antibodies and an avidin-biotin complex (Vectastain ABC Elite kit; Vector Laboratories). The signals were visualized with diaminobenzidine and the nuclei were counterstained with hematoxylin.

For dual labeling of ACTH and the AR or GR, a single staining of the AR or GR was first performed as described above. After the primary antibodies were removed by treatment with 0.1 M glycine, sections were incubated with anti-ACTH antibodies followed by alkaline phosphatase-conjugated anti-mouse immunoglobulin G (DAKO, Carpinteria, CA). The signals were visualized with 5-bromo-4-chloro-3-indolyl-phosphate and nitroblue tetrazolium.

Detection of proliferation and apoptosis of adrenal gland cells. Eight-week-old mice were injected intraperitoneally (i.p.) with the thymidine analog 5'-bromo-2'-deoxyuridine (BrdU) (30 mg/g body weight [BW]) every 12 h five times (25). Mice were fully anesthetized and their adrenal glands removed 12 h after the last injection. Incorporated BrdU was detected immunohistochemically using a mouse monoclonal anti-BrdU antibody. The proliferative index was defined as the number of BrdU-positive cells per microscopic field. Five fields per mouse were counted for each of three wild-type (WT) and three ARKO mice.

Cells undergoing apoptosis were identified by digoxigenin labeling of the free 3'OH ends of fragmented DNA by use of terminal deoxynucleotidyltransferase (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling [TUNEL] assay). Assays were performed on sections from the same tissue blocks used for BrdU immunohistochemistry. Sections were counterstained with hematoxylin to facilitate cell counting. The fraction of apoptotic cells was defined as the fraction of diaminobenzidine-positive cells per total number of cells. Five fields per mouse were counted for each of three WT and three ARKO mice.

Serum endocrine parameters. A circadian rhythm experiment and dexamethasone suppression tests were performed on 8-week-old male mice as previously described (2). For the circadian rhythm experiment, blood was collected at 08:00 or 18:00 h. For the dexamethasone suppression tests, mice were injected i.p. with different doses of dexamethasone (0, 2, or 5 µg/20 g BW) in 0.3 ml of 0.9% saline. Injections were performed between 08:00 and 08:30 h and blood was collected 6 h later. Mice were fully anesthetized and blood was collected by cardiac puncture. Plasma ACTH and serum corticosterone were measured using radioimmunoassay kits (IRMA; Mitsubishi, Tokyo, Japan) at SRL (Tokyo, Japan), according to the manufacturers’ instructions. Measurements were independently duplicated, and interassay variability and buffer dilution were corrected for by using internal correction factors.

RNA extraction and mRNA quantitation. Total mRNA was extracted from pituitary glands with TRIzol (Invitrogen) for reverse transcription-PCR (RT-PCR) and Northern blotting (35). To remove any possible DNA contamination prior to semiquantitative RT-PCR, the DNA was digested with RNase-free DNase. The digested total mRNA (2 µg) was subjected to RT using SuperScript reverse transcriptase (Invitrogen) primed by oligo(dT) primers. After first-strand cDNA synthesis, 1 ml from a 5% reaction mixture was diluted serially (2- to 128-fold). Amplification was performed with rTaqDNA polymerase (Takara) using primer pairs for GAPDH as an internal control to allow for concentration estimation (38). Expression levels of transcripts were measured using the standardized cDNA and specific primer pairs. The validity of the PCR products was confirmed by direct sequencing.

Western blot analysis. The lysates of mouse tissue and AtT-20 cells were resolved with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred onto nitrocellulose membranes (15, 39). Membranes were probed with rabbit polyclonal anti-GR antibody (M-20; Santa Cruz Biotechnology) and goat polyclonal anti-ß-actin antibody (I-19; Santa Cruz Biotechnology) as an internal control. The blots were visualized using peroxidase-conjugated anti-rabbit antibody and anti-goat antibody, together with an ECL detection kit (Amersham Biosciences). The small interfering RNA analysis used AR and control small interfering RNA (Ambion), and transfection was accomplished with the Lipofectamine 2000 system (Invitrogen).

Luciferase reporter assay. GR promoter regions (upstream regions of exons 1B, 1C, and 2) were cloned by PCR and subcloned into a luciferase reporter gene driven by a tk promoter (tk-luc). PCR primers were as follows: for 1B Fw, 5'-GGCATAGTTAGGCCACTAAAGAGA-3'; for 1B Rv, 5'-GGGAGAAGTTGCAAAGCAGA-3'; for 1C Fw, 5'-CTGGAGCAGCAAATGTCAAG-3'; for 1C Rv, 5'-AGCTCGCAAAATGGAGGAG-3'; for 2 Fw, 5'-GGATCTGGCGTCCTTTTC-3'; and for 2 Rv, 5'-CCACATTATCTCTGATCCGATT-3'.

For the luciferase reporter assay, cultured cells were transfected with the indicated plasmids using the Lipofectamine Plus reagent (Invitrogen) into 24-well plates at 40 to 50% confluence. The total amount of DNA was adjusted by supplementing with empty vector up to 1.0 µg/well. Luciferase activity was determined using a dual luciferase assay system (Promega). As a reference plasmid to normalize the transfection efficiency, 1.5 ng/well of pRL-CMV plasmid (Promega) was cotransfected in all experiments.

Statistical analysis. Values are given as the means ± standard deviations. Comparisons between two groups were made by Student's t test. P values of <0.05 were accepted as statistically significant.


   RESULTS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
High serum levels of ACTH and corticosterone in male ARKO mice. The male ARKO (ARL–/Y) mice exhibited growth retardation in comparison to WT male mice until 10 weeks of age but then showed catch-up growth over the next few weeks. Thereafter, the male ARKO mice weighed more than the WT mice and developed severe obesity (Fig. 1A) as previously reported (6, 30). Obesity to this extent was not seen for female ARKO (ARL–/L–) mice. To identify causes for the late-onset obesity in male ARKO mice, serum endocrine parameters were measured. We found that serum corticosterone levels in male ARKO mice were elevated at 8 weeks of age and became significantly higher at 13 and 20 weeks (P < 0.05 and P < 0.01, respectively) (Fig. 1B).


Figure 1
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  		FIG. 1. Hypertrophic adrenal glands with high serum levels of ACTH and corticosterone in ARKO mice. (A) Growth curves of ARKO and WT littermate mice. The floxed AR mice (female, ARL+/L+; male, ARL+/Y) were crossed with Cre-CMV transgenic mice to generate ARKO male (ARL–/Y) and female (ARL–/L–) mice (16, 30). (B) Serum corticosterone levels of ARKO and WT mice at 2, 8, 13, and 20 weeks (W) of age. (C) Plasma ACTH levels of ARKO and WT mice measured in the morning (8:00) and evening (18:00). (D) Serum corticosterone levels of ARKO and WT mice in the morning (8:00) and evening (18:00). (E) Adrenal gland weights of male and female ARKO and WT mice at 2, 8, 13, and 20 weeks of age. (F) Histology of ARKO and WT adrenal glands. All sections were stained with hematoxylin and eosin. F, zona fasciculata; G, zona glomerulosa; X, X-zone.

 
To more carefully examine the hypercorticoid state in male ARKO mice, we measured the serum levels of corticosterone and its upstream hormone ACTH. Measurements were taken both in the morning and in the evening, as these hormones exhibit a circadian rhythm. As expected, the 8-week-old WT and ARKO mice had low levels of both hormones in the morning and higher levels in the evening (Fig. 1C and D). Overall, the ARKO males tended to have high levels of ACTH and corticosterone at any time compared to WT mice. They had significantly high levels of plasma ACTH at both 8:00 and 18:00 and high levels of corticosterone at 18:00 compared to WT mice (P < 0.05) (Fig. 1C and D). However, for female ARKO mice, though serum levels of these hormones tended to be higher than in WT littermates, the differences were not statistically significant (data not shown). The obesity seen for these mice was likely the result of their hypercorticoid state, as centripetal obesity is a typical symptom of Cushing's syndrome. In the following experiments, we explored the etiology of the hypercorticoid state in the ARKO mice.

Hypertrophic adrenal glands in ARKO mice. To investigate the hypercorticoid state in the ARKO mice, we first examined the adrenal glands. The adrenal glands in the ARKO males clearly weighed more than the glands of WT mice at 13 weeks of age (Fig. 1E). This coincided with the onset of obesity and the hypercorticoid state. Likewise, in ARKO females, the adrenal glands also increased in size in comparison to what was seen for WT littermate females; however, the growth was not as pronounced as that in ARKO males (Fig. 1E). The adrenal glands of male ARKO mice were then used for subsequent experiments.

The adrenal cortex forms the major part of the gland and is divided into three layers in mammals: the zona glomerulosa, immediately beneath the capsule, followed by the zona fasciculata and the zona reticularis. The zona reticularis is replaced in rodents by the X-zone, which develops prenatally and begins to degenerate at pubertal maturity in males. In mice, corticosterone, the major glucocorticoid in rodents, is produced in the zona fasciculata, while aldosterone, the most potent mineralocorticoid, is formed in the zona glomerulosa. Hematoxylin and eosin staining of adrenal glands in 13-week-old mice revealed that the enlargement of the adrenal glands in ARKO males was caused by cellular hypertrophy of the zona fasciculata as well as by a failure of X-zone (fetal zone) regression (Fig. 1F). Since glucocorticoids are produced in the zona fasciculata, it is likely that the overproduction of corticosterone is the result of the hypertrophy in this area.

Increased proliferation and decreased apoptosis in the adrenal cortex of ARKO males. The failed regression of the X-zone in 13-week-old ARKO males raised the possibility of impaired cell death or decreased apoptosis in the adrenal cortex. Indeed, the percentage of apoptotic cells in the zona fasciculata, detected by TUNEL assay in the ARKO males, was clearly less (19.5%) than that for WT mice (33.1%) (Fig. 2A). When actively proliferating cells of the adrenal glands were counted by BrdU labeling in WT and ARKO males, 2.5 times more BrdU-labeled cells/section were found in ARKO mice. This suggests increased proliferation in the adrenal cortex of ARKO mice (Fig. 2B). Thus, the hypercorticoid state likely results from the overproduction of glucocorticoid by the hypertrophic zona fasciculata. This hypertrophy is caused by chronic exposure to high levels of ACTH.


Figure 2
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  		FIG. 2. Increased proliferation and decreased apoptosis in ARKO adrenal glands. (A) Decreased apoptosis in the ARKO adrenal glands. Histogram showing the number of TUNEL-positive cells in the zona fasciculata (Z. fasciculata). (B) Increased proliferation in the ARKO adrenal cortex. Histogram showing the number of BrdU-positive cells.

 
The HPA negative feedback system for glucocorticoid production is impaired in ARKO males. Glucocorticoid synthesis is regulated by a negative feedback loop via the HPA axis involving CRH and ACTH, produced by the hypothalamus and pituitary glands, respectively. We assessed whether the axis was intact and functioning normally in the ARKO males with a dexamethasone suppression test. As expected, serum corticosterone levels were down-regulated in 8-week-old WT mice 6 hours after i.p. injection of either 2 µg/20 g BW or 5 µg/20 g BW of dexamethasone (Fig. 3A). In ARKO mice, injection with 2 µg/20 g BW of dexamethasone did not suppress the serum levels of corticosterone. However, a dose of 5 µg/20 g BW was effective in lowering the serum levels of corticosterone in ARKO mice (Fig. 3A). This is similar to the high-dose dexamethasone suppression seen for patients with central Cushing's syndrome. Plasma ACTH levels in both the ARKO and WT males were decreased 6 h after dexamethasone injection at both the low and high doses; however, suppression was less sensitive in the ARKO males than in the WT males (Fig. 3B). No statistical difference in ACTH levels was detected between ARKO and WT males in this suppression test.


Figure 3
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  		FIG. 3. Impairment of the HPA negative feedback system of glucocorticoid production in ARKO mice. (A) Serum corticosterone levels of ARKO and WT mice in the dexamethasone suppression test. Trunk blood was collected from ARKO and WT mice 6 hours after injection with increasing doses of dexamethasone. (B) Plasma ACTH levels of ARKO and WT mice in the dexamethasone suppression test.

 
No overt abnormalities were present in the hypothalami or pituitary glands of ARKO males. The results of the suppression tests suggested that the adrenal hypertrophy of the ARKO males resulted from the hyperfunction of the hypothalami and/or pituitary glands. To address this issue, the hypothalami and pituitary glands of 8-week-old ARKO males were histologically examined. No overt abnormalities were detected in sections of the ARKO mice stained with hematoxylin and eosin (Fig. 4A). Immunohistochemical staining of the pituitary glands demonstrated similar numbers of cells expressing pituitary hormones in WT and ARKO mice (Fig. 4B). AR protein expression was detectable in several types of hormone-producing cells in the WT males but was absent in the ARKO mice (data not shown).


Figure 4
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  		FIG. 4. Histological appearance of the hypothalamus and pituitary gland in ARKO mice. (A) No clear alteration in morphology of the hypothalami or pituitary glands of ARKO mice. Sections of pituitary glands and hypothalami were stained with hematoxylin and eosin. A, anterior lobe; M, intermediate lobe; P, posterior lobe. (B) No overt abnormality in the distribution of cells expressing pituitary hormones in ARKO mice by immunohistochemical staining. LHß, luteinizing hormone ß; FSHß, follicle-stimulating hormone ß; TSHß, thyroid-stimulating hormone ß; CGA, glycoprotein hormone; PRL, prolactin; GH, growth hormone. (C) Pituitary ACTH (black/gray) colocalized with AR (brown) or GR (brown) (left) and its higher magnification (right) in WT mice as detected by immunostaining with specific antibodies. (D) No clear alterations in the GR and CRH (in the paraventricular nucleus) and {alpha}-MSH (in the arcuate nucleus) immunoreactive neurons in the hypothalami of ARKO mice.

 
Increased POMC expression and decreased GR expression in the pituitary glands is caused by AR deficiency. To address if hormone production was intact following AR inactivation, we examined the mRNA expression levels of pituitary hormones by RT-PCR. As shown in Fig. 5A, the expression of luteinizing hormone ß, follicle-stimulating hormone ß, and thyroid-stimulating hormone ß, as well as that of the orphan NRs (Nur77 and Nurr1), appeared unaltered by AR deficiency. The POMC mRNA levels, however, were clearly up-regulated in males (Fig. 5A) but not in females (Fig. 5B). The up-regulation of POMC mRNA was confirmed by Northern blot analysis (Fig. 5C). This finding is consistent with the high ACTH levels observed for the ARKO mice. In contrast, pituitary GR expression was decreased at both the mRNA and protein levels (Fig. 5A and D). Decreased GR gene expression in ARKO males was also seen for the spleen but not for the other tested tissues (Fig. 5E), suggesting tissue-specific regulation of GR expression by the AR. Interestingly, a clear decrease in the GR mRNA levels was not detected in the total brain RNA of ARKO males (Fig. 5E). Additionally, there was no alteration in the numbers of GR and CRH immunoreactive cells in the hypothalamic paraventricular nucleus in the male ARKO brain (Fig. 4D). These results suggest that the androgen/AR signaling system affects the negative feedback regulation of glucocorticoid production via pituitary GR expression. This view is further supported by the observation that colocalization of ACTH with the AR and/or GR in the pituitary glands of WT mice was detected by double immunostaining with specific antibodies (Fig. 4C).


Figure 5
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  		FIG. 5. Altered expression levels of gene transcripts involved in the HPA axis. (A) Increased POMC and decreased GR expression levels of transcripts in ARKO pituitary by semiquantitative RT-PCR. LHß, luteinizing hormone ß; FSHß, follicle-stimulating hormone ß; TSHß, thyroid-stimulating hormone ß. (B) No significant alterations of POMC and GR mRNA levels in the pituitary glands of female ARKO (ARL–/L–) mice. (C and D) Northern blot analyses showing clear up-regulation of POMC mRNA levels and down-regulation of GR mRNA levels in the ARKO pituitary. (E) Tissue-specific reduction of GR transcripts in ARKO mice. GR expression levels are down-regulated only in the spleen and pituitary in male ARKO mice.

 
Induction of the GR gene by DHT in a pituitary cell line. Finally, to confirm that GR expression levels in the pituitary glands of ARKO mice were reduced, we tested whether DHT induced GR gene expression in cultured cells. DHT treatment of the pituitary cell line AtT-20 for 4 h induced expression of the GR gene (Fig. 6A) and protein (Fig. 6B). Unexpectedly, the expression levels of POMC mRNA (Fig. 6A) and protein (Fig. 6B) were reduced. The AR effect was confirmed following treatment with an AR antagonist (Flutamide) (Fig. 6A and B) and RNA interference (Fig. 6C). Reflecting tissue-specific regulation of GR expression by the AR in intact animals, no response to DHT in the expression levels of either GR or POMC was seen in 3T3-L1 preadipocytes (Fig. 6A to C, lower panels). No consensus androgen response elements or closely related sequences are present up to –3 kb in the GR promoter. However, the intron between exons 1A and 1B of the GR gene (33) was found to counter androgen responsiveness in a transient expression assay in AtT-20 cells (Fig. 6D). This suggests that this element is responsible for androgen-induced GR expression in the pituitary.


Figure 6
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  		FIG. 6. Cell-type-specific regulation of the GR by activated AR. (A) Regulation of GR and POMC gene expression by treatment with either DHT or an AR antagonist (Flutamide) in the cultured cells as analyzed by Northern blot analysis. (B) Expression of the GR and pro-ACTH proteins was analyzed by Western blot analysis. (C) The significance of AR in the GR gene regulation was tested by AR RNA interference (with small interfering RNA [siRNA]) in the cultured cells. C siRNA, control siRNA. (D) Luciferase assay was performed with a series of the GR promoter regions in AtT-20 cells. After transfection with each of the promoter tk-luciferase vectors, the transfected cells were incubated with or without 10–7 M DHT.

 

   DISCUSSION
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Hypertrophic and hyperplastic adrenal glands are associated with a hypercorticoid state in male mice deficient for the AR. A hypercorticoid state was observed in sexually mature male mice deficient for the AR. It likely resulted from glucocorticoid overproduction by the hypertrophic and hyperplastic zona fasciculata of the adrenal gland. The ARKO mice demonstrated hyperplasia of the X-zone, which normally regresses by the time sexual maturity is attained (11). TUNEL assays and BrdU labeling confirmed that the hypertrophy and hyperplasia of adrenal glands resulted from decreased apoptosis and increased cell proliferation. Chronic ACTH stimulation causes zona fasciculata cell hypertrophy and hyperplasia, and ACTH is a potent inhibitor of apoptosis in the adrenal cortex (36). Thus, the findings for the ARKO males are consistent with exposure to high levels of ACTH.

The hypertrophic and hyperplastic adrenal glands in the ARKO mice probably resulted from high levels of serum ACTH, derived from high POMC transcript levels in the pituitaries of ARKO mice. Studies with transgenic mice expressing antisense RNA against the GR in the brain and anterior pituitary demonstrate that the GR mediates the negative feedback regulation of glucocorticoid production through HPA axis activity (26, 36). Consistent with this observation, the male ARKO mice had low pituitary GR mRNA levels but no difference in the distribution of pituitary hormone-producing cells compared to WT animals. Thus, our findings suggest that the activated AR in the pituitary gland is needed to express pituitary GR at a sufficiently high level to participate in the negative feedback regulation of glucocorticoid production. The X-zone, which is considered a fetal zone, regresses during sexual maturation and reappears after gonadectomy (11). The molecular basis underlying X-zone regression during sexual maturation remains to be investigated. However, our results raise the possibility that the activated AR in adrenal glands induces X-zone repression by the induction of apoptosis. Consequently, the identification of AR target genes expressed in the X-zone is another interesting direction to pursue.

Liganded AR augments GR gene expression in the pituitary gland. We found that GR gene expression was impaired in the pituitary glands of ARKO males. We presumed that the reduced GR levels led to increased expression of the POMC gene, with subsequent high levels of serum ACTH. This idea was supported by the observation that the suppression of ACTH production by exogenous glucocorticoids was partially impaired in the ARKO mice. Moreover, the DHT-activated AR enhanced the GR mRNA levels in a pituitary cell line but not in 3T3-L1 preadipocytes. The effect of DHT was most likely mediated by a response element in an upstream region of the GR promoter exon 1B (33). Thus, the activated AR directly induces the pituitary GR in a cell-specific manner. How this is accomplished on a molecular level remains to be elucidated.

Do androgen/AR signaling disorders link with an ACTH-dependent hypercorticoid state? A hypercorticoid state in humans is well known to cause Cushing's syndrome, in which patients suffer from a number of disorders such as centripetal obesity, facial rounding, glucose intolerance, hyperinsulinemia, and impaired lipid and bone metabolism (23). Most of these lesions are a reflection of glucocorticoid-driven gluconeogenesis. The hypercorticoid state may result from either endogenous disorders or chronic treatment with exogenous glucocorticoid. Endogenous causes of Cushing's syndrome are further classified as ACTH dependent or independent (18). The ACTH-dependent syndrome is characterized by up-regulated levels of ACTH; however, the molecular basis underlying the ACTH overproduction remains to be investigated. It is possible that sex steroids are involved, but this has not yet been fully addressed.

The male ARKO mice exhibited abnormalities similar to those seen for ACTH-dependent Cushing's syndrome patients. Since we detected up-regulation of the pituitary POMC transcript, other POMC-derived peptides might have contributed to the onset of obesity in male ARKO mice. For example, {alpha}-MSH in the neurons of the hypothalamus plays a central role in appetite control and energy homeostasis (3, 4). Although we detected no clear alteration in {alpha}-MSH immunoreactivity in the arcuate nuclei of the hypothalami of male ARKO mice, it will be of interest in future experiments to examine the melanocortin receptor system in ARKO brain. In contrast to the male ARKO mice, ARKO females did not display some of the abnormalities, such as obesity. It is possible that the lack of obesity in female ARKO mice may result from activation of estrogen receptors (ERs). ERs activated by high physiological levels of endogenous estrogens are effective in maintaining the proper levels of pituitary GR mRNAs needed to control POMC gene expression. This idea is indeed supported by the finding of unaltered levels of GR and POMC transcripts in the pituitary glands of the female ARKO mice. Moreover, estrogen treatment in female rats is shown to suppress serum levels of ACTH (27, 41). The common but gender-specific putative functions of the AR and ER in the brain have already been described in the context of mouse sexual behavior (24, 29). Though the possible ER functions remain to be studied for female ERKO mice, the present study suggests that the activated AR potentiates the negative HPA feedback regulation of glucocorticoid production through up-regulation of GR expression levels. Our study implies that the AR may be a potential therapeutic target for ACTH-dependent Cushing's syndrome.

In conclusion, the present study suggests that the androgen/AR signaling system is a negative pathway for glucocorticoid secretion in adult male mice. ARKO mice showed decreased GR expression in the pituitary glands and increased circulating ACTH and glucocorticoid. Androgens may increase the sensitivity of the HPA negative feedback loop to glucocorticoids by increasing GR expression in the pituitary gland, leading to suppression of adrenal cortical function. Thus, we presume that activated AR in the pituitary gland is a component of the negative feedback system for glucocorticoid production.


   ACKNOWLEDGMENTS
 
We thank members of the KO project team for experimental support, A. F. Parlow (NIDDK, National Hormone and Peptide Program, Torrance, CA) for kindly providing antisera, and H. Higuchi for manuscript preparation.

This work was supported in part by the Program for Promotion of Basic Research Activities for Innovative Biosciences (PROBRAIN) and priority areas from the Ministry of Education, Culture, Sports, Science and Technology (to S.K.).


   FOOTNOTES
 
* Corresponding author. Mailing address: Institute of Molecular and Cellular Biosciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan. Phone: 81-8-5841-8478. Fax: 81-3-5841-8477. E-mail: uskato{at}mail.ecc.u-tokyo.ac.jp Back

{triangledown} Published ahead of print on 30 April 2007. Back

{dagger} J.M. and T.M. contributed equally to this work. Back


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Molecular and Cellular Biology, July 2007, p. 4807-4814, Vol. 27, No. 13
0270-7306/07/$08.00+0     doi:10.1128/MCB.02039-06
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