Androgens stimulate erythropoiesis through the DNA-binding activity of the androgen receptor in non-hematopoietic cells

BACKGROUND

Androgens function through DNA and non-DNA binding-dependent signalling of the androgen receptor (AR). How androgens promote erythropoiesis is not fully understood.

DESIGN AND METHODS

To identify the androgen signalling pathway, we treated male mice lacking the second zinc finger of the DNA-binding domain of the AR (AR^ΔZF2 ) with non-aromatizable 5α-dihydrotestosterone (5α-DHT) or aromatizable testosterone. To distinguish direct hematopoietic and non-hematopoietic mechanisms, we performed bone marrow reconstitution experiments.

RESULTS

In wild-type mice, 5α-DHT had greater erythroid activity than testosterone, which can be aromatized to estradiol. The erythroid response in wild-type mice following 5α-DHT treatment was associated with increased serum erythropoietin (EPO) and its downstream target erythroferrone, and hepcidin suppression. 5α-DHT had no erythroid activity in AR^ΔZF2 mice, proving the importance of DNA binding by the AR. Paradoxically, testosterone, but not 5α-DHT, suppressed EPO levels in AR^ΔZF2 mice, suggesting testosterone following aromatization may oppose the erythroid-stimulating effects of androgens. Female wild-type mice reconstituted with AR^ΔZF2 bone marrow cells remained responsive to 5α-DHT. In contrast, AR^ΔZF2 mice reconstituted with female wild-type bone marrow cells showed no response to 5α-DHT.

CONCLUSION

Erythroid promoting effects of androgens are mediated through DNA binding-dependent actions of the AR in non-hematopoietic cells, including stimulating EPO expression.

Muscle-specific androgen receptor deletion shows limited actions in myoblasts but not in myofibers in different muscles in vivo

The aim of this study was to investigate the direct muscle cell-mediated actions of androgens by comparing two different mouse lines. The cre-loxP system was used to delete the DNA-binding activity of the androgen receptor (AR) in mature myofibers (MCK mAR(ΔZF2)) in one model and the DNA-binding activity of the AR in both proliferating myoblasts and myofibers (α-actin mAR(ΔZF2)) in another model. We found that hind-limb muscle mass was normal in MCK mAR(ΔZF2) mice and that relative mass of only some hind-limb muscles was reduced in α-actin mAR(ΔZF2) mice. This suggests that myoblasts and myofibers are not the major cellular targets mediating the anabolic actions of androgens on male muscle during growth and development. Levator ani muscle mass was decreased in both mouse lines, demonstrating that there is a myofiber-specific effect in this unique androgen-dependent muscle. We found that the pattern of expression of genes including c-myc, Fzd4 and Igf2 is associated with androgen-dependent changes in muscle mass; therefore, these genes are likely to be mediators of anabolic actions of androgens. Further research is required to identify the major targets of androgen actions in muscle, which are likely to include indirect actions via other tissues.

Expression of androgen receptor target genes in skeletal muscle

We aimed to determine the mechanisms of the anabolic actions of androgens in skeletal muscle by investigating potential androgen receptor (AR)-regulated genes in in vitro and in vivo models. The expression of the myogenic regulatory factor myogenin was significantly decreased in skeletal muscle from testosterone-treated orchidectomized male mice compared to control orchidectomized males, and was increased in muscle from male AR knockout mice that lacked DNA binding activity (AR^ΔZF2) versus wildtype mice, demonstrating that myogenin is repressed by the androgen/AR pathway. The ubiquitin ligase Fbxo32 was repressed by 12 h dihydrotestosterone treatment in human skeletal muscle cell myoblasts, and c-Myc expression was decreased in testosterone-treated orchidectomized male muscle compared to control orchidectomized male muscle, and increased in AR^ΔZF2 muscle. The expression of a group of genes that regulate the transition from myoblast proliferation to differentiation, Tceal7, p57^Kip2, Igf2 and calcineurin Aa, was increased in AR^ΔZF2 muscle, and the expression of all but p57^Kip2 was also decreased in testosterone-treated orchidectomized male muscle compared to control orchidectomized male muscle. We conclude that in males, androgens act via the AR in part to promote peak muscle mass by maintaining myoblasts in the proliferative state and delaying the transition to differentiation during muscle growth and development, and by suppressing ubiquitin ligase-mediated atrophy pathways to preserve muscle mass in adult muscle.

Normal phenotype in conditional androgen receptor (AR) exon 3-floxed neomycin-negative male mice

Androgens (testosterone and dihydrotestosterone) acting via the androgen receptor (AR) are required for male sexual differentiation, and also regulate the development of many other tissues including muscle, fat and bone. We previously generated an AR(lox) mouse line with exon 3 of the AR gene targeted by loxP sites. The deletion of exon 3 is in-frame, so only the DNA binding-dependent actions of the AR are deleted, but non-DNA binding-dependent actions are retained. This line also contained an antibiotic resistance selection cassette, neomycin (neo) in intron 3, which was also flanked by loxP sites. Hemizygous AR(lox) male mice demonstrated a phenotype of hyperandrogenization, with increased mass of androgen-dependent tissues. We hypothesized that this hyperandrogenization was likely to be due to the presence of the neo cassette. In this study, we have generated an AR(lox) neo-negative mouse line, using the EIIa-cre deleter mouse line to remove the neo cassette. Hemizygous AR(lox) neo-negative male mice have a normal phenotype, with normal body mass and normal mass of androgen-dependent tissues including the testis, seminal vesicles, kidney, spleen, heart and retroperitoneal fat. This neo-negative exon 3-targeted mouse line is the only floxed AR mouse line available to study the DNA binding-dependent actions of the AR in a tissue-specific manner, and is suitable for investigation in all tissues. This study demonstrates the importance of removing the selection cassette, which can potentially alter the phenotype of floxed mouse lines even in the absence of detectable effects on target gene expression.

Expression of Wnt signaling skeletal development genes in the cartilaginous fish, elephant shark (Callorhinchus milii)

Jawed vertebrates (Gnasthostomes) are broadly separated into cartilaginous fishes (Chondricthyes) and bony vertebrates (Osteichthyes). Cartilaginous fishes are divided into chimaeras (e.g. ratfish, rabbit fish and elephant shark) and elasmobranchs (e.g. sharks, rays and skates). Both cartilaginous fish and bony vertebrates are believed to have a common armoured bony ancestor (Class Placodermi), however cartilaginous fish are believed to have lost bone. This study has identified and investigated genes involved in skeletal development in vertebrates, in the cartilaginous fish, elephant shark (Callorhinchus milii). Ctnnb1 (β-catenin), Sfrp (secreted frizzled protein) and a single Sost or Sostdc1 gene (sclerostin or sclerostin domain-containing protein 1) were identified in the elephant shark genome and found to be expressed in a number of tissues, including cartilage. β-catenin was also localized in several elephant shark tissues. The expression of these genes, which belong to the Wnt/β-catenin pathway, is required for normal bone formation in mammals. These findings in the cartilaginous skeleton of elephant shark support the hypothesis that the common ancestor of cartilaginous fishes and bony vertebrates had the potential for making bone.

Age-dependent differences in androgen binding affinity in a family with spinal and bulbar muscular atrophy

OBJECTIVES

To investigate androgen receptor (AR) function in spinal and bulbar muscular atrophy (SBMA).

METHODS

A kindred was identified with five individuals carrying the AR gene CAG repeat expansion that causes SBMA. Androgen binding was measured in cultured genital skin fibroblasts from three affected individuals. One newborn, pre-symptomatic, individual showed normal androgen binding, but two older, symptomatic individuals showed a decrease in androgen binding affinity. This difference was not related to AR CAG repeat size, as all affected individuals in this kindred had 49 repeats (normal range 6-35). Post-mortem analysis in one subject confirmed the signs of androgen insufficiency in the testis, with marked seminiferous tubule atrophy, and the absence of germinal cells. The characteristic neuronal depletion in the anterior horn gray matter was also observed.

CONCLUSION

This report raises the possibility that age- or puberty-related changes in androgen binding could occur, which could potentially contribute to the progressive development of androgen resistance in affected men.

A physiological role for androgen actions in the absence of androgen receptor DNA binding activity

We tested the hypothesis that androgens have physiological actions via non-DNA binding-dependent androgen receptor (AR) signaling pathways in males, using our genetically modified mice that express a mutant AR with deletion of the 2nd zinc finger of the DNA binding domain (AR(ΔZF2)) that cannot bind DNA. In cultured genital skin fibroblasts, the mutant AR(ΔZF2) has normal ligand binding ability, phosphorylates ERK-1/2 in response to 1 min DHT treatment (blocked by the AR antagonist bicalutamide), but has reduced androgen-dependent nuclear localization compared to wildtype (WT). AR(ΔZF2) males have normal baseline ERK-1/2 phosphorylation, with a 1.5-fold increase in Akt phosphorylation in AR(ΔZF2) muscle vs WT. To identify physiological actions of non-DNA binding-dependent AR signaling, AR(ΔZF2) males were treated for 6 weeks with dihydrotestosterone (DHT). Cortical bone growth was suppressed by DHT in AR(ΔZF2) mice (6% decrease in periosteal and 7% decrease in medullary circumference vs untreated AR(ΔZF2) males). In conclusion, these data suggest that non-DNA binding dependent AR actions suppress cortical bone growth, which may provide a mechanism to fine-tune the response to androgens in bone.

Increased adiposity in DNA binding-dependent androgen receptor knockout male mice associated with decreased voluntary activity and not insulin resistance

In men, as testosterone levels decrease, fat mass increases and muscle mass decreases. Increased fat mass in men, in particular central obesity, is a major risk factor for type 2 diabetes, cardiovascular disease, and all-cause mortality. Testosterone treatment has been shown to decrease fat mass and increase fat-free mass. We hypothesize that androgens act directly via the DNA binding-dependent actions of the androgen receptor (AR) to regulate genes controlling fat mass and metabolism. The aim of this study was to determine the effect of a global DNA binding-dependent (DBD) AR knockout (DBD-ARKO) on the metabolic phenotype in male mice by measuring body mass, fat mass, food intake, voluntary physical activity, resting energy expenditure, substrate oxidation rates, serum glucose, insulin, lipid, and hormone levels, and metabolic gene expression levels and second messenger protein levels. DBD-ARKO males have increased adiposity despite a decreased total body mass compared with wild-type (WT) males. DBD-ARKO males showed reduced voluntary activity, decreased food intake, increased serum leptin and adiponectin levels, an altered lipid metabolism gene profile, and increased phosphorylated CREB levels compared with WT males. This study demonstrates that androgens acting via the DNA binding-dependent actions of the AR regulate fat mass and metabolism in males and that the increased adiposity in DBD-ARKO male mice is associated with decreased voluntary activity, hyperleptinemia and hyperadiponectinemia and not with insulin resistance, increased food intake, or decreased resting energy expenditure.

Ornithine decarboxylase is upregulated by the androgen receptor in skeletal muscle and regulates myoblast proliferation

The aim of this study is to determine if the Odc1 gene, which encodes ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, is directly regulated by the androgen receptor (AR) in skeletal muscle myoblasts and if Odc1 regulates myoblast proliferation and differentiation. We previously showed that expression of Odc1 is decreased in muscle from AR knockout male mice. In this study, we show in vivo that Odc1 expression is also decreased >60% in muscle from male muscle-specific AR knockout mice. In normal muscle homeostasis, Odc1 expression is regulated by age and sex, reflecting testosterone levels, as muscle of adult male mice expresses high levels of Odc1 compared with age-matched females and younger males. In vitro, expression of Odc1 is 10- and 1.5-fold higher in proliferating mouse C(2)C(12) and human skeletal muscle myoblasts, respectively, than in differentiated myotubes. Dihydrotestosterone increases Odc1 levels 2.7- and 1.6-fold in skeletal muscle cell myoblasts after 12 and 24 h of treatment, respectively. Inhibition of ODC activity in C(2)C(12) myoblasts by α-difluoromethylornithine decreases myoblast number by 40% and 66% following 48 and 72 h of treatment, respectively. In contrast, overexpression of Odc1 in C(2)C(12) myoblasts results in a 27% increase in cell number vs. control when cells are grown under differentiation conditions for 96 h. This prolonged proliferation is associated with delayed differentiation, with reduced expression of the differentiation markers myogenin and Myf6 in Odc1-overexpressing cells. In conclusion, androgens act via the AR to upregulate Odc1 in skeletal muscle myoblasts, and Odc1 promotes myoblast proliferation and delays differentiation.

Polyamines, androgens, and skeletal muscle hypertrophy

The naturally occurring polyamines, spermidine, spermine, and their precursor putrescine, play indispensible roles in both prokaryotic and eukaryotic cells, from basic DNA synthesis to regulation of cell proliferation and differentiation. The rate-limiting polyamine biosynthetic enzymes, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase, are essential for mammalian development, with knockout of the genes encoding these enzymes, Odc1 and Amd1, causing early embryonic lethality in mice. In muscle, the involvement of polyamines in muscle hypertrophy is suggested by the concomitant increase in cardiac and skeletal muscle mass and polyamine levels in response to anabolic agents including β-agonists. In addition to β-agonists, androgens, which increase skeletal mass and strength, have also been shown to stimulate polyamine accumulation in a number of tissues. In muscle, androgens act via the androgen receptor to regulate expression of polyamine biosynthetic enzyme genes, including Odc1 and Amd1, which may be one mechanism via which androgens promote muscle growth. This review outlines the role of polyamines in proliferation and hypertrophy, and explores their possible actions in mediating the anabolic actions of androgens in muscle.

DNA-binding-dependent androgen receptor signaling contributes to gender differences and has physiological actions in males and females

We used our genomic androgen receptor (AR) knockout (ARKO) mouse model, in which the AR is unable to bind DNA to: 1) document gender differences between males and females; 2) identify the genomic (DNA-binding-dependent) AR-mediated actions in males; 3) determine the contribution of genomic AR-mediated actions to these gender differences; and 4) identify physiological genomic AR-mediated actions in females. At 9 weeks of age, control males had higher body, heart and kidney mass, lower spleen mass, and longer and larger bones compared to control females. Compared to control males, ARKO males had lower body and kidney mass, higher splenic mass, and reductions in cortical and trabecular bone. Deletion of the AR in ARKO males abolished the gender differences in heart and cortical bone. Compared with control females, ARKO females had normal body weight, but 14% lower heart mass and heart weight/body weight ratio. Relative kidney mass was also reduced, and relative spleen mass was increased. ARKO females had a significant reduction in cortical bone growth and changes in trabecular architecture, although with no net change in trabecular bone volume. In conclusion, we have shown that androgens acting via the genomic AR signaling pathway mediate, at least in part, the gender differences in body mass, heart, kidney, spleen, and bone, and play a physiological role in the regulation of cardiac, kidney and splenic size, cortical bone growth, and trabecular bone architecture in females.

Oestradiol-induced spermatogenesis requires a functional androgen receptor

Spermatogenesis requires androgen but, paradoxically, oestradiol (E2) treatment stimulates spermatogenic development in gonadotrophin- and androgen-deficient hypogonadal (hpg) mice. The mechanisms of E2-induced spermatogenesis were investigated by determining intratesticular E2 levels and testis cell populations in E2-treated hpg male mice, and E2 spermatogenic actions were determined in androgen receptor-knockout (ARKO) mice. Despite increased serum E2 concentrations (150-300 pmol L(-1)), intratesticular E2 concentrations declined fivefold (P < 0.001) in E2-treated v. untreated hpg male mice. Serum FSH reached 40% of normal and total testicular numbers of known FSH-responsive Sertoli, spermatogonia and meiotic spermatocyte populations were significantly (P < 0.001) elevated 1.7-, 4- and 13-fold, respectively. However, E2 administration also increased androgen-dependent pachytene spermatocytes and post-meiotic spermatids to levels comparable with testosterone-treated hpg testes. Selective investigation of androgen receptor involvement used E2-treated ARKO mice, which were found to exhibit increased (1.6-fold; P < 0.05) intratesticular E2 concentrations and suppression of the elevated serum gonadotrophins, although FSH remained twofold higher than normal. However, testis size and total Sertoli, spermatogonia and spermatocyte numbers were not increased in E2-treated ARKO male mice. Therefore, E2-stimulated murine spermatogenic development occurs with markedly suppressed and not elevated intratesticular E2 levels and displays an absolute requirement for functional androgen receptors. We propose that this paradoxical E2 spermatogenic response is explained by predominantly extratesticular E2 actions, increasing FSH to combine with residual androgen activity in hpg testes to stimulate pre- to post-meiotic development.

Generation and analysis of an androgen-responsive myoblast cell line indicates that androgens regulate myotube protein accretion

Androgens have anabolic actions in skeletal muscle and could potentially act to: (a) increase proliferation of myoblasts; (b) delay differentiation to myotubes; and (c) induce protein accretion in post-proliferative myofibers. To identify the site of androgens action, we investigated the proliferative response of the C2C12 mouse myoblast cell line to testosterone and dihydrotestosterone (DHT) treatment. Neither androgens affected cell proliferation after up to 7 days treatment, nor was there a synergistic effect of androgens on the proliferative response of C2C12 cells to IGF-I treatment. However, proliferating C2C12 cells expressed 0.1% of the level of androgen receptor (AR) mRNA found in adult mouse gastrocnemius muscle (p<0.01). Therefore, we generated mouse C2C12 myoblast cell lines stably transfected with the mouse AR cDNA driven by the SV40 promoter (C2C12-AR). C2C12-AR cell proliferation, differentiation, and protein content were analyzed in response to androgen treatment. Our data demonstrated that androgen treatment does not alter either proliferation rate or differentiation rate of C2C12-AR cells. However, treatment of differentiated C2C12-AR myotubes with 100 nM DHT for 3 days caused a 20% increase in total protein content vs vehicle treatment (p<0.05). This effect was not observed in control C2C12 cells transfected with empty vector. These data suggest that androgens act via the AR to upregulate myotube protein content. This model cell line will be useful to further investigate the molecular mechanisms via which androgens regulate protein accretion.

A floxed allele of the androgen receptor gene causes hyperandrogenization in male mice

We previously generated a conditional floxed mouse line to study androgen action, in which exon 3 of the androgen receptor (AR) gene is flanked by loxP sites, with the neomycin resistance gene present in intron 3. Deletion of exon 3 in global AR knockout mice causes androgen insensitivity syndrome, characterized by genotypic males lacking normal masculinization. We now report that male mice carrying the floxed allele (AR(lox)) have the reverse phenotype, termed hyperandrogenization. AR(lox) mice have increased mass of androgen-dependent tissues, including kidney, (P < 0.001), seminal vesicle (P < 0.001), levator ani muscle (P = 0.001), and heart (P < 0.05). Serum testosterone is not significantly different. Testis mass is normal, histology shows normal spermatogenesis, and AR(lox) males are fertile. AR(lox) males also have normal AR mRNA levels in kidney, brain, levator ani, liver, and testis. This study reaffirms the need to investigate the potential phenotypic effects of floxed alleles in the absence of cre in tissue-specific knockout studies. In addition, this androgen hypersensitivity model may be useful to further investigate the effects of subtle perturbations of androgen action in a range of androgen-responsive systems in the male.

Current and future approaches using genetically modified mice in endocrine research

Genetically modified mouse models have been used widely to advance our knowledge in the field of endocrinology and metabolism. A number of different approaches to generate genetically modified mice are now available, which provide the power to analyze the role of individual proteins in vivo. However, there are a number of points to be considered in the use and interpretation of these models. This review discusses the advantages and disadvantages involved in the generation and use of different genetically modified mouse models in endocrine research, including conventional techniques (e.g., overexpression, knockout, and knock-in models), tissue- and/or time-specific deletion of target genes [e.g., Cre-loxP and short interfering (si)RNA transgenic approaches], and gene-trap approaches to undertake functional genomics. This review also highlights the many factors that should be considered when assessing the phenotype of these mouse models, many of which are relevant to all murine physiological studies. These approaches are a powerful means by which to dissect the function of genes and are revolutionizing our understanding of endocrine physiology and metabolism.

Continuous testosterone administration prevents skeletal muscle atrophy and enhances resistance to fatigue in orchidectomized male mice

Androgens promote anabolism in skeletal muscle; however, effects on subsequent muscle function are less well defined because of a lack of reliable experimental models. We established a rigorous model of androgen withdrawal and administration in male mice and assessed androgen regulation of muscle mass, structure, and function. Adult C57Bl/6J male mice were orchidectomized (Orx) or sham-operated (Sham) and received 10 wk of continuous testosterone (T) or control treatment (C) via intraperitoneal implants. Mass, fiber cross-sectional area (CSA), and in vitro contractile function were assessed for fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles. After 10 wk, Orx+C mice had reduced body weight gain (P < 0.05), seminal vesicle mass (P < 0.01), and levator ani muscle mass (P < 0.001) compared with Sham+C mice, and these effects were prevented with testosterone treatment. Orx+T mice had greater EDL (P < 0.01) and SOL (P < 0.01) muscle mass compared with Orx+C mice; however, median fiber CSA was not significantly altered in these muscles. EDL and SOL muscle force was greater in Sham+T compared with Orx+C mice (P < 0.05) in proportion to muscle mass. Unexpectedly, Orx+T mice had increased fatigue resistance of SOL muscle compared with Orx+C mice (P < 0.001). We used a rigorous model of androgen withdrawal and administration in male mice to demonstrate an essential role of androgens in the maintenance of muscle mass and force. In addition, we showed that testosterone treatment increases resistance to fatigue of slow- but not fast-twitch muscle.

Double-strand DNA break repair with replication slippage on two strands: a novel mechanism of deletion formation

We have characterized an unusual family with two different androgen receptor (AR) gene deletions, in which we propose a novel mechanism of deletion formation has occurred. Affected individuals have the X-linked disorder androgen insensitivity syndrome, and we previously showed that different family members have deletions of different exons of the AR gene. We have now fully sequenced the deletions from affected individuals, and confirmed the presence of different deletions in different affected family members. Most affected and heterozygote individuals have a 4,430-bp deletion of exon 5 that occurred between repeated GTGGCAT motifs in introns 4 and 5. One affected hemizygous individual has a 4,033-bp deletion of exons 6 and 7 that occurred between repeated CCTC motifs in introns 5 and 7. The intron 5 breakpoint junctions of the two deletions are only 11 bp apart. Surprisingly, the maternal grandmother of the original index case was found to be mosaic for both deletional events, as well as having the normal AR gene. Karyotyping ruled out 47,XXX trisomy, indicating triple mosaicism for the two different deleted AR alleles and a normal AR allele. This triple mosaicism must have occurred early in embryonic development, as both deletions were passed on to different children. Based on these findings, we propose a novel mechanism of deletion formation. We suggest that during AR gene replication, a double strand DNA break occurred in intron 5, and that a variant of replication slippage occurred on both newly synthesized strands between the repeat motifs of microhomology, leading to the formation of the two different AR gene deletions.

Androgen regulation of satellite cell function

Androgen treatment can enhance the size and strength of muscle. However, the mechanisms of androgen action in skeletal muscle are poorly understood. This review discusses potential mechanisms by which androgens regulate satellite cell activation and function. Studies have demonstrated that androgen administration increases satellite cell numbers in animals and humans in a dose-dependent manner. Moreover, androgens increase androgen receptor levels in satellite cells. In vitro, the results are contradictory as to whether androgens regulate satellite cell proliferation or differentiation. IGF-I is one major target of androgen action in satellite cells. In addition, the possibility of non-genomic actions of androgens on satellite cells is discussed. In summary, this review focuses on exploring potential mechanisms through which androgens regulate satellite cells, by analyzing developments from research in this area.

Increased frequency of long androgen receptor CAG repeats in male breast cancers

We have investigated the possible link between androgen hyposensitivity caused by long androgen receptor (AR) CAG repeats, and breast carcinogenesis, in men. AR gene mutations have been described in men with androgen insensitivity syndrome and breast carcinoma, and some studies have shown long CAG repeats are associated with increased risk of breast cancer in women. DNA was isolated from male breast cancer biopsies, and the AR CAG repeat sized. Forty one male breast cancer samples were studied, including one sample from a man with spinal and bulbar muscular atrophy (SBMA), which is caused by an AR CAG repeat expansion. The man with breast cancer and SBMA had 49 CAG repeats (normal range 6-35), but all other breast cancer samples had repeats within the normal range. The frequency of CAG repeats > or =24 was significantly higher in the breast cancer group (excluding the SBMA subject) than in the normal population (p<0.05), and was more marked in grade I and II tumors (p=0.001). There was no correlation between AR CAG repeat length and age at diagnosis. In conclusion, longer AR CAG repeats are more common in men with breast cancer than in the control male population. Androgen hyposensitivity, caused by long AR CAG repeats, may increase the risk of breast cancer in men.

Localization of Indian hedgehog and PTH/PTHrP receptor expression in relation to chondrocyte proliferation during mouse bone development

We have developed a useful approach to examine the pattern of gene expression in comparison to cell proliferation, using double in situ hybridization and immunofluorescence. Using this system, we examined the expression of Indian hedgehog (Ihh) and PTH/PTHrP receptor (PPR) mRNA in relation to chondrocyte proliferation during embryonic mouse bone development. Both genes are expressed strongly in prehypertrophic and early hypertrophic chondrocytes, and there is a strong correlation between upregulation of both Ihh and PPR expression and chondrocyte cell cycle arrest. At embryonic day (E14.5), PPR mRNA upregulation begins in the columnar chondrocytes just prior to cell cycle exit, but at later time points expression is only observed in the postproliferative region. In contrast, Ihh mRNA expression overlaps slightly with the region of columnar proliferating chondrocytes at all stages. This study provides further evidence that in the developing growth plate, cell cycle exit and upregulation of Ihh and PPR mRNA expression are coupled.

Expression of Stra13 during mouse endochondral bone development

We have examined the expression of the basic helix-loop-helix factor Stra13 (DEC1/Sharp2) during endochondral bone development in the mouse. Stra13 expression was examined by in situ hybridization in the tibia from E14.5-E18.5, and at post-natal day 24. At E14.5, expression of Stra13 mRNA was very low, with expression limited to scattered hypertrophic chondrocytes. At E15.5 Stra13 mRNA was present in post-mitotic hypertrophic chondrocytes, co-localizing with collagen X expression. At E16.5-E18.5, Stra13 was expressed in both the proliferating chondrocytes and in the late hypertrophic chondrocytes. At E15.5-E18.5, Stra13 expression was also observed in the primary spongiosa. Stra13 expression was also maintained in the 24-day post-natal tibia, with expression detectable only in the late hypertrophic chondrocytes. Because Stra13 has been shown to be induced by hypoxia, and the growth plate is hypoxic during embryonic development, we compared the expression pattern of Stra13 and the HIF1-alpha target gene VEGF. VEGF is expressed predominantly in the late hypertrophic chondrocytes, with lower expression in the proliferating chondrocytes. Thus, there was a large degree of overlap in the expression patterns of Stra13 and VEGF in chondrocytes during embryonic development.

Genetically modified animal models as tools for studying bone and mineral metabolism

Genetic modification of mice is a powerful tool for the study of bone development and metabolism. This review discusses the advantages and disadvantages of various approaches used in bone-related research and the contributions these studies have made to bone biology. Genetic modification of mice is a powerful tool for the study of bone development and metabolism. This review discusses the advantages and disadvantages of various approaches used in bone-related research and the contributions these studies have made to bone biology. The approaches to genetic modification included in this review are (1) overexpression of genes, (2) global gene knockouts, (3) tissue-specific gene deletion, and (4) gene knock-in models. This review also highlights issues that should be considered when using genetically modified animal models, including the rigorous control of genetic background, use of appropriate control lines, and confirmation of tissue specificity of gene expression where appropriate. This technology provides a unique and powerful way to probe the function of genes and is already revolutionizing our approach to understanding the physiology of bone development and metabolism.

The cyclin-dependent kinase inhibitor p57(Kip2) mediates proliferative actions of PTHrP in chondrocytes

Parathyroid hormone-related peptide (PTHrP) is a positive regulator of chondrocyte proliferation during bone development. In embryonic mice lacking PTHrP, chondrocytes stop proliferating prematurely, with accelerated differentiation. Because the bone phenotype of mice lacking the cyclin-dependent kinase inhibitor p57(Kip2) is the opposite of the PTHrP-null phenotype, we hypothesized that PTHrP's proliferative actions in chondrocytes might be mediated by opposing p57. We generated p57/PTHrP-null embryos, which showed partial rescue of the PTHrP-null phenotype. There was reversal of the loss of proliferative chondrocytes in most bones, with reversal of the accelerated differentiation that occurs in the PTHrP-null phenotype. p57 mRNA and protein were upregulated in proliferative chondrocytes in the absence of PTHrP. Metatarsal culture studies confirmed the action of PTHrP to decrease p57 mRNA and protein levels in a model in which parathyroid hormone (PTH), used as an analog of PTHrP, increased chondrocyte proliferation rate and the length of the proliferative domain. PTH treatment of p57-null metatarsals had no effect on proliferation rate in round proliferative chondrocytes but still stimulated proliferation in columnar chondrocytes. These studies suggest that the effects of PTHrP on both the rate and extent of chondrocyte proliferation are mediated, at least in part, through suppression of p57 expression.

Application of differential display in the identification of androgen-regulated genes

Identification of androgen-regulated genes in neurons is an important step in understanding the mechanisms involved in androgen action. The aim of the current study was to identify androgen-responsive genes in the neural cells using the technique of differential display reverse transcription polymerase chain reaction (DDRT-PCR) on the human neuroblastoma cell line, SK-N-MC. Using this analysis, 18 putatively androgen-regulated cDNA species were identified, ranging in size from 280 to 800 bp. Of these, 14 were found to be negatively regulated and 4 positively regulated by androgens. Only 12 were successfully re-amplified, and of these, 8 were found to contain multiple species of cDNA fragments. When Northern analysis was conducted using the 21 different cDNA fragments as probes, only one was found to confirm the androgen regulation demonstrated via DDRT-PCR. While this putatively regulated gene remains to be fully characterized, future studies of may provide insights into the molecular mechanisms governing androgen action in neural cells.

Novel androgen receptor gene mutations in Australian patients with complete androgen insensitivity syndrome

We have identified androgen receptor (AR) gene mutations in eight Australian subjects with complete androgen insensitivity syndrome (AIS). Four individuals, from three families, have novel mutations that introduce premature termination codons. Two siblings have the nonsense mutation Glu681X, and another subject has the nonsense mutation p.Ser884X. The other subject has a CA insertion at codon 829 (c.2847_2848insCA), causing a frameshift mutation that introduces four nonsense amino acids prior to a Stop codon. All the termination codons occur in the ligand binding domain, and cause reduced androgen binding in patient genital skin fibroblasts. Four further patients have missense mutations. One subject has two different mutations, p.Ala645Asp in the hinge region of the receptor, and p.Arg752Gln in the ligand binding domain. Both these mutations have previously been reported in patients with AIS, but the combination of these two mutations in one subject is unique. Another subject has a novel c.2533G>C transversion at the first nucleotide in exon 5, introducing the amino acid change p.Gly724Ala at a highly conserved residue in the ligand binding domain. Androgen binding is normal in fibroblasts from this subject, although other point mutations at this amino acid totally abolish binding. Two other subjects have mutations previously described as causing AIS, namely p.Arg779Trp and p.Val889Met substitutions in the ligand binding domain of the receptor. The p.Arg779Trp mutation is associated with the detection of a truncated AR protein in this patient's fibroblasts, suggesting the mutation renders the receptor susceptible to proteolysis.

Absence of transcription factor c-maf causes abnormal terminal differentiation of hypertrophic chondrocytes during endochondral bone development

In this study, we report that the transcription factor c-Maf is required for normal chondrocyte differentiation during endochondral bone development. c-maf is expressed in hypertrophic chondrocytes during fetal development (E14.5-E18.5), with maximal expression in the tibia occurring at E15.5 and E16.5, in terminally differentiated chondrocytes. In c-maf-null mice, fetal bone length is decreased approximately 10%, and hypertrophic chondrocyte differentiation is perturbed. There is an initial decrease in the number of mature hypertrophic chondrocytes at E15.5 in c-maf-null tibiae, with decreased expression domains of collagen X and osteopontin, markers of hypertrophic and terminal hypertrophic chondrocytes, respectively. By E16.5, there is an expanded domain of late hypertrophic, osteopontin-positive chondrocytes in the c-maf-/-. This accumulation of hypertrophic chondrocytes persists and is still observed at 4 weeks of age. These data suggest that c-Maf facilitates the initial chondrocyte terminal differentiation and influences the disappearance of hypertrophic chondrocytes. BrdU and TUNEL analyses show normal proliferation rate and apoptosis in the c-maf-null. There is a specific decrease in MMP-13 expression at E15.5 in the c-maf-null. MMP-13 is known to be regulated by AP-1 and may also be a target of c-Maf. Thus, cartilage is a novel system in which c-Maf acts during development, where c-Maf is required for normal chondrocyte differentiation.

p107 and p130 Coordinately regulate proliferation, Cbfa1 expression, and hypertrophic differentiation during endochondral bone development

During endochondral bone development, both the chondrogenic differentiation of mesenchyme and the hypertrophic differentiation of chondrocytes coincide with the proliferative arrest of the differentiating cells. However, the mechanisms by which differentiation is coordinated with cell cycle withdrawal, and the importance of this coordination for skeletal development, have not been defined. Through analysis of mice lacking the pRB-related p107 and p130 proteins, we found that p107 was required in prechondrogenic condensations for cell cycle withdrawal and for quantitatively normal alpha1(II) collagen expression. Remarkably, the p107-dependent proliferative arrest of mesenchymal cells was not needed for qualitative changes that are associated with chondrogenic differentiation, including production of Alcian blue-staining matrix and expression of the collagen IIB isoform. In chondrocytes, both p107 and p130 contributed to cell cycle exit, and p107 and p130 loss was accompanied by deregulated proliferation, reduced expression of Cbfa1, and reduced expression of Cbfa1-dependent genes that are associated with hypertrophic differentiation. Moreover, Cbfa1 was detected, and hypertrophic differentiation occurred, only in chondrocytes that had undergone or were undergoing a proliferative arrest. The results suggest that Cbfa1 links a p107- and p130-mediated cell cycle arrest to chondrocyte terminal differentiation.

Androgen insensitivity syndrome in the era of molecular genetics and the Internet: a point of view

The era of molecular genetics has seen the discovery of a great deal of scientific information about the androgen receptor (AR) and about the many AR mutations that have been identified in patients with Androgen Insensitivity Syndrome (AIS). In families with well-characterised mutations, carriers can now be identified and prenatal testing can be offered. An unexpected finding is that an AR mutation also causes X-linked spinobulbar muscular atrophy. The intersex community has established two influential support groups, the AIS Support Group (which has branches in the UK, North America and Australia) and the Intersex Society of North America (ISNA). It is ironic that at a time when advances in biomedical science regarding AIS are a source of pride, these support groups are accusing the medical profession of having ignored the real needs of patients with AIS. Since the support groups are willing to assist the medical profession to develop better approaches to the management of intersex disorders, a collaborative approach is likely to be mutually beneficial for patients and physicians. ISNA has alienated many doctors by advocating a radical approach, namely that surgery should not be performed to 'correct' ambiguous genitalia until the individual is old enough to express a gender preference. Many children born in developing countries have either no genital surgery to correct ambiguity, or surgery is carried out very late. Long term outcome studies, carried out in developing countries and sensitive to the cultural background, would provide information on how non-Western societies can accept genital abnormalities that would be considered unacceptable in the West.

Localization of functional domains in the androgen receptor

Functional domains of the androgen receptor (AR) have been localized through a combination of studies on naturally occurring AR gene mutations, in vitro mutagenesis studies and comparison with the structure of other members of the steroid/nuclear receptor superfamily. Two activation domains exist within the amino-terminal domain, and a ligand-dependent activation domain is present in the ligand binding domain. The poly(Gln) stretch within the amino-terminal domain may inhibit the transactivation function of the receptor. Different ligands or binding to different promoters may recruit the use of different activation domains, which may provide promoter-specific effects of receptor action. Co-activator proteins that modulate or enhance AR action have been identified, many of which interact with the ligand binding domain of the AR. Tissue-specific expression of such co-activators, and promoter-specific protein interactions, may also help control the specificity of androgen action. Target Ser residues for phosphorylation have been identified, which may be the site of action for cross-talk from protein kinase signalling pathways. However, the role of phosphorylation in AR function in general is still unclear. It is now clear that interactions occur between receptor domains, modulating functions including ligand dissociation, dimerization and transactivation. By studying the functional domains of the AR, and how they control receptor function in response to different activation signals, we are beginning to understand the mechanisms controlling the specificity of receptor action.

Intersex disorders: shedding light on male sexual differentiation beyond SRY

Male sexual differentiation involves a cascade of events initiated by the presence on the Y chromosome of the SRY gene, which causes the indifferent gonad to develop into a testis. Hormonal products of the testis, predominantly testosterone and Müllerian inhibiting substance (MIS), then control the sexual differentiation of the developing foetus. SRY is a transcription factor; however, target genes for its action have yet to be identified, because the DNA recognition sequence for SRY is found in many genes. Therefore the study of intersex disorders is being used to identify other genes active in the pathway of sexual differentiation. Genes identified as being important in the differentiation of the indifferent gonad include WT1 (abnormal in Denys Drash syndrome) and SF-1. The DSS locus may contain a gene that controls ovarian differentiation, and SOX9 (identified from campomelic dysplasia) is required for testis differentiation. In addition to playing a role in the development of the bipotential gonad, SF-1 may also activate MIS gene expression in the testis, causing regression of Müllerian structures. Luteinizing hormone and its receptor are required for Leydig cell differentiation, and the testosterone biosynthetic enzymes (P450scc, 3 beta-hydroxysteroid dehydrogenase, P45017 alpha and 17 beta-hydroxysteroid dehydrogenase) are all necessary for masculinization of external genitalia. 5 alpha-Reductase is required for the production of dihydrotestosterone, and the androgen receptor mediates the action of both testosterone and dihydrotestosterone. The identification of abnormal genes in other disorders of sexual differentiation is likely to provide further information about the factors required for testicular development and function.

Spinal and bulbar muscular atrophy: androgen receptor dysfunction caused by a trinucleotide repeat expansion

Kennedy's disease (spinal and bulbar muscular atrophy) is an X-linked form of motor neuron disease that affects adult men. The syndrome is characterized by progressive atrophy of the limb muscles, pelvic and shoulder girdles and dysphagia and dysarthria, and is caused by the degeneration of spinal and bulbar motor neurons. Kennedy's disease is caused by a trinucleotide repeat expansion of a CAG repeat in exon A of the androgen receptor gene, and is one of a group of neurological diseases caused by trinucleotide repeat expansions in different genes. The mutation in Kennedy's disease involves an increased number of glutamine residues in the amino-terminal domain of the receptor. Point mutations and deletions in the androgen receptor gene cause androgen insensitivity syndrome, however subjects with Kennedy's disease have normal virilization, although progressive gynaecomastia, testicular atrophy and infertility may occur. Androgen receptors are expressed widely in the normal brain, and in the anterior horn cells of the spinal cord; however, their role in neuronal tissue is not known, nor is it known how the androgen receptor gene mutation causes neuronal degeneration. Kennedy's disease is likely to be a 'gain of function' abnormality, so that the presence of the receptor with an increased number of glutamines is toxic to motor neurons. It is possible that the mutation alters interaction of the receptor with other neuronal transcription factors, or neuronotoxicity may occur because of a non-specific effect caused by the presence of a protein with a large homoglutamine domain. Studies of patients with Kennedy's disease have shown that expression of androgen receptor mRNA and protein in spinal cord may be decreased, as can be the affinity of the mutant receptor for androgen. In vitro studies have shown impaired transcription activation ability of the mutant androgen receptor. The age at onset of Kennedy's disease may correlate with the size of the CAG repeat, however there is a large degree of variability of age at onset between subjects with the same number of repeats. Further study of the effect of the Kennedy's disease mutation on androgen receptor function in motor neurons will allow us to increase our understanding of the pathogenesis of this disease.

Defects of androgen receptor function: from sex reversal to motor neurone disease

The androgen receptor (AR) is a ligand-dependent DNA transcription factor that binds androgens which cause masculinisation of the developing male fetus. Classical abnormalities of receptor function result in the syndrome of androgen resistance, with resultant failure of normal male differentiation. In more recent years, however, mutations in the AR gene have been described in a number of diverse clinical conditions, from male infertility to prostate and breast cancer through to a form of motor neurone disease (Kennedy's disease). This review discusses the various AR gene mutations found in androgen insensitivity syndrome (AIS) and the other conditions described above, and relates how different mutations, or disruption of different functional domains, contributes to the various phenotypes. Mutations that cause complete AIS usually disrupt the DNA or steroid binding ability of the receptor. In partial AIS, mutations generally decrease receptor affinity for ligand, affect thermostability of the protein, or affect the ability of the receptor to activate transcription of responsive genes. Isolated mutations occur in the steroid binding domain of the receptor in prostate cancer, and many cancers have an identical mutation. Similarly, in the two cases of male breast cancer in which AR gene mutations have been described, the mutations in the DNA binding domain of the receptor are alike. In Kennedy's disease a trinucleotide repeat expansion occurs in exon A of the AR gene, which appears to affect ability of the receptor to bind ligand and activate transcription, although the mechanism of neuronal degeneration remains unknown.

Androgen receptor binding studies on heterozygotes in a family with androgen insensitivity syndrome

A large family with androgen insensitivity syndrome (AIS) has been investigated, in order to detect carrier individuals and to investigate their AR binding. We have previously reported that different affected members of this family have different AR gene deletions, and, testing normal female relatives of the affected individuals, we have identified three heterozygote females all of whom carry a deletion of exon E of the AR gene. Androgen binding capacity was measured in cultured genital skin fibroblasts from normal male and female controls, the affected, and the heterozygote individuals in this family. A significant difference was found between the binding ranges for normal male foreskin and suprapubic skin fibroblasts (P < 0.005), and the three individuals with AIS had very low androgen binding capacity in their genital skin fibroblasts (2.6, 5.0, and 3.4 fmol 3HR1881/mg protein) compared to the normal range. The heterozygote females all had binding within the normal female suprapubic skin fibroblast range (12.5, 6.1, and 6.4 fmol 3HR1881/mg protein, respectively, for the three heterozygotes). Thus, we conclude that the absence of one functional AR gene in heterozygote females has not effect on AR binding capacity in cultured genital skin fibroblasts. In addition, bone mineral density was measured in the affected aunt and found to be significantly lowered at the lumbar spine (Z = -2.81) and hip (Z = -1.54); however, the role of the AR in determination of bone mineral density remains to be elucidated.

Abnormal androgen receptor binding affinity in subjects with Kennedy's disease (spinal and bulbar muscular atrophy)

We have investigated androgen-binding properties of the androgen receptor (AR) in cultured suprapubic skin fibroblasts from six subjects with Kennedy's disease (X-linked spinal and bulbar muscular atrophy). Binding of the synthetic androgen methyltrienolone (R1881) was measured in a monolayer assay, and Scatchard analysis was performed to determine the total number of binding sites and the apparent binding affinity (Kd) of the AR for androgen. Five of the six subjects investigated had an abnormal apparent binding affinity, with Kd values ranging from 0.34-11.7 nmol/L, more than 2 SD from the mean of the normal range (0.19 +/- 0.06 nmol/L). In this group of six patients, there was a significant correlation between the AR Kd and the severity of testicular atrophy and gynecomastia. The number of CAG repeats in the expanded region of exon A of the AR gene was determined in all subjects from whom suprapubic skin fibroblasts were cultured and an additional 12 subjects with Kennedy's disease. In the total group of 18 subjects investigated, there was a trend for an increasing number of CAG repeats associated with decreasing age at onset of different symptoms; however, this correlation was not statistically significant. Thus, we report for the first time a quantitative abnormality of the AR apparent binding affinity in subjects with Kennedy's disease, which appears to be related to the severity of the symptoms of androgen insensitivity.

Kennedy's disease: genetic diagnosis of an inherited form of motor neuron disease

Kennedy's disease (X-linked spinal and bulbar muscular atrophy) is an inherited form of motor neuron disease that may be diagnosed genetically using the polymerase chain reaction (PCR). This form of motor neuron disease principally affects the proximal limb girdle muscles as well as those involved with deglutition and phonation. Onset is usually late, in the fourth to fifth decades of life, and progression is slow. Moderate gynaecomastia and testicular atrophy are usually present, suggesting a defect in androgen receptor function. Being inherited in an X-linked recessive manner, only males are affected, with females as the unaffected carriers. The genetic abnormality that causes Kennedy's disease is an enlargement of the androgen receptor (AR) gene, which is located on the proximal long arm of the X chromosome. In patients with this disease, a region in the gene containing repeated CAG triplet nucleotides is approximately twice the size of that found in normal people. Using PCR to amplify this region of the AR gene, this study confirms this genetic mutation in 12 males from eight different families. All these families live on the east coast of Australia. This mutation was not found in five patients with other forms of motor neuron disease. Twelve heterozygote females, the daughters of affected males and carrier females, have also been identified. In addition, there are 14 asymptomatic and as yet untested sons of carriers, ranging in age from less than one year to over 40 years of age. Each has a 50% chance of inheriting the abnormal gene from his mother and thus developing Kennedy's disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Related individuals with different androgen receptor gene deletions

We have identified different members of one family affected by androgen insensitivity syndrome who have deletions of different exons of the X-linked androgen receptor (AR) gene. Two affected (XY) siblings have a deletion of exon E of the AR gene and their affected (XY) aunt has a normal exon E, but a deletion of exons F and G of the same gene. The mother and maternal grandmother of the children both carry the exon E deletion, but not the exon F, G deletion. Both deletions are 5 kb in length and have one breakpoint within a 200-bp region in intron 5; however, they extend in opposite directions. The probability that these two different deletions have arisen at random is extremely low, but the cause of this intriguing phenomenon remains to be found.