Androgen Receptor In Skeletal Muscle Research Articles

Lifespan Effects of Muscle-specific Androgen Receptor Overexpression on Body Composition of Male and Female Rats

Androgenic actions of gonadal testosterone are thought to be a major mechanism promoting sex differences in body composition across the lifespan. However, this inference is based largely on studies of androgen receptor (AR) function in late adolescent or emerging adult rodents. Here we assess body composition and quantify AR expression in skeletal muscle of rats at defined ages, comparing wild-type (WT) to transgenic HSAAR rats which overexpress AR in skeletal muscle. Male and female HSAAR and WT Sprague Dawley rats (N=288) underwent DEXA scanning and tissue collection at post-natal day (PND) 1, 10, 21, 42, 70, 183, 243, and 365. As expected, sex differences in body composition and muscle mass largely onset with puberty (PND-21), with no associated changes to skeletal muscle AR protein. In adulthood, HSAAR increased tibialis anterior (TA) and EDL mass in males, and reduced the expected gain in gonadal fat mass in both sexes. In WT rats, AR protein was reduced in soleus, but not TA, throughout life. Nonetheless, soleus AR protein expression was greater in males than females at all ages of sexual development, yet only at PND-70 in TA. Overall, despite muscle AR overexpression effects, results are inconsistent with major sex differences in body composition during sexual development being driven by changes in muscle AR, but rather suggest that changes in ligand promote sexual differentiation of body composition during pubertal timing. Nonetheless, increased skeletal muscle AR in adulthood can be suffcient to increase muscle mass in males, and reduce adipose in both sexes. This work was supported by an NSERC Grant awarded to D.A.M. (RGPIN 312458). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Lifespan Effects of Muscle-Specific Androgen Receptor Overexpression on Body Composition of Male and Female Rats.

Androgenic actions of gonadal testosterone are thought to be a major mechanism promoting sex differences in body composition across the lifespan. However, this inference is based on studies of androgen receptor (AR) function in late adolescent or emerging adult rodents. Here we assess body composition and AR expression in skeletal muscle of rats at defined ages, comparing wild-type (WT) to transgenic human skeletal actin-driven AR overexpression (HSAAR) rats which overexpress AR in skeletal muscle. Male and female HSAAR and WT Sprague Dawley rats (N = 288) underwent dual-energy x-ray absorptiometry (DXA) scanning and tissue collection at postnatal day (PND) 1, 10, 21, 42, 70, 183, 243, and 365. Expected sex differences in body composition and muscle mass largely onset with puberty (PND-21), with no associated changes to skeletal muscle AR protein. In adulthood, HSAAR increased tibialis anterior (TA) and extensor digitorum longus mass in males, and reduced the expected gain in gonadal fat mass in both sexes. In WT rats, AR protein was reduced in soleus, but not TA, throughout life. Nonetheless, soleus AR protein expression was greater in male rats than female rats at all ages of sexual development, yet only at PND-70 in TA. Overall, despite muscle AR overexpression effects, results are inconsistent with major sex differences in body composition during sexual development being driven by changes in muscle AR, rather suggesting that changes in ligand promote sexual differentiation of body composition during pubertal timing. Nonetheless, increased skeletal muscle AR in adulthood can be sufficient to increase muscle mass in males, and reduce adipose in both sexes.

Soy Protein Supplementation Is Not Adipogenic Or Estrogenic In Young Men When Combined With Resistance Training

PURPOSE: Sex hormone physiology (e.g., estradiol, testosterone) may be affected by soy and/or whey protein consumption. Alterations in sex hormones due to resistance training (RT) and/or protein supplementation may explain meaningful variation in adipocyte and skeletal myocyte size alterations. Consequential molecular signaling in these cell types remain unclear. Therefore, we examined effects of RT and soy (SPC), whey (WPC), or placebo (PLA) supplementation in young men. METHODS: 47 healthy, young men were partitioned into PLA, SPC, or WPC groups and completed 12 weeks of RT. Body composition, serum hormones, androgen signaling markers in myocytes, and estrogen signaling markers in adipocytes were examined using DXA, ELISA, western blotting, PCR, and immunohistochemistry. RESULTS: Testosterone increased over time, but more so in subjects consuming WPC (p<0.05). Adipocyte mRNA expression of the estrogen receptor alpha increased (p<0.05), as did hormone sensitive lipase over time (p<0.05). Skeletal muscle androgen receptor mRNA expression increased while ornithine decarboxylase mRNA decreased over time (p<0.05). Alterations in body composition, adipocyte, and myocyte morphology were not significantly different between groups (p>0.05). Changes in 17β-estradiol and testosterone explained <3% of alterations in adipocyte and myocyte size. CONCLUSIONS: These data suggest primarily RT-mediated effects with little influence of protein type and hormonal changes.

Effects of resistance training on testosterone metabolism in younger and older men

This study investigated the effects of resistance training (RT) on the metabolism of testosterone (T) in younger (n=5, 28±3yrs.) and older (n=8, 70±2yrs.) men. Experimental heavy resistance exercises (5×10RM leg presses) were performed before and after a 12-month of RT. No age differences were found in the production or metabolic clearance rate of T (determined by stable isotope dilution method), skeletal muscle androgen receptor content or serum LH concentrations due to acute or chronic RT. The T production capacity response to gonadotropin stimulation and the concentrations of the urinary T metabolites (androsterone and etiocholanolone) were lower in the older compared to younger men (p<0.05–0.01). This study further showed that RT may have acute effect on T production and clearance rates, while the exercise-induced increases in serum T appeared to be induced by decreased metabolic clearance rate of T. Attenuated T production capacity and urinary excretion of T metabolites in older men may reflect the known reduction in testicular steroidogenesis upon aging. No changes were observed in T metabolism due to RT indicating a homeostatic stability for this hormone in men of different ages.

Muscular and Systemic Correlates of Resistance Training-Induced Muscle Hypertrophy

PurposeTo determine relationships between post-exercise changes in systemic [testosterone, growth hormone (GH), insulin like grow factor 1 (IGF-1) and interleukin 6 (IL-6)], or intramuscular [skeletal muscle androgen receptor (AR) protein content and p70S6K phosphorylation status] factors in a moderately-sized cohort of young men exhibiting divergent resistance training-mediated muscle hypertrophy.MethodsTwenty three adult males completed 4 sessions•wk-1 of resistance training for 16 wk. Muscle biopsies were obtained before and after the training period and acutely 1 and 5 h after the first training session. Serum hormones and cytokines were measured immediately, 15, 30 and 60 minutes following the first and last training sessions of the study.ResultsMean fiber area increased by 20% (range: -7 to 80%; P<0.001). Protein content of the AR was unchanged with training (fold change = 1.17 ± 0.61; P=0.19); however, there was a significant correlation between the changes in AR content and fiber area (r=0.60, P=0.023). Phosphorylation of p70S6K was elevated 5 hours following exercise, which was correlated with gains in mean fiber area (r=0.54, P=0.007). There was no relationship between the magnitude of the pre- or post-training exercise-induced changes in free testosterone, GH, or IGF-1 concentration and muscle fiber hypertrophy; however, the magnitude of the post exercise IL-6 response was correlated with muscle hypertrophy (r=0.48, P=0.019).ConclusionPost-exercise increases in circulating hormones are not related to hypertrophy following training. Exercise-induced changes in IL-6 correlated with hypertrophy, but the mechanism for the role of IL-6 in hypertrophy is not known. Acute increases, in p70S6K phosphorylation and changes in muscle AR protein content correlated with muscle hypertrophy implicating intramuscular rather than systemic processes in mediating hypertrophy.

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.

Recovery after Heavy Resistance Exercise and Skeletal Muscle Androgen Receptor and Insulin-Like Growth Factor-I Isoform Expression in Strength Trained Men

The effects of heavy resistance exercise on skeletal muscle androgen receptor (AR) protein concentration and mRNAs of AR, insulin-like growth factor-I (IGF)-IEa, and mechano growth factor (MGF) expression were examined from biopsies of vastus lateralis (VL) muscle before and 48 hours after heavy resistance exercise (5 × 10 repetition maximum [RM] leg press and 4 × 10RM squats) in 8 adult strength trained men. The present exercise induced an acute decrease in maximal isometric force and increased serum total testosterone (T) and free testosterone (FT) concentrations. During 2 recovery days, maximal isometric force and subjective perception of physical fitness remained significantly lowered, whereas serum creatine kinase activity, subjective muscle soreness, and muscle swelling (i.e., thickness of VL by ultrasound) were significantly increased compared to pre-exercise values. Subjective perception of physical fitness was followed up to 7 days, and by 6 days postexercise, it was elevated above the pre-exercise level. Basal T and FT concentrations remained unaltered after the exercise. No statistically significant changes were observed in AR protein or mRNA expression, but IGF-IEa (p < 0.05) and MGF (p < 0.05) mRNA expression were increased compared to pre-exercise levels. These findings indicate that IGF-IEa and MGF responses may be related to acute regenerative processes in muscle because of exercise and may contribute to muscular adaptation to resistance exercise. Subjective perception of physical fitness suggests that recovery over a pre-exercise level of the present type of heavy resistance exercise can take approximately 6 days.

Heavy resistance exercise training and skeletal muscle androgen receptor expression in younger and older men

Effects of heavy resistance exercise on serum testosterone and skeletal muscle androgen receptor (AR) concentrations were examined before and after a 21-week resistance training period. Seven healthy untrained young adult men (YT) and ten controls (YC) as well as ten older men (OT) and eight controls (OC) volunteered as subjects. Heavy resistance exercise bouts (5 × 10 RM leg presses) were performed before and after the training period. Muscle biopsies were obtained before and 1 h and 48 h after the resistance exercise bouts from m.vastus lateralis (VL) to determine cross-sectional area of muscle fibers (fCSA) and AR mRNA expression and protein concentrations. No changes were observed in YC and OC while resistance training led to significant increases in maximal strength of leg extensors (1 RM), fCSA and lean body mass in YT and OT. Acute increases occurred in serum testosterone concentrations due to resistance exercises but basal testosterone remained unaltered. Mean AR mRNA expression and protein concentration remained unchanged after heavy resistance exercise bouts compared to pre-values. The individual pre- to post-training changes in resting (pre-exercise) AR protein concentration correlated with the changes in fCSA and lean body mass in the combined group of YT and OT. Similarly, it correlated with the changes in 1 RM in YT. Although mean AR expression did not changed due to the resistance exercise training, the present findings suggest that the individual changes of AR protein concentration in skeletal muscle following resistance training may have an impact on training-induced muscular adaptations in both younger and older men.

The Androgen Receptor Modulates Expression of Genes with Critical Roles in Muscle Development and Function

Androgen signaling through the androgen receptor (AR), a ligand-dependent transcription factor within the steroid receptor superfamily, plays an important role in the development and maintenance of many tissues. In muscle, androgens act as anabolic agents that increase both muscle mass and strength; however, a key unanswered question is the mechanism through which AR-mediated gene expression leads to these effects. To gain further insight into the mechanism of AR action in muscle, we identified AR-binding sites in primary human muscle cells using chromatin immunoprecipitation followed by tiling microarray detection. Through this analysis, we identified 32,518 potential AR-binding sites throughout the genome that were enriched upon androgen treatment. Sequence analysis of these regions indicated that approximately 90% possess a consensus androgen response element or half-site. Among the identified AR-binding sites are genes known to be directly regulated by AR, confirming the validity of our methodology. Additionally, we identified a number of novel AR targets, including genes and micro-RNAs implicated in muscle differentiation and function, suggesting a direct role for AR-mediated transcription in muscle development. Intriguingly, binding sequences for the Mef2 family of transcription factors were enriched in the AR-bound regions, and we show that several Mef2c-dependent genes are direct targets of AR, suggesting a functional interaction between Mef2c and AR in skeletal muscle. Our results provide new insights into the mechanisms by which androgens promote muscle growth and validate AR as a potential therapeutic target for sarcopenia, muscle wasting, and other androgen-related muscle disorders.

The Androgen Receptor Modulates Expression of Genes with Critical Roles in Muscle Development and Function

Androgen signaling through the androgen receptor (AR), a ligand-dependent transcription factor within the steroid receptor superfamily, plays an important role in the development and maintenance of many tissues. In muscle, androgens act as anabolic agents that increase both muscle mass and strength; however, a key unanswered question is the mechanism through which AR-mediated gene expression leads to these effects. To gain further insight into the mechanism of AR action in muscle, we identified AR-binding sites in primary human muscle cells using chromatin immunoprecipitation followed by tiling microarray detection.Throughthisanalysis,weidentified32,518potentialAR-bindingsitesthroughoutthegenomethatwereenrichedupon androgentreatment.Sequenceanalysisoftheseregionsindicatedthatapproximately90%possessaconsensusandrogenresponse element or half-site. Among the identified AR-binding sites are genes known to be directly regulated by AR, confirming the validity of our methodology. Additionally, we identified a number of novel AR targets, including genes and micro-RNAs implicated in muscle differentiation and function, suggesting a direct role for AR-mediated transcription in muscle development. Intriguingly, binding sequences for the Mef2 family of transcription factors were enriched in the AR-bound regions, and we show that several Mef2c-dependent genes are direct targets of AR, suggesting a functional interaction between Mef2c and AR in skeletal muscle. Our results provide new insights into the mechanisms by which androgens promote muscle growth and validate AR as ap otential therapeutic target for sarcopenia, muscle wasting, and other androgen-related muscle disorders.

Minireview: Thyroid Hormone Transporters: The Knowns and the Unknowns

Androgen signaling through the androgen receptor (AR), a ligand-dependent transcription factor within the steroid receptor superfamily, plays an important role in the development and maintenance of many tissues. In muscle, androgens act as anabolic agents that increase both muscle mass and strength; however, a key unanswered question is the mechanism through which AR-mediated gene expression leads to these effects. To gain further insight into the mechanism of AR action in muscle, we identified AR-binding sites in primary human muscle cells using chromatin immunoprecipitation followed by tiling microarray detection. Through this analysis, we identified 32,518 potential AR-binding sites throughout the genome that were enriched upon androgen treatment. Sequence analysis of these regions indicated that approximately 90% possess a consensus androgen response element or half-site. Among the identified AR-binding sites are genes known to be directly regulated by AR, confirming the validity of our methodology. Additionally, we identified a number of novel AR targets, including genes and micro-RNAs implicated in muscle differentiation and function, suggesting a direct role for AR-mediated transcription in muscle development. Intriguingly, binding sequences for the Mef2 family of transcription factors were enriched in the AR-bound regions, and we show that several Mef2c-dependent genes are direct targets of AR, suggesting a functional interaction between Mef2c and AR in skeletal muscle. Our results provide new insights into the mechanisms by which androgens promote muscle growth and validate AR as a potential therapeutic target for sarcopenia, muscle wasting, and other androgen-related muscle disorders. Minireview: Thyroid Hormone Transporters: The Knowns and the Unknowns W. Edward Visser, Edith C.H. Friesema, and Theo J. Visser (Mol Endocrinol, 10.1210/me.2010-0095) ABSTRACT The effects of thyroid hormone (TH) on development and metabolism are exerted at the cellular level. Metabolism and action of TH take place intracellularly, which require transport of the hormone across the plasma membrane. This process is mediated by TH transporter proteins. Many TH transporters have been identified at the molecular level, although a few are classified as specific TH transporters, including monocarboxylate transporter (MCT)8, MCT10, and organic anion-transporting polypeptide 1C1. The importance of TH transporters for physiology has been illustrated dramatically by the causative role of MCT8 mutations in males with psychomotor retardation and abnormal serum TH concentrations. Although Mct8 knockout animals have provided insight in the mechanisms underlying parts of the endocrine phenotype, they lack obvious neurological abnormalities. Thus, the pathogenesis of the neurological abnormalities in males with MCT8 mutations is not fully understood. The prospects of identifying other transporters and transporter-based syndromes promise an exciting future in the TH transporter field. T R A N S L A T I O N A L H I G H L I G H T S F R O M M O L E C U L A R E N D O C R I N O L O G YThe effects of thyroid hormone (TH) on development and metabolism are exerted at the cellular level. Metabolism and action of TH take place intracellularly, which require transport of the hormone across the plasma membrane. This process is mediated by TH transporter proteins. Many TH transporters have been identified at the molecular level, although a few are classified as specific TH transporters, including monocarboxylate transporter (MCT)8, MCT10, and organic anion-transporting polypeptide 1C1. The importance of TH transporters for physiology has been illustrated dramatically by the causative role of MCT8 mutations in males with psychomotor retardation and abnormal serum TH concentrations. Although Mct8 knockout animals have provided insight in the mechanisms underlying parts of the endocrine phenotype, they lack obvious neurological abnormalities. Thus, the pathogenesis of the neurological abnormalities in males with MCT8 mutations is not fully understood. The prospects of identifying other transporters and transporter-based syndromes promise an exciting future in the TH transporter field. T R A N S L A T I O N A L H I G H L I G H T S F R O M M O L E C U L A R E N D O C R I N O L O G Y

Research Resource: The Androgen Receptor Modulates Expression of Genes with Critical Roles in Muscle Development and Function

Androgen signaling through the androgen receptor (AR), a ligand-dependent transcription factor within the steroid receptor superfamily, plays an important role in the development and maintenance of many tissues. In muscle, androgens act as anabolic agents that increase both muscle mass and strength; however, a key unanswered question is the mechanism through which AR-mediated gene expression leads to these effects. To gain further insight into the mechanism of AR action in muscle, we identified AR-binding sites in primary human muscle cells using ChIP-on-Chip (chromatin immunoprecipitation coupled with tiling microarray detection of genomic fragments). Through this analysis, we identified 32,518 potential AR-binding sites throughout the genome that were enriched upon androgen treatment. Sequence analysis of these regions indicated that approximately 90% possess a consensus androgen response element or half-site. Among the identified AR-binding sites are genes known to be directly regulated by AR, confirming the validity of our methodology. Additionally, we identified a number of novel AR targets, including genes and micro-RNAs implicated in muscle differentiation and function, suggesting a direct role for AR-mediated transcription in muscle development. Intriguingly, binding sequences for the Mef2 family of transcription factors were enriched in the AR-bound regions, and we show that several Mef2c-dependent genes are direct targets of AR, suggesting a functional interaction between Mef2c and AR in skeletal muscle. Our results provide new insights into the mechanisms by which androgens promote muscle growth and validate AR as a potential therapeutic target for sarcopenia, muscle wasting, and other androgen-related muscle disorders.

Androgen receptor and Kennedy disease/spinal bulbar muscular atrophy

Kennedy Disease/Spinal Bulbar Muscular Atrophy (KD/SBMA) is a progressive neurodegenerative disease caused by genetic polyglutamine expansion of the androgen receptor. We have recently found that overexpression of wildtype androgen receptor in skeletal muscle of transgenic mice results in a KD/SBMA phenotype. This surprising result challenges the orthodox view that KD/SBMA requires expression of polyglutamine expanded androgen receptor within motoneurons. Theories relating to the etiology of this disease drawn from studies of human patients, cellular and mouse models are considered with a special emphasis on potential myogenic contributions to as well as the molecular etiology of KD/SBMA.

Regulation of androgen receptor expression at the onset of functional overload in rat plantaris muscle.

Skeletal muscle androgen receptor (AR) expression at the onset of functional overload (OV) has not been well described. It is also not known if overload and/or anabolic steroid differentially regulate AR expression. The purpose of this study was to examine AR gene expression at the onset of functional OV in rat plantaris muscle with and without nandrolone decanoate (ND) administration. The functional significance of AR protein induction was examined using skeletal alpha-actin promoter activity in transiently transfected CV-1 fibroblast cells. Male Sprague-Dawley rats ( approximately 125 g) were functionally overloaded for 1, 3, 7, or 21 days. A subset of animals was given an ND (6 mg/kg) injection at day 0 and then overloaded for 3 days. Control animals underwent sham surgeries. AR protein concentration increased 106 and 279% after 7 and 21 days of OV, respectively. AR mRNA increased 430% after 7 days of OV. AR protein expression in C2C12 murine myotubes subjected to 1% chronic radial stretch for 18 h was elevated 101% compared with control. ND treatment increased AR protein concentration 1,300% compared with controls, and there was no additional effect when ND and OV were combined. ND with 3 days of OV treatment increased AR mRNA expression 50% compared with control. AR overexpression in transiently transfected CV-1 fibroblast cells increased -424 bp skeletal alpha-actin promoter activity 80 to 1,800% in a dose-dependent fashion. Co-overexpression of either serum response factor (SRF) or active RhoA with AR overexpression induced a synergistic 36- and 28-fold induction of skeletal alpha-actin promoter. Cotransfection of AR, SRF, and active RhoA induced 180-fold increase in skeletal alpha-actin promoter activity. In conclusion, AR protein expression is increased after 7 days of functional OV, and this induction is regulated pretranslationally. AR induction in conjunction with SRF and RhoA signaling may be an important regulator of gene expression during overload-induced muscle growth.

Effects of castration and androgen treatment on androgen-receptor levels in rat skeletal muscles.

The effects of castration and dihydrotestosterone (DHT) treatment on levels of skeletal muscle androgen receptor (AR) were examined in three groups of adult male rats: 1) intact normal rats, 2) rats castrated at 16 wk of age, and 3) rats castrated at 16 wk of age and given DHT for 1 wk starting at week 17. All animals were killed at 18 wk of age. Castration caused a decrease (P < 0.05) in the weights of the levator ani and bulbocavernosus muscles. The administration of DHT to the castrated rats increased (P < 0.05) the weights of the levator ani and bulbocavernosus muscles. Castration caused a significant downregulation of AR levels in the bulbocavernosus (P < 0.05) but had no significant effect on AR levels in the levator ani muscle. DHT administration to the castrated group upregulated AR levels in the bulbocavernosus and levator ani muscles. The plantaris muscle did not significantly (P > 0.05) change for any of the treatments. These findings suggest that the effects of castration and androgen replacement differentially affect skeletal muscle mass and AR levels.

Androgen receptors in skeletal muscle cytosol from sheep treated with trenbolone acetate.

Female sheep were implanted with the anabolic agent trenbolone acetate and the effects of the treatment on the skeletal muscle androgen receptor studied. In two separate trials the binding capacity/mg skeletal muscle protein was reduced. In trial 1 the animals were implanted (80 mg trenbolone acetate) at the start and after 5 weeks of the trial and samples taken one week later while in the second trial samples were taken 15 days after a single implant (80 mg trenbolone acetate). The ability of a variety of steroids and steroid-like compounds to displace [3H] testosterone and [3H] methyltrienolone was also studied. The androgen receptor was also partially purified using heparin-sepharose chromatography The involvement of the anddrogen receptor in the mode of action of trenbolone is discussed.

Cytosolic androgen receptor in skeletal muscle from normal and dystrophic mice

The cytosolic androgen receptor was studied in skeletal muscle of normal mice and of mice with hereditary muscular dystrophy (129/ReJ-dy). Mouse muscle contains a cytosolic androgen receptor with high affinity ( K d 0.6 nM) and low capacity ( B max 3 fmol/mg protein). Dystrophic muscle contains a receptor with similar binding properties. The receptors from muscle of dystrophic and normal mice bind to DNA-cellulose affinity columns and are eluted with similar patterns. The reported failure of androgens to alleviate the clinical signs of murine muscular dystrophy is probably not due to decreased androgen responsiveness of the affected muscles.

Cytosolic androgen receptor in skeletal muscle from normal and testicular feminization mutant (Tfm) rats

Skeletal muscle from normal adult male rats contains a cytosolic androgen receptor that is similar to that of other androgen target tissues with regard to ligand specificity and affinity for DNA. Although the receptor is not present in many androgen target tissues from rats with testicular feminization, the receptor is present in skeletal muscle from these rats. It does not, however, bind DNA. This defect represents a previously undescribed mode of expression of the mutation and may be paradigmatic for human subjects with receptor-positive androgen resistance.

Androgen and glucocorticoid receptors in porcine skeletal muscle.

The presence of free androgen and glucocorticoid receptors was demonstrated in the rectus femoris muscle of the pig. No detectable free androgen receptors were observed in intact male pigs. Androgen receptor characteristics in castrates and gilts were related to the growth rate of the lean meat in the ham. The amount of free androgen receptor was less (P less than .05) and the androgen was more tightly bound to its receptor (P less than .05) in pigs with above-average ham growth. Moreover, a higher DNA content (P less than .01) was also found in these pigs, giving a lower (P less than .05) protein to DNA ratio. The anabolic action of androgens is likely to be mediated via androgen receptors in skeletal muscle cytosol. The amounts of free glucocorticoid receptors found were influenced by the pH of the cytosolic fraction. The relationships between cytosolic pH and both the amount of glucocorticoid receptor (positive; P less than .01) and plasma levels of cortisol (negative; P less than .05) might reflect the stress to which the pigs are exposed at slaughter.

Regulation of the androgen and glucocorticoid receptors in rat and mouse skeletal muscle cytosol.

Using a charcoal technique, we determined the relative binding affinity of some anabolic compounds for the androgen and glucocorticoid receptors in cytosol from rat skeletal muscle. Only a few of the compounds analyzed competed for the receptor-binding sites. The androgen and glucocorticoid receptors were analyzed in rat and mouse skeletal muscle cytosols by Scatchard analysis. In rats grouped according to sex and age, the cytosolic protein content was about the same in all groups, but the DNA content decreased with increased weight of the animal regardless of sex (male, female, or castrated male). The glucocorticoid receptor did not differ in concentration (2-3 pmol/g tissue) or ligand affinity (Kd, 10-40 nM) among the groups, but the androgen receptor concentration decreased with increased weight and age of the animals, more in the case of males than in the case of females or castrates. The Kd for the androgen receptor increased with age in males but was constantly about 0.2 nM for castrates or females. In adult intact rats, the androgen and glucocorticoid receptor concentrations in muscle cytosol from females were about 100 and 3000 fmol/g tissue, respectively, the corresponding values for males being about 50 and 2000 fmol/g tissue, respectively. Short term castration or adrenalectomy increased the concentration of and ligand affinity for the androgen and glucocorticoid receptors, respectively. After long term castration of male rats, the concentration of both receptors increased during 5 weeks to about the female level, only to decrease later. Neonatally castrated male rats had about the same androgen receptor concentrations and Kd values as female rats. Female mice had higher androgen receptor concentrations (approximately 700 fmol/g tissue) than rats. Intact male mice had about 200 fmol androgen receptor-binding sites/g tissue, and the same amount was found in mice bearing the testicular feminization (Tfm) mutant gene. In summary, the concentrations of androgen and glucocorticoid receptors in rat skeletal muscle are regulated at least by the testes. The presence of androgen receptors in skeletal muscle from Tfm mice is surprising and may motivate a reinvestigation of the regulation of androgen receptors in Tfm animals.