Myocyte androgen receptors increase metabolic rate and improve body composition by reducing fat mass

Metabolic benefits afforded by estradiol and testosterone in both sexes: clinical considerations

Testosterone (T) and 17β-estradiol (E2) are produced in male and female humans and are potent metabolic regulators in both sexes. When E2 and T production stops or decreases during aging, metabolic dysfunction develops and promotes degenerative metabolic and vascular disease. Here, we discuss the shared benefits afforded by E2 and T for metabolic function human females and males. In females, E2 is central to bone and vascular health, subcutaneous adipose tissue distribution, skeletal muscle insulin sensitivity, antiinflammatory immune function, and mitochondrial health. However, T also plays a role in female skeletal, vascular, and metabolic health. In males, T's conversion to E2 is fundamental to bone and vascular health, as well as prevention of excess visceral adiposity and the promotion of insulin sensitivity via activation of the estrogen receptors. However, T and its metabolite dihydrotestosterone also prevent excess visceral adiposity and promote skeletal muscle growth and insulin sensitivity via activation of the androgen receptor. In conclusion, T and E2 are produced in both sexes at sex-specific concentrations and provide similar and potent metabolic benefits. Optimizing levels of both hormones may be beneficial to protect patients from cardiometabolic disease and frailty during aging, which requires further study.

Oxidative balance score and associations with dyslipidemia and mortality among US adults: A mortality follow-up study of a cross-sectional cohort

BACKGROUND

Oxidative stress has been implicated in the pathogenesis and progression of dyslipidemia. We aimed to investigate the association between the oxidative balance score, and dyslipidemia, as well as to assess the mortality risk associated with oxidative balance score in patients with dyslipidemia.

METHODS

We performed a mortality follow-up study of a cross-sectional cohort of 26,118 adults from the National Health and Nutrition Examination Survey 1999-2018. The total oxidative balance score was calculated by 16 dietary nutrients (dietary oxidative balance score) and four lifestyle factors (lifestyle oxidative balance score). Weighted Cox proportional hazard model was applied to determine the relationship between oxidative balance score and all-cause or cardiovascular disease (CVD) mortality within the dyslipidemia group.

RESULTS

During a median follow-up of 118 months, 2448 all-cause deaths (766 CVD-related) occurred. A significant negative correlation was observed between total oxidative balance score, dietary oxidative balance score, lifestyle oxidative balance score, and dyslipidemia. The multivariable-adjusted odds ratios and 95% CIs for dyslipidemia were 0.86 (0.77-0.97), 0.80 (0.72-0.91), and 0.63 (0.56-0.70), respectively, when comparing the second, third, and fourth quartiles of total oxidative balance score to the reference lowest quartile (P for trend <0.0001). Increasing total oxidative balance score was inversely associated with all-cause (hazard ratio [HR] = 0.98, 95% CI 0.98-0.99) and CVD-specific mortality (HR = 0.98, 95% CI 0.97-0.99) in participants with dyslipidemia.

CONCLUSIONS

Oxidative balance score is inversely associated with dyslipidemia and linked to all-cause and CVD-related mortality, highlighting the potentially protective role of an antioxidant-rich diet against dyslipidemia.

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.

Higher oxidative balance scores are associated with lower nonalcoholic fatty liver disease and not with fibrosis in US adults

BACKGROUND AND AIMS

Little research have focused on the relationship between systemic oxidative stress status and NAFLD and fibrosis. The Oxidative Balance Score (OBS) is employed to evaluate whole-body lifestyle and diet exposures related to oxidative stress, with higher OBS scores implying exposure to more antioxidants. This study aimed to explore whether OBS is correlated with NAFLD and NAFLD-related fibrosis.

METHODS AND RESULTS

12,223 participants from NHANES 2003-2018 were enrolled in this study. NAFLD was defined as USFLI ≥30 and liver fibrosis was determined as FIB-4 ≥ 2.67. OBS was scored by 20 lifestyle and dietary factors. Weighted logistic regression and restricted cubic splines were used to assess the association between OBS and NAFLD and fibrosis. The prevalence of NAFLD was 29.67%. There was a significant negative correlation between OBS, dietary OBS, lifestyle OBS and NAFLD and no correlation with NAFLD-related fibrosis. Compared to the lowest quartile, the adjusted ORs for the highest quartile of OBS, lifestyle OBS, dietary OBS and NAFLD were 0.55(95%CI:0.35,0.85), 0.12(95%CI:0.08,0.16), 0.70(95%CI:0.52,0.94) respectively. In stratified analyses, lifestyle OBS was negatively associated with NAFLD across gender, dietary OBS was only negatively correlated with NAFLD in men, and any OBS was not observed to be relevant to NAFLD-related fibrosis.

CONCLUSIONS

OBS was negatively associated with NAFLD, but not with NAFLD-related fibrosis. The findings underline the significance of adhering to an antioxidant lifestyle and diet, which can help prevent NAFLD but seems to be ineffective in preventing fibrosis in individuals with NAFLD.

Androgen action on myogenesis throughout the lifespan; comparison with neurogenesis

Androgens' pleiotropic actions in promoting sex differences present not only a challenge to providing a comprehensive account of their function, but also an opportunity to gain insights by comparing androgenic actions across organ systems. Although often overlooked by neuroscientists, skeletal muscle is another androgen-responsive organ system which shares with the nervous system properties of electrochemical excitability, behavioral relevance, and remarkable capacity for adaptive plasticity. Here we review androgenic regulation of mitogenic plasticity in skeletal muscle with the goal of identifying areas of interest to those researching androgenic mechanisms mediating sexual differentiation of neurogenesis. We use an organizational-activational framework to relate broad areas of similarity and difference between androgen effects on mitogenesis in muscle and brain throughout the lifespan, from early organogenesis, through pubertal organization, adult activation, and aging. The focus of the review is androgenic regulation of muscle-specific stem cells (satellite cells), which share with neural stem cells essential functions in development, plasticity, and repair, albeit with distinct, muscle-specific features. Also considered are areas of paracrine and endocrine interaction between androgen action on muscle and nervous system, including mediation of neural plasticity of innervating and distal neural populations by muscle-produced trophic factors.

Associations of oxidative balance score and visceral adiposity index with risk of ischaemic heart disease: a cross-sectional study of NHANES, 2005-2018

OBJECTIVE

Evidence on the association of oxidative balance score (OBS) and visceral adiposity index (VAI) with risk of ischaemic heart disease (IHD) is limited. We aimed to explore the association of OBS and VAI with risk of IHD, and then examined their potential interactive effects.

DESIGN

A cross-sectional study.

SETTING

The National Health and Nutrition Examination Survey.

PARTICIPANTS

A total of 27 867 individuals aged more than 20 years were included in this study.

PRIMARY AND SECONDARY OUTCOME MEASURES

Multivariable logistic regression analyses were used to estimate ORs and 95% CIs for the associations of OBS and VAI with risk of IHD, including coronary heart disease (CHD), heart attack and angina pectoris.

RESULTS

Compared with those in the first quintile, participants with highest quintile of OBS had decreased risk of IHD (OR: 0.59, 95% CI: 0.50, 0.69), CHD (OR: 0.65, 95% CI: 0.52, 0.80), heart attack (OR: 0.53, 95% CI: 0.43, 0.66) and angina pectoris (OR: 0.63, 95% CI: 0.48, 0.82); meanwhile, those with highest quintile of VAI had increased risk of IHD (OR: 1.46, 95% CI: 1.22, 1.74), CHD (OR: 1.34, 95% CI: 1.07, 1.67), heart attack (OR: 1.55, 95% CI: 1.24, 1.94) and angina pectoris (OR: 1.40, 95% CI: 1.04, 1.87). Furthermore, we observed a stronger association between OBS and risk of IHD among participants with VAI ≥1.73 (OR: 0.50, 95% CI: 0.40, 0.62).

CONCLUSION

Our study found the negative association between OBS and risk of IHD, and positive association between VAI and risk of IHD. In addition, we found the interactive effects between VAI and OBS on the risk of IHD, underlining the importance of OBS in IHD prevention among participants with high VAI level.

Gender-specific effects of oxidative balance score on the prevalence of diabetes in the US population from NHANES

Background

The relationship between oxidative balance score (OBS) and diabetes remains poorly understood and may be gender-specific. We conducted a cross-sectional study to investigate the complex association between OBS and diabetes among US adults.

Methods

Overall, 5,233 participants were included in this cross-sectional study. The exposure variable was OBS, composed of scores for 20 dietary and lifestyle factors. Multivariable logistic regression, subgroup analysis, and restricted cubic spline (RCS) regression were applied to examine the relationship between OBS and diabetes.

Results

Compared to the lowest OBS quartile group (Q1), the multivariable-adjusted odds ratio (OR) (95% confidence interval (CI) for the highest OBS quartile group (Q4) was 0.602 (0.372-0.974) (p for trend = 0.007), and for the highest lifestyle, the OBS quartile group was 0.386 (0.223-0.667) (p for trend < 0.001). Moreover, gender effects were found between OBS and diabetes (p for interaction = 0.044). RCS showed an inverted-U relationship between OBS and diabetes in women (p for non-linear = 6e-04) and a linear relationship between OBS and diabetes in men.

Conclusions

In summary, high OBS was negatively associated with diabetes risk in a gender-dependent manner.

Social status influences relationships between hormones and oxidative stress in a cichlid fish

An individual's social environment can have widespread effects on their physiology, including effects on oxidative stress and hormone levels. Many studies have suggested that variation in oxidative stress experienced by individuals of different social statuses might be due to endocrine differences, however, few studies have evaluated this hypothesis. Here, we assessed whether a suite of markers associated with oxidative stress in different tissues (blood/plasma, liver, and gonads) had social status-specific relationships with circulating testosterone or cortisol levels in males of a cichlid fish, Astatotilapia burtoni. Across all fish, blood DNA damage (a global marker of oxidative stress) and gonadal synthesis of reactive oxygen species [as indicated by NADPH-oxidase (NOX) activity] were lower when testosterone was high. However, high DNA damage in both the blood and gonads was associated with high cortisol in subordinates, but low cortisol in dominants. Additionally, high cortisol was associated with greater production of reactive oxygen species (greater NOX activity) in both the gonads (dominants only) and liver (dominants and subordinates). In general, high testosterone was associated with lower oxidative stress across both social statuses, whereas high cortisol was associated with lower oxidative stress in dominants and higher oxidative stress in subordinates. Taken together, our results show that differences in the social environment can lead to contrasting relationships between hormones and oxidative stress.

Sex differences in neuroimmunoendocrine communication. Involvement on longevity

Endocrine, nervous, and immune systems work coordinately to maintain the global homeostasis of the organism. They show sex differences in their functions that, in turn, contribute to sex differences beyond reproductive function. Females display a better control of the energetic metabolism and improved neuroprotection and have more antioxidant defenses and a better inflammatory status than males, which is associated with a more robust immune response than that of males. These differences are present from the early stages of life, being more relevant in adulthood and influencing the aging trajectory in each sex and may contribute to the different life lifespan between sexes.

Myocytic androgen receptor overexpression does not affect sex differences in adaptation to chronic endurance exercise

Muscle-specific androgen receptor (AR) overexpression (HSAAR transgene) in sedentary male rats results in reduced adiposity, increased mitochondrial enzyme activity, and selective increase in Type 2b myofiber size. Here, we tested chronic endurance exercise interactions with this phenotype in both sexes. Across 9 weeks, rats ran 5×/week on motorized running wheels at increasing speeds and durations. Exercise reduced fat mass in all groups, but sex affected endurance exercise outcomes such that absolute lean mass increased only in females and total body mass decreased only in males. Expected sex differences were observed with males exhibiting greater total body and lean mass; absolute and relative fat mass; bone mineral density; extensor digitorum longus (EDL) myofiber size and glycolytic proportion; but lesser Type 2a and Type 1 myosin expression in tibialis anterior. Observed HSAAR outcomes were not altered by sex, with transgenic rats having greater lean mass, Type 2a myosin expression in soleus, and glycolytic myofiber size in EDL. Tibialis AR content was independently affected by sex, HSAAR, and exercise. No sex differences were observed in tibialis AR expression in wild-type rats, although HSAAR males had greater AR content than HSAAR females. We identified a moderate correlation between AR expression and glycolytic myofiber size, but not whole-body composition. Overall, results suggest myocytic AR overexpression and chronic exercise, despite sharing a similar phenotype to adaptation, are mediated by distinct mechanisms. Further, this study illustrates sex differences in adaptation to chronic endurance exercise, and suggests sex-similarity in the relationship between muscle AR and exercise response.

Adaptations in bone, lean, and total mass after forced endurance exercise are sex-dependent in rats.

Sex differences in muscle fiber-type size and proportion, lean body mass, and bone density are independent of exercise in rats.

Myocytic AR overexpression promotes lean body mass and glycolytic myofiber size in both sexes.

Skeletal muscle AR protein is elevated by chronic endurance exercise in rats, and these changes in AR content are correlated with improved glycolytic myofiber size.

Association between the Oxidative Balance Score and Telomere Length from the National Health and Nutrition Examination Survey 1999-2002

Purpose. Leukocyte telomere length (LTL) is an important biomarker of aging. The oxidative balance score (OBS) is used to assess the oxidative stress-related exposures of diet and lifestyle. This study is aimed at exploring if the OBS was associated with LTL. Methods. 3220 adults were included in this study from the National Health and Nutrition Examination Survey (NHANES) 1999-2002. LTL was assayed from leukocyte DNA. Twenty dietary and lifestyle factors were selected to score the OBS. Survey-based multivariable linear regression was conducted to assess the association between the OBS and log-transformed LTL. Results. The association between the OBS and log-transformed LTL was positive in females but not males. For females, compared with the lowest OBS category as a reference, the multivariable-adjusted beta estimate (95% confidence interval, CI) for the highest OBS category was 0.0701 (0.0205–0.1197) ( $p$ for $\text{trend}<0.01$ ), and the multivariable-adjusted beta estimate (95% CI) of the continuous OBS was 0.0039 (0.0014–0.0065). There was also the gender difference in the correlations of the dietary OBS and the lifestyle OBS with log-transformed LTL. Conclusion. There was a positive association between the OBS and LTL in females. This result suggested that diet and lifestyle might affect LTL by regulating oxidative balance.

Testosterone activates glucose metabolism through AMPK and androgen signaling in cardiomyocyte hypertrophy

Background

Testosterone regulates nutrient and energy balance to maintain protein synthesis and metabolism in cardiomyocytes, but supraphysiological concentrations induce cardiac hypertrophy. Previously, we determined that testosterone increased glucose uptake—via AMP-activated protein kinase (AMPK)—after acute treatment in cardiomyocytes. However, whether elevated glucose uptake is involved in long-term changes of glucose metabolism or is required during cardiomyocyte growth remained unknown. In this study, we hypothesized that glucose uptake and glycolysis increase in testosterone-treated cardiomyocytes through AMPK and androgen receptor (AR).

Methods

Cultured cardiomyocytes were stimulated with 100 nM testosterone for 24 h, and hypertrophy was verified by increased cell size and mRNA levels of β-myosin heavy chain (β-mhc). Glucose uptake was assessed by 2-NBDG. Glycolysis and glycolytic capacity were determined by measuring extracellular acidification rate (ECAR).

Results

Testosterone induced cardiomyocyte hypertrophy that was accompanied by increased glucose uptake, glycolysis enhancement and upregulated mRNA expression of hexokinase 2. In addition, testosterone increased AMPK phosphorylation (Thr172), while inhibition of both AMPK and AR blocked glycolysis and cardiomyocyte hypertrophy induced by testosterone. Moreover, testosterone supplementation in adult male rats by 5 weeks induced cardiac hypertrophy and upregulated β-mhc, Hk2 and Pfk2 mRNA levels.

Conclusion

These results indicate that testosterone stimulates glucose metabolism by activation of AMPK and AR signaling which are critical to induce cardiomyocyte hypertrophy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40659-021-00328-4.

Androgen-Regulated Cardiac Metabolism in Aging Men

The prevalence of cardiovascular mortality is higher in men than in age-matched premenopausal women. Gender differences are linked to circulating sex-related steroid hormone levels and their cardio-specific actions, which are critical factors involved in the prevalence and features of age-associated cardiovascular disease. In women, estrogens have been described as cardioprotective agents, while in men, testosterone is the main sex steroid hormone. The effects of testosterone as a metabolic regulator and cardioprotective agent in aging men are poorly understood. With advancing age, testosterone levels gradually decrease in men, an effect associated with increasing fat mass, decrease in lean body mass, dyslipidemia, insulin resistance and adjustment in energy substrate metabolism. Aging is associated with a decline in metabolism, characterized by modifications in cardiac function, excitation-contraction coupling, and lower efficacy to generate energy. Testosterone deficiency -as found in elderly men- rapidly becomes an epidemic condition, associated with prominent cardiometabolic disorders. Therefore, it is highly probable that senior men showing low testosterone levels will display symptoms of androgen deficiency, presenting an unfavorable metabolic profile and increased cardiovascular risk. Moreover, recent reports establish that testosterone replacement improves cardiomyocyte bioenergetics, increases glucose metabolism and reduces insulin resistance in elderly men. Thus, testosterone-related metabolic signaling and gene expression may constitute relevant therapeutic target for preventing, or treating, age- and gender-related cardiometabolic diseases in men. Here, we will discuss the impact of current evidence showing how cardiac metabolism is regulated by androgen levels in aging men.

Effects of aging and menopause on pancreatic fat fraction in healthy women population: A strobe-compliant article

Pancreatic fat fraction has been shown to increase in many pathological situations. However, pancreatic fat fraction and its physiological changes in healthy women are still unclear. The aim of this study is to investigate the effect of aging and menopause on pancreatic fat fraction in healthy female population.This was a cross-sectional study. A phantom of fat-water mixtures was established. One hundred sixty-seven healthy women (20-70 years) were recruited. Fat fraction was quantified with double-echo chemical shift magnetic resonance imaging with T1 and T2* correction. The association between measured and actual fat fractions was determined with Pearson correlation. Linear regression analysis was used to establish the calibration curve. Fat fractions were analyzed via analysis of variance.A significant positive linear correlation was revealed between the measured and actual fat fractions on the phantom (r = 0.991, P < .001). There was no significant difference in fat fractions among caput, corpus, and cauda of the pancreas. Pancreatic fat fraction remained constant during the age of 20 to 40 years (4.41 ± 0.79%) but significantly increased during the ages of 41 to 50 and 51 to 70 years (7.49 ± 1.10% and 9.43 ± 1.51%, respectively, P < .001). Moreover, pancreatic fat fractions of the healthy women aged 41 to 70 years were still significantly higher than these in the groups aged 20 to 40 years when postmenopausal healthy women were removed (P < .001). For volunteers aged 46 to 49 years, pancreatic fat fraction of the postmenopausal women was significantly increased compared with that of their premenopausal counterparts (P < .001).We found that an even distribution of pancreatic fat in healthy women, aging and menopause as 2 independent risk factors for pancreatic steatosis, a fatty infiltration in the pancreas beginning in the fifth decade in women.

Estrogen signaling increases nuclear receptor subfamily 4 group A member 1 expression and energy production in skeletal muscle cells

Estrogen deficiency has been known to associate with musculoskeletal diseases in women, based on the clinical observations of frequent susceptibility to osteoporosis and sarcopenia among postmenopausal women. In skeletal muscles, estrogen has been assumed to play physiological roles in maintaining muscle mass and strength, although its precise molecular mechanism remains to be elucidated. We have previously shown that estrogen regulates energy metabolism through the downregulation of mitochondrial uncoupling protein 3 (UCP3) in skeletal muscles, which may contribute to the prolonged exercise endurance in female mice. In the present study, we investigated the effects of estrogen on the expression levels of all members of the nuclear receptor superfamily. Microarray analysis showed that the mRNA level of nuclear receptor subfamily 4 group A member 1 (Nr4a1) was upregulated by the transduction of a recombinant adenovirus expressing constitutively active estrogen receptor α (caERα) in differentiated myoblastic C2C12 cells. Thus we assumed that NR4A1 may be an estrogen-inducible gene in myoblastic cells. We also demonstrated that caERα increases the cellular ATP content along with an increase in mitochondrial DNA content in differentiated myoblastic C2C12 cells. In contrast, the knockdown of Nr4a1 using siRNA exhibited reduced ATP generation as well as a decrease in mitochondrial DNA content. Overall, the present study indicates a crosstalk between estrogen signaling and NR4A1 in skeletal muscle cells. We consider that estrogen-dependent NR4A1 upregulation could increase efficient ATP generation in skeletal muscle cells partly through enhancing mitochondrial functions.

Sex-Dimorphic and Sex Hormone-Dependent Role of Steroid Sulfatase in Adipose Inflammation and Energy Homeostasis

Steroid sulfatase (STS), a desulfating enzyme that converts steroid sulfates to hormonally active steroids, plays an important role in the homeostasis of sex hormones. STS is expressed in the adipose tissue of both male and female mice, but the role of STS in the development and function of adipose tissue remains largely unknown. In this report, we show that the adipose expression of Sts was induced in the high-fat diet (HFD) and ob/ob models of obesity and type 2 diabetes. Transgenic overexpression of the human STS in the adipose tissue of male mice exacerbated the HFD-induced metabolic phenotypes, including increased body weight gain and fat mass, and worsened insulin sensitivity, glucose tolerance, and energy expenditure, which were accounted for by adipocyte hypertrophy, increased adipose inflammation, and dysregulation of adipogenesis. The metabolic harm of the STS transgene appeared to have resulted from increased androgen activity in the adipose tissue, and castration abolished most of the phenotypes. Interestingly, the transgenic effects were sex specific, because the HFD-fed female STS transgenic mice exhibited improved metabolic functions, which were associated with attenuated adipose inflammation. The metabolic benefit of the STS transgene in female mice was accounted for by increased estrogenic activity in the adipose tissue, whereas such benefit was abolished upon ovariectomy. Our results revealed an essential role of the adipose STS in energy homeostasis in sex- and sex hormone-dependent manner. The adipose STS may represent a therapeutic target for the management of obesity and type 2 diabetes.

The complex association between metabolic syndrome and male hypogonadism

BACKGROUND

The complex association between metabolic syndrome (MetS) and male hypogonadism is well established. A number of observational studies show that low testosterone is associated with insulin resistance and an increased risk for diabetes mellitus and MetS in men.

AIMS

To elucidate the association between MetS and male hypogonadism, present epidemiological data on the co-existence of the two comorbidities, enlighten the underlying pathophysiology and appraise the effects of testosterone supplementation therapy (TTh) and lifestyle modifications on MetS and body composition in men.

MATERIALS AND METHODS

Systematic search to PubMed and Medline databases for publications reporting data on association between MetS and male hypogonadism.

RESULTS

Both MetS and male hypogonadism have a high prevalence in the general population and are frequently co-existing e.g. in males with diabetes. Accumulating evidence from animal and human studies suggests that MetS is involved in the pathogenesis of hypogonadism in males as well as the other way around. On the other hand, there is evidence for a favorable effect of testosterone supplementation in testosterone deficient men with MetS and/or diabetes mellitus.

CONCLUSIONS

Studies with superior methodological characteristics are needed in order to establish a role for testosterone supplementation in men with MetS and/or diabetes mellitus.

Mouse models of peripheral metabolic disease

Metabolic disease risk is driven by defects in the function of cells that regulate energy homeostasis, as well as altered communication between the different tissues or organs that these cells occupy. Thus, it is desirable to use model organisms to understand the contribution of different cells, tissues and organs to metabolism. Mice are widely used for metabolic research, since well-characterised mouse strains (in terms of their genotype and phenotype) allow comparative studies and human disease modelling. Such research involves strains containing spontaneous mutations that lead to obesity and diabetes, surgically- and chemically-induced models, those that are secondary to caloric excess, genetic mutants created by transgenesis and gene knockout technologies, and peripheral models generated by Cre-Lox or CRISPR/Cas9 approaches. Focussing on obesity and type 2 diabetes as relevant metabolic diseases, we systematically review each of these models, discussing their use, limitations, and future potential.

The impact of androgen actions in neurons on metabolic health and disease

The male hormone testosterone exerts different effects on glucose and energy homeostasis in males and females. Testosterone deficiency predisposes males to visceral obesity, insulin resistance and type 2 diabetes. However, testosterone excess predisposes females to similar metabolic dysfunction. Here, we review the effects of testosterone actions in the central nervous system on metabolic function in males and females. In particular, we highlight changes within the hypothalamus that control glucose and energy homeostasis. We distinguish the organizational effects of testosterone in the programming of neural circuitry during development from the activational effects of testosterone during adulthood. Finally, we explore potential sites where androgen might be acting to impact metabolism within the central nervous system.

Physiological basis behind ergogenic effects of anabolic androgens

Anabolic androgenic steroids (AAS) are widely abused by the sporting community. Demonstrating performance enhancing effects of AAS in rigorous scientific studies is fraught with difficulty. In controlled studies, AAS have consistently been reported to increase muscle mass and strength. The clinical evidence that these anabolic effects are independent of, and additive to exercise are supported by preclinical studies suggesting that AAS and exercise affect muscle by overlapping, yet distinct mechanisms. AAS may also improve performance by their actions on other organ systems, such as the vasculature, and the erythropoietic and central nervous system, although this evidence is less strong. While most of the actions of AAS are thought to be mediated via classical androgen receptor-mediated genomic signalling, AAS may also produce rapid effects via non-genomic mechanisms.

Androgenic mechanisms of sexual differentiation of the nervous system and behavior

Testicular androgens are the major endocrine factor promoting masculine phenotypes in vertebrates, but androgen signaling is complex and operates via multiple signaling pathways and sites of action. Recently, selective androgen receptor mutants have been engineered to study androgenic mechanisms of sexual differentiation of the nervous system and behavior. The focus of these studies has been to evaluate androgenic mechanisms within the nervous system by manipulating androgen receptor conditionally in neural tissues. Here we review both the effects of neural loss of AR function as well as the effects of neural overexpression of AR in relation to global AR mutants. Although some studies have conformed to our expectations, others have proved challenging to assumptions underlying the dominant hypotheses. Notably, these studies have called into question both the primacy of direct, neural mechanisms and also the linearity of the relationship between androgenic dose and sexual differentiation of brain and behavior.

Multiple Mechanisms Linking Type 2 Diabetes and Alzheimer's Disease: Testosterone as a Modifier

Evidence in support of links between type-2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) has increased considerably in recent years. AD pathological hallmarks include the accumulation of extracellular amyloid-β (Aβ) and intracellular hyperphosphorylated tau in the brain, which are hypothesized to promote inflammation, oxidative stress, and neuronal loss. T2DM exhibits many AD pathological features, including reduced brain insulin uptake, lipid dysregulation, inflammation, oxidative stress, and depression; T2DM has also been shown to increase AD risk, and with increasing age, the prevalence of both conditions increases. In addition, amylin deposition in the pancreas is more common in AD than in normal aging, and although there is no significant increase in cerebral Aβ deposition in T2DM, the extent of Aβ accumulation in AD correlates with T2DM duration. Given these similarities and correlations, there may be common underlying mechanism(s) that predispose to both T2DM and AD. In other studies, an age-related gradual loss of testosterone and an increase in testosterone resistance has been shown in men; low testosterone levels can also occur in women. In this review, we focus on the evidence for low testosterone levels contributing to an increased risk of T2DM and AD, and the potential of testosterone treatment in reducing this risk in both men and women. However, such testosterone treatment may need to be long-term, and would need regular monitoring to maintain testosterone at physiological levels. It is possible that a combination of testosterone therapy together with a healthy lifestyle approach, including improved diet and exercise, may significantly reduce AD risk.

The role of sex steroid hormones in the pathophysiology and treatment of sarcopenia

Sex steroids influence the maintenance and growth of muscles. Decline in androgens, estrogens and progesterone by aging leads to the loss of muscular function and mass, sarcopenia. These steroid hormones can interact with different signaling pathways through their receptors. To date, sex steroid hormone receptors and their exact roles are not completely defined in skeletal and smooth muscles. Although numerous studies focused on the effects of sex steroid hormones on different types of cells, still many unexplained molecular mechanisms in both skeletal and smooth muscle cells remain to be investigated. In this paper, many different molecular mechanisms that are activated or inhibited by sex steroids and those that influence the growth, proliferation, and differentiation of skeletal and smooth muscle cells are reviewed. Also, the similarities of cellular and molecular pathways of androgens, estrogens and progesterone in both skeletal and smooth muscle cells are highlighted. The reviewed signaling pathways and participating molecules can be targeted in the future development of novel therapeutics.

Testosterone inhibits expression of lipogenic genes in visceral fat by an estrogen-dependent mechanism

The influence of the aromatase enzyme on the chronic fat-sparing effects of testosterone requires further elucidation. Our purpose was to determine whether chronic anastrozole (AN, an aromatase inhibitor) treatment alters testosterone-mediated lipolytic/lipogenic gene expression in visceral fat. Ten-month-old Fischer 344 rats (n = 6/group) were subjected to sham surgery (SHAM), orchiectomy (ORX), ORX + treatment with testosterone enanthate (TEST, 7.0 mg/wk), or ORX + TEST + AN (0.5 mg/day), with drug treatment beginning 14 days postsurgery. At day 42, ORX animals exhibited nearly undetectable serum testosterone and 29% higher retroperitoneal fat mass than SHAM animals (P < 0.001). TEST produced a ∼380-415% higher serum testosterone than SHAM (P < 0.001) and completely prevented ORX-induced visceral fat gain (P < 0.001). Retroperitoneal fat was 21% and 16% lower in ORX + TEST than SHAM (P < 0.001) and ORX + TEST + AN (P = 0.007) animals, while serum estradiol (E₂) was 62% (P = 0.024) and 87% (P = 0.010) higher, respectively. ORX stimulated lipogenic-related gene expression in visceral fat, demonstrated by ∼84-154% higher sterol regulatory element-binding protein-1 (SREBP-1, P = 0.023), fatty acid synthase (P = 0.01), and LPL (P < 0.001) mRNA than SHAM animals, effects that were completely prevented in ORX + TEST animals (P < 0.01 vs. ORX for all). Fatty acid synthase (P = 0.061, trend) and LPL (P = 0.043) mRNA levels were lower in ORX + TEST + AN than ORX animals and not different from SHAM animals but remained higher than in ORX + TEST animals (P < 0.05). In contrast, the ORX-induced elevation in SREBP-1 mRNA was not prevented by TEST + AN, with SREBP-1 expression remaining ∼117-171% higher than in SHAM and ORX + TEST animals (P < 0.01). Across groups, visceral fat mass and lipogenic-related gene expression were negatively associated with serum testosterone, but not E₂ Aromatase inhibition constrains testosterone-induced visceral fat loss and the downregulation of key lipogenic genes at the mRNA level, indicating that E₂ influences the visceral fat-sparing effects of testosterone.

Effects of sex steroids on bones and muscles: Similarities, parallels, and putative interactions in health and disease

Estrogens and androgens influence the growth and maintenance of bones and muscles and are responsible for their sexual dimorphism. A decline in their circulating levels leads to loss of mass and functional integrity in both tissues. In the article, we highlight the similarities of the molecular and cellular mechanisms of action of sex steroids in the two tissues; the commonality of a critical role of mechanical forces on tissue mass and function; emerging evidence for an interplay between mechanical forces and hormonal and growth factor signals in both bones and muscles; as well as the current state of evidence for or against a cross-talk between muscles and bone. In addition, we review evidence for the parallels in the development of osteoporosis and sarcopenia with advancing age and the potential common mechanisms responsible for the age-dependent involution of these two tissues. Lastly, we discuss the striking difference in the availability of several drug therapies for the prevention and treatment of osteoporosis, as compared to none for sarcopenia. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Distinct Etiological Roles for Myocytes and Motor Neurons in a Mouse Model of Kennedy's Disease/Spinobulbar Muscular Atrophy

Polyglutamine (polyQ) expansion of the androgen receptor (AR) causes Kennedy's disease/spinobulbar muscular atrophy (KD/SBMA) through poorly defined cellular mechanisms. Although KD/SBMA has been thought of as a motor neuron disease, recent evidence indicates a key role for skeletal muscle. To resolve which early aspects of the disease can be caused by neurogenic or myogenic mechanisms, we made use of the tet-On and Cre-loxP genetic systems to selectively and acutely express polyQ AR in either motor neurons (NeuroAR) or myocytes (MyoAR) of transgenic mice. After 4 weeks of transgene induction in adulthood, deficits in gross motor function were seen in NeuroAR mice, but not MyoAR mice. Conversely, reduced size of fast glycolytic fibers and alterations in expression of candidate genes were observed only in MyoAR mice. Both NeuroAR and MyoAR mice exhibited reduced oxidative capacity in skeletal muscles, as well as a shift in fast fibers from oxidative to glycolytic. Markers of oxidative stress were increased in the muscle of NeuroAR mice and were reduced in motor neurons of both NeuroAR and MyoAR mice. Despite secondary pathology in skeletal muscle and behavioral deficits, no pathological signs were observed in motor neurons of NeuroAR mice, possibly due to relatively low levels of polyQ AR expression. These results indicate that polyQ AR in motor neurons can produce secondary pathology in muscle. Results also support both neurogenic and myogenic contributions of polyQ AR to several acute aspects of pathology and provide further evidence for disordered cellular respiration in KD/SBMA skeletal muscle.

Testosterone and obesity

Testosterone is a key hormone in the pathology of metabolic diseases such as obesity. Low testosterone levels are associated with increased fat mass (particularly central adiposity) and reduced lean mass in males. These morphological features are linked to metabolic dysfunction, and testosterone deficiency is associated with energy imbalance, impaired glucose control, reduced insulin sensitivity and dyslipidaemia. A bidirectional relationship between testosterone and obesity underpins this association indicated by the hypogonadal-obesity cycle and evidence weight loss can lead to increased testosterone levels. Androgenic effects on enzymatic pathways of fatty acid metabolism, glucose control and energy utilization are apparent and often tissue specific with differential effects noted in different regional fat depots, muscle and liver to potentially explain the mechanisms of testosterone action. Testosterone replacement therapy demonstrates beneficial effects on measures of obesity that are partially explained by both direct metabolic actions on adipose and muscle and also potentially by increasing motivation, vigour and energy allowing obese individuals to engage in more active lifestyles. The degree of these beneficial effects may be dependent on the treatment modality with longer term administration often achieving greater improvements. Testosterone replacement may therefore potentially be an effective adjunctive treatment for weight management in obese men with concomitant hypogonadism.

The role of androgens in metabolism, obesity, and diabetes in males and females

OBJECTIVE

In men, androgen deprivation contributes to the development of metabolic syndrome and type 2 diabetes (T2D). In women, androgen excess predisposes to insulin resistance and T2D. There is a bidirectional modulation of glucose homeostasis by androgens in males and females that is analyzed in this review.

METHODS

We reviewed the literature in both rodents and humans on the role of androgens and the androgen receptor (AR) in the control of glucose and energy metabolism in health, obesity, and T2D.

RESULTS

Sex-specific activation of AR in the hypothalamus, skeletal muscle, liver, adipose tissue, and pancreatic islet β-cells accounts for maintenance or disruption in energy metabolism and glucose homeostasis.

CONCLUSIONS

We argue that AR is a target to prevent androgen-related metabolic disorders.

Contractile dysfunction in muscle may underlie androgen-dependent motor dysfunction in spinal bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is characterized by progressive muscle weakness linked to a polyglutamine expansion in the androgen receptor (AR). Current evidence indicates that mutant AR causes SBMA by acting in muscle to perturb its function. However, information about how muscle function is impaired is scant. One fundamental question is whether the intrinsic strength of muscles, an attribute of muscle independent of its mass, is affected. In the current study, we assess the contractile properties of hindlimb muscles in vitro from chronically diseased males of three different SBMA mouse models: a transgenic (Tg) model that broadly expresses a full-length human AR with 97 CAGs (97Q), a knock-in (KI) model that expresses a humanized AR containing a CAG expansion in the first exon, and a Tg myogenic model that overexpresses wild-type AR only in skeletal muscle fibers. We found that hindlimb muscles in the two Tg models (97Q and myogenic) showed marked losses in their intrinsic strength and resistance to fatigue, but were minimally affected in KI males. However, diseased muscles of all three models showed symptoms consistent with myotonic dystrophy type 1, namely, reduced resting membrane potential and deficits in chloride channel mRNA. These data indicate that muscle dysfunction is a core feature of SBMA caused by at least some of the same pathogenic mechanisms as myotonic dystrophy. Thus mechanisms controlling muscle function per se independent of mass are prime targets for SBMA therapeutics.

Managing anabolic steroids in pre-hibernating Arctic ground squirrels: obtaining their benefits and avoiding their costs

Androgens have benefits, such as promoting muscle growth, but also significant costs, including suppression of immune function. In many species, these trade-offs in androgen action are reflected in regulated androgen production, which is typically highest only in reproductive males. However, all non-reproductive Arctic ground squirrels, irrespective of age and sex, have high levels of androgens prior to hibernating at sub-zero temperatures. Androgens appear to be required to make muscle in summer, which, together with lipid, is then catabolized during overwinter. By contrast, most hibernating mammals catabolize only lipid. We tested the hypothesis that androgen action is selectively enhanced in Arctic ground squirrel muscle because of an upregulation of androgen receptors (ARs). Using Western blot analysis, we found that Arctic ground squirrels have AR in skeletal muscle more than four times that of Columbian ground squirrels, a related southern species that overwinters at approximately 0°C and has low pre-hibernation androgen levels. By contrast, AR in lymph nodes was equivalent in both species. Brain AR was also modestly but significantly increased in Arctic ground squirrel relative to Columbian ground squirrel. These results are consistent with the hypothesis that tissue-specific AR regulation prior to hibernation provides a mechanism whereby Arctic ground squirrels obtain the life-history benefits and mitigate the costs associated with high androgen production.

Androgen receptor roles in insulin resistance and obesity in males: the linkage of androgen-deprivation therapy to metabolic syndrome

Prostate cancer (PCa) is one of the most frequently diagnosed malignancies in men. Androgen-deprivation therapy (ADT) is the first-line treatment and fundamental management for men with advanced PCa to suppress functions of androgen/androgen receptor (AR) signaling. ADT is effective at improving cancer symptoms and prolonging survival. However, epidemiological and clinical studies support the notion that testosterone deficiency in men leads to the development of metabolic syndrome that increases cardiovascular disease risk. The underlying mechanisms by which androgen/AR signaling regulates metabolic homeostasis in men are complex, and in this review, we discuss molecular mechanisms mediated by AR signaling that link ADT to metabolic syndrome. Results derived from various AR knockout mouse models reveal tissue-specific AR signaling that is involved in regulation of metabolism. These data suggest that steps be taken early to manage metabolic complications associated with PCa patients receiving ADT, which could be accomplished using tissue-selective modulation of AR signaling and by treatment with insulin-sensitizing agents.

Elevated mitochondrial biogenesis in skeletal muscle is associated with testosterone-induced body weight loss in male mice

Androgen reduces fat mass, although the underlying mechanisms are unknown. Here, we examined the effect of testosterone on heat production and mitochondrial biogenesis. Testosterone-treated mice exhibited elevated heat production. Treatment with testosterone increased the expression level of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α), ATP5B and Cox4 in skeletal muscle, but not that in brown adipose tissue and liver. mRNA levels of genes involved in mitochondrial biogenesis were elevated in skeletal muscle isolated from testosterone-treated male mice, but were down-regulated in androgen receptor deficient mice. These results demonstrated that the testosterone-induced increase in energy expenditure is derived from elevated mitochondrial biogenesis in skeletal muscle.

Testosterone and glucose metabolism in men: current concepts and controversies

A wealth of observational studies show that low testosterone is associated with insulin resistance and with an increased risk of diabetes and the metabolic syndrome. Experimental studies have identified potential mechanisms by which low testosterone may lead to insulin resistance. Visceral adipose tissue is an important intermediate in this relationship. Actions of testosterone or its metabolite oestradiol on other tissues such as muscle, liver, bone or the brain, and body composition-independent effects may also play a role. However, definitive evidence from randomised controlled trials (RCTs) to clarify whether the association of low testosterone with disordered glucose metabolism is causative is currently lacking. It therefore remains possible that this association is due to reverse causation, or simply originates by association with common health and lifestyle factors. RCTs of testosterone therapy in men with or without diabetes consistently show modest metabolically favourable changes in body composition. Despite this, testosterone effects on glucose metabolism have been inconsistent. Recent evidence suggests that the hypothalamic-pituitary-testicular axis suppression in the majority of obese men with metabolic disorders is functional, and may be, at least in part, reversible with weight loss. Until further evidence is available, lifestyle measures with emphasis on weight reduction, treatment of comorbidities and optimisation of diabetic control should remain the first-line treatment in these men. Such measures, if successful, may be sufficient to normalise testosterone levels in men with metabolic disorders, who typically have only modest reductions in circulating testosterone levels.

Mitochondrial and sex steroid hormone crosstalk during aging

Decline in circulating sex steroid hormones accompanies several age-associated pathologies which may influence human healthspan. Mitochondria play important roles in biosynthesis of sex steroid hormones, and these hormones can also regulate mitochondrial function. Understanding the cross talk between mitochondria and sex steroid hormones may provide insights into the pathologies associated with aging. The aim of this review is to summarize the current knowledge regarding the interplay between mitochondria and sex steroid hormones during the aging process. The review describes the effect of mitochondria on sex steroid hormone production in the gonads, and then enumerates the contribution of sex steroid hormones on mitochondrial function in hormone responsive cells. Decline in sex steroid hormones and accumulation of mitochondrial damage may create a positive feedback loop that contributes to the progressive degeneration in tissue function during aging. The review further speculates whether regulation between mitochondrial function and sex steroid hormone action can potentially influence healthspan.

Molecular mechanisms of muscle plasticity with exercise

The skeletal muscle phenotype is subject to considerable malleability depending on use. Low-intensity endurance type exercise leads to qualitative changes of muscle tissue characterized mainly by an increase in structures supporting oxygen delivery and consumption. High-load strength-type exercise leads to growth of muscle fibers dominated by an increase in contractile proteins. In low-intensity exercise, stress-induced signaling leads to transcriptional upregulation of a multitude of genes with Ca(2+) signaling and the energy status of the muscle cells sensed through AMPK being major input determinants. Several parallel signaling pathways converge on the transcriptional co-activator PGC-1α, perceived as being the coordinator of much of the transcriptional and posttranscriptional processes. High-load training is dominated by a translational upregulation controlled by mTOR mainly influenced by an insulin/growth factor-dependent signaling cascade as well as mechanical and nutritional cues. Exercise-induced muscle growth is further supported by DNA recruitment through activation and incorporation of satellite cells. Crucial nodes of strength and endurance exercise signaling networks are shared making these training modes interdependent. Robustness of exercise-related signaling is the consequence of signaling being multiple parallel with feed-back and feed-forward control over single and multiple signaling levels. We currently have a good descriptive understanding of the molecular mechanisms controlling muscle phenotypic plasticity. We lack understanding of the precise interactions among partners of signaling networks and accordingly models to predict signaling outcome of entire networks. A major current challenge is to verify and apply available knowledge gained in model systems to predict human phenotypic plasticity.

Potential for sexual conflict assessed via testosterone-mediated transcriptional changes in liver and muscle of a songbird

Males and females can be highly dimorphic in metabolism and physiology despite sharing nearly identical genomes, and both sexes respond phenotypically to elevated testosterone, a steroid hormone that alters gene expression. Only recently has it become possible to learn how a hormone such as testosterone affects global gene expression in non-model systems, and whether it affects the same genes in males and females. To investigate the transcriptional mechanisms by which testosterone exerts its metabolic and physiological effects on the periphery, we compared gene expression by sex and in response to experimentally elevated testosterone in a well-studied bird species, the dark-eyed junco (Junco hyemalis). We identified 291 genes in the liver and 658 in the pectoralis muscle that were differentially expressed between males and females. In addition, we identified 1727 genes that were differentially expressed between testosterone-treated and control individuals in at least one tissue and sex. Testosterone treatment altered the expression of only 128 genes in both males and females in the same tissue, and 847 genes were affected significantly differently by testosterone treatment in the two sexes. These substantial differences in transcriptional response to testosterone suggest that males and females may employ different pathways when responding to elevated testosterone, despite the fact that many phenotypic effects of experimentally elevated testosterone are similar in both sexes. In contrast, of the 121 genes that were affected by testosterone treatment in both sexes, 78% were regulated in the same direction (e.g. either higher or lower in testosterone-treated than control individuals) in both males and females. Thus, it appears that testosterone acts through both unique and shared transcriptional pathways in males and females, suggesting multiple mechanisms by which sexual conflict can be mediated.

Androgen receptors in muscle fibers induce rapid loss of force but not mass: implications for spinal bulbar muscular atrophy

INTRODUCTION

Testosterone (T) induces motor dysfunction in transgenic (Tg) mice that overexpress wild-type androgen receptor (AR) in skeletal muscles. Because many genes implicated in motor neuron disease are expressed in skeletal muscle, mutant proteins may act in muscle to cause dysfunction in motor neuron disease.

METHODS

We examined contractile properties of the extensor digitorum longus (EDL) and soleus (SOL) muscles in vitro after 5 and 3 days of T treatment in motor-impaired Tg female mice.

RESULTS

Both muscles showed deficits in tetanic force after 5 days of T treatment, without losses in muscle mass, protein content, or fiber number. After 3 days of T treatment, only SOL showed a deficit in tetanic force comparable to that of 5 days of treatment. In both treatments, EDL showed slowed twitch kinetics, whereas SOL showed deficits in the twitch/tetanus ratio.

CONCLUSIONS

These results suggest calcium-handling mechanisms in muscle fibers are defective in motor-impaired mice.

Androgen receptor expression in satellite cells of the neonatal levator ani of the rat

Androgens are thought to mediate sexual differentiation of spinal nucleus of the bulbocavernosus (SNB) motoneurons via actions on androgen receptors (ARs) within their target muscles bulbocavernosus and levator ani (LA). However, the cells within these muscles which mediate masculinization of the SNB remain undefined. Until recently, myocytes were thought to be the most likely candidate cell type. However, genetic tests of AR function in myocytes have failed to support a sufficient role for these cells in producing masculine SNB morphology, suggesting the involvement of other cell types. To identify other candidate cell types in the LA, we evaluated whether satellite cells or fibroblasts express AR. Fluorescent immunohistochemistry and confocal microscopy were used to evaluate whether satellite cells and fibroblasts express AR in neonatal male and female rats in the LA and an adjacent sexually monomorphic control muscle (CM). We found that a small proportion of satellite cells in the LA express AR and that this proportion is significantly greater in the LA compared to the CM. No sex differences were found between the proportions of satellite cells expressing AR in either muscle. Less colocalization of satellite cells and AR was seen in postnatal day 3 muscle than in postnatal day 1 muscle. In contrast, only negligible amounts of fibroblasts labeled with S100A4 express AR in either the LA or the CM. Together, findings support satellite cells, but not fibroblasts, as a candidate cell type involved in the sexual differentiation of the SNB neuromuscular system.

Testosterone plus low-intensity physical training in late life improves functional performance, skeletal muscle mitochondrial biogenesis, and mitochondrial quality control in male mice

Testosterone supplementation increases muscle mass in older men but has not been shown to consistently improve physical function and activity. It has been hypothesized that physical exercise is required to induce the adaptations necessary for translation of testosterone-induced muscle mass gain into functional improvements. However, the effects of testosterone plus low intensity physical exercise training (T/PT) on functional performance and bioenergetics are unknown. In this pilot study, we tested the hypothesis that combined administration of T/PT would improve functional performance and bioenergetics in male mice late in life more than low-intensity physical training alone. 28-month old male mice were randomized to receive T/PT or vehicle plus physical training (V/PT) for 2 months. Compare to V/PT control, administration of T/PT was associated with improvements in muscle mass, grip strength, spontaneous physical movements, and respiratory activity. These changes were correlated with increased mitochondrial DNA copy number and expression of markers for mitochondrial biogenesis. Mice receiving T/PT also displayed increased expression of key elements for mitochondrial quality control, including markers for mitochondrial fission-and-fusion and mitophagy. Concurrently, mice receiving T/PT also displayed increased expression of markers for reduced tissue oxidative damage and improved muscle quality. Conclusion: Testosterone administered with low-intensity physical training improves grip strength, spontaneous movements, and respiratory activity. These functional improvements were associated with increased muscle mitochondrial biogenesis and improved mitochondrial quality control.

Turning sex inside-out: Peripheral contributions to sexual differentiation of the central nervous system

Sexual differentiation of the nervous system occurs via the interplay of genetics, endocrinology and social experience through development. Much of the research into mechanisms of sexual differentiation has been driven by an implicit theoretical framework in which these causal factors act primarily and directly on sexually dimorphic neural populations within the central nervous system. This review will examine an alternative explanation by describing what is known about the role of peripheral structures and mechanisms (both neural and non-neural) in producing sex differences in the central nervous system. The focus of the review will be on experimental evidence obtained from studies of androgenic masculinization of the spinal nucleus of the bulbocavernosus, but other systems will also be considered.

Androgens and skeletal muscle: cellular and molecular action mechanisms underlying the anabolic actions

Androgens increase both the size and strength of skeletal muscle via diverse mechanisms. The aim of this review is to discuss the different cellular targets of androgens in skeletal muscle as well as the respective androgen actions in these cells leading to changes in proliferation, myogenic differentiation, and protein metabolism. Androgens bind and activate a specific nuclear receptor which will directly affect the transcription of target genes. These genes encode muscle-specific transcription factors, enzymes, structural proteins, as well as microRNAs. In addition, anabolic action of androgens is partly established through crosstalk with other signaling molecules such as Akt, myostatin, IGF-I, and Notch. Finally, androgens may also exert non-genomic effects in muscle by increasing Ca(2+) uptake and modulating kinase activities. In conclusion, the anabolic effect of androgens on skeletal muscle is not only explained by activation of the myocyte androgen receptor but is also the combined result of many genomic and non-genomic actions.

Androgens and adipose tissue in males: a complex and reciprocal interplay

Clinical evidence shows that in males obesity is frequently associated with hypogonadism and vice versa; also, low testosterone levels have been considered a "hallmark" of metabolic syndrome in men. These observations indicate that there is a strict connection between anatomically and functionally distinct cell types such as white adipocytes and Leydig cells, that synthesize testosterone. Adipose tissue is able to control several functions of the testis through its products secreted in the bloodstream. On the other hand, circulating levels of testosterone and estradiol deeply affect adipocyte proliferation, differentiation, and fat mass distribution, hereby controlling critical metabolic functions, such as food intake, insulin sensitivity, vascular reactivity, and immunity. This paper highlights the existing clinical and experimental evidence linking androgens and adipose tissue and illustrates the consequences occurring when the balance between fat mass distribution and eugonadism is lost.

Body composition changes and inhibition of fat development in vivo implicates androgen in regulation of stem cell lineage allocation

Androgens regulate body composition in youth and declining testosterone that occurs with aging is associated with muscle wasting, increased fat mass and osteopenia. Transgenic mice with targeted androgen receptor (AR) over-expression in mesenchymal stem cells (MSC) were generated to explore the role of androgen signaling in the regulation of body composition. Transgenic males, but not females, were shorter and have reduced body weight and visceral fat accumulation. Dual-energy X-ray absorptiometry (DXA) revealed significant reductions in fat mass with a reciprocal increase in lean mass, yet no difference in food consumption or locomotor activity was observed. Adipose tissue weight was normal in brown fat but reduced in both gonadal and perirenal depots, and reduced hyperplasia was observed with smaller adipocyte size in visceral and subcutaneous white adipose tissue. Although serum leptin, adiponectin, triglyceride, and insulin levels were no different between the genotypes, intraperitoneal glucose tolerance testing (IPGTT) showed improved glucose clearance in transgenic males. High levels of the AR transgene are detected in MSCs but not in mature fat tissue. Reduced fibroblast colony forming units indicate fewer progenitor cells resident in the marrow in vivo. Precocious expression of glucose transporter 4 (GLUT4), peroxisome proliferator-activated receptor γ (PPARγ), and CCAAT enhancer-binding protein α (C/EBPα) was observed in proliferating precursor cultures from transgenic mice compared to controls. In more mature cultures, there was little difference between the genotypes. We propose a mechanism where enhanced androgen sensitivity can alter lineage commitment in vivo to reduce progenitor number and fat development, while increasing the expression of key factors to promote smaller adipocytes with improved glucose clearance.

Overexpression of androgen receptors in target musculature confers androgen sensitivity to motoneuron dendrites

The dendritic arbors of spinal motoneurons are dynamically regulated by a variety of factors, and several lines of evidence indicate that trophic interactions with the target musculature are of central importance. In highly androgen-sensitive motoneuron populations, androgens are thought to regulate motoneuron dendrites through their action at the receptor-enriched target musculature. Using rats transgenically modified to overexpress androgen receptor (AR) in skeletal muscle, we directly tested the hypothesis that the enhanced expression of AR in the target musculature can underlie the androgenic regulation of motoneuron dendritic morphology. The morphology of motoneurons innervating the quadriceps muscle was examined in wild-type (WT) rats as well as in rats that had been transgenically modified to overexpress ARs in their skeletal musculature. Motoneurons innervating the vastus lateralis muscle of the quadriceps in gonadally intact male rats, and castrated males with or without androgen replacement, were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. In WT rats, quadriceps motoneuron dendrites were insensitive to hormonal manipulation. In contrast, quadriceps motoneuron dendrites in gonadally intact transgenic males were larger than those of WT males. Furthermore, overexpression of ARs in the quadriceps muscle resulted in androgen sensitivity in dendrites, with substantial reductions in dendritic length occurring after castration; this reduction was prevented with testosterone replacement. Thus, it appears that the androgen sensitivity of motoneuron dendrites is conferred indirectly via the enrichment of ARs in the musculature.

Estrogen and androgen receptors: regulators of fuel homeostasis and emerging targets for diabetes and obesity

Because of increasing life expectancy, the contribution of age-related estrogen or androgen deficiency to obesity and type 2 diabetes will become a new therapeutic challenge. This review integrates current concepts on the mechanisms through which estrogen receptors (ERs) and androgen receptor (AR) regulate energy homeostasis in rodents and humans. In females, estrogen maintains energy homeostasis via ERα and ERβ, by suppressing energy intake and lipogenesis, enhancing energy expenditure, and ameliorating insulin secretion and sensitivity. In males, testosterone is converted to estrogen and maintains fuel homeostasis via ERs and AR, which share related functions to suppress adipose tissue accumulation and improve insulin sensitivity. We suggest that ERs and AR could be potential targets in the prevention of age-related metabolic disorders.

Microarray analysis of gene expression by skeletal muscle of three mouse models of Kennedy disease/spinal bulbar muscular atrophy

BACKGROUND

Emerging evidence implicates altered gene expression within skeletal muscle in the pathogenesis of Kennedy disease/spinal bulbar muscular atrophy (KD/SBMA). We therefore broadly characterized gene expression in skeletal muscle of three independently generated mouse models of this disease. The mouse models included a polyglutamine expanded (polyQ) AR knock-in model (AR113Q), a polyQ AR transgenic model (AR97Q), and a transgenic mouse that overexpresses wild type AR solely in skeletal muscle (HSA-AR). HSA-AR mice were included because they substantially reproduce the KD/SBMA phenotype despite the absence of polyQ AR.

METHODOLOGY/PRINCIPAL FINDINGS

We performed microarray analysis of lower hindlimb muscles taken from these three models relative to wild type controls using high density oligonucleotide arrays. All microarray comparisons were made with at least 3 animals in each condition, and only those genes having at least 2-fold difference and whose coefficient of variance was less than 100% were considered to be differentially expressed. When considered globally, there was a similar overlap in gene changes between the 3 models: 19% between HSA-AR and AR97Q, 21% between AR97Q and AR113Q, and 17% between HSA-AR and AR113Q, with 8% shared by all models. Several patterns of gene expression relevant to the disease process were observed. Notably, patterns of gene expression typical of loss of AR function were observed in all three models, as were alterations in genes involved in cell adhesion, energy balance, muscle atrophy and myogenesis. We additionally measured changes similar to those observed in skeletal muscle of a mouse model of Huntington's Disease, and to those common to muscle atrophy from diverse causes.

CONCLUSIONS/SIGNIFICANCE

By comparing patterns of gene expression in three independent models of KD/SBMA, we have been able to identify candidate genes that might mediate the core myogenic features of KD/SBMA.