Castration alters protein balance after high-frequency muscle contraction

Protein turnover in pigs: A review of interacting factors

Protein turnover defines the balance between two continuous and complex processes of protein metabolism, synthesis and degradation, which determine their deposition in tissues. Although the liver and intestine have been studied extensively for their important roles in protein digestion, absorption and metabolism, the study of protein metabolism has focused mainly on skeletal muscle tissue to understand the basis for its growth. Due to the high adaptability of skeletal muscle, its protein turnover is greatly affected by different internal and external factors, contributing to carcass lean-yield and animal growth. Amino acid (AA) labelling and tracking using isotope tracer methodology, together with the study of myofiber type profiling, signal transduction pathways and gene expression, has allowed the analysis of these mechanisms from different perspectives. Positive stimuli such as increased nutrient availability in the diet (e.g., AA), physical activity, the presence of certain hormones (e.g., testosterone) or a more oxidative myofiber profile in certain muscles or pig genotypes promote increased upregulation of translation and transcription-related genes, activation of mTORC1 signalling mechanisms and increased abundance of satellite cells, allowing for more efficient protein synthesis. However, fasting, animal aging, inactivity and stress, inflammation or sepsis produce the opposite effect. Deepening the understanding of modifying factors and their possible interaction may contribute to the design of optimal strategies to better control tissue growth and nutrient use (i.e., protein and AA), and thus advance the precision feeding strategy.

The contribution of mitochondria to age-related skeletal muscle wasting: A sex-specific perspective

As people age, their skeletal muscle (SkM) experiences a decline in mitochondrial functionality and density, which leads to decreased energy production and increased generation of reactive oxygen species. This cascade of events, in turn, might determine the loss of SkM mass, strength and quality. Even though the mitochondrial processes dysregulated by aging, such as oxidative phosphorylation, mitophagy, antioxidant defenses and mtDNA transcription, are the same in both sexes, mitochondria age differently in the SkM of men and women. Indeed, the onset and magnitude of the impairment of these processes seem to be influenced by sex-specific factors. Sexual hormones play a pivotal role in the regulation of SkM mass through both genomic and non-genomic mechanisms. However, the precise mechanisms by which these hormones regulate mitochondrial plasticity in SkM are not fully understood. Although the presence of estrogen receptors in mitochondria is recognized, it remains unclear whether androgen receptors affect mitochondrial function. This comprehensive review critically dissects the current knowledge on the interplay of sex in the aging of SkM, focusing on the role of sex hormones and the corresponding signaling pathways in shaping mitochondrial plasticity. Improved knowledge on the sex dimorphism of mitochondrial aging may lead to sex-tailored interventions that target mitochondrial health, which could be effective in slowing or preventing age-related muscle loss.

An overview on androgen-mediated actions in skeletal muscle and adipose tissue

Androgens are a class of steroid hormones primarily associated with male sexual development and physiology, but exert pleiotropic effects in either sex. They have a crucial role in various physiological processes, including the regulation of skeletal muscle and adipose tissue homeostasis. The effects of androgens are mainly mediated through the androgen receptor (AR), a ligand-activated nuclear receptor expressed in both tissues. In skeletal muscle, androgens via AR exert a multitude of effects, ranging from increased muscle mass and strength, to the regulation of muscle fiber type composition, contraction and metabolic functions. In adipose tissue, androgens influence several processes including proliferation, fat distribution, and metabolism but they display depot-specific and organism-specific effects which differ in certain context. This review further explores the potential mechanisms underlying androgen-AR signaling in skeletal muscle and adipose tissue. Understanding the roles of androgens and their receptor in skeletal muscle and adipose tissue is essential for elucidating their contributions to physiological processes, disease conditions, and potential therapeutic interventions.

Chronic Alcohol Consumption Disrupts the Skeletal Muscle Circadian Clock in Female Mice

The intrinsic skeletal muscle core clock has emerged as a key feature of metabolic control and influences several aspects of muscle physiology. Acute alcohol intoxication disrupts the core molecular clock, but whether chronic consumption, like that leading to alcoholic myopathy, is also a zeitgeber for skeletal muscle remains unknown. The purpose of this work was to determine whether chronic alcohol consumption dysregulates the skeletal muscle core molecular clock and clock-controlled genes (CCGs). C57BL/6Hsd female mice (14 weeks old) were fed a control (CON) or alcohol (EtOH) containing liquid diet for 6 weeks. Gastrocnemius muscles and serum were collected from CON and EtOH mice every 4-h for 24-h. Chronic alcohol consumption disrupted genes of the core clock including suppressing the rhythmic peak of expression of Bmal1, Per1, Per2, and Cry2. Genes involved in the regulation of Bmal1 also exhibited lower rhythmic peaks including Reverb α and Myod1. The CCGs, Dbp, Lpl, Hk2, and Hadh were also suppressed by alcohol. The nuclear expression patterns of MYOD1, DBP, and REVERBα were shifted by alcohol, while no change in BMAL1 was detected. Overall, these data indicate that alcohol disrupted the skeletal muscle core clock but whether these changes in the core clock are causative or a consequence of alcoholic myopathy requires future mechanistic confirmation.

Pharmacological hypogonadism impairs molecular transducers of exercise-induced muscle growth in humans

BACKGROUND

The relative role of skeletal muscle mechano-transduction in comparison with systemic hormones, such as testosterone (T), in regulating hypertrophic responses to exercise is contentious. We investigated the mechanistic effects of chemical endogenous T depletion adjuvant to 6 weeks of resistance exercise training (RET) on muscle mass, function, myogenic regulatory factors, and muscle anabolic signalling in younger men.

METHODS

Non-hypogonadal men (n = 16; 18-30 years) were randomized in a double-blinded fashion to receive placebo (P, saline n = 8) or the GnRH analogue, Goserelin [Zoladex (Z), 3.6 mg, n = 8], injections, before 6 weeks of supervised whole-body RET. Participants underwent dual-energy X-ray absorptiometry (DXA), ultrasound of m. vastus lateralis (VL), and VL biopsies for assessment of cumulative muscle protein synthesis (MPS), myogenic gene expression, and anabolic signalling pathway responses.

RESULTS

Zoladex suppressed endogenous T to within the hypogonadal range and was well tolerated; suppression was associated with blunted fat free mass [Z: 55.4 ± 2.8 to 55.8 ± 3.1 kg, P = 0.61 vs. P: 55.9 ± 1.7 to 57.4 ± 1.7 kg, P = 0.006, effect size (ES) = 0.31], composite strength (Z: 40 ± 2.3% vs. P: 49.8 ± 3.3%, P = 0.03, ES = 1.4), and muscle thickness (Z: 2.7 ± 0.4 to 2.69 ± 0.36 cm, P > 0.99 vs. P: 2.74 ± 0.32 to 2.91 ± 0.32 cm, P < 0.0001, ES = 0.48) gains. Hypogonadism attenuated molecular transducers of muscle growth related to T metabolism (e.g. androgen receptor: Z: 1.2 fold, P > 0.99 vs. P: 1.9 fold, P < 0.0001, ES = 0.85), anabolism/myogenesis (e.g. IGF-1Ea: Z: 1.9 fold, P = 0.5 vs. P: 3.3 fold, P = 0.0005, ES = 0.72; IGF-1Ec: Z: 2 fold, P > 0.99 vs. P: 4.7 fold, P = 0.0005, ES = 0.68; myogenin: Z: 1.3 fold, P > 0.99 vs. P: 2.7 fold, P = 0.002, ES = 0.72), RNA/DNA (Z: 0.47 ± 0.03 to 0.53 ± 0.03, P = 0.31 vs. P: 0.50 ± 0.01 to 0.64 ± 0.04, P = 0.003, ES = 0.72), and RNA/ASP (Z: 5.8 ± 0.4 to 6.8 ± 0.5, P > 0.99 vs. P: 6.5 ± 0.2 to 8.9 ± 1.1, P = 0.008, ES = 0.63) ratios, as well as acute RET-induced phosphorylation of growth signalling proteins (e.g. AKT^ser473 : Z: 2.74 ± 0.6, P = 0.2 vs. P: 5.5 ± 1.1 fold change, P < 0.001, ES = 0.54 and mTORC1^ser2448 : Z: 1.9 ± 0.8, P > 0.99 vs. P: 3.6 ± 1 fold change, P = 0.002, ES = 0.53). Both MPS (Z: 1.45 ± 0.11 to 1.50 ± 0.06%·day^-1 , P = 0.99 vs. P: 1.5 ± 0.12 to 2.0 ± 0.15%·day^-1 , P = 0.01, ES = 0.97) and (extrapolated) muscle protein breakdown (Z: 93.16 ± 7.8 vs. P: 129.1 ± 13.8 g·day^-1 , P = 0.04, ES = 0.92) were reduced with hypogonadism result in lower net protein turnover (3.9 ± 1.1 vs. 1.2 ± 1.1 g·day^-1 , P = 0.04, ES = 0.95).

CONCLUSIONS

We conclude that endogenous T sufficiency has a central role in the up-regulation of molecular transducers of RET-induced muscle hypertrophy in humans that cannot be overcome by muscle mechano-transduction alone.

Molecular Regulators of Muscle Mass and Mitochondrial Remodeling Are Not Influenced by Testosterone Administration in Young Women

Testosterone (T) administration has previously been shown to improve muscle size and oxidative capacity. However, the molecular mechanisms underlying these adaptations in human skeletal muscle remain to be determined. Here, we examined the effect of moderate-dose T administration on molecular regulators of muscle protein turnover and mitochondrial remodeling in muscle samples collected from young women. Forty-eight healthy, physically active, young women (28 ± 4 years) were assigned in a random double-blind fashion to receive either T (10 mg/day) or placebo for 10-weeks. Muscle biopsies collected before and after the intervention period were divided into sub-cellular fractions and total protein levels of molecular regulators of muscle protein turnover and mitochondrial remodeling were analyzed using Western blotting. T administration had no effect on androgen receptor or 5α-reductase levels, nor on proteins involved in the mTORC1-signaling pathway (mTOR, S6K1, eEF2 and RPS6). Neither did it affect the abundance of proteins associated with proteasomal protein degradation (MAFbx, MuRF-1 and UBR5) and autophagy-lysosomal degradation (AMPK, ULK1 and p62). T administration also had no effect on proteins in the mitochondria enriched fraction regulating mitophagy (Beclin, BNIP3, LC3B-I, LC3B-II and LC3B-II/I ratio) and morphology (Mitofilin), and it did not alter the expression of mitochondrial fission- (FIS1 and DRP1) or fusion factors (OPA1 and MFN2). In summary, these data indicate that improvements in muscle size and oxidative capacity in young women in response to moderate-dose T administration cannot be explained by alterations in total expression of molecular factors known to regulate muscle protein turnover or mitochondrial remodeling.

Binge alcohol disrupts skeletal muscle core molecular clock independent of glucocorticoids

Circadian rhythms are central to optimal physiological function, as disruption contributes to the development of several chronic diseases. Alcohol (EtOH) intoxication disrupts circadian rhythms within liver, brain, and intestines, but it is unknown whether alcohol also disrupts components of the core clock in skeletal muscle. Female C57BL/6Hsd mice were randomized to receive either saline (control) or alcohol (EtOH) (5 g/kg) via intraperitoneal injection at the start of the dark cycle [Zeitgeber time (ZT12)], and gastrocnemius was collected every 4 h from control and EtOH-treated mice for the next 48 h following isoflurane anesthetization. In addition, metyrapone was administered before alcohol intoxication in separate mice to determine whether the alcohol-induced increase in serum corticosterone contributed to circadian gene regulation. Finally, synchronized C2C12 myotubes were treated with alcohol (100 mM) to assess the influence of centrally or peripherally mediated effects of alcohol on the muscle clock. Alcohol significantly disrupted mRNA expression of Bmal1, Per1/2, and Cry1/2 in addition to perturbing the circadian pattern of clock-controlled genes, Myod1, Dbp, Tef, and Bhlhe40 (P < 0.05), in muscle. Alcohol increased serum corticosterone levels and glucocorticoid target gene, Redd1, in muscle. Metyrapone prevented the EtOH-mediated increase in serum corticosterone but did not normalize the EtOH-induced change in Per1, Cry1 and Cry2, and Myod1 mRNA expression. Core clock gene expression (Bmal, Per1/2, and Cry1/2) was not changed following 4, 8, or 12 h of alcohol treatment on synchronized C2C12 myotubes. Therefore, binge alcohol disrupted genes of the core molecular clock independently of elevated serum corticosterone or direct effects of EtOH on the muscle.NEW & NOTEWORTHY Alcohol is a myotoxin that impairs skeletal muscle metabolism and function following either chronic consumption or acute binge drinking; however, mechanisms underlying alcohol-related myotoxicity have not been fully elucidated. Herein, we demonstrate that alcohol acutely interrupts oscillation of skeletal muscle core clock genes, and this is neither a direct effect of ethanol on the skeletal muscle, nor an effect of elevated serum corticosterone, a major clock regulator.

Systemic delivery of a mitochondria targeted antioxidant partially preserves limb muscle mass and grip strength in response to androgen deprivation

Muscle mass is important for health. Decreased testicular androgen production (hypogonadism) contributes to the loss of muscle mass, with loss of limb muscle being particularly debilitating. Androgen replacement is the only pharmacological treatment, which may not be feasible for everyone. Prior work showed that markers of reactive oxygen species and markers of mitochondrial degradation pathways were higher in the limb muscle following castration. Therefore, we tested whether an antioxidant preserved limb muscle mass in male mice subjected to a castration surgery. Subsets of castrated mice were treated with resveratrol (a general antioxidant) or MitoQ (a mitochondria targeted antioxidant). Relative to the non-castrated control mice, lean mass, limb muscle mass, and grip strength were partially preserved only in castrated mice treated with MitoQ. Independent of treatment, markers of mitochondrial degradation pathways remained elevated in all castrated mice. Therefore, a mitochondrial targeted antioxidant may partially preserve limb muscle mass in response to hypogonadism.

Dynamic resistance exercise increases skeletal muscle-derived FSTL1 inducing cardiac angiogenesis via DIP2A-Smad2/3 in rats following myocardial infarction

PURPOSE

The aim of this study was to investigate the potential of dynamic resistance exercise to generate skeletal muscle-derived follistatin like-1 (FSTL1), which may induce cardioprotection in rats following myocardial infarction (MI) by inducing angiogenesis.

METHODS

Male, adult Sprague-Dawley rats were randomly divided into 5 groups (n = 12 in each group): sham group (S), sedentary MI group (MI), MI + resistance exercise group (MR), MI + adeno-associated virus (AAV)-FSTL1 injection group (MA), and MI + AAV-FSTL1 injection + resistance exercise group (MAR). The AAV-FSTL1 vector was prepared by molecular biology methods and injected into the anterior tibialis muscle. The MI model was established by ligation of the left anterior descending coronary artery. Rats in the MR and MAR groups underwent 4 weeks of dynamic resistance exercise training using a weighted climbing-up ladder. Heart function was evaluated by hemodynamic measures. Collagen volume fraction of myocardium was observed and analyzed by Masson's staining. Human umbilical vein vessel endothelial cells culture and recombinant human FSTL1 protein or transforming growth factor-β receptor 1 (TGFβR1) inhibitor treatment were used to elucidate the molecular signaling mechanism of FSTL1. Angiogenesis, cell proliferation, and disco interacting protein 2 homolog A (DIP2A) location were observed by immunofluorescence staining. The expression of FSTL1, DIP2A, and the activation of signaling pathways were detected by Western blotting. Angiogenesis of endothelial cells was observed by tubule experiment. One-way analysis of variance and Student's t test were used for statistical analysis.

RESULTS

Resistance exercise stimulated the secretion of skeletal muscle FSTL1, which promoted myocardial angiogenesis, inhibited pathological remodeling, and protected cardiac function in MI rats. Exercise facilitated skeletal muscle FSTL1 to play a role in protecting the heart. Exogenous FSTL1 promoted the human umbilical vein vessel endothelial cells proliferation and up-regulated the expression of DIP2A, while TGFβR1 inhibitor intervention down-regulated the phosphorylation level of Smad2/3 and the expression of vascular endothelial growth factor-A, which was not conducive to angiogenesis. FSTL1 bound to the receptor, DIP2A, to regulate angiogenesis mainly through the Smad2/3 signaling pathway. FSTL1-DIP2A directly activated Smad2/3 and was not affected by TGFβR1.

CONCLUSION

Dynamic resistance exercise stimulates the expression of skeletal muscle-derived FSTL1, which could supplement the insufficiency of cardiac FSTL1 and promote cardiac rehabilitation through the DIP2A-Smad2/3 signaling pathway in MI rats.

Spermidine is not an independent factor regulating limb muscle mass in mice following androgen deprivation

Maintaining a critical amount of skeletal muscle mass is linked to reduced morbidity and mortality. In males, testicular androgens regulate muscle mass with a loss of androgens being critical as it is associated with muscle atrophy. Atrophy of the limb muscles is particularly important, but the pathways by which androgens regulate limb muscle mass remain equivocal. We used microarray analysis to identify changes to genes involved with polyamine metabolism in the tibialis anterior (TA) muscle of castrated mice. Of the polyamines, the concentration of spermidine (SPD) was significantly reduced in the TA of castrated mice. To assess whether SPD was an independent factor by which androgens regulate limb muscle mass, we treated castrated mice with SPD for 8 weeks and compared them with sham operated mice. Though this treatment paradigm effectively restored SPD concentrations in the TA muscles of castrated mice, mass of the limb muscles (i.e., TA, gastrocnemius, plantaris, and soleus) were not increased to the levels observed in sham animals. Consistent with those findings, muscle force production was also not increased by SPD treatment. Overall, these data demonstrate for the first time that SPD is not an independent factor by which androgens regulate limb skeletal muscle mass. Novelty: Polyamines regulate growth in various cells/tissues. Spermidine concentrations are reduced in the limb skeletal muscle following androgen depletion. Restoring spermidine concentrations in the limb skeletal muscle does not increase limb muscle mass or force production.

A clinically relevant decrease in contractile force differentially regulates control of glucocorticoid receptor translocation in mouse skeletal muscle

Muscle atrophy decreases physical function and overall health. Increased glucocorticoid production and/or use of prescription glucocorticoids can significantly induce muscle atrophy by activating the glucocorticoid receptor, thereby transcribing genes that shift protein balance in favor of net protein degradation. Although mechanical overload can blunt glucocorticoid-induced atrophy in young muscle, those affected by glucocorticoids generally have impaired force generation. It is unknown whether contractile force alters the ability of resistance exercise to mitigate glucocorticoid receptor translocation and induce a desirable shift in protein balance when glucocorticoids are elevated. In the present study, mice were subjected to a single bout of unilateral, electrically induced muscle contractions by stimulating the sciatic nerve at 100 Hz or 50 Hz frequencies to elicit high or moderate force contractions of the tibialis anterior, respectively. Dexamethasone was used to activate the glucocorticoid receptor. Dexamethasone increased glucocorticoid signaling, including nuclear translocation of the receptor, but this was mitigated only by high force contractions. The ability of high force contractions to mitigate glucocorticoid receptor translocation coincided with a contraction-mediated increase in muscle protein synthesis, which did not occur in the dexamethasone-treated mice subjected to moderate force contractions. Though moderate force contractions failed to increase protein synthesis following dexamethasone treatment, both high and moderate force contractions blunted the glucocorticoid-mediated increase in LC3 II:I marker of autophagy. Thus, these data show that force generation is important for the ability of resistance exercise to mitigate glucocorticoid receptor translocation and promote a desirable shift in protein balance when glucocorticoids are elevated.NEW & NOTEWORTHY Glucocorticoids induce significant skeletal muscle atrophy by activating the glucocorticoid receptor. Our work shows that muscle contractile force dictates glucocorticoid receptor nuclear translocation. We also show that blunting nuclear translocation by high force contractions coincides with the ability of muscle to mount an anabolic response characterized by increased muscle protein synthesis. This work further defines the therapeutic parameters of skeletal muscle contractions to blunt glucocorticoid-induced atrophy.

Mitochondria in Sex Hormone-Induced Disorder of Energy Metabolism in Males and Females

Androgens have a complex role in the regulation of insulin sensitivity in the pathogenesis of type 2 diabetes. In male subjects, a reduction in androgens increases the risk for insulin resistance, which is improved by androgen injections. However, in female subjects with polycystic ovary syndrome (PCOS), androgen excess becomes a risk factor for insulin resistance. The exact mechanism underlying the complex activities of androgens remains unknown. In this review, a hormone synergy-based view is proposed for understanding this complexity. Mitochondrial overactivation by substrate influx is a mechanism of insulin resistance in obesity. This concept may apply to the androgen-induced insulin resistance in PCOS. Androgens and estrogens both exhibit activities in the induction of mitochondrial oxidative phosphorylation. The two hormones may synergize in mitochondria to induce overproduction of ATP. ATP surplus in the pancreatic β-cells and α-cells causes excess secretion of insulin and glucagon, respectively, leading to peripheral insulin resistance in the early phase of type 2 diabetes. In the skeletal muscle and liver, the ATP surplus contributes to insulin resistance through suppression of AMPK and activation of mTOR. Consistent ATP surplus leads to mitochondrial dysfunction as a consequence of mitophagy inhibition, which provides a potential mechanism for mitochondrial dysfunction in β-cells and brown adipocytes in PCOS. The hormone synergy-based view provides a basis for the overactivation and dysfunction of mitochondria in PCOS-associated type 2 diabetes. The molecular mechanism for the synergy is discussed in this review with a focus on transcriptional regulation. This view suggests a unifying mechanism for the distinct metabolic roles of androgens in the control of insulin action in men with hypogonadism and women with PCOS.

Exercise as a therapy for cancer-induced muscle wasting

Cancer cachexia is a progressive disorder characterized by body weight, fat, and muscle loss. Cachexia induces metabolic disruptions that can be analogous and distinct from those observed in cancer, obscuring both diagnosis and treatment options. Inflammation, hypogonadism, and physical inactivity are widely investigated as systemic mediators of cancer-induced muscle wasting. At the cellular level, dysregulation of protein turnover and energy metabolism can negatively impact muscle mass and function. Exercise is well known for its anti-inflammatory effects and potent stimulation of anabolic signaling. Emerging evidence suggests the potential for exercise to rescue muscle's sensitivity to anabolic stimuli, reduce wasting through protein synthesis modulation, myokine release, and subsequent downregulation of proteolytic factors. To date, there is no recommendation for exercise in the management of cachexia. Given its complex nature, a multimodal approach incorporating exercise offers promising potential for cancer cachexia treatment. This review's primary objective is to summarize the growing body of research examining exercise regulation of cancer cachexia. Furthermore, we will provide evidence for exercise interactions with established systemic and cellular regulators of cancer-induced muscle wasting.

Androgen depletion alters the diurnal patterns to signals that regulate autophagy in the limb skeletal muscle

Hypogonadism contributes to limb skeletal muscle atrophy by increasing rates of muscle protein breakdown. Androgen depletion increases markers of the autophagy protein breakdown pathway in the limb muscle that persist throughout the diurnal cycle. However, the regulatory signals underpinning the increase in autophagy markers remain ill-defined. The purpose of this study was to characterize changes to autophagy regulatory signals in the limb skeletal muscle following androgen depletion. Male mice were subjected to a castration surgery or a sham surgery as a control. Seven weeks post-surgery, a subset of mice from each group was sacrificed every 4 hr over a 24 hr period. Protein and mRNA from the Tibialis Anterior (TA) were subjected to Western blot and RT-PCR. Consistent with an overall increase in autophagy, the phosphorylation pattern of Uncoordinated Like Kinase 1 (ULK1) (Ser555) was elevated throughout the diurnal cycle in the TA of castrated mice. Factors that induce the progression of autophagy were also increased in the TA following androgen depletion including an increase in the phosphorylation of c-Jun N-terminal Kinase (JNK) (Thr183/Tyr185) and an increase in the ratio of BCL-2 Associated X (BAX) to B-cell lymphoma 2 (BCL-2). Moreover, we observed an increase in the protein expression pattern of p53 and the mRNA of the p53 target genes Cyclin-Dependent Kinase Inhibitor 1A (p21) and Growth Arrest and DNA Damage Alpha (Gadd45a), which are known to increase autophagy and induce muscle atrophy. These data characterize novel changes to autophagy regulatory signals in the limb skeletal muscle following androgen deprivation.

Disruptions to the limb muscle core molecular clock coincide with changes in mitochondrial quality control following androgen depletion

Androgen depletion in humans leads to significant atrophy of the limb muscles. However, the pathways by which androgens regulate limb muscle mass are unclear. Our laboratory previously showed that mitochondrial degradation was related to the induction of autophagy and the degree of muscle atrophy following androgen depletion, implying that decreased mitochondrial quality contributes to muscle atrophy. To increase our understanding of androgen-sensitive pathways regulating decreased mitochondrial quality, total RNA from the tibialis anterior of sham and castrated mice was subjected to microarray analysis. Using this unbiased approach, we identified significant changes in the expression of genes that compose the core molecular clock. To assess the extent to which androgen depletion altered the limb muscle clock, the tibialis anterior muscles from sham and castrated mice were harvested every 4 h throughout a diurnal cycle. The circadian expression patterns of various core clock genes and known clock-controlled genes were disrupted by castration, with most genes exhibiting an overall reduction in phase amplitude. Given that the core clock regulates mitochondrial quality, disruption of the clock coincided with changes in the expression of genes involved with mitochondrial quality control, suggesting a novel mechanism by which androgens may regulate mitochondrial quality. These events coincided with an overall increase in mitochondrial degradation in the muscle of castrated mice and an increase in markers of global autophagy-mediated protein breakdown. In all, these data are consistent with a novel conceptual model linking androgen depletion-induced limb muscle atrophy to reduced mitochondrial quality control via disruption of the molecular clock.

Arrdc2 and Arrdc3 elicit divergent changes in gene expression in skeletal muscle following anabolic and catabolic stimuli

Skeletal muscle is a highly plastic organ regulating various processes in the body. As such, loss of skeletal muscle underlies the increased morbidity and mortality risk that is associated with numerous conditions. However, no therapies are available to combat the loss of muscle mass during atrophic conditions, which is due in part to the incomplete understanding of the molecular networks altered by anabolic and catabolic stimuli. Thus, the current objective was to identify novel gene networks modulated by such stimuli. For this, total RNA from the tibialis anterior muscle of mice that were fasted overnight or fasted overnight and refed the next morning was subjected to microarray analysis. The refeeding stimulus altered the expression of genes associated with signal transduction. Specifically, expression of alpha arrestin domain containing 2 (Arrdc2) and alpha arrestin domain containing 3 (Arrdc3) was significantly lowered 70–85% by refeeding. Subsequent analysis showed that expression of these genes was also lowered 50–75% by mechanical overload, with the combination of nutrients and mechanical overload acting synergistically to lower Arrdc2 and Arrdc3 expression. On the converse, stimuli that suppress growth such as testosterone depletion or acute aerobic exercise increased Arrdc2 and Arrdc3 expression in skeletal muscle. While Arrdc2 and Arrdc3 exhibited divergent changes in expression following anabolic or catabolic stimuli, no other member of the Arrdc family of genes exhibited the consistent change in expression across the analyzed conditions. Thus, Arrdc2 and Arrdc3 are a novel set of genes that may be implicated in the regulation of skeletal muscle mass.

Understanding the Role of Exercise in Cancer Cachexia Therapy

Cachexia, the unintentional loss of body weight, is prevalent in many cancer types, and the associated skeletal muscle mass depletion increases patient morbidity and mortality. While anorexia can be present, cachexia is not reversible with nutritional therapies alone. Pharmacological agents have been proposed to treat this condition, but there are currently no approved treatments. Nonetheless, the hallmark characteristics associated with cancer cachexia remain viable foundations for future therapies. Regular physical activity holds a promising future as a nonpharmacological alternative to improve patient survival through cachexia prevention. Evidence suggests exercise training is beneficial during cancer treatment and survival. However, the mechanistic examination of cachectic skeletal muscle's response to exercise is both needed and justified. The primary objective of this review is to discuss the role of exercise for the prevention and treatment of cancer-associated muscle wasting. Initially, we provide an overview of systemic alterations induced by cancer and their role in the regulation of wasting processes during cachexia progression. We then discuss how exercise could alter disrupted regulatory pathways related to growth and metabolism during cancer-induced muscle atrophy. Last, we outline current exercise prescription guidelines and how exercise could be a potential behavioral therapy to curtail cachexia development in cancer patients.

Increased mitochondrial turnover in the skeletal muscle of fasted, castrated mice is related to the magnitude of autophagy activation and muscle atrophy

Androgen-deficiency promotes muscle atrophy in part by increasing autophagy-mediated muscle protein breakdown during the fasted state, but factors contributing to this remain undefined. To identify novel factors, mice were subjected to sham or castration surgery. Seven-weeks post-surgery, mice were fasted overnight, refed for 30 min, and fasted another 4.5 h before sacrifice. BNIP3-mediated turnover of mitochondria was increased within the atrophied tibialis anterior (TA) of castrated mice and related to the magnitude of muscle atrophy and autophagy activation (i.e. decreased p62 protein content), thus linking turnover of potentially dysfunctional mitochondria with autophagy-mediated atrophy. Autophagy induction was likely facilitated by AMPK activation as a stress survival mechanism since phosphorylation of AMPK (Thr172), as well as the pro survival kinases Akt (Thr308) and (ERK1/2 Thr202/Tyr204), were increased by castration. Together, these data identify a novel relationship between mitochondrial turnover in the fasted state with autophagy activation and muscle atrophy following androgen depletion.

Systemic IL-6 regulation of eccentric contraction-induced muscle protein synthesis

Systemic cytokines and contractile activity are established regulators of muscle protein turnover. Paradoxically, the IL-6 cytokine family, which shares the ubiquitously expressed membrane gp130 receptor, has been implicated in skeletal muscle's response to both contractions and cancer-induced wasting. Although we have reported that tumor-derived cachectic factors could suppress stretch-induced protein synthesis in cultured myotubes, the ability of systemic cytokines to disrupt in vivo eccentric contraction-induced protein synthesis has not been established. Therefore, we examined whether systemic IL-6 regulates basal and eccentric contraction-induced protein synthesis through muscle gp130 signaling. Systemic IL-6 overexpression was performed for 2 wk, and we then examined basal and eccentric contraction-induced protein synthesis and mammalian target of rapamycin complex 1 (mTORC1) signaling in tibialis anterior muscle of male wild-type, muscle-specific gp130 receptor knockout, and tumor-bearing ApcMin/+ mice. Systemic IL-6 overexpression suppressed basal protein synthesis and mTORC1 signaling independently of IL-6 level, which was rescued by muscle gp130 loss. Interestingly, only high systemic IL-6 levels suppressed eccentric contraction-induced protein synthesis. Systemic IL-6 overexpression in precachectic tumor-bearing ApcMin/+ mice accelerated cachexia development, which coincided with suppressed basal and eccentric contraction-induced muscle protein synthesis. The suppression of eccentric contraction-induced protein synthesis by IL-6 occurred independently of mTORC1 activation. Collectively, these findings demonstrate that basal protein synthesis suppression was more sensitive to circulating IL-6 compared with the induction of protein synthesis by eccentric contraction. However, systemic IL-6 can interact with the cancer environment to suppress eccentric contraction-induced protein synthesis independently of mTORC1 activation.

Androgens induce growth of the limb skeletal muscles in a rapamycin-insensitive manner

Signaling through the mechanistic target of rapamycin complex 1 (mTORC1) has been well defined as an androgen-sensitive transducer mediating skeletal muscle growth in vitro; however, this has yet to be tested in vivo. As such, male mice were subjected to either sham or castration surgery and allowed to recover for 7 wk to induce atrophy of skeletal muscle. Then, castrated mice were implanted with either a control pellet or a pellet that administered rapamycin (~2.5 mg·kg^-1·day^-1). Seven days postimplant, a subset of castrated mice with control pellets and all castrated mice with rapamycin pellets were given once weekly injections of nandrolone decanoate (ND) to induce muscle growth over a six-week period. Effective blockade of mTORC1 by rapamycin was noted in the skeletal muscle by the inability of insulin to induce phosphorylation of ribosomal S6 kinase 1 70 kDa (Thr389) and uncoordinated-like kinase 1 (Ser⁷⁵⁷). While castration reduced tibialis anterior (TA) mass, muscle fiber cross-sectional area, and total protein content, ND administration restored these measures to sham levels in a rapamycin-insensitive manner. Similar findings were also observed in the plantaris and soleus, suggesting this rapamycin-insensitive effect was not specific to the TA or fiber type. Androgen-mediated growth was not due to changes in translational capacity. Despite these findings in the limb skeletal muscle, rapamycin completely prevented the ND-mediated growth of the heart. In all, these data indicate that mTORC1 has a limited role in the androgen-mediated growth of the limb skeletal muscle; however, mTORC1 was necessary for androgen-mediated growth of heart muscle.

Inflammatory signalling regulates eccentric contraction-induced protein synthesis in cachectic skeletal muscle

BACKGROUND

Skeletal muscle responds to eccentric contractions (ECC) with an anabolic response that involves the induction of protein synthesis through the mechanistic target of rapamycin complex 1. While we have reported that repeated ECC bouts after cachexia initiation attenuated muscle mass loss and inflammatory signalling, cachectic muscle's capacity to induce protein synthesis in response to ECC has not been determined. Therefore, we examined cachectic muscle's ability to induce mechano-sensitive pathways and protein synthesis in response to an anabolic stimulus involving ECC and determined the role of muscle signal transducer and activator of transcription 3 (STAT3)/nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) signalling on ECC-induced anabolic signalling.

METHODS

Mechano-sensitive pathways and anabolic signalling were examined immediately post or 3 h after a single ECC bout in cachectic male Apc^Min/+ mice (n = 17; 16 ± 1% body weight loss). Muscle STAT3/NFκB regulation of basal and ECC-induced anabolic signalling was also examined in an additional cohort of Apc^Min/+ mice (n = 10; 16 ± 1% body weight loss) that received pyrrolidine dithiocarbamate 24 h prior to a single ECC bout. In all experiments, the left tibialis anterior performed ECC while the right tibialis anterior served as intra-animal control. Data were analysed by Student's t-test or two-way repeated measures analysis of variance with Student-Newman-Keuls post-hoc when appropriate. The accepted level of significance was set at P < 0.05 for all analysis.

RESULTS

Apc^Min/+ mice exhibited a cachectic muscle signature demonstrated by perturbed proteostasis (Ribosomal Protein S6 (RPS6), P70S6K, Atrogin-1, and Muscle RING-finger protein-1 (MuRF1)), metabolic (adenosine monophosphate-activated protein kinase, Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), and Cytochrome c oxidase subunit IV (COXIV)), and inflammatory (STAT3, NFκB, extracellular signal-regulated kinases 1 and 2, and P38) signalling pathway regulation. Nonetheless, mechano-sensitive signalling pathways (P38, extracellular signal-regulated kinases 1 and 2, and Protein kinase B (AKT)) were activated immediately post-ECC irrespective of cachexia. While cachexia did not attenuate ECC-induced P70S6K activation, the protein synthesis induction remained suppressed compared with healthy controls. However, muscle STAT3/NFκB inhibition increased basal and ECC-induced protein synthesis in cachectic Apc^Min/+ mice.

CONCLUSIONS

These studies demonstrate that mechano-sensitive signalling is maintained in cachectic skeletal muscle, but chronic STAT3/NFκB signalling serves to attenuate basal and ECC-induced protein synthesis.

The role of androgens in the regulation of muscle oxidative capacity following aerobic exercise training

Reduced production or bioavailability of androgens, termed hypogonadism, occurs in a variety of pathological conditions. While androgens target numerous tissues throughout the body, hypogonadism specifically reduces the ability of skeletal muscle to produce adenosine triphosphate aerobically, i.e., muscle oxidative capacity. This has important implications for overall health as muscle oxidative capacity impacts a number of metabolic processes. Although androgen replacement therapy is effective at restoring muscle oxidative capacity in hypogonadal individuals, this is not a viable therapeutic option for all who are experiencing hypogonadism. While aerobic exercise may be a viable alternative to increase muscle oxidative capacity, it is unknown whether androgen depletion affects this adaptation. To determine this, sham and castrated mice were randomized to remain sedentary or undergo 8 weeks of aerobic treadmill exercise training. All mice were fasted overnight prior to sacrifice. Though exercise increased markers of muscle oxidative capacity independent of castration (cytochrome c oxidase subunit IV and cytochrome c), these measures were lower in castrated mice. This reduction was not due to a difference in peroxisome proliferator activated receptor gamma coactivator 1 alpha protein content, as expression was increased to a similar absolute value in sham and castrated animals following exercise training. However, markers of BCL2/Adenovirus E1B 19 kDa Interacting Protein 3 (BNIP3)-mediated mitophagy were increased by castration independent of exercise. Together, these data show that exercise training can increase markers of muscle oxidative capacity following androgen depletion. However, these values are reduced by androgen depletion likely due in part to elevated BNIP3-mediated mitophagy.

Androgen-mediated regulation of skeletal muscle protein balance

Androgens significantly alter muscle mass in part by shifting protein balance in favor of net protein accretion. During various atrophic conditions, the clinical impact of decreased production or bioavailability of androgens (termed hypogonadism) is important as a loss of muscle mass is intimately linked with survival outcome. While androgen replacement therapy increases muscle mass in part by restoring protein balance, this is not a comprehensive treatment option due to potential side effects. Therefore, an understanding of the mechanisms by which androgens alter protein balance is needed for the development of androgen-independent therapies. While the data in humans suggest androgens alter protein balance (both synthesis and breakdown) in the fasted metabolic state, a predominant molecular mechanism(s) behind this observation is still lacking. This failure is likely due in part to inconsistent experimental design between studies including failure to control nutrient/feeding status, the method of altering androgens, and the model systems utilized.