Sexual Dimorphism of Skeletal Muscle in a Mouse Model of Breast Cancer: A Functional and Molecular Analysis.

Breast cancer incidence in men is statistically rare; however, given the lack of screening in males, more advanced stages at initial diagnosis result in lower 5-year survival rates for men with breast cancer compared to women. A sexual dimorphism, with respect to the effect of tumor growth on cachexia incidence and severity, has also been reported across cancer types. The purpose of this study was to examine the sexual dimorphism of breast cancer as it pertains to skeletal muscle function and molecular composition. Using female and male transgenic PyMT mice, we tested the hypothesis that the isometric contractile properties and molecular composition of skeletal muscle would be differentially affected by breast tumors. PyMT tumor-bearing mice of each sex, corresponding to maximal tumor burden, were compared to their respective controls. RNA sequencing of skeletal muscle revealed different pathway alterations that were exclusive to each sex. Further, differentially expressed genes and pathways were substantially more abundant in female tumor mice, with only minimal dysregulation in male tumor mice, each compared to their respective controls. These differences in the transcriptome were mirrored in isometric contractile properties, with greater tumor-induced dysfunction in females than male mice, as well as muscle wasting. Collectively, these data support the concept of sexually dimorphic responses to cancer in skeletal muscle and suggest that these responses may be associated with the clinical differences in breast cancer between the sexes. The identified sex-dependent pathways within the muscle of male and female mice provide a framework to evaluate therapeutic strategies targeting tumor-associated skeletal muscle alterations.

Breast cancer-associated skeletal muscle mitochondrial dysfunction and lipid accumulation is reversed by PPARG.

The peroxisome proliferator-activated receptors (PPARs) have been previously implicated in the pathophysiology of skeletal muscle dysfunction in women with breast cancer (BC) and animal models of BC. This study investigated alterations induced in skeletal muscle by BC-derived factors in an in vitro conditioned media (CM) system and tested the hypothesis that BC cells secrete a factor that represses PPAR-γ (PPARG) expression and its transcriptional activity, leading to downregulation of PPARG target genes involved in mitochondrial function and other metabolic pathways. We found that BC-derived factors repress PPAR-mediated transcriptional activity without altering protein expression of PPARG. Furthermore, we show that BC-derived factors induce significant alterations in skeletal muscle mitochondrial function and lipid accumulation, which are rescued with exogenous expression of PPARG. The PPARG agonist drug rosiglitazone was able to rescue BC-induced lipid accumulation but did not rescue effects of BC-derived factors on PPAR-mediated transcription or mitochondrial function. These data suggest that BC-derived factors alter lipid accumulation and mitochondrial function via different mechanisms that are both related to PPARG signaling, with mitochondrial dysfunction likely being altered via repression of PPAR-mediated transcription, and lipid accumulation being altered via transcription-independent functions of PPARG.

Chronic stress induced perivascular adipose tissue impairment of aortic function and the therapeutic effect of exercise.

NEW FINDINGS: What is the central question of this study? Thoracic perivascular adipose tissue (tPVAT) is known to, in part, regulate aortic function: what are the effects of unpredictable chronic mild stress (UCMS) on the tPVAT regulation of aortic function and what is the role of exercise training in alleviating the potential negative actions of UCMS on tPVAT? What is the main finding and its importance? UCMS causes tPVAT to disrupt endothelium-dependent dilatation, increases inflammatory cytokine production and diminishes tPVAT-adiponectin. Exercise training proved efficacious in preventing tPVAT-mediated disruption of aortic function. The data support a tPVAT mechanism through which chronic stress negatively impacts vascular health, which adds to our knowledge of how psychological disorders might increase the risk of cardiovascular disease. ABSTRACT: Chronic stress is a major risk for cardiovascular disease. Perivascular adipose tissue (PVAT) has been shown to regulate vascular function; however, the impact of chronic stress and the comorbidity of metabolic syndrome (MetS) on thoracic (t)PVAT is unknown. Additionally, aerobic exercise training (AET) is known to combat the pathology of MetS and chronic stress, but the role of tPVAT in these actions is also unknown. Therefore, the purpose of this study was to examine the effects of unpredictable chronic mild stress (UCMS) on the tPVAT regulation of aortic function and the preventative effect of AET. Lean (LZR) and obese (OZR) Zucker rats (16-17 weeks old) were exposed to 8 weeks of UCMS with and without treadmill exercise (AET). In LZR, UCMS impaired aortic endothelium-dependent dilatation (EDD) (assessed ex vivo by wire myography) and aortic stiffness (assessed by elastic modulus) with no change in OZR subject to UCMS. However, both LZR and OZR UCMS tPVAT impaired EDD compared to respective controls. LZR and OZR subject to UCMS had higher oxidative stress production, diminished adiponectin and impaired aortic nitric oxide levels. Divergently, UCMS induced greater inflammatory cytokine production in LZR UCMS tPVAT, but not in OZR UCMS tPVAT. AET prevented the tPVAT impairment of aortic relaxation with UCMS in LZR and OZR. Additionally, AET reduced aortic stiffness in both LZR and OZR. These beneficial effects on tPVAT regulation of the aorta are likely due to AET preservation of adiponectin, reduced oxidative stress and inflammation, and enhanced nitric oxide. UCMS impaired tPVAT-regulated aortic function in LZR, and augmented MetS-induced EDD in OZR. Conversely, AET in combination with UCMS largely preserved aortic function and the tPVAT environment, in both groups.

Aortic dysfunction in metabolic syndrome mediated by perivascular adipose tissue TNFα- and NOX2-dependent pathway.

NEW FINDINGS: What is the central question of this study? Tumour necrosis factor-α (TNFα) has been shown to impair vascular function, but the impact of thoracic aorta perivascular adipose tissue (tPVAT)-derived TNFα on tPVAT and aortic function in metabolic syndrome is unknown. What is the main finding and its importance? Release of TNFα by tPVAT causes production of reactive oxygen species in tPVAT through activation of an NADPH-oxidase 2 (NOX2)-dependent pathway, activates production of aortic reactive oxygen species and mediates aortic stiffness, potentially through matrix metalloproteinase 9 activity. Neutralization of TNFα and/or inhibition of NOX2 blocks the tPVAT-induced impairment of aortic function. These data partly implicate tPVAT NOX2 and TNFα in mediating the vascular pathology of metabolic syndrome. ABSTRACT: Perivascular adipose tissue (PVAT) is recognized for its vasoactive effects, but it is unclear how metabolic syndrome impacts thoracic aorta (t)PVAT and the subsequent effect on functional and structural aortic stiffness. Thoracic aorta and tPVAT were removed from 16- to 17-week-old lean (LZR, n = 16) and obese Zucker rats (OZR, n = 16). The OZR presented with aortic endothelial dysfunction, assessed by wire myography, and increased aortic stiffness, assessed by elastic modulus. The OZR tPVAT exudate further exacerbated the endothelial dysfunction, reducing nitric oxide and endothelium-dependent relaxation (P < 0.05). Additionally, OZR tPVAT exudate had increased MMP9 activity (P < 0.05) and further increased the elastic modulus of the aorta after 72 h of co-culture (P < 0.05). We found that the observed aortic dysfunction caused by OZR tPVAT was mediated through increased production and release of tumour necrosis factor-α (TNFα; P < 0.01), which was dependent on tPVAT NADPH-oxidase 2 (NOX2) activity. The OZR tPVAT release of reactive oxygen species and subsequent aortic dysfunction were inhibited by TNFα neutralization and/or inhibition of NOX2. Additionally, we found that OZR tPVAT had reduced activity of the active sites of the 20S proteasome (P < 0.05) and reduced superoxide dismutase activity (P < 0.01). In conclusion, metabolic syndrome causes tPVAT dysfunction through an interplay between TNFα and NOX2 that leads to tPVAT-mediated aortic stiffness by activation of aortic reactive oxygen species and increased MMP9 activity.

Hippocampus-specific deficiency of IL-15Rα contributes to greater anxiety-like behaviors in mice.

A hippocampus-specific IL15RαKO mouse (hipIl15ra fl/fl /Cre+) was generated to test the hypothesis that the targeted deletion of interleukin-15 receptor alpha (IL-15Rα) in the hippocampus contributes to altered behavior, including greater levels of anxiety and ambulatory activity. Using Cre-loxP, exons 2 and 3 of the IL-15Rα gene were excised within the hippocampus, while normal expression was maintained within the rest of the brain. In the open field test (OFT), hipIl15ra fl/fl /Cre+ spent a greater amount of time in the periphery and less time in the central portions of the chamber, and there was also a noticeable trend for decreased rearing activity; these behaviors are consistent with greater levels of anxiety-like behavior in these mice. However, there were no differences in the overall locomotor counts in the OFT when comparing hipIl15ra fl/fl /Cre+ mice to their littermate controls. These data implicate IL-15-related signaling within the hippocampus has a role in anxiety-like behavior.

Interleukin-15 directly stimulates pro-oxidative gene expression in skeletal muscle in-vitro via a mechanism that requires interleukin-15 receptor alpha.

Interleukin-15 (IL-15) signaling is heavily regulated by a high specificity IL-15 binding protein known as interleukin-15 receptor alpha (IL-15Rα). In-vivo disruption of IL-15Rα in the constitutive IL-15Rα knock-out (IL-15RαKO) mouse results in a shift towards an oxidative muscle phenotype characterized by dramatic increases in mitochondrial density. The IL-15RαKO mouse displays elevated levels of IL-15 transcript in muscle tissue, along with increased circulating levels of IL-15. As a result, it has been suggested that loss of IL-15Rα from skeletal muscle enhances muscle IL-15 secretion, and that muscle-derived IL-15 acts in an autocrine fashion to elicit pro-oxidative effects. However, this proposed mechanism of IL-15/IL-15Rα action in skeletal muscle is based primarily on in-vivo associative observations, and has yet to be explored in a direct manner. Thus, our purpose was to assess the immediate influence of IL-15Rα on the capacity of skeletal muscle to secrete and respond to IL-15, and also to determine whether IL-15 has the ability to act directly on skeletal muscle to induce pro-oxidative changes. These aims were addressed in-vitro using primary myogenic cultures derived from IL-15RαKO mice and B6129 controls, as well as cultures of the C2C12 immortalized myogenic cell line. Cultures obtained from IL-15RαKO mice displayed a diminished capacity to secrete IL-15 in relation to B6129 controls. Acute treatment of B6129-derived cultures with recombinant IL-15 increased transcriptional expression of the pro-oxidative genes PGC1α and PPARδ. IL-15 treatment failed to elicit a similar response in cultures generated from IL-15RαKO mice. Chronic treatment of C2C12 cultures with IL-15 during myogenic differentiation resulted in mature myocytes with greater mitochondrial density in relation to vehicle treated controls. Collectively, these results provide evidence that IL-15 has the capacity to act directly on skeletal muscle in a pro-oxidative manner, and that disruption of IL-15Rα ablates the ability of skeletal muscle to secrete and respond to IL-15.

Preclinical Multi-Omic Assessment of Pioglitazone in Skeletal Muscles of Mice Implanted with Human HER2/neu Overexpressing Breast Cancer Xenografts.

Breast cancer (BC) is the most prevalent cancer worldwide and is accompanied by fatigue during both active disease and remission in the majority of cases. Our lab has measured fatigue in isolated muscles from treatment-naive BC patient-derived orthotopic xenograft (BC-PDOX) mice. Here, we conducted a preclinical trial of pioglitazone in BC-PDOX mice to determine its efficacy in ameliorating BC-induced muscle fatigue, as well as its effects on transcriptomic, metabolomic, and lipidomic profiles in skeletal muscle. Methods: The pioglitazone and vehicle groups were treated orally for 4 weeks upon reaching a tumor volume of 600 mm3. Whole-animal indirect calorimetry was used to evaluate systemic metabolic states. The transcriptome was profiled using short-read bulk RNA sequencing (RNA-seq). Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to profile the metabolome and lipidome. Fast and slow skeletal muscle function were evaluated using isolated ex vivo testing. Results: Pioglitazone was associated with a significant overall decrease in metabolic rate, with no changes in substrate utilization. RNA-seq supported the downstream effects of pioglitazone on target genes and displayed considerable upregulation of mitochondrial bioenergetic pathways. Skeletal muscle metabolomic and lipidomic profiles exhibited dysregulation in response to BC, which was partially restored in pioglitazone-treated mice compared to vehicle-treated BC-PDOX mice. Despite molecular support for pioglitazone's efficacy, isolated muscle function was not affected by pioglitazone treatment. Conclusions: BC induces multi-omic dysregulation in skeletal muscle, which pioglitazone partially ameliorates. Future research should focus on profiling systemic metabolic dysfunction, identifying molecular biomarkers of fatigue, and testing alternative pioglitazone treatment regimens.

IL-15Rα deficiency leads to mitochondrial and myofiber differences in fast mouse muscles.

The purpose of this study was to determine mitochondrial changes in fast muscles from interleukin-15 receptor alpha knockout (IL-15RαKO) mice. We tested the hypothesis that fast muscles from IL-15RαKO mice would have a greater mitochondrial density and altered internal structure compared to muscles from control mice. In fast muscles from IL-15RαKO mice, mitochondrial density was 48% greater with a corresponding increase in mitochondrial DNA content. Although there were no differences in the relative size of isolated mitochondria, internal complexity was lower in mitochondria from IL-15RαKO mice. These data support an increase in mitochondrial biogenesis and provide direct evidence for a greater density and altered internal structure of mitochondria in EDL muscles deficient in IL-15Rα.

Sexual Dimorphism of Skeletal Muscle in a Mouse Model of Breast Cancer: A Functional and Molecular Analysis.

Breast cancer incidence in men is statistically rare; however, given the lack of screening in males, more advanced stages at initial diagnosis results in lower 5-year survival rates for men with breast cancer compared to women. A sexual dimorphism, with respect to the effect of tumor growth on cachexia incidence and severity, has also been reported across cancer types. The purpose of this study was to examine the sexual dimorphism of breast cancer as it pertains to skeletal muscle function and molecular composition. Using female and male transgenic PyMT mice, we tested the hypothesis that isometric contractile properties and molecular composition of skeletal muscle would be differentially affected by breast tumors. PyMT tumor-bearing mice of each sex, corresponding to maximal tumor burden, were compared to their respective controls. RNA-sequencing of skeletal muscle revealed different pathway alterations that were exclusive to each sex. Further, differentially expressed genes and pathways were substantially more abundant in female tumor mice, with only minimal dysregulation in male tumor mice, each compared to their respective controls. These differences in the transcriptome were mirrored in isometric contractile properties, with greater tumor-induced dysfunction in females than male mice, as well as muscle wasting. Collectively, these data support the concept of sexually dimorphic responses to cancer in skeletal muscle and suggest these responses may be associated with the clinical differences in breast cancer between the sexes. The identified sex-dependent pathways within muscle of male and female mice provide a framework to evaluate therapeutic strategies targeting tumor-associated skeletal muscle alterations.

Targeting the activin type IIB receptor to improve muscle mass and function in the mdx mouse model of Duchenne muscular dystrophy.

The activin receptor type IIB (ActRIIB) is a transmembrane receptor for transforming growth factor-ß superfamily members, including myostatin, that are involved in the negative regulation of skeletal muscle mass. We tested the translational hypothesis that blocking ligand binding to ActRIIB for 12 weeks would stimulate skeletal muscle growth and improve muscle function in the mdx mouse. ActRIIB was targeted using a novel inhibitor comprised of the extracellular portion of the ActRIIB fused to the Fc portion of murine IgG (sActRIIB), at concentrations of 1.0 and 10.0 mg/kg(-1) body weight. After 12 weeks of treatment, the 10.0 mg/kg(-1) dose caused a 27% increase in body weight with a concomitant 33% increase in lean muscle mass. Absolute force production of the extensor digitorum longus muscle ex vivo was higher in mice after treatment with either dose of sActRIIB, and the specific force was significantly higher after the lower dose (1.0 mg/kg(-1)), indicating functional improvement in the muscle. Circulating creatine kinase levels were significantly lower in mice treated with sActRIIB, compared with control mice. These data show that targeting the ActRIIB improves skeletal muscle mass and functional strength in the mdx mouse model of DMD, providing a therapeutic rationale for use of this molecule in treating skeletal myopathies.

Skeletal muscle gene expression in response to resistance exercise: sex specific regulation.

BACKGROUND: The molecular mechanisms underlying the sex differences in human muscle morphology and function remain to be elucidated. The sex differences in the skeletal muscle transcriptome in both the resting state and following anabolic stimuli, such as resistance exercise (RE), might provide insight to the contributors of sexual dimorphism of muscle phenotypes. We used microarrays to profile the transcriptome of the biceps brachii of young men and women who underwent an acute unilateral RE session following 12 weeks of progressive training. Bilateral muscle biopsies were obtained either at an early (4 h post-exercise) or late recovery (24 h post-exercise) time point. Muscle transcription profiles were compared in the resting state between men (n = 6) and women (n = 8), and in response to acute RE in trained exercised vs. untrained non-exercised control muscle for each sex and time point separately (4 h post-exercise, n = 3 males, n = 4 females; 24 h post-exercise, n = 3 males, n = 4 females). A logistic regression-based method (LRpath), following Bayesian moderated t-statistic (IMBT), was used to test gene functional groups and biological pathways enriched with differentially expressed genes. RESULTS: This investigation identified extensive sex differences present in the muscle transcriptome at baseline and following acute RE. In the resting state, female muscle had a greater transcript abundance of genes involved in fatty acid oxidation and gene transcription/translation processes. After strenuous RE at the same relative intensity, the time course of the transcriptional modulation was sex-dependent. Males experienced prolonged changes while females exhibited a rapid restoration. Most of the biological processes involved in the RE-induced transcriptional regulation were observed in both males and females, but sex specificity was suggested for several signaling pathways including activation of notch signaling and TGF-beta signaling in females. Sex differences in skeletal muscle transcriptional regulation might implicate a mechanism behind disproportional muscle growth in males as compared with female counterparts after RE training at the same relative intensity. CONCLUSIONS: Sex differences exist in skeletal muscle gene transcription both at rest and following acute RE, suggesting that sex is a significant modifier of the transcriptional regulation in skeletal muscle. The findings from the present study provide insight into the molecular mechanisms for sex differences in muscle phenotypes and for muscle transcriptional regulation associated with training adaptations to resistance exercise.

Pretreatment with a soluble activin type IIB receptor/Fc fusion protein improves hypoxia-induced muscle dysfunction.

Hypoxia, or reduced oxygen, occurs in a variety of clinical and environmental situations. Hypoxic exposure is associated with decreased muscle mass and a concomitant reduction in exercise capacity, although the exact mechanisms are not completely understood. The activin type IIB receptor (ActRIIB) is a receptor for transforming growth factor-beta (TGFbeta) superfamily members that are involved in the negative regulation of lean tissue mass. Given that hypoxia has negative effects on muscle mass and function and that modulation of the ActRIIB has been shown to increase muscle mass, we tested the hypothesis that pharmacological targeting of the ActRIIB for 2 wk would attenuate the loss of muscle mass and function in mice after exposure to normobaric hypoxia. ActRIIB modulation was achieved using a soluble activin receptor/Fc fusion protein (sActRIIB) in mice housed in a hypoxic chamber for 1 or 2 wk. Hypoxia induced a reduction in body weight in PBS- and sActRIIB-treated mice, although sActRIIB-treated mice remained larger throughout the hypoxic exposure. The absolute forces generated by extensor digitorum longus muscles were also significantly greater in sActRIIB- than PBS-treated mice and were more resistant to eccentric contraction-induced force drop after eccentric lengthening contractions. In summary, sActRIIB pretreatment attenuated hypoxia-induced muscle dysfunction. These data suggest that targeting the ActRIIB is an effective strategy to counter hypoxia-induced muscle dysfunction and to preacclimatize to hypoxia in clinical or high-altitude settings.

Skeletal muscle reprogramming by breast cancer regardless of treatment history or tumor molecular subtype.

Increased susceptibility to fatigue is a negative predictor of survival commonly experienced by women with breast cancer (BC). Here, we sought to identify molecular changes induced in human skeletal muscle by BC regardless of treatment history or tumor molecular subtype using RNA-sequencing (RNA-seq) and proteomic analyses. Mitochondrial dysfunction was apparent across all molecular subtypes, with the greatest degree of transcriptomic changes occurring in women with HER2/neu-overexpressing tumors, though muscle from patients of all subtypes exhibited similar pathway-level dysregulation. Interestingly, we found no relationship between anticancer treatments and muscle gene expression, suggesting that fatigue is a product of BC per se rather than clinical history. In vitro and in vivo experimentation confirmed the ability of BC cells to alter mitochondrial function and ATP content in muscle. These data suggest that interventions supporting muscle in the presence of BC-induced mitochondrial dysfunction may alleviate fatigue and improve the lives of women with BC.

Human Breast Cancer Xenograft Model Implicates Peroxisome Proliferator-activated Receptor Signaling as Driver of Cancer-induced Muscle Fatigue.

PURPOSE: This study tested the hypothesis that a patient-derived orthotopic xenograft (PDOX) model would recapitulate the common clinical phenomenon of breast cancer-induced skeletal muscle (SkM) fatigue in the absence of muscle wasting. This study additionally sought to identify drivers of this condition to facilitate the development of therapeutic agents for patients with breast cancer experiencing muscle fatigue. EXPERIMENTAL DESIGN: Eight female BC-PDOX-bearing mice were produced via transplantation of tumor tissue from 8 female patients with breast cancer. Individual hind limb muscles from BC-PDOX mice were isolated at euthanasia for RNA-sequencing, gene and protein analyses, and an ex vivo muscle contraction protocol to quantify tumor-induced aberrations in SkM function. Differentially expressed genes (DEG) in the BC-PDOX mice relative to control mice were identified using DESeq2, and multiple bioinformatics platforms were employed to contextualize the DEGs. RESULTS: We found that SkM from BC-PDOX-bearing mice showed greater fatigability than control mice, despite no differences in absolute muscle mass. PPAR, mTOR, IL6, IL1, and several other signaling pathways were implicated in the transcriptional changes observed in the BC-PDOX SkM. Moreover, 3 independent in silico analyses identified PPAR signaling as highly dysregulated in the SkM of both BC-PDOX-bearing mice and human patients with early-stage nonmetastatic breast cancer. CONCLUSIONS: Collectively, these data demonstrate that the BC-PDOX model recapitulates the expected breast cancer-induced SkM fatigue and further identify aberrant PPAR signaling as an integral factor in the pathology of this condition.

Systemic elevation of interleukin-15 in vivo promotes apoptosis in skeletal muscles of young adult and aged rats.

In this study, we tested the hypothesis that systemic elevation of IL-15 would attenuate apoptosis in skeletal muscles of aged rats. IL-15 was administered to young adult (n=6) and aged (n=6) rats for 14 days. Apoptosis was quantified using an ELISA assay and verified through TUNEL staining of muscle sections. As expected, apoptosis was greater in muscles from aged control rats, compared to age-matched control. Apoptosis was also greater in the muscles from young adult and aged rats treated with IL-15. These increases in apoptosis were associated with decreases in muscle mass of IL-15 treated rats. These data do not support our initial hypothesis and suggest that systemic elevation of IL-15 promotes apoptosis in skeletal muscle. The proposed anti-apoptotic property of IL-15 may be specific to cell-type and/or the degree of muscle pathology present; however, additional research is required to more clearly decipher its role in skeletal muscle.

Dystrophin-deficient cardiomyopathy in mouse: expression of Nox4 and Lox are associated with fibrosis and altered functional parameters in the heart.

Duchenne muscular dystrophy (DMD; dystrophin-deficiency) causes dilated cardiomyopathy in the second decade of life in affected males. We studied the dystrophin-deficient mouse heart (mdx) using high-frequency echocardiography, histomorphometry, and gene expression profiling. Heart dysfunction was prominent at 9-10months of age and showed significantly increased LV internal diameter (end systole) and decreased posterior wall thickness. This cardiomyopathy was associated with a 30% decrease in shortening fraction. Histologically, there was a 10-fold increase in connective tissue volume (fibrosis). mRNA profiling with RT-PCR validation showed activation of key pro-fibrotic genes, including Nox4 and Lox. The Nox gene family expression differed in mdx heart and skeletal muscle, where Nox2 was specifically induced in skeletal muscle while Nox4 was specifically induced in heart. This is the first report of an altered profibrotic gene expression profile in cardiac tissue of dystrophic mice showing echocardiographic evidence of cardiomyopathy.

Interleukin-15 and interleukin-15R alpha SNPs and associations with muscle, bone, and predictors of the metabolic syndrome.

The aims of this study were to examine associations between two SNPs in the human IL-15 gene and three SNPs in the IL-15Ralpha gene with predictors of metabolic syndrome and phenotypes in muscle, strength, and bone at baseline and in response to resistance training (RT). Subjects were Caucasians who had not performed RT in the previous year and consisted of a strength cohort (n=748), volumetric cohort (n=722), and serum cohort (n=544). Subjects completed 12 weeks of unilateral RT of the non-dominant arm, using their dominant arm as an untrained control. ANCOVA analyses revealed gender-specific associations with: (1) IL-15 SNP (rs1589241) and cholesterol (p=0.04), LDL (p=0.02), the homeostasis model assessment (HOMA; p=0.03), and BMI (p=0.002); (2) IL-15 SNP (rs1057972) and the pre- to post-training absolute difference in 1RM strength (p=0.02), BMI (p=0.008), and fasting glucose (p=0.03); (3) IL-15Ralpha SNP (rs2296135) and baseline total bone volume (p=0.04) and the pre- to post-training absolute difference in isometric strength (p=0.01); and 4) IL-15Ralpha SNP (rs2228059) and serum triglycerides (p=0.04), baseline whole muscle volume (p=0.04), baseline cortical bone volume (p=0.04), and baseline muscle quality (p=0.04). All associations were consistent in showing a potential involvement of the IL-15 pathway with muscle and bone phenotypes and predictors of metabolic syndrome.

Age-dependent increase in oxidative stress in gastrocnemius muscle with unloading.

Oxidative stress increases during unloading in muscle from young adult rats. The present study examined the markers of oxidative stress and antioxidant enzyme gene and protein expressions in medial gastrocnemius muscles of aged and young adult (30 and 6 mo of age) Fischer 344 x Brown Norway rats after 14 days of hindlimb suspension. Medial gastrocnemius muscle weight was decreased by approximately 30% in young adult and aged rats following suspension. When muscle weight was normalized to animal body weight, it was reduced by 12% and 22% in young adult and aged rats, respectively, after suspension. Comparisons between young adult and aged control animals demonstrated a 25% and 51% decline in muscle mass when expressed as absolute muscle weight and muscle weight normalized to the animal body weight, respectively. H(2)O(2) content was elevated by 43% while Mn superoxide dismutase (MnSOD) protein content was reduced by 28% in suspended muscles compared with control muscles exclusively in the aged animals. Suspended muscles had greater content of malondialdehyde (MDA)/4-hydroxyalkenals (4-HAE) (29% and 58% increase in young adult and aged rats, respectively), nitrotyrosine (76% and 65% increase in young adult and aged rats, respectively), and catalase activity (69% and 43% increase in young adult and aged rats, respectively) relative to control muscles. Changes in oxidative stress markers MDA/4-HAE, H(2)O(2), and MnSOD protein contents in response to hindlimb unloading occurred in an age-dependent manner. These findings are consistent with the hypotheses that oxidative stress has a role in mediating disuse-induced and sarcopenia-associated muscle losses. Our data suggest that aging may predispose skeletal muscle to increased levels of oxidative stress both at rest and during unloading.