Voluntary physical activity prevents insulin resistance in a tissue specific manner

Physical inactivity and a sedentary lifestyle are risk factors for the development of type 2 diabetes. Here, we identified the effects 8 weeks of voluntary physical activity had on the prevention of insulin resistance in mouse skeletal muscles and liver (a hallmark of T2D). To do this, 8 week old C57BL/6J mice with (RUN) and without (SED) voluntary access to running wheels were fed a standard rodent chow ad libitum for 8 weeks. In the liver, there was a 2.5-fold increase in insulin stimulated Akt^SER473 phosphorylation, and a threefold increase in insulin-stimulated (0.5 U/kg) GSK3β^SER9 phosphorylation in RUN compared to SED mice. Although not induced in skeletal muscles, there was a twofold increase in SOCS3 expression in SED compared to RUN mice in the liver. There was no difference in the glucose tolerance test between groups. This study was the first to show differences in liver insulin sensitivity after 8 weeks of voluntary physical activity, and increased SOCS3 expression in the liver of sedentary mice compared to active mice. These findings demonstrate that even in young mice that would normally be considered healthy, the lack of physical activity leads to insulin resistance representing the initial pathogenesis of impaired glucose metabolism leading to type 2 diabetes.

Interleukin-6 deficiency causes tissue-specific changes in signaling pathways in response to high-fat diet and physical activity

This study was designed to investigate the role of interleukin‐6 (IL‐6) on high‐fat diet (HFD)‐induced glucose intolerance, and the response to voluntary physical activity in the prevention of insulin resistance. Six‐week‐old wild‐type (WT) and IL‐6 knockout (KO) mice with (RUN) or without (SED) access to running wheels were fed a HFD (60% from kcal) for 4 weeks. A glucose tolerance test revealed that blood glucose levels were 25–30% higher in KO RUN compared to all other groups. In WT RUN, weight gain was positively correlated with total caloric intake; however, this correlation was absent in KO RUN. In soleus muscle, there was a 2‐fold increase in SOCS3 expression in KO RUN compared to all other groups. In gastrocnemius and plantaris muscles, Akt phosphorylation was 31% higher in WT RUN compared to WT SED, but this effect of running was absent in KO mice. Additionally, there was a 2.4‐fold increase in leptin expression in KO RUN compared to KO SED in the gastrocnemius and plantaris muscles. In the liver, there was a 2‐ to 3.8‐fold increase in SOCS3 expression in KO SED compared to all other groups, and AMPKα phosphorylation was 27% higher in WT mice (both RUN and SED) compared to KO mice (both RUN and SED). This study provides new insights into the role of the IL‐6 in metabolism and energy storage, and highlights tissue‐specific changes in early signaling pathways in response to HFD for 4 weeks. The collective findings suggest that endogenous IL‐6 is important for the prevention of insulin resistance leading to type 2 diabetes.

This study was designed to investigate the role of interleukin‐6 (IL‐6) on high‐fat diet (HFD)‐induced glucose intolerance, and the response to voluntary physical activity in the prevention of insulin resistance. This study provides new insight into the role of the IL‐6 in metabolism and energy storage, and highlights tissue‐specific changes in early signaling pathways in response to HFD for 4 weeks. The collective findings suggest that endogenous IL‐6 is important for the prevention of insulin resistance leading to type 2 diabetes.

Metformin induces apoptosis and cell cycle arrest mediated by oxidative stress, AMPK and FOXO3a in MCF-7 breast cancer cells

Recent studies have demonstrated that the anti-diabetic drug, metformin, can exhibit direct antitumoral effects, or can indirectly decrease tumor proliferation by improving insulin sensitivity. Despite these recent advances, the underlying molecular mechanisms involved in decreasing tumor formation are not well understood. In this study, we examined the antiproliferative role and mechanism of action of metformin in MCF-7 cancer cells treated with 10 mM of metformin for 24, 48, and 72 hours. Using BrdU and the MTT assay, it was found that metformin demonstrated an antiproliferative effect in MCF-7 cells that occurred in a time- and concentration- dependent manner. Flow cytometry was used to analyze markers of cell cycle, apoptosis, necrosis and oxidative stress. Exposure to metformin induced cell cycle arrest in G₀-G₁ phase and increased cell apoptosis and necrosis, which were associated with increased oxidative stress. Gene and protein expression were determined in MCF-7 cells by real time RT-PCR and western blotting, respectively. In MCF-7 cells metformin decreased the activation of IRβ, Akt and ERK1/2, increased p-AMPK, FOXO3a, p27, Bax and cleaved caspase-3, and decreased phosphorylation of p70S6K and Bcl-2 protein expression. Co-treatment with metformin and H₂O₂ increased oxidative stress which was associated with reduced cell number. In the presence of metformin, treating with SOD and catalase improved cell viability. Treatment with metformin resulted in an increase in p-p38 MAPK, catalase, MnSOD and Cu/Zn SOD protein expression. These results show that metformin has an antiproliferative effect associated with cell cycle arrest and apoptosis, which is mediated by oxidative stress, as well as AMPK and FOXO3a activation. Our study further reinforces the potential benefit of metformin in cancer treatment and provides novel mechanistic insight into its antiproliferative role.

Pro-inflammatory mediation of myoblast proliferation

Skeletal muscle satellite cell function is largely dictated by the surrounding environment following injury. Immune cell infiltration dominates the extracellular space in the injured area, resulting in increased cytokine concentrations. While increased pro-inflammatory cytokine expression has been previously established in the first 3 days following injury, less is known about the time course of cytokine expression and the specific mechanisms of cytokine induced myoblast function. Therefore, the expression of IL-1β and IL-6 at several time points following injury, and their effects on myoblast proliferation, were examined. In order to do this, skeletal muscle was injured using barium chloride in mice and tissue was collected 1, 5, 10, and 28 days following injury. Mechanisms of cytokine induced proliferation were determined in cell culture using both primary and C2C12 myoblasts. It was found that there is a ∼20-fold increase in IL-1β (p≤0.05) and IL-6 (p = 0.06) expression 5 days following injury. IL-1β increased proliferation of both primary and C2C12 cells ∼25%. IL-1β stimulation also resulted in increased NF-κB activity, likely contributing to the increased proliferation. These data demonstrate for the first time that IL-1β alone can increase the mitogenic activity of primary skeletal muscle satellite cells and offer insight into the mechanisms dictating satellite cell function following injury.

The antioxidant resveratrol down-regulates inflammation in an in-vitro model of Pseudomonas aeruginosa infection of lung epithelial cells

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that can cause severe pulmonary infection in immunocompromized individuals. During the infectious process, P. aeruginosa provokes a potent inflammatory response and induces the release of reactive oxygen species (ROS). Cells undergo oxidative stress when cellular antioxidants are unable to effectively scavenge and detoxify ROS, resulting in lung damage. Resveratrol (3,5,4'-trihydroxystilbene) is a natural polyphenolic compound with recognized antioxidant effects. We hypothesized that owing to its antioxidant activities, resveratrol can attenuate an inflammatory response in P. aeruginosa-infected cells. Lung epithelial A549 cells were pre-treated with 100 μmol/L of resveratrol for 5 h, followed by infection with P. aeruginosa. Intracellular ROS generation was used as an indicator of P. aeruginosa-induced oxidative stress, and cell surface expression of Fas receptor and activation of caspases-3 and -7 as indicators of apoptosis. We also measured the surface expression of intercellular adhesion molecule (ICAM)-1 and enzymes related to inflammation and redox signaling. Resveratrol significantly reduced ROS generation, ICAM-1, and human beta-defensin-2 expression, as well as the markers of apoptosis in A549 cells infected with P. aeruginosa, and up-regulated glutathione peroxidase, suggesting its potential therapeutic role in protecting the lungs against the deleterious effects of P. aeruginosa infection.

The IL-6 Paradox: Context Dependent Interplay of SOCS3 and AMPK

Insulin resistance is the principle step towards the progression of type 2 diabetes, and has been linked to increased circulating levels of cytokines, leading to chronic low-grade inflammation. Specifically, in chronic disease states increased IL-6 is thought to play a critical role in the regulation of insulin resistance in the peripheral tissues, and has been used as a marker of insulin resistance. There is also an endogenous up-regulation of IL-6 in response to exercise, which has been linked to improved insulin sensitivity. This leads to the question "how can elevated IL-6 lead to the development of insulin resistance, and yet also lead to increased insulin sensitivity?" Resolving the dual role of IL-6 in regulating insulin resistance/sensitivity is critical to the development of potential therapeutic interventions. This review summarizes the literature on the seemingly paradoxical role of elevated IL-6 on insulin signalling, including the activation of AMPK and the involvement of leptin and SOCS3.

Exercise training prevents development of cardiac contractile dysfunction in hypertensive TG (mREN-2)27 rats

BACKGROUND

Angiotensin-II (Ang-II) contributes to cardiac remodeling and left ventricular dysfunction. In contrast, exercise may have beneficial effects on left ventricular structure and function.

METHODS AND RESULTS

We investigated the effects of low-intensity exercise training (ET) on in vivo cardiac function in hypertensive TG (mREN-2)27 rats (Ren-2) which develop left ventricular hypertrophy and dysfunction. Ren-2 rats and Sprague Dawley (SD) controls (4-5 weeks) began treadmill exercise every day for 5-6 weeks. Cardiac function was evaluated by echocardiography. Cardiac output and stroke volume were increased by ET in both 8-wk-old SD and Ren-2. Slope of mitral deceleration time, a non-invasive measure of diastolic function, was lower in the Ren-2 rats, but not changed by ET. LV collagen deposition, as assessed by hydroxyproline assay, was not affected by rat strain or ET at 10-11 weeks of age. Left ventricular B-type natriuretic peptide mRNA levels were higher in the Ren-2 rats (100%), but not affected by ET. Both alpha (~14.5 fold) and beta (~2.5 fold) myosin heavy chain mRNA were higher in the LV of Ren-2 rats (p < 0.05), but were not changed by ET.

CONCLUSION

Low-intensity exercise training in Ren-2 rats, a model of Ang-II-mediated hypertension, maintains cardiac index and stroke volume in the presence of impaired diastolic function at 8 wks of age.

Age-related impairment of T cell-induced skeletal muscle precursor cell function

Sarcopenia is the age-associated loss of skeletal muscle mass and strength. Recent evidence suggests that an age-associated loss of muscle precursor cell (MPC) functionality contributes to sarcopenia. The objectives of the present study were to examine the influence of activated T cells on MPCs and determine whether an age-related defect in this signaling occurs. MPCs were collected from the gastrocnemius and plantaris of 3-mo-old (young) and 32-mo-old (old) animals. Splenic T cells were harvested using anti-CD3 Dynabead isolation. T cells were activated for 48 h with costimulation of 100 IU/ml interleukin-2 (IL-2) and 5 μg/ml of anti-CD28. Costimulation increased 5-bromo-2'-deoxyuridine incorporation of T cells from 13.4 ± 4.6% in control to 64.8 ± 6.0% in costimulated cells. Additionally, T cell cytokines increased proliferation on MPCs isolated from young muscle by 24.0 ± 5.7%, whereas there was no effect on MPCs isolated from aged muscle. T cell cytokines were also found to be a chemoattractant. T cells were able to promote migration of MPCs isolated from young muscle; however, MPCs isolated from aged muscle did not respond to the T cell-released chemokines. Conversely, whereas T cell-released cytokines did not affect myogenesis of MPCs isolated from young animals, there was a decrease in MPCs isolated from old animals. These data suggest that T cells may play a critical role in mediating MPC function. Furthermore, aging may alter T cell-induced MPC function. These findings have implications for developing strategies aimed at increasing MPC migration and proliferation leading to an improved regenerative capacity of aged skeletal muscle.

IGF-I activates the mouse type IIb myosin heavy chain gene

IGF-I increases skeletal muscle mass, but whether IGF-I increases type IIb myosin heavy chain (MyHC) transcriptional activity is not known. C2C12 myotubes were cultured with or without IGF-I to determine whether IGF-I increases type IIb MyHC promoter activity, and if so, what region of the promoter might IGF-I signaling regulate. At differentiation days 3 and 4, IGF-I increased type IIb MyHC mRNA and mouse 3.0-kb type IIb MyHC promoter activity. Deletion construct studies identified a potential IGF-I-responsive region between 1.25 and 1.2 kb of the type IIb MyHC promoter, which contained an exact 6-bp T-cell factor/lymphoid enhancer factor (Tcf/Lef) binding site at position -1206 to -1201. Site-specific mutation of the putative Tcf/Lef binding site reduced IGF-I-induced 1.3-kb type IIb MyHC promoter activity. To identify potential IGF-I signaling molecules, the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY-294002 were both found to markedly attenuate IGF-I activation of the 1.3-kb type IIb MyHC promoter. Downstream signaling of IGF-I can phosphorylate and inactivate GSK-3beta, thereby enhancing beta-catenin protein. The GSK-3beta inhibitor, LiCl, dramatically enhanced IGF-I induction of the 1.3-kb type IIb MyHC promoter, and constitutively active GSK-3beta attenuated IGF-I-induced 1.3-kb type IIb MyHC promoter activity. Finally, IGF-I increased nuclear beta-catenin protein, and small interfering RNA knockdown of beta-catenin attenuated IGF-I-induced 1.3-kb type IIb MyHC promoter activity and type IIb MyHC mRNA. In summary, IGF-I stimulation of C2C12 myotubes increases mouse type IIb MyHC promoter activity, likely through signaling of PI3K, GSK-3beta, beta-catenin, and a Tcf/Lef binding site at -1,206 to -1,201 bp in the promoter.

Fibroblast growth factor 2-stimulated proliferation is lower in muscle precursor cells from old rats

In aged skeletal muscle, impairments in regrowth and regeneration may be explained by a decreased responsiveness of muscle precursor cells (MPCs) to environmental cues such as growth factors. We hypothesized that impaired responsiveness to fibroblast growth factor 2 (FGF2) in MPCs from old animals would be explained by impaired FGF2 signalling. We determined that 5-bromo-2'-deoxyuridine (BrdU) incorporation and cell number increase less in MPCs from 32- compared with 3-month-old rats. In the presence of FGF2, we demonstrated that there were age-associated differential expression patterns for FGF receptor 1 and 2 mRNAs. Measurement of downstream signalling revealed that that mitogen-activated protein kinase/ERK kinase 1/2 (MEK1/2)-extracellular signal-regulated kinase 1/2, protein kinase C and p38 were FGF2-driven pathways in MPCs. Uniquely, protein kinase C signalling was shown to play the largest role in FGF2-stimulated proliferation in MPCs. c-Jun N-terminal kinase (JNK) signalling was ruled out as an FGF2-stimulated proliferation pathway in MPCs. Inhibition of JNK had no effect on FGF2 signalling to BrdU incorporation, and FGF2 treatment was associated with increased phosphorylation of p38, which inhibits, rather than stimulates, BrdU incorporation in MPCs. Surprisingly, the commonly used vehicle, dimethyl sulphoxide, rescued proliferation in MPCs from old animals. These findings provide insight for the development of effective treatment strategies that target the age-related impairments of MPC proliferation in old skeletal muscle.

Muscle precursor cells isolated from aged rats exhibit an increased tumor necrosis factor- alpha response

Improving muscle precursor cell (MPC, muscle-specific stem cells) function during aging has been implicated as a key therapeutic target for improving age-related skeletal muscle loss. MPC dysfunction during aging can be attributed to both the aging MPC population and the changing environment in skeletal muscle. Previous reports have identified elevated levels of tumor necrosis factor- alpha (TNF- alpha ) in aging, both circulating and locally in skeletal muscle. The purpose of the present study was to determine if age-related differences exist between TNF- alpha -induced nuclear factor-kappa B (NF- kappaB) activation and expression of apoptotic gene targets. MPCs isolated from 32-month-old animals exhibited an increased NF- kappaB activation in response to 1, 5, and 20 ng mL(-1) TNF- alpha, compared to MPCs isolated from 3-month-old animals. No age differences were observed in the rapid canonical signaling events leading to NF- kappaB activation or in the increase in mRNA levels for TNF receptor 1, TNF receptor 2, TNF receptorassociated factor 2 (TRAF2), or Fas (CD95) observed after 2 h of TNF- alpha stimulation. Interestingly, mRNA levels for TRAF2 and the cell death-inducing receptor, Fas (CD95), were persistently upregulated in response to 24 h TNF- alpha treatment in MPCs isolated from 32-month-old animals, compared to 3-month-old animals. Our data indicate that age-related differences may exist in the regulatory mechanisms responsible for NF- kappaB inactivation, which may have an effect on TNF- alpha-induced apoptotic signaling. These findings improve our understanding of the interaction between aged MPCs and the changing environment associated with age, which is critical for the development of potential clinical interventions aimed at improving MPC function with age.

Anti-TNF treatment reduces rat skeletal muscle wasting in monocrotaline-induced cardiac cachexia

The aim was to explore efficacy of tumor necrosis factor (TNF) inhibitors in attenuating increases in anorexia and ubiquitin proteasome pathway transcripts in cardiac cachexia, a potentially lethal condition that responds poorly to current treatments. Cardiac cachexia was rapidly induced with monocrotaline in Sprague-Dawley rats. Either soluble TNF receptor-1 or the general inhibitor of TNF production, pentoxifylline, was given to diminish TNF action on the first indication of cachexia. Animals were anesthetized with a ketamine-xylazine-acepromazine cocktail, and then skeletal muscles were removed for subsequent measurements including ubiquitin proteasome pathway transcripts and Western blots. Both soluble TNF receptor-1 and pentoxifylline attenuated losses in both body and skeletal muscle masses and also reduced increases in selected ubiquitin proteasome pathway transcripts. The action of soluble TNF receptor-1 was partly through reversal of reduced food consumption, while the effects of pentoxifylline were independent of food intake. Here we demonstrate, for the first time, that attenuation of anorexia by soluble TNF receptor-1 treatment in monocrotaline-induced cardiac cachexia is responsible for attenuating increases in some ubiquitin proteasome pathway transcripts as well as preserving body mass and attenuating loss of skeletal muscle mass.

Sirt1 increases skeletal muscle precursor cell proliferation

It is important to understand the mechanisms that control muscle precursor cell (MPC) proliferation for the development of countermeasures to offset the deleterious effects of the aging-related loss of skeletal muscle mass (and myonuclei) and the impaired ability of old muscle to regrow and regenerate. Over-expression of the NAD+-dependent histone deacetylase Sirt1 increased MPC proliferation and cell cycle progression as evidenced by increased 5-bromo-2'-deoxyuridine (BrdU) incorporation, an increase in cell number, proliferating cell nuclear antigen expression, and the phosphorylation of retinoblastoma protein. Associated with the Sirt1-mediated increase in MPC cycle progression were the bidirectional decreases and increases in the expression of the cyclin-dependent kinase inhibitors p21(Waf/Cip1) and p27(Kip1), respectively. Based upon our recent observation that lowering oxygen (O2) in culture from ambient (20%) to estimated physiological levels (5%) increased MPC proliferation, we next measured Sirt1 protein at 5% and 20% O2. Interestingly, in addition to increased proliferation in MPCs cultured at 5% O2, Sirt1 expression increased, compared to 20% O2. Using O2 levels as a platform to modulate basal Sirt1 protein, activation of Sirt1 activity with resveratrol in 20% O2 increased MPC proliferation while inhibition of Sirt1 with nicotinamide in 5% O2 lowered proliferation. For the first time, Sirt1 has been shown to increase MPC proliferation. These findings could have clinical significance since MPC proliferation has important implications in regulating skeletal muscle growth, maintenance, and repair, and the aging-related loss of skeletal muscle mass.

Age-dependent FOXO regulation of p27Kip1 expression via a conserved binding motif in rat muscle precursor cells

Previously, we have demonstrated that forkhead box O3a (FOXO3a) overexpression increased p27(Kip1) promoter activity and protein expression, whereas it decreased proliferation in muscle precursor cells (MPCs). The objectives of the present study were to 1) locate and identify FOXO regulatory elements in the rat p27(Kip1) promoter using deletion analysis of a promoter/reporter construct and 2) determine if age-related differences exist in FOXO-induced p27(Kip1) expression. The full-length (-4.0/+0.4 kb) rat p27(Kip1) promoter construct revealed that both FOXO1 and FOXO3a induced an increase in transcriptional activity. Interestingly, MPCs isolated from old animals exhibited an increased FOXO3a-induced p27(Kip1) promoter activity compared with MPCs isolated from young animals. Deletion of a 253-bp portion of the 5'-untranslated region (UTR) resulted in a significant decrease in FOXO-induced p27(Kip1) promoter expression. Site-specific mutation of a daf-16 family protein-binding element (DBE) within this 253-bp portion of the 5'-UTR also demonstrated a decrease in FOXO-induced p27(Kip1) promoter expression. These data suggest that a putative FOXO regulatory element located in the 5'-UTR of the rat p27(Kip1) gene plays a role in the age-dependent differences in FOXO3a-dependent p27(Kip1) promoter expression. These findings have implications for developing treatment strategies aimed at increasing the proliferation of MPCs and regenerative capacity of aged skeletal muscle.

p21(Cip1) expression is increased in ambient oxygen, compared to estimated physiological (5%) levels in rat muscle precursor cell culture

OBJECTIVE

While it is common practice to culture cells in the presence of ambient oxygen (approximately 21% O2), O2 level observed in the physiological environment is often much lower. Previous efforts to culture a variety of different stem cells, including muscle precursor cells (MPC), under O2 conditions that better mimic in vivo conditions have resulted in enhanced proliferation. In the present study, we hypothesized that 20% O2 in culture represents a sufficient stimulus to cause increased expression of two key negative regulators of the cell-cycle Cip/Kip family of cyclin-dependent kinase inhibitors, p21(Cip1) and p27(Kip1), in MPCs.

MATERIALS AND METHODS

MPCs were isolated from Fischer 344 x Brown Norway F(1) hybrid male rats and O2 was adjusted in culture using a tri-gas incubator.

RESULTS

5-Bromo-2'-deoxyuridine incorporation, cell number and nuclear proliferating cell nuclear antigen expression were all decreased after 48 h culture in 20% O2, compared to 5% O2. Twenty per cent O2 had no effect on either p27(Kip1) promoter activity or protein expression. Although p21(Cip1) promoter activity remained unchanged between 5% and 20% O2, there were significant increases in both p21(Cip1) mRNA and protein expression. Furthermore, 20% O2 caused an increase in p21(Cip1) mRNA stability and p53 transcription factor activity.

CONCLUSION

These findings are considered important because they reveal p21(Cip1) as a critical regulatory protein that needs to be considered when interpreting proliferation data from MPCs studied in culture. In addition, O2-dependent regulation of MPC proliferation is relevant to conditions, including sarcopenia, heart failure, cancer and muscular dystrophy, where increased oxidative stress exists.

FoxO3a preferentially induces p27Kip1 expression while impairing muscle precursor cell-cycle progression

Previous work has demonstrated that forkhead transcription factors, which include the FoxO subfamily, play a critical role in muscle atrophy by inducing expression of the atrophy-related ubiquitin ligases. The proliferation of muscle precursor cells (MPC) is also essential for skeletal muscle mass. The hypothesis was tested that the FoxO forkhead transcription factor FoxO3a hinders MPC proliferation. The present studies were designed to determine the effects of overexpression of FoxO3a on in vitro proliferation of MPCs. MPCs infected with an adenovirus for wild-type FoxO3a had decreased DNA synthesis as detected by the incorporation of 5-bromo-2' deoxyuridine. In general, cyclin-dependent kinase inhibitors, including p27(Kip1)and p21(Waf/Cip1), inhibit cell proliferation. Associated with the impaired MPC proliferation, we found an increase in the promoter activity and protein levels of the cyclin-dependent kinase inhibitor p27(Kip1), whereas there was no effect and a decrease in the promoter activity and protein levels of p21(Waf/Cip1). FoxO3a overexpression had no effect on either the phosphorylation of retinoblastoma protein (ser780) or cyclin D1 protein levels, suggesting that FoxO3a does not effect the early phase of the G(1)-S transition. In addition to its ability to induce muscle atrophy, these studies identify FoxO3a as a negative regulator of MPC proliferation. Our findings suggest that attenuating increased FoxO3a may restore MPC proliferation to prevent atrophy and improve the regenerative capacity of skeletal muscle.

Exercise-induced attenuation of obesity, hyperinsulinemia, and skeletal muscle lipid peroxidation in the OLETF rat

The Otsuka Long-Evans Tokushima fatty (OLETF) rat is a model of hyperphagic obesity in which the animals retain the desire to run voluntarily. Running wheels were provided for 4-wk-old OLETF rats for 16 wk before they were killed 5 h (WL5), 53 h (WL53), or 173 h (WL173) after the wheels were locked. Sedentary (SED) OLETF rats that were not given access to running wheels served as age-matched cohorts. Epididymal fat pad mass, adipocyte volume, and adipocyte number were 58%, 39%, and 47% less, respectively, in WL5 than SED rats. Contrary to cessation of daily running in Fischer 344 x Brown Norway rats, epididymal fat did not increase during the first 173 h of running cessation in the OLETF runners. Serum insulin and glucose levels were 77% and 29% less, respectively, in WL5 than SED rats. Oil red O staining for intramyocellular lipid accumulation was not statistically different among groups. However, lipid peroxidation levels, as determined by total trans-4-hydroxy-2-nonenal (4-HNE) and 4-HNE normalized to oil red O, was higher in epitrochlearis muscles of SED than WL5, WL53, and WL173 rats. mRNA levels of glutathione S-transferase-alpha type 4, an enzyme involved in cellular defense against electrophilic compounds such as 4-HNE, were higher in epitrochlearis muscle of WL53 than WL173 and SED rats. In contrast, 4-HNE levels in omental fat were unaltered. Epitrochlearis muscle palmitate oxidation and relative transcript levels for peroxisome proliferator-activated receptor-delta and peroxisome proliferator-activated receptor-gamma coactivator type 1 were surprisingly not different between runners and SED rats. In summary, voluntary running was associated with lower levels of lipid peroxidation in skeletal muscle without significant changes in intramyocellular lipids or mitochondrial markers in OLETF rats at 20 wk of age. Therefore, even in a genetic animal model of extreme overeating, daily physical activity promotes improved health of skeletal muscle.

Functional overload attenuates plantaris atrophy in tumor-bearing rats

BACKGROUND

Late stage cancer malignancies may result in severe skeletal muscle wasting, fatigue and reduced quality of life. Resistance training may attenuate these derangements in cancer patients, but how this hypertrophic response relates to normal muscle adaptations in healthy subjects is unknown. Here, we determined the effect of resistance training on muscle mass and myosin heavy chain (MHC) isoform composition in plantaris muscles from tumor-bearing (TB) rats.

METHODS

Age- and gender-matched Buffalo rats were used for all studies (n = 6/group). Suspensions of Morris Hepatoma MH7777 cells or normal saline were injected subcutaneously into the dorsum. Six weeks after cell implantation, muscles from TB rats were harvested, weighed and processed for ATP-independent proteasome activity assays. Once tumor-induced atrophy had been established, subgroups of TB rats underwent unilateral, functional overload (FO). Healthy, sham-operated rats served as controls. After six weeks, the extent of plantaris hypertrophy was calculated and MHC isoform compositions were determined by gel electrophoresis.

RESULTS

Six weeks of tumor growth reduced body mass and the relative masses of gastrocnemius, plantaris, tibialis anterior, extensor digitorum longus, and diaphragm muscles (p < or = 0.05). Percent reductions in body mass had a strong, negative correlation to final tumor size (r = -0.78). ATP-independent proteasome activity was increased in plantaris muscles from TB rats (p < or = 0.05). In healthy rats, functional overload (FO) increased plantaris mass ~44% compared to the contralateral control muscle, and increased the relative percentage of MHC type I and decreased the relative percentage of MHC type IIb compared to the sham-operated controls (p < or = 0.05). Importantly, plantaris mass was increased ~24% in TB-FO rats and adaptations to MHC isoform composition were consistent with normal, resistance-trained muscles.

CONCLUSION

Despite significant skeletal muscle derangements due to cancer, muscle retains the capacity to respond normally to hypertrophic stimuli. Specifically, when challenged with functional overload, plantaris muscles from TB rats displayed greater relative mass, increased percentages of MHC type I and decreased percentages of MHC type IIb. Therefore, resistance training paradigms should provide relative morphological and functional benefits to cancer patients suffering from muscle wasting.

Fundamental questions about genes, inactivity, and chronic diseases

Currently our society is faced with the challenge of understanding the biological basis for the epidemics of obesity and many chronic diseases, including Type 2 diabetes. Physical inactivity increases the relative risk of coronary artery disease by 45%, stroke by 60%, hypertension by 30%, and osteoporosis by 59%. Moreover, physical inactivity is cited as an actual cause of chronic disease by the US Centers of Disease Control. Physical activity was obligatory for survival for the Homo genus for hundreds of thousands of years. This review will present evidence that suggests that metabolic pathways selected during the evolution of the human genome are inevitably linked to physical activity. Furthermore, as with many other environmental interactions, cycles of physical activity and inactivity interact with genes resulting in a functional outcome appropriate for the environment. However, as humans are less physically active, there is a maladaptive response that leads to metabolic dysfunction and many chronic diseases. How and why these interactions occur are fundamental questions in biology. Finally, a perspective to future research in physical inactivity-gene interaction is presented. This information is necessary to provide the molecular evidence required to further promote the primary prevention of chronic diseases through physical activity, identify those molecules that will allow early disease detection, and provide society with the molecular information needed to counter the current strategy of adding physical inactivity into our lives.