Pro- and anti-apoptotic roles for IGF-I in TNF-alpha-induced apoptosis: a MAP kinase mediated mechanism

Dynamic Multiscale Regulation of Perfusion Recovery in Experimental Peripheral Arterial Disease: A Mechanistic Computational Model

In peripheral arterial disease (PAD), the degree of endogenous capacity to modulate revascularization of limb muscle is central to the management of leg ischemia. To characterize the multiscale and multicellular nature of revascularization in PAD, we have developed the first computational systems biology model that mechanistically incorporates intracellular, cellular, and tissue-level features critical for the dynamic reconstitution of perfusion after occlusion-induced ischemia. The computational model was specifically formulated for a preclinical animal model of PAD (mouse hindlimb ischemia [HLI]), and it has gone through multilevel model calibration and validation against a comprehensive set of experimental data so that it accurately captures the complex cellular signaling, cell-cell communication, and function during post-HLI perfusion recovery. As an example, our model simulations generated a highly detailed description of the time-dependent spectrum-like macrophage phenotypes in HLI, and through model sensitivity analysis we identified key cellular processes with potential therapeutic significance in the pathophysiology of PAD. Furthermore, we computationally evaluated the in vivo effects of different targeted interventions on post-HLI tissue perfusion recovery in a model-based, data-driven, virtual mouse population and experimentally confirmed the therapeutic effect of a novel model-predicted intervention in real HLI mice. This novel multiscale model opens up a new avenue to use integrative systems biology modeling to facilitate translational research in PAD.

Role of Pro-inflammatory Cytokines in Regulation of Skeletal Muscle Metabolism: A Systematic Review

Background:

Metabolic pathways perturbations lead to skeletal muscular atrophy in the cachexia and sarcopenia due to increased catabolism. Pro-inflammatory cytokines induce the catabolic pathways that impair the muscle integrity and function. Hence, this review primarily concentrates on the effects of pro-inflammatory cytokines in regulation of skeletal muscle metabolism.

Objective:

This review will discuss the role of pro-inflammatory cytokines in skeletal muscles during muscle wasting conditions. Moreover, the coordination among the pro-inflammatory cytokines and their regulated molecular signaling pathways which increase the protein degradation will be discussed.

Results:

During normal conditions, pro-inflammatory cytokines are required to balance anabolism and catabolism and to maintain normal myogenesis process. However, during muscle wasting their enhanced expression leads to marked destructive metabolism in the skeletal muscles. Proinflammatory cytokines primarily exert their effects by increasing the expression of calpains and E3 ligases as well as of Nf-κB, required for protein breakdown and local inflammation. Proinflammatory cytokines also locally suppress the IGF-1and insulin functions, hence increase the FoxO activation and decrease the Akt function, the central point of carbohydrates lipid and protein metabolism.

Conclusion:

Current advancements have revealed that the muscle mass loss during skeletal muscular atrophy is multifactorial. Despite great efforts, not even a single FDA approved drug is available in the market. It indicates the well-organized coordination among the pro-inflammatory cytokines that need to be further understood and explored.

Asymmetric cellular responses in primary human myoblasts using sera of different origin and specification

For successful growth and maintenance of primary myogenic cells in vitro, culture medium and addition of sera are the most important factors. At present it is not established as to what extent sera of different origin and composition, supplemented in media or serum-free media conditions influence myoblast function and responses to different stimuli. By assessing markers of proliferation, differentiation/fusion, quiescence, apoptosis and protein synthesis the aim of the current study was to elucidate how primary human myoblasts and myotubes are modulated by different commonly used serum using FCS (foetal calf serum), (CS-FCS charcoal-stripped FCS, a manufacturing process to remove hormones and growth factors from sera), HS (horse serum) as well as in serum free conditions (DMEM). To characterise the biological impact of the different serum, myoblasts were stimulated with Insulin (100 nM) and Vitamin D (100 nM; 1α,25(OH)2D3, 1α,25-Dihydroxycholecalciferol, Calcitriol), two factors with characterised effects on promoting fusion and protein synthesis or quiescence, respectively in human myoblasts/myotubes. We demonstrate that sera of different origin/formulation differentially affect myoblast proliferation and myotube protein synthesis. Importantly, we showed that quantifying the extent to which Insulin effects myoblasts in vitro is highly dependent upon serum addition and which type is present in the media. Upregulation of mRNA markers for myogenic fusion, Myogenin, with Insulin stimulation, relative to DMEM, appeared dampened at varying degrees with serum addition and effects on p70S6K phosphorylation as a marker of protein synthesis could not be identified unless serum was removed from media. We propose that these asymmetric molecular and biochemical responses in human myoblasts reflect the variable composition of mitogenic and anabolic factors in each of the sera. The results have implications for both the reproducibility and interpretation of results from experimental models in myoblast cells/myotubes.

The role of resveratrol on skeletal muscle cell differentiation and myotube hypertrophy during glucose restriction

Glucose restriction (GR) impairs muscle cell differentiation and evokes myotube atrophy. Resveratrol treatment in skeletal muscle cells improves inflammatory-induced reductions in skeletal muscle cell differentiation. We therefore hypothesised that resveratrol treatment would improve muscle cell differentiation and myotube hypertrophy in differentiating C2C12 myoblasts and mature myotubes during GR. Glucose restriction at 0.6 g/L (3.3 mM) blocked differentiation and myotube hypertrophy versus high-glucose (4.5 g/L or 25 mM) differentiation media (DM) conditions universally used for myoblast culture. Resveratrol (10 µM) treatment increased SIRT1 phosphorylation in DM conditions, yet did not improve differentiation when administered to differentiating myoblasts in GR conditions. Resveratrol did evoke increases in hypertrophy of mature myotubes under DM conditions with corresponding elevated Igf-I and Myhc7 gene expression, coding for the ‘slow’ type I MYHC protein isoform. Inhibition of SIRT1 via EX-527 administration (100 nM) also reduced myotube diameter and area in DM conditions and resulted in lower gene expression of Myhc 1, 2 and 4 coding for ‘intermediate’ and ‘faster’ IIx, IIa and IIb protein isoforms, respectively. Resveratrol treatment did not appear to modulate phosphorylation of energy-sensing protein AMPK or protein translation initiator P70S6K. Importantly, in mature myotubes, resveratrol treatment was able to ameliorate reduced myotube growth in GR conditions over an acute 24-h period, but not over 48–72 h. Overall, resveratrol evoked myotube hypertrophy in DM conditions while favouring ‘slower’ Myhc gene expression and acutely ameliorated impaired myotube growth observed during glucose restriction.

Omega-3 fatty acid EPA improves regenerative capacity of mouse skeletal muscle cells exposed to saturated fat and inflammation

Sarcopenic obesity is characterised by high fat mass, low muscle mass and an elevated inflammatory environmental milieu. We therefore investigated the effects of elevated inflammatory cytokine TNF-α (aging/obesity) and saturated fatty acid, palmitate (obesity) on skeletal muscle cells in the presence/absence of EPA, a-3 polyunsaturated fatty acid with proposed anti-inflammatory, anti-obesity activities. In the present study we show that palmitate was lipotoxic, inducing high levels of cell death and blocking myotube formation. Cell death under these conditions was associated with increased caspase activity, suppression of differentiation, reductions in both creatine kinase activity and gene expression of myogenic factors; IGF-II, IGFBP-5, MyoD and myogenin. However, inhibition of caspase activity via administration of Z-VDVAD-FMK (caspase-2), Z-DEVD-FMK (caspase-3) and ZIETD-KMK (caspase 8) was without effect on cell death. By contrast, lipotoxicity associated with elevated palmitate was reduced with the MEK inhibitor PD98059, indicating palmitate induced cell death was MAPK mediated. These lipotoxic conditions were further exacerbated in the presence of inflammation via TNF-α co-administration. Addition of EPA under cytotoxic stress (TNF-α) was shown to partially rescue differentiation with enhanced myotube formation being associated with increased MyoD, myogenin, IGF-II and IGFBP-5 expression. EPA had little impact on the cell death phenotype observed in lipotoxic conditions but did show benefit in restoring differentiation under lipotoxic plus cytotoxic conditions. Under these conditions Id3 (inhibitor of differentiation) gene expression was inversely linked with survival rates, potentially indicating a novel role of EPA and Id3 in the regulation of apoptosis in lipotoxic/cytotoxic conditions. Additionally, signalling studies indicated the combination of lipo- and cyto-toxic effects on the muscle cells acted through ceramide, JNK and MAPK pathways and blocking these pathways using PD98059 (MEK inhibitor) and Fumonisin B1 (ceramide inhibitor) significantly reduced levels of cell death. These findings highlight novel pathways associated with in vitro models of lipotoxicity (palmitate-mediated) and cytotoxicity (inflammatory cytokine mediated) in the potential targeting of molecular modulators of sarcopenic obesity.

The individual and combined effects of obesity- and ageing-induced systemic inflammation on human skeletal muscle properties

Background/Objectives:

The purpose of this study was to determine whether circulating pro-inflammatory cytokines, elevated with increased fat mass and ageing, were associated with muscle properties in young and older people with variable adiposity.

Subjects/Methods:

Seventy-five young (18–49 yrs) and 67 older (50–80 yrs) healthy, untrained men and women (BMI: 17–49 kg/m²) performed isometric and isokinetic plantar flexor maximum voluntary contractions (MVCs). Volume (V_m), fascicle pennation angle (FPA), and physiological cross-sectional area (PCSA) of the gastrocnemius medialis (GM) muscle were measured using ultrasonography. Voluntary muscle activation (VA) was assessed using electrical stimulation. GM specific force was calculated as GM fascicle force/PCSA. Percentage body fat (BF%), body fat mass (BFM), and lean mass (BLM) were assessed using dual-energy X-ray absorptiometry. Serum concentration of 12 cytokines was measured using multiplex luminometry.

Results:

Despite greater V_m, FPA, and PCSA (P<0.05), young individuals with BF% ⩾40 exhibited 37% less GM specific force compared to young BF%<40 (P<0.05). Older adults with BF% ⩾40 showed greater isokinetic MVC compared to older BF%<40 (P=0.019) but this was reversed when normalised to body mass (P<0.001). IL-6 correlated inversely with VA in young (r=−0.376; P=0.022) but not older adults (p>0.05), while IL-8 correlated with VA in older but not young adults (r⩾0.378, P⩽0.027). TNF-alpha correlated with MVC, lean mass, GM FPA and maximum force in older adults (r⩾0.458; P⩽0.048).

Conclusions:

The age- and adiposity-dependent relationships found here provide evidence that circulating pro-inflammatory cytokines may play different roles in muscle remodelling according to the age and adiposity of the individual.

l-glutamine Improves Skeletal Muscle Cell Differentiation and Prevents Myotube Atrophy After Cytokine (TNF-α) Stress Via Reduced p38 MAPK Signal Transduction

Tumour Necrosis Factor-Alpha (TNF-α) is chronically elevated in conditions where skeletal muscle loss occurs. As l-glutamine can dampen the effects of inflamed environments, we investigated the role of l-glutamine in both differentiating C2C12 myoblasts and existing myotubes in the absence/presence of TNF-α (20 ng · ml(-1) ) ± l-glutamine (20 mM). TNF-α reduced the proportion of cells in G1 phase, as well as biochemical (CK activity) and morphological differentiation (myotube number), with corresponding reductions in transcript expression of: Myogenin, Igf-I, and Igfbp5. Furthermore, when administered to mature myotubes, TNF-α induced myotube loss and atrophy underpinned by reductions in Myogenin, Igf-I, Igfbp2, and glutamine synthetase and parallel increases in Fox03, Cfos, p53, and Bid gene expression. Investigation of signaling activity suggested that Akt and ERK1/2 were unchanged, JNK increased (non-significantly) whereas P38 MAPK substantially and significantly increased in both myoblasts and myotubes in the presence of TNF-α. Importantly, 20 mM l-glutamine reduced p38 MAPK activity in TNF-α conditions back to control levels, with a corresponding rescue of myoblast differentiation and a reversal of atrophy in myotubes. l-glutamine resulted in upregulation of genes associated with growth and survival including; Myogenin, Igf-Ir, Myhc2 & 7, Tnfsfr1b, Adra1d, and restored atrophic gene expression of Fox03 back to baseline in TNF-α conditions. In conclusion, l-glutamine supplementation rescued suppressed muscle cell differentiation and prevented myotube atrophy in an inflamed environment via regulation of p38 MAPK. l-glutamine administration could represent an important therapeutic strategy for reducing muscle loss in catabolic diseases and inflamed ageing. J. Cell. Physiol. 9999: 231: 2720-2732, 2016. © 2016 Wiley Periodicals, Inc.

Skeletal muscle cells possess a 'memory' of acute early life TNF-α exposure: role of epigenetic adaptation

Sufficient quantity and quality of skeletal muscle is required to maintain lifespan and healthspan into older age. The concept of skeletal muscle programming/memory has been suggested to contribute to accelerated muscle decline in the elderly in association with early life stress such as fetal malnutrition. Further, muscle cells in vitro appear to remember the in vivo environments from which they are derived (e.g. cancer, obesity, type II diabetes, physical inactivity and nutrient restriction). Tumour-necrosis factor alpha (TNF-α) is a pleiotropic cytokine that is chronically elevated in sarcopenia and cancer cachexia. Higher TNF-α levels are strongly correlated with muscle loss, reduced strength and therefore morbidity and earlier mortality. We have extensively shown that TNF-α impairs regenerative capacity in mouse and human muscle derived stem cells [Meadows et al. (J Cell Physiol 183(3):330-337, 2000); Foulstone et al. (J Cell Physiol 189(2):207-215, 2001); Foulstone et al. (Exp Cell Res 294(1):223-235, 2004); Stewart et al. (J Cell Physiol 198(2):237-247, 2004); Al-Shanti et al. (Growth factors (Chur, Switzerland) 26(2):61-73, 2008); Saini et al. (Growth factors (Chur, Switzerland) 26(5):239-253, 2008); Sharples et al. (J Cell Physiol 225(1):240-250, 2010)]. We have also recently established an epigenetically mediated mechanism (SIRT1-histone deacetylase) regulating survival of myoblasts in the presence of TNF-α [Saini et al. (Exp Physiol 97(3):400-418, 2012)]. We therefore wished to extend this work in relation to muscle memory of catabolic stimuli and the potential underlying epigenetic modulation of muscle loss. To enable this aim; C2C12 myoblasts were cultured in the absence or presence of early TNF-α (early proliferative lifespan) followed by 30 population doublings in the absence of TNF-α, prior to the induction of differentiation in low serum media (LSM) in the absence or presence of late TNF-α (late proliferative lifespan). The cells that received an early plus late lifespan dose of TNF-α exhibited reduced morphological (myotube number) and biochemical (creatine kinase activity) differentiation vs. control cells that underwent the same number of proliferative divisions but only a later life encounter with TNF-α. This suggested that muscle cells had a morphological memory of the acute early lifespan TNF-α encounter. Importantly, methylation of myoD CpG islands were increased in the early TNF-α cells, 30 population doublings later, suggesting that even after an acute encounter with TNF-α, the cells have the capability of retaining elevated methylation for at least 30 cellular divisions. Despite these fascinating findings, there were no further increases in myoD methylation or changes in its gene expression when these cells were exposed to a later TNF-α dose suggesting that this was not directly responsible for the decline in differentiation observed. In conclusion, data suggest that elevated myoD methylation is retained throughout muscle cells proliferative lifespan as result of early life TNF-α treatment and has implications for the epigenetic control of muscle loss.

Longevity and skeletal muscle mass: the role of IGF signalling, the sirtuins, dietary restriction and protein intake

Advancing age is associated with a progressive loss of skeletal muscle (SkM) mass and function. Given the worldwide aging demographics, this is a major contributor to morbidity, escalating socio-economic costs and ultimately mortality. Previously, it has been established that a decrease in regenerative capacity in addition to SkM loss with age coincides with suppression of insulin/insulin-like growth factor signalling pathways. However, genetic or pharmacological modulations of these highly conserved pathways have been observed to significantly enhance life and healthspan in various species, including mammals. This therefore provides a controversial paradigm in which reduced regenerative capacity of skeletal muscle tissue with age potentially promotes longevity of the organism. This paradox will be assessed and considered in the light of the following: (i) the genetic knockout, overexpression and pharmacological models that induce lifespan extension (e.g. IRS-1/s6K KO, mTOR inhibition) versus the important role of these signalling pathways in SkM growth and adaptation; (ii) the role of the sirtuins (SIRTs) in longevity versus their emerging role in SkM regeneration and survival under catabolic stress; (iii) the role of dietary restriction and its impact on longevity versus skeletal muscle mass regulation; (iv) the crosstalk between cellular energy metabolism (AMPK/TSC2/SIRT1) and survival (FOXO) versus growth and repair of SkM (e.g. AMPK vs. mTOR); and (v) the impact of protein feeding in combination with dietary restriction will be discussed as a potential intervention to maintain SkM mass while increasing longevity and enabling healthy aging.

'From death, lead me to immortality' - mantra of ageing skeletal muscle

Skeletal muscle is a post-mitotic tissue maintained by repair and regeneration through a population of stem cell-like satellite cells. Following muscle injury, satellite cell proliferation is mediated by local signals ensuring sufficient progeny for tissue repair. Age–related changes in satellite cells as well as to the local and systemic environment potentially impact on the capacity of satellite cells to generate sufficient progeny in an ageing organism resulting in diminished regeneration. ‘Rejuvenation’ of satellite cell progeny and regenerative capacity by environmental stimuli effectors suggest that a subset of age-dependent satellite cell changes may be reversible. Epigenetic regulation of satellite stem cells that include DNA methylation and histone modifications which regulate gene expression are potential mechanisms for such reversible changes and have been shown to control organismal longevity. The area of health and ageing that is likely to benefit soonest from advances in the biology of adult stem cells is the emerging field of regenerative medicine. Further studies are needed to elucidate the mechanisms by which epigenetic modifications regulate satellite stem cell function and will require an increased understanding of stem-cell biology, the environment of the aged tissue and the interaction between the two.

Gene expression profiling of gastrocnemius of "minimuscle" mice

Few studies have investigated heterogeneity of selection response in replicate lines subjected to equivalent selection. We developed four replicate lines of mice based on high levels of voluntary wheel running (high runner or HR lines) while also maintaining four nonselected control lines. This led to the unexpected discovery of the HR minimuscle (HRmini) phenotype, recognized by a 50% reduction in hindlimb muscle mass, which became fixed in 1 of the four HR selected lines. Here, we report genome-wide expression profiling describing transcriptome differences between HRnormal and HRmini medial gastrocnemius. Consistent with the known reduction of type IIB fibers in HRmini, Myh4 gene expression was -8.82-fold less (P = 0.0001) in HRmini, which was closely associated with differences in the "calcium signaling" canonical pathway, including structural genes (e.g., Mef2c, twofold greater in HRmini, P = 0.0003) and myogenic factors (e.g., Myog, 3.8-fold greater in HRmini, P = 0.0026) associated with slow-type myofibers. The gene that determines the HRmini phenotype is known to reside in a 2.6335-Mb interval on mouse chromosome 11 and 7 genes (Myh10, Chrnb1, Acadvl, Senp3, Gabarap, Eif5a, and Clec10a) from this region were differentially expressed. Verification by real-time PCR confirmed 1.5-fold greater (P < 0.05) expression of very long chain acyl-CoA dehydrogenase (Acadvl) in HRmini. Ten other genes associated with fatty acid metabolism were also upregulated in HRmini, suggesting differences in the ability to metabolize fatty acids in HRnormal and HRmini muscles. This work provides a resource for understanding differences in muscle phenotypes in populations exhibiting high running capacity.

A Novel Role of IGF1 in Apo2L/TRAIL-Mediated Apoptosis of Ewing Tumor Cells

Insulin-like growth factor 1 (IGF1) reputedly opposes chemotoxicity in Ewing sarcoma family of tumor (ESFT) cells. However, the effect of IGF1 on apoptosis induced by apoptosis ligand 2 (Apo2L)/tumor necrosis factor (TNF-) related apoptosis-inducing ligand (TRAIL) remains to be established. We find that opposite to the partial survival effect of short-term IGF1 treatment, long-term IGF1 treatment amplified Apo2L/TRAIL-induced apoptosis in Apo2L/TRAIL-sensitive but not resistant ESFT cell lines. Remarkably, the specific IGF1 receptor (IGF1R) antibody α-IR3 was functionally equivalent to IGF1. Short-term IGF1 incubation of cells stimulated survival kinase AKT and increased X-linked inhibitor of apoptosis (XIAP) protein which was associated with Apo2L/TRAIL resistance. In contrast, long-term IGF1 incubation resulted in repression of XIAP protein through ceramide (Cer) formation derived from de novo synthesis which was associated with Apo2L/TRAIL sensitization. Addition of ceramide synthase (CerS) inhibitor fumonisin B1 during long-term IGF1 treatment reduced XIAP repression and Apo2L/TRAIL-induced apoptosis. Noteworthy, the resistance to conventional chemotherapeutic agents was maintained in cells following chronic IGF1 treatment. Overall, the results suggest that chronic IGF1 treatment renders ESFT cells susceptible to Apo2L/TRAIL-induced apoptosis and may have important implications for the biology as well as the clinical management of refractory ESFT.

Inhibitory effects of IL-6 on IGF-1 activity in skeletal myoblasts could be mediated by the activation of SOCS-3

In elderly people, low and high levels of insulin-like growth factor 1 (IGF-1) and interleukin-6 (IL-6), respectively, are well documented and may contribute to reduced muscle mass and poor muscle function of ageing and suggesting a biological interactions between IGF-1 and IL-6. However, the dual effect of IGF-1/IL-6 on skeletal muscle differentiation and proliferation has not been fully investigated. We therefore hypothesised that IL-6 impairs the biological activity of IGF-1 in skeletal muscle through inhibiting its signalling pathways, ERK1/2 and Akt. Our aim was to examine the combined effects of these factors on models of muscle wasting, with objectives to examine skeletal muscle differentiation and proliferation using the murine C2 skeletal muscle cell line. Cells were cultured with DM, IGF-1 and IL-6 alone (control treatments), or co-cultured with IGF-1/IL-6. Co-incubation of C2 cells in IGF-1 plus IL-6 resulted in maximal cell death (22 ± 4%; P < 0.005) compared with control treatments (14 ± 2.9%). This was also confirmed by cyclin D1 expression levels in co-incubation treatments (7 ± 3.5%; P < 0.05) compared with control treatments (≈ 23%). The expression levels of myogenic-specific transcriptional factor mRNAs (myoD and myogenin) were also significantly (P < 0.005) reduced by 70% and 90%, respectively, under the co-incubation regimes, compared with control treatments. Signalling investigations showed significant phosphorylation reduction by 20%, (P < 0.05) of ERK1/2 and Akt in co-incubation treatments relative to either treatment alone. Expression studies for SOCS-3 (1.6-fold ± 0.08, P < 0.05) and IRS-1 (0.65-fold ± 0.13 P < 0.005) mRNAs showed significant elevation and reduction for both genes, respectively, in co-treatments relative to control treatments. These data may suggest that IL-6 exerts its inhibitory effects on IGF-1 signalling pathways (ERK1/2 and Akt) through blocking its receptor substrate IRS-1 by SOCS-3.

Does Branched-Chain Amino Acids Supplementation Modulate Skeletal Muscle Remodeling through Inflammation Modulation? Possible Mechanisms of Action

Skeletal muscle protein turnover is modulated by intracellular signaling pathways involved in protein synthesis, degradation, and inflammation. The proinflammatory status of muscle cells, observed in pathological conditions such as cancer, aging, and sepsis, can directly modulate protein translation initiation and muscle proteolysis, contributing to negative protein turnover. In this context, branched-chain amino acids (BCAAs), especially leucine, have been described as a strong nutritional stimulus able to enhance protein translation initiation and attenuate proteolysis. Furthermore, under inflammatory conditions, BCAA can be transaminated to glutamate in order to increase glutamine synthesis, which is a substrate highly consumed by inflammatory cells such as macrophages. The present paper describes the role of inflammation on muscle remodeling and the possible metabolic and cellular effects of BCAA supplementation in the modulation of inflammatory status of skeletal muscle and the consequences on protein synthesis and degradation.

Sirtuin 1 regulates skeletal myoblast survival and enhances differentiation in the presence of resveratrol

Sirtuin 1 also known as NAD-dependent deacetylase sirtuin 1, is a protein that in humans is encoded by the Sirt1 gene. Sirt1 is an enzyme that deacetylates proteins that contribute to cellular regulation and is a key regulator of cell defenses and survival in response to stress. Deletion of Sirt1 abolishes the increase in lifespan induced by calorie restriction or sublethal cytokine stress, indicating that Sirt1 promotes longevity and survival. We have demonstrated that administration of a sublethal dose of tumour necrosis factor-α (TNF-α; 1.25 ng ml(-1)) inhibits myotube formation, and co-incubation with insulin-like growth factor I (IGF-I; 1.5 ng ml(-1)) facilitates C2 myoblast death rather than rescuing differentiation. A higher dose of TNF-α (10 ng ml(-1)) resulted in significant apoptosis, which was rescued by IGF-I (1.5 ng ml(-1); 50% rescue; P < 0.05). We aimed to investigate the role of Sirt1 in the conflicting roles of IGF-I. Quantitative real-time PCR revealed that Sirt1 expression was elevated in myoblasts following incubation of 10 ng ml(-1) TNF-α or 1.25 ng ml(-1) TNF-α plus IGF-I (fivefold and 7.2-fold increases versus control, respectively; P < 0.05). A dose of 10 ng ml(-1) TNF-α induced ∼21 ± 0.7% apoptosis, which was reduced (∼50%; P < 0.05) when administered with IGF-I. Likewise, Sirt1 expression was elevated following 10 ng ml(-1) TNF-α administration, but was reduced (∼30%; P < 0.05) in the presence of IGF-I. C2C12 myoblasts, a subclone of the C2 cell line produced for their differentiation potential and used to examine intrinsic ageing, unlike C2 cells, do not die in the presence of TNF-α and do not upregulate Sirt1. As conditions that induced the greatest myoblast stress/damage resulted in elevated Sirt1 expression, we investigated the effects of Sirt1 gene silencing. Treatment with 10 ng ml(-1) TNF-α or co-incubation with 1.25 ng ml(-1) TNF-α and 1.5 ng ml(-1) IGF-I resulted in apoptosis (20.33 ± 2.08 and 19 ± 2.65%, respectively), which was increased when myoblasts were pretreated with Sirt1 small interfering RNA (31 ± 2.65 and 27.33 ± 2.52%, respectively; P < 0.05) and was reduced (14.33 ± 3.05%, P < 0.05 and 12.78 ± 4.52%, P = 0.054) by resveratrol, which also significantly rescued the block on differentiation. In conclusion, Sirt1 expression increases in conditons of stress, potentially serving to reduce or dampen myoblast death.

Growth factor and cytokine interactions in myogenesis. Part I. The effect of TNF-alpha and IFN-gamma on IGF-I-dependent differentiation in mouse C2C12 myogenic cells

The aim of this study was to examine the potential interactions of IGF-I with TNF-alpha and IFN-gamma with regard to regulation of the myogenesis and proliferative potential of mouse C2C12 myoblasts. The stimulation of myogenesis by IGF-I (30 nmol/l) was manifested by an enhanced myoblast fusion and expression of myosin heavy chain (MHC) during the first 3 days of differentiation. IGF-I-dependent fusion and MHC expression was reduced by TNF-alpha and IFN-gamma. Both cytokines prevented the stimulatory effect of IGF-I on MyoD expression with minor modification of the myogenin level. Both TNF-alpha and IFN-gamma activated the expression of cyclin A in myoblasts restimulated to proliferation; however, when used in combination with IGF-I these cytokines prevented the rise in cyclin A induced by growth factor.

IN CONCLUSION

i) TNF-alpha and IFN-gamma reduce IGF-I-dependent myogenesis which was manifested by the reduction of myoblast fusion and MHC cellular levels, ii) Molecular mechanisms of inhibitory action of TNF-alpha and IFN-gamma on IGF-I-mediated differentiation involve a decrease in MyoD whereas myogenin level plays a minor role, iii) TNF-alpha and IFN-gamma increase the proliferative potential of myoblasts; however, they reduced the mitogenic effect of IGF-I, manifested by a decrease of IGF-I-stimulated cyclin A expression in myoblasts reinduced to proliferation. Interactions among IGF-I and proinflammatory cytokines are therefore important to establish a number of myoblasts and the onset of myogenesis during muscle regeneration.

Effects of alfacalcidol on circulating cytokines and growth factors in rat skeletal muscle

Supra-physiological levels of vitamin D induce skeletal muscle atrophy, which may be particularly detrimental in already sarcopaenic elderly. Neither the cause nor whether the atrophy is fibre type specific are known. To obtain supraphysiological levels of circulating vitamin D (1,25(OH)(2)D(3)) 27.5-month-old female Fischer(344) × Brown Norway F1 rats were orally treated for 6 weeks with vehicle or the vitamin D analogue alfacalcidol. Alfacalcidol treatment induced a 22% decrease in body mass and 17% muscle atrophy. Fibre atrophy was restricted to type IIb fibres in the low-oxidative part of the gastrocnemius medialis only (-22%; P < 0.05). There was a concomitant 1.6-fold increase in mRNA expression of the ubiquitin ligase MuRF-1 (P < 0.001), whereas those of insulin-like growth factor 1 and myostatin were not affected. Circulating IL-6 was unaltered, but leptin and adiponectin were decreased (-39%) and increased (64%), respectively. The treated rats also exhibited a reduced food intake. In conclusion, supraphysiological levels of circulating 1,25(OH)(2)D(3) cause preferential atrophy of type IIb fibres, which is associated with an increased expression of MuRF-1 without evidence of systemic inflammation. The atrophy and loss of body mass in the presence of supra-physiological levels of vitamin D are primarily due to a reduced food intake.

Insulin-like growth factor I signaling in skeletal muscle and the potential for cytokine interactions

Recent research has demonstrated that intracellular signaling components associated with several proinflammatory cytokines have the potential to interact with signaling pathways that regulate anabolic processes in skeletal muscle. This presentation and the ensuing brief review are intended to present a selection of the potential interactions between these two critical processes.

Multiple pathways to the same end: mechanisms of myonuclear apoptosis in sarcopenia of aging

Sarcopenia, the age-related decline in muscle mass and function, represents a significant health issue due to the high prevalence of frailty and disability associated with this condition. Nevertheless, the cellular mechanisms responsible for the loss of muscle mass in old age are still largely unknown. An altered regulation of myocyte apoptosis has recently emerged as a possible contributor to the pathogenesis of sarcopenia. Studies in animal models have shown that the severity of skeletal muscle apoptosis increases over the course of aging and correlates with the degree of muscle mass and strength decline. Several apoptotic pathways are operative in aged muscles, with the mitochondria- and TNF-α-mediated pathways likely being the most relevant to sarcopenia. However, despite the growing number of studies on the subject, a definite mechanistic link between myocyte apoptosis and age-related muscle atrophy has not yet been established. Furthermore, the evidence on the role played by apoptosis in human sarcopenia is still sparse. Clearly, further research is required to better define the involvement of myocyte apoptosis in the pathogenesis of muscle loss at advanced age. This knowledge will likely help in the design of more effective therapeutic strategies to preserve muscle mass into old age, thus fostering independence of the elderly population and reducing the socioeconomic burden associated with sarcopenia.

Powerful signals for weak muscles

The aim of the present review is to summarise, evaluate and critique the different mechanisms involved in anabolic growth of skeletal muscle and the catabolic processes involved in cancer cachexia and sarcopenia of ageing. This is highly relevant, since they represent targets for future promising clinical investigations. Sarcopenia is an inevitable process associated with a gradual reduction in muscle mass and strength, associated with a reduction in motor unit number and atrophy of muscle fibres, especially the fast type IIa fibres. The loss of muscle mass with ageing is clinically important because it leads to diminished functional ability and associated complications. Cachexia is widely recognised as severe and rapid wasting accompanying disease states such as cancer or immunodeficiency disease. One of the main characteristics of cancer cachexia is asthenia or lack of strength, which is directly related to the muscle loss. Indeed, apart from the speed of loss, muscle wasting during cancer and ageing share many common metabolic pathways and mediators. In healthy young individuals, muscles maintain their mass and function because of a balance between protein synthesis and protein degradation associated with rates of anabolic and catabolic processes, respectively. Muscles grow (hypertrophy) when protein synthesis exceeds protein degradation. Conversely, muscles shrink (atrophy) when protein degradation dominates. These processes are not occurring independently of each other, but are finely coordinated by a web of intricate signalling networks. Such signalling networks are in charge of executing environmental and cellular cues that ultimately determine whether muscle proteins are synthesised or degraded. Increasing our understanding for the pathways involved in hypertrophy and atrophy and particularly the interaction of these pathways is essential in designing therapeutic strategies for both prevention and treatment of muscle wasting conditions with age and with disease.

The role of systemic inflammation in age-related muscle weakness and wasting

Ageing is associated with a slow, but progressive muscle weakness, which is largely attributable to muscle wasting. A diminished function of satellite cells at old age may hamper preservation and repair from (contraction)-induced injury and contribute to the age-related muscle wasting. Satellite cell function may be affected by circulating factors, as muscle regeneration in old mice sharing the circulation of young mice is not impaired. Chronic low-grade systemic inflammation in old organisms may be that environmental factor. Indeed, the inflammatory cytokine tumor necrosis factor-alpha (TNFalpha) negatively affects the muscle regenerating capacity. TNFalpha destabilizes MyoD, a muscle-specific transcription factor involved in satellite cell proliferation and differentiation, and induces apoptosis of satellite cells, particularly at old age. Here it is proposed that some of these effects are mediated by TNFalpha-induced expression of inhibitors of differentiation proteins. Yet, the increase in TNFalpha during the normal inflammatory response helps, rather than impairs, the repair process. This apparent contradiction may be resolved by the fact that the effects of TNFalpha are concentration and time dependent. Thus, the negative effect of systemic inflammation on muscle strength at old age may only become apparent when it exceeds a certain threshold and persists for a prolonged period.