TNF-related weak inducer of apoptosis (TWEAK) is a potent skeletal muscle-wasting cytokine

A rat model of massive rotator cuff tears

Rotator cuff tears (RCTs) are the most common tendon injury seen in orthopedic patients. Massive RCT does not heal spontaneously and results in poor clinical outcomes. Muscle atrophy and fatty infiltration in rotator cuff muscles are major complications of chronic massive RCT and are thought to be the key factors responsible for the failure of attempted massive RCT repair. However, the pathophysiology of rotator cuff muscle atrophy and fat infiltration remains largely unknown, and no small animal model has been shown to reproduce the histologic and molecular changes seen in massive RCT. In this article, we report a novel rat massive RCT model, in which significant and consistent muscle atrophy and fat infiltration were observed in the rotator cuff muscles after rotator cuff tendon transection and denervation. The supraspinatus and infraspinatus muscle lost 25.4% and 28.9% of their wet weight 2 weeks after complete tendon transection, respectively. Six weeks after surgery, the average wet weight of supraspinatus and infraspinatus muscles decreased 13.2% and 28.3%, respectively. Significant fat infiltration was only observed in infraspinatus 6 weeks after tendon transection.

MicroRNAs in skeletal muscle: their role and regulation in development, disease and function

Maintaining skeletal muscle function throughout the lifespan is a prerequisite for good health and independent living. For skeletal muscle to consistently function at optimal levels, the efficient activation of processes that regulate muscle development, growth, regeneration and metabolism is required. Numerous conditions including neuromuscular disorders, physical inactivity, chronic disease and ageing are associated with perturbations in skeletal muscle function. A loss or reduction in skeletal muscle function often leads to increased morbidity and mortality either directly, or indirectly, via the development of secondary diseases such as diabetes, obesity, cardiovascular and respiratory disease. Identifying mechanisms which influence the processes regulating skeletal muscle function is a key priority. The discovery of microRNAs (miRNAs) provides a new avenue that will extend our knowledge of factors controlling skeletal muscle function. miRNAs may also improve our understanding and application of current therapeutic approaches as well as enable the identification of new therapeutic strategies and targets aimed at maintaining and/or improving skeletal muscle health. This review brings together the latest developments in skeletal muscle miRNA biology and focuses on their role and regulation under physiological and patho-physiological conditions with an emphasis on: myogenesis, hypertrophy, atrophy and regeneration; exercise and nutrition; muscle disease, ageing, diabetes and obesity.

Targeted ablation of TRAF6 inhibits skeletal muscle wasting in mice

TRAF6 expression is enhanced during muscle atrophy and induces activation of signal transduction cascades that promote muscle wasting.

Skeletal muscle wasting is a major human morbidity, and contributes to mortality in a variety of clinical settings, including denervation and cancer cachexia. In this study, we demonstrate that the expression level and autoubiquitination of tumor necrosis factor (α) receptor adaptor protein 6 (TRAF6), a protein involved in receptor-mediated activation of several signaling pathways, is enhanced in skeletal muscle during atrophy. Skeletal muscle–restricted depletion of TRAF6 rescues myofibril degradation and preserves muscle fiber size and strength upon denervation. TRAF6 mediates the activation of JNK1/2, p38 mitogen-activated protein kinase, adenosine monophosphate–activated protein kinase, and nuclear factor κB, and induces the expression of muscle-specific E3 ubiquitin ligases and autophagy-related molecules in skeletal muscle upon denervation. Inhibition of TRAF6 also preserves the orderly pattern of intermyofibrillar and subsarcolemmal mitochondria in denervated muscle. Moreover, depletion of TRAF6 prevents cancer cachexia in an experimental mouse model. This study unveils a novel mechanism of skeletal muscle atrophy and suggests that TRAF6 is an important therapeutic target to prevent skeletal muscle wasting.

Regulation of skeletal muscle growth by the IGF1-Akt/PKB pathway: insights from genetic models

A highly conserved signaling pathway involving insulin-like growth factor 1 (IGF1), and a cascade of intracellular components that mediate its effects, plays a major role in the regulation of skeletal muscle growth. A central component in this cascade is the kinase Akt, also called protein kinase B (PKB), which controls both protein synthesis, via the kinases mammalian target of rapamycin (mTOR) and glycogen synthase kinase 3β (GSK3β), and protein degradation, via the transcription factors of the FoxO family. In this paper, we review the composition and function of this pathway in skeletal muscle fibers, focusing on evidence obtained in vivo by transgenic and knockout models and by muscle transient transfection experiments. Although this pathway is essential for muscle growth during development and regeneration, its role in adult muscle response to mechanical load is less clear. A full understanding of the operation of this pathway could help to design molecularly targeted therapeutics aimed at preventing muscle wasting, which occurs in a variety of pathologic contexts and in the course of aging.

Identification of genes that elicit disuse muscle atrophy via the transcription factors p50 and Bcl-3

Skeletal muscle atrophy is a debilitating condition associated with weakness, fatigue, and reduced functional capacity. Nuclear factor-kappaB (NF-κB) transcription factors play a critical role in atrophy. Knockout of genes encoding p50 or the NF-κB co-transactivator, Bcl-3, abolish disuse atrophy and thus they are NF-κB factors required for disuse atrophy. We do not know however, the genes targeted by NF-κB that produce the atrophied phenotype. Here we identify the genes required to produce disuse atrophy using gene expression profiling in wild type compared to Nfkb1 (gene encodes p50) and Bcl-3 deficient mice. There were 185 and 240 genes upregulated in wild type mice due to unloading, that were not upregulated in Nfkb1^−/− and Bcl-3^−/− mice, respectively, and so these genes were considered direct or indirect targets of p50 and Bcl-3. All of the p50 gene targets were contained in the Bcl-3 gene target list. Most genes were involved with protein degradation, signaling, translation, transcription, and transport. To identify direct targets of p50 and Bcl-3 we performed chromatin immunoprecipitation of selected genes previously shown to have roles in atrophy. Trim63 (MuRF1), Fbxo32 (MAFbx), Ubc, Ctsl, Runx1, Tnfrsf12a (Tweak receptor), and Cxcl10 (IP-10) showed increased Bcl-3 binding to κB sites in unloaded muscle and thus were direct targets of Bcl-3. p50 binding to the same sites on these genes either did not change or increased, supporting the idea of p50:Bcl-3 binding complexes. p65 binding to κB sites showed decreased or no binding to these genes with unloading. Fbxo9, Psma6, Psmc4, Psmg4, Foxo3, Ankrd1 (CARP), and Eif4ebp1 did not show changes in p65, p50, or Bcl-3 binding to κB sites, and so were considered indirect targets of p50 and Bcl-3. This work represents the first study to use a global approach to identify genes required to produce the atrophied phenotype with disuse.

Tumor necrosis factor-α regulates distinct molecular pathways and gene networks in cultured skeletal muscle cells

Background

Skeletal muscle wasting is a debilitating consequence of large number of disease states and conditions. Tumor necrosis factor-α (TNF-α) is one of the most important muscle-wasting cytokine, elevated levels of which cause significant muscular abnormalities. However, the underpinning molecular mechanisms by which TNF-α causes skeletal muscle wasting are less well-understood.

Methodology/Principal Findings

We have used microarray, quantitative real-time PCR (QRT-PCR), Western blot, and bioinformatics tools to study the effects of TNF-α on various molecular pathways and gene networks in C2C12 cells (a mouse myoblastic cell line). Microarray analyses of C2C12 myotubes treated with TNF-α (10 ng/ml) for 18h showed differential expression of a number of genes involved in distinct molecular pathways. The genes involved in nuclear factor-kappa B (NF-kappaB) signaling, 26s proteasome pathway, Notch1 signaling, and chemokine networks are the most important ones affected by TNF-α. The expression of some of the genes in microarray dataset showed good correlation in independent QRT-PCR and Western blot assays. Analysis of TNF-treated myotubes showed that TNF-α augments the activity of both canonical and alternative NF-κB signaling pathways in myotubes. Bioinformatics analyses of microarray dataset revealed that TNF-α affects the activity of several important pathways including those involved in oxidative stress, hepatic fibrosis, mitochondrial dysfunction, cholesterol biosynthesis, and TGF-β signaling. Furthermore, TNF-α was found to affect the gene networks related to drug metabolism, cell cycle, cancer, neurological disease, organismal injury, and abnormalities in myotubes.

Conclusions

TNF-α regulates the expression of multiple genes involved in various toxic pathways which may be responsible for TNF-induced muscle loss in catabolic conditions. Our study suggests that TNF-α activates both canonical and alternative NF-κB signaling pathways in a time-dependent manner in skeletal muscle cells. The study provides novel insight into the mechanisms of action of TNF-α in skeletal muscle cells.

Genetic ablation of TWEAK augments regeneration and post-injury growth of skeletal muscle in mice

Impairment in the regeneration process is a critical determinant for skeletal muscle wasting in chronic diseases and degenerative muscle disorders. Inflammatory cytokines are known to cause significant muscle wasting, however, their role in myofiber regeneration is less clear. In this study we have investigated the role of tumor necrosis factor-like weak inducer of apoptosis (TWEAK) in skeletal muscle regeneration in vivo. Our results show that expression levels of TWEAK and its receptor Fn14 are significantly increased in skeletal muscles of mice after injury. Genetic deletion of TWEAK increased the fiber cross-sectional area and levels of embryonic isoform of myosin heavy chain in regenerating tibial anterior muscle. Conversely, muscle-specific transgenic overexpression of TWEAK reduced the fiber cross-sectional area and levels of the embryonic myosin heavy chain in regenerating muscle. TWEAK induced the expression of several inflammatory molecules and increased interstitial fibrosis in regenerating muscle. Genetic ablation of TWEAK suppressed, whereas overexpression of TWEAK increased, the activation of nuclear factor-kappa B without affecting the activation of Akt or p38 kinase in regenerating myofibers. Primary myoblasts from TWEAK-null mice showed enhanced differentiation in vitro, whereas myoblasts from TWEAK-Tg mice showed reduced differentiation compared with wild-type mice. Collectively, our study suggests that TWEAK negatively regulates muscle regeneration and that TWEAK is a potential therapeutic target to enhance skeletal muscle regeneration in vivo.

Methods and potential biomarkers for the evaluation of endothelial dysfunction in chronic kidney disease: a critical approach

The impressive cardiovascular morbidity and mortality of chronic kidney disease (CKD) patients is attributable in a significant proportion to endothelial dysfunction (ED), arterial stiffness, and vascular calcifications. Abnormal vascular reactivity in these patients is more pronounced compared with other high-risk populations, but remains undiagnosed in the usual clinical setting. We briefly review the most important causes and risk factors of ED, oxidative stress, and inflammation related to arterial stiffness. We describe the main methods of ED investigation and the importance of using potential biomarkers together with classic techniques for a more comprehensive assessment of this condition. These methods include evaluation of: forearm blood flow by plethysmography, skin microcirculation by laser Doppler, and flow-mediated vasodilation by Doppler ultrasound imaging. Applanation tonometry is an easy-to-handle tool that allows a clinically reliable assessment of arterial stiffness and is also useful in quantifying endothelium-dependent and -independent vascular reactivity. We also discuss the diagnostic and therapeutic impact of new markers of ED in the CKD population. Improvement of endothelial function is an important challenge for clinical practice, and there are relatively few therapeutical strategies available. Therefore, a combined biomarker and bedside investigational approach could be a starting point for developing optimal therapeutic tools.

Genetics vs. entropy: longevity factors suppress the NF-kappaB-driven entropic aging process

Molecular studies in model organisms have identified potent longevity genes which can delay the aging process and extend the lifespan. Longevity factors promote stress resistance and cellular survival. It seems that the aging process itself is not genetically programmed but a random process involving the loss of molecular fidelity and subsequent accumulation of waste products. This age-related increase in cellular entropy is compatible with the disposable soma theory of aging. A large array of host defence systems has been linked to the NF-kappaB system which is an ancient signaling pathway specialized to host defence, e.g. functioning in immune system. Emerging evidence demonstrates that the NF-kappaB system is activated during aging. Oxidative stress and DNA damage increase with aging and elicit a sustained activation of the NF-kappaB system which has negative consequences, e.g. chronic inflammatory response, increase in apoptotic resistance, decline in autophagic cleansing, and tissue atrophy, i.e. processes that enhance the aging process. We will discuss the role of NF-kappaB system in the pro-aging signaling and will emphasize that several longevity factors seem to be inhibitors of NF-kappaB signaling and in that way they can suppress the NF-kappaB-driven entropic host defence catastrophe.

The TWEAK-Fn14 system is a critical regulator of denervation-induced skeletal muscle atrophy in mice

The TNF-related cytokine TWEAK promotes skeletal muscle atrophy that is associated with classical disuse syndromes.

Skeletal muscle atrophy occurs in a variety of clinical settings, including cachexia, disuse, and denervation. Inflammatory cytokines have been shown to be mediators of cancer cachexia; however, the role of cytokines in denervation- and immobilization-induced skeletal muscle loss remains unknown. In this study, we demonstrate that a single cytokine, TNF-like weak inducer of apoptosis (TWEAK), mediates skeletal muscle atrophy that occurs under denervation conditions. Transgenic expression of TWEAK induces atrophy, fibrosis, fiber-type switching, and the degradation of muscle proteins. Importantly, genetic ablation of TWEAK decreases the loss of muscle proteins and spared fiber cross-sectional area, muscle mass, and strength after denervation. Expression of the TWEAK receptor Fn14 (fibroblast growth factor–inducible receptor 14) and not the cytokine is significantly increased in muscle upon denervation, demonstrating an unexpected inside-out signaling pathway; the receptor up-regulation allows for TWEAK activation of nuclear factor κB, causing an increase in the expression of the E3 ubiquitin ligase MuRF1. This study reveals a novel mediator of skeletal muscle atrophy and indicates that the TWEAK–Fn14 system is an important target for preventing skeletal muscle wasting.

Decreased soluble TWEAK levels predict an adverse prognosis in patients with chronic stable heart failure

AIMS

Tumour necrosis factor (TNF)-like weak inducer of apoptosis (sTWEAK) is a multifunctional cytokine that has recently been implicated in cardiovascular disease. The aim of this study was to define the plasma levels of sTWEAK in patients with stable chronic heart failure and evaluate the possibility of a prognostic impact of sTWEAK.

METHODS AND RESULTS

sTWEAK levels in plasma samples from 364 patients with systolic heart failure were compared with 36 control patients. The median levels of sTWEAK in heart failure patients were significantly lower than those of the control group (217 pg/mL, interquartile range 136-311 vs. 325 pg/mL, interquartile range 250-394 pg/mL). Moreover, sTWEAK levels were lower in patients with ischaemic cardiomyopathy vs. dilated cardiomyopathy and correlated significantly with functional NYHA class. Patients with plasma levels below a ROC-derived cut-off value of 227 pg/mL had a significantly higher mortality rate after 4 years. Upon univariate and multivariate analyses, sTWEAK levels below 227 pg/mL emerged as an independent predictor of subsequent death.

CONCLUSION

In contrast to other cytokines shown to be increased in heart failure patients, plasma levels of sTWEAK are significantly reduced in chronic stable heart failure. In addition, lower plasma levels of sTWEAK predict an adverse prognosis independent of established risk markers such as NT-proBNP.

Persistent inflammation as a catalyst for other risk factors in chronic kidney disease: a hypothesis proposal

Because inflammation by now is a "traditional" finding that predicts poor outcome and cardiovascular events in the vast majority of patients with ESRD, it could be argued that inflammatory biomarkers should not longer be considered "novel" risk factors. In this review, we forward the hypothesis that, in addition to putative direct proatherogenic effects, persistent inflammation may serve as a catalyst and, in the toxic uremic milieu, modulate the effects of other concurrent vascular and nutritional risk factors. We discuss some recent observational studies, suggesting that the presence of persistent inflammation magnifies the risk for poor outcome via mechanisms related to self-enhancement of the inflammatory cascade and exacerbation of both the wasting and the vascular calcification processes. Because persistent inflammation may be the silent culprit of other commonly observed pathophysiologic alterations in chronic kidney disease, it is imperative that inflammatory markers be regularly monitored and therapeutic attempts be made to target persistent low-grade inflammation in this patient group.

The role of cytokines in cancer cachexia

PURPOSE OF REVIEW

The present investigation is devoted to uncovering the different signaling pathways - particularly transcriptional factors - involved in muscle wasting.

RECENT FINDINGS

Although the search for the cachectic factor(s) started a long time ago, and although many scientific and economic efforts have been devoted to its discovery, we are still a long way from knowing the whole truth. In this review we describe recent findings about the tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, TWEAK and myostatin actions in cancer cachexia models.

SUMMARY

The main aim of the present review is to summarize and evaluate the different molecular mechanisms and catabolic mediators (mainly cytokines) involved in cancer cachexia since they may represent targets for future promising clinical investigations.

Cytokines, atherogenesis, and hypercatabolism in chronic kidney disease: a dreadful triad

The term cytokine clusters denotes a copious family of molecules and correspondent receptors implicated in numerous processes mediating health and disease. In the context of chronic kidney disease (CKD), generation and metabolism of most of these cytokines are disturbed. Available evidence suggests that cytokine imbalances contribute to the progression of common CKD complications, such as atherosclerosis, mineral-bone disease, and protein-energy wasting via pleiotropic effects. The belief that cytokine CKD research is solely represented by interleukins (IL) and tumor-necrosis factors (TNF) (mainly IL-6 and TNF-alpha) is a common misconception among nephrologists. We here explore recent findings concerning the pathophysiological role of various cytokines in uremic complications, and discuss how cytokines could be used as novel potential therapeutic targets in CKD. At the same time, we provide a brief overview of current discoveries in the main transforming growth factors and chemokines.

Genomic profiling of messenger RNAs and microRNAs reveals potential mechanisms of TWEAK-induced skeletal muscle wasting in mice

Background

Skeletal muscle wasting is a devastating complication of several physiological and pathophysiological conditions. Inflammatory cytokines play an important role in the loss of skeletal muscle mass in various chronic diseases. We have recently reported that proinflammatory cytokine TWEAK is a major muscle-wasting cytokine. Emerging evidence suggests that gene expression is regulated not only at transcriptional level but also at post-transcriptional level through the expression of specific non-coding microRNAs (miRs) which can affect the stability and/or translation of target mRNA. However, the role of miRs in skeletal muscle wasting is unknown.

Methodology/Principal Findings

To understand the mechanism of action of TWEAK in skeletal muscle, we performed mRNA and miRs expression profile of control and TWEAK-treated myotubes. TWEAK increased the expression of a number of genes involved in inflammatory response and fibrosis and reduced the expression of few cytoskeletal gene (e.g. Myh4, Ankrd2, and TCap) and metabolic enzymes (e.g. Pgam2). Low density miR array demonstrated that TWEAK inhibits the expression of several miRs including muscle-specific miR-1-1, miR-1-2, miR-133a, miR-133b and miR-206. The expression of a few miRs including miR-146a and miR-455 was found to be significantly increased in response to TWEAK treatment. Ingenuity pathway analysis showed that several genes affected by TWEAK are known/putative targets of miRs. Our cDNA microarray data are consistent with miRs profiling. The levels of specific mRNAs and miRs were also found to be similarly regulated in atrophying skeletal muscle of transgenic mice (Tg) mice expressing TWEAK.

Conclusions/Significance

Our results suggest that TWEAK affects the expression of several genes and microRNAs involved in inflammatory response, fibrosis, extracellular matrix remodeling, and proteolytic degradation which might be responsible for TWEAK-induced skeletal muscle loss.

Effects of TWEAK (TNF superfamily member 12) on differentiation, metabolism, and secretory function of human primary preadipocytes and adipocytes

Expansion of adipose tissue mass by hypertrophy and hyperplasia is the hallmark of obesity. An automated cDNA screen was established to identify secreted human proteins with an inhibitory effect on adipocyte differentiation and, thereby, a potential inhibitory effect on adipose tissue growth. A member of the TNF superfamily, TNF-like weak inducer of apoptosis (TWEAK; TNF superfamily 12) was identified by means of high-throughput screening with the lipophilic dye Nile Red as an inhibitor of murine adipocyte differentiation and, subsequently, also of human adipocyte differentiation. TWEAK inhibited lipid deposition in a dose-dependent manner without causing cytotoxic effects. This inhibitory action was mimicked by an agonistic antibody of the TWEAK receptor. The TWEAK receptor (fibroblast growth factor inducible 14; CD266) was expressed on human primary preadipocytes and mature adipocytes. Knockdown of TWEAK receptor by short-hairpin RNA abolished the inhibitory effect of TWEAK on cell differentiation, demonstrating that the effects of TWEAK are mediated by its specific receptor. Inhibition of differentiation was the result of interference at an early step of transcriptional activation as assessed by decreased peroxisome proliferator-activated receptor-gamma, CCAAT enhancer-binding protein alpha (C/EBPalpha), and CCAAT enhancer-binding protein beta (C/EBPbeta) mRNA expression. In contrast to TNFalpha, basal and insulin-stimulated glucose uptake and lipolysis of terminally differentiated mature adipocytes and secretion of proinflammatory cytokines were not altered in the presence of TWEAK, and nuclear factor kappa B activity was only weakly induced. We conclude from our findings that TWEAK and the corresponding agonistic antibody have the potential to prevent adipose tissue growth without adversely influencing central metabolic pathways or proinflammatory cytokine secretion in adipose tissue.

TWEAK is a positive regulator of cardiomyocyte proliferation

AIMS

Proliferation of mammalian cardiomyocytes stops during the first weeks after birth, preventing the heart from regenerating after injury. Recently, several studies have indicated that induction of cardiomyocyte proliferation can be utilized to regenerate the mammalian heart. Thus, it is important to identify novel factors that can induce proliferation of cardiomyocytes. Here, we determine the effect of TNF-related weak inducer of apoptosis (TWEAK) on cardiomyocytes, a cytokine known to regulate proliferation in several other cell types.

METHODS AND RESULTS

Stimulation of neonatal rat cardiomyocytes with TWEAK resulted in increased DNA synthesis, increased expression of the proliferative markers Cyclin D2 and Ki67, and downregulation of the cell cycle inhibitor p27KIP1. Importantly, TWEAK stimulation resulted also in mitosis (H3P), cytokinesis (Aurora B), and increased cardiomyocyte numbers. Loss of function experiments revealed that re-induction of proliferation was dependent on tumour necrosis factor receptor superfamily member 12A (FN14) signalling. Downstream signalling was mediated through activation of extracellular signal-regulated kinases and phosphatidylinositol 3-kinase as well as inhibition of glycogen synthase kinase-3beta. In contrast to neonatal cardiomyocytes, TWEAK had no effect on adult rat cardiomyocytes due to developmental downregulation of its receptor FN14. However, adenoviral expression of FN14 enabled efficient induction of cell cycle re-entry in adult cardiomyocytes after TWEAK stimulation.

CONCLUSION

Our data establish TWEAK as a positive regulator of cardiomyocyte proliferation.

Tumor necrosis factor-like weak inducer of apoptosis stimulation of glioma cell survival is dependent on Akt2 function

Malignant gliomas are the most common primary brain tumors. Despite intensive clinical investigation and significant technical advances in surgical and radiation treatment, the impact on clinical outcome for patients with malignant gliomas is disappointing. We have previously shown that tumor necrosis factor-like weak inducer of apoptosis (TWEAK), a member of the tumor necrosis factor superfamily, can stimulate glioma cell survival via binding to the Fn14 receptor, activation of the NF-kappaB pathway, and upregulation of BCL-X(L) gene expression. Here, we show that TWEAK treatment of glioma cells leads to phosphorylation of Akt and BAD. TWEAK stimulation results in the phosphorylation of both Akt1 and Akt2. However, small interfering RNA (siRNA)-mediated depletion of either Akt1 or Akt2 showed that BAD serine 136 phosphorylation is dependent specifically on Akt2 function. Depletion of Akt2 expression by siRNA also abrogates TWEAK-stimulated glioma cell survival, whereas no effect on glioma cell survival was observed after siRNA-mediated depletion of Akt1 expression. Surprisingly, although siRNA-mediated depletion of BAD in glioma cells abrogates cytotoxic- and chemotherapy-induced apoptosis, TWEAK still displays a strong protective effect, suggesting that BAD serine 136 phosphorylation plays a minor role in TWEAK-Akt2-induced glioma cell survival. We also report here that AKT2 gene expression levels increased with glioma grade and inversely correlate with patient survival. Additionally, immunohistochemical analysis showed that Akt2 expression positively correlates with Fn14 expression in glioblastoma multiforme specimens. We hypothesize that the TWEAK-Fn14 signaling axis functions, in part, to enhance glioblastoma cell survival by activation of the Akt2 serine/threonine protein kinase.

Myostatin reduces Akt/TORC1/p70S6K signaling, inhibiting myoblast differentiation and myotube size

Myostatin is a negative regulator of skeletal muscle size, previously shown to inhibit muscle cell differentiation. Myostatin requires both Smad2 and Smad3 downstream of the activin receptor II (ActRII)/activin receptor-like kinase (ALK) receptor complex. Other transforming growth factor-beta (TGF-beta)-like molecules can also block differentiation, including TGF-beta(1), growth differentiation factor 11 (GDF-11), activins, bone morphogenetic protein 2 (BMP-2) and BMP-7. Myostatin inhibits activation of the Akt/mammalian target of rapamycin (mTOR)/p70S6 protein synthesis pathway, which mediates both differentiation in myoblasts and hypertrophy in myotubes. Blockade of the Akt/mTOR pathway, using small interfering RNA to regulatory-associated protein of mTOR (RAPTOR), a component of TOR signaling complex 1 (TORC1), increases myostatin-induced phosphorylation of Smad2, establishing a myostatin signaling-amplification role for blockade of Akt. Blockade of RAPTOR also facilitates myostatin's inhibition of muscle differentiation. Inhibition of TORC2, via rapamycin-insensitive companion of mTOR (RICTOR), is sufficient to inhibit differentiation on its own. Furthermore, myostatin decreases the diameter of postdifferentiated myotubes. However, rather than causing upregulation of the E3 ubiquitin ligases muscle RING-finger 1 (MuRF1) and muscle atrophy F-box (MAFbx), previously shown to mediate skeletal muscle atrophy, myostatin decreases expression of these atrophy markers in differentiated myotubes, as well as other genes normally upregulated during differentiation. These findings demonstrate that myostatin signaling acts by blocking genes induced during differentiation, even in a myotube, as opposed to activating the distinct "atrophy program." In vivo, inhibition of myostatin increases muscle creatine kinase activity, coincident with an increase in muscle size, demonstrating that this in vitro differentiation measure is also upregulated in vivo.

TNF-like weak inducer of apoptosis (TWEAK) activates proinflammatory signaling pathways and gene expression through the activation of TGF-beta-activated kinase 1

TWEAK, TNF-like weak inducer of apoptosis, is a relatively recently identified proinflammatory cytokine that functions through binding to Fn14 receptor in target cells. Although TWEAK has been shown to modulate several biological responses, the TWEAK-induced signaling pathways remain poorly understood. In this study, we tested the hypothesis that TAK1 (TGF-beta-activated kinase 1) is involved in TWEAK-induced activation of NF-kappaB and MAPK and expression of proinflammatory protein. TWEAK increased the phosphorylation and kinase activity of TAK1 in cultured myoblast and fibroblast cells. The activation of NF-kappaB was significantly inhibited in TAK1-deficient (TAK1(-/-)) mouse embryonic fibroblasts (MEF) compared with wild-type MEF. Deficiency of TAK1 also inhibited the TWEAK-induced activation of IkappaB kinase and the phosphorylation and degradation of IkappaBalpha protein. However, there was no difference in the levels of p100 protein in TWEAK-treated wild-type and TAK1(-/-) MEF. Furthermore, TWEAK-induced transcriptional activation of NF-kappaB was significantly reduced in TAK1(-/-) MEF and in C2C12 myoblasts transfected with a dominant-negative TAK1 or TAK1 short interfering RNA. TAK1 was also required for the activation of AP-1 in response to TWEAK. Activation of JNK1 and p38 MAPK, but not ERK1/2 or Akt kinase, was significantly inhibited in TAK1(-/-) MEF compared with wild-type MEF upon treatment with TWEAK. TWEAK-induced expression of proinflammatory genes such as MMP-9, CCL-2, and VCAM-1 was also reduced in TAK1(-/-) MEF compared with wild-type MEF. Furthermore, the activation of NF-kappaB and the expression of MMP-9 in response to TWEAK involved the upstream activation of Akt kinase. Collectively, our study demonstrates that TAK1 and Akt are the important components of TWEAK-induced proinflammatory signaling and gene expression.

Metabolic abnormalities in chronic kidney disease that contribute to cardiovascular disease, and nutritional initiatives that may diminish the risk

PURPOSE OF REVIEW

Chronic kidney disease (CKD) is associated with a wide range of severe metabolic and nutritional disturbances that directly or indirectly contribute to left ventricular hypertrophy, ischemic heart disease, vascular calcification, heart failure and other manifestations of cardiovascular disease (CVD). The CVD mortality rate in CKD patients is far higher than in the general population, and CKD is today recognized as one of the most important risk factors for CVD. In this review, we discuss the links between metabolic abnormalities and CVD in CKD patients and nutritional initiatives that may reduce this risk.

RECENT FINDINGS

Certain nontraditional risk factors, such as protein-energy wasting, inflammation, and biomarkers reflecting bone and mineral disorders, are strong predictors of CVD mortality in CKD patients. Although several small nutritional intervention studies have been performed and nutritional guidelines have been introduced in order to minimize metabolic disorders and improve nutritional status, they have so far not been proven to reduce morbidity nor mortality.

SUMMARY

Although the pathophysiological mechanisms involved in the markedly increased CVD risk of CKD patients are becoming more evident, still few nutritional randomized controlled studies have been conducted in this high-risk patient group.

Additive effects of soluble TWEAK and inflammation on mortality in hemodialysis patients

BACKGROUND AND OBJECTIVES

Chronic kidney disease (CKD) is characterized by an exceptionally high mortality rate, primarily due to cardiovascular disease. Reduced soluble TNF-like weak inducer of apoptosis (sTWEAK) plasma levels have been reported both in patients with subclinical atherosclerosis and CKD.

DESIGN, PARTICIPANTS, & MEASUREMENTS

A cross-sectional study was conducted in 218 prevalent patients (121 men; 63 +/- 14 yr) undergoing hemodialysis (HD). sTWEAK levels in relation with the patients' outcome were studied.

RESULTS

sTWEAK plasma levels were 208 [(165 to 272) pg/ml, median interquartile range], significantly lower than healthy controls (P < 0.0001). sTWEAK was negatively associated with inflammatory markers, such as C-reactive protein and IL-6. Overall mortality was assessed after an average follow-up of 31 mo, during which 81 patients died. After controlling for potential confounding variables, patients in the upper tertile of sTWEAK plasma levels had an increased risk of cardiovascular and all-cause mortality. A significant interaction effect between sTWEAK and IL-6 levels was found [synergy index: 2.19 (0.80, 5.93)]. Thus, the association of sTWEAK with mortality was strongest in patients with inflammation (defined as IL-6 > 7.0 pg/ml), in whom high sTWEAK strongly predicted cardiovascular and all-cause mortality. These results were confirmed in a second cohort of HD patients.

CONCLUSIONS

The concurrent presence of elevated sTWEAK plasma concentrations and an inflammatory environment have additive effects on mortality in HD patients. Further studies on the potential different role of sTWEAK in health and disease are warranted.

Tumor necrosis factor-related weak inducer of apoptosis augments matrix metalloproteinase 9 (MMP-9) production in skeletal muscle through the activation of nuclear factor-kappaB-inducing kinase and p38 mitogen-activated protein kinase: a potential role of MMP-9 in myopathy

Destruction of skeletal muscle extracellular matrix is an important pathological consequence of many diseases involving muscle wasting. However, the underlying mechanisms leading to extracellular matrix breakdown in skeletal muscle tissues remain unknown. Using a microarray approach, we investigated the effect of tumor necrosis factor-related weak inducer of apoptosis (TWEAK), a recently identified muscle-wasting cytokine, on the expression of extracellular proteases in skeletal muscle. Among several other matrix metalloproteinases (MMPs), we found that the expression of MMP-9, a type IV collagenase, was drastically increased in myotubes in response to TWEAK. The level of MMP-9 was also higher in myofibers of TWEAK transgenic mice. TWEAK increased the activation of both classical and alternative nuclear factor-kappaB (NF-kappaB) signaling pathways. Inhibition of NF-kappaB activity blocked the TWEAK-induced production of MMP-9 in myotubes. TWEAK also increased the activation of AP-1, and its inhibition attenuated the TWEAK-induced MMP-9 production. Overexpression of a kinase-dead mutant of NF-kappaB-inducing kinase or IkappaB kinase-beta but not IkappaB kinase-alpha significantly inhibited the TWEAK-induced activation of MMP-9 promoter. The activation of MMP-9 also involved upstream recruitment of TRAF2 and cIAP2 proteins. TWEAK increased the activity of ERK1/2, JNK1, and p38 MAPK. However, the inhibition of only p38 MAPK blocked the TWEAK-induced expression of MMP-9 in myotubes. Furthermore the loss of body and skeletal muscle weights, inflammation, fiber necrosis, and degradation of basement membrane around muscle fibers were significantly attenuated in Mmp9 knock-out mice on chronic administration of TWEAK protein. The study unveils a novel mechanism of skeletal muscle tissue destruction in pathological conditions.

Molecular characterization of cytokine TWEAK and its receptor Fn14 in pig (Sus scrofa)

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a member of the tumor necrosis factor superfamily (TNFSF). The interaction of TWEAK with its receptor fibroblast growth factor-inducible 14 (Fn14) regulates multiple cellular responses, including stimulation of proliferation, migration, apoptosis, angiogenesis, and induction of proinflammatory cytokines. This paper reports for the first time the molecular cloning of porcine TWEAK and Fn14 by EST and RACE strategies. The full-length cDNA of porcine TWEAK is 1327bp, including an open reading frame (ORF) of 747bp. Its genomic DNA consists of seven exons and six introns and is approximately 10kb in size by computer-assisted analysis. Sequence similarity at the amino acid level between porcine TWEAK and human or mouse was 95 and 92%, respectively. The full-length cDNA of porcine Fn14 contains 691bp, of which 390bp are the ORF. Sequence similarity at the amino acid level between porcine Fn14 and human, or mouse, or frog was 95, 93 and 64%, respectively. Real-time quantitative PCR (Q-PCR) analysis revealed that both TWEAK and Fn14 are constitutively expressed in various tissues in pig. Our results suggest that the TWEAK-Fn14 pathway is evolutionarily highly conserved. It will be helpful for investigation on the biological role of the TWEAK/Fn14 system in this important animal model. Furthermore, it provides insight into the molecular evolution of the emerging TWEAK and Fn14 families.

TNF induction of atrogin-1/MAFbx mRNA depends on Foxo4 expression but not AKT-Foxo1/3 signaling

Murine models of starvation-induced muscle atrophy demonstrate that reduced protein kinase B (AKT) function upregulates the atrophy-related gene atrogin-1/MAFbx (atrogin). The mechanism involves release of inhibition of Forkhead transcription factors, namely Foxo1 and Foxo3. Elevated atrogin mRNA also corresponds with elevated TNF in inflammatory catabolic states, including cancer and chronic heart failure. Exogenous tumor necrosis factor (TNF) increases atrogin mRNA in vivo and in vitro. We used TNF-treated C2C12 myotubes to test the hypothesis that AKT-Foxo1/3 signaling mediates TNF regulation of atrogin mRNA. Here we confirm that exposure to TNF increases atrogin mRNA (+125%). We also confirm that canonical AKT-mediated regulation of atrogin is active in C2C12 myotubes. Inhibition of phosphoinositol-3 kinase (PI3K)/AKT signaling with wortmannin reduces AKT phosphorylation (-87%) and increases atrogin mRNA (+340%). Activation with insulin-like growth factor (IGF) increases AKT phosphorylation (+126%) and reduces atrogin mRNA (-15%). Although AKT regulation is intact, our data suggest it does not mediate TNF effects on atrogin. TNF increases AKT phosphorylation (+50%) and stimulation of AKT with IGF does not prevent TNF induction of atrogin mRNA. Nor does TNF appear to signal through Foxo1/3 proteins. TNF has no effect on Foxo1/3 mRNA or Foxo1/3 nuclear localization. Instead, TNF increases nuclear Foxo4 protein (+55%). Small interfering RNA oligos targeted to two distinct regions of Foxo4 mRNA reduce the TNF-induced increase in atrogin mRNA (-34% and -32%). We conclude that TNF increases atrogin mRNA independent of AKT via Foxo4. These results suggest a mechanism by which inflammatory catabolic states may persist in the presence of adequate growth factors and nutrition.

Signaling in muscle atrophy and hypertrophy

Muscle performance is influenced by turnover of contractile proteins. Production of new myofibrils and degradation of existing proteins is a delicate balance, which, depending on the condition, can promote muscle growth or loss. Protein synthesis and protein degradation are coordinately regulated by pathways that are influenced by mechanical stress, physical activity, availability of nutrients, and growth factors. Understanding the signaling that regulates muscle mass may provide potential therapeutic targets for the prevention and treatment of muscle wasting in metabolic and neuromuscular diseases.

Nuclear factor-kappa B signaling in skeletal muscle atrophy

Skeletal muscle atrophy/wasting is a serious complication of a wide range of diseases and conditions such as aging, disuse, AIDS, chronic obstructive pulmonary disease, space travel, muscular dystrophy, chronic heart failure, sepsis, and cancer. Emerging evidence suggests that nuclear factor-kappa B (NF-kappaB) is one of the most important signaling pathways linked to the loss of skeletal muscle mass in various physiological and pathophysiological conditions. Activation of NF-kappaB in skeletal muscle leads to degradation of specific muscle proteins, induces inflammation and fibrosis, and blocks the regeneration of myofibers after injury/atrophy. Recent studies employing genetic mouse models have provided strong evidence that NF-kappaB can serve as an important molecular target for the prevention of skeletal muscle loss. In this article, we have outlined the current understanding regarding the role of NF-kappaB in skeletal muscle with particular reference to different models of muscle wasting and the development of novel therapy.

The TWEAK-Fn14 cytokine-receptor axis: discovery, biology and therapeutic targeting

TWEAK is a multifunctional cytokine that controls many cellular activities including proliferation, migration, differentiation, apoptosis, angiogenesis and inflammation. TWEAK acts by binding to Fn14, a highly inducible cell-surface receptor that is linked to several intracellular signalling pathways, including the nuclear factor-kappaB (NF-kappaB) pathway. The TWEAK-Fn14 axis normally regulates various physiological processes, in particular it seems to play an important, beneficial role in tissue repair following acute injury. Furthermore, recent studies have indicated that TWEAK-Fn14 axis signalling may contribute to cancer, chronic autoimmune diseases and acute ischaemic stroke. This Review provides an overview of TWEAK-Fn14 axis biology and summarizes the available data supporting the proposal that both TWEAK and Fn14 should be considered as potential targets for the development of novel therapeutics.

CCK-induced pancreatic growth is not limited by mitogenic capacity in mice

In mice fed trypsin inhibitor (camostat) to elevate endogenous CCK, pancreatic growth plateaus by 7 days. It is unknown whether this represents the maximum growth capacity of the pancreas. To test the ability of CCK to drive further growth, mice were fed chow containing camostat (0.1%) for 1 wk, then fed standard chow for 1 wk, and finally returned to the camostat diet for a week. Pancreatic mass increased to 245% of initial value (iv) following 1 wk of camostat feeding, decreased to 147% iv following a 1 wk return to normal chow, and increased to 257% iv with subsequent camostat feeding. Camostat feeding was associated with significant increases in circulating CCK and changes in pancreatic mass were paralleled by changes in protein and DNA content. Moreover, regression of the pancreas following camostat feeding was associated with changes in the expression of the autophagosome marker LC3. Pancreatic protein synthetic rates were 130% of control after 2 days on camostat but were equivalent to control after 7 days. Changes in the phosphorylation of 4E-BP1 and S6, downstream effectors of mammalian target of rapamycin (mTOR), paralleled changes in protein synthetic rates. Cellular content of Akt, an upstream activating kinase of mTOR, decreased after 7 days of camostat feeding whereas expression of the E3 ubiquitin-ligases and the cell cycle inhibitor p21 increased after 2 days. These results indicate that CCK-stimulated growth of the pancreas is not limited by acinar cell mitogenic capacity but is due, at least in part, to inhibition of promitogenic Akt signaling.

Tumor necrosis factor-alpha augments matrix metalloproteinase-9 production in skeletal muscle cells through the activation of transforming growth factor-beta-activated kinase 1 (TAK1)-dependent signaling pathway

We have investigated the effect of tumor necrosis factor-alpha (TNF-alpha) on the production of extracellular matrix-degrading proteases in skeletal muscles. Using microarray, quantitative PCR, Western blotting, and zymography, we found that TNF-alpha drastically increases the production of matrix metalloproteinase (MMP)-9 from C2C12 myotubes. In vivo administration of TNF-alpha in mice increased the transcript level of MMP-9 in skeletal muscle tissues. Although TNF-alpha activated all the three MAPKs (i.e. ERK1/2, JNK, and p38), inhibition of ERK1/2 or p38 but not JNK blunted the TNF-alpha-induced production of MMP-9 from myotubes. Inhibition of Akt also inhibited the TNF-alpha-induced production of MMP-9. TNF-alpha increased the activation of transcription factors NF-kappaB and AP-1 but not SP-1 in myotubes. Overexpression of a dominant negative inhibitor of NF-kappaB or AP-1 blocked the TNF-alpha-induced expression of MMP-9 in myotubes. Similarly, point mutations in AP-1- or NF-kappaB-binding sites in MMP-9 promoter inhibited the TNF-alpha-induced expression of a reporter gene. TNF-alpha increased the activity of transforming growth factor-beta-activating kinase-1 (TAK1). Furthermore, overexpression of a dominant negative mutant of TAK1 blocked the TNF-alpha-induced expression of MMP-9 and activation of NF-kappaB and AP-1. Our results also suggest that TNF-alpha induces MMP-9 expression in muscle cells through the recruitment of TRAF-2, Fas-associated protein with death domain, and TNF receptor-associated protein with death domain but not NIK or TRAF-6 proteins. We conclude that TAK1-mediated pathways are involved in TNF-alpha-induced MMP-9 production in skeletal muscle cells.

Systemic Inflammation and Skeletal Muscle Dysfunction in Chronic Obstructive Pulmonary Disease: State of the Art and Novel Insights in Regulation of Muscle Plasticity

Systemic inflammation is a recognized hallmark of chronic obstructive pulmonary disease pathogenesis. Although the origin and mechanisms responsible for the persistent chronic inflammatory process remain to be elucidated, it is recognized that it plays an important role in skeletal muscle pathology as observed in chronic obstructive pulmonary disease and several other chronic inflammatory disorders. This article describes state-of-the-art knowledge and novel insights in the role of inflammatory processes on several aspects of inflammation-related skeletal muscle pathology and offers new insights in therapeutic perspectives.

Inflammation dampened by gelatinase A cleavage of monocyte chemoattractant protein-3

Tissue degradation by the matrix metalloproteinase gelatinase A is pivotal to inflammation and metastases. Recognizing the catalytic importance of substrate-binding exosites outside the catalytic domain, we screened for extracellular substrates using the gelatinase A hemopexin domain as bait in the yeast two-hybrid system. Monocyte chemoattractant protein-3 (MCP-3) was identified as a physiological substrate of gelatinase A. Cleaved MCP-3 binds to CC-chemokine receptors-1, -2, and -3, but no longer induces calcium fluxes or promotes chemotaxis, and instead acts as a general chemokine antagonist that dampens inflammation. This suggests that matrix metalloproteinases are both effectors and regulators of the inflammatory response.