Signal transduction pathways involved in proteolysis-inducing factor induced proteasome expression in murine myotubes

Mfat-1 ameliorates cachexia after hypoxic-ischemic brain damage in mice by protecting the hypothalamus-pituitary-adrenal axis

AIMS

Cachexia, a metabolic syndrome, affects 21 % of patients suffering from ischemic encephalopathy. However, the specific mechanism and prevention measures are still unclear. Omega-3 polyunsaturated fatty acids (n-3 PUFAs) have been proven to reduce inflammatory cytokine levels during ischemic events, but whether they have a protective effect against cachexia after hypoxic-ischemic brain damage (HIBD) remains unclear.

MAIN METHODS

C57BL/6J wild-type and mfat-1 transgenic male mice were treated with and without HIBD. One day after HIBD, the epididymal white fat, gastrocnemius muscle and hypothalamus were weighed and analyzed the phenotypic changes. RNA sequencing was applied to gastrocnemius muscle to identify differential genes and pathways in HIBD groups. The effect of HPA axis on cachexia post-HIBD was examined via adrenalectomy, dexamethasone (0.1 mg/kg), and corticosterone injection (100 mg/kg).

KEY FINDINGS

The results showed that the incidence of cachexia in mfat-1 mice, which produce high proportion of n-3 PUFAs, was significantly lower than that in wild-type mice post-HIBD. Cachexia-related factors, such as inflammation, muscle atrophy and lipid metabolism were significantly improved in mfat-1 HIBD. RNA sequencing revealed that catabolic and proteasome pathways were significantly downregulated. In hypothalamus, inflammatory cytokines, lipid peroxidation levels were reduced. Corticosterone, glucocorticoid receptor, and dexamethasone suppression test all showed that mfat-1 improved the dysfunction of the HPA axis post-HIBD. The present study elucidated for the first time that mfat-1 reduced HIBD-induced hyperactivation of the HPA axis in mice by reducing inflammation and oxidative stress and contributed to the reduction of metabolic imbalance in peripheral tissues.

SIGNIFICANCE

Our study provides mechanistic information for the development of intervention strategies to prevent cachexia.

Towards Drug Repurposing in Cancer Cachexia: Potential Targets and Candidates

As a multifactorial and multiorgan syndrome, cancer cachexia is associated with decreased tolerance to antitumor treatments and increased morbidity and mortality rates. The current approaches for the treatment of this syndrome are not always effective and well established. Drug repurposing or repositioning consists of the investigation of pharmacological components that are already available or in clinical trials for certain diseases and explores if they can be used for new indications. Its advantages comparing to de novo drugs development are the reduced amount of time spent and costs. In this paper, we selected drugs already available or in clinical trials for non-cachexia indications and that are related to the pathways and molecular components involved in the different phenotypes of cancer cachexia syndrome. Thus, we introduce known drugs as possible candidates for drug repurposing in the treatment of cancer-induced cachexia.

Role of Oxidative Stress as Key Regulator of Muscle Wasting during Cachexia

Skeletal muscle atrophy is a pathological condition mainly characterized by a loss of muscular mass and the contractile capacity of the skeletal muscle as a consequence of muscular weakness and decreased force generation. Cachexia is defined as a pathological condition secondary to illness characterized by the progressive loss of muscle mass with or without loss of fat mass and with concomitant diminution of muscle strength. The molecular mechanisms involved in cachexia include oxidative stress, protein synthesis/degradation imbalance, autophagy deregulation, increased myonuclear apoptosis, and mitochondrial dysfunction. Oxidative stress is one of the most common mechanisms of cachexia caused by different factors. It results in increased ROS levels, increased oxidation-dependent protein modification, and decreased antioxidant system functions. In this review, we will describe the importance of oxidative stress in skeletal muscles, its sources, and how it can regulate protein synthesis/degradation imbalance, autophagy deregulation, increased myonuclear apoptosis, and mitochondrial dysfunction involved in cachexia.

Anti-Differentiation Effect of Oncogenic Met Receptor in Terminally-Differentiated Myotubes

Activation of the hepatocyte growth factor/Met receptor is involved in muscle regeneration, through promotion of proliferation and inhibition of differentiation in myogenic stem cells (MSCs). We previously described that the specific expression of an oncogenic version of the Met receptor (Tpr-Met) in terminally-differentiated skeletal muscle causes muscle wasting in vivo. Here, we induced Tpr-Met in differentiated myotube cultures derived from the transgenic mouse. These cultures showed a reduced protein level of myosin heavy chain (MyHC), increased phosphorylation of Erk1,2 MAPK, the formation of giant sacs of myonuclei and the collapse of elongated myotubes. Treatment of the cultures with an inhibitor of the MAPK kinase pathway or with an inhibitor of the proteasome increased the expression levels of MyHC. In addition, the inhibition of the MAPK kinase pathway prevented the formation of myosacs and myotube collapse. Finally, we showed that induction of Tpr-Met in primary myotubes was unable to produce endoreplication in their nuclei. In conclusion, our data indicate that multinucleated, fused myotubes may be forced to disassemble their contractile apparatus by the Tpr-Met oncogenic factor, but they resist the stimulus toward the reactivation of the cell cycle.

Pharmacological strategies in lung cancer-induced cachexia: effects on muscle proteolysis, autophagy, structure, and weakness

Cachexia is a relevant comorbid condition of chronic diseases including cancer. Inflammation, oxidative stress, autophagy, ubiquitin-proteasome system, nuclear factor (NF)-κB, and mitogen-activated protein kinases (MAPK) are involved in the pathophysiology of cancer cachexia. Currently available treatment is limited and data demonstrating effectiveness in in vivo models are lacking. Our objectives were to explore in respiratory and limb muscles of lung cancer (LC) cachectic mice whether proteasome, NF-κB, and MAPK inhibitors improve muscle mass and function loss through several molecular mechanisms. Body and muscle weights, limb muscle force, protein degradation and the ubiquitin-proteasome system, signaling pathways, oxidative stress and inflammation, autophagy, contractile and functional proteins, myostatin and myogenin, and muscle structure were evaluated in the diaphragm and gastrocnemius of LC (LP07 adenocarcinoma) bearing cachectic mice (BALB/c), with and without concomitant treatment with NF-κB (sulfasalazine), MAPK (U0126), and proteasome (bortezomib) inhibitors. Compared to control animals, in both respiratory and limb muscles of LC cachectic mice: muscle proteolysis, ubiquitinated proteins, autophagy, myostatin, protein oxidation, FoxO-1, NF-κB and MAPK signaling pathways, and muscle abnormalities were increased, while myosin, creatine kinase, myogenin, and slow- and fast-twitch muscle fiber size were decreased. Pharmacological inhibition of NF-κB and MAPK, but not the proteasome system, induced in cancer cachectic animals, a substantial restoration of muscle mass and force through a decrease in muscle protein oxidation and catabolism, myostatin, and autophagy, together with a greater content of myogenin, and contractile and functional proteins. Attenuation of MAPK and NF-κB signaling pathway effects on muscles is beneficial in cancer-induced cachexia.

Pancreatic cancer cachexia: a review of mechanisms and therapeutics

Over the last decade, we have gained new insight into the pathophysiology of cachexia associated with pancreatic cancer. Unfortunately, its treatment is complex and remains a challenge. Pancreatic cancer cachexia is a multifactorial syndrome characterized by uncompensated adipose tissue and skeletal muscle loss in the setting of anorexia that leads to progressive functional impairment. This paper will review the current concepts of pancreatic cancer cachexia, its assessment and pathophysiology as well as current and future treatments. The successful management of pancreatic cancer cachexia will likely require a multimodal approach that includes nutritional support and combination pharmaceutical interventions.

Mitochondrial dysfunction and therapeutic approaches in respiratory and limb muscles of cancer cachectic mice

NEW FINDINGS

What is the central question of this study? We explored whether experimental cancer-induced cachexia may alter mitochondrial respiratory chain (MRC) complexes and oxygen uptake in respiratory and peripheral muscles,and whether signalling pathways, proteasome and oxidative stress influence that process. What is the main finding and what is its importance? In cancer cachectic mice, MRC complexes and oxygen consumption were decreased in the diaphragm and gastrocnemius. Blockade of nuclear factor-κB and mitogen-activated protein kinase actions partly restored the muscle mass and force and corrected the MRC dysfunction,while concomitantly reducing tumour burden. Antioxidants improved mitochondrial oxygen consumption without eliciting effects on the loss of muscle mass and force or the tumour size,whereas bortezomib reduced tumour burden without influencing muscle mass and strength or MRC function. Abnormalities in mitochondrial content, morphology and function have been reported in several muscle-wasting conditions. We specifically explored whether experimental cancer-induced cachexia may alter mitochondrial respiratory chain (MRC) complexes and oxygen uptake in respiratory and peripheral muscles, and whether signalling pathways, proteasomes and oxidative stress may influence that process. We evaluated complex I, II and IV enzyme activities (specific activity assays) and MRC oxygen consumption (polarographic measurements) in diaphragm and gastrocnemius of cachectic mice bearing the LP07 lung tumour, with and without treatment with N-acetylcysteine, bortezomib and nuclear factor-κB (sulfasalazine) and mitogen-activated protein kinases (MAPK, U0126) inhibitors (n = 10 per group for all groups). Whole-body and muscle weights and limb muscle force were also assessed in all rodents at baseline and after 1 month. Compared with control animals, cancer cachectic mice showed a significant reduction in body weight gain, smaller sizes of the diaphragm and gastrocnemius, lower muscle strength, decreased activity of complexes I, II and IV and decreased oxygen consumption in both muscles. Blockade of nuclear factor-κB and MAPK actions restored muscle mass and force and corrected the MRC dysfunction in both muscles, while partly reducing tumour burden. Antioxidants improved mitochondrial oxygen uptake without eliciting significant effects on the loss of muscle mass and force or tumour size, whereas the proteasome inhibitor reduced tumour burden without significantly influencing muscle mass and strength or mitochondrial function. In conclusion, nuclear factor-κB and MAPK signalling pathways modulate muscle mass and performance and MRC function of respiratory and limb muscles in this model of experimental cancer cachexia, thus offering targets for therapeutic intervention.

Curcumin inhibits Rift Valley fever virus replication in human cells

Rift Valley fever virus (RVFV) is an arbovirus that is classified as a select agent, an emerging infectious virus, and an agricultural pathogen. Understanding RVFV-host interactions is imperative to the design of novel therapeutics. Here, we report that an infection by the MP-12 strain of RVFV induces phosphorylation of the p65 component of the NFκB cascade. We demonstrate that phosphorylation of p65 (serine 536) involves phosphorylation of IκBα and occurs through the classical NFκB cascade. A unique, low molecular weight complex of the IKK-β subunit can be observed in MP-12-infected cells, which we have labeled IKK-β2. The IKK-β2 complex retains kinase activity and phosphorylates an IκBα substrate. Inhibition of the IKK complex using inhibitors impairs viral replication, thus alluding to the requirement of an active IKK complex to the viral life cycle. Curcumin strongly down-regulates levels of extracellular infectious virus. Our data demonstrated that curcumin binds to and inhibits kinase activity of the IKK-β2 complex in infected cells. Curcumin partially exerts its inhibitory influence on RVFV replication by interfering with IKK-β2-mediated phosphorylation of the viral protein NSs and by altering the cell cycle of treated cells. Curcumin also demonstrated efficacy against ZH501, the fully virulent version of RVFV. Curcumin treatment down-regulated viral replication in the liver of infected animals. Our data point to the possibility that RVFV infection may result in the generation of novel versions of host components (such as IKK-β2) that, by virtue of altered protein interaction and function, qualify as unique therapeutic targets.

Role of Ca2+ in proteolysis-inducing factor (PIF)-induced atrophy of skeletal muscle

Proteolysis-inducing factor (PIF) induces muscle loss in cancer cachexia through a high affinity membrane bound receptor. This study investigates the mechanism by which the PIF receptor communicates to intracellular signalling pathways. C(2)C(12) murine myoblasts were used as a model using PIF purified from MAC16 tumours. Calcium imaging was determined using fura-4-acetoxymethyl ester (Fura-4-AM). PIF induced a rapid rise in Ca(2+)(i), which was completely attenuated by a anti-receptor antibody, or peptides representing 20 mers of the N-terminus of the PIF receptor. Other agents catabolic for skeletal muscle including angiotensin II (AngII) tumour necrosis factor-α (TNF-α) and lipopolysaccharide (LPS) also induced a rise in Ca(2+)(i), but this was not attenuated by anti-PIF-receptor antibody. The rise in Ca(2+)(i) induced by PIF and AngII was completely attenuated by the Zn(2+) chelator D-myo-inositol-1,2,6-triphosphate, and this was reversed by administration of exogenous Zn(2+). The Ca(2+)(i) rise induced by PIF was independent of the presence of extracellular Ca(2+), and attenuated by the Ca(2+) pump inhibitor thapsigargin, suggesting that the Ca(2+)(i) rise was due to release from intracellular stores. This rise in Ca(2+)(i) induced by PIF was attenuated by both the phospholipase C inhibitor U73122 and 2-APB, an inhibitor of the inositol 1,4,5-triphosphate receptor, suggesting the involvement of a G-protein. Binding of the PIF to its receptor in skeletal muscle triggers a rise in Ca(2+)(i), which initiates a signalling cascade leading to a depression in protein synthesis, and an increase in protein degradation.

Decreased NADPH oxidase expression and antioxidant activity in cachectic skeletal muscle

BACKGROUND: Cancer cachexia is the progressive loss of skeletal muscle protein that contributes significantly to cancer morbidity and mortality. Evidence of antioxidant attenuation and the presence of oxidised proteins in patients with cancer cachexia indicate a role for oxidative stress. The level of oxidative stress in tissues is determined by an imbalance between reactive oxygen species production and antioxidant activity. This study aimed to investigate the superoxide generating NADPH oxidase (NOX) enzyme and antioxidant enzyme systems in murine adenocarcinoma tumour-bearing cachectic mice. METHODS: Superoxide levels, mRNA levels of NOX enzyme subunits and the antioxidant enzymes superoxide dismutase (SOD), glutathione peroxidise (GPx) and catalase was measured in the skeletal muscle of mice with cancer and cancer cachexia. Protein expression levels of NOX enzyme subunits and antioxidant enzyme activity was also measured in the same muscle samples. RESULTS: Superoxide levels increased 1.4-fold in the muscle of mice with cancer cachexia, and this was associated with a decrease in mRNA of NOX enzyme subunits, NOX2, p40(phox) and p67(phox) along with the antioxidant enzymes SOD1, SOD2 and GPx. Cancer cachexia was also associated with a 1.3-fold decrease in SOD1 and 2.0-fold decrease in GPx enzyme activity. CONCLUSION: Despite increased superoxide levels in cachectic skeletal muscle, NOX enzyme subunits, NOX2, p40(phox) and p67(phox), were downregulated along with the expression and activity of the antioxidant enzymes. Therefore, the increased superoxide levels in cachectic skeletal muscle may be attributed to the reduction in the activity of endogenous antioxidant enzymes.

Mechanism of activation of dsRNA-dependent protein kinase (PKR) in muscle atrophy

The role of Ca(2+) in the activation of PKR (double-stranded-RNA-dependent protein kinase), which leads to skeletal muscle atrophy, has been investigated in murine myotubes using the cell-permeable Ca(2+) chelator BAPTA/AM (1,2-bis (o-aminphenoxy) ethane-N,N,N',N'-tetraacetic acid tetra (acetoxymethyl) ester). BAPTA/AM effectively attenuated both the increase in total protein degradation, through the ubiquitin-proteasome pathway, and the depression of protein synthesis, induced by both proteolysis-inducing factor (PIF) and angiotensin II (Ang II). Since both protein synthesis and degradation were attenuated this suggests the involvement of PKR. Indeed BAPTA/AM attenuated both the activation (autophosphorylation) of PKR and the subsequent phosphorylation of eIF2alpha (eukaryotic initiation factor 2alpha) in the presence of PIF, suggesting the involvement of Ca(2+) in this process. PIF also induced an increase in the activity of both caspases-3 and -8, which was attenuated by BAPTA/AM. The increase in caspase-3 and -8 activity was shown to be responsible for the activation of PKR, since the latter was completely attenuated by the specific caspase-3 and -8 inhibitors. These results suggest that Ca(2+) is involved in the increase in protein degradation and decrease in protein synthesis by PIF and Ang II through activation of PKR by caspases-3 and -8.

New hopes from old drugs: revisiting DNA-binding small molecules as anticancer agents

Most of the anticancer chemotherapeutic drugs that are broadly and successfully used today are DNA-damaging agents. Targeting of DNA has been proven to cause relatively potent and selective destruction of tumor cells. However, the clinical potential of DNA-damaging agents is limited by the adverse side effects and increased risk of secondary cancers that are consequences of the agents' genotoxicity. In this review, we present evidence that those agents capable of targeting DNA without inducing DNA damage would not be limited in these ways, and may be as potent as DNA-damaging agents in the killing of tumor cells. We use as an example literature data and our own research of the well-known antimalarial drug quinacrine, which binds to DNA without inducing DNA damage, yet modulates a number of cellular pathways that impact tumor cell survival.

The survival-promoting peptide Y-P30 enhances binding of pleiotrophin to syndecan-2 and -3 and supports its neuritogenic activity

Y-P30 is a polypeptide produced by peripheral blood mononuclear cells of the maternal immune system during pregnancy. The peptide passes the blood-placenta barrier and accumulates in neurons of the developing infant brain, where it enhances survival of thalamic neurons and displays neuritogenic activities. In this study, we identify pleiotrophin (PTN) and syndecan-2 and -3 as direct binding partners of Y-P30. PTN is known to promote neurite outgrowth of thalamic neurons due to its association with the proteoglycan syndecan-3. Via spontaneous oligomerization Y-P30 can capture large macromolecular complexes containing PTN and potentially syndecans. Accordingly, the neuritogenic activity of Y-P30 in thalamic primary cultures requires the presence of PTN in the media and binding to syndecans. Thus, we propose that the neurite outgrowth promoting actions of Y-P30 during brain development are essentially based on its association with the PTN/syndecan signaling complex. This identifies a new mechanism of communication between the nervous and the immune system that might directly affect the wiring of the brain during development.

The dermcidin gene in cancer: role in cachexia, carcinogenesis and tumour cell survival

PURPOSE OF REVIEW

The diverse protein products of the dermcidin gene are relevant to immunity, cancer cell progression and cancer cachexia. This article evaluates recent developments/controversies around dermcidin.

RECENT FINDINGS

Dermcidin has recently been shown to act as a survival/proliferation factor in hepatoma and prostate cancer cell lines. Recent studies suggest that the Y-P30 subunit of the dermcidin polypeptide offers a survival advantage in such cancer cells. Nevertheless, the relevance of Y-P30 to cancer growth in vivo, and mechanisms of action remain unknown. In mice, tumour cells appear to glycosylate the Y-P30 subunit, transforming it into a potent skeletal muscle proteolysis-inducing factor. Recent work has described a receptor and signal transduction pathways for murine glycosylated proteolysis-inducing factor. The absence of classical N-glycosylation sites in the human proteolysis-inducing factor peptide and the lack of specific tools for the detection of the key carbohydrate moieties conferring the proteolysis-inducing activity, however, remain barriers to confirming glycosylated proteolysis-inducing factor as a pro-cachectic factor in humans.

SUMMARY

There is a growing body of evidence illustrating dermcidin as an oncogene and Y-P30 as a survival factor. The biology of murine proteolysis-inducing factor as a pro-cachectic factor continues to evolve; however, its role in human biology remains speculative.

Involvement of phosphoinositide 3-kinase and Akt in the induction of muscle protein degradation by proteolysis-inducing factor

In the present study the role of Akt/PKB (protein kinase B) in PIF- (proteolysis-inducing factor) induced protein degradation has been investigated in murine myotubes. PIF induced transient phosphorylation of Akt at Ser473 within 30 min, which was attenuated by the PI3K (phosphoinositide 3-kinase) inhibitor LY294002 and the tyrosine kinase inhibitor genistein. Protein degradation was attenuated in myotubes expressing a dominant-negative mutant of Akt (termed DNAkt), compared with the wild-type variant, whereas it was enhanced in myotubes containing a constitutively active Akt construct (termed MyrAkt). A similar effect was observed on the induction of the ubiquitin–proteasome pathway. Phosphorylation of Akt has been linked to up-regulation of the ubiquitin–proteasome pathway through activation of NF-κB (nuclear factor κB) in a PI3K-dependent process. Protein degradation was attenuated by rapamycin, a specific inhibitor of mTOR (mammalian target of rapamycin), when added before, or up to 30 min after, addition of PIF. PIF induced transient phosphorylation of mTOR and the 70 kDa ribosomal protein S6 kinase. These results suggest that transient activation of Akt results in an increased protein degradation through activation of NF-κB and that this also allows for a specific synthesis of proteasome subunits.

Dermcidin expression in hepatic cells improves survival without N-glycosylation, but requires asparagine residues

Proteolysis-inducing factor, a cachexia-inducing tumour product, is an N-glycosylated peptide with homology to the unglycosylated neuronal survival peptide Y-P30 and a predicted product of the dermcidin gene, a pro-survival oncogene in breast cancer. We aimed to investigate whether dermcidin is pro-survival in liver cells, in which proteolysis-inducing factor induces catabolism, and to determine the role of potentially glycosylated asparagine residues in this function. Reverse cloning of proteolysis-inducing factor demonstrated ∼100% homology with the dermcidin cDNA. This cDNA was cloned into pcDNA3.1+ and both asparagine residues removed using site-directed mutagenesis. In vitro translation demonstrated signal peptide production, but no difference in molecular weight between the products of native and mutant vectors. Immunocytochemistry of HuH7 cells transiently transfected with V5-His-tagged dermcidin confirmed targeting to the secretory pathway. Stable transfection conferred protection against oxidative stress. This was abrogated by mutation of both asparagines in combination, but not by mutation of either asparagine alone. These findings suggest that dermcidin may function as an oncogene in hepatic as well as breast cells. Glycosylation does not appear to be required, but the importance of asparagine residues suggests a role for the proteolysis-inducing factor core peptide domain.

Anorexia–Cachexia syndrome in cancer: implications of the ubiquitin–proteasome pathway

INTRODUCTION

Malnutrition is a common problem in cancer patients. Its incidence varies according to disease stage (between 15 and 90%) and is considered a possible prognostic factor for therapeutic response and survival. It is also one of the causes contributing to the increase in morbidity and mortality in patients. Tumor cachexia is defined as a nutritional defect caused by tumor growth in the patient and presents as a significant weight loss. This weight loss is mainly caused by a degradation of skeletal muscle proteins.

CONCLUSION

The ubiquitin-proteasome system is the most important pathway of protein degradation. As a regulatory system governing protein half-life, it is involved in the regulation of the cell cycle, signal transmission, immune system response, apoptosis, and oncogenesis. Knowledge of the molecular pathways involved in the induction of cancer-associated cachexia will favor a more rational approach to its treatment as well as possible quality of life and survival benefit for the patient.

Mechanism of induction of muscle protein degradation by angiotensin II

Angiotensin I and II have been shown to directly induce protein degradation in skeletal muscle through an increased activity and expression of the ubiquitin-proteasome proteolytic pathway. This investigation determines the role of the nuclear transcription factor nuclear factor-kappaB (NF-kappaB) in this process. Using murine myotubes as a surrogate model system both angiotensin I and II were found to induce activation of protein kinase C (PKC), with a parabolic dose-response curve similar to the induction of total protein degradation. Activation of PKC was required for the induction of proteasome expression, since calphostin C, a highly specific inhibitor of PKC, attenuated both the increase in total protein degradation and in proteasome expression and functional activity increased by angiotensin II. PKC is known to activate I-kappaB kinase (IKK), which is responsible for the phosphorylation and subsequent degradation of I-kappaB. Both angiotensin I and II induced an early decrease in cytoplasmic I-kappaB levels followed by nuclear accumulation of NF-kappaB. Using an NF-kappaB luciferase construct this was shown to increase transcriptional activation of NF-kappaB regulated genes. Maximal luciferase expression was seen at the same concentrations of angiotensin I/II as those inducing protein degradation. Total protein degradation induced by both angiotensin I and II was attenuated by resveratrol, which prevented nuclear accumulation of NF-kappaB, confirming that activation of NF-kappaB was responsible for the increased protein degradation. These results suggest that induction of proteasome expression by angiotensin I/II involves a signalling pathway involving PKC and NF-kappaB.

Induction of protein degradation in skeletal muscle by a phorbol ester involves upregulation of the ubiquitin-proteasome proteolytic pathway

Although muscle atrophy is common to a number of disease states there is incomplete knowledge of the cellular mechanisms involved. In this study murine myotubes were treated with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to evaluate the role of protein kinase C (PKC) as an upstream intermediate in protein degradation. TPA showed a parabolic dose-response curve for the induction of total protein degradation, with an optimal effect at a concentration of 25 nM, and an optimal incubation time of 3 h. Protein degradation was attenuated by co-incubation with the proteasome inhibitor lactacystin (5 microM), suggesting that it was mediated through the ubiquitin-proteasome proteolytic pathway. TPA induced an increased expression and activity of the ubiquitin-proteasome pathway, as evidenced by an increased functional activity, and increased expression of the 20S proteasome alpha-subunits, the 19S subunits MSS1 and p42, as well as the ubiquitin conjugating enzyme E2(14k), also with a maximal effect at a concentration of 25 nM and with a 3 h incubation time. There was also a reciprocal decrease in the cellular content of the myofibrillar protein myosin. TPA induced activation of PKC maximally at a concentration of 25 nM and this effect was attenuated by the PKC inhibitor calphostin C (300 nM), as was also total protein degradation. These results suggest that stimulation of PKC in muscle cells initiates protein degradation through the ubiquitin-proteasome pathway. TPA also induced degradation of the inhibitory protein, I-kappaBalpha, and increased nuclear accumulation of nuclear factor-kappaB (NF-kappaB) at the same time and concentrations as those inducing proteasome expression. In addition inhibition of NF-kappaB activation by resveratrol (30 microM) attenuated protein degradation induced by TPA. These results suggest that the induction of proteasome expression by TPA may involve the transcription factor NF-kappaB.

Diminished ERK 1/2 and p38 MAPK phosphorylation in skeletal muscle during sepsis

Sepsis induces weight loss and the loss of skeletal muscle proteins, in part through an inhibition of protein synthesis secondary to an inhibition of the key steps controlling mRNA translation in skeletal muscle. We have previously shown that sepsis decreases the phosphorylation of eIF4E. The present study examines the phosphorylation of Erk 1/2 MAPK and p38 MAPK in skeletal muscle of rats with a chronic (5-day) intra-abdominal septic abscess. Mnk1 catalyzes the phosphorylation of eIF4E, and Mnk1 is activated by phosphorylation via Erk1/2 MAPK and p38 MAPK. Sepsis resulted in a significant decrease in the steady-state phosphorylation of Erk 1/2 and p38 MAPKs compared with sterile inflammation. To examine the mediators responsible for decreased phosphorylation of Erk 1/2 and p38 MAPKs, rats were treated with TNF binding protein (TNFbp) or infused for 24 h with TNF. Treatment of septic rats with TNFbp resulted in an increase in the phosphorylation of both Erk 1/2 and p38 MAPKs in skeletal muscle. This was associated with enhanced phosphorylation of eIF4E. In contrast, constant intravenous infusion of TNF-alpha for 24 h resulted in a complete inhibition of p38 MAPK phosphorylation while Erk 1/2 MAPK phosphorylation was increased. The net effect was a modest increase in eIF4E phosphorylation. The results suggest altered regulation of Erk 1/2 and p38 MAPK signal translation pathways by endogenously produced TNF, or some compound dependent on TNF may modulate, in part, the phosphorylation state of eIF4E in skeletal muscle during sepsis.

NF-kappaB mediates proteolysis-inducing factor induced protein degradation and expression of the ubiquitin-proteasome system in skeletal muscle

Loss of skeletal muscle in cancer cachexia has a negative effect on both morbidity and mortality. The role of nuclear factor-κB (NF-κB) in regulating muscle protein degradation and expression of the ubiquitin–proteasome proteolytic pathway in response to a tumour cachectic factor, proteolysis-inducing factor (PIF), has been studied by creating stable, transdominant-negative, muscle cell lines. Murine C₂C₁₂ myoblasts were transfected with plasmids with a CMV promoter that had mutations at the serine phosphorylation sites required for degradation of I-κBα, an NF-κB inhibitory protein, and allowed to differentiate into myotubes. Proteolysis-inducing factor induced degradation of I-κBα, nuclear accumulation of NF-κB and an increase in luciferase reporter gene activity in myotubes containing wild-type, but not mutant, I-κBα proteins. Proteolysis-inducing factor also induced total protein degradation and loss of the myofibrillar protein myosin in myotubes containing wild-type, but not mutant, plasmids at the same concentrations as those causing activation of NF-κB. Proteolysis-inducing factor also induced increased expression of the ubiquitin–proteasome pathway, as determined by ‘chymotrypsin-like’ enzyme activity, the predominant proteolytic activity of the β-subunits of the proteasome, protein expression of 20S α-subunits and the 19S subunits MSS1 and p42, as well as the ubiquitin conjugating enzyme, E2_14k, in cells containing wild-type, but not mutant, I-κBα. The ability of mutant I-κBα to inhibit PIF-induced protein degradation, as well as expression of the ubiquitin–proteasome pathway, confirms that both of these responses depend on initiation of transcription by NF-κB.

Signal-transduction networks and the regulation of muscle protein degradation

Protein degradation in muscle functions in maintaining normal physiological homeostasis and adapting to new homeostatic states, and is required for muscle wasting or atrophy in various pathological states. The interplay between protein synthesis and degradation to maintain homeostasis is complex and responds to a variety of autocrine and intercellular signals from neuronal inputs, hormones, cytokines, growth factors and other regulatory molecules. The intracellular events that connect extracellular signals to the molecular control of protein degradation are incompletely understood, but likely involve interacting signal-transduction networks rather than isolated pathways. We review some examples of signal-transduction systems that regulate protein degradation, including effectors of proteolysis inducing factor (PIF), insulin and insulin-like growth factor (IGF) and their receptors, and fibroblast growth factor (FGF) and its receptors.

Mechanism of the Attenuation of Proteolysis-Inducing Factor Stimulated Protein Degradation in Muscle by β-Hydroxy-β-Methylbutyrate

The leucine metabolite beta-hydroxy-beta-methylbutyrate (HMB) prevents muscle protein degradation in cancer-induced weight loss through attenuation of the ubiquitin-proteasome proteolytic pathway. To investigate the mechanism of this effect, the action of HMB on protein breakdown and intracellular signaling leading to increased proteasome expression by the tumor factor proteolysis-inducing factor (PIF) has been studied in vitro using murine myotubes as a surrogate model of skeletal muscle. A comparison has been made of the effects of HMB and those of eicosapentaenoic acid (EPA), a known inhibitor of PIF signaling. At a concentration of 50 mumol/L, EPA and HMB completely attenuated PIF-induced protein degradation and induction of the ubiquitin-proteasome proteolytic pathway, as determined by the "chymotrypsin-like" enzyme activity, as well as protein expression of 20S proteasome alpha- and beta-subunits and subunit p42 of the 19S regulator. The primary event in PIF-induced protein degradation is thought to be release of arachidonic acid from membrane phospholipids, and this process was attenuated by EPA, but not HMB, suggesting that HMB might act at another step in the PIF signaling pathway. EPA and HMB at a concentration of 50 mumol/L attenuated PIF-induced activation of protein kinase C and the subsequent degradation of inhibitor kappaBalpha and nuclear accumulation of nuclear factor kappaB. EPA and HMB also attenuated phosphorylation of p42/44 mitogen-activated protein kinase by PIF, thought to be important in PIF-induced proteasome expression. These results suggest that HMB attenuates PIF-induced activation and increased gene expression of the ubiquitin-proteasome proteolytic pathway, reducing protein degradation.

The molecular basis of skeletal muscle atrophy

Skeletal muscle atrophy attributable to muscular inactivity has significant adverse functional consequences. While the initiating physiological event leading to atrophy seems to be the loss of muscle tension and a good deal of the physiology of muscle atrophy has been characterized, little is known about the triggers or the molecular signaling events underlying this process. Decreases in protein synthesis and increases in protein degradation both have been shown to contribute to muscle protein loss due to disuse, and recent work has delineated elements of both synthetic and proteolytic processes underlying muscle atrophy. It is also becoming evident that interactions among known proteolytic pathways (ubiquitin-proteasome, lysosomal, and calpain) are involved in muscle proteolysis during atrophy. Factors such as TNF-alpha, glucocorticoids, myostatin, and reactive oxygen species can induce muscle protein loss under specified conditions. Also, it is now apparent that the transcription factor NF-kappaB is a key intracellular signal transducer in disuse atrophy. Transcriptional profiles of atrophying muscle show both up- and downregulation of various genes over time, thus providing further evidence that there are multiple concurrent processes involved in muscle atrophy. The purpose of this review is to synthesize our current understanding of the molecular regulation of muscle atrophy. We also discuss how ongoing work should uncover more about the molecular underpinnings of muscle wasting, particularly that due to disuse.

Role of protein kinase C and NF-kappaB in proteolysis-inducing factor-induced proteasome expression in C(2)C(12) myotubes

Proteolysis-inducing factor (PIF) is a sulphated glycoprotein produced by cachexia-inducing tumours, which initiates muscle protein degradation through an increased expression of the ubiquitin–proteasome proteolytic pathway. The role of kinase C (PKC) in PIF-induced proteasome expression has been studied in murine myotubes as a surrogate model of skeletal muscle. Proteasome expression induced by PIF was attenuated by 4α-phorbol 12-myristate 13-acetate (100 nM) and by the PKC inhibitors Ro31-8220 (10 μM), staurosporine (300 nM), calphostin C (300 nM) and Gö 6976 (200 μM). Proteolysis-inducing factor-induced activation of PKC_α, with translocation from the cytosol to the membrane at the same concentration as that inducing proteasome expression, and this effect was attenuated by calphostin C. Myotubes transfected with a constitutively active PKC_α (pCO₂) showed increased expression of proteasome activity, and a longer time course, compared with their wild-type counterparts. In contrast, myotubes transfected with a dominant-negative PKC_α (pKS1), which showed no activation of PKC_α in response to PIF, exhibited no increase in proteasome activity at any time point. Proteolysis-inducing factor-induced proteasome expression has been suggested to involve the transcription factor nuclear factor-κB (NF-κB), which may be activated through PKC. Proteolysis-inducing factor induced a decrease in cytosolic I-κBα and an increase in nuclear binding of NF-κB in pCO₂, but not in pKS1, and the effect in wild-type cells was attenuated by calphostin C, confirming that it was mediated through PKC. This suggests that PKC may be involved in the phosphorylation and degradation of I-κBα, induced by PIF, necessary for the release of NF-κB from its inactive cytosolic complex.