Transcription factors and muscle cachexia: is there a therapeutic target?

Effect of Isoflavone on Muscle Atrophy in Ovariectomized Mice

BACKGROUND

Sarcopenia, characterized by muscle mass decline due to aging or other causes, is exacerbated by decreased estrogen levels after menopause in women. Isoflavones, a class of flavonoids acting on estrogen receptors, may have beneficial effects on metabolic disorders. We examined these effects in ovariectomized mice fed a high-fat, high-sucrose diet (HFHSD).

METHODS

At 7 weeks old, female C57BL6/J mice (18-20 g, n = 12) underwent bilateral ovariectomy (OVX), and were then fed a high-fat, high-sucrose diet starting at 8 weeks of age. Half of the mice received isoflavone water (0.1%). Metabolic analyses, including glucose and insulin tolerance tests, were conducted. Muscle analysis involved grip strength assays, next-generation sequencing, quantitative RT-PCR, and western blotting of skeletal muscle after euthanizing the mice at 14 weeks old. Additionally, 16S rRNA gene sequence analysis of the gut microbiota was performed.

RESULTS

The results demonstrated that isoflavone administration did not affect body weight, glucose tolerance, or lipid metabolism. In contrast, isoflavone-treated mice had higher grip strength. Gene expression analysis of the soleus muscle revealed decreased Trim63 expression, and western blotting showed inactivation of muscle-specific RING finger protein 1 in isoflavone-treated mice. Gut microbiota analysis indicated higher Bacteroidetes and lower Firmicutes abundance in the isoflavone group, along with increased microbiota diversity. Gene sets related to TNF-α signaling via NF-κB and unfolded protein response were negatively associated with isoflavones.

CONCLUSIONS

Isoflavone intake alters gut microbiota and increases muscle strength, suggesting a potential role in improving sarcopenia in menopausal women.

Identification of a novel immune-related transcriptional regulatory network in sarcopenia

BACKGROUND

Sarcopenia is highly prevalent in elderly individuals and has a significant adverse effect on their physical health and quality of life, but the mechanisms remain unclear. Studies have indicated that transcription factors (TFs) and the immune microenvironment play a vital role in skeletal muscle atrophy.

METHODS

RNA-seq data of 40 muscle samples were downloaded from the GEO database. Then, differentially expressed genes (DEGs), TFs(DETFs), pathways(DEPs), and the expression of immune gene sets were identified with limma, edgeR, GO, KEGG, ORA, GSVA, and ssGSEA. Furthermore, the results above were integrated into coexpression analysis by Pearson correlation analysis (PCA). Significant coexpression patterns were used to construct the immune-related transcriptional regulatory network by Cytoscape and potential medicine targeting the network was screened by Connectivity Map. Finally, the regulatory mechanisms and RNA expression of DEGs and DETFs were identified by multiple online databases and RT‒qPCR.

RESULTS

We screened 808 DEGs (log2 fold change (FC) > 1 or <  - 1, p < 0.05), 4 DETFs (log2FC > 0.7 or <  - 0.7, p < 0.05), 304 DEPs (enrichment scores (ES) > 1 or <  - 1, p < 0.05), and 1208 differentially expressed immune genes sets (DEIGSs) (p < 0.01). Based on the results of PCA (correlation coefficient (CC) > 0.4 or <  - 0.4, p < 0.01), we then structured an immune-related network with 4 DETFs, 9 final DEGs, 11 final DEPs, and 6 final DEIGSs. Combining the results of online databases and in vitro experiments, we found that PAX5-SERPINA5-PI3K/Akt (CC ≤ 0.444, p ≤ 0.004) was a potential transcriptional regulation axis, and B cells (R = 0.437, p = 0.005) may play a vital role in this signal transduction. Finally, the compound of trichostatin A (enrichment = -0.365, specificity = 0.4257, p < 0.0001) might be a potential medicine for sarcopenia based on the PubChem database and the result of the literature review.

CONCLUSIONS

We first identified immune-related transcriptional regulatory network with high-throughput RNA-seq data in sarcopenia. We hypothesized that PAX5-SERPIAN5-PI3K/Akt axis is a potential mechanism in sarcopenia and that B cells may play a vital role in this signal transduction. In addition, trichostatin A might be a potential medicine for sarcopenia.

Malnutrition in Gastrointestinal Disorders: Detection and Nutritional Assessment

All patients with significant gastrointestinal disease should be clinically assessed for protein calorie malnutrition by using the Subjective Global Assessment. Blood tests for anemia, electrolytes, calcium, phosphorus, magnesium, ferritin, vitamin B₁₂, and folate should be considered for assessment of major micronutrients. Where malabsorption or inflammatory bowel disease is diagnosed, bone mineral density using dual beam x-ray absorptiometry, 25-OH vitamin D levels, and measurement of other vitamins and trace elements should be considered. In addition, in at-risk patients, vitamin and trace element clinical deficiency syndromes should be considered during patient assessment.

Nutritional influences on age-related skeletal muscle loss

Age-related muscle loss impacts on whole-body metabolism and leads to frailty and sarcopenia, which are risk factors for fractures and mortality. Although nutrients are integral to muscle metabolism the relationship between nutrition and muscle loss has only been extensively investigated for protein and amino acids. The objective of the present paper is to describe other aspects of nutrition and their association with skeletal muscle mass. Mechanisms for muscle loss relate to imbalance in protein turnover with a number of anabolic pathways of which the mechanistic TOR pathway and the IGF-1–Akt–FoxO pathways are the most characterised. In terms of catabolism the ubiquitin proteasome system, apoptosis, autophagy, inflammation, oxidation and insulin resistance are among the major mechanisms proposed. The limited research associating vitamin D, alcohol, dietary acid–base load, dietary fat and anti-oxidant nutrients with age-related muscle loss is described. Vitamin D may be protective for muscle loss; a more alkalinogenic diet and diets higher in the anti-oxidant nutrients vitamin C and vitamin E may also prevent muscle loss. Although present recommendations for prevention of sarcopenia focus on protein, and to some extent on vitamin D, other aspects of the diet including fruits and vegetables should be considered. Clearly, more research into other aspects of nutrition and their role in prevention of muscle loss is required.

Identification of microRNAs involved in dexamethasone-induced muscle atrophy

MicroRNAs (miRNAs), a novel class of post-transcriptional gene regulators, have been demonstrated to be involved in several cellular processes regulating the expression of protein-coding genes. To investigate the mechanisms of miRNA-mediated regulation during the process of muscle atrophy, we performed miRNA microarray hybridization between normal differentiated C2C12 cells and dexamethasone (DEX)-treated C2C12 cells. We observed that 11 miRNAs were significantly up-regulated and six miRNAs were down-regulated in the differentiated C2C12 cells after being treated with DEX. Stem-loop real-time RT-PCR confirmed the differential expression of six selected miRNAs (miR-1, miR-147, miR-322, miR-351, and miR-503*, miR-708). miRNA potential target prediction was accomplished using TargetScan, and many target genes related to muscle growth and atrophy have been reported in previous studies. The results of the current study suggested the potential roles of these differentially expressed miRNAs in skeletal muscle atrophy.

The frequency of 4 common gene polymorphisms in nonagenarians, centenarians, and average life span individuals

Single nucleotide polymorphisms of angiotensin-converting enzyme (ACE) such as rs1799752, nuclear factor kappa B (NFkB) such as rs28362491 and cholesteryl ester transport protein (CETP) such as rs708272 (TaqB1) and rs5882 (I405V) were evaluated in nonagenarians, centenarians, and average life span individuals (controls). The study population (n = 307; 190 nonagenarians, 12 centenarians and 105 middle-aged controls) was genotyped for ACE, NFkB, and CETP genetic variants. The age of nonagenarian and centenarian group ranged between 90 and 111 years; centenarians and controls age ranged from 99 to 111, and from 18 to 80 years, respectively. The I carriers of ACE I/D gene were fewer in nonagenarians compared to centenarians (37.6% vs 62.5%, P = .016). The I carriers of ACE gene were more frequent in centenarians compared to controls (62% vs 41%, P = .045). No differences in frequency of common NFkB and CETP genotypes between patients with exceptional longevity and middle-aged patients were observed.

Sarcopenia and cachexia: the adaptations of negative regulators of skeletal muscle mass

Recent advances in our understanding of the biology of muscle, and how anabolic and catabolic stimuli interact to control muscle mass and function, have led to new interest in the pharmacological treatment of muscle wasting. Loss of muscle occurs as a consequence of several chronic diseases (cachexia) as well as normal aging (sarcopenia). Although many negative regulators [Atrogin-1, muscle ring finger-1, nuclear factor-kappaB (NF-κB), myostatin, etc.] have been proposed to enhance protein degradation during both sarcopenia and cachexia, the adaptation of mediators markedly differs among these conditions. Sarcopenic and cachectic muscles have been demonstrated to be abundant in myostatin- and apoptosis-linked molecules. The ubiquitin-proteasome system (UPS) is activated during many different types of cachexia (cancer cachexia, cardiac heart failure, chronic obstructive pulmonary disease), but not many mediators of the UPS change during sarcopenia. NF-κB signaling is activated in cachectic, but not in sarcopenic, muscle. Some studies have indicated a change of autophagic signaling during both sarcopenia and cachexia, but the adaptation remains to be elucidated. This review provides an overview of the adaptive changes in negative regulators of muscle mass in both sarcopenia and cachexia.

Cancer cachexia: traditional therapies and novel molecular mechanism-based approaches to treatment

The complex syndrome of cancer cachexia (CC) that occurs in 50% to 80% cancer patients has been identified as an independent predictor of shorter survival and increased risk of treatment failure and toxicity, contributing to the mortality and morbidity in this population. CC is a pathological state including a symptom cluster of loss of muscle (skeletal and visceral) and fat, manifested in the cardinal feature of emaciation, weakness affecting functional status, impaired immune system, and metabolic dysfunction. The most prominent feature of CC is its non-responsiveness to traditional treatment approaches; randomized clinical trials with appetite stimulants, 5-HT3 antagonists, nutrient supplementation, and Cox-2 inhibitors all have failed to demonstrate success in reversing the metabolic abnormalities seen in CC. Interventions based on a clear understanding of the mechanism of CC, using validated markers relevant to the underlying metabolic abnormalities implicated in CC are much needed. Although the etiopathogenesis of CC is poorly understood, studies have proposed that NFkB is upregulated in CC, modulating immune and inflammatory responses induce the cellular breakdown of muscle, resulting in sarcopenia. Several recent laboratory studies have shown that n-3 fatty acid may attenuate protein degradation, potentially by preventing NFkB accumulation in the nucleus, preventing the degradation of muscle proteins. However, clinical trials to date have produced mixed results potentially attributed to timing of interventions (end stage) and utilizing outcome markers such as weight which is confounded by hydration, cytotoxic therapies, and serum cytokines. We propose that selective targeting of proteasome activity with a standardized dose of omega-3-acid ethyl esters, administered to cancer patients diagnosed with early stage CC, in addition to a standard intervention with nutritionally adequate diet and appetite stimulants, will alter metabolic abnormalities by downregulating NFkB, preventing the breakdown of myofibrillar proteins and resulting in increasing serum protein markers, lean body mass, and functional status.

Cellular mechanisms and local progenitor activation to regulate skeletal muscle mass

Skeletal muscle hypertrophy is a result of increased load, such as functional and stretch-overload. Activation of satellite cells and proliferation, differentiation and fusion are required for hypertrophy of overloaded skeletal muscles. On the contrary, a dramatic loss of skeletal muscle mass determines atrophy settings. The epigenetic changes involved in gene regulation at DNA and chromatin level are critical for the opposing phenomena, muscle growth and atrophy. Physiological properties of skeletal muscle tissue play a fundamental role in health and disease since it is the most abundant tissue in mammals. In fact, protein synthesis and degradation are finely modulated to maintain an appropriate muscle mass. When the molecular signaling is altered muscle wasting and weakness occurred, and this happened in most common inherited and acquired disorders such as muscular dystrophies, cachexia, and age-related wasting. To date, there is no accepted treatment to improve muscle size and strength, and these conditions pose a considerable anxiety to patients as well as to public health. Several molecules, including Magic-F1, myostatin inhibitor, IGF, glucocorticoids and microRNAs are currently investigated to interfere positively in the blueprint of skeletal muscle growth and regeneration.

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

OBJECTIVE

The concept of skeletal muscle homeostasis--often viewed as the net balance between two separate processes, namely protein degradation and protein synthesis--are not occurring independently of each other, but are finely co-ordinated by a web of intricate signalling networks.

MATERIALS AND METHODS

Using rodent muscle cell lines we have investigated TNF-alpha/IGF-I interactions, in an attempt to mimic and understand mechanisms underlying the wasting process.

RESULTS AND CONCLUSION

When myoblast cells are incubated with TNF-alpha (10 ng ml(- 1)) maximal damage ( approximately 21% +/- 0.7 myoblast death, p < 0.05) was induced. Co-incubation of TNF-alpha (10 ng ml(- 1)) with IGF-I resulted in cell survival ( approximately 50% reduction in myoblast death, p < 0.05), however, myotube formation was not evident. In contrast, a novel role of IGF-I has been identified whereby co-incubation of muscle cells with IGF-I (1.5 ng ml(- 1)) and a non-apoptotic dose of TNF-alpha (1.25 ng ml(- 1); sufficient to block differentiation) unexpectedly were shown not to rescue a block on differentiation but to facilitate significant myoblast death (p < 0.05). Interestingly, pre-administration of PD98059, a MAPK signal-blocking agent followed by co-incubation of 1.25 ng ml(- 1) TNF-alpha and 1.5 ng ml(- 1) IGF-I, reduced death to baseline levels (p < 0.05). We show for the first time that IGF-I can be apoptotic in the absence of TNF-alpha-induced cell death.

Role of glucocorticoids in the molecular regulation of muscle wasting

OBJECTIVE

To review glucocorticoid-regulated molecular mechanisms of muscle wasting.

DESIGN

Review of recent literature describing the role of glucocorticoids in the regulation of proteolytic mechanisms, transcription factors, and nuclear cofactors in skeletal muscle during various catabolic conditions.

MAIN RESULTS

Catabolic doses of glucocorticoids induce muscle atrophy both in vivo and in vitro by stimulating protein breakdown and inhibiting protein synthesis. Signaling pathways that regulate muscle protein synthesis at the translational level are inhibited by glucocorticoids. Glucocorticoids increase the expression and activity of the ubiquitin-proteasome pathway, a major proteolytic mechanism of muscle atrophy. The expression and activity of muscle wasting-related transcription factors, including C/EBPbeta and delta and Forkhead box O 1, 3, and 4, as well as the nuclear cofactor p300, are up-regulated by glucocorticoid excess.

CONCLUSIONS

Muscle wasting in various catabolic conditions is, at least in part, regulated by glucocorticoids. The role of glucocorticoids in muscle wasting is complex and reflects regulation at the molecular level of multiple mechanisms influencing both synthesis and degradation of muscle proteins.

Intracellular signaling during skeletal muscle atrophy

A variety of conditions lead to skeletal muscle atrophy including muscle inactivity or disuse, multiple disease states (i.e., cachexia), fasting, and age-associated atrophy (sarcopenia). Given the impact on mobility in the latter conditions, inactivity could contribute in a secondary manner to muscle atrophy. Because different events initiate atrophy in these different conditions, it seems that the regulation of protein loss may be unique in each case. In fact differences exist between the regulation of the various atrophy conditions, especially sarcopenia, as evidenced in part by comparisons of transcriptional profiles as well as by the unique triggering molecules found in each case. By contrast, recent studies have shown that many of the intracellular signaling molecules and target genes are similar, particularly among the atrophies related to inactivity and cachexia. This review focuses on the most recent findings related to intracellular signaling during muscle atrophy. Key findings are discussed that relate to signaling involving muscle ubiquitin ligases, the IGF/PI3K/Akt pathway, FOXO activity, caspase-3 activity, and NF-kappaB signaling, and an attempt is made to construct a unifying picture of how these data can be connected to better understand atrophy. Once more detailed cellular mechanisms of the atrophy process are understood, more specific interventions can be designed for the attenuation of protein loss.

A novel ubiquitin-binding protein ZNF216 functioning in muscle atrophy

The ubiquitin-proteasome system (UPS) is critical for specific degradation of cellular proteins and plays a pivotal role on protein breakdown in muscle atrophy. Here, we show that ZNF216 directly binds polyubiquitin chains through its N-terminal A20-type zinc-finger domain and associates with the 26S proteasome. ZNF216 was colocalized with the aggresome, which contains ubiquitinylated proteins and other UPS components. Expression of Znf216 was increased in both denervation- and fasting-induced muscle atrophy and upregulated by expression of constitutively active FOXO, a master regulator of muscle atrophy. Mice deficient in Znf216 exhibited resistance to denervation-induced atrophy, and ubiquitinylated proteins markedly accumulated in neurectomized muscle compared to wild-type mice. These data suggest that ZNF216 functions in protein degradation via the UPS and plays a crucial role in muscle atrophy.

The evidence against malnutrition as a prominent problem for chronic dialysis patients

A large number of dialysis patients have been classified as being malnourished because of anthropometric measurements and laboratory values that are similar to those in patients with protein-energy malnutrition. Malnutrition implies that abnormalities can be reversed with better nutrition. However, this approach of providing more nutrients in the diet has been unsuccessful in reversing these changes, suggesting that a diagnosis of malnutrition is generally a misdiagnosis for dialysis patients In this review we discuss mechanisms that cause a loss of protein stores, muscle mass, albumin, and other plasma proteins in dialysis patients. We also review the shortcomings of techniques that are used to measure the nutritional status of these patients.

Disruption of either the Nfkb1 or the Bcl3 gene inhibits skeletal muscle atrophy

The intracellular signals that mediate skeletal muscle protein loss and functional deficits due to muscular disuse are just beginning to be elucidated. Previously we showed that the activity of an NF-kappaB-dependent reporter gene was markedly increased in unloaded muscles, and p50 and Bcl-3 proteins were implicated in this induction. In the present study, mice with a knockout of the p105/p50 (Nfkb1) gene are shown to be resistant to the decrease in soleus fiber cross-sectional area that results from 10 days of hindlimb unloading. Furthermore, the marked unloading-induced activation of the NF-kappaB reporter gene in soleus muscles from WT mice was completely abolished in soleus muscles from Nfkb1 knockout mice. Knockout of the B cell lymphoma 3 (Bcl3) gene also showed an inhibition of fiber atrophy and an abolition of NF-kappaB reporter activity. With unloading, fast fibers from WT mice atrophied to a greater extent than slow fibers. Resistance to atrophy in both strains of knockout mice was demonstrated clearly in fast fibers, while slow fibers from only the Bcl3(-/-) mice showed atrophy inhibition. The slow-to-fast shift in myosin isoform expression due to unloading was also abolished in both Nfkb1 and Bcl3 knockout mice. Like the soleus muscles, plantaris muscles from Nfkb1(-/-) and Bcl3(-/-) mice also showed inhibition of atrophy with unloading. Thus both the Nfkb1 and the Bcl3 genes are necessary for unloading-induced atrophy and the associated phenotype transition.

Cellular mechanisms causing loss of muscle mass in kidney disease

In stable adults or patients with kidney disease, the daily turnover of cellular proteins is very large, amounting to the quantity of protein in 1 to 1.5 kg of muscle. Consequently, even a small but persistent increase in protein degradation or decrease in protein synthesis leads to a substantial loss of muscle mass. In chronic kidney disease, the pathway that degrades muscle protein is the ubiquitin-proteasome system. We tested whether either of two complications of chronic kidney disease, metabolic acidosis or insulin resistance accelerates the loss of muscle protein. Metabolic acidosis activates the ubiquitin-proteasome system and this can explain an large number of clinical conditions in which metabolic acidosis also causes loss of muscle protein. Insulin deficiency as a model of insulin resistance also activates the ubiquitin-proteasome system. Both complications also activate caspase-3 and we found that this protease performs a critical initial step in breaking down the complex structure of muscle to provide actin, myosin and fragments of these proteins as substrates for the ubiquitin-proteasome system. Defects in insulin signalling processes can activate both caspase-3 and the ubiquitin-proteasome system to degrade muscle protein. Understanding mechanisms that activate protein breakdown will lead to therapies that successfully prevent the loss of muscle mass in patients with kidney disease.

Multifaceted roles of TNF-alpha in myoblast destruction: a multitude of signal transduction pathways

In catabolic conditions, such as cancer cachexia, a balance favouring a cytokine environment culminates in muscle destruction. Utilising an in vitro model to mimic muscle wasting, we elucidate here the multifaceted roles that one such cytokine, TNF-alpha, invokes in the degeneration process. Treatment of C2 skeletal myoblasts with TNF-alpha not only suppresses morphological and biochemical differentiation, but following an initial wave of proliferation, and of survival (24 h), induces apoptosis. Investigating the mechanisms underlying these diverse actions of TNF-alpha, we demonstrate that cell replication is dependent on rapid and sustained activation of MAP kinase. Map kinase is not, however, central to the death process, which is associated with a progressive rise in caspase-8 activity, and is accompanied by sustained activation of JNK1 and transient activation of JNK2. Caspase inhibition caused a dose responsive reduction in cell death, while inhibition of the JNKs caused a significant increase in apoptosis. We further report that PI3 kinase is not involved in conferring early protection against TNF-alpha-induced death. By contrast, inhibition of NF-kappaB in the presence of TNF-alpha culminates in increased cell cycle progression, decreased gadd45beta expression and significant and precociously increased cell death, when compared with TNF-alpha alone. Our results begin to characterise the mechanisms underlying the acute mitogenic and anti-apoptotic roles of TNF-alpha, which appear to be defined by a balance between MAP kinase, Jun kinase (JNK), NF-kappaB and gadd45beta. They establish that inhibition of any one of these molecules, as may occur following caspase activation, could eliminate vital stem cells required for skeletal muscle regeneration during chronic catabolic conditions.

Catabolic mediators as targets for cancer cachexia

The cachexia syndrome, characterized by a marked weight loss, anorexia, asthenia and anaemia, is invariably associated with the growth of a tumour and leads to a malnutrition status caused by the induction of anorexia or decreased food intake. In addition, the competition for nutrients between the tumour and the host results in an accelerated catabolism state, which promotes severe metabolic disturbances in the patient. The search for the cachectic factor(s) started a long time ago, and many scientific and economic efforts have been devoted to its discovery, but we are still a long way from a complete answer. The present review aims to evaluate the different molecular mechanisms and catabolic mediators (both humoural and tumoural) that are involved in cancer cachexia and to discuss their potential as targets for future clinical investigations.

Increased expression of the ubiquitin-proteasome pathway in murine myotubes by proteolysis-inducing factor (PIF) is associated with activation of the transcription factor NF-kappaB

Proteolysis-inducing factor (PIF), isolated from a cachexia-inducing murine tumour, has been shown to stimulate protein breakdown in C(2)C(12) myotubes. The effect was attenuated by the specific proteasome inhibitor lactacystin and there was an elevation of proteasome 'chymotrypsin-like' enzyme activity and expression of 20S proteasome alpha-subunits at concentrations of PIF between 2 and 16 nM. Higher concentrations of PIF had no effect. The action of PIF was attenuated by eicosapentaenoic acid (EPA) (50 microM). At a concentration of 4 nM, PIF induced a transient decrease in IkappaBalpha levels after 30 min incubation, while no effect was seen at 20 nM PIF. The level of IkappaBalpha, an NF-kappaB inhibitory protein, returned to normal after 60 min. Depletion of IkappaBalpha from the cytosol was not seen in myotubes pretreated with EPA, suggesting that the NF-kappaB/IkappaB complex was stabilised. At concentrations between 2 and 8 nM, PIF stimulated an increased nuclear migration of NF-kappaB, which was not seen in myotubes pretreated with EPA. The PIF-induced increase in chymotrypsin-like enzyme activity was also attenuated by the NF-kappaB inhibitor peptide SN50, suggesting that NF-kappaB may be involved in the PIF-induced increase in proteasome expression. The results further suggest that EPA may attenuate protein degradation induced by PIF, at least partly, by preventing NF-kappaB accumulation in the nucleus.

Skeletal muscle metabolism in physiology and in cancer disease

Skeletal muscle is a tissue of high demand and it accounts for most of daily energy consumption. The classical concept of energy metabolism in skeletal muscle has been profoundly modified on the basis of studies showing the influence of additional factors (i.e., uncoupling proteins (UCPs) and peroxisome proliferator activated receptors (PPARs)) controlling parameters, such as substrate availability, cellular enzymes, carrier proteins, and proton leak, able to affect glycolysis, nutrient oxidation, and protein degradation. This extremely balanced system is greatly altered by cancer disease that can induce muscle cachexia with significant deleterious consequences and results in muscle wasting and weakness, delaying or preventing ambulation, and rehabilitation in catabolic patients.

Mechanisms activating proteolysis to cause muscle atrophy in catabolic conditions

The daily turnover of cellular proteins is the same as the amount of protein contained in 1 to 1.5 kg of muscle. Consequently, even a small but persistent increase in protein degradation or decrease in protein synthesis results in substantial loss of muscle mass, as shown in patients with trauma, sepsis, or kidney failure. Activation of the ubiquitin-proteasome proteolytic system in muscle is the major pathway contributing to loss of muscle mass in catabolic illnesses. At least 3 signals have been identified as causing loss of muscle mass: acidosis, defective insulin action, and glucocorticoids. The influence of inflammatory cytokines on this system in muscle is more complicated because cytokines can suppress the system unless glucocorticoids are present. An initial reaction that breaks down muscle appears to involve caspases. Such information could lead to therapies that successfully prevent the loss of muscle mass in catabolic illnesses.

A method to increase the number of growth hormone receptors at the surface of cells

The number of growth hormone receptors (GHRs) per cell are regulated and this feature plays a major role in the hormone responsiveness of the body. We previously observed in transfected Chinese hamster lung cells that GHR availability is determined by three factors: endocytosis (75%), shedding (10%), and other undetermined mechanisms (15%). The endocytosis depends on an active ubiquitin conjugation system. In addition, this process is ligand-independent. Here, we show that this principle is useful to increase the abundance of GHRs at the cell surface of cells using a combination of inhibitors. In theory, an inhibitor that targets the ubiquitin conjugation specific for the GHR, would suffice, as almost 80% of the removal rate depends on this mechanism. As the molecular mechanism is unknown yet, we used a general inhibitor of proteasome action. Unfortunately, such an inhibitor stimulates the shedding process severalfold. Our data show that the combination of a general proteasome inhibitor and a matrix metalloprotease inhibitor results in an almost twofold increase in functional GHRs at the cell surface, and generate new perspectives to increase the sensitivity of cells for growth hormone.

Nutrition in CAPD: serum bicarbonate and the ubiquitin-proteasome system in muscle

BACKGROUND

Metabolic acidosis in chronic renal failure (CRF) induces loss of lean body mass while elimination of acidosis during a one year trial improved anthropometric indices in continuous ambulatory peritoneal dialysis (CAPD) patients. In rats with CRF, the mechanisms causing loss of lean body mass have been linked to acidosis-induced destruction of the essential, branched-chain amino acids (BCAA) and activation of the ubiquitin-proteasome system that degrades muscle protein; the latter response includes increased transcription of the ubiquitin gene.

METHOD

Our aim was to determine if increasing the serum bicarbonate (HCO3) concentration of CAPD patients would improve their nutritional status, increase plasma BCAA levels, and reduce ubiquitin mRNA in their muscle as an index of suppressed activity of the ubiquitin-proteasome system. Eight, stable, long-term CAPD patients underwent vastus lateralis muscle biopsy before being randomized to continue 35 mmol/L lactate dialysate or convert to a 40 mmol/L lactate dialysate. After four weeks, measurements were repeated.

RESULTS

Serum HCO3 increased in all patients and final values did not differ statistically between the two groups so results for all patients were combined. Weight and body mass index increased significantly as did plasma BCAA. Muscle levels of ubiquitin mRNA decreased significantly; serum tumor necrosis factor-alpha (TNF-alpha) also decreased.

CONCLUSION

Our results indicate that even a small correction of serum HCO3 improves nutritional status, and provide evidence for down-regulation of BCAA degradation and muscle proteolysis via the ubiquitin-proteasome system. Whether acidosis and inflammatory cytokines (such as, TNF-alpha) interact to impair nutrition is unknown.

Mechanisms activated by kidney disease and the loss of muscle mass

The daily turnover of cellular proteins is large, with amounts equivalent to the protein contained in 1.0 to 1.5 kg of muscle. Consequently, even a small, persistent increase in the rate of protein degradation or decrease in protein synthesis will result in substantial loss of muscle mass. Activation of protein degradation in the ubiquitin-proteasome system is the mechanism contributing to loss of muscle mass in kidney disease. Because other catabolic conditions also stimulate this system to cause loss of muscle mass, the identification of activating signals is of interest. A complication of kidney disease, metabolic acidosis, activates this system in muscle by a process that requires glucocorticoids. The influence of inflammatory cytokines on this system in muscle is more complicated, as evidence indicates that cytokines suppress the system, but glucocorticoids block the effect of cytokines to slow protein breakdown in the system. New information identifying mechanisms that activate protein breakdown and the rebuilding of muscle fibers would lead to therapies that successfully prevent the loss of muscle mass in kidney disease and other catabolic illnesses.