Regulation of proteolysis

Effects of fasting and refeeding on protein and glucose metabolism in Arctic charr

Refeeding, following a period of food deprivation will often lead to compensatory growth. Although many studies have focused on molecular mechanisms behind this accelerated growth response in fish, little is known on the roles of protein and metabolism. We also assessed, for the first time, the potential roles of miRNAs in regulating compensatory growth. Artcic charr, Salvelinus alpinus, a northern freshwater species, was deprived of food for 101 days and then fed to satiety for 126 days. The refeeding period resulted in compensatory growth, with a partial compensation of body mass. The feed deprivation period lead to a decrease in hepatosomatic index (HSI) and intestinal somatic index (ISI). HSI and ISI were then gradually replenished during early refeeding, following a lag phase prior to the compensatory growth response. mRNA transcripts regulating protein degradation via the autophagy pathway (Cathepsin D and Cathepsin L) in muscle were upregulated during feed restriction and downregulated after refeeding, which could allow for greater protein accretion in muscle, facilitating compensatory growth. Transcript levels from the ubiquitin proteasome pathway (Mafbx and Murf1) and the calpain system (Calpain 7 and Calpastatin) suggested that these pathways were not involved in regulating compensatory growth. Furthermore, we've shown that miRNAs (miR-29a and miR-223) could be involved in fish glycogen homeostasis during the early stages of refeeding. These findings provide a deeper understanding of the molecular mechanisms regulating growth in fish.

Effects of short-term energy deficit on muscle protein breakdown and intramuscular proteolysis in normal-weight young adults

The effects of short-term energy deficit (ED) on direct measures of muscle proteolysis and the intracellular mechanisms by which muscle proteins are degraded at rest and following aerobic exercise are not well described. This study evaluated the effects of a short-term diet-induced ED, on muscle fractional breakdown rate (FBR), intramuscular 26S proteasome activity, caspase-3 activation, and PSMA2 and MAFbx expression at rest, in the postabsorptive state, and following a single bout of moderate aerobic exercise (45 min at 65% peak oxygen uptake). Six men and 4 women participated in two 10-day diet interventions: weight maintenance (WM) followed by ED (80% estimated energy requirements). Dietary protein (1.5 g·kg−1·day−1) intake was constant for WM and ED. Mixed muscle FBR, proteasome activity, and intracellular proteolytic factor expression were measured using stable isotope methodology, fluorescent enzyme activity assays, and Western blotting, respectively. Overall, FBR and caspase-3 activation increased 60% and 11%, respectively, in response to ED (P < 0.05), but were not influenced by exercise. During ED, 26S proteasome α-subunit PSMA2 expression was 25% higher (P < 0.05) after exercise compared with rest. Exercise did not influence PSMA2 expression during WM, and MAFbx expression and 26S proteasome activity were not affected by ED or exercise. These data illustrate the effects of short-term, moderate ED on muscle protein degradation. In the context of skeletal muscle integrity during weight loss interventions, this work demonstrates a need for further investigations aimed at mitigating muscle loss associated with energy deficit imposed for intentional reduction of total body weight.

Effects of Fasting and Refeeding on Expression of Atrogin-1 and Akt/FOXO Signaling Pathway in Skeletal Muscle of Chicks

This experiment was conducted to study the effects of fasting and refeeding on expression of the atrogin-1 and Akt/FOXO signaling pathway in skeletal muscle of chicks. Chicks were fasted for 24 h and refed for 2 h. Atrogin-1 mRNA expression was increased by fasting, and their increment was reduced by refeeding. Phosphorylations of Akt and FOXO1 were not decreased by fasting, but, they were increased by refeeding. These results indicate that refeeding stimulates phosphorylation of Akt/FOXO, resulting in a decrease in atrogin-1 expression in skeletal muscle of chicks.

MULAN related gene (MRG): a potential novel ubiquitin ligase activator of NF-kB involved in immune response in Atlantic salmon (Salmo salar)

Nuclear factor-kB (NF-kB) is a transcription factor that plays a central role in the regulation of a variety of genes including many involved in bacterial and viral infections. NF-kB is normally sequestered by inhibitory proteins (IkBs) in the cytoplasm of non-stimulated cells. The degradation of IkBs by the ubiquitin proteasome pathway releases NF-kB allowing its translocation to the nucleus where it regulates gene transcription. The Mitochondrial Ubiquitin Ligase Activator of NF-kB, (MULAN), is an E3 ubiquitin ligase involved in controlling activation of NF-kB, and regulating mitochondrial dynamics and apoptosis. We report the characterisation of a novel piscine-specific MULAN related gene (MRG) sequence, its mRNA tissue distribution and expression following in vivo and in vitro challenges. MRG cDNA was identified in Atlantic salmon and its sequence encodes a predicted protein of 274 amino acids. The mRNA of MRG was expressed in multiple tissues, with the highest abundance head kidney. An Aeromonas salmonicida bacterial challenge increased expression of this gene in head kidney, liver and gill tissue at 6 h and 24 h. In vitro stimulation of a salmonid cell line indicated MRG was increased in expression following stimulation with LPS, PolyI:C and recombinant trout IL-1β for 4 h and 24 h. These results suggest an active role of MRG in the activation of the NF-kB pathway during early immune responses.

Responses to nutrients in farm animals: implications for production and quality

It is well known that any quantitative (energy and protein levels) and qualitative (nature of the diet, nutrient dynamic) changes in the feeding of animals affect metabolism. Energy expenditure and feed efficiency at the whole-body level, nutrient partitioning between and within tissues and organs and, ultimately, tissue and organ characteristics are the major regulated traits with consequences on the quality of the meat and milk produced. Recent progress in biology has brought to light important biological mechanisms which explain these observations: for instance, regulation by the nutrients of gene expression or of key metabolic enzyme activity, interaction and sometimes cross-regulation or competition between nutrients to provide free energy (ATP) to living cells, indirect action of nutrients through a complex hormonal action, and, particularly in herbivores, interactions between trans-fatty acids produced in the rumen and tissue metabolism. One of the main targets of this nutritional regulation is a modification of tissue insulin sensitivity and hence of insulin action. In addition, the nutritional control of mitochondrial activity (and hence of nutrient catabolism) is another major mechanism by which nutrients may affect body composition and tissue characteristics. These regulations are of great importance in the most metabolically active tissues (the digestive tract and the liver) and may have undesirable (i.e. diabetes and obesity in humans) or desirable consequences (such as the production of fatty liver by ducks and geese, and the production of fatty and hence tasty meat or milk with an adapted fatty acid profile).

Catabolic signaling pathways, atrogenes, and ubiquitinated proteins are regulated by the nutritional status in the muscle of the fine flounder

A description of the intracellular mechanisms that modulate skeletal muscle atrophy in early vertebrates is still lacking. In this context, we used the fine flounder, a unique and intriguing fish model, which exhibits remarkably slow growth due to low production of muscle-derived IGF-I, a key growth factor that has been widely acknowledged to prevent and revert muscle atrophy. Key components of the atrophy system were examined in this species using a detailed time-course of sampling points, including two contrasting nutritional periods. Under basal conditions high amounts of the atrogenes MuRF-1 and Atrogin-1 were observed. During fasting, the activation of the P38/MAPK and Akt/FoxO signaling pathways decreased; whereas, the activation of the IκBα/NFκB pathway increased. These changes in signal transduction activation were concomitant with a strong increase in MuRF-1, Atrogin-1, and protein ubiquitination. During short-term refeeding, the P38/MAPK and Akt/FoxO signaling pathways were strongly activated, whereas the activation of the IκBα/NFκB pathway decreased significantly. The expression of both atrogenes, as well as the ubiquitination of proteins, dropped significantly during the first hour of refeeding, indicating a strong anti-atrophic condition during the onset of refeeding. During long-term refeeding, Akt remained activated at higher than basal levels until the end of refeeding, and Atrogin-1 expression remained significantly lower during this period. This study shows that the components of the atrophy system in skeletal muscle appeared early in the evolution of vertebrates and some mechanisms have been conserved, whereas others have not. These results represent an important achievement for the area of fish muscle physiology, showing an integrative view of the atrophy system in a non-mammalian species and contributing to novel insights on the molecular basis of muscle growth regulation in earlier vertebrates.

Muscle-specific RING finger (MuRF) cDNAs in Atlantic salmon (Salmo salar) and their role as regulators of muscle protein degradation

The selection of proteins destined for degradation by the ubiquitin-proteasome pathway is coordinated by E3 ubiquitin ligases (E3Ub). One group of E3Ubs is described as muscle-specific RING finger (MuRF) molecules. In mammals, these proteins are believed to be central to targetting of muscle proteins for degradation during physiological perturbations such as starvation and inflammatory responses. In fish, the diversity of MuRF sequences is unexplored as is the expression of their mRNAs. In this study, three MuRF1 cDNAs, denoted as MuRF1a, MuRF1b, and MuRF1c, and a single MuRF2 were identified and characterized in Atlantic salmon. The MuRF1 sequences are highly conserved and encode predicted proteins of 349, 350, and 353 amino acids, whereas MuRF2 encodes a longer protein of 462 amino acids. The evolutionary relationship of these sequences with other fish and mammalian molecules shows that MuRF1a and 1b may have arisen from a recent salmonid duplication. The mRNA of MuRFs was expressed in multiple tissues, with highest abundance in white muscle tissue followed by the heart. The expression of MuRFs was modulated after both starvation and immune challenge. Starvation increased expression of all MuRF mRNAs in white muscle, with the greatest increase found in MuRF1a. A proinflammatory stimulation increased expression of MuRF mRNA in muscle and other tissues indicating a role of these proteins in protein degradation during inflammation.

Ghrelin and its analogues, BIM-28131 and BIM-28125, improve body weight and regulate the expression of MuRF-1 and MAFbx in a rat heart failure model

Cardiac cachexia is a serious complication of chronic heart failure with a prevalence of 10–16% and poor prognosis. There are no current therapy options for cardiac cachexia. Ghrelin is the natural ligand for the GHS-1a-receptor and a potential target for conditions associated with cachexia. Ghrelin has been shown to increase weight in several species. The GHS-1a-receptor is not only found in the brain, but also in other tissues, including the myocardium. Human clinical trials with native ghrelin in cardiac cachexia demonstrated increases in appetite, weight and cardiac output.

Down-regulation of the ubiquitin-proteasome proteolysis system by amino acids and insulin involves the adenosine monophosphate-activated protein kinase and mammalian target of rapamycin pathways in rat hepatocytes

The purpose of this work was to examine whether changes in dietary protein levels could elicit differential responses of tissue proteolysis and the pathway involved in this response. In rats fed with a high protein diet (55%) for 14 days, the liver was the main organ where adaptations occurred, characterized by an increased protein pool and a strong, meal-induced inhibition of the protein breakdown rate when compared to the normal protein diet (14%). This was associated with a decrease in the key-proteins involved in expression of the ubiquitin-proteasome and autophagy pathway gene and a reduction in the level of hepatic ubiquitinated protein. In hepatocytes, we demonstrated that the increase in amino acid (AA) levels was sufficient to down-regulate the ubiquitin proteasome pathway, but this inhibition was more potent in the presence of insulin. Interestingly, AICAR, an adenosine monophosphate-activated protein kinase (AMPK) activator, reversed the inhibition of protein ubiquination induced by insulin at high AA concentrations. Rapamycin, an mammalian target of rapamycin (mTOR) inhibitor, reversed the inhibition of protein ubiquination induced by a rise in insulin levels with both high and low AA concentrations. Moreover, in both low and high AA concentrations in the presence of insulin, AICAR decreased the mTOR phosphorylation, and in the presence of both AICAR and rapamycin, AICAR reversed the effects of rapamycin. These results demonstrate that the inhibition of AMPK and the activation of mTOR transduction pathways, are required for the down-regulation of protein ubiquitination in response to high amino acid and insulin concentrations.

Effects of heat exposure on Akt/S6K1 signaling and expression of genes related to protein and energy metabolism in chicken (Gallus gallus) pectoralis major muscle

In order to improve understanding of the heat-induced changes in muscle growth, we determined the expression of genes related to protein and energy metabolism in the pectoralis major muscle of chickens. We also explored the protein kinase B (PKB also called Akt)/p70 S6 kinase (S6K1)/S6 pathway that mediates anabolic signals thereby regulating metabolism and hypertrophic/atrophic balance. Four-week-old chickens were exposed to 32 or 22 degrees C for 1 week. Chickens from both groups were then fasted for 16 h or left fed, and submitted to an oral administration of glucose-arginine to induce an anabolic response (30-min treatment) or left untreated. High ambient temperature and the associated decrease in feed intake modified the expression of certain energy-related genes (e.g. -40% for PGC-1alpha) and protein metabolism (e.g. about +80% for atrogin-1), but the expression of several muscle metabolism-related genes considered here was unchanged. The capacity for muscle protein synthesis, i.e. RNA/protein ratio, was reduced in warm conditions (approximately -20%). Slightly lower activation of S6 induced by glucose-arginine treatment was found at 32 degrees C compared to 22 degrees C, which might indicate somewhat lower efficiency of mRNA translation. Analysis of glucose/insulin balance suggested changes in glucose metabolism under heat exposure. However, this remains to be characterized.

Protein synthesis is lowered while 20S proteasome activity is maintained following acclimation to low temperature in juvenile spotted wolffish(Anarhichas minor Olafsen)

The effects of temperature on protein metabolism have been studied mostly with respect to protein synthesis. Temperature generally has a parabolic effect on protein synthesis with a maximum rate being observed at optimal growth temperature. The effect of temperature on protein degradation is poorly understood. The 20S proteasome is mainly responsible for the degradation of short-lived and oxidatively modified proteins and has been recently identified as a potentially good proxy for protein degradation in fish. The aim of this experiment was to examine the relationships between the rate of protein synthesis, activity of the 20S proteasome, oxidative stress markers and antioxidant capacity in white muscle of juvenile spotted wolffish(Anarhichas minor) acclimated at three temperatures (4, 8 and 12°C). The rate of protein synthesis was lower at 4°C than at 8°C while it was intermediate at 12°C. Despite the decrease of protein synthesis at low temperature, the activity of 20S proteasome activity was maintained high in fish acclimated at lower temperature (4°C), reaching levels 130% of that of fish acclimated at 8°C when measured at a common temperature. The oxidative stress markers TBARS and protein-carbonyl content did not change among temperature groups, but reduced glutathione concentration was higher in cold-acclimated fish, suggesting a higher antioxidant capacity in this group. Our data suggest that lower growth rate in cold temperature results from both high 20S proteasome activity and a reduced rate of protein synthesis.

Daily variations in dietary lysine content alter the expression of genes related to proteolysis in chicken pectoralis major muscle

Amino acids are known to be anabolic factors that affect protein metabolism, but the response of animals to daily amino acid changes is little understood. We aimed to test the effects of feeding birds with alternations of diets varying in lysine content on the expression of genes related to proteolysis in chicken muscle. Cyclic feeding programs with 2 diets, each given for 24 h during 48-h cycles, were carried out from 10 d of age. Three programs were used: 1) control treatment with continuous distribution of a complete diet containing standard medium lysine level (ML; 11.9 g/kg); 2) alternation of diets with high (HL) and low (LL) lysine levels; 3) alternation of ML and LL diets, where LL = 70%, ML = 100%, HL = 130% of standard lysine level. The Pectoralis major muscles were sampled after 2 wk of cyclic feeding. Measurements included the expression patterns of 6 genes involved in proteolysis, and mammalian target of rapamycin and Forkhead box-O transcription factor (FoxO) signaling. Cathepsin B, m-calpain, and E3 ubiquitin ligases Muscle Ring Finger-1 and Muscle Atrophy F box were significantly overexpressed in chickens transiently fed the LL diet, whereas the mRNA levels of 20S proteasome C2 subunit and ubiquitin remained unchanged. Modifications of E3 ubiquitin ligase expression can be partly explained by significant changes in FoxO phosphorylation with cyclic dietary treatments. Our results suggest timing-sensitive regulation of proteolysis in chicken muscle according to dietary treatment and a high metabolism capacity to compensate for changes in amino acid supply, which might be used for nutritional purposes.

Pressure support ventilation attenuates ventilator-induced protein modifications in the diaphragm

Introduction

Controlled mechanical ventilation (CMV) induces profound modifications of diaphragm protein metabolism, including muscle atrophy and severe ventilator-induced diaphragmatic dysfunction. Diaphragmatic modifications could be decreased by spontaneous breathing. We hypothesized that mechanical ventilation in pressure support ventilation (PSV), which preserves diaphragm muscle activity, would limit diaphragmatic protein catabolism.

Methods

Forty-two adult Sprague-Dawley rats were included in this prospective randomized animal study. After intraperitoneal anesthesia, animals were randomly assigned to the control group or to receive 6 or 18 hours of CMV or PSV. After sacrifice and incubation with ¹⁴C-phenylalanine, in vitro proteolysis and protein synthesis were measured on the costal region of the diaphragm. We also measured myofibrillar protein carbonyl levels and the activity of 20S proteasome and tripeptidylpeptidase II.

Results

Compared with control animals, diaphragmatic protein catabolism was significantly increased after 18 hours of CMV (33%, P = 0.0001) but not after 6 hours. CMV also decreased protein synthesis by 50% (P = 0.0012) after 6 hours and by 65% (P < 0.0001) after 18 hours of mechanical ventilation. Both 20S proteasome activity levels were increased by CMV. Compared with CMV, 6 and 18 hours of PSV showed no significant increase in proteolysis. PSV did not significantly increase protein synthesis versus controls. Both CMV and PSV increased protein carbonyl levels after 18 hours of mechanical ventilation from +63% (P < 0.001) and +82% (P < 0.0005), respectively.

Conclusions

PSV is efficient at reducing mechanical ventilation-induced proteolysis and inhibition of protein synthesis without modifications in the level of oxidative injury compared with continuous mechanical ventilation. PSV could be an interesting alternative to limit ventilator-induced diaphragmatic dysfunction.

Feeding status regulates the polyubiquitination step of the ubiquitin-proteasome-dependent proteolysis in rainbow trout (Oncorhynchus mykiss) muscle

In mammals, the ubiquitin-proteasome proteolytic pathway is a major route of protein degradation and has been shown to be regulated by the feeding status via the protein kinase B (PKB)-Forkehead box-O transcription factor signaling pathway-mediated transcription regulation of atrophy-related ubiquitin ligases, atrogin1 and muscle RING finger 1. In contrast, in rainbow trout (Oncorhynchus mykiss), the activity of the proteasome in muscle was not affected during starvation-induced muscle degradation. The aim of this study was therefore to explore the molecular basis for this lack of induction of this proteolytic route during starvation. In this study, rainbow trout were food deprived for 7 and 14 d, refed ad libitum, and the effect of the nutritional status was assessed on the different steps involved in the regulation of the ubiquitin-proteasome system in muscle. We observed that starvation reduced the phosphorylation of PKB and enhanced the expression of atrogin1 in muscle, whereas refeeding led to the opposite effects. The level of polyubiquitinated proteins in muscle increased to over 2 times the initial value on d 0 after 14 d of starvation and decreased significantly at 12 h after refeeding, but there were no major changes in the activity of the main proteasomal peptidases (chymotrypsin-like and trypsin-like). Altogether, these results indicate that in rainbow trout muscle, the polyubiquitination step of the ubiquitin-proteasome route is regulated by the feeding status similarly to what is observed in mammals.

Effect of prolonged intravenous glucose and essential amino acid infusion on nitrogen balance, muscle protein degradation and ubiquitin-conjugating enzyme gene expression in calves

Background

Intravenous infusions of glucose and amino acids increase both nitrogen balance and muscle accretion. We hypothesised that co-infusion of glucose (to stimulate insulin) and essential amino acids (EAA) would act additively to improve nitrogen balance by decreasing muscle protein degradation in association with alterations in muscle expression of components of the ubiquitin-proteasome proteolytic pathway.

Methods

We examined the effect of a 5 day intravenous infusions of saline, glucose, EAA and glucose + EAA, on urinary nitrogen excretion and muscle protein degradation. We carried out the study in 6 restrained calves since ruminants offer the advantage that muscle protein degradation can be assessed by excretion of 3 methyl-histidine and multiple muscle biopsies can be taken from the same animal. On the final day of infusion blood samples were taken for hormone and metabolite measurement and muscle biopsies for expression of ubiquitin, the 14-kDa E2 ubiquitin conjugating enzyme, and proteasome sub-units C2 and C8.

Results

On day 5 of glucose infusion, plasma glucose, insulin and IGF-1 concentrations were increased while urea nitrogen excretion and myofibrillar protein degradation was decreased. Co-infusion of glucose + EAA prevented the loss of urinary nitrogen observed with EAA infusions alone and enhanced the increase in plasma IGF-1 concentration but there was no synergistic effect of glucose + EAA on the decrease in myofibrillar protein degradation. Muscle mRNA expression of the ubiquitin conjugating enzyme, 14-kDa E2 and proteasome sub-unit C2 were significantly decreased, after glucose but not amino acid infusions, and there was no further response to the combined infusions of glucose + EAA.

Conclusion

Prolonged glucose infusion decreases myofibrillar protein degradation, prevents the excretion of infused EAA, and acts additively with EAA to increase plasma IGF-1 and improve net nitrogen balance. There was no evidence of synergistic effects between glucose + EAA infusion on muscle protein degradation or expression of components of the ubiquitin-proteasome proteolytic pathway.

Mechanisms through which sulfur amino acids control protein metabolism and oxidative status

Amino acids regulate protein synthesis and breakdown (i.e., protein turnover) and consequently protein deposition, which corresponds to the balance between the two processes. Elucidating the mechanisms involved in such regulation is important from fundamental and applied points of view since it can provide a basis to optimize amino acid requirements and to control protein mass, body composition and so forth. Amino acids, which have long been considered simply as precursors of protein synthesis, are now recognized to exert other significant influences; that is, they are precursors of essential molecules, act as mediators or signal molecules and affect numerous functions. For example, amino acids act as mediators of metabolic pathways in the same manner as certain hormones. Thus, they modulate the activity of intracellular protein kinases involved in the regulation of metabolic pathways such as mRNA translation. We provide here an overview of the roles of amino acids as regulators of protein metabolism, by focusing particularly on sulfur amino acids. The potential importance of methionine as a "nutrient signal" is discussed in the light of recent findings. Emphasis is also placed on mechanisms controlling oxidative status since sulfur amino acids are involved in the synthesis of intracellular antioxidants (glutathione, taurine etc.) and in the methionine sulfoxide reductase antioxidant system.

The energetics of wasting diseases

PURPOSE OF REVIEW

Unintentional weight loss or wasting continues to be a nutritional problem in the treatment of HIV/AIDS, cancer and many other diseases. Although wasting is a general sign of energy imbalance, the relative contribution of increased energy demands and decreased energy intake remains incompletely understood.

RECENT FINDINGS

Until the development of the method using doubly labeled water for measuring total energy expenditure, it was difficult to obtain unbiased estimates of total energy expenditure, particularly in clinical populations. Although costly, this technique provides greater insight in the energy requirements of clinical populations and the relative contributions of energy expenditure, energy intake and disease to wasting diseases. Although data are available in only a small portion of wasting diseases, the data suggest that even in the presence of increased demands for resting metabolic rate, total energy expenditure is often reduced because of decreased physical activity.

SUMMARY

Wasting is often the result of endocrine disorders accompanying the disease process itself. This, coupled with reduced energy intake, is often the primary contributor to wasting.

Hepatocellular expression of glutamine synthetase: an indicator of morphogen actions as master regulators of zonation in adult liver

Glutamine synthetase (GS) has long been known to be expressed exclusively in pericentral hepatocytes most proximal to the central veins of liver lobuli. This enzyme as well as its peculiar distribution complementary to the periportal compartment for ureogenesis plays an important role in nitrogen metabolism, particularly in homeostasis of blood levels of ammonium ions and glutamine. Despite this fact and intensive studies in vivo and in vitro, many aspects of the regulation of its activity on the protein and on the genetic level remained enigmatic. Recent experimental advances using transgenic mice and new analytic tools have revealed the fundamental role of morphogens such as wingless-type MMTV integration site family member signals (Wnt), beta-catenin, and adenomatous polyposis coli in the regulation of this particular enzyme. In addition, novel information concerning the structure of transcription factor binding sites within regulatory regions of the GS gene and their interactions with signalling pathways could be collected. In this review we focus on all aspects of the regulation of GS in the liver and demonstrate how the new findings have changed our view of the determinants of liver zonation. What appeared as a simple response of hepatocytes to blood-derived factors and local cellular interactions must now be perceived as a fundamental mechanism of adult tissue patterning by morphogens that were considered mainly as regulators of developmental processes. Though GS may be the most obvious indicator of morphogen action among many other targets, elucidation of the complex regulation of the expression of the GS gene could pave the road for a better understanding of the mechanisms involved in patterning of liver parenchyma. Based on current knowledge we propose a new concept of how morphogens, hormones and other factors may act in concert, in order to restrict gene expression to small subpopulations of one differentiated cell type, the hepatocyte, in different anatomical locations. Although many details of this regulatory network are still missing, and an era of exciting new discoveries is still about to come, it can already be envisioned that similar mechanisms may well be active in other organs contributing to the fine-tuning of organ-specific functions.

Opposed growth factor signals control protein degradation in muscles of Caenorhabditis elegans

In addition to contractile function, muscle provides a metabolic buffer by degrading protein in times of organismal need. Protein is also degraded during adaptive muscle remodeling upon exercise, but extreme degradation in diverse clinical conditions can compromise function(s) and threaten life. Here, we show how two independent signals interact to control protein degradation. In striated muscles of Caenorhabditis elegans, reduction of insulin-like signaling via DAF-2 insulin/IGF receptor or its intramuscular effector PtdIns-3-kinase (PI3K) causes unexpected activation of MAP kinase (MAPK), consequent activation of pre-existing systems for protein degradation, and progressive impairment of mobility. Degradation is prevented by mutations that increase signal downstream of PI3K or by disruption of autocrine signal from fibroblast growth factor (FGF) via the FGF receptor and its effectors in the Ras-MAPK pathway. Thus, the activity of constitutive protein degradation systems in normal muscle is minimized by a balance between directly interacting signaling pathways, implying that physiological, pathological, or therapeutic alteration of this balance may contribute to muscle remodeling or wasting.

Atrophy and hypertrophy of skeletal muscles: structural and functional aspects

This review summarizes current information on structural and functional changes that occur during muscle atrophy and hypertrophy. Most published studies consider an increase in total mass of a muscle as hypertrophy, whereas a decrease in total mass of a muscle is referred to as atrophy. In hypertrophy, the rate of synthesis is much higher than the rate of degradation of muscle contractile proteins, leading to an increase in the size or volume of an organ due to enlargement of existing cells. When a muscle remains in disuse for a long period, the rate of degradation of contractile proteins becomes greater than the rate of replacement, resulting in muscle atrophy. This defect may occur as a result of lack of nutrition, loss of nerve supply, micro-gravity, ageing, systemic disease, prolonged immobilization or disuse. An understanding of the specific modifications that occur during muscle atrophy and hypertrophy may facilitate the development of novel techniques, as well as new therapies for affected muscles.

Inhibition of TRIP1/S8/hSug1, a component of the human 19S proteasome, enhances mitotic apoptosis induced by spindle poisons

Mitotic spindle poisons (e.g., Taxol and vinblastine), used as chemotherapy drugs, inhibit mitotic spindle function, activate the mitotic spindle checkpoint, arrest cells in mitosis, and then cause cell death by mechanisms that are poorly understood. By expression cloning, we identified a truncated version of human TRIP1 (also known as S8, hSug1), an AAA (ATPases associated with diverse cellular activities) family ATPase subunit of the 19S proteasome regulatory complex, as an enhancer of spindle poison-mediated apoptosis. Stable expression of the truncated TRIP1/S8/hSug1 in HeLa cells [OP-TRIP1(88-406)] resulted in a decrease of measurable cellular proteasome activity, indicating that OP-TRIP1(88-406) had a dominant-negative effect on proteasome function. OP-TRIP1(88-406) revealed an increased apoptotic response after treatment with spindle poisons or with proteasome inhibitors. The increased apoptosis coincided with a significant decrease in expression of BubR1, a kinase required for activation and maintenance of the mitotic spindle checkpoint in response to treatment with spindle poisons. Small interfering RNA (siRNA)-mediated knockdown of TRIP1/S8/hSug1 resulted in a reduction of general proteasome activity and an increase in mitotic index. The siRNA treatment also caused increased cell death after spindle poison treatment. These results indicate that inhibition of TRIP1/S8/hSug1 function by expression of a truncated version of the protein or by siRNA-mediated suppression enhances cell death in response to spindle poison treatment. Current proteasome inhibitor drugs in trial as anticancer agents target elements of the 20S catalytic subcomplex. Our results suggest that targeting the ATPase subunits in 19S regulatory complex in the proteasome may enhance the antitumor effects of spindle poisons.

Altered responses in skeletal muscle protein turnover during aging in anabolic and catabolic periods

One of the most important effects of aging is sarcopenia, which is associated with impaired locomotion and general weakness. In addition, there is increased susceptibility to illness in aging, which often results in muscle wasting episodes. In such instances, the mobilization of muscle proteins provides free amino acids that are used for energetic purpose, the synthesis of acute phase proteins, and the immune response. However, since muscle protein mass is already depleted, the ability of the aged organism to recover from stress is impaired. Therefore, elucidating the mechanisms that result in sarcopenia is of obvious importance. Age-related changes in protein synthesis and proteolysis are rather small and our current methodology does not enable one to establish unequivocally whether sarcopenia results from depressed protein synthesis, increased proteolysis or both. By contrast, in anabolic and catabolic periods, a number of dysregulations in muscle protein turnover became clearly apparent. The aim of this review is to provide an overview of such altered responses to nutrients and catabolic treatments, which may ultimately contribute to explain sarcopenia. This includes impaired recovery in catabolic states, impaired anabolic effects of nutrients, in particular leucine, and a lack of regulation of the ubiquitin-proteasome proteolytic system. These alterations are discussed with respect to modifications in the insulin/IGF-1 axis and glucocorticoid related effects.

Whole-body and hindlimb protein breakdown are differentially altered by feeding in neonatal piglets

The high rate of muscle protein accretion in neonates is sustained by the marked increase in muscle protein synthesis in response to feeding. Little is known about the role of proteolysis in the regulation of protein accretion in response to feeding during the neonatal period. To determine the feeding-induced response of protein breakdown at the whole-body level and in the hindlimb of neonates, 10- and 28-d-old piglets that had been food deprived overnight were infused (7 h) with [1-13C]phenylalanine and [ring-2H4]tyrosine during an initial food deprivation period (3 h), followed by a feeding period (4 h). During feeding, endogenous flux of phenylalanine decreased (P < 0.01) in both the whole body and the hindlimb. Feeding reduced (P < 0.01) whole-body proteolysis but increased hindlimb proteolysis (P = 0.04), suggesting that tissues other than the hindlimb are involved in the reduction in whole-body proteolysis during feeding. Overnight food deprivation resulted in a net mobilization of phenylalanine from whole-body proteins (P < 0.01) but not hindlimb proteins. These responses were unaffected by age. The results suggest that the hindlimb requires a continuous supply of free amino acids to sustain the high rate of muscle protein turnover in neonates and that adaptive mechanisms provide free amino acids to sustain skeletal muscle protein accretion in early postnatal life when the amino acid supply is limited.

Folding Efficiency Is Rate-limiting in Dopamine D4 Receptor Biogenesis

Dopamine receptors are G protein-coupled receptors that are critically involved in locomotion, reward, and cognitive processes. The D2 class of dopamine receptors (DRD2, -3, and -4) is the target for antipsychotic medication. DRD4 has been implicated in cognition, and genetic studies have found an association between a highly polymorphic repeat sequence in the human DRD4 coding region and attention deficit hyperactivity disorder. Using DRD4 as a model, we show that antipsychotics can function as potent pharmacological chaperones up-regulating receptor expression and can also rescue a non-functional DRD4 folding mutant. This chaperone-mediated up-regulation involves reduced degradation by the 26 S proteasome; likely via the stabilization of newly synthesized receptor in the endoplasmic reticulum. Dopamine itself can function as a chaperone when shuttled into the cell by means of the dopamine transporter. Furthermore, different repeat variants of DRD4 display differential sensitivity to this chaperone effect. These data suggest that folding efficiency may be rate-limiting for dopamine receptor biogenesis and that this efficiency differs between receptor variants. Consequently, the clinical profile of dopaminergic ligands, including antipsychotics, may include their ability to serve as pharmacological chaperones.

Activation of ubiquitin-proteasome pathway is involved in skeletal muscle wasting in a rat model with biliary cirrhosis: potential role of TNF-alpha

Hepatic cirrhosis is associated with negative nitrogen balance and loss of lean body mass. This study aimed to identify the specific proteolytic pathways activated in skeletal muscles of cirrhotic rats. TNF-alpha can stimulate muscle proteolysis; therefore, a potential relationship between TNF-alpha and muscle wasting in liver cirrhosis was also evaluated. Cirrhosis was induced by bile duct ligation (BDL) in male adult Sprague-Dawley rats. mRNA and protein levels of various targets were determined by RT-PCR and Western blotting, respectively. The proteolytic rate was measured ex vivo using isolated muscles. Compared with sham-operated controls, BDL rats had an increased degradation rate of muscle proteins and enhanced gene expression of ubiquitin, 14-kDa ubiquitin carrier protein E2, and the proteasome subunits C2 and C8 (P < 0.01). The muscle protein levels of free ubiquitin and conjugated ubiquitin levels were also elevated (P < 0.01). However, there was no difference between the two groups with regard to cathepsin and calpain mRNA levels. Cirrhotic muscle TNF-alpha levels were increased and correlated positively with free and conjugated ubiquitin (P < 0.01). We conclude that the ubiquitin-proteasome system is involved in muscle wasting of rats with BDL-induced cirrhosis. TNF-alpha might play a role in mediating activation of this proteolytic pathway, probably through a local mechanism.

Gene profiling in muscle of severely burned children: Age- and sex-dependent changes1

BACKGROUND

Thermal injury is associated with a pronounced catabolic response in skeletal muscle. This study identifies gene expression changes in skeletal muscle of thermally injured girls and boys using high-density oligonucleotide arrays.

METHODS

Six burned children with a mean age of 8.3 +/- 1.3 years and TBSA burn size covering 51 +/- 6% admitted to our hospital with in 48 h of injury and six cleft lip and cleft palate patients were studied. Total RNA was isolated, in vitro transcribed, and hybridized to HG-U95 Av.2 Affymetrix arrays. Messenger RNA expression patterns of controls and burn patients were compared using Affymetrix GeneChip Analysis Suite 5.2 and dChip.

RESULTS

Statistical analysis of the 12,625 genes on each array showed a significant increase in the expression of 77 genes in burn children and a decrease in 21 genes when compared to controls (P < 0.05). We found three genes in burned males and two genes in burned females with decreased expression in muscle compared to controls. Chromosomes 1, 2, 7, 12, and 16 showed genes with increased expression in muscle from burned children, while chromosomes 3, 7, 8, 19, and 22 had genes with decreased expression. Categories of genes affected were related to metabolism, proliferation, transcription/translation, immune response, stress response, angiogenesis, and signal transduction.

CONCLUSIONS

Genes that are differentially expressed in skeletal muscle of burned children, but whose function in muscle is unknown, include those related to various transcription factors and those known to encode proteins involved in signaling pathways. Further analysis is required to achieve the ultimate goal of making functionally relevant conclusions about the molecular pathology of burn injury.

Control of the Size of the Human Muscle Mass

This review is divided into two parts, the first dealing with the cell and molecular biology of muscle in terms of growth and wasting and the second being an account of current knowledge of physiological mechanisms involved in the alteration of size of the human muscle mass. Wherever possible, attempts have been made to interrelate the information in each part and to provide the most likely explanation for phenomena that are currently only partially understood. The review should be of interest to cell and molecular biologists who know little of human muscle physiology and to physicians, physiotherapists, and kinesiologists who may be familiar with the gross behavior of human muscle but wish to understand more about the underlying mechanisms of change.

New insights into the assembly of the periaxonemal structures in mammalian spermatozoa

Disruption of Ube2b in the mouse has revealed that the regular and symmetric organization of the fibrous sheath of the sperm flagella is dependent on expression of the ubiquitin-conjugating enzyme UBE2B. These data could cast light on how a component of the ubiquitin-proteasome pathway participates in the assembly of flagellar periaxonemal structures. Data in the literature support the notion of involvement of ubiquitin-proteasome pathways in the assembly of cytoskeletal components in somatic cells. This review attempts to integrate recent knowledge regarding flagellar components that could be related to proteasome components and, therefore, could be targets of UBE2B in the spermatid. An attempt is made to characterize the human flagellar anomalies of infertile patients, which are the closest to those of Ube2b-deficient mice. These new insights regarding the assembly of mammalian sperm flagella provide a basis for studying the ontogenesis of flagellar accessory structures and suggest leads for medical and genetic investigations.

Spermatid nuclear and sperm periaxonemal anomalies in the mouse Ube2b null mutant

Ube2b (yeast Ubc2b/Rad6 homolog) null mice were described previously. Ube2b encodes the murine ubiquitin conjugating enzyme mHR6B. Ube2b(-/-) mice were shown to present male infertility and their sperm head shape anomalies suggested that Ube2b may be involved in the replacement of nuclear proteins during spermatid chromatin condensation. Apoptosis of spermatocytes suggested additional targets of Ube2b during spermatogenesis. Consistently, we found Ube2b transcription in both meiotic and postmeiotic stages by in situ hybridization. Immuno-electron microscopy revealed that transition proteins 1 and 2, protamines 1 and 2, and actin appear normally distributed during morphogenesis of Ube2b(-/-) spermatid heads. Surprisingly, electron microscopy revealed a particular sperm flagellum phenotype characterized by an abnormal distribution of periaxonemal structures. Flagellar anomalies of Ube2b null mice were previously described in infertile men indicating a possible genetic pathway for flagellar periaxonemal assembly in human.

Increased transcription of ubiquitin-proteasome system components: molecular responses associated with muscle atrophy

Muscle atrophy is a common consequence of catabolic conditions like kidney failure, cancer, sepsis, and acute diabetes. Loss of muscle protein is due primarily to activation of the ubiquitin-proteasome proteolytic system. The proteolytic responses to catabolic signals include increased levels of mRNA that encode various components of the system. In the case of two genes, the proteasome C3 subunit and ubiquitin UbC, the higher levels of mRNA result from increased transcription but the mechanisms of transactivation differ between them. This review summaries the evidence that cachectic signals activate a program of selective transcriptional responses in muscle that frequently occurs coordinately with increased protein destruction.

Role of proteasomes in the degradation of short-lived proteins in human fibroblasts under various growth conditions

Degradation of proteins in the cells occurs by proteasomes, lysosomes and other cytosolic and organellar proteases. It is believed that proteasomes constitute the major proteolytic pathway under most conditions, especially when degrading abnormal and other short-lived proteins. However, no systematic analysis of their role in the overall degradation of truly short-lived cell proteins has been carried out. Here, the degradation of short-labelled proteins was examined in human fibroblasts by release of trichloroacetic acid-soluble radioactivity. The kinetics of degradation was decomposed into two, corresponding to short- and long-lived proteins, and the effect of proteasomal and lysosomal inhibitors on their degradation, under various growth conditions, was separately investigated. From the degradation kinetics of proteins labelled for various pulse times it can be estimated that about 30% of newly synthesised proteins are degraded with a half-life of approximately 1h. These rapidly degraded proteins should mostly include defective ribosomal products. Deprivation of serum and confluent conditions increased the degradation of the pool of long-lived proteins in fibroblasts without affecting, or affecting to a lesser extent, the degradation of the pool of short-lived proteins. Inhibitors of proteasomes and of lysosomes prevented more than 80% of the degradation of short-lived proteins. It is concluded that, although proteasomes are responsible of about 40-60% of the degradation of short-lived proteins in normal human fibroblasts, lysosomes have also an important participation in the degradation of these proteins. Moreover, in confluent fibroblasts under serum deprivation, lysosomal pathways become even more important than proteasomes in the degradation of short-lived proteins.

A novel C-terminal proteolytic processing of cytosolic pyruvate kinase, its phosphorylation and degradation by the proteasome in developing soybean seeds

Cytosolic pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40) is an important glycolytic enzyme, but the post-translational regulation of this enzyme is poorly understood. Sequence analysis of the soybean seed enzyme suggested the potential for two phosphorylation sites: site-1 (FVRKGS220DLVN) and site-2 (VLTRGGS407TAKL). Sequence- and phosphorylation state-specific antipeptide antibodies established that cytosolic pyruvate kinase (PyrKinc) is phosphorylated at both sites in vivo. However, by SDS-PAGE, the phosphorylated polypeptides were found to be smaller (20-51 kDa) than the full length (55 kDa). Biochemical separations of seed proteins by size exclusion chromatography and sucrose-density gradient centrifugation revealed that the phosphorylated polypeptides were associated with 26S proteasomes. The 26S proteasome particle in developing seeds was determined to be of approximately 1900 kDa. In vitro, the 26S proteasome degraded associated PyrKinc polypeptides, and this was blocked by proteasome-specific inhibitors such as MG132 and NLVS. By immunoprecipitation, we found that some part of the phosphorylated PyrKinc was conjugated to ubiquitin and shifted to high molecular mass forms in vivo. Moreover, recombinant wild-type PyrKinc was ubiquitinated in vitro to a much greater extent than the S220A and S407A mutant proteins, suggesting a link between phosphorylation and ubiquitination. In addition, during seed development, a progressive accumulation of a C-terminally truncated polypeptide of approximately 51 kDa was observed that was in parallel with a loss of the full-length 55 kDa polypeptide. Interestingly, the C-terminal 51 kDa truncation showed not only pyruvate kinase activity but also activation by aspartate. Collectively, the results suggest that there are two pathways for PyrKinc modification at the post-translational level. One involves partial C-terminal truncation to generate a 51 kDa pyruvate kinase subunit which might have altered regulatory properties and the other involves phosphorylation and ubiquitin conjugation that targets the protein to the 26S proteasome for complete degradation.

Ubiquitin-proteasome-dependent muscle proteolysis responds slowly to insulin release and refeeding in starved rats

The central role of the ubiquitin-proteasome system in the loss of skeletal muscle protein in many wasting conditions has been well established. However, it is unclear what factors are responsible for the suppression of this system during periods of protein gain. Thus, the aim of these studies was to examine the short-term effects of insulin release and nutrients on skeletal muscle protein turnover in young rats starved for 48 h, and then infused intravenously with amino acids (AA), or fed an oral diet. Forty-eight hours of starvation (i.e. prolonged starvation in young rats) decreased muscle protein synthesis and increased proteasome-dependent proteolysis. Four-hour AA infusion and 4 h of refeeding increased plasma insulin release and AA concentrations, and stimulated muscle protein synthesis, but had no effect on either total or proteasome-dependent proteolysis, despite decreased plasma corticosterone concentrations. Both muscle proteasome-dependent proteolysis and the rate of ubiquitination of muscle proteins were not suppressed until 10 h of refeeding. The temporal response of these two measurements correlated with the normalised expression of the 14-kDa E2 (a critical enzyme in substrate ubiquitination in muscle) and the expression of the MSS1 subunit of the 19S regulatory complex of the 26S proteasome. In contrast, the starvation-induced increase in mRNA levels for 20S proteasome subunits was normalised by refeeding within 24 h in muscle, and 6 h in jejunum, respectively. In conclusion, unlike protein synthesis, skeletal muscle proteasome-dependent proteolysis is not acutely responsive in vivo to insulin, AA, and/or nutrient intake in refed starved rats. This suggests that distinct and perhaps independent mechanisms are responsible for the nutrient-dependent regulation of protein synthesis and ubiquitin-proteasome-dependent proteolysis following a prolonged period of catabolism. Furthermore, factors other than the expression of ubiquitin-proteasome pathway components appear to be responsible for the suppression of skeletal muscle proteasome-dependent proteolysis by nutrition.

Signalling pathways that mediate skeletal muscle hypertrophy and atrophy

Atrophy of skeletal muscle is a serious consequence of numerous diseases, including cancer and AIDS. Successful treatments for skeletal muscle atrophy could either block protein degradation pathways activated during atrophy or stimulate protein synthesis pathways induced during skeletal muscle hypertrophy. This perspective will focus on the signalling pathways that control skeletal muscle atrophy and hypertrophy, including the recently identified ubiquitin ligases muscle RING finger 1 (MuRF1) and muscle atrophy F-box (MAFbx), as a basis to develop targets for pharmacologic intervention in muscle disease.

Activation of Ras and the mitogen-activated protein kinase pathway promotes protein degradation in muscle cells of Caenorhabditis elegans

To discover and study intracellular signals that regulate proteolysis in muscle, we have employed transgenic strains of Caenorhabditis elegans that produce a soluble LacZ reporter protein limited to body-wall and vulval muscles. This reporter protein is stable in well-fed wild-type animals, but its degradation is triggered upon a shift to 25 degrees C in a strain carrying a temperature-sensitive activating mutation in the Ras oncogene homologue let-60. These mutants are not physiologically starved, inasmuch as growth rates are normal at 25 degrees C. Ras-induced degradation is not prevented by the presence of cycloheximide added at or before the temperature shift and thus uses preexisting proteolytic systems and signaling components. Furthermore, degradation is triggered when adult animals are shifted to conditions of 25 degrees C, confirming that Ras acutely promotes protein degradation in muscles whose developmental history is normal. Reduction-of-function mutations in the downstream protein kinase Raf (lin-45), MEK (mek-2), or mitogen-activated protein kinase (MAPK) (mpk-1) prevent Ras-induced protein degradation, whereas activated MPK-1 is sufficient to trigger degradation, indicating that this kinase cascade is the principal route by which Ras signaling triggers protein degradation in muscle. This pathway is activated in hypodermal cells by the LET-23 epidermal growth factor receptor homologue, but an activating mutation in let-23 does not promote proteolysis in muscle. Starvation-induced LacZ reporter degradation is unaffected by reduction-of-function mutations in Ras, Raf, MEK, or MAPK, implying that Ras activation and starvation trigger proteolysis by mechanisms that are at least partially independent. This is the first evidence that Ras-Raf-MEK-MAPK signaling activates protein degradation in differentiated muscle.

Development and evaluation of a sandwich ELISA for quantification of the 20S proteasome in human plasma

Because quantification of the 20S proteasome by functional activity measurements is difficult and inaccurate, we have developed an indirect sandwich enzyme-linked immunosorbent assays (ELISA) for quantification of the 20S proteasome in human plasma. This sandwich ELISA uses a combination of a monoclonal antibody (mcp 20) recognizing the C2-beta subunit of human 20S proteasome (Mr approximately 30,000) and a polyclonal rabbit anti-20S antibody which labels different subunits of the complex. The detection limit of the assay was established as 10 ng/ml (n=10, mean of zero standard+2 S.D.) and the recovery rate ranged from 96% to 104%. The within-run and between-run coefficients of variation (CV) ranges were 2.8-3.3 and 3.0-3.4, respectively. Using serial dilutions of plasma to which various amounts of purified 20S proteasome were added, a linear dose-response was observed between 102 and 2050 ng/ml with a slope of 1.004 and a coefficient of determination r(2) of 0.99. In a preliminary experiment performed on a limited number of patients, the present assay was used to quantify the 20S proteasome in plasma from healthy subjects (n=11) and from a limited number of patients with various diseases (two patients with each of the following diagnoses: acute myeloid leukaemia, chronic myeloproliferative syndromes, Hodgkin's disease and solid tumors). The average concentration of 20S proteasome in plasma from normal subjects was found to be 2319+/-237 ng/ml (n=11). With reference to this normal range, the plasma proteasome concentration was found to be increased in most of these pathological state and as high as 1200% when solid tumors had been detected. For patients with Hodgkin's disease, the changes were more variable whereas in patients with chronic lymphocytic leukaemia, the proteasome concentration was raised during the acute phase of disease and decreased during therapy. We suggest that this robust, accurate and highly reproducible assay could be used to quantify proteasome in human plasma and investigate its value as a biological marker for various malignant and nonmalignant diseases.

Torbafylline (HWA 448) inhibits enhanced skeletal muscle ubiquitin-proteasome-dependent proteolysis in cancer and septic rats

The development of new pharmacological approaches for preventing muscle wasting in cancer is an important goal because cachectic patients display a reduced response to chemotherapy and radiotherapy. Xanthine derivatives such as pentoxifylline inhibit tumour necrosis factor-alpha (TNF) production, which has been implicated in the signalling of muscle wasting. However, the effect of pentoxifylline has been inconclusive in clinical trials. We report here the first direct evidence that daily injections of torbafylline (also known as HWA 448), another xanthine derivative, had no effect by itself on muscle proteolysis in control healthy rats. In cancer rats, the drug blocked the lipopolysaccharide-induced hyperproduction of TNF and prevented muscle wasting. In these animals HWA 448 suppressed the enhanced proteasome-dependent proteolysis, which is sensitive to the proteasome inhibitor MG132, and the accumulation of high-molecular-mass ubiquitin (Ub) conjugates in the myofibrillar fraction. The drug also normalized the enhanced muscle expression of Ub, which prevails in the atrophying muscles from cancer rats. In contrast, HWA 448 did not reduce the increased expression of either the 14 kDa Ub conjugating enzyme E2 or the ATPase and non-ATPase subunits of the 19 S regulatory complex of the 26 S proteasome, including the non-ATPase subunit S5a, which recognizes polyUb degradation signals. Finally, the drug also prevented muscle wasting in septic rats (which exhibit increased TNF production), and was much more potent than pentoxifylline or other xanthine derivatives. Taken together, the data indicate that HWA 448 is a powerful inhibitor of muscle wasting that blocks enhanced Ub-proteasome-dependent proteolysis in situations where TNF production rises, including cancer and sepsis.

Atrogin-1, a muscle-specific F-box protein highly expressed during muscle atrophy

Muscle wasting is a debilitating consequence of fasting, inactivity, cancer, and other systemic diseases that results primarily from accelerated protein degradation by the ubiquitin-proteasome pathway. To identify key factors in this process, we have used cDNA microarrays to compare normal and atrophying muscles and found a unique gene fragment that is induced more than ninefold in muscles of fasted mice. We cloned this gene, which is expressed specifically in striated muscles. Because this mRNA also markedly increases in muscles atrophying because of diabetes, cancer, and renal failure, we named it atrogin-1. It contains a functional F-box domain that binds to Skp1 and thereby to Roc1 and Cul1, the other components of SCF-type Ub-protein ligases (E3s), as well as a nuclear localization sequence and PDZ-binding domain. On fasting, atrogin-1 mRNA levels increase specifically in skeletal muscle and before atrophy occurs. Atrogin-1 is one of the few examples of an F-box protein or Ub-protein ligase (E3) expressed in a tissue-specific manner and appears to be a critical component in the enhanced proteolysis leading to muscle atrophy in diverse diseases.

Role of ubiquitin-proteasome-dependent proteolytic process in degradation of muscle protein from diabetic rabbits

The activity of ATP, ubiquitin (Ub)-dependent proteases partially purified from skeletal muscle (psoas) from alloxan diabetic rabbits was determined at different periods of insulin deficiency. Two days after alloxan injection, no change was observed in the activity of ATP, Ub-dependent proteases, but this activity increased 3 and 5 days after diabetes induction, attaining 181% of control values on the 5th day. However, after this early rise, the activity of muscle ATP, Ub-dependent proteases decreased, returning to values that did not differ significantly from controls 7 and 10 days after alloxan injection. After 15 days, the activity of these proteases was 57% lower than in muscle from control rabbits. Both the initial increase and the subsequent fall in the activity of the enzymes were prevented by insulin treatment of alloxan diabetic rabbits. The data suggest that Ub-proteasome-dependent proteolysis have an important role in the control of muscle protein degradation and may be regulated by insulin.

Weightlessness as an accelerated model of nutritional disturbances

Food intake and eating patterns, body functions and composition are significantly altered by short-duration space flight. Prolonged missions lasting weeks or months further aggravate these changes, and are responsible for acute or chronic physical impairments at return to ground conditions. Current projects of missions to Mars, resulting in 2 years of microgravity conditions, stress the critical need for the development of optimal nutritional programs and physical countermeasures to prevent body mass and function alterations. This review outlines ground models of microgravity simulation, summarizes the major effects of weightlessness on body composition, protein metabolism, hormonal pattern, and muscle function, and addresses contradictory findings related to the oxidative stress secondary to space flight. Potential countermeasures, such as nutrient intake and physical conditioning, as well as areas of interest for future research both in ground and space medicine, are discussed.