Activation of the ubiquitin pathway in rat skeletal muscle by catabolic doses of glucocorticoids

Gene expression profile of duodenal epithelial cells in response to chronic metabolic acidosis

Chronic metabolic acidosis (CMA) affects ion transport, permeability, and metabolism of the intestinal absorptive cells. Most effects of CMA on the intestine are long-term adaptations at genomic level. To identify the CMA-regulated genes, the Illumina's microarray featuring high-performance BeadArray technology was performed on RNA samples from the rat duodenal epithelial cells exposed to long-standing acidemia. After 21 days of CMA, we found 423 transcripts upregulated and 261 transcripts downregulated. Gene ontology analysis suggested effects of CMA on cellular processes, such as cell adhesion, proliferation, fuel metabolism, and biotransformation. Interestingly, 27 upregulated transcripts (e.g., Aqp1, Cacnb1, Atp1a2, Kcnab2, and Slc2a1) and 13 downregulated transcripts (e.g., Slc17a7, Slc9a4, and Slc30a3) are involved in the absorption of water, ions, and nutrients. Some upregulated genes, such as Slc38a5 and Slc1a7 encoding glutamine transporters, may be parts of the total body adaptation to alleviate negative nitrogen balance. Therefore, the present results provided a novel genome-wide information for further investigations of the mechanism of CMA effect on the intestine.

Enterotoxigenic Escherichia coli modulates host intestinal cell membrane asymmetry and metabolic activity

Enterotoxigenic Escherichia coli (ETEC) is a common cause of travelers' and postweaning diarrhea in humans and swine, respectively. The extent to which ETEC damages host cells is unclear. Experiments are presented that probe the ability of porcine ETEC isolates to induce apoptosis and cell death in porcine intestinal epithelial cells. Quantification of host phosphatidylserine exposure following ETEC infection suggested that ETEC induced changes in plasma membrane asymmetry, independent of the expression of the heat-labile enterotoxin. Significant host cell death was not observed. ETEC infection also caused a drastic inhibition of host esterase activity, as measured by calcein fluorescence. While ETEC infection resulted in activation of host caspase 3, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling of DNA double-strand breakage, indicative of late stages of apoptosis, was not observed. Camptothecin-induced apoptosis markedly increased subsequent ETEC adherence. Transfer of cell-free supernatants from apoptotic cells to bacterial inocula prior to infection of naïve cells increased the transcriptional activity of the regulatory region upstream of the K88ac operon and promoted subsequent adherence to host cells.

Adaptations to short-term muscle unloading in young and aged men

INTRODUCTION

The purpose of this investigation was to determine whether young (21.7 yr) and aged (68.5 yr) men experienced similar responses to 7 d of muscle unloading (N = 10 per group).

METHODS

Unilateral lower limb suspension (ULLS) was used to impose muscle unloading of the knee extensors. To compare the effects of unloading on aged and young men, a repeated-measures factorial ANOVA was used to assess those effects on isometric strength, as well as strength, total work, and average power during isokinetic contractions conducted at 0.53, 1.05, and 2.09 rad.s(-1).

RESULTS

Data showed that at slower speeds of movement, only a main effect of unloading was identified with young and aged men displaying similar and significant (P < 0.05) ULLS-induced decrements in strength, work, and power. The decrease in isometric strength correlated well with loss of electromyographic activity of contracting muscles (r = 0.79, P = 0.0002). At higher speeds of isokinetic contractions, not only was a main effect of age detected (young > aged), but it was also revealed that aged men, but not young men, experienced significant unloading-induced declines in muscle performance. Moreover, unloading resulted in a significant increase in plasma cortisol, a potent catabolic hormone, only among aged men. In contrast to other variables assessed, muscle endurance, quantified during 30 repetitions completed at 3.14 rad.s(-1), did not differ between age groups, nor was it altered by unloading.

CONCLUSION

These findings suggest that young and aged men respond differently to muscle unloading, but in assessing muscle performance, these differences are manifested only during faster contractile velocities.

A proteome map of murine heart and skeletal muscle

The balance of hypertrophy and atrophy is critical for the adaptation of cardiac and skeletal muscle mass to the demands of the environment and when deregulated can cause disease. Here we have used a proteomics approach to generate protein reference maps for the mouse heart and skeletal muscle, which provide a molecular basis for future functional and pathophysiological studies. The reference map provides information on molecular mass, pI, and literature data on function and localization, to facilitate the identification of proteins based on their migration in 2-D gels. In total, we have identified 351 cardiac and 284 skeletal muscle protein spots, representing 249 and 214 different proteins, respectively. In addition, we have visualized the protein pattern of mouse heart and skeletal muscle at defined conditions comparing knockout (KO) animals deficient in the sarcomeric protein titin (a genetic atrophy model) and control littermates. We found 20 proteins that were differently expressed linking titin's kinase region to the heat-shock- and proteasomal stress response. Taken together, the established reference maps should provide a suitable tool to relate protein expression and PTM to cardiovascular and skeletal muscle disease using the mouse as an animal model.

Testosterone protects against dexamethasone-induced muscle atrophy, protein degradation and MAFbx upregulation

Administration of glucocorticoids in pharmacological amounts results in muscle atrophy due, in part, to accelerated degradation of muscle proteins by the ubiquitin-proteasome pathway. The ubiquitin ligase MAFbx is upregulated during muscle loss including that caused by glucocorticoids and has been implicated in accelerated muscle protein catabolism during such loss. Testosterone has been found to reverse glucocorticoid-induced muscle loss due to prolonged glucocorticoid administration. Here, we tested the possibility that testosterone would block muscle loss, upregulation of MAFbx, and protein catabolism when begun at the time of glucocorticoid administration. Coadministration of testosterone to male rats blocked dexamethasone-induced reduction in gastrocnemius muscle mass and upregulation of MAFbx mRNA levels. Administration of testosterone together with dexamethasone also prevented glucocorticoid-induced upregulation of MAFbx mRNA levels and protein catabolism in C2C12 myotube expressing the androgen receptor. Half-life of MAFbx was not altered by testosterone, dexamethasone or the combination. Testosterone blocked dexamethasone-induced increases in activity of the human MAFbx promotor. The findings indicate that administration testosterone prevents glucocorticoid-induced muscle atrophy and suggest that this results, in part at least, from reductions in muscle protein catabolism and expression of MAFbx.

Post-ischaemic restituted intestinal mucosa is more resistant to further ischaemia than normal mucosa in the pig

OBJECTIVE

Ischaemic preconditioning may protect the intestine from subsequent prolonged ischaemia. This study evaluates whether a much longer initial ischaemia, encountered clinically, may modify intestinal resistance to further ischaemia in a pig model.

MATERIAL AND METHODS

After cross-clamping of the superior mesenteric artery for 1 h, the intestine was either reperfused for 8 h or a second cross-clamping for 1 h was performed at 4 h of reperfusion. Based on microarray analysis of intestinal samples at 1, 4 and 8 h of reperfusion, mRNA of selected genes was measured with QRT-PCR.

RESULTS

The first ischaemic period caused exfoliation of surface epithelial cells from the basement membrane comprising about 90 % of the villi tips, a marked increase in permeability and depletion of ATP. The second ischaemic challenge caused about 30 % less denudation of the basement membrane (p = 0.008), no increase in permeability (p = 0.008) and less depletion of ATP (p = 0.039). mRNAs for superoxide dismutase 2, heat shock proteins and signal transducer and activator of transcription 3, which may protect against ischaemia/reperfusion injury, were up-regulated throughout the reperfusion period. mRNAs for matrix metalloproteinase 1, connexin 43 and peripheral myelin 22, which may be associated with cell migration or tight junctions, showed a particular up-regulation at 4 h of reperfusion.

CONCLUSION

One hour of initial ischaemia followed by 4 h of reperfusion is associated with increased intestinal resistance to further ischaemia. The differential regulation of genes identified in this study provides working hypotheses for mechanisms behind this observation.

Leptin levels and body composition of mice selectively bred for high voluntary locomotor activity

Selective breeding produced four replicate lines of high-runner (HR) mice that run on wheels for approximately 2.7 times more revolutions per day than four unselected control lines. Previous studies found that HR mice of both sexes have lower body fat (isotope dilution at 15 wk of age) and that males (females not studied) have smaller retroperitoneal fat pads (17 wk). HR mice also exhibit elevated plasma corticosterone and insulin-stimulated glucose uptake by some hindlimb muscles but apparently do not differ in circulating insulin or glucose levels (males at 18 wk). Given their lower body fat and higher activity levels, we hypothesized that HR mice would have lower circulating leptin levels than controls. Female mice were given wheel access for 6 d at 7 wk of age, as part of the routine wheel testing for the selective breeding protocol, and then were killed after one additional week without wheels to reduce possible acute effects of activity on leptin. As hypothesized, serum leptin levels were significantly lower in HR mice. ANCOVA indicated that leptin was strongly positively correlated with both total body fat (measured by ether extraction) and body mass change from weaning, but HR mice still had significantly lower adjusted leptin levels (ANCOVA). Within HR lines but not within control lines, individual variation in leptin levels was negatively correlated with amount or speed of wheel running measured a week before being killed. Growth from weaning to euthanasia and body dry mass were lower in HR mice than in controls, but absolute dry masses of the ventricles, liver, gut, and uterus plus ovaries did not significantly differ, nor did percentage of the total dry mass as fat. HR mice offer a novel model for studying the causes and consequences of physiologically relevant variations in serum leptin.

Lactate and glycerol released to the intestinal lumen reflect mucosal injury and permeability changes caused by strangulation obstruction

BACKGROUND

The present study evaluates whether microdialysis of glycerol and lactate reflects mucosal injury and permeability changes after strangulation obstruction of the pig small intestine.

METHODS

Strangulation obstruction was induced by tightening a rubber band around a small bowel loop until its venous pressure increased to a level just below diastolic aortic pressure (partial strangulation), or further until cessation of flow in the main feeding artery (total strangulation). Mucosal injury and permeability of marker molecules from blood to lumen and vice versa was compared to release of glycerol and lactate to the intestinal lumen.

RESULTS

Mucosal injury, hyperpermeability, and release of glycerol were more pronounced after total than after partial strangulation. In animals with partial strangulation there was a complete restitution of the surface epithelium, and luminal glycerol and lumen-to-blood permeability of polyethylene glycol 4000 remained low. Such animals showed a sustained elevation of lactate and blood-to-lumen permeability of fluorescein isothiocyanate dextran after 2 h of partial strangulation, but a decline to baseline levels of these parameters in animals with 1 h partial strangulation.

CONCLUSION

Microdialysis of lactate and glycerol in the intestinal lumen may be used to assess structural and functional changes of the intestinal mucosa after strangulation obstruction.

Interleukin-18 controls energy homeostasis by suppressing appetite and feed efficiency

Circulating levels of the cytokine interleukin 18 (IL-18) are elevated in obesity. Here, we show that administration of IL-18 suppresses appetite, feed efficiency, and weight regain in food-deprived male and female C57BL/6J mice. Intraperitoneal vs. intracerebroventricular routes of IL-18 administration had similar potency and did not promote formation of a conditioned taste aversion (malaise-like behavior). Mice partially (Il18(+/-)) or totally (Il18(-/-)) deficient in IL-18 were hyperphagic by young adulthood, with null mutants then becoming overweight by the fifth month of life. Adult Il18(-/-) mice gained 2- to 3-fold more weight than WT mice per unit energy consumed of low- or high-fat diet. Indirect calorimetry revealed reduced energy expenditure in female Il18(-/-) mice and increased respiratory exchange ratios [volume of carbon dioxide production (VCO(2))/volume of oxygen consumption (VO(2))] in mutants of both sexes. Hyperphagia continued in maturity, with overeating greatest during the mid- to late-dark cycle. Relative white fat-pad mass of Il18(-/-) mice was approximately 2- to 3-fold greater than that of WT, with gonadal, mesenteric, and inguinal depots growing most. The data suggest that endogenous IL-18 signaling modulates food intake, metabolism, and adiposity during adulthood and might be a central or peripheral pharmacological target for controlling energy homeostasis.

Proteasome proteolytic activity in skeletal muscle is increased in patients with sepsis

Patients with sepsis in the ICU (intensive care unit) are characterized by skeletal muscle wasting. This leads to muscle dysfunction that also influences the respiratory capacity, resulting in prolonged mechanical ventilation. Catabolic conditions are associated with a general activation of the ubiquitin-proteasome pathway in skeletal muscle. The aim of the present study was to measure the proteasome proteolytic activity in both respiratory and leg muscles from ICU patients with sepsis and, in addition, to assess the variation of proteasome activity between individuals and between duplicate leg muscle biopsy specimens. When compared with a control group (n=10), patients with sepsis (n=10) had a 30% (P<0.05) and 45% (P<0.05) higher proteasome activity in the respiratory and leg muscles respectively. In a second experiment, ICU patients with sepsis (n=17) had a 55% (P<0.01) higher proteasome activity in the leg muscle compared with a control group (n=10). The inter-individual scatter of proteasome activity was larger between the patients with sepsis than the controls. We also observed a substantial intra-individual difference in activity between duplicate biopsies in several of the subjects. In conclusion, the proteolytic activity of the proteasome was higher in skeletal muscle from patients with sepsis and multiple organ failure compared with healthy controls. It was shown for the first time that respiratory and leg muscles were affected similarly. Furthermore, the variation in proteasome activity between individuals was more pronounced in the ICU patients for both muscle types, whereas the intra-individual variation between biopsies was similar for ICU patients and controls.

Altered expression of genes regulating skeletal muscle mass in the portacaval anastomosis rat

We examined the temporal relationship between portacaval anastomosis (PCA), weight gain, changes in skeletal muscle mass and molecular markers of protein synthesis, protein breakdown, and satellite cell proliferation and differentiation. Male Sprague-Dawley rats with end to side PCA (n=24) were compared with sham-operated pair-fed rats (n=24). Whole body weight, lean body mass, and forelimb grip strength were determined at weekly intervals. The skeletal muscle expression of the ubiquitin proteasome system, myostatin, its receptor (the activin 2B receptor) and its signal, cyclin-dependent kinase inhibitor (CDKI) p21, insulin-like growth factor (IGF)-I and its receptor (IGF-I receptor-alpha), and markers of satellite cell proliferation and differentiation were quantified. PCA rats did not gain body weight and had lower lean body mass, forelimb grip strength, and gastrocnemius muscle weight. The skeletal muscle expression of the mRNA of ubiquitin proteasome components was higher in PCA rats in the first 2 wk followed by a lower expression in the subsequent 2 wk (P<0.01). The mRNA and protein of myostatin, activin 2B receptor, and CDKI p21 were higher, whereas IGF-I and its receptor as well as markers of satellite cell function (proliferating nuclear cell antigen, myoD, myf5, and myogenin) were lower at weeks 3 and 4 following PCA (P < 0.05). We conclude that PCA resulted in uninhibited proteolysis in the initial 2 wk. This was followed by an adaptive response in the later 2 wk consisting of an increased expression of myostatin that may have contributed to reduced muscle protein synthesis, impaired satellite cell function, and lower skeletal muscle mass.

Time-Course Changes of Hormones and Cytokines by Lipopolysaccharide and Its Relation with Anorexia

We assessed the time course effects of lipopolysaccaride (LPS) on food intake, cytokines, and hormones in rats and evaluated the relation between LPS-induced anorexia and its possible causative factors. Food intake was reduced 2 h after LPS injection (500 microg/kg, intraperitoneally) and remained decreased for 24 h. Plasma TNF-alpha and IL-6 levels increased by LPS administration at 0.5 and 2 h, and at 2 and 4 h, respectively. Plasma leptin and glucose levels were elevated at 8 and 16 h, and insulin levels were elevated at 2, 4, 8, and 16 h in the LPS-injected group, as compared to the counterpart controls. IL-6 levels in the CSF were elevated at 2 and 4 h. Hypothalamic cytokines tended to increase as early as 0.5 h after LPS injection and remained increased until 16 h. LPS-induced anorexia was attenuated in insulin-deficient STZ rats and was abolished by insulin treatment. The hypothalamic expression of NPY, a target of insulin's anorexic effect, was decreased 2 h after LPS administration, and central NPY injection (3 nM) prevented LPS-induced anorexia. In conclusion, cytokines, insulin, and leptin levels evidence different time courses by LPS administration. In LPS-induced anorexia, insulin may constitute a newly found causative factor, whereas leptin appears to be uninvolved in an early period in rats.

Intercellular localization of occludins and ZO-1 as a solute transport barrier of the mesothelial monolayer

A hyperpermeable state has been observed in patients on long-term peritoneal dialysis. To understand the causes of the structural or functional changes and the progression of the fibrotic process, it is important to determine which region of the peritoneum exhibits these changes. The objectives of this study were to determine the solute permeability associated with cell-cell adhesion of human peritoneal mesothelial cells (HPMCs), to study the relationship between solute permeability and localizations of tight junction-associated proteins (TJPs: occludins and ZO-1), and to assess the effect of exogenous H2O2 supplementation. HPMCs were cultured on a Transwell until the transmesothelial electrical resistance (TER) reached a plateau. Solute permeation tests were conducted using fluorescein isothiocyanate - labeled dextran (molecular weight: 4, 10, 70, and 150 kDa) to calculate the solute permeability coefficient (SPC). Localization of TJPs was observed by a confocal laser scanning microscope after immunofluorescent staining. TER levels increased steadily, beginning at 97.5 +/- 0.7 ohms.cm2 and leveling off at 128 +/- 3.6 ohms.cm2 (n = 4). This was accompanied by the confluence of cells and the appearance of localized TJPs. SPC levels of the HPMC monolayer on the Transwell were reduced compared to those of the Transwell itself, indicating that the HPMC monolayer provided resistance against solute permeation. Exogenous H2O2 supplementation revealed an increased permeability accompanied with delocalization of TJPs, particularly occludins. The delocalization of occludins and ZO-1 at the intercellular space led to a decrease in intercellular binding capacity and thus triggered an increase in the solute permeability.

Creatine intake attenuates corticosteroid-induced impairment of voluntary running in hamsters

Myopathy is a well-known side effect of corticosteroid therapy. Creatine monohydrate (Cr) supplementation has been shown to increase fat-free mass and muscular function. This study aimed to investigate if Cr administration could offset the deleterious functional effects of high doses of steroids. Fifty-six male Syrian golden hamsters were randomized among 4 groups: GI (n = 10), subcutaneous (s.c.) and intraperitoneal (i.p.) saline; GII (n = 10), s.c. saline and i.p. Cr (600 mg·kg–1·d–1); GIII (n = 18), s.c. dexamethasone (7.5 mg·kg–1·d–1) and i.p. saline; and GIV (n = 18), s.c. dexamethasone and i.p. Cr. Daily voluntary running was measured using activity wheels for 18 d. At the end of the study, statistically significant differences in running were observed between all groups, except for GI versus GII (GI, 8878 ± 2737 m; GII, 9145 ± 2000 m; GIII, 4289 ± 2623 m; GIV, 6339 ± 2345 m). Dexamethasone led to a significant decrease in cross-sectional area of type II fibers of the medial gastrocnemius. The cross-sectional area of type I fibers was significantly larger in GIV than in GIII. In conclusion, Cr administration attenuated the impairment of daily spontaneous running of hamsters receiving a high dose of corticosteroids. Additional research is needed to clarify the clinical implications of this finding.

Muscle atrophy in Titin M-line deficient mice

We investigated the response to deletion of the titin M-line region in striated muscle, using a titin knockout model and a range of techniques that include histology, in situ hybridization, electron microscopy, and 2D gel analysis. We found that the loss of titin's kinase domain and binding sites for myomesin and MURF-1 causes structural changes in the sarcomere that proceed from the M-line to the Z-disc and ultimately result in disassembly of the sarcomere. Disassembly goes along with central localization of nuclei (a hallmark for muscular dystrophy), up-regulation of heat-shock proteins, and induction of proteasome activity. While fiber type composition does not change in soleus and extensor digitorum longus muscle, fiber size is reduced. Animals die from complications of muscle atrophy at five weeks of age. In addition to the structural importance of the titin M-line region in any striated muscle, our data show how differences in M-line composition between heart and skeletal muscle affect sarcomere stability and function.

Quantification of hormone-induced atrophy of large myotubes from C2C12and L6 cells: atrophy-inducible and atrophy-resistant C2C12myotubes

Myofiber atrophy is the final outcome of muscle wasting induced by catabolic factors such as glucocorticoids and thyroid hormones. We set up an in vitro system to define the catabolic reaction based on myotube atrophy. Both mouse C2C12and rat L6 cells were used. C2C12myotube formation was improved by replacing horse serum with the serum substitute Ultroser G. A new method was developed to quantify size changes of large (0.5–1 mm) myotubes only, excluding remaining myoblasts and small myotubes. Dexamethasone reduced myotube size by 30% in L6 but not in C2C12myotubes. Expression of the glucocorticoid receptor was twofold higher in L6 myotubes than in C2C12myotubes. In both cell lines, 3,3′,5-triiodo-l-thyronine (T3) did not induce a significant size reduction. Expression of the major T3receptor (T3Rβ1) was higher in L6 myotubes. We investigated whether the changes in myotube size are related to changes in atrogin-1 expression, as this enzyme is thought to be a key factor in the initiation of muscle atrophy. Dexamethasone induced a twofold increase of atrogin-1 mRNA; again, only L6 myotubes were susceptible. Interestingly, atrogin-1 expression in Ultroser G-fused C2C12myotubes was lower than that in horse serum-fused myotubes. Furthermore, dexamethasone treatment increased atrogin-1 expression only in horse serum-fused myotubes but not in Ultroser G-fused myotubes. Ultroser G-induced fusion may result in atrophy-resistant C2C12myotubes. Therefore, C2C12myotubes offer an ideal system in which to study skeletal muscle atrophy because, depending on differentiation conditions, C2C12cells produce atrophy-inducible and atrophy-resistant myotubes.

Modulations of muscle protein metabolism by branched-chain amino acids in normal and muscle-atrophying rats

It has been shown that BCAAs, especially leucine, regulate skeletal muscle protein metabolism. However, it remains unclear how BCAAs regulate muscle protein metabolism and lead to anabolism in vivo. We examined muscle protein synthesis rate and breakdown rate simultaneously during BCAA infusion in muscle atrophy models as well as in normal healthy rats. Corticosterone-treated rats and hindlimb-immobilized rats were used as muscle atrophy models. Muscle protein synthesis rate and breakdown rate were measured as phenylalanine kinetics across the hindlimb. In anesthetized normal rats, BCAAs stimulated muscle protein synthesis despite low insulin concentration and did not suppress muscle protein breakdown. In corticosterone-treated rats, BCAAs failed to restore inhibited muscle protein synthesis, but reduced muscle protein breakdown. Immobilization of hindlimb increased muscle protein breakdown within a day. BCAAs did not change muscle protein metabolism, although essential amino acids (EAAs) suppressed muscle protein breakdown in hindlimb-immobilized rats. We also evaluated changes of fractional synthesis rate (FSR) of skeletal muscle protein during infusion of leucine alone or EAAs for 4 h in anesthetized normal rats. FSR showed a transient increase at 15-30 min of leucine infusion and then declined, whereas FSR stayed elevated throughout EAA infusion. We concluded that 1) BCAAs primarily stimulate muscle protein synthesis in normal rats independently of insulin; 2) EAAs are required to maintain the BCAA stimulation of muscle protein synthesis; and 3) The effects of BCAAs on muscle protein metabolism differ between atrophy models.

Dexamethasone upregulates the expression of the nuclear cofactor p300 and its interaction with C/EBPbeta in cultured myotubes

Muscle wasting during sepsis and other catabolic conditions is, at least in part, mediated by glucocorticoids and is associated with upregulated transcription of multiple genes in the ubiquitin-proteasome proteolytic pathway. In addition to transcription factors, nuclear cofactors, including p300, regulate gene transcription. We tested the hypothesis that glucocorticoids upregulate the expression of p300 in muscle cells. Treatment of cultured L6 myotubes, a rat skeletal muscle cell line, with dexamethasone resulted in a dose- and time-dependent increase in p300 protein and mRNA levels. Surprisingly, the effect of dexamethasone on p300 levels was not inhibited by the glucocorticoid receptor (GR) antagonist RU38486 and RU38486 exerted an agonist effect on p300, increasing its expression. Co-immunoprecipitation showed that treatment of the myotubes with dexamethasone resulted in protein-protein interaction between p300 and C/EBPbeta, but not C/EBPdelta. The present results suggest that glucocorticoids upregulate the expression of p300 and its interaction with C/EBPbeta in skeletal muscle. Increased expression and activity of p300 may be involved in the regulation of gene transcription in glucocorticoid-dependent muscle wasting.

Monoamine oxidase-A is a major target gene for glucocorticoids in human skeletal muscle cells

Skeletal myopathy is a common complication of endogenous and exogenous glucocorticoid excess, yet its pathogenetic mechanisms remain unclear. There is accumulating evidence that mitochondrial dysfunction and oxidative stress are involved in this process. To explore the glucocorticoid-induced transcriptional adaptations that may affect mitochondrial function in skeletal muscle, we studied gene expression profiles in dexamethasone-treated primary human skeletal myocytes using a cDNA microarray, which contains 501 mitochondria-related genes. We found that monoamine oxidase A (MAO-A) was the most significantly up-regulated gene. MAO-A is the primary enzyme metabolizing catecholamines and dietary amines, and its role in skeletal muscle remains largely unexplored. Dexamethasone induced dose- and time-dependent increases of MAO-A gene and protein expression, while its effects on MAO-B were minimal. Both the glucocorticoid receptor (GR) and the Sp1 transcription factor were required for dexamethasone-induced MAO-A mRNA expression, as blockade of the GR with RU 486 or ablation of Sp1 binding with mithramycin abrogated MAO-A mRNA induction. The observed dexamethasone effect was biologically functional, as this steroid significantly increased MAO-mediated hydrogen peroxide production. We suggest that MAO-A-mediated oxidative stress can lead to cell damage, representing a novel pathogenetic mechanism for glucocorticoid-induced myopathy and a potential target for therapeutic intervention.

Salmon spawning migration and muscle protein metabolism: the August Krogh principle at work

The August Krogh principle, stating that for any particular question in biology, nature holds an ideal study system, was applied by choosing the anorexic, long-distance migration of salmon as a model to analyze protein degradation and amino acid metabolism. Reexamining an original study done over 20 years ago on migrating sockeye salmon (Oncorhynchus nerka), data on fish migration and starvation are reviewed and a general model is developed on how fish deal with muscle proteolysis. It is shown that lysosomal activation and degradation of muscle protein by lysosomal cathepsins, especially cathepsin D and sometimes cathepsin L, are responsible for the degradation of muscle protein during fish migration, maturation and starvation. This strategy is quite the opposite to mammalian muscle wasting, including starvation, uremia, cancer and others, where the ATP-ubiquitin proteasome in conjunction with ancillary systems, constitutes the overwhelming pathway for protein degradation in muscle. In mammals, the lysosome plays a bit part, if any. In contrast, the proteasome plays at best a subordinate role in muscle degradation in piscine systems. This diverging strategy is put into the context of fish metabolism in general, with its high amino acid turnover, reliance on amino acids as oxidative substrates and flux of amino acids from muscle via the liver into gonads during maturation. Brief focus is placed on structure, function and evolution of the key player in fishes: cathepsin D. The gene structure of piscine cathepsin D is outlined, focusing on the existence of duplicate, paralogous, cathepsin D genes in some species and analyzing the relationship between a female and liver-specific aspartyl protease and fish cathepsin Ds. Evolutionary relationships are developed between different groups of piscine cathepsins, aspartyl proteases and other cathepsins. Finally, based on specific changes in muscle enzymes in fish, including migrating salmon, common strategies of amino acid and carbon flux in fish muscle are pointed out, predicting some metabolic concepts that would make ideal application grounds for the August Krogh principle.

Muscle-specific expression of IGF-1 blocks angiotensin II-induced skeletal muscle wasting

Advanced congestive heart failure is associated with activation of the renin-angiotensin system and skeletal muscle wasting. We previously showed that angiotensin II infusion in rats produces cachexia secondarily to increased muscle proteolysis and also decreases levels of circulating and skeletal muscle IGF-1. Here we show that angiotensin II markedly downregulates phospho-Akt and activates caspase-3 in skeletal muscle, leading to actin cleavage, an important component of muscle proteolysis, and to increased apoptosis. These changes are blocked by muscle-specific expression of IGF-1, likely via the Akt/mTOR/p70S6K signaling pathway. We also demonstrate that mRNA levels of the ubiquitin ligases atrogin-1 and muscle ring finger-1 are upregulated in angiotensin II-infused WT, but not in IGF-1-transgenic, mice. These findings strongly suggest that angiotensin II downregulation of IGF-1 in skeletal muscle is causally related to angiotensin II-induced wasting. Because the renin-angiotensin system is activated in many catabolic conditions, our findings have broad implications for understanding mechanisms of skeletal muscle wasting and provide a rationale for new therapeutic approaches.

Skeletal Muscle wasting and contractile performance in septic rats

We investigated the temporal effects of sepsis on muscle wasting and function in order to study the contribution of wasting to the decline in muscle function; we also studied the fiber-type specificity of this muscle wasting. Sepsis was induced by injecting rats intraperitoneally with a zymosan suspension. At 2 h and at 2, 6, and 11 days after injection, muscle function was measured using in situ electrical stimulation, Zymosan injection induced severe muscle wasting compared to pair-fed and ad libitum fed controls. At 6 days, isometric force-generating capacity was drastically reduced in zymosan-treated rats. We conclude that this was fully accounted fo by the reduction of muscle mas. At day 6, we also observed increased activity of the 20S proteasome in gastrocnemius but not soleus muscle from septic rats. In tibialis anterior but not in soleus, muscle wasting occurred in a fiber-type specific fashion, i.e., the reduction in cross-sectional area was significantly smaller in type 1 than type 2A and 2B/X fibers. These findings suggest that both the inherent function of a muscle and the muscle fiber-type distribution affect the responsiveness to catabolic signals.

Glucocorticoid-induced osteoporosis: from basic mechanisms to clinical aspects

Glucocorticoid (GC)-induced osteoporosis (GCOP) is the most common cause of osteoporosis in adults aged 20-45 years as well as the most common cause of iatrogenic osteoporosis. GC excess, either endogenous or exogenous, induces bone loss in 30-50% of cases. Indeed, bone loss leading to fractures is perhaps the most incapacitating, sometimes partially irreversible, complication of GC therapy. Nevertheless, GCOP is often underdiagnosed and left untreated. The following article provides an update on the cellular and molecular mechanisms implicated in the pathophysiology of GC-induced bone loss, as well as some guidelines on diagnostic, preventive and therapeutic strategies for this medical condition, in an effort to promote a better knowledge and greater awareness of GCOP by both the patient and the physician.

Multiple types of skeletal muscle atrophy involve a common program of changes in gene expression

Skeletal muscle atrophy is a debilitating response to starvation and many systemic diseases including diabetes, cancer, and renal failure. We had proposed that a common set of transcriptional adaptations underlie the loss of muscle mass in these different states. To test this hypothesis, we used cDNA microarrays to compare the changes in content of specific mRNAs in muscles atrophying from different causes. We compared muscles from fasted mice, from rats with cancer cachexia, streptozotocin-induced diabetes mellitus, uremia induced by subtotal nephrectomy, and from pair-fed control rats. Although the content of >90% of mRNAs did not change, including those for the myofibrillar apparatus, we found a common set of genes (termed atrogins) that were induced or suppressed in muscles in these four catabolic states. Among the strongly induced genes were many involved in protein degradation, including polyubiquitins, Ub fusion proteins, the Ub ligases atrogin-1/MAFbx and MuRF-1, multiple but not all subunits of the 20S proteasome and its 19S regulator, and cathepsin L. Many genes required for ATP production and late steps in glycolysis were down-regulated, as were many transcripts for extracellular matrix proteins. Some genes not previously implicated in muscle atrophy were dramatically up-regulated (lipin, metallothionein, AMP deaminase, RNA helicase-related protein, TG interacting factor) and several growth-related mRNAs were down-regulated (P311, JUN, IGF-1-BP5). Thus, different types of muscle atrophy share a common transcriptional program that is activated in many systemic diseases.

Glucocorticoid Receptor and Ubiquitin Expression after Repeated Eccentric Exercise

INTRODUCTION/PURPOSE

Eccentric exercise causes muscle proteolysis that may be attenuated with repeated exercise. Therefore, this study determined the effect of repeated bouts of eccentric exercise on ubiquitin (UBI), ubiquitin conjugating enzyme (E2), and 20S proteasome (20S) and glucocorticoid receptor (GR) mRNA and protein expression, myofibrillar protein content, DNA content, caspase-3 activity, serum skeletal muscle troponin-I (sTnI) and cortisol (CORT), and muscle strength.

METHODS

Nine males underwent two identical eccentric exercise bouts (BT1 and BT2) 3 wk apart involving seven sets of 10 repetitions at 150% one-repetition maximum of the dominant knee extensors. Blood and muscle biopsy samples were obtained before and at 6 and 24 h postexercise whereas muscle strength was assessed before and at 24, 48, and 72 h postexercise. Data were analyzed with separate 2 x 3 and 2 x 4 factorial ANOVA (P < 0.05).

RESULTS

Decrements in strength and increased soreness occurred at 24 and 48 h postexercise for both bouts (P < 0.05); however, the changes for BT1 were greater than BT2. Serum CORT and sTnI were greater immediately after and at 6, 24, and 48 h postexercise for both bouts; however, the differences in BT1 were greater than BT2 (P < 0.05). Caspase-3 activity and the mRNA and protein levels of UBI, E2, 20S, and GR were increased at 6 and 24 h postexercise, and these differences were greater for BT1 than BT2 (P < 0.05). For BT1, DNA and myofibrillar protein content decreases were apparent at 24 h postexercise (P < 0.05) but not in BT2.

CONCLUSION

These results indicate that muscle injury occurring from an initial bout of eccentric exercise seems to decrease muscle strength and myofibrillar protein, possibly due to apoptosis and up-regulation of glucocorticoid receptor mediated increases in UBI-proteolytic pathway activity, all of which appear to be tempered with a repeated eccentric exercise bout.

The synergistic effects of hypoxia/reoxygenation or tissue acidosis and bacteria on intestinal epithelial cell apoptosis

BACKGROUND

Clinical data indicate that gut perfusion deficits must be rectified within 24 hours after traumatic injury to decrease organ failure and death. Ischemia/reperfusion injury to the gut causes enterocyte apoptosis (Apo), which may contribute to intestinal barrier failure. The temporal response of enterocyte Apo to acidosis and hypoxia/reoxygenation (H/R) in vitro is unknown. The purpose of this study was to examine the effect of various time points of acidosis or H/R on enterocyte apoptosis and monolayer integrity in an in vitro model.

METHODS

Caco-2 cell monolayers were made acidic (Dulbecco's modified Eagle's medium, pH 6.9) by hydrochloric acid or exposed to 95% nitrogen/5% carbon dioxide (hypoxia) and then 21% oxygen (reoxygenation). Escherichia coli C-25 were added to the apical media in subsets. Apo and necrosis were quantified by flow cytometry. Permeability was determined by fluorescein isothiocyanate-dextran. Transepithelial electrical resistance (TEER) indexed monolayer.

RESULTS

Extracellular acidosis and C-25 significantly increased apoptosis of Caco-2 cells at 18 hours (extracellular acidosis [EC] + C-25, 14.5 +/- 3.0; control, 3.8 +/- 0.8; p < 0.001 by analysis of variance). Similarly, the H/R + C-25 group showed a significant increase in apoptosis at 12 hours (H/R + C-25 vs. control, 22.86 +/- 2.12 vs. 3.74 +/- 0.7; p < 0.001 by analysis of variance). The permeability difference was not significant for EC + C-25 versus control at 18 hours (0.68 +/- 0.25 vs. 0.43 +/- 0.0.0.36, respectively; p > 0.05). The H/R + C-25 group had a profound increase in permeability over control at 12 hours (10.8 +/- 0.5 vs. 2.1 +/- 0.3, respectively; p < 0.001). The TEER was significantly lowered for EC versus control at 18 hours (458 +/- 1.5 vs. 468 +/- 8.2) and at 0, 6, and 18 hours for EC + C-25 (409 +/- 28.1, 443 +/- 16.8, and 438 +/- 8.9 vs. 455 +/- 6.5, 467 +/- 6.5, and 469 +/- 8.2, respectively). There was no significant change in the H/R and H/R + C-25 groups.

CONCLUSION

Synergism of H/R or tissue acidosis and bacteria caused increased Apo, TEER, and permeability in vitro.

Glucocorticoid-induced skeletal muscle atrophy is associated with upregulation of myostatin gene expression

The mechanisms by which excessive glucocorticoids cause muscular atrophy remain unclear. We previously demonstrated that dexamethasone increases the expression of myostatin, a negative regulator of skeletal muscle mass, in vitro. In the present study, we tested the hypothesis that dexamethasone-induced muscle loss is associated with increased myostatin expression in vivo. Daily administration (60, 600, 1,200 micro g/kg body wt) of dexamethasone for 5 days resulted in rapid, dose-dependent loss of body weight (-4.0, -13.4, -17.2%, respectively, P < 0.05 for each comparison), and muscle atrophy (6.3, 15.0, 16.6% below controls, respectively). These changes were associated with dose-dependent, marked induction of intramuscular myostatin mRNA (66.3, 450, 527.6% increase above controls, P < 0.05 for each comparison) and protein expression (0.0, 260.5, 318.4% increase above controls, P < 0.05). We found that the effect of dexamethasone on body weight and muscle loss and upregulation of intramuscular myostatin expression was time dependent. When dexamethasone treatment (600 micro g. kg-1. day-1) was extended from 5 to 10 days, the rate of body weight loss was markedly reduced to approximately 2% within this extended period. The concentrations of intramuscular myosin heavy chain type II in dexamethasone-treated rats were significantly lower (-43% after 5-day treatment, -14% after 10-day treatment) than their respective corresponding controls. The intramuscular myostatin concentration in rats treated with dexamethasone for 10 days returned to basal level. Concurrent treatment with RU-486 blocked dexamethasone-induced myostatin expression and significantly attenuated body loss and muscle atrophy. We propose that dexamethasone-induced muscle loss is mediated, at least in part, by the upregulation of myostatin expression through a glucocorticoid receptor-mediated pathway.

Insulin-like growth factor I prevents corticosteroid-induced diaphragm muscle atrophy in emphysematous hamsters

The aim of this study was to evaluate whether recombinant human insulin-like growth factor I (rhIGF-I) could attenuate or prevent diaphragm (DIA) fiber atrophy with corticosteroid (CS) administration to emphysematous (EMP) hamsters. DIA muscle IGF-I responses to CS administration with and without exogenous rhIGF-I administration were evaluated. Three groups were studied: 1) EMP; 2) EMP + triamcinolone (T; 0.4 mg.kg-1.day-1 im); and 3) EMP + T + IGF-I (600 microg/day by constant infusion). After 4 wk, the DIA was analyzed histochemically and biochemically (IGF-I mRNA levels by RT-PCR and endogenous and exogenous IGF-I peptide levels immunochemically). Body weights of EMP-T progressively decreased, while those of EMP and EMP-T-IGF-I remained stable despite similarly reduced food intake in both T groups. DIA weight was reduced with T but preserved with rhIGF-I infusion. DIA fiber proportions were similar among the groups. The cross-sectional areas of types I, IIa, and IIx fibers were reduced (17 to 31%) with T administration but unchanged with rhIGF-I infusion. DIA IGF-I mRNA levels were similar across all groups. By contrast, the endogenous DIA IGF-I levels were reduced (41%) in the EMP-T-IGF-I animals. Total DIA IGF-I levels (endogenous + exogenous) were still significantly reduced. IGF-I immunoreactivity confirmed this reduction in all DIA fibers. We conclude that DIA fiber atrophy with T was completely prevented by exogenous rhIGF-I administration. This effect was likely mediated by the pharmacological influences of exogenously administered rhIGF-I. We speculate that this results from increased bioavailability of free IGF-I to react with muscle receptors. Reduced endogenous IGF-I levels in the DIA likely reflect a negative-feedback influence. These results may have important clinical implications for treatment options to offset the adverse effects of CS on the respiratory muscles in patients with chronic lung disorders.

Cortisol treatment affects glucocorticoid receptor and glucocorticoid-responsive genes in the liver of rainbow trout

We investigated whether longer-term cortisol exposure modified hepatic glucocorticoid receptor (GR) status and tissue responsiveness to cortisol stimulation in rainbow trout. Fish were given intraperitoneal implants of cortisol (50mg/kg body mass) and this led to elevated plasma cortisol levels mimicking chronically stressed salmonids. There was significantly higher hepatic GR mRNA abundance, despite a drop in GR protein content in the liver of cortisol-treated fish. The tissue responsiveness to cortisol stimulation was apparent from the higher plasma glucose concentration and liver glycogen content. Also, the higher phosphoenolpyruvate carboxykinase (PEPCK) mRNA abundance, a key glucocorticoid-responsive gene, by cortisol suggests activation of the GR signalling pathway. There was no significant effect of cortisol treatment on liver PEPCK, alanine aminotransferase, aspartate aminotransferase, and lactate dehydrogenase activities compared to the sham fish. The higher heat shock protein (hsp) 90 mRNA abundance and a corresponding elevation in this protein and constitutive hsp70 (hsc70) protein content in the cortisol-treated fish reflects a role for glucocorticoids in the hepatic stress response process. Taken together, the molecular and biochemical responses evident in the liver of trout imply changes favouring tissue responsiveness to glucocorticoids and may be a mechanism to offset GR protein downregulation evident with chronic cortisol stimulation in rainbow trout.

Autoregulation of glucocorticoid receptor by cortisol in rainbow trout hepatocytes

We used primary cultures of trout hepatocytes and a physiological dose of cortisol (100 ng/ml), mimicking stressed levels in salmonid fish, to address the impact of glucocorticoid stimulation on glucocorticoid receptor (GR) mRNA abundance and protein content. Cortisol significantly elevated GR mRNA content over a 24-h period; this increase was abolished by actinomycin D, suggesting transcriptional control of GR. However, cortisol significantly decreased GR protein content, leading us to hypothesize that lower GR protein content may be regulating GR mRNA abundance. Indeed, treatment of hepatocytes with MG-132, a proteasomal inhibitor shown to prevent GR degradation by cortisol, abolished cortisol-mediated GR mRNA upregulation. Also, geldanamycin, a heat shock protein 90-specific inhibitor, abolished the GR mRNA increase evident with cortisol but did not modify cortisol-induced increases in abundance of mRNA for phosphoenolpyruvate carboxykinase, a glucocorticoid-responsive gene, or hepatocyte glucose release. Together, our results suggest a negative feedback loop for GR gene regulation by cortisol in trout hepatocytes. The autoregulation of GR may be a crucial step in the physiological stress response process, especially in modulating energy-dependent processes that are glucocorticoid dependent, including gluconeogenesis.

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.

Adverse and beneficial functions of proteolytic enzymes in skeletal muscle. An overview

Proteolytic enzymes (proteases) comprise a family of enzymes which hydrolyse protein or peptide substrates in the generalised process of intracellular protein degradation, a process essential for the normal functioning of all cells. Proteases may also have a wide range of additional functions, including metabolic control of physiologically active oligopeptides or precursor protein forms, antigen presentation/recognition by the major histocompatibility complex in the cellular immune response, as well as in digestion, blood clotting, complement activation, etc. In this article, the nomenclature and classification of proteolytic enzymes in skeletal muscle, and their role in normal muscle physiological processes have been reviewed, including exercise, muscle development and ageing. Although proteases play an important role in normal muscle functioning, in pathological situations the enzymes may themselves be regarded as 'toxic agents' in terms of their damaging effects on muscle tissue. Muscle damage resulting from inappropriate activity of proteolytic enzymes in muscle wasting associated with muscular dystrophies, denervation atrophy, inflammatory myopathies, cancer, sepsis, diabetes and alcoholism have been reviewed. In addition, evidence that the adverse effects of drugs known to induce muscle wasting, such as corticosteroids, (or beneficial effects of growth promoting drugs) may be mediated via proteolytic enzymes is also reviewed.

Divergent regulation of proteasomes by insulin-like growth factor I and growth hormone in skeletal muscle of rats made catabolic with dexamethasone

Insulin-like growth factor I (IGF-I) and growth hormone (GH) exert their anabolic actions by increasing protein synthesis, but only IGF-I has been reported to impede protein breakdown. Using a model of myofibrillar catabolism produced by dexamethasone (Dex) we have reported that IGF-I down-regulates Dex-induced mRNAs for Ubiquitin (Ub) and Ub-conjugating enzymes (E2) in skeletal muscle, whereas GH had no significant effect. In the present study, we used the same model to determine whether IGF-I (0.35 mg/100 g BW) and/or GH (0.3 mg/100 g BW) have effects on proteasome subunit mRNAs in skeletal muscles of rats treated with Dex (0.5 mg/100 g BW) for 3 days. Dex caused significant increases in C-2, -3, and -8 proteasome subunit mRNAs (6.0-, 4.0-, and 6.6-fold increases, respectively). Injections of IGF-I in Dex-treated animals caused significant suppression of transcripts for C-2, -3, and -8 (32%, 42%, and 40%, respectively). GH restored the serum IGF-I levels in Dex treated animals, but caused further increases in proteasome subunit mRNAs (C-2, 35%; C-3, 34.5%; C-8, 33%; C-6, 42%; C-5, 32%; C-9, 37%). Administration of IGF-I in the Dex/GH-treated animals decreased the mRNAs of proteasome subunits in a manner and degree similar to those observed in the Dex/IGF-I group. Surprisingly, injection of GH alone in normal animals increased proteasome subunit mRNAs in skeletal muscle (C-2, 85%; C-3, 109%; C-8, 91%). This effect of GH on proteasome subunit mRNAs was also observed in liver. These findings suggest, therefore, that suppression of Dex-induced expression of proteasome subunit mRNAs in skeletal muscle is one of the mechanisms by which IGF-I exerts its antiproteolytic activity in catabolic states. On the other hand, the biological function of GH in regulating proteasome subunits needs further investigation.

Dietary supplementation with creatine monohydrate prevents corticosteroid-induced attenuation of growth in young rats

Corticosteroids are used as chemotherapeutic agents in many medical conditions, despite many common and potentially serious side effects. Supplementation with creatine monohydrate (CrM) can increase strength and lean body mass in humans and, therefore, may be a viable countermeasure to the side effects of corticosteroids. Therefore, the purpose of this study was to determine if CrM could prevent the attenuation of growth associated with corticosteroid administration. Forty male Sprague-Dawley rats were randomized to the following groups: control (CON, n = 10), 7 mg methylprednisolone x kg(-1) x week(-1) (PRED, n = 10), 2% CrM in diet (CD, n = 10), or CrM and methylprednisolone (CD-PRED, n = 10). Animals received either a weekly sham injection (saline; CON and CD) or an injection of methylprednisolone (PRED and CD-PRED) for 6 weeks. At the completion of the 6th week, body composition was determined and skeletal muscles were collected. Weight gain was attenuated in PRED as compared with all other groups (P < 0.05). Muscle total creatine and phosphocreatine were greater in the extensor digitorum longus in the CD and CD-PRED groups as compared with the CON and PRED groups (P < 0.05); however, total creatine and phosphocreatine in the soleus were not different. Mean fiber area was greater in type II fibers from the extensor digitorum longus in the CD and CD-PRED groups as compared with the CON and PRED groups (P < 0.05); no treatment effect was seen in the soleus. In conclusion, CrM supplementation prevented the attenuation of growth associated with corticosteroids and also increased type II muscle fiber area. These results could have important clinical implications for several patient populations commonly treated with corticosteroids, and further work is required to determine the specific mechanisms underlying the physiological effects that were observed.

Neurohormonal factors in the development of catabolic/anabolic imbalance and cachexia

Mechanisms that lead to cachexia are still poorly understood. The neurohormonal changes that occur in severe disease states may cause an imbalance between protein synthesis and degradation at the cellular level, followed by muscle wasting. Here, we review actions of angiotensin II, TNF-alpha, corticosteroids, insulin-like growth factor-I (IGF-I), and the IGF binding proteins, factors that may each contribute to the metabolic imbalance. The complex endocrine, autocrine and intracellular interactions between these factors will be described with examples from patient, rat and cell culture studies. Moreover, some of the data supporting that each of these hormones may directly affect cellular protein degradation mechanisms will be reviewed. Knowledge on these regulatory mechanisms will facilitate the development of new pharmaceutical strategies to treat cachexia.

Ubiquitin-conjugating enzyme E214k/HR6B is dispensable for increased protein catabolism in muscle of fasted mice

Activated skeletal muscle proteolysis in catabolic states has been linked to an upregulation of the ATP-ubiquitin-dependent proteolytic system. Previous studies suggested that the N-end rule pathway is primarily responsible for the bulk of skeletal muscle proteolysis. The activity of this pathway is dependent on the 14-kDa ubiquitin-conjugating enzyme E2(14k) (HR6B) and the ubiquitin protein ligase Ubr1. To address the requirement of E2(14k) in muscle proteolysis, we examined muscle protein metabolism in wild-type (WT) mice and mice lacking the E2(14k) gene (KO) in fed and fasted (48 h) states. Baseline body weight, muscle mass, and protein content were similar, and these parameters decreased similarly upon fasting in the two genotypes. There were also no effects of genotype on the rate of proteolysis in soleus muscle. The fasting-induced increase in the amount of ubiquitinated proteins was the same in WT and KO mice. The absence of any significant effect of loss of E2(14k) function was not due to a compensatory induction of the closely related isoform HR6A. Total intracellular concentration of E2(14k) and HR6A in the WT mice was 290 +/- 40 nM, but the level in the KO mice (reflecting the level of HR6A) was 110 +/- 9 nM. This value is about threefold the apparent Michaelis-Menten constant (K(m)) of E2(14k) (approximately 40 nM) for stimulating conjugation in muscle extracts. Because the HR6A isoform has a K(m) of 16 nM for stimulating conjugation, the HR6A levels in the muscles of KO mice appear sufficient for supporting conjugation mediated by this pathway during fasting.

Gut hormone PYY(3-36) physiologically inhibits food intake

Food intake is regulated by the hypothalamus, including the melanocortin and neuropeptide Y (NPY) systems in the arcuate nucleus. The NPY Y2 receptor (Y2R), a putative inhibitory presynaptic receptor, is highly expressed on NPY neurons in the arcuate nucleus, which is accessible to peripheral hormones. Peptide YY(3-36) (PYY(3-36)), a Y2R agonist, is released from the gastrointestinal tract postprandially in proportion to the calorie content of a meal. Here we show that peripheral injection of PYY(3-36) in rats inhibits food intake and reduces weight gain. PYY(3-36) also inhibits food intake in mice but not in Y2r-null mice, which suggests that the anorectic effect requires the Y2R. Peripheral administration of PYY(3-36) increases c-Fos immunoreactivity in the arcuate nucleus and decreases hypothalamic Npy messenger RNA. Intra-arcuate injection of PYY(3-36) inhibits food intake. PYY(3-36) also inhibits electrical activity of NPY nerve terminals, thus activating adjacent pro-opiomelanocortin (POMC) neurons. In humans, infusion of normal postprandial concentrations of PYY(3-36) significantly decreases appetite and reduces food intake by 33% over 24 h. Thus, postprandial elevation of PYY(3-36) may act through the arcuate nucleus Y2R to inhibit feeding in a gut-hypothalamic pathway.

Effects of high doses of glucocorticoids on free amino acids, ribosomes and protein turnover in human muscle

BACKGROUND

Treatment with glucocorticosteroids causes a negative nitrogen balance, but the kinetic mechanisms responsible for this catabolic effect are controversial. We investigated the effects of 60 mg day(-1) prednisolone on protein synthesis and degradation in human skeletal muscle.

MATERIALS AND METHODS

Healthy adults (n = 9) were studied in the postabsorptive state, before and after 3 days of prednisolone treatment. The L-[ring 2,6(-3)H(5)]-phenylalanine tracer technique, concentration and size distribution of the ribosomes, mRNA content of the ubiquitin-proteasome pathway components in muscle, phenylalanine flux across the leg, and the free amino acid concentrations in skeletal muscle were used to study muscle protein metabolism.

RESULTS

The concentrations of most amino acids in arterial blood increased after prednisolone. There were also increased effluxes of phenylalanine, asparagine, arginine, alanine, methionine and isoleucine from the leg. The rate of protein degradation, as measured by the appearance rate (Ra) of phenylalanine, increased by 67% (P = 0.023) which, together with a doubling of the net release of phenylalanine from the leg (P = 0.007), indicated accelerated protein degradation. The pathway was not identified but there was no significant increase in mRNAs' encoding components of the ubiquitin-proteasome pathway. There was a 6% reduction in polyribosomes (P = 0.007), suggesting a decrease in the capacity for protein synthesis, although there was no measured decrease in the rate of protein synthesis.

CONCLUSIONS

These findings indicate that high doses of prednisolone lead to a sharp increase in net protein catabolism, which depends more on enhanced protein breakdown, and an uncertain effect on protein synthesis. The mechanisms stimulating proteolysis and the pathway stimulated to increase muscle protein degradation should be explored.

The ubiquitin-proteasome pathway regulates the availability of the GH receptor

GH promotes not only longitudinal growth in children but is active throughout life in protein, fat, and carbohydrate metabolism. The multiple actions of GH start when GH binds to the cell surface-expressed GH receptor. Effectiveness of the hormone depends both on its presence in the circulation and the availability of receptors at the cell surface of target cells. In this study, we examined the role of the ubiquitin-proteasome pathway in regulating GH receptor availability. We show that receptor turnover is rapid, and almost 3-fold prolonged in the internalization-deficient mutant GH receptor (F327A). Using a monovalent GH antagonist, B2036, we could quantify the internalization of the nonactivated receptor. By comparing internalization of the receptor with shedding of the GH-binding protein, we show that in Chinese hamster lung cell lines, internalization followed by lysosomal degradation is the major pathway for receptor degradation and that the ubiquitin-proteasome pathway controls this process. Inhibition of endocytosis resulted in a 200% increase in receptor availability at the cell surface at steady state.

Poly(adenosine 5'-diphosphate) ribose polymerase activation as a cause of metabolic dysfunction in critical illness

Poly(adenosine 5'-diphosphate) ribose polymerase is a nuclear enzyme activated in response to genotoxic stress induced by a variety of DNA damaging agents. Several oxygen and nitrogen-centered free radicals, notably peroxynitrite, are strong inducers of DNA damage and poly(adenosine 5'-diphosphate) ribose polymerase activation in vitro and in vivo. Activation of this nuclear enzyme depletes the intracellular stores of its substrate nicotinamide adenine dinucleotide, slowing the rate of glycolysis, mitochondrial electron transport and adenosine triphosphate formation. This process triggers a severe energetic crisis within the cell, leading to acute cell dysfunction and cell necrosis. Poly(adenosine 5'-diphosphate) ribose polymerase also plays an important role in the regulation of inflammatory cascades, through a functional association with various transcription factors and transcription co-activators. Recent works identified this enzyme as a critical mediator of cellular metabolic dysfunction, inflammatory injury, and organ damage in conditions associated with overwhelming oxidative stress, including systemic inflammation, circulatory shock, and ischemia-reperfusion. Accordingly, pharmacological inhibitors of poly(adenosine 5'-diphosphate) ribose polymerase protect against cell death and tissue injury in such conditions, and may therefore represent novel therapeutic tools to limit multiple organ damage and dysfunction in critically ill patients.

Molecular regulation of muscle cachexia: it may be more than the proteasome

Muscle cachexia induced by sepsis, severe injury, cancer, and a number of other catabolic conditions is mainly caused by increased protein degradation, in particular breakdown of myofibrillar proteins. Ubiquitin-proteasome-dependent proteolysis is the predominant mechanism of muscle protein loss in these conditions, but there is evidence that several other regulatory mechanisms may be important as well. Some of those mechanisms are reviewed in this article and they include pre-, para-, and postproteasomal mechanisms. Among preproteasomal mechanisms, mediators, receptor binding, signaling pathways, activation of transcription factors, and modification of proteins are important. Several paraproteasomal mechanisms may influence the trafficking of ubiquitinated proteins and their interaction with the proteasome, including the expression and activity of the COP9 signalosome, the carboxy terminus of heat shock protein 70-interacting protein (CHIP) and valosin-containing protein (VCP). Finally, because the proteasome does not degrade proteins completely into free amino acids but into peptides, postproteasomal degradation of peptides by the giant protease tripeptidyl peptidase II (TPP II) and various aminopeptidases is important in muscle catabolism. Thus, multiple mechanisms and regulatory steps may influence the breakdown of ubiquitinated muscle proteins by the 26S proteasome.

What do we really know about the ubiquitin-proteasome pathway in muscle atrophy?

Studies of many different rodent models of muscle wasting have indicated that accelerated proteolysis via the ubiquitin-proteasome pathway is the principal cause of muscle atrophy induced by fasting, cancer cachexia, metabolic acidosis, denervation, disuse, diabetes, sepsis, burns, hyperthyroidism and excess glucocorticoids. However, our understanding about how muscle proteins are degraded, and how the ubiquitin-proteasome pathway is activated in muscle under these conditions, is still very limited. The identities of the important ubiquitin-protein ligases in skeletal muscle, and the ways in which they recognize substrates are still largely unknown. Recent in-vitro studies have suggested that one set of ubquitination enzymes, E2(14K) and E3(alpha), which are responsible for the 'N-end rule' system of ubiquitination, plays an important role in muscle, especially in catabolic states. However, their functional significance in degrading different muscle proteins is still unclear. This review focuses on the many gaps in our understanding of the functioning of the ubiquitin-proteasome pathway in muscle atrophy, and highlights the strengths and limitations of the different experimental approaches used in such studies.

Genetic defects in acetylcholine signalling promote protein degradation in muscle cells of Caenorhabditis elegans

A myosin-lacZ fusion, expressed in 103 muscle cells of Caenorhabditis elegans, reports on how proteolysis in muscle is controlled by neural and intramuscular signals. Upon acute starvation, the fusion protein is degraded in the posterior 63 cells of the body-wall muscle, but remains stable in 32 anterior body-wall muscles and 8 vulval muscle cells. This distinction correlates with differences in the innervation of these cells. Reporter protein in the head and vulval muscles becomes labile upon genetic ‘denervation’ in mutants that have blocks in pre-synaptic synthesis or release of acetylcholine (ACh) or post-synaptic reception at nicotinic ACh receptors (nAChR), whereas protein in all 103 muscles is stabilized by the nicotinic agonist levamisole in the absence of ACh production. Levamisole does not stabilize muscle protein in nAChR mutants that are behaviorally resistant to levamisole. Neural inputs thus exert negative control over the proteolytic process in muscle by stimulating muscle nicotinic ACh receptors.

Hypoxic signal transduction in critical illness

Derangements in tissue perfusion occur during critical illness, and the resulting deficit in oxygen delivery may play an important role in the pathogenesis of hemorrhagic and septic shock. Cells and organisms have developed a variety of adaptive strategies to maintain adequate energy production to maintain normal cellular function under hypoxic conditions. Recent studies from our laboratory suggest that certain proinflammatory cytokines, which are likely to be elaborated during or after shock, can interfere with the ability of cells to adapt to hypoxia, and thereby contribute to the development of organ system dysfunction.

Induction of a catabolic state in rats by dexamethasone: dose or time dependency?

BACKGROUND

Daily injections of dexamethasone (DEX) given to adult rats are a recognized but nonstandardized model of stress. The aim of this work was to establish a reproducible and accurate model of stress in adult rats by chronic injection of DEX in order to standardize it. For this purpose, the effect of the duration of treatment and the effect of DEX dose were tested. To help understand the mechanisms of the catabolic effect of DEX, the study was extended to the metabolism of glutamine (GLN). In experiment 1, 60 male Sprague-Dawley rats (3 months old) were divided into 8 groups of 6 rats: groups G3, G5, G7, and G9 received 1.50 mg/kg/d of DEX by intraperitoneal (i.p.) injection for 3, 5, 7, or 9 days, respectively. Groups G3PF, G5PF, G7PF, or G9PF were pair-fed to groups G3, G5, G7, or G9, respectively. Group AL (n = 12) was healthy rats fed ad libitum.

RESULTS

In treated rats, nitrogen balance reached its lowest value at day 5. After 9 days treatment by DEX, the catabolic state was reduced. An increase in GLN-synthetase activity and a decrease in muscle GLN content were related to DEX per se not to DEX-induced anorexia. In experiment 2, 25 rats were divided into 5 groups of 5 animals. Groups G0.75, G1.50, and G2.50 received 0.75, 1.50, and 2.50 mg/kg/d, respectively, of DEX by i.p. injection for 5 days. Group PF was pair-fed to group G2.50 and group AL was control rats.

RESULTS

DEX induced a decrease in nitrogen balance that was dose-independent. GLN-synthetase activity was increased maximally in gastrocnemius by 0.75 mg/kg.

CONCLUSIONS

Five days of treatment by DEX and a dose of 0.75 mg/kg/d induced a marked catabolic state.

Stimulation of myofibrillar protein degradation and expression of mRNA encoding the ubiquitin-proteasome system in C(2)C(12) myotubes by dexamethasone: effect of the proteasome inhibitor MG-132

Addition of the synthetic glucocorticoid, dexamethasone (Dex) to serum-deprived C(2)C(12) myotubes elicited time- and concentration-dependent changes in N(tau)-methylhistidine (3-MH), a marker of myofibrillar protein degradation. Within 24 h, 100 nM Dex significantly decreased the cell content of 3-MH and increased release into the medium. Both of these responses had increased in magnitude by 48 h and then declined toward basal values by 72 h. The increase in the release of 3-MH closely paralleled its loss from the cell protein. Furthermore, Dex also decreased the 3-MH:total cell protein ratio, suggesting that myofibrillar proteins were being preferentially degraded. Incubation of myotubes with the peptide aldehyde, MG-132, an inhibitor of proteolysis by the (ATP)-ubiquitin (Ub)-dependent proteasome, prevented both the basal release of 3-MH (>95%) and the increased release of 3-MH into the medium in response to Dex (>95%). Northern hybridization studies demonstrated that Dex also elicited similar time- and concentration-dependent increases in the expression of mRNA encoding two components (14 kDa E(2) Ub-conjugating enzyme and Ub) of the ATP-Ub-dependent pathway. The data demonstrate that Dex stimulates preferential hydrolysis of myofibrillar proteins in C(2)C(12) myotubes and suggests that the ATP-Ub-dependent pathway is involved in this response.