Influence of sepsis in rats on muscle protein turnover in vivo and in tissue incubated under different in vitro conditions

Inhibition of glycogen synthase kinase 3[beta] activity with lithium in vitro attenuates sepsis-induced changes in muscle protein turnover

Loss of lean body mass is a characteristic feature of the septic response, and the mechanisms responsible for this decrease and means of prevention have not been fully elucidated. The present study tested the hypothesis that in vitro treatment of skeletal muscle with lithium chloride (LiCl), a glycogen synthase kinase (GSK) 3 inhibitor, would reverse both the sepsis-induced increase in muscle protein degradation and inhibition of protein synthesis. Sepsis decreased GSK-3[beta] phosphorylation and increased GSK-3[beta] activity, under basal conditions. Sepsis increased muscle protein degradation, with a concomitant increase in atrogin 1 and MuRF1 mRNA and 26S proteosome activity. Incubation of septic muscle with LiCl completely reversed the increased GSK-3[beta] activity and decreased proteolysis to basal nonseptic values, but only partially reduced proteosome activity and did not diminish atrogene expression. Lithium chloride also did not ameliorate the sepsis-induced increase in LC3-II, a marker for activated autophagy. In contrast, LiCl increased protein synthesis only in nonseptic control muscle. The inability of septic muscle to respond to LiCl was independent of its ability to reverse the sepsis-induced increase in eukaryotic initiation factor (eIF) 2B[varepsilon] phosphorylation, decreased eIF2B activity, or the reduced phosphorylation of FOXO3, but instead was more closely associated with the continued suppression of mTOR (mammalian target of rapamycin) kinase activity (e.g., reduced phosphorylation of 4E-BP1 and S6). These data suggest that in vitro lithium treatment, which inhibited GSK-3[beta] activity, (a) effectively reversed the sepsis-induced increase in proteolysis, but only in part by a reduction in the ubiquitin-proteosome pathway and not by a reduction in autophagy; and (b) was ineffective at reversing the sepsis-induced decrease in muscle protein synthesis. This lithium-resistant state seems mediated at the level of mTOR and not eIF2/eIF2B. Hence, use of GSK-3[beta] inhibitors in the treatment of sepsis may not be expected to fully correct the imbalance in muscle protein turnover.

Hyperglycemia contributes to cardiac dysfunction in a lipopolysaccharide-induced systemic inflammation model

OBJECTIVES

Hyperglycemia is frequently observed in nondiabetic patients during acute illness. Furthermore, intensive insulin therapy significantly reduces mortality and morbidity due to several critical illnesses, including cardiac or infectious diseases. The purpose of this study was to determine whether cardiac function is affected by hyperglycemia and its treatment with insulin.

DESIGN

Lipopolysaccharide (LPS) was administered intravenously to rats, with or without the administration of insulin with glucose.

SETTING

University Medical Center research laboratory.

SUBJECTS

Male Wistar rats.

INTERVENTIONS

In this study, we determined the effect of hyperglycemia and insulin therapy on cardiac function in an LPS-induced systemic inflammation model.

MEASUREMENTS AND MAIN RESULTS

Levels of serum cytokines, nitrate/nitrite, and high-mobility group box 1 protein after LPS treatment were measured in hyperglycemic rats and those treated with insulin. The following parameters were examined to assess cardiac function in Langendorff-perfused hearts: left ventricular developed pressure, left ventricular end-diastolic pressure, and left ventricular pressure development during isovolumetric contraction (+dP/dtmax) and relaxation (-dP/dtmin). We observed that levels of cytokines, nitrate/nitrite, and high-mobility group box 1 significantly increased. However, treatment of hyperglycemic rats with insulin was associated with significantly less severe disease as assessed by cytokine levels. Furthermore, hyperglycemia was associated with decreased +dP/dtmax and -dP/dtmin in Langendorff-perfused hearts of hyperglycemic rats, whereas insulin treatment improved these parameters.

CONCLUSIONS

Hyperglycemia was associated with the induction of various inflammatory mediators and an inhibition of cardiac function. Treatment of hyperglycemia with insulin protected against inflammation and cardiac dysfunction in a rat model of LPS-induced systemic inflammation. This improvement is likely because of the neutralization of deleterious effects associated with hyperglycemia and the specific actions of insulin on the inflammatory response.

Acute oral leucine administration stimulates protein synthesis during chronic sepsis through enhanced association of eukaryotic initiation factor 4G with eukaryotic initiation factor 4E in rats

Sepsis induces the loss of muscle proteins by impairing skeletal muscle protein synthesis through an inhibition of messenger RNA (mRNA) translation initiation. Amino acids and Leu (Leu) in particular stimulate mRNA translation initiation. The experiments were designed to test the effects of Leu on potential signal transduction pathways that may be important in accelerating mRNA translation initiation in skeletal muscle of rats with chronic (5-6 d) septic intra-abdominal abscess. Gastrocnemius from male Sprague Dawley rats gavaged with Leu or water were sampled 5-6 d following development of an intra-abdominal sterile or septic abscess. Gavage with Leu stimulated protein synthesis and enhanced the assembly of the active eukaryotic initiation factor (eIF)4G-eIF4E complex. Increased assembly of the active eIF4G-eIF4E complex was associated with a robust rise in phosphorylation of eIF4G(Ser(1108)) and a decreased assembly of inactive eIF4E binding protein-1 (4E-BP1)-eIF4E complex in both sterile inflammatory and septic rats. The reduced assembly of 4E-BP1-eIF4E complex was associated with an increase in phosphorylation of 4E-BP1 in the gamma-form following Leu gavage. Phosphorylation of 70-kDa ribosomal protein S6 kinase on Thr(389) was also increased following Leu gavage, as well as the phosphorylation of mammalian target of rapamycin on Ser(2448) or Ser(2481). In contrast, phosphorylation of protein kinase B (PKB) on Thr(308) or Ser(473) was not augmented following Leu gavage in septic rats. We conclude that Leu stimulates a PKB-independent signal pathway elevating the eIF4G-eIF4E complex assembly through increased phosphorylation of eIF4G and decreased association of 4E-BP1 with eIF4E in skeletal muscle during sepsis.

Hormone, cytokine, and nutritional regulation of sepsis-induced increases in atrogin-1 and MuRF1 in skeletal muscle

Various atrophic stimuli increase two muscle-specific E3 ligases, muscle RING finger 1 (MuRF1) and atrogin-1, and knockout mice for these "atrogenes" display resistance to denervation-induced atrophy. The present study determined whether increased atrogin-1 and MuRF1 mRNA are mediated by overproduction of endogenous glucocorticoids or inflammatory cytokines in adult rats and whether atrogene expression can be downregulated by anabolic agents such as insulin-like growth factor (IGF)-I and the nutrient-signaling amino acid leucine. Both atrogin-1 and MuRF1 mRNA in gastrocnemius was upregulated dose and time dependently by endotoxin. Additionally, peritonitis produced by cecal ligation and puncture increased atrogin-1 and MuRF1 mRNA in gastrocnemius (but not soleus or heart) by 8 h, which was sustained for 72 and 24 h, respectively. Whereas the sepsis-induced increase in atrogin-1 expression was completely prevented by IGF-I, the increased MuRF1 was not altered. In contrast to the IGF-I effect, the sepsis-induced increased mRNA of both atrogenes was unresponsive to either acute or repetitive administration of leucine. Whereas exogenous infusion of TNF-alpha increased atrogin-1 and MuRF1 in gastrocnemius, pretreatment of septic rats with the TNF antagonist TNF-binding protein did not prevent increased expression of either atrogene. Similarly, whereas dexamethasone increased atrogene expression, pretreatment with the glucocorticoid receptor antagonist RU-486 failed to ameliorate the sepsis-induced increase in atrogin-1 and MuRF1. Thus, under in vivo conditions in mature adult rats, the sepsis-induced increase in muscle atrogin-1 and MuRF1 mRNA appears both glucocorticoid and TNF independent and is unresponsive to leucine.

Free radical-mediated skeletal muscle dysfunction in inflammatory conditions

Loss of functional capacity of skeletal muscle is a major cause of morbidity in patients with a number of acute and chronic clinical disorders, including sepsis, chronic obstructive pulmonary disease, heart failure, uremia, and cancer. Weakness in these patients can manifest as either severe limb muscle weakness (even to the point of virtual paralysis), respiratory muscle weakness requiring mechanical ventilatory support, and/or some combination of these phenomena. While factors such as nutritional deficiency and disuse may contribute to the development of muscle weakness in these conditions, systemic inflammation may be the major factor producing skeletal muscle dysfunction in these disorders. Importantly, studies conducted over the past 15 years indicate that free radical species (superoxide, hydroxyl radicals, nitric oxide, peroxynitrite, and the free radical-derived product hydrogen peroxide) play an key role in modulating inflammation and/or infection-induced alterations in skeletal muscle function. Substantial evidence exists indicating that several free radical species can directly alter contractile protein function, and evidence suggests that free radicals also have important effects on sarcoplasmic reticulum function, on mitochondrial function, and on sarcolemmal integrity. Free radicals also modulate activation of several proteolytic pathways, including proteosomally mediated protein degradation and, at least theoretically, may also influence pathways of protein synthesis. As a result, free radicals appear to play an important role in regulating a number of downstream processes that collectively act to impair muscle function and lead to reductions in muscle strength and mass in inflammatory conditions.

Metabolic pathways implicated in the kinetic impairment of muscle glutamine homeostasis in adult and old glucocorticoid-treated rats

An impairment of muscle glutamine metabolism in response to dexamethasone (DEX) occurs with aging. To better characterize this alteration, we have investigated muscle glutamine release with regard to muscle glutamine production (net protein breakdown, de novo glutamine synthesis) in adult and old glucocorticoid-treated rats. Male Sprague-Dawley rats (3 or 24 mo old) were divided into seven groups: three groups received 1.5 mg/kg of DEX once a day by intraperitoneal injection for 3, 5, or 7 days; three groups were pair fed to the three treated groups, respectively; and one control group of healthy rats was fed ad libitum. Muscle glutamine synthetase activity increased earlier in old rats (day 3) than in adult rats (day 7), whereas an increase in muscle glutamine release occurred later in old rats (day 5) than in adult DEX-treated rats (day 3). Consequently, muscle glutamine concentration decreased later in old rats (day 5) than in adults (day 3). Finally, net muscle protein breakdown increased only in old DEX-treated rats (day 7). In conclusion, the impairment of muscle glutamine metabolism is due to a combination of an increase in glutamine production and a delayed increase in glutamine release.

Role of ubiquitin-proteasome pathway in skeletal muscle wasting in rats with endotoxemia

OBJECTIVE

To investigate the mechanism of muscle protein breakdown under endotoxemia condition.

DESIGN

Randomized, controlled, animal experiment in a hospital institute.

SETTING

Experimental laboratory.

INTERVENTION

Either saline or endotoxin (Escherichia coli O(55)B(5), 10 mg/kg) were administered into the peritoneal cavity in rats.

MEASUREMENTS AND MAIN RESULTS

The rate of total protein breakdown was increased by 29% and 61% in extensor digitorum longus muscle at 2 hrs and 6 hrs, whereas the myofibrillar proteolytic rate was increased by 155%, 222%, and 40% at 2 hrs, 6 hrs, and 12 hrs, respectively, in the endotoxin treatment group compared with that of the pair-fed normal control group. Meanwhile, compared with the normal control group, the level of 2.4-kilobase (kb) messenger RNA (mRNA) for ubiquitin in extensor digitorum longus muscle in rats was increased by 153% and 470% at 2 hrs and 6 hrs. There were 87% and 117% increases in 1.2-kb mRNA for E2-14K, and 89% and 168% increase in RC2 mRNA expression in extensor digitorum longus muscle in endotoxemic rats than normal control rats at 2 hrs and 6 hrs after injection of endotoxin peritoneally. The tumor necrosis factor-alpha and interleukin-6 concentrations in rat plasma progressively increased after endotoxin treatment, but tumor necrosis factor-alpha peaked at the 2-hr time point, whereas interleukin-6 peaked at 12 hrs. Endotoxin administration resulted in a marked increase in endotoxin level at 2 hrs and 6 hrs. No significant change was observed in soleus muscle after endotoxin injection. A significantly positive correlation was found between the net release of 3-methylhistidine and respective values of endotoxin, intensity of mRNA expression of 2.-kb ubiquitin, 1.2-kb E2-14K, and subunit RC2 in extensor digitorum longus muscle (r =.9882, .9731, .9653, .9814, p <.05). However, no significant correlation was seen between tumor necrosis factor-alpha or interleukin-6 and respective values of 3-methylhistidine, mRNA expression of 2.4-kb ubiquitin, 1.2-kb E2-14K, and subunit RC2 (r =.3580, .4521, .5277, .4931, p >.05; r =.3950, .1767, .2136, .2519, p >.05, respectively.) in soleus muscle.

CONCLUSIONS

Endotoxemia can induce enhancement of skeletal muscle protein breakdown, mainly involving myofibrillar protein and white, fast-twitch extensor digitorum longus muscle. Ubiquitin-proteasome proteolytic pathway plays an important and major role in skeletal muscle proteolysis. Endotoxin, tumor necrosis factor-alpha, and interleukin-6 can directly or indirectly regulate muscle protein breakdown.

The relationship between skeletal muscle proteolysis and ubiquitin-proteasome proteolytic pathway in burned rats

Negative nitrogen balance and accelerated muscle protein breakdown are characteristics of burn injury. The mechanism by which muscle proteolysis occurs may be activation of the ubiquitin-proteasome pathway, but needs to be further elucidated. The aim of this study was to gain more insight into the role of ubiquitin-proteasome pathway in muscle proteolysis, after burn injury in a rat burn injury model. The proteolytic rates and mRNA expression of ubiquitin, E2-14K, and subunit RC2 in extensor digital longus (EDL) and soleus (SOL) muscle were determined by amino acid analyzer and Northern blot, respectively. The results were as follows: the total and myofibrillar proteolytic rate of EDL muscle increased markedly, especially at 12 and 24h post-burn. The levels of 2.4kb mRNA for ubiquitin, 1.2kb mRNA for E2-14K (a rate-limiting and regulated enzyme for conjugation of ubiquitin with protein substrate) and mRNA for subunit RC2 (the largest subunit of 20S proteasome) predominantly increased in EDL muscle after the stimulation of burn injury. No significant changes in proteolytic rate and transcription of ubiquitin, E2-14K, and subunit RC2 in SOL muscle were observed. There was a significantly positive correlation between the proteolytic rate and the levels of 2.4kb mRNA for ubiquitin, 1.2kb mRNA for E2-14K, or mRNA for subunit RC2. The results indicated that muscle wasting after burn injury was mainly due to the accelerated breakdown of myofibrils, and EDL muscle was more sensitive to burn injury than SOL muscle. The activation of ubiquitin-proteasome pathway was one reason for the enhanced protein catabolism in skeletal muscle. This is the first demonstration of upregulated expression of E2-14K and subunit RC2 in muscle, in response to burn injury, and it provides a clue to reduce muscle wasting by specifically inhibiting the specific enzymes or subunits involved in the enhancement of the activity of ubiquitin-proteasome pathway after burn injury.

Expression and binding activity of the glucocorticoid receptor are upregulated in septic muscle

We examined the influence of sepsis, induced by cecal ligation and puncture in rats, on the protein and gene expression and hormone binding activity of the glucocorticoid receptor (GR) in skeletal muscle. Sepsis resulted in increased GR mRNA and protein levels and upregulated hormone binding activity in extensor digitorum longus and soleus muscles. Scatchard analysis suggested that the increased GR hormone binding activity reflected an increased number of hormone binding sites, whereas receptor affinity for glucocorticoids was unchanged. The GR antagonist RU-38486 blocked the sepsis-induced increase in GR expression and hormone binding activity, implicating a positive regulatory effect of glucocorticoids on GR expression and binding activity under the present experimental conditions. The results suggest that glucocorticoid-dependent metabolic changes in skeletal muscle during sepsis may reflect not only high circulating glucocorticoid levels but increased amounts and hormone binding activity of the GR as well.

Peripheral blood mononuclear cells exhibit hypercatabolic activity in response to thermal injury correlating with diminished MHC I expression

BACKGROUND

Muscle wasting is one of the major consequences of severe injury or infection. Although the mechanisms underlying this hypercatabolic state are not completely characterized, it was hypothesized that other cells in the body would be similarly affected. In particular, we sought to determine whether lymphoid cell populations experienced increased protein turnover after burn injury in a fashion analogous to that seen in skeletal muscle.

METHODS

BALB/c mice received either a 20% total body surface area burn or a control sham treatment. At days 1, 2, and 7 after treatment, skeletal muscle, peripheral blood, spleen, and lymph nodes were harvested from both groups. Protein synthesis and degradation rates were measured using 14C-phenylalanine incorporation and tyrosine release. Lymphocyte subpopulations (CD4 and CD8 T cells, macrophages, and B cells) and expression of major histocompatibility complex I (MHC I) molecules were assessed by flow cytometry.

RESULTS

The burn model used in this study resulted in increased skeletal muscle protein turnover in the first 2 days after injury. Protein synthetic and degradation rates of peripheral blood mononuclear cells (PBMNCs) in burned mice also demonstrated comparable changes, but persisted through day 7. Splenocytes showed similar hypercatabolic effects, whereas lymph node cells showed no change. Cell viability analysis confirmed that the observed alterations were not caused by cell death. MHC I expression was depressed in tandem with the increased catabolic rate in PBMNCs.

CONCLUSION

This study demonstrates that various lymphoid populations undergo protein catabolic changes similar to those characteristically observed in skeletal muscle, and these correlated with diminished MHC I expression. Moreover, PBMNCs exhibited prolonged sensitivity to burn injury, of a duration exceeding that observed in skeletal muscles or other lymphoid tissues.

Sepsis in mice stimulates muscle proteolysis in the absence of IL-6

We tested the role of interleukin-6 (IL-6) in sepsis-induced muscle proteolysis by determining ubiquitin mRNA levels and protein breakdown rates in incubated extensor digitorum longus muscles from septic and sham-operated IL-6 knockout and wild-type mice. In addition, the effect of treatment of mice with human recombinant IL-6 on muscle protein breakdown rates was determined. Finally, protein breakdown rates were measured in myotubes treated for up to 48 h with different concentrations of IL-6. Sepsis in wild-type mice resulted in an approximately ninefold increase in plasma IL-6 levels, whereas IL-6 was not detectable in plasma of sham-operated or septic IL-6 knockout mice. Total and myofibrillar muscle protein breakdown rates were increased by approximately 30% and threefold, respectively, in septic IL-6 wild-type mice with an almost identical response noted in septic IL-6 knockout mice. Ubiquitin mRNA levels determined by dot blot analysis were increased during sepsis in muscles from both IL-6 knockout and wild-type mice, although the increase was less pronounced in IL-6 knockout than in wild-type mice. Treatment of normal mice or of cultured L6 myotubes with IL-6 did not influence protein breakdown rates. The present results suggest that IL-6 does not regulate muscle proteolysis during sepsis.

Effects of alimentary whole proteins versus their small peptide hydrolysates on liver and skeletal muscle during the acute inflammation phase in the rat

Acute inflammation induces changes in liver proteins with an increase in synthesis of positive acute-phase proteins such as alpha1-acid glycoprotein (alpha1-AGP) and a decrease in synthesis of negative acute-phase proteins such as albumin. This is associated with muscle wasting, mediated by increased proteolysis and impaired protein synthesis. As protein metabolism can be altered in other situations (malnutrition, growth) by the form of the dietary nitrogen, we studied the effects of the molecular form of nitrogen on liver and skeletal muscle adaptation, looking at gene expression for two acute-phase proteins (albumin and alpha1-AGP) and a number of muscle proteins (alpha1-actin, ubiquitin and C9 proteasome subunit). Two groups of 24 Wistar rats (250 g) were injected S/C with 0.125 ml turpentine/rat and were fed one of two liquid diets. These diets had caloric, nitrogen, carbohydrate and lipid content but differed in the molecular form of the nitrogen source (whole protein [WP] versus peptide hydrolysate [PH]). Liver and muscle adaptation were studied at 18, 42 or 66 h after turpentine injection. Weight, deoxyribonucleic acid and protein content of the liver were significantly higher with the WP diet than with the PH diet at 42 h and 66 h. There was more alpha1-AGP messenger ribonucleic acid (mRNA) at 18 h and less albumin mRNA at 42 h. Thus, the PH diet causes a more rapid increase in alpha1-AGP mRNA content and a smaller decrease in albumin mRNA content after turpentine injection than the WP diet. However, the changes in plasma acute-phase proteins (albumin and alpha1-AGP) were similar with the two diets. In skeletal muscle, there was no change in mRNA levels for the C9 proteasome subunit at any time point with both diets compared to the controls. However, there were greater ubiquitin mRNA levels at 18|h and less alpha-actin mRNA levels at 18 h, 42 h and 66 h following turpentine injection in the two dietary groups than in the controls. These results suggest that the molecular form of nitrogen ingested regulates hepatic gene transcription or mRNA stability of acute-phase proteins, during the early period of inflammation, but did not affect the expression of muscle proteins, which was altered by turpentine injection. Post-transcriptional control of acute-phase protein genes may contribute to the maintenance of similar plasma levels.

The anabolic effects of IGF-1 in skeletal muscle after burn injury are not caused by increased cell volume

BACKGROUND

In a recent report, insulin-like growth factor 1 (IGF-1) stimulated protein synthesis and inhibited protein breakdown in skeletal muscle after bum injury. The mechanism of the anabolic effects of IGF-1 in skeletal muscle is not known. We tested the hypotheses that IGF-1 stimulates protein synthesis and inhibits protein breakdown in skeletal muscle secondary to cell swelling and that cell swelling in itself induces an anabolic response in muscle tissue.

METHODS

Extensor digitorum longus muscles from control and burned rats were incubated in the absence or presence of 1 microg/mL of IGF-1. Protein synthesis and breakdown rates were determined by measuring incorporation of 14C-phenylalanine into protein and net release of tyrosine, respectively. Cell volume was measured by determining wet and dry weight and by using 3H-mannitol as an extracellular marker.

RESULTS

IGF-1 stimulated protein synthesis and inhibited protein breakdown in muscles from nonburned and burned rats without influencing cell volume. Incubating muscles in hypo-osmotic medium increased cell volume by 17% and inhibited protein breakdown by 14% but did not influence protein synthesis.

CONCLUSIONS

The anabolic effects of IGF-1 in skeletal muscle are not caused by increased cell volume. The results differ from those reported previously in liver cells in which the anabolic effects of IGF-1 were associated with cell swelling. The role of changes in cell volume in the regulation of protein metabolism may be different in skeletal muscle than in other tissues.

IGF-I stimulates protein synthesis but does not inhibit protein breakdown in muscle from septic rats

Sepsis is associated with reduced protein synthesis and increased protein degradation in skeletal muscle. We examined the effects of insulin-like growth factor I (IGF-I) on protein synthesis and breakdown in muscles from nonseptic and septic rats. Sepsis was induced by cecal ligation and puncture; control rats were sham operated. Extensor digitorum longus muscles were incubated in the absence or presence of IGF-I at concentrations ranging from 100 ng/ml to 10 micrograms/ml. Total and myofibrillar protein breakdown rates were measured as net release of tyrosine and 3-methylhistidine, respectively. Protein synthesis was determined by measuring incorporation of [U-14C]phenylalanine into protein. IGF-I stimulated protein synthesis in a dose-dependent fashion in muscles from both sham-operated and septic rats, with a maximal effect seen at a hormone concentration between 500 and 1,000 ng/ml. IGF-I inhibited total and myofibrillar protein breakdown in muscles from sham-operated rats, whereas in muscles from septic rats, IGF-I had no effect on protein breakdown, even at high concentrations. The results suggest that protein breakdown in skeletal muscle becomes resistant to IGF-I during sepsis and that this resistance reflects a postreceptor defect.

Regulation of peptide-chain initiation in muscle during sepsis by interleukin-1 receptor antagonist

The mechanism by which interleukin-1 (IL-1) regulates protein synthesis in skeletal muscle during hypermetabolic sepsis in rats was investigated. Treatment of septic rats with a specific interleukin-1 receptor antagonist (IL-1ra) prevented the sepsis-induced inhibition of protein synthesis and translational efficiency in gastrocnemius. Analysis of ribosomal subunits revealed that the increase in free 40S and 60S ribosomal subunits observed in septic rats was prevented by infusion of IL-1ra, indicating peptide-chain initiation was maintained at control values. The failure of sepsis to inhibit peptide-chain initiation after infusion of IL-1ra correlated with a maintenance of the epsilon-subunit of eukaryotic initiation factor (eIF) 2B (eIF-2B epsilon) protein at control values. The alterations in the eIF-2B epsilon protein content in gastrocnemius of septic rats treated with or without IL-1ra were associated with corresponding changes in the abundance of eIF 2B epsilon mRNA. The results provide evidence that infusion of IL-1ra attenuates the sepsis-induced inhibition of protein synthesis by preventing the inhibition of peptide-chain initiation and downregulation of eIF-2B expression during sepsis.

The effect of endotoxin on skeletal muscle protein gene expression in the rat

Sepsis is associated with net breakdown of skeletal muscle protein, mediated partly by reduced rates of muscle protein synthesis. This study investigated the role of altered gene expression for specific muscle proteins in mediating reduced protein synthesis in a rat model of acute severe sepsis. Adult rats were given a single sublethal intraperitoneal dose of endotoxin (bacterial lipopolysaccharide). Protein, RNA and DNA contents of muscle were measured and changes in expression of mRNA in tibialis anterior and extensor digitorum longus muscles were detected by quantification of Northern blots at 6, 24, 48 and 72 hr after endotoxin and in animals starved for 24 hr. Results showed that at 24 hr after endotoxin there was a loss of about 14% of muscle protein content. No reduction in mRNA was found at any time point for beta-myosin heavy chain (MHC), fast-MHC, alpha-actin, skeletal muscle troponin or carbonic anhydrase III (CA III); rather, at 48 hr there was increased expression of beta-MHC (224 +/- 123% control) and CA III (202 +/- 56%). Blocking TNF-alpha by pre-treatment with a monoclonal antibody did not appear to influence this. Total RNA content of muscle was reduced to 67% of the control values 24 hr after LPS, although this was no different to pair-fed animals starved for 24 hr. It is concluded that reduced protein synthesis in skeletal muscle in early acute sepsis is not primarily associated with reduced muscle protein gene expression.

Muscle wasting in a rat model of long-lasting sepsis results from the activation of lysosomal, Ca2+ -activated, and ubiquitin-proteasome proteolytic pathways

We studied the alterations in skeletal muscle protein breakdown in long lasting sepsis using a rat model that reproduces a sustained and reversible catabolic state, as observed in humans. Rats were injected intravenously with live Escherichia coli; control rats were pair-fed to the intake of infected rats. Rats were studied in an acute septic phase (day 2 postinfection), in a chronic septic phase (day 6), and in a late septic phase (day 10). The importance of the lysosomal, Ca2+ -dependent, and ubiquitin-proteasome proteolytic processes was investigated using proteolytic inhibitors in incubated epitrochlearis muscles and by measuring mRNA levels for critical components of these pathways. Protein breakdown was elevated during the acute and chronic septic phases (when significant muscle wasting occurred) and returned to control values in the late septic phase (when wasting was stopped). A nonlysosomal and Ca2+ -independent process accounted for the enhanced proteolysis, and only mRNA levels for ubiquitin and subunits of the 20 S proteasome, the proteolytic core of the 26 S proteasome that degrades ubiquitin conjugates, paralleled the increased and decreased rates of proteolysis throughout. However, increased mRNA levels for the 14-kD ubiquitin conjugating enzyme E2, involved in substrate ubiquitylation, and for cathepsin B and m-calpain were observed in chronic sepsis. These data clearly support a major role for the ubiquitin-proteasome dependent proteolytic process during sepsis but also suggest that the activation of lysosomal and Ca2+ -dependent proteolysis may be important in the chronic phase.

Energy-ubiquitin-dependent muscle proteolysis during sepsis in rats is regulated by glucocorticoids

Recent studies suggest that sepsis-induced increase in muscle proteolysis mainly reflects energy-ubiquitin-dependent protein breakdown. We tested the hypothesis that glucocorticoids activate the energy-ubiquitin-dependent proteolytic pathway in skeletal muscle during sepsis. Rats underwent induction of sepsis by cecal ligation and puncture or were sham-operated and muscle protein breakdown rates were measured 16 h later. The glucocorticoid receptor antagonist RU 38486 or vehicle was administered to groups of septic and sham-operated rats. In other experiments, dexamethasone (2.5 or 10 mg/kg) was injected subcutaneously in normal rats. Total and myofibrillar proteolysis was determined in incubated extensor digitorum longus muscles as release of tyrosine and 3-methylhistidine, respectively. Energy-dependent proteolysis was determined in incubated muscles depleted of energy with 2-deoxyglucose and 2,4-dinitrophenol. Levels of muscle ubiquitin mRNA and free and conjugated ubiquitin were determined by Northern and Western blot, respectively. RU 38486 inhibited the sepsis-induced increase in total and myofibrillar energy-dependent protein breakdown rates and blunted the increase in ubiquitin mRNA levels and free ubiquitin. Some, but not all, sepsis-induced changes in ubiquitin protein conjugates were inhibited by RU 38486. Injection of dexamethasone in normal rats increased energy-dependent proteolysis and ubiquitin mRNA levels. The results suggest that glucocorticoids regulate the energy-ubiquitin-dependent proteolytic pathway in skeletal muscle during sepsis.

Modulation of skeletal muscle protein synthesis by amino acids and insulin during sepsis

Effects of different concentrations of insulin and amino acids on protein synthesis in skeletal muscle of young, fed septic rats were determined in the perfused rat hindlimb. Rates of protein synthesis in gastrocnemius were measured by incorporation of [3H]-phenylalanine into protein. Perfusion of hindlimb muscles from young, fed control rats with medium containing either insulin and a complete mixture of amino acids at plasma concentration (1x) or a mixture of amino acids at 10-fold (10x) plasma concentration resulted in an approximately twofold stimulation of the rate of protein synthesis. The effect of amino acids on protein synthesis was partly accounted for by elevated concentrations of branched-chain amino acids ([BCAA] leucine, isoleucine, and valine). In young, fed septic rats, the rate of protein synthesis in muscle perfused with buffer containing the normal concentration of amino acids was reduced 40% as compared with control levels (P < .05). In contrast to controls, addition of insulin (1,000 microU/mL) did not augment protein synthesis in muscle from young, fed septic rats perfused with the complete mixture of amino acids. Addition of insulin 10,000 microU/mL stimulated protein synthesis approximately 80% in gastrocnemius of septic rats (P < .05). However, the rate of protein synthesis remained less than that observed in young, fed control rats at similar insulin concentrations. Perfusion with medium containing 10x plasma amino acids stimulated protein synthesis approximately fourfold in young, fed septic rats as compared with control animals. In contrast to controls, BCAA at 10x plasma concentration did not augment protein synthesis in young, fed septic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Is muscle protein turnover regulated by intracellular glutamine during sepsis?

BACKGROUND

Low muscle glutamine levels during sepsis are associated with reduced protein synthesis and elevated protein breakdown, in particular myofibrillar protein breakdown. It is not known if this is a causal or coincidental relationship. We tested the hypothesis that muscle protein turnover rates are directly regulated by glutamine.

METHODS

Paired extensor digitorum longus muscles from nonseptic (sham-operated) and septic rats (16 hours after cecal ligation and puncture) were incubated in the absence or presence of 15 mmol glutamine/L. The effect of glutamine was tested in unsupplemented medium or in medium containing 1 mU/mL of insulin or a mixture of amino acids at normal plasma concentrations. Protein synthesis was measured as incorporation of 14C-phenylalanine into protein; total and myofibrillar protein breakdown was determined by measuring tyrosine and 3-methylhistidine, respectively.

RESULTS

Muscles accumulated intracellular glutamine well above normal concentrations in the presence of 15 mmol glutamine/L. In spite of this, protein synthesis was not affected by glutamine, neither when muscles were incubated in unsupplemented medium nor in medium containing insulin or amino acid mixture. Total protein breakdown was not influenced by glutamine when muscles were incubated in unsupplemented medium or with insulin but was reduced by glutamine in the presence of an amino acid mixture. Myofibrillar protein breakdown was unaffected by glutamine in unsupplemented medium and in medium containing insulin but was increased by glutamine in the presence of amino acid mixture.

CONCLUSION

Reduced muscle protein synthesis and increased myofibrillar protein breakdown during sepsis are probably not caused by the low intracellular glutamine levels noticed in this condition.

Amrinone prevents muscle protein wasting during chronic sepsis

The time course for the effects of sepsis on rates of protein synthesis, RNA contents, and translational efficiencies was measured in mixed muscles of rat hindlimb perfused in vitro 3, 5, and 10 days after induction of sepsis. Furthermore, the effect of daily injections of amrinone (5 mg.kg-1.day-1) on muscle protein synthesis was investigated. On day 3 of sepsis, decreased rates of protein synthesis in muscle from untreated septic animals or septic rats treated with amrinone resulted from a reduced food intake. When food intake became normalized to control after 5 days, rates of protein synthesis in untreated septic rats remained depressed. Treatment of septic animals with amrinone for 5 days prevented the sepsis-induced inhibition of protein synthesis by abolishing the inhibition of peptide-chain initiation and restoring translational efficiency to control values. In contrast, amrinone treatment of control rats for 5 days did not cause an accretion of muscle protein or augment protein synthesis. Ten days after induction of sepsis, there were no differences in rates of protein synthesis, RNA content, or translational efficiency in septic animals compared with control or amrinone-treated septic rats. Thus, amrinone prevented the sepsis-induced abnormalities in skeletal muscle protein synthesis.

Sepsis stimulates nonlysosomal, energy-dependent proteolysis and increases ubiquitin mRNA levels in rat skeletal muscle

We tested the role of different intracellular proteolytic pathways in sepsis-induced muscle proteolysis. Sepsis was induced in rats by cecal ligation and puncture; controls were sham operated. Total and myofibrillar proteolysis was determined in incubated extensor digitorum longus muscles as release of tyrosine and 3-methylhistidine, respectively. Lysosomal proteolysis was assessed by using the lysosomotropic agents NH4Cl, chloroquine, leupeptin, and methylamine. Ca(2+)-dependent proteolysis was determined in the absence or presence of Ca2+ or by blocking the Ca(2+)-dependent proteases calpain I and II. Energy-dependent proteolysis was determined in muscles depleted of ATP by 2-deoxyglucose and 2.4-dinitrophenol. Muscle ubiquitin mRNA and the concentrations of free and conjugated ubiquitin were determined by Northern and Western blots, respectively, to assess the role of the ATP-ubiquitin-dependent proteolytic pathway. Total and myofibrillar protein breakdown was increased during sepsis by 50 and 440%, respectively. Lysosomal and Ca(2+)-dependent proteolysis was similar in control and septic rats. In contrast, energy-dependent total and myofibrillar protein breakdown was increased by 172% and more than fourfold, respectively, in septic muscle. Ubiquitin mRNA was increased severalfold in septic muscle. The results suggest that the increase in muscle proteolysis during sepsis is due to an increase in nonlysosomal energy-dependent protein breakdown, which may involve the ubiquitin system.

Is the metabolic response to sepsis in skeletal muscle different in infants and adults? An experimental study in rats

In this study we compared the effect of sepsis on muscle protein metabolism in infant (3 to 4 weeks) and adult (3 to 4 months) rats. Sepsis was induced by cecal ligation and puncture (CLP). Control animals underwent sham operation. Sixteen hours after CLP or sham operation, metabolic studies were performed in incubated intact extensor digitorum longus muscles from infant rats or in strips of the same muscle from adult rats. Protein synthesis rate was determined as incorporation of 3H-phenylalanine into protein; total and myofibrillar protein breakdown rates were determined as release of tyrosine and 3-methylhistidine, respectively. Mortality rate following CLP was similar in both age groups. Basal protein synthesis rate was 3 times higher, total protein breakdown rate was 50% higher, and myofibrillar protein breakdown rate was 3 times higher in infant than in adult animals. However, the relative changes in protein turnover rates induced by sepsis were similar in infant and adult rats: protein synthesis rate decreased by approximately 30%, total protein breakdown increased by 40% to 50%, and myofibrillar protein breakdown increased severalfold. The data suggest that despite prominent differences in basal protein turnover rates between infant and adult rats, the effect of sepsis on muscle protein metabolism is not age dependent.

Muscle protein breakdown during endotoxemia in rats and after treatment with interleukin-1 receptor antagonist (IL-1ra)

The purpose of this study was to examine the effect of endotoxemia on muscle protein degradation and to test the hypothesis that muscle proteolysis during endotoxemia is regulated by interleukin-1 (IL-1). Both total and myofibrillar protein breakdown rates in incubated extensor digitorum longus muscles were increased after the subcutaneous injection of 0.1 or 1.0 mg/kg endotoxin in rats. The endotoxin-induced increase in muscle protein breakdown was blunted by IL-1 receptor antagonist, administered intraperitoneally at a total dose of 45 or 105 mg/kg. Results suggest that endotoxemia in rats gives rise to sepsislike changes in muscle protein breakdown. Increased muscle protein breakdown during endotoxemia may be regulated, at least in part, by IL-1.

Tissue metabolite levels in different types of skeletal muscle during sepsis

The effect of sepsis on energy and metabolite levels in the white, fast-twitch extensor digitorum longus (EDL) and the red, slow-twitch soleus (SOL) muscles was studied in rats. Sepsis was induced by cecal ligation and puncture (CLP). Control rats were sham-operated. Sixteen hours later, metabolite levels in muscle tissue were determined. Adenosine triphosphate (ATP) levels and energy charge were reduced during sepsis in SOL, but were unchanged in EDL muscles. In contrast, phosphocreatine (PCr) concentration was reduced during sepsis in EDL, but not in SOL. Tissue glycogen levels were reduced and lactate concentrations were increased in both muscles during sepsis. Results suggest that sepsis affects energy metabolism differently in different types of skeletal muscle. Tissue lactate accumulation may be consistent with muscle hypoperfusion following CLP, although other mechanisms may also be involved.