Activation of caspase-3 is an initial step triggering accelerated muscle proteolysis in catabolic conditions

With trauma, sepsis, cancer, or uremia, animals or patients experience accelerated degradation of muscle protein in the ATP-ubiquitin-proteasome (Ub-P'some) system. The initial step in myofibrillar proteolysis is unknown because this proteolytic system does not break down actomyosin complexes or myofibrils, even though it degrades monomeric actin or myosin. Since cytokines or insulin resistance are common in catabolic states and will activate caspases, we examined whether caspase-3 would break down actomyosin. We found that recombinant caspase-3 cleaves actomyosin, producing a characteristic, approximately 14-kDa actin fragment and other proteins that are degraded by the Ub-P'some. In fact, limited actomyosin cleavage by caspase-3 yields a 125% increase in protein degradation by the Ub-P'some system. Serum deprivation of L6 muscle cells stimulates actin cleavage and proteolysis; insulin blocks these responses by a mechanism requiring PI3K. Cleaved actin fragments are present in muscles of rats with muscle atrophy from diabetes or chronic uremia. Accumulation of actin fragments and the rate of proteolysis in muscle stimulated by diabetes are suppressed by a caspase-3 inhibitor. Thus, in catabolic conditions, an initial step resulting in loss of muscle protein is activation of caspase-3, yielding proteins that are degraded by the Ub-P'some system. Therapeutic strategies could be designed to prevent these events.     

X-chromosome linked inhibitor of apoptosis protein inhibits muscle proteolysis in insulin-deficient mice

Loss of muscle protein is a serious complication of catabolic diseases and contributes substantially to patients' morbidity and mortality. This muscle loss is mediated largely by the activation of the ubiquitin-proteasome system; however, caspase-3 catalyzes an initial step in this process by cleaving actomyosin into small protein fragments that are rapidly degraded by the proteasome-dependent proteolytic pathway. We hypothesized that X-chromosome linked inhibitor of apoptosis protein (XIAP), an endogenous caspase-3 inhibitor, would block this first step in the cleavage of actomyosin that would make XIAP a candidate for treating muscle wasting. To determine if XIAP could attenuate muscle protein degradation, we used a recombinant lentivirus (Len-XIAP) encoding the full-length human XIAP cDNA to express XIAP in vivo. In muscle of streptozotocin-treated insulin-deficient mice, total muscle protein degradation, caspase-3 activity, and myofibril destruction were increased while XIAP was decreased. Overexpression of XIAP in these mice attenuated the excessive muscle protein degradation. Increased proteasome activity, caspase-3 activity and myofibril protein breakdown were all reduced. The ability of XIAP to prevent the loss of muscle protein suggests that XIAP could be a therapeutic reagent for muscle atrophy in catabolic diseases.     

Powerful activation of skeletal muscle actomyosin ATPase by goniodomin A is highly sensitive to troponin/tropomyosin complex

Goniodomin A has been shown to cause the conformational change of actin to modify actomyosin ATPase activity. Goniodomin A induced a potent stimulation of the actomyosin ATPase activities of the actin-myosin reconstituted system and natural actomyosin in the range of 10(-8) to 10(-7) M. When the concentration was increased above 10(-7) M, actomyosin ATPase activity was decreased. Interestingly, the troponin/tropomyosin complex caused a concentration-dependent inhibition of the goniodomin A-induced stimulation of actomyosin ATPase activity. In the presence of a high concentration of the troponin/tropomyosin complex, goniodomin A decreased actomyosin ATPase activity in a concentration-dependent manner. The enhancement of the ATPase activity of troponin/tropomyosin-free natural actomyosin by goniodomin A was larger than that obtained with natural actomyosin. Goniodomin A at lower concentrations enhanced the superprecipitation of natural actomyosin but decreased it at higher concentrations. The ATPase activity of skeletal muscle myofibrils and the contractile response of skinned fibers to Ca(2+) were never activated and were decreased by this compound, suggesting an inhibition by the troponin/tropomyosin complex. In the far ultraviolet circular dichroism, goniodomin A above 10(-8) M increased the negative ellipticity at 220 nm, suggesting an increase in the alpha-helical content of actin. These results suggest that goniodomin A increases and decreases actomyosin ATPase activity, probably through the stimulatory and inhibitory sites on actin, respectively. It is also suggested that the troponin/tropomyosin complex binds to actin to inhibit the goniodomin A-induced enhancement of actomyosin ATPase activity, probably by affecting the stimulatory site on the molecule.     

Inflammatory and protein metabolism signaling responses in human skeletal muscle after burn injury

Severe burn injuries lead to a prolonged hypercatabolic state resulting in dramatic loss of skeletal muscle mass. Postburn muscle loss is well documented but the molecular signaling cascade preceding atrophy is not. The purpose of this study is to determine the response to burn injury of signaling pathways driving muscle inflammation and protein metabolism. Muscle biopsies were collected in the early flow phase after burn injury from the vastus lateralis of a noninjured leg in patients with 20 to 60% TBSA burns and compared with uninjured, matched controls. Circulating levels of proinflammatory cytokines were also compared. Immunoblotting was performed to determine the protein levels of key signaling components for translation initiation, proteolysis, and tumor necrosis factor/nuclear factor kappa B (NFκB)and interleukin (IL)-6/STAT3 signaling. Burn subjects had significantly higher levels of circulating proinflammatory cytokines, with no difference in muscle STAT3 activity and lower NFκB activity. No differences were found in any translational signaling components. Regarding proteolytic signaling in burn, calpain-2 was 47% higher, calpastatin tended to be lower, and total ubiquitination was substantially higher. Surprisingly, a systemic proinflammatory response 3 to 10 days postburn did not lead to elevated muscle STAT3 or NFκB signaling. Signaling molecules governing translation initiation were unaffected, whereas indices of calcium-mediated proteolysis and ubiquitin-proteasome activity were upregulated. These novel findings are the first in humans to suggest that the net catabolic effect of burn injury in skeletal muscle (ie, atrophy) may be mediated, at least during the early flow phase, almost entirely by an increased proteolytic activity in the absence of suppressed protein synthesis signaling.     

Potent stimulation of myofilament force and adenosine triphosphatase activity of canine cardiac muscle through a direct enhancement of troponin C Ca++ binding by MCI-154, a novel cardiotonic agent

In the present study we have analyzed a likely biochemical mechanism underlying the Ca++-sensitizing action of MCI-154 (6-[4-(4'-pyridyl)aminophenyl)-4,5-dihydro-3(2H)-pyridazinone hydrochloride), a novel cardiotonic agent, on the contractile protein system. MCI-154 (10(-7) to 10(-4) M) enhanced the tension development induced by -log molar-free Ca++ concentration (pCa) 5.8 in chemically skinned fiber from the canine right ventricular muscle in a concentration-dependent manner. At pCa 7.0, MCI-154 (10(-7) to 10(-4) M) markedly increased adenosine triphosphatase (ATPase) activities of canine myofibrils and reconstituted actomyosin. In myofibrils and reconstituted actomyosin, MCI-154 (10(-7) to 10(-4) M) caused a parallel shift of the pCa-ATPase activity relation curve to the left without affecting the maximum activity, suggesting an increase in Ca++ sensitivity. MCI-154 (10(-8) to 10(-4) M) had little effect on actin-activated, Mg++, Ca++ and (K+, EDTA)-ATPase activities of myosin. Ca++ binding to cardiac myofibrils or purified cardiac troponin was increased by 10(-4) M MCI-154. These results suggest that MCI-154 enhances Ca++ binding to cardiac troponin C to elevate the Ca++ sensitivity of myofilaments and thus may cause a positive inotropic action in cardiac muscle. MCI-154 may provide a valuable tool for studying the molecular mechanism by which Ca++ regulates the contractile system.     

Endogenous glucocorticoids and impaired insulin signaling are both required to stimulate muscle wasting under pathophysiological conditions in mice

Muscle wasting is associated with a number of pathophysiologic conditions, including metabolic acidosis, diabetes, sepsis, and high angiotensin II levels. Under these conditions, activation of muscle protein degradation requires endogenous glucocorticoids. As the mechanism(s) underlying this dependence on glucocorticoids have not been identified, we analyzed the effects of glucocorticoids on muscle wasting in a mouse model of acute diabetes. Adrenalectomized, acutely diabetic mice given a physiologic dose of glucocorticoids exhibited decreased IRS-1–associated PI3K activity in muscle and progressive muscle atrophy. These responses were related to increased association of PI3K with the glucocorticoid receptor (GR). In mice with muscle-specific GR deletion (referred to as MGRKO mice), acute diabetes minimally suppressed IRS-1–associated PI3K activity in muscle and did not cause muscle atrophy. However, when a physiologic dose of glucocorticoids was given to mice with muscle-specific IR deletion, muscle protein degradation was accelerated. Fluorescence resonance energy transfer and an in vitro competition assay revealed that activated GRs competed for PI3K, reducing its association with IRS-1. Reexpression of WT GRs or those with a mutation in the nuclear localization signal in the muscle of MGRKO mice indicated that competition for PI3K was a prominent mechanism underlying reduced IRS-1–associated PI3K activity. This nongenomic influence of the GR contributes to activation of muscle protein degradation. We therefore conclude that stimulation of muscle proteolysis requires 2 events, increased glucocorticoid levels and impaired insulin signaling.     

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.     

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.     

Sensitivity and protein turnover response to glucocorticoids are different in skeletal muscle from adult and old rats. Lack of regulation of the ubiquitin-proteasome proteolytic pathway in aging.

We studied glucocorticoid-induced muscle wasting and subsequent recovery in adult (7-mo-old) and old (22-mo-old) rats, since the increased incidence of various disease states may result in glucocorticoids hypersecretion in aging. Adult and old rats received dexamethasone in their drinking water and were then allowed to recover. Muscle wasting occurred more rapidly in old rats and the recovery of muscle mass was impaired, suggesting that glucocorticoids may be involved in the emergence of muscle atrophy with advancing age. According to measurements in incubated epitrochlearis muscles, dexamethasone-induced muscle wasting mainly resulted from increased protein breakdown in the adult, but from depressed protein synthesis in the aged animal. Increased expression of cathepsin D, m-calpain, and ubiquitin was observed in the muscles from both dexamethasone-treated adult and old rats. By contrast, the disappearance of the stimulatory effect of glucocorticoids on protein break-down in aging occurred along with a loss of ability of steroids to enhance the expression of the 14-kD ubiquitin carrier protein E2, which is involved in protein substrates ubiquitinylation, and of subunits of the 20 S proteasome (the proteolytic core of the 26 S proteasome that degrades ubiquitin conjugates). Thus, if glucocorticoids play any role in the progressive muscle atrophy seen in aging, this is unlikely to result from an activation of the ubiquitin-proteasome proteolytic pathway.     

Effects of the calcium antagonists perhexiline and cinnarizine on vascular and cardiac contractile protein function

The weakly basic, lipophilic Ca++ antagonists perhexiline and cinnarizine have been compared with the calmodulin inhibitor W-7 and the cardiotonics Vardax and APP-201-533 for the ability to modulate Ca++-dependent contractile protein interactions directly, as well as Ca++-calmodulin-mediated myosin light chain phosphorylation, in arterial actomyosin or cardiac myofibrils. Both perhexiline and cinnarizine inhibited arterial myosin P-light chain phosphorylation and superprecipitation of arterial actomyosin over the concentration range of 10 to 200 microM. Concomitant inhibition of arterial superprecipitation and phosphorylation by perhexiline (IC50 = 33 microM) and cinnarizine (IC50 = 60 microM) was similar to W-7 (IC50 = 35 microM), and was characterized by a rightward shift in the pCa superprecipitation and pCa-light chain phosphorylation relationships, depressed maximum activity and attenuation by 2 microM exogenous calmodulin. However, whereas inhibition of superprecipitation and P-light chain phosphorylation by W-7 was equal at different Mg++ concentrations, relatively greater inhibition with perhexiline and less inhibition with cinnarizine was apparent as the free Mg++ concentration was lowered. In cardiac myofibrils prepared from both bovine and canine ventricles, perhexiline stimulated Mg-adenosine triphosphatase (ATPase) activity and cinnarizine was without effect, whereas W-7 significantly depressed ATPase activity. Perhexiline was 10-fold more potent and 3-fold more efficacious than either Vardax or APP-201-533 in canine cardiac myofibrils. Whereas APP-201-533 increased Ca++ sensitivity and maximum ATPase activity (Vmax), perhexiline increased Ca++ sensitivity, but not Vmax, and W-7 depressed both parameters.(ABSTRACT TRUNCATED AT 250 WORDS)     

Imaging skeletal muscle using second harmonic generation and coherent anti-Stokes Raman scattering microscopy

We describe experimental results on label free imaging of striated skeletal muscle using second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) microscopy. The complementarity of the SHG and CARS data makes it possible to clearly identify the main sarcomere sub-structures such as actin, myosin, acto-myosin, and the intact T-tubular system as it emanates from the sarcolemma. Owing to sub-micron spatial resolution and the high sensitivity of the CARS microscopy technique we were able to resolve individual myofibrils. In addition, key organelles such as mitochondria, cell nuclei and their structural constituents were observed revealing the entire structure of the muscle functional units. There is a noticeable difference in the CARS response of the muscle structure within actin, myosin and t-tubule areas with respect to laser polarization. We attribute this to a preferential alignment of the probed molecular bonds along certain directions. The combined CARS and SHG microscopy approach yields more extensive and complementary information and has a potential to become an indispensable method for live skeletal muscle characterization.     

Burn injury upregulates the activity and gene expression of the 20 S proteasome in rat skeletal muscle.

There is evidence that burn injury stimulates ubiquitin-proteasome-dependent protein breakdown in skeletal muscle. In this proteolytic pathway, protein substrates are conjugated to multiple molecules of ubiquitin, whereafter they are recognized, unfolded and degraded by the multicatalytic 26 S protease complex. The 20 S proteasome is the catalytic core of the 26 S protease complex. The influence of burn injury on the expression and activity of the 20 S proteasome has not been reported. We tested the hypothesis that burn injury increases 20 S proteasome activity and the expression of mRNA for the 20 S proteasome subunits RC3 and RC7. Proteolytic activity of isolated 20 S proteasomes, assessed as activity against fluorogenic peptide substrates, was increased in extensor digitorum longus muscles from burned rats. Northern-blot analysis revealed that the expression of mRNA for RC3 and RC7 was increased by 100% and 80% respectively following burn injury. Increased activity and expression of the 20 S proteasome in muscles from burned rats support the concept that burn-induced muscle cachexia is at least, in part, regulated by the ubiquitin-proteasome proteolytic pathway.     

Indomethacin preserves muscle mass and reduces levels of E3 ligases and TNF receptor type 1 in the gastrocnemius muscle of tumor-bearing mice

Tumor-induced skeletal muscle wasting involves tumor necrosis factor (TNF) and the ubiquitin-proteasome pathway of muscle protein degradation. In this study, growth of the colon-26 adenocarcinoma in mice was associated with diminished gastrocnemius muscle mass and increased muscle levels of actin, ubiquitin-conjugated proteins, free ubiquitin, E3 ubiquitin ligases, and the type 1 TNF receptor (TNFR1). Indomethacin at 1 or 5 mg/kg/day reduced tumor growth and muscle levels of TNFR1. However, only the 5 mg dose of indomethacin reduced muscle wasting and muscle levels of the E3 ligases and actin. These data suggest that the beneficial effects of indomethacin in the treatment of tumor-induced skeletal muscle wasting may involve inhibition of TNF- and ubiquitin-mediated pathways of muscle protein degradation. These data also demonstrate that E3 ligases, which are involved in disuse atrophy, also are associated with tumor-induced skeletal muscle wasting.     

Zelluläre Regulation des Anabolismus und Katabolismus der Skelettmuskulatur bei Immobilität, im Alter und bei kritisch Kranken

Skeletal muscle atrophy is associated with situations of acute and chronical illness, such as sepsis, surgery, trauma and immobility. Additionally, it is a common problem during the physiological process of aging. The myofibrillar proteins myosin and actin, which are essential for muscle contraction, are the major targets during the process of protein degradation. This leads to a general loss of muscle mass, muscle strength and to increased muscle fatigue. In critically ill or immobile patients skeletal muscle atrophy is accompanied by enhanced inflammation, reduced wound healing, weaning complications and difficulties in mobilisation. During aging it results in falls, fractures, physical injuries and loss of mobility. Relating to the primary stimulators - hormones, muscle lengthening, stress, inflammation, neuronal activity - research is now focusing on the investigation of the signal transduction pathways, which influence protein synthesis and protein degradation during skeletal muscle atrophy.     

Xestoquinone, a novel cardiotonic agent activates actomyosin ATPase to enhance contractility of skinned cardiac or skeletal muscle fibers

Xestoquinone (XQN), a novel cardiotonic principle from the sea sponge Xestospongia sapra, enhanced Ca+(+)-induced tension development of chemically skinned fibers from guinea pig cardiac muscle, even at both free Ca++ concentrations as low as -log molar free Ca++ (pCa) 9 to 8. In skinned fibers from guinea pig skeletal muscle, XQN (10 microM) also increased developed tension with a similar Ca++ dependence to that for cardiac fibers. In contrast to the unique Ca+(+)-dependence of XQN effects, the reference drug sulmazole enhanced Ca+(+)-induced tension development of skinned cardiac fibers at pCa 6.6 but did not affect it at pCa 8. In natural actomyosin from canine cardiac muscle, as well as in that from rabbit skeletal muscle, XQN (1-30 microM) enhanced the rate and extent of superprecipitation. Moreover, XQN produced a concentration-dependent increase in the myofibrillar ATPase activity of canine cardiac muscle, even at very low free Ca++ concentrations below the normal threshold for ATPase activation (pCa 9-8). The natural actomyosin ATPase activity of chicken smooth muscle was not influenced by XQN (up to 30 microM). In cardiac myofibrils, no significant difference was observed between the bound 45Ca+(+)-pCa relationship curves in the presence and absence of XQN (10 microM). Furthermore, XQN (30 microM) did not cause or potentiate Ca+(+)-induced Ca++ release from cardiac sarcoplasmic reticulum vesicles. These observations suggest that XQN directly activates actomyosin ATPase activity of cardiac and skeletal myofibrils, thus producing an enhanced superprecipitation activity as well as an increase in skinned fiber contractility.(ABSTRACT TRUNCATED AT 250 WORDS)     

Potent inhibition of arterial smooth muscle tonic contractions by the selective myosin II inhibitor, blebbistatin

Blebbistatin is reported to be a selective and specific small molecule inhibitor of the myosin II isoforms expressed by striated muscles and nonmuscle (IC(50) = 0.5-5 microM) but is a poor inhibitor of purified turkey smooth muscle myosin II (IC(50) approximately 80 microM). We found that blebbistatin potently (IC(50) approximately 3 microM) inhibited the actomyosin ATPase activities of expressed "slow" [smooth muscle myosin IIA (SMA)] and "fast" [smooth muscle myosin IIB (SMB)] smooth muscle myosin II heavy-chain isoforms. Blebbistatin also inhibited the KCl-induced tonic contractions produced by rabbit femoral and renal arteries that express primarily SMA and the weaker tonic contraction produced by the saphenous artery that expresses primarily SMB, with an equivalent potency comparable with that identified for nonmuscle myosin IIA (IC(50) approximately 5 microM). In femoral and saphenous arteries, blebbistatin had no effect on unloaded shortening velocity or the tonic increase in myosin light-chain phosphorylation produced by KCl but potently inhibited beta-escin permeabilized artery contracted with calcium at pCa 5, suggesting that cell signaling events upstream from KCl-induced activation of cross-bridges were unaffected by blebbistatin. It is noteworthy that KCl-induced contractions of chicken gizzard were less potently inhibited (IC(50) approximately 20 microM). Adult femoral, renal, and saphenous arteries did not express significant levels of nonmuscle myosin. These data together indicate that blebbistatin is a potent inhibitor of smooth muscle myosin II, supporting the hypothesis that the force-bearing structure responsible for tonic force maintenance in adult mammalian vascular smooth muscle is the cross-bridge formed from the blebbistatin-dependent interaction between actin and smooth muscle myosin II.     

Insulin-like growth factor-I inhibits lysosomal and proteasome-dependent proteolysis in skeletal muscle after burn injury

Previous studies suggest that insulin-like growth factor-I (IGF-I) inhibits burn-induced muscle wasting mainly by reducing muscle protein degradation. The intracellular mechanisms of this effect of IGF-I are not known. In the present study, we examined the influence of IGF-I on individual proteolytic pathways in muscles from burned rats. Extensor digitorum longus muscles from burned rats were incubated with specific blockers of lysosomal, calcium-calpain-dependent, and ubiquitin-proteasome-dependent proteolytic pathways in the absence or presence of IGF-I. In addition, cathepsin B and L activities and 20S proteasome activity were determined. IGF-I inhibited lysosomal and ubiquitin-proteasome-dependent protein breakdown in skeletal muscle from burned rats by 70 and 90%, respectively, but did not influence calcium-calpain-dependent protein breakdown. The hormone blocked the burn-induced increase in cathepsin B and L activities but did not reduce 20S proteasome activity. Results are important because they provide novel information about intracellular mechanisms by which IGF-I inhibits the catabolic response to burn injury in skeletal muscle.     

Proteolysis in illness-associated skeletal muscle atrophy: from pathways to networks

Improvements in health in the past decades have resulted in increased numbers of the elderly in both developed and developing regions of the world. Advances in therapy have also increased the prevalence of patients with chronic and degenerative diseases. Muscle wasting, a feature of most chronic diseases, is prominent in the elderly and contributes to both morbidity and mortality. A major research goal has been to identify the proteolytic system(s) that is responsible for the degradation of proteins that occurs in muscle atrophy. Findings over the past 20 years have clearly confirmed an important role of the ubiquitin proteasome system in mediating muscle proteolysis, particularly that of myofibrillar proteins. However, recent observations have provided evidence that autophagy, calpains and caspases also contribute to the turnover of muscle proteins in catabolic states, and furthermore, that these diverse proteolytic systems interact with each other at various levels. Importantly, a number of intracellular signaling pathways such as the IGF1/AKT, myostatin/Smad, PGC1, cytokine/NFκB, and AMPK pathways are now known to interact and can regulate some of these proteolytic systems in a coordinated manner. A number of loss of function studies have identified promising therapeutic approaches to the prevention and treatment of wasting. However, additional biomarkers and other approaches to improve early identification of patients who would benefit from such treatment need to be developed. The current data suggests a network of interacting proteolytic and signaling pathways in muscle. Future studies are needed to improve understanding of the nature and control of these interactions and how they work to preserve muscle function under various states of growth and atrophy.     

Cytoprotective effect of sodium orthovanadate on ischemia/reperfusion-induced injury in the rat heart involves Akt activation and inhibition of fodrin breakdown and apoptosis

In a rat model of myocardial ischemic infarction, sodium orthovanadate rescued cells from ischemia/reperfusion injuries. Rats underwent 30 min of myocardial ischemia by occluding the left coronary artery followed by 24 h of reperfusion. Post-treatment with orthovanadate reduced infarct size in a dose-dependent manner. Orthovanadate treatment also ameliorated contractile dysfunction of the left ventricle 72 h after reperfusion. The cytoprotective action of orthovanadate treatment was closely associated with inhibition of fodrin breakdown. Since orthovanadate is a potent inhibitor for protein tyrosine phosphatases, thereby activating tyrosine kinases and phosphatidylinositol 3-kinase (PI3K) pathways, we investigated activities of protein kinase B (Akt), a downstream target of PI3K in cardiomyocytes. Orthovanadate-induced cytoprotection was associated with partial restoration of reduced Akt activity following myocardial infarction. Restoration of Akt activity by orthovanadate treatment correlated positively with increased phosphorylation of glycogen synthase kinase-3beta and Bad in cardiomyocytes. Furthermore, orthovanadate treatment inhibited caspase-3 activation induced by ischemia. Taken together, orthovanadate post-treatment rescued cardiomyocytes from ischemia/reperfusion injuries via Akt activation and inhibition of fodrin breakdown, thereby inhibiting apoptosis.