Importance of the ATP-ubiquitin-proteasome pathway in the degradation of soluble and myofibrillar proteins in rabbit muscle extracts

Expression of a calpastatin transgene slows muscle wasting and obviates changes in myosin isoform expression during murine muscle disuse

Muscle wasting is a prominent feature of several systemic diseases, neurological damage and muscle disuse. The contribution of calpain proteases to muscle wasting in any instance of muscle injury or disease has remained unknown because of the inability to specifically perturb calpain activity in vivo. We have generated a transgenic mouse with muscle-specific overexpression of calpastatin, which is the endogenous inhibitor of calpains, and induced muscle atrophy by unloading hindlimb musculature for 10 days. Expression of the transgene resulted in increases in calpastatin concentration in muscle by 30- to 50-fold, and eliminated all calpain activity that was detectable on zymograms. Muscle fibres in ambulatory, transgenic mice were smaller in diameter, but more numerous, so that muscle mass did not differ between transgenic and non-transgenic mice. This is consistent with the role of the calpain-calpastatin system in muscle cell fusion that has been observed in vitro. Overexpression of calpastatin reduced muscle atrophy by 30 % during the 10 day unloading period. In addition, calpastatin overexpression completely prevented the shift in myofibrillar myosin content from slow to fast isoforms, which normally occurs in muscle unloading. These findings indicate that therapeutics directed toward regulating the calpain-calpastatin system may be beneficial in preventing muscle mass loss in muscle injury and disease.

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

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

Myofibril-bound serine protease and its endogenous inhibitor in mouse: extraction, partial characterization and effect on myofibrils

The protein content of muscle is determined by the relative rates of synthesis and degradation. The balance between this process determines the number of functional contractile units within each muscle cell. Myofibril-bound protease, protease M previously reported in mouse skeletal muscle could be solubilized from the myofibrillar fraction by salt and acid treatment and partially purified by Mono Q and Superose 12 chromatography. Isolated protease M activity in vitro on whole myofibrils resulted in myosin, actin, troponin T, alpha-actinin and tropomyosin degradation. Protease M is serine type and was able to hydrolyze trypsin-type synthetic substrates but not those of chymotrypsin type. In gel filtration chromatography, protease M showed Mr 120.0 kDa. The endogenous inhibitor (MHPI) is a glycoprotein (110.0 kDa) that efficiently blocks the protease M-dependent proteolysis of myofibrillar proteins in a dose-dependent way, as shown by electrophoretic analysis and synthetic substrates assays. Protease M-Inhibitor system would be implicated in myofibrillar proteins turnover.

Increased muscle proteolysis after local trauma mainly reflects macrophage-associated lysosomal proteolysis

Rat gastrocnemius showed increased protein degradation (+75-115%) at 48 h after traumatic injury. Injured muscle showed increased cathepsin B activity (+327%) and mRNA encoding cathepsin B (+670%), cathepsin L (+298%), cathepsin H (+159%), and cathepsin C (+268%). In in situ hybridization, cathepsin B mRNA localized to the mononuclear cell infiltrate in injured muscle, and only background levels of hybridization were observed either over muscle cells in injured tissue or in uninjured muscle. Immunogold/electron microscopy showed specific staining for cathepsin B only in lysosome-like structures in cells of the mononuclear cell infiltrate in injured muscle. Muscle cells were uniformly negative in the immunocytochemistry. Matrix metalloproteinase-9 (granulocyte-macrophage gelatinase) mRNA and activity were not present in uninjured muscle but were expressed after trauma. There was no activation of the ATP-ubiquitin-proteasome-dependent proteolytic pathway in injured muscle, by contrast to diverse forms of muscle wasting where the activity of this system and the expression of genes encoding ubiquitin and proteasome elements rise. These results suggest that proteolytic systems of the muscle cells remain unstimulated after local injury and that lysosomal enzymes of the inflammatory infiltrated cells are likely to be the major participant in protein catabolism associated with local trauma.

Mechanisms activated by kidney disease and the loss of muscle mass

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

Construction and analysis of mouse strains lacking the ubiquitin ligase UBR1 (E3alpha) of the N-end rule pathway

The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. In the yeast Saccharomyces cerevisiae, the UBR1-encoded ubiquitin ligase (E3) of the N-end rule pathway mediates the targeting of substrate proteins in part through binding to their destabilizing N-terminal residues. The functions of the yeast N-end rule pathway include fidelity of chromosome segregation and the regulation of peptide import. Our previous work described the cloning of cDNA and a gene encoding the 200-kDa mouse UBR1 (E3alpha). Here we show that mouse UBR1, in the presence of a cognate mouse ubiquitin-conjugating (E2) enzyme, can rescue the N-end rule pathway in ubr1Delta S. cerevisiae. We also constructed UBR1(-/-) mouse strains that lacked the UBR1 protein. UBR1(-/-) mice were viable and fertile but weighed significantly less than congenic +/+ mice. The decreased mass of UBR1(-/-) mice stemmed at least in part from smaller amounts of the skeletal muscle and adipose tissues. The skeletal muscle of UBR1(-/-) mice apparently lacked the N-end rule pathway and exhibited abnormal regulation of fatty acid synthase upon starvation. By contrast, and despite the absence of the UBR1 protein, UBR1(-/-) fibroblasts contained the N-end rule pathway. Thus, UBR1(-/-) mice are mosaics in regard to the activity of this pathway, owing to differential expression of proteins that can substitute for the ubiquitin ligase UBR1 (E3alpha). We consider these UBR1-like proteins and discuss the functions of the mammalian N-end rule pathway.

Fiber type-specific expression of major proteolytic systems in fast- to slow-transforming rabbit muscle

The present study investigates the role of two major proteolytic systems in transforming rabbit and rat muscles. The fast-to-slow transformation of rabbit muscle by chronic low-frequency stimulation (CLFS) induces fast-to-slow transitions of intact, mature fibers and replacement of degenerating fibers by newly formed slow fibers. Ubiquitination, an indicator of the ATP-dependent proteasome system, and calpain activity were measured in homogenates of control and stimulated extensor digitorum longus muscles. Calpain activity increased similarly (approximately 2-fold) in stimulated rat and rabbit muscles. CLFS had no effect on protein ubiquitination in rat muscle but led to elevations in ubiquitin protein conjugates in rabbit muscle. Immunohistochemistry was used to study the distribution of micro-calpain and m-calpain and of ubiquitinated proteins in myosin heavy chain-based fiber types. The findings suggest that both proteolytic systems are involved in fiber transformation and replacement. Transforming mature fibers displayed increases in micro-calpain and accumulation of ubiquitin protein conjugates. The majority of these fibers were identified as type IIA. Enhanced ubiquitination was also observed in degenerating and necrotic fibers. Such fibers additionally displayed elevated m-calpain levels. Conversely, p94, the skeletal muscle-specific calpain, decayed rapidly after stimulation onset and was hardly detectable after 4 days of CLFS.

Muscle cachexia: current concepts of intracellular mechanisms and molecular regulation

OBJECTIVE

To review present knowledge of intracellular mechanisms and molecular regulation of muscle cachexia.

SUMMARY BACKGROUND DATA

Muscle cachexia, mainly reflecting degradation of myofibrillar proteins, is an important clinical feature in patients with severe injury, sepsis, and cancer. The catabolic response in skeletal muscle may result in muscle wasting and weakness, delaying or preventing ambulation and rehabilitation in these patients and increasing the risk for pulmonary complications.

RESULTS

Muscle cachexia, induced by severe injury, sepsis, and cancer, is associated with increased gene expression and activity of the calcium/calpain- and ubiquitin/proteasome-proteolytic pathways. Calcium/calpain-regulated release of myofilaments from the sarcomere is an early, and perhaps rate-limiting, component of the catabolic response in muscle. Released myofilaments are ubiquitinated in the N-end rule pathway, regulated by the ubiquitin-conjugating enzyme E2(14k) and the ubiquitin ligase E3 alpha, and degraded by the 26S proteasome.

CONCLUSIONS

An understanding of the mechanisms regulating muscle protein breakdown is important for the development of therapeutic strategies aimed at treating or preventing muscle cachexia in patients with severe injury, sepsis, cancer, and perhaps other catabolic conditions as well.

Effects of corticosterone on connectin content and protein breakdown in rat skeletal muscle

We examined the effects of a glucocorticoid, corticosterone, on calpain activity, connectin content and protein breakdown in rat muscle. The results indicated that calpain activity was increased by corticosterone and thus breakdown of connectin was stimulated followed by increased breakdown of skeletal muscle protein.

Identification of a myofibril-bound serine protease and its endogenous inhibitor in mouse skeletal muscle

Myofibrillar proteins, like all other intracellular proteins, are in a dynamic state of continual degradation and resynthesis. The proteolytic system responsible for degrading myofibrillar proteins in skeletal muscle is not well defined. A proteolytic activity associated to myofibrils was found in mouse skeletal muscle, as show electrophoretic patterns, and denominated by us, as protease M. During incubation of whole myofibrils at 37 degrees C, myosin heavy chain, alpha actinin, actin and troponin T suffered degradation. These effects were inhibited selectively by serine protease inhibitors (soybean trypsin inhibitor, di-isopropyl phosphofluoridate, phenylmethanesulfonyl fluoride). Using myofibrils as protease M source, azocaseinolytic activity was also detected. Endogenous inhibitor and various compounds effects on protease M activity were also quantified by trichloroacetic acid soluble products formation, using radiolabeled myofibrils. An endogenous trypsin inhibitor isolated from the muscle cytoplasmic fraction could inhibit protease M activity on myofibrillar proteins and on azocasein. While K(+) increased protease M activity, the presence of Ca(2+) did not show any effect. Data presented in this study suggest that reported protease M may be implicated in myofibrillar degradation in vivo and isolated endogenous inhibitor may provide a mechanism to control its action in mouse skeletal muscle.

Skeletal muscle apoptosis after burns is associated with activation of proapoptotic signals

Critical illness is associated with muscle wasting and muscle weakness. Using burn injury as a model of local and systemic inflammatory response, we tested the hypothesis that thermal injury causes apoptosis in muscle. After a 40% body surface area burn to rats, abdominal muscles beneath the burn and limb muscles distant from the burn were examined for apoptosis at varying times after burn. Ladder assay, ELISA, and histological methods showed evidence of apoptosis in the abdominal muscles within 4-12 h with peak changes occurring at 3-7 days. Maximal apoptosis was also evident at distant limb muscles at 3-7 days. Investigation of proapoptotic pathways indicated mitochondrial membrane potential to be altered by 1 h after burn. Starting at 15 min after burn, cytochrome c was released from the mitochondria into the cytosol, followed by increased activity of caspase-3, starting at 6 h after burn. These studies suggest that mitochondria and caspase-mediated apoptotic pathways may be an additional mechanism of muscle weight loss in burns and may be potential therapeutic targets for prevention of muscle wasting.

The ubiquitin-proteasome proteolytic pathway in heart vs skeletal muscle: effects of acute diabetes

The ubiquitin-proteasome system is thought to play a major role in normal muscle protein turnover and to contribute to diabetes-induced protein wasting in skeletal muscle. However, its importance in cardiac muscle is not clear. We measured heart muscle mRNA for ubiquitin and for the C2 and C8 proteasomal subunits, the amount of free ubiquitin and the proteasome chymotrypsin-like proteolytic activity in control and diabetic rats. Results were compared to those in skeletal muscle (rectus). Heart ubiquitin, C2 and C8 subunit mRNA and proteolytic activity were significantly greater than in skeletal muscle (P </= 0.05). This suggests that the ubiquitin proteasomal pathway may also be important for normal heart muscle turnover. Diabetes increased ubiquitin mRNA by approximately 50% in heart (P < 0.03) and by approximately 100% in skeletal muscle (P < 0.005). It remained high after 3 days of insulin treatment in both tissues. C2 and C8 subunit mRNA did not change with diabetes or insulin treatment. Diabetes did not change the amount of free ubiquitin or the proteasomal (lactacystin-inhibitable) chymotrypsin-like peptidase activity in heart or skeletal muscle. In conclusions, gene expression for several components of the ubiquitin-proteasome proteolytic pathway is significantly higher in cardiac than in skeletal muscle, as is the proteasome chymotrypsin-like peptidase activity. Diabetes increases the expression of ubiquitin but not C2 or C8 subunit mRNA, nor does it significantly alter the amount of free ubiquitin or the proteasome chymotrypsin-like peptidase activity. The rate-limiting step of enhanced protein degradation in diabetic rat heart and skeletal muscle may be located at ubiquitin conjugation and/or its binding to proteasome, not at the ubiquitin availability or the proteasome itself.

Calpain activity in fast, slow, transforming, and regenerating skeletal muscles of rat

Fiber-type transitions in adult skeletal muscle induced by chronic low-frequency stimulation (CLFS) encompass coordinated exchanges of myofibrillar protein isoforms. CLFS-induced elevations in cytosolic Ca(2+) could activate proteases, especially calpains, the major Ca(2+)-regulated cytosolic proteases. Calpain activity determined by a fluorogenic substrate in the presence of unaltered endogenous calpastatin activities increased twofold in low-frequency-stimulated extensor digitorum longus (EDL) muscle, reaching a level intermediate between normal fast- and slow-twitch muscles. micro- and m-calpains were delineated by a calpain-specific zymographical assay that assessed total activities independent of calpastatin and distinguished between native and processed calpains. Contrary to normal EDL, structure-bound, namely myofibrillar and microsomal calpains, were abundant in soleus muscle. However, the fast-to-slow conversion of EDL was accompanied by an early translocation of cytosolic micro-calpain, suggesting that myofibrillar and microsomal micro-calpain was responsible for the twofold increase in activity and thus involved in controlled proteolysis during fiber transformation. This is in contrast to muscle regeneration where m-calpain translocation predominated. Taken together, we suggest that translocation is an important step in the control of calpain activity in skeletal muscle in vivo.

Insulin suppresses the increased activities of lysosomal cathepsins and ubiquitin conjugation system in burn-injured rats

BACKGROUND

Burn injury results in increased rate of skeletal muscle protein degradation. In vitro studies on incubated muscles indicate that increased rate of protein degradation is due to activation of multiple proteolytic systems, but the supporting evidence is of an indirect nature. The present study was carried out to investigate the role of various lysosomal cathepsins, ubiquitin conjugation, and proteasome systems in accelerated proteolysis, and the effect of insulin in burn-induced muscle wasting syndrome.

MATERIALS AND METHODS

Fifteen to twenty percent total body surface area scald burn injury was inflicted on the shaved dorsum of young growing rats. Insulin-treated rats received a daily single subcutaneous injection for 3 days (0.25-1.0 U/day). The rate of ubiquitin conjugation to endogenous proteins and exogenously added (125)I-lysozyme and the activities of various proteases were measured in muscle homogenates.

RESULTS

Burn injury resulted in increased rate of ubiquitin conjugation to endogenous proteins and (125)I-lysozyme. Activities of cathepsins B, C, H, and L were also up-regulated following burn injury. When the burn-injured rats were treated with insulin, the increased rate of ubiquitin conjugation and cathepsin activities were suppressed to the control levels.

CONCLUSIONS

The increased ubiquitin conjugation and lysosomal cathepsins contribute to accelerated protein degradation in burn-injured rats and insulin suppresses the muscle protein degradation at least in part by suppressing the activities of lysosomal cathepsins and of ubiquitin conjugation system.

Glucocorticoids induce proteasome C3 subunit expression in L6 muscle cells by opposing the suppression of its transcription by NF-kappa B

Muscle wasting in catabolic conditions results from activation of the ubiquitin-proteasome proteolytic pathway by a process that requires glucocorticoids and is generally associated with increased levels of mRNAs encoding components of this proteolytic system. In L6 muscle cells, dexamethasone stimulates proteolysis and increases the amount of the proteasome C3 subunit protein by augmenting its transcription. Transfection studies with human C3 promoter-luciferase reporter genes and electrophoretic mobility shift assays revealed that a NF-kappaB.protein complex containing Rel A is abundant in L6 muscle cell nuclei. Glucocorticoids stimulate C3 subunit expression by antagonizing the interaction of this NF-kappaB protein with an NF-kappaB response element in the C3 subunit promoter region. Dexamethasone also increased the cytosolic amounts of the NF-kappaB p65 subunit and the IkappaBalpha inhibitor proteins in L6 cells. Incubation of L6 cells with a cytokine mixture not only increased the amount of activated NF-kappaB but also decreased C3 promoter activity and lowered endogenous C3 subunit mRNA. Thus, NF-kappaB is a repressor of C3 proteasome subunit transcription in muscle cells, and glucocorticoids stimulate C3 subunit expression by opposing this suppressor action.

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.

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.

Regulation of cardiac myocyte protein turnover and myofibrillar structure in vitro by specific directions of stretch

We have examined how different degrees (0.5%, 1.0%, 2.5%, 5.0%, and 10.0%) and directions of stretch regulate the turnover and accumulation of contractile proteins in cultured neonatal cardiac myocytes (NCMs). In pulse-chase experiments, stellate-shaped NCMs with random arrays of myofibrils (MFs) exhibited a threshold response to stretch. With respect to unstretched controls, the turnover of the contractile protein pool was suppressed 50% to 100% in stellate NCMs stretched 1.0% to 5.0% and was unaltered in stellate NCMs stretched 0.5% or 10.0%. The posttranslational metabolism of myosin heavy chain (MHC) and actin was regulated in parallel with the total contractile protein pool. The turnover of the cytoplasmic protein pool remained unchanged in response to stretch. NCMs plated onto an aligned matrix of type I collagen expressed an elongated, rod-like cell shape. The MFs of these cells were distributed in parallel with one another along a single unique axis. The tissue-like pattern of organization of these cultures made it possible to assay how specific directions of stretch affected cardiac protein turnover and MF organization. In pulse-chase experiments, stretch in parallel with the MFs did not alter the turnover of the total contractile protein pool, the cytoplasmic protein pool, MHC, or actin. The total cellular concentration of MHC and actin remained constant, and MF alignment was not overtly affected. In contrast, even modest degrees of stretch across the short axis of the MFs suppressed total contractile protein turnover, the turnover of MHC and actin, and promoted the accumulation of these MF subunits. The parallel alignment of MFs deteriorated in myocytes stretched greater than 5%. The characteristic response of aligned myocytes to stretch was not affected by the contractile state of the cells. Isoproterenol (ISO) treatment in concert with stretch in parallel with the MFs modestly accelerated contractile protein turnover. Conversely, contractile protein turnover was suppressed in cells treated with ISO and stretched across the short axis of the MFs. Contractile arrest with nifedipine (NIFED) accelerated total myofibrillar protein turnover. Stretch across the short axis, but not in parallel with the MFs, suppressed protein turnover in cells treated with NIFED. The turnover of the cytosolic proteins remained constant under all conditions assayed. These data suggest that specific directions of stretch may play a crucial role in regulating MF organization and the metabolism of contractile proteins in the cardiac myocyte.

Ubiquitin conjugation by the N-end rule pathway and mRNAs for its components increase in muscles of diabetic rats

Insulin deficiency (e.g., in acute diabetes or fasting) is associated with enhanced protein breakdown in skeletal muscle leading to muscle wasting. Because recent studies have suggested that this increased proteolysis is due to activation of the ubiquitin-proteasome (Ub-proteasome) pathway, we investigated whether diabetes is associated with an increased rate of Ub conjugation to muscle protein. Muscle extracts from streptozotocin-induced insulin-deficient rats contained greater amounts of Ub-conjugated proteins than extracts from control animals and also 40-50% greater rates of conjugation of (125)I-Ub to endogenous muscle proteins. This enhanced Ub-conjugation occurred mainly through the N-end rule pathway that involves E2(14k) and E3alpha. A specific substrate of this pathway, alpha-lactalbumin, was ubiquitinated faster in the diabetic extracts, and a dominant negative form of E2(14k) inhibited this increase in ubiquitination rates. Both E2(14k) and E3alpha were shown to be rate-limiting for Ub conjugation because adding small amounts of either to extracts stimulated Ub conjugation. Furthermore, mRNA for E2(14k) and E3alpha (but not E1) were elevated 2-fold in muscles from diabetic rats, although no significant increase in E2(14k) and E3alpha content could be detected by immunoblot or activity assays. The simplest interpretation of these results is that small increases in both E2(14k) and E3alpha in muscles of insulin-deficient animals together accelerate Ub conjugation and protein degradation by the N-end rule pathway, the same pathway activated in cancer cachexia, sepsis, and hyperthyroidism.

Autoregulation of actin synthesis requires the 3'-UTR of actin mRNA and protects cells from actin overproduction

Monomeric (G) actin was shown to be involved in inhibiting its own synthesis by an autoregulatory mechanism that includes enhanced degradation of the actin mRNA [Bershadsky et al., 1995; Lyubimova et al., 1997]. We show that the 3'-untranslated region (3'-UTR) of beta-actin mRNA, but not its 5'-untranslated region, is important for this regulation. The level of full-length beta-actin mRNA in cells was reduced when actin filaments were depolymerized by treatment with latrunculin A and elevated when actin polymerization was induced by jasplakinolide. By contrast, the level of actin mRNA lacking the 3'-UTR remained unchanged when these drugs modulated the dynamics of actin assembly in the cell. Moreover, the transfection of cells with a construct encoding the autoregulation-deficient form of beta-actin mRNA led to very high levels of actin expression compared with transfection with the control actin construct and was accompanied by characteristic changes in cell morphology and the structure of the actin cytoskeleton. These results suggest that the autoregulatory mechanism working via the 3'-UTR of actin mRNA is involved in controlling the maintenance of a defined pool of actin monomers that could be necessary for the proper organization of the microfilament system and the cytoskeleton-mediated signaling.

Sepsis stimulates release of myofilaments in skeletal muscle by a calcium-dependent mechanism

Sepsis is associated with a pronounced catabolic response in skeletal muscle, mainly reflecting degradation of the myofibrillar proteins actin and myosin. Recent studies suggest that sepsis-induced muscle proteolysis may reflect ubiquitin-proteasome-dependent protein breakdown. An apparently conflicting observation is that the ubiquitin-proteasome pathway does not degrade intact myofibrils. Thus, it is possible that actin and myosin need to be released from the myofibrils before they can be ubiquitinated and degraded by the proteasome. We tested the hypothesis that sepsis results in disruption of Z-bands, increased expression of calpains, and calcium-dependent release of myofilaments in skeletal muscle. Sepsis induced in rats by cecal ligation and puncture resulted in increased gene expression of micro-calpain, m-calpain, and p94 and in Z-band disintegration in the extensor digitorum longus muscle. The release of myofilaments from myofibrillar proteins was increased in septic muscle. This response to sepsis was blocked by treating the rats with dantrolene, a substance that inhibits the release of calcium from intracellular stores to the cytoplasm. The present results provide evidence that sepsis is associated with Z-band disintegration and a calcium-dependent release of myofilaments in skeletal muscle. Release of myofilaments may be an initial and perhaps rate-limiting component of sepsis-induced muscle breakdown.

A new model of cancer cachexia: contribution of the ubiquitin-proteasome pathway

A new model of cachexia is described in which muscle protein metabolism related to the ubiquitin-proteasome pathway was investigated. Cloning of the colon-26 tumor produced a cell line, termed R-1, which induced cytokine (noninterleukin-1beta, interleukin-6 and tumor necrosis factor-alpha)-independent cachexia. Implantation of R-1 cells in mice elicited significant (20-30%) weight loss and decreased blood glucose by 70%, and adipose tissue levels declined by 95% and muscle weights decreased by 20-25%. Food intake was unaffected. The decrease in muscle weight reflected a decline in insoluble, but not soluble, muscle protein that was associated with a significant increase in net protein degradation. The rate of ubiquitin conjugation of proteins was significantly elevated in muscles of cachectic mice. Furthermore, the proteasome inhibitor lactacystin blocked the increase in protein breakdown but had no significant effect on proteolysis. Several markers of the ubiquitin-proteasome pathway, E2(14k) mRNA and E2(14k) protein and ubiquitin-protein conjugates, were not elevated. Future investigations with this new model should gain further insights into the mechanisms of cachexia and provide a background to evaluate novel and more efficacious therapies.

Adaptation of the ubiquitin-proteasome proteolytic pathway in cancer cachexia

The ubiquitin-proteasome proteolytic pathway is of major importance in the breakdown of skeletal muscle proteins. The first step in this pathway is the covalent attachment of polyubiquitin chains to the targeted protein. Polyubiquitinylated proteins are then recognized and degraded by the 26S proteasome complex. In this review, we critically analyze recent findings in the regulation of ubiquitinylation of protein substrates and of their subsequent proteasome-dependent degradation in animal models of cancer cachexia. In particular, we discuss the influence of various mediators (anorexia, hormones, prostaglandins, cytokines, and proteolysis-inducing factor) in signaling the activation of ubiquitin-proteasome proteolysis in skeletal muscle. These findings have lead to new concepts that are starting to be used for preventing cachexia in cancer and other wasting diseases.

Dynamic distribution and formation of a para-sarcomeric banding pattern of prosomes during myogenic differentiation of satellite cells in vitro

Myogenesis proceeds by fusion of proliferating myoblasts into myotubes under the control of various transcription factors. In adult skeletal muscle, myogenic stem cells are represented by the satellite cells which can be cultured and differentiate in vitro. This system was used to investigate the subcellular distribution of a particular type of prosomes at different steps of the myogenic process. Prosomes constitute the MCP core of the 26S proteasomes but were first observed as subcomplexes of the untranslated mRNPs; recently, their RNase activity was discovered. A monoclonal antibody raised against the p27K subunit showed that the p27K subunit-specific prosomes move transiently into the nucleus prior to the onset of myoblast fusion into myotubes; this represents possibly one of the first signs of myoblast switching into the differentiation pathway. Prior to fusion, the prosomes containing the p27K subunit return to the cytoplasm, where they align with the gradually formed lengthwise-running desmin-type intermediate filaments and the microfilaments, co-localizing finally with the actin bundles. The prosomes progressively form discontinuous punctate structures which eventually develop a pseudo-sarcomeric banding pattern. In myotubes just formed in vitro, the formation of this pattern seems to preceed that produced by the muscle-specific sarcomeric (alpha)-actin. Interestingly, this pattern of prosomes of myotubes in terminal in vitro differentiation was very similar to that of prosomes observed in vivo in foetal and adult muscle. These observations are discussed in relation to molecular myogenesis and prosome/proteasome function.

Changes in 20S proteasome activity during ageing of the LOU rat

Muscular functions decline and muscle mass decreases during ageing. In the rat, there is a 27% decrease in muscle protein between 18 and 34 months of age. We examined age-related changes in the proteasome-dependent proteolytic pathway in rats at 4, 18, 24, 29 and 34 months of age. The three best characterised activities of the proteasome (chymotrypsin-like, trypsin-like and peptidylglutamyl peptide hydrolase) increased to 29 months and then decreased in the senescent animal. These variations in activity were accompanied by an identical change in the quantity of 20S proteasome measured by Western blot, whereas the S4 subunit of the 19S regulator and the quantity of ubiquitin-linked proteins remained constant. mRNA of subunits C3, C5, C9, and S4 increased in the senescent animal, but ubiquitin mRNA levels were unchanged. These findings suggest that the 20S proteasome may be partly responsible for the muscular atrophy observed during ageing in the rat.

DNA-antiviral vaccines: new developments and approaches--a review

Current vaccines can be divided into "live," "recombinant" and "killed" vaccines. Live vaccines are traditionally composed of attenuated viruses or bacteria, selected for their reduced pathogenicity. Recombinant vaccines, driven by a viral or bacterial vector express foreign antigens, or only recombinant proteins injected as antigen. Killed vaccines consist of inactivated whole pathogens. But all these traditional vaccines have some disadvantages: Attenuated live vaccine are able to undergo mutation and as mutated viruses or bacteria can now provoke the diseases against which the vaccine should protect the organism. A further disadvantage of live vaccines is the possibility of shedding which is a real problem especially in veterinary medicine. Clearly, there is a need for better vaccines to protect against diseases without the disadvantages associated with vaccines presently in use. Modern vaccines might be characterized as safe, no risk of reversion to pathogenicity, and they should be stable without the necessity of a "cold chain." Production should be simple, standardized and inexpensive. Vaccine development has now been improved by the ability to use direct inoculations of plasmid DNA encoding viral or bacterial proteins. One of the major benefits of DNA-vaccines, variously termed "DNA-, genetic- or nucleic acid-immunization," is the endogenous synthesis of the encoded protein. Therefore DNA vaccines mimic natural infection and provoke both strong humoral and cellular immune response. This review summarizes new developments and approaches of DNA vaccination and explains the construction of expression plasmids as well as possible mechanisms of immune responses.

Role of calpain in skeletal-muscle protein degradation

Although protein degradation is enhanced in muscle-wasting conditions and limits the rate of muscle growth in domestic animals, the proteolytic system responsible for degrading myofibrillar proteins in skeletal muscle is not well defined. The goals of this study were to evaluate the roles of the calpains (calcium-activated cysteine proteases) in mediating muscle protein degradation and the extent to which these proteases participate in protein turnover in muscle. Two strategies to regulate intracellular calpain activities were developed: overexpression of dominant-negative m-calpain and overexpression of calpastatin inhibitory domain. To express these constructs, L8 myoblast cell lines were transfected with LacSwitch plasmids, which allowed for isopropyl beta-D-thiogalactoside-dependent expression of the gene of interest. Inhibition of calpain stabilized fodrin, a well characterized calpain substrate. Under conditions of accelerated degradation (serum withdrawal), inhibition of m-calpain reduced protein degradation by 30%, whereas calpastatin inhibitory domain expression reduced degradation by 63%. Inhibition of calpain also stabilized nebulin. These observations indicate that calpains play key roles in the disassembly of sarcomeric proteins. Inhibition of calpain activity may have therapeutic value in treatment of muscle-wasting conditions and may enhance muscle growth in domestic animals.

Rates of ubiquitin conjugation increase when muscles atrophy, largely through activation of the N-end rule pathway

The rapid loss of muscle mass that accompanies many disease states, such as cancer or sepsis, is primarily a result of increased protein breakdown in muscle, and several observations have suggested an activation of the ubiquitin-proteasome system. Accordingly, in extracts of atrophying muscles from tumor-bearing or septic rats, rates of 125I-ubiquitin conjugation to endogenous proteins were found to be higher than in control extracts. On the other hand, in extracts of muscles from hypothyroid rats, where overall proteolysis is reduced below normal, the conjugation of 125I-ubiquitin to soluble proteins decreased by 50%, and treatment with triiodothyronine (T3) restored ubiquitination to control levels. Surprisingly, the N-end rule pathway, which selectively degrades proteins with basic or large hydrophobic N-terminal residues, was found to be responsible for most of these changes in ubiquitin conjugation. Competitive inhibitors of this pathway that specifically block the ubiquitin ligase, E3alpha, suppressed most of the increased ubiquitin conjugation in the muscle extracts from tumor-bearing and septic rats. These inhibitors also suppressed ubiquitination in normal extracts toward levels in hypothyroid extracts, which showed little E3alpha-dependent ubiquitination. Thus, the inhibitors eliminated most of the differences in ubiquitination under these different pathological conditions. Moreover, 125I-lysozyme, a model N-end rule substrate, was ubiquitinated more rapidly in extracts from tumor-bearing and septic rats, and more slowly in those from hypothyroid rats, than in controls. Thus, the rate of ubiquitin conjugation increases in atrophying muscles, and these hormone- and cytokine-dependent responses are in large part due to activation of the N-end rule pathway.

The N-end rule pathway catalyzes a major fraction of the protein degradation in skeletal muscle

In skeletal muscle, overall protein degradation involves the ubiquitin-proteasome system. One property of a protein that leads to rapid ubiquitin-dependent degradation is the presence of a basic, acidic, or bulky hydrophobic residue at its N terminus. However, in normal cells, substrates for this N-end rule pathway, which involves ubiquitin carrier protein (E2) E214k and ubiquitin-protein ligase (E3) E3alpha, have remained unclear. Surprisingly, in soluble extracts of rabbit muscle, we found that competitive inhibitors of E3alpha markedly inhibited the 125I-ubiquitin conjugation and ATP-dependent degradation of endogenous proteins. These inhibitors appear to selectively inhibit E3alpha, since they blocked degradation of 125I-lysozyme, a model N-end rule substrate, but did not affect the degradation of proteins whose ubiquitination involved other E3s. The addition of several E2s or E3alpha to the muscle extracts stimulated overall proteolysis and ubiquitination, but only the stimulation by E3alpha or E214k was sensitive to these inhibitors. A similar general inhibition of ubiquitin conjugation to endogenous proteins was observed with a dominant negative inhibitor of E214k. Certain substrates of the N-end rule pathway are degraded after their tRNA-dependent arginylation. We found that adding RNase A to muscle extracts reduced the ATP-dependent proteolysis of endogenous proteins, and supplying tRNA partially restored this process. Finally, although in muscle extracts the N-end rule pathway catalyzes most ubiquitin conjugation, it makes only a minor contribution to overall protein ubiquitination in HeLa cell extracts.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

Determinants of protein turnover in health and disease

Protein synthesis, protein degradation, and amino acid oxidation are tightly regulated to preserve lean body mass in healthy individuals. An adaptative response to a reduction in dietary protein in normal adults is decreased branched-chain amino acid oxidation which increases the availability of amino acids. In nephrosis, reduced branched-chain amino acid oxidation decreases amino acid requirements and helps to compensate for urinary protein loss. Conversely, uremia and other catabolic diseases are associated with muscle wasting resulting from activation of the ubiquitin-proteasome proteolytic pathway and branched-chain ketoacid dehydrogenase, the rate-limiting enzyme for branched-chain amino acid catabolism. By understanding the processes responsible for muscle wasting in catabolic states, therapeutic interventions may be designed to improve protein balance.

DNA immunization: ubiquitination of a viral protein enhances cytotoxic T-lymphocyte induction and antiviral protection but abrogates antibody induction

DNA immunization can induce cytotoxic T lymphocytes (CTL), antibodies, and protection against microbial challenge. The underlying mechanisms remain obscure and must be understood to permit rational manipulation and optimization of the technique. We set out to enhance the intracellular degradation of a viral antigen, with the intent of improving antigen entry into, and presentation by, the class I major histocompatibility complex pathway. We achieved this goal by cotranslational ubiquitination of a plasmid-encoded viral antigen, lymphocytic choriomeningitis virus (LCMV) nucleoprotein (NP). We show that native NP is very stable in cell culture, while the ubiquitinated product is so rapidly degraded that it is barely detectable. This rapid degradation leads to more efficient sensitization of target cells in an in vitro cytotoxicity assay, consistent with enhanced antigen presentation, and both degradation and target cell recognition are blocked by a proteasome inhibitor. We have used the plasmid for in vivo studies and find that, remarkably, ubiquitination leads to a complete abrogation of antibody responses, presumably because the encoded protein is so rapidly and completely degraded that insufficient antigen remains to interact appropriately with B cells. In contrast, in vivo CTL induction is improved by ubiquitination of NP. That CTL are induced at all by this rapidly degraded protein may shed light on the mechanism by which CTL are induced by DNA immunization; it has been suggested that CTL induction following intramuscular DNA injection results not from antigen presentation by cells taking up and expressing the DNA but rather from uptake of soluble protein by specialized antigen-presenting cells (APC). It appears to us unlikely that the ubiquitinated protein could function in this manner, since it is so rapidly degraded in vitro and fails to induce antibodies in vivo. Finally, the ubiquitinated protein confers markedly enhanced protection against LCMV challenge. Mice immunized with a plasmid encoding NP show approximately 100-fold reductions in virus titers compared to controls, while mice immunized with a plasmid encoding the ubiquitinated NP show reductions in virus load of at least 5 x 10(4)- to 5 x 10(5)-fold. This is by far the most effective DNA vaccine that we have yet designed. Ubiquitination therefore may improve DNA immunization, but caution is warranted, since immunity to many microbes depends on induction of good humoral immunity, and we show here that this may be prevented by ubiquitination of the encoded protein.

TGF-alpha-induced breakdown of stress fibers and degradation of tropomyosin in NRK cells is blocked by a proteasome inhibitor

Treatment of NRK cells with TGF-alpha in the presence of serum initiates disassembly of cytoskeletal stress fibers and suppresses the synthesis of tropomyosin isoforms (TMs) 1, 2, and 3 but not TMs 4 and 5 (Cooper et al., Cancer Res. 47, 4493-4500, 1987). In order to determine how the loss of tropomyosin is induced and what role it plays in cytoskeletal disruption, the turnover of tropomyosin was studied in the presence of the transforming growth factor and protease inhibitors. Cells were pulse-labeled with [35S]methionine and chased in the absence or the presence of the growth factor. It was found that TMs 1, 2, and 3 are degraded at about twice the rate of TMs 4 and 5 in control cells and that the rate of degradation of TMs 1-3 is accelerated by the growth factors. Degradation of TMs in control and growth factor-treated cells is blocked by a membrane-permeable inhibitor of cysteine proteases (LLnL) that acts upon calpains and proteasomes, and the cells maintain a flattened shape with a normal complement of stress fibers. Application of inhibitors that block calpains but not proteasomes does not block TM degradation. Treatments (suspension culture or cytochalasin B) that disrupt stress fibers without application of the growth factors also accelerate TM degradation, suggesting that acceleration of TM degradation is a consequence of its release from stress fibers during their breakdown. The normally more rapid turnover of the TM isoforms 1-3 that are lost in the phenotypically transformed cells could serve to facilitate the cytoskeletal reorganization that follows the activation of signal transduction pathways by the transforming growth factors observed in this study or during other rearrangements of the cytoskeleton such as occur during cell migration or mitosis.

Ubiquitin-dependent destruction of topoisomerase I is stimulated by the antitumor drug camptothecin

Topoisomerase I (TOP1) relaxes superhelical DNA through a breakage/rejoining reaction in which the active site tyrosine links covalently to a 3' phosphate at the break site as a transient intermediate. The antitumor drug camptothecin (CPT) and its analogs inhibit the rejoining step of the breakage/rejoining reaction, which traps the enzyme in covalent linkage with DNA (the cleavable complex). Little is known about the fate of cellular TOP1 trapped in the cleavable complex. We have analyzed TOP1 in mammalian cell lines treated with CPT. When CPT-treated cells were lysed with either SDS or alkali and analyzed by Western blotting, greater than 90% of the TOP1 was linked to DNA. Nuclease treatment of the cell lysate to remove the covalently linked DNA from TOP1 revealed a distinct ladder of higher molecular weight bands having properties indicative of multi-ubiquitin (Ub) conjugates of TOP1. Approximately 5-10% of TOP1 was present as these conjugates within minutes of CPT treatment. Consistent with ubiquitination, TOP1 was not modified in ts85 cells at the restrictive temperature for its thermolabile ubiquitin-activating enzyme (E1). Because conjugation with ubiquitin can mark proteins for destruction by the 26S proteasome, we analyzed TOP1 protein levels during prolonged CPT treatment. TOP1 protein levels were reduced to about 25% during CPT treatments of 2-4 h resulting from increased destruction, with the half-life dropping from 10-16 h down to 1-2 h. The destruction of TOP1, like the formation of Ub-TOP1 conjugates, was not observed in ts85 cells at the restrictive temperature. The destruction of TOP1 was also prevented in cells treated with MG-132 and lactacystin, specific inhibitors of the 26S proteasome. Finally, the multi-Ub conjugates of TOP1 were observed whether or not aphidicolin was included in cotreatment with CPT, indicating that replication fork activity was not involved in making TOP1 a substrate for ubiquitination. These results demonstrate that independent of DNA replication, the TOP1 cleavable complex is ubiquitinated and destroyed in cells treated with antitumor drugs that block the religation step of the TOP1 reaction.

Inhibitors of the proteasome reduce the accelerated proteolysis in atrophying rat skeletal muscles

Several observations have suggested that the enhanced proteolysis and atrophy of skeletal muscle in various pathological states is due primarily to activation of the ubiquitin-proteasome pathway. To test this idea, we investigated whether peptide aldehyde inhibitors of the proteasome, N-acetyl-leucyl-leucyl-norleucinal (LLN), or the more potent CBZ-leucyl-leucyl-leucinal (MG132) suppressed proteolysis in incubated rat skeletal muscles. These agents (e.g., MG132 at 10 microM) inhibited nonlysosomal protein breakdown by up to 50% (P < 0.01), and this effect was rapidly reversed upon removal of the inhibitor. The peptide aldehydes did not alter protein synthesis or amino acid pools, but improved overall protein balance in the muscle. Upon treatment with MG132, ubiquitin-conjugated proteins accumulated in the muscle. The inhibition of muscle proteolysis correlated with efficacy against the proteasome, although these agents could also inhibit calpain-dependent proteolysis induced with Ca2+. These inhibitors had much larger effects on proteolysis in atrophying muscles than in controls. In the denervated soleus undergoing atrophy, the increase in ATP-dependent proteolysis was reduced 70% by MG132 (P < 0.001). Similarly, the rise in muscle proteolysis induced by administering thyroid hormones was reduced 40-70% by the inhibitors. Finally, in rats made septic by cecal puncture, the increase in muscle proteolysis was completely blocked by MG132. Thus, the enhanced proteolysis in many catabolic states (including denervation, hyperthyroidism, and sepsis) is due to a proteasome-dependent pathway, and inhibition of proteasome function may be a useful approach to reduce muscle wasting.