The 20S/26S proteasomal pathway of protein degradation in muscle tissue

Curcumin may induce lipolysis via proteo-stress in Huh7 human hepatoma cells

Curcumin has been shown to have anti-obesity effects in animal studies. Although several molecular mechanisms of action have been reported, the initial or upstream molecular events remain to be revealed. In this study, we found that curcumin or heat shock treatment up-regulated the expression of adipose triglyceride lipase (ATGL) in Huh7 hepatoma cells, which resulted in acceleration of lipolysis. Interestingly, perturbation of protein homeostasis was seen in curcumin-treated cells, as detected by formation of numerous ubiquitinated proteins and conjugated proteins with p62 (SQSTM). Curcumin activated the protein expression of molecular chaperones, such as heat shock protein (HSP)40 and HSP70. Pre-treatment of the cells with 4-phenylbutyric acid, a chemical chaperone, suppressed proteo-stress induced by curcumin and reduced its lipolysis effect. Importantly, the cytotoxicity of curcumin was markedly alleviated when intracellular triglyceride was consumed by the polyphenol. Thus, energy supplementation from lipolysis may play substantial roles in adaptation and survival of curcumin-exposed cells. To support this notion, the cytotoxicity of curcumin was aggravated in ATGL-knockdown cells. Curcumin decreased intracellular ATP for activating AMP-activated protein kinase, which initiates catabolic pathways including ATGL-dependent lipolysis. Taken together, we propose a hypothesis that curcumin induces lipolysis to compensate for ATP reduction due to its proteo-stress effects.

Oxidative stress, neurodegeneration, and the balance of protein degradation and protein synthesis

Oxidative stress occurs in a variety of disease settings and is strongly linked to the development of neuron death and neuronal dysfunction. Cells are equipped with numerous pathways to prevent the genesis, as well as the consequences, of oxidative stress in the brain. In this review we discuss the various forms and sources of oxidative stress in the brain and briefly discuss some of the complexities in detecting the presence of oxidative stress. We then focus the review on the interplay between the diverse cellular proteolytic pathways and their roles in regulating oxidative stress in the brain. Additionally, we discuss the involvement of protein synthesis in regulating the downstream effects of oxidative stress. Together, these components of the review demonstrate that the removal of damaged proteins by effective proteolysis and the synthesis of new and protective proteins are vital in the preservation of brain homeostasis during periods of increased levels of reactive oxygen species. Last, studies from our laboratory and others have demonstrated that protein synthesis is intricately linked to the rates of protein degradation, with impairment of protein degradation sufficient to decrease the rates of protein synthesis, which has important implications for successfully responding to periods of oxidative stress. Specific neurodegenerative diseases, including Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, and stroke, are discussed in this context. Taken together, these findings add to our understanding of how oxidative stress is effectively managed in the healthy brain and help elucidate how impairments in proteolysis and/or protein synthesis contribute to the development of neurodegeneration and neuronal dysfunction in a variety of clinical settings.

Targeting β-tubulin:CCT-β complexes incurs Hsp90- and VCP-related protein degradation and induces ER stress-associated apoptosis by triggering capacitative Ca2+ entry, mitochondrial perturbation and caspase overactivation

We have previously demonstrated that interrupting the protein–protein interaction (PPI) of β-tubulin:chaperonin-containing TCP-1β (CCT-β) induces the selective killing of multidrug-resistant cancer cells due to CCT-β overexpression. However, the molecular mechanism has not yet been identified. In this study, we found that CCT-β interacts with a myriad of intracellular proteins involved in the cellular functions of the endoplasmic reticulum (ER), mitochondria, cytoskeleton, proteasome and apoptosome. Our data show that the targeted cells activate both the heat-shock protein 90 (Hsp90)-associated protein ubiquitination/degradation pathway to eliminate misfolded proteins in the cytoplasm and the valosin-containing protein (VCP)-centered ER-associated protein degradation pathway to reduce the excessive levels of unfolded polypeptides from the ER, thereby mitigating ER stress, at the onset of β-tubulin:CCT-β complex disruption. Once ER stress is expanded, ER stress-associated apoptotic signaling is enforced, as exhibited by cellular vacuolization and intracellular Ca²⁺ release. Furthermore, the elevated intracellular Ca²⁺ levels resulting from capacitative Ca²⁺ entry augments apoptotic signaling by provoking mitochondrial perturbation and caspase overactivation in the targeted cells. These findings not only provide a detailed picture of the apoptotic signaling cascades evoked by targeting the β-tubulin:CCT-β complex but also demonstrate a strategy to combat malignancies with chemoresistance to Hsp90- and VCP-related anticancer agents.

Skeletal muscle fiber type and myofibrillar proteins in relation to meat quality

Although numerous studies have reported the relationships among muscle fiber characteristics, lean meat content and meat quality, controversial perspectives still remain. Conventional histochemical classifications may be involved in a high level of error, subjectivity and it could not clearly explain variety of myofibrillar protein isoforms. Therefore, more information is needed on how different factors, such as species, breeds, gender, nutrient conditions, physiological state of animals, and environment factors, affect ultimate meat quality in order to evaluate these uncertainness. Unfortunately, there is little information that completely covers with relationship among the muscle fiber types, myofibrillar proteins and enzymatic proteolysis. In addition to the perspective of postmortem metabolism, protein quality control in skeletal muscle and proteolytic degradation of muscle proteins during postmortem period could help to clarify this relationship. Therefore, the present review will focus on muscle fiber types, typing methods, muscle proteins and meat quality, and will summarize aspects of enzymatic view of proteasome.

Trends in Postmortem Aging in Fish: Understanding of Proteolysis and Disorganization of the Myofibrillar Structure

Postmortem tenderization is caused by enzymatic degradation of key structural proteins in myofibrils as well as in extracellular matrix, and of proteins involved in intermyofibrillar linkages and linkages between myofibrils and the sarcolemma. The function of these proteins is to maintain the structural integrity of myofibrils. Current data indicate that calpains and cathepsins may be responsible for degradation of these proteins. Other phenomena occurring in cells postmortem (pH drop, sarcoplasmic Ca2+ increase, osmotic pressure rise, oxidative processes) may act in synergy with proteases. Our understanding of the underlying mechanisms of muscle degradation should be improved for an accurate evaluation of the postmortem muscle changes and consequently of the fish quality.

Paradigms of growth in fish

Most fish are indeterminate growers with white muscle making up the majority of the acquired bulk. Within the muscle, the myofibrillar fraction accounts for almost two-thirds of the protein synthetic activity, implying that it is accretion of myofibrillar proteins that makes the single most important contribution to fish growth. Fish muscle growth itself is not linear and occurs through a combination of hyperplasia and hypertrophy in post-juvenile stages. Superimposed on periodicity of growth in length and mass can be other phases governed by lunar, reproductive or circannual cycles. Data on fish growth are discussed in the framework of site-specific muscle abundance, metabolic and functional zonation of muscle, proliferation and differentiation of satellite cells and the contribution of myofibrillar proteins. Hormonal control of muscle growth is described against the backdrop of plasma availability of myogens (insulin, IGF-I, growth hormone), distribution and dynamics of their respective receptors, and their interactions. Important contributions of the 'supply side' are discussed with hormones regulating amino acid resorption from the intestine, intestinal growth, liver processing and amino acid uptake by the muscle. Data are also interpreted from metabolic angles, to explain lipolytic and nitrogen-sparing effects of growth hormones, and lipogenic effects of insulin and high protein diets. Finally, special attention is devoted to the multifaceted roles of arginine in fish growth, as precursor, intermediate and hormone secretagogue.

Catalytic activities of the 20 S proteasome, a multicatalytic proteinase complex

The proteasome, a multisubunit, multicatalytic proteinase complex, is attracting growing attention as the main intracellular, extralysosomal, proteolytic system involved in ubiquitin-(Ub) dependent and Ub-independent intracellular proteolysis. Its involvement in the mitotic cycle, and control of the half-life of most cellular proteins, functions absolutely necessary for cell growth and viability, make it an attractive target for researchers of intracellular metabolism and an important target for pharmacological intervention. The proteasome belongs to a new mechanistic class of proteases, the N-terminal nucleophile hydrolases, where the N-terminal threonine residue functions as the nucleophile. This minireview focuses on the three classical catalytic activities of the proteasome, designated chymotrypsin-like, trypsin-like, and peptidyl-glutamyl-peptide hydrolyzing in eukaryotes and also the activities of the more simple Archaebacteria and Eubacteria proteasomes. Other catalytic activities of the proteasome and their possible origin are also examined. The specificity of the catalytic components toward synthetic substrates, natural peptides, and proteins and their relationship to the catalytic centers are reviewed. Some unanswered questions and future research directions are suggested.

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.

The role of cytokines in the catabolic consequences of infection and injury

During infection and injury a series of metabolic events are activated that leads to a state of negative nitrogen balance and significant loss of lean body mass. This process is characterized by marked anorexia, net whole body protein breakdown, and liver anabolism. This host response initially is beneficial to the body because it helps it to fight disease and enhance healing. However, if such imbalance is maintained for long periods, it will invariably produce significant loss of lean body mass that may lead to a series of untoward clinical events. The role of the proximate cytokines, tumor necrosis factor (TNF), interleukin-1 (IL-1), and interleukin-6 (IL-6) as well as glucocorticoids as important mediators of many pathophysiological manifestations of infection and injury has been studied extensively. However, the involvement of other mediators, at least in skeletal muscle proteolysis during sepsis has been hypothesized, because blockade of glucocorticoids, TNF, IL-1, and IL-6 reduces but does not normalize protein breakdown rates nor does the direct application of these mediators to skeletal muscle in vitro enhance proteolysis. Furthermore other studies have suggested that the lymphokine, interferon-gamma (IFN-gamma, type II interferon or immune interferon), produces fever and enhances thermogenesis, body weight loss, and skeletal muscle depletion in rodents in a manner similar to that seen with TNF and IL-1. Cytokines appear to be major components of the host metabolic response during infection and injury. However, neither all the cytokines involved nor the exact mechanisms underlying their metabolic effects are completely understood. The regulation of muscle protein synthesis and breakdown, which largely determines the development of cachexia, appears to depend on the delicate balance between a number of regulatory substances including cytokines, glucocorticoids, catecholamines, insulin, and insulin-like growth factors.

Crustacean muscle plasticity: molecular mechanisms determining mass and contractile properties

Two crustacean models for understanding molecular mechanisms of muscle plasticity are reviewed. Metabolic changes underlying muscle protein synthesis and degradation have been examined in the Bermuda land crab, Gecarcinus lateralis. During proecdysis, the claw closer muscle undergoes a programmed atrophy, which results from a highly controlled breakdown of myofibrillar proteins by Ca(2+)-dependent and, possibly, ATP/ubiquitin-dependent proteolytic enzymes. The advantage of this model is that there is neither fiber degeneration nor contractile-type switching, which often occurs in mammalian skeletal muscles. The second model uses American lobster, Homarus americanus, to understand the genetic regulation of fiber-type switching. Fibers in the claw closer muscles undergo a developmentally-regulated transformation as the isomorphic claws of larvae and juveniles differentiate into the heteromorphic cutter and crusher claws of adults. This switching occurs at the boundary between fast- and slow-fiber regions, and thus the transformation of a specific fiber is determined by its position within the muscle. The ability to predict fiber switching can be exploited to isolate and identify putative master regulatory factors that initiate and coordinate the expression of contractile proteins.

Purification and characterization of proteasome from ostrich liver

The proteasome (EC 3.4.99.46) is a high molecular mass (approximately 700 kDa) multisubunit enzyme complex which is the focus of worldwide research in order to identify the structure, mechanism of action and specificity of the complex. The purpose of the present study was to investigate the tryptic, chymotryptic and peptidylglutamyl-peptide hydrolysing (PGPH) activities of ostrich liver proteasome. The proteasome was purified from ostrich liver by employing ammonium sulphate fractionation, followed by three sequential chromatographic steps on Toyopearl Super Q-650 S, Sephadex G-150 and phenyl-Toyopearl columns. Temperature and pH optima were examined and the effect of inhibitors, detergents, fatty acids and cations on the peptidase activities was determined. Ostrich proteasome exhibited a relative M(r) of approximately 665,000 using non-denaturing gradient PAGE and dissociated into the characteristic "ladder" associated with the proteasome subunits during SDS-PAGE. The pH optima for the peptidase activities were found to be slightly alkaline (tryptic activity) and neutral (chymotryptic-like and PGPH activities). Ostrich liver proteasome was found to be activated in terms of the PGPH activity by fatty acids and SDS, whereas the chymotryptic and tryptic-like activities were differentially inhibited. Ostrich proteasome, in its inhibition by monovalent cations, was similar to the proteasomes extracted from other sources. The specificity of the proteasome appears to be very broad, although it lacks aminopeptidase activity. The yield compared favourably with similar extraction procedures which have been reported. On the basis of the physicochemical and kinetic properties which ostrich liver proteasome exhibited, it can be safely concluded that it corresponds well with the proteasomes isolated from many other sources.

The ubiquitin-proteasome pathway: review of a novel intracellular mechanism of muscle protein breakdown during sepsis and other catabolic conditions

SUMMARY BACKGROUND DATA

Patients with sepsis and other catabolic conditions, such as severe trauma, cancer, and fasting, suffer significant loss of body protein, the majority of which originates from skeletal muscle. Recent evidence suggests that muscle protein breakdown during sepsis is caused by upregulated activity in the ubiquitin-proteasome pathway and is associated with increased expression of the ubiquitin gene.

PURPOSE

The purpose of the study was to review the role of the ubiquitin-proteasome pathway in the regulation of muscle proteolysis during sepsis and other catabolic conditions.

REVIEW

Proteins that are degraded by the ubiquitin-proteasome mechanism are first conjugated to ubiquitin, a 76-amino-acid, highly conserved residue. Ubiquitinated proteins are recognized by the 26S proteasome, which is a large proteolytic complex consisting of the 19S cap complex and the 20S proteasome. The 20S proteasome is a cylindrical particle composed of four stacked rings, making it look like a barrel. The rings form a "tunnel" in which the target proteins are hydrolyzed, after which ubiquitin is released to be reused in the proteolytic pathway. A unique feature of the ubiquitin-proteasome proteolytic pathway is its energy dependency.

CONCLUSIONS

An understanding of the molecular regulation of protein metabolism in patients with sepsis and other catabolic conditions is important because it may form the basis for improved treatment in the future.