Amino Acids and Insulin Control Autophagic Proteolysis through Different Signaling Pathways in Relation to mTOR in Isolated Rat Hepatocytes

Dual targeting of AKT and mammalian target of rapamycin: a potential therapeutic approach for malignant peripheral nerve sheath tumor

The mammalian target of rapamycin (mTOR) pathway may constitute a potential target for the treatment of malignant peripheral nerve sheath tumors (MPNST). However, investigations of other cancers suggest that mTOR blockade can paradoxically induce activation of prosurvival, protumorigenic signaling molecules, especially upstream AKT. Consequently, we hypothesized that dual phosphatidylinositol 3-kinase (PI3K)/AKT-mTOR blockade might be applicable for MPNST treatment. Expression of activated mTOR downstream targets (p4EBP1 and pS6RP) and pAKT was evaluated immunohistochemically in a tissue microarray of human MPNSTs (n = 96) and benign neurofibromas (n = 31). Results were analyzed by Wilcoxon rank-sum tests. mTOR and AKT pathways in human MPNST cell lines, and the effects of rapamycin (mTOR inhibitor), LY294002 (dual PI3K/mTOR inhibitor), and PI-103 (potent dual PI3K/AKT-mTOR inhibitor) on pathway activation were evaluated by Western blot. Effects on cell growth were evaluated via MTS and colony formation assays. Cell cycle progression and apoptosis were assessed by propidium iodide/fluorescence-activated cell sorting staining and Annexin V assays. Acridine orange staining/fluorescence-activated cell sorting analysis, electron microscopy, and Western blot evaluated autophagy induction. p4EBP1, pS6Rp, and pAKT levels were found to be significantly higher in MPNST versus neurofibroma (P < 0.05 for all markers). mTOR and AKT pathways were found to be highly activated in MPNST cell lines. MPNST cells were sensitive to rapamycin; however, rapamycin enhanced pAKT and peIF4E expression. PI-103 abrogated MPNST cell growth and induced G(1) cell cycle arrest potentially through repression of cyclin D1. PI-103 did not elicit apoptosis but significantly induced autophagy in MPNST cells. These results suggest further study of combined PI3K/AKT and mTOR inhibition as a novel therapy for patients harboring MPNST.

All-you-can-eat: autophagy in neurodegeneration and neuroprotection

Autophagy is the major pathway involved in the degradation of proteins and organelles, cellular remodeling, and survival during nutrient starvation. Autophagosomal dysfunction has been implicated in an increasing number of diseases from cancer to bacterial and viral infections and more recently in neurodegeneration. While a decrease in autophagic activity appears to interfere with protein degradation and possibly organelle turnover, increased autophagy has been shown to facilitate the clearance of aggregation-prone proteins and promote neuronal survival in a number of disease models. On the other hand, too much autophagic activity can be detrimental as well and lead to cell death, suggesting the regulation of autophagy has an important role in cell fate decisions. An increasing number of model systems are now available to study the role of autophagy in the central nervous system and how it might be exploited to treat disease. We will review here the current knowledge of autophagy in the central nervous system and provide an overview of the various models that have been used to study acute and chronic neurodegeneration.

Cytosolic LC3 ratio as a quantitative index of macroautophagy

Macroautophagy, an intracellular bulk degradation process and a typical form of autophagy in eukaryotes, is sensitive to physiological regulation, such as the supply and deprivation of nutrients. Microtubule-associated protein 1 light chain 3 (LC3), a mammalian homologue of yeast Atg8, plays a critical role in macroautophagy formation and is considered a suitable marker for this process. In mammalian cells, there is a limitation for biochemical and morphological methods to monitor autophagy within a short period of time. During analysis of the subcellular distribution of LC3, we found that the cytosolic fraction contains not only a precursor form (LC3-I), but also an apparently active form, denoted as LC3-IIs. Both LC3-I and LC3-IIs in the cytosolic fraction, and thus the LC3-IIs/I ratio (designated the cytosolic LC3 ratio), were more responsive to amino acids than monitoring LC3-II or the LC3-II/I ratio in the total homogenate, and remarkably reflected the total proteolytic flux in fresh rat hepatocytes and the cultured H4-II-E cell line. Thus, in addition to representing a sensitive index of macroautophagy, examining the cytosolic LC3 ratio is an easy and quick quantitative method for monitoring the regulation of this process in hepatocytes and H4-II-E cells.

Assaying of autophagic protein degradation

Macroautophagy is a three-step process: (1) autophagosomes form and mature, (2) the autophagosomes fuse with lysosomes, and (3) the autophagic cargo is degraded in the lysosomes. It is this lysosomal degradation of the autophagic cargo that constitutes the autophagic flux. As in the case of metabolic pathways, the steady-state concentration of the intermediary autophagic structures alone is insufficient for investigating the flux. Assaying the degradation of long-lived proteins as described in this chapter is one of the methods that can be used to measure autophagic flux.

Amino acid regulation of TOR complex 1

TOR complex 1 (TORC1), an oligomer of the mTOR (mammalian target of rapamycin) protein kinase, its substrate binding subunit raptor, and the polypeptide Lst8/GbetaL, controls cell growth in all eukaryotes in response to nutrient availability and in metazoans to insulin and growth factors, energy status, and stress conditions. This review focuses on the biochemical mechanisms that regulate mTORC1 kinase activity, with special emphasis on mTORC1 regulation by amino acids. The dominant positive regulator of mTORC1 is the GTP-charged form of the ras-like GTPase Rheb. Insulin, growth factors, and a variety of cellular stressors regulate mTORC1 by controlling Rheb GTP charging through modulating the activity of the tuberous sclerosis complex, the Rheb GTPase activating protein. In contrast, amino acids, especially leucine, regulate mTORC1 by controlling the ability of Rheb-GTP to activate mTORC1. Rheb binds directly to mTOR, an interaction that appears to be essential for mTORC1 activation. In addition, Rheb-GTP stimulates phospholipase D1 to generate phosphatidic acid, a positive effector of mTORC1 activation, and binds to the mTOR inhibitor FKBP38, to displace it from mTOR. The contribution of Rheb's regulation of PL-D1 and FKBP38 to mTORC1 activation, relative to Rheb's direct binding to mTOR, remains to be fully defined. The rag GTPases, functioning as obligatory heterodimers, are also required for amino acid regulation of mTORC1. As with amino acid deficiency, however, the inhibitory effect of rag depletion on mTORC1 can be overcome by Rheb overexpression, whereas Rheb depletion obviates rag's ability to activate mTORC1. The rag heterodimer interacts directly with mTORC1 and may direct mTORC1 to the Rheb-containing vesicular compartment in response to amino acid sufficiency, enabling Rheb-GTP activation of mTORC1. The type III phosphatidylinositol kinase also participates in amino acid-dependent mTORC1 activation, although the site of action of its product, 3'OH-phosphatidylinositol, in this process is unclear.

Glutamine increases autophagy under Basal and stressed conditions in intestinal epithelial cells

BACKGROUND & AIMS

Glutamine plays a protective role in intestinal cells during physiologic stress; however, the protection mechanisms are not fully understood. Autophagy functions in bulk degradation of cellular components, but has been recognized recently as an important mechanism for cell survival under conditions of stress. We therefore sought to see if glutamine's actions involve the induction of autophagy in intestinal cells and, if so, the mechanisms that underlie this action.

METHODS

Formation of microtubule-associated protein light chain 3 (LC3)-phospholipid conjugates (LC3-II) in rat intestinal epithelial IEC-18 cells and human colonic epithelial Caco-2(BBE) cells was determined by Western blotting and localized by confocal microscopy. Activation of mammalian target of rapamycin (mTOR) pathway, mitogen-activated protein (MAP) kinases, caspase-3, and poly (ADP-ribose) polymerase were monitored by Western blotting.

RESULTS

Glutamine increased LC3-II as well as the number of autophagosomes. Glutamine-induced LC3-II formation was paralleled by inactivation of mTOR and p38 MAP kinase pathways, and inhibition of mTOR and p38 MAP kinase allowed LC3-II induction in glutamine-deprived cells. Under glutamine starvation, LC3-II recovery after heat stress or the increase under oxidative stress was blunted significantly. Glutamine depletion increased caspase-3 and poly (ADP-ribose) polymerase activity after heat stress, which was inhibited by treatment with inhibitors of mTOR and p38 MAP kinase.

CONCLUSIONS

Glutamine induces autophagy under basal and stressed conditions, and prevents apoptosis under heat stress through its regulation of the mTOR and p38 MAP kinase pathways. We propose that glutamine contributes to cell survival during physiologic stress by induction of autophagy.

Cardioprotection requires taking out the trash

Autophagy is a critical cellular housekeeping process that is essential for removal of damaged or unwanted organelles and protein aggregates. Under conditions of starvation, it is also a mechanism to break down proteins to generate amino acids for synthesis of new and more urgently needed proteins. In the heart, autophagy is upregulated by starvation, reactive oxygen species, hypoxia, exercise, and ischemic preconditioning, the latter a well-known potent cardioprotective phenomenon. The observation that upregulation of autophagy confers protection against ischemia/reperfusion injury and inhibition of autophagy is associated with a loss of cardioprotection conferred by pharmacological conditioning suggests that the pathway plays a key role in enhancing the heart's tolerance to ischemia. While many of the antecedent signaling pathways of preconditioning are well-defined, the mechanisms by which preconditioning and autophagy converge to protect the heart are unknown. In this review we discuss mechanisms that potentially underlie the linkage between cardioprotection and autophagy in the heart.

Autophagy: a target for therapeutic interventions in myocardial pathophysiology

BACKGROUND

Autophagy is a major degradative and highly conserved process in eukaryotic cells that is activated by stress signals. This self-cannibalisation is activated as a response to changing environmental conditions, cellular remodelling during development and differentiation, and maintenance of homeostasis.

OBJECTIVE

To review autophagy regarding its process, molecular mechanisms and regulation in mammalian cells, and its role in myocardial pathophysiology.

RESULTS/CONCLUSION

Autophagy is a multistep process regulated by diverse, intracellular and/or extracellular signalling complexes and pathways. In the heart, normally, autophagy occurs at low basal levels, where it represents a homeostatic mechanism for the maintenance of cardiac function and morphology. However, in the diseased heart the functional role of the enhanced autophagy is unclear and studies have yielded conflicting results. Recently, it was shown that during myocardial ischemia autophagy promotes survival by maintaining energy homeostasis. Also, rapamycin was demonstrated to prevent cardiac hypertrophy. In heart failure, upregulation of autophagy acts as an adaptive response that protects cells from hemodynamic stress. In addition, sirolimus-eluting stents have been shown to lower re-stenosis rates in patients with coronary artery disease after angioplasty. Thus, this mechanism can become a major target for therapeutic intervention in heart pathophysiology.

Macroautophagy: the key ingredient to a healthy diet?

Dietary compounds can influence the risk of cancer and other diseases through diverse mechanisms which include the activation or inhibition of macroautophagy. Macroautophagy is a catabolic process for the lysosomal degradation and recycling of cytoplasmic constituents which has been implicated in several pathologies, including cancer and neurodegeneration. In some instances, macroautophagy acts to suppress tumor formation and neural degeneration. Thus, it may be feasible to design diets, supplements or therapeutics that can alter the level of macroautophagy within cells to prevent or treat disease. While critical questions still need to be answered before we can safely and effectively implement such a strategy, we provide here a review of the literature regarding dietary constituents that have a demonstrated macroautophagy-modulating function.

Isolation of hyperactive mutants of mammalian target of rapamycin

The mammalian target of rapamycin (mTOR) is a Ser/Thr kinase that plays essential roles in the regulation of a wide array of growth-related processes such as protein synthesis, cell sizing, and autophagy. mTOR forms two functionally distinct complexes, termed the mTOR complex 1 (mTORC1) and 2 (mTORC2); only the former of which is inhibited by rapamycin. Based on the similarity between the cellular responses caused by rapamycin treatment and by nutrient starvation, it has been widely accepted that modulation in the mTORC1 activity in response to nutrient status directs these cellular responses, although direct evidence has been scarce. Here we report isolation of hyperactive mutants of mTOR. The isolated mTOR mutants exhibited enhanced kinase activity in vitro and rendered cells refractory to the dephosphorylation of the mTORC1 substrates upon amino acid starvation. Cells expressing the hyperactive mTOR mutant displayed larger cell size in a normal growing condition and were resistant to cell size reduction and autophagy induction in an amino acid-starved condition. These results indicate that the activity of mTORC1 actually directs these cellular processes in response to nutrient status and confirm the biological functions of mTORC1, which had been proposed solely from loss-of-function analyses using rapamycin and (molecular)genetic techniques. Additionally, the hyperactive mTOR mutant did not induce cellular transformation of NIH/3T3 cells, suggesting that concomitant activation of additional pathways is required for tumorigenesis. This hyperactive mTOR mutant will be a valuable tool for establishing physiological consequences of mTOR activation in cells as well as in organisms.

Amino acid regulation of autophagosome formation

Amino acids are not only substrates for various metabolic pathways, but can also serve as signaling molecules controlling signal transduction pathways. One of these signaling pathways is mTOR-dependent and is activated by amino acids (leucine in particular) in synergy with insulin. Activation of this pathway inhibits autophagy. Because activation of mTOR-mediated signaling also stimulates protein synthesis, it appears that protein synthesis and autophagic protein degradation are reciprocally controlled by the same signaling pathway. Recent developments indicate that amino acid-stimulated mTOR-dependent signaling is subject to complex regulation. The mechanism by which amino acids stimulate mTORdependent signaling (and other signaling pathways), and its molecular connection with the autophagic machinery, is still unknown.

Autophagy in the liver

A great part of our current understanding of mammalian macroautophagy is derived from studies of the liver. The term "autophagy" was introduced by Christian de Duve in part based on ultrastructural changes in rat liver following glucagon injection. Subsequent morphological, biochemical, and kinetics studies of autophagy in the liver defined the basic process of autophagosome formation, maturation, and degradation and the regulation of autophagy by hormones, phosphoinositide 3-kinases, and mammalian target of rapamycin. It is now clear that macroautophagy in the liver is important for the balance of energy and nutrients for basic cell functions, the removal of misfolded proteins resulting from genetic mutations or pathophysiological stimulations, and the turnover of major subcellular organelles such as mitochondria, endoplasmic reticulum, and peroxisomes under both normal and pathophysiological conditions. Disturbance of autophagy function in the liver could thus have a major impact on liver physiology and liver disease.

Rapamycin inhibits polyglutamine aggregation independently of autophagy by reducing protein synthesis

Accumulation of misfolded proteins and protein assemblies is associated with neuronal dysfunction and death in several neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's disease (HD). It is therefore critical to understand the molecular mechanisms of drugs that act on pathways that modulate misfolding and/or aggregation. It is noteworthy that the mammalian target of rapamycin inhibitor rapamycin or its analogs have been proposed as promising therapeutic compounds clearing toxic protein assemblies in these diseases via activation of autophagy. However, using a cellular model of HD, we found that rapamycin significantly decreased aggregation-prone polyglutamine (polyQ) and expanded huntingtin and its inclusion bodies (IB) in both autophagy-proficient and autophagy-deficient cells (by genetic knockout of the atg5 gene in mouse embryonic fibroblasts). This result suggests that rapamycin modulates the levels of misfolded polyQ proteins via pathways other than autophagy. We show that rapamycin reduces the amount of soluble polyQ protein via a modest inhibition of protein synthesis that in turn significantly reduces the formation of insoluble polyQ protein and IB formation. Hence, a modest reduction in huntingtin synthesis by rapamycin may lead to a substantial decrease in the probability of reaching the critical concentration required for a nucleation event and subsequent toxic polyQ aggregation. Thus, in addition to its beneficial effect proposed previously of reducing polyQ aggregation/toxicity via autophagic pathways, rapamycin may alleviate polyQ disease pathology via its effect on global protein synthesis. This finding may have important therapeutic implications.

New insights into the mechanisms of macroautophagy in mammalian cells

Macroautophagy is a self-digesting pathway responsible for the removal of long-lived proteins and organelles by the lysosomal compartment. Parts of the cytoplasm are first segregated in double-membrane-bound autophagosomes, which then undergo a multistep maturation process including fusion with endosomes and lysosomes. The segregated cytoplasm is then degraded by the lysosomal hydrolases. The discovery of ATG genes has greatly enhanced our understanding of the mechanisms of this pathway. Two novel ubiquitin-like protein conjugation systems were shown to function during autophagosome formation. Autophagy has been shown to play a role in a wide variety of physiological processes including energy metabolism, organelle turnover, growth regulation, and aging. Impaired autophagy can lead to diseases such as cardiomyopathy and cancer. This review summarizes current knowledge about the formation and maturation of autophagosomes, the role of macroautophagy in various physiological and pathological conditions, and the signaling pathways that regulate this process in mammalian cells.

Autophagy: process and function

Autophagy is an intracellular degradation system that delivers cytoplasmic constituents to the lysosome. Despite its simplicity, recent progress has demonstrated that autophagy plays a wide variety of physiological and pathophysiological roles, which are sometimes complex. Autophagy consists of several sequential steps--sequestration, transport to lysosomes, degradation, and utilization of degradation products--and each step may exert different function. In this review, the process of autophagy is summarized, and the role of autophagy is discussed in a process-based manner.

[Cellular regulation of anabolism and catabolism in skeletal muscle during immobilisation, aging and critical illness]

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.

Developmental commitment in Dictyostelium discoideum

Upon starvation, Dictyostelium discoideum cells halt cell proliferation, aggregate into multicellular organisms, form migrating slugs, and undergo morphogenesis into fruiting bodies while differentiating into dormant spores and dead stalk cells. At almost any developmental stage cells can be forced to dedifferentiate when they are dispersed and diluted into nutrient broth. However, migrating slugs can traverse lawns of bacteria for days without dedifferentiating, ignoring abundant nutrients and continuing development. We now show that developing Dictyostelium cells revert to the growth phase only when bacteria are supplied during the first 4 to 6 h of development but that after this time, cells continue to develop regardless of the presence of food. We postulate that the cells' inability to revert to the growth phase after 6 h represents a commitment to development. We show that the onset of commitment correlates with the cells' loss of phagocytic function. By examining mutant strains, we also show that commitment requires extracellular cyclic AMP (cAMP) signaling. Moreover, cAMP pulses are sufficient to induce both commitment and the loss of phagocytosis in starving cells, whereas starvation alone is insufficient. Finally, we show that the inhibition of development by food prior to commitment is independent of contact between the cells and the bacteria and that small soluble molecules, probably amino acids, inhibit development during the first few hours and subsequently the cells become unable to react to the molecules and commit to development. We propose that commitment serves as a checkpoint that ensures the completion of cooperative aggregation of developing Dictyostelium cells once it has begun, dampening the response to nutritional cues that might inappropriately block development.

Regulation of macroautophagy by mTOR and Beclin 1 complexes

Macroautophagy or autophagy is a vacuolar degradative pathway terminating in the lysosomal compartment after forming a cytoplasmic vacuole or autophagosome that engulfs macromolecules and organelles. The original discovery that ATG (AuTophaGy related) genes in yeast are involved in the formation of autophagosomes has greatly increased our knowledge of the molecular basis of autophagy, and its role in cell function that extends far beyond non-selective degradation. The regulation of autophagy by signaling pathways overlaps the control of cell growth, proliferation, cell survival and death. The evolutionarily conserved TOR (Target of Rapamycin) kinase complex 1 plays an important role upstream of the Atg1 complex in the control of autophagy by growth factors, nutrients, calcium signaling and in response to stress situations, including hypoxia, oxidative stress and low energy. The Beclin 1 (Atg6) complex, which is involved in the initial step of autophagosome formation, is directly targeted by signaling pathways. Taken together, these data suggest that multiple signaling checkpoints are involved in regulating autophagosome formation.

Regulation of various proteolytic pathways by insulin and amino acids in human fibroblasts

Intracellular protein degradation is a regulated process with several proteolytic pathways. Although regulation of macroautophagy has been investigated in some detail in hepatocytes and in few other cells, less is known on this regulation in other cells and proteolytic pathways. We show that in human fibroblasts insulin and amino acids reduce protein degradation by different signalling pathways and that this inhibition proceeds in part via the mammalian target of rapamycin, especially with amino acids, which probably increase lysosomal pH. Moreover, the regulatory amino acids (Phe, Arg, Met, Tyr, Trp and Cys) are partially different from other cells. Finally, and in addition to macroautophagy, insulin and amino acids modify, to different extents and sometimes in opposite directions, the activities of other proteolytic pathways.

Amino acids and insulin act additively to regulate components of the ubiquitin-proteasome pathway in C2C12 myotubes

Background

The ubiquitin-proteasome system is the predominant pathway for myofibrillar proteolysis but a previous study in C2C12 myotubes only observed alterations in lysosome-dependent proteolysis in response to complete starvation of amino acids or leucine from the media. Here, we determined the interaction between insulin and amino acids in the regulation of myotube proteolysis

Results

Incubation of C2C12 myotubes with 0.2 × physiological amino acids concentration (0.2 × PC AA), relative to 1.0 × PC AA, significantly increased total proteolysis and the expression of 14-kDa E2 ubiquitin conjugating enzyme (p < 0.05). The proteasome inhibitor MG132 blocked the rise in proteolysis observed in the 0.2 × PC AA media. Addition of insulin to the medium inhibited proteolysis at both 0.2 and 1.0× PC AA and the expression of 14-kDa E2 proteins and C2 sub unit of 20 S proteasome (p < 0.05). Incubation of myotubes with increasing concentrations of leucine in the 0.2 × PC AA media inhibited proteolysis but only in the presence of insulin. Incubation of rapamycin (inhibitor of mTOR) inhibited amino acid or insulin-dependent p70 S6 kinase phosphorylation, blocked (P < 0.05) the inhibitory effects of 1.0 × PC AA on protein degradation, but did not alter the inhibitory effects of insulin or leucine

Conclusion

In a C2C12 myotube model of myofibrillar protein turnover, amino acid limitation increases proteolysis in a ubiquitin-proteasome-dependent manner. Increasing amino acids or leucine alone, act additively with insulin to down regulate proteolysis and expression of components of ubiquitin-proteasome pathway. The effects of amino acids on proteolysis but not insulin and leucine, are blocked by inhibition of the mTOR signalling pathway.

An obligatory requirement for the heterotrimeric G protein Gi3 in the antiautophagic action of insulin in the liver

Heterotrimeric G proteins of the G(i) class have been implicated in signaling pathways regulating growth and metabolism under physiological and pathophysiological conditions. Knockout mice carrying inactivating mutations in both of the widely expressed Galpha(i) class genes, Galpha(i2) and Galpha(i3), demonstrate shared as well as gene-specific functions. The presence of a single active allele of Galpha(i3) is sufficient for embryonic development, whereas at least one allele of Galpha(i2) is required for extrauterine life. Mice lacking both Galpha(i2) and Galpha(i3) are massively growth-retarded and die in utero. We have used biochemical and cell biological methods together with in situ liver perfusion experiments to study Galpha(i) isoform-specific functions in Galpha(i2)- and Galpha(i3)-deficient mice. The subcellular localization of Galpha(i3) in isolated mouse hepatocytes depends on the cellular metabolic status. Galpha(i3) localizes to autophagosomes upon starvation-induced autophagy and distributes to the plasma membrane upon insulin stimulation. Analysis of autophagic proteolysis in perfused mouse livers showed that mice lacking Galpha(i3) are deficient in the inhibitory action of insulin. These data indicate that Galpha(i3) is crucial for the antiautophagic action of insulin and suggest an as-yet-unrecognized function for Galpha(i3) on autophagosomal membranes.

Signalling and autophagy regulation in health, aging and disease

It has become clear in recent years that autophagy not only serves to produce amino acids for ongoing protein synthesis and to produce substrates for energy production when cells become starved but autophagy is also able to eliminate defective cell structures and for this reason the process may be implicated in several diseased states. Autophagy is controlled by complex signalling pathways, including that used by insulin. In these pathways, phosphatidylinositol 3-kinases and the protein kinase mTOR play important roles.

Autophagy: A cell repair mechanism that retards ageing and age-associated diseases and can be intensified pharmacologically

The process of ageing denotes a post-maturational deterioration of cells and organisms with the passage of time, an increased vulnerability to challenges and prevalence of age-associated diseases, and a decreased ability to survive. Causes may be found in an enhanced production of reactive oxygen species (ROS) and oxidative damage and not completed housekeeping, with an accumulation of altered ROS-hypergenerating organelles in older cells. It has been shown that autophagy is the only tier of defence against the accumulation of effete mitochondria and peroxisomes; that functioning of autophagy declines with increasing age and determinates cell and individual lifespan; that autophagy can be intensified by drugs; and that the pharmacological intensification of autophagy may be a big step towards retardation of ageing and prevention and therapy of age-associated diseases including neurodegeneration.

Suppression of myofibrillar proteolysis in chick skeletal muscles by α-ketoisocaproate

We previously reported that L-leucine suppresses myofibrillar proteolysis in chick skeletal muscles. In the current study, we compared the effects of L- and D-enantiomers of leucine on myofibrillar proteolysis in skeletal muscle of chicks. We also assessed whether leucine itself or its metabolite, alpha-ketoisocaproate (alpha-KIC), mediates the effects of leucine. Food-deprived (24 h) chicks were orally administered 225 mg/100 g body weight L-leucine, D-leucine or alpha-KIC and were sacrificed after 2 h. L-Leucine administration had an obvious inhibitory effect on myofibrillar proteolysis (plasma N(tau)-methylhistidine concentration) in chicks while D-leucine and alpha-KIC were much more effective. We also examined the expression of the proteolytic-related genes (ubiquitin, proteasome, m-calpain and cathepsin B) by real-time PCR of cDNA in chick skeletal muscles. Ubiquitin mRNA expression was decreased by D-leucine and alpha-KIC but not L-leucine. Proteasome and m-calpain mRNA expressions as well as cathepsin B mRNA expression were likewise decreased by L-leucine, D-leucine and alpha-KIC. These results indicate that D-leucine and alpha-KIC suppress proteolytic-related genes, resulting in an decrease in myofibrillar proteolysis while L-leucine is much less effective in skeletal muscle of chicks, may be explain by conversion of D-leucine to alpha-KIC.

Nutrient control of macroautophagy in mammalian cells

A growing number of evidences indicate a strict causality between the reduction of autophagic functionality and aging. In this context the preservation of a proper autophagic response is of paramount importance to preserve the cellular processes in aging cell. Nutrients availability, especially for amino acids, is the most physiological key regulator of macroautophagy. In mammalian cells the knowledge of the mechanism and the underlying regulation of macroautophagy has been greatly improved in recent years and we focus on the role of nutrients, in particular on their involvement in preventing cellular aging through the modulation of autophagy. This review covers the main features of macroautophagy regulation by nutrients, in particular amino acids as well as glucose and vitamins, and its mechanisms, focusing primarily on the mammalian hepatocyte, which has been extensively utilized to dissect signaling pathways underlying the regulation of macroautophagy.

Silencing mammalian target of rapamycin signaling by small interfering RNA enhances rapamycin-induced autophagy in malignant glioma cells

The mammalian target of rapamycin (mTOR) plays a central role in regulating the proliferation of malignant glioma cells, and mTOR-specific inhibitors such as rapamycin analogs are considered as promising therapy for malignant gliomas. However, the efficacy of mTOR inhibitors alone in the treatment of patients with malignant gliomas is only modest, potentially because these agents rather than acting as mTOR kinase inhibitors instead interfere with the function of only mTOR/raptor (regulatory-associated protein of mTOR) complex and thus do not perturb all mTOR functions. The purpose of this study was to determine whether global inhibition of the mTOR molecule enhances the antitumor effect of rapamycin on malignant glioma cells. We showed that rapamycin induced autophagy and that inhibition of autophagy by small interfering RNA (siRNA) directed against autophagy-related gene Beclin 1 attenuated the cytotoxicity of rapamycin in rapamycin-sensitive tumor cells, indicating that the autophagy was a primary mediator of rapamycin's antitumor effect rather than a protective response. Exogenous expression of an mTOR mutant interfering with its kinase activity markedly enhanced the incidence of rapamycin-induced autophagy. Moreover, silencing of mTOR with siRNA augmented the inhibitory effect of rapamycin on tumor cell viability by stimulating autophagy. Importantly, not only rapamycin-sensitive malignant glioma cells with PTEN mutations but also rapamycin-resistant malignant glioma cells with wild-type PTEN were sensitized to rapamycin by mTOR siRNA. These results indicate that rapamycin-induced autophagy is one of the agent's antitumor effects and that silencing or inhibiting mTOR kinase activity could enhance the effectiveness of rapamycin.

Insulin resistance accelerates muscle protein degradation: Activation of the ubiquitin-proteasome pathway by defects in muscle cell signaling

Conditions such as acidosis, uremia, and sepsis are characterized by insulin resistance and muscle wasting, but whether the insulin resistance associated with these disorders contributes to muscle atrophy is unclear. We examined this question in db/db mice with increased blood glucose despite high levels of plasma insulin. Compared with control littermate mice, the weights of different muscles in db/db mice and the cross-sectional areas of muscles were smaller. In muscle of db/db mice, protein degradation and activities of the major proteolytic systems, caspase-3 and the proteasome, were increased. We examined signals that could activate muscle proteolysis and found low values of both phosphatidylinositol 3 kinase (PI3K) activity and phosphorylated Akt that were related to phosphorylation of serine 307 of insulin receptor substrate-1. To assess how changes in circulating insulin and glucose affect muscle protein, we treated db/db mice with rosiglitazone. Rosiglitazone improved indices of insulin resistance and abnormalities in PI3K/Akt signaling and decreased activities of caspase-3 and the proteasome in muscle leading to suppression of proteolysis. Underlying mechanisms of proteolysis include increased glucocorticoid production, decreased circulating adiponectin, and phosphorylation of the forkhead transcription factor associated with increased expression of the E3 ubiquitin-conjugating enzymes atrogin-1/MAFbx and MuRF1. These abnormalities were also corrected by rosiglitazone. Thus, insulin resistance causes muscle wasting by mechanisms that involve suppression of PI3K/Akt signaling leading to activation of caspase-3 and the ubiquitin-proteasome proteolytic pathway causing muscle protein degradation.

Intracellular network of phosphatidylinositol 3-kinase, mammalian target of the rapamycin/70 kDa ribosomal S6 kinase 1, and mitogen-activated protein kinases pathways for regulating mycobacteria-induced IL-23 expression in human macrophages

We previously demonstrated that Mycobacterium tuberculosis (M. tbc)-induced interleukin (IL)-12 expression is negatively regulated by the phosphatidylinositol 3-kinase (PI3K) and extracellular signal-regulated kinase (ERK) 1/2 pathways in human monocyte-derived macrophages (MDMs). To extend these studies, we examined the nature of the involvement of toll-like receptors (TLRs) and intracellular signalling pathways downstream from PI3K in M. tbc-induced IL-23 expression in human MDMs. M. tbc-induced Akt activation and IL-23 expression were essentially dependent on TLR2. Blockade of the mammalian targets of rapamycin (mTOR)/70 kDa ribosomal S6 kinase 1 (S6K1) pathway by the specific inhibitor rapamycin greatly enhanced M. tbc-induced IL-12/IL-23 p40 (p40) and IL-23 p19 (p19) mRNA and IL-23 protein expression. In sharp contrast, p38 mitogen-activated protein kinase (MAPK) inhibition abrogated the p40 and p19 mRNA and IL-23 protein expression induced by M. tbc. Furthermore, the inhibition of PI3K-Akt, but not ERK 1/2 pathway, attenuated M. tbc-induced S6K1 phosphorylation, whereas PI3K inhibition enhanced p38 phosphorylation and apoptosis signal-regulating kinase 1 activity during exposure to M. tbc. Although the negative or positive regulation of IL-23 was not reversed by neutralization of IL-10, it was significantly modulated by blocking TLR2. Collectively, these findings provide new insight into the homeostatic mechanism controlling type 1 immune responses during mycobacterial infection involving the intracellular network of PI3K, S6K1, ERK 1/2 and p38 MAPK pathways in a TLR2-dependent manner.

Targets for antiageing drugs

The process of ageing denotes a postmaturational deterioration of cells and organisms with the passage of time, an increased vulnerability to challenges and a decreased ability to survive. Causes may be found in oxidative damage and incomplete cell maintenance, and both of these factors may be considered the primary targets of any antiageing therapy. Levels of lipophylic anti-oxidant defences may be the limiting factor of longevity in ad-libitum fed animals. An appropriate administration of lipophylic antioxidants (e.g., polyunsaturated fatty acids, vitamin E) and the stimulation of cytomembrane and organelle renewal, by calorie restriction and pharmacological intensification of autophagy, are currently available interventions that may extend longevity and retard the onset, as well as aid the treatment, of most age-associated diseases.

The amino acid sensitive TOR pathway from yeast to mammals

The target of rapamycin (TOR) is an ancient effector of cell growth that integrates signals from growth factors and nutrients. Two downstream effectors of mammalian TOR, the translational components S6K1 and 4EBP1, are commonly used as reporters of mTOR activity. The conical signaling cascade initiated by growth factors is mediated by PI3K, PKB, TSC1/2 and Rheb. However, the process through which nutrients, i.e., amino acids, activate mTOR remains largely unknown. Evidence exists for both an intracellular and/or a membrane bound sensor for amino acid mediated mTOR activation. Research in eukaryotic models, has implicated amino acid transporters as nutrient sensors. This review describes recent advances in nutrient signaling that impinge on mTOR and its targets including hVps34, class III PI3K, a transducer of nutrient availability to mTOR.

Autophagic stress in neuronal injury and disease

Autophagy is the regulated process by which cytoplasmic organelles and long-lived proteins are delivered for lysosomal degradation. Increased numbers of autophagosomes and autolysosomes often represent prominent ultrastructural features of degenerating or dying neurons. This morphology is characteristic not only of neurons undergoing pathologic degeneration, but also during developmental programmed cell death of some neuronal populations. In recent years, a growing number of reports highlight potentially important roles for autophagy-related processes in relation to protein aggregation, regulated cell death pathways, and neurodegeneration. While starvation-induced autophagy involves nonselective bulk degradation of cytoplasm, mechanisms that regulate selective targeting of damaged organelles form an emerging area. As the study of autophagy evolves from physiologic homeostasis to pathologic situations, consideration of terminology and definitions becomes important. Increased autophagic vacuoles do not necessarily correlate with increased autophagic activity or flux. Instead, the striking accumulation of autophagic vacuoles in dying or degenerating neurons likely reflects an imbalance between the rates of autophagic sequestration and completion of the degradative process. In other words, these cells can be thought of as undergoing "autophagic stress." The concept of autophagic stress may reconcile apparently conflicting roles of autophagy-related processes in adaptive, homeostatic responses and in pathways of neurodegeneration and cell death.

Cell hydration and mTOR-dependent signalling

Insulin- and amino acid-induced signalling by the mammalian target of rapamycin (mTOR) involves hyperphosphorylation of the p70 ribosomal S6 protein kinase (p70S6-kinase) and the eukaryotic initiation factor 4E (eIF4E) binding protein 4E-BP1 and contributes to regulation of protein metabolism. This review considers the impact of cell hydration on mTOR-dependent signalling. Although hypoosmotic hepatocyte swelling in some instances activates p70S6-kinase, the hypoosmolarity-induced proteolysis inhibition in perfused rat liver is insensitive to mTOR inhibition by rapamycin. Likewise, swelling-dependent proteolysis inhibition by insulin and swelling-independent proteolysis inhibition by leucine, a potent activator of p70S6-kinase and 4E-BP1 hyperphosphorylation, in perfused rat liver is insensitive to rapamycin, indicating that at least rapamycin-sensitive mTOR signalling is not involved. Hyperosmotic dehydration in different cell types produces inactivation of signalling components around mTOR, thereby attenuating insulin-induced glucose uptake, glycogen synthesis, and lipogenesis in adipocytes, and MAP-kinase phosphatase MKP-1 expression in hepatoma cells. Direct inactivation of mTOR, stimulation of the AMP-activated protein kinase, and the destabilization of individual proteins may impair mTOR signalling under dehydrating conditions. Further investigation of the crosstalk between the mTOR pathway(s) and hyperosmotic signalling will improve our understanding about the contribution of cell hydration changes in health and disease and will provide further rationale for fluid therapy of insulin-resistant states.

Microtubules facilitate autophagosome formation and fusion of autophagosomes with endosomes

Nutrient deprivation of eukaryotic cells provokes a variety of stress responses, including autophagy. Autophagy is carried out by autophagosomes which sequester cytosolic components and organelles for degradation after fusion with protease-containing endosomes. To determine the role of microtubules in autophagy, we used nocodazole and vinblastine to disrupt microtubules and independently measured formation and fusion of autophagsosomes in primary rat hepatocytes. By measuring the translocation of GFP-LC3, an autophagosomal marker, to autophagosomes and the lipidation of GFP-LC3, we quantified the rate and magnitude of autophagosome formation. Starvation increased both the rate of autophagosome formation over the basal level and the total number of autophagosomes per cell. Maximal autophagosome formation required an intact microtubule network. Fusion of autophagosomes with endosomes, assayed by acquisition of protease-inhibitor sensitivity as well as overlap with LysoTracker Red-positive endosomes, required intact microtubules. Live-cell imaging demonstrated that autophagosomes were motile structures, and their movement also required microtubules. Interestingly, vinblastine stimulated autophagosome formation more than twofold before any discernable change in the microtubule network was observed. Stimulation of autophagosome formation by vinblastine was independent of nutrients and mTOR activity but was inhibited by depletion of the Autophagy proteins Atg5 and Atg6, known to be required for autophagy.

In yeast, loss of Hog1 leads to osmosensitivity of autophagy

In mammalian liver, proteolysis is regulated by the cellular hydration state in a microtubule- and p38(MAPK) (p38 mitogen-activated protein kinase)-dependent fashion. Osmosensing in liver cells towards proteolysis is achieved by activation of integrin receptors. The yeast orthologue of p38(MAPK) is Hog1 (high-osmolarity glycerol 1), which is involved in the hyperosmotic-response pathway. Since it is not known whether starvation-induced autophagy in yeast is osmosensitive and whether Hog1 is involved in this process, we performed fluorescence microscopy experiments. The hog1Delta cells exhibited a visible decrease of autophagy in hypo-osmotic and hyperosmotic nitrogen-starvation medium as compared with normo-osmolarity, as determined by GFP (green fluorescent protein)-Atg8 (autophagy-related 8) fluorescence. Western blot analysis of GFP-Atg8 degradation showed that WT (wild-type) cells maintained a stable autophagic activity over a broad osmolarity range, whereas hog1Delta cells showed an impaired autophagic actitivity during hypo- and hyper-osmotic stress. In [3H]leucine-pre-labelled yeast cells, the proteolysis rate was osmodependent only in hog1Delta cells. Neither maturation of pro-aminopeptidase I nor vitality was affected by osmotic stress in either yeast strain. In contrast, rapamycin-dependent autophagy, as measured by degradation of GFP-Atg8, did not significantly respond to hypo-osmotic or hyperosmotic stress in hog1Delta or WT cells. We conclude that Hog1 plays a role in the stabilization machinery of nitrogen-deprivation-induced autophagy in yeast cells during ambient osmolarity changes. This could be an analogy to the p38(MAPK) pathway in mammalian liver, where osmosensing towards p38(MAPK) is required for autophagy regulation by hypo-osmotic or amino-acid-induced cell swelling. A phenotypic difference is observed in rapamycin-induced autophagy, which does not seem to respond to extracellular osmolarity changes in hog1Delta cells.

Autophagy and signaling: their role in cell survival and cell death

Macroautophagy is a vacuolar, self-digesting mechanism responsible for the removal of long-lived proteins and damaged organelles by the lysosome. The discovery of the ATG genes has provided key information about the formation of the autophagosome, and about the role of macroautophagy in allowing cells to survive during nutrient depletion and/or in the absence of growth factors. Two connected signaling pathways encompassing class-I phosphatidylinositol 3-kinase and (mammalian) target of rapamycin play a central role in controlling macroautophagy in response to starvation. However, a considerable body of literature reports that macroautophagy is also a cell death mechanism that can occur either in the absence of detectable signs of apoptosis (via autophagic cell death) or concomitantly with apoptosis. Macroautophagy is activated by signaling pathways that also control apoptosis. The aim of this review is to discuss the signaling pathways that control macroautophagy during cell survival and cell death.

Leucine suppresses myofibrillar proteolysis by down-regulating ubiquitin-proteasome pathway in chick skeletal muscles

In skeletal muscle, amino acids, together with hormones, are key regulators of protein metabolism. Leucine, in particular, has inhibitory effects of protein degradation in skeletal muscles, but the mechanisms are poorly understood. The present study addressed the role of leucine as a regulator of myofibrillar proteolysis in cultured chick myotubes and chick skeletal muscles, and aimed to determine which cellular responses regulate the process. In chick myotubes, leucine suppressed myofibrillar proteolysis (as measured by N(tau)-methylhistidine release), while also decreasing ubiquitin and proteasome C2 subunit mRNA. Oral administration of leucine also suppressed myofibrillar proteolysis (as measured by plasma N(tau)-methylhistidine concentration), while also decreasing proteasome C2 subunit mRNA in chick skeletal muscle. Leucine activated the phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) (but not the mammalian target of rapamycin) inhibition of these pathways and increased myofibrillar proteolysis, ubiquitin and proteasome C2 subunit mRNA. Thus, an important component of muscle proteolysis inhibition by leucine, through the PI3K and PKC, is its ability to suppress transcription of the ubiquitin and proteasome C2 subunit, and degradation of myofibrillar protein.

Macroautophagy--a novel Beta-amyloid peptide-generating pathway activated in Alzheimer's disease

Macroautophagy, which is a lysosomal pathway for the turnover of organelles and long-lived proteins, is a key determinant of cell survival and longevity. In this study, we show that neuronal macroautophagy is induced early in Alzheimer's disease (AD) and before β-amyloid (Aβ) deposits extracellularly in the presenilin (PS) 1/Aβ precursor protein (APP) mouse model of β-amyloidosis. Subsequently, autophagosomes and late autophagic vacuoles (AVs) accumulate markedly in dystrophic dendrites, implying an impaired maturation of AVs to lysosomes. Immunolabeling identifies AVs in the brain as a major reservoir of intracellular Aβ. Purified AVs contain APP and β-cleaved APP and are highly enriched in PS1, nicastrin, and PS-dependent γ-secretase activity. Inducing or inhibiting macroautophagy in neuronal and nonneuronal cells by modulating mammalian target of rapamycin kinase elicits parallel changes in AV proliferation and Aβ production. Our results, therefore, link β-amyloidogenic and cell survival pathways through macroautophagy, which is activated and is abnormal in AD.

Isoleucine, a blood glucose-lowering amino acid, increases glucose uptake in rat skeletal muscle in the absence of increases in AMP-activated protein kinase activity

Leucine and isoleucine were shown to stimulate insulin-independent glucose uptake in skeletal muscle cells in vitro. In this study, we examined the effects of leucine and isoleucine on blood glucose in food-deprived rats and on glucose metabolism in skeletal muscle in vivo. Furthermore, we investigated the possible involvement of the energy sensor, 5'-AMP-activated protein kinase (AMPK), in the modulation of glucose uptake in skeletal muscle, which is independent of insulin, and also in leucine- or isoleucine-stimulated glucose uptake. Oral administration of isoleucine, but not leucine, significantly decreased the plasma glucose concentration. An i.v. bolus of 2-[1,2-3H]-deoxyglucose (2-[3H]DG) was administered to calculate glucose uptake. Glucose uptake in the skeletal muscle did not differ after leucine administration, but glucose uptake in the muscles of rats administered isoleucine was 73% greater than in controls, suggesting that isoleucine increases skeletal muscle glucose uptake in vivo. On the contrary, in the skeletal muscles, administration of leucine but not isoleucine significantly increased [U-14C]-glucose incorporation into glycogen compared with controls. AMPK alpha1 activity in skeletal muscle was not affected by leucine or isoleucine administration. However, isoleucine, but not leucine, significantly decreased AMPK alpha2 activity. The decrease in AMPK alpha2 activity was thought to be due to decreases in AMP content and the AMP:ATP ratio, which were related to the isoleucine administration. This is the first report of isoleucine stimulating glucose uptake in rat skeletal muscle in vivo, and these results indicate that there might be a relation between the reduction in blood glucose and the increase in skeletal muscle glucose uptake that occur with isoleucine administration in rats. The alterations in glucose metabolism caused by isoleucine may result in an improvement of the availability of ATP in the absence of increases in AMP-activated protein kinase activity in skeletal muscle.

Inhibition of Akt signaling and enhanced ERK1/2 activity are involved in induction of macroautophagy by triterpenoid B-group soyasaponins in colon cancer cells

Triterpenoid B-group soyasaponins have been found to induce macroautophagy in human colon cancer cells at concentrations obtainable through consumption of legume foodstuffs. In the present studies the mechanism(s) for this autophagy-inducing action of soyasaponins was evaluated by measuring changes in signal transduction pathways associated with autophagy. Specifically, inhibition of the Akt signaling pathway and enhanced activity of ERK1/2 have previously been implicated in controlling induction of macroautophagy in mammalian cancer cells. Here we show that these pathways are also involved in B-group soyasaponin-induced macroautophagy, as changes in enzyme activities preceded significant increases in autophagic activity. The autophagic capacity of HCT-15 cells was significantly increased by 6 h post-saponin exposure, which led us to measure alterations in signaling events that preceded this time point. We determined that exposure to B-group soyasaponins suppressed Akt activity maximally by 50%, which was associated with a reduction in the activating phosphorylation of the Akt-serine473 residue. In addition, ERK1/2 activity was significantly increased by 60%, and was determined to be necessary for B-group soyasaponin-induced autophagy. The raf-1 kinase has been identified as a potential point of cross-talk between the Akt and ERK1/2 signaling cascades. Following B-group soyasaponin treatment activity of raf-1 was significantly increased by a maximal 200%, suggesting that this enzyme in part modulates the enhanced ERK1/2 activity. These results provide new insights into the signaling events that control induction of autophagy by B-group soyasaponins in human colon cancer cells and suggest that soyasaponins warrant further study as potential colon cancer chemopreventive agents.

Synergistic Augmentation of Rapamycin-Induced Autophagy in Malignant Glioma Cells by Phosphatidylinositol 3-Kinase/Protein Kinase B Inhibitors

The mammalian target of rapamycin (mTOR) is a downstream effector of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway and a central modulator of cell proliferation in malignant gliomas. Therefore, the targeting of mTOR signaling is considered a promising therapy for malignant gliomas. However, the mechanisms underlying the cytotoxic effects of a selective mTOR inhibitor, rapamycin, on malignant glioma cells are poorly understood. The purpose of this study was thus to elucidate how rapamycin exerts its cytotoxic effects on malignant glioma cells. We showed that rapamycin induced autophagy but not apoptosis in rapamycin-sensitive malignant glioma U87-MG and T98G cells by inhibiting the function of mTOR. In contrast, in rapamycin-resistant U373-MG cells, the inhibitory effect of rapamycin was minor, although the phosphorylation of p70S6 kinase, a molecule downstream of mTOR, was remarkably inhibited. Interestingly, a PI3K inhibitor, LY294002, and an Akt inhibitor, UCN-01 (7-hydroxystaurosporine), both synergistically sensitized U87-MG and T98G cells as well as U373-MG cells to rapamycin by stimulating the induction of autophagy. Enforced expression of active Akt in tumor cells suppressed the combined effects of LY294002 or UCN-01, whereas dominant-negative Akt expression was sufficient to increase the sensitivity of tumor cells to rapamycin. These results indicate that rapamycin exerts its antitumor effect on malignant glioma cells by inducing autophagy and suggest that in malignant glioma cells a disruption of the PI3K/Akt signaling pathway could greatly enhance the effectiveness of mTOR inhibitors.

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

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

Nonsynaptic localization of the excitatory amino acid transporter 4 in photoreceptors

Excitatory amino acid transporters (EAATs) are involved in regulating extracellular glutamate levels at synaptic regions in the CNS. EAAT1, 2, 3, and 5 have been found in the mammalian retina, but the presence of EAAT4 has remained controversial. Recently, we found a high level of EAAT4 mRNA in the human retina, and this observation lead us to examine whether EAAT4 was expressed in the mammalian retina. Immunoblotting studies showed the presence of EAAT4-immunoreactive proteins in human and mouse retinas, corresponding to EAAT4 monomers and dimers. Immunohistochemistry revealed that EAAT4 was localized in rod and cone photoreceptor outer segments in the human retina, and in the outer and inner segments of mouse and ground squirrel retinas. In no case was EAAT4 found in the outer plexiform layer or in any other layer in the retina. EAAT4 expression by photoreceptors was confirmed by immunoblotting a purified rod outer segment preparation, which showed the presence of a 50-kDa EAAT4-immunoreactive protein. In addition, the EAAT4-associated protein, GTRAP41, was found in the human, mouse, and squirrel retinas as well as in the rod outer segment preparation. Further immunocytochemical and co-immunoprecipitation experiments demonstrated that GTRAP41 was colocalized and interacted in vivo with EAAT4. Importantly, glutamate uptake and drug inhibition experiments showed that an EAAT4-like glutamate uptake system is present in the rod outer segments. Finally, we examined whether glutamate signaling mediated by EAAT4 can modulate rod outer segment phagocytosis by the retinal pigment epithelium. Results of the present study show that EAAT4 is present in the outer segments, a nonsynaptic region of photoreceptors, where it might provide a feedback mechanism for sensing extracellular glutamate or serve as an outer barrier to prevent glutamate from escaping from the retina.

Regulation of mTOR by amino acids and resistance exercise in skeletal muscle

Resistance exercise disturbs skeletal muscle homeostasis leading to activation of catabolic and anabolic processes within the muscle cell. A current challenge of exercise biology is to describe the molecular mechanisms of regulation by which contractile activity stimulates net protein breakdown during exercise and net protein synthesis during recovery. Muscle growth is optimized by combining exercise and appropriate nutritional strategies, such as amino acid (AA) and carbohydrate ingestion. The effects are integrated at the level of one central regulatory protein, mTOR (mammalian target of rapamycin). mTOR is a complex protein integrating signals of the energetic status of the cell and environmental stimuli to control protein synthesis, protein breakdown and therefore cell growth. mTOR is known to be activated by insulin, and the mechanisms involved are well documented. The ways by which exercise and AA lead to mTOR activation remain partially unclear. Exercise and AA use different signalling pathways upstream of mTOR. Exercise seems to recruit partially the same pathway as insulin, whereas AA could act more directly on mTOR. During resistance exercise, the activity of mTOR could be acutely blunted by AMP-activated protein kinase (AMPK), thus inhibiting protein synthesis and enhancing AA availability for energy metabolism. During recovery, the inhibition of mTOR by AMPK is suppressed, and its activation is maximized by the presence of AA. There appears to be a requirement for a minimal concentration of plasma insulin to stimulate muscle protein synthesis in response to resistance exercise and AA ingestion.

In rat hepatocytes glucagon increases mammalian target of rapamycin phosphorylation on serine 2448 but antagonizes the phosphorylation of its downstream targets induced by insulin and amino acids

The major function of mammalian target of rapamycin (mTOR) is the control of cell growth. Insulin and amino acids regulate the mTOR pathway, and both are needed to promote its maximal activation. To further understand mTOR regulation by insulin and amino acids, we have studied the enzyme in primary cultures of hepatocytes. We show that insulin increases mTOR phosphorylation on Ser2448, a consensus phosphorylation site for protein kinase B (PKB). Ser2448 phosphorylation is also increased by amino acids, although they do not activate PKB. Furthermore, insulin and amino acids have an additive effect, indicating that they act through distinct pathways. We also show that phosphorylation of Ser2448 does not seem to modulate in vitro phosphorylation of eukaryotic initiation factor 4E-binding protein 1 by mTOR. However, stimulation of hepatocytes with insulin and amino acids leads to an increase in mTOR kinase activity. Rapamycin has no effect on insulin-, glucagon-, and 8-(4-chlorophenylthio)adenosine-cAMP-induced amino acid transport. Surprisingly, glucagon and 8-(4-chlorophenylthio)adenosine-cAMP, which do not activate PKB, stimulate the phosphorylation on Ser2448 of mTOR. However, glucagon inhibits amino acid- and insulin-induced activation of ribosomal S6 protein kinase 1 and phosphorylation of the translational repressor eukaryotic initiation factor 4E-binding protein 1. Our results demonstrate that glucagon, which is not able to activate but rather inhibits the mTOR pathways, stimulates the phosphorylation of mTOR on Ser2448. This finding suggests that phosphorylation of this site might not be sufficient for mTOR kinase activity but is likely to be involved in other functions.

Programmed Autophagy in the Drosophila Fat Body Is Induced by Ecdysone through Regulation of the PI3K Pathway

Eukaryotic cells catabolize their own cytoplasm by autophagy in response to amino acid starvation and inductive signals during programmed tissue remodeling and cell death. The Tor and PI3K signaling pathways have been shown to negatively control autophagy in eukaryotes, but the mechanisms that link these effectors to overall animal development and nutritional status in multicellular organisms remain poorly understood. Here, we reveal a complex regulation of programmed and starvation-induced autophagy in the Drosophila fat body. Gain-of-function genetic analysis indicated that ecdysone receptor signaling induces programmed autophagy whereas PI3K signaling represses programmed autophagy. Genetic interaction studies showed that ecdysone signaling downregulates PI3K signaling and that this represents the effector mechanism for induction of programmed autophagy. Hence, these studies link hormonal induction of autophagy to the regulatory function of the PI3K signaling pathway in vivo.