Amino acid control of proteolysis in perfused livers of synchronously fed rats. Mechanism and specificity of alanine co-regulation

Autophagy: molecular machinery, regulation, and implications for renal pathophysiology

Autophagy is a cellular process of "self-eating." During autophagy, a portion of cytoplasm is enveloped in double membrane-bound structures called autophagosomes, which undergo maturation and fusion with lysosomes for degradation. At the core of the molecular machinery of autophagy is a specific family of genes or proteins called Atg. Originally identified in yeast, Atg orthologs are now being discovered in mammalian cells and have been shown to play critical roles in autophagy. Traditionally, autophagy is recognized as a cellular response to nutrient deprivation or starvation whereby cells digest cytoplasmic organelles and macromolecules to recycle nutrients for self-support. However, studies during the last few years have indicated that autophagy is a general cellular response to stress. Interestingly, depending on experimental conditions, especially stress levels, autophagy can directly induce cell death or act as a mechanism of cell survival. In this review, we discuss the molecular machinery, regulation, and function of autophagy. In addition, we analyze the recent findings of autophagy in renal systems and its possible role in renal pathophysiology.

Amino acid sensing and mTOR regulation: inside or out?

mTOR (mammalian target of rapamycin) plays a key role in determining how growth factor, nutrient and oxygen levels modulate intracellular events critical for the viability and growth of the cell. This is reflected in the impact of aberrant mTOR signalling on a number of major human diseases and has helped to drive research to understand how TOR (target of rapamycin) is itself regulated. While it is clear that amino acids can affect TOR signalling, how these molecules are sensed by TOR remains controversial, perhaps because cells use different mechanisms as environmental conditions change. Even the question of whether they have an effect inside the cell or at its surface remains unresolved. The present review summarizes current ideas and suggests ways in which some of the models proposed might be unified to produce an amino acid detection system that can adapt to environmental change.

Potential antiproteolytic effects of L-leucine: observations of in vitro and in vivo studies

The purpose of present review is to describe the effect of leucine supplementation on skeletal muscle proteolysis suppression in both in vivo and in vitro studies. Most studies, using in vitro methodology, incubated skeletal muscles with leucine with different doses and the results suggests that there is a dose-dependent effect. The same responses can be observed in in vivo studies. Importantly, the leucine effects on skeletal muscle protein synthesis are not always connected to the inhibition of skeletal muscle proteolysis. As a matter of fact, high doses of leucine incubation can promote suppression of muscle proteolysis without additional effects on protein synthesis, and low leucine doses improve skeletal muscle protein ynthesis but have no effect on skeletal muscle proteolysis. These research findings may have an important clinical relevancy, because muscle loss in atrophic states would be reversed by specific leucine supplementation doses. Additionally, it has been clearly demonstrated that leucine administration suppresses skeletal muscle proteolysis in various catabolic states. Thus, if protein metabolism changes during different atrophic conditions, it is not surprising that the leucine dose-effect relationship must also change, according to atrophy or pathological state and catabolism magnitude. In conclusion, leucine has a potential role on attenuate skeletal muscle proteolysis. Future studies will help to sharpen the leucine efficacy on skeletal muscle protein degradation during several atrophic states.

Impact of polysol, a newly developed preservation solution, on cold storage of steatotic rat livers

Chronic shortage of donor organs has led to acceptance of steatotic livers as grafts, although there is a higher risk of primary graft dysfunction. We herein report the beneficial impact of Polysol, a newly developed preservation solution, on cold storage of steatotic rat livers. Dietary hepatic steatosis was induced in Wistar rats by 2-day fasting and subsequent 3-day re-feeding with a fat-free, carbohydrate-rich diet. Fatty livers were retrieved, flushed and then stored at 4 degrees C for 24 hours with either HTK or Polysol. Functional integrity of the grafts was evaluated by isolated reperfusion with oxygenated Krebs-Henseleit buffer at 37 degrees C for 45 minutes in both groups. Polysol preservation resulted in significant reductions of not only parenchymal (AST (IU/L); 6728+/-824 in HTK vs. 3107+/-718 in Polysol; P < 0.001) but also mitochondrial (GLDH (IU/L); 3189+/-773 vs. 1282+/-365; P < 0.01) enzyme release throughout reperfusion. Moreover, PVP (16.9+/-2.7 vs. 7.8+/-1.5 mmHg; P < 0.05), hepatic O2 consumption (0.291+/-0.047 vs. 1.056+/-0.053 micromol/g liver/min; P < 0.001), tissue ATP content (0.695+/-0.086 vs. 1.340+/-0.157 micromol/g dry-liver; P < 0.005), bile production (0.79+/-0.11 vs. 4.08+/-0.66 microL/g liver/45-min; P < 0.001), malondialdehyde into the perfusate (1.922+/-0.198 vs. 0.573+/-0.094 nmol/L; P < 0.0001) and wet/dry-weight ratio of the liver tissues (5.20+/-0.31 vs. 3.85+/-0.15; P < 0.005) were all better preserved by Polysol. In line with these benefits, electron microscopy revealed that Polysol preservation substantially suppressed deleterious mitochondrial alterations in steatotic livers. In conclusion, cold storage using Polysol resulted in significantly better integrity and function of steatotic livers. Polysol, therefore, may be a new alternative especially for "marginal" organs.

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.

Human muscle protein synthesis is modulated by extracellular, not intramuscular amino acid availability: a dose-response study

To test the hypothesis that muscle protein synthesis (MPS) is regulated by the concentration of extracellular amino acids, we investigated the dose-response relationship between the rate of human MPS and the concentrations of blood and intramuscular amino acids. We increased blood mixed amino acid concentrations by up to 240 % above basal levels by infusion of mixed amino acids (Aminosyn 15, 44-261 mg kg-1 h-1) in 21 healthy subjects, (11 men 10 women, aged 29 +/- 2 years) and measured the rate of incorporation of D5-phenylalanine or D3-leucine into muscle protein and blood and intramuscular amino acid concentrations. The relationship between the fold increase in MPS and blood essential amino acid concentration ([EAA], mM) was hyperbolic and fitted the equation MPS = (2.68 x [EAA])/(1.51 + [EAA]) (P < 0.01). The pattern of stimulation of myofibrillar, sarcoplasmic and mitochondrial protein was similar. There was no clear relationship between the rate of MPS and the concentration of intramuscular EAAs; indeed, when MPS was increasing most rapidly, the concentration of intramuscular EAAs was below basal levels. We conclude that the rates of synthesis of all classes of muscle proteins are acutely regulated by the blood [EAA] over their normal diurnal range, but become saturated at high concentrations. We propose that the stimulation of protein synthesis depends on the sensing of the concentration of extracellular, rather than intramuscular EAAs.

Differential regulation of protein dynamics in splanchnic and skeletal muscle beds by insulin and amino acids in healthy human subjects

To determine the in vivo effect of amino acids (AAs) alone or in combination with insulin on splanchnic and muscle protein dynamics, we infused stable isotope tracers of AAs in 36 healthy subjects and sampled from femoral artery and vein and hepatic vein. The subjects were randomized into six groups and were studied at baseline and during infusions of saline (group 1), insulin (0.5 mU. kg(-1). min(-1)) (group 2), insulin plus replacement of AAs (group 3) insulin plus high-dose AAs (group 4), or somatostatin and baseline replacement doses of insulin, glucagon and GH plus high dose of AAs (group 5) or saline (group 6). Insulin reduced muscle release of AAs mainly by inhibition of protein breakdown. Insulin also enhanced AA-induced muscle protein synthesis (PS) and reduced leucine transamination. The main effect of AAs on muscle was the enhancement of PS. Insulin had no effect on protein dynamics or leucine transamination in splanchnic bed. However, AAs reduced protein breakdown and increased synthesis in splanchnic bed in a dose-dependent manner. AAs also enhanced leucine transamination in both splanchnic and muscle beds. Thus insulin's anabolic effect was mostly on muscle, whereas AAs acted on muscle as well as on splanchnic bed. Insulin achieved anabolic effect in muscle by inhibition of protein breakdown, enhancing AA-induced PS, and reducing leucine transamination. AAs largely determined protein anabolism in splanchnic bed by stimulating PS and decreasing protein breakdown.

Human protein metabolism: its measurement and regulation

The body's protein mass not only provides architectural support for cells but also serves vital roles in maintaining their function and survival. The whole body protein pool, as well as that of individual tissues, is determined by the balance between the processes of protein synthesis and degradation. These in turn are regulated by interactions among hormonal, nutritional, neural, inflammatory, and other influences. Prolonged changes in either the synthetic or degradative processes (or both) that cause protein wasting increase morbidity and mortality. The application of tracer kinetic methods, combined with measurements of the activity of components of the cellular signaling pathways involved in protein synthesis and degradation, affords new insights into the regulation of both protein synthesis and breakdown in vivo. These insights, including those from studies of insulin, insulin-like growth factor I, growth hormone, and amino acid-mediated regulation of muscle and whole body protein turnover, provide opportunities to develop and test therapeutic approaches with promise to minimize or prevent these adverse health consequences.

Latency, duration and dose response relationships of amino acid effects on human muscle protein synthesis

The components of the stimulatory effect of food on net deposition of protein are beginning to be identified and separated. One of the most important of these appears to be the effect of amino acids per se in stimulating muscle anabolism. Amino acids appear to have a linear stimulatory effect within the range of normal diurnal plasma concentrations from postabsorptive to postprandial. Within this range, muscle protein synthesis (measured by incorporation of stable isotope tracers of amino acids into biopsied muscle protein) appears to be stimulated approximately twofold; however, little further increase occurs when very high concentrations of amino acids (>2.5 times the normal postabsorptive plasma concentration) are made available. Amino acids provided in surfeit of the ability of the system to synthesize protein are disposed of by oxidation, ureagenesis and gluconeogenesis. The stimulatory effect of amino acids appears to be time dependent; a square wave increase in the availability of amino acids causes muscle protein synthesis to be stimulated and to fall back to basal values, despite continued amino acid availability. The relationship between muscle protein synthesis and insulin availability suggests that most of the stimulatory effects occur at low insulin concentrations, with large increases having no effect. These findings may have implications for our understanding of the body's requirements for protein. The maximal capacity for storage of amino acids as muscle protein probably sets an upper value on the extent to which amino acids can be stored after a single meal.

Tissue-specific regulation of mitochondrial and cytoplasmic protein synthesis rates by insulin

In vivo studies have reported conflicting effects of insulin on mixed tissue protein synthesis rates. To test the hypothesis that insulin has differential effects on synthesis rates of various protein fractions in different organs, we infused miniature swine (n = 8 per group) with saline, insulin alone (at 0.7 mU/kg(-1). min(-1)), or insulin plus an amino acid mixture for 8 h. Fractional synthesis rate (FSR) of mitochondrial and cytoplasmic proteins in liver, heart, and skeletal muscle, as well as myosin heavy chain (MHC) in muscle, were measured using L-[1-(13)C]leucine as a tracer. The FSR of mitochondrial and cytoplasmic proteins were highest in liver, followed by heart and then muscle. Mitochondrial FSR in muscle was higher during insulin and insulin plus amino acid infusions than during saline. Insulin had no significant effect on FSR of MHC in muscle. In contrast, FSR of both mitochondrial and cytoplasmic proteins were not stimulated by insulin in liver. Insulin also did not increase FSR of mitochondrial in heart, whereas insulin and amino acid stimulated FSR of cytoplasmic protein. In conclusion, insulin stimulates the synthesis of muscle mitochondrial proteins, with no significant stimulatory effect on synthesis of sarcoplasmic and MHC. These results demonstrate that insulin has different effects on synthesis rates of specific protein fractions in the liver, heart, and skeletal muscle.

Embryo growth and amino acid concentration profiles of broiler breeder eggs, embryos, and chicks after in ovo administration of amino acids

Two experiments were conducted to evaluate the effect of in ovo amino acid (AA) injections in broiler breeder eggs on AA utilization of embryos. All AA used in these experiments were pure crystalline AA in free-base form. Treatments in Experiment 1 comprised 1) control eggs (no injection), 2) 0.5 mL sterile-distilled water injected eggs, and 3) eggs injected with an AA solution suspended in 0.5 mL sterile-distilled water. Injections were administered into the yolk at Day 7 of incubation. At hatch, chicks were killed and bled, and plasma AA concentration was determined. Plasma AA concentration of hatched chicks decreased (P < 0.05) when water was injected. In addition, all AA from eggs injected with AA, except Glu and Lys, were decreased (P < 0.05) at hatch as compared to control eggs. However, AA pattern was not affected by in ovo water injection, but the AA ratio to Lys was reduced by in ovo AA injection. Experiment 2 was conducted to evaluate whole internal egg AA concentrations over incubation time in the presence or absence of in ovo AA administration. Treatments in Experiment 2 comprised 1) control eggs (no injection), and 2) eggs injected with a AA solution at Day 7 of incubation. The AA contents of embryo, yolk, albumen, and allantoic and amnion fluids were analyzed over time during incubation (Days 0, 7, 14, and 19 of incubation). On Day 14 of incubation, there were no differences in AA contents of all tissues between the control group and the group injected with AA on Day 7 of incubation. On Day 19 of incubation, AA contents of embryo, yolk, albumen, and allantoic and amnion fluids were increased (P < 0.05) as mediated by in ovo administration of AA at Day 7 of incubation. These results suggest that in ovo administration of AA may increase AA concentrations in chicken embryos and other egg contents.

Amino acid effects on translational repressor 4E-BP1 are mediated primarily by L-leucine in isolated adipocytes

Previous studies indicated that amino acids may activate the protein kinase activity of the target of rapamycin (TOR) and thereby augment and/or mimic the effects of insulin on protein synthesis, p70(S6k) phosphorylation, and multicellular clustering in adipocytes. To identify the individual amino acids responsible for these effects, the present study focused on the TOR substrate and translational repressor 4E-BP1. A complete mixture of amino acids stimulated the phosphorylation of 4E-BP1, decreasing its association with eukaryotic initiation factor eIF-4E. Studies on subsets of amino acids and individual amino acids showed that L-leucine was the amino acid responsible for most of the effects on 4E-BP1 phosphorylation; however, the presence of other amino acids was required to observe a maximal effect. The stimulatory effect of leucine was stereospecific and not mimicked by other branched chain amino acids but was mimicked by the leucine metabolite alpha-ketoisocaproate (alpha-KIC). The effect of alpha-KIC, but not leucine, was attenuated by the transaminase inhibitor (aminooxy)acetate. The latter result indicates that the effects of alpha-KIC required its conversion to leucine. Half-maximal stimulation of 4E-BP1 phosphorylation occurred at approximately 430 microM; therefore, the response was linear within the range of circulating concentrations of leucine found in various nutritional states.

Protective properties of amino acids in liver preservation: effects of glycine and a combination of amino acids on anaerobic metabolism and energetics

BACKGROUND/AIMS/METHODS

In this study, we investigated the hepatoprotective effects of three storage solutions containing glycine (180 mM), glycylglycine (180 mM), and a mixture of 20 amino acids (combined concentration of 180 mM) on energy metabolism and levels of glucose and lactate (as an index of glycolytic flux) in rat livers. All effects were compared to those of livers flushed/stored with a modified University of Wisconsin solution.

RESULTS

Glycine-treatment showed no improvement in liver energetics (ATP, ADP, AMP) and lactate accumulation; this solution had the lowest buffering capacity of the four tested (approximately 30% of the University of Wisconsin solution). The glycylglycine solution had the highest buffering capacity of the four solutions tested (including University of Wisconsin solution). Complete titration of the glycine-, combined amino acids-, and University of Wisconsin solutions (from 8.0 to pH = 6.0) resulted in a minor decrease in glycylglycine buffer pH; pH dropped by 0.2 pH units. In glycylglycine-treated livers, energetics showed an improvement over the first 1 h cold storage; ATP and 'energy charge' values remained high and ADP levels (and consequently total adenylate contents) were 0.7-2.4 micro mol/g greater than livers stored in University of Wisconsin solution. A 2-fold increase in lactate accumulation suggested that the improvement in liver energetics for the glycylglycine buffer was due to maintained flux through glycolysis brought about by enhanced buffering capacity. The solution containing a combination of amino acids exhibited maximum maintenance of liver energetics via increased glycolytic flux, despite its slightly inferior buffering capacity (85% of University of Wisconsin solution). ATP levels were maintained over the first 2 h storage and ADP levels (and consequently, total adenylate contents) were 1.2-2.1 micro mol/g greater than University of Wisconsin solution-treated livers during the entire 24 h storage period. Energy charge values for livers treated with the combination of amino acids were also significantly higher than with glycine-, glycylglycine- and University of Wisconsin solution-treatment; even at 24 h, energy charge was 0.36 (comparable to only 4 h storage in University of Wisconsin solution).

CONCLUSIONS

Our data suggest that a combination of amino acids may be required for maximum protection of the liver, and furthermore there may be several independent mechanisms, including buffering capacity, responsible for cytoprotection of the liver during cold storage.

Evidence for the detrimental role of proteolysis during liver preservation in humans

BACKGROUND/AIMS

Proteolysis may persist in the liver allograft during cold storage. The aim of this study was to determine the significance of proteolysis within liver allografts stored at 4 degrees C in University of Wisconsin preservation fluid.

METHODS

Thirty recipients of 32 liver allografts were studied prospectively. Amino acid content of the preservation fluid was analyzed at the end of cold storage and was correlated to graft and patient outcome after transplantation.

RESULTS

Analysis of the preservation fluid showed the presence of free amino acids, the profile of which was different from that of stored liver parenchyma. Concentrations of amino acids (alanine, cysteine, leucine, isoleucine, methionine, lysine, ornithine, and threonine) and transaminases (alanine aminotransferase and aspartate aminotransferase) in the preservation fluid correlated with the duration of cold ischemia. Indexes of graft dysfunction (serum alanine aminotransferase and aspartate aminotransferase peaks and prothrombin rate) correlated with concentrations of cysteine, alanine, isoleucine, leucine, methionine, lysine, ornithine, and threonine, whereas enzyme concentrations in the fluid were not predictive of graft dysfunction.

CONCLUSIONS

These data suggest that liver proteolysis occurs during cold storage and may have a detrimental effect on the outcome after transplantation. The measurement of the amino acids in the preservation fluid at the end of the cold storage period could help to identify the most severely damaged organs.

Involvement of a cystatin-alpha-sensitive cysteine proteinase in the degradation of native L-lactate dehydrogenase and serum albumin by rat liver or kidney lysosomes

A lysosomal cysteine proteinase plays a critical role in the lysosomal degradation of native L-lactate dehydrogenase. The effects of a recombinant cystatin alpha and other cysteine proteinase inhibitors on the degradation of unlabeled native L-lactate dehydrogenase by total lysosomal enzymes were examined in vitro. L-Lactate dehydrogenase was inactivated, its 35-kDa subunit disappeared and the final amino acid degradation products were produced during an incubation with disrupted lysosomes in vitro. These processes were all markedly suppressed by a small amount of cystatin alpha without inhibiting the activities of the known lysosomal cysteine proteinases, cathepsins B, H, L, and J. These results suggest that a cysteine proteinases, which is highly sensitive to cystatin alpha and distinct from other known lysosomal cysteine proteinases, is involved in the lysosomal degradation of native L-lactate dehydrogenase. An L-lactate-dehydrogenase-inactivating enzyme in the extracts of lysosomes was partially purified. It was separated from cathepsin J using a Sephacryl S-200 gel-filtration column and was further separated from cathepsin H by DEAE-Sephadex A-50 anion-exchange column chromatography. The inactivation and degradation of L-lactate dehydrogenase by this partially purified enzyme were all markedly suppressed by a low level of cystatin alpha without inhibiting the activities of both cathepsins B and L. The degradation of rat serum albumin by the partially purified enzyme was also inhibited by the same concentration of cystatin alpha. It is concluded that a cystatin-alpha-sensitive cysteine proteinase, other than cathepsins B, H, L and J, is present in lysosomes and functions in the lysosomal degradation of at least native L-lactate dehydrogenase and serum albumin.

Multiphasic control of proteolysis by leucine and alanine in the isolated rat hepatocyte

Autophagically mediated proteolysis in the perfused rat liver is under complex multiphasic control by a small group of amino acids dominated by leucine. Because there have been no prior reports of such regulation in the isolated hepatocyte, our goal was to determine whether it is a manifestation of interactions between diverse cells in the intact liver or, alternatively, the expression of a unique control mechanism within a single population of cells. Hepatocytes were isolated from livers of ad libitum-fed rats and incubated with cycloheximide at low density (approximately 10(6) cells/ml) for the determination of valine release. As in perfusion experiments with synchronously fed rats, proteolytic responses to leucine in cells from fed rats were mediated through two inhibitory mechanisms that alternated randomly on a day-to-day basis. The first (L) represented a typical multiphasic dose-response with low- and high-concentration inhibition separated by a sharp zonal loss of inhibition that could be abolished by alanine. The second (H) mediated inhibition only at high concentrations. It disappeared after 24 h of starvation, leaving L as the prevailing mode. The findings indicate that both macroautophagy and the multiphasic mechanism for regulating it coexist in a single population of hepatocytes, making the cells suitable for studies aimed at defining the putative plasma membrane site of leucine recognition.

4-Amino-6-methylhept-2-enoic acid: a leucine analogue and potential probe for localizing sites of proteolytic control in the hepatocyte

A recent analysis of leucine analogues has suggested that the carboxyl group is not required for mediating low concentration proteolytic inhibition in liver cells. In designing a probe to localize the regulatory site(s), we tested this hypothesis by synthesizing an analogue with a 2-carbon insert between the carboxyl and alpha-carbon. The Wittig product, a trans olefin, was fully active. Surprisingly, low concentration activity was lost when the double bond was eliminated by hydrogenation although some inhibitory effectiveness at high concentrations was evident. Since the double bond extends the carboxyl group away from the alpha-carbon, the results support the above hypothesis as well as the feasibility of adding functional groups to the carboxyl end of leucine.

Glycine cytoprotection during lethal hepatocellular injury from adenosine triphosphate depletion

Glycine protects renal tubule cells from cell death during adenosine triphosphate (ATP) depletion. Although the liver plays a key role in glycine metabolism, information is lacking regarding the effects of glycine on lethal hepatocellular injury. Thus, the aim of this study was to determine the potential cytoprotective role of glycine during ATP depletion of rat hepatocytes. Metabolic inhibition with 2.5 mmol/L potassium cyanide (KCN) was used to produce ATP depletion. Hepatocyte suspensions treated with KCN had a 2-hour viability of 5.9% +/- 2.0%, whereas cells treated with KCN in the presence of 2.0 mmol/L glycine had a viability of 80.2% +/- 1.5%, which was virtually identical to controls (81.5% +/- 1.9%). Glycine cytoprotection was dose dependent and amino acid specific. The cytoprotective effect of glycine was not mediated by protein synthesis, glycine mitochondrial metabolism, cytosolic acidosis, or preservation of either intracellular cellular glutathione or ATP. However, glycine did decrease total cellular proteolysis by 18% +/- 2%, 25% +/- 3%, and 33% +/- 1% after 1, 2, and 3 hours of KCN treatment, respectively (P less than 0.01). Inhibition of proteolysis by glycine was dose dependent over the same range as its cytoprotection. The results suggest that glycine protects against hepatocellular injury by inhibiting degradative proteolytic activity. It was concluded that proteolysis may be an important mechanism contributing to lethal injury of hepatocytes during ATP depletion.