Multiphasic control of hepatic protein degradation by regulatory amino acids. General features and hormonal modulation

A randomized phase II study of SM-88 plus methoxsalen, phenytoin, and sirolimus in patients with metastatic pancreatic cancer treated in the second line and beyond

BACKGROUND

This trial explores SM-88 used with methoxsalen, phenytoin, and sirolimus (MPS) in pretreated metastatic pancreatic ductal adenocarcinoma (mPDAC) METHODS: Forty-nine patients were randomized to daily 460 or 920 mg oral SM-88 with MPS (SM-88 Regimen). The primary endpoint was objective response rate (RECIST 1.1).

RESULTS

Thirty-seven patients completed ≥ one cycle of SM-88 Regimen (response evaluable population). Disease control rate (DCR), overall survival (OS), and progression-free survival (PFS) did not differ significantly between dose levels. Stable disease was achieved in 9/37 patients (DCR, 24.3%); there were no complete or partial responses. Quality-of-life (QOL) was maintained and trended in favor of 920 mg. SM-88 Regimen was well tolerated; a single patient (1/49) had related grade 3 and 4 adverse events, which later resolved. In the intention-to-treat population of 49 patients, the median overall survival (mOS) was 3.4 months (95% CI: 2.7-4.9 months). Those treated in the second line had an mOS of 8.1 months and a median PFS of 3.8 months. Survival was higher for patients with stable versus progressive disease (any line; mOS: 10.6 months vs. 3.9 months; p = 0.01).

CONCLUSIONS

SM-88 Regimen has a favorable safety profile with encouraging QOL effects, disease control, and survival trends. This regimen should be explored in the second-line treatment of patients with mPDAC.

CLINICALTRIALS

gov Identifier: NCT03512756.

Oligomerization of Selective Autophagy Receptors for the Targeting and Degradation of Protein Aggregates

The selective targeting and disposal of solid protein aggregates are essential for cells to maintain protein homoeostasis. Autophagy receptors including p62, NBR1, Cue5/TOLLIP (CUET), and Tax1-binding protein 1 (TAX1BP1) proteins function in selective autophagy by targeting ubiquitinated aggregates through ubiquitin-binding domains. Here, we summarize previous beliefs and recent findings on selective receptors in aggregate autophagy. Since there are many reviews on selective autophagy receptors, we focus on their oligomerization, which enables receptors to function as pathway determinants and promotes phase separation.

Non-Canonical Cell Death Induced by p53

Programmed cell death is a vital biological process for multicellular organisms to maintain cellular homeostasis, which is regulated in a complex manner. Over the past several years, apart from apoptosis, which is the principal mechanism of caspase-dependent cell death, research on non-apoptotic forms of programmed cell death has gained momentum. p53 is a well characterized tumor suppressor that controls cell proliferation and apoptosis and has also been linked to non-apoptotic, non-canonical cell death mechanisms. p53 impacts these non-canonical forms of cell death through transcriptional regulation of its downstream targets, as well as direct interactions with key players involved in these mechanisms, in a cell type- or tissue context-dependent manner. In this review article, we summarize and discuss the involvement of p53 in several non-canonical modes of cell death, including caspase-independent apoptosis (CIA), ferroptosis, necroptosis, autophagic cell death, mitotic catastrophe, paraptosis, and pyroptosis, as well as its role in efferocytosis which is the process of clearing dead or dying cells.

Regulation of autophagy by amino acids and MTOR-dependent signal transduction

Amino acids not only participate in intermediary metabolism but also stimulate insulin-mechanistic target of rapamycin (MTOR)-mediated signal transduction which controls the major metabolic pathways. Among these is the pathway of autophagy which takes care of the degradation of long-lived proteins and of the elimination of damaged or functionally redundant organelles. Proper functioning of this process is essential for cell survival. Dysregulation of autophagy has been implicated in the etiology of several pathologies. The history of the studies on the interrelationship between amino acids, MTOR signaling and autophagy is the subject of this review. The mechanisms responsible for the stimulation of MTOR-mediated signaling, and the inhibition of autophagy, by amino acids have been studied intensively in the past but are still not completely clarified. Recent developments in this field are discussed.

Chronic α-hydroxyisocaproic acid treatment improves muscle recovery after immobilization-induced atrophy

Muscle disuse atrophy is observed routinely in patients recovering from traumatic injury and can be either generalized resulting from extended bed rest or localized resulting from single-limb immobilization. The present study addressed the hypothesis that a diet containing 5% α-hydroxyisocaproic acid (α-HICA), a leucine (Leu) metabolite, will slow the loss and/or improve recovery of muscle mass in response to disuse. Adult 14-wk-old male Wistar rats were provided a control diet or an isonitrogenous isocaloric diet containing either 5% α-HICA or Leu. Disuse atrophy was produced by unilateral hindlimb immobilization ("casting") for 7 days and the contralateral muscle used as control. Rats were also casted for 7 days and permitted to recover for 7 or 14 days. Casting decreased gastrocnemius mass, which was associated with both a reduction in protein synthesis and S6K1 phosphorylation as well as enhanced proteasome activity and increased atrogin-1 and MuRF1 mRNA. Although neither α-HICA nor Leu prevented the casting-induced muscle atrophy, the decreased muscle protein synthesis was not observed in α-HICA-treated rats. Neither α-HICA nor Leu altered the increased proteasome activity and atrogene expression observed with immobilization. After 14 days of recovery, muscle mass had returned to control values only in the rats fed α-HICA, and this was associated with a sustained increase in protein synthesis and phosphorylation of S6K1 and 4E-BP1 of previously immobilized muscle. Proteasome activity and atrogene mRNA content were at control levels after 14 days and not affected by either treatment. These data suggest that whereas α-HICA does not slow the loss of muscle produced by disuse, it does speed recovery at least in part by maintaining an increased rate of protein synthesis.

The variability of autophagy and cell death susceptibility: Unanswered questions

Impaired autophagic machinery is implicated in a number of diseases such as heart disease, neurodegeneration and cancer. A common denominator in these pathologies is a dysregulation of autophagy that has been linked to a change in susceptibility to cell death. Although we have progressed in understanding the molecular machinery and regulation of the autophagic pathway, many unanswered questions remain. How does the metabolic contribution of autophagy connect with the cell’s history and how does its current autophagic flux affect metabolic status and susceptibility to undergo cell death? How does autophagic flux operate to switch metabolic direction and what are the underlying mechanisms in metabolite and energetic sensing, metabolite substrate provision and metabolic integration during the cellular stress response? In this article we focus on unresolved questions that address issues around the role of autophagy in sensing the energetic environment and its role in actively generating metabolite substrates. We attempt to provide answers by explaining how and when a change in autophagic pathway activity such as primary stress response is able to affect cell viability and when not. By addressing the dynamic metabolic relationship between autophagy, apoptosis and necrosis we provide a new perspective on the parameters that connect autophagic activity, severity of injury and cellular history in a logical manner. Last, by evaluating the cell’s condition and autophagic activity in a clear context of regulatory parameters in the intra- and extracellular environment, this review provides new concepts that set autophagy into an energetic feedback loop, that may assist in our understanding of autophagy in maintaining healthy cells or when it controls the threshold between cell death and cell survival.

Involvement of protein kinase C activation in L-leucine-induced stimulation of protein synthesis in l6 myotubes

Effects of leucine and related compounds on protein synthesis were studied in L6 myotubes. The incorporation of [(3)H]tyrosine into cellular protein was measured as an index of protein synthesis. In leucine-depleted L6 myotubes, leucine and its keto acid, alpha-ketoisocaproic acid (KIC), stimulated protein synthesis, while D-leucine did not. Mepacrine, an inhibitor of both phospholipases A(2) and C, canceled stimulatory actions of L-leucine and KIC on protein synthesis. Neither indomethacin, an inhibitor of cyclooxygenase, nor caffeic acid, an inhibitor of lipoxygenase, diminished their stimulatory actions, suggesting no involvement of arachidonic acid metabolism. Conversely, 1-O-hexadecyl-2-O-methylglycerol, an inhibitor of proteinkinase C, significantly canceled the stimulatory actions of L-leucine and KIC on protein synthesis, suggesting an involvement of phosphatidylinositol degradation and activation of protein kinase C. L-Leucine caused a rapid activation of protein kinase C in both cytosol and membrane fractions of the cells. These results strongly suggest that both L-leucine and KIC stimulate protein synthesis in L6 myotubes through activation of phospholipase C and protein kinase C.

Vitamin E as a novel enhancer of macroautophagy in rat hepatocytes and H4-II-E cells

Autophagy is an intracellular bulk degradation process induced by nutrient starvation, and contributes to macromolecular turnover and rejuvenation of cellular organelles. We demonstrated that vitamin E was a novel nutritional enhancer of autophagy in freshly isolated rat hepatocytes and rat hepatoma H4-II-E cells. Supplementation of fresh hepatocytes with vitamin E (up to 100 microM) increased proteolysis significantly in the presence or absence of amino acids in a dose-dependent manner. The cytosolic LC3 ratio, a newly established index of autophagic flux, was significantly increased by vitamin E, strongly suggesting that the possible site of action is the LC3 conversion step, an early step in autophagosome formation. A typical antioxidant, alpha-lipoic acid, exerted autophagy suppression, while H(2)O(2) stimulated autophagy. It is conceivable that autophagy was stimulated by oxidative stress and this stimulation was cancelled by cellular antioxidative effects. However, in our studies, vitamin E could have enhanced autophagy over-stimulation by H(2)O(2), rather than suppress it. From these results, using a new cytosolic LC3 ratio, vitamin E increases autophagy by accelerating LC3 conversion through a new signaling pathway, emerging as a novel enhancer of autophagy.

Effects of alfa-hydroxy-isocaproic acid on body composition, DOMS and performance in athletes

Background

Alfa-Hydroxy-isocaproic acid (HICA) is an end product of leucine metabolism in human tissues such as muscle and connective tissue. According to the clinical and experimental studies, HICA can be considered as an anti-catabolic substance. The present study investigated the effects of HICA supplementation on body composition, delayed onset of muscle soreness (DOMS) and physical performance of athletes during a training period.

Methods

Fifteen healthy male soccer players (age 22.1+/-3.9 yr) volunteered for the 4-week double-blind study during an intensive training period. The subjects in the group HICA (n = 8) received 583 mg of sodium salt of HICA (corresponding 500 mg of HICA) mixed with liquid three times a day for 4 weeks, and those in the group PLACEBO (n = 7) received 650 mg of maltodextrin mixed with liquid three times a day for the same period. According to a weekly training schedule, they practiced soccer 3 - 4 times a week, had strength training 1 - 2 times a week, and had one soccer game during the study. The subjects were required to keep diaries on training, nutrition, and symptoms of DOMS. Body composition was evaluated with a dual-energy X-ray absorptiometry (DXA) before and after the 4-week period. Muscle strength and running velocity were measured with field tests.

Results

As compared to placebo, the HICA supplementation increased significantly body weight (p < 0.005) and whole lean body mass (p < 0.05) while fat mass remained constant. The lean body mass of lower extremities increased by 400 g in HICA but decreased by 150 g in PLACEBO during the study. This difference between the groups was significant (p < 0.01). The HICA supplementation decreased the whole body DOMS symptoms in the 4^th week of the treatment (p < 0.05) when compared to placebo. Muscle strength and running velocity did not differ between the groups.

Conclusion

Already a 4-week HICA supplementation of 1.5 g a day leads to small increases in muscle mass during an intensive training period in soccer athletes.

Flow cytometric analysis of autophagy in living mammalian cells

Autophagy is a major intracellular catabolic pathway induced in response to amino acid starvation. Recent findings implicate it in diverse physiological/pathophysiological events, such as protein and organelle turnover, development, aging, pathogen infection, cell death, and neurodegeneration. However, experimental methods to monitor this process in mammalian cells are limited because of the deficiency of autophagic markers. Recently, MAP1-LC3 (LC3), a mammalian homolog of the yeast ubiquitin-like (UBL) protein Atg8, has been shown to selectively incorporate into the autophagosomal membrane, thus serving as a unique bona fide marker of autophagosomes in mammals. Thus, the autophagic activity can be largely determined by GFP-LC3/LC3, predominantly associated with autophagosomes (when LC3 is conjugated to phosphatidylethanolamine), both biochemically and microscopically. However, current methods to quantify autophagic activity using LC3 are time consuming, labor intensive, and require expertise in accurate interpretation. In this chapter we describe the use of flow cytometry and fluorescence-activated cell sorting (FACS) as a new assay designed to quantify autophagy in cells stably expressing GFP-LC3. Flow cytometry is a well-established technique for performing quantitative fluorescence measurements, allowing quick, accurate, and simultaneous determination of many parameters in cell subpopulations. Here flow cytometry and FACS were used to quantify the turnover of GFP-LC3 (reflecting an autophagic flux) as a reliable and simple assay to measure autophagic activity in living mammalian cells.

The role of AMPK and mTOR in nutrient sensing in pancreatic beta-cells

The AMP-activated protein kinase (AMPK) is a central regulator of the energy status of the cell, based on its unique ability to respond directly to fluctuations in the ratio of AMP:ATP. Because glucose and amino acids stimulate insulin release from pancreatic beta-cells by the regulation of metabolic intermediates, AMPK represents an attractive candidate for control of beta-cell function. Here, we show that inhibition of AMPK in beta-cells by high glucose inversely correlates with activation of the mammalian Target of Rapamycin (mTOR) pathway, another cellular sensor for nutritional conditions. Forced activation of AMPK by AICAR, phenformin, or oligomycin significantly blocked phosphorylation of p70S6K, a downstream target of mTOR, in response to the combination of glucose and amino acids. Amino acids also suppressed the activity of AMPK, and this at a minimum required the presence of leucine and glutamine. It is unlikely that the ability of AMPK to sense both glucose and amino acids plays a role in regulation of insulin secretion, as inhibition of AMPK by amino acids did not influence insulin secretion. Moreover, activation of AMPK by AICAR or phenformin did not antagonize glucose-stimulated insulin secretion, and insulin secretion was also unaffected in response to suppression of AMPK activity by expression of a dominant negative AMPK construct (K45R). Taken together, these results suggest that the inhibition of AMPK activity by glucose and amino acids might be an important component of the mechanism for nutrient-stimulated mTOR activity but not insulin secretion in the beta-cell.

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.

Regulation of skeletal muscle proteolysis by amino acids

Skeletal muscle is the major reservoir of body protein that can be mobilized in a number of muscle wasting conditions, that include kidney failure. Increased proteolysis in such conditions provides free amino acids that are used for acute-phase protein synthesis or that are degraded for energy purposes. Amino acids act as signals to regulate both protein synthesis and protein breakdown. We review the current but limited information available on the regulation of proteolytic systems in muscle cells. In particular, recent data have shown that amino acid deprivation in C2C12 myotubes stimulates autophagic sequestration by mechanisms that implicate the Apg system through a class III phosphoinositide-3'-kinase (PI3K III ) signaling cascade.

Inhibition of proteasome activity by selected amino acids

Cellular protein homeostasis is a balance between synthesis and degradation. Protein degradation is regulated by hormones (eg, insulin) and nutrients (eg, amino acids). Certain amino acids are capable of decreasing cellular protein degradation, with evidence that this is mediated through altered lysosomal function. However, proteasomes, the major cytosolic protein degrading machinery, are being shown to play a central role in the control of protein turnover in the cell. In this study we show that the amino acids, isoleucine, leucine, tyrosine, phenylalanine, tryptophan, lysine, and arginine are capable of inhibiting the chymotrypsin-like activity of the proteasome in a dose-dependent manner. Leucine, tyrosine, and phenylalanine have a substantial effect at normal serum concentrations. The effect was greater in a proteasome preparation derived from muscle compared to a similar preparation from liver. On the assumption that amino acid-induced alterations in cellular protein degradation reflect the inhibitory changes in proteasomal activity shown here, we may conclude that amino acid control of cellular protein degradation is mediated, at least in part, through proteasomes.

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.

Interaction of glucagon and epinephrine in the control of hepatic glucose production in the conscious dog

Epinephrine increases net hepatic glucose output (NHGO) mainly via increased gluconeogenesis, whereas glucagon increases NHGO mainly via increased glycogenolysis. The aim of the present study was to determine how the two hormones interact in controlling glucose production. In 18-h-fasted conscious dogs, a pancreatic clamp initially fixed insulin and glucagon at basal levels, following which one of four protocols was instituted. In G + E, glucagon (1.5 ng x kg(-1) x min(-1); portally) and epinephrine (50 ng x kg(-1) x min(-1); peripherally) were increased; in G, glucagon was increased alone; in E, epinephrine was increased alone; and in C, neither was increased. In G, E, and C, glucose was infused to match the hyperglycemia seen in G + E ( approximately 250 mg/dl). The areas under the curve for the increase in NHGO, after the change in C was subtracted, were as follows: G = 661 +/- 185, E = 424 +/- 158, G + E = 1178 +/- 57 mg/kg. Therefore, the overall effects of the two hormones on NHGO were additive. Additionally, glucagon exerted its full glycogenolytic effect, whereas epinephrine exerted its full gluconeogenic effect, such that both processes increased significantly during concurrent hormone administration.

Reduced amino acid availability inhibits muscle protein synthesis and decreases activity of initiation factor eIF2B

We have examined the effect of a hemodialysis-induced 40% reduction in plasma amino acid concentrations on rates of muscle protein synthesis and breakdown in normal swine. Muscle protein kinetics were measured by tracer methodology using [(2)H(5)]phenylalanine and [1-(13)C]leucine and analysis of femoral arterial and venous samples and tissue biopsies. Net amino acid release by muscle was accelerated during dialysis. Phenylalanine utilization for muscle protein synthesis was reduced from the basal value of 45 +/- 8 to 25 +/- 6 nmol x min(-1) x 100 ml leg(-1) between 30 and 60 min after start of dialysis and was stimulated when amino acids were replaced while dialysis continued. Muscle protein breakdown was unchanged. The signal for changes in synthesis appeared to be changes in plasma amino acid concentrations, as intramuscular concentrations remained constant throughout. The changes in muscle protein synthesis were accompanied by a reduction or stimulation, respectively, in the guanine nucleotide exchange activity of eukaryotic initiation factor (eIF)2B following hypoaminoacidemia vs. amino acid replacement. We conclude that a reduction in plasma amino acid concentrations below the normal basal value signals an inhibition of muscle protein synthesis and that corresponding changes in eIF2B activity suggest a possible role in mediating the response.

Leucine is a direct-acting nutrient signal that regulates protein synthesis in adipose tissue

In freshly isolated rat adipocytes, leucine or its analog norleucine activates the mammalian target of rapamycin (mTOR)-signaling pathway. This results in phosphorylation of the ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1), two proteins involved in the initiation phase of protein synthesis. The purpose of the studies reported herein was to address the question of whether or not these in vitro effects of leucine and norleucine on adipocytes could be extended to the intact animal and to other tissues. To accomplish this, food-deprived (18 h) male Sprague-Dawley rats were orally administered solutions (2.5 ml/100 g body wt) containing normal saline (0.9% NaCl), a carbohydrate mixture (26.2% D-glucose and 26.2% sucrose), leucine (5.4%), or norleucine (5.4%). The protein synthetic responses of adipose tissue were measured and compared with those of other tissues. In addition, S6K1 and 4E-BP1 phosphorylation was measured, as was the plasma concentration of insulin and tissue ATP concentrations. Leucine administration stimulated protein synthesis in adipose tissue, gastrocnemius, and kidney but not in liver and heart. Norleucine stimulated protein synthesis in all of the tissues tested but, in contrast to leucine, without affecting plasma insulin concentrations. The carbohydrate meal had no effect on protein synthesis in any tissue tested but elicited a robust increase in plasma insulin. These findings provide support for a role of leucine as a direct-acting nutrient signal for stimulation of protein synthesis in adipose tissue as well as other select tissues. In adipose tissue, the effects of the different treatment conditions on the acute regulation of protein synthesis closely correlated with changes in phosphorylation of S6K1 and 4E-BP1; however, this correlation did not exist in all tissues examined. This result implies that leucine or norleucine may acutely stimulate protein synthesis, at least in some tissues, by a mechanism that is independent of both S6K1 and 4E-BP1 phosphorylation.

What are the dietary protein requirements of physically active individuals? New evidence on the effects of exercise on protein utilization during post-exercise recovery

Exercise and physical activity increase energy expenditure up to 10-fold. This brief review will focus on the effect of exercise on protein requirements. Evidence has accumulated that amino acids are oxidized as substrates during prolonged submaximal exercise. In addition, studies have determined that both endurance and resistance training exercise increase skeletal muscle protein synthesis and breakdown in the post-exercise recovery period. Studies using nitrogen balance have further confirmed that protein requirements for individuals engaged in regular exercise are increased. The current recommended intakes of protein for strength and endurance athletes are 1.6 to 1.7 g/kg and 1.2 to 1.4 g/kg per day, respectively. Presently, most athletes consume an adequate amount of protein in their diet. The timing and nutritional content of the post-exercise meal, although often overlooked, are known to have synergistic effects on protein accretion after exercise. New evidence suggests that individuals engaging in strenuous activity consume a meal rich in amino acids and carbohydrate soon after the exercise bout or training session.

Metabolic regulation by leucine of translation initiation through the mTOR-signaling pathway by pancreatic beta-cells

Recent findings have demonstrated that the branched-chain amino acid leucine can activate the translational regulators, phosphorylated heat- and acid-stable protein regulated by insulin (PHAS-I) and p70 S6 kinase (p70S6k), in an insulin-independent and rapamycin-sensitive manner through mammalian target of rapamycin (mTOR), although the mechanism for this activation is undefined. It has been previously established that leucine-induced insulin secretion by beta-cells involves increased mitochondrial metabolism by oxidative decarboxylation and allosteric activation of glutamate dehydrogenase (GDH). We now show that these same intramitochondrial events that generate signals for leucine-induced insulin exocytosis are required to activate the mTOR mitogenic signaling pathway by beta-cells. Thus, a minimal model consisting of leucine and glutamine as substrates for oxidative decarboxylation and an activator of GDH, respectively, confirmed the requirement for these two metabolic components and mimicked closely the synergistic interactions achieved by a complete complement of amino acids to activate p70s6k in a rapamycin-sensitive manner. Studies using various leucine analogs also confirmed the close association of mitochondrial metabolism and the ability of leucine analogs to activate p70s6k. Furthermore, selective inhibitors of mitochondrial function blocked this activation in a reversible manner, which was not associated with a global reduction in ATP levels. These findings indicate that leucine at physiological concentrations stimulates p70s6k phosphorylation via the mTOR pathway, in part, by serving both as a mitochondrial fuel and an allosteric activator of GDH. Leucine-mediated activation of protein translation through mTOR may contribute to enhanced beta-cell function by stimulating growth-related protein synthesis and proliferation associated with the maintenance of beta-cell mass.

Leucine, glutamine, and tyrosine reciprocally modulate the translation initiation factors eIF4F and eIF2B in perfused rat liver

Leucine, glutamine, and tyrosine, three amino acids playing key modulatory roles in hepatic proteolysis, were evaluated for activation of signaling pathways involved in regulation of liver protein synthesis. Furthermore, because leucine signals to effectors that lie distal to the mammalian target of rapamycin, these downstream factors were selected for study as candidate mediators of amino acid signaling. Using the perfused rat liver as a model system, we observed a 25% stimulation of protein synthesis in response to balanced hyperaminoacidemia, whereas amino acid imbalance due to elevated concentrations of leucine, glutamine, and tyrosine resulted in a protein synthetic depression of roughly 50% compared with normoaminoacidemic controls. The reduction in protein synthesis accompanying amino acid imbalance became manifest at high physiologic concentrations and was dictated by the guanine nucleotide exchange activity of translation initiation factor eIF2B. Paradoxically, this phenomenon occurred concomitantly with assembly of the mRNA cap recognition complex, eIF4F as well as activation of the 70-kDa ribosomal S6 kinase, p70(S6k). Dual and reciprocal modulation of eIF4F and eIF2B was leucine-specific because isoleucine, a structural analog, was ineffective in these regards. Thus, we conclude that amino acid imbalance, heralded by leucine, initiates a liver-specific translational fail-safe mechanism that deters protein synthesis under unfavorable circumstances despite promotion of the eIF4F complex.

Structural requirement of leucine for activation of p70 S6 kinase

The addition of leucine induced activation of p70S6k in amino acid-depleted H4IIE cells. Whereas the activation of p70S6k by leucine was transient, the complete amino acid stimulated p70S6k more persistently. The effect of leucine on p70S6k was sensitive to rapamycin, but less sensitive to wortmannin. Using various amino acids and derivatives of leucine, we found that the chirality, the structure of the four branched hydrocarbons, and the primary amine are required for the ability of leucine to stimulate p70S6k, indicating that the structural requirement of leucine to induce p70S6k activation is very strict and precise. In addition, some leucine derivatives exhibited the ability to stimulate p70S6k and the other derivatives acted as inhibitors against the leucine-induced activation of p70S6k.

Regulation of translational effectors by amino acid and mammalian target of rapamycin signaling pathways. Possible involvement of autophagy in cultured hepatoma cells

Amino acid deprivation of Chinese hamster ovary cells overexpressing human insulin receptors results in deactivation of p70 S6 kinase (p70) and dephosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), which become unresponsive to insulin; readdition of amino acids restores these responses in a rapamycin-sensitive manner, suggesting that amino acids and mammalian target of rapamycin signal through common effectors. Contrarily, withdrawal of medium amino acids from the hepatoma cell line H4IIE does not abolish the ability of insulin to stimulate p70 and 4E-BP1. The addition of 3-methyladenine (3MA) to H4IIE cells deprived of amino acids inhibited the increment in protein degradation caused by amino acid withdrawal nearly completely at 10 mM and also strongly inhibited the ability of insulin to stimulate p70 and 4E-BP1 at 10 mM. Treatment of H4IIE cells with 3MA did not alter the ability of insulin to activate tyrosine phosphorylation, phosphoinositide 3-kinase, or mitogen-activated protein kinase. In conclusion, the ability of H4IIE cells to maintain the insulin responsiveness of the mammalian target of rapamycin-dependent signaling pathways impinging on p70 and 4E-BP1 without exogenous amino acids reflects the generation of amino acids endogenously through a 3MA-sensitive process, presumably autophagy, a major mechanism of facultative protein degradation in liver.

Threonine and methionine are limiting amino acids for protein synthesis in patients with AIDS

This study was conducted to identify the most rate-limiting amino acids for whole-body protein synthesis in acquired immunodeficiency syndrome (AIDS) patients. We postulated that an essential amino acid that would be rate limiting in AIDS should have a low basal plasma concentration and should remain at a low level during amino acid infusion. Seven male AIDS patients (median age 37 y, CD4 cell count: 76 mm-3) without any clinically active opportunistic infection during the month before the experiment were infused intravenously with a complete amino acid-glucose mixture for 2.5 h. Eight healthy volunteers were used as controls. Before the infusion, the concentrations of most free essential amino acids (methionine, threonine, histidine, isoleucine, leucine and tryptophan) were significantly lower (P < 0.05) in AIDS patients than in controls. Most plasma free essential amino acids increased significantly during infusion. However, the absolute increase above basal levels for threonine, valine, lysine, (P < 0.05) and methionine (P < 0.073) was smaller in AIDS patients than in control subjects. Thus, threonine and possibly methionine may be rate limiting for whole-body protein synthesis in AIDS patients, suggesting that there are selective amino acid requirements in patients with AIDS.

Insulin acts intracellularly on proteasomes through insulin-degrading enzyme

Insulin decreases cellular protein degradation, but the mechanism of this action is poorly understood. We have shown that insulin can have an inhibitory effect on the action of the proteasome in vitro, which requires the presence of insulin degrading enzyme (IDE). In this study we have used an antibody which inhibits the activity of IDE to show that IDE is required for insulin inhibition of protein degradation in intact cells. The anti-IDE antibody blocked the insulin effect on cellular degradation of proteins prelabeled with radioactive amino acids. The anti-IDE antibody also decreased insulin inhibition of proteasome degradation of a specific substrate in intact cells. These data suggest that insulin works intracellularly via IDE to inhibit protein degradation by the proteasome. Thus, IDE may function as an intracellular mediator for insulin effects on protein degradation. This is a novel signal transduction mechanism for peptide hormones.

Phosphorylation of ribosomal protein S6 is inhibitory for autophagy in isolated rat hepatocytes

In rat hepatocytes, autophagy is known to be inhibited by amino acids. Insulin and cell swelling promote inhibition by amino acids. Each of the conditions leading to inhibition of autophagic proteolysis was found to be associated with phosphorylation of a 31-kDa protein that we identified as ribosomal protein S6. A combination of leucine, tyrosine, and phenylalanine, which efficiently inhibits autophagic proteolysis, was particularly effective in stimulating S6 phosphorylation. The relationship between the percentage inhibition of proteolysis and the degree of S6 phosphorylation was linear. Thus, inhibition of autophagy and phosphorylation of S6 are under the control of the same signal transduction pathway. Stimulation of S6 phosphorylation by the presence of amino acids was due to activation of S6 kinase and not to inhibition of S6 phosphatase. The inhibition by amino acids of both autophagic proteolysis and autophagic sequestration of electro-injected cytosolic [14C]sucrose was partially prevented by rapamycin, a compound known to inhibit activation of p70 S6 kinase. In addition, rapamycin partially inhibited the rate of protein synthesis. We conclude that the fluxes through the autophagic and protein synthetic pathways are regulated in an opposite manner by the degree to which S6 is phosphorylated. Possible mechanisms by which S6 phosphorylation can cause inhibition of autophagy are discussed.

Anabolic effects of insulin and amino acids in promoting nitrogen accretion in postoperative patients

The present study evaluates the effect of insulin and amino acids on nitrogen balance in the immediate postoperative period in moderately stressed patients who have undergone major abdominal surgical operations. Patients were randomly assigned into two large groups (n = 16 each) and four subgroups (n = 8 each). Groups ICON and IINS received an infusion of total parenteral nutrition solution containing 0.25 g of nitrogen per kilogram per day with a calorie:nitrogen (C:N) ratio of 150:1. Groups IICON and IIINS received twice the load of nitrogen with a C:N ratio of 75:1. Groups IINS AND IIINS received an additional continuous infusion of insulin at a rate of 1.0 mU/kg per minute for 7 days. During the total parenteral nutrition period, the patients were kept NPO, and 24-hour urine output was collected for estimation of total nitrogen excretion and nitrogen balance. Net nitrogen excretion (grams per kilogram per day) averaged 0.143 +/- 0.06 in IINS and 0.23 +/- 0.08 in ICON (p < .05) and 0.178 +/- 0.6 in IIINS and 0.25 +/- 0.10 in IICON (p < .05). Nitrogen balance (grams per day) was positive in the four groups: +0.65 +/- 3.8 in IINS and +6.74 +/- 2.94 in ICON (p < .05), and +14.4 +/- 2.61 in IIINS and +11.63 +/- 6.44 in IICON (p = not significant). The average nitrogen incorporation (percent per day) was: 41.3 +/- 6.2 in IINS and 14.6 +/- 20.1 in ICON (p < .05), and 58.3 +/- 4.5 in IIINS and 38.7 +/- 26.2 in IICON (p = not significant).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Insulin action on protein metabolism

On the basis of the preceding observations, the following sequence of events can be postulated during insulin deficiency or excess. The main feature of insulin deficiency is the disruption of protein balance in muscle that rapidly leads to emaciation and wasting. Muscle protein degradation is greatly enhanced while increased amino acid availability maintains protein synthesis. In splanchnic tissues, both degradation and synthesis are increased but with an altered pattern, so that the levels of some proteins are increased (e.g. proteins of the acute-phase response), while those of others are decreased (e.g. albumin). As a result, intracellular protein content in liver is maintained but secretion of plasma proteins is abnormal. In healthy subjects, an acute increase in insulin concentration, as occurs after a meal, leads to a rapid suppression of protein breakdown in the splanchnic area. If hyperinsulinaemia is not supported by an exogenous amino acid supply, as might occur during a protein-free meal or experimentally during euglycaemic hyperinsulinaemic clamping, the plasma as well as muscle free amino acid concentration drops, owing to reduced splanchnic release. With reduced amino acid availability, insulin is not anabolic in muscle. If amino acid concentrations are maintained at normal or high levels, e.g. following a mixed meal, a net protein deposition in muscle may occur, primarily because of a stimulation of synthesis and possibly owing to inhibition of breakdown.

Cell swelling and the sensitivity of autophagic proteolysis to inhibition by amino acids in isolated rat hepatocytes

In the isolated perfused rat liver, autophagic proteolysis is inhibited by hypo-osmotic perfusion media [Häussinger, D., Hallbrucker, C., vom Dahl, S., Lang, F. & Gerok, W. (1990) Biochem. J. 272, 239-242]. Here we report that in isolated hepatocytes, incubated in the absence of amino acids to ensure maximal proteolytic flux, proteolysis was not inhibited by hypo-osmolarity while the synthesis of glycogen from glucose, a process known to be very sensitive to changes in cell volume [Baquet, A., Hue, L., Meijer, A. J., van Woerkom, G. M. & Plomp, P. J. A. M. (1990) J. Biol. Chem. 265, 955-959], was stimulated under identical conditions. However, in isolated hepatocytes, hypo-osmolarity increased the sensitivity of autophagic proteolysis to inhibition by low concentrations of amino acids. The anti-proteolytic effect of hypo-osmolarity in our experiments was not due to stimulation of amino-acid transport into the hepatocytes: neither the consumption of most amino acids, nor the rate of urea synthesis was appreciably affected by hypo-osmotic incubation conditions. In the course of these studies we also found that hypo-osmolarity increased the affinity of protein synthesis for amino acids. In the presence of amino acids the intracellular level of ATP was not much affected. However, because of cell swelling under these conditions the intracellular concentration of ATP decreased. It is proposed that a small part of the inhibition of proteolysis by amino acids may be due to this fall in ATP concentration.

Regulation of RNA degradation in cultured rat hepatocytes: effects of specific amino acids and insulin

The regulation of RNA degradation by specific amino acids and insulin was investigated in cultured rat hepatocytes from fed rats previously injected in vivo with [6-(14)C]orotic acid. The effects of three groups of amino acids were compared to those of a complete amino acid mixture. The first one consisted of the eight amino acids (leucine, proline, glutamine, histidine, phenylalanine, tyrosine, methionine, tryptophan) previously found to be particularly effective in the control of proteolysis. The two other groups were defined from our study with single additions of amino acids, one consisting of proline, asparagine, glutamine, alanine, phenylalanine, and leucine and the other including the latter group with serine, histidine, and tyrosine. The results showed that these three groups were able to strongly inhibit deprivation-induced RNA breakdown at one and ten times normal plasma concentrations but to a lower extent than the complete amino acid mixture. Six amino acids (proline, asparagine, glutamine, alanine, phenylalanine, leucine) inhibited individually RNA degradation by more than 20%. However, the deletions of proline, asparagine, glutamine, or alanine from the group of these six amino acids were not followed by a loss of inhibitory effect. On the contrary, an important loss of inhibition was observed when leucine and phenylalanine were deleted. Furthermore, only these two amino acids exhibited an additive inhibitory effect. Thus leucine and phenylalanine could be considered as important inhibitors of RNA breakdown in cultured rat hepatocytes. Finally, insulin which had no significant effect on RNA degradation in the absence of amino acids, was able to potentiate the inhibitory effect of different amino acid groups.

Leucine as a regulator of whole body and skeletal muscle protein metabolism in humans

Leucine has been proposed as an in vivo regulator of protein metabolism, although the evidence for this in humans remains inconclusive. To test this hypothesis, we infused either L-leucine (154 +/- 1 mumol.kg-1 x h-1) or saline intravenously in six healthy men in two separate studies. L-Leucine infusion increased plasma concentrations of leucine and alpha-ketoisocaproate from 112 +/- 6 and 38 +/- 3 mumol/l to 480 +/- 27 (P < 0.001) and 94 +/- 13 mumol/l (P < 0.001), respectively, without any significant change in circulating insulin or C peptide levels. Leucine infusion decreased plasma concentrations of several amino acids and decreased whole body valine flux and valine oxidation (using L-[1-13C]valine as a tracer) and phenylalanine flux (using [2H5]-phenylalanine as a tracer). According to arteriovenous differences across the leg, the net balance of phenylalanine, valine, and lysine shifted toward greater retention during leucine infusion, whereas alanine balance did not change. Valine release and phenylalanine release from the leg (estimated from the dilution of respective tracers) decreased, indicating inhibition of protein degradation by leucine infusion. We conclude that leucine decreases protein degradation in humans and that this decreased protein degradation during leucine infusion contributes to the decrease in plasma essential amino acids. This study suggests a potential role for leucine as a regulator of protein metabolism in humans.

Degradation of rat liver cytochromes P450 3A after their inactivation by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine: characterization of the proteolytic system

The suicide substrate 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4- dihydropyridine (DDEP) inactivates rat liver cytochrome P450 (P450) 3A isozymes through prosthetic heme alkylation of the apoprotein in a mechanism-based fashion, which marks them for rapid proteolysis. In this article, through the use of epitope-specific monoclonal antibodies, we show that both 3A1 and 3A2 isozymes are targeted for proteolysis. Furthermore, using intact rats, isolated rat hepatocytes, and rat liver subcellular fractions supplemented with ATP and MgCl2, as well as various proteolytic inhibitors as probes, we now report that the hepatic cytosolic ubiquitin-dependent proteolytic system rather than hepatic lysosomes is involved in the rapid degradation of DDEP-induced heme alkylated P450s 3A.

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.

Role of leucine and other amino acids in regulating protein metabolism in vivo

The present study examines the independent effects of amino acids and leucine in modulating insulin's effect on leucine kinetics in 24-h fasted conscious dogs during an experimental period where insulin was infused at 600 mU.kg-1.h-1. Group I (n = 7) received saline, group II (n = 10) received sequential infusions of L-leucine at 0, 1, 3, and 1 mumol.kg-1.min-1 each lasting for 90 min, and group III (n = 6) received L-amino acids with doses of L-leucine matching those of group II. Plasma leucine (mumol/l) was 120 +/- 5 basally and 135 +/- 23 and 129 +/- 12 during the infusion of 3.0 mumol.kg-1.min-1 in groups II and III compared with 40 +/- 3 in group I. Leucine rate of appearance (mumol.kg-1.min-1) was 3.5 +/- 0.3 during the basal period and was suppressed 80% in both groups II and III as compared with 40% in group I (P less than 0.01). Leucine oxidation (basal = 0.7 +/- 0.15 mumol.kg-1.min-1) dropped 20% in group I but increased to threefold basal in group II and twofold in group III (P less than 0.05). Nonoxidative rate of disposal (basal = 2.6 +/- 0.2 mumol.kg-1.min-1) dropped 25% in group I and 55% in group II but did not change in group III. These data show that, in addition to insulin, amino acids and particularly leucine cause a marked suppression of proteolysis. Availability of all amino acids to prevent hypoaminoacidemia is necessary to sustain basal rates of protein synthesis. The infusion of leucine alone resulted in significant stimulation of leucine oxidation.

Rapid determination of amino acids by high-performance liquid chromatography: release of amino acids by perfused rat liver

Perfused rat liver can be considered as one of the most suitable ex vivo models for studies of liver metabolism. To assess the possible effect of L-carnitine and some of its acyl esters on proteolysis in the rat liver, the amino acid derivatization and high-performance liquid chromatographic separation of Tapuhi et al. [Anal. Biochem., 115 (1981) 123] was modified.