Assessment of cell-signaling pathways in the regulation of mammalian target of rapamycin (mTOR) by amino acids in rat adipocytes

S6 kinase 1 at the central node of cell size and ageing

Genetic evidence in living organisms from yeast to plants and animals, including humans, unquestionably identifies the Target Of Rapamycin kinase (TOR or mTOR for mammalian/mechanistic) signal transduction pathway as a master regulator of growth through the control of cell size and cell number. Among the mTOR targets, the activation of p70 S6 kinase 1 (S6K1) is exquisitely sensitive to nutrient availability and rapamycin inhibition. Of note, in vivo analysis of mutant flies and mice reveals that S6K1 predominantly regulates cell size versus cell proliferation. Here we review the putative mechanisms of S6K1 action on cell size by considering the main functional categories of S6K1 targets: substrates involved in nucleic acid and protein synthesis, fat mass accumulation, retrograde control of insulin action, senescence program and cytoskeleton organization. We discuss how S6K1 may be involved in the observed interconnection between cell size, regenerative and ageing responses.

Leucine and resistance training improve hyperglycemia, white adipose tissue loss, and inflammatory parameters in an experimental model of type 1 diabetes

BACKGROUND

Loss of white adipose tissue (WAT), associated with type 1 diabetes (DM1), contributes to increased chronic systemic inflammation.

AIM

The aim of this study was to investigate the effects of leucine supplementation and resistance training (RT) in attenuating WAT loss and improving inflammatory parameters and glucose metabolism in DM1 rats.

METHODS

Thirty-two male Wistar rats were distributed into four groups: DA (sedentary and supplemented with non-essential amino acids (NEAA)), DL (sedentary and supplemented with leucine), DTA (submitted to RT and supplemented with NEAA) and DTL (submitted to RT and supplemented with leucine). DM1 was induced by streptozotocin (STZ). An 8-week period of RT consisted of climbing a ladder with a progressively increased load, and supplementation was offered in the feed.

RESULTS

Glycemia, polyphagia and polydipsia were lower in DL, DTA and DTL groups compared with the DA group by approximately 20% ( p<.0001), 28% ( p=.004) and 64% ( p<.0001), respectively. Weight of total WAT and retroperitoneal adipose tissue (RPAT) were higher by approximately 21% ( p=.01) and 54% ( p=.0004), respectively, in DL, DTA and DTL groups compared with DA. However, gene expression of adiponectin and leptin in RPAT was only increased by RT (DTA and DTL) compared with DA and DL by approximately 93% ( p<.0001) and 78% ( p=.0002), respectively. Similarly, the levels of adiponectin in the serum, tissue IL-10 (RPAT) and serum IL-10 were only increased in DTA and DTL compared with DA and DL by approximately 31% ( p=.03), 45% ( p=.0009) and 35% ( p=.003), respectively.

CONCLUSIONS

Both interventions, isolated or together, reduced hyperglycemia and excessive loss of WAT, but RT was the main factor responsible for attenuating inflammation.

Role of GCN2-Independent Signaling Through a Noncanonical PERK/NRF2 Pathway in the Physiological Responses to Dietary Methionine Restriction

Restricting availability of essential amino acids (EAAs) limits aminoacylation of tRNAs by their cognate EAAs and activates the nutrient-sensing kinase, general control nonderepressible 2 (GCN2). Activated GCN2 phosphorylates eukaryotic initiation factor 2 (eIF2), altering gene-specific translation and initiating a transcriptional program collectively described as the integrated stress response (ISR). Central GCN2 activation by EAA deprivation is also linked to an acute aversive feeding response. Dietary methionine restriction (MR) produces a well-documented series of physiological responses (increased energy intake and expenditure, decreased adiposity, and increased insulin sensitivity), but the role of GCN2 in mediating them is unknown. Using Gcn2(-/-) mice, we found that the absence of GCN2 had no effect on the ability of MR to reduce body weight or adiposity, increase energy intake and expenditure, increase hepatic transcription and release of fibroblast growth factor 21, or improve insulin sensitivity. Interestingly, hepatic eIF2 phosphorylation by MR was uncompromised in Gcn2(-/-) mice. Instead, protein kinase R-like endoplasmic reticulum (ER) kinase (PERK) was activated in both intact and Gcn2(-/-) mice. PERK activation corresponded with induction of the ISR and the nuclear respiratory factor 2 antioxidant program but not ER stress. These data uncover a novel glutathione-sensing mechanism that functions independently of GCN2 to link dietary MR to its metabolic phenotype.

Leucine supplementation of a chronically restricted protein and energy diet enhances mTOR pathway activation but not muscle protein synthesis in neonatal pigs

Suboptimal nutrient intake represents a limiting factor for growth and long-term survival of low-birth weight infants. The objective of this study was to determine if in neonates who can consume only 70 % of their protein and energy requirements for 8 days, enteral leucine supplementation will upregulate the mammalian target of rapamycin (mTOR) pathway in skeletal muscle, leading to an increase in protein synthesis and muscle anabolism. Nineteen 4-day-old piglets were fed by gastric tube 1 of 3 diets, containing (kg body weight(-1) · day(-1)) 16 g protein and 190 kcal (CON), 10.9 g protein and 132 kcal (R), or 10.8 g protein + 0.2 % leucine and 136 kcal (RL) at 4-h intervals for 8 days. On day 8, plasma AA and insulin levels were measured during 6 post-feeding intervals, and muscle protein synthesis rate and mTOR signaling proteins were determined at 120 min post-feeding. At 120 min, leucine was highest in RL (P < 0.001), whereas insulin, isoleucine and valine were lower in RL and R compared to CON (P < 0.001). Compared to RL and R, the CON diet increased (P < 0.01) body weight, protein synthesis, phosphorylation of S6 kinase (p-S6K1) and 4E-binding protein (p-4EBP1), and activation of eukaryotic initiation factor 4 complex (eIF4E · eIF4G). RL increased (P ≤ 0.01) p-S6K1, p-4EBP1 and eIF4E · eIF4G compared to R. In conclusion, when protein and energy intakes are restricted for 8 days, leucine supplementation increases muscle mTOR activation, but does not improve body weight gain or enhance skeletal muscle protein synthesis in neonatal pigs.

Phenylalanine sensitive K562-D cells for the analysis of the biochemical impact of excess amino acid

Although it is recognized that the abnormal accumulation of amino acid is a cause of the symptoms in metabolic disease such as phenylketonuria (PKU), the relationship between disease severity and serum amino acid levels is not well understood due to the lack of experimental model. Here, we present a novel in vitro cellular model using K562-D cells that proliferate slowly in the presence of excessive amount of phenylalanine within the clinically observed range, but not phenylpyruvate. The increased expression of the L-type amino acid transporter (LAT2) and its adapter protein 4F2 heavy chain appeared to be responsible for the higher sensitivity to phenylalanine in K562-D cells. Supplementation with valine over phenylalanine effectively restored cell proliferation, although other amino acids did not improve K562-D cell proliferation over phenylalanine. Biochemical analysis revealed mammalian target of rapamycin complex (mTORC) as a terminal target of phenylalanine in K562-D cell proliferation, and supplementation of valine restored mTORC1 activity. Our results show that K562-D cell can be a potent tool for the investigation of PKU at the molecular level and to explore new therapeutic approaches to the disease.

Metabolic effects of milk protein intake strongly depend on pre-existing metabolic and exercise status

Milk protein intake has recently been suggested to improve metabolic health. This Perspective provides evidence that metabolic effects of milk protein intake have to be regarded in the context of the individual’s pre-existing metabolic and exercise status. Milk proteins provide abundant branched-chain amino acids (BCAAs) and glutamine. Plasma BCAAs and glutamine are increased in obesity and insulin resistance, but decrease after gastric bypass surgery resulting in weight loss and improved insulin sensitivity. Milk protein consumption results in postprandial hyperinsulinemia in obese subjects, increases body weight of overweight adolescents and may thus deteriorate pre-existing metabolic disturbances of obese, insulin resistant individuals.

The impact of cow's milk-mediated mTORC1-signaling in the initiation and progression of prostate cancer

Prostate cancer (PCa) is dependent on androgen receptor signaling and aberrations of the PI3K-Akt-mTORC1 pathway mediating excessive and sustained growth signaling. The nutrient-sensitive kinase mTORC1 is upregulated in nearly 100% of advanced human PCas. Oncogenic mTORC1 signaling activates key subsets of mRNAs that cooperate in distinct steps of PCa initiation and progression. Epidemiological evidence points to increased dairy protein consumption as a major dietary risk factor for the development of PCa. mTORC1 is a master regulator of protein synthesis, lipid synthesis and autophagy pathways that couple nutrient sensing to cell growth and cancer. This review provides evidence that PCa initiation and progression are promoted by cow´s milk, but not human milk, stimulation of mTORC1 signaling. Mammalian milk is presented as an endocrine signaling system, which activates mTORC1, promotes cell growth and proliferation and suppresses autophagy. Naturally, milk-mediated mTORC1 signaling is restricted only to the postnatal growth phase of mammals. However, persistent consumption of cow´s milk proteins in humans provide highly insulinotropic branched-chain amino acids (BCAAs) provided by milk´s fast hydrolysable whey proteins, which elevate postprandial plasma insulin levels, and increase hepatic IGF-1 plasma concentrations by casein-derived amino acids. BCAAs, insulin and IGF-1 are pivotal activating signals of mTORC1. Increased cow´s milk protein-mediated mTORC1 signaling along with constant exposure to commercial cow´s milk estrogens derived from pregnant cows may explain the observed association between high dairy consumption and increased risk of PCa in Westernized societies. As well-balanced mTORC1-signaling plays an important role in appropriate prostate morphogenesis and differentiation, exaggerated mTORC1-signaling by high cow´s milk consumption predominantly during critical growth phases of prostate development and differentiation may exert long-term adverse effects on prostate health. Attenuation of mTORC1 signaling by contemporary Paleolithic diets and restriction of dairy protein intake, especially during mTORC1-dependent phases of prostate development and differentiation, may offer protection from the most common dairy-promoted cancer in men of Western societies.

Excessive Leucine-mTORC1-Signalling of Cow Milk-Based Infant Formula: The Missing Link to Understand Early Childhood Obesity

Increased protein supply by feeding cow-milk-based infant formula in comparison to lower protein content of human milk is a well-recognized major risk factor of childhood obesity. However, there is yet no conclusive biochemical concept explaining the mechanisms of formula-induced childhood obesity. It is the intention of this article to provide the biochemical link between leucine-mediated signalling of mammalian milk proteins and adipogenesis as well as early adipogenic programming. Leucine has been identified as the predominant signal transducer of mammalian milk, which stimulates the nutrient-sensitive kinase mammalian target of rapamycin complex 1 (mTORC1). Leucine thus functions as a maternal-neonatal relay for mTORC1-dependent neonatal β-cell proliferation and insulin secretion. The mTORC1 target S6K1 plays a pivotal role in stimulation of mesenchymal stem cells to differentiate into adipocytes and to induce insulin resistance. It is of most critical concern that infant formulas provide higher amounts of leucine in comparison to human milk. Exaggerated leucine-mediated mTORC1-S6K1 signalling induced by infant formulas may thus explain increased adipogenesis and generation of lifelong elevated adipocyte numbers. Attenuation of mTORC1 signalling of infant formula by leucine restriction to physiologic lower levels of human milk offers a great chance for the prevention of childhood obesity and obesity-related metabolic diseases.

Leucine signaling in the pathogenesis of type 2 diabetes and obesity

Epidemiological evidence points to increased dairy and meat consumption, staples of the Western diet, as major risk factors for the development of type 2 diabetes (T2D). This paper presents a new concept and comprehensive review of leucine-mediated cell signaling explaining the pathogenesis of T2D and obesity by leucine-induced over-stimulation of mammalian target of rapamycin complex 1 (mTORC1). mTORC1, a pivotal nutrient-sensitive kinase, promotes growth and cell proliferation in response to glucose, energy, growth factors and amino acids. Dairy proteins and meat stimulate insulin/insulin-like growth factor 1 signaling and provide high amounts of leucine, a primary and independent stimulator for mTORC1 activation. The downstream target of mTORC1, the kinase S6K1, induces insulin resistance by phosphorylation of insulin receptor substrate-1, thereby increasing the metabolic burden of β-cells. Moreover, leucine-mediated mTORC1-S6K1-signaling plays an important role in adipogenesis, thus increasing the risk of obesity-mediated insulin resistance. High consumption of leucine-rich proteins explains exaggerated mTORC1-dependent insulin secretion, increased β-cell growth and β-cell proliferation promoting an early onset of replicative β-cell senescence with subsequent β-cell apoptosis. Disturbances of β-cell mass regulation with increased β-cell proliferation and apoptosis as well as insulin resistance are hallmarks of T2D, which are all associated with hyperactivation of mTORC1. In contrast, the anti-diabetic drug metformin antagonizes leucine-mediated mTORC1 signaling. Plant-derived polyphenols and flavonoids are identified as natural inhibitors of mTORC1 and exert anti-diabetic and anti-obesity effects. Furthermore, bariatric surgery in obesity reduces increased plasma levels of leucine and other branched-chain amino acids. Attenuation of leucine-mediated mTORC1 signaling by defining appropriate upper limits of the daily intake of leucine-rich animal and dairy proteins may offer a great chance for the prevention of T2D and obesity, as well as other epidemic diseases of civilization with increased mTORC1 signaling, especially cancer and neurodegenerative diseases, which are frequently associated with T2D.

Deptor knockdown enhances mTOR Activity and protein synthesis in myocytes and ameliorates disuse muscle atrophy

Deptor is an mTOR binding protein that affects cell metabolism. We hypothesized that knockdown (KD) of Deptor in C2C12 myocytes will increase protein synthesis via stimulating mTOR-S6K1 signaling. Deptor KD was achieved using lentiviral particles containing short hairpin (sh)RNA targeting the mouse Deptor mRNA sequence, and control cells were transfected with a scrambled control shRNA. KD reduced Deptor mRNA and protein content by 90%, which increased phosphorylation of mTOR kinase substrates, 4E-BP1 and S6K1, and concomitantly increased protein synthesis. Deptor KD myoblasts were both larger in diameter and exhibited an increased mean cell volume. Deptor KD increased the percentage of cells in the S phase, coincident with an increased phosphorylation (S807/S811) of retinoblastoma protein (pRb) that is critical for the G(1) to S phase transition. Deptor KD did not appear to alter basal apoptosis or autophagy, as evidenced by the lack of change for cleaved caspase-3 and light chain (LC)3B, respectively. Deptor KD increased proliferation rate and enhanced myotube formation. Finally, in vivo Deptor KD (~50% reduction) by electroporation into gastrocnemius of C57/BL6 mice did not alter weight or protein synthesis in control muscle. However, Deptor KD prevented atrophy produced by 3 d of hindlimb immobilization, at least in part by increasing protein synthesis. Thus, our data support the hypothesis that Deptor is an important regulator of protein metabolism in myocytes and demonstrate that decreasing Deptor expression in vivo is sufficient to ameliorate muscle atrophy.

Methionine deprivation regulates the S6K1 pathway and protein synthesis in avian QM7 myoblasts without activating the GCN2/eIF2 alpha cascade

Amino acids modulate mRNA translation through the 70 kDa ribosomal protein S6 kinase (S6K1) and the general control nondepressible 2 protein kinase (GCN2)/eukaryotic initiation factor 2 alpha eIF2 alpha pathways. The aim of the present study was therefore to explore the signaling cascades potentially modulated by methionine availability in quail muscle QM7 myoblasts using media providing all other amino acids. Methionine deprivation caused a lower S6K1 phosphorylation compared with control (Ctl) cells. Supplying the methionine-deprived media with L- and DL-methionine isomers restored S6K1 phosphorylation to the levels observed in Ctl cells. Methionine also regulated downstream S6K1 targets (i.e. ribosomal protein S6 and eukaryotic elongation factor 2), modulated translation preinitiation complex (PIC) assembly, and stimulated protein synthesis. Replacing the lacking methionine with D-methionine or its hydroxyanalog [2-hydroxy-(4-methylthio) butanoic acid] did not restore S6K1 activation or protein synthesis. Conversely, the S6K1 pathway was activated by a methionine precursor, the ketoanalog of methionine. Methionine availability regulated the GCN2/eIF2 alpha pathway. However, our results indicate that methionine deprivation led to lower protein synthesis without activating eIF2 alpha phosphorylation, a process known to limit the formation of the 43S PIC. Using the amino acid alcohol methioninol did not decrease S6K1 phosphorylation or activity and did not alter the regulation of protein synthesis by methionine. These findings suggest that methionine exerts an effect on S6K1 signaling and protein synthesis in avian QM7 myoblasts through a mechanism partly independent of the global regulation via tRNA charging.

Leucine metabolism in regulation of insulin secretion from pancreatic beta cells

Leucine, a branched-chain amino acid that must be supplied in the daily diet, plays an important role in controlling protein synthesis and regulating cell metabolism in various cell types. In pancreatic beta cells, leucine acutely stimulates insulin secretion by serving as both metabolic fuel and allosteric activator of glutamate dehydrogenase to enhance glutaminolysis. Leucine has also been shown to regulate gene transcription and protein synthesis in pancreatic islet beta cells via both mTOR-dependent and -independent pathways at physiological concentrations. Long-term treatment with leucine has been shown to improve insulin secretory dysfunction of human diabetic islets via upregulation of certain key metabolic genes. In vivo, leucine administration improves glycemic control in humans and rodents with type 2 diabetes. This review summarizes and discusses the recent findings regarding the effects of leucine metabolism on pancreatic beta-cell function.

Wnt signalling and the control of cellular metabolism

At the cellular level, the biological processes of cell proliferation, growth arrest, differentiation and apoptosis are all tightly coupled to appropriate alterations in metabolic status. In the case of cell proliferation, this requires redirecting metabolic pathways to provide the fuel and basic components for new cells. Ultimately, the successful co-ordination of cell-specific biology with cellular metabolism underscores multicellular processes as diverse as embryonic development, adult tissue remodelling and cancer cell biology. The Wnt signalling network has been implicated in all of these areas. While each of the Wnt-dependent signalling pathways are being individually delineated in a range of experimental systems, our understanding of how they integrate and regulate cellular metabolism is still in its infancy. In the present review we reassess the roles of Wnt signalling in functionally linking cellular metabolism to tissue development and function.

The sensing of essential amino acid deficiency in the anterior piriform cortex, that requires the uncharged tRNA/GCN2 pathway, is sensitive to wortmannin but not rapamycin

Animals detect and reject their first essential/indispensable amino acid (IAA) deficient meal within 20min; this IAA sensing requires an intact anterior piriform cortex (APC). In the biochemical responses to IAA deficiency in the APC we have shown that: uncharged tRNA is the primary sensor; IAA transport is increased; and signaling, including the extracellular-regulated kinase (ERK1/2), is activated. The mammalian target of rapamycin (mTOR) is a potential AA sensor and is regulated by AA transport. Previously, the inhibitors, rapamycin for mTOR, wortmannin for phosphoinositide 3 kinase (PI3K) and PD98059 for ERK, each blocked the upregulation of the System A transporter in IAA depleted APC neurons. Here we injected these same inhibitors into the APC and measured intake of an IAA deficient diet. Rapamycin had no effect on the rejection of the IAA deficient diet, but wortmannin increased ERK activation and intake of the deficient diet before 40min and PD98059 acted after 40min to increase the second meal. While the specific wortmannin target involved in blocking the behavioral response remains unclear, we conclude that mTOR is dispensable for sensing IAA deficiency in the APC, and that ERK is associated with the secondary learned responses to IAA deficient diets.

Early responses of insulin signaling to high-carbohydrate and high-fat overfeeding

Background

Early molecular changes of nutritionally-induced insulin resistance are still enigmatic. It is also unclear if acute overnutrition alone can alter insulin signaling in humans or if the macronutrient composition of the diet can modulate such effects.

Methods

To investigate the molecular correlates of metabolic adaptation to either high-carbohydrate (HC) or high-fat (HF) overfeeding, we conducted overfeeding studies in 21 healthy lean (BMI < 25) individuals (10 women, 11 men), age 20-45, with normal glucose metabolism and no family history of diabetes. Subjects were studied first following a 5-day eucaloric (EC) diet (30% fat, 50% CHO, 20% protein) and then in a counter balanced manner after 5 days of 40% overfeeding of both a HC (20% fat, 60% CHO) diet and a HF (50% fat, 30% CHO) diet. At the end of each diet phase, in vivo insulin sensitivity was assessed using the hyperinsulinemic-euglycemic clamp technique. Ex vivo insulin action was measured from skeletal muscle tissue samples obtained 15 minutes after insulin infusion was initiated.

Results

Overall there was no change in whole-body insulin sensitivity as measured by glucose disposal rate (GDR, EC: 12.1 ± 4.7; HC: 10.9 ± 2.7; HF: 10.8 ± 3.4). Assessment of skeletal muscle insulin signaling demonstrated increased tyrosine phosphorylation of IRS-1 (p < 0.001) and increased IRS-1-associated phosphatidylinositol 3 (PI 3)-kinase activity (p < 0.001) following HC overfeeding. In contrast, HF overfeeding increased skeletal muscle serine phosophorylation of IRS-1 (p < 0.001) and increased total expression of p85α (P < 0.001).

Conclusion

We conclude that acute bouts of overnutrition lead to changes at the cellular level before whole-body insulin sensitivity is altered. On a signaling level, HC overfeeding resulted in changes compatible with increased insulin sensitivity. In contrast, molecular changes in HF overfeeding were compatible with a reduced insulin sensitivity.

Threonine-deficient diets induced changes in hepatic bioenergetics

Diets deficient in an indispensable amino acid are known to suppress food intake in rats. Few studies were focused at understanding how amino acid-deficient diets may elicit biochemical changes at the mitochondrial level. The goal of this study was to evaluate mitochondrial function in rats fed diets with 0.00, 0.18, 0.36, and 0.88% threonine (Thr) (set at 0, 30, 60, and 140% of Thr requirement for growth). Here, it is described for the first time that Thr-deficient diets induce a specific uncoupling of mitochondria in liver, especially with NADH-linked substrates, not observed in heart (except for Thr-devoid diet). The advantage of this situation would be to provide ATP to support growth and maintenance when high-quality protein food (or wealth of high-quality food in general) is available, whereas Thr-deficient diets (or deficient-quality protein food) promote the opposite, increasing mitochondrial uncoupling in liver. The uncoupling with NADH substrates would favor the use of nutrients as energy sources with higher FADH-to-NADH ratios, such as fat, minimizing the first irreversible NADH-dependent catabolism of many amino acids, including Thr, thus enhancing the use of the limiting amino acid for protein synthesis when a low quality protein source is available.

Rescuing 3T3-L1 adipocytes from insulin resistance induced by stimulation of Akt-mammalian target of rapamycin/p70 S6 kinase (S6K1) pathway and serine phosphorylation of insulin receptor substrate-1: effect of reduced expression of p85alpha subunit of phosphatidylinositol 3-kinase and S6K1 kinase

Phosphorylation of insulin receptor substrate-1 (IRS-1) on serine residues has been recognized as a mechanism responsible for a diminution of insulin action and insulin resistance. Potential approaches to improve insulin sensitivity may include interference with and/or reduction in expression of certain signaling intermediates that participate in the pathogenesis of insulin resistance. In this study, we transduced fully differentiated 3T3-L1 adipocytes with a constitutively active myristoylated Akt that led to hyperactivation of mammalian target of rapamycin and p70 S6 kinase (S6K1), increased serine phosphorylation of IRS-1, and reduction in insulin-stimulated phosphatidylinositol (PI) 3-kinase activity and glucose transport. We then reduced expression of the PI 3-kinase regulatory subunit, p85alpha, or expression of S6K1 kinase using small interfering RNA transfections, which led to a reduction in p85alpha expression of 70% at 48 h (P < 0.05) and S6K1 of 49% (P < 0.05). Reduction in expression of either p85alpha or S6K1 achieved with small interfering RNA in the presence of myristoylated Akt rescued 3T3-L1 adipocytes from the insulin resistance induced by serine phosphorylation of IRS-1 and completely restored insulin-stimulated activation of PI 3-kinase and glucose uptake. We conclude that reduction in expression of p85alpha or S6K1 could represent therapeutic targets to mitigate insulin resistance.

Insulin signaling in skeletal muscle and liver of neonatal pigs during endotoxemia

Sepsis has been associated with tumor necrosis factor alpha (TNF-alpha) and nitric oxide (NO) overproduction, insulin resistance, and a profound suppression of muscle protein synthesis. However, lesser suppression of muscle protein synthesis in neonatal pigs occurs in response to endotoxin (LPS) when glucose and amino acids are provided. We hypothesize that the LPS-induced TNF-alpha and NO overproduction down-regulates insulin signaling pathway activation in neonatal pigs in the presence of fed levels of insulin, glucose, and amino acids. In skeletal muscle, inducible NOS activity was increased in response to LPS infusion, but phosphorylation of the insulin receptor, insulin receptor substrate-1 (IRS-1), p42/p44 mitogen-activated protein kinase (MAPK), and protein kinase B, the association of IRS-1 with phosphatidylinositol 3-kinase (PI 3-kinase), and constitutive NOS activity were not altered. In liver, activation of the insulin receptor, IRS-1, and PI 3-kinase were not affected by LPS, but p42 MAPK phosphorylation was increased. The absence of a down-regulation in the insulin signaling cascade in muscle despite the LPS-induced increase in TNF-alpha and muscle iNOS, may contribute to the near-maintenance of muscle protein synthesis rates in the presence of glucose and amino acids in LPS-infused neonatal pigs.

Re-evaluating the roles of proposed modulators of mammalian target of rapamycin complex 1 (mTORC1) signaling

Signaling through mammalian target of rapamycin complex 1 (mTORC1) is stimulated by amino acids and insulin. Insulin inactivates TSC1/2, the GTPase-activator complex for Rheb, and Rheb.GTP activates mTORC1. It is not clear how amino acids regulate mTORC1. FKBP38 (immunophilin FK506-binding protein, 38 kDa), was recently reported to exert a negative effect on mTORC1 function that is relieved by its binding to Rheb.GTP. We confirm that Rheb binds wild type FKBP38, but inactive Rheb mutants showed contrasting abilities to bind FKBP38. We were unable to observe any regulation of FKBP38/mTOR binding by amino acids or insulin. Furthermore, FKBP38 did not inhibit mTORC1 signaling. The translationally controlled tumor protein (TCTP) in Drosophila was recently reported to act as the guanine nucleotide-exchange factor for Rheb. We have studied the role of TCTP in mammalian TORC1 signaling and its control by amino acids. Reducing TCTP levels did not reproducibly affect mTORC1 signaling in amino acid-replete/insulin-stimulated cells. Moreover, overexpressing TCTP did not rescue mTORC1 signaling in amino acid-starved cells. In addition, we were unable to see any stable interaction between TCTP and Rheb or mTORC1. Accumulation of uncharged tRNA has been previously proposed to be involved in the inhibition of mTORC1 signaling during amino acid starvation. To test this hypothesis, we used a Chinese hamster ovary cell line containing a temperature-sensitive mutation in leucyl-tRNA synthetase. Leucine deprivation markedly inhibited mTORC1 signaling in these cells, but shifting the cells to the nonpermissive temperature for the synthetase did not. These data indicate that uncharged tRNA(Leu) does not switch off mTORC1 signaling and suggest that mTORC1 is controlled by a distinct pathway that senses the availability of amino acids. Our data also indicate that, in the mammalian cell lines tested here, neither TCTP nor FKBP38 regulates mTORC1 signaling.

Basal Activation of p70S6K Results in Adipose-specific Insulin Resistance in Protein-tyrosine Phosphatase 1B–/– Mice

Although protein-tyrosine phosphatase 1B (PTP-1B) is a negative regulator of insulin action, adipose tissue from PTP-1B-/- mice does not show enhanced insulin-stimulated insulin receptor phosphorylation. Investigation of glucose uptake in isolated adipocytes revealed that the adipocytes from PTP-1B-/- mice have a significantly attenuated insulin response as compared with PTP-1B+/+ adipocytes. This insulin resistance manifests in PTP-1B-/- animals older than 16 weeks of age and could be partially rescued by adenoviral expression of PTP-1B in null adipocytes. Examination of adipose signaling pathways found that the basal p70S6K activity was at least 50% higher in adipose from PTP-1B-/- mice compared with wild type animals. The increased basal activity of p70S6K in PTP-1B-/- adipose correlated with decreases in IR substrate-1 protein levels and insulin-stimulated Akt/protein kinase B activity, explaining the decrease in insulin sensitivity even as insulin receptor phosphorylation was unaffected. The insulin resistance of the of the PTP-1B-/- adipocytes could also be rescued by treatment with rapamycin, suggesting that in adipose the loss of PTP-1B results in basal activation of mTOR (mammalian target of rapamycin) complex 1 leading to a tissue-specific insulin resistance.

Mechanisms of Food Intake Repression in Indispensable Amino Acid Deficiency

Animals reject diets that lead to indispensable amino acid (IAA) depletion or deficiency. This behavior is adaptive, as continued IAA depletion is incompatible with maintenance of protein synthesis and survival. Following rejection of the diet, animals begin foraging for a better IAA source and develop conditioned aversions to cues associated with the deficient diet. These responses require a sensory system to detect the IAA depletion and alert the appropriate neural circuitry for the behavior. The chemosensor for IAA deprivation is in the highly excitable anterior piriform cortex (APC) of the brain. Recently, the well-conserved general AA control non-derepressing system of yeast was discovered to be activated by IAA deprivation via uncharged tRNA in mammalian APC. This system provides the sensory limb of the mechanism for recognition of IAA depletion that leads to activation of the APC, diet rejection, and subsequent adaptive strategies.

Leucine in food mediates some of the postprandial rise in plasma leptin concentrations

In vitro, leptin secretion is regulated at the level of mRNA translation by the rapamycin-sensitive mammalian target of rapamycin (mTOR) and its agonist leucine (Leu). Studies were conducted on meal-trained rats to evaluate the potential physiological relevance of these in vitro findings and the role of Leu in affecting rises in plasma leptin observed after a meal. In the first study, we correlated changes in plasma insulin and Leu to mTOR-signaling pathway activation and plasma leptin at different times during meal feeding. Rapid rises in plasma insulin and Leu, along with mTOR signaling (phosphorylation of eIF4G, S6K1, rpS6, and 4E-BP1) in adipose tissue were observed during the 3-h meal and declined thereafter. Plasma leptin rose more slowly, peaking at 3 h, and was inhibited by rapamycin (0.75 mg/kg) pretreatment. In another experiment, oral Leu or norleucine was provided instead of a meal. Leu and norleucine stimulated a rise in plasma leptin; however, the magnitude was less than the response to a complete meal. In a third study, rats were provided a meal that lacked Leu, branched-chain amino acids, or all amino acids. Stimulation of leptin secretion was reduced approximately 40% in animals provided the Leu-deficient meal. Further reductions were not observed by removing the other amino acids. Thus Leu appears to regulate most of the effects of dietary amino acids on the postprandial rise in plasma leptin but is responsible only for part of the leptin response to meal feeding.

Molecular mechanisms of insulin resistance: serine phosphorylation of insulin receptor substrate-1 and increased expression of p85alpha: the two sides of a coin

Initial attempts to unravel the molecular mechanism of insulin resistance have strongly suggested that a defect responsible for insulin resistance in the majority of patients lies at the postreceptor level of insulin signaling. Subsequent studies in insulin-resistant animal models and humans have consistently demonstrated a reduced strength of insulin signaling via the insulin receptor substrate (IRS)-1/phosphatidylinositol (PI) 3-kinase pathway, resulting in diminished glucose uptake and utilization in insulin target tissues. However, the nature of the triggering event(s) remains largely enigmatic. Two separate, but likely, complementary mechanisms have recently emerged as a potential explanation. First, it became apparent that serine phosphorylation of IRS proteins can reduce their ability to attract PI 3-kinase, thereby minimizing its activation. A number of serine kinases that phosphorylate serine residues of IRS-1 and weaken insulin signal transduction have been identified. Additionally, mitochondrial dysfunction has been suggested to trigger activation of several serine kinases, leading to a serine phosphorylation of IRS-1. Second, a distinct mechanism involving increased expression of p85alpha has also been found to play an important role in the pathogenesis of insulin resistance. Conceivably, a combination of both increased expression of p85alpha and increased serine phosphorylation of IRS-1 is needed to induce clinically apparent insulin resistance.

Leucine activates pancreatic translational machinery in rats and mice through mTOR independently of CCK and insulin

Feeding stimulates pancreatic digestive enzyme synthesis at the translational level, and this is thought to be mediated by hormones and neurotransmitters. However, BCAAs, particularly leucine, stimulate protein synthesis in several tissues. We investigated whether BCAA stimulated the translational machinery in murine pancreas and whether their effects were independent of hormones. Rats and mice were administered (i.g. gavage) individual BCAA at 1.35 mg/g (body weight) and rat isolated pancreatic acini were incubated with BCAA under different conditions. Activation of translation initiation factors and total protein synthesis were analyzed. BCAA gavage stimulated the phosphorylation of the initiation factor 4E (eIF4E) binding protein 1 (4E-BP1) and the ribosomal protein S6 kinase (S6K), with leucine being the most effective. Leucine also increased the association of the initiation factors eIF4E and eIF4G, but did not affect the activity of the guanine nucleotide exchange factor eIF2B, nor total protein synthesis. BCAA acted independently of insulin signaling on isolated pancreatic acini from diabetic rats. The ability of leucine to promote phosphorylation of 4E-BP1 and S6K as well as enhance the assembly of the eIF4F complex was unimpaired in CCK-deficient mice. Finally, rapamycin (0.75 mg/kg) administered to rats 2 h before leucine gavage inhibited the phosphorylation of S6 and 4E-BP1 induced by leucine. We conclude that leucine may participate, as a signal as well as a substrate, in activating the translational machinery in pancreatic acinar cells independently of hormonal effects and that this action is through the mTOR pathway.

Role of amino acids in insulin signaling in adipocytes and their potential to decrease insulin resistance of adipose tissue

Recently, our knowledge concerning the role of amino acids in signal transduction in mammals has greatly improved. This significant advance is mainly due to the remarkable discovery that the mammalian target of rapamycin (mTOR) protein kinase, known to be activated in response to a large number of hormones, growth factors and cytokines, is also under the tight control of branched-chain amino acids. Actually, both inputs are necessary to fully activate the mTOR pathway, the main function of which is to increase cell size, via the regulation of translational processes. However, amino acids are able to modulate other biological effects and appear to have unexpected actions, as evidenced by our recent work in rat adipocytes. The aim of this review is to summarize novel findings on the role of mTOR and amino acids in insulin signaling in adipocytes. A possible beneficial impact of the use of amino acids in the treatment of insulin resistance is discussed, and hypotheses about the molecular mechanisms underlying their effect are proposed.

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.

Inhibition of the mammalian target of rapamycin (mTOR) by rapamycin increases chemosensitivity of CaSki cells to paclitaxel

Paclitaxel, a potent anti-neoplastic agent, has been found to be effective against several tumours, including cervical cancer. However, the exact mechanism underlying the cytotoxic effects of pacitaxel, especially in the survival-signalling pathway, is poorly understood. The aim of this study was to investigate the molecular pathway of the cytotoxic effect of paclitaxel in human cervical cancer cell lines. Four human cervical cancer cell lines were treated for 24 h with various concentration of paclitaxel, and the sensitivity was analysed by an MTT assay. The cell cycle progression and sub-G1 population were analysed by flow cytometry. Apoptosis was further measured by DNA fragmentation and microscope examination. The protein expression was determined by Western blot analysis. Our results showed that HeLa cells demonstrated the highest sensitivity to paclitaxel, whereas CaSki cells showed the lowest. In cervical cancer cells, paclitaxel induced apoptosis through an intrinsic pathway with prior G2/M arrest. In addition, we showed that paclitaxel downregulated the phosphorylation of Akt in both HeLa and CaSki cells. Interestingly, in CaSki cells, which were more suggestive of a resistant phenotype, paclitaxel induced the activation of mTOR as a downstream target of Akt. Pre-treatment with rapamycin inhibited activation of mTOR signalling and significantly enhanced the sensitivity of CaSki cells to paclitaxel by increasing apoptotic cell death. This effect was mediated, at least partly, through caspase activation. Overall, paclitaxel exerts its anti-tumour effects on cervical cancer cells by inducing apoptosis through intrinsic pathway, and rapamycin targeted to mTOR can sensitise paclitaxel-resistant cervical cancer cells.

Nutritional upregulation of p85alpha expression is an early molecular manifestation of insulin resistance

AIMS/HYPOTHESIS

We sought to define early molecular alterations associated with nutritionally induced insulin resistance in humans.

METHODS

Insulin sensitivity was assessed using a hyperinsulinaemic-euglycaemic clamp in eight healthy women while on an isocaloric diet and after 3 days of overfeeding (50% above eucaloric diet). Expression of phosphatidylinositol (PI) 3-kinase subunits p85alpha and p110 was assessed and measurements were made of IRS-1-associated PI 3-kinase activity, tyrosine and serine phosphorylation of IRS-1, and serine and threonine phosphorylation of p70S6 kinase. Measurements were made in skeletal muscle biopsies obtained before and after overfeeding.

RESULTS

Three days of overfeeding resulted in a reduction of insulin sensitivity accompanied by: (1) increased expression of skeletal muscle p85alpha; (2) an alteration in the ratio of p85alpha to p110; (3) a decrease in the amount of IRS-1-associated p110; and (4) a decrease in PI 3-kinase activity. Increases in expression of p85alpha and in the p85alpha:p110 ratio demonstrated a highly significant inverse correlation with insulin sensitivity, and changes in PI 3-kinase activity correlated with changes in insulin sensitivity. Tyrosine and serine phosphorylation of IRS-1 and serine and threonine phosphorylation of p70S6 kinase were unaffected by 3 days of overfeeding.

CONCLUSIONS/INTERPRETATION

We identified a novel mechanism of nutritionally induced insulin resistance in healthy women of normal weight. We conclude that increased expression of p85alpha may be one of the earliest molecular alterations in the mechanism of the insulin resistance associated with overfeeding.

Modulation of arginine metabolic pathways as the potential anti-tumor mechanism of recombinant arginine deiminase

Arginine deiminase (ADI), currently in clinical trials, has various biological activities including anti-proliferation, anti-angiogenesis and inhibition of inducible nitric oxide synthase (iNOS). To recognize limitations and therapeutic applications, the mechanism of ADI modulation of arginine metabolic pathways was investigated. MCF-7 and A549 cells have notable different sensitivity to recombinant ADI (rADI) and express diverse argininosuccinate synthase (AS) activity, which regenerates arginine. Due to compartmentalization of arginine, utilization of arginine for protein synthesis occurs from either the intracellular arginine pool or the citrulline-arginine-regeneration pathway, whereas for polyamine synthesis, utilization is only from the intracellular arginine pool. Modulating AS activity or introducing rADI intracellularly to reduce intracellular arginine regeneration may expand therapeutic applications of rADI.

Nutritional homeostasis and indispensable amino acid sensing: a new solution to an old puzzle

Indispensable amino acids are neither synthesized nor stored in animals and are rapidly depleted when not provided by the diet. To maintain homeostasis, organisms must sense deficiency of an indispensable amino acid and implement a repletion strategy. In rats and birds, the anterior piriform cortex houses the detector, but its mechanism has evaded description for >50 years. Recently, rapid detection of amino acid depletion was shown behaviorally when naïve animals, pre-fed a low nitrogen diet, terminated their first deficient meal within 20 min. The general amino acid control system of yeast, which is activated by amino acid deprivation via deacylated tRNA, was found to be active in rodent brain, showing conservation of amino acid sensory mechanisms across eukaryotic species.

The mammalian target of rapamycin-p70 ribosomal S6 kinase but not phosphatidylinositol 3-kinase-Akt signaling is responsible for fibroblast growth factor-9-induced cell proliferation

Fibroblast growth factor-9 (FGF9) is a potent mitogen that stimulates normal and cancer cell proliferation though the signaling mechanism is not fully understood. In this study, we aimed to unravel the signaling cascades mediate FGF9 actions in human uterine endometrial stromal cell. Our results demonstrate that the mitogenic effect of FGF9 is transduced via two parallel but additive signaling pathways involving mammalian target of rapamycin (mTOR) and extracellular signal-regulated kinase. Activation of mTOR by FGF9 induces p70 ribosomal S6 kinase (S6K1) phosphorylation, cyclin expression, and cell proliferation, which are independent of phosphatidylinositol 3-kinase and Akt. Coimmunoprecipitation analysis demonstrates that mTOR physically associates with S6K1 upon FGF9 treatment, whereas ablation of mTOR activity using RNA interference or pharmacological inhibitor blocks S6K1 phosphorylation and cell proliferation induced by FGF9. Further study demonstrates that activation of mTOR is regulated by a phospholipase Cgamma-controlled calcium signaling pathway. These studies provide evidence to demonstrate, for the first time, that a novel signaling cascade involving phospholipase Cgamma, calcium, mTOR, and S6K1 is activated by FGF9 in a receptor-specific manner.

Nutrient sensing and metabolic decisions

Cells have several sensory systems that detect energy and metabolic status and adjust flux through metabolic pathways accordingly. Many of these sensors and signaling pathways are conserved from yeast to mammals. In this review, we bring together information about five different nutrient-sensing pathways (AMP kinase, mTOR, PAS kinase, hexosamine biosynthesis and Sir2), highlighting their similarities, differences and roles in disease.

Regulation of adipocyte differentiation and insulin action with rapamycin

Here, we demonstrated that inhibition of mTOR with rapamycin has negative effects on adipocyte differentiation and insulin signaling. Rapamycin significantly reduced expression of most adipocyte marker genes including PPARgamma, adipsin, aP2, ADD1/SREBP1c, and FAS, and decreased intracellular lipid accumulation in 3T3-L1 and 3T3-F442A cells, suggesting that rapamycin would affect both lipogenesis and adipogenesis. Contrary to the previous report that suppressive effect of rapamycin on adipogenesis is limited to the clonal expansion, we revealed that its inhibitory effect persisted throughout the process of adipocyte differentiation. Thus, it is likely that constitutive activation of mTOR might be required for the execution of adipogenic programming. In differentiated 3T3-L1 adipocytes, chronic treatment of rapamycin blunted the phosphorylation of AKT and GSK, which is stimulated by insulin, and reduced insulin-dependent glucose uptake activity. Taken together, these results suggest that rapamycin not only prevents adipocyte differentiation by decrease of adipogenesis and lipogenesis but also downregulates insulin action in adipocytes, implying that mTOR would play important roles in adipogenesis and insulin action.

N-(methylamino)isobutyric acid inhibits proliferation of CFSC-2C hepatic stellate cells

Activation of hepatic stellate cells (HSCs) involves the induction of ECM protein synthesis and rapid cell proliferation. Thus, agents that interfere with either process could potentially mitigate the development of liver disease by reducing the synthesis of proteins associated with fibrosis or by reducing the number of activated HSC. Previously, we described that the non-metabolizable amino acid analog N-(methylamino)isobutyric acid (MeAIB) reduced hepatic collagen content of rats in a model of CCl(4)-induced liver injury, and in vitro studies using CFSC-2G cells indicated that MeAIB directly reduced collagen synthesis. However, the MeAIB-mediated reduction of hepatic collagen, in vivo, following liver injury was associated with a decrease in hepatic alpha-smooth muscle actin (alpha-SMA) which suggested that MeAIB also inhibited the activation of HSCs. Because HSC activation is inseparable from proliferation, the purpose of this study was to examine the effect of MeAIB treatment on the proliferation of HSCs in an in vitro model utilizing CFSC-2G cell cultures. In these studies, MeAIB effectively inhibited the proliferation of CFSC-2G cells by interfering with the progression of the cells through the G(1)-phase of the cell cycle which delayed entry into S-phase. MeAIB prevented the phosphorylation of p70S6 kinase (p70S6K) at Thr389 and reduced the phosphorylation at Thr421/Ser424. Because p70S6K is required for G(1)-cell cycle progression and is known to be regulated by nutrient availability, this correlates well with MeAIB interfering with the proliferation of CFSC-2G HSCs. In addition, the rate of protein synthesis was reduced by MeAIB treatment following mitogenic stimulation, which agrees with a p70S6K-mediated reduction in translation. These data are consistent with MeAIB inhibiting the proliferation of CFSC-2G cells by altering the mitogen activated pathway(s) leading to phosphorylation of p70S6K by a yet to be described mechanism.

Electron microscopic and biochemical study of the effects of rapamycin on glycogen autophagy in the newborn rat liver

The effects of rapamycin on glycogen autophagy in the newborn rat liver were studied using biochemical determinations, electron microscopy, and morphometric analysis. Rapamycin increased the fractional volume of hepatocytic autophagic vacuoles, the liver lysosomal glycogen-hydrolyzing activity of acid glucosidase, the degradation of glycogen inside the autophagic vacuoles, and decreased the activity of acid mannose 6-phosphatase. These findings suggest that rapamycin, a known inhibitor of the mammalian target of rapamycin (mTOR) signaling, induces glycogen autophagy in the newborn rat hepatocytes. mTOR may participate in the regulation of this process.

Mammalian target of rapamycin regulates IRS-1 serine 307 phosphorylation

Insulin signaling can be negatively regulated by phosphorylation of serine 307 of the insulin receptor substrate (IRS)-1. Rapamycin, an inhibitor of the kinase mTOR, can prevent serine 307 phosphorylation and the development of insulin resistance. We further investigated the role of mTOR in regulating serine 307 phosphorylation, demonstrating that serine 307 phosphorylation in response to insulin, anisomycin, or tumor necrosis factor was quantitatively and temporally associated with activation of mTOR and could be inhibited by rapamycin. Amino acid stimulation activated mTOR and resulted in IRS-1 serine 307 phosphorylation without activating PKB or JNK. Okadaic acid, an inhibitor of the phosphatase PP2A, activated mTOR and stimulated the phosphorylation of serine 307 in a rapamycin-sensitive manner, indicating serine 307 phosphorylation requires mTOR activity but not PP2A, suggesting that mTOR itself may be responsible for phosphorylating serine 307. Finally, we demonstrated that serine 307 phosphorylated IRS-1 is detected primarily in the cytosolic fraction.

Regulation of global and specific mRNA translation by oral administration of branched-chain amino acids

The importance of branched-chain amino acids as nutrient regulators of protein synthesis in skeletal muscle was recognized more than 20 years ago. Of the branched-chain amino acids, leucine in particular was shown to play a central role in promoting muscle protein synthesis. However, it was only recently that the mechanism(s) involved in the stimulation of protein synthesis by leucine has begun to be defined. Studies performed in our laboratory during the past few years have revealed that oral administration of leucine to fasted rats enhances protein synthesis in association with increased phosphorylation of two proteins downstream of the protein kinase referred to as the mammalian target of rapamycin (mTOR). These proteins, eukaryotic initiation factor eIF4E binding protein (4E-BP)1 and ribosomal protein S6 kinase S6K1, control in part the step in translation initiation involving the binding of mRNA to the 40S ribosomal subunit. In theory the translation of all mRNAs can be regulated through such mechanisms, however, some mRNAs are more sensitive to the changes than others, resulting in modulation of gene expression through altered patterns of translation of specific mRNAs. Moreover, although a basal amount of plasma insulin is required for leucine to enhance signaling downstream of mTOR, the concentration observed in plasma of fasted rats is sufficient to observe maximal changes in phosphorylation of 4E-BP1 and S6K1.

Role of mTOR Signalling in the Control of Translation Initiation and Elongation by Nutrients

Protein synthesis requires nutrients both as precursors (amino acids) and as a source of energy, since this process consumes a high proportion of cellular metabolic energy. Recent work has shown that both types of nutrients directly influence the activities of components of the translational machinery in mammalian cells. Amino acids positively regulate signalling through the mammalian target of the rapamycin (mTOR) pathway, although the degree of dependency on external amino acids varies between cell types. mTOR signalling modulates several key components involved in mRNA translation, in particular (via repressor proteins) the cap-binding initiation factor eIF4E, the ribosomal protein S6 kinases, and elongation factor eEF2. The branched-chain amino acid leucine is the most effective one in most cell types. It is currently unclear how mammalian cells sense prevailing amino acid levels, although this may involve intracellular amino acids. Cellular ATP levels can also influence mTOR activity. The activities of some translation factors are modulated by mTOR-independent mechanisms. Examples include the regulation of eEF2 by cellular energy levels, which may be controlled via the AMP-activated protein kinase, and the activity of the guanine nucleotide-exchange factor eIF2B, which is modulated by amino acids and metabolic fuels.

Regulation of protein synthesis by branched-chain amino acids in vivo

Recent advances in the understanding of mRNA translation have facilitated molecular studies on the regulation of protein synthesis by nutrients and the interplay between nutrients and hormonal signals. Numerous reports have established that, in skeletal muscle, the branched-chain amino acids (BCAAs) have the unique ability to initiate signal transduction pathways that modulate translation initiation. Of the BCAAs, leucine is the most potent. Oral administration of leucine to food-deprived rats enhances muscle protein synthesis, in part, through activation of the mRNA binding step of translation initiation. Interestingly, leucine signaling in skeletal muscle differs from that in liver, suggesting that the responses may be tissue specific. The purpose of this paper was to briefly review the current knowledge of how BCAAs act as regulators of protein synthesis in physiologically important tissues, with particular focus on the mechanisms by which BCAAs regulate translation initiation.

Individuality: the barrier to optimal immunosuppression

Immunosuppressive therapy aims to protect transplanted organs from host responses. Individuals have unique repertoires of responses to foreign antigens and toxic reactions to immunosuppressants; the former determining the type or intensity of rejection reactions and the latter influencing the severity of iatrogenic effects. Because existing agents target molecules that are widely distributed in tissues, new strategies must selectively block lymphoid cells only, disrupt alloresponses but not innate immune responses, interact synergistically with other agents, facilitate the homeostatic process that naturally leads to graft acceptance and ideally only interrupt donor-specific responses. Approaches presently under investigation aim to alter cell trafficking, or selectively deviate the maturation of antigen-presenting cells or inhibit lymphocyte-activation cascades - events that are crucial to rejection responses.

Potential role of leucine metabolism in the leucine-signaling pathway involving mTOR

Leucine has been shown to stimulate adipose tissue protein synthesis in vivo as well as leptin secretion, protein synthesis, hyper-plastic growth, and tissue morphogenesis in in vitro experiments using freshly isolated adipocytes. Recently, others have proposed that leucine oxidation in the mitochondria may be required to activate the mammalian target of rapamycin (mTOR), the cytosolic Ser/Thr protein kinase that appears to mediate some of these effects. The first irreversible and rate-limiting step in leucine oxidation is catalyzed by the branched-chain alpha-keto acid dehydrogenase (BCKD) complex. The activity of this complex is regulated acutely by phosphorylation of the E1alpha-subunit at Ser293 (S293), which inactivates the complex. Because the alpha-keto acid of leucine regulates the activity of BCKD kinase, it has been suggested as a potential target for leucine regulation of mTOR. To study the regulation of BCKD phosphorylation and its potential link to mTOR activation, a phosphopeptide-specific antibody recognizing this site was developed and characterized. Phospho-S293 (pS293) immunoreactivity in liver corresponded closely to diet-induced changes in BCKD activity state. Immunoreactivity was also increased in TREMK-4 cells after the induction of BCKD kinase by a drug-inducible promoter. BCKD S293 phosphorylations in adipose tissue and gastrocnemius (which is mostly inactive in vivo) were similar. This suggests that BCKD complex in epididymal adipose tissue from food-deprived rats is mostly inactive (unable to oxidize leucine), as is the case in muscle. To begin to test the leucine oxidation hypothesis of mTOR activation, the dose-dependent effects of orally administered leucine on acute activation of S6K1 (an mTOR substrate) and BCKD were compared using the pS293 antibodies. Increasing doses of leucine directly correlated with increases in plasma leucine concentration. Phosphorylation of S6K1 (Thr389, the phosphorylation site leading to activation) in adipose tissue was maximal at a dose of leucine that increased plasma leucine approximately threefold. Changes in BCKD phosphorylation state required higher plasma leucine concentrations. The results seem more consistent with a role for BCKD and BCKD kinase in the activation of leucine metabolism/oxidation than in the activation of the leucine signal to mTOR.

Regulation of targets of mTOR (mammalian target of rapamycin) signalling by intracellular amino acid availability

In mammalian cells, amino acids affect the phosphorylation state and function of several proteins involved in mRNA translation that are regulated via the rapamycin-sensitive mTOR (mammalian target of rapamycin) pathway. These include ribosomal protein S6 kinase, S6K1, and eukaryotic initiation factor 4E-binding protein, 4E-BP1. Amino acids, especially branched-chain amino acids, such as leucine, promote phosphorylation of 4E-BP1 and S6K1, and permit insulin to further increase their phosphorylation. However, it is not clear whether these effects are exerted by extracellular or intracellular amino acids. Inhibition of protein synthesis is expected to increase the intracellular level of amino acids, whereas inhibiting proteolysis has the opposite effect. We show in the present study that inhibition of protein synthesis by any of several protein synthesis inhibitors tested allows insulin to regulate 4E-BP1 or S6K1 in amino-acid-deprived cells, as does the addition of amino acids to the medium. In particular, insulin activates S6K1 and promotes initiation factor complex assembly in amino-acid-deprived cells treated with protein synthesis inhibitors, but cannot do so in the absence of these compounds. Their effects occur at concentrations commensurate with their inhibition of protein synthesis and are not due to activation of stress-activated kinase cascades. Inhibition of protein breakdown (autophagy) impairs the ability of insulin to regulate 4E-BP1 or S6K1 under such conditions. These and other data presented in the current study are consistent with the idea that it is intracellular amino acid levels that regulate mTOR signalling.

Regulation of mammalian translation factors by nutrients

Protein synthesis requires both amino acids, as precursors, and a substantial amount of metabolic energy. It is well established that starvation or lack of nutrients impairs protein synthesis in mammalian cells and tissues. Branched chain amino acids are particularly effective in promoting protein synthesis. Recent work has revealed important new information about the mechanisms involved in these effects. A number of components of the translational machinery are regulated through signalling events that require the mammalian target of rapamycin, mTOR. These include translational repressor proteins (eukaryotic initiation factor 4E-binding proteins, 4E-BPs) and protein kinases that act upon the small ribosomal subunit (S6 kinases). Amino acids, especially leucine, positively regulate mTOR signalling thereby relieving inhibition of translation by 4E-BPs and activating the S6 kinases, which can also regulate translation elongation. However, the molecular mechanisms by which amino acids modulate mTOR signalling remain unclear. Protein synthesis requires a high proportion of the cell's metabolic energy, and recent work has revealed that metabolic energy, or fuels such as glucose, also regulate targets of the mTOR pathway. Amino acids and glucose modulate a further important regulatory step in translation initiation, the activity of the guanine nucleotide-exchange factor eIF2B. eIF2B controls the recruitment of the initiator methionyl-tRNA to the ribosome and is activated by insulin. However, in the absence of glucose or amino acids, insulin no longer activates eIF2B. Since control of eIF2B is independent of mTOR, these data indicate the operation of additional, and so far unknown, regulatory mechanisms that control eIF2B activity.

Metabolic and autocrine regulation of the mammalian target of rapamycin by pancreatic beta-cells

Mammalian target of rapamycin (mTOR) is a serine and threonine protein kinase that regulates numerous cellular functions, in particular, the initiation of protein translation. mTOR-mediated phosphorylation of both the translational repressor eukaryotic initiation factor 4E binding protein-1 and p70 S6 kinase are early events that control the translation initiation process. Rapamycin, an inhibitor of mTOR, is a potent immunosuppressant due, in part, to its ability to interfere with T-cell activation at the level of translation, and it has gained a prominent role in preventing the development and progression of rejection in pancreatic islet transplant recipients. The characterization of the insulin signaling cascade that modulates mTOR in insulin-sensitive tissues has been a major focus of investigation. Recently, the ability of nutrients, in particular the branched-chain amino acid leucine, to activate mTOR independent of insulin by a process designated as nutrient signaling has been identified. The beta-cell expresses components of the insulin signaling cascade and utilizes the metabolism of nutrients to affect insulin secretion. These combined transduction processes make the beta-cell an unique cell to study metabolic and autocrine regulation of mTOR signaling. Our studies have described the ability of insulin and IGFs in concert with the nutrients leucine, glutamine, and glucose to modulate protein translation through mTOR in beta-cells. These findings suggest that mitochondria-derived factors, ATP in particular, may be responsible for nutrient signaling. The significance of these findings is that the optimization of mitochondrial function is not only important for insulin secretion but may significantly impact the growth and proliferation of beta-cells through these mTOR signaling pathways.

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.

Invited Review: Role of insulin in translational control of protein synthesis in skeletal muscle by amino acids or exercise

Protein synthesis in skeletal muscle is modulated in response to a variety of stimuli. Two stimuli receiving a great deal of recent attention are increased amino acid availability and exercise. Both of these effectors stimulate protein synthesis in part through activation of translation initiation. However, the full response of translation initiation and protein synthesis to either effector is not observed in the absence of a minimal concentration of insulin. The combination of insulin and either increased amino acid availability or endurance exercise stimulates translation initiation and protein synthesis in part through activation of the ribosomal protein S6 protein kinase S6K1 as well as through enhanced association of eukaryotic initiation factor eIF4G with eIF4E, an event that promotes binding of mRNA to the ribosome. In contrast, insulin in combination with resistance exercise stimulates translation initiation and protein synthesis through enhanced activity of a guanine nucleotide exchange protein referred to as eIF2B. In both cases, the amount of insulin required for the effects is low, and a concentration of the hormone that approximates that observed in fasting animals is sufficient for maximal stimulation. This review summarizes the results of a number of recent studies that have helped to establish our present understanding of the interactions of insulin, amino acids, and exercise in the regulation of protein synthesis in skeletal muscle.

p53 activation results in rapid dephosphorylation of the eIF4E-binding protein 4E-BP1, inhibition of ribosomal protein S6 kinase and inhibition of translation initiation

p53 is an important regulator of cell cycle progression and apoptosis, and inactivation of p53 is associated with tumorigenesis. Although p53 exerts many of its effects through regulation of transcription, this protein is also found in association with ribosomes and several mRNAs have been identified that are translationally controlled in a p53-dependent manner. We have utilized murine erythroleukemic cells that express a temperature-sensitive p53 protein to determine whether p53 also functions at the level of translation. The data presented here demonstrate that p53 causes a rapid decrease in translation initiation. Analysis of several potential mechanisms for regulating protein synthesis shows that p53 has selective effects on the phosphorylation of the eIF4E-binding protein, 4E-BP1, and the activity of the p70 ribosomal protein S6 kinase. These data provide evidence that modulation of translational activity constitutes a further mechanism by which the growth inhibitory effects of p53 may be mediated.