Recent advances in the understanding of amino acid regulation of gene expression

Acute Effects of Dietary Protein Consumption on the Postprandial Metabolic Response, Amino Acid Levels and Circulating MicroRNAs in Patients with Obesity and Insulin Resistance

The post-nutritional intervention modulation of miRNA expression has been previously investigated; however, post-acute dietary-ingestion-related miRNA expression dynamics in individuals with obesity and insulin resistance (IR) are unknown. We aimed to determine the acute effects of protein ingestion from different dietary sources on the postprandial metabolic response, amino acid levels, and circulating miRNA expression in adults with obesity and IR. This clinical trial included adults with obesity and IR who consumed (1) animal-source protein (AP; calcium caseinate) or (2) vegetable-source protein (VP; soy protein isolate). Glycaemic, insulinaemic, and glucagon responses, amino acid levels, and exosomal microRNAs isolated from plasma were analysed. Post-AP ingestion, the area under the curve (AUC) of insulin (p = 0.04) and the plasma concentrations of branched-chain (p = 0.007) and gluconeogenic (p = 0.01) amino acids increased. The effects of different types of proteins on the concentration of miRNAs were evaluated by measuring their plasma circulating levels. Compared with the baseline, the AP group presented increased circulating levels of miR-27a-3p, miR-29b-3p, and miR-122-5p (p < 0.05). Subsequent analysis over time at 0, 30, and 60 min revealed the same pattern and differences between treatments. We demonstrated that a single dose of dietary protein has acute effects on hormonal and metabolic regulation and increases exosomal miRNA expression in individuals with obesity and IR.

Relationship between the amino acid release kinetics of feed proteins and nitrogen balance in finishing pigs

In current nutrition requirements of swine, although the protein diets are formulated based on the ileal digestibility of protein and amino acid (AA), there is a difference in nitrogen utilisation among various protein diets, which might be related to the AA release kinetics. To evaluate the relationship between AA release kinetics of feed proteins and nitrogen balance in finishing pigs, pigs were fed diets based on casein (CAS) or corn gluten meal (CGM) at normal or low-protein concentrations, and the AA release patterns were assessed. A 2 × 2 full factorial experimental design was used. 24 pigs (Duroc × Landrace × Yorkshire) with an initial weight of 67.0 ± 1.8 kg were randomly assigned to consume a normal-protein casein-based diet (N.CAS, 10% CP), normal-protein corn gluten meal-based diet (N.CGM, 10% CP), low-protein casein-based diet (L.CAS, 8.5% CP), or low-protein corn gluten meal-based diet (L.CGM, 8.5% CP) for 14 days (n = 6 per group; pigs housed and fed separately). The low-protein diets were associated with a more rapid release of AAs in the early stages of gastric digestion than the normal-protein diets. The N.CAS and L.CAS diets were associated with a peak AA release at approximately 4 h during trypsin digestion, whereas N.CGM and L.CGM were at approximately 16 h. The N.CAS diet was associated with the least dispersed release curves and lowest synchronisation indexes, implying that it was associated with the best AA release synchronism, whereas the L.CGM diet was on the contrary. The nitrogen intake (NI), faecal nitrogen, urine nitrogen (UN), total nitrogen, net protein utilisation and apparent biological value (ABV) of protein of pigs fed the L.CAS or L.CGM diets were lower than those fed the N.CAS or N.CGM diets (P < 0.05). Notably, there was a difference in NI (P < 0.05) and trends with respect to UN and ABV (0.05 < P < 0.1), but no differences in retained nitrogen or apparent nitrogen digestibility between pigs fed the N.CAS or L.CAS diets and those fed the N.CGM or L.CGM diets. Pigs fed the N.CAS or N.CGM diets had higher serum concentrations of UN than pigs fed the L.CAS or L.CGM diets (P < 0.05), but there were no differences in serum total protein, albumin, triglyceride, glucose, alanine transaminase, or aspartate aminotransferase between the groups. In addition, there was an interaction between protein level and protein source on serum globulin (P < 0.05). Therefore, the diet with a better AA release synchronism can improve protein utilisation efficiency in finishing pigs and to reduce environmental pollution.

New Insight into Diabetes Management: From Glycemic Index to Dietary Insulin Index

BACKGROUND

Despite efforts to control hyperglycemia, diabetes management is still challenging. This may be due to focusing on reducing hyperglycemia and neglecting the importance of hyperinsulinemia; while insulin resistance and resultant hyperinsulinemia preceded diabetes onset and may contribute to disease pathogenesis.

OBJECTIVE

The present narrative review attempts to provide a new insight into the management of diabetes by exploring different aspects of glycemic index and dietary insulin index.

RESULTS

The current data available on this topic is limited and heterogeneous. Conventional diet therapy for diabetes management is based on reducing postprandial glycemia through carbohydrate counting, choosing foods with low-glycemic index and low-glycemic load. Since these indicators are only reliant on the carbohydrate content of foods and do not consider the effects of protein and fat on the stimulation of insulin secretion, they cannot provide a comprehensive approach to determine the insulin requirements.

CONCLUSION

Selecting foods based on carbohydrate counting, glycemic index or glycemic load are common guides to control glycemia in diabetic patients, but neglect the insulin response, thus leading to failure in diabetes management. Therefore, paying attention to insulinemic response along with glycemic response seems to be more effective in managing diabetes.

Analysis of amino acids in human tears by hydrophilic interaction liquid chromatography and quadrupole orbitrap mass spectrometry

Amino acids in human tears play certain physiological roles and their determination is challenging due to complicated chemical properties.

Synthesis and biological properties of radiohalogenated α,α-disubstituted amino acids for PET and SPECT imaging of amino acid transporters (AATs)

Fluorine-18 and iodine-123 labeled nonnatural alicyclic and methyl branched disubstituted α,α-amino acids are a diverse and useful class of tumor imaging agents suitable for positron emission tomography and single photon emission computed tomography. These tracers target the increased expression of the cell membrane amino acid transporter systems L, ASC, and A exhibited by many human tumor cells. The most established clinical use for these radiolabeled amino acids is imaging primary and recurrent gliomas and primary, recurrent, and metastatic prostate cancer. This review focuses on the synthesis, radiolabeling, and amino acid transport mechanism of a series of nonnatural fluorine-18 and iodine-123 labeled analogs of 1-aminocyclobutane-1-carboxylic acid, 1-aminocyclopentane-1-carboxylic acid, α-aminoisobutyric acid, and α-methylaminoisobutyric acid.

Expressão gênica de IGF-I e GHR no fígado e no músculo do peito de codornas de corte suplementadas com diferentes níveis de metionina em duas gerações sucessivas

RESUMO Este estudo foi desenvolvido com o objetivo de avaliar a expressão gênica do fator de crescimento semelhante à insulina I (IGF-I) e do receptor do hormônio do crescimento (GHR) no fígado e no músculo do peito de codornas de corte, alimentadas com dietas contendo diferentes níveis de suplementação de metionina, em duas gerações sucessivas. Foram utilizadas codornas dos 22 aos 42 dias de idade, distribuídas em três e cinco tratamentos na primeira e na segunda geração, respectivamente. Ao final, as aves foram abatidas por deslocamento cervical, sendo coletados fígado e músculo do peito para extração de RNA total. O cDNA foi amplificado usando primers específicos para os genes analisados. Os resultados mostraram que a expressão dos genes GHR e IGF-I sofreu influência da suplementação. No quinto tratamento, em que apenas a primeira geração recebeu uma suplementação acima do padrão das exigências para o período, houve uma expressão significativamente maior do GHR tanto no músculo do peito como no fígado e igualmente do IGF-I no músculo, levando a concluir que o excesso de metionina na dieta torna-se tóxica para as aves. Apesar de a expressão dos genes ter sofrido influência da adição de metionina nos níveis estudados, não foi observada diferença no consumo alimentar, na conversão alimentar e no peso das aves.

Algorithms to Improve the Prediction of Postprandial Insulinaemia in Response to Common Foods

Dietary patterns that induce excessive insulin secretion may contribute to worsening insulin resistance and beta-cell dysfunction. Our aim was to generate mathematical algorithms to improve the prediction of postprandial glycaemia and insulinaemia for foods of known nutrient composition, glycemic index (GI) and glycemic load (GL). We used an expanded database of food insulin index (FII) values generated by testing 1000 kJ portions of 147 common foods relative to a reference food in lean, young, healthy volunteers. Simple and multiple linear regression analyses were applied to validate previously generated equations for predicting insulinaemia, and develop improved predictive models. Large differences in insulinaemic responses within and between food groups were evident. GL, GI and available carbohydrate content were the strongest predictors of the FII, explaining 55%, 51% and 47% of variation respectively. Fat, protein and sugar were significant but relatively weak predictors, accounting for only 31%, 7% and 13% of the variation respectively. Nutritional composition alone explained only 50% of variability. The best algorithm included a measure of glycemic response, sugar and protein content and explained 78% of variation. Knowledge of the GI or glycaemic response to 1000 kJ portions together with nutrient composition therefore provides a good approximation for ranking of foods according to their “insulin demand”.

Sequential metabolic phases as a means to optimize cellular output in a constant environment

Temporal changes of gene expression are a well-known regulatory feature of all cells, which is commonly perceived as a strategy to adapt the proteome to varying external conditions. However, temporal (rhythmic and non-rhythmic) changes of gene expression are also observed under virtually constant external conditions. Here we hypothesize that such changes are a means to render the synthesis of the metabolic output more efficient than under conditions of constant gene activities. In order to substantiate this hypothesis, we used a flux-balance model of the cellular metabolism. The total time span spent on the production of a given set of target metabolites was split into a series of shorter time intervals (metabolic phases) during which only selected groups of metabolic genes are active. The related flux distributions were calculated under the constraint that genes can be either active or inactive whereby the amount of protein related to an active gene is only controlled by the number of active genes: the lower the number of active genes the more protein can be allocated to the enzymes carrying non-zero fluxes. This concept of a predominantly protein-limited efficiency of gene expression clearly differs from other concepts resting on the assumption of an optimal gene regulation capable of allocating to all enzymes and transporters just that fraction of protein necessary to prevent rate limitation. Applying this concept to a simplified metabolic network of the central carbon metabolism with glucose or lactate as alternative substrates, we demonstrate that switching between optimally chosen stationary flux modes comprising different sets of active genes allows producing a demanded amount of target metabolites in a significantly shorter time than by a single optimal flux mode at fixed gene activities. Our model-based findings suggest that temporal expression of metabolic genes can be advantageous even under conditions of constant external substrate supply.

Unaffected arm muscle hypercatabolism in dysphagic subacute stroke patients: the effects of essential amino acid supplementation

Alterations in muscle protein turnover of the unaffected side of stroke patients could contribute to physical disability. We investigated whether hypercatabolic activity occurred in unaffected arm muscle and whether supplemented essential amino acids (EAAs) could limit muscle hypercatabolism (MH). Thirty-eight dysphagic subacute stroke subjects (<3 months after acute event) (29 males+9 females; 69.7±11.4 yrs) were enrolled and randomized to receive 8 g/day EAAs (n=19; EAA group) or isocaloric placebo (maltodextrin; n=19, Plac group). Before randomization, all patients had their arterial (A) and venous (V) amino acids measured and muscle (A-V) differences calculated in the unaffected arm. Eight matched and healthy subjects served as controls. When compared to healthy controls, the entire stroke population showed significant muscle release (=negative value A-V) of the amino acid phenylalanine (phenyl-) indicating a prevalence of MH. Moreover, randomized EAA and Plac groups had similar rates of MH. After 38 days from the start of the protocol, the EAA group but not the Plac group had MH converted to balanced protein turnover or anabolic activity. We concluded that muscle protein metabolism of the unaffected arm of dysphagic subacute stroke individuals could be characterized by MH which can be corrected by supplemented EAAs.

Effects of individual branched-chain amino acids deprivation on insulin sensitivity and glucose metabolism in mice

OBJECTIVE

We recently discovered that leucine deprivation increases hepatic insulin sensitivity via general control nondepressible (GCN) 2/mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) pathways. The goal of the present study was to investigate whether the above effects were leucine specific or were also induced by deficiency of other branched chain amino acids including valine and isoleucine.

METHODS

Following depletion of BCAAs, changes in metabolic parameters and the expression of genes and proteins involved in regulation of insulin sensitivity and glucose metabolism were analyzed in mice and cell lines including human HepG2 cells, primary mouse hepatocytes and a mouse myoblast cell line C2C12.

RESULTS

Valine or isoleucine deprivation for 7 days has similar effect on improving insulin sensitivity as leucine, in wild type and insulin-resistant mice models. These effects are possibly mediated by decreased mTOR/S6K1 and increased AMPK signaling pathways, in a GCN2-dependent manner. Similar observations were obtained in in vitro studies. In contrast to leucine withdrawal, valine or isoleucine deprivation for 7 days significantly decreased fed blood glucose levels, possibly due to reduced expression of a key gluconeogenesis gene, glucose-6-phosphatase. Finally, insulin sensitivity was rapidly improved in mice 1 day following maintenance on a diet deficient for any individual BCAAs.

CONCLUSIONS

Our results show that while improvement on insulin sensitivity is a general feature of BCAAs depletion, individual BCAAs have specific effects on metabolic pathways, including those that regulate glucose level. These observations provide a conceptual framework for delineating the molecular mechanisms that underlie amino acid regulation of insulin sensitivity.

Ratio of lysine to methionine alters expression of genes involved in milk protein transcription and translation and mTOR phosphorylation in bovine mammary cells

This study was conducted to determine the optimum ratio of lysine and methionine (Lys:Met) to enhance milk protein concentration in vitro, focusing on the regulation of genes related to the JAK2-STAT5 and the mammalian target of rapamycin (mTOR) signaling pathways. A preliminary dose response study revealed that casein concentration peaked (2.5-2.7 ppm) at a supplemental Lys concentration of 1.2 mM and Met at 0.5 mM. At the peak casein concentration cell proliferation rate also was higher. Furthermore, the expression of CSN1S1, CSN1S2, CSN2, CSN3, LALBA, JAK2, STAT5, and MTOR was upregulated with both Lys and Met compared with the control. A subsequent experiment was conducted as a 5 × 3 factorial design with supplemental Lys plus Met at different ratios. When the supplemental concentration of Lys was 1.2 mM and Met was 0.4 mM (∼3:1), the concentration of casein peaked. Therefore, we measured gene expression, mTOR protein expression, and phosphorylated mTOR (p-mTOR) in cultures incubated with 3:1 Lys:Met (Lys&Met). Expression of CSN1S1 and LALBA were the most highly expressed genes (P < 0.01). The upregulation of CSN2, CSN3, CSN1S2 isoforms (P < 0.01) and JAK2, ELF5, mTOR (P < 0.05) was also observed. Total mTOR protein expression was greater (P < 0.05) with Lys alone and also Lys&Met. However, Lys&Met resulted in the greatest (P < 0.05) p-mTOR. Results suggest that peak concentration of casein at a supplemental 3:1 Lys:Met is driven in part via upregulation of the mRNA expression of components of the JAK-STAT and mTOR pathways.

Cellular and animal indispensable amino acid limitation responses and health promotion. Can the two be linked? A critical review

Cellular growth repression can mediate positive health outcomes by improving resistance while delaying the manifestation and decelerating the progression, of chronic diseases. Sensing systems that respond to amino acid limitation are, the general control non-derepressible kinase 2 (GCN2), the mammalian target of rapamycin (mTOR; namely mammalian target of rapamycin complex 1), the extracellular signal-regulated kinase (ERK)-mitogen-activated protein kinase pathway, the adenosine 5-mono-phosphate-activated protein kinase system. GCN2 particularly, under limiting essential amino acid conditions, activates the integrated stress response (ISR) causing selective up- /down-regulation of pro-survival/pro-apoptotic genes, respectively, rendering beneficial adaptation responses to amino acid limitation. This review attempts to bridge the link between molecular events and mechanisms observed at the cellular level with the potential health benefits possibly achieved at the whole organism level. The article describes mechanisms of essential amino acid sensing and provides a discussion on relevant research that suggests a potential role of essential amino acid sensing for promoting health.

Effect of excess levels of lysine and leucine in wheat-based, amino acid-fortified diets on the mRNA expression of two selected cationic amino acid transporters in pigs

An experiment was conducted to evaluate the effect of excess levels of Leu and Lys on the expression of b(0,+) and CAT-1 mRNA in jejunum, liver and the muscles Longissimus dorsi (LDM) and Semitendinosus (STM). Twenty pigs with an average initial BW of 16.4 ± 1.7 kg were used in a Randomized Complete Block. Dietary treatments (T) were as follows: T1, basal diet; T2, basal plus 3.5 g l-Lys/kg diet; T3, basal plus 1.5 g l-Leu/kg diet; T4, basal plus 3.5 g l-Lys plus 1.5 g l-Leu/kg diet. Diets in T1 and T3 met 100% the requirement of Lys for pigs within the 10 to 20 kg body weight range; diets in T2 and T4 contained 35% excess of Lys. Also, diets in T1 and T2 supplied 104%, whereas diets in T3 and T4 supplied 116% the requirement of Leu. The expression of b(0,+) in jejunum was reduced (p = 0.002) because of the supplementation of l-Leu, but l-Lys supplementation had no effect (p = 0.738). In contrast, the expression of b(0,+) in STM (p = 0.012) and liver (p = 0.095) was reduced by the high level of Lys, but Leu had no effect (p > 0.100). CAT-1 expression in STM increased by high Lys (p = 0.023) and Leu (p = 0.007) levels. In liver, the expression of CAT-1 substantially increased (p = 0.001) because of Lys. In conclusion, excess levels of dietary Lys and Leu affect the expression of cationic amino acid transporters, and this effect varies depending on the studied tissue.

Leucine deprivation increases hepatic insulin sensitivity via GCN2/mTOR/S6K1 and AMPK pathways

OBJECTIVE

We have previously shown that serum insulin levels decrease threefold and blood glucose levels remain normal in mice fed a leucine-deficient diet, suggesting increased insulin sensitivity. The goal of the current study is to investigate this possibility and elucidate the underlying cellular mechanisms.

RESEARCH DESIGN AND METHODS

Changes in metabolic parameters and expression of genes and proteins involved in regulation of insulin sensitivity were analyzed in mice, human HepG2 cells, and mouse primary hepatocytes under leucine deprivation.

RESULTS

We show that leucine deprivation improves hepatic insulin sensitivity by sequentially activating general control nonderepressible (GCN)2 and decreasing mammalian target of rapamycin/S6K1 signaling. In addition, we show that activation of AMP-activated protein kinase also contributes to leucine deprivation-increased hepatic insulin sensitivity. Finally, we show that leucine deprivation improves insulin sensitivity under insulin-resistant conditions.

CONCLUSIONS

This study describes mechanisms underlying increased hepatic insulin sensitivity under leucine deprivation. Furthermore, we demonstrate a novel function for GCN2 in the regulation of insulin sensitivity. These observations provide a rationale for short-term dietary restriction of leucine for the treatment of insulin resistance and associated metabolic diseases.

Amino acids from Mytilus galloprovincialis (L.) and Rapana venosa molluscs accelerate skin wounds healing via enhancement of dermal and epidermal neoformation

Wound healing consists of re-epithelialization, contraction and formation of granulation and scar tissue. Amino acids from proteins are involved in these events, but their exact roles are not well understood. The present study was undertaken to investigate the anti-inflammatory effects of some amino acids from two molluscs, Mytilus galloprovincialis (L.) (Mediterranean mussel) and Rapana venosa (hard shell-clam) employed in induced skin burn injuries in Wistar rats. The treatment was evaluated in terms of essential amino acids composition which rendered the extracts very efficient in healing skin burns. The healing process was examined by periodic acid Schiff's, Verhoeff's Van Gieson and immunohistochemistry stains for collagen IV, CD 34 and CD 117 antibodies. According to the obtained results, as expressed by histological studies, the most abundant blood vessels, collagen fibres, basal and stem cells were found only for treated animals with amino acids from Rapana venosa extracts. The rich composition of amino acids from the two molluscs merits consideration as therapeutic agents in the treatment of skin burns.

Resistance training increases muscle mitochondrial biogenesis in patients with chronic kidney disease

BACKGROUND AND OBJECTIVES

Muscle wasting, a common complication in chronic kidney disease (CKD), contributes to poor outcomes. Mitochondrial biogenesis is critical for the maintenance of skeletal muscle function and structural integrity. The present study--a secondary analysis from a published randomized controlled trial--examined the effect of resistance exercise training on skeletal muscle mitochondrial (mt)DNA copy number and determined its association with skeletal muscle phenotype (muscle mass and strength).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Twenty-three patients with moderate-to-severe CKD were randomized to resistance training (n = 13) or an attention-control (n = 10) group for 12 weeks. After a run-in period of a low-protein diet that continued during the intervention, mtDNA copy number in the vastus lateralis muscle was estimated by quantitative real-time PCR at baseline and 12 weeks.

RESULTS

Participants mean age was 64 +/- 10 (SD) years and median (interquartile range, IQR) GFR 27.5 (37.0) ml/min. There were no differences between groups at baseline. Median (IQR) mtDNA copy number was 13,713 (10,618). There was a significant increase in muscle mtDNA with exercise compared with controls (1306 [13306] versus -3747 [15467], P = 0.01). The change in muscle mtDNA copy number was positively correlated with previously reported changes in types I and II muscle fiber cross-sectional area.

CONCLUSIONS

In this pilot study, resistance training was highly effective in enhancing mitochondrial content in patients with moderate-to-severe CKD. This finding suggests that the mitochondrial dysfunction observed with chronic disease could potentially be restored with this exercise modality and should be investigated further.

Nutrient modulation of insulin secretion

The presence of different nutrients regulates the beta-cell response to secrete insulin to maintain glucose in the physiological range and appropriate levels of fuels in different organs and tissues. Glucose is the only nutrient secretagogue capable of promoting alone the release of insulin release. The mechanisms of Insulin secretion are dependent or independent of the closure of ATP-sensitive K(+) channels. In addition, insulin secretion in response to glucose and other nutrients is modulated by several hormones as incretins, glucagon, and leptin. Fatty acids (FAs), amino acids, and keto acids influence secretion as well. The exact mechanism for which nutrients induce insulin secretion is complicated because nutrient signaling shows one of the most complex transduction systems, which exists for the reason that nutrient have to be metabolized. FAs in the absence of glucose induce FA oxidation and insulin secretion in a lesser extent. However, FAs in the presence of glucose produce high concentration of malonyl-CoA that repress FA oxidation and increase the formation of LC-CoA amplifying the insulin release. Long-term exposure to fatty acids and glucose results in glucolipotoxicity and decreases in insulin release. The amino acid pattern produced after the consumption of a dietary protein regulates insulin secretion by generating anaplerotic substrates that stimulates ATP synthesis or by activating specific signal transduction mediated by mTOR, AMPK, and SIRT4 or modulating the expression of genes involved in insulin secretion. Finally, dietary bioactive compounds such as isoflavones play an important role in the regulation of insulin secretion.

Amino acid metabolism, insulin secretion and diabetes

In addition to the primary stimulus of glucose, specific amino acids may acutely and chronically regulate insulin secretion from pancreatic beta-cells in vivo and in vitro. Mitochondrial metabolism is crucial for the coupling of glucose, alanine, glutamine and glutamate recognition with exocytosis of insulin granules. This is illustrated by in vitro and in vivo observations discussed in the present review. Mitochondria generate ATP (the main coupling messenger in insulin secretion) and other factors that serve as sensors for the control of the exocytotic process. The main factors that mediate the key amplifying pathway over the Ca(2+) signal in nutrient-stimulated insulin secretion are nucleotides (ATP, GTP, cAMP and NADPH), although metabolites have also been proposed, such as long-chain acyl-CoA derivatives and glutamate. In addition, after chronic exposure, specific amino acids may influence gene expression in the beta-cell, which have an impact on insulin secretion and cellular integrity. Therefore amino acids may play a direct or indirect (via generation of putative messengers of mitochondrial origin) role in insulin secretion.

Mechanisms through which sulfur amino acids control protein metabolism and oxidative status

Amino acids regulate protein synthesis and breakdown (i.e., protein turnover) and consequently protein deposition, which corresponds to the balance between the two processes. Elucidating the mechanisms involved in such regulation is important from fundamental and applied points of view since it can provide a basis to optimize amino acid requirements and to control protein mass, body composition and so forth. Amino acids, which have long been considered simply as precursors of protein synthesis, are now recognized to exert other significant influences; that is, they are precursors of essential molecules, act as mediators or signal molecules and affect numerous functions. For example, amino acids act as mediators of metabolic pathways in the same manner as certain hormones. Thus, they modulate the activity of intracellular protein kinases involved in the regulation of metabolic pathways such as mRNA translation. We provide here an overview of the roles of amino acids as regulators of protein metabolism, by focusing particularly on sulfur amino acids. The potential importance of methionine as a "nutrient signal" is discussed in the light of recent findings. Emphasis is also placed on mechanisms controlling oxidative status since sulfur amino acids are involved in the synthesis of intracellular antioxidants (glutathione, taurine etc.) and in the methionine sulfoxide reductase antioxidant system.

The GCN2 eIF2alpha kinase regulates fatty-acid homeostasis in the liver during deprivation of an essential amino acid

Metabolic adaptation is required to cope with episodes of protein deprivation and malnutrition. GCN2 eIF2alpha kinase, a sensor of amino acid deficiency, plays a key role in yeast and mammals in modulating amino acid metabolism as part of adaptation to nutrient deprivation. The role of GCN2 in adaptation to long-term amino acid deprivation in mammals, however, is poorly understood. We found that expression of lipogenic genes and the activity of fatty acid synthase (FAS) in the liver are repressed and lipid stores in adipose tissue are mobilized in wild-type mice upon leucine deprivation. In contrast, GCN2-deficient mice developed liver steatosis and exhibited reduced lipid mobilization. Liver steatosis in Gcn2(-/-) mice was found to be caused by unrepressed expression of lipogenic genes, including Srebp-1c and Fas. Thus, our study identifies a novel function of GCN2 in regulating lipid metabolism during leucine deprivation in addition to regulating amino acid metabolism.

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.

Sulfur amino acid restriction induces the pi class of glutathione S-transferase expression in primary rat hepatocytes

The regulation of genes by amino acids is attracting increasing attention. In the present study, we investigated the restriction of expression of the pi class of glutathione S-transferase (GST Yp) by sulfur amino acids. Hepatocytes isolated from male Sprague-Dawley rats were cultured with L-15-based medium containing low (LSAA; 0.1 mmol/L L-methionine and 0.1 mmol/L L-cysteine) or high (HSAA; 0.5 mmol/L L-methionine and 0.2 mmol/L L-cysteine) amounts of sulfur amino acids for up to 6 d. Cellular protein contents did not differ between LSAA- and HSAA-treated cells over the entire period. In contrast, glutathione concentrations were suppressed by the LSAA medium and on d 6 were only 20% of those of HSAA-treated cells (P < 0.05). As shown by immunoblot analysis, GST Yp protein levels were greater in LSAA-treated cells than in HSAA-treated cells (P < 0.05). The induction of GST Yp by L-methionine and L-cysteine restriction was not affected by insulin and dexamethasone, but the latter suppressed GST Yp expression (P < 0.05). LSAA increased GST Yp mRNA levels and GST activity toward ethacrynic acid (P < 0.05). GST Yp induction occurred only in cells with a limited supply of L-methionine; restriction of L-isoleucine, L-leucine, L-lysine, and L-phenylalanine had no significant effect. In contrast with the induction of GST Yp, the expression of the GST isoforms Ya and Yb was not changed by amino acid restriction. In conclusion, hepatic GST Yp gene expression is upregulated by a limited availability of sulfur amino acids.

Higher urinary excretion of essential amino acids in preterm infants fed protein hydrolysates

AIM

Protein hydrolysates have been introduced in preterm formulae, but it is not clear whether they are needed for the feeding of preterm infants. We designed a randomized, controlled trial to test the effects of a preterm formula with hydrolysed cow's milk proteins on short-term growth and urinary and plasma amino acids levels.

METHODS

Infants with a birthweight < or = 1750 g and gestational age < or = 34 wk fed a conventional preterm infant formula (formula B) or a hydrolysed formula (formula A). Weight was measured daily; length, head circumference, mid-arm circumference and total skinfold thickness were measured weekly. Blood and urine were analysed for amino acid concentrations at start, 14 and 28 d.

RESULTS

Twenty-one infants met the criteria for randomization. The daily feeding volumes were: formula A 172.8 +/- 5.6 vs formula B 170.1 +/- 2.8 ml/kg/d. Infants fed with formula A showed slower weight gain (17.4 +/- 3.4 vs 20.5 +/- 3.3 g/kg/d; p = 0.045) and lower mean change in Z-scores for weight (-0.18 +/- 0.16 vs 0.00 +/- 0.09; p = 0.009) and for head circumference (-0.06 +/- 0.13 vs 0.06 +/- 0.13; p = 0.049). After 14 d, infants receiving formula A had statistically significant higher urinary levels of essential amino acids compared to infants receiving formula B.

CONCLUSION

Our results support the hypothesis of less nutritional value of hydrolysed versus conventional preterm formulae. Higher renal excretion of essential amino acids may be one of the mechanisms involved. These findings must be confirmed by further studies with larger sample sizes and protein hydrolysates with different degrees of hydrolysis.

Transcriptional regulation of the LAT-1/CD98 light chain

LAT-1/CD98 amino acid transporter expression and activity are induced in hepatic cells deprived of arginine. The promoter dependency of this regulation was investigated. LAT-1 expression, in contrast to that of CD98 heavy chain 4F2, was actinomycin D sensitive in cells cultured without arginine. Transient transfection analysis with promoter reporter constructs including the 2 kbp LAT-1 promoter or a sub-sequence containing multiple potential amino acid response elements failed to show significant amino acid sensitivity in various cell types. Chromatin-dependency did not appear to account for this result as hepatic cell clones stably transfected with the promoter constructs showed little or no arginine or leucine responsive promoter activity. These studies suggest that while amino acid sensitivity of LAT-1 expression is transcriptionally regulated, cis elements within the proximal promoter do not directly mediate this regulation. Understanding mechanisms by which this gene responds to amino acid availability will contribute to our knowledge of how eukaryotic cells sense and respond to their environment.

Evaluation of the effects of the dietary intake of proteins and amino acids by DNA microarray technology

The DNA microarray technique has been increasingly utilized in various fields of life sciences. It allows us to analyze the expression levels of thousands of genes simultaneously. The high productivity will facilitate the evaluation of changes in amino acid metabolism and their consequences in response to dietary proteins and amino acids. We compared the expression profiles by the GeneChip system in the liver and other tissues among three groups of rats fed with a 12% casein, a 12% gluten or a protein-free diet. Feeding the gluten or the protein-free diet up- or down-regulated a few hundred genes in the liver compared to the casein diet. Although some of the genes were already known to respond to changes in the protein nutritional state, the majority was newly identified responders. This paper also discusses the possibility of a use this technology for safety evaluation of excessive intake of dietary components, especially of amino acids.