Recent advances on molecular mechanisms involved in amino acid control of gene expression

Increased protein intake affects pro-opiomelanocortin (POMC) processing, immune function and IGF signaling in peripheral blood mononuclear cells of home-dwelling old subjects using a genome-wide gene expression approach

Background

Adequate protein intake among older adults is associated with better health outcomes such as immune function and metabolic regulation of skeletal muscle, but conflicting results make it difficult to define the optimal intake. To further understand the impact of protein intake on metabolic processes, the aim of the study was to explore genome-wide gene expression changes in peripheral blood mononuclear cells (PBMCs) in home-dwelling old subjects after increased protein intake for 12 weeks.

Method

In a parallel double-blind randomized controlled intervention study, subjects (≥ 70 years) received a protein-enriched milk (2 × 20 g protein/day, n = 14, mean (±SD) age 76.9 ± 4.9 years) or an isocaloric carbohydrate drink (n = 17, mean (±SD) age 77.7 ± 4.8 years) for breakfast and evening meal for 12 weeks. PBMCs were isolated before and after the intervention. Microarray analysis was performed using Illumina technology. Serum levels of gut peptides and insulin growth factor (IGF)-1 were also measured.

Results

In total 758 gene transcripts were regulated after increased protein intake, and 649 gene transcripts were regulated after intake of carbohydrates (p < 0.05). Forty-two of these genes were overlapping. After adjusting for multiple testing, 27 of the 758 gene transcripts were regulated (FDR, q-value < 0.25) after protein intake. Of these 25 were upregulated and two downregulated. In particular, genes and signaling pathways involved in pro-opiomelanocortin (POMC) processing, immune function, and IGF signaling were significantly altered.

Conclusions

PBMCs can be used to study gene expression changes after long-term protein intake, as many signaling pathways were regulated after increased protein intake. The functional significance of these findings needs to be further investigated.

Trial registration

ClinicalTrials.gov, ID no. NCT02218333. The study was registered on August 18, 2014.

Insulin and amino acids both strongly participate to the regulation of protein metabolism

PURPOSE OF REVIEW

The application of tracer kinetic methods, combined with measurements of the activity of components of the cellular signaling pathways involved in protein synthesis and degradation, affords new insights into the regulation of skeletal muscle protein metabolism in vivo in humans. Feeding is associated with an increase in protein synthesis and a decrease in proteolysis. These changes are mediated by feeding-induced increases in plasma concentrations of both nutrients and hormones.

RECENT FINDINGS

Recent studies definitely demonstrated that insulin and amino acids directly interacted in promoting postprandial anabolism. However, the contribution of amino acids was abolished in old individuals in whom only insulin action persisted. There was a line of evidence that the effect of amino acids originates from leucine, which should not be viewed simply as a building block for protein synthesis, but as a signal in the regulation of cell functions. Although their cellular signaling pathways do not completely overlap, insulin and amino acids both activate the translation initiation of protein synthesis. Insulin presumably inhibits skeletal muscle protein degradation through a decrease in the activity of the ubiquitin proteasome-dependent pathway.

SUMMARY

Whether or not amino acids modify insulin action and have specific effects on proteolysis has not yet been documented. At the molecular level, amino acids such as insulin modulate gene expression. Such studies are needed to gain a better understanding of the interactions between insulin and amino acids in the regulation of skeletal muscle protein anabolism.

Arginine increases growth hormone gene expression in rat pituitary and GH3 cells

The effect of arginine (Arg) and Ornitargin (OT) [a compound containing the aminoacids Arg, citrulline (Cit) and ornithine (Orn)] administration upon growth hormone (GH) gene expression was studied both in vivo and in vitro (hemipituitaries and GH3 cells) by Northern blot analysis. For in vivo studies, adult male Wistar rats were anesthetized, subjected to i.v. infusion of 200 microl of 150 mM NaCl (control group), Arg (15 or 150 mg) or OT (15 mg of Arg, 1 mg of Cit and 4 mg of Orn) at a rate of 20 microl/min, and killed 50 min thereafter. For the in vitro studies, hemipituitaries or GH3 cells were incubated in 1 ml of appropriate medium containing Arg (15 or 150 mg) or OT (15 mg of Arg, 1 mg of Cit and 4 mg of Orn) for 60 min. The pituitaries of the in vivo and in vitro studies and GH3 cells were subsequently processed for RNA extraction. Total RNA was subjected to electrophoresis in agarose (1%)/formaldehyde gel, transferred to a nylon membrane and subjected to hybridization with a rat GH (32)P-cDNA, and (32)P-18S rRNA probe to correct for the variability in RNA loading. After autoradiography of the membrane, the abundance of GH mRNA and 18S rRNA bands was quantified by densitometry. The in vivo study demonstrated that Arg and OT infusion induced a 2.3-fold increase in GH mRNA expression, which could result from the Arg-mediated inhibition of somatostatin release. In addition, in vitro Arg, but not OT, induced GH gene expression in hemipituitaries and GH3 cells, indicating that the aminoacid can act per se at the pituitary somatotrope level. In conclusion, our data show for the first time that arginine stimulates GH gene expression in parallel to its recognized GH-releasing activity.

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

In mammals, the impact of nutrients on gene expression has become an important area of research. Because amino acids have multiple and important functions, their homeostasis has to be finely maintained. However, amino acidemia can be affected by certain nutritional conditions or various forms of stress. Consequently, mammals must adjust several of the physiological functions involved in the adaptation to amino acid availability by regulating expression of numerous genes. It has been shown that amino acids alone can modify the expression of target genes. However, understanding of amino acid-dependent control of gene expression has just started to emerge. This review focuses on recent advances in the understanding of mechanisms involved in the amino acid control of gene expression.

Nutritional control of mRNA stability is mediated by a conserved AU-rich element that binds the cytoplasmic shuttling protein HuR

The cationic amino acid transporter, Cat-1, is a high affinity transporter of the essential amino acids, arginine and lysine. Expression of the cat-1 gene increases during nutritional stress as part of the adaptive response to starvation. Amino acid limitation induces coordinate increases in stability and translation of the cat-1 mRNA, at a time when global protein synthesis decreases. It is shown here that increased cat-1 mRNA stability requires an 11 nucleotide AU-rich element within the distal 217 bases of the 3'-untranslated region. When this 217-nucleotide nutrient sensor AU-rich element (NS-ARE) is present in a chimeric mRNA it confers mRNA stabilization during amino acid starvation. HuR is a member of the ELAV family of RNA-binding proteins that has been implicated in regulating the stability of ARE-containing mRNAs. We show here that the cytoplasmic concentration of HuR increases during amino acid starvation, at a time when total cellular HuR levels decrease. In addition, RNA gel shift experiments in vitro demonstrated that HuR binds to the NS-ARE and binding was dependent on the 11 residue AU-rich element. Moreover, HuR binding to the NS-ARE in extracts from amino acid-starved cells increased in parallel with the accumulation of cytoplasmic HuR. It is proposed that an adaptive response of cells to nutritional stress results in increased mRNA stability mediated by HuR binding to the NS-ARE.

Metabolic bases of amino acid requirements in acute diseases

Acute diseases are characterized by a catabolic state, resulting in a negative nitrogen balance and muscle wasting. Increasing protein intake often proves to have little effect in limiting muscle protein loss. This suggests a qualitative inadequacy of the usual nutritional supports to meet the amino acid requirements of the critically ill patient. Therefore, it can be assumed that the additional intake of limiting amino acids would allow the sparing of muscle proteins. The aim of this review is to examine whether metabolic and kinetics studies using labelled amino acids can help identify the pathways activated in injury and their specific amino acid requirements. The kinetics of cysteine, arginine and glutamine, which are mainly cited as conditionally indispensable in stress situations, are presented. Moreover, amino acids can act as mediators or signal molecules and modulate numerous functions. The optimal conditions allowing the best expression of these activities are discussed.