Dietary Methionine Restriction Regulates Liver Protein Synthesis and Gene Expression Independently of Eukaryotic Initiation Factor 2 Phosphorylation in Mice

GCN2 drives diurnal patterns in the hepatic integrated stress response and maintains circadian rhythms in whole body metabolism during amino acid insufficiency

Disruptions in circadian rhythms are associated with an increased risk of developing metabolic diseases. General control nonderepressible 2 (GCN2), a primary sensor of amino acid insufficiency and activator of the integrated stress response (ISR), has emerged as a conserved regulator of the circadian clock in multiple organisms. The objective of this study was to examine diurnal patterns in hepatic ISR activation in the liver and whole body rhythms in metabolism. We hypothesized that GCN2 activation cues hepatic ISR signaling over a natural 24-h feeding-fasting cycle. To address our objective, wild-type (WT) and whole body Gcn2 knockout (GCN2 KO) mice were housed in metabolic cages and provided free access to either a control or leucine-devoid diet (LeuD) for 8 days in total darkness. On the last day, blood and livers were collected at CT3 (CT = circadian time) and CT15. In livers of WT mice, GCN2 phosphorylation followed a diurnal pattern that was guided by intracellular branched-chain amino acid concentrations (r2 = 0.93). Feeding LeuD to WT mice increased hepatic ISR activation at CT15 only. Diurnal oscillations in hepatic ISR signaling, the hepatic transcriptome including lipid metabolic genes, and triglyceride concentrations were substantially reduced or absent in GCN2 KO mice. Furthermore, mice lacking GCN2 were unable to maintain circadian rhythms in whole body energy expenditure, respiratory exchange ratio, and physical activity when fed LeuD. In conclusion, GCN2 activation functions to maintain diurnal ISR activation in the liver and has a vital role in the mechanisms by which nutrient stress affects whole body metabolism.NEW & NOTEWORTHY This work reveals that the eIF2 kinase GCN2 functions to support diurnal patterns in the hepatic integrated stress response during natural feeding and is necessary to maintain circadian rhythms in energy expenditure, respiratory exchange ratio, and physical activity during amino acid stress.

Computational modeling of the synergistic role of GCN2 and the HPA axis in regulating the integrated stress response in the central circadian timing system

The circadian timing system and integrated stress response (ISR) systems are fundamental regulatory mechanisms that maintain body homeostasis. The central circadian pacemaker in the suprachiasmatic nucleus (SCN) governs daily rhythms through interactions with peripheral oscillators via the hypothalamus-pituitary-adrenal (HPA) axis. On the other hand, ISR signaling is pivotal for preserving cellular homeostasis in response to physiological changes. Notably, disrupted circadian rhythms are observed in cases of impaired ISR signaling. In this work, we examine the potential interplay between the central circadian system and the ISR, mainly through the SCN and HPA axis. We introduce a semimechanistic mathematical model to delineate SCN's capacity for indirectly perceiving physiological stress through glucocorticoid-mediated feedback from the HPA axis and orchestrating a cellular response via the ISR mechanism. Key components of our investigation include evaluating general control nonderepressible 2 (GCN2) expression in the SCN, the effect of physiological stress stimuli on the HPA axis, and the interconnected feedback between the HPA and SCN. Simulation revealed a critical role for GCN2 in linking ISR with circadian rhythms. Experimental findings have demonstrated that a Gcn2 deletion in mice leads to rapid re-entrainment of the circadian clock following jetlag as well as to an elongation of the circadian period. These phenomena are well replicated by our model, which suggests that both the swift re-entrainment and prolonged period can be ascribed to a reduced robustness in neuronal oscillators. Our model also offers insights into phase shifts induced by acute physiological stress and the alignment/misalignment of physiological stress with external light-dark cues. Such understanding aids in strategizing responses to stressful events, such as nutritional status changes and jetlag.NEW & NOTEWORTHY This study is the first theoretical work to investigate the complex interaction between integrated stress response (ISR) sensing and central circadian rhythm regulation, encompassing the suprachiasmatic nucleus (SCN) and hypothalamus-pituitary-adrenal (HPA) axis. The findings carry implications for the development of dietary or pharmacological interventions aimed at facilitating recovery from stressful events, such as jetlag. Moreover, they provide promising prospects for potential therapeutic interventions that target circadian rhythm disruption and various stress-related disorders.

The role of lipoylation in mitochondrial adaptation to methionine restriction

Dietary methionine restriction (MR) is associated with a spectrum of health-promoting benefits. Being conducive to prevention of chronic diseases and extension of life span, MR can activate integrated responses at metabolic, transcriptional, and physiological levels. However, how the mitochondria of MR influence metabolic phenotypes remains elusive. Here, we provide a summary of cellular functions of methionine metabolism and an overview of the current understanding of effector mechanisms of MR, with a focus on the aspect of mitochondria-mediated responses. We propose that mitochondria can sense and respond to MR through a modulatory role of lipoylation, a mitochondrial protein modification sensitized by MR.

The integrated stress response in metabolic adaptation

The integrated stress response (ISR) refers to signaling pathways initiated by stress-activated eIF2α kinases. Distinct eIF2α kinases respond to different stress signals, including amino acid deprivation and mitochondrial stress. Such stress-induced eIF2α phosphorylation attenuates general mRNA translation and, at the same time, stimulates the preferential translation of specific downstream factors to orchestrate an adaptive gene expression program. In recent years, there have been significant new advances in our understanding of ISR during metabolic stress adaptation. Here, I discuss those advances, reviewing among others the ISR activation mechanisms in response to amino acid deprivation and mitochondrial stress. In addition, I review how ISR regulates the amino acid metabolic pathways and how changes in the ISR impact the physiology and pathology of various disease models.

Rescue of Methionine Dependence by Cobalamin in a Human Colorectal Cancer Cell Line

Methionine dependence is a characteristic of most cancer cells where they are unable to proliferate when the essential amino acid methionine is replaced with its precursor homocysteine in the growing media. Normal cells, on the other hand, thrive under these conditions and are referred to as methionine-independent. The reaction that adds a methyl group from 5-methyltetrahydrofolate to homocysteine to regenerate methionine is catalyzed by the enzyme methionine synthase with the cofactor cobalamin (vitamin B12). However, decades of research have shown that methionine dependence in cancer is not due to a defect in the activity of methionine synthase. Cobalamin metabolism has been tied to the dependent phenotype in rare cell lines. We have identified a human colorectal cancer cell line in which the cells regain the ability to proliferation in methionine-free, L-homocystine-supplemented media when cyanocobalamin is supplemented at a level of 1 µg/mL. In human SW48 cells, methionine replacement with L-homocystine does not induce any measurable increase in apoptosis or reactive oxygen species production in this cell line. Rather, proliferation is halted, then restored in the presence of cyanocobalamin. Our data show that supplementation with cyanocobalamin prevents the activation of the integrated stress response (ISR) in methionine-deprived media in this cell line. The ISR-associated cell cycle arrest, characteristic of methionine-dependence in cancer, is also prevented, leading to the continuation of proliferation in methionine-deprived SW48 cells with cobalamin. Our results highlight differences between cancer cell lines in the response to cobalamin supplementation in the context of methionine dependence.

The integrated stress response protects against ER stress but is not required for altered translation and lifespan from dietary restriction in Caenorhabditis elegans

The highly conserved integrated stress response (ISR) reduces and redirects mRNA translation in response to certain forms of stress and nutrient limitation. It is activated when kinases phosphorylate a key residue in the alpha subunit of eukaryotic translation initiation factor 2 (eIF2). General Control Nonderepressible-2 (GCN2) is activated to phosphorylate eIF2α by the presence of uncharged tRNA associated with nutrient scarcity, while protein kinase R-like ER kinase-1 (PERK) is activated during the ER unfolded protein response (UPRER). Here, we investigated the role of the ISR during nutrient limitation and ER stress with respect to changes in protein synthesis, translationally driven mRNA turnover, and survival in Caenorhabditis elegans. We found that, while GCN2 phosphorylates eIF2α when nutrients are restricted, the ability to phosphorylate eIF2α is not required for changes in translation, nonsense-mediated decay, or lifespan associated with dietary restriction (DR). Interestingly, loss of both GCN2 and PERK abolishes increased lifespan associated with dietary restriction, indicating the possibility of other substrates for these kinases. The ISR was not dispensable under ER stress conditions, as demonstrated by the requirement for PERK and eIF2α phosphorylation for decreased translation and wild type-like survival. Taken together, results indicate that the ISR is critical for ER stress and that other translation regulatory mechanisms are sufficient for increased lifespan under dietary restriction.

Physical activity of mice on dietary sulfur amino acid restriction is influenced by age of diet initiation and biological sex

Sulfur amino acid restriction (SAAR)-the reduction of methionine and cysteine concentrations either in the diet or by genetic manipulation-promotes health span and extends lifespan, but its effects on physical activity remain unclear. We investigated whether age of diet initiation and biological sex could influence physical activity in mice fed either a control diet (CF, 0.86% methionine w/w) or SAAR (0.12% methionine w/w). Quadriceps femoris muscle mass is smaller in SAAR than in CF mice. Young mice fed a chronic SAAR diet at 8 weeks of age exhibited improved wire hang and running wheel activities compared to young CF mice, while aged mice showed comparable results. The effects of chronic SAAR on physical activity was mildly influenced by sex as observed in middle-aged male SAAR mice who showed minor improvements than CF males while middle-aged females displayed no discernible effects. Muscle mass is minimally affected by changes in markers of protein synthesis, autophagy and atrophy. Improvements to physical activity in young SAAR mice could be partially attributed to increased skeletal muscle mitochondrial activity. Furthermore, SAAR in C2C12 myotubes increased citrate synthase protein expression and enhanced succinyl dehydrogenase enzyme activity compared to CF myotubes. Overall, our data reveal that SAAR can improve mouse physical activity without compromising muscle proteostasis. This is partially due to enhanced mitochondrial activity, but the effects are influenced by age of diet initiation and sex.

GCN2 is required to maintain core body temperature in mice during acute cold

Nonshivering thermogenesis in rodents requires macronutrients to fuel the generation of heat during hypothermic conditions. In this study, we examined the role of the nutrient sensing kinase, general control nonderepressible 2 (GCN2) in directing adaptive thermogenesis during acute cold exposure in mice. We hypothesized that GCN2 is required for adaptation to acute cold stress via activation of the integrated stress response (ISR) resulting in liver production of FGF21 and increased amino acid transport to support nonshivering thermogenesis. In alignment with our hypothesis, female and male mice lacking GCN2 failed to adequately increase energy expenditure and veered into torpor. Mice administered a small molecule inhibitor of GCN2 were also profoundly intolerant to acute cold stress. Gcn2 deletion also impeded liver-derived FGF21 but in males only. Within the brown adipose tissue (BAT), acute cold exposure increased ISR activation and its transcriptional execution in males and females. RNA sequencing in BAT identified transcripts that encode actomyosin mechanics and transmembrane transport as requiring GCN2 during cold exposure. These transcripts included class II myosin heavy chain and amino acid transporters, critical for maximal thermogenesis during cold stress. Importantly, Gcn2 deletion corresponded with higher circulating amino acids and lower intracellular amino acids in the BAT during cold stress. In conclusion, we identify a sex-independent role for GCN2 activation to support adaptive thermogenesis via uptake of amino acids into brown adipose.NEW & NOTEWORTHY This paper details the discovery that GCN2 activation is required in both male and female mice to maintain core body temperature during acute cold exposure. The results point to a novel role for GCN2 in supporting adaptive thermogenesis via amino acid transport and actomyosin mechanics in brown adipose tissue.

Kinetic proteomics identifies targeted changes in liver metabolism and the ribo-interactome by dietary sulfur amino acid restriction

Dietary sulfur amino acid restriction (SAAR) protects against diet-induced obesity, extends healthspan, and coincides with an overall reduction in hepatic protein synthesis. To explore the underpinnings of SAAR-induced slowed growth and its impact on liver metabolism and proteostasis, we resolved changes in hepatic mRNA and protein abundances and compared synthesis rates of individual liver proteins. To achieve this, adult male mice were provided deuterium-labeled drinking water while freely consuming either a regular-fat or high-fat diet that was SAA restricted. Livers from these mice and their respective dietary controls were used to conduct transcriptomic, proteomic, and kinetic proteomic analyses. We found that remodeling of the transcriptome by SAAR was largely agnostic to dietary fat content. Shared signatures included activation of the integrated stress response alongside alterations in metabolic processes impacting lipids, fatty acids, and amino acids. Changes to the proteome correlated poorly with the transcriptome, and yet, functional clustering of kinetic proteomic changes in the liver during SAAR revealed that the management of fatty acids and amino acids were altered to support central metabolism and redox balance. Dietary SAAR also strongly influenced the synthesis rates of ribosomal proteins and ribosome-interacting proteins regardless of dietary fat. Taken together, dietary SAAR alters the transcriptome and proteome in the liver to safely manage increased fatty acid flux and energy use and couples this with targeted changes in the ribo-interactome to support proteostasis and slowed growth.

Artesunate targets cellular metabolism to regulate the Th17/Treg cell balance

INTRODUCTION

Metabolic reprogramming is one of the important mechanisms of cell differentiation, and different cells have different preferences for energy sources. During the differentiation of naive CD4 + T cells into Th17 and Treg cells, these cells show specific energy metabolism characteristics. Th17 cells depend on enhanced glycolysis, fatty acid synthesis, and glutaminolysis. In contrast, Treg cells are dependent on oxidative phosphorylation, fatty acid oxidation, and amino acid depletion. As a potent antimalarial drug, artesunate has been shown to modulate the Th17/Treg imbalance and regulate cell metabolism.

METHODOLOGY

Relevant literatures on ART, cellular metabolism, glycolysis, lipid metabolism, amino acid metabolism, CD4 + T cells, Th17 cells, and Treg cells published from January 1, 2010 to now were searched in PubMed database.

CONCLUSION

In this review, we will highlight recent advances in which artesunate can restore the Th17/Treg imbalance in disease states by altering T-cell metabolism to influence differentiation and lineage selection. Data from the current study show that few studies have focused on the effect of ART on cellular metabolism. ART can affect the metabolic characteristics of T cells (glycolysis, lipid metabolism, and amino acid metabolism) and interfere with their differentiation lineage, thereby regulating the balance of Th17/Treg and alleviating the symptoms of the disease.

Cysteine restriction-specific effects of sulfur amino acid restriction on lipid metabolism

Decreasing the dietary intake of methionine exerts robust anti-adiposity effects in rodents but modest effects in humans. Since cysteine can be synthesized from methionine, animal diets are formulated by decreasing methionine and eliminating cysteine. Such diets exert both methionine restriction (MR) and cysteine restriction (CR), that is, sulfur amino acid restriction (SAAR). Contrarily, SAAR diets formulated for human consumption included cysteine, and thus might have exerted only MR. Epidemiological studies positively correlate body adiposity with plasma cysteine but not methionine, suggesting that CR, but not MR, is responsible for the anti-adiposity effects of SAAR. Whether this is true, and, if so, the underlying mechanisms are unknown. Using methionine- and cysteine-titrated diets, we demonstrate that the anti-adiposity effects of SAAR are due to CR. Data indicate that CR increases serinogenesis (serine biosynthesis from non-glucose substrates) by diverting substrates from glyceroneogenesis, which is essential for fatty acid reesterification and triglyceride synthesis. Molecular data suggest that CR depletes hepatic glutathione and induces Nrf2 and its downstream targets Phgdh (the serine biosynthetic enzyme) and Pepck-M. In mice, the magnitude of SAAR-induced changes in molecular markers depended on dietary fat concentration (60% fat >10% fat), sex (males > females), and age-at-onset (young > adult). Our findings are translationally relevant as we found negative and positive correlations of plasma serine and cysteine, respectively, with triglycerides and metabolic syndrome criteria in a cross-sectional epidemiological study. Controlled feeding of low-SAA, high-polyunsaturated fatty acid diets increased plasma serine in humans. Serinogenesis might be a target for treating hypertriglyceridemia.

Rates of protein synthesis are maintained in brain but reduced in skeletal muscle during dietary sulfur amino acid restriction

Dietary interventions such as sulfur amino acid restriction (SAAR) target multiple drivers of aging, and show promise for preventing or delaying the onset of chronic diseases. SAAR promotes metabolic health and longevity in laboratory animals. The effects of SAAR on proteostasis remain relatively unexplored. We previously reported that SAAR promotes mitochondrial proteostatic maintenance, despite suppression of global protein synthesis, in two peripheral tissues, the liver and skeletal muscle. However, the brain, a tissue vulnerable to age-related neurodegenerative diseases due to the loss of proteostasis, has not been thoroughly studied. Therefore, we sought to reveal proteostatic responses in the brains of mice fed SAAR for 35 days. Here, we demonstrate that male C57Bl/6J mice fed two levels of SAAR maintained rates of protein synthesis in all sub-cellular fractions of the pre-frontal cortex. In comparison, rates of skeletal muscle protein synthesis in SAAR fed mice were slower than control-fed mice. To gain mechanistic insight, we examined several key nutrient/energy sensitive signaling proteins: AMP-activated protein kinase (AMPK), eukaryotic initiation factor 2 (eIF2), and ribosomal protein S6 (rpS6). SAAR had minimal to modest effects on the total abundance and phosphorylation of these proteins in both tissues. Our results indicate that the pre-frontal cortex in brain is resistant to perturbations in protein synthesis in mice fed SAAR, unlike skeletal muscle, which had a reduction in global protein synthesis. The results from this study demonstrate that proteostatic control in brain is of higher priority than skeletal muscle during dietary SAAR.

The Origins, Evolution, and Future of Dietary Methionine Restriction

The original description of dietary methionine restriction (MR) used semipurified diets to limit methionine intake to 20% of normal levels, and this reduction in dietary methionine increased longevity by ∼30% in rats. The MR diet also produces paradoxical increases in energy intake and expenditure and limits fat deposition while reducing tissue and circulating lipids and enhancing overall insulin sensitivity. In the years following the original 1993 report, a comprehensive effort has been made to understand the nutrient sensing and signaling systems linking reduced dietary methionine to the behavioral, physiological, biochemical, and transcriptional components of the response. Recent work has shown that transcriptional activation of hepatic fibroblast growth factor 21 (FGF21) is a key event linking the MR diet to many but not all components of its metabolic phenotype. These findings raise the interesting possibility of developing therapeutic, MR-based diets that produce the beneficial effects of FGF21 by nutritionally modulating its transcription and release.

Activation and execution of the hepatic integrated stress response by dietary essential amino acid deprivation is amino acid specific

Dietary removal of an essential amino acid (EAA) triggers the integrated stress response (ISR) in liver. Herein, we explored the mechanisms that activate the ISR and execute changes in transcription and translation according to the missing EAA. Wild‐type mice and mice lacking general control nonderepressible 2 (Gcn2) were fed an amino acid complete diet or a diet devoid of either leucine or sulfur amino acids (methionine and cysteine). Serum and liver leucine concentrations were significantly reduced within the first 6 h of feeding a diet lacking leucine, corresponding with modest, GCN2‐dependent increases in Atf4 mRNA translation and induction of selected ISR target genes (Fgf21, Slc7a5, Slc7a11). In contrast, dietary removal of the sulfur amino acids lowered serum methionine, but not intracellular methionine, and yet hepatic mRNA abundance of Atf4, Fgf21, Slc7a5, Slc7a11 substantially increased regardless of GCN2 status. Liver tRNA charging levels did not correlate with intracellular EAA concentrations or GCN2 status and remained similar to mice fed a complete diet. Furthermore, loss of Gcn2 increased the occurrence of ribosome collisions in liver and derepressed mechanistic target of rapamycin complex 1 signal transduction, but these changes did not influence execution of the ISR. We conclude that ISR activation is directed by intracellular EAA concentrations, but ISR execution is not. Furthermore, a diet devoid of sulfur amino acids does not require GCN2 for the ISR to execute changes to the transcriptome.

Specific amino acids regulate Sestrin2 mRNA and protein levels in an ATF4-dependent manner in C2C12 myocytes

BACKGROUND

Sestrin2 is a conserved protein in several species, and its expression is upregulated in cells under various environmental stresses. Sestrin2 content is involved in the function of mechanistic target of rapamycin complex 1 (mTORC1) in mouse embryonic fibroblasts and C2C12 cells.

METHODS

C2C12 cells were treated with amino acid-free DMEM (AF-DMEM) for 5 h. The effects of the addition of specific amino acids to AF-DMEM on Sestrin2 mRNA and protein expression were examined using RT-qPCR and western blotting, respectively. The mechanism by which amino acids regulate Sestrin2 mRNA expression was examined using blocking and siRNA experiments.

RESULTS

AF-DMEM increased the mRNA and protein levels of both Sestrin2 and activating transcription factor 4 (ATF4). The addition of a specific amino acid changed Sestrin2 mRNA and protein levels. The response pattern of Sestrin2 to specific amino acids was similar to that of ATF4. ATF4 siRNA reduced Sestrin2 mRNA levels. AF-DMEM increased eukaryotic initiation factor 2α (eIF2α) phosphorylation as early as 10 min after the treatment; however, ATF4 and Sestrin2 were increased 300 min after the treatment. Nuclear factor erythroid 2-related factor 2 and pancreatic and duodenal homeobox 1 siRNA did not affect ATF4 and Sestrin2 mRNA expression.

CONCLUSIONS

Specific Amino acids regulate Sestrin2 levels in an ATF4-dependent manner in C2C12 cells.

GENERAL SIGNIFICANCE

The results of the present study indicate that amino acids regulate levels of Sestrin2, which might cause phenotypic alterations, including mTORC1 activity, in C2C12 cells.

Targeting the methionine addiction of cancer

Methionine is the initiator amino acid for protein synthesis, the methyl source for most nucleotide, chromatin, and protein methylation, and the carbon backbone for various aspects of the cellular antioxidant response and nucleotide biosynthesis. Methionine is provided in the diet and serum methionine levels fluctuate based on dietary methionine content. Within the cell, methionine is recycled from homocysteine via the methionine cycle, which is linked to nutrient status via one-carbon metabolism. Unlike normal cells, many cancer cells, both in vitro and in vivo, show high methionine cycle activity and are dependent on exogenous methionine for continued growth. However, the molecular mechanisms underlying the methionine dependence of diverse malignancies are poorly understood. Methionine deprivation initiates widespread metabolic alterations in cancer cells that enable them to survive despite limited methionine availability, and these adaptive alterations can be specifically targeted to enhance the activity of methionine deprivation, a strategy we have termed "metabolic priming". Chemotherapy-resistant cell populations such as cancer stem cells, which drive treatment-resistance, are also sensitive to methionine deprivation, suggesting dietary methionine restriction may inhibit metastasis and recurrence. Several clinical trials in cancer are investigating methionine restriction in combination with other agents. This review will explore new insights into the mechanisms of methionine dependence in cancer and therapeutic efforts to translate these insights into enhanced clinical activity of methionine restriction in cancer.

Metabolic benefits of methionine restriction in adult mice do not require functional methionine sulfoxide reductase A (MsrA)

Methionine restriction (MR) extends lifespan and improves several markers of health in rodents. However, the proximate mechanisms of MR on these physiological benefits have not been fully elucidated. The essential amino acid methionine plays numerous biological roles and limiting its availability in the diet directly modulates methionine metabolism. There is growing evidence that redox regulation of methionine has regulatory control on some aspects of cellular function but interactions with MR remain largely unexplored. We tested the functional role of the ubiquitously expressed methionine repair enzyme methionine sulfoxide reductase A (MsrA) on the metabolic benefits of MR in mice. MsrA catalytically reduces both free and protein-bound oxidized methionine, thus playing a key role in its redox state. We tested the extent to which MsrA is required for metabolic effects of MR in adult mice using mice lacking MsrA. As expected, MR in control mice reduced body weight, altered body composition, and improved glucose metabolism. Interestingly, lack of MsrA did not impair the metabolic effects of MR on these outcomes. Moreover, females had blunted MR responses regardless of MsrA status compared to males. Overall, our data suggests that MsrA is not required for the metabolic benefits of MR in adult mice.

Methionine restriction exposes a targetable redox vulnerability of triple-negative breast cancer cells by inducing thioredoxin reductase

PURPOSE

Tumor cells are dependent on the glutathione and thioredoxin antioxidant pathways to survive oxidative stress. Since the essential amino acid methionine is converted to glutathione, we hypothesized that methionine restriction (MR) would deplete glutathione and render tumors dependent on the thioredoxin pathway and its rate-limiting enzyme thioredoxin reductase (TXNRD).

METHODS

Triple (ER/PR/HER2)-negative breast cancer (TNBC) cells were treated with control or MR media and the effects on reactive oxygen species (ROS) and antioxidant signaling were examined. To determine the role of TXNRD in MR-induced cell death, TXNRD1 was inhibited by RNAi or the pan-TXNRD inhibitor auranofin, an antirheumatic agent. Metastatic and PDX TNBC mouse models were utilized to evaluate in vivo antitumor activity.

RESULTS

MR rapidly and transiently increased ROS, depleted glutathione, and decreased the ratio of reduced glutathione/oxidized glutathione in TNBC cells. TXNRD1 mRNA and protein levels were induced by MR via a ROS-dependent mechanism mediated by the transcriptional regulators NRF2 and ATF4. MR dramatically sensitized TNBC cells to TXNRD1 silencing and the TXNRD inhibitor auranofin, as determined by crystal violet staining and caspase activity; these effects were suppressed by the antioxidant N-acetylcysteine. H-Ras-transformed MCF-10A cells, but not untransformed MCF-10A cells, were highly sensitive to the combination of auranofin and MR. Furthermore, dietary MR induced TXNRD1 expression in mammary tumors and enhanced the antitumor effects of auranofin in metastatic and PDX TNBC murine models.

CONCLUSION

MR exposes a vulnerability of TNBC cells to the TXNRD inhibitor auranofin by increasing expression of its molecular target and creating a dependency on the thioredoxin pathway.

Hepatic Nfe2l2 Is Not an Essential Mediator of the Metabolic Phenotype Produced by Dietary Methionine Restriction

The principal sensing of dietary methionine restriction (MR) occurs in the liver, where it activates multiple transcriptional programs that mediate various biological components of the response. Hepatic Fgf21 is a key target and essential endocrine mediator of the metabolic phenotype produced by dietary MR. The transcription factor, Nfe2l2, is also activated by MR and functions in tandem with hepatic Atf4 to transactivate multiple, antioxidative components of the integrated stress response. However, it is unclear whether the transcriptional responses linked to Nfe2l2 activation by dietary MR are essential to the biological efficacy of the diet. Using mice with liver-specific deletion of Nfe2l2 (Nfe2l2^fl/(Alb)) and their floxed littermates (Nfe2l2^fl/fl) fed either Control or MR diets, the absence of hepatic Nfe2l2 had no effect on the ability of the MR diet to increase FGF21, reduce body weight and adiposity, and increase energy expenditure. Moreover, the primary elements of the hepatic transcriptome were similarly affected by MR in both genotypes, with the only major differences occurring in induction of the P450-associated drug metabolism pathway and the pentose glucuronate interconversion pathway. The biological significance of these pathways is uncertain but we conclude that hepatic Nfe2l2 is not essential in mediating the metabolic effects of dietary MR.

Physiologic Responses to Dietary Sulfur Amino Acid Restriction in Mice Are Influenced by Atf4 Status and Biological Sex

BACKGROUND

Dietary sulfur amino acid restriction (SAAR) improves body composition and metabolic health across several model organisms in part through induction of the integrated stress response (ISR).

OBJECTIVE

We investigate the hypothesis that activating transcription factor 4 (ATF4) acts as a converging point in the ISR during SAAR.

METHODS

Using liver-specific or global gene ablation strategies, in both female and male mice, we address the role of ATF4 during dietary SAAR.

RESULTS

We show that ATF4 is dispensable in the chronic induction of the hepatokine fibroblast growth factor 21 while being essential for the sustained production of endogenous hydrogen sulfide. We also affirm that biological sex, independent of ATF4 status, is a determinant of the response to dietary SAAR.

CONCLUSIONS

Our results suggest that auxiliary components of the ISR, which are independent of ATF4, are critical for SAAR-mediated improvements in metabolic health in mice.

The acute transcriptional responses to dietary methionine restriction are triggered by inhibition of ternary complex formation and linked to Erk1/2, mTOR, and ATF4

The initial sensing of dietary methionine restriction (MR) occurs in the liver where it activates an integrated stress response (ISR) that quickly reduces methionine utilization. The ISR program is regulated in part by ATF4, but ATF4's prototypical upstream regulator, eIF2α, is not acutely activated by MR. Bioinformatic analysis of RNAseq and metabolomics data from liver samples harvested 3 h and 6 h after initiating MR shows that general translation is inhibited at the level of ternary complex formation by an acute 50% reduction of hepatic methionine that limits formation of initiator methionine tRNA. The resulting ISR is induced by selective expression of ATF4 target genes that mediate adaptation to reduced methionine intake and return hepatic methionine to control levels within 4 days of starting the diet. Complementary in vitro experiments in HepG2 cells after knockdown of ATF4, or inhibition of mTOR or Erk1/2 support the conclusion that the early induction of genes by MR is partially dependent on ATF4 and regulated by both mTOR and Erk1/2. Taken together, these data show that initiation of dietary MR induces an mTOR- and Erk1/2-dependent stress response that is linked to ATF4 by the sharp, initial drop in hepatic methionine and resulting repression of translation pre-initiation.

The role of liver metabolism in compensatory-growth piglets induced by protein restriction and subsequent protein realimentation

The aim of this work was to study the role of hepatic metabolism of compensatory growth in piglets induced by protein restriction and subsequent protein realimentation. Thirty-six weaned piglets were randomly distributed in a control group and a treatment group. The control group piglets were fed with a normal protein level diet (18.83% CP) for the entire experimental period (day 1-28). The treatment group piglets were fed with a protein-restriction diet (13.05% CP) for day 1 to day 14, and the diet was restored to normal protein level diet for day 15 to day 28. RNA-seq is used to analyze samples of liver metabolism on day 14 and day 28, respectively. Hepatic RNA-sequencing analysis revealed that some KEGG signaling pathways involved in glycolipid metabolism (eg, "AMPK signaling pathway," "insulin signaling pathway," and "glycolysis or gluconeogenesis") were significantly enriched on day 14 and day 28. On day 14, protein restriction promoted hepatic lipogenesis by increasing the genes expression level of ACACA, FASN, GAPM, and SREBP1C, decreasing protein phosphorylation levels of AMPKɑ and ACC in AMPK signaling pathway. In contrast, on day 28, protein realimentation promoted hepatic gluconeogenesis by increasing the concentration of G6Pase and PEPCK, decreasing protein phosphorylation levels of IRS1, Akt, and FoXO1 in insulin signaling pathway. In addition, protein realimentation activated the GH-IGF1 axis between the liver and skeletal muscle. Overall, these findings revealed the importance of liver metabolism in achieving compensatory growth.

Dietary Protein and Amino Acid Deficiency Inhibit Pancreatic Digestive Enzyme mRNA Translation by Multiple Mechanisms

BACKGROUND & AIMS

Chronic amino acid (AA) deficiency, as in kwashiorkor, reduces the size of the pancreas through an effect on mammalian target of rapamycin complex 1 (mTORC1). Because of the physiological importance of AAs and their role as a substrate, a stimulant of mTORC1, and protein synthesis, we studied the effect of acute protein and AA deficiency on the response to feeding.

METHODS

ICR/CD-1 mice were fasted overnight and refed for 2 hours with 4 different isocaloric diets: control (20% Prot); Protein-free (0% Prot); control (AA-based diet), and a leucine-free (No Leu). Protein synthesis, polysomal profiling, and the activation of several protein translation factors were analyzed in pancreas samples.

RESULTS

All diets stimulated the Protein Kinase-B (Akt)/mTORC1 pathway, increasing the phosphorylation of the kinase Akt, the ribosomal protein S6 (S6) and the formation of the eukaryotic initiation factor 4F (eIF4F) complex. Total protein synthesis and polysome formation were inhibited in the 0% Prot and No Leu groups to a similar extent, compared with the 20% Prot group. The 0% Prot diet partially reduced the Akt/mTORC1 pathway and the activity of the guanine nucleotide exchange factor eIF2B, without affecting eIF2α phosphorylation. The No Leu diet increased the phosphorylation of eIF2α and general control nonderepressible 2, and also inhibited eIF2B activity, without affecting mTORC1. Essential and nonessential AA levels in plasma and pancreas indicated a complex regulation of their cellular transport mechanisms and their specific effect on the synthesis of digestive enzymes.

CONCLUSIONS

These studies show that dietary AAs are important regulators of postprandial digestive enzyme synthesis, and their deficiency could induce pancreatic insufficiency and malnutrition.

CORP: The use of deuterated water for the measurement of protein synthesis

The use of deuterium oxide (D₂O) has greatly expanded the scope of what is possible for the measurement of protein synthesis. The greatest asset of D₂O labeling is that it facilitates the measurement of synthesis rates over prolonged periods of time from single proteins through integrated tissue-based measurements. Because the ease of administration, the method is amenable for use in a variety of models and conditions. Although the method adheres to the same rules as other isotope methods, the flexibility can create conditions that are not the same as other approaches and thus requires careful execution to maintain validity and reliability. For this CORP article, we provide a history that gave rise to the method and discuss the advantages and disadvantages of the method, the critical assumptions, guidelines, and best practices based on instrumentation, models, and experimental design. The goal of this CORP article is to propagate additional use of D₂O in a manner that produces reliable and valid data.

Mechanisms underlying reduced weight gain in intestinal fatty acid-binding protein (IFABP) null mice

Intestinal-fatty acid binding protein (IFABP; FABP2) is a 15-kDa intracellular protein abundantly present in the cytosol of the small intestinal (SI) enterocyte. High-fat (HF) feeding of IFABP-/- mice resulted in reduced weight gain and fat mass relative to wild-type (WT) mice. Here, we examined intestinal properties that may underlie the observed lean phenotype of high fat-fed IFABP-/- mice. No alterations in fecal lipid content were found, suggesting that the IFABP-/- mice are not malabsorbing dietary fat. However, the total excreted fecal mass, normalized to food intake, was increased for the IFABP-/- mice relative to WT mice. Moreover, intestinal transit time was more rapid in the IFABP-/- mice. IFABP-/- mice displayed a shortened average villus length, a thinner muscularis layer, reduced goblet cell density, and reduced Paneth cell abundance. The number of proliferating cells in the crypts of IFABP-/- mice did not differ from that of WT mice, suggesting that the blunt villi phenotype is not due to alterations in proliferation. IFABP-/- mice were observed to have altered expression of genes and proteins related to intestinal structure, while immunohistochemical analyses revealed increased staining for markers of inflammation. Taken together, these studies indicate that the ablation of IFABP, coupled with high-fat feeding, leads to changes in gut motility and morphology, which likely contribute to the relatively leaner phenotype occurring at the whole-body level. Thus, IFABP is likely involved in dietary lipid sensing and signaling, influencing intestinal motility, intestinal structure, and nutrient absorption, thereby impacting systemic energy metabolism.NEW & NOTEWORTHY Intestinal fatty acid binding protein (IFABP) is thought to be essential for the efficient uptake and trafficking of dietary fatty acids. In this study, we demonstrate that high-fat-fed IFABP-/- mice have an increased fecal output and are likely malabsorbing other nutrients in addition to lipid. Furthermore, we observe that the ablation of IFABP leads to marked alterations in intestinal morphology and secretory cell abundance.

Muscle-specific changes in protein synthesis with aging and reloading after disuse atrophy

BACKGROUND

Successful strategies to halt or reverse sarcopenia require a basic understanding of the factors that cause muscle loss with age. Acute periods of muscle loss in older individuals have an incomplete recovery of muscle mass and strength, thus accelerating sarcopenic progression. The purpose of the current study was to further understand the mechanisms underlying the failure of old animals to completely recover muscle mass and function after a period of hindlimb unloading.

METHODS

Hindlimb unloading was used to induce muscle atrophy in Fischer 344-Brown Norway (F344BN F1) rats at 24, 28, and 30 months of age. Rats were hindlimb unloaded for 14 days and then reloaded at 24 months (Reloaded 24), 28 months (Reloaded 28), and 24 and 28 months (Reloaded 24/28) of age. Isometric torque was determined at 24 months of age (24 months), at 28 months of age (28 months), immediately after 14 days of reloading, and at 30 months of age (30 months). During control or reloaded conditions, rats were labelled with deuterium oxide (D₂ O) to determine rates of muscle protein synthesis and RNA synthesis.

RESULTS

After 14 days of reloading, in vivo isometric torque returned to baseline in Reloaded 24, but not Reloaded 28 and Reloaded 24/28. Despite the failure of Reloaded 28 and Reloaded 24/28 to regain peak force, all groups were equally depressed in peak force generation at 30 months. Increased age did not decrease muscle protein synthesis rates, and in fact, increased resting rates of protein synthesis were measured in the myofibrillar fraction (Fractional synthesis rate (FSR): %/day) of the plantaris (24 months: 2.53 ± 0.17; 30 months: 3.29 ± 0.17), and in the myofibrillar (24 months: 2.29 ± 0.07; 30 months: 3.34 ± 0.11), collagen (24 months: 1.11 ± 0.07; 30 months: 1.55 ± 0.14), and mitochondrial (24 months: 2.38 ± 0.16; 30 months: 3.20 ± 0.10) fractions of the tibialis anterior (TA). All muscles increased myofibrillar protein synthesis (%/day) in Reloaded 24 (soleus: 3.36 ± 0.11, 5.23 ± 0.19; plantaris: 2.53 ± 0.17, 3.66 ± 0.07; TA: 2.29 ± 0.14, 3.15 ± 0.12); however, in Reloaded 28, only the soleus had myofibrillar protein synthesis rates (%/day) >28 months (28 months: 3.80 ± 0.10; Reloaded 28: 4.86 ± 0.19). Across the muscles, rates of protein synthesis were correlated with RNA synthesis (all muscles combined, R² = 0.807, P < 0.0001).

CONCLUSIONS

These data add to the growing body of literature that indicate that changes with age, including following disuse atrophy, differ by muscle. In addition, our findings lead to additional questions of the underlying mechanisms by which some muscles are maintained with age while others are not.

Fibroblast Growth Factor 21 and the Adaptive Response to Nutritional Challenges

The Fibroblast Growth Factor 21 (FGF21) is considered an attractive therapeutic target for obesity and obesity-related disorders due to its beneficial effects in lipid and carbohydrate metabolism. FGF21 response is essential under stressful conditions and its metabolic effects depend on the inducer factor or stress condition. FGF21 seems to be the key signal which communicates and coordinates the metabolic response to reverse different nutritional stresses and restores the metabolic homeostasis. This review is focused on describing individually the FGF21-dependent metabolic response activated by some of the most common nutritional challenges, the signal pathways triggering this response, and the impact of this response on global homeostasis. We consider that this is essential knowledge to identify the potential role of FGF21 in the onset and progression of some of the most prevalent metabolic pathologies and to understand the potential of FGF21 as a target for these diseases. After this review, we conclude that more research is needed to understand the mechanisms underlying the role of FGF21 in macronutrient preference and food intake behavior, but also in β-klotho regulation and the activity of the fibroblast activation protein (FAP) to uncover its therapeutic potential as a way to increase the FGF21 signaling.

Methionine restriction activates the integrated stress response in triple-negative breast cancer cells by a GCN2- and PERK-independent mechanism

Transformed cells are often selectively susceptible to depletion of the amino acid methionine, which induces growth arrest and/or apoptosis. In non-transformed cells, amino acid deficiency is sensed by two stress-activated kinases, general control nonderepressible 2 (GCN2) and protein kinase R-like endoplasmic reticulum kinase (PERK), which phosphorylate and inactivate elongation initiation factor 2 α (eIF2α), thereby suppressing global mRNA translation and inducing activated transcription factor (ATF4). ATF4 and its downstream transcriptional targets including Sestrin-2 constitute an adaptive integrated stress response. We postulated that methionine depletion activates the integrated stress response in breast cancer cells by a GCN2- and/or PERK-dependent mechanism and that selective disruption of one or both of these kinases would enhance the therapeutic activity of methionine restriction. Here we demonstrate that methionine restriction induces eIF2α phosphorylation and enhances ATF4 gene expression and protein levels of ATF4 and Sestrin-2 in triple (ER/PR/HER2)-negative breast cancer (TNBC) cells. However, knockdown of GCN2, PERK or both in TNBC cells did not prevent induction of ATF4 or Sestrin-2 by methionine restriction. In contrast, deletion of GCN2 in murine embryonic fibroblasts abrogated ATF4 and Sestrin-2 induction in response to methionine restriction. Moreover, knockdown of GCN2, PERK or both did not affect TNBC cell growth or apoptosis in response to methionine restriction. Overall, our findings point to a GCN2- and PERK-independent mechanism(s) by which methionine restriction activates the integrated stress response in TNBC cells. Elucidation of this pathway(s) could lead to strategies to enhance the therapeutic response of methionine restriction.

Dietary Sulfur Amino Acid Restriction and the Integrated Stress Response: Mechanistic Insights

Dietary sulfur amino acid restriction, also referred to as methionine restriction, increases food intake and energy expenditure and alters body composition in rodents, resulting in improved metabolic health and a longer lifespan. Among the known nutrient-responsive signaling pathways, the evolutionary conserved integrated stress response (ISR) is a lesser-understood candidate in mediating the hormetic effects of dietary sulfur amino acid restriction (SAAR). A key feature of the ISR is the concept that a family of protein kinases phosphorylates eukaryotic initiation factor 2 (eIF2), dampening general protein synthesis to conserve cellular resources. This slowed translation simultaneously allows for preferential translation of genes with special sequence features in the 5' leader. Among this class of mRNAs is activating transcription factor 4 (ATF4), an orchestrator of transcriptional control during nutrient stress. Several ATF4 gene targets help execute key processes affected by SAAR such as lipid metabolism, the transsulfuration pathway, and antioxidant defenses. Exploration of the canonical ISR demonstrates that eIF2 phosphorylation is not necessary for ATF4-driven changes in the transcriptome during SAAR. Additional research is needed to clarify the regulation of ATF4 and its gene targets during SAAR.

Nutritional Regulation of Gene Expression: Carbohydrate-, Fat- and Amino Acid-Dependent Modulation of Transcriptional Activity

The ability to detect changes in nutrient levels and generate an adequate response to these changes is essential for the proper functioning of living organisms. Adaptation to the high degree of variability in nutrient intake requires precise control of metabolic pathways. Mammals have developed different mechanisms to detect the abundance of nutrients such as sugars, lipids and amino acids and provide an integrated response. These mechanisms include the control of gene expression (from transcription to translation). This review reports the main molecular mechanisms that connect nutrients’ levels, gene expression and metabolism in health. The manuscript is focused on sugars’ signaling through the carbohydrate-responsive element binding protein (ChREBP), the role of peroxisome proliferator-activated receptors (PPARs) in the response to fat and GCN2/activating transcription factor 4 (ATF4) and mTORC1 pathways that sense amino acid concentrations. Frequently, alterations in these pathways underlie the onset of several metabolic pathologies such as obesity, insulin resistance, type 2 diabetes, cardiovascular diseases or cancer. In this context, the complete understanding of these mechanisms may improve our knowledge of metabolic diseases and may offer new therapeutic approaches based on nutritional interventions and individual genetic makeup.

Impact of Glutathione Peroxidase-1 (Gpx1) Genotype on Selenoenzyme and Transcript Expression When Repleting Selenium-Deficient Mice

Glutathione peroxidase (Gpx1) is the major selenoprotein in most tissues in animals. Knockout (KO) of Gpx1 decreases Gpx1 activity to near zero and substantially reduces liver selenium (Se) levels, but has no overt effects in otherwise healthy mice. To investigate the impact of deletion of Gpx1 on Se metabolism, Se flux, and apparent Se requirements, KO, Gpx1 heterozygous (Het), and Gpx1 wild-type (WT) mice were fed Se-deficient diet for 17 weeks, then repleted with graded levels of Se (0-0.3 μg Se/g as Na₂SeO₃) for 7 days, and selenoprotein activities and transcripts were determined in blood, liver, and kidney. Se deficiency decreased the activities of plasma Gpx3, liver Gpx1, liver Txnrd, and liver Gpx4 to 3, 0.3, 11, and 50% of WT Se-adequate levels, respectively, but the Gpx1 genotype had no effect on growth or changes in activity or expression of selenoproteins other than Gpx1. Se repletion increased selenoprotein transcripts to Se-adequate levels after 7 days; Se response curves and apparent Se requirements for selenoprotein transcripts were similar to those observed in studies starting with Se-adequate mice. With short-term Se repletion, selenoenzyme activities resulted in three Se response curve patterns: (1) liver and kidney Gpx1, Gpx4, and Txnrd activities were sigmoidal or hyperbolic with breakpoints (0.08-0.19 μg Se/g) that were double those observed in studies starting with Se-adequate mice; (2) red blood cell Gpx1 activity was not significantly changed; and (3) plasma Gpx3 activity only increased substantially with 0.3 μg Se/g. Plasma Gpx3 is secreted from kidney. In this short-term study, kidney Gpx3 mRNA reached plateau levels at 0.1 μg Se/g, and other kidney selenoenzyme activities reached plateau levels at ≤ 0.2 μg Se/g, so sufficient Se should have been present in kidney. Thus, the delayed increase in plasma Gpx3 activity suggests that newly synthesized and secreted kidney Gpx3 is preferentially retained in kidney or rapidly cleared by binding to basement membranes in kidney or in other tissues. This repletion study shows that loss of capacity to incorporate Se into Gpx1 in Gpx1 KO mice does not dramatically alter expression of other Se biomarkers, nor the short-term flux of Se from intestine to liver to kidney.

Homeostatic sensing of dietary protein restriction: A case for FGF21

Restriction of dietary protein intake increases food intake and energy expenditure, reduces growth, and alters amino acid, lipid, and glucose metabolism. While these responses suggest that animals 'sense' variations in amino acid consumption, the basic physiological mechanism mediating the adaptive response to protein restriction has been largely undescribed. In this review we make the case that the liver-derived metabolic hormone FGF21 is the key signal which communicates and coordinates the homeostatic response to dietary protein restriction. Support for this model centers on the evidence that FGF21 is induced by the restriction of dietary protein or amino acid intake and is required for adaptive changes in metabolism and behavior. FGF21 occupies a unique endocrine niche, being induced when energy intake is adequate but protein and carbohydrate are imbalanced. Collectively, the evidence thus suggests that FGF21 is the first known endocrine signal of dietary protein restriction.

Short-term methionine deprivation improves metabolic health via sexually dimorphic, mTORC1-independent mechanisms

Obesity and diabetes are major challenges to global health, and there is an urgent need for interventions that promote weight loss. Dietary restriction of methionine promotes leanness and improves metabolic health in mice and humans. However, poor long-term adherence to this diet limits its translational potential. In this study, we develop a short-term methionine deprivation (MD) regimen that preferentially reduces fat mass, restoring normal body weight and glycemic control to diet-induced obese mice of both sexes. The benefits of MD do not accrue from calorie restriction, but instead result from increased energy expenditure. MD promotes increased energy expenditure in a sex-specific manner, inducing the fibroblast growth factor (Fgf)-21-uncoupling protein (Ucp)-1 axis only in males. Methionine is an agonist of the protein kinase mechanistic target of rapamycin complex (mTORC)-1, which has been proposed to play a key role in the metabolic response to amino acid-restricted diets. In our study, we used a mouse model of constitutive hepatic mTORC1 activity and demonstrate that suppression of hepatic mTORC1 signaling is not required for the metabolic effects of MD. Our study sheds new light on the mechanisms by which dietary methionine regulates metabolic health and demonstrates the translational potential of MD for the treatment of obesity and type 2 diabetes.-Yu, D., Yang, S. E., Miller, B. R., Wisinski, J. A., Sherman, D. S., Brinkman, J. A., Tomasiewicz, J. L., Cummings, N. E., Kimple, M. E., Cryns, V. L., Lamming, D. W. Short-term methionine deprivation improves metabolic health via sexually dimorphic, mTORC1-independent mechanisms.

Dietary and Endocrine Regulation of Endogenous Hydrogen Sulfide Production: Implications for Longevity

SIGNIFICANCE

Hydrogen sulfide (H₂S) at the right concentration is associated with numerous health benefits in experimental organisms, ranging from protection from ischemia/reperfusion injury to life span extension. Given the considerable translation potential, two major strategies have emerged: supplementation of exogenous H₂S and modulation of endogenous H₂S metabolism. Recent Advances: Recently, it was reported that hepatic H₂S production capacity is increased in two of the best-characterized mammalian models of life span extension, dietary restriction, and hypopituitary dwarfism, leading to new insights into dietary and hormonal regulation of endogenous H₂S production together with broader changes in sulfur amino acid (SAA) metabolism with implications for DNA methylation and redox status.

CRITICAL ISSUES

Here, we discuss the role of dietary SAAs and growth hormone (GH)/thyroid hormone (TH) signaling in regulation of endogenous H₂S production largely via repression of H₂S generating enzymes cystathionine γ-lyase (CGL) and cystathionine β-synthase (CBS) on the level of gene transcription, as well as reciprocal regulation of GH and TH signaling by H₂S itself. We also discuss plasticity of CGL and CBS gene expression in response to environmental stimuli and the potential of the microbiome to impact overall H₂S levels.

FUTURE DIRECTIONS

The relative contribution of increased H₂S to health span or lifespan benefits in models of extended longevity remains to be determined, as does the mechanism by which such benefits occur. Nonetheless, our ability to control H₂S levels using exogenous H₂S donors or by modifying the endogenous H₂S production/consumption equilibrium has the potential to improve health and increase "shelf-life" across evolutionary boundaries, including our own. Antioxid. Redox Signal. 28, 1483-1502.

Effects of single amino acid deficiency on mRNA translation are markedly different for methionine versus leucine

Although amino acids are known regulators of translation, the unique contributions of specific amino acids are not well understood. We compared effects of culturing HEK293T cells in medium lacking either leucine, methionine, histidine, or arginine on eIF2 and 4EBP1 phosphorylation and measures of mRNA translation. Methionine starvation caused the most drastic decrease in translation as assessed by polysome formation, ribosome profiling, and a measure of protein synthesis (puromycin-labeled polypeptides) but had no significant effect on eIF2 phosphorylation, 4EBP1 hyperphosphorylation or 4EBP1 binding to eIF4E. Leucine starvation suppressed polysome formation and was the only tested condition that caused a significant decrease in 4EBP1 phosphorylation or increase in 4EBP1 binding to eIF4E, but effects of leucine starvation were not replicated by overexpressing nonphosphorylatable 4EBP1. This suggests the binding of 4EBP1 to eIF4E may not by itself explain the suppression of mRNA translation under conditions of leucine starvation. Ribosome profiling suggested that leucine deprivation may primarily inhibit ribosome loading, whereas methionine deprivation may primarily impair start site recognition. These data underscore our lack of a full understanding of how mRNA translation is regulated and point to a unique regulatory role of methionine status on translation initiation that is not dependent upon eIF2 phosphorylation.

Lessons from animal nutritionists: dietary amino acid requirement studies and considerations for healthy aging studies

Dietary restriction (DR) increases median life span and protects against age-related disease. Improved longevity can be achieved by restriction of dietary energy, protein, or amino acids (AAs), such as methionine (Met). Met requirements have been defined using methodologies that measure the dose response to Met when all other dietary variables are held constant and with outcomes focused on protein turnover. Here, we cover protein and sulfur AA requirements and discuss the terms "deficient," "optimal," and "excess" and how these need to be considered. We additionally discuss the effect of methyl-donating compounds on sulfur AA metabolism and outcomes. We will discuss how the mechanistic target of rapamycin complex 1 (mTORC1) signaling network regulates protein turnover, lipogenesis and cell growth, proliferation, differentiation, and metabolism in response to hormones, AAs, and cellular energy status. Inhibition of mTORC1 signaling with rapamycin or genetic mutation increases median life span in model organisms, and mTORC1 inhibition may be responsible for some of the life span-extending effects of DR. Finally, we discuss how the sulfur AAs may regulate aspects of reactive oxygen species (ROS) mitigation. Overall, we suggest that approaches evaluating AA intake need to consider whole-body protein synthesis and measures related to tissue-specific and whole-body metabolism that have been associated with longevity.

Physiological and Molecular Mechanisms of Methionine Restriction

Methionine restriction (MR) has been studied extensively over the last 25 years for its role in altering metabolic hallmarks of disease. Animals subjected to MR, display changes in metabolic flexibility demonstrated by increases in energy expenditure, glucose tolerance, and lifespan. These changes have been well characterized in a number of model systems and significant progress has been made in understanding how hepatic fibroblast growth factor 21 links MR to several components of its metabolic phenotype. Despite these advances, a complete understanding of mechanisms engaged by dietary MR remains elusive. In this review, we offer a brief history of MR and its known mechanisms associated with stress, metabolism, and lifespan extension. We consider the role of epigenetics in the response of animals to MR and propose a novel epigenetic pathway involving the regulation of microRNAs during MR.