UCP1 is an essential mediator of the effects of methionine restriction on energy balance but not insulin sensitivity

Insulin at the intersection of thermoregulation and glucose homeostasis

Mammals are protected from changes in environmental temperature by altering energetic processes that modify heat production. Insulin is the dominant stimulus of glucose uptake and metabolism, which are fundamental for thermogenic processes. The purpose of this work was to determine the interaction of ambient temperature induced changes in energy expenditure (EE) on the insulin sensitivity of glucose fluxes. Short-term and adaptive responses to thermoneutral temperature (TN, ∼28 °C) and room (laboratory) temperature (RT, ∼22 °C) were studied in mice. This range of temperature does not cause detectable changes in circulating catecholamines or shivering and postabsorptive glucose homeostasis is maintained. We tested the hypothesis that a decrease in EE that occurs with TN causes insulin resistance and that this reduction in insulin action and EE is reversed upon short term (<12h) transition to RT. Insulin-stimulated glucose disposal (Rd) and tissue-specific glucose metabolic index were assessed combining isotopic tracers with hyperinsulinemic-euglycemic clamps. EE and insulin-stimulated Rd are both decreased (∼50%) in TN-adapted vs RT-adapted mice. When RT-adapted mice are switched to TN, EE rapidly decreases and Rd is reduced by ∼50%. TN-adapted mice switched to RT exhibit a rapid increase in EE, but whole-body insulin-stimulated Rd remains at the low rates of TN-adapted mice. In contrast, whole body glycolytic flux rose with EE. This higher EE occurs without increasing glucose uptake from the blood, but rather by diverting glucose from glucose storage to glycolysis. In addition to adaptations in insulin action, 'insulin-independent' glucose uptake in brown fat is exquisitely sensitive to thermoregulation. These results show that insulin action adjusts to non-stressful changes in ambient temperature to contribute to the support of body temperature homeostasis without compromising glucose homeostasis.

Dietary sulfur amino acid restriction in humans with overweight and obesity: a translational randomized controlled trial

BACKGROUND

Dietary sulfur amino acid restriction (SAAR) improves metabolic health in animals. In this study, we investigated the effect of dietary SAAR on body weight, body composition, resting metabolic rate, gene expression profiles in white adipose tissue (WAT), and an extensive blood biomarker profile in humans with overweight or obesity.

METHODS

N = 59 participants with overweight or obesity (73% women) were randomized stratified by sex to an 8-week plant-based dietary intervention low (~ 2 g/day, SAAR) or high (~ 5.6 g/day, control group) in sulfur amino acids. The diets were provided in full to the participants, and both investigators and participants were blinded to the intervention. Outcome analyses were performed using linear mixed model regression adjusted for baseline values of the outcome and sex.

RESULTS

SAAR led to a ~ 20% greater weight loss compared to controls (β 95% CI - 1.14 (- 2.04, - 0.25) kg, p = 0.013). Despite greater weight loss, resting metabolic rate remained similar between groups. Furthermore, SAAR decreased serum leptin, and increased ketone bodies compared to controls. In WAT, 20 genes were upregulated whereas 24 genes were downregulated (FDR < 5%) in the SAAR group compared to controls. Generally applicable gene set enrichment analyses revealed that processes associated with ribosomes were upregulated, whereas processes related to structural components were downregulated.

CONCLUSION

Our study shows that SAAR leads to greater weight loss, decreased leptin and increased ketone bodies compared to controls. Further research on SAAR is needed to investigate the therapeutic potential for metabolic conditions in humans.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT04701346, registered Jan 8th 2021, https://www.

CLINICALTRIALS

gov/study/NCT04701346.

Insulin at the Intersection of Thermoregulation and Glucose Homeostasis

Mammals are protected from changes in environmental temperature by altering energetic processes that modify heat production. Insulin is the dominant stimulus of glucose uptake and metabolism, which are fundamental for thermogenic processes. The purpose of this work was to determine the interaction of ambient temperature induced changes in energy expenditure (EE) on the insulin sensitivity of glucose fluxes. Short-term and adaptive responses to thermoneutral temperature (TN, ~28°C) and room (laboratory) temperature (RT, ~22°C) were studied in mice. This range of temperature does not cause detectable changes in circulating catecholamines or shivering and postabsorptive glucose homeostasis is maintained. We tested the hypothesis that a decrease in EE that occurs with TN causes insulin resistance and that this reduction in insulin action and EE is reversed upon short term (<12h) transition to RT. Insulin-stimulated glucose disposal (Rd) and tissue specific glucose uptake were assessed combining isotopic tracers with hyperinsulinemic-euglycemic clamps. EE and insulin-stimulated Rd are both decreased (~50%) in TN-adapted vs RT-adapted mice. When RT-adapted mice are switched to TN, EE rapidly decreases and Rd is reduced by ~50%. TN-adapted mice switched to RT exhibit a rapid increase in EE, but whole body insulin-stimulated Rd remains at the low rates of TN-adapted mice. In contrast, whole body glycolytic flux rose with EE. This higher EE occurs without increasing glucose uptake from the blood, but rather by diverting glucose from glucose storage to glycolysis. In addition to adaptations in insulin action, 'insulin-independent' glucose uptake in brown fat is exquisitely sensitive to thermoregulation. These results show that insulin action adjusts to non-stressful changes in ambient temperature to contribute to the support of body temperature homeostasis without compromising glucose homeostasis.

Short-term Pre-operative Methionine Restriction Induces Browning of Perivascular Adipose Tissue and Improves Vein Graft Remodeling in Mice

Short-term preoperative methionine restriction (MetR) shows promise as a translatable strategy to modulate the body's response to surgical injury. Its application, however, to improve post-interventional vascular remodeling remains underexplored. Here, we find that MetR protects from arterial intimal hyperplasia in a focal stenosis model and adverse vascular remodeling after vein graft surgery. RNA sequencing reveals that MetR enhances the brown adipose tissue phenotype in arterial perivascular adipose tissue (PVAT) and induces it in venous PVAT. Specifically, PPAR-α was highly upregulated in PVAT-adipocytes. Furthermore, MetR dampens the post-operative pro-inflammatory response to surgery in PVAT-macrophages in vivo and in vitro . This study shows for the first time that the detrimental effects of dysfunctional PVAT on vascular remodeling can be reversed by MetR, and identifies pathways involved in browning of PVAT. Furthermore, we demonstrate the potential of short-term pre-operative MetR as a simple intervention to ameliorate vascular remodeling after vascular surgery.

Preservation of mitochondrial membrane potential is necessary for lifespan extension from dietary restriction

Dietary restriction (DR) increases lifespan in many organisms, but its underlying mechanisms are not fully understood. Mitochondria play a central role in metabolic regulation and are known to undergo changes in structure and function in response to DR. Mitochondrial membrane potential (Δψ_m) is the driving force for ATP production and mitochondrial outputs that integrate many cellular signals. One such signal regulated by Δψ_m is nutrient-status sensing. Here, we tested the hypothesis that DR promotes longevity through preserved Δψ_m during adulthood. Using the nematode Caenorhabditis elegans, we find that Δψ_m declines with age relatively early in the lifespan, and this decline is attenuated by DR. Pharmacologic depletion of Δψ_m blocked the longevity and health benefits of DR. Genetic perturbation of Δψ_m and mitochondrial ATP availability similarly prevented lifespan extension from DR. Taken together, this study provides further evidence that appropriate regulation of Δψ_m is a critical factor for health and longevity in response to DR.

Dietary Methionine Restriction Promotes Fat Browning and Attenuates Hepatic Lipid Accumulation in High-Choline-Fed Mice Associated with the Improvement of Thyroid Function

This study aims to explore the influences of a methionine-restricted diet (MRD) on fat browning and hepatic lipid accumulation in mice fed with a high-choline diet (HCD) and their possible mechanisms. ICR mice were randomly divided into three groups and fed with a normal diet (0.86% methionine + 0.20% choline, ND), HCD (0.86% methionine + 1.20% choline), or MRD (0.17% methionine + 1.20% choline) for 90 consecutive days. We found that MRD reduced body weight and fat mass; increased heat production and ambulatory locomotor activity; reduced hepatic and plasma lipid levels, hepatic fatty infiltration area, and adipocyte volume in white and brown adipose tissue; promoted fat browning, especially upregulated gene and protein expression levels of uncoupling protein 1 (UCP1); and promoted fat catabolism and inhibited fat anabolism in the liver and adipose tissue. Moreover, MRD increased antioxidant defenses and reduced inflammatory cytokine levels in the thyroid, blood, and liver. Furthermore, MRD improved thyroid morphological structure, promoted the synthesis and secretion of thyroid hormones, and enhanced the actions of thyroid hormones on its receptor organs (liver and adipose tissue). These findings suggested that MRD promoted fat browning and attenuated hepatic lipid accumulation in HCD mice associated with the improvement of thyroid function.

The Association between Methionine Intake and Diabetes in Chinese Adults-Results from the China Health and Nutrition Survey

This study aimed to evaluate the association between methionine intake and diabetes prevalence in Chinese adults and explore whether the association was source-specific. Data from 12,849 adults aged ≥20 years old were used from the China Health and Nutrition Survey during 1997-2011. Diabetes was diagnosed as self-reported and/or when blood tests results met the diagnostic criteria. A 3-day, 24-h recall was used to assess different sources of methionine. Multivariable mixed linear regression was used to examine the associations. Across the quartiles of total methionine intake, the odds ratio (ORs, 95% CI) of diabetes were 1.00, 1.49 (1.21 to 1.82), 1.72 (1.37 to 2.15), and 2.53 (1.97 to 3.23). In the subgroup analysis, similar trends were observed in both animal and plant methionine. There was a significant interaction between urbanization and diabetes. The positive association was only significant in those who lived in low or medium urbanization areas. The ORs (95% CI) were 1.00, 1.27 (0.85 to 1.88), 1.56 (1.01 to 2.39), and 1.79 (1.09 to 2.95) for medium urbanization, respectively. No interaction was identified when stratified by different methionine sources. In conclusion, methionine intake was positively associated with diabetes independent of food source, and it was modified by urbanization levels.

Methionine restriction - Association with redox homeostasis and implications on aging and diseases

Methionine is an essential amino acid, involved in the promotion of growth, immunity, and regulation of energy metabolism. Over the decades, research has long focused on the beneficial effects of methionine supplementation, while data on positive effects of methionine restriction (MR) were first published in 1993. MR is a low-methionine dietary intervention that has been reported to ameliorate aging and aging-related health concomitants and diseases, such as obesity, type 2 diabetes, and cognitive disorders. In addition, MR seems to be an approach to prolong lifespan which has been validated extensively in various animal models, such as Caenorhabditis elegans, Drosophila, yeast, and murine models. MR appears to be associated with a reduction in oxidative stress via so far mainly undiscovered mechanisms, and these changes in redox status appear to be one of the underlying mechanisms for lifespan extension and beneficial health effects. In the present review, the association of methionine metabolism pathways with redox homeostasis is described. In addition, the effects of MR on lifespan, age-related implications, comorbidities, and diseases are discussed.

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.

Impacts of essential amino acids on energy balance

BACKGROUND

Obesity develops due to an imbalance in energy homeostasis, wherein energy intake exceeds energy expenditure. Accumulating evidence shows that manipulations of dietary protein and their component amino acids affect the energy balance, resulting in changes in fat mass and body weight. Amino acids are not only the building blocks of proteins but also serve as signals regulating multiple biological pathways.

SCOPE OF REVIEW

We present the currently available evidence regarding the effects of dietary alterations of a single essential amino acid (EAA) on energy balance and relevant signaling mechanisms at both central and peripheral levels. We summarize the association between EAAs and obesity in humans and the clinical use of modifying the dietary EAA composition for therapeutic intervention in obesity. Finally, similar mechanisms underlying diets varying in protein levels and diets altered of a single EAA are described. The current review would expand our understanding of the contribution of protein and amino acids to energy balance control, thus helping discover novel therapeutic approaches for obesity and related diseases.

MAJOR CONCLUSIONS

Changes in circulating EAA levels, particularly increased branched-chain amino acids (BCAAs), have been reported in obese human and animal models. Alterations in dietary EAA intake result in improvements in fat and weight loss in rodents, and each has its distinct mechanism. For example, leucine deprivation increases energy expenditure, reduces food intake and fat mass, primarily through regulation of the general control nonderepressible 2 (GCN2) and mammalian target of rapamycin (mTOR) signaling. Methionine restriction by 80% decreases fat mass and body weight while developing hyperphagia, primarily through fibroblast growth factor 21 (FGF-21) signaling. Some effects of diets with different protein levels on energy homeostasis are mediated by similar mechanisms. However, reports on the effects and underlying mechanisms of dietary EAA imbalances on human body weight are few, and more investigations are needed in future.

Adaptations Supporting Plasma Methionine on a Limited-Methionine, High-Cystine Diet Alter the Canine Plasma Metabolome Consistent with Interventions that Extend Life Span in Other Species

BACKGROUND

Using indicator amino acid oxidation methodology, the mean dietary requirement of adult dogs for methionine (Met) was estimated to be ∼66% of the current recommended allowance. Dogs fed a diet formulated to provide the estimated mean Met requirement for 32 wk maintained plasma Met, seemingly supported by betaine oxidation.

OBJECTIVE

To gain a better understanding of the metabolic changes that were associated with supporting plasma Met when dogs were fed a limited Met diet over 32 wk, we analyzed plasma samples taken from that study using a data-driven metabolomics approach.

METHODS

Labrador retrievers (20 females/13 males; mean age: 4.9 y; range: 2.0-7.9 y) were fed semi-purified, nutritionally complete diets. After 4 wk of feeding a control diet (DL-Met; 1.37 g/1000 kcal), 17 dogs remained on this diet and 16 were transitioned to a test diet formulated to the estimated mean Met requirement (0.55 g/1000 kcal), with dietary total sulfur amino acid maintained with additional l-cystine (Cys). Dogs were individually fed diets to maintain a stable body weight at an ideal body condition score for 32 wk. Plasma samples from fasted blood collected at baseline and 8 and 32 wk were analyzed using untargeted metabolic profiling.

RESULTS

Analysis of metabolites (n = 593) confirmed our primary findings (increased Met, betaine, and dimethylglycine). Metabolite changes consistent with repartitioning choline to support Met cycling included reduced pools of lipids derived via phosphatidylethanolamine N-methyltransferase and enhanced fatty acid oxidation. Some changes were consistent with metabolomics studies reported in other species that used interventions known to extend life span (caloric- and Met-restricted diets or feeding strategy).

CONCLUSIONS

Changes in the plasma metabolome were consistent with reported adaptations to support Met-dependent activities. We propose that feeding a limited-Met, high-Cys diet using the estimated mean Met requirement in adult Labrador retrievers alters regulation of the Met cycle, thereby altering metabolism, similar to interventions that extend life span.

Metabolomic Analysis Reveals Changes in Plasma Metabolites in Response to Acute Cold Stress and Their Relationships to Metabolic Health in Cold-Acclimatized Humans

Cold exposure results in activation of metabolic processes required for fueling thermogenesis, potentially promoting improved metabolic health. However, the metabolic complexity underlying this process is not completely understood. We aimed to analyze changes in plasma metabolites related to acute cold exposure and their relationship to cold-acclimatization level and metabolic health in cold-acclimatized humans. Blood samples were obtained before and acutely after 10–15 min of ice-water swimming (<5 °C) from 14 ice-water swimmers. Using mass spectrometry, 973 plasma metabolites were measured. Ice-water swimming induced acute changes in 70 metabolites. Pathways related to amino acid metabolism were the most cold-affected and cold-induced changes in several amino acids correlated with cold-acclimatization level and/or metabolic health markers, including atherogenic lipid profile or insulin resistance. Metabolites correlating with cold-acclimatization level were enriched in the linoleic/α-linolenic acid metabolic pathway. N-lactoyl-tryptophan correlated with both cold-acclimatization level and cold-induced changes in thyroid and parathyroid hormones. Acute cold stress in cold-acclimatized humans induces changes in plasma metabolome that involve amino acids metabolism, while the linoleic and α-linolenic acid metabolism pathway seems to be affected by regular cold exposure. Metabolites related to metabolic health, thermogenic hormonal regulators and acclimatization level might represent prospective molecular factors important in metabolic adaptations to regular cold exposure.

The Role of Reduced Methionine in Mediating the Metabolic Responses to Protein Restriction Using Different Sources of Protein

Dietary protein restriction and dietary methionine restriction (MR) produce a comparable series of behavioral, physiological, biochemical, and transcriptional responses. Both dietary regimens produce a similar reduction in intake of sulfur amino acids (e.g., methionine and cystine), and both diets increase expression and release of hepatic FGF21. Given that FGF21 is an essential mediator of the metabolic phenotype produced by both diets, an important unresolved question is whether dietary protein restriction represents de facto methionine restriction. Using diets formulated from either casein or soy protein with matched reductions in sulfur amino acids, we compared the ability of the respective diets to recapitulate the metabolic phenotype produced by methionine restriction using elemental diets. Although the soy-based control diets supported faster growth compared to casein-based control diets, casein-based protein restriction and soy-based protein restriction produced comparable reductions in body weight and fat deposition, and similar increases in energy intake, energy expenditure, and water intake. In addition, the prototypical effects of dietary MR on hepatic and adipose tissue target genes were similarly regulated by casein- and soy-based protein restriction. The present findings support the feasibility of using restricted intake of diets from various protein sources to produce therapeutically effective implementation of dietary methionine restriction.

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.

Implementation of dietary methionine restriction using casein after selective, oxidative deletion of methionine

Dietary methionine restriction (MR) is normally implemented using diets formulated from elemental amino acids (AA) that reduce methionine content to ∼0.17%. However, translational implementation of MR with elemental AA-based diets is intractable due to poor palatability. To solve this problem and restrict methionine using intact proteins, casein was subjected to mild oxidation to selectively reduce methionine. Diets were then formulated using oxidized casein, adding back methionine to produce a final concentration of 0.17%. The biological efficacy of dietary MR using the oxidized casein (Ox Cas) diet was compared with the standard elemental MR diet in terms of the behavioral, metabolic, endocrine, and transcriptional responses to the four diets. The Ox Cas MR diet faithfully reproduced the expected physiological, biochemical, and transcriptional responses in liver and inguinal white adipose tissue. Collectively, these findings demonstrate that dietary MR can be effectively implemented using casein after selective oxidative reduction of methionine.

Low protein diet and methyl-donor supplements modify testicular physiology in mice

The link between male diet and sperm quality has received significant investigation. However, the impact diet and dietary supplements have on the testicular environment has been examined to a lesser extent. Here, we establish the impact of a sub-optimal low protein diet (LPD) on testicular morphology, apoptosis and serum fatty acid profiles. Furthermore, we define whether supplementing a LPD with specific methyl donors abrogates any detrimental effects of the LPD. Male C57BL6 mice were fed either a control normal protein diet (NPD; 18% protein; n = 8), an isocaloric LPD (LPD; 9% protein; n = 8) or an LPD supplemented with methyl donors (MD-LPD; choline chloride, betaine, methionine, folic acid, vitamin B12; n = 8) for a minimum of 7 weeks. Analysis of male serum fatty acid profiles by gas chromatography revealed elevated levels of saturated fatty acids and lower levels of mono- and polyunsaturated fatty acids in MD-LPD males when compared to NPD and/or LPD males. Testes of LPD males displayed larger seminiferous tubule cross section area when compared to NPD and MD-LPD males, while MD-LPD tubules displayed a larger luminal area. Furthermore, TUNNEL staining revealed LPD males possessed a reduced number of tubules positive for apoptosis, while gene expression analysis showed MD-LPD testes displayed decreased expression of the pro-apoptotic genes Bax, Csap1 and Fas when compared to NPD males. Finally, testes from MD-LPD males displayed a reduced telomere length but increased telomerase activity. These data reveal the significance of sub-optimal nutrition for paternal metabolic and reproductive physiology.

Low Protein Diets and Energy Balance: Mechanisms of Action on Energy Intake and Expenditure

Low protein diets are associated with increased lifespan and improved cardiometabolic health primarily in rodents, and likely improve human health. There is strong evidence that moderate to severe reduction in dietary protein content markedly influences caloric intake and energy expenditure, which is often followed by a decrease in body weight and adiposity in animal models. While the neuroendocrine signals that trigger hyperphagic responses to protein restriction are better understood, there is accumulating evidence that increased sympathetic flux to brown adipose tissue, fibroblast growth factor-21 and serotonergic signaling are important for the thermogenic effects of low protein diets. This mini-review specifically focuses on the effect of low protein diets with variable carbohydrate and lipid content on energy intake and expenditure, and the underlying mechanisms of actions by these diets. Understanding the mechanisms by which protein restriction influences energy balance may unveil novel approaches for treating metabolic disorders in humans and improve production efficiency in domestic animals.

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.

Effects of dietary restriction on metabolic and cognitive health

Life expectancy in most developed countries has been rising over the past century. In the UK alone, there are about 12 million people over 65 years old and centenarians have increased by 85% in the past 15 years. As a result of the ageing population, which is due mainly to improvements in medical treatments, public health, improved housing and lifestyle choices, there is an associated increase in the prevalence of pathological conditions, such as metabolic disorders, type 2 diabetes, cardiovascular and neurodegenerative diseases, many types of cancer and others. Statistics suggest that nearly 54% of elderly people in the UK live with at least two chronic conditions, revealing the urgency for identifying interventions that can prevent and/or treat such disorders. Non-pharmacological, dietary interventions such as energetic restriction (ER) and methionine restriction (MR) have revealed promising outcomes in increasing longevity and preventing and/or reversing the development of ageing-associated disorders. In this review, we discuss the evidence and mechanisms that are involved in these processes. Fibroblast growth factor 1 and hydrogen sulphide are important molecules involved in the effects of ER and MR in the extension of life span. Their role is also associated with the prevention of metabolic and cognitive disorders, highlighting these interventions as promising modulators for improvement of health span.

Manipulation of Dietary Amino Acids Prevents and Reverses Obesity in Mice Through Multiple Mechanisms That Modulate Energy Homeostasis

Reduced activation of energy metabolism increases adiposity in humans and other mammals. Thus, exploring dietary and molecular mechanisms able to improve energy metabolism is of paramount medical importance because such mechanisms can be leveraged as a therapy for obesity and related disorders. Here, we show that a designer protein-deprived diet enriched in free essential amino acids can 1) promote the brown fat thermogenic program and fatty acid oxidation, 2) stimulate uncoupling protein 1 (UCP1)-independent respiration in subcutaneous white fat, 3) change the gut microbiota composition, and 4) prevent and reverse obesity and dysregulated glucose homeostasis in multiple mouse models, prolonging the healthy life span. These effects are independent of unbalanced amino acid ratio, energy consumption, and intestinal calorie absorption. A brown fat-specific activation of the mechanistic target of rapamycin complex 1 seems involved in the diet-induced beneficial effects, as also strengthened by in vitro experiments. Hence, our results suggest that brown and white fat may be targets of specific amino acids to control UCP1-dependent and -independent thermogenesis, thereby contributing to the improvement of metabolic health.

Dietary Methionine Restriction Signals to the Brain Through Fibroblast Growth Factor 21 to Regulate Energy Balance and Remodeling of Adipose Tissue

OBJECTIVE

Restricting dietary methionine to 0.17% in mice increases energy expenditure (EE), reduces fat deposition, and improves metabolic health by increasing hepatic fibroblast growth factor 21 (FGF21). The goal of this study was to compare each of these responses in mice with the coreceptor for FGF21 deleted in either adipose tissue or the brain.

METHODS

Methionine-restriction (MR) diets were fed to age-matched cohorts of mice with the coreceptor for FGF21 deleted in either adipose tissue or the brain. The physiological and transcriptional responses to MR were compared in the respective cohorts.

RESULTS

Tissue-specific deletion of the FGF21 coreceptor in adipose tissue did not abrogate the ability of dietary MR to increase  EE and reduce fat deposition. Tissue-specific deletion of the FGF21 coreceptor from the brain produced mice that were unable to respond to the effects of MR on  EE or the remodeling of adipose tissue.

CONCLUSIONS

The increase in FGF21 produced by dietary MR acts primarily in the brain to produce its physiological effects on energy balance. In contrast, the effects of MR on hepatic gene expression were intact in both models, supporting a mechanism that directly links detection of reduced methionine in the liver to transcriptional mechanisms that alter gene expression in the liver.

The effect of short-term methionine restriction on hydrogen peroxide metabolism in Fischer-344 rat skeletal muscle mitochondria

Skeletal muscle, a significant contributor to resting energy expenditure and reactive oxygen species, may play a critical role in body-weight regulation and aging processes. Methionine restriction (MR) is a dietary intervention which extends lifespan, lowers body-weight and enhances energy expenditure in rodents, all of which have been linked to mitochondrial function in various tissues including liver, kidney, heart and brown adipose tissue; however, mitochondrial responses to MR in skeletal muscle is largely unknown. Given the importance of skeletal muscle on energy metabolism and aging-related processes, we investigated if there are changes in skeletal muscle mitochondrial energetics in response to MR. Although MR lowers body-weight in rats, neither respiration, proton leak nor hydrogen peroxide metabolism were altered in isolated skeletal muscle mitochondria. This suggests that mitochondrial function in skeletal muscle remains conserved while MR alters metabolism in other tissues.

FGF21 Signals Protein Status to the Brain and Adaptively Regulates Food Choice and Metabolism

Reduced dietary protein intake induces adaptive physiological changes in macronutrient preference, energy expenditure, growth, and glucose homeostasis. We demonstrate that deletion of the FGF21 co-receptor βKlotho (Klb) from the brain produces mice that are unable to mount a physiological response to protein restriction, an effect that is replicated by whole-body deletion of FGF21. Mice forced to consume a low-protein diet exhibit reduced growth, increased energy expenditure, and a resistance to diet-induced obesity, but the loss of FGF21 signaling in the brain completely abrogates that response. When given access to a higher protein alternative, protein-restricted mice exhibit a shift toward protein-containing foods, and central FGF21 signaling is essential for that response. FGF21 is an endocrine signal linking the liver and brain, which regulates adaptive, homeostatic changes in metabolism and feeding behavior during protein restriction.

Perspective: Methionine Restriction-Induced Longevity-A Possible Role for Inhibiting the Synthesis of Bacterial Quorum Sensing Molecules

Methionine restriction (MR) extends lifespans in multiple species through mechanisms that include enhanced oxidative stress resistance and inhibition of insulin/insulin-like growth factor I (IGF-I) signaling. Methionine and S-adenosylmethionine (SAM) are the essential precursors of bacterial quorum sensing (QS) molecules, and therefore, MR might also affect bacterial communication to prevent enteric bacterial infection as well as chronic inflammation, which contributes to lifespan prolongation. Here, we discuss the influence of MR on oxidative stress resistance and inhibition of insulin/IGF-I cell signaling and further propose a potential mechanism involving bacterial QS inhibition for lifespan extension. Unraveling the connection between MR and inhibition of QS provides new strategies for combating infectious diseases, resulting in enriched understanding of MR-induced lifespan extension.

The road ahead for health and lifespan interventions

Aging is a modifiable risk factor for most chronic diseases and an inevitable process in humans. The development of pharmacological interventions aimed at delaying or preventing the onset of chronic conditions and other age-related diseases has been at the forefront of the aging field. Preclinical findings have demonstrated that species, sex and strain confer significant heterogeneity on reaching the desired health- and lifespan-promoting pharmacological responses in model organisms. Translating the safety and efficacy of these interventions to humans and the lack of reliable biomarkers that serve as predictors of health outcomes remain a challenge. Here, we will survey current pharmacological interventions that promote lifespan extension and/or increased healthspan in animals and humans, and review the various anti-aging interventions selected for inclusion in the NIA's Interventions Testing Program as well as the ClinicalTrials.gov database that target aging or age-related diseases in humans.

Methionine Restriction and Cancer Biology

The essential amino acid, methionine, is important for cancer cell growth and metabolism. A growing body of evidence indicates that methionine restriction inhibits cancer cell growth and may enhance the efficacy of chemotherapeutic agents. This review summarizes the efficacy and mechanism of action of methionine restriction on hallmarks of cancer in vitro and in vivo. The review highlights the role of glutathione formation, polyamine synthesis, and methyl group donation as mediators of the effects of methionine restriction on cancer biology. The translational potential of the use of methionine restriction as a personalized nutritional approach for the treatment of patients with cancer is also discussed.

Sexually Dimorphic Effects of Dietary Methionine Restriction are Dependent on Age when the Diet is Introduced

OBJECTIVE

Restricting dietary methionine to 0.17% in male mice increases energy expenditure, reduces fat deposition, and improves metabolic health. The goal of this work was to compare each of these responses in postweaning male and female mice and in physically mature male and female mice.

METHODS

Methionine-restricted (MR) diets were fed to age-matched cohorts of male and female mice for 8 to 10 weeks beginning at 8 weeks of age or beginning at 4 months of age. The physiological and transcriptional responses to MR were compared in the respective cohorts.

RESULTS

Dietary MR produced sexually dimorphic changes in body composition in young growing animals, with males preserving lean at the expense of fat and females preserving fat at the expense of lean. The effects of MR on energy balance were comparable between sexes when the diet was initiated after attainment of physical maturity (4 months), and metabolic and endocrine responses were also comparable between males and females after 8 weeks on the MR diet.

CONCLUSIONS

The sexually dimorphic effects of MR are limited to nutrient partitioning between lean and fat tissue deposition in young, growing mice. Introduction of the diet after physical maturity produced comparable effects on growth and metabolic responses in male and female mice.

Dietary sulfur amino acid restriction upregulates DICER to confer beneficial effects

Objective

Dietary restriction (DR) improves health and prolongs lifespan in part by upregulating type III endoribonuclease DICER in adipose tissue. In this study, we aimed to specifically test which missing dietary component was responsible for DICER upregulation.

Methods

We performed a nutrient screen in mouse preadipocytes and validated the results in vivo using different kinds of dietary interventions in wild type or genetically modified mice and worms, also testing the requirement of DICER on the effects of the diets.

Results

We found that sulfur amino acid restriction (i.e., methionine or cysteine) is sufficient to increase Dicer mRNA expression in preadipocytes. Consistently, while DR increases DICER expression in adipose tissue of mice, this effect is blunted by supplementation of the diet with methionine, cysteine, or casein, but not with a lipid or carbohydrate source. Accordingly, dietary methionine or protein restriction mirrors the effects of DR. These changes are associated with alterations in serum adiponectin. We also found that DICER controls and is controlled by adiponectin. In mice, DICER plays a role in methionine restriction-induced upregulation of Ucp1 in adipose tissue. In C. elegans, DR and a model of methionine restriction also promote DICER expression in the intestine (an analog of the adipose tissue) and prolong lifespan in a DICER-dependent manner.

Conclusions

We propose an evolutionary conserved mechanism in which dietary sulfur amino acid restriction upregulates DICER levels in adipose tissue leading to beneficial health effects.

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DICER is upregulated in adipose tissue by dietary sulfur amino acid restriction.

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Adiponectin and DICER co-regulate each other in adipocytes.

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Methionine restriction requires DICER to promote adipose tissue browning.

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DICER is upregulated in Caenorhabditis elegans intestine upon dietary restriction.

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Methionine restriction requires DICER to prolong lifespan in C. elegans.

The effect of short-term methionine restriction on glutathione synthetic capacity and antioxidant responses at the whole tissue and mitochondrial level in the rat liver

Dietary methionine restriction (MR) where methionine is the sole source of sulfur amino acid increases lifespan in diverse species. Methionine restricted rodents experience a decrease in glutathione (GSH), a major antioxidant, in several tissues, which is paradoxical to longevity interventions because tissues with low GSH might experience more oxidative damage. Liver plays a key role in GSH synthesis and here we examined how MR influences GSH metabolism in the liver. We also hypothesised that low GSH might be subsidized by compensatory pathway(s) in the liver. To investigate GSH synthesis and antioxidant responses, Fischer-344 rats were given either a MR diet or a control diet for 8 weeks. Based on γ-glutamylcysteine synthetase activity, GSH synthetic capacity did not respond to low dietary methionine availability. Tissue level protein and lipid oxidation markers do not support elevated oxidative damage, despite low GSH availability. Whole tissue and mitochondrial level responses to MR differed. Specifically, the activity of glutathione reductase and thioredoxin reductase increase in whole liver tissue which might offset the effects of declined GSH availability whereas mitochondrial GSH levels were unperturbed by MR. Moreover, enhanced proton leak in liver mitochondria by MR (4 week) presumably diminishes ROS production. Taken together, we suggest that the effect of low GSH in liver tissue is subsidized, at least in part, by increased antioxidant activity and possibly by enhanced mitochondrial proton leak.

Metabolites as regulators of insulin sensitivity and metabolism

The cause of insulin resistance in obesity and type 2 diabetes mellitus (T2DM) is not limited to impaired insulin signalling but also involves the complex interplay of multiple metabolic pathways. The analysis of large data sets generated by metabolomics and lipidomics has shed new light on the roles of metabolites such as lipids, amino acids and bile acids in modulating insulin sensitivity. Metabolites can regulate insulin sensitivity directly by modulating components of the insulin signalling pathway, such as insulin receptor substrates (IRSs) and AKT, and indirectly by altering the flux of substrates through multiple metabolic pathways, including lipogenesis, lipid oxidation, protein synthesis and degradation and hepatic gluconeogenesis. Moreover, the post-translational modification of proteins by metabolites and lipids, including acetylation and palmitoylation, can alter protein function. Furthermore, the role of the microbiota in regulating substrate metabolism and insulin sensitivity is unfolding. In this Review, we discuss the emerging roles of metabolites in the pathogenesis of insulin resistance and T2DM. A comprehensive understanding of the metabolic adaptations involved in insulin resistance may enable the identification of novel targets for improving insulin sensitivity and preventing, and treating, T2DM.

Selenium Deficiency Is Associated with Pro-longevity Mechanisms

Selenium (Se) is an essential trace element because of its presence in selenoproteins in the form of selenocysteine residue. Both Se deficiency, which compromises selenoprotein functions, and excess Se, which is toxic, have been associated with altered redox homeostasis and adverse health conditions. Surprisingly, we found that, although Se deficiency led to a drastic decline in selenoprotein expression, mice subjected to this dietary regimen for their entire life had normal lifespans. To understand the molecular mechanisms involved, we performed systemic analyses at the level of metabolome, transcriptome, and microRNA profiling. These analyses revealed that Se deficiency reduced amino acid levels, elevated mononucleotides, altered metabolism, and activated signaling pathways linked to longevity-related nutrient sensing. The data show that the metabolic control associated with nutrient sensing coordinately responds to suppressed selenoprotein functions, resulting in normal lifespan under Se deficiency.

MetAP2 inhibition increases energy expenditure through direct action on brown adipocytes

Inhibitors of methionine aminopeptidase 2 (MetAP2) have been shown to reduce body weight in obese mice and humans. The target tissue and cellular mechanism of MetAP2 inhibitors, however, have not been extensively examined. Using compounds with diverse chemical scaffolds, we showed that MetAP2 inhibition decreases body weight and fat mass and increases lean mass in the obese mice but not in the lean mice. Obesity is associated with catecholamine resistance and blunted β-adrenergic receptor signaling activities, which could dampen lipolysis and energy expenditure resulting in weight gain. In the current study, we examined effect of MetAP2 inhibition on brown adipose tissue and brown adipocytes. Norepinephrine increases energy expenditure in brown adipose tissue by providing fatty acid substrate through lipolysis and by increasing expression of uncoupled protein-1 (UCP1). Metabolomic analysis shows that in response to MetAP2 inhibitor treatment, fatty acid metabolites in brown adipose tissue increase transiently and subsequently decrease to basal or below basal levels, suggesting an effect on fatty acid metabolism in this tissue. Treatment of brown adipocytes with MetAP2 inhibitors enhances norepinephrine-induced lipolysis and energy expenditure, and prolongs the activity of norepinephrine to increase ucp1 gene expression and energy expenditure in norepinephrine-desensitized brown adipocytes. In summary, we showed that the anti-obesity activity of MetAP2 inhibitors can be mediated, at least in part, through direct action on brown adipocytes by enhancing β-adrenergic-signaling-stimulated activities.

Methionine Restriction Partly Recapitulates the Sympathetically Mediated Enhanced Energy Expenditure Induced by Total Amino Acid Restriction in Rats

Total amino acid (AA) restriction promotes hyperphagia and energy expenditure. We determined whether (i) methionine restriction mimics the effects of total AA restriction, (ii) methionine supplementation attenuates these responses, and iii) sympathetic signaling mediates such effects. Rats were injected with either vehicle (V) or 6-hydroxydopamine (S) to induce chemical sympathectomy, and then randomized to four diets: 16% AA (16AA), 5% AA (5AA), 16% AA-methionine (16AA-Met), and 5% AA+methionine (5AA+Met). Propranolol or ondansetron were injected to examine the role of sympathetic and serotonergic signaling, respectively. 5AA, 5AA+Met, and 16AA-Met increased the food conversion rate for 1–3 weeks in the V and S groups, and increased mean energy expenditure in V group,; the magnitude of these changes was attenuated in the S group. Propranolol decreased the energy expenditure of V16AA, V5AA, and V5AA+Met and of S5AA, S5AA+Met, and S16AA-Met, whereas ondansetron decreased the energy expenditure in only the S groups. Compared to 16AA, the other V groups had reduced body weights from days 7–11 onwards and decreased lean masses throughout the study and the other S groups had decreased body weights and lean masses from day 14 onwards. Total AA restriction enhanced the energy expenditure and reduced the weight and lean mass; these effects were partly recapitulated by methionine restriction and were sympathetically mediated.

Regulation of metabolic health by essential dietary amino acids

Although the beneficial effects of calorie restriction (CR) on health and aging were first observed a century ago, the specific macronutrients and molecular processes that mediate the effect of CR have been heavily debated. Recently, it has become clear that dietary protein plays a key role in regulating both metabolic health and longevity, and that both the quantity and quality - the specific amino acid composition - of dietary protein mediates metabolic health. Here, we discuss recent findings in model organisms ranging from yeast to mice and humans regarding the influence of dietary protein as well as specific amino acids on metabolic health, and the physiological and molecular mechanisms which may mediate these effects. We then discuss recent findings which suggest that the restriction of specific dietary amino acids may be a potent therapy to treat or prevent metabolic syndrome. Finally, we discuss the potential for dietary restriction of specific amino acids - or pharmaceuticals which harness these same mechanisms - to promote healthy aging.

Mitochondria Inspire a Lifestyle

Tucked inside our cells, we animals (and plants, and fungi) carry mitochondria, minuscule descendants of bacteria that invaded our common ancestor 2 billion years ago. This unplanned breakthrough endowed our ancestors with a convenient, portable source of energy, enabling them to progress towards more ambitious forms of life. Mitochondria still manufacture most of our energy; we have evolved to invest it to grow and produce offspring, and to last long enough to make it all happen. Yet because the continuous generation of energy is inevitably linked to that of toxic free radicals, mitochondria give us life and give us death. Stripping away clutter and minutiae, here we present a big-picture perspective of how mitochondria work, how they are passed on virtually only by mothers, and how they shape the lifestyles of species and individuals. We discuss why restricting food prolongs lifespan, why reproducing shortens it, and why moving about protects us from free radicals despite increasing their production. We show that our immune cells use special mitochondria to keep control over our gut microbes. And we lay out how the fabrication of energy and free radicals sets the internal clocks that command our everyday rhythms-waking, eating, sleeping. Mitochondria run the show.

Metabolic Regulation of Methionine Restriction in Diabetes

Although the effects of dietary methionine restriction have been investigated in the physiology of aging and diseases related to oxidative stress, the relationship between methionine restriction (MR) and the development of metabolic disorders has not been explored extensively. This review summarizes studies of the possible involvement of dietary methionine restriction in improving insulin resistance, glucose homeostasis, oxidative stress, lipid metabolism, the pentose phosphate pathway (PPP), and inflammation, with an emphasis on the fibroblast growth factor 21 and protein phosphatase 2A signals and autophagy in diabetes. Diets deficient in methionine may be a useful nutritional strategy in patients with diabetes.

Methionine restriction leads to hyperhomocysteinemia and alters hepatic H2S production capacity in Fischer-344 rats

Dietary methionine restriction (MR) increases lifespan in several animal models. Despite low dietary intake of sulphur amino acids, rodents on MR develop hyperhomocysteinemia. On the contrary, MR has been reported to increase H₂S production in mice. Enzymes involved in homocysteine metabolism also take part in H₂S production and hence, in this study, the impact of MR on hyperhomocysteinemia and H₂S production capacity were investigated using Fischer-344 rats assigned either a control or a MR diet for 8 weeks. The MR animals showed elevated plasma homocysteine accompanied with a reduction in liver cysteine content and methylation potential. It was further found that MR decreased cystathionine-β-synthase (CBS) activity in the liver, however, MR increased hepatic cystathionine-γ-lyase (CGL) activity which is the second enzyme in the transsulfuration pathway and also participates in regulating H₂S production. The relative contribution of CGL in H₂S production increased concomitantly with the increased CGL activity. Additionally, hepatic mercaptopyruvate-sulphur-transferase (MPST) activity also increased in response to MR. Taken together, our results suggest that reduced CBS activity and S-Adenosylmethionine availability contributes to hyperhomocysteinimia in MR animals. Elevated CGL and MPST activities may provide a compensatory mechanism for maintaining hepatic H₂S production capacity in response to the decreased CBS activity.

Sensing and signaling mechanisms linking dietary methionine restriction to the behavioral and physiological components of the response

Dietary methionine restriction (MR) is implemented using a semi-purified diet that reduces methionine by ∼80% and eliminates dietary cysteine. Within hours of its introduction, dietary MR initiates coordinated series of transcriptional programs and physiological responses that include increased energy intake and expenditure, decreased adiposity, enhanced insulin sensitivity, and reduction in circulating and tissue lipids. Significant progress has been made in cataloguing the physiological responses to MR in males but not females, but identities of the sensing and communication networks that orchestrate these responses remain poorly understood. Recent work has implicated hepatic FGF21 as an important mediator of MR, but it is clear that other mechanisms are also involved. The goal of this review is to explore the temporal and spatial organization of the responses to dietary MR as a model for understanding how nutrient sensing systems function to integrate complex transcriptional, physiological, and behavioral responses to changes in dietary composition.

The Components of Age-Dependent Effects of Dietary Methionine Restriction on Energy Balance in Rats

OBJECTIVE

Dietary methionine restriction (MR) improves biomarkers of metabolic health, in part through coordinated increases in energy intake and energy expenditure (EE). Some metabolic benefits of dietary MR are secondary to its effects on energy balance, so this study's purpose was to examine how age at initiation of MR influences its effects on energy balance and body composition.

METHODS

Energy balance was examined in rats provided control or MR diets for 9 months after weaning or in rats between 6 and 12 months of age.

RESULTS

Rats provided the control diet for 9 months after weaning increased their body weight (BW) and fat mass by five- and eightfold, respectively, while BW and fat accumulation in the MR group were reduced to 50% of that of controls. In adult rats fed the respective diets between 6 and 12 months of age, dietary MR increased energy intake by ∼23%, but the 15% increase in EE was sufficient to prevent increases in BW or fat mass.

CONCLUSIONS

Dietary MR produces comparable increases in EE in young, growing animals and in mature animals, but young animals continue to deposit new tissue because of the proportionately larger effect of MR on energy intake relative to maintenance requirements.

Dietary Quercetin Attenuates Adipose Tissue Expansion and Inflammation and Alters Adipocyte Morphology in a Tissue-Specific Manner

Chronic inflammation in adipose tissue may contribute to depot-specific adipose tissue expansion, leading to obesity and insulin resistance. Dietary supplementation with quercetin or botanical extracts containing quercetin attenuates high fat diet (HFD)-induced obesity and insulin resistance and decreases inflammation. Here, we determined the effects of quercetin and red onion extract (ROE) containing quercetin on subcutaneous (inguinal, IWAT) vs. visceral (epididymal, EWAT) white adipose tissue morphology and inflammation in mice fed low fat, high fat, high fat plus 50 μg/day quercetin or high fat plus ROE containing 50 μg/day quercetin equivalents for 9 weeks. Quercetin and ROE similarly ameliorated HFD-induced increases in adipocyte size and decreases in adipocyte number in IWAT and EWAT. Furthermore, quercetin and ROE induced alterations in adipocyte morphology in IWAT. Quercetin and ROE similarly decreased HFD-induced IWAT inflammation. However, quercetin and red onion differentially affected HFD-induced EWAT inflammation, with quercetin decreasing and REO increasing inflammatory marker gene expression. Quercetin and REO also differentially regulated circulating adipokine levels. These results show that quercetin or botanical extracts containing quercetin induce white adipose tissue remodeling which may occur through inflammatory-related mechanisms.

Disease prevention and delayed aging by dietary sulfur amino acid restriction: translational implications

Sulfur amino acids (SAAs) play numerous critical roles in metabolism and overall health maintenance. Preclinical studies have demonstrated that SAA-restricted diets have many beneficial effects, including extending life span and preventing the development of a variety of diseases. Dietary sulfur amino acid restriction (SAAR) is characterized by chronic restrictions of methionine and cysteine but not calories and is associated with reductions in body weight, adiposity and oxidative stress, and metabolic changes in adipose tissue and liver resulting in enhanced insulin sensitivity and energy expenditure. SAAR-induced changes in blood biomarkers include reductions in insulin, insulin-like growth factor-1, glucose, and leptin and increases in adiponectin and fibroblast growth factor 21. On the basis of these preclinical data, SAAR may also have similar benefits in humans. While little is known of the translational significance of SAAR, its potential feasibility in humans is supported by findings of its effectiveness in rodents, even when initiated in adult animals. To date, there have been no controlled feeding studies of SAAR in humans; however, there have been numerous relevant epidemiologic and disease-based clinical investigations reported. Here, we summarize observations from these clinical investigations to provide insight into the potential effectiveness of SAAR for humans.

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.

The role of suppression of hepatic SCD1 expression in the metabolic effects of dietary methionine restriction

Dietary methionine restriction (MR) produces concurrent increases in energy intake and expenditure, but the proportionately larger increase in energy expenditure (EE) effectively limits weight gain and adipose tissue accretion over time. Increased hepatic fibroblast growth factor-21 (FGF21) is essential to MR-dependent increases in EE, but it is unknown whether the downregulation of hepatic stearoyl-coenzyme A desaturase-1 (SCD1) by MR could also be a contributing factor. Global deletion of SCD1 mimics cold exposure in mice housed at 23 °C by compromising the insular properties of the skin. The resulting cold stress increases EE, limits fat deposition, reduces hepatic lipids, and increases insulin sensitivity by activating thermoregulatory thermogenesis. To examine the efficacy of MR in the absence of SCD1 and without cold stress, the biological efficacy of MR in Scd1^-/- mice housed near thermoneutrality (28 °C) was evaluated. Compared with wild-type mice on the control diet, Scd1^-/- mice were leaner, had higher EE, lower hepatic and serum triglycerides, and lower serum leptin and insulin. Although dietary MR increased adipose tissue UCP1 expression, hepatic Fgf21 messenger RNA, 24 h EE, and reduced serum triglycerides in Scd1^-/- mice, it failed to reduce adiposity or produce any further reduction in hepatic triglycerides, serum insulin, or serum leptin. These findings indicate that even when thermal stress is minimized, global deletion of SCD1 mimics and effectively masks many of the metabolic responses to dietary MR. However, the retention of several key effects of dietary MR in this model indicates that SCD1 is not a mediator of the biological effects of the diet.

Cutting back on the essentials: Can manipulating intake of specific amino acids modulate health and lifespan?

With few exceptions, nutritional and dietary interventions generally impact upon both old-age quality of life and longevity. The life prolonging effects, commonly observed with dietary restriction reportedly are linked to alterations in protein intake and specifically limiting the dietary intake of certain essential amino acids. There is however a paucity of data methodically evaluating the various essential amino acids on health- and lifespan and the mechanisms involved. Rodent diets containing either lower methionine content, or tryptophan, than that found in commercially available chow, appear to elicit beneficial effects. It is unclear whether all of these favorable effects associated with restricted intake of methionine and tryptophan are due to their specific unique properties or if restriction of other essential amino acids, or proteins in general, may produce similar results. Considerably more work remains to be done to elucidate the mechanisms by which limiting these vital molecules may delay the onset of age-associated diseases and improve quality of life at older ages.

Nutrient sensing and metabolic changes after methionine deprivation in primary muscle cells of turbot (Scophthalmus maximus L.)

The low methionine content in plant-based diets is a major limiting factor for feed utilization by animals. However, the molecular consequences triggered by methionine deficiency have not been well characterized, especially in fish species, whose metabolism is unique in many aspects and important for aquaculture industry. In the present study, the primary muscle cells of turbot (Scophthalmus maximus L.) were isolated and treated with or without methionine for 12 h in culture. The responses of nutrient sensing pathways, the proteomic profiling of metabolic processes, and the expressions of key metabolic molecules were systematically examined. Methionine deprivation (MD) suppressed target of rapamycin (TOR) signaling, activated AMP-activated protein kinase (AMPK) and amino acid response (AAR) pathways. Reduced cellular protein synthesis and increased protein degradation by MD led to increased intracellular free amino acid levels and degradations. MD also reduced glycolysis and lipogenesis while stimulated lipolysis, thus resulted in decreased intracellular lipid pool. MD significantly enhanced energy expenditure through stimulated tricarboxylic acid (TCA) cycle and oxidative phosphorylation. Collectively, our results identified a comprehensive set of transcriptional, proteomic, and signaling responses generated by MD and provided the molecular insight into the integration of cell homeostasis and metabolic controls in fish species.

Direct comparison of methionine restriction with leucine restriction on the metabolic health of C57BL/6J mice

The effects of methionine restriction (MR) in rodents are well established; it leads to decreased body and fat mass, improved glucose homeostasis and extended lifespan, despite increased energy intake. Leucine restriction (LR) replicates some, but not all, of these effects of MR. To determine any differences in metabolic effects between MR and LR, this study compared 8 weeks of MR (80% restriction), LR (80% restriction) and control diet in 10-month-old C57BL/6J male mice. Body composition, food intake and glucose homeostasis were measured throughout the study and biochemical analyses of white adipose tissue (WAT) and liver were performed. MR and LR decreased body and fat mass, increased food intake, elevated lipid cycling in WAT and improved whole-body glucose metabolism and hepatic insulin sensitivity in comparison to the control diet. MR produced more substantial effects than LR on body mass and glucose homeostasis and reduced hepatic lipogenic gene expression, which was absent with the LR diet. This could be a result of amino acid-specific pathways in the liver responsible for FGF21 stimulation (causing varied levels of FGF21 induction) and Akt activation. In summary, LR is effective at improving metabolic health; however, MR produces stronger effects, suggesting they activate distinct signalling pathways.

Low protein-induced increases in FGF21 drive UCP1-dependent metabolic but not thermoregulatory endpoints

Dietary protein restriction increases adipose tissue uncoupling protein 1 (UCP1), energy expenditure and food intake, and these effects require the metabolic hormone fibroblast growth factor 21 (FGF21). Here we test whether the induction of energy expenditure during protein restriction requires UCP1, promotes a resistance to cold stress, and is dependent on the concomitant hyperphagia. Wildtype, Ucp1-KO and Fgf21-KO mice were placed on control and low protein (LP) diets to assess changes in energy expenditure, food intake and other metabolic endpoints. Deletion of Ucp1 blocked LP-induced increases in energy expenditure and food intake, and exacerbated LP-induced weight loss. While LP diet increased energy expenditure and Ucp1 expression in an FGF21-dependent manner, neither LP diet nor the deletion of Fgf21 influenced sensitivity to acute cold stress. Finally, LP-induced energy expenditure occurred even in the absence of hyperphagia. Increased energy expenditure is a primary metabolic effect of dietary protein restriction, and requires both UCP1 and FGF21 but is independent of changes in food intake. However, the FGF21-dependent increase in UCP1 and energy expenditure by LP has no effect on the ability to acutely respond to cold stress, suggesting that LP-induced increases in FGF21 impact metabolic but not thermogenic endpoints.

An integrative analysis of tissue-specific transcriptomic and metabolomic responses to short-term dietary methionine restriction in mice

Dietary methionine restriction (MR) produces a coordinated series of transcriptional responses in peripheral tissues that limit fat accretion, remodel lipid metabolism in liver and adipose tissue, and improve overall insulin sensitivity. Hepatic sensing of reduced methionine leads to induction and release of fibroblast growth factor 21 (FGF21), which acts centrally to increase sympathetic tone and activate thermogenesis in adipose tissue. FGF21 also has direct effects in adipose to enhance glucose uptake and oxidation. However, an understanding of how the liver senses and translates reduced dietary methionine into these transcriptional programs remains elusive. A comprehensive systems biology approach integrating transcriptomic and metabolomic readouts in MR-treated mice confirmed that three interconnected mechanisms (fatty acid transport and oxidation, tricarboxylic acid cycle, and oxidative phosphorylation) were activated in MR-treated inguinal adipose tissue. In contrast, the effects of MR in liver involved up-regulation of anti-oxidant responses driven by the nuclear factor, erythroid 2 like 2 transcription factor, NFE2L2. Metabolomic analysis provided evidence for redox imbalance, stemming from large reductions in the master anti-oxidant molecule glutathione coupled with disproportionate increases in ophthalmate and its precursors, glutamate and 2-aminobutyrate. Thus, cysteine and its downstream product, glutathione, emerge as key early hepatic signaling molecules linking dietary MR to its metabolic phenotype.

FGF21 Mediates the Thermogenic and Insulin-Sensitizing Effects of Dietary Methionine Restriction but Not Its Effects on Hepatic Lipid Metabolism

Dietary methionine restriction (MR) produces a rapid and persistent remodeling of white adipose tissue (WAT), an increase in energy expenditure (EE), and enhancement of insulin sensitivity. Recent work established that hepatic expression of FGF21 is robustly increased by MR. Fgf21^-/- mice were used to test whether FGF21 is an essential mediator of the physiological effects of dietary MR. The MR-induced increase in energy intake and EE and activation of thermogenesis in WAT and brown adipose tissue were lost in Fgf21^-/- mice. However, dietary MR produced a comparable reduction in body weight and adiposity in both genotypes because of a negative effect of MR on energy intake in Fgf21^-/- mice. Despite the similar loss in weight, dietary MR produced a more significant increase in in vivo insulin sensitivity in wild-type than in Fgf21^-/- mice, particularly in heart and inguinal WAT. In contrast, the ability of MR to regulate lipogenic and integrated stress response genes in liver was not compromised in Fgf21^-/- mice. Collectively, these findings illustrate that FGF21 is a critical mediator of the effects of dietary MR on EE, remodeling of WAT, and increased insulin sensitivity but not of its effects on hepatic gene expression.

Concentration-dependent linkage of dietary methionine restriction to the components of its metabolic phenotype

OBJECTIVE

Restricting dietary methionine to 0.17% produces a series of physiological responses through coordinated transcriptional effects in liver and adipose tissue. The goal of the present work was to determine the threshold concentrations above and below 0.17% at which the beneficial responses to 0.17% dietary methionine are preserved.

METHODS

Diets were formulated to restrict methionine to different degrees, followed by evaluation of the transcriptional and physiological responses to the different diets.

RESULTS

Restriction of dietary methionine to 0.25%, but not 0.34%, was partially effective in reproducing the metabolic phenotype produced by restriction of methionine to 0.17%, while restriction of methionine to 0.12% reproduced the responses produced by restriction to 0.17% but failed to support growth and caused excessive weight loss. Restriction beyond 0.12% initiated responses characteristic of essential amino acid deprivation including food aversion and rapid weight loss.

CONCLUSIONS

Restriction of dietary methionine to levels above 0.25% was without effect, while restriction to levels below 0.12% produced responses characteristic of essential amino acid deprivation. In addition, although restriction of dietary methionine to 0.12% did not evoke essential amino acid deprivation responses, it provided insufficient methionine to support growth. The ideal range of dietary methionine restriction was from 0.17% to 0.25%.