Role of beta-adrenergic receptors in the hyperphagic and hypermetabolic responses to dietary methionine restriction

History and future of leptin: Discovery, regulation and signaling

The cloning of leptin 30 years ago in 1994 was an important milestone in obesity research. Prior to the discovery of leptin, obesity was stigmatized as a condition caused by lack of character and self-control. Mutations in either leptin or its receptor were the first single gene mutations found to cause severe obesity, and it is now recognized that obesity is caused mostly by a dysregulation of central neuronal circuits. Since the discovery of the leptin-deficient obese mouse (ob/ob) the cloning of leptin (ob aka lep) and leptin receptor (db aka lepr) genes, we have learned much about leptin and its action in the central nervous system. The first hope that leptin would cure obesity was quickly dampened because humans with obesity have increased leptin levels and develop leptin resistance. Nevertheless, leptin target sites in the brain represent an excellent blueprint to understand how neuronal circuits control energy homeostasis. Our expanding understanding of leptin function, interconnection of leptin signaling with other systems and impact on distinct physiological functions continues to guide and improve the development of safe and effective interventions to treat metabolic illnesses. This review highlights past concepts and current emerging concepts of the hormone leptin, leptin receptor signaling pathways and central targets to mediate distinct physiological functions.

Preoperative methionine restriction induces perivascular adipose tissue browning and improves vein graft remodeling in male mice

Short-term preoperative methionine restriction (MetR) is a promising translatable strategy to mitigate surgical injury response. However, its application to improve post-interventional vascular remodeling remains underexplored. Here we find that MetR protects from arterial intimal hyperplasia in a focal stenosis model and pathologic vascular remodeling following vein graft surgery in male mice. RNA sequencing reveals that MetR enhances browning in arterial (thoracic aorta) perivascular adipose tissue (PVAT) and induces it in venous (caval vein) PVAT. Specifically, Ppara is highly upregulated in PVAT-adipocytes upon MetR. Furthermore, MetR dampens the postoperative pro-inflammatory response to surgery in PVAT-macrophages in vivo and in vitro. This study shows that the detrimental effects of dysfunctional PVAT on vascular remodeling can be reversed by MetR, and identifies pathways involved in MetR-induced browning of PVAT. Furthermore, we demonstrate the potential of short-term preoperative MetR as a simple intervention to ameliorate vascular remodeling after vascular surgery.

Cysteine depletion triggers adipose tissue thermogenesis and weight-loss

Dietary interventions such as caloric restriction (CR)1 and methionine restriction2 that prolong lifespan induce the 'browning' of white adipose tissue (WAT), an adaptive metabolic response that increases heat production to maintain health3,4. However, how diet influences adipose browning and metabolic health is unclear. Here, we identified that weight-loss induced by CR in humans5 reduces cysteine concentration in WAT suggesting depletion of this amino-acid may be involved in metabolic benefits of CR. To investigate the role of cysteine on organismal metabolism, we created a cysteine-deficiency mouse model in which dietary cysteine was eliminated and cystathionine γ-lyase (CTH)6, the enzyme that synthesizes cysteine was conditionally deleted. Using this animal model, we found that systemic cysteine-depletion causes drastic weight-loss with increased fat utilization and browning of adipose tissue. The restoration of dietary cysteine in cysteine-deficient mice rescued weight loss together with reversal of adipose browning and increased food-intake in an on-demand fashion. Mechanistically, cysteine deficiency induced browning and weight loss is dependent on sympathetic nervous system derived noradrenaline signaling via β3-adrenergic-receptors and does not require UCP1. Therapeutically, in high-fat diet fed obese mice, one week of cysteine-deficiency caused 30% weight-loss and reversed inflammation. These findings thus establish that cysteine is essential for organismal metabolism as removal of cysteine in the host triggers adipose browning and rapid weight loss.

Long term methionine restriction: Influence on gut microbiome and metabolic characteristics

The Methionine restriction (MR) diet has been shown to delay aging and extend lifespan in various model organisms. However, the long-term effects of MR diet on the gut microbiome composition remain unclear. To study this, male mice were started on MR and control diet regimens at 6 months and continued until 22 months of age. MR mice have reduced body weight, fat mass percentage, and bone mineral density while having increased lean mass percentage. MR mice also have increased insulin sensitivity along with increasing indirect calorimetry markers such as energy expenditure, oxygen consumption, carbon dioxide production, and glucose oxidation. Fecal samples were collected at 1 week, 18 weeks, and 57 weeks after the diet onset for 16S rRNA amplicon sequencing to study the gut microbiome composition. Alpha and beta diversity metrics detected changes occurring due to the timepoint variable, but no changes were detected due to the diet variable. The results from LEfSe analysis surprisingly showed that more bacterial taxa changes were linked to age rather than diet. Interestingly, we found that the long-term MR diet feeding induced smaller changes compared to short-term feeding. Specific taxa changes due to the diet were observed at the 1 or 18-week time points, including Ileibacterium, Odoribacter, Lachnoclostridium, Marinifilaceae, and Lactobacillaceae. Furthermore, there were consistent aging-associated changes across both groups, with an increase in Ileibacterium and Erysipelotrichaceae with age, while Eubacterium_coprostanoligenes_group, Ruminococcaceae, Peptococcaceae, and Peptococcus decreased with age.

Multi-Omics Analysis Reveals Sphingomyelin Accumulation, Glycerolipids Loss, and Disorders of Lipid Metabolism Regulated by Leucine Deprivation in the Liver of Mice

SCOPE

Branched-chain amino acids, especially leucine, have been reported to play a role in regulating lipid metabolism. This study aims to examine the effects of leucine deprivation on hepatic lipid metabolism.

METHODS AND RESULTS

C57BL/6 mice are fed with a chow diet (control group, n = 8) or a leucine-free diet (-Leu group, n = 8) for 7 days. Histology, lipidomics, targeted metabolomics, and transcriptomics are performed to analyze the liver tissue. Compared to control group, -Leu group exhibits a notably reduced liver weight, accompanied by hepatic injury, and disorders of lipid metabolism. The level of sphingomyelin (SM) is significantly increased in the liver of -Leu group, while the glycerolipids (GL) level is significantly decreased. The expression of sphingomyelin synthase 1 (SGMS1) is upregulated by leucine deprivation in a time-dependent manner, leading to hepatic SM accumulation. Moreover, leucine deprivation results in hepatic GL loss via suppressing fatty acid synthase (FASN) and acetyl-CoA carboxylase 1 (ACC1) expression.

CONCLUSION

The findings demonstrate that leucine deprivation results in abnormal lipid metabolism in the liver, mainly manifested as SM accumulation and GL loss. These results provide insights into the role of leucine in regulating lipid metabolism.

[WITHDRAWN] Activation of transsulfuration pathway to maintain cysteine is a thermogenic checkpoint for the conservation of energy

The authors have requested that this preprint be removed from Research Square.

Chowing down: diet considerations in rodent models of metabolic disease

Diet plays a substantial role in the etiology, progression, and treatment of chronic disease and is best considered as a multifaceted set of modifiable input variables with pleiotropic effects on a variety of biological pathways spanning multiple organ systems. This brief review discusses key issues related to the design and conduct of diet interventions in rodent models of metabolic disease and their implications for interpreting experiments. We also make specific recommendations to improve rodent diet studies to help better understand the role of diet on metabolic physiology and thereby improve our understanding of metabolic disease.

Methionine-Restricted Diet: A Feasible Strategy Against Chronic or Aging-Related Diseases

Dietary methionine restriction (MR) has been associated with multifaceted health-promoting effects. MR is conducive to prevention of several chronic diseases and cancer, and extension of lifespan. A growing number of studies on new phenotypes and mechanisms of MR have become available in the past five years, especially in angiogenesis, neurodegenerative diseases, intestinal microbiota, and intestinal barrier function. In this review, we summarize the characteristics and advantages of MR, and current knowledge on the physiological responses and effects of MR on chronic diseases and aging-associated pathologies. Potential mechanisms, in which hydrogen sulfide, fibroblast growth factor 21, gut microbiota, short-chain fatty acids, and so on are involved, are discussed. Moreover, directions for epigenetics and gut microbiota in an MR diet are presented in future perspectives. This review comprehensively summarizes the novel roles and interpretations of the mechanisms underlying MR in the prevention of chronic diseases and aging.

Dietary Methionine and Total Sulfur Amino Acid Restriction in Healthy Adults

OBJECTIVES

Dietary restriction of methionine (Met) and cysteine (Cys) delays the aging process and aging-related diseases, improves glucose and fat metabolism and reduces oxidative stress in numerous laboratory animal models. Little is known regarding the effects of sulfur amino acid restriction in humans. Thus, our objectives were to determine the impact of feeding diets restricted in Met alone (MetR) or in both Met and Cys (total sulfur amino acids, SAAR) to healthy adults on relevant biomarkers of cardiometabolic disease risk.

DESIGN

A controlled feeding study.

SETTING AND PARTICIPANTS

We included 20 healthy adults (11 females/9 males) assigned to MetR or SAAR diet groups consisting of three 4-wk feeding periods: Control period; low level restriction period (70% MetR or 50% SAAR); and high level restriction period (90% MetR or 65% SAAR) separated by 3-4-wk washout periods.

RESULTS

No adverse effects were associated with either diet and level of restriction and compliance was high in all subjects. SAAR was associated with significant reductions in body weight and plasma levels of total cholesterol, LDL, uric acid, leptin, and insulin, BUN, and IGF-1, and increases in body temperature and plasma FGF-21 after 4 weeks (P<0.05). Fewer changes occurred with MetR including significant reductions in BUN, uric acid and 8-isoprostane and an increase in FGF-21 after 4 weeks (P<0.05). In the 65% SAAR group, plasma Met and Cys levels were significantly reduced by 15% and 13% respectively (P<0.05).

CONCLUSION

These results suggest that many of the short-term beneficial effects of SAAR observed in animal models are translatable to humans and support further clinical development of this intervention.

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.

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.

Age and Sex: Impact on adipose tissue metabolism and inflammation

Age associated chronic inflammation is a major contributor to diseases with advancing age. Adipose tissue function is at the nexus of processes contributing to age-related metabolic disease and mediating longevity. Hormonal fluctuations in aging potentially regulate age-associated visceral adiposity and metabolic dysfunction. Visceral adiposity in aging is linked to aberrant adipogenesis, insulin resistance, lipotoxicity and altered adipokine secretion. Age-related inflammatory phenomena depict sex differences in macrophage polarization, changes in T and B cell numbers, and types of dendritic cells. Sex differences are also observed in adipose tissue remodeling and cellular senescence suggesting a role for sex steroid hormones in the regulation of the adipose tissue microenvironment. It is crucial to investigate sex differences in aging clinical outcomes to identify and better understand physiology in at-risk individuals. Early interventions aimed at targets involved in adipose tissue adipogenesis, remodeling and inflammation in aging could facilitate a profound impact on health span and overcome age-related functional decline.

A planetary health perspective on synthetic methionine

Methionine is an amino acid that humans and farm animals must derive from food. This metabolite, a tightly regulated resource in ecosystems, has become a mass commodity in the global economy, with well over 1 million tons being produced annually from petroleum to fortify livestock feed. Viewed from the standpoint of planetary health, anthropogenic methionine synthesis is an important enabler of low-cost animal protein production, with interdependent but unexamined effects on human health and ecosystems. At a time when agrochemical engineering is shifting the way sulphur is assimilated and moves up our food chain, research suggests that dietary methionine restriction alone captures many healthspan benefits noted with calorie restriction. As such, methionine synthesis is an excellent exemplar of planetary scale anthropogenic activity that manifests at the molecular scale of cellular metabolism, with potential systemic effects on human health. In this Personal View we establish the scale and historical trajectory of the methionine industry and provide a preliminary model for tracing this amino acid through the food supply into the human body. We draw together insights across disparate publications of applied animal agriculture, human nutrition, and biomedical research to call for cross-disciplinary dialogue on responsible use of methionine-augmentation technologies.

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.

Effects of Weight Loss and Moderate-Protein, High-Fiber Diet Consumption on the Fasted Serum Metabolome of Cats

Feline obesity elicits a plethora of metabolic responses leading to comorbidities, with potential reversal during weight loss. The specific metabolic alterations and biomarkers of organ dysfunction are not entirely understood. Untargeted, high-throughput metabolomic technologies may allow the identification of biological components that change with weight status in cats, increasing our understanding of feline metabolism. The objective of this study was to utilize untargeted metabolomic techniques to identify biomarkers and gain mechanistic insight into the serum metabolite changes associated with reduced food intake and weight loss in overweight cats. During a four-wk baseline period, cats were fed to maintain body weight. For 18 wk following baseline, cats were fed to lose weight at a rate of ~1.5% body weight/wk. Blood serum metabolites were measured at wk 0, 1, 2, 4, 8, 12, and 16. A total of 535 named metabolites were identified, with up to 269 of them being altered (p- and q-values < 0.05) at any time point. A principal component analysis showed a continual shift in metabolite profile as weight loss progressed, with early changes being distinct from those over the long term. The majority of lipid metabolites decreased with weight loss; however, ketone bodies and small lipid particles increased with weight loss. The majority of carbohydrate metabolites decreased with weight loss. Protein metabolites had a variable result, with some increasing, but others decreasing with weight loss. Metabolic mediators of inflammation, oxidative stress, xenobiotics, and insulin resistance decreased with weight loss. In conclusion, global metabolomics identified biomarkers of reduced food intake and weight loss in cats, including decreased markers of inflammation and/or altered macronutrient metabolism.

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.

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.

Effect of Methionine Restriction on Aging: Its Relationship to Oxidative Stress

Enhanced oxidative stress is closely related to aging and impaired metabolic health and is influenced by diet-derived nutrients and energy. Recent studies have shown that methionine restriction (MetR) is related to longevity and metabolic health in organisms from yeast to rodents. The effect of MetR on lifespan extension and metabolic health is mediated partially through a reduction in oxidative stress. Methionine metabolism is involved in the supply of methyl donors such as S-adenosyl-methionine (SAM), glutathione synthesis and polyamine metabolism. SAM, a methionine metabolite, activates mechanistic target of rapamycin complex 1 and suppresses autophagy; therefore, MetR can induce autophagy. In the process of glutathione synthesis in methionine metabolism, hydrogen sulfide (H₂S) is produced through cystathionine-β-synthase and cystathionine-γ-lyase; however, MetR can induce increased H₂S production through this pathway. Similarly, MetR can increase the production of polyamines such as spermidine, which are involved in autophagy. In addition, MetR decreases oxidative stress by inhibiting reactive oxygen species production in mitochondria. Thus, MetR can attenuate oxidative stress through multiple mechanisms, consequently associating with lifespan extension and metabolic health. In this review, we summarize the current understanding of the effects of MetR on lifespan extension and metabolic health, focusing on the reduction in oxidative stress.

Still Living Better through Chemistry: An Update on Caloric Restriction and Caloric Restriction Mimetics as Tools to Promote Health and Lifespan

Caloric restriction (CR), the reduction of caloric intake without inducing malnutrition, is the most reproducible method of extending health and lifespan across numerous organisms, including humans. However, with nearly one-third of the world’s population overweight, it is obvious that caloric restriction approaches are difficult for individuals to achieve. Therefore, identifying compounds that mimic CR is desirable to promote longer, healthier lifespans without the rigors of restricting diet. Many compounds, such as rapamycin (and its derivatives), metformin, or other naturally occurring products in our diets (nutraceuticals), induce CR-like states in laboratory models. An alternative to CR is the removal of specific elements (such as individual amino acids) from the diet. Despite our increasing knowledge of the multitude of CR approaches and CR mimetics, the extent to which these strategies overlap mechanistically remains unclear. Here we provide an update of CR and CR mimetic research, summarizing mechanisms by which these strategies influence genome function required to treat age-related pathologies and identify the molecular fountain of youth.

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.

Recent advances in understanding the role of leptin in energy homeostasis

The hormone leptin plays a critical role in energy homeostasis, although our overall understanding of acutely changing leptin levels still needs improvement. Several developments allow a fresh look at recent and early data on leptin action. This review highlights select recent publications that are relevant for understanding the role played by dynamic changes in circulating leptin levels. We further discuss the relevance for our current understanding of leptin signaling in central neuronal feeding and energy expenditure circuits and highlight cohesive and discrepant findings that need to be addressed in future studies to understand how leptin couples with physiological adaptations of food intake and energy expenditure.

Methionine abrogates the renoprotective effect of a low-protein diet against diabetic kidney disease in obese rats with type 2 diabetes

Dietary interventions, including a low-protein diet (LPD) and methionine (Met) restriction, have shown longevity, anti-aging and metabolic health effects. We previously reported that the LPD has a renoprotective effect against diabetic kidney disease (DKD) in rats with type 2 diabetes and obesity. However, it is unclear whether the beneficial effect of the LPD is mediated by low-Met intake or how Met is related to the pathogenesis for DKD. We herein show that the addition of Met with the LPD abrogates the beneficial effects induced by the LPD such as anti-oxidative stress, anti-inflammation and anti-fibrosis, in diabetic kidney. Additionally, the increased levels of S-adenosylmethionine (SAM) in renal tubular cells, which are associated with the reduced expression of glycine N-methyltransferase (Gnmt) and non-restricted Met intake, contributes to the activation of mechanistic target of rapamycin complex 1 (mTORC1) and impaired autophagy, in diabetic kidney. Moreover, starvation-induced autophagy was suppressed in renal cortex of Gnmt null mice and amino acid free-induced autophagy was also suppressed by administration of SAM in cultured HK-2 cells. A LPD could exert a renoprotective effect through the suppression of mTORC1 and restoration of autophagy, which is associated with reduced levels of SAM due to low-Met intake, in diabetic kidney.

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 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.

Nutrient Sensing and Redox Balance: GCN2 as a New Integrator in Aging

Aging is a complex process in which the accumulation of molecular, cellular, and organism dysfunction increases the probability of death. Several pieces of evidence have revealed a contribution of stress responses in aging and in aging-related diseases, in particular, the key role of signaling pathways associated to nutritional stress. Here, we review the possible interplay between amino acid sensing and redox balance maintenance mediated by the nutritional sensor general control nonderepressive 2 (GCN2). We discuss this new dimension of nutritional stress sensing consequences, standing out GCN2 as a central coordinator of key cellular processes that assure healthy homeostasis in the cell, raising GCN2 as a novel interesting target, that when activated, could imply pleiotropic benefits, particularly GCN2 intervention and its new unexplored therapeutic role as a player in the aging process.

The impact of dietary protein intake on longevity and metabolic health

Lifespan and metabolic health are influenced by dietary nutrients. Recent studies show that a reduced protein intake or low-protein/high-carbohydrate diet plays a critical role in longevity/metabolic health. Additionally, specific amino acids (AAs), including methionine or branched-chain AAs (BCAAs), are associated with the regulation of lifespan/ageing and metabolism through multiple mechanisms. Therefore, methionine or BCAAs restriction may lead to the benefits on longevity/metabolic health. Moreover, epidemiological studies show that a high intake of animal protein, particularly red meat, which contains high levels of methionine and BCAAs, may be related to the promotion of age-related diseases. Therefore, a low animal protein diet, particularly a diet low in red meat, may provide health benefits. However, malnutrition, including sarcopenia/frailty due to inadequate protein intake, is harmful to longevity/metabolic health. Therefore, further study is necessary to elucidate the specific restriction levels of individual AAs that are most effective for longevity/metabolic health in humans.

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.

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.

Dietary R, S-1,3-butanediol diacetoacetate reduces body weight and adiposity in obese mice fed a high-fat diet

The dietary R-3-hydroxybutyrate- R-1,3-butanediol monoester increases resting energy expenditure (REE) and markers of brown and white adipose thermogenesis in lean mice. The purpose of this investigation was to determine whether the ketone ester, R, S-1,3-butanediol diacetoacetate (BD-AcAc2), increases energy expenditure and markers of adipose tissue thermogenesis in the context of high-fat diet (HFD)-induced obesity. Thirty-five-week-old male C57BL/6J mice were placed on an ad libitum HFD (45% kcal) for 10 wk. The mice were then randomized to 1 of 3 groups ( n = 10 per group) for an additional 12 wk: 1) control (Con), continuous HFD, 2) pair-fed (PF) to ketone ester (KE); and 3) KE: HFD+30% energy from BD-AcAc2. Mean energy intake throughout the study was ∼26% lower in the KE compared to the Con group (8.2 ± 0.5 vs. 11.2 ± 0.7 kcal/d; P < 0.05). Final body weight (26.8 ± 3.6 vs. 34.9 ± 4.8 g; P < 0.001) and fat mass (5.2 ± 1.2 vs. 11.3 ± 4.5 g; P < 0.001) of the KE group was significantly lower than PF, despite being matched for energy provisions. Differences in body weight and adiposity were accompanied by higher REE and total energy expenditure in the KE group compared to PF after adjustment for lean body mass and fat-mass ( P = 0.001 and 0.007, respectively). Coupled or uncoupled mitochondrial respiratory rates in skeletal muscle were not different among groups, but markers of mitochondrial uncoupling and thermogenesis (uncoupling protein-1, deiodinase-2, and peroxisome proliferator-activated receptor γ coactivator-1α) were higher in interscapular brown adipose tissue (BAT) of mice receiving the KE diet. The absence of mitochondrial uncoupling in skeletal muscle and increased markers of mitochondrial uncoupling in BAT suggest that BD-AcAc2 initiates a transcriptional signature consistent with BAT thermogenesis in the context of HFD-induced obesity.-Davis, R. A. H., Deemer, S. E., Bergeron, J. M., Little, J. T., Warren, J. L., Fisher, G., Smith, D. L., Jr., Fontaine, K. R., Dickinson, S. L., Allison, D. B., Plaisance, E. P. Dietary R, S-1,3-butanediol diacetoacetate reduces body weight and adiposity in obese mice fed a high-fat diet.

Methionine Restriction and Life-Span Extension

It has been known for almost a century that caloric restriction can extend the life span of rodents and many other types of animals. Approximately 25 years ago, it was found that a methionine-restricted (MR) diet could replace a caloric-restricted diet with the result of extending the life span of animals. This chapter summarizes the effects of MR on the reversal of diabetes, obesity, and other aspects of the metabolic syndrome, as well as extending the normal life span. The most effective way to restrict methionine in the body, using orally-delivered methioninase, is also explored.

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.

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.

Browning of white adipose tissue induced by the ß3 agonist CL-316,243 after local and systemic treatment - PK-PD relationship

Transformation of white adipose tissue (WAT) to a brown adipose tissue-like (BAT-like) phenotype has emerged as an attractive approach against obesity e.g. using g ß3 adrenergic receptor agonists. These could however, produce side-effects following systemic exposure. The present study explored the possibility of local use of CL-316,243 - a selective ß3 agonist - to circumvent this problem. Rats treated s.c. for 2 weeks (0.3 and 1 mg/kg) showed decreased inguinal fat pad (IFP) weight/volume, increased UCP-1 staining and expressed BAT-like features in H&E stained micrographs. Interscapular BAT increased in weight/volume. In contrast, local treatment into the IFP was not efficacious in terms of weight/volume, despite slight increases in UCP-1 staining and changes in histological features. After local treatment, the exposure of the IFP was lower than after systemic treatment. In turn higher local doses (0.5 and 5 mg/ml) were then tested which produced a strong trend for decreased volume of the IFP, a significant increase in UCP-1 staining, and also a decrease in adipocytes size but increased number. However, after this treatment the systemic exposure was in the same range as following systemic treatment. In conclusion, we saw no evidence for the possibility of converting inguinal WAT to a BAT-phenotype solely through local activation of ß3 receptors. This is in concert with our in vitro experiments which detected direct effects of PPARγ agonists at the gene/protein expression and functional level, but were unable to detect any effect of CL-316,243.

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.

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 Tryptophan Restriction Dose-Dependently Modulates Energy Balance, Gut Hormones, and Microbiota in Obesity-Prone Rats

OBJECTIVE

To determine the effects of graded dietary restriction of tryptophan on food intake, energy expenditure, body composition, gut hormones, and select fecal bacterial populations in obesity-prone rats.

METHODS

Obesity-prone rats were randomized to isocaloric diets with varying degrees of tryptophan restriction: control (100% requirements), 70% tryptophan (70TRP), 40% tryptophan (40TRP), or 10% tryptophan (10TRP) for 21 days. The sympathetic system was challenged with a subcutaneous injection of propranolol on days 15 to 17. Measurements included food intake, energy expenditure, body composition, metabolic hormones, and fecal concentrations of select bacteria.

RESULTS

Moderate tryptophan restriction (70TRP) induced thermogenesis without altering body composition, whereas severe degrees of restriction (40TRP, 10TRP) produced profound hypophagia and decreased energy expenditure and body weight. The thermogenic effects of moderate tryptophan restriction were sympathetically mediated. Severe tryptophan restriction decreased fasting circulating concentrations of glucose, insulin, C-peptide, and leptin, but increased glucagon, pancreatic polypeptide, and glucagon-like peptide-1. Severe tryptophan restriction decreased fecal concentrations of Enterobacteriaceae, Lactobacillus, Bacteroides, and Clostridium coccoides while increasing Roseburia groups.

CONCLUSIONS

Our findings demonstrate that dietary tryptophan restriction dose-dependently modulates energy balance, with severe restriction causing hypophagia and weight loss and moderate restriction promoting sympathetically driven thermogenesis as well as concurrent changes in gut microbiota and hormones.

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.

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.

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.

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%.

Browning of white adipose tissue: lessons from experimental models

Beige or brite (brown-in-white) adipocytes are present in white adipose tissue (WAT) and have a white fat-like phenotype that when stimulated acquires a brown fat-like phenotype, leading to increased thermogenesis. This phenomenon is known as browning and is more likely to occur in subcutaneous fat depots. Browning involves the expression of many transcription factors, such as PR domain containing 16 (PRDM16) and peroxisome proliferator-activated receptor (PPAR)-γ, and of uncoupling protein (UCP)-1, which is the hallmark of thermogenesis. Recent papers pointed that browning can occur in the WAT of humans, with beneficial metabolic effects. This fact indicates that these cells can be targeted to treat a range of diseases, with both pharmacological and nutritional activators. Pharmacological approaches to induce browning include the use of PPAR-α agonist, adrenergic receptor stimulation, thyroid hormone administration, irisin and FGF21 induction. Most of them act through the induction of PPAR-γ coactivator (PGC) 1-α and the consequent mitochondrial biogenesis and UCP1 induction. About the nutritional inducers, several compounds have been described with multiple mechanisms of action. Some of these activators include specific amino acids restriction, capsaicin, bile acids, Resveratrol, and retinoic acid. Besides that, some classes of lipids, as well as many plant extracts, have also been implicated in the browning of WAT. In conclusion, the discovery of browning in human WAT opens the possibility to target the adipose tissue to fight a range of diseases. Studies have arisen showing promising results and bringing new opportunities in thermogenesis and obesity control.

Pleiotropic responses to methionine restriction

Methionine restriction (MR) extends lifespan across different species. The main responses of rodent models to MR are well-documented in adipose tissue (AT) and liver, which have reduced mass and improved insulin sensitivity, respectively. Recently, molecular mechanisms that improve healthspan have been identified in both organs during MR. In fat, MR induced a futile lipid cycle concomitant with beige AT accumulation, producing elevated energy expenditure. In liver, MR upregulated fibroblast growth factor 21 and improved glucose metabolism in aged mice and in response to a high-fat diet. Furthermore, MR also reduces mitochondrial oxidative stress in various organs such as liver, heart, kidneys, and brain. Other effects of MR have also been reported in such areas as cardiac function in response to hyperhomocysteinemia (HHcy), identification of molecular mechanisms in bone development, and enhanced epithelial tight junction. In addition, rodent models of cancer responded positively to MR, as has been reported in colon, prostate, and breast cancer studies. The beneficial effects of MR have also been documented in a number of invertebrate model organisms, including yeast, nematodes, and fruit flies. MR not only promotes extended longevity in these organisms, but in the case of yeast has also been shown to improve stress tolerance. In addition, expression analyses of yeast and Drosophila undergoing MR have identified multiple candidate mediators of the beneficial effects of MR in these models. In this review, we emphasize other in vivo effects of MR such as in cardiovascular function, bone development, epithelial tight junction, and cancer. We also discuss the effects of MR in invertebrates.