Cellular and molecular remodeling of inguinal adipose tissue mitochondria by dietary methionine restriction

Sulfur Amino Acid Restriction alters mitochondrial function depending on tissue, sex, and Methionine sulfoxide reductase A (MsrA) status

Methionine restriction (MR) has been shown to affect mitochondrial function including altering oxygen consumption, reactive oxygen species (ROS) generation, Complex expression, and oxidative damage. The sulfur-containing amino acid methionine can become oxidized forming methionine sulfoxide which can lead to changes in protein function and signaling. Methionine sulfoxide reductases are endogenous enzymes capable of reducing methionine sulfoxide, with Methionine sulfoxide reductase A (MsrA) being ubiquitously expressed in mammals. Here we investigated if the effects of MR on mitochondrial function required functional MsrA in the liver and kidney which are the major tissues involved in sulfur biochemistry and both highly express MsrA. Moreover, MsrA is endogenously found in the mitochondria thereby providing potential mechanisms linking diet to mitochondrial phenotype. We found sex-specific changes in oxygen consumption of isolated mitochondria and females showed changes with MR in a tissue-dependent manner - increased in liver and decreased in kidney. Loss of MsrA increased or decreased oxygen consumption depending on the tissue and which portion of the electron transport chain was being tested. In general, males had few changes in either tissue regardless of MR or MsrA status. Hydrogen peroxide production was increased in the kidney with MR regardless of sex or MsrA status. However, in the liver, production was increased by MR in females and only slightly higher with loss of MsrA in both sexes. Mitochondrial Complex expression was found to be largely unchanged in either tissue suggesting these effects are driven by regulatory mechanisms and not by changes in expression. Together these results suggest that sex and MsrA status do impact the mitochondrial effects of MR in a tissue-specific manner.

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.

Sexually dimorphic effects of methionine sulfoxide reductase A (MsrA) on murine longevity and health span during methionine restriction

Methionine restriction (MR) extends lifespan in various model organisms, and understanding the molecular effectors of MR could expand the repertoire of tools targeting the aging process. Here, we address to what extent the biochemical pathway responsible for redox metabolism of methionine plays in regulating the effects of MR on lifespan and health span. Aerobic organisms have evolved methionine sulfoxide reductases to counter the oxidation of the thioether group contained in the essential amino acid methionine. Of these enzymes, methionine sulfoxide reductase A (MsrA) is ubiquitously expressed in mammalian tissues and has subcellular localization in both the cytosol and mitochondria. Loss of MsrA increases sensitivity to oxidative stress and has been associated with increased susceptibility to age-associated pathologies including metabolic dysfunction. We rationalized that limiting the available methionine with MR may place increased importance on methionine redox pathways, and that MsrA may be required to maintain available methionine for its critical uses in cellular homeostasis including protein synthesis, metabolism, and methylation. Using a genetic mutant mouse lacking MsrA, we tested the requirement for this enzyme in the effects of MR on longevity and markers of healthy aging late in life. When initiated in adulthood, we found that MR had minimal effects in males and females regardless of MsrA status. MR had minimal effect on lifespan with the exception of wild-type males where loss of MsrA slightly increased lifespan on MR. We also observed that MR drove an increase in body weight in wild-type mice only, but mice lacking MsrA tended to maintain more stable body weight throughout their lives. We also found that MR had greater benefit to males than females in terms of glucose metabolism and some functional health span assessments, but MsrA generally had minimal impact on these metrics. Frailty was also found to be unaffected by MR or MsrA in aged animals. We found that in general, MsrA was not required for the beneficial effects of MR on longevity and health span.

Effects of dietary methionine restriction on age-related changes in perivascular and beiging adipose tissues in the mouse

OBJECTIVE

Perivascular adipose tissue (PVAT) regulates vascular health. Dietary methionine restriction (MetR) impacts age-related adiposity, and this study addresses its effects in PVAT.

METHODS

Male C57BL/6 mice at 8, 52, and 102 weeks of age were fed a standard (0.86%) or low-methionine (0.12%) diet for 52 weeks in 8-week-old and 52-week-old mice and for 15 weeks in 102-week-old mice.

RESULTS

Mice with dietary MetR were resistant to weight gain and maintained a healthy blood profile. Aging increased lipid accumulation, and MetR reversed this phenotype. Notch signaling in inguinal white adipose tissue (iWAT) was decreased by MetR but increased in gonadal white adipose tissue. However, the Notch phenotype of brown adipose tissue (BAT) was not affected by MetR. Uncoupling protein 1 (UCP1) was increased in PVAT, iWAT, and BAT by MetR when initiated in young mice, but this effect was lost in middle-aged mice.

CONCLUSIONS

Lipid in mouse PVAT peaked at 1 year of age, consistent with peak body mass. MetR reduced body weight, normalized metabolic parameters, and decreased lipid in PVAT in all age cohorts. Mice fed a MetR diet from early maturity to 1 year of age displayed an increased thermogenic adipocyte phenotype in iWAT, PVAT, and BAT, all tissues with thermogenic capacity.

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.

Dietary protein and amino acid restriction: Roles in metabolic health and aging-related diseases

The prevalence of obesity is a worldwide phenomenon in all age groups and is associated with aging-related diseases such as type 2 diabetes, as well metabolic and cardiovascular diseases. The use of dietary restriction (DR) while avoiding malnutrition has many profound beneficial effects on aging and metabolic health, and dietary protein or specific amino acid (AA) restrictions, rather than overall calorie intake, are considered to play key roles in the effects of DR on host health. Whereas comprehensive reviews of the underlying mechanisms are limited, protein restriction and methionine (Met) restriction improve metabolic health and aging-related neurodegenerative diseases, and may be associated with FGF21, mTOR and autophagy, improved mitochondrial function and oxidative stress. Circulating branched-chain amino acids (BCAAs) are inversely correlated with metabolic health, and BCAAs and leucine (Leu) restriction promote metabolic homeostasis in rodents. Although tryptophan (Trp) restriction extends the lifespan of rodents, the Trp-restricted diet is reported to increase inflammation in aged mice, while severe Trp restriction has side effects such as anorexia. Furthermore, inadequate protein intake in the elderly increases the risk of muscle-centric health. Therefore, the restriction of specific AAs may be an effective and executable dietary manipulation for metabolic and aging-related health in humans, which warrants further investigation to elucidate the underlying mechanisms.

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.

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.

High-protein diet more effectively reduces hepatic fat than low-protein diet despite lower autophagy and FGF21 levels

BACKGROUND AND AIMS

Non-alcoholic fatty liver disease (NAFLD) is becoming increasingly prevalent and nutrition intervention remains the most important therapeutic approach for NAFLD. Our aim was to investigate whether low- (LP) or high-protein (HP) diets are more effective in reducing liver fat and reversing NAFLD and which mechanisms are involved.

METHODS

19 participants with morbid obesity undergoing bariatric surgery were randomized into two hypocaloric (1500-1600 kcal/day) diet groups, a low protein (10E% protein) and a high protein (30E% protein), for three weeks prior to surgery. Intrahepatic lipid levels (IHL) and serum fibroblast growth factor 21 (FGF21) were measured before and after the dietary intervention. Autophagy flux, histology, mitochondrial activity and gene expression analyses were performed in liver samples collected during surgery.

RESULTS

IHL levels decreased by 42.6% in the HP group, but were not significantly changed in the LP group despite similar weight loss. Hepatic autophagy flux and serum FGF21 increased by 66.7% and 42.2%, respectively, after 3 weeks in the LP group only. Expression levels of fat uptake and lipid biosynthesis genes were lower in the HP group compared with those in the LP group. RNA-seq analysis revealed lower activity of inflammatory pathways upon HP diet. Hepatic mitochondrial activity and expression of β-oxidation genes did not increase in the HP group.

CONCLUSIONS

HP diet more effectively reduces hepatic fat than LP diet despite of lower autophagy and FGF21. Our data suggest that liver fat reduction upon HP diets result primarily from suppression of fat uptake and lipid biosynthesis.

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.

Weight Loss and Concomitant Adipose Autophagy in Methionine-Restricted Obese Mice is Not Dependent on Adiponectin or FGF21

OBJECTIVE

Identifying novel approaches to combat obesity is important to improve health span. It was hypothesized that methionine restriction (MR) will induce weight loss in obese mice by reducing adipose tissue mass caused by increased energy expenditure and reprogramming of adipose tissue homeostasis. The roles of adiponectin (ADIPOQ) and fibroblast growth factor 21 (FGF21) during weight loss in MR mice were also tested.

METHODS

Diet-induced obese (DIO) male C57BL/6J (wild type), Adipoq-deficient (Adipoq knockout [KO]), Fgf21-KO, and Adipoq-Fgf21 double-KO mice were used. Following a switch to high-fat control (DIO-CF, 60% fat/0.86% methionine) or MR (DIO-MR, 60% fat/0.12% methionine) diet, physiological parameters were measured, and inguinal and perigonadal adipose tissues were examined.

RESULTS

Obese mice subjected to MR showed loss of body weight and adiposity, increased energy expenditure, and improved glucose tolerance that were independent of the actions of ADIPOQ and FGF21. MR induced reduction of circulating lipids, glucose, insulin, leptin, and insulin like growth factor 1 and increased β-hydroxybutyrate, ADIPOQ, and FGF21 concentrations. In fat, MR upregulated protein levels of adipose triglyceride lipase, apoptosis-inducing factor, lysosomal-associated membrane proteins 1 and 2, autophagy-related protein 5, beclin-1, and light chain 3B I and II.

CONCLUSIONS

MR reduction of adipose tissue mass in obese mice is associated with elevated lipolysis, apoptosis, and autophagy and occurs independently of the actions of ADIPOQ and FGF21.

Bone Marrow Adiposity: Basic and Clinical Implications

The presence of adipocytes in mammalian bone marrow (BM) has been recognized histologically for decades, yet, until recently, these cells have received little attention from the research community. Advancements in mouse transgenics and imaging methods, particularly in the last 10 years, have permitted more detailed examinations of marrow adipocytes than ever before and yielded data that show these cells are critical regulators of the BM microenvironment and whole-body metabolism. Indeed, marrow adipocytes are anatomically and functionally separate from brown, beige, and classic white adipocytes. Thus, areas of BM space populated by adipocytes can be considered distinct fat depots and are collectively referred to as marrow adipose tissue (MAT) in this review. In the proceeding text, we focus on the developmental origin and physiologic functions of MAT. We also discuss the signals that cause the accumulation and loss of marrow adipocytes and the ability of these cells to regulate other cell lineages in the BM. Last, we consider roles for MAT in human physiology and disease.

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.

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.

Growth differentiation factor 15 contributes to marrow adipocyte remodeling in response to the growth of leukemic cells

Background

The adipocyte remodeling, including of the morphological change, might indicate special pathological function. Our previous study found that the morphological remodeling of larger marrow adipocytes into small marrow adipocytes correlates with a poor prognosis for acute myeloid leukemia (AML) patients. However, the mechanisms contributed to the marrow adipocyte remodeling are still poorly understood.

Methods

GDF15 expression was analyzed by RT-qPCR and western blotting assays in the leukemic cells. The enhancing and antibody neutralization tests in vitro were employed to evaluate the effect of GDF15 on the morphology of mature adipocytes. CCK8 test was used to detect the proliferation of leukemic cells after co-cultivation with small marrow adipocytes. Flow cytometry was used to analysis the proportion of cell cycle of leukemic cells. Immunofluorescence staining and linear analysis were applied to verify the GDF15 expression and the relationship between GDF15 and small marrow adipocytes in AML patients.

Results

In this study, we found that leukemic cell lines not only expressed significantly higher growth differentiation factor 15 (GDF15) than the other three cytokines associated with adipocyte differentiation in RNA level but also secreted GDF15 factor. Furthermore, the in vitro experiments demonstrated that GDF15 was involved in the conversion of small marrow adipocytes from larger marrow adipocytes. Correspondingly, the leukemic cells proliferated more rapidly through regulating the cell cycle when co-cultured with GDF15-induced small marrow adipocytes. The immunofluorescence staining on the bone marrow sections of AML patients further exhibited that GDF15 was partly produced by leukemic cells. The positive correlation between the concentration of GDF15 in the marrow aspirates and the number and the volume of small marrow adipocytes might suggest the contribution of GDF15 in AML patients (r = 0.72, r = 0.67).

Conclusions

GDF15 secreted by leukemic cells was involved in the morphological remodeling of marrow adipocytes, which can in turn promote leukemic cell growth, indicating that GDF15 may be a promising treatment target for AML patients.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0738-y) contains supplementary material, which is available to authorized users.

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.

Bone marrow adipocytes

Adipocytes were identified in human bone marrow more than a century ago, yet until recently little has been known about their origin, development, function or interactions with other cells in the bone marrow. Little functional significance has been attributed to these cells, a paradigm that still persists today. However, we now know that marrow adipose tissue increases with age and in response to a variety of physiologic induction signals. Bone marrow adipocytes have recently been shown to influence other cell populations within the marrow and can affect whole body metabolism by the secretion of a defined set of adipokines. Recent research shows that marrow adipocytes are distinct from white, brown and beige adipocytes, indicating that the bone marrow is a distinct adipose depot. This review will highlight recent data regarding these areas and the interactions of marrow adipose tissue (MAT) with cells within and outside of the bone marrow.

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.

Role of GCN2-Independent Signaling Through a Noncanonical PERK/NRF2 Pathway in the Physiological Responses to Dietary Methionine Restriction

Restricting availability of essential amino acids (EAAs) limits aminoacylation of tRNAs by their cognate EAAs and activates the nutrient-sensing kinase, general control nonderepressible 2 (GCN2). Activated GCN2 phosphorylates eukaryotic initiation factor 2 (eIF2), altering gene-specific translation and initiating a transcriptional program collectively described as the integrated stress response (ISR). Central GCN2 activation by EAA deprivation is also linked to an acute aversive feeding response. Dietary methionine restriction (MR) produces a well-documented series of physiological responses (increased energy intake and expenditure, decreased adiposity, and increased insulin sensitivity), but the role of GCN2 in mediating them is unknown. Using Gcn2(-/-) mice, we found that the absence of GCN2 had no effect on the ability of MR to reduce body weight or adiposity, increase energy intake and expenditure, increase hepatic transcription and release of fibroblast growth factor 21, or improve insulin sensitivity. Interestingly, hepatic eIF2 phosphorylation by MR was uncompromised in Gcn2(-/-) mice. Instead, protein kinase R-like endoplasmic reticulum (ER) kinase (PERK) was activated in both intact and Gcn2(-/-) mice. PERK activation corresponded with induction of the ISR and the nuclear respiratory factor 2 antioxidant program but not ER stress. These data uncover a novel glutathione-sensing mechanism that functions independently of GCN2 to link dietary MR to its metabolic phenotype.