Impaired mitochondrial fatty acid oxidation and insulin resistance in aging: novel protective role of glutathione

An Integrated Understanding of the Rapid Metabolic Benefits of a Carbohydrate-Restricted Diet on Hepatic Steatosis in Humans

A carbohydrate-restricted diet is a widely recommended intervention for non-alcoholic fatty liver disease (NAFLD), but a systematic perspective on the multiple benefits of this diet is lacking. Here, we performed a short-term intervention with an isocaloric low-carbohydrate diet with increased protein content in obese subjects with NAFLD and characterized the resulting alterations in metabolism and the gut microbiota using a multi-omics approach. We observed rapid and dramatic reductions of liver fat and other cardiometabolic risk factors paralleled by (1) marked decreases in hepatic de novo lipogenesis; (2) large increases in serum β-hydroxybutyrate concentrations, reflecting increased mitochondrial β-oxidation; and (3) rapid increases in folate-producing Streptococcus and serum folate concentrations. Liver transcriptomic analysis on biopsy samples from a second cohort revealed downregulation of the fatty acid synthesis pathway and upregulation of folate-mediated one-carbon metabolism and fatty acid oxidation pathways. Our results highlight the potential of exploring diet-microbiota interactions for treating NAFLD.

Investigation of the urinary metabolic variations and the application in bladder cancer biomarker discovery

Urine metabolomics have been used to identify biomarkers for clinical diseases. However, inter-individual variations and effect factors need to be further evaluated. In our study, we explored the urine metabolome in a cohort of 203 health adults, 6 patients with benign bladder lesions, and 53 patients with bladder cancer (BCa) using liquid chromatography coupled with high resolution mass spectrometry. Inter-individual analysis of both healthy controls and BCa patients showed that the urine metabolome was relatively stable. Further analysis indicated that sex and age affect inter-individual variations in urine metabolome. Metabolic pathways such as tryptophan metabolism, the citrate cycle, and pantothenate and CoA biosynthesis were found to be related to sex and age. To eliminate age and sex interference, additional BCa urine metabolomic biomarkers were explored using age and sex-matched urine samples (Test group: 44 health adults vs. 33 patients with BCa). Metabolic profiling of urine could significantly differentiate the cases with cancer from the controls and high-grade from low-grade BCa. A metabolite panel consisting of trans-2-dodecenoylcarnitine, serinyl-valine, feruloyl-2-hydroxyputrescine, and 3-hydroxynonanoyl carnitine were discovered to have good predictive ability for BCa with an area under the curve (AUC) of 0.956 (cross validation: AUC = 0.924). A panel of indolylacryloylglycine, N₂ -galacturonyl-L-lysine, and aspartyl-glutamate was used to establish a robust model for high- and low-grade BCa distinction with AUC of 0.937 (cross validation: AUC = 0.891). External sample (26 control vs. 20 BCa) validation verified the acceptable accuracy of these models for BCa detection. Our study showed that urinary metabolomics is a useful strategy for differential analysis and biomarker discovery.

Glycine Increases Insulin Sensitivity and Glutathione Biosynthesis and Protects against Oxidative Stress in a Model of Sucrose-Induced Insulin Resistance

Oxidative stress and redox status play a central role in the link between insulin resistance (IR) and lipotoxicity in metabolic syndrome. This mechanistic link may involve alterations in the glutathione redox state. We examined the effect of glycine supplementation to diet on glutathione biosynthesis, oxidative stress, IR, and insulin cell signaling in liver from sucrose-fed (SF) rats characterized by IR and oxidative stress. Our hypothesis is that the correction of glutathione levels by glycine treatment leads to reduced oxidative stress, a mechanism associated with improved insulin signaling and IR. Glycine treatment decreases the levels of oxidative stress markers in liver from SF rats and increases the concentrations of glutathione (GSH) and γ-glutamylcysteine and the amount of γ-glutamylcysteine synthetase (γ-GCS), a key enzyme of GSH biosynthesis in liver from SF rats. In liver from SF rats, glycine also decreases the insulin-induced phosphorylation of insulin receptor substrate-1 (ISR-1) in serine residue and increases the phosphorylation of insulin receptor β-subunit (IR-β) in tyrosine residue. Thus, supplementing diets with glycine to correct GSH deficiency and to reduce oxidative stress provides significant metabolic benefits to SF rats by improving insulin sensitivity.

Carnitine palmitoyltransferase 1C regulates cancer cell senescence through mitochondria-associated metabolic reprograming

Cellular senescence is a fundamental biological process that has profound implications in cancer development and therapeutics, but the underlying mechanisms remain elusive. Here we show that carnitine palmitoyltransferase 1C (CPT1C), an enzyme that catalyzes carnitinylation of fatty acids for transport into mitochondria for β-oxidation, plays a major role in the regulation of cancer cell senescence through mitochondria-associated metabolic reprograming. Metabolomics analysis suggested alterations in mitochondria activity, as revealed by the marked decrease in acylcarnitines in senescent human pancreatic carcinoma PANC-1 cells, indicating low CPT1C activity. Direct analyses of mRNA and protein show that CPT1C is significantly reduced in senescent cells. Furthermore, abnormal mitochondrial function was observed in senescent PANC-1 cells, leading to lower cell survival under metabolic stress and suppressed tumorigenesis in a mouse xenograft model. Knock-down of CPT1C in PANC-1 cells induced mitochondrial dysfunction, caused senescence-like growth suppression and cellular senescence, suppressed cell survival under metabolic stress, and inhibited tumorigenesis in vivo. Further, CPT1C knock-down suppressed xenograft tumor growth in situ. Silencing of CPT1C in five other tumor cell lines also caused cellular senescence. On the contrary, gain-of-function of CPT1C reversed PANC-1 cell senescence and enhanced mitochondrial function. This study identifies CPT1C as a novel biomarker and key regulator of cancer cell senescence through mitochondria-associated metabolic reprograming, and suggests that inhibition of CPT1C may represent a new therapeutic strategy for cancer treatment through induction of tumor senescence.

Pathophysiology of mitochondrial lipid oxidation: Role of 4-hydroxynonenal (4-HNE) and other bioactive lipids in mitochondria

Mitochondrial lipids are essential for maintaining the integrity of mitochondrial membranes and the proper functions of mitochondria. As the "powerhouse" of a cell, mitochondria are also the major cellular source of reactive oxygen species (ROS). Oxidative stress occurs when the antioxidant system is overwhelmed by overproduction of ROS. Polyunsaturated fatty acids in mitochondrial membranes are primary targets for ROS attack, which may lead to lipid peroxidation (LPO) and generation of reactive lipids, such as 4-hydroxynonenal. When mitochondrial lipids are oxidized, the integrity and function of mitochondria may be compromised and this may eventually lead to mitochondrial dysfunction, which has been associated with many human diseases including cancer, cardiovascular diseases, diabetes, and neurodegenerative diseases. How mitochondrial lipids are oxidized and the underlying molecular mechanisms and pathophysiological consequences associated with mitochondrial LPO remain poorly defined. Oxidation of the mitochondria-specific phospholipid cardiolipin and generation of bioactive lipids through mitochondrial LPO has been increasingly recognized as an important event orchestrating apoptosis, metabolic reprogramming of energy production, mitophagy, and immune responses. In this review, we focus on the current understanding of how mitochondrial LPO and generation of bioactive lipid mediators in mitochondria are involved in the modulation of mitochondrial functions in the context of relevant human diseases associated with oxidative stress.

PARP1-mediated PPARα poly(ADP-ribosyl)ation suppresses fatty acid oxidation in non-alcoholic fatty liver disease

BACKGROUND & AIMS

PARP1 is a key mediator of cellular stress responses and critical in multiple physiological and pathophysiological processes of cells. However, whether it is involved in the pathogenesis of non-alcoholic fatty liver disease (NAFLD) remains elusive.

METHODS

We analysed PARP1 activity in the liver of mice on a high fat diet (HFD), and samples from NAFLD patients. Gain- or loss-of-function approaches were used to investigate the roles and mechanisms of hepatic PARP1 in the pathogenesis of NAFLD.

RESULTS

PARP1 is activated in fatty liver of HFD-fed mice. Pharmacological or genetic manipulations of PARP1 are sufficient to alter the HFD-induced hepatic steatosis and inflammation. Mechanistically we identified peroxisome proliferator-activated receptor α (PPARα) as a substrate of PARP1-mediated poly(ADP-ribosyl)ation. This poly(ADP-ribosyl)ation of PPARα inhibits its recruitment to target gene promoters and its interaction with SIRT1, a key regulator of PPARα signaling, resulting in suppression of fatty acid oxidation upregulation induced by fatty acids. Moreover, we show that PARP1 is a transcriptional repressor of PPARα gene in human hepatocytes, and its activation suppresses the ligand (fenofibrate)-induced PPARα transactivation and target gene expression. Importantly we demonstrate that liver biopsies of NAFLD patients display robust increases in PARP activity and PPARα poly(ADP-ribosyl)ation levels.

CONCLUSIONS

Our data indicate that PARP1 is activated in fatty liver, which prevents maximal activation of fatty acid oxidation by suppressing PPARα signaling. Pharmacological inhibition of PARP1 may alleviate PPARα suppression and therefore have therapeutic potential for NAFLD.

LAY SUMMARY

PARP1 is activated in the non-alcoholic fatty liver of mice and patients. Inhibition of PARP1 activation alleviates lipid accumulation and inflammation in fatty liver of mice.

Aging, the Medical Subspecialties, and Career Development: Where We Were, Where We Are Going

Historically, the medical subspecialties have not focused on the needs of older adults. This has changed with the implementation of initiatives to integrate geriatrics and aging research into the medical and surgical subspecialties and with the establishment of a home for internal medicine specialists within the annual American Geriatrics Society (AGS) meeting. With the support of AGS, other professional societies, philanthropies, and federal agencies, efforts to integrate geriatrics into the medical and surgical subspecialties have focused largely on training the next generation of physicians and researchers. They have engaged several subspecialties, which have followed parallel paths in integrating geriatrics and aging research. As a result of these combined efforts, there has been enormous progress in the integration of geriatrics and aging research into the medical and surgical subspecialties, and topics once considered to be geriatric concerns are becoming mainstream in medicine, but this integration remains a work in progress and will need to adapt to changes associated with healthcare reform.

Can heavy isotopes increase lifespan? Studies of relative abundance in various organisms reveal chemical perspectives on aging

Stable heavy isotopes co-exist with their lighter counterparts in all elements commonly found in biology. These heavy isotopes represent a low natural abundance in isotopic composition but impose great retardation effects in chemical reactions because of kinetic isotopic effects (KIEs). Previous isotope analyses have recorded pervasive enrichment or depletion of heavy isotopes in various organisms, strongly supporting the capability of biological systems to distinguish different isotopes. This capability has recently been found to lead to general decline of heavy isotopes in metabolites during yeast aging. Conversely, supplementing heavy isotopes in growth medium promotes longevity. Whether this observation prevails in other organisms is not known, but it potentially bears promise in promoting human longevity.

Protein Requirements during Aging

Protein recommendations for elderly, both men and women, are based on nitrogen balance studies. They are set at 0.66 and 0.8 g/kg/day as the estimated average requirement (EAR) and recommended dietary allowance (RDA), respectively, similar to young adults. This recommendation is based on single linear regression of available nitrogen balance data obtained at test protein intakes close to or below zero balance. Using the indicator amino acid oxidation (IAAO) method, we estimated the protein requirement in young adults and in both elderly men and women to be 0.9 and 1.2 g/kg/day as the EAR and RDA, respectively. This suggests that there is no difference in requirement on a gender basis or on a per kg body weight basis between younger and older adults. The requirement estimates however are ~40% higher than the current protein recommendations on a body weight basis. They are also 40% higher than our estimates in young men when calculated on the basis of fat free mass. Thus, current recommendations may need to be re-assessed. Potential rationale for this difference includes a decreased sensitivity to dietary amino acids and increased insulin resistance in the elderly compared with younger individuals.

Metabolomics in diabetes, a review

Metabolomics is a promising approach for the identification of chemical compounds that serve for early detection, diagnosis, prediction of therapeutic response and prognosis of disease. Moreover, metabolomics has shown to increase the diagnostic threshold and prediction of type 2 diabetes. Evidence suggests that branched-chain amino acids, acylcarnitines and aromatic amino acids may play an early role on insulin resistance, exposing defects on amino acid metabolism, β-oxidation, and tricarboxylic acid cycle. This review aims to provide a panoramic view of the metabolic shifts that antecede or follow type 2 diabetes. Key messages BCAAs, AAAs and acylcarnitines are strongly associated with early insulin resistance. Diabetes risk prediction has been improved when adding metabolomic markers of dysglycemia to standard clinical and biochemical factors.

Skeletal muscle-specific eukaryotic translation initiation factor 2α phosphorylation controls amino acid metabolism and fibroblast growth factor 21-mediated non-cell-autonomous energy metabolism

The eukaryotic translation initiation factor 2α (eIF2α) phosphorylation-dependent integrated stress response (ISR), a component of the unfolded protein response, has long been known to regulate intermediary metabolism, but the details are poorly worked out. We report that profiling of mRNAs of transgenic mice harboring a ligand-activated skeletal muscle-specific derivative of the eIF2α protein kinase R-like ER kinase revealed the expected up-regulation of genes involved in amino acid biosynthesis and transport but also uncovered the induced expression and secretion of a myokine, fibroblast growth factor 21 (FGF21), that stimulates energy consumption and prevents obesity. The link between the ISR and FGF21 expression was further reinforced by the identification of a small-molecule ISR activator that promoted Fgf21 expression in cell-based screens and by implication of the ISR-inducible activating transcription factor 4 in the process. Our findings establish that eIF2α phosphorylation regulates not only cell-autonomous proteostasis and amino acid metabolism, but also affects non-cell-autonomous metabolic regulation by induced expression of a potent myokine.

Postprandial insulin action relies on meal composition and hepatic parasympathetics: dependency on glucose and amino acids: Meal, parasympathetics & insulin action

Insulin sensitivity (IS) increases following a meal. Meal composition affects postprandial glucose disposal but still remains unclear which nutrients and mechanisms are involved. We hypothesized that gut-absorbed glucose and amino acids stimulate hepatic parasympathetic nerves, potentiating insulin action. Male Sprague-Dawley rats were 24 h fasted and anesthetized. Two series of experiments were performed. (A) IS was assessed before and after liquid test meal administration (10 ml.kg(-1), intraenteric): glucose + amino acids + lipids (GAL, n=6); glucose (n=5); amino acids (n=5); lipids (n=3); glucose + amino acids (GA, n=9); amino acids + lipids (n=3); and glucose + lipids (n=4). (B) Separately, fasted animals were submitted to hepatic parasympathetic denervation (DEN); IS was assessed before and after GAL (n=4) or GA administration (n=4). (A) Both GAL and GA induced significant insulin sensitization. GAL increased IS from 97.9±6.2 mg glucose/kg bw (fasting) to 225.4±18.3 mg glucose/kg bw (P<0.001; 143.6±26.0% potentiation of IS); GA increased IS from 109.0±6.6 to 240.4±18.0 mg glucose/kg bw (P<0.001; 123.1±13.4% potentiation). None of the other meals potentiated IS. (B) GAL and GA did not induce a significant insulin sensitization in DEN animal. To achieve maximal insulin sensitization following a meal, it is required that gut-absorbed glucose and amino acids trigger a vagal reflex that involves hepatic parasympathetic nerves.

Metabolites related to gut bacterial metabolism, peroxisome proliferator-activated receptor-alpha activation, and insulin sensitivity are associated with physical function in functionally-limited older adults

Identification of mechanisms underlying physical function will be important for addressing the growing challenge that health care will face with physical disablement in the expanding aging population. Therefore, the goals of the current study were to use metabolic profiling to provide insight into biologic mechanisms that may underlie physical function by examining the association between baseline and the 6-month change in serum mass spectrometry-obtained amino acids, fatty acids, and acylcarnitines with baseline and the 6-month change in muscle strength (leg press one repetition maximum divided by total lean mass, LP/Lean), lower extremity function [short physical performance battery (SPPB)], and mobility (400 m gait speed, 400-m), in response to 6 months of a combined resistance exercise and nutritional supplementation (whey protein or placebo) intervention in functionally-limited older adults (SPPB ≤ 10; 70–85 years, N = 73). Metabolites related to gut bacterial metabolism (cinnamoylglycine, phenol sulfate, p-cresol sulfate, 3-indoxyl sulfate, serotonin, N-methylproline, hydrocinnamate, dimethylglycine, trans-urocanate, valerate) that are altered in response to peroxisome proliferator-activated receptor-alpha (PPAR-α) activation (α-hydroxyisocaproate, α-hydroxyisovalerate, 2-hydroxy-3-methylvalerate, indolelactate, serotonin, 2-hydroxypalmitate, glutarylcarnitine, isobutyrylcarnitine, cinnamoylglycine) and that are related to insulin sensitivity (monounsaturated fatty acids: 5-dodecenoate, myristoleate, palmitoleate; γ-glutamylamino acids: γ-glutamylglutamine, γ-glutamylalanine, γ-glutamylmethionine, γ-glutamyltyrosine; branched-chain amino acids: leucine, isoleucine, valine) were associated with function at baseline, with the 6-month change in function or were identified in backward elimination regression predictive models. Collectively, these data suggest that gut microbial metabolism, PPAR-α activation, and insulin sensitivity may be involved in mechanisms that underlie physical function in functionally-limited older adults.

Nrf2 affects the efficiency of mitochondrial fatty acid oxidation

Transcription factor Nrf2 (NF-E2 p45-related factor 2) regulates the cellular redox homoeostasis and cytoprotective responses, allowing adaptation and survival under conditions of stress. The significance of Nrf2 in intermediary metabolism is also beginning to be recognized. Thus this transcription factor negatively affects fatty acid synthesis. However, the effect of Nrf2 on fatty acid oxidation is currently unknown. In the present paper, we report that the mitochondrial oxidation of long-chain (palmitic) and short-chain (hexanoic) fatty acids is depressed in the absence of Nrf2 and accelerated when Nrf2 is constitutively active. Addition of fatty acids stimulates respiration in heart and liver mitochondria isolated from wild-type mice. This effect is significantly weaker when Nrf2 is deleted, whereas it is stronger when Nrf2 activity is constitutively high. In the absence of glucose, addition of fatty acids differentially affects the production of ATP in mouse embryonic fibroblasts from wild-type, Nrf2-knockout and Keap1 (Kelch-like ECH-associated protein 1)-knockout mice. In acute tissue slices, the rate of regeneration of FADH2 is reduced when Nrf2 is absent. This metabolic role of Nrf2 on fatty acid oxidation has implications for chronic disease conditions including cancer, metabolic syndrome and neurodegeneration.

Effect of increasing glutathione with cysteine and glycine supplementation on mitochondrial fuel oxidation, insulin sensitivity, and body composition in older HIV-infected patients

BACKGROUND

HIV-infected patients are reported to have impaired oxidation of fatty acids despite increased availability, suggesting a mitochondrial defect. We investigated whether diminished levels of a key mitochondrial antioxidant, glutathione (GSH), was contributing to defective fatty acid oxidation in older HIV-infected patients, and if so, the metabolic mechanisms contributing to GSH deficiency in these patients.

METHODS

In an open-label design, 8 older GSH-deficient HIV-infected males were studied before and after 14 days of oral supplementation with the GSH precursors cysteine and glycine. A combination of stable-isotope tracers, calorimetry, hyperinsulinemic-euglycemic clamp, and dynamometry were used to measure GSH synthesis, fasted and insulin-stimulated (fed) mitochondrial fuel oxidation, insulin sensitivity, body composition, anthropometry, forearm-muscle strength, and lipid profiles.

RESULTS

Impaired synthesis contributed to GSH deficiency in the patients and was restored with cysteine plus glycine supplementation. GSH improvement was accompanied by marked improvements in fasted and fed mitochondrial fuel oxidation. Associated benefits included improvements in insulin sensitivity, body composition, anthropometry, muscle strength, and dyslipidemia.

CONCLUSIONS

This work identifies 2 novel findings in older HIV-infected patients: 1) diminished synthesis due to decreased availability of cysteine and glycine contributes to GSH deficiency and can be rapidly corrected by dietary supplementation of these precursors and 2) correction of GSH deficiency is associated with improvement of mitochondrial fat and carbohydrate oxidation in both fasted and fed states and with improvements in insulin sensitivity, body composition, and muscle strength. The role of GSH on ameliorating metabolic complications in older HIV-infected patients warrants further investigation.

Serum glycine is associated with regional body fat and insulin resistance in functionally-limited older adults

BACKGROUND

Metabolic profiling may provide insight into biologic mechanisms related to age-related increases in regional adiposity and insulin resistance.

OBJECTIVES

The objectives of the current study were to characterize the association between mid-thigh intermuscular and subcutaneous adipose tissue (IMAT, SCAT, respectively) and, abdominal adiposity with the serum metabolite profile, to identify significant metabolites as further associated with the homeostasis model assessment of insulin resistance (HOMA-IR), and, to develop a HOMA-IR associated metabolite predictor set representative of regional adiposity, in 73 functionally-limited (short physical performance battery ≤10; SPPB) older adults (age range, 70-85 y).

METHODS

Fasting levels of 181 total metabolites, including amino acids, fatty acids and acylcarnitines were measured with use of an untargeted mass spectrometry-based metabolomic approach. Multivariable-adjusted linear regression was used in all analyses.

RESULTS

Thirty-two, seven and one metabolite(s) were found to be associated with IMAT, abdominal adiposity and, SCAT, respectively, including the amino acid glycine, which was positively associated with SCAT and, negatively associated with both IMAT and abdominal adiposity. Glycine and four metabolites found to be significantly associated with regional adiposity were additionally associated with HOMA-IR. Separate stepwise regression models identified glycine as a HOMA-IR associated marker of both IMAT (model R(2) = 0.51, p<0.0001) and abdominal adiposity (model R(2) = 0.41, p<0.0001).

CONCLUSION

Our findings for a positive association between glycine with SCAT but, a negative association between glycine with IMAT and abdominal adiposity supports the hypothesis that SCAT metabolic processes are different from that found in other fat depots. In addition, because of the significant associations found between glycine with HOMA-IR, IMAT, SCAT and abdominal adiposity, our results suggest glycine as a serum biomarker of both insulin sensitivity and regional fat mass in functionally-limited older adults.