Lalia, Antigoni Z.

Insulin is a key regulator of amino acid metabolism. Many plasma amino acids, including lysine and its metabolite, α-aminoadipic acid (α-AA), a predictor for developing diabetes, are elevated in insulin resistance (IR). In 18 overweight women with IR and polycystic ovary syndrome compared with 12 lean control women, high physiological insulin during a euglycemic clamp failed to normalize many elevated amino acid metabolites, including branched-chain and aromatic amino acids, α-aminobutyric acid, and lysine, but normalized α-AA. To understand the underpinnings of differential responses of lysine and its metabolic product α-AA to high physiological insulin in IR compared with control participants, we developed a kinetic model using [α-15N1]-lysine and [13C1]-α-AA as tracers and measured the two tracers simultaneously in α-AA by innovative mass spectrometry. High insulin increased lysine conversion to α-AA in the IR and control groups but failed to normalize plasma lysine concentrations in IR due to a decrease in lysine metabolic clearance rate (MCR). In contrast, despite higher conversion rates of lysine to α-AA by high insulin, α-AA concentration decreased in IR because of the sustained greater MCR of α-AA. The abnormal amino acids and metabolites, even while on high physiological insulin, could potentially explain many functional derangements in IR. Article Highlights

There is evidence that the anabolic response to exercise is blunted in skeletal muscle in older adults. Older adults also exhibit reduced mitochondrial content and lower oxidative capacity. Since mitochondria provide chemical energy for muscle protein synthesis, it is possible that age‐related changes to mitochondrial physiology may contribute to anabolic resistance. We evaluated this possibility by measuring mitochondrial function and anabolic responses to exercise in 12 young (19–34 yrs) and 12 older (67–83 yrs) adults. Serial vastus lateralis biopsies were collected during an infusion of 13 C 6 phenylalanine to measure muscle protein synthesis at baseline and 16 hours after a single bout of unaccustomed resistance exercise (unilateral knee extension, 8 sets, 10 repetitions at 70% of 1‐repetition maximum). Real‐time PCR and RNA sequencing were used to evaluate the transcriptional response to acute exercise in muscle tissue. Respiratory capacity of isolated mitochondria was measured by high‐resolution respirometry. Mitochondrial respiratory capacity was significantly lower in older adults (4.67±0.47 pmol/s/ug) compared to young (6.60±0.70 pmol/s/ug, p=0.024). Postabsorptive protein synthesis rates were similar in young and old for mixed muscle (Y: 0.067±0.005 %/hr, O: 0.104±0.030 %/hr, p=0.13), mitochondrial (Y: 0.076±0.014 %/hr, O: 0.089±0.017 %/hr, p=0.58), and myofibrillar fractions (Y: 0.073±0.005 %/hr, O: 0.072±0.015 %/hr, p=0.94). Exercise increased protein synthesis similarly in young and old for mixed muscle (Fold changes Y: 1.45±0.16, O: 1.46±0.18, p=0.98), mitochondrial (Y: 0.90±0.28, O: 1.74±0.41, p=0.11), and myofibrillar (Y: 1.35±0.13, O: 1.37±0.18, p=0.93). These data demonstrate older adults have lower mitochondrial respiratory capacity yet exhibit similar anabolic responses to resistance exercise compared to young. Protein synthesis does not appear limited by the decreased mitochondrial capacity with age.

Aging is associated with altered mitochondrial physiology and exercise responsiveness in skeletal muscle. Activation of the unfolded protein response in the endoplasmic reticulum (UPR ER ) and mitochondria (UPR MT ) is linked with exercise‐induced adaptations in skeletal muscle, but the possibility that these processes impact exercise responsiveness and mitochondrial function in older humans has not been fully elucidated. The purpose of this study was to evaluate age‐related changes in UPR ER and UPR MT gene transcripts and protein abundance in response to a single bout of exercise. Muscle biopsies were obtained from 12 young (27±5 yr) and 12 older (75±5 yr) well‐matched, healthy men and women before exercise and 18h following a single bout of unilateral leg extension. Before exercise, a gene set enrichment analysis (GSEA) revealed that 27 genes associated with the UPR ER were upregulated in the older subjects compared to the young (enrichment false discovery rate [FDR] = 0.0842). After exercise, GSEA indicated upregulated UPR ER genes in both the young (FDR = 0.0405) and older subjects (FDR = 0.0619), however, pathway activation analysis revealed significantly greater activation of UPR ER and UPR MT gene transcripts post‐exercise in the younger subjects ( P < 0.05). In contrast, age did not affect key UPR‐regulatory protein abundance 18h post‐exercise. Interestingly, statistically significant correlations were observed in the young subjects between the mitochondrial transcriptional coactivator, peroxisome proliferator‐activated receptor gamma coactivator 1‐alpha (PGC‐1α) and key regulatory genes associated with the UPR ER (GRP78, R = 0.67; ATF6, R = 0.70) and the UPR MT (HSPE1, R = 0.61; HSPD1, R = 0.55; and HSPA9, R = 0.79), which were not evident in the older subjects. In conclusion, transcriptomic analysis revealed an age‐related decline in the adaptive UPR ER and UPR MT transcriptional response following a single bout of exercise that could potentially explain, in part, impaired exercise responsiveness and mitochondrial dysfunction with age. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

AbstractOmega-3 polyunsaturated fatty acids (n3-PUFA) are well recognized for their potent triglyceride-lowering effects, but the potential influence of these bioactive lipids on other biological processes, particularly in the context of healthy aging, remains unknown. With the goal of gaining new insight into some less well-characterized biological effects of n3-PUFAs in healthy older adults, we performed metabolomics of fasting peripheral blood plasma collected from 12 young adults and 12 older adults before and after an open-label intervention of n3-PUFA (3.9 g/day, 2.7 g eicosapentaenoic [EPA], 1.2 g docosahexaenoic [DHA]). Proton nuclear magnetic resonance (1H-NMR) based lipoprotein subclass analysis revealed the expected reduction in total triglyceride (TG), but also demonstrated that n3-PUFA supplementation reduced very low-density lipoprotein (VLDL) particle number, modestly increased high-density lipoprotein (HDL) cholesterol, and shifted the composition of HDL subclasses. Further metabolite profiling by 1H-NMR and mass spectrometry revealed pronounced changes in phospholipids, cholesterol esters, diglycerides, and triglycerides following n3-PUFA supplementation. Furthermore, significant changes in hydroxyproline, kynurenine, and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF) following n3-PUFA supplementation provide further insight into some less well-recognized biological effects of n3-PUFA supplementation, including possible effects on protein metabolism, the kynurenine pathway, and glucose metabolism.

Omega-3 polyunsaturated fatty acids (n-3 PUFA) of marine origin, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have been long studied for their therapeutic potential in the context of type 2 diabetes, insulin resistance, and glucose homeostasis. Glaring discordance between observations in animal and human studies precludes, to date, any practical application of n-3 PUFA as nutritional therapeutics against insulin resistance in humans. Our objective in this review is to summarize current knowledge and provide an up-to-date commentary on the therapeutic value of EPA and DHA supplementation for improving insulin sensitivity in humans. We also sought to discuss potential mechanisms of n-3 PUFA action in target tissues, in specific skeletal muscle, based on our recent work, as well as in liver and adipose tissue. We conducted a literature search to include all preclinical and clinical studies performed within the last two years and to comment on representative studies published earlier. Recent studies support a growing consensus that there are beneficial effects of n-3 PUFA on insulin sensitivity in rodents. Observational studies in humans are encouraging, however, the vast majority of human intervention studies fail to demonstrate the benefit of n-3 PUFA in type 2 diabetes or insulin-resistant non-diabetic people. Nevertheless, there are still several unanswered questions regarding the potential impact of n-3 PUFA on metabolic function in humans.

Context:Numerous factors are purported to influence insulin sensitivity including age, adiposity, mitochondrial function, and physical fitness. Univariate associations cannot address the complexity of insulin resistance or the interrelationship among potential determinants.Objective:The objective of the study was to identify significant independent predictors of insulin sensitivity across a range of age and adiposity in humans.Design, Setting, and Participants:Peripheral and hepatic insulin sensitivity were measured by two stage hyperinsulinemic-euglycemic clamps in 116 men and women (aged 19–78 y). Insulin-stimulated glucose disposal, the suppression of endogenous glucose production during hyperinsulinemia, and homeostatic model assessment of insulin resistance were tested for associations with 11 potential predictors. Abdominal subcutaneous fat, visceral fat (AFVISC), intrahepatic lipid, and intramyocellular lipid (IMCL) were quantified by magnetic resonance imaging and spectroscopy. Skeletal muscle mitochondrial respiratory capacity (state 3), coupling efficiency, and reactive oxygen species production were evaluated from muscle biopsies. Aerobic fitness was measured from whole-body maximum oxygen uptake (VO2 peak), and metabolic flexibility was determined using indirect calorimetry.Results:Multiple regression analysis revealed that AFVISC (P &lt; .0001) and intrahepatic lipid (P = .002) were independent negative predictors of peripheral insulin sensitivity, whereas VO2 peak (P = .0007) and IMCL (P = .023) were positive predictors. Mitochondrial capacity and efficiency were not independent determinants of peripheral insulin sensitivity. The suppression of endogenous glucose production during hyperinsulinemia model of hepatic insulin sensitivity revealed percentage fat (P &lt; .0001) and AFVISC (P = .001) as significant negative predictors. Modeling homeostatic model assessment of insulin resistance identified AFVISC (P &lt; .0001), VO2 peak (P = .001), and IMCL (P = .01) as independent predictors.Conclusion:The reduction in insulin sensitivity observed with aging is driven primarily by age-related changes in the content and distribution of adipose tissue and is independent of muscle mitochondrial function or chronological age.

SummaryMitochondrial dysfunction is often observed in aging skeletal muscle and is implicated in age‐related declines in physical function. Early evidence suggests that dietary omega‐3 polyunsaturated fatty acids (n‐3 PUFAs) improve mitochondrial function. Here, we show that 10 weeks of dietary eicosapentaenoic acid (EPA) supplementation partially attenuated the age‐related decline in mitochondrial function in mice, but this effect was not observed with docosahexaenoic acid (DHA). The improvement in mitochondrial function with EPA occurred in the absence of any changes in mitochondrial abundance or biogenesis, which was evaluated from RNA sequencing, large‐scale proteomics, and direct measurements of muscle mitochondrial protein synthesis rates. We find that EPA improves muscle protein quality, specifically by decreasing mitochondrial protein carbamylation, a post‐translational modification that is driven by inflammation. These results demonstrate that EPA attenuated the age‐related loss of mitochondrial function and improved mitochondrial protein quality through a mechanism that is likely linked with anti‐inflammatory properties of n‐3 PUFAs. Furthermore, we demonstrate that EPA and DHA exert some common biological effects (anticoagulation, anti‐inflammatory, reduced FXR/RXR activation), but also exhibit many distinct biological effects, a finding that underscores the importance of evaluating the therapeutic potential of individual n‐3 PUFAs.

OBJECTIVE Dietary n-3 polyunsaturated fatty acids, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), prevent insulin resistance and stimulate mitochondrial biogenesis in rodents, but the findings of translational studies in humans are thus far ambiguous. The aim of this study was to evaluate the influence of EPA and DHA on insulin sensitivity, insulin secretion, and muscle mitochondrial function in insulin-resistant, nondiabetic humans using a robust study design and gold-standard measurements. RESEARCH DESIGN AND METHODS Thirty-one insulin-resistant adults received 3.9 g/day EPA+DHA or placebo for 6 months in a randomized double-blind study. Hyperinsulinemic-euglycemic clamp with somatostatin was used to assess hepatic and peripheral insulin sensitivity. Postprandial glucose disposal and insulin secretion were measured after a meal. Measurements were performed at baseline and after 6 months of treatment. Abdominal fat distribution was evaluated by MRI. Muscle oxidative capacity was measured in isolated mitochondria using high-resolution respirometry and noninvasively by magnetic resonance spectroscopy. RESULTS Compared with placebo, EPA+DHA did not alter peripheral insulin sensitivity, postprandial glucose disposal, or insulin secretion. Hepatic insulin sensitivity, determined from the suppression of endogenous glucose production by insulin, exhibited a small but significant improvement with EPA+DHA compared with placebo. Muscle mitochondrial function was unchanged by EPA+DHA or placebo. CONCLUSIONS This study demonstrates that dietary EPA+DHA does not improve peripheral glucose disposal, insulin secretion, or skeletal muscle mitochondrial function in insulin-resistant nondiabetic humans. There was a modest improvement in hepatic insulin sensitivity with EPA+DHA, but this was not associated with any improvements in clinically meaningful outcomes.