K Sreekumaran Nair

Mayo Clinic - Rochester, Рочестер, Minnesota, United States

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Publications (307)1830.8 Total impact

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    ABSTRACT: Context: Poor glycemic control in individuals with type 1 diabetes (T1D) is associated with both micro and macrovascular complications but good glycemic control does not fully prevent the risk of these complications. 2. Objective: To determine whether T1D with good glycemic control have persistent abnormalities of metabolites and pathways that exist in T1D with poor glycemic control 3. Design: We compared plasma metabolites in T1D with poor (HbA1c ≥8.5%, T1D(-) and good (HbA1c of < 6.5%, T1D(+) glycemic control to non-diabetic controls (ND). 4. Setting: Clinical Research Unit. 5. Patients or other participants: T1D with poor (n=14), T1D(-) and good, T1D(+) (n=15) glycemic control and matched (for age, sex, and BMI) ND participants. 6. Intervention(s): No intervention. 7. Main outcome measure(s): Comparison of qualitative and quantitative profiling of metabolome was performed. 8. Results: In T1D(-), 347 known metabolites belonging to 38 metabolic pathways involved in cholesterol, Vitamin-D, tRNA, amino acids (AA), bile acids, urea, TCA cycle, immune response and eicosanoids were different from ND. In T1D(+),154 known metabolites belonging to 26 pathways including glycolysis, gluconeogenesis, bile acids, tRNA biosynthesis, AA, BCAA, retinol and Vitamin-D metabolism remained altered from ND. Targeted measurements of amino acid metabolites, TCA and free fatty acids showed directional changes similar to the untargeted metabolomics approach. 9. Conclusions: Comprehensive metabolomic profiling identified extensive metabolomic abnormalities in T1D with poor glycemic control. Chronic good glycemic control failed to normalize many of these perturbations suggesting a potential role for these persistent abnormalities in many complications in T1D.
    No preview · Article · Jan 2016 · Journal of Clinical Endocrinology & Metabolism
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    ABSTRACT: Insulin plays pivotal role in cellular fuel metabolism in skeletal muscle (SM). Despite being the primary site of energy metabolism, the underlying mechanism on how insulin deficiency deranges SM mitochondrial physiology remains to be fully understood. Here we report important link between altered skeletal muscle proteome homeostasis and mitochondrial physiology during insulin deficiency. Deprivation of insulin in treated streptozotocin diabetic mice decreased mitochondrial ATP production, reduced coupling and phosphorylation efficiency and increased oxidant emission in skeletal muscle. Proteomic survey revealed that the mitochondrial derangements during insulin deficiency were related to, increased mitochondrial protein degradation and decreased protein synthesis, resulting in reduced abundance of proteins involved in mitochondrial respiration and β-oxidation. However, a paradoxical up-regulation of proteins involved in cellular uptake of fatty acids triggered an accumulation of incomplete fatty acid oxidation products in skeletal muscle. These data implicate a mismatch of β-oxidation and fatty acid uptake as a mechanism leading to increased oxidative stress in diabetes. This notion was supported by elevated oxidative stress in cultured myotubes exposed to palmitate in the presence of a β-oxidation inhibitor. Together, these results indicate that insulin deficiency alters the balance of proteins involved in fatty acid transport and oxidation in skeletal muscle, leading to impaired mitochondrial function and increased oxidative stress.
    No preview · Article · Dec 2015 · Diabetes
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    ABSTRACT: 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 syndrome or the interrelationship among potential determinants. Objective: 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 (19-78 years). Insulin-stimulated glucose disposal (GIR), the suppression of endogenous glucose production during hyperinsulinemia (EGPSUP), and HOMA-IR were tested for associations with 11 potential predictors. Abdominal subcutaneous fat (AFSQ), visceral fat (AFVISC), intrahepatic lipid (IHL) and intramyocellular lipid (IMCL) were quantified by magnetic resonance imaging and spectroscopy. Skeletal muscle mitochondrial respiratory capacity (State 3), coupling efficiency (RCR) and reactive oxygen species production (ROS) were evaluated from muscle biopsies. Aerobic fitness was measured from whole-body maximum oxygen uptake (VO2 peak), and metabolic flexibility (ΔRQ) was determined using indirect calorimetry. Results: Multiple regression analysis revealed that AFVISC (p<0.0001) and IHL (p=0.002) were independent negative predictors of peripheral insulin sensitivity, while VO2 peak (p=0.0007) and IMCL (p=0.023) were positive predictors. Mitochondrial capacity and efficiency were not independent determinants of peripheral insulin sensitivity. The EGPSUP model of hepatic insulin sensitivity revealed % fat (p<0.0001) and AFVISC (p=0.001) as significant negative predictors. Modeling HOMA-IR identified AFVISC (p<0.0001), VO2 peak (p=0.001), and IMCL (p=0.01) 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.
    No preview · Article · Dec 2015 · The Journal of Clinical Endocrinology and Metabolism
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    ABSTRACT: Muscle weakness and myopathy are observed in vitamin D deficiency and chronic renal failure, where concentrations of the active vitamin D3 metabolite, 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3), are low. To evaluate the mechanism of action of 1α,25(OH)2D3 in skeletal muscle, we examined mitochondrial oxygen consumption, dynamics and biogenesis, and changes in expression of nuclear genes encoding mitochondrial proteins in human skeletal muscle cells following treatment with 1α,25(OH)2D3. Mitochondrial oxygen consumption rate (OCR) increased in 1α,25(OH)2D3-treated cells. Vitamin D3 metabolites lacking a 1α-hydroxyl group (vitamin D3, 25-hydroxyvitamin D3, and 24R,25-dihydroxyvitamin D3) decreased or failed to increase OCR. 1α-Hydroxyvitamin D3 did not increase OCR. In 1α,25(OH)2D3-treated cells, mitochondrial volume and branching, and expression of the pro-fusion protein, optic atrophy 1 (OPA1) increased, whereas, expression of the pro-fission proteins, fission 1 (Fis1) and dynamin 1-like (Drp1) decreased. Phosphorylated pyruvate dehydrogenase (S293 PDH) and PDH kinase 4 (PDK4) decreased in 1α,25(OH)2D3-treated cells. There was a trend to increased PDH activity in 1α,25(OH)2D3-treated cells (P = 0.09). 83 nuclear mRNAs encoding mitochondrial proteins were changed following 1α,25(OH)2D3-treatment; notably PDK4 mRNA decreased, and PDP2 mRNA increased. MYC, MAPK13 and EPAS1 mRNAs, which encode proteins that regulate mitochondrial biogenesis, were increased following 1α,25(OH)2D3-treatment. Vitamin D receptor-dependent changes in the expression of 1947 mRNAs encoding proteins involved in muscle contraction, focal adhesion, and integrin, JAK/STAT, MAPK, growth factor and p53 signaling pathways were observed following 1α,25(OH)2D3-treatment. Five micro-RNAs were induced or repressed by 1α,25(OH)2D3. 1α,25(OH)2D3 regulates mitochondrial function, dynamics and enzyme function, which are likely to influence muscle strength.
    No preview · Article · Nov 2015 · Journal of Biological Chemistry
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    ABSTRACT: Caloric restriction (CR) improves insulin sensitivity and reduces the incidence of diabetes in obese individuals. The underlying mechanisms whereby CR improves insulin sensitivity are not clear. We evaluated the effect of 16-weeks of CR on whole-body insulin sensitivity by pancreatic clamp before and after CR in 11 obese participants (BMI=35 kg/m(2)) in comparison with a matched control period (CON, N=9, BMI=34 kg/m(2)). Compared to CON, CR increased the glucose infusion rate needed to maintain euglycemia during hyperinsulinemia indicating enhancement of peripheral insulin sensitivity. This improvement in insulin sensitivity was not accompanied by changes in skeletal muscle mitochondrial oxidative capacity or oxidant emissions, nor was there changes in skeletal muscle ceramide, diacylglycerol, or amino acid metabolite levels. However, CR lowered insulin-stimulated thioredoxin-interacting protein (TXNIP) levels and enhanced non-oxidative glucose disposal. These results support a role for TXNIP in mediating the improvement in peripheral insulin sensitivity after CR. © 2015 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered.
    No preview · Article · Aug 2015 · Diabetes
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    ABSTRACT: Decline in mitochondrial DNA (mtDNA) copy number, function, and accumulation of mutations and deletions have been proposed to contribute to age-related physical decline, based on cross sectional studies in genetically unrelated individuals. There is wide variability of mtDNA and functional measurements in many population studies and therefore we assessed mitochondrial function and physical function in 18 families of grandmothers, mothers, and daughters who share the same maternally inherited mtDNA sequence. A significant age-related decline in mtDNA copy number, mitochondrial protein expression, citrate synthase activity, cytochrome c oxidase content, and VO2 peak were observed. Also, a lower abundance of SIRT3, accompanied by an increase in acetylated skeletal muscle proteins, was observed in grandmothers. Muscle tissue-based full sequencing of mtDNA showed greater than 5% change in minor allele frequency over a lifetime in two locations, position 189 and 408 in the noncoding D-loop region but no changes were noted in blood cells mtDNA. The decline in oxidative capacity and muscle function with age in three generations of women who share the same mtDNA sequence are associated with a decline in muscle mtDNA copy number and reduced protein deacetylase activity of SIRT3. © The Author 2015. Published by Oxford University Press on behalf of The Gerontological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.
    No preview · Article · Aug 2015 · The Journals of Gerontology Series A Biological Sciences and Medical Sciences
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    ABSTRACT: Exercise provides many health benefits, including improved metabolism, cardiovascular health, and cognition. We have shown previously that FNDC5, a type I transmembrane protein, and its circulating form, irisin, convey some of these benefits in mice. However, recent reports questioned the existence of circulating human irisin both because human FNDC5 has a non-canonical ATA translation start and because of claims that many human irisin antibodies used in commercial ELISA kits lack required specificity. In this paper we have identified and quantitated human irisin in plasma using mass spectrometry with control peptides enriched with heavy stable isotopes as internal standards. This precise state-of-the-art method shows that human irisin is mainly translated from its non-canonical start codon and circulates at ∼3.6 ng/ml in sedentary individuals; this level is increased to ∼4.3 ng/ml in individuals undergoing aerobic interval training. These data unequivocally demonstrate that human irisin exists, circulates, and is regulated by exercise. Copyright © 2015 Elsevier Inc. All rights reserved.
    Full-text · Article · Aug 2015 · Cell metabolism
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    ABSTRACT: Stable isotope-labeled amino acids have long been used to measure the fractional synthesis rate of proteins, although the mass spectrometry platforms used for such analyses have changed throughout the years. More recently, tandem mass spectrometers such as triple quadrupoles have been accepted as the standard platform for enrichment measurement due to their sensitivity and the enhanced specificity offered by multiple reaction monitoring (MRM) experiments. The limit in the utility of such platforms for enrichment analysis occurs when measuring very low levels of enrichment from small amounts of sample, particularly proteins isolated from two-dimensional gel electrophoresis (2D-GE), where interference from contaminant ions impacts the sensitivity of the measurement. We therefore applied a high-resolution orbitrap mass spectrometer to the analysis of [ring-(13)C6]-phenylalanine enrichment in individual muscle proteins isolated with 2D-GE. Comparison of samples analyzed on both platforms revealed that the high-resolution MS has significantly improved sensitivity relative to the triple quadrupole MS at very low-level enrichments due to its ability to resolve interferences in the m/z dimension. At higher enrichment levels, enrichment measurements from the orbitrap platform showed significant correlation (R (2) > 0.5) with those of the triple quadrupole platform. Together, these results indicate that high-resolution MS platforms such as the orbitrap are not only as capable of performing isotope enrichment measurements as the more commonly preferred triple quadrupole instruments, but offer unparalleled advantages in terms of mass accuracy and sensitivity in the presence of similar-mass contaminants.
    No preview · Article · Apr 2015 · Analytical and Bioanalytical Chemistry
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    ABSTRACT: Objective To examine the association of gene variants of uncoupling proteins (UCP)-2 and −3 with obesity and gastrointestinal (GI) traits.Methods In 255 overweight or obese adults, the associations of gene variants in UCP-2 (−3474, rs659366) and UCP-3 (rs1626521, rs2075577, rs15763) with body weight (BW) and GI traits were studied. Gene variants were genotyped by TaqMan® assay. The associations of genotypes with BW and GI traits (gastric emptying, gastric volume, satiety by buffet meal, satiation by nutrient drink test and GI hormones) were assessed using ANOVA corrected for false detection rate (FDR).ResultsA novel UCP-3 gene variant, rs1626521, was identified; it was associated with BW (P = 0.039), waist circumference (P = 0.035), and significantly higher postprandial gastric volume (P = 0.003) and calories ingested at buffet meal (P = 0.006, both significant with FDR). In a subgroup of 11 participants, rs1626521 was also associated with reduced mitochondrial bioenergetics efficiency in skeletal muscle (P = 0.051). In an in vitro study in HEK293 cells, rs1626521 reduced UCP-3 protein expression (P = 0.049). Associations detected between other genotypes and GI traits were nonsignificant with FDR.ConclusionsA newly identified functional variant (rs1626521) in UCP-3 affects postprandial gastric functions and satiety and may contribute to weight gain and alter human mitochondrial function.
    No preview · Article · Mar 2015 · Obesity
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    ABSTRACT: Prior studies have reported that elevated concentrations of several plasma amino acids (AA), particularly branched chain (BCAA) and aromatic AA predict the onset of type 2 diabetes. We sought to test the hypothesis that circulating BCAA, aromatic AA and related AA metabolites decline in response to the use of insulin sensitizing agents in overweight/obese adults with impaired fasting glucose or untreated diabetes. We performed a secondary analysis of a randomized, double-blind, placebo, controlled study conducted in twenty five overweight/obese (BMI ~30kg/m(2)) adults with impaired fasting glucose or untreated diabetes. Participants were randomized to three months of pioglitazone (45mg per day) plus metformin (1000mg twice per day, N=12 participants) or placebo (N=13). We measured insulin sensitivity by the euglycemic-hyperinsulinemic clamp and fasting concentrations of AA and AA metabolites using ultra-pressure liquid chromatography tandem mass spectrometry before and after the three-month intervention. Insulin sensitizer therapy that significantly enhanced insulin sensitivity reduced 9 out of 33 AA and AA metabolites measured compared to placebo treatment. Moreover, insulin sensitizer therapy significantly reduced three functionally clustered AA and metabolite pairs: i) phenylalanine/tyrosine, ii) citrulline/arginine, and iii) lysine/α-aminoadipic acid. Reductions in plasma concentrations of several AA and AA metabolites in response to three months of insulin sensitizer therapy support the concept that reduced insulin sensitivity alters AA and AA metabolites. Copyright © 2015 Elsevier Inc. All rights reserved.
    No preview · Article · Jan 2015 · Metabolism: clinical and experimental
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    ABSTRACT: The notion that mitochondria contribute to obesity-induced insulin resistance is highly debated. Therefore, we determined if obese (BMI=33 kg/m(2)), insulin-resistant women with polycystic ovary syndrome had aberrant skeletal muscle mitochondrial physiology compared to lean, insulin-sensitive women (BMI=23 kg/m(2)). Maximal whole-body and mitochondrial oxygen consumption were not different between obese and lean women. However, obese women exhibited lower mitochondrial coupling and phosphorylation efficiency and elevated mitochondrial H2O2 (mtH2O2) emissions compared to lean women. We further evaluated the impact of 12-weeks of aerobic exercise on obesity-related impairments in insulin sensitivity and mitochondrial energetics in the fasted state and following a high-fat, mixed meal. Exercise training reversed obesity-related mitochondrial derangements as evidenced by enhanced mitochondrial bioenergetics efficiency and decreased mtH2O2 production. A concomitant increase in catalase antioxidant activity and decreased DNA oxidative damage indicate improved cellular redox status and a potential mechanism contributing to improved insulin sensitivity. mtH2O2 emissions were refractory to a high-fat meal at baseline but after exercise mtH2O2 emissions increased following the meal, which resembles previous findings in lean individuals. We demonstrate obese women exhibit impaired mitochondrial bioenergetics in the form of decreased efficiency and impaired mtH2O2 emissions, while exercise effectively restores mitochondrial physiology toward that of lean, insulin-sensitive individuals. © 2015 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered.
    No preview · Article · Jan 2015 · Diabetes
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    ABSTRACT: Context: Skeletal muscle from sedentary older adults exhibits reduced mitochondrial abundance and oxidative capacity (OXPHOS). Objective: The primary objective was to determine whether eight weeks of combined training (CT) has more robust effect than, endurance training (ET) or resistance training (RT) on mitochondrial physiology in healthy young (18-30 y) and older (≥65 y) adults. Intervention: Thirty-four young and 31 older adults were randomized to eight weeks of ET, RT, control/CT. Control subjects completed eight weeks of no exercise (control) followed by eight weeks of CT. Body composition, skeletal muscle strength, and peak oxygen uptake were measured before and after the intervention. Vastus lateralis muscle biopsies were obtained before and 48 h after the intervention. Mitochondrial physiology was evaluated by high-resolution respirometry, and expression of mitochondrial proteins and transcription factors by quantitative PCR and immunoblotting. Results: ET and CT significantly increased oxidative capacity and expression of mitochondrial proteins and transcription factors. All training modalities improved body composition, cardiorespiratory fitness, and skeletal muscle strength. CT induced the most robust improvements in mitochondrial related outcomes and physical characteristics despite lower training volumes for the ET and RT components. Importantly, most of the adaptations to training occurred independent of age. Conclusion: Collectively, these results demonstrate that both ET and CT increase muscle mitochondrial abundance and capacity. Although CT induced the most robust improvements in the outcomes measured. In conclusion, CT provides a robust exercise regimen to improve muscle mitochondrial outcomes and physical characteristics independent of age.
    Full-text · Article · Jan 2015 · Journal of Clinical Endocrinology & Metabolism
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    ABSTRACT: Acute aerobic exercise increases reactive oxygen species and could potentially damage proteins, but exercise training (ET) enhances mitochondrial respiration irrespective of age. Here, we report a differential impact of ET on protein quality in young and older participants. Using mass spectrometry we measured oxidative damage to skeletal muscle proteins before and after 8 weeks of ET and find that young but not older participants reduced oxidative damage to both total skeletal muscle and mitochondrial proteins. Young participants showed higher total and mitochondrial derived semitryptic peptides and 26S proteasome activity indicating increased protein degradation. ET however, increased the activity of the endogenous antioxidants in older participants. ET also increased skeletal muscle content of the mitochondrial deacetylase SIRT3 in both groups. A reduction in the acetylation of isocitrate dehydrogenase 2 was observed following ET that may counteract the effect of acute oxidative stress. In conclusion aging is associated with an inability to improve skeletal muscle and mitochondrial protein quality in response to ET by increasing degradation of damaged proteins. ET does however increase muscle and mitochondrial antioxidant capacity in older individuals, which provides increased buffering from the acute oxidative effects of exercise. © The Author 2014. Published by Oxford University Press on behalf of The Gerontological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.
    Full-text · Article · Dec 2014 · The Journals of Gerontology Series A Biological Sciences and Medical Sciences
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    ABSTRACT: Maintenance of musculoskeletal function in older adults is critically important for preserving cardiorespiratory function and health span. Aerobic endurance training (ET) improves skeletal muscle metabolic function including age-related declines in muscle mitochondrial function. To further understand the underlying mechanism of enhanced muscle function with ET, we profiled the gene transcription (mRNA levels) patterns by gene array and determined the canonical pathways associated with skeletal muscle aging in a cross-sectional study involving vastus lateralis muscle biopsy samples of four subgroups (young and old, trained, and untrained). We first analyzed the sedentary individuals and then sought to identify the pathways impacted by long-term ET (>4 years) and determined the age effect. We found that skeletal muscle aging in older sedentary adults decreased mitochondrial genes and pathways involved in oxidative phosphorylation while elevating pathways in redox homeostasis. In older adults compared to their younger counterparts who chronically perform ET however, those differences were absent. ET did, however, impact nearly twice as many genes in younger compared to older participants including downregulation of gene transcripts involved in protein ubiquitination and the ERK/MAPK pathways. This study demonstrates that in individuals who are chronically endurance trained, the transcriptional profile is normalized for mitochondrial genes but aging impacts the number of genes that respond to ET including many involved in protein homeostasis and cellular stress.
    Full-text · Article · Dec 2014
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    ABSTRACT: We, as representatives of scientific organizations devoted to improving health care and advancing research, reaffirm that it is the mission of our respective medical journals to report and disseminate data from scientific investigation, evolving medical care, and innovative treatments. We believe these reports serve to unite basic scientists, clinical investigators, and medical professionals regardless of their country of origin, ethnic group, or political leaning. We believe that these efforts achieve the common goal of advancing scientific discoveries that lead to improved health of people worldwide. On the basis of our goals and principles, our respective journals will refrain from publishing articles addressing political issues that are outside of either research funding or health care delivery.
    Preview · Article · Oct 2014 · Endocrine Practice
  • Alice Y. Chang · Tumpa Dutta · Surendra Dasari · K. Sreekumaran Nair

    No preview · Article · Oct 2014 · Journal of Women's Health
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    F González · K. Sreekumaran Nair · E Basal · D M Bearson · J M Schimke · H E Blair
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    ABSTRACT: We determined the effect of hyperandrogenemia as observed in polycystic ovary syndrome (PCOS) on fasting and glucose-stimulated proatherogenic inflammation markers in lean healthy reproductive-age women. Sixteen lean healthy ovulatory reproductive-age women were treated with 130 mg of DHEA or placebo (n=8 each) for 5 days. Interleukin-6 (IL-6) mRNA and IL-6 release from mononuclear cells (MNC), plasma IL-6 and C-reactive protein (CRP), and MNC-derived (matrix metalloproteinase-2) MMP-2 protein were quantified in the fasting state and 2 h after glucose ingestion, before and after treatment. Before treatment, subjects receiving dehydroepinadrosterone (DHEA) or placebo exhibited no differences in androgens, or any proatherogenic inflammation markers while fasting and after glucose ingestion. Compared with placebo, DHEA administration raised levels of testosterone, androstenedione, and DHEA-sulfate (DHEA-S), and increased the percent change from baseline in fasting IL-6 mRNA, IL-6 release, plasma IL-6, and CRP and MMP-2 protein. However, there were no differences in any of the proatherogenic inflammation markers following glucose ingestion after DHEA administration. We conclude that in lean reproductive-age women, proatherogenic inflammation in the fasting state increases after raising circulating androgens to levels observed in PCOS. However, this hyperandrogenemia-induced MNC activation does not provoke a similar response to subsequent glucose ingestion.
    Full-text · Article · Sep 2014 · Hormone and Metabolic Research
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    ABSTRACT: Context: Insulin and essential amino acids (EAA) regulate skeletal muscle protein synthesis, yet their independent effects on mitochondrial protein synthesis (MiPS) and oxidative function remain to be clearly defined. Objective: Determine the effects of high or low insulin with or without EAA on MiPS. Design: Thirty participants were randomized to three groups of 10 each with each participant studied twice. Study groups comprised of 1) Low and High Insulin 2) Low Insulin with and without EAA or 3) High Insulin with and without EAA. Setting: In-patient Clinical Research Unit. Participants: Eligibility included 18-45 years old, BMI<25 kg/m(2) and free of diseases and medications that may impair mitochondrial function. Intervention: Low (∾6 μ U/mL) and high (∾40 μ U/mL) insulin levels were maintained by intravenous insulin infusion during a somatostatin clamp while maintaining euglycemia (4.7-5.2 mM) and replacing growth hormone and glucagon. EAA infusion was 5.4% Nephramine. L-[ring(13)C6]-phenylalanine was infused and muscle needle biopsies were performed. Main Outcomes: Muscle MiPS, oxidative enzymes and plasma amino acid metabolites. Results: MiPS and oxidative enzyme activities did not differ between low and high insulin (MiPS: 0.07±0.009 vs. 0.07±0.006%/hr p=0.86) or between EAA and saline during low insulin (MiPS: 0.05±0.01 vs. 0.07±0.01 p=0.5). During high insulin, EAA in comparison with saline increased MiPS (0.1±0.01 vs. 0.06±0.01 p<0.05) and cytochrome-c-oxidase activity (p<0.05) but not citrate synthase (p=0.27). EAA infusion decreased (p<0.05) glucose infusion rates needed to maintain euglycemia during low (∾40%) and high insulin (∾24%). Conclusion: EAA increased MiPS and oxidative enzyme activity only with high insulin concentrations.
    No preview · Article · Sep 2014 · Journal of Clinical Endocrinology & Metabolism
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    ABSTRACT: Aim Fasting is characterised by profound changes in energy metabolism including progressive loss of body proteins. The underlying mechanisms are however unknown and we therefore determined the effects of a 72-hour-fast on human skeletal muscle protein metabolism and activation of mammalian target of rapamycin (mTOR), a key regulator of cell growth. Methods Eight healthy male volunteers were studied twice: in the postabsorptive state and following 72 hours of fasting. Regional muscle amino acid kinetics was measured in the forearm using amino acid tracers. Signaling to protein synthesis and breakdown were assessed in skeletal muscle biopsies obtained during non-insulin and insulin stimulated conditions on both examination days. Results Fasting significantly increased forearm net phenylalanine release and tended to decrease phenylalanine rate of disappearance. mTOR phosphorylation was decreased by ∼50% following fasting, together with reduced downstream phosphorylation of 4EBP1, ULK1 and rpS6. In addition, the insulin stimulated increase in mTOR and rpS6 phosphorylation was significantly reduced after fasting indicating insulin resistance in this part of the signaling pathway. Autophagy initiation is in part regulated by mTOR through ULK1 and fasting increased expression of the autophagic marker LC3B-II by ∼30%. p62 is degraded during autophagy but was increased by ∼10% during fasting making interpretation of autophagic flux problematic. MAFbx and MURF1 ubiquitin ligases remained unaltered after fasting indicating no change in protesomal protein degradation. Conclusions Our results show that during fasting increased net phenylalanine release in skeletal muscle is associated to reduced mTOR activation and concomitant decreased downstream signaling to cell growth.
    Full-text · Article · Jul 2014 · PLoS ONE
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    ABSTRACT: Background & aims: Amino acid (AA) availability is critical to maintain protein homeostasis and reduced protein intake causes a decline in protein synthesis. Citrulline, an amino acid metabolite, has been reported to stimulate muscle protein synthesis in malnourished rats. Methods: To determine whether citrulline stimulates muscle protein synthesis in healthy adults while on a low-protein diet, we studied 8 healthy participants twice in a cross-over study design. Following a 3-days of low-protein intake, either citrulline or a non-essential AA mixture (NEAA) was given orally as small boluses over the course of 8 h. [ring-(13)C6] phenylalanine and [(15)N] tyrosine were administered as tracers to assess protein metabolism. Fractional synthesis rates (FSR) of muscle proteins were measured using phenylalanine enrichment in muscle tissue fluid as the precursor pool. Results: FSR of mixed muscle protein was higher during the administration of citrulline than during NEAA (NEAA: 0.049 ± 0.005; citrulline: 0.060 ± 0.006; P = 0.03), while muscle mitochondrial protein FSR and whole-body protein turnover were not different between the studies. Citrulline administration increased arginine and ornithine plasma concentrations without any effect on glucose, insulin, C-peptide, and IGF-1 levels. Citrulline administration did not promote mitochondria protein synthesis, transcripts, or citrate synthesis. Conclusions: Citrulline ingestion enhances mixed muscle protein synthesis in healthy participants on 3-day low-protein intake. This anabolic action of citrulline appears to be independent of insulin action and may offer potential clinical application in conditions involving low amino acid intake.
    No preview · Article · May 2014 · Clinical Nutrition

Publication Stats

14k Citations
1,830.80 Total Impact Points


  • 1995-2015
    • Mayo Clinic - Rochester
      • • Department of Endocrinology, Diabetes, Metabolism and Nutrition
      • • Department of Internal Medicine
      Рочестер, Minnesota, United States
  • 2014
    • University of Michigan
      • Department of Ophthalmology and Visual Sciences
      Ann Arbor, Michigan, United States
  • 2008
    • Wake Forest School of Medicine
      Winston-Salem, North Carolina, United States
    • Duke University Medical Center
      • Division of Endocrinology, Metabolism, and Nutrition
      Durham, North Carolina, United States
    • Rochester College
      Rochester, New York, United States
  • 2005
    • Aarhus University
      • Medical Research Laboratories
      Aarhus, Central Jutland, Denmark
  • 2002
    • Washington University in St. Louis
      • Department of Medicine
      San Luis, Missouri, United States
  • 1999
    • Aarhus University Hospital
      • Department of Endocrinology and Internal Medicine
      Aarhus, Central Jutland, Denmark
  • 1992-1999
    • University of Vermont
      • Department of Medicine
      Burlington, Vermont, United States
  • 1998
    • Abbott Laboratories
      • Abbott Laboratories
      North Chicago, Illinois, United States
  • 1994-1998
    • University of Vermont Medical Center
      Burlington, Vermont, United States
  • 1996
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 1987-1990
    • University of Rochester
      • • Department of Medicine
      • • Division of Hospital Medicine
      Rochester, New York, United States
    • Monroe Community College
      Rochester, New York, United States