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1.  Identification of the Amino Acids 300–600 of IRS-2 as 14-3-3 Binding Region with the Importance of IGF-1/Insulin-Regulated Phosphorylation of Ser-573 
PLoS ONE  2012;7(8):e43296.
Phosphorylation of insulin receptor substrate (IRS)-2 on tyrosine residues is a key event in IGF-1/insulin signaling and leads to activation of the PI 3-kinase and the Ras/MAPK pathway. Furthermore, phosphorylated serine/threonine residues on IRS-2 can induce 14-3-3 binding. In this study we searched IRS-2 for novel phosphorylation sites and investigated the interaction between IRS-2 and 14-3-3. Mass spectrometry identified a total of 24 serine/threonine residues on IRS-2 with 12 sites unique for IRS-2 while the other residues are conserved in IRS-1 and IRS-2. IGF-1 stimulation led to increased binding of 14-3-3 to IRS-2 in transfected HEK293 cells and this binding was prevented by inhibition of the PI 3-kinase pathway and an Akt/PKB inhibitor. Insulin-stimulated interaction between endogenous IRS-2 and 14-3-3 was observed in rat hepatoma cells and in mice liver after an acute insulin stimulus and refeeding. Using different IRS-2 fragments enabled localization of the IGF-1-dependent 14-3-3 binding region spanning amino acids 300–600. The 24 identified residues on IRS-2 included several 14-3-3 binding candidates in the region 300–600. Single alanine mutants of these candidates led to the identification of serine 573 as 14-3-3 binding site. A phospho-site specific antibody was generated to further characterize serine 573. IGF-1-dependent phosphorylation of serine 573 was reduced by inhibition of PI 3-kinase and Akt/PKB. A negative role of this phosphorylation site was implicated by the alanine mutant of serine 573 which led to enhanced phosphorylation of Akt/PKB in an IGF-1 time course experiment. To conclude, our data suggest a physiologically relevant role for IGF-1/insulin-dependent 14-3-3 binding to IRS-2 involving serine 573.
PMCID: PMC3422239  PMID: 22912850
2.  Hyperglycemia Impairs Proteasome Function by Methylglyoxal 
Diabetes  2009;59(3):670-678.
The ubiquitin-proteasome system is the main degradation machinery for intracellularly altered proteins. Hyperglycemia has been shown to increase intracellular levels of the reactive dicarbonyl methylglyoxal (MGO) in cells damaged by diabetes, resulting in modification of proteins and alterations of their function. In this study, the influence of MGO-derived advanced glycation end product (AGE) formation on the activity of the proteasome was investigated in vitro and in vivo.
MGO-derived AGE modification of proteasome subunits was analyzed by mass spectrometry, immunoprecipitation, and Western blots. Proteasome activity was analyzed using proteasome-specific fluorogenic substrates. Experimental models included bovine retinal endothelial cells, diabetic Ins2Akita mice, glyoxalase 1 (GLO1) knockdown mice, and streptozotocin (STZ)-injected diabetic mice.
In vitro incubation with MGO caused adduct formation on several 20S proteasomal subunit proteins. In cultured endothelial cells, the expression level of the catalytic 20S proteasome subunit was not altered but proteasomal chymotrypsin-like activity was significantly reduced. In contrast, levels of regulatory 19S proteasomal proteins were decreased. In diabetic Ins2Akita, STZ diabetic, and nondiabetic and diabetic G101 knockdown mice, chymotrypsin-like activity was also reduced and MGO modification of the 20S-β2 subunit was increased.
Hyperglycemia-induced formation of MGO covalently modifies the 20S proteasome, decreasing its activity in the diabetic kidney and reducing the polyubiquitin receptor 19S-S5a. The results indicate a new link between hyperglycemia and impairment of cell functions.
PMCID: PMC2828656  PMID: 20009088
3.  Insulin Sensitivity Is Reflected by Characteristic Metabolic Fingerprints - A Fourier Transform Mass Spectrometric Non-Targeted Metabolomics Approach 
PLoS ONE  2010;5(10):e13317.
A decline in body insulin sensitivity in apparently healthy individuals indicates a high risk to develop type 2 diabetes. Investigating the metabolic fingerprints of individuals with different whole body insulin sensitivity according to the formula of Matsuda, et al. (ISIMatsuda) by a non-targeted metabolomics approach we aimed a) to figure out an unsuspicious and altered metabolic pattern, b) to estimate a threshold related to these changes based on the ISI, and c) to identify the metabolic pathways responsible for the discrimination of the two patterns.
Methodology and Principal Findings
By applying infusion ion cyclotron resonance Fourier transform mass spectrometry, we analyzed plasma of 46 non-diabetic subjects exhibiting high to low insulin sensitivities. The orthogonal partial least square model revealed a cluster of 28 individuals with alterations in their metabolic fingerprints associated with a decline in insulin sensitivity. This group could be separated from 18 subjects with an unsuspicious metabolite pattern. The orthogonal signal correction score scatter plot suggests a threshold of an ISIMatsuda of 15 for the discrimination of these two groups. Of note, a potential subgroup represented by eight individuals (ISIMatsuda value between 8.5 and 15) was identified in different models. This subgroup may indicate a metabolic transition state, since it is already located within the cluster of individuals with declined insulin sensitivity but the metabolic fingerprints still show some similarities with unaffected individuals (ISI >15). Moreover, the highest number of metabolite intensity differences between unsuspicious and altered metabolic fingerprints was detected in lipid metabolic pathways (arachidonic acid metabolism, metabolism of essential fatty acids and biosynthesis of unsaturated fatty acids), steroid hormone biosyntheses and bile acid metabolism, based on data evaluation using the metabolic annotation interface MassTRIX.
Our results suggest that altered metabolite patterns that reflect changes in insulin sensitivity respectively the ISIMatsuda are dominated by lipid-related pathways. Furthermore, a metabolic transition state reflected by heterogeneous metabolite fingerprints may precede severe alterations of metabolism. Our findings offer future prospects for novel insights in the pathogenesis of the pre-diabetic phase.
PMCID: PMC2955523  PMID: 20976215
4.  Medium Chain Acylcarnitines Dominate the Metabolite Pattern in Humans under Moderate Intensity Exercise and Support Lipid Oxidation 
PLoS ONE  2010;5(7):e11519.
Exercise is an extreme physiological challenge for skeletal muscle energy metabolism and has notable health benefits. We aimed to identify and characterize metabolites, which are components of the regulatory network mediating the beneficial metabolic adaptation to exercise.
Methodology and Principal Findings
First, we investigated plasma from healthy human subjects who completed two independent running studies under moderate, predominantly aerobic conditions. Samples obtained prior to and immediately after running and then 3 and 24 h into the recovery phase were analyzed by a non-targeted (NT-) metabolomics approach applying liquid chromatography-qTOF-mass spectrometry. Under these conditions medium and long chain acylcarnitines were found to be the most discriminant plasma biomarkers of moderately intense exercise. Immediately after a 60 min (at 93% VIAT) or a 120 min run (at 70% VIAT) a pronounced, transient increase dominated by octanoyl-, decanoyl-, and dodecanoyl-carnitine was observed. The release of acylcarnitines as intermediates of partial β-oxidation was verified in skeletal muscle cell culture experiments by probing 13C-palmitate metabolism. Further investigations in primary human myotubes and mouse muscle tissue revealed that octanoyl-, decanoyl-, and dodecanoyl-carnitine were able to support the oxidation of palmitate, proving more effective than L-carnitine.
Medium chain acylcarnitines were identified and characterized by a functional metabolomics approach as the dominating biomarkers during a moderately intense exercise bout possessing the power to support fat oxidation. This physiological production and efflux of acylcarnitines might exert beneficial biological functions in muscle tissue.
PMCID: PMC2902514  PMID: 20634953
5.  Metabonomic fingerprints of fasting plasma and spot urine reveal human pre-diabetic metabolic traits 
Metabolomics  2010;6(3):362-374.
Impaired glucose tolerance (IGT) which precedes overt type 2 diabetes (T2DM) for decades is associated with multiple metabolic alterations in insulin sensitive tissues. In an UPLC-qTOF-mass spectrometry-driven non-targeted metabonomics approach we investigated plasma as well as spot urine of 51 non-diabetic, overnight fasted individuals aiming to separate subjects with IGT from controls thereby identify pathways affected by the pre-diabetic metabolic state. We could clearly demonstrate that normal glucose tolerant (NGT) and IGT subjects clustered in two distinct groups independent of the investigated metabonome. These findings reflect considerable differences in individual metabolite fingerprints, both in plasma and urine. Pre-diabetes associated alterations in fatty acid-, tryptophan-, uric acid-, bile acid-, and lysophosphatidylcholine-metabolism, as well as the TCA cycle were identified. Of note, individuals with IGT also showed decreased levels of gut flora-associated metabolites namely hippuric acid, methylxanthine, methyluric acid, and 3-hydroxyhippuric acid. The findings of our non-targeted UPLC-qTOF-MS metabonomics analysis in plasma and spot urine of individuals with IGT vs NGT offers novel insights into the metabolic alterations occurring in the long, asymptomatic period preceding the manifestation of T2DM thereby giving prospects for new intervention targets.
Electronic supplementary material
The online version of this article (doi:10.1007/s11306-010-0203-1) contains supplementary material, which is available to authorized users.
PMCID: PMC2899018  PMID: 20676218
Metabolomics; UPLC-qTOF-MS; Diabetes; Pre-diabetes; Impaired glucose tolerance

Results 1-5 (5)