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Skeletal Muscle Insulin Resistance in Type 2 Diabetes Investigated by Two Approaches:
Studies of Insulin Signaling into Glycogen Synthase
and Proteome Analysis


Kurt Højlund  


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Accepted by: Faculty of Health Sciences University of Southern Denmark
Defended on: November 14, 2003
Official opponents: Henrik Vestergård , Flemming Pociot , Ole Skøtt
Tutors: Henning Beck-Nielsen , Jørgen F. P. Wojtaszewski , Peter Mose Larsen and Stephen J. Fey

Published in the PhD Database: March 18, 2004


English abstract
The PhD dissertation consists of a review and four papers. It is based on experiments carried out during my employment at the Diabetes Research Centre, Odense University Hospital, Denmark, in the period from 1999 to 2002.

Impaired insulin activation of glycogen synthase (GS), a key enzyme in the regulation of glycogen synthesis, plays an important pathophysiological role in the development of insulin resistance in skeletal muscle and hence type 2 diabetes (T2DM). The aim of the PhD study was to elucidate the molecular mechanisms underlying skeletal muscle insulin resistance in T2DM by two approaches: 1) To investigate the effect of insulin on proximal and distal components of the insulin signalling cascade (IRS-1/PI3K/Akt/GSK-3) and the activation of GS by dephosphorylation of specific serine residues. Moreover, to study other enzymes that may regulate GS activity. 2) To search for alterations in the expression and post-translational modification of proteins in skeletal muscle of patients with T2DM by proteome analysis.

We found that impaired insulin activation of GS in T2DM was not caused by defects in the insulin signalling cascade, but rather due to increased phosphorylation of GS at NH 2 -terminal sites, which was not regulated by insulin. We observed no abnormalities in the expression or activity of AMP-activated protein kinase that could explain this defect. Insulin-mediated down-regulation of PP2A protein content was associated with a normal insulin action on glucose storage, glucose and lipid oxidation, but was absent in skeletal muscle of patients with T2DM. Using proteome analysis we identified eight potential markers of skeletal muscle insulin resistance in T2DM. The data suggest a role for increased cellular stress and perturbations in the mitochondrial function including ATP synthesis.

We conclude that future studies of the molecular mechanisms responsible for impaired insulin activation of GS and skeletal muscle insulin resistance should include the potential role of mitochondrial dysfunction and increased cellular stress for the development of T2DM.



Danish abstract
Ph.D. afhandlingen består af en oversigt samt fire artikler og udgår fra Diabetesforskningscentret, Odense Universitetshospital.

En defekt i insulinaktiveringen af glykogensyntasen (GS), nøgleenzymet i glykogensyntesen, spiller en væsentlig patofysiologisk rolle for udviklingen af insulinresistens i skeletmuskulaturen og dermed type 2 diabetes (T2DM). Formålet med Ph.D. studiet var at belyse de underliggende cellulære mekanismer bag muskulær insulinresistens ved T2DM ved to strategier: 1) At undersøge insulinvirkning på enzymer i insulinsignalkaskaden (IRS-1/PI3K/Akt/GSK-3) og på aktiveringen af GS ved defosforylering af specifikke seriner. Herunder at studere andre enzymer som kunne påvirke GS-aktivitet. 2) At finde ændringer i ekspression og posttranslationel modifikation af proteiner i skeletmuskulatur hos patienter med T2DM ved proteomanalyse.

Vi fandt at nedsat insulinaktivering af GS ved T2DM ikke skyldes defekter i insulinsignalkaskaden, men en øget fosforylering af seriner i den N-terminale ende af GS. Vi fandt ingen ændringer i AMP-aktiveret protein kinase, som kunne forklare denne defekt. Insulinmedieret nedregulering af PP2A proteinmængde var relateret til normal insulineffekt på glukose og fedtmetabolismen, men var ikke tilstede hos patienter med T2DM. Ved proteomanalyse fandt vi otte potentielle proteinmarkører for muskulær insulinresistens ved T2DM, som tyder på øget cellulær stress og forstyrrelser i den mitokondrielle funktion, herunder specielt ATP-dannelsen.

Vi konkluderer at fremtidige studier af de cellulære mekanismer bag defekt insulinaktivering af GS og muskulær insulinresistens også bør fokusere på betydningen af mitokondriel dysfunktion og øget intracellulær stress for udviklingen af T2DM.