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.