Overt type 2 diabetes mellitus is the most prevalent form of diabetes, accounting for approximately 90% of the 20.8 million cases in the United States. Unlike type 1 diabetes mellitus, type 2 diabetes mellitus is most commonly caused by a defect in insulin action on cells rather than by a defect in the insulin itself1. It has recently been shown that overactivity of glycogen synthase kinase 3 (GSK-3) may be implicated in the impaired insulin action2. Although GSK-3 is a distal element of the insulin signaling pathway (which includes insulin receptor substrate 1 or IRS-1), when it is overactive, this affects both insulin signaling and the removal of glucose from the blood stream by skeletal muscle.
Firstly, when GSK-3 is activated it phosphorylates glycogen synthase (GS) and, as GS is inactive in its phosphorylated form, this inhibits glycogen synthesis. Glycogen is synthesized from glucose and this process is essential to removing excess glucose from the cells and reducing the hyperglycemia that hallmarks diabetes. Secondly, GSK-3’s active form plays a role in phosphorylation of the serine 307 (Ser307) molecule on IRS-1 (although it cannot directly do this and it possibly works through a kinase known as JNK). This causes a conformational change that prevents the insulin receptor from phosphorylating IRS-1 on tyrosine residues. Tyrosine phosphorylation activates IRS-1 allowing it to interact with and activate the next enzyme in the pathway, PI3-kinase, and continue the downstream process that brings glucose into the cell for glycogen synthesis3.
Henriksen’s paper outlines the results of a study that investigated the inhibition of GSK-3 on skeletal muscle from overtly type 2 diabetic Zucker Diabetic Fatty (ZDF) rats. Inhibition of GSK-3 activity increased the uptake of glucose into the cell and the activation of IRS-1 by tyrosine phosphorylation.
1. American Diabetes Association
2. Role of Glycogen Synthase Kinase-3 in Insulin Resistance and Type 2 Diabetes. Erik J. Henriksen and Betsy B. Dokken
3. Short-term in vitro inhibition of glycogen synthase 3 potentiates insulin signaling in type I skeletal muscle of Zucker Diabetic Fatty rats. Erik J. Henriksen and Mary K. Teachey
In this study, male lean Zucker and type 2 diabetic Zucker Diabetic Fatty (ZDF) rats at twelve weeks of age were used. The rats were deeply anesthetized with pentobarbital sodium, and soleus muscle strips were prepared and incubated in the absence or presence of insulin (5 mU/ml) and in the absence or presence of the GSK-3 inhibitor CT98014 (1 µM), which has been used previously by the same research group. Muscles were used for the assessment of GS activity, the magnitude of insulin signaling function, and glucose transport activity.
The ratio of GS activity without glucose 6-phosphate to that in the presence of 5 mM of glucose 6-phosphate was used as the measure of GS activity. To assess the...