Diabetes mellitus may be caused by a deficiency in either the amount of circulating insulin or the ability of cells to respond to the hormone. The long term objectives of this project are to delineate mechanisms involved in insulin action and insulin resistance. It can be inferred that insulin binding to a receptor on the plasma membrane generates a signal(s) (not yet identified) that is transmitted to the inside of the cell and causes changes in metabolic processes. Insulin resistance can be caused by a defect at a step after receptor binding, as is the case in denervated skeletal muscle.
One aim i s to identify the defect in the denervated muscle responsible for its resistance to the hormone. We have evidence that increased protein phosphorylation is involved, and will try to identify the kinase and/or phosphatase responsible.
Other aims are to determine if motor neurons maintain the ability of muscle to respond to insulin by releasing trophic substances or by promoting muscle activity. More complete understanding of the steps in the pathway of insulin action in normal cells might be necessary to define post-receptor defects. We propose to continue our investigation of the mechanism by which insulin stimulates the dephosphorylation and activation of glycogen synthase (GS). Glucose transport-dependent and -independent pathways exist for insulin-stimulated dephosphorylation of GS in fat cells.
Aims are to find out if these 2 pathways exist in muscle. GS is phosphorylated on several sites and we plan to determine which sites are affected by insulin. Such information is essential for understanding the mechanism of GS activation, and might provide clues concerning the mediators of insulin action. Insulin is known to stimulate the phosphorylation of some proteins. Increased phosphorylation of an appropriate protein might trigger dephosphorylation of others. For example, inhibitor 2 (I-2) is a regulatory protein which, when in a dephosphorylated form, is a potent inhibitor of Type I phosphatase. Phosphorylation of I-2 on an appropriate site results in phosphatase activation. We have evidence that insulin stimulates the phosphorylation of I-2 in fat cells.
An aim i s to determine if the hormonal effect involves sites in I-2 that affect its phosphatase inhibitory activity. Another aim is to determine if the effects of other agents, such as glucagon and adrenergic agonists, on protein phosphorylation could be mediated, in part, through changes in I-2 phosphorylation.
Showing the most recent 10 out of 72 publications
