Molecular biology of insulin resistance

Insulin resistance is an essential feature of a great variety of clinical disorders, like diabetes mellitus, obesity, essential hypertension, and is primarily due to a defect in hormone action at the cellular level. In the past decade application of novel research techniques including recombinant DNA technology have paved the way to understand the mechanisms of insulin action and its alterations at the molecular level. The first step in insulin action is the activation of the insulin receptor. The insulin receptor is a tetrameric protein consisting of two extracellular alpha- and two transmembrane beta-subunits. Binding of insulin to the alpha-subunit causes autophosphorylation of the intracellular beta-subunit region on tyrosine residues thereby activating the receptor. How the hormonal signal is subsequently transduced within the cell is still quiet unclear. The activated insulin receptor appears to couple to cytosolic receptor substrates which can affect different signaling cascades eliciting the pleiotropic hormone response on cell metabolism and growth. Most proteins involved in the signal transduction pathway of insulin are not known yet, but each of them might play a role in the various forms of insulin resistance. Taking the insulin receptor as an exemplary protein involved in insulin action we review molecular mechanisms regulating insulin receptor activity, gene expression, and the role of natural occurring insulin receptor gene mutations in patients with insulin resistant diabetes mellitus. It is outlined how the combination of both clinical medicine and molecular biology not only helps to understand insulin action and the pathogenesis of insulin resistance, but also leads to new avenues in the differential diagnosis, therapy, and possibly prevention of this heterogenous but most frequent metabolic and endocrine disorder.