Alterations of Insulin Signaling in Type 2 Diabetes: a Review of the Current Evidence from Humans

A generally accepted view posits that insulin resistant condition in type 2 diabetes is caused by defects at one or several levels of the insulin-signaling cascade in skeletal muscles, adipose tissue and liver, that quantitatively constitute the bulk of the insulin-responsive tissues. Hence, the gradual uncovering of the biochemical events defining the intracellular signaling of insulin has been quickly followed by clinical studies on humans attempting to define the molecular defect(s) responsible for the establishment of the insulin resistant state. While the existence of molecular defects within the insulin signal transduction machinery is undisputed, contrasting data exist on what is the principal molecular alteration leading to insulin resistance. Such discrepancies in the literature may depend on: 1) different subject characteristics, 2) methodological differences 3) small cohorts of subjects, and - not least - 4) intrinsic limitations in studying every detail of the insulin signaling cascade. Here, we review the studies on humans exploring the defects of the insulin signaling cascade generated by insulin resistance and type 2 diabetes, focusing on muscle and adipose tissue - which account for most of the glucose disposal capacity of the body - with focus on the unresolved discrepancies present in the literature.