Pathophysiology
Clinical meaning
Type 2 diabetes mellitus results from the interplay between insulin resistance in target tissues and progressive pancreatic beta-cell dysfunction, a concept described as the 'ominous octet' of pathophysiological defects. Insulin resistance: In normal physiology, insulin binds to the insulin receptor (a tyrosine kinase receptor) on target cells, triggering autophosphorylation and activation of the IRS-1/PI3K/Akt signaling cascade, which promotes GLUT4 transporter translocation to the cell membrane for glucose uptake (primarily in skeletal muscle and adipose tissue), stimulates glycogen synthesis in liver and muscle, and suppresses hepatic gluconeogenesis. In insulin resistance, this signaling pathway is impaired at multiple levels: (1) serine phosphorylation of IRS-1 (instead of tyrosine phosphorylation) by inflammatory kinases (JNK, IKK-beta) inactivates the insulin signaling cascade; (2) excess free fatty acids from visceral adipose lipolysis activate protein kinase C (PKC) in muscle and liver, further impairing insulin receptor signaling; (3) adipose tissue dysfunction shifts adipokine secretion toward pro-inflammatory mediators (TNF-alpha, IL-6, resistin) and away from protective adiponectin, creating a chronic low-grade inflammatory state that perpetuates insulin resistance. The result is decreased glucose uptake in skeletal muscle (accounting for ~75% of...