Insulin: The Receptor and Signaling Pathways

The insulin receptor is a receptor tyrosine kinase composed of disulfide-linked α/β dimers forming a transmembrane heterotetramer. It contains two extracellular α subunits and two membrane-spanning β subunits. The α subunits inhibit the inherent tyrosine kinase activity of the β subunits until insulin binding releases this inhibition, triggering autophosphorylation and receptor activation.

Insulin binding to the α subunits stimulates the β subunit's tyrosine kinase activity, allowing them to phosphorylate each other. This autophosphorylation and transphosphorylation activates the receptor, which then phosphorylates intracellular proteins such as Shc and IRS. These phosphorylated proteins interact with downstream effectors like MAP kinase and PI3-kinase, extending the signaling cascade.

PI3-kinase interacts with IRS proteins to generate PIP3, which activates and anchors Akt in the membrane. Akt2 isoform controls essential downstream steps for glucose uptake in skeletal muscle and adipose tissue. This signaling promotes GLUT4 translocation to the plasma membrane, facilitating glucose entry into insulin-responsive tissues.

GLUT4, predominantly expressed in insulin-responsive tissues like adipocytes and skeletal muscle, translocates rapidly to the plasma membrane following insulin receptor activation. This translocation facilitates inward glucose transport. Insulin signaling also decreases GLUT4 endocytosis, increasing its plasma membrane residence time and maximizing glucose uptake capacity.

Once inside cells, glucose is phosphorylated to glucose-6-phosphate by hexokinases. Glucose-6-phosphate can then enter the glycolytic or pentose phosphate pathway for energy production. Alternatively, it can be isomerized to glucose-1-phosphate and stored as glycogen in muscle and liver tissues.

The number of insulin receptors per cell varies significantly depending on tissue type and metabolic function. Erythrocytes have approximately 40 receptors, while adipocytes and hepatocytes have up to 300,000 receptors per cell. This variation reflects the differential insulin sensitivity and glucose uptake capacity of different tissues.

Adipocytes, skeletal muscle, and hepatocytes are the primary insulin-responsive tissues. These cells express high numbers of insulin receptors and contain abundant GLUT4 transporters. They are responsible for the majority of glucose uptake and storage following insulin stimulation, playing central roles in glucose homeostasis and energy metabolism.