Figure 1: Insulin or insulin-like growth factor 1 (IGF-1) binds to its receptor and activates the receptor tyrosine kinase. The receptor undergoes autophosphorylation and provides a binding site for the insulin receptor substrate (IRS) proteins. Once bound, these IRS proteins undergo phosphorylation on tyrosine residues. These phosphorylated tyrosine residues provide a docking site for the p85 regulatory subunit of the Class I phosphatidylinositol 3-kinase. In turn, the p110 catalytic subunit is released, becomes activated and catalyzes the production of phosphatidylinositol (3, 4, 5)-triphosphate (PIP3) from phosphatidylinositol (3, 4)-biphosphate (PIP2). PIP3 then activates protein kinase B (Akt). Akt then serves as a branch point for a variety of downstream signaling pathways. Insulin and IGF-1 can also stimulate cell growth through the mitogen activated protein kinase pathway/extracellular signal related kinase (MEK/ERK) pathway.