IGF-1

Insulin-like growth factor 1 (IGF-1, somatomedin C) is a 70-amino-acid peptide hormone produced predominantly by hepatocytes in response to growth-hormone stimulation, with additional local production in muscle, bone, cartilage, and other peripheral tissues. It is the principal downstream mediator of GH's anabolic effects on tissue growth, protein synthesis, and chondrocyte/osteoblast proliferation, and is the standard pharmacodynamic readout in growth-hormone secretagogue research because it integrates the pulsatile GH signal into a stable circulating concentration measurable by routine immunoassay. IGF-1 acts through the type-1 IGF receptor (IGF-1R), a transmembrane tyrosine kinase with strong structural homology to the insulin receptor; circulating IGF-1 is largely bound to IGFBPs (notably IGFBP-3 with the acid-labile subunit), which regulate bioavailability. The IGF-1 / IGF-1R / PI3K / Akt / mTOR cascade drives much of the protein-synthetic effect attributed to growth hormone, while parallel MAPK signalling drives proliferative effects. Synthetic analogues such as IGF-1 LR3 (long-arginine-3 IGF-1, reduced IGFBP affinity, extended half-life) are studied in tendon and skeletal-muscle research; mecasermin (recombinant human IGF-1) is approved for severe primary IGF-1 deficiency.