IGFs (insulin like growth factors) are predominantly produced by the liver, although a variety of tissues produce the IGFs at distinctive times. The IGFs belong to the Insulin gene family, which also contains insulin and relaxin.
[1] The IGFs are similar by structure and function to insulin, but have a much higher growth-promoting activity than insulin. IGF-I expression is regulated by growth hormone.
[2] IGF-I signal through the tyrosine kinase type I receptor (IGF-IR).
[3] Mature IGFs are generated by proteolytic processing of inactive precursor proteins, which contain N-terminal and C-terminal propeptide regions.
[4] Recombinant human IGF-I and IGF-II are globular proteins containing 70 and 67 amino acids, respectively, and three intra-molecular disulfide bonds.