Insulin-like Growth Factor 1 (IGF-1 or IGF-I), also known as Somatomedin C, is the dominant effector of Growth Hormone (GH) and is structurally homologous to Proinsulin. Human IGF-1 is synthesized as two precursor isoforms with N- and alternative C‑terminal propeptides (1). These isoforms are differentially expressed by various tissues (1). The 7.6 kDa mature IGF‑1 is identical between isoforms and is generated by proteolytic removal of the N- and C-terminal regions. Mature human IGF-1 shares 94% and 96% amino acid (aa) sequence identity with the mouse and rat orthologs, respectively (2). GH stimulates the production of IGF-1 in most tissues (3). Hepatocytes produce circulating IGF-1, while local IGF-1 is produced by many other tissues in which it has paracrine effects (1). IGF-1 induces the proliferation, migration, and differentiation of a wide variety of cell types during development and postnatally (4, 5). IGF-1 regulates glucose, fatty acid, and protein metabolism, steroid hormone activity, and cartilage and bone metabolism (6-11). It plays an important role in muscle regeneration and tumor progression (1, 12, 13). IGF-1 binds IGF1R, IGF2R, and the Insulin Receptor, although its effects are mediated primarily by IGF1R (14). IGF-1 also binds with strong affinity to IGF binding proteins (IGFBPs), which regulate the availability and biological activities of IGF-1 (15, 16).
Long R3 IGF-1 (LR3 IGF-1) is a 9.2 kDa synthetic analog of IGF-1 that is generated by modifying the aa sequence for mature human IGF-1. These modifications include the substitution of an Arg for Glu at position 3 of the mature IGF-1 sequence and the addition of a thirteen aa N-terminal extension, which is derived from methionyl porcine Growth Hormone (17). These aa changes generate a protein that is still capable of binding to IGF-1 and Insulin receptors, but shows considerably lower affinity binding to IGFBPs compared to wild-type IGF-1 (17, 18). As a result, LR3 IGF-1 has an increased half-life and displays increased biological potency compared to IGF-1 (17-22).
