Research overview
IGF-1 LR3 (Long R3 IGF-1, also written LR3IGF-I) is an engineered form of human insulin-like growth factor 1 (IGF-1), developed in Adelaide, Australia, in the early 1990s. It has arginine in place of glutamic acid at position 3 and a 13-amino-acid extension at the start of the chain, mostly taken from pig growth hormone, and it was first produced in E. coli bacteria (Francis et al., J Mol Endocrinol 1992). These changes mean it binds very poorly to IGF-binding proteins, which bind IGF-1 and hold back its activity. In cells that make these binding proteins it was more potent than IGF-1, but in cells that do not, it was less potent. In rats it is cleared from the blood faster than IGF-1, not more slowly, yet it was still more potent for several measures (Tomas et al., J Endocrinol 1996).
Most published work is in cell culture, where it is widely used as a growth-factor supplement, including in Chinese hamster ovary (CHO) cells used to make antibodies. Animal studies, mainly in rats and pigs, looked at gut growth, body and organ weight, and blood sugar. There are no published human studies, so its effects, handling in the body and safety in people are unknown, and it is not a licensed medicine. Supplied for laboratory research use only.
Mechanism of action
IGF-1 LR3 binds and activates the IGF-1 receptor, a receptor tyrosine kinase on the cell surface that switches on signalling inside the cell for growth, division and protein synthesis. Because it binds IGF-binding proteins very weakly, more of it is free to reach the receptor than with native IGF-1. In rat muscle cells it was more potent than IGF-1 at stimulating protein and DNA synthesis; in rats it had stronger effects on body and organ weight, and in pigs a stronger, longer-lasting fall in blood sugar, despite faster clearance from the blood.
