Insulin-like growth factor-1 (IGF-1) is a peptide hormone under control of the growth hormone. IGF-1 promotes cell proliferation, growth and survival. Six specific binding proteins, IGFBP 1-6, allow for a more nuanced control of IGF activity.
The IGF-1 receptor (IGF-1R) is a transmembrane tyrosine kinase protein. IGF-1-induced receptor activation results in autophosphorylation followed by an enhanced capability to activate downstream pathways. Activated IGF-1R phosphorylates SHC and IRS-1. SHC along with adapter molecules GRB2 and SOS forms a signaling complex that activates the Ras/Raf/MEK/ERK pathway. ERK translocation to the nucleus results in the activation of transcriptional regulators ELK-1, c-Jun and c-Fos which induce genes that promote cell growth and differentiation. IRS-1 activates pathways for cell survival via the PI3K/PDK1/AKT/BAD pathway. IRS-1 also activates pathways for cell growth via the PI3K/PDK1/p70RSK pathway. IGF-1 also signals via the JAK/STAT pathway by inducing tyrosine phosphorylation of JAK-1, JAK-2 and STAT-3. SOCS proteins are able to inhibit the JAKs thereby inhibiting this pathway.
The adapter protein GRB10 interacts with IGF-IR. GRB10 also binds the E3 ubiquitin ligase NEDD4 and promotes ligand stimulated ubiquitination, internalization, and degradation of the IGF-IR as a means of long-term attenuation of signaling.