IGF-1 is a growth hormone that plays a role in a variety of
biological events. It is produced primarily by hepatocytes, serving an
endocrine function. It is also produced by many other cells, where it may act
in an autocrine or paracrine manner. It binds two tyrosine kinase receptors,
the IGF-1 receptor (IGF1R) and the insulin receptor, to initiate downstream
events like the AKT and PI3K signal transduction pathways. This triggers cell
proliferation and protects cells from apoptosis. IGF-1 also interacts with
seven IGF-binding proteins (IGFBP-1 through IGFBP7), influencing IGF-1 binding
to IGF1R and increasing IGF-1 half-life to closely regulate IGF-1 signaling.
For example, IGFBP-3 binds over 90% of the total IGF in serum in a complex of
IGF, IGFBP, and an acid-labile subunit. This ternary complex greatly stabilizes
IGF in the circulation, changing the half-life from minutes to hours. Proteases
also facilitate IGF-1R binding by cleaving IGFBPs to modify their affinity for
IGF or completely eliminate the IGFBP. The interactions of IGF, IGFBP, IGFBP
proteases, and IGF receptors are referred to as the IGF axis.
The IGF axis affects many primary physiological and
pathological processes, including development, growth, metabolic regulation,
tumorigenesis, atherosclerosis, and angiogenesis. Its ability to inhibit
apoptosis and stimulate cell growth and proliferation plays a significant role
in prenatal development, growth to adulthood, and metabolic control. Serum
levels of IGF-1 have been reported to increase from birth to puberty, followed
by a slow decline through adulthood. IGF-1 also induces amino acid uptake, protein
synthesis, and glucose utilization. In the brain, IGF-1 acts as a neurotrophic
factor to promote neurogenesis and neuronal survival. Exercise increases levels
of IGF-1 in blood serum, indicating it plays a role as a key mediator of
exercise-induced neurogenesis.
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