Biology · BEGINNER LESSON
IGF
IGF stands for insulin-like growth factor. It is a family of related peptides, mainly IGF-I and IGF-II, that help control growth, cell survival, and cell differentiation through receptors, binding proteins, and local tissue signaling.
What IGF is and what is measured
Insulin-like growth factors, or IGFs, are small peptide signals in the body. The main members named in the sources are IGF-I and IGF-II. What researchers often measure is the amount of IGF in blood or in a tissue, along with related partners such as IGF-binding proteins (IGFBPs), especially IGFBP-3. In blood, most IGF is not free; it is carried in complexes, so measured levels reflect both the ligand itself and how much is bound and available to tissues. Source evidence shows that circulating IGF-I is usually low in free form and is mostly protein-bound. [0,1,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC10706945Role of the Insulin-like Growth Factor (IGF) Axis in Diseases [2] Insulin-like growth factor 1 (IGF-1): a growth hormone - PMC - NIH [3] The Insulin-like Growth Factor System: Towards Clinical Applications
How IGF works
IGFs act in several ways. Endocrine signaling means a signal made in one place, such as the liver, travels through blood to distant tissues. Paracrine signaling means the signal acts on nearby cells, and autocrine signaling means the same cell both makes and responds to the signal. The sources say the liver is the main source of circulating IGF-I under growth hormone (GH) control, while many tissues also make IGFs locally. IGFs mainly act through the IGF-I receptor, a receptor protein on cells that triggers internal signaling cascades such as RAS/MAPK and PI3K/AKT. These pathways help regulate cell proliferation, differentiation, survival, and glucose and lipid metabolism. IGF-I also has some ability to bind the insulin receptor because of structural similarity to insulin. [0,1,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC10706945Role of the Insulin-like Growth Factor (IGF) Axis in Diseases [2] Insulin-like growth factor 1 (IGF-1): a growth hormone - PMC - NIH [3] The Insulin-like Growth Factor System: Towards Clinical Applications
How binding proteins shape IGF activity
IGFBPs are a family of six high-affinity binding proteins, named IGFBP-1 through IGFBP-6. They bind IGFs tightly and usually reduce immediate access of IGF to its receptor, but in some settings they can enhance IGF action. IGFBP-3 is the main circulating binding protein, and with acid labile subunit it forms a ternary complex that lengthens IGF half-life in serum. The sources also note that binding proteins can act in ways that are partly independent of IGF receptors, so their effects are not always simple blocking. [0,1,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC10706945Role of the Insulin-like Growth Factor (IGF) Axis in Diseases [2] Insulin-like growth factor 1 (IGF-1): a growth hormone - PMC - NIH [3] The Insulin-like Growth Factor System: Towards Clinical Applications
Limitations and why context matters
IGF biology is complex because many components vary by tissue, cell environment, nutrition, sex steroids, cytokines, and growth hormone. The sources stress that the same IGF signal can have different effects in different tissues. They also note that dysregulation of the IGF system is linked with disease risk and progression, including cancer and Alzheimer’s disease, but that does not mean a single blood value alone explains a disease. IGF action depends on free versus bound hormone, receptor type, local production, and the surrounding cellular milieu. [0,1,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC10706945Role of the Insulin-like Growth Factor (IGF) Axis in Diseases [2] Insulin-like growth factor 1 (IGF-1): a growth hormone - PMC - NIH [3] The Insulin-like Growth Factor System: Towards Clinical Applications
SEE THE IDEA
How IGF signaling is controlled
Make IGF-I and IGF-II; the liver supplies much of circulating IGF-I under growth hormone control.
IGFBPs carry IGF in blood and often limit immediate receptor access; IGFBP-3 is a major carrier.
Cell-surface receptor that starts signaling after IGF binds.
Signals influence growth, survival, differentiation, and metabolism.
Production -> transport by binding proteins -> receptor binding -> internal signaling -> cell response
IGF activity depends on where it is made, how much is bound, and which receptor-linked pathways are available in the tissue.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.