Key Takeaways
- •IGF-1 forms a single chain peptide with 70 amino acids across four domains: B, C, A, and D.
- •Growth hormone from the pituitary, along with insulin, promotes IGF-1 production in the liver and other tissues.
- •During puberty, serum IGF-1 levels rise 2-3 fold in males and females, driven by increased pulsatile growth hormone secretion and elevated sex steroids.
IGF-1 Structure and Basic Features
IGF-1 forms a single chain peptide with 70 amino acids across four domains: B, C, A, and D. The A and B domains show structural homology to insulin's A and B chains, sharing 50% sequence similarity. Domain C resembles the connecting C-peptide in proinsulin, while domain D has no equivalent in insulin.
Growth hormone from the pituitary, along with insulin, promotes IGF-1 production in the liver and other tissues. This peptide circulates at high levels in blood, with synthesis peaking during postnatal development.
During puberty, serum IGF-1 levels rise 2-3 fold in males and females, driven by increased pulsatile growth hormone secretion and elevated sex steroids. IGF-1 functions as an endocrine hormone through the bloodstream and as a paracrine or autocrine growth factor in local tissues.
Circulation and Regulation by Binding Proteins
Unlike insulin, which moves freely in blood, IGFs travel bound to IGF-binding proteins (IGFBPs). Plasma free IGF-1 levels depend on six high-affinity IGFBPs (IGFBPs-1 to -6), each around 30 kDa in size.
These IGFBPs bind 99% of circulating IGF-1, controlling its interaction with the IGF-1 receptor (IGF-1R). In humans, about 80% of IGF-1 circulates with the IGFBP-3/acid labile subunit complex.
All IGFBPs limit IGF action by trapping the peptides, yet IGFBP-1, -3, and -5 can enhance it under certain conditions. Normal growth requires precise IGF levels, managed by free IGF availability through IGFBPs. In tissues, IGF releases from IGFBP complexes via proteolysis or IGFBP attachment to the extracellular matrix.
Abnormal IGF-1 circulation causes issues: excess from growth hormone overproduction leads to acromegaly, while low levels from inactive growth hormone receptors result in Laron dwarfism. For detailed peptide terms, check the Peptide Glossary.
IGF-1 Receptors and Signaling
IGF-1 attaches to cell surface receptors including IGF-1R, IGF-2R, and insulin receptor (IR) with varying affinities. Structural parallels exist between IGF ligands and insulin, as well as between IGF-1R and IR, allowing system cross-talk.
IGF-1R forms hybrid receptors with IR isoforms IR-A or IR-B, though their cellular roles remain unclear. Binding to the extracellular α-subunit alters conformation, triggering tyrosine phosphorylation on β-subunits.
This boosts receptor kinase activity, activating pathways that control proliferation and survival. Major routes include mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), and phosphatidylinositide-3-kinase (PI3-K)/Akt-1.
Studies like alanine scanning mutagenesis, antibody work, and receptor chimeras reveal IGF-1 uses residues on the molecule's opposite surface for receptor binding. Activation sparks signaling cascades promoting cell proliferation, differentiation, migration, and anti-apoptosis protection.
IGF-2 Structure and Fetal Expression
IGF-2 consists of a 67-amino-acid single-chain polypeptide, mainly produced in the liver. NMR spectroscopy confirms its tertiary structure mirrors IGF-1's, with four domains (A-D).
IGF-2's B and A domains share 50% sequence similarity with insulin's B and A chains. Its three intramolecular disulfide bonds align with those in proinsulin.
IGF-2 expresses paternally in fetus and placenta, essential for placental development and fetal growth. Its synthesis avoids growth hormone dependence and peaks during fetal stages.
The IGF2 gene uses four promoters (P1-P4); only P1 acts biallelically, while others face methylation-based imprinting, limiting expression to one allele. IGF-2 variants differ in molecular weight, but the 7.5 kDa form binds receptors most effectively.
IGF-2 Receptors and Physiological Roles
IGF-2 binds multiple receptors: high affinity for IGF-2R and IR-A, moderate for IR-B, low for IGF-1R, and also IR-A/IGF-1R heterodimers. IGF-2R, or cation-independent mannose-6-phosphate receptor (IGF-2R/M-6-P), differs structurally from IGF-1R.
This monomeric glycoprotein spans the membrane with 15 extracellular repeats (one IGF-2 site, two M-6-P sites), a 23-amino-acid transmembrane domain, and 163-amino-acid intracellular region. It binds IGF-1 lowly and IGF-2 highly, ignoring insulin.
Lacking a signaling domain, IGF-2R internalizes and degrades IGF-2, clearing it from circulation. It also handles M-6-P proteins for proliferation and lysosomal enzymes. IGF-2 prefers IGFBP-2 and -6.
IGF-2 drives muscle and bone development, placental and fetal growth. Evidence links it to tumor growth via increased cell proliferation in culture. Tools like the Reconstitution Calculator aid research on such compounds.
Key Takeaways on IGFs
Both IGF-1 and IGF-2 share insulin-like structures and influence growth through complex binding and signaling. Dysregulation leads to disorders like acromegaly or Laron dwarfism for IGF-1, and supports fetal development for IGF-2.
Researchers value precise handling; explore our catalog for related research materials. Reference: Shabanpoor, F., Separovic, F., & Wade, J. D. (2009). The human insulin superfamily of polypeptide hormones. Vitamins & Hormones, 80, 1-31.
