IGF-1 Research Guide: Receptor Signalling and the Binding Protein Problem
What IGF-1 is
Insulin-like growth factor 1 is a 70-residue polypeptide produced largely by the liver under growth hormone stimulation, though many tissues also produce it locally. It is named for its structural resemblance to proinsulin — the two share a characteristic fold and a degree of receptor cross-reactivity, which is why insulin and IGF-1 biology are permanently entangled in the literature.
| Property | Detail |
|---|---|
| Class | Protein hormone (growth factor) |
| Length | 70 amino acids |
| Structural family | Insulin superfamily |
| Principal receptor | IGF-1 receptor (IGF-1R), a receptor tyrosine kinase |
| Regulation | Six IGF-binding proteins (IGFBP-1 to 6) |
| Upstream driver | Growth hormone, primarily hepatic |
| Typical supplied form | Lyophilized powder, 1 mg per vial |
The GH–IGF-1 axis
Growth hormone released from the pituitary acts on the liver, the liver produces IGF-1, and circulating IGF-1 feeds back to suppress further GH release. That loop is the reason IGF-1 is frequently measured as a proxy for GH status: GH itself is released in pulses and is awkward to sample, whereas IGF-1 is comparatively stable in circulation.
It also explains why IGF-1 turns up in studies of growth hormone secretagogues. Compounds studied for their effect on GH release are often assessed partly through what happens downstream in the IGF-1 axis rather than through GH measurement alone.
Why the binding proteins matter
This is the part that separates a serious treatment of IGF-1 from a superficial one. The overwhelming majority of circulating IGF-1 is not free. It is bound to one of six IGF-binding proteins, predominantly IGFBP-3 in a ternary complex with an acid-labile subunit. Bound IGF-1 cannot engage its receptor.
The consequences are significant. Binding proteins extend IGF-1’s half-life from minutes to hours, they buffer it so that receptor exposure is not driven by momentary fluctuations, and they act as a regulatory layer in their own right — several IGFBPs have receptor-independent activities of their own. Any experimental system involving IGF-1 has to account for what binding proteins are present, because total concentration and bioavailable concentration are very different numbers.
Receptor signalling
IGF-1R is a receptor tyrosine kinase structurally similar to the insulin receptor. Ligand binding triggers autophosphorylation and recruitment of adaptor proteins, feeding two principal downstream routes:
- PI3K–Akt — the branch most associated with protein synthesis, cell survival and metabolic effects, including downstream mTOR signalling.
- Ras–MAPK — the branch most associated with proliferation and differentiation.
Because IGF-1R and the insulin receptor are close relatives and can form hybrid receptors, and because IGF-1 has measurable affinity for the insulin receptor at higher concentrations, clean attribution of an effect to one receptor over the other requires deliberate experimental design.
Where IGF-1 sits relative to the secretagogues
Peptides such as CJC-1295, Ipamorelin and Tesamorelin are studied for their effects upstream — on growth hormone release from the pituitary. IGF-1 is the downstream mediator. Studying them addresses different points on the same axis.
| Compound | Position on the axis | What it acts on |
|---|---|---|
| Tesamorelin | Upstream | GHRH receptor on pituitary somatotrophs |
| CJC-1295 | Upstream | GHRH receptor, extended duration |
| Ipamorelin | Upstream | Growth hormone secretagogue receptor (ghrelin receptor) |
| IGF-1 | Downstream | IGF-1 receptor on target tissues |
Handling notes
IGF-1 is a folded protein, not a short linear peptide, and that changes the handling calculus. Its three disulfide bonds hold the tertiary structure that receptor binding depends on. Denaturation is therefore a real failure mode rather than a theoretical one: an IGF-1 preparation can be chemically present and functionally inactive.
- Reconstitute gently — no shaking, no hard vortexing. Foaming is particularly damaging to folded proteins.
- Avoid repeated freeze–thaw cycles; aliquot before the first freeze.
- Avoid alkaline conditions, which promote disulfide scrambling.
- Vials are typically 1 mg rather than 10 mg, so reconstitution volumes and concentration calculations differ from the rest of the catalogue — 1 mg in 1 mL is 1 mg/mL, not 10.