IGF-1 LR3

IGF-1 LR3 (Insulin-like Growth Factor-1 Long R3) is a modified analogue of human IGF-1 incorporating two structural changes that dramatically extend its biological activity: a 13 amino acid N-terminal extension and a Glu substitution at position 3 (replacing Arg3). Together, these modifications reduce IGF binding protein (IGFBP) affinity by approximately 1000-fold compared to native IGF-1, enabling sustained, reproducible IGF-1 receptor (IGF-1R) signalling in research systems where native IGF-1 would be rapidly sequestered by endogenous IGFBPs.

The IGFBP sequestration problem is fundamental to understanding why IGF-1 LR3 was developed. In blood and in cell culture media containing serum, greater than 99% of native IGF-1 is bound by IGFBPs — predominantly IGFBP-3 in a ternary complex with ALS (acid-labile subunit). This leaves less than 1% as free bioavailable peptide. The effective free concentration of native IGF-1 in serum-containing cell culture depends on the IGFBP composition of the specific serum lot used, creating batch-to-batch variability that makes quantitative IGF-1R pharmacology unreliable. IGF-1 LR3 bypasses this sequestration, providing a defined free concentration that enables reproducible dose-response characterisation across experiments and serum lots.

IGF-1R is a receptor tyrosine kinase (RTK) that forms a constitutive alpha2-beta2 heterotetrameric structure at the cell surface. IGF-1 LR3 binding to the L1 and CR domains of the IGF-1R alpha subunits induces conformational changes that activate the intracellular beta subunit tyrosine kinase domains, leading to trans-autophosphorylation at multiple tyrosine residues (Tyr1158, Tyr1162, Tyr1163 in the activation loop; Tyr1328 and Tyr1334 in the C-terminal domain). This activates two major downstream signalling cascades.

The PI3K/Akt/mTOR pathway is activated through IRS-1 (insulin receptor substrate-1) scaffold proteins. IGF-1R phosphorylation recruits IRS-1 via its PTB domain, and IRS-1 is subsequently phosphorylated at multiple tyrosines, creating docking sites for the p85 regulatory subunit of PI3K. PI3K generates PIP3, recruiting Akt and PDK1 to the plasma membrane. Akt phosphorylation at Thr308 (PDK1) and Ser473 (mTORC2) produces fully active Akt with over 100 substrates including TSC2 (mTORC1 activation), FOXO transcription factors (nuclear export, preventing atrophy gene expression), BAD (anti-apoptotic), and GSK3beta (inactivation). mTORC1 activates protein synthesis through S6K1 and 4E-BP1 phosphorylation.

The Ras/MAPK/ERK proliferative pathway operates via Shc/Grb2/SOS adapter complex activation downstream of IGF-1R, driving Ras-GTP loading and the Raf-MEK-ERK kinase cascade. ERK activation drives transcription of immediate early genes (c-Fos, c-Myc, Egr-1) promoting cell cycle progression from G1 to S phase.

Research applications: skeletal muscle anabolism (C2C12 myotubes — myotube diameter, MHC expression, protein synthesis by puromycin SUnSET); cancer cell IGF-1R pharmacology (MCF-7, LNCaP, DU145 — phospho-Akt, phospho-ERK, proliferation, survival); adipocyte differentiation (3T3-L1 preadipocytes — lipid accumulation, adipogenic gene expression); neuronal survival (SH-SY5Y, primary neurons — phospho-Akt, caspase-3, neurite length); and osteoblast/chondrocyte differentiation and matrix production.

Reconstitution: dissolve in 0.1% acetic acid (acidified water) at 0.1mg/mL stock, then dilute in neutral buffer or media. Do not reconstitute directly in neutral or alkaline pH — precipitation may occur. MW: approximately 9.1 kDa (83 amino acids). Store lyophilised at -20°C. Reconstituted acidic stock stable at 4°C for 2 weeks. For laboratory and analytical research purposes only.

For serum-free reconstitution: dissolve IGF-1 LR3 in 0.1% acetic acid at 0.1mg/mL, then dilute in neutral PBS or culture media immediately before use. Do not reconstitute directly at neutral pH — precipitation may occur. Working concentrations for most cell-based assays: 1-100 nM. Positive controls: insulin at 100nM (activates IR and IGF-1R), EGF at 10ng/mL (EGFR-mediated proliferation, MAPK/ERK control). Key selectivity check: use IGF-1R-selective antibody (alpha-IR3) at 10ug/mL to confirm IGF-1R-dependence of observed effects versus insulin receptor cross-activity at higher concentrations. MW: approximately 9.1 kDa. Store lyophilised at -20°C. For laboratory and analytical research purposes only.