IGF-1 LR3 is an 83-residue IGF-I analog built to escape IGF-binding proteins. This guide covers how that design produces its potency, what the cell and rat studies show, what is actually known about its persistence, and how to test and handle it.
IGF-1 LR3 (Long R3 IGF-I) is an 83-amino-acid analog of human insulin-like growth factor I. It comes from a family of IGF-I analogs that a group at the CSIRO Division of Human Nutrition in Adelaide, Australia, characterized in 1992 as reagents for studying how IGF-I works [1]. The analog is useful because it binds poorly to the IGF-binding proteins that normally hold IGF-I in check. For the same reason, results obtained with LR3 cannot simply be read as results for native IGF-I. This guide covers the biology behind the design, what each line of evidence shows and in which model, the gaps in what is known, and practical points on testing and handling.
Native IGF-I is a 70-amino-acid polypeptide related in structure to insulin. It acts through the IGF-1 receptor, a receptor tyrosine kinase. When IGF-I binds the receptor's alpha subunit, the beta subunit's kinase is activated and phosphorylates adaptor proteins such as the insulin receptor substrates (IRS) and SHC. These adaptors recruit the p85 subunit of PI 3-kinase, GRB2 and SHP2, which switch on the PI 3-kinase pathway and the Ras-MAP kinase pathway. Those pathways drive IGF-I's effects on cell proliferation, differentiation and survival [7].
Six high-affinity IGF-binding proteins (IGFBPs) sit between IGF-I and its receptor. By holding IGFs away from the receptor, they can inhibit IGF-stimulated growth, differentiation and survival. When IGFBPs are cleaved by proteases, that inhibition can be released. Binding to cell surfaces and extracellular matrix can concentrate IGFs near the receptor instead. Several IGFBPs also have actions that do not involve IGF at all [6]. In any system that contains IGFBPs, including serum-supplemented media, many cultured cell lines and all intact animals, the amount of IGF-I added is not the amount the receptor sees.
The clue came from a naturally occurring truncated form. Des(1-3)IGF-I, which lacks the first three N-terminal residues, was about 7 times more potent than IGF-I at stimulating protein synthesis in L6 myoblasts. That extra potency did not come with stronger binding to the IGF-I receptors on those cells [3]. Something other than receptor affinity was responsible, and the N-terminus was implicated.
The Adelaide group then expressed a series of IGF-I fusion analogs in E. coli [1]. Each carried the first 11 amino acids of methionyl porcine growth hormone, followed by Val-Asn, attached to the N-terminus of IGF-I. That makes a 13-residue extension. The variants were named for the IGF-I sequence that followed: Long IGF-I kept the authentic sequence, while Long [Gly3]-IGF-I and Long [Arg3]-IGF-I had glutamate-3 replaced by glycine or arginine. The authors also noted that the hydrophobic extension appeared to help the protein fold correctly during production [1].
The original characterization is the most important paper for interpreting LR3 data [1]. In L6 rat myoblasts, all of the analogs were more potent than IGF-I at stimulating protein and DNA synthesis and at inhibiting protein breakdown. In cell lines that secrete IGFBPs into their medium, the order of potency was Long [Arg3]-IGF-I, roughly equal to des(1-3)IGF-I, then Long [Gly3]-IGF-I, then Long IGF-I, then IGF-I. In chicken embryo fibroblasts, which secrete no detectable IGFBPs, Long [Arg3]-IGF-I was less potent than IGF-I. The authors concluded that the N-terminal analogs gain potency by avoiding IGFBPs, not by activating the receptor more strongly.
This means that LR3's potency depends on the system. It is a property of the experiment, not a fixed multiple of IGF-I. In a medium rich in binding proteins, LR3 can look far more active than IGF-I. In a clean, IGFBP-free system, the difference can shrink or reverse. Statements that LR3 is a set number of times more potent than IGF-I do not hold up without naming the system.
LR3 also does not escape every IGFBP effect. In L6 cells, which make no detectable IGFBP-3, added recombinant porcine IGFBP-3 suppressed proliferation stimulated by either IGF-I or LR3, pointing to an IGF-independent action of the binding protein. The same IGFBP-3 suppressed IGF-I-stimulated differentiation but not LR3-stimulated differentiation [10]. Finally, in H35 hepatoma cells, where IGFs act through the insulin receptor, the Long analogs kept a relative potency similar to what they showed in L6 myoblasts [1]. LR3 can therefore signal through the insulin receptor in some cell types, which matters for metabolic readouts.
In growing female rats given peptides by implanted osmotic pump for 14 days, IGF-I increased weight gain, nitrogen retention and food conversion efficiency. LR3 was substantially more potent: a much smaller amount produced effects similar to the highest IGF-I level tested. Both peptides generally preserved body proportions. Muscle protein synthesis and myofibrillar protein breakdown both rose slightly, so the gain in nitrogen retention could not be explained by the protein accretion rates calculated from those measures. Human growth hormone, infused in the same study, did not stimulate body growth [5].
Earlier work by the same group with full-length and truncated IGF-I helps put this in context. In diabetic rats, IGF peptides improved weight gain, nitrogen retention and muscle protein synthesis without correcting glucosuria. Insulin increased carcass fat and the IGF peptides did not, which suggests that the IGFs mainly stimulated lean tissue [2]. In rats moved to a low-protein diet, the fractional rate of muscle protein synthesis rose by 31% and 26% with the low and high IGF-I levels and by 21% with des(1-3)IGF-I. Kidney weight also rose by 16% in the high-IGF-I and des(1-3)IGF-I groups [4], a reminder that these peptides act on organs other than muscle.
For mechanistic interpretation, the IGF1-Akt pathway controls muscle protein synthesis through mTOR and GSK3-beta, and protein breakdown through the FoxO transcription factors. Genetic models show the pathway is essential for muscle growth during development and regeneration. Its role in the adult response to mechanical load is less clear [8].
LR3 is widely described online as having a half-life of a day or more. We have not found primary data in the sources reviewed here that support a specific figure, and one analytical study complicates the picture. In rats given a single administration, intact LR3 disappeared rapidly after 4 hours. Several N-terminally truncated products appeared, including Des(1)-, Des(1-10)- and Des(1-11)-LongR3-IGF-I, and the Des(1-11) form remained detectable for up to 16 hours. The same products formed when LR3 was incubated in human whole blood in vitro [11]. Any sustained activity seen in vivo may therefore come partly from fragments. Time-course studies should use an assay that can tell the intact protein from its truncation products, such as LC-MS, rather than an immunoassay alone.
LR3 is best established as a medium supplement. In serum-free cultures of two Chinese hamster ovary cell lines making recombinant cytokine receptors, LR3 sustained viability under production conditions better than insulin, the growth factor most commonly used in serum-free CHO media [9]. In experimental cell biology, a clean design includes the following:
Because LR3 is a recombinant protein that must fold correctly, identity testing matters more than it does for a short peptide. Anti-doping laboratories have documented the problems. One black-market vial contained LR3 carrying a C-terminal six-histidine tag attached through a Leu-Glu linker, most likely a by-product of biochemical production [12]. Another study found abundant oxidized forms in black-market products and recommended monitoring both the native and the mono-oxidized protein. It also noted that none of these IGF-I analogs has ever been approved for use in humans [11]. A useful certificate therefore shows the intact mass, which catches tags and truncations, and an HPLC purity profile, and it should also address oxidation (+16 Da per methionine sulfoxide). Northbridge Research Labs has every batch independently tested by a third-party laboratory, and published certificates are listed on our COA page.
IGF-1 LR3 is supplied as a white lyophilized powder. It should be stored sealed at -20°C and is soluble in sterile water. At low concentrations, proteins of this size can adsorb to plastic and glass. Many labs therefore prepare a more concentrated stock and dilute it into medium containing carrier protein, provided the carrier is compatible with the assay. Divide stocks into single-use aliquots, avoid repeated freeze-thaw, and let the vial reach room temperature before opening to keep moisture out of the remaining powder.
Note: IGF-1 LR3 from Northbridge Research Labs is for laboratory research use only, not for human or veterinary use. It is not an approved drug, and this article describes published research only.
Studied compound
IGF-1 LR3
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Research Use Only: The information in this article is for educational and research purposes only. All products mentioned are intended for laboratory research use only and are not approved for human or veterinary use.