# How does IGF-1 LR3 work? Binding proteins, the IGF-1 receptor and what cells showed Updated 2026-10-16. In cell experiments, IGF-1 LR3 switches on the same receptor as natural IGF-I, called the type 1 IGF receptor. The main difference is that carrier proteins, which hold IGF-I in the blood and around cells, grip IGF-1 LR3 much less. The 1992 authors concluded that this weaker grip, not a better fit with the receptor, explains why IGF-1 LR3 worked more strongly in cells that release those carriers. | | | | --- | --- | | Receptor | Type 1 IGF receptor (IGF-1R), a receptor with tyrosine kinase activity | | Key difference from IGF-I | Much lower affinity for IGF binding proteins (Gajanandana et al., 1998) | | Foundational paper | Francis et al., Journal of Molecular Endocrinology, 1992 ([PMID 1378742](https://pubmed.ncbi.nlm.nih.gov/1378742/)) | | Pathways described downstream | PI3K-Akt and MAPK/ERK, presumed from IGF-I biology (Dominikowski et al., 2026) | | Binding proteins in blood | Six; more than 80% of IGF-1 is carried with IGFBP-3 and an acid-labile subunit ([FDA label for Increlex](https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/021839s033lbl.pdf)) | | Human half-life | Not established in the sources opened | Think of a crowded train station where most travellers are held at a barrier until their platform opens. Your blood treats IGF-I, a hormone that carries growth signals, in a similar way: carrier proteins hold most of it back and let it go slowly, in the right place. IGF-1 LR3 is a changed copy built to slip past that barrier. This page belongs to [the IGF-1 LR3 hub](/learn/igf-1-lr3). Start with [what IGF-1 LR3 is](/learn/igf-1-lr3/what-is-igf-1-lr3) if the name is new. Below you will find how the receptor and the carriers fit together, what the first cell experiments measured, and what animal studies added. A short section states what is not known about how long the analogue lasts. ## How does IGF-1 LR3 work? IGF-1 LR3 uses the same receptor as IGF-I, the type 1 IGF receptor. A receptor is a lock on the surface of a cell, and a matching molecule acts as the key. The big difference is that carrier proteins called IGF binding proteins hold IGF-1 LR3 much less than they hold natural IGF-I. A 2026 review says IGF-1 LR3 is generally presumed to send the same signals inside the cell as IGF-I, along routes named PI3K-Akt and MAPK/ERK. The review notes the weaker grip may change the size or timing of that signal. This is a presumption from IGF-I biology, not a human measurement. A direct receptor test exists. In 2001, Devi and colleagues measured phosphorylation of the IGF-1 receptor. Phosphorylation is a chemical tag that marks the receptor as switched on. They found that a carrier protein called IGF-binding protein 3 reduced this tagging by IGF-I, IGF-II, des(1-3)IGF-I and Long(R3)IGF-I over a similar range of concentrations. ## What are IGF binding proteins? IGF binding proteins (IGFBPs) are carrier proteins that hold IGF-I in the blood and in body tissues. In blood, IGF-1 is bound to six different IGFBPs. More than 80% of it travels as a group with IGFBP-3 and a partner protein called the acid-labile subunit. These facts come from the FDA label for mecasermin, an approved IGF-1 medicine. The label adds that, in one rare condition with very low IGFBP-3, the body clears IGF-1 faster. That shows the carriers affect how long IGF-1 stays in the blood. Think of the carriers as a waiting room. IGF-I sits there bound, and only some of it is free to meet the receptor at any moment. Devi and colleagues concluded that IGFBP-3 works mostly by holding IGFs and their analogues out in the surrounding fluid, not by touching the receptor itself. This is why researchers built analogues that carriers grip poorly. If the carriers cannot hold the analogue, an experiment tests the receptor side more directly. The 1992 paper was written to separate those two effects. ![Small circles held inside cup-shaped carriers, labelled Carrier protein and Held back, beside other circles that move past empty cups, labelled Slips past, to reach a Y-shaped structure on a cell labelled Receptor.](/assets/content/edu-carrier-proteins-hold-or-release.png) In blood, more than 80% of IGF-1 travels bound to carrier proteins, mostly IGFBP-3. IGF-1 LR3 is gripped by these carriers much less than natural IGF-I. Sources: FDA label for Increlex; Gajanandana et al., Journal of Endocrinology, 1998. ## Why was IGF-1 LR3 more potent than IGF-I in cells? In L6 rat myoblasts, a line of young muscle cells, all the long analogues were more potent than IGF-I. They did more to build protein and DNA, and more to slow protein breakdown. In cell lines that release IGF binding proteins, the authors ranked them. Long [Arg3]-IGF-I and des(1-3)IGF-I came first, then Long [Gly3]-IGF-I, then Long IGF-I, with IGF-I last. Francis and colleagues concluded that the extra potency comes from changes in how the analogues interact with binding proteins. Potency here means how strong an effect a set amount produces. The control experiments matter more than the headline. In chicken embryo fibroblasts, a cell line with no binding proteins detected in the surrounding fluid, Long [Arg3]-IGF-I was less potent than IGF-I, not more. In H35 hepatoma cells, a line of liver tumour cells where the IGFs act through the insulin receptor, the long analogues kept about the same potency compared with IGF-I as in the L6 cells. Put together, the cells that made binding proteins showed a boost, and the cells that did not make them showed none. That pattern points at the carriers, not at a better fit with the receptor. The table below sets out the three cell settings. ## What have animal studies shown about IGF-1 LR3? Animal studies show that results depend on the species and the setting. In rats made diabetic with a drug called streptozotocin, two analogues that carriers grip poorly, des(1-3)IGF-I and LR3-IGF-I, were 2.5 to 3 times more potent than IGF-I at restoring growth. Other insulin-dependent processes in liver, muscle and fat were not restored. In guinea pigs given a 7-day infusion, several organs grew heavier compared with body weight, yet overall growth did not increase. These are rodent results. They do not show that people would respond the same way. In 2025, a study of fetal sheep that were growing too slowly gave IGF-1 LR3 by infusion for 1 week. Growth did not improve, and amino acids in the blood fell. The same group reported earlier that, in normal fetal sheep, a 1-week infusion raised body weight but lowered insulin and the release of insulin in response to sugar. In 5XFAD mice, which are bred to model Alzheimer's disease, long R3 IGF-1 given through the nose changed features of amyloid plaques in the cortex, the brain's outer layer. It did not protect thinking and memory. Each study asked a narrow question in one animal model. None measured a human response, and the sheep and mouse papers were not designed to describe use in people. [The side-effects page](/learn/igf-1-lr3/igf-1-lr3-side-effects) records what is documented about safety. ## How long does IGF-1 LR3 last in the body? No human half-life for IGF-1 LR3 was found in the sources checked. Half-life is the time the body takes to clear half of a substance. A 2026 review lists the half-life as not documented. It also says no peer-reviewed human study has reported how the analogue was given. So that review cannot supply a measured time for people. The 1992 paper studied cells grown in dishes, not people. It compared how strong the effects were, not how fast a body clears the molecule. A result for mecasermin cannot fill that gap, because mecasermin is a different molecule. There is a reason to expect IGF-1 LR3 to behave differently from natural IGF-I. The mecasermin label says low IGFBP-3 makes the body clear IGF-1 faster, so carrier binding affects how fast IGF-1 leaves the blood. That expectation is not a measured number for IGF-1 LR3, and the half-life of mecasermin must not be copied onto it. A 1998 paper built a test that measures IGF-1 LR3 in blood plasma, and tried it on plasma from cattle. The summary reports how well the test worked, not a half-life, and no people were involved. [What studies show about IGF-1 LR3](/learn/igf-1-lr3/igf-1-lr3-study-findings) sets out the wider evidence, and [the guide to what peptides do](/learn/peptide-science/what-do-peptides-do) explains signalling molecules in general. ## Comparison The three cell settings in the 1992 paper by Francis and colleagues. The table records what the authors reported. It does not describe any result in people. | Cell setting | IGF binding proteins | What was reported | What it suggests | | --- | --- | --- | --- | | L6 rat myoblasts (young muscle cells) | Not stated per cell line in the abstract; the potency order is given for cell lines that release binding proteins | All long analogues were more potent than IGF-I for building protein and DNA and for slowing protein breakdown | Extra potency appears when binding proteins are around | | H35 hepatoma cells (liver tumour cells) | IGFs act through the insulin receptor in these cells | The long analogues kept about the same potency compared with IGF-I as in L6 cells | The potency pattern held in a second cell type | | Chicken embryo fibroblasts | No binding proteins detected in the surrounding fluid | Long [Arg3]-IGF-I was less potent than IGF-I | Without binding proteins, the extra potency did not appear | ## Frequently asked questions ### Does IGF-1 LR3 bind the IGF-1 receptor? Cell studies treat it as acting at the type 1 IGF receptor, like IGF-I. A 2001 study by Devi and colleagues measured the receptor being switched on by Long(R3)IGF-I. A 2026 review calls similar signalling inside the cell a general presumption, not a human measurement. ### Why does a weak grip by binding proteins matter? Binding proteins hold IGF-I in the blood and in tissues. In the 1992 cell work, an analogue they grip poorly was more potent where binding proteins were present. It was less potent where none were detected. ### Is the half-life of IGF-1 LR3 known in people? No. A 2026 review in Frontiers in Endocrinology lists the half-life as not documented. No human study of how the body handles IGF-1 LR3 was found in the sources opened. ### How is the mechanism of IGF-1 LR3 different from mecasermin? Mecasermin has the same sequence as human IGF-1, so the same carrier proteins bind it, according to its FDA label. IGF-1 LR3 was built so that carriers bind it much less, as a 1998 study measured. [What IGF-1 LR3 is](/learn/igf-1-lr3/what-is-igf-1-lr3) compares the three molecules. ## Citations - Francis GL, Ross M, Ballard FJ, et al.. Journal of Molecular Endocrinology. 1992;8(3):213-223. DOI: 10.1677/jme.0.0080213 - Gajanandana O, Irvine K, Grant PA, et al.. Journal of Endocrinology. 1998;156(3):407-414. DOI: 10.1677/joe.0.1560407 - Devi GR, Graham DL, Oh Y, Rosenfeld RG. Growth Hormone & IGF Research. 2001;11(4):231-239. DOI: 10.1054/ghir.2001.0231 - Tomas FM, Knowles SE, Owens PC, et al.. Biochemical Journal. 1993;291(3):781-786. DOI: 10.1042/bj2910781 - Conlon MA, Tomas FM, Owens PC, et al.. Journal of Endocrinology. 1995;146(2):247-253. DOI: 10.1677/joe.0.1460247 - White A, Stremming J, Wesolowski SR, et al.. American Journal of Physiology Endocrinology and Metabolism. 2025;328(1):E116-E125. DOI: 10.1152/ajpendo.00259.2024 - Engel et al.. Journal of Alzheimer's Disease. 2025;103(1):113-126. DOI: 10.1177/13872877241299056 - Dominikowski A, Rękoś Z, Olejarz M, et al.. Frontiers in Endocrinology. 2026;17:1822475. DOI: 10.3389/fendo.2026.1822475 --- Canonical page: https://www.formlaboratories.com/learn/igf-1-lr3/how-igf-1-lr3-works Form Laboratories - Perth-based research-use-only peptide supplier, shipping Australia-wide. - Site guide for agents: https://www.formlaboratories.com/llms.txt - Sitemap: https://www.formlaboratories.com/sitemap.xml - Full catalogue: https://www.formlaboratories.com/shop - Contact: hello@formlaboratories.com All materials are supplied for laboratory research use only - not for human or veterinary use, and nothing on this site is medical advice.