IGF-1 LR3

What IGF-1 LR3 is

IGF-1 LR3 is a lab-made protein used as a research tool. It has been tested in cells in a dish and in small animals such as rats and guinea pigs. Nobody has run a human trial program on it, and it is not a medicine.

The full name is Long R3 Insulin-like Growth Factor-1. Some papers write LR3-IGF-I or Long-R3-IGF-I. It is a changed copy of IGF-1 (insulin-like growth factor 1) from humans, a natural growth signal. It is recombinant, which means microbes are engineered to make it. Most of it is made in the bacterium Escherichia coli 12.

Natural IGF-1 has 70 amino acids, the building blocks of protein. In the blood, carrier proteins soak most of it up. LR3 was designed to slip past them. It differs from natural IGF-1 in two ways 12:

1. A longer front end. Thirteen extra amino acids are joined to the N-terminal end of the whole IGF-1 sequence. Eleven come from the front end of pig growth hormone. The other two are a short link.
2. One swap. At position 3, arginine takes the place of glutamic acid.

The finished protein is a chimera, a blend of two sources, 83 amino acids long.

In the early 1990s, a team led by Francis, Ross and Ballard studied it in Adelaide, at CSIRO’s Division of Human Nutrition. They looked at how it binds the IGF-binding proteins (IGFBPs). Humans have six, IGFBP-1 through IGFBP-6. The two changes together made LR3 stick to binding proteins much less. Its grip on the type-1 IGF receptor (IGF-1R) was somewhat weaker too. Even so, in many test systems it was more potent than natural IGF-1 129.

This matters because over 99% of the IGF-1 in serum is bound to IGFBPs. Weaken that bond and far more of the protein is free and active for any cell that carries IGF-1R.

Labs use LR3 widely in cell culture. When the goal of an experiment is to switch on IGF-1R, it serves as a stronger stand-in for natural IGF-1. Lab supply houses sell it as a research chemical. The label limits it strictly to in vitro use, meaning in a dish. No agency in the US, the EU or the UK has licensed it as a medicine. Nobody markets it as a treatment for people.

How much research there is

All of the evidence is preclinical. It covers the makeup of the protein, cell-culture studies and drug studies in small animals. There is no human trial program for LR3 itself.

Question	Answer
Level of evidence	Preclinical only. Studies of the recombinant protein, cells in a dish, and small animals
Main use in the lab	A standard cell-culture reagent for switching on IGF-1R. Used in models of muscle formation, fibroblast growth and organ growth
How it is built	Natural IGF-1 (70 amino acids) plus a 13-amino-acid N-terminal extension and the Arg3 swap. 83 amino acids in all
Grip on binding proteins	Much weaker than natural IGF-1. The size of the drop depends on the binding protein and the test
Grip on IGF-1R	Somewhat weaker than natural IGF-1. One paper put it at about 3 times weaker 9
Half-life	The papers cited here give no blood half-life for LR3. Free natural IGF-1 in humans lasts about 10 to 12 min
Human trials	LR3 has none. Mecasermin, which is natural recombinant human IGF-1, has approval for primary IGF-1 deficiency that is severe
Approval	Not FDA approved, and no UK medicine license either. Sold as a research chemical, only for use in vitro. Mecasermin (rhIGF-1) is a different product, approved for GH insensitivity of the Laron type
Sport	The WADA Prohibited List bans IGF-1 and IGF-1 analogs by name (S2, Peptide Hormones, Growth Factors)

How it works

LR3 uses the same receptor as natural IGF-1. What differs is how the body handles it. The longer front end and the Arg3 swap were added on purpose. Together they give the practical gains that make it useful in cell culture and in live-animal (in vivo) studies 13.

It binds IGF-1R

• The receptor. IGF-1R is a tyrosine kinase, a kind of signaling enzyme. It is built from four chains of two types, held together by disulfide links. Its structure closely resembles the insulin receptor.
• The first steps. IGF-1 or LR3 binds the alpha subunits on the outside of the cell. The beta subunit inside the cell then adds phosphate tags to its own tyrosine kinase region. That draws in adaptor proteins called insulin receptor substrates (IRS) 4.
• Two branches. From there the signal splits. One branch is PI3K, Akt and mTOR. It handles protein building, glucose uptake and cell survival. The other is Ras, Raf, MEK and ERK, also called the MAP kinase branch. It handles cell division and growth in cell numbers.
• The branches can be pulled apart. Coolican and colleagues showed this with drugs in L6A1 myoblasts, a line of young muscle cells. MAP kinase drove the cells to multiply. PI3K drove them to mature. IGFs can turn on both at once 5.
• LR3 still fits the receptor. In receptor-binding tests, LR3 gripped IGF-1R about 3 times less well than natural IGF-1. So its extra potency does not come from a tighter grip on the receptor 9.

It slips past the binding proteins

In serum and in the fluid around cells, nearly all natural IGF-1 is bound to the six IGFBPs, which hold it tightly. The great bulk sits in a three-part (ternary) complex of about 150 kDa. The other two parts are IGFBP-3 and the acid-labile subunit (ALS).

These proteins work as a reservoir and as a throttle on IGF-1R. They limit how much free IGF-1 is on hand. They also carry IGF-1 to the tissues it acts on 6.

• How LR3 avoids them. In natural IGF-1, an acidic amino acid called Glu3 touches the binding pocket of the IGFBPs. The Arg3 swap replaces it. The water-repelling N-terminal extension adds to the effect. Together they cut LR3’s affinity for IGFBPs sharply 2.
• Proof that this explains the potency. Francis and colleagues compared cell systems. Where cells were actively releasing IGFBPs, LR3 pushed DNA and protein synthesis harder than natural IGF-1. Where no IGFBPs were present, the edge collapsed. This is direct evidence that LR3’s extra strength in cells comes from dodging IGFBPs. It does not come from any built-in change in the IGF-1R signal 2.
• Background. Firth and Baxter wrote the standard review of how IGFBPs act on cells. It sums up the biology that the design takes advantage of 6.

How long it lasts

Free natural IGF-1 is gone quickly. Guler and colleagues estimated how long 125I-IGF-I lasts in humans. They gave a half-life for each of three states 7:

• free: 10 to 12 minutes
• bound in two-part (binary) IGFBP complexes: 20 to 30 minutes
• held in the ternary IGFBP-3/ALS complex: 12 to 15 hours

LR3 binds IGFBPs weakly, so little of it ends up in the ternary reservoir. The papers cited in this guide do not report a blood half-life for LR3. What they do report is that binding proteins slow the move of natural IGF-1 from blood to tissue, and they are not expected to slow LR3 the same way 9.

For an experiment, this means more of the dose is free to reach IGF-1R. The animal studies cited here gave it by continuous infusion 38.

What the studies found

Most LR3 papers describe work in cells or small animals. It is not a medicine, and there is no human clinical trial program.

Cells in a dish

LR3 is used heavily as a reference agonist, a standard compound that turns a receptor on, in cell studies of IGF-1R.

• Potency. The founding studies by Francis and colleagues came out in 1992. They tested a range of cell lines. Among them were H-35 hepatoma cells, fibroblasts from chick embryos, and L6 myoblasts from rats. LR3 IGF-I and its related analogs with N-terminal changes boosted DNA synthesis, protein synthesis and cell growth at concentrations 2 to 10-fold lower than natural IGF-1. In fibroblast systems with no IGFBPs, that gap vanished 12.
• Muscle cell research. Coolican and colleagues used L6A1 myoblasts to tease apart two IGF-driven signals. The growth signal ran through MAP kinase and the maturing signal through PI3K 5.
• Why the outputs match. Adams and colleagues reviewed the structure of IGF-1R. Their review gives the receptor-level background. It helps explain why LR3 and natural IGF-1 produce similar results in cells even though the body handles them differently 4.
• As a reagent. For cell culture, LR3 usually comes freeze-dried (lyophilized). It is mixed into media with little or no serum. Because IGFBPs barely affect it, one important cause of batch-to-batch variation is removed.

Muscle and growth in animals

One preclinical result keeps turning up. Microgram for microgram, LR3 IGF-I builds tissue more strongly in live animals than natural IGF-1.

• Diabetic rats. Tomas and colleagues used rats made diabetic with streptozotocin. Long R3 IGF-I and IGF-I each restored growth. Each raised RNA levels and the rate of muscle protein synthesis by up to 50%. Mole for mole, LR3 was the stronger of the two. Neither copied the full glucose-lowering pattern of insulin 8.
• Steroid-treated rats. The same group had worked earlier with rats given dexamethasone, a steroid drug. That study had already shown that IGF-I variants with a weaker grip on IGFBPs were especially good at reversing steroid-caused muscle wasting 9.
• Guinea pigs. Conlon and colleagues infused LR3 IGF-I under the skin for seven days. It favored growth of the adrenal glands, gut, kidneys and spleen. Oddly, it also lowered blood levels of the animals’ own IGF-I, IGF-II and IGFBPs. That drop is an indirect sign of the shifted IGFBP-binding balance the design is meant to use 3.

These animal data sets are still the most cited reference point for LR3. Scientists use them to interpret muscle and organ-growth results in tissue studied outside the body (ex vivo).

How the two changes affect binding proteins

Work on LR3’s behavior with IGFBPs is central to why it acts differently from natural IGF-1.

In 1992 the Francis, Ross and Ballard team published a paper on fusion-protein analogs that took the design apart. It tested the N-terminal extension alone, the Arg3 swap alone, and the two combined 2. It reported two things:

• The two changes added up, roughly, in lowering IGFBP binding.
• In cell systems that released no detectable IGFBPs, the analogs lost their extra potency in vitro.

That confirms the point directly. The analog is stronger because it evades IGFBPs, not because it engages the receptor differently.

Firth and Baxter’s full review of IGFBPs gives the molecular background for reading these results. It covers three things that control IGF-1 supply in natural tissue. They are the cutting of IGFBPs by enzymes, changes made to IGFBPs after they are built, and their contacts with the cell surface 6.

Cancer signaling concerns

Any talk about switching on IGF-1R has to deal with cancer.

• The pathway and tumors. IGF-1 and IGF-1R are tied to cell growth, resistance to cell death, and spread in a wide range of tumor types. Population studies show that people in the top quarters for blood IGF-1 have a modestly higher risk of several common cancers.
• Pollak, Nature Reviews Cancer. This review pulls together the evidence on insulin and IGF-1 signaling in tumor growth. It sets out the reason to block IGF-1R with drugs as a cancer treatment plan. Blocking is the opposite of switching the receptor on 10.
• Clemmons, Nature Reviews Drug Discovery. This review takes up the reverse problem. It covers how IGF-1 activity is turned up or down for different medical uses, and how risk is weighed against benefit in cancer 11.
• Travis and colleagues, 2016. This meta-analysis used data on individual participants. It pooled 2 cross-sectional studies and 17 prospective ones, with over 10,500 cases of prostate cancer. It confirmed that higher blood IGF-1 is independently linked to later prostate cancer 12.

These data are the basis for the standard research-use framing. LR3 is a potent IGF-1R agonist, and its power to drive cell division is well documented. That is exactly why it is not given to humans. The exception is strictly regulated clinical studies, such as those run in the past in Laron syndrome 13.

Safety

LR3 is a research reagent, not a treatment for people. The points here concern lab handling and what can be inferred.

• No human safety data. No controlled human clinical trial of IGF-1 LR3 exists. Published views on its safety are drawn from two places. One is animal drug studies. The other is clinical experience with natural rhIGF-1 (mecasermin). Neither is LR3 itself.
• It drives cell division. As a strong IGF-1R agonist, LR3 makes cells that carry IGF-1R multiply. That is what makes it useful in cell culture. It is also the source of concern about giving it without a clear target.
• Low blood sugar in animals. At doses above natural levels, IGF-1 and its analogs lower blood sugar the way insulin does in preclinical species. Clemmons’ review sums up the overlap with the insulin receptor that explains this 11.
• Storage. Suppliers set the storage terms for the freeze-dried peptide. Typical terms are minus 20 °C for the long term. After mixing in a sterile buffered liquid, they are 2 to 8 °C for short-term use. Repeated freezing and thawing breaks down proteins of the IGF family.

Legal status in the US

• Not FDA approved. No US agency has licensed IGF-1 LR3 as a medicine. Neither has the EU or the UK. Mecasermin (rhIGF-1) is a different product. It has approval for GH insensitivity of the Laron type.
• Banned in sport. IGF-1 and its analogs are banned at all times. The rule is section S2.3 of the WADA Prohibited List (Growth Factors and Growth Factor Modulators). For athletes tested under the WADA code, IGF-1 LR3 is a banned substance.
• Research use. Laboratory suppliers label it only for research in vitro.

This guide treats IGF-1 LR3 only as a research reagent. Nothing in it is guidance on giving it to people.

Limits of the research

1. Preclinical only. Rodent and cell-culture work makes up most of the data. A handful of labs produced it, chiefly CSIRO Adelaide and its partners. Independent repeats across varied models are still limited.
2. Species and tissue differences. LR3’s pattern of IGFBP binding was worked out mostly against human IGFBPs. How it reacts with IGFBPs of other species has not been mapped evenly. That may affect how results carry from one model to another.
3. Potency depends on the test. Reported gains over natural IGF-1 run from 2× to 100×. The number changes with whether the system releases IGFBPs, which IGFBPs are there, and what outcome is measured.
4. Cancer-risk data are about the body’s own IGF-1, not LR3. The population link between high blood IGF-1 and cancer risk comes from measurements of natural IGF-1. No such data exist for LR3, because no group of people has been exposed to it 12.
5. It is not mecasermin. Data from treating people who have Laron syndrome with recombinant IGF-1 (mecasermin, sold as Increlex) do not carry over to LR3. LR3 is a different molecule, and the body handles it differently 13.

IGF-1 LR3 is a well-described lab tool for cells and small animals. No human trial has tested whether it is safe or what it does in people.

References

Selected peer-reviewed references. Not exhaustive. All citations DOI-verified via PubMed where indicated.

1. King R, Wells JR, Krieg P, Snoswell M, Brazier J, Bagley CJ, Wallace JC, Ballard FJ, Ross M, Francis GL. Production and characterization of recombinant insulin-like growth factor-I (IGF-I) and potent analogues of IGF-I, with Gly or Arg substituted for Glu3, following their expression in Escherichia coli as fusion proteins. J Mol Endocrinol. 1992;8(1):29-41. DOI: 10.1677/jme.0.0080029. PMID: 1311930.
2. Francis GL, Ross M, Ballard FJ, Milner SJ, Senn C, McNeil KA, Wallace JC, King R, Wells JR. Novel recombinant fusion protein analogues of insulin-like growth factor (IGF)-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency. J Mol Endocrinol. 1992;8(3):213-223. DOI: 10.1677/jme.0.0080213. PMID: 1378742.
3. Conlon MA, Tomas FM, Owens PC, Wallace JC, Howarth GS, Ballard FJ. Long R3 insulin-like growth factor-I (IGF-I) infusion stimulates organ growth but reduces plasma IGF-I, IGF-II and IGF binding protein concentrations in the guinea pig. J Endocrinol. 1995;146(2):247-253. DOI: 10.1677/joe.0.1460247. PMID: 7561636.
4. Adams TE, Epa VC, Garrett TP, Ward CW. Structure and function of the type 1 insulin-like growth factor receptor. Cell Mol Life Sci. 2000;57(7):1050-1093. DOI: 10.1007/PL00000744. PMID: 10961344.
5. Coolican SA, Samuel DS, Ewton DZ, McWade FJ, Florini JR. The mitogenic and myogenic actions of insulin-like growth factors utilize distinct signaling pathways. J Biol Chem. 1997;272(10):6653-6662. DOI: 10.1074/jbc.272.10.6653. PMID: 9045696.
6. Firth SM, Baxter RC. Cellular actions of the insulin-like growth factor binding proteins. Endocr Rev. 2002;23(6):824-854. DOI: 10.1210/er.2001-0033. PMID: 12466191.
7. Guler HP, Zapf J, Schmid C, Froesch ER. Insulin-like growth factors I and II in healthy man. Estimations of half-lives and production rates. Acta Endocrinol (Copenh). 1989;121(6):753-758. DOI: 10.1530/acta.0.1210753. PMID: 2558477.
8. Tomas FM, Knowles SE, Owens PC, Chandler CS, Francis GL, Ballard FJ. Insulin-like growth factor-I and more potent variants restore growth of diabetic rats without inducing all characteristic insulin effects. Biochem J. 1993;291(Pt 3):781-786. DOI: 10.1042/bj2910781. PMID: 7683875.
9. Tomas FM, Knowles SE, Owens PC, Chandler CS, Francis GL, Read LC, Ballard FJ. Insulin-like growth factor-I (IGF-I) and especially IGF-I variants are anabolic in dexamethasone-treated rats. Biochem J. 1992;282(Pt 1):91-97. DOI: 10.1042/bj2820091. PMID: 1371669.
10. Pollak M. Insulin and insulin-like growth factor signalling in neoplasia. Nat Rev Cancer. 2008;8(12):915-928. DOI: 10.1038/nrc2536. PMID: 19029956.
11. Clemmons DR. Modifying IGF1 activity: an approach to treat endocrine disorders, atherosclerosis and cancer. Nat Rev Drug Discov. 2007;6(10):821-833. DOI: 10.1038/nrd2359. PMID: 17906644.
12. Travis RC, Appleby PN, Martin RM, Holly JMP, Albanes D, Black A, et al. A meta-analysis of individual participant data reveals an association between circulating levels of IGF-I and prostate cancer risk. Cancer Res. 2016;76(8):2288-2300. DOI: 10.1158/0008-5472.CAN-15-1551. PMID: 26921328.
13. Laron Z. Insulin-like growth factor 1 (IGF-1): a growth hormone. Mol Pathol. 2001;54(5):311-316. DOI: 10.1136/mp.54.5.311. PMID: 11577173.