IGF-1: The GH-IGF-1 Axis and Mechanisms in Research
Understanding IGF-1 and Its Biological Actions
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Introduction to IGF-1
Insulin-like Growth Factor-1 (IGF-1), also known as Somatomedin-C, is a polypeptide hormone in the insulin family. It consists of 70 amino acids with a molecular weight of 7.6 kDa. Structurally, IGF-1 shares similarities with insulin, featuring A and B chains connected by three disulfide bonds. In adults, the primary source of IGF-1 is the liver, although other tissues also produce it locally.
IGF-1 functions as a potent mitogen, regulating cellular proliferation, differentiation, and survival. It exerts anabolic effects on skeletal muscle, bone, cartilage, and connective tissue, and contributes to systemic growth and tissue repair.
Growth Hormone–IGF-1 Axis
Growth hormone (GH), secreted by the anterior pituitary, stimulates hepatic production of IGF-1. GH release is regulated by hypothalamic signals, including GHRH (stimulatory), somatostatin (inhibitory), and ghrelin. Once released, IGF-1 acts via endocrine, autocrine, and paracrine pathways to influence nearly all tissues, including skeletal muscle, cartilage, bone, skin, liver, kidney, and the central nervous system.
Binding proteins (IGFBPs) modulate IGF-1 bioavailability. Six IGFBPs have been identified, with IGFBP-3 carrying the majority (80–90%) of circulating IGF-1. IGFBP-1 is regulated by insulin, while IGFBP-3 is influenced by GH and, to a lesser extent, IGF-1 itself.
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Mechanisms of Action
IGF-1 signals primarily through the IGF-1 receptor (IGF-1R), a transmembrane tyrosine kinase. Activation of IGF-1R initiates downstream signaling cascades, including:
- PI3K–Akt–mTOR pathway, promoting protein synthesis, cell survival, and hypertrophy.
- MAPK pathway, regulating proliferation and differentiation.
Through these mechanisms, IGF-1 enhances muscle hypertrophy, satellite cell activation, bone remodeling, and cartilage repair. It also supports lipid metabolism in muscle while conserving glucose.
Documented Physiological and Clinical Roles
Research has demonstrated multiple actions of IGF-1, including:
- Promotion of muscle and bone growth.
- Enhancement of tendon, ligament, and soft tissue repair.
- Cardiovascular support through anti-inflammatory and antioxidant effects.
- Neuroprotection and stimulation of adult neurogenesis.
- Improved glucose handling and insulin sensitivity.
- Maintenance of bone mineral density.
- Modulation of immune and inflammatory responses.
IGF-1 and Aging
Circulating IGF-1 levels decline progressively after midlife, a phenomenon termed somatopause. Low IGF-1 is associated with sarcopenia, reduced mobility, and impaired antioxidant defenses. Higher levels are correlated with increased muscle strength and better physical function in older adults.
In addition, IGF-1 has an anti-inflammatory role, as low concentrations are linked to elevated CRP and IL-6.
Neurological Effects
Within the central nervous system, IGF-1 supports neuronal growth, survival, and differentiation. It enhances hippocampal neurogenesis, promotes myelination, and improves neuronal resistance to injury. Reduced IGF-1 signaling is associated with cognitive decline, while higher circulating levels correlate with improved processing speed and motor performance.
Role in Muscle Growth and Repair
IGF-1 is essential for skeletal muscle adaptation. It promotes proliferation of myoblasts, differentiation into mature fibers, and activation of satellite cells following injury. In animal models, local administration of IGF-1 accelerates muscle repair after trauma, although some studies noted fibrosis in high-dose treatments.
Metabolic Disorders
Diabetes
The GH–IGF-1 axis is disrupted in type 1 diabetes, characterized by GH hypersecretion, low IGF-1, and altered IGFBPs.
Obesity and Metabolic Syndrome
IGF-1 supports adipocyte differentiation and regulates metabolism within fat tissue. It suppresses lipolysis and helps maintain adipocyte function. Dysregulation of IGF-1 in obesity contributes to adipocyte stress and metabolic dysfunction.
Cardiovascular Health
IGF-1 exerts protective vascular effects by reducing oxidative stress, stabilizing plaques, and preventing further atherosclerotic development. Low IGF-1 levels are associated with increased risk of ischemic heart disease, stroke, and heart failure. However, both very low and very high IGF-1 levels are linked to increased overall mortality, suggesting a U-shaped relationship.
Bone Health
IGF-1, alongside GH, is critical for skeletal development and bone maintenance. Higher IGF-1 levels are associated with improved bone mineral density, particularly in older women.
IGF-1 LR3
A recombinant derivative, IGF-1 LR3, is engineered for enhanced metabolic stability and potency.
- Contains 83 amino acids, with a molecular weight of 9.1 kDa.
- Substitution at position 3 (arginine for glutamic acid) increases activity.
- Addition of 13 amino acids extends half-life to 20–30 hours.
- Reduced affinity for IGFBPs enhances receptor availability.
- Demonstrates approximately threefold potency compared to native IGF-1.
IGF-1 LR3: Production and Research Findings
IGF-1 LR3 (Long R3 IGF-1) is a recombinant, non-glycosylated polypeptide. It is expressed in Escherichia coli, and chromatographic methods are then used to fold and purify it into the active form that binds the IGF-1 receptor (IGF-1R). Historically, IGF-1 itself was described as the “sulfation factor” and its activity as NSILA (non-suppressible insulin-like activity) before the name somatomedin C was adopted.
In cell and animal research, IGF-1 LR3 shows the same biological activities as endogenous IGF-1: it increases glucose and amino acid uptake, raises RNA and protein synthesis, and inhibits protein breakdown, leading to higher cellular protein content. In muscle cell models it has been reported to drive both hypertrophy (enlargement of existing cells) and hyperplasia (formation of new cells and fibers).
Protein Metabolism in Livestock Models
In 1999, Hill and colleagues studied IGF-1 LR3 in underfed beef heifers. Animals given IGF-1 LR3 intravenously preserved skeletal and whole-body protein, showed lower plasma amino acids and glucose, and had reduced endogenous IGF-1 and IGF-2, while IGF-binding protein levels rose significantly.
Atherosclerotic Plaque Models
In 2011, von der Thüsen and colleagues examined IGF-1 and IGF-1 LR3 in a mouse model of atherosclerosis. Medium conditioned by M1-polarized macrophages suppressed IGF-1 signaling: it lowered IGF-1, increased IGF-binding protein-3 expression, promoted vascular smooth muscle cell apoptosis and upregulated matrix-degrading enzymes. IGF-1 LR3 counteracted these effects, increased vascular smooth muscle content in advanced plaques and reduced the likelihood of intra-plaque hemorrhage by more than 50%.
IGF-1 and Age-Related Change
Lower IGF-1 levels are repeatedly linked with age-related loss of lean mass and greater fat accumulation. A study of 21 healthy men aged 61 to 81 years supported this association between low IGF-1 and aging-related changes in body composition. Recombinant IGF-1 has also been investigated in models of peripheral neuropathy, including motor axon degeneration.
REFERENCES
- Kawai M, Rosen CJ. The IGF-I regulatory system and its impact on skeletal and energy homeostasis. J Cell Biochem. 2010 Sep 1;111(1):14-9. doi: 10.1002/jcb.22678. Erratum in: J Cell Biochem. 2012 Apr;113(4):1447. PMID: 20506515; PMCID: PMC3276304.
- Jonsson, K., Wiberg, K., Ljunghall, S. et al. Insulin-like Growth Factor I Does Not Stimulate Bone Resorption in Cultured Neonatal Mouse Calvarial Bones. Calcif Tissue Int 59, 366–370 (1996). https://doi.org/10.1007/s002239900141
- Tsukazaki T et al., on TGF-β and IGF-I axis in chondrocytes — likely available via journal archives (e.g., Experimental Cell Research), but not readily found free online.
- Musaro A et al., IGF-1 induces skeletal myocyte hypertrophy through calcineurin… (Nature, 1999) — accessible through Nature archives with institutional access.
- Lu H et al., macrophage-derived IGF-1 and muscle repair — FASEB Journal; accessible via institutional or FASEB website.
- Aguirre GA et al., IGF-1 deficiency and metabolic syndrome — Journal of Translational Medicine; open-access version available on publisher’s site.
- Higashi Y et al., IGF-1 regulation of glutathione peroxidase in endothelial cells — reviewable via Biochimica et Biophysica Acta archives.
- Clemmons DR., glucose homeostasis — Hormone Research, likely accessible via journal platform.
- Kaushal K et al., IGF and inflammation in metabolic syndrome — Diabetes Care, searchable through PubMed or journal site.
- Hill RA, Hodgkinson SC, Sauerwein H, Davis SR, Bass JJ. Action of long(R3)-insulin-like growth factor-1 on protein metabolism in beef heifers. Domest Anim Endocrinol. 1999;16(4):219-229.
- von der Thüsen JH, Borensztajn KS, Moimas S, et al. IGF-1 has plaque-stabilizing effects in atherosclerosis by altering vascular smooth muscle cell phenotype. Am J Pathol. 2011;178(2):924-934.
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