Insulin-like growth factor 1 (IGF-1), also called somatomedin C, is a
hormone similar in
molecular structure to
insulin which plays an important role in childhood growth, and has
anabolic effects in adults.[5]
In the 1950s IGF-1 was called "
sulfation factor" because it stimulated sulfation of cartilage in vitro,[6] and in the 1970s due to its effects it was termed "nonsuppressible insulin-like activity" (NSILA).[7]
IGF-1 is a
protein that in humans is encoded by the IGF1gene.[8][9] IGF-1 consists of 70
amino acids in a single chain with three intramolecular disulfide bridges. IGF-1 has a molecular weight of 7,649
daltons.[10] In dogs, an ancient mutation in IGF1 is the primary cause of the
toy phenotype.[11]
IGF-1 is produced primarily by the
liver. Production is stimulated by
growth hormone (GH). Most of IGF-1 is bound to one of 6 binding proteins (IGF-BP). IGFBP-1 is regulated by insulin. IGF-1 is produced throughout life; the highest rates of IGF-1 production occur during the
pubertal growth spurt.[12] The lowest levels occur in infancy and old age.[13][14]
A synthetic analog of IGF-1,
mecasermin, is used for the treatment of
growth failure in children with severe IGF-1 deficiency.[15]
Cyclic glycine-proline (cGP) is a metabolite of hormone insulin-like growth factor-1 (IGF-1). It has a cyclic structure, lipophilic nature, and is enzymatically stable which makes it a more favourable candidate for manipulating the binding-release process between IGF-1 and its binding protein, thereby normalising IGF-1 function.[16]
The
polypeptide hormone IGF-1 is synthesized primarily in the
liver upon stimulation by
growth hormone (GH). It is a key mediator of anabolic activities in numerous tissues and cells, such as growth hormone-stimulated growth,
metabolism and protein translation.[17] Due to its participation in the GH-IGF-1 axis it contributes among other things to the maintenance of muscle strength, muscle mass, development of the skeleton and is a key factor in brain, eye and lung development during fetal development.[18]
A deficiency of IGF-1 is associated with the increased risks of
insulin resistance,
glucose intolerance,
diabetes type 2, as well as cardiovascular morbidity and mortality.[17][19] Studies have shown the importance of the GH-IGF-1 axis in directing development and growth, where mice with a IGF-1 deficiency had a reduced body- and tissue mass. Mice with an excessive expression of IGF-1 had an increased mass.[19]
The levels of IGF-1 in the body vary throughout life, depending on age, where peaks of the hormone is generally observed during puberty and the
postnatal period. After puberty, when entering the third decade of life, there is a rapid decrease in IGF-1 levels due to the actions of GH. Between the third and eight decade of life, the IGF-1 levels decrease gradually, but unrelated to functional decline.[18] However, protein intake is proven to increase IGF-1 levels.[20]
IGF-1 is a primary mediator of the effects of
growth hormone (GH). Growth hormone is made in the
anterior pituitary gland, is released into the blood stream, and then stimulates the
liver to produce IGF-1. IGF-1 then stimulates systemic body growth, and has growth-promoting effects on almost every
cell in the body, especially skeletal
muscle,
cartilage,
bone,
liver,
kidney,
nerve,
skin,
hematopoietic, and
lung cells. In addition to the
insulin-like effects, IGF-1 can also regulate cellular
DNA synthesis.[21]
IGF-1 binds to at least two cell surface
receptor tyrosine kinases: the
IGF-1 receptor (IGF1R), and the
insulin receptor. Its primary action is mediated by binding to its specific receptor, IGF1R, which is present on the surface of many cell types in many tissues. Binding to the IGF1R initiates intracellular signaling. IGF-1 is one of the most potent natural activators of the
AKTsignaling pathway, a stimulator of cell growth and proliferation, and a potent inhibitor of
programmed cell death .[22][23] The IGF-1 receptor and insuline receptor are two closely related members of a transmembrane tetrameric tyrosine kinase receptor family. They control vital brain functions, such as survival, growth, energy metabolism, longevity, neuroprotection and neuroregeneration.[24]
IGF-1 binds and activates its own receptor,
IGF-1R, through the cell surface expression of Receptor Tyrosine Kinase's (RTK's), and further signals through multiple intracellular transduction cascades. IGF-1R is the critical role-playing inducer in modulating the metabolic effects of IGF-1 for cellular senescence and survival. At a localized target cell, IGF-1R elicits the mediation of paracrine activity. After its activation the initiation of intracellular signaling occurs inducing a magnitude of signaling pathways. An important mechanistic pathway involved in mediating a cascade affect regulated by phosphatidylinositol-3 kinase (
PI3K) and its downstream partner, mTOR (mammalian Target of Rapamycin). Rapamycin binds with the enzyme
FKBPP12 to inhibit the mTORC1 complex. mTORC2 remains unaffected and responds by up-regulating AKT, driving signals through the inhibited mTORC1. Phosphorylation of Eukaryotic translation initiation factor 4E (
EIF4E) by mTOR suppresses the capacity of Eukaryotic translation initiation factor 4E-binding protein 1 (
EIF4EBP1) to inhibit EIF4E and slow metabolism.[25][26] A mutation in the signaling pathway PI3K-AKT-mTOR is a big factor in the formation of tumors found predominantly on skin, internal organs, and secondary lymph nodes (Kaposi sarcoma).[26]
Metabolic effects
As a major growth factor, IGF-1 is responsible for stimulating growth of all cell types, and causing significant metabolic effects.[27] One important metabolic effect of IGF-1 is its ability to signal cells that sufficient nutrients are available for cells to undergo hypertrophy and cell division.[28] These signals also enable IGF-1 to inhibit cell apoptosis and increase the production of cellular proteins.[28] IGF-1 receptors are ubiquitous, which allows for metabolic changes caused by IGF-1 to occur in all cell types.[27] IGF-1's metabolic effects are far-reaching and can coordinate protein, carbohydrate, and fat metabolism in a variety of different cell types.[27] The regulation of IGF-1's metabolic effects on target tissues is also coordinated with other hormones such as growth hormone and insulin.[29]
Related growth factors
IGF-1 exists within the insulin/insulin-like growth factor (IGF) signaling system. The system consists of three ligands (
insulin, IGF-1 and
IGF-2, 2
tyrosine kinase receptors (
insulin receptor and
IGF-1R receptor) and six ligand binding proteins (IGFBP 1-6).[30] It plays an essential role in proliferation, survival, regulation of cell growth and affects almost every organ system in the body.[31]
Similarly to IGF-1,
IGF-2 is mainly produced in the liver. After release into circulation it stimulates growth and cell proliferation. IGF-2 is thought to be a fetal growth factor, as it is essential for a normal embryonic development and is highly expressed in embryonic and neonatal tissues.[32]
A
splice variant of IGF-1 sharing an identical mature region, but with a different E domain is known as mechano-growth factor (MGF).[33]
Disorders
Laron syndrome
Patients with severe primary insulin-like growth factor-1 deficiency (IGFD), called
Laron syndrome (LS) or Laron dwarfism, may be treated with
Mecasermin (brand name Increlex). This is a synthetic analog of IGF-1 which is approved for the treatment of
growth failure.[34]
Laron syndrome does not respond at all to
growth hormone treatment due to a lack of GH receptors. The FDA has grouped these diseases into a disorder called severe primary IGF deficiency. Patients with severe primary IGFD typically present with normal to high GH levels, height below 3
standard deviations (SD), and IGF-1 levels below 3 SD.[35] Severe primary IGFD includes patients with mutations in the GH receptor, post-receptor mutations or IGF mutations, as previously described. As a result, these patients cannot be expected to respond to GH treatment.[36]
People with Laron syndrome have very low rates of both cancer and
diabetes.[37]
Acromegaly
Acromegaly is a
syndrome that results in the
anterior pituitary gland producing excess
growth hormone (GH). A number of disorders may increase the pituitary's GH output, although most commonly it involves a tumor called
pituitary adenoma, derived from a distinct type of cell (
somatotrophs). It leads to anatomical changes and metabolic dysfunction caused by both an elevated GH and elevated IGF-1 levels.[38]
High level of IGF-1 in acromegaly is related to an increased risk of some cancers, particularly colon cancer and thyroid cancer.[39]
IGF-1 levels can be analyzed and used by physicians as a
screening test for
growth hormone deficiency,
acromegaly and
gigantism.[35] However IGF-1 was proved to be a bad diagnostic screening test for growth hormone deficiency (GHD). Therefore, IGF-1 should not be used alone as a screening test for GHD.[40]
The ratio of IGF-1 and insulin-like growth factor 1 binding protein-3 (IGFBP-3) can be analyzed and used as a diagnostic tool for growth-hormone related disorders.[41]
Interpretation of IGF-1 levels is complicated by the wide normal ranges, and marked variations by age, sex, and pubertal stage. Clinically significant conditions and changes may be masked by the wide normal ranges. Sequential measurement over time is often useful for the management of several types of pituitary disease, undernutrition, and growth problems.[42]
Several studies have shown associations between high levels of IGF-1 and an increased risk of tumor development. With an increase in serum IGF-1 levels of 100 ng/ml, there was a corresponding increase in the risk of colorectal cancer with 69%. High levels of IGF-1 were also associated with a 65% risk increase in breast cancer, 49% increase in prostate cancer and 106% in lung cancer.[48]
It has been suggested that consumption of IGF-1 in
dairy products could increase cancer risk, particularly
prostate cancer.[49][50] However, a 2018 review by the Committee on Carcinogenicity of Chemicals in Food, Consumer Products and the Environment (COC) concluded that there is "insufficient evidence to draw any firm conclusions as to whether exposure to dietary IGF-1 is associated with an increased incidence of cancer in consumers".[50] Certain dairy processes such as
fermentation are known to significantly decrease IGF-1 concentrations.[51]
A mutation in the signaling pathway PI3K-AKT-mTOR is a factor in the formation of
tumors found predominantly on skin, internal organs, and secondary lymph nodes (Kaposi sarcoma).[52]
Diabetes
Low IGF-1 levels are shown to increase the risk of developing
type 2 diabetes and
insulin resistance.[53] On the other hand, a high IGF-1 bioavailability in diabetes patients may delay or prevent the inception of diabetes-associated complications. A normal functioning IGF-1 mechanism reduces the occurrence of diabetes complications associated with lower IGF-1 levels, as it improves impaired small blood vessel function.[54]
Mortality
A 2022 review found that both high and low levels of IGF‐1 increase mortality risk, whilst a mid‐range (120–160 ng/ml) is associated with the lowest mortality.[48]
Mecasermin is a complex consisting of recombinant human IGF-1 and recombinant human IGF-binding protein-3.[58] The complex is used for the long-term treatment in children with growth failure, where they suffer from severe IGF-1 deficiency unresponsive to GH. Children with growth failure were given 0,12 mg/kg subcutaneous mecasermin two times a day over a period with a mean duration of 4,4 years (range: 0,04-12,5 years). During the first year of treatment the height velocity of the children increased from a mean of 2,8 cm/year at baseline to a mean of 8,0 cm/year. The mean growth velocities continued to remain above baseline for up to 8 years.[59]
Several companies have evaluated administering recombinant human IGF-1 (rhIGF-1) in clinical trials for
type 1 diabetes. These patients, despite having increased GH secretion, have low levels of circulating IGF-1 and therefore may benefit from rhIGF-1 therapy.[60] Results shows that a rhIGF-1 therapy two times a day in adults with type 1 diabetes increased the circulating IGF-1. This was with a reciprocal decrease in IGF-2 and an elevation of IGFBP-2.[60]
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