Insulin-like growth factor 1
|, IGF-I, IGF1A, IGFI, MGF, insulin like growth factor 1, IGF|
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.
IGF-1 is a protein that in humans is encoded by the IGF1 gene. 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.
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. The lowest levels occur in infancy and old age.[medical citation needed] Protein intake increases IGF-1 levels in humans under age 65, independent of total calorie consumption.
- 1 Synthesis and circulation
- 2 Mechanism of action
- 3 Related growth factors
- 4 Disorders
- 5 Use as a diagnostic test
- 6 Possible causes of elevated IGF-1 levels
- 7 Use as a therapeutic agent
- 8 Clinical trials
- 9 Society and culture
- 10 History of name
- 11 See also
- 12 References
- 13 External links
Synthesis and circulation
IGF-1 is produced primarily by the liver as an endocrine hormone as well as in target tissues in a paracrine/autocrine fashion. Production is stimulated by growth hormone (GH) and can be retarded by undernutrition, growth hormone insensitivity, lack of growth hormone receptors, or failures of the downstream signaling pathway post GH receptor including SHP2 and STAT5B. Approximately 98% of IGF-1 is always bound to one of 6 binding proteins (IGF-BP). IGFBP-3, the most abundant protein, accounts for 80% of all IGF binding. IGF-1 binds to IGFBP-3 in a 1:1 molar ratio. 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. The lowest levels occur in infancy and old age.[medical citation needed]
Protein intake increases IGF-1 levels in humans, independent of total calorie consumption. Factors that are known to cause variation in the levels of growth hormone (GH) and IGF-1 in the circulation include: insulin levels, genetic make-up, the time of day, age, sex, exercise status, stress levels, nutrition level and body mass index (BMI), disease state, ethnicity, estrogen status and xenobiotic intake.
Mechanism of action
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.
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 AKT signaling pathway, a stimulator of cell growth and proliferation, and a potent inhibitor of programmed cell death . The IGF-1 receptor seems to be the "physiologic" receptor because it binds IGF-1 with significantly higher affinity than insulin receptor does. IGF-1 activates the insulin receptor at approximately 0.1 times the potency of insulin. Part of this signaling may be via IGF1R/Insulin Receptor heterodimers (the reason for the confusion is that binding studies show that IGF1 binds the insulin receptor 100-fold less well than insulin, yet that does not correlate with the actual potency of IGF1 in vivo at inducing phosphorylation of the insulin receptor, and hypoglycemia).[medical citation needed]
Insulin-like growth factor 1 receptor (IGF-1R) and other tyrosine kinase growth factor receptors signal through multiple pathways. A key pathway is regulated by phosphatidylinositol-3 kinase (PI3K) and its downstream partner, the mammalian target of rapamycin (mTOR). Rapamycins complex with FKBPP12 to inhibit the mTORC1 complex. mTORC2 remains unaffected and responds by upregulating Akt, driving signals through the inhibited mTORC1. Phosphorylation of eukaryotic initiation factor 4e (eif-4E) [4EBP] by mTOR inhibits the capacity of 4EBP to inhibit eif-4E and slow metabolism.[medical citation needed]
Insulin-like growth factor 1 has been shown to bind and interact with all seven IGF-1 binding proteins (IGFBPs): IGFBP1, IGFBP2, IGFBP3, IGFBP4, IGFBP5, IGFBP6, and IGFBP7.[medical citation needed] Some IGFBPs are inhibitory. For example, both IGFBP-2 and IGFBP-5 bind IGF-1 at a higher affinity than it binds its receptor. Therefore, increases in serum levels of these two IGFBPs result in a decrease in IGF-1 activity.[medical citation needed]
Related growth factors
IGF-1 is closely related to a second protein called "IGF-2". IGF-2 also binds the IGF-1 receptor. However, IGF-2 alone binds a receptor called the "IGF-2 receptor" (also called the mannose-6 phosphate receptor). The insulin-like growth factor-II receptor (IGF2R) lacks signal transduction capacity, and its main role is to act as a sink for IGF-2 and make less IGF-2 available for binding with IGF-1R. As the name "insulin-like growth factor 1" implies, IGF-1 is structurally related to insulin, and is even capable of binding the insulin receptor, albeit at lower affinity than insulin.
Rare diseases characterized by inability to make or respond to IGF-1 produce a distinctive type of growth failure. One such disorder, termed Laron dwarfism 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. 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.
Acromegaly is a syndrome that results when the anterior pituitary gland produces 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. High level of IGF-1 in acromegaly is related to an increased risk of some cancers, particularly colon cancer and thyroid cancer.
Use as a diagnostic test
IGF-1 levels can be measured in the blood in 10-1000 ng/ml amounts. As levels do not fluctuate greatly throughout the day for an individual person, IGF-1 is used by physicians as a screening test for growth hormone deficiency and excess in acromegaly and gigantism.
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 management over time is often useful for the management of several types of pituitary disease, undernutrition, and growth problems.
Possible causes of elevated IGF-1 levels
- acromegaly (especially when GH is also high)
- high-protein diet 
- high glycemic-index diet
- dairy products (except for cheese) consumption
- delayed puberty
- IGF-1 assay problems
- some rare tumors (i.e. carcinoids) secreting IGF-1
Use as a therapeutic agent
Patients with severe primary insulin-like growth factor-1 deficiency (IGFD), called Laron syndrome, may be treated with either IGF-1 alone or in combination with IGFBP-3. Mecasermin (brand name Increlex) is a synthetic analog of IGF-1 which is approved for the treatment of growth failure. IGF-1 has been manufactured recombinantly on a large scale using both yeast and E. coli.
Several companies have evaluated administering recombinant IGF-1 in clinical trials for type 1 diabetes, type 2 diabetes, amyotrophic lateral sclerosis, severe burn injury and myotonic muscular dystrophy.
Results of clinical trials evaluating the efficacy of IGF-1 in type 1 diabetes and type 2 diabetes showed reduction in hemoglobin A1C levels and daily insulin consumption.[medical citation needed] However the sponsor discontinued the program due to an exacerbation of diabetic retinopathy, coupled with a shift in corporate focus towards oncology.
Two clinical studies of IGF-1 for ALS were conducted and although one study demonstrated efficacy the second was equivocal,[medical citation needed] and the product was not submitted for approval to the FDA.
Small molecules that upregulate IGF-1
Society and culture
In December 2006 a version of IGF-1 marketed by Insmed was found to infringe on patents licensed by Tercica which also sold a version of IGF-1; Tercica then sought to get a U.S. district court judge to ban sales of Iplex. To settle patent infringement charges and resolve all litigation between the two companies, in March 2007 Insmed agreed to withdraw Iplex from the U.S. market, leaving Tercica's Increlex as the sole version of IGF-1 available in the United States at that time.
Numerous sources have claimed that Deer Antler Spray, purportedly extracted from cervid sources, contains IGF-1. Credence to this claim comes from the fact that deer's antlers grow extremely rapidly and that the associated cellular factors can similarly aid in skeletal healing in humans. However, since IGF-1 is a protein, it is rapidly broken down in the gastrointestinal tract; and even if it were not, its large molecular weight and high hydrophilicity prevents it from being absorbed by intestinal tissue. In spite of this, sprays and pills claiming to be 'deer antler velvet extracts' are freely available on the market.
In September 2013, the headquarters of SWATS, a well-known distributor of deer antler spray and other controversial products, was raided and ordered to shut down by Alabama's attorney general citing "numerous serious and willful violations of Alabama’s deceptive trade practices act".
History of name
In the 1950s IGF-1 was called "sulfation factor" because it stimulated sulfation of cartilage in vitro, and in the 1970s due to its effects it was termed "nonsuppressible insulin-like activity" (NSILA).
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