Insulin-like growth factor
The insulin-like growth factors (IGFs) are proteins with high sequence similarity to insulin. IGFs are part of a complex system that cells use to communicate with their physiologic environment. This complex system (often referred to as the IGF "axis") consists of two cell-surface receptors (IGF1R and IGF2R), two ligands (Insulin-like growth factor 1 (IGF-1) and Insulin-like growth factor 2 (IGF-2)), a family of six high-affinity IGF-binding proteins (IGFBP-1 to IGFBP-6), as well as associated IGFBP degrading enzymes, referred to collectively as proteases.
IGF1/GH Axis 
The IGF "axis" is also commonly referred to as the Growth Hormone/IGF1 Axis. Insulin-like growth factor 1 (IGF-1) is mainly secreted by the liver as a result of stimulation by growth hormone (GH). IGF-1 is important for both the regulation of normal physiology, as well as a number of pathological states, including cancer. The IGF axis has been shown to play roles in the promotion of cell proliferation and the inhibition of cell death (apoptosis). Insulin-like growth factor 2 (IGF-2) is thought to be a primary growth factor required for early development while IGF-1 expression is required for achieving maximal growth. Gene knockout studies in mice have confirmed this, though other animals are likely to regulate the expression of these genes in distinct ways. While IGF-2 may be primarily fetal in action it is also essential for development and function of organs such as the brain, liver and kidney.
Factors that are known to cause variation in the levels of GH and IGF-1 in the circulation include an individuals genetic make-up, the time of day, their age, sex, exercise status, stress levels, genetics, nutrition level and body mass index (BMI), disease state, race, estrogen status and xenobiotic intake.
IGF-I has an involvement in regulating neural development including neurogenesis, myelination, synaptogenesis, and dendritic branching and neuroprotection after neuronal damage. Increased serum levels of IGF-I in children link to higher IQ.
IGF-1 shapes the development of the cochlea through controlling apoptosis. Its deficit can cause hearing loss. Serum level of it also underlies a correlation between short height and reduced hearing abilities particularly around 3–5 years of age, and at age 18 (late puberty).
IGF receptors 
The IGFs are known to bind the IGF-1 receptor, the insulin receptor, the IGF-2 receptor, the insulin-related receptor and possibly other receptors. The IGF-1 receptor is the "physiological" receptor—IGF-1 binds to it at significantly higher affinity than it binds the insulin receptor. Like the insulin receptor, the IGF-1 receptor is a receptor tyrosine kinase—meaning the receptor signals by causing the addition of a phosphate molecule on particular tyrosines. The IGF-2 receptor only binds IGF-2 and acts as a "clearance receptor"—it activates no intracellular signaling pathways, functioning only as an IGF-2 sequestering agent and preventing IGF-2 signaling.
Organs and tissues affected by IGF-1 
Since many distinct tissue types express the IGF-1 receptor, IGF-1's effects are diverse. It acts as a neurotrophic factor, inducing the survival of neurons. It causes skeletal muscle hypertrophy, by inducing protein synthesis, and by blocking muscle atrophy. It is protective for cartilage cells, and is associated with activation of osteocytes, and thus may be an anabolic factor for bone. Since at high concentrations it is capable of activating the insulin receptor, it can also complement for the effects of insulin.
IGF-Binding Proteins 
IGF-1 and IGF-2 are regulated by a family of proteins known as the IGF-Binding Proteins. These proteins help to modulate IGF action in complex ways that involve both inhibiting IGF action by preventing binding to the IGF-1 receptor as well as promoting IGF action possibly through aiding in delivery to the receptor and increasing IGF half-life. Currently, there are six characterized IGF Binding Proteins (IGFBP-1 to IGFBP-6). There is currently significant data suggesting that IGFBPs play important roles in addition to their ability to regulate IGFs.
Diseases affected by IGF 
Studies of recent interest show that the Insulin/IGF axis play an important role in aging. Nematodes, fruit-flies and other organisms have an increased life span when the gene equivalent to the mammalian insulin is knocked out. It is somewhat difficult to relate this finding to the mammal, however, because in the smaller organism there are many genes (at least 37 in the nematode) that are "insulin-like" or "IGF-1-like", whereas in the mammals insulin-like proteins comprise only seven members (insulin, IGFs, relaxins, EPIL, and relaxin-like factor) and have apparently distinct roles with some but less crosstalk. Simpler organisms typically have fewer receptors (only one known in the nematode) and the roles of these other insulins are unknown. Additionally, these animals do not have specialized organs (Islets of Langerhans), which sense insulin in response to glucose homeostasis. Moreover, IGF1 affects lifespan in nematodes by causing dauer formation, a developmental stage of C. elegans larva. There is no mammalian correlate. Therefore it is an open question as to whether either IGF-1 or insulin in the mammal may perturb aging, although there is the suggestion that dietary restriction phenomena may be related.
Other studies are beginning to uncover the important role the IGFs play in diseases such as cancer and diabetes, showing for instance that IGF-1 stimulates growth of both prostate and breast cancer cells. Researchers are not in complete agreement about the degree of cancer risk that IGF-1 poses.
See also 
- Insulin-like growth factor 1
- Insulin-like growth factor 2
- Growth hormone treatment
- HGH controversies
- Insulin/IGF/Relaxin family
- Gunnell, David; Miller, LL; Rogers, I; Holly, JM; Alspac Study, Team (11/01/2005). "Association of Insulin-like Growth Factor I and Insulin-like Growth Factor-Binding Protein-3 with Intelligence Quotient Among 8- to 9-Year-Old Children in the Avon Longitudinal Study of Parents and Children". Pediatrics 116 (5): e681. doi:10.1542/peds.2004-2390. PMID 16263982.
- Welch, D; Dawes, PJ. (2007). "Childhood hearing is associated with growth rates in infancy and adolescence". Pediatr Res 62 (4): 495–8. doi:10.1203/PDR.0b013e3181425869. PMID 17667854.
- Pierce SB, Costa M, Wisotzkey R, Devadhar S, Homburger SA, Buchman AR, Ferguson KC, Heller J, Platt DM, Pasquinelli AA, Liu LX, Doberstein SK, Ruvkun G (2001). "Regulation of DAF-2 receptor signaling by human insulin and ins-1, a member of the unusually large and diverse C. elegans insulin gene family". Genes & Development 15 (6): 672–86. doi:10.1101/gad.867301. PMC 312654. PMID 11274053.
- Cohen P, Peehl DM, Lamson G, Rosenfeld RG (1991). "Insulin-like growth factors (IGFs), IGF receptors, and IGF-binding proteins in primary cultures of prostate epithelial cells". Journal of Clinical Endocrinology and Metabolism 73 (2): 401–7. doi:10.1210/jcem-73-2-401. PMID 1713219.
- Lippman ME (1993). "The development of biological therapies for breast cancer". Science 259 (5095): 631–2. doi:10.1126/science.8430312. PMID 8430312.
- Papa V, Gliozzo B, Clark GM, McGuire WL, Moore D, Fujita-Yamaguchi Y, Vigneri R, Goldfine ID, Pezzino V (1993). "Insulin-like growth factor-I receptors are overexpressed and predict a low risk in human breast cancer". Cancer Research 53 (16): 3736–40. PMID 8339284.
- Scarth JP (2006). "Modulation of the growth hormone-insulin-like growth factor (GH-IGF) axis by pharmaceutical, nutraceutical and environmental xenobiotics: an emerging role for xenobiotic-metabolizing enzymes and the transcription factors regulating their expression. A review". Xenobiotica 36 (2–3): 119–218. doi:10.1080/00498250600621627. PMID 16702112.
- Woods AG, Guthrie KM, Kurlawalla MA, Gall CM (1998). "Deafferentation-induced increases in hippocampal insulin-like growth factor-1 messenger RNA expression are severely attenuated in middle aged and aged rats". Neuroscience 83 (3): 663–8. doi:10.1016/S0306-4522(97)00539-3. PMID 9483550.