In 1957, Salmon and Daughaday discovered that during their research on growth hormone (GH), the serum of rats after pituitary removal stimulated the infiltration of sulfation into cultured cartilage, but not directly in the culture medium. The addition of GH had no effect, leading them to conclude that GH does not directly stimulate cartilage growth but acts through a "sulfation factor," later named the growth regulator. In 1963, Froesch et al. identified a portion of insulin-like activity in serum that was not suppressed by insulin antiserum, which was soluble in acidified ethanol and termed "nonsuppressible insulin-like activity" (NSILA). By 1972, Pieron and Temin isolated a factor from bovine serum that stimulated cell division and called it "proliferative stimulating activity." As molecular biology advanced, NSILA (I and II) were purified in 1978 and found to resemble proinsulin, leading to the naming of insulin-like growth factors I and II (IGF-I and IGF-II). It was confirmed that "sulfation factor" and "proliferative stimulating activity" belong to the same family as IGF.
The IGF family includes two low-molecular-weight peptides (IGF-I and IGF-II), two specific receptors, and six binding proteins. IGF-I is a 70-amino-acid single-chain basic protein with a molecular weight of 7,649 Da, while IGF-II is a 67-amino-acid weakly acidic protein with a molecular weight of 7,471 Da. Both are heat-resistant and share about 70% homology, resembling human proinsulin by around 50%. Their biological effects are mediated through specific cell surface receptors. Two distinct IGF receptors have been identified: IGF-I receptor (similar to the insulin receptor) and IGF-II receptor (also known as the mannose-6-phosphate receptor). These receptors differ in structure and ligand affinity. IGFs also bind to binding proteins (IGFBPs) in extracellular fluids, forming inactive complexes. Six IGFBPs (IGFBP-1 to -6) have been identified, all being small peptides with high affinity for IGFs but not for insulin.
IGFs play critical roles in growth and development. While IGF-I and IGF-II have similar structures, their in vivo functions differ. IGF-I promotes cell proliferation and protein/DNA synthesis, often produced locally by various tissues. IGF-II is more active during fetal development and is not regulated by GH. Studies show IGFs regulate trophoblast invasion and implantation during early pregnancy by enhancing extracellular matrix adhesion. IGF-I levels increase during pregnancy, and cord blood studies suggest its role in fetal nutrition. Genetic studies reveal that IGF-I and IGF-II are essential for normal embryonic growth, with mutations causing severe developmental defects.
Research on IGFs has expanded significantly, with applications in treating growth disorders like Laron syndrome. IGF-I therapy has shown promise in improving growth in GH-insensitive patients. Current understanding suggests that IGFs are central to growth regulation, interacting with GH and other factors. Despite extensive research, many aspects of IGF function remain to be explored, especially in human systems.
References:
1. Salmon WD, Daughaday WH. A hormonally controlled serum factor which stimulates sulfate incorporation by cartilage in vitro. J Lab Clin Med, 1957.
2. Froesch ER. Antibody-suppressible and nonsuppressible insulin-like activities in human serum and their physiologic significance. J Clin Invest, 1963.
3. Pierson RW Jr. Partial purification from calf serum of a fraction with multiplication-stimulating activity. J Cell Physiol, 1972.
4. Grudice IC. Insulin-like growth factors and ovarian follicular development. Endocrine Reviews, 1992.
5. Froesch ER. Action of insulin-like growth factors. Ann Rev Physiol, 1985.
6. Stylianopoulou F. Pattern of the insulin-like growth factor II gene expression during rat embryogenesis. Development, 1988.
7. Irving JA, Lala PK. Function role of cell surface integrins on human trophoblast cell migration. Exp Cell Res, 1995.
8. Kniss DA. Insulin-like growth factors: Regulation of glucose and amino acid transport. J Reprod Med, 1994.
9. Liu Baoying, Wang Huixin. Progress in insulin growth factor research. Foreign Medicine* Molecular Biology, 1996.
10. Gao Min, Qu Xinzhong, Li Guilin. Effects of growth factors and hormones on the fetus. Foreign Medicine* Maternal and Child Health Care, 1996.
11. Steven D. The growth hormone/insulin-like growth factor axis in intrauterine growth retardation. Endocrinology, 1996.
12. Tong Tanjun. Molecular basis of malnutrition affecting children’s growth. Progress in Physiological Sciences, 1995.
13. Julie Baker, Liu JP, Robertson EJ, et al. Role of insulin-like growth factors in embryonic and postnatal growth. Cell, 1993.
14. Liu JP, Baker J, Perkins AS, et al. Mice carrying null mutations of the genes encoding insulin-like growth factor I and type 1 IGF receptor. Cell, 1993.
15. Philippe F, Backel Jauw, Louis E, Underwood et al. Prolonged treatment with recombinant insulin-like growth factor I in children with growth hormone insensitivity syndrome. J Clin Endocrinol Metab, 1996.
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