IGF-II/IGF-IIE

Since the discovery of IGF-I and IGF-II in three experiments from 1957 to 1978, an axis of regulation centered on IGFs has been gradually discovered and studied. Since the circulating IGFs have a negative feedback regulation on the secretion of hypothalamic growth factors, the axis is called the growth hormone-insulin-like growth factor axis. This axis, like other axes, also has important physiological functions, regulating cell growth, differentiation, and apoptosis at the molecular level, as well as tissue differentiation, growth, and tumorigenesis. In addition to growth factors, this axis includes two peptide growth factors. IGF-I and IGF-II are polypeptides of 70 and 67 amino acids, respectively, formed by cross-linking three disulfide bonds into small B, C and D regions, which have 70% identical sequences and pass the IGF-I receptor and the IGF-II receptor function. The amino acid sequence and structure of IGF is highly homologous to insulin. The IGF-I receptor shares two alpha and two beta subunits as the insulin receptor. The intracellular region of the beta subunit has tyrosine kinase activity that catalyzes the phosphorylation of tyrosine residues of the self and intracellular receptor substrates and initiates intracellular signaling pathways. As a transmembrane protein, the IGF-II receptor is a fructose-6-phosphate receptor and has no tyrosine kinase activity. Its main function is to internalize and degrade extracellular IGF-II. Most of the IGF-I in the circulation and body fluids is secreted by the liver and regulated by growth hormone. The level of IGF-I in body fluids is lowest in children, peaks in puberty, and gradually decreases in adulthood; the level of IGF-II does not change much in human life.