The insulin-like growth factor (IGF) family comprises three peptides, (insulin, IGF-I and IGF-II), six binding proteins (ICFBPs 1-6) and two receptors with inherent tyrosine kinase activity (insulin and IGF-I receptors), and one receptor with no known signalling function (IGF-II/M- 6-P receptor). They are all widely expressed and play an essential role in growth and development of all tissues, as well as in the normal functioning of differentiated tissues. The gene for IGF-I is highly conserved throughout vertebrate evolution; the salmon IGF-I gene encodes a peptide very similar to mammalian IGF-I. The biological actions of IGFs in mammalian tissues are modulated by locally produced IGFBPs. Cancer cells express IGFBPs in different combinations. Human salivary gland cancers express IGFBPs 3,4,5, whose release from the cell is regulated by growth factors and cytokines including interferon and tumor necrosis factor. These alterations in IGFBP release may mediate the inhibitory action of these cytokines on cancer cells. During the early stages of diabetic kidney disease there are hemodynamic changes that are partially induced by IGF-I which accumulates in the kidney due to increased IGFBP expression. Long-term disease is also associated with alterations in expression of certain ICFBPs and these changes may be associated with the glomerular basement membrane thickening seen at this stage of the disease. The IGFs also play an important role in the immune system. IGF administration to rodents and monkeys induces changes in the immune system including splenic enlargement and increased % splenic T cells. Thus, the IGFs (in combination with growth hormone) may prove to be useful agents in certain immune deficiency syndromes.
