From Insulin and Insulin-Like Activity to the Insulin Superfamily of Growth-Promoting Peptides: A 20th-Century Odyssey

In 1941, Gellhorn reported that administration of human blood to hypophysectomized/adrenodemedullated rats caused a fall in blood sugar. This was among the early demonstrations that human blood possesses glucose-lowering or insulin-like activity (ILA). Gellhorn assumed he had detected only insulin. During the 1960s, however, it became evident that plasma ILA contained at least two components: one, suppressible ILA (SILA), was inactivated by anti-insulin antibody and was therefore considered to be indistinguishable from pancreatic insulin; the other, nonsuppressible ILA (NSILA), was unaffected by anti-insulin antibody. Subsequent work resolved NSILA into insulin-like growth factors I and II (IGF-I and IGF-II), two 7.5 kilodalton peptides with potent mitogenic properties; established their identity with the somatomedins; and investigated both their therapeutic potential and role in the pathogenesis of neoplastic and other human diseases. Insulin and the IGFs exhibit striking homologies in amino acid composition and some degree of overlap in their signaling pathways and actions. Moreover, insulin-like proteins have been identified not only in all vertebrate classes but also in molluscs, insects, and worms. These observations are the basis for the hypothesis that the genes encoding vertebrate insulins and IGFs and invertebrate insulin-like molecules evolved from a common ancestral gene, and for the concept of an insulin superfamily of growth-promoting peptides.