Studies of signalling by the receptors for insulin and insulin-like growth factor I

Insulin and insulin-like growth factor I (IGF-I) are structurally homologous vertebrate peptide hormones which elicit their effects by binding to specific membrane receptors. Insulin primarily regulates metabolism whereas IGF-I primarily regulates growth. Immunological and biochemical studies of their receptors and analyses of the sequences of receptor cDNA show the receptors are quite similar. Both are membrane-spanning heterotetramers containing a hormone-stimulatable tyrosine kinase activity in the cytoplasmic domain. Although the exact molecular mechanism whereby these receptors mediate the generation of a signal is not known, many studies show the kinase is essential for their ligands' actions.;Differences in the receptors' intracellular signalling could account for the different physiological roles of the two hormones. To determine whether there are differences in the receptors' signalling, we studied the receptors in Chinese hamster ovary (CHO) cells stably expressing either human insulin (CHO-HIR) or human IGF-I (CHO-IGFIR) receptor cDNA. We first showed, using CHO-IGFIR, that a putative IGF-I receptor cDNA did, in fact, encode a functional IGF-I receptor. We then studied biological responses in both cell lines and found there are no differences in the abilities of the insulin and IGF-I receptors to signal metabolic and mitogenic responses.;Anti-receptor antibodies are used extensively to probe the structure and function of the two receptors. Some reports suggest that certain insulin-mimetic antibodies do not stimulate receptor kinase activity. We showed, using CHO-HIR, that these antibodies do stimulate receptor kinase activity. We also showed these antibodies mimic insulin's stimulation of the tyrosine phosphorylation of certain described cellular proteins. Like insulin, insulin-mimetic antibodies require receptor kinase activity.;We then studied IGF-I receptor signalling in more detail. Using CHO-IGFIR, we showed a monoclonal antibody to the IGF-I receptor which blocks IGF-I binding and biological responses inhibits IGF-I-stimulation of the receptor kinase. We also showed that this antibody blocks IGF-II biological responses by inhibiting IGF-II-stimulation of the IGF-I receptor kinase but does not inhibit IGF-II binding. Furthermore, we showed the IGF-I receptor mediates IGF-I and IGF-II-stimulated tyrosine phosphorylation of the type I phosphatidylinositol kinase.