Mitogenic Activity and Receptor Reactivity of Hybrid Molecules Containing Portions of the Insulin-like Growth Factor I (IGF-I), IGF-II, and Insulin Molecules

Insulin and the insulin-like growth factors IGF-I and IGF-II are thought to exert their mitogenic effects in cultured chick embryo fibroblasts and human skin fibroblasts via IGF receptors rather than via insulin receptors. These effects appear to be mediated by the type I subtype of IGF receptor, which is structurally similar to the insulin receptor and exhibits significant cross-reactivity with insulin. As a first step in our longrange goal of defining those features of the IGF-I and IGF-II molecules that confer enhanced mitogenic activity and reactivity with these mitogenic type I IGF receptors, we have prepared two hybrid insulin-IGF molecules and examined their mitogenic and binding activities: (1) A27-insulin, containing an elongated 27- residue A-chain (in which the 6-residue D-domain of IGF-II was added to the carboxy-terminus of the 21-residue A-chain of insulin) combined with the B-chain of insulin; and (2) A_insulin-B_IGF-I, containing the A-chain of insulin and the synthetic 30-residue B-domain of IGF-I. Both hybrid molecules stimulated DNA synthesis and inhibited ¹²⁵I-IGF-I binding to type I IGF receptors in both chick embryo and human fibroblast cultures. A27- insulin had considerably greater mitogenic potency and binding potency than A_insulin-B_IGF-I. Neither hybrid molecule was more potent in these assays than insulin, indicating that the presence of D_IGF-II,or B_IGF-I, by itself was not sufficient to increase the mitogenic potency of insulin in fibroblasts. By contrast, A_insulin-B_IGF-I showed enhanced reactivity with an antiserum to IGF-I. A27-insulin retained significant insulin-like metabolic activity despite the presence of the D-domain of IGF-II.