Structural and immunological characterization of insulin-like growth factor II binding to IM-9 cells

The structural and immunological properties of the insulin-like growth factor II (IGF-II) receptor on IM-9 lymphoblasts were studied using a combination of competitive binding and affinity cross-linking techniques as well as with a panel of polyclonal and monoclonal antireceptor antibodies. Unlike IGF-II binding to the classical type II IGF receptor, [125I] IGF-II binding to IM-9 cells was potently inhibited not only by unlabeled IGF-II, but also by insulin (50% inhibition of binding at 2.5 and 1.5 nM, respectively). Affinity cross-linking of [125I] IGF-II to intact cells, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, demonstrated that the overwhelming majority of IGF-II binding was to a type I receptor (apparent mol wt, greater than 300,000 unreduced and 135,000 reduced), with minimal binding to a type II receptor (apparent mol wt, 220,000 unreduced and 240,000 reduced). After preincubation with five different antireceptor antibodies, inhibition of [125I] IGF-II binding was comparable to inhibition of [125I]insulin binding. These studies demonstrate that in IM-9 cells, the majority of IGF-II binding is to a type I receptor with high affinity for both insulin and IGF-II. Whether this is an atypical insulin receptor or a unique type I receptor remains to be established.