High-affinity insulin binding to an atypical insulin-like growth factor-I receptor in human breast cancer cells

We studied the nature of insulin receptor binding in MCF-7 breast cancer cells. In both intact cells and solubilized receptor preparations, high-affinity insulin binding was seen. However, unlabeled insulin-like growth factor-I (IGF-I) was five-fold more potent in inhibiting 125I-insulin binding than insulin itself. With monoclonal antibodies to the insulin receptor, 30% of 125I-insulin binding was inhibited. In contrast when alpha-IR3, a monoclonal antibody that recognizes typical IGF-I receptor, was employed over 60% of 125I-insulin binding was inhibited. The B29-MAB-125I-insulin photoprobe was then cross-linked to MCF-7 membranes. Cross-linking was inhibited by both unlabeled insulin and IGF-I. Further, the B29-MAB-125I-insulin photoprobe cross-linked to MCF-7 membranes was strongly immunoprecipitated by alpha-IR3. Employing sequential affinity chromatography with insulin-Affi-gel followed by insulin receptor monoclonal antibody agarose, atypical insulin binding activity was separated from insulin receptor binding activity. This atypical receptor had intrinsic tyrosine kinase activity. Both insulin and IGF-I stimulated the phosphorylation of the receptor's beta subunit. In MCF-7 cells both IGF-I and insulin stimulated [3H]thymidine incorporation; alpha-IR3 blocked all of the IGF-I effect but only 50-60% of the insulin effect. This study demonstrates in MCF-7 cells that, in addition to typical insulin and IGF-I receptors, there is another receptor that binds both insulin and IGF-I with high affinity.