Proinsulin Binds with High Affinity the Insulin Receptor Isoform A and Predominantly Activates the Mitogenic Pathway

Proinsulin is generally regarded as an inactive prohormone because of its low metabolic activity. However, proinsulin appears to regulate embryo development in animal models. In this study, we evaluated whether proinsulin may differentially bind to and activate the two insulin receptor (IR) isoforms (IR-A and IR-B), because IR-A is a relatively low-specificity receptor that is prevalent in fetal and cancer cells and is able to mediate the growth effects of IGF-II. Mouse R− fibroblasts devoid of IGF-I receptor (IGF-IR) and stably transfected with cDNA encoding either human IR-A or IR-B (R− /IR-A and R− /IR-B cells) were used. Three human cancer cell lines were also studied. We found that proinsulin stimulated phosphorylation of IR-A with an EC50 of 4.5 ± 0.6 nm and displaced [125I]insulin from IR-A with a similar EC50. In contrast, proinsulin EC50 values for stimulation of IR-B phosphorylation and for [125I]insulin displacement from IR-B were approximately 7-fold higher. Proinsulin did not bind or activate...