EVIDENCE THAT TYPE-II INSULIN-LIKE GROWTH-FACTOR RECEPTOR IS COUPLED TO CALCIUM GATING SYSTEM

In competent Balb/c 3T3 cells primed with epidermal growth factor (primed competent cells), insulin-like growth factor-II (IGF-II) stimulated calcium influx in a concentration dependent manner with the ED50 of 450 pM. When receptor-bound (125I]IGF-II was cross-linked by use of disuccinimidyl suberate, a 240 K-Da protein was radiolabeled. Excess amount of unlabeled IGF-II inhibited the affinity-labeling of the 240 K-Da protein. To further examine whether IGF-II stimulates calcium influx by acting on the type II IGF receptor, we employed polyclonal antibody raised against rat type II IGF receptor, R-II-PAB1. This antibody immunoprecipitated the type II IGF receptor and inhibited IGF-II binding in Balb/c 3T3 cell membrane without affecting IGF-I binding. In primed competent cells, R-II-PAB1 elicited an agonistic action in stimulating [3H]thymidine incorporation. Under the same condition, R-II-PAB1 elicited a marked stimulation of calcium influx. These results suggest that, in Balb/c 3T3 cells, 1) relatively low concentrations of IGF-II act mainly on the type II IGF receptor; 2) the type II IGF receptor is coupled to a calcium gating system; and 3) binding of a ligand to the type II IGF receptor leads to the stimulation of DNA synthesis.