Global modulation of the epidermal growth factor receptor is triggered by occupancy of only a few receptors. Evidence for a binary regulatory system in normal human fibroblasts

We investigated the effect of epidermal growth factor (EGF) pretreatment on binding to its own receptor. We found that EGF specifically induces a rapid, reversible, and global change in the affinity of surface EGF receptors. Occupancy of only a few (less than 1%) was sufficient to reduce the affinity of the majority of surface receptors by 10 min and a maximal response required only 5% occupancy. The rate at which EGF receptor affinity decreased was essentially independent of the extent of receptor occupancy and occurred with a t 1/2 between 2-2.5 min. Surface receptors remained in the lower affinity state as long as EGF remained present. Removal of EGF resulted in the restoration of receptor affinity with a t 1/2 of about 20 min. Kinetic analyses revealed that the alteration in apparent affinity was due to changes in both the association and dissociation rate constants as well as an increase in the specific internalization rate of the receptor. Treatment of cells with phorbol esters produced a similar affinity drop, but depletion of intracellular protein kinase C did not affect the affinity change induced by EGF. Thus, phorbol esters and EGF mediate their effects through different pathways. EGF reduced the affinity of its own receptors in a variety of cell types including Chinese hamster ovary cells expressing transfected human EGF receptors. Our observations are consistent with the hypothesis that occupancy of a few receptors on EGF naive cells triggers a global modification/phosphorylation of surface receptors which results in the observed change in affinity. This system is independent of protein kinase C and probably serves to regulate the activity of the EGF receptor.