INTERNALIZATION AND PROCESSING OF THE EGF RECEPTOR IN THE INDUCTION OF DNA-SYNTHESIS IN CULTURED FIBROBLASTS - ENDOCYTIC ACTIVATION HYPOTHESIS

The addition of EGF (epidermal growth factor) to cultured murine 3T3 cells produces a decrease in EGF binding activity with concomitant internalization and degradation of the initially bound EGF. When the EGF receptor on cultured 3T3 cells was affinity labeled with high specific activity 125I-EGF and the fate of the affinity labeled EGF-receptor complex was determined, the loss in binding activity was accounted for by receptor internalization and subsequent proteolytic processing of the EGF receptor molecules in the lysosomes. Studies of the effects of EGF concentration on EGF binding by cells, EGF-induced receptor internalization and EGF-induced stimulation of 3H-thymidine uptake into cellular DNA show that there is a direct correlation between EGF-induced receptor internalization and EGF-induced stimulation of DNA synthesis, but not between EGF binding and EGF-induced stimulation of DNA synthesis. This correlation is lost at high EGF concentrations where stimulation of DNA synthesis is suboptimal. Optimal stimulation of DNA synthesis requires a minimum of 6 h of incubation of EGF with cells; the suboptimal stimulation of DNA synthesis at high EGF concentration is intensified when the period of incubation of EGF with cells is less than 6 h. The data are consistent with a model of hormone signal transmission by Endocytic Activation, wherein the activation of EGF-induced processes requires constant EGF-induced internalization of receptor for a requisite 6-8 h period as an obligatory step in production of second messenger in the action of this hormone.