Endocytosed Epidermal Growth Factor (EGF) Receptors Contribute to the EGF-Mediated Growth Arrest in A431 Cells by Inducing a Sustained Increase in p21/CIP1

Abstract

We investigated the ability of endocytosed activated epidermal growth factor receptors (EGFR) to induce expression of the cyclin-interacting protein p21/CIP1 in A431 cells. Transforming growth factor α (TGFα) and EGF both induced tyrosine phosphorylation, induction of p21/CIP1, and thereby inhibition of DNA synthesis. TGFα is released from the EGFR when the TGFα–EGFR complex encounters low pH upon endocytosis. Consistently, we found more rapid dephosphorylation of the EGFR and less induction of p21/CIP1 by TGFα than by EGF. This difference was abolished upon neutralizing endosomal pH by the carboxylic ionophore monensin or the proton ATPase inhibitor bafilomycin A1. When surface-bound TGFα was removed by acid stripping and endosomal pH was neutralized with bafilomycin A1, TGFα stimulated EGFR tyrosine phosphorylation, induced p21/CIP1, and inhibited DNA synthesis. This strongly suggests that p21/CIP1 can be induced by endocytosed, activated EGFR and that endocytosed EGFR can affect cell growth.