Endocytosed epidermal growth factor (EGF) receptors contribute to the EGF-mediated growth arrest in A431 cells by inducing a sustained increase in p21/CIP1

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 alpha (TGFalpha) and EGF both induced tyrosine phosphorylation, induction of p21/CIP1, and thereby inhibition of DNA synthesis. TGFalpha is released from the EGFR when the TGFalpha-EGFR complex encounters low pH upon endocytosis. Consistently, we found more rapid dephosphorylation of the EGFR and less induction of p21/CIP1 by TGFalpha 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 TGFalpha was removed by acid stripping and endosomal pH was neutralized with bafilomycin A1, TGFalpha 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.