Protein-tyrosine Phosphatase-mediated Decrease of Epidermal Growth Factor and Platelet-derived Growth Factor Receptor Tyrosine Phosphorylation in High Cell Density Cultures

Contact-induced growth inhibition is a characteristic feature of normal cells grown in monolayer. The importance of reversible tyrosine phosphorylation in mitogenic signaling, together with earlier reports of increased levels of protein-tyrosine phosphatases (PTPs) in densely cultured cells, has led to the proposal that PTPs may be involved in mediating contact inhibition of cell growth. We have compared net levels of ligand-induced tyrosine phosphorylation of the epidermal growth factor (EGF) receptor in mink lung epithelial cells cultured under sparse or dense conditions. The levels of net tyrosine phosphorylation of the stimulated EGF receptor was found to be more than 4-fold higher in sparse cultures. This difference was greatly reduced when cells were pretreated with the PTP inhibitor phenyl arsine oxide. Monitoring of dephosphorylation rates in vivo of the stimulated EGF receptors revealed increased EGF receptor-directed PTP activity in dense cultures. The platelet-derived growth factor β-receptor, expressed in stably transfected porcine aortic endothelial cells, also displayed lower levels of ligand induced net tyrosine phosphorylation in cells from dense cultures. This density-dependent difference in tyrosine phosphorylation was reduced by pretreatment of cultures with the PTP inhibitor orthovanadate. A PTP-mediated decrease of the in vivo net levels of ligand induced tyrosine phosphorylation of EGF and platelet-derived growth factor receptors in cells at high density have thus been demonstrated. Loss of this previously unnoticed regulatory pathway may be involved in cellular transformation.