Modulation of epidermal growth factor-dependent protein phosphorylation in cell membrane preparations by receptor down regulation

We have reported previously 6] that epidermal growth factor (EGF)-induced down regulation of EGF receptors in normal rat kidney (NRK) cells results in a selective decrease in the in vitro EGF-dependent ³²P-phosphorylation of two membrane phosphoproteins of Mr I70K and Mr I50K. In this report, we further characterized the modulation of ³²P-phosphorylation of the 170K- and 150K-dalton proteins by down regulation with EGF in NRK cells. While EGF binding to its receptors was a necessary condition to induce loss of EGF-dependent phosphorylation of the 170K- and 150K-dalton proteins, it was not sufficient. Thus, reduction in the temperature of the incubation of cells with EGF from 37°C to 4°C abolished the loss of EGF-dependent phosphorylation of the 170K- and 150K-dalton membrane proteins. When EGF was removed from the medium the EGF-dependent phosphorylation of the 170K- and l50K-dalton proteins was quickly replenished; by 3 hr one-half of the “down regulated” phosphorylation was restored. All EGF-dependent phosphorylating capacity of the 170K- and l50K-dalton protein bands returned by 6 hr after removal of the growth factor. The loss of EGF-dependent phosphorylation of the 170K- and I50K-dalton proteins occurred at physiological EGF concentrations (0.25–25 ng/ml) that span the concentration range which is mitogenic for NRK cells. Exposure of confluent nondividing NRK cells to 1 ng/ml EGF, followed by incubation for 5 hr at 37°C. led to a 50% reduction in the EGF-dependent phosphorylation of the 170K- and 150K-dalton proteins. Maximal reduction (～95%) in the EGF-dependent phosphorylation of the 170K- and 150K-dalton proteins was noted with 10 ng/ml EGF for 5 hr. The EGF-induced loss of EGF-dependent phosphorylation was specific: several other growth factors did not produce phosphorylation loss of the 170K-