| Abstract: | Monoclonal antibodies against phosphotyrosine were used to study tyrosine phosphorylation in human epidermal carcinoma A431 cells in vivo. Incubation of A431 cells with the epidermal growth factor (EGF) leads to tyrosine phosphorylation of the EGF receptor; the phosphotyrosine content in cellular EGF receptors increases 50-100-fold in the presence of the growth factor. The maximum level of the receptor autophosphorylation is reached on the 5th min and is then held constant during 90-min incubation with EGF. After preincubation of A431 cells with phorbol-12-myristoyl-13-acetate (PMA) or calcium ionophore A23187 the receptor autophosphorylation decreases significantly. After addition of A23187 and EGTA to the preincubation medium the phosphotyrosine content in cellular EGF receptors stimulated by the growth factor reaches the control level i.e., that observed in the absence of the ionophore. After preincubation of cells in the presence of phorbol ester and H-7 (protein kinase C inhibitor) the level of EGF receptor autophosphorylation does not practically differ from that of control. |
