Modulation of type α transforming growth factor receptors by a phorbol ester tumor promoter

Epidermal growth factor (EGF) and an EGF‐like transforming growth factor (eTGF) from retrovirally transformed cells bind to a common receptor type in A431 cells. We have investigated the effects of the tumor promoter phorbol myristate acetate [PMA] on EGF/eTGF receptors in intact A431 cells. Treatment with PMA at 37°C induces a complete loss of high‐affinity (K d = 35–50 pM) binding sites for eTGF and EGF on the cell surface of A431 cells. This effect is half‐maximal at 0.1 nM PMA, exhibits rapid kinetics, and persists for at least 4 hr in the presence of PMA. eTGF and PMA added to intact A431 cells induce the phosphorylation of immunoprecipitable 170kd EGF /eTGF receptors. The EGF/ eTGF receptor isolated from control cells was found to contain phosphoserine and phosphothreonine. PMA and eTGF caused a marked increase in the level of these two phosphoamino acids. In addition, eTGF but not PMA caused the appearance of phosphotyrosine in the EGF/eTGF receptor in vivo. We conclude that the tumor‐promoting phorbol diester regulates both the affinity and phosphorylation state of the A431 cell receptor for the type α transforming growth factors, eTGF and EGF.