Differential mechanism for the inhibition of epidermal growth factor binding to its receptor on mouse keratinocytes by anthones and phorbol esters

Abstract

1,8-Dlhydroxy-3-methyl-9-anthrone (chrysarobin), a potent anthrone tumor promoter, reduced [¹²⁵I] epidermal growth factor (EGF) binding to its receptor in primary epidermal cells from SENCAR mice maintained In low Ca²⁺ containing medium. The time course for this effect with chrysarobin was different from that of 12-O-tetradecanoylphorbol-13-acetate (TPA). Maximum inhibition of [¹²⁵I]EGF binding was observed at 18 h versus 1 h respectively. Scatchard analyses revealed that the inhibition by chrysarobin was due to a decrease in the number of bothhigh- and low-affinity classes of EGF receptors. In contrast, TPA caused a rapid inhibition of EGF binding, primarily due to a loss of high-affinity receptors. The mechanism by which chrysarobin inhibited the binding of EGF to its receptor involved neither direct activation nor membrane translocatlon of epidermal protein kinase C, whereas the rapid decrease in EGFbinding induced by TPA was consistent with its ability to activate protein kinase C. Structure—activity relationships for EGF binding inhibition by anthrones revealed that inhibition was inversely proportional to chain length at the C^10-position which correlated closely with oxidation rate and skin tumorpromoting activity. α-Tocopherol wasable to block partially the effect of chrysarobin but not TPA on EGF binding. These results suggest that oxidation at position C¹⁰ is at least partially responsible for the inhibition of EGF binding induced by chrysarobin. Furthermore, these studies support the hypothesis that changes in EGF receptor binding and/or function may play a role in skin tumor promotion by diverse classes of promoting agents.