Tumor promoter-induced inhibition of epidermal growth factor binding to cultured mouse primary epidermal cells

The effect of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and other diterpene derivatives on the binding of epidermal growth factor (EGF) to primary cultures of mouse epidermal cells was studied. ¹²⁵I-EGF was used to study the specific binding of the growth factor to monolayer cultures of mouse epidermal cells grown under low-calcium culture conditions (0.06 mM Ca²⁺). Under these growth conditions, nonspecific binding did not exceed 10%. Initially, TPA decreased the binding of ¹²⁵I-EGF to cells. However, when cells were incubated continuously in TPA plus EGF (0.25 ng/ml) for 19 hr, there was more EGF bound to the TPA-treated cells than to control cells. This phenomenon was not observed at high (5 ng/ml) EGF concentrations. Scatchard analysis of specific ¹²⁵l-EGF binding at 4° after a 1-hr pretreatment of the cells with TPA at 37° converted a curvilinear plot to a linear plot. TPA induced a 25% decrease in the number of receptors per cell and eliminated binding of EGF to a class of high-affinity receptors. Preincubation of cells in TPA at 37° for up to 13 hr followed by Scatchard analysis at 4° showed that the curvilinear plot was restored and that the effects of TPA were partially reversible. TPA did not alter the rate at which bound EGF was degraded. However, at low EGF concentrations, TPA reduced the amount of EGF that was metabolized. The greater amount of EGF bound to TPA-treated cells over controls after long-term incubation was due to the presence of larger amounts of whole EGF in the media of TPA-treated cells at a time when the cells have regained their ability to bind EGF. A series of diterpene derivatives of different abilities to act as tumor promoters and hyperplasia-inducing agents were tested for their ability to influence EGF binding. The abilities of members of this series to decrease EGF binding and prevent degradation of EGF correlated more with their potentials to induce hyperplasia than with their tumor-promoting potentials. The ability of these diterpene derivatives to induce DNA synthesis with EGF synergistically may depend on the transient sparing of the EGF from degradation and subsequent binding of the spared EGF.