Effects of cell density and phorbol esters on the expression of epidermal growth factor receptors

Previously, it has been shown that the binding of epidermal growth factor (EGF) by a wide range of cells decreases as cell density increases. In this report, we demonstrate that KB cells treated chronically with phorbol esters continue to exhibit decreases in EGF receptor binding as cell density increases. This finding suggests that protein kinase-C may not be essential for density-induced down regulation of EGF receptors, since phorbol esters are known to down regulate protein kinase-C. We also report that short-term and long-term effects of phorbol esters on the binding of EGF are affected by density. As shown previously for several cell lines, the phorbol ester 12-0-tetradecanoylphorbol-13-acetate transiently reduces EGF binding. We now show that the magnitude of this reduction diminishes as cell density increases. In addition, we determined that long-term treatment of KB cells with phorbol ester increases EGF binding. Again, this effect is diminished at high cell densities. Finally, we report that the increases in EGF binding induced by long-term treatment with phorbol esters are due to increases in the number of EGF receptors.