Differential inhibition of epidermal growth factor binding by photoactivated psoralens and the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate in cells overexpressing protein kinase C

A rat fibroblast cell line, R6PKC3, that stably overexpresses the β-1 form of protein kinase C was used to analyze sensitivity to inhibitors of epidermal growth factor (EGF) binding. R6PKC3 cells overexpresses protein kinase C activity 53-fold relative to non-overexpressing control R6C1 cells. Inhibition of EGF binding by the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA) and photoactivated psoralens was compared in these cells. We found that 125I-EGF bound both of the cell lines and was rapidly internalized in a temperature-dependent process and metabolized. Binding of EGF to the R6 cells overexpressing protein kinase C was markedly less than binding to R6C1 control cells. In both of the cell lines, TPA and photoactivated psoralens inhibited 125I-EGF binding but the response of these cells to these inhibitors was distinct. R6PKC3 cells were markedly more sensitive to TPA and were resistant to recovery from TPA-induced inhibition of 125I-EGF binding when compared to control cells. These differences were not observed in other subclones of cells overexpressing protein kinase C, suggesting that they may be unique to R6PKC3 cells. In contrast, no major differences in sensitivity to photoactivated psoralens were observed in R6C1 and R6PKC3 cells. These data indicate that TPA and photoactivated psoralens inhibit 125I-EGF binding to these cell lines by distinct mechanisms.