Protein kinase C and non-functional EGF receptor in K-ras transformed cells

The aim of this study was to examine the mechanism by which K-ras transformation leads to the loss of epidermal growth factor (EGF) binding. Triton X-100 solubilized membranes from K-ras transformed cells were not capable of binding [¹²⁵I]-EGF in vitro, indicating that the loss of EGF binding was not due to improper insertion of EGF receptor (EGF-R) into the membrane of the transformed cells. Phosphoamino acid analysis of EGF-R from phosphorylated membranes of normal cells showed enhancement of phosphotyrosine after EGF stimulation. However, the EGF-R from phosphorylated membranes of transformed cells showed enhancement of phosphoserine and phosphothreonine level in the presence of EGF. We speculated that EGF-R in transformed cells might be modulated by serine- and threonine-specific kinases such as protein kinase C. Our hypothesis was supported by the finding that normal cells had a low basal protein kinase C activity and tumor promotor, such as 12-0-tetradecanoyl phorbol 13-acetate (TPA), activated this enzyme 3.3-fold. In contrast, transformed cells had a high constitutive protein kinase C activity in the absence of TPA.