Overexpression of Protein Kinase C Alpha-subtype in Swiss/3T3 Fibroblasts Causes Loss of Both High and Low Affinity Receptor Numbers for Epidermal Growth Factor

The full length cDNA coding for the alpha form of protein kinase C (PKC) was introduced into Swiss/3T3 cells using a retroviral expression system. This has enabled the generation of a series of cell lines stably expressing high levels of PKC alpha enzyme, as well as the appropriate control cell lines, carrying an integrated vector but lacking PKC alpha cDNA insert. PKC alpha-overexpressing cell lines did not display a transformed morphology nor were they capable of growth in soft agar. However, these cells exhibited enhanced growth rates especially under low serum conditions. Using Scatchard plot analysis of 125I-epidermal growth factor (EGF) binding to its receptor on the surface of PKC alpha-overproducing cells revealed reduction in EGF receptor numbers when compared to control cells, without change in affinities of remaining receptors. These data indicated that reduced receptor numbers for EGF were among the phenotypic changes occurring in these cells underlying their diminished dependence on external factors for growth. Furthermore, we provide evidence that reduced EGF receptor numbers found on the cell surface of PKC alpha-overproducing cells resulted from the decreased biosynthesis of EGF receptor molecules, which correlated also with lower levels of mRNA transcripts coding for the EGF receptor found in these cells. Hence, our studies imply that PKC alpha is involved in a cellular mechanism regulating the expression of EGF receptor molecules in Swiss/3T3 cells. Thus, deregulation of PKC alpha, i.e. by increasing its expression levels in specific cells may affect, in turn the expression of cell surface receptors including the EGF receptor. Similar molecular mechanisms may be involved in initial stages of neoplasia and tumor promotion.