Epidermal growth factor receptor: Regulation of cellular proliferation and gene expression

Abstract

Binding of epidermal growth factor (EGF) to the EGF receptor stimulates the tyrosine kinase activity of the receptor and initiates a signal transduction cascade culminating in a mitogenic response. In many tumor derived cell lines which overexpress EGF receptor, exposure to EGF results in growth inhibition. The mechanism for this is unclear. This work involves analysis of growth inhibition by EGF, mechanisms of EGF receptor overexpression and regulation of the EGF receptor gene. The first two studies utilize a cell line (PC-10) that overexpresses EGF receptor without gene amplification. PC-10 cells appear to adopt a novel mechanism to overexpress EGF receptor; one that involves stabilization of the receptor message rather than the more common gene amplification. PC-10 cells were found to be killed by EGF in a cell density dependent manner. However, chronic exposure to EGF subsequently allowed proliferation under conditions which previously resulted in cell death. These "adapted" cells had similar levels of EGF receptor on the cell surface and similar EGF binding parameters. The tyrosine kinase activity of the receptor in response to EGF in the adapted cells was significantly reduced both in vitro and in vivo. Evidence was also found that the signal transduction cascade initiated by EGF was altered by adaptation. These data provide evidence for a unique mechanism for EGF receptor-overexpressing cells to survive EGF toxicity, one that involves a reduction in the tyrosine kinase activity of the EGF receptor in the absence of a decrease in the number of EGF receptor. Finally, additional studies were carried out on the response of the EGF receptor gene to activation of protein kinase C (PKC) in A549 cells. Activation of PKC resulted in increased levels of EGF receptor mRNA. No induction of a transfected reporter gene containing EGF receptor regulatory DNA could be seen. Repression of the reporter gene, however, was consistently seen. The cis-element for repression was mapped to 233 base pairs in the EGF receptor regulatory region. Additional data support the hypothesis that a previously characterized repressor protein called GCF may be responsible for this repression.