Selective utilization of ectopic epidermal growth factor receptor signaling in estrogen receptor positive breast cancer cells

Overexpression of the transmembrane tyrosine kinase epidermal growth factor receptor (EGFR) correlates with absence of estrogen receptor (ER) and poor prognosis in breast cancer, suggesting that this molecule may play a role in escape from dependence on estrogen for growth. To test this hypothesis, we have stably transfected the human EGFR into two ER positive human breast cancer cell lines, MCF-7 cells and a clone of MCF-7 cells previously transfected with the EGFR ligand transforming growth factor alpha.;Expression of the EGFR is modulated in vitro in both cell lines in response to culture conditions. Cells maintain elevated transfected EGFR expression upon estrogen withdrawal, but growth in medium containing estrogen results in loss of expression at the level of both protein and messenger RNA. Although the transfected DNA is not lost, long-term growth in the presence of estrogen is associated with hypermethylation of the integrated plasmid. Loss of expression is not a consequence of nonspecific repression of the heterologous promoter. Changes in EGFR expression are reversible, implying that this phenomenon occurs in response to environmental conditions.;Transfected cells retain ER expression as well as the ability to induce estrogen-responsive genes upon hormone treatment, indicating that the EGFR overexpression does not interfere with ER function. Growth of cells overexpressing EGFR in the absence of estrogen is enhanced compared to control transfectants; however, growth in the presence of estrogen is inhibited. This effect may be mediated through induction of polypeptide factors capable of binding EGFR, because EGFR ligands inhibit growth of EGFR overexpressing cells. These results suggest that overexpression of EGFR in the absence of estrogen confers a selective advantage upon cells but is deleterious when estrogen is present.;The instability of EGFR expression in the presence of estrogen has complicated study of the interactions between these two pathways. In an attempt to develop a system that may be useful in delineating the interactions between ER and EGFR in future studies, an inducible gene expression system controlled by tetracycline was used to regulate expression of the EGFR-related c-erbB-2 gene in MCF-7 cells.