Transient over-expression of estrogen receptor-a in breast cancer cells promotes cell survival and estrogen-independent growth

Estrogen receptor-a (ERa) positive breast cancer frequently responds to inhibitors of ERa activity, such as tamoxifen, and/or to aromatase inhibitors that block estrogen biosynthesis. However, many patients become resistant to these agents through mechanisms that remain unclear. Previous studies have shown that expression of ERa in ERa-negative breast cancer cell lines frequently inhibits their growth. In order to determine the consequence of ERa over-expression in ERa-positive breast cancer cells, we over-expressed ERa in the MCF-7 breast cancer cell line using adenovirus gene transduction. ERa over-expression led to ligand-independent expression of the estrogen-regulated genes pS2 and PR and growth in the absence of estrogen. Interestingly, prolonged culturing of these cells in estrogen-free conditions led to the outgrowth of cells capable of growth in cultures from ERa transduced, but not in control cultures. From these cultures a line, MLET5, was established which remained ERa-positive, but grew in an estrogen-independent manner. Moreover, MLET5 cells were inhibited by anti-estrogens showing that ERa remains important for their growth. Gene expression microarray analysis comparing MCF-7 cells with MLET5 highlighted apoptosis as a major functional grouping that is altered in MLET5 cells, such that cell survival would be favoured. This conclusion was further substantiated by the demonstration that MLET5 show resistance to etoposide-induced apoptosis. As the gene expression microarray analysis also shows that the apoptosis gene set differentially expressed in MLET5 is enriched for estrogen-regulated genes, our findings suggest that transient over-expression of ERa could lead to increased cell survival and the development of estrogen-independent growth, thereby contributing to resistance to endocrine therapies in breast cancer patients.