Persistent and non-persistent changes in gene expression result from long-term estrogen exposure of MCF-7 breast cancer cells.

Life-long estrogen exposure is recognized as a major risk factor for the development of breast cancer. While the initial events in the regulation of gene expression by estrogen have been described in detail, far less is known of the role of estrogen in the long-term regulation of gene expression. In this study, we investigated the effects of long-term exposure of MCF-7 breast cancer cells to 1nM 17β-estradiol on gene expression with the goal of distinguishing between gene expression that is continually reliant on estrogen receptor (ER) function as opposed to secondary and persistent effects that are downstream of ER. To assess the direct involvement of ER in the differential gene expression of long-term estrogen exposed (LTEE) cells in comparison with that of control cells, we exposed cultures to the selective estrogen receptor modulator raloxifene (RAL). cDNA microarray analysis showed that exposure to RAL inhibited expression of numerous characterized estrogen-regulated genes, including PGR, GREB1, and PDZK1. Genes that were increased in expression in LTEE cells yet were unaffected by RAL exposure included the aryl hydrocarbon receptor (AHR) and numerous other genes that were not previously reported to be regulated by estrogen. Epigenetic regulation was evident for the AHR gene; AhR transcript levels remained elevated for several cell passages after the removal of estrogen. Signal transducer and activator of transcription 1 (STAT1); STAT1-regulated genes including ISG15, IFI27, and IFIT1; and MHC class I genes were also up-regulated in LTEE cells and were unaffected by RAL exposure. STAT1 is commonly overexpressed in breast and other cancers, and is associated with increased resistance to radiation and chemotherapy. This is the first study to relate estrogen exposure to increased STAT1 expression in breast cancer cells, an effect that may represent an additional role of estrogen in the pathogenesis of breast cancer.