Activation of Mitogen-Activated Protein Kinase in Estrogen

Breast cancer presents as either estrogen receptor A (ERA) positive or negative, with ERA+ tumors responding to antiestrogen therapy and having a better prognosis. By themselves, mRNA expression signatures of estrogen regulation in ERA+ breast cancer cells do not account for the vast molecular differences observed between ERA+ and ERA cancers. In ERA tumors, overexpression of epidermal growth factor receptor (EGFR) or c-erbB-2, leading to increased growth factor signaling, is observed such that mitogen-activated protein (MAP) kinase (MAPK) is significantly hyperactivated compared with ERA+ breast cancer. In ERA+/progesterone receptor–positive, estrogen-dependent MCF-7 breast cancer cells, we stably overexpressed EGFR or constitutively active erbB-2, Raf, or MAP/extracellular signalregulated kinase kinase, resulting in cell lines exhibiting hyperactivation of MAPK, estrogen-independent growth, and the reversible down-regulation of ERA expression. By global mRNA profiling, we found a ‘‘MAPK signature’’ of f400 genes consistently up-regulated or down-regulated in each of the MAPK+ cell lines. In several independent profile data sets of human breast tumors, the in vitro MAPK signature was able to accurately distinguish ER+ from ER tumors. In addition, our in vitro mRNA profile data revealed distinct mRNA signatures specific to either erbB-2 or EGFR activation. A subset of breast tumor profiles was found to share extensive similarities with either the erbB-2-specific or the EGFRspecific signatures. Our results confirm that increased MAPK activation causes loss of ERA expression and suggest that hyperactivation of MAPK plays a role in the generation of the ERA phenotype in breast cancer. These MAPK+ cell lines are excellent models for investigating the underlying mechanisms behind the ERA phenotype. (Cancer Res 2006; 66(7): 3903-11)