Mammary fibroblasts induce a downregulation in the level of cellular ERa and promote a more rapidly proliferating hormone-independent state in ER + breast cancer cells. Importantly, the cross-over to hormone-independent growth is associated with acquisition of an EMT phenotype, which has been previously correlated to Butacaine invasive disease and poor prognosis. EMT was concurrent with the disruption of E2/ERa signaling and the occurrence of a more invasive phenotype. We also found ERa to be downregulated such that it was no longer the prime determinant of tumor growth, underscoring this as a potential mechanism by which tumors become resistant to endocrine therapy. Of interest, a recent report has shown a similar association between low ERa and EMT and poor prognosis in human uterine endometrial carcinoma. The therapeutic implication is that although women may be diagnosed as ER +, some will not respond favorably to endocrine therapies because growth of their tumor is regulated by the nature of their tumor microenvironment. Therefore, it may be advantageous to simultaneously inhibit paracrine factors produced by a fibroblast-enriched tumor microenvironment in combination with the use of anti-estrogen or other endocrine therapies. In analyzing fibroblast-cancer cell interactions for molecular targets using protein microarrays, we found this interaction to result in the expression of a set of regulatory proteins, including MCSF, SDF-1, and MMP family proteins that have been individually associated with cancer activated fibroblasts and invasive disease. Our DBeQ time-course experiments show that these represent early signaling events in the fibroblast-cancer cell relationship and suggest that it is the combined external signaling of both fibroblasts and cancer cells that contributes to the extracellular milieu that drives this EMT phenotype. Notably, while the presence of fibroblasts increased tumor cell invasiveness, the proteins secreted into conditioned medium after co-culture also greatly affected the properties of normal mammary epithelial cells.