AbstractAcquisition of invasive characteristics is a hallmark of breast carcinoma progression. During this phenomenon, Ets‐1 transcription factor overexpression is induced and associated with breast cancer invasiveness, and poor prognosis. We hypothesized that Ets‐1 transcription factor could be the orchestrator of a genetic program inducing the expression of genes necessary for cell motility, as postulated by the tumor microenvironment invasion model. We aimed at elucidating the role of Ets‐1 in the molecular control of mammary cancer cell invasion and aggressiveness within their matrix environment. To that purpose, mouse mammary tumor MMT epithelial cells were engineered to stably overexpress Ets‐1, or the dominant negative Ets‐1 DNA Binding domain. The biological function of Ets‐1 was assessed in three‐dimensional extracellular matrix systems recreating a microenvironmental architecture resembling in vivo geometric constraints. Ets‐1 overexpression provided MMT cells with a motile and invasive phenotype, leading to cell scattering, and impairing multicellular organization in matrix‐mimicking gels. We evidenced that Ets‐1 promoted HGF/SF activation, and the expression of its receptor, c‐Met. Ets‐1 also orchestrated switches in integrin expression pattern, towards a pro‐migratory and malignant phenotype. Moreover, Ets‐1 concomitantly triggered matrix metalloproteinases (MMP) expression and activation, thus contributing to cell scattering. Functional relevance of these observations was confirmed with blocking antibodies or MMP inhibitors. Our data highlight a critical role for Ets‐1 in the orchestration of a network of molecular and phenotypic events, converging to enhance malignant features and invasion by mammary cancer cells of their environment. Ets‐1 overexpression hence appears as a probable key step for breast cancer progression. J. Cell. Physiol. 215: 782–793, 2008. © 2008 Wiley‐Liss, Inc.