Breast cancer progression is likely a multistep process involving the activation and inactivation of a number of genes. Previously, we showed that the mRNA coding for Fra-1, a FOS family member and an AP-1 transcription factor component, was highly expressed in the more invasive estrogen receptor negative (ER-) breast cancer cell lines. We used a tet-off system to stably overexpress Fra-1 in MCF7 ER+ cells and evaluate the impact of Fra-1 on this aggressive phenotype. Conversely, Fra-1 was silenced in highly invasive ER-MDA-MB231 cells using RNA interference. We report that in both systems the Fra-1 expression level was positively associated with cell proliferation, cell motility and invasiveness assessed in vitro. In addition, Fra-1 inhibition in fibroblastoid ER- cells, which formed colonies with large stellate projections in Matrigel, resulted in morphological changes. Cells acquired an epithelioid shape and had a spherical appearance in Matrigel. Fra-1 regulated several genes, implicated in invasion, angiogenesis and cell proliferation independently of beta1-integrin activation, and directly induced MMP-1 and MMP-9 promoter activity. These overall results show that high Fra-1 expression is associated with a more malignant cell phenotype and suggest that Fra-1 could have a pivotal role in breast cancer progression.