Understanding the mechanisms of tumor cell invasion is essential for our attempts to prevent cancer deaths. We screened by DNA microarrays the c-Jun-and transformation-related gene expression changes in S-adenosylmethionine decarboxylase (AdoMetDC)-overexpressing mouse fibroblasts that are highly invasive in vivo, and their derivatives expressing a tetracycline-inducible dominant-negative mutant of c-Jun (TAM67) or c-Jun shRNA. Among the small set of target genes detected were integrins a6 and b7, cathepsin L and thymosin b4, all upregulated in the AdoMetDC-transformed cells and downregulated upon reversal of transformation by TAM67 or c-Jun shRNA. The upregulation of integrin a6 subunit, pairing with integrin b1, endowed the transformed cells with the capability to attach to basement membrane laminin and to spread. Further, inhibition of integrin a6 or b1 function with neutralizing antibodies blocked the invasiveness of AdoMetDC-transformants and human HT-1080 fibrosarcoma cells in three-dimensional Matrigel. Moreover, immunohistochemical analyses showed strong integrin a6 staining in high-grade human fibrosarcomas. Our data show that c-Jun can regulate all three key steps of invasion: cell adhesion (integrin a6), basement membrane/extracellular matrix degradation (cathepsin L) and cell migration (thymosin b4). In addition, this is the first study to associate integrin b7, known as a leukocyte-specific integrin binding to endothelial/epithelial cell adhesion molecules, with the transformed phenotype in cells of nonleukocyte origin. As tumor cell invasion is a prerequisite for metastasis, the observed critical role of integrin a6b1 in fibrosarcoma cell invasion/spreading allures testing antagonists to integrin a6b1, alone or combined with inhibitors of cathepsin L and thymosin b4, as chemotherapeutic agents. ' 2009 UICC