Focal adhesion proteins as markers of malignant transformation and prognostic indicators in breast carcinoma

Focal adhesion kinase (FAK) is one of the central signaling molecules found at focal adhesion sites, which are specific areas on the cell membrane where cells attach to extracellular matrix proteins. Focal adhesion kinase interacts with multiple signaling and adaptor molecules and effects several signaling pathways. Overexpression of FAK and its substrate c-Src has been implicated in malignant transformation and acquisition of an invasive tumor phenotype of different tissues. Overexpression of the multidomain protein paxillin, which is also a FAK ligand and a c-Src substrate, has been associated with less malignant tumor behavior. The purpose of this study was to analyze the involvement of integrin signaling molecules FAK, c-Src, and paxillin in malignant transformation of the breast epithelium. Using phosphospecific antibodies FAK-pY³⁹⁷ and Src-pY⁴¹⁶, we demonstrated that neither activation of FAK nor activation of c-Src correlates with development of invasive tumor properties. However, activation of both FAK and c-Src correlates with malignant transformation. We further demonstrated that overexpression of paxillin also correlates with malignant transformation and is a marker of a less invasive tumor phenotype. Using tissue microarray, we demonstrated that expression and activation of paxillin inversely correlated with lymph node metastases and lymphovascular invasion, respectively. No correlation between paxillin expression and activation and tumor grade, estrogen, progesterone, and Her2/Neu receptor expression was found. In summary, focal adhesion proteins FAK and c-Src can be used as markers of malignant transformation in epithelial cells but not invasive phenotype, whereas expression and activation of paxillin may represent a good prognosticator in breast carcinoma.