Tumor cell invasiveness correlates with changes in integrin expression and localization

In nontumorigenic mammary epithelial cells (EpH4), transforming growth factor-beta (TGF beta 1) causes cell cycle arrest/apoptosis, but induces epitheliomesenchymal transition (EMT) in Ha-Ras-transformed EpH4 cells (EpRas). EMT is closely correlated with late-stage tumor progression and results in fibroblastic, migratory cells displaying a mesenchymal gene expression program (FibRas). EpRas and FibRas cells showed strongly increased cell substrate adhesion to fibronectin, collagens I/IV and laminin 1. Furthermore, Ras transformation caused enhanced or de-novo expression of the integrin subunits beta 1, alpha 2 and alpha 3, or alpha 5 and alpha 6, respectively, the latter subunits being even more strongly expressed in FibRas cells. Importantly, polarized EpRas cells expressed integrin subunits beta 1 and beta 6 at distinct (apical and lateral) membrane domains, while FibRas cells coexpressed these integrins and alpha 5 at the entire plasma membrane. During EMT, EpRas cells formed an alpha 5 beta 1 complex and deposited its ligand fibronectin into the extracellular matrix. Function-blocking alpha 5 antibodies attenuated migration, and caused massive apoptosis in EpRas cells undergoing TGF beta 1-induced EMT in collagen gels, but failed to affect EpRas- or FibRas-derived structures. We conclude that functional alpha 5b1 integrin is centrally implicated in EMT induction. Importantly, FibRas cells also failed to deposit the alpha 6 beta 4 ligand laminin 5, suggesting that alpha 6 beta 4 is no longer functional after EMT and replaced by mesenchymal integrins such as alpha 5 beta 1.