Tumor cell invasiveness correlates with changes in integrin expression and localization

Abstract

In nontumorigenic mammary epithelial cells (EpH4), transforming growth factor-β (TGFβ1) 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 β1, α2 and α3, or α5 and α6, respectively, the latter subunits being even more strongly expressed in FibRas cells. Importantly, polarized EpRas cells expressed integrin subunits β1 and α6 at distinct (apical and lateral) membrane domains, while FibRas cells coexpressed these integrins and α5 at the entire plasma membrane. During EMT, EpRas cells formed an α5β1 complex and deposited its ligand fibronectin into the extracellular matrix. Function-blocking α5 antibodies attenuated migration, and caused massive apoptosis in EpRas cells undergoing TGFβ1-induced EMT in collagen gels, but failed to affect EpRas- or FibRas-derived structures. We conclude that functional α5β1 integrin is centrally implicated in EMT induction. Importantly, FibRas cells also failed to deposit the α6β4 ligand laminin 5, suggesting that α6β4 is no longer functional after EMT and replaced by mesenchymal integrins such as α5β1.