VCAM-1 inhibits TGFβ stimulated epithelial-mesenchymal transformation by modulating Rho activity and stabilizing intercellular adhesion in epicardial mesothelial cells

Regulation of epithelial-mesenchymal transformation (EMT) is of central importance both in normal development and in disease. During heart development, cells of the superficial epicardial mesothelium undergo EMT to give rise to precursor cells of the coronary vasculature and cardiac fibroblasts. Here we report that the α₄β₁ integrin ligand, VCAM-1, inhibits EMT of chick epicardial mesothelial cells stimulated by TGFβ isoforms. We further investigated the molecular basis of this inhibition using cultured chick embryonic and rat adult epicardial mesothelial cells. We observed that VCAM-1 increased cortical actin filaments at intercellular junctions and reduced stress fibers across epicardial cells. VCAM-1 inhibited stress fiber formation by TGFβ1, TGFβ2, TGFβ3 and lysophosphatidic acid and altered Rho activity stimulated by TGFβ3. This was accompanied by an increase in tyrosine phosphorylation of p190RhoGAP. All three TGFβ isoforms weakened intercellular adhesion, reduced membrane localization of β-catenin and E-cadherin and stimulated epicardial EMT in chick hearts. Each of these effects was restricted by simultaneous VCAM-1 treatment. Our data support the hypothesis that VCAM-1 can alter epicardial EMT at two key points: it limits Rho-dependent events such as stress fiber formation and it maintains the association of β-catenin and E-cadherin with the adherens junction.