Integrin α3β1-dependent β-catenin phosphorylation links epithelial Smad signaling to cell contacts

Injury-initiated epithelial to mesenchymal transition (EMT) depends on contextual signals from the extracellular matrix, suggesting a role for integrin signaling. Primary epithelial cells deficient in their prominent laminin receptor, alpha 3 beta 1, were found to have a markedly blunted EMT response to TGF-beta 1. A mechanism for this defect was explored in alpha 3-null cells reconstituted with wild-type (wt) alpha 3 or point mutants unable to engage laminin 5 (G163A) or epithelial cadherin (E-cadherin; H245A). After TGF-beta 1 stimulation, wt epithelial cells but not cells expressing the H245A mutant internalize complexes of E-cadherin and TGF-beta 1 receptors, generate phospho-Smad2 (p-Smad2)-pY654-beta-catenin complexes, and up-regulate mesenchymal target genes. Although Smad2 phosphorylation is normal, p-Smad2-pY654-beta-catenin complexes do not form in the absence of alpha 3 or when alpha 3 beta 1 is mainly engaged on laminin 5 or E-cadherin in adherens junctions, leading to attenuated EMT. These. findings demonstrate that alpha 3 beta 1 coordinates cross talk between beta-catenin and Smad signaling pathways as a function of extracellular contact cues and thereby regulates responses to TGF-beta 1 activation.