Integrin [alpha]3[beta]1-dependent [beta]-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 upregulate 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.