Re-establishment of cell-cell contacts and beta-catenin membrane localization in Colo205 colorectal tumor cells treated with the Src inhibitor bosutinib (SKI-606) Free

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The cell-cell adhesion molecule, E-cadherin is a tumor suppressor protein located in adherens junctions of epithelia, which when completely functional, suppresses tumor growth and inhibits invasion. The cytoplasmic domain of E-cadherin acts as a tether in anchoring E-cadherin to the actin cytoskeleton via catenin family members including the transcription factor beta-catenin. The protein tyrosine kinase Src associates with adherens junctions and is known to phosphorylate members of this signaling complex, including p120^ctn and beta-catenin. Inhibition of Src kinase activity by small molecules can increase beta-catenin localization to the plasma membrane and association with E-cadherin. We show here that Colo205 colorectal tumor cells, which grow on tissue culture plates as a mixture of loosely attached and suspended cells, form dense multicellular aggregates when treated with the dual Src/Abl kinase inhibitor bosutinib, but not when treated with the Abl kinase inhibitor imatinib. In untreated cells, E-cadherin and beta-catenin exhibit diffuse staining, while bosutinib treatment induces localization of E-cadherin and beta-catenin to the cell-cell junctions of the newly formed aggregates. In contrast to previous observations where treatment of other colorectal tumor lines with bosutinib caused complete abrogation of beta-catenin tyrosine phosphorylation, only modest inhibition of beta-catenin tyrosine phosphorylation was observed, even though tyrosine phosphorylation of a number of cellular proteins was dramatically reduced by bosutinib. No tyrosine phosphorylation of E-cadherin was seen in the absence of bosutinib. These results suggest that another target protein is involved in re-establishment of E-cadherin function upon inhibition of Src family kinases in Colo205 cells.