Induction of tyrosine phosphorylation and association of β-catenin with EGF receptor upon tryptic digestion of quiescent cells at confluence

Normal human breast epithelial (HBE) cells which reached confluence ceased growth and tightly adhered to each other, forming a monolayer. In quiescent cells thus arrested by density, E-cadherin colocalized and coimmunoprecipitated with alpha- and beta-catenins in the boundary region between adjacent cells. By contrast, immunocytostaining and Western blot analyses revealed that E-cadherin colocalized and coprecipitated with beta-catenin but not with alpha-catenin in exponentially growing cells at low density. As a comparable amount of alpha-catenin was detected in the total cell lysate of cells at different densities, it is suggested that alpha-catenin is present but dissociates from the E-cadherin-beta-catenin complex in growing cells. beta-Catenin was tyrosine phosphorylated in growing cells at low density but not in quiescent cells at confluence. Tyrosine phosphorylation of beta-catenin was concomitantly induced with association of beta-catenin with EGF receptor (EGFR) when quiescent cells at confluence were dissociated into single cells by tryptic digestion, being accompanied by dissociation of alpha-catenin from E-cadherin. Both tyrosine phosphorylation and association of beta-catenin with EGFR were inhibited by tyrphostin, a specific inhibitor of the EGFR tyrosine kinase, whereas dissociation of alpha-catenin from E-cadherin was not. The results suggest that tyrosine phosphorylation of beta-catenin is achieved by EGFR upon tryptic digestion of cells and concurrent with but independent of dissociation of alpha-catenin from E-cadherin. beta-Catenin thus phosphorylated at tyrosine is suggested to play the role in preventing alpha-catenin once dissociated from reassociating with E-cadherin until cells reach confluence.