INDUCTION OF TYROSINE PHOSPHORYLATION AND ASSOCIATION OF BETA-CATENINWITH EGF RECEPTOR UPON TRYPTIC DIGESTION OF QUIESCENT CELLS AT CONFLUENCE

Normal human breast epithelial (HBE) cells which reached confluence ce ased 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 reg ion between adjacent cells, By contrast, immunocytostaining and Wester n blot analyses revealed that E-cadherin colocalized and coprecipitate d with beta-catenin but not with alpha-catenin in exponentially growin g cells at low density. As a comparable amount of alpha-catenin was de tected in the total cell lysate of cells at different densities, it is suggested that alpha-catenin is present but dissociates from the E-ca dherin-beta-catenin complex in growing cells, beta-Catenin was tyrosin e phosphorylated in growing cells at low density but not in quiescent cells at confluence, Tyrosine phosphorylation of beta-catenin was conc omitantly 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 associatio n of beta-catenin with EGFR were inhibited by tyrphostin, a specific i nhibitor of the EGFR tyrosine kinase, whereas dissociation of beta-cat enin from E-cadherin was not, The results suggest that tyrosine phosph orylation of beta-catenin is achieved by EGFR upon tryptic digestion o f cells and concurrent with but independent of dissociation of alpha-c atenin from E-cadherin, beta-Catenin thus phosphorylated at tyrosine i s suggested to play the role in preventing alpha-catenin once dissocia ted from reassociating with E-cadherin until cells reach confluence.