Geldanamycin abrogates ErbB2 association with proteasome-resistant beta-catenin in melanoma cells, increases beta-catenin-E-cadherin association, anddecreases beta-catenin-sensitive transcription

beta -catenin undergoes both serine and tyrosine phosphorylation. Serine ph osphorylation in the amino terminus targets beta -catenin for proteasome de gradation, whereas tyrosine phosphorylation in the COOH terminus influences interaction with E-cadherin. We examined the tyrosine phosphorylation stat us of beta -catenin in melanoma cells expressing proteasome-resistant beta -catenin, as well as the effects that perturbation of beta -catenin tyrosin e phosphorylation had on its association with E-cadherin and on its transcr iptional activity. beta -catenin is tyrosine phosphorylated in three melano ma cell lines and associates with both the ErbB2 receptor tyrosine kinase a nd the LAR receptor tyrosine phosphatase. Geldanamycin, a drug which destab ilizes ErbB2, caused rapid cellular depletion of the kinase and loss of its association with beta -catenin without perturbing either LAR or beta -cate nin levels or LAR/beta -catenin association. Geldanamycin also stimulated t yrosine dephosphorylation of beta -catenin and increased beta -catenin/E-ca dherin association, resulting in substantially decreased cell motility. Gel danamycin also decreased the nuclear beta -catenin level and inhibited beta -catenin-driven transcription, as assessed using two different beta -caten in-sensitive reporters and the endogenous cyclin D1 gene. These findings we re confirmed by transient transfection of two beta -catenin point mutants, Tyr-654Phe and Tyr-654Glu, which, respectively, mimic the dephosphorylated and phosphorylated states of Tyr-654, a tyrosine residue contained within t he beta -catenin-ErbB2-binding domain. These data demonstrate that the func tional activity of proteasome-resistant beta -catenin is regulated further by geldanamycin-sensitive tyrosine phosphorylation in melanoma cells.