H-ras activation promotes cytoplasmic accumulation and phosphoinositide 3-OH kinase association of beta-catenin in epidermal keratinocytes

The mechanisms underlying downregulation of the cadherin/catenin complexes and beta-catenin signaling during tumor progression are not fully understoo d. We have analyzed the effect of oncogenic H-Ras on E-cadherin/catenin com plex formation/stabilization and beta-catenin distribution in epidermal ker atinocytes, Microinjection or stable expression of V12Ras into keratinocyte s promotes the loss of E-cadherin and alpha-catenin and relocalization of b eta-catenin to the cytoplasm and nucleus. Moreover, these effects are depen dent on PI3K (phosphoinositide 3-OH kinase) activity. Interestingly, a stro ng association of p85 alpha and p110 alpha subunits of PI3K with beta-caten in is induced in V12Ras-expressing keratinocytes, and in vitro binding assa ys show a direct interaction between beta-catenin and p85 alpha. Overexpres sion of either V12Ras or constitutively active p110 alpha induces metabolic stabilization of beta-catenin and promotes its accumulation in cytoplasmic and nuclear pools. In addition. the interaction of beta-catenin with the a denomatous polyposis coli protein is blocked in V12Ras and p110 alpha trans formants though no changes in glycogen synthase kinase 3 beta activity coul d be detected. Nevertheless, in V12Ras transformants the in vivo phosphoryl ation of beta-catenin in Ser residues is strongly decreased. These results indicate that H-Ras activation induces the relocalization and cytoplasmic s tabilization of beta-catenin by a mechanism involving its interaction with PI3K.