Mechanisms of cytoplasmic β-catenin accumulation and its involvement in tumorigenic activities mediated by oncogenic splicing variant of the receptor originated from Nantes tyrosine kinase

The beta-catenin pathway plays a critical role in the pathogenesis of certain types of cancers. To gain insight into mechanisms by which altered receptor tyrosine kinases regulate cytoplasmic beta-catenin accumulation, the effect of an oncogenic receptor originated from Nantes (RON) variant on beta-catenin accumulation and the role of beta-catenin in RON-mediated tumorigenic activities were studied. In NIH3T3 cells harboring oncogenic variant RON Delta 160, increased beta-catenin accumulation with tyrosine phosphorylation and nuclear translocation was observed. Overexpression of RON Delta 160 also resulted in increased expression of beta-catenin target genes c-myc and cyclin D1. By analyzing cellular proteins that regulate beta-catenin stabilities, it was found that RON Delta 160 activates the protein disheveled (DVL) and inactivates glycogen synthase kinase-3 beta by Ser-9 residue phosphorylation. These effects were channeled by RON Delta 160-activated PI 3-kinase-AKT pathways that are sensitive to specific inhibitors, such as wortmannin, but not to other chemical inhibitors. Silencing RON Delta 160 expression by specific small interfering RNA blocked not only beta-catenin expression but also c-myc and cyclin D1 expression, suggesting that RON expression is required for the activation of the beta-catenin signaling pathway. Moreover, it was found that knockdown of the beta-catenin gene expression by small interfering RNA techniques reduces significantly the RON Delta 160-mediated NIH3T3 cell proliferation, focus-forming activities and anchorage-independent growth. Thus, the oncogenic RON variant regulates beta-catenin stabilities through activation of DVL and inactivation of glycogen synthase kinase-3 beta. The activated beta-catenin cascade is one of the pathways involved in tumorigenic activities mediated by the oncogenic RON variant.