Dissection of the RET/β-catenin interaction in the TPC1 thyroid cancer cell line

The RET receptor tyrosine kinase is a member of the cadherin superfamily and plays a pivotal role in cell survival, differentiation and proliferation. Currently, 12 ret/ptc chimeric oncogenes, characterized by the fusion between the intracellular domain of RET and different activating genes, which can cause ligand-independent dimerization and constitutive activation, have been described. β-catenin is usually involved in the maintenance of cell-to-cell adhesion and mediates the Wnt/β-catenin pathway important during embryogenesis and in cellular malignant transformation. Recently, a novel mechanism of RET-mediated function through the β-catenin pathway has been reported in multiple endocrine neoplasia type 2 and in sporadic thyroid carcinomas. Here, we investigated the effects of the ZD6474, a small molecule RET-inhibitor, on RET/β-catenin interaction. We confirmed the ZD6474 mediated-inhibition of recombinant RET kinase and of growth of cells expressing RET/PTC. Interestingly, we firstly observed reduced cellular mobility and changed morphology of TPC1 treated cells suggesting that RET-inhibitor could affect β-catenin cellular distribution as resulted in its co-immunoprecipitation with E-cadherin. We further investigated this hypothesis showing that TPC1 treated cells displayed predominantly β-catenin cytosolic localization. Surprisingly, RET and β-catenin co-immunoprecipitated in both ZD6474-treated and untreated TPC1 cells, suggesting that RET/β-catenin interaction might not be affected by RET kinase inactivation. All together these results suggest that RET kinase activation is crucial for β-catenin stabilization (pY654), localization and its signaling pathway activation but not for β-catenin/RET physical interactions, in human papillary thyroid carcinomas. In conclusion, ZD6474, by inhibiting RET kinase, down-modulates β-catenin pathway leading its recruitment to the membrane by E-cadherin.