Abstract #4418: Dissection of the RET/beta-catenin interaction in the thyroid cancer line TPC1
Authors: Carmen Tartari, Carla Donadoni, Elisa Manieri, Luca Mologni, Carlo Gambacorti-Passerini
Journal: Cancer Research (2009)

AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO

The RET receptor tyrosine kinase is a member of the cadherin superfamily and plays a pivotal role in cell survival, differentiation and proliferation. Somatic chromosomal rearrangements, which represent the most frequent genetic alteration in papillary thyroid carcinomas, generate different chimeric oncogenes designated as RET/PTC. Currently, 12 rearranged forms of RET referred to as RET/PTC 1-9, PCM1/RET, ELKS/RET, and RFP/RET have been isolated from sporadic and radiation-associated PTCs. Chimeric proteins are characterized by the fusion between the intracellular domain of RET and different activating genes which can cause ligand-independent dimerization and constitutive activation. Beta-catenin is usually involved in the maintenance of cell-to-cell adhesion and mediates the Wnt/beta-catenin signal transduction pathway important during embryogenesis and in cellular malignant transformation. Recently, a novel mechanism of RET-mediated function through the beta-catenin signaling pathway has been reported in multiple endocrine neoplasia type 2 (MEN 2) and in sporadic thyroid carcinomas (Gujral et al. Cancer Res, 2008). These data suggest that RET/PTC forms could mediate oncogenic transformation through the beta-catenin signaling pathway in papillary thyroid carcinomas. Here we investigated the effects of the ZD6474, a small molecule RET-inhibitor, on RET/ beta-catenin interaction. First, we verified the inhibitory effect of ZD6474 on recombinant RET kinase domain and on the growth of cells expressing RET/PTC fusion proteins. As expected, TPC1 cells displayed reduced level of proliferation under increasing amount of ZD6474 inhibitor compared to RET/PTC-negative NPA thyroid cancer cell line, confirming that ZD6474 blocks the enzymatic activity of RET-derived oncoproteins. Surprisingly, RET and beta-catenin co-immunoprecipitated in both ZD6474-treated and untreated TPC1 cells, suggesting that RET/beta-catenin interaction may not be affected by RET kinase inactivation. In addition, we demonstrated that RET/PTC was able to stabilize beta-catenin by tyrosine phosphorylation leading to its nuclear accumulation through the phosphorylation of beta-catenin residues Y142 and Y654. All together these results suggest that ZD6474, by inhibiting RET kinase, down-modulates beta-catenin signaling pathway. Furthermore, ZD6474 induced a block of cell migration and a change of cell morphology in RET/PTC-transfected NIH3T3 cells; further experiments showed that unphosphorylated beta-catenin was retained on the cell membrane by cadherin-catenin complex formation in ZD6474-treated cells. In conclusion, RET kinase activation is crucial for beta-catenin localization and its signaling pathway activation but not for beta-catenin/RET physical interactions, in human papillary thyroid carcinomas. Further members of the beta-catenin/RET complex are now under study.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 4418.