The β-Catenin Axis Integrates Multiple Signals Downstream from RET/Papillary Thyroid Carcinoma Leading to Cell Proliferation

RET/papillary thyroid carcinoma (RET/PTC) oncoproteins result from the in-frame fusion of the RET receptor tyrosine kinase domain with protein dimerization motifs encoded by heterologous genes. Here, we show that RET/PTC stimulates the β-catenin pathway. By stimulating PI3K/AKT and Ras/extracellular signal–regulated kinase (ERK), RET/PTC promotes glycogen synthase kinase 3β (GSK3β) phosphorylation, thereby reducing GSK3β-mediated NH2-terminal β-catenin (Ser33/Ser37/Thr41) phosphorylation. In addition, RET/PTC physically interacts with β-catenin and increases its phosphotyrosine content. The increased free pool of S/T(nonphospho)/Y(phospho)β-catenin is stabilized as a result of the reduced binding affinity for the Axin/GSK3β complex and activates the transcription factor T-cell factor/lymphoid enhancer factor. Moreover, through the ERK pathway, RET/PTC stimulates cyclic AMP–responsive element binding protein (CREB) phosphorylation and promotes the formation of a β-catenin-CREB-CREB-binding protein/p300 transcriptional complex. Transcriptional complexes containing β-catenin are recruited to the cyclin D1 promoter and a cyclin D1 gene promoter reporter is active in RET/PTC–expressing cells. Silencing of β-catenin by small interfering RNA inhibits proliferation of RET/PTC–transformed PC Cl3 thyrocytes, whereas a constitutively active form of β-catenin stimulates autonomous proliferation of thyroid cells. Thus, multiple signaling events downstream from RET/PTC converge on β-catenin to stimulate cell proliferation. [Cancer Res 2009;69(5):1867–76]