Cross-talk between the Hippo and Wnt pathways has been implicated recently in breast cancer development, but key intersections have yet to be fully defined. Here we report that WBP2, a transcription coactivator that binds the Hippo pathway transcription factor YAP/TAZ, contributes to Wnt signaling and breast cancer pathogenesis. Clinically, overexpression of WBP2 in breast cancer specimens correlated with malignant progression and poor patient survival. In breast cancer cells, nuclear entry and interaction of WBP2 with β-catenin was stimulated by Wnt3A, thereby activating TCF-mediated transcription and driving malignant invasive character. Mechanistic investigations showed WBP2 levels were controlled by the E3 ligase ITCH, which bound and target WBP2 for ubiquitin-dependent proteasomal degradation. Accordingly, ITCH silencing could elevate WBP2 levels. Wnt signaling upregulated WBP2 by disrupting ITCH-WBP2 interactions via EGFR-mediated tyrosine phosphorylation of WBP2 and TAZ/YAP competitive binding. Conversely, ITCH-mediated downregulation of WBP2 inhibited TCF/β-catenin transcription, in vitro transformation, and in vivo tumorigenesis. We identified somatic mutations in ITCH, which impaired its ability to degrade WBP2 and to block its function in cancer, even while retaining binding capacity to WBP2. Thus, the Wnt pathway appeared to engage WBP2 primarily by affecting its protein stability. Our findings show how WBP2/ITCH signaling functions to link the intricate Wnt and Hippo signaling networks in breast cancer. Cancer Res; 76(21); 6278-89. ©2016 AACR.

WBP2 transcription coactivator is an emerging oncoprotein and a key node of convergence between EGF and Wnt signaling pathways. Understanding how WBP2 is regulated has important implications for cancer therapy. WBP2 is tightly controlled by post-translational modifications, including phosphorylation and ubiquitination, leading to changes in subcellular localization, protein-protein interactions, and protein turnover. As the function of WBP2 is intricately linked to YAP and TAZ, we hypothesize that WBP2 is negatively regulated by the Hippo tumor suppressor pathway. Indeed, MST is demonstrated to negatively regulate WBP2 expression in a kinase-dependent but LATSindependent manner. This was observed in the majority of the breast cancer cell lines tested. The effect of MST was enhanced by SAV and concomitant with the inhibition of the transcription co-activation, in vitro and in vivo tumorigenesis activities of WBP2, resulting in good prognosis in xenografts. Downregulation of WBP2 by MST involved miRNA but not proteasomal or lysosomal degradation. Our data support the existence of a novel MST-Dicer signaling axis, which in turn regulates both WBP2 CDS-and UTR-targeting miRNAs expression, including miR-23a. MiR-23a targets the 3′UTR of WBP2 mRNA directly. Significant inverse relationships between WBP2 and MST or miR23a expression levels in clinical specimens were observed. In conclusion, WBP2 is a target of the Hippo/MST kinase; MST is identified as yet another rheostat in the regulation of WBP2 and its oncogenic function. The findings have implications in targeted therapeutics and precision medicine for breast cancer.