The molecular mechanism of TAZ-induced mammary tumorigenesis
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TAZ (Transcriptional co-activator with PDZ-binding motif) is a WW-domain containing protein recently identified as a downstream component of the Hippo tumor suppressor pathway and mediator of biologically important processes (mesenchymal stem cell differentiation, embryonic stem cell renewal, mechanotransduction). Recently, loss of LATS1/2, a negative regulator of TAZ, has been observed in ~50% of breast cancers. However, whether and how TAZ is also involved in breast cancer has not been investigated. Therefore, this study explores the cellular functions of TAZ in breast cancer and the underlying molecular mechanisms. The cellular functions of TAZ were investigated using overexpression studies in an immortalized mammary epithelial cell line (MCF10A). Compared to control vector-only expressing cells (MCF10A-WPI) TAZ overexpression (MCF10A-TAZ) promotes enhanced cell proliferation, cell migration and cell-ECM adhesion, induces transformation and the epithelial-mesenchymal transition, and confers resistance to chemotherapeutics (paclitaxel, cisplatin). Together, these findings strongly suggest TAZ functions as an oncogene in the development, progression and drug resistance of breast cancer. As a transcriptional co-activator, TAZ likely mediates these cellular functions through the transcriptional activation of downstream genes. By screening a 44K human genome microarray we have identified and characterized Cyr61 and CTGF, mediators of paclitaxel resistance, and BMP4, a regulator of cell migration. Through stable shRNA-mediated knockdown, we show that loss of Cyr61/CTGF expression in MCF10A-TAZ cells can rescue TAZ-induced paclitaxel resistance. Similarly, shRNA-mediated knockdown of BMP4 can significantly attenuate TAZ-induced cell migration. Therefore, these findings demonstrate that Cyr61/CTGF and BMP4 are functionally significant mediators of TAZ-induced paclitaxel resistance and cell migration, respectively. The clinical relevance of our in vitro findings were also validated by immunohistochemistry using tissue microarrays containing human breast cancer samples. TAZ levels were highly expressed in 66.6% of clinical samples further suggesting TAZ may be an important oncogene in breast cancer. Our study has characterized TAZ as an oncogene in breast cancer and elucidated two novel mechanisms underlying paclitaxel resistance and cell migration. These findings highlight the importance of TAZ during the development, progression and drug resistance of breast cancers and the potential use of TAZ as a therapeutic target to treat TAZ-expressing breast cancers.