Regulation of hippo signaling by PTPN14 and KIBRA and the role of TAZ in mammary development and tumorigenesis
Denson, Kayla Erin
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A key feature of the mammary gland is that it doesn’t fully develop until after birth. This allows for the use of mouse models as unique tools to study development and organ specificity. Processes involved in mammary gland development and the acquisition of breast cancer are closely related, which bolsters the importance of studying the gland’s epithelial cells, in particular, since these are the cells undergoing the most changes throughout a female’s life. The Hippo signaling pathway regulates cellular proliferation and survival, thus exerting profound effects on normal cell fate and tumorigenesis. Pivotal effectors of this pathway are YAP/TAZ, transcriptional co-activators whose dysfunction contributes to epithelialto- mesenchymal transition and malignant transformation. Therefore, it is of great importance to decipher the mechanisms underlying the regulations of YAP/TAZ at various levels. Here, we report that non-receptor tyrosine phosphatase 14 (PTPN14) interacts with the KIBRA protein. The interaction between PTPN14 and KIBRA is through the PPxY domain of PTPN14 and the WW domain of KIBRA. PTPN14 and KIBRA can induce LATS1 activation independently and cooperatively. Interestingly, activation of LATS1 by PTPN14 is dependent on the C-terminus of PTPN14 and independent of the upstream mammalian STE-20-like kinase (MST) proteins. Furthermore, we demonstrate that PTPN14 increases the LATS1 protein stability. Last, overexpression of KIBRA rescues the increased cell migration and aberrant three-dimensional morphogenesis induced by knockdown of PTPN14, and this rescue is mediated through the activation of the upstream LATS1 kinase and subsequent cytoplasmic sequestration of YAP. In summary, these results indicate a potential regulatory role of PTPN14 in the Hippo pathway and demonstrate another layer of regulation in the YAP oncogenic function. Additionally, we report that the loss of function of KIBRA, but not PTPN14, induces growth factor-independent growth in MCF10A cells in 3D cultures. This growth can be attributed to the ability of shKIBRA cells to induce the expression of Amphiregulin (AREG) and activate EGFR signaling, through the activation of YAP. In addition, these shKIBRA cells show abnormal acini formation in 3D cultures, increased cell migration ability, and anchorage-independent growth. In order to study Hippo signaling in mammary gland development and breast cancer, we created a tissue-specific inducible transgenic mouse model, overexpressing TAZ 4SA in the mammary gland. Finally, we report that TAZ overexpression causes accelerated branching morphogenesis during puberty, which show high expression of Ki67, as well as activated AKT and ERK1/2. TAZ 4SA mammary glands also grow larger, accumulating fat, when compared to control mice. This fat phenotype is not restricted to the mammary gland, but can be found throughout the body cavity. Weight and fat gain is partially reversible, upon inactivation of TAZ 4SA and may be influenced by the balance between canonical and alternative Wnt signaling. Finally, TAZ 4SA mice are able to form mammary tumors when treated with a carcinogen. Our data reveal a relationship between Hippo signaling, mammary gland development, breast cancer, and obesity and we have provided a novel mouse model to be utilized in related studies.