Defining mechanisms directing YAP/TAZ-mediated tumorigenesis
Hiemer, Samantha Elizabeth
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Dysregulated Hippo pathway signaling promotes the onset of aggressive cancers through the induced nuclear activity of yes-associated protein (YAP) and transcriptional co-activator with PDZ binding motif (TAZ) (YAP/TAZ). Uncontrolled nuclear YAP/TAZ activity evokes tumor-initiating properties in a range of epithelial-derived cancers, including oral and breast cancers, but their downstream targets and mechanisms of action are unclear. Recent studies have suggested that the pro-tumorigenic roles for YAP/TAZ relate to their convergence with growth factor signaling pathways. Based on these previous studies, I hypothesized that YAP/TAZ driven transcription contributes to carcinoma progression, and that cooperation with transforming growth factor β (TGFβ)-induced signals promotes aggressive oncogenic traits. In this thesis I show that dysregulated YAP localization precedes oral squamous cell carcinoma (OSCC) development, and that nuclear YAP/TAZ activity drives cell proliferation, survival, and migration in vitro, and is required for tumor growth and metastasis in vivo. Global gene expression studies in OSCC cells revealed that YAP/TAZ-mediated gene expression correlates with expression changes that occur in human OSCCs identified by “The Cancer Genome Atlas” (TCGA), many of which encode cell cycle and survival regulators. By exploring the relationship with growth factor signaling, I found that YAP/TAZ induce pro-tumorigenic events by converging with TGFβ-induced signals, particularly in breast cancer cells where TGFβ is known to promote metastatic properties. My observations indicated that YAP/TAZ are necessary for maintaining and promoting TGFβ-induced tumorigenic phenotypes in breast cancer cells, and that these phenotypes result from the cooperative activity of YAP/TAZ, the TEA domain family of transcription factors (TEADs), and TGFβ-activated SMAD2/3 in the nucleus. Genome-wide expression analyses indicated that YAP/TAZ, TEADs, and TGFβ-induced signals coordinate a specific pro-tumorigenic transcriptional program. Importantly, genes cooperatively regulated by these complexes, such as the novel targets neuronal growth regulator 1 (NEGR1) and urothelial cancer associated 1 (UCA1), are necessary to maintain tumorigenic activity in metastatic breast cancer cells. Nuclear YAP/TAZ also cooperate with TGFβ signaling to promote phenotypic and transcriptional changes in non-tumorigenic cells to overcome TGFβ-mediated growth inhibition. This work thus defines novel roles for YAP/TAZ in cancer, offering molecular mechanisms that may be useful for identifying and targeting YAP/TAZ-driven cancers.