YAP-regulated epithelial-fibroblast crosstalk

According to the Centers for Disease Control, cancer is one of the leading causes of death in the United States. Characterized as a disease that develops as a result of an unstable genome, cancer is known to arise from numerous spontaneous mutations in the DNA of cells. Recent evidence shows that cancer cells within tumors are not self-reliant; rather, they progress along with other cells in their surrounding environment. Tumor cells recruit neighboring cells that, like the cancer cells, also become deregulated, forming the tumor stroma that aids in tumor progression. Within the stroma, cancer-associated fibroblasts (CAFs) play a vital role in the progression of cancer. Recent studies have found an important link between increased matrix stiffness surrounding the tumor and the invasion of the tumor. Thus, it is proposed that as the matrix stiffens, the tumor takes on more aggressive phenotypes. The transcriptional regulators YAP and TAZ (YAP/TAZ), key effectors of the Hippo pathway, are known to respond and influence matrix stiffening. In stiff matrix environments YAP/TAZ accumulate in the nucleus, and can drive transcriptional events. CAF's from late stage breast cancers have been found to exhibit increased YAP expression and increased ability to remodel and stiffen the extracellular matrix. Whether YAP or TAZ in these CAFs influences the metastatic properties of tumor cells is unclear. The present study aims to establish a link between YAP/TAZ activity in CAFs and cancer migration and invasion. We hypothesized that high nuclear activity of YAP/TAZ in fibroblasts would lead to non-autonomous signals that increase epithelial migration, and conversely that signals originating from epithelial cells affect YAP regulation in fibroblasts. We obtained CAFs from oral squamous cell carcinomas (OSCC) at various stages, and interestingly found that when CAFs obtained from stage III and stage IV tumors were co-cultured with OSCC cells they had the ability to cause OSCC cell migration. This CAF-induced migration was dependent on YAP/TAZ in the CAFs, as YAP/TAZ knockdown repressed this crosstalk. To gain insight into the mechanisms driving this process, transwell migration assays were conducted using NIH-3T3 fibroblasts engineered to overexpress YAP, or mutants of YAP, in doxycycline-inducible manner. We found that expression of YAP in NIH-3T3 cells, particularly a nuclear-localized YAP mutant, promoted the ability for OSCC cells to migrate in co-culture experiments. Media conditioned from these cells was sufficient to recapitulate this phenotype, suggesting that secreted factors from these fibroblasts may act as a signal that promotes migration. This activity of YAP was dependent on the ability for YAP to bind to the TEAD transcription factors, a major mediator of YAP transcriptional activity. Together these results indicate that nuclear YAP activity in fibroblasts can modulate the migration of neighboring cancer cells, suggesting that YAP plays a key role in stroma-cancer crosstalk during cancer progression