Thyroid cancer is the most common endocrine malignancy, accounting for over 52,000 new diagnoses each year. Advanced papillary thyroid cancer (PTC) and anaplastic thyroid cancer (ATC) are aggressive subtypes of thyroid cancer, and have limited therapeutic options. PTC and ATC are thus the leading causes of endocrine cancer death. Activating mutations of the MAP kinase (MAPK) pathway are common in PTC and ATC, however, drug resistance to MAPK-directed therapies is a major problem. As the most downstream node of the MAPK pathway, inhibition of ERK1/2 has the potential to reduce bypass signaling and overcome resistance. I have shown combined BRAF and ERK inhibition blocks pathway reactivation and inhibits growth in vitro and in vivo. Further, I have shown that BRAF inhibition paradoxically increases invasion in a PTC model of acquired resistance (resistance that occurs slowly over a long period of time) and in ATC models of intrinsic resistance (resistance that occurs upfront), which is blocked by dual inhibition of BRAF and ERK. An emerging mechanism of resistance to targeted therapies involves cells exhibiting a more invasive phenotype to survive targeted therapies. My preliminary data show that BRAF inhibition increases the expression and secretion of the ECM protein fibronectin (FN1). Further, supplementation of FN1 to the media phenocopies BRAF inhibitor treatment by increasing invasion, which can also be blocked by inhibition of ERK. Finally, conditioned media from resistant cells treated with a BRAF inhibitor increases the invasiveness of sensitive cells. Taken together, I hypothesize that BRAF inhibition increases the production and secretion of FN1 through ERK reactivation to promote a pro-invasive autocrine secretome allowing cells to invade and survive. Thus, the goals of this proposal are to 1) determine the role of ERK and FN1 in an invasive phenotype in response to BRAF inhibition, 2) identify the pro-invasive secretome in response to BRAF inhibition, and 3) determine the role of an invasive phenotype in response to BRAF inhibition in vivo. The expected outcomes will identify key drivers of an invasive phenotype and will inform novel treatment strategies to circumvent resistance to MAPK- directed therapies and prevent invasion and metastasis. The completion of the proposed research will help the National Cancer Institute fulfill their mission to support cancer research and training in the fundamental sciences.

In order for cells to grow and survive in response to targeted therapies, emerging studies are showing these cells become more invasive. Consistent with this, I have shown that cells resistant to inhibition of BRAF, a key pro-tumor driver in thyroid cancer, become more invasive in response to BRAF inhibitor treatment. Therefore, the goal of this project is to define an invasive phenotype in response to BRAF inhibition and determine its role in invasion and metastasis to inform better treatment options for patients who do not respond to current therapies.