SAT-574 Elucidating the Role of Nuclear Focal Adhesion Kinase (FAK) in Thyroid Cancer

Abstract

Background: Advanced papillary (PTC) and Anaplastic Thyroid Cancer (ATC) are the leading causes of endocrine cancer deaths. Thus there is a critical need to identify novel targeted therapies for these patients. Focal Adhesion Kinase (FAK) and Src are overexpressed in thyroid cancer and represent promising therapeutic targets. However, Src inhibition has had limited success in the clinic due to development of resistance. A possible mechanism of resistance may be through FAK which is a nonreceptor tyrosine kinase that functions as a kinase and scaffold/adaptor protein to regulate growth, survival, migration, and invasion. FAK is thought to be primarily autophosphorylated at tyrosine 397 (Y397) in response to integrin or growth factor receptor signaling, which results in the recruitment of Src and downstream signaling pathways. FAK has been found to go to the nucleus where it causes increased survival, tumor growth, and immune evasion. Hypothesis: We hypothesized that localization of FAK to the nucleus is a mechanism of resistance to Src inhibition, and that dual inhibition of FAK and Src will overcome resistance to Src inhibition and lead to decreased invasion in thyroid cancer. Results: We found that BRAF-mutant thyroid cancer cells resistant to the Src inhibitor, dasatinib (DasRes cells) exhibited increased tumor volume when injected orthotopically into the thyroid gland of nude mice. Immunohistochemical of pY397 FAK showed an increased level of active nuclear FAK in the DasRes tumors compared to the control tumors. Interestingly, we found that combined FAK and Src inhibition led to decreased levels of active and total nuclear FAK in DasRes cells, which correlated with decreased invasion and growth in DasRes BRAF-mutant cells compared to either single agent. Given the decrease in invasion in DasRes cells in response to combined FAK and Src inhibition, we then asked if loss of FAK localization to the nucleus leads to decreased invasion. To address this hypothesis, we transduced empty vector (EV), WT FAK, or a FAK Nuclear localization mutant into BRAF-mutant thyroid cancer cells with Crispr-Cas9 Knockout (KO) of FAK. Interestingly, we found that preventing FAK from localizing to the nucleus caused increased invasion and decreased proliferation compared to KO FAK thyroid cells transduced with WT FAK. Conclusions: Taken together, this data indicates that a mechanism of resistance to Src inhibition could be through FAK nuclear localization and that inhibiting both FAK and Src leads to decreased levels of nuclear FAK, invasion, and growth. However, localization of FAK to the nucleus appears to be responsible for proliferation while cytosolic FAK contributes to invasion. Thus this data provides a rationale for targeting FAK and Src in thyroid cancer to inhibit invasion of FAK at focal adhesion sites and to inhibit proliferation through decreased nuclear FAK.