Abstract 1204: FAK inhibition selectively promotes tumor cell apoptosis in three-dimensional environments and suppresses tumor cell metastasis

Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase that canonically promotes signaling downstream of integrin receptors. FAK expression and activity are elevated during breast cancer progression and is associated with a poor clinical prognosis. FAK inhibitors are in clinical trials and we found that oral administration of a small molecule FAK inhibitor blocks orthotopic breast and ovarian tumor growth in mice without general toxicity (Walsh et al. and Tanjoni et al., in press). Our studies are the first to show that FAK inhibition can suppress spontaneous breast to lung metastasis using a syngenic mouse model system. FAK inhibition also prevented the metastasis of MDA-MB-231 human breast carcinoma containing activating mutations in KRAS and BRAF. Surprisingly, we found that FAK inhibition did not limit cell proliferation or promote cell death in breast or ovarian carcinoma cells in adherent two-dimensional culture. However, when the same cells were grown under three-dimensional (3D) conditions as spheroids, low concentrations of FAK inhibitors promoted caspase-3 activation and cell death. Spheroid-mediated FAK inhibition was associated with dephosphorylation of the FAK substrate p130Cas and the inhibition of the small GTPase Rap1. Spheroid-mediated cell death upon FAK inhibition was associated with increased entosis, a process of cell-cell engulfment. Integrin activation can counter-act entosis and we evaluated cells expressing wildtype (WT) or kinase-dead (KD) FAK for effects on β1 integrin activation via fibronectin binding as analyzed by flow cytometry. Whereas cells expressing WT FAK showed high levels of fibronectin binding, KD FAK-expressing cells showed 10-fold reduced levels of fibronectin binding with equal surface β1 integrin expression. Taken together, our results support the hypothesis that loss of integrin activation associated with FAK-p130Cas-Rap1 inhibition triggers entosis within spheroids. Ongoing studies are focused on determining whether FAK-mediated signaling promotes β1 integrin activation that affects the lodging, clearance rate, or survival of tumor cells upon metastasis to the lungs. There is a great need for drugs that might prevent tumor progression and metastasis. Culturing breast carcinoma cells as spheroids better mimics the growth and survival conditions of tumor cells dissociated from a primary tumor and undergoing metastasis. Unfortunately, tumor spheroids usually show increased resistance to various chemo-therapeutic drugs and this may be one reason limiting drug efficacy in vivo. In contrast, FAK inhibitors selectively effect spheroid survival and also prevent tumor growth and metastasis in vivo. Our studies may have uncovered a potential “Achilles heel” of breast carcinoma tumor growth in relying on FAK signaling to provide a non-canonical survival pathway through inside-out integrin activation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1204.