Abstract GMM-024: TARGETING THE FORMIN-ASSEMBLED ACTIN CYTOSKELETON AS AN ANTI-INVASION STRATEGY IN OVARIAN CANCER SPHEROIDS

Due to high rates of recurrent metastasis during EOC progression, there is a critical unmet need to limit advancement of metastatic lesions present at diagnosis and upon initial recurrence. In non-hematogenous dissemination, cancer cells metastasize within the peritoneum and attach to visceral organs. Single cells, multi-cell clusters, or highly invasive multi-cellular spheroids are shed into peritoneal fluid from primary tumors, or are released by mechanical disruption of peritoneal metastases during cytoreductive surgery. Peritoneal EOC metastasis is not halted upon surgical removal of primary or metastatic tumors; it is a dynamic, longitudinal process of recurring tumors in the peritoneal cavity/pleural space. Recurrent peritoneal metastasis likely occurs via tumor cells suspended in the accumulating ascites fluid attaching to and invading the peritoneal mesothelium. Thus, EOC spheroids promote recurrent peritoneal metastasis and must be targeted for therapeutic intervention. We wished to assess the therapeutic utility of small molecules called intramimics (IMM) and Diaphanous-autoregulatory domain (DAD) in EOC spheroid peritoneal metastasis models. Both IMMs and DADs stimulate mDia2 formin function, and are thus classified as agonists. Formins are intracellular cytoskeletal nanomachines that assemble G-actin monomers into F-actin filaments. Formin-mediated F-actin assembly regulates stress fiber formation and generates forces to deform membranes into protrusive structures (i.e., lamellae, filopodia) to enable cell migration. While this process is necessary for normal cell migration (i.e., wound healing, development), it is abnormally invoked in metastasis. mDia proteins also support tumor cell-cell interactions by promoting the assembly of the F-actin architecture underlying cell-cell adherens junctions (AJs) that link cells of epithelial sheets together. IMMs and DADs specifically agonize mDia2 formins, keeping the nanomachine “on”, and in so doing, promote constitutive F-actin assembly within cells. Therefore, we hypothesize that mDia2 formin agonism would effectively block EOC spheroid invasion in 3D matrices by halting single cell invasive egress from spheroids. Using SKOV-3 and OVCA429 adenocarcinoma EOC cells, we demonstrated that mDia2 protein associates with proteins underlying AJ complexes. Specifically, mDia2 protein associates with alpha and beta catenins, yet not E- or N-cadherins. We further show that siRNA-mediated depletion of mDia2 alters the subcellular localization of AJ-associated proteins, impacting F-actin accumulation at the AJ and enhancing spheroid invasive dissemination. In contrast, mDia agonism using IMM and mDia DAD small molecules effectively inhibits EOC spheroid invasion, disallowing single cell egress from spheroids. Collectively, these results indicate a role for mDia formins in regulating invasive egress from EOC spheroids. Thus, there may be therapeutic utility to targeting mDia formin cytoskeleton effectors in halting EOC spheroid invasive egress. Citation Format: Krista M. Pettee, Yuqi Zhang, Kathryn M. Eisenmann. TARGETING THE FORMIN-ASSEMBLED ACTIN CYTOSKELETON AS AN ANTI-INVASION STRATEGY IN OVARIAN CANCER SPHEROIDS [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr GMM-024.