Abstract 1354: The role of RALA in soft tissue sarcoma tumor growth and metastasis
Authors: Sizemore Steven T. , Sizemore Gina M. , Shakya Reena , Amaya Peter , Hammer Anisha M. , Rice Alexander H. , Chalmers Jeffrey J. , Ostrowski Michael C. , Chakravarti Arnab 
Published: July 2017

Abstract
Soft tissue sarcomas (STS) are a diverse collection of cancers of mesenchymal origin arising from the connective and supportive tissues of the body. While localized STS are well managed by surgery and radiation; metastasis, particularly to the lung, is frequent. More than 30% of adult STS patients develop lung metastases and the 5-year survival for these patients is a dismal 16%. Treatment options for metastatic STS are limited, thus there is an urgent unmet need for a better understanding of the key molecular pathways that drive metastatic spread in STS and identification of inhibitors of these pathways for clinical application. Through analysis of gene expression data from metastatic STS patient samples, we identified decreased expression of PPP2R1B as a hallmark of metastatic STS. To directly test its function as a suppressor of tumor growth and metastasis in STS, PPP2R1B was stably over-expressed in HT1080 cells, a model of metastatic STS. PPP2R1B expression almost completely abolished HT1080 tumor growth in nude mice. PPP2R1B is a subunit of the PP2A protein phosphatase complex that negatively regulates numerous cancer signaling pathways. However, the functional consequences of decreased PPP2R1B expression in STS are unknown. A combination of high-throughput and targeted approaches were utilized to identify 37 phosphoproteins that are significantly dephosphorylated following PPP2R1B expression in HT1080 cells. One of these phosphoproteins, the small GTPase RALA, exhibited decreased phosphorylation on Ser194 following PPP2R1B expression. RALA is significantly prognostic of STS metastasis and is elevated in more aggressive STS subtypes relative to less aggressive subtypes and normal tissue. RALA knockdown in HT1080 significantly slowed tumor growth and decreased the incidence of pulmonary metastasis, mirroring PPP2R1B overexpression. Importantly, RALA is an actionable therapeutic target for improved treatment of STS. Aurora A inhibitors indirectly inhibit RALA function by preventing RALA Ser194 phosphorylation by aurora A. We found that RALA expression and activity predicted response of STS cell lines to aurora A inhibition. Excitingly, the aurora A inhibitor alisertib nearly eradicated growth of HT1080 tumors in vivo. Exploration of the biological mechanisms through which RALA regulates STS metastasis identified regulation of vesicular traffic as a likely critical function of RALA in this process. These findings identify PPP2R1B, RALA, and aurora A as members of a key molecular pathway that drives STS progression and advocate the use of treatments targeting this pathway to improve outcome for STS patients with advanced disease.
Citation Format: Steven T. Sizemore, Gina M. Sizemore, Reena Shakya, Peter Amaya, Anisha M. Hammer, Alexander H. Rice, Jeffrey J. Chalmers, Michael C. Ostrowski, Arnab Chakravarti. The role of RALA in soft tissue sarcoma tumor growth and metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1354. doi:10.1158/1538-7445.AM2017-1354