Abstract 4505: Activating transcription factor-6α dependent regulation of Rheb-mTOR and Notch signaling contributes to chemoresistance in osteosarcoma

The standard treatment for patients with newly diagnosed osteosarcoma (OS) consists of surgery in combination with multi-agent chemotherapy. However, the optimal treatment strategy for patients with refractory and/or metastatic disease is yet to be defined and continues to be a persistent challenge in osteosarcoma treatment. The high degree of genetic aberrations and tumor heterogeneity has impeded the identification and testing of new and effective therapeutic targets. Therefore, finding and characterizing cellular mechanisms, which contribute to OS chemoresistance could be one promising strategy for designing therapies that can change the outlook for patients with this disease. Due to the well-known role of unfolded protein response (UPR) in promoting chemoresistance in solid tumors our objective here is to examine how this promotes chemoresistance in patients with OS. Using in silico pathway analysis of gene expression datasets from patients with OS we found that protein processing in the ER was one of the pathways that was significantly enriched in OS tumors compared to normal tissue. We then examined whether the UPR sensors ATF6α, PERK and IRE-1α were also activated in OS cells in vitro. Using western blotting, qPCR and immunofluorescence we confirmed that all three UPR pathways were activated in human OS cell lines. However we found that only ATF6α activation significantly enhanced chemoresistance to cisplatin, irinotecan and combinatorial treatment with cisplatin and the mTOR inhibitor rapamycin. This occurred via inhibition of Bax activation, suppression of RHEB-mTOR and NOTCH signaling. Our findings highlight a novel mechanism of chemo-resistance in OS. Furthermore, retrospective analysis of banked OS patient samples for ATF6α expression followed by a preliminary multivariate analysis using Cox regression model showed that high levels of nuclear ATF6α, was an independent prognostic indicator while also taking into account the presence of metastases and tumor site. Moreover, we found that while 50% patients with high nuclear ATF6α levels had a poor histologic response to treatment only 32% patients with low nuclear ATF6α expression suggesting that ATF6α activation could be associated with resistance to chemotherapy and poor outcome for OS patients. Our findings emphasize a role for ATF6α as the nexus of a versatile signaling network that regulates pathways that are crucial for the pathogenesis and therapy resistance of osteosarcoma. Hence therapeutic targeting of the ATF6α pathway holds promise as an innovative and effective treatment strategy for OS. Citation Format: Suma Yarapureddy, Pooja Hingorani, Saravana Kumar, Omar Asad, Jazmine Abril, Veena Sharath, Paul Dickman, Aparna C. Sertil. Activating transcription factor-6α dependent regulation of Rheb-mTOR and Notch signaling contributes to chemoresistance in osteosarcoma [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 4505. doi:10.1158/1538-7445.AM2017-4505