Abstract 191: Paracrine role of endothelial cells in HER3-mediated colon cancer cell survival

INTRODUCTION: Colorectal cancer (CRC) remains the second-leading cause of cancer-related death in the United States. Median survival of patients with metastatic CRC (mCRC) is ~2.5 years and patients often become resistance to systemic therapy within 1 year of treatment. Therefore, a better understanding of the regulation of CRC cell survival pathways is necessary in the development of new therapeutic strategies that will improve outcomes for patients with mCRC. We have previously demonstrated that endothelial cells (ECs) from the liver, the most common site of CRC metastases, secrete soluble factors in the conditioned medium (CM) that, in turn, increase the cancer stem cell phenotype in CRC cells. OBJECTIVES: In this study, we sought to (1) elucidate the paracrine role of liver ECs in mediating CRC cell growth and chemoresistance, and (2) determine the mechanism(s) involved. METHODS: Primary ECs from non-malignant liver parenchyma were isolated and confirmed. CRC cells were incubated with either their own CM (control) or CM from ECs. The effect of CM on CRC cell growth was determined by the MTT assay. The effect on chemoresistance was determined by fluorescence-activated cell sorting (FACS)-based assay for apoptosis after treating CRC cells with 5-fluorouracil (5-FU) either in control CM or EC CM. Effects of CM on CRC tumor growth in vivo was determined using a subcutaneous (subQ) xenograft tumor model. CRC cells were pretreated with control or EC CM and then injected subQ in an inoculation mixture of Matrigel and concentrated CM. Tumor burden was assessed over time by bioluminescence with in vitro imaging system (IVIS) and by measuring tumor volume with calipers. RESULTS: CM from liver ECs significantly increased cell growth and chemoresistance in CRC cells in vitro via activating the AKT pathway. We identified human epidermal growth factor receptor 3 (HER3, also known as ERBB3) being the only receptor tyrosine kinase (RTK) activated by EC CM. Furthermore, we found that inhibition of HER3, either by a HER3 inhibitor or by HER3 siRNAs, blocked EC CM-induced AKT activation, cell growth, and chemoresistance in CRC cells. In our in vivo tumor model, CRC cells treated with EC CM formed larger tumors with higher growth rates than that with control CM. CONCLUSIONS: Our results demonstrated a paracrine role of liver ECs in promoting cell growth and chemoresistance via activating HER3-AKT in CRC cells. Identification of the liver EC-secreted factor(s) for activating CRC-associated HER3 is currently under study. This study suggested a potential strategy of treating mCRC patients with HER3 antibodies/inhibitors that are already being assessed in clinical trials for various cancer types. Citation Format: Rui Wang, Rajat Bhattacharya, Fan Fan, Xiangcang Ye, Delphine Boulbes, Lee M. Ellis. Paracrine role of endothelial cells in HER3-mediated colon cancer cell survival [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 191.