Abstract 3177: Liver endothelium secreted LRG1 is a novel ligand of HER3 to promote metastatic colorectal cancer growth

Abstract

Abstract Background: Liver is the most common site of developing distant metastasis in colorectal cancer (CRC). Patients with metastatic CRC (mCRC) have a 5-year survival rate at 14%. We previously showed that liver endothelial cells (EC), a key component of the liver microenvironment, secrete soluble factors to promote CRC growth and chemoresistance via activating human epidermal growth factor receptor (ERBB3, also known as HER3). However, we found that the ECs activated HER3 by a previously unknown mechanism that is independent of the established HER3 ligand neuregulins. Identifying a new HER3 ligand will potentially give rise to novel therapeutic target for treating patients with mCRC. Methods: We first fractionated conditioned medium (CM) from liver ECs by fast protein liquid chromatography. The specific fractions that activated HER3 and AKT in CRC cells were subjected to mass spectrometry (MS) for protein identification. In parallel, we used a His-tagged HER3 Extracellular domain (ECD) and pulled down EC-secreted factors that directly bound to HER3 and then subjected to protein identification by MS. Candidate proteins identified in both MS analyses were subjected for further validations by siRNA or immunoprecipitation depletion from EC CM to determine their roles in HER3 activation and CRC growth. Results: We identified a specific fraction of liver EC CM increased phosphorylation of HER3 and AKT in CRC cells (determined by Western blotting). Meanwhile, the HER3-ECD affinity binding assay showed that HER3 ECD-depleted EC CM could no longer activate HER3 or promote growth, and the soluble factor(s) eluted from HER3-ECD increased phosphorylation of HER3 and AKT, and promoted growth in CRC cells. Results from both MS analyses revealed that LRG1 is a potential ligand that binds to and activate HER3 in CRC cells. We depleted LRG1 from EC CM, either by siRNA knockdown LRG1 in ECs or immunoprecipitation, and found that LRG1-depleted EC CM could no longer activate HER3 and AKT in CRC cells, and LRG1-depleted EC CM failed to promote growth of CRC cell in vitro and CRC xenografts in vivo. Conclusions: We identified LRG1 as a new HER3 ligand and demonstrated that liver EC-secreted LRG1 mediates EC paracrine effects on activating HER3 and promoting CRC cell growth. Our findings suggest an oncogenic role of EC-secreted LRG1 in the surrounding liver microenvironment of mCRC, and highlighted a potential of blocking LRG1 with monoclonal antibodies for treating patients with mCRC. Citation Format: Moeez Ghani Rathore, Wei Zhang, Michel'le Wright, Jordan Winter, Yamu Li, Zhenghe Wang, Rui Wang. Liver endothelium secreted LRG1 is a novel ligand of HER3 to promote metastatic colorectal cancer growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3177.