Liver Endothelium Microenvironment Promotes HER3-mediated Cell Growth in Pancreatic Ductal Adenocarcinoma

~90% metastatic pancreatic ductal adenocarcinoma (mPDAC) occurs in the liver, and the 5-year survival rate for patients with mPDAC is only at 3%. The liver has a unique endothelial cell (EC)-rich microenvironment, and preclinical studies showed that ECs promote cancer cell survival pathways by secreting soluble factors in a paracrine fashion in other types of cancer. However, the effects of liver ECs on mPDAC have not been elucidated. In this study, we used primary liver ECs and determined that liver EC-secreted factors containing conditioned medium (CM) increased PDAC cell growth, compared to control CM from PDAC cells. Using an unbiased receptor tyrosine kinase array, we identified human epidermal growth factor receptor 3 (HER3, also known as ErbB3) as a key mediator of liver EC-induced growth in PDAC cells with HER3 expression (HER3 +ve). We found that EC-secreted neuregulins activated the HER3-AKT signaling axis, and that depleting neuregulins from EC CM or blocking HER3 with an antibody, seribantumab, attenuated EC-induced functions in HER3 +ve PDAC cells, but not in cells without HER3 expression. Furthermore, we determined that EC CM increased PDAC xenograft growth in vivo, and that seribantumab blocked EC-induced growth in xenografts with HER3 expression. These findings elucidated a paracrine role of liver ECs in promoting PDAC cell growth, and identified the HER3-AKT axis as a key mediator in EC-induced functions in HER3 +ve PDAC cells. As over 70% mPDAC express HER3, this study highlights the potential of using HER3-targeted therapies for treating patients with HER3 +ve mPDAC.