Abstract 93: Soluble mesothelin acts as a signaling molecule

Abstract Mesothelin (MSLN) is a cell surface glycoprotein that is expressed by most pancreatic ductal adenocarcinoma (PDAC) and is used as a target for anti-cancer therapy. MSLN expression appears to make pancreatic cancer more aggressive. Previously, we found that MSLN KO PDAC cells grow in culture and as subcutaneous tumors similarly to wild-type (WT) cells but form intraperitoneal metastases poorly. MSLN pre-cursor (pre-MSLN) is a 71-kD glycosylphosphatidylinositol (GPI)-anchored protein that is cleaved intracellularly to make the 31-kD secreted protein Megakaryocyte Potentiating Factor (MPF), and a 40-kD mature MSLN (mMSLN) that retains membrane attachment. The mMSLN can be shed from the cell surface by the action of specific proteases, resulting in release of soluble MSLN (sMSLN) to both the tumor microenvironment and serum. The goal of this study is to see if sMSLN promotes cancer cell aggressiveness. We transduced MSLN KO pancreas cancer cells to express: 1) a mutant, secreted-only pre-MSLN (includes WT MPF) with a C-terminal truncation (MSLNΔ599) that prevents GPI-linkage of MSLN to the membrane, and 2) truncated pre-MSLN consisting of MPF only. Results show that complementation of KO cells with MPF or MSLNΔ599 does not rescue the MSLN KO phenotype, while MSLNΔ599 bearing a Y318A point mutation that disrupts the binding of MSLN to MUC16/CA125 does restore growth of peritoneal metastasis. We performed RNA-deep sequencing on MSLN KO cells exposed to WT or Y318A sMSLN from conditioned medium. We identified 2000 genes that were differentially expressed across the treatment groups. Exposure to sMSLN resulted in changes in expression of cell adhesion and sterol biosynthetic synthesis pathway genes. Individual genes that were top hits were also identified and are under investigation. Exposure to sMSLN changes the transcriptomic program of pancreas cancer cells and may promote peritoneal colonization. Citation Format: Theressa Ewa, Leela Avula, Sarah Joseph, Michael Rudloff, Samantha Sevilla, Vishal Koparde, Xianyu Zhang, Christine Alewine. Soluble mesothelin acts as a signaling molecule [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 93.