LAUSR

The cell-surface glycoprotein mesothelin (MSLN) is overexpressed in pancreatic adenocarcinoma (PDAC) and is positively correlated with tumor aggressiveness. MSLN knock-out (KO) PDAC cells have delayed growth of peritoneal metastases in mice. Complementation with wild-type MSLN rescued the phenotype, although complementation with MSLN containing a Y318A point mutation to ablate interaction with MSLN binding partner MUC-16 could not. Mature MSLN is shed into the tumor microenvironment to generate soluble MSLN (sMSLN). We hypothesized that sMSLN would promote peritoneal metastasis. We engineered MSLN C-terminal truncation mutants (∆599-WT and ∆599-Y318A) that lack the membrane attachment site and exist only in soluble form. Parent and MSLN KO PDAC cells were transduced with the vector constructs. These cells were injected into athymic nude mice and peritoneal tumor growth was measured 6 weeks later. MSLN KO cells complemented with ∆599-WT MSLN exhibited poor peritoneal tumor growth, while ∆599-Y318A resulted in growth rescue. We considered that sMSLN might exert a dominant negative effect because it competes for MUC-16 binding with membrane-bound MSLN but cannot facilitate cell-to-cell interaction. Expression of ∆599-WT inhibited tumor growth in parent cells, while ∆599-Y318A failed to do so. The activity of ∆599-Y318A in KO cells remained unexplained. We hypothesized that levels of mircoRNA-198, a purported tumor suppressor regulated by MSLN, might be altered by ∆599 expression and correlate with phenotype rescue. However, no change in microRNA-198 expression was observed in ∆599-expressing cells. We are continuing to investigate the mechanism for a novel activity of sMSLN that is unrelated to MUC-16 binding. Citation Format: Neel M. Panchwagh, Theressa Ewa, Leela Rani Avula, Sarah Joseph, Chin-Hsien Tai, Samantha Sevilla, Vishal Koparde, Xianyu Zhang, Christine Alewine. Role of soluble mesothelin in peritoneal metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1280.