Simultaneous Establishment of Autologous Colorectal Cancer and Mesothelial Stromal Cell Lines from Malignant Ascites Reveals a Mesothelial‐Stromal <scp>FGFR3</scp> Axis as a Potential Vulnerability in Peritoneal Metastasis

Colorectal cancer (CRC) with peritoneal dissemination remains a major therapeutic challenge because of poor prognosis and limited treatment options. Experimental models that accurately recapitulate tumor-mesothelial interactions are scarce. Here, we report the establishment of a novel autologous paired model comprising a CRC cell line (OMUCR-1) and matched cancer-associated mesothelial cells (CAmeso), both simultaneously derived from the malignant ascites of the same patient. Lineage marker analysis using qPCR demonstrated that OMUCR-1 selectively expressed epithelial markers (EPCAM, KRT20), whereas CAmeso strongly expressed mesothelial-mesenchymal markers (ACTA2, MSLN) and lacked epithelial marker expression. These mutually exclusive expression patterns confirm that the two cell populations are phenotypically distinct and rule out cross-contamination. OMUCR-1 displayed strong tumorigenic capacity across multiple transplantation models. CAmeso enhanced CRC cell migration and invasion in vitro, and co-transplantation with OMUCR-1 resulted in larger tumors enriched with αSMA-positive stromal components. RNA sequencing of co-injected xenografts revealed increased expression of murine stromal Fgfr3. Treatment with the FGFR inhibitor BGJ398 reduced tumor growth and decreased stromal FGFR3-positive components, suggesting that stromal FGFR3 may represent a potential microenvironmental vulnerability in CRC with peritoneal dissemination. This autologous CRC-mesothelial system provides a physiologically relevant platform for dissecting tumor-stroma interactions in peritoneal metastasis and may advance stromal-targeted therapeutic strategies.