Epithelial–mesenchymal transition (EMT) promotes invasion and metastasis of pancreatic ductal adenocarcinoma (PDAC). However, the importance of its reverse process, mesenchymal–epithelial transition (MET), for PDAC remains unclear. We aimed to characterize the histological finding “focal differentiation” in PDAC at perineural invasion sites in the context of MET and to investigate the role of Schwann cells in inducing tumor MET. Tumor differentiation and immunohistochemical expressions of E-cadherin, SMAD3, and vimentin at perineural invasion sites were examined in 168 PDAC tissues. Four PDAC cell lines were co-cultured with Schwann cells to investigate cell morphology, motility, or EMT-related markers using immunocytochemistry and quantitative PCR. Of 168 tumors, 124 (74%) showed focal differentiation with enhanced E-cadherin membrane expression (P < 0.001) and decreased nuclear accumulation of SMAD3 (P < 0.001). Among 115 PDACs harboring grade 1/2 tumor, tumors with focal differentiation showed worse survival compared to those without focal differentiation (P = 0.019). PDAC cells co-cultured with Schwann cells demonstrated a sheet-like appearance, increased E-cadherin expression, decreased expressions of SMAD3 and vimentin, and reduced cell motility. In conclusion, MET-like change is induced by Schwann cells, suggesting that Schwann cells contribute to PDAC colonization in pancreatic nerves through activating the MET machinery inside tumor cells in the pancreatic tumor microenvironment.
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