During peritoneal dialysis (PD), mesothelial cells undergo mesothelial-to-mesenchymal transition (MMT), a process associated with peritoneal-membrane dysfunction. Because TGF-β1 can induce MMT, we evaluated the efficacy of TGF-β1-blocking peptides in modulating MMT and ameliorating peritoneal damage in a mouse model of PD. Exposure of the peritoneum to PD fluid induced fibrosis, angiogenesis, functional impairment, and the accumulation of fibroblasts. In addition to expressing fibroblast-specific protein-1 (FSP-1), some fibroblasts co-expressed cytokeratin, indicating their mesothelial origin. These intermediate-phenotype (Cyto⁺/FSP-1⁺) fibroblasts had features of myofibroblasts with fibrogenic capacity. PD fluid treatment triggered the appearance of CD31⁺/FSP-1⁺ and CD45⁺/FSP-1⁺ cells, suggesting that fibroblasts also originate from endothelial cells and from cells recruited from bone marrow. Administration of blocking peptides significantly ameliorated fibrosis and angiogenesis, improved peritoneal function, and reduced the number of FSP-1⁺ cells, especially in the Cyto⁺/FSP-1⁺ subpopulation. Conversely, overexpression of TGF-β1 in the peritoneum by adenovirus-mediated gene transfer led to a marked accumulation of fibroblasts, most of which derived from the mesothelium. Taken together, these results demonstrate that TGF-β1 drives the peritoneal deterioration induced by dialysis fluid and highlights a role of TGF-β1-mediated MMT in the pathophysiology of peritoneal-membrane dysfunction.

SUMMARY: Epithelial-to-mesenchymal transition (EMT) occurs in fibrotic diseases affecting the kidney, liver and lung, and in the peritoneum of patients undergoing peritoneal dialysis. EMT in the peritoneum is linked to peritoneal membrane dysfunction, and its establishment limits the effectiveness of peritoneal dialysis. The molecular regulation of EMT in the peritoneum is thus of interest from basic and clinical perspectives. Treatment of primary human mesothelial cells (MCs) with effluent from patients undergoing peritoneal dialysis induced a genuine EMT, characterized by downregulated E-cadherin and cytokeratin expression, cell scattering, and spindle-like morphology. This EMT was replicated by co-stimulation with transforming growth factor (TGF)-β1 and interleukin (IL)-1β. Retroviral overexpression of a mutant inhibitor of kappaB (IκB) demonstrated that NF-κB activation is required for E-cadherin and cytokeratin downregulation during EMT. Pre-treatment with the MAP kinase kinase (MEK)-1/2 inhibitor U0126 showed that cytokine-triggered NF-κB nuclear translocation and transcriptional activity are mediated by activation of extracellular regulated kinase (ERK). Cytokine-mediated induction of mRNA expression of the transcription factor Snail1, a repressor of E-cadherin expression and a potent inducer of EMT, was prevented by blockade of ERK or NF-κB. Finally, blockade of ERK/NF-κB signaling in ex vivo MCs that were cultured from peritoneal dialysis effluents reverted cells to an epithelioid morphology, upregulated E-cadherin and cytokeratin expression, and downregulated Snail1 expression. Modulation of the ERK/NF-κB/Snail1 pathway may provide a means of counteracting the progressive structural and functional deterioration of the peritoneal membrane during peritoneal dialysis.

Peritoneal dissemination is the primary metastatic route of ovarian cancer (OvCa), and is often accompanied by the accumulation of ascitic fluid. The peritoneal cavity is lined by mesothelial cells (MCs), which can be converted into carcinoma‐associated fibroblasts (CAFs) through mesothelial‐to‐mesenchymal transition (MMT). Here, we demonstrate that MCs isolated from ascitic fluid (AFMCs) of OvCa patients with peritoneal implants also undergo MMT and promote subcutaneous tumour growth in mice. RNA sequencing of AFMCs revealed that MMT‐related pathways – including transforming growth factor (TGF)‐β signalling – are differentially regulated, and a gene signature was verified in peritoneal implants from OvCa patients. In a mouse model, pre‐induction of MMT resulted in increased peritoneal tumour growth, whereas interfering with the TGF‐β receptor reduced metastasis. MC‐derived CAFs showed activation of Smad‐dependent TGF‐β signalling, which was disrupted in OvCa cells, despite their elevated TGF‐β production. Accordingly, targeting Smad‐dependent signalling in the peritoneal pre‐metastatic niche in mice reduced tumour colonization, suggesting that Smad‐dependent MMT could be crucial in peritoneal carcinomatosis. Together, these results indicate that bidirectional communication between OvCa cells and MC‐derived CAFs, via TGF‐β‐mediated MMT, seems to be crucial to form a suitable metastatic niche. We suggest MMT as a possible target for therapeutic intervention and a potential source of biomarkers for improving OvCa diagnosis and/or prognosis. © 2017 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.