Hiroyuki Tomita

Mesothelial cells promote peritoneal invasion and metastasis of ascites-derived ovarian cancer cells through spheroid formation

Patients with epithelial ovarian cancer (EOC) are often diagnosed with peritoneal metastasis and ascites, the accumulation of intraperitoneal fluid containing nonmalignant cells. However, the interactions between EOC and nonmalignant cells before peritoneal metastasis remain unclear. To investigate this, whole EOC spheroids were observed using a multiphoton microscope, and their invasion ability was assessed. Mesothelial cells were identified as notable components of ascites through morphological assessment, immunohistochemical/immunofluorescence staining, and single-cell RNA sequencing analyses. Almost all EOC cells were spheroids, with 60% containing mesothelial cells. EOC cells quickly generate aggregated spheroids with mesothelial cells, and these aggregated cancer-mesothelial spheroids (ACMSs) invade collagen or mesothelial layers. Mesothelial cells forming ACMSs initiated the invasion. RNA sequencing analysis revealed marked RNA expression changes in mesothelial cells, whereas the changes in EOC cells were minor. Transforming growth factor–β1–stimulated mesothelial cells showed increased invadopodium formation along with fascin-1 up-regulation. These findings suggest that EOC cells alter mesothelial cells through ACMSs, thereby elucidating the rapid spread of EOC in the abdominal cavity.

Metastatic Voyage of Ovarian Cancer Cells in Ascites with the Assistance of Various Cellular Components

Epithelial ovarian cancer (EOC) is the most lethal gynecologic malignancy and has a unique metastatic route using ascites, known as the transcoelomic root. However, studies on ascites and contained cellular components have not yet been sufficiently clarified. In this review, we focus on the significance of accumulating ascites, contained EOC cells in the form of spheroids, and interaction with non-malignant host cells. To become resistant against anoikis, EOC cells form spheroids in ascites, where epithelial-to-mesenchymal transition stimulated by transforming growth factor-β can be a key pathway. As spheroids form, EOC cells are also gaining the ability to attach and invade the peritoneum to induce intraperitoneal metastasis, as well as resistance to conventional chemotherapy. Recently, accumulating evidence suggests that EOC spheroids in ascites are composed of not only cancer cells, but also non-malignant cells existing with higher abundance than EOC cells in ascites, including macrophages, mesothelial cells, and lymphocytes. Moreover, hetero-cellular spheroids are demonstrated to form more aggregated spheroids and have higher adhesion ability for the mesothelial layer. To improve the poor prognosis, we need to elucidate the mechanisms of spheroid formation and interactions with non-malignant cells in ascites that are a unique tumor microenvironment for EOC.