Spontaneous spheroid budding from monolayers: a potential contribution to ovarian cancer dissemination

Ovarian cancer is the most lethal gynaecologic cancer, in large part because of its early dissemination and rapid development of chemotherapy resistance. Spheroids are clusters of tumor cells found in the peritoneal fluid of patients that are thought to promote this dissemination. Current models suggest that spheroids form by aggregation of single tumor cells shed from the primary tumor. Here, we demonstrate that spheroids can also form by budding directly as adherent clusters from a monolayer. Formation of budded spheroids correlated with expression of vimentin and lack of cortical E-cadherin. We also found that compared to cells grown in monolayers, cells grown as spheroids acquired progressive resistance to the chemotherapy drugs Paclitaxel and Cisplatin. This resistance could be completely reversed by dissociating the spheroids. Our observations highlight a previously unappreciated mode of spheroid formation that might have implications for tumor dissemination and chemotherapy resistance in patients, and suggest that this resistance might be reversed by spheroid dissociation.