Tumorigenicity of Ascites-Derived Tumor Cells: Insights Into the Molecular Mechanisms of Ovarian Cancer Progression and Therapy Resistance

Background: Ovarian cancer (OvCa) cells disseminate primarily intraperitoneally. Here, detached tumor cell aggregates (spheroids) from the primary tumor are generally regarded as “metastatic units”, which exhibit a survival benefit, probably due to the protective microenvironment and their unique molecular characteristics. Hence, current therapeutic concepts such as classical chemotherapy are not sufficient preventing growth and spread of OvCa spheroids. Methods: In the current study we analyzed the cellular composition of ascites from ovarian cancer patients using flow cytometry and the tumorigenic potential of the different subpopulations in an intraperitoneal mouse model. Comparative transcriptome analyses (RNAseq) from ascites-derived tumor cells spheroids (n=10) vs. tumor samples from different metastatic sites (n=30) were further performed in order to identify key molecular players responsible for the special cellular characteristics of OvCa spheroids. Results: In vitro culture of ascites-derived cells gave rise to two different subpopulations: an adherent cell population (ADs) including mainly CD90+ cells with highly proliferative rates in vitro but no tumorigenic potential in vivo, and a non-adherent cell population (NADs) containing principally EpCAM+/CD24+ cells with low proliferative potential in vitro. NADs included cell aggregates and single cells, the first showing a high content (> 80%) of tumor cells (EpCAM+/CD24+). Enriched tumor cell spheroids from the ascites using cell strainers showed higher tumorigenic potential in vivo in comparison to the original ascites-derived cell population. Interestingly, the different metastatic spread patterns observed in the mice resembled the tumor dissemination pattern found in the corresponding patients. RNAseq analyses from tumor-spheroids revealed up-regulation of genes involved in chemoresistance (TGM1, HSPAs, MT1s), cell-adhesion and cell barrier (PKP3, CLDNs, PPL) and the oxidative phosphorylation (OXPHOS) process compared to the solid tumor tissue samples. Additionally, down-regulation of extracellular matrix components and angiogenesis-related genes could be observed. Targeting OXPHOS by metformin treatment led to reduced viability of ascites-derived spheroids from OvCa patients, showing to some extent a synergistic effect with cisplatin treatment. Conclusions: the actual study contributes to a better understanding of the biology of ovarian cancer spheroids and to the identification of new treatment opportunities in advanced ovarian cancer.