Spheroid architecture strongly induces miR-221/222 expression and promotes oxidative phosphorylation and survival of mobile tumor cells through a mechanism that includes restriction of miR-9 expression

Tumor cell spheroids are three dimensional multicellular structures that form during the expansive growth of carcinoma cells. Spheroids support tumor metastasis as vehicles of dissemination, promoting growth and survival of bulk tumor and cancer stem cells within the mobile tumor cell population. Deciphering how spheroid architecture affects tumor cell phenotype will be essential for the development of therapeutics to inhibit transperitoneal metastasis and the development of peritoneal carcinomatosis by ovarian cancers. We investigated how spheroid formation directly affects OXPHOS activity and microRNA expression in a cultured ovarian carcinoma cell system. The rate of oxidative phosphorylation/respiration per cell in spheroids was nearly double that of the same cell type growing in suspension as single cells. Cells growing as spheroids showed greatly enhanced expression of miR-221/222, an oncomiR that targets multiple tumor suppressor genes, promotes invasion, as well as reduced expression of miR-9, which targets mitochondrial tRNA-modification enzymes and inhibits OXPHOS. Consistent with the greater efficiency of ATP generation afforded by OXPHOS phosphorylation, tumor cells growing as spheroids injected into the nutrient-poor environment of the murine peritoneum survived longer than the cells growing in suspension as loosely associated aggregates. The data suggest that in addition to the reported effects of spheroid formation on cancer cell growth and phenotype, including promotion of stem cell generation, spheroid architecture increases the OXPHOS activity of constituent tumor cells. During the mobile phase of metastasis, when ovarian tumor cells disperse through nutrient-poor environments such as the peritoneum, enhanced OXPHOS activity afforded by spheroid architecture would enhance survival and thereby contribute to metastatic potential.