Background: Circular RNAs (circRNAs) play a critical role in cancer. Emerging evidence has shown circ-Foxo3, a circRNA, was dysregulated in a variety of tumor types. However, the exact role of circ-Foxo3  in bladder cancer has never been studied.
Methods: We measured the expression level of circ-Foxo3 in human and murine bladder cancer tissues and in various human bladder cancer cell lines. We induced bladder cancer in mice by a carcinogen N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN). circ-Foxo3 was overexpressed in mice by lentiviral gene transfer and in cultured cells via overexpression plasmid. The effect of circ-Foxo3 on apoptosis was examined via apoptotic marker staining, Western blot, and flow cytometry. We further characterized the interaction between circ-Foxo3 and miR-191 and its functional impact on bladder cancer cells.
Results: circ-Foxo3 was downregulated in bladder cancer in vivo and in vitro, and was upregulated in response to apoptotic stress. Overexpression of circ-Foxo3 promoted bladder cancer cell apoptosis in BBN mice and in human bladder cancer cell lines. miR-191-5p suppressed circ-Foxo3 expression and the pro-apoptotic effect of circ-Foxo3  in bladder cancer cells via directly targeting the 3ʹ-untranslated region (3ʹ-UTR) of circ-Foxo3 .
Conclusion: circ-Foxo3 was downregulated in bladder cancer in vivo and in vitro, and promoted bladder cancer apoptosis via direct interaction with miR-191. circ-Foxo3 could be a potential therapeutic target for bladder cancer.
Keywords: bladder cancer, circular RNA, apoptosis, circ-Foxo3, miR-191