Induction of tumor apoptosis through a circular RNA enhancing Foxo3 activity

Abstract

Circular RNAs are a class of non-coding RNAs that are receiving extensive attention. Despite reports showing circular RNAs acting as microRNA sponges, the biological functions of circular RNAs remain largely unknown. We show that in patient tumor samples and in a panel of cancer cells, circ-Foxo3 was minimally expressed. Interestingly, during cancer cell apoptosis, the expression of circ-Foxo3 was found to be significantly increased. We found that silencing endogenous circ-Foxo3 enhanced cell viability, whereas ectopic expression of circ-Foxo3 triggered stress-induced apoptosis and inhibited the growth of tumor xenografts. Also, expression of circ-Foxo3 increased Foxo3 protein levels but repressed p53 levels. By binding to both, circ-Foxo3 promoted MDM2-induced p53 ubiquitination and subsequent degradation, resulting in an overall decrease of p53. With low binding affinity to Foxo3 protein, circ-Foxo3 prevented MDM2 from inducing Foxo3 ubiquitination and degradation, resulting in increased levels of Circular RNAs are a large class of non-coding RNAs that are circularized by joining free 3'-to 5'-ends, forming a circular structure. 1-4Although circular RNAs were initially characterized over 30 years ago, their functions in mammalian cells are still largely unknown. Most circular RNAs are predominantly found in the cytoplasm and contain exons, known as circRNAs.5 A relatively smaller group of circular RNAs that contain both exons and introns are known as EIciRNAs, and are predominantly found in the nucleus. 6 Recent studies have indicated that some circular RNAs contain miRNA binding sites and may function as sponges to arrest miRNA functions. 7,8 It has further been reported that EIciRNAs increase the transcription of their parental genes.9 Recently, we showed that the circular RNA circ-Foxo3 could function by binding to proteins in related signal pathways. 10,11 In the present study, we used computational approach to elucidate the interaction of circ-Foxo3 with MDM2 and p53. The RING-finger domain in the carboxyl terminal of the MDM2 is known to bind RNA specifically in a sequence-specific manner, 12 whereas p53 interacts with RNA via its C-terminal regulatory domain. 13 Our study comprised of computer-aided RNA structure modeling of circ-Foxo3 employing minimum free energy algorithm and machine translation system followed by its molecular interaction with MDM2 (RING-finger domain) and p53 (C-terminal regulatory domain) that includes docking, scoring, clustering, and refinement of the most promising models. The interaction was further confirmed by an approach of molecular experiments to explicate the biological functions of circ-Foxo3. ResultsDecreased expression of circ-Foxo3 in tumors and cancer cells. Downregulation of Foxo3 is often observed in cancer development.14,15 Both circ-Foxo3 and Foxo3 mRNA are encoded by the FOXO3 gene. 16 We found that the levels of circ-Foxo3 in tumor specimen were significantly lower than in the adjacent benign tissue (Figure 1a). We examined circ-Foxo3 expression and detected s...

circCPA4 acts as a prognostic factor and regulates the proliferation and metastasis of glioma

“…Recent studies suggest that circRNAs play vital roles in tumour procession, including cell apoptosis, angiogenesis and cell cycle progression. 12,13 And circRNAs could be promising prognostic biomarkers and therapeutic targets in tumour treatment. 14,15 But the expression and function of circRNAs in glioma are still not clear.…”

PLCA8 suppresses breast cancer apoptosis by activating the PI3k/AKT/NF‐κB pathway

“…Apoptosis is considered to be the major mechanism of programmed cell death, and the maintenance of normal physiology and cellular homeostasis occurs through different mechanisms . Apoptosis regulates diverse cellular processes, including cell proliferation, the cell cycle and survival, and the results of previous studies have suggested that the inhibition of PI3K/AKT signalling induces apoptosis …”

“…Apoptosis is considered to be the major mechanism of programmed cell death, and the maintenance of normal physiology and cellular homeostasis occurs through different mechanisms. [11][12][13] Apoptosis regulates diverse cellular processes, including cell proliferation, the cell cycle and survival, 14,15 and the results of previous studies have suggested that the inhibition of PI3K/AKT signalling induces apoptosis. 16,17 In this study, we aimed to determine the expression profile and elucidate the pathological functions of PLAC8 during BC progression, and our results revealed a relationship between endogenous PLAC8 expression and PI3K/AKT pathway activity.…”

Roles of the circular RNA circ-Foxo3 in breast cancer progression

“…Specifically, this group found that circFoxo3 was downregulated in patient tumor samples and in a group of cancer cells, whereas it was upregulated in the cancer cells when the cells were induced to undergo apoptosis. 2 The authors showed that silencing endogenous circFoxo3 produced an opposite effect, while ectopic circ-Foxo3 induced cell apoptosis and inhibit progression of tumor xenografts. Interestingly, ectopic circ-Foxo3 played roles in increasing Foxo3 protein levels.…”