Foxo3 circular RNA retards cell cycle progression via forming ternary complexes with p21 and CDK2

Abstract

Most RNAs generated by the human genome have no protein-coding ability and are termed non-coding RNAs. Among these include circular RNAs, which include exonic circular RNAs (circRNA), mainly found in the cytoplasm, and intronic RNAs (ciRNA), predominantly detected in the nucleus. The biological functions of circular RNAs remain largely unknown, although ciRNAs have been reported to promote gene transcription, while circRNAs may function as microRNA sponges. We demonstrate that the circular RNA circ-Foxo3 was highly expressed in non-cancer cells and were associated with cell cycle progression. Silencing endogenous circ-Foxo3 promoted cell proliferation. Ectopic expression of circ-Foxo3 repressed cell cycle progression by binding to the cell cycle proteins cyclin-dependent kinase 2 (also known as cell division protein kinase 2 or CDK2) and cyclin-dependent kinase inhibitor 1 (or p21), resulting in the formation of a ternary complex. Normally, CDK2 interacts with cyclin A and cyclin E to facilitate cell cycle entry, while p21works to inhibit these interactions and arrest cell cycle progression. The formation of this circ-Foxo3-p21-CDK2 ternary complex arrested the function of CDK2 and blocked cell cycle progression.

The emerging role and clinical implication of human exonic circular RNA

“…Another study by the same research group showed that circ-Foxo3 could also bind to the cell cycle proteins cyclin-dependent kinase 2 (CDK2) and cyclin-dependent kinase inhibitor 1 (p21), resulting in the formation of a ternary complex. 49 The formation of this circ-Foxo3-p21-CDK2 ternary complex arrested the function of CDK2 and blocked cell cycle progression, adding support to circ-Foxo3 acting as a protein's scaffold to modulate certain cellular activities. In summary, circ-Foxo3 seems to be able to bind to many different proteins.…”

CircRNAs in the brain

“…17,21 Second, pioneering studies have now showed functionality of specific circRNAs in both normal physiology and disease. 18,[22][23][24][25] Third, circRNAs are cytoplasmic, whereas most (if not all) undesired splicing products accumulate at the site of transcription. 26 Fourth, circRNAs are evolutionarily conserved at both their sequence level and their pattern of expression, in particular in the brain.…”

“…CircFOXO3 inhibits cell cycle progression by forming a ternary complex with cyclin-dependent kinase 2 (CDK2) and cyclin-dependent kinase inhibitor 1 (p21). 23 CircH-IPK3 directly binds and inhibits miR-124 activity, and silencing it results in arrested cell growth. 22 In addition, we recently reported that the splicing factor muscleblind (mbl) hosts a circRNA which is involved in the auto-regulation of this RNA-binding protein.…”

“…48 Indeed, circRNAs have already been shown to be good biomarkers for different types of cancers. 23,24,49 Future prospects and concluding remarks Many neuronal genes generate multiple circRNAs. These "hotspots" for circRNA production usually involve splicing with one specific exon, either at the splice donor or splice acceptor.…”

Induction of tumor apoptosis through a circular RNA enhancing Foxo3 activity

“…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.…”