A Circular RNA from the MDM2 Locus Controls Cell Cycle Progression by Suppressing p53 Levels

Abstract

Circular RNAs (circRNAs) are a class of noncoding RNAs produced by a noncanonical form of alternative splicing called back-splicing. To investigate a potential role of circRNAs in the p53 pathway, we analyzed RNA sequencing (RNA-seq) data from colorectal cancer cell lines (HCT116, RKO, and SW48) that were untreated or treated with a DNA-damaging agent. Surprisingly, unlike the strong p53-dependent induction of hundreds of p53-induced mRNAs upon DNA damage, only a few circRNAs were upregulated from p53-induced genes. circ-MDM2, an annotated circRNA from the MDM2 locus, was one of the handful of circRNAs that originated from a p53-induced gene. Given the central role of MDM2 in suppressing p53 protein levels and p53 activity, we investigated the function of circ-MDM2. Knocking down circ-MDM2 with small interfering RNAs (siRNAs) that targeted circ-MDM2 did not alter MDM2 mRNA or MDM2 protein levels but resulted in increased basal p53 levels and growth defects in vitro and in vivo. Consistent with these results, transcriptome profiling showed increased expression of several direct p53 targets, reduced retinoblastoma protein (Rb) phosphorylation, and defects in G1-S progression upon silencing circ-MDM2. Our results on the initial characterization of circ-MDM2 identify a new player from the MDM2 locus that suppresses p53 levels and cell cycle progression.

Comprehensive landscape and future perspectives of circular RNAs in colorectal cancer

“…Finally, circRNA formation is induced by certain drugs [ 83 , 124 , 125 ], chemokines [ 94 ] and stress [ 80 , 126 ] (Fig. 4 f).…”

“…4 f). Chaudhary et al observed the upregulation of hsa_circ_0027492 (circMDM2) in three p53 wild-type CRC lines (HCT116, RKO and SW48 cells) after treatment with the DNA-damaging agent doxorubicin, and the overexpression of circMDM2 was induced in a p53-dependent manner and was not restricted to CRC cells [ 125 ]. Shen et al observed the downregulation of circ_0026344 following costimulation with CCL20 and CXCL8 (two well-known chemokines), leading to EMT in CRC cells [ 94 ].…”

“…Importantly, circRNAs may regulate the expression of cell cycle-related proteins (e.g., p53 and p16) [ 72 , 125 ], apoptosis-related proteins (e.g., Bax and Bcl-2) [ 97 , 163 ] and EMT-related proteins (e.g., MMP2, MMP-9, and E-cadherin) [ 164 – 166 ] to mediate cell cycle arrest, apoptosis, and EMT. Additionally, circRNAs directly regulate CRC-associated signaling pathways, such as the Wnt/β-catenin pathway [ 76 , 78 , 167 ] and PTEN/Akt/mTOR pathway [ 168 , 169 ].…”

Hsa_circ_0043278 Inhibits Tumorigenesis and is Downregulated in Colorectal Cancer

“… 24 Chaudhary et al demonstrated that circ-MDM2, which was from the MDM2 locus and identified as DNA damage-regulated circRNA, could inhabit the expression level of p53 and regulate the cell cycle to promote tumorigenesis in CRC. 25 Here, we identified hsa_circ_0043278 as a suppressor in CRC, with a significantly low expression level in tissues samples and cell lines. In addition, we demonstrated that hsa_circ_0043278 inhibited the proliferation and migration of HCT116 and SW620 cells while promoting apoptosis.…”

Practical guide for circular RNA analysis: Steps, tips, and resources

“…Other important observations are: (a) the number of binding sites of an RBP in a circRNA, (b) the possibility that the RBP multimerizes (and hence the circRNA:RBP ratios can deviate from 1:1), and (c) the possibility that a proportion of the RBP content in the cell is already bound to other targets and only a smaller, free proportion of the total RBP pool is available for the circRNA. The study of circRNPs is still in its infancy, so it may be necessary to adapt and develop specific methods to more rigorously study circRNPs (Chaudhary et al, 2020; Du et al, 2017; Hanniford et al, 2020; A. Huang, Zheng, Wu, Chen, & Huang, 2020).…”