rtcisE2F drives liver TIC self-renewal and metastasis via m6A-modulated mRNA stability of E2F6 and E2F3

Background: Liver tumor initiating cells (TICs) harbor self-renewal and differentiation capacities, and well contribute to liver tumorigenesis, metastasis and heterogeneity. However, the molecular mechanisms of liver TIC self-renewal are unclear. N6-methyladenosine is the most abundant modification of RNA molecules, and is involved in RNA stability and translation, but the molecular mechanisms of m6A regulation remain largely unknown. Methods: circRNA expression was detected by in situ hybridization, fluorescence in situ hybridization, quantitative real-time PCR and Northern blot. Target gene expression was examined by microarray analyses, quantitative real-time PCR and Western blot. CRISPR, CRISPR interference (CRISPRi) and short-hairpin RNA (shRNA) were used for circRNA/target gene knockout and knockdown. Liver TICs were enriched through sphere formation and FACS using CD133 as a marker, and liver TIC activity was assessed by tumor propagation, sphere formation, tumor-initiating, and transwell assays. Quantitative real-time PCR and Northern blot were used to determine mRNA stability. RNA-protein interactions were examined by RNA pulldown, RNA immunoprecipitation, Tagged RNA affinity purification (TRAP) and electrophoretic mobility shift assays (EMSA). Results: Here, we identified a functional rt-circRNA, termed rtcisE2F, that is highly expressed in liver cancer and liver TICs. rtcisE2F plays essential roles in the self-renewal and activities of liver TICs. rtcisE2F targets E2F6 and E2F3 mRNAs, attenuates mRNA turnover, and increases E2F6/E2F3 expression. Mechanistically, rtcisE2F functions as a scaffold of m6A reader IGF2BP2 and E2F6/E2F3 mRNA, promotes the association of E2F6/E2F3 mRNAs with IGF2BP2, and then inhibits their association with another m6A reader, YTHDF2. IGF2BP2 inhibits E2F6/E2F3 mRNA decay, whereas YTHDF2 promotes E2F6/E2F3 mRNA decay. By switching m6A readers, rtcisE2F enhances E2F6/E2F3 mRNA stability. E2F6 and E2F3 are both required for liver TIC self-renewal and Wnt/β-catenin activation, and inhibition of these pathways is a potential strategy for preventing liver tumorigenesis and metastasis. Conclusion: This work identified rtcisE2F as a key modulator in liver cancer and liver TICs, providing evidence for the biological function of rt-circRNA and unveiling a new regulatory layer for liver TIC self-renewal. rtcisE2F is involved in E2F6/E2F3 stability by switching their binding to the m6A readers IGF2BP2 and YTHDF2, providing a competitive mechanism between RNA molecules and m6A readers. Both E2F6 and E2F3 are required for liver TIC self-renewal and serve as therapeutic targets for liver TIC elimination.