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

BackgroundLiver 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. MethodscircRNA 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). ResultsHere, 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/{beta}-catenin activation, and inhibition of these pathways is a potential strategy for preventing liver tumorigenesis and metastasis. ConclusionThis 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.