Circ-FOXO3 inhibits triple-negative breast cancer progression via interaction with WHSC1

Abstract Background . Circular RNAs (circRNAs), a subclass of non-coding RNAs characterized by covalently closed continuous loops, play a key role in tumorigenesis and aggressiveness. However, the potential molecular mechanism of circRNAs in triple-negative breast cancer (TNBC) remains largely unknown. Exploring their roles and mechanisms in TNBC progression may help identify new diagnostic markers and therapeutic targets. Methods . Differentially expressed circRNAs were identified by RNA sequencing. The expression of circ-FOXO3 was evaluated using Real-time quantitative RT-PCR and RNA in situ hybridization. The impacts of circ-FOXO3 on the TNBC progression were investigated by in vitro and in vivo functional assays. The underlying mechanism of circ-FOXO3 was revealed by Western blot, RNA pull down, mass spectrometry, RNA immunoprecipitation, luciferase assays and rescue experiments. Results . Here, we identified circ-FOXO3 as a tumor suppressor in TNBC progression. Circ-FOXO3 was predominantly expressed in the cytoplasm and dramatically downregulated in TNBC tissues. Notably, circ-FOXO3 expression was significantly lower in blood samples from patients with TBNC. Low circ-FOXO3 expression in TNBC tissues and blood was associated with lymph node metastasis and unfavorable outcomes in patients with TNBC. In addition, circ-FOXO3 overexpression inhibited the growth, invasion, and metastasis of TNBC cells both in vitro and in vivo . Moreover, we demonstrated that circ-FOXO3 interacts with Wolf-Hirschhorn syndrome candidate 1 (WHSC1) and inhibits WHSC1 activity, resulting in the inhibition of H3K36me2 modifications at the Zeb2 promoter, ultimately inhibiting Zeb2 expression and halting cancer progression. Conclusion . Taken together, these results reveal the tumor-suppressive functions of circ-FOXO3 in inhibiting WHSC1-mediated H3K36me2 modification of Zeb2, suggesting that circ-FOXO3 could serve as a potential novel predictive prognostic biomarker and therapeutic target for TNBC.