EZH2 inhibitors-mediated epigenetic reactivation of FOSB inhibits triple-negative breast cancer progress

Background: Triple-negative breast Cancer (TNBC) is the most aggressive subtype of breast Cancer that lacks expression of Estrogen Receptor (ER) and Progesterone Receptor (PR) and the human epidermal growth factor receptor 2 (HER2) gene. Chemotherapy remains the standard of care for TNBC treatment, but considerable patients are very resistant to chemotherapy. Mutations or aberrant upregulation of EZH2 occur frequently, and EZH2 Inhibitor (EZH2i) showed some preclinic antitumor effects in TNBC.

Methods: RNA-seq data of 3 TNBC cell lines either treated with 2 μM GSK343, or stably transduced with shEHZ2, compared to untreated controls (GSE112378) were analyzed by Limma R package. The Kaplan-Meier plotter (KM plotter) database was used to assess the relevance of FOSB mRNA expression to relapse-free survival (RFS) in TNBC. Cell number counting and colony formation assays were used to detect the biological effect of FOSB on the growth of TNBC cells in vitro. The effect of FOSB on TNBC tumor growth in vivo was investigated in a mice tumor xenograft model. Luciferase reporter and chromatin immunoprecipitation (Chip) assays were used to determine the regulatory roles of C/EBPβ on FOSB expression.

Results: We found that FOSB, a member of the activator protein-1 complex, was a direct downstream target of EZH2. FOSB was significantly decreased in TNBC samples and associated with better relapse-free survival (RFS). EZH2-mediated histone 3 trimethylated on lysine 27 (H3K27me3), a marker of silent chromatin conformation, at the FOSB promoter inhibited it expression. Depletion of FOSB in TNBC cells promoted cell proliferation in vitro and tumor growth in vitro by inactivating the p53 pathway and conferred resistant to EZH2 Inhibitor (EZH2i). Mechanistically, EZH2i promotes the shift from H3K27me3 to H3K27ac at the FOSB promoter, and recruits the transcription factor C/EBPβ to activate FOSB gene transcription.

Conclusion: Together, our results suggest that EZH2-mediated epigenetic inactivation of FOSB promotes TNBC expression and demonstrate that reactivation of FOSB expression by C/EBPβ underlies the anti-TNBC action of EZH2is.