The E2F1 corepressor BIN1 is a novel tumor suppressor RB1-binding cofactor for cell-cycle arrest and genomic instability

Abstract

The bridging integrator 1 protein (BIN1) exhibits tumor suppressor activities through multiple mechanisms, including by acting as a corepressor of the adenovirus E2-promoter binding cellular factor 1 (E2F1) transcription factor and sensitizing cells to genotoxic stress. By inhibiting E2F1-dependent cell-cycle progression, BIN1 displays an overlapping function with the well-known E2F1 corepressor retinoblastoma protein 1 (RB1). However, whether RB1 and BIN1 collaborate to inhibit E2F1 activity and whether E2F1 is involved in BIN1-dependent sensitization to DNA-damaging agents remain unresolved. Here, I report that BIN1 physically interacts with RB1 to cooperatively repress E2F1-mediated transactivation and that BIN1 interferes with the early stages of DNA damage responses in a manner dependent on BIN1/E2F1 interaction. BIN1 efficiently bound hypophosphorylated RB1, particularly when BIN1 contained exon 10. This was functionally meaningful because BIN1 inhibited serum-induced RB1 phosphorylation by blocking the physical binding of cyclin D1 to RB1 and stabilized hypophosphorylated RB1 on an E2F-sensitive gene promoter. BIN1 and RB1 cooperatively suppressed the transcription of a number of E2F1-sensitive genes, including ATM Ser/Thr kinase (ATM) and BRCA1, and reduced cellular DNA-end joining. Because BIN1 inhibits cellular poly(ADP-ribosyl)ation, simultaneous activation of RB1 and BIN1 induced ‘synthetic lethality’ in multiple cancer cell lines regardless of ‘BRCAness’. Furthermore, even in the absence of DNA damage, depletion of BIN1 enhanced E2F1-dependent ATM autophosphorylation, formation of the MRE11A/RAD50/NBS1 DNA-repair–promoting complex, and phosphorylation of histone H2AX (forming γH2AX, a biomarker of double-stranded DNA breaks). Similar to BIN1 reduction, RB1 deficit increased cancer-cell resistance to cisplatin. Our study reveals that by physically binding to RB1, BIN1 acts as a novel RB1 cofactor and stabilizes RB1 on an E2F-sensitive promoter. Additionally, we unveil that one of the likely mechanisms by which the tumor suppressors BIN1 and RB1 induce growth arrest is attributable to facilitating accumulation of genomic instability via inhibition of E2F1-dependent DNA-repair–promoting activities.