RAD6B is a major mediator of triple negative breast cancer cisplatin resistance: Regulation of translesion synthesis/Fanconi anemia crosstalk and BRCA1 independence

https://doi.org/10.1016/j.bbadis.2019.165561Get rights and content
Under an Elsevier user license
open archive

Highlights

  • Rad6B overexpression is associated with poor survival of TNBC patients.

  • Rad6B inhibitor SMI#9 sensitizes BRCA1 wild-type and mutant TNBCs to cisplatin.

  • Rad6B inhibitor SMI#9 attenuates cisplatin-induced FANCD2, POL η and γH2AX levels.

  • Rad6B modulates cisplatin-induced γH2AX via H2AX monoubiquitination.

  • Rad6B inhibition/silencing impairs homologous recombination.

Abstract

Triple negative breast cancer (TNBC) is an aggressive breast cancer subtype with few therapy options besides chemotherapy. Although platinum-based drugs have shown initial activity in BRCA1-mutated TNBCs, chemoresistance remains a challenge. Here we show that RAD6B (UBE2B), a principal mediator of translesion synthesis (TLS), is overexpressed in BRCA1 wild-type and mutant TNBCs, and RAD6B overexpression correlates with poor survival. Pretreatment with a RAD6-selective inhibitor, SMI#9, enhanced cisplatin chemosensitivity of BRCA1 wild-type and mutant TNBCs. SMI#9 attenuated cisplatin-induced PCNA monoubiquitination (TLS marker), FANCD2 (Fanconi anemia (FA) activation marker), and TLS polymerase POL η. SMI#9-induced decreases in γH2AX levels were associated with concomitant inhibition of H2AX monoubiquitination, suggesting a key role for RAD6 in modulating cisplatin-induced γH2AX via H2AX monoubiquitination. Concordantly, SMI#9 inhibited γH2AX, POL η and FANCD2 foci formation. RAD51 foci formation was unaffected by SMI#9, however, its recruitment to double-strand breaks was inhibited. Using the DR-GFP-based assay, we showed that RAD6B silencing or SMI#9 treatment impairs homologous recombination (HR) in HR-proficient cells. DNA fiber assays confirmed that restart of cisplatin-stalled replicating forks is inhibited by SMI#9 in both BRCA1 wild-type and mutant TNBC cells. Consistent with the in vitro data, SMI#9 and cisplatin combination treatment inhibited BRCA1 wild-type and mutant TNBC growth as compared to controls. These RAD6B activities are unaffected by BRCA1 status of TNBCs suggesting that the RAD6B function in TLS/FA crosstalk could occur in HR-dependent and independent modes. Collectively, these data implicate RAD6 as an important therapeutic target for TNBCs irrespective of their BRCA1 status.

Keywords

Triple negative breast cancer
BRCA1
FANCD2
POL η
H2AX
Cisplatin
Ubiquitination
Small molecule inhibitor
Homologous recombination
Replication restart

Cited by (0)