Synergistic loss of prostate cancer cell viability by coinhibition of HDAC and PARP.

UNLABELLED Tumors with BRCA germline mutations are defective in repairing DNA double-strand breaks (DSB) through homologous recombination (HR) pathways, making them sensitive to PARP inhibitors (PARPi). However, BRCA germline mutations are rare in prostate cancer limiting the ability to therapeutically target these pathways. This study investigates whether histone deacetylase (HDAC) inhibitors (HDACi), reported to modulate DSB repair pathways in sporadic cancers, can downregulate DSB repair pathways and sensitize prostate cancer cells to PARPi. Prostate cancer cells cotreated with the HDAC inhibitor, suberoylanilide hydroxamic acid (SAHA) and the PARPi, olaparib, demonstrated a synergistic decrease in cell viability compared with single-agent treatment (combination index < 0.9), whereas normal prostatic cells did not. Similarly, clonogenicity was significantly decreased after cotreatment. Flow cytometric cell-cycle analysis and Annexin-V staining revealed significant apoptosis upon treatment with SAHA+olaparib. This coincided with increased DNA damage observed by immunofluorescence microscopy analysis of γH2AX foci, a marker of DSBs. In addition, immunoblot analysis showed a significant and persistent increase in nuclear γH2AX levels. Both SAHA and olaparib downregulated the expression of HR-related proteins, BRCA1 and RAD51, whereas SAHA + olaparib had an additive effect on RAD51. Silencing RAD51 sensitized prostate cancer cells to SAHA and olaparib alone. Collectively, cotreatment with HDACi and PARPi downregulated HR-related protein expression and concomitantly increased DNA damage, resulting in prostate cancer cell death. IMPLICATIONS These findings provide a strong rationale for supporting the use of combined HDAC and PARP inhibition in treating advanced prostate cancer.