Synergistic Loss of Prostate Cancer Cell Viability by Coinhibition of HDAC and PARP

Abstract

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 SAHAþolaparib. This coincided with increased DNA damage observed by immunofluorescence microscopy analysis of gH2AX foci, a marker of DSBs. In addition, immunoblot analysis showed a significant and persistent increase in nuclear gH2AX levels. Both SAHA and olaparib downregulated the expression of HR-related proteins, BRCA1 and RAD51, whereas SAHA þ 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.

DNA-Repair Defects and Olaparib in Metastatic Prostate Cancer

“…21,37 We observed clinical responses to olaparib in patients with metastatic, castration-resistant prostate cancer and defective PALB2 or HDAC2 . HDAC inhibition has been reported to result in sensitivity to PARP inhibition, 28,38 but the clinical relevance of HDAC genomic loss is not known.…”

A Novel Use of Olaparib for the Treatment of Metastatic Castration‐Recurrent Prostate Cancer

“…The PARPs are a family of enzymes responsible for repairing single‐strand DNA breaks through base excision repair pathways (Figure ) . Olaparib inhibits the PARP1, PARP2, and PARP3 enzymes and exhibits antitumor effects by inhibiting the repair of DNA strand breaks, leading to an accumulation of DNA single‐strand breaks and, ultimately, cell death (Figure ) . DNA double‐strand breaks are repaired through two mechanisms: nonhomologous end joining, which is error prone, and homologous recombination (HR) .…”

“…Olaparib inhibits the PARP1, PARP2, and PARP3 enzymes and exhibits antitumor effects by inhibiting the repair of DNA strand breaks, leading to an accumulation of DNA single‐strand breaks and, ultimately, cell death (Figure ) . DNA double‐strand breaks are repaired through two mechanisms: nonhomologous end joining, which is error prone, and homologous recombination (HR) . BRCA1/2 and RAD51 assist with DNA repair through HR, so mutations in these genes lead to inactivation of homologous DNA repair and accrual of double‐strand DNA breaks, making these cells sensitive to the effects of PARP inhibitors .…”

The inducible E3 ubiquitin ligases SIAH1 and SIAH2 perform critical roles in breast and prostate cancers

“…The SIAH2-dependent proteasomal degradation of NCOR may also contribute to cancer development and chemoresistance [12]. Since the pharmacological inhibition of NCOR-associated transcriptional repressors with HDACi blocks cancer-relevant signaling and the growth of transformed breast and prostate cells [25,[76][77][78][79], the elimination of NCOR by SIAH2 may affect the growth of such cells. Additional work is required to generate experimental support for this hypothesis.…”