Epigenetic targeting of DNA damage response (DDR)-related mechanisms to overcome acquired cisplatin resistance of tumor cells

Mechanisms of the DNA damage response (DDR) are considered as useful targets to overcome tumor cell resistance. We investigated the impact of various classes of histone deacetylase inhibitors (HDACi) (i.e. broad-spectrum HDACi (vorinostat), class I HDACi (entinostat), preferential class IIb HDAC6i (ricolinostat) and dual HDAC class I/IIb inhibitors (HDAC1/6i)) on mechanisms of the DDR using parental (J82^WT) and cisplatin (CisPt)-resistant bladder carcinoma cells (J82^CisR). Mono-treatment with entinostat revealed relatively low genotoxic and DDR-activating potency, while showing similar antiproliferative, cytotoxic and pro-apoptotic activities as the other HDACi. Despite its low DNA double-strand break (DSB) forming potency with mono-treatment, entinostat conferred the highest synergistic cytotoxicity (CI ≤ 0.8) in J82^CisR following co-treatment with cisplatin. Notably, this effect is independent of Pt-(GpG)-DNA-intrastrand-crosslink formation. Entinostat increased the level of CisPt-induced DNA strand-breaks, promotes replication and transcription blockage, pRPA32 foci formation, PARP- and caspase-7 cleavage, influenced the activation of DDR-related factors (e.g. γH2AX, pp53, pRPA32, pKap1), reverted CisPt-induced p53 acetylation and blocked drug-stimulated mRNA expression of multiple DSB (HR, NHEJ)- and BER repair-related factors. Summarizing, the class I-selective HDACi entinostat reveals the lowest DDR-activating potency in mono-treatment and the highest anticancer efficacy in combination with CisPt, likely involving inhibition of HDAC3. This is due to amplification of replicative and transcriptional stress caused by CisPt treatment as well as interference with mechanisms of DDR and DNA repair, eventually promoting apoptosis. Thus, epigenetic targeting of DDR-related death pathways by class I HDACi is useful to overcome acquired CisPt resistance of tumor cells.