Poly (ADP-ribose) polymerase inhibition synergizes with the NF-kB inhibitor DHMEQ to kill hepatocellular carcinoma cells

Poly (ADP-ribose) polymerase (PARP) enzymes play a key role in the cellular machinery responsible for DNA repair. Dehydroxymethylepoxyquinomicin (DHMEQ), a new inhibitor of NF-?B, induces oxidative stress and DNA damage. The effects of DHMEQ in combination with Olaparib (PARP inhibitor) were studied on hepatocellular carcinoma (HCC) cells. The DHMEQ-Olaparib combination synergistically inhibited cell viability, cell proliferation and colony formation of Hep3B, but had additive effects on Huh7 cells. The synergistic effects of the combination correlated with increased apoptosis, caspase 3/7 activity and PARP cleavage. There was an induction of an endoplasmic reticulum (ER) stress response with significant up-regulation of CHOP and TRB3 genes and splicing of XBP1 mRNA in Hep3B cells but not in Huh7 cells. Silencing of the TRB3 mRNA in Hep3B cells reversed the reduction in viability caused by DHMEQ-Olaparib treatment, while depletion of unspliced XBP1 mRNA in DHMEQ-Olaparib-treated Huh7 cells reduced viability. ROS production was increased after DHMEQ-Olaparib treatment of Hep3B, which caused DNA damage by an accumulation of ?H2AX, increased AKT phosphorylation and reduced cell viability. The combination reduced Rad51 nuclear foci in Hep3B cells (not Huh7 cells), and silencing of Rad51 enhanced sensitivity of Huh7 cells to the DHMEQ-Olaparib combination. Knockdown of AKT in Hep3B cells restored the number of Rad51 nuclear foci after DHMEQ-Olaparib treatment. In summary, the DHMEQ-Olaparib combination induced ROS production, which killed HCC cells via DNA damage that could not be repaired by Rad51. Summary: PARPs and NF-?B are frequently deregulated in HCC. The DHMEQ-Olaparib combination exerted synergistic anti-tumour effects on HCC cells through ROS production via DNA damage that could not be repaired by Rad51. This suggested that the DHMEQ-Olaparib combination could be used to treat tumours that were resistant to Olaparib treatment. © 2014 Elsevier B.V.