NF-kappa B mediates radio-sensitization by the PARP-1 inhibitor, AG-014699

The stress-inducible transcription factor, nuclear factor (NF)-kappa B induces genes involved in proliferation and apoptosis. Aberrant NF-kappa B activity is common in cancer and contributes to therapeutic-resistance. Poly(ADPribose) polymerase-1 (PARP-1) is activated during DNA strand break repair and is a known transcriptional co-regulator. Here, we investigated the role of PARP-1 function during NF-kappa B activation using p65 small interfering RNA (siRNA), PARP siRNA or the potent PARP-1 inhibitor, AG-014699. Survival and apoptosis assays showed that NF-kappa B p65(-/-) cells were more sensitive to ionizing radiation (IR) than p65(+/+) cells. Co-incubation with p65 siRNA, PARP siRNA or AG-014699 radio-sensitized p65(+/+), but not p65(-/-) cells, demonstrating that PARP-1 mediates its effects on survival via NF-kappa B. Single-strand break (SSB) repair kinetics, and the effect SSB repair inhibition by AG-014699 were similar in p65(+/+) and p65(-/-) cells. As preventing SSB repair did not radio-sensitize p65(-/-) cells, we conclude that radio-sensitization by AG-014699 is due to downstream inhibition of NF-kappa B activation, and independent of SSB repair inhibition. PARP-1 catalytic activity was essential for IR-induced p65 DNA binding and NF-kappa B-dependent gene transcription, whereas for tumor necrosis factor (TNF)-alpha-treated cells, PARP-1 protein alone was sufficient. We hypothesize that this stimulus-dependent differential is mediated via stimulation of the poly(ADP-ribose) polymer, which was induced following IR, not TNF-alpha. Targeting DNA damage-activated NF-kappa B using AG-014699 may therefore overcome toxicity observed with classical NF-kappa B inhibitors without compromising other vital inflammatory functions. These data highlight the potential of PARP-1 inhibitors to overcome NF-kappa B-mediated therapeutic resistance and widens the spectrum of cancers in which these agents may be utilized. Oncogene (2012) 31, 251-264; doi:10.1038/onc.2011.229; published online 27 June 2011