Suppression of NF-κB Activity by Parthenolide Induces X-Ray Sensitivity through Inhibition of Split-Dose Repair in TP53 Null Prostate Cancer Cells

We have shown that parthenolide, a sesquiterpene lactone, is a radiation sensitizer for human CGL1 hybrid cells that have constitutively activated NF-kappa B and wild-type p53. Since many malignant cells have nonfunctional p53, we investigated whether parthenolide could alter the X-ray sensitivity of PC-3 prostate cancer cells, a p53 null cell line with constitutively activated NF-kappa B. The addition of 5 mu M parthenolide induced non-apoptotic cell death, inhibited PC-3 proliferation, and increased the population doubling time from 23 +/- 1 h to 49 +/- 4 h. Parthenolide also inhibited constitutive and radiation-induced NF-kappa B binding activity and enhanced the X-ray sensitivity of these p53 null PC-3 cells by a dose modification factor of 1.7. Cell cycle analysis of PC-3 cells treated with parthenolide showed only small alterations in G(1) and G(2)/M cells, and these appeared to be insufficient to explain the observed radiosensitization. Split-dose studies using clinically relevant 2- and 4-Gy fractions demonstrated that parthenolide completely inhibited split-dose repair in PC-3 cells. We hypothesized that inhibition of NF-kappa B activity by parthenolide was responsible for the observed X-ray sensitization and inhibition of split-dose repair. To test this hypothesis, we knocked down the expression of NF-kappa B p65 protein, an active component of NF-kappa B in both PC-3 and CGL1 cells, by siRNA. Inhibition of NF-kappa B activity by knockdown of p65 increased radiation sensitivity and completely inhibited split-dose repair in both cell lines in a nearly identical manner as parthenolide treatment alone. Treating p65-depleted PC-3 cells with 5 mu M parthenolide did not further increase their radiation sensitivity or the inhibition of split-dose repair. We propose that the suppression of radiation-induced NF-kappa B activity by parthenolide leads to X-ray sensitization through inhibition of split-dose repair in p53 null PC-3 prostate cancer cells. (C) 2009 by Radiation Research Society