Suppression of NF-kappaB 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-kappaB 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-kappaB. The addition of 5 microM 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-kappaB 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 G1 and G2/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-kappaB 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-kappaB p65 protein, an active component of NF-kappaB in both PC-3 and CGL1 cells, by siRNA. Inhibition of NF-kappaB 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 microM parthenolide did not further increase their radiation sensitivity or the inhibition of split-dose repair. We propose that the suppression of radiation-induced NF-kappaB activity by parthenolide leads to X-ray sensitization through inhibition of split-dose repair in p53 null PC-3 prostate cancer cells.
Watson
Christopher
C
Department of Radiation Oncology, Indiana University School of Medicine, Indianapolis, Indiana 46202, USA.
Miller
Douglas A
DA
Chin-Sinex
Helen
H
Losch
Adam
A
Hughes
William
W
Sweeney
Christopher
C
Mendonca
Marc S
MS
D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000494:pharmacology; D002453:Cell Cycle; D045744:Cell Line, Tumor; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D008297:Male; D008954:Models, Biological; D016328:NF-kappa B / Q000378:metabolism*; D011471:Prostatic Neoplasms / Q000188:drug therapy* / Q000473:pathology*; D034741:RNA, Small Interfering / Q000378:metabolism; D011836:Radiation Tolerance; D012717:Sesquiterpenes / Q000494:pharmacology*; D051996:Transcription Factor RelA / Q000378:metabolism; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*; D014965:X-Rays
D000894:Anti-Inflammatory Agents, Non-Steroidal; D016328:NF-kappa B; D034741:RNA, Small Interfering; D012717:Sesquiterpenes; C495901:TP53 protein, human; D051996:Transcription Factor RelA; D016159:Tumor Suppressor Protein p53; C002669:parthenolide