2688

Abstract

Parthenolide, a major active component of the traditional herbal medicine, feverfew, has been shown to have anti-inflammatory, anti-tumor properities and increases tumor sensitivity to radiation. However, how parthenolide enhances tumor sensitivity to radiation therapy are unknown. PC3 human prostate cancer cells and PZ-HPV-7 human papilloma virus 18 (HPV-18) transformed non-tumorigenic human prostate epithelial cells were used in the studies described. Cellular survival was assessed by either clonogenic end-points or MTT assay. Western blot analysis was employed to determine the protein levels of manganese superoxide dismutase (MnSOD) and Bcl-Xl compared to actin. MRNA levels of MnSOD and Bcl-Xl were assessed by mRNA isolation and RT-PCR. DNA binding of nuclear factor kappa B (NFkB) family members were analyzed by electrophoretic mobility shift assay. Chromatin immunoprecipitation assay (ChIP) was used to directly identify the regulatory region of the MnSOD gene occupied by NF-kB family members. In this study, we demonstrate that parthenolide sensitizes prostate cancer PC3 cells, but not non-tumorigenic prostate epithelial PZ-HPV-7 cells, to radiation. Radiation induces mRNA and protein levels of Bcl-Xl and MnSOD, two anti-apoptotic genes in PC3 cells, but only induces Bcl-Xl in PZ-HPV-7 cells. The presence of parthenolide suppresses Bcl-Xl and MnSOD induction by radiation in PC3 cells but not in PZ-HPV-7 cells. We further demonstrate that parthenolide inhibits radiation-induced NF-kB activation in PC3 cells but not in the PZ-HPV-7 cells. The effect of parthenolide on NF-kB is upstream of the nuclear translocation of NF-kB family members because when radiation-induced increase of IkB kinase complex (IKC) activity is inhibited in PC3 cells, radiation-induced NF-kB DNA binding activity is also inhibited. Parthenolide inhibits radiation-induced binding of RelA and RelB to the MnSOD intronic enhancer (I2E) region, confirming that parthenolide suppresses both the classical and alternative pathways of NF-kB activation. Together, these results demonstrate that inhibition of NF-kB activation and its subsequent transcriptional function on MnSOD expression in prostate cancer cells is, in part responsible for the selective radiosensitization effect of parthenolide.