Celastrol potentiates radiotherapy by impairment of DNA damage processing in human prostate cancer.
OBJECTIVE
Celastrol is an active ingredient of traditional herbal medicine and has recently been identified as a potent natural proteasome inhibitor. In the present study, we evaluated the radiosensitizing potential of celastrol in the human prostate cancer PC-3 model.


METHODS
Clonogenic assays were performed to determine the radiosensitizing effect of celastrol. Apoptosis was examined by flow cytometry using Annexin V and propidium iodide staining and by a caspase-3 activation assay. DNA damage processing was examined by immunofluorescent staining and Western blot for phosphorylated H2AX (gammaH2AX). The PC-3 xenograft model in the athymic nude mouse was used for the determination of the in vivo efficacy of celastrol combined with radiotherapy. The tumor samples were also analyzed for apoptosis and angiogenesis.


RESULTS
Celastrol sensitized PC-3 cells to ionizing radiation (IR) in a dose- and schedule-dependent manner, in which pretreatment with celastrol for 1 h followed by IR achieved maximal radiosensitization. Celastrol significantly prolonged the presence of IR-induced gammaH2AX and increased IR-induced apoptosis. Celastrol, combined with fractionated radiation, significantly inhibited PC-3 tumor growth in vivo without obvious systemic toxicity. The combination treatment increased gammaH2AX levels and apoptosis, induced cleavage of poly(adenosine diphosphate-ribose)polymerase and Mcl-1, and reduced angiogenesis in vivo compared with either treatment alone.


CONCLUSIONS
Celastrol sensitized PC-3 cells to radiation both in vitro and in vivo by impairing DNA damage processing and augmenting apoptosis. Celastrol might represent a promising new adjuvant regimen for the treatment of hormone-refractory prostate cancer.
Dai
Yao
Y
Department of Radiation Oncology, Comprehensive Cancer Center, University of Michigan, Ann Arbor, MI 48109-5637, USA.
DeSano
Jeffrey T
JT
Meng
Yang
Y
Ji
Qing
Q
Ljungman
Mats
M
Lawrence
Theodore S
TS
Xu
Liang
L
D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D053148:Caspase 3 / Q000378:metabolism; D045744:Cell Line, Tumor; D004249:DNA Damage / Q000187:drug effects*; D004307:Dose-Response Relationship, Radiation; D004789:Enzyme Activation; D005260:Female; D005434:Flow Cytometry; D006657:Histones / Q000378:metabolism; D006801:Humans; D008297:Male; D051379:Mice; D008819:Mice, Nude; D009389:Neovascularization, Pathologic / Q000517:prevention & control; D053978:Pentacyclic Triterpenes; D011471:Prostatic Neoplasms / Q000098:blood supply / Q000235:genetics / Q000532:radiotherapy*; D011838:Radiation-Sensitizing Agents / Q000627:therapeutic use*; D011897:Random Allocation; D014315:Triterpenes / Q000627:therapeutic use*; D023041:Xenograft Model Antitumor Assays / Q000379:methods
C468783:H2AX protein, human; D006657:Histones; D053978:Pentacyclic Triterpenes; D011838:Radiation-Sensitizing Agents; D014315:Triterpenes; D053148:Caspase 3; C050414:celastrol