title: Combination effect of epigenetic regulation and ionizing radiation in colorectal cancer cells.
abstract: Exposure of cells to ionizing radiation (IR) induces, not only, activation of multiple signaling pathways that play critical roles in cell fate determination, but also alteration of molecular pathways involved in cell death or survival. Recently, DNA methylation has been established as a critical epigenetic process involved in the regulation of gene expression in cancer cells, suggesting that DNA methylation inhibition may be an effective cancer treatment strategy. Because alterations of gene expression by DNA methylation have been considered to influence radioresponsiveness, we investigated the effect of a DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine (5-aza-dC), on radiosensitivity. In addition, we investigated the underlying cellular mechanisms of combination treatments of ionizing irradiation (IR) and 5-aza-dC in human colon cancer cells. Colon cancer cell lines were initially tested for radiation sensitivity by IR in vitro and were treated with two different doses of 5-aza-dC. Survival of these cell lines was measured using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and clonogenic assays. The effects of 5-aza-dC along with irradiation on cell growth, cell cycle distribution, apoptosis, and apoptosis-related gene expression were examined. Combination irradiation treatment with 5-aza-dC significantly decreased growth activity compared with irradiation treatment alone or with 5-aza-dC treatment alone. The percentage of HCT116 cells in the sub-G1 phase and their apoptotic rate was increased when cells were treated with irradiation in combination with 5-aza-dC compared with either treatment alone. These observations were strongly supported by increased caspase activity, increased comet tails using comet assays, and increased protein levels of apoptosis-associated molecules (caspase 3/9, cleaved PARP). Our data demonstrated that 5-aza-dC enhanced radiosensitivity in colon cancer cells, and the combination effects of 5-aza-dC with radiation showed greater cellular effects than that of single treatment, suggesting that the combination of 5-aza-dC and radiation has the potential to become a clinical strategy for the treatment of cancer.
authors.affiliation: Research Center, Dongnam Institute of Radiological & Medical Sciences (DIRAMS), Busan, South Korea.
authors.forename: Joong-Gook
authors.initials: JG
authors.lastname: Kim
authors.affiliation: Research Center, Dongnam Institute of Radiological & Medical Sciences (DIRAMS), Busan, South Korea.
authors.forename: Jin-Han
authors.initials: JH
authors.lastname: Bae
authors.affiliation: Research Center, Dongnam Institute of Radiological & Medical Sciences (DIRAMS), Busan, South Korea.
authors.forename: Jin-Ah
authors.initials: JA
authors.lastname: Kim
authors.affiliation: Research Center, Dongnam Institute of Radiological & Medical Sciences (DIRAMS), Busan, South Korea.
authors.forename: Kyu
authors.initials: K
authors.lastname: Heo
authors.affiliation: Research Center, Dongnam Institute of Radiological & Medical Sciences (DIRAMS), Busan, South Korea; Department of Radiation Oncology, Korea Institute of Radiological and Medical Sciences, Seoul, Korea.
authors.forename: Kwangmo
authors.initials: K
authors.lastname: Yang
authors.affiliation: Research Center, Dongnam Institute of Radiological & Medical Sciences (DIRAMS), Busan, South Korea.
authors.forename: Joo Mi
authors.initials: JM
authors.lastname: Yi
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000235:genetics / Q000528:radiation effects; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism; D001374:Azacitidine / Q000031:analogs & derivatives* / Q000494:pharmacology; D045744:Cell Line, Tumor; D015179:Colorectal Neoplasms / Q000188:drug therapy* / Q000235:genetics / Q000378:metabolism / Q000532:radiotherapy*; D003131:Combined Modality Therapy / Q000379:methods; D019175:DNA Methylation / Q000187:drug effects / Q000235:genetics / Q000528:radiation effects; D000077209:Decitabine; D044127:Epigenesis, Genetic / Q000187:drug effects* / Q000235:genetics / Q000528:radiation effects*; D005260:Female; D016193:G1 Phase / Q000187:drug effects / Q000235:genetics / Q000528:radiation effects; D045325:HCT116 Cells; D006801:Humans; D051379:Mice; D016513:Mice, SCID; D011836:Radiation Tolerance / Q000187:drug effects / Q000235:genetics; D011839:Radiation, Ionizing
chemical_list: D051017:Apoptosis Regulatory Proteins; D000077209:Decitabine; D001374:Azacitidine