title: Zerumbone increases oxidative stress in a thiol-dependent ROS-independent manner to increase DNA damage and sensitize colorectal cancer cells to radiation.
abstract: Locally advanced rectal cancers are treated with neoadjuvant chemoradiation therapy followed by surgery. In a minority (~20%) of patients, no tumor is present at the time of surgery; these patients with a complete pathologic response (pathCR) to neoadjuvant therapy have better treatment outcomes. Unfortunately, the inherent radioresistance of colorectal cancer (CRC) cells dictates that the majority of patients do not achieve a pathCR. Efforts to improve these odds have fueled the search for novel, relatively less-toxic radiosensitizers with distinct molecular mechanism(s) and broad-spectrum anticancer activities. Here, we use zerumbone, a sesquiterpene from the edible ginger (Zingiber zerumbet Smith), to enhance radiosensitivity of CRC cells. Short exposure to zerumbone (7 h) profoundly sensitized CRC cells, independent of their p53 or k-RAS status. Zerumbone enhanced radiation-induced cell cycle arrest (G2/M), increased radiation-induced apoptosis, but induced little apoptosis by itself. Zerumbone significantly enhanced radiation-induced DNA damage, as evident by delayed resolution of post-irradiation nuclear γH2AX foci, whereas zerumbone treatment alone did not induce γH2AX foci formation. Zerumbone pretreatment inhibited radiation-induced nuclear expression of DNA repair proteins ataxia-telangiectasia mutated (ATM) and DNA-PKcs. Interestingly, zerumbone-mediated radiosensitization did not involve reactive oxygen species (ROS), but was mediated through depletion of cellular glutathione (GSH). Ability of only thiol-based antioxidants to abrogate zerumbone-mediated radiosensitization further corroborated this hypothesis. The α,β-unsaturated carbonyl group in zerumbone was found to be essential for its bioactivity as zerumbone analog α-Humulene that lacks this functional group, could neither radiosensitize CRC cells, nor deplete cellular GSH. Our studies elucidate novel mechanism(s) of zerumbone's ability to enhance CRC radiosensitivity.
authors.affiliation: Department of Experimental Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, Texas, 77030.
authors.forename: Amit
authors.initials: A
authors.lastname: Deorukhkar
authors.forename: Niharika
authors.initials: N
authors.lastname: Ahuja
authors.forename: Armando-Lopez
authors.initials: AL
authors.lastname: Mercado
authors.forename: Parmeswaran
authors.initials: P
authors.lastname: Diagaradjane
authors.forename: Uma
authors.initials: U
authors.lastname: Raju
authors.forename: Nalini
authors.initials: N
authors.lastname: Patel
authors.forename: Pranshu
authors.initials: P
authors.lastname: Mohindra
authors.forename: Nga
authors.initials: N
authors.lastname: Diep
authors.forename: Sushovan
authors.initials: S
authors.lastname: Guha
authors.forename: Sunil
authors.initials: S
authors.lastname: Krishnan
mesh_terms: D017209:Apoptosis; D002453:Cell Cycle / Q000187:drug effects / Q000528:radiation effects; D018797:Cell Cycle Proteins / Q000378:metabolism; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects / Q000528:radiation effects; D015179:Colorectal Neoplasms / Q000235:genetics* / Q000378:metabolism*; D004249:DNA Damage / Q000187:drug effects / Q000528:radiation effects; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000528:radiation effects; D005978:Glutathione / Q000378:metabolism*; D045325:HCT116 Cells; D019073:HT29 Cells; D006801:Humans; D018384:Oxidative Stress / Q000187:drug effects; D011838:Radiation-Sensitizing Agents / Q000737:chemistry / Q000494:pharmacology*; D017382:Reactive Oxygen Species / Q000378:metabolism*; D012717:Sesquiterpenes / Q000737:chemistry / Q000494:pharmacology*
chemical_list: D018797:Cell Cycle Proteins; D011838:Radiation-Sensitizing Agents; D017382:Reactive Oxygen Species; D012717:Sesquiterpenes; C403304:zerumbone; D005978:Glutathione
keywords: Colorectal cancer; DNA repair; glutathione; radiation; sesquiterpene