Histone deacetylase (HDAC) inhibitors are undergoing clinical evaluation for cancer therapy. Because HDAC modulates chromatin structure and gene expression, parameters considered to influence radioresponse, we have investigated the effects of the HDAC inhibitor MS-275 on the radiosensitivity of two human tumor cell lines (DU145 prostate carcinoma and U251 glioma). Acetylation status of histones H3 and H4 was determined as a function of time after MS-275 addition to and removal from culture medium. Histone acetylation increased by 6 h after MS-275 addition, reaching a maximum between 24 and 48 h of exposure; providing fresh drug-free medium then resulted in a decrease in histone acetylation that began by 6 h and approached untreated levels by 16 h. Treatment of cells with MS-275 for 48 h followed by irradiation had little or no effect on radiation-induced cell death. However, exposure to MS-275 before and after irradiation resulted in an increase in radiosensitivity with dose enhancement factors of 1.9 and 1.3 for DU145 and U251 cells, respectively. This MS-275 treatment protocol did not result in a redistribution of the cells into a more radiosensitive phase of the cell cycle or in an increase in apoptosis. However, MS-275 did modify the time course of ␥H2AX expression in irradiated cells. Whereas there was no significant difference in radiation-induced ␥H2AX foci at 6 h, the number of cells expressing ␥H2AX foci was significantly greater in the MS-275-treated cells at 24 h after irradiation. These results indicate that MS-275 can enhance radiosensitivity and suggest that this effect may involve an inhibition of DNA repair.
Valproic acid (VA) is a well-tolerated drug used to treat seizure disorders and has recently been shown to inhibit histone deacetylase (HDAC). Because HDAC modulates chromatin structure and gene expression, parameters considered to influence radioresponse, we investigated the effects of VA on the radiosensitivity of human brain tumor cells grown in vitro and in vivo. The human brain tumor cell lines SF539 and U251 were used in our study. Histone hyperacetylation served as an indicator of HDAC inhibition. The effects of VA on tumor cell radiosensitivity in vitro were assessed using a clonogenic survival assay and ␥H2AX expression was determined as a measure of radiation-induced DNA double strand breaks. The effect of VA on the in vivo radioresponse of brain tumor cells was evaluated according to tumor growth delay analysis carried out on U251 xenografts. Irradiation at the time of maximum VA-induced histone hyperacetylation resulted in significant increases in the radiosensitivity of both SF539 and U251 cells. Key words: valproic acid; radiosensitization; histone deacetylase; in vivo; murine Valproic acid (VA), an 8-carbon branched-chained fatty acid, has well-established efficacy in the treatment of epilepsy and other seizure disorders. 1,2 Its broad-spectrum anticonvulsant activity has been suggested to result from a combination of mechanisms including the increase in ␥-aminobutyric acid, a decrease in ␥-hydroxybutyric acid and a direct interaction with the neuronal membrane blocking voltage dependent sodium channels. 3 Because of its effectiveness, oral bioavailability and generally low toxicity profile VA has been frequently used as a chronic anti-epileptic therapy. 4 Whereas generally free of other major side effects, VA is teratogenic in humans and rodents. In each case administration of VA during pregnancy can result in major malformations, dysmorphic syndromes and an estimated risk of neural tube defects of 1-3%. 5 Structure activity studies using VA and VA-related compounds, however, showed that the teratogenic effects could be separated from the anticonvulsant actions. 6,7 Moreover, the teratogenic activity of VA was attributed to reduced rate of cell proliferation resulting in a disruption in neuroepithelial-mesenchymal interactions. 8 Attempts to define the mechanism responsible for the neural tube defects led to in vitro studies using neuroblastoma cell lines, which showed that VA inhibited cell proliferation and induced morphological differentiation. 9 Subsequent investigations supported a potential anti-cancer action with VA reported to inhibit the proliferation of a variety of human tumor cells grown in vitro and as in vivo xenografts. 10 -12 Valproic acid was shown recently to be an effective inhibitor of histone deacetylase (HDAC). 6,11 Using VA and VA analogs, Phiel et al. 6 showed that the teratogenic but not the anti-epileptic activity of VA is likely due to HDAC inhibition. Moreover, their results along with previous reports 11 indicate that the anti-tumor effects of VA are likely the resu...
Aberrant DNA hypermethylation is a frequent finding in tumor cells, which has suggested that inhibition of DNA methylation may be an effective cancer treatment strategy. Because DNA methylation affects gene expression and chromatin structure, parameters considered to influence radioresponse, we investigated the effects of the DNA methylation inhibitor zebularine on the radiosensitivity of human tumor cells. Three human tumor cell lines were used in this study (MiaPaCa, DU145, and U251) and the methylation status of three genes frequently hypermethylated in tumor cells (RASSF1A, HIC-1, and14-3-3r) was determined as a function of zebularine exposure. Zebularine resulted in DNA demethylation in a time-dependent manner, with the maximum loss of methylation detected by 48 hours. Treatment of cells with zebularine for 48 hours also resulted in an increase in radiosensitivity with dose enhancement factors of >1.5. As a measure of radiation-induced DNA damage, gH2AX expression was determined.Whereas zebularine had no effect on radiation-induced gH2AX foci at 1hour, the number of gH2AX foci per cell was significantly greater in the zebularine-treated cells at 24 hours after irradiation, suggesting the presence of unrepaired DNA damage. Zebularine administration to mice reactivated gene expression in U251xenografts; irradiation of U251tumors in mice treated with zebularine resulted in an increase in radiation-induced tumor growth delay.These results indicate that zebularine can enhance tumor cell radiosensitivity in vitro and in vivo and suggest that this effect may involve an inhibition of DNA repair.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.