DNMT (DNA methyltransferase) inhibitors radiosensitize human cancer cells by suppressing DNA repair activity

Abstract

Background Histone modifications and DNA methylation are two major factors in epigenetic phenomenon. Unlike the histone deacetylase inhibitors, which are known to exert radiosensitizing effects, there have only been a few studies thus far concerning the role of DNA methyltransferase (DNMT) inhibitors as radiosensitizers. The principal objective of this study was to evaluate the effects of DNMT inhibitors on the radiosensitivity of human cancer cell lines, and to elucidate the mechanisms relevant to that process. Methods A549 (lung cancer) and U373MG (glioblastoma) cells were exposed to radiation with or without six DNMT inhibitors (5-azacytidine, 5-aza-2'-deoxycytidine, zebularine, hydralazine, epigallocatechin gallate, and psammaplin A) for 18 hours prior to radiation, after which cell survival was evaluated via clonogenic assays. Cell cycle and apoptosis were analyzed via flow cytometry. Expressions of DNMT1, 3A/3B, and cleaved caspase-3 were detected via Western blotting. Expression of γH2AX, a marker of radiation-induced DNA double-strand break, was examined by immunocytochemistry. Results Pretreatment with psammaplin A, 5-aza-2'-deoxycytidine, and zebularine radiosensitized both A549 and U373MG cells. Pretreatment with psammaplin A increased the sub-G1 fraction of A549 cells, as compared to cells exposed to radiation alone. Prolongation of γH2AX expression was observed in the cells treated with DNMT inhibitors prior to radiation as compared with those treated by radiation alone. Conclusions Psammaplin A, 5-aza-2'-deoxycytidine, and zebularine induce radiosensitivity in both A549 and U373MG cell lines, and suggest that this effect might be associated with the inhibition of DNA repair.

Radiation-induced epigenetic DNA methylation modification of radiation-response pathways

“…Epigenetic therapies targeting DNA methylation are under study and have been shown to sensitize cells to radiation. 4,[15][16][17] Treatment of cells with the DNA methyltransferase inhibitors psammaplin A or zebularine partially abrogated the G 2 phase arrest, increased apoptosis, and reduced the ability to repair DNA double strand breaks in cells exposed to 6 Gy radiation. 4 The DNA damage response is complex and involves changes to the cell cycle, activation of DNA repair pathways, and induction of apoptosis or, alternatively, senescence.…”

“…4,[15][16][17] Treatment of cells with the DNA methyltransferase inhibitors psammaplin A or zebularine partially abrogated the G 2 phase arrest, increased apoptosis, and reduced the ability to repair DNA double strand breaks in cells exposed to 6 Gy radiation. 4 The DNA damage response is complex and involves changes to the cell cycle, activation of DNA repair pathways, and induction of apoptosis or, alternatively, senescence. Although it has been previously shown that irradiation affects global changes in DNA methylation, it was not known whether these changes were enriched in pathways directly involved in the radiation response.…”

“…3 Other evidence suggesting DNA methylation plays a role in radiation resistance have been studies showing treatment with 5-azacitidine to reduce global methylation has a radiosensitizing effect. 4,5 DNA methylation profiling technology with the Illumina HumanMethylation450 array allows us to query >450 000 loci within the genome and to cover 99% of RefSeq genes. 6 When we analyzed the number of genes showing altered DNA methylation at each time point and radiation dose, there was significant overlap between doses at each time point (P < 0.05; Fig.…”

Epigenetic therapy approaches in non-small cell lung cancer: Update and perspectives

“…46 Several proven or potential DNMTi (azacitidine, decitabine, zebularine, hydralazine, epigallocatechin gallate, and psammaplin A) caused radiosensitization in the NSCLC cell line A549. 47 …”

Epigenetic mechanisms in cancer: push and pull between kneaded erasers and fate writers

“…For example, psammaplin A, isolated from Psammaplysilla sponge, improved radiation-induced cell destruction in astrocytoma U373MG and lung cancer A549 cell lines. 67 Both DNMT3A and DNMT1 were depleted in U373MG and A549 cancer cells after treatment with either psammaplin A or zebularine. 67 Other natural products, including cyclopamine, curcumin, epigallocatechin 3-gallate, genistein, resveratrol, zerumbone, norcantharidin, and arsenic trioxide have been well reviewed for their ability to target SHH signaling.…”