Abstract
Although methyltransferase has been recognized as a major element that governs the epigenetic regulation of the genome during temozolomide (TMZ) chemotherapy in glioblastoma multiforme (GBM) patients, its regulatory effect on glioblastoma chemoresistance has not been well defined. This study investigated whether DNA methyltransferase (DNMT) expression was associated with TMZ sensitivity in glioma cells and elucidated the underlying mechanism. DNMT expression was analyzed by western blotting. miR-20a promoter methylation was evaluated by methylation-specific PCR. Cell viability and apoptosis were assessed using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) and TdT-mediated dUTP-biotin nick end labeling assays, respectively. The results showed that compared with parental U251 cells, DNMT1 expression was downregulated, miR-20a promoter methylation was attenuated and miR-20a levels were elevated in TMZ-resistant U251 cells. Methyltransferase inhibition by 5-aza-2′-deoxycytidine treatment reduced TMZ sensitivity in U251 cells. In U251/TM cells, DNMT1 expression was negatively correlated with miR-20a expression and positively correlated with TMZ sensitivity and leucine-rich repeats and immunoglobulin-like domains 1 expression; these effects were reversed by changes in miR-20a expression. DNMT1 overexpression induced an increase in U251/TM cell apoptosis that was inhibited by the miR-20a mimic, whereas DNMT1 silencing attenuated U251/TM cell apoptosis in a manner that was abrogated by miR-20a inhibitor treatment. Tumor growth of the U251/TM xenograft was inhibited by pcDNA-DNMT1 pretreatment and boosted by DNMT1-small hairpin RNA pretreatment. In summary, DNMT1 mediated chemosensitivity by reducing methylation of the microRNA-20a promoter in glioma cells.
Highlights
Glioblastoma multiforme (GBM) is the most common malignant primary brain tumor in adults and is one of the most aggressive human tumors
This study investigated whether DNA methyltransferase (DNMT) expression was associated with TMZ sensitivity in glioma cells and elucidated the underlying mechanism
The results showed that compared with parental U251 cells, DNMT1 expression was downregulated, miR-20a promoter methylation was attenuated and miR-20a levels were elevated in TMZ-resistant U251 cells
Summary
Glioblastoma multiforme (GBM) is the most common malignant primary brain tumor in adults and is one of the most aggressive human tumors. Its therapeutic schemes represent a difficult problem for patients. At present, alkylating agents are the most popular and effective drugs for GBM chemotherapy. A bioavailable imidazotetrazine derivative of dacarbazine (temozolomide; TMZ) penetrates the blood–brain barrier[1] and has been demonstrated to possess broadspectrum antitumor activity chemotherapeutic following oral administration. TMZ can efficiently inhibit the proliferation of glioma cells and induce apoptosis.[2] In systemic circulation, TMZ undergoes rapid chemical decomposition to its active compound MTIC(5-(3-methyltriazen-1-yl) imidazole-4-carboximide) and subsequently causes guanine methylation at the O6-position.[3] In turn, this modification yields DNA-
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