A New Epigenetic Mechanism of Temozolomide Action in Glioma Cells

Abstract

Temozolomide (TMZ) is an oral alkylating chemotherapeutic agent that prolongs the survival of patients with glioblastoma (GBM). Despite that high TMZ potential, progression of disease and recurrence are still observed. Therefore a better understanding of the mechanism of action of this drug is necessary and may allow more durable benefit from its anti-glioma properties. Using nucleotide post-labelling method and separation on thin-layer chromatography we measured of global changes of 5-methylcytosine (m5C) in DNA of glioma cells treated with TMZ. Although m5C is not a product of TMZ methylation reaction of DNA, we analysed the effects of the drug action on different glioma cell lines through global changes at the level of the DNA main epigenetic mark. The first effect of TMZ action we observed is DNA hypermethylation followed by global demethylation. Therefore an increase of DNA methylation and down regulation of some genes expression can be ascribed to activation of DNA methyltransferases (DNMTs). On the other hand hypomethylation is induced by oxidative stress and causes uncontrolled expression of pathologic protein genes. The results of brain tumours treatment with TMZ suggest the new mechanism of modulation epigenetic marker in cancer cells. A high TMZ concentration induced a significant increase of m5C content in DNA in the short time, but a low TMZ concentration at longer time hypomethylation is observed for whole range of TMZ concentrations. Therefore TMZ administration with low doses of the drug and short time should be considered as optimal therapy.

Hypoxia-Mediated Epigenetic Regulation of Stemness in Brain Tumor Cells

“…Our study thus implies the pivotal role of hypoxia in maintenance of pluripotency in cancer cells, and further elaborates on the epigenetic mechanism activated in gliomas. With TMZ (the most effective drug for gliomas), being reported to alter the global methylation status of cancer cells , future work to elaborate upon the possible effects of TMZ on regulation of pluripotency markers in hypoxia would be pertinent. Also several unanswered questions need to be addressed like identifying effectors that mediate recruitment of TET proteins to the regulatory regions of pluripotency genes, differential regulation by TET paralogues, and functional implications of 5‐hmC gain at the regulatory regions of pluripotency genes.…”

In vitro single-strand DNA damage and cancer cell cytotoxicity effects of Temozolomide

“…The study is carried out to check DNA nicking on various DNA substrates like plasmids, bacterial and HeLa DNA with TMZ at various conditions that is at different concentrations and temperature. Earlier reports indicate that the concentration range of drug TMZ varied from 0.3 mM to 2.5 mM [25]. Results interpret the three forms of DNA as supercoiled DNA as the lower band, the nicked or relaxed DNA as the uppermost band and linearized DNA in the middle.…”

“…Hence, experiments are carried out at various time points using TMZ as DNA damaging drug and plasmid DNA substrate. The similar experimental methods are reported to use plasmid DNA substrate to establish nincking activity of anticancer drugs [25]. Literature suggests that prolonged exposure of TMZ is able to induce cellular stress and cytotoxicity [36].…”

“…However, it is also suggested that TMZ does not produce direct (frank) single strand breaks, they are observed in cells as incomplete BER, and in vitro as a product of slow DNA hydrolysis due to alkylation causing a weakening of the glycosidic bond [7,[20][21][22][23][24][25][26]. In BER pathway, DNA ligases are necessary to seal the gaps or single-stranded damage occurred in DNA.…”

A mathematical model of low grade gliomas treated with temozolomide and its therapeutical implications

“…During subsequent cycles of replication, the futile mismatch repair system is initiated and O 6 MeG is incorrectly paired with thymine instead of cytosine, eventually inducing cell death, see e.g. [13,14]. A phase III trial showed the efficacy of TMZ for high-grade gliomas [15] and since then it has been used routinely for patients with newly diagnosed glioblastoma (the most malignant type of glioma) [16].…”