Repair gene O⁶‐methylguanine‐DNA methyltransferase is controlled by SP1 and up‐regulated by glucocorticoids, but not by temozolomide and radiation

Abstract

Therapy of malignant glioma relies on treatment with the O -methylating agent temozolomide (TMZ) concomitant with ionizing radiation followed by adjuvant TMZ. For the treatment of recurrences, DNA chloroethylating drugs are also used. The main killing lesion induced by these drugs is O -alkylguanine. Since this damage is repaired by O -methylguanine-DNA methyltransferase (MGMT), the repair enzyme represents a most important factor of drug resistance, limiting the therapy of malignant high-grade gliomas. Although MGMT has been shown to be transcriptionally up-regulated in rodents following genotoxic stress, it is still unclear whether human MGMT is subject to up-regulation. Here, we addressed the question whether MGMT in glioma cells is enhanced following alkylating drugs or ionizing radiation, using promoter assays. We also checked the response of glioma cell lines to dexamethasone. In a series of experiments, we found no evidence that the human MGMT promoter is significantly up-regulated following treatment with TMZ, the chloroethylating agent nimustine or radiation. It was activated, however, by dexamethasone. Using deletion constructs, we further show that the basal level of MGMT is mainly determined by the transcription factor SP1. The high amount of SP1 sites in the MGMT promoter likely prevents transcriptional up-regulation following genotoxic stress by neutralizing inducible signals. The regulation of MGMT by miRNAs plays only a minor role, as shown by DICER knockdown experiments. Since high dose dexamethasone concomitant with temozolomide is frequently used in glioblastoma therapy, induction of the MGMT gene through glucocorticoids in MGMT promoter unmethylated cases might cause further elevation of drug resistance, while radiation and alkylating drugs seem not to induce MGMT at transcriptional level.

Mifepristone Overcomes Tumor Resistance to Temozolomide Associated with DNA Damage Repair and Apoptosis in an Orthotopic Model of Glioblastoma

“…It has recently been documented that MGMT expression is associated with SP1 expression in glioma cell lines. Since knockdown of SP1 strongly reduced MGMT protein expression, SP1 is one of the main factors regulating MGMT [34]. MGMT expression also depends on p53, a transcription factor that sequesters SP1 and prevents its binding to MGMT [34].…”

“…Since knockdown of SP1 strongly reduced MGMT protein expression, SP1 is one of the main factors regulating MGMT [34]. MGMT expression also depends on p53, a transcription factor that sequesters SP1 and prevents its binding to MGMT [34]. Additionally, there is evidence of mifepristone increasing apoptosis due to p53 activation in several cancer cell lines [35,36].…”

Antagonism of Dexamethasone to Radiation Bystander Effect and Its Re-transmission

“…For example, in the treatment of glioblastoma, dexamethasone can reduce the efficacy of radiotherapy. Aasland D found that radiotherapy could not activate the MGMT promoter of glioblastoma, while dexamethasone induced MGMT gene in cases of unmethylation of MGMT promoter, and may lead to further increase in temozolomide resistance, and reduce the effect of radiotherapy [11]. Luedi MM reported that dexamethasone reduced temozolomide-induced apoptosis in glioma stem cells and reduced radiation sensitivity.…”

O6-Methylguanine-DNA Methyltransferase (MGMT): Challenges and New Opportunities in Glioma Chemotherapy

“…Boldogh et al analyzed the mechanism of human MGMT expression induction, they found that protein kinase C-mediated signaling played an important role, involving activation of AP-1 sites on MGMT promoter by TPA (55). Aasland et al further identified that human MGMT promoter can be induced by glucocorticoids, but not by genotoxic stress, in human malignant glioblastoma cells (56). They put forward that a cluster of SP1 sites in human MGMT promoter prevented transcriptional up-graduation and overshadowed activation signals from other weaker transcriptional factors.…”