Enhanced anti-tumor effect of zoledronic acid combined with temozolomide against human malignant glioma cell expressing O6-methylguanine DNA methyltransferase.

Junya Fukai,Fumiaki Koizumi,Naoyuki Nakao

doi:10.1371/journal.pone.0104538

Junya Fukai, Fumiaki Koizumi + Show 1 more

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https://doi.org/10.1371/journal.pone.0104538

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Abstract

Temozolomide (TMZ), a DNA methylating agent, is widely used in the adjuvant treatment of malignant gliomas. O6-methylguanine-DNA methyltranferase (MGMT), a DNA repair enzyme, is frequently discussed as the main factor that limits the efficacy of TMZ. Zoledronic acid (ZOL), which is clinically applied to treat cancer-induced bone diseases, appears to possess direct anti-tumor activity through apoptosis induction by inhibiting mevalonate pathway and prenylation of intracellular small G proteins. In this study, we evaluated whether ZOL can be effectively used as an adjuvant to TMZ in human malignant glioma cells that express MGMT. Malignant glioma cell lines, in which the expression of MGMT was detected, did not exhibit growth inhibition by TMZ even at a longer exposure. However, combination experiment of TMZ plus ZOL revealed that a supra-additive effect resulted in a significant decrease in cell growth. In combined TMZ/ZOL treatment, an increased apoptotic rate was apparent and significant activation of caspase-3 and cleavage of poly-(ADP-ribose) polymerase were observed compared with each single drug exposure. There were decreased amounts of Ras-GTP, MAPK and Akt phosphorylation and MGMT expression in the ZOL-treated cells. Subcutanous xenograft models showed significant decrease of tumor growth with combined TMZ/ZOL treatment. These results suggest that ZOL efficaciously inhibits activity of Ras in malignant glioma cells and potentiates TMZ-mediated cytotoxicity, inducing growth inhibition and apoptosis of malignant glioma cells that express MGMT and resistant to TMZ. Based on this work, combination of TMZ with ZOL might be a potential therapy in malignant gliomas that receive less therapeutic effects of TMZ due to cell resistance.

Highlights

Glioblastoma multiforme (GBM) is the frequent form of malignant glioma, the most common primary brain tumor, and is characterized by poor prognosis
Stupp et al revealed a statistically significant survival benefit for GBM patients treated with radiotherapy (RT) plus concomitant and adjuvant temozolomide (TMZ) chemotherapy, which currently represents the standard of care for newly diagnosed GBM patients [1,2,3]
T98G and LN-18 did not exhibit significant growth inhibition with less than 100 mM TMZ, as clinically achievable concentration, even at a longer exposure, whereas the 50% growth inhibition was reached at 72 hours on LN-229, in which methylguanineDNA methyltransferase (MGMT) was not expressed (Fig. 1D)

Summary

Introduction

Glioblastoma multiforme (GBM) is the frequent form of malignant glioma, the most common primary brain tumor, and is characterized by poor prognosis. Methylation of the O6 position of guanine (O6MeG) will activate mismatch repair (MMR) mechanisms and DNA damage signaling pathways, leading to G2/M cell cycle arrest and eventually to induction of cell death [4,5,6]. O6MeG lesions can be rapidly repaired by the cellular DNA-repair protein O6-methylguanineDNA methyltransferase (MGMT) which is expressed in about 50% of GBM patients [4]. Through this mechanism, MGMT expression can cause TMZ resistance in tumor cells; otherwise, the loss of MMR system should be considered as the other mode of TMZ tolerance in GBM [7,8].

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