gms | German Medical Science

Objective: In the management of malignant glioma, the resistance to chemotherapeutic agents by the tumor cells could be a big problem. Although temozolomide (TMZ) has been used as a major compound in the treatment for this kind of tumor, the average survival time has been extended only a few months. While O6-methylguanine-DNA methyltransferase (MGMT) is a well-known TMZ resistance mechanism, some of the glioblastomas shows low MGMT expression. Even glioblastomas with decreased MGMT expression come to acquire resistance to TMZ, elucidation of the mechanism of TMZ resistance caused by factors other than MGMT is therefore necessary. Previous studies have shown that TMZ induces prolonged arrest of human glioma cells in the G2/M phase of the cell cycle followed by a senescence-like phenomenon or mitotic catastrophe. These findings suggest that the G2 checkpoint is linked to DNA repair mechanisms. In the present study, based on the above stated background, we aimed to elucidate the mechanism of TMZ resistance in glioblastomas with low MGMT expression and thereby to solve this problem.

Methods: We generated TMZ-resistant (TR) clones with low MGMT expression by repeatedly administrating TMZ to human glioma U87MG cells. We evaluated TMZ-induced cell cycle arrest, expression of MSH6 which is a key component of mismatch repair system (MMR), and effect of Chk1- and Cdc2-inhibitors in TR-clones.

Results: While TR clones obtained after 2-3 months exposure to TMZ (TR-s) underwent transient G2 arrest after TMZ treatment, longer (4-6 months) exposure to TMZ enriched the proportion of TR-clones that underwent only minimal G2 arrest following TMZ treatment (TR-l). The expression of p-chk1, p-chk2, and p-cdc2, which are key event in TMZ-induced G2 arrest, were not observed after TMZ treatment in TR-l clones. MGMT was not detectable, however, MSH6, a major MMR component, was reduced in TR-l clones. None of Chk1- and Cdc2- inhibitors could resensitize TR-l clones to TMZ.

Conclusion: Our result suggested that longer drug treatment may induce the development of cells highly resistant to DNA damaging agent(s) by means of MMR modification. Further investigation on the mechanism of acquired resistance to TMZ by glioma cells could provide a key to developing rational drug treatment. It has been shown that the development of new treatments for MMR-deficient tumors is required, on the other hand, in order to achieve great treatment results, it might be important to intensify treatment before the tumor develops a MMR deficiency.