Minimally cytotoxic doses of temozolomide produce radiosensitization in human glioblastoma cells regardless of MGMT expression1

• Published in: Molecular cancer therapeutics 2010-05, Vol.9 (5), p.1208-1218
• Main Authors: Bobola, Michael S., Kolstoe, Douglas D., Blank, A., Silber, John R.
• Format: Article
• Language: English
• ISSN: 1535-7163; 1538-8514
• DOI: 10.1158/1535-7163.MCT-10-0010

Concurrent treatment with the methylating agent temozolomide (TMZ) during radiotherapy (RT) has yielded the first significant improvement in survival of adult glioblastomas (GBMs) in the last three decades. However, improved survival is observed in a minority of patients, most frequently those whose tumors display CpG methylation of the MGMT (O 6 -methylguanine-DNA methyltransferase) promoter, and adult GBMs remain invariably fatal. Some, though not all, pre-clinical studies have shown that TMZ can increase radiosensitivity in GBM cells that lack MGMT, the sole activity in human cells that removes O 6 -meG from DNA. Here, we systematically examined the TMZ dose dependence of radiation killing in established GBM cell lines that differ in ability to remove O 6 -meG or tolerate its lethality. Our results show that minimally cytotoxic doses of TMZ can produce dose-dependent radiosensitization in MGMT-deficient cells, MGMT-proficient cells, and MGMT-deficient cells that lack mismatch repair, a process that renders cells tolerant of the lethality of O 6 -meG. In cells that either possess or lack MGMT activity, radiosensitization requires exposure to TMZ before but not after radiation, and is accompanied by formation of double-strand breaks within 45 min of radiation. Moreover, suppressing alkyladenine-DNA glycosylase, the only activity in human cells that excises 3-meA from DNA, reduces the TMZ dose dependence of radiosensitization, indicating that radiosensitization is mediated by 3-meA as well as by O 6 -meG. These results provide novel information on which to base further mechanistic study of radiosensitization by TMZ in human GBM cells, and to develop strategies to improve the outcome of concurrent TMZ-RT.