Brca2/Xrcc2 dependent HR, but not NHEJ, is required for protection against O6-methylguanine triggered apoptosis, DSBs and chromosomal aberrations by a process leading to SCEs

Abstract O 6 -methylguanine (O 6 MeG) is a highly critical DNA adduct induced by methylating carcinogens and anticancer drugs such as temozolomide, streptozotocine, procarbazine and dacarbazine. Induction of cell death by O 6 MeG lesions requires mismatch repair (MMR) and cell proliferation and is thought to be dependent on the formation of DNA double-strand breaks (DSBs) or, according to an alternative hypothesis, direct signaling by the MMR complex. Given a role for DSBs in this process, either homologous recombination (HR) or non-homologous end joining (NHEJ) or both might protect against O 6 MeG. Here, we compared the response of cells mutated in HR and NHEJ proteins to temozolomide and…

MGMT: Key node in the battle against genotoxicity, carcinogenicity and apoptosis induced by alkylating agents

O(6)-methylguanine-DNA methyltransferase (MGMT) plays a crucial role in the defense against alkylating agents that generate, among other lesions, O(6)-alkylguanine in DNA (collectively termed O(6)-alkylating agents [O(6)AA]). The defense is highly important, since O(6)AA are common environmental carcinogens, are formed endogenously during normal cellular metabolism and possibly inflammation, and are being used in cancer therapy. O(6)AA induced DNA damage is subject to repair, which is executed by MGMT, AlkB homologous proteins (ABH) and base excision repair (BER). Although this review focuses on MGMT, the mechanism of repair by ABH and BER will also be discussed. Experimental systems, in wh…

Temozolomide- and fotemustine-induced apoptosis in human malignant melanoma cells: response related to MGMT, MMR, DSBs, and p53

Malignant melanomas are highly resistant to chemotherapy. First-line chemotherapeutics used in melanoma therapy are the methylating agents dacarbazine (DTIC) and temozolomide (TMZ) and the chloroethylating agents BCNU and fotemustine. Here, we determined the mode of cell death in 11 melanoma cell lines upon exposure to TMZ and fotemustine. We show for the first time that TMZ induces apoptosis in melanoma cells, using therapeutic doses. For both TMZ and fotemustine apoptosis is the dominant mode of cell death. The contribution of necrosis to total cell death varied between 10 and 40%. The O(6)-methylguanine-DNA methyltransferase (MGMT) activity in the cell lines was between 0 and 1100 fmol m…

DNA repair in defence against genotoxin-induced apoptosis

Apoptosis in malignant glioma cells triggered by the temozolomide-induced DNA lesion O6-methylguanine

Methylating drugs such as temozolomide (TMZ) are widely used in the treatment of brain tumours (malignant gliomas). The mechanism of TMZ-induced glioma cell death is unknown. Here, we show that malignant glioma cells undergo apoptosis following treatment with the methylating agents N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and TMZ. Cell death determined by colony formation and apoptosis following methylation is greatly stimulated by p53. Transfection experiments with O(6)-methylguanine-DNA methyltransferase (MGMT) and depletion of MGMT by O(6)-benzylguanine showed that, in gliomas, the apoptotic signal originates from O(6)-methylguanine (O(6)MeG) and that repair of O(6)MeG by MGMT prevent…