Role of MGMT in protecting against cyclophosphamide-induced toxicity in cells and animals

Abstract

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that protects cells from the biological consequences of alkylating agents by removing alkyl groups from the O(6)-position of guanine. Cyclophosphamide and ifosfamide are oxazaphosphorines used clinically to treat a wide variety of cancers; however, the role of MGMT in recognizing DNA damage induced by these agents is unclear. In vitro evidence suggests that MGMT may protect against the urotoxic oxazaphosphorine metabolite, acrolein. Here, we demonstrate that Chinese hamster ovary cells transfected with MGMT are protected against cytotoxicity following treatment with chloroacetaldehyde (CAA), a neuro- and nephrotoxic metabolite of cyclophosphamide and ifosfamide. The mechanism by which MGMT recognizes damage induced by acrolein and CAA is unknown. CHO cells expressing a mutant form of MGMT (MGMT(R128A)), known to have >1000-fold less repair activity towards alkylated DNA while maintaining full active site transferase activity towards low molecular weight substrates, exhibited equivalent CAA- and acrolein-induced cytotoxicity to that of CHO cells transfected with plasmid control. These results imply that direct reaction of acrolein or CAA with the active site cysteine residue of MGMT, i.e. scavenging, is unlikely a mechanism to explain MGMT protection from CAA and acrolein-induced toxicity. In vivo, no difference was detected between Mgmt-/- and Mgmt+/+ mice in the lethal effects of cyclophosphamide. While MGMT may be important at the cellular level, mice deficient in MGMT are not significantly more susceptible to cyclophosphamide, acrolein or CAA. Thus, our data does not support targeting MGMT to improve oxazaphosphorine therapy.

Cross-Linking of the DNA Repair Protein O6-Alkylguanine DNA Alkyltransferase to DNA in the Presence of Antitumor Nitrogen Mustards

“…The presence of AGT renders E. coli strains more sensitive to killing by chlorambucil (27) but no effect of AGT status on drug sensitivity was seen in a studies of HeLa or CHO cells (25). Clinical studies have also yielded conflicting results on the role of AGT levels in the outcome of therapy with cyclophosphamide [reviewed (24)]. Recently, it was shown that MGMT does not protect against mutations induced by cyclophosphamide in mice (28).…”

The cytogenetic action of ifosfamide, mesna, and their combination on peripheral rabbit lymphocytes: an in vivo/in vitro cytogenetic study

“…The method used was the study of Sister Chromatid Exchanges (SCEs). IFO is a powerful and effective drug performing exceptionally in anticancer therapy, on the other hand it induces chromatid fragility and genotoxicity even in healthy tissue cells (Chen et al 2007;Hansen et al 2007;Lialiaris et al 2010a). As previously stated, Mesna can help to avoid haemorrhagic cystitis induced following administration of IFO.…”

DNA Repair Mechanisms

“…Metilasyon ajanları, kloroetilasyon ajanları) DNA, RNA ve protein gibi hücresel moleküllere alkil grupları transfer ederek biyolojik etkilerini ortaya koymaktadırlar. Bu ajanlar; DNA omurgası içindeki fosfodiester gruplarının oksijeni ile birlikte, nükleik asitlerin nitrojen ve oksijeni içeren nükleofilik bölgeleriyle reaksiyona girerler 11 . Metilasyon ve kloroetilasyon ajanlarının her ikisi de bir unimoleküler nükleofilik substitüsyon reaksiyonu (NS1 reaksiyonu) aracılığıyla hücresel makromoleküllere zarar verir ve bu nedenden dolayı onlar DNA'daki oksijen atomlarına doğru güçlü bir elektrofilik afiniteye sahiptirler.…”

“…Metilasyon ajanlarının hücre DNA'sında oluşturduğu O6-MeG eklentisi, MGMT (O6-metilguanin-DNA-metiltransferaz) isimli bir enzim tarafından tamir edilir ve metilasyon kaldırılır 2,13,16 . MGMT memeli hücre ve dokularında DNA alkilasyon ajanlarının zararlı etkilerine karşı koruyucu bir rol oynamaktadır 11,15 . MGMT molekülü, alkil eklentilerini kendi üzerindeki sistein amino asidine transfer ederek alkilasyonun mutajenik etkisini ortadan kaldırmaktadır 6,7,11 (Şekil3).…”

“…MGMT memeli hücre ve dokularında DNA alkilasyon ajanlarının zararlı etkilerine karşı koruyucu bir rol oynamaktadır 11,15 . MGMT molekülü, alkil eklentilerini kendi üzerindeki sistein amino asidine transfer ederek alkilasyonun mutajenik etkisini ortadan kaldırmaktadır 6,7,11 (Şekil3).…”