Protecting the genome: defence against nucleotide glycation and emerging role of glyoxalase I overexpression in multidrug resistance in cancer chemotherapy

Glycation of nucleotides in DNA forms AGEs (advanced glycation end-products). Nucleotide AGEs are: the imidazopurinone derivative dG-G {3-(2´-deoxyribosyl)-6,7-dihydro-6,7-dihydroxyimidazo[2,3-b]purin-9(8)one}, CMdG (N₂-carboxymethyldeoxyguanosine) and gdC (5-glycolyldeoxycytidine) derived from glyoxal, dG-MG {6,7-dihydro-6,7-dihydroxy-6-methylimidazo-[2,3-b]purine-9(8)one}, dG-MG₂ [N₂,7-bis-(1-hydroxy-2-oxopropyl)deoxyguanosine] and CEdG [N₂-(1-carboxyethyl)deoxyguanosine] derived from methylglyoxal, and dG-3DG [N₂-(1-oxo-2,4,5,6-tetrahydroxyhexyl)deoxyguanosine] derived from 3-deoxyglucosone and others. Glyoxal and methylglyoxal induce multi-base deletions, and base-pair substitutions – mostly occurring at G:C sites with G:C→C:G and G:C→T:A transversions. Suppression of nucleotide glycation by glyoxalase I and aldehyde reductases and dehydrogenases, and base excision repair, protects and recovers DNA from damaging glycation. The effects of DNA glycation may be most marked in diabetes and uraemia. Mutations arising from DNA glycation may explain the link of non-dietary carbohydrate intake to incidence of colorectal cancer. Overexpression of glyoxalase I was found in drug-resistant tumour cells and may be an example of an undesirable effect of the enzymatic protection against DNA glycation. Experimental overexpression of glyoxalase I conferred resistance to drug-induced apoptosis. Glyoxalase I-mediated drug resistance was found in human leukaemia and lung carcinoma cells. Methylglyoxal-mediated glycation of DNA may contribute to the cytotoxicity of some antitumour agents as a consequence of depletion of NAD⁺ by poly(ADP-ribose) polymerase, marked increases in triosephosphate concentration and increased formation of methylglyoxal. S-p-Bromobenzylglutathione cyclopentyl diester is a cell-permeable glyoxalase I inhibitor. It countered drug resistance and was a potent antitumour agent against lung and prostate carcinoma. Glyoxalase I overexpression was also found in invasive ovarian cancer and breast cancer.