Methylglyoxal-Histone Adducts
As the primary protein component of the nucleosome, histones play a critical role in packaging DNA and regulating chromatin dynamics. Histones contain an unusually large number of lysine and arginine residues, particularly in their N-terminal tail. These residues serve as sites of numerous enzymatically controlled post-translational modifications (PTMs) that alter the histone-DNA interaction and serve as a binding site for proteins involved in DNA transcription, replication, and repair. Histone lysine and arginine residues may also be subject to nonenzymatic modification through reaction with endogenous and exogenous electrophilic molecules. Such modifications may interfere with physiological PTM function, leading to potentially toxic results. This led Vanderbilt University Basic Science investigators Larry Marnett and James Galligan (now at the University of Arizona) to explore the ability of methylglyoxal (MGO), a ubiquitous electrophilic aldehyde generated as a by-product of glycolysis, to modify histones. Their first experiments revealed that the chromatin from seven different cell lines contained adducts of MGO and arginine at concentrations similar to those of some enzymatically-generated PTMs known to regulate chromatin function. Incubating the cells in medium containing high levels of glucose increased the levels of one of the MGO-arginine adducts. The researchers next used CRISPR/Cas technology to create a cell line in which the gene for glyoxylase-1 (GLO1) had been knocked out (GLO1-/-). Although GLO-1 is the primary MGO detoxifying enzyme, its deletion had no effect on baseline levels of MGO in GLO1-/-cells. These cells did, however, exhibit increased sensitivity to MGO toxicity, consistent with the fact that exposure to exogenous MGO led to higher peak levels in GLO1-/-cells than wild-type cells. Further work demonstrated that exposure of GLO1-/-cells to MGO led to increased formation of MGO-arginine adducts in chromatin, and the investigators were able to identify 23 MGO-arginine and 5 MGO-lysine adducts in histones isolated from MGO-treated GLO1-/-cells. They also showed that exposure of GLO1-/-cells to MGO led to interference with enzymatic histone PTM formation, and RNA-Seq analysis demonstrated that GLO1 knockout, especially when combined with MGO exposure, resulted in altered transcription of numerous genes. The researchers concluded that MGO can form adducts with histones, and that these adducts have the potential to disrupt physiological PTM-driven chromatin regulation. The findings are of particular interest in light of reports that MGO levels are frequently increased in diabetic patients and correlate with the severity of diabetic nephropathy, one of the major life-threatening complications of diabetes. The work is published in the journal Proceedings of the National Academy of Sciences [J. Galligan, et al., (2018) Proc. Natl. Acad. Sci. U.S.A., published online August 27, DOI: 10.1073/pnas.1802901115].