Abstract
Aberrant protein modifications play an important role in the pathophysiology of many human diseases, in terms of both dysfunction of physiological modifications and the formation of pathological modifications by reaction of proteins with endogenous electrophiles. Recent studies have identified a chemical homolog of lysine acetylation, N6-formyllysine, as an abundant modification of histone and chromatin proteins, one possible source of which is the reaction of lysine with 3′-formylphosphate residues from DNA oxidation. Using a new liquid chromatography-coupled to tandem mass spectrometry method to quantify all N6-methyl-, -acetyl- and -formyl-lysine modifications, we now report that endogenous formaldehyde is a major source of N6-formyllysine and that this adduct is widespread among cellular proteins in all compartments. N6-formyllysine was evenly distributed among different classes of histone proteins from human TK6 cells at 1–4 modifications per 104 lysines, which contrasted strongly with lysine acetylation and mono-, di-, and tri-methylation levels of 1.5-380, 5-870, 0-1400, and 0-390 per 104 lysines, respectively. While isotope labeling studies revealed that lysine demethylation is not a source of N6-formyllysine in histones, formaldehyde exposure was observed to cause a dose-dependent increase in N6-formyllysine, with use of [13C,2H2]-formaldehyde revealing unchanged levels of adducts derived from endogenous sources. Inhibitors of class I and class II histone deacetylases did not affect the levels of N6-formyllysine in TK6 cells, and the class III histone deacetylase, SIRT1, had minimal activity (<10%) with a peptide substrate containing the formyl adduct. These data suggest that N6-formyllysine is refractory to removal by histone deacetylases, which supports the idea that this abundant protein modification could interfere with normal regulation of gene expression if it arises at conserved sites of physiological protein secondary modification.
Highlights
In addition to physiological secondary modifications, proteins are subjected to reactions with endogenous electrophiles generated by oxidative stress, inflammation, and normal cell metabolic processes [1,2,3,4,5]
Formaldehyde emerged as an alternative source given the high reactivity of formaldehyde toward primary amines, such as the side chain of lysine, and the potential for endogenous oxidation to convert a formaldehyde-derived carbinolamine to a stable formamide (Figure 1)
We have demonstrated in vitro and in cells that formaldehyde exposure leads to formation of N6formyllysine residues in proteins
Summary
In addition to physiological secondary modifications, proteins are subjected to reactions with endogenous electrophiles generated by oxidative stress, inflammation, and normal cell metabolic processes [1,2,3,4,5]. The resulting adducts, which can alter protein function and lead to protein degradation, are associated with a variety of pathological processes and human diseases [1,2,3,4,5,8] Among these pathological adducts, N6-formylation of lysine has recently emerged as an abundant protein modification [5,9,10,11]. Considering that formaldehyde reacts with amines to give a carbinolamine intermediate that is Author Summary
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