Regulation of Chromatin Assembly and Cell Transformation by Formaldehyde Exposure in Human Cells

Abstract

Background:Formaldehyde (FA) is an environmental and occupational chemical carcinogen. Recent studies have shown that exogenous FA causes only a modest increase in DNA adduct formation compared with the amount of adducts formed by endogenous FA, raising the possibility that epigenetic mechanisms may contribute to FA-mediated carcinogenicity.Objectives:We investigated the effects of FA exposure on histone modifications and chromatin assembly. We also examined the role of defective chromatin assembly in FA-mediated transcription and cell transformation.Methods:Cellular fractionation and Western blot analysis were used to measure the levels of histone modifications in human bronchial epithelial BEAS-2B cells and human nasal RPMI2650 cells in the presence of FA. Chromatin immunoprecipitation (ChIP) and micrococcal nuclease (MNase) digest assays were performed to examine the changes in chromatin assembly and accessibility after FA exposure. RNA sequencing (RNA-seq) and real-time polymerase chain reaction (PCR) were used to examine transcriptional dysregulation. Finally, anchorage-independent cell growth ability was tested by soft agar assay following FA exposure.Results:Exposure to FA dramatically decreased the acetylation of the N-terminal tails of cytosolic histones. These modifications are important for histone nuclear import and subsequent chromatin assembly. Histone proteins were depleted in both the chromatin fraction and at most of the genomic loci tested following FA exposure, suggesting that FA compromises chromatin assembly. Moreover, FA increased chromatin accessibility and altered the expression of hundreds of cancer-related genes. Knockdown of the histone H3.3 gene (an H3 variant), which mimics inhibition of chromatin assembly, facilitated FA-mediated anchorage-independent cell growth.Conclusions:We propose that the inhibition of chromatin assembly represents a novel mechanism of cell transformation induced by the environmental and occupational chemical carcinogen FA. https://doi.org/10.1289/EHP1275

Formaldehyde Exposure and Epigenetic Effects: A Systematic Review

“…The ability of FA to react with histone lysine residues reported by Yoshida et al [30] and Chen et al [31] are in line with previous results obtained by Lu et al [45] in a simplified in vitro model employing H4 isolated from calf thymus tissues and human recombinant H4 purified after expression in E. Coli. The authors, in fact, demonstrated that the FA-induced lysine adducts on histone may impair post-transcriptional histone acetylation and deacetylation balance.…”

“…In this context, FA may impact post-transcriptional modifications disturbing the recruitment of regulatory complexes and evolving into an aberrant chromatin assembly that may facilitate anchorage-independent growth of cells. These data suggest that compromising chromatin assembly, through inhibition of lysine acetylation, may dysregulate cancer-related gene expression, as a possible new mechanism underlying FA-induced carcinogenesis [31]. The identification of FA reactive sites on histones is an initial step in understanding additional mechanisms of FA toxicity and carcinogenicity [59].…”

“…Formaldehyde-induced epigenetic changes, such as DNA methylation [21,28,29], histone modifications [30][31][32], as well as alterations in non-coding microRNAs (MiRNAs) [33][34][35][36] have been investigated both in occupational and experimental settings ( Table 1). The following paragraphs will attempt to summarize such effects with the aim to point out currently available data and critical issues that require further epidemiological research in order to also conceive epigenetic contribution to risk assessment and management strategies.…”

“…These molecular changes have recently attracted attention because they have been linked to many diseases including cancer [45]. In regard to histone modifications, no human or animal data are currently available and only few in vitro studies investigated these epigenetic effects [30][31][32]. Yoshida et al [30] examined histone modifications in human lung adenocarcinoma epithelial A549 cells following an acute (10-600 min) treatment with FA with a focus on histone 3 (H3).…”

“…Chen et al [31] demonstrated FA's (0.5 mM) ability to form adducts on lysine residues of histone proteins in human bronchial epithelial BEAS-2B cells. The formation of such FA-histone adducts prevented the acetylation of the same site by histone acetyltransferase as demonstrated by the dramatic decrease of the lysine acetylation of the cytosolic fraction of histones H3 and H4, specifically at H3K9, H3K14, and H4K12 residues.…”