Abstract
The differentiation of monocytes into macrophages and dendritic cells involves mechanisms for activation of the innate immune system in response to inflammatory stimuli, such as pathogen infection and environmental cues. Epigenetic reprogramming is thought to play an important role during monocyte differentiation. Complementary to cell surface markers, the characterization of monocytic cell lineages by mass spectrometry based protein/histone expression profiling opens a new avenue for studying immune cell differentiation. Here, we report the application of mass spectrometry and bioinformatics to identify changes in human monocytes during their differentiation into macrophages and dendritic cells. Our data show that linker histone H1 proteins are significantly down-regulated during monocyte differentiation. Although highly enriched H3K9-methyl/S10-phos/K14-acetyl tri-modification forms of histone H3 were identified in monocytes and macrophages, they were dramatically reduced in dendritic cells. In contrast, histone H4 K16 acetylation was found to be markedly higher in dendritic cells than in monocytes and macrophages. We also found that global hyperacetylation generated by the nonspecific histone deacetylase HDAC inhibitor Apicidin induces monocyte differentiation. Together, our data suggest that specific regulation of inter- and intra-histone modifications including H3 K9 methylation, H3 S10 phosphorylation, H3 K14 acetylation, and H4 K16 acetylation must occur in concert with chromatin remodeling by linker histones for cell cycle progression and differentiation of human myeloid cells into macrophages and dendritic cells.
Highlights
From the ‡Department of Biochemistry, Loma Linda University, Loma Linda, California 92354; §Department of Pharmacology and Toxicology, UTMB at Galveston, Texas 77554 ; ¶Singapore Institute for Clinical Sciences, Agency for Science, Technology and Research (A*STAR), Singapore 117609
One half was harvested and presumed to be macrophage cells (MC) and the other half was further treated with interleukin-4 (IL4) (10 ng/ml) for an additional 18 h to differentiate them into dendritic cells (DC)
Monocytes were treated with granulocyte-macrophage colony-stimulating factor (GMCSF) (50 ng/ mL) and interleukin-2 (IL4) (10 ng/mL) for 6 days to differentiate them into dendritic cells (DC)
Summary
Previous studies demonstrated that histone H1s are differentially expressed and incorporated into chromatin during embryonic stem cell differentiation and reprogramming to pluripotency [2]. More than being accumulated after differentiation, the three histone H1 isoforms, H1.3, H1.4, and H1.5, are required for embryonic stem cell differentiation as demonstrated by in vivo H1.3/H1.4/H1.5 triple null experiments [3]. As a follow up to our proteomics studies on epigenetic networks in U937 cell differentiation [8], we have performed proteomics studies on primary human monocyte differentiation. In this report, using proteomics and bioinformatics tools in lieu of microarray analysis of gene expression, we describe the presence of unique protein expression profiles, the linker histones, in monocyte differentiation into macrophages and dendritic cells. Our findings suggest a switch from H3 tri-modification and linker histone expression to histone H4 K16 acetylation occurs during the monocyteto-dendritic cell transition
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