Post-translational Methylation of High Mobility Group Box 1 (HMGB1) Causes Its Cytoplasmic Localization in Neutrophils

Abstract

High mobility group box 1 (HMGB1) protein plays multiple roles in transcription, replication, and cellular differentiation. HMGB1 is also secreted by activated monocytes and macrophages and passively released by necrotic or damaged cells, stimulating inflammation. HMGB1 is a novel antigen of antineutrophil cytoplasmic antibodies (ANCA) observed in the sera of patients with ulcerative colitis and autoimmune hepatitis, suggesting that HMGB1 is secreted from neutrophils to the extracellular milieu. However, the actual distribution of HMGB1 in the cytoplasm of neutrophils and the mechanisms responsible for it are obscure. Here we show that HMGB1 in neutrophils is post-translationally mono-methylated at Lys 42 . The methylation alters the conformation of HMGB1 and weakens its DNA binding activity, causing it to become largely distributed in the cytoplasm by passive diffusion out of the nucleus. Thus, post-translational methylation of HMGB1 causes its cytoplasmic localization in neutrophils. This novel pathway explains the distribution of nuclear HMGB1 to the cytoplasm and is important for understanding how neutrophils release HMGB1 to the extracellular milieu. High mobility group box 1 (HMGB1)2 protein is one of the most abundant nonhistone chromosomal proteins in eukaryotic organisms. The primary sequences of HMGB1 in various higher organisms, from birds to mammals, show more than 90% homology with each other (1). The protein has multiple roles in transcription, replication, and cellular differentiation (2, 3). HMGB1 interacts with several transcription factors, thereby allowing them to perform their cellular roles. The phenotype of Hmgb1 knock-out mice confirmed the functional importance of HMGB1 as a regulator of transcription: they die shortly after birth and show a defect in the transcriptional control exerted by the glucocorticoid receptor (4). The subcellular distribution of the protein is tissue-specific: HMGB1 is located in both the nuclei and the cytoplasm of different tissues, such as lymphoid tissue, testis, neurons, and hepatocytes (5). Wang et al. (6) identified HMGB1 as a late mediator of endotoxin lethality in mice and showed that monocytes and macrophages stimulated by lipopolysaccharide (LPS), tumor necrosis factor (TNF) or interleukin-1 (IL-1) secrete HMGB1 in a delayed response. Patients with sepsis show an increased serum level of HMGB1, which is correlated with the severity of infection (7). Moreover, HMGB1 in monocytes and macrophages is extensively acetylated upon activation by LPS, causing localization of the protein to the cytosol (8). Cytosolic HMGB1 is then concentrated into secretory lysosomes and secreted when the cells receive an appropriate second signal (9). The recent discovery of extracellular HMGB1 as a proinflammatory mediator has been supported by a number of studies. In addition, HMGB1 is passively released from the nucleus to the extracellular milieu by cells that die as a result of necrosis or damage (10).Our previous studies showed that HMGB1 and HMGB2 are novel antigens of a...

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“…A previous report has shown that methylation at Lys42 alters the conformation of HMGB1 and reduces its affinity to DNA, causing it to become largely distributed in the cytoplasm by passive diffusion out of nucleus (Ito et al, 2007). Thus Lys112 methylation may facilitate the translocation in a similar way.…”

“…Experiments in Schistosoma mansoni conclusively demonstrated that acetylation and phosphorylation played roles in cellular trafficking, culminating with its secretion to the extracellular milieu de Abreu da Silva et al, 2011). In addition, it has been reported that single post translational methylation at lysine 42 of HMGB1 in neutrophil altered its conformation and weakened its DNA binding activity, causing it translocated to cytoplasm by passively diffusion out of nucleus (Ito et al, 2007).…”

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