Chromatin-specific remodeling by HMGB1 and linker histone H1 silences proinflammatory genes during endotoxin tolerance

Epigenetic silencing of tumor necrosis factor alpha (TNF-α) and interleukin lβ (IL-1β) transcription occurs in blood leukocytes of animals and humans after the initiation of severe systemic inflammation (SSI). We previously reported that the epigenetic signature requires induction of NF-κB factor RelB, which directs histone H3K9 dimethylation, disrupts assembly of transcription activator NF-κB p65, and induces a sustained switch from the euchromatin to heterochromatin. Here, we report the novel findings that intracellular high mobility group box 1 protein (HMGB1) and nucleosome linker histone H1 protein are necessary components of endotoxin-mediated silencing of TNF-α in THP-1 human promonocytes. HMGB1 binds the TNF-α promoter during transcription silencing and promotes assembly of the repressor Re1B. Depletion of HMGB1 by small interfering RNA results in dissociation of RelB from the promoter and partially restores TNF-α transcription. Histone H1, which typically displaces HMGB1 from nucleosomal DNA, also binds concomitantly with HMGB1 to the heterochromatin of the silenced TNF-α promoter. Combined knockdown of HMGB1 and H1 restores binding of the transcriptionally active NF-κB p65 and reestablishes TNF-α mRNA levels. Chromatin reimmunoprecipitation experiments demonstrate that HMGB1 and H1 are likely recruited to TNF-α sequences independently and that their binding correlates with histone H3K9 dimethylation, as inhibition of histone methylation blocks HMGB1 and H1 binding. Moreover, HMGB1- and H1-mediated chromatin modifications are gene specific during endotoxin silencing in that they also bind and repress acute proinflammatory IL-1β, while no binding nor repression of antiinflammatory IκBα is observed. Finally, we find that H1 and HMGB1 bind to the TNF-α a promoter in human leukocytes obtained from patients with SSI. We conclude proinflammatory HMGB1 and structural nucleosome linker H1 couple as a component of the epigenetic complex that silences acute proinflammatory TNF-α during the assembly of heterochromatin in the SSI phenotype.