Epigenetic silencing of
tumor necrosis factor alpha (
TNF-alpha) and
interleukin 1beta (IL-1beta) 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-kappaB factor RelB, which directs
histone H3K9 dimethylation, disrupts assembly of transcription activator
NF-kappaB 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-alpha in THP-1 human promonocytes.
HMGB1 binds the
TNF-alpha promoter during transcription silencing and promotes assembly of the repressor RelB. Depletion of
HMGB1 by
small interfering RNA results in dissociation of RelB from the promoter and partially restores
TNF-alpha transcription.
Histone H1, which typically displaces
HMGB1 from nucleosomal
DNA, also binds concomitantly with
HMGB1 to the
heterochromatin of the silenced
TNF-alpha promoter. Combined knockdown of
HMGB1 and H1 restores binding of the transcriptionally active
NF-kappaB p65 and reestablishes
TNF-alpha mRNA levels.
Chromatin reimmunoprecipitation experiments demonstrate that
HMGB1 and H1 are likely recruited to
TNF-alpha 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-1beta, while no binding nor repression of antiinflammatory
IkappaBalpha is observed. Finally, we find that H1 and
HMGB1 bind to the
TNF-alpha 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-alpha during the assembly of
heterochromatin in the SSI phenotype.