RelB Sustains IκBα Expression during Endotoxin Tolerance

Abstract

Transcription factors and chromatin structural modifiers induce clinically relevant epigenetic modifications of blood leukocytes during severe systemic inflammation (SSI) in humans and animals. These changes affect genes with distinct functions, as exemplified by the silencing of a set of acute proinflammatory genes and the sustained expression of a group of antimicrobial and anti-inflammatory genes. This paradigm is closely mimicked in the THP-1 human promonocyte cell model of lipopolysaccharide (LPS) endotoxin tolerance. We previously reported that LPS-induced de novo expression of RelB is required for generating tolerance to interleukin-1␤ (IL-1␤) and tumor necrosis factor alpha (TNF-␣) expression. RelB represses transcription by binding with heterochromatic protein 1 ␣ (HP1␣) to the proximal promoters of IL-1␤ and TNF-␣. In contrast, we report herein that RelB is required for sustained expression of anti-inflammatory IB␣ in LPS-tolerant THP-1 cells. RelB transcription activation requires binding to the IB␣ proximal promoter along with NF-B p50 and is associated with an apparent dimer exchange with p65. We also observed that RelB induced during human SSI binds to the IB␣ proximal promoter of circulating leukocytes. We conclude that RelB functions as a dual transcription regulator during LPS tolerance and human SSI by activating and repressing innate immunity genes.Alterations in gene expression occur in circulating and tissue monocytes and neutrophils during severe systemic inflammation (SSI), such as sepsis, and in cell models of SSI, such as lipopolysaccharide (LPS)-mediated tolerance in THP-1 promonocytes (30). These genes have distinct functions, including persistent repression of a set of acute proinflammatory genes that initiate SSI and sustained expression of a group of anti-inflammatory and antimicrobial genes (7,10,17,28).The formation of the transcriptionally active NF-B p65/p50 heterodimer on cognate DNA sequences plays a crucial role in initiating the expression of many innate immunity genes with acute proinflammatory functions that participate in SSI (24). We reported that disruption of the NF-B p65/p50 heterodimer formation at the proximal interleukin-1␤ (IL-1␤) and tumor necrosis factor alpha (TNF-␣) promoters occurs during epigenetic transcription repression of these genes in LPS tolerance and in human SSI (6, 9). We also discovered that de novo LPS induction of NF-B transcription factor RelB is required for sustaining repression of IL-1␤ and TNF-␣ transcription in the THP-1 cell model of LPS tolerance (31). The repressor effect of RelB is coupled to the heterochromatin silencing marks of dimethylation of histone H3 lysine 9 and heterochromatin protein 1 ␣ (HP1␣) binding (9). This epigenetic repressor profile also occurs in LPS-tolerant circulating leukocytes obtained from humans with SSI (31).The mechanisms controlling the sustained expression of antiinflammatory genes during endotoxin tolerance are unclear. One such critically important immune modulator is IB␣.IB␣ has the most efficient inhi...

Epigenetics and Bacterial Infections

“…As mentioned previously, opening the chromatin at target genes during this acute phase involves histone phosphorylation and acetylation. However, sustained exposure to LPS or subsequent LPS challenge activates a pathway leading to permanent gene repression, as characterized for TNF-a and IL1-b (El Gazzar et al 2008;Chen et al 2009). This relies on a change in the composition of NF-kB transcription factor at the proximal promoters of TNF-a and IL1-b, with a shift from activating p65-p50 to repressive RelB-p50, owing to the up-regulation of RelB expression by p65-p50.…”

Gene-Specific Epigenetic Regulation in Serious Infections with Systemic Inflammation

“…In SSI, the activator p65 targets TNF ␣ and IL-1 ␤ and feeds forward to induce RelB as an activator of a distinct target module, e.g. I B inhibitor-␣ (I B ␣ ) [22] . Thus, these two feedforward loops from p65 generate RelB as a dual-function mediator of distinct targets with different functions and provide one essential mechanism for gene-specific programming that requires integrating transcription factors with epigenetic histone and DNA modifiers.…”

The NF-κB Factor RelB and Histone H3 Lysine Methyltransferase G9a Directly Interact to Generate Epigenetic Silencing in Endotoxin Tolerance

“…We discovered that de novo induction of NF-B transcription factor RelB after TLR4 stimulation is necessary and sufficient for silencing transcription of TNF␣ and IL-1␤ in the SSI phenotype (8,19). We also found that RelB can function in the same cell type as a dual transcription regulator in the SSI phenotype to deactivate transcription of acute proinflammatory genes while activating transcription of the NF-B regulator IB␣ (20). RelB also participates in constitutive silencing of inflammatory genes in fibroblasts by a process that supports regional methylation of CpG DNA (21), and when normally silenced fibroblasts are rendered RelB Ϫ / Ϫ , they become responsive to LPS (22).…”