Epigenetic Silencing of Tumor Necrosis Factor α during Endotoxin Tolerance*

Abstract

Sustained silencing of potentially autotoxic acute proinflammatory genes like tumor necrosis factor α (TNFα) occurs in circulating leukocytes following the early phase of severe systemic inflammation. Aspects of this gene reprogramming suggest the involvement of epigenetic processes. We used THP-1 human promonocytes, which mimic gene silencing when rendered endotoxin-tolerant in vitro, to test whether TNFα proximal promoter nucleosomes and transcription factors adapt to an activation-specific profile by developing characteristic chromatin-based silencing marks. We found increased TNFα mRNA levels in endotoxin-responsive cells that was preceded by dissociation of heterochromatin-binding protein 1α, demethylation of nucleosomal histone H3 lysine 9 (H3(Lys⁹)), increased phosphorylation of the adjacent serine 10 (H3(Ser¹⁰)), and recruitment of NF-κB RelA/p65 to the TNFα promoter. In contrast, endotoxintolerant cells repressed production of TNFα mRNA, retained binding of heterochromatin-binding protein 1α, sustained methylation of H3(Lys⁹), reduced phosphorylation of H3(Ser¹⁰), and showed diminished binding of NF-κB RelA/p65 to the TNFα promoter. Similar levels of NF-κB p50 occurred at the TNFα promoter in the basal state, during active transcription, and in the silenced phenotype. RelB, which acts as a repressor of TNFα transcription, remained bound to the promoter during silencing. These results support an immunodeficiency paradigm where epigenetic changes at the promoter of acute proinflammatory genes mediate their repression during the late phase of severe systemic inflammation.