Identification and Characterization of Enhancers Controlling the Inflammatory Gene Expression Program in Macrophages. Mus musculus

Enhancers determine tissue-specific gene expression programs. Enhancers are marked by high histone H3 lysine 4 mono-methylation (H3K4me1) and by the acetyl-transferase p300, which has allowed genome-wide enhancer identification. However, the regulatory principles by which subsets of enhancers become active in specific developmental and/or environmental contexts are unknown. We exploited inducible p300 binding to chromatin to identify, and then mechanistically dissect, enhancers controlling endotoxin-stimulated gene expression in macrophages. In these enhancers, binding sites for the lineage-restricted and constitutive Ets protein PU.1 coexisted with those for ubiquitous stress-inducible transcription factors such as NF-kappaB, IRF, and AP-1. PU.1 was required for maintaining H3K4me1 at macrophage-specific enhancers. Reciprocally, ectopic expression of PU.1 reactivated these enhancers in fibroblasts. Thus, the combinatorial assembly of tissue- and signal-specific transcription factors determines the activity of a distinct group of enhancers. We suggest that this may represent a general paradigm in tissue-restricted and stimulus-responsive gene regulation. Overall design: Chromatin immuno-precipitations of p300, PU.1 and mono-methylated H3 lysine 4 followed by multiparallel sequencing were performed in bone marrow-derived macrophages. Experiments for p300 and PU.1 were also carried out in cells treated for 2hrs with lipopolysaccharide (LPS). In case of p300, reads obtained from two different biological replicates were merged.