Researchers at The Wistar Institute say they have discovered that Early Growth Response 1 (EGR1), a protein that turns on and off specific genes during blood cell development, inhibits expression of pro-inflammatory genes in macrophages. As part of their function to protect the body against pathogens, macrophages play a major role in initiation, maintenance, and resolution of inflammation.

The discovery expands the understanding of how macrophages are set off and deactivated in the inflammatory process, which is critical in many normal and pathological conditions, according to the scientists, who published their study “EGR1 is a gatekeeper of inflammatory enhancers in human macrophages” in Science Advances.

“Monocytes and monocyte-derived macrophages originate through a multistep differentiation process. First, hematopoietic stem cells generate lineage-restricted progenitors that eventually develop into peripheral, postmitotic monocytes. Second, blood-circulating monocytes undergo differentiation into macrophages, which are specialized phagocytic cells capable of tissue infiltration,” write the investigators.

“While monocytes mediate some level of inflammation and cell toxicity, macrophages boast the widest set of defense mechanisms against pathogens and elicit robust inflammatory responses. Here, we analyze the molecular determinants of monocytic and macrophagic commitment by profiling the EGR1 transcription factor. EGR1 is essential for monopoiesis and binds enhancers that regulate monocytic developmental genes such as CSF1R.”

“However, differentiating macrophages present a very different EGR1 binding pattern. We identify novel binding sites of EGR1 at a large set of inflammatory enhancers, even in the absence of its binding motif. We show that EGR1 repressive activity results in suppression of inflammatory genes and is mediated by the NuRD corepressor complex.”

“By deepening the understanding of the role of EGR1, we shed light on the fundamental process of macrophage maturation, which is required for many aspects of the immune response including inflammation,” said Alessandro Gardini, PhD, assistant professor in the Gene Expression & Regulation Program at The Wistar Institute and senior author on the study. “Our data suggest EGR1 acts as a master regulator of inflammation in macrophages.”

The multi-step maturation of macrophages from progenitor cells in the bone marrow requires the concerted action of critical transcription factors that regulate expression of specific genes. EGR1 is one of these factors but its function remained elusive.

In response to tissue damage and infection, white blood cells of the immune system called monocytes can leave the bloodstream and infiltrate tissues, where they undergo an elaborate developmental program and mature into macrophages. Macrophages have the ability to “eat” pathogens, promote inflammation and elicit pathogen-specific immune responses.

The molecular mechanisms underlying this maturation process are not well defined. The same set of transcription factors acting in early monocyte development were thought to be involved in the conversion of monocytes to macrophages.

Gardini and colleagues used a model to recreate differentiation of monocytes to macrophages in vitro and performed a systematic genomic analysis of the role of EGR1 in this process. They found that EGR1 binds to different DNA regulatory regions in late-differentiating macrophages as opposed to progenitor cells differentiating into monocytes.

The lab previously uncovered a mechanism whereby EGR1 regulates gene expression in monocytes and macrophages by interacting with enhancers. These are short regulatory DNA sequences that, when bound by specific transcription factors, augment the expression of the associated genes.

In the new study, researchers found that EGR1 represses inflammatory enhancers in developing and mature macrophages, blunting their activation and the immune response.

“Our results suggest that the role of EGR1 in modulating inflammation may extend beyond development of blood cells and be relevant to the control of inflammation in health and disease conditions,” said Avery Zucco, PhD, a postdoctoral researcher in the Gardini lab and co-first author of the study.

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