The transcription factor EGR2 is the molecular linchpin connecting STAT6 activation to the late, stable epigenomic program of alternative macrophage polarization

Abstract

Macrophages polarize into functionally distinct subtypes while responding to microenvironmental cues. The identity of proximal transcription factors (TFs) downstream from the polarization signals are known, but their activity is typically transient, failing to explain the long-term, stable epigenomic programs developed. Here, we mapped the early and late epigenomic changes of interleukin-4 (IL-4)-induced alternative macrophage polarization. We identified the TF, early growth response 2 (EGR2), bridging the early transient and late stable gene expression program of polarization. EGR2 is a direct target of IL-4-activated STAT6, having broad action indispensable for 77% of the induced gene signature of alternative polarization, including its autoregulation and a robust, downstream TF cascade involving PPARG. Mechanistically, EGR2 binding results in chromatin opening and the recruitment of chromatin remodelers and RNA polymerase II. Egr2 induction is evolutionarily conserved during alternative polarization of mouse and human macrophages. In the context of tissue resident macrophages, Egr2 expression is most prominent in the lung of a variety of species. Thus, EGR2 is an example of an essential and evolutionarily conserved broad acting factor, linking transient polarization signals to stable epigenomic and transcriptional changes in macrophages.

Unorthodox Transcriptional Mechanisms of Lipid-Sensing Nuclear Receptors in Macrophages: Are We Opening a New Chapter?

“…Recently, the synthetic agonist-and antagonist-insensitive (so-called pharmacologically non-responsive) fractions of LXRa and b cistromes were also identified in non-polarized murine immortalized BMDMs further confirming those opinions that the ligand insensitive fraction of lipid sensing nuclear receptor cistromes is general rather than cell type or nuclear receptorspecific phenomenon (86). It has been previously described that IL-4 can enhance the ligand-dependent activity of PPARg in human and murine alternatively polarized macrophages through three different mechanisms including EGR2 transcription factor-dependent activation of its expression, induction of endogenous ligand producing mechanisms, and direct protein-protein interaction with IL-4-activated STAT6 (87)(88)(89)(90). Nevertheless, a significant contradiction can be observed between the PPARg-dependency of alternative macrophage polarization and PPARg ligand-activated gene expression signature.…”

“…It has been previously described that IL-4 can enhance the ligand-dependent activity of PPARγ in human and murine alternatively polarized macrophages through three different mechanisms including EGR2 transcription factor-dependent activation of its expression, induction of endogenous ligand producing mechanisms, and direct protein-protein interaction with IL-4-activated STAT6 ( 87 – 90 ). Nevertheless, a significant contradiction can be observed between the PPARγ-dependency of alternative macrophage polarization and PPARγ ligand-activated gene expression signature.…”

The epigenetic pioneer EGR2 initiates DNA demethylation in differentiating monocytes at both stable and transient binding sites

“…EGR2 associates with the NAB2 corepressor 34 and early work in a murine model for macrophage differentiation suggested that EGR2 represses neutrophil-specific genes during monocytic maturation 35 , 36 . However, NAB2 was also identified as an essential co-factor for the recruitment of the mediator subunit INST13 to EGR1/2-bound enhancer elements in human monocyte/MAC 37 and more recent studies in mice have implicated EGR2 in the regulation of transcriptional networks in alternatively activated MAC 38 downstream of STAT6 39 . EGR2 is also required for the upregulation of key factors driving T cell anergy 40 , and associated with genes upregulated during in vitro differentiation of human MO into MAC 25 .…”

Mechanisms underlying divergent responses of genetically distinct macrophages to IL-4

“…Out of the Early Growth Response (EGR) family members only Egr2 is expressed in unstimulated BMDMs and rapidly induced after IL-4 stimulation (Figure 4D, S4C). Egr2 has also been associated with late IL-4 enhancer activation in a recent study (Daniel et al, 2020). Examination of the Egr2 locus indicates IL-4 induced binding of STAT6 and PPARg to a set of upstream super enhancers that gain H3K27ac and RNA Pol2 signal after IL-4 stimulation (Figure 4E).…”

“…In macrophages, IL-4 drives an ‘alternatively activated’ program of gene expression associated with inhibition of inflammatory responses and promotion of wound repair (Gordon and Martinez, 2010). The immediate transcriptional response to IL-4 is mediated by activation of STAT6 (Goenka and Kaplan, 2011; Ostuni et al, 2013), which rapidly induces the expression of direct target genes that include effector proteins such as Arginase 1 ( Arg1 ) and transcription factors like PPARg (Daniel et al, 2018; Huang et al, 1999) and EGR2 (Daniel et al, 2020). However, the extent to which natural genetic variation influences the program of alternative macrophage activation has not been systematically evaluated.…”