Appropriate innate and adaptive immune responses are essential for protection and resolution against chemical, physical or biological insults. Immune cell polarization is fundamental in orchestrating distinct phases of inflammation, specifically acute phase responses followed by resolution and tissue repair. Dysregulation of immune cell and inflammatory responses is a hallmark of multiple diseases encompassing atherosclerosis, rheumatoid arthritis, psoriasis and metabolic syndromes. A master transcriptional mediator of diverse inflammatory signaling and immune cell function is NF-κB, and altered control of this key regulator can lead to an effective switch from acute to chronic inflammatory responses. Members of the nuclear receptor (NR) superfamily of ligand-dependent transcription factors crosstalk with NF-κB to regulate immune cell function(s). Within the NR superfamily the NR4A1-3 orphan receptors have emerged as important regulators of immune cell polarization and NF-κB signaling. NR4A receptors modulate NF-κB activity in a dynamic fashion, either repressing or enhancing target gene expression leading to altered inflammatory outcome. Here we will discuss the pivotal role NR4A’s receptors play in orchestrating immune cell homeostasis through molecular crosstalk with NF-κB. Specifically, we will examine such NR4A/NF-κB interactions within the context of distinct cell phenotypes, including monocyte, macrophage, T cells, endothelial, and mesenchymal cells, which play a role in inflammation-associated disease. Finally, we review the therapeutic potential of altering NR4A/NF-κB interactions to limit hyper-inflammatory responses in vivo.

Macrophages play a pivotal role in atherosclerotic plaque development. Recent evidence has suggested the glucagon-like peptide-1 receptor (GLP-1R) agonist, liraglutide, can attenuate pro-inflammatory responses in macrophages. We hypothesized that liraglutide could limit atherosclerosis progression in vivo via modulation of the inflammatory response. Human THP-1 macrophages and bone marrow-derived macrophages, from both wild-type C57BL/6 (WT) and apolipoprotein E null mice (ApoE^−/−) were used to investigate the effect of liraglutide on the inflammatory response in vitro. In parallel, ApoE^−/− mice were fed a high-fat (60% calories from fat) high-cholesterol (1%) diet for 8 weeks to induce atherosclerotic disease progression with/without daily 300 μg/kg liraglutide administration for the final 6 weeks. Macrophages were analysed for MΦ1 and MΦ2 macrophage markers by Western blotting, RT-qPCR, ELISA and flow cytometry. Atherosclerotic lesions in aortae from ApoE^−/− mice were analysed by en face staining and monocyte and macrophage populations from bone marrow derived cells analysed by flow cytometry. Liraglutide decreased atherosclerotic lesion formation in ApoE^−/− mice coincident with a reduction in pro-inflammatory and increased anti-inflammatory monocyte/macrophage populations in vivo. Liraglutide decreased IL-1beta in MΦ0 THP-1 macrophages and bone marrow-derived macrophages from WT mice and induced a significant increase in the MΦ2 surface marker mannose receptor in both MΦ0 and MΦ2 macrophages. Significant reduction in total lesion development was found with once daily 300 μg/kg liraglutide treatment in ApoE^−/− mice. Interestingly, liraglutide inhibited disease progression at the iliac bifurcation suggesting that it retards the initiation and development of disease. These results corresponded to attenuated MΦ1 markers (CCR7, IL-6 and TNF-alpha), augmented MΦ2 cell markers (Arg-1, IL-10 and CD163) and finally decreased MΦ1-like monocytes and macrophages from bone marrow-derived cells. This data supports a therapeutic role for liraglutide as an atheroprotective agent via modulating macrophage cell fate towards MΦ2 pro-resolving macrophages.

Dysregulation of inflammatory responses is a hallmark of multiple diseases such as atherosclerosis and rheumatoid arthritis. As constitutively active transcription factors NR4A nuclear receptors function to control the magnitude of inflammatory responses and in chronic inflammatory disease can be protective or pathogenic. Within this study we demonstrate TLR4 stimulation using the endotoxin lipopolysaccharide (LPS) rapidly enhances NR4A1-3 expression in human and murine, primary and immortalized myeloid cells with concomitant gene transcription and protein secretion of MIP-3α, a central chemokine implicated in numerous pathologies. Deficiency of NR4A2 and NR4A3 human and murine myeloid cells reveals both receptors function as positive regulators of enhanced MIP-3α expression. In contrast within the same cell types and conditions, altered NR4A activity leads to suppression of LPS induced MCP-1 gene and protein expression. An equivalent pattern of inflammatory gene regulation is replicated in TNFα treated myeloid cells. We show NF-κB is the critical regulator of NR4A1-3, MIP-3α and MCP-1 during TLR4 stimulation in myeloid cells and highlight a parallel mechanism whereby NR4A activity can repress or enhance NF-κB target gene expression simultaneously. Mechanistic insight reveals that NR4A2 does not require DNA binding capacity in order to enhance or repress NF-κB target gene expression simultaneously, and establishes a role for NF-κB family member Relb as a novel NR4A target gene involved in the positive regulation of MIP-3α. Thus, our data reveals a dynamic role for NR4A receptors concurrently enhancing and repressing NF-κB activity in myeloid cells leading to altered transcription of key inflammatory mediators