The nuclear factor-κB (NF-κB) transcription factor family has a crucial role in rapid responses to stress and pathogens. We show that the NF-κB subunit RelB is functionally associated with the aryl hydrocarbon receptor (AhR) and mediates transcription of chemokines such as IL-8 via activation of AhR and protein kinase A. RelB physically interacts with AhR and binds to an unrecognized RelB/AhR responsive element of the IL-8 promoter linking two signaling pathways to activate gene transcription. We found a time-dependent recruitment of AhR to the RelB/AhR responsive element site of IL-8 mediated by the AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) and via activation of protein kinase A. Furthermore, NF-κB-binding sites that are preferentially recognized by RelB/p52 are a target for RelB/AhR complexes without addition of any stimuli, implicating the endogenous function of the AhR. RelB/AhR complexes are also found to bind on xenobiotic responsive element, and RelB drastically increases the 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced xenobiotic responsive element reporter activity. The interaction of RelB with AhR signaling, and AhR with NF-κB RelB signaling pathways represent a new mechanism of cross talk between the two transcription factors.

The aryl hydrocarbon receptor (AhR) is involved in the regulation of immune responses, T-cell differentiation, and immunity. Here, we show that inflammatory stimuli such as LPS induce the expression of AhR in human dendritic cells (DC) associated with an AhR-dependent increase of CYP1A1 (cytochrome P4501A1). In vivo data confirmed the elevated expression of AhR by LPS and the LPS-enhanced 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated induction of CYP1A1 in thymus of B6 mice. Inhibition of nuclear factor-κB (NF-κB) repressed both normal and LPS-enhanced, TCDD-inducible, AhR-dependent gene expression and canonical pathway control of RelA-regulated AhR-responsive gene expression. LPS-mediated induction of AhR was NF-κB-dependent, as shown in mouse embryonic fibroblasts (MEFs) derived from Rel null mice. AhR expression and TCDD-mediated induction of CYP1A1 was significantly reduced in RelA-deficient MEF compared with wild type MEF cells and ectopic expression of RelA restored the expression of AhR and induction of CYP1A1 in MEF RelA null cells. Promoter analysis of the human AhR gene identified three putative NF-κB-binding elements upstream of the transcription start site. Mutation analysis of the AhR promoter identified one NF-κB site as responsible for mediating the induction of AhR expression by LPS and electrophoretic shift assays demonstrated that this NF-κB motif is recognized by the RelA/p50 heterodimer. Our results show for the first time that NF-κB RelA is a critical component regulating the expression of AhR and the induction of AhR-dependent gene expression in immune cells illustrating the interaction of AhR and NF-κB signaling.

Objective—: Exposure to dioxins has been shown to contribute to the development of inflammatory diseases, such as atherosclerosis. Macrophage-mediated inflammation is a critical event in the initiation of atherosclerosis. Previously, we showed that treatment of macrophages with 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD) leads to aryl hydrocarbon receptor (AhR)–dependent activation of inflammatory mediators and the formation of cholesterol-laden foam cells. However, the mechanisms responsible for the formation of atherosclerotic lesions mediated through AhR have not been identified. Methods and Results—: An in vitro macrophage and an apolipoprotein E (ApoE)−/− mouse model were used to determine whether chemokines and their receptors are responsible for the AhR-mediated atherogenesis. Exposure of ApoE−/− mice to TCDD caused a time-dependent progression of atherosclerosis, which was associated with induction of inflammatory genes, including interleukin-8, as well as F4/80 and matrix metalloproteinase-12. A high-fat diet enhanced the TCDD-mediated inflammatory response and aggravated the formation of complex atheromas. Treatment with a CXCR2 inhibitor and an AhR antagonist reduced the TCDD-induced progression of early atherosclerotic lesions in ApoE−/− mice. Conclusion—: The results suggest that CXCR2 mediates the atherogenic activity of environmental pollutants, such as dioxins, and contributes to the development of atherosclerosis through the induction of a vascular inflammatory response by activating the AhR-signaling pathway.

Exposure to particulate matter air pollution causes inflammatory responses and is associated with the progression of atherosclerosis and increased cardiovascular mortality. Macrophages play a key role in atherogenesis by releasing proinflammatory cytokines ...