A new cross-talk between the aryl hydrocarbon receptor and RelB, a member of the NF-kappaB family

Abstract

The discovery of the new crosstalk between the aryl hydrocarbon receptor (AhR) and the NF-kappaB subunit RelB may extend our understanding of the biological functions of the AhR and at the same time raises a number of questions, which will be addressed in this review. The characteristics of this interaction differ from that of AhR with RelA in that the latter appears to be mostly negative unlike the collaborative interactions of AhR/RelB. The AhR/RelB dimer is capable of binding to DNA response elements including the dioxin response element (DRE) as well as NF-kappaB binding sites supporting the activation of target genes of the AhR as well as NF-kappaB pathway. Further studies show that AhR/RelB complexes can be found not only in lymphoid cells but also in a human hepatoma cell line (HepG2) or breast cancer cell line (MDA-MB-231). RelB has been implicated in carcinogenesis of breast cancer for instance and RelB is known to be a critical factor for the function and differentiation of dendritic cells; interestingly the participation of AhR in both processes has been suggested recently, which offers the great potential to expand the scope of the physiological roles of the AhR. There is evidence indicating that RelB may serve as a pro-survival factor, including its ability to promote "inflammation resolution" besides the association of RelB with inflammatory disorders. Based on such information, a hypothesis has been proposed in this review that AhR together with RelB functions as a coordinator of inflammatory responses.

Figure 1

Nuclear protein extracts from control (C), FSK- (F), or TCDD-stimulated (T) HepG2 cells were incubated with ³²P-labeled oligonucleotide containing a DRE consensus element of the CYP1A1 promoter. Nuclear proteins were extracted after treatment (Treat.) of HepG2 cells (ATCC, Manassas, VA) with 10 nM TCDD or 5 μM FSK for 2h. TCDD (>99% purity) was originally obtained from Dow Chemical Co. (Midland, MI) and FSK was purchased from Sigma (St. Louis, MO). A possible binding of AhR, ARNT, and RelB was identified by supershift analyses using AhR-, ARNT-, or RelB-specific antibodies (Ab). Monoclonal ARNT (Santa Cruz Biotech Inc, Santa Cruz, CA), RelB (Active Motif, Carlsbad, CA) and polyclonal AhR (Novus Biologicals, Littleton, CO) antibodies were used for supershift analysis. To confirm specificity a 100-fold excess of unlabeled DRE consensus oligonucleotide was added. A 100-fold molar excess of unlabeled oligonucleotides was added as competitor (Compet.).

Figure 2

C57BL/6 wild type (wt) and AhR homozygous null mice (AhR^-/-) were treated with 50 μg LPS E. coli 055:B5 (Sigma L4005, St. Louis, MO). In addition AhR^-/- mice were treated with 15 μg/kg TCDD. After 6 hrs animals were sacrificed and tissues were prepared for RNA analysis by real time PCR. Values of KC and prostaglandin endoperoxide H synthase-2 (PGHS-2 or COX-2) mRNA are normalized to the expression of β-actin. C57BL/6J (BL6) and AhR null (AhR^-/-) mice were originally purchased from The Jackson Laboratory (Bar Habor, ME). * significantly different from LPS-treated wild type mice p≤0.05

Figure 3

Putative crosstalk between the classical pathway of NF-κB and AhR activation. Ligand engagement of specific membrane receptors (TNFR, IL-1R or TLR) triggers phosphorylation and activation of IKKβ. Free NF-κB translocates to the nucleus and activates target genes via p50/RelA dimers. Activated free RelA may also bind to ligand-activated AhR generating transcriptionally inactive complexes that could suppress the classical activation pathways of NF-κB [89] and AhR [26] or positively regulate gene expression as shown for c-myc [90]. IL-1R, IL-1 receptor; NEMO, NF-κB essential modulator; TLR, Toll like receptor; TNFR, TNF receptor.

Figure 4

New mechanism of cross talk between AhR and RelB. Ligand-activated or unliganded AhR activated by PKA translocates into the nucleus and interacts with RelB and occupy RelB/AhR-response elements (RelBAhRE) of promoters as in the case of IL-8 (alternative AhR/RelB pathway). AhR agonists induce the recruitment of AhR/ARNT complexes to DRE-responsive promoters such as CYP1a1 (classical AhR/ARNT pathway). Alternative pathway of NF-κB activation through the LTβR, RANK, CD40, or BAFFR leads to the release of active NIK which phosphorylates IKK. After processing of p100, RelB/p52 heterodimers are generated and activate target genes. BAFFR, B-cell activating factor receptor; FSK, forskolin; IKK, IKB kinase; LTβR, Lymphotoxin β receptor; NIK, NF-κB-inducing kinase; NF-κB, nuclear factor-κB; PKA, Protein kinase A; RANK, receptor activator of NF-κB. From Vogel et al. [18] reproduced with permission from Molecular Endocrinology 2008.