The aryl-hydrocarbon receptor (AhR) is a ligand-dependent basic helix-loop-helix/Per-Arnt-Sim domain transcription factor. It resides in the cytoplasm in a complex with two molecules of heat shock protein 90, HBV X-associated protein 2, and p23. On ligand activation, AhR translocates to the nucleus and heterodimerizes with AhR nuclear translocator. The AhR–AhR nuclear translocator heterodimer binds to a consensus sequence (TNGCGG) known as the dioxin responsive element (DRE) and drives the transcription of its target genes (reviewed by Hankinson1 and Petrulis and Perdew2). Most of the well-characterized AhR target genes belong to phase I and II enzyme families involved in xenobiotic metabolism, principally Cyp1a1, Cyp1a2, Cyp1b1, glutathione-S-transferase Ya, NAD(P)H: quinone oxido-reductase 1, and UDP-glucuronosyl transferase 1.3
Although several genes have been characterized as regulated by AhR in a DRE-dependent manner, alternative modes of receptor function and novel target genes must be identified to adequately explain the wide spectrum of pathophysiological effects associated with AhR. An emerging aspect of transcription factor biology is the ability of various factors to interact with members of different signaling pathways. Recent reports focusing on receptor crosstalk have highlighted the ability of AhR to influence the activity of other proteins involved in gene regulation, including nuclear factor κB,4 estrogen receptor,5–7 and TGF-β1.8
The constitutive androstane receptor (CAR), also known as Nr1i3, is a member of the nuclear receptor family. It is found in the cytoplasm in a complex with heat shock protein 90 and CAR cytoplasmic retention protein.9 A unique feature of CAR is that it can be activated by two distinct mechanisms. Ligands such as 1,4-bis[2-(3,5,-dichloropyridyloxy)]benzene (TCPOBOP) can directly bind to CAR and activate the receptor.10 Alternately, CAR activity can be induced indirectly by a phenobarbital-responsive protein phosphatase-2A–dependent signaling cascade.11 Activated CAR undergoes nuclear translocation, heterodimerizes with 9-cis retinoic acid receptor to bind its response element, and drives the transcription of its target genes. Using cell culture models, it has been demonstrated that activation of the glucocorticoid receptor can up-regulate transcriptional activity at the CAR promoter12 and that IL-1β–mediated nuclear factor κB activation inhibits this up-regulation by interfering with chromatin remodeling.13
In the current study, we have identified CAR to be an AhR target gene. Previously, it has been shown that both AhR and CAR play a significant role in response to exogenous stimuli as well as pathophysiological events in the liver. Both receptors induce numerous xenobiotic metabolism enzymes. AhR is also known to affect vascular development, as demonstrated by persistent ductus venosus and microvasculature abnormalities in the livers of AhR-null mice.14 Recently, it has been demonstrated that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure severely impairs the regenerative ability of partially excised mouse livers,15 an effect most likely mediated through AhR. On the other hand, TCDD is also known to be a potent tumor promoter in mouse liver.16 CAR is similarly involved in a number of physiological processes in the liver, including bilirubin metabolism and hepatocyte proliferation. As shown in this study, AhR activation increases CAR messenger RNA (mRNA) in the liver as well as extrahepatic tissues and follows a temporal pattern similar to Cyp1A1, a known AhR target gene. This increase in CAR mRNA correlates with an increase in the transcriptional activity of CAR. Because a broad range of compounds can activate AhR, an AhR-mediated increase in CAR activity could potentially lead to unexpected effects on drug metabolism. Considering the importance of AhR and CAR in liver biology, knowledge of their interaction will be useful in interpreting the observations made in relation to these two receptors.