Regulation of the activity and expression of aryl hydrocarbon receptor by ethanol in mouse hepatic stellate cells.

Activation of hepatic stellate cells has been suggested as the central event in alcoholic fibrosis and cirrhosis (Ahrendt et al., 2000). In the normal liver, stellate cells remain in a quiescent state. When liver is damaged by alcohol and/or other hepatotoxic agents, the quiescent stellate cells are transformed into proliferative, fibrogenic, and contractile myofibroblasts (Maher, 1997). These dramatic phenotypic changes involve global reprogramming of gene expression. Currently, a number of genes, whose encoded proteins control cell proliferation and differentiation, have been suggested to either promote or inhibit hepatic stellate cell activation and liver fibrosis induced by various stimuli, such as viral infection and heavy long-term alcohol consumption (Mann and Marra, 2010). Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and was initially recognized as a regulator of the expression of xenobiotic-metabolizing enzymes (Fujii-Kuriyama and Mimura, 2005). Recent studies suggest that AhR also regulates the expression of genes involved in cell death, fibrogenesis, proliferation, and differentiation. For example, it has been shown that treatment of animals with exogenous AhR ligands induces liver hyperplasia (Nebert and Karp, 2008), while AhR-null knockout mice are more susceptible to spontaneous apoptosis and fibrosis in mouse liver (Schmidt et al., 1996). However, the mechanisms underlying the pathological effects induced by activation and deletion of AhR have not been fully defined. There are indications that exposure to ethanol (EtOH) might affect the expression and activity of AhR and its target genes. For example, EtOH stimulates the production of arachidonic acid metabolites in hepatocytes (Enomoto et al., 2000; Nanji et al., 1993) and tryptophan metabolites of the kynurenine pathway (Badawy et al., 2009). These compounds are able to bind AhR and thus induce AhR target gene expression. The molecular processes by which AhR regulates gene expression have been clearly described (Fujii-Kuriyama and Mimura, 2005). In the absence of ligands, AhR exists predominantly in the cytosolic compartment in association with a chaperone complex (Hsp90/XAP2/p23). Upon binding to ligands, AhR translocates to the nucleus and forms a heterodimer with AhR nuclear translocator already present in the nucleus (Hankinson, 1995). This heterodimer binds to consensus regulatory sequences (xenobiotic response elements [XREs]) located upstream in the promoter of the target genes. Although hundreds of genes throughout the genome contain XREs in their promoter regions, the regulatory effect of AhR on these genes varies with ligands, cell types, and the cell’s oxidative status. For example, different AhR ligands have been reported to selectively induce the expression of different genes, and therefore either provoke (Marlowe and Puga, 2005; Park et al., 2005) or suppress (Marlowe et al., 2004; Paajarvi et al., 2005; Pang et al., 2008) cell death and growth. More recently, we reported that the gene expression profile induced by the AhR ligand benzo(a)pyrene (BaP) varied greatly in cells with or without overexpression of the hydrogen peroxide scavenger catalase (Wang et al., 2009). In this article, we studied the impact of EtOH on the expression and activity of AhR in mouse hepatic stellate cells (MHSCs). Our data demonstrate that exposure of MHSCs to EtOH induces AhR translocation to the nucleus and up-regulates its target genes cytochrome P450 (CYP) 1A1 and 1B1. In addition, we observed that EtOH down-regulated AhR expression. Chronic exposure to 50 mM EtOH kept AhR expression at a low level, but diminished the inducibility of AhR target gene CYP1A1 and 1B1 in response to the AhR ligand BaP, as well as in response to an acute higher dose EtOH challenge. These findings contribute to our understanding of the mechanism underlying the synergistic toxicity induced by EtOH and polycyclic aromatic hydrocarbons (PAHs).