The activated aryl hydrocarbon receptor synergizes mitogen-induced murine liver hyperplasia

Environmental Ligands of the Aryl Hydrocarbon Receptor and Their Effects in Models of Adult Liver Progenitor Cells

The toxicity of environmental and dietary ligands of the aryl hydrocarbon receptor (AhR) in mature liver parenchymal cells is well appreciated, while considerably less attention has been paid to their impact on cell populations exhibiting phenotypic features of liver progenitor cells. Here, we discuss the results suggesting that the consequences of the AhR activation in the cellular models derived from bipotent liver progenitors could markedly differ from those in hepatocytes. In contact-inhibited liver progenitor cells, the AhR agonists induce a range of effects potentially linked with tumor promotion. They can stimulate cell cycle progression/proliferation and deregulate cell-to-cell communication, which is associated with downregulation of proteins forming gap junctions, adherens junctions, and desmosomes (such as connexin 43, E-cadherin, β-catenin, and plakoglobin), as well as with reduced cell adhesion and inhibition of intercellular communication. At the same time, toxic AhR ligands may affect the activity of the signaling pathways contributing to regulation of liver progenitor cell activation and/or differentiation, such as downregulation of Wnt/β-catenin and TGF-β signaling, or upregulation of transcriptional targets of YAP/TAZ, the effectors of Hippo signaling pathway. These data illustrate the need to better understand the potential role of liver progenitors in the AhR-mediated liver carcinogenesis and tumor promotion.

Ah Receptor Pathway Intricacies; Signaling Through Diverse Protein Partners and DNA-Motifs

The Ah receptor is a transcription factor that modulates gene expression via interactions with multiple protein partners; these are reviewed, including the novel NC-XRE pathway involving KLF6.

Ah receptor-mediated suppression of liver regeneration through NC-XRE-driven p21Cip1 expression

Previous studies in hepatocyte-derived cell lines and the whole liver established that the aryl hydrocarbon receptor (AhR) can disrupt G1-phase cell cycle progression following exposure to persistent AhR agonists, such as TCDD (dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin). Growth arrest was attributed to inhibition of G1-phase cyclin-dependent kinase 2 (CDK2) activity. The present study examined the effect of TCDD exposure on liver regeneration following 70% partial hepatectomy in mice lacking the Cip/Kip inhibitors p21(Cip1) or p27(Kip1) responsible for regulating CDK2 activity. Assessment of the regenerative process in wild-type, p21(Cip1) knockout, and p27(Kip1) knockout mice confirmed that TCDD-induced inhibition of liver regeneration is entirely dependent on p21(Cip1) expression. Compared with wild-type mice, the absence of p21(Cip1) expression completely abrogated the TCDD inhibition, and accelerated hepatocyte progression through G1 phase during the regenerative process. Analysis of the transcriptional response determined that increased p21(Cip1) expression during liver regeneration involved an AhR-dependent mechanism. Chromatin immunoprecipitation studies revealed that p21(Cip1) induction required AhR binding to the newly characterized nonconsensus xenobiotic response element, in conjunction with the tumor suppressor protein Kruppel-like factor 6 functioning as an AhR binding partner. The evidence also suggests that AhR functionality following partial hepatectomy is dependent on a p21(Cip1)-regulated signaling process, intimately linking AhR biology to the G1-phase cell cycle program.

The tumor suppressor Kruppel-like factor 6 is a novel aryl hydrocarbon receptor DNA binding partner

The aryl hydrocarbon receptor (AhR) is a ligand-mediated basic helix-loop-helix transcription factor of the Per/Arnt/Sim family that regulates adaptive and toxic responses to a variety of chemical pollutants, including polycyclic aromatic hydrocarbons and halogenated aromatic hydrocarbons, most notably 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Ligand activation leads to AhR nuclear translocation and binding to a xenobiotic response element (XRE) in association with the Arnt to regulate gene expression. Several recent genome-wide transcriptional studies identified numerous AhR target genes that lack the canonical XRE recognition site in the promoter regions. Characterization of one such target gene, the plasminogen activator inhibitor 1, identified a novel nonconsensus XRE (NC-XRE) that confers TCDD responsiveness independently of the Arnt protein. Studies reported here show that the NC-XRE is a recognition site for the AhR and a new binding partner, the Kruppel-like factor (KLF) family member KLF6. In vivo chromatin immunoprecipitations and in vitro DNA binding studies demonstrate that the AhR and KLF6 proteins form an obligatory heterodimer necessary for NC-XRE binding. Mutational analyses show that the protein-protein interactions involve the AhR C terminus and KLF6 N terminus, respectively. Moreover, NC-XRE binding depends on the 5' basic region in KLF6 rather than the previously characterized zinc finger DNA binding domain. Collectively, the results unmask a novel AhR signaling mechanism distinct from the canonical XRE-driven process that will enrich our future understanding of AhR biology.

Identification of stanniocalcin 2 as a novel aryl hydrocarbon receptor target gene

Proper hepatocyte function is vital for survival; thus, unrepaired destruction of the parenchymal tissue leading to liver decompensation is devastating. Therefore, understanding the homeostatic process regulating liver regeneration is clinically important, and evidence that the aryl hydrocarbon receptor (AhR) can promote cell survival after intrinsic apoptotic stimuli is integral to the regenerative process. The current study uses primary hepatocytes to identify survival mechanisms consistent with normal AhR biology. Taking advantage of the Cre-lox system to manipulate AhR status, we designed a comprehensive microarray analysis to identify immediate and direct changes in the transcriptome concomitant with the loss of the AhR. As a result, we identified a unique data set with minimal overlap, compared with previous array studies, culminating in the identification of Stanniocalcin 2 (Stc2) as a novel receptor target gene previously reported to have a cytoprotective role in endoplasmic reticulum stress. The Stc2 promoter contains multiple putative xenobiotic response elements clustered in a 250-bp region that was shown to recruit the AhR by chromatin immunoprecipitation. Of interest, Stc2 gene expression is refractory to classic exogenous AhR agonists, but responds to cellular stress in an AhR-dependent mechanism consistent with a process promoting cell survival.

p-Anilinoaniline enhancement of dioxin-induced CYP1A1 transcription and aryl hydrocarbon receptor occupancy of CYP1A1 promoter: role of the cell cycle

The aryl hydrocarbon receptor (AhR) is targeted by ubiquitination for degradation by the proteasome shortly after its activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In silico screening identified p-anilinoaniline (pAA) as a putative inhibitor of an E2 ligase that partners with an E3 ligase implicated in AhR ubiquitination. We investigated whether pAA could modify AhR-dependent activation of its target gene CYP1A1. pAA (1-200 μM) alone did not affect AhR content, or stimulate CYP1A1 mRNA accumulation in human mammary epithelial MCF10A cultures. However, pretreatment with ≥100 μM pAA suppressed TCDD-induced CYP1A1 activation and AhR degradation via its functioning as an AhR antagonist. At a lower concentration (25 μM), pAA cotreatment increased TCDD-induced CYP1A1 mRNA accumulation, without inhibiting AhR turnover or altering CYP1A1 mRNA half-life. Whereas TCDD alone did not affect MCF10A proliferation, 25 μM pAA was cytostatic and induced a G(1) arrest that lasted ∼7 h and induced an S phase arrest that peaked 5 to 8 h later. TCDD neither affected MCF10A cell cycle progression nor did it alter pAA effects on the cell cycle. The magnitude of CYP1A1 activation depended upon the time elapsed between pAA pretreatment and TCDD addition. Maximal AhR occupancy of the CYP1A1 promoter and accumulation of CYP1A1 heterogeneous nuclear RNA and mRNA occurred when pAA-pretreated cultures were exposed to TCDD in late G(1) and early/mid S phase. TCDD-mediated induction of CYP2S1 was also cell cycle-dependent in MCF10A cultures. Similar studies with HepG2 cultures indicated that the cell cycle dependence of CYP1A1 induction is cell context-dependent.

Non-additive hepatic gene expression elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) co-treatment in C57BL/6 mice

Interactions between environmental contaminants can lead to non-additive effects that may affect the toxicity and risk assessment of a mixture. Comprehensive time course and dose-response studies with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), non-dioxin-like 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) and their mixture were performed in immature, ovariectomized C57BL/6 mice. Mice were gavaged once with 30 μg/kg TCDD, 300 mg/kg PCB153, a mixture of 30 μg/kg TCDD with 300 mg/kg PCB153 (MIX) or sesame oil vehicle for 4,12, 24,72 or 168 h. In the 24h dose-response study, animals were gavaged with TCDD (0.3,1, 3, 6, 10, 15, 30, 45 μg/kg), PCB153 (3,10, 30, 60, 100, 150, 300, 450 mg/kg), MIX (0.3+3, 1+10, 3+30, 6+60, 10+100, 15+150, 30+300, 45 μg/kg TCDD+450 mg/kg PCB153, respectively) or vehicle. All three treatments significantly increased relative liver weights (RLW), with MIX eliciting significantly greater increases compared to TCDD and PCB153 alone. Histologically, MIX induced hepatocellular hypertrophy, vacuolization, inflammation, hyperplasia and necrosis, a combination of TCDD and PCB153 responses. Complementary lipid analyses identified significant increases in hepatic triglycerides in MIX and TCDD samples, while PCB153 had no effect on lipids. Hepatic PCB153 levels were also significantly increased with TCDD co-treatment. Microarray analysis identified 167 TCDD, 185 PCB153 and 388 MIX unique differentially expressed genes. Statistical modeling of quantitative real-time PCR analysis of Pla2g12a, Serpinb6a, Nqo1, Srxn1, and Dysf verified non-additive expression following MIX treatment compared to TCDD and PCB153 alone. In summary, TCDD and PCB153 co-treatment elicited specific non-additive gene expression effects that are consistent with RLW increases, histopathology, and hepatic lipid accumulation.