Toxicogenomic evaluation of long-term hepatic effects of TCDD in immature, ovariectomized C57BL/6 mice

Aryl hydrocarbon receptor activation alters immune cell populations in the lung and bone marrow during coronavirus infection

Respiratory viral infections are one of the major causes of illness and death worldwide. Symptoms associated with respiratory infections can range from mild to severe, and there is limited understanding of why there is large variation in severity. Environmental exposures are a potential causative factor. The aryl hydrocarbon receptor (AHR) is an environment-sensing molecule expressed in all immune cells. Although there is considerable evidence that AHR signaling influences immune responses to other immune challenges, including respiratory pathogens, less is known about the impact of AHR signaling on immune responses during coronavirus (CoV) infection. In this study, we report that AHR activation significantly altered immune cells in the lungs and bone marrow of mice infected with a mouse CoV. AHR activation transiently reduced the frequency of multiple cells in the mononuclear phagocyte system, including monocytes, interstitial macrophages, and dendritic cells in the lung. In the bone marrow, AHR activation altered myelopoiesis, as evidenced by a reduction in granulocyte-monocyte progenitor cells and an increased frequency of myeloid-biased progenitor cells. Moreover, AHR activation significantly affected multiple stages of the megakaryocyte lineage. Overall, these findings indicate that AHR activation modulates multiple aspects of the immune response to a CoV infection. Given the significant burden of respiratory viruses on human health, understanding how environmental exposures shape immune responses to infection advances our knowledge of factors that contribute to variability in disease severity and provides insight into novel approaches to prevent or treat disease.NEW & NOTEWORTHY Our study reveals a multifaceted role for aryl hydrocarbon receptor (AHR) signaling in the immune response to coronavirus (CoV) infection. Sustained AHR activation during in vivo mouse CoV infection altered the frequency of mature immune cells in the lung and modulated emergency hematopoiesis, specifically myelopoiesis and megakaryopoiesis, in bone marrow. This provides new insight into immunoregulation by the AHR and extends our understanding of how environmental exposures can impact host responses to respiratory viral infections.

Roles for B[a]P and FICZ in subchondral bone metabolism and experimental temporomandibular joint osteoarthritis via the AhR/Cyp1a1 signaling axis

Bone loss due to smoking represents a major risk factor for fractures and bone osteoporosis. Signaling through the aryl hydrocarbon receptor (AhR) and its ligands contributes to both bone homeostasis and inflammatory diseases. It remains unclear whether the same AhR signaling axis affects the temporomandibular joint (TMJ). The aim of this study was to investigate possible mechanisms which mediate bone loss in the TMJ due to smoking. In particular, whether benzo[a]pyrene (B[a]P), a carcinogen of tobacco smoke, induces expression of the AhR target gene, Cyp1a1, in mandibular condyles. Possible functions of an endogenous ligand of FICZ, were also investigated in a TMJ-osteoarthritis (OA) mouse model. B[a]P was administered orally to wild-type and AhR-/- mice and bone metabolism was subsequently examined. TMJ-OA was induced in wild-type mice with forceful opening of the mouth. Therapeutic functions of FICZ were detected with μCT and histology. Exposure to B[a]P accelerated bone loss in the mandibular subchondral bone. This bone loss manifested with osteoclastic bone resorption and upregulated expression of Cyp1a1 in an AhR-dependent manner. In a mouse model of TMJ-OA, FICZ exhibited a dose-dependent rescue of mandibular subchondral bone loss by repressing osteoclast activity. Meanwhile, in vitro, pre-treatment with FICZ reduced RANKL-mediated osteoclastogenesis. B[a]P regulates mandibular subchondral bone metabolism via the Cyp1a1. The AhR ligand, FICZ, can prevent TMJ-OA by regulating osteoclast differentiation.

Fenofibrate, a peroxisome proliferator-activated receptor-alpha agonist, blocks steatosis and alters the inflammatory response in a mouse model of inflammation-dioxin interaction

2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin; TCDD) is an environmental contaminant that elicits a variety of toxic effects, many of which are mediated through activation of the aryl hydrocarbon receptor (AhR). Interaction between AhR and the peroxisome proliferator-activated receptor-alpha (PPAR-α), which regulates fatty acid metabolism, has been suggested. Furthermore, with recognition of the prevalence of inflammatory conditions, there is current interest in the potential for inflammatory stress to modulate the response to environmental agents. The aim of this work was to assess the interaction of TCDD with hepatic inflammation modulated by fenofibrate, a PPAR-α agonist. Female, C57BL/6 mice were treated orally with vehicle or fenofibrate (250 mg/kg) for 13 days, and then were given vehicle or 30 μg/kg TCDD. Four days later, the animals received an i.p. injection of lipopolysaccharide-galactosamine (LPS-GalN) (0.05x10⁷ EU/kg and 500 mg/kg, respectively) to incite inflammation, or saline as vehicle control. After 4 h, the mice were euthanized, and blood and liver samples were collected for analysis. Livers of animals treated with TCDD with or without LPS-GalN had increased lipid deposition, and this effect was blocked by fenofibrate. In TCDD/LPS-GalN-treated mice, fenofibrate caused an increase in plasma activity of alanine aminotransferase, a marker of hepatocellular injury. TCDD reduced LPS-GalN-induced apoptosis, an effect that was prevented by fenofibrate pretreatment. LPS-GalN induced an increase in the concentration of interleukin-6 in plasma and accumulation of neutrophils in liver. TCDD exposure enhanced the former response and inhibited the latter one. These results suggest that fenofibrate counteracts the changes in lipid metabolism induced by TCDD but increases inflammation and liver injury in this model of inflammation-TCDD interaction.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) dysregulates hepatic one carbon metabolism during the progression of steatosis to steatohepatitis with fibrosis in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a persistent environmental contaminant, induces steatosis that can progress to steatohepatitis with fibrosis, pathologies that parallel stages in the development of non-alcoholic fatty liver disease (NAFLD). Coincidently, one carbon metabolism (OCM) gene expression and metabolites are often altered during NAFLD progression. In this study, the time- and dose-dependent effects of TCDD were examined on hepatic OCM in mice. Despite AhR ChIP-seq enrichment at 2 h, OCM gene expression was not changed within 72 h following a bolus dose of TCDD. Dose-dependent repression of methionine adenosyltransferase 1A (Mat1a), adenosylhomocysteinase (Achy) and betaine-homocysteine S-methyltransferase (Bhmt) mRNA and protein levels following repeated treatments were greater at 28 days compared to 8 days. Accordingly, levels of methionine, betaine, and homocysteic acid were dose-dependently increased, while S-adenosylmethionine, S-adenosylhomocysteine, and cystathionine exhibited non-monotonic dose-dependent responses consistent with regulation by OCM intermediates and repression of glycine N-methyltransferase (Gnmt). However, the dose-dependent effects on SAM-dependent metabolism of polyamines and creatine could not be directly attributed to alterations in SAM levels. Collectively, these results demonstrate persistent AhR activation disrupts hepatic OCM metabolism at the transcript, protein and metabolite levels within context of TCDD-elicited progression of steatosis to steatohepatitis with fibrosis.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters hepatic polyunsaturated fatty acid metabolism and eicosanoid biosynthesis in female Sprague-Dawley rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor (AhR) agonist that elicits a broad spectrum of dose-dependent hepatic effects including lipid accumulation, inflammation, and fibrosis. To determine the role of inflammatory lipid mediators in TCDD-mediated hepatotoxicity, eicosanoid metabolism was investigated. Female Sprague-Dawley (SD) rats were orally gavaged with sesame oil vehicle or 0.01-10 μg/kg TCDD every 4 days for 28 days. Hepatic RNA-Seq data was integrated with untargeted metabolomics of liver, serum, and urine, revealing dose-dependent changes in linoleic acid (LA) and arachidonic acid (AA) metabolism. TCDD also elicited dose-dependent differential gene expression associated with the cyclooxygenase, lipoxygenase, and cytochrome P450 epoxidation/hydroxylation pathways with corresponding changes in ω-6 (e.g. AA and LA) and ω-3 polyunsaturated fatty acids (PUFAs), as well as associated eicosanoid metabolites. Overall, TCDD increased the ratio of ω-6 to ω-3 PUFAs. Phospholipase A2 (Pla2g12a) was induced consistent with increased AA metabolism, while AA utilization by induced lipoxygenases Alox5 and Alox15 increased leukotrienes (LTs). More specifically, TCDD increased pro-inflammatory eicosanoids including leukotriene LTB4, and LTB3, known to recruit neutrophils to damaged tissue. Dose-response modeling suggests the cytochrome P450 hydroxylase/epoxygenase and lipoxygenase pathways are more sensitive to TCDD than the cyclooxygenase pathway. Hepatic AhR ChIP-Seq analysis found little enrichment within the regulatory regions of differentially expressed genes (DEGs) involved in eicosanoid biosynthesis, suggesting TCDD-elicited dysregulation of eicosanoid metabolism is a downstream effect of AhR activation. Overall, these results suggest alterations in eicosanoid metabolism may play a key role in TCDD-elicited hepatotoxicity associated with the progression of steatosis to steatohepatitis.

Protective roles of FICZ and aryl hydrocarbon receptor axis on alveolar bone loss and inflammation in experimental periodontitis

AIM

The aryl hydrocarbon receptor (AhR)-ligand axis has been shown to be involved in inflammatory diseases and bone homeostasis. However, the activation of AhR signalling pathway and the possible functions of AhR ligands in periodontitis are underexplored. This study investigated the expression of the AhR target gene cytochrome P450 subfamily B member 1 (CYP1B1) and the functions and mechanisms of the AhR ligand 6 formylindolo[3,2-b]carbazole (FICZ) in periodontitis.

MATERIALS AND METHODS

CYP1B1 expression was detected in human periodontitis samples, mice with ligature-induced periodontitis and lipopolysaccharide (LPS)-induced inflammation in periodontal ligament cells (PDLCs) in vitro. FICZ was administered topically or systemically. The therapeutic functions of FICZ were detected via qPCR, micro-computed tomography and immunohistochemistry. Finally, the mechanisms of AhR signalling in periodontitis were investigated by cell assays.

RESULTS

CYP1B1 expression was downregulated in periodontitis. FICZ rescued the alveolar bone loss and mitigated the inflammatory cytokines in periodontitis mice. In vitro, FICZ pre-treatment reduced the LPS-induced inflammation in PDLCs via the increased phosphorylation of STAT3. Additionally, FICZ prompted the mineralization of PDLCs via activation of the Wnt/β-catenin signalling pathway.

CONCLUSION

AhR signalling pathway is suppressed in periodontitis and the AhR ligand FICZ can prevent periodontitis.

The aryl hydrocarbon receptor: an environmental sensor integrating immune responses in health and disease

The environment, diet, microbiota and body's metabolism shape complex biological processes in health and disease. However, our understanding of the molecular pathways involved in these processes is still limited. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that integrates environmental, dietary, microbial and metabolic cues to control complex transcriptional programmes in a ligand-specific, cell-type-specific and context-specific manner. In this Review, we summarize our current knowledge of AHR and the transcriptional programmes it controls in the immune system. Finally, we discuss the role of AHR in autoimmune and neoplastic diseases of the central nervous system, with a special focus on the gut immune system, the gut-brain axis and the therapeutic potential of targeting AHR in neurological disorders.

Mechanisms of Environmental Contributions to Fatty Liver Disease

PURPOSE

Fatty liver disease (FLD) affects over 25% of the global population and may lead to liver-related mortality due to cirrhosis and liver cancer. FLD caused by occupational and environmental chemical exposures is termed "toxicant-associated steatohepatitis" (TASH). The current review addresses the scientific progress made in the mechanistic understanding of TASH since its initial description in 2010.

RECENT FINDINGS

Recently discovered modes of actions for volatile organic compounds and persistent organic pollutants include the following: (i) the endocrine-, metabolism-, and signaling-disrupting chemical hypotheses; (ii) chemical-nutrient interactions and the "two-hit" hypothesis. These key hypotheses were then reviewed in the context of the steatosis adverse outcome pathway (AOP) proposed by the US Environmental Protection Agency. The conceptual understanding of the contribution of environmental exposures to FLD has progressed significantly. However, because this is a new research area, more studies including mechanistic human data are required to address current knowledge gaps.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces peripheral blood abnormalities and plasma cell neoplasms resembling multiple myeloma in mice

Although epidemiologic studies have suggested a possible association between occupational exposures to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the risk of development of multiple myeloma, definitive evidence in support of this association is lacking. In the present study, we employed the Vk*Myc mouse model of multiple myeloma to assess the impact of TCDD exposure on multiple myeloma pathogenesis. TCDD induced splenomegaly and multiple peripheral blood abnormalities, including anemia and high serum IgG levels. In addition, TCDD triggered bone lytic lesions, as well as renal tubular casts, a phenomenon associated with human myeloma kidney disease. Even in wild-type C57BL/6 mice, TCDD increased serum IgG levels, induced anemia, and increased plasma cell presence in the spleen and bone marrow, hallmarks of benign monoclonal gammopathy. Lastly, TCDD induced AKT activation and the DNA damage response, key pathogenic events in myeloma pathogenesis, in animal spleen and/or bone marrow. These data indicate that TCDD accelerates monoclonal gammopathy development and promotes progression to multiple myeloma in genetically-predisposed mice. This work offers the first direct experimental evidence establishing TCDD as an environmental risk factor for monoclonal gammopathy of undetermined significance and multiple myeloma.

Comparison of Hepatic NRF2 and Aryl Hydrocarbon Receptor Binding in 2,3,7,8-Tetrachlorodibenzo-p-dioxin-Treated Mice Demonstrates NRF2-Independent PKM2 Induction

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces hepatic oxidative stress following activation of the aryl hydrocarbon receptor (AhR). Our recent studies showed TCDD induced pyruvate kinase muscle isoform 2 (Pkm2) as a novel antioxidant response in normal differentiated hepatocytes. To investigate cooperative regulation between nuclear factor, erythroid derived 2, like 2 (Nrf2) and the AhR in the induction of Pkm2, hepatic chromatin immunoprecipitation sequencing (ChIP-seq) analyses were integrated with RNA sequencing (RNA-seq) time-course data from mice treated with TCDD for 2-168 hours. ChIP-seq analysis 2 hours after TCDD treatment identified genome-wide NRF2 enrichment. Approximately 842 NRF2-enriched regions were located in the regulatory region of differentially expressed genes (DEGs), whereas 579 DEGs showed both NRF2 and AhR enrichment. Sequence analysis of regions with overlapping NRF2 and AhR enrichment showed over-representation of either antioxidant or dioxin response elements, although 18 possessed both motifs. NRF2 exhibited negligible enrichment within a closed Pkm chromatin region, whereas the AhR was enriched 29-fold. Furthermore, TCDD induced Pkm2 in primary hepatocytes from wild-type and Nrf2-null mice, indicating NRF2 is not required. Although NRF2 and AhR cooperate to regulate numerous antioxidant gene expression responses, the induction of Pkm2 by TCDD is independent of reactive oxygen species-mediated NRF2 activation.

Chronic exposure of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces an obesogenic effect in C57BL/6J mice fed a high fat diet

Contaminant involvement in the pathophysiology of obesity is widely recognized. It has been shown that low dose and chronic exposure to endocrine disruptor compounds (EDCs) potentiated diet- induced obesity. High and acute exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a persistent organic pollutant (POP) and an EDC with anti-estrogenic property, causes wasting syndrome . However at lower doses, the TCDD metabolic effects remain poorly understood. We investigated the obesogenic effect during chronic exposure of TCDD at 1μg/kg body weight (bw)/week in adult C57BL/6J mice fed with a high fat diet (HFD) and exposed from 10 to 42 weeks old to TCDD or equal volume of vehicle by intragastric gavage. Under these conditions, TCDD was obesogenic in adult mice (7% in males and 8% in females), which was linked to fat mass. A sex effect was observed in the fat mass distribution in adipose tissue and in the hepatic triglyceride content evolution. In visceral fat pad weight, we observed a decrease (11%) in males and an increase (14%) in females. The hepatic triglyceride content increase (41%) in females only. TCDD failed to induce any change in plasma parameters regarding glucose and lipid homeostasis. Messenger ribonucleic acid (mRNA) levels involved in adipose tissue and hepatic metabolism, inflammation, xenobiotic metabolism and endocrine disruption were differently regulated between males and females. In conclusion, these results provide new evidence that dioxin, a POP and EDC can be obesogenic for adult mice with multi-organ effects.

Lipidomic Evaluation of Aryl Hydrocarbon Receptor-Mediated Hepatic Steatosis in Male and Female Mice Elicited by 2,3,7,8-Tetrachlorodibenzo-p-dioxin

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces hepatic steatosis mediated by the aryl hydrocarbon receptor. To further characterize TCDD-elicited hepatic lipid accumulation, mice were gavaged with TCDD every 4 days for 28 days. Liver samples were examined using untargeted lipidomics with structural confirmation of lipid species by targeted high-resolution MS/MS, and data were integrated with complementary RNA-Seq analyses. Approximately 936 unique spectral features were detected, of which 379 were confirmed as unique lipid species. Both male and female samples exhibited similar qualitative changes (lipid species) but differed in quantitative changes. A shift to higher mass lipid species was observed, indicative of increased free fatty acid (FFA) packaging. For example, of the 13 lipid classes examined, triglycerides increased from 46 to 48% of total lipids to 68-83% in TCDD treated animals. Hepatic cholesterol esters increased 11.3-fold in male mice with moieties consisting largely of dietary fatty acids (FAs) (i.e., linolenate, palmitate, and oleate). Phosphatidylserines, phosphatidylethanolamines, phosphatidic acids, and cardiolipins decreased 4.1-, 5.0-, 5.4- and 7.4-fold, respectively, while ceramides increased 6.6-fold. Accordingly, the integration of lipidomic data with differential gene expression associated with lipid metabolism suggests that in addition to the repression of de novo fatty acid synthesis and β-oxidation, TCDD also increased hepatic uptake and packaging of lipids, while inhibiting VLDL secretion, consistent with hepatic fat accumulation and the progression to steatohepatitis with fibrosis.

Fibrin(ogen) drives repair after acetaminophen-induced liver injury via leukocyte αMβ2 integrin-dependent upregulation of Mmp12

BACKGROUND & AIMS

Acetaminophen (APAP)-induced liver injury is coupled with activation of the blood coagulation cascade and fibrin(ogen) accumulation within APAP-injured livers of experimental mice. We sought to define the role of fibrin(ogen) deposition in APAP-induced liver injury and repair.

METHODS

Wild-type, fibrinogen-deficient mice, mutant mice with fibrin(ogen) incapable of binding leukocyte αMβ2 integrin (Fibγ390-396A mice) and matrix metalloproteinase 12 (Mmp12)-deficient mice were fasted, injected with 300mg/kg APAP i.p. and evaluated at a range of time-points. Plasma and liver tissue were analyzed. Rescue of Fibγ390-396A mice was carried out with exogenous Mmp12. To examine the effect of the allosteric leukocyte integrin αMβ2 activator leukadherin-1 (LA-1), APAP-treated mice were injected with LA-1.

RESULTS

In wild-type mice, APAP overdose increased intrahepatic levels of high molecular weight cross-linked fibrin(ogen). Anticoagulation reduced early APAP hepatotoxicity (6h), but increased hepatic injury at 24h, implying a protective role for coagulation at the onset of repair. Complete fibrin(ogen) deficiency delayed liver repair after APAP overdose, evidenced by a reduction of proliferating hepatocytes (24h) and unresolved hepatocellular necrosis (48 and 72h). Fibγ390-396A mice had decreased hepatocyte proliferation and increased multiple indices of liver injury, suggesting a mechanism related to fibrin(ogen)-leukocyte interaction. Induction of Mmp12, was dramatically reduced in APAP-treated Fibγ390-396A mice. Mice lacking Mmp12 displayed exacerbated APAP-induced liver injury, resembling Fibγ390-396A mice. In contrast, administration of LA-1 enhanced hepatic Mmp12 mRNA and reduced necrosis in APAP-treated mice. Further, administration of recombinant Mmp12 protein to APAP-treated Fibγ390-396A mice restored hepatocyte proliferation.

CONCLUSIONS

These studies highlight a novel pathway of liver repair after APAP overdose, mediated by fibrin(ogen)-αMβ2 integrin engagement, and demonstrate a protective role of Mmp12 expression after APAP overdose.

LAY SUMMARY

Acetaminophen overdose leads to activation of coagulation cascade and deposition of high molecular weight cross-linked fibrin(ogen) species in the liver. Fibrin(ogen) is required for stimulating liver repair after acetaminophen overdose. The mechanism whereby fibrin(ogen) drives liver repair after acetaminophen overdose requires engagement of leukocyte αMβ2 integrin and subsequent induction of matrix metalloproteinase 12.

From the Cover: Coagulation-Driven Hepatic Fibrosis Requires Protease Activated Receptor-1 (PAR-1) in a Mouse Model of TCDD-Elicited Steatohepatitis

Emerging evidence supports a role for environmental chemical exposure in the pathology of non-alcoholic fatty liver disease (NAFLD), a disease process tightly linked to increased activity of the blood coagulation cascade. Exposure of C57BL/6 mice to the persistent environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) recapitulates features of the NAFLD spectrum, including steatosis, hepatic injury, inflammation, and fibrosis. We assessed coagulation cascade activation, and determined the role of the thrombin receptor protease activated receptor-1 (PAR-1) in experimental TCDD-elicited NAFLD. Chronic exposure to TCDD (30 µg/kg every 4 days for 28 days) was associated with intrahepatic coagulation, indicated by increased plasma thrombin-antithrombin levels and hepatic fibrin(ogen) deposition. PAR-1 deficiency diminished TCDD-elicited body weight loss and relative liver weight was reduced in TCDD-exposed PAR-1^-/- mice compared with TCDD-exposed wild-type mice. PAR-1 deficiency did not affect TCDD-induced hepatic steatosis or hepatocellular injury, as indicated by serum alanine aminotransferase activity. Despite a lack of effect on these 2 features of NAFLD pathology, PAR-1 deficiency was associated with a reduction in hepatic inflammation evident in liver histopathology, and reflected by a reduction in serum levels of the proinflammatory cytokine interleukin-6. Moreover, TCDD-driven hepatic collagen deposition was markedly reduced in PAR-1-deficient mice. These results indicate that experimental TCDD-elicited steatohepatitis is associated with coagulation cascade activation and PAR-1-driven hepatic inflammation and fibrosis.

Dose-Dependent Metabolic Reprogramming and Differential Gene Expression in TCDD-Elicited Hepatic Fibrosis

We have previously shown that in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-elicited NAFLD progression, central carbon, glutaminolysis, and serine/folate metabolism are reprogrammed to support NADPH production and ROS defenses. To further investigate underlying dose-dependent responses associated with TCDD-induced fibrosis, female C57BL/6 mice were gavaged with TCDD every 4 days (d) for 28 d or 92 d. RNA-Seq, ChIP-Seq (2 h), and 28 d metabolomic (urine, serum, and hepatic extract) analyses were conducted with complementary serum marker assessments at 92 d. Additional vehicle and 30 µg/kg treatment groups were allowed to recover for 36 d following the 92-d treatment regimen to examine recovery from TCDD-elicited fibrosis. Histopathology revealed dose-dependent increases in hepatic fat accumulation, inflammation, and periportal collagen deposition at 92 days, with increased fibrotic severity in the recovery group. Serum proinflammatory and profibrotic interleukins-1β, -2, -4, -6, and -10, as well as TNF-α and IFN-γ, exhibited dose-dependent induction. An increase in glucose tolerance was observed with a concomitant 3.0-fold decrease in hepatic glycogen linked to increased ascorbic acid biosynthesis and proline metabolism, consistent with increased fibrosis. RNA-Seq identified differential expression of numerous matrisome genes including an 8.8-fold increase in Tgfb2 indicating myofibroblast activation. Further analysis suggests reprogramming of glycogen, ascorbic acid, and amino acid metabolism in support of collagen deposition and the use of proline as a substrate for ATP production via the proline cycle. In summary, we demonstrate that glycogen, ascorbic acid, and amino acid metabolism are also reorganized to support remodeling of the extracellular matrix, progressing to hepatic fibrosis in response to chronic injury from TCDD.

2,3,7,8-Tetrachlorodibenzo-p-Dioxin Alters Lipid Metabolism and Depletes Immune Cell Populations in the Jejunum of C57BL/6 Mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor agonist that elicits dose-dependent hepatic fat accumulation and inflammation that can progress to steatohepatitis. To investigate intestine-liver interactions that contribute to TCDD-elicited steatohepatitis, we examined the dose-dependent effects of TCDD (0.01, 0.03, 0.1, 0.3, 1, 3, 10, or 30 µg/kg) on jejunal epithelial gene expression in C57BL/6 mice orally gavaged every 4 days for 28 days. Agilent 4x44K whole-genome microarray analysis of the jejunal epithelium identified 439 differentially expressed genes (|fold change| ≥ 1.5, P1(t) ≥ 0.999) across 1 or more doses, many related to lipid metabolism and immune system processes. TCDD-elicited differentially expressed genes were associated with lipolysis, fatty acid/cholesterol absorption and transport, the Kennedy pathway, and retinol metabolism, consistent with increased hepatic fat accumulation. Moreover, several major histocompatibility complex (MHC) class II genes (H2-Aa, H2-Ab1, H2-DMb1, Cd74) were repressed, coincident with decreased macrophage and dendritic cell levels in the lamina propria, suggesting migration of antigen-presenting cells out of the intestine. In contrast, hepatic RNA-Seq analysis identified increased expression of MHC class II genes, as well as chemokines and chemokine receptors involved in macrophage recruitment (Ccr1, Ccr5, Ccl5, Cx3cr1), consistent with hepatic F4/80 labeling and macrophage infiltration into the liver. Collectively, these results suggest TCDD elicits changes that support hepatic lipid accumulation, macrophage migration, and the progression of hepatic steatosis to steatohepatitis.

RNA-Seq versus oligonucleotide array assessment of dose-dependent TCDD-elicited hepatic gene expression in mice

BACKGROUND

Dose-dependent differential gene expression provides critical information required for regulatory decision-making. The lower costs associated with RNA-Seq have made it the preferred technology for transcriptomic analysis. However, concordance between RNA-Seq and microarray analyses in dose response studies has not been adequately vetted.

RESULTS

We compared the hepatic transcriptome of C57BL/6 mice following gavage with sesame oil vehicle, 0.01, 0.03, 0.1, 0.3, 1, 3, 10, or 30 μg/kg TCDD every 4 days for 28 days using Illumina HiSeq RNA-Sequencing (RNA-Seq) and Agilent 4 × 44 K microarrays using the same normalization and analysis approach. RNA-Seq and microarray analysis identified a total of 18,063 and 16,403 genes, respectively, that were expressed in the liver. RNA-Seq analysis for differentially expressed genes (DEGs) varied dramatically depending on the P1(t) cut-off while microarray results varied more based on the fold change criteria, although responses strongly correlated. Verification by WaferGen SmartChip QRTPCR revealed that RNA-Seq had a false discovery rate of 24% compared to 54% for microarray analysis. Dose-response modeling of RNA-Seq and microarray data demonstrated similar point of departure (POD) and ED50 estimates for common DEGs.

CONCLUSIONS

There was a strong correspondence between RNA-Seq and Agilent array transcriptome profiling when using the same samples and analysis strategy. However, RNA-Seq provided superior quantitative data, identifying more genes and DEGs, as well as qualitative information regarding identity and annotation for dose response modeling in support of regulatory decision-making.

Comparisons of differential gene expression elicited by TCDD, PCB126, βNF, or ICZ in mouse hepatoma Hepa1c1c7 cells and C57BL/6 mouse liver

The aryl hydrocarbon receptor (AhR) is a promiscuous receptor activated by structurally diverse synthetic and natural compounds. AhR activation may lead to ligand-specific changes in gene expression despite similarities in mode of action. Therefore, differential gene expression elicited by four structurally diverse, high affinity AhR ligands (2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 10nM, 30 μg/kg), 3,3',4,4',5-pentachlorobiphenyl (PCB126; 100nM, 300μg/kg), β-naphthoflavone (βNF; 10 μM, 90 mg/kg), and indolo[3,2-b]carbazole (ICZ; 1μM)) in mouse Hepa1c1c7 hepatoma cells and C57BL/6 mouse liver samples were compared. A total of 288, 183, 119, and 131 Hepa1c1c7 genes were differentially expressed (|fold-change|≥ 1.5, P1(t)≥ 0.9999) by TCDD, βNF, PCB126, and ICZ, respectively. Only ∼35% were differentially expressed by all 4 ligands in Hepa1c1c7 cells. In vivo, 661, 479, and 265 hepatic genes were differentially expressed following treatment with TCDD, βNF, and PCB126, respectively. Similar to Hepa1c1c7 cells, ≤ 34% of gene expression changes were common across all ligands. Principal components analysis identified time-dependent gene expression divergence. Comparisons of ligand-elicited expression between Hepa1c1c7 cells and mouse liver identified only 11 common gene expression changes across all ligands. Although metabolism may explain some ligand-specific gene expression changes, PCB126, βNF, and ICZ also elicited divergent expression compared to TCDD, suggestive of selective AhR modulation.