Nuclear factor-E2-related factor 2 is a major determinant of bile acid homeostasis in the liver and intestine

Beneficial effects of hepatic cyclooxygenase-2 expression against cholestatic injury after common bile duct ligation in mice

BACKGROUND AND AIMS

Cyclooxygenase-2 (COX-2) is involved in different liver diseases, but little is known about the significance of COX-2 in cholestatic injury. This study was designed to elucidate the role of COX-2 expression in hepatocytes during the pathogenesis of obstructive cholestasis.

METHODS

We used genetically modified mice constitutively expressing human COX-2 in hepatocytes. Transgenic mice (hCOX-2-Tg) and their wild-type (Wt) littermates were either subjected to a mid-abdominal laparotomy or common bile duct ligation (BDL) for 2 or 5 days. Then, we explored the mechanisms underlying the role of COX-2 and its derived prostaglandins in liver function, and the synthesis and excretion of bile acids (BA) in response to cholestatic liver injury.

RESULTS

After BDL, hCOX-2-Tg mice showed lower grades of hepatic necrosis and inflammation than Wt mice, in part by a reduced hepatic neutrophil recruitment associated with lower mRNA levels of pro-inflammatory cytokines. Furthermore, hCOX-2-Tg mice displayed a differential metabolic pattern of BA synthesis that led to an improved clearance after BDL-induced accumulation. In addition, an enhanced response to the BDL-induced oxidative stress and hepatic apoptosis was observed. In vitro experiments using hepatic cells that stably express hCOX-2 confirmed the cytoprotective role of prostaglandin E2 against BA toxicity.

CONCLUSIONS

Taken together, our data indicate that constitutive expression of COX-2 in hepatocytes ameliorates cholestatic liver injury in mice by reducing inflammation and cell damage and by modulating BA metabolism, pointing to a role for COX-2 as a defensive response against cholestasis-derived BA accumulation and injury.

Sex-specific effects of gut microbiome on shaping bile acid metabolism

Gut microbiome is a group of microorganisms that plays important roles in contributing to health and diseases. These bacterial compositions have been demonstrated to impact bile acids (BAs) profiles, either by directly metabolizing primary BAs to secondary BAs or indirect ways through host metabolism by influencing BAs synthesis, transportation and conjugation in liver. It has been observed sexually dimorphic gut microbiome and bile acids composition, with variations in expression levels of bile acid metabolizing genes in the liver. However, associations between sex-specific differences in gut microbiome and BAs profiles are not well understood. This study aimed to investigate whether gut microbiome could influence BAs profiles in host in a sexspecific manner. We transplanted cecum feces of male and female C57BL/6 mice to male mice and measured BAs concentrations in feces, serum and liver samples 7 days after fecal transplantation. We found different BAs profiles between mice with male and female gut microbiome, including altering levels and proportions of secondary BAs. We also observed varied expression levels of genes related to bile acid metabolism in the liver and distal ileum. Our results highlight sex-specific effects of gut microbiome on shaping bile acid metabolism through gut bacteria and regulation of host genes.

Activation of Nrf2/HO-1 signaling pathway exacerbates cholestatic liver injury

Nuclear factor erythroid 2-related factor-2 (Nrf2) antioxidant signaling is involved in liver protection, but this generalization overlooks conflicting studies indicating that Nrf2 effects are not necessarily hepatoprotective. The role of Nrf2/heme oxygenase-1 (HO-1) in cholestatic liver injury (CLI) remains poorly defined. Here, we report that Nrf2/HO-1 activation exacerbates liver injury rather than exerting a protective effect in CLI. Inhibiting HO-1 or ameliorating bilirubin transport alleviates liver injury in CLI models. Nrf2 knockout confers hepatoprotection in CLI mice, whereas in non-CLI mice, Nrf2 knockout aggravates liver damage. In the CLI setting, oxidative stress activates Nrf2/HO-1, leads to bilirubin accumulation, and impairs mitochondrial function. High levels of bilirubin reciprocally upregulate the activation of Nrf2 and HO-1, while antioxidant and mitochondrial-targeted SOD2 overexpression attenuate bilirubin toxicity. The expression of Nrf2 and HO-1 is elevated in serum of patients with CLI. These results reveal an unrecognized function of Nrf2 signaling in exacerbating liver injury in cholestatic disease.

Nonalcoholic steatohepatitis and mechanisms by which it is ameliorated by activation of the CNC-bZIP transcription factor Nrf2

Non-alcoholic steatohepatitis (NASH) represents a global health concern. It is characterised by fatty liver, hepatocyte cell death and inflammation, which are associated with lipotoxicity, endoplasmic reticulum (ER) stress, mitochondrial dysfunction, iron overload and oxidative stress. NF-E2 p45-related factor 2 (Nrf2) is a transcription factor that combats oxidative stress. Remarkably, Nrf2 is downregulated during the development of NASH, which probably accelerates disease, whereas in pre-clinical studies the upregulation of Nrf2 inhibits NASH. We now review the scientific literature that proposes Nrf2 downregulation during NASH involves its increased ubiquitylation and proteasomal degradation, mediated by Kelch-like ECH-associated protein 1 (Keap1) and/or β-transducin repeat-containing protein (β-TrCP) and/or HMG-CoA reductase degradation protein 1 (Hrd1, also called synoviolin (SYVN1)). Additionally, downregulation of Nrf2-mediated transcription during NASH may involve diminished recruitment of coactivators by Nrf2, due to increased levels of activating transcription factor 3 (ATF3) and nuclear factor-kappaB (NF-κB) p65, or competition for promoter binding due to upregulation of BTB and CNC homology 1 (Bach1). Many processes that downregulate Nrf2 are triggered by transforming growth factor-beta (TGF-β), with oxidative stress amplifying its signalling. Oxidative stress may also increase suppression of Nrf2 by β-TrCP through facilitating formation of the DSGIS-containing phosphodegron in Nrf2 by glycogen synthase kinase-3. In animal models, knockout of Nrf2 increases susceptibility to NASH, while pharmacological activation of Nrf2 by inducing agents that target Keap1 inhibits development of NASH. These inducing agents probably counter Nrf2 downregulation affected by β-TrCP, Hrd1/SYVN1, ATF3, NF-κB p65 and Bach1, by suppressing oxidative stress. Activation of Nrf2 is also likely to inhibit NASH by ameliorating lipotoxicity, inflammation, ER stress and iron overload. Crucially, pharmacological activation of Nrf2 in mice in which NASH has already been established supresses liver steatosis and inflammation. There is therefore compelling evidence that pharmacological activation of Nrf2 provides a comprehensive multipronged strategy to treat NASH.

Plumbagin ameliorates bile duct ligation-induced cholestatic liver injury in rats

Plumbagin, a natural bicyclic naphthoquinone, has diverse pharmacological properties and biological benefits against a number of disorders, including liver disease. Though plumbagin's hepatoprotective potential attracts attention, currently no experimental evidence exists on its effectiveness against cholestatic liver injury. The present study investigated its hepatoprotection in the rat model of extrahepatic cholestasis using Bile Duct Ligation (BDL). We found that daily plumbagin supplementation protected the liver from cholestatic damage. Hepatoprotective actions of plumbagin were accompanied by reduction of Transforming Growth Factor β1 (TGF-β1)/Smad, High Mobility Group Box-1 (HMGB1)/Toll-Like Receptor-4 (TLR4), Hypoxia-Inducible Factor-1α (HIF-1α), Aryl Hydrocarbon Receptor (AhR), Heat Shock Protein 90 (HSP90), caveolin-1, NF-κB/AP-1, Dynamin Related Protein-1 (Drp1), malondialdehyde level, Interleukin-1β (IL-1β), p62/SQSTM1, and caspase 3 as well as increase of Farnesoid X Receptor (FXR), bile acid efflux transporters, glutathione, LC3-II, Beclin1, and nuclear NF-E2-Related Factor-2 (Nrf2) and Transcription Factor EB (TFEB). The activation of nuclear Nrf2 caused by plumbagin correlated well with the improvement in bile acid retention, liver histology, serum biochemical, ductular reaction, mitochondrial dysfunction, oxidative stress, inflammation, apoptosis, impaired autophagy, and fibrosis, involving interplay of multiple intracellular signaling pathways. Plumbagin is likely a candidate drug to protect the liver from cholestatic damages. Despite the promising findings from this study, translational implication of plumbagin on cholestatic liver injury warrants further investigation.

Nrf2 deficiency causes hepatocyte dedifferentiation and reduced albumin production in an experimental extrahepatic cholestasis model

The transcription factor Nrf2 modulates the initiation and progression of a number of diseases including liver disorders. We evaluated whether Nrf2 mediates hepatic adaptive responses to cholestasis. Wild-type and Nrf2-null mice were subjected to bile duct ligation (BDL) or a sham operation. As cholestasis progressed to day 15 post-BDL, hepatocytes in the wild-type mice exhibited a tendency to dedifferentiate, indicated by the very weak expression of hepatic progenitor markers: CD133 and tumor necrosis factor-like weak induced apoptosis receptor (Fn14). During the same period, Nrf2 deficiency augmented this tendency, manifested by higher CD133 expression, earlier, stronger, and continuous induction of Fn14 expression, and markedly reduced albumin production. Remarkably, as cholestasis advanced to the late stage (40 days after BDL), hepatocytes in the wild-type mice exhibited a Fn14+ phenotype and strikingly upregulated the expression of deleted in malignant brain tumor 1 (DMBT1), a protein essential for epithelial differentiation during development. In contrast, at this stage, hepatocytes in the Nrf2-null mice entirely inhibited the upregulation of DMBT1 expression, displayed a strong CD133+/Fn14+ phenotype indicative of severe dedifferentiation, and persistently reduced albumin production. We revealed that Nrf2 maintains hepatocytes in the differentiated state potentially via the increased activity of the Nrf2/DMBT1 pathway during cholestasis.

18β-Glycyrrhetinic Acid Protects against Cholestatic Liver Injury in Bile Duct-Ligated Rats

18β-Glycyrrhetinic acid is a nutraceutical agent with promising hepatoprotective effects. Its protective mechanisms against cholestatic liver injury were further investigated in a rodent model of extrahepatic cholestasis caused by Bile Duct Ligation (BDL) in rats. The daily oral administration of 18β-Glycyrrhetinic acid improved liver histology, serum biochemicals, ductular reaction, oxidative stress, inflammation, apoptosis, impaired autophagy, and fibrosis. 18β-Glycyrrhetinic acid alleviated the BDL-induced hepatic and systemic retention of bile acids, matrix-producing cell activation, hepatic collagen deposition, Transforming Growth Factor beta-1/Smad activation, malondialdehyde elevation, glutathione reduction, High Mobility Group Box-1/Toll-Like Receptor-4 activation, NF-κB activation, inflammatory cell infiltration/accumulation, Interleukin-1β expression, Signal Transducer and Activator of Transcription-1 activation, Endoplasmic Reticulum stress, impairment autophagy, and caspase 3 activation. Conversely, the protein expression of Sirt1, Farnesoid X Receptor, nuclear NF-E2-Related Factor-2, Transcription Factor EB, bile acid efflux transporters, and LC3-II, as well as the protein phosphorylation of AMP-Activated Protein Kinase, was promoted in 18β-Glycyrrhetinic acid-treated BDL rats. The hepatoprotective effects of 18β-Glycyrrhetinic acid in the present investigation correlated well with co-activation and possible interactions among Sirt, FXR, and Nrf2. The concurrent or concomitant activation of Sirt1, FXR, and Nrf2 not only restored the homeostatic regulation of bile acid metabolism, but also alleviated oxidative stress, inflammation, apoptosis, impaired autophagy, and fibrosis.

Oltipraz ameliorates the progression of steatohepatitis in Nrf2-null mice fed a high-fat diet

Oltipraz, a synthetic dithiolethione, has chemopreventive effect through nuclear factor erythroid 2-related factor 2 (Nrf2) activation. Nrf2 is known to be involved in the development of experimental steatohepatitis in rodents. In this study, to evaluate the effect of oltipraz on lipid and bile acid metabolism, wild-type and Nrf2-null mice were fed the standard diet (containing 4% soybean oil) with or without oltipraz. Based on these results, we examined the effect of oltipraz on the experimental steatohepatitis in high-fat diet (containing 4% soybean oil and 20% lard) fed Nrf2-null mice. Oltipraz induced hepatic mRNA expression of peroxisome proliferator-activated receptor α, carnitine palmityl transferase 1, and bile salt export pump by Nrf2 independent mechanisms. In Nrf2-null mice fed a high-fat diet for 12 weeks, moderate to severe inflammation and fibrosis were observed. Oral administration of oltipraz suppressed the degree of inflammation and fibrosis in Nrf2-null mouse liver fed a high-fat diet. These histopathological findings approximately corresponded to the data of mRNA expression of tumor necrosis factor α, monocyte chemoattractant protein-1, Timp-1, and collagen type 1α1. These results indicated that oltipraz administration ameliorated liver injury by Nrf2 independent manner in a model of steatohepatitis generated by Nrf2-null mice with high-fat diet.

Advances of natural activators for Nrf2 signaling pathway on cholestatic liver injury protection: a review

Cholestasis is a common manifestation of obstruction of bile flow in various liver diseases. If the bile acid accumulation is not treated in time, it will further lead to hepatocyte damage, liver fibrosis and ultimately to cirrhosis, which seriously affects human life. The pathogenesis of cholestatic liver injury is very complicated, mainly including oxidative stress and inflammation. Nuclear factor erythroid 2-related factor 2 (Nrf2) is an important transcription factor responsible for upregulating expression of various genes with cytoprotective functions. Nrf2 activation has been proved to inhibit oxidative stress and inflammatory reaction, modulate bile acid homeostasis, and alleviate fibrosis during cholestasis. Therefore, Nrf2 emerges as a potential therapeutic target for cholestatic liver injury. In recent years, natural products with various biological activities including anti-inflammatory, anti-oxidant, anti-tumor and anti-fibrotic effects have received growing attention for being hepatoprotective agents. Natural products like asiatic acid, diosmin, rutin, and so forth have shown significant potential in activating Nrf2 pathway which can lead to attenuate cholestatic liver injury. Therefore, this paper emphasizes the effect of Nrf2 signaling pathway on alleviating cholestasis, and summarizes recent evidence about natural Nrf2 activators with hepatoprotective effect in various models of cholestatic liver injury, thus providing theoretical reference for the development of anti-cholestatic drug.

Bicyclol Alleviates Signs of BDL-Induced Cholestasis by Regulating Bile Acids and Autophagy-Mediated HMGB1/p62/Nrf2 Pathway

Cholestasis is a liver disease characterized by the accumulation of toxic bile salts, bilirubin, and cholesterol, resulting in hepatocellular damage. Recent findings have revealed several key steps of cholestasis liver injury including the toxicity of bile acids and accumulation of proinflammatory mediator. In this study, we investigated the protective effect of bicyclol in cholestasis caused by bile duct ligation (BDL), as well as relevant mechanisms. Bicyclol attenuated liver damage in BDL mice by increasing the levels of hydrophilic bile acid such as α-MCA and β-MCA, regulating bile acid-related pathways and improving histopathological indexes. High-mobility group box 1 (HMGB1) is an extracellular damage-associated molecular pattern molecule which can be used as biomarkers of cells and host defense. Bicyclol treatment decreased extracellular release of HMGB1. In addition, HMGB1 is also involved in regulating autophagy in response to oxidative stress. Bicyclol promoted the lipidation of LC3 (microtubule-associated protein 1 light chain 3)-Ⅱ to activate autophagy. The nuclear factor, E2-related factor 2 (Nrf2) and its antioxidant downstream genes were also activated. Our results indicate that bicyclol is a promising therapeutic strategy for cholestasis by regulating the bile acids and autophagy-mediated HMGB1/p62/Nrf2 pathway.

ASBT(SLC10A2): A promising target for treatment of diseases and drug discovery

Bile acids has gradually become a new focus in various diseases, and ASBT as a transporter responsible for the reabsorption of ileal bile acids, is a key hinge associated to the bile acids-cholesterol balance and bile acids of enterohepatic circulation. The cumulative studies have also shown that ASBT is a promising target for treatment of liver, gallbladder, intestinal and metabolic diseases. This article briefly reviewed the process of bile acids enterohepatic circulation, as well as the regulations of ASBT expression, covering transcription factors, nuclear receptors and gut microbiota. In addition, the relationship between ASBT and various diseases were discussed in this paper. According to the structural classification of ASBT inhibitors, the research status of ASBT inhibitors and potential ASBT inhibitors of traditional Chinese medicine (such resveratrol, jatrorrhizine in Coptis chinensis) were summarized. This review provides a basis for the development of ASBT inhibitors and the treatment strategy of related diseases.

Can Prohibitin 1 be a Safeguard against liver disease?

Prohibitin (PHB) 1 is involved in multiple regulatory pathways in liver disease to protect hepatocytes, and its function is associated with subcellular localization. PHB1 located in the nucleus, cytoplasm and the mitochondrial inner membrane has anti-oxidative stress and anti-inflammatory effects in hepatitis and cirrhosis, which can protect liver cells from damage caused by inflammatory factors and reactive oxygen species (ROS) stimulation. The low expression of PHB1 located in the nucleus of liver cancer cells inhibits the proliferation and metastasis of liver cancer; thus, PHB1 exhibits the function of a tumor suppressor gene. Understanding the mechanisms of PHB1 in liver diseases may be useful for further research on the disease and may provide new ideas for the development of targeted therapeutic drugs in the future. Therefore, this review puts forward an overview of the role of PHB1 and its protective mechanism in liver diseases.

Cholestasis Differentially Affects Liver Connexins

Connexins are goal keepers of tissue homeostasis, including in the liver. As a result, they are frequently involved in disease. The current study was set up to investigate the effects of cholestatic disease on the production of connexin26, connexin32 and connexin43 in the liver. For this purpose, bile duct ligation, a well-known trigger of cholestatic liver injury, was applied to mice. In parallel, human hepatoma HepaRG cell cultures were exposed to cholestatic drugs and bile acids. Samples from both the in vivo and in vitro settings were subsequently subjected to assessment of mRNA and protein quantities as well as to in situ immunostaining. While the outcome of cholestasis on connexin26 and connexin43 varied among experimental settings, a more generalized repressing effect was seen for connexin32. This has also been observed in many other liver pathologies and could suggest a role for connexin32 as a robust biomarker of liver disease and toxicity.

Constitutive Activation of Nrf2 in Mice Expands Enterogenesis in Small Intestine Through Negative Regulation of Math1

BACKGROUND & AIMS

Notch signaling coordinates cell differentiation processes in the intestinal epithelium. The transcription factor Nrf2 orchestrates defense mechanisms by regulating cellular redox homeostasis, which, as shown previously in murine liver, can be amplified through signaling crosstalk with the Notch pathway. However, interplay between these 2 signaling pathways in the gut is unknown.

METHODS

Mice modified genetically to amplify Nrf2 in the intestinal epithelium (Keap1^f/f::VilCre) were generated as well as pharmacological activation of Nrf2 and subjected to phenotypic and cell lineage analyses. Cell lines were used for reporter gene assays together with Nrf2 overexpression to study transcriptional regulation of the Notch downstream effector.

RESULTS

Constitutive activation of Nrf2 signaling caused increased intestinal length along with expanded cell number and thickness of enterocytes without any alterations of secretory lineage, outcomes abrogated by concomitant disruption of Nrf2. The Nrf2 and Notch pathways in epithelium showed inverse spatial profiles, where Nrf2 activity in crypts was lower than villi. In progenitor cells of Keap1^f/f::VilCre mice, Notch downstream effector Math1, which regulates a differentiation balance of cell lineage through lateral inhibition, showed suppressed expression. In vitro results demonstrated Nrf2 negatively regulated Math1, where 6 antioxidant response elements located in the regulatory regions contributed to this repression.

CONCLUSIONS

Activation of Nrf2 perturbed the dialog of the Notch cascade though negative regulation of Math1 in progenitor cells, leading to enhanced enterogenesis. The crosstalk between the Nrf2 and Notch pathways could be critical for fine-tuning intestinal homeostasis and point to new approaches for the pharmacological management of absorptive deficiencies.

Effects of ablation and activation of Nrf2 on bile acid homeostasis in male mice

The role of nuclear factor erythroid 2-related factor 2 (Nrf2) in bile acid (BA) homeostasis remains controversial. In this study, activation of Nrf2 was achieved either pharmacologically by CDDO-imidazolide (CDDO-Im) or genetically through a "gene dose-response" model consisting of Nrf2-null, wild-type (WT), Keap1-knockdown (Keap1-KD), and Keap1-hepatocyte knockout (Keap1-HKO) mice. In WT mice, CDDO-Im increased bile flow and decreased hepatic BAs, which was associated with a down-regulation of the canalicular BA efflux transporter Bsep and an increase in biliary BA excretion. In contrast, hepatic Bsep and biliary BA excretion were not altered in Keap1-KD or Keap1-HKO mice, suggesting that Nrf2 is not important for regulating Bsep or BA-dependent bile flow. In contrast, hepatic Mrp2 and Mrp3 were up-regulated by both pharmacological and genetic activations of Nrf2. Furthermore, ileal BA transporters (Asbt and Ostβ) and cholesterol transporters (Abcg5 and Abcg8) were down-regulated by both pharmacological and genetic activations of Nrf2, suggesting a role of Nrf2 in intestinal absorption of BAs and cholesterol. In Nrf2-null mice, CDDO-Im down-regulated hepatic BA uptake transporters (Ntcp, Oatp1a1, and Oatp1b2), leading to a 39-fold increase of serum BAs. To conclude, the present study demonstrates that activation of Nrf2 in mice up-regulates Mrp2 and Mrp3 in the liver and down-regulates BA and cholesterol transporters in the intestine.

Exercise protects against ethanol-induced damage in rat heart and liver through the inhibition of apoptosis and activation of Nrf2/Keap-1/HO-1 pathway

PROPOSE

Understanding the protective effect of exercise against ethanol-induced toxicity through the oxidative stress signaling pathway, apoptosis, and cholesterol metabolism is important to prevent development of cardiovascular diseases.

METHODS

Thirty-two male Wistar rats were randomly divided into four equal groups as follow: control, exercise training (ET), ethanol (4 g/kg of body weight/day) and ET + ethanol. The ET and ET + Ethanol groups ran on the treadmill at 65% maximum running speed for 60 min for five sessions per week for eight weeks. The ethanol and ET + Ethanol groups received ethanol for eight weeks. At the end of the study, animals were anesthetized and blood and tissues were sampled to examine the biochemical and molecular evaluation.

RESULTS

The results showed that the antioxidant enzymes activity decreased and MDA levels increased in the heart and liver of animals in ethanol group compared to control group. The levels of these oxidative biomarkers improved by ET in ET + Ethanol group compared to ethanol group. It showed that ET could protect the heart and liver against oxidative damage induced by ethanol through up-regulating the expression of the Nrf2/Keap-1/HO-1 pathway. ET could exert a cardioprotective effect on ethanol-induced apoptosis through down-regulating the Bax and the caspase-3 and via up-regulating the Bcl-2 expression in the heart. ET could also improve the impairment of cholesterol metabolism induced by ethanol.

CONCLUSION

Exercise can protect against ethanol-induced toxicity through moderating the expression of genes which are involved in oxidative status, apoptosis and cholesterol metabolism.

HGF induces protective effects in α-naphthylisothiocyanate-induced intrahepatic cholestasis by counteracting oxidative stress

Cholestasis is a clinical syndrome common to a large number of hepatopathies, in which either bile production or its transit through the biliary tract is impaired due to functional or obstructive causes; the consequent intracellular retention of toxic biliary constituents generates parenchyma damage, largely via oxidative stress-mediated mechanisms. Hepatocyte growth factor (HGF) and its receptor c-Met represent one of the main systems for liver repair damage and defense against hepatotoxic factors, leading to an antioxidant and repair response. In this study, we evaluated the capability of HGF to counteract the damage caused by the model cholestatic agent, α-naphthyl isothiocyanate (ANIT). HGF had clear anti-cholestatic effects, as apparent from the improvement in both bile flow and liver function test. Histology examination revealed a significant reduction of injured areas. HGF also preserved the tight-junctional structure. These anticholestatic effects were associated with the induction of basolateral efflux ABC transporters, which facilitates extrusion of toxic biliary compounds and its further alternative depuration via urine. The biliary epithelium seems to have been also preserved, as suggested by normalization in serum GGT levels, CFTR expression and cholangyocyte primary cilium structure our results clearly show for the first time that HGF protects the liver from a cholestatic injury.

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

Cholestatic liver disease results increased production of reactive aldehydes and an atypical periportal hepatic antioxidant response

Cholangiopathies such as primary sclerosing cholangitis (PSC) are chronic liver diseases characterized by increased cholestasis, biliary inflammation and oxidative stress. The objective of this study was to elucidate the impact of cholestatic injury on oxidative stress-related factors. Using hepatic tissue and whole cell liver extracts (LE) isolated from 11-week old C57BL/6J (WT) and Mdr2^KO mice, inflammation and oxidative stress was assessed. Concurrently, specific targets of carbonylation were assessed in LE prepared from murine groups as well as from normal and human patients with end-stage PSC. Identified carbonylated proteins were further evaluated using bioinformatics analyses. Picrosirius red staining revealed extensive fibrosis in Mdr2^KO liver, and fibrosis colocalized with increased periportal inflammatory cells and both acrolein and 4-HNE staining. Western blot analysis revealed elevated periportal expression of antioxidant proteins Cbr3, GSTμ, Prdx5, TrxR1 and HO-1 but not GCLC, GSTπ or catalase in the Mdr2^KO group when compared to WT. From immunohistochemical analysis, increased periportal reactive aldehyde production colocalized with elevated staining of Cbr3, GSTμ and TrxR1 but surprisingly not with Nrf2. Mass spectrometric analysis revealed an increase in carbonylated proteins in the Mdr2^KO and PSC groups compared to respective controls. Gene ontology and KEGG pathway analysis of carbonylated proteins revealed a propensity for increased carbonylation of proteins broadly involved in metabolic processes as well more specifically in Rab-mediated signal transduction, lysosomes and the large ribosomal subunit in human PSC. Western blot analysis of Rab-GTPase expression revealed no significant differences in Mdr2^KO mice when compared to WT livers. In contrast, PSC tissue exhibited decreased levels of Rabs 4, 5 and increased abundance of Rabs 6 and 9a protein. Results herein reveal that cholestasis induces stage-dependent increases in periportal oxidative stress responses and protein carbonylation, potentially contributing to pathogenesis in Mdr2^KO. Furthermore, during early stage cholestasis, there is cell-specific upregulation of some but not all, antioxidant proteins.

Protective effect of herbal medicine Huangqi decoction against chronic cholestatic liver injury by inhibiting bile acid-stimulated inflammation in DDC-induced mice

BACKGROUND

Huangqi decoction (HQD), a classic traditional herbal medicine, has been used for liver fibrosis, but its effect on intrahepatic chronic cholestatic liver injury remains unknown.

PURPOSE

In the present study, we investigated the hepatoprotective effect of HQD and the underlying molecular mechanisms in 3, 5-diethoxycarbonyl-1, 4-dihydroxychollidine (DDC)-induced chronic cholestatic mice.

METHODS

The DDC-induced cholestatic mice were administrated HQD for 4 or 8 weeks. Serum biochemistry and morphology were investigated. The serum and liver bile acid (BA) levels were detected by ultra performance liquid chromatography-tandem mass spectrometry. The liver expression of BA metabolizing enzymes and transporters, and inflammatory and fibrotic markers was measured by real-time polymerase chain reaction, western blotting, and immunohistochemistry.

RESULTS

HQD treatment for 4 or 8 weeks ameliorated DDC-induced liver injury by improving impaired hepatic function and tissue damage. HQD treatment for 8 weeks further decreased the liver expression of cytokeratin 19, tumor growth factor (TGF)-β, collagen I, and α-smooth muscle actin, and ameliorated ductular reaction and liver fibrosis. HQD markedly decreased the accumulation of serum and liver BA. The expression of BA-metabolizing enzymes, cytochrome P450 2b10 and UDP glucuronosyltransferase 1 A1, and multidrug resistance-associated protein 2, Mrp3, and Mrp4 involved in BA homeostasis was increased by 4 weeks of HQD treatment. The expression of BA uptake transporter Na⁺-taurocholate cotransporting polypeptide was decreased and that of Mrp4 was increased after 8 weeks of HQD treatment. Nuclear factor-E2-related factor-2 (Nrf2) was remarkably induced by HQD treatment. Additionally, HQD treatment for 8 weeks decreased the liver expression of inflammatory factors, interleukin (IL)-6, IL-1β, tumor necrosis factor-α, monocyte chemoattractant protein-1, and intracellular adhesion molecule-1. HQD suppressed the nuclear factor (NF)-κB pathway.

CONCLUSION

HQD protected mice against chronic cholestatic liver injury and biliary fibrosis, which may be associated with the induction of the Nrf2 pathway and inhibition of the NF-κB pathway, ameliorating BA-stimulated inflammation.

Hepatic Autophagy Deficiency Compromises Farnesoid X Receptor Functionality and Causes Cholestatic Injury

Autophagy is important for hepatic homeostasis, nutrient regeneration, and organelle quality control. We investigated the mechanisms by which liver injury occurred in the absence of autophagy function. We found that mice deficient in autophagy because of the lack of autophagy-related gene 7 or autophagy-related gene 5, key autophagy-related genes, manifested intracellular cholestasis with increased levels of serum bile acids, a higher ratio of tauromuricholic acid/taurocholic acid in the bile, increased hepatic bile acid load, abnormal bile canaliculi, and altered expression of hepatic transporters. In determining the underlying mechanism, we found that autophagy sustained and promoted the basal and up-regulated expression of farnesoid X receptor (Fxr) in the fed and starved conditions, respectively. Consequently, expression of Fxr and its downstream genes, particularly bile salt export pump, and the binding of FXR to the promoter regions of these genes, were suppressed in autophagy-deficient livers. In addition, codeletion of nuclear factor erythroid 2-related factor 2 (Nrf2) in autophagy deficiency status reversed the FXR suppression. Furthermore, the cholestatic injury of autophagy-deficient livers was reversed by enhancement of FXR activity or expression, or by Nrf2 deletion. Conclusion: Together with earlier reports that FXR can suppress autophagy, our findings indicate that autophagy and FXR form a regulatory loop and deficiency of autophagy causes abnormal FXR functionality, leading to the development of intracellular cholestasis and liver injury.

Evidence that decreased expression of sinusoidal bile acid transporters accounts for the inhibition by rapamycin of bile flow recovery following liver ischemia

Rapamycin is employed as an immunosuppressant following organ transplant and, in patients with hepatocellular carcinoma, to inhibit cancer cell regrowth following liver surgery. Preconditioning the liver with rapamycin to induce the expression of antioxidant enzymes is a potential strategy to reduce ischemia reperfusion (IR) injury. However, pre-treatment with rapamycin inhibits bile flow, especially following ischemia. The aim was to investigate the mechanisms involved in this inhibition. In a rat model of segmental hepatic ischemia and reperfusion, acute administration of rapamycin by intravenous injection did not inhibit the basal rate of bile flow. Pre-treatment of rats with rapamycin for 24 h by intraperitoneal injection inhibited the expression of mRNA encoding the sinusoidal influx transporters Ntcp, Oatp1 and 2 and the canalicular efflux transporter Bsep, and increased expression of canalicular Mrp2. Dose-response curves for the actions of rapamycin on the expression of Bsep and Ntcp in cultured rat hepatocytes were biphasic, and monophasic for effects on Oatp1. In cultured tumorigenic H4IIE liver cells, several bile acid transporters were not expressed, or were expressed at very low levels compared to hepatocytes. In H4IIE cells, rapamycin increased expression of Ntcp, Oatp1 and Mrp2, but decreased expression of Oatp2. It is concluded that the inhibition of bile flow recovery following ischemia observed in rapamycin-treated livers is principally due to inhibition of the expression of sinusoidal bile acid transporters. Moreover, in tumorigenic liver tissue the contribution of tumorigenic hepatocytes to total liver bile flow is likely to be small and is unlikely to be greatly affected by rapamycin.

Hepatoprotective Activity of the Ethanolic Extract of Polygonum multiflorum Thunb. against Oxidative Stress-Induced Liver Injury

Oxidative stress is an important pathological mechanism in various liver diseases. Polygonum multiflorum Thunb. (PM) can be used for the treatment of diseases associated with aging, hyperlipidemia, and oxidative stress in traditional Chinese medicine. In this study, we examined the hepatoprotective effects of the ethanolic extract of PM (PME) in in vitro and in vivo models. The PME induced expression of antioxidant-response-element- (ARE-) related genes in HepG2 cells showed a dose-dependent manner. Pretreatment of HepG2 cell with PME suppressed H₂O₂- and acetaminophen- (APAP-) induced cellular reactive oxygen species (ROS) generation and cytotoxicity. In APAP-induced mouse liver injury, pretreatment with PME also showed ability to increase the survival rate and reduce the severity of liver injury. Treatment with PME attenuated bile duct ligation-induced extrahepatic cholestatic liver injury and further increased multidrug resistance protein 4 (MRP4) and reduced organic anion-transporting polypeptide (OATP) expression. Furthermore, increased nuclear translocation of the nuclear factor erythroid 2-related factor 2 (Nrf2) was observed after treatment with PME in both in vivo models. In conclusion, the current study showed the hepatoprotective activity of PME by regulating the redox state in liver injury through Nrf2 activation and controlling hepatic bile acid homeostasis in obstructive cholestasis, through bile acid transporter expression modulation.

Mechanisms of Triptolide-Induced Hepatotoxicity and Protective Effect of Combined Use of Isoliquiritigenin: Possible Roles of Nrf2 and Hepatic Transporters

Triptolide (TP), the main bioactive component of Tripterygium wilfordii Hook F, can cause severe hepatotoxicity. Isoliquiritigenin (ISL) has been reported to be able to protect against TP-induced liver injury, but the mechanisms are not fully elucidated. This study aims to explore the role of nuclear transcription factor E2-related factor 2 (Nrf2) and hepatic transporters in TP-induced hepatotoxicity and the reversal protective effect of ISL. TP treatment caused both cytotoxicity in L02 hepatocytes and acute liver injury in mice. Particularly, TP led to the disorder of bile acid (BA) profiles in mice livers. Combined treatment of TP with ISL effectively alleviated TP-induced hepatotoxicity. Furthermore, ISL pretreatment enhanced Nrf2 expressions and nuclear accumulations and its downstream NAD(P)H: quinine oxidoreductase 1 (NQO1) expression. Expressions of hepatic P-gp, MRP2, MRP4, bile salt export pump, and OATP2 were also induced. In addition, in vitro transport assays identified that neither was TP exported by MRP2, OATP1B1, or OATP1B3, nor did TP influence the transport activities of P-gp or MRP2. All these results indicate that ISL may reduce the hepatic oxidative stress and hepatic accumulations of both endogenous BAs and exogenous TP as well as its metabolites by enhancing the expressions of Nrf2, NQO1, and hepatic influx and efflux transporters. Effects of TP on hepatic transporters are mainly at the transcriptional levels, and changes of hepatic BA profiles are very important in the mechanisms of TP-induced hepatotoxicity.

Physiological Regulation of Drug Metabolism and Transport: Pregnancy, Microbiome, Inflammation, Infection, and Fasting

This article is a report on a symposium entitled "Physiological Regulation of Drug Metabolism and Transport" sponsored by the American Society for Pharmacology and Experimental Therapeutics and held at the Experimental Biology 2017 meeting in Chicago, IL. The contributions of physiologic and pathophysiological regulation of drug-metabolizing enzymes and transporters to interindividual variability in drug metabolism are increasingly recognized but in many cases are not well understood. The presentations herein discuss the phenomenology, consequences, and mechanism of such regulation. CYP2D6 transgenic mice were used to provide insights into the mechanism of regulation of this enzyme in pregnancy, via hepatocyte nuclear factor 4α, small heterodimer partner, and retinoids. Regulation of intestinal and hepatic drug-processing enzymes by the intestinal microbiota via tryptophan and its metabolites was investigated. The potential impact of parasitic infections on human drug metabolism and clearance was assessed in mice infected with Schistosoma mansoni or Plasmodium chabaudi chabaudi AS, both of which produced widespread and profound effects on murine hepatic drug-metabolizing enzymes. Finally, the induction of Abcc drug efflux transporters by fasting was investigated. This was demonstrated to occur via a cAMP, protein kinase A/nuclear factor-E2-related factor 2/Sirtuin 1 pathway via antioxidant response elements on the Abcc genes.

Organic Solute Transporter α-β Protects Ileal Enterocytes From Bile Acid-Induced Injury

BACKGROUND & AIMS

Ileal bile acid absorption is mediated by uptake via the apical sodium-dependent bile acid transporter (ASBT), and export via the basolateral heteromeric organic solute transporter α-β (OSTα-OSTβ). In this study, we investigated the cytotoxic effects of enterocyte bile acid stasis in Ostα-/- mice, including the temporal relationship between intestinal injury and initiation of the enterohepatic circulation of bile acids.

METHODS

Ileal tissue morphometry, histology, markers of cell proliferation, gene, and protein expression were analyzed in male and female wild-type and Ostα-/- mice at postnatal days 5, 10, 15, 20, and 30. Ostα-/-Asbt-/- mice were generated and analyzed. Bile acid activation of intestinal Nrf2-activated pathways was investigated in Drosophila.

RESULTS

As early as day 5, Ostα-/- mice showed significantly increased ileal weight per length, decreased villus height, and increased epithelial cell proliferation. This correlated with premature expression of the Asbt and induction of bile acid-activated farnesoid X receptor target genes in neonatal Ostα-/- mice. Expression of reduced nicotinamide adenine dinucleotide phosphate oxidase-1 and Nrf2-anti-oxidant responsive genes were increased significantly in neonatal Ostα-/- mice at these postnatal time points. Bile acids also activated Nrf2 in Drosophila enterocytes and enterocyte-specific knockdown of Nrf2 increased sensitivity of flies to bile acid-induced toxicity. Inactivation of the Asbt prevented the changes in ileal morphology and induction of anti-oxidant response genes in Ostα-/- mice.

CONCLUSIONS

Early in postnatal development, loss of Ostα leads to bile acid accumulation, oxidative stress, and a restitution response in ileum. In addition to its essential role in maintaining bile acid homeostasis, Ostα-Ostβ functions to protect the ileal epithelium against bile acid-induced injury. NCBI Gene Expression Omnibus: GSE99579.

Huangqi Decoction Alleviates Alpha-Naphthylisothiocyanate Induced Intrahepatic Cholestasis by Reversing Disordered Bile Acid and Glutathione Homeostasis in Mice

Intrahepatic cholestasis is a serious symptom of liver disorders with limited therapies. In this study, we investigated the efficacy of Huangqi decoction (HQD), a two-herb classic traditional Chinese medicine (TCM), in the treatment of alpha-naphthylisothiocyanate (ANIT)-induced intrahepatic cholestasis in mice. HQD treatment ameliorated impaired hepatic function and tissue damage. A metabolomics study revealed that the endogenous metabolites significantly affected by HQD were related to bile acid (BA) biosynthesis and glutathione metabolism pathways. HQD treatment decreased the intrahepatic accumulation of cytotoxic BAs, normalized serum BA levels, and increased biliary and urinary BA excretion. Additionally, HQD restored the hepatic glutathione content and suppressed reactive oxygen species (ROS) in cholestatic mice. Protein and gene analysis revealed that HQD increased the expression of the hepatic metabolizing enzymes cytochrome P450 (CYP) 2B10 and UDP glucuronosyltransferase family 1 member A1 (UGT1A1), as well as multidrug resistance-associated protein 2 (Mrp2), Mrp3, and Mrp4, which play crucial roles in BA homeostasis. Further, HQD increased the protein expression of glutamate-cysteine ligase, which is involved in the synthesis of glutathione. Importantly, HQD increased the nuclear expression of nuclear factor-E2-related factor-2 (Nrf2). In conclusion, HQD protects against intrahepatic cholestasis by reversing the disordered homeostasis of BAs and glutathione.

Rifampicin-induced injury in HepG2 cells is alleviated by TUDCA via increasing bile acid transporters expression and enhancing the Nrf2-mediated adaptive response

Bile acid transporters and the nuclear factor erythroid 2-related factor (Nrf-2)-mediated adaptive response play important roles in the development of drug-induced liver injury (DILI). However, little is known about the contribution of the adaptive response to rifampicin (RFP)-induced cell injury. In this study, we found RFP decreased the survival rate of HepG2 cells and increased the levels of lactate dehydrogenase (LDH), alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (AKP), γ-glutamyl-transferase (γ-GT), total bilirubin (TBIL), direct bilirubin (DBIL), indirect bilirubin (IBIL), total bile acid (TBA) and adenosine triphosphate (ATP) in the cell culture supernatants in both a concentration- and a time-dependent manner. RFP increased the expression levels of bile acid transporter proteins and mRNAs, such as bile salt export pump (BSEP), multidrug resistance protein 1 (MDR1), multidrug resistance-associated protein 2 (MRP2), Na+/taurocholate cotransporter (NTCP), organic anion transporting protein 2 (OATP2), organic solute transporter β (OSTβ) and Nrf2. Following the transient knockdown of Nrf2 and treatment with RFP, the expression levels of the BSEP, MDR1, MRP2, NTCP, OATP2 and OSTβ proteins and mRNAs were decreased to different degrees. Moreover, the cell survival was decreased, whereas the LDH level in the cell culture supernatant was increased. Overexpression of the Nrf2 gene produced the opposite effects. Treatment with tauroursodeoxycholic acid (TUDCA) increased the expression levels of the bile acid transporters and Nrf2, decreased the expression levels of glucose-regulated protein 78 (GRP78), PKR-like ER kinase (PERK), activating transcription factor 4 (ATF4), and C/EBP-homologous protein (CHOP), and inhibited RFP-induced oxidative stress. Moreover, TUDCA reduced cell apoptosis, increased cell survival and decreased the levels of LDH, ALT, AST, AKP, γ-GT, TBIL, DBIL, IBIL, TBA and ATP in the cell culture supernatant. Therefore, TUDCA alleviates RFP-induced injury in HepG2 cells by enhancing bile acid transporters expression and the Nrf2-mediated adaptive response.

Naturally Occurring Nrf2 Activators: Potential in Treatment of Liver Injury

Oxidative stress plays a major role in acute and chronic liver injury. In hepatocytes, oxidative stress frequently triggers antioxidant response by activating nuclear erythroid 2-related factor 2 (Nrf2), a transcription factor, which upregulates various cytoprotective genes. Thus, Nrf2 is considered a potential therapeutic target to halt liver injury. Several studies indicate that activation of Nrf2 signaling pathway ameliorates liver injury. The hepatoprotective potential of naturally occurring compounds has been investigated in various models of liver injuries. In this review, we comprehensively appraise various phytochemicals that have been assessed for their potential to halt acute and chronic liver injury by enhancing the activation of Nrf2 and have the potential for use in humans.

Glutathione antioxidant pathway activity and reserve determine toxicity and specificity of the biliary toxin biliatresone in zebrafish

Biliatresone is an electrophilic isoflavone isolated from Dysphania species plants that has been causatively linked to naturally occurring outbreaks of a biliary atresia (BA)-like disease in livestock. Biliatresone has selective toxicity for extrahepatic cholangiocytes (EHCs) in zebrafish larvae. To better understand its mechanism of toxicity, we performed transcriptional profiling of liver cells isolated from zebrafish larvae at the earliest stage of biliatresone-mediated biliary injury, with subsequent comparison of biliary and hepatocyte gene expression profiles. Transcripts encoded by genes involved in redox stress response, particularly those involved in glutathione (GSH) metabolism, were among the most prominently up-regulated in both cholangiocytes and hepatocytes of biliatresone-treated larvae. Consistent with these findings, hepatic GSH was depleted at the onset of biliary injury, and in situ mapping of the hepatic GSH redox potential using a redox-sensitive green fluorescent protein biosensor showed that it was significantly more oxidized in EHCs both before and after treatment with biliatresone. Pharmacological and genetic manipulation of GSH redox homeostasis confirmed the importance of GSH in modulating biliatresone-induced injury given that GSH depletion sensitized both EHCs and the otherwise resistant intrahepatic cholangiocytes to the toxin, whereas replenishing GSH level by N-acetylcysteine administration or activation of nuclear factor erythroid 2-like 2 (Nrf2), a transcriptional regulator of GSH synthesis, inhibited EHC injury.

CONCLUSION

These findings strongly support redox stress as a critical contributing factor in biliatresone-induced cholangiocyte injury, and suggest that variations in intrinsic stress responses underlie the susceptibility profile. Insufficient antioxidant capacity of EHCs may be critical to early pathogenesis of human BA. (Hepatology 2016;64:894-907).

Mechanisms of activation of the transcription factor Nrf2 by redox stressors, nutrient cues, and energy status and the pathways through which it attenuates degenerative disease

Nuclear factor-erythroid 2 p45-related factor 2 (Nrf2) regulates the basal and stress-inducible expression of a battery of genes encoding key components of the glutathione-based and thioredoxin-based antioxidant systems, as well as aldo-keto reductase, glutathione S-transferase, and

NAD(P)H

quinone oxidoreductase-1 drug-metabolizing isoenzymes along with multidrug-resistance-associated efflux pumps. It therefore plays a pivotal role in both intrinsic resistance and cellular adaptation to reactive oxygen species (ROS) and xenobiotics. Activation of Nrf2 can, however, serve as a double-edged sword because some of the genes it induces may contribute to chemical carcinogenesis by promoting futile redox cycling of polycyclic aromatic hydrocarbon metabolites or confer resistance to chemotherapeutic drugs by increasing the expression of efflux pumps, suggesting its cytoprotective effects will vary in a context-specific fashion. In addition to cytoprotection, Nrf2 also controls genes involved in intermediary metabolism, positively regulating those involved in NADPH generation, purine biosynthesis, and the β-oxidation of fatty acids, while suppressing those involved in lipogenesis and gluconeogenesis. Nrf2 is subject to regulation at multiple levels. Its ability to orchestrate adaptation to oxidants and electrophiles is due principally to stress-stimulated modification of thiols within one of its repressors, the Kelch-like ECH-associated protein 1 (Keap1), which is present in the cullin-3 RING ubiquitin ligase (CRL) complex CRLKeap1. Thus modification of Cys residues in Keap1 blocks CRLKeap1 activity, allowing newly translated Nrf2 to accumulate rapidly and induce its target genes. The ability of Keap1 to repress Nrf2 can be attenuated by p62/sequestosome-1 in a mechanistic target of rapamycin complex 1 (mTORC1)-dependent manner, thereby allowing refeeding after fasting to increase Nrf2-target gene expression. In parallel with repression by Keap1, Nrf2 is also repressed by β-transducin repeat-containing protein (β-TrCP), present in the Skp1-cullin-1-F-box protein (SCF) ubiquitin ligase complex SCFβ-TrCP. The ability of SCFβ-TrCP to suppress Nrf2 activity is itself enhanced by prior phosphorylation of the transcription factor by glycogen synthase kinase-3 (GSK-3) through formation of a DSGIS-containing phosphodegron. However, formation of the phosphodegron in Nrf2 by GSK-3 is inhibited by stimuli that activate protein kinase B (PKB)/Akt. In particular, PKB/Akt activity can be increased by phosphoinositide 3-kinase and mTORC2, thereby providing an explanation of why antioxidant-responsive element-driven genes are induced by growth factors and nutrients. Thus Nrf2 activity is tightly controlled via CRLKeap1 and SCFβ-TrCP by oxidative stress and energy-based signals, allowing it to mediate adaptive responses that restore redox homeostasis and modulate intermediary metabolism. Based on the fact that Nrf2 influences multiple biochemical pathways in both positive and negative ways, it is likely its dose-response curve, in terms of susceptibility to certain degenerative disease, is U-shaped. Specifically, too little Nrf2 activity will lead to loss of cytoprotection, diminished antioxidant capacity, and lowered β-oxidation of fatty acids, while conversely also exhibiting heightened sensitivity to ROS-based signaling that involves receptor tyrosine kinases and apoptosis signal-regulating kinase-1. By contrast, too much Nrf2 activity disturbs the homeostatic balance in favor of reduction, and so may have deleterious consequences including overproduction of reduced glutathione and NADPH, the blunting of ROS-based signal transduction, epithelial cell hyperplasia, and failure of certain cell types to differentiate correctly. We discuss the basis of a putative U-shaped Nrf2 dose-response curve in terms of potentially competing processes relevant to different stages of tumorigenesis.

A protective mechanism of licorice (Glycyrrhiza uralensis): isoliquiritigenin stimulates detoxification system via Nrf2 activation

ETHNOPHARMACOLOGICAL RELEVANCE

Licorice (Glycyrrhizae radix), the root of Glycyrrhiza uralensis Fisch. (Leguminosae), is mainly used to moderate the characteristics of toxic herbs in Traditional Chinese Medicine, which could be partly interpreted as detoxification. However, the underlying mechanism is still not fully elucidated. Nuclear factor erythroid 2-related factor 2 (Nrf2) plays a key role in the protection against toxic xenobiotics. In our previous research, we have identified that extracts from Glycyrrhiza uralensis induced the expression of Nrf2 nuclear protein and its downstream genes. This research aims to screen the most potent Nrf2 inducer isolated from Glycyrrhiza uralensis and examine its effect on Nrf2 signaling pathway and detoxification system.

MATERIALS AND METHODS

Four compounds derived from Glycyrrhiza uralensis (glycyrrhetinic acid, liquiritigenin, isoliquiritigenin and liquiritin) were screened by ARE-luciferase reporter. The most potent ARE-luciferase inducer was chosen to further examine its effect on Nrf2 and detoxification genes in HepG2 cells. The role of Nrf2-dependent mechanism was tested by using Nrf2 knockout mice (Nrf2 KO) and Nrf2 wild-type mice (Nrf2 WT).

RESULTS

ARE-luciferase reporter assay showed these four compounds were all potent Nrf2 inducers, and isoliquiritigenin was the most potent inducer. Isoliquiritigenin significantly up-regulated the expression of Nrf2 and its downstream detoxification genes UDP-glucuronosyltransferase 1A1 (UGT1A1), glutamate cysteine ligase (GCL), multidrug resistance protein 2 (MRP2) and bile salt export pump (BSEP) in vitro and in vivo. Additionally, isoliquiritigenin showed Nrf2-dependent transactivation of UGT1A1, GCLC and MRP2.

CONCLUSIONS

Isoliquiritigenin, isolated from Glycyrrhiza uralensis, stimulates detoxification system via Nrf2 activation, which could be a potential protective mechanism of licorice.

Activation of a novel c-Myc-miR27-prohibitin 1 circuitry in cholestatic liver injury inhibits glutathione synthesis in mice

AIMS

We showed that chronic cholestatic liver injury induced the expression of c-Myc but suppressed that of glutamate-cysteine ligase (GCL, composed of catalytic and modifier subunits GCLC and GCLM, respectively). This was associated with reduced nuclear antioxidant response element (ARE) binding by nuclear factor-erythroid 2 related factor 2 (Nrf2). Here, we examined whether c-Myc is involved in this process.

RESULTS

Similar to bile duct ligation (BDL), lithocholic acid (LCA) treatment in vivo induced c-Myc but suppressed GCL subunits expression at day 14. Nrf2 expression and Nrf2 ARE binding fell markedly. However, Nrf2 heterodimerization with MafG was enhanced by LCA, which prompted us to examine whether LCA treatment in vivo altered proteins that bind to ARE using biotinylated ARE in pull-down assay followed by proteomics. LCA treatment enhanced c-Myc but lowered prohibitin 1 (PHB1) binding to ARE. This was a result of c-Myc-mediated induction of microRNA 27a/b (miR27a/b), which target both PHB1 and Nrf2 to reduce their expression. Knockdown of c-Myc or miR27a/b attenuated LCA-mediated suppression of Nrf2, PHB1, and GCL subunit expression, whereas overexpression of PHB1 protected against the fall in Nrf2 and GCL subunits. Both c-Myc and PHB1 directly interact with Nrf2 but c-Myc lowers Nrf2 binding to ARE while PHB1 enhances it.

INNOVATION

This is the first work that shows how activation of this circuit in cholestatic liver injury inhibits GCL expression.

CONCLUSIONS

LCA feeding and BDL activate c-Myc-miR27a/b-PHB1 circuit, with the consequence of inhibiting Nrf2 expression and ARE binding, resulting in decreased reduced glutathione synthesis and antioxidant capacity.

Individual bile acids have differential effects on bile acid signaling in mice

Bile acids (BAs) are known to regulate BA synthesis and transport by the farnesoid X receptor in the liver (FXR-SHP) and intestine (FXR-Fgf15). However, the relative importance of individual BAs in regulating these processes is not known. Therefore, mice were fed various doses of five individual BAs, including cholic acid (CA), chenodeoxycholic acid (CDCA), deoxoycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA) in their diets at various concentrations for one week to increase the concentration of one BA in the enterohepatic circulation. The mRNA of BA synthesis and transporting genes in liver and ileum were quantified. In the liver, the mRNA of SHP, which is the prototypical target gene of FXR, increased in mice fed all concentrations of BAs. In the ileum, the mRNA of the intestinal FXR target gene Fgf15 was increased at lower doses and to a higher extent by CA and DCA than by CDCA and LCA. Cyp7a1, the rate-limiting enzyme in BA synthesis, was decreased more by CA and DCA than CDCA and LCA. Cyp8b1, the enzyme that 12-hydroxylates BAs and is thus responsible for the synthesis of CA, was decreased much more by CA and DCA than CDCA and LCA. Surprisingly, neither a decrease in the conjugated BA uptake transporter (Ntcp) nor increase in BA efflux transporter (Bsep) was observed by FXR activation, but an increase in the cholesterol efflux transporter (Abcg5/Abcg8) was observed with FXR activation. Thus in conclusion, CA and DCA are more potent FXR activators than CDCA and LCA when fed to mice, and thus they are more effective in decreasing the expression of the rate limiting gene in BA synthesis Cyp7a1 and the 12-hydroxylation of BAs Cyp8b1, and are also more effective in increasing the expression of Abcg5/Abcg8, which is responsible for biliary cholesterol excretion. However, feeding BAs do not alter the mRNA or protein levels of Ntcp or Bsep, suggesting that the uptake or efflux of BAs is not regulated by FXR at physiological and pharmacological concentrations of BAs.

Schisandra chinensis regulates drug metabolizing enzymes and drug transporters via activation of Nrf2-mediated signaling pathway

Drug metabolizing enzymes (DMEs) and drug transporters are regulated via epigenetic, transcriptional, posttranscriptional, and translational and posttranslational modifications. Phase I and II DMEs and drug transporters play an important role in the disposition and detoxification of a large number of endogenous and exogenous compounds. The nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a critical regulator of a variety of important cytoprotective genes that are involved in disposition and detoxification of xenobiotics. Schisandra chinensis (SC) is a commonly used traditional Chinese herbal medicine that has been primarily used to protect the liver because of its potent antioxidative and anti-inflammatory activities. SC can modulate some DMEs and drug transporters, but the underlying mechanisms are unclear. In this study, we aimed to explore the role of Nrf2 in the regulatory effect of SC extract (SCE) on selected DMEs and drug transporters in human hepatocellular liver carcinoma cell line (HepG2) cells. The results showed that SCE, schisandrin A, and schisandrin B significantly increased the expression of NAD(P)H: Nicotinamide Adenine Dinucleotide Phosphate-oxidase or:quinone oxidoreductase 1, heme oxygenase-1, glutamate–cysteine ligase, and glutathione S-transferase A4 at both transcriptional and posttranscriptional levels. Incubation of HepG2 cells with SCE resulted in a significant increase in the intracellular level of glutathione and total glutathione S-transferase content. SCE significantly elevated the messenger ribonucleic acid and protein levels of P-glycoprotein and multidrug resistance-associated protein 2 and 4, whereas the expression of organic anion transporting peptide 1A2 and 1B1 was significantly downregulated by SCE. Knockdown of Nrf2 by small interfering ribonucleic acid attenuated the regulatory effect of SCE on these DMEs and drug transporters. SCE significantly upregulated Nrf2 and promoted the translocation of Nrf2 from cytoplasm to the nuclei. Additionally, SCE significantly suppressed the expression of cytosolic Kelch-like ECH-associated protein 1 (the repressor of Nrf2) and remarkably increased Nrf2 stability in HepG2 cells. Taken together, our findings suggest that the hepatoprotective effects of SCE may be partially ascribed to the modulation of DMEs and drug transporters via Nrf2-mediated signaling pathway. SCE may alter the pharmacokinetics of other coadministered drugs that are substrates of these DMEs and transporters and thus cause unfavorable herb–drug interactions.

Differential Fmo3 gene expression in various liver injury models involving hepatic oxidative stress in mice

Flavin-containing monooxygenase-3 (FMO3) catalyzes metabolic reactions similar to cytochrome P450 monooxygenase, however, most metabolites of FMO3 are considered non-toxic. Recent findings in our laboratory demonstrated Fmo3 gene induction following toxic acetaminophen (APAP) treatment in mice. The goal of this study was to evaluate Fmo3 gene expression in other diverse mouse models of hepatic oxidative stress and injury. Fmo3 gene regulation by Nrf2 was also investigated using Nrf2 knockout (Nrf2 KO) mice. In our studies, male C57BL/6J mice were treated with toxic doses of hepatotoxicants or underwent bile duct ligation (BDL, 10 days). Hepatotoxicants included APAP (400 mg/kg, 24-72 h), alpha-naphthyl isothiocyanate (ANIT; 50 mg/kg, 2-48 h), carbon tetrachloride (CCl4; 10 or 30 μL/kg, 24 and 48 h) and allyl alcohol (AlOH; 30 or 60 mg/kg, 6 and 24 h). Because oxidative stress activates nuclear factor (erythroid-derived 2)-like 2 (Nrf2), additional studies investigated Fmo3 gene regulation by Nrf2 using Nrf2 knockout (Nrf2 KO) mice. At appropriate time-points, blood and liver samples were collected for assessment of plasma alanine aminotransferase (ALT) activity, plasma and hepatic bile acid levels, as well as liver Fmo3 mRNA and protein expression. Fmo3 mRNA expression increased significantly by 43-fold at 12 h after ANIT treatment, and this increase translates to a 4-fold change in protein levels. BDL also increased Fmo3 mRNA expression by 1899-fold, but with no change in protein levels. Treatment of mice with CCl4 decreased liver Fmo3 gene expression, while no change in expression was detected with AlOH treatment. Nrf2 KO mice are more susceptible to APAP (400mg/kg, 72 h) treatment compared to their wild-type (WT) counterparts, which is evidenced by greater plasma ALT activity. The Fmo3 mRNA and protein expression increased in Nrf2 KO mice after APAP treatment. Collectively, not all hepatotoxicants that produce oxidative stress alter Fmo3 gene expression. Along with APAP, toxic ANIT treatment in mice markedly increased Fmo3 gene expression. While BDL increased the Fmo3 mRNA expression, the protein level did not change. The discrepancy with Fmo3 induction in cholestatic models, ANIT and BDL, is not entirely clear. Results from Nrf2 KO mice with APAP suggest that the transcriptional regulation of Fmo3 during liver injury may not involve Nrf2.

Loss of Nrf2 in mice evokes a congenital intrahepatic shunt that alters hepatic oxygen and protein expression gradients and toxicity

The transcription factor Nrf2 (Nfe2l2 nuclear factor, erythroid 2-like 2) regulates gene expression directly, controlling pharmacological and toxicological responses. These processes may also be influenced by the structure of the hepatic vasculature, which distributes blood flow through compartmentalized microenvironments to maintain organismal stability. Castings of the hepatic portal vasculature of albino C57BL/6J but not ICR Nrf2(-/-) mice revealed a congenital intrahepatic shunt that was present in two thirds of Nrf2-disrupted mice. This shunt directly connected the portal vein to the inferior vena cava and displayed characteristics of a patent ductus venosus. Immunohistochemistry revealed that Nrf2(-/-) mice with an intrahepatic shunt manifest changes to hepatic oxygen and protein expression gradients when compared with wild-type (WT) and non-shunted Nrf2(-/-) mice. Centrilobular hypoxia found in WT and Nrf2(-/-) mice without shunts was reduced in Nrf2(-/-) livers with a shunt. Hepatic protein expression of phosphoenolpyruvate carboxykinase (Pepck), normally confined to the periportal zone, exhibited both periportal and centrilobular zonal expression in livers from Nrf2(-/-) mice with an intrahepatic shunt. Centrilobular expression of Cytochrome P450 2E1 (Cyp2e1) was diminished in shunted Nrf2(-/-) livers compared with WT and Nrf2(-/-) livers without shunts. The intrahepatic shunt in Nrf2(-/-) mice was further found to diminish acetaminophen hepatoxicity compared with WT and Nrf2(-/-) non-shunted mice following a 6 h challenge with 250 mg/kg acetaminophen. The presence of an intrahepatic shunt influences several physiological and pathophysiological properties of Nrf2(-/-) mice through changes in blood flow, hepatic oxygenation, and protein expression that extent beyond loss of canonical transactivation of Nrf2 target genes.

Role of Nrf2 in the alteration of cholesterol and bile acid metabolism-related gene expression by dietary cholesterol in high fat-fed mice

Nuclear factor-E2-related factor 2 (Nrf2) is a regulator of lipid metabolism as well as various cytoprotective enzymes and may be involved in the pathogenesis of non-alcoholic fatty liver disease. Although, bile acids affect lipid metabolism, the role of Nrf2 in bile acid metabolism remains unclear. In this study, it was tested how Nrf2 modulates lipid and bile acid homeostasis in liver in response to changes of cholesterol absorption under high-fat diet using Nrf2-null mice. Eight-week-old male wild-type and Nrf2-null mice (n = 6/group) were divided into three groups fed the following diets: 1) control diet containing 4% soybean oil and 16% lard, 2) control diet plus ezetimibe, 3) control diet plus cholesterol. Blood and livers were removed after 4 weeks feeding. High cholesterol diet increased hepatic expression of liver X receptor α target genes related to fatty acid metabolism (FAS, ACC1, SREBP-1c, SCD-1c and CD36), cholesterol transport (Abcg5/abcg8) and bile acid synthesis (Cyp7a1) in wild type mice. However, these genes were not induced in Nrf2-null mice. These findings suggest that Nrf2 has a relation to liver X receptor α and controls the regulation of gene expressions related to lipid and bile acid metabolism.

Fasting induces nuclear factor E2-related factor 2 and ATP-binding Cassette transporters via protein kinase A and Sirtuin-1 in mouse and human

AIMS

The purpose of this study was to determine whether 3'-5'-cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) and Sirtuin-1 (SIRT1) dependent mechanisms modulate ATP-binding Cassette (ABC) transport protein expression. ABC transport proteins (ABCC2-4) are essential for chemical elimination from hepatocytes and biliary excretion. Nuclear factor-E2 related-factor 2 (NRF2) is a transcription factor that mediates ABCC induction in response to chemical inducers and liver injury. However, a role for NRF2 in the regulation of transporter expression in nonchemical models of liver perturbation is largely undescribed.

RESULTS

Here we show that fasting increased NRF2 target gene expression through NRF2- and SIRT1-dependent mechanisms. In intact mouse liver, fasting induces NRF2 target gene expression by at least 1.5 to 5-fold. In mouse and human hepatocytes, treatment with 8-Bromoadenosine-cAMP, a cAMP analogue, increased NRF2 target gene expression and antioxidant response element activity, which was decreased by the PKA inhibitor, H-89. Moreover, fasting induced NRF2 target gene expression was decreased in liver and hepatocytes of SIRT1 liver-specific null mice and NRF2-null mice. Lastly, NRF2 and SIRT1 were recruited to MAREs and Antioxidant Response Elements (AREs) in the human ABCC2 promoter.

INNOVATION

Oxidative stress mediated NRF2 activation is well described, yet the influence of basic metabolic processes on NRF2 activation is just emerging.

CONCLUSION

The current data point toward a novel role of nutrient status in regulation of NRF2 activity and the antioxidant response, and indicates that cAMP/PKA and SIRT1 are upstream regulators for fasting-induced activation of the NRF2-ARE pathway.

Notch-Nrf2 axis: regulation of Nrf2 gene expression and cytoprotection by notch signaling

The Notch signaling pathway enables regulation and control of development, differentiation, and homeostasis through cell-cell communication. Our investigation shows that Notch signaling directly activates the Nrf2 stress adaptive response pathway through recruitment of the Notch intracellular domain (NICD) transcriptosome to a conserved Rbpjκ site in the promoter of Nrf2. Stimulation of Notch signaling through Notch ligand expression in cells and by overexpression of the NICD in Rosa(NICD/-)::AlbCre mice in vivo induces expression of Nrf2 and its target genes. Continuous and transient NICD expression in the liver produces a Notch-dependent cytoprotective response through direct transcriptional activation of Nrf2 signaling to rescue mice from acute acetaminophen toxicity. This response can be reversed upon genetic disruption of Nrf2. Morphological studies showed that the characteristic phenotype of high-density intrahepatic bile ducts and enlarged liver in Rosa(NICD/-)::AlbCre mice could be at least partially reversed after Nrf2 disruption. Furthermore, the liver and bile duct phenotypes could be recapitulated with constitutive activation of Nrf2 signaling in Keap1(F/F)::AlbCre mice. It appears that Notch-to-Nrf2 signaling is another important determinant in liver development and function and promotes cell-cell cytoprotective signaling responses.

Zederone, a Sesquiterpene from Curcuma elata Roxb, is Hepatotoxic in Mice

The present study aimed to investigate the hepatotoxicity of zederone isolated from Curcuma elata in mice. Adult male mice were intraperitoneally injected with a single dose of zederone (50-300 mg/kg body weight [BW]). Twenty-four hours after the injection, zederone induced liver enlargement with scattered white foci over the organ. The medium lethal dose (LD50) value at 24 hours of zederone was approximately 223 mg/kg BW. Hepatic centrilobular necrosis with marked increases in plasma alanine transaminase activity and total bilirubin levels was observed. Zederone at a dose of 200 mg/kg BW markedly decreased the activity of superoxide dismutase and the hepatic glutathione content, whereas the activity of catalase was not altered. The compound at this dose also increased the messenger RNA (mRNA) expression of Cyp2b10 and Cyp3a11, which are the main drug-metabolizing enzymes in the liver. The mRNA expression of proinflammatory cytokine tumor necrosis factor α was increased. The nuclear factor-E2-related factor 2 protein, which is the transcription factor regulating the antioxidant gene expression, was decreased. The histopathology of massive hepatic centrilobular necrosis with an increase in the expression of cytochrome P450 (Cyp) suggests that the possible potentiation of zederone-induced hepatotoxicity implicated the induction of Cyps, which leads to the formation of biological reactive metabolites and that cause the oxidative stress and liver cell injuries.

Oleanolic acid alters bile acid metabolism and produces cholestatic liver injury in mice

Oleanolic acid (OA) is a triterpenoids that exists widely in plants. OA is effective in protecting against hepatotoxicants. Whereas a low dose of OA is hepatoprotective, higher doses and longer-term use of OA produce liver injury. This study characterized OA-induced liver injury in mice. Adult C57BL/6 mice were given OA at doses of 0, 22.5, 45, 90, and 135 mg/kg, s.c., daily for 5 days, and liver injury was observed at doses of 90 mg/kg and above, as evidenced by increases in serum activities of alanine aminotransferase and alkaline phosphatase, increases in serum total bilirubin, as well as by liver histopathology. OA-induced cholestatic liver injury was further evidenced by marked increases of both unconjugated and conjugated bile acids (BAs) in serum. Gene and protein expression analysis suggested that livers of OA-treated mice had adaptive responses to prevent BA accumulation by suppressing BA biosynthetic enzyme genes (Cyp7a1, 8b1, 27a1, and 7b1); lowering BA uptake transporters (Ntcp and Oatp1b2); and increasing a BA efflux transporter (Ostβ). OA increased the expression of Nrf2 and its target gene, Nqo1, but decreased the expression of AhR, CAR and PPARα along with their target genes, Cyp1a2, Cyp2b10 and Cyp4a10. OA had minimal effects on PXR and Cyp3a11. Taken together, the present study characterized OA-induced liver injury, which is associated with altered BA homeostasis, and alerts its toxicity potential.

Role of the Nrf2-ARE pathway in liver diseases

The liver is a central organ that performs a wide range of functions such as detoxification and metabolic homeostasis. Since it is a metabolically active organ, liver is particularly susceptible to oxidative stress. It is well documented that liver diseases including hepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma are highly associated with antioxidant capacity. NF-E2-related factor-2 (Nrf2) is an essential transcription factor that regulates an array of detoxifying and antioxidant defense genes expression in the liver. It is activated in response to electrophiles and induces its target genes by binding to the antioxidant response element (ARE). Therefore, the roles of the Nrf2-ARE pathway in liver diseases have been extensively investigated. Studies from several animal models suggest that the Nrf2-ARE pathway collectively exhibits diverse biological functions against viral hepatitis, alcoholic and nonalcoholic liver disease, fibrosis, and cancer via target gene expression. In this review, we will discuss the role of the Nrf2-ARE pathway in liver pathophysiology and the potential application of Nrf2 as a therapeutic target to prevent and treat liver diseases.