7-ketocholesterol is an endogenous modulator for the arylhydrocarbon receptor

Tr1 cell-mediated protection against autoimmune disease by intranasal administration of a fusion protein targeting cDC1 cells

Curative therapies against autoimmune diseases are lacking. Indeed, most of the currently available treatments are only targeting symptoms. We have developed a novel strategy for a therapeutic vaccine against autoimmune diseases based on intranasal administration of a fusion protein tolerogen, which consists of a mutant, enzymatically inactive, cholera toxin A1 (CTA1)-subunit genetically fused to disease-relevant high-affinity peptides and a dimer of D-fragments from protein A (DD). The CTA1 R7K mutant - myelin oligodendrocyte glycoprotein (MOG), or proteolipid protein (PLP) - DD (CTA1R7K-MOG/PLP-DD) fusion proteins effectively reduced clinical symptoms in the experimental autoimmune encephalitis model of multiple sclerosis. The treatment induced Tr1 cells, in the draining lymph node, which produced interleukin (IL)-10 and suppressed effector clusters of differentiation 4+ T-cell responses. This effect was dependent on IL-27 signaling because treatment was ineffective in bone marrow chimeras lacking IL-27Ra within their hematopoietic compartment. Single-cell RNA sequencing of dendritic cells in draining lymph nodes demonstrated distinct gene transcriptional changes of classic dendritic cells 1, including enhanced lipid metabolic pathways, induced by the tolerogenic fusion protein. Thus, our results with the tolerogenic fusion protein demonstrate the possibility to vaccinate and protect against disease progression by reinstating tolerance in multiple sclerosis and other autoimmune diseases.

Evidence for crosstalk between the aryl hydrocarbon receptor and the translocator protein in mouse lung epithelial cells

Cellular homeostasis requires the use of multiple environmental sensors that can respond to a variety of endogenous and exogenous compounds. The aryl hydrocarbon receptor (AHR) is classically known as a transcription factor that induces genes that encode drug metabolizing enzymes when bound to toxicants such as 2,3,7,8-tetrachlorodibenzo-ρ-dioxin (TCDD). The receptor has a growing number of putative endogenous ligands, such as tryptophan, cholesterol, and heme metabolites. Many of these compounds are also linked to the translocator protein (TSPO), an outer mitochondrial membrane protein. Given a portion of the cellular pool of the AHR has also been localized to mitochondria and the overlap in putative ligands, we tested the hypothesis that crosstalk exists between the two proteins. CRISPR/Cas9 was used to create knockouts for AHR and TSPO in a mouse lung epithelial cell line (MLE-12). WT, AHR-/-, and TSPO-/- cells were then exposed to AHR ligand (TCDD), TSPO ligand (PK11195), or both and RNA-seq was performed. More mitochondrial-related genes were altered by loss of both AHR and TSPO than would have been expected just by chance. Some of the genes altered included those that encode for components of the electron transport system and the mitochondrial calcium uniporter. Both proteins altered the activity of the other as AHR loss caused the increase of TSPO at both the mRNA and protein level and loss of TSPO significantly increased the expression of classic AHR battery genes after TCDD treatment. This research provides evidence that AHR and TSPO participate in similar pathways that contribute to mitochondrial homeostasis.

Acupuncture at Taichong and Zusanli points exerts hypotensive effect in spontaneously hypertensive rats by metabolomic analysis

BACKGROUND

The development of hypertension affects several target organs, the kidneys being one of them. Acupuncture has been used to treat hypertension for a long time. Several mechanisms of acupuncture on hypotensive effect have been reveled, while the effects of acupuncture on the alterations in renal cortex from a metabolomic perspective are still unclear.

METHODS

Twelve male Wistar rats served as the control group (Wistar Group). Twenty-four male spontaneously hypertensive rats (SHR) were randomly divided into two groups: the model group (SHR Group) and the acupuncture group (AC Group). In the AC Group, milli-needle acupuncture was used to puncture the bilateral Taichong (LR3) and Zusanli (ST36) points. Blood pressure values were measured weekly and the rats were euthanized after three weeks. Renal cortical tissues were collected for non-targeted and targeted metabolomic analyses.

RESULTS

Acupuncture reduced blood pressure values in rats (Compared with the SHR Group, P < 0.001). Thirteen metabolites with significant differences and three metabolic pathways were screened by untargeted metabolomics. The SHR Group was compared with the Wistar Group and AC Group both involving metabolites and pathways related to bile acid metabolism. Furthermore, targeted metabolomics quantification of four bile acids, Cholic acid (CA), Allocholic acid (ACA), Deoxycholic acid (DCA) and Chenodeoxycholic acid (CDCA), revealed that all bile acid concentrations were relatively high in the SHR Group, except for ACA.

CONCLUSION

This study indicate that abnormal bile acid metabolism may be an independent risk factor the development of hypertension. Acupuncture at Taichong and at Zusanli points effectively modulated bile acids metabolism in SHR renal cortex tissues to exert a hypotensive effect, and CA may be able to be a new target for the treatment of hypertension.

Attenuation of 7-ketocholesterol- and 7β-hydroxycholesterol-induced oxiapoptophagy by nutrients, synthetic molecules and oils: Potential for the prevention of age-related diseases

Age-related diseases for which there are no effective treatments include cardiovascular diseases; neurodegenerative diseases such as Alzheimer's disease; eye disorders such as cataract and age-related macular degeneration; and, more recently, Severe Acute Respiratory Syndrome (SARS-CoV-2). These diseases are associated with plasma and/or tissue increases in cholesterol derivatives mainly formed by auto-oxidation: 7-ketocholesterol, also known as 7-oxo-cholesterol, and 7β-hydroxycholesterol. The formation of these oxysterols can be considered as a consequence of mitochondrial and peroxisomal dysfunction, leading to increased in oxidative stress, which is accentuated with age. 7-ketocholesterol and 7β-hydroxycholesterol cause a specific form of cytotoxic activity defined as oxiapoptophagy, including oxidative stress and induction of death by apoptosis associated with autophagic criteria. Oxiaptophagy is associated with organelle dysfunction and in particular with mitochondrial and peroxisomal alterations involved in the induction of cell death and in the rupture of redox balance. As the criteria characterizing 7-ketocholesterol- and 7β-hydroxycholesterol-induced cytotoxicity are often simultaneously observed in major age-related diseases (cardiovascular diseases, age-related macular degeneration, Alzheimer's disease) the involvement of these oxysterols in the pathophysiology of the latter seems increasingly likely. It is therefore important to better understand the signalling pathways associated with the toxicity of 7-ketocholesterol and 7β-hydroxycholesterol in order to identify pharmacological targets, nutrients and synthetic molecules attenuating or inhibiting the cytotoxic activities of these oxysterols. Numerous natural cytoprotective compounds have been identified: vitamins, fatty acids, polyphenols, terpenes, vegetal pigments, antioxidants, mixtures of compounds (oils, plant extracts) and bacterial enzymes. However, few synthetic molecules are able to prevent 7-ketocholesterol- and/or 7β-hydroxycholesterol-induced cytotoxicity: dimethyl fumarate, monomethyl fumarate, the tyrosine kinase inhibitor AG126, memantine, simvastatine, Trolox, dimethylsufoxide, mangafodipir and mitochondrial permeability transition pore (MPTP) inhibitors. The effectiveness of these compounds, several of which are already in use in humans, makes it possible to consider using them for the treatment of certain age-related diseases associated with increased plasma and/or tissue levels of 7-ketocholesterol and/or 7β-hydroxycholesterol.

Transcriptomic Changes Associated with Loss of Cell Viability Induced by Oxysterol Treatment of a Retinal Photoreceptor-Derived Cell Line: An In Vitro Model of Smith-Lemli-Opitz Syndrome

Smith–Lemli–Opitz Syndrome (SLOS) results from mutations in the gene encoding the enzyme DHCR7, which catalyzes conversion of 7-dehydrocholesterol (7DHC) to cholesterol (CHOL). Rats treated with a DHCR7 inhibitor serve as a SLOS animal model, and exhibit progressive photoreceptor-specific cell death, with accumulation of 7DHC and oxidized sterols. To understand the basis of this cell type specificity, we performed transcriptomic analyses on a photoreceptor-derived cell line (661W), treating cells with two 7DHC-derived oxysterols, which accumulate in tissues and bodily fluids of SLOS patients and in the rat SLOS model, as well as with CHOL (negative control), and evaluated differentially expressed genes (DEGs) for each treatment. Gene enrichment analysis and compilation of DEG sets indicated that endoplasmic reticulum stress, oxidative stress, DNA damage and repair, and autophagy were all highly up-regulated pathways in oxysterol-treated cells. Detailed analysis indicated that the two oxysterols exert their effects via different molecular mechanisms. Changes in expression of key genes in highlighted pathways (Hmox1, Ddit3, Trib3, and Herpud1) were validated by immunofluorescence confocal microscopy. The results extend our understanding of the pathobiology of retinal degeneration and SLOS, identifying potential new druggable targets for therapeutic intervention into these and other related orphan diseases.

Pomegranate seed oil and bitter melon extract supplemented in diet influence the lipid profile and intensity of peroxidation in livers of SPRD rats exposed to a chemical carcinogen

Despite promising health effects of pomegranate seed oil (PSO) and bitter melon extract (BM) used for centuries as food and traditional medicine, neither mechanism of action nor safety has been fully recognized. This study aimed to evaluate the influence of diet supplementation with PSO and BM on fatty acid, conjugated fatty acid and cholesterol content in rat' livers, since liver is crucial for lipid metabolism. Oxidation indicators (malondialdehyde, oxysterols and tocopherols) were also determined. Lipid profiles did not reveal the presence of punicic acid, while other conjugated dienes and trienes, including rumenic acid, were determined. Both supplementation and exposition to carcinogen significantly increased cholesterol and reduced selected oxysterols levels, simultaneously increasing malondialdehyde content in animals suffering from cancer. Impact of PSO and BM on oxidative status varied depending on carcinogen exposure and coexisting neoplastic process, which is important, due to the growing interest in their use in prevention and therapy of various diseases, including cancer.

How Ah Receptor Ligand Specificity Became Important in Understanding Its Physiological Function

Increasingly, the aryl hydrocarbon receptor (AHR) is being recognized as a sensor for endogenous and pseudo-endogenous metabolites, and in particular microbiota and host generated tryptophan metabolites. One proposed explanation for this is the role of the AHR in innate immune signaling within barrier tissues in response to the presence of microorganisms. A number of cytokine/chemokine genes exhibit a combinatorial increase in transcription upon toll-like receptors and AHR activation, supporting this concept. The AHR also plays a role in the enhanced differentiation of intestinal and dermal epithelium leading to improved barrier function. Importantly, from an evolutionary perspective many of these tryptophan metabolites exhibit greater activation potential for the human AHR when compared to the rodent AHR. These observations underscore the importance of the AHR in barrier tissues and may lead to pharmacologic therapeutic intervention.

Esterification of 4β-hydroxycholesterol and other oxysterols in human plasma occurs independently of LCAT

The acyltransferase LCAT mediates FA esterification of plasma cholesterol. In vitro studies have shown that LCAT also FA-esterifies several oxysterols, but in vivo evidence is lacking. Here, we measured both free and FA-esterified forms of sterols in 206 healthy volunteers and 8 individuals with genetic LCAT deficiency, including familial LCAT deficiency (FLD) and fish-eye disease (FED). In the healthy volunteers, the mean values of the ester-to-total molar ratios of the following sterols varied: 4β-hydroxycholesterol (4βHC), 0.38; 5,6α-epoxycholesterol (5,6αEC), 0.46; 5,6β-epoxycholesterol (5,6βEC), 0.51; cholesterol, 0.70; cholestane-3β,5α,6β-triol (CT), 0.70; 7-ketocholesterol (7KC), 0.75; 24S-hydroxycholesterol (24SHC), 0.80; 25-hydroxycholesterol (25HC), 0.81; 27-hydroxycholesterol (27HC), 0.86; and 7α-hydroxycholesterol (7αHC), 0.89. In the individuals with LCAT deficiency, the plasma levels of the FA-esterified forms of cholesterol, 5,6αEC, 5,6βEC, CT, 7αHC, 7KC, 24SHC, 25HC, and 27HC, were significantly lower than those in the healthy volunteers. The individuals with FLD had significantly lower FA-esterified forms of 7αHC, 24SHC, and 27HC than those with FED. It is of note that, even in the three FLD individuals with negligible plasma cholesteryl ester, substantial amounts of the FA-esterified forms of 4βHC, 5,6αEC, 7αHC, 7KC, and 27HC were present. We conclude that LCAT has a major role in the FA esterification of many plasma oxysterols but contributes little to the FA esterification of 4βHC. Substantial FA esterification of 4βHC, 5,6αEC, 7αHC, 7KC, and 27HC is independent of LCAT.

β-Naphthoflavone, an exogenous ligand of aryl hydrocarbon receptor, disrupts zinc homeostasis in human hepatoma HepG2 cells

Recent studies have demonstrated a relationship between the disruption of zinc homeostasis and the onset of diseases. However, little is known about the factors that disrupt zinc homeostasis. Here, we investigated the effects of β-naphthoflavone, an exogenous ligand of aryl hydrocarbon receptor (AHR), on intracellular zinc levels. Human hepatoma HepG2 cells were treated with β-naphthoflavone for 3 days, and intracellular labile and total zinc levels were assessed through flow cytometry and inductively coupled plasma atom emission spectroscopy, respectively. The mRNA levels of zinc transporters were determined by real-time PCR. Treatment of cells with β-naphthoflavone induced a decrease in intracellular labile zinc in a dose-dependent manner, with significantly decreased levels observed at 1 µM compared with controls. Additionally, intracellular total zinc levels demonstrated a decreasing trend with 10 µM β-naphthoflavone. Zinc pyrithione recovered the decrease in intracellular labile zinc levels induced by β-naphthoflavone, while zinc sulfate had no effect. Moreover, significant decreases in the mRNA levels of zinc transporters ZnT10 and ZIP5 were observed in response to 10 µM β-naphthoflavone. These results demonstrated that β-naphthoflavone has the potential to disrupt zinc homeostasis in hepatocytes. Although the underlying mechanism remains to be determined, suppression of zinc transporter transcription through AHR activation may be involved in the β-naphthoflavone-induced disruption of intracellular zinc levels.

11β-Hydroxysteroid dehydrogenases control access of 7β,27-dihydroxycholesterol to retinoid-related orphan receptor γ

Oxysterols previously were considered intermediates of bile acid and steroid hormone biosynthetic pathways. However, recent research has emphasized the roles of oxysterols in essential physiologic processes and in various diseases. Despite these discoveries, the metabolic pathways leading to the different oxysterols are still largely unknown and the biosynthetic origin of several oxysterols remains unidentified. Earlier studies demonstrated that the glucocorticoid metabolizing enzymes, 11β-hydroxysteroid dehydrogenase (11β-HSD) types 1 and 2, interconvert 7-ketocholesterol (7kC) and 7β-hydroxycholesterol (7βOHC). We examined the role of 11β-HSDs in the enzymatic control of the intracellular availability of 7β,27-dihydroxycholesterol (7β27OHC), a retinoid-related orphan receptor γ (RORγ) ligand. We used microsomal preparations of cells expressing recombinant 11β-HSD1 and 11β-HSD2 to assess whether 7β27OHC and 7-keto,27-hydroxycholesterol (7k27OHC) are substrates of these enzymes. Binding of 7β27OHC and 7k27OHC to 11β-HSDs was studied by molecular modeling. To our knowledge, the stereospecific oxoreduction of 7k27OHC to 7β27OHC by human 11β-HSD1 and the reverse oxidation reaction of 7β27OHC to 7k27OHC by human 11β-HSD2 were demonstrated for the first time. Apparent enzyme affinities of 11β-HSDs for these novel substrates were equal to or higher than those of the glucocorticoids. This is supported by the fact that 7k27OHC and 7β27OHC are potent inhibitors of the 11β-HSD1-dependent oxoreduction of cortisone and the 11β-HSD2-dependent oxidation of cortisol, respectively. Furthermore, molecular docking calculations explained stereospecific enzyme activities. Finally, using an inducible RORγ reporter system, we showed that 11β-HSD1 and 11β-HSD2 controlled RORγ activity. These findings revealed a novel glucocorticoid-independent prereceptor regulation mechanism by 11β-HSDs that warrants further investigation.

Oxysterols and nuclear receptors

Oxysterols are derivatives of cholesterol and an important regulator of cholesterol metabolism, in part due to their role as ligands for nuclear receptors, such as the liver X receptors. Oxysterols are also known to be ligands for the RAR-related orphan receptors, involved in normal T cell differentiation. However, increasing evidence supports a role for oxysterols in the progression of several diseases. Here, we review recent developments in oxysterol research, highlighting the biological functions that oxysterols exert through their target nuclear receptors: the liver X receptors, estrogen receptors, RAR-related orphan receptors and the glucocorticoid receptor. We also bring the regulation of the immune system into the context of interaction between oxysterols and nuclear receptors, discussing the effect of such interaction on the pro-inflammatory function of macrophages and the development of T cells. Finally, we examine the impact that oxysterols have on various disease models, including cancer, Alzheimer's disease and atherosclerosis, stressing the role of nuclear receptors if previously identified. This review underscores the need to consider the multifaceted roles of oxysterols in terms of multiple receptor engagements and selective modulation of these receptors.

The cholesterol biosynthesis pathway regulates IL-10 expression in human Th1 cells

The mechanisms controlling CD4+ T cell switching from an effector to an anti-inflammatory (IL-10+) phenotype play an important role in the persistence of chronic inflammatory diseases. Here, we identify the cholesterol biosynthesis pathway as a key regulator of this process. Pathway analysis of cultured cytokine-producing human T cells reveals a significant association between IL-10 and cholesterol metabolism gene expression. Inhibition of the cholesterol biosynthesis pathway with atorvastatin or 25-hydroxycholesterol during switching from IFNγ+ to IL-10+ shows a specific block in immune resolution, defined as a significant decrease in IL-10 expression. Mechanistically, the master transcriptional regulator of IL10 in T cells, c-Maf, is significantly decreased by physiological levels of 25-hydroxycholesterol. Strikingly, progression to rheumatoid arthritis is associated with altered expression of cholesterol biosynthesis genes in synovial biopsies of predisposed individuals. Our data reveal a link between sterol metabolism and the regulation of the anti-inflammatory response in human CD4+ T cells.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Actein alleviates 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated cellular dysfunction in osteoblastic MC3T3-E1 cells

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to affect bone metabolism. We evaluated the protective effects of the triterpene glycoside actein from the herb black cohosh against TCDD-induced toxicity in MC3T3-E1 osteoblastic cells. We found that TCDD significantly reduced cell viability and increased apoptosis and autophagy in MC3T3-E1 osteoblastic cells (P < .05). In addition, TCDD treatment resulted in a significant increase in intracellular calcium concentration, mitochondrial membrane potential collapse, reactive oxygen species (ROS) production, and cardiolipin peroxidation, whereas pretreatment with actein significantly mitigated these effects (P < .05). The effects of TCDD on extracellular signal-related kinase (ERK), aryl hydrocarbon receptor, aryl hydrocarbon receptor repressor, and cytochrome P450 1A1 levels in MC3T3-E1 cells were significantly inhibited by actein. The levels of superoxide dismutase, ERK1, and nuclear factor kappa B mRNA were also effectively restored by pretreatment with actein. Furthermore, actein treatment resulted in a significant increase in alkaline phosphatase (ALP) activity and collagen content, as well as in the expression of genes associated with osteoblastic differentiation (ALP, type I collagen, osteoprotegerin, bone sialoprotein, and osterix). This study demonstrates the underlying molecular mechanisms of cytoprotection exerted by actein against TCDD-induced oxidative stress and osteoblast damage.

Resveratrol, Potential Therapeutic Interest in Joint Disorders: A Critical Narrative Review

Trans-resveratrol (t-Res) is a natural compound of a family of hydroxystilbenes found in a variety of spermatophyte plants. Because of its effects on lipids and arachidonic acid metabolisms, and its antioxidant activity, t-Res is considered as the major cardioprotective component of red wine, leading to the “French Paradox” health concept. In the past decade, research on the effects of resveratrol on human health has developed considerably in diverse fields such as cancer, neurodegenerative and cardiovascular diseases, and metabolic disorders. In the field of rheumatic disorders, in vitro evidence suggest anti-inflammatory, anti-catabolic, anti-apoptotic and anti-oxidative properties of t-Res in various articular cell types, including chondrocytes and synoviocytes, along with immunomodulation properties on T and B lymphocytes. In preclinical models of osteoarthritis and rheumatoid arthritis, resveratrol has shown joint protective effects, mainly mediated by decreased production of pro-inflammatory and pro-degradative soluble factors, and modulation of cellular and humoral responses. Herein, we comprehensively reviewed evidence supporting a potential therapeutic interest of t-Res in treating symptoms related to rheumatic disorders.

Free Radical-derived Oxysterols: Novel Adipokines Modulating Adipogenic Differentiation of Adipose Precursor Cells

CONTEXT

Increased oxidative stress in adipose tissue emerges as an inducer of obesity-linked insulin resistance. Here we tested whether free-radical derived oxysterols are formed by, and accumulate in, human adipocytes. Moreover, we asked whether increased accumulation of oxysterols characterizes the adipose cells of obese patients with type 2 diabetes (T2D) (OBT2D) compared with lean, nondiabetic controls (CTRLs). Finally, we studied the effects of the free radical-derived oxysterols on adipogenic differentiation of adipose-derived stem cells (ASCs).

MAIN OUTCOME MEASURES

Adipocytes and ASCs were isolated from sc abdominal adipose tissue biopsy in four OBT2D and four CTRL subjects. Oxysterols in adipocytes were detected by gas chromatography/mass spectrometry. The cellular and molecular effects of oxysterols were then evaluated on primary cultures of ASCs focusing on cell viability, adipogenic differentiation, and "canonical" WNT and MAPK signaling pathways.

RESULTS

7-ketocholesterol (7κ-C) and 7β-hydroxycholesterol were unambiguously detected in adipocytes, which showed higher oxysterol accumulation (P < .01) in OBT2D, as compared with CTRL individuals. Notably, the accumulation of oxysterols in adipocytes was predicted by the adipose cell size of the donor (R² = 0.582; P < .01). Challenging ASCs with free radical-derived type I (7κ-C) and type II (5,6-Secosterol) oxysterols led to a time- and concentration-dependent decrease of cell viability. Meaningfully, at a non-toxic concentration (1μM), these bioactive lipids hampered adipogenic differentiation of ASCs by sequential activation of WNT/β-catenin, p38-MAPK, ERK1/2, and JNK signaling pathways.

CONCLUSION

Free radical-derived oxysterols accumulate in the "diabetic" fat and may act as novel adipokines modulating the adipogenic potential of undifferentiated adipose precursor cells.

Redox Control of Multidrug Resistance and Its Possible Modulation by Antioxidants

Clinical efficacy of anticancer chemotherapies is dramatically hampered by multidrug resistance (MDR) dependent on inherited traits, acquired defence against toxins, and adaptive mechanisms mounting in tumours. There is overwhelming evidence that molecular events leading to MDR are regulated by redox mechanisms. For example, chemotherapeutics which overrun the first obstacle of redox-regulated cellular uptake channels (MDR1, MDR2, and MDR3) induce a concerted action of phase I/II metabolic enzymes with a temporal redox-regulated axis. This results in rapid metabolic transformation and elimination of a toxin. This metabolic axis is tightly interconnected with the inducible Nrf2-linked pathway, a key switch-on mechanism for upregulation of endogenous antioxidant enzymes and detoxifying systems. As a result, chemotherapeutics and cytotoxic by-products of their metabolism (ROS, hydroperoxides, and aldehydes) are inactivated and MDR occurs. On the other hand, tumour cells are capable of mounting an adaptive antioxidant response against ROS produced by chemotherapeutics and host immune cells. The multiple redox-dependent mechanisms involved in MDR prompted suggesting redox-active drugs (antioxidants and prooxidants) or inhibitors of inducible antioxidant defence as a novel approach to diminish MDR. Pitfalls and progress in this direction are discussed.

A possible role of aryl hydrocarbon receptor in spontaneous preterm birth

Preterm birth (PTB) is defined as birth before 37 weeks of gestation and is a leading cause of neonatal mortality and morbidity. To date, the etiology of spontaneous PTB (sPTB) remains unclear; however, intrauterine bacterial infection-induced inflammation is considered to be one of the major triggers. Aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor. Upon activation, AhR signaling mediates many biological processes. AhR is abundantly expressed in human placentas, primarily in trophoblasts, and several fetal organs and tissues. The activation of AhR signaling can modulate inflammatory responses via promoting production of pro-inflammatory cytokines by the placenta and fetal membranes. These cytokines could enhance expression and/or activity of cyclooxygenase-2 (COX2) in human trophoblasts and amniotic epithelia, which in turn stimulate synthesis and release of prostaglandins (PGs; e.g., PGE2 and PGF2α). Given the discovery of a number of natural and endogenous AhR ligands in human, we hypothesize that in a subset of patients with high AhR expression in placentas and fetal membranes, repeated exposure to these AhR ligands hyperactivates AhR, inducing hyperactivation of the cytokines/COX2/PGs pathway, resulting in myometrial contractions, ultimately leading to sPTB. We further hypothesize that hyperactivation of this AhR pathway can induce sPTB either directly or in synergy with the bacterial infection. Proof of this hypothesis may provide a novel mechanism underlying sPTB.

Aryl hydrocarbon receptor ligands in cancer: friend and foe

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that is best known for mediating the toxicity and tumour-promoting properties of the carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin, commonly referred to as ‘dioxin’. AHR influences the major stages of tumorigenesis — initiation, promotion, progression and metastasis — and physiologically relevant AHR ligands are often formed during disease states or during heightened innate and adaptive immune responses. Interestingly, ligand specificity and affinity vary between rodents and humans. Studies of aggressive tumours and tumour cell lines show increased levels of AHR and constitutive localization of this receptor in the nucleus. This suggests that the AHR is chronically activated in tumours, thus facilitating tumour progression. This Review discusses the role of AHR in tumorigenesis and the potential for therapeutic modulation of its activity in tumours.

Involvement of oxysterols in age-related diseases and ageing processes

Ageing is accompanied by increasing vulnerability to major pathologies (atherosclerosis, Alzheimer's disease, age-related macular degeneration, cataract, and osteoporosis) which can have similar underlying pathoetiologies. All of these diseases involve oxidative stress, inflammation and/or cell death processes, which are triggered by cholesterol oxide derivatives, also named oxysterols. These oxidized lipids result either from spontaneous and/or enzymatic oxidation of cholesterol on the steroid nucleus or on the side chain. The ability of oxysterols to induce severe dysfunctions in organelles (especially mitochondria) plays key roles in RedOx homeostasis, inflammatory status, lipid metabolism, and in the control of cell death induction, which may at least in part contribute to explain the potential participation of these molecules in ageing processes and in age related diseases. As no efficient treatments are currently available for most of these diseases, which are predicted to become more prevalent due to the increasing life expectancy and average age, a better knowledge of the biological activities of the different oxysterols is of interest, and constitutes an important step toward identification of pharmacological targets for the development of new therapeutic strategies.

Immunological Relevance of the Coevolution of IDO1 and AHR

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor initially identified because of its role in controlling the cellular response to environmental molecules. More recently, AHR has been shown to play a crucial role in controlling innate and adaptive immune responses through several mechanisms, one of which is the regulation of tryptophan metabolism. Indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO) are considered rate-limiting enzymes in the tryptophan catabolism and play important roles in the regulation of the immunity. Moreover, AHR and IDO/TDO are closely interconnected: AHR regulates IDO and TDO expression, and kynurenine produced by IDO/TDO is an AHR agonist. In this review, we propose to examine the relationship between AHR and IDO/TDO and its relevance for the regulation of the immune response in health and disease.

11β-Hydroxysteroid dehydrogenase-1 is involved in bile acid homeostasis by modulating fatty acid transport protein-5 in the liver of mice

11β-Hydroxysteroid dehydrogenase-1 (11β-HSD1) plays a key role in glucocorticoid receptor (GR) activation. Besides, it metabolizes some oxysterols and bile acids (BAs). The GR regulates BA homeostasis; however, the impact of impaired 11β-HSD1 activity remained unknown. We profiled plasma and liver BAs in liver-specific and global 11β-HSD1-deficient mice. 11β-HSD1-deficiency resulted in elevated circulating unconjugated BAs, an effect more pronounced in global than liver-specific knockout mice. Gene expression analyses revealed decreased expression of the BA-CoA ligase Fatp5, suggesting impaired BA amidation. Reduced organic anion-transporting polypeptide-1A1 (Oatp1a1) and enhanced organic solute-transporter-β (Ostb) mRNA expression were observed in livers from global 11β-HSD1-deficient mice. The impact of 11β-HSD1-deficiency on BA homeostasis seems to be GR-independent because intrahepatic corticosterone and GR target gene expression were not substantially decreased in livers from global knockout mice. Moreover, Fatp5 expression in livers from hepatocyte-specific GR knockout mice was unchanged. The results revealed a role for 11β-HSD1 in BA homeostasis.

Ligand promiscuity of aryl hydrocarbon receptor agonists and antagonists revealed by site-directed mutagenesis

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that can be activated by structurally diverse chemicals. To examine the mechanisms responsible for the promiscuity in AhR ligand binding, we determined the effects of mutations within the AhR ligand-binding domain (LBD) on the activity of diverse AhR ligands. Site-directed mutagenesis identified Ile319 of the mouse AhR and, to a lesser extent, Phe318 as residues involved in ligand-selective modulation of AhR transformation using a panel of 12 AhR ligands. These ligands could be categorized into four distinct structurally related groups based on their ability to activate AhR mutants at position 319 in vitro. The mutation I319K was selectively activated by FICZ and not by other examined ligands in vitro and in cell culture. F318L and F318A mutations resulted in the conversion of AhR agonists β-naphthoflavone and 3-methylcholanthrene, respectively, into partial agonists/antagonists. Hsp90 binding to the AhR was decreased with several mutations and was inversely correlated with AhR ligand-binding promiscuity. Together, these data define overlapping amino acid residues within the AhR LBD involved in the selectivity of ligand binding, the agonist or antagonist mode of ligand binding, and hsp90 binding and provide insights into the ligand diversity of AhR activators.

Aryl hydrocarbon receptor deficiency causes dysregulated cellular matrix metabolism and age-related macular degeneration-like pathology

The aryl hydrocarbon receptor (AhR) is a nuclear receptor that regulates xenobiotic metabolism and detoxification. Herein, we report a previously undescribed role for the AhR signaling pathway as an essential defense mechanism in the pathogenesis of early dry age-related macular degeneration (AMD), the leading cause of vision loss in the elderly. We found that AhR activity and protein levels in human retinal pigment epithelial (RPE) cells, cells vulnerable in AMD, decrease with age. This finding is significant given that age is the most established risk factor for development of AMD. Moreover, AhR(-/-) mice exhibit decreased visual function and develop dry AMD-like pathology, including disrupted RPE cell tight junctions, accumulation of RPE cell lipofuscin, basal laminar and linear-like deposit material, Bruch's membrane thickening, and progressive RPE and choroidal atrophy. High-serum low-density lipoprotein levels were also observed in AhR(-/-) mice. In its oxidized form, this lipoprotein can stimulate increased secretion of extracellular matrix molecules commonly found in deposits from RPE cells, in an AhR-dependent manner. This study demonstrates the importance of cellular clearance via the AhR signaling pathway in dry AMD pathogenesis, implicating AhR as a potential target, and the mouse model as a useful platform for validating future therapies.

Use of natural AhR ligands as potential therapeutic modalities against inflammatory disorders

The aim of this review is to discuss research involving ligands for the aryl hydrocarbon receptor (AhR) and their role in immunomodulation. While activation of the AhR is well known for its ability to regulate the biochemical and toxic effects of environmental chemicals, more recently an exciting discovery has been made indicating that AhR ligation can also regulate T-cell differentiation, specifically through activation of Foxp3(+) regulatory T cells (Tregs) and downregulation of the proinflammatory Th17 cells. Such findings have opened new avenues of research on the possibility of targeting the AhR to treat inflammatory and autoimmune diseases. Specifically, this review will discuss the current research involving natural and dietary AhR ligands. In addition, evidence indicating the potential use of these ligands in regulating inflammation in various diseases will be highlighted. The importance of the AhR in immunological processes can be illustrated by expression of this receptor on a majority of immune cell types. In addition, AhR signaling pathways have been reported to influence a number of genes responsible for mediating inflammation and other immune responses. As interest in the AhR and its ligands increases, it seems prudent to consolidate current research on the contributions of these ligands to immune regulation during the course of inflammatory diseases.

Aryl hydrocarbon receptor regulates the cholesterol biosynthetic pathway in a dioxin response element-independent manner

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor. Activation of AhR mediates the expression of target genes (e.g., CYP1A1) by binding to dioxin response element (DRE) sequences in their promoter region. To understand the multiple mechanisms of AhR-mediated gene regulation, a microarray analysis on liver isolated from ligand-treated transgenic mice expressing a wild-type (WT) Ahr or a DRE-binding mutant Ahr (A78D) on an ahr-null background was performed. Results revealed that AhR DRE binding is not required for the suppression of genes involved in cholesterol synthesis. Quantitative reverse-transcription polymerase chain reaction performed on both mouse liver and primary human hepatocyte RNA demonstrated a coordinated repression of genes involved in cholesterol biosynthesis, namely, HMGCR, FDFT1, SQLE, and LSS after receptor activation. An additional transgenic mouse line was established expressing a liver-specific Ahr-A78D on a Cre(Alb)/Ahr(flox/flox) background. These mice displayed a similar repression of cholesterol biosynthetic genes, compared to Ahr(flox/flox) mice, further indicating that the observed modulation is AhR specific and occurs in a DRE-independent manner. Elevated hepatic transcriptional levels of the genes of interest were noted in congenic C57BL/6J-Ah(d) allele mice, when compared to the WT C57BL/6J mice, which carry the Ah(b) allele. Down-regulation of AhR nuclear translocator levels using short interfering RNA in a human cell line revealed no effect on the expression of cholesterol biosynthetic genes. Finally, cholesterol secretion was shown to be significantly decreased in human cells after AhR activation.

CONCLUSION

These data firmly establish an endogenous role for AhR as a regulator of the cholesterol biosynthesis pathway independent of its DRE-binding ability, and suggest that AhR may be a previously unrecognized therapeutic target.

Dioxins: diagnostic and prognostic challenges arising from complex mechanisms

Dioxins are ubiquitous environmental challenges to humans, with a pervasiveness that arises from 200 years of rapid industrialization and mechanization of Western societies and which is now extending into the developing world. In spite of their penetrance of the human biota, these compounds are poorly understood in terms of their true physiological potential for harm, and the mechanisms by which they impact cellular and organ level function are only recently becoming clear. Emerging awareness that chronic exposures to toxins may have generational and subtle effects on the outcomes of diseases such as cancer and diabetes, which are already multifactorial and highly complex, creates the context for the current review paper. Here, we summarize dioxin exposure paradigms and the resulting physiological effects that have been documented in animals and humans. Novel insights into potential endogenous end exogenous ligands, as well as the mechanisms by which these ligands impact acute and chronic cellular processes, are discussed. We develop the idea that the diagnosis of dioxin exposure, the subtleties of the cellular effects of the compounds and prognosis of the long-term effects of exposure are problems requiring that researchers leverage the power of genomics and epigenetics. However, the continuation of longitudinal epidemiological studies and the development of a firmer basis from which to extrapolate animal studies will be critical in ensuring optimal insight from these resource-intensive techniques.

Salinomycin inhibits prostate cancer growth and migration via induction of oxidative stress

BACKGROUND

We have shown that a sodium ionophore monensin inhibits prostate cancer cell growth. A structurally related compound to monensin, salinomycin, was recently identified as a putative cancer stem cell inhibitor.

METHODS

The growth inhibitory potential of salinomycin was studied in a panel of prostate cells. To get insights into the mechanism of action, a variety of assays such as gene expression and steroid profiling were performed in salinomycin-exposed prostate cancer cells.

RESULTS

Salinomycin inhibited the growth of prostate cancer cells, but did not affect non-malignant prostate epithelial cells. Salinomycin impacted on prostate cancer stem cell functions as evidenced by reduced aldehyde dehydrogenase activity and the fraction of CD44(+) cells. Moreover, salinomycin reduced the expression of MYC, AR and ERG, induced oxidative stress as well as inhibited nuclear factor-κB activity and cell migration. Furthermore, profiling steroid metabolites revealed increased levels of oxidative stress-inducing steroids 7-ketocholesterol and aldosterone and decreased levels of antioxidative steroids progesterone and pregnenolone in salinomycin-exposed prostate cancer cells.

CONCLUSION

Our results indicate that salinomycin inhibits prostate cancer cell growth and migration by reducing the expression of key prostate cancer oncogenes, inducing oxidative stress, decreasing the antioxidative capacity and cancer stem cell fraction.

Differential ligand-dependent activation and a role for Y322 in aryl hydrocarbon receptor-mediated regulation of gene expression

The aryl hydrocarbon receptor (AHR) mediates the toxic effects of halogenated aromatic hydrocarbons (HAHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), 2,3,4,7,8-pentachlorodibenzofuran (2,3,4,7,8-PeCDF) and 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-TCDF). Non-traditional activators, including omeprazole (Omp), are thought to regulate AHR action through phosphorylation rather than binding to the receptor. In this study, we examined the ability of these compounds to induce AHR-dependent regulation of cytochrome P450 1A1 (CYP1A1) and CYP1B1 in T-47D human breast cancer cells. The role of Y322, a residue implicated in Omp-dependent activation of AHR was also investigated. All four compounds induced CYP1A1 and CYP1B1 mRNA expression, with Omp differing from the HAHs. Chromatin immunoprecipitation assays revealed ligand- and gene-selectivity in the recruitment patterns of AHR coactivators. We also found that residue Y322 of human AHR was important for maximum activation of AHR by 2,3,7,8-TCDD and 2,3,4,7,8-PeCDF, but required for 2,3,7,8-TCDF and Omp in an AHR-deficient MCF-7 human breast cancer cell line. In summary, this study provides evidence for context- and ligand-selective differences in coactivator recruitment in AHR-regulated gene expression and reveal an important role of Y322 in AHR activation.

The aryl hydrocarbon receptor ligand ITE inhibits TGFβ1-induced human myofibroblast differentiation

Fibrosis can occur in any human tissue when the normal wound healing response is amplified. Such amplification results in fibroblast proliferation, myofibroblast differentiation, and excessive extracellular matrix deposition. Occurrence of these sequelae in organs such as the eye or lung can result in severe consequences to health. Unfortunately, medical treatment of fibrosis is limited by a lack of safe and effective therapies. These therapies may be developed by identifying agents that inhibit critical steps in fibrotic progression; one such step is myofibroblast differentiation triggered by transforming growth factor-β1 (TGFβ1). In this study, we demonstrate that TGFβ1-induced myofibroblast differentiation is blocked in human fibroblasts by a candidate endogenous aryl hydrocarbon receptor (AhR) ligand 2-(1'H-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE). Our data show that ITE disrupts TGFβ1 signaling by inhibiting the nuclear translocation of Smad2/3/4. Although ITE functions as an AhR agonist, and biologically persistent AhR agonists, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, cause severe toxic effects, ITE exhibits no toxicity. Interestingly, ITE effectively inhibits TGFβ1-driven myofibroblast differentiation in AhR(-/-) fibroblasts: Its ability to inhibit TGFβ1 signaling is AhR independent. As supported by the results of this study, the small molecule ITE inhibits myofibroblast differentiation and may be useful clinically as an antiscarring agent.

CYP1A1 induction in the colon by serum: involvement of the PPARα pathway and evidence for a new specific human PPREα site

Background

We previously showed that blood serum induced cytochrome P450 1A1 (CYP1A1) monooxygenase expression in vitro.

Objective

Our purpose was (i) to identify the molecular mechanism involved and (ii) to characterize the inducer compound(s) in serum involved at least in part.

Methods

Serum was fractionated on hydrophobic columns. PPARα involvement was demonstrated by gene reporter assays, DNA mutagenesis and EMSA. Gene expression was evaluated by qRT-PCR. Serum samples were analyzed using HS-SPME-GC-MS.

Results

The inductive effect of serum did not depend on the AhR pathway and was enhanced by cotransfection of PPARα cDNA. Mutations in the PPAR response elements of the CYP1A1 gene promoter suppressed this effect. One of the PPRE sites appeared highly specific for human PPARα, an unreported PPRE property. A link was found between CYP1A1 inducibility and serum hydrophobic compounds. Characterization of sera showed that hexanal, a metabolite produced by peroxidation of linoleic acid, was involved in CYP1A1 induction by serum, possibly along with other serum entities.

Conclusion

We demonstrate that serum induces CYP1A1 via the PPARα pathway and that hexanal is one of the serum inducers. The two PPRE sites within the CYP1A1 promoter are functional and one of them is specific for PPARα.

Curcumin induces paraoxonase 1 in cultured hepatocytes in vitro but not in mouse liver in vivo

Paraoxonase 1 (PON1) is an enzyme that is mainly synthesised in the liver and protects LDL from oxidation, thereby exhibiting antiatherogenic properties. Using a luciferase reporter gene assay, we tested curcumin for its ability to induce PON1 in Huh7 hepatocytes in culture. Curcumin ( ≥ 10 μmol/l) dose-dependently induced PON1 transactivation in Huh7 cells. However, dietary supplementation of female B6C3F1 mice with curcumin (500 mg/kg diet) for 2 weeks did not increase the hepatic PON1 mRNA and protein levels. No curcumin was detectable in the plasma of the 12 h fasted mice. In conclusion, curcumin may be a potent PON1 inducer in cultured cells in vitro, but not in the liver of curcumin-fed mice because of its low concentrations in vivo.

Breast cancer stem-like cells are inhibited by a non-toxic aryl hydrocarbon receptor agonist

Background

Cancer stem cells (CSCs) have increased resistance to cancer chemotherapy. They can be enriched as drug-surviving CSCs (D-CSCs) by growth with chemotherapeutic drugs, and/or by sorting of cells expressing CSC markers such as aldehyde dehydrogenase-1 (ALDH). CSCs form colonies in agar, mammospheres in low-adherence cultures, and tumors following xenotransplantation in Scid mice. We hypothesized that tranilast, a non-toxic orally active drug with anti-cancer activities, would inhibit breast CSCs.

Methodology/Findings

We examined breast cancer cell lines or D-CSCs generated by growth of these cells with mitoxantrone. Tranilast inhibited colony formation, mammosphere formation and stem cell marker expression. Mitoxantrone-selected cells were enriched for CSCs expressing stem cell markers ALDH, c-kit, Oct-4, and ABCG2, and efficient at forming mammospheres. Tranilast markedly inhibited mammosphere formation by D-CSCs and dissociated formed mammospheres, at pharmacologically relevant concentrations. It was effective against D-CSCs of both HER-2+ and triple-negative cell lines. Tranilast was also effective in vivo, since it prevented lung metastasis in mice injected i.v. with triple-negative (MDA-MB-231) mitoxantrone-selected cells. The molecular targets of tranilast in cancer have been unknown, but here we demonstrate it is an aryl hydrocarbon receptor (AHR) agonist and this plays a key role. AHR is a transcription factor activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polycyclic aromatic hydrocarbons and other ligands. Tranilast induced translocation of the AHR to the nucleus and stimulated CYP1A1 expression (a marker of AHR activation). It inhibited binding of the AHR to CDK4, which has been linked to cell-cycle arrest. D-CSCs expressed higher levels of the AHR than other cells. Knockdown of the AHR with siRNA, or blockade with an AHR antagonist, entirely abrogated the anti-proliferative and anti-mammosphere activity of tranilast. Thus, the anti-cancer effects of tranilast are AHR dependent.

Conclusion/Significance

We show that tranilast is an AHR agonist with inhibitory effects on breast CSCs. It is effective against CSCs of triple-negative breast cancer cells selected for anti-cancer drug resistance. These results suggest it might find applications in the treatment of breast cancer.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

Potential health-modulating effects of isoflavones and metabolites via activation of PPAR and AhR

Isoflavones have multiple actions on cell functions. The most prominent one is the activation of estrogen receptors. Other functions are often overlooked, but are equally important and explain the beneficial health effects of isoflavones. Isoflavones are potent dual PPARα/γ agonists and exert anti-inflammatory activity, which may contribute to the prevention of metabolic syndrome, atherosclerosis and various other inflammatory diseases. Some isoflavones are potent aryl hydrocarbon receptor (AhR) agonists and induce cell cycle arrest, chemoprevention and modulate xenobiotic metabolism. This review discusses effects mediated by the activation of AhR and PPARs and casts a light on the concerted action of isoflavones.

7-ketocholesterol and 5,6-secosterol modulate differently the stress-activated mitogen-activated protein kinases (MAPKs) in liver cells

Enhanced oxidative stress is a common feature of liver diseases and contributes to chronic liver disease (CLD) progression by inducing fibrogenesis during liver regeneration. Peroxidation products of cholesterol metabolism, named oxysterols, are new and reliable markers of oxidative stress in vivo. Patients affected by CLDs present high plasma levels of oxysterols, raising the question of the origin and biological relevance of these compounds in the pathophysiology of chronic liver damage. The aim of this study was to examine the molecular basis of the biological effects of oxysterols on liver-derived cells, HepG2 and Huh7. Cells were treated with different concentrations (10(-9) to 10(-5) M) of 7-ketocholesterol used as a reference, and 5,6-secosterol, a recently discovered oxysterol. FACS investigations, caspase-3 activation, and Sytox Green immunofluorescent assay showed that pathological concentrations of oxysterols induced necrosis (30-50%) after 48 h of treatment. The two analyzed compounds displayed a similar, but not identical, behavior. In fact, 5,6-secosterol, but not 7-ketocholesterol, induced cell senescence. Notably, low concentrations of 5,6-secosterol caused a sustained activation of ERK1/2, inducing cell proliferation, this unexpected behavior should be better characterized by further studies. Since enhanced oxidative stress is known to worsen liver chronic hepatitis and frequently results in overall decreased cellular survival, our data suggest the important and different role oxysterols may have in interfering with physiological liver tissue regeneration in injured human liver. Antioxidant treatment may provide a highly specific and effective mean to counteract the common consequences of oxidative stress on chronic hepatitis, such as fibrosis/cirrhosis and liver failure.

The pleiotropy of dioxin toxicity--xenobiotic misappropriation of the aryl hydrocarbon receptor's alternative physiological roles

The aryl hydrocarbon receptor is a signal regulated transcription factor that has best been characterised as regulating the xenobiotic response to a variety of planar aromatic hydrocarbons. There is compelling evidence that it mediates most, if not all, of the toxic effects of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin). Dioxin exposure results in a wide variety of toxic outcomes including severe wasting syndrome, chloracne, thymic involution, severe immune suppression, reduced fertility, hepatotoxicity, teratogenicity, tumour promotion and death. The pleiotropy of toxic outcomes implies the disruption of a wide range of normal physiological functions. The aryl hydrocarbon receptor has developmentally restricted expression as well as developmental defects in gene-targeted mice. It has a wide range of target genes that do not fit into the classical xenobiotic metabolising gene battery and has recently been shown to interact with NF-kappa B and the estrogen receptor. There is also evidence for its activation in the absence of exogenous ligand, all of which point to various roles outside xenobiotic metabolism. Ligands so far identified display differential activation potential with respect to receptor activity. This article addresses activities of the aryl hydrocarbon receptor that are outside the xenobiotic response. Known physiological roles are discussed as well as how their disruption contributes to the pleiotropic toxicity of TCDD.

12(R)-Hydroxy-5(Z),8(Z),10(E),14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid derivative, is an activator of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) is a ligand-regulated transcription factor that can be activated by structurally diverse chemicals, ranging from environmental carcinogens to dietary metabolites. Evidence supporting a necessary role for the AHR in normal biology has been established; however, identification of key endogenous ligand/activator remains to be established. Here, we report the ability of 12(R)-hydroxy-5(Z),8(Z),10(E), 14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid metabolite produced by either a lipoxygenase or cytochrome P-450 pathway, to act as a potent indirect modulator of the AHR pathway. In contrast, structurally similar HETE isomers failed to demonstrate significant activation of the AHR. Electrophoretic mobility shift assays, together with ligand competition binding experiments, have demonstrated the inability of 12(R)-HETE to directly bind or directly activate the AHR to a DNA binding species in vitro. However, cell-based xenobiotic-responsive element-driven luciferase reporter assays indicate the ability of 12(R)-HETE to modulate AHR activity, and quantitation of induction of an AHR target gene confirmed 12(R)-HETE's ability to activate AHR-mediated transcription, even at high nanomolar concentrations in human hepatoma (HepG2)- and keratinocyte (HaCaT)-derived cell lines. One explanation for these results is that a metabolite of 12(R)-HETE is acting as a direct ligand for the AHR. However, several known metabolites failed to exhibit AHR activity. The ability of 12(R)-HETE to activate AHR target genes required receptor expression. These results indicate that 12(R)-HETE can serve as a potent activator of AHR activity and suggest that in normal and inflammatory disease conditions in skin, 12(R)-HETE is produced, perhaps leading to AHR activation.

Activation of the aryl hydrocarbon receptor by the calcium/calmodulin-dependent protein kinase kinase inhibitor 7-oxo-7H-benzimidazo[2,1-a]benz[de]isoquinoline-3-carboxylic acid (STO-609)

This study was designed to analyze the effects of the Ca2+/calmodulin-dependent protein kinase kinase (CaMKK) inhibitor STO-609 (7-oxo-7H-benzimidazo[2,1-a]benz[de]isoquinoline-3-carboxylic acid) toward the aryl hydrocarbon receptor (AhR) pathway because Ca2+/calmodulin-dependent protein kinase (CaMK) Ialpha, known as a downstream CaMKK effector, has been recently shown to contribute to the AhR cascade. STO-609 failed to alter up-regulation of the AhR target CYP1A1 in response to the potent AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in MCF-7 cells. STO-609, used at a 25 muM concentration known to fully inhibit CaMKK activity, was surprisingly found to markedly induce CYP1A1 expression and activity by itself in MCF-7 cells; it similarly up-regulated various other AhR target genes in human macrophages. STO-609-related CYP1A1 induction was prevented by chemical inhibition or small interfering RNA-mediated knockdown expression of AhR. Moreover, STO-609 was demonstrated to physically interact with the ligand-binding domain of AhR, as assessed by TCDD binding competition assay, and to induce AhR translocation to the nucleus. As already reported for AhR agonists, STO-609 triggered the increase of [Ca2+](i) and activation of CaMKIalpha, whose inhibition through the use of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester or the CaMK inhibitor KN-93 (2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine), respectively, prevented STO-609-mediated CYP1A1 activity induction. Taken together, these results demonstrate that the CaMKK inhibitor STO-609 can act as an AhR ligand and, in this way, fully activates the Ca2+/CaMKIalpha/AhR cascade. Such data, therefore, make unlikely any contribution of CaMKK activity to the AhR pathway and, moreover, suggest that caution may be required when using STO-609 as a specific inhibitor of CaMKKs.

Red clover isoflavones biochanin A and formononetin are potent ligands of the human aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AhR) activation affects the cell cycle and drives cells to apoptosis. Thus, selective AhR modulators (SAhRMs) have previously been implicated in cancer therapy and prevention, particularly for hormone-dependent cancers. In the present study, isoflavones a remedy used to ameliorate menopausal complaints were tested for their potential in transactivating AhR in order to investigate the biological function of red clover isoflavones. The results were compared to the transactivation potentials of other flavonoids and plant-derived indole compounds. We found that the isoflavones biochanin A and formononetin were potent AhR agonists in vitro, with EC(50) values of 2.5 x 10(-7) and 1.3 x 10(-7)mol/l, respectively. These isoflavones are 10 times more potent compared to the indole compounds indole-3-carbinol and diindolylmethane, publicised as powerful AhR agonists with EC(50) values of 5.8 x 10(-6) and 1.1 x 10(-6)mol/l, respectively. Because activated AhR crosstalks with estrogen receptor alpha, future risk-benefit assessments of isoflavones should take into consideration their AhR transactivating potential.

Antagonistic and agonistic effects of indigoids on the transformation of an aryl hydrocarbon receptor

Halogenated and polycyclic aromatic hydrocarbons, exogenous ligands of the aryl hydrocarbon receptor (AhR), cause various toxicological effects through the transformation of the AhR. In this study, we investigated the antagonistic effects of indigoids on the transformation in addition to their agonistic ones. In a cell-free system, indigoids induced the transformation dose-dependently, but suppressed the transformation induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin and the binding of 3-methylcholanthrene to the AhR. In mouse hepatoma Hepa-1c1c7 cells, indigoids, especially indirubin, suppressed the transformation and expression of CYP1A1 by inhibiting the translocation of AhR into the nucleus. When orally administered to mice at 10mg/kg BW/day for three successive days, indigoids did not induce AhR transformation and expression of the CYP1A subfamily in the liver, while indirubin and indigo upregulated quinone reductase activity. These results indicate that indigoids are able to bind to the AhR as ligands and exhibit antagonistic effects at lower concentrations in mammalian cells.