The bioflavonoid galangin blocks aryl hydrocarbon receptor activation and polycyclic aromatic hydrocarbon-induced pre-B cell apoptosis

Inhibition of Cancer Development by Natural Plant Polyphenols: Molecular Mechanisms

Malignant tumors remain one of the main sources of morbidity and mortality around the world. A chemotherapeutic approach to cancer treatment poses a multitude of challenges, primarily due to the low selectivity and genotoxicity of the majority of chemotherapeutic drugs currently used in the clinical practice, often leading to treatment-induced tumors formation. Highly selective antitumor drugs can largely resolve this issue, but their high selectivity leads to significant drawbacks due to the intrinsic tumor heterogeneity. In contrast, plant polyphenols can simultaneously affect many processes that are involved in the acquiring and maintaining of hallmark properties of malignant cells, and their toxic dose is typically much higher than the therapeutic one. In the present work we describe the mechanisms of the action of polyphenols on cancer cells, including their effects on genetic and epigenetic instability, tumor-promoting inflammation, and altered microbiota.

Pectin-type polysaccharide from galangal: An efficient emulsifier to construct the emulsion-based delivery system for galangal flavonoids

Galangal is rich in flavonoids and polysaccharides but underutilized. In this study, galangal flavonoids and polysaccharides (GP-HN and GP-UN) were obtained by segmented extraction, used for chemical composition determination/structural characterization, and constructed for the emulsion delivery system. The results showed that galangin accounted for 71.45 % of total flavonoids. GP-HN and GP-UN were prepared by enzymatic-assisted high-temperature and ultrasonic extraction, which were low-molecular-weight pectin-type polysaccharides mainly constructed by galacturonic acid, galactose, and arabinose. GP-UN was the best emulsifier due to interfacial activities, emulsifying properties, interfacial resistance to bile salts displacement abilities, and anti-lipid digestion abilities of GPs. GP-UN emulsion could stably deliver flavonoids. This study presented a method for orderly reorganizing flavonoids and polysaccharides, guiding for utilization of whole bioactive components in galangal.

Reduced Haematopoietic Output in Automobile Mechanics and Sprayers with Chronic Exposure to Petrochemicals: A Case-Control Study in Cape Coast, Ghana

Background

Automobile mechanics and sprayers are at a higher risk of exposure to hazardous chemicals such as polycyclic aromatic hydrocarbons and heavy metals which may cause adverse health outcomes. This study aimed to use reticulocyte count as an indirect measure of the haematological output in automobile mechanics and sprayers in the Cape Coast Metropolis, Ghana.

Method

This cross-sectional study recruited 130 participants: 90 cases (57 automobile mechanics and 33 automobile sprayers) and 40 controls (nonautomobile workers). Venous blood samples were drawn from the participants and examined for full blood count and absolute reticulocyte count. Semi-structured questionnaire was used to collect demographic and occupational safety information from participants.

Results

75.6% of cases had never received occupational safety training. Whereas 35.1% of automobile mechanics routinely siphoned fuel, 36.4% of automobile sprayers never used nose masks in the discharge of their duties. Controls had significantly higher WBC counts compared to mechanics (p = 0.0001; 5.04 ± 1.7 versus 3.81 ± 1.1), or sprayers (p = 0.0004; 5.04 ± 1.7 versus 3.74 ± 0.9). Lymphocyte, monocyte, and platelet counts were also significantly higher in controls compared to cases. Whereas RBC counts were significantly higher in controls compared to automobile mechanics (4.85 versus 4.66; p = 0.034), haemoglobin levels were significantly higher in automobile sprayers compared to controls (15.13 versus 14.1 g/dl; p = 0.0126). Absolute reticulocyte count was significantly higher in controls compared to cases [p < 0.0001; (56.88 ± 32.14) × 10⁹/L (controls) versus (25.31 ± 15.75) × 10⁹/L (sprayers) or (33.27 ± 24.42) × 10⁹/L (mechanics)]. Among the cases however, only RBC counts were significantly lower in automobile mechanics compared to automobile sprayers (p = 0.0088; 4.66 ± 0.4 versus 4.85 ± 0.5).

Conclusion

It was evident that both automobile mechanics and sprayers had significantly reduced haematopoietic output. Occupational safety training is not given priority and must be addressed.

Activation of the Aryl Hydrocarbon Receptor Leads to Resistance to EGFR TKIs in Non-Small Cell Lung Cancer by Activating Src-mediated Bypass Signaling

Purpose: The aryl hydrocarbon receptor (AhR) has been generally recognized as a ligand-activated transcriptional factor that responds to xenobiotic chemicals. Recent studies have suggested that the expression of AhR varies widely across different cancer types and cancer cell lines, but its significance in cancer treatment has yet to be clarified.Experimental Design: AhR expression in non-small cell lung cancer (NSCLC) was determined by Western blotting and IHC staining. In vitro and in vivo functional experiments were performed to determine the effect of AhR on sensitivity to targeted therapeutics. A panel of biochemical assays was used to elucidate the underlying mechanisms.Results: A high AhR protein level indicated an unfavorable prognosis for lung adenocarcinoma. Inhibition of AhR signaling sensitized EGFR tyrosine kinase inhibitors (TKIs) in NSCLC cells that express high level of endogenous AhR protein. Notably, activation of AhR by pharmacologic and molecular approaches rendered EGFR-mutant cells resistant to TKIs by restoring PI3K/Akt and MEK/Erk signaling through activation of Src. In addition, we found that AhR acts as a protein adaptor to mediate Jak2-Src interaction, which does not require the canonical transcriptional activity of AhR.Conclusions: Our results reveal a transcription-independent function of AhR and indicate that AhR may act as a protein adaptor that recruits kinases bypassing EGFR and drives resistance to TKIs. Accordingly, targeting Src would be a strategy to overcome resistance to EGFR TKIs in AhR-activated NSCLC. Clin Cancer Res; 24(5); 1227-39. ©2017 AACR.

Mechanisms and therapeutic prospects of polyphenols as modulators of the aryl hydrocarbon receptor

Polyphenolic AhR modulators displayed concentration-, XRE-, gene-, species- and cell-specific agonistic/antagonistic activity.

The role of the aryl hydrocarbon receptor in the development of cells with the molecular and functional characteristics of cancer stem-like cells

BACKGROUND

Self-renewing, chemoresistant breast cancer stem cells are believed to contribute significantly to cancer invasion, migration and patient relapse. Therefore, the identification of signaling pathways that regulate the acquisition of stem-like qualities is an important step towards understanding why patients relapse and towards development of novel therapeutics that specifically target cancer stem cell vulnerabilities. Recent studies identified a role for the aryl hydrocarbon receptor (AHR), an environmental carcinogen receptor implicated in cancer initiation, in normal tissue-specific stem cell self-renewal. These studies inspired the hypothesis that the AHR plays a role in the acquisition of cancer stem cell-like qualities.

RESULTS

To test this hypothesis, AHR activity in Hs578T triple negative and SUM149 inflammatory breast cancer cells were modulated with AHR ligands, shRNA or AHR-specific inhibitors, and phenotypic, genomic and functional stem cell-associated characteristics were evaluated. The data demonstrate that (1) ALDH(high) cells express elevated levels of Ahr and Cyp1b1 and Cyp1a1, AHR-driven genes, (2) AHR knockdown reduces ALDH activity by 80%, (3) AHR hyper-activation with several ligands, including environmental ligands, significantly increases ALDH1 activity, expression of stem cell- and invasion/migration-associated genes, and accelerates cell migration, (4) a significant correlation between Ahr or Cyp1b1 expression (as a surrogate marker for AHR activity) and expression of stem cell- and invasion/migration-associated gene sets is seen with genomic data obtained from 79 human breast cancer cell lines and over 1,850 primary human breast cancers, (5) the AHR interacts directly with Sox2, a master regulator of self-renewal; AHR ligands increase this interaction and nuclear SOX2 translocation, (6) AHR knockdown inhibits tumorsphere formation in low adherence conditions, (7) AHR inhibition blocks the rapid migration of ALDH(high) cells and reduces ALDH(high) cell chemoresistance, (8) ALDH(high) cells are highly efficient at initiating tumors in orthotopic xenografts, and (9) AHR knockdown inhibits tumor initiation and reduces tumor Aldh1a1, Sox2, and Cyp1b1 expression in vivo.

CONCLUSIONS

These data suggest that the AHR plays an important role in development of cells with cancer stem cell-like qualities and that environmental AHR ligands may exacerbate breast cancer by enhancing expression of these properties.

Molecular structure and anti-proliferative effect of galangin in HCT-116 cells: In vitro study

Galangin is a naturally occurring plant flavonoid with potential anticancer activity. In present work, the Becke three-parameter hybrid exchange functional method and the Lee-Yang-Parr correction functional methods were used to investigate the structural properties of galangin. The structure-activity relationship analysis has been performed to determine its antioxidant pharmacophore by using density functional theory method and quantum chemical calculations. The free radical scavenging activities of galangin were analyzed with the use of 2, 2-diphenyl-1-picrylhydrazyl and compared with Vitamin C as a control. Galangin decreased the cell proliferation rate in HCT-116 cells and showed concentration- and time-dependent response. Galangin significantly increase the inhibitory effect on HCT-116 clonogenicity and promotes cell cycle arrest at the G2/M or G1 phase, as confirmed by flow cytometry analysis.

Naturally occurring marine brominated indoles are aryl hydrocarbon receptor ligands/agonists

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the toxic and biological effects of structurally diverse chemicals, including the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). As part of a larger effort to identify the full spectrum of chemicals that can bind to and activate the AhR, we have examined the ability of several naturally occurring marine-derived brominated indoles and brominated (methylthio)indoles (collectively referred to as brominated indoles) to bind to the AhR and stimulate AhR-dependent gene expression. Incubation of mouse, rat, and guinea pig recombinant cell lines containing a stably transfected AhR-responsive luciferase reporter gene with eight brominated indoles revealed that all compounds stimulated luciferase reporter gene activity, although some species-specific differences were observed. All compounds induced significantly more luciferase activity when incubated with cells for 4 h as compared to 24 h, demonstrating that these compounds are transient activators of the AhR signaling pathway. Three of the brominated indoles induced CYP1A1 mRNA in human HepG2 cells in vitro and Cyp1a mRNA in zebrafish embryos in vivo. The identification of the brominated indoles as direct ligands and activators/agonists of the AhR was confirmed by their ability to compete with [(3)H]TCDD for binding to the AhR and to stimulate AhR transformation and DNA binding in vitro. Taken together, these results indicate that marine-derived brominated indoles are members of a new class of naturally occurring AhR agonists.

Forest biorefinery: Potential of poplar phytochemicals as value-added co-products

The global forestry industry after experiencing a market downturn during the past decade has now aimed its vision towards the integrated biorefinery. New business models and strategies are constantly being explored to re-invent the global wood and pulp/paper industry through sustainable resource exploitation. The goal is to produce diversified, innovative and revenue generating product lines using on-site bioresources (wood and tree residues). The most popular product lines are generally produced from wood fibers (biofuels, pulp/paper, biomaterials, and bio/chemicals). However, the bark and other tree residues like foliage that constitute forest wastes, still remain largely an underexploited resource from which extractives and phytochemicals can be harnessed as by-products (biopharmaceuticals, food additives and nutraceuticals, biopesticides, cosmetics). Commercially, Populus (poplar) tree species including hybrid varieties are cultivated as a fast growing bioenergy crop, but can also be utilized to produce bio-based chemicals. This review identifies and underlines the potential of natural products (phytochemicals) from Populus species that could lead to new business ventures in biorefineries and contribute to the bioeconomy. In brief, this review highlights the importance of by-products/co-products in forest industries, methods that can be employed to extract and purify poplar phytochemicals, the potential pharmaceutical and other uses of >160 phytochemicals identified from poplar species - their chemical structures, properties and bioactivities, the challenges and limitations of utilizing poplar phytochemicals, and potential commercial opportunities. Finally, the overall discussion and conclusion are made considering the recent biotechnological advances in phytochemical research to indicate the areas for future commercial applications from poplar tree species.

The role of the aryl hydrocarbon receptor in normal and malignant B cell development

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor historically studied for its role in environmental chemical-mediated toxicity and carcinogenicity. In the last 5 years, however, it has become clear that the AhR, presumably activated by endogenous ligand(s), plays an important role in immune system development and function. Other articles in this edition summarize AhR function during T cell and antigen-presenting cell development and function, including the effects of AhR activation on dendritic cell function, T cell skewing, inflammation, and autoimmune disease. Here, we focus on AhR expression and function during B cell differentiation. Studies exploiting immunosuppressive environmental chemicals to probe the role of the AhR in humoral immunity are also reviewed to illustrate the multiple levels at which a “nominally activated” AhR could control B cell differentiation from the hematopoietic stem cell through the pro-B cell, mature B cell, and antibody-secreting plasma cell stages. Finally, a putative role for the AhR in the basic biology of B cell malignancies, many of which have been associated with exposure to environmental AhR ligands, is discussed.

CCAAT/ enhancer-binding protein β activation by capsaicin contributes to the regulation of CYP1A1 expression, mediated by the aryl hydrocarbon receptor

BACKGROUND AND PURPOSE

Capsaicin, a constituent of peppers, has been linked to the suppression of tumorigenesis and carcinogenesis. The influence of capsaicin on cytochrome P450 (CYP) 1A1, which is involved in metabolism of carcinogens, and the underlying mechanisms remain unclear. Here, we examined the effect of capsaicin on CYP1A1 expression in mouse hepatoma cells.

EXPERIMENTAL APPROACH

Murine hepatoma Hepa-1c1c7 cells were incubated with capsaicin and/or 3-methylcholanthrene (3-MC). Effects of capsaicin on CYP1A1 levels were determined by analysing mRNA expression, transcription activity and protein expression. Regulation of CYP1A1 was investigated by determining transcriptional factor expression, activation and binding activity with cotreatment with target signal antagonists.

KEY RESULTS

Capsaicin alone slightly induced CYP1A1 activity, mRNA expression, protein level and promoter activity. Treatment with transient receptor potential vanilloid type-1 receptor (TRPV1) or aryl hydrocarbon receptor (AhR) antagonist decreased induction of CYP1A1 expression by capsaicin. Additionally, capsaicin significantly inhibited 3-MC-induced CYP1A1 mRNA and protein level and xenobiotic response element-luciferase activity. Capsaicin also inhibited 3-MC-induced AhR transactivation and nuclear localization of AhRs. Moreover, capsaicin increased Ca(2+) /calmodulin (CaM)-dependent protein kinase (CaMK) and CCAAT/ enhancer-binding protein β (C/EBPβ) activation, downstream of TRPV1 receptors. Capsaicin-induced C/EBPβ activation inhibited induction of CYP1A1 mRNA and protein by 3-MC.

CONCLUSIONS AND IMPLICATIONS

Capsaicin alone weakly induced CYP1A1 expression, and 3-MC-induced CYP1A1 levels were suppressed by capsaicin. Activation of C/EBPβ and inhibition of 3-MC-induced AhR transactivation by capsaicin contributed to the suppression of CYP1A1 expression. Capsaicin has a potential chemopreventive effect through inhibiting induction of CYP1A1 by poly aryl hydrocarbons.

Dyrk1a activates antioxidant NQO1 expression through an ERK1/2-Nrf2 dependent mechanism

BACKGROUND AND AIMS

Among cardiovascular risk factor, people with Down syndrome have a lower plasma homocysteine level. In a previous study, we have shown that DYRK1A (dual-specificity tyrosine-(Y)-phosphorylation regulated kinase 1a), a serine/threonine kinase found on human chromosome 21, is implicated on homocysteine metabolism regulation. Indeed, mice that overexpress in liver this kinase have a lower plasma homocysteine level concomitant with an increased hepatic S-adenosyhomocysteine hydrolase (SAHH) activity, which depends on the activation of NAD(P)H:quinone oxidoreductase-1 (NQO1). Since NQO1 gene transcription is under the control of NRF2 and AhR, the aim of the present study was to analyze the effect of DYRK1A overexpression in mice onto NRF2 and AhR signaling pathways.

METHODS

Effects of DYRK1A overexpression were examined in mice overexpressing Dyrk1a treated with an inhibitor, harmine, by real-time quantitative reverse-transcription polymerase reaction and western blotting.

RESULTS

We found that overexpression of DYRK1A increases the nuclear NRF2 quantity, concomitant with the activation of ERK1/2. We also show that the overexpression of Dyrk1a has no effect on PI3K/AKT activation, and AhR signaling pathway in liver of mice.

CONCLUSIONS

Our results reveal a link between DYRK1A and NRF2 signaling pathway.

Proximal events in 7,12-dimethylbenz[a]anthracene-induced, stromal cell-dependent bone marrow B cell apoptosis: stromal cell-B cell communication and apoptosis signaling

Intercellular communication is an essential process in stimulating lymphocyte development and in activating and shaping an immune response. B cell development requires cell-to-cell contact with and cytokine production by bone marrow stromal cells. However, this intimate relationship also may be responsible for the transfer of death-inducing molecules to the B cells. 7,12-Dimethylbenz[a]anthracene (DMBA), a prototypical polycyclic aromatic hydrocarbon, activates caspase-3 in pro/pre-B cells in a bone marrow stromal cell-dependent manner, resulting in apoptosis. These studies were designed to examine the hypothesis that an intrinsic apoptotic pathway is activated by DMBA and that the ultimate death signal is a DMBA metabolite generated by the stromal cells and transferred to the B cells. Although a loss of mitochondrial membrane potential did not occur in the DMBA/stromal cell-induced pathway, cytochrome c release was stimulated in B cells. Caspase-9 was activated, and formation of the apoptosome was required to support apoptosis, as demonstrated by the suppression of death in Apaf-1(fog) mutant pro-B cells. Investigation of signaling upstream of the mitochondria demonstrated an essential role for p53. Furthermore, DMBA-3,4-dihydrodiol-1,2-epoxide, a DNA-reactive metabolite of DMBA, was sufficient to upregulate p53, induce caspase-9 cleavage, and initiate B cell apoptosis in the absence of stromal cells, suggesting that production of this metabolite by the stromal cells and transfer to the B cells are proximal events in triggering apoptosis. Indeed, we provide evidence that metabolite transfer from bone marrow stromal cells occurs through membrane exchange, which may represent a novel communication mechanism between developing B cells and stromal cells.

Metabolites of galangin by 2,3,7,8-tetrachlorodibenzo-p-dioxin-inducible cytochrome P450 1A1 in human intestinal epithelial Caco-2 cells and their antagonistic activity toward aryl hydrocarbon receptor

Galangin, a dietary flavonoid, inhibited cytochrome P450 1A1 (CYP1A1) expression induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This inhibitory activity remained after permeating human intestinal epithelial Caco-2 cell monolayers, but was reduced when galangin permeated TCDD-pretreated Caco-2 cells. The present study tested whether TCDD affected the intestinal metabolism of flavonoids. LC-MS/MS analyses showed that galangin and two galangin glucuronoconjugates were reduced 0.7-fold, whereas kaempferol (a galangin oxidate) and kaempferol glucuronoconjugate were increased 1.5-fold by permeating TCDD-pretreated Caco-2 cells, as compared to untreated Caco-2 cells. An assay using recombinant human CYP1A1 and the CYP1A1 inhibitor alpha-naphthoflavone revealed that CYP1A1 oxidized galangin to kaempferol. These results indicated that galangin was metabolized to kaempferol by TCDD-inducible CYP1A1 in Caco-2 cells. A previous study revealed that kaempferol had much weaker inhibitory activity than galangin toward TCDD-induced CYP1A1 expression. Therefore, the oxidative metabolism of galangin to kaempferol in TCDD-pretreated Caco-2 cells implicated reduction in the inhibitory activity of galangin.

Suppression mechanisms of flavonoids on aryl hydrocarbon receptor-mediated signal transduction

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates biological and toxicological effects by binding to its agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Previously we demonstrated that flavonoids suppressed the TCDD-induced DNA-binding activity of the AhR in a structure-dependent manner. In this study, we investigated the mechanisms by which flavonoids suppressed the AhR-mediated signal transduction in mouse hepatoma Hepa-1c1c7 cells. Flavones and flavonols suppressed the TCDD-induced nuclear translocation of the AhR and dissociation of its partner proteins, heat shock protein 90 and X-associated protein 2, whereas flavanones and catechins did not. Flavonoids of all these four subclasses suppressed the phosphorylation of both AhR and Arnt and the formation of a heterodimer consisting of these proteins. Since certain flavonoids are known to inhibit mitogen-activated protein kinases (MAPKs), we confirmed the contribution of MAPK/ERK kinase (MEK) to the AhR-mediated signal transduction by using U0126, an inhibitor of MEK1/2. U0126 suppressed TCDD-induced phosphorylation of the AhR and Arnt followed by the DNA-binding activity of the AhR. Flavanones and catechins suppressed the TCDD-induced phosphorylation of ERK1/2. The inhibition of MEK/ERK phosphorylation is one of the mechanisms by which flavanones and catechins suppress the AhR-mediated signal transduction in Hepa-1c1c7 cells.

Integrated approach to immunotoxicity: electron transfer, reactive oxygen species, antioxidants, cell signaling, and receptors

As with all body organs, the immune system is subjected to attack by a variety of toxins. Serious consequences can result because the immune organs serve as a defense against infective agents. The toxins, both organic and inorganic, fall into a large variety of classes, such as metals, therapeutic drugs, industrial chemicals, pollutants, pesticides, fuels, herbicides and abused drugs. Although the mode of action is multifaceted, our focus is on electron transfer (ET), reactive oxygen species (ROS), antioxidants (AOs), cell signaling, and receptors. It is significant that the toxins or their metabolites incorporate ET functionalities capable of redox cycling with resultant generation of ROS and accompanying oxidative stress.

Phytochemical regulation of UDP-glucuronosyltransferases: implications for cancer prevention

Uridine 5'-diphospho-glucuronosyltransferases (UGTs) are Phase II biotransformation enzymes that metabolize endogenous and exogenous compounds, some of which have been associated with cancer risk. Many phytochemicals have been shown to induce UGTs in humans, rodents, and cell culture systems. Because UGTs maintain hormone balance and facilitate excretion of potentially carcinogenic compounds, regulation of their expression and activity may affect cancer risk. Phytochemicals regulate transcription factors such as the nuclear factor-erythroid 2-related factor 2 (Nrf2), aryl hydrocarbon, and pregnane X receptors as well as proteins in several signal transduction cascades that converge on Nrf2 to stimulate UGT expression. This induction can be modified by several factors, including phytochemical dose and bioavailability and interindividual variation in enzyme expression. In this review, we summarize the knowledge of dietary modulation of UGTs, particularly by phytochemicals, and discuss the potential mechanisms by which phytochemicals regulate UGT transcription.

Base excision repair pathway is involved in the repair of lesions generated by flavonoid-enriched fractions of pepper tree (Schinus terebinthifolius, Raddi) stem bark

Cell-free and bacterial assays indicate that flavonoid-enriched fractions and the flavonoids of pepper tree stem bark from Schinus terebinthifolius Raddi have genotoxic rather than antigenotoxic properties. In the present report, we have examined the ability of flavonoid-enriched fractions to damage plasmid DNA and the repair pathways involved in the recognition of these DNA lesions. High concentrations of two flavonoid-enriched fractions were able to break phosphodiester bonds in DNA. In addition, studies using bacterial strains deficient in nucleotide excision repair and base excision repair (BER) enzymes indicated that the flavonoid-enriched fractions generated lesions that were substrates for enzymes belonging to the BER pathway. In addition, in vitro studies indicated that the DNA damage produced by the flavonoid-enriched fractions was also a substrate for exonuclease III and that the phosphodiester breakage was amplified by copper ions. These results indicate that flavonoids from the pepper tree (Schinus terebinthifolius, Raddi) generate lesions on DNA that are potential targets of FPG and MutY glycosylase from the BER pathway. Chromatographic and spectral analyses helped to support the hypothesis that the flavonoids of the Brazilian pepper tree bark are the main factors involved in the fraction's damage potential. The isolated flavonoids from Fraction II were also tested in vitro and support the oxidative damage potential of these flavonoids.

Interaction between the aryl hydrocarbon receptor and its antagonists, flavonoids

Flavonoids have been reported to be dietary antagonists of an aryl hydrocarbon receptor (AhR). However, little is known about the molecular mechanism on their antagonistic effects. In this study, the inhibitory effect of flavonoids on ligand binding to the AhR and interaction between flavonoids and the AhR complex (AhRc) were investigated in each flavonoid subclass. Flavone, flavonol, and flavanone but not catechin inhibited the specific binding between the AhR and 3-methylcholanthrene dose-dependently, indicating that the former three subclasses possibly act as competitive antagonists of the AhR. However, catechin in addition to the former three subclasses directly interacted with the AhRc by surface plasmon resonance analysis. The dissociation constant values showed an inverse correlation with the suppressive effect on the DNA binding activity. These results suggest that flavone, flavonol, and flavanone act as competitive antagonists of the AhR, while catechin associates with the AhRc and indirectly exhibits its antagonistic effects.

Modulation of aryl hydrocarbon receptor transactivation by carbaryl, a nonconventional ligand

Carbaryl (1-naphthyl-N-methylcarbamate), a widely used carbamate insecticide, induces cytochrome P450 1A gene expression in mammalian cells. This activity is usually mediated by the interaction of the compound with the aryl hydrocarbon receptor. However, it has been proposed that this mechanism does not apply to carbaryl because its structure differs from that of typical aryl hydrocarbon receptor ligands. We show here that carbaryl promotes activation of target genes in a yeast-based bioassay expressing both aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator. By contrast, carbaryl acted as a competitive inhibitor, rather than as an agonist, in a simplified yeast system, in which aryl hydrocarbon receptor nuclear translocator function is bypassed by fusing aryl hydrocarbon receptor to a heterologous DNA binding domain. This dual action of carbaryl, agonist and partial antagonist, was also observed by comparing carbaryl response in two vertebrate cell lines. A yeast two-hybrid assay showed that the mammalian coactivator cAMP response element-binding protein readily interacts with aryl hydrocarbon receptor bound to its canonical ligand beta-naphthoflavone, but not with the carbaryl-aryl hydrocarbon receptor complex. We propose that carbaryl interacts with aryl hydrocarbon receptor, but that its peculiar structure imposes a substandard configuration on the aryl hydrocarbon receptor ligand-binding domain that prevents interaction with key coactivators and activates transcription without the need for aryl hydrocarbon receptor nuclear translocator. This effect may be relevant in explaining its physiological effects in exposed animals, and may help to predict its effects, and that of similar compounds, in humans. Our data also identify the aryl hydrocarbon receptor/cAMP response element-binding protein interaction as a molecular target for the identification and development of new aryl hydrocarbon receptor antagonists.

Growth of a human mammary tumor cell line is blocked by galangin, a naturally occurring bioflavonoid, and is accompanied by down-regulation of cyclins D3, E, and A

Introduction

This study was designed to determine if and how a non-toxic, naturally occurring bioflavonoid, galangin, affects proliferation of human mammary tumor cells. Our previous studies demonstrated that, in other cell types, galangin is a potent inhibitor of the aryl hydrocarbon receptor (AhR), an environmental carcinogen-responsive transcription factor implicated in mammary tumor initiation and growth control. Because some current breast cancer therapeutics are ineffective in estrogen receptor (ER) negative tumors and since the AhR may be involved in breast cancer proliferation, the effects of galangin on the proliferation of an ER^-, AhR^high line, Hs578T, were studied.

Methods

AhR expression and function in the presence or absence of galangin, a second AhR inhibitor, α-naphthoflavone (α-NF), an AhR agonist, indole-3-carbinol, and a transfected AhR repressor-encoding plasmid (FhAhRR) were studied in Hs578T cells by western blotting for nuclear (for instance, constitutively activated) AhR and by transfection of an AhR-driven reporter construct, pGudLuc. The effects of these agents on cell proliferation were studied by ³H-thymidine incorporation and by flow cytometry. The effects on cyclins implicated in mammary tumorigenesis were evaluated by western blotting.

Results

Hs578T cells were shown to express high levels of constitutively active AhR. Constitutive and environmental chemical-induced AhR activity was profoundly suppressed by galangin as was cell proliferation. However, the failure of α-NF or FhAhRR transfection to block proliferation indicated that galangin-mediated AhR inhibition was either insufficient or unrelated to its ability to significantly block cell proliferation at therapeutically relevant doses (IC₅₀ = 11 μM). Galangin inhibited transition of cells from the G₀/G₁ to the S phases of cell growth, likely through the nearly total elimination of cyclin D3. Expression of cyclins A and E was also suppressed.

Conclusion

Galangin is a strong inhibitor of Hs578T cell proliferation that likely mediates this effect through a relatively unique mechanism, suppression of cyclin D3, and not through the AhR. The results suggest that this non-toxic bioflavonoid may be useful as a chemotherapeutic, particularly in combination with agents that target other components of the tumor cell cycle and in situations where estrogen receptor-specific therapeutics are ineffective.

Influence of galangin on HL-60 cell proliferation and survival

The effect of galangin, a flavonol component of India root spice and the 'herbal' medicine propolis, on HL-60 human leukaemia cell survival is characterised. Galangin (1-100 microM) exerted an antiproliferative effect that, with dose and exposure longevity, was progressively associated with an elevated hypodiploid DNA content and expression of the active form of caspase-3, principally prior to membrane damage. At >or=50 microM, plasmamembrane phosphatidylserine exposure was observed. There was no evidence for intracellular oxidative stress as an orchestrator of cytotoxicity and significant phagocyte-like differentiation was not detected. We discuss whether such cytotoxicity will be therapeutically exploitable or contribute to cancer prevention within a pharmacological or dietary context.

Protective Effects of Tea Melanin against 2,3,7,8-Tetrachlorodibenzo-p-dioxin-Induced Toxicity: Antioxidant Activity and Aryl Hydrocarbon Receptor Suppressive Effect

We examined the protective ability of tea melanin against 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced toxicity in C57BL6J mice. Reduced tea melanin (RTM) and non-reduced tea melanin (NRTM) were incorporated to distinguish anti-oxidant activity from alternative pathways. The mice were given a single oral dose of TCDD (100 microg/kg body weight) and then they were administered daily with NRTM or RTM (40 mg/kg, p.o.) for next 14 d. RTM protected the animals against TCDD-induced lipid peroxidation, inhibition of glutathione peroxidase, alteration in reduced and oxidized glutathione concentrations, loss of body weight, and increased relative liver weight. NRTM was less effective as compared to RTM because of its inferior antioxidant activity, but it still displayed a strong protective effect against TCDD toxicity owing to its similar suppression of the activity of the aryl hydrocarbon receptor. Both NRTM and RTM suppressed the expression of CYP1A1 gene and prevented the activation of cytochrome P450 isozyme in the livers of animals exposed to TCDD. These results suggest that tea melanin might be a potential agent offering dual protection against the development of TCDD-induced oxidative stress.

Genotoxicity of phytoestrogens

Plant extracts containing phytohormones are very popular as 'alternative' medicine for many kinds of diseases. They are especially favored by women who enter menopause and are concerned about the side effects of hormone replacement therapy. However, adverse health effects of phytoestrogens have often been ignored. This review examines the literature on genotoxicity and apoptotic effects of phytohormones. Genistein, coumestrol, quercetin, zearalenone, and resveratrol exerted genotoxic effects in in vitro test systems. Other phytoestrogens such as lignans, the isoflavones daidzein and glycetein, anthocyanidins, and the flavonol fisetin exhibited only weak or no effects in vitro. However, some metabolites of daidzein showed a genotoxic activity in vitro. Practically all of the phytoestrogens exhibit pro-apoptotic effects in some cell systems. Further investigations regarding dose-response-relationships and other aspects relevant for extrapolation to human exposure seem necessary. Until then, care may be advised in taking concentrated phytohormones. Nevertheless, the intake of substantial amounts of plant-food in a normal diet constitutes an important, individual contribution to cancer prevention.

Effects of Flavonoids on Cell Proliferation and Caspase Activation in a Human Colonic Cell Line HT29: An SAR Study

A library of 42 natural and synthetic flavonoids has been screened for their effect on cell proliferation and apoptosis in a human colonic cell line (HT-29). Examples of different classes of flavonoids have been screened, and the effects of hydroxylation, methoxylation and/or C-alkylation at various positions in the A- and B-rings have been assessed. Flavones and flavonols possess greater antiproliferative activity than chalcones and flavanones. With respect to structural modification of flavonoids, C-isoprenylation was by far the most effective, with substitution at the 8-position and longer chains, such as geranyl giving the best results. Finally, most compounds that significantly reduced cell survival also increased caspase activity, suggesting that at least part of their antiproliferative activity might be attributable to an apoptotic response.

Early steps in bilirubin-mediated apoptosis in murine hepatoma (Hepa 1c1c7) cells are characterized by aryl hydrocarbon receptor-independent oxidative stress and activation of the mitochondrial pathway

Unconjugated bilirubin (UCB), the end product of heme catabolism, causes apoptosis in cells of the central nervous system, endothelial cells, and hepatotoma cells. However, the molecular mechanisms that contribute to UCB cytotoxicity remain unclear. The purpose of this study was to characterize the sequence of early events leading to UCB-mediated cytotoxicity in murine hepatoma Hepa 1c1c7 cells. In the present study, UCB (5-50 microM) was found to markedly increase the intracellular generation of reactive oxygen species (ROS) in a concentration-dependent manner, which is significantly elevated by 30 min post-treatment. This generation of ROS by UCB is not dependent on aryl hydrocarbon receptor (Ahr) signaling, as cells deficient in the Ahr (C12 cells) or the Ahr nuclear translocator protein (Arnt; C4 cells) were as efficient at generating ROS as wild type (WT) Hepa 1c1c7 cells. Mitochondrial membrane depolarization, evaluated with the lipophilic cationic dye, JC-1, occurred at least by 2 h after treatment with 50 muM UCB. Analysis of the caspase cascade demonstrated that activation of caspase-9 preceded activation of caspase-3. No conversion of procaspase-2 to active caspase-2 was detected in this study. These results demonstrate that UCB-mediated apoptosis in Hepa 1c1c7 cells is associated with increased oxidative stress and that caspase-9, and definitely not caspase-2, is the initiator caspase for apoptosis in UCB-treated Hepa 1c1c7 cells.

Differential effects of fluoranthene and benzo[a]pyrene in MCF-7 cells

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants, which are suspected carcinogens and may affect the reproductive system as potential endocrine disruptors. Therefore, we tested fluoranthene (FL) and benzo[a]pyrene (BaP) on human breast cancer cell line (MCF-7 cells) to determine possible toxic effects. The cells were incubated in the presence of medium, medium containing 0.1% dimethylsulfoxide (DMSO) as vehicle, or in the presence of FL (10, 50, and 100 microg/ml), BaP (10, 50, and 100 microg/ml), 17beta-estradiol (E2; 5 microg/ml and 500 ng/ml), or tamoxifen (Tx; 5 microg/ml and 500 ng/ml). After 24 h, FL (100 microg/ml), BaP (100 microg/ml), or Tx (5 microg/ml) killed significant numbers of cells. After 72 h, FL (50 and 100 microg/ml), BaP (100 microg/ml), E2 (5 microg/ml), or Tx (5 microg/ml and 500 ng/ml) decreased MCF-7 cell viability significantly as demonstrated by the MTT assay. Measurement of DNA synthesis was conducted using 3H-thymidine incorporation into MCF-7 cell DNA for 72 h. After 72 h, BaP (10, 50, and 100 microg/ml) and Tx (5 microg/ml and 500 ng/ml) significantly decreased DNA synthesis in MCF-7 cells. FL did not significantly alter 3H-thymidine incorporation into the cells. While higher concentration of E2 (5 microg/ml) decreased 3H-thymidine incorporation, the lower concentration of E2 (500 ng/ml) increased cell proliferation. Apoptotic response was tested by in situ fluorescence staining of cells incubated for 72 h in media containing 0.1% DMSO, or vehicle containing FL (10 microg/ml), BaP (10 microg/ml), E2 (500 ng/ml), or Tx (500 ng/ml). Microscopic examination demonstrated presence of apoptosis with BaP (10 microg/ml) and Tx (500 ng/ml), but not with FL (10 microg/ml) and E2 (500 ng/ml). The cell cycle analysis using flow cytometry demonstrated that E2 (500 ng/ml) did not significantly change the progression of MCF-7 cells after 72 h of incubation. However, FL (10 microg/ml) only suppressed G2/M phase. Tx (500 ng/ml) blocked G0/G1, S, and G2/M phases, and BaP (10 microg/ml) suppressed the G0/G1 phase. These data suggest that BaP on MCF-7 cells is growth inhibitory and apoptotic, whereas the toxic effects of FL are not exerted through apoptosis.

Synthesis and Biological Properties of New Stilbene Derivatives of Resveratrol as New Selective Aryl Hydrocarbon Modulators

We developed new stilbene derivatives of resveratrol (E)-1-(4'-hydroxyphenyl)-2-(3,5-dihydroxyphenyl)ethene) selective for AhR and devoid of affinity for ER. Among the 24 stilbenes synthesized, all display a higher affinity than resveratrol for AhR. (E)-1-(4'-Trifluoromethylphenyl)-2-(3,5-ditrifluoromethylphenyl)ethene (4e), (E)-1-(4'-methoxyphenyl)-2-(3,5-dichlorophenyl)ethene (4j), and (E)-1-(4'-chlorophenyl)-2-(3,5-dichlorophenyl)ethene (4b) are selective, high-affinity AhR antagonists with, respective, K(i)s of 2.1, 1.4, and 1.2 nM. (E)-1-(4'-Trifluoromethylphenyl)-2-(3,5-dichlorophenyl)ethene (4i) displays a K(i) of 0.2 nM and is a selective and high-affinity agonist on AhR.

Pigments in green tea leaves (Camellia sinensis) suppress transformation of the aryl hydrocarbon receptor induced by dioxin

Environmental contaminants such as dioxins enter the body mainly through diet and cause various toxicities through transformation of the aryl hydrocarbon receptor (AhR). We previously reported that certain natural flavonoids at the dietary level suppress the AhR transformation induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In this study, we identified lutein and chlorophyll a and b from green tea leaves as the novel antagonists for AhR. These active compounds suppressed AhR transformation dose-dependently with the 50% inhibitory concentration (IC(50)) values against 0.1 nM TCDD-induced AhR transformation at 3.2, 5.0, and 5.9 microM, respectively. (-)-Epigallocatechin gallate, which is the most abundant flavonoid in green tea leaves, also showed stronger suppressive effects than did other major tea components, with the IC(50) value of 1.7 microM. Thus, these pigments of green tea leaves have the potential to protect from dioxin toxicity through the suppression of AhR transformation.

Environmental chemical-induced pro/pre-B cell apoptosis: analysis of c-Myc, p27Kip1, and p21WAF1 reveals a death pathway distinct from clonal deletion

Polycyclic aromatic hydrocarbons (PAH) are common environmental pollutants that suppress the immune system in part by inducing pro/pre-B cell apoptosis. The PAH-induced death signaling pathway resembles the signaling cascade activated during clonal deletion and modeled by B cell receptor cross-linking or by dexamethasone exposure of immature surface Ig(+) B cells in that apoptosis is mediated by NF-kappa B down-regulation. Because a PAH-induced, clonally nonrestricted deletion of B cells would have important implications for B cell repertoire development, the nature of the PAH-induced intracellular death signal was studied further. Particular emphasis was placed on the roles of growth arrest and c-Myc, p27(Kip1), and p21(WAF1) expression, because all of these elements contribute to clonal deletion. As in clonal deletion models, and as predicted by the down-regulation of NF-kappa B, PAH-induced death of pro/pre-B cells was at least partially dependent on c-Myc down-regulation. Furthermore, whereas dexamethasone induced a G(0)/G(1) cell cycle arrest, PAH had no effect on pro/pre-B cell growth, indicating that growth arrest and apoptosis occur by separable signaling pathways in this early phase of B cell development. Finally, in contrast to clonal deletion, PAH-induced pro/pre-B cell death was not dependent on p27(Kip1) or p21(WAF1) up-regulation but did coincide with p53 induction. These results distinguish the PAH-induced apoptosis pathway from that activated during clonal deletion and indicate that signaling cascades leading to growth arrest and/or apoptosis in pro/pre-B cells differ from those active at later B cell developmental stages.

Species-specific transcriptional activity of synthetic flavonoids in guinea pig and mouse cells as a result of differential activation of the aryl hydrocarbon receptor to interact with dioxin-responsive elements

To investigate possible species-specificity of aryl hydrocarbon receptor (AhR)-mediated signal transduction pathways, activities of 2,3,7,8-tetrochlorodibenzo-p-dioxin (TCDD) and six synthetic flavonoids were evaluated in mouse hepatoma and guinea pig adenocarcinoma cells transfected with an AhR-responsive luciferase reporter. Rank order potency in these two cell lines was similar for the ability of these flavonoids to antagonize TCDD-induced reporter gene expression. However, in the presence of flavone alone, a species-specific difference in agonist activity was observed. In guinea pig cells, several flavonoids demonstrated agonist activity up to 50% of the maximum TCDD response. In mouse cells, however, no significant agonist activity was observed at the same concentrations based on luciferase enzyme activity, protein expression, and mRNA analysis. Moreover, competitive ligand-binding assays, using [(3)H]TCDD in cytosolic fractions, demonstrated that 3'-methoxy-4'-nitroflavone had a similar IC(50) in both recombinant cell lines, suggesting that the flavone has similar binding affinity to receptors from both species. However, electrophoretic mobility shift assay using the cytosolic fractions demonstrated that this flavone elicited binding to the DRE by guinea pig but not mouse AhR complex. The dependence of the AhR in this differential interaction was further demonstrated using in vitro synthesized guinea pig and mouse Ah receptors and mouse Arnt. Together, these data suggest that the differential agonist/antagonist activity of these flavone derivatives is caused by the efficacy of these flavonoids in eliciting an AhR conformation that recognizes regulatory response elements in a species-specific manner.

Effects of flavonoids on cisplatin-induced apoptosis of HL-60 and L1210 leukemia cells

Effects of three flavonoids, quercetin (QU), galangin (GA), and chrysin (ChR) on cisplatin (cis-Pt)-induced apoptosis of human promyelocytic leukemia HL-60 cells and murine leukemia L1210 cells were investigated. The quantitative analysis of apoptotic DNA fragmentation was used to show that preincubation of cells with flavonoids can influence cis-Pt-induced apoptosis in different way. ChR had no effect, QU enhanced, and GA reduced apoptotic DNA fragmentation. It is also shown that combined treatment with QU and cis-Pt showed synergistic effect, however, GA combined with cis-Pt exhibited antagonism on cytotoxicity in L1210 murine leukemia cells. We assume that tested flavonoids affect the important biological activities connected with cancer chemotherapy and chemoprevention as they differently modulated the sensitivity of cells to cis-Pt treatment. QU is presented as pro-apoptotic agent and GA as agent with anti-apoptotic potential.

PCB-induced oxidative stress in endothelial cells: modulation by nutrients

There is an increasing body of evidence suggesting that exposure to Superfund chemicals may have adverse consequences on many organ systems, as well as carcinogenic and atherogenic effects. This is particularly true for polyhalogenated aromatic hydrocarbons such as the polychlorinated biphenyls (PCBs). The vascular endothelium, which is constantly exposed to blood components including environmental contaminants, is extremely vulnerable to chemical insult as well as necrotic and apoptotic injury. Our recent studies suggest that certain PCBs, especially coplanar PCBs, can compromise normal functions of vascular endothelial cells by activating oxidative stress-sensitive signaling pathways and subsequent proinflammatory events critical in the pathology of atherosclerosis and cardiovascular disease. Our findings suggest that an increase in the level of cellular oxidative stress is a significant event in PCB-mediated endothelial cell dysfunction and that nutrients can modulate PCB-induced oxidative stress and endothelial toxicity. We have demonstrated that the dietary fat linoleic acid, the parent unsaturated fatty acid of the omega-6 family, can increase endothelial dysfunction induced by selected PCBs, probably by contributing to oxidative stress and as the result of the production of toxic metabolites called leukotoxins. The subsequent imbalance in the overall cellular oxidant/antioxidant status can activate oxidative stress- or redoxsensitive transcription factors, which in turn promote gene expression for inflammatory cytokines and adhesion molecules, intensifying the inflammatory response and endothelial cell dysfunction. Our data also suggest that antioxidant nutrients such as vitamin E can protect against endothelial cell damage mediated by PCBs or polyunsaturated dietary fats by interfering with oxidative stress-sensitive and proinflammatory signaling pathways. The concept that nutrition can modify or ameliorate the toxicity of Superfund chemicals is provocative and warrants further study as the implications for human health are significant. The information from such studies could be used to develop dietary recommendations and nutritional interventions for populations at high risk for exposure to PCBs, including communities living near Superfund sites and those exposed via occupation or diet.

Anti-genotoxicity of galangin as a cancer chemopreventive agent candidate

Flavonoids are polyphenolic compounds that are present in plants. They have been shown to possess a variety of biological activities at non-toxic concentrations in organisms. Galangin, a member of the flavonol class of flavonoid, is present in high concentrations in medicinal plants (e.g. Alpinia officinarum) and propolis, a natural beehive product. Results from in vitro and in vivo studies indicate that galangin with anti-oxidative and free radical scavenging activities is capable of modulating enzyme activities and suppressing the genotoxicity of chemicals. These activities will be discussed in this review. Based on our review, galangin may be a promising candidate for cancer chemoprevention.

The role of polycyclic aromatic hydrocarbon metabolism in dimethylbenz[a]anthracene-induced pre-B lymphocyte apoptosis

Previous studies indicated that two prototypic PAH, benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA), suppress the developing immune system by inducing apoptosis in bone marrow pre-B lymphocytes. In bone marrow cultures consisting of pre-B cells growing on bone marrow stromal cell monolayers, pre-B cell apoptosis was shown to be dependent on the aryl hydrocarbon receptor/transcription factor (AhR) expressed in stromal cells. However, it was not determined if AhR activation alone is sufficient or if DMBA metabolism is required for induction of a stromal cell-derived apoptosis signal. To address these issues we assessed: 1) the ability of poorly metabolized AhR ligands to induce pre-B cell apoptosis and 2) the capacity for and the mechanism through which an early DMBA metabolite induces pre-B cell apoptosis. Three poorly metabolized AhR ligands, 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3,3',4,4',5-pentachlorobiphenyl, and 3,3',4,4'-tetrachlorobiphenyl failed to induce pre-B cell apoptosis in bone marrow cultures, indicating that AhR activation alone is not sufficient to induce apoptosis and suggesting a role for PAH metabolism in induction of an apoptosis signal. Consistent with this hypothesis, DMBA-3, 4-dihydrodiol, an early DMBA metabolite, induced significant pre-B cell apoptosis. The ability of DMBA-3,4-dihydrodiol to activate the AhR, inhibition of DMBA-3,4-dihydrodiol-induced apoptosis by alpha-naphthoflavone, and the significantly lower levels of DMBA-3, 4-dihydrodiol-induced apoposis in pre-B cell populations maintained on AhR(-) stromal cells strongly support a role for the AhR in DMBA-3,4-dihydrodiol-induced apoptosis. Of two DMBA-metabolizing enzymes evaluated, CYP1A1 and CYP1B1, the latter appeared to be the more likely to play a role in DMBA-induced apoptosis. These data confirm a role for the AhR in PAH-induced pre-B cell apoptosis, indicate a role for DMBA metabolism, and suggest a feedback loop in which at least one product of DMBA metabolism augments AhR signaling, leading to induction of an apoptosis stimulus.