Acute cytotoxicity of the chemical carcinogen 2-acetylaminofluorene in cultured rat liver epithelial cells

In silico cytotoxicity assessment on cultured rat intestinal cells deduced from cellular impedance measurements

Early and reliable identification of chemical toxicity is of utmost importance. At the same time, reduction of animal testing is paramount. Therefore, methods that improve the interpretability and usability of in vitro assays are essential. xCELLigence's real-time cell analyzer (RTCA) provides a novel, fast and cost effective in vitro method to probe compound toxicity. We developed a simple mathematical framework for the qualitative and quantitative assessment of toxicity for RTCA measurements. Compound toxicity, in terms of its 50% inhibitory concentration IC₅₀ on cell growth, and parameters related to cell turnover were estimated on cultured IEC-6 cells exposed to 10 chemicals at varying concentrations. Our method estimated IC₅₀ values of 113.05, 7.16, 28.69 and 725.15 μM for the apparently toxic compounds 2-acetylamino-fluorene, aflatoxin B1, benzo-[a]-pyrene and chloramphenicol in the tested cell line, in agreement with literature knowledge. IC₅₀ values of all apparent in vivo non-toxic compounds were estimated to be non-toxic by our method. Corresponding estimates from RTCA's in-built model gave false positive (toxicity) predictions in 5/10 cases. Taken together, our proposed method reduces false positive predictions and reliably identifies chemical toxicity based on impedance measurements. The source code for the developed method including instructions is available at https://git.zib.de/bzfgupta/toxfit/tree/master.

Induction of an altered lipid phenotype by two cancer promoting treatments in rat liver

Changes in lipid metabolism have been associated with tumor promotion in rat liver. Similarities and differences of lipid parameters were investigated using the mycotoxin fumonisin B1 (FB1) and the 2-acetylaminofluorene/partial hepatectomy (AAF/PH) treatments as cancer promoters in rat liver. A typical lipid phenotype was observed, including increased membranal phosphatidylethanolamine (PE) and cholesterol content, increased levels of C16:0 and monounsaturated fatty acids in PE and phosphatidylcholine (PC), as well as a decrease in C18:0 and long-chained polyunsaturated fatty acids in the PC fraction. The observed lipid changes, which likely resulted in changes in membrane structure and fluidity, may represent a growth stimulus exerted by the cancer promoters that could provide initiated cells with a selective growth advantage. This study provided insight into complex lipid profiles induced by two different cancer promoting treatments and their potential role in the development of hepatocyte nodules, which can be used to identify targets for the development of chemopreventive strategies against cancer promotion in the liver.

Areca nut components affect COX-2, cyclin B1/cdc25C and keratin expression, PGE2 production in keratinocyte is related to reactive oxygen species, CYP1A1, Src, EGFR and Ras signaling

AIMS

Chewing of betel quid (BQ) increases the risk of oral cancer and oral submucous fibrosis (OSF), possibly by BQ-induced toxicity and induction of inflammatory response in oral mucosa.

METHODS

Primary gingival keratinocytes (GK cells) were exposed to areca nut (AN) components with/without inhibitors. Cytotoxicity was measured by 3-(4,5-dimethyl- thiazol- 2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. mRNA and protein expression was evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR) and western blotting. PGE2/PGF2α production was measured by enzyme-linked immunosorbent assays.

RESULTS

Areca nut extract (ANE) stimulated PGE2/PGF2α production, and upregulated the expression of cyclooxygenase-2 (COX-2), cytochrome P450 1A1 (CYP1A1) and hemeoxygenase-1 (HO-1), but inhibited expression of keratin 5/14, cyclinB1 and cdc25C in GK cells. ANE also activated epidermal growth factor receptor (EGFR), Src and Ras signaling pathways. ANE-induced COX-2, keratin 5, keratin 14 and cdc25C expression as well as PGE2 production were differentially regulated by α-naphthoflavone (a CYP 1A1/1A2 inhibitor), PD153035 (EGFR inhibitor), pp2 (Src inhibitor), and manumycin A (a Ras inhibitor). ANE-induced PGE2 production was suppressed by piper betle leaf (PBL) extract and hydroxychavicol (two major BQ components), dicoumarol (a

NAD(P)H

Quinone Oxidoreductase--NQO1 inhibitor) and curcumin. ANE-induced cytotoxicity was inhibited by catalase and enhanced by dicoumarol, suggesting that AN components may contribute to the pathogenesis of OSF and oral cancer via induction of aberrant differentiation, cytotoxicity, COX-2 expression, and PGE2/PGF2α production.

CONCLUSIONS

CYP4501A1, reactive oxygen species (ROS), EGFR, Src and Ras signaling pathways could all play a role in ANE-induced pathogenesis of oral cancer. Addition of PBL into BQ and curcumin consumption could inhibit the ANE-induced inflammatory response.

Mutagenicity test using Vibrio harveyi in the assessment of water quality from mussel farms

This work analyses the mutagenicity of seawater from mussel farms using the Vibrio harveyi mutagenicity test and its relationship with the accumulated pollutants and the development of gonadal neoplasia in mussels. Histological disorders identified as germinoma were observed in the gonad of Mytilus galloprovincialis during the period of study. The prevalence of this pathology is significantly correlated with certain levels of pollutants accumulated in mussels, mainly of PAHs and PCBs, whose toxic equivalents were calculated as EROD induction equivalency. The mutagenicity and toxicity of the water surrounding mussel's farms is clearly correlated with the pollutants accumulated and with the neoplasia prevalence in mussels. Such correlations are corroborated by a multivariate analysis. Our results conclude with the utility of V. harveyi test as an optimal and rapid method in the monitoring of the quality of the water from mussel farms and as a tool to control the risks of pollution on mussel production and its safety for human food.

Cellular Proliferation, Cell Death, and Liver Histology in Gambusia affinis After Dietary Exposure to Benzidine and 2-Aminofluorene

Chronic exposure to arylamines through diet and/or smoking has been associated with genetic changes and tumorigenesis. Cellular proliferation, apoptosis, and histological changes in liver tissue were investigated in Gambusia affinis ( G affinis) after chronic dietary exposure to 6.9 mM and 0.069 mM concentrations of benzidine (BZ), 2-aminofluorene (2AF), and their combination for 4, 8, and 12 weeks, respectively. The proliferation assay indicated non–dose-dependent increases in cellular proliferation over the controls for all treatment groups at 4 and 12 weeks but not at 8 weeks except for the low dose of 2AF. The apoptosis assay showed effects in the low-dose group of 2AF and BZ at 4 weeks only. Hematoxylin/eosin staining of liver tissue revealed an increase in oval/spindle cell proliferation and altered foci formation in the treated groups compared with controls. These results demonstrate a mammalian-like response to 2AF and BZ in G affinis liver.

Aryl hydrocarbon receptor-dependent induction of the NADPH oxidase subunit NCF1/p47 phox expression leading to priming of human macrophage oxidative burst

Polycyclic aromatic hydrocarbons such as benzo(a)pyrene (BaP) are toxic environmental contaminants known to regulate gene expression through activation of the aryl hydrocarbon receptor (AhR). In the present study, we demonstrated that acute treatment by BaP markedly increased expression of the NADPH oxidase subunit gene neutrophil cytosolic factor 1 (NCF1)/p47(phox) in primary human macrophages; NCF1 was similarly up-regulated in alveolar macrophages from BaP-instilled rats. NCF1 induction in BaP-treated human macrophages was prevented by targeting AhR, through its chemical inhibition or small interference RNA-mediated down-modulation of its expression. BaP moreover induced activity of the NCF1 promoter sequence, containing a consensus AhR-related xenobiotic-responsive element (XRE), and electrophoretic mobility shift assays and chromatin immunoprecipitation experiments indicated that BaP-triggered binding of AhR to this XRE. Finally, we showed that BaP exposure resulted in p47(phox) protein translocation to the plasma membrane and in potentiation of phorbol myristate acetate (PMA)-induced superoxide anion production in macrophages. This BaP priming effect toward NADPH oxidase activity was inhibited by the NADPH oxidase specific inhibitor apocynin and the chemical AhR inhibitor alpha-naphtoflavone. These results indicated that BaP induced NCF1/p47(phox) expression and subsequently enhanced superoxide anion production in PMA-treated human macrophages, in an AhR-dependent manner; such an NCF1/NADPH oxidase regulation by polycyclic aromatic hydrocarbons may participate in deleterious effects toward human health triggered by these environmental contaminants, including atherosclerosis and smoking-related diseases.

The spectrum of enzymes involved in activation of 2-aminoanthracene varies with the metabolic system applied

The aim of this study was to estimate the involvement of cytochrome P450s (CYPs) in the metabolic activation of 2-aminoanthracene (2AA) by use of metabolic systems such as liver S9 or hepatocytes from untreated and beta-naphthoflavone (BNF)- or phenobarbital (PB)-treated rats. Metabolic activation was determined in the Salmonella reverse mutation assay (Ames test). Unexpectedly, both enzyme inducers, BNF and PB, significantly decreased the mutagenicity of 2AA activated by S9 fractions. 2AA mutagenicity was detected in the presence of cytochrome P450 inhibitors such as alpha-naphthoflavone (ANF), clotrimazole and N-benzylimidazole to study the contribution of CYP isoenzymes to the activation process. ANF significantly decreased the activation of 2AA by S9 from untreated rats. In contrast, ANF significantly increased the metabolic activation of 2AA by S9 from BNF- and PB-treated rats. The enhanced mutagenicity was not altered by co-incubation with clotrimazole and ANF. Pre-incubation of 2AA in the presence of N-benzylimidazole significantly increased the activation of 2AA by S9 from BNF- and PB-treated rats, which suggests that CYPs play minor role in 2AA metabolic activation by rat liver S9 fractions. In contrast with the results described above, BNF treatment of rats significantly enhanced the activation of 2AA by hepatocytes. ANF attenuated the extent of this activation suggesting that different enzymes play a major role in the activation processes in these metabolic systems. Our results indicate that identification of mutagenic hazard by use of the Ames test may depend on the metabolic system applied.

Inhibition of carcinogen-bioactivating cytochrome P450 1 isoforms by amiloride derivatives

We examined the effects of amiloride derivatives, especially 5-(N-ethyl-N-isopropyl)amiloride (EIPA), on the activity of cytochrome P450 (CYP) 1 isoforms, known to metabolize carcinogenic polycyclic aromatic hydrocarbons (PAHs), such as benzo(a)pyrene (BP), into mutagenic metabolites and whose cellular expression can be induced through interaction of PAHs with the arylhydrocarbon receptor. EIPA was found to cause a potent and dose-dependent inhibition of CYP1-related ethoxyresorufine O-deethylase (EROD) activity in both liver cells and microsomes. It also markedly reduced activity of human recombinant CYP1A1 enzyme through a competitive mechanism; activities of other human CYP1 isoforms, i.e. CYP1A2 and CYP1B1, were also decreased. However, EIPA did not affect BP-mediated induction of CYP1A1 mRNA and protein levels in rat liver cells, likely indicating that EIPA does not block activation of the arylhydrocarbon receptor by PAHs. Inhibition of CYP1 activity by EIPA was associated with a decreased metabolism of BP, a reduced formation of BP-derived DNA adducts and a diminished BP-induced apoptosis in liver cells. The present data suggest that amiloride derivatives, such as EIPA, may be useful for preventing toxicity of chemical carcinogens, such as PAHs, through inhibition of CYP1 enzyme activity.

Identification of Na+/H+exchange as a new target for toxic polycyclic aromatic hydrocarbons in liver cells

The ubiquitous environmental pollutants polycyclic aromatic hydrocarbons are responsible for important carcinogenic and apoptotic effects, whose mechanisms are still poorly understood, owing to the multiplicity of possible cellular targets. Among these mechanisms, alterations of ionic homeostasis have been suggested. In this work, the effects of benzo(a)pyrene [B(a)P] on pHi were tested in the rat liver F258 epithelial cell line, using the fluoroprobe carboxy-SNARF-1. After a 48-h treatment, B(a)P (50 nM) induced an alkalinization, followed by an acidification after 72 h and the development of apoptosis. Determinations of pH(i) recovery following an acid load showed an increased acid efflux at 48 h. Cariporide inhibited both the early alkalinization and the increased acid efflux, thus suggesting the involvement of Na+/H+ exchanger 1 (NHE1). Besides, alpha-naphtoflavone (alpha-NF), an inhibitor of CYP1A1-mediated B(a)P metabolism, prevented all pH(i) changes, and NHE1 activation was blocked by the antioxidant thiourea, which inhibited CYP1A1 metabolism-dependent H2O2 production. Regarding B(a)P-induced apoptosis, this was prevented by alpha-NF and bongkrekic acid, an inhibitor of mitochondria-dependent apoptosis. Interestingly, apoptosis was significantly reduced by cariporide. Taken together, our results indicate that B(a)P, via H2O2 produced by CYP1A1-dependent metabolism, induces an early activation of NHE1, resulting in alkalinization; this appears to play a significant role in mitochondria-dependent B(a)P-induced apoptosis.

Induction of glutathione-S-transferases in primary cultured digestive gland acini from the mollusk bivalve Pecten maximus (L.): application of a new cellular model in biomonitoring studies

In the last three decades, marine invertebrates have been used to monitor environmental health conditions and potential pollution, e.g. in the Mussel Watch Program. The whole animal or specific organs are used to determine contamination levels and disturbances. In the present study, a new in vitro cell culture model was validated for pollution monitoring. A commercial species, the scallop Pecten maximus, was tested for the presence and induction of phase II glutathione-S-transferase (GST) enzymes. These activities were monitored for a year, and the results were found to be consistent with those in the literature. Tributyltin, ethylmethane sulfonate and the water-soluble fraction of crude oil were assayed in, in vitro induction studies. A rapid increase of GST activities was observed within 24 h with all compounds tested, and a time- as well as a dose-response was established. This in vitro cell culture model seems suitable for routine use to predict the effects of pollutants on whole organisms within an ecosystem and in fisheries.

Pregnane X receptor-dependent and -independent effects of 2-acetylaminofluorene on cytochrome P450 3A23 expression and liver cell proliferation

The arylamide 2-acetylaminofluorene (AAF) is a powerful carcinogen displaying a marked promoting activity, also known to regulate expression of liver detoxifying proteins. In this study we identified CYP3A23, a major inducible cytochrome P-450 (CYP) isoform, as an AAF target in hepatocytes. Indeed, exposure to AAF of primary rat hepatocytes resulted in a marked up-regulation of CYP3A23 expression at both mRNA and protein levels. Using CYP3A23 reporter gene constructs, we further demonstrated that AAF activated the CYP3A23 Direct Repeat 3 (DR3) promoter element interacting with the nuclear pregnane X receptor (PXR). Moreover, the PXR antagonist ecteinascidin-743 fully suppressed AAF-related CYP3A23 induction. Low doses of AAF inhibiting DNA synthesis in hepatocytes however failed to trigger PXR-related CYP3A23 induction and PXR-negative epithelial liver cells remained sensitive to the mito-inhibitory effects of AAF. Such data indicate that AAF up-regulates CYP3A23 through PXR activation but does not require PXR for exerting its carcinogenic promoting properties based on inhibition of cell growth.