Deregulation of cancer-related pathways in primary hepatocytes derived from DNA repair-deficient Xpa-/-p53+/- mice upon exposure to benzo[a]pyrene

Downregulation of Stem-Loop Binding Protein by Nicotine via α7-Nicotinic Acetylcholine Receptor and Its Role in Nicotine-Induced Cell Transformation

The use of electronic-cigarettes (e-cigs) has increased substantially in recent years, particularly among the younger generations. Liquid nicotine is the main component of e-cigs. Previous studies have shown that mice exposed to e-cig aerosols developed lung adenocarcinoma and bladder hyperplasia. These findings implicated a potential role for e-cig aerosols and nicotine in cancer development, although the underlying mechanisms are not fully understood. Here we report that exposure to liquid nicotine or nicotine aerosol generated from e-cig induces downregulation of Stem-loop binding protein (SLBP) and polyadenylation of canonical histone mRNAs in human bronchial epithelial cells and in mice lungs. Canonical histone mRNAs typically do not end in a poly(A) tail and the acquisition of such a tail via depletion of SLBP has been shown to causes chromosome instability. We show that nicotine-induced SLBP depletion is reversed by an inhibitor of α7-nicotinic acetylcholine receptors (α7-nAChR) or siRNA specific for α7-nAChR, indicating a nAChR-dependent reduction of SLBP by nicotine. Moreover, PI3K/AKT pathway is activated by nicotine exposure and CK2 and probably CDK1, 2 kinases well known for their function for SLBP phosphorylation and degradation, are shown to be involved, α7-nAChR-dependently, in nicotine-induced SLBP depletion. Importantly, nicotine-induced anchorage-independent cell growth is attenuated by inhibition of α7-nAChR and is rescued by overexpression of SLBP. We propose that the SLBP depletion and polyadenylation of canonical histone mRNAs via activation of α7-nAChR and a series of downstream signal transduction pathways are critical for nicotine-induced cell transformation and potential carcinogenesis.

Transcriptomic effect marker patterns of genotoxins - a comparative study with literature data

Microarray approaches are frequently used experimental tools which have proven their value for example in the characterization of the molecular mode of action of toxicologically relevant compounds. In a regulatory context, omics techniques are still not routinely used, amongst others due to lacking standardization in experimental setup and data processing, and also due to issues with the definition of adversity. In order to exemplarily determine whether consensus transcript biomarker signatures for a certain toxicological endpoint can be derived from published microarray datasets, we here compared transcriptome data from human HepaRG hepatocarcinoma cells treated with different genotoxins, based on re-analyzed datasets extracted from the literature. Comparison of the resulting data show that even with similarly-acting compounds in the same cell line, considerable variation was observed with respect to the numbers and identities of differentially expressed genes. Greater concordance was observed when considering the whole data sets and biological functions associated with the genes affected. The present results highlight difficulties and possibilities in inter-experiment comparisons of omics data and underpin the need for future efforts towards improved standardization to facilitate the use of omics data in risk assessment. Existing omics datasets may nonetheless prove valuable in establishing biological context information essential for the development of adverse outcome pathways.

Involvement of p38 MAPK pathway in benzo(a)pyrene-induced human hepatoma cell migration and invasion

The objective of this study was to investigate the potential role of p38 mitogen-activated protein kinases (MAPK) in benzo(a)pyrene (BaP)-induced hepatoma cell migration and invasion. Western blot assay was applied to detect the expression of proteins. qRT-PCR assay was used to measure the expression of mRNA. Wound healing assay and Transwell invasion assay were performed to evaluate cell migratory ability and cell invasive ability, respectively. Our data showed that BaP exposure increased the expression of p-p38 protein in human hepatoma HepG2 cells. Exposure to BaP facilitated HepG2 cell migration and invasion, which could be blocked by p38 MAPK inhibitors. In addition, BaP exposure induced upregulation of MMP9 mRNA expression, which was modulated by p-p38. In conclusion, p38 MAPK pathway was involved in BaP-induced hepatoma cell migration and invasion.

The effects of acetaldehyde exposure on histone modifications and chromatin structure in human lung bronchial epithelial cells

As the primary metabolite of alcohol and the most abundant carcinogen in tobacco smoke, acetaldehyde is linked to a number of human diseases associated with chronic alcohol consumption and smoking including cancers. In addition to direct DNA damage as a result of the formation of acetaldehyde-DNA adducts, acetaldehyde may also indirectly impact proper genome function through the formation of protein adducts. Histone proteins are the major component of the chromatin. Post-translational histone modifications (PTMs) are critically important for the maintenance of genetic and epigenetic stability. However, little is known about how acetaldehyde-histone adducts affect histone modifications and chromatin structure. The results of protein carbonyl assays suggest that acetaldehyde forms adducts with histone proteins in human bronchial epithelial BEAS-2B cells. The level of acetylation for N-terminal tails of cytosolic histones H3 and H4, an important modification for histone nuclear import and chromatin assembly, is significantly downregulated following acetaldehyde exposure in BEAS-2B cells, possibly due to the formation of histone adducts and/or the decrease in the expression of histone acetyltransferases. Notably, the level of nucleosomal histones in the chromatin fraction and at most of the genomic loci we tested are low in acetaldehyde-treated cells as compared with the control cells, which is suggestive of inhibition of chromatin assembly. Moreover, acetaldehyde exposure perturbs chromatin structure as evidenced by the increase in general chromatin accessibility and the decrease in nucleosome occupancy at genomic loci following acetaldehyde treatment. Our results indicate that regulation of histone modifications and chromatin accessibility may play important roles in acetaldehyde-induced pathogenesis. Environ. Mol. Mutagen. 59:375-385, 2018. © 2018 Wiley Periodicals, Inc.

Regulation of Chromatin Assembly and Cell Transformation by Formaldehyde Exposure in Human Cells

BACKGROUND

Formaldehyde (FA) is an environmental and occupational chemical carcinogen. Recent studies have shown that exogenous FA causes only a modest increase in DNA adduct formation compared with the amount of adducts formed by endogenous FA, raising the possibility that epigenetic mechanisms may contribute to FA-mediated carcinogenicity.

OBJECTIVES

We investigated the effects of FA exposure on histone modifications and chromatin assembly. We also examined the role of defective chromatin assembly in FA-mediated transcription and cell transformation.

METHODS

Cellular fractionation and Western blot analysis were used to measure the levels of histone modifications in human bronchial epithelial BEAS-2B cells and human nasal RPMI2650 cells in the presence of FA. Chromatin immunoprecipitation (ChIP) and micrococcal nuclease (MNase) digest assays were performed to examine the changes in chromatin assembly and accessibility after FA exposure. RNA sequencing (RNA-seq) and real-time polymerase chain reaction (PCR) were used to examine transcriptional dysregulation. Finally, anchorage-independent cell growth ability was tested by soft agar assay following FA exposure.

RESULTS

Exposure to FA dramatically decreased the acetylation of the N-terminal tails of cytosolic histones. These modifications are important for histone nuclear import and subsequent chromatin assembly. Histone proteins were depleted in both the chromatin fraction and at most of the genomic loci tested following FA exposure, suggesting that FA compromises chromatin assembly. Moreover, FA increased chromatin accessibility and altered the expression of hundreds of cancer-related genes. Knockdown of the histone H3.3 gene (an H3 variant), which mimics inhibition of chromatin assembly, facilitated FA-mediated anchorage-independent cell growth.

CONCLUSIONS

We propose that the inhibition of chromatin assembly represents a novel mechanism of cell transformation induced by the environmental and occupational chemical carcinogen FA. https://doi.org/10.1289/EHP1275.

Evaluation of the LacZ reporter assay in cryopreserved primary hepatocytes for In vitro genotoxicity testing

Assessment of genotoxic potential is an important step in the safety evaluation of chemical substances. Under most regulatory jurisdictions, the first tier of testing comprises a standard battery of in vitro genotoxicity tests in bacterial and mammalian cells. However, the mammalian cell tests commonly used exhibit a relatively high rate of misleading positive results, which may lead to unnecessary in vivo testing. We previously established a proof-of-concept for the LacZ reporter assay in proliferating primary hepatocytes as a promising alternative genotoxicity test. Here, cryopreserved instead of freshly isolated hepatocytes were used and the assay was evaluated in more detail. We examined the effect of cryopreservation on phenotype and metabolic capacity of the LacZ hepatocytes, and assessed the predictive performance of the assay by testing a set of substances comprising true positive, true negative, and misleading positive substances. Additionally, a historical negative control database was created and the type of mutations induced was analyzed for two of the substances tested. Our findings indicate that proliferating cryopreserved primary hepatocytes derived from LacZ plasmid mice retain their hepatocyte-specific characteristics and metabolic competence. Furthermore, we demonstrate that both gene mutations and genome rearrangements due to large deletions can be detected with the LacZ reporter assay. The assay seems to have a lower rate of misleading positive test results compared to the assays currently used. Together, our findings strongly support the use of the LacZ reporter assay in cryopreserved primary hepatocytes as follow-up to the standard in vitro test battery for genotoxicity testing. Environ. Mol. Mutagen. 57:643-655, 2016. © 2016 Wiley Periodicals, Inc.

Screening a mouse liver gene expression compendium identifies modulators of the aryl hydrocarbon receptor (AhR)

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the biological and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), dioxin-like compounds (DLC) as well as some drugs and endogenous tryptophan metabolites. Short-term activation of AhR can lead to hepatocellular steatosis, and chronic activation can lead to liver cancer in mice and rats. Analytical approaches were developed to identify biosets in a genomic database in which AhR activity was altered. A set of 63 genes was identified (the AhR gene expression biomarker) that was dependent on AhR for regulation after exposure to TCDD or benzo[a]pyrene and includes the known AhR targets Cyp1a1 and Cyp1b1. A fold-change rank-based test (Running Fisher's test; p-value ≤ 10(-4)) was used to evaluate the similarity between the AhR biomarker and a test set of 37 and 41 biosets positive or negative, respectively for AhR activation. The test resulted in a balanced accuracy of 95%. The rank-based test was used to identify factors that activate or suppress AhR in an annotated mouse liver/mouse primary hepatocyte gene expression database of ∼ 1850 comparisons. In addition to the expected activation of AhR by TCDD and DLC, AhR was activated by AP20189 and phenformin. AhR was suppressed by phenobarbital and 1,4-Bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP) in a constitutive activated receptor (CAR)-dependent manner and pregnenolone-16α-carbonitrile in a pregnane X receptor (PXR)-dependent manner. Inactivation of individual genes in nullizygous models led to AhR activation (Pxr, Ghrhr, Taf10) or suppression (Ahr, Ilst6st, Hnf1a). This study describes a novel screening strategy for identifying factors in mouse liver that perturb AhR in a gene expression compendium.

Drug-induced endoplasmic reticulum and oxidative stress responses independently sensitize toward TNFα-mediated hepatotoxicity

Drug-induced liver injury (DILI) is an important clinical problem. Here, we used a genomics approach to in detail investigate the hypothesis that critical drug-induced toxicity pathways act in synergy with the pro-inflammatory cytokine tumor necrosis factor α (TNFα) to cause cell death of liver HepG2 cells. Transcriptomics of the cell injury stress response pathways initiated by two hepatoxicants, diclofenac and carbamazepine, revealed the endoplasmic reticulum (ER) stress/translational initiation signaling and nuclear factor-erythroid 2 (NF-E2)-related factor 2 (Nrf2) antioxidant signaling as two major affected pathways, which was similar to that observed for the majority of ∼80 DILI compounds in primary human hepatocytes. Compounds displaying weak or no TNFα synergism, namely ketoconazole, nefazodone, and methotrexate, failed to synchronously induce both pathways. The ER stress induced was primarily related to protein kinase R-like ER kinase (PERK) and activating transcription factor 4 (ATF4) activation and subsequent expression of C/EBP homologous protein (CHOP), which was all independent of TNFα signaling. Identical ATF4 dependent transcriptional programs were observed in primary human hepatocytes as well as primary precision-cut human liver slices. Targeted RNA interference studies revealed that whereas ER stress signaling through inositol-requiring enzyme 1α (IRE1α) and activating transcription factor 6 (ATF6) acted cytoprotective, activation of the ER stress protein kinase PERK and subsequent expression of CHOP was pivotal for the onset of drug/TNFα-induced apoptosis. Whereas inhibition of the Nrf2-dependent adaptive oxidative stress response enhanced the drug/TNFα cytotoxicity, Nrf2 signaling did not affect CHOP expression. Both hepatotoxic drugs enhanced expression of the translational initiation factor EIF4A1, which was essential for CHOP expression and drug/TNFα-mediated cell killing. Our data support a model in which enhanced drug-induced translation initiates PERK-mediated CHOP signaling in an EIF4A1 dependent manner, thereby sensitizing toward caspase-8-dependent TNFα-induced apoptosis.

Mechanisms of amiodarone and valproic acid induced liver steatosis in mouse in vivo act as a template for other hepatotoxicity models

Liver injury is the leading cause of drug-induced toxicity. For the evaluation of a chemical compound to induce toxicity, in this case steatosis or fatty liver, it is imperative to identify markers reflective of mechanisms and processes induced upon exposure, as these will be the earliest changes reflective of disease. Therefore, an in vivo mouse toxicogenomics study was completed to identify common pathways, nuclear receptor (NR) binding sites, and genes regulated by three known human steatosis-inducing compounds, amiodarone (AMD), valproic acid (VPA), and tetracycline (TET). Over 1, 4, and 11 days of treatment, AMD induced changes in clinical chemistry parameters and histopathology consistent with steatosis. Common processes and NR binding sites involved in lipid, retinol, and drug metabolism were found for AMD and VPA, but not for TET, which showed no response. Interestingly, the pattern of enrichment of these common pathways and NR binding sites over time was unique to each compound. Eleven biomarkers of steatosis were identified as dose responsive and time sensitive to toxicity for AMD and VPA. Finally, this in vivo mouse study was compared to an AMD rat in vivo, an AMD mouse primary hepatocyte, and a VPA human primary hepatocyte study to identify concordance for steatosis. We conclude that concordance is found on the process level independent of species, model or dose*time point.

In vitro effects of aldehydes present in tobacco smoke on gene expression in human lung alveolar epithelial cells

Tobacco smoke consists of thousands of harmful components. A major class of chemicals found in tobacco smoke is formed by aldehydes, in particular formaldehyde, acetaldehyde and acrolein. The present study investigates the gene expression changes in human lung alveolar epithelial cells upon exposure to formaldehyde, acrolein and acetaldehyde at sub-cytotoxic levels. We exposed A549 cells in vitro to aldehydes and non-aldehyde chemicals (nicotine, hydroquinone and 2,5-dimethylfuran) present in tobacco smoke and used microarrays to obtain a global view of the transcriptomic responses. We compared responses of the individual aldehydes with that of the non-aldehydes. We also studied the response of the aldehydes when present in a mixture at relative concentrations as present in cigarette smoke. Formaldehyde gave the strongest response; a total of 66 genes were more than 1.5-fold differentially expressed mostly involved in apoptosis and DNA damage related processes, followed by acetaldehyde (57 genes), hydroquinone (55 genes) and nicotine (8 genes). For acrolein and the mixture only one gene was upregulated involved in oxidative stress. No gene expression effect was found for exposure to 2,5-dimethylfuran. Overall, aldehyde responses are primarily indicative for genotoxicity and oxidative stress. These two toxicity mechanisms are linked to respiratory diseases such as cancer and COPD, respectively. The present findings could be important in providing further understanding of the role of aldehydes emitted from cigarette smoke in the onset of pulmonary diseases.

Genome-Wide Functional and Stress Response Profiling Reveals Toxic Mechanism and Genes Required for Tolerance to Benzo[a]pyrene in S. cerevisiae

Benzo[a]pyrene (BaP) is a ubiquitous, potent, and complete carcinogen resulting from incomplete organic combustion. BaP can form DNA adducts but other mechanisms may play a role in toxicity. We used a functional toxicology approach in S. cerevisiae to assess the genetic requirements for cellular resistance to BaP. In addition, we examined translational activities of key genes involved in various stress response pathways. We identified multiple genes and processes involved in modulating BaP toxicity in yeast which support DNA damage as a primary mechanism of toxicity, but also identify other potential toxicity pathways. Gene ontology enrichment analysis indicated that DNA damage and repair as well as redox homeostasis and oxidative stress are key processes in cellular response to BaP suggesting a similar mode of action of BaP in yeast and mammals. Interestingly, toxicant export is also implicated as a potential novel modulator of cellular susceptibility. In particular, we identified several transporters with human orthologs (solute carrier family 22) which may play a role in mammalian systems.

Pharmacokinetics explain in vivo/in vitro discrepancies of carcinogen-induced gene expression alterations in rat liver and cultivated hepatocytes

Cultivated hepatocytes represent a well-established in vitro system. However, the applicability of hepatocytes in toxicogenomics is still controversially discussed. Recently, an in vivo/in vitro discrepancy has been described, whereby the non-genotoxic rat liver carcinogen methapyrilene alters the expression of the metabolizing genes SULT1A1 and ABAT, as well as the DNA damage response gene GADD34 in vitro, but not in vivo. If the collagen sandwich cultures of hepatocytes really produce false-positive data, this would compromise its application in toxicogenomics. To revisit the putative in vivo/in vitro discrepancy, we first analyzed and modeled methapyrilene concentrations in the portal vein of rats. The relatively short half-life of 2.8 h implies a rapid decrease in orally administered methapyrilene in vivo below concentrations that can cause gene expression alterations. This corresponded to the time-dependent alteration levels of GADD34, ABAT and SULT1A1 RNA in the liver: RNA levels are altered 1, 6 and 12 h after methapyrilene administration, but return to control levels after 24 and 72 h. In contrast, methapyrilene concentrations in the culture medium supernatant of primary rat hepatocyte cultures decreased slowly. This explains why GADD34, ABAT and SULT1A1 were still deregulated after 24 h exposure in vitro, but not in vivo. It should also be considered that the earliest analyzed time point in the previous in vivo studies was 24 h after methapyrilene administration. In conclusion, previously observed in vitro/in vivo discrepancy can be explained by different pharmacokinetics present in vitro and in vivo. When the in vivo half-life is short, levels of some initially altered genes may have returned to control levels already 24 h after administration.

Screening for drug-induced hepatotoxicity in primary mouse hepatocytes using acetaminophen, amiodarone, and cyclosporin a as model compounds: an omics-guided approach

Drug-induced hepatotoxicity is a leading cause of attrition for candidate pharmaceuticals in development. New preclinical screening methods are crucial to predict drug toxicity prior to human studies. Of all in vitro hepatotoxicity models, primary human hepatocytes are considered as 'the gold standard.' However, their use is hindered by limited availability and inter-individual variation. These barriers may be overcome by using primary mouse hepatocytes. We used differential in gel electrophoresis (DIGE) to study large-scale protein expression of primary mouse hepatocytes. These hepatocytes were exposed to three well-defined hepatotoxicants: acetaminophen, amiodarone, and cyclosporin A. Each hepatotoxicant induces a different hepatotoxic phenotype. Based on the DIGE results, the mRNA expression levels of deregulated proteins from cyclosporin A-treated cells were also analyzed. We were able to distinguish cyclosporin A from controls, as well as acetaminophen and amiodarone-treated samples. Cyclosporin A induced endoplasmic reticulum (ER) stress and altered the ER-Golgi transport. Moreover, liver carboxylesterase and bile salt sulfotransferase were differentially expressed. These proteins were associated with a protective adaptive response against cyclosporin A-induced cholestasis. The results of this study are comparable with effects in HepG2 cells. Therefore, we suggest both models can be used to analyze the cholestatic properties of cyclosporin A. Furthermore, this study showed a conserved response between primary mouse hepatocytes and HepG2 cells. These findings collectively lend support for use of omics strategies in preclinical toxicology, and might inform future efforts to better link preclinical and clinical research in rational drug development.

Benzo[a]pyrene-induced transcriptomic responses in primary hepatocytes and in vivo liver: toxicokinetics is essential for in vivo-in vitro comparisons

The traditional 2-year cancer bioassay needs replacement by more cost-effective and predictive tests. The use of toxicogenomics in an in vitro system may provide a more high-throughput method to investigate early alterations induced by carcinogens. Recently, the differential gene expression response in wild-type and cancer-prone Xpa (-/-) p53 (+/-) primary mouse hepatocytes after exposure to benzo[a]pyrene (B[a]P) revealed downregulation of cancer-related pathways in Xpa (-/-) p53 (+/-) hepatocytes only. Here, we investigated pathway regulation upon in vivo B[a]P exposure of wild-type and Xpa (-/-) p53 (+/-) mice. In vivo transcriptomics analysis revealed a limited gene expression response in mouse livers, but with a significant induction of DNA replication and apoptotic/anti-apoptotic cellular responses in Xpa (-/-) p53 (+/-) livers only. In order to be able to make a meaningful in vivo-in vitro comparison we estimated internal in vivo B[a]P concentrations using DNA adduct levels and physiologically based kinetic modeling. Based on these results, the in vitro concentration that corresponded best with the internal in vivo dose was chosen. Comparison of in vivo and in vitro data demonstrated similarities in transcriptomics response: xenobiotic metabolism, lipid metabolism and oxidative stress. However, we were unable to detect cancer-related pathways in either wild-type or Xpa (-/-) p53 (+/-) exposed livers, which were previously found to be induced by B[a]P in Xpa (-/-) p53 (+/-) primary hepatocytes. In conclusion, we showed parallels in gene expression responses between livers and primary hepatocytes upon exposure to equivalent concentrations of B[a]P. Furthermore, we recommend considering toxicokinetics when modeling a complex in vivo endpoint with in vitro models.

Benzo[a]pyrene (BP) DNA adduct formation in DNA repair-deficient p53 haploinsufficient [Xpa(-/-)p53(+/-)] and wild-type mice fed BP and BP plus chlorophyllin for 28 days

We have evaluated DNA damage (DNA adduct formation) after feeding benzo[a]pyrene (BP) to wild-type (WT) and cancer-susceptible Xpa(-/-)p53(+/-) mice deficient in nucleotide excision repair and haploinsufficient for the tumor suppressor p53. DNA damage was evaluated by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC/ES-MS/MS), which measures r7,t8,t9-trihydroxy-c-10-(N (2)-deoxyguanosyl)-7,8,9,10-tetrahydrobenzo[a]pyrene (BPdG), and a chemiluminescence immunoassay (CIA), using anti-r7,t8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE)-DNA antiserum, which measures both BPdG and the other stable BP-DNA adducts. When mice were fed 100 ppm BP for 28 days, BP-induced DNA damage measured in esophagus, liver and lung was typically higher in Xpa(-/-)p53(+/-) mice, compared with WT mice. This result is consistent with the previously observed tumor susceptibility of Xpa(-/-)p53(+/-) mice. BPdG, the major DNA adduct associated with tumorigenicity, was the primary DNA adduct formed in esophagus (a target tissue in the mouse), whereas total BP-DNA adducts predominated in higher levels in the liver (a non-target tissue in the mouse). In an attempt to lower BP-induced DNA damage, we fed the WT and Xpa(-/-)p53(+/-) mice 0.3% chlorophyllin (CHL) in the BP-containing diet for 28 days. The addition of CHL resulted in an increase of BP-DNA adducts in esophagus, liver and lung of WT mice, a lowering of BPdG in esophagi of WT mice and livers of Xpa(-/-)p53(+/-) mice and an increase of BPdG in livers of WT mice. Therefore, the addition of CHL to a BP-containing diet showed a lack of consistent chemoprotective effect, indicating that oral CHL administration may not reduce PAH-DNA adduct levels consistently in human organs.

Dissecting modes of action of non-genotoxic carcinogens in primary mouse hepatocytes

Under REACH, the European Community Regulation on chemicals, the testing strategy for carcinogenicity is based on in vitro and in vivo genotoxicity assays. Given that non-genotoxic carcinogens are negative for genotoxicity and chronic bioassays are no longer regularly performed, this class of carcinogens will go undetected. Therefore, test systems detecting non-genotoxic carcinogens, or even better their modes of action, are required. Here, we investigated whether gene expression profiling in primary hepatocytes can be used to distinguish different modes of action of non-genotoxic carcinogens. For this, primary mouse hepatocytes were exposed to 16 non-genotoxic carcinogens with diverse modes of action. Upon profiling, pathway analysis was performed to obtain insight into the biological relevance of the observed changes in gene expression. Subsequently, both a supervised and an unsupervised comparison approach were applied to recognize the modes of action at the transcriptomic level. These analyses resulted in the detection of three of eight compound classes, that is, peroxisome proliferators, metalloids and skin tumor promotors. In conclusion, gene expression profiles in primary hepatocytes, at least in rodent hepatocytes, appear to be useful to detect some, certainly not all, modes of action of non-genotoxic carcinogens.

Proliferating primary hepatocytes from the pUR288 lacZ plasmid mouse are valuable tools for genotoxicity assessment in vitro

Safety assessments of substances with regard to genotoxicity are generally based on a combination of in vitro and in vivo tests. These tests are performed according to a (tiered) test strategy whereby a positive result in vitro usually triggers further testing in vivo. A low specificity and high frequency of irrelevant positive results associated with most in vitro mammalian cell genotoxicity assays necessitates the design and validation of suitable alternatives. As such, we examined the feasibility of culturing primary hepatocytes from the pUR288 lacZ reporter mouse, and moreover, using established cultures to reliably assess genotoxic activity in vitro. Initial studies characterizing the metabolic capacity of proliferating lacZ primary hepatocytes indicated that these cells retained at least some activities important for xenobiotic metabolism: cytochrome P450 1A1 enzyme activities were markedly increased in the hepatocytes after exposure to benzo[a]pyrene, and also UDP-glucuronosyl transferase and glutathione-S-transferase activities, both Phase II enzymes, were detected. Increasing levels of phosphorylated p53 at residue serine 389 after ultraviolet treatment indicated a properly functioning p53, one of the criteria for an effective new test system. Four genotoxic substances with different mechanisms of genotoxicity, i.e., benzo[a]pyrene, bleomycin, etoposide, and cyclophosphamide, were tested in the lacZ rescue assay. For etoposide and cyclophosphamide, the induction of mutant colonies was rather low. Exposure to benzo[a]pyrene and bleomycin, however, yielded a clear concentration-dependent induction of the lacZ mutant frequency. Based on our preliminary observations, proliferating lacZ primary hepatocytes are a promising new tool for the assessment of genotoxic hazard.