Linking gene expression to mechanisms of toxicity

Gonadal disruption after single dose exposure of prothioconazole and prothioconazole-desthio in male lizards (Eremias argus)

Prothioconazole (PTC) is a widely used triazole fungicide with low toxicity, and its desulfurization metabolite, prothioconazole-desthio (PTC-d), is reported to have higher reproductive toxicity to mammals. However, little is known about the reproductive toxicity, much less endocrine disrupting effect, of these two chemicals on reptiles. In this study, we investigated the effects of single dose of PTC/PTC-d (100 mg kg^-1 body weight) exposure on the pathomorphism of testes and epididymides, serum sex steroid hormones (testosterone and 17β-estradiol) and transcription of steroidogenic-related genes (STARD, cyp11A, cyp17, cyp19A, 17β-HSD, 3β-HSD, AR and ER-α) in gonads of male lizards (Eremias argus). Although structural disorder existed in PTC-d exposure group, severe gonadal disruption, especially suppression of spermatogenesis was only observed in testis after PTC treatment, which consequently led to the lack of spermatozoa in epididymal ducts. Consistent with this result, T/E₂ value in PTC exposure was elevated to a significant higher level compared with control and continually increased over time, while T/E₂ value in the PTC-d exposure group slightly increased only at 12 h. These results demonstrated a more serious disruption of PTC on male lizard gonads than PTC-d. In addition, the expression of cyp17 gene was inhibited at 6 h, however, was induced at 12 h, and exhibited negative correlations with STARD, cyp11A and 3β-HSD after PTC exposure at each timepoint. In PTC-d group, the expression of STARD and 3β-HSD were significantly down-regulated, in contrast, cyp11A and cyp17 were up-regulated, and each gene showed consistent changes over time. For 17β-HSD, no significance was observed in both treated groups. This study was the first to compare the gonadal disruption of PTC and PTC-d in male lizards and elucidated that these two chemicals influenced the physiological function of male gonads through differential transcriptional modulation.

Energy of the Lowest Unoccupied Molecular Orbital, Thiol Reactivity, and Toxicity of Three Monobrominated Water Disinfection Byproducts

Disinfection of drinking water protects public health against waterborne pathogens. However, during disinfection, toxic disinfection byproducts (DBPs) are formed. Exposure to DBPs was associated with increased risk of bladder cancer in humans. DBPs are generated at concentrations below their carcinogenic potencies; it is unclear how exposure leads to adverse health outcomes. We used computational estimates of the energy of the lowest unoccupied molecular orbital (ELUMO) to predict thiol reactivity and additive toxicity among soft electrophile DBPs. Bromoacetic acid (BAA) was identified as non-thiol-reactive, which was supported by in chemico and in vitro data. Bromoacetonitrile (BAN) and bromoacetamide (BAM) were thiol-reactive. Genotoxicity induced by these compounds was reduced by increasing the thiol pool with N-acetyl L-cysteine (NAC), while NAC had little effect on BAA. BAN and BAM shared depletion of glutathione (GSH) or cellular thiols as a molecular initiating event (MIE), whereas BAA induces toxicity through another pathway. Binary mixtures of BAM and BAN expressed a potentiating effect in genotoxicity. We found that soft electrophile DBPs could be an important predictor of common mechanism groups that demonstrated additive toxicity. In silico estimates of ELUMO could be used to identify the most relevant DBPs that are the forcing factors of the toxicity of finished drinking waters.

Epigenetics changes associated to environmental triggers in autoimmunity

Autoimmune diseases (AIDs) are chronic conditions initiated by the loss of immunological tolerance to self-antigens and represent a heterogeneous group of disorders that affect specific target organs or multiple organs in different systems. While the pathogenesis of AID remains unclear, its aetiology is multifunctional and includes a combination of genetic, epigenetic, immunological and environmental factors. In AIDs, several epigenetic mechanisms are defective including DNA demethylation, abnormal chromatin positioning associated with autoantibody production and abnormalities in the expression of RNA interference (RNAi). It is known that environmental factors may interfere with DNA methylation and histone modifications, however, little is known about epigenetic changes derived of regulation of RNAi. An approach to the known environmental factors and the mechanisms that alter the epigenetic regulation in AIDs (with emphasis in systemic lupus erythematosus, the prototype of systemic AID) are showed in this review.

Chlorogenic acid against carbon tetrachloride-induced liver fibrosis in rats

This study examined the effects of chlorogenic acid (CGA) on liver fibrosis induced by carbon tetrachloride (CCl(4)) and explored the possible mechanisms of action. Liver fibrosis was induced in male Sprague-Dawley (SD) rats by the injection of 40% CCl(4) subcutaneously twice a week for eight weeks. At the same time, CGA (60 and 30mg/kg) was administered intragastrically once daily to a subset of rats. Upon pathological examination, the CGA-treated rats showed significantly reduced liver damage and symptoms of liver fibrosis. The expression of collagen I and collagen III mRNA was increased markedly by the CCl(4) treatment but this increase was suppressed by CGA. As compared with the CGA-treated group, the expression of bcl-2, vascular endothelial growth factor (VEGF), and transforming growth factor (TGF-beta1) mRNA was increased in CCl(4) group, whereas Bax mRNA expression decreased. The expression of Bax and bcl-2 protein was confirmed by western blotting. Intragastric administration of CGA reduced the protein expression of alpha-smooth muscle actin (alpha-SMA) and glucose-regulated proteins 78 and 94 (GRP78 and GRP94) in rats injured by treatment with CCl(4). Our data indicate that CGA can efficiently inhibit CCl(4)-induced liver fibrosis in rats. Therefore, CGA could be an effective drug for preventing liver fibrosis.

The Endoplasmic Reticulum in Xenobiotic Toxicity

The endoplasmic reticulum (ER) is involved in an array of cellular functions that play important roles in xenobiotic toxicity. The ER contains the majority of cytochrome P450 enzymes involved in xenobiotic metabolism, as well as a number of conjugating enzymes. In addition to its role in drug bioactivation and detoxification, the ER can be a target for damage by reactive intermediates leading to cell death or immune-mediated toxicity. The ER contains a set of luminal proteins referred to as ER stress proteins (including GRP78, GRP94, protein disulfide isomerase, and calreticulin). These proteins help regulate protein processing and folding of membrane and secretory proteins in the ER, calcium homeostasis, and ER-associated apoptotic pathways. They are induced in response to ER stress. This review discusses the importance of the ER in molecular events leading to cell death following xenobiotic exposure. Data showing that the ER is important in both renal and hepatic toxicity will be discussed.

Potential mechanisms of estrogen quinone carcinogenesis

There is a clear association between the excessive exposure to estrogens and the development of cancer in hormone-sensitive tissues (breast, endometrium). It has become clear that there are likely multiple overlapping mechanisms of estrogen carcinogenesis. One major pathway is the extensively studied hormonal pathway, by which estrogen stimulates cell proliferation through nuclear estrogen receptor (ER)-mediated signaling, thus resulting in an increased risk of genomic mutations during DNA replication. A similar "nongenomic pathway", potentially involving newly discovered membrane-associated ERs, also appears to regulate extranuclear estrogen signaling pathways. This perspective is focused on a third pathway involving the metabolism of estrogens to catechols mediated by cytochrome P450 and further oxidation of these catechols to estrogen o-quinones. Oxidative enzymes, metal ions, and in some cases molecular oxygen can catalyze o-quinone formation, so that these electrophilic/redox-active quinones can cause damage within cells by alkylation and/or oxidation of cellular proteins and DNA in many tissues. It appears that the endogenous estrogen quinones primarily form unstable N3-adenine or N7-guanine DNA adducts, ultimately resulting in mutagenic apurinic sites. In contrast, equine estrogen quinones, formed from estrogens present in popular hormone replacement therapy prescriptions, generate a variety of DNA lesions, including bulky stable adducts, apurinic sites, DNA strand cleavage, and oxidation of DNA bases. DNA damage induced by these equine quinones is significantly increased in cells containing ERs, leading us to hypothesize a mechanism involving ER binding/alkylation by the catchol/quinone, resulting in a "Trojan horse". The "Trojan horse" carries the highly redox-active catechol to estrogen -sensitive genes, where high amounts of reactive oxygen species are generated, causing selective DNA damage. Our data further suggest that other key protein targets for estrogen o-quinones could be redox-sensitive enzymes (i.e, GST P1-1, QR). These proteins are involved in stress response cascades that are known to contribute to the regulation of cell proliferation and apoptosis. Finally, it has been shown that catechol estrogens can transform breast epithelial cells into a tumorigenic phenotype and that these transformed cells had differential gene expression of several genes involved in oxidative stress. Given the direct link between excessive exposure to estrogens, metabolism of estrogens, and increased risk of breast cancer, it is crucial that factors that affect the formation, reactivity, and cellular targets of estrogen quinoids be thoroughly explored.

Decreased protein and mRNA expression of ER stress proteins GRP78 and GRP94 in HepG2 cells over-expressing CYP2E1

CYP2E1 causes oxidative stress mediated cell death; the latter is one mechanism for endoplasmic reticulum (ER) stress in the cell. Unfolded proteins accumulate during ER stress and ER resident proteins GRP78 and GRP94 protect cells against ER dysfunction. We examined the possible role of GRP78 and GRP94 as protective factors against CYP2E1-mediated toxicity in HepG2 cells expressing CYP2E1 (E47 cells). E47 cells expressed high levels of CYP2E1 protein and catalytic activity which is associated with increased ROS generation, lipid peroxidation and the elevated presence of ubiquinated and aggregated proteins as compared to control HepG2 C34 cells which do not express CYP2E1. The mRNA and protein expression of GRP78 and GRP94 were decreased in E47 cells compared to the C34 cells, which may explain the accumulation of ubiquinated and aggregated proteins. Expression of these GRP proteins was induced with the ER stress agent thapsigargin in E47 cells, and E47 cells were more resistant to the toxicity caused by thapsigargin and calcimycin, possibly due to this upregulation and also because of the high expression of GSH and antioxidant enzymes in E47 cells. Antioxidants such as trolox and N-acetylcysteine increased GRP78 and GRP94 levels in the E47 cells, suggesting that CYP2E1- derived oxidant stress was responsible for down regulation of these GRPs in the E47 cells. Thapsigargin mediated toxicity was decreased in cells treated with the antioxidant trolox indicating a role for oxidative stress in this toxicity. These results suggest that CYP2E1 mediated oxidative stress downregulates the expression of GRP proteins in HepG2 cells and oxidative stress is an important mechanism in causing ER dysfunction in these cells.

Disruption of the endoplasmic reticulum by cytotoxins in LLC-PK1 cells

Prior induction of an endoplasmic reticulum stress response results in protection against reactive cytotoxins in the LLC-PK1 cell line. The purpose of this investigation was to determine therefore if the endoplasmic reticulum was disrupted by iodoacetamide, tert-butylhydroperoxide or sulfamethoxazole hydroxylamine. Toxic concentrations of the three toxins caused a dramatic loss of GRP94 protein within 3-8h of exposure, while induction of GRP78 and calreticulin occurred at 8 and 24h following exposure. There was no evidence of cytosolic elevation of calcium and neither dantrolene nor xestospongin were able to block the cytotoxicity of IDAM and TBHP. Exposure to the toxins led to DNA degradation and cleavage of procaspase-12. There was only evidence of procaspase-3 cleavage after TBHP exposure. These results demonstrate that the ER is disrupted by the reactive cytotoxins examined in LLC-PK1cells and suggest that the cytoprotection against low to moderate concentrations of cytotoxins observed following endoplasmic reticulum stress protein induction is likely due to a mechanism other than maintenance of calcium homeostasis.

Cytoprotection following endoplasmic reticulum stress protein induction in continuous cell lines

Prior induction of an endoplasmic reticulum stress response has been associated with an increased tolerance to cellular toxins in in vitro systems, primarily involving renal and neuronal cells. Reactive intermediates are involved in toxicity in many tissues, therefore, we wished to determine if cytoprotection after induction of an endoplasmic reticulum stress response was a general phenomenon in other cell types. A stress response was induced by tunicamycin in a human hepatocyte cell line (HepG2), a rat hepatocyte cell line (H4IIE), a porcine kidney cell line (LLC-PK1), and a human lymphocyte cell line (K562). Induction of the endoplasmic reticulum stress proteins GRP78, GRP94, calreticulin and protein disulfide isomerase was assessed by immunoblotting. Cytotoxicity was assessed 24 hr after a 3 hr exposure to iodoacetamide, tert-butylhydrogenperoxide, menadione, or sulfamethoxazole hydroxylamine, or after a 2 hr exposure to N-acetyl-p-benzoquinoneimine, the reactive metabolite of acetaminophen. Induction of endoplasmic reticulum stress proteins in LLC-PK1 cells resulted in a 2-6 times increase in the concentration of all the cytotoxins required to cause a 50% decrease in cell viability at 24 hr. In contrast, tunicamycin pretreatment only resulted in a 1.7-times increase for iodo-acetamide in HepG2 cells and a 2.2-times increase for N-acetyl-p-benzoquinoneimine in the H4IIE cells, but had no effect on the other toxins tested. Induction of endoplasmic reticulum stress proteins in K562 cells did not alter susceptibility to any toxins tested. Our results indicate that protection afforded by the induction of an endoplasmic reticulum stress response is dependent on the cell type and may be toxin specific. These results suggest that either the molecular pathways of cell death for individual toxins are different between cell types and toxins, or that the function of endoplasmic reticulum stress proteins are dependent on the cell type.

Effects of lipopolysaccharide-stimulated inflammation and pyrazole-mediated hepatocellular injury on mouse hepatic Cyp2a5 expression

Murine hepatic cytochrome P450 2a5 (Cyp2a5) is induced during hepatotoxicity and hepatitis, however, the specific regulatory mechanisms have not been determined. We compared the influence of acute inflammation elicited in vivo by bacterial endotoxin lipopolysaccharide (LPS) and liver injury caused by the hepatotoxin pyrazole on hepatic Cyp2a5 expression in mice. Pyrazole treatment resulted in statistically significant increases in levels of Cyp2a5 mRNA, protein and catalytic activity by 540, 273 and 711%, respectively (P<0.05). In LPS-treated livers Cyp2a5 expression was significantly reduced compared to controls at the mRNA (46%) protein (35%), and activity (23%) levels (P<0.05). Treatment of mice with recombinant murine interleukin-1 beta and interleukin-6 had no significant effect on Cyp2a5 mRNA and protein levels. Liver injury, as assessed by serum alanine aminotransferase, was greater with pyrazole than with LPS treatment (609 vs 354% of control levels respectively). ER stress, determined by hepatic glucose regulated protein 78 (grp78) levels, was greater with pyrazole (185% of controls) than with LPS (128% of controls). In pyrazole-treated liver, overexpression of immunoreactive grp78 protein revealed that ER stress was localized to pericentral hepatocytes in which Cyp2a5 was induced. Evidence of glycogen loss and membrane damage in these cells was suggestive of oxidative damage. Moreover, vitamin E attenuated Cyp2a5 induction by pyrazole in vivo. These results suggest that induction of Cyp2a5 that has been observed in mouse models of hepatitis and hepatoxicity may be related to oxidative injury to the endoplasmic reticulum of pericentral hepatocytes rather than exposure to pro-inflammatory cytokines.

Custom cDNA microarrays; technologies and applications

Technologies designed to characterise genes and their products on a discovery scale are now having an impact on many areas of biology, including toxicology. A number of platforms exist which measure changes in expression of potentially thousands of genes simultaneously. These approaches, when applied to toxicology, are termed 'toxicogenomics' and promise to greatly facilitate mechanism-based research on toxicant action with the longer term possibility of assisting in the identification of potential toxicity issues earlier in the development of new pharmaceutical, agrochemical and chemical products. An example of such a platform developed in our laboratory is ToxBlot II, a custom microarray containing cDNAs representing 12564 human genes chosen on the basis of their potential relevance to a broad range of toxicities. ToxBlot II can assist in characterising many outcomes including processes as diverse as immune system response, receptor biology, signal transduction, protein modification, membrane transport, growth and development, metabolism, oxidative stress and regulation of the cell cytoskeleton. Furthermore, ToxBlot II allows the simultaneous expression profiling of genes representing entire cellular pathways, facilitating a very detailed investigation of potential mechanisms of toxicity. Our laboratory is applying this and other custom microarrays to many areas of toxicology, including endocrine disruption, receptor biology, stress response and the effect of toxicants on immune function. Such approaches can be particularly valuable when used in conjunction with 'functional genomics' such as transgenic or knockout models.

DNA microarrays and toxicogenomics: applications for ecotoxicology?

Toxicogenomics attempts to define how the regulation and expression of genes mediate the toxicological effects associated with exposure to a chemical. DNA microarrays are rapidly becoming one of the tools of choice for large-scale toxicogenomic studies. An approach in modern toxicogenomics has been to classify toxicity based on gene transcriptional patterns; comparing the transcriptional responses of a chemical with unknown toxicity to those for which the transcriptional profiles and toxicological endpoints have been well characterized. Recent evidence suggests that gene expression microarrays may be instrumental in defining mechanisms of action of toxicants. However, several assumptions are inherent to a toxicogenomic-based approach in toxicology, many of which remain to be validated. Gene expression profiling using DNA microarrays represents a snapshot of the gene transcriptional responses occurring at a particular time and within a particular tissue. Toxicity, on the other hand, represents a continuum of possible effects governed by both temporal and spatial factors that are inextricably contingent upon the exposure conditions. The perceived toxicological properties of any chemical are dependent on the route, dose, and duration of the exposure, and as such, gene expression patterns are also subject to these variables. Correct interpretation of DNA microarray data for the assessment of the toxicological properties of chemicals will require that temporal and spatial gene expression profiles be accounted for. These considerations are further compounded in ecotoxicological studies, during which altered gene expression patterns induced from exposure to an anthropogenic substance must be discernible over and above the complex effects that phenotypic, genotypic, and environmental variables have on gene expression. To this end, the greatest utility of DNA microarrays in the field of ecotoxicology may be in predicting the toxicological modes of action of anthropogenic substances on host physiology, particularly in non-model organisms. Predictable and accurate assessment of the impacts of a chemical substance in ecotoxicology will require that classical toxicological endpoints be used to validate any effects predicted based on gene expression profiling. Validated expression profiling may subsequently find utility in ecotoxicological-based computer simulation models, such as the Biotic Ligand Model (BLM), in which gene expression information may be integrated with geochemical, pharmacokinetic, and physiological data to accurately assess and predict toxicity of metals to aquatic organisms.

Differential heat shock gene hsp70-1 response to toxicants revealed by in vivo study of lungs in transgenic mice

Members of heat shock proteins (Hsp70) family have been considered to respond to a large variety of stressful conditions. But it was suggested that, in pulmonary cells, Hsp response depends more closely on the type of stimulus. The lungs are critical organs potentially subjected to air pollution affecting respiratory function and, therefore, these organs are of particular interest with regard to the stress response. To investigate the stress dependence of Hsp70 response in lungs, we created transgenic mice where the firefly luciferase reporter gene is under the control of the murine hsp70-1 promoter and exposed them to different sublethal toxic conditions. For each condition, the level of transgene induction and pulmonary toxicity were assessed. We found that hsp70-1 promoter was stimulated by heat shock and cadmium but not by ozone, paraquat, and parathion, even if these chemicals induced respiratory distress and lung inflammation. Similar observations were made when expression of the endogenous hsp70-1 gene was analyzed, indicating that our transgenic model was accurately detecting hsp70-1 induction. Thereby, it appeared that hsp70-1 response is selective and depends on signaling pathways triggered by the toxicants rather than by their pathologic toxicity per se. Furthermore, because all the chemicals used in our study have been previously described to increase the level of oxidative stress, it indicates that there is no direct and simple correlation between hsp70-1 response and the level of oxidative stress, but more specific oxidative patterns should be involved in Hsp regulation.

Toxicogenomics; transcript profiling and potential application to chemical allergy

Novel transcript profiling technologies allow simultaneous measurement of the changes in expression of many hundreds or many thousands of genes. The availability of these methods has brought about revolutionary changes in many areas of investigative biology, where analyses of patterns of gene expression, rather than of individual genes, are being employed. The application of these technologies to toxicology (toxicogenomics) offers new opportunities for both mechanistic toxicity research and predictive toxicology. Here we provide an overview of the basic approaches used in this field. The development of a series of toxicology-specific microarrays in our own laboratory is discussed, together with an example of one area of mechanistic research, chemical allergy, where we believe judicious application of toxicogenomics will make an important contribution.