Detoxification function of the Arabidopsis sulphotransferase AtSOT12 by sulphonation of xenobiotics

Cytosolic sulphotransferases have been implicated in inactivation of endogenous steroid hormones and detoxification of xenobiotics in human and animals. Yet, the function of plant sulphotransferases in xenobiotic sulphonation and detoxification has not been reported. In this study, we show that the Arabidopsis sulphotransferase AtSOT12 could sulphonate the bacterial‐produced toxin cycloheximide. Loss‐of‐function mutant sot12 exhibited hypersensitive phenotype to cycloheximide, and expression of AtSOT12 protein in yeast cells conferred resistance to this toxic compound. AtSOT12 exhibited broad specificity and could sulphonate a variety of xenobiotics including phenolic and polycyclic compounds. Enzyme kinetics analysis indicated that AtSOT12 has different selectivity for simple phenolics with different side chains, and the position of the side chain in the simple phenolic compounds affects substrate binding affinity and catalytic efficiency. We proposed that the broad specificity and induced production of AtSOT12 may have rendered this enzyme to not only modify endogenous molecules such as salicylic acid as we previously reported, but also sulphonate pathogen‐produced toxic small molecules to protect them from infection. Sulphonation of small molecules in plants may constitute a rapid way to inactivate or change the physiochemical properties of biologically active molecules that could have profound effects on plant growth, development and defence.     

In utero-initiated cancer: the role of reactive oxygen species

It is becoming more evident that not only can drugs and environmental chemicals interfere with normal fetal development by causing structural malformations, such as limb defects, but that xenobiotic exposure during development can also cause biochemical and functional abnormalities that may ultimately lead to cancer later on in life. Fetal toxicity may be partly mediated by the embryonic bioactivation of xenobiotics to free radical intermediates that can lead to oxidative stress and potentially lead, in some cases, to carcinogenesis. Using a number of examples, this review will focus on the role of reactive oxygen species (ROS) in the mechanisms pertaining to in utero initiated cancers.     

Human and animal hepatocytes in vitro with extrapolation in vivo

Human and animal hepatocytes are now being used as an in vitro technique to aid drug discovery by predicting the in vivo metabolic pathways of drugs or new chemical entities (NCEs), identifying drug-metabolizing enzymes and predicting their in vivo induction. Because of the difficulty of establishing whether the cytotoxic susceptibility of human hepatocytes to xenobiotics/drugs in vitro could be used to predict in vivo human hepatotoxicity, a comparison of the susceptibility of the hepatocytes of human and animal models to six chemical classes of drugs/xenobiotics in vitro have been related to their in vivo hepatotoxicity and the corresponding activity of their metabolizing enzymes. This study showed that the cytotoxic effectiveness of 16 halobenzenes towards rat hepatocytes in vitro using higher doses and short incubation times correlated well with rat hepatotoxic effectiveness in vivo with lower doses/longer times. The hepatic/hepatocyte xenobiotic metabolizing enzyme activities of various animal species and human have been reviewed for use by veterinarians and research scientists. Where possible, recommendations have been made regarding which animal hepatocyte model is most applicable for modeling the susceptibility to xenobiotic induced hepatotoxicity of those humans with slow versus rapid metabolizing enzyme polymorphisms. These recommendations are based on the best human fit for animal drug/xenobiotic metabolizing enzymes in terms of activity, kinetics and substrate/inhibitor specificity. The use of human hepatocytes from slow versus rapid metabolizing individuals for drug metabolism/cytotoxicity studies; and the research use of freshly isolated rat hepatocytes and "Accelerated Cytotoxicity Mechanism Screening" (ACMS) techniques for identifying drug/xenobiotic reactive metabolites are also described. Using these techniques the molecular hepatocytotoxic mechanisms found in vitro for seven classes of xenobiotics/drugs were found to be similar to the rat hepatotoxic mechanisms reported in vivo.     

Physiology and toxicology of hormone-disrupting chemicals in higher plants

Higher plants are exposed to natural environmental organic chemicals, associated with plant–environment interactions, and xenobiotic environmental organic chemicals, associated with anthropogenic activities. The effects of these chemicals result not only from interaction with metabolic targets, but also from interaction with the complex regulatory networks of hormone signaling. Purpose-designed plant hormone analogues thus show extensive signaling effects on gene regulation and are as such important for understanding plant hormone mechanisms and for manipulating plant growth and development. Some natural environmental chemicals also act on plants through interference with the perception and transduction of endogenous hormone signals. In a number of cases, bioactive xenobiotics, including herbicides that have been designed to affect specific metabolic targets, show extensive gene regulation effects, which are more in accordance with signaling effects than with consequences of metabolic effects. Some of these effects could be due to structural analogies with plant hormones or to interference with hormone metabolism, thus resulting in situations of hormone disruption similar to animal cell endocrine disruption by xenobiotics. These hormone-disrupting effects can be superimposed on parallel metabolic effects, thus indicating that toxicological characterisation of xenobiotics must take into consideration the whole range of signaling and metabolic effects. Hormone-disruptive signaling effects probably predominate when xenobiotic concentrations are low, as occurs in situations of residual low-level pollutions. These hormone-disruptive effects in plants may thus be of importance for understanding cryptic effects of low-dosage xenobiotics, as well as the interactive effects of mixtures of xenobiotic pollutants.     

Overview of Toxicity Data and Risk Assessment Methods for Evaluating the Chemical Effects of Depleted Uranium Compounds

In the United States, depleted uranium is handled or used in several chemical forms by both governmental agencies and private industry (primarily companies producing and machining depleted uranium metal for military applications). Human exposure can occur as a result of handling these compounds, routine low-level effluent releases to the environment from processing facilities, or materials being accidentally released from storage locations or during processing or transportation. Exposure to uranium can result in both chemical and radiological toxicity, but in most instances chemical toxicity is of greater concern. This article discusses the chemical toxic effects from human exposure to depleted uranium compounds that are likely to be handled during the long-term management and use of depleted uranium hexafluoride (UF₆) inventories in the United States. It also reviews representative publications in the toxicological literature to establish appropriate reference values for risk assessments. Methods are described for evaluating chemical toxicity caused by chronic low-level exposure and acute exposure. Example risk evaluations are provided for illustration. Preliminary results indicate that chemical effects of chronic exposure to uranium compounds under normal operating conditions would be negligibly small. Results also show that acute exposures under certain accident conditions could cause adverse chemical effects among the populations exposed.     

A novel variant of mouse MATE-1 H+/organic cation antiporter with a long hydrophobic tail

Mammalian multidrug and toxic compound extrusion 1 (MATE1) are polyspecific H⁺-coupled exporters of organic cations (OCs) and responsible for excretion of metabolic waste products and xenobiotics. Here, we report a novel variant of mouse MATE1, mMATE1b, that has a long carboxyl terminal hydrophobic tail homologous to other MATE1 transporter proteins. Mouse MATE1b mediates tetraethylammonium (TEA) uptake with properties similar to that of mMATE1 and is localized in renal brush border membranes. Thus, mMATE1b is a functional variant of mMATE1 and seems to be the true counterpart to other MATE1 transporters.     

Can antioxidants be beneficial in the treatment of lead poisoning?

Recent studies have shown that lead causes oxidative stress by inducing the generation of reactive oxygen species, reducing the antioxidant defense system of cells via depleting glutathione, inhibiting sulfhydryl-dependent enzymes, interfering with some essential metals needed for antioxidant enzyme activities, and/or increasing susceptibility of cells to oxidative attack by altering the membrane integrity and fatty acid composition. Consequently, it is plausible that impaired oxidant/antioxidant balance can be partially responsible for the toxic effects of lead. Where enhanced oxidative stress contributes to lead-induced toxicity, restoration of a cell's antioxidant capacity appears to provide a partial remedy. Several studies are underway to determine the effect of antioxidant supplementation following lead exposure. Data suggest that antioxidants may play an important role in abating some hazards of lead. To explain the importance of using antioxidants in treating lead poisoning the following topics are addressed: (i) Oxidative damage caused by lead poisoning; (ii) conventional treatment of lead poisoning and its side effects; and (iii) possible protective effects of antioxidants in lead toxicity.     

Two faces of nitric oxide: implications for cellular mechanisms of oxygen toxicity

Recent investigations have elucidated some of the diverse roles played by reactive oxygen and nitrogen species in events that lead to oxygen toxicity and defend against it. The focus of this review is on toxic and protective mechanisms in hyperoxia that have been investigated in our laboratories, with an emphasis on interactions of nitric oxide (NO) with other endogenous chemical species and with different physiological systems. It is now emerging from these studies that the anatomical localization of NO release, which depends, in part, on whether the oxygen exposure is normobaric or hyperbaric, strongly influences whether toxicity emerges and what form it takes, for example, acute lung injury, central nervous system excitation, or both. Spatial effects also contribute to differences in the susceptibility of different cells in organs at risk from hyperoxia, especially in the brain and lungs. As additional nodes are identified in this interactive network of toxic and protective responses, future advances may open up the possibility of novel pharmacological interventions to extend both the time and partial pressures of oxygen exposures that can be safely tolerated. The implications of a better understanding of the mechanisms by which NO contributes to central nervous system oxygen toxicity may include new insights into the pathogenesis of seizures of diverse etiologies. Likewise, improved knowledge of NO-based mechanisms of pulmonary oxygen toxicity may enhance our understanding of other types of lung injury associated with oxidative or nitrosative stress.     

The effect of different solvents on the ATP/ADP content and growth properties of HeLa cells

Testing of the effects of xenobiotics in cultured cells often requires the use of organic solvents to effect suspension of the test agents in cell culture media. However, the toxic effects of the solvents themselves may introduce artifacts, which obscure interpretation of the experimental results. In this article, the toxicity of different solvents commonly used for solvation of a variety of xenobiotic agents was studied. We show that ethanol, acetone, isooctane, methanol, and hexane were considerably less toxic than the more commonly used solvent, DMSO, when ATP content and growth rates of HeLa cells exposed to these solvents was measured. © 1998 John Wiley & Sons, Inc. J Biochem Toxicol 13: 11–15, 1999     

The relevance of xenobiotic metabolism in the interindividual susceptibility to chemicals

Biotransformation enzymes may catalyze either detoxication or bioactivation reactions; indeed, many xenobiotics exert their toxic effects after metabolic activation to electrophilic chemicals, interacting with nucleophilic sites on cellular macromolecules. On the other hand, by increasing xenobiotic hydrophilicity, the drug-metabolizing enzymes favors excretion of lipophilic chemicals, not allowing their bioaccumulation up to toxic levels. The expression of the enzymes of the drug-metabolizing system is modulated by genetic, pathological, developmental, environmental and dietary factors. Genetic polymorphism resulting in interindividual and interethnic variation in xenobiotic metabolism is responsible for differences in the susceptibility to chemical-induced toxicity and carcinogenicity, allowing the identification of people at increased risk. Moreover, differences in drug metabolism may correspond to variability in drug response during pharmacological therapy, which can be manifest either as adverse reactions or as a lack of benefit.     

Structure of a human carcinogen-converting enzyme,SULT1A1. Structural and kinetic implications of substrate inhibition

Sulfonation catalyzed by sulfotransferase enzymes plays an important role in chemical defense mechanisms against various xenobiotics but also bioactivates carcinogens. A major human sulfotransferase, SULT1A1, metabolizes and/or bioactivates many endogenous compounds and is implicated in a range of cancers because of its ability to modify diverse promutagen and procarcinogen xenobiotics. The crystal structure of human SULT1A1 reported here is the first sulfotransferase structure complexed with a xenobiotic substrate. An unexpected finding is that the enzyme accommodates not one but two molecules of the xenobiotic model substrate p-nitrophenol in the active site. This result is supported by kinetic data for SULT1A1 that show substrate inhibition for this small xenobiotic. The extended active site of SULT1A1 is consistent with binding of diiodothyronine but cannot easily accommodate beta-estradiol, although both are known substrates. This observation, together with evidence for a disorder-order transition in SULT1A1, suggests that the active site is flexible and can adapt its architecture to accept diverse hydrophobic substrates with varying sizes, shapes and flexibility. Thus the crystal structure of SULT1A1 provides the molecular basis for substrate inhibition and reveals the first clues as to how the enzyme sulfonates a wide variety of lipophilic compounds.     

Palmitate exacerbates bisphenol A toxicity via induction of ER stress and mitochondrial dysfunction

Combined exposure to dietary nutrients and environmental chemicals may elicit significantly different physiological effects than single exposures. Exposure to dietary saturated fats and environmental toxins is a physiologically-significant dual exposure that is particularly associated with lower socioeconomic status, potentially placing these individuals at heightened risk of xenobiotic toxicities. However, no prior studies have examined interactions between specific lipids and environmental xenobiotics in modulating cellular health. Using primary mouse embryonic fibroblasts, we have discovered that prior exposure to the saturated fatty acid, palmitate, exacerbates cellular toxicity associated with the industrial plasticizer, bisphenol A (BPA). Cell death upon BPA exposure following palmitate pre-treatment was greater than that occurring with either exposure alone. Mechanistically, cell death was preceded by increased endoplasmic reticulum stress and loss of mitochondrial membrane potential in palmitate plus BPA exposed cells, leading to increased caspase-3 cleavage and subsequent apoptosis. Interestingly, inclusion of the unsaturated fatty acid, oleate, along with palmitate during the pre-treatment period completely abrogated the ER stress, mitochondrial toxicity, and cell death induced by subsequent exposure to BPA. Thus, our data identify for the first time an important interaction between a fatty acid and an environmental toxin and have implications for developing nutritional interventions to mitigate the deleterious effects of such xenobiotic exposures.     

Preventive Efficacy of Bulk and Nanocurcumin Against Lead-Induced Oxidative Stress in Mice

Chronic lead exposure is associated with several health disorders in humans and animals. Lead exposure leads to the generation of reactive oxygen species and depletes body antioxidant enzymes causing damage to various macromolecules and ultimately cell death. Curcumin has been widely recognized to protect against metal toxicity but has major limitations of reduced bioavailability. Nanoencapsulation of curcumin could be an effective strategy to combat lead induced toxic manifestations. The present study investigates the protective efficacy of bulk and nanocurcumin against lead-induced toxicity. Swiss albino mice were daily exposed to lead acetate (25 mg/kg, i.p.) alone and after 1 h treated either with curcumin (15 mg/kg, orally) or nanocurcumin (15 mg/kg, orally) for two consecutive weeks. The preventive efficacy of nanocurcumin was evaluated against various altered biochemical variables suggestive of oxidative stress and lead accumulation in blood and soft tissues. Coadministration of nanocurcumin with lead restored the altered δ-aminolevulinic acid dehydratase activity, glutathione (reduced and oxidized) levels, and also decreased reactive oxygen species, and thiobarbituric acid reactive substances levels. Nanocurcumin due to its possible chelating property and enhanced bioavailability efficiently removed lead from blood and soft tissues compared to bulk curcumin. Results demonstrate the enhanced preventive efficacy of nanocurcumin and suggest an interesting and novel approach for better treatment of lead toxicity.     

Biomarkers as Predictors in Health and Ecological Risk Assessment

Biomarkers are measurable biological parameters that change in response to xenobiotic exposure and other environmental or physiological stressors, and can be indices of toxicant exposure or effects. If the biomarkers are sufficiently specific and well characterized, they can have great utility in the risk assessment process by providing an indication of the degree of exposure of humans or animals in natural populations to a specific xenobiotic or class of xenobiotics. Most biomarkers are effective as indices of exposure, but adequate information is rarely available on the appropriate dose-response curves to have well-described biomarkers of effect that can be widely applicable to additional populations. Specific examples of acetylcholinest-erase inhibition following exposure to organophosphorus insecticides are cited from experiments in both mammals (rats) and fish. These experiments have indicated that the degree of inhibition can be readily influenced by endogenous (e.g., age) and exogenous (e.g., chemical exposures) factors, and that the degree of inhibition is not readily correlated with toxicological effects. Caution is urged, therefore, in an attempt to utilize biomarkers in the risk assessment process until more complete documentation is available on the specificity, sensitivity, and time course of changes, and on the impact of multiple exposures or the time of exposures.     

Xenobiotic-induced apoptosis: significance and potential application as a general biomarker of response

The process of apoptosis, often coined programmed cell death, involves cell injury induced by a variety of stimuli including xenobiotics and is morphologically, biochemically, and physiologically distinct from necrosis. Apoptotic death is characterized by cellular changes such as cytoplasm shrinkage, chromatin condensation, and plasma membrane asymmetry. This form of cell suicide is appealing as a general biomarker of response in that it is expressed in multiple cell systems (e.g. immune, neuronal, hepatal, intestinal, dermal, reproductive), is conserved phylogenetically (e.g. fish, rodents, birds, sheep, amphibians, roundworms, plants, humans), is modulated by environmentally relevant levels of chemical contaminants, and indicates a state of stress of the organism. Further, apoptosis is useful as a biomarker as it serves as a molecular control point and hence may provide mechanistic information on xenobiotic stress. Studies reviewed here suggest that apoptosis is a sensitive and early indicator of acute and chronic chemical stress, loss of cellular function and structure, and organismal health. Examples are provided of the application of this methodology in studies of health of lake trout (Salvelinus namaycush) in the Laurentian Great Lakes.     

The use of cultured hepatocytes to investigate the metabolism of drugs and mechanisms of drug hepatotoxicity

Hepatotoxins can be classified as intrinsic when they exert their effects on all individuals in a dose-dependent manner, and as idiosyncratic when their effects are the consequence of an abnormal metabolism of the drug by susceptible individuals (metabolic idiosyncrasy) or of an immune-mediated injury to hepatocytes (allergic hepatitis). Some xenobiotics are electrophilic, and others are biotransformed by the liver into highly reactive metabolites that are usually more toxic than the parent compound. This activation process is the key to many hepatotoxic phenomena. Mitochondria are a frequent target of hepatotoxic drugs, and the alteration of their function has immediate effects on the energy balance of cells (depletion of ATP). Lipid peroxidation, oxidative stress, alteration of Ca(2+) homeostasis, and covalent binding to cell macromolecules are the molecular mechanisms that are frequently involved in the toxicity of xenobiotics. Against these potential hazards, cells have their own defence mechanisms (for example, glutathione, DNA repair, suicide inactivation). Ultimately, toxicity is the balance between bioactivation and detoxification, which determines whether a reactive metabolite elicits a toxic effect. The ultimate goal of in vitro experiments is to generate the type of scientific information needed to identify compounds that are potentially toxic to man. For this purpose, both the design of the experiments and the interpretation of the results are critical.]     

CAR and PXR: the xenobiotic-sensing receptors

The xenobiotic receptors CAR and PXR constitute two important members of the NR1I nuclear receptor family. They function as sensors of toxic byproducts derived from endogenous metabolism and of exogenous chemicals, in order to enhance their elimination. This unique function of CAR and PXR sets them apart from the steroid hormone receptors. In contrast, the steroid receptors, exemplified by the estrogen receptor (ER) and glucocorticoid receptor (GR), are the sensors that tightly monitor and respond to changes in circulating steroid hormone levels to maintain body homeostasis. This divergence of the chemical- and steroid-sensing functions has evolved to ensure the fidelity of the steroid hormone endocrine regulation while allowing development of metabolic elimination pathways for xenobiotics. The development of the xenobiotic receptors CAR and PXR also reflect the increasing complexity of metabolism in higher organisms, which necessitate novel mechanisms for handling and eliminating metabolic by-products and foreign compounds from the body. The purpose of this review is to discuss similarities and differences between the xenobiotic receptors CAR and PXR with the prototypical steroid hormone receptors ER and GR. Interesting differences in structure explain in part the divergence in function and activation mechanisms of CAR/PXR from ER/GR. In addition, the physiological roles of CAR and PXR will be reviewed, with discussion of interactions of CAR and PXR with endocrine signaling pathways.     

Increased Reactive Oxygen Species Production in the Brain After Repeated Low-Dose Pesticide Paraquat Exposure in Rats. A Comparison with Peripheral Tissues

The pesticide paraquat (PQ) was found to be a suitable xenobiotic to model Parkinson’s disease. The reactive oxygen species (ROS) production was suggested to be the main cause of PQ toxicity but very few evidences were found for its generation in the brain in vivo after ip administration. We compared the effects of PQ-induced ROS generation between the brain structures and the peripheral tissues using two different hydroxyl radical generation markers. Repeated but not single ip PQ administration increased the levels of ROS in the striatal homogenates but, when measured in the extracellular microdialysis filtrate, no change was observed. The increased dopamine release was detected in the striatum after the fourth PQ administration and its basal levels were decreased. A single treatment with the pesticide did not influence ROS production in the lungs or kidneys but repeated intoxication decreased its levels. These results suggest that repeated, systemic administration of a low dose of PQ triggers intracellular ROS formation in the brain and can cause slowly progressing degenerative processes, without the toxic effects in the peripheral tissues.