Xenobiotic biotransformation/bioactivation in organogenesis-stage conceptual tissues: implications for embryotoxicity and teratogenesis

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Development and Uses of Offline and Web-Searchable Metabolism Databases - The Case of Benzo[a]pyrene

BACKGROUND

The present work describes development of offline and web-searchable metabolism databases for drugs, other chemicals, and physiological compounds using human and model species, prompted by the large amount of data published after year 1990. The intent was to provide a rapid and accurate approach to published data to be applied both in science and to assist therapy.

METHODS

Searches for the data were done using the Pub Med database, accessing the Medline database of references and abstracts. In addition, data presented at scientific conferences (e.g., ISSX conferences) are included covering the publishing period beginning with the year 1976.

RESULTS

Application of the data is illustrated by the properties of benzo[a]pyrene (B[a]P) and its metabolites. Analysis show higher activity of P450 1A1 for activation of the (-)- isomer of trans-B[a]P-7,8-diol, while P4501B1 exerts higher activity for the (+)- isomer. P450 1A2 showed equally low activity in the metabolic activation of both isomers.

CONCLUSION

The information collected in the databases is applicable in prediction of metabolic drug-drug and/or drug-chemical interactions in clinical and environmental studies. The data on the metabolism of searched compound (exemplified by benzo[a]pyrene and its metabolites) also indicate toxicological properties of the products of specific reactions. The offline and web-searchable databases had wide range of applications (e.g. computer assisted drug design and development, optimization of clinical therapy, toxicological applications) and adjustment in everyday life styles.

Impairment of preimplantation and postimplantation embryonic development through intrinsic apoptotic processes by ginsenoside Rg1 in vitro and in vivo

Ginsenoside Rg1, which is the most abundant compound found in Asian ginseng (Panax ginseng), has demonstrated various pharmacological actions, including neuroprotective, immune-stimulatory, and antidiabetic effects. Pregnant women, especially in the Asian community, consume ginseng as a nutritive supplement. Thus, the effects of ginsenoside-Rg1 on embryonic development need to be investigated, such as in a mouse model. As previous investigations have found that ginsenoside Rg1 appears to either trigger or prevent apoptosis in different cell lines, the effects of this agent on apoptosis remain to be clarified. In this study, we investigated whether ginsenoside Rg1 exerts a hazardous effect on mouse blastocysts and/or affects subsequent embryonic development in vitro and in vivo. Blastocysts treated with 25-100 μM ginsenoside Rg1 exhibited significant induction of apoptosis and a corresponding decrease in the inner cell mass (ICM) cell number. Importantly, the implantation rate was lower among ginsenoside Rg1-treated blastocysts compared to untreated controls. Moreover, embryo transfer assays revealed that blastocysts treated with 100 μM ginsenoside Rg1 exhibited increased resorption of postimplantation embryos and decreased weight among surviving fetuses. In vivo, intravenous injection of mice with ginsenoside Rg1 (2, 4, or 6 mg/kg body weight/day) for 4 days was associated with increased apoptosis of blastocyst-stage embryos and negatively impacted early embryonic development. Further experiments revealed that these effects may reflect the ability of ginsenoside Rg1 to trigger oxidative stress-mediated intrinsic apoptotic signaling. Our in vitro results indicate that ginsenoside　Rg1 treatment increases intracellular oxidative stress, decreases mitochondrial membrane potential, increases the Bax/Bcl-2 ratio, and activates caspase-9 and caspase-3, but not caspase-8. Taken together, our study results strongly suggest that ginsenoside Rg1 induces apoptosis and impairs the early preimplantation and postimplantation development of mouse embryos, both in vitro and in vivo.

Bioaccumulation and ROS generation in Coontail Ceratophyllum demersum L. exposed to phenanthrene

Phenanthrene bioaccumulation, induction free radicals and their consequent biochemical responses in coontail (Ceratophyllum demersum L.) were examined. Plants were exposed to different levels (0.01, 0.02, 0.05, 0.07 and 0.1 mg/l) of phenanthrene for 10 days. Results showed that the phenanthrene concentration in the plants was exponentially correlated to exposure concentration (R (2) = 0.958) and phenanthrene exposure significantly increased the total free radicals and superoxide anion in the plants. The activities of antioxidant enzymes and the contents of glutathione were determined. The superoxide dismutase (SOD) activity and reduced glutathione (GSH) content were inhibited, while the catalase (CAT), peroxidase (POD), glutathione-s-transferase (GST) activities and oxidized glutathione (GSSG) content were significantly induced. Changes in the contents of chlorophyll and malondialdehyde (MDA) indicated that the MDA content was enhanced after phenanthrene exposure and the contents of chlorophyll were significantly increased in the 0.01 mg/l group. These experimental data demonstrated that the bioaccumulation of phenanthrene induced the production of free radicals and ROS, and changed the antioxidant defense system, ultimately resulting in oxidative damage in C. demersum.

Receptor- and reactive intermediate-mediated mechanisms of teratogenesis

Drugs and environmental chemicals can adversely alter the development of the fetus at critical periods during pregnancy, resulting in death, or in structural and functional birth defects in the surviving offspring. This process of teratogenesis may not be evident until a decade or more after birth. Postnatal functional abnormalities include deficits in brain function, a variety of metabolic diseases, and cancer. Due to the high degree of fetal cellular division and differentiation, and to differences from the adult in many biochemical pathways, the fetus is highly susceptible to teratogens, typically at low exposure levels that do not harm the mother. Insights into the mechanisms of teratogenesis come primarily from animal models and in vitro systems, and involve either receptor-mediated or reactive intermediate-mediated processes. Receptor-mediated mechanisms involving the reversible binding of xenobiotic substrates to a specific receptor are exemplified herein by the interaction of the environmental chemical 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or "dioxin") with the cytosolic aryl hydrocarbon receptor (AHR), which translocates to the nucleus and, in association with other proteins, binds to AH-responsive elements (AHREs) in numerous genes, initiating changes in gene transcription that can perturb development. Alternatively, many xenobiotics are bioactivated by fetal enzymes like the cytochromes P450 (CYPs) and prostaglandin H synthases (PHSs) to highly unstable electrophilic or free radical reactive intermediates. Electrophilic reactive intermediates can covalently (irreversibly) bind to and alter the function of essential cellular macromolecules (proteins, DNA), causing developmental anomalies. Free radical reactive intermediates can enhance the formation of reactive oxygen species (ROS), resulting in oxidative damage to cellular macromolecules and/or altered signal transduction. The teratogenicity of reactive intermediates is determined to a large extent by the balance among embryonic and fetal pathways of xenobiotic bioactivation, detoxification of the xenobiotic reactive intermediate, detoxification of ROS, and repair of oxidative macromolecular damage.

CYP1B1, a developmental gene with a potential role in glaucoma therapy

The association of CYP1B1 gene alterations in primary congenital glaucoma individuals has been known for about a decade. Recent evidence has shown the involvement of CYP1B1 mutations in a number of forms of glaucoma and anterior segment disorders. This suggests a wide role for CYP1B1 in ocular physiology. Histochemical studies of eyes from individuals with primary congenital glaucoma revealed abnormalities in the anterior chamber angle, the region between the cornea and the iris, containing the trabecular meshwork. The cells of the trabecular meshwork serve as a filter to allow drainage of the aqueous humour, the fluid formed by the ciliary body that fills the anterior chamber. Mutations in CYP1B1 that affect its activity have frequently been shown to influence development of the trabecular meshwork, and it is thought that CYP1B1, a monooxygenase, acts to form or degrade some endobiotic compound that is necessary for proper development of the filtering structures. The rapidly developing area of stem cell research suggests a potential therapeutic approach for glaucomas resulting from deleterious mutations in CYP1B1, that is, the transfer of stem cells, differentiated to a specific lineage, containing wild-type CYP1B1 to specific regions of the eye, where they will develop into normal cells of that region and rectify the defect.

Oxidative stress in developmental origins of disease: teratogenesis, neurodevelopmental deficits, and cancer

In the developing embryo and fetus, endogenous or xenobiotic-enhanced formation of reactive oxygen species (ROS) like hydroxyl radicals may adversely alter development by oxidatively damaging cellular lipids, proteins and DNA, and/or by altering signal transduction. The postnatal consequences may include an array of birth defects (teratogenesis), postnatal functional deficits, and diseases. In animal models, the adverse developmental consequences of in utero exposure to agents like thalidomide, methamphetamine, phenytoin, benzo[a]pyrene, and ionizing radiation can be modulated by altering pathways that control the embryonic ROS balance, including enzymes that bioactivate endogenous substrates and xenobiotics to free radical intermediates, antioxidative enzymes that detoxify ROS, and enzymes that repair oxidative DNA damage. ROS-mediated signaling via Ras, nuclear factor kappa B and related transducers also may contribute to altered development. Embryopathies can be reduced by free radical spin trapping agents and antioxidants, and enhanced by glutathione depletion. Further modulatory approaches to evaluate such mechanisms in vivo and/or in embryo culture have included the use of knockout mice, transgenic knock-ins and mutant deficient mice with altered enzyme activities, as well as antisense oligonucleotides, protein therapy with antioxidative enzymes, dietary depletion of essential cofactors and chemical enzyme inhibitors. In a few cases, measures anticipated to be protective have conversely enhanced the risk of adverse developmental outcomes, indicating the complexity of development and need for caution in testing therapeutic strategies in humans. A better understanding of the developmental effects of ROS may provide insights for risk assessment and the reduction of adverse postnatal consequences.

Bioaccumulation and oxidative stress in submerged macrophyte Ceratophyllum demersum L. upon exposure to pyrene

Laboratory experiments were carried out to investigate pyrene bioaccumulation and its consequent biological responses in submerged macrophyte Ceratophyllum demersum. Plants were exposed to different levels (0.01, 0.02, 0.05, 0.07, 0.1 mg/L) of pyrene for 10 days, and the pyrene content, and total free radicals in plant were analyzed. The pyrene concentration in plant was highly correlated to exposure concentration (R(2)=0.990). Electron paramagnetic resonance (EPR) analysis revealed that pyrene exposure significantly increased total free radicals in the plants. A strong positive correlation (R(2)=0.956) between O(2)(*-) generation and pyrene contents implied that pyrene exposure induced reactive oxygen species (ROS) and led to oxidative stress in C. demersum. The activities of antioxidant enzymes and the contents of glutathione were determined. Change in the contents of malondialdehyde (MDA) was also studied. Results indicated that the bioaccumulation of pyrene resulted in the changes of the antioxidant defense system and the production of ROS with the oxidative stress, ultimately induced damnification in C. demersum.

Effect of Placental Fatty Acid Metabolism and Regulation by Peroxisome Proliferator Activated Receptor on Pregnancy and Fetal Outcomes

Fatty acids, particularly the omega-3 and omega-6 essential fatty acids (EFAs), are considered critical nutritional sources for the developing fetus. The placenta governs the fetal supply of fatty acids via two processes: transport and metabolism. Placental fatty acid metabolism can play a critical role in guiding pregnancy and fetal outcome. EFAs can be metabolized to important cell signaling molecules in placenta by several major isoform families including: the Cytochrome P450 subfamily 4A (CYP4A); Cyclooxygenases (COXs); and Lipoxygenases (LOXs). Peroxisome proliferator-activated nuclear receptors (PPARs) have been demonstrated to regulate a number of placental fatty acid/lipid homeostasis-related proteins (e.g., metabolizing enzymes and transporters). The present review summarizes research on the molecular and functional relevance of fatty acid metabolizing enzymes and the role of PPARs in regulating their expression in the mammalian placenta. Elucidating the pathways of placental fatty acid metabolism and the regulatory processes governing these pathways is critical for advancing our understanding of the role of placenta in supplying EFAs to the developing fetus and the potential implications on pregnancy and fetal outcome. A more complete understanding of placental fatty acid disposition may also provide a basis for nutritional/pharmacological interventions to ameliorate the risk of adverse pregnancy and/or fetal outcomes.

Changes in hepatic cytosolic glutathione S-transferase activity and expression of its class-P during prenatal and postnatal period in rats treated with aflatoxin B1

The effect of aflatoxin B1 (AFB1) on the expression of glutathione S-transferase-P (GST-P) which is the major isoform of GST in developmental stages has been investigated in rat liver during prenatal and postnatal stages. Following administration of AFB1 (0, 0.5, 1.0, 2.0, 3.0 or 4.0 mg/kg bw) injected I.P on day 8.5 of gestation the number of dead or reabsorbed fetuses and malformed embryos were recorded. Then the fetal livers were processed for measurement of total GST and GST-P activities, using 1-chloro-2,4-dinitrobenzene (CDNB) and ethacrynic acid (ETA) as substrates respectively. RT-PCR using rat GST-P specific primers was performed on mRNA extracted from livers. Besides, the effects of AFB1 on hepatic GST and GST-P were assessed in groups of suckling rats directly injected with the toxin. The results show that a single dose of AFB1 (1.0 or 2.0 mg/kg bw) caused approximately 50-60% depletion in fetal liver GST towards CDNB. Postnatal experiments revealed that liver GST (using CDNB as substrate) was significantly induced (approximately 40%) in suckling rats injected with a single dose of AFB1 (3.0 mg AFB1/kg) 24 h before killing. Liver GST-P expression was unaffected due to AFB1 exposures of rats before and after the birth. This finding was substantiated by western blotting and RT-PCR techniques. These data suggest that AFB1-related induction in rat liver total GST after birth may be implicated in protective mechanisms against AFB1. In contrast, inhibition of this enzyme in fetal liver following placental transfer of the carcinogen may explain high susceptibility of fetal cells to trans-plancental aflatoxins. Furthermore, lack of influence of AFB1 on GST-P expression in developmental stages can role out the involvement of this class of GST in AFB1 biotransformation.

Expression of CYP4A isoforms in developing rat placental tissue and rat trophoblastic cell models

Maintaining fatty acid homeostasis during pregnancy is critical for normal fetal development. As an organ that controls nutrient supply from the mother to the fetus, the placenta plays a significant role in guiding fatty acid transfer to the developing fetus. The cytochrome P450 4A (CYP4A) subfamily of metabolizing enzymes is a group of structurally and functionally conserved proteins that are specialized in the omega/omega-1 hydroxylation of saturated and unsaturated fatty acids and their derivatives. To understand the function of the CYP4A system in the placenta and its significance in maintaining fetal fatty acid homeostasis, information about the placental expression of individual CYP4A isoforms is required. In the present study, we have elucidated the temporal and spatial patterns of expression of the four known rat CYP4A isoforms (CYP4A1, CYP4A2, CYP4A3, and CYP4A8) in the junctional and labyrinthine zones of the developing rat chorioallantoic placenta as well as two rat trophoblastic cell lines, HRP-1 and Rcho-1, using semi-quantitative RT-PCR and immunohistochemical analyses. The mRNA from the four rat CYP4A isoforms was detected in the developing rat placenta with CYP4A1 exhibiting the strongest expression (4A1 > 4A2 >> 4A3 approximately equal to 4A8). CYP4A1 was also detected by immunohistochemical staining in the developing rat placenta. We also observed CYP4A1 in both HRP-1 and Rcho-1 cells by RT-PCR, suggesting the utility of these cells as in vitro tools to study the effects of xenobiotics on placental fatty acid metabolism. Establishing the expression of CYP4A isoforms in these tissues and cell models provides a framework for further investigation of their functional and physiological significance in guiding proper fetal development.

The peroxynitrite pathway in development: phenytoin and benzo[a]pyrene embryopathies in inducible nitric oxide synthase knockout mice

Nitric oxide generated by nitric oxide synthases (NOSs) can react with reactive oxygen species (ROS), forming peroxynitrite, which may contribute to the ROS-initiated macromolecular damage implicated in the embryopathic effects of both endogenous and drug-enhanced oxidative stress. Inducible NOS (iNOS) is nonconstitutive in most tissues, and its embryonic expression and developmental importance are unknown. Herein, during organogenesis (Gestational Days 9 and 10), wild-type B6129PF2 embryos in culture were highly susceptible to the ROS-initiating teratogens phenytoin and benzo[a]pyrene, whereas iNOS knockout embryos were substantially but not completely protected (p < .05), implicating iNOS in the embryopathic mechanism. However, in contrast to prostaglandin H synthase-catalyzed teratogen bioactivation and ROS formation, which occurs within the embryo, in vivo iNOS expression was limited to placental tissue. These results suggest that the diffusion of nitric oxide from placental progenitor tissue (ectoplacental cone) to embryonic target tissues contributes to the embryopathic effects of ROS-initiating teratogens in embryo culture, which may constitute a mechanism by which embryonic determinants of ROS-mediated teratogenesis can be modulated by maternal extra-embryonic processes.

The development of a higher throughput reactive intermediate screening assay incorporating micro-bore liquid chromatography–micro-electrospray ionization–tandem mass spectrometry and glutathione ethyl ester as an in vitro conjugating agent

An in vitro reactive intermediate screening assay, incorporating the use of the close analog of glutathione, glutathione ethyl ester (GSH-EE) as a conjugating agent, was developed to identify compounds that form reactive intermediates in an in vitro metabolite generating system. The biological assay consisted of substrate [s] = 10 microM, human liver microsomes, an NADPH generating system and glutathione ethyl ester. Conjugates were extracted from the biological matrix using a combination of protein precipitation and a semi-automated 96-well plate solid phase extraction (SPE) procedure. A micro-bore liquid chromatography-micro-electrospray ionization-tandem mass spectrometry (microLC-microESI-MS/MS) method detected glutathione ethyl ester conjugates using selected reaction monitoring (SRM) to simultaneously monitor for multiple MH+ to [MH - 129]+ transitions, where the 129 mass unit (Da) represents the neutral loss of the pyroglutamate moiety from GSH-EE. The multiple MH+ to [MH - 129]+ transitions (SRM mass table) were generated for potential reactive intermediates of each compound. Glutathione (GSH) and GSH-EE conjugate standards were used to evaluate MS detection sensitivity. Based on direct comparison of standard curve data, an approximate 10-fold increase in sensitivity was observed for conjugates containing GSH-EE moiety versus GSH. In vitro experiments were conducted using literature substrates acetaminophen, rosiglitazone, clozapine, diclofenac and either GSH-EE or GSH as a reactive intermediate conjugating agent. An increase in detection sensitivity was observed for each GSH-EE conjugate and in the case of acetaminophen-GSH-EE the peak area increase was approximately 80-fold. Twelve drug compounds, each having known biotransformation mechanisms, were used to further test the detection capabilities of the assay and establish a concordance to literature data. When GSH was used in the assay, conjugates were detected for 4 out of the 12 test compounds (33%). When GSH-EE was used in the assay, conjugates were detected for 10 out of the 12 test compounds (83%).

Assessment of metabolites and AhR and CYP1A1 mRNA expression subsequent to prenatal exposure to inhaled benzo(a)pyrene

Few studies have focused on environmental aerosol contaminant, mechanistically-based, dose-related neurotoxicity with respect to development of the central nervous system. To fill this important data gap and to highlight possible mechanistic pathways, a study was undertaken to determine metabolite concentrations associated with the transplacental disposition of inhaled benzo(a)pyrene (B(a)P) and the resulting effects on the status of aryl hydrocarbon receptor (AhR), and cytochrome P450 1A1 (CYP1A1) mRNA in preweaning F1 generation animals. In this study, laparotomy on GD 8 was performed on timed-pregnant rats followed by dosing via nose-only exposure for 4h a day for 10 days (GD 11-GD 20) to three concentrations of a B(a)P: carbon black aerosol (25, 75 and 100 microg/m(3)). A dose-dependent decrease in birth index was observed in the B(a)P exposed group as compared to the controls (P<0.05). Analysis of cerebrocortical extracts from F1 generation pups revealed a dose-dependent (P<0.05) increase in total B(a)P metabolites. Analysis of cerebrocortical and hippocampal mRNA developmental expression profiles for AhR and CYP1A1 using 18sRNA as the internal standard, revealed that inhaled B(a)P upregulates AhR during the first postnatal month. The present study suggest that prenatal exposure to inhaled B(a)P upregulates hepatic aryl hydrocarbon receptor dependent mechanisms in the F1 generation. Hepatic upregulation of the aryl hydrocarbon receptor may modulate the potential for benzo(a)pyrene toxicity via the activation of cytochrome P450 and the subsequent deposition of lipophillic metabolites to developing central nervous system structures such as cerebral cortex and hippocampus.

Anticonvulsant activity, teratogenicity and pharmacokinetics of novel valproyltaurinamide derivatives in mice

1 The purpose of this study was to synthesize novel valproyltaurine (VTA) derivatives including valproyltaurinamide (VTD), N-methyl-valproyltaurinamide (M-VTD), N,N-dimethyl-valproyltaurinamide (DM-VTD) and N-isopropyl-valproyltaurinamide (I-VTD) and evaluate their structure-pharmacokinetic-pharmacodynamic relationships with respect to anticonvulsant activity and teratogenic potential. However, their hepatotoxic potential could not be evaluated. The metabolism and pharmacokinetics of these derivatives in mice were also studied. 2 VTA lacked anticonvulsant activity, but VTD, DM-VTD and I-VTD possessed anticonvulsant activity in the Frings audiogenic seizure susceptible mice (ED(50) values of 52, 134 and 126 mg kg(-1), respectively). 3 VTA did not have any adverse effect on the reproductive outcome in the Swiss Vancouver/Fnn mice following a single i.p. injection of 600 mg kg(-1) on gestational day (GD) 8.5. VTD (600 mg kg(-1) at GD 8.5) produced an increase in embryolethality, but unlike valproic acid, it did not induce congenital malformations. DM-VTD and I-VTD (600 mg kg(-1) at GD 8.5) produced a significant increase in the incidence of gross malformations. The incidence of birth defects increased when the length of the alkyl substituent or the degree of N-alkylation increased. 4 In mice, N-alkylated VTDs underwent metabolic N-dealkylation to VTD. DM-VTD was first biotransformed to M-VTD and subsequently to VTD. I-VTD's fraction metabolized to VTD was 29%. The observed metabolic pathways suggest that active metabolites may contribute to the anticonvulsant activity of the N-alkylated VTDs and reactive intermediates may be formed during their metabolism. In mice, VTD had five to 10 times lower clearance (CL), and three times longer half-life than I-VTD and DM-VTD, making it a more attractive compound than DM-VTD and I-VTD for further development. VTD's extent of brain penetration was only half that observed for the N-alkylated taurinamides suggesting that it has a higher intrinsic activity that DM-VTD and I-VTD. 5 In conclusion, from this series of compounds, although VTD caused embryolethality, this compound emerged as the most promising new antiepileptic drug, having a preclinical spectrum characterized by the highest anticonvulsant potential, lowest potential for teratogenicity and favorable pharmacokinetics.

Gender differences in microsomal metabolic activation of hepatotoxic clivorine in rat

The gender differences in the in vitro microsomal metabolic activation of hepatotoxic clivorine, a representative naturally occurring hepatotoxic otonecine type pyrrolizidine alkaloid, in Sprague-Dawley rats and their relation to the gender differences in susceptibility to clivorine intoxication were reported in the present study. Clivorine-induced liver damage in the male rat via metabolic activation to form the reactive pyrrolic ester followed by covalent binding to liver tissue constituents has been reported previously by our research group. The present study demonstrated, for the first time, that cytochromes p450 3A1 and 3A2, which are constitutively expressed in the male rat, might play a significant role in the metabolic activation of clivorine in the rat. Thus, in the male rat, the metabolic activation by liver microsomes to form the reactive pyrrolic ester was found as the only direct metabolic pathway of clivorine followed by subsequent formation of the toxic tissue-bound pyrroles leading to hepatotoxicity. In the case of the female rat, a less significant metabolic activation was observed, whereas the formations of two novel nonpyrrolic metabolites were determined as the predominant biotransformations. None of the four cDNA-expressed rat enzymes (cytochrome p450 2C12, 2E1, 3A1, 3A2) tested could catalyze the formation of these two new metabolites. Furthermore, the female rat (LD(50) = 114 +/- 9 mg/kg, i.p.) was found to be significantly less susceptible to clivorine intoxication than the male rat (LD(50) = 91 +/- 3 mg/kg, i.p.). Therefore, the results suggested that a significantly lower metabolic activation due to the lack of cytochrome p450 3A1 and p450 3A2 activities mainly accounted for the smaller susceptibility of the female rat to clivorine intoxication.

Development of an in vitro system detecting pro-embryotoxin

An in vitro system for detection of embryotoxins has been developed by using primary cultures of embryo fibroblasts. Various embryotoxins, including benzo[a]pyrene and thalidomide, have trivial cytotoxicity in embryo fibroblast systems, which is at least in part due to a lack of capacity for metabolic activation. Introduction of steps for microsomal pre-incubation and calcium-precipitation prior to chemical contact resulted in the clear appearance of embryotoxicity toward thalidomide and benzo[a]pyrene. This pre-incubation method will offer advantages for the detection of embryotoxins, which require maternal metabolic activation, and for understanding the mechanisms of their metabolic activations.

Comparative expression of two alpha class glutathione S-transferases in human adult and prenatal liver tissues

The ability of the fetus to detoxify transplacental drugs and chemicals can be a critical determinant of teratogenesis and developmental toxicity. Developmentally regulated expression of alpha class glutathione S-transferases (GSTs) is of particular interest, since these isozymes have high activity toward peroxidative byproducts of oxidative injury that are linked to teratogenesis. The present study was initiated to examine the expression and catalytic activities of alpha class GST isozymes in human prenatal liver. Northern analysis demonstrated the presence of hGSTA1 and/or A2 (hGSTA1/2) and hGSTA4 steady-state mRNAs in second trimester prenatal livers. Western blotting of prenatal liver proteins provided corroborating evidence via detection of an hGSTA1/2-reactive protein in both cytosol and mitochondria and of hGSTA4-4-reactive protein in mitochondria alone. Catalytic studies demonstrated that prenatal liver cytosolic GSTs were active toward 1-chloro-2,4-dinitrobenzene (a general GST reference substrate), delta5-androstene-3,17-dione (relatively specific for hGSTA1-1), and 4-hydroxynonenal, a highly mutagenic alpha,beta-unsaturated aldehyde produced during oxidative damage and a substrate for hGSTA4-4. Total GSH-peroxidase and GST-dependent peroxidase activities were 9- and 18-fold higher, respectively, in adult liver than in prenatal liver. Multiple tissue array analyses demonstrated considerable tissue-specific and developmental variation in GST mRNA expression. In summary, our results demonstrate the presence of two important alpha class GSTs in second trimester human prenatal tissues, and indicate that mitochondrial targeting of GST may represent an important pathway for removal of cytotoxic products in prenatal liver. Furthermore, the relatively inefficient prenatal reduction of hydroperoxides may underlie an increased susceptibility to maternally transferred pro-oxidant drugs and chemicals.

Variability in aflatoxin B(1)-macromolecular binding and relationship to biotransformation enzyme expression in human prenatal and adult liver

Studies of transplacental transfer of aflatoxin B(1) (AFB(1)) suggest that the developing human fetus may be a sensitive target for AFB(1) injury. Because AFB(1) requires metabolic activation to the reactive AFB(1)-8,9-exo-epoxide (AFBO) to exert its carcinogenic effects, ontogenic and interindividual differences in AFB(1) biotransformation enzymes may underlie susceptibility to AFB(1)-induced cell injury. The present study was initiated to compare the rates of in vitro AFB(1)-DNA and AFB(1)-protein adduct formation among a panel of 10 adult and 10 second-trimester prenatal livers and to examine the relationship among AFB(1) metabolizing enzyme expression and AFB(1) binding. Mixtures of cytosolic and microsomal proteins from prenatal and adult livers catalyzed the formation of AFB(1)-DNA and AFB(1)-protein adducts at relatively similar rates, although greater individual variability in AFB(1) adduct formation was observed in adult tissues. Extensive interindividual variation among adult tissues was observed in the expression of the AFB(1) activation enzymes cytochrome P4501A2 (CYP1A2), CYP3A4/5, and lipoxygenase (LO). Prenatal CYP3A7 expression was also highly variable. LO expression was eightfold higher in prenatal liver tissues than adults, whereas the expression of the AFBO detoxification enzyme microsomal epoxide hydrolase was twofold higher in adult liver. The levels of the polymorphic glutathione S-transferase M1 (hGSTM1-1), which may potentially protect against AFBO injury, were higher in the hGSTM1-1-expressing tissues of adults in relation to prenatal livers. In general, there was not a strong relationship among AFB(1)-DNA or AFB(1)-protein adduct formation and expression levels of individual AFB(1) metabolizing enzymes. In summary, despite the presence of marked individual and ontogenic differences in the expression of AFB(1) metabolizing enzymes, human second trimester prenatal liver tissues compared to adults do not exhibit a marked sensitivity to the in vitro formation of macromolecular AFB(1) adducts.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. III: pharmacokinetic and pharmacodynamic considerations

We review pharmacokinetic and pharmacodynamic factors that should be considered in the design and interpretation of developmental neurotoxicity studies. Toxicologic effects on the developing nervous system depend on the delivered dose, exposure duration, and developmental stage at which exposure occurred. Several pharmacokinetic processes (absorption, distribution, metabolism, and excretion) govern chemical disposition within the dam and the nervous system of the offspring. In addition, unique physical features such as the presence or absence of a placental barrier and the gradual development of the blood--brain barrier influence chemical disposition and thus modulate developmental neurotoxicity. Neonatal exposure may depend on maternal pharmacokinetic processes and transfer of the xenobiotic through the milk, although direct exposure may occur through other routes (e.g., inhalation). Measurement of the xenobiotic in milk and evaluation of biomarkers of exposure or effect following exposure can confirm or characterize neonatal exposure. Physiologically based pharmacokinetic and pharmacodynamic models that incorporate these and other determinants can estimate tissue dose and biologic response following in utero or neonatal exposure. These models can characterize dose--response relationships and improve extrapolation of results from animal studies to humans. In addition, pharmacologic data allow an experimenter to determine whether exposure to the test chemical is adequate, whether exposure occurs during critical periods of nervous system development, whether route and duration of exposure are appropriate, and whether developmental neurotoxicity can be differentiated from direct actions of the xenobiotic.

Impact of cellular metabolism on the biological effects of benzo[a]pyrene and related hydrocarbons

Polycyclic aromatic hydrocarbons are ubiquitous contaminants in the environment. Benzo[a]pyrene (BaP), a prototypical member of this class of chemicals, has been extensively studied for its toxic effects in laboratory animals and human populations. BaP toxicity is often mediated by oxidative metabolism to reactive intermediates that interact with macromolecules leading to alterations in target cell structure and function. More recent evidence suggests that disruption of cellular signaling pathways involved in the regulation of growth and differentiation contribute significantly to the toxicity of BaP and its metabolites. This review summarizes recent advances in our understanding of biological mechanisms of BaP toxicity at the molecular level, and the role of metabolic intermediates in carcinogenesis, atherogenesis, and teratogenesis.

Developmental ontogeny of NAD+ kinase in the rat conceptus

The ubiquitous NAD+ kinase (NADK) is the only known enzyme to catalyze formation of NADP+ from NAD+. The capacity to maintain an adequate supply of NADP(H) has important implications for development because of its requirement as a cofactor and electron donor in biosynthesis and detoxication reactions. Modulation of NADK may directly influence NADP(H) concentrations and cell sensitivity to embryotoxicants. Measurable activities of NADK were not detected in gestational day (GD) 10 rat conceptuses. By GD 11, specific activities of 1.8 and 7.0 pmol NADP+/min/microg protein were measured in embryos and visceral yolk sacs (VYSs), respectively. The VYS specific activities decreased thereafter to 0.5 pmol NADP+/min/microg protein by GD 18. Specific activities of NADK in placenta increased from 1.3 pmol NADP+/min/microg protein on GD 11 to 32.7 pmol NADP+/min/microg protein on GD 15. Specific activities in the liver increased from 1.7 pmol NADP+/min/microg protein on GD 15 to 51.1 pmol NADP+/min/microg protein on GD 21. NADK specific activities were also determined in other developmentally relevant tissues such as the heart and the brain. In the heart, NADK activity was at its lowest just before birth while in the brain it peaked at 5.4 pmol NADP+/min/microg protein just prior to birth. In the lung, activity increased from 0.9 pmol NADP+/min/microg protein on GD 17 to 5.9 pmol NADP+/min/microg protein on GD 21. However, activities dropped in the kidney from 2.0 pmol NADP+/min/microg protein on GD 17 to 1.1 pmol NADP+/min/microg protein on GD 21. These results demonstrate dramatic temporal and spatial variations in NADK activity. Tissue variations in NADK activities may reflect alterations in functional needs for cofactors during differentiation and a cooperation between tissues to optimize detoxification capacity. This is particularly important when chemical exposure during pregnancy disrupts pyridine nucleotide redox status and the conceptus must rely on NADK to provide additional NADP(H).

Evaluation of benzo(a)pyrene-induced DNA damage in human endothelial cells using alkaline single cell gel electrophoresis

The alkaline version of the 'comet assay' was used to evaluate DNA damage in human umbilical vein endothelial cells (HUVEC) exposed to 0.1, 1.0, or 10 microM benzo(a)pyrene for 90min. The genotoxicity was monitored in HUVEC pretreated with the Ah-receptor agonist beta-naphthoflavone (BNF), previously shown to induce cytochrome P4501A1 (CYP1A1) activity in these cells, and in vehicle-treated HUVEC with only constitutive levels of this enzyme. Increased DNA damage was observed only in cells that had been exposed to 10 microM benzo(a)pyrene, cells exposed to BNF being subjected to the most extensive damage. The CYP1A/B-inhibitor alpha-naphthoflavone (ANF) reduced the benzo(a)pyrene-induced DNA-damage in the BNF-treated HUVEC to the same level as in the uninduced cells. The fact that benzo(a)pyrene induced DNA damage in vehicle-treated HUVEC suggests that there may be at least one alternative route of bioactivation for benzo(a)pyrene in these cells. Consequently, judging from the present results it seems as if tobacco-related polycyclic aromatic hydrocarbons (PAHs) may disrupt the function of the endothelial lining in blood vessels with low monooxygenase activity. It is proposed that exposure to Ah receptor agonists via, for example, tobacco smoke, may enhance the DNA-damaging effects of smoke-related genotoxic PAHs in human endothelial cells. The role of PAHs in endothelial dysfunction of tobacco smokers should therefore be further studied.

Antioxidant and GSH-related enzyme response to a single teratogenic exposure to the anticonvulsant phenytoin: temporospatial evaluation

BACKGROUND

It has been proposed that the anticonvulsant drug phenytoin (PHT) requires bioactivation to reactive intermediate(s) to achieve its recognized teratogenic potential and that embryonal detoxification power may play a fundamental role in the teratogenic response. On this basis, we sought to investigate the potential effects of a teratogenic exposure to PHT on the activities of antioxidant and GSH-related detoxifying enzymes in gestational murine tissues.

METHODS

Pregnant Swiss mice were injected intraperitoneally with 0 (vehicle) or 65 mg/kg of PHT on gestation day (GD) 12 (plug day = GD 1). Biochemical determinations, including activities of glutathione transferase, glutathione peroxidase, glutathione reductase, glyoxalase I, glyoxalase II, catalase, and superoxide dismutase, were carried out on maternal and embryonic/fetal livers and in placentas on GD 14 and 19.

RESULTS

The major findings of this study show that (1) organogenesis-stage conceptal tissues have detectable levels of all the tested enzymes; (2) most of the embryonic liver and placental enzymes investigated undergo a significant induction within 48 hr (GD 14) after PHT administration; and (3) in the same tissues a down-regulation of enzyme activities is noted near term (GD 19).

CONCLUSIONS

Overall, these findings show that teratogenic exposure to PHT is associated with a modulation of reactive-intermediates-scavenging enzyme activities, and provide further support for role of generation of reactive intermediates in PHT-induced teratogenesis.

Effect of in utero and postnatal exposure to environmental tobacco smoke on the developmental expression of pulmonary cytochrome P450 monooxygenases

Pulmonary cytochrome P450 monooxygenases metabolize xenobiotic chemicals, including those found in environmental tobacco smoke (ETS). Exposure to ETS beginning at birth has been shown to induce the P450 CYP1A1 by seven days of life. The effects of perinatal exposure to ETS of the rat lung on the expression of CYP1A1, 1B1, 2B1, and NADPH cytochrome P450 reductase were measured using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). Timed pregnant dams and their pups were exposed to aged and diluted sidestream cigarette smoke (ADSS) as a surrogate for ETS for four hours/ day from gestational day 5 through postnatal day 21. For all genes analyzed, mRNA could be detected in the fetal lung beginning at gestational day 17 but were not altered by ADSS. In contrast, intraperitoneal injection of dams with beta-naphthoflavone significantly elevated both CYP1A1 and 1B1 at gestational day 21, indicating that these genes are inducible. Continued exposure to ADSS significantly induced CYP1A1 but not other P450 genes as early as one day after birth.. We conclude that (1) ADSS induces pulmonary CYP1A1 in the first day of life; (2) fetal cytochrome P450 genes are not induced by maternal exposure to ADSS; and (3) in the fetal lung, CYP1A1 and 1B1 can be induced by beta-naphthoflavone.

Maternal exposure to benzo[a]pyrene alters development of T lymphocytes in offspring

Childhood cancer has been increasing significantly over the past two decades in the United States, suggesting that environmental exposures may be playing a causative role. One such cause may be maternal smoking during pregnancy. Suspected carcinogens in cigarette smoke and environmental pollution include N-nitrosamines and polycyclic aromatic hydrocarbons, which may be several micrograms per exposure. Previously, we have shown that mouse progeny of mothers exposed to benzo[a]pyrene (B[a]P) during midpregnancy had abnormalities in their humoral and cell-mediated immune response. Immunodeficiency was detectable during gestation, at one week after birth and persisted for 18 months. Tumor incidences in progeny were eight to 10-fold higher than in controls. The present study compared frequencies of CD4+, CD8+, V gamma 2+, and V beta 8+ T cells in progeny following in utero exposure to B[a]P. The significant reduction in newborn CD4+CD8+, CD4+CD8+V beta 8+ thymocytes and CD4+ splenocytes from 1-week-old progeny, suggests that B[a]P induces abnormal changes in developing T cells. These early alterations may lead to postnatal T cell suppression, thus providing a more suitable environment for the growth of tumors later in life. These results suggest that developmental immunosuppression mediated by B[a]P may play a critical role in the relationship between maternal exposures and childhood carcinogenesis.

Free radical intermediates of phenytoin and related teratogens. Prostaglandin H synthase-catalyzed bioactivation, electron paramagnetic resonance spectrometry, and photochemical product analysis

Phenytoin and related xenobiotics can be bioactivated by embryonic prostaglandin H synthase (PHS) to a teratogenic free radical intermediate. The mechanism of free radical formation was evaluated using photolytic oxidation with sodium persulfate and by EPR spectrometry. Characterization of the products by mass spectrometry suggested that phenytoin photolyzes to a nitrogen-centered radical that rapidly undergoes ring opening to form a carbon-centered radical. PHS-1 was incubated with teratogen (phenytoin, mephenytoin, trimethadione, phenobarbital, and major metabolites) or its vehicle and the free radical spin trap alpha-phenyl-N-t-butylnitrone, and incubations were analyzed by EPR spectrometry. There was no alpha-phenyl-N-t-butylnitrone radical adduct in control incubations. For phenytoin, a putative unstable nitrogen-centered radical adduct and a stable carbon-centered radical adduct were detected. Free radical spin adducts also were detected for all other teratogens and metabolites except carbamazepine. The PHS inhibitor eicosatetraynoic acid abolished the free radical EPR signal. Incubation of 2'-deoxyguanosine with phenytoin and PHS-1 resulted in a 5-fold increase in its oxidation to 8-hydroxy-2'-deoxyguanosine. This is the first direct chemical evidence for PHS-catalyzed bioactivation of phenytoin and related teratogens to a free radical intermediate that initiates DNA oxidation, which may constitute a common molecular mechanism of teratologic initiation.

Gestational exposure to chlorpyrifos: apparent protection of the fetus?

Previous studies have shown that, in general, young, postnatal animals are more sensitive than adults to the toxic effects of anticholinesterase (antiChE) pesticides. Paradoxically, often fetal brain cholinesterase (ChE) is less inhibited than maternal brain after gestational exposure to an antiChE, presumably due to placental and fetal detoxification of the antiChE. The present investigation was designed to study selected toxicokinetic and toxicodynamic factors surrounding the toxicity of chlorpyrifos (CPF; [O,O'-diethyl O-3,5,6-trichloro-2-pyridyl] phosphorothionate) in pregnant rats dosed repeatedly or singly during late gestation. Dams were dosed daily (po) with CPF in corn oil (0 or 7 mg/kg) on gestational days (GD) 14 to 18. Animals were euthanized at 2 to 120 h after the last dose and tissues were collected for enzyme analysis. Using this dosing regimen, we found that (1) the time of maximal ChE inhibition was the same (i.e., 5-10 h after dosing) for both maternal and fetal brain, (2) the degree of fetal brain ChE inhibition was 4.7 times less than maternal brain inhibition, and (3) the detoxification potential (i.e., carboxylesterase and chlorpyrifos-oxonase) of the fetal tissues was very low compared to the maternal tissues. A separate group of experiments showed that if pregnant dams received only one oral dose of 7 or 10 mg/kg CPF on GD18, the degree of ChE inhibition in the fetal brain was comparable to the maternal brain ChE inhibition. Taking into consideration the net increase (more than fourfold) in fetal brain ChE activity from GD14 to 18 in control animals, and the fact that maternal brain ChE was inhibited more than fetal brain ChE only in a repeated-dosing regimen, we conclude that the fetus is not genuinely protected from the toxic effects of a given dose of CPF. We propose that fetal brain ChE is simply able to recover more fully between each dose as compared to maternal brain ChE, giving the illusion that the fetal compartment is less affected than the maternal compartment.

Cytochrome-P450-dependent biotransformation of xenobiotics in human and rodent embryonic tissues

Profound species differences and developmental stage differences as well as a lack of solid data prevent broad, sweeping generalizations in terms of statements that can be made concerning the prenatal expression of individual P450 isoforms. It is clear, however, that several of such isoforms are expressed at levels that can be toxicologically significant. At present, the greatest interest appears to be in P450s 1A1, 1B1, 2E1, and 3A7, each of which has been reported to be expressed at toxicologically significant levels or at least at potentially toxicologically significant levels during organogenesis. Reports of the expression of other P450 isoforms at later stages of gestation also have appeared in the recent literature.

Xenobiotic-metabolizing cytochrome P450 enzymes in the human feto-placental unit: role in intrauterine toxicity

Practically all lipid-soluble xenobiotics enter the conceptus through placental transfer. Many xenobiotics, including a number of clinically used drugs, are known to cause unwanted effects in the embryo or fetus, including in utero death, initiation of birth defects, and production of functional abnormalities. It is well established that numerous xenobiotics are not necessarily toxic as such, but are enzymatically transformed in the body to reactive and toxic intermediates. The cytochrome P450 (CYP) enzymes are known to catalyze oxidative metabolism of a vast number of compounds, including many proteratogens, procarcinogens, and promutagens. About 20 xenobiotic-metabolizing CYP forms are known to exist in humans. Most of these forms are most abundant in the liver, but examples of exclusively extrahepatic CYP forms also exist. Unlike rodents, the liver of the human fetus and even embryo possesses relatively well-developed metabolism of xenobiotics. There is experimental evidence for the presence of CYP1A1, CYP1B1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7 in the fetal liver after the embryonic phase (after 8 to 9 weeks of gestation). Significant xenobiotic metabolism occurs also during organogenesis (before 8 weeks of gestation). Also, some fetal extrahepatic tissues, most notably the adrenal, contain substantial levels of CYP enzymes. The full-term human placenta is devoid of many CYP activities present in liver. Placental CYP1A1 is highly inducible by maternal cigarette smoking. Other forms present in full-term placenta include CYP4B1 and CYP19 (steroid aromatase), which also contribute to the oxidation of some xenobiotics. At earlier stages of pregnancy, the placenta may express a wider array of CYP genes, including CYP2C, CYP2D6, and CYP3A7. Due to the small size of the fetus and low abundance of CYPs in placenta, the contribution of feto-placental metabolism to overall gestational pharmacokinetics of drugs is probably minor. In contrast, several toxic outcomes have been ascribed to altered metabolic patterns in the feto-placental unit, including a putative association between reduced placental oxidative capacity and birth defects. Examples of human teratogens that are substrates for CYP enzymes include thalidomide, phenytoin, ethanol, and several hormonal agents. Recent studies have improved our understanding of the expression and regulation of individual CYP genes in the fetus and placenta, and the stage is set for applying this knowledge with more precision to the role of xenobiotic metabolism in abnormal intrauterine development in humans.

Oxidative damage in chemical teratogenesis

The teratogenicity of many xenobiotics is thought to depend at least in part upon their bioactivation by embryonic cytochromes P450, prostaglandin H synthase (PHS) and lipoxygenases (LPOs) to electrophilic and/or free radical reactive intermediates that covalently bind to or oxidize cellular macromolecules such as DNA, protein and lipid, resulting in in utero death or teratogenesis. Using as models the tobacco carcinogens benzo[a]pyrene (B[a]P) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), the anticonvulsant drug phenytoin, structurally related anticonvulsants (e.g. mephenytoin, nirvanol, trimethadione, dimethadione) and the sedative drug thalidomide, we have examined the potential teratologic relevance of free radical-initiated, reactive oxygen species (ROS)-mediated oxidative molecular target damage, genotoxicity (micronucleus formation) and DNA repair in mouse and rabbit models in vivo and in embryo culture, and in vitro using purified enzymes or cultured rat skin fibroblasts. These teratogens were bioactivated by PHS and LPOs to free radical reactive intermediary metabolites, characterized by electron spin resonance spectrometry, that initiated ROS formation, including hydroxyl radicals, which were characterized by salicylate hydroxylation. ROS-initiated oxidation of DNA (8-hydroxy-2'-deoxyguanosine formation), protein (carbonyl formation), glutathione (GSH) and lipid (peroxidation), and embryotoxicity were shown for phenytoin, its major hydroxylated metabolite 5-(p-hydroxyphenyl)-5-phenylhydantoin [HPPH], thalidomide, B[a]P and NNK in vivo and/or in embryo culture, the latter indicating a teratologically critical role for embryonic, as distinct from maternal, processes. DNA oxidation and teratogenicity of phenytoin and thalidomide were reduced by PHS inhibitors. Oxidative macromolecular lesions and teratogenicity also were reduced by the free radical trapping agent phenylbutylnitrone (PBN), and the antioxidants caffeic acid and vitamin E. In embryo culture, addition of superoxide dismutase (SOD) to the medium enhanced embryonic SOD activity, and SOD or catalase blocked the oxidative lesions and embryotoxicity initiated by phenytoin and B[a]P, suggesting a major contribution of ROS, as distinct from covalent binding, to the teratologic mechanism. In in vivo studies, other antioxidative enzymes like GSH peroxidase, GSH reductase and glucose-6-phosphate dehydrogenase (G6PD) were similarly protective. Even untreated G6PD-deficient mice had enhanced embryopathies, indicating a teratological role for endogenous oxidative stress. In cultured fibroblasts, B[a]P, NNK, phenytoin and HPPH initiated DNA oxidation and micronucleus formation, which were inhibited by SOD. Oxidation of DNA may be particularly critical, since transgenic mice with +/- or -/- deficiencies in the p53 tumor suppressor gene, which facilitates DNA repair, are more susceptible to phenytoin and B[a]P teratogenicity. Even p53-deficient mice treated only with normal saline showed enhanced embryopathies, suggesting the teratological importance of endogenous oxidative stress, as observed with G6PD deficiency. These results suggest that oxidative macromolecular damage may play a role in the teratologic mechanism of xenobiotics that are bioactivated to a reactive intermediate, as well in the mechanism of embryopathies occurring in the absence of xenobiotic exposure.

Chemical teratogenesis in humans: biochemical and molecular mechanisms

In this review, an attempt has been made to summarize our current understanding of the mechanisms whereby certain chemicals cause birth defects. The chemicals selected for consideration were those that have been designated as established or recognized human teratogens. It is clear that our current understanding of mechanisms whereby these agents cause teratogenic effects (birth defects) can vary dramatically from one agent to the next. Extremes include the folic acid antagonists, which are now well established as agents that produce birth defects by virtue of potent inhibition of dihydrofolate reductase as a primary biochemical mechanism. An example at the other extreme is ethanol, for which very few definitive statements can be made with regard to teratogenic mechanisms, and the probability exists that a large number of interacting, contributory mechanisms can be invoked. For nearly all chemical teratogens, the critical links in the chains of events between the initial, primary biochemical and molecular mechanistic event (e.g. dihydrofolate reductase inhibition) and the manifestations of specific abnormalities (pathogenic mechanisms) remain to be delineated. This will provide an enormous challenge for investigators for years to come.

Expression of CYP2E1 during embryogenesis and fetogenesis in human cephalic tissues: implications for the fetal alcohol syndrome

Reverse transcription and the polymerase chain reaction (RT-PCR) with oligonucleotide primers designed to target cDNA nucleotides 1241-1357 corresponding to exons 8 (3' end) and 9 (5' end) in human genomic CYP2E1 detected consistently strong signals in 9 of 10 prenatal human brains. Cephalic tissues analyzed were between 54 and 78 days of gestation. RT-PCR signals for expression of CYP2E1 in corresponding human hepatic or adrenal tissues were weaker or, with only 2 exceptions, undetectable. Attempts to approximate levels of P4502E1 mRNA with Northern blots and RNase protection assays indicated that levels in human prenatal whole brain tissues tended to increase as a function of gestational age but, at the early stages investigated, were far lower than the constitutive levels in hepatic tissues of adult humans or male rats. Localized, P4502E1-dependent cephalic bioactivation of ethanol, with associated generation of several reactive chemical species, could contribute significantly to the etiology of neuroembryotoxic effects of prenatal ethanol exposure.

Evidence for embryonic prostaglandin H synthase-catalyzed bioactivation and reactive oxygen species-mediated oxidation of cellular macromolecules in phenytoin and benzo[a]pyrene teratogenesis

A mouse embryo culture model was used to determine whether embryonic prostaglandin H synthase (PHS)-catalyzed bioactivation and resultant oxidative damage to embryonic protein and DNA may constitute a molecular mechanism mediating phenytoin and benzo[a]pyrene teratogenesis. Embryos were explanted from CD-1 mouse dams on gestational day 9.5 (vaginal plug = day 1) and incubated for either 4 h (biochemistry) or 24 h (embryotoxicity) at 37 degrees C in medium containing either phenytoin (20 micrograms/ml, 80 microM), benzo[a]pyrene (10 microM), or their respective vehicles. As previously observed with phenytoin (Mol. Pharmacol.48: 112-120, 1995), embryos incubated with benzo[a]pyrene showed decreases in anterior neuropore closure, turning, yolk sac diameter, and somite development (p < .05). Addition of the antioxidative enzyme superoxide dismutase (SOD) substantially enhanced embryonic SOD activity (p < .05) and completely inhibited benzo[a]pyrene embryotoxicity (p < .05). Substantial PHS was detected in day 9.5 embryos using SDS/PAGE, anti-PHS antibody, and alkaline phosphatase-conjugated donkey anti-goat IgG. Embryonic protein oxidation was detected by the reaction of 0.5 mM 2,4-dinitrophenylhydrazine with protein carbonyl groups. This method was first validated by using a known hydroxyl radical-generating system consisting of vanadyl sulfate and H2O2, with bovine serum albumin or embryonic protein as the target. Embryonic proteins were characterized by SDS/PAGE, anti-dinitrophenyl antisera, and peroxidase-labeled goat anti-donkey IgG. Using enhanced chemiluminescence, the number and content of oxidized protein bands detected between 25 and 200 kDa were substantially increased by both phenytoin and benzo[a]pyrene. Addition of the reducing agent dithiothreitol, or SOD or catalase, decreased protein oxidation in phenytoin-exposed embryos. Both phenytoin (Mol. Pharmacol.48: 112-120, 1995) and benzo[a]pyrene enhanced embryonic DNA oxidation, determined by the formation of 8-hydroxy-2'-deoxyguanosine, as measured by high-performance liquid chromatography (HPLC) (p < .05). Phenytoin also enhanced the oxidation of embryonic glutathione (GSH) to its GSSG disulfide, as measured by HPLC (p < .05). These results provide direct evidence that, in the absence of maternal or placental processes, embryonic PHS-catalyzed bioactivation and reactive oxygen species-mediated oxidation of embryonic protein, thiols, and DNA may constitute a molecular mechanism mediating phenytoin and benzo[a]pyrene teratogenesis.

Biochemical toxicology of chemical teratogenesis

Although exposure during pregnancy to many drugs and environmental chemicals is known to cause in utero death of the embryo of fetus, or initiate birth defects (teratogenesis) in the surviving offspring, surprisingly, little is known about the underlying biochemical and molecular mechanisms, or the determinants of teratological susceptibility, particularly in humans. In vitro and in vivo studies based primarily on rodent models suggest that many potential embryotoxic xenobiotics are actually proteratogens that must be bioactivated by enzymes such as the cytochromes P450 and peroxidases such as prostaglandin H synthase to teratogenic reactive intermediary metabolites. These reactive intermediates generally are electrophiles or free radicals that bind covalently (irreversibly) to, or directly of indirectly oxidize, embryonic cellular macromolecules such as DNA, protein, and lipid, irreversibly altering cellular function. Target oxidation, known as oxidase stress, often appears to be mediated by reactive oxygen species (ROS) such as hydroxyl radicals. The precise nature of the teratologically relevant molecular targets remains to be established, as do the relative conditions of the various types of macromolecular lesions. Teratological suseptibility appears to be determined in part by a balance among pathways of maternal xenobiotic elimination, embryonic xenobiotic bioactivation and detoxification of the xenobiotic reactive intermediate, direct and indirect pathways for the detoxification of ROS (cytoprotection), and repair of macromolecular lesions. Due largely to immature or otherwise compromised embryonic pathways for detoxification, Cytoprotection, and repair, the embryo is relatively susceptible to reactive intermediates, and teratogenesis via this mechanism can occur from exposure to therapeutic concentrations of drugs, or supposedly safe concentrations of environmental chemicals. Greater insight into the mechanisms involved in human reactive intermediate-mediated teratogenicity, and the determinants of individual teratological susceptibility, will be necessary to reduce the unwarranted embryonic attrition from xenobiotic exposure.

Demonstration of benzo(a)pyrene-induced DNA damage in mice by alkaline single cell gel electrophoresis: evidence for strand breaks in liver but not in lymphocytes and bone marrow

Alkaline single cell gel electrophoresis (also known as the 'comet assay') was used to measure DNA strand breaks and alkali-labile sites in peripheral lymphocytes, bone marrow and liver cells of C57BL/6 mice orally exposed to benzo(a)pyrene. Although this polycyclic aromatic hydrocarbon is a well-known genotoxic agent, little is known about to what extent it actually induces DNA strand breaks in peripheral lymphocytes and other tissues after in vivo exposure. Significant and dose-related damage was observed in liver cells after three days of exposure (lowest observed effect level being 3 x 100 mg benzo(a)pyrene/kg b.wt. No such damage could be observed in the lymphocytes and bone marrow cells even after administration of 3 x 150 mg benzo(a)pyrene/kg b.wt. The reference substance cyclophosphamide produced pronounced DNA damage in lymphocytes and bone marrow cells already in a single dose of 100 mg/kg b.wt. The present mouse study questions the usability of DNA strand breaks in peripheral lymphocytes as an indicator of benzo(a)pyrene-induced genotoxicity.

Phenytoin covalent binding and embryopathy in mouse embryos co-cultured with maternal hepatocytes from mouse, rat, and rabbit

The anticonvulsant drug phenytoin is teratogenic in a variety of species including humans. Traditional embryo culture studies have employed the addition of 9000 g supernatant (S-9) or microsomal fractions from induced rat or mouse liver as an exogenous bioactivating system to approximate a maternal contribution. However, cellular fractions, unlike cultured intact hepatocytes, may themselves be embryotoxic, and do not reflect the in vivo balance of bioactivation and detoxification. To evaluate in vitro the known in vivo differential species susceptibility to phenytoin teratogenesis, day 9.5 (day of plug = day 1) mouse embryos either were cultured alone for 24 hr or were co-cultured with hepatocytes from maternal mice, rats or male rabbits, thereby exposing the embryos to the effects of potential species-specific phenytoin metabolism. In the absence of hepatocytes, phenytoin embryotoxicity was concentration dependent (0, 10, 20 and 60 micrograms/mL), with decreases in embryonic growth, reflected by reduced yolk sac diameter and crown rump length, apparent within the maternal therapeutic range (20 micrograms/mL). Covalent binding of the radiolabeled drug to live embryonic tissue was significantly higher than in control embryos previously killed by fixation, suggesting that the embryo can bioactivate phenytoin to a toxic reactive intermediate. Mouse embryos grew equally well with hepatocytes from all three species, indicating interspecies tissue compatibility. The addition of rat and rabbit hepatocytes, but not mouse hepatocytes, significantly enhanced the phenytoin-induced impairment of mouse embryonic development, as demonstrated by reductions in somite number. The phenytoin-induced impairment of mouse embryonic growth was not enhanced by the addition of rat or rabbit hepatocytes, while mouse hepatocytes conferred protection. The covalent binding of phenytoin to extracellular proteins in the culture medium was not enhanced by the addition of mouse hepatocytes. These results suggest that mouse embryos intrinsically can bioactivate phenytoin to a toxic reactive intermediate, with embryopathic consequences. The protection conferred by maternal mouse hepatocytes suggests a species-specific maternal biochemical balance favouring detoxification that is not shared by rat and rabbit hepatocytes, which enhanced phenytoin embryopathy. Thus, while phenytoin teratogenicity likely involves embryonic bioactivation, maternal determinants may contribute variably to teratologic susceptibility in a species-specific manner.

DNA oxidation as a potential molecular mechanism mediating drug-induced birth defects: phenytoin and structurally related teratogens initiate the formation of 8-hydroxy-2'-deoxyguanosine in vitro and in vivo in murine maternal hepatic and embryonic tissues

A considerable number of teratogens, including the anticonvulsant drug phenytoin and structurally related drugs and environmental chemicals, may be bioactivated by peroxidases, such as prostaglandin H synthase (PHS) and lipoxygenases (LPOs), to a reactive free radical intermediate that initiates birth defects. However, the molecular targets of the reactive free radical intermediates mediating chemical teratogenesis, and hence the fundamental determinants of susceptibility, are poorly understood. In these studies, a teratogenic dose of phenytoin (65 mg/kg), when injected into pregnant CD-1 mice during organogenesis on gestational day 12, initiated the oxidation of DNA in maternal hepatic and embryonic nuclei, forming 8-hydroxy-2'-deoxyguanosine. Significant maternal and embryonic DNA oxidation occurred at 6 and 3 h, respectively, suggesting relative embryonic deficiencies in free radical-related cytoprotective enzymes, although the rates appeared similar. Maximal DNA oxidation in both maternal and embryonic tissues occurred at 6 h, presumably reflecting the balance of DNA oxidation and repair, the latter of which appeared similar in both tissues. Inhibition of phenytoin-initiated embryonic DNA oxidation by the free radical spin trapping agent alpha-phenyl-N-t-butylnitrone (41.5 mg/kg), and by acetylsalicylic acid (10 mg/kg), an inhibitor of the cyclooxygenase component of PHS, was consistent with the previously reported reduction by these inhibitors of phenytoin-initiated murine birth defects. In vitro studies using a horseradish peroxidase (0.5 mg/ml)-H2O2 (5.45 micrograms/ml) bioactivating system for drug-initiated oxidation of 2'-deoxyguanosine (3.74 mM), indicated that the potency of xenobiotic-initiated formation of 8-hydroxy-2'-deoxyguanosine for the structurally related drugs and metabolites phenytoin, 5-(p-hydroxyphenyl)-5-phenylhydantoin, trimethadione, dimethadione, l-mephenytoin, l-nirvanol, d-nirvanol (80 microM each), or thalidomide (64 microM), reflected their murine teratogenic potency. Given the relatively low activities of cytochromes P450, compared to PHS and LPOs, in human and rodent embryonic tissues, these data support the potential teratological importance of peroxidase-catalysed bioactivation of xenobiotics with structural similarities to phenytoin. These studies provide the first evidence that peroxidase-catalysed embryonic DNA oxidation may constitute a critical molecular mechanism mediating the teratogenicity of phenytoin and related drugs and environmental chemicals, and suggest the potential teratological importance of additional embryonic processes, such as DNA repair and tumor suppressor genes, as determinants of susceptibility.

Xenobiotic metabolism in humans during early pregnancy: peroxidase-mediated oxidation and bioactivation of 2-aminofluorene

Arylamines such as 2-aminofluorene (2-AF) are known teratogens and transplacental carcinogens in laboratory animal species. Although exposure of women to arylamines is likely to occur during pregnancy, how these chemicals are metabolized by the enzymes from the human conceptual tissues is currently unknown. Highly purified preparations of peroxidase isolated from human intrauterine conceptual tissues at 8 weeks of gestation were used to study in vitro metabolism of 2-AF. The oxidation of 2-AF was examined spectrophotometrically whereas the bioactivation was assessed from the covalent binding to protein and DNA using [3H] 2-AF. Using guaiacol as a model substrate, the purified preparations of peroxidase used exhibited a specific activity of 15-20 micromol/min/mg protein. 2-AF oxidation was found to be enzymatic in nature. Kinetic data obtained under optimal assay conditions yielded a Km = 41 microM for 2-AF, 8.33 microM for H2O2, and a Vmax=1.2 micromol 2-AF oxidized/min/mg protein. Under optimal assay conditions, the covalent binding of reactive intermediates to protein and DNA (nmol equivalent/min/mg enzyme/mg bovine serum albumin or calf thymus DNA) was observed at the rate of about 3.75 +/- 0.39 and 1.90 +/- 0.11 respectively. A significant decline in the rate of both oxidation and bioactivation of 2-AF was observed in the presence of classical peroxidase inhibitors, KCN and NaN3.

Expression of functional cytochrome P4501A1 in human embryonic hepatic tissues during organogenesis

Investigations with chemical inhibitors and with inhibitory antibodies specific for cytochrome P4501A-catalyzed ethoxyresorufin (ethoxyphenoxazone) O-deethylation and 2-acetylaminofluorene (N-2-fluorenylacetamide) ring hydroxylation indicated that cytochrome(s) P450 of the 1A subfamily was functionally expressed in human embryonic hepatic tissues at very early stages (days 50-60) of gestation. Lack of detectable capacity of hepatic microsomal enzymes to catalyze either N-hydroxylation of 2-acetylaminofluorene or O-demethylation of methoxyresorufin indicated that functional cytochrome P4501A2 is expressed minimally or negligibly in human embryonic hepatic tissues. By contrast, profound inhibition of the ring hydroxylation of 2-acetylaminofluorene and of the O-deethylation of ethoxyresorufin by 7,8-benzoflavone as well as by anti-cytochrome P4501A1 antibodies indicated the presence of significant levels of functional cytochrome P4501A1 in hepatic microsomes of human embryos. Using the reverse transcriptase-linked polymerase chain reaction with specific oligonucleotide primers, we also detected significant expression of cytochrome P4501A1 mRNA in human embryonic livers. Polymerase chain reaction amplification, cloning and sequencing of the corresponding cDNA provided evidence that the cytochrome P4501A1 mRNA expressed in human embryonic tissues was identical to that expressed in adult human tissues. The results of the study have important implications in terms of the embryotoxic effects of chemicals that are known to be substrates, inhibitors or inducers of cytochrome P4501A1 and to which pregnant women are exposed.

Time-dependent and tissue-specific variations of glutathione transferase activity during gestation in the mouse

Glutathione transferases (GSTs; EC. 2.1.5.18) activity was measured in maternal liver and conceptal tissues during gestation. In maternal liver, maximum activity was found at gestational day (GD) 9 after which it slowly decreased up to the end of gestation. The placental GSTs activity at GD18 was three times lower than that found at GD14. Conversely, fetal liver GSTs at GD14 was about 75% that at GD18. It was also observed that GSTs activity at GD9 and GD10 was higher in visceral yolk sac than in embryo proper. Substrate specificity measurements, SDS PAGE analysis and HPLC runs, carried out on GSH-affinity purified fractions, revealed that with the progress of gestation in maternal liver an increase in pi class GSTs subunit occurs, with a concomitant decrease in alpha class GSTs. With respect to the time of gestation, a significant change in alpha, mu and pi class GSTs expression also occurred in fetal liver and in chorioallantoic placenta. It was concluded that during gestation the GSTs system is subjected to a time-dependent and tissue-specific modulation which may play a protective role against developmental toxicants.

Peroxidative xenobiotic oxidation by partially purified peroxidase and lipoxygenase from human fetal tissues at 10 weeks of gestation

1. Present study reports the ability of partially purified peroxidase and lipoxygenase from human fetal tissues at 10 weeks of gestation to oxidize selected xenobiotics in vitro. 2. Peroxidase was found to oxidize four different chemicals in the presence of H2O2. Sodium azide and potassium cyanide inhibited peroxidase activity towards guaiacol in a concentration-dependent manner. 3. The dioxygenase and co-oxidase activities of lipoxygenase towards linoleic acid and four model xenobiotics, respectively, were observed. Both the catalytic activities of lipoxygenase were significantly inhibited by < 1.0 microM nordihydroguaiaretic acid. 4. These findings suggest that peroxidase and lipoxygenase may be important pathways for peroxidative xenobiotic oxidation in human fetal tissues.