Attenuation of 2-methoxyethanol and methoxyacetic acid-induced digit malformations in mice by simple physiological compounds: implications for the role of further metabolism of methoxyacetic acid in developmental toxicity

Methoxyacetic acid inhibits histone deacetylase and impairs axial elongation morphogenesis of mouse gastruloids in a retinoic acid signaling-dependent manner

BACKGROUND

Teratogenic potential has been linked to various industrial compounds. Methoxyacetic acid (MAA) is a primary metabolite of the widely used organic solvent and plasticizer, methoxyethanol and dimethoxyethyl phthalate, respectively. Studies using model animals have shown that MAA acts as the proximate teratogen that causes various malformations in developing embryos. Nonetheless, the molecular mechanisms by which MAA exerts its teratogenic effects are not fully understood.

METHODS

Gastruloids of mouse P19C5 pluripotent stem cells, which recapitulate axial elongation morphogenesis of gastrulation-stage embryos, were explored as an in vitro model to investigate the teratogenic action of MAA. Morphometric parameters of gastruloids were measured to evaluate the morphogenetic effect, and transcript levels of various developmental regulator genes were examined to assess the impact on gene expression patterns. The effects of MAA on the level of retinoic acid (RA) signaling and histone deacetylase activity were also measured.

RESULTS

MAA reduced axial elongation of gastruloids at concentrations comparable to the teratogenic plasma level (5 mM) in vivo. MAA at 4 mM significantly altered the expression profiles of developmental regulator genes. In particular, it upregulated the RA signaling target genes. The concomitant suppression of RA signaling using a pharmacological agent alleviated the morphogenetic effect of MAA. MAA at 4 mM also significantly reduced the activity of purified histone deacetylase protein.

CONCLUSIONS

MAA impaired axial elongation morphogenesis in a RA signaling-dependent manner in mouse gastruloids, possibly through the inhibition of histone deacetylase.

Testicular effect of a mixture of 2-methoxyethanol and 2-ethoxyethanol in rats

BACKGROUND

2-Methoxyethanol (ME) and 2-ethoxyethanol (EE) represent a large group of chemicals which are used separately or as mixtures. These compounds exert multidirectional toxic effects. The present studies aimed to demonstrate the effects of ME and EE alone and their mixture on the reproductive organs in the rats.

METHODS

Male Wistar rats were treated subcutaneously with ME and EE alone (1.25-5.0mM/kg/day) or with their mixture (1:1) for 4 weeks. After completion of the experiment, the testes, epididymides, and prostate were weighed. In post-mitochondrial supernatant of the testes, the level of total protein, non-protein and protein sulfhydryl groups, malondialdehyde, total antioxidant status, and glutathione peroxidase and glutathione reductase activities were determined.

RESULTS

Exposure to ME alone resulted in a dose-dependent decrease in the organ weights, the total protein, non-protein and protein sulfhydryl groups. EE alone led to less marked alterations. Co-exposure to ME and EE caused alterations similar as in the rats treated with ME alone.

CONCLUSIONS

Marked testicular atrophy, decrease in epididymis and prostate weights are predominant effects of the repeated exposure to relatively low doses of ME and EE. A decrease in the total protein level, and protein sulfhydryl groups may be responsible for testicular atrophy. A significant depletion of non-protein sulfhydryl groups and occasionally elevated glutathione peroxidase activity may indicate that ME and EE resulted in disturbances of pro-oxidant/antioxidant balance. The study suggests that testicular toxicity in male rats co-exposed to ME and EE is mainly caused by the former compound.

Effects of ethylene glycol monomethyl ether and its metabolite, 2-methoxyacetic acid, on organogenesis stage mouse limbs in vitro

Exposure to ethylene glycol monomethyl ether (EGME), a glycol ether compound found in numerous industrial products, or to its active metabolite, 2-methoxyacetic acid (2-MAA), increases the incidence of developmental defects. Using an in vitro limb bud culture system, we tested the hypothesis that the effects of EGME on limb development are mediated by 2-MAA-induced alterations in acetylation programming. Murine gestation day 12 embryonic forelimbs were exposed to 3, 10, or 30 mM EGME or 2-MAA in culture for 6 days to examine effects on limb morphology; limbs were cultured for 1 to 24 hr to monitor effects on the acetylation of histones (H3K9 and H4K12), a nonhistone protein, p53 (p53K379), and markers for cell cycle arrest (p21) and apoptosis (cleaved caspase-3). EGME had little effect on limb morphology and no significant effects on the acetylation of histones or p53 or on biomarkers for cell cycle arrest or apoptosis. In contrast, 2-MAA exposure resulted in a significant concentration-dependent increase in limb abnormalities. 2-MAA induced the hyperacetylation of histones H3K9Ac and H4K12Ac at all concentrations tested (3, 10, and 30 mM). Exposure to 10 or 30 mM 2-MAA significantly increased acetylation of p53 at K379, p21 expression, and caspase-3 cleavage. Thus, 2-MAA, the proximate metabolite of EGME, disrupts limb development in vitro, modifies acetylation programming, and induces biomarkers of cell cycle arrest and apoptosis

Evaluation of an alternative in vitro test battery for detecting reproductive toxicants

The application of alternative methods in developmental and reproductive toxicology is challenging in view of the complexity of mechanisms involved. A battery of complementary test systems may provide a better prediction of developmental and reproductive toxicity than single assays. We tested twelve compounds with varying mechanisms of toxic action in an assay battery including 24 CALUX transcriptional activation assays, mouse cardiac embryonic stem cell test, ReProGlo assay, zebrafish embryotoxicity assay, and two CYP17 and two CYP19 activity assays. The battery correctly detected 11/12 compounds tested, with one false negative occurring, which could be explained by the absence of the specific mechanism of action of this compound in the battery. Toxicokinetic modeling revealed that toxic concentrations were in the range expected from in vivo reproductive toxicity data. This study illustrates added value of combining assays that contain complementary biological processes and mechanisms, increasing predictive value of the battery over individual assays.

The mechanism of ethylene glycol ether reproductive and developmental toxicity and evidence for adverse effects in humans

Numerous experimental studies have established that only a few among the large family of ethylene glycol ethers (EGEs) elicit toxicity on reproduction in either gender. Notable are the monomethyl (EGME) and monoethyl (EGEE) ethers and their respective acetate esters whose production volumes have dramatically declined. Oxidation to the respective monoalkoxy acids is a prerequisite for toxicity. The most potent EGE reproductive toxicant is EGME (via 2-methoxyacetic acid; MAA), which elicits developmental phase-specific insults on either conceptus or on testes. Toxicity at either target site is markedly attenuated by simple physiological compounds such as acetate, formate, glycine, D-glucose and serine. Lack of solid EGME occupational exposure data and the need to improve the scientific foundations for animal data extrapolations, prompted the development of physiologically based pharmacokinetic (PBPK) models for pregnancy application. Interspecies (mouse-rat) and different exposure routes (including inhalation) were experimentally validated. Such PBPK models were then extrapolated to potential occupational exposures, using rather limited human MAA pharmacokinetic data. PBPK model predictions of human blood levels upon simulated inhalation exposure to the 5 ppm threshold limit value (TLV) for 8 h were approximately 60 microM were well below those causing adverse effects in pregnant mice or rats. This conclusion concurs with the lack of objective analytical chemistry data for EGME/MAA in occupational settings, regardless of the potential route of exposure. There are no exposure data that can be linked in a cause-and-effect association to adverse human reproductive outcomes.

Toxicity review of ethylene glycol monomethyl ether and its acetate ester

Ethylene glycol monomethyl ether (EGME) and its acetate ester (EGMEA) are highly flammable, colorless, moderately volatile liquids with very good solubility properties. They are used in paints, lacquers, stains, inks and surface coatings, silk-screen printing, photographic and photo lithographic processes, for example, in the semiconductor industry, textile and leather finishing, production of food-contact plastics, and as an antiicing additive in hydraulic fluids and jet fuel. EGME and EGMEA are efficiently absorbed by inhalation as well as via dermal penetration. Dermal absorption may contribute substantially to the total uptake following skin contact with liquids or vapours containing EGME or EGMEA. EGMEA is rapidly converted to EGME in the body and the two substances are equally toxic in animals. Therefore, the two substances should be considered as equally hazardous to man. Effects on peripheral blood, testes, and sperm have been reported at occupational exposure levels ranging between 0.4 and 10 ppm EGME in air, and with additional, possibly substantial, dermal exposure. Severe malformations and disturbed hematopoiesis have been linked with exposure to EGME and EGMEA at unknown, probably high, levels. Embryonic deaths in monkeys and impaired spermatogenesis in rabbits have been reported after daily oral doses of 12 and 25 mg per kg body weight, respectively. In several studies, increased frequency of spontaneous abortions, disturbed menstrual cycle, and subfertility have been demonstrated in women working in the semiconductor industry. The contribution of EGME in relation to other exposure factors in the semiconductor industry is unclear.

Development of a physiologically based pharmacokinetic model of 2-methoxyethanol and 2-methoxyacetic acid disposition in pregnant rats

An accurate description of developing embryos' exposure to a xenobiotic is a desirable component of mechanism-based risk assessments for humans exposed to potential developmental toxicants during pregnancy. 2-Methoxyethanol (2-ME), a solvent used in the manufacture of semiconductors, is embryotoxic and teratogenic in all species tested including nonhuman primates. 2-Methoxyacetic acid (2-MAA) is the primary metabolite of 2-ME and the proximate embryotoxic agent. The objective of the work described here was to adapt an existing physiologically based pharmacokinetic (PBPK) model for 2-ME and 2-MAA kinetics during midorganogenesis in mice to rats on gestation days (GD) 13 and 15. Blood and tissue data were analyzed using the extrapolated PBPK model that was modified to simulate 2-ME and 2-MAA kinetics in maternal plasma and total embryo tissues in pregnant rats. The original mouse model was simplified by combining the embryos and placenta with the richly perfused tissue compartment. The model includes a description of the growth of the developing embryo and changes in the physiology of the dam during pregnancy. Biotransformation pathways of 2-ME to either ethylene glycol (EG) or to 2-MAA were described as first-order processes based on the data collected from rats by Green et al., (Occup. Hyg. 2, 67-75, 1996). Tissue partition coefficients (PCs) for 2-ME and 2-MAA were determined for a variety of maternal tissues and the embryos. Model simulations closely reflected the biological measurement of 2-ME and 2-MAA concentrations in blood and embryo tissue following gavage or iv administration of 2-ME or 2-MAA. The PBPK model for rats as described here is well suited for extrapolation to pregnant women and for assessment of 2-MAA dosimetry under various conditions of possible human exposure to 2-ME.

Effects of combinations of methanol and formic acid on rat embryos in culture

Acute human exposure to methanol (MeOH) results in elevated serum concentrations of both MeOH and formic acid. In order to better assess the risk of adverse developmental effects of MeOH exposure in humans, the effects of the combination of formate and MeOH, in addition to the individual toxicity profiles for MeOH and formate, need to be established. Gestational day 9 rat embryos were exposed to various concentrations of MeOH and formate in whole embryo culture (WEC) for 48 hr and the degree of embryotoxicity was evaluated using developmental score (DEVSC) as the parameter of comparison across exposure combinations. After establishing embryo toxicity of the individual compounds in previous studies, concentrations of MeOH and formate were chosen which would produce similar DEVSCs, and isoboles were plotted joining the equivalently toxic doses. These mixtures would be expected to have similar toxicity to the MeOH or formate concentrations according to a dose-addition model. The responses of embryos to the selected concentrations along each isobole were measured and tested for linearity to determine the nature of any interaction between the two agents. The concentrations of MeOH and formate used separately and in combination ranged from 0 to 8.75 mg/ml MeOH and 0 to 1.51 mg/ml formate. Increasing concentrations of either MeOH or formate resulted in significant decreases in DEVSC. Exposure to combinations of MeOH and formate had less effect on DEVSC than would be expected based on simple toxicity additivity. This observation was also true for embryonic crown-rump length, head length, and somite number. These results suggest that the embryotoxicity observed following low level exposure to MeOH is mechanistically different from that observed following exposure to formate.

Developmental toxicity of diglyme by inhalation in the rat

Diglyme (Diethylene glycol dimethyl ether, CAS No. 111-96-6) is a glycol ether which has been used in solvent formulations. To assess the potential developmental toxicity of this chemical, groups of pregnant Crl:CD BR rats were exposed to either 0 (control, room air only), 25, 100, or 400 ppm diglyme by inhalation for 6 hrs/day for Days 7 through 16 or gestation (day on which the copulation plug was detected was designation Day 1 G). All female rats were euthanized on day 21G and the fetuses were examined. An additional group of rats was treated with 25 ppm 2-methoxethanol (2ME) to serve as a positive control and for comparison of relative potencies. Maternal toxicity evident as depressed feed consumption at 400 ppm and increased liver weights at 100 ppm. There were no dams in the 400 ppm group with live fetuses (all litters consisted on resorbed conceptuses). Embryo viability was unaffected by concentrations of diglyme as high as 100 ppm. 2ME produced increased liver weights and depressed feed consumption at 25 ppm. Embryo-fetal toxicity was evident as a concentration-related decrease in fetal weight at diglyme concentrations as high as 100 ppm (and with 2ME). There were no fetuses derived from the 400 ppm diglyme-treated dams. A low incidence of structural malformations was observed in all diglyme groups (as well as with 2ME). The incidence of variations, (primarily delayed skeletal ossification and rudimentary ribs) was increased in the 25 and 100 ppm diglyme groups. The incidence and severity in the diglyme and 2ME groups exposed to 25 ppm was essentially the same suggesting similar potency for producing structural variations. In this study, diglyme was embryolethal at 400 ppm; a level that otherwise was only marginally toxic to the dam. Maternal and fetal toxicity also were demonstrated at 100 ppm. Although the fetal defects detected following diglyme exposure at 25 ppm were not significantly different from control values (with the exception of the incidence of skeletal developmental variations), the pattern, type, and incidence of variations were similar to those seen at 100 ppm, suggesting that 25 ppm was an effect level that approaches the lower end of the developmental toxicity response curve. Therefore, the no-observable-effect level (NOEL) for diglyme exposure in the dam is 25 ppm and a NOEL was not clearly demonstrated for the conceptus.

Serine-enhanced restoration of 2-methoxyethanol-induced dysmorphogenesis in the rat embryo and near-term fetus

Effects of serine on restorative growth were characterized by comparing embryo/fetal responses after maternal exposure to 2-methoxyethanol (ME) and ME + serine by gavage on gestation day (gd) 13, a day of heightened limb sensitivity. Paws (gd 20) and limb buds (gd 15) were examined after ME alone at 50, 100, and 250 mg/kg, and after ME (either 100 or 250 mg ME/kg) + serine (1734 mg serine/kg) administered within minutes (0 hr) to 24 hr after ME. Paw development was not altered after ME at 100 mg/kg, but was highly sensitive to 250 mg ME/kg with all fetuses and litters exhibiting defects (ectrodactyly, syndactyly, and short digit) in the preaxial region. In contrast, the limb bud displayed dose-related incidences of abnormalities after maternal treatment with the low and high levels of ME. The condensing (precartilaginous, pentadactyl pattern) and noncondensing (undifferentiated mesenchymal cells) regions exhibited changes in their size, number, and location. Serine administration after 250 mg ME/kg was effective in reducing the occurrence of paw dysmorphogenesis with its protection potency inversely related to its delay of administration (i.e., 0% paw defect incidence after 0-hr delay, 25% after 4-hr delay, 41-45% after 8- and 12-hr delays, and 76% after 24-hr delay). The occurrences of limb bud pattern disturbances produced by ME were also markedly decreased by serine cotreatment. Higher incidences of embryonic defects versus those of fetal defects demonstrate that restorative growth followed ME exposure. Serine attenuation of ME teratogenicity appears to emanate from enhanced restorative growth so that tissue damage, which otherwise would be expressed as a defect at parturition, is repaired and replaced to resume development of the limb toward its normal structure.

Multiple chemical exposures: synergism vs. individual exposure levels

Exposure to single chemicals is known to produce congenital malformations in both pregnant animals and humans exposed at sufficiently high intensity. However, real life involves multiple, simultaneous exposures. Using as a database the 43 multiple chemical exposure studies located by Nelson (Teratology 49:33-71; 1994) where synergism was reported, we explored the degree to which such concerns may be realistic from the viewpoint of the current standard developmental toxicity safety evaluation process. Focusing on the assessment of the lowest tested dose of a given agent participating in synergistic activity as compared to its threshold level for eliciting toxicity when administered alone, we found that while the availability of adequate data was limited, all cases, with the possible exception of one, demonstrated synergistic toxic expression only when at least one, and usually both, compounds were used at or above their individual threshold for toxicity. These findings suggest that in animals such phenomena of synergistic chemical interactions are likely to occur only when at least one and more likely both agents are administered at or above their individual threshold for toxicity. To the extent animal studies are predictive of human developmental hazards due to single chemical exposures, available data do not establish multiple chemical exposures as a major human developmental concern.

The theoretical and empirical case for in vitro developmental toxicity screens, and potential applications

In vitro assays for the screening of developmental toxicity potential have been under development for approximately 15 years. During that period, we have learned that assays consisting of primary cultures of embryonic tissues or cells, intact embryos in culture, or free-living embryos are capable of distinguishing between mammalian developmental toxicants and nondevelopmental toxicants with an accuracy of > or = 80%. Despite this level of performance, there is still considerable reluctance among the scientific community to employ these assays for preliminary screening. In this paper, I review the theoretical basis for the predictiveness of these assays, outline the empirical data indicating their utility in screening toxicants, discuss the major limitations of in vitro assays and how they can be managed, and suggest applications for in vitro pre-screens. The embryo-derived assays should work because they continue to develop in vitro, and the underlying cellular and molecular processes driving this development are the same as those in the mammalian embryo in situ, and therefore, susceptible to the same insults. The assays do work, as specific mechanisms of developmental toxicity have been demonstrated in vitro, and because extensive validation studies have shown them to be highly concordant with traditional in vivo screens. The assays are inherently limited by the fact that they do not include all the levels of complexity of the maternal-embryonic unit; however, these limitations can be minimized by thoughtful assay selection, study design, and interpretation. Potential applications are suggested that complement but do not replace in vivo testing. Pre-screens will make product development more efficient and add to our knowledge about the developmental toxicity of previously untested compounds. In vivo screening would still be conducted on all classes of substances that are currently tested for developmental toxicity; however, fewer chemicals with high likelihood of being developmentally toxic, and therefore not appropriate for further commercial consideration, would be evaluated in these costly screens.

Repeated high dose oral exposure or continuous subcutaneous infusion of 2-methoxyacetic acid does not suppress humoral immunity in the mouse

2-Methoxyethanol (ME) has been shown to be immunosuppressive in rats but not mice, with oxidation of ME to 2-methoxyacetic acid (MAA) being a prerequisite for immunosuppression. MAA is more rapidly cleared by mice than rats, consequently this study was designed to determine if increasing the bioavailability of MAA in mice might play a role in this species difference. Female B6C3F1 mice were given MAA by oral multiple daily high doses or by continuous subcutaneous infusion via mini-osmotic pumps. Humoral immunity was evaluated in MAA-exposed mice using the plaque-forming cell (PFC) response to either sheep red blood cells (SRBC) or trinitrophenyl-lipopolysaccharide (TNP-LPS). Female F344 rats were also used to compare the effects of multiple daily MAA exposure on these humoral immune responses. Rats and mice were dosed orally twice a day for 4 days by gavage with MAA at dosages ranging from 40-320 mg/kg/day and 240-1920 mg/kg/day, respectively. All animals were immunized on the first day of dosing and body and lymphoid organ weights and PFC responses to SRBC or TNP-LPS were evaluated 4 days later. While body weights in rats were unaffected, thymus weights were reduced at all dosages of MAA and spleen weights were reduced at 160 or 320 mg/kg/day. PFC responses to SRBC and TNP-LPS were suppressed in rats at dosages of 160 and 320 mg/kg/day. In contrast, thymus weights of mice were reduced only at 960 mg/kg/day or greater, with no effect on spleen or body weights. Furthermore, neither the PFC response to SRBC nor the response to TNP-LPS was suppressed in mice exposed to any oral dosage of MAA. In the continuous infusion study, mice were subcutaneously implanted with mini-osmotic pumps containing MAA which was delivered at 840 mg/kg/day over a 7-day period. Continuous exposure to MAA via mini-osmotic pumps did not suppress the PFC response to either SRBC or TNP-LPS, but rather significantly enhanced the response to TNP-LPS. These results indicate that mice are insensitive to MAA even at the high dosages given as a bolus or continuously over 1 week. The data further support earlier work, which suggested that the observed difference between rats and mice for MAA-induced immunosuppression appears to be unrelated to the availability of MAA to target lymphoid tissue in these rodent species.

Physiologically based pharmacokinetics of methoxyacetic acid: dose-effect considerations in C57BL/6 mice

Methoxyacetic acid (MAA), a weak acid with a pKa of 3.57, was used to test the broad hypothesis that distribution of weak acids in maternal and fetal tissues is determined principally by the pKa of the acid and the pH values of tissue and fluid compartments and to examine tissue dose-teratogenesis relationships, as well as administered dose-teratogenesis relationships. Five related experimental studies were conducted in pregnant C57BL/6CrIBR mice: a conventional dose-response study of developmental toxicity and transplacental pharmacokinetics in mice, a second dose-response study in which reproductive outcomes in litters from individual dams were related to individual pharmacokinetic behavior, a protein-binding experiment, an embryo tissue localization study, and determination of pH in maternal and embryonic compartments after exposure to MAA. MAA was administered intraperitoneally at 9:00 a.m. on day 10 of gestation, at doses ranging from 88 to 164 mg/kg. Localization within the forelimb bud of the embryo, an MAA target site, was determined by computerized image analysis of the distribution of radiolabeled MAA. The kinetic predictions of a physiologically based model incorporating tissue pH values and MAA pKa agreed well with observed concentrations at the lowest dose. However, at intermediate and higher doses, concentrations in both maternal and embryonic tissues were consistently underestimated. MAA was bound neither to maternal plasma proteins nor to embryonic proteins. Intermediate and higher doses of MAA caused dose-dependent transient depressions in tissue pH, but these were not of sufficient duration to bring predicted tissue concentrations into congruence with the concentrations observed. Distribution of MAA within the forelimb bud was broadly consistent with the pH hypothesis, but MAA concentration was not increased in the distal postaxial sector that is the site of the precursor cells of the missing digits. Internal exposure to MAA, defined as the area under the maternal plasma or embryo concentration curve (AUC), was not proportional to administered dose, but AUC-response relationships generated by the group and individual dose-response studies were comparable. While AUC may be a useful measure of effective MAA dose, it cannot be accurately predicted at teratogenic doses of this agent by the model as it is presently structured.

Use of mechanistic information in risk assessment for toxic chemicals

Risk assessment (RA) for toxic chemicals is assumed to be a scientific activity providing a framework of principles for the complication and evaluation of all available scientific information and the rational extrapolation to human health effects in as quantitative terms as possible and with a high degree of certainty. Sensible public health decisions are made more certain through the use of mechanistic information throughout the 4 steps in RA: hazard identification, dose-response assessment, exposure (dose) assessment and risk characterisation. Examples of the use of mechanistic information to assess risks of systemic, developmental/reproductive and neurotoxic effects show how to move away from the presently used threshold/no observable adverse effect/uncertainty factor default methodology towards an evaluation based on all available scientific data. The experience gained in cancer RA in the use of metabolic and tissue binding (receptor) models as well as physiologically based pharmacokinetic (PBPK) and pharmacodynamic (PBPD) models can be transferred to non-cancer RA. A good example is the use of a PBPK model for the hepatoxicity of chloroform. As in cancer RA, as default positions are replaced by biological data the risk assessments become less uncertain when extrapolating between species. Combining information on tissue dosimetry and response data can also provide an estimate of variability within populations, which is impossible with present default type methodology but essential for adequate risk characterisation. Unlike the cancer field there is no single hypothesis for the mechanism of action for the multitude of non-cancer end-points studied.

Species and strain comparisons of immunosuppression by 2-methoxyethanol and 2-methoxyacetic acid

2-Methoxyethanol (ME) and its principal metabolite 2-methoxyacetic acid (MAA) have been shown in our laboratory to be immunosuppressive in male Fischer 344 rats. In this study several strains of 12-week-old female rats and mice were used to compare the immunosuppressive activity of equimolar concentrations of ME and MAA on the trinitrophenyl-lipopolysaccharide (TNP-LPS) antibody plaque-forming cell (PFC) response, which we previously demonstrated to be a sensitive end point. Female inbred Lewis, Fischer 344 and Wistar/Furth, and outbred Sprague-Dawley rats were dosed by gavage with either ME or MAA at dosages of 0.33 to 2.64 mmol/kg/day for 10 consecutive days. Female inbred C3H and C57BL/6J, hybrid B6C3F1, and outbred CD-1 mice were similarly dosed with equimolar dosages of 0.66 to 5.28 mmol/kg/day ME or MAA. All animals were immunized on day 9 of dosing and PFC responses evaluated 3 days later. Suppression of the PFC response was observed in all strains of rats at 2.64 mmol/kg/day ME or MAA. Lewis and Wistar/Furth rats were found to be the most sensitive strains with suppression at levels as low as 0.66 mmol/kg/day ME or MAA. While ME and MAA dosing resulted in suppression of the TNP PFC response in all the rat strains tested, such treatment did not suppress this PFC response in any of the mouse strains examined. These results indicate that under the conditions of this study rats, but not mice, are immunosuppressed by ME and MAA exposure, and that the susceptibility to immunosuppression differs among rat strains.

Interactions in developmental toxicology: a literature review and terminology proposal

Developmental toxicologists have investigated the interactive effects from concurrent exposures to a variety of chemical and physical agents, including therapeutic drugs, industrial agents, and some biological organisms or their toxins. Of approximately 160 reports of concurrent exposures reviewed in this paper, about one third report no interactive effects (including additive effects--usually referring to response--as opposed to dose-additivity); another one third report antagonistic effects, and the final third report potentiative or synergistic effects. The quality of the studies is highly variable. Frequently, only small numbers of animals were included, and very few dose levels were evaluated. Maternal toxicity was rarely discussed. Time-effect relationships were examined infrequently. In addition, these studies are also inconsistent in the use of terms to describe interactive effects, and more than 90% of the terms were not in harmony with currently accepted definitions in toxicology. Because interaction studies will continue to be important in the future, this paper proposes uniform usage of terms for additivity and interactions in developmental toxicology: additivity (the combined effect of two or more developmental toxicants approximates the sum of the effects of the agents administered separately); antagonism (the combined effect of two or more agents, one or more of which are present at doses that would be developmentally toxic if given individually, is significantly less than the sum of the effects of the agents administered separately); potentiation (the increased effect of a developmental toxicant by concurrent action of another agent at a dose that is not developmentally toxic); synergism (the combined effect of two or more developmental toxicants is significantly greater than the sum of the effects of each agent administered alone); and, interaction if more precise terminology does not apply.

Differences between rats and mice in the immunosuppressive activity of 2-methoxyethanol and 2-methoxyacetic acid

Previous studies from this laboratory have demonstrated that 2-methoxyethanol (ME) and its principal metabolite 2-methoxyacetic acid (MAA) are immunosuppressive in young adult male Fischer 344 rats. In the present study, the immunosuppressive potential of ME and MAA was evaluated in young adult female Fischer 344 rats and C57BL/6J mice. Rats and mice were dosed by gavage with either ME or MAA in water, at dosages ranging from 50-400 mg/kg/day, for 10 consecutive days. Rats and mice were examined for alterations in body, spleen and thymus weights and mitogen-induced proliferation of splenic lymphocytes in vitro; separate groups were employed for the antibody plaque-forming cell (PFC) response to trinitrophenyl-lipopolysaccharide (TNP-LPS). Rats dosed at 100-400 mg/kg/day ME and rats dosed at 50-400 mg/kg/day MAA had decreased thymus weights in the absence of decreased body or spleen weights. Lymphoproliferative (LP) responses to concanavalin A (Con A), phytohemagglutinin (PHA), pokeweed mitogen (PWM) and Salmonella typhimurium mitogen (STM) were all reduced in rats treated with all dosages of ME. Rats treated with MAA displayed similar reductions in these LP responses except that the responses to PWM and STM in rats dosed at 50 mg/kg/day were not reduced. In contrast to the effects of ME and MAA on these end points in the rat, no thymic involution or suppression of LP responses were observed in mice dosed at 50-400 mg/kg/day. The PFC response to TNP-LPS was suppressed in rats dosed with either ME or MAA at dosages of 100-400 mg/kg/day. ME and MAA, however, failed to suppress the PFC response in mice immunized with TNP-LPS. These results indicate that unlike Fischer 344 rats, C57BL/6J mice are insensitive to the immunosuppressive effects of ME and MAA at the dosages employed in this study. Whether the different sensitivities of these two rodent species to ME- and MAA-induced immunosuppression are due to immunologic, pharmacokinetic or metabolic differences within each species remains to be determined.

In vitro approaches to the elucidation of mechanisms of chemical teratogenesis

This article describes some of the contributions that in vitro methods have made to our progress, albeit slow, toward understanding mechanisms of chemical teratogenesis. Emphasis is given to the painstaking and time consuming nature of approaches required to elucidate mechanisms. The examples considered are cyclophosphamide, 2-methoxyethanol, and retinoids. Some of the newer methods that take advantage of the recent advances in molecular biology and analytical chemistry have already been applied to studies on teratogenic mechanisms. Prospects for the 1990s are excellent and promise more rapid progress than during the past decade toward unraveling the mysteries of normal developmental biology. That knowledge in turn should be immediately applicable for investigations on developmental toxicant-induced abnormal development.

2-Methoxyacetic acid dosimetry-teratogenicity relationships in CD-1 mice exposed to 2-methoxyethanol

The teratogen 2-methoxyethanol (2-ME), an industrial solvent, was administered to pregnant CD-1 mice either as a single subcutaneous (sc) bolus dose (100-250 mg/kg) or via constant-rate infusion from sc implanted osmotic minipumps (34.7 or 69.4 mg/kg/hr for up to 12 hr) on gestation Day 11, when embryonic paw development is maximally sensitive to perturbation by this agent. The sc entry route most closely reflects likely human exposures via dermal penetration, while bolus and constant-rate infusion administrations were contrasted to mimic potential occupational exposure scenarios. The pharmacokinetic profiles of 2-methoxyacetic acid (2-MAA), the proximate toxic metabolite of 2-ME, were quantitated, generating peak concentration (Cmax) and total 2-MAA exposure values (24-hr area under the concentration-time curve; AUC) in the maternal plasma, extraembryonic fluid, and embryo. The total 2-ME dose (mg/kg) required to achieve similar 2-MAA levels (Cmax or AUC) in these compartments was 2- to 3-fold higher by constant-rate infusion than by bolus injection; therefore, no simple association existed between 2-MAA levels and the total 2-ME dose, when the dose rate was not considered. Similarly, there was no good correlation between the combined total 2-ME doses and the fetal malformation rate, although clear dose-response patterns for paw malformations were observed in litters and fetuses for each individual dosing regimen. However, the combined 2-MAA pharmacokinetic data from each of the dosing regimens demonstrated that during the phase of maximum susceptibility of paw morphogenesis to disruption by 2-MAA (from gd 11 to gd 11.5), a strong linear correlation existed between fetal malformation incidence and 2-MAA AUC levels in either maternal plasma or embryonic compartments (linear correlation coefficient, r2 0.91-0.92). The correlation with Cmax was less favorable (r2 0.74-0.81) over the dose range studied. In a further experiment designed to investigate the importance of AUC vs Cmax regarding 2-ME teratogenicity, infusion of 2-ME (34.7 mg/kg/hr for 8 hr) beginning 2.5 hr after bolus loading (175 mg/kg) provided an increased 24-hr 2-MAA AUC without increased Cmax. This resulted in greater than 70% of the fetuses having various digit malformations (micro-, syn-, ectro-, and polydactyly), compared to only 32-35% of fetuses with mostly stunted digits when either dose was applied singularly. These data support total 2-MAA exposure (AUC levels), rather than peak 2-MAA concentrations, as the principle determinant of teratogenesis following exposure to 2-ME.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Methoxyethanol metabolism in pregnant CD-1 mice and embryos

Upon oxidation to 2-methoxyacetic acid (2-MAA), 2-methoxyethanol (2-ME) causes malformations in all animal species that have been examined. Commonly, 2-MAA is thought to be the proximate toxicant. However, our previous studies with [1,2-14C]2-ME and the present data obtained with [1-14C]2-MAA, [2-14C]2-ME and [methoxy-14C]2-ME revealed that metabolism beyond 2-MAA occurs. Regardless of the 14C position, dams exhaled approximately 5% of the radioactivity administered as a single teratogenic oral dose (3.3 mmol/kg on Gestation Day [gd] 11) as 14CO2. With all isotopic variants urine contained 70-80% of the dose within 24 hr after administration and 13-18% in the next 24 hr. Three labeled products were resolved using HPLC: an unidentified Peak A (12-18% of dose), 2-MAA (approximately 50%), and the glycine conjugate of 2-MAA (approximately 25%). Short-term (4 hr) whole embryo culture on gd 11 with 3 mM 2-MAA and a tracer dose of [1-14C]2-MAA, [2-14C]2-MAA, or [methoxy-14C]2-MAA showed that 14CO2 evolved from the former two substrates, while there was none detectable from the latter. The data indicate that dams metabolized [methoxy-14]2-MAA to 14CO2, while embryos apparently did not. The production of labeled CO2 from [2-14C]2-ME suggests that 2-methoxyacetyl approximately CoA (the precursor for amino acid conjugation with glycine) entered into the tricarboxylic acid (TCA) cycle. This interpretation is supported by the inhibition of 14CO2 evolution elicited by fluoroacetate (0.1 or 1.0 mM) and sodium acetate (5 mM). It is not yet clear whether entry of 2-methoxyacetyl approximately CoA as a "false substrate" in the TCA cycle is of significance for the embryotoxic effects of 2-ME/2MAA.

Protection against 2-methoxyethanol-induced teratogenesis by serine enantiomers: studies of potential alteration of 2-methoxyethanol pharmacokinetics

Several simple physiological compounds attenuate the teratogenic effects of 2-methoxyethanol (2-ME) when coadministered with 2-ME to mice. The mechanism of this protective action, however, has not been elucidated. Alteration of the kinetics of 2-ME and its oxidation product 2-methoxyacetic acid (2-MAA), the putative ultimate toxicant, was considered. D-Serine, the most efficacious attenuator, and L-serine (both 16.5 mmol/kg po) were examined for their abilities to mitigate 2-ME teratogenicity and to alter the disposition of an oral or sc bolus dose of 2-ME (3.3 mmol/kg containing 6 microCi 2-[methoxy-14C]ethanol) given to CD-1 mice on Gestation Day 11. L-Serine reduced the incidence of malformed fetuses from greater than or equal to 72% to 26-28%, while only 18 and 9% of fetuses were affected after coadministration of D-serine with sc and po 2-ME, respectively. Changes in the metabolism of orally administered 2-[14C]ME were specific to each enantiomer. D-Serine reduced the amount of 2-methoxy-N-acetylglycine eliminated in the urine to 70-75% of values observed with 2-ME alone, and concurrently increased the amount of urinary 2-MAA. L-Serine induced an initially higher rate of 14CO2 exhalation. Both enantiomers delayed gastrointestinal absorption of 2-ME, and significantly reduced 2-MAA levels in maternal plasma during the first hour after dosing. This resulted in a nonsignificant decrease (10-17%) in total embryonic exposure to 2-MAA. However, when 2-ME was injected sc, maternal plasma 2-ME/2-MAA pharmacokinetics were not affected by serine. In addition, dosing with 2.3 and 1.3 mmol 2-ME/kg sc alone showed that the embryo 2-MAA exposure levels which cause malformations in less than or equal to 35% fetuses were considerably lower than those measured following serine plus 3.3 mmol 2-ME/kg (po or sc). These data infer that serine does not protect against 2-ME-induced teratogenicity by altering 2-ME pharmacokinetics and reducing 2-MAA levels in the embryo.

Evaluation of 2-methoxyacetic acid toxicity on mouse germ cells by flow cytometry

Flow cytometric (FCM) DNA content measurements were carried out on testicular monocellular suspensions obtained from mice exposed per os to a single dose of 50, 100, 300, 600, and 900 mg/kg body weight (b.w.) of 2-methoxyacetic acid (MAA) in order to investigate its cytotoxic action on germ cells. The effects of MAA were evaluated 2, 7, 14, 28, and 45 d after treatment in terms of altered cell type ratios in FCM fluorescence distribution histograms. Testis weight and histological tissue sections were also analyzed. MAA induced marked changes in the relative percentages of tetraploid and haploid cells, indicating the occurrence of cytotoxic damage on primary spermatocytes. Multiparameter FCM analysis showed that, besides its action on nucleic acid synthesis, MAA can also affect the cellular energy metabolism reflected in an altered mitochondrial mass distribution on round spermatids surviving the MAA treatment. This study demonstrates that rapid and unique FCM procedures can be usefully applied in reproductive toxicology.

Attenuation of 2-methoxyethanol-induced testicular toxicity in the rat by simple physiological compounds

2-Methoxyethanol (2-ME) is an industrial solvent which is toxic to both male and female reproductive systems of laboratory animals. Earlier data have demonstrated that the developmental toxicity of 2-ME can be attenuated by simple physiological compounds such as serine, acetate, sarcosine, glycine, and D-glucose. The present experiments were designed to evaluate the same compounds for their ability to ameliorate the testicular toxicity that occurs in rats after 2-ME exposure. The extent of testicular damage was assessed by quantitating daily sperm production (DSP) on Day 24 following a single dose of 2-ME (6.6 mmol/kg, 500 mg/kg). Serine completely eliminated 2-ME-induced decreases in DSP, while glucose was without effect. Acetate, sarcosine, and glycine were of similar efficacy resulting in DSP that was significantly greater than that observed in rats which received 2-ME alone. Histopathological studies revealed that 2-ME treatment resulted in stage-specific degeneration of late stage pachytene spermatocytes 24 hr after treatment. No apparent degenerative changes occurred after concurrent treatment with serine. Similarly, serine also prevented the decreased number of spermatids in the lumina of the seminiferous tubules on Day 24 after 2-ME exposure alone. All of the compounds utilized in this study are linked to oxidation pathways involving tetrahydrofolic acid as a catalyst for one-carbon moiety transfer into purine and pyrimidine bases which are necessary precursors for DNA and RNA synthesis. The ability of these compounds to attenuate the testicular toxicity of 2-ME may result from their ability to donate one-carbon units which can be used in purine base biosynthesis. Reduced availability of bases would be expected to affect late stage pachytene spermatocytes which are known to be undergoing rapid RNA synthesis.

The possible role of one-carbon moieties in 2-methoxyethanol and 2-methoxyacetic acid-induced developmental toxicity

The ethylene glycol ether, 2-methoxyethanol (2-ME), is rapidly (less than 1 hr) oxidized to 2-methoxyacetic acid (2-MAA). Both agents are selectively embryotoxic and equipotent in causing digit malformations when given to CD-1 mice on gestation Day 11. Previous observations have shown that simple physiological compounds such as formate, acetate, glycine, and glucose ameliorate the embryotoxicity of 2-ME. A common link for all of the attenuating agents may be oxidation pathways involving tetrahydrofolic acid (THF) as a catalyst for one-carbon transfer into purine and pyrimidine bases. In the present study serine at 16.5 mmol/kg, which reacts directly with THF, was as effective as formate in almost completely eliminating digit malformations resulting from treatment with 2-ME. Unlike formate, serine was equally effective against 2-MAA-induced dysmorphogenesis and the attenuating efficacy remained unchanged when serine administration was delayed for up to 8 hr after 2-ME or 2-MAA exposure. The protective effect of sarcosine, which is an intermediate in a pathway leading from choline to glycine and a structural analog of 2-MAA, was also determined. Both concomitant (43, 16.5, or 3.3 mmol/kg) and delayed (16.5 mmol/kg at 6 hr) sarcosine administration resulted in significantly less 2-ME-induced paw dysmorphogenesis. In addition, acetate administration was delayed for increasing intervals after 2-ME to determine the time at which attenuation would no longer occur, and acetate was effective for as long as 12 hr after 2-ME. These results support our hypothesis that 2-MAA, which has a long biological half-life, may interfere with the availability of one-carbon units for incorporation into purine and pyrimidine bases. Alterations in availability of these precursors might be expected to affect DNA and/or RNA synthesis and thereby influence normal cellular proliferation and differentiation in the developing embryo.

Mechanistic approaches in the study of testicular toxicity: agents that directly affect the testis

Toxicants which affect the male reproductive system can act indirectly via alteration of the complex hormonal control of the testes, or directly by virtue of their chemical reactivity. Both categories of agents may require metabolic biotransformation to attain activity, and the site of activation may play a significant role in the modulation of testicular toxicity. The modes of action of 2 well known direct-acting male reproductive toxicants are discussed here. The use of a combination of in vivo and in vitro systems to distinguish the site and mechanism of action of these chemicals, ethylene glycol monomethyl ether and dinitrobenzene, is described. Because of the complexity of the reproductive process and of the interaction between gonadal and extragonadal metabolism, the use of multiple endpoints and multiple test systems can lend a greater degree of precision and confidence to our conclusions.

Inhibition of DNA synthesis in mouse whole embryo culture by 2-methoxyacetic acid and attenuation of the effects by simple physiological compounds

Gestation day 11 CD-1 mouse embryos were cultured in serum containing or serum-free media for 6 h in the presence of 2-methoxyacetic acid (2-MAA), the proximate teratogen arising from 2-methoxyethanol. The rate of DNA synthesis was determined following exposure of embryos to [3H]thymidine during the final hour of culture. 2-MAA (25 mM) produced a 50% reduction of [3H]thymidine incorporation into embryos cultured in serum-containing medium, whereas only 5 mM 2-MAA were required for a similar inhibitory effect in embryos cultured in serum-free medium. When embryos were concomitantly exposed to 2-MAA (5 mM) and to formate (1 mM), acetate (5 mM), or sarcosine (2 mM), a significant attenuation of the 2-MAA-induced inhibition of [3H]thymidine incorporation occurred. The findings suggest that these agents, which also attenuated developmental toxicity in vivo, may compete with 2-MAA actions which inhibit DNA synthesis in the embryo.

The relationship of embryotoxicity to disposition of 2-methoxyethanol in mice

Paw development of CD-1 mice is uniquely sensitive to 2-methoxyethanol (ME) given by gavage (po) on gestation day (gd) 11 (copulation plug day = gd 0). The relation between induction of paw dysmorphogenesis and disposition of po ME (3.3 or 4.6 mmol/kg) in the maternal and conceptus compartments was investigated. The expression of digit malformations depends on metabolism of ME to methoxyacetic acid (MAA). ME and MAA were equipotent in causing teratogenicity. Alcohol dehydrogenase (ADH) catalyzes the initial rate-limiting oxidation that leads to embryotoxicity. The ADH inhibitor 4-methylpyrazole (0.12 or 1.2 mmol/kg) or ethanol (43.3 mmol/kg, single dose concomitant with ME or additional ethanol 5 and 10 hr later) reduced the incidence of malformations 60-100%, depending on the dosing regimen. Elimination of 14C from 1,2-14C-ME occurred predominantly via urine where 80% of a teratogenic dose was excreted and 6% appeared in CO2. Oxidation of ME to MAA was nearly complete after 1 hr when approximately 90% of 14C in maternal plasma and conceptus coeluted with authentic 14C-MAA upon HPLC. 14C-MAA levels in embryos were 1.2 X those in plasma 1 and 6 hr after dosing, although by 6 hr concentrations had declined to approximately 50% of 1-hr values. Concomitant ethanol did not affect 14C kinetics as measured in maternal blood after oral 14C-ME, but retarded ME conversion to MAA by about 2 hr. Furthermore, embryo 14C-MAA levels then reached only 50% of the peak in embryos from dams dosed with ME alone, an effect that coincided with less 14C incorporation into macromolecules synthesized by the embryo within 6 hr. These data imply that the attenuation of digit malformations by concomitant ethanol may be explained by changes in MAA disposition. However, delayed ethanol (5 and 10 hr after 3.3 mmol ME/kg) reduced teratogenicity by 25%, although MAA was present in the embryo up to 5 hr. Dams given 14C-MAA by iv injection had higher 14C blood levels than after MAA po but their offspring had fewer digit malformations. Peak and steady-state plasma levels of MAA as well as embryo concentrations of the chemical do not appear to determine the embryotoxic outcome whereas further metabolism of MAA does.