Modulation of the embryotoxicity and cytotoxicity elicited by 7-hydroxy-2-acetylaminofluorene and acetaminophen via deacetylation

Glutathione during embryonic development

BACKGROUND

Glutathione (GSH) is a ubiquitous, non-protein biothiol in cells. It plays a variety of roles in detoxification, redox regulation and cellular signaling. Many processes that can be regulated through GSH are critical to developing systems and include cellular proliferation, differentiation and apoptosis. Understanding how GSH functions in these aspects can provide insight into how GSH regulates development and how during periods of GSH imbalance how these processes are perturbed to cause malformation, behavioral deficits or embryonic death.

SCOPE OF REVIEW

Here, we review the GSH system as it relates to events critical for normal embryonic development and differentiation.

MAJOR CONCLUSIONS

This review demonstrates the roles of GSH extend beyond its role as an antioxidant but rather GSH acts as a mediator of numerous processes through its ability to undergo reversible oxidation with cysteine residues in various protein targets. Shifts in GSH redox potential cause an increase in S-glutathionylation of proteins to change their activity. As such, redox potential shifts can act to modify protein function on a possible longer term basis. A broad group of targets such as kinases, phosphatases and transcription factors, all critical to developmental signaling, is discussed.

GENERAL SIGNIFICANCE

Glutathione regulation of redox-sensitive events is an overlying theme during embryonic development and cellular differentiation. Various stresses can change GSH redox states, we strive to determine developmental stages of redox sensitivity where insults may have the most impactful damaging effect. In turn, this will allow for better therapeutic interventions and preservation of normal developmental signaling. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Maternal occupational exposure to polycyclic aromatic hydrocarbons and risk of neural tube defect-affected pregnancies

BACKGROUND

This study evaluated whether there is an association between maternal occupational exposure to polycyclic aromatic hydrocarbons (PAHs) and neural tube defects (NTDs) in offspring. This is the first such study of which the authors are aware.

METHODS

Data were analyzed from 1997 to 2002 deliveries in the National Birth Defects Prevention Study, a large population-based case-control study in the United States. Maternal interviews yielded information on jobs held in the month before through 3 months after conception. Three industrial hygienists blinded to case or control status assessed occupational exposure to PAHs. Crude and adjusted odds ratios (ORs) with 95% confidence intervals (CIs) were estimated using unconditional logistic regression.

RESULTS

Of the 520 mothers of children with NTDs, 5.0% were classified as exposed to occupational PAHs, as were 3.5% of the 2989 mothers of controls. The crude OR for PAH exposure was 1.43 (95% CI, 0.92-2.22) for any NTD and 1.71 (95% CI, 1.03-2.83) for spina bifida. Adjusted ORs were smaller in magnitude and not significant. Among women who were normal weight or underweight, the crude OR for spina bifida was 3.13 (95% CI, 1.63-6.03) and adjusted OR was 2.59 (95% CI, 1.32-5.07). Based on estimated cumulative exposure, a statistically significant dose-response trend was observed for spina bifida; however, it was attenuated and no longer significant after adjustment.

CONCLUSION

Maternal occupational exposure to PAHs may be associated with increased risk of spina bifida in offspring among women who are normal weight or underweight. Other comparisons between PAHs and NTDs were consistent with no association.

Maternal occupational exposure to polycyclic aromatic hydrocarbons: effects on gastroschisis among offspring in the National Birth Defects Prevention Study

BACKGROUND

Exposure to polycyclic aromatic hydrocarbons (PAHs) occurs in many occupational settings. There is evidence in animal models that maternal exposure to PAHs during pregnancy is associated with gastroschisis in offspring; however, to our knowledge, no human studies examining this association have been conducted.

OBJECTIVE

Our goal was to conduct a case-control study assessing the association between estimated maternal occupational exposure to PAHs and gastroschisis in offspring.

METHODS

Data from gastroschisis cases and control infants were obtained from the population-based National Birth Defects Prevention Study for the period 1997-2002. Exposure to PAHs was assigned by industrial hygienist consensus, based on self-reported maternal occupational histories from 1 month before conception through the third month of pregnancy. Logistic regression was used to determine the association between estimated occupational PAH exposure and gastroschisis among children whose mothers were employed for at least 1 month during the month before conception through the third month of pregnancy.

RESULTS

The prevalence of estimated occupational PAH exposure was 9.0% in case mothers (27 of 299) and 3.6% in control mothers (107 of 2,993). Logistic regression analyses indicated a significant association between occupational PAHs and gastroschisis among mothers ≥ 20 years of age [odds ratio (OR) = 2.53; 95% confidence interval (CI): 1.27, 5.04] after adjusting for maternal body mass index, education, gestational diabetes, and smoking. This association was not seen in mothers < 20 years (OR = 1.14; 95% CI: 0.55, 2.33), which is notable because although young maternal age is the strongest known risk factor for gastroschisis, most cases are born to mothers ≥ 20 years.

CONCLUSION

Our findings indicate an association between occupational exposure to PAHs among mothers who are ≥ 20 years and gastroschisis. These results contribute to a body of evidence that PAHs may be teratogenic.

Oral administration of paclitaxel affects the distribution and metabolism of 2-aminofluorene in various tissues of Sprague-Dawley rats

To evaluate the question of whether or not paclitaxel affects the distribution and metabolism of chemical carcinogens such as 2-aminofluorene (AF) on Sprague-Dawley rats were examined. The AF, acetylated AF and AF metabolites were determined and examined by using high performance liquid chromatography. After having received AF only, AF with paclitaxel at the same time and paclitaxel pretreated for 24 h then treated with AF for 24 h, urine, stool and tissues such as liver, kidneys, stomach, colon, bladder and blood were collected and assayed for AF and its metabolites. Compared to the control group, paclitaxel caused an increase of the metabolites excreted in urine and stool. The major metabolite excreted in urine and stool was 9-OH-AAF. The liver is the major metabolism center and the major residual metabolite of AF in the liver was also 9-OH-AAF.

Spatial glutathione and cysteine distribution and chemical modulation in the early organogenesis-stage rat conceptus in utero

Glutathione (GSH), cysteine, and other low-molecular-weight thiols (LMWT) play a vital role in the detoxication of xenobiotics and endogenous chemicals. Differential alterations of LMWT status in various cell types of the developing embryo may underlie cell-specific sensitivity or resistance to xenobiotics and contribute to embryotoxicity. This study describes the spatial and temporal distribution of LMWTs in rat conceptuses and alterations produced by the non-teratogenic GSH modulator, acetaminophen (APAP). Pregnant female rats were given 125, 250, or 500 mg/kg APAP (po) on gestational day 9. Conceptal LMWT was localized histochemically using mercury orange in cryosections, and GSH and cysteine concentrations were measured by HPLC analysis. Mercury orange histofluorescence revealed a non-uniform distribution of LMWT in untreated conceptal tissues, with strongest staining observed in the ectoplacental cone (EPC), visceral yolk sac (VYS), and embryonic heart. Less intense staining was observed in the neuroepithelium. Following treatment with APAP, tissue-associated LMWT decreased dramatically except in the EPC, while exocoelomic fluid LMWT, and LMWT within embryonic lumens, increased. Exposure to 250 mg/kg APAP decreased embryonic GSH after 6 and 24 h by 46% and 38%, respectively. Acetaminophen (500 mg/kg) decreased embryonic and VYS cysteine content by 54% and 83%, respectively, after 24 h. Acetaminophen alters the spatial distribution of LMWT in rat conceptuses, particularly with respect to cysteine. The mobilization of cysteine following chemical insult may influence the ability of conceptal cells to maintain normal GSH status due to reduced availability of cysteine for de novo GSH synthesis.

Development of a morphologically-based scoring system for postimplantation New Zealand White rabbit embryos

Rodent whole-embryo culture (WEC) systems are well-established, as are several corresponding morphological scoring systems. Recently, WEC techniques for rabbits have been developed, creating the need for a morphological evaluation system in this species. Consequently, we developed a gestational-age-based quantitative morphology evaluation system for rabbit embryos. Detailed descriptions of 21 embryonic structures, as collected from gestational day (gd) 9-13 rabbit embryos, formed the basis for this evaluation system. These descriptions were then developed into specific criteria for assigning numerical scores to quantify the degree of development of each embryonic structure. The overall morphologic score was calculated as the average of the individual structure scores. To make the system as informative as possible, the numerical scale of the scoring system was gestationally age-based (i.e., range of potential scores was 9.0-13.0). The scoring system was then applied in the evaluation of New Zealand White (NZW) rabbit embryos explanted on gd 9 and cultured for 48 hr. Embryos grown in vitro developed normally, but at a slightly slower rate in vitro than in vivo, as evidenced by the lower morphology score (10.4 in vitro, 11.0 in vivo) and measures of growth (somite number, total protein, and head length). This work firmly establishes the normal archetype of embryonic development in the gd 9-13 NZW rabbit and provides an important tool for the advancement of mechanistic studies of rabbit embryos developing both in vivo and in vitro.

Evaluation of various toxicants in rabbit whole-embryo culture using a new morphologically-based evaluation system

In an effort to advance the use of whole-embryo culture (WEC) techniques in the rabbit, we recently developed a gestational-age-based quantitative morphologic evaluation system for rabbit embryos. In the current study, we applied this new morphological scoring system to assess the development of rabbit gestational day (gd) 9 embryos exposed for 48 hr in WEC to the teratogens ethanol (EtOH, 154 mM), 6-aminonicotinamide (6AN, 0.15 mM), and methoxyacetic acid (MAA, 5.0 mM), and the nonteratogen penicillin G (PG, 2.0 mM). Each teratogen at the concentration tested markedly inhibited morphological development, as indicated by significantly lower morphologic scores (10.1+/-0.05, EtOH; 10.2+/-0.05, 6AN; and 9.8, MAA) relative to controls (10.6+/-0.04), and resulted in an increased percentage of malformed embryos (53%, EtOH; 57%, 6AN; 90%, MAA; and 3%, control). Embryonic growth, as measured by head length, somite number, and total embryonic protein, was significantly decreased by each teratogen. The abnormalities produced by teratogen exposure, which included brain, somite, and facial defects, were often similar to those produced following in vivo exposure in rabbits and rodents, and/or in vitro exposure in rodents. In contrast to the teratogen exposure groups, PG had no effect on embryo growth parameters, or on malformation rate (6%), although a slight but statistically significant decrease in morphology score (10.5+/-0.03) was noted. Our preliminary studies demonstrate the usefulness of the morphology evaluation system by quantifying graded differences in development, and indicate that rabbit WEC may be a useful adjunct to rodent WEC in gaining insights regarding differential interspecies sensitivity.

Modulation of intracellular glutathione and cysteine metabolism in bovine oviduct epithelial cells cultured In vitro

The objective of this study was to determine how alterations in intracellular thiol levels of oviduct epithelium occur in response to chemical or environmental conditions that could result in oxidative stress. Bovine oviducts were classified as follicular (F) or luteal (L) according to the reproductive stage of the ovary. Epithelial cells were harvested from the ampulla (AMP) and isthmus (ISTH) region of each oviduct, suspended in culture medium, and then plated into collagen-coated culture plates and grown to confluency. Basal levels of intracellular cysteine (Cys) and glutathione (GSH) were determined in oviduct epithelial cells and found to range from 0.36 to 0.46 pmol/ microg protein for Cys and from 5.3 to 6.4 pmol/ microg protein for GSH. Oxidized Cys values ranged from 21% to 39% of total Cys, whereas the oxidized GSH levels observed were from 21% to 28% of total GSH except in luteal ISTH, where they were significantly lower (6%). Confluent cells were exposed to GSH-depleting agents, <FONT SIZE=-1>L-buthionine-S,R-sulfoximine (BSO) or diethyl maleate (DEM), at doses ranging from 10 to 5000 microM. Both compounds depleted GSH in a dose-dependent manner, and 500 microM concentrations were chosen for subsequent studies with each compound. Cys levels in BSO (500 microM)-treated oviduct epithelial cells were transiently elevated over control values during the initial 5-h incubation; there was then a decrease in Cys levels by AMP but not ISTH oviduct epithelial cells. BSO-treated oviduct epithelial cells displayed a continued depletion of GSH over the incubation period and by 24 h were depleted by 38% to 61%. These results demonstrate a difference in GSH turnover in oviduct epithelial cells associated with reproductive stage. Exposure to DEM (500 microM) caused a decline in both Cys and GSH levels, which were partially restored after DEM removal. In general, L-staged oviduct epithelial cells were observed to be more competent at replenishing thiol stores than F-staged oviduct epithelial cells. Results from this study suggest that reproductive stage and region influence intracellular oviduct epithelium thiol status and therefore may affect how this tissue responds to chemicals or environmental conditions leading to oxidative stress.

Formation of glutathione adducts and 2-aminofluorene from 2-nitrosofluorene in postimplantation rat conceptuses in vitro

The formation of glutathione (GSH) adducts and 2-aminofluorene (AF), GSH-derived metabolic products from 2-nitrosofluorene (NOF), was examined as a possible mechanism of GSH-mediated protection from NOF embryotoxicity in the gestational day 10 (GD 10) rat conceptus in vitro. When added to whole embryo culture medium, NOF produced dose-dependent decreases in growth parameters and increases in the incidence of axial rotation defects in embryos cultured for 26 h. Culture of GD 10 rat conceptuses in 50 microM NOF for 24 h following 2 h pretreatment with an irreversible inhibitor of glutathione disulfide reductase, 1,3-bis(2-chloroethyl)1-nitrosourea (BCNU, 25 microM) did not result in statistically significant differences in morphology or biochemical parameters compared to NOF alone; viability, however, was decreased relative to controls. Nearly equal amounts of GS-AF(I), a stable S-oxide conjugate of GSH with NOF, AF, a GSH-dependent reaction product of NOF, and the parent NOF were recovered following short-term incubation of conceptuses with NOF (100 microM) in serum-free medium. Stimulation of GSH synthesis with the cysteine prodrug 2-oxothiazolidine-4-carboxylate (OTC, 5 mM) resulted in a significant increase in AF concentrations (205% of control) and a decrease in NOF (50% of control) after 30 min. Sixty-minute exposure to the GSH depletor, diethylmaleate (DEM, 500 microM), resulted in apparent reductions in both GS-AF(I) and AF by 36% and 34%, respectively, though these reductions were not statistically significant. Treatment with 25 microM BCNU for 2 h, followed by exposure to 100 microM NOF in serum-free medium resulted in a significant decrease in AF to 76% of control concomitant with lower GSH levels relative to NOF treatment alone. Exposure of conceptuses to 50 microM NOF in complete medium following pretreatment with BCNU resulted in a reduction of GSH levels in the visceral yolk sac after 3 h and in embryos after 5 h relative to controls. These data demonstrate that the intracellular protective effects of GSH toward NOF embryotoxicity may act through a nonenzymatic mechanism of direct formation of GSH-NOF adducts in the day 10 rat conceptus in vitro, followed by the GSH-mediated conversion to a less toxic metabolite, AF.

Identification of proximate toxicant for ethylene glycol developmental toxicity using rat whole embryo culture

The effects of ethylene glycol (EG) and its metabolite, glycolic acid (GA), were compared by culturing day 10.5 rat conceptuses for 46 h in media containing 0.5, 2.5, 12.5, 25 or 50 mM EG or GA. EG up to 50 mM was essentially without effect, whereas > or = 12.5 mM GA inhibited embryo growth and development. Craniofacial dysmorphogenesis was observed in 70% of the 12.5 mM GA embryos (0% in controls). To determine if GA toxicity in vitro was an indirect effect of medium acidification, embryos were cultured in 12.5 mM GA (pH 6.7), 12.5 mM sodium glycolate (pH 7.4), or in control medium (pH 7.4 or 6.7). The percentage of dysmorphic embryos was 67% for the 12.5 mM GA (pH 6.7) group, 58% for the sodium glycolate (pH 7.4) group, 8% in the pH 6.7 controls, and 0% in the pH 7.4 controls. These results suggest that GA, not parent EG, is the active toxicant for EG-induced developmental toxicity and that acidification of culture medium pH plays only a minor role in GA's effects in vitro. The identification of GA as the active toxicant is important for the risk assessment of EG because GA exhibits dose-rate-dependent, nonlinear kinetics in vivo.

Lindane embryotoxicity and differential alteration of cysteine and glutathione levels in rat embryos and visceral yolk sacs

The lindane embryotoxicity and associated changes in cysteine (CYS) and glutathione (GSH) status have been investigated in the early organogenesis-stage rat conceptus utilizing whole embryo culture techniques. Direct exposure of gestational day 10 (GD 10) conceptuses to lindane (50, 100, 200, 300, and 400 microM) in the culture medium resulted in a dose- and time-dependent increase in mortality (88% at 400 microM), frequency, and severity of malformations and in decreased growth parameters. Protein and DNA contents of embryo and visceral yolk sac (VYS), likewise decreased significantly as lindane concentrations increased. Lindane exposures greater than 100 microM produced abnormal axial rotation, pooled blood on lateral cephalic surfaces, cephalic edema, and decreased VYS vasculature. Histologic sections showed a variety of abnormalities, including distended anterior cardinal veins, thinning of the neuroepithelium in forebrain and hindbrain regions, and abnormal branchial arch development. CYS and GSH levels in the VYS were not significantly affected by 100 microM lindane exposure during a 5-h incubation period on GD 10 and GD 11. In contrast, CYS and GSH levels in lindane-exposed embryos remained unchanged while control levels continued to increase with gestational age. At 5 h, treated embryos showed a significant depletion of CYS (GD 10, 22%; GD 11, 35%) and GSH (GD 10, 41%; GD 11, 24%) relative to controls. Selective lindane-induced depletion of embryonic GSH suggests involvement of the glutathione redox cycle in lindane embryotoxicity.

Chloroquine embryotoxicity in the postimplantation rat conceptus in vitro

The embryotoxicity of the antimalarial drug chloroquine (CQ) was evaluated in vitro using the rat whole embryo culture system. CQ was found to be embryotoxic and dysmorphogenic when added directly to the culture media containing gestational day (GD) 10 rat conceptuses. Twenty-six-hr exposure to CQ elicited dose-related decreases in embryonic crown-rump length, protein and DNA contents and increases in the incidence of morphologically abnormal embryos. At 30 microM CQ, embryonic protein content was decreased to 67% and DNA content to 58% of control while the incidence of morphological abnormalities rose to 100%. Abnormal axial rotation, micro-ophthalmia, and selective cephalic hypoplasia were the most common developmental abnormalities observed. Visceral yolk sac (VYS) vasculature and blood pigmentation were also decreased in a dose-dependent manner, as was VYS DNA content (80% of control at 30 microM). VYS protein content, however, showed an alternate pattern of response, decreasing to 87% of control at 10 microM CQ but increasing to 125% of control at 30 microM. Histologic evaluation revealed that the cytoplasm of the VYS endoderm epithelium was distended due to vacuolization produced by CQ exposure. In the embryo proper, CQ inhibited cranial neural tube development and altered the morphology of cranial neural crest cells. These observations document the in vitro embryotoxicity of CQ and suggest altered VYS histiotrophic nutrition as well as direct embryonic effects as possible mechanisms of CQ embryotoxicity.

Chemical teratogenesis

This review has briefly summarized what is currently known concerning the mechanisms whereby several groups of chemicals regarded as "recognized" human teratogens elicit their respective teratogenic effects. It is evident that the extent of our understanding of mechanisms for individual chemicals varies dramatically from that of a reasonably good understanding for methotrexate and other folic acid antagonists to that of virtually no understanding for the most widely recognized human teratogen, thalidomide. Even with methotrexate, however, much remains to be learned pertaining to mechanisms--i.e., the critical links in the chain of events between dihydrofolate reductase inhibition and the manifestation of specific abnormalities. Nevertheless, we can take some comfort in being able to say that we understand the primary causative mechanism. For thalidomide, as well as several others the chemical represents both a shame and a challenge--a challenge that should be addressed with our most serious efforts.

Cytochrome P450-dependent bioactivation of prodysmorphogens in cultured conceptuses

These investigations were undertaken to determine the extent to which tissues of cultured rat conceptuses contain cytochrome P450 isoforms in sufficient quantities to significantly influence the capacity of certain chemicals to elicit dysmorphogenic effects in vitro. Investigations with highly sensitive probe substrates/inhibitors and with immunologic methods enabled the detection of at least four separate P450 isoforms in tissues of the visceral yolk sac, ectoplacental cone, and embryo proper. One of the isoforms was identified as P450IA1 and was found to be inducible by polycyclic aromatic hydrocarbons in all three tissues. Other isoforms exhibited properties differing from characterized adult rat hepatic isoforms. Each of the isoforms was detectable in conceptuses on gestational days 10, 11, 12, and 14 and was present in the highest concentrations in the visceral yolk sac. Conceptal P450IA1 catalyzed the conversion of dysmorphogenically inactive 2-acetylaminofluorene to 7-hydroxy-2-acetylaminofluorene, a proximate dysmorphogen. Investigations with microinjections suggested that visceral yolk sac hydroxylation was largely responsible for the bioactivation reaction in vitro. The same isoform exhibited no capacity to influence the dysmorphogenic activity of cyclophosphamide. The results demonstrated that tissues of cultured rat conceptuses may contain P450 isoforms in sufficient amounts to markedly influence the dysmorphogenic activity of substrates of the corresponding isoforms.

Acetaminophen toxicity to cultured rat embryos

We tested the effects of acetaminophen on cultured rat embryo development. When added directly to culture media at 300 microM, a concentration approximately twice the human therapeutic blood level, acetaminophen caused abnormalities in the cultured embryos. Sera from both rats and monkeys following gavage with acetaminophen were also toxic to cultured embryos. The sera toxicities were related to acetaminophen concentrations, and the toxicity could be removed by serum dialysis. With regard to the metabolism of acetaminophen, glutathione levels in the yolk sac decreased in a concentration related fashion with addition of the drug. Also, buthionine sulfoximine, an inhibitor of glutathione synthesis, appeared to enhance acetaminophen embryo toxicity, and N-acetylcysteine, a glutathione precursor, appeared to protect embryos from acetaminophen toxicity. These results suggested that acetaminophen embryo toxicity resulted from direct exposure of embryos to acetaminophen and not a maternal metabolite.

Abnormal neurulation induced by 7-hydroxy-2-acetylaminofluorene and acetaminophen: evidence for catechol metabolites as proximate dysmorphogens

Direct additions to culture media of either acetaminophen (APAP) or 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) resulted in abnormal closure of the anterior neuropores of cultured rat embryos in the absence of an exogenous bioactivation system. Concentrations required to produce a 50% incidence of the defect were approximately 500 and 250 microM for APAP and 7-OH-AAF, respectively. Losses of viability were not evident at these concentrations but 7-OH-AAF elicited a somewhat greater effect on growth parameters and generalized embryotoxicity. Transplacental induction with 3-methylcholanthrene (MC) of P450IA1 in subsequently cultured rat embryos did not detectably alter the capacity of APAP or 7-OH-AAF to effect embryotoxicity or neuropore closure. However, additions to the culture medium of exogenous hepatic bioactivating systems (S9) from MC-induced vs phenobarbital (PB)-induced adult rats produced profoundly different effects on neuropore closure. Coincubation with S9 from MC-induced rats reduced the incidence of 7-OH-AAF-elicited abnormal neuropores from 45 to 19%, whereas coincubation with S9 from PB-induced rats increased the incidence to 77%. Coincubation with MC-induced S9 produced no statistically significant effect on APAP-elicited neuropore abnormalities but, with PB-induced S9, resulted in a significant increase from 60 to 86%. Additions of 3-OH-APAP (0.1-0.2 mM) but not N-acetyl-p-benzoquinoneimine (NAPQI, 0.1-0.5 mM) to the culture medium elicited the typical neuropore abnormality. Experiments with APAP and 7-OH-AAF as substrates demonstrated that embryonic enzymes catalyzed their conversion to the corresponding catechols. Considered together, the results provided evidence that embryonic conversion of APAP or 7-OH-AAF to the corresponding catechol metabolites may be instrumental in effecting the abnormal anterior neuropore closure observed after exposure of embryos to the respective parent compounds.

Immunodetection, immunoinhibition, immunoquantitation and biochemical analyses of cytochrome P-450IA1 in tissues of the ratoffceptus during the progression of organogenesis

Polyclonal antibodies raised against the adult form of rat hepatic cytochrome P-450IA1 were used to immunologically detect, inhibit and quantitate an analogous isozymic form(s) in various tissues of the rat conceptus during the progression of organogenesis. Tissues investigated were the embryo proper, the visceral yolk sac and the ectoplacental cone/chorioallantoic placenta. Studies were performed on conceptuses from day 10 (day of conception = day 0) to day 14 of gestation. Ethoxyphenoxazone deethylation, benzo[a]pyrene (BaP) hydroxylation, and ring- and N-hydroxylation of 2-acetylamino-fluorene (AAF) were utilized in assessments of cytochrome P-450IA1-dependent monooxygenase activities during the same gestational period. In untreated conceptuses, cytochrome P-450IA1 could not be detected immunologically in any of the three tissues at any stage of gestation investigated. The deethylation reaction was quantifiable in embryos and yolk sacs of untreated conceptuses, but was not inhibited by cytochrome anti-P-450IA1 antibodies, alpha-naphthoflavone or metyrapone. Treatment of pregnant rats with 40 mg/kg of 3-methylcholanthrene 48 hr prior to removal of the conceptuses resulted in marked increases in measured enzymatic activities as well as in readily immunodetectable cytochrome P-450IA1. Inducibility for the deethylase was greatest in the visceral yolk sac (3-8x), was evident in the embryo proper (2-3x) but was minimal in the ectoplacental cone (1.5-2x). Much greater induction (up to 70x) was observed with BaP and AAF as substrates. Induced activities were inhibited effectively (70-100% inhibition) by cytochrome anti-P-450IA1 antibodies and by alpha-naphthoflavone but not by metyrapone. Inducibility increased as a function of gestational age in the ectoplacental cone/chorioallantoic placenta but reached maxima on day 12 in the embryo and visceral yolk sac. A good correlation between antibody/alpha-naphthoflavone-inhibited enzymatic activities and quantities of immuno-detectable cytochrome P-450IA1 was also apparent. The results indicate that cytochrome P-450IA1, or a very closely related isoform(s), is both inducible and enzymatically functional in tissues of the conceptus throughout organogenesis and have important implications for the potential effects of bioactivatable proteratogens.

Microinjection of cultured rat embryos: a new technique for studies in chemical dysmorphogenesis

The utility of a new technique for exposure of cultured whole rat embryos to potential dysmorphogens was demonstrated with nitrosofluorene (NF), a cytotoxic and mutagenic metabolite of 2-acetylaminofluorene (AAF). At an initial concentration in the culture medium of 41 microM, NF produced a 100% incidence of defects in axial rotation with no significant effect on prosencephalic development, consistent with previous reports. This route of exposure was also associated with a significant decrease in yolk sac vasculature and protein content. However, when 2 to 20 ng of NF was microinjected directly into the amniotic space, the predominant malformation observed was prosencephalic hypoplasia. Injection of 10 ng NF resulted in approximately equivalent decreases in viability as 41 microM NF dissolved in the culture medium, but produced only a 41% incidence of rotation defects and a 27% incidence of open neural tubes in the rhombencephalic region. The protein content of injected conceptuses was significantly reduced in the embryo, but not in the visceral yolk sac. When 10 ng of NF was injected inside the yolk sac but outside the amnion, the incidence of abnormal rotation was increased to 75%, and the severity of prosencephalic hypoplasia as well as the incidence of neural tube abnormalities was attenuated. The protein content of both the embryo and yolk sac was significantly decreased relative to that of the controls. The data are consistent with the suggestion that NF elicits defects in axial rotation primarily via its effects on the visceral yolk sac and demonstrate the capacity of this technique to provide insights into mechanistic aspects of chemical dysmorphogenesis.

Influence of electrophilic character and glutathione depletion on chemical dysmorphogenesis in cultured rat embryos

To examine the importance of reduced intracellular glutathione (GSH) in the modulation of dysmorphogenesis and to gain insight into the electrophilic character of the embryotoxic intermediates generated in the rat embryo from N-acetoxy-2-acetylaminofluorene (AAAF) and acetaminophen (APAP) in cultured embryos, the effects of GSH depletion on the embryotoxicity, dysmorphogenesis and covalent binding of these agents were examined. Both AAAF (90 microM) and APAP (500 microM) produced concentration-dependent, statistically significant (P less than or equal to 0.05) decreases in embryonic length as well as embryonic and visceral yolk sac protein content when rat embryos were exposed in vitro between days 10 and 11 of gestation. The predominant malformations observed upon exposure to AAAF and APAP were prosencephalic hypoplasia and abnormal neurulation respectively. Exposure of conceptuses to [3H]APAP followed by separation and fractionation of the cellular RNA, DNA and protein via density gradient centrifugation resulted in detectable binding in fractions that contained protein, but not DNA or RNA. This suggested that the rat conceptus is capable of bioactivating APAP to a soft electrophile that selectively arylates protein. In contrast, conceptuses exposed to [3H]AAAF exhibited detectable binding to RNA, DNA and protein, indicative of conversion to both hard and soft electrophiles. Depletion of GSH was accomplished by pretreating conceptuses with 500 microM L-buthionine-S,R-sulfoximine (BSO) from the start of the culture period (day 9.5) until the morning of day 10. When conceptuses were depleted previously of GSH by BSO, exposure to APAP resulted in significant potentiation (relative to APAP alone) of the observed embryotoxicity. These conceptuses displayed further decreases in both embryonic size and protein content of the embryo and yolk sac, as well as increased incidence of abnormally open anterior neuropores and increased binding (3-fold) of [3H]APAP to protein. In contrast, pretreatment with BSO did not potentiate the AAAF-elicited decreases in embryonic size or protein content, nor the severity of prosencephalic hypoplasia, although a slight increase in binding of [3H]AAAF to DNA was observed. Taken together, these data are consistent with the concept that abnormal neurulation elicited by APAP results from the generation of one or more soft electrophilic species, whereas elicitation of prosencephalic hypoplasia by AAAF appears to be a consequence of conversion to a relatively hard electrophile(s).