Role of glutathione and hsp 70 in the acquisition of thermotolerance in postimplantation rat embryos

Studies were initiated to determine the extent to which reduced glutathione (GSH) may be involved in the capacity of cultured rat embryos to develop heat-induced tolerance to the deleterious effects of exposure to high temperatures (heat shock). Investigations of the modulation of dysmorphogenic responses of embryos to heat shock (43 degrees C, 30 min) as well as to the expression of the hsp70 gene and subsequent formation of hsps indicated that the acquisition of thermotolerance by rat embryos could be significantly influenced by the inhibition of GSH synthesis. Treatment of conceptuses with L-buthionine-S,R-sulfoximine (BSO) reduced intracellular GSH concentrations and compromised the capacity of embryos to mount a thermotolerance response as assessed by alterations in indices of growth and development. Embryonic thermotolerance elicited by preexposure to 42 degrees C for 30 min was accompanied by increases in GSH to levels greater than those measured in control embryos at 37 degrees C just prior to the subsequent 43 degrees C heat exposure. Expression of hsp70 mRNA was detectable soon after elevation of the temperature to 42 degrees C and reached its highest level of accumulation 1.5 hr after the 43 degrees C heat shock. BSO treatment had little if any effect on hsp70 message levels or on the synthesis of hsp70. The fact that BSO-treatment attenuated the thermotolerance response but did not produce a decrease in hsp70 RNA or the synthesis of hsp70 suggests that hsp70 alone is not sufficient to confer thermotolerance upon cultured rat embryos.

Morphological differences elicited by two weak acids, retinoic and valproic, in rat embryos grown in vitro

We compared in rat whole-embryo culture the morphological changes elicited by valproic acid (VPA) with those elicited by trans-retinoic acid (RA). Rat embryos explanted on day 9.5 of gestation were treated on day 10 with RA or VPA at concentrations producing equivalent reductions in embryonic protein. The concentrations selected for morphological assessment by scanning and transmission electron microscopy, 2.3 and 800 microM, respectively, for RA and VPA, produced approximately a 50% incidence of abnormally open anterior neuropores in initial range-finding experiments in the culture system. Protein and DNA analyses were also performed on corresponding groups of embryos at three different doses. With concurrent control groups used as reference standards, the two treatment groups were compared for differences in external and internal morphology, protein and DNA contents, and growth indices. While certain variables responded similarly in the two treatment groups, e.g., the growth variables, protein and DNA contents, each drug produced selective morphological effects. Whereas treatment with RA produced underdeveloped branchial arches, symmetrically cleft cranial defects resulting in openings in rhombencephalic and prosencephalic regions, and exteriorized neural tissue in the caudal neuropore region, VPA produced irregular clefts with wavy margins along the entire length of the neural tube, and an open caudal neuropore without eversion of the neuroepithelium, while producing no detectable effect on the branchial arches. The similar effects of these two drugs on protein and DNA contents suggest comparable degrees of overall toxicity; however, the dissimilar effects on neural tube and branchial arches, coupled with the large difference in concentration of the drug required to produce the effects, add to the evidence that their mechanisms for elicitation of abnormal development are qualitatively different.

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.

cAMP-dependent regulation of P450-catalyzed dealkylation in rat conceptal tissues

Exposures of cultured whole rat conceptuses to varying concentrations of dibutyryl cyclic AMP or isobutylmethylxanthine, alone or in combination, resulted in significant increases in rates of cytochrome P450-dependent depentylation of pentoxyphenoxazone in cell-free preparations. Lesser increases in rates of debenzylation of benzyloxyphenoxazone were also observed. In cultured whole conceptuses, basal depentylase and debenzylase activities in the visceral yolk sac were approximately sixfold higher than in the embryo. The ectoplacental cone and decidual tissues exhibited no detectable depentylase activity. Only the visceral yolk sac exhibited increased depentylase activity in response to dibutyryl cyclic AMP and isobutylmethylxanthine. Inhibitory antibodies raised against adult hepatic P450s IIB1, IIC11, and IA1 failed to significantly inhibit the yolk sac depentylase activities of noncultured conceptuses. The results suggested that the conceptal depentylation reaction may be catalyzed by a unique P450 isoform(s) that is not expressed during adult life.

Effects of QA 208-199 and its metabolite 209-668 on embryonic development in vitro after microinjection into the exocoelomic space or into the amniotic cavity of cultured rat conceptuses

The objective of this study was to determine the direct embryotoxic effects in vitro of N-hydroxy-N-methyl-7-propoxy-2-naphthalene-ethanamine (QA 208-199, QAB) and of one of its metabolites, 7-propoxy-naphthalene-2-ylacetic acid (209-668, QAA), after circumventing the bioconverting conceptual membranes. The compounds were, therefore, microinjected either into the exocoelomic space or into the amniotic cavity of rat conceptuses of 10 d at prenatal age at doses of up to 84.9 ng (QAA) and 180 ng (QAB) per conceptus respectively. The conceptuses were subsequently cultured for 28 h after which their development was assessed. QAB produced marginal effects on embryonic differentiation only after microinjection of the compound into the amniotic cavity. Dysmorphogenic effects, however, occurred in a dose-dependent fashion after either exocoelomic or intraamniotic microinjections of the compound. The frequencies and types of anomalies were similar after either exposure route and consisted predominantly of anomalies associated with axial rotation. QAA also impaired embryonic differentiation at only the high dose level of 84.9 ng per embryo and after intraamniotic injections only. Dysmorphogenic effects were observed in all experimental groups, although the differences were not statistically significant when compared with the concomitant controls. An increased proportion of anomalies observed were in the cephalic region as compared to the defects produced by QAB. These data suggest that QAA most probably is not the QAB metabolite responsible for the embryotoxic action of QAB in vitro. Furthermore, the results tend to confirm the suggested involvement of the visceral yolk sac membrane in mediating QAB embryotoxicity.

Dysmorphogenesis elicited by microinjected acetaminophen analogs and metabolites in rat embryos cultured in vitro

Direct additions of acetaminophen (APAP), 3,5-dimethylacetaminophen, 3-hydroxyacetaminophen or 3-methoxyacetaminophen to the medium of cultured embryos each produced an increased incidence of morphologically similar, abnormally open anterior neuropores. Approximate concentrations required to produce an equal incidence were 0.5 mM, 1.0 mM, 0.1 mM and 0.75 mM, respectively. In contrast, 2.6-dimethylacetaminophen and N-acetyl-p-benzoquinoneimine failed to produce elevated incidences of abnormal neurulation unaccompanied by marked growth retardation. However, with intra-amniotic microinjections, 3-hydroxyacetaminophen and N-acetyl-p-benzoquinoneimine were roughly equipotent for eliciting abnormal neurulation, whereas 3-methoxyacetaminophen required greater than 30-fold higher concentrations. This suggests that N-acetyl-p-benzoquinoneimine does not readily transit the visceral yolk sac and would likely not be a major factor in APAP-elicited neural tube abnormalities unless generated in target tissues. The differential effects produced by two dimethylated (2.6 and 3.5) APAP analogs further suggest that sulfhydryl oxidation is associated more closely than sulfhydryl conjugation with the neurulation defect. Intra-amniotic microinjections of large quantities (3500 ng) of 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) or APAP failed to produce the specific neurulation defect. Microinjections of 7-OH-AAF into the exocoelomic cavity effected the characteristic abnormal neurulation. Conversion by conceptal homogenates of 7-OH-AAF was roughly 7- to 8-fold more rapid than conversion of APAP to respective catechol metabolites, and specific activities in yolk sac tissues were greater than those in the embryo. Rates of conceptal conversion to the quinoneimine were approximately 2- to 3-fold lower than catechol generation.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate specificities and functions of the P450 cytochromes

Currently, the major recognized biochemical functions of members of the large superfamily of P450 hemoproteins (referred to commonly as the cytochromes P450) include catalyses of the monooxygenations of a wide variety of endogenous and exogenous lipophilic chemicals. Substrates that have attracted the greatest attention thus far are steroids, fatty acids, eicosanoids, retinoids, other endogenous lipids, therapeutic agents, pesticides/herbicides, chemical carcinogens, industrial chemicals and other environmental contaminants and toxic xenobiotic organics of low molecular weight. Commonly, monooxygenation of such substrates results in the generation of metabolites capable of producing biological effects that are profoundly different (qualitatively as well as quantitatively) from those elicitable by the parent chemical per se. P45OXIX-dependent conversion of testosterone to estradiol-17 beta provides a dramatic example. Thus, these hemoproteins serve as extremely important but, as yet, largely unpredictable regulators of the biological effects producible by endobiotics as well as by xenobiotics. Current focus is on the identification and acquisition of sequence information on hereto unidentified and/or uncharacterized P450 isoforms and ascertainment of the specific functions of specific, individual isoforms. The regulation of quantities and activities of such isoforms in specific species/tissues, understandably, is also of great current interest. This interest has been further intensified by recent results indicating that substrate specificity associated with one P450 may not be the same as the corresponding isoform derived from a different animal species. Recent technological advances promise to greatly hasten the acquisition of knowledge concerning the functions of these important hemoproteins.

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).

Occurrence and inducibility of cytochrome P450IIIA in maternal and fetal rats during prenatal development

The purpose of this study was to quantify cytochrome P450IIIA1 in fetal and maternal livers of uninduced and pregnenolone-16 alpha-carbonitrile (PCN) induced rats during the course of prenatal development. The activities and levels of P450IIIA in hepatic microsomes from maternal rats and fetuses at 15-21 days of gestation were measured by triacetyloleandomycin (TAO) inhibited debenzylation of (benzyloxy)phenoxazone and by immunoassay with defined antiserum specific for P450IIIA. P450IIIA was not detectable (less than 10 pmol/mg for maternal microsomes and less than 2 pmol/mg for fetal microsomes) by immunoassay in uninduced maternal or fetal livers. In hepatic microsomes from PCN-induced dams, values ranged from 59.3 to 116 micrograms P450IIIA1/mg of protein during the same gestational period. Changes in debenzylase activity of 15.9-46.5 pmol of resorufin (mg of protein)-1 min-1 were consistent with these findings as were the changes in TAO-inhibitable debenzylase activity. In the transplancentally induced fetal liver, debenzylase activity increased steadily from 0.19 pmol of resorufin mg-1 min-1 at day 15 to 9.34 pmol of resorufin mg-1 min-1 at day 21 and was paralleled by the TAO-inhibitable activity that ranged from 0.09 pmol of resorufin mg-1 min-1 at day 15 to 3.33 pmol of resorufin mg-1 min-1 at day 21. The amount of immunoreactive P450IIIA1 also increased from 0.5 to 28.7 micrograms/mg of microsomal protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Bioactivation in chemical teratogenesis

Within the past decade, interest has increased markedly in the elucidation of mechanisms whereby drugs and other chemicals can alter the normal developmental pattern of the developing conceptus. This has, in large measure, been attributable to the recent availability of methods for the successful long-term culture of whole embryos as well as various embryonic tissues (e.g. limb buds). These preparations have enabled a more straightforward investigation of the direct effects of chemicals on the conceptus per se, without the complicating and frequently confounding participation of maternal factors. The demonstration that exogenous metabolic preparations could be incorporated into such culture systems has enabled investigators to pursue questions about the nature of proximate and ultimate chemical species responsible for producing abnormal morphogenesis. Demonstrations of the capacity of the early conceptus to effect profound dysmorphogenic bioactivation provide additional relevance to such questions. Elucidation of the identity of the chemical species represents a first and necessary step in unravelling the pathogenic mechanism. Control of their rates of generation and inactivation or elimination are probable major determinants of incidence/severity of chemically induced embryotoxicity. Future investigations of these phenomena promise to yield key contributions to the discovery of mechanisms in chemical teratogenesis.

Biotransformation, estrogenicity, and steroid structure as determinants of dysmorphogenic and generalized embryotoxic effects of steroidal and nonsteroidal estrogens

A series of nine chemicals of varying structure and estrogenicity was investigated for biochemical determinants of their relative capacities to alter normal embryonic growth and developmental patterns during organogenesis in rats. In order to circumvent the potentially confounding influences of maternal factors, the direct effects of steroidal and nonsteroidal estrogens on cultured whole embryos were compared at concentrations producing readily measurable embryotoxicity but low embryolethality (2-20%). Nonsteroidal estrogens included were diethylstilbestrol (DES), hexestrol (HES), E,E-dienestrol (alpha-DIES), and tamoxifen (TAM). Steroidal estrogens were estradiol 17 beta (E2), estrone (E1), and 17 alpha-ethinylestradiol 17 beta (EE). For comparative purposes, the effects of two essentially nonestrogenic phenols, Z,Z-dienestrol (beta-DIES) and phenol, were also studied. TAM, a weak estrogen which also exhibits antiestrogenic properties, was studied for possible interactive effects with potent estrogens. Prosencephalic hypoplasia was the abnormality most consistently observed and was elicited by each of the chemicals investigated. Embryotoxicity was neither attenuated by TAM nor related to estrogenic potency or steroidal structure, but was strongly and unpredictably influenced by biotransformational determinants. Presence of a cytochrome P450-dependent oxidizing system in the culture medium resulted in marked increases in embryotoxicity of E1, E2, and phenol, only minor increases for beta-DIES and alpha-DIES, but in strikingly decreased effects of EE, TAM, and HES. It produced no statistically significant differences in effects of DES. The results obtained were compatible with the concept that effects of these agents on growth and development during the earlier stages of organogenesis are independent of steroid structure or estrogenic activity but strongly dependent upon pathways and rates of biotransformation of some (but not all) of the parent chemicals.

Studies of embryotoxic mechanisms of niridazole: evidence that oxygen depletion plays a role in dysmorphogenicity

Previous study has shown that niridazole (NDZ) is dysmorphogenic to rat embryos between days 10 and 11 under culture conditions including 5% oxygen. Other studies have found that reductive embryonic biotransformation is required but that covalent binding is not a major basis of this embryotoxicity. In research presented here, NDZ exposure of homogenates prepared from day 10 rat embryos resulted in stimulation of oxygen uptake from incubation media. Further studies showed that a large percentage of this increased oxygen uptake was associated with the generation of superoxide anion radical and hydrogen peroxide. These findings led us to hypothesize that redox cycling forms the basis of the in vitro dysmorphogenicity of NDZ. The basic premise of this hypothesis is that as a result of redox cycling, oxygen is depleted from the sensitive tissues of embryos. In order to investigate it, we devised a technique for carefully controlling and monitoring oxygen tensions in embryo cultures. We found that when oxygen concentrations of 4% were established, a highly significant incidence of asymmetric defects resulted. These defects appeared analogous to those induced by NDZ exposure, consisting of asymmetric necrosis of mesenchymal tissue near the cephalic end of the neural tube and thinning of the neuroepithelium on the right. We concluded that the hypoxia induced by redox cycling of NDZ and related nitroheterocycles represents a major embryotoxic principle of action.

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

Acetaminophen (APAP) and 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) each produced a similar incidence of, as well as a qualitatively similar, abnormal closure of the anterior neuropore at similar concentrations when added to the medium of cultured rat embryos. At concentrations producing a 50-65% incidence of abnormal neurulation, the affected embryos displayed relatively complete embryonic development as assessed from measurements of protein, axial rotation, and embryonic length. The neural tube defect produced by these agents consisted of elevated neural folds remaining separated by approximately 45 degrees as well as the presence of a mitotically active neural epithelium. In contrast, the nonacetylated structures, p-aminophenol (PAP) and 7-hydroxyaminofluorene (7-OH-AF), were embryotoxic at concentrations 10-fold lower than the corresponding acetylated compounds; each produced a greater incidence of abnormal axial rotation and a greater decrease in embryonic protein than APAP or 7-OH-AAF. In addition, the embryos exposed to PAP or 7-OH-AF were morphologically and histologically dissimilar to those exposed to the acetylated compounds. The neural folds of the latter remained elevated and in apposition, but lacked complete fusion of the folds of neural epithelium and were accompanied by marked cytotoxicity. Addition of active deacetylase sources (guinea pig liver microsomes or commercially obtained, purified carboxylic-ester hydrolase) to the culture medium of conceptuses exposed to 7-OH-AAF or APAP resulted in an increased embryotoxicity which was indistinguishable from that produced by the nonacetylated compounds alone. The increases in toxicity were effectively blocked by the addition of paraoxon, indicating that catalysis of the deacetylation of APAP and 7-OH-AAF was the causative factor. No evidence could be found for deacetylation of 7-OH-AAF or APAP mediated by the Day 10 conceptus itself. When examined for cytotoxicity in F9 embryonal carcinoma cells, APAP and 7-OH-AAF each produced observable cell death only if reduced glutathione (GSH) had previously been depleted and if a deacetylase source were present; this cytotoxicity was also blocked by addition of paraoxon. The nonacetylated metabolites were directly cytotoxic, although GSH depletion greatly increased the incidence of cell death. Therefore, deacetylation of APAP and 7-OH-AAF produced an increase in generalized embryotoxicity and cytotoxicity relative to abnormal neurulation, suggesting that APAP and 7-OH-AAF are capable of eliciting neural tube defects via a mechanism(s) that is distinguishable from the generalized embryotoxicity or cytotoxicity produced by their nonacetylated counterparts.

Studies of mechanisms of niridazole-elicited embryotoxicity: evidence against a major role for covalent binding

Studies reported here were designed to examine the hypothesis that covalent binding of reactive intermediates to macromolecules of the conceptus represents a major mechanism for the embryotoxicity of niridazole (NDZ). The roles of embryonic thiol content and oxygenation on: 1) malformation incidence; 2) reductive metabolism; and 3) covalent binding to embryonic macromolecules of metabolites resulting from reductive biotransformation of NDZ were studied. Results were compared with those from studies with the nondysmorphogenic analog of NDZ, 4'-methylniridazole (MNDZ). Day 10 rat embryos were pretreated for 5 hours in vitro with either L-buthionine-S, R-sulfoximine (BSO) or N-acetylcysteine (NAC) to modulate their glutathione (GSH) content. BSO reduced GSH levels, but NAC was ineffective. Following pretreatment, embryos were cultured for an additional 15 hours in the presence of [14C]NDZ or [14C]MNDZ with an initial oxygen concentration of 5%. At the end of the culture period (day 11, AM), those embryos with active heartbeat and vitelline circulation were examined for asymmetric malformations. Drug metabolites were subjected to multiple extractions from the culture medium and subjected to quantitative high-performance liquid chromatography (HPLC) analysis. Homogenates of the embryos were extracted with trichloroacetic acid (TCA) to estimate the covalent binding of radiolabeled parent compound/metabolites. Autoradiographic analyses were performed on other embryos. BSO pretreatment, which reduces embryonic GSH tissue levels, dramatically increased both the conversion of NDZ to 1-thiocarbamoyl-2-imidazolidinone (TCI) (generated via reductive metabolism of NDZ) and covalently bound label but failed to increase embryotoxicity. NAC, by contrast, did not significantly affect embryonic GSH levels, TCI generation, or covalent binding. Because both rates of metabolism of NDZ to TCI and covalent binding could vary independently of malformation incidence, we concluded that they do not represent critical mechanistic factors for the embryotoxicity of NDZ and related nitroheterocycles.

Developmental aspects of P450IIIA: prenatal activity and inducibility

We have used antiserum of defined specificity as well as a specific inducers and inhibitors of P450IIIA1(2) to determine the fetal occurrence and inducibility of this enzyme in rats. Apparently absent from uninduced fetal rat liver (or present in extremely low amounts) cytochrome P450IIIA1(2) becomes increasingly inducible as a function of gestational age. In adult rats, it is now apparent that there are at least two inducible members and one male-specific constitutive member of the IIIA subfamily. The ontogenesis of these enzymes from 2 weeks post partum to puberty has also been determined. The male-specific occurrence of P450IIIA2 subject to testosterone imprinting and maintenance has been proposed. Inconsistencies persist, however. Waxman et al. have proposed the perinatal occurrence in male and female rats with subsequent suppression in females, whereas others have not detected P450IIIA1(2) in uninduced perinatal rat liver. These differences remain unresolved and reflect the difficulties in defining the individual enzyme specificities for various substrates and of antiserum reactivity. Approaches recently applied to investigations of the IIB subfamily of cytochromes P-450 should contribute greatly to the elucidation of factors governing the ontogenesis of IIIA in rats and humans. Recently, cDNA probes capable of discriminating P450IIB1 and P450IIB2 (commonly referred to as P450s b and e, respectively) were utilized to discriminate the developmental regulation of these immune cross-reactive enzymes. cDNA probes specific for the constitutive and inducible P450IIIA enzymes should clarify the P450IIIA ontogeny in rats. However, in light of regulatory differences among the human and rat members of P450IIIA, it is apparent that the extrapolation of human biotransforming potential from results of animal models must be approached with great caution.

On the substrate specificity of cytochrome P450IIIA1

The instability of the solubilized/purified form, the lack of catalytic activity of the stabilized, macrolide-complexed form, and the compromised catalytic activity of the decomplexed form of steroid-inducible cytochrome P450IIIA1 motivated further investigations of the substrate specificity of this isozyme. A major complementary goal was to identify reactions utilizable as sensitive, specific diagnostic probes for the detection and partial characterization of this isozyme in tissues for which isolation and purification are not practical (e.g., extrahepatic, embryonic tissues, etc.). The approach utilized a combination of a specific, purified inducer, specific inhibitors including triacetyloleandomycin and inhibitory antibodies, and diagnostic probe substrates including the phenoxazone ethers, testosterone, warfarin, 2-acetylaminofluorene, estradiol-17 beta and benzo[a]pyrene. The results obtained indicated that steroid-inducible, rat hepatic P450IIIA1 would catalyze minimal or no O-dealkylation of methoxy-, ethoxy- or pentoxyphenoxazone but catalyzed rapid O-debenzylation of benzyloxyphenoxazone. Hydroxylation of testosterone was specific for the beta face of the molecule at the 2-, 6-, 15- and 16-positions with no detectable conversion to androstenedione and minimal hydroxylation on the alpha face. Both the R- and S-enantiomers of warfarin were attacked at positions 9 and 10, and these reactions appeared to be specific to isozymes of the IIIA family. Aromatic hydroxylation of estradiol-17 beta was efficiently catalyzed, particularly at the 2-position. Hydroxylations of 2-acetylaminofluorene at positions 5 and 7 were catalyzed at relatively rapid rates, but N-hydroxylation of the same substrate was not catalyzed effectively. Hydroxylation of benzo[a]pyrene occurred preferentially at carbon 3 with much lesser activity at carbon 9 and little or no detectable attack at positions 7 or 1. The results indicated that the 2 beta- and 15 beta-hydroxylation of testosterone and the 10-hydroxylation of warfarin would serve as the most useful probes thus far available for detection of the presence of functional P450IIIA1 isozymes in tissues for which isolation and purification are impractical. The results also indicated a very broad, yet selective substrate specificity for the steroid-inducible P450IIIA1.

Cytochrome P-450-dependent biotransformation of a series of phenoxazone ethers in the rat conceptus during early organogenesis: evidence for multiple P-450 isoenzymes

Using highly sensitive probe-substrate analyses, investigations of drug biotransformation in tissues of the rat conceptus during an early stage of organogenesis revealed that three separate tissue components each contained P-450 isozymes capable of catalyzing the monooxygenation of foreign organic chemicals. Tissues of the embryo proper contained constitutive P450(s) that catalyzed readily measurable O-depentylation and O-debenzylation of pentoxyphenoxazone and benzyloxyphenoxazone, respectively, but no measurable O-demethylation of methoxyphenoxazone and barely detectable O-deethylation of ethoxyphenoxazone. Higher specific activities for the O-depentylation and O-debenzylation reactions were measured in preparations of the yolk sac and this organ also appeared to contain constitutive P450(s) for the readily detectable O-deethylation of ethoxyphenoxazone. The O-demethylation of methoxyphenoxazone could not be detected in the yolk sac. Only the O-debenzylation reaction could be detected in tissues of the ectoplacental cone. Treatment of conceptuses in utero with 3-methycholantherene (MC) resulted in significantly increased rates of O-deethylation reactions in preparations of yolk sac and embryo but not ectoplacental cone. Demethylation was not detectable in the same preparations. Treatment with phenobarbital, pregnenolone-16 alpha-carbonitrile, or isosafrole produced no observable effect on any of the reactions studied. Carbon monoxide (CO:O2 = 80:20 versus N2:O2 = 80:20) markedly inhibited all reaction rates and inhibition could be reversed by replacement of CO with N2. Deethylation and debenzylation were inhibited by anti-P450IA1 IgG after MC induction but were not affected by the same IgG fraction in untreated conceptuses. Depentylation reactions were not inhibited by anti-P450IA1 or anti-P450IIB1/2 antibodies under any of the conditions used. Deethylation was strongly inhibited by 1.0 microM 7,8-benzoflavone in tissues from MC-treated but not untreated conceptus. Metyrapone (0.1 mM) failed to significantly inhibit any of the measurable conceptus-catalyzed depentylation reaction. The results indicated the presence of four (or more) functional P450 isozymes in tissues of the conceptus during organogenesis, a constitutive depentylase(s) in the yolk sac and embryo, a constitutive deethylase(s) present in the yolk sac, an MC-inducible deethylase(s) in the embryo and yolk sac, and constitutive debenzylase(s) present in all three tissues. No O-demethylation was detectable in any of the three tissues, even after in utero exposure to inducers.(ABSTRACT TRUNCATED AT 400 WORDS)

Contrasting effects of estradiol-17 beta and 17 alpha-ethinyl estradiol-17 beta on cultured whole embryos

Estradiol-17 beta (E2) and 17 alpha-ethinyl estradiol-17 beta (EE) were compared in terms of their relative capacities to alter growth and developmental patterns of cultured whole embryos during the early stages of organogenesis. Embryos exhibited a notable differential susceptibility to the embryotoxic effects of parents E2 vs EE when these estrogens were added directly to the media at the onset of the culture period. At initial concentrations of 0.1 mM, E2 failed to produce statistically significant effects whereas EE elicited marked embryotoxicity. Inclusion of a P-450-dependent biotransformation system in the culture media resulted in a significant attenuation of the embryotoxic effects of parent E2 vs EE when these estrogens were added directly to the media at the onset of the culture period. At initial concentrations of 0.1 mM, E2 failed to produce statistically embryotoxicity by hepatic S9. The divergent results produced by the two steroids could not be attributed to differences in rates of catecholestrogen generation in the culture medium or by the conceptuses. The results demonstrate definitive dissimilarities between the effects of two steroidal estrogens on developmental parameters and document marked differences in the effects of biotransformation on their embryotoxic potential. The data strongly suggest that the embryotoxicity of these steroids is not mediated via interactions with estrogen receptors. Additionally, the data show that the differential capacity of these two steroids to produce embryotoxic effects is diametrically opposite to earlier reported patterns of their carcinogenic potential in the Syrian hamster kidney.

Glutathione status and the incidence of neural tube defects elicited by direct acting teratogens in vitro

Valproic acid (VPA), cytochalasin D (CD) and 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) each caused abnormal closure of the anterior neuropore in rat embryos cultured in vitro in the absence of an exogenous bioactivation system. Morphological comparisons showed that although all three compounds prevented normal neural tube closure, each did so in a distinctive manner. Modulation of GSH in cultured rat conceptuses was evaluated to determine whether common responses occurred relative to the ability of different chemicals to elicit neural tube defects. Malformation incidence in embryos (10-14 somites) varied widely following exposure to CD (44%), 7-OH-AAF (29%) or VPA (17%). The incidence of CD-elicited malformations was increased by 50% following GSH depletion by L-buthionine-S, R-sulfoximine (BSO) and was decreased by nearly 60% when the cysteine pro-drug 2-oxothiazolidine-4-carboxylate (OTC) was added to the culture medium. GSH modulation also exerted significant effects on the incidence of abnormal neurulation caused by VPA or 7-OH-AAF. A relatively low incidence of open neural tubes produced by VPA or 7-OH-AAF alone was shown to be a function of the state of maturation in the embryos. Conceptuses cultured in the presence of VPA or 7-OH-AAF from an earlier gestational age (6-10 somites) showed 2-3 fold increases in the number of embryos with open neural tubes. Differential alterations in protein and DNA content were observed among embryos and yolk sacs after various treatments indicating possible differences in the site of embryotoxicity. These results demonstrate the role of GSH status on the capacity of three chemically diverse compounds to elicit abnormal neurulation in cultured rat embryos and suggest some possible mechanisms by which normal neurulation may be compromised.

Niridazole metabolism by rat embryos in vitro

We report the results of studies on the reductive activation of the schistosomicidal agent, niridazole (NDZ). Intact rat embryos in vitro reduced this compound, generating a stable metabolite in the presence of 5% O2. By contrast, embryo and yolk sac homogenates or liver microsomes appeared to require anaerobiasis. Malformation incidence--specifically, axial asymmetry--showed a strong correlation with nitroreductase activity rates when the latter were modulated by oxygen tension. Data presented here suggest that when embryos are exposed to NDZ under conditions of low oxygen in vitro, redox cycling ensues with molecular oxygen serving to oxidize early reduction products. This process continues, regenerating the parent compound until oxygen is depleted locally. The basis of this localized depletion is unknown, but inability of the immature supply system to replete oxygen or demand by precociously aerobic tissues may be involved. Once local anaerobiasis is attained, further reduction could generate toxic metabolites capable of covalently binding cellular macromolecules. Localized hypoxia represents another potential mechanism of dysmorphogenesis.

Embryotoxicity elicited by inhibition of gamma-glutamyltransferase by Acivicin and transferase antibodies in cultured rat embryos

Acivicin (also known as AT-125) and IgG isolated from goat anti-gamma-glutamyltransferase antiserum were used to inhibit the activity of gamma-glutamyltransferase (GGT, EC 2.3.2.2) in rat conceptuses cultured from Days 10 to 11 of gestation. Inhibition of GGT by either Acivicin or anti-GGT IgG produced embryotoxicity and malformations, although each compound produced a unique spectrum of effects. Acivicin, at an initial concentration in the culture medium of 5 microM, produced a marked decrease in yolk sac vasculature and was associated with embryonic malformations such as neural tube necrosis, microophthalmia, and cephalic edema after 24 hr exposure. These malformations were accompanied by significant decreases in both embryonic and yolk sac protein, yolk sac GGT activity, as well as embryonic glutathione (GSH) levels. In contrast, anti-GGT IgG produced no apparent effects on yolk sac vasculature or protein after exposure of conceptuses to an initial concentration of 50 micrograms IgG/ml culture medium, even though equal inhibition of yolk sac GGT (30%) was achieved by each inhibitor. Exposure to IgG (50 micrograms/ml) for 24 hr was associated with decreased embryonic protein; decreased levels of GSH in the embryo were observed after both 3 and 24 hr. The dichotomy of effects on the yolk sac by the two compounds indicates that Acivicin produced these effects by mechanisms other than by GGT inhibition alone. These results demonstrate that inhibition of GGT in rat embryos undergoing organogenesis can elicit embryotoxic effects and produce alterations in GSH levels. The capacity of the anti-GGT antibody to inhibit the GGT activity in the yolk sac (while having no apparent effect on yolk sac morphology), and yet influence the embryo by decreasing protein and GSH levels, underscores the important role of the yolk sac during the highly sensitive stages of organogenesis.

Conversion of estradiol-17 beta to reactive embryotoxic intermediates by cytochrome P-450-dependent bioactivating systems

P-450-dependent enzyme systems added to media of cultured rat embryos markedly increased the embryotoxicity of estradiol-17 beta. Increases were markedly attenuated by omission of NADPH, omission of enzyme, substitution of female for male rat liver as enzyme source, d) replacement of N2 with CO or replacement of estradiol-17 beta with diethylstilbestrol. Embryotoxicity correlated well (r = 0.84) with catecholestrogen generating activities. Addition of a catechol-methylating system failed to modify embryotoxicity even though large quantities of methoxyestrogens were formed. The results document that endogenous estrogen can be converted by P-450 to embryotoxic intermediates and suggest that reactive proximate metabolites are precatechols, perhaps epoxyenones.

Regulation of intracellular glutathione in rat embryos and visceral yolk sacs and its effect on 2-nitrosofluorene-induced malformations in the whole embryo culture system

The dysmorphogenic effects of 2-nitrosofluorene (NF) in vitro were modulated in Day 10 rat embryos by agents which regulate intracellular glutathione (GSH) levels. The incidence of abnormal axial rotation caused by NF alone increased in a dose-dependent manner at NF concentrations in excess of 25 microM. No effects were observed at 15 microM NF and doses of 100 microM resulted in a 100% incidence of mortality. L-Buthionine-S,R-sulfoximine (BSO), an inhibitor of GSH synthesis, produced malformations (50%) in embryos exposed to 15 microM NF but produced no additional effects on embryos at higher NF concentrations. BSO treatment alone resulted in a greater than 50% decrease in GSH content in visceral yolk sacs and had a lesser but likewise significant effect (15% decrease) on the GSH content of embryos. Protein content was inversely affected as embryonic levels were increased by 20% and yolk sac levels were unchanged. When BSO was added in combination with NF at the onset of the culture period, embryonic GSH decreased in a dose-dependent manner, suggesting a relatively low rate of embryonic GSH turnover that could be increased by addition of an exogenous substrate capable of forming adducts with and removing GSH from the cells. 2-Oxothiazolidine-4-carboxylate (OTC), a compound which is enzymatically modified to provide an additional source of intracellular cysteine and increase GSH synthesis, produced no significant changes in embryonic or yolk sac GSH when added alone to the culture medium. When OTC (5 mM) was added in combination with NF, however, NF-elicited malformations were eliminated. This was also the case at 100 microM NF in which OTC not only prevented malformations but completely protected embryos against the loss in viability. The GSH and protein levels were indistinguishable from controls when OTC and NF were added simultaneously except for the 41 microM NF dose at which a highly significant increase in both embryonic and yolk sac protein was observed. This study clearly demonstrates the potential importance of GSH in the modulation of chemical dysmorphogenesis and provides an important new tool for the study of mechanisms of developmental toxicity.

Embryotoxicity induced by diethylstilbestrol in vitro

The embryotoxic potential of diethylstilbestrol (DES) was examined in a whole embryo culture system containing a P-450-dependent bioactivating system. Sprague-Dawley rat embryos were explanted on day 10 and cultured for 24 hours. Concentration-dependent effects of DES on embryonic growth parameters, viability, and embryotoxicity were observed. Concentrations of DES greater than 0.26 mM (final concentration) produced 100% embryolethality, while those below 0.15 mM were without significant effects. At a final concentration of 0.19 mM, DES produced only a slight increase in embryolethality. The same concentration elicited a marked increase in observed embryotoxicity, including prosencephalic hypoplasia, incomplete axial rotation, and open neural tubes. In addition, reductions in embryonic length, somite number, and protein and DNA content were observed. An exogenous P-450-dependent hepatic biotransforming (catechol-generating) system failed to alter either the incidence of observed toxic effects or measured growth parameters. Likewise, exposure of cultured embryos to 20% carbon monoxide (CO) failed to reduce DES-induced embryotoxicity, indicating a lack of participation of an endogenous P-450-dependent embryonic bioactivating system. Arachidonic acid (0.20 mM) and/or indomethacin (0.50 mM) also had no observable effect on DES-induced embryotoxicity, suggesting that prostaglandin synthase was not involved in the embryotoxic activity of DES, as has been proposed to explain its carcinogenic effect. The antioxidants N-acetylcysteine (1.14 mM) and alpha-tocopherol (0.08 mM) failed to protect against DES-induced embryotoxicity, while the anti-estrogen tamoxifen (up to 0.85 mM) actually enhanced this effect of DES in culture. The DES analogs Z,Z-dienestrol (DIES, 0.10 mM) and hexestrol (HES, 0.48 mM) were both embryotoxic in vitro. The presence of an exogenous P-450-dependent hepatic biotransforming system appeared to protect against HES-induced embryolethality but had little effect upon DIES-induced embryotoxicity. The results were consistent with a direct effect of DES independent of either estrogenicity or exogenously generated metabolites.

Differential glutathione depletion by L-buthionine-S,R-sulfoximine in rat embryo versus visceral yolk sac in vivo and in vitro

L-Buthionine-S,R-sulfoximine (L-BSO), a selective inhibitor of glutathione synthesis, exhibited the capacity to deplete embryonic and visceral yolk sac glutathione (GSH) after administration in vivo and also after addition of L-BSO to a whole embryo culture system. Administration of L-BSO to pregnant dams at 2.5 or 18 hr prior to the explantation of day 10 conceptuses resulted in greater than 55% GSH depletion relative to control values in both the yolk sac and the embryo proper. The levels of GSH returned to normal in all embryos and in the corresponding visceral yolk sacs pretreated at 2.5 hr (but not at 18 hr) after culturing for 24 hr in L-BSO-free medium. GSH content was significantly lower in yolk sacs but not in embryos of conceptuses cultured for 24 hr in medium containing 1 mM L-BSO. This treatment, however, significantly increased the amount of protein and DNA in the embryo. The differential sensitivity of the yolk sac versus embryo and the demonstration of the ability to modulate tissue levels of GSH during organogenesis promise to provide important new tools for the study of embryonic protective mechanisms in chemical teratogenesis.

Modulation of the embryotoxicity in vitro of reactive metabolites of 2-acetylaminofluorene by reduced glutathione and ascorbate and via sulfation

To provide insights into mechanisms whereby reactive intermediary metabolites of foreign chemicals elicit teratogenic and embryotoxic effects, we initiated investigations of the capacity of physiologic factors to modulate the effects of embryotoxic metabolites of 2-acetylaminofluorene (AAF). The whole embryo culture system was utilized in order to avoid potentially confounding maternal factors. Reduced glutathione (GSH) effectively protected cultured embryos from the embryolethal effects of N-acetoxy-2-AAF (AAAF) and also reduced the severity of AAAF-elicited malformations although the percentage of embryos exhibiting malformations was not affected significantly. GSH also reduced the embryolethality of 2-nitrosofluorene (NF) as well as the percentage of NF-elicited axial rotation defects. Ascorbate protected against the embryolethality of both AAAF and NF and exhibited significant protection in terms of the capacity of NF to cause flexure abnormalities. However, significant protection against NF-elicited prosencephalic deformities was not detected. N-Acetylcysteine and alpha-tocopherol each failed to produce significant protection, even at the highest concentrations utilized. Enzymatic sulfation of N-hydroxy-AAF (N-OH-AAF) markedly increased the incidence of observable malformations. Synthesized N-sulfonyloxy-AAF also elicited a high incidence of malformations at relatively low concentrations when added to the culture medium. Malformations elicited, however, resembled those produced by NF rather than by AAAF. The results suggest that endogenous metabolic systems can play a critical role as determinants of both the quantitative and the qualitative capacity of foreign organic chemicals to produce embryotoxic or teratogenic effects via the generation of reactive intermediates.

Valproate hydroxylation by human fetal tissues and embryotoxicity of metabolites

The oxidative biotransformation of sodium valproate was studied in liver, lung, brain, and adrenal homogenates from human conceptuses with gestational ages ranging from 50 to 77 days. Analyses of metabolites by GC/MS indicated the formation of 3-hydroxy-, 4-hydroxy-, and 5-hydroxyvalproic acid, with hydroxylation occurring preferentially at the 4- position. The adrenal homogenate was consistently the most active fetal tissue studied, with rates of hydroxylation similar to those in rat and macaque liver homogenates. Reaction rates in the fetal adrenal homogenate were approximately four times those in fetal liver and approximately 10 times the rates of the same reactions measured in fetal brain and lung. Although valproic acid itself (0.8 mmol/L) was highly embryotoxic to cultured whole rat embryos, none of the hydroxylated metabolites produced by human fetal tissues exhibited significant embryotoxicity at equimolar concentrations. This suggests that hydroxylation of valproic acid in human fetal tissues is a process of detoxification, and implies that valproic acid is a direct-acting teratogen.

Phase II biotransformation of carcinogens/atherogens in cultured aortic tissues and cells. II. Glucuronidation of 3-hydroxy-benzo(a)pyrene

Avian aortic tissues contain active UDP-glucuronosyltransferase(s) (EC 2.4.1.17) which catalyze(s) the glucuronidation of 3-hydroxybenzo(a)pyrene and p-nitrophenol. Activities were compared in abdominal segments (susceptible to the atheroma-promoting effects of polynuclear aromatic hydrocarbons) vs. thoracic segments (susceptible to the atheroma-initiating effects of polynuclear aromatic hydrocarbons). Activities measured in the abdominal segments were approximately 8-9-fold higher than those measured in thoracic segments from the same cockerels. Surprisingly, pretreatment of cockerels with phenobarbital, but not 3-methylcholanthrene, resulted in small (60-120%) but consistent increases in glucuronosyltransferase activities in both aortic segments. Activities were readily detectable in microsomal fractions of both segments and in cultured smooth muscle cells derived from aortic segments, but were not detectable in cultured endothelial cells. Avian aortic microsomal glucuronosyltransferases exhibited minimal or no response to the effects of several common activators of hepatic microsomal glucuronosyltransferases.

Carbon monoxide inhibits monooxygenation by the conceptus and embryotoxic effects of proteratogens in vitro

Evidence that Day 10 to 11 rat conceptuses can contain sufficient quantities of xenobiotic-oxidizing P-450(s) to catalyze the conversion of proteratogens to reactive teratogenic intermediates is presented. Quantities of reactive intermediates generated by conceptuses from 2-acetylaminofluorene (AAF) were sufficient to result in the appearance of gross morphologic abnormalities with a high incidence of neural tube anomalies. Carbon monoxide effectively inhibited the conversion of AAF to hydroxylated metabolites by rat conceptuses, and also inhibited the capacity of AAF to elicit neural tube anomalies in cultured embryos. Inhibition by CO of either parameter was observed only with conceptuses exposed in utero to the potent P-450 inducer, 3-methylcholanthrene.

Phase II biotransformation of carcinogens/atherogens in cultured aortic tissues and cells. I. Sulfation of 3-hydroxy-benzo(a)pyrene

The capacity of polynuclear aromatic hydrocarbons to elicit profound effects on the development of avian aortic atheromata has raised questions regarding the biotransformation of polynuclear aromatic hydrocarbons in the target (aortic) tissue. Results of this investigation demonstrate the capacity of aortic enzymes to affect the sulfoconjugation of 3-hydroxy-benzo(a)pyrene and describe several characteristics of the aortic sulfotransferase activity. Conjugating activities measured in avian aortic tissues were approximately 10-20% of those assayed in corresponding preparations of avian hepatic tissues under the same reaction conditions. Activities were measured in homogenates, in a series of homogenate subfractions, in whole organ cultures, in cultured aortic endothelial cells, and in cultured aortic smooth muscle cell preparations. Sulfoconjugation was localized in the cytosolic fraction and kinetics in this fraction yielded a range of apparent Km values from 9 to 16 microM (mean = 11.8 +/- 3.1, n = 4) and a range of apparent Vmax values from 281 to 457 pmol/mg of protein/30 min (mean = 360 +/- 49, n = 4). Abdominal and thoracic segments of the aorta exhibited virtually identical specific activities. Also, activities assayed in cultured aortic smooth muscle cells were similar to those measured in cultured aortic endothelial cells. Capacity to generate adenosine 3'-phosphate 5'-phosphosulfate (PAPS) appeared to limit the reaction rate as judged by comparative investigations with PAPS and a PAPS-generating system. Aortic sulfatases actively hydrolyzed benzo(a)pyrene-3-O-sulfate. The sulfatase activity appeared to partially mask sulfotransferase activities measured in organ and cell culture preparations and in particulate subfractions of cellular homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytosolic activation of hematin-dependent microsomal monooxygenase activity in the lung

Additions of micromolar concentrations of hematin to washed rat pulmonary microsomal preparations resulted in marked (5-7-fold) increases in the NADPH-dependent generation of phenolic metabolites of benzo[a]pyrene (BaP). 9-Hydroxy-BaP was identified as the major reaction product. Additions of pulmonary cytosolic fractions to microsomes produced no measurable effect but cytosol and hematin added together elicited 25-30-fold increases in total phenolic products. Cytosolic fractions from other tissues, including rat kidneys and perfused rat livers, were also highly effective in enhancing the hematin-mediated increases in monooxygenase activity. However, cytosol from human placental tissues was only minimally effective when either pulmonary or placental microsomes were utilized as enzyme source. Superoxide dismutase and catalase (alone or in combination) had no measurable effect on hematin-mediated increases. Horseradish peroxidase effectively inhibited the hematin-dependent reactions but hematin-independent reactions were inhibited with equal effectiveness. Carbon monoxide profoundly inhibited all hematin-mediated increases in metabolite formation. The activating cytosolic component was non-dialyzable, inactivated by trypsin and heat, and eluted in the void volume from Sephadex G-150 columns. This suggested that the cytosolic factor(s) responsible for the increased hematin-dependent oxidation was a protein(s) with a high molecular weight or perhaps an aggregate or oligomer of proteinaceous material. HPLC profiles indicated a major effect on the generation of phenolics; quinones were also increased but only minimal increases in diols were observed. Results were consistent with the hypothesis that hematin-mediated increases in pulmonary monooxygenase activity result from an increased association of a small pool of pulmonary P-450-apoprotein(s) with the hematin prosthetic group to result in increased levels of an unidentified holocytochrome(s) with a relatively high substrate turnover number. The current data suggest a quaternary interaction among P-450 apoprotein(s), heme prosthetic group, reaction products (particularly 3-hydroxy-BaP) and a cytosolic protein(s). We postulate that the mechanism of action of the cytosolic factor is to facilitate the interaction of hematin with the apocytochrome.

On the capacity of nitroheterocyclic compounds to elicit an unusual axial asymmetry in cultured rat embryos

A series of nitroheterocyclic compounds with antimicrobial and radiosensitizing properties was tested for embryotoxicity in cultured Sprague-Dawley rat embryos, and their effects were compared with various other five-membered heterocycles. Nifuroxime, furazolidone, nitrofurazone, niridazole, 2-nitroimidazole, and ronidazole each elicited a striking malformation characterized by asymmetrical, right-sided hypoplasia when coincubated with embryos for 26 hr. Minimum concentrations required to elicit this unusual defect ranged from 0.01 mM with nifuroxime and furazolidone to 0.5 mM with ronidazole and were roughly correlated with single electron redox potentials; i.e., agents with relatively high redox potentials were generally more effective than those with low potentials. Nitrofurantoin failed to elicit this unusual malformation but exhibited an extremely steep dose-response curve for embryolethality. Metronidazole and 4-nitroimidazole, nitroheterocyclic agents with relatively low redox potentials, did not produce the asymmetric abnormality nor were they highly embryotoxic, even at concentrations up to 2 mM. 2-Amino-5-nitrothiazole and 4'-methylniridazole also failed to evoke the asymmetric malformation but produced significant embryotoxicity as manifested by decreased growth parameters and elicitation of other kinds of malformations. Heterocyclic compounds not bearing a nitro group (furosemide, 2-aminothiazole, and 2-aminoimidazole) failed to elicit axial asymmetry at concentrations up to 1.0 mM but produced other signs of embryotoxicity at the highest concentrations tested. The results suggest that the presence of a reducible nitro group is critical for generation of the unusual malformation and that the single-electron redox potential of the nitro group plays a dominant but not exclusive role in the capacity of these chemicals to evoke axial asymmetry in cultured rat embryos.

The toxicity of niridazole in rat embryos in vitro

The antischistosomal drug niridazole (NDZ) was found to be teratogenic in a concentration-dependent manner from 10 to 50 micrograms/ml (47-233 microM) in rat embryos cultured from day 10 to day 11. A striking malformation consisting of axial asymmetry in which the right side of the embryo showed severe necrosis was the predominant malformation. The drug showed significantly greater dysmorphogenic activity at low (5%) compared to high (20%) oxygen tensions in cultures. Coincubation of embryos and NDZ with an exogenous source of metabolic enzymes and cofactors (NADPH) failed to modify teratogenicity. Inclusion of CO in the culture atmosphere significantly reduced the malformation incidence as did preincubation of the drug with S-9 and cofactors in medium with low O2 tension. Treatment of gravida with NDZ up to and including the maternal-lethal dose failed to result in observable malformations despite the use of several routes of exposure. These data lead us to hypothesize that rat embryos are capable of performing the reductive activation of NDZ in a fashion analogous to the schistosome but that reductive extraembryonic metabolism may result in teratogenic bioinactivation.

Generation of reactive dysmorphogenic intermediates by rat embryos in culture: effects of cytochrome P-450 inducers

We have investigated the capacity of cultured whole rat embryos to convert 2-acetylaminofluorene (AAF) to reactive metabolites capable of eliciting dysmorphogenic effects in the same embryos. Cultured embryos (Sprague-Dawley) were exposed to AAF for periods of 2 or 24 hr, after which metabolites were isolated from the culture medium and identified with HPLC. Embryotoxic effects were evaluated in the same embryos. Day 10 embryos preexposed in utero to pregnenolone-16 alpha-carbonitrile (PCN) exhibited marked increases in capacity to convert AAF to a variety of hydroxylated metabolites. 3-Methylcholanthrene (3MC) was also a very effective inducer in utero but Aroclor 1254 (PCB), and isosafrole (ISF) evoked only minimal induction while phenobarbital (PB) was not demonstrably effective. Exogenously added hepatic postmitochondrial supernatant (S9) fractions from adult male rats pretreated with PCB, 3MC, or ISF exhibited induced monooxygenase activities as well as increased capacity to convert AAF to dysmorphogenic intermediates in the culture system. PB and PCN displayed much lesser effects. PCN was a very effective inducer of hepatic monooxygenases of pregnant rats but, when this tissue was utilized as an enzyme source, no significant increase in malformations was observed. Embryos with relatively high monooxygenase activities also displayed a high incidence of embryonic abnormalities when cocultured with AAF. Malformation incidence was strongly correlated with hydroxy metabolite generation, suggesting that induction in utero of P-450-dependent, embryonic monooxygenases resulted in the production of embryotoxic metabolites by the embryos own enzymes. The data also indicated that endogenous bioactivation (within the conceptus) was considerably more effective than bioactivation effected by an exogenous (hepatic) enzyme source.

The effect of oxygen concentration on the teratogenicity of salicylate, niridazole, cyclophosphamide, and phosphoramide mustard in rat embryos in vitro

Comparisons were made of rat embryos cultured at 5% or 20% oxygen in the presence of salicylate (SAL), cyclophosphamide (CP), niridazole (NDZ), or phosphoramide mustard (PM). Multiple regression analyses were used to compare the effects of drug concentration, oxygen concentration, and the product of drug times oxygen concentration on malformation incidence, viability, and protein content of embryos cultured for 24 hours. Drug concentration significantly affected malformation incidence or severity and protein content (P less than 0.001) for the four drugs tested. Oxygen concentration significantly affected protein content for the four compounds (P less than 0.001) but affected malformation incidence only with NDZ. Furthermore, the interaction of oxygen concentration and drug concentration significantly affected the malformation incidence in the presence of NDZ (P less than 0.001), and protein content (P less than 0.001) and viability (P less than 0.001) in the presence of CP. The pattern of significant effects of the independent variables (drug concentration, oxygen concentration, and drug times oxygen concentration) is consistent with the hypotheses of oxygen-dependent metabolism (or lack of metabolism) of the drugs in question. NDZ, which is thought to be converted to reactive intermediates by an oxygen-inhibited nitroreductase, was more toxic at reduced oxygen tension. CP, which is activated by an oxygen-dependent P-450 system, was more toxic with increased oxygen tension. Significant effects of the independent variables on embryos exposed to SAL or PM were consistent with the effects on control embryos, notably, increased protein content with increased oxygen.

Analysis of metabolites of 2-acetylaminofluorene generated in an embryo culture system. Relationship of biotransformation to teratogenicity in vitro

2-Acetylaminofluorene (AAF) produced abnormal, open neural tubes in cultured whole rat embryos only in the presence of an added, NADPH-dependent monooxygenase system. Reactive intermediary metabolites, including N-hydroxy-AAF, N-hydroxy-2-aminofluorene, 2-nitrosofluorene and N-acetoxy-AAF, each elicited embryonic malformations under culture conditions, but a statistically significant increase in the incidence of abnormal neurulation was not observed. Using [14C]AAF and high pressure liquid chromatography (HPLC) separation techniques, the biotransformation of AAF was studied under conditions in which embryos and the monooxygenase system were coincubated. The major metabolites produced cochromatographed with 5-hydroxy-AAF, 7-hydroxy-AAF, 9-hydroxy-AAF and 3-hydroxy-AAF. Other metabolic products also were detected. The embryonic effects of these major AAF metabolites were tested singly and in combination in the embryo culture system. Addition of 7-hydroxy-AAF to the embryo culture system resulted in open neural tubes in the absence of an added monooxygenase system. Other individual ring-hydroxylated metabolites produced retarded growth, but neurulation appeared normal. Ring-hydroxylated metabolites, added to the embryo culture system in combination in the same proportions as were formed during biotransformation in culture, also produced a marked increase in incidence of neural tube defects in the absence of an exogenous (added) biotransforming system. In combination with 3-, 5- and 9-hydroxy-AAF, 7-hydroxy-AAF exposure (86 microM) resulted in a 47% incidence of abnormal, open neural tubes. When tested individually, higher concentrations of 7-hydroxy-AAF (104 microM) produced a lower percentage of malformed embryos (13%). The results suggested that 7-hydroxy-AAF was principally responsible for the neural tube defects caused by AAF following monooxygenase-dependent bioactivation, but that other metabolites also appeared to contribute to the observed effect.

Effects of 3-methylcholanthrene and phenobarbital on the capacity of embryos to bioactivate teratogens during organogenesis

Pregnant Sprague-Dawley rats were divided into four groups and given ip injections of 3-methylcholanthrene (MC) in corn oil, corn oil only, phenobarbital (PB) in Hank's balanced salt solution (HBSS), or HBSS only. Maternal animals were killed on Day 10 of gestation, and embryos from each group were explanted in medium containing cyclophosphamide (CP), 2-acetylaminofluorene (AAF), or dimethylsulfoxide vehicle. After a 24-hr culture period, embryos from dams treated with HBSS, corn oil, or PB/HBSS exhibited no increase in abnormalities (as compared with controls) when either CP or AAF were added to the media. However, embryos transplacentally preexposed to MC and subsequently treated during culturing with AAF (but not CP) exhibited striking increases in malformation incidence. Commonly observed malformations included abnormally open neural tubes, abnormal flexure rotation, and prosencephalic defects. Homogenates of Day 10 embryos transplacentally preexposed to MC exhibited readily measurable oxidative biotransformation of AAF as assessed with HPLC. Biotransformation of AAF by embryos from the other three groups was virtually undetectable. Incorporation of exogenously supplemented bioactivating systems from livers of mature animals indicated that postmitochondrial supernatant fractions (S-9) from male, MC-pretreated rats effectively catalyzed the conversion of AAF (but not CP) to embryotoxic metabolites. Conversely, hepatic S-9 from adult, male, PB-pretreated rats was highly effective in converting CP (but not AAF) to embryotoxic metabolites. The results indicated the inducerspecific occurrence of embryonic bioconversion of AAF to embryotoxic metabolites via MC-inducible, P-450-dependent, embryonic enzyme systems.

The embryotoxicity of adriamycin in rat embryos in vitro

Adriamycin (ADR) is a widely used and highly valued antineoplastic agent but chronic treatment is limited by cardiotoxicity. To investigate its embryotoxic potential, cultured rat embryos were exposed to ADR at concentrations ranging from 0.4 to 1.2 microM. Within this range, ADR elicited decreases in growth parameters (somite numbers, embryonic length, and protein and DNA content), malformations involving the prosencephalic region, and embryolethality at the higher concentrations. Embryotoxicity was significantly increased (p less than 0.05) when cultures included cofactors for cytochrome P-450-dependent biotransformation and a hepatic microsomal preparation (S-9) prepared from animals pretreated with 3-methylcholanthrene or a mixture of polychlorinated biphenyls (Aroclor 1254). When S-9 from control animals or from rats pretreated with phenobarbital was used, significant increases in ADR-elicited embryotoxicity were not observed. Substitution of NADH for NADPH as a cofactor reduced the incidence of malformations from 100 to 60% at ADR concentrations of 0.5 microM. Increasing O2 concentrations partially counteracted the embryotoxic effects of ADR. Several other agents [including various antioxidants, compounds bearing free sulfhydryl groups, coenzyme Q10, and superoxide dismutase (with or without catalase)] that prevent or reduce the cardiotoxicity of ADR without impairing its antineoplastic properties, failed to modify the embryotoxicity significantly. This suggested that the embryopathic and antineoplastic properties of ADR may share a common mechanism which is distinct from that responsible for cardiotoxicity.

Focal smooth muscle proliferation in the aortic intima produced by an initiation-promotion sequence

Human atherosclerotic fibrous plaques display a clonal character similar to many benign neoplasms. We report here that chickens treated with an initiation-promotion sequence developed focal intimal smooth muscle lesions in the thoracic aorta that resemble early forms of atherosclerosis. Scanning electron microscopy revealed small mound-like lesions protruding from an intact endothelium in birds treated with an initiating dose of 7,12-dimethylbenz[a]anthracene (Me2BA) followed by twice weekly injections of the alpha 1-selective adrenergic agonist methoxamine for 20 weeks. Intimal lesion foci were composed of densely packed modified smooth muscle cells, abundant extracellular matrix, and occasional mononuclear cells (possibly monocytes). There was no ultrastructural evidence of lipid accumulation or alteration of the underlying media. These intimal lesions appeared in aortic segments of treated chickens in a pattern similar to that observed in classical experiments of multistage tumorigenesis in epidermis and other tissues. The treatment with Me2BA followed by methoxamine produced more focal lesions per thoracic segment and more segments per group with lesions than did treatment with either Me2BA or methoxamine alone. Thoracic intimal foci were absent from untreated and vehicle-treated groups. In contrast, the growth of a spontaneously arising atheroma in the distal abdominal aorta was not demonstrably affected by the initiation-promotion regimen. Likewise, weekly injections of Me2BA for 23 weeks, while greatly enhancing abdominal atheroma growth, produced no thoracic lesions. These results provide evidence that focal proliferation of intimal smooth muscle cells, a critical early event in atherogenesis, can be produced by an initiation-promotion treatment sequence.

Regulation of aromatic oxidation of estradiol-17 beta in maternal hepatic, fetal hepatic and placental tissues: comparative effects of a series of inducing agents

The effects of nine separate inducers of cytochrome P-450-dependent monooxygenases on the hydroxylation of estradiol-17 beta (E2) were investigated in near-term pregnant rats. Isosafrole exhibited highly effective inducing properties in the maternal liver (20-fold and 5-fold increases in 4- and 2-hydroxylase activities respectively). Pregnenolone 16 alpha-carbonitrile produced approx 20- and 30-fold increases in measured respective rates of 4- and 2-hydroxylase activities in fetal hepatic tissues; isosafrole produced only 2-fold increases in the same reaction. Only minor changes or slight increases in estrogen hydroxylation rates were observed in maternal hepatic, fetal hepatic or placental tissues following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or other potent 3-methylcholanthrene (MC)-like inducing agents (beta-naphthoflavone, MC, caffeine). Phenobarbital exhibited relatively weak inducing properties and exposure of pregnant rats to ethanol from days 3-19 of gestation was without statistically significant effects on the parameters investigated. Rat placentas exhibited extremely low estrogen hydroxylase activities irrespective of pre-exposure of pregnant rats to the inducers studied. The results suggested separate regulatory controls for estrogen 2- and 4-monooxygenase activities even though relatively high correlation between the two reaction were generally observed in all three tissues.

Stereoselective and regioselective hydroxylation of warfarin and selective O-dealkylation of phenoxazone ethers in human placenta

The oxidative metabolism of warfarin and a series of phenoxazone ethers was studied in two groups of human placentas which exhibited high or low levels of aryl hydrocarbon hydroxylase (AHH). Warfarin metabolism was stereoselective (mean R/S = 2.48) for the R-enantiomer and regioselective for the 6- and 8- positions in the high AHH group whereas warfarin metabolism in the low AHH group displayed no significant overall stereoselectivity (mean R/S = 1.24) and was regioselective for the 7- position. The high AHH group metabolized the methyl, ethyl, propyl and butyl ethers of phenoxazone rapidly, while the low AHH group catalyzed their biotransformation at very low or negligible rates. Neither group detectably metabolized phenoxazone or pentyloxyphenoxazone whereas both groups metabolized benzyloxyphenoxazone at low but similar rates. Rates of warfarin R-6 and R-8 hydroxylation were highly correlated with metabolism of benzo(alpha)pyrene (r = 0.99) and the C1-C4 phenoxazone ethers (r greater than 0.87), but poorly correlated with metabolism of benzyloxyphenoxazone (r less than 0.50). These data support the use of warfarin and the phenoxazone ethers as sensitive biochemical probes for P-450 isozymes in human extrahepatic tissues. They indicate the presence of a multiplicity of xenobiotic metabolizing P-450's in placental tissue which has not been exposed to inducing agents that elevate AHH.

Interaction of alpha and beta adrenergic stimulation on aortic ornithine decarboxylase activity

The interaction between beta and alpha adrenergic agonists on regulation of cockerel aortic ornithine decarboxylase (ODC) activity was examined. The beta adrenergic agonist isoproterenol both reduced basal aortic ODC activity and prevented induction of the decarboxylase by the alpha adrenergic agonist methoxamine. 3-Isobutyl-1- methylxanthine (IBMX) similarly reduced basal ODC activity and blocked induction of the enzyme by methoxamine. When given ten minutes before or after methoxamine, isoproterenol prevented aortic ODC induction, but not large sustained increases in blood pressure evoked by the alpha adrenergic agonist. In contrast, when injected three hours after methoxamine, isoproterenol had no effect on already elevated levels of enzyme activity. Addition of isoproterenol (10(-7)M), IBMX (1 mM) or dibutyryl cAMP (2.5 mM) to isolated aortic segments cultured in minimal salts-glucose media evoked decreases in basal levels of ODC activity resembling those observed in the intact animal. These results suggest that the balance between alpha and beta adrenergic stimulation may be an important feature of the regulation of polyamine biosynthesis in artery wall cells.

Teratogenicity in vitro of two deacetylated metabolites of N-hydroxy-2-acetylaminofluorene

In previous studies [E. Faustman-Watts, J. C. Greenaway, M. J. Namkung, A. G. Fantel, and M. R. Juchau (1983) Teratology 27, 19-28] an embryo culture system was utilized to investigate the role of biotransformation in the embryotoxicity of 2-acetylaminofluorene. For this investigation, the capacity of two deacetylated metabolites of N-hydroxy-2-acetylaminofluorene (N-OH-AAF) to produce malformations in cultured whole rat embryos is reported. The relative capacities of N-hydroxy-2-aminofluorene (N-OH-AF) and 2-nitrosofluorene (NF) to elicit embryotoxic effects, including embryolethality, malformations, growth retardation, and alterations in macromolecular content, were assessed and compared with effects produced by N-OH-AAF and bioactivated 2-acetylaminofluorene (AAF). Qualitatively similar patterns of malformations were produced by NF and N-OH-AF. At initial concentrations greater than 60 microM, both deacetylated compounds caused abnormalities in axial rotation (flexure), decreased viability, and decreases in embryonic DNA and protein content. Both chemicals were active in the absence of a bioactivating system. AAF produced a different spectrum of defects, and was active only in the presence of a complete monooxygenase system. The malformations produced by bioactivated AAF included abnormally open neural tubes; flexure abnormalities were rarely observed. The primary defect elicited by N-OH-AAF was prosencephalic hypoplasia. This chemical was active without an added bioactivating system. Temporal studies demonstrated that exposure of embryos to NF (128 microM) for as little as 2 hr was sufficient to elicit embryotoxic effects. None of the individual metabolites appeared to be solely responsible for the interruptions of neural tube closure produced by bioactivated AAF.

Embryotoxic effects of salicylates: role of biotransformation

The three major metabolites of salicylate, o- hydroxyhippurate ( salicylglycine , salicyluric acid), 2,5-dihydroxybenzoate (gentisic acid), and 2,3-dihydroxybenzoate, were examined for their capacities to elicit dysmorphogenesis, embryolethality , and growth retarding effects in an embryo culture system. The effects were compared with those produced by the parent salicylate. At the highest concentrations tested (1.9 mM), none of the three metabolites produced significant increases in the number of malformed embryos or in embryolethality . At the same concentration, all three agents reduced crown-rump lengths and somite numbers slightly but significantly (p less than 0.01), and the dihydroxy metabolites also reduced the embryonic protein content (p less than 0.01). In contrast, the parent salicylate produced large increases in embryolethality ( embryolethality in controls was 6% or less) and malformed embryos at equivalent or lower concentrations. Preincubation of the parent salicylate with various biotransforming systems did not affect embryotoxicity significantly. The most rapid biotransformation of salicylate in vitro was achieved with mitochondrial preparations of monkey kidney as the enzyme source but quantities metabolized were not sufficient to prevent malformations in the culture system. Increased serum protein concentration in the culture medium, however, markedly reduced the capacity of added salicylate to cause malformations. An examination of the kinetics of the dysmorphogenic effects of parent salicylate indicated that 5 hr of exposure elicited nonsignificant increases in numbers of malformations. A significant malformation rate was produced by 9 hr of exposure. In contrast, effects on embryonic growth parameters and embryolethality were greatest after a 24-hr exposure period. The results strongly suggest that the parent salicylate, rather than generated metabolites, was primarily or solely responsible for the malformations observed and that the duration of exposure of embryos to unmetabolized salicylate may be the critical factor for determining teratogenic outcome.

Mutagenic, cytotoxic, and teratogenic effects of 2-acetylaminofluorene and reactive metabolites in vitro

The embryotoxic, mutagenic, and cytotoxic properties of 2-acetylaminofluorene (AAF) and two of its reactive metabolites, N-acetoxy-2-acetylaminofluorene (AAAF) and 2-nitrosofluorene (NF) were assessed in vitro. A combined embryo culture/biotransformation system was used to determine the ability of these compounds to produce embryonic malformations, growth retardation, and/or embryolethality. Salmonella typhimurium auxotrophs (his-) were utilized to measure the mutagenic and cytotoxic potentials of these compounds. The parent compound, AAF, did not produce embryonic malformations or mutagenicity in the absence of an added cytochrome P-450-dependent monooxygenase system. Both metabolites produced each of the measured toxic effects without supplementation of a bioactivation system. However, the three chemicals each elicited a different spectrum of malformations. Bioactivated AAF produced neural tube abnormalities, whereas embryos treated with AAAF primarily exhibited prosencephalic malformations, and NF produced abnormalities of axial rotation or flexure. NF was approximately ten times more potent than AAAF as a direct-acting mutagen but only slightly more active in producing embryonic malformations in vitro. The results indicated that differential effects on the various measured parameters could be produced by these chemicals. The results indicated further that neither NF nor AAAF appeared to be individually responsible for the neural tube abnormalities generated by biotransformed AAF.