Cytochrome P450-dependent bioactivation of prodysmorphogens in cultured conceptuses

Embryonic turkey liver: activities of biotransformation enzymes and activation of DNA-reactive carcinogens

Avian embryos are a potential alternative model for chemical toxicity and carcinogenicity research. Because the toxic and carcinogenic effects of some chemicals depend on bioactivation, activities of biotransformation enzymes and formation of DNA adducts in embryonic turkey liver were examined. Biochemical analyses of 22-day in ovo turkey liver post-mitochondrial fractions revealed activities of the biotransformation enzymes 7-ethoxycoumarin de-ethylase (ECOD), 7-ethoxyresorufin de-ethylase (EROD), aldrin epoxidase (ALD), epoxide hydrolase (EH), glutathione S-transferase (GST), and UDP-glucuronyltransferase (GLUT). Following the administration of phenobarbital (24 mg/egg) on day 21, enzyme activities of ECOD, EROD, ALD, EH and GLUT, but not of GST, were increased by two-fold or higher levels by day 22. In contrast, acute administration of 3-methylcholanthrene (5 mg/egg) induced only ECOD and EROD activities. Bioactivation of structurally diverse pro-carcinogens was also examined using (32)P-postlabeling for DNA adducts. In ovo exposure of turkey embryos on day 20 of gestation to 2-acetylaminofluorene (AAF), 4,4'-methylenebis(2-chloroaniline) (MOCA), benzo[a]pyrene (BaP), and 2-amino-3,8-dimethylimidazo[4,5- f]quinoxaline (MeIQx) resulted in the formation of DNA adducts in livers collected by day 21. Some of the DNA adducts had (32)P-postlabeling chromatographic migration patterns similar to DNA adducts found in livers from Fischer F344 rats exposed to the same pro-carcinogens. We conclude that 21-day embryonic turkey liver is capable of chemical biotransformation and activation of genotoxic carcinogens to form DNA adducts. Thus, turkey embryos could be utilized to investigate potential chemical toxicity and carcinogenicity.

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

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

Female reproductive and developmental toxicology: overview and current approaches

In recent years, concern about possible female reproductive and developmental toxicity due to environmental contaminants, such as PCBs, has been growing. Because this area of toxicology had not been emphasized prior to this time, there are many gaps in current knowledge about female developmental and reproductive toxicology and only a limited number of validated tests to assay effects of toxicants on various parts of the reproductive and developmental cycle. This article reviews the current state of knowledge on this topic and also explores a variety of techniques for assessing female reproductive and developmental toxicity. These include an assay of the state of intercellular communication among the embryo, fetus and placenta; protocols for assessing toxicity in early pregnancy; and techniques for evaluating the role of glutathione in protecting the conceptus from xenobiotics.

Induction in vitro and complete coding region sequence of cytochrome P4501A1 cDNA from cultured whole rat conceptuses during early organogenesis

Exposures of cultured whole rat conceptuses during organogenesis to 3-methylcholanthrene (MC; 0.025-25 microM), 5,6-benzoflavone (BNF; 5-100 microM) or benz[a]anthracene (BA; 5-100 microM) were effected by placement of each of these "MC-type" inducing agents in the culture medium at the time of explantation on day 9.5 of gestation. Conceptuses were then cultured for 48 hr and evaluated on day 11.5 for increased expression of inducible conceptal cytochrome P450 (P450). The three agents each elicited concentration-dependent increases in 7,8-benzoflavone (ANF)-inhibitable ethoxyresorufin O-deethylase (EROD) activities and increased P4501A1 mRNA as detected by primer-specific reverse transcriptase-polymerase chain reaction (RT-PCR) in cell-free preparations of the treated, cultured conceptuses. At effective inducing concentrations, dysmorphogenic or other embryotoxic effects were not detectable. At 20 microM concentrations, the three agents exhibited roughly equal induction that was approximately equivalent in magnitude (6- to 13-fold) to that achieved previously with exposures to MC in utero. Additions to the culture medium of 2.5 to 10 microM concentrations of dexamethasone (DEX) did not alter significantly the magnitude of MC-elicited induction in vitro. Repeated full-length sequencing of an RT-PCR-amplified cDNA revealed a coding region sequence identical to that reported for the P4501A1 sequence from adult rat liver. The results provide a basis for investigations, in the absence of maternal influences, of the regulation of mammalian conceptal P4501A1 in intact tissues during organogenesis, a gestational period critical in terms of the dysmorphogenic and other embryotoxic effects of foreign organic chemicals. The results are also pertinent to studies of embryotoxicity, particularly to the transplacental carcinogenicity, mutagenicity and dysmorphogenicity of P4501A1 substrates.

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.

An immunohistochemical investigation into the presence and localization of cytochrome P-450 isozymes in organogenesis-stage rat conceptal tissue

To investigate the presence and localization of a variety of xenobiotic biotransforming isozymes of the cytochrome P-450 superfamily in the organogenesis-stage rat conceptal tissue, pregnant female rats were dosed with one of two inducing agents, 3-methylcholanthrene [3MC, 40 mg/kg body weight, ip, day 7 post coitum (pc)] and phenobarbital (PB, 40 mg/kg, ip, days 5, 6, 7 and 8 pc), or with their vehicles (3MC, olive oil; PB, 0.9% NaCl) as controls. The conceptuses were allowed to grow either in vivo, or in vitro, using the whole embryo culture system, from days 9.5 to 11.5 pc. The embryos and isolated visceral yolk sacs were submitted to immunohistochemical investigation using light microscopy. The livers of the dams served as positive controls. Polyclonal and monoclonal antibodies raised against a variety of cytochrome P-450 isozymes were used in the alkaline phosphatase-anti-alkaline phosphatase enzyme immune complex method. All pre-induced dam livers showed positive staining with all polyclonal and monoclonal antibodies tested. The presence of P450IA1 was detected in the visceral yolk sac of both ex vivo and cultured conceptuses, preinduced in utero with 3MC, with the appropriate polyclonal antibodies but not with the monoclonal antibodies. P450IIB1/2 was detected in the visceral yolk sac of both ex vivo and cultured conceptuses, pre-induced in utero by phenobarbital, with the appropriate polyclonal antibodies, but not with the monoclonal antibodies. No staining was seen in any embryo proper, with any vehicle-treated conceptal tissue, or with antibodies raised against P-450s IIE1, IIIA or IVA. Our results support the hypothesis that the organogenesis-stage rat conceptus contains, in the visceral yolk sac, a 3MC-inducible P-450 isozyme similar, but not identical, to adult IA1. They also provide evidence that a PB-inducible isozyme similar, but not identical, to adult IIB1/2, is present in the visceral yolk sac at this stage of conceptus development.

Mechanisms of glucagon-induced increases in rates of cytochrome P450-dependent pentoxyphenoxazone O-depentylation in cultured rat conceptuses

Inclusions of glucagon (1.0 or 2.0 microM, final concentrations) in the media of cultured whole rat conceptuses resulted in concentration-dependent increases in measured rates of O-depentylation of pentoxyphenoxazone in cell-free preparations of conceptal tissues. Enzymic activities were assayed 24 h after initial exposure of the conceptuses to glucagon on day 10 of gestation. Glucagon elicited increases in tissue levels of cAMP that were parallel to those produced by 3-isobutyl-1-methylxanthine over the same time period. Tissue cAMP levels were maximal after 2 h, rapidly returned to control levels and were also equal to background levels in controls after the 24 h culture period. Dibutyryl cAMP, 3-isobutyl-1-methylxanthine, theophylline, and RO201724, a cAMP-selective phosphodiesterase inhibitor, each produced 75 to 100% increases in O-dealkylase activity. Dibutyryl cGMP and two phosphodiesterase inhibitors, enoximone (cGMP-inhibited) and zaprinast (cGMP-specific), each failed to produce statistically significant increases in O-depentylase activity. The O-depentylase was tentatively identified as a conceptus-specific P450 cytochrome that is synthesized predominantly in tissues of the visceral yolk sac. The results indicated that glucagon may upregulate a unique, xenobiotic-biotransforming P450(s) via a long-term mechanism(s) specifically involving tissue cAMP.

Human embryonic cytochrome P450S: phenoxazone ethers as probes for expression of functional isoforms during organogenesis

Human embryonic tissues were investigated during the period of organogenesis with a combination of substrate probes, selective inhibitors and immunoprobes in terms of their capacity to express functional P450 isoforms. A series of phenoxazone ethers utilized as substrate probes revealed that human embryonic hepatic, pulmonary, renal, adrenal and cardiac tissues each contained a complement of functional P450 isoforms when analyzed between days 50 and 60 of gestation. Preparations of each of these tissues contained isoforms capable of catalyzing O-demethylation, O-deethylation, O-depentylation and O-debenzylation of the respective phenoxazone ethers. Investigations with chemical inhibitors and inhibitory antibodies as well as comparisons with vector-expressed, human P450 isoforms suggested that isoforms of P450 subfamilies 1A, 2B, 2C or 3A were not major contributors to any of the observed reactions. The P450-dependent reactions studied exhibited several unexpected and unusual characteristics including a preference for NADH over NADPH as the initial electron donor. Results were consistent with the concept that conceptal-specific P450 isoforms participate in the human embryonic O-dealkylation/debenzylation probe reactions investigated.

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.