An in vitro test for teratogens: its practical application

Androgenic and Teratogenic Effects of Iodoacetic Acid Drinking Water Disinfection Byproduct in Vitro and in Vivo

Iodoacetic acid (IAA) is the most genotoxic iodinated disinfection byproduct known in drinking water. Previous studies have shown that IAA may be an endocrine disruptor. However, whether IAA has reproductive and developmental toxicity remains unclear. In this study, the reproductive and developmental toxicity of IAA was evaluated using a battery of in vitro and in vivo reproductive/developmental toxicity screening tests. The results of E-Screen, uterotrophic, and H295R steroidogenesis assays were negative. The Hershberger bioassay revealed that IAA could induce significant increases in absolute and relative weights of paired Cowper's glands. Moreover, there was an increasing trend in the relative weights of the ventral prostate. The micromass test showed that IAA could inhibit the differentiation of midbrain and limb bud cells. A reproductive/developmental toxicity screening test showed that IAA resulted in significantly increased relative weights of testis and seminal vesicles plus coagulating glands in parental male rats, with a dose-response relationship. IAA could not only induce head congestion in offspring but also decrease litter weight, viability index, and anogenital distance index of male pups on postnatal day 4. All these results indicated that IAA had reproductive and developmental toxicity.

Evaluation of embryotoxic potential of olaquindox and vitamin a in micromass culture and in rats

Limb bud (LB) and central nerve system (CNS) cells were prepared from 12.5 day old pregnant female Crj:CD (SD) rats and treated with olaquindox and vitamin A. Cytotoxicity and inhibition on differentiation were measured in each cell. Three doses of olaquindox (4, 21 and 100 mgkg) , and 0.2 and 75 mg/kg of vitamin A were administered to pregnant rat for 11 days from 6(th) to 16(th) of pregnancy. IC50 values of olaquindox for proliferation and differentiation in CNS cell were 22.74 and 28.32 μg/ml and 79.34 and 23.29 μg/ml in LB cell and those values of vitamin A were 8.13 and 5.94 μg/ml in CNS cell and 0.81 and 0.05 μg/ml in LB cell, respectively. Mean body weights of pregnant rats were decreased at high dose of olaquindox (110 mg/kg) but relative ovary weight, number of corpus lutea, and number of implantation were not changed. Resorption and dead fetus were increased at high dose of olaquindox, and relative ovary weight, the number of corpus lutea and implantation, and sex ratio of male to female were not significantly changed in all dose of olaquindox. Mean fetal and placenta weights were significantly (p < 0.01) decreased in rats of high group. Seven fetuses out of 103 showed external anomaly like bent tail, and 10 out of 114 fetuses showed visceral anomalies at high group. The ossification of sternebrae and metacarpals were significantly (p < 0.01) increased by low and middle dose of olaquindox but it was significantly (p < 0.01) prohibited by high dose of olaquindox. In rats treated with vitamin A, the resorption and dead fetus were increased by high dose. Mean fetal weights were significantly (p < 0.01) increased by low dose but significantly (p < 0.01) decreased by high dose. Thirty four fetuses out of 52 showed external anomaly; bent tail (1) , cranioarchschisis (14) , exencephaly (14) , dome shaped head (22) , anophthalmia (15) , brcahynathia (10) and others (19) . Forty five fetuses out of 52 showed soft tissue anomaly; cleft palate (42/52) and anophthalmia (22/52) by high dose of vitamin A. Sixty one fetuses out of 61 (85.2%) showed skull anomaly; defect of frontal, partial and occipital bone (21/61) , defect of palatine bone (52/61) and others (50/61) . In summary, we support that vitamin A is strong teratogen based on our micromass and in vivo data, and olaquindox has a weak teratogenic potential in LB cell but not in CNS cell. We provide the in vivo evidence that a high dose of olaquindox could have weak embryotoxic potential in rats.

Comparison of embryotoxicity of ESBO and phthalate esters using an in vitro battery system

Epoxidized soy bean oil (ESBO) and phthalate esters have been used as a plasticizer in polyvinyl chloride products. In this study, the embryotoxicity of ESBO and phthalate esters, namely, diethyl hexyl phthalate (DEHP), butylbenzyl phthalate (BBP) and dibutyl phthalate (DBP) was evaluated using short-term in vitro battery system, such as the whole embryo, midbrain and limb bud culture systems. Whole embryos at gestation day 9.5 were cultured for 48 h and the morphological scoring was measured. The cytotoxic effect and differentiation for mid-brain (MB) and limb bud (LB) cell were assessed by 50% inhibition concentration (IC(50)) with neutral red uptake and hematoxylin-stained foci (MB) or Alcian Blue staining (LB), respectively. In the whole embryo culture assay, ESBO (83, 250 and 750 microg/ml) exerted no toxic effect on growth and development of the embryo, whereas phthalate esters (1, 10, 100 microg/ml for DEHP, 10, 100, 1,000 microg/ml for BBP and DBP) inhibited growth and development dose dependently. In mid-brain and limb bud culture, the IC(50) of differentiation and cytotoxicity in BBP was 412.24 and 231. 76 microg/ml for mid-brain, and 40.13 and 182.38 microg/ml for limb bud, respectively. The IC(50) of differentiation and cytotoxicity in DBP was 27.47 and 44.53 microg/ml for mid-brain, and 21.21 and 25.54 microg/ml for limb bud cells, respectively. The lower IC(50) in both cells was obtained from DBP when compared to BBP. From these results, limb bud cells responded more sensitively to BBP and DBP than mid-brain cells. The IC(50) of limb bud cell differentiation and cytotoxicity in DBP is 1.9 and 7.1 less than that of BBP. However, any alteration in cytotoxicity and differentiation was observed with ESBO treatment. These studies suggested that ESBO is not embryotoxic; however, DEHP, BBP and DBP exhibit embryotoxic potential at high concentration.

Effects of dibutyl phthalate and monobutyl phthalate on cytotoxicity and differentiation in cultured rat embryonic limb bud cells; protection by antioxidants

This present study was undertaken to examine the effects of DBP and its metabolite mono-n-butyl phthalate (MBuP) on cytotoxicity and differentiation in cultured rat embryonic limb bud cells. When limb bud cells extracted from rats on gestation d 12.5 were treated with DBP or MBuP for 96 h, induction of cytotoxicity and inhibition of cell differentiation were observed in a concentration-dependent manner. However, MBuP elicited a toxic effect at higher concentrations than DBP. The IC50 values of DBP for cytotoxicity (measured by neutral red uptake) and cell differentiation (measured by alcian blue staining) were 25.54 microg/ml (91.75 microM) and 21.21 microg/ml (76.20 microM), respectively. The IC50 values of MBuP for cytotoxicity and cell differentiation were 307.24 microg/ml (1.38 mM) and 142.61 microg/ml (0.64 mM), respectively. in order to determine whether free radicals are related to induction of cytotoxicity and inhibition of differentiation by DBP in limb bud cells, DBP was coadministered with several antioxidants, including catalase and vitamin E acetate to limb bud cells. Cotreatment with catalase and vitamin E acetate decreased induction of cytotoxicity and inhibition of differentiation by DBP in limb bud cells. However, these compounds did not show any protective effect against MBuP. Results indicate that DBP and MBuP induced developmental toxicity in rat embryonic limb bud cells and suggest that this effect of DBP might be exerted through oxidative stress.

Evaluation of P19 cells for studying mechanisms of developmental toxicity: application to four direct-acting alkylating agents

P19 cells are pluripotent murine embryonic carcinoma (EC) cells that can be induced by all-trans-retinoic acid (RA) to differentiate into cells that are biochemically and morphologically similar to cells of the central nervous system. We have established these cells as a reproducible cell system to evaluate potential effects of agents disrupting neuronal differentiation. The viability of P19 cells was assessed using a neutral red assay. Uptake of [3H]-gamma-amino butyric acid ([3H]GABA) was assessed as a marker of neuronal differentiation. We observed significant increases in [3H]GABA over time, corresponding with the appearance of cells with neuronal morphologies. 2,4-Diaminobutyric acid, a specific inhibitor of high affinity neuronal GABA uptake, reduced [3H]GABA uptake by approximately 75%. Additionally, [3H]GABA uptake in cells treated with dimethylsulfoxide (DMSO), which differentiate into mesodermal derivatives, was approximately 25% of uptake observed in RA-exposed, neuronally differentiated P19 cells. The morphology of P19 cell cultures correlated with [3H]GABA uptake: high [3H]GABA uptake was only observed in cultures with distinct neuronal morphologies. These results suggest that [3H]GABA uptake is a good indicator of neuronal differentiation in P19 cells. The responsiveness of P19 cells to developmental toxicants was assessed by comparing effects in P19 cells with effects observed previously in primary cultures of differentiating rat embryo midbrain (CNS) cells. Alkylating agents chosen for this investigation include methylnitrosourea (MNU), ethylnitrosourea (ENU), methyl methanesulfonate (MMS), and ethyl methanesulfonate (EMS). The rank order of potency of these alkylating agents in the CNS cells was MMS > MNU > ENU > EMS. With the exception of ENU, concentrations that caused effects on growth and differentiation in the P19 cells were very comparable to those causing similar effects in CNS cell cultures. Our results with P19 cells suggest that this EC cell line may also be a useful in vitro cell system for studying mechanisms of developmental toxicity, with the advantages of being an established cell line.

The Comparative Testing of Eight Coded Chemicals in the Rat Limb Bud Micromass and Rat Embryo Culture Systems

When cultured at high density, mesenchymal cells from rat limb buds proliferate and differentiate into chondrocytes. Inhibition of this in vitro chondrogenic process has been used for the preliminary evaluation of teratogenic potential. Alternatively, intact post-implantation rat embryos, maintained in short-term culture, provide a system for the in vitro study of abnormal development not limited to the skeletal system. Both systems isolate the test agent from maternal metabolism and pharmacokinetic restraints. In this study, drug-associated selective inhibition of alcian blue uptake by cartilage proteoglycans, in micromass cultures of limb bud cells prepared from 13-day-old rat embryos, was used to assess teratogenic potential in vitro following exposure for 48 hours to eight coded compounds (acetylsalicylic acid, isoniazid. penicillin G, saccharine, vincristine sulphate, 6-aminonicotinamide, retinoic acid, and amaranth). Following drug exposure, cultures were incubated for another 96 hours, and the cells were then fixed and stained with 0.5% alcian blue. Bound dye was then extracted and quantitated. In parallel cultures, cell viability was measured by neutral red uptake, and protein content was assayed by using the bicinchoninic acid method. Except for retinoic acid and vincristine sulphate, the maximum test concentration was 1000μg/ml. Inhibition of alcian blue uptake (> 50%) was noted at 0.001μg/ml vincristine sulphate, 0.5/μg/ml retinoic acid and 5μg/ml 6-aminonicotinamide, demonstrating that strong teratogens inhibit differentiation in micromass cultures at lower concentrations than those which affect limb cell viability. When the same eight compounds were tested in a 24-hour embryo culture model, dysmorphogenesis was evident at 0.005μg/ml vincristine sulphate, 0.1μg/ml retinoic acid and 0.3μg/ml 6-aminonicotinamide. For the other five chemicals, little or no toxicity was noted up to the maximum test concentration in either model. We conclude that the two test systems, both based on the developing rat embryo, are consistent with each other, and that either of them would be useful for the preliminary screening of potential teratogens.

Characterization of cytoskeletal and neuronal markers in micromass cultures of rat embryonic midbrain cells

Micromass cultures of rat embryonic midbrain cells were characterized with regard to the immunolocalization of neuronal and cytoskeletal markers. Cells taken from gestational day-12 embryos and cultured for 5 days in vitro comprise at least two morphologically distinct cells types: fibroblast-like cells and neurons. Antibodies to the following markers yielded preferential staining of neuronal cells: A2B5 (GQ ganglioside), gamma-aminobutyric acid (GABA), microtubule-associated protein 2 (MAP2), MAP5, neuron-specific enolase (NSE), neural cell adhesion molecule (N-CAM), and tau. Antibodies to beta-tubulin, c-neu, MAP1, and neurofilament (NF-H) stained both neuronal and fibroblast-like cells. Antibodies to glial fibrillary acidic protein (GFAP) and vimentin failed to immunoreact with any cells in day-5 CNS cultures. SDS-PAGE and Western analysis were employed to determine the specificity of the antibodies and determine the electrophoretic profiles of the markers. We conclude that the pattern of neuronal differentiation in CNS micromass cultures exhibits certain similarities to that observed in vivo. In addition, certain markers identified in this study may be of potential utility as (1) biomarkers of chemically-induced developmental neurotoxicity, and (2) indicators of differential toxicity toward the diverse cell types that comprise the mammalian central nervous system.

Effects of benzimidazole analogs on cultures of differentiating rodent embryonic cells

Micromass cell culture systems for rat embryo midbrain (CNS) and limb bud (LB) cells were employed to assess the in vitro developmental toxicity of the benzimidazole analogs, mebendazole (MBZ), thiabendazole (TBZ), and nocodazole (NCZ), in addition to the classic microtubule inhibitor, colchicine. Comparison was made to albendazole (ABZ), a previously studied benzimidazole anthelmintic. Two parameters for assessing developmental toxicity were measured: differentiation and cytotoxicity. The relative potencies of the benzimidazole analogs in the micromass system (NCZ greater than MBZ approximately ABZ much greater than TBZ) mirrored their effectiveness in an assay for in vitro inhibition of mammalian tubulin polymerization. Colchicine also exhibits a high affinity for mammalian tubulin and was a potent inhibitor of cell proliferation, chondrogenesis, and neuronal differentiation. Immunofluorescent staining of Day 1 LB cultures with a monoclonal antibody to beta-tubulin revealed that these agents elicited mitotic arrest. Many anti-tubulin agents are teratogenic in rats and their in vivo developmental toxicity may reflect perturbation of microtubular structure or function. With the exception of TBZ, these agents should be considered potential developmental toxicants since they inhibit cell growth and differentiation of micromass cultures at nanomolar concentrations.

Effects of albendazole and albendazole sulfoxide on cultures of differentiating rodent embryonic cells

Micromass cell culture systems for rat embryo midbrain (CNS) and limb bud (LB) cells were employed to assess the in vitro developmental toxicity of the human and veterinary anthelmintic albendazole (ABZ) and its sulfoxide metabolite (SOABZ). ABZ is reported to be teratogenic in rats, and is extensively metabolized to the sulfoxide derivative. It has been postulated that SOABZ is the reactive metabolite responsible for albendazole's developmental toxicity and anthelmintic activity in vivo. Three parameters for assessing developmental toxicity were measured: cell growth, differentiation, and cytotoxicity. CNS and LB cultures were equivalent in their sensitivities to both ABZ and SOABZ. ABZ was approximately 50-fold more potent than SOABZ. Immunohistochemical determinations of tubulin organization revealed that both ABZ and its sulfoxide metabolite elicit an accumulation of cells in the mitotic phase of the cell cycle. Since ABZ is one of the most potent agents tested in the micromass system to date, this anthelmintic should be considered a potential developmental toxicant.

Chemically induced growth inhibition and cell cycle perturbations in cultures of differentiating rodent embryonic cells

Ethylnitrosourea (ENU) is a proven animal teratogen, although the mechanism of its developmental toxicity is unknown. The micromass rat embryo midbrain (CNS) and limb bud (LB) cultures were used in an effort to determine potential mechanisms by which ENU may exert its teratogenic effect. When cultured at high cell densities, both cell types undergo several rounds of replication while differentiating into discrete foci of neuronal cells and chondrocytes, respectively. Differentiation was monitored after 5 days by staining with hematoxylin (CNS) and alcian blue (LB). Our objectives were to (1) apply flow cytometry technology to the micromass cultures and (2) determine how ENU disrupts the normal growth, differentiation, and cell cycling of these cultures. Dose-dependent decreases in cell attachment and viability were observed in the first 24 hr after ENU exposure. Exposed cultures also exhibited dose-dependent growth inhibition over 5 days in culture as determined by cell counts. Flow cytometric cell cycle analysis of treated cultures revealed a dose-related accumulation of CNS cells in late G1/early S. Treated LB cells also displayed dose-related cell cycle changes with cells accumulating throughout the S phase. The concentration-dependent changes in both the CNS and the LB cell cycle profiles were observed in the attached cell populations which had greater than 94 +/- 3% viability at all ENU concentrations tested. This suggests that flow cytometric analysis allowed description of cellular alterations that would have been overlooked if only cell viability had been examined. Our examinations suggest that the effects of ENU on cell differentiation are related to its early effects on cell attachment, cell cycling, and cell proliferation.

Examination of a rodent limb bud micromass assay as a prescreen for developmental toxicity

The mouse limb bud micromass assay is one of many short-term tests proposed as preliminary screens for potential developmental toxicity. Previous efforts to validate this assay have used too few "nonteratogens." The purpose of this study was to examine additional compounds, most of which, based on the literature, were perceived to have low potential for developmental toxicity in vivo. In addition, a method of data analysis was sought that would identify selective developmental toxins in the micromass assay, i.e., those that are effective at dosages not maternally toxic. The concentration of each of 23 compounds that produced a 50% inhibition (IC50) of radiolabeled thymidine (T) and sulfate (S) incorporation was determined and used to calculate a T/S ratio. The T/S ratio may be a useful measure of developmental hazard, since T incorporation measures toxicity toward a general cell function (DNA synthesis) and S incorporation measures mainly toxicity toward a developmentally specific cell activity (chondroitin sulfate synthesis). All compounds tested produced T/S ratios of less than 2.0. Since 22 of these 23 compounds are classified as "nonteratogens" or nonselective developmental toxins in vivo, a low T/S ratio in this in vitro assay system may be capable of discriminating potential for developmental hazard in vivo.

In vitro screening for anticonvulsant-induced teratogenesis in neural primary cultures and cell lines

To establish inherent potential for the induction of neural tube defects the ability of selected anticonvulsant agents to interfere with cell division has been established in vitro using an antiproliferative assay in clonal cell lines and a cytotoxicity assay using primary cultures of cerebral cortex neurons at different stages of development. In order to evaluate the relative toxicities of these agents their in vitro effects were determined at 2-3 times the plasma therapeutic level. By these procedures valproate and the benzodiazepines, diazepam and clonazepam, exerted a potent antiproliferative action which could not be attributed to increased cytotoxicity. In contrast phenytoin was markedly cytotoxic but was without an antiproliferative action. This cytotoxicity was most pronounced during the periods of extensive fibre outgrowth. When compared to epidemiological and animal study data, agents which inhibited cell proliferation within twice therapeutic concentration were consistently associated with major neural tube malformations. However phenytoin, found to be positive in the cell cytotoxicity assay, is not associated with neural tube malformations but rather is primarily associated with mental retardation. Thus assessment of antiproliferative activity of anticonvulsant drugs may be one criterion for identification of teratogenic potential during neurulation.

Toxic effects of cyclophosphamide in differentiating chicken limb bud cell culture using rat liver 9,000 g supernatant or rat liver cells as an activation system: an in vitro short-term test for proteratogens

Cells from 4-day chicken embryo limb buds plated as micromass cultures differentiate and form cartilage nodules after a 5- to 6-day growth period. The innate ability of these cells to biotransform compounds, such as cyclophosphamide (CP), into reactive metabolites is apparently inadequate. Protocols used rat liver S9 from Aroclor 1254-pretreated (PCB) rats or hepatocytes from control rats or polychlorinated biphenyls (PCB)-pretreated rats and were added to micromass cultures with CP causing concentration-related toxicity in the cell cultures. Coculturing chicken limb bud cells with PCB-hepatocytes was the most efficient method, yielding an IC50 of 2 micrograms CP per ml. Toxic CP metabolites accumulated in the medium of PCB-hepatocyte cultures and were quite stable. The toxicity of bioactivated CP was nearly identical for both proliferation and cartilage proteoglycan accumulation.

Estimating chemical developmental hazard in a chicken embryo limb bud micromass system

A chicken limb bud micromass system measuring the production of cartilage proteoglycans and the incorporation of radiolabelled thymidine and leucine was used to calculate potential developmental hazard. This hazard index was based upon the 50% inhibitory concentration (IC50) for these parameters and enabled classification of substances according to their intrinsic ability to inhibit differentiation at concentrations which were not cytotoxic. All-trans-retinoic acid, a potent teratogen, inhibited cartilage proteoglycan synthesis at a concentration which was more than 100 times lower than those affecting the incorporation of 14C-thymidine and 14C-leucine. On the other hand, sodium valproate, salicylate and isoniazid inhibited cartilage proteoglycan synthesis only near concentrations which were also cytotoxic. The results from testing with chicken limb bud micromass compared favorably, both with respect to effective concentrations and ratios describing developmental hazard to those from rodent micromass systems.

In vitro metabolism of teratogens by differentiating rat embryo cells

Rapid and accurate prediction of teratogenic hazard had been achieved using cultures of differentiating limb mesenchyme (LB) and midbrain (CNS) cells from 13-day-old rat embryos. In this study we have used these cultures to examine the role of metabolism in the in vitro teratogenic activity of diphenylhydantoin (DPH) and cyclophosphamide (CPA). Two approaches were used. The first involved modulation of cytochrome P-450 activity by co-incubation in vitro with a variety of inhibitors at concentrations that were non-cytotoxic to the cells. This enhanced the toxicity of DPH by 13-82% in LB and by 3-52% in CNS cells. Benzimidazole and ellipticine caused the greatest enhancement and SKF 525A the least. DPH appears to be the proximate teratogen and there appear to be embryo-tissue cytochrome P-450s that assist in its detoxification. Following prior transplacental induction, CPA was toxic in vitro to LB cells from beta-naphthoflavone-pretreated mothers. CPA was non-toxic in cells of control, phenobarbitone- or 3-methylcholanthrene-treated embryos. Thus there appear to be inducible levels of cytochrome P-448 in embryo cells. In the second approach, positive immunocytochemical staining of the cells with both monoclonal and polyclonal P-450 antibodies identified phenobarbitone, beta-naphthoflavone- and 3-methylcholanthrene-inducible cytochrome P-450s at a constitutive level. Cytochromes P-448 (beta-naphthoflavone type) and P-450 (phenobarbitone type, PB3 fraction) were inducible, confirming that cytochrome P-450s are in fact present in the embryo cells.