Teratogenicity of nitrofen (2,4-dichloro-4'-nitrodiphenyl ether) and its effects on thyroid function in the rat

Genotoxic and mutagenic studies of teratogens in developing rat and mouse

In this review, genotoxic and mutagenic effects of teratogenic chemical agents in both rat and mouse have been reviewed. Of these chemicals, 97 are drugs and 33 are pesticides or belong to other groups. Large literature searches were conducted to determine the effects of chemicals on chromosome abnormalities, sister chromatid exchanges, and micronucleus formation in experimental animals such as rats and mice. In addition, studies that include unscheduled DNA synthesis, DNA adduct formations, and gene mutations, which help to determine the genotoxicity or mutagenicity of chemicals, have been reviewed. It has been estimated that 46.87% of teratogenic drugs and 48.48% of teratogenic pesticides are positive in all tests. So, all of the teratogens involved in this group have genotoxic and mutagenic effects. On the other hand, 36.45% of the drugs and 21.21% of the pesticides have been found to give negative results in at least one test, with the majority of the tests giving positive results. However, only 4.16% of the drugs and 18.18% of the pesticides were determined to give negative results in the majority of the tests. Among tests with major negative results, 12.50% of the teratogenic drugs and 12.12% of the teratogenic pesticides were negative in all conducted tests.

New Insights into Congenital Diaphragmatic Hernia - A Surgeon's Introduction to CDH Animal Models

In recent decades, new research into the developmental defects and pathophysiological basis of congenital diaphragmatic hernia (CDH) has revealed opportunities for the development of innovative therapies. Importantly, the use of animal models to represent this anomaly in the laboratory has resulted in the discovery of many important genetic, epigenetic, and other molecular contributors to this condition. In this review, the most commonly used and newly devised animal models of CDH are presented to familiarize the reader with the latest innovations in the basic sciences.

Nitrofen induces apoptosis independently of retinaldehyde dehydrogenase (RALDH) inhibition

BACKGROUND

Nitrofen is a diphenyl ether that induces congenital diaphragmatic hernia (CDH) in rodents. Its mechanism of action has been hypothesized as inhibition of the retinaldehyde dehydrogenase (RALDH) enzymes with consequent reduced retinoic acid signaling.

METHODS

To determine if nitrofen inhibits RALDH enzymes, a reporter gene construct containing a retinoic acid response-element (RARE) was transfected into HEK-293 cells and treated with varying concentrations of nitrofen in the presence of retinaldehyde (retinal). Cell death was characterized by caspace-cleavage microplate assays and terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assays. Ex vivo analyses of cell viability were characterized in fetal rat lung explants using Live/Dead staining. Cell proliferation and apoptosis were assessed using fluorescent immunohistochemistry with phosphorylated histone and activated caspase antibodies on explant tissues. Nile red staining was used to identify intracellular lipid droplets.

RESULTS

Nitrofen-induced dose-dependent declines in RARE-reporter gene expression. However, similar reductions were observed in control-reporter constructs suggesting that nitrofen compromised cell viability. These observed declines in cell viability resulted from increased cell death and were confirmed using two independent assays. Ex vivo analyses showed that mesenchymal cells were particularly susceptible to nitrofen-induced apoptosis while epithelial cell proliferation was dramatically reduced in fetal rat lung explants. Nitrofen treatment of these explants also showed profound lipid redistribution, primarily to phagocytes.

CONCLUSIONS

The observed declines in nitrofen-associated retinoic acid signaling appear to be independent of RALDH inhibition and likely result from nitrofen induced cell death/apoptosis. These results support a cellular apoptotic mechanism of CDH development, independent of RALDH inhibition.

Linking animal models to human congenital diaphragmatic hernia

BACKGROUND

Congenital diaphragmatic hernia (CDH) is a major life-threatening malformation, occurring in approximately 1 in 3,000 live births. Over the years, different animal models have been used to gain insight into the etiology of this complex congenital anomaly and to develop treatment strategies. However, to date the pathogenic mechanism is still not understood, and treatment remains difficult because of the associated pulmonary hypoplasia and pulmonary hypertension.

METHODS

In this review, data available from several animal models will be discussed. The retinoic acid signaling pathway (RA pathway, retinoid pathway) will be addressed as a developmental pathway that is potentially disrupted in the pathogenesis of CDH. Furthermore, genetic factors involved in diaphragm and lung development will be discussed.

CONCLUSIONS

With this review article, we aim to provide a concise overview of the current most important experimental genetic data available in the field of CDH.

Mechanisms of action of the congenital diaphragmatic hernia-inducing teratogen nitrofen

Congenital diaphragmatic hernia (CDH) is a developmental anomaly that results in significant mortality and morbidity. The underlying etiology is poorly understood. Insights will arise from an understanding of the mechanisms by which the teratogen nitrofen induces CDH in rodent models. In this study, we use in vitro cell assays in conjunction with whole animal rodent studies to test hypotheses regarding nitrofen's mechanism of action. The first component examined the interaction of nitrofen with various aspects of the retinoid signaling pathway including uptake proteins, binding proteins, receptors, conversion, and degradation enzymes. The second component examined the interactions of nitrofen and vitamins A, C, and E to test the hypothesis that nitrofen was functioning as an antioxidant to interfere with retinoid signaling. Third, we performed a series of experiments examining the interaction of nitrofen and thyroid signaling. Collectively, the data suggest that the primary aspect of retinoid signaling affected by nitrofen is via inhibition of the rate-limiting enzymes controlling retinoic acid synthesis. Retinoid signaling perturbations do not appear to involve oxidative effects of nitrofen. Any substantial roles of nitrofen-induced perturbations of thyroid hormone signaling or receptor function are not supported.

Vitamin A deficiency (VAD), teratogenic, and surgical models of congenital diaphragmatic hernia (CDH)

Congenital diaphragmatic hernia (CDH) is a congenital malformation that occurs with a frequency of 0.08 to 0.45 per 1,000 births. Children with CDH are born with the abdominal contents herniated through the diaphragm and exhibit an associated pulmonary hypoplasia which is frequently accompanied by severe morbidity and mortality. Although the etiology of CDH is largely unknown, considerable progress has been made in understanding its molecular mechanisms through the usage of genetic, teratogenic, and surgical models. The following review focuses on the teratogenic and surgical models of CDH and the possible molecular mechanisms of nitrofen (a diphenyl ether, formerly used as an herbicide) in both induction of CDH and pulmonary hypoplasia. In addition, the mechanisms of other compounds including several anti-inflammatory agents that have been linked to CDH will be discussed. Furthermore, this review will also explore the importance of vitamin A in lung and diaphragm development and the possible mechanisms of teratogen interference in vitamin A homeostasis. Continued exploration of these models will bring forth a clearer understanding of CDH and its molecular underpinnings, which will ultimately facilitate development of therapeutic strategies.

The co-occurrence of congenital diaphragmatic hernia, esophageal atresia/tracheoesophageal fistula, and lung hypoplasia

BACKGROUND

Two severe birth defects, congenital diaphragmatic hernia (CDH) and esophageal atresia (EA) with or without tracheoesophageal fistula (TEF), have traditionally been analyzed separately in epidemiological studies. Lung hypoplasia (LH), part of the CDH spectrum, is not usually associated with EA/TEF, yet both are foregut malformations.

METHODS

We conducted an epidemiological study of two combinations of the defects in the population of 3,318,966 live births and stillbirths monitored from 1983 to 1996 by the California Birth Defects Monitoring Program (CBDMP).

RESULTS

A total of 433 cases had a Bochdalek type CDH/LH (0.13 per 1000 births), 893 had EA/TEF (0.27 per 1000 births), and 646 had LH (0.19 per 1000 births). Among them, 18 cases had CDH/LH with EA/TEF (0.005 per 1000 births), and 53 had EA/TEF and LH (0.02 per 1000 births); both prevalences are significantly higher than expected. Sixteen of 17 cases of CDH/LH with EA/TEF, and 34 of 40 cases of EA/TEF with LH were stillborn or died; 72% and 74%, respectively, had an autopsy. The male to female sex ratios were 1.43 and 1.13, respectively. In both groups, infants had similar proportions of additional severe defects, except for genitourinary and anal defects and syndromes/associations, which were more prevalent in the EA/TEF with LH group. We reviewed human studies and experimental animal models for factors reported to cause any combination of the defects.

CONCLUSIONS

Several genetic and environmental factors could affect the significant co-occurrence of the defects. Future studies should include storage of patients' biological materials for DNA analysis, karyotyping, and environmental exposure evaluation.

Mutagenicity of sulfoscanate: a comparative study

The mutagenic activity of sulfoscanate (SSC) (4-isothiocyanate-4'-nitrodiphenyl sulphide) has been compared with that of the following reported drugs: (a) nitroscanate (NSC) (4-isothiocyanate-4'-nitrodiphenyl ether) which is a veterinary anthelmintic drug and (b) amoscanate (ASC) (4-isothiocyanate-4'-nitrodiphenyl amine) which is effective against schistosomes. SSC has been found to be a very potent mutagen towards TA98 and TA100 inducing 26.0 and 475.5revertants/nmole, respectively. NSC was found to induce mutations at a rate of 11.1 and 21.5revertants/nmole in TA98 and TA100, respectively. ASC was found to be non-mutagenic as such, but the urine of animals given the drug displayed mutagenicity. When SSC was tested in TA98/1,8-DNP(6), deficient in O-acetyltransferase, the activity decreased to 10.0revertants/nmole. However, in case of NSC the mutagenic activity was reduced to 0.24revertants/nmole, indicating the importance of O-acetyltransferase in generating N-acetoxyarylamine. In TA98NR, deficient in nitroreductase, the mutagenicity of SSC and NSC was totally absent. The positional isomers of SSC, 4-isothiocyanate-3'-nitro- and 4-isothiocyanate-2'-nitrodiphenyl sulphide, were found to be non-mutagenic in both TA98 and TA100. Our comparison of the mutagenic activity of SSC, NSC and ASC indicates that the pattern of activity is SSC>NSC>ASC.

Altered gene level expression of thyroid hormone receptors alpha-1 and beta-1 in the lung of nitrofen-induced diaphragmatic hernia

PURPOSE

Thyroid receptor alpha-1 (TR-alpha1) and thyroid receptor beta-1 (TR-beta1) are thought to be essential for the fetal and postnatal development of the lung. The authors investigated gene level expression of TR-alpha1 and TR-beta1 in the lung of nitrofen-induced congenital diaphragmatic hernia (CDH) using reverse transcription polymerase chain reaction (RT-PCR).

METHODS

CDH was induced in pregnant rats after administration of 100 mg nitrofen on day 9.5 of gestation. Cesarean section was performed on day 21 of gestation. The fetuses were divided into 3 groups: normal controls (n = 16), nitrofen-induced CDH (n = 16), and nitrofen-treated without CDH (n = 16). mRNA was extracted from the left lung in each group. RT-PCR was performed to evaluate mRNA expressions of TR-alpha1 and TR-beta1. Levels of mRNA were expressed as a ratio of the band density divided by that of beta-actin, a house-keeping gene.

RESULTS

TR-alpha1 mRNA expression was decreased significantly in CDH lung (1.618 +/- 0.148) compared with controls (2.658 +/- 0.251; P <.01) and nitrofen-treated without CDH lung (2.232 +/- 0.193; (P <.05). TR-beta1 mRNA expression also was significantly decreased in CDH lung (2.223 +/- 0.270) compared with controls (3.569 +/- 0.262; P <.01) and nitrofen-treated without CDH lung (3.235 +/- 0.299; P <.05).

CONCLUSION

These data suggest that the downregulation of thyroid hormone signaling pathway through altered expression of TR-alpha1 and TR-beta1 during lung morphogenesis may be a contributory factor in the pathogenesis of pulmonary hypoplasia in nitrofen-induced CDH.

Hormonal modulation of fetal pulmonary development: relevance for the fetus with diaphragmatic hernia

Antenatal hormonal modulation of pulmonary growth has been successfully introduced in clinical practice to reduce the incidence of respiratory distress syndrome (RDS) of preterm born infants. However, a certain amount of reserve to repeat courses should be taken into account because of possible adverse effects of antenatal administration of glucocorticoids. Although in experimental animals thyroid hormones given alone were not shown to have stimulatory effects on pulmonary development, there was an apparent synergistic effect with corticosteroids. Yet, such effects have not been substantiated in clinical trials. Whereas in cases of congenital diaphragmatic hernia (CDH) in utero tracheal occlusion could stimulate fetal lung growth and modulation, the enhancement of type II cell differentiation is more likely to be achieved with antenatal exposure to hormonal therapies. However, there is still no firm scientific basis for either of these two treatment modalities in CDH. Yet, antenatal hormonal modulation is now soon to be tested in an extensive multi-center clinical trial. In this review, the current status of antenatal hormonal modulation of pulmonary growth will be described and its potential role in the treatment of CDH will be discussed.

Maternal and developmental toxicity of halogenated 4'-nitrodiphenyl ethers in mice

In an ongoing effort to delineate structure-activity relationships in the developmental toxicity of diphenyl ethers, we evaluated the maternal and developmental toxicity of 10 diphenyl ethers related to the herbicide nitrofen. All possible trichlorophenyl 4'-nitrophenyl ethers were evaluated, as were the 2,4-difluorophenyl and 2,4-dibromophenyl 4'-nitrophenyl ethers. We also evaluated bifenox and chlomethoxyfen, which are 2,4-dichlorophenyl congeners with meta-substituents on the 4'-nitrophenyl ring. Nitrofen (2,4-dichlorophenyl 4'-nitrophenyl ether) was included for comparison. Identity of the halogen affected the postnatal (but not prenatal) mortality induced by 2,4-dihalogenated 4'-nitrophenyl ethers. The presence of 3'-substituents on the 4'-nitrophenyl ring reduced both pre- and postnatal toxicity of 2,4-dichlorinated congeners. Among chlorinated 4'-nitrophenyl congeners without meta-substituents on the nitrophenyl ring, the position of chlorine substituents strongly affected the congener's potential for inducing prenatal vs. postnatal syndromes. All congeners increased liver to body weight ratios in unmated females, but such increases were not well-correlated with either prenatal or postnatal embryotoxicity.

Thyroid hormones in the pathogenesis of lung hypoplasia and immaturity induced in fetal rats by prenatal exposure to nitrofen

BACKGROUND/PURPOSE

Nitrofen is believed to act on prenatally exposed fetuses by changing maternal or fetal thyroid hormone physiology. The aim of this study was to determine whether the amounts of circulating and lung tissue T3 and T4 are decreased in rat fetuses with nitrofen-induced pulmonary hypoplasia and diaphragmatic hernia.

METHODS

Timed-pregnant rats were given 100 mg of nitrofen in oil on gestational day 9.5, and their fetuses were recovered on the 21st day. Lung weight to body weight ratio was determined. Hormonal studies consisted in measurement of plasma T3, T4, and TSH, and of T3, T4, and DNA in lung tissue. Suitable groups of control fetuses prenatally exposed to oil were used for comparison.

RESULTS

The lungs of nitrofen-treated fetuses were hypoplastic and those who had congenital diaphagmatic hernia were even more so. Nitrofen treatment led to decreased plasma T3 and T4 levels without TSH changes. T3 and T4 in lung tissue were apparently decreased in treated fetuses when expressed by weight, but these differences disappeared when expressed by DNA (cell content).

CONCLUSIONS

Lung hypoplasia and immaturity induced by nitrofen treatment are not related to decreased levels of thyroid hormones in tissue near term. This should be kept in mind when proposing hormonal treatment for prenatal induction of lung maturation.

Genotoxicity of potential metabolites of nitroscanate--an antischistosomal drug

The potential metabolites of nitroscanate(4-isothiocyanato-4'-nitrodiphenyl ether) such as 4-amino-4'-nitrodiphenyl ether (ANDE), 4-acetamido-4'-nitrodiphenyl ether (AcNDE), 4-acetamido-4'-nitrosodiphenyl ether (4-N = 0), 4-acetamido-4'-hydroxylaminodiphenyl ether (4-NHOH), 4-acetamido-4'-acetohydroxamicdiphenyl ether [4-N(OH)Ac], 4-acetamido-4'-formohydroxamicdiphenyl ether [4-N(OH)CHO] and 4-acetamido-4'-acetylaceto-hydroxamicdiphenyl ether [4-N(OAc)Ac] were synthesized and investigated in the standard Salmonella mutagenicity test using TA98, TA98NR, TA98/1,8-DNP6, TA100 and TA100NR as indicator strains, in the presence and absence of hepatic S9. The relative order of activity among nitro and its reduction products, 4-N = 0 and 4-NHOH in TA98 and TA100 was 4-N = 0 > 4-NHOH > AcNDE. In nitroreductase deficient strain TA98NR, AcNDE was inactive, but expressed a slight activity in TA100NR while 4-N = 0 and 4-NHOH showed a large increase in specific activity in both the strains. In O-acetyltransferase deficient strain TA98/1,8-DNP6, AcNDE was inactive, while 4-N = O and 4-NHOH showed a sharp fall in activity. The hydroxylamine derived products with an activity order 4-N(OAc)Ac > 4-N(OH)CHO > 4-N(OH)Ac in both TA98 and TA100, showed 3-6 times increase in the specific activity for the latter two compounds in the presence of S9 mix, which was inhibited in the presence of paraoxan, indicating N-deacylation as an important metabolic activation pathway. Except the 4-NO in TA100, the observed mutagenicity of nitroscante (NSC) was higher than those of potential metabolites and the nor-isothiocyanato derivative 4'-nitrodiphenyl ether, thereby showing that -NCS function has a potentiating effect on the mutagenicity of this drug.

Mutagenicity of nitroscanate, an antischistosomal drug

Nitroscanate (NSC) was found to be a direct acting mutagen in the Ames Salmonella tester strains TA100 and TA98 and this activity increased further in the presence of rat liver S9 mix. It was inactive in TA98NR and TA100NR, and weakly active in TA98/1,8-DNP6. A substantial fall in drug induced mutagenicity by pentachlorophenol, an inhibitor of acetyltransferase, in TA98, TA100 and YG1024 suggests the initial bioconversion of a nitro group to hydroxylamine and its further activation to the ultimate N-acetoxyarylamine. The refrigerated DMSO solution of the drug in the plate incorporation assay and freshly prepared solutions using the pre-incubation procedure indicated a fall in mutagenicity owing to the conversion of NSC to N,N'-bis-4-(p-nitrophenoxy)phenyl thiourea (NFPT). The drastic reduction in mutagenicity in the presence of 4-amino-4'-nitrodiphenyl ether (ANDE) and 4-aminodiphenyl ether (ADE) was also attributed to the conversion of NSC to the corresponding thiourea, a non-mutagen. The negligible mutagenicity of ANDE and its absence in ADE and 4-isothiocyanate diphenyl ether (ITDE) suggests that the mutagenicity of NSC is due to the nitro group, and the -NCS function is responsible for enhanced mutagenicity over nor-isothiocyanate 4-nitrodiphenyl ether (NDE).

Embryonic cell death patterns associated with nitrofen-induced congenital diaphragmatic hernia

The offspring of pregnant Sprague-Dawley rats exposed to nitrofen on gestational day 9.5 develop left-sided congenital diaphragmatic hernia (CDH). Twenty-four hours after treatment, on day 10.5, supravital staining with Nile blue sulfate and histological examination showed bilateral excessive cell death in cervical somites 2 through 4. After 48 hours, on day 11.5, cell death was absent in the cervical somites but was apparent in the mesoderm adjacent to the somites in the septum transversum and in the developing sympathetic ganglia adjacent to the dorsal aortae. Cell death was not apparent in the foregut or lung primordia on either day 10.5 or 11.5. The incidence of nitrofen-exposed embryos with such patterns of cell death closely paralleled that of left-sided CDH in similarly treated day 21.5 fetuses. Control animals treated with olive oil had normal programmed cell death patterns in the regions of interest and had no evidence of CDH on day 21.5. It is possible that these patterns of excessive cell death early in gestation may play a role in the genesis of diaphragmatic hernia. Mesoderm derived from cervical somites 3 through 5 contributes to the diaphragmatic anlage and forms the major portion of the muscle of the diaphragm. Because nitrofen damages mesodermal cell populations in cervical somites 2 through 4 and in the mesenchyme adjacent to the septum transversum 24 to 48 hours after administration, the authors propose that damage to these populations may reduce progenitor cells needed to populate the diaphragmatic anlage, thereby hindering pleuro-peritoneal canal closure.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin-converting enzyme activity is increased in lungs of rats with pulmonary hypoplasia and congenital diaphragmatic hernia

Lung hypoplasia (LH) and pulmonary hypertension are responsible for the high mortality rate in congenital diaphragmatic hernia (CDH) patients. Angiotensin-converting enzyme (ACE) plays a role in the regulation of pulmonary vascular resistance in the postnatal period and might be involved in the development of pulmonary hypertension of the newborn. A study was made of the development of ACE activity spectrophotometrically in a rat model of LH and CDH. It was previously shown that the lungs in this model are hypoplastic and the muscularization of the pulmonary vascular bed is increased. CDH was induced in fetal rats by oral administration of 115 mg/kg Nitrofen to the mother on day 10.5 of pregnancy. Fetuses were delivered by hysterotomy on days 19, 20, 21, and 22. Nitrofen-exposed rats showed significantly lower lung weights and not statistically significant lower total ACE activities than in controls. ACE activity expressed per milligram lung wet weight and per milligram protein was significantly increased compared to controls. ACE converts angiotensin I to the vasoconstrictor angiotensin II, and it inactivates the vasodilator bradykinin. Increased ACE activity may therefore contribute to pulmonary hypertension. Whether ACE and angiotensin II levels are increased in human newborns with a diaphragmatic defect and whether they contribute to the development of persistent pulmonary hypertension has not been studied up till now.

The kidney in the fetal rat model of congenital diaphragmatic hernia induced by nitrofen

This paper explores whether there is a correlation between kidney and lung growths in an experimental model of congenital diaphragmatic hernia (CDH) induced by intragastric administration of Nitrofen (115 mg/kg) in olive oil on time-dated pregnant Wistar rats at the 9th day of gestation. For comparison we used pregnant rats treated with olive oil alone. Twenty-nine normal fetuses from 3 control rats and 24 left CDH fetuses from 6 Nitrofen rats were studied. Fetal (3.6 +/- 0.8 v 4.9 +/- 0.4 g, P < .001) and total lung (2% +/- 0.5% v 2.6% +/- 0.3% of body weight, P < .001) weights were significantly decreased in animals with CDH. Kidneys were also smaller in CDH animals although not significantly (0.7% +/- 0.1% v 0.8% +/- 0.1% of body weight, P = .05) and were also histologically immature. Regression of kidney weight on body weight for both groups yielded regression lines that were identical at analysis of covariance and all data points from the CDH group were within the control group 95% confidence limits. After converting raw data into lung/body and kidney/body weight ratios, no inverse correlation suggesting a feedback mechanism of growth regulation between both organs could be found. Since nitrofen acts through modifications of the thyroid hormone status in both dam and fetus, altered maturation of several organs should be expected although some of them, like the lung, are the leading targets. The present CDH rodent model is probably different from the human malformation in spite of the striking anatomic similarities between them.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental toxicity of diphenyl ether herbicides in nestling American kestrels

Beginning the day after hatching, American kestrel (Falco sparverius) nestlings were orally dosed for 10 consecutive days with 5 microliters/g of corn oil (controls) or one of the diphenyl ether herbicides (nitrofen, bifenox, or oxyfluorfen) at concentrations of 10, 50, 250, or 500 mg/kg in corn oil. At 500 mg/kg, nitrofen resulted in complete nestling mortality, bifenox in high (66%) mortality, and oxyfluorfen in no mortality. Nitrofen at 250 mg/kg reduced nestling growth as reflected by decreased body weight, crown-rump length, and bone lengths including humerus, radius-ulna, femur, and tibiotarsus. Bifenox at 250 mg/kg had less effect on growth than nitrofen, but crown-rump, humerus, radius-ulna, and femur were significantly shorter than controls. Liver weight as a percent of body weight increased with 50 and 250 mg/kg nitrofen. Other manifestations of impending hepatotoxicity following nitrofen ingestion included increased hepatic GSH peroxidase activity in all nitrofen-treated groups, and increased plasma enzyme activities for ALT, AST, and LDH-L in the 250-mg/kg group. Bifenox ingestion resulted in increased hepatic GSH peroxidase activity in the 50- and 250-mg/kg groups. Nitrofen exposure also resulted in an increase in total plasma thyroxine (T4) concentration. These findings suggest that altricial nestlings are more sensitive to diphenyl ether herbicides than young or adult birds of precocial species.

Relative teratogenicity of nitrofen analogs in mice: unchlorinated, monochlorinated, and dichlorinated-phenyl ethers

To determine the role of number and position of chlorine substituents on the developmental toxicity of diphenyl ether analogs of nitrofen, we have evaluated one unchlorinated, three monochlorinated, and five dichlorinated-phenyl 4'-nitrophenyl ethers with respect to effects on liver weight and on maintenance of pregnancy in females, and with respect to postnatal survival and the occurrence of small or absent Harderian glands in offspring. None of the diphenyl ethers evaluated in these experiments was as active as nitrofen with respect to any parameter evaluated. Both the position and the number of chlorine substituents affected toxicity, but no simple relationship between number or position of chlorine substituents and either maternal or fetal endpoints was established.

Teratogenic effects of nitrofen on cellular and functional maturation of the rat lung

The herbicide nitrofen was administered to pregnant Sprague-Dawley and Fischer-344 rats on Days 10-13 of gestation (po, 20 or 40 mg/kg daily) and the effects of maturation of the perinatal lung were evaluated. Nitrofen interfered with the ontogenetic acquisition of lung cells as DNA, RNA, and protein content were subnormal. The hypoplastic lungs in the newborns were associated with structural deficits, resulting in a profound reduction of surface area available for gas exchange and depressed lung compliance. Other factors which influence pulmonary function and systemic delivery of oxygen were also considered. Adrenal catecholamines, which play an important role in surfactant production and fluid resorption in the lung during the transition to air-breathing, were markedly reduced. In addition, red blood cell concentration was significantly diminished. Taken together, these results suggest that the neonatal mortality observed in the nitrofen-treated rats is likely associated with respiratory distress caused by a number of cellular and functional aberrations. These include (a) hypoplasia and structural defects in the lung leading to deficient pulmonary function, (b) deficits in adrenal catecholamines potentially impeding the transition of the lung to air-breathing, and (c) impaired systemic delivery of oxygen due to reduced hemoglobin concentration.

Effects of prenatal nitrofen exposure on cardiac structure and function in the rat

The herbicide nitrofen was administered to pregnant Fischer-344 and Sprague-Dawley rats on Days 10-13 of gestation (po, 20 or 40 mg/kg daily) and its effects on cardiac structure and function were investigated in the offspring. In the 21-day fetuses, nitrofen did not influence intrauterine growth or basal heart rate. In contrast, the herbicide produced a marked depression of heart rate and abnormal electrocardiographic (ECG) profiles in the newborn rats, in conjunction with labored respiratory movements and a profound increase in postnatal mortality. A few animals displayed cardiac ventricular septal defects and diaphragmatic hernias but these malformations did not appear to be associated with the ECG changes. The chronotropic deficiencies seen in the nitrofen-treated pups were reversible by acute hyperoxia (40% oxygen). These results suggest that the teratogenic effects of nitrofen on cardiac physiology and on postnatal mortality cannot be accounted for solely by specific gross anatomical damages to the rat heart and diaphragm; rather, other more subtle morphological and physiological factors which contribute to improper systemic delivery and cellular utilization of oxygen may be involved.

Further characterization of the distribution and metabolism of nitrofen in the pregnant rat

Nitrofen (2,4-dichloro-4'-nitrodiphenyl ether) is an herbicide with potent teratogenic activity in rodent species. The present study was an extension of previous efforts to characterize the distribution and metabolism of nitrofen in pregnant rats. Following a single p.o. exposure to radiolabeled compound on day 10 of pregnancy, maternal and embryonic tissues were collected at intervals from 1.5 to 72 hours. Radioactivity was accumulated and retained in maternal fat for over 72 hours. Peak levels were reached in other maternal organs at 3-12 hours. The half-life in maternal plasma was estimated to be 42 hours. Radioactivity was first detected in the embryonic compartment at 3 hours and continued to increase through the 72-hour time point. HPLC analysis indicated that the parent compound is initially deposited in maternal fat and after 48 hours redistributes to other maternal organs and to the embryo. The 5-hydroxy derivative was the major nitrofen metabolite found in maternal tissues, while the 4'-amino and 4'-acetylamine derivatives were found at lower levels and all exhibited single-phase kinetics. The parent compound alone was found in the embryo, and levels increased gradually as nitrofen redistributed from the fat at 48 hours after exposure. The results of this and other studies of nitrofen metabolism in pregnant rats suggest that its teratogenicity is not mediated via generation of mutagenic intermediates through nitroreduction of the parent compound. Rather, the embryo is exposed to the parent compound alone and appears to be a deep compartment for accumulation of nitrofen.

Strain differences in response of Sprague-Dawley and Long Evans Hooded rats to the teratogen nitrofen

Administration of nitrofen (2,4-dichloro-4'-nitrodiphenyl ether) during organogenesis in rodents produces neonatal lethality accompanied by lung hypoplasia, diaphragmatic hernias, heart anomalies, and hydronephrosis. Different strains of rats, Long Evans Hooded (LEH) and Sprague-Dawley (SD), are reported to have different malformation responses to prenatal exposure, which could be due to true strain differences, to different levels and times of exposure, or to the use of different methods for detecting visceral malformations. In the present study, LEH, SD, and "virus-antibody-negative" SD (VAN-SD) rats were identically exposed to 0, 6.25, 12.5, or 25 mg/kg/day of nitrofen by gavage in corn oil on days 6 15 of gestation. At term, half of the litter was examined by the Wilson method of razorblade sectioning and the remainder by a modified Staples method of fresh visceral examination. The two methods were equally sensitive for detecting diaphragm, kidney, and lung anomalies, whereas heart malformations were more frequently identified with fresh visceral examination. The frequency of total malformations did not vary across strains at any dose, but there were substantial differences in the pattern of malformations in each strain. SD and VAN-SD rats responded similarly for all malformations, but had significantly higher incidences of diaphragm and lung anomalies than LEH rats. Conversely, LEH rats had significantly elevated levels of kidney anomalies compared to SD and VAN-SD rats, whereas frequency of heart malformations was low and comparable across strains. These results suggest that true strain differences exist in the pattern of malformation produced by prenatal exposure to nitrofen that may be based on genetic differences in embryonic susceptibility.

Role of structure in diphenyl ether teratogenesis

The fetotoxicity of nitrofen and 5 analogous diphenyl ethers was compared in mice, using survival on postnatal day 5 to evaluate prenatal and perinatal toxicity. The effect of each chemical on the Harderian gland weight of surviving pups was also assessed, since decreased Harderian gland weight is seen in nitrofen-exposed pups at levels of treatment that do not cause mortality. All treatments were percutaneous. 2,4,5-Trichlorophenyl 4'-nitrophenyl ether was significantly more fetotoxic than nitrofen, but 2,4,6-trichlorophenyl 4'-nitrophenyl ether (CNP) was not fetotoxic at the doses used. Neither of the 2 monochlorinated 4'-nitrophenyl ethers nor the unchlorinated 4'-nitrophenyl ether were fetotoxic at the doses tested. Absent Harderian glands resulted only from exposure to nitrofen (71% of litters) or to 2,4,5-trichlorophenyl 4'-nitrophenyl ether (22% of litters), which were also the only compounds to decrease Harderian gland weight significantly.

Mutagenicity of N-arylacetohydroxamic acids and their O-glucosides derived from chlorinated 4-nitrobiphenyl ethers

The mutagenic activity of N-arylacetohydroxamic acids, their O-acetates, their O-glucosides, and N-arylhydroxylamines, derived from chlorinated 4-nitrobiphenyl ethers (CNBs), was tested in the Salmonella reversion assay. N-Arylhydroxylamines were mutagenic by themselves; however, other compounds containing an N-acetyl group showed mutagenic activity in the presence of guinea pig liver S9. The mutagenic activation of the glucosides of N-arylacetohydroxamic acids was caused by Ms but not by S10.5, whereas their aglycones, N-arylacetohydroxamic acids, were activated to mutagens by both the fractions. The mutagenic activation of these compounds was inhibited by bis(p-nitrophenyl)phosphate, which indicates that enzymatic deacetylation is a crucial step in the mutagenic activation. Analysis of metabolites of the O-glucosides of N-arylacetohydroxamic acids by h.p.l.c. indicates that the corresponding deacetylated O-glucosides are primary metabolites, which decomposed to amino and azoxy (via hydroxylamine) derivatives, and that the deacetylating activity of S9 locates exclusively in Ms.

Effects on the fetal rat intestine of maternal malnutrition and exposure to nitrofen (2,4-dichlorophenyl-p-nitrophenyl ether)

The effects of maternal protein-energy malnutrition and exposure to nitrofen on selected aspects of intestinal morphology and function were studied in the fetal rat. Pregnant rats were fed, throughout gestation, diets containing 24% or 6% casein as the sole source of protein. Reduced total food intake produced protein-energy malnutrition (PEM). Each diet group was divided in half and gavaged with either 12.5 mg nitrofen in corn oil/kg/day or corn oil carrier only from days 7 to 21 of gestation. Body weight, intestinal weight, length, and diameter were measured as were villus length (VL), villus width (VW), and number of villi per length of intestine (VMM). Protein (horseradish peroxidase) and lipid absorption were studied histochemically. Lactase and dipeptidase activities were determined in proximal, medial, and distal thirds of the intestine. Results showed that the restricted maternal diet resulted in reduced fetal body weight (BW), intestinal weight (IW) and length (IL), reduced IW/BW and IW/IL ratios, VH, and VMM. The VW was reduced only in the distal third. Protein and lipid absorption were unaffected. Lactase and dipeptidase activities were reduced. Maternal nitrofen exposure resulted in reduced body weight, intestinal size, and lipid absorption, with some evidence of interaction with the diet effects on enzyme activities. It is concluded that effects of maternal malnutrition were extensive, but that nitrofen exposure, at this dosage level, is not likely to contribute to the postnatal fetal mortality rate in either adequately nourished or malnourished rats.