Transfection of cultured fish cells with exogenous DNA

We have applied to fish cell cultures the techniques used routinely to introduce exogenous genes into cultured mammalian cells. Using calcium phosphate-mediated transfection, a plasmid containing bacterial aminoglycoside phosphotransferase under the influence of the simian virus 40 early promoter and polyadenylation signal was introduced into several fish cell lines. The plasmid was expressed in these cells in a stable manner, with transfection occurring at a frequency comparable to that seen with mammalian cells. These results suggest that plasmids constructed for use in mammalian cells may be used efficiently in fish systems without further modification and indicate that the advantages of transfection procedures utilized in mammalian systems can also be applied to fish.

Chlorophyllin intervention reduces aflatoxin-DNA adducts in individuals at high risk for liver cancer

Residents of Qidong, People's Republic of China, are at high risk for development of hepatocellular carcinoma, in part from consumption of foods contaminated with aflatoxins. Chlorophyllin, a mixture of semisynthetic, water-soluble derivatives of chlorophyll that is used as a food colorant and over-the-counter medicine, has been shown to be an effective inhibitor of aflatoxin hepatocarcinogenesis in animal models by blocking carcinogen bioavailability. In a randomized, double-blind, placebo-controlled chemoprevention trial, we tested whether chlorophyllin could alter the disposition of aflatoxin. One hundred and eighty healthy adults from Qidong were randomly assigned to ingest 100 mg of chlorophyllin or a placebo three times a day for 4 months. The primary endpoint was modulation of levels of aflatoxin-N(7)-guanine adducts in urine samples collected 3 months into the intervention measured by using sequential immunoaffinity chromatography and liquid chromatography-electrospray mass spectrometry. This aflatoxin-DNA adduct excretion product serves as a biomarker of the biologically effective dose of aflatoxin, and elevated levels are associated with increased risk of liver cancer. Adherence to the study protocol was outstanding, and no adverse events were reported. Aflatoxin-N(7)-guanine could be detected in 105 of 169 available samples. Chlorophyllin consumption at each meal led to an overall 55% reduction (P = 0.036) in median urinary levels of this aflatoxin biomarker compared with those taking placebo. Thus, prophylactic interventions with chlorophyllin or supplementation of diets with foods rich in chlorophylls may represent practical means to prevent the development of hepatocellular carcinoma or other environmentally induced cancers.

Application of matrix solid-phase dispersion in the determination of dibenzo[a,l]pyrene content of experimental animal diets used in a large-scale tumor study

A method utilizing matrix solid-phase dispersion (MSPD) was developed for isolation and determination of dibenzo[a,l]pyrene (DBP) in experimental rainbow-trout diets used in a large-scale carcinogenesis study. A 0.5 g sample of moist ration containing 0-225 ppm DBP (dry basis) was mixed with 2 g C18 sorbent and benzo[a]pyrene internal standard was added to the mixture. Extraction and clean-up were accomplished in a single step by extracting the sample mixture with hexane-benzene 4:1 from a cartridge containing 2 g Florisil. DBP was quantified by HPLC on a C5 bonded phase column with fluorescence detection. Mean analytical recovery of DBP from control diet spiked at three concentration levels was 101 to 107% with relative standard deviations of 1 to 7%. The limit of detection of DBP was equivalent to 0.014 ppm in the ration. Application of the method to verification of DBP levels in trout rations from the carcinogenesis study is described. Control ration (0 ppm DBP) was screened for possible DBP contamination and none was found. This is the first report on analysis of DBP in experimental animal diets.

beta-Catenin mutation in rat colon tumors initiated by 1,2-dimethylhydrazine and 2-amino-3-methylimidazo[4,5-f]quinoline, and the effect of post-initiation treatment with chlorophyllin and indole-3-carbinol

Carcinogens 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 1,2-dimethylhydrazine (DMH) induce colon tumors in the rat that contain mutations in beta-catenin, but the pattern of mutation differs from that found in human colon cancers. In both species, mutations affect the glycogen synthase kinase-3beta consensus region of beta-catenin, but whereas they directly substitute critical Ser/Thr phosphorylation sites in human colon cancers, the majority of mutations cluster around Ser33 in the rat tumors. Two dietary phytochemicals, chlorophyllin and indole-3-carbinol, given post-initiation, shifted the pattern of beta-catenin mutations in rat colon tumors induced by IQ and DMH. Specifically, 17/39 (44%) of the beta-catenin mutations in groups given carcinogen plus modulator were in codons 37, 41 and 45, and substituted critical Ser/Thr residues directly, as seen in human colon cancers. None of the tumors from groups given carcinogen alone had mutations in these codons. Interestingly, many of the mutations that substituted critical Ser/Thr residues in beta-catenin were from a single group given DMH and 0.001% chlorophyllin, in which a statistically significant increase in colon tumor multiplicity was observed compared with the group given DMH only. These tumors had marked over-expression of cyclin D1, c-myc and c-jun mRNA and c-Myc and c-Jun proteins were strongly elevated compared with tumors containing wild-type beta-catenin. The results indicate that the pattern of beta-catenin mutations in rat colon tumors can be influenced by exposure to dietary phytochemicals administered post-initiation, and that the mechanism might involve the altered expression of beta-catenin/Tcf/Lef target genes.

Post-initiation effects of chlorophyllin and indole-3-carbinol in rats given 1,2-dimethylhydrazine or 2-amino-3-methyl- imidazo

Chlorophyllin (CHL) is a water-soluble derivative of chlorophyll, the ubiquitous pigment in green and leafy vegetables, whereas indole-3-carbinol (I3C) is present in cruciferous vegetables such as cabbage, broccoli and cauliflower. In rats initiated with 1,2-dimethylhydrazine (DMH), CHL and I3C reportedly promoted or enhanced the incidence of colon tumors when they were administered after, or during and after the carcinogen exposure, respectively. The same compounds given post-initiation inhibited the formation of colonic aberrant crypts induced by heterocyclic amines, such as 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), but tumor suppression was not examined in the latter studies. In the present investigation, male F344 rats were treated with IQ or DMH during the first 5 weeks of a 1 year study; IQ was given in the diet (0.03%), whereas DMH was administered once a week by s.c. injection (20 mg/kg body wt). Beginning 1 week after the last dose of IQ or DMH until sacrifice, rats received 0.001, 0.01 or 0.1% (w/v) CHL in the drinking water or 0.001, 0.01 or 0.1% I3C in the diet. Compared with controls given carcinogen alone, 0.1% I3C treatment suppressed the multiplicity of IQ-induced colon tumors, and CHL inhibited in a dose-related manner the incidence of IQ-induced liver tumors. However, 0.001% CHL increased significantly the multiplicity of DMH-induced colon tumors while having no effect on the colon tumors induced by IQ. These results indicate that both the choice of carcinogen as well as the dose of the tumor modulator can be important determinants of the events that occur during post-initiation exposure to CHL or I3C. Based on the present findings and data in the literature, it is possible for CHL and I3C to act as tumor promoters or anticarcinogens, depending upon the test species, initiating agent and exposure protocol.

Neoplasia in zebrafish (Danio rerio) treated with N-methyl-N'-nitro-N-nitrosoguanidine by three exposure routes at different developmental stages

We exposed embryos (83 hours postfertilizaton) and fry (3 weeks posthatch) to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) by immersion in aqueous solutions of 0-10 ppm for 1 hour (embryo) or 0-2 ppm for 24 hours (fry). Zebrafish embryos were microinjected with MNNG at levels of 0 or 96 ng/egg. Diets containing 0-2,000 ppm MNNG were fed to juvenile zebrafish for 3 months beginning at 2 months posthatch. Fish were sampled for histopathologic study at 6-12 months after initiation of carcinogen exposure. Embryos and fry were both quite responsive to MNNG; however, juvenile zebrafish were remarkably refractory to MNNG-induced neoplasia. Principal target organs in zebrafish treated as embryos with MNNG were liver and testis, with hepatocellular adenoma the most prevalent hepatic neoplasm. A variety of mesenchymal neoplasms occurred in zebrafish following embryo exposure to MNNG, including chondroma, hemangioma, hemangiosarcoma, leiomyosarcoma, and rhabdomyosarcoma. Testis and blood vessels were primary target organs for MNNG following fry exposure, with seminoma, hemangioma, hemangiosarcoma, and various other epithelial and mesenchymal neoplasms occurring. The zebrafish is a responsive, cost-effective lower vertebrate model system in which to study mechanisms of carcinogenesis.

Neoplasia in zebrafish (Danio rerio) treated with 7,12-dimethylbenz[a]anthracene by two exposure routes at different developmental stages

Using zebrafish, Danio rerio, initial pioneering work in the 1960s revealed carcinogen responsiveness of fish, yet very few subsequent tumorigenesis investigations have utilized this species. We exposed embryos (60 hours postfertilization) and fry (3 week posthatch) to 7,12-dimethylbenz[a]anthracene (DMBA) by immersion in aqueous solutions for 24 hours, at concentrations of 0-1 or 0-5 ppm (mg/L), respectively. Juvenile zebrafish 2 months posthatch were fed a diet containing 0-1,000 ppm DMBA for 4 months. Fish were sampled for histologic evaluation at 7-12 months after the onset of carcinogen treatment. Fry were most responsive to DMBA and showed the widest diversity of target tissues and histologic types of neoplasia, having several types of epithelial, mesenchymal, and neural neoplasia. The principal target tissues for carcinogenic response were liver following embryo or fry exposure, with gill and blood vessel the second and third most responsive tissues in fry. Intestine was the primary target and gill a secondary target in fish that received dietary DMBA as juveniles. These studies indicate that young zebrafish are most responsive to DMBA, showing a greater diversity of neoplasm types than rainbow trout. Thus, zebrafish are a valuable model system in which to study mechanistic aspects of the carcinogenesis process.

Purification and characterization of cationic chymotrypsin from the pancreas of the Arabian camel (Camelus dromedarius)

This study reports on the purification and characterization of a cationic enzyme with chymotryptic activity from camel pancreas. The enzyme was purified 52-fold in a 48% yield by a three-step chromatographic procedure consisting of anion-exchange, cation-exchange and affinity chromatographies. The purified enzyme was homogeneous on gel isoelectric focusing and on SDS gel electrophoresis. Its isoelectric point was estimated to be 7.3 and its molecular mass was found to be 23,600 Da. The enzyme was identified as a cationic chymotrypsin according to its physiochemical properties, substrate specificity and susceptibility to inhibition. It was active towards esters of aromatic amino acids but much less active towards a leucine ester. In all cases, the k(cat) values of the camel enzyme were less than the corresponding values of bovine chymotrypsin A. It also showed a lower level of kininase activity. Camel chymotrypsin was more susceptible than its bovine equivalent to inhibition by soybean trypsin inhibitor and aprotinin. It showed the same pH optimum as bovine chymotrypsin A for its esterolytic activity, but was more dependent on CaCl2 for long-term stability.

Purification and some properties of camel carboxypeptidase B

A carboxypeptidase B-like enzyme was purified 116-fold with a recovery of activity of 29% from a crude extract of camel pancreas by a four-step procedure consisting of two anion exchange chromatographies in succession, gel filtration and hydrophobic interaction chromatography. The enzyme was homogeneous on SDS and non-denaturing gel electrophoresis and on gel isoelectric focusing. Its molecular mass was found to be 31.5 kDa and its isoelectric point was estimated as 6.1. It was active towards a number of substrates that are cleaved by carboxypeptidases B from other species and was also susceptible to inhibition by inhibitors of such enzymes. The camel enzyme showed a pH optimum of 8.0 and it was seen to be a relatively potent kininase in vitro. The enzyme purified in this study was very similar to carboxypeptidases B isolated from other species in size, charge, substrate specificity and susceptibility to inhibition and thus it can be identified as camel carboxypeptidase B.

Inhibition of dibenzo[a,l]pyrene-induced multi-organ carcinogenesis by dietary chlorophyllin in rainbow trout

Cancer chemoprevention by dietary chlorophyllin (CHL) was investigated in a rainbow trout multi-organ tumor model. In study 1, duplicate groups of 130 juvenile trout were treated for 2 weeks with control diet, 500 p.p.m. dibenzo[a,l]pyrene (DB[a,l]P) or 500 p.p.m. DB[a,l]P + 2052 p.p.m. CHL, then returned to control diet. DB[a,l]P alone proved somewhat toxic but induced high tumor incidences in liver (61%), stomach (91%) and swimbladder (53%) 11 months after initiation. CHL co-feeding abrogated DB[a,l]P acute toxicity and reduced tumor incidences to 18% in liver, 34% in stomach and 3% in swimbladder (P Collapse

Key Words

• chlorophyllin
• aflatoxin b1
• heterocyclic amine
• trout
• salmonella assay
• dna adduct
• anticarcinogen

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MESH Headings

• Animals
• Anticarcinogenic Agents/administration & dosage
• Anticarcinogenic Agents/pharmacology
• Benzopyrenes/toxicity
• Carcinogens/toxicity
• Chlorophyllides/administration & dosage
• Chlorophyllides/pharmacology
• Diet
• Mutagenicity Tests
• Neoplasms, Experimental/chemically induced
• Neoplasms, Experimental/prevention & control
• Oncorhynchus mykiss

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Grants

• ES00210 NIEHS NIH HHS
• P30 ES003850 NIEHS NIH HHS
• R29 CA065525 NCI NIH HHS
• ES03850 NIEHS NIH HHS
• R01 CA065525 NCI NIH HHS
• ES04766 NIEHS NIH HHS
• R01 CA065525-06A1 NCI NIH HHS
• P30 ES000210 NIEHS NIH HHS

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Affiliation(s)

A P Reddy Department of Environmental Toxicology, Oregon State University, Corvallis, OR 97331, USA
U Harttig
M C Barth
W M Baird
M Schimerlik
J D Hendricks
G S Bailey

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Potency of dietary indole-3-carbinol as a promoter of aflatoxin B1-initiated hepatocarcinogenesis: results from a 9000 animal tumor study

Indole-3-carbinol (I3C), a metabolite of glucobrassicin found in cruciferous vegetables, is documented as acting as a modulator of carcinogenesis and, depending on timing and dose of administration, it may promote hepatocarcinogenesis in some animal models. In this study we demonstrate that, when given post-initiation, dietary I3C promotes aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in the rainbow trout model at levels as low as 500 p.p.m. Trout embryos (approximately 9000) were initiated with 0, 25, 50, 100, 175 or 250 p.p.b. AFB1 by a 30 min immersion. Experimental diets containing 0, 250, 500, 750, 1000 or 1250 p.p.m. I3C were administered starting at 3 months and fish were sampled for liver tumors at 11-13 months. Promotion at the level of tumor incidence was statistically significant for all dietary levels, except 250 p.p.m. Relative potency for promotion markedly increased at dietary levels >750 p.p.m. We propose that more than one mechanism could be involved in promotion and that both estrogenic and Ah receptor-mediated pathways could be active. The estrogenicity of I3C, measured as its ability to induce vitellogenin (an estrogen biomarker in oviparous vertebrates) was evident at the lowest dietary level (250 p.p.m.), whereas CYPIA (a P450 isozyme induced through the Ah receptor pathway) was not induced until dietary levels of 1000 p.p.m. Therefore, at lower dietary levels, promotion by I3C in this model could be explained by estrogenic activities of I3C acid derivatives, as it is known that estrogens promote hepatocarcinogenesis in trout. Much stronger promotion was observed at high dietary I3C levels (1000 and 1250 p.p.m.), at which levels both CYP1A and vitellogenin were induced.

Experimental hepatic tumorigenicity by environmental hydrocarbon dibenzo[a,l]pyrene

There is an evident need of low-cost vertebrates to be used in experimental carcinogenesis. Medaka (Oryzias latipes) provide a useful vertebrate model system for investigating tissue tropism of carcinogens and the action mechanisms of environmental contaminants posing a potential risk to human health. Juvenile medaka 2 months of age fed diets containing 100 ppm (dry weight basis) dibenzo[a,l]pyrene (DBP) for 28 days responded with hepatic neoplasia predominately of hepatocellular origin. When sampled 9 months following the termination of carcinogen exposure, medaka showed 26% incidence of neoplasia and 25% hepatic neoplasia, compared with 8% total neoplasia and 0% hepatic neoplasia in control fish. The predominant spontaneous neoplasms in this group of medaka were ovarian germ cell tumors. Hepatic neoplasia occurred at a higher incidence in female DBP-treated medaka than in males (11/29 vs 5/36). Nonneoplastic lesions observed in the livers of DBP-exposed fish included spongiosis hepatis, globular hyaline eosinophilic cytoplasmic inclusions in hepatocytes, foci of hepatocellular degeneration, extensive cytomegaly, and karyomegaly of hepatocytes. No activating exon I mutations in the one ras protooncogene examined were detected among six liver neoplasms. These results indicate that medaka are sensitive to the tumorigenic effects of the environmental carcinogen DBP, administered by dietary exposure.

Molecular cloning, sequence analysis and expression distribution of rainbow trout (Oncorhynchus mykiss) cystatin C

Cystatin C is one of a family of proteinase inhibitors of cathepsins and other cysteine proteinases. Among warm-blooded vertebrates, small functional regions of cystatin amino acid sequences are well conserved among species, but major portions of cystatin amino acid sequences vary evolutionarily. Although considerable attention has been given to mammalian and avian cystatins, little data exist on cystatins from other vertebrates. A cDNA clone for trout cystatin C was isolated from a lambda gt11 cDNA library of rainbow trout (Oncorhynchus mykiss) liver. An apparently full-length cDNA clone of 674 bp encoding 132 amino acid residues was obtained. Sequence analysis indicated that trout cystatin C contains an N-terminal signal sequence extension of 21 amino acids and a mature sequence of 111 amino acid residues, with amino acid residues conserved in functional regions relative to mammalian and avian cystatin C. Using cloned cDNA as a probe, we investigated expression of the cystatin C gene in trout tissues, several cell lines of trout liver or liver tumor, and cell cultures of liver tumor origin. Cystatin C mRNA was in high abundance in trout embryo tissue, a tumor-derived liver cell line and some normal adult tissues. Southern hybridization analysis indicated one copy of the trout cystatin C gene per haploid genome, and sequence comparisons indicated considerable divergence in large portions of the coding region of the trout cystatin C gene relative to a variety of species.

Chemoprotection by natural chlorophylls in vivo: inhibition of dibenzo[a,l]pyrene-DNA adducts in rainbow trout liver

Naturally occurring chlorophylls (Chl) have shown anti-mutagenic activity but little is known about their chemoprotective properties in vivo. This study examined the effect of Chl on formation in vivo of DNA adducts by the potent environmental carcinogen dibenzo[a,l]pyrene (DBP), using rainbow trout as the animal model. Fingerling trout were fed diets containing 200 p.p.m. DBP alone or with one of the following preparations incorporated at 3000 p.p.m. total chlorins: purified pheophytin a (Phe a) (94%); semi-purified Chl a (77%, 23% Phe a), commercial Chl a (88%, 12% other Chl a-related compounds); crude spinach extract (53% Chl a, 19% Chl b, 14% Phe a, 9% carotenoids); commercial Cu-chlorophyllin (55% chlorins, 45% neutral salts), as a known inhibitory control. After 2 weeks dietary treatment, the animals were killed and organs were collected. Stable DBP-DNA adducts from liver were quantified after 33P-post-labeling and separation by reversed-phase HPLC. Total DBP-DNA adducts in the DBP-only group were 2.46 +/- 0.32 adducts/10(6) nucleotides. All chlorophyll treatment groups showed significantly lower adduct levels (P < 0.001, Tukey's HSD test), as follows: crude spinach extract, 0.64 +/- 0.14; semi-pure Chl a, 0.5 +/- 0.11; commercial Chl a, 1.26 +/- 0.17; Phe a, 0.95 +/- 0.01; chlorophyllin, 0.78 +/- 0.09. The various treatments suppressed DBP-DNA adducts essentially uniformly across the HPLC profile, which is consistent with complex formation and reduced carcinogen uptake as the predominant protective mechanism. The chlorophyll-mediated reduction in DBP-DNA adducts in vivo is the first demonstration of anti-genotoxic activity of these common dietary phytochemicals in any vertebrate animal model.

Molecular dosimetry in fish: quantitative target organ DNA adduction and hepatocarcinogenicity for four aflatoxins by two exposure routes in rainbow trout

Rainbow trout, a species highly sensitive to aflatoxins, was used to investigate the relative carcinogenicities of four structurally related aflatoxins in terms of their target organ DNA binding characteristics. Tritiated syntheses were carried out, DNA binding dose-response curves were established, and liver DNA binding indices were calculated for the four aflatoxins following a 2-week dietary fry exposure protocol. The results indicated that adduct levels increased linearly with dietary dose concentration, with relative DNA binding indices of 20.7, 20.3, 2.35, and 2.22 x 10(3) (pmoles aflatoxin mg-1 DNA)/(pmoles aflatoxin g-1 diet) for aflatoxin B1 (AFB1), aflatoxicol (AFL), aflatoxin M1 (AFM1), and aflatoxicol M1 (AFLM1), respectively. A similar protocol used over 7200 trout fry averaging 1.2 g initial body weight to establish full carcinogen dose-response curves for each aflatoxin, along with a single-dose estimate of DNA binding index within the tumor study animals. Owing to trout sensitivity a total of 180 micrograms or less of each aflatoxin was required. Data analyzed on logit incidence vs. Ln dose coordinates generated four curves which were modeled as parallel in slope over most or all dose ranges studied. By this analysis, relative tumorigenic potencies were: AFB1 1.00; AFL 0.936; AFM1 0.086; and AFLM1 0.041. When data were plotted as logit incidence vs. Ln adducts (effective dose received), all aflatoxin adducts described the same dose-response curve; that is, they were equally tumorigenic, except those from AFLM1, which were 2-3 fold less potent. Therefore, by these molecular dose studies, differences in tumorigenicity among the four dietary aflatoxins are largely or entirely accounted for by differences in uptake and metabolism leading to DNA adduction, rather than any inherent differences in tumor initiating potency per DNA adduct.

Purification and partial characterization of camel anionic chymotrypsin

An anionic chymotrypsin-like enzyme was isolated from a crude extract of camel pancreas by a three-step procedure consisting of anion-exchange chromatography, gel filtration, and hydrophobic interaction chromatography. The purified enzyme was homogeneous on native and SDS gel electrophoresis and on gel isoelectric focusing. Its molecular mass was estimated as 28.5 kDa and its isoelectric point was found to be 4.4. The enzyme differed markedly from bovine chymotrypsin A in its substrate specificity, showing considerably lower values of the specificity constant for its action on tyrosine, tryptophan, and phenylalanine esters. Its pH optimum was found to be 7.8. It showed lower kininase activity and was more susceptible to inhibition by a number of inhibitors than the bovine cationic chymotrypsin. On the other hand, the camel enzyme showed a much greater hydrolytic activity than the bovine enzyme toward a leucine ester. In terms of its size, charge, and substrate specificity the camel enzyme was very similar to anionic chymotrypsins that have been isolated from other species and thus appears to be a camel anionic chymotrypsin.

Inhibition of 2-amino-3-methylimidazo[4,5-f]quinoline-DNA adducts by indole-3-carbinol: dose-response studies in the rat colon

Indole-3-carbinol (I3C) inhibits the formation of colonic aberrant crypt foci and DNA adducts in rats given heterocyclic amine colon carcinogens, such as 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Mechanism studies indicate that I3C induces cytochromes P4501A1 and 1A2 (CYP1A1 and CYP1A2), isozymes that respectively metabolize IQ via ring hydroxylation or activate the carcinogen by N-hydroxylation. The present study examined the dose-response for induction of CYP1A1 versus CYP1A2 by I3C, and compared the profiles of induction with the dose-response for inhibition of IQ-DNA adducts in the colon of the F344 rat. Dietary equivalent doses of I3C in the range 100-1000 p.p.m. increased in a dose-related manner both ethoxyresorufin O-deethylase (EROD) and methoxyresorufin O-demethylase (MROD) activities in the liver and colonic mucosa, and Western blots showed a corresponding induction of CYP1A1 and CYP1A2 proteins. However, dietary equivalent doses of I3C in the range 10-25 p.p.m. (i) reduced hepatic EROD and MROD activities and CYP1A protein levels compared with controls, (ii) increased the ratio of CYP1A2 versus CYP1A1, and (iii) activated IQ to a more potent mutagen when liver microsomes from rats given I3C were used for metabolic activation in the Salmonella assay. Rats given a single oral dose of I3C shortly before administering IQ (5 mg/kg body wt, p.o.) exhibited dose-related inhibition of colonic IQ-DNA adducts in the range 25-100 p.p.m. I3C, reaching 95% inhibition at doses > or = 100 p.p.m. I3C, but IQ-DNA adducts were elevated slightly at the lowest I3C dose as compared with the controls. The possible significance of the low versus high dose effects of I3C are discussed in the context of human dietary exposures to I3C and the reported chemopreventive mechanisms of I3C in vivo.

CYP1A induction by beta-naphthoflavone, Aroclor 1254, and 2,3,7,8-tetrachlorodibenzo-p-dioxin and its influence on aflatoxin B1 metabolism and DNA adduction in zebrafish

This study investigated the inductive response of cytochrome P4501A (CYP1A) in the zebrafish (Danio rerio) following exposure to Aroclor 1254, beta-naphthoflavone (betaNF), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and then investigated TCDD modulation of aflatoxin B1 (AFB1) metabolism and hepatic AFB1-DNA adduction. Aroclor 1254 fed at 500 ppm for 1 to 9 days or intraperitoneal (ip) injection of 75-200 mg Aroclor 1254/kg body weight failed to induce CYP1A protein or associated 7-ethoxyresorufin-O-deethylase (EROD) activity. By contrast, dietary betaNF at 500 ppm for 3 or 7 days induced CYP1A protein and EROD activity approximately threefold above controls. A single ip injection of 150 mg/kg betaNF showed maximal induction of CYP1A protein and EROD activity near 24 hr, both of which decreased to control levels during the next 6 days. Single ip administration of 25, 50, 100, or 150 mg betaNF/kg body weight provided dose-responsive increases in CYP1A and EROD activity. Dietary exposure to 0.75 ppm TCDD for 3 days also significantly induced CYP1A and EROD. The effect of TCDD on the metabolism of [3H]AFB1 in zebrafish was then investigated. The major [3H]AFB1 metabolites excreted in water over 24 hr in the control group were aflatoxicol, aflatoxicol-glucuronide, and parent AFB1. By contrast, the predominant metabolites in the TCDD-pretreated group were aflatoxicol-M1-glucuronide, aflatoxicol, aflatoxin M1 plus aflatoxicol-M1 (unresolved), aflatoxicol-glucuronide, and parent AFB1. Surprisingly, hepatic AFB1-DNA adduction was approximately fourfold higher in the TCDD treated group than in controls. This significant difference could not be explained by increased capacity for bioactivation of AFB1 as measured by an in vitro AFB1-exo-8, 9-epoxide trapping assay. However, it was demonstrated that both control and induced zebrafish have high capacity to bioactivate aflatoxin M1 to a reactive intermediate, such that secondary bioactivation of this genotoxic intermediate may be responsible for the increased DNA binding.

Zebrafish (Danio rerio) p53 tumor suppressor gene: cDNA sequence and expression during embryogenesis

Three methods were used in succession to screen a whole adult zebrafish cDNA library for expressed p53-like genes. The sequences of the resultant clones describe an open reading frame 1122 nucleotides in length, with another 43 and 940 bases of 5' and 3' untranslated sequence, respectively. The deduced amino acid sequence of the zebrafish p53 protein is 63% identical to that of trout and 48% identical to that of human p53. Two of the three zebrafish clones overlap to span the entire reported cDNA sequence and are identical in their deduced amino acid sequence over their coincident length. The third clone contains a conservative amino acid change, as well as an inserted amino acid subsequently found to be at the junction of exons 2 and 3, suggestive of alternative splicing in the p53 mRNA for this species. Northern analysis demonstrated a zebrafish p53-related transcript to be present and most abundant in zygotes and early-cleavage embryos less than 1 hour after fertilization, thereafter declining to barely detectable levels at 48 hours. A similar temporal expression was detected for the zebrafish L-myc, known to be present in maternally derived RNA, whereas zebrafish N-myc and the zebrafish homologue of the murine T gene were not detectable prior to the onset of zygotic transcription.

In vivo aflatoxin B1 metabolism and hepatic DNA adduction in zebrafish (Danio rerio)

The zebrafish (Danio rerio) is assuming prominence in developmental genetics research. By comparison, little is known of tumorigenesis and nothing is known of carcinogen metabolism in this species. This study evaluated the ability of zebrafish to metabolize a well-characterized human carcinogen, aflatoxin B1 (AFB1), to phase I and phase II metabolites and assessed hepatic AFB1-DNA adduction in vivo. Fish i.p. injected with 50-400 micrograms [3H]AFB1/kg body wt displayed a linear dose response for hepatic DNA binding at 24 hr. AFB1-DNA adduct levels among treatments showed no statistical difference over the period from 1 to 21 days after injection, suggesting poor adduct repair in this species. DNA binding in female fish was 1-7-fold higher than that in males (p < 0.01). An in vitro AFB1 metabolism assay verified that zebrafish liver extracts oxidize AFB1 to the 8,9-epoxide proximate electrophile (Km = 79.0 +/- 16.4 microM, Vmax = 11.7 +/- 1.4 pmol/min/mg protein at 28 degrees C). The excretion of AFB1 and its metabolites was also examined by HPLC. As is typical of other fish studied, major metabolites excreted were aflatoxicol (AFL) and aflatoxicol-glucuronide (AFL-g), followed by unreacted AFB1. AFL appeared as early as 5 min after injection, whereas AFL-g was a significant metabolite after 18 hr. This study shows that in vivo administration of AFB1 to zebrafish results in moderate adduction of the carcinogen to liver DNA and that zebrafish have the capacity for both phase I and phase II metabolism of AFB1. The approximate fourfold difference between rainbow trout and zebrafish AFB1-DNA covalent binding index appears insufficient to explain the relative resistance of zebrafish to dietary AFB1 hepatocarcinogenicity.

Organ specific, protocol dependent modulation of 7,12-dimethylbenz[a]anthracene carcinogenesis in rainbow trout (Oncorhynchus mykiss) by dietary ellagic acid

This study investigated pre-initiation and post-initiation effects of dietary ellagic acid (EA) on 7,12-dimethylbenz[a]anthracene (DMBA) multi-organ carcinogenesis in rainbow trout (Oncorhynchus mykiss). EA at 100, 250 (study 2), 1000 and 2000 (study 1) p.p.m. suppressed stomach adenopapilloma incidence by 33, 60, 70 and 78% (P < or = 0.001), respectively, as well as tumor multiplicity (P < 0.01) and size (P < 0.001) when fed continuously following DMBA initiation. However, continuous EA feeding also produced modest (250 p.p.m.) to extensive (1000, 2000 p.p.m.) growth rate suppression in these studies. Retrospective logistic regression modeling of the data allowed separation of growth-related from non-growth-related inhibitory effects. By this approach: (i) tumor development showed a similarly strong dependence (same regression slope) on animal growth rate in all treatment groups; (ii) EA-mediated reduction in mean population growth contributed to suppressed stomach tumor response above 250 p.p.m. EA; and (iii) even at high, toxic doses EA displayed inhibitory mechanisms additional to, and distinct from, growth suppression effect. The effects of post-initiation EA were organ specific. Chronic EA treatment significantly suppressed swim-bladder as well as stomach tumor incidence at doses > or = 1000 p.p.m., but increased liver tumor incidence at doses > or = 250 p.p.m. Three protocols examined EA effects on the initiation process. EA fed at 1000 p.p.m. concurrently with 750 p.p.m. dietary DMBA for 7 weeks modestly reduced stomach tumor incidence (from 85 to 78%, P < 0.05) and multiplicity (from 6.3 +/- 4.3 to 4.9 +/- 2.9, P < 0.01), but did not alter swim-bladder or liver response. The effect of EA pretreatment prior to DMBA single-dose initiation by gill uptake was also examined. When fed for 1 week prior to initiation, 2000 p.p.m. EA again imposed a small reduction in stomach adenoma incidence (from 88 to 78%; P < 0.05) and multiplicity (from 5.5 +/- 3.2 to 4.4 +/- 3.2; P < 0.01). However, when EA was pre-fed for 3 weeks instead of 1 week, protection in the stomach was lost and response in liver and swim-bladder significantly increased. In sum, these studies demonstrate that EA influence on DMBA tumorigenesis in this multi-organ model is highly protocol dependent and organ specific. Post-initiation dietary EA consistently suppressed stomach tumor development in trout, at EA doses far lower than those required for protection in rodents. At higher doses, however, EA also displayed toxicity and a potential in some protocols to enhance tumor response in other organs.

Fish models for environmental carcinogenesis: the rainbow trout

Progress over the past 30 years has revealed many strengths of the rainbow trout as an alternative model for environmental carcinogenesis research. These include low rearing costs, an early life-stage ultrasensitive bioassay, sensitivity to many classes of carcinogen, a well-described tumor pathology, responsiveness to tumor promoters and inhibitors, and a mechanistically informative nonmammalian comparative status. Low-cost husbandry, for example, has permitted statistically challenging tumor study designs with up to 10,000 trout to investigate the quantitative interrelationships among carcinogen dose, anticarcinogen dose, DNA adduct formation, and final tumor outcome. The basic elements of the trout carcinogen bioassay include multiple exposure routes, carcinogen response, husbandry requirements, and pathology. The principal known neoplasms occur in liver (mixed hepatocellular/cholangiocellular adenoma and carcinoma, hepatocellular carcinoma), kidney (nephroblastoma), swim bladder (adenopapilloma), and stomach (adenopapilloma). Trout possess a complex but incompletely characterized array of cytochromes P450, transferases, and other enzymic systems for phase I and phase II procarcinogen metabolism. In general, trout exhibit only limited capacity for DNA repair, especially for removal of bulky DNA adducts. This factor, together with a high capacity for P450 bioactivation and negligible glutathione transferase-mediated detoxication of the epoxide, accounts for the exceptional sensitivity of trout to aflatoxin B1 carcinogenesis. At the gene level, all trout tumors except nephroblastoma exhibit variable and often high incidences of oncogenic Ki-ras gene mutations. Mutations in the trout p53 tumor suppressor gene have yet to be described. There are many aspects of the trout model, especially the lack of complete organ homology, that limit its application as a surrogate for human cancer research. Within these limitations, however, it is apparent that trout and other fish models can serve as highly useful adjuncts to conventional rodent models in the study of environmental carcinogenesis and its modulation. For some problems, fish models can provide wholly unique approaches.

The model Ah-receptor agonist beta-naphthoflavone inhibits aflatoxin B1-DNA binding in vivo in rainbow trout at dietary levels that do not induce CYP1A enzymes

beta-Naphthoflavone (BNF), a well-known Ah-receptor agonist, has been believed to inhibit aflatoxin B1 (AFB1) carcinogenesis in rats and rainbow trout primarily through induction of the cytochrome P450 1A (CYP1A) enzyme subfamily and consequent diversion of AFB1 to the less carcinogenic phase I metabolite aflatoxin M1 (AFM1). This study investigates the dose responsive effects of dietary BNF treatment on CYP1A induction. AFM1 formation, AFB1-8,9-epoxide formation and AFB1-DNA binding in the trout model. Pre-feeding diet containing 10-200 p.p.m. BNF after AFB1 i.p. injection provided dose-dependent induction of CYP1A-dependent ethoxyresorufin-O-deethylase (EROD) activity and inhibition of in vivo AFB1-DNA binding. However, most of the observable inhibition of DNA adduction (45% inhibition) had occurred at 10 p.p.m. BNF without detectable EROD induction; higher doses of BNF up to 200 p.p.m. induced EROD > 6-fold but provided only another 15% inhibition of DNA adduction in vivo. When in vitro AFB1-DNA binding was assessed using liver microsomes from trout fed 10-100 p.p.m. BNF, induced microsomal EROD activity correlated moderately with reduction of in vitro AFB1-DNA binding activity. However, BNF treatment in a low dose range (0.2-10 p.p.m.) also strongly inhibited in vivo hepatic AFB1-DNA binding (69% inhibition at 5 p.p.m. BNF in this experiment), in a dose-dependent manner, in the complete absence of detectable EROD induction. The microsomes from 5 p.p.m. BNF-treated trout had no more EROD activity than control microsomes, and no less capacity for catalyzing AFB1-DNA binding in vitro than control microsomes. Thus, the potent inhibition of hepatic AFB1-DNA binding in vivo by 5 p.p.m. BNF was a result of neither CYP1A enzyme induction nor irreversibly reduced catalytic capacity for AFB1-8,9-epoxide formation. Direct analysis of AFB1 metabolites formed in vitro by liver microsomes from trout fed 10, 100 and 500 p.p.m. BNF showed that low dietary BNF (10 p.p.m.) neither induced microsomal CYP1A-mediated AFM1 formation nor altered AFB1-8,9-epoxide formation compared to the control. By comparison, 100 and 500 p.p.m. BNF pretreatment significantly elevated microsome-catalyzed AFM1 formation in vitro (P < 0.001), and this increase was highly correlated with increased EROD activity (r2 = 0.999, P < 0.001). Upon in vitro addition, BNF was found to be a potent inhibitor of microsome-mediated AFB1-8,9-exo-epoxide formation (IC50 = 2.6 +/- 0.1 microM) and AFB1-DNA binding (inhibition constant Ki = 3.03 +/- 0.25 microM). These findings indicate that CYP1A enzyme induction can contribute modestly to BNF protection against AFB1 in this species both in vivo and in vitro at higher BNF doses, but does not do so at lower doses. Instead, enzyme inhibition by BNF against AFB1 8,9-epoxidation appears to be the predominant protective mechanism at higher BNF doses, and the sole protective mechanism at low doses, in the rainbow trout. These findings demonstrate that mechanisms of chemoprevention can change with anticarcinogen dose, and caution that even potent induction of phase I or phase II activities does not assure that pathway to be a predominant protective mechanism in vivo.

Dehydroepiandrosterone is a complete hepatocarcinogen and potent tumor promoter in the absence of peroxisome proliferation in rainbow trout

Dehydroepiandrosterone (DHEA), fed for 30 weeks to rainbow trout after initiation with the hepatocarcinogen aflatoxin B1 (AFB1), produced a dose-dependent enhancement of carcinogenesis as measured by increased tumor incidence, multiplicity and size. Significant enhancement was observed at 222 p.p.m., which corresponds to a daily dosage one-half that previously administered to humans in clinical trials. DHEA was also capable of acting as a complete carcinogen in this model, producing liver tumors at doses as low as 222-444 p.p.m. Tumors isolated from trout treated with DHEA alone contained mutations in Ki-ras, primarily codon 12[1] G-->A transitions, providing the first suggestive evidence that DHEA could be a genotoxic carcinogen. The carcinogenicity of DHEA in trout is independent of peroxisome proliferation, as measurements of peroxisomal beta-oxidation and catalase activity support previous observations that trout, like humans, are weak responders to peroxisome proliferators.

Protection by chlorophyllin and indole-3-carbinol against 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced DNA adducts and colonic aberrant crypts in the F344 rat

The most abundant heterocyclic amine in fried ground beef, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), induces colon carcinomas in the male F344 rat. The potential chemopreventive effects of two compounds, namely, the 'interceptor molecule' chlorophyllin (CHL) and a modulator of carcinogen activation, indole-3-carbinol (I3C), were examined in a PhIP colon carcinogenesis model. During weeks 3 and 4 of a 16-week study, F344 rats were given PhIP by oral gavage (50 mg/kg body weight, alternating days). Inhibitors were given either before and during PhIP exposure, after PhIP treatment, or continuously for 16 weeks. Treatment of rats with 0.1% CHL in the drinking water inhibited the formation of aberrant crypt foci (ACF) with > or = 4 crypts/focus, from 1.4 +/- 0.9 in controls to 0.7 +/- 0.3 following post-initiation CHL treatment, and to 0.3 +/- 0.5 in rats given CHL continuously for 16 weeks (mean +/- SD; P < 0.05). Potent inhibition of PhIP-induced ACF occurred following initiation, post-initiation and continuous exposure to 0.1% I3C in the diet. Using the initiation protocol, I3C completely inhibited the induction of the ACF with > or = 4 crypts/focus. In a separate experiment, rats were given 0.1% CHL in the drinking water or 0.1% I3C in the diet for 4 weeks. At the end of week 3, animals received 50 mg PhIP/kg body weight by single oral gavage and PhIP-DNA adducts were quantified in the colon and several other tissues by 32P-postlabeling analysis. In addition, the urine and feces were collected to study the effects of inhibitor treatment on PhIP metabolism and excretion. No significant protection against PhIP-DNA adduct formation was detected in the colon after CHL dosing, nor was a consistent pattern of CHL inhibition observed in several other tissues. In contrast, I3C shifted the time-course of adducts in all tissue; compared with controls, adducts were increased by I3C at 6 h but decreased at 24 h and 7 days following PhIP treatment. Analysis of urine metabolites revealed that I3C and CHL decreased the excretion of unmetabolized PhIP and 4'-hydroxy- << PhIP but increased the phase II detoxification products PhIP-4'-O-glucuronide and PhIP-4'- sulfate. In the feces, the elimination of unmetabolized PhIP was increased from 54.5% in controls to approximately 67% in CHL-treated rats and decreased to 28% in rats given I3C (P < 0.05). These results support a protective role for CHL and I3C against PhIP-induced colon carcinogenesis through mechanisms which alter the uptake or metabolism of the carcinogen, and by suppression in the post-initiation phase.

Mechanisms of indole-3-carbinol (I3C) anticarcinogenesis: inhibition of aflatoxin B1-DNA adduction and mutagenesis by I3C acid condensation products

Possible inhibitory mechanisms of indole-3-carbinol (I3C) against aflatoxin B1 (AFB1), a potent hepatocarcinogen, were examined in rainbow trout. In the Salmonella assay using a trout post-mitochondrial activation system, I3C itself was not an antimutagen against AFB1. The study also evaluated: the antimutagenic ability of I3C oligomers; an acid reaction mixture (RXM) of I3C, generated at low pH to simulate I3C products formed under acidic conditions of the stomach; 3,3-diindolylmethane (I33'), the major derivative of I3C found in trout liver; and 5,6,11,12,17,18- hexahydrocyclononal [1,2-b:4,5-b':7,8-b"]triindole , the cyclic trimer of I3C (CT), a derivative of I3C in liver and one of the major components of RXM. Concentrations of 3.5 microM and greater of I33', CT or RXM showed about 80% inhibition compared with the control. Higher concentrations (70 microM) of the various I3C oligomers also inhibited (to a maximum of 55%) mutagenesis of synthetic AFB1-8,9-epoxide added to the Salmonella assay, in the absence of activating enzymes. I33' inhibited total microsome catalysed AFB1-DNA binding in vitro in an apparently non-competitive manner (Kis = 27.6 +/- 9.4 microM, Kii = 37.5 +/- 32.2 microM). These results suggest that the anticarcinogenic effect of I3C against AFB1 in rainbow trout, and perhaps other species, is due in part to inhibition of AFB1 bioactivation enzymes and to scavenging of the activated AFB1-8,9-epoxide.

Induction of hepatic CYP1A by indole-3-carbinol in protection against aflatoxin B1 hepatocarcinogenesis in rainbow trout

This study examined the significance of hepatic cytochrome P4501A (CYP1A) induction in the inhibition of aflatoxin B1 (AFB1)-DNA adduction by indole-3-carbinol (I3C) in rainbow trout. I3C, fed prior to [3H]AFB1 exposure, provided dose-dependent inhibition of hepatic AFB1-DNA binding, which appeared to vary inversely with hepatic CYP1A-mediated ethoxyresorufin O-deethylase (EROD) activity (r = -0.81, P = 0.051). However, 1000 ppm dietary 13C inhibited AFB1-DNA adduction without detectably inducing CYP1A protein or EROD activity. Dietary I3C was found to inhibit AFB1-DNA adduction by approximately 50%, whether [3H]AFB1 was injected ip 1, 2, 3, 5 or 7 days after the onset of I3C feeding, yet hepatic EROD activity was only transiently induced over this period and was not correlated with AFB1-DNA inhibition. Microsome-catalysed AFB1-DNA binding in vitro did correlate inversely with EROD activity in microsomes from control- and I3C-treated trout (r = -0.955, P = 0.01), but data obtained using microsomes from beta-naphthoflavone-treated trout suggest that this observation may not be indicative of a cause-and-effect relationship. I3C-mediated reduction in covalent binding was not due to I3C derivatives in the microsomal preparation or to reduced CYP protein levels, but may reflect a lower microsomal catalytic capacity for AFB1 epoxidation as a result of enzyme inactivation. In addition, the major I3C derivative found in liver, 3,3'-diindolylmethane, has been shown to be a non-competitive inhibitor of EROD, and of enzymes that catalyse AFB1 epoxidation. These findings indicate little, if any, role for CYP1A induction in the inhibition of AFB1 carcinogenicity in rainbow trout by levels of I3C likely to be encountered in cruciferous vegetables.

Mechanisms of tumor modulation by indole-3-carbinol. Disposition and excretion in male Fischer 344 rats

This study describes the disposition and excretion of indole-3-carbinol (I3C), a natural dietary tumor modulator and candidate chemopreventive agent, in male Fisher 344 rats after continuous dietary or a single oral administration. Steady-state urinary and fecal excretion were attained 40 and 112 hr, respectively, after commencing continuous exposure. These two routes accounted for approximately 75% of the administered dose, of which 77% appeared in feces. After 7 days of 2,000 ppm dietary I3C, a mean of 1,154 microM I3C eq was found in liver, of which 17% was present as extractable, unbound I3C derivatives. Total equivalents in liver decreased to 643 and 411 microM 24 and 48 hr later, respectively, for animals returned to control diet. Mean levels of I3C eq in lung decreased from 436 to 219 microM, and blood levels decreased from 320 to 208 microM over the same 48-hr period. After administration of 1 mmol/kg radioinert I3C (a comparable daily dose as in the feeding study) for 6 days, animals were given 1 mmol/kg [3H]I3C. Mean liver levels were 257, 283, and 541 microM I3C eq at 1.5, 3, and 6 hr after dosing, and these levels represented 0.97%, 1.34%, and 2.45% of the total I3C dose administered, respectively. Concentrations of I3C eq changed little in blood, kidney, tongue, or lung over this time period. HPLC analysis of ethyl acetate extracts of liver from rats given an oral dose revealed 24 distinct [3H]I3C-derived peaks. Two of the predominant peaks were identified as 3,3'-diindolylmethane (I33', a linear dimer of I3C) and [2-(indol-3-ylmethyl)-indol-3-yl]indol-3-ylmethane (LT, a linear trimer). A novel I3C metabolite was identified as 1-(3-hydroxymethyl)-indolyl-3-indolylmethane (HI-IM). Hepatic levels of these metabolites and three major, but unidentified, products were between 1.0 and 13.1 microM; highest levels were observed at 6 hr or, for HI-IM, at 1.5 hr postdosing. Parent I3C was not detected in liver extracts, whereas the potent Ah receptor agonist 3,2-b-indolocarbazole (ICZ) was estimated at 1.6 nM. These data suggest that neither I33', LT, or ICZ alone reach sufficient hepatic concentration to account for cytochrome P450IA induction by dietary I3C, or provide effective inhibition of microsomal bioactivation of the hepatocarcinogen aflatoxin B1; however, the total hepatic mixture of I3C derivatives may be sufficient to provide both modulatory responses in the rat.

The anticarcinogen 3,3'-diindolylmethane is an inhibitor of cytochrome P-450

Dietary indole-3-carbinol inhibits carcinogenesis in rodents and trout. Several mechanisms of inhibition may exist. We reported previously that 3,3'-diindolylmethane, an in vivo derivative of indole-3-carbinol, is a potent noncompetitive inhibitor of trout cytochrome P450 (CYP) 1A-dependent ethoxyresorufin O-deethylase with Ki values in the low micromolar range. We now report a similar potent inhibition by 3,3'-diindolylmethane of rat and human CYP1A1, human CYP1A2, and rat CYP2B1 using various CYP-specific or preferential activity assays. 3,3'-Diindolylmethane also inhibited in vitro CYP-mediated metabolism of the ubiquitous food contaminant and potent hepatocarcinogen, aflatoxin B1. There was no inhibition of cytochrome c reductase. In addition, we found 3,3'-diindolylmethane to be a substrate for rat hepatic microsomal monooxygenase(s) and tentatively identified a monohydroxylated metabolite. These observations indicate that 3,3'-diindolylmethane can inhibit the catalytic activities of a range of CYP isoforms from lower and higher vertebrates in vitro. This broadly based inhibition of CYP-mediated activation of procarcinogens may be an indole-3-carbinol anticarcinogenic mechanism applicable to all species, including humans.

Carcinogenicity of dietary dimethylnitrosomorpholine, N-methyl-N'-nitro-N-nitrosoguanidine, and dibromoethane in rainbow trout

Eighteen-mo feeding trials of rainbow trout were used to test the carcinogenicity of 5 chemicals in this species. A single exposure level was used for each substance. The doses and chemicals tested were 1,556 ppm 2,6-dimethylnitrosomorpholine (DMNM), 500 ppm N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 2,000 ppm 1,2-dibromoethane (DBE), 2,000 ppm 1,1-dichloroethylene (DCE), and 200 ppm cyclophosphamide (CP). Liver and/or glandular stomach neoplasms were produced by DMNM (liver and stomach), MNNG (stomach), and DBE (chiefly, stomach tumors). In addition, DMNM produced a low incidence of swimbladder papillomas and caused testicular atrophy in 50% of treated males. DCE and CP produced no neoplasms at the exposure levels used. No evidence of other chronic toxicity was seen for any of the 5 compounds.

Inhibition of in vitro aflatoxin B1-DNA binding in rainbow trout by CYP1A inhibitors: alpha-naphthoflavone, beta-naphthoflavone and trout CYP1A1 peptide antibody

Rainbow trout cytochrome P450 (CYP)1A detoxifies aflatoxin B1 (AFB1) to aflatoxin M1 (AFM1), whereas CYP2K1 activates AFB1 to AFB1-8,9-epoxide. We report that alpha-naphthoflavone (ANF) and beta-naphthoflavone (BNF) both strongly inhibit CYP1A-mediated ethoxyresorufin O-deethylase (EROD) activity (Ki = 9.1 +/- 0.8 and 7.6 +/- 1.1 nM, respectively). These inhibitors (selective for mammalian CYP1A at low concentrations), as well as rabbit polyclonal antibody to a trout CYP1A1 peptide (residues 277-294), also strongly inhibited trout microsome-catalyzed AFB1-DNA binding and lauric acid (omega-1) hydroxylation in vitro, reactions previously established to be CYP2K1-dependent. ANF at 0.5, 5, 50 and 500 microM inhibited liver microsome-catalyzed AFB1-DNA binding by 22, 58, 84 and 91%, respectively, whereas BNF at the same concentrations inhibited 22, 74, 78 and 81%, respectively. The CYP1A1 peptide and CYP2K1 polyclonal antibodies (10 mg IgG/mg microsomal protein) inhibited AFB1-DNA binding by 84 and 66%, respectively, compared with pre-immune IgG. Lauric acid (omega-1) hydroxylation was inhibited 61% by 5 microM ANF, 69% by 5 microM BNF and 100% by either antibody at 12 mg IgG/mg microsomal protein. These results demonstrate that mammalian CYP1A inhibitors also inhibit trout microsomal AFB1-DNA binding and lauric acid (omega-1) hydroxylation, catalyzed primarily by CYP2K1. In the absence of evidence that trout CYP1A can catalyze AFB1-DNA binding, the results suggest configuration similarities at, or near, the active sites for these two fish enzymes that result in antibody crossreaction and loss of the inhibitor specificity observed with mammalian CYP1A.

Regulation of hepatic cytochrome P4501A by indole-3-carbinol: transient induction with continuous feeding in rainbow trout

This study investigated the kinetics of hepatic cytochrome P-4501A (CYP1A) induction in rainbow trout by indole-3-carbinol (I3C), a natural tumour modulator from cruciferous vegetables, and its low pH reaction products 3,3'-diindolylmethane (I33'), 5,6,11,12,17,18-hexahydrocyclononal[1,2-b:4,5-b':7,8-b"]triindo le cyclic trimer (CT), and the unresolved I3C acid reaction mixture (RXM). RXM, CT and I33' were potent inducers of total embryonic CYP1A following direct microinjection, and of fingerling hepatic CYP1A following ip exposure, whereas I3C itself produced only a transient and relatively weak induction. It is also reported for the first time that dietary I3C induced hepatic CYP1A and its associated ethoxyresorufin O-deethylase (EROD) activity in trout but, again, the induction was weak and transient even with continuous I3C feeding. Mechanism studies and mixed exposures with the Ah agonist beta-naphthoflavone indicated that transient induction by I3C was not due to diet ageing, but appears to involve inactivation of the Ah inductive pathway and irreversible inactivation of CYP1A-mediated EROD activity by I3C-derived metabolites. Thus, I3C derivatives exhibit dual capacities for CYP1A induction and inhibition in trout.

Quantitative carcinogenesis and dosimetry in rainbow trout for aflatoxin B1 and aflatoxicol, two aflatoxins that form the same DNA adduct

Two exposure protocols were used to establish complete dose-response relationships for the hepatic carcinogenicity and DNA adduction in vivo of aflatoxin B1 (AFB1) and aflatoxicol (AFL) in rainbow trout. By passive egg exposure, AFL was taken up less well than AFB1, but was more efficiently sequestered into the embryo itself, to produce an embryonic DNA binding curve that was linear with carcinogen dose and with a DNA binding index three-fold greater than AFB1. Both aflatoxins produced the same phenotypic response, predominantly mixed hepatocellular/cholangiocellular carcinoma. Tumor responses as logit [incidence] vs. In [dose] were parallel-offset, non-linear responses showing a three-fold greater carcinogenic potency for AFL at all doses examined (i.e. 3 times more AFB1 than AFL required to produce an equivalent liver tumor incidence). By molecular dosimetry analysis (logit [incidence] vs. In [DNA adducts]), the two data sets were coincident, indicating that, per DNA adduct formed in vivo in total embryonic DNA, these two aflatoxins were equally efficient in tumor initiation. By dietary fry exposure, both carcinogens produced linear DNA binding dose responses in liver, but with an AFL target organ DNA binding index only 1.14 times that of AFB1 by this exposure route. The tumor dose-response curves also did not exhibit the three-fold difference shown by embryo exposure, but were closely positioned non-linear curves. Since the DNA binding indices differed by only 14%, the resulting molecular dosimetry curves for AFL and AFB1 by dietary exposure were similar to the tumor response curves. These results indicate that differing exposure routes produced differing relative carcinogenicity estimates based on doses applied, as a result of protocol-dependent differences in AFL and AFB1 pharmacokinetic behaviors, but that potency comparisons based on molecular dose received were similar for the two protocols. By comparison with standard DNA adducts produced in vitro using the dimethyloxirane-produced 8,9-epoxides of AFB1 and AFL, we conclude that > 99% of AFL-DNA adducts produced in vivo were identical to those produced by AFB1. Thus similar molecular dosimetry responses should be expected under all exposure protocols in which the two parent carcinogens do not exhibit differing toxicities to the target organ.

Effect of ammoniation of aflatoxin B1-contaminated cottonseed feedstock on the aflatoxin M1 content of cows' milk and hepatocarcinogenicity in the trout bioassay

The effectiveness of ammonia in inactivating aflatoxins in contaminated cottonseed was investigated. Two aflatoxin-contaminated cottonseed lots were treated separately using an atmospheric pressure, ambient temperature ammoniation procedure (APAT) or a high pressure, high temperature ammoniation procedure (HPHT), and incorporated into dairy cow rations. Isocalorific diets containing 25% defatted, dried milk from cows fed aflatoxin-contaminated cottonseed without or with APAT or HPHT treatment, or an aflatoxin-free human grade commercial milk powder, were then fed for 12 months to rainbow trout (Oncorhynchus mykiss). Aflatoxin M1 (AFM1) concentrations in milk powders without and with seed treatment were: APAT, 85 and < 0.05 microgram/kg; HPHT, 32 and < 0.05 microgram/kg. In the APAT experiment, trout consuming the diet containing milk from cows fed the aflatoxin-contaminated cottonseed had a 42% incidence of hepatic tumours; APAT cottonseed treatment reduced this to 2.5%. Positive controls were included to demonstrate trout responsiveness. AFB1 fed continuously for 12 months at 4 micrograms/kg resulted in a 34% tumour incidence, whereas positive controls fed 20 micrograms AFB1/kg, 80 micrograms AFM1/kg, or 800 micrograms AFM1/kg for 2 wk and killed 9 months later had a 37, 5.7 and 50% incidence of tumours, respectively. These data demonstrate that APAT ammonia treatment of aflatoxin-contaminated dairy cattle cottonseed feedstock abolished the detectable transfer of AFM1 or AFB1 into milk powder, and greatly reduced the carcinogenic risk posed by any carry-over of aflatoxins or their derivatives into milk. In addition, the results confirm AFM1 to be a lower level hepatocarcinogen in comparison with AFB1 in the trout carcinogenicity assay. In the separate HPHT experiment, no tumours were observed in the livers of trout fed diets containing milk from either the ammonia-treated or untreated source, or the control diet containing 8 micrograms AFM1/kg. Positive controls fed 64 micrograms AFB1/kg for 2 wk exhibited a 29% tumour incidence 12 months later. Thus in this experiment, neither AFM1 at 8 micrograms/kg nor any HPHT-derived aflatoxin derivatives that might have been carried over into milk, represented a detectably carcinogenic hazard to trout.

Purification and some properties of a hydrophilic prokallikrein and a hydrophobic prokallikrein from sheep kidney cortex

1. A hydrophilic and a hydrophobic prokallikrein were purified from sheep kidney cortex and some of their biochemical properties were compared. 2. The effects of several inhibitors on the activity of the hydrophilic and hydrophobic kallikreins were investigated. 3. The thermal stability and response of the hydrophobic kallikrein towards various detergents were also studied.

Dose-dependent carcinogenicity and frequent Ki-ras proto-oncogene activation by dietary N-nitrosodiethylamine in rainbow trout

While the experimental data upon which current concepts in mechanistically based risk assessment and molecular epidemiology are grounded derive almost entirely from rodent models, fish models have several attributes (e.g., low background incidence, extremely low cost tumor studies, nonmammalian comparative status for extrapolation of mechanisms to humans) that make them valuable adjuncts for addressing these concepts. This report provides an initial characterization of the dose dependency of dietary N-nitrosodiethylamine (DEN) hepatocarcinogenicity in Shasta strain rainbow trout (Oncorhynchus mykiss) and the potential of DEN to elicit ras proto-oncogene activation in this species. Carcinogen was administered in the diet at five concentrations for 12 months. Necropsies were performed at 9, 12, and 18 months, the latter on fish maintained on control diet for 6 months after cessation of DEN exposure. The incidence of hepatic neoplasms at the lower dietary concentrations (< or = 70 ppm) did not consistently exceed that for control groups, which were higher in this particular study (2%) than expected (historically 0.1%). For the higher DEN concentrations, a linear relationship between the hepatic tumor incidence (expressed as log odds, log [p/(1-p)], where p = proportion of fish bearing tumors), and the logarithm of total cumulative dose was observed, with response being independent of the length of time (9 or 12 months) during which the dose was accumulated. The dose-response curve for fish maintained an additional 6 months postexposure was shifted toward higher incidence but was parallel to the curve for fish killed at cessation of exposure. The model predicts that doubling the dose will produce somewhat more than a doubling of the odds (p/(100-p)) for tumor incidence and that the odds for lesions 6 months postexposure will be approximately double those at cessation of exposure. Comparison of these results with previous studies using rats suggests an overall similarity in dose-response curves, with trout being somewhat less sensitive than rats to DEN hepatocarcinogenesis. To examine the molecular basis for DEN carcinogenesis in this species, seven liver tumors induced separately by short-term DEN treatment were probed by 3'-mismatch primer polymerase chain reaction analysis for evidence of Ki-ras proto-oncogene activating point mutations. A very high proportion (6/7) of tumors was found to carry codon 12 GGA-->AGA mutations, whereas no codon 61 mutants were detected in this sample.(ABSTRACT TRUNCATED AT 400 WORDS)

Anticarcinogenic activity of indole-3-carbinol acid products: ultrasensitive bioassay by trout embryo microinjection

The relative contribution of indole-3-carbinol (I3C) and its acid condensation products to the anticarcinogenic activity of this crucifer phytochemical has been studied using trout embryo microinjection. I3C was treated with 0.07 N HCl to give a reaction mixture (RXM) comprising < 0.5% parent compound and over 20 products, the most prevalent being the dimer 3,3'-diindolylmethane (I33') and a related cyclic trimer (CT). RXM, I33' or CT was injected into embryos with [3H]aflatoxin B1 (AFB1) and total embryonic DNA was isolated 1, 3, or 10 days postinjection. Compared with controls given AFB1 alone, I3C failed to inhibit carcinogen-DNA binding at any time point. In contrast I33', CT, and RXM inhibited AFB1-DNA binding by an average of 37, 51, and 65%, respectively. Coinjection of AFB1 and 350 microM I3C, RXM, or I33' into trout embryos reduced AFB1-induced hepatocarcinogenesis after 1 year from 43.4% in positive controls to 36.0, 12.2 (P < 0.05), and 24.6% (P < 0.05), respectively. No tumor data were obtained in the AFB1 plus CT group due to poor survival of the embryos posthatching. These results indicate that acid condensation products, not the parent compound, represent the anticarcinogenic species in trout and that their formation in the stomach is a likely prerequisite for I3C anticarcinogenesis.

A survey of kininase, tyrosine esterase, kininogenase and arginine esterase activities in some snake venoms

Kininase (kinin-degrading), tyrosine esterase, kininogenase (kinin-releasing) and arginine esterase activities of various crotalid, viperid and elapid venoms were measured. Wide ranges of those enzymatic activities were recorded for the crotalid and viperid venoms but no activities were detected in the Naja venoms.

Indole-3-carbinol induces a rat liver glutathione transferase subunit (Yc2) with high activity toward aflatoxin B1 exo-epoxide. Association with reduced levels of hepatic aflatoxin-DNA adducts in vivo

Aflatoxin B1 (AFB1), a metabolite of the grain mold Aspergillus flavus, is a potent hepatocarcinogen and widespread contaminant of human food supplies. AFB1-induced tumors or preneoplastic lesions in experimental animals can be inhibited by cotreatment with several compounds, including indole-3-carbinol (I3C), a component of cruciferous vegetables, and the well-known Ah receptor agonist beta-naphthoflavone (BNF). This study examines the influence of these two agents on the AFB1-glutathione detoxication pathway and AFB1-DNA adduction in rat liver. After 7 days of feeding approximately equally inhibitory doses of I3C (0.2%) or BNF (0.04%) alone or in combination, male Fischer 344 rats were administered [3H]AFB1 (0.5 mg/kg, 480 microCi/kg) intraperitoneally and killed 2 hr later. All three experimental diets inhibited in vivo AFB1-DNA adduction (BNF, 46%; I3C, 68%; combined, 51%). Based on Western blots using antibodies specific for the glutathione S-transferase (GST), subunit Yc2 (subunit 10) appeared to be substantially elevated by the diets containing I3C (I3C diet, 4.0-fold increase in band density; combined diet, 2.8-fold). The BNF diet appeared to elevate Yc2 to a lesser extent (2.2-fold increase in band density).(ABSTRACT TRUNCATED AT 250 WORDS)

Indole-3-carbinol and beta-naphthoflavone induction of aflatoxin B1 metabolism and cytochromes P-450 associated with bioactivation and detoxication of aflatoxin B1 in the rat

Aflatoxin B1 (AFB1) is a highly hepatotoxic and hepatocarcinogenic secondary metabolite of the grain mold Aspergillus flavus and related fungi. Indole-3-carbinol (I3C), found in cruciferous vegetables, can both inhibit and promote AFB1-induced carcinogenesis. We have examined the influence of dietary treatment with I3C and the well-known Ah receptor agonist beta-naphthoflavone (BNF) on the relative levels of different cytochrome P-450 (CYP) isoforms known to metabolize AFB1 in male Fischer 344 rats. After 7 days of feeding 0.2% I3C or 0.04% BNF, alone or in combination, the relative levels of hepatic CYP1A1, 1A2, 2B1/2, 2C11, and 3A were assessed by laser densitometry of Western blots. Both diets containing I3C markedly increased band densities of CYP1A1 (up to 24-fold), 1A2 (3.1-fold), and 3A1/2 (3.8-fold), and had lesser effects on the levels of 2B1/2 (1.8-fold) and no effect on CYP2C11. BNF also strongly increased band densities of CYP1A1 (12-fold) and 1A2 (2.7-fold), but had no effect on the levels of CYP2B1/2 or 3A1/2 band densities, and repressed those of CYP2C11 (2-fold). In addition, we examined the in vitro hepatic microsomal metabolism of AFB1 at 16, 124, and 512 microM substrate levels. Diets containing I3C elevated initial rates of AFM1 (a detoxication product) production 18.6- to 19.2-fold over control at 16 microM AFB1, which declined to 7.8- to 9.5-fold at 512 microM AFB1. The BNF-only diet gave similar, but less dramatic effects (5.9-fold at 16 microM AFB1, 3.5-fold at 512 microM AFB1).(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term, high-dose dietary exposure of rainbow trout to butylated hydroxyanisole is non-carcinogenic

The hepatocarcinogenic and/or promotional properties of butylated hydroxyanisole (BHA) were tested in rainbow trout. Four groups of 100, 21-day-old trout embryos were exposed to 0.5 ppm aqueous aflatoxin B1 (AFB1) for 30 min, and four similar groups were sham treated. After hatching, swim-up, and the onset of vigorous feeding behavior (2 weeks pest swim-up), duplicate groups of 60 AFB1-treated and sham-treated fry were started on a test diet containing 0.6% (6000 ppm) BHA, and the other duplicate groups of treated and untreated fry were fed the control Oregon Test Diet (OTD). After 8 months of feeding BHA or OTD, the fish were necropsied for tumor detection, with particular attention given to the stomach and liver. No tumors were seen in the livers or stomachs of the sham-treated fish fed OTD or BHA, showing that BHA is not carcinogenic to rainbow trout under the conditions of this experiment. Promotional results were equivocal, with one tank of fish having a higher hepatic tumor incidence, but the other the same as the positive AFB1 control. When the tanks were combined, however, there was no statistical difference between the two groups.