Toxicokinetics and effects of PCBs in Arctic fish: a review of studies on Arctic charr

In a series of environmentally realistic laboratory experiments, toxicokinetics and effects of polychlorinated biphenyls (PCBs) were studied in the Arctic charr (Salvelinus alpinus). Winter fasting and emaciation, which are common among Arctic charr living in high latitudes, resulted in a redistribution of the lipophilic PCBs from lipid-storing tissue such as the muscle, to vital organs that must be considered sensitive toward PCB (liver and brain). This redistribution was accompanied by a significant potentiation of the hepatic cytochrome P-450 (CYP) 1A biomarker response, from low activities in October (within those measured in uncontaminated charr) to a high, probably maximum, induction in May. Performance studies demonstrated a clear effect of environmentally realistic PCB levels on endocrine mechanisms, immune function, and seawater preadaptation (smoltification) in charr that had been feed deprived for several months after contamination with Aroclor 1254, whereas a high PCB dose exerted only minor, if any, effects in charr that had been fed after contamination. These results demonstrate that emaciation results in decreased dose-response relationships in fish, and indicate that arctic animals undergoing seasonal cycles of "fattening" and emaciation may be extra sensitive toward persistent, lipophilic organochlorines. Pilot studies on Arctic charr from Bj?rn?ya Island revealed marked CYP1A biomarker responses and an upregulation of genes involved in cellular homeostatic mechanisms in charr from Lake Ellasj?en (high PCB levels).     

Cytochromes P-4501A, P-4503A and P-4502B in liver and heart of Mugil capito treated with CYP1A inducers

Hepatic microsomes of Aroclor 1254-treated Mugil capito showed a single protein band detected in immunoblot with monoclonal antibody 1-12-3 to teleost (scup) CYP1A. The hepatic CYP1A like protein was induced with dose dependency after exposure of the fish to β-naphthoflavone (BNF) as well as to Aroclor 1254. The induced mullet hepatic CYP1A protein was confined to a distinct fraction obtained by DE-52 anion exchange chromatography, and its relative content in that fraction increased in fish that were treated with higher doses of inducer. EROD (7-ethoxyresorufin O-deethylase) activity in hepatic microsomes from mullet treated with various doses of BNF correlated significantly (r(2)=0.81502, P<0.01) with CYP1A content. Treatment of the mullet with low dose of Aroclor 1254 (25 mg/kg) induced only traces of CYP1A in liver microsomes (5.1±4.8 mg/kg). However, in mullet treated with the high dose of Aroclor 1254 (100 mg/kg) there was a dramatic induction in CYP1A content (408±275 pmol/mg) and this hemoprotein comprised about 83% of the total P-450 content of liver microsomes. The total level of P-450, although induced in the liver tissue, was not induced in heart tissue of Aroclor 1254 treated mullet. On the other hand, P-4501A was induced in treated mullet to a level that comprised almost all of the cardiac P-450 content. EROD activity in the heart tissue of induced mullet was characterized by low V(max) and high K(m) values (K(m)=2.35 mM, V(max)=39.5 pmol/min per mg) compared to the values recorded for the enzyme from the liver (K(m)=1.0 mM, V(max)=288.0 pmol/min per mg). Cardiac CYP1A with low catalytic activity and repression of CYP-types other then CYP1A in heart of CYP1A induced fish may be part of a mechanism aimed to preserve crucial levels of electron donors and molecular oxygen in cardiac muscle of fish exposed to CYP1A inducers.     

Induction of hepatic mixed function oxidase activity in trout (Salvelinus fontinalis) by aroclor 1254 and some aromatic hydrocarbon PCB replacements

Groups of brook trout (Salvelinus fontinalis) were fed a polychlorinated biphenyl (PCB) mixture (Aroclor 1254) or three PCB replacements based on phenylxylylethane or diisopropylnaphthalene mixtures at dose levels of 30 and 100 μg·g^?1. Both dose levels of all the materials tested induced hepatic ethoxyresorufin O-deethylase activity; ethoxycoumarin O-deethylase activity was induced by Aroclor 1254 and one of the phenylxylylethane mixtures. Benzphetamine N-demethylase activity was not induced by any treatment. Effects on other indices of MFO induction, such as protein content and cytochrome P-450 content, were variable. The fish accumulated from 13 to 46% of the Aroclor 1254 dose but only 2 to 7% of the PCB replacements. All components of the phenylxylylethane mixtures were accumulated but some major components of the diisopropylnaphthalenes were not.     

Inhibitory effect of Aroclor 1254 on aflatoxin-initiated carcinogenesis in rainbow trout and mutagenesis using a Salmonella/trout hepatic activation system

Duplicate lots of 140 rainbow trout (Salmo gairdneri) were fed either control diet (CD) or 100 ppm Aroclor 1254 (a polychlorinated biphenyl—PCB) for 3 mth followed by initiation of liver carcinomas with 20 ppb dietary aflatoxin B₁ (AFB₁) for 2 wk. At 8 and 12 mth after AFB₁ treatment, fish were sampled and tumor incidence determined. Trout that were prefed PCB showed a 45% inhibition in tumor incidence at 12 mth, when compared to those prefed CD. Throughout the experiment fish were sampled to determine the time relationship of PCB bioaccumulation. A rapid uptake of PCB into total body fat was seen, with concentration of 594 ppm at the time of AFB₁ exposure. Liver benzo[a]pyrehe monooxygenase (B[a]PM) activity was also induced at the time AFB₁ exposure began. The Ames mutagenesis assay was used to determine the effects of in vitro Aroclor 1254 on AFB₁-induced mutagenesis using a trout liver preparation. A PCB dose-related inhibition was observed with a 67% inhibition at the highest dose tested (500 μg Aroclor 1254/plate). Proposed mechanisms of the inhibition of AFB₁-induced carcinogenesis/mutagenesis are discussed.     

The effect of polychlorinated biphenyls on liver gluconeogenic enzyme activities in rats during exposure to cold

Rats were fed diets containing either 0, 75, or 400 ppm Aroclor 1254. after 2 weeks on the diets, 4 animals per group were placed in the cold at 4° for an additional 2 weeks. Rats maintained at 4° gained less weight and had decreased adipose tissue weights as compared with animals at 25°. Ingestion of Aroclor 1254, a polychlorinated biphenyl (PCB), increased liver weights in the cold and at ambient temperature. Blood glucose was also significantly decreased in the cold-exposed rats fed 400 ppm PCB. At both temperatures, PCB decreased the specific activity (μmoles/g liver/min) of phosphoenol-pyruvate carboxykinase (PEPck) and fructose-1,6-diphosphatase (FDPase). Cold exposure significantly incresed the activity of PEPck and glucose-6-phosphatase (G-6-Pase) in rats fed PCB. When the activities were expressed as total μmoles/100 g body weight/min to correct for PCB-induced hepatomegaly, no significant differences were observed in the key gluconeogenic enzymes for PCB-fed rats at 25°. At 4°, FDPase and G-6-Pase activities were significantly increased with PCB feeding. Malic enzyme was significantly increased with PCB at both temperatures, regardless of the method of calculation. PCB ingestion had no effect on mitochondrial pyruvate metabolism. These studies indicate that ingestion of PCB (as Aroclor 1254) does not alter the response of the gluconeogenic enzymes to cold exposure.     

Interactive effects of hypoxia and PCB co‐exposure on expression of CYP1A and its potential regulators in Atlantic croaker liver

Although marine and coastal environments which are contaminated with xenobiotic organic compounds often become hypoxic during the summer, the interactive effects of hypoxia and xenobiotic exposure on marine species such as teleost fishes remain poorly understood. The expression and activity of monooxygenase enzyme cytochrome P450‐1A (CYP1A) in fishes are upregulated by exposure to polychlorinated biphenyls (PCBs), whereas they are down‐regulated during hypoxia exposure. We investigated the interactive effects of hypoxia and PCB co‐exposure on hepatic CYP1A expression in Atlantic croaker and on potential regulators of CYP1A. Croaker were exposed to hypoxia (1.7 mg/L dissolved oxygen), 3,3′,4,4′‐tetrachlorobiphenyl (PCB 77, dose: 2 and 8 µg/g body weight), and Aroclor 1254 (a common PCB mixture, dose: 0.5 and 1 µg/g body weight), alone and in combination for 4 weeks. PCB 77 exposure markedly increased hepatic CYP1A mRNA and protein expression, and ethoxyresorufin‐O‐deethylase (EROD, an indicator of CYP1A enzyme) activity and increased endothelial nitric oxide synthase (eNOS) protein expression. PCB 77 treatment also increased interleukin‐1β (IL‐1β, a cytokine) mRNA levels and protein carbonyl (PC, an indicator of reactive oxygen species, ROS) contents. These marked PCB 77‐ and Aroclor 1254‐induced increases in CYP1A mRNA levels and EROD activity were significantly attenuated by co‐exposure to hypoxia, whereas the increases in hepatic eNOS protein and IL‐1β mRNA expression, and PC contents were augmented by hypoxia co‐exposure. The results suggest that biotransformation of organic xenobiotics by CYP1A is reduced in fish during co‐exposure to hypoxia and is accompanied by alterations in eNOS, ROS, and IL‐1β levels.     

Hepatic microsomal desulfuration and dearylation of chlorpyrifos and parathion in fingerling channel catfish: lack of effect from Aroclor 1254

Channel catfish were treated intraperitoneally with 100 mg Aroclor 1254/kg body weight and sacrificed at 96 h to observe the effects of this cytochrome P450 1A (CYP1A) inducer on chlorpyrifos and parathion metabolism. In the initial experiment, hepatic microsomal ethoxyresorufin-O-deethylase (EROD) activity of the Aroclor-treated fish was significantly induced but no effects on desulfuration or dearylation of chlorpyrifos or parathion were evident. In the second experiment, Aroclor 1254 did not alter total hepatic microsomal P450s content, but significantly induced hepatic EROD and CYP1A. There were no evident effects to other hepatic CYP isoforms recognized by anti-trout CYP2K1, CYP2M1 and CYP3A27. These experiments indicate that Aroclor 1254 did not induce the P450s responsible for metabolism of the phosphorothionate insecticides.     

Induction of hepatic cytochromes P450 in dogs exposed to a chronic low dose of polychlorinated biphenyls.

Induction of cytochrome P450 isoforms, specifically CYP1A1, and their catalytic activities are potential biomarkers of environmental contamination by polychlorinated biphenyls (PCBs). In this study, dogs were exposed to 25 ppm or 5 ppm Aroclor 1248 (PCB mixture) daily in their diet for 10 or 20 weeks, respectively. Relative to controls, hepatic microsomes from dogs dosed with PCBs had higher levels of CYP1A1 detected in immunoblots and higher levels of EROD activity, but low levels of induction for CYP2B and PROD activity. Concentrations of 96 PCB congeners in serum and liver were evaluated using capillary chromatography. Results showed that all dogs exposed to PCB mixtures had higher levels of PCB in serum and liver. Dogs preferentially sequestered highly chlorinated PCB congeners in liver relative to serum. With these experiments, we demonstrated that EROD activity was a potentially sensitive marker of PCB exposure at 5 and 25 ppm. Furthermore, CYP1A1 and EROD activity were maximally induced in dogs consuming dietary concentrations only 2.5 times the maximal permissible level for human food (FDA). The value of CYP1A1 induction as a biomarker of PCB exposure was tenuous because neither CYP1A1 levels nor EROD activity correlated with total PCB body burden. However, a small subset of congeners were identified in liver that may strongly influence EROD and PROD induction. Finally, two dogs in the 25 ppm dose group were fasted for 48 h. After 24 h of fasting, several new congeners appeared in the serum and remained in the serum for the remainder of the fast. The fast caused a 293% increase in PCB concentration in serum. This increase has strong implications regarding mobilization of toxic PCBs in wildlife during fasting (e.g., migration, hibernation).     

Induction of hepatic mixed function oxidases in trout by polychlorinated biphenyls and butylated monochlorodiphenyl ethers.

Brook trout (Salvelinus fontinalis) were fed either a polychlorinated biphenyl (PCB) (Aroclor 1254) or a replacement material based on butylated monochlorodiphenyl ethers (formerly Dow Chemical Co. XFS-4169L, now sold as Dielectric Fluid C4) over a 15-day period to yield whole body concentrations (assuming complete retention of dose fed) of 200 μg/g, and the induction of hepatic mixed function oxidase (MFO) enzymes was studied. Concentrations of microsomal protein and cytochrome P-450 were significantly higher in fish fed Aroclor 1254 than in controls, and O-de-ethylase activity was increased fourfold. Trout fed XFS-4169L showed only slight increases over control values in most components of the MFO system; only the cytochrome P-450 concentration was significantly increased and O-deethylase activity was unchanged. About 25% of the Aroclor 1254 fed was deposited in fillets of the trout; there was no obvious selectivity of components. Much less XFS-4169L was deposited; the components most efficiently retained represented only about 8% of the dose fed, and one of its major components, 4-sec-butyl-4′-chlorodiphenyl ether, was scarcely retained at all.     

Aroclor 1254 inhibits tryptophan hydroxylase activity in rat brain

Previous studies have shown that oral exposure of rats to polychlorinated biphenyls (PCBs) results in reduced 5-hydroxytryptamine (5-HT) concentrations in certain brain regions. In the present study, we investigated whether the PCB mixture Aroclor 1254 (0.33 mg/g body weight as a single oral dose) can inhibit the activity of tryptophan hydroxylase (TPH), the rate-limiting enzyme in 5-HT synthesis, and reduce 5-HT concentrations in selected brain areas. In two separate experiments, Aroclor 1254 exposure consistently reduced TPH activity in the brainstem (7.2 and 8.7%), frontal cortex (17.4 and 14.8%), and hypothalamus (10.7 and 9.4%) without altering the rats food intake or growth. Moreover, Aroclor 1254 accumulation in the frontal cortex demonstrated a negative correlation with TPH activity (correlation coefficient –0.82). In addition, 5-HT concentrations decreased in the brainstem and frontal cortex after Aroclor 1254 exposure by 9.1 and 19.7%, respectively. These results suggest that the Aroclor 1254-induced decreases in 5-HT concentrations in certain areas of the rat brain are due to inhibition of TPH activity, similar to our recent observations in Atlantic croaker, and that TPH is one of the targets of PCB neurotoxicity in both fish and mammals.     

Polychlorinated biphenyl dynamics in hudson river striped bass. I. Accumulation in early life history stages

Polychlorinated biphenyl (PCB) burdens in larval striped bass, Morone saxatilis, from the Hudson River estuary and laboratory determinations of PCB accumulation in larval and juvenile striped bass are presented.Hudson River fish contained variable amounts of PCBs, from below detection (0.05 gm/g dry weight) to 7.07 μ/g. More PCBs similar to Aroclor 1254 than Aroclor 1016 were found. Larval and young-of-year fish exposed to [¹⁴C]Aroclor 1254 ([¹⁴C]PCB) in water and sorbed to suspended particles and sediments accumulated PCBs at linear rates during the first 24 h of exposure. Larvae exposed in river water accumulated body burdens related to exposure concentrations. Similar accumulation rates and body burdens in larvae exposed in filtered (0.45 μm pore size) river water showed that accumulation resulted from the uptake of dissolved PCB. PCB accumulation in larvae exposed to [¹⁴C]PCB on suspended particles resulted from uptake of PCB desorbed from particles. PCB burdens in larvae exposed to sediment-sorbed [¹⁴C]PCB resulted from desorption of sediment-bound PCB and maintenance of solubility equilibrium with the water.The linear relationship between the initial PCB concentration in water and accumulated PCB in fish from each exposure suggested that accumulation resulted from equilibrium partitioning between water and fish. There are indications that PCB burdens in river fish reflect the solubility-equilibrium based accumulation determined in these studies.     

Aroclor 1254-induced cytotoxicity in catecholaminergic CATH.a cells related to the inhibition of NO production

The neuronal nitric oxide synthase (nNOS) specific inhibitor, 7-nitroindazole (7-NI), and the nitric oxide (NO) donor (S-nitroso-N-acetylpenicillarnine, SNAP) were used to study the role of NO in polychlorinated biphenyl (PCB: Aroclor 1254)-induced cytotoxicity in the immortalized dopaminergic cell line (CATH.a cells), derived from the central nervous system of mice. Treatment of the dopaminergic cells with various concentrations of Aroclor 1254 (0.5-10 microg/ml), a commercial PCB mixture, showed significant cytotoxicity as evaluated by lactate dehydrogenase (LDH) release and assessment of cell viability, depending on the concentration used. We also observed that Aroclor 1254 treatment reduced the level of nNOS expression. Furthermore, the cytotoxicity of Aroclor 1254 was augmented by 10 microM of 7-NI, which alone did not produce cytotoxicity, while it was protected by treatment with SNAP. Depending on the concentrations of Aroclor 1254 used, intracellular dopamine and dihydroxyphenylacetic acid (DOPAC) concentrations were significantly decreased. Therefore, these results suggest that PCBs have the potential for dopaminergic neurotoxicity, which may be related with the PCBs-mediated alteration of NO production originating from nNOS at least in part.     

Enhanced polychlorinated biphenyl lesions in Moloney leukemia virus-infected mice.

The hepatic toxicity produced by polychlorinated biphenyls (PCB) was enhanced in mice that were inoculated with an oncogenic virus, Moloney leukemia virus (MLV). Whenever there was neoplastic involvement of the spleen by MLV, the hepatic lesions produced by PCB were more pronounced than in those of non-MLV inoculated mice. Mice were exposed to PCB Aroclors, 1254, 1242, and 1221 for six months. Aroclors 1254 and 1242 were hepatotoxic with Aroclor 1254 causing death. Aroclor 1221 did not affect the mice. Liver weights in mice that were fed PCBs for six months and then maintained on a PCB-free diet for an additional three months were comparable with those of non-PCB exposed mice. These results suggest that the PCB-produced hepatic lesions (noncirrhotic) regenerate after removal of PCB from the diet. Polychlorinated biphenyls did not affect (promote or induce) the oncogenesis of MLV in this study.     

Differential in vitro neurotoxicity of the flame retardant PBDE-99 and of the PCB Aroclor 1254 in human astrocytoma cells

Polybrominated diphenyl ethers (PBDEs) are an important class of flame retardants. Because of their presence in maternal milk and their structural similarity to polychlorinated biphenyls (PCBs), concern has been raised on their possible developmental neurotoxicity. Aim of the present study was to investigate the in vitro effects of PBDE-99 (2,2', 4,4', 5-pentabromodiphenyl ether) on astroglial cells (human 132-1N1 astrocytoma cells) and comparing it with those of the PCB mixture Aroclor 1254. Both PBDE-99 and Aroclor 1254 caused a concentration-dependent inhibition of MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) reduction, however, only the latter increased lactate dehydrogenase (LDH) release or cell death, assessed by the trypan blue assay. PBDE-99 caused translocation of the three protein kinase C (PKC) isozymes (alpha, epsilon, zeta) present in 132-1N1 astrocytoma cells, while Aroclor 1254 affected only PKCalpha and epsilon translocation. However, pre-incubation with the PKC inhibitor GF109203X or PKC down-regulation by the phorbol ester PMA, had minimal or no effect on PBDE-99 or Aroclor 1254-induced cytotoxicity. Similarly, the calcium chelator BAPTA-AM, the tyrosine kinase inhibitor genistein, and the MEK (mitogen activated protein kinase kinase) inhibitor PD98059 had no effect on PBDE-99 and Aroclor 1254 cytoxicity. On the other hand, the phosphatidylinositol 3 kinase (PI-3K) inhibitor LY290042 enhanced PBDE-99 toxicity, but did not affect Aroclor 1254. Because of the involvement of PI-3K in apoptotic cell death, the ability of PBDE-99 and Aroclor 1254 to induce apoptosis in astrocytoma cells was investigated. PBDE-99, but not Aroclor 1254, caused apoptotic cell death in astrocytoma cells, assessed by the TUNEL method and by Hoechst 33258 staining, via a p53 dependent mechanism. These results suggest that PBDE-99 and Aroclor 1254 exert differential cytotoxic effects on human astroglial cells.     

Cytochrome P450 mono-oxygenase gene expression and protein activity in cultures of adult cardiomyocytes of the rat

There are a substantial number of drugs acting either directly or indirectly on the heart, but surprisingly, little is known about the metabolic capacity of heart muscle cells. We therefore investigated the gene expression and protein activity of cytochrome P450 isozymes in cultures of adult cardiomyocytes of the rat. Semi-quantitative CYP gene expression pattern suggests CYP1A1 and CYP2B1/2 to be key players in cardiomyocytes and upon treatment with Aroclor 1254 approximate 4 fold inductions could be observed for both gene families, when compared with appropriate controls. The mRNA expression of most genes was sustained for prolonged periods of time, e.g. up to 120 h in culture and in the case of the CYP3A1 gene an approximate 10 fold induction was observed at the higher Aroclor 1254 dose level (10 microM) in 24 h old cultures. The constitutively expressed genes, e.g. CYP2C11 and CYP2E1 are expressed throughout the entire culture period (5 days) and did not respond to Aroclor 1254 treatment. CYP4A1 was mainly expressed in freshly isolated cardiomyocytes of control animals and its expression declined rapidly in culture. There was good agreement between gene expression and translated protein activity using 7-ethoxyresorufin and testosterone as substrates. The data reported herein should foster the routine use of freshly isolated and cultivated cardiomyocytes for drug profiling and toxicity studies.     

Alkoxyresorufin Metabolism in White-Footed Mice at Relevant Environmental Concentrations of Aroclor 1254

Recent investigations have detected polychlorinated biphenyl(PCB) body burdens in wild white-footed mice (Peromyscus Ieucopus)captured at hazardous waste sites. Insufficient informationis currently available to interpret the toxicological significanceof these body burdens. In an effort to provide this information,we investigated hepatic changes and PCB body burdens in whitefootedmice following a 21-day dietary exposure to a PCB mixture, Aroclor1254. Dietary concentrations tested were 0, 2.5, 25, 50, and100 mg Aroclor 1254/kg diet (reported as ppm). Liver weightswere significantly increased at all concentrations except 2.5ppm. Ethoxyresorufin O-dealkylase (EROD) activity, an aryl hydrocarbonhydroxylase-type substrate, was significantly increased at allPCB concentrations, but the dose-response tended to plateauabove 25 ppm. Pentoxyresorufin O-dealkylase (PROD) activity,a putative phenobarbital-type substrate, was significantly increasedin a dose-dependent manner at 25 ppm PCB and above, with noplateau response. Pentobarbital sleep time was significantlydecreased at 25 ppm, but not at 2.5 ppm. Results indicate white-footedmice undergo a mixed-type in duction pattern following exposureto Aroclor 1254, with EROD the most sensitive indicator of PCBexposure. This investigation identified a no observed effectconcentration for liver weights and PROD activity at 2.5 ppmin the diet which is equivalent to a body burden of 2.0 mg Aroclor1254/kg wet wt of mice; the no observed effect concentrationfor EROD is below these levels. These results support the useof EROD, PROD, and liver weight as biomarkers of PCB exposurein field-captured rodents.     

Potentiation of carbon tetrachloride hepatotoxicity in rats by pretreatment with polychlorinated biphenyls.

Pretreatment of male rats with Aroclor 1254 at a dose of 25 mg/kg i.p. for 6 days resulted in potentiation of the hepatotoxicity of inhaled carbon tetrachloride (CCl4) as evidenced by a decrease in liver glucose-6-phosphatase and elevations of serum glutamic oxalacetic transaminase (SGOT), serum glutamic pyruvic transaminase (SGPT), isocitrate dehydrogenase, and sorbitol dehydrogenase. Aroclor 1254 alone did not demonstrate hepatotoxicity. Aroclor 1254 administration resulted in large increases in cytochrome c reductase, cytochrome P-450 (448) AND P-Nitroanisole demethylation. Subsequent exposure to CCl4 vapor resulted in over 70% decreases in the latter two parameters. The potentiation was dose-dependent with a dose of 5 mg/kg or higher being effective. Aroclor 1260 administration gave results similar to those of Aroclor 1254, but Aroclor 1221 enhanced CCl4 toxicity to a lesser extent.     

Metabolism of nicotine and induction of CYP1A forms in precision-cut rat liver and lung slices.

The aim of this study was to investigate xenobiotic metabolism and induction of cytochrome P450 (CYP) forms in precision-cut rat liver and lung slices, employing nicotine as a model compound. Freshly cut rat liver and lung slices metabolised nicotine to the major metabolite cotinine. Observed Km values for cotinine formation in liver and lung slices were 323 and 41.7 microM, respectively, with corresponding V(max) values of 47.2 and 3.21 pmol/min/mg protein, respectively. Rat liver and lung slices were cultured for 48 h with Aroclor 1254, benzo(a)pyrene, nicotine and cotinine. Both Aroclor 1254 and benzo(a)pyrene produced a marked induction of CYP1A-dependent 7-ethoxyresorufin O-deethylase activity in both liver and lung slices. However, while nicotine induced 7-ethoxyresorufin O-deethylase activity in lung slices, but not in liver slices, cotinine did not induce enzyme activity in either liver or lung slices. Overall, while higher rates of nicotine metabolism were observed in rat liver slices, nicotine-induced CYP1A form induction was observed in lung slices. These results demonstrate the usefulness of precision-cut tissue slices for studying tissue differences in xenobiotic metabolism and CYP form induction.     

Effect of dose on the inhibition of carcinogenesis/mutagenesis by Aroclor 1254 in rainbow trout fed aflatoxin B1

Prior studies have shown that Aroclor 1254 (PCB) differentially alters the incidence of aflatoxin B1 (AFB1) induced hepatocellular carcinomas in trout, depending upon the time of PCB administration relative to AFB1 exposure (Shelton et al., 1983). When fed simultaneously with AFB1, PCB inhibits carcinoma incidence. We investigated the effect of AFB1 and PCB dose on this inhibition. Duplicate tanks of 100 rainbow trout were fed AFB1 at concentrations of 1, 4, or 8 ppb, either with or without the addition of 50 ppm PCB. Other groups were fed 4 ppb AFB1 + 5 ppm PCB, 50 ppm PCB alone, or control diet alone. After 9 and 12 mo, 40 and 60 fish per tank, respectively, were sampled to determine the incidence of liver tumors. The results show a parallel inhibition of the AFB1-tumor dose-response curve by the presence of 50 ppm PCB. Fish fed 4 ppb AFB1 + 5 ppm PCB showed slight inhibition in response when compared with 4ppb AFB1 alone. Also, livers from fish fed 50 ppm PCB were used to prepare S20 for use in the Salmonella mutagenesis assay. These livers were less efficient in converting AFB1 to a mutagen, when compared to control S20. The AFB1-mutagenesis dose-response curve was again shifted parallel to the right of the curve generated using control S20. These results suggest that the inhibitory action is at least partly at the level of carcinogen activation. The finding of parallel, as opposed to proportional, inhibition with varying carcinogen exposure for certain classes of inhibitors may have important implications for inhibition of environmental carcinogenesis at low levels of carcinogen exposure.