Effect of cigarette smoke exposure and mutant Kras overexpression on pancreatic cell proliferation

Pancreatic cancer is the fourth leading cause of cancer-associated mortality. The major risk factor for pancreatic cancer is cigarette smoking. Kras mutations are commonly observed in human pancreatic cancers. The present study examined the hypothesis that exposure to cigarette smoke and overexpression of a mutant Kras gene in the pancreas affects pancreatic cell proliferation in mice. Mice overexpressing the mutant Kras gene (KRas^G12D) in the pancreas as well as wild-type mice were exposed to environmental tobacco smoke for 2 weeks. Overexpression of mutant Kras increased cell proliferation in pancreatic ductal, acinar and islet cells. Notably, cigarette smoke exposure decreased cell proliferation in pancreatic ductal and acinar cells, and had no effect in islet cells. Cigarette smoke did not affect pancreatic protein levels of tumor necrosis factor (TNF)α, p53, or cyclin D₁, but mutant Kras overexpression slightly decreased TNFα and p53 protein levels. Therefore, pancreatic cell proliferation in mice overexpressing mutant Kras is associated with the later development of pancreatic tumors, but effects of cigarette smoke on pancreatic cell proliferation do not provide a good model for human pancreatic carcinogenesis.

Dietary antioxidants in the prevention of hepatocarcinogenesis: a review

In this review, the role of dietary antioxidants in the prevention of hepatocarcinogenesis is examined. Both human and animal models are discussed. Vitamin C, vitamin E, and selenium are antioxidants that are essential in the human diet. A number of non-essential chemicals also contain antioxidant activity and are consumed in the human diet, mainly as plants or as supplements, including beta-carotene, ellagic acid, curcumin, lycopene, coenzyme Q(10), epigallocatechin gallate, N-acetyl cysteine, and resveratrol. Although some human and animal studies show protection against carcinogenesis with the consumption of higher amounts of antioxidants, many studies show no effect or an enhancement of carcinogenesis. Because of the conflicting results from these studies, it is difficult to make dietary recommendations as to whether consuming higher amounts of specific antioxidants will decrease the risk of developing hepatocellular carcinoma.

Effects of cigarette smoke on the activation of oxidative stress-related transcription factors in female A/J mouse lung

Cigarette smoke contains a high concentration of free radicals and induces oxidative stress in the lung and other tissues. Several transcription factors are known to be activated by oxidative stress, including nuclear factor-kappaB (NF-kappaB), activator protein-1 (AP-1), and hypoxia-inducible factor (HIF). Studies were therefore undertaken to examine whether cigarette smoke could activate these transcription factors, as well as other transcription factors that may be important in lung carcinogenesis. Female A/J mice were exposed to cigarette smoke for 2, 5, 10, 15, 20, 42, or 56 d (6 hr/d, 5 d/wk). Cigarette smoke did not increase NF-kappaB activation at any of these times, but NF-kappaB DNA binding activity was lower after 15 d and 56 d of smoke exposure. The DNA binding activity of AP-1 was lower after 10 d and 56 d but was not changed after 42 d of smoke exposure. The DNA binding activity of HIF was quantitatively increased after 42 d of smoke exposure but decreased after 56 d. Whether the activation of other transcription factors in the lung could be altered after exposure to cigarette smoke was subsequently examined. The DNA binding activities of FoxF2, myc-CF1, RORE, and p53 were examined after 10 d of smoke exposure. The DNA binding activities of FoxF2 and p53 were quantitatively increased, but those of myc-CF1 and RORE were unaffected. These studies show that cigarette smoke exposure leads to quantitative increases in DNA binding activities of FoxF2 and p53, while the activations of NF-kappaB, AP-1, and HIF are largely unaffected or reduced.

Effect of dietary selenium and cigarette smoke on pulmonary cell proliferation in mice

The objective of this study was to determine if dietary selenium could inhibit pulmonary cell proliferation in control and cigarette smoke-exposed female A/J mice. Selenium in the form of sodium selenite was supplemented to purified diets similar to the AIN-93M diet to yield 0.15, 0.5, or 2.0 mg selenium/kg diet. After 3 weeks, mice in each dietary group were divided into two subgroups; one used as control, whereas the other was exposed to cigarette smoke for five consecutive days. Mice from both groups were euthanized 3 days later. Mice were administered bromodeoxyuridine in the drinking water starting 5 days before the initiation of the smoke exposure and continuing until they were euthanized. After euthanasia, the left lung lobe was processed for histology and cell proliferation analysis. Cigarette smoke increased cell proliferation in the terminal bronchioles and large airways, but not in alveoli. High-selenium diets inhibited cell proliferation in the alveoli, terminal bronchioles and large airways areas in both control and smoke-exposed mice. Increasing the dietary selenium level led to increased selenium levels in the blood and lung, and increased glutathione peroxidase (GPx) activity in the lung. Cytochrome P-450 1A1 protein levels in the lung were increased by cigarette smoke but were not affected by dietary selenium. It is concluded that dietary selenium inhibits pulmonary cell proliferation in both control and cigarette smoke-exposed mice, indicating that selenium is inhibiting cell proliferation independently of smoke exposure, and that this inhibition may be related to selenium concentration and GPx activity in the lung.

Role of Oxidative Stress in Peroxisome Proliferator-Mediated Carcinogenesis

In this review, the evidence about the role of oxidative stress in the induction of hepatocellular carcinomas by peroxisome proliferators is examined. The activation of PPAR-alpha by peroxisome proliferators in rats and mice may produce oxidative stress, due to the induction of enzymes like fatty acyl coenzyme A (CoA) oxidase (AOX) and cytochrome P-450 4A1. The effect of peroxisome proliferators on the antioxidant defense system is reviewed, as is the effect on endpoints resulting from oxidative stress that may be important in carcinogenesis, such as lipid peroxidation, oxidative DNA damage, and transcription factor activation. Peroxisome proliferators clearly inhibit several enzymes in the antioxidant defense system, but studies examining effects on lipid peroxidation and oxidative DNA damage are conflicting. There is a profound species difference in the induction of hepatocellular carcinomas by peroxisome proliferators, with rats and mice being sensitive, whereas species such as nonhuman primates and guinea pigs are not susceptible to the effects of peroxisome proliferators. The possible role of oxidative stress in these species differences is also reviewed. Overall, peroxisome proliferators produce changes in oxidative stress, but whether these changes are important in the carcinogenic process is not clear at this time.

Role of the p50 subunit of NF-kappaB in vitamin E-induced changes in mice treated with the peroxisome proliferator, ciprofibrate

Peroxisome proliferators (PPs) are a diverse class of chemicals, which cause a dramatic increase in the size and number of hepatic peroxisomes in rodents and eventually lead to the development of hepatic tumors. Nuclear factor-kappaB (NF-kappaB) is a transcription factor activated by reactive oxygen and is involved in cell proliferation and apoptosis. Previously we found that the peroxisome proliferator ciprofibrate (CIP) activates NF-kappaB and that dietary vitamin E decreases CIP-induced NF-kappaB DNA binding. We, therefore, hypothesized that inhibition of NF-kappaB by vitamin E is necessary for effects of vitamin E on CIP-induced cell proliferation and the inhibition of apoptosis by CIP. Sixteen B6129 female mice (p50+/+) and twenty mice deficient in the p50 subunit of NF-kappaB (p50-/-) were fed a purified diet containing 10 or 250mg/kg vitamin E (alpha-tocopherol acetate) for 28 days. At that time, half of the mice were placed on the same diet with 0.01% CIP for 10 days. CIP treatment increased the DNA binding activity of NF-kappaB and cell proliferation, but had no significant effect on apoptosis. Compared to wild-type mice, the p50-/- mice had lower NF-kappaB activation, higher basal levels of cell proliferation and apoptosis, and a lower ratio of reduced glutathione to oxidized glutathione (GSH/GSSG). There was approximately a 60% reduction in cell proliferation in the CIP-treated p50-/- mice fed higher vitamin E in comparison to the p50-/- mice fed lower vitamin E. Dietary vitamin E also inhibited the DNA binding activity of NF-kappaB, increased apoptosis, and increased the GSH/GSSG ratio. This study shows the effects of vitamin E on cell growth parameters do not appear to be solely through decreased NF-kappaB activation, suggesting that vitamin E is acting by other molecular mechanisms.

Effect of dietary selenium on the promotion of hepatocarcinogenesis by 3,3', 4,4'-tetrachlorobiphenyl and 2,2', 4,4', 5,5'-hexachlorobiphenyl

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that have promoting activity in the liver. PCBs induce oxidative stress, which may influence carcinogenesis. Epidemiological studies strongly suggest an inverse relationship between dietary selenium (Se) and cancer. Despite evidence linking Se deficiency to hepatocellular carcinoma and liver necrosis, the underlying mechanisms for Se cancer protection in the liver remain to be determined. We examined the effect of dietary Se on the tumor promoting activities of two PCBs congeners, 3,3', 4,4'-tetrachlorobiphenyl (PCB-77) and 2,2', 4,4', 5,5'-hexachlorobiphenyl (PCB-153) using a 2-stage carcinogenesis model. An AIN-93 torula yeast-based purified diet containing 0.02 (deficient), 0.2 (adequate), or 2.0 mg (supplemental) selenium/kg diet was fed to Sprague-Dawley female rats starting ten days after administering a single dose of diethylnitrosamine (150 mg/kg). After being fed the selenium diets for 3 weeks, rats received four i.p. injections of either PCB-77 or PCB-153 (150 micromol/kg) administered every 14 days. The number of placental glutathione S-transferase (PGST)-positive foci per cm(3) and per liver among the PCB-77-treated rats was increased as the Se dietary level increased. Unlike PCB-77, rats receiving PCB-153 did not show the same Se dose-response effect; nevertheless, Se supplementation did not confer protection against foci development. However, the 2.0 ppm Se diet reduced the mean focal volume, indicating a possible protective effect by inhibiting progression of preneoplastic lesions into larger foci. Cell proliferation was not inhibited by Se in the liver of the PCB-treated groups. Se did not prevent the PCB-77-induced decrease of hepatic Se and associated reduction in glutathione peroxidase (GPx) activity. In contrast, thioredoxin reductase (TrxR) activity was not affected by the PCBs treatment or by Se supplementation. These findings indicate that Se does not inhibit the number of PGST-positive foci induced during promotion by PCBs, but that the size of the lesions may be inhibited. The effects of Se on altered hepatic foci do not correlate with its effects on GPx and TrxR.

Role of oxidative stress in the promoting activities of pcbs

PCBs are organic pollutants that persist and bioaccumulate in the environment. These chemicals induce and promote liver tumors in rodents. Previous studies have shown that they increase oxidative stress in the liver, including lipid peroxidation, oxidative DNA damage, and NF-κB activation. The objective of these studies was to determine if the promoting activities of PCBs could be inhibited by dietary antioxidants (vitamin E, selenium, or phytochemicals) or by knocking out the p50 subunit of NF-κB. In the antioxidant studies, female rats were first injected with DEN (150 mg/kg) and then administered 4 biweekly i.p. injections (300 μmol/kg/injection) of PCB-77, PCB-153, or vehicle; the number and volume of placental glutathione S-transferase (PGST)-positive foci were then quantified. Vitamin E did not influence the promoting activities of PCBs. Increasing dietary selenium above the recommended intake increased the number of foci induced but decreased their volume. Most of the phytochemicals examined (N-acetyl cysteine, β-carotene, resveratrol, EGCG) had no significant effect on the promoting activity of PCB-77. Ellagic acid increased and lycopene decreased the number of foci; ellagic acid, CoQ(10), and curcumin decreased the volume of foci. In the NF-κB knockout study, male mice were first injected with DEN (90 mg/kg); controls not receiving DEN were also studied. Both p50 -/- and wild-type mice were then injected biweekly 20 times with PCB-153 (300 (μmol/kg). In DEN-treated and DEN + PCB-treated mice, the incidence of tumors was lower in the p50 -/- mice than in wild-type mice. In mice receiving PCB-153, the tumor incidence and tumor volume were higher. The volume of tumors that were positive for glutamine synthetase was increased in mice administered PCB-153. This study shows that the promotion of hepatocarcinogenesis by PCBs is largely unaffected by dietary antioxidants but is diminished when NF-κB activation is impaired by the absence of the p50 subunit.

Effect of phenobarbital on hepatic cell proliferation and apoptosis in mice deficient in the p50 subunit of NF-kappaB

Phenobarbital (PB) is a nongenotoxic tumor promoter in the liver. One mechanism by which PB may exert its tumor promoting activity is by inducing oxidative stress. We previously found that PB administration increased hepatic NF-kappaB DNA binding activity. In this study we examined the hypothesis that the effects of PB on cell proliferation and apoptosis are dependent on NF-kappaB. We used a mouse model that is deficient in the p50 subunit of NF-kappaB; previous studies had found that p50-/- mice were less sensitive to the induction of hepatic cell proliferation by PCBs or peroxisome proliferators. Mice (p50-/- and wild-type B6129) were fed a control diet or one containing 0.05% PB for 3, 10 or 34 days. At the end of the experiment, the mice were euthanized and livers removed and processed. PB increased cell proliferation at 3 and 10 days (but not at 34 days), but the deletion of the NF-kappaB p50 subunit did not inhibit these increases. p50-/- Mice had higher cell proliferation at the 3 day (only in mice fed PB) and 34-day timepoints. PB decreased hepatocyte apoptosis after 3 days, slightly decreased it after 10 days, and did not affect it after 34 days. The deletion of the NF-kappaB p50 subunit did not influence PB's effect on apoptosis. In p50-/- mice, apoptosis was increased after 3 or 10 days compared to wild-type mice, but no effect was seen after 34 days. The hepatic expression of the NF-kappaB-regulated gene TNF-alpha correlated more with the hepatic cell proliferation data than with hepatic apoptosis, and was not decreased by the deletion of the p50 subunit. These findings show that the p50 subunit of NF-kappaB is not required for the alteration of hepatocyte proliferation or apoptosis by PB up to 34 days after its administration.

Proteome analysis of chemically induced mouse liver tumors with different genotype

Mouse liver tumors frequently harbor mutations in Ha-ras, B-raf, or Ctnnb1 (encoding beta-catenin). We conducted a proteome analysis with protein extracts from normal mouse liver and from liver tumors which were induced by a single injection of N-nitrosodiethylamine (DEN) as initiator followed by multiple injections of two different polychlorinated biphenyls (PCBs) as tumor promoters, or corn oil as a control. Liver tumors were stratified into two classes: they were either mutated in Ctnnb1 and positive for the marker glutamine synthetase (GS(+)), or they lacked Ctnnb1 mutations and were therefore GS-negative (GS(-)). Proteome analysis by 2-DE and MS revealed 98 significantly deregulated proteins, 44 in GS(+) and 54 in GS(-) tumors. Twelve of these proteins showed expression changes in both tumor types, but only seven of them were deregulated in the same direction. Several of the identified enzymes could be assigned to fundamental metabolic or other cellular pathways with characteristically different alterations in GS(+) and GS(-) tumors such as ammonia and amino acid turnover, cellular energy supply, and calcium homeostasis. Our data suggest that GS(+) and GS(-) tumor cells show a completely different biology and use divergent evolutionary strategies to gain a selective advantage over normal hepatocytes.

The effect of dietary glycine on the hepatic tumor promoting activity of polychlorinated biphenyls (PCBs) in rats

Polychlorinated biphenyls (PCBs) are ubiquitious lipophilic environmental pollutants. Some of the PCB congeners and mixtures of congeners have tumor promoting activity in rat liver. The mechanism of their activity is not fully understood and is likely to be multifactorial. The aim of this study was to investigate if the resident liver macrophages, Kupffer cells, are important in the promoting activity of PCBs. The hypothesis of this study was that the inhibition of Kupffer cell activity would inhibit hepatic tumor promotion by PCBs in rats. To test our hypothesis, we studied the effects of Kupffer cell inhibition by dietary glycine (an inhibitor of Kupffer cell secretory activity) in a rat two-stage hepatocarcinogenesis model using 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153, a non-dioxin-like PCB) or 3,3',4,4'-tetrachlorobiphenyl (PCB-77, a dioxin-like PCB) as promoters. Diethylnitrosamine (DEN, 150 mg/kg) was administered to female Sprague-Dawley rats, which were then placed on an unrefined diet containing 5% glycine (or casein as nitrogen control) starting two weeks after DEN administration. On the third day after starting the diets, rats received PCB-77 (300 micromol/kg), PCB-153 (300 micromol/kg), or corn oil by i.p. injection. The rats received a total of 4 PCB injections, administered every 14 days. The rats were euthanized on the 10th day after the last PCB injection, and the formation of altered hepatic foci expressing placental glutathione S-transferase (PGST) and the rate of DNA synthesis in these foci and in the normal liver tissue were determined. Glycine did not significantly affect foci number or volume. PCB-153 did not significantly increase the focal volume, but increased the number of foci per liver, but only in the rats not fed glycine; PCB-77 increased both the foci number and their volume in both glycine-fed and control rats. Glycine did not alter the PCB content of the liver, but did increase the activity of 7-benzyloxyresorufin O-dealkylase (BROD) in liver microsomes from PCB-153 treated rats. However, glycine did not affect the induction of ethoxyresorufin O-dealkylase activity by PCB-77 in liver microsomes. Glycine diminished hepatocyte proliferation in PGST-positive foci, but not in normal tissue. Overall these results do not support the hypothesis that dietary glycine inhibits the promoting activities of PCBs. The observations that PCB-153 increased the number of foci per liver in control rats but not glycine-fed rats and that dietary glycine reduced cell proliferation in PGST-positive foci, however, do not allow us to completely rule out a role for dietary glycine. But the data overall indicate that Kupffer cells likely do not contribute to the tumor promoting activities of PCB-77 and PCB-153.

Suppression of peroxisomal enzyme activities and cytochrome P450 4A isozyme expression by congeneric polybrominated and polychlorinated biphenyls

The purpose of this study was to determine the effects of PCBs and PBBs on peroxisome proliferator-activated receptor-alpha-(PPARalpha-) associated enzyme activities or protein levels. Male Sprague-Dawley rats were administered a single IP injection (150 mu mol/kg) of either 3,3',4,4'-tetrabromobiphenyl, 3,3',4,4'-tetrachlorobiphenyl, 3,3',5,5'-tetrabromobiphenyl, 2',3,3',4,5-pentachlorobiphenyl, 3,3',4,4',5-pentachlorobiphenyl, 2,2',3,3',5,5'-hexachlorobiphenyl, or 3,3',4,4',5,5'-hexabromobiphenyl in corn oil (10 ml/kg). One week later, the activities of catalase, peroxisomal fatty acyl-CoA oxidase, and peroxisomal beta-oxidation as well as cytochrome P450 4A (CYP4A) protein content were determined in subcellular liver fractions. None of the peroxisomal enzyme activities were significantly increased by any of the halogenated biphenyl congeners tested. Except for minor (approx. 25%) increases in the total CYP4A content following treatment with 2,2',3,3',5,5'-hexachlorobiphenyl and 3,3',5,5'-tetrabromobiphenyl, CYP4A protein contents were not increased by any treatment. The two Ah receptor agonists, 3,3',4,4'-tetrabromobiphenyl and 3,3',4,4',5-pentachlorobiphenyl, significantly diminished the liver content of CYP4A proteins and activities of the peroxisomal enzymes studied. Since a range of congeners with different biologic and toxicologic activities were selected for this study, it may be concluded that the polyhalogenated biphenyls do not induce peroxisome proliferation in the male rat, but rather certain members of this class of compounds down regulate peroxisome-associated enzymes. Since PCBs and PBBs do not increase enzyme activities and expression of proteins associated with PPARalpha, these agents are therefore exerting their carcinogenic and promoting activities by some other mechanism.

Vitamin E and NF‐κB Activation: A Review

Nuclear factor (NF-kappaB)(1) is a eukaryotic transcription factor that may be activated by oxidative stress. Because of this hypothesis, the effect of vitamin E on NF-kappaB activation has been examined in many studies, using both in vivo and in vitro models. Most of these studies have observed that vitamin E inhibits the activation of NF-kappaB, with the greatest inhibition seen with the succinate form. Vitamin E may be inhibiting NF-kappaB by reducing oxidative stress or through one of its nonantioxidant functions; this is not clear at the present time. It also is not known if the inhibition of NF-kappaB is necessary for any of vitamin E's effects on gene expression and the resulting physiological effects.

PCB 153, a non-dioxin-like tumor promoter, selects for beta-catenin (Catnb)-mutated mouse liver tumors

Polychlorinated biphenyls (PCBs) are ubiquitous environmental toxicants which act as liver tumor promoters in rodents and can be classified as either dioxin-like or non-dioxin (phenobarbital [PB])-like inducers of cytochrome P-450. Since we have previously shown that tumor promotion by PB leads to clonal outgrowth of beta-catenin (Catnb)-mutated but not Ha-ras-mutated mouse liver tumors, we were interested to know whether the non-dioxin-like tumor promoter 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) shows the same selective pressure during tumor promotion. Male B6129SF2/J mice were given a single injection of N-nitrosodiethylamine (90 mg/kg body weight) at 9 weeks of age, followed by 39 weeks of treatment with PCB 153 (20 biweekly ip injections of 300 mumol/kg body weight) or corn oil as a control. Animals were killed 15 weeks after the last PCB 153 injection and liver tumors were identified by immunohistochemical staining of glutamine synthetase (GS) and analyzed for Catnb, Ha-ras, and B-raf mutations. Quantitative analyses revealed that GS-positive tumors were much larger and more frequent in livers from PCB 153-treated mice than in control animals, whereas GS-negative tumors were similar in both groups. Almost 90% (34/38) of all tumors from PCB 153-treated animals contained Catnb mutations, which compares to approximately 45% (17/37) of tumors in the control group. Ha-ras- and B-raf-mutated liver tumors were rare and not significantly different between treatment groups. These results clearly indicate that PCB 153 strongly selects for Catnb-mutated, GS-positive liver tumors, which is similar to the known action of PB, a prototypical tumor promoter in rodent liver.

Inhibition of hepatocarcinogenesis by the deletion of the p50 subunit of NF-kappaB in mice administered the peroxisome proliferator Wy-14,643

Wy-14,643 (WY) is a hypolipidemic drug that induces hepatic peroxisome proliferation and tumors in rodents. We previously showed that peroxisome proliferators increase NF-kappaB DNA binding activity in rats, mice, and hepatoma cell lines, and that mice deficient in the p50 subunit of NF-kappaB had much lower cell proliferation in response to the peroxisome proliferator ciprofibrate. In this study we examined the promotion of hepatocarcinogenesis by WY in the p50 knockout (-/-) mice. The p50 -/- and wild type mice were first administered diethylnitrosamine (DEN) as an initiating agent. Mice were then fed a control diet or a diet containing 0.05% WY for 38 weeks. Wild-type mice receiving DEN only developed a low incidence of tumors, and the majority of wild-type mice receiving both DEN and WY developed tumors. However, no tumors were seen in any of the p50 -/- mice. Cell proliferation and apoptosis were measured in hepatocytes by BrdU labeling and the TUNEL assay, respectively. Treatment with DEN + WY increased both cell proliferation and apoptosis in both the wild-type and p50 -/- mice; DEN treatment alone has no effect. In the DEN/WY-treated mice, cell proliferation and apoptosis were slightly lower in the p50 -/- mice than in the wild-type mice. These data demonstrate that NF-kappaB is involved in the promotion of hepatic tumors by the peroxisome proliferator WY; however, the difference in tumor incidence could not be attributed to alterations in either cell proliferation or apoptosis.

Comparison of different initiation protocols in the resistant hepatocyte model

Several models in rat liver have been developed to study multistage carcinogenesis, including the Solt-Farber resistant hepatocyte model. In this model, initiation consists of either a necrogenic dose of a hepatocarcinogen or a non-necrogenic dose in conjunction with partial hepatectomy (PH). As an alternative to PH, we investigated two different procedures: fasting for 96 h followed by refeeding, or the use of one-day-old neonates. Male Fisher 344 rats were injected p.o. with diethylnitrosamine (DEN) (0, 20, or 100 mg/kg) 24 h after refeeding or PH (controls received DEN alone with no proliferative stimulus). For the neonatal group, male and female Fisher 344 rats were treated with DEN (0 or 20 mg/kg, i.p.) at one day of age. All initiated animals were treated at the same age (11 weeks) with the following selection agents: three daily doses of 2-acetylaminofluorene (AAF) (30 mg/kg), followed by a single dose of carbon tetrachloride (2 ml/kg), followed by three additional daily treatments of AAF (30 mg/kg). Rats were euthanized 2 weeks after the last AAF injection. The PH, neonatal male, and neonatal female groups receiving DEN developed more gamma-glutamyl transpeptidase (GGT)-positive foci per cubic centimeter and foci per liver as compared to untreated rats receiving the same proliferative stimulus, whereas the fasting/refeeding group and the group receiving no proliferative stimulus did not. All DEN-treated groups receiving one of the proliferative stimuli had more foci per cubic centimeter than the DEN-treated group receiving no proliferative stimulus. The volume fractions of GGT-positive foci in the PH/DEN and neonatal male/DEN groups were higher than those of both the DEN-treated group receiving no proliferative stimulus and the groups receiving the same proliferative stimulus without DEN. In neonatal females-receiving DEN, the volume fraction was not different from either neonatal females not receiving DEN or DEN-treated rats receiving no proliferative stimulus. The volume fraction in the fasting/refeeding group was increased when DEN was administered at 100 mg/kg but not at 20 mg/kg. We conclude that the use of male neonatal rats can replace the PH in the Solt-Farber protocol. Fasting/refeeding was also effective at increasing the volume fraction of GGT-positive foci, but only at the higher dose of DEN.

Dietary vitamin E does not inhibit the promotion of liver carcinogenesis by polychlorinated biphenyls in rats

In this study, the effect of dietary vitamin E on the hepatic tumor-promoting activity of PCB-77 and PCB-153 in female Sprague-Dawley rats (175-200 g) was investigated. One week after diethylnitrosamine injection, rats were fed purified diets containing 10, 50, or 250 mg/kg vitamin E in the form of alpha-tocopheryl acetate. Starting 1 wk later, we injected rats i.p. with vehicle (corn oil) or PCB-77 or PCB-153 (300 mumol/kg) every 14 d for 4 injections. All rats were killed 10 d after the last PCB injection. The number and volume of placental glutathione S-transferase (PGST)-positive foci were increased by PCB-77 but not by PCB-153. Vitamin E did not affect the induction of PGST-positive foci. PCB-77, but not PCB-153, increased hepatic NF-kappaB activity. In conclusion, dietary vitamin E supplementation does not protect against the induction of altered hepatic focal lesions by PCBs.

Effects of vitamin E on the NF-κB pathway in rats treated with the peroxisome proliferator, ciprofibrate

Peroxisome proliferators (PPs) are a diverse group of nongenotoxic compounds, which induce hepatic tumors in rodents. The mechanisms leading to hepatic tumors have not been elucidated, but oxidative stress may play a role in the process. Previous studies in our laboratory have shown that peroxisome proliferators activate the transcription factor nuclear factor-kappa B (NF-kappaB) and that this activation is mediated at least in part by oxidative stress. We therefore hypothesized that increased dietary vitamin E would decrease NF-kappaB DNA binding in rodents treated with ciprofibrate (CIP). In this study, 36 male Sprague-Dawley rats were fed a purified diet containing varying levels of vitamin E (10, 50, 250 ppm alpha-tocopherol acetate). After 28 days on the purified diet, seven animals per vitamin E group received 0.01% CIP in the diet for 10 days. Electrophoretic mobility shift assays (EMSAs) showed that CIP treatment increased DNA binding of NF-kappaB. Increased dietary alpha-tocopherol acetate inhibited CIP-induced NF-kappaB DNA binding. Because NF-kappaB translocates to the nucleus upon the phosphorylation and degradation of inhibitor of IkappaB, we also used Western blots to measure cytosolic protein levels of IkappaBalpha and IkappaBbeta, and the IkappaB kinases, IKKalpha and IKKbeta. IkappaBalpha protein levels were decreased in all three CIP-treated groups, with the 10 ppm vitamin E diet also decreasing IkappaBalpha levels in control rats. No difference in IkappaBbeta protein levels was observed among any of the groups. The CIP-treated rats generally had lower protein levels of IKKalpha and IKKbeta. This study supports our working hypothesis that an increased antioxidant environment can inhibit CIP-mediated NF-kappaB induction.

Effect of 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153) on hepatocyte proliferation and apoptosis in mice deficient in the p50 subunit of the transcription factor NF-kappaB

Polychlorinated biphenyls (PCBs) are a group of synthetic chemicals that induce and promote liver tumors in rodents. We previously showed hepatic nuclear factor kappaB (NF-kappaB) activation and increased hepatocyte proliferation in PCB-treated rats. In this study, the role of NF-kappaB in hepatocyte proliferation and apoptosis after PCB administration was analyzed in wild-type mice and in mice deficient in the NF-kappaB p50 subunit (p50-/-). In a 2-day study, mice received a single intraperitoneal (ip) injection of corn oil or PCB-153. Hepatic NF-kappaB DNA binding activity and cell proliferation were increased by PCB-153 in wild-type mice but not in p50-/- mice. In a 21-day study, mice received six ip injections of corn oil or PCB-153 (twice weekly for 3 weeks) and were euthanized 4 days after the last injection. In this study, NF-kappaB DNA binding activity was not increased after PCB-153 treatment in wild-type or p50-/- mice. Cell proliferation was significantly increased in the wild-type mice treated with PCB-153; in the p50-/- mice treated with PCB-153, cell proliferation was greater than in untreated mice but less than in wild-type mice treated with PCB-153. The livers of p50-/- mice showed greater apoptosis than those of wild-type mice; PCB-153 decreased apoptosis in p50-/- mice, with higher inhibition in the 21-day study than in the 2-day study. RNase protection assays indicated that PCB-153 decreased the mRNA level of cyclin A2, B1, B2, and C in the 2-day study, but not in the 21-day study; however, it did not affect cyclin D1 and D2 mRNA levels at either time point. Cyclin D1 protein levels were not affected by PCB-153. Taken together, these data indicate that the absence of the NF-kappaB p50 subunit alters the proliferative and apoptotic changes in mouse liver in the response to PCB-153.

Initiating activity of 4-chlorobiphenyl metabolites in the resistant hepatocyte model

We recently reported that several mono- to tetrachlorinated biphenyls have initiating activity in the livers of Fischer 344 rats. In the present study, we investigated the metabolic activation of one of those compounds, 4-chlorobiphenyl (PCB 3). Monohydroxy (400 micromol/kg), dihydroxy (200 micromol/kg), and quinone (100 micromol/kg) metabolites of PCB 3 were evaluated for their initiating activity. Fischer 344 male rats were fasted for 4 days; 24 h after feeding again, they were injected (ip) with metabolites, vehicle, or diethylnitrosamine (DEN, 20 or 40 mg/kg). All animals were treated with selection agents as follows: three daily p.o. doses of 2-acetylaminofluorene (2-AAF, 30 mg/kg), followed by a single p.o. dose of carbon tetrachloride (2 ml/kg) and three additional daily treatments of 2-AAF. Rats were killed 2 weeks after the last 2-AAF intubation. Livers were evaluated for changes in morphology, and the number and volume of gamma-glutamyl transpeptidase-positive foci were measured. Of the metabolites tested, only one monohydroxy and one quinoid metabolite showed initiating activity. The metabolic activation of PCB 3, therefore, proceeds via parahydroxylation and oxidation to the ortho 3,4-quinone, the ultimate carcinogen. This is the first report to demonstrate that specific PCB metabolites possess initiating activity in the rodent liver in vivo. The results support the conclusion that 4-OH PCB 3 and 3,4-BQ PCB 3 act as proximate and ultimate carcinogenic metabolites resulting from the bioactivation of PCB 3 in rat liver.

Cell proliferation and apoptosis are altered in mice deficient in the NF-kappaB p50 subunit after treatment with the peroxisome proliferator ciprofibrate

We previously showed that the peroxisome proliferator ciprofibrate increases hepatic NF-kappaB DNA binding activity in rats, mice, and hepatoma cell lines. Here, we analyzed the response to ciprofibrate in mice that lack the NF-kappaB p50 gene (p50-/-). Wild-type and p50-/- mice were fed a diet with or without 0.01% ciprofibrate for 10 days. NF-kappaB DNA binding activity was present and increased after ciprofibrate treatment in wild-type mice, but was not detected in p50-/- mice. The untreated p50-/- mice had a higher level of hepatic cell proliferation, as measured by BrdU labeling, than did untreated wild-type mice. However, the increase in proliferation was greater in ciprofibrate-fed wild-type mice than in ciprofibrate-fed p50-/- mice. The apoptotic index was low in wild-type mice in the presence or absence of ciprofibrate. Apoptosis was increased in untreated p50-/- mice compared to wild-type mice; apoptosis was reduced in p50-/- mice after ciprofibrate feeding. The c-Jun and JunB mRNA levels were higher in untreated p50-/- mice than in untreated control mice; c-Jun mRNA levels increased, whereas JunB mRNA levels decreased in both groups after ciprofibrate treatment. The c-Jun and JunB protein levels were the same in untreated wild-type and p50-/- mice and increased in both groups after ciprofibrate treatment. Several apoptosis-related mRNAs were higher in untreated p50-/- mice compared to untreated control mice; expression of these genes increased in both groups after ciprofibrate treatment. These data indicate that NF-kappaB contributes to the proliferative and apoptotic changes that occur in the liver in response to ciprofibrate.

Effect of a single dose of polychlorinated biphenyls on hepatic cell proliferation and the DNA binding activity of NF-kappaB and AP-1 in rats

Polychlorinated biphenyls (PCBs) are environmental pollutants that, because of their persistence and biomagnification, raise concerns about the health consequences of long-term exposure. PCB mixtures induce hepatocellular carcinomas in rodents, but the mechanism of their promoting activity is not clear. Previous studies have shown that oxidative stress occurs after PCB administration, with the induction of lipid peroxidation and oxidative DNA damage, which may contribute to their promoting activity. In this study, we examined whether the oxidative stress-sensitive transcription factors NF-kappaB or AP-1 were activated by PCBs in the liver. Male Sprague-Dawley rats were injected i.p. with corn oil, 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153, 30, 150, or 300 micromol/kg), 3,3',4,4'-tetrachlorobiphenyl (PCB-77, 30, 150, or 300 micromol/kg), or both PCBs (each 30 or 150 micromol/kg). Rats were euthanized 2, 6, or 24 h, or 2, 6, and 10 d after the PCB injection. Electrophoretic mobility shift assays (EMSAs) were performed to determine NF-kappaB and AP-1 DNA binding activities. The highest NF-kappaB DNA binding activity was observed in rats receiving higher doses of PCB-153 (150 and 300 micromol/kg), with peak activation occurring 2 d after injection. AP-1 activation was not detected at any timepoint. Hepatocyte proliferation, as measured by the labeling index, was increased only in groups receiving the highest dose of PCB-153 or the combination of two PCBs (150 micromol/kg each) at day 2, and not by any other PCB treatment at any timepoint. These results show that PCB-153, but not PCB-77, can induce hepatocyte proliferation and hepatic NF-kappaB activation after a single dose.

Polychlorinated biphenyls as initiators in liver carcinogenesis: resistant hepatocyte model

A modified Solt-Farber protocol was established to investigate the potential initiating activity of lower chlorinated polychlorinated biphenyl (PCB) congeners in rat liver. Two different studies were conducted in male Fisher 344 rats. PCBs investigated were PCB3, PCB12, PCB38, and PCB77 in study 1 and PCB15, PCB52, PCB77, and the combination of PCB52 and PCB77 in study 2. Rats were subjected to partial hepatectomy followed by a single dose of the suspected initiating agent, diethylnitrosamine, or vehicle. Two weeks later all groups received selection treatment consisting of three daily doses of 2-acetylaminofluorene (2-AAF) and then a single dose of carbon tetrachloride, followed by three additional daily treatments of 2-AAF via gavage. Rats were killed 2 weeks after the last treatment of 2-AAF, and the number and volume of gamma-glutamyltranspeptidase (GGT)-positive foci were determined. Among the PCBs tested, PCB3, PCB15, PCB52, and PCB77 significantly increased the number of GGT-positive foci per cm(3) of liver and per liver. Only PCB3 and PCB15 increased the volume fraction of GGT-positive foci. Histopathologic analysis of hematoxylin- and eosin-stained liver sections showed that rats with significantly increased GGT-positive foci also had extensive cellular alteration. This effect was not seen in nonselection groups. We conclude that, under the conditions and time courses of these experiments, several PCBs have initiating activity in male Fischer 344 rats.

Vitamin E inhibits hepatic NF-kappaB activation in rats administered the hepatic tumor promoter, phenobarbital

Phenobarbital (PB) is an efficacious hepatic tumor promoter. Although the promoting activity of PB is likely related to altered cell proliferation or apoptosis, the induction of an oxidative stress environment may also be important. PB has been shown to activate the transcription factor nuclear factor-kappaB (NF-kappaB). In this study, we hypothesized that PB-induced NF-kappaB activation can be decreased by dietary vitamin E in rats. Male Sprague-Dawley rats (n = 39) were fed a purified diet with varying levels of dietary vitamin E (10, 50 or 250 mg/kg of dl-alpha-tocopherol acetate) for 28 d, at which time 8 rats per level of dietary vitamin E were fed the same diet with 500 mg/kg PB for 10 d. In the rats fed the low vitamin E diet, PB increased NF-kappaB DNA binding, but it did not affect NF-kappaB activation in rats fed higher levels of vitamin E (50 and 250 mg/kg). Vitamin E may decrease the oxidative stress created by PB by also enhancing other antioxidants; therefore, we also measured hepatic glutathione S-transferase, glutathione peroxidase, glutathione reductase, superoxide dismutase, catalase and NAD(P)H:quinone reductase (DT-diaphorase) activities and glutathione and ascorbic acid concentrations. Increased dietary alpha-tocopherol did not affect the antioxidants and antioxidant enzymes altered by PB treatment. Thus, the effect of alpha-tocopherol acetate on NF-kappaB activation does not appear to be mediated by alterations in the antioxidant system. These results demonstrate that the activation of NF-kappaB, a transcription factor that affects cell proliferation- and apoptosis-related gene expression, can be inhibited by dietary vitamin E.

Moderate iron overload enhances lipid peroxidation in livers of rats, but does not affect NF-kappaB activation induced by the peroxisome proliferator, Wy-14,643

It has been hypothesized that high concentrations of tissue iron may enhance carcinogenesis induced by free radical mechanisms. Wy-14,643 is a peroxisome proliferator that is hepatocarcinogenic in rats. Tumor induction may result in part from excessive production of reactive oxygen species, particularly H(2)O(2). The purpose of this study was to examine the effect of iron status on oxidative stress and NF-kappaB activation in livers of rats treated with Wy-14,643. Forty-eight male Sprague-Dawley rats were fed one of four diets (20, 45, 650, 1500 mg Fe/kg diet) for 28 d. At the time of tissue collection, liver iron ranged from 1.4 to 9.9 micro mol/g wet tissue in the diet groups. Wy-14,643 (0 or 0.1 g/100 g diet) was added to the diet for the final 10 d of the study. Wy-14,643 doubled the liver weight/body weight ratio (P = 0.0001), which was also increased by iron supplementation (P < 0.01). Iron supplementation increased thiobarbituric acid reactive substances and/or conjugated dienes, but there was no synergism between Wy 14,643 and iron on lipid peroxidation measures. The hepatic DNA binding activity of NF-kappaB was increased in rats administered Wy-14,643. However, differences in liver iron concentration did not alter activation of NF-kappaB in untreated rats or in those treated with Wy-14,643. DNA double-strand breakage was not affected by iron or Wy-14,643. In summary, although moderate changes in iron status altered liver lipid peroxidation, iron did not significantly increase oxidative stress induced by a hepatocarcinogenic peroxisome proliferator.

Peroxisome proliferators do not activate the transcription factors AP-1, early growth response-1, or heat shock factors 1 and 2 in rats or hamsters

Peroxisome proliferators (PPs) cause hepatomegaly, peroxisome proliferation, and hepatocarcinogenesis in rats and mice, whereas hamsters are less responsive to these compounds. PPs increase peroxisomal beta-oxidation and P4504A subfamily activity, which have been hypothesized to result in oxidative stress. Work in our laboratory indicated that differential modulation of the redox-sensitive transcription factor NF-kappaB may contribute to the resulting difference in species susceptibility following PP administration. Therefore, we hypothesized that other redox-sensitive transcription factors such as AP-1, early growth response gene 1 (Egr-1), and heat-shock factors 1 and 2 (HSF1/2) may also be alternatively activated in differentially susceptible species. Accordingly, we measured the activation of these transcription factors using gel mobility shift assays, with hepatic nuclear extracts derived from rats and Syrian hamsters fed two doses of three peroxisome proliferators (dibutyl-phthalate [DBP], gemfibrozil and Wy-14,643) for 6, 34, or 90 days. Although changes were observed at various time points, no consistent, dose-responsive changes were observed in the DNA binding activities of these transcription factors following PP treatment. The lack of increased binding of AP-1, Egr-1, and HSFs suggests that these factors are not involved in increased cell proliferation following PP administration, although we cannot rule out that these factors are activated at earlier time points than those examined in this study.

Effect of 3,3',4,4'-tetrachlorobiphenyl and 2,2',4,4',5,5'-hexachlorobiphenyl on the induction of hepatic lipid peroxidation and cytochrome P-450 associated enzyme activities in rats

Polychlorinated biphenyls (PCBs) are environmental contaminants that have been widely used for various industrial purposes. In spite of numerous studies on PCBs, however, their mechanism of toxicity remains unknown. The role of cytochrome P-450 in PCBs induced hepatic lipid peroxidation is controversial. Therefore, the present study was undertaken to study the mechanism of action of two PCBs and their role in cytochrome P-450 induction and lipid peroxidation, determined in vivo and during the incubation of subcellular fractions. We also examined whether agonist/antagonist activities between the two PCBs were occurring. Two PCBs were studied: 3,3',4,4'-tetrachlorobiphenyl (PCB-77), a non-ortho-substituted, coplanar PCB; and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153), a di-ortho-substituted, non-planar PCB. Groups of male Sprague-Dawley rats were given a single i.p. injection of one of the two PCBs (at doses of 30, 150, or 300 micromol/kg), both PCBs (at doses of 30 or 150 micromol/kg), or vehicle alone. Rats were sacrificed after 2, 6, or 24 h; or 2, 6, or 10 days. Cytochrome P-450 induction occurred as early as 2 h with PCB-77 and 24 h with PCB-153. Significant increases in thiobarbituric acid reactive substances (TBARS) content in liver tissue occurred 2, 6 and 10 days after treatment with PCB-77 and PCB-153; it was unclear whether these PCBs were synergistic in their induction of TBARS formation. Liver microsomal fractions incubated with NADPH only showed increased TBARS formation at the highest doses of PCB-77 and PCB-153 after 6 days. The results indicate that both PCBs induced cytochrome P-450 enzymes and enhanced lipid peroxidation in liver and subcellular fractions but with different potencies and onsets of action. The results also indicate a larger time difference between cytochrome P-450 induction and lipid peroxidation for PCB-77. Thus, both PCB-77 and PCB-153 are toxic to cells, but may act via different mechanisms to induce their effects.

Regulation of cell proliferation, apoptosis, and transcription factor activities during the promotion of liver carcinogenesis by polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are environmental pollutants that are complete carcinogens and tumor promoters in the liver. The mechanisms of their promoting activities are not clear, but one possible mechanism is the induction of oxidative stress. In the present study we evaluated the ability of two PCB congeners to activate the oxidative stress-responsive transcription factors nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1), as well as hepatocyte cell proliferation and apoptosis, which are influenced by activation of these transcription factors, in rat liver. Two transcription factors not activated by oxidative stress, signal transducers and activators of transcription 3 and 5 (STAT3 and STAT5), were also examined. All the animals in this study received a single dose of diethylnitrosamine (150 mg/kg) followed by four biweekly injections of 3,3',4,4'-tetrachlorobiphenyl (PCB-77) or 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153) (100 or 300 micromol/kg), or both PCBs (100 micromol/kg each). Ten days after the last PCB injection, all animals were euthanized; 3 days before euthanasia all animals were implanted with Alzet osmotic pumps containing 5-bromo-2'-deoxyuridine (BrdU). The number of placental glutathione S-transferase (PGST)-positive foci were increased in rats administered PCBs, with the highest increase seen in rats administered PCB-77. The number of foci in rats administered both PCBs was intermediate between the numbers seen with either PCB-77 or PCB-153, indicating that a synergistic effect did not occur. There was a significant increase in NF-kappaB and AP-1 binding activities in hepatic nuclear extracts from rats receiving the high dose of PCB-77 or PCB-153 and in rats receiving both PCBs. In contrast, the DNA binding activities of STAT3 and STAT5 were decreased in rats administered PCBs. Cell proliferation in both focal and nonfocal hepatocytes was increased by PCB-77 but was not affected by PCB-153. Apoptotic indexes, as quantified by the TUNEL method, were increased in both focal and nonfocal hepatocytes by PCB-77 but were decreased in focal hepatocytes by PCB-153. This study shows that both PCBs alone or in combination can increase the DNA binding activities of NF-kappaB and AP-1, whereas the DNA binding activities of STAT3 and STAT5 are decreased. The induction of altered hepatic foci appears to be related to compensatory cell proliferation in PCB-77-treated rats, whereas the inhibition of apoptosis appears to be important in PCB-153-treated rats.

Peroxisome proliferator perfluorodecanoic acid alters glutathione and related enzymes

Previously we have shown that treatment with the peroxisome proliferator perfluorodecanoic acid (PFDA) significantly increased hepatic reduced glutathione (GSH) content without altering the activity of selenium-glutathione peroxidase. In this study we examined some potential mechanisms by which PFDA treatment increases GSH levels. Male Sprague-Dawley rats were given a single injection of 0, 8.8, 17.5, and 35 mg PFDA in corn oil per kg body weight. Twelve days later the effects of PFDA on the activities of enzymes associated with GSH synthesis, utilization, and regeneration were assessed. The results showed that in a dose-dependent manner, PFDA treatment significantly decreased the activity of gamma-glutamylcysteine synthetase, while the activities of NADPH-generating enzymes, malic enzyme, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase were increased. PFDA treatment also dose dependently decreased cytosolic, but not microsomal, glutathione S-transferase activity, and the activity of glutathione reductase was decreased by the highest dose of PFDA. The data obtained suggest that increased hepatic GSH levels following PFDA treatment may result from increased regeneration and/or decreased utilization.

Differential activation of hepatic NF-kappaB in rats and hamsters by the peroxisome proliferators Wy-14,643, gemfibrozil, and dibutyl phthalate

Nuclear factor-kappaB (NF-kappaB) is an oxidative stress-activated transcription factor involved in the regulation of cell proliferation and apoptosis. We found previously that the peroxisome proliferator ciprofibrate activates NF-kappaB in the livers of rats and mice. These species are sensitive to the hepatocarcinogenic effects of peroxisome proliferators, whereas other species such as Syrian hamsters are not. In the present study we examined the effects of 3 different peroxisome proliferators on NF-kappaB activation in rats and Syrian hamsters. The peroxisome proliferators Wy-14,643, gemfibrozil, and dibutyl phthalate were administered to animals for 6, 34, or 90 days. NF-kappaB activity was determined using electrophoretic mobility-shift assays and confirmed using supershift assays. Wy-14,643 increased the DNA binding activity of NF-kappaB at all 3 time points in rats and produced the highest activation of the 3 chemicals tested. Gemfibrozil and dibutyl phthalate increased NF-kappaB activation to a lesser extent in rats and not at all times. There were no differences in hepatic NF-kappaB levels between control hamsters and hamsters treated with any of the peroxisome proliferators. This study demonstrates species-specific differences in hepatic NF-kappaB activation by peroxisome proliferators.

Effects of peroxisome proliferators on antioxidant enzymes and antioxidant vitamins in rats and hamsters

Peroxisome proliferators (PPs) cause hepatomegaly, peroxisome proliferation, and hepatocarcinogenesis in rats and mice, whereas hamsters are less responsive to PPs. PPs increase the activities of enzymes involved in peroxisomal beta-oxidation and omega-hydroxylation of fatty acids, which has been hypothesized to result in oxidative stress. The hypothesis of this study was that differential modulation of antioxidant enzymes and vitamins might account for differences in species susceptibility to PPs. Accordingly, we measured the activities of DT-diaphorase and superoxide dismutase (SOD) and the hepatic content of ascorbic acid and alpha-tocopherol in male Sprague-Dawley rats and Syrian hamsters fed 2 doses of 3 known peroxisome proliferators (dibutyl phthalate [DBP], gemfibrozil, and [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio]acetic acid (Wy-14,643) for 6, 34, or 90 days. In untreated animals, the activity of DT-diaphorase was much higher in hamsters than in rats, but the control levels of SOD, ascorbic acid and alpha-tocopherol were similar. In rats and hamsters treated with Wy-14,643, we observed decreases in alpha-tocopherol content and total SOD activity. DT-diaphorase was decreased in activity following Wy-14,643 treatment in rats at all time points and doses, but only sporadically affected in hamsters. Rats and hamsters treated with DBP demonstrated increased SOD activity at 6 days; however, in the rat, DBP decreased SOD activity at 90 days and alpha-tocopherol content was decreased throughout. In gemfibrozil treated rats and hamsters, a decrease in alpha-tocopherol content and an increase in DT-diaphorase activity were observed. In either species, no consistent trend was observed in total ascorbic acid content after treatment with any of the PPs. In conclusion, these data suggest that both rats and hamsters are compromised in antioxidant capabilities following PP treatment and additional hypotheses for species susceptibility should be considered.

Polychlorinated biphenyl-induced effects on metabolic enzymes, AP-1 binding, vitamin E, and oxidative stress in the rat liver

Environmental pollutants, such as polychlorinated biphenyls (PCBs), may induce drug metabolism and may be substrates for the induced metabolic enzymes. Both processes may lead to oxidative stress. The goal of this study was to determine the influence of polychlorinated biphenyls, selected as inducers and substrates of drug metabolism, on oxidative events within the liver over a 3-week time course. Male and female Sprague-Dawley rats received two ip injections per week of 4-chlorobiphenyl, 2,4,4'-trichlorobiphenyl, 3,4,5-trichlorobiphenyl, 3,3',4,4'-tetrachlorobiphenyl (PCB 77), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), or both PCB 77 and 153 (100 micromol/kg/injection) and were euthanized at the end of 1, 2, or 3 weeks. Hepatic cytochrome P450 1A1 (EROD) activity, DT-diaphorase activity, AP-1 DNA-binding activity, conjugated dienes, and alpha-tocopherol (vitamin E) as well as alpha-tocopheryl quinone (oxidized vitamin E) were determined. While the lower chlorinated biphenyls (at these doses and times) showed little or no effect on these oxidative stress parameters, both CYP 1A1 and DT-diaphorase activities were significantly increased in both male and female rats receiving PCB 77, a ligand for the aryl hydrocarbon receptor. In addition, the DNA-binding activity of the transcription factor AP-1 was increased in rats treated with PCB 77 or PCB 153. Within the lipid fraction there was no significant increase observed in conjugated diene concentrations, but there was a significant increase in alpha-tocopheryl quinone upon treatment with all PCBs tested. These data indicate that alpha-tocopheryl quinone may be a sensitive marker for PCB exposure and is possibly increased by a wide range of PCBs.

Effects of peroxisome proliferators on glutathione and glutathione-related enzymes in rats and hamsters

Peroxisomeproliferators (PPs) cause hepatomegaly, peroxisome proliferation, and hepatocarcinogenesis in rats and mice. Conversely, hamsters are less responsive to these compounds. PPs increase peroxisomal beta-oxidation and P4504A subfamily activity, which has been hypothesized to result in oxidative stress. We hypothesized that differential modulation of glutathione-related defenses could account for the resulting difference in species susceptibility following PP administration. Accordingly, we measured glutathione S-transferase (GST), glutathione peroxidase (GPx), and glutathione reductase (GR) activities, and total glutathione (GSH) in male Sprague-Dawley rats and Syrian hamsters fed two doses of three known peroxisome proliferators [dibutylphthalate (DBP), gemfibrozil, and Wy-14,643] for 6, 34, or 90 days. In rats, decreases in GR, GST, and selenium-dependent GPx were observed following PP treatment at various time points. In hamsters, we observed higher basal levels of activities for GR, GST, and selenium-dependent GPx compared to rats. In addition, hamsters showed decreases in GR and GST activities following PP treatment. Interestingly, selenium-dependent GPx activity was increased in hamsters following treatment with Wy-14,643 and DBP. Treatment for 90 days with Wy-14,643 resulted in no change in GPx1 mRNA in rats and increased GPx1 mRNA in hamsters. Sporadic changes in total GSH and selenium-independent GPx were observed in both species. This divergence in the hydrogen peroxide detoxification ability between rats and hamsters could be a contributing factor in the proposed oxidative stress mechanism of PPs observed in responsive and nonresponsive species.

Activation of hepatic NF-kappaB by the herbicide Dicamba (2-methoxy-3,6-dichlorobenzoic acid) in female and male rats

Nuclear factor-kappaB is a transcription factor that is activated in many different cell types by pathologic stimuli, such as reactive oxygen intermediates. One class of hepatocarcinogens, the peroxisome proliferators, may produce reactive oxygen intermediates, and one potent peroxisome proliferator, ciprofibrate, was recently reported to activate nuclear factor-kappaB. In this study, we investigated whether Dicamba, a broad leaf herbicide and peroxisome proliferator, could activate nuclear factor-KB in the livers of rats. Female and male Sprague Dawley rats (n = 4) were fed diets containing either 0, 1, or 3% Dicamba or 0.01% ciprofibrate for 7 days. As expected, the potent peroxisome proliferator, ciprofibrate, significantly increased fatty acyl CoA oxidase, peroxisomal beta-oxidation, and catalase activities in male rats and, except for catalase, also in female rats. Dicamba significantly increased peroxisomal fatty acyl CoA oxidase, peroxisomal beta-oxidation, and catalase activities, but decreased the activity of the cytosolic antioxidant enzyme, Se-dependent glutathione peroxidase, in both female and male rats. Dicamba increased nuclear factor-kappaB binding in the nuclear protein of livers from male rats fed both the 1 and 3% Dicamba diets. However, the highest binding was seen in nuclear protein from female rats fed 3% Dicamba. Both supershift and cold competition assays confirmed that this DNA binding activity was specific for nuclear factor-kappaB. Our results in this study suggest that the herbicide and peroxisome proliferator Dicamba has the ability to activate nuclear factor-kappaB.

Effect of extracellular matrix on the expression of peroxisome proliferation associated genes in cultured rat hepatocytes

The purpose of this study was to determine whether the extracellular matrix used in hepatocyte culture would alter gene expression induced by the peroxisome proliferator ciprofibrate (CIP). We compared the activities and mRNA levels of two enzymes associated with peroxisome proliferation-fatty acyl-CoA oxidase (FAO), the first enzyme in the peroxisomal beta-oxidation pathway, and lauric acid hydroxylase (LAH), which represents the activity of the cytochrome P450 4A subfamily-in rat hepatocytes cultured on different plates: plastic, collagen-coated, thin and thick Matrigel, and collagen gel plates. CIP increased FAO activity about fivefold in collagen gel plates and sixfold in thick Matrigel plates compared to a fourfold increase in other plates; LAH was increased about threefold in thin Matrigel plates and fourfold in thick Matrigel and collagen gel plates compared to only a twofold increase in plastic and collagen-coated plates. The mRNA level for FAO was highest in hepatocytes cultured on collagen gel and Matrigel plates compared to those cultured on plastic and collagen-coated plates. The P-450 4A1 mRNA expression, however, was highest in the collagen gel plates, with lower expression in the thick Matrigel, collagen-coated and plastic plates. DNA synthesis and the DNA binding activity of the transcription factor AP-1 were also examined in response to epidermal growth factor (EGF) and CIP. Without the addition of EGF, DNA synthesis was significantly higher on collagen-coated plates than on collagen gel plates. The DNA binding activity of AP-1 was also induced after 24 hr culture in collagen-coated plates, whereas it was not detected in collagen gel plates. After the addition of EGF, the DNA binding activity of AP-1 was increased in both collagen-coated plates and collagen gel plates. CIP did not increase the DNA binding activity of AP-1 in either plate. These results demonstrate that components of the extracellular matrix influence the induction of peroxisome proliferator-induced enzyme activities and mRNA levels by CIP, with the highest induction seen in collagen gel and thick Matrigel plates. Furthermore, the induction of cell proliferation and AP-1 DNA binding activity are influenced by the extracellular matrix.

Activation of nuclear factor-kappaB by the peroxisome proliferator ciprofibrate in H4IIEC3 rat hepatoma cells and its inhibition by the antioxidants N-acetylcysteine and vitamin E

Peroxisome proliferators are a class of hepatic carcinogens in rodents and are proposed to act in part by increasing reactive oxygen species such as hydrogen peroxide. We previously showed that treatment of rats with ciprofibrate, a peroxisome proliferator, results in increased hepatic nuclear factor-kappaB (NF-kappaB) DNA binding activity. In this study, we have examined the link between peroxisome proliferators and NF-kappaB activation in hepatoma cell lines to test whether increased nuclear NF-kappaB levels activate NF-kappaB-regulated genes and to determine the mechanism of NF-kappaB activation. Electrophoretic mobility shift assays demonstrated NF-kappaB induction by ciprofibrate in peroxisome proliferator-responsive H4IIEC3 rat hepatoma cells but not in peroxisome proliferator-insensitive HepG2 human hepatoma cell lines. In addition, we found that stably transfected NF-kappaB-regulated reporter genes were activated by ciprofibrate in H4IIEC3 cells. This reporter gene activation was blocked by the antioxidants N-acetylcysteine and vitamin E. These studies suggest that hepatocytes are at least partially responsible for peroxisome proliferator-mediated hepatic NF-kappaB activation, and support the possibility that this activation is dependent upon reactive oxygen species.

Expression of the hydrogen peroxide-generating enzyme fatty acyl CoA oxidase activates NF-kappaB

Peroxisome proliferators are a class of hepatic carcinogens in rodents and have been proposed to act in part by increasing oxidative stress. Fatty acyl CoA oxidase (FAO), which is highly induced by peroxisome proliferators, is the hydrogen peroxide-generating enzyme of the peroxisomal beta-oxidation pathway. We previously showed that the treatment of rats and mice with the peroxisome proliferator ciprofibrate resulted in increased hepatic NF-kappaB activity and suggested that this effect may be secondary to the action of H2O-generating enzymes. To test this possibility directly, we have determined whether transient overexpression of FAO, in the absence of peroxisome proliferators, leads to NF-kappaB activation. Here, we show that FAO overexpression in Cos-1 cells, in the presence of an H2O-generating substrate, can activate a NF-kappaB regulated reporter gene. Electrophoretic mobility shift assays further demonstrated that FAO expression increases nuclear NF-kappaB DNA binding activity in a dose-dependent manner. The antioxidants vitamin E and catalase can inhibit this activation. These results indicate that FAO mediates, at least in part, peroxisome proliferator-induced NF-kappaB activation.

Promoting activity of the herbicide dicamba (2-methoxy-3, 6-dichlorobenzoic acid) in two stage hepatocarcinogenesis

Our goal was to examine whether dicamba, a widely-used broad leaf herbicide, has promoting activity in two-stage hepatocarcinogenesis. Female Sprague Dawley rats were given a single dose of diethylnitrosamine and then diets containing dicamba, or phenobarbital, or both for six months. The number and volume of placental glutathione-S-transferase-positive, glucose-6-phosphatase-negative or ATPase-negative foci were quantified in the liver. Dicamba alone did not increase the number or volume of these altered hepatic foci. Dicamba did, however, show a significant effect on the number or volume of certain markers in animals also treated with phenobarbital. These data show that dicamba in combination with other promoters may have weak promoting activity in two-stage hepatocarcinogenesis in the rat.

Stimulation of the DNA binding activity of AP-1 by the peroxisome proliferator ciprofibrate and eicosanoids in cultured rat hepatocytes

Peroxisome proliferators induce hepatic peroxisome proliferation and hepatic tumors in rodents. These chemicals increase the expression of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolize lipids, including eicosanoids. Peroxisome proliferators also induce increased cell proliferation in vivo. However, peroxisome proliferators are only weakly mitogenic and are not comitogenic with epidermal growth factor (EGF) in cultured hepatocytes. Our earlier studies found that the peroxisome proliferator ciprofibrate is comitogenic with eicosanoids. We therefore hypothesized that the comitogenicity of the peroxisome proliferator ciprofibrate and eicosanoids may result from a synergistic increase of the DNA binding activity of AP-1. Primary rat hepatocytes were cultured on collagen gels in serum-free L-15 medium with ciprofibrate, eicosanoids, and/or growth factors. The DNA binding activity of AP-1 was determined in nuclear protein extracts by electrophoretic mobility shift assay. The DNA binding activity of AP-1 was not induced by ciprofibrate or eicosanoids alone, but the addition of eicosanoids along with ciprofibrate increased the induction of DNA binding activity of AP-1 at 30 min and 2 h after exposure. The combination of ciprofibrate and PGF2alpha blocked the inhibitory effect of transforming growth factor (TGF)-beta on the DNA binding activity of AP-1 induced by EGF. These results show that the peroxisome proliferator ciprofibrate and eicosanoids co-stimulate the DNA binding activity of AP-1 and suggest that changes in eicosanoid concentrations may modulate mitogenic signal transduction pathways by the peroxisome proliferator ciprofibrate.

Inhibition of cigarette smoke-related lipophilic DNA adducts in rat tissues by dietary oltipraz

The present study investigated the effects of dietary oltipraz on cigarette smoke-related lipophilic DNA adduct formation. Female Sprague-Dawley rats were exposed daily to sidestream cigarette smoke in a whole-body exposure chamber 6 h/day for 4 consecutive weeks. One group of rats was maintained on control diet while another group received the same diet supplemented with either a low (167 p.p.m.) or high (500 p.p.m.) dose of oltipraz, starting 1 week prior to initiation of smoke exposure until the end of the experiment. Analysis of lipophilic DNA adducts by the nuclease P1-mediated 32P-post-labeling showed up to five smoke-related adducts. Adduct no. 5 predominated in both the lung and the heart while adduct nos 3 and 2 predominated in the trachea and bladder, respectively. Quantitative analysis revealed that the total adduct level was the highest in lungs (270+/-68 adducts/10(10) nucleotides), followed by trachea (196+/-48 adducts/10(10) nucleotides), heart (141+/-22 adducts/10(10) nucleotides) and bladder (85+/-16 adducts/10(10) nucleotides). High dose oltipraz treatment reduced the adduct levels in lungs and bladder by >60%, while the reduction in lungs in the low-dose group was approximately 35%. In trachea, the effect of low and high dietary oltipraz on smoke DNA adduction was equivocal, while smoke-related DNA adducts in the heart were minimally inhibited by high-dose oltipraz. In a repeat experiment that employed a 3-fold lower dose of cigarette smoke, oltipraz (500 p.p.m.) was found to inhibit the formation of DNA adducts in rat lungs and trachea by 80 and 65%, respectively. These data clearly demonstrate a high efficacy of oltipraz in inhibiting the formation of cigarette smoke-induced DNA adducts in the target tissues.

Effect of the peroxisome proliferator ciprofibrate on lipid peroxidation and 8-hydroxydeoxyguanosine formation in transgenic mice with elevated hepatic catalase activity

Peroxisome proliferators are a group of non-genotoxic hepatic carcinogens which have been proposed to act by increasing oxidative damage in the liver. To test this hypothesis, we have produced a transgenic mouse line that has elevated catalase activity specifically in the liver. In this study, we have examined if catalase overexpression influences the induction of lipid peroxidation or oxidative DNA damage, two mechanisms which have been hypothesized to be important in the carcinogenesis by peroxisome proliferators. Transgenic mice or non-transgenic litter mates were fed either 0.01% ciprofibrate or a control diet for 21 days. The activities of fatty acyl CoA oxidase and lauric acid hydroxylase were not significantly affected by catalase overexpression, although the ratio of fatty acyl CoA oxidase to catalase was significantly decreased in transgenic animals. Hepatic lipid peroxidation was estimated by quantifying the concentrations of malondialdehyde and conjugated dienes. Ciprofibrate treatment did not affect either endpoint, but catalase overexpression increased the concentrations of malondialdehyde (in untreated mice only) and conjugated dienes (in both untreated and ciprofibrate-fed mice). Oxidative DNA damage was estimated by quantifying 8-hydroxydeoxyguanosine (8-OHdG) by high-performance liquid chromatography/electrochemical detection. Ciprofibrate treatment significantly increased hepatic 8-OHdG concentrations, in agreement with several previous studies, but catalase overexpression did not significantly affect them, although 8-OHdG concentrations were decreased 50% in untreated mice. These results imply that the metabolism of hydrogen peroxide by catalase is not an important factor in the development of hepatic lipid peroxidation. The decrease in hepatic 8-OHdG in untreated transgenic mice and the increase seen after ciprofibrate administration imply that hydrogen peroxide is important in the formation of 8-OHdG. While the lack of decreased 8-OHdG levels in ciprofibrate-treated transgenic mice does not support this conclusion, it is possible that catalase levels were not sufficiently high to affect this endpoint. Transgenic mice with higher hepatic catalase activities may be required to resolve this issue.

Liver-specific catalase expression in transgenic mice inhibits NF-kappaB activation and DNA synthesis induced by the peroxisome proliferator ciprofibrate

Peroxisome proliferators are a group of non-genotoxic hepatic carcinogens that have been proposed to act by increasing oxidative damage in the liver. To test this hypothesis, we have examined if hepatic catalase overexpression in peroxisome proliferator-treated mice influences the induction of cell proliferation or the activation of transcription factors involved in cell proliferation. Transgenic mice or non-transgenic littermates were fed either 0.01% ciprofibrate or a control diet for 21 days. Fatty acyl CoA oxidase activity was not significantly affected by catalase overexpression, although the ratio of fatty acyl CoA oxidase to catalase was significantly decreased in transgenic animals. The labeling index in hepatocytes was significantly increased by ciprofibrate in non-transgenic mice, but catalase overexpression significantly inhibited this increase. Ciprofibrate increased the activation of nuclear factor (NF)-kappaB in non-transgenic mice, but this increase was inhibited by catalase overexpression. Ciprofibrate also increased AP-1 activation, but catalase overexpression did not significantly inhibit this increase, although AP-1 activation was 40% lower in transgenic mice. These results support the hypothesis that active oxygen plays a role in the induction of cell proliferation by the peroxisome proliferator ciprofibrate and therefore may be important in the carcinogenicity of these agents.