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.

Hepatic expression of acute-phase protein genes during carcinogenesis induced by peroxisome proliferators

Concern exists regarding peroxisome proliferator (PP) xenobiotic exposure because many PPs are potent hepatocarcinogens in rodents. The mechanism of carcinogenicity induced by PPs is atypical compared with those of other hepatocarcinogens in that the former appears to involve alterations in expression of PP-activated receptor (PPAR) target genes rather than direct mutagenicity. To begin to identify some of these genes, we used differential display to compare mRNA expression between hepatic adenomas and adjacent non-tumor liver from rats fed the potent PP Wy-14643 (WY) for 78 wk. Here, we report increased expression of the acute-phase protein (APP) gene alpha-1 antitrypsin (AT) and decreased expression of alpha2-urinary globulin in the tumors. Similar changes were seen in hepatic adenomas induced by a diethylnitrosamine and phenobarbital protocol, indicating a lack of specificity for PP-induced tumors. Additional APP genes, including ceruloplasmin, haptoglobin, beta-fibrinogen, and alpha1-acid glycoprotein were also upregulated in WY-induced tumors but were downregulated in the livers of rats administered a different PP for 13 wk. Mice treated with either WY or di(2-ethylhexyl) phthalate for 3 wk had decreased hepatic AT expression but increased expression of ceruloplasmin and haptoglobin. PPARalpha-null mice showed no hepatic APP gene alteration after PP treatment but had higher basal expression than did wild-type controls. We conclude that PPARalpha activation by several different PPs leads to dysregulation of hepatic APP gene expression in rats and mice. This dysregulation may indicate alterations in cytokine signaling networks regulating both APP gene expression and hepatocellular proliferation.

Inhibition of ciprofibrate-induced hepatocarcinogenesis in the rat by dimethylthiourea, a scavenger of hydroxyl radical

DNA damage caused by oxidative stress is considered to play an important role in peroxisome proliferator-induced hepatocarcinogenesis in rats and mice. In this study, we investigated the effect of dimethylthiourea (DMTU), a known hydroxyl radical scavenger, on ciprofibrate-induced hepatocarcinogenesis. Male F-344 rats were fed a diet containing 0.025% ciprofibrate and given daily intraperitoneal injections of DMTU (5 days a week) at a dose of 50 or 250 mg/kg body weight for 60 weeks at which time the study was terminated. Livers from all animals were analyzed grossly and microscopically for incidence, number and type of tumors. All rats given ciprofibrate alone developed altered areas, neoplastic nodules (NN) and hepatocellular carcinomas (HCC). Combined administration of ciprofibrate and DMTU resulted in inhibition of tumor development. In the group given higher doses of DMTU the incidence of NN was 100% and HCC 0%. The number of tumors per liver also significantly decreased (p<0.001). At lower dose levels DMTU caused significant reduction in the number of tumors per liver (p<0. 05) and a slight reduction (29%) in the incidence of HCC. The inhibitory effect of DMTU on ciprofibrate-induced hepatocarcinogenesis could be explained by hydroxyl radical scavenging properties of DMTU, resulting in decreased free radical induced DNA damage.

Peroxisome proliferators: mechanisms of adverse effects in rodents and molecular basis for species differences

Peroxisome proliferators (PPs), such as diethylhexylphthalate (DEHP), constitute a diverse class of chemicals with many therapeutic, industrial and environmental applications. In rodents, PPs are nongenotoxic hepatocarcinogens, raising concerns regarding the potential of PPs to harm human health. However, humans differ from rodents in their response to PPs and the weight of evidence supports the supposition that PPs do not pose a carcinogenic risk to humans. The effects of PPs in the rodent are mediated by peroxisome proliferator activated receptor alpha (PPARalpha). PPARalpha predominates in the liver whereas another isoform PPARgamma predominates in adipose tissue and in the immune system. This tissue-specific pattern of PPARalpha expression is consistent with a role for PPARalpha but not PPARgamma or PPARbeta in PP-induced rodent hepatocarcinogenesis. Humans, marmosets and guinea-pigs appear refractory or less responsive to the adverse liver effects of PPs. However, humans give a therapeutic response to the fibrate PPs via an alteration in lipid metabolism mediated by PPARalpha. Such marked species differences may be explained by quantity of PPARalpha and/or the quality of the PPARalpha-mediated response. The lower expression of full-length functional PPARalpha in humans could be attributed to the presence of a truncated, inactive form of PPARalpha, which appears to be present in most individuals examined to date. In addition, there are species differences in sequence and responsiveness of the acyl CoA oxidase (ACO) gene promoter, suggesting that even in the presence of sufficient PPARalpha, the human equivalent of rodent genes associated with peroxisome proliferation may remain inactive.

Dietary glycine prevents the development of liver tumors caused by the peroxisome proliferator WY-14,643

Previous studies demonstrated that dietary glycine prevents elevated rates of cell proliferation following treatment with the peroxisome proliferator and liver carcinogen WY-14,643. Since increased cell replication is associated with the development of hepatic cancer caused by peroxisome proliferators, glycine may have anti-cancer properties. Therefore, experiments were designed to test the hypothesis that dietary glycine would inhibit the hepatocarcinogenic effect of WY-14,643. Male F344 rats were fed four different NIH 07-based diets: 5% glycine; 5% valine for nitrogen balance (control); 0.1% WY-14,643 + 5% valine (WY-14,643); 0.1% WY-14,643 + 5% glycine (WY-14,643 + glycine). Food consumption did not differ among the groups, but WY-14,643-fed rats weighed 10-25% less than expected based on previous studies. Serum glycine levels were elevated 4-5-fold by glycine-containing diets; however, the 10-fold increase in peroxisomal enzyme activity caused by WY-14,643 was unaffected by the addition of 5% glycine to the diet. After 22 weeks, livers from rats fed WY-14,643 had a similar incidence and multiplicity of proliferative lesions (foci and adenomas) to those fed WY-14,643 + glycine. Moreover, cell proliferation in the surrounding 'normal' parenchyma (labeling index approximately 4%) and foci (labeling index approximately 50%) did not differ between WY-14,643 and WY-14,643 + glycine-fed rats. However, after 51 weeks of dietary exposure to WY-14,643, glycine prevented formation of small (0-5 mm diameter) tumors by 23% and inhibited the development of medium size (5-10 mm) tumors by 64%. Furthermore, glycine prevented the formation of the largest tumors (>10 mm) by nearly 80%. Thus, glycine did not inhibit early foci formation; however, it significantly decreased their ability to progress to tumors. Moreover, the inhibitory effect of glycine was greater with increasing tumor size. These studies demonstrate that dietary glycine prevents the development of hepatic tumors caused by the peroxisome proliferator WY-14,643 consistent with the idea that it may be an effective chemopreventive agent.

Investigation of the potential of conjugated linoleic acid (Cla) to cause peroxisome proliferation in rats

Conjugated linoleic acid (CLA) is reported as having several beneficial effects including anticarcinogenic, cholesterol-lowering and anti-atherogenic properties; however, CLA has also been reported as a putative peroxisome proliferator in mice. In this study the ability of CLA to cause peroxisome proliferation in the rat, as measured by accepted enzyme markers was investigated. Male Wistar rats were fed a semi-purified diet containing 0.0, 1.5 or 5.0 energy % CLA for 4 weeks. A positive control group were given 250 mg clofibrate/kg by gavage for 4 days. Hepatic cyanide-insensitive palmitoyl coenzyme A (PCoA) oxidase and carnitine acetyl transferase (CAT) activities and total cytochrome P450 (CYP) levels were measured. CLA had no effect on body weight or liver/body weight ratios, but clofibrate significantly increased mean liver/body weight ratio by 41.6%. Clofibrate-treated rats showed typical changes with increases in hepatic PCoA oxidase and CAT activity (5.8-fold and 22.8-fold) and in total CYP (1.66-fold) compared with control. There were no differences between the control group and the groups fed CLA for either the peroxisomal enzymes or total CYP. These results suggest that CLA does not act in the rat as a classical peroxisome proliferator and that there may be a species difference in the effects on rat and mice.

Comparison of the effects of various peroxisome proliferators on peroxisomal enzyme activities, DNA synthesis, and apoptosis in rat and human hepatocyte cultures

Peroxisome proliferators (PPs) are a class of rodent nongenotoxic hepatocarcinogens that cause hepatocyte peroxisome proliferation, increased DNA synthesis, and decreased spontaneous apoptosis. We examined the effects of various PPs such as the hypolipidemic agents clofibric acid (CLO), bezafibrate (BEZA), ciprofibrate (CIPRO), and nafenopin (NAFE) and the plasticizer di-(2-ethylhexyl)phthalate (DEHP) on the various parameters in vitro in rat and human hepatocyte cultures. In rat hepatocyte cultures, after 72 h of treatment with the various PPs at 100-500 microM, a compound-dependent increase in acyl CoA oxidase (ACO) and carnitine acetyl transferase (CAT) activities, markers of peroxisome proliferation, was observed with the following potencies: CIPRO = NAFE > BEZA > CLO > DEHP. A minor (120-150%), but significant, no concentration-dependent increase in DNA synthesis and a marked, no compound-dependent and, with the exception of NAFE, no concentration-dependent 60-80% decrease in spontaneous apoptosis was observed with all tested compounds (50-250 microM) after 48 h of treatment. Inhibition of spontaneous apoptosis in PP-treated versus control rat hepatocyte cultures was also observed morphologically. Furthermore, PPs inhibited transforming growth factor beta (TGFbeta)-induced apoptosis but not tumor necrosis factor alpha (TNFalpha)/alpha Amanitine (alphaAma)-induced apoptosis in rat hepatocyte cultures. In human hepatocyte cultures, the various PPs at 50-500 microM did not affect peroxisomal enzyme activities, DNA synthesis, or spontaneous and induced (TGFbeta or TNFalpha/alphaAma) apoptosis. The compound-dependent peroxisome proliferation but no compound-dependent disruption of the mitogenic/apoptotic balance elicited by PPs in primary rat hepatocyte cultures supports the hypothesis that oxidative stress is directly linked to the hepatocarcinogenic potential of a given PP in rodents and that disruption of the mitogenic/apoptotic balance contributes to the development of PP-induced hepatocarcinogenesis. In addition, the absence of effects of all PPs on both peroxisome proliferation-associated parameters and mitogenic/apoptotic balance supports the hypothesis that human liver cells are refractory to PP-induced hepatocarcinogenesis.

The extracellular signal-regulated kinase pathway contributes to mitogenic and antiapoptotic effects of peroxisome proliferators in vitro

Peroxisome proliferators are a class of nongenotoxic rodent hepatocarcinogens thought to induce tumors by altering the balance between mitosis and apoptosis. Previous studies suggest mitogenic growth factors that act through the extracellular signal-regulated kinase (ERK) pathway, including insulin and epidermal growth factor (EGF), modulate peroxisome proliferator-activated receptor alpha activation as well as the mitogenic activity of peroxisome proliferators. We have investigated whether the ERK pathway plays a role in regulating the growth and survival altering properties of peroxisome proliferators in primary mouse hepatocytes. Exposure of hepatocytes to Wy-14,643 and trichloroacetate resulted in a dose-dependent phosphorylation and activation of ERK. Peroxisome proliferator-induced ERK phosphorylation was blocked when cells were pretreated with the MEK (ERK kinase) inhibitor, PD098059, or the phosphatidyl-inositol 3-kinase (PI3K) inhibitors, LY294002 and apigenin, suggesting that both MEK and PI3K are involved in the initial response. The pathway leading to peroxisome proliferator-induced ERK activation is different than that induced by phorbol ester or EGF, since the PI3K inhibitors had no effect on ERK phosphorylation induced by these agents. Under defined culture conditions, Wy-14,643 increased the level of BrdU incorporation in primary hepatocytes and suppressed the incidence of apoptosis induced by transforming growth factor beta 1. In contrast, concentrations of PD098059 that block Wy-14,643-induced ERK phosphorylation also blocked the stimulation of DNA replicative synthesis and suppression of apoptosis by Wy-14,643. These studies indicate that activation of the ERK pathway through a PI3K-dependent mechanism may play a significant role in the tumor-promoting properties of peroxisome proliferators.

Chronic peroxisome proliferation and hepatomegaly associated with the hepatocellular tumorigenesis of di(2-ethylhexyl)phthalate and the effects of recovery

This study compared the levels of cell proliferation and peroxisome proliferation in rodent liver with tumor incidence, to provide more information on the relationship between these events following chronic exposure. Fischer 344 rats were treated with 0, 100, 500, 2500, or 12,500 ppm DEHP, and B6C3F1 mice were treated with 0, 100, 500, 1500, or 6000 ppm DEHP in the diet for up to 104 weeks. Additional groups of rats and mice received the highest concentration for 78 weeks and then the control diet for an additional 26 weeks (recovery groups). Animals were terminated at weeks 79 and 105 for histopathologic examination. Elevated palmitoyl CoA oxidation activity and higher liver-to-body weight ratios were observed for the 2500- and 12,500-ppm groups of rats, and for the 500-, 1500-, and 6000-ppm groups of mice at Week 105. No increase in palmitoyl CoA oxidation activity was evident in the recovery group, and relative liver weights were near control levels following recovery. No hepatic cell proliferation was detected at Weeks 79 or 105 in either species although preliminary data indicated that cell proliferation did occur within the first 13 weeks of exposure. A significantly higher incidence of hepatocellular tumors was only observed for the 2500- and 12,500-ppm group and its recovery group of rats, and for the 500-, 1500-, and 6000-ppm groups and the recovery group of mice. The tumor incidences were reduced for the recovery groups compared with the groups fed DEHP continuously for 104 weeks. The data indicate that high levels of peroxisome proliferation and hepatomegaly are associated with DEHP hepatocarcinogenesis in rodent liver, and that the tumorigenic process may be arrested by cessation of DEHP treatment, suggesting that extended treatment with DEHP acts to promote tumor growth.

Effects of di-isononyl phthalate (DINP) on peroxisomal markers in the marmoset-DINP is not a peroxisome proliferator

In the present study the systemic toxic potential of di-isononyl phthalate (DINP) was assessed in a 13-week study in marmosets. Particular attention was given to its potential for hepatic peroxisome proliferation. Three groups of four male and four female marmosets received DINP, by oral gavage administration, at dosages of 100, 500 or 2500 mg/kg/day for 13 weeks. A fourth group served as a concurrent Control group and received the vehicle (1% methylcellulose and 0.5% Tween) only. A fifth group received clofibrate at a dosage of 500 mg/kg/day to provide a positive Control for liver peroxisome activity. At the end of the treatment period, the animals were killed and their livers were removed. 3000 x g supernatant and microsomal subcellular fractions were prepared from homogenised liver by differential centrifugation. The peroxisomal marker enzyme activity, cyanide-insensitive palmitoyl CoA oxidase, was assayed in the former, while cytochrome P450 concentration and lauric acid 11- and 12-hydroxylase activities (selective for CYP2E1 and 4A, respectively) were assayed in the microsomes. No statistically significant changes were seen in any of these parameters measured following DINP treatment, compared with the Control. Clofibrate treatment resulted in an approximately 100% increase (p < 0.01) in both male and female marmoset cyanide-insensitive palmitoyl CoA oxidase activity and a similar increase (p < 0.05) in male (only) lauric acid 11-hydroxylase activity. No other changes were statistically significant at the 5% level. These data provide no evidence that DINP was acting as a peroxisome proliferator when administered to marmosets under the conditions of the study.

Dose-response hapatotoxicity of the peroxisome proliferator, perfluorodecanoic acid and the relationship to phospholipid metabolism in rats

Perfluorodecanoic acid (PFDA) is a potent peroxisome proliferator that causes hepatotoxicity but lacks tumor-promoting activity in rats. We previously showed that a single dose of PFDA at 50 mg/kg (approximately LD50) causes an elevation in liver phosphocholine (PCho) and other effects related to phospholipid metabolism. In this study, we examined metabolic effects in the dose range 2-50 mg/kg in rats. At doses < or =20 mg/kg, PFDA is significantly less hepatotoxic than the LD50 as manifested by electron microscopy and measurements of daily food consumption and body weight. At 50 mg/kg rat serum tumor necrosis factor (TNF)-alpha concentration was increased 8-fold, while at 15 mg/kg there was no apparent increase in this cytokine. This lower dose, however, induces metabolic effects similar to those seen at the LD50. Liver fatty acyl-CoA oxidase activity showed a dose-dependent increase from 5-25 mg/kg PFDA. Treatments at 15 and 50 mg/kg caused a significant increase in liver phosphatidylcholine (28 and 66%) and phosphatidylethanolamine (31 and 74%). Both doses caused a significant increase in liver PCho but did not affect liver ATP levels, as manifested in 31P nuclear magnetic resonance (NMR) spectra from rat livers in vivo. These data suggest that the increase in liver [PCho] observed following PFDA exposure in rats represents a specific metabolic response, rather than a broad-range hepatotoxic effect.

A cancer risk assessment of di(2-ethylhexyl)phthalate: application of the new U.S. EPA Risk Assessment Guidelines

The current United States Environmental Protection Agency (EPA) classification of di(2-ethylhexyl)phthalate (DEHP) as a B2 "probable human" carcinogen is based on outdated information. New toxicology data and a considerable amount of new mechanistic evidence were used to reconsider the cancer classification of DEHP under EPA's proposed new cancer risk assessment guidelines. The total weight-of-evidence clearly indicates that DEHP is not genotoxic. In vivo administration of DEHP to rats and mice results in peroxisome proliferation in the liver, and there is strong evidence and scientific consensus that, in rodents, peroxisome proliferation is directly associated with the onset of liver cancer. Peroxisome proliferation is a transcription-mediated process that involves activation by the peroxisome proliferator of a nuclear receptor in rodent liver called the peroxisome proliferator-activated receptor (PPARalpha). The critical role of PPARalpha in peroxisomal proliferation and carcinogenicity in mice is clearly established by the lack of either response in mice genetically modified to remove the PPARalpha. Several mechanisms have been proposed to explain how, in rodents, peroxisome proliferation can lead to the formation of hepatocellular tumors. The general consensus of scientific opinion is that PPARalpha-induced mitogenesis and cell proliferation are probably the major mechanisms responsible for peroxisome proliferator-induced hepatocarcinogenesis in rodents. Oxidative stress appears to play a significant role in this increased cell proliferation. It triggers the release of TNFalpha by Kupffer cells, which in turn acts as a potent mitogen in hepatocytes. Rats and mice are uniquely responsive to the morphological, biochemical, and chronic carcinogenic effects of peroxisome proliferators, while guinea pigs, dogs, nonhuman primates, and humans are essentially nonresponsive or refractory; Syrian hamsters exhibit intermediate responsiveness. These differences are explained, in part, by marked interspecies variations in the expression of PPARalpha, with levels of expression in humans being only 1-10% of the levels found in rat and mouse liver. Recent studies of DEHP clearly indicate a nonlinear dose-response curve that strongly suggests the existence of a dose threshold below which tumors in rodents are not induced. Thus, the hepatocarcinogenic effects of DEHP in rodents result directly from the receptor-mediated, threshold-based mechanism of peroxisome proliferation, a well-understood process associated uniquely with rodents. Since humans are quite refractory to peroxisomal proliferation, even following exposure to potent proliferators such as hypolipidemic drugs, it is concluded that the hepatocarcinogenic response of rodents to DEHP is not relevant to human cancer risk at any anticipated exposure level. DEHP should be classified an unlikely human carcinogen with a margin of exposure (MOE) approach to risk assessment. The most appropriate and conservative point of reference for assessing MOEs should be 20 mg/kg/day, which is the mouse NOEL for peroxisome proliferation and increased liver weight. Exposure of the general human population to DEHP is approximately 30 microg/kg body wt/day, the major source being from residues in food. Higher exposures occur occupationally [up to about 700 microg/kg body wt/day (mainly by inhalation) based on current workplace standards] and through use of certain medical devices [e.g., up to 457 microg/kg body wt/day for hemodialysis patients (intravenous)], although these have little relevance because the routes of exposure bypass critical activation enzymes in the gastrointestinal tract.

Beneficial effects of fibrates on apolipoprotein A-I metabolism occur independently of any peroxisome proliferative response

BACKGROUND

In humans, fibrates are frequently used normolipidemic drugs. Fibrates act by regulating genes involved in lipoprotein metabolism via activation of the peroxisome proliferator-activated receptor-alpha (PPARalpha) in liver. In rodents, however, fibrates induce a peroxisome proliferation, leading to hepatomegaly and possibly hepatocarcinogenesis. Although this peroxisome proliferative response appears not to occur in humans, it remains controversial whether the beneficial effects of fibrates on lipoprotein metabolism can occur dissociated from such undesirable peroxisomal response. Here, we assessed the influence of fenofibrate on lipoprotein metabolism and peroxisome proliferation in the rabbit, an animal that, contrary to rodents and similar to humans, is less sensitive to peroxisome proliferators.

METHODS AND RESULTS

First, we demonstrate that in normal rabbits, fenofibrate given at a high dose for 2 weeks does not influence serum concentrations or intestinal mRNA levels of the HDL apolipoprotein apoA-I. Therefore, the study was continued with human apoA-I transgenic rabbits that overexpress the human apoA-I gene under control of its homologous promoter, including its PPAR-response elements. In these animals, fenofibrate increases serum human apoA-I concentrations via an increased expression of the human apoA-I gene in liver. Interestingly, liver weight or mRNA levels and activity of fatty acyl-CoA oxidase, a rate-limiting and marker enzyme of peroxisomal beta-oxidation, remain unchanged after fenofibrate.

CONCLUSIONS

Expression of the human apoA-I transgene in rabbit liver suffices to confer fibrate-mediated induction of serum apoA-I. Furthermore, these data provide in vivo evidence that the beneficial effects of fibrates on lipoprotein metabolism occur mechanistically dissociated from any deleterious activity on peroxisome proliferation and possibly hepatocarcinogenesis.

Effect of D-tagatose on liver weight and glycogen content of rats

D-tagatose is an incompletely absorbed ketohexose (stereoisomer of D-fructose) which has potential as an energy-reduced alternative sweetener. In an earlier 90-day toxicity study, rats fed diets with 10, 15 and 20% D-tagatose exhibited increased liver weights, but no histopathological alterations. To determine whether there might be any toxicological relevance to this effect, three studies were conducted in male, adult Sprague-Dawley rats. In the first study, four groups received Purina diet (group A), Purina diet with 20% D-tagatose (group B), SDS diet (group C), or SDS diet with 20% D-tagatose (group D). For groups A and B, the 28-day treatment period was followed by a 14-day recovery period (Purina diet). Food remained available to all animals until the time of sacrifice. Groups of 10 rats were killed on days 14 (groups A and B), 28 (groups A-D), and 42 (groups A and B). Body weights, as well as weights of wet and lyophilized livers, were determined. The lyophilized livers collected on day 28 from groups A and B were analyzed for protein, total lipid, glycogen, DNA, and residual moisture. By day 14, relative wet liver weights had increased by 23% in group B. On day 28, the increase was 38% in group B and 44% in group D. At the end of the recovery period, the increase had diminished to 14% in group B. On day 28, liver glycogen content (in %) was significantly increased, and liver protein, lipid, and DNA contents were significantly decreased in group B compared to group A. Total amounts per liver of protein, total lipid, glycogen, and DNA were significantly increased. In the second study, four groups of 20 rats each received SDS diet with 0, 5, 10, and 20% D-tagatose for 29-31 days. The food was available until the time of sacrifice. At termination, plasma was obtained from 10 rats/group for clinicochemical analyses. Five rats/group were subjected to whole-body perfusion, followed by processing of livers for qualitative and quantitative electron microscopic examination. Livers of 6 rats/group were analyzed for acyl-CoA oxidase and laurate 12-hydroxylase (cytochrome P450 4A1) activity, DNA synthesis (Ki-67 index), and number of nuclei per unit area of tissue. Liver weights were significantly increased in linear relation to the D-tagatose intake. Plasma transaminases (but not glutamyl transferase and alkaline phosphatase) were increased in the high-dose group. Except for glycogen accumulation, no ultrastructural changes were seen on electron microscopic examination of livers of the control and high-dose groups. Morphometric analysis confirmed the increase of glycogen and the absence of alterations of endoplasmatic reticulum, mitochondria, and Golgi apparatus. The Ki-67 index did not differ between the groups. A dose-related decrease of the number of nuclei per unit area signified some hepatocellular hypertrophy. Acyl-CoA oxidase and CYP4A1 activity were significantly increased in the mid- and high-dose groups, but these increases were small and not accompanied by electron-microscopic evidence of peroxisome proliferation. In the third study, four groups received SDS diet (groups A and C) or SDS diet with 5% D-tagatose (groups B and D). All animals were killed on day 28. Groups A and B were fasted for 24 h before sacrifice; groups C and D had food available until sacrifice. Liver weights and liver composition were measured as in Study 1. Relative wet and dry liver weights were increased in response to the treatment in rats killed under the fed condition, but not in rats killed under the fasted condition. The livers of the treated rats (group D) had an increased glycogen content in comparison to the controls (group C). Taken together, these results demonstrate that D-tagatose at dietary levels of 5-20% increases liver glycogen deposition and relative liver weights in nonfasting rats. In fasted rats the 5% dose level is the no-effect level. (ABSTRACT TRUNCATED)

The peroxisome proliferator (PP) response element upstream of the human acyl CoA oxidase gene is inactive among a sample human population: significance for species differences in response to PPs

Peroxisome proliferators (PP) cause peroxisome proliferation, associated with rodent hepatocyte growth perturbation and hepatocarcinogenesis. However, in humans this class of non-genotoxic carcinogens does not appear to have the same adverse effects. The peroxisome proliferator-activated receptor alpha (PPARalpha) mediates the effects of PPs in rodents via peroxisome proliferator response elements (PPREs) upstream of PP-responsive genes such as acyl coenzyme A oxidase (ACO). When the human ACO promoter was cloned previously, it was found to be active and to contain a consensus PPRE (-1918 AGGTCA C TGGTCA -1906). To confirm and extend those original findings, we isolated a 2 kb genomic fragment of the ACO gene promoter from a human liver biopsy and used it to create a beta-galactosidase reporter gene plasmid. The human ACO promoter reporter plasmid was added to both Hepalclc7 and NIH 3T3 cells together with a plasmid expressing mPPARa and assessed for its ability to drive PP-mediated gene transcription. The human ACO promoter fragment was inactive, unlike the equivalent rat ACO promoter fragment used as a positive control. The PPRE within our cloned fragment of the human ACO promoter differed at three positions (5'-AGGTCA G CTGTCA-3') from the previously published active human ACO promoter. Next, we studied the frequency of the inactive versus the active human PPRE within the human population. Using a PCR strategy, we isolated and analysed genomic DNA fragments from 22 unrelated human individuals and from the human hepatoma cell line HepG2. In each case, the PPRE contained the inactive sequence. These data show that the human ACO gene promoter found in a sample human population is inactive. This may explain at the genomic level the lack of response of humans to some of the adverse effects of the PP class of non-genotoxic hepatocarcinogens.

Carcinogenic aspect of xenobiotic molecules belonging to the peroxisome proliferator family

It is known that a short-term exposure of rat, mice or incubation of hepatic cells with fibrate molecules leads to increase in peroxisome number and cell hyperplasia. Further, long-term incubation of cells (at least a year) show transformed characteristics with foci and nodules. To explain the hepatocarcinogenic effect of peroxisome proliferators in rodents we studied the effect of peroxisome proliferators on rat liver oncogenes expression. Earlier, we reported an increase in liver and kidney mRNA level of c-myc and N-myc. Since several metabolic genes are activated by PPAR (peroxisome proliferators activated receptor) through a PPRE (peroxisome proliferator response element), we suggest the involvment of PPAR in oncogene activation, because of the presence of PPRE in the N-myc 5'-upstream region. We showed by flow cytometric analysis that ciprofibrate increased the size of rat Fao derived cell line and the activity of palmitoyl CoA oxidase, a peroxisome proliferation enzyme marker for studying peroxisome proliferation was increased. The above effects which can contribute to hepatocarcinogenesis seem to be restricted to rat and mice, which show strong response to peroxisome proliferators. Indeed, no changes are observed in weak responsive species such as humans (using hepatic derived cell lines) and guinea pig. These data provide arguments for the non-carcinogenic effect of this xenobiotic class in human especially when sensitive, or normal individuals are exposed either to hypolipidaemic agents of the fibrate family.

Peroxisome proliferator-activated receptor alpha: role in rodent liver cancer and species differences

Peroxisome proliferators (PPs) are chemicals of industrial and pharmaceutical importance that elicit liver carcinogenesis by a non-genotoxic mechanism. One of the intriguing properties of PPs is that the pleiotropic effects of these compounds (including increased DNA synthesis and peroxisome proliferation) are seen in rats and mice only, but not humans. It is important to determine the risks to humans of environmental and therapeutic exposure to these compounds by understanding the mechanisms of non-genotoxic hepatocarcinogenesis in rodents. To understand this apparent lack of human susceptibility, attention has focused on the peroxisome proliferator-activated receptor alpha (PPARalpha), which appears to mediate the effects of PPs in rodents. It is also known to mediate the hypolipidaemic effects that fibrate drugs exert on humans with elevated plasma cholesterol and triglyceride levels. Human PPARalphas share many functional characteristics with the rodent receptors, in that they can be transcriptionally activated by PPs and regulate specific gene expression. However, one key difference is that PPARalpha is less abundant in human than in rodent liver, which has led to the suggestion that species differences result from quantitative differences in gene expression. In this review we describe the effects of PPs and what is known of the molecular mechanisms of action and species differences with respect to rodents and man. Attention will be given to differences in the amounts of PPARalpha between species as well as the 'qualitative' aspects of PPARalpha-mediated gene regulation which might also explain the activation of some genes and not of others in human liver by PPs.

Kupffer cell oxidant production is central to the mechanism of peroxisome proliferators

Increased cell proliferation most likely plays a key role in peroxisome proliferator-induced liver cancer. Recently, Kupffer cells were shown to be responsible for Wy-14,643-induced cell proliferation. However, the mechanism by which peroxisome proliferators activate Kupffer cells is unknown. Since gut-derived endotoxin is a known activator of Kupffer cells, the hypothesis that it is involved was evaluated. Increased cell proliferation and peroxisome induction were unaffected by gut sterilization. Moreover, endotoxin was not detectable in portal blood following treatment with Wy-14,643. Therefore, it is concluded that gut-derived endotoxin is not responsible for Kupffer cell activation. To test the hypothesis that Kupffer cells are activated by Wy-14,643 directly, Kupffer cell superoxide production was measured following treatment in vitro. Wy-14,643 increased superoxide production in a dose-dependent manner (0.1 and 50 microM) with half-maximal stimulation at 2.5 microM. Diethylhexylphthalate (DEHP) and ethylhexanol did not increase superoxide production even at doses 50 times higher than Wy-14,643; however, monoethylhexylphthalate (MEHP) activated superoxide production as effectively as Wy-14,643 with half-maximal stimulation at 5 microM. Treatment with Wy-14,643 for 21 days caused a 2-fold increase in Kupffer cell superoxide production while DEHP did not. Pretreatment of Kupffer cells with staurosporine (0.01-10 pM) completely blocked generation of superoxide demonstrating that protein kinase C is required. Moreover, Wy-14,643 increased Kupffer cell protein kinase C activity 3-fold. Pretreatment of Kupffer cells with the amino acid glycine (0.01-3 mM), which blunts calcium signaling, inhibited Wy-14,643-stimulated superoxide production and increased protein kinase C activity completely. These data are consistent with the hypothesis that potent peroxisome proliferators (Wy-14,643 and MEHP) directly activate Kupffer cell production of oxidants via mechanisms involving protein kinase C. Further, peroxisome proliferator treatments that sustain elevated rates of cell proliferation (e.g. Wy-14,643) activate Kupffer cell superoxide production following long-term dietary treatment supporting the hypothesis that Kupffer cell-derived oxidants are involved in peroxisome proliferator-induced neoplasia.

Peroxisome proliferator-activated receptors (PPARS) and carcinogenesis

Peroxisome proliferators (PPs) are an important group of chemicals that include certain hypolipidemic drugs, plasticizers and pollutants. Many of these agents are known rodent liver tumor promoters and debate exists as to whether humans are at increased cancer risk following exposure to PPs. Research over the last decade has focused on determining the biochemical and molecular mechanisms by which peroxisome proliferators exert their effects, in the hope that this controversy will be settled. PPs regulate gene expression via a steroid hormone receptor, the peroxisome proliferator-activated receptor (PPAR). At least three subtypes of PPAR (alpha, beta and gamma) have been cloned from several species, including humans. These receptors have been implicated in tumor promotion, cellular differentiation, and apoptosis. In the present article, the current understanding of how PPARs are involved in tumorigenesis, and what this may mean to human risk assessment, will be discussed.

Peroxisome proliferator-activated receptor (PPAR) alpha-regulated growth responses and their importance to hepatocarcinogenesis

Peroxisome proliferators (PPs) are a class of non-genotoxic rodent hepatocarcinogens that act by perturbing liver growth regulation. We have demonstrated previously that PPs suppress both spontaneous rat hepatocyte apoptosis and that induced by exogenous stimuli such as transforming growth factor-beta1 (TGF beta1). More recently, we have demonstrated that PPs can suppress apoptosis induced by more diverse stimuli such as DNA damage or ligation of Fas, a receptor related to the tumour necrosis factor alpha (TNF alpha) family of cell surface receptors. PPs transcriptionally activate the peroxisome proliferator activated receptor-alpha, PPAR alpha, a member of the nuclear hormone receptor superfamily. We investigated whether activation of PPAR alpha mediates the suppression of rat hepatocyte apoptosis induced by PPs. We isolated a naturally occurring variant form of PPAR alpha (hPPAR alpha-6/29) from human liver by PCR cloning. hPPAR alpha-6/29 shared the ability of mPPAR alpha to bind to DNA but, unlike mPPAR alpha, could not be activated by PPs. Furthermore, hPPAR alpha-6/29 could act as a dominant negative regulator of PPAR-mediated gene transcription. When introduced into primary rat liver cell cultures by transient transfection, hPPAR alpha-6/29 prevented the suppression of hepatocyte apoptosis by the PP nafenopin, but not that seen in response to phenobarbitone (PB), a non-genotoxic carcinogen whose action does not involve PPAR alpha. The suppression of hepatocyte apoptosis was abrogated completely even though only 30% of hepatocytes were transfected, suggesting the involvement of a soluble factor. Recent data have suggested that TNF alpha, perhaps released by liver Kupffer cells in response to PPs, may play a key role in mediating the effects of PPs on hepatocyte growth regulation.

Peroxisome proliferator-activated receptor (PPAR) alpha activation and its consequences in humans

Amphipathic carboxylates collectively defined as peroxisome proliferators (PP) induce in rodents a pleiotropic effect, mediated by the peroxisome proliferator-activated receptor alpha (PPAR alpha). Treatment with PP results in rodents in hypolipidemia, peroxisome proliferation and liver hypertrophy and hyperplasia leading to non-genotoxic hepatocarcinogenesis. In contrast to rodents, the hypolipidemic effect exerted by PP in humans is not accompanied by peroxisome proliferation nor by induction of peroxisomal beta-oxidation or other activities induced by PP in rodents. Non-responsiveness in humans may be ascribed to a missing liver component in the PPAR alpha transduction pathway specifically involved with transcriptional modulation of chromosomal PPAR alpha responsive genes. Hence, biological effects exerted by PP in the human liver are likely to be mediated by a transduction pathway independent of PPAR alpha.

Inhibition of gap junctional intercellular communication by perfluorinated fatty acids is dependent on the chain length of the fluorinated tail

Perfluorinated fatty acids (PFFAs), such as perfluorooctanoic acid (PFOA) and perfluorodecanoic acid (PFDA), are known peroxisome proliferators and hepatocarcinogens. A causal link between an increase in the oxidative stress by peroxisomes and tumor promotion has been proposed to explain the hepatocarcinogenicity of PFOA and PFDA. However, the down-regulation of gap junctional intercellular communication (GJIC) has also been linked to the tumor-promoting properties of many carcinogens. Therefore, the effect of PFFAs on GJIC in WB-rat liver epithelial cells was determined. The chain length of the PFFAs tested for an effect on GJIC ranged from 2 to 10, 16 and 18 carbons. Carbon lengths of 7 to 10 inhibited GJIC in a dose-response fashion, whereas carbon lengths of 2 to 5, 16 and 18 did not appreciably inhibit GJIC. Inhibition occurred within 15 min and was reversible, with total recovery from inhibition occurring within 30 min after the removal of the compound from the growth medium. This short time of inhibition suggests that GJIC was modified at the post-translational level. Also, this short time period was not long enough for peroxisome proliferation. The post-translational modification of the gap junction proteins was not a consequence of altered phosphorylation as determined by Western blot analysis. Perfluorooctanesulfonic acid also inhibited GJIC in a dose-response fashion similar to PFDA, indicating that the determining factor of inhibition was probably the fluorinated tail, which required 7-10 carbons. Our results suggest that PFFAs could potentially act as hepatocarcinogens at the level of gap junctions in addition to or instead of through peroxisome proliferation.

1,1,1-Trifluoro-2,2-dichloroethane (HCFC-123) and 1,1,1-trifluoro-2-bromo-2-chloroethane (halothane) cause similar biochemical effects in rats exposed by inhalation for five days

1,1,1-Trifluoro-2,2-dichloroethane (HCFC-123) and 1,1,1-trifluoro-2-bromo-2 chloroethane (halothane) are gases with anesthetic properties. HCFC-123 is used as a refrigerant, fire extinquishing agent, and solvent, while halothane is a clinical anesthetic. Much information is available on chronic toxicity of HCFC-123 in animals, while the information available for halothane is from short-term animal exposures or chronic, low level human exposures. Thus, there is little biochemical information available on similar endpoints for these two chemicals, which share common metabolites. In the present study, male rats were exposed to 5000 ppm HCFC-123, 5000 ppm halothane, or room air for 6 hr per day for 5 consecutive days. Rats exposed to both test compounds gained little or no weight during the study. Liver weights were slightly decreased in the rats exposed to HCFC-123 and halothane compared to controls. The serum triglycerides were decreased to approximately 20% of control level in rats exposed to both HCFC-123 and halothane, and serum cholesterol was decreased to less than 80% of control by both compounds. Both test compounds increased hepatic beta-oxidation by approximately 3-fold over control, and HCFC-123 caused a significant increase in hepatic cytochrome P450 content, while the increase in cytochrome P450 was not statistically significant in the halothane-treated rats. The results indicate that HCFC-123 and halothane share not only common metabolic pathways, but also several common biological effects, specifically those associated with peroxisome proliferation. These data indicate that human experience with halothane may be useful in the risk assessment of HCFC-123.

Role of peroxisome proliferator-activated receptor alpha in altered cell cycle regulation in mouse liver

The mechanisms underlying peroxisome proliferator-induced hepatocarcinogenesis are unclear but are mediated by the peroxisome proliferator-activated receptor alpha (PPARalpha). To determine the role of PPARalpha in the mechanisms of hepatocarcinogenesis, the effect of Wy-14,643 on expression patterns of acyl CoA oxidase (ACO) and proteins involved in cell proliferation in the PPARalpha-null mouse were evaluated. ACO, CDK-1, CDK-2, CDK-4, PCNA and c-myc proteins were significantly increased in wild-type mice fed Wy-14,643 for 5 weeks or 11 months, as compared with controls. This effect was not observed in Wy-14,643-treated PPARalpha-null mice. Expression patterns of cyclin B1, cyclin D, cyclin E and p53 were not different in any of the groups. mRNAs encoding CDK-1, CDK-4, cyclin D1 and c-myc were also increased in wild-type mice fed Wy-14,643 but not in PPARalpha-null mice. These results indicate that the increase in CDK-1, CDK-4 and c-myc may be caused by an increase in transcription that is mediated directly or indirectly by PPARalpha. Thus PPARalpha-dependent alterations in cell cycle regulatory proteins induced by peroxisome proliferators are likely to contribute to the hepatocarcinogenicity of peroxisome proliferators.