Non-genotoxic hepatocarcinogens stimulate DNA synthesis and their withdrawal induces apoptosis, but in different hepatocyte populations

Role of Nrf2 in 1,2-dichloropropane-induced cell proliferation and DNA damage in the mouse liver

1,2-Dichloropropane (1,2-DCP) is recognized as the causative chemical of occupational cholangiocarcinoma in printing workers in Japan. However, the cellular and molecular mechanisms of 1,2-DCP-induced carcinogenesis remains elusive. The present study investigated cellular proliferation, DNA damage, apoptosis, and expression of antioxidant and proinflammatory genes in the liver of mice exposed daily to 1,2-DCP for 5 weeks, and the role of nuclear factor erythroid 2-related factor 2 (Nrf2) in these responses. Wild-type and Nrf2-knockout (Nrf2-/-) mice were administered 1,2-DCP by gastric gavage, and then the livers were collected for analysis. Immunohistochemistry for BrdU or Ki67 and TUNEL assay revealed that exposure to 1,2-DCP dose-dependently increased proliferative cholangiocytes, whereas decreased apoptotic cholangiocytes in wild-type mice but not in Nrf2-/- mice. Western blot and quantitative real-time PCR showed that exposure to 1,2-DCP increased the levels of DNA double-strand break marker γ-H2AX and mRNA expression levels of NQO1, xCT, GSTM1, and G6PD in the livers of wild-type mice in a dose-dependent manner, but no such changes were noted in Nrf2-/- mice. 1,2-DCP increased glutathione levels in the liver of both the wild-type and Nrf2-/- mice, suggesting that an Nrf2-independent mechanism contributes to 1,2-DCP-induced increase in glutathione level. In conclusion, the study demonstrated that exposure to 1,2-DCP induced proliferation but reduced apoptosis in cholangiocytes, and induced double-strand DNA breaks and upregulation of antioxidant genes in the liver in an Nrf2-dependent manner. The study suggests a role of Nrf2 in 1,2-DCP-induced cell proliferation, antiapoptotic effect, and DNA damage, which are recognized as key characteristics of carcinogens.

Viral hepatitis induces hepatocellular cancer: what can we learn from epidemiology comparing iran and worldwide findings?

CONTEXT

Several risk factors play the role in the development of hepatocellular carcinoma (HCC) from which chronic hepatitis B and C infections are the most important ones. DNA integration of hepatitis viruses alters the function of critical genes promoting malignant transformation of virus-infected liver cells.

EVIDENCE ACQUISITION

There are remarkable geographic differences in prevalence of chronic viral hepatitis and incidence of HCC. Middle Eastern countries are characterized by a moderate to high prevalence rate of chronic viral hepatitis in the population. This review discusses about epidemiologic findings of hepatitis B and C infections, and HCC, as well as focuses on Middle East countries, particularly Iran. We provide an overview about risk factors, prevention and treatment, and bring up the role of HCC induced by chronic viral hepatitis.

RESULTS

Vaccination against hepatitis B virus (HBV) in the early childhood is highly effective to lower infection rates, substantially. For hepatitis C, adequate hygiene when dealing with human blood and screening programs for blood donors can mainly reduce infection rates. As HCC is strongly associated with chronic viral hepatitis, prevention against the infection is crucial for preventing against HCC too.

CONCLUSIONS

Although prevention and treatment of chronic hepatitis B and C have improved within the last decades even in high-risk countries, effective and sustainable reduction of these infections still needs more actions.

Characterization of a cell culture model for clinically aggressive hepatocellular carcinoma induced by chronic hypoxia

We demonstrated in an in vitro model (human HepG2 liver cells) that chronic hypoxia induced gene expression is associated with an aggressive phenotype in patients with hepatocellular carcinoma (HCC). The aim of this study was to characterize this model further using gene expression microarray, real-time PCR and immunocytochemistry. Subsequently, pathway analysis software was used to identify relevant processes. After examination, we selected 2% O2 during 72 h as conditions to study chronic hypoxia. The most affected signaling is centered on TGF-β1 and PPARα/RXRα. Cells at 2% O2 showed a shift in expression of Epithelial-to-Mesenchymal-Transition (EMT) related genes. Furthermore, a downregulation of liver specific detoxification pathways including cytochrome P450's and glutathione-S-transferases was observed. Both up- and downregulation events within different signaling cascades indicated a cellular adaptation and the onset of a new equilibrium. The prominent role of TGF-β1- and PPARα/RXRα signaling and cell motility pathways warrants their further investigation for therapeutic targets in HCC.

Experimental mouse models for hepatocellular carcinoma research

Every year almost 500,000 new patients are diagnosed with hepatocellular carcinoma (HCC), a primary malignancy of the liver that is associated with a poor prognosis. Numerous experimental models have been developed to define the pathogenesis of HCC and to test novel drug candidates. This review analyses several mouse models useful for HCC research and points out their advantages and weaknesses. Chemically induced HCC mice models mimic the injury-fibrosis-malignancy cycle by administration of a genotoxic compound alone or, if necessary, followed by a promoting agent. Xenograft models develop HCC by implanting hepatoma cell lines in mice, either ectopically or orthotopically; these models are suitable for drug screening, although extrapolation should be considered with caution as multiple cell lines must always be used. The hollow fibre assay offers a solution for limiting the number of test animals in xenograft research because of the ability for implanting multiple cell lines in one mouse. There is also a broad range of genetically modified mice engineered to investigate the pathophysiology of HCC. Transgenic mice expressing viral genes, oncogenes and/or growth factors allow the identification of pathways involved in hepatocarcinogenesis.

PPARα Agonist-Induced Rodent Tumors: Modes of Action and Human Relevance

Widely varied chemicals--including certain herbicides, plasticizers, drugs, and natural products--induce peroxisome proliferation in rodent liver and other tissues. This phenomenon is characterized by increases in the volume density and fatty acid oxidation of these organelles, which contain hydrogen peroxide and fatty acid oxidation systems important in lipid metabolism. Research showing that some peroxisome proliferating chemicals are nongenotoxic animal carcinogens stimulated interest in developing mode of action (MOA) information to understand and explain the human relevance of animal tumors associated with these chemicals. Studies have demonstrated that a nuclear hormone receptor implicated in energy homeostasis, designated peroxisome proliferator-activated receptor alpha (PPARalpha), is an obligatory factor in peroxisome proliferation in rodent hepatocytes. This report provides an in-depth analysis of the state of the science on several topics critical to evaluating the relationship between the MOA for PPARalpha agonists and the human relevance of related animal tumors. Topics include a review of existing tumor bioassay data, data from animal and human sources relating to the MOA for PPARalpha agonists in several different tissues, and case studies on the potential human relevance of the animal MOA data. The summary of existing bioassay data discloses substantial species differences in response to peroxisome proliferators in vivo, with rodents more responsive than primates. Among the rat and mouse strains tested, both males and females develop tumors in response to exposure to a wide range of chemicals including DEHP and other phthalates, chlorinated paraffins, chlorinated solvents such as trichloroethylene and perchloroethylene, and certain pesticides and hypolipidemic pharmaceuticals. MOA data from three different rodent tissues--rat and mouse liver, rat pancreas, and rat testis--lead to several different postulated MOAs, some beginning with PPARalpha activation as a causal first step. For example, studies in rodent liver identified seven "key events," including three "causal events"--activation of PPARalpha, perturbation of cell proliferation and apoptosis, and selective clonal expansion--and a series of associative events involving peroxisome proliferation, hepatocyte oxidative stress, and Kupffer-cell-mediated events. Similar in-depth analysis for rat Leydig-cell tumors (LCTs) posits one MOA that begins with PPARalpha activation in the liver, but two possible pathways, one secondary to liver induction and the other direct inhibition of testicular testosterone biosynthesis. For this tumor, both proposed pathways involve changes in the metabolism and quantity of related hormones and hormone precursors. Key events in the postulated MOA for the third tumor type, pancreatic acinar-cell tumors (PACTs) in rats, also begin with PPARalpha activation in the liver, followed by changes in bile synthesis and composition. Using the new human relevance framework (HRF) (see companion article), case studies involving PPARalpha-related tumors in each of these three tissues produced a range of outcomes, depending partly on the quality and quantity of MOA data available from laboratory animals and related information from human data sources.

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.

Administration of ciprofibrate to lactating mothers induces PPARalpha-signaling pathway in the liver and kidney of suckling rats

It is well known that the hypolipidemic drug ciprofibrate induces peroxisome proliferation in rodent liver, which in turn leads to the oxidative stress, and modifies some parameters related to cell proliferation and apoptosis. The administration of ciprofibrate to rats during the lactating period determined in their pups significant modifications in hepatic peroxisome enzyme activities, induction of the PPARalpha-target gene, Cyp4a10, and perturbation in cell proliferation and apoptosis, which affected the size of the liver. Moreover, this modification was associated to about two-fold induction of mRNA-PPARalpha. On the contrary, in the kidney, although a similar two-fold up-regulation of PPARalpha was detected, the induction of both peroxisomal enzyme activities and Cyp4a10 were weak, and no alterations were detected, neither in cell cycle nor in the size of the tissue. Our results indicate that the response to ciprofibrate is stronger in the liver than in the kidney of newborn rats.

Modes of action and species-specific effects of di-(2-ethylhexyl)phthalate in the liver

The industrial plasticizer di-(2-ethylhexyl)phthalate (DEHP) is used in manufacturing of a wide variety of polyvinyl chloride (PVC)-containing medical and consumer products. DEHP belongs to a class of chemicals known as peroxisome proliferators (PPs). PPs are a structurally diverse group of compounds that share many (but perhaps not all) biological effects and are characterized as non-genotoxic rodent carcinogens. This review focuses on the effect of DEHP in liver, a primary target organ for the pleiotropic effects of DEHP and other PPs. Specifically, liver parenchymal cells, identified herein as hepatocytes, are a major cell type that are responsive to exposure to PPs, including DEHP; however, other cell types in the liver may also play a role. The PP-induced increase in the number and size of peroxisomes in hepatocytes, so called 'peroxisome proliferation' that results in elevation of fatty acid metabolism, is a hallmark response to these compounds in the liver. A link between peroxisome proliferation and tumor formation has been a predominant, albeit questioned, theory to explain the cause of a hepatocarcinogenic effect of PPs. Other molecular events, such as induction of cell proliferation, decreased apoptosis, oxidative DNA damage, and selective clonal expansion of the initiated cells have been also been proposed to be critically involved in PP-induced carcinogenesis in liver. Considerable differences in the metabolism and molecular changes induced by DEHP in the liver, most predominantly the activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)alpha, have been identified between species. Both sexes of rats and mice develop adenomas and carcinomas after prolonged feeding with DEHP; however, limited DEHP-specific human data are available, even though exposure to DEHP and other phthalates is common in the general population. This likely constitutes the largest gap in our knowledge on the potential for DEHP to cause liver cancer in humans. Overall, it is believed that the sequence of key events that are relevant to DEHP-induced liver carcinogenesis in rodents involves the following events whereby the combination of the molecular signals and multiple pathways, rather than a single hallmark event (such as induction of PPARalpha and peroxisomal genes, or cell proliferation) contribute to the formation of tumors: (i) rapid metabolism of the parental compound to primary and secondary bioactive metabolites that are readily absorbed and distributed throughout the body; (ii) receptor-independent activation of hepatic macrophages and production of oxidants; (iii) activation of PPARalpha in hepatocytes and sustained increase in expression of peroxisomal and non-peroxisomal metabolism-related genes; (iv) enlargement of many hepatocellular organelles (peroxisomes, mitochondria, etc.); (v) rapid but transient increase in cell proliferation, and a decrease in apoptosis; (vi) sustained hepatomegaly; (vii) chronic low-level oxidative stress and accumulation of DNA damage; (viii) selective clonal expansion of the initiated cells; (ix) appearance of the pre-neoplastic nodules; (x) development of adenomas and carcinomas.

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.

Growth and Division of Peroxisomes

Peroxisomes are ubiquitous subcellular organelles, which are highly dynamic and display large plasticity in response to cellular and environmental conditions. Novel proteins and pathways that mediate and control peroxisome formation, growth, and division continue to be discovered, and the cellular machineries that act together to regulate peroxisome number and size are under active investigation. Here, advances in the field of peroxisomal dynamics and proliferation in mammals and yeast are reviewed. The authors address the signals, conditions, and proteins that affect, regulate, and control the number and size of this essential organelle, especially the components involved in the division of peroxisomes. Special emphasis is on the function of dynamin-related proteins (DRPs), on Fis1, a putative adaptor for DRPs, on the role of the Pex11 family of peroxisomal membrane proteins, and the cytoskeleton.

Nitrofurazone-induced gene expressions in rat hepatocytes and their modification by N-acetylcysteine

The antibiotic nitrofurazone (NF) at a subtoxic dose has been shown to increase hepatocyte DNA synthesis with no preceding cell damage or necrosis. This was suppressed by concomitant administration of the antioxidant N-acetylcysteine (NAC), which suggests that free radical production is involved in the process. In this study, male F344 rats were given a single oral subtoxic dose of NF to investigate the changes in genes implicated in hepatocyte proliferation between 1 and 20h postdose by real-time PCR. Some rats were also given NAC to examine the involvement of free radicals. There were transient and sequential increases in mRNA levels of c-myc and c-jun shortly after the administration, followed by tumor necrosis factor-alpha (TNF-alpha), transforming growth factor-alpha (TGF-alpha), c-Ha-ras, and cyclin E. The increases were blocked by concomitant administration of NAC. In contrast, there were no NF-specific increases in c-fos, hepatocyte growth factor, epidermal growth factor or cyclin D1 mRNAs. These results indicate that the induction of hepatocyte proliferation by NF is triggered by free radicals, with a pathway involving increases in c-jun, c-myc, TNF-alpha, TGF-alpha, c-Ha-ras, and cyclin E. The results also indicate that NF-induced proliferation resembles that of other mitogens.

Differential display in rat livers treated for 13 weeks with phenobarbital implicates a role for metabolic and oxidative stress in nongenotoxic carcinogenicity

Hepatic enzyme inducers such as phenobarbital are often nongenotoxic rodent hepatocarcinogens. Currently, nongenotoxic hepatocarcinogens can only be definitively identified through costly and extensive long-term, repeat-dose studies (e.g., 2-year rodent carcinogenicity assays). Although liver tumors caused by these compounds are often not found to be relevant to human health, the mechanism(s) by which they cause carcinogenesis are not well understood. Toxicogenomic technologies represent a new approach to understanding the molecular bases of toxicological liabilities such asnongenotoxic carcinogenicity early in the drug discovery/development process. Microarrays have been used to identify mechanistic molecular markers of nongenotoxic rodent hepatocarcinogenesis in short-term, repeat-dose preclinical safety studies. However, the initial "noise" of early adaptive changes may confound mechanistic interpretation of transcription profiling data from short-term studies, and the molecular processes triggered by treatment with a xenobiotic agent are likely to change over the course of long-term treatment. Here, we describe the use of a differential display technology to understand the molecular mechanisms related to 13 weeks of dosing with the prototype rodent nongenotoxic hepatocarcinogen, phenobarbital. These findings implicate a continuing role for oxidative stress in nongenotoxic carcinogenicity.An Excel data file containing raw data is available in full at http://taylorandfrancis.metapress.com/openurl.asp?genre=journal&issn=0192-6233. Click on the issue link for 33(1), then select this article. A download option appears at the bottom of this abstract. The file contains raw data for all gene changes detected by AFLP, including novel genes and genes of unknown function; sequences of detected genes; and animal body and liver weight ratios. In order to access the full article online, you must either have an individual subscription or a member subscription accessed through www.toxpath.org.

Long term phenobarbital administration does not promote the multiplication of hepatocytes replicating after single cyproterone acetate administration

Cyproterone acetate (CPA) is a synthetic antiandrogenic compound which is widely used in clinic but suspected to be hepatocarcinogenic. CPA is also mitogenic in rat liver. Using genetic labeling of dividing cells, we examined whether hepatocytes dividing in response to acute CPA administration could give rise to preneoplastic foci after administration of a tumor promoter: phenobarbital. CPA was administered orally to rats and dividing hepatocytes were genetically labeled using retroviral vectors carrying the beta-galactosidase gene. After labeling rats were given phenobarbital for 10 months and sacrificed. The presence of beta-galactosidase labeled hepatocytes as well as preneoplastic hepatocytes was assessed by immunohistochemistry. Genetic labeling of hepatocytes was obtained in all animals. At the end of phenobarbital administration, no hepatic tumors were observed. Preneoplastic foci were not increased in treated animals as compared to control rats. Moreover beta-galactosidase positive hepatocytes were never detected in preneoplastic foci. Finally, the size of the beta-galactosidase positive clusters was smaller in treated animals as compared to control rats. We conclude that acute CPA administration is not carcinogenic in rat liver and does not initiate preneoplastic hepatocytes capable to give rise to foci after phenobarbital promotion. Therefore the mitogenic property of CPA is distinct from its putative carcinogenic activity. Finally, analysis of the size of beta-galactosidase positive cells clusters demonstrate that phenobarbital does not induce hepatocyte proliferation in rats.

Interaction networks: coordinating responses to xenobiotic exposure

In the last decade the increased usage of '-omic' technologies, plus the sequencing of over 800 complete genomes has led to a vast increase in the amount of information available to the researcher for examining cellular responses to xenobiotics. Much effort has been put into the identification and analysis of expression profiles associated with pathobiological conditions and/or xenobiotic exposure. These profiles are commonly used in two applications. Firstly, comparative profile experiments are used to classify pathobiological states and for the screening of novel chemical entities to predict their action(s) on the body. Secondly, mechanistic investigations will gain information on the molecular mechanisms underlying toxic responses/pathobiological states. During the course of such analysis it has become increasingly clear that a series of highly refined interaction networks exist within the body, regulating both the sensitivity and selectivity of the body's response to pathobiological states/xenobiotic exposure. These interaction networks exist at several levels: Firstly, within individual cells, the interaction between factors that transmit xenobiotics signals will determine the overall cellular response. Secondly, intraorgan communication occurs between the different cell types/sub-types which makes up an organ, coordinating the overall organ response. Finally, interorgan interactions provide axes of response through the body.

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.

Gene expression changes in rat liver following exposure to liver growth agents: role of Kupffer cells in xenobiotic-mediated liver growth

Many xenobiotics are known to cause liver enlargement and hepatocarcinogenesis in rats, although the molecular mechanisms that underlie this effect remain largely undefined. Human exposure to several of these compounds, including glucocorticoids and peroxisome proliferators may be significant, due to their use in both pharmaceutical and industrial processes. It is therefore important to elucidate the molecular mechanisms underlying this abnormal liver enlargement in rats, as this will enable more accurate extrapolation of the possible outcomes of human exposure. Male Sprague-Dawley rats were dosed with the peroxisome proliferator Wy-14,643 and changes in liver gene expression examined using subtractive suppression hybridisation examined either 12 of 24hr later. Twenty-five transcripts were identified which showed differential gene expression in liver following exposure to Wy-14,643. Biochemical indices of liver growth (DNA synthesis, apoptosis) showed that these changes correlated with the initiation of liver enlargement. Rats were next treated with either Wy-14,643, cyproterone acetate and dexamethasone, chemically and mechanistically-distinct hepatomegalic compounds. Carboxylesterase and Kupffer cell receptor mRNA levels were seen to alter in a qualitatively similar fashion for all three compounds, and in a liver specific fashion. In addition, these changes correlated with a decrease in the density of Kupffer cells within the liver, which are known to release mitogenic cytokines, and have been linked to Wy-14,643-induced cell proliferation. We therefore propose that Kupffer cells play a role in a general mechanism of xenobiotic-mediated liver enlargement.

Chromatin sphingomyelin changes in cell proliferation and/or apoptosis induced by ciprofibrate

It has been shown that neutral-sphingomyelinase and sphingomyelin-synthase activities are present in chromatin and they modify the sphingomyelin (SM) content. The activity of the first enzyme is stimulated and the second inhibited, when the hepatocytes enter into the S-phase after partial hepatectomy, thus suggesting that ceramide may have a pivotal role in cell proliferation. An opposite function was attributed to ceramide in hepatocytes which undergo apoptosis after lobular ligature. In order to clarify this point, a model was developed in which the same liver cells undergo proliferation followed by induced apoptosis. To this purpose, the rats were treated for 7 days with ciprofibrate and then left without treatment for 4 days. During the treatment, the peroxisome enzyme markers increase their activity and the number of proliferating cells increases, reaching a maximum after 3 days of treatment, as shown by the number of cells positive for the proliferating cell nuclear antigen. At the same time, the chromatin sphingomyelinase activity reaches the maximum, while a similar increase is not found in the cytoplasm or in the isolated nuclei. On the contrary, SM-synthase activity is depressed in chromatin, but not in the nuclei in which a peak is shown after 3 days of ciprofibrate treatment. After drug withdrawal, the hepatocytes undergo apoptosis as confirmed by the increase of Bax and tissue transglutaminase (tTGase) expression; the chromatin SM increases as a consequence of an increase of SM-synthase activity. It can be hypothesised that chromatin SM may have a role in cell duplication by influencing the chromatin structure stability.

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.

Transcription profiling distinguishes dose-dependent effects in the livers of rats treated with clofibrate

Peroxisome proliferators such as the fibrates act via the peroxisome proliferator activated receptor (PPAR)-alpha as hypolipidemic agents. Many peroxisome proliferators are also nongenotoxic hepatic carcinogens and hepatotoxicants in rodents. We performed transcription profiling using cDNA microarrays on livers of rats treated for 5 days with 3 doses of the peroxisome proliferator clofibrate. All 3 doses had hepatic effects as assessed by liver to body weight ratio, alanine aminotransferase (ALT) increases and histopathology examination. Analysis of the transcription profiling data identified changes in the expression of many genes within several mechanistic pathways that support existing hypotheses regarding peroxisome proliferator mediated carcinogenicity. Additionally, the transcription profiling, histopathology, and clinical chemistry results suggested a biphasic response to clofibrate. These findings provide insight into the pathogenesis of toxic and carcinogenic effects of clofibrate in rodents and demonstrate the ability of cDNA microarrays to provide information regarding mechanisms of toxicity identified during the drug development process.

Peroxisome proliferation as a biomarker in environmental pollution assessment

Peroxisome proliferators comprise a heterogeneous group of compounds known for their ability to cause massive proliferation of peroxisomes and liver carcinogenesis in rodents. In recent years it has become evident that other animals may be threatened by peroxisome proliferators, in particular aquatic organisms living in coastal and estuarine areas. These animals are exposed to a variety of pollutants of industrial, agricultural and urban origin which are potential peroxisome proliferators. Both laboratory and field studies have shown that phthalate ester plasticizers, PAHs and oil derivatives, PCBs, certain pesticides, bleached kraft pulp and paper mill effluents, alkylphenols and estrogens provoke peroxisome proliferation in different fish or bivalve mollusc species. The response appears to be mediated by peroxisome-proliferator activated receptors, members of the nuclear receptor family, recently cloned in fish. Based on these results it is proposed that peroxisome proliferation could be used as a biomarker of exposure to a variety of pollutants in environmental pollution assessment. This is illustrated by a case study in which mussels, used worldwide as sentinels of environmental pollution, were transplanted from reference to contaminated areas and vice versa. In mussels native to an area polluted with PAHs and PCBs, peroxisomal acyl-CoA oxidase (AOX) activity and peroxisomal volume density were 2-3 fold and 5-fold higher, respectively, compared to the reference site. When animals were transplanted to the polluted station, with increased concentration of organic xenobiotics, a concomitant significant increase of AOX was recorded. Conversely, in animals transplanted to the cleaner station, AOX activity and peroxisomal volume density decreased significantly. These results indicate that peroxisome proliferation is a rapid (i.e., two days) and reversible response to pollution in mussels. Before peroxisome proliferation can be implemented as a biomarker in biomonitoring programs, a well-defined protocol should be established and validated in intercalibration and quality assurance programmes. Furthermore, the influence of biotic and abiotic factors, some of which are known to affect peroxisome proliferation (season, tide level, interpopulation and interindividual variability), should be taken into consideration. The possible hepatocarcinogenic effects as well as the potential adverse effects on reproduction, development, and growth of peroxisome proliferators are unknown in aquatic organisms, thus providing a challenge for future investigations.

Regulation of cell proliferation and cell death by peroxisome proliferators

Peroxisome proliferators cause increases in liver mass in rodents, linked to changes in cell proliferation and cell death of hepatocytes. These effects are reversible upon cessation of treatment. The underlying mechanism of the response in rodent liver is complex, but clearly dependent on activation of the nuclear receptor PPARalpha. Other signaling pathways have been implicated in this response, but evidence is mixed. Differing sensitivity among various species to effects of peroxisome proliferators has been associated with differences in PPARalpha expression and function. Changes in cell proliferation and cell death in neoplastic hepatocytes also have been found in liver tumors caused by long-term treatment with peroxisome proliferators.

Advances in understanding the regulation of apoptosis and mitosis by peroxisome-proliferator activated receptors in pre-clinical models: relevance for human health and disease

Peroxisome proliferator activated receptors (PPARs) are a family of related receptors implicated in a diverse array of biological processes. There are 3 main isotypes of PPARs known as PPARalpha, PPARbeta and PPARgamma and each is organized into domains associated with a function such as ligand binding, activation and DNA binding. PPARs are activated by ligands, which can be both endogenous such as fatty acids or their derivatives, or synthetic, such as peroxisome proliferators, hypolipidaemic drugs, anti-inflammatory or insulin-sensitizing drugs. Once activated, PPARs bind to DNA and regulate gene transcription. The different isotypes differ in their expression patterns, lending clues on their function. PPARalpha is expressed mainly in liver whereas PPARgamma is expressed in fat and in some macrophages. Activation of PPARalpha in rodent liver is associated with peroxisome proliferation and with suppression of apoptosis and induction of cell proliferation. The mechanism by which activation of PPARalpha regulates apoptosis and proliferation is unclear but is likely to involve target gene transcription. Similarly, PPARgamma is involved in the induction of cell growth arrest occurring during the differentiation process of fibroblasts to adipocytes. However, it has been implicated in the regulation of cell cycle and cell proliferation in colon cancer models. Less in known concerning PPARbeta but it was identified as a downstream target gene for APC/beta-catenin/T cell factor-4 tumor suppressor pathway, which is involved in the regulation of growth promoting genes such as c-myc and cyclin D1. Marked species and tissue differences in the expression of PPARs complicate the extrapolation of pre-clinical data to humans. For example, PPARalpha ligands such as the hypolipidaemic fibrates have been used extensively in the clinic over the past 20 years to treat cardiovascular disease and side effects of clinical fibrate use are rare, despite the observation that these compounds are rodent carcinogens. Similarly, adverse clinical responses have been seen with PPARgamma ligands that were not predicted by pre-clinical models. Here, we consider the response to PPAR ligands seen in pre-clinical models of efficacy and safety in the context of human health and disease.

Hepatocellular peroxisome proliferation and DNA synthesis in Wistar rats treated with herbicide fluazifop

The aim of this study was to determine the effect of herbicide fluazifop, on the early occurring changes in rat liver regarded as hepatic markers of peroxisome proliferators (PPs). Fluazifop was administered orally to male Wistar rats at increasing doses from 5.6 to 891 mg/kg body weight per day for 1, 2, 4, 7 and 14 consecutive days and peroxisome proliferation, induction of some peroxisome-associated enzymes and mitogenesis (S-phase, M-phase and percentage of binucleated hepatocytes) were studied. Short-term treatment of rats with fluazifop resulted in hepatomegaly due to time dependent proliferation of smooth endoplasmic reticulum (SER) and peroxisomes. The increase in the number of peroxisomes in the hepatocytes was supported by an increase in peroxisomal palmitoyl-CoA oxidation and catalase activity. In contrast to other PPs fluazifop induced low rate of rcplicative DNA synthesis and did not affect mitoses (M-phase). DNA synthesis was accompanied by the appearance of binucleated hepatocytes. Thus, we can conclude that fluazifop produces in male Wistar rats hepatomegaly due to cellular hypertrophy. The threshold dose for palmitoyl-CoA oxidation and DNA synthesis was 112 and 223 mg/kg body weight per day, respectively. The value for hepatomegaly and catalase activity was 56 mg/kg body weight per day. The results presented in this paper demonstrated that fluazifop can be classified as a weak rodent PPs.

Senescence-associated decline in hepatic peroxisomal enzyme activities corresponds with diminished levels of retinoid X receptor alpha, but not peroxisome proliferator-activated receptor alpha

Aging is associated with alterations in hepatic peroxisomal metabolism and susceptibility to hepatocarcinogenicity produced by agonists of peroxisome proliferator-activated receptor alpha (PPAR alpha). Mechanisms involved in these effects are not well understood. However, as a heterodimer with retinoid X receptor alpha (RXR alpha), PPAR alpha regulates transcription of genes involved in oxidative stress, cell proliferation and apoptosis. Modulating these important cell functions as a result of aging may be responsible for altered hepatic peroxisomal responses in the senescent liver. Therefore, we investigated hepatic apoptosis, and peroxisomal beta-oxidation activity, a major source of H(2)O(2), as well as the activity of the peroxisomal anti-oxidant enzyme catalase, in male Fischer-344 rats of four age groups (4, 10, 50 and 100 week old). We further quantified protein levels of both PPAR alpha and RXR alpha in these animals. Data show that peroxisomal beta-oxidation and catalase activities were significantly lower in livers of the 100 week old animals compared with other age groups, while percentage of apoptotic hepatocytes were identical in all animal age groups. However, aging had no effect on hepatic PPAR alpha protein levels. In the senescent group, the level of decline in both peroxisomal enzyme activities of 30% was surprisingly similar to the decline observed in the hepatic expression of the RXR alpha protein. Results from this study suggest that alterations in peroxisomal metabolism observed in the senescent liver may be a result of the decline in the availability of RXR alpha receptor, and not the primary PPAR alpha receptor. On the other hand, PPAR alpha-independent mechanisms appear to play a role in controlling apoptosis in the senescent liver.

Mechanisms involved in responses to the poroxisome proliferator WY-14,643 on gap junctional intercellular communication in V79 hamster fibroblasts

WY-14,643, a potent hepatic peroxisome proliferator, decreased gap junctional intercellular communication when used at low and intermediate concentrations (1 nM to 10 microM) in the V79 Chinese hamster fibroblast cell line. It did not decrease intercellular communication in early passage Syrian hamster embryo fibroblasts. The mechanism of WY-14,643-induced suppression of intercellular communication was studied by preexposure of V79 cells to inhibitors of protein kinase C, mitogen-activated protein kinases, phosphatidylinositol 3-kinase, or mammalian target-of-rapamycin before addition of WY-14,643. Only protein kinase C, particularly the delta isoform, appeared involved in the inhibition of communication by WY-14,643. Also clofibrate-induced suppression of GJIC in V79 cells appeared to involve protein kinase Cdelta. Furthermore, WY-14,643 did not cause any activation of the mitogen-activated protein kinases p44/42, p38, or Jun N-terminal kinase. When WY-14,643 was used at a higher concentration (100 microM), intercellular communication was increased both in V79 and Syrian hamster embryo cells. This effect was inhibited by preexposure of V79 cells to brefeldin A. Thus, there may be a pool of connexins in the Golgi complex that could be recruited to the plasma membrane upon exposure to high concentrations of WY-14,643.

In vivo cell lineage analysis in cyproterone acetate-treated rat liver using genetic labeling of hepatocytes

Genetic labeling using recombinant retroviruses is a powerful strategy for the study of cell lineage in the liver. However, this type of vector is only able to infect dividing cells. The synthetic steroid cyproterone acetate (CPA) is mitogenic and carcinogenic in the adult rat liver. In this study, we used retroviral vectors carrying the nuclear targeted beta-galactosidase gene to selectively label and follow the fate of hepatocytes dividing on administration of CPA. Labeled cells as well as those in mitosis were preferentially located around the portal tract, whereas apoptotic bodies were predominant in the pericentral area. Labeled hepatocytes did not disappear after apoptosis, suggesting a preferential elimination of nontransduced cells. The presence of labeled binucleated hepatocytes showed the persistence of a binucleation process. Finally, we performed long-term analysis of labeled cells in transgenic animals tolerant for beta-galactosidase and treated with 2-acetylaminofluorene (2-AAF) to promote the growth of CPA-initiated hepatocytes. The presence of beta-galactosidase-positive hepatocyte clones showed that hepatocytes divided during treatment with 2-AAF. Only 3% of beta-galactosidase clones were positive for the placental form of glutathione S-transferase (GSTp), indicating the absence of a preferential appearance of preneoplastic foci in the population of beta-galactosidase-labeled hepatocytes. In conclusion, our results show that the mitogenic and tumor-initiating activities of CPA are directed toward different hepatocyte populations.

Efficient retroviral gene transfer to the liver in vivo using nonpolypeptidic mitogens

Recombinant retroviral vectors are attractive tools for achieving sustained expression of a therapeutic gene in the liver. However, cell division is required for efficient transduction with these vectors. Here we report that two widely used liver mitogens, triiodothyronin (T3) and cyproterone acetate (CPA), enable hepatocyte transduction with recombinant retroviral vectors delivered in vivo into the bloodstream. Treatment with T3 as well as CPA, alone or in combination, resulted in an increase in hepatocyte replication predominantly around the portal tract. The mitogenic activity made it possible to transduce hepatocytes in the same location. Moreover, when administered together, the two drugs synergized and the transduction level reached 5% of hepatocytes. This transduction level is compatible with clinical applications for a number of inherited liver diseases. Since these two compounds have a long history of safe clinical use, we propose that these liver mitogens may have potential for clinical application in liver-directed gene therapy.

The functions of cytokines and their uses in toxicology

Cytokines are critical controllers of cell, and hence tissue, growth, migration, development and differentiation. The family includes the inflammatory cytokines such as the interleukins and interferons, growth factors such as epidermal and hepatocyte growth factor and chemokines such as the macrophage inflammatory proteins, MIP-1alpha and MIP-1beta. They do not include the peptide and steroid hormones of the endocrine system. Cytokines have important roles in chemically induced tissue damage repair, in cancer development and progression, in the control of cell replication and apoptosis, and in the modulation of immune reactions such as sensitization. They have the potential for being sensitive markers of chemically induced perturbations in function but from a toxicological point of view, the detection of cytokine changes in the whole animal is limited by the fact that they are locally released, with plasma measures being generally unreliable or irrelevant, and they have short half lives which require precise timing to detect. Even where methodology is adequate the interpretation of the downstream effects of high, local concentrations of a particular cytokine is problematic because of their interdependence and the pleiotropism of their action. A range of techniques exist for their measurement including those dependent upon antibodies specific for the respective cytokines, but with the introduction of genomic and proteomic technology, a more complete study of cytokine changes occurring under the influence of chemical toxicity should be possible. Their further study, as markers of chemical toxicity, will undoubtedly lead to a greater understanding of how synthetic molecules perturb normal cell biology and if, and how, this can be avoided by more intuitive molecular design in the future.

Cell biology of peroxisomes and their characteristics in aquatic organisms

The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.

Inhibition of gap-junctional-intercellular communication in intact rat liver by nongenotoxic hepatocarcinogens

Many nongenotoxic hepatocarcinogens can induce cell proliferation, and inhibit apoptosis and gap-junctional-intercellular communication (GJIC). GJIC, the movement of small molecules (less than 1.2 kD) through membrane channels, is important in regulating cellular homeostasis and differentiation. The inhibition of hepatic GJIC can increase cell proliferation and possibly, inhibit apoptosis. In this study, the relationship between hepatic GJIC, proliferation, and apoptosis was examined in rats treated for 7 days with tumor-promoting doses of the nongenotoxic hepatocarcinogens phenobarbital (PB; 800 ppm), pregnenolone-16alpha-carbonitrile (PCN; 1000 ppm), and Aroclor 1254 (PCB; 100 ppm). In addition, 3-methylcholanthrene (3MC) was included as a negative control. PB, PCN, and PCB increased parenchymal-cell proliferation and inhibited hepatic apoptosis, while no alteration in these growth parameters was observed in 3MC-treated rats. GJIC, as measured by fluorescent-dye transfer through intact liver, was decreased nearly 50% by PB, PCN, and PCB, yet no effect on GJIC was observed in liver from 3MC-treated rats. These data indicate that compounds that inhibit GJIC in liver may be nongenotoxic hepatocarcinogens, which occurs simultaneously during increased cell proliferation and inhibited apoptosis.

Apoptosis, mitosis and cyclophilin-40 expression in regressing peroxisome proliferator-induced adenomas

Chronic exposure to peroxisome proliferators (PP), including certain industrial and pharmaceutical chemicals, causes liver cancer in rodents. Continuous exposure to PP is needed for tumor development since the frequency of hepatocellular neoplasms is decreased in animals returned to control diet. To determine cellular and molecular events responsible for enhanced growth in PP-induced liver tumors, we evaluated the relationships of WY-14,643 levels, apoptosis, mitosis and cyclophilin-40 (Cyp-40) expression in regressing tumors induced by WY-14,643, a potent PP. Male F344 rats were fed WY-14,643 (0.1%) in the diet for 43 weeks and then switched to control diet for 2, 3, 5 or 36 days. Mean serum and hepatic concentrations of WY-14,643 were decreased as early as 2 days following removal of WY-14,643 as compared with rats continuously fed WY-14,643. Adenomas from rats maintained on WY-14,643 markedly compressed surrounding parenchyma. Evidence of adenoma regression was observed by 3 days of WY-14,643 withdrawal and was characterized by loss of compression. Decreased compression corresponded to increases in the apoptotic index and decreases in the mitotic index in regressing adenomas at 2, 3, and 5 days following the switch to control diet. Cyclophilins are multifunctional receptor proteins involved in numerous signal transduction pathways, including those mediated by cyclosporin, a liver tumor promoter in rats. Cyp-40 expression was markedly increased in adenomas from continuously exposed rats, but expression returned to levels similar to surrounding parenchyma in adenomas after 5 days of WY-14,643 withdrawal. Taken together, these results indicate that WY-14, 643-induced adenomas regress rapidly following withdrawal of the PP in association with declining liver WY-14,643 levels, suggesting that peroxisome proliferator-activated receptor alpha may mediate PP-induced alterations in mitogenic and/or apoptotic regulation in growing tumors, in conjunction with alterations in Cyp-40 signal transduction.

Suppression of mouse hepatocyte apoptosis by peroxisome proliferators: role of PPARalpha and TNFalpha

Peroxisome proliferators (PPs) are a diverse group of nongenotoxic chemicals that in rodents cause hepatic peroxisome proliferation, liver enlargement, increased replicative DNA synthesis and suppression of apoptosis. The effects of PPs in vivo can be reproduced in vitro where PPs can induce mouse hepatocyte DNA synthesis and suppress both spontaneous apoptosis and that induced by transforming growth factor beta (TGFbeta). In vitro, high concentrations (>500 U/ml) of exogenous tumour necrosis factor (TNFalpha) [M. Rolfe, N.H. James, R.A. Roberts, TNF suppresses apoptosis and induces S-phase in rodent hepatocytes: a mediator of the hepatocarcinogenicity of peroxisome proliferators?, Carcinogenesis 18 (1997) 2277-2280] are also able to stimulate hepatocyte DNA synthesis and suppress apoptosis, implicating TNFalpha in mediating or permitting the liver growth response to PPs. Here, using cultured mouse hepatocytes isolated from PPARalpha null mice, we have examined the role of the peroxisome proliferator activated receptor alpha (PPARalpha) in mediating the suppression of apoptosis caused by PPs. In addition we have investigated further the role of TNFalpha in mediating the rodent response to PPs. The PP nafenopin (50 microM) was unable to stimulate DNA synthesis measured by bromodeoxyuridine incorporation in these PPARalpha null mouse hepatocytes (96% of control), unlike epidermal growth factor, a growth factor used as a positive control. In assays of apoptosis using H33258 staining of chromatin condensation, nafenopin was unable to suppress either spontaneous or TGFbeta1-induced apoptosis. In contrast, high concentrations of TNFalpha (>500 U/ml) were able to both stimulate DNA synthesis (204% of control) and suppress apoptosis in PPARalpha null hepatocytes (40% and 38% of control for spontaneous and TGFbeta1-induced apoptosis respectively). However, TNFalpha could not stimulate beta-oxidation of palmitoyl CoA in either PPARalpha null mouse or B6C3F1 (PPARalpha wild type) mouse hepatocytes. These data confirm the dependence of the response to PPs on PPARalpha by demonstrating that PPARalpha mediates the suppression of hepatocyte apoptosis in response to PPs. In addition, the data provide evidence that high concentrations of TNFalpha can modulate DNA synthesis and apoptosis in the absence of PPs and PPARalpha. Thus, in vivo, physiological levels of TNFalpha may be permissive for a PPARalpha-dependent growth response to PPs.

Apoptosis and cell proliferation in rat hepatocytes induced by barbiturates

To examine the effect on cell population in hepatocytes of phenobarbital (PB) and other barbiturates, PB, allobarbital (ALB), barbital sodium (BS) and barbituric acid (BA) were given orally to male rats for 7 consecutive days. Although there was no apparent change in non-promoting BA, hepatomegaly was induced by PB, BS and ALB, which are promoters of hepatocarcinogenesis. In PB- and BS-treated livers, hepatomegaly was attributable to hepatocyte proliferation and enzyme induction. In ALB-treated liver, it was attributable to enzyme induction. The level of cell proliferation was reduced to less than the control values following withdrawal of PB, ALB and BS. It seemed that the degree of suppression of cell proliferation following withdrawal of these compounds correlated to the degree of cell proliferation (PB>BS>ALB) during treatment. In PB-treated liver, apoptosis was induced during treatment, serving to eliminate the excess of hepatocytes. This suggests that short-term administration of PB neither induced suppression of apoptosis nor disturbed homeostasis of hepatocyte populations.

Sex-specific induction of apoptosis by cyproterone acetate in primary rat hepatocytes

The synthetic steroid cyproterone acetate (CPA) has been reported to be hepatogenotoxic in female rats depending on sex-specific expression of a hydroxysteroid sulfotransferase (HST) which is involved in the bioactivation of CPA to reactive metabolites. In the present study the ability of CPA to initiate apoptosis in rat hepatocytes in vitro was investigated with respect to sex-specific effects and dependency on HST activity. Incubation of primary hepatocytes of female rats with CPA (0.1-30 microM) caused a strong increase in percent of cells undergoing apoptosis. The lowest concentration leading to apoptosis was 0.3 microM. In contrast, hepatocytes isolated from male rats showed a very weak response at high exposure to CPA (30 microM) only. Treatment with transforming growth factor-beta1 induced high levels of apoptotis in hepatocytes of both genders. Megestrol acetate and chlormadinone acetate, two structural analogues of CPA with a much lower genotoxic potency, did not induce apoptosis. Pre-addition of 10 or 50 microM dehydroepiandrosterone (DHEA), a known inhibitor of hepatic HST, almost completely inhibited CPA-induced apoptosis in hepatocytes of female rats. Using similar test concentrations, DHEA also reduced CPA-induced DNA excision repair as measured in the unscheduled DNA synthesis test. The results suggest that apoptosis induction is directly related to DNA damage induced by HST-dependent CPA metabolites.

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.

The perturbation of apoptosis and mitosis by drugs and xenobiotics

Drugs such as the barbiturate phenobarbitone and fibrate hypolipidaemic agents, in addition to a range of chemicals of environmental and industrial significance, are able to perturb rodent tissue homeostasis, leading to tissue enlargement. Many of these xenobiotics are rodent nongenotoxic carcinogens since they do not damage DNA, yet cause tumours in the rat and mouse. These nongenotoxic carcinogens display both species and tissue specificity; for example, rat and mouse hepatocytes display S-phase induction and a suppression of apoptosis in response to drugs such as phenobarbitone or the hypolipidaemic peroxisome proliferators (PPs). In contrast, human hepatocytes or other types of rodent cells are refractory to these effects. However, in the absence of a discrete mechanism of action, the clear species differences preclude extrapolation of rodent data to provide an accurate human risk assessment. Recent data have demonstrated that PPs activate the PP-activated receptor alpha in rodent liver, leading to enzyme induction, stimulation of S-phase, and a suppression of apoptosis. How these acute effects may lead to hepatocarcinogenesis and the relevance of this for humans will be discussed.

Mechanism of action of the nongenotoxic peroxisome proliferators: role of the peroxisome proliferator-activator receptor alpha

Peroxisome proliferators are a diverse group of chemicals that include several therapeutically used drugs (e.g., hypolipidemic agents), plasticizers and organic solvents used in the chemical industry, herbicides, and naturally occurring hormones. As the name implies, peroxisome proliferators cause an increase in the number and size of peroxisomes in the liver, kidney, and heart tissue of susceptible species, such as rats and mice. Long-term administration of peroxisome proliferators can cause liver cancer in these animals, a response that has been the central issue of research on peroxisome proliferators for many years. Peroxisome proliferators are representative of the class of nongenotoxic carcinogens that cause cancer through mechanisms that do not involve direct DNA damage. The fact that humans are frequently exposed to these agents makes them of particular concern to government regulatory agencies responsible for assuring human safety. Whether frequent exposure to peroxisome proliferators represents a hazard to humans is unknown; however, increased cancer risk has not been shown to be associated with long-term therapeutic administration of the hypolipidemic drugs gemfibrozil, fenofibrate, and clofibrate. To make sound judgments regarding the safety of peroxisome proliferators, the validity of extrapolating results from rodent bioassays to humans must be based on the agents' mechanism of action and species differences in biologic activity and carcinogenicity. The peroxisome proliferator-activated receptor alpha (PPARalpha), a member of the nuclear receptor superfamily, has been found to mediate the activity of peroxisome proliferators in mice. Gene-knockout mice lacking PPARalpha are refractory to peroxisome proliferation and peroxisome proliferator-induced changes in gene expression. Furthermore, PPARalpha-null mice are resistant to hepatocarcinogenesis when fed a diet containing a potent nongenotoxic carcinogen WY-14,643. Recent studies have revealed that humans have considerably lower levels of PPARalpha in liver than rodents, and this difference may, in part, explain the species differences in the carcinogenic response to peroxisome proliferators.

Hepatocyte growth factor induces hepatocyte proliferation in vivo and allows for efficient retroviral-mediated gene transfer in mice

Recombinant retroviral vectors are an attractive means of transferring genes into the liver because they integrate into the host cell genome and result in permanent gene expression. However, efficient in vivo gene transfer is limited by the requirement of active cell division for integration. Traditional approaches to induce liver proliferation have the disadvantage of inducing hepatocellular injury by delivery of toxins or by surgical partial hepatectomy. As a nontraumatic alternative, we show that exogenous hepatocyte growth factor (HGF) is a powerful and safe mitogen for the mature intact murine liver when delivered continuously into the portal vein. A 5-day infusion of human HGF (5 mg/kg/d) resulted in > 140% increase in relative liver mass, which returned to normal in 4 to 5 weeks. This clearly shows that an exogenous growth factor can induce robust liver proliferation in vivo. In addition, we show that the HGF-induced proliferation was independent of interleukin-6, an essential cytokine involved in liver regeneration after partial hepatectomy. When recombinant retroviral vectors were infused in combination with HGF, 30% of hepatocytes were stably transduced with no indication of hepatic injury or histopathology. These results show the ability to obtain a clinically relevant transduction efficiency with retroviral vectors in vivo without the prior induction of liver injury. The level of hepatic gene transfer achieved has the potential to be curative for a large number of genetic liver diseases.