Rodent toxicity and nongenotoxic carcinogenesis: knowledge-based human risk assessment based on molecular mechanisms

It is necessary to determine whether chemicals or drugs have the potential to pose a threat to human health. Chemicals that can damage DNA are detected in short-term assays, but the detection of nongenotoxic carcinogens relies upon bioassays in laboratory animals. However, there are marked differences between rodents and humans in response to nongenotoxic carcinogens, which makes the relevance of rodent data to human risk assessment questionable. Here, we address the background issues concerning rodent nongenotoxic carcinogenesis and then focus upon peroxisome proliferators, chloroform, and dioxins as examples of toxicants that cause rodent-specific oxidative stress, cell proliferation, and the suppression of apoptosis. In the case of peroxisome proliferators and dioxins, this response is receptor-mediated. The evidence presented suggests that, at least for some toxicants, the molecular mechanisms of the rodent carcinogenic responses do not operate in humans; this is discussed in the context of human risk assessment. Finally, consideration is given to incorporating mechanism-based information into risk assessment for regulatory purposes.

Guided plate wave scattering at vertical stiffeners and its effect on source location

This paper examines guided wave transmission characteristics of plate stiffeners and their influence on the performance of acoustic noise source location. The motivation for this work is the detection of air leaks in manned spacecraft. The leaking air is turbulent and generates noise that can be detected by a contact-coupled acoustic array to perform source location and find the air leak. Transmission characteristics of individual integral stiffeners are measured across a frequency range of 50-400kHz for both high and low aspect-ratio rectangular stiffeners, and comparisons are made to model predictions which display generally good agreement. It is demonstrated that operating in frequency ranges of high plate wave stiffener transmission significantly improves the reliability of noise source location in the plate. A protocol is presented to enable the selection of an optimal frequency range for leak location.

An animal evaluation of a paste of chitosan glutamate and hydroxyapatite as a synthetic bone graft material

The objective of this study was to develop a synthetic bone graft in a paste form. Reported here are the results of the evaluation of a paste of chitosan glutamate (Protosan) and hydroxyapatite (referred to as a paste) used in a critical size defect model in rats. Eight-millimeter--diameter cranial defects were made in rat calvaria following a protocol approved by the animal review committee. Five groups were studied: (1) empty control, (2) defect filled with paste only, (3) defect filled with the paste containing bone-marrow aspirate, (4) defect filled with paste containing BMP-2, and (5) defect filled with paste containing osteoblasts cultured from bone-marrow aspirate. The sacrifice intervals were 9 and 18 weeks. Calvaria containing the defect were harvested, and the bone mineral density (BMD) was determined by dual energy X-ray absorptiometry. Push-out strength measurements were also performed. The BMD values of empty control were significantly lower than those of other groups at both 9 and 18 weeks. The mechanical properties, that is, push-out strengths and area under the push-out load and displacement were not significantly different between the samples. Histological examination of Goldner-trichromestained undecalcified sections showed the presence of mineralized bone spicules in the defect areas that were more prominent in those filled with paste and osteoblasts cultured from bone-marrow aspirate. Hence, this study demonstrated that the paste of chitosan glutamate and hydroxyapatite-containing osteoblasts cultured from bone-marrow aspirate would be an effective material to repair bone defects.

PPAR alpha and the regulation of cell division and apoptosis

Peroxisome proliferators (PPs) such as the hypolipidaemic drug, nafenopin and the phthalate plasticiser 2-diethylhexylphthalate induce rodent hepatocyte cell proliferation and suppress apoptosis leading to tumours. PPs act via the nuclear hormone receptor peroxisome proliferator activated receptor alpha (PPAR alpha) which directly regulates genes implicated in the response to PPs such as the peroxisomal gene acyl CoA oxidase. As expected for xenobiotics that perturb proliferation, PPs alter expression of cell cycle regulatory proteins. However, the ability to alter expression of cyclins and cyclin-dependent kinases is shared by physiological hepatic mitogens such as epidermal growth factor and is thus unlikely to be specific to the PP-induced aberrant growth associated with hepatocarcinogenesis. Recent evidence suggests that the response of hepatocytes to PPs is not only dependent upon PPAR alpha but also on the trophic environment provided by nonparenchymal cells and by cytokines such as tumour necrosis factor alpha. Additionally, the ability of PPs to suppress apoptosis and induce proliferation depends upon survival signalling mediated by p38 mitogen activated protein kinase. The cross talk between PPAR alpha-mediated transcription, survival signalling and cell cycle will be discussed with particular emphasis on relevance to toxicology.

Randomized clinical trial of laparoscopic versus open abdominal rectopexy for rectal prolapse

BACKGROUND

The objectives of this study were to compare both subjective clinical outcomes and the objective stress response of laparoscopic and open abdominal rectopexy in patients with full-thickness rectal prolapse. Abdominal rectopexy for patients with rectal prolapse is well suited for a laparoscopic approach as no resection or anastomosis is necessary.

METHODS

Forty patients with a full-thickness rectal prolapse were randomized before operation to a laparoscopic group and an open group. They agreed to conform to a clinical pathway (CP) of liquid diet (CP1) and full mobility (CP2) on day 1, solid diet (CP3) on day 2 and discharge (CP4) before day 5. Their compliance was monitored by an assessor blinded to the operative group, who also rated pain and mobility. Patient-controlled morphine use was documented. Neuroendocrine and immune stress response and respiratory function were measured.

RESULTS

Some 75 per cent of all clinical pathway objectives of early recovery were achieved in the laparoscopic group compared with 37 per cent in the open group (P < 0.01). Significant differences in favour of laparoscopy were noted with regard to narcotic requirements, and pain and mobility scores. Differences in objective measures of stress response favouring laparoscopy were found for urinary catecholamines, interleukin 6, serum cortisol and C-reactive protein. No differences were noted in respiratory function but significant respiratory morbidity was greater in the open group (P < 0.05). None of the measured outcomes, subjective or objective, favoured the open group apart from operating time, which was significantly shorter (153 versus 102 min; P < 0.01).

CONCLUSION

This study has demonstrated significant subjective and objective differences in favour of a laparoscopic technique for abdominal rectopexy. The advantages were all short term but no evidence of any adverse effect on longer-term outcomes was observed.

Scientific analysis of the proposed uses of the T25 dose descriptor in chemical carcinogen regulation

The uncertainties that surround the methods used for risk assessment of exposure to carcinogens have been highlighted by a recent document advocating an approach based on the T25 dose (the dose giving a 25% incidence of cancer in an appropriately designed animal experiment). This method relies on derivation of the T25 dose then assesses risk at the exposure dose using proportionality provided by a linear extrapolation (T25/linear). To promote discussion of the scientific issues underlying methods for the risk assessment of chemical carcinogens, the European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) hosted a one-day workshop in Brussels on 10 November 2000. Several invited presentations were made to participants, including scientists from regulatory authorities, industry and academia. In general, it was felt that there was sufficient basis for using the T25 dose as an index of carcinogenic potency and hence as part of the hazard assessment process. However, the use of the T25 in risk assessment has not been validated. The T25/linear and other extrapolation methods based on metrics such as LED 10 assume linearity which may be invalid. Any risk calculated using the T25/linear method would provide a precise risk figure similar to the output obtained from the Linearised Multistage (LMS) method formerly used by the Environmental Protection Agency (EPA) in the United States of America. Similarity of output does not provide validation but rather reflects their reliance on similar mathematical approaches. In addition to the T25 issue, evidence was provided that using two separate methods (linearised non-threshold model for genotoxic carcinogens; no-observable-effect level with a safety factor (NOEL/SF) method for all other toxicity including non-genotoxic carcinogens) is not justified. Since the ultimate purpose of risk assessment is to provide reliable information to risk managers and the public, there was strong support at the workshop for harmonisation of approaches to risk assessment for all genotoxic and nongenotoxic carcinogens. In summary, the T25 method has utility for ranking potency to focus efforts in risk reduction. However, uncertainties such as the false assumption of precision and non-linearity in the dose-response curve for tumour induction raise serious concerns that caution against the use of T25/linear method for predicting human cancer risk.

Results following trapeziometacarpal arthroplasty of the thumb

A retrospective follow-up study was performed on patients with degenerative joint disease (DJD) who underwent trapeziometacarpal arthroplasty of the thumb with 3-week immobilization and without the use of K-wire fixation. Pre- and post-operative pain, activities of daily living (ADLs), grip strength, and pinch strength were compared. Data were collected on 25 hands in 23 patients, 7 hands with full trapezium resections and 18 with hemi-trapezium resections. The median age was 60 years, with a range of 39 to 73 years, and the median follow-up period was 1 year 11 months, with a range of 3 months to 11 years. Grip and pinch strength were measured pre- and postoperatively. Pain was assessed on a visual analog scale (VAS), and ADLs were assessed by means of a 15-item survey. Both pain and ADLs were evaluated postoperatively with recall of preoperative status. Following surgery, all thumbs were immobilized in a static splint for 3 weeks and then allowed progressive use. Median improvements in hemi-trapezium resections included grip, 22.5 lb; pinch, 4.7 lb; and ADLs, 33%. Pain was reduced a median of 7.0 cm on the VAS. Median improvements in full trapezium resection included grip, 29.5 lb; pinch, 0 lb; ADLs, 60%; and pain reduction, 8 cm on the VAS. This follow-up study suggests that satisfactory results can be achieved in pain reduction, strength, and ADLs with an immobilization period of only 3 weeks and without the use of K-wires following carpometacarpal (CMC) arthroplasty.

A comparison of breast cancer secondary prevention activities and satisfaction with access and communication issues in women 50 and over

BACKGROUND

Between 1950 and 1990, the incidence of breast cancer increased about 52% and the mortality rate increased 4%. Prevention programs (mammograms and clinical breast exams) can positively affect both cost control and mortality rates. Balancing the costs of preventive screening against the potential savings is a part of an ongoing debate centering on the age at which women should have yearly mammograms. Yet, if all agencies agree that women aged 50 and over should receive yearly mammograms, then why are so many women aged 50 and over not being screened?

METHODS

Using previously validated instruments, this study surveyed residents of Spokane County, Washington. Respondents (1,850 returned of 2,600) were compared over time by demographic characteristics and by insurance type to identify any significant differences between those who had preventative screens and those who did not. Issues involving access to screening and communication with healthcare providers were also examined.

RESULTS

Factors that affect whether women receive preventative screening include insurance type, provider type, long waiting times, and poor communication among the doctor, the staff, and the patient.

CONCLUSION

The most important determinant to whether preventative screening is being conducted is the relationship between the patient and their healthcare provider.

Role of cytokines in non-genotoxic hepatocarcinogenesis: cause or effect?

Chemicals with the potential to cause cancer through damaging DNA can be readily identified in a range of in vitro screens that detect genotoxicity. However, many carcinogens are non-genotoxic yet cause rodent tumours, particularly in the liver. Some non-genotoxic carcinogens such as the peroxisome proliferators (PPs) act directly to cause liver growth and proliferation, whereas others such as carbon tetrachloride cause liver damage, followed by regenerative hyperplasia. Current data support a role for cytokines such as tumour necrosis factor alpha (TNFalpha) and interleukin 1 (IL1) in hepatocarcinogenesis. However, these data give rise to conflicting hypotheses; in some experimental models, TNFalpha appears to mediate damage, whereas in others it is postulated to play a role in tissue repair. Recently, we have shown that TNFalpha acting via TNFalpha receptor 1 and p38 MAP kinase suppresses hepatocyte apoptosis. However, when new protein synthesis is disabled, TNFalpha becomes a death signal. An understanding of the role of cytokines in rodent hepatocarcinogenesis will allow the development of markers that can be used to identify, at an early stage, those chemicals with the potential to induce rodent tumours.

Identification of a possible association between carbon tetrachloride-induced hepatotoxicity and interleukin-8 expression

Hepatotoxicants can elicit liver damage by various mechanisms that can result in cell necrosis and death. The changes induced by these compounds can vary from gross alterations in DNA repair mechanisms, protein synthesis, and apoptosis, to more discrete changes in oxidative damage and lipid peroxidation. However, little is known of the changes in gene expression that are fundamental to the mechanisms of hepatotoxicity. We have used DNA microarray technology to identify gene transcription associated with the toxicity caused by the hepatotoxicant carbon tetrachloride. Labeled poly A+ RNA from cultured human hepatoma cells (HepG2) exposed to carbon tetrachloride for 8 hours was hybridized to a human microarray filter. We found that 47 different genes were either upregulated or downregulated more than 2-fold by the hepatotoxicant compared with dimethyl formamide, a chemical that does not cause liver cell damage. The proinflammatory cytokine interleukin-8 (IL-8) was upregulated over 7-fold compared with control on the array, and this was subsequently confirmed at 1 hour and 8 hours by Northern blot analyses. We also found that carbon tetrachloride caused a time-dependent increase in interleukin-8 protein release in HepG2 cells, which was paralleled by a decrease in cell viability. These data demonstrate that carbon tetrachloride causes a rapid increase in IL-8 mRNA expression in HepG2 cells and that this increase correlates with a later and significant increase in the levels of interleukin-8 protein. These results illustrate the potential of microarray technology in the identification of novel gene changes associated with toxic processes.

PPAR alpha-mediated responses in the rodent liver: a holistic biochemical view

Carcinogenesis through the direct action of genotoxic, DNA damaging chemicals is an established and well-studied paradigm. As yet there are no short term tests available for non-genotoxic rodent carcinogens that do not damage DNA but cause liver tumours in long term rodent bioassays. A key aim is to develop short term in vitro screens for the detection of nongenotoxic carcinogens, and this requires knowledge of the mode or mechanism of action of this class of chemicals. The largest and most chemically diverse family of non-genotoxic hepatocarcinogens is the peroxisome proliferators (PPs) such as hypolipidaemic fibrate drugs, plasticizers used in clingwrap/medical tubing and certain pesticides and solvents. PPs mediate their biological responses via activation of the transcription factor PPAR alpha (peroxisome proliferator activated receptor alpha), a member of the nuclear hormone receptor superfamily. PPAR alpha activation is responsible for the pleiotropic effects of PPs in rodent liver such as the induction of enzymes of the b-oxidation pathway, hepatocyte DNA synthesis, liver enlargement and tumourigenesis. Although much is known, we are far from defining the key cell cycle regulating targets of PPs, due perhaps to past limitations of technology. The technology of proteomics allows quantitative measurement of the expression levels of potentially thousands of individual genes at the protein level on exposure to toxic insult. This is predicted to revolutionise the way many biological systems are investigated. Here we review the current knowledge of proteins involved in the response to peroxisome proliferators and describe the impact of proteomics in this field.

Role of hepatic non-parenchymal cells in the response of rat hepatocytes to the peroxisome proliferator nafenopin in vitro

Induction of liver cancer by peroxisome proliferators such as nafenopin is frequently associated with increased liver growth, increased DNA synthesis and suppression of apoptosis. The cytokine, tumour necrosis factor alpha (TNF alpha), and non-parenchymal liver cells have been implicated in mediating the hepatic response to peroxisome proliferators. Here, we have investigated the dependency of the hepatocyte response to peroxisome proliferators on non-parenchymal cells, a major source of hepatic cytokines. Addition of non-parenchymal cells, or conditioned medium from non-parenchymal cell cultures, increased DNA synthesis (220% and 270% of control, respectively) and suppressed transforming growth factor beta(1)-induced hepatocyte apoptosis (32% and 54% of control, respectively). Removal of non-parenchymal cells from normal hepatocyte cultures prevented both the nafenopin- and TNF alpha-induced increase in DNA synthesis and suppression of hepatocyte apoptosis; this response was restored by returning non-parenchymal cells to the purified hepatocytes. TNF alpha was detected in the medium of non-parenchymal cell (3-15 pg/ml) and normal hepatocyte cultures (25-100 pg/ml) by bioassay using L929 cells. However, the contribution of TNF alpha released from non-parenchymal cells was small compared with that released spontaneously by hepatocytes. Nafenopin significantly increased the release of TNF alpha from non-parenchymal cells to 56 +/- 18 pg/ml, but had little effect on TNF alpha release by hepatocytes. However, the concentration of exogenous TNF alpha required to elicit a response in hepatocytes was 100 pg/ml and above. These data provide evidence that hepatic non-parenchymal cells are permissive for the growth response of hepatocytes in vitro to peroxisome proliferators and this may be mediated, at least in part by TNF alpha. However, the levels of TNF alpha released spontaneously or in response to peroxisome proliferators are insufficient per se to induce a growth response.

PPARalpha-dependent alteration of GRP94 expression in mouse hepatocytes

The adverse effects of the peroxisome proliferators (PPs), a class of rodent nongenotoxic hepatocarcinogens, include suppression of apoptosis, induction of hepatocyte proliferation, and liver enlargement which eventually leads to tumours. The response to PPs is mediated by the peroxisome proliferator activated receptor alpha (PPARalpha). We carried out proteomic analyses of PP-treated hepatocytes from wild-type and PPARalpha-null mice to identify the molecular pathways underlying the adverse effects of PPs. We have identified eighteen protein spots exhibiting differential expression in PP-treated wild-type mouse hepatocytes. Several proteins involved in lipid metabolism pathways, but also ATP synthase beta subunit, which are regulated by PPs were identified. In addition, both 2D silver-stained gels and Western blotting analysis indicated that the anti-apoptotic glucose-regulated protein 94 (GRP94) is consistently overexpressed upon stimulation with PPs, providing us with novel insights into the anti-apoptotic mechanism activated by PPs.

Transforming growth factor-beta(1) induces apoptosis in rat FaO hepatoma cells via cytochrome c release and oligomerization of Apaf-1 to form a approximately 700-kd apoptosome caspase-processing complex

In mammalian cells, non receptor-mediated apoptosis occurs via the cytochrome c-dependent assembly of a approximately 700-kd apoptotic protease-activating factor 1 (Apaf-1)/caspase-9 containing apoptosome complex. This initiates the postmitochondrial-mediated effector caspase cascade. We now show that receptor mediated transforming growth factor beta(1) (TGF-beta(1))-induced apoptosis in rat hepatoma cells is accompanied by processing and activation of caspases-2, -3, -7, and -8. Furthermore, we show that caspase activation is mediated via the release of cytochrome c and the oligomerization of Apaf-1 into an approximately 700-kd apoptosome complex. Similarly, in vitro activation of hepatoma cell lysates with 2'-deoxyadenosine 5'-triphosphate (dATP) results in the formation of the approximately 700-kd apoptosome complex, which recruits and processes caspases-3 and -7. Z-VAD.FMK [benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethylketone], the pan-caspase inhibitor totally inhibits dATP-stimulated caspase activation but does not block the assembly of the large Apaf-1 containing apoptosome complex. However, the recruitment and subsequent processing of caspases-3 and -7 to the apoptosome is blocked. Similarly, in intact cells, although Z-VAD.FMK blocked TGF-beta(1)-induced apoptosis, it did not prevent the oligomerization of Apaf-1 into the apoptosome. However, recruitment and processing of caspases-3 and -7 were prevented by Z-VAD.FMK. These data show that TGF-beta(1) induces apoptosis via release of cytochrome c and activation of the Apaf-1 apoptosome complex, which initiates the caspase cascade.

A dominant negative form of IKK2 prevents suppression of apoptosis by the peroxisome proliferator nafenopin

Peroxisome proliferators (PPs) are a class of non-genotoxic chemicals that cause rodent liver enlargement and hepatocarcinogenesis. In primary rat hepatocyte cultures, PPs suppress spontaneous apoptosis and that induced by a number of pro-apoptotic stimuli such as transforming growth factor-beta(1). Tumour necrosis factor alpha (TNF-alpha) and the transcription factor NFkappaB have been implicated in the mode of action of PPs. TNF-alpha signalling to NFkappaB is thought to be responsible for many of the effects elicited by this cytokine. NFkappaB regulates gene expression in immunity, stress responses and the inhibition of apoptosis. Activation of NFkappaB requires the successive action of NFkappaB-inducing kinase and the phosphorylation of NFkappaB inhibitory proteins (IkappaB) by an IkappaB kinase (IKK) complex. The IKK2 subunit of IkappaB kinase is thought to be essential for NFkappaB activation and prevention of apoptosis. To determine whether IKK2 plays a role in the suppression of apoptosis by PPs, we expressed a dominant negative form of IKK2 (IKK2dn) in primary rat hepatocyte cultures. Infection with an adenovirus construct expressing IKK2dn caused apoptosis in control primary rat hepatocytes in the absence of exogenous TNF-alpha. Moreover, IKK2dn-induced apoptosis could not be rescued by addition of TNF-alpha or the peroxisome proliferator nafenopin. These results demonstrate a requirement for intracellular signalling pathways mediated by IKK2 in the suppression of apoptosis by the PP class of hepatocarcinogens.

Proteomic analysis of differential protein expression in primary hepatocytes induced by EGF, tumour necrosis factor alpha or the peroxisome proliferator nafenopin

Peroxisome proliferators are nongenotoxic rodent-liver carcinogens that have been shown to cause both an induction of hepatocyte proliferation and a suppression of apoptosis. Both epidermal growth factor (EGF) and the peroxisome proliferator nafenopin induce DNA replication in primary rat hepatocyte cultures, but apparently through different signalling pathways. However, both EGF and nafenopin require tumour necrosis factor alpha (TNFalpha) signalling to induce DNA replication. By examining proteins isolated from rat primary hepatocyte cultures using two-dimensional gel electrophoresis and mass spectrometry, we found that proteins showing an altered expression pattern in response to nafenopin differed from those showing altered expression in response to EGF. However, many proteins showing altered expression upon stimulation with TNFalpha were common to both the EGF and nafenopin responses. These proteome profiling experiments contribute to a better understanding of the molecular mechanisms involved in the response to peroxisome proliferators. We found 32 proteins with altered expression upon stimulation with nafenopin, including muscarinic acetylcholine receptor 3, intermediate filament vimentin and the beta subunit of the ATP synthase. These nonperoxisomal protein targets offer insights into the mechanisms of peroxisome proliferator-induced carcinogenesis in rodents and provide opportunities to identify toxicological markers to facilitate early identification of nongenotoxic carcinogens.

The nongenotoxic hepatocarcinogens diethylhexylphthalate and methylclofenapate induce DNA synthesis preferentially in octoploid rat hepatocytes

Diethylhexylphthalate (DEHP), a rodent carcinogen, and 1,4-dichlorobenzene (DCB), a noncarcinogen in rat liver, are potent hepatomitogens. We have reported previously that 7-day dosing with DEHP induced a higher bromodeoxyuridine labeling index (LI) in binuclear octoploid (2x4N) rat hepatocytes than did DCB, suggesting that induction of DNA synthesis in 2x4N hepatocytes might represent a more substantial carcinogenic risk. We compared 2 additional rodent hepatocarcinogens, methylclofenapate (MCP) and phenobarbitone, with ethylene thiourea (ETU), a noncarcinogenic hepatomitogen in rat. All 3 chemicals increased hepatic LI; the 8N population had the highest LI, but only the carcinogens increased LI in the 2x4N and 4N populations. To identify the target population for induction of DNA synthesis, we used a 1-hour pulse label at the peak of induction. The results were consistent with the 7-day data, and again the highest LI was in the 8N population. The nongenotoxic rodent carcinogens MCP and DEHP induced a significant increase in the LI in the 2x4N population, whereas ETU and DCB did not. These data support the hypothesis that increased DNA synthesis within the minority 2x4N population may be more significant for subsequent hepatocarcinogenesis.

Nafenopin causes protein kinase C-mediated serine phosphorylation and loss of function of connexin 32 protein in rat hepatocytes without aberrant expression or localization

The characteristics and mechanism of the inhibition of connexin-mediated gap junctional communication by the non-genotoxic rodent hepatocarcinogen, nafenopin, has been studied in rat hepatocytes. Nafenopin caused a time- and concentration-dependent inhibition of dye coupling in hepatocytes as assessed by transfer of microinjected lucifer yellow. A half-maximum inhibitory effect of nafenopin occurred at approximately 50 microM, which was not cytotoxic. The inhibitory effect was reversible since a significant recovery of communication was observed 3 h after removal of the chemical. The protein kinase inhibitor Gö6976 prevented the inhibition of dye coupling, but a tyrosine kinase inhibitor (genistein) did not. Connexin 32 and 26 protein expression, as assessed by immunoblotting, was similar in nafenopin-treated hepatocytes compared to controls, with the exception that in a 10-h culture with nafenopin, the level of connexin 26 was elevated compared to controls. Immunohistochemistry indicated that the localization of plaques containing connexin 32 was not affected in hepatocytes by nafenopin. Immunoprecipitated connexin 32 was, however, detected by an anti-phosphoserine antibody following nafenopin treatment, but not in controls. This serine phosphorylation was prevented in the presence of Gö6976. The results give further support for a role of protein kinase C in the post-translational inactivation of connexin 32 function in rat hepatocytes by nafenopin.

Species differences in response to diethylhexylphthalate: suppression of apoptosis, induction of DNA synthesis and peroxisome proliferator activated receptor alpha-mediated gene expression

Diethylhexylphthalate (DEHP) is a phthalate plasticizer that belongs to the peroxisome proliferator (PP) class of rodent nongenotoxic hepatocarcinogens. Previously, we have shown that MEHP (a principal metabolite of DEHP and the proximal PP) induced DNA synthesis and suppressed apoptosis in rat but not in human hepatocytes in vitro. Here, we present further studies of species differences in response to DEHP. In rats, 4 days of exposure to DEHP (950 mg/kg per day by gavage) induced peroxisomal beta-oxidation, DNA synthesis and suppressed apoptosis. In contrast, there was no response of guinea pig liver to DEHP. In rat hepatocytes in vitro, MEHP (250, 500 and 750 microM) induced peroxisomal beta-oxidation, DNA synthesis and suppressed apoptosis. In contrast to the pleiotropic response noted in rat hepatocytes, there was no response of human hepatocytes to 250, 500 or 750 microM MEHP. PPs activate the peroxisome proliferator activated receptor alpha (PPARalpha) that binds to DNA at peroxisome proliferator response elements (PPREs) within the promoters of PP-responsive genes such as rat acyl CoA oxidase (ACO). However, the human ACO gene promoter differs at three bases within the PPRE from the rat ACO promoter and appears refractory to PPs. To address species differences in response to DEHP at the molecular level, we used promoter-reporter gene assays to compare the ability of MEHP to induce gene expression from the rat or the human ACO promoter. MEHP gave a concentration-dependent increase in reporter gene expression from the rat ACO gene promoter with either mouse or human PPARalpha. In contrast, the human ACO promoter was unable to drive MEHP-induced gene transcription irrespective of the species origin of PPARalpha. These data provide further weight of evidence at the cellular and molecular levels for a lack of risk to human health from the phthalate DEHP.

Species differences in peroxisome proliferation; mechanisms and relevance

Peroxisome proliferators are a class of structurally diverse chemicals, which induce liver carcinogenesis in rodents through interaction and activation of the Peroxisome Proliferator-Activated Receptor alpha (PPARalpha). PPARalpha agonists elicit a powerful pleiotropic response, which include hypolipidaemia. We have examined the response of species that are classically unresponsive to peroxisome proliferators. Whereas hamster responds to PPARalpha agonists by hepatomegaly and induction of marker genes, the guinea pig does not undergo hepatomegaly or induction of marker genes, such as CYP4A13. Both the hamster and the guinea pig have PPARalpha, and the guinea pig receptor has been characterised to be fully functional, as demonstrated in reporter gene expression assays. However, the guinea pig PPARalpha is expressed at low levels in liver, and the currently favoured hypothesis to explain species differences in hepatic peroxisome proliferation invokes the low level of PPARalpha as the principal determinant of species responsiveness. However, the demonstration that guinea pigs and humans undergo hypolipidaemia induced by PPARalpha-agonists calls into question the mode of action of PPARalpha agonists in "non-responsive" species.

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 proliferation in nongenotoxic carcinogenesis: species differences and role of PPARalpha

Peroxisome proliferators (PPs) are nongenotoxic rodent hepatocarcinogens that cause liver enlargement and hepatocarcinogenesis associated with peroxisome proliferation, induction of hepatocyte DNA synthesis and suppression of apoptosis. Acyl CoA oxidase (ACO) is a key enzyme of peroxisomal beta-oxidation and its transcriptional activation by PPs is often used as marker for the rodent response. PPs activate the peroxisome proliferator activated receptor-alpha, PPARalpha. Recent data suggest a role for tumour necrosis factor alpha (TNFalpha). This cytokine appears to be permissive for a PPARalpha-dependent growth response to PPs. Humans and guinea pigs appear to be nonresponsive to the adverse effects of PPs noted in rodents. These species differences can be attributed to reduced quantity of full length functional PPARalpha in human liver and evidence supports the presence of a truncated form of PPARalpha, hPPARalpha8/14 in human liver. In addition, species differences could be attributed to qualitative differences in the PPARalpha-mediated response because the promoter for human ACO differs in sequence and activity from the rat equivalent. These data contribute to our understanding of how chemicals may cause tumours in rodents and how this response may differ in humans.

Tumour necrosis factor alpha (TNFalpha): role in suppression of apoptosis by the peroxisome proliferator nafenopin

The peroxisome proliferator (PPs) class of non-genotoxic rodent hepatocarcinogens induce mouse hepatocyte DNA synthesis and suppress apoptosis. This phenotype can be reproduced in vitro using exogenous tumour necrosis factor alpha (TNFalpha), suggesting a role for TNFalpha in mediating the liver growth response to PPs. In hepatocytes isolated from the peroxisome proliferator activated receptor alpha (PPARalpha) null mouse, PPs are unable to stimulate DNA synthesis or to suppress either spontaneous or TGFbeta1-induced apoptosis. However, the ability of TNFalpha to modulate hepatocyte survival and growth is unaltered, suggesting that TNFalpha acts independently or downstream of PPARalpha to mediate the growth changes associated with PPARalpha activation. Since PPARalpha is a ligand activated transcription factor, we determined if TNFalpha gene expression was altered by PP treatment during an early time window preceding PP-induced growth changes. However there was no induction of TNFalpha expression by nafenopin over the constitutive levels noted in control cultured cells. In summary, TNFalpha acts downstream or independently of PPARalpha to mediate the suppression of apoptosis and induction of DNA synthesis by PPs. In this in vitro model, the PP nafenopin do not appear to mediate de novo TNFalpha gene expression suggesting that the response to nafenopin may be mediated by bioactivation or release of pre-existing TNFalpha protein from Kupffer cells.

The role of protein kinase B and mitogen-activated protein kinase in epidermal growth factor and tumor necrosis factor alpha-mediated rat hepatocyte survival and apoptosis

Perturbation of hepatocyte growth regulation is associated with a number of liver diseases such as fibrosis and cancer. These diseases are mediated by a network of growth factors and cytokines that regulate the induction of hepatocyte proliferation and apoptosis. In this study, we have investigated the role of signaling pathways activated by tumor necrosis factor alpha (TNF-alpha) and epidermal growth factor (EGF) in the regulation of apoptosis induced by transforming growth factor beta(1) (TGF-beta(1)), because this physiological factor is believed to regulate spontaneous apoptosis in the liver. We show that pretreatment with (10 ng/mL) EGF or (25 ng/mL) TNF-alpha can suppress TGF-beta(1)-induced apoptosis by 73% and 50%, respectively, in isolated rat hepatocytes. However, suppression of TGF-beta(1)-induced apoptosis by EGF and TNF-alpha occurs via different protein kinase signaling pathways. Using specific inhibitors, we show that suppression of apoptosis by EGF is dependent on activation of phosphoinositide 3-kinase (PI 3-kinase) and the extracellular signal regulated kinase (ERK) mitogen-activated protein (MAP) kinase pathways, but not p38 MAP kinase. In contrast, suppression of TGF-beta(1)-induced apoptosis by TNF-alpha does not require PI 3-kinase and protein kinase B (PKB or Akt)-mediated pathways, but is dependent on ERK and p38 MAP kinase activity. These data contribute to our understanding of the intracellular survival signals that play a role in normal liver homeostasis and in diverse pathological conditions.

Induction of DNA replication by peroxisome proliferators is independent of both tumour necrosis factor (alpha) priming and EGF-receptor tyrosine kinase activity

Peroxisome proliferators (PPs) cause hepatocyte proliferation and tumorigenesis in rodent liver. PPs induce hepatocyte DNA synthesis although the mechanism is unclear. Tumour necrosis factor (alpha) (TNF(alpha)) and epidermal growth factor (EGF) have been implicated in mediating this growth response since these factors induce a threefold and 17.2-fold increase, respectively, in DNA synthesis in rat primary hepatocyte cultures. Previously, others have suggested that TNF(alpha) acts as a primer to sensitise hepatocytes to the proliferative effects of growth factors. Indeed, here we show that costimulation with TNF(alpha) and a suboptimal (4-20% of optimal) concentration of EGF permits an 11.7-fold increase in DNA synthesis in rat primary hepatocyte cultures. The PP nafenopin induced a 2. 3-fold increase in DNA synthesis but there was no further increase upon co-administration of either TNF(alpha) or a suboptimal concentration of EGF. Furthermore, there was no gross dysregulation of the CDK and cyclin protein expression profile upon stimulation with nafenopin. Using a specific epidermal growth factor receptor tyrosine kinase inhibitor (4-(3-chloro-4-fluorophenylamino)-7-methoxy-6- (3-?1-pyrolidino)-propoxyquinazoline, EGFR-TKI), we show that signalling through EGF-R is not required for nafenopin-induced DNA synthesis. The EGFR-TKI also prevented progression into S phase upon stimulation with TNF(alpha), but DNA synthesis was not reduced to control levels, indicating that TNF(alpha) has a mitogenic activity in the absence of EGF signalling. Therefore, although TNF(alpha) can act as a priming factor for growth factors such as EGF, nafenopin does not appear to act via this mechanism.

Role for tumor necrosis factor alpha receptor 1 and interleukin-1 receptor in the suppression of mouse hepatocyte apoptosis by the peroxisome proliferator nafenopin

Peroxisome proliferators (PPs) cause rodent liver enlargement and tumors. In vitro, PPs induce rat and mouse hepatocyte DNA synthesis and suppress apoptosis, a response mimicked by exogenous tumor necrosis factor alpha (TNFalpha). Here, we determine the role of TNF receptor 1 (TNFR1), TNF receptor 2 (TNFR2), and nuclear factor kappa beta (NFkappaB) in the response of mouse hepatocytes to the PP, nafenopin. Nafenopin (50 micromol/L) induced DNA synthesis as measured by bromodeoxyuridine (BrdU) incorporation, suppressed cell death as measured by Hoechst 33258 staining, induced peroxisomal beta-oxidation as measured by cyanide insensitive palmitoyl CoA oxidation (PCO) and caused activation of nuclear factor kappa beta (NFkappaB) as determined by electrophoretic mobility gel shift assay (EMSA). The induction of DNA synthesis and the suppression of apoptosis in response to nafenopin was abrogated completely by blocking antibodies to TNFR1 but not to TNFR2. In contrast, the induction of peroxisomal beta-oxidation by nafenopin was not blocked by the anti-TNFR1 antibody. Next, we evaluated the response of hepatocytes to interleukin-1 (IL-1), another proinflammatory cytokine. IL-1alpha (2.5 ng/mL) and, to a lesser extent, IL-1beta (5 ng/mL), shared the ability of TNFalpha to induce DNA synthesis and suppress apoptosis. In addition, anti-IL-1 receptor, type 1/p80 (IL-1R) antibodies were able to abrogate the response to nafenopin. IL-1alpha was still able to perturb hepatocyte growth in the presence of the anti-TNFR1 antibody suggesting that IL-1alpha acts independently rather than by elaborating TNFalpha. In summary, these data provide additional evidence for a role for hepatic cytokines in the perturbation of hepatocyte growth by PPs such as nafenopin.

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.

Suppression of apoptosis and induction of DNA synthesis in vitro by the phthalate plasticizers monoethylhexylphthalate (MEHP) and diisononylphthalate (DINP): a comparison of rat and human hepatocytes in vitro

Diethylhexylphthalate (DEHP) and diisononylphthalate (DINP) are plasticizers with many important commercial, industrial and medical applications. However, both DEHP and DINP are rodent peroxisome proliferators (PPs), a class of compounds that cause rodent liver tumours associated with peroxisome proliferation, induction of hepatic DNA synthesis and the suppression of apoptosis. Despite these effects in the rodent, humans appear to be nonresponsive to the adverse effects of PPs. Previously, we have shown that the fibrate hypolipidaemic peroxisome proliferator, nafenopin, induced DNA synthesis and suppressed apoptosis in rat but not in human hepatocytes. In this work, we have examined species differences in the response of rat and human hepatocytes to DEHP and DINP in vitro. In rat hepatocytes in vitro, both DINP and MEHP (a principle metabolite of DEHP and the proximal peroxisome proliferator) caused a concentration-dependent induction of DNA synthesis and suppression of both spontaneous and transforming growth factor beta1 (TGFbeta1)-induced apoptosis. Similarly, both MEHP and DINP caused a concentration-dependent induction of peroxisomal beta-oxidation although the response to DINP was less robust. In contrast to the pleiotropic response noted in rat hepatocytes, neither DINP nor MEHP caused an induction of beta-oxidation, stimulation of DNA synthesis and suppression of apoptosis in human hepatocytes cultured from three separate donors. These data provide evidence for species differences in the hepatic response to the phthalates DEHP and DINP, confirming that human hepatocytes appear to be refractory to the hepatocarcinogenic effects of PPs first noted in rodents.

Species differences in sequence and activity of the peroxisome proliferator response element (PPRE) within the acyl CoA oxidase gene promoter

In rats and mice, peroxisome proliferators (PP) cause liver enlargement, hepatocarcinogenesis and peroxisome proliferation associated with induction of enzymes such as acyl CoA oxidase (ACO). However, humans appear to be non-responsive to the adverse effects of PPs such as ACO induction. PPs activate the peroxisome proliferator activated receptor alpha (PPARalpha) that binds to DNA at peroxisome proliferator response elements (PPREs) within the promoters of PP-responsive genes. When the human ACO promoter was cloned previously (Varanasi et al., 1996. Journal of Biological Chemistry, 271, 2147-2155), it was reported to contain a PPRE (5' AGGTCA C TGGTCA 3') that bound PPARalpha and could be activated in vitro by Wyeth-14,643 (at >1 mM) or DEHP (at > 1.5 mM). In contrast, when we cloned the ACO gene promoter from a human liver biopsy, it was non-responsive to PPs and differed at three positions (5' AGGTCA G CTGTCA 3') from that reported previously (Woodyatt et al., 1999. Carcinogenesis, 20, 369-375). Subsequent to this, Varanasi et al. re-sequenced their constructs and obtained the same sequence as we have described (Varanasi et al., 1998. Journal of Biological Chemistry, 273, 30832). However, the observation that the errant sequence (5' AGGTCA C TGGTCA 3') was able to bind PPARalpha still remained since it appears that this sequence was used by Varanasi et al. (1996) to design oligonucleotides for their DNA binding analyses. Thus, if the 5' AGGTCA C TGGTCA 3' sequence did exist in some individuals, it could be active. To address this, we used site-directed mutagenesis to create a promoter fragment that contained the errant sequence. This reporter gene was transfected into NIH3T3 cells together with a plasmid expressing mPPARalpha, and assessed for its ability to drive PP-mediated gene transcription using a non-toxic concentration of Wyeth-14,643 (100 microM). This human ACO promoter was also inactive, unlike the equivalent rat ACO promoter fragment used as a positive control. Next, we used site directed mutagenesis to convert the PPRE found in the active rat ACO promoter (3' AGGACA A AGGTCA 5') to our inactive human sequence (AGGTCA G CTGTCA). This human PPRE was unable to drive PP-induced gene transcription even in the context of the rat ACO promoter suggesting that the activity of the rat promoter is conferred principally by the PPRE sequence, even though it may be enhanced by flanking sequences. These data confirm that neither the native nor the errant human ACO gene PPRE can respond to PPs. The absence of a responsive PPRE contributes to our understanding of the lack of response of humans to some of the adverse effects of the PP class of non-genotoxic hepatocarcinogens.

Addition of peroxisome proliferator-activated receptor alpha to guinea pig hepatocytes confers increased responsiveness to peroxisome proliferators

The fibrate drugs, such as nafenopin and fenofibrate, show efficacy in hyperlipidemias but cause peroxisome proliferation and liver tumors in rats and mice via nongenotoxic mechanisms. However, humans and guinea pigs appear refractory to these adverse effects. The peroxisome proliferator (PP)-activated receptor alpha (PPAR alpha) mediates the effects of PPs by heterodimerizing with the retinoid X receptor (RXR) to bind to DNA at PP response elements (PPREs) upstream of PP-regulated genes, such as acyl-CoA oxidase. Hepatic expression of PPAR alpha in guinea pigs and humans is low, suggesting that species differences in response to PPs may be due at least in part to quantity of PPAR alpha. To test this hypothesis, we introduced mouse PPAR alpha and its heterodimerization partner, RXR alpha, into guinea pig hepatocytes by transient transfection and determined responsiveness to the PP nafenopin by cyanide-insensitive palmitoyl-CoA oxidation (CIPCO). Expression of the mRNA for mouse PPAR alpha in transfected guinea pig hepatocytes was verified using species-specific PCR. In guinea pig hepatocytes transfected with control plasmids and treated with 50 microM nafenopin in the absence or presence of the RXR ligand, 9-cis-retinoic acid (5 microM) gave only a 1.7 +/- 1.5- or 3.3 +/- 1.5-fold induction in CIPCO, respectively. However, addition of ligands to hepatocytes co-transfected with both mPPAR alpha and RXR gave a strong induction of CIPCO (14.8 +/- 8.6-fold). Mouse, human, and guinea pig PPAR alpha showed equivalent function in the CIPCO assays. Thus, quantity of PPAR alpha plays a significant role in the lack of response to PPs in guinea pigs. In humans, however, lack of PPAR alpha may be only one factor dictating lack of response because recent data show that the human acyl-CoA oxidase gene lacks a functional PP response element.

G1-arrested FaO cells re-enter the cell cycle upon stimulation with the rodent non-genotoxic hepatocarcinogen nafenopin

The peroxisome proliferators are rodent non-genotoxic hepatocarcinogens that suppress apoptosis and induce DNA replication, cell proliferation and liver tumours. In order to investigate the effect of peroxisome proliferators on cell cycle progression, we arrested the well-differentiated rat hepatoma cell line FaO in the G1 phase of the cell cycle. Under these conditions, CDK2 and CDK4 protein expression remained unchanged compared with proliferating cells, but expression of cyclin D1 and p27(KIP1) was down-regulated and cyclin E accumulated in the inactive form. G1-arrested cells were able to enter the cell cycle on addition of exogenous growth factors such as epidermal growth factor (EGF) or hepatocyte growth factor (HGF) and replicate their DNA within 12 to 24 h of re-stimulation. Upon release from G1 arrest, CDK2 protein expression was down-regulated and, surprisingly, p27(KIP1) expression was restored. Cyclin D1 and phosphorylated cyclin E accumulated at 12 h but were degraded by 24 h after addition of EGF. Importantly, the peroxisome proliferator nafenopin and tumour necrosis factor alpha were able to induce DNA replication. Thus, the profile of expression of cell cycle regulatory proteins upon stimulation with nafenopin is comparable with that induced by growth factors such as EGF.

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.

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.

Application of near-infrared spectroscopy for nondestructive analysis of Avicel powders and tablets

The purpose of this study was to use near-infrared spectroscopy (NIRS) as a nondestructive technique to (a) differentiate three Avicel products (microcrystalline cellulose [MCC] PH-101, PH-102, and PH-200) in powdered form and in compressed tablets with and without 0.5% w/w magnesium stearate as a lubricant; (b) determine the magnesium stearate concentrations in the tablets; and (c) measure hardness of tablets compressed at several compression forces. Diffuse reflectance NIR spectra from Avicel powders and tablets (compression forces ranging from 0.2 to 1.2 tons) were collected and distance scores calculated from the second-derivative spectra were used to distinguish the different Avicel products. A multiple linear regression model was generated to determine magnesium stearate concentrations (from 0.25 to 2% w/w), and partial least squares (PLS) models were generated to predict hardness of tablets. The NIRS technique could distinguish between the three different Avicel products, irrespective of lubricant concentration, in both the powdered form and in the compressed tablets because of the differences in the particle size of the Avicel products. The percent error for predicting the lubricant concentration of tablets ranged from 0.2 to 10% w/w. The maximum percent error of prediction of hardness of tablets compressed at the various compression forces was 8.8% for MCC PH-101, 5.3% for MCC PH-102, and 4.6% for MCC PH-200. The NIRS nondestructive technique can be used to predict the Avicel type in both powdered and tablet forms as well as to predict the lubricant concentration and hardness.

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.

The peroxisome proliferator nafenopin does not suppress hepatocyte apoptosis in guinea-pig liver in vivo nor in human hepatocytes in vitro

In rats and mice, nafenopin is a nongenotoxic hepatocarcinogen, which induces hepatic DNA synthesis and enzyme induction both in vivo and in hepatocyte cultures in vitro. However, humans and guinea-pigs are considered to be non-responsive to the liver growth effects of peroxisome proliferators (PPs). The ability to stimulate cell replication coupled with the ability to suppress apoptosis is thought to underpin the carcinogenicity of nongenotoxic carcinogens such as PPs. Previous studies in this laboratory have shown that in rats in vivo and in vitro nafenopin suppressed spontaneous hepatocyte apoptosis and that induced by the physiological negative growth regulator transforming growth factors beta1 (TGFbeta1). In addition nafenopin suppressed apoptosis in cultured hepatocytes from guinea-pig and hamster. The effects of PPs on apoptosis in human hepatocyte cultures is not known. To correlate these previous in vitro findings to the known species differences in hepatocarcinogenicity of PPs we have investigated the effects of nafenopin on guinea-pig liver growth in vivo. Also, we have examined the effects of nafenopin on apoptosis in cultures of human hepatocytes, a valuable model for human risk assessment. Nafenopin did not inhibit either spontaneous or TGFbeta1 induced apoptosis in human hepatocytes in vitro. Administration of nafenopin to guinea-pigs in vivo produced none of the changes seen previously in responsive species, such as rats and mice. There was no change in liver/body weight ratio, peroxisomal volume of hepatocytes or DNA synthesis as determined by incorporation of bromodeoxyuridine and there was no suppression of apoptosis. The lack of response to nafenopin in guinea-pigs in vivo and human hepatocytes in vitro provides further evidence that these species may be refractory to the liver growth effects of PPs despite the ability of guinea-pigs and humans to respond to PPs by alterations in lipid metabolism. The data presented add to our overall understanding of species differences in response to the PP class of rodent nongenotoxic carcinogens.

Suppression of hepatocyte apoptosis and induction of DNA synthesis by the rat and mouse hepatocarcinogen diethylhexylphlathate (DEHP) and the mouse hepatocarcinogen 1,4-dichlorobenzene (DCB)

Nongenotoxic rodent hepatocarcinogens do not damage DNA but cause liver tumours in the rat and mouse, associated with the induction of hepatic DNA synthesis. Previously, we have demonstrated that nongenotoxic hepatocarcinogens such as phenobarbitone and the peroxisome proliferator (PP), nafenopin, also suppress rat hepatocyte apoptosis. The nongenotoxic chemicals 1,4-dichlorobenzene (DCB) and the PP, diethylhexyl phthalate (DEHP), both induce high levels of DNA synthesis in rat liver in vivo, but only DEHP is hepatocarcinogenic in this species. Here, we investigate whether the difference in rat carcinogenicity of these two hepatic mitogens may be due to differences in their ability to suppress hepatocyte apoptosis. In rat hepatocytes in vitro, MEHP (the active metabolite of DEHP) induced DNA synthesis 2.5-fold (P = 0.001) and suppressed 10- and 4-fold, respectively both spontaneous (P = 0.0008) and transforming growth factor beta1 (TGFbeta1)-induced (P = 0.0001) apoptosis. DCB gave a small (1.7-fold) increase in DNA synthesis (P = 0.03) and a small (1.7- to 2-fold) suppression of both spontaneous (P = 0.022) and TGFbeta1-induced (P = 0.015) apoptosis. We next analysed the induction of DNA synthesis and the suppression of apoptosis in rat liver in vivo. Both DEHP and DCB were able to induce DNA synthesis although, as seen in vitro, the induction by DCB (4.2-fold; P = 0.023) was less marked than that with DEHP (13.4-fold; P = 0.007). Similarly, DEHP and DCB were both able to suppress rat hepatocyte apoptosis in vivo but the magnitude of the suppression was comparable; apoptosis was reduced to undetectable levels in four out of five animals with DCB and three out of five with DEHP. Since both chemicals suppressed apoptosis and induced DNA synthesis in rat liver but, overall, DCB was less potent, the disparate hepatocarcinogenic potential of these two chemicals could arise from differences in the magnitude of growth perturbation. To test this hypothesis, we repeated the studies in mouse, a species where both DCB and DEHP are hepatocarcinogenic. Both in vitro and in vivo, DCB and DEHP/MEHP were able to suppress apoptosis and induce hepatocyte DNA synthesis in the mouse with comparable potencies. The data support the hypothesis that the carcinogenicity of nongenotoxic hepatocarcinogens is associated strongly with the ability to perturb hepatocyte growth regulation. However, the ability to effect such changes is not unique to nongenotoxic carcinogens and is common to some noncarcinogenic chemicals, such as DCB, suggesting that the growth perturbation may need to exceed a threshold for carcinogenesis.

Perturbation of rodent hepatocyte growth control by nongenotoxic hepatocarcinogens: mechanisms and lack of relevance for human health (review)

During the development of new industrial and pharmaceutical chemicals, it is necessary to determine whether they are potential carcinogens. However, there are no short-term tests available for nongenotoxic carcinogens that do not damage DNA yet cause tumours in rodent bioassays. The peroxisome proliferators (PPs) constitute a diverse class of nongenotoxic carcinogens that include chemicals of therapeutic, industrial and environmental importance such as hypolipidaemic fibrate drugs, clingwrap/medical tubing plasticizers and certain pesticides and solvents. PPs induce DNA synthesis and suppress apoptosis in rat and mouse hepatocytes, leading to tumour formation. In addition to altering hepatocyte growth and survival, PPs cause peroxisome proliferation and the induction of enzymes of the beta-oxidation pathway. PPs mediate their biological responses in rodents via activation of the nuclear hormone receptor PPARalpha (peroxisome proliferator activated receptor alpha) which regulates expression of the genes associated with response to PPs. The mechanisms through which normally quiescent hepatocytes are recruited into the cell cycle currently remain obscure. However, it is probable that expression of hepatic cytokines by hepatic macrophages (Kupffer cells) may be involved. In common with other classes of nongenotoxic carcinogen, there are remarkable species differences in response to PPs; humans respond to the fibrate hypolipidaemic PPs via a reduction in serum cholesterol but appear refractory to the adverse effects of PPs such as hepatic peroxisome proliferation, DNA synthesis and tumour formation. The molecular basis of the observed species differences in response to PPs is unclear at present, but recent data support a quantitative hypothesis wherein PPARalpha expression levels are sufficient in humans to mediate hypolipidaemia, but too low for transcriptional regulation of the full battery of genes associated with the adverse effects seen in rodents such as peroxisome proliferation, liver enlargement and tumours. A more detailed understanding of the mechanisms through which these chemicals cause tumours in rodents and how humans may differ will assist in extrapolation of rodent data to human risk assessment.

The rodent non-genotoxic hepatocarcinogen nafenopin suppresses apoptosis preferentially in non-cycling hepatocytes but also elevates CDK4, a cell cycle progression factor

Rodent non-genotoxic hepatocarcinogens such as nafenopin suppress spontaneous and transforming growth factor beta1 (TGFbeta1)-induced rat hepatocyte apoptosis as well as inducing DNA synthesis. We wished to determine if these two processes are associated. In primary rat hepatocytes, nafenopin suppressed apoptosis from 1.9 to 0.63% but more apoptotic bodies were bromodeoxyuridine (BrdU)-labelled (0.35%) than predicted statistically from a random distribution of apoptosis within the cycling and non-cycling populations (0.10%). In contrast, TGFbeta1 induced hepatocyte apoptosis (7.8%) but fewer hepatocytes were BrdU-labelled (0.29%) than predicted (0.82%). Western blot analyses showed that nafenopin and TGFbeta1 had opposing effects on cyclin-dependent kinase 4 (CDK4) protein: nafenopin elevated CDK4 compared with controls, whereas TGFbeta1 caused a reduction. These data suggest that non-genotoxic hepatocarcinogens suppress apoptosis in the non-cycling population of hepatocytes and elevate CDK4 levels, possibly allowing potentially tumourigenic cells to enter the cell cycle.

Plasma transferrin receptor levels and indices of erythropoiesis and iron status in healthy term infants

PURPOSE

The goal of this study was to determine if the postnatal changes in plasma transferrin receptor (TfR) levels in healthy infants were associated with changes in erythropoiesis or iron status.

SUBJECTS AND METHODS

Longitudinal blood samples were obtained monthly from healthy term infants fed iron-fortified formula for the first 7 months and analyzed for plasma TfR and indices of erythropoiesis and iron status.

RESULTS

Plasma TfR level rose during the first 2 months of life (p < 0.002). When examined for its association with indices of erythropoiesis, plasma TfR was negatively associated with hemoglobin (Hb) (p < 0.01), and positively associated with plasma erythropoietin (EPO) concentration (p < 0.005) and absolute reticulocyte count (p < 0.005). Plasma TfR was not associated with erythrocyte protoporphyrin. Although indices of iron status were not suggestive of iron deficiency, plasma TfR was negatively associated with plasma ferritin, Tf saturation, and plasma iron, and positively associated with total iron binding capacity (TIBC) (p < 0.0001 for all).

CONCLUSIONS

Increases in plasma TfR levels were observed during normal infancy. The increases in plasma TfR levels correlate with increases in erythropoiesis without evidence for functional iron deficiency.

The rodent nongenotoxic hepatocarcinogen and peroxisome proliferator nafenopin inhibits intercellular communication in rat but not guinea-pig hepatocytes, perturbing S-phase but not apoptosis

Previously, we have shown that the peroxisome proliferator (PP), nafenopin, induces S-phase in rat hepatocytes and suppresses apoptosis in hepatocytes from both rat and guinea-pig. Here, we confirm and extend these findings by defining the time course of growth perturbation and by correlating this with species differences in loss of gap junctional intercellular communication (GJIC). GJIC is associated with nongenotoxic carcinogenesis, possibly reflecting a tumour suppresser role of the connexins. Fluorescence microscopy of Hoechst 33258-stained rat or guinea-pig hepatocyte monolayers showed 1% apoptosis during the first 8 h of culture, peaking to 2-2.5% at 20-24 h. Nafenopin suppressed apoptosis compared with controls in both rat and guinea-pig, measured at 20 h and 24 h onwards, respectively. The induction of S-phase in rat hepatocytes by nafenopin could be detected as early as 4 h after compound addition whereas S-phase was not altered by nafenopin in guinea-pig hepatocytes. Intercellular communication as measured by intercellular transfer of microinjected Lucifer Yellow CH was observed during the first 14 h of primary rat hepatocyte culture peaking at a maximum value of 88 +/- 3.0% after 7 h. In hepatocyte cultures from guinea-pig, dye-coupling levels were maintained between 88 +/- 3.0 and 93 +/- 3.0% within 2-10 h of culture and by 12 h showed only a slight decrease to 72 +/- 3.0%. In the rat, significant inhibition was observed at 4 h after administration of nafenopin since GJIC was reduced by 20 +/- 5% compared with vehicle control. By contrast, in the presence of nafenopin, the level of dye-coupling between guinea-pig hepatocytes did not decrease but remained between 85 +/- 5 and 93 +/- 3.0%, similar to that observed in control guinea-pig cultures. The data obtained contribute to our understanding of the role of GJIC inhibition in the perturbation of cell survival and proliferation caused by nongenotoxic hepatocarcinogens.

The non-genotoxic hepatocarcinogen nafenopin suppresses rodent hepatocyte apoptosis induced by TGFbeta1, DNA damage and Fas

The suppression of apoptosis may contribute to the carcinogenicity of the peroxisome proliferators (PPs), a class of non-genotoxic rodent hepatocarcinogens. Our previous work demonstrated that the PP nafenopin suppressed both spontaneous and transforming growth factor beta1 (TGFbeta1)-induced hepatocyte apoptosis both in vivo and in vitro. Here, we extend these observations by demonstrating the ability of nafenopin to suppress apoptosis induced by other major candidates for the signalling of cell death in the liver. Treatment of rat or mouse hepatocyte monolayers with TGFbeta1 or the DNA damaging drugs etoposide or hydroxyurea induced high levels of apoptosis. Western blot analysis did not support a role for either p53 or p21waf1 in etoposide-induced apoptosis in rat hepatocytes. Treatment of mouse hepatocytes with an agonistic anti-Fas antibody also resulted in an induction of high levels of apoptosis. Pre-addition and continued exposure to nafenopin suppressed apoptosis induced by all three stimuli. Overall, our studies demonstrate that the ability of nafenopin to protect hepatocytes from apoptosis is not restricted to species or apoptotic stimulus. It is possible, therefore, that the PPs may suppress apoptosis by acting on diverse signalling pathways. However, it seems more likely that nafenopin suppresses hepatocyte apoptosis elicited by each death stimulus by impinging on a core apoptotic mechanism.