The natural history and dose-response characteristics of enzyme-altered foci in rat liver following phenobarbital and diethylnitrosamine administration

Factors to consider in the use of stem cells for pharmaceutic drug development and for chemical safety assessment

Given the reality of the inadequacies of current concepts of the mechanisms of chemical toxicities, of the various assays to predict toxicities from current molecular, biochemical, in vitro and animal bioassays, and of the failure to generate efficacious and safe chemicals for medicines, food supplements, industrial, consumer and agricultural chemicals, the recent NAS Report, "Toxicity Testing in the 21st Century: A Vision and a Strategy", has drawn attention to a renewed examination of what needs to be done to improve our current approach for better assessment of potential risk to human health. This "Commentary" provides a major paradigm challenge to the current concepts of how chemicals induce toxicities and how these various mechanisms of toxicities can contribute to the pathogenesis of some human diseases, such as birth defects and cancer. In concordance with the NAS Report to take "... advantage of the on-going revolution in biology and biotechnology", this "Commentary" supports the use of human embryonic and adult stem cells, grown in vitro under simulated "in vivo niche conditions". The human being should be viewed "as greater than the sum of its parts". Homeostatic control of the "emergent properties" of the human hierarchy, needed to maintain human health, requires complex integration of endogenous and exogenous signaling molecules that control cell proliferation, differentiation, apoptosis and senescence of stem, progenitor and differentiated cells. Currently, in vitro toxicity assays (mutagenesis, cytotoxicity, epigenetic modulation), done on 2-dimensional primary rodent or human cells (which are always mixtures of cells), on immortalized or tumorigenic rodent or human cell lines do not represent normal human cells in vivo [which do not grow on plastic and which are in micro-environments representing 3 dimensions and constantly interacting factors]. In addition, with the known genetic, gender, and developmental state of cells in vivo, any in vitro toxicity assay will need to mimic these conditions in vitro. More specifically, while tissues contain a few stem cells, many progenitor/transit cells and terminally differentiated cells, it should be obvious that both embryonic and adult stem cells would be critical "target" cells for toxicity testing. The ultimate potential for in vitro testing of human stem cells will to try to mimic a 3-D in vitro micro-environment on multiple "organ-specific and multiple genotypic/gender "adult stem cells. The role of stem cells in many chronic diseases, such as cancer, birth defects, and possibly adult diseases after pre-natal and early post-natal exposures (Barker hypothesis), demands toxicity studies of stem cells. While alteration of gene expression ("toxico-epigenomics") is a legitimate endpoint of these toxicity studies, alteration of the quantity of stem cells during development must be serious considered. If the future utility of human stem cells proves to be valid, the elimination of less relevant, expensive and time-consuming rodent and 2-D human in vitro assays will be eliminated.

A preliminary operational classification system for nonmutagenic modes of action for carcinogenesis

This article proposes a system of categories for nonmutagenic modes of action for carcinogenesis. The classification is of modes of action rather than individual carcinogens, because the same compound can affect carcinogenesis in more than one way. Basically, we categorize modes of action as: (1) co-initiation (facilitating the original mutagenic changes in stem and progenitor cells that start the cancer process) (e.g. induction of activating enzymes for other carcinogens); (2) promotion (enhancing the relative growth vs differentiation/death of initiated clones (e.g. inhibition of growth-suppressing cell-cell communication); (3) progression (enhancing the growth, malignancy, or spread of already developed tumors) (e.g. suppression of immune surveillance, hormonally mediated growth stimulation for tumors with appropriate receptors by estrogens); and (4) multiphase (e.g., "epigenetic" silencing of tumor suppressor genes). A priori, agents that act at relatively early stages in the process are expected to manifest greater relative susceptibility in early life, whereas agents that act via later stage modes will tend to show greater susceptibility for exposures later in life.

Adventures in hepatocarcinogenesis

Neoplasia is a heritably altered, relatively autonomous growth of tissue. Hepatocarcinogenesis, the pathogenesis of neoplasia in liver, as modeled in the rat exhibits three distinct, quantifiable stages: initiation, promotion, and progression. Simple mutations and/or epigenetic alterations may result in the irreversible stage of initiation. The stage of promotion results from selective enhancement of cell replication and selective inhibition of cellular apoptosis of initiated cells dependent on the genetic and/or epigenetic alterations of the latter. The irreversible stage of progression results from initial karyotypic alterations that evolve into greater degrees of genomic instability. The initial genomic alteration in the transition from promotion to progression may involve primarily epigenetic mechanisms driven by epigenetic and genetic alterations fixed during the stage of promotion.

Hormesis and dose-response-mediated mechanisms in carcinogenesis: evidence for a threshold in carcinogenicity of non-genotoxic carcinogens

Recently the idea of hormesis, a biphasic dose-response relationship in which a chemical exerts opposite effects dependent on the dose, has attracted interest in the field of carcinogenesis. With non-genotoxic agents there is considerable experimental evidence in support of hormesis and the present review highlights current knowledge of dose-response effects. In particular, several in vivo studies have provided support for the idea that non-genotoxic carcinogens may inhibit hepatocarcinogenesis at low doses. Here, we survey the examples and discuss possible mechanisms of hormesis using phenobarbital, 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), alpha-benzene hexachloride (alpha-BHC) and other non-genotoxins. Furthermore, the effects of low and high doses of non-genotoxic and genotoxic compounds on carcinogenesis are compared, with especial attention to differences in mechanisms of action in animals and possible application of the dose-response concept to cancer risk assessment in humans. Epigenetic processes differentially can be affected by agents that impinge on oxidative stress, DNA repair, cell proliferation, apoptosis, intracellular communication and cell signaling. Non-genotoxic carcinogens may target nuclear receptors, cause aberrant DNA methylation at the genomic level and induce post-translational modifications at the protein level, thereby impacting on the stability or activity of key regulatory proteins, including oncoproteins and tumor suppressor proteins. Genotoxic agents, in contrast, cause genetic change by directly attacking DNA and inducing mutations, in addition to temporarily modulating the gene activity. Carcinogens can elicit a variety of changes via multiple genetic and epigenetic lesions, contributing to cellular carcinogenesis.

Dose and litter allocations in the design of teratological studies for detecting hormesis

BACKGROUND

Hormesis is being recognized in the field of toxicology due to the stimulating effects of some toxic compounds at low exposure levels. Therefore, it is desirable that experimental designs for toxicological studies be flexible enough to aid in the detection of hormetic effects. Current designs may still not have enough power to do this.

METHODS

A simulation study was conducted to determine teratological study designs that would yield more power over standard designs in detecting hormesis. Developmental toxicity endpoints of interest are the number of dead/resorbed or malformed fetuses in a litter. The simulation designs mimic teratological experiments in terms of sample size and number of dose levels. Modified designs with even dose spacing at low levels and reallocated litters are investigated to determine the power of hormetic detection.

RESULTS

Designs with reallocated litters (with more litters at low exposure levels than at high levels) and even dose spacing have more power than those with equal litters per group and uneven dose spacing.

CONCLUSIONS

Through appropriate modifications of current experimental designs, such as reallocation of litters and even dose spacing, we can better detect hormetic effects.

Detailed low-dose study of 1,1-bis(p-chlorophenyl)-2,2,2- trichloroethane carcinogenesis suggests the possibility of a hormetic effect

To obtain information on the effects of nongenotoxic carcinogens at low doses for human cancer risk assessment, the carcinogenic potential of the organochlorine insecticide, 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), in the liver was assessed in F344 rats. In experiment 1, 240 male animals, 21 days old, were administered 0, 0.5, 1.0, 2.0, 5.0, 20, 100 and 500 ppm DDT in the diet for 16 weeks. Experiment 2 was conducted to elucidate the carcinogenic potential of DDT at lower levels using 180 rats given doses of 0, 0.005, 0.01, 0.1, 0.2 and 0.5 ppm. The livers of all animals were immunohistochemically examined for expression of glutathione S-transferase placental form (GST-P), putative preneoplastic lesions. Quantitative values for GST-P-positive foci in the liver were increased dose-dependently in rats given 20 ppm DDT and above with statistical significance as compared with the concurrent control value. In contrast, doses of 0.005 and 0.01 ppm were associated with a tendency for decrease below the control value, although not significantly. Western blotting analysis show that cytochrome P-450 3A2 (CYP3A2) protein expression tended to decrease at 0.005 and 0.01 ppm, a good correlation being observed with the change in the number of GST-P-positive foci. These findings suggest that a DDT hepatocarcinogenicity may show nonlinear response, that is, hormetic response at low doses. Furthermore, since CYP3A2 protein expression appears to be important for the effects of phenobarbital and the alpha-isomer of benzene hexachloride, mRNAs for IL-1 receptor type 1 (IL-1R1) and TNF-alpha receptor type 1 (TNFR1) whose ligands have roles not only in downregulating CYP3A2 expression but also in inducing antiproliferative effect or apoptosis in hepatocyte were examined. Increase was observed at low doses of DDT. Oxidative stress in liver DNA, assessed in terms of 8-hydroxydeoxyguanosine as a marker, was also decreased. These findings suggest that the possible hormetic effect that was observed in our detailed low-dose study of DDT carcinogenesis, although not statistically significant, may be linked to levels of oxidative stress and proinflammatory cytokines.

Hazard assessment of chemical carcinogens: the impact of hormesis

The recent report of reductions in the number and area of preneoplastic hepatic lesions in response to low doses of the tumor promoter phenobarbital provides important new support for the existence of hormetic responses to carcinogens. The presence of hormetic responses to carcinogenic agents and the corollary that beneficial doses of these compounds can be determined have several implications for the bioassay and hazard assessment of carcinogens as well as for public policy regulating exposure to these agents. To be adequately sensitive to detect and quantify hormetic or other non-linear dose-response functions, current study designs must be modified to include lower doses and sufficiently large numbers of animals. Short- or medium-term animal studies are a cost-effective means of addressing these needs and have been used recently to describe a classical hormetic response to the non-genotoxic carcinogen phenobarbital. These basic changes should be supported by a continuing emphasis on mechanistic research and the development of biologically based quantitative models of toxicant action. Linking these models with physiologically based pharmacokinetic model descriptions of target dose holds the greatest promise for improving the description of the dose-response curve at low doses. These approaches are generally encouraged by the USEPA in the form of The 1996 Proposed Carcinogen Risk Assessment Guidelines. However, there remain substantial questions regarding integration of the concept of hormesis into hazard testing and public policy that require careful consideration. Herein, we explore the issues that surround testing for hormetic responses and the implications for public policy.

An FDA review of sulfamethazine toxicity

Recently, changes have been proposed in the criteria historically used in the evaluation of the applicability to humans of some of the results obtained from the rodent carcinogenicity bioassay data. These questions center on the suitability of the rodent model for agents that exert their toxic effects via specific enzyme interactions and endocrine mechanisms which appear to be inoperative within humans. Within the U.S. Food and Drug Administration (FDA), this issue has been brought to the forefront of concern with the recent application for a New Animal Drug Application for sulfamethazine (SMZ). A panel of FDA experts from the National Center for Toxicological Research (NCTR), the Center for Veterinary Medicine (CVM), and the Center for Food Safety and Applied Nutrition has reviewed the sum of the scientific evidence available on the toxicology of SMZ. They noted that, in previous feeding studies at NCTR, high doses of SMZ were associated with significant incidences of thyroid tumors in mice and rats. The panel also notes that the tumorigenic activity of SMZ in rodents was due to its goitrogenic activity, resulting in constant stimulation of the thyroid by TSH. Humans, on the other hand, were found to be insensitive to the SMZ-like inhibition of thyroid function. Further, apart from X-irradiation and radioactive iodine, there are no other physical or chemical agents known to cause thyroid tumors in humans. Thus, the expert panel concludes that the best scientific information available indicates that elevated levels of TSH and the consequent thyroid tumors would not be produced under approved use conditions of SMZ. This conclusion is in agreement with recommendations made by three other panels, viz. the World Health Organization, the U.S. Environmental Protection Agency, and CVM, which also evaluated the public health risk of SMZ.

Non-carcinogenicity, but dose-related increase in preneoplastic hepatocellular lesions, in a two-year feeding study of phenobarbital sodium in male F344 rats

Phenobarbital sodium (PB) was administered at dietary levels of 0 (control), 8, 30, 125 and 500 ppm to groups of 20 male F344/DuCrj rats for 104 weeks. There were no treatment-related clinical signs or adverse effects on survival rate, body weights, food consumption, and haematology or blood biochemistry data. Statistically significant increases of relative liver weights were found in the 500 and 125 ppm, but not the 30 and 8 ppm groups. Quantitative analysis of glutathione S-transferase placental form positive (GST-P+) hepatocyte foci/areas revealed clear increases limited to the 500 and 125 ppm groups. Western blotting revealed CYP2B1, 2C6 and 3A2 proteins to be also increased only with these high doses. In addition, significant increase of regenerative hepatocellular hyperplasias was noted in the 500 ppm group. No hepatocellular adenomas were observed, but a hepatocellular carcinoma arose in single rats of the 8 ppm and 125 ppm groups. No treatment-related changes were found in any other organs or tissues. Thus, under the experimental conditions used, the highest dose of PB (500 ppm) was not carcinogenic in male F344 rats. Furthermore, increase in putative preneoplastic proliferative hepatocytic lesions was only noted with 500 and 125 ppm.

Can the concept of hormesis Be generalized to carcinogenesis?

The concept of hormesis (i.e., low-dose stimulation/high-dose inhibition) has been shown to be widely generalizable with respect to chemical class, animal model, gender, and biological end point. The public health implication of this lack of linearity in the low-dose area of the dose-response curve raises the question of whether low doses of carcinogens will reduce cancer risk. Articles relating to the process of carcinogenesis (i.e., initiation, promotion, tumor development, and progression) were obtained from a recently developed chemical hormesis database and evaluated for their evidence of hormesis. Numerous examples in well-designed studies indicate that U- or J-shaped dose-response relationships exist with respect to various biomarkers of carcinogenesis in different animal models of both sexes. Examples of such J-shaped dose-response relationships in each stage of the process of carcinogenesis were selected for detailed toxicological examination. These results have important implications for both the hazard assessment of carcinogens and cancer risk assessment procedures.

Short- and intermediate-term carcinogenicity testing--a review. Part 1: the prototypes mouse skin tumour assay and rat liver focus assay

Carcinogenicity testing is by far the most expensive and time-consuming study type of toxicology. For many years, the lifetime exposure with the maximum tolerated dose in two rodent species has been the gold standard of carcinogenicity testing of pharmaceuticals. Major change was introduced by the Fourth International Conference on Harmonization in July 1997; a chronic rodent bioassay in one species and a short-term carcinogenicity assay are regarded as sufficient for registration. Such requirements provide the opportunity to redirect the vast resources previously spent on the lifetime study in the second species. Numerous experimental protocols for short- and intermediate-term carcinogenicity testing in many target tissues have been available for years. The first part of this review describes the basic principles of short- and intermediate-term carcinogenicity testing using the examples of the widely used mouse skin tumour assay and the rat liver foci assay. In the context of these experimental models, the discrimination and quantification of initiating and promoting activity and the use of preneoplastic lesions as endpoints in carcinogenicity testing are described. The review includes the limitations of the models with regard to the extrapolation from effects observed in animal experiments to a potential exposure of humans.

Implications of hormesis on the bioassay and hazard assessment of chemical carcinogens

Hormesis has been defined as a dose-response relationship which depicts improvement in some endpoint (increased metabolic rates, reduction in tumor incidence, etc.) at low doses of a toxic compound followed by a decline in the endpoint at higher doses. The existence of hormetic responses to carcinogenic agents has several implications for the bioassay and hazard assessment of carcinogens. To be capable of detecting and statistically testing for hormetic or other nonlinear dose-response functions, current study designs must be modified to include lower doses and sufficiently large numbers of animals. In addition, improved statistical methods for testing nonlinear dose-response relationships will have to be developed. Research integrating physiologically-based pharmacokinetic model descriptions of target dose with mechanistic data holds the greatest promise for improving the description of the dose-response curve at low doses. The 1996 Proposed Carcinogen Risk Assessment Guidelines encourage the use of mechanistic data to improve the descriptions of the dose-response curve at low doses, but do not distinguish between the types of nonlinear dose-response curves. Should this refined approach lead to substantial support for hormesis in carcinogenic processes, future guidelines will need to provide guidance on establishing safe doses and communicating the results to the public.

Dose-related increases in quantitative values for altered hepatocytic foci and cytochrome P-450 levels in the livers of rats exposed to phenobarbital in a medium-term bioassay

The dose-response relationship between liver tumor promoting activity and cytochrome P-450 (CYP) induction by phenobarbital sodium (PB) was investigated using the liver medium-term bioassay system of Ito. Two weeks after a single dose of N-nitrosodiethylamine (DEN) (200 mg/kg body weight, i.p.), rats were given PB at dietary levels of 500, 250, 125, 60, 30, 15 and 8 parts per million (ppm) for 6 weeks. All rats were subjected to partial hepatectomy at week 3, and were killed at week 8. Quantitative values for glutathione S-transferase placental form positive hepatocytic (GST-P+) foci were increased in the high dose groups dose-dependently. In contrast, the values in the low dose groups were rather lower than that of the control. CYP2B1, 2C6 and 3A2 were predominantly immunostainable in hepatocytes around the central vein. While Western blotting revealed CYP2B1 and 2C6 proteins to be increased with strict dose-dependence, CYP3A2 was only elevated at high doses. Thus, a good correlation between increase of GST-P+ foci and CYP3A2 induction was observed, as well as with CYP2B1 and 2C6 in high dose groups.

Epigenetic carcinogens: evaluation and risk assessment

Regulatory policies in the U.S. have been developed based upon a single model of cancer causation, which assumes chemical-induced genetic alterations. Such a model predicts some degree of cancer risk even at extremely low exposure levels. Many chemicals that produce tumors in experimental animals have been shown to act by epigenetic mechanisms that do not involve an attack by the chemical on DNA leading to subsequent genetic alteration. Such indirect mechanisms require prolonged exposures to high levels of chemicals for the production of tumors. For chemicals that are carcinogenic in this manner, the cancer mechanism would not be operative at exposures below a threshold at which the relevant cellular effect does not occur. Also, in contrast to DNA-reactive mechanisms, epigenetic effects may be unique to the rodent species used for testing. Certain chemical tumorigens have been well studied and provide examples for the use of mechanistic information in risk assessment. Butylated hydroxyanisole and saccharin are nongenotoxic food additives for which no risk to humans is predicted based upon low exposure levels and the likelihood that humans are either insensitive or much less sensitive to the tumorigenic effects found in rodent test species. For another non-genotoxic food additive d-limonene, the mechanism that underlies kidney tumor development in male rats is not expected to be operative in humans at all. The pharmaceutical phenobarbital represents a large group of non-genotoxic liver microsome enzyme inducers, which produce liver cancer in mice at levels that are near to therapeutic doses in humans. Epidemiology studies have not shown phenobarbital-related tumors in humans, indicating that humans may be less sensitive to the effects of phenobarbital. The mechanistic considerations involved in the risk assessment of these agents demonstrate that humans are not at risk from current exposure levels of many epigenetic carcinogens.

Development of a sensitive in vitro method for identifying tumor promoters

The identification and characterization of nongenotoxic carcinogens represents a significant challenge to toxicologists. In vitro methods for identifying tumor promoters with suitable sensitivity and specificity have been particularly elusive. Experiments are described which suggest that the human promyelocytic leukemia cell line HL-60 provides a sensitive indicator of promoter-induced changes to gene regulation and expression. As a result of differentiation these cells undergo a transition from a non-phagocytic suspension culture to an attached fibroblast-like culture which exhibits high phagocytic activity. Fluorescent latex particles were used as sensors to highlight the phagocytic phenotype and permitted the use of flow cytometry to automatically quantitate particle internalization. To evaluate specificity, HL-60 cells were treated with a series of phorbol esters covering a range of in vivo tumor promoting activity. Results indicate that this family of compounds induces HL-60 cells to differentiate in proportion to their in vivo promoting activity. To closely assess the sensitivity of the phagocytic endpoint, HL-60 cells were treated with picogram levels of 12-O-tetradecanoyl phorbol-13-acetate (TPA), whereupon increments as low as 50 pg of TPA per ml caused statistically significant increases in phagocytic activity. The experiments described herein suggest that in vitro differentiation of HL-60 cells may reflect the promoter-dependent modifications to gene expression that are observed in vivo during the promotion phase of carcinogenesis. The described method may represent a sensitive promoter screening assay which is both rapid and economical.

The role of modulated gap junctional intercellular communication in epigenetic toxicology

The normal development and health of all multicellular organisms, including the human being, depend on the adaptive maintenance of the integrity of the genetic information (e.g., DNA protective and repair mechanisms), as well as of the homeostatic and cybernetic regulatory systems within and between tissues. The primary focus of the past and current toxicological studies and risk assessment practices has been to ascertain and predict the "genotoxicity" of various physical and chemical agents. The paradigm of "carcinogen as mutagen," while valuable for stimulating studies of the detection of mutagens and of their potential role in "causing" somatic and germ line diseases, has tended to blunt research on the role of nongenotoxic mechanisms in disease causation. This brief analysis will emphasize the need to consider the role of modulated gap junctional intercellular communication (GJIC) in any biological risk assessment model. It is based on the following assumptions and facts. Because gap junctions exist in all metazoans, they have been associated with the regulation of cell proliferation, development, differentiation, and the adaptive function of both excitable and nonexcitable coupled cells. A highly evolutionarily conserved family of genes codes for proteins (connexins), which, as hexameric units (connexons), form membrane-associated channels of gap junctions. Cells coupled by gap junctions will have their ions and small regulatory molecules equilibrated. Regulation of GJIC can be at the transcriptional, translational, or posttranslational levels. Transient down or up regulation of GJIC can be induced by endogenous or exogenous chemicals via many mechanisms at any of these three levels. Stable abnormal regulation has been associated with activated oncogenes, and normal regulation has been associated with several tumor suppressor genes. The dysfunction of these gap junctions might play a role in the actions of various toxic chemicals that have cell type/tissue/organ specificity. This could bring about distinct clinical consequences, such as embryo lethality or teratogenesis, reproductive dysfunction in the gonads, neurotoxicity of the central nervous system, hyperplasia of the skin, and tumor promotion of initiated tissue. Modulation of GJIC should be viewed as a scientific basis of "epigenetic toxicology" because the alteration of intercellular communication would alter the internal physiological state of the cell. The inhibition of GJIC is a necessary component of mitogenesis (a necessary component of the multistage carcinogenic process). The modulation of GJIC can have both toxicological, as well as therapeutic potential.

Tumor-promoting effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and phenobarbital in initiated weanling Sprague-Dawley rats: a quantitative, phenotypic, and ras p21 protein study

In an initiation-promotion protocol, female weanling Sprague-Dawley rats were initiated with 10 mg/kg nitrosodiethylamine and promotion was started after 30 days. Promotion regimens were as follows: 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD; 150 ppt in diet) continuously until day 450; phenobarbital (PB; 500 ppm in diet) until day 170; PB until day 170, followed by TCDD until day 240; and PB until day 170, followed by a basal diet (BD) until day 240 and subsequently TCDD from days 240 to 450. TCDD fed to initiated rats had a promoting effect on the development of adenosine triphosphatase-negative altered hepatocellular foci (AHF). At 450 days, the volume fraction of liver occupied by AHF was increased in initiated rats given TCDD continuously and in those given PB followed by TCDD, whereas the mean volume of AHF was significantly larger in initiated rats given TCDD continuously. PB and TCDD promoted similar phenotypes of AHF as seen in hemotoxylin and eosin-stained sections, but the eosinophilic phenotype most closely correlated with the development of hepatocellular neoplasms. The protooncogene product ras p21 protein was present in the majority of PB- and TCDD-promoted AHF, hepatocellular adenomas, and hepatocellular carcinomas. Eosinophilic AHF and ras p21 protein expression most closely correlated with neoplastic development, suggesting that these cell populations, when promoted, may be at greater risks for developing into neoplasms.

Dose response of promotion by butylated hydroxyanisole in chemically initiated tumours of the rat forestomach

The antioxidant food preservative butylated hydroxyanisole (BHA) was tested in an initiation-promotion protocol in which male F344 rats (6 wk old), 27 per group, were gavaged with a single dose of 200 mg N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)/kg. After 3 wk on control diet, test diets containing 0, 60, 300, 1000, 3000, 6000 or 12,000 ppm BHA were fed until termination of the experiment at approximately 110 wk, at which time most animals had died with stomach tumours. MNNG caused a high incidence of tumours in the glandular stomach and forestomach of all groups. Administration of 12,000 and 6000 ppm BHA, but not 3000 ppm or lower doses, caused statistically significant increases in the time-related incidence of MNNG-induced forestomach tumours as analyzed by life table analysis. BHA had no effect on the incidence of tumours in the glandular stomach or oesophagus. Tumour incidences in other organs were not related to BHA dose. No increase in hyperplasia in the oesophagus was evident in the high-dose BHA-treated animals compared with the MNNG-only group. This study provides corroboration that BHA affects only forestomach tumorigenesis and that the dose for enhancement of tumorigenesis is at least 1500-fold greater than human exposure.

Altered foci of hepatocytes in rats initiated with diethylnitrosamine after prolonged fasting

The influence of fasting on the potential of diethylnitrosamine (DEN) to initiate liver carcinogenesis was tested in a medium-term assay using the development of putative preneoplastic altered foci of hepatocytes (AFH) as the endpoint. Male Wistar rats fasted for 48 hr were given a single ip injection of DEN (200 mg/kg body weight). Partial hepatectomies were carried out at wk 3 and the rats were killed at wk 8. Fasted rats exhibited a small increase in the numbers of AFH with glutathione S-transferase in the placental form and eosinophilic AFH when compared with non-fasted animals. However, after a 6-wk exposure to 0.05% sodium phenobarbital in the diet, there were no differences in the numbers of AFH between fasted and non-fasted animals. Fasting also increased DEN-dependent centrilobular cell necrosis and specifically drug metabolism as indicated in vivo by a decreased time of paralysis of the lower limbs induced by zoxazolamine (40 mg/kg body weight, ip) and by an unaltered sleeping time induced by sodium pentobarbital (40 mg/kg body weight, ip). The results indicate that although fasting during the initiation stage of carcinogenesis increases DEN hepatoxicity, it does not interfere quantitatively with the development of liver preneoplastic lesions.

Effect of the peroxisome proliferator perfluorodecanoic acid on the promotion of two-stage hepatocarcinogenesis in rats

This study was conducted to determine if the peroxisome proliferator perfluorodecanoic acid (PFDA) has promoting activity in two-stage hepatocarcinogenesis. Because PFDA is a non-competitive inhibitor of the peroxisomal bifunctional enzyme and thus inhibits the peroxisomal beta pathway, we hypothesized that PFDA may not have promoting activity as do other peroxisome proliferators, because hydrogen peroxide production is inhibited. Twenty-four hours after partial hepatectomy, female Sprague-Dawley rats were given an initiating dose of 10 mg/kg diethylnitrosamine by gavage. The rats were divided into five groups that received monthly i.p. injections of 0.0, 0.05, 0.50 or 5.0 mg/kg PFDA in corn oil or were placed on diets that contained either 0.01% ciprofibrate or 0.05% phenobarbital for 9 or 18 months. Both ciprofibrate and the highest dose of PFDA increased the activity of the peroxisomal enzyme fatty acyl CoA oxidase. PFDA treatment did not increase the tumor incidence or the number of altered hepatic foci at 9 or 18 months, although the mean volume of foci was increased at 9 months. Ciprofibrate increased the incidence of hepatocellular carcinomas at 18 months but did not increase the number or volume of altered hepatic foci at 9 or 18 months. Phenobarbital increased the number and volume of foci but did not influence the tumor incidence. The results of this investigation indicate that PFDA is not a promoter of hepatocarcinogenesis.

Alterations in populations of GST-p-immunoreactive single hepatocytes and hepatocellular foci after a single injection of N-nitrosodiethylamine with or without phenobarbital promotion in male F344/NCr rats

The fate of placental glutathione S-transferase (GST-P)-immunoreactive hepatocytes, detectable in livers of rats soon after treatment with N-nitrosodiethylamine (DEN), was examined sequentially with or without phenobarbital (PB) promotion. Group 1 male F344/NCr rats were administered a single i.p. injection of 200 mg DEN per kg body weight at 5 weeks of age. Group 2 rats were given 500 ppm PB in the diet two weeks after the DEN treatment. Groups of six rats were sequentially sacrificed 16, 42, 70, 126 and 238 days after DEN injection. In DEN-treated rats, GST-P immunoreactive hepatocytes (single cells and multiple cell foci) were detectable 16 days after DEN, the total numbers decreasing by day 70 and thereafter rising again. In the early stages the proportion of single immunoreactive hepatocytes was prominent, but with time a gradual increase in small GST-P+ hepatocellular foci and larger foci became evident. Feeding of PB to rats for 16-238 days after a single DEN injection resulted in increases of both single cells and foci, especially foci composed of more than three hepatocytes. The growth response was increasingly pronounced with time. Adenomas or carcinomas were only observed at 126 or 238 days. Numbers of GST-P+ foci far exceeded the numbers of foci visible in hematoxylin-eosin (H & E) stained sections, and a few H & E foci were negative for GST-P. Many GST-P+ foci smaller than ten cells were composed of histologically normal hepatocytes. Almost all GST-P+ foci identifiable in H&E stained sections were larger than ten cells, consisted of clear cells (in both groups) or mixed (clear-eosinophilic) cells in PB-exposed rats, and appeared to be evenly distributed throughout the three zones of the liver. These results suggest that the promotive effect of PB is most evident as an increase in larger hepatocyte populations composed of more than three GST-P+ hepatocytes, rather than in increasing the populations of single GST-P immunoreactive cells. PB may cause clonal expansion of these single GST-P reactive hepatocytes. This study provides evidence for the hypothesis that some of the GST-P reactive hepatocytes are initiated cells.

Persistent proliferation of normal hepatocytes and promotion of preneoplastic development by N-nitrosodibenzylamine in rats

In a traditional long-term study N-nitrosodibenzylamine (NDBzA) was proven to be noncarcinogenic, but recently the substance was found to produce genotoxic lesions in hepatocytes. Our own experiments have shown that relatively low single doses of NDBzA cause liver hypertrophy and additive proliferation of hepatocytes in rats. Both effects are known from well-documented promoters and non-genotoxic carcinogens, respectively, in rodents. Investigation of NDBzA in an initiation-promotion assay (IP assay) showed it to cause an increase in the number and size of preneoplastic liver cell foci. This occurred only after initiation with diethylnitrosamine, but not when 2-acetylaminofluorene was used. Another property of NDBzA is its sustained mitotic stimulation of extrafocal hepatocytes. This is inconsistent with their adaptive loss of susceptibility to mitogens in IP assays using other promoters of hepatocarcinogenesis. The following conclusions can be drawn. First, "differential inhibition" of the proliferation of extrafocal hepatocytes, in contrast to the selective mitostimulation of preneoplastic cells, is obviously no prerequisite for cancer development. Second, primary mitogenicity of a compound in short-term studies can be a useful indicator for tumorigenic potential. In the case of NDBzA the data available at present are still insufficient to classify it unequivocally in terms of genotoxic or nongenotoxic carcinogenicity.

Endogenous and exogenous modulation of gap junctional intercellular communication: toxicological and pharmacological implications

During the evolution of single-celled organisms to multicellular metazoans, a family of highly conserved genes coding for proteins (connexins), which as hexameric units (connexins), has evolved to form intercellular channels (gap junctions). These gap junctions allow ions and small molecular weight molecules to flow between coupled cells, thereby facilitating synchronization of electrotonic or metabolic cooperation. Control of cell proliferation, cell differentiation and adaptive responses of differentiated cells have been speculated to be biological roles of gap junctions. The regulation of these gap junctions can occur at the transcriptional, translational and posttranslational levels. Transient downregulation by endogenous or exogenous chemicals can bring about adaptive or maladaptive consequences depending on circumstances. Stable abnormal regulation of gap junction function has been associated with the activation of several oncogenes. Several tumor suppressor genes have also been associated with the up-regulation of gap junction function. Since gap junctions exist in all organs of the multi-cellular organisms, the dysfunction of these gap junctions by various toxic chemicals which have cell type/tissue/organ specificity could bring about very distinct clinical consequences, such as embryo lethality or teratogenesis, reproductive dysfunction in the gonads, neurotoxicity of the CNS system, hyperplasia of the skin, and tumor promotion of initiated tissue. Understanding how many non-mutagenic chemicals might alter normal gap junction function should form the basis of "epigenetic" toxicology. On the other hand, restoring normal gap junction function to cells which have dysfunctional intercellular communication could be the basis for a new approach for therapeutic pharmaceuticals.

Rat liver foci study on coexposure with 50 Hz magnetic fields and known carcinogens

A study was performed to investigate possible interactions by magnetic fields (MF) with the processes of initiation and promotion of chemically induced preneoplastic lesions in rat liver. Male Sprague-Dawley rats were subjected to a 70% partial hepatectomy followed after 24 h by i.p. injection of diethylnitrosamine (DENA) as a tumour initiator. Starting one week after the DENA-treatment phenobarbital (PB) was given to promote growth of enzymatically altered foci of liver cells. MF was applied immediately after the partial hepatectomy and continued until sacrifice after 12 weeks of PB exposure. Homogenous horizontal AC magnetic fields with a frequency of 50 Hz and flux densities of 0.5 mu T or 0.5 mT were used. The rats coexposed with MF and DENA plus PB did not gain weight as much as the rats exposed to the chemical agents only. The MF-exposure also resulted in a slight reduction in size and numbers of the focal lesions. The results suggest an interaction of MF with the processes of chemical carcinogenesis either as a result of stress or depending on effects on the proliferation of preneoplastic cells.

Dose-response relationships in promotion of rat hepatocarcinogenesis by 17 alpha-ethinylestradiol

One of the critical issues in risk assessment for chemical carcinogens is the evaluation of dose-response relationships for tumor promoters. In the studies reported here we have systematically investigated dose-response relationships for the liver tumor-promoting actions of 17 alpha-Ethinylestradiol (EE2) following a single injection of diethylnitrosamine (200 mg/kg) to ovariectomized female rats. Parameters measured included tumor incidence, gamma-glutamyltranspeptidase (GGT) positive foci, serum prolactin and serum EE2. The length of tumor promotion ranged from 30 to 60 wk. Results showed a linear increase in GGT-positive foci between doses of 16 and 90 micrograms EE2 kg/d for 30 wk. This was associated with corresponding increases in liver tumor incidence at 60 wk. Seventy-five percent of the animals had either hepatocellular adenoma or hepatocellular carcinoma in the group promoted with 90 micrograms EE2/kg for 60 wk. No liver tumors were evident in either controls or animals receiving estrogen only. Serum prolactin concentrations were elevated in all estrogen-treated groups. In summary, our studies have evaluated dose-response relationships for GGT-positive foci and tumor incidence in a two-stage model for hepatocarcinogenesis using EE2 as the promoting agent.

Relative toxicity and tumor-promoting ability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PCDF), and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) in hairless mice

2,3,7,8-Tetrachlorodibenzo-p-dixoin 2,3,4,7,8-pentachlorodibenzofuran (PCDF), and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) are highly toxic members of a class of environmental contaminants, the polychlorinated aromatic hydrocarbons (PCAH), which exhibit a similar and highly characteristic spectrum of toxic effects. For purposes of risk assessment, it is important to be able to make accurate estimates of the relative potency of these and related compounds. Previous investigations have indicated that, in acute exposure or in vitro studies, PCDF is approximately 0.1 times as toxic and HCDF is approximately 0.01 times as toxic as TCDD. In this study, we compared the relative toxicity and tumor-promoting abilities of TCDD, PCDF, and HCDF in hairless mouse skin. Female hairless mice (HRS/J hr/hr) were treated dermally with the initiator MNNG, then dosed twice weekly for 20 weeks with acetone, TCDD (2.5-10 ng/mouse/dose), PCDF (25-100 ng/mouse/dose), or HCDF (250-1000 ng/mouse/dose) as promoter. TCDD, PCDF, and HCDF were all potent promoters for the induction of squamous cell papillomas. There was, however, no difference in the incidence or multiplicity of papilloma formation between groups. The same doses of the three PCAH, in the absence of initiator, induced no skin papillomas. TCDD produced a significant increase in liver:body weight ratio (p less than 0.001) at all doses and a decrease in thymus:body weight ratio at a dose of 10 ng (p less than 0.001). Mice treated with PCDF and HCDF had marked thymic and splenic involution, liver hypertrophy, mucous cell hyperplasia in the fundic portion of the glandular stomach, and loss of body weight. PCDF and HCDF produced a greater incidence and severity of dermatotoxic effects than TCDD. Based on data for dermal toxicity and changes in body weight and organ weights, PCDF is estimated to be 0.2 to 0.4 times, and HCDF 0.08 to 0.16 times, as toxic as TCDD following repeated dermal exposure. Therefore, toxic equivalence factors generated using data from acute and/or in vitro studies may underestimate the risk from repeated low-dose exposures to these compounds.

Increase in hepatocyte and nuclear volume and decrease in the population of binucleated cells in preneoplastic foci of rat liver: a stereological study using the nucleator method

Gamma-glutamyltranspeptidase-positive hepatocyte foci were produced in female rats given a single dose of diethylnitrosamine neonatally after birth and, after weaning, a diet containing phenobarbitone for 30 wk. The nucleator method, a new stereological approach, provided an efficient, unbiased estimate of mean cell volume in focal lesions and extrafocal areas. It also provided an unbiased sample of cells to estimate hepatocyte nuclear volume and the percentage of binucleated cells. The results showed an increase in the mean volume of mononucleated cells--from 4,700 micron3 in extrafocal areas to 12,700 micron2 in foci--and of binucleated cells--from 6,900 micron3 to 25,000 micron3. This demonstrated the hypertrophic effect of the carcinogenic treatment in focal lesions. A striking reduction in the proportion of binucleated cells was also observed in the preneoplastic lesions. Nuclear volume measurements from mononucleated and binucleated hepatocytes were used to assess ploidy. An apparent increase in nuclear ploidy, with no change in cellular ploidy, was noted in focal tissue when compared with nonfocal tissue. This appeared to be caused by an increase in mononucleated tetraploid cells and a reduction in binucleated cells with two diploid nuclei, indicating an altered mitotic mechanism in focal lesions. The significance of these changes in cell volume, apparent ploidy levels and binuclearity in preneoplastic foci is discussed in relation to the hepatocarcinogenic process.

Effect of the antihistamine, methapyrilene, as an initiator of hepatocarcinogenesis in female rats

The antihistamine methapyrilene was examined for its ability to initiate hepatocarcinogenesis in rats. Rats were first subjected to partial hepatectomy and then were intubated with one of four doses (30, 100, 200 or 300 mg/kg) of methapyrilene hydrochloride (or an equivalent amount of water for controls, or 10 mg diethylnitrosamine/kg for positive controls). Rats were then fed 0.05% phenobarbital in the diet for 3, 6 or 9 months. The number and volume of altered hepatic foci were quantified with the histochemical markers gamma-glutamyl transpeptidase, glucose-6-phosphatase, and ATPase. The number of foci induced was increased 2- to 4-fold by the highest dose of methapyrilene at all 3 time points, but the only statistically significant increase was produced by the 200 mg/kg dose after 3 months of promotion. This study shows that methapyrilene may act as a weak initiator.

Modification by sodium L-ascorbate, butylated hydroxytoluene, phenobarbital and pepleomycin of lesion development in a wide-spectrum initiation rat model

Rats were treated for 1 week each with 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN), 0.2% N-bis(2-hydroxypropyl)-nitrosamine (DHPN) and 0.2% N-ethyl-N-hydroxyethylnitrosamine (EHEN) in the drinking water, and then administered diet containing 5% sodium L-ascorbate (Na-AsA), 1% butylated hydroxytoluene (BHT) or 0.05% phenobarbital (PB), or weekly intraperitoneal injections of 2 mg of pepleomycin per kg body weight until week 36. Histopathological examination revealed that all exerted significant modulation effects on tumor development in the various target organs. Na-AsA was found to inhibit liver but promote renal pelvis and bladder carcinogenesis. BHT similarly decreased liver and enhanced bladder lesion development. PB, in contrast promoted hepatocarcinogenesis. However both PB and BHT were associated with increased incidences of adenomas and adenocarcinomas of the thyroid. Thus the wide-spectrum initiation model allowed confirmation of site-specific modification potential and in addition demonstrated potentiation of kidney and bladder carcinogenesis promotion by pepleomycin.

Significance of sequential cellular changes inside and outside foci of altered hepatocytes during hepatocarcinogenesis

A variety of phenotypic cellular changes emerge in the liver of different species prior to the appearance of hepatocellular adenomas and carcinomas induced by carcinogenic agents (chemicals, radiation, hepadna viruses) or develop "spontaneously." Foci of altered hepatocytes have been studied most extensively in rats treated with chemical carcinogens; they are considered preneoplastic lesions and have been used in several laboratories as endpoints in carcinogenicity testing. The principles and problems of the morphological classification of foci of altered hepatocytes are presented. In addition to the 4 types of foci generally accepted (clear, acidophilic, basophilic and mixed cell foci), further subtypes (intermediate cell foci) or other types of foci, namely tigroid cell foci and amphophilic cell foci, have more recently been separated as distinct pathomorphological entities. Whereas the amphophilic foci might result from a modulation of clear and acidophilic cell foci, the tigroid cell foci apparently represent a stage in a separate cell lineage leading to hepatocellular adenomas. It remains open whether the tigroid cell foci may also progress to carcinomas. Extrafocal phenotypic changes of hepatocytes might also be involved in hepatocarcinogenesis. The cellular phenotypes within foci also depend strongly, among many other factors, on the dose and duration of the carcinogenic treatment. Cytomorphological, cytochemical, microbiochemical and stereological studies suggest that the predominant sequence of cellular changes during hepatocarcinogenesis leads from the clear and acidophilic cell foci storing glycogen in excess through mixed cell foci and nodules to basophilic cell populations prevailing in hepatocellular carcinomas. A multitude of metabolic aberrations is associated with the sequential cellular changes. Aberrations in carbohydrate metabolism are particularly prominent and might be causally related to the neoplastic transformation of the hepatocytes.

Morphological and stereological characterization of hepatic foci of cellular alteration in control Fischer 344 rats

Quantitative evaluation of altered hepatocellular foci (AHF), followed by stereological analysis was performed on standard hematoxylin and eosin-stained liver sections from control Fischer 344 (F344) rats of both sexes from seven 2-yr carcinogenicity studies conducted by the National Toxicology Program (NTP). Liver samples were collected at 6, 9, 12, 15, 18, and/or 24 months on study. Although AHF had a broad spectrum of morphological features, they could be classified into the following 5 types using previously published criteria: basophilic, eosinophilic, clear, vacuolated, mixed cell foci. Approximately 50% of the animals had foci at 6 months, and the incidence reached nearly 100% at 15 months in both sexes. The number, size and volume fraction of AHF increased with age in both sexes; these changes were most evident for basophilic and clear cell foci. The number of basophilic foci was significantly greater in females than in males while clear cell foci were more numerous in males. This sex difference was observed at each time point. Mean number of all types of AHF in males and females at 24 months was 547 and 460 per cubic centimeter of liver, respectively. Despite the high incidence of AHF in control rats, the incidence of hepatocellular neoplasms is low. The implication is that most foci do not progress to neoplasia in control F344 rats used in 2-yr studies.

Use of rat liver altered focus models for testing chemicals that have completed two-year carcinogenicity studies

Partial hepatectomy (PH) and neonatal rat short-term liver focus models were used to examine the effects of selected chemicals that had been previously tested in the National Toxicology Program (NTP) 2-yr carcinogenicity studies. C.I. Solvent Yellow 14, monuron, chlorendic acid, and 4-hydroxyacetanilide were tested for initiating and promoting activity in the PH model. Chlorendic acid, 4,4'-oxydianiline, 1-amino-2,4-dibromoanthraquinone (ADBAQ), and 4-hydroxyacetanilide were similarly tested in a neonatal rat liver focus model. With the exception of 4-hydroxyacetanilide which was not carcinogenic in the NTP studies, all chemicals tested showed clear evidence of hepatocarcinogenicity. While none of the chemicals showed initiating activity in either the PH or neonatal models, promoting activity, as indicated by increased number, size, or volume fraction of histochemically detected hepatic foci of cellular alteration, was evident for all chemicals with previously demonstrated hepatocarcinogenicity. Liver tumor incidence was documented at 14 months in the PH model and at 300 days in the neonatal model. On the basis of the results obtained from these few chemicals, it is suggested that the use of short-term rat liver focus models may represent a reliable means for identifying chemicals with hepatocarcinogenic potential.

Defining foci of cellular alteration in short-term and medium-term rat liver tumor models

Evaluation of foci of cellular alteration is utilized in short-term and mid-term rat liver models to assess chemicals for potential carcinogenic effects. Such foci are morphologically defined and readily identified by a variety of histological stains. No single morphological marker appears capable of identifying all foci because of their phenotypic heterogeneity. Since most rodent hepatocarcinogens induce an increase in the size and/or the number of foci prior to tumor appearance, foci are believed to be precursors to hepatic tumors. The size, number, phenotype and conversion rate of foci to tumors is chemical dependent. Despite certain limitations, the quantitation of rat liver foci provides a useful means to study the mechanism of action of selected chemicals and assess their carcinogenic potential.

Critical parameters in the quantitation of the stages of initiation, promotion, and progression in one model of hepatocarcinogenesis in the rat

Critical parameters in the quantitation of altered hepatic foci (AHF) developing during multistage hepatocarcinogenesis in the rat include: 1) the enumeration of AHF induced by test agents as well as those AHF occurring spontaneously in livers of untreated animals; 2) the volume percentage or fraction of the liver occupied by all AHF as a reflection of the total number of altered cells within the liver and the degree of tumor promotion which has occurred; and 3) the phenotype of individual AHF as determined by multiple markers with serial sections. These parameters, especially the number of AHF, should be corrected by the presence of spontaneous AHF which increase with the age of the animal, more so in males than females. While accurate estimation of the background level of spontaneous AHF can be important in demonstrating that a carcinogenic agent does not possess the ability to increase the numbers of AHF above the background level, a better method to distinguish the effectiveness and relative potencies of agents as initiators or promoters is reviewed. The relative effectiveness of four different markers--gamma-glutamyltranspeptidase (GGT), a placental form of glutathione S-transferase (GST), canalicular ATPase, and glucose 6-phosphatase (G6Pase)--was described for the chemicals C.I. Solvent Yellow 14 and chlorendic acid as promoting agents in males and females. C.I. Solvent Yellow 14 is a more effective promoting agent in females than males, and AHF exhibit extremely low numbers scored by GGT. On the other hand, the numbers of AHF present in livers of male rats promoted by this agent are more than twice those seen in livers of female animals, possibly owing to the effectiveness of this agent as an initiator in the male but not the female. Very few AHF, especially in the male, are scored by GGT during chlorendic acid promotion. The distribution of phenotypes with these markers also differs in the spontaneous AHF appearing in the livers of animals fed 0.05% phenobarbital on either a crude NIH-07 or AIN-76 purified diet. Such studies emphasize the extreme dependence of the promoting stage of hepatocarcinogenesis on environmental factors of sex, diet, and the molecular nature of the promoting agent itself. The hallmark of the final stage of progression in the development of hepatocellular carcinomas is aneuploidy, which may be reflected by phenotypic heterogeneity within individual AHF, termed foci-in-foci. The implications of such quantitative analyses during hepatocarcinogenesis induced by specific agents in relation to the specific action of the agent at one or more of the stages of hepatocarcinogenesis are discussed.

Unusual histochemical pattern in preneoplastic hepatic foci characterized by hyperactivity of several enzymes

In a stop-experiment using the hepatocarcinogen N-nitrosomorpholine, as well as glycogenotic and related lesions, hepatocellular foci with a different histochemical pattern were identified. The outstanding features of these hepatic foci, which may progress to hepatocellular adenoma, were increased activities of mitochondrial glycerol-3-phosphate dehydrogenase (mG3PD), glycogen synthase, pyruvate kinase and glucose-6-phosphatase detected by enzyme histochemistry. Since no decrease in activity of any of the enzymes examined were seen in these foci, compared with normal liver, the term enzymatically hyperactive focus (EHF) is proposed for this type of lesion. Only at the stage of overtly nodular growth did these lesions exhibit some of the characteristic changes seen in nodules developing from glycogenotic foci, namely elevated activities of glucose-6-phosphate dehydrogenase, gamma-glutamyl transferase and glutathione-S-transferase P as well as decreased activities of adenosine-triphosphatase, glucose-6-phosphatase and adenylate cyclase. Some of these enzymes have been used widely in morphometric studies as markers for preneoplastic and neoplastic lesions. The inability to detect early EHF may lead to an underestimation of preneoplastic liver lesions in quantitative studies. Although there are apparent differences in the histochemical patterns of glycogen storing foci and early EHF, these differences tend to disappear during progression to overtly neoplastic lesions. In studies comparing the phenotypic alterations in different types of preneoplastic hepatic lesions, the recognition of EHF may contribute to the distinction of obligatory from facultative phenomena during transformation.

Rat liver foci and in vitro assays to detect initiating and promoting effects of chlorinated ethanes and ethylenes

Nine chlorinated aliphatics (CAs) were examined in a rat liver foci assay for tumor initiating and promoting activities. In this model, young adult male Osborne Mendel rats were first subjected to a partial hepatectomy, the test chemical was then administered at the maximum tolerated dose in the initiation or promotion phase in conjunction with diethylnitrosamine (DEN; 30 mg/kg b.w.) or phenobarbital (PB; 0.05 percent, w/w, in the diet), and gamma glutamyltranspeptidase (GGT) was used as a putative preneoplastic indicator. When administered in the promotion protocol after initiation with DEN, 1,1-dichloroethane, 1,1,2-trichloroethane (1,1,2-TCE), 1,1,2,2-tetrachloroethane (1,1,2,2-TTCE), tetrachloroethylene (TTCY), and hexachloroethane induced significant increases in GGT+-foci above control levels. 1,1,2,2-TTCE, TTCY, and 1,1,2-TCE also induced significant increases in GGT+-foci when administered in the promotion protocol without DEN initiation. Two variants of GGT+-foci were observed: the classical type associated with PB promotion, and the other, which was more diffuse, less intensely stained, resembling foci undergoing redifferentiation and associated with CAs. A number of CAs were also genotoxic in short-term in vitro tests. Taken together, the studies suggest that CAs may be complete carcinogens in vivo with weak initiating activity and stronger promoting activity.

Multistage carcinogenesis: implications for risk estimation

In undertaking a quantitative estimation of carcinogenesis risk, it is essential to keep in mind that carcinogenesis is a multistage process, and that each stage can be affected by different classes of risk factors. Furthermore, different mechanisms are involved in the various stages of carcinogenesis. Thus, a dose-response analysis of one given factor cannot provide an accurate estimation of carcinogenic risk. Carcinogenic risk estimation is usually undertaken for a specific chemical or group of chemicals; however, the concept of multistage carcinogenesis is based on biological processes and not on the mechanisms of action of the agents involved. It is therefore important to consider three related, but different, factors involved in carcinogenesis: stage, agent, and activity of agent. This is especially important in developing a short-term test for stage-related risk factors, such as tumor-promoting agents. For this reason, carcinogens should not be classified according to only one chemical activity. This article briefly reviews the cellular and molecular mechanisms involved in multistage carcinogenesis, and discusses their implications for risk estimation. Special consideration is given to the effect of treatment frequency on the response of tumor-promoting agents, as seen in long-term tests in experimental animals. It is proposed that exposure frequency be taken into account together with exposure dose.

An approach to the determination of the relative potencies of chemical agents during the stages of initiation and promotion in multistage hepatocarcinogenesis in the rat

The potency of carcinogenic agents in eliciting neoplastic lesions has long been a concern of investigators in the field of oncology. This paper describes a method, based on quantitative stereologic calculations, to estimate the relative potency of chemicals as initiating and/or promoting agents. The parameters defined in this paper are: (a) Initiation index = no. foci induced X liver-1 X [mmole/kg body weight]-1; and (b) Promotion index = Vf/Vc X mmol-1 X wk-1. These parameters have been calculated for a number of chemical agents, based both on data from this laboratory and others published in the literature. Neither parameter varied significantly with the dose of two different initiating agents used in this study. The range of promotion indices extended over more than eight orders of magnitude, whereas that of the initiation indices was much less variable. Such parameters may be useful as quantitative estimates of the potency of hepatocarcinogenic agents not only in rodents, but potentially in quantitative risk estimations in the human.

Effect of tryptophan on isolated hepatocytes of rats

The addition of tryptophan to adult rat hepatocyte cultures stimulated DNA synthesis. The increase in DNA synthesis as measured by 3H-thymidine incorporation into DNA was observed on treatment of the cultures with tryptophan for 48 h but also as short as for 6 h in comparison with control cultures. An increase was also apparent at 30 h which was maintained for up to 48 h post treatment with tryptophan. The increase in DNA synthesis by tryptophan cannot be attributed to cell injury or to increased DNA degradation. Of the degradative enzymes added after harvesting the hepatocytes, only DNase decreased incorporation of 3H-thymidine. The observed effect was specific for tryptophan since treatment with kynurenine, isoleucine, methionine or serine failed to show a significant effect. Pretreatment of cultured hepatocytes with hydroxyurea prevented the tryptophan stimulated increase in DNA synthesis suggesting that the latter was due to replicative and not to reparative DNA synthesis. Experiments performed with the addition of diethylnitrosamine also alluded to tryptophan's role in replicative DNA synthesis. The mechanism of tryptophan-induced DNA synthesis is discussed.

A cytochemical study of the livers of rats treated with diethylnitrosamine/phenobarbital, with benzidine/phenobarbital, with phenobarbital, or with clofibrate

Male Sprague-Dawley rats were treated with clofibrate (CLOF) in the diet for 2 years or with 4 i.p. injections of either diethylnitrosamine (DEN) or benzidine (BZ) followed by phenobarbital (PB) in the diet for 67 weeks, or just with PB for 41 weeks. Animals were killed at frequent intervals, some while still on treatment and others after 3 or 6 months withdrawal of treatment. The livers were subjected to cytochemical measurements of the parenchyma, foci, nodules and carcinomas. The parenchyma of the CLOF groups showed, in general, increases in glucose-6-phosphate dehydrogenase (G-6PD), alpha-glycerophosphate dehydrogenase (alpha-GPD), 5'-nucleotidase (5'-Nu), acid phosphatase (AP) and catalase and decreases in uricase and glutathione (GSH). CLOF induced a low incidence of GSH positive foci; nodules showed universally lower levels of catalase and GSH. In the DEN/PB and BZ/PB groups the parenchyma showed increases (even before PB treatment started) in G-6PD and in gamma-glutamyl transpeptidase (gamma-GT) and decreases in GSH. DEN raised and BZ lowered 5'-Nu. Neither initiator affected alpha-GPD. Both initiators caused a high incidence of foci positive for G-6PD and for gamma-GT; nodules induced by DEN/PB were mainly positive for gamma-GT and showed an erratic response to the other parameters. Carcinomas, found only after DEN/PB, were all positive for G-6PD and, with one exception, all were negative for alpha-GPD, 5'-Nu, AP and GSH. All changes regressed within 3 months of withdrawal of CLOF but not after withdrawal of PB from DEN-initiated animals. In conclusion G-6PD, alpha-GPD and 5'-Nu may be useful histocytochemical parameters for studying the precarcinogenic hepatic changes and nodules induced by peroxisome proliferators and by genotoxic hepatocarcinogens.

Overdose toxicity studies versus threshold: elements of biology must be incorporated into risk assessment

The estimation of risk and evaluation of risk/benefit are traditionally and by necessity oriented at the current state of science and technology to ensure contemporary rights of safety. This demands a careful development in the methodology of hazard identification and risk assessment and its continuous updating and involvement. Compounds, e.g. erythrosine, 2,4,5-T, TCDD, and antioxidants, have still to be judged case-by-case, taking into account all available information on dosage, effect, kinetics, and mechanism of action, i.e., matters of biology rather than of mathematics alone. Considerations of mechanism of action and kinetics, especially recognition of low-dose/high-affinity assumptions in vitro, are necessary. This might lead to a new view upon thresholds which appears to apply for promoters. As accepted in pharmacology the dose-dependent magnitude of response observed in vivo is often a composite effect. Composite effects in toxicology can be viewed as having similar characteristics in vivo as in vitro in terms of potency, slope, maximal efficay and variability. Composite vectors can be antagonistic, leading to toxic and carcinogenic results, but also to protection. A continuous updating of scientific expertise supported by own experimental work is required for the regulator.