Quantitative stereological evaluation of four histochemical markers of altered foci in multistage hepatocarcinogenesis in the rat

Dietary antioxidants in the prevention of hepatocarcinogenesis: a review

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

Characteristic gene expression profiles in the progression from liver cirrhosis to carcinoma induced by diethylnitrosamine in a rat model

Background

Liver cancr is a heterogeneous disease in terms of etiology, biologic and clinical behavior. Very little is known about how many genes concur at the molecular level of tumor development, progression and aggressiveness. To explore the key genes involved in the development of liver cancer, we established a rat model induced by diethylnitrosamine to investigate the gene expression profiles of liver tissues during the transition to cirrhosis and carcinoma.

Methods

A rat model of liver cancer induced by diethylnitrosamine was established. The cirrhotic tissue, the dysplasia nodules, the early cancerous nodules and the cancerous nodules from the rats with lung metastasis were chosen to compare with liver tissue of normal rats to investigate the differential expression genes between them. Affymetrix GeneChip Rat 230 2.0 arrays were used throughout. The real-time quantity PCR was used to verify the expression of some differential expression genes in tissues.

Results

The pathological changes that occurred in the livers of diethylnitrosamine-treated rats included non-specific injury, fibrosis and cirrhosis, dysplastic nodules, early cancerous nodules and metastasis. There are 349 upregulated and 345 downregulated genes sharing among the above chosen tissues when compared with liver tissue of normal rats. The deregulated genes play various roles in diverse processes such as metabolism, transport, cell proliferation, apoptosis, cell adhesion, angiogenesis and so on. Among which, 41 upregulated and 27 downregulated genes are associated with inflammatory response, immune response and oxidative stress. Twenty-four genes associated with glutathione metabolism majorly participating oxidative stress were deregulated in the development of liver cancer. There were 19 members belong to CYP450 family downregulated, except CYP2C40 upregulated.

Conclusion

In this study, we provide the global gene expression profiles during the development and progression of liver cancer in rats. The data obtained from the gene expression profiles will allow us to acquire insights into the molecular mechanisms of hepatocarcinogenesis and identify specific genes (or gene products) that can be used for early molecular diagnosis, risk analysis, prognosis prediction, and development of new therapies.

Potential inhibitory effects of D-allose, a rare sugar, on liver preneoplastic lesion development in F344 rat medium-term bioassay

D-allose, the C-3 epimer of d-glucose, is a monosaccharide present in minute quantities in nature and a rare sugar. The effects of D-allose on diethyl nitrosamine (DEN)-induced hepatocarcinogenesis were examined in male F344 rats by a rat medium-term bioassay based on the two-step model of hepatocarcinogenesis (experiment 1). In addition, a DNA microarray analysis was employed to clarify possible mechanisms of action of D-allose (experiment 2). The antioxidation potential of D-allose solution itself or of serum in rats treated with D-allose was also examined directly by measuring Cu(+)-reducing antioxidation power (experiment 3). Furthermore, to investigate the effects of D-allose in vivo under conditions of oxidative stress, it was administered with a choline-deficient, L-amino acid-defined diet (CDAA) in the medium-term liver carcinogenesis bioassay (experiment 4). Experiment 1 demonstrated no effects of D-allose on the development of glutathione S-transferase placental form (GST-P) positive foci in the liver. From DNA microarray analysis, several mRNA markers were found to be altered with functions related to apoptosis and cell proliferation (experiment 2), although D-allose itself and serum in vivo exhibited no antioxidation power directly (experiment 3). When D-allose was administered with the CDAA diet, decreases in the area and number of GST-P positive foci were noted with P values of 0.158 for area (%) and 0.061 for number (/cm(2)) (experiment 4). These results suggest the potential inhibitory effect of D-allose on liver carcinogenesis, particularly under oxidative stress conditions.

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.

Evaluation of tumour promoting potency of fish borne toxaphene residues, as compared to technical toxaphene and UV-irradiated toxaphene

In this study the potential impact of food chain-based biotransformation and physico-chemical weathering of toxaphene on its tumour promoting potential was investigated in vitro and in vivo. Human exposure to toxaphene is mainly through consumption of contaminated fish, therefore fish-borne residues of toxaphene (cod liver extract, CLE) were prepared by exposing cod to technical toxaphene (TT) for 63 days. UV-irradiated toxaphene (uvT) was included to represent a physico-chemical weathered toxaphene mixture. In vitro, TT, uvT and CLE all showed a dose- and time-dependent inhibition of gap junctional intercellular communication (GJIC) with a relative potency of CLE>TT=uvT. Tumour promoting potency was further studied in vivo in a medium term two-stage initiation/promotion bioassay in female Sprague-Dawley rats, using an increase in altered hepatic foci positive for glutathione-S-transferase-P (AHF-GST-P) as read out. No increase in AHF-GST-P occurred following exposure to either TT, uvT, or CLE, except for the positive control group (2,3,7,8-TCDD). Based on this study the no observed adverse effect level (NOAEL) for tumour promoting potency is at least 12.5mg/kg/week, or higher for CLE. Considering current human exposure levels in Europe it is doubtful that consumption of fish at current levels of toxaphene contamination give rise to human health risk.

Gene expression profile related to the progression of preneoplastic nodules toward hepatocellular carcinoma in rats

In this study, we investigated the time course gene expression profile of preneoplastic nodules and hepatocellular carcinomas (HCC) to define the genes implicated in cancer progression in a resistant hepatocyte model. Tissues that included early nodules (1 month, ENT-1), persistent nodules (5 months, ENT-5), dissected HCC (12 months), and normal livers (NL) from adult rats were analyzed by cDNA arrays including 1185 rat genes. Differential genes were derived in each type of sample (n = 3) by statistical analysis. The relationship between samples was described in a Venn diagram for 290 genes. From these, 72 genes were shared between tissues with nodules and HCC. In addition, 35 genes with statistical significance only in HCC and with extreme ratios were identified. Differential expression of 11 genes was confirmed by comparative reverse transcription-polymerase chain reaction, whereas that of 2 genes was confirmed by immunohistochemistry. Members involved in cytochrome P450 and second-phase metabolism were downregulated, whereas genes involved in glutathione metabolism were upregulated, implicating a possible role of glutathione and oxidative regulation. We provide a gene expression profile related to the progression of nodules into HCC, which contributes to the understanding of liver cancer development and offers the prospect for chemoprevention strategies or early treatment of HCC.

A software package to improve image quality and isolation of objects of interest for quantitative stereology studies of rat hepatocarcinogenesis

In the studies of quantitative stereology of rat hepatocarcinogenesis, we have used image analysis technology (automatic particle analysis) to obtain data such as liver tissue area, size and location of altered hepatic focal lesions (AHF), and nuclei counts. These data are then used for three-dimensional estimation of AHF occurrence and nuclear labeling index analysis. These are important parameters for quantitative studies of carcinogenesis, for screening and classifying carcinogens, and for risk estimation. To take such measurements, structures or cells of interest should be separated from the other components based on the difference of color and density. Common background problems seen on the captured sample image such as uneven light illumination or color shading can cause severe problems in the measurement. Two application programs (BK_Correction and Pixel_Separator) have been developed to solve these problems. With BK_Correction, common background problems such as incorrect color temperature setting, color shading, and uneven light illumination background, can be corrected. With Pixel_Separator different types of objects can be separated from each other in relation to their color, such as seen with different colors in immunohistochemically stained slides. The resultant images of such objects separated from other components are then ready for particle analysis. Objects that have the same darkness but different colors can be accurately differentiated in a grayscale image analysis system after application of these programs.

Three-dimensional reconstruction from serial sections in PC-Windows platform by using 3D_Viewer

Three-dimensional (3D) reconstruction from serial sections allows identification of objects of interest in 3D and clarifies the relationship among these objects. 3D_Viewer, developed in our laboratory for this purpose, has four major functions: image alignment, movie frame production, movie viewing, and shift-overlay image generation. Color images captured from serial sections were aligned; then the contours of objects of interest were highlighted in a semi-automatic manner. These 2D images were then automatically stacked at different viewing angles, and their composite images on a projected plane were recorded by an image transform-shift-overlay technique. These composition images are used in the object-rotation movie show. The design considerations of the program and the procedures used for 3D reconstruction from serial sections are described. This program, with a digital image-capture system, a semi-automatic contours highlight method, and an automatic image transform-shift-overlay technique, greatly speeds up the reconstruction process. Since images generated by 3D_Viewer are in a general graphic format, data sharing with others is easy. 3D_Viewer is written in MS Visual Basic 6, obtainable from our laboratory on request.

Combined supplementation of vanadium and beta-carotene suppresses placental glutathione S-transferase-positive foci and enhances antioxidant functions during the inhibition of diethylnitrosamine-induced rat liver carcinogenesis

BACKGROUND AND AIM

The present study was designed to investigate the chemopreventive effects of combined vanadium (V; 0.5 p.p.m.) and beta-carotene (BC; 120 mg/kg of basal diet) on diethylnitrosoamine (DEN)-induced and phenobarbital (PB)-promoted rat hepatocarcinogenesis.

METHODS

All rats were subjected to two-thirds partial hepatectomy (PH) at the fourth week. After PH they were administered either trioctanoin alone (groups A', B', C' and D') or a single injection of DEN in trioctanoin at a dose of 10 mg/kg of body weight (groups A, B, C and D). Two weeks after the DEN treatment PB was administered (0.05% in basal diet) to all the DEN-treated rats and continued until the end of the experiment. Supplementation of V (groups B and B'), BC (groups C and C') or both V and BC (groups D and D') at the doses stated previously were started 4 weeks before DEN administration (at week 0) and continued until the 16th week.

RESULTS

It was observed that in the DEN-treated and PB-promoted group (group A) the expression of the numbers and areas of the placental form of glutathione S-transferase (GST-P)-positive altered hepatic foci (AHF) was maximum. Treatment with V (group B) and BC (group C) significantly reduced the expression of GST-P-positive AHF by 29.5% and 42.8%, respectively. An additive protection action (65.7%) was noticed in group D, which received both V and BC for the entire period of the experiment. It was also observed that supplementation of V and BC for the entire period of the experiment significantly reduced the number and size of the hyperplastic nodules, while the combination treatment worked as an additive effect, reducing the number and size of the hyperplastic nodules to 22% from 89%. Moreover, a significantly reduced level of cytosolic glutathione (P < 0.001) and glutathione-S-transferase (P < 0.001) activity and stabilization of aerobic metabolism and hepatic architecture of the cells as compared with carcinogen control were observed in the V + BC-treated group.

CONCLUSION

The present study suggests that V, an essential trace element, may be useful in combination with BC, an antioxidant, in the inhibition of experimentally induced rat hepatocarcinogenesis.

An improved stereologic method for three-dimensional estimation of particle size distribution from observations in two dimensions and its application

Single enzyme-altered hepatocytes; altered hepatic foci (AHF); and nodular lesions have been implicated, respectively in the processes of initiation, promotion, and progression in rodent hepatocarcinogenesis. Qualitative and quantitative analyses of such lesions have been utilized both to identify and to determine the potency of initiating, promoting, and progressor agents in rodent liver. Of a number of possible parameters determined in the study of such lesions, estimation of the number of foci or nodules in the liver is very important. The method of Saltykov has been used for estimating the number of AHF in rat liver. However, in practice, the Saltykov calculation has at least two weak points: (a) the size class range is limited to 12, which in many instances is too narrow to cover the range of AHF data obtained; and (b) under some conditions, the Saltykov equation generates negative values in several size classes, an obvious impossibility in the real world. In order to overcome these limitations in the Saltykov calculations, a study of the particle size distribution in a wide-range, polydispersed sphere system was performed. A stereologic method, termed the 25F Association method, was developed from this study. This method offers 25 association factors that are derived from the frequency of different-sized transections obtained from transecting a spherical particle, thus expanding the size class range to be analyzed up to 25, which is sufficiently wide to encompass all rat AHF found in most cases. This method exhibits greater flexibility, which allows adjustments to be made within the calculation process when NA((k,k)), the net number of transections from the same size spheres, was found to be a negative value, which is not possible in real situations. The reliability of the 25F Association method was tested thoroughly by computer simulation in both monodispersed and polydispersed sphere systems. The test results were compared with the original Saltykov method. We found that the 25F Association method yielded a better estimate of the total number of spheres in the three-dimensional tissue sample as well as the detailed size distribution information. Although the 25F Association method was derived from the study of a polydispersed sphere system, it can be used for continuous size distribution sphere systems. Application of this method to the estimation of parameters of preneoplastic foci in rodent liver is presented as an example of its utility. An application software program, 3D_estimation.exe, which uses the 25F Association method to estimate the number of AHF in rodent liver, has been developed and is now available at the website of this laboratory.

Menhaden oil inhibited gamma-glutamyltransferase-positive altered hepatic foci in female Sprague-Dawley rats

Feeding menhaden oil, high in n-3 fatty acids, or a mixture of lard and corn oil with a polyunsaturated-to-monounsaturated fatty acid ratio of 1:1 was hypothesized to inhibit promotion of hepatocarcinogenesis in rats by decreasing hepatic prostaglandin (PG) levels. Ten-day-old female Sprague-Dawley rats were injected with diethylnitrosamine (DEN, 15 mg/kg body wt ip). At 4 wk of age, rats were fed fumonisin B1(50 mg/kg diet) for 5 wk in diets containing 14% lard + 6% corn oil, 10% lard + 10% corn oil, 14% menhaden oil + 6% corn oil, and 7% menhaden oil + 13% corn oil. Plasma alanine aminotransferase activity was 20% lower in rats fed 10% lard than in rats fed the other diets (P < 0.05). In menhaden oil-fed rats, total plasma cholesterol concentrations decreased 26% (P < 0.05) and hepatic phospholipid C20:5n-3, C22:5n-3, and C22:6n-3 fatty acid concentrations increased compared with lard-fed rats. Hepatic n-3 fatty acids were threefold greater in rats fed 10% lard than in rats fed 14% lard. The liver-associated natural killer cell activity in rats fed menhaden oil was 58% lower than in rats fed lard (P < 0.03). Rats fed lard had threefold (P < 0.05) greater area of _-glutamyltransferase-positive altered hepatic foci (AHF) than did rats fed menhaden oil. There was no significant difference in placental glutathione S-transferase-positive AHF among the groups. Hepatic PGF2alpha production was 60-80% greater in rats fed 14% lard than in rats fed the other diets (P < 0.05). Hepatic PGE2 was 48% less in rats fed 14%; menhaden oil than in rats fed 14% lard (P < 0.05). Although gamma-glutamyltransferase-positive focal area was inhibited by menhaden oil, only 14% menhaden oil inhibited PGE2. Feeding 10% lard inhibited PGF2alpha, but not the development of AHF. Therefore, decreased hepatic PGs did not explain the inhibition of carcinogenesis.

Effects of griseofulvin in medium-term liver carcinogenesis assay and peripheral blood micronucleus test in rat

Published data have suggested a possible link between the tumor promoting activity and the aneugenic properties of griseofulvin. The present study was conducted to explore this relationship. Griseofulvin was evaluated both for its potential promoting activity in liver carcinogenesis in partially hepatectomized F344 male rats initiated by diethylnitrosamine and for its genotoxic potential in the peripheral blood micronucleus assay. Rats were treated daily with 2,000 mg/kg body weight by oral gavage for 12 weeks in the medium-term carcinogenesis bioassay. GST-P-positive foci (mean number and surface area) and altered cell foci were compared in the liver of rats treated with griseofulvin alone, diethylnitrosamine alone,and griseofulvin in addition to diethylnitrosamine by using immunohistochemical and histopathological evaluation, respectively. This evaluation allowed the conclusion that griseofulvin did not initiate the carcinogenic process but rather had a potential in the liver for tumor promoting activity. Griseofulvin was found to be negative in the rat peripheral blood micronucleus test when given at a daily oral dose of 2,000 mg/kg body weight for at least 3 weeks.

Histological organization in hepatocyte organoid cultures

Hepatocytes and other cellular elements isolated by collagenase perfusion of the liver and maintained in defined culture conditions undergo a series of complex changes, including apoptosis and cell proliferation, to reconstruct tissue with specific architecture. Cultures in collagen-coated pleated surface roller bottles, with hepatocyte growth medium medium and in the presence of hepatocyte growth factor (HGF) and epidermal growth factor (EGF), form characteristic and reproducible tissue architecture composed of a superficial layer of biliary epithelial cells, an intermediate layer of connective tissue and hepatocytes, and a basal layer of endothelial cells. Dexamethasone, EGF, and HGF are required for the complete histological organization. Analysis of the structures formed demonstrates that the receptor tyrosine kinase ligands HGF and EGF are required for the presence, growth, and phenotypic maturation of the biliary epithelium on the surface of the cultures and for the formation of connective tissue in the cultures. Dexamethasone, in the presence of HGF and EGF, was required for the phenotypic maturation of hepatocytes. The results demonstrate the role of these molecules for the formation and phenotypic maturation of specific histological elements of the liver and suggest roles for these signaling molecules in the formation and structure of the in vivo hepatic architecture.

Regulation of 2,3,7,8-Tetrachlorodibenzo-p-dioxin-Induced Tumor Promotion by 17β-Estradiol in Female Sprague–Dawley Rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatocarcinogen in female but not in male rats. In an initiation-promotion model, ovariectomy inhibits TCDD-induced cell replication and reduces both TCDD-induced tumor formation and the promotion of enzyme-altered hepatocellular foci (AHF). The aim of this study was to determine the involvement of the ovarian hormone 17 beta-estradiol in the induction of cell proliferation and development of putative preneoplastic AHF following chronic exposure to TCDD. Diethylnitrosamine (DEN)-initiated ovariectomized (OVX) female rats were treated with TCDD for 20 or 30 weeks in the presence and absence of 17 beta-estradiol administered continuously by implanted 90-day release pellets. Following 20 weeks of treatment, cell proliferation in TCDD-treated rats was decreased regardless of ovarian hormones. Following 30 weeks of exposureTCDD, only significantly induced cell proliferation in OVX rats receiving supplemental 17 beta-estradiol. These data demonstrate that the the transitory mitoinhibition of cell replication by TCDD is not hormonally responsive, but that induction of cell replication at later time points is. TCDD exposure resulted in elevated AHF expressing gamma-glutamyltranspeptidase (GGT) in intact, but not OVX rats at both time points. TCDD also induced GGT-positive AHF in 17 beta-estradiol-supplemented OVX rats. TCDD induced AHF expressing the placental form of glutathione-S-transferase (PGST) in both intact and OVX rats regardless of 17 beta-estradiol exposure, indicating that the modulating effect of 17 beta-estradiol on AHF was specific to the GGT-positive phenotype.

Enhancement of hepatocarcinogenesis initiated with diethylnitrosamine or N-nitrosobis(2-hydroxypropyl)amine by a choline-deficient, L-amino acid-defined diet administered prior to the carcinogen exposure in rats

Effects of pre-administration of a choline-deficient, L-amino acid-defined (CDAA) diet on hepatocarcinogenesis initiated with diethylnitrosamine (DEN) or N-nitrosobis(2-hydroxypropyl)amine (BHP) in rats were investigated. A pre-administrating period was set as 1 week, because CDAA diet induces liver injuries by this time-point. In a time-course study, male Fischer 344 rats, 6 weeks old, received a 1-week pre-administration of choline-supplemented, L-amino acid-defined (CSAA) or CDAA diet, DEN at a dose of 100 mg/kg body weight by a single intraperitoneal injection, then CSAA or CDAA diet for up to 8 weeks, and were sacrificed 4, 6 and 8 weeks after DEN. CDAA diet administered only after DEN significantly increased the numbers of glutathione S-transferase placental form (GST-P)-positive lesions 4, 6 and 8 weeks after DEN and their sizes 6 and 8 weeks after DEN. CDAA diet administered both before and after DEN similarly increased the numbers and sizes of GST-P-positive lesions, but with a significantly greater degree than obtained by the diet administered only after DEN. In a dose response study, rats received vechicle or DEN, at a dose of 0.001, 0.01, 0.1, 1, 10, 20, 50, 100 or 200 mg/kg body weight, 1 week after the commencement of CSAA or CDAA diet, and sacrificed 8 weeks after vehicle or DEN. The significant increases of the numbers of GST-P-positive lesions were obtained after 50-200 mg/kg body weight of DEN under the CSAA diet administration, whereas those were detected after 10-200 mg/kg under CDAA diet administration. Sizes became significantly larger with only 200 mg/kg body weight of DEN in the CSAA case but with 50-200 mg/kg in the CDAA case. Male Wistar rats received a 1-week pre-administration of CSAA or CDAA diet, vehicle or BHP, at a dose of 600 or 1200 mg/kg body weight, by a single intraperitoneal injection, then CSAA or CDAA diet for 8 weeks, and were then sacrificed. The numbers of GST-P-positive lesions demonstrated significant increment with 1200 mg/kg body weight of BHP by CDAA diet administered only after BHP and, to a significantly greater degree, by the diet administered both before and after BHP. While CDAA diet administered only after BHP did not alter the sizes of GST-P-positive lesions, the diet administered both before and after 600 and 1200 mg/kg body weight of BHP significantly increased the sizes of the lesions. These results indicate that the pre- plus post-administration of CDAA diet enhances hepatocarcinogenesis initiated with DEN or BHP, more than the post-administration only, thus providing a sensitive model to detect weak liver carcinogenic potency of environmental chemicals.

Selective loss of nucleoside carrier expression in rat hepatocarcinomas

Evidence that hepatoma cell lines show differential expression of concentrative nucleoside transporters (CNT1 and CNT2) prompted us to study the transporter proteins in 2 models of hepatocarcinogenesis, the chemically induced Solt and Farber model and the albumin-SV40 large T antigen (Alb-SV40) transgenic rat. CNT1 expression was lower in tumor biopsy specimens from Alb-SV40 rat livers than in normal tissue. Immunocytochemistry revealed that the CNT1 protein was indeed absent in the tumor lesions. CNT1 was also absent in a cell line, L25, derived from the Alb-SV40 transgenic rat liver tumors, whereas another cell line, L37, derived from the normal-appearing parenchyma, retained the expression of both carrier isoforms. The protein expression correlated with the nucleoside transport properties of these cell lines. Moreover, although CNT2 expression was highly dependent on the growth characteristics of the 2 cell lines, as was CNT1 (albeit to a lower extent) in L37 cells, it was not expressed in L25 cells at any stage of cell growth. In contrast to the transgenic model of hepatocarcinogenesis, in the chemically induced tumors the expression of CNT2 was lower, although still detectable. In summary, these data indicate that hepatocarcinogenesis leads to a selective loss or diminished expression of nucleoside carrier isoforms, a feature that may be relevant to our understanding of the molecular basis of the bioavailability of those drugs that are nucleoside derivatives and may be substrates of these carriers. The transport properties and isoform-expression profile of the L25 and L37 cell lines make them suitable hepatocyte culture models with which to study nucleoside transport processes and drug sensitivity.

Effects of intermittent exposures of aflatoxin B(1) on hepatic and testicular glutathione S-transferase in rats

Glutathione S-transferase (GST) plays a major role in the detoxification of the potent hepatocarcinogen aflatoxin B(1) (AFB(1)). This study evaluated the effects of intermittent exposures to AFB(1) on hepatic and testicular GST in rats. Male Fischer 344 rats were fed diets containing AFB(1) (0, 0.01, 0.04, 0.4 and 1.6 ppm) intermittently at 4-week intervals up to 20 weeks. The control animals were fed an AFB(1)-free NIH-31 diet. Rats consuming diets with 0.01 ppm AFB(1) did not show the induction of hepatic or testicular GST activity. Intermittent exposures to AFB(1) at concentrations of 0.04-1.6 ppm significantly increased the GST activities. The increase of the enzyme activity was proportional to the dose and length of AFB(1) exposure.

Animal studies addressing the carcinogenicity of TCDD (or related compounds) with an emphasis on tumour promotion

Dioxin and certain structurally related compounds increase the incidence of liver neoplasms in rodents upon chronic bioassay. Short-term studies indicate the lack of direct DNA-damaging effects including covalent binding to DNA; however, secondary mechanisms may be important in the observed carcinogenicity as these chemicals affect a number of pathways necessary for maintenance of normal growth control and differentiation status. Studies with TCDD in the mouse skin support a lack of initiating activity but an ability to promote the growth of previously initiated lesions indicative of a promoting agent. Mouse skin tumour promotion studies indicate that Ah receptor activation may be involved in promotion by TCDD and selected structurally related compounds. While the mechanism of carcinogenicity induced by TCDD is unknown, the processes involved have a no-effect level, which in the rat liver is at an exposure level below 10 ng TCDD/kg/day. At least for the rodent liver, the relative effective dose for cytochrome P450 induction is not a good indicator of promotion potency. Studies on liver tumour promotion in the female rat liver support a non-genotoxic mechanism for the induction of neoplasms by TCDD. The ability of TCDD to enhance proliferation and inhibit apoptotic processes in focal hepatic lesions further supports an indirect mechanism of carcinogenicity.

Dietary oat lipids-induced novel DNA modifications and suppression of altered hepatic foci formation

Previous studies have shown that the presence of several hepatic I-compounds, i.e., age-dependent covalent DNA modifications, is related to the presence of a natural ingredient, i.e., oats, in the diet. To demonstrate the biological significance of these novel DNA modifications, the effect of oat lipids on tumor initiation and promotion was examined in a rat liver tumor model. Female Sprague-Dawley rats were treated with a single dose of diethylnitrosamine, a hepatic carcinogen, 24 hours after a 70% partial hepatectomy, then subjected to dietary phenobarbital promotion. Diets containing 10% oat lipids or corn oil were given during the initiation or the promotion stage of the tumorigenesis. At the end of the feeding, hepatic I-compounds were measured by 32P postlabeling, and the number and volume of enzyme-altered hepatic foci, which served as preneoplastic markers, were measured in serial sections of liver by the method of quantitative stereology. Rats receiving oat lipids-supplemented diets had five- to sixfold higher levels of I-compounds in their liver DNA than those receiving control diets. Meanwhile, rats receiving diets containing oat lipids during promotion had significantly smaller numbers and reduced volume of altered hepatic foci compared with those fed the control diet containing corn oil. These observations support the hypothesis that some I-compounds, e.g., the oats-specific I-compounds, are novel DNA modifications related to nutrient metabolism. The diet containing oat lipids may have chemopreventive activities, as demonstrated in this model system.

Inhibition of early-phase exogenous and endogenous liver carcinogenesis in transgenic rats harboring a rat glutathione S-transferase placental form gene

Hepatocarcinogenesis initiated with N-nitrosodiethylamine (DEN) and that initiated by feeding of a choline-deficient, L-amino acid-defined (CDAA) diet were compared in transgenic male Wistar rats harboring a rat glutathione S-transferase placental form (GST-P) gene (GST-P-Tg rats) and non-transgenic (N-Tg) rats. Eight-week-old GST-P-Tg and N-Tg rats were administered DEN intraperitoneally at 100 mg/kg body weight, subjected to a selection procedure with 2-acetylaminofluorene and CCl4, and killed at the end of weeks 5 and 12. Other groups were fed the CDAA diet for 12 weeks and killed. Five weeks after the DEN treatment, numbers and sizes of gamma-glutamyltransferase (GGT)- or GST-P-positive lesions and 8-hydroxyguanine (8-OHG) levels in the livers were significantly less in GST-P-Tg rats than in N-Tg rats. The lesion numbers were unchanged between the ends of weeks 5 and 12 in GST-P-Tg rats, but decreased in N-Tg rats. The lesion sizes were increased in GST-P-Tg rats, but unchanged in N-Tg rats. While the proliferating cell nuclear antigen labeling indices (PCNA L.I.) in and surrounding the lesions were decreased, more prominently in GST-P-Tg rats than in N-Tg rats, the 8-OHG levels were also decreased but similarly in both cases. After 12 weeks on the CDAA diet, the lesion incidences, numbers and sizes, 8-OHG levels, PCNA L.I. in and surrounding the lesions, and liver injury were significantly less in GST-P-Tg rats than in N-Tg rats. These results indicate that insertion of a rat GST-P transgene alters the early phase of exogenous and endogenous rat hepatocarcinogenesis, presumably due to enhanced detoxification by GST-P expressed both transiently during the initiation and chronically in the altered hepatocyte populations.

Initiation and promotion of altered hepatic foci in female rats and inhibition of cell-cell communication by the imidazole fungicide prochloraz

The imidazole fungicide prochloraz (1-[N-propyl-N-2-(2,4,6-trichlorophenoxy) ethyl carbamoyl] imidazole) was investigated for its ability to inhibit gap junctional intercellular communication in the scrape-loading/dye-transfer assay in IAR 20 rat liver epithelial cells and for effects on the initiation and promotion stages of hepatocarcinogenesis. Female Sprague-Dawley rats initiated with N-nitrosodiethylamine 24-hr after partial hepatectomy were administered prochloraz five days a week by oral gavage (30 or 150 mg/kg) for 10 weeks. Altered hepatic foci (AHF) were analyzed by quantitative stereology from liver sections stained for gamma-glutamyltranspeptidase (GGT) and glutathione S-transferase P (GST-P). The fungicide was also studied for its ability to initiate the development of GGT-positive AHF in rat liver. The in vitro studies showed prochloraz to be an inhibitor of cell-cell communication in the test system used. In the in vivo studies, prochloraz showed no effect on the initiation of GGT-positive foci. However, significant increases in the percentage of liver tissue occupied by GGT-positive AHF and the number of GST-P-positive AHF per cm3 in initiated animals were recorded in the low dose group. The present data suggest that prochloraz acts as a weak tumour promoter of hepatocarcinogenesis but does not initiate this process.

Comparison of effect of tumor promoter treatments on DNA methylation status and gene expression in B6C3F1 and C57BL/6 mouse liver and in B6C3F1 mouse liver tumors

The effects of different liver tumor-promoting treatments (i.e., a choline-devoid, methionine-deficient (CMD) diet, phenobarbital (PB), or both) on Ha-ras and raf methylation status and expression were determined in mouse strains with different susceptibilities to liver tumor formation: the relatively sensitive B6C3F1 and the relatively resistant C57BL/6. Additionally, B6C3F1 mouse liver tumors, spontaneous or PB induced, were assessed for alterations in global DNA methylation status and expression of Ha-ras and raf. The CMD diet led to hypomethylation of Ha-ras and raf after 12 wk of administration in B6C3F1 and C57BL/6 mice. At this early phase of tumor promotion, the frequency of increased expression of both Ha-ras and raf mRNAs was higher in the B6C3F1 but not the C57BL/6 mice. This is a mechanism that may, in part, underlie the heightened sensitivity of the B6C3F1 mouse to liver tumorigenesis. Subpopulations of B6C3F1 mouse liver tumors displayed altered global methylation status, with both hypomethylation and hypermethylation evident. Carcinomas were significantly more hypomethylated than adenomas. The level of raf mRNA was not changed in spontaneous or PB-induced B6C3F1 mouse liver tumors. Increased expression of Ha-ras was evident in some spontaneous B6C3F1 liver tumors and in most of the PB-induced liver tumors. These experiments support the concept that altered DNA methylation plays a key role in tumorigenesis and indicate that the high propensity of the B6C3F1 mice to liver tumorigenesis may be due, in part, to a decreased ability to maintain normal methylation status.

Quantitation of multistage carcinogenesis in rat liver

A well characterized model of multistage carcinogenesis is that of hepatocarcinogenesis in the rat. The histopathology as well as the cell and molecular biology of the stages of initiation, promotion, and progression have been elucidated to varying degrees in this system. Putatively single initiated hepatocytes are identified by their expression of the ubiquitous marker of hepatocarcinogenesis, glutathione-S-transferase pi (GSTP). 0.5-1.0 x 10(6) GSTP-positive "initiated" hepatocytes developed within 14 days after initiation with a subcarcinogenic dose of diethylnitrosamine (DEN). Approximately 1% of these cells develop clonally into altered hepatic foci (AHF) in animals administered promoting agents, such as phenobarbital, chronically for 4-8 mo. Hepatocytes within AHF during the stage of promotion exhibit normal diploid karyotypes but various phenotypes depending on the chemical nature of the promoting agent. Continued administration of the promoting agent results in the infrequent development of hepatocellular carcinomas; however, administration of a complete carcinogen or a progressor agent during the stage of promotion results in substantial numbers of hepatic neoplasms. In order to quantitate the development of the stage of progression more accurately, markers selective for this stage have been sought. Transforming growth factor-alpha (TGF-alpha) appears to be such a marker of progression. About 500 TGF-alpha-positive lesions develop spontaneously following initiation and continued promotion, usually within GSTP-positive AHF, but administration of a single dose of a progressor agent such as ethylnitrosourea may increase this number 3-fold or more. Some agents such as gamma radiation and hydroxyurea, when administered as single or a few closely spaced multiple doses, result in no increased number in TGF-alpha-positive lesions but a markedly enhanced increase in their growth rate. By monitoring gene expression using quantitative stereology, the stages of hepatocarcinogenesis can be analyzed and quantified in sufficient detail so that the animal data can be utilized in biomathematical modeling to develop more accurate models for estimation of human cancer risks.

Comparison of experimental and theoretical parameters of the Moolgavkar-Venzon-Knudson incidence function for the stages of initiation and promotion in rat hepatocarcinogenesis

Mathematical descriptions of complex biological phenomena, such as cancer, require an experimental format that faithfully recapitulates the biological process. In addition, the biological process must dictate the parameters in the mathematical formula. Evidence from the epidemiology of several human cancers and from experimental carcinogenesis in several organ systems indicates that cancer is a multistage process. The initiation-promotion-progression format of experimental carcinogenesis mimics the development of cancer in humans and other animals. In rats, the altered hepatic focus model of hepatocarcinogenesis has been well characterized and, coupled with the method of quantitative stereology, permits accurate determination of the number and the volume fraction of such altered foci per liver. The placental isozyme of glutathione S-transferase (PGST) is reportedly the best single marker of preneoplasia in the rat liver. Recently, single hepatocytes expressing PGST have been proposed as putatively initiated cells. Quantitation of individual hepatic cells and altered hepatic foci expressing PGST in the livers of rats subjected to an initiation-promotion protocol permits determination of the congruence of the Moolgavkar-Venzon-Knudson (MVK) model with experimental data. The best fit of the MVK model for the preneoplastic stages of hepatocarcinogenesis assumes that all hepatocytes are susceptible and that single hepatocytes expressing PGST are the initiated cell population for the focal lesions that express PGST. Further refinement of the initiation-promotion-progression model to permit accurate quantitation of early malignant conversion should allow a more complete analysis of the congruence of the MVK model for human cancer risk determination. In addition, the MVK model may be extended to other model systems and to human cancers in which early preneoplasia can be quantitated. Furthermore, the use of a more biologically based risk-assessment protocol, such as the MVK model rather than the stochastic one-hit model presently used, would permit incorporation of the present knowledge on the pathogenesis of cancer. To apply experimental data to a mathematical model that reflects the biological processes underlying human cancer development will require integration of the cell kinetics and experimental data to a mathematical model that reflects the biological processes underlying human cancer development including the pharmacokinetic and pharmacodynamic properties of the treatment chemicals.

Development and persistence of placental glutathione-S-transferase-positive foci in livers of male F344 rats exposed to o-nitrotoluene

In a previous 13-week study of o-nitrotoluene, a chemical-related increase in liver weight, hepatocellular vacuolization, and oval cell hyperplasia in male F344 rats was reported. In this study, the occurrence and change in number and size of hepatic foci in male F344 rats fed a diet containing 5000 ppm o-nitrotoluene or a control diet for 13 weeks, 26 weeks, and 13 weeks followed by a 13-week recovery period (26-week stop-exposure) were evaluated. The livers were stained immunohistochemically for placental glutathione S-transferase (PGST), a marker of hepatic preneoplasia, and quantified stereologically using computer-assisted image analysis. Exposure to o-nitrotoluene induced PGST-positive (PGST-positive (PGST+) liver foci in all treatment groups. The 26-week continuous-exposure group produced more PGST+ liver foci (961.4 foci/cm3 versus 445.4 foci/cm3) and greater mean focus volume (4.34 microns3 versus 1.34 microns3) than the 13-week continuous-exposure group. In the 26-week stop-exposure group, there were fewer PGST+ liver foci (181.4 foci/cm3) than observed with continuous exposure at 13 weeks or 26 weeks; however, the mean focal volume in the stop-exposure group at 26 weeks (5.33 microns3) was greater than that at 13 weeks (1.34 microns3) or 26 weeks of continuous exposure (4.34 microns3). These findings demonstrate that (1) PGST+ foci are observed after only 13 weeks of exposure to o-nitrotoluene; (2) the number and size of foci increase with continued exposure for 26 weeks; and (3) although the number of PGST+ foci decreases with time after chemical exposure is discontinued, many PGST+ foci do not regress but increase in size during the recovery of 13 weeks. The persistence and increase in size of these foci, even in the absence of chemical exposure, suggest the potential for a hepatocarcinogenic effect in long-term studies for o-nitrotoluene.

The potential for the use of cell proliferation and oncogene expression as intermediate markers during liver carcinogenesis

Intense research using animal models has indicated that chemically-induced rat liver cancer proceeds through multiple, distinct stages that can be characterised morphologically and biochemically. Primary human liver cancer, with hepatitis B and other environmental factors such as poor nutrition and food contaminating mycotoxins as contributing etiological factors, is one of the major causes of cancer deaths in African, Asian and some Western countries. Recent advances in surgical and diagnostic techniques have also allowed the identification of potential morphological precursors of primary human liver cancer, and suggested a model consistent with the concepts of initiation--promotion--progression as in the rat. The expression of proliferating cell nuclear antigen (PCNA), silver-staining nucleolar organiser regions (AgNOR), oncogenes and the tumor suppressor gene p53 in preneoplastic and neoplastic lesions of rat and human livers is presently reviewed. This undertaking is an attempt to evaluate whether the current knowledge regarding molecular mechanisms of carcinogenesis is sufficient to permit the use of these molecular parameters as 'intermediate' markers in studies of risk assessment and cancer prevention, without having to resort to tumor appearance as an end-point.

The quantitation of altered hepatic foci during multistage hepatocarcinogenesis in the rat: transforming growth factor alpha expression as a marker for the stage of progression

The experimental three-stage hepatocarcinogenesis protocol of initiation, promotion, and progression, coupled with the analytical technique of stereology, permits quantitative analysis of the carcinogenic process, including the derivation of biologically based risk assessment models. The aberrant expression of the placental isozyme of glutathione S-transferase (PGST) is an efficient marker for initiated, preneoplastic, and neoplastic hepatocytes. Putatively initiated cells and their clonal progeny can be identified, enumerated, and their growth characteristics determined on the basis of their aberrant expression of this protein. A lack of suitable markers has made the identification and quantitation of hepatocytes in the early stage of progression more difficult. One characteristic of cells in the stage of progression is the evolution of relatively autonomous growth. The alteration of growth factor signalling pathways may provide one mechanism for this observation. The expression of transforming growth factor alpha (TGF alpha) is seen in many malignancies. The initiation-promotion-progression protocol has been used to induce progression in the rat liver. The focal expression of TGF alpha was found to correlate with areas of progression in rats subjected to this protocol. The ability to identify and quantitate cells in the stage of progression should facilitate application of the Moolgavkar-Venzon-Knudson model for assessing human risk from carcinogens active at each of these three stages. Validation of this model will require determination of the number and growth characteristics of hepatocytes in the stage of progression.

Soybean isoflavone extract suppresses early but not later promotion of hepatocarcinogenesis by phenobarbital in female rat liver

The antioxidant and anticarcinogenic activities of soybean isoflavone extracts were investigated in female F344/rats. Diethylnitrosamine (DEN, 15 mg/kg body wt) as a cancer initiator was injected intraperitoneally into 120 female F344/N rats at 10 days of age, and at weaning, phenobarbital (PB, 500 mg/kg diet) was fed to one-half of the rats. Soybean isoflavones were extracted in acetone-0.1 N HCl and analyzed by high-performance liquid chromatography, and two levels of soybean isoflavones (920 and 1,840 mumol/kg diet) were fed during PB treatment for 3 and 11 months. Control rats were fed diets without PB and with or without isoflavones. The effect of soybean isoflavone extract on hepatic glutathione peroxidase was measured, and development of gamma-glutamyltransferase (GGT)-positive (GGT+) and placental glutathione transferase (PGST)-positive (PGST+) altered hepatic foci (AHF) was analyzed by computerized stereology. Soybean isoflavone extract providing 920 or 1,840 mumol/kg diet normalized total heptic glutathione peroxidase activity, which was suppressed about 17% by PB (p < 0.05), and both doses of isoflavone extract suppressed PB promotion of hepatocarcinogenesis, decreasing the volume occupied by GGT+ and PGST+ AHF (p < 0.05) after three months. After 11 months of PB promotion, isoflavone extract at 920 mumol/kg diet decreased PGST+ AHF compared with the PB-fed group, but neither dose of isoflavone extract suppressed development of GGT+ AHF compared with the group fed PB alone. Furthermore the control group fed isoflavone extract at 1,840 mumol/kg diet showed greater development of GGT+ and PGST+ AHF than the group fed the basal diet alone. Therefore soybean isoflavones may be anticarcinogenic, but their margin of safety is relatively narrow, with a cancer-promoting dose of 1,840 mumol/kg in female F344/N rats initiated with DEN.

Multiple mechanisms are responsible for altered expression of gap junction genes during oncogenesis in rat liver

Although several abnormalities in gap junction (GJ) structure and/or function have been described in neoplasms, the molecular mechanisms responsible for many of the alterations remain unknown. The identification of a family of GJ proteins, termed connexins, prompted this study of connexin32 (Cx32), connexin26 (Cx26) and connexin43 (Cx43) expression during rat hepatocarcinogenesis. Using antibody, cDNA and cRNA probes, we investigated connexin mRNA and protein expression in preneoplastic and neoplastic rat livers. In normal liver, Cx32 is expressed in hepatocytes throughout the hepatic acinus, Cx26 is restricted to periportal hepatocytes, and Cx43 is expressed by mesothelial cells forming Glisson's capsule. Most preneoplastic altered hepatic foci generated by diethylnitrosamine (DEN) initiation and either phenobarbital (PB) or 2,3,7,8-dichlorodibenzo-p-dioxin (TCDD) promotion exhibited decreased Cx32 or increased Cx26 staining. Foci from either protocol failed to display Cx43 immunoreactivity. In the majority of PB-promoted foci, Cx32 immunoreactivity decreased independently of changes in mRNA abundance. Continuous thymidine labeling, following cessation of PB promotion, showed that downregulation of Cx32 staining is reversible in foci that are promoter-dependent for growth, but irreversible in lesions that are promoter-independent for growth. Hepatic neoplasms from rats initiated with DEN and promoted with PB or TCDD also displayed modified connexin expression. While all 24 neoplasms studied were deficient in normal punctate Cx32 and Cx26 staining, altered cellular localization of these proteins was apparent in some tumors. Immunoblotting of crude tissue extracts revealed that neoplasms with disordered Cx32 staining showed immunoreactive bands with altered electrophoretic mobility. These observations show that hepatomas may downregulate Cx32 expression through changes in the primary structure of Cx32 or by post-translational modifications. Northern blotting of total tumor mRNAs failed to demonstrate consistent changes in the abundance of Cx32, Cx26 or Cx43 transcripts. Some tumors expressed steady-state transcripts without observable immunoreactivity, indicating that some hepatomas downregulate connexin immunoreactivity independently of mRNA abundance. Increased levels of Cx43 mRNA and protein were found in several neoplasms, but immunostaining was always localized to nonparenchymal cells. Areas of bile duct proliferation and cholangiomas displayed Cx43 staining, whereas, cholangiocarcinomas were deficient in immunoreactivity. These findings show that alterations in the expression of connexins, by either downregulation or differential induction, represent common modifications during hepatocarcinogenesis. Although our results imply that connexins represent useful markers for the boundary between tumor promotion and progression, preneoplastic and neoplastic rat hepatocytes fail to use a common mechanism to modify connexin expression.

Studies of tamoxifen as a promoter of hepatocarcinogenesis in female Fischer F344 rats

Tamoxifen, an antiestrogen used in the treatment of breast cancer, was assessed for carcinogenic potential in the two-stage model of experimental hepatocarcinogenesis. Groups of female Fisher F344 rats were initiated with a non-necrogenic, subcarcinogenic dose of diethylnitrosamine (DEN; 10 mg/kg, po) and fed tamoxifen at a concentration of 250 mg per kg of AIN-76A diet for 6 or 15 months. The livers of these animals exhibited an increase in size and number of altered hepatic foci compared with those animals which were initiated with DEN but not exposed to tamoxifen. This finding indicates that tamoxifen may have a carcinogenic potential in the rat liver. After 6 months of treatment, neoplastic nodules were observed in 3/8 rats in the DEN-initiated, tamoxifen-treated group. In the initiated group provided with tamoxifen for 15 months, neoplastic nodules were observed in 7/8 rats and hepatocellular carcinomas in 3/8 rats. The serum level of tamoxifen in these rats was 200-300 ng/ml. The ratio of tamoxifen, 4-hydroxy tamoxifen, and N-desmethyl tamoxifen was 1:0.1:0.5-1 in the serum. When adjusted for age-related weight increases, the serum and liver levels of tamoxifen and its N-desmethyl metabolite did not change over the 15 months. In the rat liver, the level of tamoxifen and its N-desmethyl metabolite was 10-29 micrograms/g liver after 6 or 15 months of chronic dietary administration. The ratio of tamoxifen:4-hydroxy tamoxifen:N-desmethyl tamoxifen was 1:0.1.3-3.3 in the liver. Therefore, the liver had 20- to 30-fold more tamoxifen and 4-hydroxy tamoxifen and at least 100-fold more N-desmethyl tamoxifen than the serum (assuming 1 gram of tissue is equivalent to 1 ml of serum). These results indicate that tamoxifen is a promoting agent for the rat liver at serum levels found in patients given the usual therapeutic course of tamoxifen. The high concentrations of tamoxifen attained in the rat liver indicate that actions other than its known estrogenicity for liver could contribute to its promoting action. In addition, these results indicate that the pharmacodynamic differences in tamoxifen metabolism in rats and humans and at low versus high doses should be determined. Thus, the therapeutic indications for tamoxifen should be balanced by the potential risk it may present as a promoting agent in mammalian liver.

Number of simultaneously expressed enzyme alterations correlates with progression of N-ethyl-N-hydroxyethylnitrosamine-induced hepatocarcinogenesis in rats

Preneoplastic and neoplastic liver cell lesions, induced by EHEN (N-ethyl-N-hydroxyethylnitrosamine) in rats, were investigated to establish the numbers of simultaneously expressed altered enzyme phenotypes within the lesion cells. The lesions were divided into 5 classes on the basis of altered expression in one or more of the following 5 enzymes: glutathione S-transferase placental form, glucose-6-phosphate dehydrogenase, glucose-6-phosphatase, adenosine triphosphatase, and gamma-glutamyl transpeptidase. Class 1 lesions contained cells expressing one altered enzyme. Similarly, class 2, 3, 4 and 5 lesions had cells simultaneously expressing 2, 3, 4, and 5 enzyme alterations, respectively. Four histopathological categories of lesions, ACF (altered cell foci) (274 lesions), HN (hyperplastic nodules) (47 lesions), HCC (hepatocellular carcinomas) (99 lesions) and THC (transplanted hepatocellular carcinomas) (5 lesions) were studied. Proliferation potential was assessed in terms of 5-bromo-2'-deoxyuridine (BrdU) incorporation. The distribution profiles of classes 1 to 5 showed a clear reciprocal change from low class (1 to 2 enzymes) predominance in ACF to high class (4 to 5 enzymes) predominance in HN. Increase of BrdU labeling indices was clearly correlated with progression from HN to HCC. Only a small population of class 5 ACF showed a high BrdU labeling index, indicating particular potential for further development. Thus, the stages of EHEN-induced neoplasia were found to be characterized by gradual increase in the number of altered enzyme phenotypes, with acquisition of proliferative potential being associated with further progression towards malignant conversion.

Three-dimensional analysis of glutathione S-transferase placental form-positive lesion development in early stages of rat hepatocarcinogenesis

The development of glutathione S-transferase placental form (GST-P)-positive lesions in the rat liver, from single cells to foci, and their locations within liver lobules were examined by a combined stereological approach for calculation of three-dimensional (3-D) data and by 3-D computer graphics for reconstruction of lesions. Two weeks after initiation with diethylnitrosamine, the rats were divided into two groups. Animals in group 1 were given 2-acetylaminofluorene, and animals in group 2 were given basal diet for 6 weeks. Partial hepatectomy (PH) was performed at week 3. The GST-P-positive single cells increased in number per liver after PH in group 1, but not in group 2, where a plateau level was maintained. The number of GST-P-positive foci per liver in group 2 also reached an almost constant low value after 4 weeks. In contrast, foci in group 1 increased greatly after PH. A 3-D reconstruction, performed with a computer graphics system using up to 180 sections at 10 microns intervals, revealed the single cells to be distributed at random. Those that grew into foci were also not preferentially localized in any particular zone of the hepatic lobule. When foci within the same lobule came into contact, they underwent fusion. The present results thus indicate that only a small proportion of GST-P-positive single cells develops into foci, and that their growth is independent of zonal factors within individual lobules in early stages of rat hepatocarcinogenesis.

Spontaneous altered hepatocellular foci (AHF) in captive aged male cotton rats (Sigmodon hispidus)

Spontaneous altered hepatocellular foci (AHF) of basophilic type were observed in adult male cotton rats. The histological features of these foci were similar to those observed in Fischer 344 rats. However, an immunohistochemical technique with antibody to glutathione S-transferase placental form (GST-P) failed to stain these foci. Also normal bile ducts were not immunoreactive for GST-P. The presence of a gall bladder and of non-GST-P immunoreactive liver foci and bile ducts suggests that these cotton rats are phylogenetically closer to mice than rats.

Relative tumour promoting activity of three polychlorinated biphenyls in rat liver

The relative tumour promoting activity of three structurally and toxicologically diverse polychlorinated biphenyls (3,4,5,3',4'-penta- 2,3,4,3',4'-penta- and 2,4,5,2',4',5'-hexachlorobiphenyl) was measured in an initiation/promotion assay in nitrosodiethylamine-initiated female Sprague-Dawley rats. The congeners under study were administered by once-weekly subcutaneous injections for 20 weeks. Evaluation of the development of gamma-glutamyl transpeptidase (GGT)- and glutation transferase P (GST-P)-positive hepatic foci showed that all congeners promoted altered hepatic foci, although 3,4,5,3',4'-pentachlorobiphenyl was far more potent. The volume fraction of the liver occupied by GGT-positive tissue in the 3,4,5,3',4'-pentachlorobiphenyl-treated animals (100 micrograms/kg per week) was 23%, while the volume fractions of altered liver tissue in the rats treated with 2,3,4,3',4'-pentachlorobiphenyl (5000 micrograms/kg per week) and 2,4,5,2',4',5'-hexaCB (20,000 micrograms/kg per week) were 1.2 and 2.3, respectively. The enhancement of GGT- and GST-P-positive foci was accompanied by an increased incidence of histological changes in the livers.

Quantitative comparison of initiation and mutation phenotypes in hepatocytes of the analbuminemic rat

The potential relationship between mutagenesis and carcinogenesis has been examined in the Nagase analbuminemic rat treated with a single dose of benzo[a]pyrene, an incomplete liver carcinogen. The apparent mutation rate at the albumin locus was calculated by determining the number of hepatocytes expressing a cross-reactive product of albumin in analbuminemic rats treated with benzo[a]pyrene. The rate of initiation, the first stage in carcinogenesis, was determined by assessing the number of hepatocytes expressing the placental isozyme of glutathione S-transferase (PGST) after administration of benzo[a]pyrene. Since the expression of PGST may represent hepatocellular changes independent of initiation, promotion with phenobarbital was employed to clonally expand those putatively initiated hepatocytes expressing PGST. With immunohistochemical measures to assess changes in albumin expression, a threefold increase in the number of hepatocytes expressing albumin was detected after administration of benzo[a]pyrene in Nagase analbuminemic rats. A more than five-fold increase in altered hepatic foci (AHF) exhibiting increased PGST expression was observed in animals given benzo[a]pyrene treatment followed by phenobarbital, compared with those given benzo[a]pyrene alone. The number of albumin-expressing single hepatocytes detected was of the same order of magnitude as the number of individual hepatocytes and AHF expressing PGST, suggesting that similar events may be involved in their formation. Since 3 x 10(6) single hepatocytes expressing albumin were found in the analbuminemic rat liver after a single administration of benzo[a]pyrene, while less than 2 x 10(4) AHF expressing PGST were observed, formation of individual hepatocytes expressing albumin was a far more frequent event than clonal expansion of initiated hepatocytes in the Nagase analbuminemic rat. However, the number of loci of PGST expression including AHF and single hepatocytes is comparable to that of single hepatocytes expressing albumin.

Effects of modifying agents on conformity of enzyme phenotype and proliferative potential in focal preneoplastic and neoplastic liver cell lesions in rats

Development of preneoplastic lesions in the rat liver under the influence of various modifiers was investigated with particular attention to changes in simultaneous expression of altered enzyme phenotype within the lesions (conformity) and proliferation potential. Degree of conformity of marker enzymes such as glutathione S-transferase placental form (GST-P), glucose-6-phosphate dehydrogenase (G6PD), glucose-6-phosphatase, adenosine triphosphatase and gamma-glutamyltranspeptidase was compared with levels of 5-bromo-2-deoxyuridine labeling. After initiation with diethylnitrosamine, rats were administered the hepatopromoter sodium phenobarbital (PB, 0.05%), the antioxidant ethoxyquin (EQ, 0.5%), or a peroxisome proliferator, clofibrate (CF, 1.0%) or di(2-ethylhexyl)-phthalate (0.3%) and killed at week 16 or 32. The PB promoting regimen was clearly associated with increase in the numbers of high conformity class lesions simultaneously expressing three to five enzymes, and elevated proliferation potential. The inhibitor, EQ, in contrast, brought about a time-dependent decrease in conformity so that only 1 or 2 alterations were most commonly observed at week 32. Lesion populations in the peroxisome proliferator- and especially CF-treated cases were characterized by obvious dissociation between degree of conformity and proliferative status. Such treatment-dependent differences were not always correlated with the size of the lesion. The results thus suggested that the conformity and proliferation potential of preneoplastic lesions are dependent on modification treatment. Overall, GST-P was found to be the most reliable marker, although G6PD was less influenced in the peroxisome proliferator cases.

Attenuation of preneoplastic lesion development by dietary protein intervention: apparent persistence and regression

The effects of feeding high protein diets that promote the development of aflatoxin B1 (AFB1)-induced gamma-glutamyl transpeptidase positive (GGT+) preneoplastic lesions were examined in Fischer 344 (F344) rats. After administering AFB1 for 2 weeks (initiation), animals were fed diets over four successive 3-week periods (promotion). Either a low (5% casein) or high (20% casein) protein diet was fed for 3, 6, or 9 weeks before switching to the opposite diet to determine whether progressively longer periods of feeding the initial diet caused preneoplastic foci to become more refractory to the intervention effects of the second diet. The results from animals consuming the 20% casein diet for progressively longer periods suggest that longer exposure to the high protein diet progressively enhances the potential for future lesion growth. Results from animals consuming the 5% casein diet for progressively longer periods suggest that longer exposure to the inhibitory low protein diet progressively inhibits the potential for future lesion growth. These results suggest that a high protein diet is a potent promoter of preneoplastic growth and that progressively longer exposure to a particular promotive environment increasingly attenuates foci response to future dietary intervention.

The toxicity of purified cellulose in studies with laboratory animals

The available literature has been reviewed for studies in laboratory animals using purified cellulose, as the production of purified cellulose may result in trace organochlorine contamination. The chronic ingestion of purified cellulose over the entire lifespans in rats and mice does not result in any increase in spontaneous disease or neoplasia. Further, purified cellulose does not display promotional activity in the mammary gland, the colon, or the bladder of rats and does not significantly alter the absorption or the metabolism of dietary components. No adverse effects were found on reproduction or neonate development in rats and mice. Therefore, no adverse health effects in humans are expected from exposure to purified cellulose.

Glutathione transferases and cancer

The glutathione transferases, a family of multifunctional proteins, catalyze the glutathione conjugation reaction with electrophilic compounds biotransformed from xenobiotics, including carcinogens. In preneoplastic cells as well as neoplastic cells, specific molecular forms of glutathione transferase are known to be expressed and have been known to participate in the mechanisms of their resistance to drugs. In this article, following a brief description of recently identified molecular forms, we review new findings regarding the respective molecular forms involved in carcinogenesis and anticancer drug resistance, with particular emphasis on Pi class forms in preneoplastic tissues. The rat Pi class form, GST-P (GST 7-7), is strongly expressed not only in hepatic foci and hepatomas, but also in initiated cells that occur at the very early stages of chemical hepatocarcinogenesis, and is regarded as one of the most reliable markers for preneoplastic lesions in the rat liver. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-responsive element-like sequences have been identified in upstream regions of the GST-P gene, and oncogene products c-jun and c-fos are suggested to activate the gene. The Pi-class forms possess unique enzymatic properties, including broad substrate specificity, glutathione peroxidase activity toward lipid hydroperoxides, low sensitivity to organic anion inhibitors, and high sensitivity to active oxygen species. The possible functions of Pi class glutathione transferases in neoplastic tissues and drug-resistant cells are discussed.

Effects of alpha-tocopherol, phenobarbital, and butylated hydroxyanisole during promotion of diethylnitrosamine-initiated rat hepatocarcinogenesis

The promotion-suppressing ability of two antioxidants was measured to determine the role of oxidative stress in hepatocarcinogenesis. Four-day-old female F344/N rats were dosed with diethylnitrosamine (10 mg/kg). After weaning, they were fed semipurified diets with and without 500 ppm alpha-tocopherol, or the same two diets containing 500 ppm phenobarbital, or 5,000 ppm butylated hydroxyanisole (BHA) for 3 or 11 months. By 11 months, phenobarbital-fed groups had eaten 30% more than other groups did (p less than 0.05), suggesting a role for increased caloric intake in phenobarbital promotion. Phenobarbital and BHA significantly reduced body weights and increased liver weights compared with control rats. After three months, alpha-tocopherol significantly suppressed mean volume of placental glutathione S-transferase (PGST)-positive altered hepatic foci (AHF), regardless of xenobiotic treatment. Phenobarbital increased and BHA decreased the numbers of AHF compared with those of the control group. After 11 months, mean focal volume was significantly suppressed by BHA compared with that of the control group, and phenobarbital increased the total volume of AHF [PGST-positive plus gamma-glutamyltransferase (GGT)-positive AHF] compared with rats fed either control or BHA diets. BHA treatment also increased hepatic glutathione levels by 40% compared with control and rats fed phenobarbital. In conclusion, alpha-tocopherol had only a slight, early effect to suppress promotion of hepatocarcinogenesis. BHA suppressed some indices of promotion at both times and increased hepatic glutathione; however, BHA's toxicity (which suppressed body weight) may also be a factor in its supposable promotion-inhibitory effects.

Phenobarbital increases rat hepatic prostaglandin F2 alpha, glutathione S-transferase activity and oxidative stress

Eight-week-old female F344/N rats were fed 3.0 or 6.0% of calories (kcal%) as linoleate with or without 0.05% phenobarbital (PB) for 35 days. PB treatment increased glutathione S-transferase (GST) activity by 80% and prostaglandin (PG) F2 alpha levels 4-fold (p less than 0.05). PB decreased hepatic alpha-tocopherol significantly. Hepatic linoleate was decreased by PB in rats fed 6 kcal% but not 3 kcal% linoleate. Increased dietary linoleate had no significant effect on hepatic PGF2 alpha or alpha-tocopherol levels or GST activity. This study suggests that PB hepatotoxicity and tumor-promoting ability may be mediated, at least in part, by PGF2 alpha. PB's effect on PGF2 alpha could be a result of both GST-mediated prostaglandin synthesis and oxidative stress. The removal of significant amounts of hepatic alpha-tocopherol during oxidative stress induced by PB might diminish endogenous inhibition of hepatic PG synthesis by a-tocopherol.

Tumor promotion as a target for estrogen/antiestrogen effects in rat hepatocarcinogenesis

Tamoxifen is a well-tolerated palliative and adjuvant treatment for human breast cancer and requires continuous, long-term administration for optimal therapeutic effectiveness. A two-stage model of experimental hepatocarcinogenesis, based upon the natural history of cancer development, has been employed to assess the carcinogenic potential of tamoxifen. In this study, the effectiveness of tamoxifen both as an initiator and a promoter in hepatocarcinogenesis was assessed in female F-344 rats. Tamoxifen was tested as an initiator at a single intragastric dose of 40 mg/kg, followed by promotion with 0.05% phenobarbital. The number and size of the resulting altered hepatic lesions were quantified, and tamoxifen was found to lack initiating action at the dose tested. Other groups of animals were initiated with a nonnecrogenic, subcarcinogenic dose of diethylnitrosamine (10 mg/kg) and were fed tamoxifen at either 250 or 500 mg/kg in the AIN-76A purified diet for 6 months. The livers of these animals showed an increase in the size and number of altered hepatic lesions compared with those animals that were initiated but not exposed to tamoxifen; this indicates that tamoxifen acts as a tumor promoter in the rat liver. The promotion index of tamoxifen, a measure of relative potency, was less than one-tenth that of ethinyl estradiol and more than four times that of phenobarbital, an agent commonly employed as a representative promoting agent in experimental carcinogenesis. Since tamoxifen lacked initiating activity in the rat liver at the dose tested, the mechanism of tumor induction in long-term feeding studies by tamoxifen may be due to its promotion of spontaneously initiated hepatocytes. The chronic therapeutic use of tamoxifen should therefore be limited by the potential carcinogenic risk of this agent as an effective tumor promoter.

Histochemical and stereological analysis of putative preneoplastic hepatic lesions

Histochemistry is a valuable tool in the analysis of altered hepatic foci. These lesions contain alterations in the activities of certain enzymes, including gamma-glutamyl transpeptidase (GGT), placental glutathione-S-transferase (PGST), glucose-6-phosphatase (G6Pase), and ATPase, or in certain cellular functions, such as the ability to store iron. The appearance of altered hepatic foci has been found to correlate with the later appearance of hepatocellular carcinomas in rodents. The markers PGST and GGT are the most sensitive at detecting altered hepatic foci induced by most chemicals, but are unable to detect altered hepatic foci induced by some agents, such as peroxisome proliferators. Other markers, such as ATPase or G6Pase, should therefore be used in combination with PGST or GGT in identifying altered hepatic foci. The strain of rat used and the type of diet fed also influence the number of altered hepatic foci induced and the enzyme markers seen. The number of foci per cm2 and the diameters of altered hepatic foci in histochemically-stained tissue sections can easily be quantified. The number of foci per cm2, however, does not give a reliable estimate of the number of altered hepatic foci induced because larger altered hepatic foci are more likely to be transected. The equations of quantitative stereology therefore should be used to transform the data to obtain the number of foci induced per cm3 or per liver, the average volume of individual foci, and the percent of the liver volume occupied by altered hepatic foci. In conclusion, the use of histochemistry to identify preneoplastic lesions and the use of quantitative stereology to estimate their number and volume allow accurate and sensitive quantitation of altered hepatic foci.