Liver tumor-promoting activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in TCDD-sensitive and TCDD-resistant rat strains

Risk assessment of dioxins is currently based on induction of liver tumors in rats. The toxicity of dioxins is characterized by large sensitivity differences among animal species and even strains of the same species, which complicates the risk assessment. The significance of these differences in dioxin-induced carcinogenicity is not known. We therefore studied the liver tumor-promoting activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the sensitive Long-Evans (L-E) and the resistant Han/Wistar (H/W) rats differing >1000-fold in their sensitivity to the acute lethaity of TCDD. Female rats were partially hepatectomized, initiated with nitrosodiethylamine, and treated with TCDD for 20 weeks. Altered hepatic foci (AHF) were stereologically quantitated using glutathione S-transferase P as a marker. AHF were significantly (P < 0.001) and dose dependently increased in L-E rats at 10 and 100 ng/kg/day, but in H/W rats only at 1000 ng/kg/day and above, indicating a remarkable (approximately 100-fold) sensitivity difference between L-E and H/W rats. The same sensitivity difference but 10-fold less foci were observed between nonhepatectomized/noninitiated L-E and H/W rats. Induction of AHF was related to hepatotoxicity but not to cytochrome P4501A1 activity in the liver. Liver TCDD concentrations were similar in both strains. H/W rats are exceptionally resistant to induction of AHF by TCDD, and the resistance is associated with an altered transactivation domain of the aryl hydrocarbon receptor. Genetic differences may account for significant interindividual/intraspecies sensitivity differences in dioxin-induced carcinogenesis. Understanding the role of transactivation domain of the aryl hydrocarbon receptor in carcinogenesis is therefore likely to improve dioxin risk assessment.

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.

Promotion of altered hepatic foci by 2,3',4,4',5-pentachlorobiphenyl in Sprague-Dawley female rats

The tumor promotion potential of 2,3',4,4',5-pentachlorobiphenyl (PCB-118) was studied in a two-stage initiation/promotion bioassay in female Sprague-Dawley rats. The animals were initiated by intraperitoneal administration of N-nitrosodiethylamine after partial hepatectomy. After 5 weeks of recovery, the promotion period commenced by once-weekly subcutaneous administrations of PCB-118 at six dose levels (10, 40, 160, 640, 2500, and 10,000 microg/kg body weight/week) for 20 weeks. In addition, three of these dose levels (40, 640, and 10,000 microg/kg body weight/week) were administered for 52 weeks. Evaluation of hepatic foci positive for glutathione S-transferase P demonstrated that the mono-ortho chlorine substituted congener PCB-118 significantly increased the number of foci/cm3 of liver in the two highest dose groups after 20 weeks, but did not significantly increase the percentage of the liver occupied by foci. After 52 weeks of treatment, both the percentage and the number of foci/cm3 were significantly increased in the highest dose group. A toxic equivalency factor based on foci development during 20 weeks of treatment would be less than 0.00002. Altered relative liver and thymus weights were observed after treatment with both substances as well as an induction of methyl cholanthrene- and phenobarbital-inducible isoenzymes of cytochrome P450 monooxygenase. These results show that PCB-118 has a potency to enhance foci growth in rat liver, although the potency is low compared to that of structurally related compounds.

Interaction of 3,4,5,3',4'-pentachlorobiphenyl and 2,4,5,2',4',5'-hexachlorobiphenyl in promotion of altered hepatic foci in rats

This study was undertaken to investigate tumour promoting interactions of 2,4,5,2',4',5'-hexachlorobiphenyl (PCB 153) and 3,4,5,3',4'-pentachlorobiphenyl (PCB 126) in female Sprague-Dawley rats. Five weeks before the promotion treatment, the rats were partially hepatectomized and initiated with nitrosodiethylamine. The test substances were administered by weekly, subcutaneous injections for 20 weeks. The results from this study suggest that treatment with a combination of these two congeners causes a more than additive effect on the formation of gamma-glutamyltranspeptidase-positive hepatic foci. Co-exposure to PCB 126 and PCB 153 caused a dose-dependent reduction of the PCB 153-induced CYP2B1/B2-activity in these livers.

Liver tumour promoting activity of 3,4,5,3',4'-pentachlorobiphenyl and its interaction with 2,3,7,8-tetrachlorodibenzo-p-dioxin

This study was undertaken to compare the tumour promoting effects induced by 3,4,5,3',4'-pentachlorobiphenyl (PCB 126) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In addition, interactive effects in rats treated with combinations of PCB 126 and TCDD were studied. Partially hepatectomized female Sprague-Dawley rats were initiated with nitrosodiethylamin. After 5 weeks of recovery the promotion treatment started and continued for 20 weeks. The results from the present study demonstrate that PCB 126 elicit approximately 10% of TCDD's tumour promoting activity measured as enhancement of the development of gamma-glutamyl-transpeptidase-positive altered hepatic foci in the liver. The factor required for the PCB to match the response of TCDD was adopted as a toxic equivalency factor and was in this case 0.1, which is the same as the factor suggested by Ahlborg et al. (1994). In the groups treated with a mixture of PCB 126 and TCDD the tumour promoting effect indicated an additive response. This result suggests that PCB 126 and TCDD act by the same mechanistical pathway, which in turn, supports that the toxic equivalency factor-concept can be used for TCDD-like tumour promoters.

Enhancement of altered hepatic foci in rat liver and inhibition of intercellular communication in vitro by the pyrethroid insecticides fenvalerate, flucythrinate and cypermethrin

Male Sprague-Dawley rats dosed with N-nitrosodiethylamine (NDEA) 24 h after two-thirds partial hepatectomy were treated with the pyrethroid insecticides fenvalerate, flucythrinate or cypermethrin in the diet for 20 weeks. Altered hepatic foci were analyzed by quantitative stereology from paraffin-embedded sections stained for gamma-glutamyltranspeptidase (GGT) or glutathione S-transferase P (GST-P). The present results demonstrate that the pyrethroids tested all enhance the development of NDEA-initiated, GGT-positive foci in rat liver at non-hepatotoxic doses. On the contrary, the volume fractions of GST-P-positive foci were not elevated as compared to the control group. The three pyrethroids tested all inhibited the transfer of Lucifer Yellow CH between WB-F344 rat liver epithelial cells in culture, supporting the increase of GGT-positive foci and suggesting that these substances can act as tumour promoters. The discrepancy between the results from analyses using GGT or GST-P as markers emphasizes the importance of understanding the mechanism underlying the expression of different markers for preneoplastic lesions and the importance of such effects in tumour promotion.

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.

The insecticide endosulfan and its two stereoisomers promote the growth of altered hepatic foci in rats

The non-genotoxic, chlorinated cyclodiene insecticide endosulfan was studied for its ability to act as a tumour promoter in a two-stage, altered hepatic foci bioassay in male Sprague-Dawley rats. Two stereoisomers of endosulfan, alpha-endosulfan (ENDO alpha) and beta-endosulfan (ENDO beta) were used, as well as a commercially-occurring mixture of the alpha- and beta-isomers (ENDO alpha beta). The animals were initiated by intraperitoneal injection of nitrosodiethylamine 24 h after a two-thirds-partial hepatectomy. Five weeks later the animals were transferred to diets containing 30, 100 and 300 p.p.m. of either ENDO alpha beta, ENDO alpha or ENDO beta. The study was terminated 25 weeks after initiation and the development of foci of gamma-glutamyltranspeptidase-positive hepatocytes was evaluated by stereological methods. The results show that endosulfan and its two stereoisomers promote the development of altered hepatic foci, suggesting that endosulfan is a tumour-promoting agent acting by clonal expansion of initiated cells.

Relative liver tumour promoting activity and toxicity of some polychlorinated dibenzo-p-dioxin- and dibenzofuran-congeners in female Sprague-Dawley rats

The polychlorinated dibenzo-p-dioxins/dibenzofurans 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) were studied for liver tumour promoting activity in a medium-term altered foci assay in nitrosamine-initiated female Sprague-Dawley rats. The congeners under study were administered by weekly subcutaneous injections at three dose levels for 20 weeks. Evaluation of gamma-glutamyltranspeptidase (GGT+), altered hepatic foci development, showed that all congeners studied acted as potent promoters of hepatocarcinogenesis. TCDD and PeCDD were virtually equipotent as enhancers of foci development while PeCDF displayed approximately ten per cent of the activity of the dioxins. Analysis of the dioxin- and furan-congeners by gas chromatography/mass spectroscopy (GC/MS) technique showed that the retention of PeCDD and PeCDF in liver tissue was approximately 7 and 20 times, respectively, as high as the retention of TCDD. Based on the concentration of the respective congener in liver tissue, PeCDD and PeCDF were 0.14 and 0.007 times as active as TCDD as promoters of foci development. The dose related enhancement of GGT+ foci development induced by the PCDD/PCDF congeners was accompanied by an increased incidence of histological changes in the liver.

Modulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin and phenobarbital-induced promotion of hepatocarcinogenesis in rats by the type of diet and vitamin A deficiency

This study was undertaken to investigate the influence of dietary vitamin A deficiency and type of diet on tetrachlorodibenzo-p-dioxin (TCDD)- and phenobarbital-induced liver tumor promotion in rats. Female Sprague-Dawley rats were partially hepatectomized and subsequently initiated with nitrosodiethylamine. One week later the rats were allocated to five different dietary regimens for the duration of the study: three purified (casein-based) diets containing 200, 1200, and 10000 IU vitamin A per kilogram, respectively, and two conventional (cereal-based) rat diets containing 2000 and 14000 IU vitamin A per kilogram, respectively. After an additional 4 weeks, groups of rats on each dietary regimen were started on one of four different promoter treatments: 0.07 micrograms TCDD/kg/week (sc); 0.7 micrograms TCDD/kg/week (sc); 500 ppm phenobarbital in the drinking water or vehicle only (arachis oil, sc). The study was terminated after 16 weeks of promoter treatment. Sections of liver were stained for gamma-glutamyltranspeptidase (GGT) activity and GGT-positive altered hepatic foci (AHF) were evaluated by stereological methods. All factors studied (TCDD, phenobarbital, dietary vitamin A content, and the type of diet) were shown to influence AHF development significantly. As expected, TCDD and phenobarbital enhanced foci development. Vitamin A deficiency enhanced foci development in its own right and increased the TCDD-induced response markedly. Dietary vitamin A content did not modulate phenobarbital promotion of AHF in the same manner. The enhancement of TCDD-induced effects on foci development by vitamin A deprivation was accompanied by an increased incidence of histological changes marking degeneration in the liver (e.g., oval cell hyperplasia) and accentuation of other TCDD-related toxic responses. In addition, the groups of rats maintained on the cereal-based diets and subjected to the various promoter/vitamin A regimens exhibited significantly higher AHF incidence as compared to correspondingly treated rats fed the purified, casein-based diets. In conclusion, vitamin A deficiency alone may promote hepatocarcinogenesis and enhance the promoting effect of TCDD treatment. However, TCDD-induced depletion of hepatic vitamin A stores was not implicated as a major cause of promotion by TCDD. Nevertheless, vitamin A deficiency brought about by TCDD alone may well act as a promotive stimulus concertedly with an as yet unidentified cellular mechanism in TCDD-induced liver tumor promotion. The differential effects of the two types of diets recorded in the study remain undocumented.

Promotion of altered hepatic foci development in rat liver, cytochrome P450 enzyme induction and inhibition of cell-cell communication by DDT and some structurally related organohalogen pesticides

The organochlorine pesticide 1,1'-(2,2,2-trichloroethylidene) bis(4-chlorobenzene) (DDT) and four structural analogues (bromopropylate, chlorobenzilate, dicofol and fenarimol) were investigated for their ability to inhibit gap junctional intercellular communication both in the Chinese hamster V79 metabolic co-operation assay and in the scrape-loading/dye-transfer assay in WB-F344 rat liver epithelial cells. The pesticides were also studied for their ability to enhance the development of gamma-glutamyltranspeptidase-positive altered hepatic foci and induce cytochrome P450 monooxygenase isoenzymes in nitrosamine-initiated male Sprague-Dawley rats. The in vitro studies showed all organohalogens except fenarimol to be potent inhibitors of cell-cell communication in both test systems used. Concomitant results were recorded in the in vivo study. Thus, all potent inhibitors of intercellular communication were found to enhance significantly foci development and fenarimol was again without any significant effect. All pesticides studied were shown to be potent inducers of the phenobarbital-inducible cytochrome P450b isoenzyme and to cause hepatomegaly. Thus, no strict correlation between cytochrome P450b induction/liver growth and tumour promotion-related effects in vivo and in vitro was apparent for these organohalogen pesticides in the present study.

Mechanistic studies on the DDT-induced inhibition of intercellular communication

Two structurally unrelated compounds, 1,1'-(2,2,2-trichloroethylidene) bis(4-chlorobenzene) (DDT) and 12-O-tetradecanoylphorbol-13-acetate (TPA), are both potent inhibitors of cell-cell communication in vitro as well as tumour promoters in vivo. There is evidence that TPA acts via a specific receptor mechanism involving activation of protein kinase C (pkC). The mechanism of action of DDT has been discussed in terms of membrane perturbation, increased intracellular calcium, interaction with calmodulin and decreased cAMP levels. In the present study the objective was to examine the potential role of pkC activation in DDT-induced inhibition of intercellular communication in cultured cells. The V79 metabolic cooperation assay was used for measuring intercellular communication. Furthermore, the effects of DDT on the activity of partially purified pkC from V79 cells was measured, as was the interaction of DDT with the phorbol ester/DAG-binding site on the pkC enzyme. Results from the biochemical studies showed that DDT neither activates pkC nor binds to the phorbol ester/DAG-binding site, as measured by displacement of PDBU binding. Using the metabolic cooperation assay it was demonstrated that pretreatment with TPA made cells refractory, i.e. a second application of TPA did not inhibit cell-cell communication. DDT added to cells down-regulated with TPA inhibited cell-cell communication, even though these cells were refractive to TPA. This result further supports the hypothesis that DDT and TPA inhibit intercellular communication primarily by different pathways. At non-cytotoxic concentrations, pkC inhibitors (H7, W7 and palmitoyl carnitine) did not affect the TPA- or DDT-induced inhibition of cell-cell communication in the V79 metabolic cooperation assay. Quercetin, a pkC inhibitor which has been reported to eliminate DDT- or TPA-induced inhibition of intercellular communication, was investigated in an in vivo study that measured promotion of enzyme-altered foci in DEN-treated rat liver. Quercetin co-administered with DDT did not act as an antipromoter.

Effects of tetradecanoyl phorbol acetate, pyrethroids and DDT in the V79

The effects of the pyrethroids flucythrinate, cyfluthrin, bioallethrin and resmethrin on metabolic cooperation between V79 cells were investigated. Addition of flucythrinate to cocultures of 6-thioguanine-resistant and 6-thioguanine-sensitive V79 cells significantly increased the mutant cell recovery, indicating inhibition of intercellular communication. No such effect was observed by the other pyrethroids tested. To compare the modes of action of TPA-, DDT-, and pyrethroid-induced inhibition of intercellular communication, co-exposure experiments were undertaken. Addition of TPA, together with increasing doses of fenvalerate or flucythrinate, produced a synergistic response. Various combinations of fenvalerate-, flucythrinate- and DDT-exposure gave results in accordance with an additive response. The result suggest different pathways of action for TPA and the insecticides investigated in this study.

Chlorobenzilate-induced effects on enzyme-altered foci in rat liver and intercellular communication in rat liver WB-F344 epithelial cells

The mouse liver carcinogen chlorobenzilate (CB), a DDT-related pesticide, was investigated for enhancement of enzyme altered foci incidence in partially hepatectomized, diethyl-nitrosamine-initiated rats. In this in vivo experiment, CB administered per os (25 or 100 mg/kg per day for 10 weeks) enhanced foci incidence at the high dose level. In order to study potential mechanisms involved, CB was investigated for inhibition of gap-junctional intercellular communication in rat liver epithelial WB-F344 cells and Chinese hamster V79 cells in vitro. CB abolished dye transfer in WB-F344 cells and inhibited metabolic cooperation in V79 cells. Two CB metabolites were unable to induce such tumor promotion related effects. The results of this investigation provide support for the involvement of an epigenetic, tumor promoting mechanism in CB-induced liver tumors in laboratory animals.

Tumour promotion related effects by the cyclodiene insecticide endosulfan studied in vitro and in vivo

The cyclodiene insecticide endosulfan is structurally related to the tumour promoting pesticides chlordane and heptachlor. Divergent conclusions have been reported regarding the carcinogenic activity of endosulfan. In this study we have investigated if endosulfan and four of its metabolites possess tumour promotion related effects. Two in vitro test systems detecting inhibition of intercellular communication were used; the Chinese hamster lung fibroblast (V79) metabolic cooperation assay and a scrape loading/dye transfer assay using rat liver WB epithelial cells. At non-cytotoxic concentrations, technical grade endosulfan, analytical grade endosulfan (alpha- and beta-isomers and an alpha beta-isomer mixture) and endosulfan-sulfate inhibited gap junctional communication in both assay systems. In addition, the metabolite endosulfan-ether was effective in the rat liver WB epithelial cells. Endosulfan was also studied for enhancement of gamma-glutamyl transpeptidase positive enzyme altered foci incidence in partially hepatectomized, nitrosodiethylamine-initiated male Sprague-Dawley rats. However, endosulfan administered orally (1 or 5 mg/kg/day) five days a week for ten weeks did not enhance enzyme altered foci incidence. These apparently contradictory results with regard to possible tumour promoting activity of endosulfan are discussed in relation to metabolism, systemic toxicity and tissue/species specificity in tumour promotion.

Inhibition of metabolic cooperation in vitro and enhancement of enzyme altered foci incidence in rat liver by the pyrethroid insecticide fenvalerate

The synthetic pyrethroids cypermethrin, delta-methrin, fenvalerate, permethrin, and the fenvalerate metabolite p-chlorophenylisovaleric acid were investigated for inhibition of gap-junctional intercellular communication in vitro in the Chinese hamster lung fibroblast (V79) metabolic cooperation assay. Fenvalerate was furthermore studied for enhancement of gamma-glutamyl transpeptidase-positive enzyme altered foci incidence in partially hepatectomized, nitrosodiethylamine-initiated male Sprague Dawley rats. The in vitro studies showed that fenvalerate and p-chlorophenylisovaleric acid were inhibitors of intercellular communication at non-cytotoxic concentrations while cypermethrin, deltamethrin, and permethrin were inactive. In the in vivo study in rat liver, fenvalerate administered p.o. (75 mg/kg/day) 5 days a week for 10 weeks induced significantly more foci per cm3 and a larger percentage of liver tissue occupied by foci tissue compared to a vehicle control group. Analysis of size distributions of foci in fenvalerate- and vehicle-treated rats showed elevated foci incidences in fenvalerate-treated rats at all foci sizes. Fenvalerate induced no hepatotoxic effects as judged by plasma transaminase activities and histopathology. The results of this study suggest fenvalerate to be a potential tumour promoter.

Calmodulin involvement in TPA and DDT induced inhibition of intercellular communication

The organochlorine pesticide DDT is a liver tumour promoter and a potent inhibitor of intercellular communication. Present knowledge of the mechanism by which DDT inhibits intercellular communication is limited but it has been suggested that increased intracellular free calcium induced by DDT could be of importance. As the effects of calcium are closely associated with the multifunctional protein calmodulin (CaM) in most cells the potential binding of DDT to CaM and subsequent effects on CaM-stimulated Ca2+/Mg2+-ATPase activity were studied. DDT inhibited CaM-stimulated Ca2+/Mg2+-ATPase activity and bound to CaM in a manner similar to established CaM-inhibitors. Subsequently an in vitro assay for measuring inhibition of metabolic cooperation between 6-thioguanine (TG)-sensitive and TG-resistant Chinese hamster (V79) cells was used to investigate the possible involvement of CaM in the regulation of intercellular communication. Calmidazolium (CzM), a potent CaM inhibitor, was tested alone or in combination with the tumour promoters 12-O-tetradecanoyl phorbol-13-acetate (TPA) or DDT known inhibitors of intercellular communication. The results showed that CzM alone was without effect with regard to inhibition of metabolic cooperation but potentiated the response induced by TPA, an effect not noticed with DDT. These results suggest different mechanisms of action of TPA and DDT on metabolic cooperation and support the hypothesis that with calcium CaM may be of importance for drug-induced inhibition of intercellular communication and tumour promotion.

Interaction between quercetin, TPA and DDT in the V79 metabolic cooperation assay

An in vitro assay measuring inhibition of metabolic cooperation between 6-thioguanine sensitive and 6-thioguanine resistant Chinese hamster (V79) cells in co-culture was used to detect chemically induced inhibition of gap-junctional intercellular communication. Inhibition of this cellular process by xenobiotics has been suggested to be an important event in tumour promotion. This study was undertaken to determine the effect on metabolic cooperation by the bioflavonoid quercetin alone and in co-exposure experiments with two recognized tumour promoters, the phorbol ester TPA and the organochlorine pesticide DDT. Furthermore, co-exposure experiments with TPA and DDT were performed. Quercetin alone did not affect metabolic cooperation at noncytotoxic doses. Treatment of the cells with either TPA, DDT or TPA together with DDT caused significant inhibition of metabolic cooperation. This effect was dose-dependently decreased by addition of quercetin. These findings suggest that quercetin inhibits or compensates a common effect induced by both TPA and DDT. Treatment of the cells with a fixed dose of TPA and increasing doses of DDT, or a fixed dose of DDT and increasing doses of TPA, caused significantly higher recovery of mutant cells than a calculated additive response. The data indicate that TPA and DDT induce a synergistic response with respect to affecting intercellular communication. The results suggest that there are different pathways of action for TPA and DDT.