Carcinogenicity of TCDD in laboratory animals: implications for risk assessment

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

Liver tumor promotion by 2,3,7,8-tetrachlorodibenzo-p-dioxin is dependent on the aryl hydrocarbon receptor and TNF/IL-1 receptors

We set out to better understand the signal transduction pathways that mediate liver tumor promotion by 2,3,7,8-tetrachlorodibenzo-p-dioxn ("dioxin"). To this end, we first employed congenic mice homozygous for either the Ahr(b1) or Ahr(d) alleles (encoding an aryl hydrocarbon receptor (AHR) with high or low binding affinity for dioxin, respectively) and demonstrated that hepatocellular tumor promotion in response to dioxin segregated with the Ahr locus. Once we had genetic evidence for the importance of AHR signaling, we then asked if tumor promotion by dioxin was influenced by "interleukin-1 (IL-1)-like" inflammatory cytokines. The importance of this question arose from our earlier observation that aspects of the acute hepatocellular toxicity of dioxin are dependent upon IL1-like cytokine signaling. To address this issue, we employed a triple knock-out (TKO) mouse model with null alleles at the loci encoding the three relevant receptors for tumor necrosis factors α and β and IL-1α and IL-1β (i.e., null alleles at the Tnfrsf1a, Tnfrsf1b, and Il-1r1 loci). The observation that TKO mice were resistant to the tumor promoting effects of dioxin in liver suggests that inflammatory cytokines play an important step in dioxin mediated liver tumor promotion in the mouse. Collectively, these data support the idea that the mechanism of dioxin acute hepatotoxicity and its activity as a promoter in a mouse two stage liver cancer model may be similar, i.e., tumor promotion by dioxin, like acute hepatotoxicity, are mediated by the linked action of two receptor systems, the AHR and the receptors for the "IL-1-like" cytokines.

Protective Effects of Vitamin A and Vitamin E Succinate against 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-Induced Body Wasting, Hepatomegaly, Thymic Atrophy, Production of Reactive Oxygen Species and DNA Damage in C57BL/6J Mice

The protective effect of vitamin A and vitamin E succinate against 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced acute toxicity and measures of oxidative stress was studied. Ten mice were treated with either vitamin A (50 mg/kg every other day for eight days) or vitamin E succiante (150 mg/kg/day followed by a dose of 40 mg/kg/day for five additional days). Half of each of the above groups of animals received TCDD on day 4. Five mice received corn oil or TCDD alone. After five days of TCDD treatment, antioxidant combination treatment with vitamin A and TCDD or vitamin E succinate and TCDD resulted in a significant reduction in indicators of acute toxicity including the decrease in total body and thymus weight as compared to TCDD alone (P<0.05). The combination treatment produced also a significant reduction in the increase in liver weight as compared to TCDD only (P<0.05). Following one day of treatment with 50 microg TCDD/kg, vitamin A and vitamin E succinate produced a significant decrease in the production of superoxide anion by peritoneal lavage cells (P<0.05) and in DNA-single strand breaks in the same cells (P<0.05) as assessed by the reduction of cytochrome c and the alkaline elution technique, respectively. A significant decrease in DNA-single strand breaks in peritoneal lavage cells was observed following 5 days treatment with 50 microg TCDD/kg (P<0.05). The results indicate a potential role for oxidative stress in the acute toxicity of TCDD and a protective effect for vitamin A and vitamin E succinate in the overall toxicity of TCDD including measures of oxidative stress.

Effects of the porphyrinogenic compounds hexachlorobenzene and 3,5-diethoxycarbonyl-1,4-dihydrocollidine on polyamine metabolism

The naturally occurring polyamines--putrescine, spermidine and spermine--are organic cations present in all living cells and essential for cell growth and differentiation. The aim of the present study was to extend the investigations on the effects of porphyrinogenic compounds on polyamine metabolism. This was achieved by studying putrescine, spermidine and spermine levels in a model of acute porphyria, i.e. 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-induced porphyria, and in a model of non-acute porphyria, i.e. hexachlorobenzene (HCB)-induced porphyria. HCB administration to female Wistar rats for 7, 14, 21, 28 and 56 days did not alter polyamine levels in liver, even though rats presented clear signs of HCB-induced porphyria. In contrast to HCB, DDC treatment resulted in a remarkable increase in putrescine levels in the liver of female and male Sprague-Dawley rats. This increase was due, at least in part, to ornithine decarboxylase (ODC) activation. DDC induction of putrescine levels did not show organ specificity, since it could also be seen in adrenal gland. Interestingly, the deregulation of polyamine biosynthesis occurred concomitantly with the deregulation of the heme biosynthetic pathway. In addition to porphyria, it is known that DDC intoxication affects several proteins of the hepatocyte cytoskeleton. It is suggested that DDC-induced increase in ODC activity and putrescine levels may be an early event contributing to alter the cytoskeleton.

Selected lesions of dioxin in laboratory rodents

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) has been the subject of intensive investigations in laboratory animals during the last 2 decades. Toxicity studies have been conducted in several species of rodents and include several carcinogenicity studies as well as numerous mechanistic studies initiated to elucidate dioxin's mode of action, as both a carcinogen and a toxicant. Hepatotoxicity is a primary effect of dioxin. There has been an increase in hepatocellular tumors reported in both rats and mice exposed to dioxin. In addition to neoplastic changes, dioxin causes a spectrum of toxic changes in the liver. Additional neoplastic changes include subcutaneous fibrosarcomas and thyroid follicular cell tumors in both rats and mice and histiocytic sarcomas in mice. Dioxin causes developmental effects in the palate and kidney of mice. Changes in the female reproductive tract include ovarian atrophy, sertoliform hyperplasia, and Sertoli cell tumors. Dosing in utero results in gross malformations of the external genetalia. The effects of dioxin on the rodent model of endometriosis are described. In males, there are lowered sperm counts in the epididymis and minor testicular effects following gestational administration of dioxin. Both estrogenic and antiestrogenic-like effects have been ascribed to dioxin in laboratory animals; these activities are the result of dioxin-specific pathways resulting in the same end points as classic reproductive toxicants.

Mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) reduces circulating melatonin levels in the rat

We have previously shown that the prototype for halogenated aromatic hydrocarbons, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), diminishes serum melatonin concentration at the same dose in both the most TCDD-susceptible (Long-Evans, Turku AB; L-E) and the most TCDD-resistant (Han/Wistar, Kuopio; H/W) rat strain. The change developed within 24 h and persisted for at least 28 days after TCDD exposure; was independent of the time of day and was not associated with any morphological damage to the pineal gland. In the present study, we investigated the mechanism of this endocrine effect. Despite a 40-50% decrease in circulating melatonin levels, the pineal content of melatonin, serotonin and 5-hydroxyindole acetic acid remained unaltered and the rate-limiting enzyme of pineal melatonin biosynthesis, N-acetyltransferase, displayed only a relatively minor suppression in activity (30%) in TCDD-treated L-E rats. Likewise, TSDD did not influence the ability of pineal glands from L-E rats to synthesize and secrete melatonin in ex vivo or in vitro experiments. TCDD accelerated the disappearance of exogenous melatonin from the serum in both rat strains. This enhancement probably did not originate in the liver, because liver perfusion studies revealed that even control rat livers were capable of total melatonin clearance in spite of the fact that the melatonin concentration far exceeded physiological levels. Urine excretion of the normal main metabolite of melatonin, 6-hydroxymelatoninsulfate, was reduced by TCDD treatment in both strains. This was accompanied by an altered HPLC pattern of metabolites, especially in H/W rats. We conclude that TCDD decreases serum melatonin levels in rats by enhancing the peripheral, evidently extrahepatic, metabolism of the hormone.