Interactive effects of three structurally different polychlorinated biphenyls in a rat liver tumor promotion bioassay

Follicular Epithelial Cell Hypertrophy Induced by Chronic Oral Administration of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin in Female Harlan Sprague—Dawley Rats

2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD) affects the thyroid morphologically and/or functionally in adult animals. Recently, the National Toxicology Program conducted a 2-year gavage study of TCDD in female Harlan Sprague—Dawley rats. The only treatment-related alterations found in thyroid follicles were decreased luminal size and increased height of the follicular epithelial cells, without prominent protrusion into the lumen. The present study elucidated the nature of these follicular lesions. Thyroid glands of 10 rats each from the control, high (100 ng/kg/day)-dose, and stop-study (100 ng/kg/day, 30 weeks; vehicle to study termination) groups in the 2-year study were evaluated microscopically. Twenty randomly selected follicles were measured morphometrically in each animal. TCDD treatment significantly decreased the mean ratio of luminal/epithelial areas and increased the mean sectional epithelial height of the high-dose group compared to controls. Thyroid sections were immunostained with antibody against minichromosome maintenance (MCM) proteins, a novel cell-cycle biomarker. The MCM labeling index of the high-dose group was significantly higher than that of the control; however, the TUNEL labeling index was also higher in the high-dose group than the control. All data from the stop group were comparable to those from controls. These results indicate that the follicular cell response was hypertrophic and reversible. This information should contribute to diagnosis of nonneoplastic thyroid follicular lesions in rats.

Protective role of estrogen receptor-alpha on lower chlorinated PCB congener-induced DNA damage and repair in human tumoral breast cells

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants. Much of the research has focused on the carcinogenic potential of higher chlorinated PCBs, but accumulative evidence has shown that lower chlorinated PCB congeners have initiating and promoting activities. The goal of this study was to examine the potential of lower chlorinated PCBs, including 2,2',5,5'-tetrachlorobiphenyl (PCB52) and 3,3',4,4'-tetrachlorobiphenyl (PCB77), to induce DNA damage and apoptosis in human MDA-MB-231 (MDA) and MCF-7 breast cancer cells. Results confirmed that treatment of cells with PCB52 and PCB77 resulted in oxidative stress and caspase-dependent apoptosis in both MDA and MCF-7 cells. We noticed that at non-cytotoxic concentrations PCB52 and PCB77-induced decreases in intracellular NAD(P)H in MDA cells but not in MCF-7 cells. Further investigation confirmed that decreases in intracellular NAD(P)H in PCB-treated MDA cells are primarily due to reduction in intracellular NAD(+) pool mediated by poly(ADP-ribose)polymerase-1 activation through formation of DNA strand breaks. Antagonism was observed between PCB52 and PCB77 for the effect on induction of DNA strand breaks in MDA cells. Overall, this evidence demonstrates that at non-cytotoxic concentrations, lower chlorinated PCB congeners are capable of inducing oxidative DNA lesions in ERalpha(-)/MDA cells but not in ERalpha(+)/MCF-7 cells and that functional ERalpha plays a protective role in modulating the PCB-induced DNA damage in human breast cancer cells.

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.

Carcinogenicity of "non-dioxinlike" polychlorinated biphenyls

Complex technical mixtures of polychlorinated biphenyls (PCBs) cause liver and thyroid neoplasms in rodents, whereas very few data are available on the carcinogenic potency of single non-dioxinlike (NDL) PCB congeners. In most genotoxicity assays technical PCB mixtures and individual congeners were inactive, suggesting that PCBs act as indirect, nongenotoxic carcinogens. Various mechanisms, including suppression of apoptosis in preneoplastic cells or inhibition of intercellular communication, have been suggested to be active in liver tumor promotion by PCBs. A decrease in thyroid hormone levels after PCB treatment has been suggested to play a role in the development of thyroid neoplasms in rats; however, other mechanisms may also be involved. Results from a chronic carcinogenicity study in rats indicate that not the dose of total PCBs but the total TCDD or toxic equivalents (TEQs) associated with "dioxinlike" (DL) constituents within a technical mixture are mainly if not exclusively responsible for the development of liver neoplasms in female rats. Quantitative comparison reveals almost identical dose-response curves for the total TEQs in various technical PCB mixtures and for TCDD as inducers of hepatic neoplasms in female rats. Tumor promotion experiments have shown, however, that, after initiation with a genotoxic carcinogen, technical PCB mixtures and individual DL-and NDL-PCBs act as liver tumor promoters in rodents. Based on these data, a weak carcinogenic potency of individual NDL-PCB congeners cannot be excluded. In epidemiological studies, increased mortality from cancers of the liver, gallbladder, biliary tract, gastrointestinal tract, and from brain cancer and malignant melanoma were observed in workers exposed to a series of technical PCB mixtures. A significant association between PCB concentrations in adipose tissue and non-Hodgkins lymphoma was found in another study. While in all human studies mixed exposure to DL-and NDL-PCBs occurred, no comprehensive data are available on the relative contribution of NDL-PCBs to the overall external and/or internal PCB exposure in those cohorts.

Understanding population and individual risk assessment: the case of polychlorinated biphenyls

Decisions about how to improve or protect the public health can be, and sometimes necessarily are, made on imprecise science. The regulation of potential human carcinogens in the environment entails a population-risk assessment process intended to reduce risks to less than one additional cancer in 100,000 or 1,000,000 persons. These risk assessment processes, however, may be miscommunicated or misinterpreted in the context of individual cancer risks by scientists, regulators, the lay media, and the public. This commentary will review methods for establishing a causal relationship between carcinogen exposures and cancer risk. It will use the case of polychlorinated biphenyls (PCB) as an example of how to place scientific data into the context of human exposure and cancer risk. PCBs are widespread environmental contaminants and most people have detectable levels of PCBs in their bodies. The primary source for exposure in the general population is through the diet. PCBs are carcinogens in experimental animal models, but how this information can be extrapolated to human risk remains uncertain. PCB experimental studies provide data that are used to regulate and control human exposure, although the epidemiologic evidence fails to establish PCBs as human carcinogens. Thus, what is used for population-risk assessment may not be appropriate for individual-risk assessment or concluding that a causal relationship exists between PCB exposure and cancer risk. The hazards from a carcinogen designated by regulatory and review agencies as a "probable" human carcinogen is often misunderstood out of context about the magnitude of the risk and in what settings. How scientists communicate their results in scientific articles can strongly influence how others interpret their data. Misunderstandings from both the use of regulatory and review-agency opinions and the conclusions espoused by scientists occur in the media, among private physicians counseling their patients about cancer risk, and in the legal settings where plaintiffs seek compensation for exposure and alleged harm (or future harm). This can lead to false conclusions about what caused a cancer in a specific patient, undue anxiety about future cancer risk, inappropriate cancer screening, and attendant increased morbidity due to increased uses of the medical system and complication rates from medical procedures. The communication of research findings by scientists must be presented with caution, resisting the temptation to extrapolate, inappropriately, research data to the general population.

Advances in analytical techniques for polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans and dioxin-like PCBs

Analytical techniques for the determination of polychorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF) and dioxin-like PCBs (DLPCB) are reviewed. The focus of the review is on recent advances in methodology and analytical procedures. The paper also reviews toxicology, the development of toxic equivalent factors (TEF) and the determination of toxic equivalent quantity (TEQ) values. Sources, occurrence and temporal trends of PCDD/PCDF are summarized to provide examples of levels and concentration ranges for the methods and techniques reviewed.

Induction of ROS formation, poly(ADP-ribose) polymerase-1 activation, and cell death by PCB126 and PCB153 in human T47D and MDA-MB-231 breast cancer cells

The primary purpose of this research is to investigate whether exposure to polychlorinated biphenyls (PCBs), i.e. PCB153 and PCB126, is associated with induction of reactive oxygen species (ROS), poly(ADP-ribose) polymerase-1 (PARP-1) activation, and cell death in human T47D and MDA-MB-231 breast cancer cells. Results indicated that PCB153 and PCB126 induced concentration- and time-dependent increases in cytotoxic response and ROS formation in both T47D and MDA-MB-231 cells. At non-cytotoxic concentrations both PCB153 and PCB126 induced decreases in intracellular NAD(P)H and NAD+ in T47D and MDA-MB-231 cells where T47D cells were more resistant to PCB-induced reduction in intracellular NAD(P)H than MDA-MB-231 cells. Further investigation indicated that three specific PARP inhibitors completely blocked PCB-induced decreases in intracellular NAD(P)H in both T47D and MDA-MB-231 cells. These results imply that decreases in intracellular NAD(P)H in PCB-treated cells may be, in part, due to depletion of intracellular NAD+ pool mediated by PARP-1 activation through formation of DNA strand breaks. Overall, the extent of cytotoxic response, ROS formation, and PARP-1 activation generated in T47D and MDA-MB-231 cells was greater for PCB153 than for PCB126. In addition, the cytotoxicity induced by PCB153 and PCB126 in both T47D and MDA-MB-231 cells was completely blocked by co-treatment of catalase, dimethylsulfoxide, cupper (I)-/iron (II)-specific chelators, and CYP1A/2B inhibitors. This evidence suggests the involvement of ROS, Cu(I), Fe(II), and CYP1A/2B enzymes in mediating the induction of cell death by PCB153 and PCB126. Further, antagonism was observed between PCB126 and PCB153 for effects on cytotoxic response and ROS formation in T47D and MDA-MB-231 cells. Antagonism was also observed between PCB153 and PCB126 in the induction of NAD(P)H depletion at lower concentration (<10 microM) in T47D cells, but not in MDA-MB-231 cells. In conclusions, results from our investigation suggest that ROS formation induced by PCBs is a significant determinant factor in mediating the DNA damage and cell death in human breast cancer cells. The data also suggests that the status of estrogen receptor alpha may play a role in modulating the PCB-induced oxidative DNA damage and cell death in human breast cancer cells.

PCB 153, a non-dioxin-like tumor promoter, selects for beta-catenin (Catnb)-mutated mouse liver tumors

Polychlorinated biphenyls (PCBs) are ubiquitous environmental toxicants which act as liver tumor promoters in rodents and can be classified as either dioxin-like or non-dioxin (phenobarbital [PB])-like inducers of cytochrome P-450. Since we have previously shown that tumor promotion by PB leads to clonal outgrowth of beta-catenin (Catnb)-mutated but not Ha-ras-mutated mouse liver tumors, we were interested to know whether the non-dioxin-like tumor promoter 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) shows the same selective pressure during tumor promotion. Male B6129SF2/J mice were given a single injection of N-nitrosodiethylamine (90 mg/kg body weight) at 9 weeks of age, followed by 39 weeks of treatment with PCB 153 (20 biweekly ip injections of 300 mumol/kg body weight) or corn oil as a control. Animals were killed 15 weeks after the last PCB 153 injection and liver tumors were identified by immunohistochemical staining of glutamine synthetase (GS) and analyzed for Catnb, Ha-ras, and B-raf mutations. Quantitative analyses revealed that GS-positive tumors were much larger and more frequent in livers from PCB 153-treated mice than in control animals, whereas GS-negative tumors were similar in both groups. Almost 90% (34/38) of all tumors from PCB 153-treated animals contained Catnb mutations, which compares to approximately 45% (17/37) of tumors in the control group. Ha-ras- and B-raf-mutated liver tumors were rare and not significantly different between treatment groups. These results clearly indicate that PCB 153 strongly selects for Catnb-mutated, GS-positive liver tumors, which is similar to the known action of PB, a prototypical tumor promoter in rodent liver.

Polychlorinated biphenyls (PCBs): routes of exposure and effects on human health

The polychlorinated biphenyls (PCBs) are synthetic organochlorine chemicals that were useful industrial products in the past, but their production was ended because they persist in both the environment and living organisms. The PCBs are mixtures of up to 209 different components (congeners), depending on the number and position of chlorines around the biphenyl ring. The PCBs are fat-soluble substances to which everyone is exposed through ingesting animal fats, inhalation, or dermal contact. Exposure to PCBs suppresses the immune system, thereby increasing the risk of acquiring several human diseases. Both ortho-substituted and coplanar (dioxin-like) congeners are tumor promoters that enhance the effects of other carcinogenic substances. PCB exposure, especially during fetal and early life, reduces IQ and alters behavior. The PCBs alter thyroid and reproductive function in both males and females and increase the risk of developing cardiovascular and liver disease and diabetes. Women are at high risk of giving birth to infants of low birth weight, who are at high lifetime risk for several diseases. As knowledge of their toxic effects has grown faster than environmental levels have declined, PCBs remain dangerous contaminants.

Dioxin-like components in human breast milk collected from Hong Kong and Guangzhou

The H4IIE rat hepatoma cell line was employed as a cell model to screen 7-ethoxyresorufin O-deethylase (EROD)-TCDD equivalents (EROD-TEQ) of human breast milk samples collected from Hong Kong and Guangzhou, China. The screening methods employed a 96-well plate spectrofluorometer-EROD assay. For cell-line validation, our results demonstrated a dose-dependent increase in the Ah receptor-mediated response (i.e., CYP1A1 mRNA and EROD) of the cells upon exposure to a number of known Ah receptor agonists, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzothiophene, benzo[a]pyrene, and beta-naphthaflavone. TCDD induced CYP1A1 mRNA and EROD was in a close positive correlation (r=0.98). For the screening of dioxin-like compounds, breast milk samples collected during lactation weeks 3-5 were used. One hundred (from Hong Kong) and 48 (from Guangzhou) breast milk samples were assayed, of which 65% and 68% of the samples, respectively, showed detectable dioxin-like activities using the H4IIE cell EROD screening method. For sixty-five samples from Hong Kong the mean EROD-TEQ values ranged from 58.1 to 96.5 pg/g of milk fat for those aged 21-36 years while 32 samples from Guangzhou had mean values of 98.8-202.1 pg/g of milk fat. In comparisons of the EROD-TEQ values for different age groups from both cities, there were no significant differences (P<0.05). However, the mean and median EROD-TEQ values of the Guangzhou population were in general higher than those of the Hong Kong population. The results of the present study indicate that it is feasible to use the H4IIE cell-line as a model for screening dioxin-like compounds in human breast milk. In addition, the method is rapid and cost-effective, particularly for a routine and high-throughput sample screening analysis, compared to the costly and time-intensive chemical analytical techniques.

Effect of a single dose of polychlorinated biphenyls on hepatic cell proliferation and the DNA binding activity of NF-kappaB and AP-1 in rats

Polychlorinated biphenyls (PCBs) are environmental pollutants that, because of their persistence and biomagnification, raise concerns about the health consequences of long-term exposure. PCB mixtures induce hepatocellular carcinomas in rodents, but the mechanism of their promoting activity is not clear. Previous studies have shown that oxidative stress occurs after PCB administration, with the induction of lipid peroxidation and oxidative DNA damage, which may contribute to their promoting activity. In this study, we examined whether the oxidative stress-sensitive transcription factors NF-kappaB or AP-1 were activated by PCBs in the liver. Male Sprague-Dawley rats were injected i.p. with corn oil, 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153, 30, 150, or 300 micromol/kg), 3,3',4,4'-tetrachlorobiphenyl (PCB-77, 30, 150, or 300 micromol/kg), or both PCBs (each 30 or 150 micromol/kg). Rats were euthanized 2, 6, or 24 h, or 2, 6, and 10 d after the PCB injection. Electrophoretic mobility shift assays (EMSAs) were performed to determine NF-kappaB and AP-1 DNA binding activities. The highest NF-kappaB DNA binding activity was observed in rats receiving higher doses of PCB-153 (150 and 300 micromol/kg), with peak activation occurring 2 d after injection. AP-1 activation was not detected at any timepoint. Hepatocyte proliferation, as measured by the labeling index, was increased only in groups receiving the highest dose of PCB-153 or the combination of two PCBs (150 micromol/kg each) at day 2, and not by any other PCB treatment at any timepoint. These results show that PCB-153, but not PCB-77, can induce hepatocyte proliferation and hepatic NF-kappaB activation after a single dose.

Mammary gland differentiation in female rats after prenatal exposure to 3,3',4,4',5-pentachlorobiphenyl

Recently we reported finding that prenatal exposure to a relatively low dose of 3,3',4,4',5-pentachlorobiphenyl (PCB126) increases the rate of 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary carcinoma, while a high dose decreases it. One of the most important factors determining sensitivity of the mammary gland to neoplastic stimuli is its stage of differentiation at the time of exposure to the carcinogenic agent. Hence, to verify a biphasic dose-response relationship (enhancement of carcinogenesis at low dose, and inhibition at high dose), we investigated the effects of prenatal exposure to PCB126 on mammary gland differentiation. Female SD rats were injected (i.g.) with 25 pg, 2.5 ng, 250 ng, 7.5 microg of PCB126/kg, or the vehicle, on days 13-19 postconception. In 50-day-old offspring, regardless of the day of exposure to DMBA, only the 7.5 microg group showed statistically significant high levels of PCB126 in the fatty tissue of their mammary glands. Fifty-day-old female offspring of the 250 ng group showed apparent inhibition of the normal differentiation of terminal end buds (TEB) to alveolar buds and lobules (ABL), while those of the 7.5 microg group showed mammary gland hypoplasia. Expression levels of the estrogen receptor-alpha (ER) in TEBs and the ER mRNA in mammary glands were higher in the 7.5 microg, 250 ng, 2.5 ng groups. Proliferating cell nuclear antigen (PCNA) expression in TEBs of 50-day-old rats was statistically significantly higher in the 250 ng group and lower in the 7.5 microg group. In the developing mammary gland, TEBs are considered the most susceptible to mammary carcinogenesis, while ABLs are relatively protected from mammary carcinogenesis. Thus, prenatal exposure to a relatively low dose of PCB126 induced an alteration of mammary gland differentiation that might potentially increase the risk of DMBA-induced mammary carcinoma.

The retinoid signaling system--a target in dioxin toxicity

This review summarizes the available data on the effects of dioxins on retinoid levels, retinoid-related enzyme activities, and toxicological endpoints that have been correlated to retinoid effects. Similarities between dioxin toxicity and retinoid deficiency as well as retinoid excess are pointed out. Several possible levels of interaction between the dioxin and the retinoid signaling pathways are discussed, including the involvement of the Ah receptor, altered retinoic acid homeostasis, and an altered set point for retinoid storage. A hypothesis for the effect of dioxins on retinoids is suggested. In this hypothesis, comprising two cascades of effects on the molecular level, the effect of dioxins on retinoic acid levels is central.

Regulation of cell proliferation, apoptosis, and transcription factor activities during the promotion of liver carcinogenesis by polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are environmental pollutants that are complete carcinogens and tumor promoters in the liver. The mechanisms of their promoting activities are not clear, but one possible mechanism is the induction of oxidative stress. In the present study we evaluated the ability of two PCB congeners to activate the oxidative stress-responsive transcription factors nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1), as well as hepatocyte cell proliferation and apoptosis, which are influenced by activation of these transcription factors, in rat liver. Two transcription factors not activated by oxidative stress, signal transducers and activators of transcription 3 and 5 (STAT3 and STAT5), were also examined. All the animals in this study received a single dose of diethylnitrosamine (150 mg/kg) followed by four biweekly injections of 3,3',4,4'-tetrachlorobiphenyl (PCB-77) or 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153) (100 or 300 micromol/kg), or both PCBs (100 micromol/kg each). Ten days after the last PCB injection, all animals were euthanized; 3 days before euthanasia all animals were implanted with Alzet osmotic pumps containing 5-bromo-2'-deoxyuridine (BrdU). The number of placental glutathione S-transferase (PGST)-positive foci were increased in rats administered PCBs, with the highest increase seen in rats administered PCB-77. The number of foci in rats administered both PCBs was intermediate between the numbers seen with either PCB-77 or PCB-153, indicating that a synergistic effect did not occur. There was a significant increase in NF-kappaB and AP-1 binding activities in hepatic nuclear extracts from rats receiving the high dose of PCB-77 or PCB-153 and in rats receiving both PCBs. In contrast, the DNA binding activities of STAT3 and STAT5 were decreased in rats administered PCBs. Cell proliferation in both focal and nonfocal hepatocytes was increased by PCB-77 but was not affected by PCB-153. Apoptotic indexes, as quantified by the TUNEL method, were increased in both focal and nonfocal hepatocytes by PCB-77 but were decreased in focal hepatocytes by PCB-153. This study shows that both PCBs alone or in combination can increase the DNA binding activities of NF-kappaB and AP-1, whereas the DNA binding activities of STAT3 and STAT5 are decreased. The induction of altered hepatic foci appears to be related to compensatory cell proliferation in PCB-77-treated rats, whereas the inhibition of apoptosis appears to be important in PCB-153-treated rats.

Polychlorinated biphenyl congeners as markers of toxic equivalents of polychlorinated biphenyls, dibenzo-p-dioxins and dibenzofurans in breast milk

In breast milk, concentrations of polychlorinated biphenyls (PCBs) are higher than those of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), making PCB analyses less time-consuming and expensive. We searched for PCB "markers" of PCDD/DF concentrations, by studying associations between concentrations of PCB and PCDD/DFs (expressed as toxic equivalents, TEQs) in breast milk from 27 women (primiparas, 22-35 years). These women donated breast milk in 1996-1999 together with 183 other primiparas from Uppsala County, Sweden. Regression analyses showed that both dioxin-like and non-dioxin-like penta- to hepta-chlorinated PCBs could be used as markers of TEQ concentrations in this group of women, in some cases after age adjustment of the regressions. The strong positive association between concentrations of dioxin-like PCB/DD/DFs and non-dioxin-like PCBs will in future epidemiological studies make it difficult to separate Ah receptor-dependent effects from non-Ah receptor-dependent effects. With the use of regression equations and concentrations in breast milk samples collected in 1994, TEQ concentrations were estimated in the 1994 samples. Comparisons between estimated and measured concentrations indicated that associations between concentrations of marker substances and TEQs should be determined separately within each study population, in order to obtain reliable TEQ exposure assessments from PCB markers.

Contribution of planar (0-1 ortho) and nonplanar (2-4 ortho) fractions of Aroclor 1260 to the induction of altered hepatic foci in female Sprague-Dawley rats

The hepatic tumor promoting activity of the planar 0-1 ortho ( approximately 9.7% w/w) and the nonplanar 2-4 ortho ( approximately 90.3% w/w) fraction of the commercial PCB mixture Aroclor 1260 was studied using a medium-term two-stage initiation/promotion bioassay in female Sprague-Dawley rats. Fractionation was carried out on an activated charcoal column. The composition of the effluent from the column was tested by GC-ECD. The absence of planar compounds in the 2-4 ortho fraction was confirmed by GC-MS analysis. The dioxin-like toxic potency of the fractions was determined with the DR-CALUX assay. The animal experiment was started with the initiation procedure (diethylnitrosamine injection, 30 mg/kg body wt ip, 24 h after (2)/(3) hepatectomy), followed 6 weeks later by the promotion treatment, which consisted of a weekly subcutaneous injection during 20 weeks. Exposure groups (n = 10) received the following treatments (dose/kg body wt/week): Aroclor 1260 (10 mg), 0-1 ortho fraction (0.97 mg), 2-4 ortho fraction (1, 3, or 9 mg), a reconstituted 0-4 ortho fraction (9.97 mg), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153; 1 or 9 mg), 2,3,7,8-TCDD (1 microg; positive control) or corn oil (1 ml; vehicle control). One group did not receive a promotion treatment. All exposure groups exhibited a significantly increased volume fraction of the liver occupied by hepatic foci positive for the placental form of glutathione-S-transferase-p compared to the corn oil control, except for the groups treated with 0-1 ortho fraction and 1 mg PCB 153/kg body wt/week. Approximately 80% of the total tumor promoting capacity of the reconstituted 0-4 ortho fraction could be explained by the 2-4 ortho PCB fraction while the 0-1 ortho fraction had only a negligible contribution. These results suggest that the majority of the tumor promotion potential of PCB mixtures resides in the non-dioxin-like fraction, which is not taken into account in the toxic equivalency factor (TEF) approach for risk assessment of PCBs. This may result in an underestimation of the tumor promotion potential of environmental PCB mixtures.

Change of bone tissue composition and impaired bone strength in rats exposed to 3,3',4,4',5-pentachlorobiphenyl (PCB126)

The aim of this study was to compare effects of estrogen depletion (ovariectomy) and exposure to 3,3',4,4',5-pentachlorobiphenyl (PCB126) on bone strength and bone tissue composition in the rat. Half of the rats were ovariectomized (n=20) and the remainder were sham-operated. Ten of the ovariectomized rats and ten of the sham operated were exposed to PCB126 (ip injections) for 3 months (total dose, 384 microgram/kg bodyweight), while those remaining received the vehicle. The humerus and femur were used for analysis of torsional strength and biochemical studies, respectively. Both sham-operated and ovariectomized animals showed a significantly shorter bone length, lower water content and a decreased torsional stiffness when exposed to PCB126. Sham-operated rats exposed to PCB126 had lower maximum torque when compared with sham operated controls. The PCB126-exposed rats also exhibited a significantly lower collagen concentration, but showed a higher pyridinoline concentration of cortical bone. PCB126 exposure decreased the hepatic level of vitamin A but increased vitamin A levels in serum and kidneys. Ovariectomy per se increased bone length and organic content and decreased the inorganic content significantly, but did not affect any of the tested biomechanical parameters. In conclusion, this study showed that the common environmental pollutant PCB126 impaired bone strength and altered bone composition. It is hypothesized that these effects might partly be explained by PCB-induced retinoid disturbances.

Relative potency values derived from hepatic vitamin A reduction in male and female Sprague-Dawley rats following subchronic dietary exposure to individual polychlorinated dibenzo-p-dioxin and dibenzofuran congeners and a mixture thereof

This study investigated the potency of individual polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) to reduce hepatic vitamin A in the rat. Dose-response relationships were determined following long-term dietary exposure to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7, 8-pentachlorodibenzofuran, 1,2,3,4,8-pentachlorodibenzofuran, 1,2,3, 7,8-pentachlorodibenzofuran, 1,2,3,6,7,8-hexachlorodibenzofuran, 1,2, 3,7,8-pentachlorodibenzo-p-dioxin, octachlorodibenzo-p-dioxin, octachlorodibenzofuran, or mixtures of some of these congeners. The aim was to estimate vitamin A-related relative potency (REP) values for each congener in relation to that of TCDD and to investigate if these values were in accordance with REP values estimated for the subchronic toxicity observed in the same study. An additional aim was to investigate if the effect on hepatic vitamin A levels was additive compared to the effect of the individual congeners. The obtained results demonstrate that hepatic vitamin A reduction occurs as a consequence of long-term low-level exposure to 2,3,7, 8-substituted but not to non-2,3,7,8-substituted congeners. Female rats were slightly more responsive to this effect as judged from the lower EC50 values for all the congeners in this sex. The vitamin A-related REP values were similar for female and male rats and were in good agreement with the estimated REP values for subchronic toxicity in the same animals. The vitamin A effect of the individual congeners in the mixture tended to be somewhat less than pure additive for male rats and very close to pure additive for female rats. In conclusion, the presented data show that reduction of hepatic vitamin A is a sensitive marker of an altered retinoid homeostasis following long-term low-dose exposure to dioxin-like compounds, which essentially conforms to their assumed additive mechanism of action.

Induction of altered hepatic foci by a mixture of dioxin-like compounds with and without 2,2',4,4',5,5'-hexachlorobiphenyl in female Sprague-Dawley rats

The hepatic tumor-promoting activity of a mixture of polyhalogenated aromatic hydrocarbons (PHAHs) was studied in a medium term two-stage initiation/promotion bioassay in female Sprague-Dawley rats. The PHAH mixture contained 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1, 2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,4,7, 8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), 2,3',4,4',5-pentachlorobiphenyl (PCB 118), 2,3,3',4,4', 5-hexachlorobiphenyl (PCB 156), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) and covered >90% of the total toxic equivalents (TEQ) present in Baltic herring. To determine possible interactive effects of di-ortho-substituted PCBs, the PHAH mixture was tested with (PHAH+) and without (PHAH-) PCB 153. Rats were initiated by a diethylnitrosamine injection (30 mg/kg body wt i.p.) 24 h after a partial 23 hepatectomy. Six weeks after initiation, the PHAH mixtures were administered once a week by subcutaneous injections for 20 weeks. Treatment with the PHAH mixtures caused liver enlargement and an increased activity of the hepatic cytochrome P4501A1/2 and P4502B1/2. All PHAH exposure groups exhibited an increased occurrence of hepatic foci positive for the placental form of glutathione-S-transferase. In the PHAH-group dosed 1 microgram TEQ/kg body wt/week, the volume fraction of the liver occupied by foci was significantly lower compared to the TEQ equivalent dosed TCDD group (3.8 vs 8.7%). The volume fraction was significantly increased in the groups treated with 0.5, 1, or 2 micrograms TEQ/kg body wt/week of the PHAH+ mixture (4.5, 5.2, and 6.6%, respectively) compared to the corn oil group (2.0%), but to a lower extent than expected on basis of the TEQ doses. Overall, the TEQ-based administered dose overestimated the observed tumor-promoting effects of this PHAH mixture. The applicability of the toxic equivalency factor concept, the role of differences in toxicokinetic properties and interactive effects of PCB 153 on hepatic deposition of the dioxin-like congeners are discussed.