Chronic toxicity and carcinogenicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin displays a distinct dose/time toxicity threshold (c×t=k) and a life-prolonging subthreshold effect

Repeated exposure to the irrigative wastewater in Shijiazhuang induced precancerous lesion associated with cytochrome P450

In the present study, the carcinogenic effects of the wastewater sample collected from the Dongming Canal in Shijiazhuang city were first detected by the rat medium-term liver bioassay. The experiment contained five groups: a negative control group, a DEN-alone group, 25% wastewater, 50% wastewater, and 100% wastewater. The body weight of rats decreased significantly as the dose increased. Morphologically, we also found that the damage of the hepatic lobule was more serious and the proliferation of liver cells was more obvious as the dose increased. In addition, we observed a significantly increased liver organ coefficient in rat. With the increase in dose, the damage of the hepatocytes was more serious, which was manifested in significantly elevated of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and gammaglutamyl transfer peptide enzyme (γ-GT). And, the irrigative wastewater significantly increased GST-p in the liver of rats at both the transcriptional and translational levels dose-dependently, eventually causing precancerous lesions in the liver tissues. CYP1A1 and CYP1B1 expressions in the rat liver cells at the level of transcription and translation were also significantly increased dose-dependently. Our data clearly demonstrated that the irrigative wastewater had a carcinogenetic effect that was associated with CYP1A1 and CYP1B1. The risk of carcinogenic potential to human health might be due to joint action and accumulative effects over a long period of exposure. We can also concluded that the medium-term liver bioassay could be used as an effective method for evaluating the carcinogenicity of complex water mixtures such as irrigative wastewater.

Critique of dose response in carcinogenesis

A few landmarks in the development of dose response in toxicology are presented, with an explanation of why dose should only be considered on a logarithmic scale. Examples are shown, illustrating that the current practice of labeling dose-response curves for carcinogenesis as supralinear, linear or sublinear, is meaningless unless the dose-response scales are defined. Since many reports labeling such curves as supralinear, linear, or sublinear are carried out with dose on a linear scale, the scientific significance of the shape of the curve is obscured. Examples of dose-response curves for carcinogenesis from 2-acetylaminofluorene, N-nitrosodiethylamine, aflatoxins, and radium are shown. In addition, more than 500 National Toxicology Program Technical Reports (NTP-TR) on carcinogenicity were examined; from this database, three groups of studies were selected. The first group consisted of those studies in which the lowest dose produced no tumors and the study had a positive dose response. The second group consisted of those studies with three or more doses, with a positive dose response producing tumors, but in which there were no tumors in the control group. The third group of more than 50 studies was from NTP-TR-00 to NTP-TR-52 that had only two data points with a positive dose response. These studies were all evaluated on the Rozman et al. scale, since it conforms to the laws of nature and allows evaluation of all doses. It was observed that virtually all of these NTP-TR carcinogenicity studies show a linear response when dose is on this logarithmic scale; a clear threshold for carcinogenicity is typical for nearly all of these chemicals. An exponential dose-response curve was a better fit for a few, but experimental error could account for this deviation from linearity. It is pointed out that there is strong experimental evidence that the mere presence of DNA adducts does not necessarily lead to tumor production. Hormesis probably applies to carcino-genesis and proof of this will require abandoning the no threshold concept. Experiments showing that cumulative dose is a better metric than daily dose may require reevaluating almost all carcinogenicity studies.

Hormesis depends upon the life-stage and duration of exposure: Examples for a pesticide and a nanomaterial

Tests to assess toxic effects on the reproduction of adult C. elegans after 72h exposure for two chemicals, (3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)), also known as diuron, and silver nanoparticles (Ag NPs) indicated potential, although not significant hormesis. Follow up toxicity tests comparing the potential hormesis concentrations with controls at high replication confirmed that the stimulatory effect was repeatable and also statistically significant within the test. To understand the relevance of the hormesis effects for overall population fitness, full life-cycle toxicity tests were conducted for each chemical. When nematodes were exposed to DCMU over the full life-span, the hormesis effect for reproduction seen in short-term tests was no longer evident. Further at the putative hormesis concentrations, a negative effect of DCMU on time to maturation was also seen. For the Ag NPs, the EC50 for effects on reproduction in the life-cycle exposure was substantially lower than in the short-term test, the EC50s estimated by a three parameter log logistic model being 2.9mg/L and 0.75mg/L, respectively. This suggests that the level of toxicity for Ag NPs for C. elegans reproduction is dependant on the life stage exposed and possibly the duration of the exposure. Further, in the longer duration exposures, hormesis effects on reproduction seen in the short-term exposures were no longer apparent. Instead, all concentrations reduced both overall brood size and life-span. These results for both chemicals suggest that the hormesis observed for a single endpoint in short-term exposure may be the result of a temporary reallocation of resources between traits that are not sustained over the full life-time. Such reallocation is consistent with energy budget theories for organisms subject to toxic stress.

A comparison of Reduced Life Expectancy (RLE) model with Haber's Rule to describe effects of exposure time on toxicity

The Reduced Life Expectancy (RLE) Model (LC50 = [ln(NLE) - ln(LT50)]/d) has been proposed as an alternative to Haber's Rule. The model is based on a linear relationship between LC50 (Lethal Exposure Concentration) and lnLT50 (Lethal Exposure Time) and uses NLE (Normal Life Expectancy) as a limiting point as well as a long term data point (where d is a constant). The purposes of this paper were to compare the RLE Model with Haber's Rule with available toxicity data and to evaluate the strengths and weaknesses of each approach. When LT50 is relatively short and LC50 is high, Haber's Rule is consistent with the RLE model. But the difference between the two was evident in the situation when LT50 is relatively long and LC50 is low where the RLE model is a marked departure from Haber's Rule. The RLE Model can be used to appropriately evaluate long term effects of exposure.

Evaluation of effects of long term exposure on lethal toxicity with mammals

The relationship between exposure time (LT50) and lethal exposure concentration (LC50) has been evaluated over relatively long exposure times using a novel parameter, Normal Life Expectancy (NLT), as a long term toxicity point. The model equation, ln(LT50) = aLC50(ν) + b, where a, b and ν are constants, was evaluated by plotting lnLT50 against LC50 using available toxicity data based on inhalation exposure from 7 species of mammals. With each specific toxicant a single consistent relationship was observed for all mammals with ν always <1. Use of NLT as a long term toxicity point provided a valuable limiting point for long exposure times. With organic compounds, the Kow can be used to calculate the model constants a and v where these are unknown. The model can be used to characterise toxicity to specific mammals and then be extended to estimate toxicity at any exposure time with other mammals.

From dioxin to drug lead--the development of 2,3,7,8-tetrachlorophenothiazine

Polychlorinated dibenzo-p-dioxins are persistent environmental pollutants. The most potent congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), causes a wasting syndrome and is a potent carcinogen and immunosuppressant in the rat at high doses. However, low doses cause opposite effects to some of those observed at higher doses, resulting in chemoprevention, stimulation of the immune system, and longevity in experimental animals. The new TCDD analogue, 2,3,7,8-tetrachlorophenothiazine (TCPT), was developed to take advantage of the low-dose effects of dioxins that have potential application as therapeutics. Its development marked a deviation from the traditional scope of phenothiazine drug design by deriving biological effects from aryl substituents. TCPT was synthesized in three steps. The key ring-closing step was performed utilizing a Buchwald-Hartwig amination to provide TCPT in 37% yield. Its potency to induce CYP1A1 activity over 24 h was 370 times lower than that of TCDD in vitro. The elimination half-life of the parent compound in serum was 5.4 h in the rat and 2.7 h in the guinea pig, compared to 11 and 30 days, respectively, for TCDD. These initial findings clearly differentiate TCPT from TCDD and provide the basis for further studies of its potential as a drug lead.

Overview of exposure, toxicity, and risks to children from current levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds in the USA

Studies of children indicate that exposure of the general population to low levels of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) does not result in any clinical evidence of disease, although accidental exposure to high levels either before or after birth have led to a number of developmental deficits. Breast-fed infants have higher exposures than formula-fed infants, but studies consistently find that breast-fed infants perform better on developmental neurologic tests than their formula-fed counterparts, supporting the well-recognized benefits of breast feeding. Children receive higher exposures to PCDD/Fs from food than adults on a body-weight basis but those exposures are below the World Health Organization's tolerable daily intake. Laboratory rodents appear to be at least an order of magnitude more sensitive than humans to the aryl hydrocarbon receptor-mediated effects of these substances, which makes them poor surrogates for predicting quantitative risks but makes them good models for establishing safe levels of human exposure by organizations mandated to protect public health. Any exposure limit for PCDD/Fs based on developmental toxicity in sensitive laboratory animals can be expected to be especially protective of human health, including the health of infants and children. Because body burdens and environmental levels continue to decline, it is unlikely that children alive today in the USA will experience exposures to PCDD/Fs that are injurious to their health.

A Weight-of-Evidence Analysis of the Cancer Dose-Response Characteristics of 2,3,7,8-Tetrachlorodibenzodioxin (TCDD)

Cancer risk assessment for TCDD and other compounds must focus on the cancer dose-response relationship and corresponding potency for the range of human doses before it can have relevance to the human exposure environment. Major differences of opinion exist over whether the dose-response curve for TCDD and other dioxin congeners is non-linear (incorporating a threshold dose region below which tumors are unlikely to be elicited) or linear (implying that any exposure has a statistical likelihood of causing cancer). The World Health Organization and others strongly support a non-linear dose-response relationship for TCDD and cancer, whereas USEPA characterizes the dose-response function as linear. This review critically summarizes the available information on TCDD dose-response relationship for cancer utilizing a weight-of-evidence approach. This assessment concludes that the available data support a non-linear dose-response relationship as being most likely and appropriate for human cancer risk assessment, i.e., the evidence suggests that a biological threshold exists in the dose-response. While proof of a threshold is not absolute, and never can be, the level of certainty for TCDD is substantial because of the concordance of many lines of evidence and the consistency of repeated observations pointing to non-linearity.

Validation of Haber's Rule (dose×time=constant) in rats and mice for monochloroacetic acid and 2,3,7,8-tetrachlorodibenzo-p-dioxin under conditions of kinetic steady state

Haber's Rule and associated time to coma after monochloroacetic acid (MCA) exposure in male Sprague-Dawley (SD) rats and time to death after 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in female Sprague-Dawley rats and male A/J mice were investigated at isoeffective or nearly isoeffective doses. Animals exposed to MCA received either single bolus intravenous (iv) doses or a loading dose rate via the iv route followed by a maintenance dose rate through subcutaneously implanted osmotic mini pumps. For TCDD, rats received a loading dose rate via bolus oral gavage followed by maintenance dose rates through iv injection every fourth day until death. Mice received both loading and maintenance (once a week) dose rates via oral gavage. Different dosing regimens were employed to demonstrate that the key to Haber's Rule lies not in the route of administration but in conducting experiments under conditions of kinetic steady state. Single doses of MCA produced inconsistent time responses but a reasonably constant c x t product (7657+/-391 mg/kg x min) which was not anticipated although it should have been expected because MCA's elimination half-life (2 h) is twice as long as its time to coma ( approximately 1h). Generation of kinetic steady state by infusion of MCA after iv injection of a loading dose rate resulted in a consistently decreasing time response with increasing dose which diminished the variability in the c x t (dose x time)=k relationship (8032+/-136 mg/kg x min). Both acute and chronic toxicity of TCDD under conditions of kinetic steady state yielded consistent time responses with inverse proportionality between dose and time leading to robust c x t=k products in both rats (1060+/-82 microg/kg x day) and mice (80+/-2 mg/kg x day).

Hormesis and risk assessment

It is postulated in this paper that at low doses all chemicals have hormetic/hormoligotic (beneficial) effects in living organisms. It has been known since Paracelsus that at high doses all chemicals are toxic. The combination of low and high dose effects can be empirically described by a beta-curve or an inverted beta-curve. A mathematical method is suggested to determine the maximum of the beta-curve or the minimum of the inverted beta-curve, yielding a point estimate for risk assessment.

Human exposure to dioxins from food, 1999-2002

In response to aggressive attempts to control dioxin emissions over the last 35 years, human exposures to dioxins from the environment have declined significantly. The primary source of human exposure to dioxins at present is food. The sources of dioxins in food are not well understood and are probably varied. Data on the levels of dioxins measured in various foods for samples collected from 2000 to 2002 have recently been released by the US Food and Drug Administration as part of its Total Diet Study. Data on samples collected in 1999, and released in 2002, are also available. Based on those data and on the US Department of Agriculture's most recent food consumption survey (1994-1996 & 1998 Continuing Survey of Food Intakes by Individuals), estimates of dioxin intake for the total US population and for three age groups of children were obtained. Results show that the most recent mean dietary exposures for all groups are below 2 pg TEQ/kg BW/day, the tolerable daily intake established for dioxins by the World Health Organization. Between 1999 and 2002 mean dioxin intakes from food appear to have decreased, but when estimates are adjusted based on a standardized limit of detection and evaluating only those {congenerxfood} combinations common to all 4 years, no trend is apparent. When dioxin concentrations below the limit of detection are represented by one-half the limit, approximately 5% of the intake estimates for 2-year-olds and 1% of the intake estimates for 6-year-olds exceed the tolerable daily intake by about 10%, although such upper-percentile estimates should not be equated with excess risk. When non-detectable dioxin values are set to zero (i.e., when only dioxin values actually measured are used), only 1% of intake estimates exceed the tolerable daily intake for 2-year-olds. As expected, about 50% of daily dietary dioxin intake by the total US population is attributable to meat and dairy products, based on the same food group classifications used by the National Academy of Sciences' Committee on the Implications of Dioxin in the Food Supply. This information may be useful for targeting future risk management activities.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD) alter body weight by decreasing insulin-like growth factor I (IGF-I) signaling

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) affects glycemia due to reduced gluconeogenesis; when combined with a reduction in feed intake, this culminates in decreased body weight. We investigated the effects of steady-state levels of TCDD (loading dose rates of 0.0125, 0.05, 0.2, 0.8, and 3.2 microg/kg) or approximately isoeffective dose rates of 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD) (loading dose rates of 0.3125, 1.25, 5, 20, and 80 microg/kg) on body weight, phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression and activity, and circulating concentrations of insulin, glucose, and insulin-like growth factor-I (IGF-I), and expression of hepatic phosphorylated AMP kinase-alpha (p-AMPK) protein in female Sprague-Dawley rats (approximately 250 gm) at 2, 4, 8, 16, 32, 64, and 128 days after commencement of treatment. At the 0.05 and 1.25 microg/kg loading dose rates of TCDD and HxCDD, respectively, there was a slight increase in body weight as compared to controls, whereas at the 3.2 and 80 microg/kg loading dose rates of TCDD and HxCDD, respectively, body weight of the rats was significantly decreased. TCDD and HxCDD also inhibited PEPCK activity in a dose-dependent fashion, as demonstrated by reductions in PEPCK mRNA and protein. Serum IGF-I levels of rats treated initially with 3.2 microg/kg TCDD or 80 microg/kg HxCDD started to decline at day 4 and decreased to about 40% of levels seen in controls after day 16, remaining low for the duration of the study. Eight days after initial dosing, hepatic p-AMPK protein was increased in a dose-dependent manner with higher doses of TCDD and HxCDD. There was no effect with any dose of TCDD or HxCDD on circulating insulin or glucose levels. In conclusion, doses of TCDD or HxCDD that began to inhibit body weight in female rats also started to inhibit PEPCK, inhibited IGF-I, while at the same time inducing p-AMPK.