Some implications for quantitative risk assessment if hormesis exists

The existence of hormesis should impact quantitative risk assessment in at least seven fundamental ways. (1) The dose-response models for bioassay and epidemiological data should have greater flexibility to fit the observed shape of the dose-response data and no longer be forced to always be linearly increasing at low doses. (2) Experimental designs should be altered to provide greater opportunity to identify the hormetic component of a dose-response relationship. (3) Rather than a lifetime average daily dose or its analog for shorter time periods, dose scales or metrics should be used that reflect the age or time dependence of the dose level. (4) Low-dose risk characterization should include the likelihood of beneficial effects and the likelihood that a dose level has reasonable certainty of no appreciable adverse health effects. (5) Exposure assessments should make greater efforts to characterize the distribution of actual doses from exposure rather than just upper bounds. (6) Uncertainty characterizations should be expanded to include both upper and lower bounds, and there should be an increased explicit use of expert judgement and weight-of-evidence based distributional analyses reflecting more of the available relevant dose-response information and alternative risk characterizations. (7) Risk should be characterized in terms of the net effect of a dose on health rather than a dose's effect on a single factor affecting health - for example, risk would be better expressed in terms of mortality from all causes combined rather than a specific type of fatal disease.

Vitamin E modulation of dieldrin-induced hepatic focal lesion growth in mice

The effect of vitamin E on dieldrin-induced hepatic focal lesion growth in male B6C3F1 mice previously treated with diethylnitrosamine (DEN) was investigated. After hepatic focal lesions were formed, mice were placed into one of the following treatment groups: Group 1, 50 mg vitamin E/kg diet (control NIH-07 diet); Group 2, 10 mg dieldrin/kg NIH-07 diet; Group 3, 10 mg dieldrin and 450 mg vitamin E/kg NIH-07 diet; and Group 4, 450 mg vitamin E/kg NIH-07 diet. Mice were killed and necropsied after 30 and 60 d of dietary treatment. The effect of treatment on lesion growth was examined by measuring the number of focal lesions per liver and the relative hepatic focal lesion volume. In addition, the possible cellular mechanism of focal hepatocyte growth was investigated by examining both focal DNA synthesis and apoptosis. Dieldrin treatment alone (Group 2) increased the focal lesion volume, focal lesion number, and focal lesion labeling index. Supplementation with vitamin E (Group 3) blocked this effect. Vitamin E supplementation to the diet alone (Group 4) also enhanced focal lesion growth and increased the number of lesions per liver, the relative focal volume, and the labeling index in hepatic focal lesions. Interestingly, vitamin E supplementation inhibited apoptosis in normal liver but did not produce an observable decrease in apoptosis in hepatic focal lesions. The present study showed that dieldrin (Group 2) or vitamin E supplementation alone (Group 4) promoted the growth of hepatic focal lesions in mice. However, when vitamin E is supplemented to dieldrin-fed mice (Group 3), there is an inhibition of hepatic focal lesion growth.

Modeling to incorporate defense mechanisms into the estimation of dose responses

Several adverse health effects (including cancer and noncancer effects) may be the result of an imbalance between exogenous and endogenous invading substances and defense mechanisms. In these cases the probability of an adverse effect depends on how much the exposure to a substance increases or decreases the number of defenders or their efficiency as well as increasing or decreasing the number of invaders. Rather than using a dose scale such as parts per million or milligram/kilogram/day in these cases, dose-response models can directly incorporate the impact of defense mechanisms by using a dose scale that corresponds to the number of invaders that break through the defenders and become free to do their damage. The number of breakthroughs at a specific age, the cumulative number of breakthroughs by a specific age, or the cumulative number of breakthroughs in a window of time would usually be the appropriate age-dependent dose. Although a lifetime average daily dose level can be used as a surrogate for an age-dependent dose in simplistic dose-response models, the age-dependent dose itself can be used in more biologically based models that include time, reflect the key role of feedback mechanisms, and treat the human body as an age-dependent dynamic system responding to internal and external stimuli and not as a system at equilibrium. Some illustrative biologic examples of defense mechanisms and invader-defender interactions are presented. Several numerical examples are given in which the dose incorporates the age-dependent effects of a substance on the number of invaders, the number of defenders, and/or the defenders' efficiencies.

The role of oxidative stress in chemical carcinogenesis

Oxidative stress results when the balance between the production of reactive oxygen species (ROS) overrides the antioxidant capability of the target cell; oxidative damage from the interaction of reactive oxygen with critical cellular macromolecules may occur. ROS may interact with and modify cellular protein, lipid, and DNA, which results in altered target cell function. The accumulation of oxidative damage has been implicated in both acute and chronic cell injury including possible participation in the formation of cancer. Acute oxidative injury may produce selective cell death and a compensatory increase in cell proliferation. This stimulus may result in the formation of newly initiated preneoplastic cells and/or enhance the selective clonal expansion of latent initiated preneoplastic cells. Similarly, sublethal acute oxidative injury may produce unrepaired DNA damage and result in the formation of new mutations and, potentially, new initiated cells. In contrast, sustained chronic oxidative injury may lead to a nonlethal modification of normal cellular growth control mechanisms. Cellular oxidative stress can modify intercellular communication, protein kinase activity, membrane structure and function, and gene expression, and result in modulation of cell growth. We examined the role of oxidative stress as a possible mechanism by which nongenotoxic carcinogens may function. In studies with the selective mouse liver carcinogen dieldrin, a species-specific and dose-dependent decrease in liver antioxidant concentrations with a concomitant increase in ROS formation and oxidative damage was seen. This increase in oxidative stress correlated with an increase in hepatocyte DNA synthesis. Antioxidant supplementation prevented the dieldrin-induced cellular changes. Our findings suggest that the effect of nongenotoxic carcinogens (if they function through oxidative mechanisms) may be amplified in rodents but not in primates because of rodents' greater sensitivity to ROS. These results and findings reported by others support a potential role for oxidative-induced injury in the cancer process specifically during the promotion stage.

Role of oxidative stress in the mechanism of dieldrin's hepatotoxicity

The production of reactive oxygen species (ROS) by toxic chemicals has been implicated in acute and chronic disease states, including cancer. This increase in cellular ROS can lead to a state of oxidative stress. Many compounds selectively induce hepatic tumors in mice but not rats. The mechanism for the induction of hepatic cancer by these compounds and the observed species selectivity of this effect are not known but may be related to the induction of oxidative stress. Dieldrin is one such compound and is used in the present study to characterize the relationship between oxidative stress and the observed selective hepatotoxicity of dieldrin in mice. It was found that dieldrin induced oxidative stress in the mouse but not the rat, and the observed oxidative stress correlated with the induction of DNA S-phase synthesis. This evidence suggests that the induction of oxidative stress may be a mechanism by which dieldrin and other mouse specific compounds selectively induce their hepatic toxic effects in mice.

Reversibility of promoter induced hepatic focal lesion growth in mice

The effect of cessation of phenobarbital and dieldren treatment on hepatic focal lesion growth in male B6C3F1 mice was investigated. Following induction of lesions by diethylnitrosamine, mice were placed on control NIH-07 diet (control diet) or NIH-07 diet containing either dieldrin (10.0 mg/kg diet) or phenobarbital (500 mg/kg diet). Mice were sacrificed after 30 and 60 days of dietary treatment. Two additional groups of mice were fed either the dieldren- or phenobarbital-containing diet for 30 days followed by feeding of NIH-07-only diet for an additional 30 days. The effect of treatment and removal of dieldrin or phenobarbital on lesion growth was examined by measuring both the number of focal lesions per liver and the relative volume of focal lesions. In addition, the rate of cell proliferation and programmed cell death in focal lesion growth was investigated by examining DNA synthesis and apoptosis in the focal lesions. Dietary dieldrin or phenobarbital increased the number of focal lesions and the focal lesion volume. In both dieldrin- and phenobarbital-treated mice, an increased number of eosinophilic lesions were seen. The focal lesion volume was increased in both eosinophilic and basophilic lesions. Dieldrin and phenobarbital treatment also increased the DNA synthetic labeling index in both eosinophilic and basophilic lesions. Removal of dieldrin or phenobarbital from the diet after 30 days of promoter treatment decreased the total number and volume of hepatic focal lesions. The labeling index of the focal lesions was also decreased in these mice. At the terminal sacrifice, the percentage of apoptotic cells in focal lesions was higher in mice fed dieldrin- or phenobarbital-containing diets for the entire 60 days than in mice returned to control diet for the last 30 days. Eosinophilic lesions were more dependent on the presence of a promoting stimulus than the basophilic lesions. These data indicate that induction and maintenance of the growth of some preneoplastic lesions in the mouse may be dependent upon continuous tumor promoter treatment.

Selective dieldrin promotion of hepatic focal lesions in mice

Chronic exposure to a number of chlorinated pesticides, including dieldrin, results in an increased incidence and/or multiplicity of hepatocellular neoplasia in mice, with no such effect in similarly treated rats. One possible explanation of this observed selective carcinogenicity is species-specific hepatic tumor promotion. In the present study we examined the dose-response effect of dieldrin (at several doses) on focal lesion growth (tumor promotion), hepatocyte apoptosis and DNA synthesis in rat and mouse liver. Preneoplastic focal hepatic lesions were produced by diethylnitrosamine (DEN). After the lesions developed, mice and rats were placed into one of the following dose groups: control (NIH-07 diet) or 0.1, 1.0 or 10.0 mg dieldrin/kg diet. Increased focal lesion volume, number of foci per liver and focal DNA synthetic labeling index were observed in 10 mg dieldrin/kg diet-treated mice, but not in similarly treated rats. Dieldrin at dietary concentrations of 0.1 and 1.0 mg/kg diet produced an increase in the number of preneoplastic lesions (0.1 mg/kg diet at 7 days only) and focal volume (0.1 mg/kg diet at 7 and 30 days, 1.0 mg/kg diet at 30 days), but these concentrations did not increase focal DNA labeling index. At dietary concentrations of 0.1, 1.0 and 10 mg dieldrin/kg diet no significant change in lesion percent volume, number of preneoplastic lesions per liver or preneoplastic lesion DNA labeling index was seen in treated rats compared with control rats. Apoptosis, a form of programed cell death, was not decreased in foci by any concentration of dieldrin in either rats or mice. Thus our results suggest that dieldrin may function as a mouse-specific tumor promoter through increased lesion DNA synthesis.

Dose dependence of phenobarbital promotion of preneoplastic hepatic lesions in F344 rats and B6C3F1 mice: effects on DNA synthesis and apoptosis

Phenobarbital (PB), a non-genotoxic hepatocarcinogen in rodents, has been studied extensively but its mechanism of carcinogenic action is unclear. PB appears to function as a tumor promoter by selectively inducing the growth of preneoplastic hepatocytes. In the present study, the comparative effects of PB at tumor-promoting and non-promoting doses were examined in male B6C3F1 mice and male F344 rats. In addition, the mechanism by which PB produced the selective induction of preneoplastic cell growth (increased DNA synthesis/cell proliferation and/or decreased apoptosis) was investigated. Preneoplastic focal lesions were produced using diethylnitrosamine (DEN). After the lesions were histologically apparent, mice and rats were fed PB (10, 100, or 500 mg/kg NIH-07 diet) or control diet and sampled after 7, 30 and 60 days of treatment In both mice and rats, 100 and 500 mg PB/kg increased the number and the relative volume of focal lesions. In rats and mice, 10 mg PB/kg did not enhance focal lesion growth. The preneoplastic lesions that clonally expanded due to phenobarbital treatment were predominantly eosinophilic in appearance. In addition, DNA synthesis in focal hepatocytes was significantly increased in the 100 and 500 mg PB/kg diet. In PB-treated mice and rats, there also was a significant decrease in the rates of apoptosis in focal hepatocytes. Therefore, our data showed that PB at doses of 100 and 500 mg/kg diet promoted focal hepatic lesion growth both by increasing DNA synthesis and cell proliferation and by decreasing the rate of apoptosis.

Subchronic effects of dieldrin and phenobarbital on hepatic DNA synthesis in mice and rats

Dieldrin, an organochlorine pesticide, has been shown to be hepatocarcinogenic in mice but not rats. Phenobarbital, in contrast, induces hepatic tumors in both mice and rats. Previous studies have shown that acute dietary exposure of rats or mice to either dieldrin or phenobarbital produces several liver changes, including centrilobular hypertrophy, induction of hepatic cytochrome P450, and increased liver weight. The present study examined the subchronic effect of dieldrin (0.1, 1.0, 3.0, 10.0 mg dieldrin/kg diet) and phenobarbital (10, 50, 100, 500 mg phenobarbital/kg diet) on the induction of hepatic DNA synthesis and hepatocyte lethality in male B6C3F1 mice and male F344 rats. Eight-week-old animals were treated as above and evaluated for hepatic DNA synthesis after 7, 14, 21, 28, and 90 days of continual treatment to dieldrin or phenobarbital. Maximal induction of hepatic DNA synthesis in mice was seen at the 14-, 21-, and 28-day sampling times. In rats, no significant increase in hepatic DNA synthesis or hepatocyte lethality was observed at any dose of dieldrin investigated. Phenobarbital produced a significant increase in hepatic DNA synthesis in both rat and mouse liver following 7 days of treatment. The induction of DNA synthesis in rat liver was transient, with the labeling index returning to control levels by 14 days of treatment. In contrast, mice treated with phenobarbital showed a significant increase in hepatic DNA synthesis throughout the treatment. In both mice and rats, dieldrin and phenobarbital induced hepatic DNA synthesis selectively in the centrilobular region of the hepatic lobule. The lack of an increase in serum enzymes indicative of hepatic damage and the absence of liver histopathology in mice or rats fed dieldrin or phenobarbital indicate that the induction of DNA synthesis was not mediated by a cytolethal, compensatory hyperplastic response, suggesting a mitogenic mechanism. Therefore, the species-specific induction of hepatic DNA synthesis by either dieldrin or phenobarbital correlated with the previously observed species-specific induction of hepatic cancer by these two compounds.

Oxidative stress in nongenotoxic carcinogenesis

The induction of oxidative stress in the target tissue has been proposed as a possible mechanism of action for nongenotoxic carcinogens. A variety of nongenotoxic hepatocarcinogens including peroxisome proliferators, organochlorines, barbiturates, and metals have been shown to produce an increase in reactive oxygen species (ROS) in the liver. Our group has examined the induction of oxidative stress by the organochlorine mouse hepatic carcinogen, dieldrin. Using a salicylate spin trap assay, dieldrin was found to produce mouse liver-specific increases in ROS in cultured hepatocytes. Increased amounts of hepatic 8-hydroxy-2'-deoxyguanosine and malondialdehyde (MDA) and decreased levels of cellular antioxidants were also seen in cultured mouse hepatocytes following dieldrin treatment. In subchronically dieldrin-treated mice and rats, hepatic vitamin E (Vit E) was decreased correlated with dieldrin dose. While Vit E levels were decreased in both rats and mice, the normal lower levels of Vit E in the mouse resulted in a subsequent oxidative stress, evidenced by an increase in MDA formation in the mouse liver. Dieldrin also produced a dose-dependent increase in DNA synthesis in the mouse (not the rat) following subchronic treatment. These effects seen in both cells in culture and in vivo were species specific, organ specific, and dose dependent which directly correlated with the observed pattern of cancer induction for dieldrin in rodents (mouse liver-specific). These findings support a possible role for the induction of oxidative stress in nongenotoxic hepatic carcinogenesis possibly through modulation of gene expression.

Challenges to default assumptions stimulate comprehensive realism as a new tier in quantitative cancer risk assessment

The current practice in carcinogen risk assessment of using a linearized multistage model and assuming low-dose linearity is based on several false premises. In many cases linearity at low doses would not be expected based on the interaction between the multiple components in the carcinogenic process. The two-stage growth models, involving multiple mutations and cell birth and death rates, provide one means of exploring these interactions. In addition, if carcinogenesis is considered to be the imbalance between invading substances and defense mechanisms, then the cancer probability depends on how much the substance increases or decrease the number of defenders or their efficiency as well as increasing or decreasing the number of invaders. Challenges to low-dose linearity and other default assumptions have stimulated the development of new risk assessment methodologies as have the need for more realistic estimates of risk, better uncertainty characterization, and greater utilization of cost-benefit analyses, and other tools for risk management decision making. "Comprehensive realism" is an emerging quantitative weight-of-evidence risk assessment methodology which is designed to reflect all of the relevant and available information and the current state of knowledge about the health risks associated with a substance.

Effect of dietary antioxidants on dieldrin-induced hepatotoxicity in mice

An increasing number of reports suggest that oxidative stress plays a role in the toxicity of various xenobiotics, including organochlorine pesticides and drugs such as phenobarbital. Antioxidants appear to be protective against the damage induced by an acute dose of endrin, supporting the theory of a role for reactive oxygen in the toxicity of this class of compounds. The current study examined the effects of the dietary administration of vitamin C (400 mg/kg diet) or vitamin E (200 mg DL-alpha-tocopherol acetate/kg diet) on hepatotoxicity induced by subchronic (7 or 28 days) feeding of dieldrin (1, 3 and 10 mg/kg diet) to male B6C3F1 mice. Hepatoxicity induced by feeding of dieldrin for 28 days was evidenced by liver enlargement, hypertrophy of centrolobular hepatocytes, induction of hepatic ethoxyresorufin O-deethylase activity, and increased DNA synthesis in hepatocytes, particularly in centrolobular hepatocytes. Neither vitamin inhibited the dose-dependent increase in liver/body weight ratios, hypertrophy of centrolobular hepatocytes, or induction of hepatic ethoxyresorufin O-deethylase. Vitamin E, however, inhibited hepatic DNA synthesis at all dietary intakes of dieldrin, while vitamin C was inhibitory at 1 and 3, but stimulatory at 10 mg dieldrin per kg diet. The major changes in DNA labeling occurred in the centrolobular zones, but were not consistently inhibited by vitamins C or E. The ability of antioxidant vitamins to inhibit dieldrin-induced hepatic DNA synthesis suggests oxidative stress is involved in the toxicity of this compound; however, the inability of these vitamins to prevent all hepatotoxic changes indicates other factors are also involved.

Another flaw in the linearized multistage model upper bounds on human cancer potency

The statistical methodology used by the EPA and other federal agencies to characterize human cancer potencies based on animal experiments greatly exaggerates the estimates and bounds for human cancer risks in some instances. The current methodology incorporates simplified assumptions and approximations that fail to properly assess the impact of quantitative differences in human and experimental animal background transition rates from stage to stage in the multistage carcinogenic process. Because the majority of tumorigenic responses in rats and mice occur in organs with a high background tumor incidence and, hence, high background stage transition rates, the current simplified methodology often significantly overstates human risk. This newly recognized flaw in conjunction with the several previously recognized flaws in the current characterizations of human cancer potencies argues strongly for a change away from the current characterizations based solely on a default screening methodology to a more comprehensive, biologically based risk assessment methodology.