Limitations of multimedia models for use in environmental decision making.

The United States currently is engaged in a complex, multi-billion dollar effort to cleanup a legacy of both privately- and federally-owned hazardous waste sites. Decisions regarding the best approach for remediation of these sites often are based on the analysis of potential risks to human health and the environment. A cornerstone of such analysis is the frequent use of computerized multimedia environmental transport models, to evaluate the large quantities of information necessary to understand the present and future implications of contamination at a site. One barrier to wide-spread use of this analytical procedure is the view that results obtained using computer models are highly dependent on user input, and therefore, subject to manipulation. It is widely recognized that for decisions to be both credible and implementable, the public must have confidence in both the scientific basis for judgments involved and the decision processes employed (NRC, 1983). Our purpose in this article is to overview the difficulties associated with application of multimedia models to real world problems and the contribution these models can make to technically sound estimates of exposure and risk.

Determinability of model parameters in a two-stage deterministic cancer model.

We investigate the determinability of model parameters for a two-stage cancer model, using as an example chemically induced skin cancer in mice. When the time course of the papilloma number and cancer rate are known, the rate of creation of initiated cells and their net-growth rate can be uniquely determined. These two high-level parameters are sufficient to uniquely simulate the experimental papilloma data. However, the mitotic and death rates of initiated and transformed cells cannot be determined from the available experimental data. The rate of creation of transformed cells and their net-growth rate cannot be determined independently. Thus, although the deterministic two-stage cancer model can simulate the kinetics of papilloma formation and skin cancer data, many of the basic underlying biological parameters (i.e., mitotic and death rates) cannot be uniquely determined from the usually available biological data.

Mathematical modeling of skin papilloma data in SENCAR mice.

Published data from SENCAR mice were analyzed to elucidate mechanisms underlying the formation of chemically induced skin papillomas. The experimental data followed a two-step protocol using 7,12-dimethylbenz[a]anthracene (DMBA) for initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA) for promotion. The two-mutation model of Ellwein and Cohen was used to simulate the data. In this model, probabilities of stem cell birth and death along with initiation and increased cell division are allowed to vary in time. We incorporated a pulse dose with exponential decay to represent the effect of DMBA on the probability of initiation over time, and a summation of pulse doses to represent the repeated topical applications of TPA and its effect on cell division and cell death. We fitted the model to the papilloma and carcinoma data sequentially, and indirectly determined the dependence of model parameters on the administered dose of DMBA and TPA. The occurrence of chemically induced epidermal hyperplasia is a fundamental assumption in our fits of the papilloma data. We found a delay in the onset of normal and initiated cell death relative to the enhancement of cell division rates. In addition, the model parameters display a nonlinear dependence on the dose of DMBA or TPA administered.

Biological model of ED01 hepatocarcinogenesis.

The ED01 bioassay on 2-acetylaminofluorene (2-AAF)-induced hepatocarcinogenesis in BALB/c mice was analyzed using a biologically based, two-mutation model of the oncogenic process. Computer simulations indicate that 2-AAF-induced hepatocarcinogenesis has both genotoxic and promotional components. However, contrary to the current paradigm, we find that the genotoxic component of 2-AAF plays a minor role in its carcinogenicity. The ED01 hepatocarcinogenicity can be explained almost entirely to result from the promotional influence of 2-AAF on an existing pool of spontaneously initiated cells.

Wild type p53 reduces the size of tumors caused by bovine leukemia virus-infected cells.

Bat lung (BAT(2)CL6) cells infected with bovine leukemia virus (BLV) cause malignant tumors in nude mice that after 6 weeks subcutaneous growth, have an average volume of 0.3m(3). Uninfected bat lung cells (Tb 1 Lu) produce small benign neoplasms that average 0.003 cm(3). BAT(2)CL6 cells were transfected in vitro with expression vectors that produce wild type human or mutant p53. Production of human p53 in transfected BAT(2)CL6 cells was confirmed by immunoprecipitation of p53 and by immunohistochemical staining using anti-human p53 monoclonal antibodies. BAT(2)CL6 cells transfected with wild type p53 produced tumors in nude mice averaging 0.03 cm(3) whereas cells transfected with mutant p53 yielded tumors averaging 0.3cm(3). BAT(2)CL6 cell tumors after 1 week subcutaneous growth were transfected in situ with the wild type p53 gene. At 6 weeks tumor volume of in situ transfected tumors was similar to those resulting from cells transfected in vitro. Histopathologic examination and immunochemical staining of tumors produced in nude mice after wild type p53 treatment showed no significant differences when compared to tumors produced by untreated BAT(2)CL6 cells. Therefore, it is likely that the tumors produced by p53 treated-cells arose from cells that escaped transfection. The reduction of tumor size by restoration of wild type 53 may prove to be a useful therapy for BLV-induced tumors.

Effectiveness of Purge-and-Trap for Measurement of Volatile Organic Compounds in Aged Soils.

The U.S. EPA-recommended method for measurement of trace levels of volatile organic compounds (VOCs) in soil, purge-and-trap, measures the readily desorbable organic contaminants from soil pore spaces and external soil surfaces. It does not, however, measure contamination that has diffused into internal micropores of soil matrix. Thus, the purge-and-trap method measures only a small fraction of total soil contaminants, especially in long-contaminated soils, where ∼90-99% of contamination may be in the interior of the soil matrix. We compared three methods for determination of VOCs in aged field samples: purge-and-trap, methanol immersion, and hot solvent extraction. Hot solvent extraction proved to be much more effective than the U.S. EPA-approved purge-and-trap technique. For three long-contaminated soils containing such VOCs as trichloroethene, benzene, toluene, chloroform, methylene chloride, and cis-1,1-dichloroethylene, recovery from purge-and-trap ranged between 1.5 and 41.3% that of hot solvent extraction. Our data show that purge-and-trap may not be the best methodology for measuring soil VOCs concentrations, particularly in aged soils. It is clear from this and previous studies that the best overall choice for soil VOCs measurements is hot solvent extraction. These results also indicate the inefficiency of purge-and-trap as a method for evaluating vapor extraction remediation technology. Our results suggest that the EPA should review the use of the purge-and-trap method for measuring VOCs concentrations in soils.

Increased p53 site-specific DNA binding in cells producing mutant p53.

Chemotherapeutic and DNA-damaging agents were found to increase p53 site-specific DNA binding in human breast and rat liver epithelial WBrasII cells which produce mutant p53. Increased p53 site-specific DNA binding by chemotherapeutic or DNA-damaging agents was also induced in transfected Saos-2 cells producing wild type or transforming mutant p53. Therefore, exposure of cells containing a transforming p53 mutant to chemotherapeutic or DNA-damaging agents may potentially enhance their transformation state and tumorigenic potential.

Stabilization of the p53 gene product in two bovine leukemia virus infected cell lines.

Fetal lamb kidney cells (FLK) and bat lung (BAT2CL6) cells that continuously produce bovine leukemia virus (BLV) were found to cause malignant tumors in nude mice. Uninfected bat lung cells (Tb 1 Lu) produced a small benign neoplasm. Pulse chase studies showed that the p53 gene product in BAT2CL6 cells was more stable compared with p53 in Tb 1 Lu cells. Mono-clonal antibody studies suggested that a mutant form of the p53 protein was produced in BLV-infected cells. Heteroduplex mapping studies of the p53 gene from BLV-infected cells also suggested that a mutation in p53 had occurred. Stabilization of the p53 gene product in BLV-infected cells may contribute to the progression of tumor virulence.

Co-exposure to gasoline vapor decreases benzene metabolism in Fischer-344 rats.

The metabolic interactions of benzene and gasoline vapor were investigated in male Fischer-344 rats. A closed chamber gas-uptake exposure system was used to obtain inhalation uptake curves for benzene alone and benzene in the presence of gasoline vapor. Exposure to benzene as a component of gasoline vapor resulted in a decrease of benzene metabolism. A physiologically based pharmacokinetic model of benzene metabolism was used to quantitatively determine the extent of the inhibitory effect of gasoline vapor on benzene metabolism. This observed inhibitory effect cannot be accounted for by the presence of toluene in gasoline vapor.

Validation of a terrestrial food chain model.

An increasingly important topic in risk assessment is the estimation of human exposure to environmental pollutants through pathways other than inhalation. The Environmental Protection Agency (EPA) has recently developed a computerized methodology (EPA, 1990) to estimate indirect exposure to toxic pollutants from Municipal Waste Combuster emissions. This methodology estimates health risks from exposure to toxic pollutants from the terrestrial food chain (TFC), soil ingestion, drinking water ingestion, fish ingestion, and dermal absorption via soil and water. Of these, one of the most difficult to estimate is exposure through the food chain. This paper estimates the accuracy of the EPA methodology for estimating food chain contamination. To our knowledge, no data exist on measured concentrations of pollutants in food grown around Municipal Waste Incinerators, and few field-scale studies have been performed on the uptake of pollutants in the food chain. Therefore, to evaluate the EPA methodology, we compare actual measurements of background contaminant levels in food with estimates made using EPA's computerized methodology. Background levels of contaminants in air, water, and soil were used as input to the EPA food chain model to predict background levels of contaminants in food. These predicted values were then compared with the measured background contaminant levels. Comparisons were performed for dioxin, pentachlorophenol, polychlorinated biphenyls, benzene, benzo(a)pyrene, mercury, and lead.

Promotion as a factor in carcinogenesis.

Promotion is any factor which results in the increased cellular replication of initiated or transformed cells. We argue that cytotoxicity is not a necessary component of promotion and that, therefore, the existence of a threshold for promotion is unlikely.

Quantitative correlation of carcinogenic potency with four different classes of short-term test data.

The search for a short-term mutagenicity test to identify potential carcinogens has yielded over 100 assay systems, including the Ames test. This paper continues the investigation of Travis et al. into the prediction of carcinogenic potency of known mouse carcinogens using different classes of short-term toxicologic data. We used four classes of short-term test data (mutation, toxicity, reproduction and tumorigenicity) from the Registry of Toxic Effects of Chemical Substances (RTECS) database. We conclude that mutation data alone are poor predictors of carcinogenic potency, accounting for only 21% of the observed variability in experimentally-determined mouse TD50 values. We further conclude that batteries of toxicologic data containing varying types of short-term data are excellent predictors of the carcinogenic potency of known mouse carcinogens: four types of short-term data account for 82% of the observed variability in experimentally-obtained mouse TD50 values. By using all available toxicological data, we obtained a strong correlation between toxicologic assay results and carcinogenic potency of known mouse carcinogens.

Interspecies scaling of anesthetic potency.

Anesthetic potency data for 11 volatile anesthetics were correlated against body weight in multiple mammalian species, including man. The results indicate that the alveolar concentration necessary to produce anesthesia is approximately constant across species. Because alveolar ventilation rates scale with the 0.75 power of body weight, this implies that administered dose measured in mg/kg0.75/day produces the same anesthetic effect in all species. This analysis provides further support for the use of a mg/kg0.75/day interspecies scaling metric for acute toxic effects.

Diethylnitrosamine-induced hepatocarcinogenesis in rats: a theoretical study.

Multiple data sets on hepatocarcinogenesis in rats resulting from pulse (single) or continuous exposure to diethylnitrosamine (DEN) are analyzed within the framework of a two-mutation carcinogenesis model in order to identify the underlying biological processes that control the pharmacodynamics of DEN-induced liver cancer. Our findings indicate: (1) Predictions of the two-mutation oncogenic model are consistent with empirical data on DEN-induced hepatocarcinogenesis. (2) The probability of the first genetic alteration (initiation) is linearly dependent on applied dose and decays exponentially following a pulse (single) dose or cessation of exposure. (3) The probability of initiation is proportional to the number of O4-ethyldeoxythymidine DNA adducts resulting from DEN exposure, indicating that these adducts are the likely promutagenic lesions in DEN-induced hepatocarcinogenesis. (4) The mitotic rates of initiated and transformed cells are nonlinear with dose. (5) The average growth rate of initiated hepatocytes as a function of DEN dose is related to Druckery's slope. (6) The probability of the second genetic event (transformation) is independent of applied dose, suggesting that it is the result of a spontaneous genetic alteration.

Benzo-a-pyrene: environmental partitioning and human exposure.

A multimedia transport model was used to evaluate the environmental partitioning of benzo-a-pyrene (BaP). Measured and predicted environmental concentrations were used to estimate the accumulation of BaP in the food chain and the subsequent extent of human exposure from inhalation and ingestion. Results show that BaP partitions mainly into soil (82%) and sediment (17%) and that the food chain is the dominant pathway of human exposure, accounting for about 97% of the total daily intake of BaP. Inhalation and consumption of contaminated water are only minor pathways of human exposure. The long-term average daily intake of BaP by the general population of the U.S. is estimated to be 2.2 micrograms (micrograms) per day. Cigarette smoking and indoor activities do not substantially increase human exposure to BaP relative to exposures to background levels of BaP present in the environment. Since the increased lifetime risk associated with human exposure to background levels of BaP is 3.5 x 10(-4), we conclude that ingestion of food items contaminated with BaP may pose a serious health threat to the U.S. population.

Human exposure to dioxin.

Because 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent chemical carcinogen evaluated by the U.S. Environmental Protection Agency (EPA), many people fear that exposure to even small amounts of TCDD could lead to serious health effects. Ambient measurements confirm that environmental TCDD contamination is widespread. The public is concerned about TCDD exposure from such diverse sources as municipal solid waste incinerators, pulp and paper mills, and contaminated fish and soil. This paper evaluates several critical issues including: (i) the extent of background contamination; (ii) accumulation in the food chain and the potential for human exposure from ingesting contaminated food items; (iii) the magnitude of TCDD emissions into the U.S. environment, and the relative contribution of various known TCDD sources to the total TCDD load; and (iv) setting environmental standards for TCDD.