The induction of malignant neoplasms in the thyroid gland of the rat

Consideration of both genotoxic and nongenotoxic mechanisms in predicting carcinogenic potential

Bacterial and cell culture genotoxicity assays have proven to be valuable in the identification of DNA reactive carcinogens because mutational events that alter the activity or expression of growth control genes are a key step in carcinogenesis. The addition of metabolizing enzymes to these assays have expanded the ability to identify agents that require metabolic activation. However, chemical carcinogenesis is a complex process dependent on toxicokinetics and involving at least steps of initiation, promotion and progression. Identification of those carcinogens that are activated in a manner unique to the whole animal, such as 2,6-dinitrotoluene, require in vivo genotoxicity assays. There are many different classes of non-DNA reactive carcinogens ranging from the potent promoter 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) that acts through a specific receptor, to compounds that alter growth control, such as phenobarbital. Many compounds, such as saccharin, appear to exhibit initiating, promotional and/or carcinogenic activity as events secondary to induced cytotoxicity and cell proliferation seen only at the chronic lifetime maximum tolerated doses mandated in rodent bioassays. Simple plus/minus vs. carcinogen/noncarcinogen comparisons used to validate the predictivity of bacterial and cell culture genotoxicity assays have revealed that a more comprehensive analysis will be required to account for the carcinogenicity of so many diverse chemical agents. Predictive assays and risk assessments for the numerous types of nongenotoxic carcinogens will require understanding of their mechanism of action, reasons for target organ and species specificity, and the quantitative dose-response relationships between endpoints such as induced cell proliferation and carcinogenic potential.

Nongenotoxic carcinogens in the regulatory environment

The biological activity of many carcinogens is to directly induce mutational events, thereby altering the information encoded in the DNA. Short-term tests for potential carcinogens and risk assessment models generally rely on the assumption that the agent in question will operate through a genotoxic mechanism. However, carcinogenesis is a multistep process, and it is increasingly clear that the primary biological effect for many carcinogenic chemicals involves events other than direct DNA reactivity. For many experimental rodent models as well as human cancers, nongenotoxic mechanisms appear to be the driving force in the formation of tumors. Many of these nongenotoxic mechanisms are highly species-specific. Thus, it is increasingly important to ask if the rodent model applies to the human situation at all, in addition to the examination of appropriate, hypothetical, mathematical risk assessment models. More research is now being focused to better define the mechanisms by which the many distinctly different classes of nongenotoxic carcinogens are acting. This understanding will become the basis for new predictive assays and more realistic risk assessment models. If specific conditions are met, then a no observed effect level with a safety factor may be the most appropriate risk model for some carcinogens.

Goitrogens and thyroid follicular cell neoplasia: evidence for a threshold process

Thyroid neoplasia can result from many different causes. These include low iodine diets, subtotal thyroidectomy, radioactive iodine, natural goitrogens such as rape seed and cabbage, chemotherapeutic agents such as sulfathiazole, and pesticides such as amitrole. All of these appear to act through either direct or indirect interference with thyroid hormone synthesis. Decreased circulating levels of thyroid hormones in the blood result in increased release of thyroid-stimulating hormone by the anterior pituitary gland. This, in turn, results in hypertrophy and hyperplasia of the thyroid without a corresponding increase in blood thyroid hormone levels. Hyperplasia of the pituitary is also observed due to increased functional demand for continued production of thyroid-stimulating hormone. After prolonged stimulation of the pituitary/thyroid axis, hyperplasia may progress to neoplasia. Cessation of exposure prior to the induction of neoplasia results in a return to the normal state. It is clear that some degree of thyroid inhibition can be accommodated within the bounds of the normal feedback mechanism without the induction of either hyperplasia or neoplasia. A threshold for thyroid follicular neoplasia is therefore indicated.