The degranulation test. Six tests for carcinogenicity

A Comprehensive, Integrated Review and Evaluation of the Scientific Evidence Relating to the Safety of the Herbicide 2,4-D

The safety of 2,4-D to farm and forestry workers, commercial applicators and the general public has been of continuing concern because certain epidemiological studies of groups potentially exposed to 2,4-D have suggested a relationship between 2,4-D use and increased risk of soft tissue sarcoma, Hodgkin's disease or non-Hodgkin's lymphoma. This review on 2,4-D is unique in that the approach taken was to integrate data from worker exposure studies, whole animals, metabolic and other relevant laboratory studies with the epidemiological findings to assess the extent to which there is scientific support for the hypothesis that 2,4-D exposure is associated with any increased risk of human cancer.

The case-control epidemiological studies that have been the source of the cancer risk hypothesis are inconclusive. Problems in assessing exposure based on patients' memories make these studies difficult to interpret. Cohort studies of exposed workers do not generally support the specific hypothesis that 2,4-D causes cancer. Taken together, the epidemiological studies provide, at best, only weak evidence of an association between 2,4-D and the risk of cancer.

Importantly, the cancer hypothesis is not supported by other data. A critical evaluation of the exposure data indicates that exposure to 2,4-D in user groups is intermittent and much lower than the doses tested chronically in long-term animal studies that have not shown evidence of tumor induction. Moreover, the structure of 2,4-D does not suggest it would be a carcinogen. 2,4-D is a simple organic acid, that is largely excreted unchanged, and there is no evidence that it is metabolized to critically reactive metabolites or accumulates in tissues. This evidence is supported by a large body of negative studies on genotoxicity, which taken together with the metabolic studies, clearly indicates that 2,4-D is highly unlikely to be a genotoxic carcinogen. Furthermore, 2,4-D has no known hormonal activity and does not induce proliferative changes in any tissue or organ, indicating that it does not possess any of the characteristics of non-genotoxic animal carcinogens. Thus the available mechanistic studies provide no plausible basis for a hypothesis of carcinogenicity.

In this review, data relating to potential neurotoxicity, immunotoxicity and reproductive toxicity also were evaluated. There is no evidence that 2,4-D adversely affects the immune system and neurotoxic and reproductive effects only have been associated with high toxic doses that would not be encountered by 2,4-D users.

Historical exposures to 2,4-D by user groups, particularly farmers, forestry workers and commercial applicators, would be higher than those sustained under present rigorous standards for application which involve the use of protective clothing and other measures to reduce exposure. Proposed label changes indicate that in the future exposures will be even further reduced. Viewed in this context, the available data indicate that the potential public health impact of 2,4-D, including the risk of human cancer, was negligible in the past and would be expected to be even smaller in the present and future.

Effects of toxic chemicals on the release of pyrimidine compounds in cell culture

Exposure of hamster embryo cells and BF lymphoblastoid cells to 18 known toxic substances and four nominally nontoxic substances results in the release of pyrimidines (and their nucleosides) into the culture medium. The extent of release is dependent on the specific chemical and the specific cells present in the assay. BF cells are not affected by exposure to benzo(a)pyrene, while the hamster embryo cells exhibit enhanced excretion on exposure to benzo(a)pyrene. This difference in response may be due to the difference in endogenous aryl hydrocarbon hydroxylase (BaP) activity. In contrast, diethylstilbestrol, which is metabolized by a peroxidase-mediated enzyme system, causes enhanced excretion in both cell types. Direct alkylating agents and Ni(+2) salts also cause enhanced excretion in both cell types. We have used concentrations of chemicals that give a 5% enhanced excretion as the criterion of low-dose response. Within the range of concentrations tested, chromate induces enhanced excretion in BF cells but not the HEC cells, and Pb(+2) induces enhanced excretion in HEC cells but not the BF cells. Benzene, dimethylnitrosamine, and Mg(+2) did not affect either cell type. 7,12-Dimethylbenzo(a)anthracene, anthracene, benzo(a)anthracene, phenylazoaniline, N-methyl, N-nitroso, N'-nitroguanidine, dioxane, and pyrene cause enhanced excretion in the hamster embryo cells while benzo(e)pyrene, ZnSO4 and cholesterol do not cause enhanced excretion in the hamster embryo cells. Of those chemicals causing enhanced excretion, the concentration range bracketing 5% enhanced excretion approximated low-dose exposures reported to result in toxic responses like cancer, teratogenesis or pulmonary disease.

Toxicological aspects of cyclamate and cyclohexylamine

In the late 1960s the artificial sweetener cyclamate was implicated as a bladder carcinogen in rats. This finding and other concerns about its safety ultimately led to a ban on cyclamate in the U.S. and restrictions on its use in many other countries. Since that time, the carcinogenic potential of cyclamate and cyclohexylamine, its principal metabolite, has been reevaluated in a group of well-controlled, well-designed bioassays that have failed to substantiate the earlier findings. This review of the published and unpublished literature on cyclamate attempts to evaluate the carcinogenicity question and other important aspects of the toxicity of cyclamate and cyclohexylamine, including their effects on various organ systems, their genotoxic potential, and their effects on reproduction. In addition, the physiological disposition of cyclamate is reviewed, with particular attention directed toward the site and extent of its conversion to cyclohexylamine.

Studies on the degranulation test for carcinogens

The radiometric assay of degranulation of the hepatic endoplasmic reticulum by chemical carcinogens has been re-examined. Both 1,2,3,4,- and 1,25,6-dibenzanthracenes caused degranulation of rough membranes in vitro; with acetamidofluorenes and naphthylamines the carcinogenic analogues caused moderately greater degranulation. Degranulation by 1,2,3,4-dibenzantracene was rapid and was maximal after 5 min incubation. Pretreatment of animals with phenobarbital or methylcholanthrene increased the fraction of rough membranes, but these were not fully granulated. The assay is limited in specificity and sensitivity because the 1.35 M sucrose gradient does not effectively separate rough and smooth membranes, and sedimented membranes are contaminated with aggregates of free ribosomes.