Chemical carcinogenesis – mode of action to inform quantitative human risk

The progressive public and governmental awareness that we live in a chemical world, where both natural and manmade substances impose specific risks to living organisms, raised the need for regulatory and interventional measures to protect human health. Naturally, regulatory measures depend on identification of the hazard and the risk imposed by suspected chemicals. In the absence of human data, laboratory models have been used to identify target organs, adverse effects, and critical toxicological doses, all of which contribute to subsidize specific regulations. A case study is presented to illustrate how basic research can contribute to regulatory measures aiming to control human cancer risk due to environmental contaminants. Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is a substituted urea herbicide acting through inhibition of plant photosynthesis. In a 2-yr long experiment, Wistar rats fed diuron at 2,500 ppm (about 157 mg/kg.day for both sexes) developed urothelial carcinomas in the urinary bladder. The non-observed effect level (NOEL) for carcinogenicity was 25 ppm (1.35 mg/kg.day), based on urothelial hyperplasia and tumors [1]. Accordingly, the U.S. Environmental Protection Agency evaluated diuron as a “known/likely carcinogen to humans” [1,2]. Despite this classification, the quantitative risk associated to diuron was considered of no concern [2] and the herbicide is worldwide applied on main agricultural crops such as soy, cotton and sugar cane, among other uses. The impact of chronic low-dose exposure to diuron on human health has not been determined. The standard procedure for identifying chemical carcinogens is the long-term bioassay with rodents. Although presenting a 40-year long history of usefulness, this 2-year assay bears important limitations, such as high cost and operational complexity, and also because it was not designed to provide information about the toxicological mode of action (MoA) of chemicals. As for any experiment with laboratory animals, extrapolating the results to the human situation is not a simple task, and may compromise human risk assessment. To inform risk assessment, a framework was proposed for systematic and detailed evaluation of the MoA in laboratory animals and for evaluating the human relevance of experimental results [3-5]. The genotoxic/mutagenic potentials of diuron are accepted to be negative [1,6,7]. Therefore, the carcinogenic MoA of diuron in the rat urinary bladder should be predominantly non-genotoxic. In an early study, rats fed diuron at 2,500 ppm during 20 weeks developed urothelial necrosis – observed ultrastructurally -, regenerative cell proliferation and urothelial hyperplasia [6]. To test the hypothesis that urinary crystals and amorphous precipitates could act as microabrasives to the mucosa causing cell death and regenerative hyperplasia that lead to urothelial carcinogenesis, rats were fed 2,500 ppm of diuron either with or without NH4Cl 10,000 to acidify urine during 15 to 30 weeks [7]. Groups fed diuron+NH4 showed reduced amount of urinary solids in the urine; nevertheless, urothelial necrosis and simple hyperplasia (SH), a stochastic preneoplastic urothelial lesion, were observed both in diuronand diuron+NH4Cl treated animals. Therefore, chemically induced cytotoxicity and not abrasion by urinary solids seems to be the initial key event for diuron-induced rat urothelial carcinogenesis. These urothelial lesions start at the very 1 day of exposure to 2,500 ppm and encompass cell swelling followed by necrosis, exfoliation and piling up of cells indicative of hyperplasia [8]. A recent study indicated that 500 ppm can also induce urothelial citotoxicity [9]. Regarding dose-response and the molecular pathways involved, rats were fed diuron at different concentrations Correspondence: [email protected] Department of Pathology – Faculty of Medicine, São Paulo State University, UNESP, Botucatu, São Paulo, Brazil de Camargo BMC Proceedings 2013, 7(Suppl 2):K10 http://www.biomedcentral.com/1753-6561/7/S2/K10