Dose and temporal effects on gene expression profiles of urothelial cells from rats exposed to diuron.

Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is a substituted urea herbicide that at high dietary levels (2500 ppm) induces rat urinary bladder hyperplasia after 20 weeks of exposure and neoplasia after 2 years. The effects on the urothelium after short-term exposure have not been described. The present 7-day study evaluated the dose-dependency of urothelial alterations in the urinary bladder using light microscopy, scanning electron microscopy, and genome-wide transcriptional profiling. Male Wistar rats were fed 0, 125, 500, 2500 ppm diuron for 7 days. The urinary bladder and isolated urothelial cells of these animals were processed for microscopic examination and gene expression profiling, respectively. No significant treatment-related morphologic effects were observed. The number of differentially expressed genes (DEGs) in the exposed groups increased with diuron levels. Diuron-altered genes involved in cell-to-cell interactions and tissue organization were identified in all treatment groups. After 7 days of diuron exposure, transcriptional responses were observed in the urothelium in the absence of clear morphologic changes. These morphological findings are different from those observed in a previous study in which 20 weeks of diuron exposure was associated with simple hyperplasia secondary to the persistent cytotoxicity and necrosis associated with continuous cellular regeneration. Comparison of the gene expression profiles of rats exposed to the 2500 ppm carcinogenic diuron dose for 7 days versus 20 weeks revealed few similarities between these two time points at the gene or pathway level. Taken together, these data provide insight into the dose- and temporal-dependent morphological and transcriptional changes associated with diuron exposure that may lead to the development of tumors in the rat urinary bladder.

Transcriptional profile of diuron-induced toxicity on the urinary bladder of male Wistar rats to inform mode of action.

Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is a substituted urea herbicide that induces rat urinary bladder urothelial tumors at high dietary levels (2500 ppm). The specific mode of action and molecular alterations triggered by diuron, however, have not been clarified. The present study evaluated the dose-dependent effects of mucosal alterations and transcriptional changes in the urinary bladder of rats exposed to diuron. Six-week-old male Wistar rats were treated with 0, 60, 125, 1250, and 2500 ppm of diuron in the diet for 20 weeks. Histologic examination showed urothelial hyperplasia present in rats treated with either 1250 or 2500 ppm of diuron but not 60 or 125 ppm. Comprehensive gene expression analyses of urothelial cell RNA were conducted using Affymetrix microarrays. The numbers of differentially expressed transcripts between each treatment group and control increased with diuron dose. Based on similar histology and gene expression responses, the treatment groups were regrouped into a high-dose (1250 and 2500 ppm) and low-dose group (60 and 125 ppm). These data suggest that persistent exposure to high dietary concentrations of diuron induces oxidative stress, increases cellular metabolism, and enhances cell death that is associated with sustained urothelial hyperplasia.

Cytotoxicity and regenerative proliferation as the mode of action for diuron-induced urothelial carcinogenesis in the rat.

Diuron, a substituted urea herbicide, is carcinogenic to the urinary bladder of rats at high dietary levels. Its proposed carcinogenic mode of action (MOA) includes urothelial cytotoxicity and necrosis followed by regenerative cell proliferation and sustained urothelial hyperplasia. Cytotoxicity could be induced either by urinary solids or by chemical toxicity by diuron and/or metabolites excreted in the urine. Diuron was not genotoxic in a previous single-cell gel (comet) assay, but possible cross-linking activity remained to be evaluated. The present study explored the MOA of diuron and the effect of urinary acidification on the development of urothelial lesions. Male Wistar rats were fed diuron (2500 ppm, about 130 mg/kg of body weight) either with or without NH(4)Cl 10,000 ppm to acidify the urine. Reversibility of urothelial changes was also examined. The animals were euthanized after 15, 25, or 30 weeks. Diuron-fed rats had urinary amorphous precipitate and magnesium ammonium phosphate crystals similar to control animals. Groups treated with diuron + NH(4)Cl showed decreased urinary pH and reduced amounts of urinary crystals and precipitate. Urothelial necrosis and simple hyperplasia were observed by light microscopy and scanning electron microscopy both in diuron- and in diuron + NH(4)Cl-treated groups. Cytotoxicity and proliferative changes were mostly reversible. A modified comet assay developed in vitro with Chinese hamster ovary cells showed that diuron did not induce DNA cross-links. These data suggest that cytotoxicity with consequent regenerative cell proliferation is the predominant MOA for diuron rat urothelial carcinogenesis, the cytotoxicity being chemically induced and not due to urinary solids.