Evaluation of the carcinogenic potential of pesticides. 1. Acifluorfen

Generation and characterization of human U-2 OS cell lines with the CRISPR/Cas9-edited protoporphyrinogen oxidase IX gene

In humans, disruptions in the heme biosynthetic pathway are associated with various types of porphyrias, including variegate porphyria that results from the decreased activity of protoporphyrinogen oxidase IX (PPO; E.C.1.3.3.4), the enzyme catalyzing the penultimate step of the heme biosynthesis. Here we report the generation and characterization of human cell lines, in which PPO was inactivated using the CRISPR/Cas9 system. The PPO knock-out (PPO-KO) cell lines are viable with the normal proliferation rate and show massive accumulation of protoporphyrinogen IX, the PPO substrate. Observed low heme levels trigger a decrease in the amount of functional heme containing respiratory complexes III and IV and overall reduced oxygen consumption rates. Untargeted proteomics further revealed dysregulation of 22 cellular proteins, including strong upregulation of 5-aminolevulinic acid synthase, the major regulatory protein of the heme biosynthesis, as well as additional ten targets with unknown association to heme metabolism. Importantly, knock-in of PPO into PPO-KO cells rescued their wild-type phenotype, confirming the specificity of our model. Overall, our model system exploiting a non-erythroid human U-2 OS cell line reveals physiological consequences of the PPO ablation at the cellular level and can serve as a tool to study various aspects of dysregulated heme metabolism associated with variegate porphyria.

β-catenin Mutations Are Not Involved in Early-stage Hepatocarcinogenesis Induced by Protoporphyrinogen Oxidase Inhibitors in Mice

We previously reported the contribution of constitutive androstane receptor (CAR) in cytotoxicity-related hepatocarcinogenesis induced by oxadiazon (OX) or acifluorfen (ACI), two pesticides categorized as protoporphyrinogen oxidase (PROTOX) inhibitors. The molecular characteristics of preneoplastic and neoplastic lesions induced by OX and ACI were immunohistochemically compared to those by phenobarbital (PB), a typical CAR activator, in wild-type (WT) and CAR knockout (CARKO) mice after diethylnitrosamine initiation. We focused on changes in β-catenin and its transcriptional product glutamine synthetase (GS). In PB-promoted foci and adenomas, nuclear accumulation of mutated β-catenin was increased with high frequency. PB treatment also increased the multiplicity and area of GS-positive foci and adenomas in WT mice. No foci and adenomas showed nuclear accumulation of β-catenin and expression of GS in CARKO mice, similar to both genotypes of mice treated with OX and ACI. Interestingly, hepatocellular carcinoma induced in ACI-treated WT mice showed nuclear accumulation of β-catenin and was positive for GS. Our results indicated that β-catenin mutations were not involved in early-stage hepatocarcinogenesis induced by PROTOX inhibitors in mice, although activation of β-catenin and CAR is important in PB-induced tumorigenesis. The significant differences in molecular profiles suggested involvements of multiple mode of actions for hepatocarcinogenesis induced by PROTOX inhibitors.

Epidemiological studies on the distribution and determinants of biliary tract cancer

With the help of my colleagues, I have been conducting epidemiological studies on biliary tract cancer (BTC), including gallbladder cancer (GBC) and extrahepatic bile duct cancer (BDC), in Japan and Chile for about 19 years. Clustered areas with high mortality rates, especially for female GBC were found to correspond with places or prefectures in Japan that were famous for rice production. The roles of known risk factors, such as gallstones and cholecystitis, were examined, but no single factor was implicated in the high mortality rates for GBC in these areas. A working hypothesis, called the "rice production hypothesis" was formulated; this initial hypothesis was replaced by a new multifactorial causation hypothesis: GBC is more likely to occur in individuals with a genetic susceptibility and a past history of gallstones or cholecystitis who are exposed to geographically specific environmental factors, such as agricultural chemicals. On the basis of various analytical studies, it is concluded that a certain agricultural chemical was responsible for the occurrence of GBC. At the time of writing, no evidence has been obtained to disprove our hypothesis. We have also conducted international collaborative studies in Chile, which has the highest mortality rate for GBC in the world. Bile from Chileans was found to have a higher mutagenic activity than that from Japanese subjects; Chileans with a history of constipation or a habit of consuming red chilli pepper had a high risk of developing GBC, if they also had gallstone(s). The presence of a regional difference in p53 mutagenesis was also observed.

Liver preneoplastic changes in mice treated with the herbicide fomesafen

1. Effect of the diphenyl ether herbicide fomesafen on liver preneoplastic changes and porphyrin biosynthesis was examined in male C57BL/6J mice (0.23% in the diet for 14 months) and ICR mice (0.3% in the diet for 50 weeks). Fomesafen treatment resulted in preneoplastic changes (liver nodules and foci of altered hepatocytes) in both strains, uroporphyria developed only in ICR mice. 2. Iron pretreatment (600 mg/kg as a single dose) accelerated the development of fomesafen-induced preneoplastic changes in both mouse strains. The number of foci containing altered hepatocytes, as well as the number and size of liver nodules, were increased in iron-pretreated animals. 3. A single injection of iron induced marked uroporphyria in C57BL/6J mice after 14 months (liver porphyrin content 102 nmol/g). This uroporphyria was further potentiated by fomesafen administration (208 nmol/g). 4. In ICR mice, liver histology was apparently normal after a 3 month recovery from fomesafen treatment (0.32% for 9 months). Liver porphyrin content (260 nmol/g) started to decrease immediately after fomesafen withdrawal, but was still significantly elevated after 3 months (5 nmol/g), as compared to controls (1 nmol/g). 5. It is concluded that the toxicological evaluation of fomesafen should focus on liver porphyrin biosynthesis.

Evaluation of the carcinogenic potential of pesticides. 3. Aliette

Aliette, a fungicide compound, was evaluated for carcinogenic potential by the Health Effects Division of the Office of Pesticide Programs using a consensus peer review process and EPA's guidelines for risk assessment. Aliette was categorized as a group C (possible human) carcinogen based upon evidence of an increased incidence of combined benign and malignant urinary bladder tumors in a single study involving male Charles River (CR) CD rats. The bladder tumors occurred only at the unusually high top dose level of aliette that was tested (40,000/30,000 ppm). The compound was not carcinogenic in female CR-CD rats in the same study, or in CD-1 mice of either sex in a second study. Monosodium phosphite, the main urinary metabolite of aliette, was also not carcinogenic in male or female CR-CD rats. Aliette was not demonstrated to be genotoxic. No structural analogues of aliette were identified. The mechanism of action for the production of bladder tumors was not identified; however, it did not appear to involve a genotoxic effect, a carcinogenic effect of metabolites, or the formation of renal stones. The data were not found to be sufficient to quantify human cancer risk from aliette.

Evaluation of the carcinogenic potential of pesticides. 2. Methidathion

The carcinogen potential of methidathion, a dimethoxyorganic phosphorus pesticide and cholinesterase inhibitor, was evaluated by the Health Effects Division of the Office of Pesticide Programs using a consensus peer review process and the EPA's guidelines for risk assessment. Methidathion was categorized as a Group C (possible human) carcinogen based upon evidence of an increased incidence of benign and malignant hepatocellular tumors, alone and in combination, in a single study involving male Chr-CD-1 mice. The compound was not carcinogenic in female Chr-CD-1 mice in the same study or in Sprague-Dawley rats of either sex in a second study. Methidathion was not genotoxic in a variety of in vitro or in vivo tests designed to detect DNA damage, chromosome aberrations, gene mutations, and sister chromatid exchange. Although methidathion was identified as being structurally similar to two other organophosphate insecticides, prothidathion and lythidathion, no toxicological data were available on either of these agents for comparative purposes. The biological information on methidathion was reviewed by the agency's FIFRA Scientific Advisory Panel who agreed with the category C designation for methidathion. The data were not found to be sufficient to quantify human risk to methidathion.