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In northeast Thailand, the traditional habit of eating ground, raw freshwater and salt-fermented fish on a daily basis results in a local population repeatedly exposed to both liver fluke (Opisthorchis viverrini) infection and consuming nitrosamine-contaminated food from early in life. Epidemiological studies have revealed a coincident high prevalence of cholangiocarcinoma in this region and we have demonstrated in animal models that dietary contamination with nitrosamines and Opisthorchiasis are strong predisposing factors for cholangiocarcinogenesis. Thus all Syrian golden hamsters receiving a combination of subcarcinogenic doses of dimethylnitrosamine (DMN) and infection with flukes developed cholangiocarcinomas, while chemical administration or fluke infection alone did not cause cancer. Synergistic induction by chemical carcinogens and liver fluke infection was found to be related to levels of exposure to both. In this two-stage carcinogenesis model, nitrosamines are considered to act as genotoxicants exerting carcinogenic effects, while the liver flukes are assumed to play epigenetic roles. In our studies of biliary pathology related to worm burden in humans we found that while most of the subjects had worms, only a minority (25%) demonstrated a pathology of adenomatous hyperplasia, which is believed to predispose bile ducts to subsequent development of carcinomas, indicating the possible role of flukes as promoters. Biliary changes in nontumorous areas of hepatectomy specimens, including fibrosis (with or without adenomatous hyperplasia) which is found in most cases, and dysplasia in the fibrotic ducts indicate a conversion event in carcinogenesis: other factors may be required to aggravate the simple proliferation lesion so that they subsequently change to carcinomas. Furthermore, commonality in tumor phenotypes and expressions of ras p21 in both fluke related and non-fluke-related cholangiocarcinomas suggest that some similar mechanisms might be operating, at least in the relatively late stages of this multistage carcinogenesis involving the bile ducts.

Cholangiocarcinoma (CCA) is one of the most prevalent cancers in north-east Thailand and has been associated with infestation by the liver fluke Opisthorchis viverrini (OV). Two samples of 12-h overnight urine (after dosing with 500 mg proline and 200 mg ascorbic acid or 500 mg proline alone) were collected from about 100 inhabitants in five contrasting incidence areas for CCA and hepatocellular carcinoma. The incidences of CCA and hepatocellular carcinoma were not correlated with either the amount of NPRO or other nitrosamino acids, endogenous nitrosation potential (difference in NPRO levels between proline dose and proline and ascorbic acid dose), or nitrate level. However, when urinary levels of nitrosamino acids were compared in subjects living in high-risk areas, subjects who were positive for OV antibody excreted significantly more (p less than 0.01) NPRO (12.3 +/- 18.7 micrograms/12 h) after proline ingestion than those who were negative 3.5 +/- 3.2 micrograms/12 h). After ingestion of ascorbic acid, the NPRO levels in the positive subjects were significantly reduced (p less than 0.01) to 2.4 +/- 2.0 micrograms/12 h, suggesting that endogenous nitrosation of proline was inhibited. Thus, endogenous nitrosation potential estimated from the difference of NPRO and sum of nitrosamino acids excreted in the two urine samples was significantly higher in subjects positive for the OV antibody. In addition, of the representative food samples and beverages consumed frequently in high-risk areas for CCA, fermented fish and pork contained N-nitrosodimethylamine (0-26 micrograms/kg), N-nitrosopyrrolidine (0-117 micrograms/kg) and N-nitrosopiperidine (0-23 micrograms/kg).(ABSTRACT TRUNCATED AT 250 WORDS)