Chronic
inflammation induced by repeated
infection with Opisthorchis viverrini has been postulated to be a risk factor for
cholangiocarcinoma. To clarify the mechanism of
carcinogenesis induced by repeated O.viverrini
infection, we investigated the timecourse of
8-nitroguanine and
8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) formation,
inducible nitric oxide synthase (iNOS) expression,
nitric oxide production and pathological features in hamsters with two (2-IF) or three (3-IF) O.viverrini
infections. Inflammatory cell infiltration triggered by repeated
infection (3-IF > 2-IF > 1-IF) was earlier than by single
infection (1-IF). HPLC coupled with an electrochemical detector revealed that
8-oxodG level in the liver was the highest on day 3 in 3-IF and day 7 in 2-IF, earlier than that on day 21 in 1-IF. Notably, a double immunofluorescence study revealed that formation of
8-nitroguanine and
8-oxodG appeared to increase in the epithelium of bile ducts in the order 3-IF > 2-IF > 1-IF after the decrease in inflammatory cells. This may be explained by the fact that repeated
infection increased iNOS expression in the epithelium of bile ducts in the order 3-IF > 2-IF > 1-IF on day 90.
Proliferating cell nuclear antigen accumulated in the epithelium of bile ducts on day 90 after repeated O.viverrini
infection, supporting the hypothesis that cell proliferation was promoted by
inflammation-mediated DNA damage. In conclusion, more frequent O.viverrini
infection can induce the expression of iNOS not only in inflammatory cells but also in the epithelium of bile ducts and subsequently cause nitrosative and oxidative damage to
nucleic acids, which may participate in the initiation and/or promotion steps of
cholangiocarcinoma development.