Abstract: Purpose : To study the effect s of drinking water contaminated with microcystins (MC) on cell proliferation and apoptosis during the course of hepatocarcinogenesis ,and to probe the mechanism of promoting hepatocarcinogenesis of microcystic toxins. Methods : 50 healthy Wistar rat s were randomly divided into 4 groups :group A (normal cont rol) ; group B (t reated with DEN only) ; group C (t reated with DEN and MC contaminated water) ; group D (t reated with DEN and microcystins i. p ) . The PCNA , Bcl22 and Bax protein expression in preneoplastic liver tissue were detected using immunohistochemical assays. Results : The average integrated optical densities (AIOD) of PCNA in the experimental groups were 0. 0033 (group B) , 0. 0036 ( group C ) and 0. 0088 (group D) respectively , and they were all significantly increased as compared to group A (0. 0013) . But as comparing to group B , only group D had significantly higher PCNA expression. Up2regulation of Bcl22 and down-regulation of Bax expression were found in group C and group D. Conclusion : Uncont rolled cell proliferation caused by unbalanced expression of Bcl-2 and Bax gene may be one of the mechanisms of hepatocarcinoma promoted by microcystins.

Key words: Microcystin toxins, gene expression, liver neoplasms, drinking water