Objective:To study the changes of cell cycle phase and cell cycle phase regulation protein during arsenic trioxide induced apoptosis in hepatoma cells

and to explore mechanisms of arsenic trioxide inhibited HCC.Methods:Cultured in vitre

HCC SMMC-7721 were treated 72h with 2μmol/L arsenic trioxide.The cell cycle and apoptosis index were detected by flow cytometry（FCM）;the morphological changes were observed by transmission electron microscope.The immunohistochemistry was used to detect the protein expression of p21、cyclin D

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cyclinA.Results:SMMC-7721 cell cycle was arrested in G

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and S phase by arsenic trioxide.Compared with control in the course of apoptosis induced by arsenic trioxide p21 protein expressions rose significantly（control:1.128;arsenic trioxide:3.794）

while cyclinD

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cyclinA protein expressions decreased significantly（control 1:3.647

2.833;arsenic trioxide:1.133

1.179）.Conclusion:Arsenic trioxide could disturb cell cycle progression of SMMC-7721 and induce apoptosis.The mechanism may relate to that arsenic trioxide up-regulate expression of p21 and down-regulate expression of cyclinD

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cyclinA.