Background Hepatitis B virus(HBV)X protein(HBx)and p53 could mutually down-regulate at transcriptional level andHBx could bind with p53 protein within its transactivation domain and inhibit the function of p53 protein.In recent years,effects of arsenic trioxide(As_2O_3)on the expression of p53 protein have been widely studied,while little is known aboutthe activity of p53 protein.This study was undertaken to delineate the effect of HBV X gene and As_2O_3 on p53 proteinexpression(level and activity)in HepG2 cells by small hairpin RNA(shRNA)-mediated RNA interference(RNAi)technique.Methods Cell line HepG2 and cells with stable expression of HBV X gene(HepG2-X)were treated with 2 μmol/L As_2O_3,with corresponding untreated cells serving as controls.Cell lysates and nuclear extracts were extracted.Total level andthe relative activity of p53 protein were detected by modified enzyme-linked immunosorbent assay(ELISA).HBV X genesequence-specific shRNA expression vector(pXi-1 and pXi-2)and sequence-unrelated control(pXi-3)were transfectedinto HepG2-X.Single cell clone with stable expression of shRNA was selected and exposed to propagating culture.Theeffect of As_2O_3 on p53 protein expression and activity was re-observed.Results Total p53 protein level was up-regulated and its relative activity ratio was enhanced by As_2O_3 in HepG2 andHepG2-X cells.The total p53 protein level induced by As_2O_3 was up-regulated by HBV X gene expression,while itsrelative activity was significantly suppressed.The suppression was removed after HBV X gene expression wasrepressed by shRNA.Conclusions As_2O_3 up-regulates p53 protein expression and enhance its activity.HBV X up-regulates As_2O_3induced-p53 protein expression while suppresses its activity. Background Hepatitis B virus (HBV) X protein (HBx) and p53 could be down-regulated at transcriptional level and HBx could bind with p53 protein within its transactivation domain and inhibit the function of p53 protein. Recent years, effects of arsenic trioxide (As_2O_3 ) on the expression of p53 protein have been widely studied, while little is known about the activity of p53 protein. This study was undertaken to delineate the effect of HBV X gene and As_2O_3 on p53 proteinexpression (level and activity) in HepG2 cells by small hairpin RNA (shRNA) -mediated RNA interference (RNAi) technique. Methods Cell line HepG2 and cells with stable expression of HBV X gene (HepG2-X) were treated with 2 μmol / L As_2O_3, with corresponding untreated cells serving as controls. Cell lysates and nuclear extracts were extracted.Total level and the relative activity of p53 protein were detected by modified enzyme-linked immunosorbent assay (ELISA). HBV X genesequence-specific shRNA expression vector (pXi-1 and pXi-2) and seq uence-unrelated control (pXi-3) were transfectedinto HepG2-X.Single cell clone with stable expression of shRNA was selected and exposed to propagating culture. The effect of As_2O_3 on p53 protein expression and activity was re-observed. Results Total p53 protein level was up-regulated and its relative activity ratio was enhanced by As_2O_3 in HepG2 andHepG2-X cells.The total p53 protein level induced by As_2O_3 was up-regulated by HBV X gene expression, while its relative activity was significantly suppressed. The suppression was removed after HBV X gene expression wasrepressed by shRNA.Conclusions As_2O_3 up-regulates p53 protein expression and enhance its activity. HBV X up-regulates As_2O_3induced-p53 protein expression while suppresses its activity.