AIM:To investigate the effect of zinc protoporphyrin IX on the response of hepatoma cells to cisplatin and the possible mechanism involved.METHODS:Cytotoxicity was determined using the3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay.Apoptosis was determined by a flow cytometric assay.Western blotting was used to measure protein expression.Heme oxygenase(HO)-1 activity was measured by determining the level of bilirubin generated in isolated microsomes.Reactive oxygen species(ROS)production was monitored by flow cytometry.Caspase-3 activity was measured with a colorimetric assay kit.Mice were inoculated with 1×107 tumor cells subcutaneously into the right flanks.All mice were sacrificed 6 wk after the first treatment and tumors were weighed and measured.RESULTS:Overexpression of HO-1 in HepG2 cell line was associated with increased chemoresistance to cisdiaminedichloroplatinum(cisplatin;CDDP)compared to other cell lines in vitro.Inhibition of HO-1 expression or activity by zinc protoporphyrin IX(ZnPP IX)markedly augmented CDDP-mediated cytotoxicity towards all liver cancer cell lines in vitro and in vivo.In contrast,induction of HO-1 with hemin increased resistance of tumor cells to CDDP-mediated cytotoxicity in vitro and in vivo.Furthermore,cells treated with ZnPP IX plus CDDP exhibited marked production of intracellular ROS and caspase-3 activity,which paralleled the incidence of cell apoptosis,whereas hemin decreased cellular ROS and caspase-3 activity induced by CDDP.CONCLUSION:ZnPP IX increases cellular sensitivity and susceptibility of liver cancer cell lines to CDDP and this may represent a mechanism of increasing ROS. AIM: To investigate the effect of zinc protoporphyrin IX on the response of hepatoma cells to cisplatin and the possible mechanism involved. METHODS: Cytotoxicity was determined using the 3- (4,5-dimethylthiazol-2-yl) -2,5-diphenyltetrazolium bromide assay. Apoptosis was determined by a flow cytometric assay. Western blotting was used to measure protein expression. Heme oxygenase (HO) -1 activity was measured by determining the level of bilirubin generated in isolated microsomes.Reactive oxygen species (ROS) production was monitored by flow cytometry. Caspase-3 activity was measured with a colorimetric assay kit. Mice were inoculated with 1 × 107 tumor cells subcutaneously into the right flanks. All mice were sacrificed 6 weeks after the first treatment and tumors were weighed and measured .RESULTS: Overexpression of HO-1 in HepG2 cell line was associated with increased chemoresistance to cisdiaminedichloroplatinum (cisplatin; CDDP) compared to other cell lines in vitro. Inhibition of HO-1 expression or activit y by zinc protoporphyrin IX (ZnPP IX) markedly augmented CDDP-mediated cytotoxicity towards all liver cancer cell lines in vitro and in vivo. In contrast, induction of HO-1 with hemin increased resistance of tumor cells to CDDP-mediated cytotoxicity in vitro and in vivo.Furthermore, cells treated with ZnPP IX plus CDDP potential marked production of intracellular ROS and caspase-3 activity, which paralleled the incidence of cell apoptosis, while hemin decreased cellular ROS and caspase-3 activity induced by CDDP.CONCLUSION: ZnPP IX increases cellular sensitivity and susceptibility of liver cancer cell lines to CDDP and this may represent a mechanism of increasing ROS.