Objective   To investigate the possible apoptotic pathway in breast cancer daunorubicin-resistant subline

MCF-7/Adr cell line.   Methods   Drug-resistance cell line MCF-7/Adr expressing functional P-gp and its wild type MCF-7/wt cell line were used for comparison. The concentration of hydrogen peroxide (H

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) that may lead to loss of mitochondrial function was determined using MTT assay. The cell morphology and the mitochondrial ultrasr

ucture were observed by invert and transmission electron microscopy

respectively. The apoptosis of the two cell lines was determined with DNA gel electrophoresis and Annexin V and PI double-stained flow cytometry. Mitochondrial membrane potential (MMP) and the function of P-gp were assessed with rhodamine 123 (Rh123) staining intensity by confocal microscopy. The release of cytochrome C from mitochondria was observed by Western blotting.   Results   The mitochondrial function of MCF-7/Adr was irreversibly damaged at the concentration of 4 mmol/L H

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. The mitochondria were vacuolated and their cristae were swollen. The percentage of MCF-7/Adr cell apoptosis was significantly higher than that of MCF-7/wt

96.52％ and 18.57％

respectively (P0.01). Only 800 and 1 600 Da DNA fragmentation was seen. The number of mitochondria and membrane potential of MCF-7/Adr mitochondria decreased as compared with those of MCF-7/wt. H

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at the concentration of 4 mmol/L induced release of cytochrome C from MCF-7/Adr mitochondria to cytoplasma

but little was detected in MCF-7/wt.   Conclusion   MCF-7/Adr cells expressing functional P-gp are more sensitive to H

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-mediated mitochondrial apoptosis. It may attribute to the changes of mitochondrial structure and function in MCF-7/Adr cells. These may suggest that some way which may modify mitochondrial characteristics may be benificial to overcome the multidrug resistance related to P-gp.