Mitochondrial localization of P-glycoprotein in the human breast cancer cell line MCF-7/ADM and its functional characterization

The current view of multidrug resisitance is that overexpression of membrane P-glycoprotein (P-gp) is a major causative factor. However, the controversial presence of subcellular P-gp may also participate in the drug resistance. In this study, we sought to investigate the localization and functional characterization of P-gp in mitochondria isolated from MCF-7 and doxorubicin-resistant MCF-7 (MCF-7/ADM) cells. Mitochondria were isolated and purified from the MCF-7 cell line and its resistant cells MCF-7/ADM. We used electron microscopy, western blot analysis and confocal microscopy to demonstrate the localization of P-gp in the mitochondria of MCF-7/ADM cells. Flow cytometry was used to evaluated the efflux function of mitochondrial P-gp in the presence or absence of the P-gp inhibitor cyclosporine A (CsA). Mitochondria were isolated and purified successfully and were analyzed by electron microscopy. Western blotting demonstrated the expression of P-gp in the cell membrane and purified mitochondria from MCF-7/ADM cells but not from sensitive MCF-7 cells. Immunofluorescence analysis using confocal microscopy demonstrated the localization of P-gp [labeled with green fluorescence (FITC)] to the mitochondria [labeled with red fluorescence (Mitotracker Deep red 633)] of MCF-7/ADM cells and that was absent in MCF-7 cells. Rho123 (a mitochondrial fluorescent probe) accumulation was largely reduced and efflux was strongly increased in the mitochondria of MCF-7/ADM cells compared to those of MCF-7 cells (P<0.01), and these were completely reversed in the presence of the P-gp inhibitor CsA (P<0.01). No significant changes were observed in the mitochondria of MCF-7 cells (P>0.05). P-gp is expressed in the mitochondria of doxorubicin-resistant MCF-7 cells and has an efflux function. It could be involved in multidrug resistance at the subcellular site by pumping out anticancer drugs from mitochondria to protect the function of mitochondria.