Evidence for cytoplasmic P-glycoprotein location associated with increased multidrug resistance and resistance to chemosensitizers

A new human myeloma cell line, 8226/MDR10V, was selected from a P-glycoprotein-positive cell line, 8226/Dox40, in the continuous presence of doxorubicin and verapamil. MDR10V cells are 13-fold more resistant to doxorubicin and 4-fold more resistant to vincristine than the parent cell line, Dox40. Chemosensitizers are also less effective in reversing resistance in the MDR10V compared to the Dox40 cells. Despite higher resistance to cytotoxic agents, MDR10V expresses 40% less P-glycoprotein in the plasma membrane compared to Dox40; however, total cellular P-glycoprotein is the same in both cell lines. Confocal immunofluorescence microscopy shows 2.5-fold more P-glycoprotein in the cytoplasm of MDR10V cells as compared to Dox40 cells. The cytoplasmic location of P-glycoprotein in the MDR10V cells is associated with a redistribution of doxorubicin. In Dox40 cells, doxorubicin is concentrated in the nucleus, whereas in MDR10V cells, 90% of doxorubicin is found in the cytoplasm. In the presence of equivalent intracellular doxorubicin, there was a decrease in DNA-protein crosslinks in the MDR10V cell line compared to the Dox40 cell line. This finding is in agreement with the intracellular doxorubicin fluorescence studies showing less doxorubicin in the nuclei of MDR10V cells compared to Dox40 cells. Verapamil is less effective in increasing doxorubicin accumulation in the nuclei of MDR10V cells compared to Dox40 cells. Processing of P-glycoprotein from the endoplasmic reticulum to the medial Golgi was identical between the two cell lines as determined by endoglycosidase H sensitivity of newly sensitized P-glycoprotein. No mutations were found in MDR1 cDNA from MDR10V cells compared to Dox40 cells. These results suggest that resistance to chemosensitizing agents plus cytotoxic drugs is associated with a redistribution of P-glycoprotein from the plasma membrane to the cytoplasm, which in turn reduces the amount of cytotoxic drug reaching the nucleus.