Detection of P-glycoprotein in the nuclear envelope of multidrug resistantcells

P-glycoprotein is a plasma membrane efflux pump which is responsible for mu ltidrug resistance of many cancer cell lines. A number of studies have demo nstrated the presence of P-glycoprotein molecules, besides on the plasma me mbrane, also in intracellular sites, such as the Golgi apparatus and the nu cleus. In this study, the presence and function of P-glycoprotein in the nu clear membranes of human breast cancer cells (MCF-7 WT) and their multidrug resistant variants (MCF-7 DX) were investigated. Electron and confocal mic roscopy immunolabelling experiments demonstrated the presence of P-glycopro tein molecules in the nuclear membranes of MCF-7 DX cells. Moreover, the la belling pattern was strongly dependent on pH values of the incubation buffe r. At physiological pH (7.2), a strong labelling was detected in the cytopl asm and the nuclear matrix in both sensitive and resistant MCF-7 cells. By raising the pH to 8.0, the P-glycoprotein molecules were easily detected in the cytoplasm (transport vesicles and Golgi apparatus), plasma and nuclear membranes exclusively in MCF-7 DX cells. Furthermore, drug uptake and effl ux studies, performed by flow cytometry on isolated nuclei in the presence of the P-glycoprotein inhibitor cyclosporin A, suggested the presence of a functional P-glycoprotein in the nuclear membrane, but not in the nuclear m atrix, of drug resistant cells. Therefore, P-glycoprotein in the nuclear en velope seems to represent a further defense mechanism developed by resistan t cells against antineoplastic agents.