Detection of soluble P-glycoprotein in culture media and extracellular fluids

Multidrug resistance (MDR) is a unique phenomenon in cancer patients and is commonly associated with an overexpression of the human MDR gene mdr1, which encodes an energy-dependent Mr 180 kDa membrane bound protein, known as P-glycoprotein. P-glycoprotein serves as a membrane efflux to pump the drugs out of the cancer cells. Western blot analysis, using a newly generated monoclonal antibody F4 which recognizes specifically an extracellular epitope of human MDR1 P-glycoprotein, reveals that soluble P-glycoprotein is detected in the cultured media of viable adriamycin-resistant human ovarian carcinoma 2780AD cells, whereas those of the drug-sensitive parent A2780 cells contain no detectable level of soluble P-glycoprotein. Soluble P-glycoprotein also is detected in extracellular fluids of cancer patients, such as malignant ascites and serum, and is not detectable in serum samples of normal healthy individuals. The Mr of soluble P-glycoprotein is the same as that of membrane bound P-glycoprotein. The presence of soluble P-glycoprotein in extracellular fluids may provide the basis for its use as a quantitative parameter of MDR and as a means to lessen or reverse MDR.