Expression of the human multidrug resistance cDNA in insect cells generates a high activity drug-stimulated membrane ATPase.

Drug-resistant tumor cells actively extrude a variety of chemotherapeutic agents by the action of the multi-drug resistance (MDR1) gene product, the plasma membrane P-glycoprotein. In this report we show that the expression of the human MDR1 gene in cultured Sf9 insect cells via a baculovirus vector generates a high activity vanadate-sensitive membrane ATPase. This ATPase is markedly stimulated by drugs known to interact with the P-glycoprotein, such as vinblastine and verapamil, and the ability of the various drugs to stimulate the ATPase corresponds to their previously observed affinity for this transporter. The drug-stimulated ATPase is not present in uninfected or mock-infected Sf9 cells, and its appearance correlates with the appearance of the MDR1 gene product detected with a monoclonal anti-MDR protein antibody and by labeling with 8-azido-ATP. The drug-induced ATPase requires magnesium ions, does not utilize ADP or AMP as substrates, exhibits a half-maximal activation at about 0.5 mM MgATP, and its maximal activity (about 3-5 mumol/mg MDR protein/min) approaches that of the well characterized ion transport ATPases. These results provide the first direct demonstration of a high capacity drug-stimulated ATPase activity of the human multidrug resistance protein and offer a new and simple assay for the investigation of functional interactions of various drugs with this clinically important enzyme.