EXPRESSION AND FUNCTION OF MULTIDRUG-RESISTANCE P-GLYCOPROTEIN IN A CULTURED NATURAL-KILLER CELL-RICH POPULATION REVEALED BY MRK16 MONOCLONAL-ANTIBODY AND AHC-52

Natural killer (NK) cells have been reported recently to be the highest in expressing multidrug resistance (MDR) P-glycoprotein among normal mature lymphoid cells. Using a cultured NK cell-rich population, we have examined the expression and function of P-glycoprotein, in particular its role in NK cell-mediated cytotoxicity, by employing two MDR-reversing agents (nicardipine and AHC-52, a nicardipine analog almost devoid of calcium channel blocking activity) and monoclonal antibody against P-glycoprotein (MRK-16). The expression of P-glycoprotein was detected by flaw cytometry and polymerase chain reaction of reverse transcribed mRNA. P-glycoprotein was functional in terms of rhodamine dye excretion and its susceptibility to the MDR-reversing agents. Since the concentration of nicardipine required for 50% inhibition (IC50) of rhodamine dye excretion (2 mu M) was close to that of AHC-52 (5 mu M), it was suggested that their inhibitory effects were not due to calcium channel blocking activity, and that AHC-52 is a selective inhibitor for P-glycoprotein. The IC50 Of nicardipine for NK cell-mediated cytotoxicity (33 mu M) was also close to that of AHC-52 (26 mu M), indicating that P-glycoprotein is involved in NK cell-mediated cytotoxicity. In support of this, MRK16 inhibited NK cell-mediated cytotoxicity in a concentration-dependent manner. Both binding of target cells to NK cells and post-binding events were affected by AHC-52, suggesting that P-glycoprotein is involved in several steps in NK cell-mediated cytotoxicity.