Mechanisms of Multidrug Resistance in HL60 Cells: Evidence That a Surface Membrane Protein Distinct from P-Glycoprotein Contributes to Reduced Cellular Accumulation of Drug1

III .6(1cells exhibiting a 140-fold increase in resistance to vincristine contain three surface membrane proteins with molecular weights of 210,000 (P210), 180,000 (HISO), and 150,000 (P150) which are highly phosphorylated in vivo and in an in vitro system in the presence of Mn2* and IT-^PJATP. These phosphorylated proteins are either absent or present in very low levels in membranes of drug-sensitive cells. Growth of the vincristine-resistant isolate in the absence of drug results in a decrease in the level of resistance and a major reduction in the phosphorylation of P210 and P180. The phosphorylation of P150 is not altered in the revertant which still exhibits substantial levels of resistance. Further studies show that P210 and P180 are highly reactive with a monoclonal antibody against P-glycoprotein. These two proteins are present in only very low levels in revertant cells. The monoclonal antibody exhibits no reactivity with PI 50. In 111.6(1cells isolated for a 25-fold increase in vincristine resistance proteins reactive with P-glycoprotein monoclonal antibody are essentially absent. PISO is however highly phosphorylated in these cells. Additional experiments using lectin binding of 32P-labeled proteins demonstrates that PI 50 has properties distinct from P210 and P180. Analysis of drug uptake patterns in the vincristineresistant isolates and the revertant shows that resistance is related to a reduced intracellular accumulation of drug. Reduced accumulation of vincristine is also found in III,60 cells isolated for resistance to Adriamycin. These cells are devoid of P-glycoprotein but contain phosphoryl ated P150. These results suggest that proteins P150, P180, and P210 may con tribute to multidrug resistance in 111.60cells through a mechanism which involves reduced cellular accumulation of drug. P180 and P210 are structurally related whereas P150 is distinct from these two proteins.