EFFECTS OF MEMBRANE-POTENTIAL VERSUS PH(I) ON THE CELLULAR RETENTION OF DOXORUBICIN ANALYZED VIA A COMPARISON BETWEEN CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR (CFTR) AND MULTIDRUG-RESISTANCE (MDR) TRANSFECTANTS

Recently (Wei et al., Biophys J 69: 883-895, 1995), several 3T3/hu cys tic fibrosis transmembrane conductance regulator (CFTR) transfectant c lones were found to exhibit a low-level multidrug resistance (MDR) phe notype. This phenotype is similar, but not identical to that found for MDR transfectants not previously exposed to chemotherapeutic drugs. B oth MDR and CFTR transfectants are depolarized (exhibit lower plasma m embrane Delta Psi), but the former have alkaline pH(i) whereas the lat ter are acidic. It has been proposed (Roepe er al., Biochemistry 32: 1 1042-11056, 1993) that both decreased Delta Psi and increased pH(i) co ntribute to altered cellular retention of chemotherapeutic drugs in MD R tumor cells, but the relative contribution of each to altered cellul ar drug accumulation, drug retention, and drug efflux has not been stu died in detail. We therefore examined doxorubicin transport for hu CFT R and mu MDR 1 transfectants using sensitive continous monitoring of f luorescence techniques. Both CFTR and MDR transfectants exhibited sign ificantly reduced doxorubicin accumulation, relative to drug-sensitive control cells. Plots of the initial rate of accumulation versus doxor ubicin concentration were linear for the control cells and the CFTR an d MDR transfectants between 0.1 to 0.5 mu M drug, but better fit by a quadratic between 0.1 to 1.5 mu M drug. The slopes of these curves wer e proportional to measured Delta Psi. Low-level selection of either CF TR or MDR transfectants with chemotherapeutic drug did not decrease fu rther the initial rate of drug accumulation or change Delta Psi. Accum ulation experiments for control cells performed in the presence of var ious concentrations of K+ further suggests that the rate of accumulati on is related to Delta Psi. By measuring the kinetics of doxorubicin r elease for CFTR and MDR transfectants preloaded with drug, we conclude d that alkaline pH(i) perturbations are more important for determining relative intracellular binding efficiency. We also concluded, similar to the case previously made for MDR protein (Roepe, Biochemistry 31: 12555-12564, 1992) that CFTR overexpression does not enhance the rate of drug efflux. These data better define the role of lowered Delta Psi and elevated pH(i) in altering the cellular retention of doxorubicin in MDR tumor cells.