INCREASED EFFLUX OF VINCRISTINE, BUT NOT OF DAUNORUBICIN, ASSOCIATED WITH THE MURINE MULTIDRUG-RESISTANCE PROTEIN (MRP)

The multidrug resistance protein (MRP) is a membrane protein that medi ates altered transport of cytotoxic drugs. Although MRP overexpression has been described in doxorubicin-selected human tumor cell lines, th e murine PC-V10 and PC-V40 cell lines are members of the only reported series of vincristine-selected cell lines that overexpress mrp. Weste rn blotting, using an antiserum developed against human MRP, demonstra ted high-level expression of murine MRP primarily in the plasma membra nes in each of the vincristine-selected cell lines. Only PC-V160, sele cted for high level resistance, demonstrated concomitant overexpressio n of the P-glycoprotein. As compared with parental cells, each of the drug selected cell lines demonstrated an energy-dependent, decreased n et accumulation of vincristine without any changes in the initial rate s of vincristine influx. However, there was an enhanced rate of vincri stine loss, 2.3-fold from the PC-V40 cell line and 3.9-fold from the P C-V160 cell line. Selective plasma membrane permeabilization with digi tonin equalized vincristine accumulation among the parental, the PC-V4 0, and the PC-V160 cell lines. No intracellular pH differences were de tected among the cell lines. Despite high-level MRP expression, daunor ubicin accumulation and the rate of daunorubicin loss in the PC-V40 ce lls were the same as that observed in parental PC4 cells. Fluorescence microscopy demonstrated no difference in the pattern of subcellular d aunorubicin accumulation between parental and PC-V40 cells. These stud ies demonstrate that murine MRP, overexpressed and found predominantly in the plasma membrane of vincristine-selected PC-V40 cells, is assoc iated with an energy-dependent increased efflux of vincristine, but no t with efflux or altered distribution of daunorubicin. Copyright (C) 1 996 Elsevier Science Inc.