Multidrug resistance in MCF-7 human breast cancer cells is associated with increased expression of nucleoside transporters and altered uptake of adenosine.

The rate of adenosine uptake and the corresponding expression of nucleoside transporters were studied in several MCF-7 human breast-cancer cell lines that express different levels of multidrug resistance (MDR). Kinetic studies of adenosine transport in these cell lines revealed that the mean apparent Km and Vmax values for the nucleoside transporters increased with increasing MDR. The apparent Km and the apparent Vmax of Adriamycin-resistant (ADR10) cell lines were respectively 3.2- and 1.8- fold those of Adriamycin-sensitive wild-type (WT) cells (P less than 0.001). A partially revertant cell line (ADR10rev) that was derived from the ADR10 line and was partially sensitive to Adriamycin exhibited apparent Km and Vmax parameters that lay between those of the ADR10 and WT cells (P less than 0.001 vs ADR10 cells; P less than 0.05 vs WT cells). ADR10 cell membranes bound greater than 4 times more of the nucleoside transporter blockers [3H]-nitrobenzylthioinosine [( 3H]-NBI) and [3H]-dipyridamole [( 3H]-DPR) than did WT cell membranes per unit protein (P less than 0.0001). Scatchard analysis revealed a 2-3 times greater density for nucleoside transporters in ADR10 membranes as compared with those in WT membranes. ADR10rev membranes bound less [3H]-NBI and [3H]-DPR than did ADR10 membranes (P less than 0.001), but they bound more of the blockers than did WT membranes (P less than 0.05). A 2.5-h exposure to 200 nM phorbol-12,13-dibutyrate (PDBu), which activates protein kinase C (PKC) and induces WT cells to exhibit a 4-fold increased transient MDR phenotype, increased the apparent Km of WT cells for adenosine transport by greater than 2 times (P less than 0.001) to a value close to that found for the ADR10 cells. An identical exposure of ADR10 cells to PDBu produced no significant effect. The apparent Km of ADR10rev cells was increased 1.4 times by a 2.5-h PDBu exposure. None of the cell lines were affected by a 2.5-h exposure to 200 nM phorbol-13,10-diacetate (PDA), a much less active phorbol, or vehicle. These results suggest that MDR in MCF-7 cells is associated with changes in nucleoside transport, including both the number of transporters and their rate of transport, and that such changes can be partially mimicked by stimulation of PKC.