Decreased melphalan accumulation in a human breast cancer cell line selected for resistance to melphalan

An in vitro model of acquired melphalan resistance was developed by serial incubation of an MCF-7 human breast cancer cell line in increasing concentrations of melphalan. The resulting derivative cell line, MelR MCF-7, was 30-fold resistant to melphalan. Uptake studies demonstrated decreased initial melphalan accumulation in MelR MCF-7 cells. Inverse-reciprocal plots of initial melphalan uptake revealed a 4-fold decrease in the apparent Vmax of MelR MCF-7 compared with WT MCF-7 (516 amol cell^-1 min^-1 vs 2110 amol cell^-1 min^-1 respectively) as well as a decrease in the apparent Kt (36 μM vs 70 μM respectively). Two amino acid transporters have previously been identified as melphalan transporters: system L, which is sodium-independent and inhibited by 2-amino-bicyclo[2,2,l]heptane-2-carboxylic acid (BCH), and system ASC which is sodium dependent and unaffected by BCH. At low concentrations of melphalan (3-30 μm), 1mM BCH competition eliminated the differences between the two cell lines, thus implicating an alteration of the system L transporter in the transport defect in the resistant cells. MelR MCF-7 cells were also evaluated for glutathione-mediated detoxification mechanisms associated with melphalan resistance. There was no difference between MelR MCF-7 and WT MCF-7 in glutathione content, glutathione-S-transferase activity and expression of pi class glutathione S-transferase RNA. In addition, buthionine sulfoximine did not reverse melphalan resistance in MelR MCF-7 cells. Therefore, MelR MCF-7 cells provide an in vitro model of transport-mediated melphalan resistance in human breast cancer cells.