Down-Regulation of CD98 in Melphalan-Resistant Myeloma Cells with Reduced Drug Uptake

Although melphalan has been used as a therapeutic reagent for multiple myeloma, many patients become refractory. To elucidate the mechanism of resistance to melphalan, we generated a melphalan-resistant myeloma cell line, KHM-11EMS, by treating a parental line, KHM-11, with a mutagen, ethylmethanesulfonate. KHM-11EMS is 55 times more resistant to melphalan. (gamma)-Glutamylcysteine synthetase, P-glycoprotein, multidrug-resistance-associated protein, lung-resistance-related protein and the Bcl-2 family of proteins were not responsible for the drug resistance in KHM-11EMS. Intracellular incorporation of melphalan to myeloma cells was determined by using [14C]-labeled melphalan. Accumulation of melphalan in KHM-11EMS was 43% of KHM-11, while the efflux rates were comparable in both cell lines. The uptake of melphalan was inhibited by the addition of L-phenylalanine, indicating that melphalan is incorporated through the L-phenylalanine transporter as reported previously. Expression of CD98, which was recently cloned as an L-phenylalanine transporter, was 6-fold decreased in KHM-11EMS, suggesting that CD98 may be correlated with the incorporation of melphalan. CD98 expression and incorporation of melphalan were analyzed in fresh purified myeloma cells from 5 patients. All myeloma cells from 4 cases expressed CD98 at a high level and incorporated melphalan. However, tumor cells from 1 case expressed CD98 at low levels and did not incorporate melphalan. Taken together, reduced melphalan uptake could be responsible for the drug resistance in KHM-11EMS, and down-regulation of CD98 may be related to this phenomenon. Further investigation of the correlation between impaired drug uptake and downregulation of CD98 in myeloma cells should be important to understand the mechanism of resistance to melphalan.