Molecular and cellular characterization of imexon-resistant RPMI8226/I myeloma cells

Imexon is an aziridine-containing iminopyrrolidone with selective growth-inhibitory potency for multiple myeloma. Our previous research indicates that imexon induces mitochondrial alterations, oxidative stress, and apoptosis. This drug represents an interesting model drug with a nonmyelosuppressive profile to study the basic mechanisms leading to antitumor activity and resistance. The major purpose of this study was to characterize an imexon-resistant RPMI8226/I cell line that was developed from RPMI8226 cells by continuous exposure to imexon. No significant differences were observed in the sensitivity to several cytotoxic drugs, including mitoxantrone, mitomycin C, melphalan, methotrexate, cytarabine, cisplatin, vincristine, and paclitaxel, in the imexon-resistant cells. However, RPMI8226/I cells were cross-resistant to arsenic trioxide, doxorubicin, fluorouracil, etoposide, irinotecan, and especially IFN-alpha. The data from DNA microarray and Western blot analyses indicated that the levels of antiapoptotic proteins Bcl-2 and thioredoxin-2, which reside mainly in the mitochondria, are increased in RPMI8226/I cells. In addition, increased levels of lung resistance protein were detected in imexon-resistant cells. Expression of P-glycoprotein was not detected in RPMI8226/I cells. No loss of mitochondrial membrane potential or increase in the levels of reactive oxygen species was observed in RPMI8226/I cells after exposure to imexon; however, the levels of glutathione are increased in the RPMI8226/I cells. Transmission electron microscopy revealed significant changes in the mitochondrial morphology of RPMI8226/I cells, whereas no ultrastructural changes were observed in other cellular compartments. Imexon-resistant RPMI8226/I myeloma cells appear to have a unique mechanism of resistance that is associated with morphological alterations of mitochondria, increased protection against oxidative stress, elevated levels of glutathione, and enhanced expression of antiapoptotic mitochondrial proteins.