Cloning and Characterization of a Tipifarnib-Resistant BCR/ABL-Positive Cell Line, K562/RR.

The results of preclinical studies and clinical trials have suggested that Tipifarnib, a farnesyltransferase inhibitor, might be effective for treating various hematological disorders, including chronic myeloid leukemia. However, period for which Tipifarnib is effective may be short term, indicating that drug resistance is an important problem for patients being treated with Tipifarnib. To analyze the mechanisms of resistance to Tipifarnib, we have generated a new Tipifarnib-resistant BCR/ABL-positive cell line, K562/RR. The IC50 of Tipifarnib was 7.4-fold higher in K562/RR than in the parental cell line K562. The level of unprocessed HDJ-2, which is a substrate of farnesyltransferase, was increased by Tipifarnib treatment in K562/RR as well as in K562 cells. This result suggests that Tipifarnib inhibits protein farnesylation in K562/RR cells in a same manner and that mechanisms independent of farnesyltransferase activity are involved in the acquisition of resistance to Tipifarnib in K562/RR cells. While the levels of cleaved caspase 3, cleaved caspase 7, cleaved caspase 9 and cleaved PARP were significantly increased in K562 cells, those were not changed in K562/RR cells with Tipifarnib treatment. Furthermore, Tipifarnib-induced decrease in the level of phospho-Bcl-2 was not observed in K562/RR cells, indicating that induction of apoptosis by Tipifarnib is much less in K562/RR than in K562 cells. Tipifarnib significantly increased the percentage of S phase cells and decreased that of G1 phase cells in K562 cells, which was probably caused by activation of G2/M check point. In contrast, addition of Tipifarnib had no remarkable effect on cell cycle in K562/RR cells, suggesting that Tipifarnib-mediated induction of cell cycle blockage is also abrogated in K562/RR cells. From DNA microarray analyses using a cDNA microarray comprising 25,000 genes, we identified 5 genes whose expression levels were higher in K562/RR than in K562 cells. These genes include _-globin, calcium channel Caveolin 2 and FEN1, which is involved in DNA replication and repair, and CUGBP2, which may affect expression of cyclogenase 1. These results suggest that various mechanisms are involved in Tipifarnib resistance in K562/RR cells and that this cell line is a useful model for investigating the farnesyltransferase activity-independent mechanisms of resistance to Tipifarnib.