Extracellular Signal-Regulated Protein Kinase 5 Mediates Resistance of Human Chronic Myeloid Leukemia K562 Cells to Imatinib.

Mutations in the Abl kinase domain and Bcr-Abl gene amplification have been recognised as the most common mechanisms of resistance to Imatinib (Gleevec) in chronic myeloid leukemia (CML). However, Imatinib resistance by Bcr-Abl independent mechanisms have also been reported. The extracellular signal-regulated protein kinase 5 (Erk5) signalling pathway is involved in cell survival and cell cycle control functions. Interestingly, it was recently reported that ERK5 is important in CML (Buschbeck M., 2005). Bcr/Abl was found to increase the level of Erk5 by stabilizing the protein thus contributing to increased cell survival. In this study we asked whether small interfering RNA (siRNA)-mediated depletion of endogenous Erk5 or overexpression of wild type Erk5 would affect the survival of Imatinib-treated human K562 CML cells. These cells express Bcr-Abl with a wild type kinase domain. Fluorescence activated cell sorting analyses of propidium iodide-stained K562 cells revealed that treatment with 1 μM Imatinib for 24 hrs led to an increase of the apoptotic cell population from 9 to 27 %. The number of cells, arrested in G_0/1 phase was also elevated following Imatinib treatment from 66 to 85 %. Overexpression of wild type Erk5 in K562 cells reduced the frequency of Imatinib-induced apoptosis from 27 to 18 % without affecting cell cycle distribution of the living cells. After having established that overexpression of Erk5 renders cells partially resistant to Imatinib treatment we tested the effect of siRNA-mediated depletion of endogenous Erk5 on the viability of K562 cells challenged with Imatinib. Indeed, 42% of K562 cells transfected with siRNA against Erk5 underwent apoptosis after Imatinib treatment. Imatinib induced apoptosis in only 20% of cells transfected with a control siRNA. Furthermore, Erk5 depletion strongly inhibited cell cycle progression in Imatinib-treated K562 cells. Thus, the cell populations in S phase and mitosis were reduced from 22 to 16 % and from 24 to 9 %, respectively. Taken together, our data and those of Buschbeck et al. suggest that ERK5 levels may strongly affect cell survival during Imatinib treatment of CML cells.