Although imatinib therapy of chronic myelogenous leukemia is effective, the resistance to imatinib challenges the treatment of this disease. Therefore, search of novel drugs to overcome imatinib resistance is a critical issue in clinic. Withaferin A (WA), an extract of Withania somniferia, exhibits anticancer activity on a number of solid tumors. In this study, we investigate the effect of WA on imatinib-sensitive and -resistant CML cells.

WA at low concentrations induces autophagy in imatinib- sensitive K562 cells. Co-treatment of chloroquine suppresses autophagy and switches WA-treated K562 cells to apoptosis. This data indicates that autophagy protects K562 cells from apoptosis induced by WA. However, we find that WA triggers caspase activation and apoptosis in imatinib-resistant T315I-positive cells and this effect is associated with downregulation of Akt activity. Treatment of the AKT inhibitor LY294002 also causes apoptosis in imatinib-resistant T315I- positive cells. Ectopic expression of constitutively active Akt reverses WA-induced apoptosis and caspase activation in imatinib-resistant T315I-positive cells. Molecular study demonstrates that WA represses the Akt signaling pathway by decreasing Akt expression. We find that WA causes Akt degradation through the ubiquitin- and proteasome-dependent pathway. More importantly, WA also induces AKT downregulation and apoptosis in primary CML cells.

Taken together, our results suggest that imatinib-resistant T315I-positive cells are more addicted to Akt-dependent survival pathway and are more sensitive to WA. Therefore, WA can be useful for the treatment of imatinib-resistant CML.

Citation Information: Clin Cancer Res 2010;16(14 Suppl):A4.