Chronic myeloid leukemia (CML) is a myeloproliferative disease characterized by the BCR/ABL oncogene and an increased survival of leukemic cells. The BCR-ABL tyrosine kinase inhibitor imatinib has successfully been introduced as a treatment of CML. However, resistance after an intitial response is common in patients with advanced disease, and it is not yet clear if responses in early disease phases will be durable. Therefore, current studies focus on novel potential drug-targets in CML cells. We have recently identified heme oxygenase-1 (HO-1) as a novel BCR/ABL-dependent survival-molecule in primary CML cells. In this study, we analyzed signal transduction pathways underlying BCR/ABL-induced expression of HO-1 and evaluated the role of HO-1 as a potential new target of drug therapy. We found that the PI3-kinase inhibitor LY294002 and MEK inhibitor PD98059 downregulate expression of HO-1 in CML cells. In addition, constitutively active Ras- and Akt -mutants were found to promote expression of HO-1 in Ba/F3 cells, further supporting the involvement of the PI3-kinase/Akt as well as the MAPK pathway in regulating HO-1 expression. To establish a role for HO-1 in survival of CML cells, expression of HO-1 was silenced by siRNAs which resulted in apoptosis of K562 cells. Next, HO-1 was targeted in CML cells by pegylated zinc protoporphyrin (PEG-ZnPP), a competitive inhibitor of HO-1. Exposure to PEG-ZnPP resulted in growth inhibition and induction of apoptosis in primary CML cells as well as in the CML-derived cell lines K562 and KU812 with IC50 values ranging between 1–10 μM. The growth-inhibitory effects of PEG-ZnPP were not only observed in CML cells responsive to imatinib, but also in imatinib-resistant K562 cells and Ba/F3 cells expressing various imatinib-resistant mutants of BCR/ABL (T315I, E255K, M351T, Y253F, Q252H, H396P). Moreover, imatinib and PEG-ZnPP were found to exert synergistic growth inhibitory effects on imatinib-resistant leukemic cells. Together, these data suggest that HO-1 represents a novel drug target in cells expressing BCR/ABL, including those with resistance to imatinib.

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