PMC:3940921 / 1520-5691 JSONTXT

{"project":"2_test","denotations":[{"id":"24590311-18519952-79065493","span":{"begin":790,"end":791},"obj":"18519952"},{"id":"24590311-11756187-79065494","span":{"begin":849,"end":850},"obj":"11756187"},{"id":"24590311-12623848-79065495","span":{"begin":956,"end":957},"obj":"12623848"},{"id":"24590311-10910924-79065496","span":{"begin":979,"end":980},"obj":"10910924"},{"id":"24590311-20204543-79065497","span":{"begin":1015,"end":1016},"obj":"20204543"},{"id":"24590311-21859733-79065498","span":{"begin":1078,"end":1079},"obj":"21859733"},{"id":"24590311-19878872-79065499","span":{"begin":1093,"end":1094},"obj":"19878872"},{"id":"24590311-17213283-79065500","span":{"begin":1206,"end":1207},"obj":"17213283"},{"id":"24590311-19047139-79065501","span":{"begin":1209,"end":1210},"obj":"19047139"},{"id":"24590311-11032729-79065502","span":{"begin":1988,"end":1990},"obj":"11032729"},{"id":"24590311-17875757-79065503","span":{"begin":2311,"end":2313},"obj":"17875757"},{"id":"24590311-22566373-79065504","span":{"begin":2412,"end":2414},"obj":"22566373"},{"id":"24590311-22571447-79065505","span":{"begin":2440,"end":2442},"obj":"22571447"},{"id":"24590311-22566605-79065506","span":{"begin":2470,"end":2472},"obj":"22566605"},{"id":"24590311-22555973-79065507","span":{"begin":2893,"end":2895},"obj":"22555973"},{"id":"24590311-23138275-79065508","span":{"begin":2897,"end":2899},"obj":"23138275"},{"id":"24590311-21195056-79065509","span":{"begin":3111,"end":3113},"obj":"21195056"},{"id":"24590311-20305692-79065510","span":{"begin":3115,"end":3117},"obj":"20305692"},{"id":"24590311-20068223-79065511","span":{"begin":3119,"end":3121},"obj":"20068223"},{"id":"24590311-19157685-79065512","span":{"begin":3309,"end":3311},"obj":"19157685"}],"text":"Introduction\nChronic myeloid leukemia (CML) is a myeloproliferative neoplasm that arises in a hematopoietic stem cell. It is characterized by a reciprocal translocation between the long arms of chromosomes 9 and 22. The resulting fusion gene BCR-ABL, encodes a constitutively active protein tyrosine kinase Bcr-Abl, which is responsible for the pathogenesis of CML. Current first-line therapy for CML is imatinib mesylate (imatinib, Glivec; Novartis Pharmaceuticals), a small molecule tyrosine kinase inhibitor (TKI) specifically designed to inhibit the Bcr-Abl tyrosine kinase. Imatinib has dramatically improved the treatment of CML, inducing durable responses in the majority of patients with chronic phase CML. Nevertheless, two key issues remain—a number of patients develop resistance1 and primitive CML stem cells are insensitive to imatinib.2 The most common mechanisms of resistance are mutations in the kinase domain that affect imatinib binding,3 BCR-ABL amplification4 and altered drug efflux or influx.5 Second and third generation TKIs such as dasatinib, nilotinib6 and ponatinib7 demonstrate clinical efficacy in some cases of imatinib resistance; however, CML stem cells remain insensitive.8, 9 This highlights the need to find alternative therapeutic strategies to overcome resistance and eliminate the CML stem cell.\nThe proteasome is an enzymatic complex that has a key role in regulating cellular processes through selective degradation of intracellular proteins. There are three distinct enzymatic activities associated with the proteasome—chymotrypsin-like (CT-L), trypsin-like (T-L) and caspase-like (C-L)—mediated by subunits β5, β2 and β1, respectively. Upon exposure to interferon (IFN)-γ and tumor necrosis factor-α, an alternative form of the proteasome is formed, referred to as the immunoproteasome. The immunoproteasome expresses subunits LMP7, MECL1 and LMP2 in place of β5, β2 and β1, altering the proteasome to favor the generation of antigenic peptides.10 Over the last decade, the proteasome has emerged as a therapeutic target in hematopoietic malignancies. Bortezomib, the first-in-class proteasome inhibitor (PI) validated the proteasome as a therapeutic target and has provided significant advancement in the treatment of multiple myeloma (MM)11 and mantle cell lymphoma.12 Clinical benefit has also been seen with bortezomib-based combinations for non-Hodgkin's lymphoma,13 myelodysplastic syndromes14 and acute myeloid leukemia.15 Following bortezomib's success, there are a number of next generation PIs with improved pharmacological properties in clinical trials. The next generation compound carfilzomib is an epoxyketone-based inhibitor that binds irreversibly to the proteasome. Carfilzomib has recently been approved by the FDA for the treatment of relapsed/refractory MM and demonstrates greater efficacy and fewer side effects than bortezomib.16, 17\nA number of studies support a potential role for the use of PIs in CML. In vitro studies demonstrated that bortezomib alone and in combination with kinase inhibitors is effective in imatinib-resistant CML cells.18, 19, 20 In addition, we have previously shown that BCR-ABL activity is associated with increased proteasome activity, and that CML cell lines are more susceptible to PIs than normal counterparts.21 In this study, we evaluate the activity of carfilzomib alone and in combination with TKIs imatinib and nilotinib, using imatinib-sensitive and -resistant CML models. We demonstrate a downregulation of phosphorylated ERK and accumulation of Abelson interactor proteins 1 and 2 (ABI 1/2), along with induction of apoptosis and inhibition of proliferation by carfilzomib in imatinib-sensitive and -resistant cell lines and CD34+38−-enriched CML stem cells. We show that the combination of carfilzomib with imatinib or nilotinib results in synergistic effects, even in imatinib-resistant cell lines. Finally, we demonstrate that the immunoproteasome is a major constituent of the total proteasome in the majority of CML cell lines and primary CML cells and that the presence of immunoproteasome subunits is associated with an increased sensitivity to carfilzomib."}