Inhibition of protein kinase CK2 with the clinical-grade small ATP-competitive compound CX-4945 or by RNA interference unveils its role in acute myeloid leukemia cell survival, p53-dependent apoptosis and daunorubicin-induced cytotoxicity.

BACKGROUND: The involvement of protein kinase CK2 in sustaining cancer cell survival could have implications also in the resistance to conventional and unconventional therapies. Moreover, CK2 role in blood tumors is rapidly emerging and this kinase has been recognized as a potential therapeutic target. Phase I clinical trials with the oral small ATP-competitive CK2 inhibitor CX-4945 are currently ongoing in solid tumors and multiple myeloma. METHODS: We have analyzed the expression of CK2 in acute myeloid leukemia and its function in cell growth and in the response to the chemotherapeutic agent daunorubicin We employed acute myeloid leukemia cell lines and primary blasts from patients grouped according to the European LeukemiaNet risk classification. Cell survival, apoptosis and sensitivity to daunorubicin were assessed by different means. p53-dependent CK2-inhibition-induced apoptosis was investigated in p53 wild-type and mutant cells. RESULTS: CK2a was found highly expressed in the majority of samples across the different acute myeloid leukemia prognostic subgroups as compared to normal CD34+ hematopoietic and bone marrow cells. Inhibition of CK2 with CX-4945, K27 or siRNAs caused a p53-dependent acute myeloid leukemia cell apoptosis. CK2 inhibition was associated with a synergistic increase of the cytotoxic effects of daunorubicin. Baseline and daunorubicin-induced STAT3 activation was hampered upon CK2 blockade. CONCLUSIONS: These results suggest that CK2 is over expressed across the different acute myeloid leukemia subsets and acts as an important regulator of acute myeloid leukemia cell survival. CK2 negative regulation of the protein levels of tumor suppressor p53 and activation of the STAT3 anti-apoptotic pathway might antagonize apoptosis and could be involved in acute myeloid leukemia cell resistance to daunorubicin.

Protein kinase CK2 inhibition down modulates the NF-κB and STAT3 survival pathways, enhances the cellular proteotoxic stress and synergistically boosts the cytotoxic effect of bortezomib on multiple myeloma and mantle cell lymphoma cells.

CK2 is a pivotal pro-survival protein kinase in multiple myeloma that may likely impinge on bortezomib-regulated cellular pathways. In the present study, we investigated CK2 expression in multiple myeloma and mantle cell lymphoma, two bortezomib-responsive B cell tumors, as well as its involvement in bortezomib-induced cytotoxicity and signaling cascades potentially mediating bortezomib resistance. In both tumors, CK2 expression correlated with that of its activated targets NF-κB and STAT3 transcription factors. Bortezomib-induced proliferation arrest and apoptosis were significantly amplified by the simultaneous inhibition of CK2 with two inhibitors (CX-4945 and K27) in multiple myeloma and mantle cell lymphoma cell lines, in a model of multiple myeloma bone marrow microenvironment and in cells isolated from patients. CK2 inhibition empowered bortezomib-triggered mitochondrial-dependent cell death. Phosphorylation of NF-κB p65 on Ser529 (a CK2 target site) and rise of the levels of the endoplasmic reticulum stress kinase/endoribonuclease Ire1α were markedly reduced upon CK2 inhibition, as were STAT3 phospho Ser727 levels. On the contrary, CK2 inhibition increased phospho Ser51 eIF2α levels and enhanced the bortezomib-dependent accumulation of poly-ubiquitylated proteins and of the proteotoxic stress-associated chaperone Hsp70. Our data suggest that CK2 over expression in multiple myeloma and mantle cell lymphoma cells might sustain survival signaling cascades and can antagonize bortezomib-induced apoptosis at different levels. CK2 inhibitors could be useful in bortezomib-based combination therapies.

Protein kinase CK2 protects multiple myeloma cells from ER stress-induced apoptosis and from the cytotoxic effect of HSP90 inhibition through regulation of the unfolded protein response.

PURPOSE: Protein kinase CK2 promotes multiple myeloma cell growth by regulating critical signaling pathways. CK2 also modulates proper HSP90-dependent client protein folding and maturation by phosphorylating its co-chaperone CDC37. Because the endoplasmic reticulum (ER) stress/unfolded protein response (UPR) is central in myeloma pathogenesis, we tested the hypothesis that the CK2/CDC37/HSP90 axis could be involved in UPR in myeloma cells. EXPERIMENTAL DESIGN: We analyzed CK2 activity upon ER stress, the effects of its inactivation on the UPR pathways and on ER stress-induced apoptosis. The consequences of CK2 plus HSP90 inhibition on myeloma cell growth in vitro and in vivo and CK2 regulation of HSP90-triggered UPR were determined. RESULTS: CK2 partly localized to the ER and ER stress triggered its kinase activity. CK2 inhibition reduced the levels of the ER stress sensors IRE1α and BIP/GRP78, increased phosphorylation of PERK and EIF2α, and enhanced ER stress-induced apoptosis. Simultaneous inactivation of CK2 and HSP90 resulted in a synergic anti-myeloma effect (combination index = 0.291) and in much stronger alterations of the UPR pathways as compared with the single inhibition of the two molecules. Cytotoxicity from HSP90 and CK2 targeting was present in a myeloma microenvironment model, on plasma cells from patients with myeloma and in an in vivo mouse xenograft model. Mechanistically, CK2 inhibition led to a reduction of IRE1α/HSP90/CDC37 complexes in multiple myeloma cells. CONCLUSIONS: Our results place CK2 as a novel regulator of the ER stress/UPR cascades and HSP90 function in myeloma cells and offer the groundwork to design novel combination treatments for this disease.