Down-Regulation of NF-κB and STAT3 Signaling Pathways Upon Protein Kinase CK2 Inhibition Empowers Bortezomib-Induced Multiple Myeloma Cell-Death

Abstract 1849

Multiple Myeloma (MM) malignant plasma cells can be induced to die by blocking the proteasome. Bortezomib (BZ), a first-in class proteasome inhibitor of wide clinical use in MM patients, causes MM cell apoptosis through different mechanisms; however, the means of resistance to its effects are poorly recognized. Our group identified protein kinase CK2 as a critical survival molecule for MM cells (Piazza FA et al., 2006, Blood, 108(5):1698–707). This kinase regulates pivotal apoptosis-related pathways in cancer cells; however, it is currently unknown whether CK2 could be involved downstream proteasome inhibition. Intriguingly, phase I clinical trials are currently ongoing with an oral ATP-competitive CK2 inhibitor in MM and other tumors. We have here sought to investigate whether CK2 takes part in BZ-induced MM cell apoptosis and we studied whether blocking CK2 could influence pro-survival signalling pathways, which could account for MM cell resistance to BZ and chemotherapy.

MM cell lines U-266, RPMI-8226 and INA-6, human bone marrow stromal cells and freshly isolated plasma cells from patients were cultured and exposed to BZ and CK2 inhibitors K27 and CX4945 for different time points. Annexin V and propidium iodide staining, evaluation of mitochondrial membrane potential depolarization and western blot (WB) analysis of PARP cleavage and apoptosis-related proteins expression were the assays employed to assess cell growth and viability upon the different treatments.

We found that the rate of BZ-induced MM cell apoptosis was significantly increased by the simultaneous inhibition of CK2 and the proteasome in all the MM models tested and mitochondrial membrane potential measurements revealed that CK2 inhibition enhanced BZ-triggered intrinsic apoptotic cascade. Importantly, the combination of CK2 inhibitors and BZ resulted in a synergic growth-suppressive action. WB and RT-PCR analysis revealed that survival-signalling pathways associated with STAT3 and NF-κB were activated by BZ, which also caused a rise in the levels of the unfolded protein response-associated kinase/endoribonuclease IRE1α. These effects could represent unwanted side consequences of BZ treatment and could lend MM cells the ability to escape the cytotoxic effects of this drug. CK2 inhibition produced a strong reduction of phospho Ser 536 and phospho Ser 529 p65 NF-κB subunit, phospho Ser 727 STAT3 and IRE1α levels in MM cells. Remarkably, the simultaneous treatment with BZ and CK2 inhibitors was accompanied by a significant reduction of BZ-triggered p65 NF-κB and STAT3 activation and IRE1α protein levels.

These results indicate that protein kinase CK2 protects from BZ-induced apoptosis and modulates pivotal signaling pathways in MM cells, such as the NF-κB and STAT3 cascades, which could otherwise be exploited in the selection of BZ-resistant MM cell clones. Our findings suggest that CK2 inhibition could offer a rational therapeutic option when designing novel BZ-based anti-MM combination therapies.