Targeting IKK Inhibits Multiple Myeloma (MM) Cell Growth in the Bone Marrow Microenvironment.

NF-κB is a transcriptional factor promoting tumor cell growth and inhibition of apoptosis via regulating expression of proteins modulating cell cycle and anti-apoptosis. NF-κB is constitutively associated with an inhibitor (IκB), which is phosphorylated by IκB kinase (IKK) upon cell stimulation (ie, TNFα) and subsequently degraded by ubiquitin-proteasome pathway, thereby allowing NF-κB to translocate to nucleus. We have previously shown that an IKK inhibitor PS-1145 partially (20–50%) inhibits MM cell proliferation; however, it inhibits both IL-6 secretion from BMSCs triggered by MM cell adhesion and proliferation of MM cells adherent to BMSCs. Targeting IKKβ is therefore a possible therapeutic option for inhibition of MM cell growth in the bone marrow microenvironment by downregulating NF-κB activity.

In this study, we further delineated the biologic significance of IKK inhibition in MM cells using IKKβ specific inhibitor MLN120B (Millennium Pharmaceuticals, Cambridge, MA). MLN120B induced 60–90% growth inhibition in cells from the MM cell lines RPMI8226, RPMI/Dox40, RPMI/LR5, U266, and INA-6; on the other hand, it induced only 25–30% inhibition in MM.1S and MM.1R cells, assessed by 72 h tritiated-thymidine uptake. Importantly, neither IL-6 nor IGF-1 overcomes the growth inhibitory effect of MLN120B in both MM.1S and U266 cells. Interestingly, MLN120B significantly augmented TNFα-induced cytotoxicity in MM.1S cells. We next examined whether MLN120B could enhance the cytotoxicity of other agents. MLN120B augmented growth inhibition triggered by doxorubicin and melphalan in RPMI8226 and IL-6 dependent INA-6 cell line. We therefore studied growth inhibitory effect of MLN120B in the presence of bone marrow stromal cells (BMSCs). MLN120B inhibited 70–80% of constitutive IL-6 secretion from BMSCs, without toxicity. Importantly, MLN120B almost completely blocked stimulation of MM.1S, U266 and INA-4 cell growth and IL-6 secretion from BMSCs induced by binding of tumor cells to BMSCs. Finally, MLN120B overcame the protective effect of BMSCs, cell adhesion mediated drug resistance, against dexamethasone in MM.1S cells. Taken together, our data demonstrate that an IKKβ inhibitor induces cytotoxicity in MM cells in the BM milieu, providing the framework for clinical trials of these novel agents to improve patient outcome in MM.