Bortezomib induces canonical nuclear factor-κB activation in multiple myeloma cells

Bortezomib is a proteasome inhibitor with remarkable preclinical and clinical antitumor activity in multiple myeloma (MM) patients. The initial rationale for its use in MM was inhibition of nuclear factor (NF)-kappa B activity by blocking proteasomal degradation of inhibitor of kappa B alpha (I kappa B alpha). Bortezomib inhibits inducible NF-kappa B activity; however, its impact on constitutive NF-kappa B activity in MM cells has not yet been defined. In this study, we demonstrate that bortezomib significantly down-regulated I kappa B alpha expression and triggered NF-kappa B activation in MM cell lines and primary tumor cells from MM patients. Importantly, no inhibition of p65 ( RelA) nuclear translocation was recognized after bortezomib treatment in a murine xenograft model bearing human MM cells. Bortezomib-induced NF-kappa B activation was mediated via the canonical pathway. Moreover, other classes of proteasome inhibitors also induced I kappa B alpha down-regulation associated with NF-kappa B activation. Molecular mechanisms whereby bortezomib induced I kappa B alpha down-regulation were further examined. Bortezomib triggered phosphorylation of I kappa B kinase (IKK beta) and its upstream receptor-interacting protein 2, whereas IKK beta inhibitor MLN120B blocked bortezomib-induced I kappa B alpha down-regulation and NF-kappa B activation, indicating receptor-interacting protein 2/IKK beta signaling plays crucial role in bortezomib-induced NF-kappa B activation. Moreover, IKK beta inhibitors enhanced bortezomib-induced cytotoxicity. Our studies therefore suggest that bortezomib-induced cytotoxicity cannot be fully attributed to inhibition of canonical NF-kappa B activity in MM cells. (Blood. 2009; 114: 1046-1052)