NF-Kb Localization in Multiple Myeloma Plasmacells and Mesenchimal Cells

Several reports have clearly demonstrated that the activation of Nuclear factor-kappa B (NF-kB) is essential for the pathogenesis of MM because it regulates the expression of many proteins involved in cell survival, proliferation, and resistance to chemotherapeutic drugs. We therefore analyzed by immunohistochemistry, immunofluorescence and western blot analysis the nuclear localization of NF-κB in MM cells derived from 60 different patients with MM at presentation and in relapse, as well as in three myeloma cell lines (XG1, RPMI 8226, KMS-18). Surprisingly, nuclear localization (the active form) of NF-κB was detected in only one MM sample from a refractory MM patient and in two samples from relapsed patients, while all the other samples, including the MM cell lines, exclusively express the cytoplasmic (inactive) form of NF-κB. We next analyzed the NF-κB status in mesenchymal cells from MM patients and we found that NF-κB was clearly present in the nucleus. We have also analyzed the sensitivity of MM primary cells to different doses of the proteasome inhibitor Bortezomib, which is described to antagonize NF-κB activity. We found a consistent dose- and time-dependent antitumor activity against both chemoresistant and chemosensitive myeloma cells in all the samples analyzed, independently of NF-κB localization. In conclusion, contrary to the current dogma that indicates a constitutive activation of NF-kB in MM cells, we demonstrated that in a steady state, especially in patients at diagnosis, NF-kB resides in the cytoplasm (the inactive form) but the cells maintain the sensitivity to Bortezomib, thus indicating that block of IkB degradation is not the only mechanism responsible for proteasome inhibitor induced-apoptosis.