Impaired Proteasomal Capacity and ER Stress as Therapeutic Targets Against Multiple Myeloma

Multiple Myeloma (MM) is an aggressive, debilitating and deadly hematological malignancy, arising from the clonal expansion of plasma cells at multiple sites in the bone marrow. Recently, MM proved sensitive to a new class of drugs, proteasome inhibitors (PI), currently in phase 4 clinical trial. Understanding the molecular bases by which PI induce apoptosis selectively in MM cells promises to help identify novel molecular strategies to combat MM. We explored the mechanisms underlying sensitivity of MM to PI on plasma cells differentiated in vitro from the murine B lymphoma I.29µ+ and ex vivo from primary mouse B cells. We found that in both models apoptotic sensitivity to PI directly correlates with the Ig-synthetic load. Surprisingly, as apoptosis increases, the relative amount of proteasomal subunits and the resulting proteolytic activity decrease dramatically, thus denying the demand for a higher degradative capacity when antibody production becomes maximal. Pulse-chase assays confirmed that proteasomes are overloaded in differentiating plasma cells. The excessive load for the reduced proteolytic capacity causes accumulation of poly-ubiquitinated proteins and stabilization of endogenous proteasomal substrates such as the UPR mediator Xbp1, the NF-kB inhibitor Ik-Balpha, and the pro-apoptotic Bcl-2 relatives Bim and Bax, two proteins critical in limiting B lymphocyte lifespan and activity. Accumulation of these proteins critically exaggerates endoplasmic reticulum (ER) stress, thus predisposing plasma cells to apoptosis upon treatment with PI. Importantly, a similar scenario can be reproduced in HeLa cells, a non-B tumoral line, by driving Ig heavy chain over-expression under an inducible promoter, leading to proteasomal overload, apoptotic sensitivity to proteasome inhibitors, and eventually spontaneous apoptosis, establishing a cause-effect relationship between synthetic load and cell death. Our results unveil a novel developmental program enabling plasma cells to count the integral of produced Ig, linking death to protein production, thus ending the humoral immune response upon accomplishment of its goal. Based on our data, we propose that the high efficacy of PI against MM is due, to a significant extent, to overloading the cell’s degradative capacity, a key component of the stress response, already challenged by misfolded chains generated as a side product of intense Ig synthesis. This model provides a framework for attempting to achieve tumour cell destruction through modulation of stress in MM.