The proteasome load vs. capacity balance determines apoptotic sensitivity of multiple myeloma cells to proteasome inhibition

Proteasome inhibitors (PI) are effective against multiple myeloma (MM), but the mechanisms of action and bases of individual susceptibility remain unclear. Recent work linked PI sensitivity to protein synthesis and proteasome activity, raising the question whether different levels of proteasome expression and workload underlie PI sensitivity in MM cells (MMC). Exploiting human MM lines characterized by differential PI sensitivity, we report that highly sensitive MMC express lower proteasome levels and higher proteasomal workload than relatively PI-resistant MMC, resulting in the accumulation of poly-ubiquitinated proteins at the expense of free ubiquitin (proteasome stress). Manipulating proteasome expression or workload alters apoptotic sensitivity to PI, demonstrating a cause-effect relationship between proteasome stress and apoptotic responses in MMC. Intracellular immunostaining in primary, patient-derived MMC reveals that poly-ubiquitinated proteins hallmark neoplastic plasma cells, in positive correlation with Ig content, both intra- and inter-patient. Moreover, overall proteasome activity of primary MMC inversely correlates with apoptotic sensitivity to PI. Altogether, our data indicate that the balance between proteasome workload and degradative capacity represents a critical determinant of apoptotic sensitivity of MMC to PI, potentially providing a framework for identifying indicators of responsiveness and designing novel combination therapies.