A proteasome inhibitor, bortezomib (BTZ), was initially reported as an inhibitor of the NF-κB pathway, which plays a critical role in the pathogenesis of multiple myeloma (MM). The NF-κB activity of MM cells is mediated via two distinguishable pathways, canonical and non-canonical, which showed opposing action after BTZ treatment in MM cells. Recent studies of proteasome inhibition in MM cells reveal that accumulation of unfolded proteins in the endoplasmic reticulum (ER), so called ER stress, triggered several pro-apoptotic factors and cell stress such as Reactive Oxygen Species (ROS) accumulation, which is considered to be the main mechanism of action of BTZ-induced apoptosis. Several factors associated with ER stress and unfolded protein response (UPR), have been identified with sensitivity of BTZ treatment. Low levels of XBP1, ATF3, and ATF4, which regulate UPR and ER stress-induced apoptosis, have been observed in poor responders to BTZ treatment, and three other genes, KLF9, Nampt, and CDK5, are associated with response to BTZ-containing therapy. These findings contribute to a better understanding of the mechanism of BTZ-induced apoptosis in MM cells, and further study is needed to develop potential predictive biomarkers of efficacy of BTZ-containing therapy.

Myeloma (MM) cells and osteoclasts create a highly acidic milieu in MM bone lesions by proton produced by osteoclasts and lactate by MM cells. Although bendamustine exerts clinical activity against MM, the potential mechanisms of action of bendamustine remain largely unknown. In the present study, we therefore explored anti-MM activity of bendamustine with special reference to its effects on MM cells in acidic conditions. Bendamustine at the range of 25 to 100 μM dose-dependently induced death in INA6 and TSPC-1 MM cells. However, bendamustine did not impair the viability of RPMI8226, U266 and OPC cells even at 100 μM. In acidic conditions at pH 6.8 or lower, the bendamustine’s cytotoxic effects were further enhanced in INA6 and TSPC-1 cells, and triggered in RPMI8226 and MM.1S cells which were resistant to bendamustine at pH 7.4. Intriguingly, bendamustine reduced Pim-2 protein levels in MM cells and enhanced its anti-MM activity in combination with the Pim inhibitor SMI-16a, preferentially in acidic conditions. These results demonstrate that anti-MM effects of bendamustine are augmented in acidic conditions, but considerably vary among MM cells, and that Pim inhibition further enhances the bendamustine’s anti-MM activity in acidic conditions.