Inhibition of Autophagy with 3-Methyladenine Potentiates Bortezomib-Induced Apoptosis of Myeloma Cell Via up Regulation of Mitochondrial Reactive Oxygen Species

Backgrounds : Autophagy plays an important role in the pathogenesis of multiple myeloma. Autophagy usually acts on a pro-survival mechanism, and cooperates with the ubiquitin proteasome system in the homeostasis of myeloma cells by degrading excessive and misfolded proteins for energy recycling. Therefore, the inhibition of autophagy could effectively induce death and could synergize with proteasome inhibitors in myeloma cells. In this study, we investigated the synergistic mechanism of apoptosis with bortezomib (BTZ) and autophagy inhibitors in the MM cells.

Methods : We evaluated the change of autophagy, apoptosis, mitochondrial ROS and cellular level of redox enzymes, including peroxiredoxin (Prx), thioredoxin (Trx), thioredoxin reductase (Trx-R) using human myeloma cell lines, MM.1S and MM.1R during BTZ treatment. To study the status of oxidation for redox enzyme, sulfinic acid (SO2) or multimeric form of Prx, Trx and Trx-R by western blot were measured using non-reducing or reducing gel.

Results : Mitochondrial over cytosolic ROS of MM cells was increased significantly after 24 hour of BTZ (2.5 nM) treatment. Apoptosis of MM cell after BTZ treatment was increased in concordance with mitochondrial ROS increment of MM cells. N-acetylcystein (NAC) reversed BTZ-induced mitochondrial ROS elevation and apoptosis of MM cells as well. Increased expressions of cleaved caspase-9 and cleaved caspase-3 were also observed during BTZ-induced MM cell apoptosis. LC3-II expression was elevated along with increment of mitochondrial ROS and apoptosis of MM cells after BTZ treatment. Oxidation of PRX4 and TRX2 was observed during BTZ-induced apoptosis of MM cells. After treatment of NAC, LC3-II and PRX4/TRX2 expression was reversed. Inhibition of autophagy with 3-Methyladenine (3-MA) not only resulted in a further increase in BTZ-induced apoptosis but also induced mitochondrial ROS in MM cells.

Conclusions : Autophagy is induced during BTZ treatment in MM cells. Our experiment showed crosstalk between autophagy and mitochondrial ROS and its redox enzymes during BTZ-induced MM cell apoptosis. Inhibition of autophagy with 3-MA potentiates BTZ-induced apoptosis of MM cells via up regulation of mitochondrial ROS. Our results provide new perspective on the cellular mechanism of action of BTZ and support the synergism of BTZ with autophagy inhibitor for an improved therapy of the multiple myeloma.