Bortezomib induces AMPK-dependent autophagosome formation uncoupled from apoptosis in drug resistant cells

The proteasome inhibitor bortezomib is an effective anti-cancer agent for the plasma cell malignancy multiple myeloma but clinical response is hindered by the emergence of drug resistance through unknown mechanisms. Drug sensitive myeloma cells were exposed to bortezomib to generate drug resistant cells that displayed a significant increase in subunits of the energy sensor AMP-activated protein kinase (AMPK). AMPK activity in resistant cells was increased and bortezomib resistant cells contained a similar to 4-fold greater level of autophagosomes than drug sensitive cells. Real-time measurements indicated that bortezomib reduced the oxygen consumption rate in drug sensitive cells more readily than in resistant cells. Genetic ablation of AMPK activity reduced the bortezomib effect on autophagy. The autophagy-related gene (Atg) 5 is required for autophagosome formation and enhances cellular susceptibility to apoptotic stimuli. Atg5 knockout eliminated bortezomib-induced autophagosome formation and reduced susceptibility to bortezomib. Bortezomib treatment of myeloma cells lead to ATG5 cleavage through a calpain-dependent manner while calpain inhibition or a calpain-insensitive Atg5 mutant promoted bortezomib-resistance. In contrast, AICAR, an AMPK activator, enhanced bortezomib-induced cleavage of ATG5 and increased bortezomib-induced killing. Taken together, the results demonstrate that ATG5 cleavage provokes apoptosis and represents a molecular link between autophagy and apoptosis with therapeutic implications.