Proteasome inhibitors induce AMPK activation via CaMKKβ in human breast cancer cells

The purpose of present study is to examine the mechanism of the 5'-AMP-activated protein kinase (AMPK) activation induced by proteasome inhibitors. AMPK activation and ubiquitin proteasome system (UPS) inhibition have gained great attention as therapeutic strategies for the treatment of certain types of cancers. While AMPK serves as a master regulator of cellular metabolism, UPS regulates protein homeostasis. However, the relationship between these two important pathways is not very clear. We observe that proteasome inhibition leads to AMPK activation in human breast cancer cells. siRNA transfection, western blotting, qPCR, and proteasomal inhibition assays were used to study the mechanism of proteasome inhibitor-induced AMPK activation using human triple-negative breast cancer, lung, and cervical cancer cell lines. We report that a variety of proteasome inhibitors activate AMPK in all the tested different cancer cell lines. Our data using liver kinase B1-deficient cancer cells suggest that proteasome inhibitor-induced AMPK activation is primarily mediated by Calcium/Calmodulin-dependent kinase kinase β (CaMKKβ). This hypothesis is supported by that pharmacological or genetic inhibition of CaMKKβ leads to a decrease in proteasome inhibitor-induced AMPK activation. Additionally, the AMPK-activating function of the FDA-approved proteasome inhibitor bortezomib depends on an increase in intracellular calcium levels as calcium chelation abrogates its induced AMPK activation. Finally, bortezomib-mediated upregulation in CaMKKβ levels is due to its enhanced protein synthesis. These data suggest that proteasome inhibitors indirectly activate AMPK in human cancer cells primarily via Ca(2+)-CaMKKβ-dependent pathway.