AMPK promotes Notch1 stability to potentiate hypoxia-induced breast cancer stemness and drug resistance

Hypoxia is a hall mark of solid tumor microenvironment and contributes to tumor progression and therapy failure. The developmentally important Notch pathway is implicated in cellular response of cancer cells to hypoxia. Yet, the mechanisms that potentiate Notch signaling under hypoxia are not fully understood. Hypoxia is also a stimulus for AMP-activated protein kinase (AMPK), a major cellular energy sensor. In this study, we investigated if AMPK interacts with the Notch pathway and influences the hypoxia-response of breast cancer cells. Activating AMPK with pharmacological agent or genetic approaches led to an increase in the levels of cleaved Notch1 and elevated Notch signaling in invasive breast cancer cell lines. In contrast, inhibition or depletion of AMPK reduced the amount of cleaved Notch1. Significantly, we show that the hypoxia-induced increase in cleaved Notch1 levels requires AMPK activation. Probing into the mechanism, we demonstrate that AMPK activation impairs the interaction between cleaved Notch1 and its ubiquitin ligase, Itch/AIP4 through the tyrosine kinase Fyn. Under hypoxia, the AMPK-Fyn axis promotes inhibitory phosphorylation of Itch which abrogates its interaction with substrates, thus stabilizing cleaved Notch1 by reducing its ubiquitination and degradation. We further show that inhibition of AMPK alleviates the hypoxia-triggered, Notch-mediated stemness and drug resistance phenotype. Breast cancer patient samples also showed co-expression of hypoxia/AMPK/Notch gene signature. Our work thus establishes AMPK as a key component in the adaptation of breast cancer cells to hypoxia, and proposes therapeutic inhibition of AMPK to mitigate the hypoxia-triggered aggressiveness.