Alkaline intracellular pH activates AMPK-mTORC2 signaling to promote cell survival during growth factor limitation

Abstract

mTORC2 controls cell metabolism and promotes cell survival, yet its upstream regulation by diverse cellular cues remains poorly defined. While considerable evidence indicates that mTORC1 but not mTORC2 responds dynamically to amino acid levels, several studies reported activation of mTORC2 signaling by amino acids, a paradox that remains unresolved. Following amino acid starvation, we noted that addition of a commercial amino acid solution but not re-feeding with DMEM containing amino acids increased mTORC2 signaling. Interestingly, the pH of the amino acid solution was [~] 10. These key observations enabled us to discover that alkaline intracellular pH (pHi) represents a previously unknown activator of mTORC2. Using a fluorescent pH-sensitive dye (cSNARF-1-AM) coupled to live-cell imaging, we demonstrate that alkaline extracellular pH (pHe) increases intracellular pHi, which increases mTORC2 catalytic activity and downstream signaling to Akt. Alkaline pHi also activates AMPK, a sensor of energetic stress. Functionally, alkaline pHi attenuates apoptosis caused by growth factor withdrawal, which requires AMPK in part and mTOR in full. Collectively, these findings reveal that alkaline pHi increases AMPK-mTORC2 signaling to promote cell survival during growth factor limitation. As elevated pHi represents an under-appreciated hallmark of cancer cells, alkaline pH sensing by AMPK-mTORC2 may contribute to tumorigenesis. One Sentence SummaryAlkaline intracellular pH activates mTORC2