Autophagy, Glucose Starvation & V-ATPase Signaling
Mike Forgac & Christina McGuire and a model of regulated assembly of the V-ATPase, which they have been studying.
In order for cells to maintain homeostasis, it is crucial that they properly sense and respond to nutrients in their environment. During nutrient starvation, the cell halts growth and proliferation and initiates a process called autophagy in order to maintain homeostasis. Autophagy allows for the recycling of the cell’s building blocks to promote cell survival during these periods of nutrient stress. One of the major signaling molecules that activates autophagy is AMP-activated protein kinase (AMPK).
Christina McGuire a PhD student in the Biochemistry Program studying in Mike Forgac's lab is focused on the relationship between AMPK and the vacuolar H(+)-ATPase (V-ATPase) during glucose starvation. The V-ATPase is a proton pump involved in many normal and disease processes, and it was recently found to be required for proper activation of AMPK during glucose starvation through an unknown mechanism. Our work in mammalian cells indicates that acute glucose starvation increases V-ATPase activity by increasing the assembly of its two domains, the peripheral V1 domain and the membrane-embedded V0 domain, in a process called reversible assembly. These findings change our current understanding of the reversible assembly model, where glucose starvation in a yeast system was shown to induce disassembly of the pump. This is the first finding suggesting that the yeast V-ATPase responds differently to nutrient cues than the V-ATPase of higher eukaryotes.
These differences may arise from different energy requirements of the fungal and mammalian systems. In yeast, glucose starvation likely activates dissociation of the V-ATPase as a way to conserve cellular stores of ATP. By contrast, mammalian cells activate autophagy upon glucose starvation. Because autophagy requires the activity of the V-ATPase to acidify lysosomes for proper degradation and recycling of cellular material, the observed increase in V-ATPase-dependent acidification of lysosomes likely contributes to proper autophagic flux.
Additionally, we examined the interaction between the V-ATPase and AMPK during glucose starvation. Because AMPK activation occurs prior to any detectable increase in lysosomal V-ATPase activity, it is unlikely that increased assembly plays a role in AMPK activation. By contrast, the starvation-induced increases in V-ATPase activity and assembly are prevented by the AMPK inhibitor dorsomorphin and by the phosphoinositide 3-kinase (PI3K) and Akt inhibitors LY294002 and MK2206, respectively. These results suggest that these signaling pathways are involved in increasing V-ATPase assembly and activity during glucose starvation. Together, this work provides novel insight into how V-ATPase activity is controlled in response to nutrient availability. These results are of relevance to the study of a number of human diseases in which autophagy and nutrient sensing are dysregulated, including cancer, diabetes, and various neurological disorders.
McGuire CM, Forgac M. 2018. Glucose starvation increases V-ATPase assembly and activity in mammalian cells through AMP kinase and phosphatidylinositide 3-kinase/Akt signaling. J Biol. Chem. 293: 9113-9123. Abstract
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