Hexokinase-II Positively Regulates Glucose Starvation-Induced Autophagy through TORC1 Inhibition

Abstract

Summary Hexokinase-II (HK-II) catalyzes the first step of glycolysis and also functions as a protective molecule, however, its role in protective autophagy has not been determined. Results showed that inhibition of HK-II diminished, while overexpression of HK-II potentiated, autophagy induced by glucose deprivation in cardiomyocyte and non-cardiomyocyte cells. Immunoprecipitation studies revealed that HK-II binds to and inhibits the autophagy suppressor, mTOR complex 1 (TORC1), and this binding was increased by glucose deprivation. The TOS motif, a scaffold sequence responsible for binding TORC1 substrates, is present in HK-II and mutating it blocked its ability to bind to TORC1 and regulate protective autophagy. The transition from glycolysis to autophagy appears to be regulated by a decrease in glucose-6 phosphate. We suggest that HK-II binds TORC1 as a decoy substrate and provides a previously unrecognized mechanism for switching cells from a metabolic economy based on plentiful energy, to one of conservation, under starvation.

Hexokinase II integrates energy metabolism and cellular protection: Akting on mitochondria and TORCing to autophagy

“…Thus, in the absence of glucose, HK-II binds mTORC1 via its TOS motif, acting as a decoy substrate to facilitate autophagy. 117 As discussed earlier, mTORC1 activation stimulates HK-II expression under growth conditions conferring metabolic support while HK-II provides negative feedback to mTORC1 under starvation to stimulate autophagy. This dual regulation for HK-II and mTOR provides an adaptive mechanism to preserve cellular homeostasis dependent on metabolic status.…”

“…116 Thus, in response to ischemia, in which Akt activity and mitoHK-II are decreased (resulting in increased cytosolic HK-II), HK-II would stimulate these cytosolic events to preserve cellular homeostasis. (4) As discussed later, non-mitochondrial HK-II facilitates autophagy induced by glucose withdrawal, 117 suggesting regulation of energy conservation by HK-II.…”

“…A series of experiments demonstrated that HK-II binds to mTORC1 in the absence of glucose, decreasing mTORC1 activity. 117 This interaction was mediated by an mTOR signaling motif (TOS motif), identified at positions 199-203 (FDIDI) in HK-II but absent in HK-I. The sequence is found in various species, including mouse, rat, horse, boar and human HK-II, suggesting that the TOS motif in HK-II is well conserved.…”

“…[155][156][157][158][159][160] Although it has been widely accepted that autophagy is induced in response to metabolic suppression, the molecular links between metabolic and autophagic pathways have not been fully elucidated. Recently, we made the surprising discovery that HK-II facilitates autophagy in response to glucose deprivation to protect cells 117 (Figure 4).…”

“…We found that, in contrast to 2-DG, the glucose analog 5-thio-glucose, which binds to HK-II but cannot be phosphorylated, does not inhibit autophagy induced by glucose deprivation. 117 This suggests that phosphorylated glucose, but not glucose binding, is the key factor responsible for suppressing the autophagic effect of HK-II. To support this hypothesis, it was demonstrated that expression of a kinase-dead HK-II mutant (in the absence of endogenous HK-II) stimulated autophagy that was insensitive to inhibition by glucose, while expression of WT HK-II retained glucose dependent regulation.…”