Editors' ChoiceCellular Metabolism

Hexokinase-II Integrates Glycolysis and Autophagy

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Sci. Signal.  25 Feb 2014:
Vol. 7, Issue 314, pp. ec55
DOI: 10.1126/scisignal.2005208

Glucose deprivation inhibits the activity of the kinase complex mTORC1 in cells, thereby reducing anabolic metabolism and stimulating autophagy to increase catabolic metabolism. Hexokinase enzymes catalyze the phosphorylation of glucose to glucose 6-phosphate (G6P) and are key enzymes in glycolytic metabolism. Whereas hexokinase-I (HK-I) is ubiquitous, hexokinase-II (HK-II) is abundant in heart, adipose, and skeletal muscle and is increased in many tumors. Roberts et al. observed that neonatal rat ventricular myocytes (NRVMs) exposed to the HK inhibitor 2-deoxyglucose (2-DG) or those with knockdown of HK-II, but not HK-I, did not exhibit the increase in autophagy when cultured in glucose-free medium. Furthermore, knockdown of HK-II, but not HK-I, increased the proportion of cells that underwent apoptosis in response to glucose deprivation. Overexpression of HK-II, but not HK-I, increased basal and glucose deprivation–induced autophagy and promoted cell survival in response to glucose deprivation. Analysis of enzymes that exhibit altered activity in response to glucose deprivation indicated that mechanistic target of rapamycin (mTOR) and not Akt or the ATP-sensing enzyme AMPK was involved in the effects of HK-II on autophagy. NRVMs in which HK-II was knocked down or those exposed to 2-DG maintained the phosphorylation of mTOR complex 1 (mTORC1) substrates in response to glucose deprivation, suggesting that HK-II inhibits mTORC1 activity. Although overexpression of a kinase-dead HK-II in cells with endogenous HK-II enhanced autophagy in response to glucose-limiting conditions, introduction of the kinase-dead form into cells in which HK-II had been knocked down did not potentiate autophagy, suggesting that glycolytic activity is important for the response but that there was a kinase-independent function as well. Indeed, exposing NRVMs to 5-thio-glucose, an HK inhibitor that cannot be phosphorylated by HK, inhibited mTORC1 activity and promoted autophagy, suggesting that the HK substrate G6P (or phosphorylated 2-DG) prevented HK-II from inhibiting mTORC1 activity. HK-II coimmunoprecipitated with components of mTORC1 in NRVMs. This interaction was increased by glucose deprivation and reduced by knocking down Raptor, a component of mTORC1, or by exposing cells to 2-DG. HK-II and mTORC1 also interacted in ex vivo glucose-deprived adult mouse hearts, and these hearts also exhibited increased markers of autophagic flux and reduced phosphorylation of mTORC1 substrates. Mouse, rat, and human HK-II contain a consensus TOS motif that binds to Raptor. Overexpression of HK-II with a mutation to the TOS motif, which prevented the interaction with mTORC1, failed to enhance autophagy and inhibit apoptosis in glucose-deprived NRVMs. As Kundu highlights, G6P functions as a key entry point for three metabolic pathways—the glycolytic pathway, the pentose phosphate pathway, and the glycogen synthesis pathway—and the Roberts et al. study connects these pathways to the regulation of autophagy and mTORC1 through G6P-mediated regulation of HK-II. The identification of HK-II as an integrator of metabolic state in the heart may have implications for various types of cardiomyopathy associated with metabolic disease.

D. J. Roberts, V. P. Tan-Sah, E. Y. Ding, J. M. Smith, S. Miyamoto, Hexokinase-II positively regulates glucose starvation-induced autophagy through TORC1 inhibition. Mol. Cell 53, 521–533 (2014). [PubMed]

M. Kundu, Too sweet for autophagy: Hexokinase inhibition of mTORC1 activates autophagy. Mol. Cell 53, 517–518 (2014). [PubMed]