Mitochondrial hexokinase II (HKII) and phosphoprotein enriched in astrocytes (PEA15) form a molecular switch governing cellular fate depending on the metabolic state

Abstract

The metabolic state of a cell is a key determinant in the decision to live and proliferate or to die. Consequently, balanced energy metabolism and the regulation of apoptosis are critical for the development and maintenance of differentiated organisms. Hypoxia occurs physiologically during development or exercise and pathologically in vascular disease, tumorigenesis, and inflammation, interfering with homeostatic metabolism. Here, we show that the hypoxia-inducible factor (HIF)-1-regulated glycolytic enzyme hexokinase II (HKII) acts as a molecular switch that determines cellular fate by regulating both cytoprotection and induction of apoptosis based on the metabolic state. We provide evidence for a direct molecular interactor of HKII and show that, together with phosphoprotein enriched in astrocytes (PEA15), HKII inhibits apoptosis after hypoxia. In contrast, HKII accelerates apoptosis in the absence of PEA15 and under glucose deprivation. HKII both protects cells from death during hypoxia and functions as a sensor of glucose availability during normoxia, inducing apoptosis in response to glucose depletion. Thus, HKII-mediated apoptosis may represent an evolutionarily conserved altruistic mechanism to eliminate cells during metabolic stress to the advantage of a multicellular organism.cell death | endogenous tolerance | fluorescence lifetime imaging microscopy-FRET | mitochondria | preconditioning B alanced energy metabolism and regulation of apoptosis are of vital importance to all organisms (1, 2). Therefore, energy metabolism and regulation of apoptosis are interdependent (3). In cells, metabolism and apoptosis converge at mitochondria, thereby integrating pathways responsible for endogenous tolerance against substrate deprivation (4). All differentiated multicellular organisms have evolved strategies to promote survival when deprived of metabolic substrates, such as during hypoxia (5). Therefore, elucidating the underlying molecular mechanisms by which metabolism and apoptosis are coregulated may lead to novel therapeutic strategies for both acute and chronic diseases.The transcription factor hypoxia-inducible factor (HIF)-1 is a key regulator in the adaptation to hypoxia and the resultant energy depletion, orchestrating the cellular response to hypoxic conditions (5-7). Induction of HIF-1 leads to the transcriptional regulation of a multitude of genes, ultimately resulting in a hypoxia-tolerant state of the cell (7). HIF-1 also links hypoxia and glycolysis (8) via complex and incompletely understood mechanisms. HIF-1 adapts cellular metabolism to hypoxic conditions during development (7) or exercise (9) and thereby prevents death of tumor cells and primary cells under various conditions of disease (5-7, 10). In addition, HIF-1 controls innate immunity by regulating glycolysis in cells of the immune system (11, 12). Finally, by controlling the expression of members of the glycolytic cascade, including hexokinase II (HKII) (7, 8), HIF-1 contributes to a proliferative metabolism (13).Mitochondrial glycolytic h...

Mitochondrial localization of TIGAR under hypoxia stimulates HK2 and lowers ROS and cell death

“…2B). Interestingly, the glucose dependence of mitochondrial localization has also been reported for HK2, which is localized to the mitochondria in the presence of glucose but moves to the cytosol when glucose is removed (2,12). Consistently, we also found that the removal of glucose resulted in some dissociation of HK2 and a mitochondrial marker (mtHsp70), particularly in 0.1% O 2 , although even under these conditions, some HK2 remained at the mitochondria (Fig.…”

“…Although HK2 can protect cells from hypoxic cell death, this activity is lost in the absence of glucose (12), conditions under which HK2 becomes dissociated from the mitochondria (2). Consistently, we have also shown that the translocation of TIGAR to the mitochondria and the binding to HK2 depend on glucose availability.…”

“…siRNA and TIGAR constructs were described (Santa Cruz) (16) or generated for this study. HK2 constructs were kindly provided by Philipp Mergenthaler (Berlin, Germany) (44). Commercially available and the described Anti-TIGAR antibody (16) were used for protein analysis.…”

“…Furthermore, mitochondrial HK2 can directly inhibit apoptosis through regulation of the mitochondrial permeability transition pore that, in part, reflects the ability to block the recruitment of proapoptotic proteins such as Bax and Bak (7)(8)(9)(10)(11). The interaction of HK2 with the phosphoprotein PEA15 is also required for protection from apoptosis under hypoxia (12).…”

The Balance of Glucose Metabolism and Cell Death: Uncovering Protective and Lethal Effects of Hexokinase-2

“…New insight into the mechanisms of HK2-mediated protection has been provided by a recent study by Mergenthaler et al published in The Proceedings of the National Academy of Sciences [9]. Here, overexpression of HK2 was shown to protect cultured neurons against combined Oxygen-Glucose Deprivation (OGD) as well as just Oxygen Deprivation (OD) alone.…”