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

Abstract

The p53-inducible protein TIGAR (Tp53-induced Glycolysis and Apoptosis Regulator) functions as a fructose-2,6-bisphosphatase (Fru-2,6-BPase), and through promotion of the pentose phosphate pathway, increases NADPH production to help limit reactive oxygen species (ROS). Here, we show that under hypoxia, a fraction of TIGAR protein relocalized to mitochondria and formed a complex with hexokinase 2 (HK2), resulting in an increase in HK2 activity. Mitochondrial localization of TIGAR depended on mitochondrial HK2 and hypoxia-inducible factor 1 (HIF1α) activity. The ability of TIGAR to function as a Fru-2,6-BPase was independent of HK2 binding and mitochondrial localization, although both of these activities can contribute to the full activity of TIGAR in limiting mitochondrial ROS levels and protecting from cell death.cancer metabolism | glycolysis | oxidative stress | low oxygen T he uptake and metabolism of glucose is fundamental for energy production and supporting various anabolic pathways that produce the macromolecules required for cell growth and proliferation. Flux through the different metabolic pathways can be modulated in response to varying growth conditions and demands, as seen in cancer cells, which adopt high rates of aerobic glycolysis that is characterized by extensive glucose uptake and high levels of lactate production (1).Glycolysis can be regulated by changes in the expression or activity of enzymes that catalyze various steps of the pathways. Allosteric regulation or covalent modification of many metabolic enzymes has been described, and the concentration of the active enzyme can also be controlled through changes in transcription, splicing, or protein stability. The subcellular localization of some glycolytic enzymes has also been shown to play an important role in regulating their functions (2, 3). In the first step of glucose metabolism, hexokinases (HK) generate glucose-6-phosphate, which can then be further used through glycolysis to produce energy, diverted into the pentose phosphate pathway (PPP) to produce NADPH and anabolic intermediates or converted to glycogen for storage. HK2 is thought to play a key role in promoting anabolic pathways and is frequently overexpressed in cancers (4). Both HK1 and HK2 show cytoplasmic and-through interaction with voltage-dependent anion-selective channel (VDAC)-mitochondrial localization. The presence of HK at the mitochondria provides a close proximity to intramitochondrial ATP production, which helps to couple glycolysis and oxidative phosphorylation (5). This mitochondrial HK also helps to limit mitochondrial reactive oxygen species (ROS) production by maintaining local ADP levels (6). 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 (7-11). The interaction of HK2 with the phosphoprotein PEA15 is also required for protection from apoptosis under hypoxia (12).Severa...

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