Pluripotent Stem Cell Metabolism and Mitochondria: Beyond ATP

Figure 1

Oxygen regulation of ESC metabolism and epigenetic landscape. Relative to atmospheric oxygen (20%), physiological oxygen (1–5%) reduces the content of mitochondrial DNA (mtDNA) and mitochondrial electron transport chain (ETC) gene expression in pluripotent stem cells [11]. These mitochondria consume less oxygen and respire less than those at atmospheric oxygen generating less ATP through glucose-derived oxidative phosphorylation (OXPHOS). Mitochondrial OXPHOS from glutamine- and fatty acid-derived carbon is still an active pathway in pluripotent stem cells; atmospheric oxygen increases the consumption of glutamine and its oxidation in the mitochondria [77, 93]. Pluripotent stem cells rely heavily on glycolysis, followed by the conversion of pyruvate to lactate, which recycles the NAD⁺ required for the rapid continuation of glycolysis. Per carbon, glycolysis is less efficient than OXPHOS at generating ATP; however, should there be a sufficient flux of glucose, then enough ATP can readily be formed. At physiological oxygen, glycolytic flux is increased relative to atmospheric oxygen resulting in significantly more lactate production [11, 77, 128]. Several mechanisms direct glucose-derived carbon towards either lactate or alanine and away from mitochondrial OXPHOS. Under physiological oxygen conditions, the hypoxic inducible factors (HIFs) are stabilised; targets of transcription factor HIF2α include glucose transporter 1 (GLUT1) [128] which increases glucose transport into the cell and pyruvate dehydrogenase kinase (PDK) which inhibits the conversion of pyruvate to acetyl-CoA by pyruvate dehydrogenase (PDH) in the mitochondrion. Uncoupling protein 2 (UCP2), an inner mitochondrial membrane protein, blocks the import of pyruvate into the mitochondria in human PSC [84]. Glutamine and fatty acids stimulate UCP2, decreasing pyruvate oxidation, which in turn facilitates glutamine and fatty acid oxidation and the maintenance of a rapid glycolytic flux [187, 188]. The flux of metabolic reactions in PSCs is increased at physiological oxygen [93] as is amino acid turnover [11, 189]. Increased serine and glycine consumption at physiological oxygen may feed into the folate and methionine cycles, collectively known as one carbon metabolism. One carbon metabolism, glycolysis, and the tricarboxlyic acid (TCA) cycle generate intermediate metabolites that act as cofactors for epigenetic modifying enzymes. Threonine and methionine metabolism in mouse [5] and human [4] PSCs, respectively, generate S-adenosylmethionine (SAM) which is a methyl donor for histone methyl transferases (HMT). Glucose-derived acetyl coenzyme A (acetyl-CoA), synthesised in the TCA cycle or from threonine metabolism [5], acts as a cofactor for histone acetyltransferases (HAT), modulating hESC histone acetylation and plausibly maintains pluripotency [88]. Glutamine metabolism increases the αKG:succinate ratio, leading to DNA demethylation by ten-eleven translocation (TET) activity, which then stimulates the mouse naïve pluripotency network [83]. In primed human ESC, an increased αKG:succinate ratio induces differentiation [100]. In human ESC, physiological oxygen causes a euchromatic state within NANOG, OCT4, and SOX2 hypoxic response elements (HREs) allowing the binding of HIF2α and the upregulation of the pluripotency network [109]. HIFα is stabilised at physiological [160, 167] and atmospheric oxygen [170] due to the action of mitochondrial ROS [161, 168, 169]. Stabilised HIFα protein upregulates glycolytic flux through glycolytic gene expression [147], increases cellular glucose import, and upregulates pluripotency [109]. The proximity of the mitochondria to the nucleus facilitates a ROS-nucleus signalling axis in the form of H₂O₂, plausibly through the HIF family of transcription factors. Concurrently, antioxidant production is increased at physiological oxygen [175]. Glutathione (GSH) from glutaminolysis, and NADPH from either glutaminolysis or the pentose phosphate pathway, protect the cell from increased levels of ROS. Thick arrows and bold text indicate increased flux/transcription. Metabolic regulators of chromatin-modifying enzymes are highlighted in red. Circles attached to chromatin in the nucleus represent epigenetic modifications: acetylated (green); 5mC (red); 5hmC (blue).