Metabolic regulation in pluripotent stem cells during reprogramming and self-renewal

Abstract

Small, rapidly dividing pluripotent stem cells (PSCs) have unique energetic and biosynthetic demands compared with typically larger, quiescent differentiated cells. Shifts between glycolysis and oxidative phosphorylation with PSC differentiation or reprogramming to pluripotency are accompanied by changes in cell cycle, biomass, metabolite levels, and redox state. PSC and cancer cell metabolism are overtly similar, with metabolite levels influencing epigenetic/genetic programs. Here, we discuss the emerging roles for metabolism in PSC self-renewal, differentiation, and reprogramming.

Figure 1. Key Differences in Metabolism Between PSCs and Differentiated Cells

Energy metabolism shifts from glycolysis to OXPHOS with differentiation, or from OXPHOS to glycolysis with reprogramming to pluripotency. Glycolytic flux is elevated in PSCs (left panel) to provide ATP and intermediate metabolites through the pentose phosphate pathway for nucleotide and lipid biosynthesis. Glycolysis regulating enzymes, such as lactate dehydrogenase and hexokinase, are highly expressed in PSCs, which also consume oxygen through a functional electron transport chain. PSCs rely more on glycolysis for energy because respiration is lower and less coupled to energy production than in differentiated cells. Pyruvate entry into mitochondria is limited by an inactive pyruvate dehydrogenase complex or by high expression of uncoupling protein 2. The mitochondrial membrane potential is partially maintained by hydrolysis of ATP in the complex V ATP synthase. Less reactive oxygen species is present in PSCs from lower respiration and elevated antioxidant enzymes. These features establish a reduced cellular redox environment (blue text) that becomes more oxidative (orange text) with differentiation. The tricarboxylic acid cycle in PSCs provides intermediate metabolites such as citrate and α-ketoglutarate that are siphoned for lipid and amino acid biosynthesis. Key: ETC, electron transport chain; FA, fatty acid; G-6-P: glucose-6-phosphate; GSH; glutathione; GSSG, glutathione disulfide; mtDNA, mitochondrial DNA; NAD: nicotinamide adenine dinucleotide; NADP: nicotinamide adenine dinucleotide phosphate; PPP, pentose phosphate pathway; R-5-P, ribose-5-phosphate; ROS, reactive oxygen species; TCA, tricarboxylic acid cycle. Elevated metabolic pathways are indicated with bold letters and arrows.

Figure 2. Metabolic, Signaling, Genetic, and Epigenetic Network Crosstalk

Glucose enters glycolysis (orange panel) and is metabolized to lactate. The expression and activity of glycolytic regulating enzymes are controlled by mTOR and PI3K signaling pathways (crosstalk). Reprogramming factors c-MYC and LIN28 promote glucose metabolism and c-MYC also suppresses respiration (crosstalk). Alternatively, pyruvate can be converted into two molecules of acetyl-CoA and enter the TCA cycle. Acetyl-CoA can traffic to the nucleus as a substrate for acetyltransferase enzymes (crosstalk). TCA cycle intermediates can be siphoned into amino acid or lipid biosynthesis pathways, or can participate in the control of gene expression. For example, α-ketoglutarate (α-KG) is a cofactor for Jumonji-domain containing histone demethylases (crosstalk). Methionine adenosyltransferase, an enzyme in amino acid biosynthesis, also converts methionine and ATP into S-adenosylmethionine (SAM) for DNA methyltransferases and other transmethylation reactions (crosstalk). The NAD⁺/NADH ratio reflects cellular redox state and regulates deacetylase enzymes, such as the Sirtuins (crosstalk). Reactive oxygen species (ROS) are required for lineage-specific differentiation and are regulated by the p38 MAPK stress signaling pathway (crosstalk). Lipid biosynthesis supports membrane formation but also provides signaling molecules including arachidonic acid (AA) and docosahexanoic acid (DHA) (crosstalk). Altogether, metabolites, the redox environment, and metabolism-regulated signaling cascades communicate with the nucleus (grey panel) to directly impact chromatin structure and gene expression programs that may control PSC self-renewal, differentiation, and reprogramming. (Blue buttons indicate nucleus and signaling control of metabolic pathways; orange buttons indicate metabolic pathways that influence gene expression changes within the nucleus.)