Proposed scheme of glucose and fatty acid metabolic pathways in shGTPBP3 cells.

<p>Glucose uptake is regulated by the translocation of the glucose transporters (e.g., GLUT1) to the cell membrane. In glycolysis, the first regulatory step that commits glucose to catabolism is controlled by phosphofructokinase (PFK1), whereas the last step is catalyzed by pyruvate kinase to produce pyruvate and ATP. Pyruvate is either oxidized (glucose oxidation) or converted to lactate by lactate dehydrogenase (LDH). When glycolysis is coupled to glucose oxidation, pyruvate enters the mitochondria by means of the mitochondrial pyruvate carrier (MPC) and is converted into acetyl CoA by pyruvate dehydrogenase (PDH), which is regulated by the action of pyruvate kinase (PDK) and pyruvate phosphatase. Finally, acetyl CoA incorporates into the tricarboxylic acid cycle (TCAC). NADH and FADH₂ generated by TCAC enzymes are funneled into the mitochondrial electron transport chain through Complex I and Complex II, respectively. Long chain fatty acids use the carnitine shuttle, which includes carnitine palmitoyltransferase I (CPTI), to cross the mitochondrial membranes. Fatty acid oxidation (FAO) produces acetyl-CoA, which incorporates to the TCAC, and NADH and FADH₂, which are used in the electron transport chain via Complex I and ETF-ubiquinone oxidoreductase, respectively. Fatty acid and glucose metabolism can regulate each other by the Randle Cycle [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144273#pone.0144273.ref075" target="_blank">75</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144273#pone.0144273.ref117" target="_blank">117</a>]. In shGTPBP3 cells, where Complex I is impaired, AMPK activation results in down-regulation of MPC1 (one of the two subunits that form MPC) and up-regulation of UCP2. The increase of UCP2 in shGTPBP3 cells could favor the export of malate and oxaloacetate (OAA) out of mitochondria and promote FAO. The decrease of MPC may also be part of an adaptive response that invokes FAO and oxidation of branched-chain amino acids to provide reducing equivalents to the OXPHOS system. However, insufficient recycling of NADH to NAD⁺, due to the low activity of Complex I, may limit the effects of the metabolic reprogramming on the OXPHOS capacity. This situation, together with a reverse operation of Complex V aimed to prevent a drastic drop of the membrane potential, could explain the low ATP levels in shGTPBP3 cells. These reduced ATP levels and the uncoupling of glycolysis from pyruvate oxidation, which can l increase proton and lactate production could be detrimental for tissues like the heart. In the figure, red and green lines indicate regulatory actions.