Saturation of the mitochondrial NADH shuttles drives aerobic glycolysis in proliferating cells

Proliferating cells exhibit a metabolic phenotype known as "aerobic glycolysis," which is characterized by an elevated rate of glucose fermentation to lactate irrespective of oxygen availability. Although several theories have been proposed, a rationalization for why proliferating cells seemingly waste glucose carbon by excreting it as lactate remains elusive. Using the NCI-60 cell lines, we determined that lactate excretion is strongly correlated with the activity of mitochondrial NADH shuttles, but not proliferation. Quantifying the fluxes of the malate-aspartate shuttle (MAS), the glycerol 3-phosphate shuttle (G3PS), and lactate dehydrogenase under various conditions demonstrated that proliferating cells primarily transform glucose to lactate when glycolysis outpaces the mitochondrial NADH shuttles. Increasing mitochondrial NADH shuttle fluxes decreased glucose fermentation but did not reduce the proliferation rate. Our results reveal that glucose fermentation, a hallmark of cancer, is a secondary consequence of MAS and G3PS saturation rather than a unique metabolic driver of cellular proliferation.

Mitochondrial lactate metabolism: history and implications for exercise and disease

Mitochondrial structures were probably observed microscopically in the 1840s, but the idea of oxidative phosphorylation (OXPHOS) within mitochondria did not appear until the 1930s. The foundation for research into energetics arose from Meyerhof's experiments on oxidation of lactate in isolated muscles recovering from electrical contractions in an O2 atmosphere. Today, we know that mitochondria are actually reticula and that the energy released from electron pairs being passed along the electron transport chain from NADH to O2 generates a membrane potential and pH gradient of protons that can enter the molecular machine of ATP synthase to resynthesize ATP. Lactate stands at the crossroads of glycolytic and oxidative energy metabolism. Based on reported research and our own modelling in silico, we contend that lactate is not directly oxidized in the mitochondrial matrix. Instead, the interim glycolytic products (pyruvate and NADH) are held in cytosolic equilibrium with the products of the lactate dehydrogenase (LDH) reaction and the intermediates of the malate-aspartate and glycerol 3-phosphate shuttles. This equilibrium supplies the glycolytic products to the mitochondrial matrix for OXPHOS. LDH in the mitochondrial matrix is not compatible with the cytoplasmic/matrix redox gradient; its presence would drain matrix reducing power and substantially dissipate the proton motive force. OXPHOS requires O2 as the final electron acceptor, but O2 supply is sufficient in most situations, including exercise and often acute illness. Recent studies suggest that atmospheric normoxia may constitute a cellular hyperoxia in mitochondrial disease. As research proceeds appropriate oxygenation levels should be carefully considered.

Malate-aspartate shuttle inhibitor aminooxyacetic acid blocks lipopolysaccharides-induced activation of BV2 microglia

NADH shuttles, including malate-aspartate shuttle (MAS) and glycerol-3-phosphate shuttle, mediate the transfer of the reducing equivalents of cytosolic NADH into mitochondria. In our current study, we used BV2 microglia as a cellular model to determine the roles of NADH shuttles in lipopolysaccharides (LPS)-induced microglial activation. We found that aminooxyacetic acid (AOAA), a widely used MAS inhibitor, significantly attenuated LPS-induced increases in the levels of nitric oxide-a hallmarker of microglial activation. Our Western Blot assays also showed that AOAA blocked the LPS-induced increases in the protein levels of iNOS, TNF-α and COX-2. Furthermore, we found that AOAA decreased LPS-induced nuclear translocation of NF-κB. Collectively, our study has suggested that AOAA may be a new agent for inhibiting microglial activation. Our study has also suggested that MAS may be a novel target for modulating microglial activation under pathological conditions.