Evidence for the oxidation of glycolytic NADH by the malate-aspartate shuttle in Ehrlich ascites tumor cells

The malate-aspartate shuttle (Borst cycle): How it started and developed into a major metabolic pathway

This article presents a personal and critical review of the history of the malate-aspartate shuttle (MAS), starting in 1962 and ending in 2020. The MAS was initially proposed as a route for the oxidation of cytosolic NADH by the mitochondria in Ehrlich ascites cell tumor lacking other routes, and to explain the need for a mitochondrial aspartate aminotransferase (glutamate oxaloacetate transaminase 2 [GOT2]). The MAS was soon adopted in the field as a major pathway for NADH oxidation in mammalian tissues, such as liver and heart, even though the energetics of the MAS remained a mystery. Only in the 1970s, LaNoue and coworkers discovered that the efflux of aspartate from mitochondria, an essential step in the MAS, is dependent on the proton-motive force generated by the respiratory chain: for every aspartate effluxed, mitochondria take up one glutamate and one proton. This makes the MAS in practice uni-directional toward oxidation of cytosolic NADH, and explains why the free NADH/NAD ratio is much higher in the mitochondria than in the cytosol. The MAS is still a very active field of research. Most recently, the focus has been on the role of the MAS in tumors, on cells with defects in mitochondria and on inborn errors in the MAS. The year 2019 saw the discovery of two new inborn errors in the MAS, deficiencies in malate dehydrogenase 1 and in aspartate transaminase 2 (GOT2). This illustrates the vitality of ongoing MAS research.

SIRT3-dependent GOT2 acetylation status affects the malate-aspartate NADH shuttle activity and pancreatic tumor growth

The malate-aspartate shuttle is indispensable for the net transfer of cytosolic NADH into mitochondria to maintain a high rate of glycolysis and to support rapid tumor cell growth. The malate-aspartate shuttle is operated by two pairs of enzymes that localize to the mitochondria and cytoplasm, glutamate oxaloacetate transaminases (GOT), and malate dehydrogenases (MDH). Here, we show that mitochondrial GOT2 is acetylated and that deacetylation depends on mitochondrial SIRT3. We have identified that acetylation occurs at three lysine residues, K159, K185, and K404 (3K), and enhances the association between GOT2 and MDH2. The GOT2 acetylation at these three residues promotes the net transfer of cytosolic NADH into mitochondria and changes the mitochondrial NADH/NAD(+) redox state to support ATP production. Additionally, GOT2 3K acetylation stimulates NADPH production to suppress ROS and to protect cells from oxidative damage. Moreover, GOT2 3K acetylation promotes pancreatic cell proliferation and tumor growth in vivo. Finally, we show that GOT2 K159 acetylation is increased in human pancreatic tumors, which correlates with reduced SIRT3 expression. Our study uncovers a previously unknown mechanism by which GOT2 acetylation stimulates the malate-aspartate NADH shuttle activity and oxidative protection.

Pyruvate kinase type M2 and its role in tumor growth and spreading

Proliferating cells and tumor cells in particular express the pyruvate kinase isoenzyme type M2 (M2-PK). Within the tumor metabolome M2-PK regulates the proportions of glucose carbons that are channelled to synthetic processes (inactive dimeric form) or used for glycolytic energy production (highly active tetrameric form, a component of the glycolytic enzyme complex). In tumor cells, the dimeric form of M2-PK (Tumor M2-PK) is always predominant. The dimerization is caused by direct interaction of M2-PK with certain oncoproteins. The switch between the tetrameric and dimeric form of M2-PK allows tumor cells to survive in environments with varying oxygen und nutrient supply.

Subcellular localization of enzyme activities in chemostat-grown murine myeloma cells

As part of the development of structured models for the metabolism of myeloma cells in suspension culture, a study was made of the subcellular localization of key enzymes of glucose and glutamine metabolism. Steady state chemostat cultures of the mouse myeloma SP2/0-Ag14 were used as a reproducible source of biomass. Homogenates of the cells, obtained via mechanical disruption, were separated into a mitochondrial and a cytosolic fraction via differential centrifugation. The following conclusions are drawn: (1) approximately one fifth of the hexokinase activity of cell-free homogenates is associated with the mitochondria; (2) a malate-aspartate shuttle may operate for oxidation of cytosolic NADH, as indicated by high levels of malate dehydrogenase and aspartate aminotransferase in both particulate and soluble fractions; (3) the pentose phosphate pathway and isocitrate dehydrogenase may contribute to the provision of cytosolic NADPH; (4) phosphoenolpyruvate carboxykinase and pyruvate kinase, which are present in high activities, are exclusively cytosolic and probably play a key role in glutamine metabolism; (5) oxidation of glutamine via these enzymes leads to the formation of pyruvate that enters the same pool as pyruvate generated by glycolysis. As a result, lactate and alanine formation can occur from both glucose and glutamine.

The role of phosphometabolites in cell proliferation, energy metabolism, and tumor therapy

A common characteristic of tumor cells is the constant overexpression of glycolytic and glutaminolytic enzymes. In tumor cells the hyperactive hexokinase and the partly inactive pyruvate kinase lead to an expansion of all phosphometabolites from glucose 6-phosphate to phosphoenolpyruvate. In addition to the glycolytic phosphometabolites, synthesis of their metabolic derivatives such as P-ribose-PP, NADH, NADPH, UTP, CTP, and UDP-N-acetyl glucosamine is also enhanced during cell proliferation. Another phosphometabolite derived from P-ribose-PP, AMP, inhibits cell proliferation. The accumulation of AMP inhibits both P-ribose-PP-synthetase and the increase in concentration of phosphometabolites derived from P-ribose-PP. In cells with low glycerol 3-phosphate and malate-aspartate shuttle capacities the inhibition of the lactate dehydrogenase by low NADH levels leads to an inhibition of glycolytic ATP production. Several tumor-therapeutic drugs reduce NAD and NADH levels, thereby inhibiting glycolytic energy production. The role of AMP, NADH, and NADPH levels in the success of chemotherapeutic treatment is discussed.

The role of phosphometabolites in cell proliferation, energy metabolism, and tumor therapy

A common characteristic of tumor cells is the constant overexpression of glycolytic and glutaminolytic enzymes. In tumor cells the hyperactive hexokinase and the partly inactive pyruvate kinase lead to an expansion of all phosphometabolites from glucose 6-phosphate to phosphoenolpyruvate. In addition to the glycolytic phosphometabolites, synthesis of their metabolic derivatives such as P-ribose-PP, NADH, NADPH, UTP, CTP, and UDP-N-acetyl glucosamine is also enhanced during cell proliferation. Another phosphometabolite derived from P-ribose-PP, AMP, inhibits cell proliferation. The accumulation of AMP inhibits both P-ribose-PP-synthetase and the increase in concentration of phosphometabolites derived from P-ribose-PP. In cells with low glycerol 3-phosphate and malate-aspartate shuttle capacities the inhibition of the lactate dehydrogenase by low NADH levels leads to an inhibition of glycolytic ATP production. Several tumor-therapeutic drugs reduce NAD and NADH levels, thereby inhibiting glycolytic energy production. The role of AMP, NADH, and NADPH levels in the success of chemotherapeutic treatment is discussed.

Effect of extracellular AMP on cell proliferation and metabolism of breast cancer cell lines with high and low glycolytic rates

In differentiated tissues, such as muscle and brain, increased adenosine monophosphate (AMP) levels stimulate glycolytic flux rates. In the breast cancer cell line MCF-7, which characteristically has a constantly high glycolytic flux rate, AMP induces a strong inhibition of glycolysis. The human breast cancer cell line MDA-MB-453, on the other hand, is characterized by a more differentiated metabolic phenotype. MDA-MB-453 cells have a lower glycolytic flux rate and higher pyruvate consumption than MCF-7 cells. In addition, they have an active glycerol 3-phosphate shuttle. AMP inhibits cell proliferation as well as NAD and NADH synthesis in both MCF-7 and MDA-MB-453 cells. However, in MDA-MB-453 cells glycolysis is slightly activated by AMP. This disparate response of glycolytic flux rate to AMP treatment is presumably caused by the fact that the reduced NAD and NADH levels in AMP-treated MDA-MB-453 cells reduce lactate dehydrogenase but not cytosolic glycerol-3-phosphate dehydrogenase reaction. Due to the different enzymatic complement in MCF-7 cells, proliferation is inhibited under glucose starvation, whereas MDA-MB-453 cells grow under these conditions. The inhibition of cell proliferation correlates with a reduction in glycolytic carbon flow to synthetic processes and a decrease in phosphotyrosine content of several proteins in both cell lines.

In vitro effect of extracellular AMP on MCF-7 breast cancer cells: inhibition of glycolysis and cell proliferation

MCF-7 human breast cancer cells propagated in vitro were treated with adenosine derivatives added to the culture medium. The effects on cell proliferation, glycolysis, and glutaminolysis were investigated. Of all adenosine derivatives tested, AMP was the most efficient inhibitor of cell proliferation. In AMP-treated cells, DNA synthesis decreased, whereas RNA and protein syntheses rose normally with time. In terms of carbohydrate metabolism, lactate production from glucose was drastically reduced; therefore, most of lactate produced must have been derived from glutamine. Increases in the enzyme activities involved in glutamate degradation and in the malate-aspartate shuttle were observed. In contrast, actual glycolytic flux rates declined, whereas key glycolytic enzyme activities increased. Metabolites such as fructose 1,6-bisphosphate and pyruvate accumulated in AMP-arrested cells. Based on the lowered NAD level in the AMP-treated cells, lactate dehydrogenase, but not malate dehydrogenase, was impaired; thereby the whole of glycolysis was inhibited. In compensation, glutamine catabolism was increased. NAD concentrations fell drastically because of the known inhibition of P-ribose-PP synthesis through heightened intracellular AMP levels. A hypothetical metabolic scheme to explain these results and to show how extracellular AMP may influence carbohydrate metabolism and cell proliferation is presented.

Network analysis of intermediary metabolism using linear optimization. I. Development of mathematical formalism

Analysis of metabolic networks using linear optimization theory allows one to quantify and understand the limitations imposed on the cell by its metabolic stoichiometry, and to understand how the flux through each pathway influences the overall behavior of metabolism. A stoichiometric matrix accounting for the major pathways involved in energy and mass transformations in the cell was used in our analysis. The auxiliary parameters of linear optimization, the so-called shadow prices, identify the intermediates and cofactors that cause the growth to be limited on each nutrient. This formalism was used to examine how well the cell balances its needs for carbon, nitrogen, and energy during growth on different substrates. The relative values of glucose and glutamine as nutrients were compared by varying the ratio of rates of glucose to glutamine uptakes, and calculating the maximum growth rate. The optimum value of this ratio is between 2-7, similar to experimentally observed ratios. The theoretical maximum growth rate was calculated for growth on each amino acid, and the amino acids catabolized directly to glutamate were found to be the optimal nutrients. The importance of each reaction in the network can be examined both by selectively limiting the flux through the reaction, and by the value of the reduced cost for that reaction. Some reactions, such as malic enzyme and glutamate dehydrogenase, may be inhibited or deleted with little or no adverse effect on the calculated cell growth rate.

Influence of the malate-aspartate shuttle on oxidative metabolism in synaptosomes

beta-Methyleneaspartate, a specific inhibitor of aspartate aminotransferase (EC 2.6.1.1.), was used to investigate the role of the malate-aspartate shuttle in rat brain synaptosomes. Incubation of rat brain cytosol, "free" mitochondria, synaptosol, and synaptic mitochondria, with 2 mM beta-methyleneaspartate resulted in inhibition of aspartate aminotransferase by 69%, 67%, 49%, and 76%, respectively. The reconstituted malate-aspartate shuttle of "free" brain mitochondria was inhibited by a similar degree (53%). As a consequence of the inhibition of the aspartate aminotransferase, and hence the malate-aspartate shuttle, the following changes were observed in synaptosomes: decreased glucose oxidation via the pyruvate dehydrogenase reaction and the tricarboxylic acid cycle; decreased acetylcholine synthesis; and an increase in the cytosolic redox state, as measured by the lactate/pyruvate ratio. The main reason for these changes can be attributed to decreased carbon flow through the tricarboxylic acid cycle (i.e., decreased formation of oxaloacetate), rather than as a direct consequence of changes in the NAD+/NADH ratio. Malate/glutamate oxidation in "free" mitochondria was also decreased in the presence of 2 mM beta-methyleneaspartate. This is probably a result of decreased glutamate transport into mitochondria as a result of low levels of aspartate, which are needed for the exchange with glutamate by the energy-dependent glutamate-aspartate translocator.

The function of redox shuttles during aerobic glycolysis in two strains of Ehrlich ascites tumor cells

The function of glycerophosphate and malate-aspartate shuttles during glucose metabolism in two strains of Ehrlich ascites tumor cells was evaluated by several experimental approaches. The activities of the enzymes involved in these shuttle systems were assayed in the cytosolic and mitochondrial compartments after cell fractionation by the digitonin method. The glycerophosphate shuttle can be ruled out because of the lack of relevant enzymatic activities, and the failure of glucose to increase rotenone-inhibited respiration. Analysis of glycolytic flux in the presence of aminooxyacetate indicates that the activity of malate-aspartate shuttle may be very low. Balance studies of glucose uptake and lactate production suggest the existence of other pathways for the reoxidation of cytosolic NADH, which are acetyl-CoA dependent. Estimation of citrate synthase and ATP citrate lyase, in addition to the observed high activity of malate dehydrogenase, suggests a malate-citrate shuttle.

Evidence for the occurrence of the malate-citrate shuttle in intact Ehrlich ascites tumor cells

A possible activity of the malate-citrate shuttle has been investigated in Ehrlich ascites cells by testing the effects of 1,2,3-benzenetricarboxylic acid, an inhibitor of the malate-citrate exchange, and (-)-hydroxycitrate, an inhibitor of the citrate cleavage enzyme, on the glucose-dependent oxidation-reduction rates of pyridine nucleotides and cytochrome b as well as two inhibitors glycolyzing cells. Moreover, to quantitate such an activity, the effects of these two inhibitors have been compared with those induced under the same experimental conditions by aminooxyacetate, an inhibitor of the malate-aspartate shuttle which is known to operate in this strain of ascites tumor. Both benzenetricarboxylic acid and hydroxycitrate are able to increase the reduction of pyridine nucleotides, which follows glucose addition to whole cells, to about the same extent. A much more pronounced effect is elicited by aminooxyacetate under the same condition. When n-butylmalonate is added to slow down the flux of glycolytic reducing equivalents to the respiratory chain via the malate-aspartate shuttle, benzenetricarboxylic acid or hydroxycitrate promotes an ATP-driven reversal of electron transfer. Indeed, the glucose induced reduction of cytochrome b becomes sensitive to oligomycin and the ATP level is raised significantly with respect to the value of uninhibited cells. It is concluded that the malate-citrate shuttle operates in Ehrlich ascites cells, although with a substantially lower activity with respect to the malate-aspartate shuttle.

[Aerobic glycolysis of tumor cells]

A high aerobic glycolysis (aerobic lactate production) is the most significant feature of the energy metabolism of rapidly growing tumor cells. Several mechanisms, which may be different in different cell lines, seem to be involved in this characteristic of energy metabolism of the tumor cell. Changes in the cell membrane leading to increased uptake and utilization of glucose, a high level of fetal types of isoenzymes, a decreased number of mitochondria and a reduced capacity to metabolize pyruvate are some factors which must be taken into consideration. It is not possible to favour one of them at the present time.