The role of the alpha-glycerophosphate shuttle in the reoxidation of cytosolic NADH in Ehrlich ascites tumour cells

Mitochondrial FAD-linked Glycerol-3-phosphate Dehydrogenase: A Target for Cancer Therapeutics

Imbalances in cellular redox state are frequently observed in cancer cells, and contribute significantly to cancer progression and apoptotic resistance. Hydrogen peroxide (H₂O₂) is one reactive oxygen species (ROS) that is produced in excess within cancer cells. In this study, we investigated the mitochondrial glycerol-3-phosphate-dependent (GPD2) ROS production in PC-3 cells and demonstrated the importance of excessive H₂O₂ production on their survival. By exploiting the abnormal H₂O₂ production of PC-3 cells, we initiated a high-throughput screening of the Canadian Compound Collection, composed of 29,586 small molecules, targeting the glycerophosphate-dependent H₂O₂ formation in PC-3 cells. Eighteen compounds were identified to have significant inhibitory activity. These compounds have not been previously characterized as inhibitors of the enzyme. Six of these compounds were further analyzed in PC-3 cells and dose response studies displayed an inhibitory and anti-oxidative potency that ranged from 1 µM to 30 µM. The results presented here demonstrate that inhibitors of mitochondrial GPD2 activity elicit anti-proliferative effects on cancer cells.

The function and the role of the mitochondrial glycerol-3-phosphate dehydrogenase in mammalian tissues

Mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) is not included in the traditional textbook schemes of the respiratory chain, reflecting the fact that it is a non-standard, tissue-specific component of mammalian mitochondria. But despite its very simple structure, mGPDH is a very important enzyme of intermediary metabolism and as a component of glycerophosphate shuttle it functions at the crossroads of glycolysis, oxidative phosphorylation and fatty acid metabolism. In this review we summarize the present knowledge on the structure and regulation of mGPDH and discuss its metabolic functions, reactive oxygen species production and tissue and organ specific roles in mammalian mitochondria at physiological and pathological conditions.

Substrate-dependent utilization of the glycerol 3-phosphate or malate/aspartate redox shuttles by Ehrlich ascites cells

The rate of transfer of reducing equivalents from cytoplasm to mitochondria has been examined in Ehrlich ascites tumour cells incubated in the presence of lactate. The flux of reducing equivalents was determined from the rate of metabolism of reduced intermediates that are oxidized within the cytosol. The magnitude of the flux of reducing equivalents was dependent on both the concentration of added lactate and the presence of carbohydrate. The rate of flux was twice as great in the presence of glucose and four times as high when glucose and lactate were added together as when lactate was the only added substrate. Fructose was less effective than glucose in stimulating reducing equivalent flux. In the presence of glucose or fructose, there was a substantial accumulation of hexose phosphates, dihydroxyacetone phosphate and glycerol 3-phosphate. Rotenone, an inhibitor of NADH dehydrogenase, and amino-oxyacetate, which inhibits the malate/aspartate shuttle, were powerful suppressors of reducing equivalent flux from lactate as sole substrate, but were much less potent in the presence of carbohydrate. Antimycin substantially inhibited reducing equivalent flux from all combinations of added substrates, consistent with its ability to block oxidation of reducing equivalents transferred by both the malate/aspartate and glycerol 3-phosphate shuttles. The glycerol 3-phosphate shuttle represents around 80% of the maximum total observed activity but is active only while glycolytic intermediates are present to provide the necessary substrates of the shuttle. This Ehrlich ascites cell line has an essentially similar total reducing equivalent shuttle capacity to that of isolated hepatocytes.

alpha-Glycerophosphate shuttle in a clonal beta-cell line

It has been proposed that the alpha-glycerophosphate (alpha-GOP) shuttle plays a crucial role in regulation of glycolysis in beta-cells by linking reoxidation of cytosolic NADH to formation of ATP in the electron transport chain (J. Biol. Chem. 265: 8287, 1981). Direct evidence for this suggestion is still lacking, however. In this work the operation of the alpha-GOP shuttle was investigated in the insulin-secreting cell line HIT-T15. The constituent enzymes of the pathway were found to be present in HIT cells. Flavin-linked alpha-GOP dehydrogenase was associated with the mitochondrial fraction, whereas NAD+-dependent alpha-GOP dehydrogenase was localized in the cytosol. In the presence of amobarbital (used to preserve the function of the alpha-GOP shuttle under conditions where oxidation of NADH by the respiratory chain was blocked), glucose increased insulin secretion, O2 consumption, and the cell [ATP]/[ADP] when compared with amobarbital alone. These results indicate that the alpha-GOP shuttle contributes to ATP generation in HIT cells and that its activation may be necessary for the initiation of insulin secretion by glucose.

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.

Human glycerol kinase deficiency: an inborn error of compartmental metabolism

Twelve individuals have been described with glycerol kinase deficiency. Five of these individuals are adults who were noted incidentally to have pseudohypertriglyceridemia. Six of these individuals are children who manifest a clinical complex which includes adrenal hypoplasia/insufficiency and developmental delay. Another child has intermittent coma, a normal IQ, and no evidence of adrenal insufficiency. Genetic and biochemical hypotheses are proposed to explain this clinical variability. Glycerol kinase binds specifically and reversibly to the porin, the pore-forming protein of the outer mitochondrial membrane, which also binds hexokinase. Mutations affecting any component of this kinase-binding system will alter the properties of this system. Glycerol kinase deficiency, as an inborn error of this compartmented metabolic system, offers an investigational opportunity for studying this microenvironment.

Development of rat liver and colon substrate cycle enzymes in offspring with and without prenatal exposure to a carcinogenic dose of 1,2-dimethylhydrazine

Pregnant F-344 rats were exposed by intubation to a single dose (35 mg/kg) of 1,2-dimethylhydrazine dihydrochloride or to an acetate buffer on day 14 of gestation. A detailed examination of the effects of this dose of 1,2-dimethylhydrazine on brush border enzymes in the offspring was performed. Changes in the liver and colon levels of the alpha-glycerophosphate and malate/aspartate substrate cycle enzymes were measured during the development; at days 17 and 20 of gestation and at 2, 6, 13, 20, 27, 55, 110 days and 1 year after birth. It is concluded that metabolic energy enzymes, glutamate oxaloacetate transaminase and malate dehydrogenase, are more sensitive to 1,2-dimethylhydrazine treatment than are the brush border hydrolases or NAD- and Fp-linked alpha-glycerophosphate dehydrogenases.

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.

On the mechanism of the glucose-induced ATP catabolism in ascites tumour cells and its reversal by pyruvate

Addition of glucose to Ehrlich-Landschütz ascites tumour cells preincubated for 30-60 min in phosphate-buffered Krebs-Ringer salt solution ("starved cells") resulted within 1-2 min in an approx. 90% decline of their ATP content and a massive accumulation of fructose 1,6-bisphosphate. These alterations, which took place under both aerobic and anaerobic conditions, were followed by a gradual spontaneous recovery with restoration of normal ATP and fructose 1,6-bisphosphate values. The transient derangement of the energy metabolism after glucose addition to starved ascites tumour cells by preventable by simultaneous addition of pyruvate or 2-oxobutyrate, or by preincubating the cells in the presence of glucose. The protective effect of pyruvate was duplicated by addition of phenazine methosulphate or NAD+ to the incubation medium. The data seem to warrant the conclusion that the glucose-induced ATP depletion is determined by a blockade of glycolysis at the stage of glyceraldehyde phosphate dehydrogenase caused by the failure of the cells to oxidize the NADH produced in the same reaction. The continued unrestrained action of 6-phosphofructokinase results in accumulation of fructose 1,6-bisphosphate, which constitutes a trap for the high-energy phosphate bonds of ATP. The primary metabolic disturbance appears to consist of a transient inhibition of pyruvate kinase with the resultant inability of the cells to maintain an unimpaired supply of pyruvate, as required for the lactate dehydrogenase-mediated oxidation of NADH. The regulatory mechanism underlying this phenomenon is discussed.

Superoxide dismutase content and microsomal lipid composition of tumours with different growth rates

The content of cytosolic superoxide dismutase has been determined in Morris hepatomas 3924A (fast-growing) and 44 (slow-growing) and in ascites tumour cells (Novikoff hepatoma and Ehrlich-Lettré). The enzyme is decreased in all the tumours examined. The lowest amounts were found in the tumours with the fastest growth rates. Measurements of the lipid composition and fluidity of microsomal membranes isolated from Morris hepatomas show that also these parameters are changed in relation to the growth rate. The lipid to protein ratio and the degree of fatty acid unsaturation decrease gradually from rat liver to hepatoma 44 and 3924A microsomes. The different lipid composition is reflected also by differences in the physical environment of the bilayer, as indicated by data obtained with spin-labeled fatty acids. It is proposed that the changes in the membrane lipid composition and organization are consequent to the decrease in the protective effect of cytosolic superoxide dismutase against the O2- induced lipid peroxidation.

The operation of the malate-aspartate shuttle in the reoxidation of glycolytic NADH in slices of fetal rat liver

Lactate production by liver slices from fetal rats (17th--18th day of gestation) is enhanced about two fold by aminooxyacetate, an inhibitor of aspartate transaminase (EC 2.6.1.1). Such an effect is consistent with an increase of the cytosolic NAD-redox state owing to the parallel fall in the pyruvate level, whereas the glycolytic flux does not seem to be influenced appreciably. Indeed, although the inhibitor causes a marked increase of fructose 1,6-diphosphate, glucose-6-phosphate decreases only slightly. These results suggest that in fetal rat liver the malate-aspartate shuttle is operative in the reoxidation of cytosolic NADH produced during aerobic glycolysis.

Pyruvate transport in tumour-cell mitochondria

Tumour-cell mitochondria contain a pyruvate-transporting system exhibiting the same general properties as those described in rat liver mitochondria. The Km for net pyruvate uptake in tumour-cell mitochondria is practically similar to that measured in rat liver mitochondria but the V is lower. This difference is also shown by swelling experiments. The possible implication of these observations in the context of lactate accumulation in tumour-cell is discussed.

NAD(P)H utilization in the reduction of pyruvate to lactate in a glycogen-containing subline of Ehrlich ascites tumour cells

The possible pathways of utilization of glucose-6-phosphate (G-6-P) produced from glycogen breakdown have been investigated in a glycogen-containing subline of Ehrlich ascites tumour cells. Addition of either mitochondrial inhibitors or pyruvate to ascites cells metabolizing endogenous substrates enhances the rate of lactate production. However, only in the former condition such effect is abolished by iodoacetate (IAA). In pyruvate-supplemented cells mitochondrial inhibitors cause a further increase in lactate production which becomes insensitive to IAA when the cells are depleted of endogenous substrates. Measurements of the glycogen content show that either in the presence of mitochondrial inhibitors or pyruvate there is a stimulation of glycogenolysis. Significant changes (about 10--20 fold increase) of the G-6-P level are observed only in the presence of both mitochondrial inhibitors and IAA, irrespective of pyruvate addition. However, with pyruvate the accumulation of G-6-P becomes lower if the cells are starved. The results obtained indicate that in our conditions G-6-P which is produced during glycogenolysis may be oxidized either through the Embden-Meyerhof pathway or the phosphogluconate pathway. Indeed, whereas mitochondrial inhibitors promote the utilization of this metabolite through the first route by enhancing the activity of phosphofructokinase, added pyruvate favours the other route by lowering the cytosolic NADPH/NADP+ ratio.

Glycerol phosphate shuttle in virus-transformed cells in culture

The glycerol phosphate shuttle is shown not only to be present and functional in virus-transformed cells, but its level is higher than in normal cells in culture. The increased aerobic glycolysis that has been demonstrated for these cells after transformation, therefore, is not due to an impairment of hydrogen transfer pathways.

Changes in enzyme pattern of Ehrlich ascites tumor cells following serial cultivation in media with increased (hypertonic) NaCl content

Adaptation of Ehrlich ascites tumor cells to serial cultivation in media with progressively elevated (hypertonic) NaCl content ("high NaCl"-tolerant cells) has resulted in progressive increases of the cellular activities of NAD-dependent glycerol-3-phosphate dehydrogenase (EC 1.1.1.8), NAD-dependent malate dehydrogenase (EC 1.1.1.37), glutamate--oxalacetate transaminase (EC 2.6.1.1), NAD (P)-dependent glutamate dehydrogenase (EC 1.4.1.3), NADP-dependent isocitrate dehydrogenase (EC 1.1.1.42). The activities of glutamate-pyruvate transaminase (EC 2.6.1.2.) and of glycolytic enzymes as phospho-fructokinase (EC 2.7.1.11), glyceraldehydephosphate dehydrogenase (EC 1.2.1.12) and lactate dehydrogenase (EC 1.1.1.27) were only slightly and not in progressive manner (in response to the progressive increase of the environmental NaCl concentration) affected. These changes are discussed with respect to a metabolic pattern of these "high NaCl"-tolerant cells which is compatible with increased energy requirements, especially for active cation transport. It is suggested that these increased cellular enzyme activities reflect an increased transfer of reducing equivalents across mitochondrial membranes (via the "glycerophosphate cycle and the malate-aspartate shuttle") and possibly a stimulated lipid metabolism. These alterations in the level of enzyme activities must be regarded asan adaptive cellular response to the "high NaCl" environment, since readaptation to growth in regular isotonic media resulted in a reversion to the enzyme pattern characteristic of the parent cells.