Modulation of Lactate Dehydrogenase Isozymes by Modified Base Queuine

The modified base queuine is a nutrient factor for lower and higher eukaryotes except yeast. It is synthesized in eubacteria and inserted into the wobble position of specific tRNAs (tRNA_GUN) in exchange of guanine at position 34. The tRNAs of Q family are completely modified in terminally differentiated somatic cells. However, mainly free queuine is present in embryonic and fast proliferating cells, tRNA remains Q deficient. Lactate dehydrogenase (LDH) A mRNA and LDH A protein is known to increase when cells are grown in hypoxic conditions. In the present study, the level of LDH isozymes is analyzed in different tissues of normal and cancerous (DLA) mice and the effect of queuine treatment on LDH isozyme is observed. LDH A isozyme is shown to increase in serum and liver of DLA mice. The level and activity of LDH A decreases on queuine treatment. In skeletal muscle and heart, LDH A isozyme decreases while LDH B increases in DLA mice. Queuine administration leads to change back towards normal. In case of brain, LDH A increases but LDH B decreases in DLA mice. Queuine treatment leads to decrease in A4 anaerobic isozymes of LDH. The results suggest that queuine suppresses anaerobic glycolytic pathway, which leads to tumor suppression of DLA mice.     

Hexokinase 3 dysfunction promotes tumorigenesis and immune escape by upregulating monocyte/macrophage infiltration into the clear cell renal cell carcinoma microenvironment

Purpose: This study aimed to identify the potential prognostic role of HK3 and provide clues about glycolysis and the microenvironmental characteristics of ccRCC.Methods: Based on the Cancer Genome Atlas (TCGA, n = 533) and Gene expression omnibus (GEO) (n = 127) databases, real-world (n = 377) ccRCC cohorts, and approximately 15,000 cancer samples, the prognostic value and immune implications of HK3 were identified. The functional effects of HK3 in ccRCC were analyzed in silico and in vitro.Results: The large-scale findings suggested a significantly higher HK3 expression in ccRCC tissues and the predictive efficacy of HK3 for tumor progression and a poor prognosis. Next, the subgroup survival and Cox regression analyses showed that HK3 serves as a promising and independent predictive marker for the prognosis and survival of patients with ccRCC from bioinformatic databases and real-world cohorts. Subsequently, we found that HK3 could be used to modulate glycolysis and the malignant behaviors of ccRCC cells. The comprehensive results suggested that HK3 is highly correlated with the abundance of immune cells, and specifically stimulates the infiltration of monocytes/macrophages presenting surface markers, regulates the immune checkpoint molecules PD-1 and CTLA-4 of exhaustive T cells, restrains the immune escape of tumor cells, and prompts the immune-rejection microenvironment of ccRCC.Conclusion: In conclusion, the large-scale data first revealed that HK3 could affect glycolysis, promote malignant biologic processes, and predict the aggressive progression of ccRCC. HK3 may stimulate the abundance of infiltrating monocytes/macrophages presenting surface markers and regulate the key molecular subgroups of immune checkpoint molecules of exhaustive T cells, thus inducing the microenvironmental characteristics of active anti-tumor immune responses.     

Purification and characterization of an allosteric fructose-1,6-bisphosphate aldolase from germinating mung beans (Vigna radiata)

Cytosolic fructose-1,6-P(2) (FBP) aldolase (ALD(c)) from germinated mung beans has been purified 1078-fold to electrophoretic homogeneity and a final specific activity of 15.1 micromol FBP cleaved/min per mg of protein. SDS-PAGE of the final preparation revealed a single protein-staining band of 40 kDa that cross-reacted strongly with rabbit anti-(carrot ALD(c))-IgG. The enzyme's native M(r) was determined by gel filtration chromatography to be 160 kDa, indicating a homotetrameric quaternary structure. This ALD is a class I ALD, since EDTA or Mg(2+) had no effect on its activity, and was relatively heat-stable losing 0-25% of its activity when incubated for 5 min at 55-65 degrees C. It demonstrated: (i) a temperature coefficient (Q(10)) of 1.7; (ii) an activation energy of 9.2 kcal/mol active site; and (iii) a broad pH-activity optima of 7.5. Mung bean ALD(c) is bifunctional for FBP and sedoheptulose-1,7-P(2) (K(m) approximately 17 microM for both substrates). ATP, ADP, AMP and ribose-5-P exerted inhibitory effects on the activity of the purified enzyme. Ribose-5-P, ADP and AMP functioned as competitive inhibitors (K(i) values=2.2, 3.1 and 7.5mM, respectively). By contrast, the addition of 2mM ATP: (i) reduced V(max) by about 2-fold, (ii) increased K(m)(FBP) by about 4-fold, and (iii) shifted the FBP saturation kinetic plot from hyperbolic to sigmoidal (h=1.0 and 2.6 in the absence and presence of 2mM ATP, respectively). Potent feedback inhibition of ALD(c) by ATP is suggested to help balance cellular ATP demands with the control of cytosolic glycolysis and respiration in germinating mung beans.     

Bioenergetic Mechanisms in Astrocytes May Contribute to Amyloid Plaque Deposition and Toxicity

Alzheimer disease (AD) is characterized neuropathologically by synaptic disruption, neuronal loss, and deposition of amyloid β (Aβ) protein in brain structures that are critical for memory and cognition. There is increasing appreciation, however, that astrocytes, which are the major non-neuronal glial cells, may play an important role in AD pathogenesis. Unlike neurons, astrocytes are resistant to Aβ cytotoxicity, which may, in part, be related to their greater reliance on glycolytic metabolism. Here we show that, in cultures of human fetal astrocytes, pharmacological inhibition or molecular down-regulation of a main enzymatic regulator of glycolysis, 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase (PFKFB3), results in increased accumulation of Aβ within and around astrocytes and greater vulnerability of these cells to Aβ toxicity. We further investigated age-dependent changes in PFKFB3 and astrocytes in AD transgenic mice (TgCRND8) that overexpress human Aβ. Using a combination of Western blotting and immunohistochemistry, we identified an increase in glial fibrillary acidic protein expression in astrocytes that paralleled the escalation of the Aβ plaque burden in TgCRND8 mice in an age-dependent manner. Furthermore, PFKFB3 expression also demonstrated an increase in these mice, although at a later age (9 months) than GFAP and Aβ. Immunohistochemical staining showed significant reactive astrogliosis surrounding Aβ plaques with increased PFKFB3 activity in 12-month-old TgCRND8 mice, an age when AD pathology and behavioral deficits are fully manifested. These studies shed light on the unique bioenergetic mechanisms within astrocytes that may contribute to the development of AD pathology.     

Fructose-1,6-bisphosphate as a metabolic substrate in hog ileum smooth muscle during hypoxial

Exogenously applied fructose-1,6-bisphosphate has been reported to be effective in preventing some damage to the small intestine during ischemia. To determine whether exogenously applied fructose-1,6-bisphosphate protects ileum smooth muscle from damage from hypoxia and from reoxygenation, we examined the effect of fructose-1,6-bisphosphate on the ability of hog ileum smooth muscle to maintain isometric force during hypoxia and to generate isometric force after reoxygenation in the presence of 5 mM glucose. After 180 min of hypoxia, tissues incubated with 20 mM fructose-1,6-bisphosphate maintained significantly greater levels of isometric force than tissues incubated in the absence of exogenous substrate (23% of pre-hypoxia force compared to 16%). During the first contraction following reoxygenation there was a significantly greater force generation in tissues incubated with 20 mM fructose-1,6-bisphosphate during the hypoxia period compared to tissues with no exogenous substrate included during the hypoxia period (29% of pre-hypoxia force compared to 19%). However, glucose always was a better metabolic substrate compared to fructose-1,6-bisphosphate under all experimental conditions. The presence of fructose-1,6-bisphosphate during hypoxia likely improved tissue function by fructose-1,6-bisphosphate entering the cells and acting as a glycolytic intermediate, since during a 120 min period of hypoxia, unmounted ileum smooth muscle metabolized 1,6-¹³C-fructose-1,6-bisphosphate to 3-¹³C-lactate. This conversion of 1,6-¹³C-fructose-1,6-bisphosphate to 3-¹³C-lactate was inhibited by the addition of 1 mM iodoacetic acid, a glycolytic inhibitor. We conclude that exogenously provided fructose-1,6-bisphosphate does provide modest protection of ileum smooth muscle from hypoxic damage by functioning as a glycolytic intermediate and improving the cellular energy state.This work was supported in part by NIH (HL48783 to CDH), NSF (Instrumentation Grant 8908304), and the Department of Physiology of the University of Missouri. T. Juergens was supported by the School of Medicine and the Department of Physiology of the University of Missouri.     

Caveolae and the organization of carbohydrate metabolism in vascular smooth muscle

We have previously found that glycolysis and gluconeogenesis occur in separate "compartments" of the VSM cell. These compartments may result from spatial separation of glycolytic and gluconeogenic enzymes (Lloyd and Hardin [1999] Am J Physiol Cell Physiol. 277:C1250-C1262). We have also found that an intact plasma membrane is essential for compartmentation to exist (Lloyd and Hardin [2000] Am J Physiol Cell Physiol. 278:C803-C811), suggesting that glycolysis and gluconeogenesis may be associated with distinct plasma membrane microdomains. Caveolae are one such microdomain, in which proteins of related function colocalize. Thus, we hypothesized that membrane-associated glycolysis occurs in association with caveolae, while gluconeogenesis is localized to non-caveolae domains. To test this hypothesis, we disrupted caveolae in vascular smooth muscle (VSM) of pig cerebral microvessels (PCMV) with beta methyl-cyclodextrin (CD) and examined the metabolism of [2-(13)C]glucose (a glycolytic substrate) and [1-(13)C]fructose 1,6-bisphosphate (FBP, a gluconeogenic substrate in PCMV) using (13)C nuclear magnetic resonance spectroscopy. Caveolar disruption reduced flux of [2-(13)C]glucose to [2-(13)C]lactate, suggesting that caveolar disruption partially disrupted the glycolytic pathway. Caveolae disruption may also have resulted in a breakdown of compartmentation, since conversion of [1-(13)C]FBP to [3-(13)C]lactate was increased by CD treatment. Alternatively, the increased [3-(13)C]lactate production may reflect changes in FBP uptake, since conversion of [1-(13)C]FBP to [3-(13)C]glucose was also elevated in CD-treated cells. Thus, a link between caveolar organization and metabolic organization may exist.     

The oxidized form of vitamin C,dehydroascorbic acid,regulates neuronal energy metabolism

Vitamin C is an essential factor for neuronal function and survival, existing in two redox states, ascorbic acid (AA), and its oxidized form, dehydroascorbic acid (DHA). Here, we show uptake of both AA and DHA by primary cultures of rat brain cortical neurons. Moreover, we show that most intracellular AA was rapidly oxidized to DHA. Intracellular DHA induced a rapid and dramatic decrease in reduced glutathione that was immediately followed by a spontaneous recovery. This transient decrease in glutathione oxidation was preceded by an increase in the rate of glucose oxidation through the pentose phosphate pathway (PPP), and a concomitant decrease in glucose oxidation through glycolysis. DHA stimulated the activity of glucose‐6‐phosphate dehydrogenase, the rate‐limiting enzyme of the PPP. Furthermore, we found that DHA stimulated the rate of lactate uptake by neurons in a time‐ and dose‐dependent manner. Thus, DHA is a novel modulator of neuronal energy metabolism by facilitating the utilization of glucose through the PPP for antioxidant purposes.

     

Enzymes of glycolysis and the pentose phosphate pathway during development of the rabbit stomach worm Obeliscoides cuniculi

The activities of glycolytic and other enzymes of carbohydrate metabolism were measured in free-living and parasitic stages of the rabbit stomach worm Obeliscoides cuniculi. Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, phosphoglucomutase, hexokinase, glucosephosphate isomerase, phosphofructokinase, aldolase, triosephosphate isomerase, α-glycerophosphatase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, phosphoglycerate mutase, enolase, pyruvate kinase, phosphoenol pyruvate carboxykinase, lactate dehydrogenase, alcohol dehydrogenase, and glucose-6-phosphatase activities were present in worms recovered 14, 20 and 190 days postinfection.The presence of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, and glucose-6-phosphatase indicates the possible function of a pentose phosphate pathway and a capacity for gluconeogenesis, respectively, in these worms.The ratio of pyruvate kinase (PK) to phosphoenol pyruvate carboxykinase (PEPCK) less than I in parasitic stages suggests that their most active pathway is that fixing CO₂ into phosphoenol pyruvate to produce oxaloacetate.Low levels of glucose-6-phosphate dehydrogenase, triosephosphate isomerase, PEPCK and PK were recorded in infective third-stage larvae stored at 5°C for 5 and 12 mos. The ratio of PK to PEPCK greater than 1 indicates that infective larvae preferentially utilize a different terminal pathway than the parasitic stages.