Changes in pulmonary expression of hexokinase and glucose transporter mRNAs in rats adapted to hyperoxia

The pentose phosphate pathway mediates hyperoxia-induced lung vascular dysgenesis and alveolar simplification in neonates

Dysmorphic pulmonary vascular growth and abnormal endothelial cell (EC) proliferation are paradoxically observed in premature infants with bronchopulmonary dysplasia (BPD), despite vascular pruning. The pentose phosphate pathway (PPP), a metabolic pathway parallel to glycolysis, generates NADPH as a reducing equivalent and ribose 5-phosphate for nucleotide synthesis. It is unknown whether hyperoxia, a known mediator of BPD in rodent models, alters glycolysis and the PPP in lung ECs. We hypothesized that hyperoxia increases glycolysis and the PPP, resulting in abnormal EC proliferation and dysmorphic angiogenesis in neonatal mice. To test this hypothesis, lung ECs and newborn mice were exposed to hyperoxia and allowed to recover in air. Hyperoxia increased glycolysis and the PPP. Increased PPP, but not glycolysis, caused hyperoxia-induced abnormal EC proliferation. Blocking the PPP reduced hyperoxia-induced glucose-derived deoxynucleotide synthesis in cultured ECs. In neonatal mice, hyperoxia-induced abnormal EC proliferation, dysmorphic angiogenesis, and alveolar simplification were augmented by nanoparticle-mediated endothelial overexpression of phosphogluconate dehydrogenase, the second enzyme in the PPP. These effects were attenuated by inhibitors of the PPP. Neonatal hyperoxia augments the PPP, causing abnormal lung EC proliferation, dysmorphic vascular development, and alveolar simplification. These observations provide mechanisms and potential metabolic targets to prevent BPD-associated vascular dysgenesis.

Difference in pyruvic acid metabolism between neonatal and adult mouse lungs exposed to hyperoxia

Objective

Neonatal lungs are more tolerant to hyperoxic injury than are adult lungs. This study investigated differences in the response to hyperoxic exposure between neonatal and adult mouse lungs using metabolomics analysis with capillary electrophoresis time-of-flight mass spectrometry (CE- TOFMS).

Methods

Neonatal and adult mice were exposed to 21% or 95% O₂ for four days. Subsequently, lung tissue samples were collected and analyzed by CE-TOFMS. Pyruvate dehydrogenase (PDH) enzyme activity was determined using a microplate assay kit. PDH kinase (Pdk) 1, Pdk2, Pdk3, and Pdk4 mRNA expression levels were determined using quantitative reverse transcription-polymerase chain reaction. Pdk4 protein expression was quantified by Western blotting and Pdk4 protein localization was evaluated by immunohistochemistry.

Results

Levels of 3-phosphoglyceric acid, 2-phosphoglyceric acid, phosphoenolpyruvic acid, and lactic acid were significantly elevated in the lungs of hyperoxia-exposed versus normoxia-exposed adult mice, whereas no significant differences were observed with hyperoxia exposure in neonatal mice. PDH activity was reduced in the lungs of adult mice only. Pdk4 mRNA expression levels after hyperoxic exposure were significantly elevated in adult mice compared with that in neonatal mice. Conversely, gene expression levels of Pdk1, Pdk2, and Pdk3 did not differ after hyperoxic exposure in either neonatal or adult mice. Pdk4 protein levels were also significantly increased in adult mouse lungs exposed to hyperoxia and were localized mainly to the epithelium of terminal bronchiole.

Conclusions

Specific metabolites associated with glycolysis and gluconeogenesis were altered after hyperoxia exposure in the lungs of adult mice, but not in neonates, which was likely a result of reduced PDH activity due to Pdk4 mRNA upregulation under hyperoxia.

Hyperoxia synergizes with mutant bone morphogenic protein receptor 2 to cause metabolic stress, oxidant injury, and pulmonary hypertension

Pulmonary arterial hypertension (PAH) has been associated with a number of different but interrelated pathogenic mechanisms. Metabolic and oxidative stresses have been shown to play important pathogenic roles in a variety of model systems. However, many of these relationships remain at the level of association. We sought to establish a direct role for metabolic stress and oxidant injury in the pathogenesis of PAH. Mice that universally express a disease-causing mutation in bone morphogenic protein receptor 2 (Bmpr2) were exposed to room air or to brief daily hyperoxia (95% oxygen for 3 h) for 6 weeks, and were compared with wild-type animals undergoing identical exposures. In both murine tissues and cultured endothelial cells, the expression of mutant Bmpr2 was sufficient to cause oxidant injury that was particularly pronounced in mitochondrial membranes. With the enhancement of mitochondrial generation of reactive oxygen species by hyperoxia, oxidant injury was substantially enhanced in mitochondrial membranes, even in tissues distant from the lung. Hyperoxia, despite its vasodilatory actions in the pulmonary circulation, significantly worsened the PAH phenotype (elevated right ventricular systolic pressure, decreased cardiac output, and increased pulmonary vascular occlusion) in Bmpr2 mutant animals. These experiments demonstrate that oxidant injury and metabolic stress contribute directly to disease development, and provide further evidence for PAH as a systemic disease with life-limiting cardiopulmonary manifestations.

Hyperoxia decreases glycolytic capacity, glycolytic reserve and oxidative phosphorylation in MLE-12 cells and inhibits complex I and II function, but not complex IV in isolated mouse lung mitochondria

High levels of oxygen (hyperoxia) are frequently used in critical care units and in conditions of respiratory insufficiencies in adults, as well as in infants. However, hyperoxia has been implicated in a number of pulmonary disorders including bronchopulmonary dysplasia (BPD) and adult respiratory distress syndrome (ARDS). Hyperoxia increases the generation of reactive oxygen species (ROS) in the mitochondria that could impair the function of the mitochondrial electron transport chain. We analyzed lung mitochondrial function in hyperoxia using the XF24 analyzer (extracellular flux) and optimized the assay for lung epithelial cells and mitochondria isolated from lungs of mice. Our data show that hyperoxia decreases basal oxygen consumption rate (OCR), spare respiratory capacity, maximal respiration and ATP turnover in MLE-12 cells. There was significant decrease in glycolytic capacity and glycolytic reserve in MLE-12 cells exposed to hyperoxia. Using mitochondria isolated from lungs of mice exposed to hyperoxia or normoxia we have shown that hyperoxia decreased the basal, state 3 and state3 μ (respiration in an uncoupled state) respirations. Further, using substrate or inhibitor of a specific complex we show that the OCR via complex I and II, but not complex IV was decreased, demonstrating that complexes I and II are specific targets of hyperoxia. Further, the activities of complex I (NADH dehydrogenase, NADH-DH) and complex II (succinate dehydrogenase, SDH) were decreased in hyperoxia, but the activity of complex IV (cytochrome oxidase, COX) remains unchanged. Taken together, our study show that hyperoxia impairs glycolytic and mitochondrial energy metabolism in in tact cells, as well as in lungs of mice by selectively inactivating components of electron transport system.

Coenzyme Q(1) as a probe for mitochondrial complex I activity in the intact perfused hyperoxia-exposed wild-type and Nqo1-null mouse lung

Previous studies showed that coenzyme Q(1) (CoQ(1)) reduction on passage through the rat pulmonary circulation was catalyzed by NAD(P)H:quinone oxidoreductase 1 (NQO1) and mitochondrial complex I, but that NQO1 genotype was not a factor in CoQ(1) reduction on passage through the mouse lung. The aim of the present study was to evaluate the complex I contribution to CoQ(1) reduction in the isolated perfused wild-type (NQO1(+/+)) and Nqo1-null (NQO1(-)/(-)) mouse lung. CoQ(1) reduction was measured as the steady-state pulmonary venous CoQ(1) hydroquinone (CoQ(1)H(2)) efflux rate during infusion of CoQ(1) into the pulmonary arterial inflow. CoQ(1)H(2) efflux rates during infusion of 50 μM CoQ(1) were not significantly different for NQO1(+/+) and NQO1(-/-) lungs (0.80 ± 0.03 and 0.68 ± 0.07 μmol·min(-1)·g lung dry wt(-1), respectively, P > 0.05). The mitochondrial complex I inhibitor rotenone depressed CoQ(1)H(2) efflux rates for both genotypes (0.19 ± 0.08 and 0.08 ± 0.04 μmol·min(-1)·g lung dry wt(-1) for NQO1(+/+) and NQO1(-/-), respectively, P < 0.05). Exposure of mice to 100% O(2) for 48 h also depressed CoQ(1)H(2) efflux rates in NQO1(+/+) and NQO1(-/-) lungs (0.43 ± 0.03 and 0.11 ± 0.04 μmol·min(-1)·g lung dry wt(-1), respectively, P < 0.05 by ANOVA). The impact of rotenone or hyperoxia on CoQ(1) redox metabolism could not be attributed to effects on lung wet-to-dry weight ratios, perfusion pressures, perfused surface areas, or total venous effluent CoQ(1) recoveries, the latter measured by spectrophotometry or mass spectrometry. Complex I activity in mitochondria-enriched lung fractions was depressed in hyperoxia-exposed lungs for both genotypes. This study provides new evidence for the potential utility of CoQ(1) as a nondestructive indicator of the impact of pharmacological or pathological exposures on complex I activity in the intact perfused mouse lung.

Differential responses of targeted lung redox enzymes to rat exposure to 60 or 85% oxygen

Rat exposure to 60% O(2) (hyper-60) or 85% O(2) (hyper-85) for 7 days confers susceptibility or tolerance, respectively, of the otherwise lethal effects of exposure to 100% O(2). The objective of this study was to determine whether activities of the antioxidant cytosolic enzyme NAD(P)H:quinone oxidoreductase 1 (NQO1) and mitochondrial complex III are differentially altered in hyper-60 and hyper-85 lungs. Duroquinone (DQ), an NQO1 substrate, or its hydroquinone (DQH(2)), a complex III substrate, was infused into the arterial inflow of isolated, perfused lungs, and the venous efflux rates of DQH(2) and DQ were measured. Based on inhibitor effects and kinetic modeling, capacities of NQO1-mediated DQ reduction (V(max1)) and complex III-mediated DQH(2) oxidation (V(max2)) increased by ∼140 and ∼180% in hyper-85 lungs, respectively, compared with rates in lungs of rats exposed to room air (normoxic). In hyper-60 lungs, V(max1) increased by ∼80%, with no effect on V(max2). Additional studies revealed that mitochondrial complex I activity in hyper-60 and hyper-85 lung tissue homogenates was ∼50% lower than in normoxic lung homogenates, whereas mitochondrial complex IV activity was ∼90% higher in only hyper-85 lung tissue homogenates. Thus NQO1 activity increased in both hyper-60 and hyper-85 lungs, whereas complex III activity increased in hyper-85 lungs only. This increase, along with the increase in complex IV activity, may counter the effects the depression in complex I activity might have on tissue mitochondrial function and/or reactive oxygen species production and may be important to the tolerance of 100% O(2) observed in hyper-85 rats.

Apical and basolateral localisation of GLUT2 transporters in human lung epithelial cells

Glucose concentrations of normal human airway surface liquid are ~12.5 times lower than blood glucose concentrations indicating that glucose uptake by epithelial cells may play a role in maintaining lung glucose homeostasis. We have therefore investigated potential glucose uptake mechanisms in non-polarised and polarised H441 human airway epithelial cells and bronchial biopsies. We detected mRNA and protein for glucose transporter type 2 (GLUT2) and glucose transporter type 4 (GLUT4) in non-polarised cells but GLUT4 was not detected in the plasma membrane. In polarised cells, GLUT2 protein was detected in both apical and basolateral membranes. Furthermore, GLUT2 protein was localised to epithelial cells of human bronchial mucosa biopsies. In non-polarised H441 cells, uptake of d-glucose and deoxyglucose was similar. Uptake of both was inhibited by phloretin indicating that glucose uptake was via GLUT-mediated transport. Phloretin-sensitive transport remained the predominant route for glucose uptake across apical and basolateral membranes of polarised cells and was maximal at 5–10 mM glucose. We could not conclusively demonstrate sodium/glucose transporter-mediated transport in non-polarised or polarised cells. Our study provides the first evidence that glucose transport in human airway epithelial cells in vitro and in vivo utilises GLUT2 transporters. We speculate that these transporters could contribute to glucose uptake/homeostasis in the human airway.

Mitochondrial hexokinases, novel mediators of the antiapoptotic effects of growth factors and Akt

Cell survival has been closely linked to both trophic growth factor signaling and cellular metabolism. Such couplings have obvious physiologic and pathophysiologic implications, but their underlying molecular bases remain incompletely defined. As a common mediator of both the metabolic and anti-apoptotic effects of growth factors, the serine/threonine kinase Akt - also known as protein kinase B or PKB - is capable of regulating and coordinating these inter-related processes. The glucose dependence of the antiapoptotic effects of growth factors and Akt plus a strong correlation between Akt-regulated mitochondrial hexokinase association and apoptotic susceptibility suggest a major role for hexokinases in these effects. Mitochondrial hexokinases catalyse the first obligatory step of glucose metabolism and directly couple extramitochondrial glycolysis to intramitochondrial oxidative phosphorylation, and are thus well suited to play this role. The ability of Akt to regulate energy metabolism appears to have evolutionarily preceded the capacity to control cell survival. This suggests that Akt-dependent metabolic regulatory functions may have given rise to glucose-dependent antiapoptotic effects that evolved as an adaptive sensing system involving hexokinases and serve to ensure mitochondrial homeostasis, thereby coupling metabolism to cell survival. We hypothesize that the enlistment of Akt and hexokinase in the control of mammalian cell apoptosis evolved as a response to the recruitment of mitochondria to the apoptotic cascade. The central importance of mitochondrial hexokinases in cell survival also suggests that they may represent viable therapeutic targets in cancer.

Proinflammatory interleukin-1 cytokines increase mesangial cell hexokinase activity and hexokinase II isoform abundance

Mesangial cell hexokinase (HK) activity is increased by a diverse array of factors that share both an association with pathological conditions and a common requirement for classic MAPK pathway activation. To better understand the relationship between glucose (Glc) metabolism and injury and to indirectly test the hypothesis that these changes constitute a general adaptive response to insult, we have sought to identify and characterize injury-associated factors that couple to mesangial cell HK regulation. Proinflammatory interleukin-1 (IL-1) cytokines activate the MAPK pathway and have known salutary effects in this cell type. We therefore examined their ability to influence mesangial cell HK activity, Glc utilization, MAPK pathway activation, and individual HK isoform abundance. IL-1β increased HK activity in both a time- and concentration-dependent manner: activity increased maximally by ∼50% between 12 and 24 h with an apparent EC50of 3 pM. IL-1α mimicked, but did not augment, the effects of IL-1β. Specific IL-1 receptor antagonism and selective MAPK/ERK kinase or upstream Ras inhibition prevented these increases, whereas PKC inhibition did not. Changes in HK activity were associated with both increased Glc metabolism and selective increases in HKII isoform abundance. We conclude that IL-1 cytokines can regulate cellular Glc phosphorylating capacity via an IL-1 receptor-, Ras-, and classic MAPK pathway-mediated increase in HKII abundance. These findings suggest a novel, previously undescribed mechanism whereby metabolism may be coupled to inflammation and injury.

Selective depletion of the Type I, Type II, and Type III isozymes of hexokinase in mammalian cells using small interfering RNAs

Constructs based on the pSUPER vector [Science 296 (2002) 550] and encoding small interfering RNAs specific for the Type I, Type II, or Type III isozymes of mammalian (rat) hexokinase were prepared. Transfection of Chinese hamster ovary and HeLa cells with these vectors resulted in selective depletion of the respective isozymes. A Zeocin marker was incorporated into the modified pSUPER vector, permitting isolation of stably transfected cell lines selectively depleted of the respective isozyme.

Hypoxia protects human lung microvascular endothelial and epithelial-like cells against oxygen toxicity: role of phosphatidylinositol 3-kinase

Hypoxic preconditioning is protective against oxidant-related damage in various organs, such as the heart. We previously showed that rats exposed to hypoxia also exhibit resistance to lethal pulmonary oxygen toxicity. The underlying mechanism and whether similar preconditioning is applicable to cellular models is unknown. In the present study, it was found that hypoxic pre-exposure induces a significant protective effect against hyperoxia-induced cell death in human lung microvascular endothelial cells (HLMVECs) and epithelial type II-like A549 cells. This effect of hypoxia is mediated by the phosphatidylinositol 3-kinase (PI3-K) signaling pathway because the presence of the PI3-K inhibitors, LY294002 and wortmannin, during pre-exposure to hypoxia completely blocks subsequent protection. Further, the hypoxia-dependent protection from hyperoxia was found to be associated with a 2-fold increase in PI3-K activity in hypoxia. Transient overexpression of a catalytically active class IA PI3-K p110alpha isoform also enhanced survival of A549 cells 2-fold compared with the empty vector control. These results indicate that hypoxia-induced activation of PI3-K is an important event in the acquisition of resistance against subsequent hyperoxic toxicity.

Elevated expression of hexokinase II protects human lung epithelial-like A549 cells against oxidative injury

Increased glucose utilization and hexokinase (HK)-II expression are adaptive features of lung cells exposed to hypoxia or hyperoxia. HK-II is the most regulated isoform of HK. Whether its overexpression could be protective against oxidative stress was explored in human lung epithelial-like (A549) cells. HK-II was overexpressed in A549 cells in a tetracycline-repressible retroviral vector system. Elevated expression of HK-II was confirmed by Western blot and activity measurements. Cell death caused by exposure to hyperoxia was decreased in HK-II-overexpressing cells. This effect was reversed when HK-II expression was suppressed with doxycycline. A similar protective effect was observed in HK-II-overexpressing cells after treatment with 1 mM hydrogen peroxide for 48 h. At baseline, fluorescence microscopy showed that overexpressed HK-II was localized to mitochondria. Electron microscopic studies showed that hyperoxia-exposed HK-II overexpressors had better-preserved and quantitatively smaller mitochondria than those in which the HK-II expression was suppressed or in the nontransduced A549 cells. Mitochondrial membrane potential was increased in HK-II-overexpressing cells exposed to hyperoxia compared with the nontransduced control cells under similar conditions. The present study demonstrates that HK-II protects human lung epithelial-like A549 cells against oxidative insults by protecting the mitochondria.

Glutamine protects mitochondrial structure and function in oxygen toxicity

Glutamine is an important mitochondrial substrate implicated in the protection of cells from oxidant injury, but the mechanisms of its action are incompletely understood. Human pulmonary epithelial-like (A549) cells were exposed to 95% O2 for 4 days in the absence and presence of glutamine. Cell proliferation in normoxia was dependent on glutamine, and glutamine deprivation markedly accelerated cell death in hyperoxia. Glutamine significantly increased cellular ATP levels in normoxia and prevented the loss of ATP in hyperoxia seen in glutamine-deprived cells. Mitochondrial membrane potential as assessed by flow cytometry with chloromethyltetramethylrosamine was increased by glutamine in hyperoxia-exposed A549 cells, and a glutamine dose-dependent increase in mitochondrial membrane potential was detected. Glutamine-supplemented, hyperoxia-exposed cells had a higher O2 consumption rate and GSH content. Electron and fluorescence microscopy revealed that, in hyperoxia, glutamine protected cellular structures, especially mitochondria, from damage. In hyperoxia, activity of the tricarboxylic acid cycle enzyme alpha-ketoglutarate dehydrogenase was partially protected by its indirect substrate, glutamine, indicating a mechanism of mitochondrial protection.

Hypoxia induces hexokinase II gene expression in human lung cell line A549

During adaptation to hypoxic and hyperoxic conditions, the genes involved in glucose metabolism are upregulated. To probe involvement of the transcription factor hypoxia-induced factor-1 (HIF-1) in hexokinase (HK) II expression in human pulmonary cells, A549 cells and small-airway epithelial cells (SAECs) were exposed to stimuli such as hypoxia, deferoxamine (DFO), and metal ions. The largest increase in HK-II (20-fold for mRNA and 2.5-fold for enzymatic activity) was observed in A549 cells when exposed to DFO. All stimuli selectively increased the 5.5-kb rather than 4-kb transcript in A549 cells. Cycloheximide and actinomycin D inhibited these responses. In addition, cells were transfected with luciferase reporter constructs driven by the full-length HK-II 5'-regulatory region (4.0 kb) or various deletions of that region. A549 cells transfected with the 4.0-kb construct and exposed to hypoxia or DFO increased their luciferase activity 7- and 10-fold, respectively, indicating that HK-II induction is, at least in part, due to increased gene transcription. Sixty percent of the inducible activity of the 4.0-kb construct was shown to reside within the proximal 0.5 kb. Additionally, cotransfection with a stable HIF-1 mutant and the 4.0-kb promoter construct resulted in increased luciferase activity under normoxic conditions. These results strongly suggest that HK-II is selectively regulated in pulmonary cells by a HIF-1-dependent mechanism.