Transplasma membrane redox system in HL-60 cells is modulated during TPA-induced differentiation

Cytochrome b5 reductase, a plasma membrane redox enzyme, protects neuronal cells against metabolic and oxidative stress through maintaining redox state and bioenergetics

The plasma membrane redox system (PMRS) containing NADH-dependent reductases is known to be involved in the maintenance of redox state and bioenergetics. Neuronal cells are very vulnerable to oxidative stress and altered energy metabolism linked to mitochondrial dysfunction. However, the role of the PMRS in these pathways is far from clear. In this study, in order to investigate how cytochrome b5 reductase (b5R), one of the PM redox enzymes, regulates cellular response under stressed conditions, human neuroblastoma cells transfected with b5R were used for viability and mitochondrial functional assays. Cells transfected with b5R exhibited significantly higher levels of the NAD(+)/NADH ratio, consistent with increased levels of b5R activity. Overexpression of b5R made cells more resistant to H2O2 (oxidative stress), 2-deoxyglucose (metabolic stress), rotenone and antimycin A (energetic stress), and lactacystin (proteotoxic stress), but did not protect cells against H2O2 and serum withdrawal. Overexpression of b5R induced higher mitochondrial functions such as ATP production rate, oxygen consumption rate, and activities of complexes I and II, without formation of further reactive oxygen species, consistent with lower levels of oxidative/nitrative damage and resistance to apoptotic cell death. In conclusion, higher NAD(+)/NADH ratio and consequent more efficient mitochondrial functions are induced by the PMRS, enabling them to maintain redox state and energy metabolism under conditions of some energetic stresses. This suggests that b5R can be a target for therapeutic intervention for aging and neurodegenerative diseases.

The plasma membrane redox enzyme NQO1 sustains cellular energetics and protects human neuroblastoma cells against metabolic and proteotoxic stress

The plasma membrane redox system (PMRS) of nicotinamide adenine dinucleotide (NADH)-related enzymes plays a key role in the maintenance of cellular energetics. During the aging process, neural cells are particularly sensitive to impaired energy metabolism and oxidative damage, but the involvement of the PMRS in these processes is unknown. Here, we used human neuroblastoma cells with either elevated or reduced levels of the PMRS enzyme NADH-quinone oxidoreductase 1 (NQO1) to investigate how the PMRS regulates neuronal stress responses. Cells with elevated NQO1 levels were more resistant to death induced by 2-deoxyglucose, potassium cyanide (energetic stress), and lactacystin (proteotoxic stress), but were not protected from being killed by H(2)O(2) and serum withdrawal. The NAD(+)(an oxidized form of NADH)/NADH ratio was maintained at a significantly higher level in cells overexpressing NQO1, consistent with enhanced levels of NQO1 activity. Levels of the neuroprotective transcription factors nuclear factor kappa-light-chain-enhancer of activated B cells and nuclear factor (erythroid-derived 2)-like 2, and the protein chaperone HSP70 were elevated in cells overexpressing NQO1. Cells in which NQO1 levels were decreased by RNA interference exhibited increased vulnerability to death induced by 2-deoxyglucose and lactacystin. Thus, a higher NAD(+)/NADH ratio and activation of adaptive stress response pathways are enhanced by the PMRS in neuroblastoma cells, enabling them to maintain redox homeostasis under conditions of energetic and proteotoxic stress. These findings have implications for the development of therapeutic interventions for neural tumors and neurodegenerative conditions.

Reduction of ascorbate free radical by the plasma membrane of synaptic terminals from rat brain

Synaptic plasma membranes (SPMV) decrease the steady state ascorbate free radical (AFR) concentration of 1mM ascorbate in phosphate/EDTA buffer (pH 7), due to AFR recycling by redox coupling between ascorbate and the ubiquinone content of these membranes. In the presence of NADH, but not NADPH, SPMV catalyse a rapid recycling of AFR which further lower the AFR concentration below 0.05 microM. These results correlate with the nearly 10-fold higher NADH oxidase over NADPH oxidase activity of SPMV. SPMV has NADH-dependent coenzyme Q reductase activity. In the presence of ascorbate the stimulation of the NADH oxidase activity of SPMV by coenzyme Q(1) and cytochrome c can be accounted for by the increase of the AFR concentration generated by the redox pairs ascorbate/coenzyme Q(1) and ascorbate/cytochrome c. The NADH:AFR reductase activity makes a major contribution to the NADH oxidase activity of SPMV and decreases the steady-state AFR concentration well below the micromolar concentration range.

Up-regulation of plasma membrane-associated redox activities in neuronal cells lacking functional mitochondria

Mitochondria-deficient cells (rho(o) cells) survive through enhanced glycolytic metabolism in the presence of pyruvate and uridine. The plasma membrane redox system (PMRS) contains several NAD(P)H-related enzymes and plays a key role in maintaining the levels of NAD(+)/NADH and reduced coenzyme Q. In this study, rho(o) cells were used to investigate how the PMRS is regulated under conditions of mitochondrial dysfunction. rho(o) cells exhibited a lower oxygen consumption rate and higher levels of lactate than parental cells, and were more sensitive to glycolysis inhibitors (2-deoxyglucose and iodoacetamide) than control cells. However, they were more resistant to H(2)O(2), consistent with increased catalase activity and decreased oxidative damage (protein carbonyls and nitrotyrosine). PM-associated redox enzyme activities were enhanced in rho(o) cells compared to those in control cells. Our data suggest that all PMRS enzymes and biomarkers tested are closely related to the ability of the PMs to maintain redox homeostasis. These results illustrate that an up-regulated PM redox activity can protect cells from oxidative stress as a result of an improved antioxidant capacity, and suggest a mechanism by which neurons adapt to conditions of impaired mitochondrial function.

Mouse liver plasma membrane redox system activity is altered by aging and modulated by calorie restriction

Caloric restriction (CR) is known as the only non-genetic method proven to slow the rate of aging and extend lifespan in animals. Free radicals production emerges from normal metabolic activity and generates the accumulation of oxidized macromolecules, one of the main characteristics of aging. Due to its central role in cell bioenergetics, a great interest has been paid to CR-induced modifications in mitochondria, where CR has been suggested to decrease reactive oxygen species production. The plasma membrane contains a trans-membrane redox system (PMRS) that provides electrons to recycle lipophilic antioxidants, such as α-tocopherol and coenzyme Q (CoQ), and to modulate cytosolic redox homeostasis. In the present study, we have investigated age differences in the PMRS in mouse liver and their modulation by CR. Aging induced a decrease in the ratio of CoQ10/CoQ9 and α-tocopherol in liver PM from AL-fed mice that was attenuated by CR. CoQ-dependent NAD(P)H dehydrogenases highly increased in CR old mice liver PMs. On the other hand, the CoQ-independent NADH-FCN reductase activity increased in AL-fed animals; whereas, in mice under CR this activity did not change during aging. Our results suggest that liver PMRS activity changes during aging and that CR modulates these changes. By this mechanism CR maintains a higher antioxidant capacity in liver PM of old animals by increasing the activity of CoQ-dependent reductases. Also, the putative role of PMRS in the modulation of redox homeostasis of cytosol is implicated.

Metabolism and function of coenzyme Q

Coenzyme Q (CoQ) is present in all cells and membranes and in addition to be a member of the mitochondrial respiratory chain it has also several other functions of great importance for the cellular metabolism. This review summarizes the findings available to day concerning CoQ distribution, biosynthesis, regulatory modifications and its participation in cellular metabolism. There are a number of indications that this lipid is not always functioning by its direct presence at the site of action but also using e.g. receptor expression modifications, signal transduction mechanisms and action through its metabolites. The biosynthesis of CoQ is studied in great detail in bacteria and yeast but only to a limited extent in animal tissues and therefore the informations available is restricted. However, it is known that the CoQ is compartmentalized in the cell with multiple sites of biosynthesis, breakdown and regulation which is the basis of functional specialization. Some regulatory mechanisms concerning amount and biosynthesis are established and nuclear transcription factors are partly identified in this process. Using appropriate ligands of nuclear receptors the biosynthetic rate can be increased in experimental system which raises the possibility of drug-induced upregulation of the lipid in deficiency. During aging and pathophysiological conditions the tissue concentration of CoQ is modified which influences cellular functions. In this case the extent of disturbances is dependent on the localization and the modified distribution of the lipid at cellular and membrane levels.

Characterization of keratinocyte differentiation induced by ascorbic acid: protein kinase C involvement and vitamin C homeostasis

Epidermal keratinocytes undergo differentiation in response to several stimuli to form the cornified envelope, a structure that contributes to the barrier function of skin. Although differentiation has been extensively analyzed, the precise role of vitamin C during this process is still not defined. Ascorbic acid, besides acting as a radical scavenger, has been shown to promote mesenchymal differentiation. In this study, we found that keratinocytes grown in ascorbate-supplemented medium developed a differentiated phenotype, as demonstrated by enhanced expression of marker genes and increase in cornified envelope content. The pro-differentiating effects of ascorbate were mediated by the protein-kinase-C-dependent induction of activating protein 1 DNA binding activity; indeed, down-modulation of protein kinase C activity abolished differentiation triggered by ascorbic acid. Although vitamin C appeared to regulate the same signaling pathway modulated by calcium, a classical in vitro inducer of epidermal differentiation, nonetheless terminally differentiated keratinocytes exhibited different ascorbate homeostasis and cellular antioxidant status. Indeed, we found that, unlike calcium, differentiation promoted by ascorbate was accompanied by (i) an enhanced ascorbate transport, due to overexpression of specific transporters, (ii) a great efficiency of dehydroascorbate uptake, and (iii) an increase in glutathione content with respect to proliferating cells. Ascorbic acid may be useful to promote epidermal differentiation, avoiding depletion of hydrophilic antioxidant stores.

Plasma membrane redox system in the control of stress-induced apoptosis

The plasma membrane of animal cells contains an electron transport system based on coenzyme Q (CoQ) reductases. Cytochrome b5 reductase is NADH-specific and reduces CoQ through a one-electron reaction mechanism. DT-diaphorase also reduces CoQ, although through a two-electron reaction mechanism using both NADH and NADPH, which may be particularly important under oxidative stress conditions. Because reduced CoQ protects membranes against peroxidations, and also maintains the reduced forms of exogenous antioxidants such as alpha-tocopherol and ascorbate, this molecule can be considered a central component of the plasma membrane antioxidant system. Stress-induced apoptosis is mediated by the activation of plasma membrane-bound neutral sphingomyelinase, which releases ceramide to the cytosol. Ceramide-dependent caspase activation is part of the apoptosis pathway. The reduced components of the plasma membrane antioxidant system, mainly CoQ, prevent both lipid peroxidation and sphingomyelinase activation. This results in the prevention of ceramide accumulation and caspase 3 activation and, as consequence, apoptosis is inhibited. We propose the hypothesis that antioxidant protective function of the plasma membrane redox system can be enough to protect cells against the externally induced mild oxidative stress. If this system is overwhelmed, intracellular mechanisms of protection are required to avoid activation of the apoptosis pathway.

Plasma membrane NADH-oxidoreductase system: a critical review of the structural and functional data

The observation in the early 1970s that ferricyanide can replace transferrin as a growth factor highlighted the major role plasma membrane proteins can play within a mammalian cell. Ferricyanide, being impermeant to the cell, was assumed to act at the level of the plasma membrane. Since that time, several enzymes isolated from the plasma membrane have been described, which, using NADH as the intracellular electron donor, are capable of reducing ferricyanide. However, their exact modes of action, and their physiological substrates and functions have not been solved to date. Numerous hypotheses have been proposed for the role of such redox enzymes within the plasma membrane. Examples include the regulation of cell signaling, cell growth, apoptosis, proton pumping, and ion channels. All of these roles may be a result of the function of these enzymes as cellular redox sensors. The emergence of many diverse roles for ferricyanide utilizing redox enzymes present in the plasma membrane might also, in part, be due to the numerous redox enzymes present within the membrane; the poor molecular characterization of the enzymes may be the reason for some of the diverging results reported in the literature as various researchers may be working on different enzymes. Here we review the diverse proposals given for structure and function to the plasma membrane NADH-oxidoreductase system(s) with a specific focus on those enzyme activities which can couple ferricyanide and NADH. Although they are still ill-defined enzymes, evidence is rising that they are of utmost significance for cellular regulation.

Functions of vitamin C as a mediator of transmembrane electron transport in blood cells and related cell culture models

Vitamin C (ascorbic acid) is an important physiological antioxidant. Within cells, it is practically always present in the reduced form. Several enzymatic and nonenzymatic mechanisms have been reported to maintain this status. In the extracellular environment, oxidation of ascorbate leads to loss of vitamin because the oxidized form, dehydroascorbic acid, is unstable under physiological conditions. The intermediate ascorbate free radical, although rather long-lived for a free radical, quickly disproportionates into the two other forms, also leading to loss of vitamin. Protection from loss can only be achieved by cellular regeneration mechanisms, i.e., by uptake of dehydroascorbic acid and either storage or recycling, and by plasma-membrane mediated reduction of extracellular free radical or dehydroascorbic acid. Moreover, intracellular ascorbate can also serve as an electron donor for transmembrane reduction of external electron acceptors. However, the physiological significance of this function is as yet unknown. The results presented in the literature are sometimes conflicting as to the relative contributions of these different possibilities, which seem to differ in different cell types. In this short review, the various pathways of regeneration of ascorbate and their relative contributions to the avoidance of vitamin loss in plasma or cell culture medium are discussed.

Cell membrane redox systems and transformation

Cell membrane redox systems carry electrons from intracellular donors and transport them to extracellular acceptors. This phenomenon appears to be universal. Numerous reviews have emphasized not only the bioenergetic mechanisms of redox systems but also the antioxidant defense mechanisms in which they participate. Moreover, significant progress has been made in the modulation of the membrane redox systems on cell proliferation. Because membrane redox systems play a key role in the regulation of cell growth, they need to be somehow linked into the signaling pathways resulting in either controlled or unregulated growth by both internal and external signals. Ultimately, these sequential events lead to either normal cell proliferation or cancer cell formation. However, much less is known about the involvement of membrane redox in transformation or tumorgenesis. In this review, the facts and ideas are summarized concerning the redox systems and tumorgenesis in several aspects, such as the regulation of cell growth and the effect on cell differentiation and on signaling pathways. In addition, information on a unique tumor-associated nicotinamide adenine dinucleotide (NADH) oxidase (tNOX) protein is reviewed.

Ascorbate stimulates ferricyanide reduction in HL-60 cells through a mechanism distinct from the NADH-dependent plasma membrane reductase

The impermeable oxidant ferricyanide is reduced by the plasma membrane redox system of HL-60 cells. The rate of reduction is strongly enhanced by ascorbate or dehydroascorbate. The aim of this study was to determine the mechanism by which ascorbate and dehydroascorbate accelerate ferricyanide reduction in HL-60 cells. Addition of ascorbate or dehydroascorbate to cells in the presence of ferricyanide led to the intracellular accumulation of ascorbate. Control experiments showed that extracellular ascorbate was rapidly converted to dehydroascorbate in the presence of ferricyanide. These data suggest that intracellular ascorbate originates from extracellular dehydroascorbate. Accumulation of ascorbate was prevented by inhibitors of dehydroascorbate transport into the cell. These compounds also strongly inhibited ascorbate-stimulated ferricyanide reduction in HL-60 cells. Thus, it is concluded that the stimulation of ferricyanide reduction is dependent on intracellular accumulation of ascorbate. Changing the alpha-tocopherol content of the cells had no effect on the ascorbate-stimulated ferricyanide reduction, showing that a nonenzymatic redox system utilizing alpha-tocopherol was not involved. p-Chloromercuribenzenesulfonic acid strongly affected ferricyanide reduction in the absence of ascorbate, whereas the stimulated reaction was much less responsive to this compound. Thus, it appears that at least two different membrane redox systems are operative in HL-60 cells, both capable of reducing ferricyanide, but through different mechanisms. The first system is the ferricyanide reductase, which uses NADH as its source for electrons, whereas the novel system proposed in this paper relies on ascorbate.

Ascorbate-mediated transplasma membrane electron transport in pulmonary arterial endothelial cells

Pulmonary endothelial cells are capable of reducing certain electron acceptors at the luminal plasma membrane surface. Motivation for studying this phenomenon comes in part from the expectation that it may be important both as an endothelial antioxidant defense mechanism and in redox cycling of toxic free radicals. Pulmonary arterial endothelial cells in culture reduce the oxidized forms of thiazine compounds that have been used as electron acceptor probes for studying the mechanisms of transplasma membrane electron transport. However, they reduce another commonly studied electron acceptor, ferricyanide, only very slowly by comparison. In the present study, we examined the influence of ascorbate [ascorbic acid (AA)] and dehydroascorbate [dehydroascorbic acid (DHAA)] on the ferricyanide and thiazine reductase activities of the bovine pulmonary arterial endothelial cell surface. The endothelial cells were grown on microcarrier beads so that the reduction of ferricyanide and methylene blue could be studied colorimetrically in spectrophotometer cuvettes and in flow-through cell columns. The ferricyanide reductase activity could be increased 80-fold by adding DHAA to the medium, with virtually no effect on methylene blue reduction. The DHAA effect persisted after the DHAA was removed from the medium. AA also stimulated the ferricyanide reductase activity but was less potent, and the relative potencies of AA and DHAA correlated with their relative rates of uptake by the cells. The results are consistent with the hypothesis that AA is an intracellular electron donor for an endothelial plasma membrane ferricyanide reductase and that the stimulatory effect of DHAA is the result of increasing intracellular AA. Adding sufficient DHAA to markedly increase extracellular ferricyanide reduction had little effect on the plasma membrane methylene blue reductase activity, suggesting that pulmonary arterial endothelial cells have at least two separate transplasma membrane electron transport systems.

Redox regulation of cAMP levels by ascorbate in 1,25-dihydroxy- vitamin D3-induced differentiation of HL-60 cells

1alpha,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] induces differentiation to monocyte-macrophage lineage of several leukaemic cell lines such as HL-60, U937, M1 and Mono Mac 6. Ascorbate also modulates growth and differentiation of different animal cells in culture. We have previously reported the stimulating effect of ascorbate on 1, 25-(OH)2D3-induced HL-60 cell differentiation. We show here that 1, 25-(OH)2D3 induces a transient increase in cAMP levels in these cells, and ascorbate significantly increases these cAMP levels. Ascorbate alone does not have any effect. Other cAMP-increasing agents such as isobutylmethylxanthine, forskolin and prostaglandin E2 maintain high levels of cAMP at 48 h of incubation and also enhance differentiation along the monocytic pathway induced by 1, 25-(OH)2D3, as revealed by specific differentiation markers, demonstrating the importance of cAMP in the differentiation process. It is also shown that the presence of ascorbate and its free radical (AFR) during 1,25-(OH)2D3-induced differentiation significantly decreases cytoplasmic NADH levels compared with those induced by 1,25-(OH)2D3 in HL-60 cells. The results indicate that NADH is an inhibitor of adenylate cyclase in these cells. AFR is an electron acceptor of the trans-plasma-membrane electron-transport system, and NADH is the electron donor. Through this system, ascorbate and AFR keep levels of NADH low, thereby decreasing its inhibitory effect on adenylate cyclase activity and so increasing cAMP synthesis. We also demonstrate that other ascorbate derivatives, such as ascorbate 2-phosphate and dehydroascorbate, both of which are unable to produce AFR, do not alter intracellular NADH levels during 1, 25-(OH)2D3-induced differentiation. Also, ascorbate and AFR increase specific differentiation markers (CD14 and NitroBlue Tetrazolium reduction) but neither ascorbate 2-phosphate nor dehydroascorbate show this enhancing activity. In summary, we propose that the effect of ascorbate on 1,25-(OH)2D3-induced differentiation of HL-60 cells can be explained by redox regulation of the cAMP pathway.

Multifunctional plasma membrane redox systems

All the biological membranes contain oxidoreduction systems actively involved in their bioenergetics. Plasma membrane redox systems seem to be ubiquitous and they have been related to several important functions, including not only their role in cell bioenergetics, but also in cell defense through the generation of reactive oxygen species, in iron uptake, in the control of cell growth and proliferation and in signal transduction. In the last few years, an increasing number of mechanistic and molecular studies have deeply widened our knowledge on the function of these plasma membrane redox systems. The aim of this review is to summarize what is currently known about the components and physiological roles of these systems.

Antioxidant ascorbate is stabilized by NADH-coenzyme Q10 reductase in the plasma membrane

Plasma membranes isolated from K562 cells contain an NADH-ascorbate free radical reductase activity and intact cells show the capacity to reduce the rate of chemical oxidation of ascorbate leading to its stabilization at the extracellular space. Both activities are stimulated by CoQ10 and inhibited by capsaicin and dicumarol. A 34-kDa protein (p34) isolated from pig liver plasma membrane, displaying NADH-CoQ10 reductase activity and its internal sequence being identical to cytochrome b5 reductase, increases the NADH-ascorbate free radical reductase activity of K562 cells plasma membranes. Also, the incorporation of this protein into K562 cells by p34-reconstituted liposomes also increased the stabilization of ascorbate by these cells. TPA-induced differentiation of K562 cells increases ascorbate stabilization by whole cells and both NADH-ascorbate free radical reductase and CoQ10 content in isolated plasma membranes. We show here the role of CoQ10 and its NADH-dependent reductase in both plasma membrane NADH-ascorbate free radical reductase and ascorbate stabilization by K562 cells. These data support the idea that besides intracellular cytochrome b5-dependent ascorbate regeneration, the extracellular stabilization of ascorbate is mediated by CoQ10 and its NADH-dependent reductase.

Ehrlich cell plasma membrane redox system is modulated through signal transduction pathways involving cGMP and Ca2+ as second messengers

Ehrlich cell plasma membrane ferricyanide reductase activity increased in the presence of mastoparan, a generic activator of G proteins, using either whole cells or isolated plasma membrane-fractions. Agents that increase intracellular cAMP also increased the rate of ferricyanide reduction by Ehrlich cells. For the first time, evidence is shown on a modulation of plasma membrane redox system by cGMP. In fact, permeant analogs of cGMP, dibutyryl cGMP, and 8-bromo-cGMP increased the rate of ferricyanide reduction by the Ehrlich cell plasma membrane redox system. Furthermore, specific inhibition of cGMP-phosphodiesterases by dipyridamole was also accompanied by an enhancement in the rate of ferricyanide reduction. On the other hand, treatments expected to increase cytoplasmic Ca2+ concentrations were accompanied by a remarkable stimulation of the reductase activity. Taking all these data together, it seems that the Ehrlich cell plasma membrane redox system is under a multiple and complex regulation by different signal transduction pathways involving G proteins, cyclic nucleotides, and Ca2+ ions.

Ascorbate on cell growth and differentiation

Ascorbate, an essential nutrient in humans, primates, and guinea pig, is involved in many cellular functions. Ascorbate also modulates cell growth and differentiation. Ascorbate can reduce or stimulate the growth of tumor cells, depending on the cell type. The inhibitory effect is not specific for the biological active isomer L-ascorbate, and isoascorbate and D-ascorbate are more effective in reducing cell growth than L-ascorbate. These results indicate that ascorbate has a cytotoxic effect by killing cells directly, rather a cytostatic one. However, only L-ascorbate is able to stimulate cell growth, but the mechanism of this stimulation is still unknown. L-Ascorbate stimulates the in vitro differentiation of several mesenchyme-derived cell types by altering the expression of multiple genes as the cell progresses through specific differentiation programs. Stimulation of collagen matrix at gene transcription, mRNA stabilization, hydroxylation, and secretion is a key role for L-ascorbate. L-Ascorbate also prevents cell transformation by stabilization of the differentiated state and cooperates with other agents to induce differentiation in a leukemia cell line.

Extracellular ascorbate stabilization: enzymatic or chemical process?

Ascorbate is stabilized in the presence of HL-60 cells. This stabilization has been questioned as a simple chemical effect. Further properties and controls about the enzymatic nature of this stabilization are described and discussed. Our results showed that cAMP derivatives and cAMP-increasing agents stimulated the ability of HL-60 cells to stabilize ascorbate. On the other hand, tunicamycin, a glycosylation-interfering agent, inhibited this ability. These data, together with hormonal regulation, support the hypothesis of an enzymatic redox system located at the plasma membrane as being responsible for the extracellular ascorbate stabilization by HL-60 cells.