Ferricyanide can replace pyruvate to stimulate growth and attachment of serum restricted human melanoma cells

Transplasma membrane electron transport: enzymes involved and biological function

The notion of transmembrane electron transport is usually associated with mitochondria and chloroplasts. However, since the early 1970s, it has been known that this phenomenon also occurs at the level of the plasma membrane. Ever since, evidence has accumulated for the existence of a plethora of transplasma membrane electron transport enzymes. In this review, we discuss the various enzymes known, their molecular characteristics and their biological functions.

Functional replacement of oxygen by other oxidants in articular cartilage

OBJECTIVE

Articular cartilage chondrocytes consume remarkably little O(2) in comparison with most other animal cells; glycolysis forms the principal source of ATP in this cartilage. Although not lethal for many days, imposition of anoxia immediately lowers intracellular ATP, inhibits rates of glycolysis, and prevents articular chondrocytes from producing extracellular matrix macromolecules. This study was undertaken to investigate the role of O(2) in articular chondrocyte metabolism.

METHODS

We examined the effects of oxygen and of several other classes of exogenous oxidants, i.e., 1) the dyes methylene blue and 2,6-dichlorophenol-indophenol, 2) the iron (III) complex ferricyanide, and 3) the keto-acids oxaloacetate and pyruvate (and phosphoenolpyruvate, a metabolic precursor of pyruvate), on rates of glycolysis and of sulfate incorporation by bovine articular cartilage in vitro.

RESULTS

Lactate production was lowest under conditions of anoxia and was stimulated severalfold by addition of O(2) (air-saturated medium). Under strict anoxia, other oxidants restored lactate production to rates at least comparable with those seen in aerobic controls; under aerobic conditions, they had little effect. Oxygen and all of the other oxidants examined stimulated sulfate incorporation more strongly than lactate production. The compounds that promoted glycolysis and hence sulfate incorporation in cartilage under anoxia were themselves reduced; that is, they functioned as oxidants in lieu of O(2).

CONCLUSION

For normal function, articular cartilage appears to require exogenous oxidants to stimulate glycolysis and produce ATP and extracellular matrix. Under physiologic conditions, oxygen acts as this oxidant, but its role can be adequately assumed by other agents.

Purification and characterization of a doxorubicin-inhibited NADH-quinone (NADH-ferricyanide) reductase from rat liver plasma membranes

Plasma membrane-associated redox systems play important roles in regulation of cell growth, internal pH, signal transduction, apoptosis, and defense against pathogens. Stimulation of cell growth and stimulation of the redox system of plasma membranes are correlated. When cell growth is inhibited by antitumor agents such as doxorubicin, capsaicin, and antitumor sulfonylureas, redox activities of the plasma membrane also are inhibited. A doxorubicin-inhibited NADH-quinone reductase was characterized and purified from plasma membranes of rat liver. First, an NADH-cytochrome b(5) reductase, which was doxorubicin-insensitive, was removed from the plasma membranes by the lysosomal protease, cathepsin D. After removal of the NADH-cytochrome b(5) reductase, the plasma membranes retained a doxorubicin-inhibited NADH-quinone reductase activity. The enzyme, with an apparent molecular mass of 57 kDa, was purified 200-fold over the cathepsin D-treated plasma membranes. The purified enzyme had also an NADH-coenzyme Q(0) reductase (NADH: external acceptor (quinone) reductase; EC 1.6.5.) activity. Partial amino acid sequence of the enzyme showed that it was unique with no sequence homology to any known protein. Antibody against the enzyme (peptide sequence) was produced and affinity-purified. The purified antibody immunoprecipitated both the NADH-ferricyanide reductase activity and NADH-coenzyme Q(0) reductase activity of plasma membranes and cross-reacted with human chronic myelogenous leukemia K562 cells and doxorubicin-resistant human chronic myelogenous leukemia K562R cells. Localization by fluorescence microscopy showed that the reaction was with the external surface of the plasma membranes. The doxorubicin-inhibited NADH-quinone reductase may provide a target for the anthracycline antitumor agents and a candidate ferricyanide reductase for plasma membrane electron transport.

The plasma membrane NADH oxidase of soybean has vitamin K(1) hydroquinone oxidase activity

Isolated plasma membrane vesicles and the plasma membrane NADH oxidase partially purified from soybean plasma membrane vesicles exhibited a cyanide-insensitive vitamin K(1) hydroquinone oxidase activity with isolated plasma membrane vesicles. Reduced vitamin K(1) (phylloquinol) was oxidized at a rate of about 10 nmol/min/mg protein as determined by reduced vitamin K(1) reduction or oxygen consumption. The K(m) for reduced K(1) was 350 microM. With the partially purified enzyme, reduced vitamin K(1) was oxidized at a rate of about 600 nmol/min/mg protein and the K(m) was 400 microM. When assayed in the presence of 1 mM KCN, activities of both plasma membrane vesicles and of the purified protein were stimulated (0.1 microM) or inhibited (0.1 mM) by the synthetic auxin growth factor 2, 4-dichlorophenoxyacetic acid. The findings suggest the potential participation of the plasma membrane NADH oxidase as a terminal oxidase of plasma membrane electron transport from cytosolic NAD(P)H via reduced vitamin K(1) to acceptors (molecular oxygen or protein disulfides) at the cell surface.

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.

The plasma membrane NADH oxidase of HeLa cells has hydroquinone oxidase activity

The plasma membrane NADH oxidase activity partially purified from the surface of HeLa cells exhibited hydroquinone oxidase activity. The preparations completely lacked NADH:ubiquinone reductase activity. However, in the absence of NADH, reduced coenzyme Q10 (Q10H2=ubiquinol) was oxidized at a rate of 15+/-6 nmol min-1 mg protein-1 depending on degree of purification. The apparent Km for Q10H2 oxidation was 33 microM. Activities were inhibited competitively by the cancer cell-specific NADH oxidase inhibitors, capsaicin and the antitumor sulfonylurea N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl)urea (LY181984). With coenzyme Q0, where the preparations were unable to carry out either NADH:quinone reduction or reduced quinone oxidation, quinol oxidation was observed with an equal mixture of the Q0 and Q0H2 forms. With the mixture, a rate of Q0H2 oxidation of 8-17 nmol min-1 mg protein-1 was observed with an apparent Km of 0.22 mM. The rate of Q10H2 oxidation was not stimulated by addition of equal amounts of Q10 and Q10H2. However, addition of Q0 to the Q10H2 did stimulate. The oxidation of Q10H2 proceeded with what appeared to be a two-electron transfer. The oxidation of Q0H2 may involve Q0, but the mechanism was not clear. The findings suggest the potential participation of the plasma membrane NADH oxidase as a terminal oxidase of plasma membrane electron transport from cytosolic NAD(P)H via naturally occurring hydroquinones to acceptors at the cell surface.

Effectors of the mammalian plasma membrane NADH-oxidoreductase system. Short-chain ubiquinone analogues as potent stimulators

In the presence of effectors variations in the two recognized activities of the plasma membrane NADH-oxidoreductase system were studied in separate, specific in vitro assays. We report here that ubiquinone analogues that contain a short, less hydrophobic side chain than coenzyme Q-10 dramatically stimulate the NADH-oxidase activity of isolated rat liver plasma membranes whereas they show no effect on the reductase activity of isolated membranes. If measured in assays of the NADH:ferricyanide reductase of living cultured cells these compounds have only a limited effect; the oxidase activity of whole cells is not measurable in our hands. We have furthermore identified selective inhibitors of both enzyme activities. In particular, the NADH-oxidase activity can be significantly inhibited by structural analogues of ubiquinone, such as capsaicin and resiniferatoxin. The NADH:ferricyanide reductase, on the other hand, is particularly sensitive to pCMBS, indicating the presence of a sulfhydryl group of groups at its active site. The identification of these specific effectors of the different enzyme activities of the PMOR yields further insights into the function of this system.

Proton release from HeLa cells and alkalization of cytoplasm induced by diferric transferrin or ferricyanide and its inhibition by the diarylsulfonylurea antitumor drug N-(4-methylphenylsulfonyl)-N'-(4-cholorophenyl) urea (LY181984)

Proton release from HeLa cells was stimulated by an external oxidant, potassium ferricyanide, or by the growth factor diferric transferrin. This stimulated proton release was inhibited by the antitumor sulfonylurea LY181984 [N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl)urea] over the concentration range 10 nM to 1 microM. The antitumor-inactive sulfonylurea analog LY181985 [N-(4-methylphenylsulfonyl)-N'-(phenyl)urea] was without effect at 1 microM and required 10-100 microM concentrations to inhibit proton release. Diferric transferrin-induced alkalization of the cytoplasm estimated by BCECF [2',7'-bis(2-carboxyethyl)-5,(and 6)-carboxyfluorescein] fluorescence also was inhibited by 1 microM LY181984 but not by 1 microM LY181985. The inhibited component appeared to be amiloride resistant. The proton release induced by either ferricyanide or diferric transferrin was inhibited by about 35% at a near optimal amiloride concentration of 0.2 mM or at a dimethylamiloride concentration of 0.075 mM. However, the induced proton release was inhibited further by LY181984. Conversely, when proton release was inhibited fully by LY181984 at a near optimal concentration of 10 microM (50% inhibition), increasing concentrations of amiloride or dimethylamiloride resulted in additional inhibitions of 16 and 23%, respectively. However, the inhibitions by LY181984 and the amilorides were additive, suggesting that amiloride and the sulfonylureas may act independently. Evidence for an action of the sulfonylurea in inhibiting proton efflux differently from that of the amilorides came from measurements of sodium uptake either by fluorometry or by direct measurement with 22Na+. Sodium uptake was not inhibited by either LY181984 or LY181985 in HeLa cells at concentrations of LY181984 sufficient to inhibit proton efflux by 80% or more. The results show LY181984 to be a potent inhibitor of diferric transferrin- or ferricyanide-induced proton efflux and cytoplasmic alkalization in HeLa cells and that the inhibition may involve a component of proton transport that is resistant to amiloride.

Modulation of activator protein-1 (AP-1) transcription factor and protein kinase C by hydrogen peroxide and D-alpha-tocopherol in vascular smooth muscle cells

The effects of hydrogen peroxide D-alpha-tocopherol and of D-beta-tocopherol on proliferation, protein kinase C and activator protein-1 (AP-1) activation have been studied in vascular smooth muscle cells. Cell proliferation, when activated by foetal calf serum, was inhibited by D-alpha-tocopherol. Protein kinase C activity was stimulated by hydrogen peroxide in a manner similar to phorbol myristate acetate; in the latter case, but not in the former, D-alpha-tocopherol inhibited the reaction. Hydrogen peroxide prevented phorbol-myristate-acetate-stimulated AP-1 binding to DNA but stimulated it if protein kinase C was down-regulated or inhibited. D-alpha-Tocopherol promoted AP-1 activation in quiescent cells but prevented its activation by phorbol myristate acetate. None of the described effects of D-alpha-tocopherol were shared by D-beta-tocopherol, suggesting a non-antioxidant mechanism as the basis of its action. The data show that hydrogen peroxide and D-alpha-tocopherol affect more than one element in the cell signal-transduction cascade.

Extracellular ferrireductase activity of K562 cells is coupled to transferrin-independent iron transport

The reduction of Fe3+ to Fe2+ has been established to play a critical role in the uptake of iron by many organisms. Recently, a mechanism of iron transport in the absence of transferrin (Tf) was described for the human K562 cell line and a role for ferrireductase activity was implicated in this process as well [Inman, R. S., & Wessling-Resnick, M. (1993) J. Biol. Chem. 268, 8521-8528]. The present report characterizes the extracellular reduction of ferricyanide to ferrocyanide catalyzed by K562 cells. The observation that membrane-impermeant ferricyanide competitively inhibits Tf-independent assimilation of iron from 55Fe-nitriloacetic acid indicates that this ferrireductase activity is indeed coupled to the transport mechanism. From a series of initial rate experiments, the kinetic parameters for cell surface ferrireductase activity, Vmax = 0.102 nmol min-1 (10(6) cells)-1 and Km = 6.13 microM, were determined. Neither the Vmax nor the Km of this reaction is modulated by changes in extra- or intracellular iron levels; thus, similar to Tf-independent transport activity in K562 cells, the ferrireductase activity is not regulated in response to iron levels. Transmembrane oxidoreductase activity is also reportedly involved in the control of cellular growth; however, the K562 cell ferrireductase is unresponsive to insulin and is not inhibited by the antitumor drugs adriamycin, actinomycin D, or cis-platin, observations that fail to support a role for this particular activity in cell regulation. Rather, the K562 cell ferrireductase appears to be tightly coupled to the mechanism of Tf-independent transport as demonstrated by its sensitivity to Cd2+, a specific inhibitor of non-Tf iron uptake by K562 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

NADH-ascorbate free radical and -ferricyanide reductase activities represent different levels of plasma membrane electron transport

Plasma membranes isolated from rat liver by two-phase partition exhibited dehydrogenase activities for ascorbate free radical (AFR) and ferricyanide reduction in a ratio of specific activities of 1:40. NADH-AFR reductase could not be solubilized by detergents from plasma membrane fractions. NADH-AFR reductase was inhibited in both clathrin-depleted membrane and membranes incubated with anti-clathrin antiserum. This activity was reconstituted in plasma membranes in proportion to the amount of clathrin-enriched supernatant added. NADH ferricyanide reductase was unaffected by both clathrin-depletion and antibody incubation and was fully solubilized by detergents. Also, wheat germ agglutinin only inhibited NADH-AFR reductase. The findings suggest that NADH-AFR reductase and NADH-ferricyanide reductase activities of plasma membrane represent different levels of the electron transport chain. The inability of the NADH-AFR reductase to survive detergent solubilization might indicate the involvement of more than one protein in the electron transport from NADH to the AFR but not to ferricyanide.

Hydrogen peroxide-induced c-fos expression is mediated by arachidonic acid release: role of protein kinase C

We found previously that stimulation of c-fos and c-myc mRNA expression are early events in hydrogen peroxide-induced growth in rat aortic smooth muscle (RASM) cells. In the present study, we investigated the role of phospholipase A2 (PLA2) and protein kinase C (PKC) in mediating hydrogen peroxide-induced c-fos mRNA expression in RASM cells. Mepacrine and p-bromophenacylbromide, potent inhibitors of PLA2 activity, blocked hydrogen peroxide-induced c-fos mRNA expression. Arachidonic acid, a product of PLA2 activity, stimulated the expression of c-fos mRNA with a time course similar to that of hydrogen peroxide. PKC down-regulation attenuated both hydrogen peroxide and arachidonic acid-induced c-fos mRNA expression by 50%. Nordihydroguaiaretic acid (a lipoxygenase-cytochrome P450 monooxygenase inhibitor) significantly inhibited both hydrogen peroxide and arachidonic acid-induced c-fos mRNA expression, whereas indomethacin (a cyclooxygenase inhibitor) had no effect. Together, these findings indicate that 1) hydrogen peroxide-induced c-fos mRNA expression is mediated by PLA2-dependent arachidonic acid release, 2) both PKC-dependent and independent mechanisms are involved in hydrogen peroxide-induced expression of c-fos mRNA and 3) arachidonic acid metabolism via the lipoxygenase-cytochrome P450 monooxygenase pathway appears to be required for hydrogen peroxide-induced expression of c-fos mRNA.

Stimulation of serum-free cell proliferation by coenzyme Q

Coenzyme Q added to culture media stimulates the growth of HeLa and Balb/3T3 cells in serum free conditions. The stimulation by coenzyme Q is additive to the stimulation by ferricyanide, an impermeable electron acceptor for the transplasma membrane electron transport. alpha Tocopherylquinone can also stimulate cell growth, but vitamin K1 is inactive or inhibitory. The response to coenzyme Q and ferricyanide is enhanced with insulin. A contribution to plasma membrane NADH oxidation or modification of the membrane quinone redox balance can be a basis for the growth stimulation.

A growth factor- and hormone-stimulated NADH oxidase from rat liver plasma membrane

NADH oxidase activity (electron transfer from NADH to molecular oxygen) of plasma membranes purified from rat liver was characterized by a cyanide-insensitive rate of 1 to 5 nmol/min per mg protein. The activity was stimulated by growth factors (diferric transferrin and epidermal growth factor) and hormones (insulin and pituitary extract) 2- to 3-fold. In contrast, NADH oxidase was inhibited up to 80% by several agents known to inhibit growth or induce differentiation (retinoic acid, calcitriol, and the monosialoganglioside, GM3). The growth factor-responsive NADH oxidase of isolated plasma membranes was not inhibited by common inhibitors of oxidoreductases of endoplasmic reticulum or mitochondria. As well, NADH oxidase of the plasma membrane was stimulated by concentrations of detergents which strongly inhibited mitochondrial NADH oxidases and by lysolipids or fatty acids. Growth factor-responsive NADH oxidase, however, was inhibited greater than 90% by chloroquine and quinone analogues. Addition of coenzyme Q10 stimulated the activity and partially reversed the analogue inhibition. The pH optimum for NADH oxidase was 7.0 both in the absence and presence of growth factors. The Km for NADH was 5 microM and was increased in the presence of growth factors. The stoichiometry of the electron transfer reaction from NADH to oxygen was 2 to 1, indicating a 2 electron transfer. NADH oxidase was separated from NADH-ferricyanide reductase, also present at the plasma membrane, by ion exchange chromatography. Taken together, the evidence suggests that NADH oxidase of the plasma membrane is a unique oxidoreductase and may be important to the regulation of cell growth.

Growth factor-stimulated trans plasma membrane electron transport in HL-60 cells

Electron flow across the plasma membrane of living cells and its rapid modulation by growth factors has been measured continuously through a simple assay procedure whereby the transported electrons are captured by ascorbate free radical to slow the rate of chemical oxidation of ascorbate. The assay provides a direct demonstration of electron transport to an external electron acceptor that is both physiological and impermeant. The reduction of external ascorbate free radical is stimulated by the growth factors, EGF and transferrin, and is inhibited by wheat germ agglutinin. The results demonstrate, under physiological conditions, the operation of a growth factor- and lectin-responsive electron transport system at the cell surface using a cultured human cell line.

Membrane biochemistry and chemical hepatocarcinogenesis

Biochemical membrane alterations appearing during the process of chemical carcinogenesis are described. Emphasis is put on membrane composition, structure, and biogenesis. In this presentation the knowledge gained from experimental studies of liver and skin in the process of cancer development is acknowledged. Important biochemical changes have been reported in lipid composition, fatty acid saturation, constitutional enzyme expression, receptor turnover and oligomerization. Functional consequences of the altered membrane structure is discussed within the concepts of regulation of cell proliferation, regulation of membrane receptor expression, redox control, signal transduction, drug metabolism, and multidrug resistance. Data from malignant tumours and normal tissue are addressed to evaluate the importance of the alterations for the process and for the eventual malignant transformation.

Modification of transplasma membrane oxidoreduction by SV40 transformation of 3T3 cells

Transformation of 3T3 cells by SV40 virus changes the properties of the transplasma membrane electron transport activity which can be assayed by reduction of external ferric salts. After 42 h of culture and before the growth rate is maximum, the transformed cells have a much slower rate of ferric reduction. The change in activity is expressed both by change in Km and Vmax for ferricyanide reduction. The change in activity is not based on surface charge effect or on tight coupling to proton release or on intracellular NADH concentration. With transformation by SV40 virus infection the expression of transferrin receptors increases, which correlates with greater diferric transferrin stimulation of the rate of ferric ammonium citrate reduction in transformed SV40-3T3 cells than in 3T3 cells.

Chloroquine-sensitive transplasmalemma electron transport in Tetrahymena pyriformis: a hypothesis for control of parasite protozoa through transmembrane redox

Plasma membrane electron transport was studied in a protozoan cell, Tetrahymena pyriformis, by assaying transmembrane ferricyanide reduction and the reduction of iron compounds. The rates of ferricyanide reduction varied between 0.5 and 2.5 mumol/g dry wt. per min, with a pH optimum at 7.0-7.5. Other active non-permeable electron acceptors, with redox potentials from +360 to -125 mV, were cytochrome c, hexaammine ruthenium chloride, ferric-EDTA, ammonium ferric citrate, and indigo di-, tri- and tetrasulfonates. It was found that Tetrahymena cells can reduce external electron acceptors with redox potentials at pH 7.0 down to -125 mV. Ferricyanide stimulates ciliary action. Transmembrane ferricyanide reduction by Tetrahymena was not inhibited by such mitochondrial inhibitors as antimycin A, 2-n-heptyl-4-hydroxyquinoline N-oxide, or potassium cyanide, but it responded to inhibitors of glycolysis. Transmembrane ferricyanide reduction by Tetrahymena appears to involve a plasma membrane electron transport chain similar to those of other animal cells. As in other cells, the transmembrane electron transport is associated with proton release which may be involved in internal pH control. The transmembrane redox system differs from that of mammalian cells in a 20-fold greater sensitivity to chloroquine and quinacrine. The Tetrahymena ferricyanide reduction is also inhibited by chlorpromazine and suramin. Sensitivity to these drugs indicates that the transplasma membrane electron transport and associated proton pumping may be a target for drugs used against malaria, Trypanosomes and other protozoa.

Transplasma membrane electron transport in plants

The presence of transplasma membrane electron transport in a variety of plant cells and tissues is reported. It is now agreed that this property of eukaryotic cells is of ubiquitous nature. Studies with highly purified plasma membranes have established the presence of electron transport enzymes. Two types of activities have been identified. One, termed "Standard" reductase, is of general occurrence. The other, inducible under iron deficiency and relatively more active, is "Turbo" reductase. However, the true nature of components participating in electron transport and their organization in the plasma membrane is not known. The electron transport is associated with proton release and uses intracellular NAD(P)H as substrate. The electron flow leads to changes in intracellular redox status, pH, and metabolic energy. The responsiveness of this system to growth hormones is also observed. These findings suggest a role for electron flow across the plasma membrane in cell growth and regulation of ion transport. Involvement of this system in many other cellular functions is also argued.

NADH oxidase of plasma membranes

NADH oxidase is a cyanide-resistant and hormone-responsive oxidase intrinsic to the plasma membrane of both plant and animal cells. The activity has many unique characteristics that distinguish it from other oxidases and oxidoreductases of both organelles and internal membranes and from other oxidoreductases of the plasma membrane. Among these are resistance to inhibition by cyanide, catalase, superoxide dismutase, and phenylchloromercuribenzoate. Activity is stimulated by hormones and growth factors and inhibited by quinone analogs such as piercidin, the flavin antagonist atebrin, and growth inhibiting gangliosides such as GM3. In marked contact to the NADH-ferricyanide oxidoreductase of the plasma membrane, the NADH oxidase is activated by lysophospholipids and fatty acids, products of phospholipase A2 action, in a time-dependent manner suggestive of stabilization of an activated form of the enzyme. The hormone-responsive NADH oxidase of the plasma membrane is not a peroxidase and may function as a terminal oxidase to link transfer of electrons from NADH to oxygen at the plasma membrane. The functional significance of the NADH oxidase of the plasma membrane is unknown but some relationship to growth or growth control is indicated. In both animal and plant plasma membranes, the oxidase is activated by growth factors and hormones to which the cells or tissues of origin have functional hormone or growth factor receptors. In addition, substances that inhibit the oxidase, the associated transmembrane reductase or both, inhibit growth. In transformed cells and tissues, the hormone and growth factor responsiveness of the NADH oxidase is reduced or absent. With human keratinocytes which exhibit an increased sensitivity to the antiproliferative action of both retinoic acid and calcitriol, the NADH oxidase of the plasma membrane is strongly inhibited by these agents and shows the same increased sensitivity. If transfer of electrons from NADH to oxygen across or within the eukaryotic plasma membrane is an important aspect of growth or growth control, then the hormone- and growth factor-responsive NADH oxidase associated with the plasma membrane could be of fundamental importance. Because of its low basal activity, stimulation by growth factors and hormones, and the inhibition of growth in direct proportion to inhibition of the oxidase, the activity is a candidate as a rate-limiting step in the growth process. Completely unknown is the mechanism whereby NADH oxidation and growth or growth control may be coupled. This, together with further characterization of the activity and the mechanism of loss of control with neoplastic transformation, represent important challenges for future investigations.

Diferric transferrin reduction by K562 cells. A critical study

This paper critically examines the redox activity of K562 cells (chronic myelogenous leukemia cells) and normal peripheral blood lymphocytes (PBL). Ferricyanide reduction, diferric transferrin reduction, and ferric ion reduction were measured spectrophotometrically by following the time-dependent changes of absorbance difference characteristic for ferricyanide disappearance and for the formation of ferrous ion:chelator complexes. Bathophenanthroline disulfonate (BPS) and ferrozine (FZ) were used to detect the appearance of ferrous ions in the reaction mixtures when diferric transferrin or ferric reduction was studied. Special attention was devoted to the analysis of time-dependent absorbance changes in the presence and absence of cells under different assay conditions. It was observed and concluded that: (i) FZ was far less sensitive and more sluggish than BPS for detecting ferrous ions at concentrations commonly used for BPS; (ii) FZ, at concentrations of at least 10-times the commonly used BPS concentrations, seemed to verify the results obtained with BPS; (iii) ferricyanide reduction, diferric transferrin reduction and ferric ion reduction by both K562 cells and peripheral blood lymphocytes did not differ significantly; and (iv) earlier values published for the redox activities of different cells might be overestimated, partly because of the observation published in 1988 that diferric transferrin might have loosely bound extra iron which is easily reduced. It is suggested that the specific diferric transferrin reduction by cells might be considered as a consequence of (i) changing the steady-state equilibrium in the diferric transferrin-containing solution by addition of ferrous ion chelators which effectively raised the redox potential of the iron bound in holotransferrin, and (ii) changing the steady-state equilibrium by addition of cells which would introduce, via their large and mostly negatively charged plasma membrane surface, a new phase which would favor release and reduction of the iron in diferric transferrin by a ferric ion oxidoreductase. The reduction of ferricyanide is also much slower than activities reported for other cells which may indicate reduced plasma membrane redox activity in these cells.

The biology of transferrin

The chemistry and molecular biology of transferrin is discussed. The discussion covers the genetic control of transferrin synthesis, its intracellular synthesis, intra- and extracellular transport, and its interaction with transferrin receptors. The role of transferrin in iron metabolism is evaluated, both with regard to iron uptake by transferrin as to iron uptake from transferrin by different cells. The knowledge on the biochemical mechanisms involved in iron uptake is presented, with special reference to the triple role of the acidification of endocytotic vesicles. Apart from its traditional role in iron metabolism, transferrin acts as a growth factor. A distinction of two groups of growth-stimulating properties of transferrin has been made. As an early effect, membranous and intracellular changes are initiated, possibly based on electrochemical effects on the cell. The late effects seem to relate to its role in iron transport. Interestingly, the early growth stimulating effects can be segregated from the former function of transferrin and strictly speaking neither depend on iron nor on the transferrin molecule itself. Also the trophic effect of transferrin on several cell types has been described. Hypotheses concerning the biochemical basis of this effect are presented and within this context a new hypothesis on the differential occupation of iron binding sites of serum transferrin is forwarded. Examples of the applicability of present understanding of the biology of transferrin in clinical settings are presented.

Limitations of the quantitative cytochemical assay of catechol oxidase in melanoma cells

The cytochemical quantification of catechol oxidase activity in fixed B16 melanoma cells was investigated using dopa as the substrate. Inhibitors showed that peroxidases do not significantly interfere. The kinetics of melanin formation were studied initially in solution with purified catechol oxidase. Two key parameters were identified: lag-time and the rate of melanin formation. The lag-time was taken as the time required by intermediates to reach a critical concentration at which the polymerization process starts and melanin production becomes measurable (at 640 nm). In solution, the lag-time decreases as the enzyme activity increases, particularly when the activity is very low. The rate at which melanin is formed by pure enzyme in solution is independent of dopa concentration when its activity is low but increases linearly with dopa concentration when the activity is comparatively high. In fixed melanoma cells, the lag-time decreases linearly with increases of dopa concentrations up to 20 mM; at concentrations higher than this, the lag decreases more slowly. In contrast, the rate of melanin production is unaffected by changes in dopa concentration. The lag-times of different cells lines incubated at the same substrate concentration decrease as the enzyme activity of the cells increases. The rate of melanin production seems to be affected by factors other than catechol oxidase activity, such as the intracellular organization and distribution of the enzyme.

Changes in intracellular redox and energy status during induced transplasma membrane electron transport in Cuscuta protoplasts

Extracellular reduction of ferricyanide was exhibited by isolated Cuscuta protoplasts. A larger decrease in NADH than NADPH levels of the ferricyanide-treated protoplasts pointed to the major involvement of the former as an electron donor. Glutathione levels were also found to be lowered in similarly treated tissue. The time-dependent variation in intracellular ATP levels in presence of ferricyanide supported the concept of plasma membrane ATPase activation during transplasma membrane electron transport in eukaryotes.

Ruthenium ammine complexes as electron acceptors for growth stimulation by plasma membrane electron transport

Ammineruthenium(III) complexes have been found to act as electron acceptors for the transplasmalemma electron transport system of animal cells. The active complexes hexaammineruthenium(III), pyridine pentammineruthenium(III), and chloropentaammineruthenium(III) range in redox potential (E'0) from 305 to -42 mV. These compounds also act as electron acceptors for the NADH dehydrogenase of isolated plasma membranes. Stimulation of HeLa cell growth, in the absence of calf serum, by these compounds provides evidence that growth stimulation by the transplasma membrane electron transport system is not entirely based on reduction and uptake of iron.

Decrease of NADH in yeast cells by external ferricyanide reduction

Ferricyanide reduction catalyzed by vitamin K-3 was accompanied by the decrease in intracellular (NAD(P)H concentration of yeast cells, and the rate of ferricyanide reduction depended on intracellular concentration of NADH rather than NADPH. The addition of glucose to the cell suspensions enhanced both ferricyanide reduction and intracellular NADH concentration. The catalytic action of vitamin K-3 on ferricyanide reduction was observed in the presence of NADH and plasma membrane preparations. As the toxic action of vitamin K-3 on cell growth of yeast was enhanced by addition of ferricyanide, ferricyanide reduction catalyzed by vitamin K-3 may inhibit cell growth by decreasing intracellular NADH concentration.

Characterization of a doxorubicin-resistant murine melanoma line: studies on cross-resistance and its circumvention

A B16 mouse melanoma cell line resistant to doxorubicin was obtained by continuous in vitro exposure to the drug. The ID50 for this line was 200 times higher than that for the parental cell line. The resistant cell line had some biological characteristics similar to those of the sensitive parental cell line, like saturation density and protein content. Differences were found in doubling time which was longer, cloning efficiency which was lower and DNA content which was higher in the resistant as compared to the parental line. Intracellular distribution of doxorubicin was also different having a nuclear-cytoplasmic ratio higher in sensitive than in resistant cells. Melanin content was an unstable feature in the sensitive cell line, whereas melanin was always present in resistant cells. Resistance to doxorubicin was maintained during 50 in vitro passages in the absence of the drug. Cross-resistance was found with vincristine and other anthracyclines, like daunorubicin and 4'-epi-doxorubicin but not with cis-platinum, and a new doxorubicin derivative, 4'-deoxy-4'-iodio-doxorubicin. The B16 line showed a lower resistance index to 4'-deoxy-doxorubicin and 4-demethoxy-daunorubicin (30 and 3 respectively), as compared to doxorubicin. Doxorubicin-resistance was partially circumvented by pretreatment of resistant cells with verapamil, a calcium chelating agent, and by trifluoperazine, a calmodulin-antagonist.

Modification of transmembrane electron transport activity in plasma membranes of simian virus 40 transformed pineal cells

Changes have been found in the plasma membrane enzyme system which carries out transmembrane electron transport and associated proton transport in Simian virus 40 (SV40) temperature-sensitive A (tsA) mutant-transformed rat pineal cell line, RPN209-1. This cell line was temperature-sensitive for the maintenance of transformation. RPN209-1 cells expressed the transformed phenotype (rapid growth, high cell density, and cloning in soft agar) at the permissive temperature (33 degrees C) and the nontransformed phenotype (slower growth, lower saturation density, and lower cloning efficiency in soft agar) at the nonpermissive temperature (40 degrees C). The reduction of external ferricyanide, hexaammine ruthenium and diferric transferrin was used to measure the transmembrane redox activity. The transformed RPN209-1 cells expressed a lower transmembrane redox activity, which is more sensitive to the antitumor drug adriamycin, when compared to the cells with a nontransformed phenotype. The lower transmembrane redox activity is associated with a decrease in the affinity for ferricyanide and a change in Vmax of the enzyme. Since the transformed cells have 25% lower concentration of NADH, the decrease in Vmax may be partly based on substrate limitation. Ionic strength variation in the assay media shows that the change in activity with transformation is not based on change in cell-surface change. Treatment with neuraminidase, however, indicates that sialic acid is important for enzyme activity, consistent with previous proposals that the transmembrane enzyme is a glycoprotein. The proton extrusion associated with transplasma membrane electron transport is increased in transformed cells relative to the rate of ferricyanide reduction. A relation between proton pumping transplasma membrane electron transport and growth stimulation by external oxidants is discussed.

Decrease of NADH in HeLa cells in the presence of transferrin or ferricyanide

The short-term incubation of HeLa cells in the presence of diferric transferrin or ferricyanide, which are reduced externally by the transplasma membrane reductase, produces a stoichiometric decrease in NADH and increase in NAD+, which is stimulated by insulin. The NADP/NADPH ratio does not change during 15 min incubation with the oxidants. The total pyridine nucleotide pool of HeLa cells is not affected. Incubation with apotransferrin and ferrocyanide, which cannot act as oxidants for transmembrane electron transport, does not change the pyridine nucleotide concentrations in the cells. Our results show that NADH can act as the internal electron donor for the reduction of external oxidants by the transmembrane reductase. It appears that oxidation of NADH by the transmembrane electron transport using ferricyanide or iron transferrin as external electron acceptors is sufficient to stimulate growth in HeLa cells.

Nonhaem complexes of FeIII stimulate cell attachment and growth by a mechanism different from that of serum, 2-oxocarboxylates, and haemproteins

Most cell lines, even those producing their own growth factors, need a serum supplement when growing in several commonly used media. The requirement for serum to sustain attachment and growth in RPMI 1640 and MEM has been found to be met by a range of 2-oxocarboxylates, by diverse coordination complexes of FeIII, and by a variety of haem-containing proteins including catalase. The latter directly implicates H2O2 in the serum shift-down effects. H2O2 was found to accumulate in low serum media under normal laboratory lighting conditions to levels that were shown to be sufficient, when added to freshly prepared media, to explain the depressed cell performance. With the exception of some of the nonhaem FeIII coordination complexes, substances found to stimulate cell attachment and growth were capable of scavenging H2O2. This suggests that an important function of serum and the 2-oxocarboxylates (alpha-keto acids) frequently used as "nonessential" medium additives is to remove H2O2 produced photodynamically during the storage and manipulation of media containing a high content of riboflavin. However, the nonhaem FeIII complexes with saturated coordination shells, although capable of reducing photodynamic generation of H2O2 to a greater or lesser extent, have their prime effect by an unknown, intriguing mechanism, probably based on a common redox function.

Redox-dependent activation of 5'-nucleotidase in rat liver plasma membranes

Addition of NADH, but not NAD+ or NADPH, to rat liver plasma membranes resulted in the increase of their 5'-nucleotidase activity. NADH-dependent activation of 5'-nucleotidase was significantly suppressed by atebrine, an inhibitor of NADH dehydrogenase of plasma membranes, and completely abolished by 2,4-dinitrophenol (2 X 10(-4)M) and Triton X-100 (2%). Inhibitors of electron transfer in the mitochondrial respiratory chain, rotenone and potassium cyanide, failed to affect 5'-nucleotidase activity in both the presence and absence of NADH. The data obtained give reasons to suggest a redox-dependent mechanism of 5'-nucleotidase activation in rat liver plasma membranes.

Bleomycin control of transplasma membrane redox activity and proton movement in HeLa cells

Bleomycin, tallysomycin A, tallysomycin S10b and copper-bleomycin have been tested for their capacity to inhibit the transplasma membrane electron transport and associated proton release by HeLa cells. Transplasma membrane redox activity is measured using reduction of external ferricyanide by the cells. At 75 micrograms/ml bleomycin, tallysomycin A and tallysomycin S10b gave a maximum of 65% inhibition of the ferricyanide reduction rate; half-maximum inhibition was observed at 30 micrograms/ml. The copper-bleomycin complex was slightly more effective as an inhibitor with half-maximum inhibition at 20 micrograms/ml. Survival of cells after 1 hr of drug treatment was 50% at 25 micrograms/ml for bleomycin and copper-bleomycin and at 75 micrograms/ml for tallysomycin A. Tallysomycin A and tallysomycin S10b gave 75 to 83% inhibition of ferricyanide-induced proton extrusion, respectively at 50 micrograms/ml, whereas bleomycin and copper-bleomycin appeared to be slightly less effective with 50 to 60% inhibition, respectively, at 50 micrograms/ml. In all aspects studied, which included transplasma membrane ferricyanide reduction, ferricyanide-induced proton release, and cell survival, there were significant effects by these compounds on HeLa cells in the range of 25-50 micrograms/ml.

Transplasma membrane redox stimulates HeLa cell growth

Impermeable ferricyanide stimulates the growth of HeLa cells in absence of fetal bovine serum or other growth factors. A series of impermeable oxidants with redox potentials down to -125 mV stimulate equivalent growth. All of these oxidants are reduced by the transplasma membrane electron transport system. Oxidants with redox potentials below -175 mV are not reduced by the transmembrane electron transport and do not stimulate growth. Insulin which stimulates growth in absence of serum also stimulates transmembrane ferricyanide reduction. Ferricyanide increases growth in presence of insulin. Antitumor drugs, which inhibit HeLa cell growth, inhibit the transplasma membrane redox system. Transplasma membrane electron transport is accompanied by proton release from HeLa cells.

Properties of a transplasma membrane electron transport system in HeLa cells

A transmembrane electron transport system has been studied in HeLa cells using an external impermeable oxidant, ferricyanide. Reduction of ferricyanide by HeLa cells shows biphasic kinetics with a rate up to 500 nmoles/min/g w.w. (wet weight) for the fast phase and half of this rate for the slow phase. The apparent Km is 0.125 mM for the fast rate and 0.24 mM for the slow rate. The rate of reduction is proportional to cell concentration. Inhibition of the rate by glycolysis inhibitors indicates the reduction is dependent on glycolysis, which contributes the cytoplasmic electron donor NADH. Ferricyanide reduction is shown to take place on the outside of cells for it is affected by external pH and agents which react with the external surface. Ferricyanide reduction is accompanied by proton release from the cells. For each mole of ferricyanide reduced, 2.3 moles of protons are released. It is, therefore, concluded that a transmembrane redox system in HeLa cells is coupled to proton gradient generation across the membrane. We propose that this redox system may be an energy source for control of membrane function in HeLa cells. The promotion of cell growth by ferricyanide (0.33-0.1 mM), which can partially replace serum as a growth factor, strongly supports this hypothesis.

Pyruvate regulation of growth and differentiation in primary cultures of rat tracheal epithelial cells

These studies examined the effect of exogenous pyruvate on the growth and differentiation of primary cell cultures of rat tracheal epithelial cells. The cell cultures were derived from outgrowths of tracheal explants, and require pyruvate for survival and growth in the presence of 10% FBS. In pyruvate-supplemented (2 mM) medium, the number of cells attached to the dish increased rapidly, while exfoliation of cells into the medium as well as formation of cornified envelopes were relatively low. The growth response to pyruvate was concentration-dependent in these cell cultures. In the absence of pyruvate, the extent of terminal differentiation to keratinization gradually increased. This was characterized by a cessation of growth after one week, and an increase in exfoliation until all cells had sloughed from the dish. Accompanying these changes was a marked increase in the formation of cornified envelopes. Cells undergoing DNA synthesis were present throughout 2 weeks of culture in pyruvate-deprived medium, even as the total number of cells was diminishing. Several compounds, including other 2-oxocarboxylic acids, were ineffective growth substitutes for pyruvate. These results indicate that the requirement for pyruvate is quite stringent in these cultures and that one way pyruvate promotes the growth of tracheal epithelial cells is by inhibiting terminal differentiation.

Inhibition of plasma membrane NADH dehydrogenase by adriamycin and related anthracycline antibiotics

Doxorubicin (adriamycin) is cytotoxic to cells, but the biochemical basis for this effect is unknown, although intercalation with DNA has been proposed This study suggests that the cytotoxicity of this drug may be due to inhibition of the plasma membrane redox system, which is involved in the control of cellular growth. Concentrations between 10(-6) - 10(-7) M adriamycin inhibit plasma membrane redox reactions greater than 50%. AD32, a form of adriamycin which does not intercalate with DNA, but is cytotoxic, also inhibits the plasma membrane redox system. Thus, the cytotoxic effects of adriamycin, which limit its use as a drug, may be based on the inhibition of a transplasma membrane dehydrogenase involved in a plasma membrane redox system.

Transformed liver cells have modified transplasma membrane redox activity which is sensitive to adriamycin

Electron transport across the plasma membrane is found in all cells which have been tested. This activity has been implicated in control of cellular growth, transport and hormone response. In virus transformed cells and tumor cells we find the activity is decreased and becomes sensitive to the antitumor drug adriamycin. Inhibition of transmembrane redox by adriamycin parallels cytoxicity to transformed cells.