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)

The oxidative function of diferric transferrin

There is evidence for an unexpected role of diferric transferrin as a terminal oxidase for the transplasma membrane oxidation of cytosolic NADH. In the original studies which showed the reduction of iron in transferrin by the plasma membranes NADH oxidase, the possible role of the reduction on iron uptake was emphasized. The rapid reoxidation of transferrin iron under aerobic conditions precludes a role for surface reduction at neutral pH for release of iron for uptake at the plasma membrane. The stimulation of cytosolic NADH oxidation by diferric transferrin indicates that the transferrin can act as a terminal oxidase for the transplasma membrane NADH oxidase or can bind to a site which activates the oxidase. Since plasma membrane NADH oxidases clearly play a role in cell signaling, the relation of ferric transferrin stimulation of NADH oxidase to cell control should be considered, especially in relation to the growth promotion by transferrin not related to iron uptake. The oxidase can also contribute to control of cytosolic NAD concentration, and thereby can activate sirtuins for control of ageing and growth.

A role for Na+/H+ exchangers and intracellular pH in regulating vitamin C-driven electron transport across the plasma membrane

Ascorbate (vitamin C) is the major electron donor to a tPMET (transplasma membrane electron transport) system that was originally identified in human erythrocytes. This plasma membrane redox system appears to transfer electrons from intracellular ascorbate to extracellular oxidants (e.g. non-transferrin-bound iron). Although this phenomenon has been observed in nucleated cells, its mechanism and regulation are not well understood. In the present study we have examined both facets of this phenomenon in K562 cells and primary astrocyte cultures. Using ferricyanide as the analytical oxidant we demonstrate that tPMET is enhanced by dehydroascorbate uptake via facilitative glucose transporters, and subsequent accumulation of intracellular ascorbate. Additionally, we demonstrate that this stimulation is not due to ascorbate that is released from the cells, but is dependent only on a restricted intracellular pool of the vitamin. Substrate-saturation kinetics suggest an enzyme-catalysed reaction across the plasma membrane by an as-yet-unidentified reductase that relies on extensive recycling of intracellular ascorbate. Inhibition of ascorbate-stimulated tPMET by the NHE (Na+/H+-exchanger) inhibitors amiloride and 5-(N-ethyl-N-isopropyl)amiloride, which is diminished by bicarbonate, suggests that tPMET activity may be regulated by intracellular pH. In support of this hypothesis, tPMET in astrocytes was significantly inhibited by ammonium chloride-pulse-induced intracellular acidification, whereas it was significantly stimulated by bicarbonate-induced intracellular alkalinization. These results suggest that ascorbate-dependent tPMET is enzyme-catalysed and is modulated by NHE activity and intracellular pH.

High throughput assay to detect compounds that enhance the proton permeability of Candida albicans membranes

We describe a 96-well microtiter plate format assay to detect changes in proton permeability in membranes of the pathogenic yeast, Candida albicans. Candida albicans cells were incubated with the lipophilic ester of 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), a pH-sensitive fluorescein derivative. Inside the cells, BCECF was released and trapped in the vacuole. Compounds that destroyed membrane integrity increased the pH value of the vacuole due to proton leakage into the cytoplasm. This was paralleled by an increase in BCECF fluorescence intensity, which could be quantified. The test assay was validated with amphotericin B, as well as with other membrane-active compounds known to increase membrane permeability. Possible applications and limitations of this assay in the field of antifungal drug discovery are discussed.

Carcinogenesis and the plasma membrane

Presented is a two-stage hypothesis of carcinogenesis based on: (1) plasma membrane defects that produce abnormal electron and proton efflux; and (2) electrical uncoupling of cells through loss of intercellular communication. These changes can be induced by a wide variety of stimuli including chemical carcinogens, oncoviruses, inherited and/or acquired genetic defects, and epigenetic abnormalities. The resulting loss of electron/proton homeostasis leads to decreased transmembrane potential, electrical microenvironment alterations, decreased extracellular pH, and increased intracellular pH. This produces a positive feedback loop to enhance and sustain the proton/electron efflux and loss of intercellular communication. Low transmembrane potential is functionally related to rapid cell cycling, changes in membrane structure, and malignancy. Intracellular alkalinization affects a variety of pH-sensitive systems including glycolysis, DNA synthesis, DNA transcription and DNA repair, and promotes genetic instability, accounting for the accumulation of genetic defects seen in malignancy. The abnormal microenvironment results in the selective survival and proliferation of malignant cells at the expense of contiguous normal cell populations.

A circulating form of NADH oxidase activity responsive to the antitumor sulfonylurea N-4-(methylphenylsulfonyl)-N'-(4-chlorophenyl)urea (LY181984) specific to sera from cancer patients

Our laboratory has described a drug-responsive NADH oxidase activity of the external surface of the plasma membrane of HeLa and other cancer cells, but not from normal cells, that was shed into media conditioned by the growth of cancer cells such as HeLa. The shed form of the activity exhibited the same drug responsiveness as the plasma membrane-associated form. In this study, sera from tumor-bearing and control rats, cancer patients, normal volunteers, and patients with diseases other than cancer were collected and assayed for a cancer-specific form of NADH oxidase responsive to the antitumor sulfonylurea N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl)urea (LY181984). With sera from tumor-bearing rats and cancer patients, LY181984 added at a final concentration of 1 microM either inhibited or stimulated the activity. With sera from control rats, normal volunteers, or patients with disorders other than cancer, the drug was without effect on the NADH oxidase activity of the sera. The activity altered by the antitumor sulfonylurea was present both in freshly collected sera and in sera stored frozen. Inhibition was half maximal at about 30 nM LY181984. The sulfonylurea-altered activity was found in sera of nearly 200 cancer patients including patients with solid cancers (e.g., breast, prostate, lung, ovarian) and with leukemias and lymphomas. We postulate that the serum presence of the antitumor sulfonylurea-responsive NADH oxidase represents an origin due to shedding from the patient's cancer. If so, the antitumor-responsive NADH oxidase would represent the first reported cell surface change universally associated with all forms of human cancer.

A protein disulfide-thiol interchange activity of HeLa plasma membranes inhibited by the antitumor sulfonylurea N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl) urea (LY181984)

Plasma membrane vesicles isolated from HeLa cells grown in suspension culture contain a protein disulfide-thiol interchange (protein disulfide-like) activity. The activity was estimated from the restoration of activity to inactive (scrambled) pancreatic RNAase. RNAase activity was measured either by hydrolysis of cCMP or by a decrease in acid precipitable yeast RNA. The ability of plasma membrane vesicles to restore activity to inactive (scrambled) pancreatic ribonuclease was inhibited by the antitumor sulfonylurea N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl)urea (LY181984). The activity correlated with that of a cyanide-resistant NADH oxidase also associated with the plasma membrane vesicles that exhibited a similar pattern of drug response. The activity was stimulated by reduced glutathione and inhibited by oxidized glutathione but did not depend on either for activity. The antitumor sulfonylurea-inhibited activity was greatest in the presence of reduced glutathione and least in the presence of oxidized glutathione. The antitumor sulfonylurea-inhibited activity was unaffected by a monoclonal antibody to protein disulfide isomerase. Also the antitumor sulfonylurea-inhibited activity was unaffected by peptide antisera to the consensus active site sequence of protein disulfide isomerase. Thus the antitumor sulfonylurea-inhibited activity appeared to reside with a novel cell surface protein capable of oxidation of both NADH and protein thiols and of carrying out a protein disulfide isomerase-like protein disulfide-thiol interchange activity in the absence of NADH or other external reductants.