Inhibition of NADH oxidase activity and growth of HeLa cells by the antitumor sulfonylurea, N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl) urea (LY181984) and response to epidermal growth factor

Tea catechin synergies in inhibition of cancer cell proliferation and of a cancer specific cell surface oxidase (ECTO-NOX)

The anticancer properties of tea catechins are most frequently attributed to the principal catechin (-)-epigallocatechin-3-gallate (EGCg). Efficacy was evaluated using growth of cultured HeLa cells and inhibition of the enzymatic activity of a putative cell surface tea target enzyme, a cancer-associated cell surface-located NADH oxidase (ECTO-NOX) designated tNOX. The amounts of EGCg required to inhibit by both criteria was reduced 10 times by combination with inactive catechins such as (-)-epicatechin (EC), (-)-epigallocatechin (EGC) or (-)-epicatechin-3-gallate (ECG). Various synthetic mixtures based on purified catechins and decaffeinated tea extracts treated enzymatically to reduce the ester bond-containing catechins varying in EGCg content from 0.065 to 40% were of comparable efficacy to decaffeinated green tea extracts as long as EGCg was present and the ratio of total catechins to EGCg + EGC was about 1.5. Such mixtures appear to offer potential cancer protection and therapeutic advantages over those of EGCg alone through lowered toxicity of the mixture to normal cells and for more efficient blood delivery of orally-administered catechins to a tumour site.

Preferential inhibition of the plasma membrane NADH oxidase (NOX) activity by diphenyleneiodonium chloride with NADPH as donor

The cell-surface NADH oxidase (NOX) protein of plant and animal cells will utilize both NADH and NADPH as reduced electron donors for activity. The two activities are distinguished by a differential inhibition by the redox inhibitor diphenyleneiodonium chloride (DPI). Using both plasma membranes and cells, activity with NADPH as donor was markedly inhibited by DPI at submicromolar concentrations, whereas with NADH as donor, DPI was much less effective or had no effect on the activity. The possibility of the inhibition being the result of two different enzymes was eliminated by the use of a recombinant NOX protein. The findings support the concept that NOX proteins serve as terminal oxidases for plasma membrane electron transport involving cytosolic reduced pyridine nucleotides as the natural electron donors and with molecular oxygen as the electron acceptor.

Preferential inhibition by (-)-epigallocatechin-3-gallate of the cell surface NADH oxidase and growth of transformed cells in culture

A drug-responsive and cancer-specific NADH oxidase of the mammalian plasma membrane, constitutively activated in transformed cells, was inhibited preferentially in HeLa and human mammary adenocarcinoma by the naturally-occurring catechin of green tea, (-)-epigallocatechin-3-gallate (EGCg). With cells in culture, EGCg preferentially inhibited growth of HeLa and mammary adenocarcinoma cells compared with growth of mammary epithelial cells. Inhibited cells became smaller, and cell death was accompanied by a condensed and fragmented appearance of the nuclear DNA as revealed by fluorescence microscopy with 4',6-diamidino-2-phenylindole, suggestive of apoptosis. Mammary epithelial cells recovered from EGCg treatment even at 50 microM, whereas growth of HeLa and mammary adenocarcinoma cells was inhibited by EGCg at concentrations as low as 1 microM with repeated twice-daily additions and did not recover from treatment with 50 microM EGCg. The findings correlate inhibition of cell surface NADH oxidase activity and inhibition of growth with EGCg-induced apoptosis.

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.

Oxidation of NADH by intact segments of soybean hypocotyls and stimulation by 2,4-D

Intact sections of soybean cut from regions of cell elongation of hypocotyls of etiolated soybean seedlings oxidized externally supplied NADH (NADH is an impermeant substrate). The oxidation of NADH by 1-cm intact sections was stimulated by the plant growth factor 2,4-dichlorophenoxyacetic acid (2,4-D). The optimum concentration of 2,4-D for stimulation was about 1 microM. Stimulations also were given by the naturally occurring 2,4-D analog, indole-3-acetic acid (IAA), but not by the growth inactive 2,4-D analog 2,3-dichlorophenoxyacetic acid (2,3-D). The findings confirm studies comparing inside-out and right side-out vesicles that show the 2,4-D-stimulated NADH oxidase to be located at the external cell surface. Since plant cells are unlikely to encounter NADH at their external cell surface, functions of the oxidase in reactions other than oxidation of NADH are discussed.

The sulfonylurea-inhibited NADH oxidase activity of HeLa cell plasma membranes has properties of a protein disulfide-thiol oxidoreductase with protein disulfide-thiol interchange activity

Plasma membrane vesicles of HeLa cells are characterized by a drug-responsive oxidation of NADH. The NADH oxidation takes place in an argon or nitrogen atmosphere and in samples purged of oxygen. Direct assay of protein thiols by reaction with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB; Ellman's reagent), suggests that protein disulfides may be the natural electron acceptors for NADH oxidation by the plasma membrane vesicles. In the presence of NADH, protein disulfides of the membranes were reduced with a concomitant stoichiometric increase in protein thiols. The increase in protein thiols was inhibited in parallel to the inhibition of NADH oxidation by the antitumor sulfonylurea LY181984 with an EC50 of ca. 30 nM. LY 181984, with an EC50 of 30 nM, also inhibited a protein disulfide-thiol interchange activity based on the restoration of activity to inactive (scrambled) RNase and thiol oxidation. The findings suggest that thiol oxidation, NADH-dependent disulfide reduction (NADH oxidation), and protein disulfide-thiol interchange in the absence of NADH all may be manifestations of the same sulfonylurea binding protein of the HeLa plasma membrane. A surface location of the thiols involved was demonstrated using detergents and the impermeant thiol reagent p-chloromercuriphenylsulfonic acid (PCMPS). The surface location precludes a physiological role of the protein in NADH oxidation. Rather, it may carry out some other role more closely related to a function in growth, such as protein disulfide-thiol interchange coupled to cell enlargement.