Bioreductive activation of quinones: a mixed blessing

The oxidative stress in allelopathy: Participation of prenyllipid antioxidants in the response to juglone in Chlamydomonas reinhardtii

Allelopathy is a phenomenon, where one species releases compounds able to inhibit the growth of other species. Juglone, 5-hydroxy-1,4-naphtoquinone, is an allelochemical produced by walnut trees. The main mode of juglone toxicity is the formation of semiquinone radicals, able to reduce O₂ to superoxide. Prenyllipid antioxidants such as tocopherol and plastoquinone are important for antioxidant defense in photosynthetic organisms. Here we assess their participation in the response to juglone. The impact of 20 μM juglone on the content of photosynthetic pigments and prenyllipid antioxidants in green microalga Chlamydomonas reinhardtii was measured over an incubation period of 7.5 h in low light and over 40 min under high light or in darkness. The decrease in pigment and prenyllipid content, accompanied by an increase in lipid hydroperoxides was observed over a longer incubation period with juglone. Simultaneous exposure to high light and juglone led to a pronounced decrease in carotenoids and prenyllipids, while there was no decrease in high light alone and no decrease or only a slight decrease in the series with juglone alone. The fact that semiquinone radicals are generated in juglone-exposed cells was confirmed using EPR spectroscopy. This article also shows that C. reinhardtii may be a suitable model for studies on some modes of phytotoxic action of allelochemicals.

Differential cellular responses to protein adducts of naphthoquinone and monocrotaline pyrrole

Protein-xenobiotic adducts are byproducts of xenobiotic metabolism. While there is a correlation between protein adduction and target organ toxicity, a cause and effect relationship is not often clear. Naphthoquinone (NQ) and monocrotaline pyrrole (MCTP) are two pneumotoxic electrophiles that form covalent adducts with a similar select group of proteins rich in reactive thiols. In this study, we treated human pulmonary artery endothelial cells (HPAEC) with NQ, MCTP, or preformed NQ or MCTP adducts to the protein galectin-1 (gal-1) and examined indicators of reactive oxygen species (ROS) oxidative injury, markers of apoptosis (caspase-3 and annexin V), and gene responses of cellular stress. ROS production was assayed fluorescently using CM-H(2)DCFDA. NQ adducts to gal-1 (NQ-gal) produced 183% more intracellular ROS than gal-1 alone (p < 0.0001). Caspase-3 activity and annexin V staining of phosphatidylserine were used to assess apoptotic activity in treated cells. HPAEC exposed to MCTP-gal had increases in both caspase-3 activation and membrane translocation of annexin V relative to gal-1 alone (p < 0.0001). Direct application of NQ produced significantly more ROS and induced significant caspase-3 activation, whereas MCTP did not. Human bronchial epithelial cells were also exposed to MCTP-gal and found to have significant increases in both caspase-3 activation and annexin V staining in comparison to that of gal-1 (p < 0.05). Western blot analysis showed that both NQ and MCTP significantly induced the Nrf2 mediated stress response pathway despite differences in ROS generation. ER stress was not induced by either adducts or parent compounds as seen by quantitative RT-PCR, but HOX-1 expression was significantly induced by NQ-gal and MCTP alone. Electrophile adduction to gal-1 produces different cytotoxic effects specific to each reactive intermediate.

Redox-cycling compounds can cause the permeabilization of mitochondrial membranes by mechanisms other than ROS production

The participation of reactive oxygen species (ROS) in the regulation of mitochondrial permeability transition pore (mPTP) opening by the redox-cycling compounds menadione and lucigenin was explored. The level of ROS was modulated by antioxidants, anoxia, and switching the sites of the reduction of redox cyclers, the dehydrogenases of the inner and outer mitochondrial membranes. We found that the reduction of both lucigenin and menadione in the outer mitochondrial membrane caused a strong production of ROS. However, mPTP opening was accelerated only in the presence of the cationic acceptor lucigenin. The antioxidants and scavengers of ROS that considerably decreased the level of ROS in mitochondria did not prevent or delay the mPTP opening. If the transmembrane potential under anoxia was supported by exogenous ATP or ferricyanide, the permeabilization of mitochondrial membranes by menadione or lucigenin was the same as under normoxia or even more pronounced. Under anoxia, the lucigenin-dependent permeabilization of membranes was less sensitive to mPTP antagonists than under normoxia. We conclude that the opening of the mPTP by redox cyclers may be independent of ROS and is due to the direct oxidation of mitochondrial pyridine nucleotides by menadione and the modification of critical thiols of the mPTP by the cation radical of lucigenin.

Molecular cytotoxic mechanisms of catecholic polychlorinated biphenyl metabolites in isolated rat hepatocytes

Polychlorinated biphenyl (PCB) and PCB metabolites are highly lipophilic and accumulate easily in the lipid bilayer and fat deposits of the body. The molecular cytotoxic mechanisms of these metabolites are still not understood. The aim of the present study was to compare the cytotoxicity and toxicological properties of six dihydroxylated metabolites using isolated rat hepatocytes. All of the metabolites were more cytotoxic than 4-chlorobiphenyl (4-ClBP) and less cytotoxic than phenyl hydroquinone (PHQ). The order of cytotoxic effectiveness of catecholic metabolites expressed as LC(50) (2h) was 3',4'-diCl-2,3-diOH-biphenyl>PHQ>4'-Cl-2,5-diOH-biphenyl, 4'-Cl-2,3-diOH-biphenyl>2',5'-diCl-3,4-diOH-biphenyl>2',3'-diCl-3,4-diOH-biphenyl>3',4'-diCl-3,4-diOH-biphenyl>4'Cl-3,4-diOH-biphenyl>4'-Cl-biphenyl; showing that the positions of hydroxyl and chlorine groups were important for their hepatotoxicity and that the two 2,3-diOH congeners were the most cytotoxic. Cytotoxicity for 3,4-diOH metabolites correlated with the number and position of chlorine atoms with the more chlorine atoms being more cytotoxic. The cytotoxic order of metabolites with two chlorine atoms being 2',5'>2',3'>3',4'. Borneol, an uridine diphosphate glucuronosyltransferases (UGT) inhibitor, increased the cytotoxicity of all tested metabolites; suggesting that glucuronidation was a major mechanism of elimination of these compounds. On the other hand entacapone, a catechol-O-methyl transferase (COMT) inhibitor, only increased the cytotoxicity of 3',4'-diCl-3,4-diOH-biphenyl, 3',4'-diCl-2,3-diOH-biphenyl and 4'-Cl-2,3-diOH-biphenyl. Hepatocyte GSH was depleted (oxidized and conjugated) by these metabolites before cytotoxicity ensued in a similar order of effectiveness to their cytotoxicity with PHQ being the most effective. Hepatocyte mitochondrial membrane potential also decreased before cytotoxicity ensued with a similar order of effectiveness as their cytotoxicity. These results suggest that catecholic cytotoxicity can be attributed to mitochondrial toxicity and oxidative stress. Semiquinone or benzoquinone species were also important in the cytotoxicity of catecholic metabolites.

Redox regulation of Cdc25B by cell-active quinolinediones

Intracellular reduction and oxidation pathways regulate protein functionality through both reversible and irreversible mechanisms. The Cdc25 phosphatases, which control cell cycle progression, are potential subjects of oxidative regulation. Many of the more potent Cdc25 phosphatase inhibitors reported to date are quinones, which are capable of redox cycling. Therefore, we used the previously characterized quinolinedione Cdc25 inhibitor DA3003-1 [NSC 663284 or 6-chloro-7-(2-morpholin-4-yl-ethylamino)-quinoline-5,8-dione] and a newly synthesized congener JUN1111 [7-(2-morpholin-4-yl-ethylamino)-quinoline-5,8-dione] to test the hypothesis that quinone inhibitors of Cdc25 regulate phosphatase activity through redox mechanisms. Like DA3003-1, JUN1111 selectively inhibited Cdc25 phosphatases in vitro in an irreversible, time-dependent manner and arrested cells in the G1 and G2/M phases of the cell cycle. It is noteworthy that both DA3003-1 and JUN1111 directly inhibited Cdc25B activity in cells. Depletion of glutathione increased cellular sensitivity to DA3003-1 and JUN1111, and in vitro Cdc25B inhibition by these compounds was sensitive to pH, catalase, and reductants (dithiothreitol and glutathione), consistent with oxidative inactivation. In addition, both DA3003-1 and JUN1111 rapidly generated intracellular reactive oxygen species. Analysis of Cdc25B by mass spectrometry revealed sulfonic acid formation on the catalytic cysteine of Cdc25B after in vitro treatment with DA3003-1. These results indicate that irreversible oxidation of the catalytic cysteine of Cdc25B is indeed a mechanism by which these quinolinediones inactivate this protein phosphatase.

Transfection of COS-1 cells with DT-diaphorase cDNA: role of a base change at position 609

DT-diaphorase, a homodimeric flavoenzyme, can provide for a defence mechanism against carcinogenesis mediated by dietary or environmental quinones as well as bioactivate quinone-containing chemotherapeutic drugs. Human cell lines and strains have been identified with very low or undetectable enzymatic activity and a C to T transition at nucleotide 609 of the DT-diaphorase cDNA. This single base change is predicted to result in a proline to serine change in amino acid 187. Human cells homozygous for this base transition fail to exhibit Western blot reactivity for DT-diaphorase, suggesting that this substitution results in protein instability. To directly test whether this base change affects DT-diaphorase enzymatic activity and/or protein stability in vivo, mammalian expression vectors containing DT-diaphorase cDNA with or without the nucleotide 609 base transition were transiently transfected in COS-1 cells. Co-transfection with a human growth hormone expression vector allowed normalization for transfection efficiency. COS-1 transfectants expressing the C to T base change displayed at least a tenfold reduction in DT-diaphorase activity (P < 0.001) and a two- to threefold reduction in protein levels compared with wild-type transfectants. These results are the first to detect the presence of DT-diaphorase protein coded for by the 609 base transition in mammalian cells and confirm its predicted reduced enzymatic activity.

Isolation and characterization of a laccase gene from Podospora anserina

The genome of the filamentous ascomycete Podospora anserina contains at least four non-adjacent regions that are homologous to the laccase gene of Neurospora crassa. One of these regions contains a gene (lac2) encoding a protein that displays 62% identity with the N. crassa laccase. In shaken cultures, lac2 mRNA is present at low basal levels throughout the growth phase but increases at least 20-fold at the beginning of the autolytic phase and decreases again thereafter. Addition of aromatic xenobiotics (guaiacol, hydroquinone, benzoquinone) to the medium during the growth phase results in a rapid, drastic and temporary increase in the abundance of lac2 mRNA. The promoter region of lac2 contains two sequences which display complete homology with the eukaryotic Xenobiotic Responsive Element and two sequences homologous to the eukaryotic Antioxidant Responsive Element. The identity and function of the laccase encoded by lac2 are discussed.

Redox cycling of beta-lapachone and related o-naphthoquinones in the presence of dihydrolipoamide and oxygen

Lipophilic o-naphthoquinones (beta-lapachone, CG 8-935, CG 9-442, CG 10-248, and mansonones A, C, E, and F), catalyze the oxidation of dihydrolipoamide (DHLA) by oxygen, whereas p-naphthoquinones (alpha-lapachone and menadione) are scarcely active. The greatest effects corresponded to beta-lapachone and its analogues. Quinol production was demonstrated by (a) the absorption spectrum of the reduced quinone, and (b) the effect of pH variation on the rate of quinone-catalyzed DHLA oxidation. Superoxide dismutase (SOD) inhibited the rate of cytochrome c reduction and decreased the apparent rate of oxygen consumption by several DHLA/o-naphthoquinone systems. SOD also inhibited the rate of quinol oxidation by oxygen, after quinone reduction by a stoichiometric amount of DHLA. Catalase enhanced the effect of SOD, but in its absence catalase was inactive. It is concluded that quinone-catalyzed oxidation of DHLA implies a free-radical mechanism in which the quinol and superoxide radicals play an essential role.

The cytotoxicity of mitomycin C and adriamycin in genetically engineered V79 cell lines and freshly isolated rat hepatocytes

The objective of the present study was to investigate the cytotoxicity of Adriamycin (ADR) and mitomycin C (MMC) in tumor and non-tumor cells with respect to the role of cytochrome P450 (P450). Therefore, genetically engineered V79 Chinese hamster fibroblasts expressing only single enzymes of P450 were used. SD1 and XEM2 cells expressed rat P450IIB1 and P450IA1, respectively, whereas the V79 parental cells contained no detectable P450 levels. The cytotoxicity of ADR and MMC in the V79 cell system was compared with that in freshly isolated hepatocytes from phenobarbital (PB-hepatocytes)- and beta-naphthoflavone (beta NF-hepatocytes)-induced rats. Following 24 h of exposure to ADR equal cytotoxicity was observed in V79, SD1 and XEM2 cells. Addition of metyrapone (MP, an inhibitor of P450IIB1) and alpha-naphthoflavone (alpha NF, an inhibitor of P450IA1) had no effect on the ADR-induced cytotoxicity in SD1 and XEM2 cells, respectively. Likewise, MMC was equitoxic in V79 and SD1 cells. Co-incubation of SD1 cells with MP did not alter MMC-induced cytotoxicity. MMC, however, showed a decreased cytotoxicity in XEM2 cells when compared to the parental V79 cells. Unexpectedly, the cytotoxicity of MMC in XEM2 cells was increased by alpha NF to the same level as observed in the parental V79 cells. In contrast to V79- and V79-derived cells, in freshly isolated hepatocytes from PB or beta NF-induced rats, MMC was cytotoxic (measured as lactate dehydrogenase leakage) within 3 h of incubation. ADR, however, was only cytotoxic to the hepatocytes when intracellular glutathione was first depleted by diethylmaleate. The MMC- and ADR-induced cytotoxicity was found to be more pronounced in PB-hepatocytes than in beta NF-hepatocytes. Contrary to the findings in the V79-derived cells, MP afforded complete protection against both MMC- and ADR-induced cytotoxicity in PB-hepatocytes, whereas alpha NF only partially inhibited the cytotoxicity of MMC in beta NF-hepatocytes. In conclusion, we have demonstrated that PB-inducible P450s play a role in the cytotoxicity of both MMC and ADR in freshly isolated PB-hepatocytes but that P450IIB1 does not in genetically reconstituted SD1 cells. P450IA1, however, decreased the cytotoxicity of MMC in the XEM2 cells. The ADR-induced cytotoxicity, which was observed in XEM2 cells, was not mediated by P450IA1. The present study underscores the complexity in the comparison of ADR- and MMC-induced cytotoxicities in normal and tumor cells.

Quantitative structure activity relationship for the acute cytotoxicity of 13 (bis)aziridinyl-benzoquinones: relation to cellular ATP depletion

This study was performed to establish relationships between the structure of 2,5-bis(1-aziridinyl)-1,4-benzoquinones (BABQs) bearing different substituents at the 3- and 6-position and their acute toxic effects in rat hepatocytes. The cell viability, loss of cellular glutathione (GSH+GSSG) and loss of ATP were followed during 4 h of incubation of freshly isolated hepatocytes. The toxicity of these compounds (100 microM) was predicted better by their reactivity with GSH than by their redox cycling in rat liver microsomes. The time of 50% loss of viability (LT50) correlated very well with the time of 50% depletion of ATP (AT50). LT50 could be adequately predicted by using the electronic field parameter (Ftotal) describing the electron withdrawing or donating properties for all the substituents on the quinone-nucleus. 7-(Di)halogen-substituted BABQs that all very rapidly depleted cellular glutathione showed significant differences in AT50 as well as in LT50. This suggests that alterations in ATP levels are important for explaining the differences in cytotoxicity of these compounds.

Immunomodulation by quinones. A model for the use of quinones in the treatment of inflammation

The therapeutic application of quinones in areas other than oncology, such as in chronic inflammation, has been proposed. However, because of the adverse side-effects on the function and vitality of almost all investigated cell types the therapeutical margin is small. The thiol-conjugating capacity of quinones may, however, be applied to reduce the tissue-damaging effects of stimulated neutrophils. In this paper evidence is provided that particular phenols may be used as precursor molecules of quinones. Secretory products from stimulated neutrophils can convert such phenols into quinones. As under normal conditions stimulated neutrophils are present only in inflamed tissues, quinones will be formed only at these sites.