Mechanism of tetrahydroxy-1,4-quinone cytotoxicity: involvement of CA2+ and H2O2 in the impairment of DNA replication and mitochondrial function

The role of tetrahydroxyquinone solubility on apoptosis induction in human colorectal cells

Tetrahydroxy-1,4-benzoquinone (THQ) is a highly redox-active substance that generates reactive oxygen species (ROS), which can induce apoptosis in cell culture experiments. The underlying mechanism for ROS production has previously been postulated to be the autoxidation of THQ to rhodizonic acid (RhA). However, our results suggest that the cells detoxify THQ by reducing it to hexahydroxybenzene (HHB), catalyzed by the NADPH-quinone-oxidoreductase (NQO1). Then, HHB undergoes autoxidation back to THQ, closing a redox cycle that continuously generates ROS. Only this continuous mechanism produces enough ROS to trigger apoptosis. The cell's protective measures can effectively eliminate the ROS generated by a single autoxidation of THQ to RhA because RhA is not reduced back to THQ and thus does not close a redox cycle. This also explains why only fresh THQ solutions are cytotoxic, whereas older THQ solutions, which are readily autoxidized to RhA, are not.

Development of a high-throughput screening system targeting the protein-protein interactions between PRL and CNNM

Phosphatase of regenerating liver (PRL) is an oncogenic protein that promotes tumor progression by directly binding to cyclin M (CNNM) membrane proteins and inhibiting their Mg2+ efflux activity. In this study, we have developed a high-throughput screening system to detect the interactions between PRL and CNNM proteins based on homogenous time-resolved fluorescence resonance energy transfer (HTR-FRET, HTRF). We optimized the tag sequences attached to the recombinant proteins of the CNNM4 CBS domains and PRL3 lacking the carboxyl terminal CAAX motif, and successfully detected the interaction by observing the FRET signal in the mixture of the tagged proteins and fluorophore-conjugated antibodies. Moreover, we performed compound library screening using this system and discovered several compounds that could efficiently inhibit the PRL-CNNM interaction. Characterization of one candidate compound revealed that it was relatively stable compared with thienopyridone, a known inhibitor of the PRL-CNNM interaction. The candidate compound can also inhibit PRL function in cells: suppression of CNNM-dependent Mg2+ efflux, and has sufficient in vitro drug metabolism and pharmacokinetic properties. Overall, these results demonstrate the effectiveness of this screening system for identifying novel inhibitors of the PRL-CNNM interaction, which could contribute to the development of novel anti-cancer drugs.

Oxocarbon Acids and their Derivatives in Biological and Medicinal Chemistry

The biological and medicinal chemistry of the oxocarbon acids 2,3- dihydroxycycloprop-2-en-1-one (deltic acid), 3,4-dihydroxycyclobut-3-ene-1,2-dione (squaric acid), 4,5-dihydroxy-4-cyclopentene-1,2,3-trione (croconic acid), 5,6-dihydroxycyclohex- 5-ene-1,2,3,4-tetrone (rhodizonic acid) and their derivatives is reviewed and their key chemical properties and reactions are discussed. Applications of these compounds as potential bioisosteres in biological and medicinal chemistry are examined. Reviewed areas include cell imaging, bioconjugation reactions, antiviral, antibacterial, anticancer, enzyme inhibition, and receptor pharmacology.

In vitro evaluation of the safe margin, antithrombotic and antiproliferative actions for the treatment of restenosis: Nitric oxide donor and polymers

Drug-eluting stents (DES) were developed to combat the problem of in-stent restenosis, and evaluating the biological activity from DES systems is critical for its safety and efficacy. To test the cytotoxicity of nitric oxide (NO) donor-containing polymers for their potential use in DES applications, S-nitrosoglutathione (GSNO) or in combination with poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) in an aqueous polymeric solution (PVA/PVP/GSNO) was investigated using Balb/c 3T3 and Rabbit arterial smooth muscle (RASM) cells. The sensitivity of 3T3 cells to the cytotoxicity effects induced by GSNO was higher than that of RASM cells, while RASM cells were more susceptible to alterations in membrane permeability. Cell growth assays showed that GSNO and PVA/PVP/GSNO induced antiproliferative effects in RASM cells. Moreover, the presence of polymers can reduce the cytotoxicity and enhance the antiproliferative effects of GSNO. Dose-dependent inhibition of platelet aggregation was similar for both PVA/PVP/GSNO (EC50 of 3.4 ± 2.3 µM) and GSNO (EC50 of 2.8 ± 1.1 µM) solutions. Platelet adhesion assays showed that the inhibition caused by GSNO (EC50 of 5.0 mM) was dependent on the presence of plasma. These results demonstrate that the methodology adopted here is suitable to establish safety margins and evaluate the antithrombotic potential and antiproliferative effects of NO-eluting biomaterials and polymeric solutions for the new cardiovascular devices, and also to emphasize the importance of using more specific cell lines in these evaluations.

Prooxidant action of rhodizonic acid: transition metal-dependent generation of reactive oxygen species causing the formation of 8-hydroxy-2'-deoxyguanosine formation in DNA

Rhodizonic acid, a six-membered cyclic hydroxyquinone, produced reactive oxygen species as a complex with transition metals. Addition of rhodizonic acid with ferrous ion caused an inactivation of aconitase the most sensitive enzyme to oxidative stress in permeabilized yeast cells. The iron-dependent inactivation of aconitase implies the rhodizonic acid/iron-mediated generation of reactive oxygen species. Spectrophotometric analysis of the interaction of rhodizonic acid with FeSO4 showed that addition of superoxide dismutase could inhibit the oxidation of rhodizonic acid, suggesting that reactive oxygen species produced from rhodizonic acid is superoxide radical. Rhodizonic acid further acted as a prooxidant causing a copper-dependent DNA damage. Treatment of DNA from plasmid pBR322 and calf thymus with rhodizonic acid plus copper caused strand scission and the formation of 8-hydroxy-2'-deoxyguanosine in DNA. Addition of catalase protected DNA from the rhodizonic acid-mediated strand scission, indicating that hydroxyl radical may participate in the DNA damage. Rhodizonic acid also showed a potent copper-reducing activity. These results indicate that copper ion reduced by rhodizonic acid may participate in the formation of superoxide radical that converts to hydrogen peroxide and hydroxyl radical. Other cyclic hydroxyquinones such as four-membered squaric acid and five-membered croconic acid did not show any prooxidant and reducing effects. Cytotoxic effects of tetrahydroquinone the precursor of rhodizonic acid may be related to the prooxidant properties of rhodizonic acid formed in cells.

Morphological and intracellular alterations induced by Serratia marcescens cytotoxin

In the present work, in vitro assays were used to investigate the toxicity of Serratia marcescens cytotoxin in cultured Chinese hamster ovary (CHO) cells. The time necessary to detect cellular alterations such as the onset of apoptosis, the perturbation of mitochondrial function, and cytoskeletal changes was assessed. The internalization of the cytotoxin by CHO cells was also examined. Within 10-15 min of exposure to cytotoxin, CHO cells became round, the nucleus shrank, the chromatin became more compact, and cytoplasmic blebs appeared on the cell surface. TUNEL (TdT-mediated dUTP nick end labeling) and propidium iodide staining identified some nuclei with fragmented DNA, and electrophoresis of CHO cell DNA obtained after 30-min exposure to S. marcescens toxin showed a pattern of DNA fragments typically associated with apoptosis. The cells also lost their characteristic actin organization within 10 min of exposure to cytotoxin. Lactate dehydrogenase leakage was detected after 20-min exposure to the cytotoxin and increased with time thereafter. Concomitantly, there was a time-dependent reduction in mitochondrial activity. Fluorescein-labeled S. marcescens cytotoxin was detected only on the surface of CHO cells, even after 30-min exposure to the toxin. These results show that there was no internalization of the toxin by CHO cells, and that, once bound to the cell surface, the toxin was able to induce changes in intracellular metabolism and to trigger cell death by apoptosis.

Variation in the production and distribution of substituted benzoquinone compounds among genetic strains of the confused flour beetle, Tribolium confusum

Insects often produce chemicals, such as defensive compounds, whose quantity and distribution can affect their fitness. For evolution to produce adaptations, chemical production must be genetically variable. Here we report the results of a study using high-performance liquid chromatography to quantify two important chemical secretions of the flour beetle Tribolium confusum, methyl-1, 4-benzoquinone (MBQ) and ethyl-1,4-benzoquinone (EBQ). Our results show a distinct difference in the production of the compounds among four genetically distinct strains of T. confusum (b-+, b-I, b-IV, b-Pakistan) with an unusually high amount measured for the b-Pakistan strain. By measuring internal and external benzoquinone levels separately, we were also able to detect differences in production and distribution of the compounds between the strains. Some strains secrete more of the chemicals, whereas other strains appear to sequester the compounds within their bodies. The sexes also differ in total quinone production as well as in their internal to external benzoquinone ratios, suggesting the trait is sex influenced. Finally, a consistent correlation in the amounts of MBQ to EBQ in individual beetles suggests that the substituted benzoquinones share a common precursor or pathway.

Binding of iron and inhibition of iron-dependent oxidative cell injury by the "calcium chelator" 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA)

A role for increases in intracellular calcium (Ca2+) has been suggested in the pathophysiology of various forms of oxidant-mediated cell injury. In recent studies, we found that iron bound to the Pseudomonas aeruginosa siderophore, pyochelin, augments oxidant-mediated endothelial cell injury by catalyzing the formation of hydroxyl radical (HO.). To investigate the role of Ca2+ in this process, the effects of two Ca2+ chelating agents, Fura-2 and 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA), were assessed. BAPTA, but not Fura-2, was protective against H2O2/ferripyochelin-mediated injury. Subsequent data suggested that chelation of iron rather than Ca2+ by BAPTA was most likely responsible. Spectrophotometry demonstrated that both ferrous (Fe2+) and ferric (Fe3+) iron formed a complex with BAPTA. The affinity of BAPTA for the metals was Fe3+ > Ca2+ > Fe2+. BAPTA was found to decrease markedly iron-catalyzed production of HO. and/or ferryl species when analyzed by spin trapping. Although our results do not definitively prove that BAPTA protects endothelial cells from ferripyochelin-associated damage by chelating iron, these data indicate that caution must be exercised in utilizing protective effects of intracellular "Ca2+ chelating agents" as evidence for a role of alterations in cellular Ca2+ levels in experimental conditions in which iron-mediated oxidant production is also occurring.