The chemical biology of naphthoquinones and its environmental implications

New 1,4-anthracenedione derivatives with fused heterocyclic rings: synthesis and biological evaluation

New 1,4-anthracenediones bearing fused-heterocycle rings were synthesized and evaluated as cytotoxics, antifungals and antivirals. Some of them showed GI₅₀ at the μM level.

1,4-naphthoquinones: from oxidative damage to cellular and inter-cellular signaling

Naphthoquinones may cause oxidative stress in exposed cells and, therefore, affect redox signaling. Here, contributions of redox cycling and alkylating properties of quinones (both natural and synthetic, such as plumbagin, juglone, lawsone, menadione, methoxy-naphthoquinones, and others) to cellular and inter-cellular signaling processes are discussed: (i) naphthoquinone-induced Nrf2-dependent modulation of gene expression and its potentially beneficial outcome; (ii) the modulation of receptor tyrosine kinases, such as the epidermal growth factor receptor by naphthoquinones, resulting in altered gap junctional intercellular communication. Generation of reactive oxygen species and modulation of redox signaling are properties of naphthoquinones that render them interesting leads for the development of novel compounds of potential use in various therapeutic settings.

Synthesis and cytotoxic evaluation of a series of 2-amino-naphthoquinones against human cancer cells

The cytotoxicity of a series of aminonaphthoquinones resulting from the reaction of suitable aminoacids with 1,4-naphthoquinone was assayed against SF-295 (glioblastoma), MDAMB-435 (breast), HCT-8 (colon), HCT-116 (colon), HL-60 (leukemia), OVCAR-8 (ovarian), NCI-H358M (bronchoalveolar lung carcinoma) and PC3-M (prostate) cancer cells and also against PBMC (peripheral blood mononuclear cells). The results demonstrated that all the synthetic aminonaphthoquinones had relevant cytotoxic activity against all human cancer lines used in this experiment. Five of the compounds showed high cytotoxicity and selectivity against all cancer cell lines tested (IC₅₀ = 0.49 to 3.89 µg·mL⁻¹). The title compounds were less toxic to PBMC, since IC₅₀ was 1.5 to eighteen times higher (IC₅₀ = 5.51 to 17.61 µg·mL⁻¹) than values shown by tumour cell lines. The mechanism of cell growth inhibition and structure–activity relationships remains as a target for future investigations.

Antifungal activity of naphthoquinoidal compounds in vitro against fluconazole-resistant strains of different Candida species: a special emphasis on mechanisms of action on Candida tropicalis

In recent decades, the incidence of candidemia in tertiary hospitals worldwide has substantially increased. These infections are a major cause of morbidity and mortality; in addition, they prolong hospital stays and raise the costs associated with treatment. Studies have reported a significant increase in infections by non-albicans Candida species, especially C. tropicalis. The number of antifungal drugs on the market is small in comparison to the number of antibacterial agents available. The limited number of treatment options, coupled with the increasing frequency of cross-resistance, makes it necessary to develop new therapeutic strategies. The objective of this study was to evaluate and compare the antifungal activities of three semisynthetic naphthofuranquinone molecules against fluconazole-resistant Candida spp. strains. These results allowed to us to evaluate the antifungal effects of three naphthofuranquinones on fluconazole-resistant C. tropicalis. The toxicity of these compounds was manifested as increased intracellular ROS, which resulted in membrane damage and changes in cell size/granularity, mitochondrial membrane depolarization, and DNA damage (including oxidation and strand breakage). In conclusion, the tested naphthofuranquinones (compounds 1-3) exhibited in vitro cytotoxicity against fluconazole-resistant Candida spp. strains.

Shikonin targets cytosolic thioredoxin reductase to induce ROS-mediated apoptosis in human promyelocytic leukemia HL-60 cells

Shikonin, a major active component of the Chinese herbal plant Lithospermum erythrorhizon, has been applied for centuries in traditional Chinese medicine. Although shikonin demonstrates potent anticancer efficacy in numerous types of human cancer cells, the cellular targets of shikonin have not been fully defined. We report here that shikonin may interact with the cytosolic thioredoxin reductase (TrxR1), an important selenocysteine (Sec)-containing antioxidant enzyme with a C-terminal -Gly-Cys-Sec-Gly active site, to induce reactive oxygen species (ROS)-mediated apoptosis in human promyelocytic leukemia HL-60 cells. Shikonin primarily targets the Sec residue in TrxR1 to inhibit its physiological function, but further shifts the enzyme to an NADPH oxidase to generate superoxide anions, which leads to accumulation of ROS and collapse of the intracellular redox balance. Importantly, overexpression of functional TrxR1 attenuates the cytotoxicity of shikonin, whereas knockdown of TrxR1 sensitizes cells to shikonin treatment. Targeting TrxR1 with shikonin thus discloses a previously unrecognized mechanism underlying the biological activity of shikonin and provides an in-depth insight into the action of shikonin in the treatment of cancer.

Activation of EGFR/MEK/ERK/AP-1 signaling mediated by 1,2-naphthoquinone, an atmospheric electrophile, in human pulmonary A549 cells

1,2-Naphthoquinone (1,2-NQ) is found to be an electrophile contaminated in the atmosphere. Although we found that 1,2-NQ activates epidermal growth factor receptor (EGFR) coupled to inhibition of protein tyrosine phosphatase 1B (PTP1B) activity through covalent modification of Cys121 in human epithelial A431 cells, modulation of its downstream signal transduction pathway caused by 1,2-NQ remains to be elucidated. In the present study, we examined whether 1,2-NQ could affect such cellular signaling in human pulmonary A549 cells. Exposure of A549 cells to 1,2-NQ increased EGFR phosphorylation, resulting in activation of MEK/ERK signaling that was blocked by either PD15035 or PD98059. As a result, DNA binding activity of transcription factor AP-1 was enhanced during exposure to 1,2-NQ in the cells. These results suggest that the atmospheric electrophile phosphorylates EGFR, thereby activating the MEK/ERK/AP-1 signal transduction pathway in A549 cells.

Emodin, an anthraquinone derivative, protects against gamma radiation-induced toxicity by inhibiting DNA damage and oxidative stress

PURPOSE

In the present study, we explored the modulatory effect of emodin (1,3,8-trihydroxy-6-methylanthraquinone, C(15)H(10)O(5)) against gamma radiation-induced DNA damage and oxidative stress in acellular and cellular systems, respectively.

MATERIALS AND METHODS

For cellular systems, concanavalin A (ConA)-stimulated murine splenocytes were used. Cytotoxic effect of emodin (0-400 μM), radiation (3-12 Gy) and emodin + radiation was measured by MTT [3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyltetrazolium bromide] assay. Gamma radiation (3-12 Gy)-induced production of reactive oxygen species (ROS), an increase in nitric oxide (NO) level and its inhibition by emodin were estimated by DCFDA (2',7'-dichlorofluorescein diacetate) and Griess regent, respectively. Analysis of radiation-induced apoptosis was performed using flow cytometery and acridine orange/ethidium bromide staining. DNA damage was evaluated in acellular system using pBR322 plasmid relaxation assay.

RESULTS

Emodin was able to effectively scavenge radiation- induced free radicals (ROS and NO) in murine splenocytes. Radiation-induced apoptosis and cell death was also inhibited by emodin pre-treatment. It could significantly prevent radiation-induced DNA damage.

CONCLUSIONS

Protection against gamma radiation-induced cell death and DNA damage by emodin could be attributed to its free radical scavenging activity. The present study is the first report of the radioprotective role of emodin in mammalian cells.

Kinetic and spectroscopic characterization of 1-naphthol 2-hydroxylase from Pseudomonas sp. strain C5

1-Naphthol 2-hydroxylase (1-NH) catalyzes the conversion of 1-naphthol to 1,2-dihydroxynaphthalene. 1-NH from carbaryl degrading Pseudomonas strain C5 was purified and characterized for its kinetic and spectroscopic properties. The enzyme was found to be NAD(P)H-dependent external flavin monooxygenase. Though the kinetic parameters of 1-NH from strain C5 appear to be similar to 1-NH enzyme from strains C4 and C6, however, they differ in their N-terminal sequences, mole content of flavin adenine dinucleotide (FAD), reconstitution of apoenzyme, and K i. 1-NH showed narrow substrate specificity with comparable hydroxylation efficiency on 1-naphthol and 5-amino 1-naphthol (~30 %) followed by 4-chloro 1-naphthol (~9 %). Salicylate was found to be the nonsubstrate effector. The flavin fluorescence of 1-NH was found to increase in the presence of 1-naphthol (K d = 11.3 μM) and salicylate (K d = 1027 μM). The circular dichroism (CD) spectra showed significant perturbations in the presence of NAD(P)H, whereas no changes were observed in the presence of 1-naphthol. Naphthalene, 1-chloronaphthalene, 2-napthol, and 2-naphthoic acid were found to be the mixed inhibitors. Chemical modification studies showed the probable involvement of His, Cys, and Tyr in the binding of 1-naphthol, whereas Trp was found to be involved in the binding of NAD(P)H.

Glutathione-mediated reversibility of covalent modification of ubiquitin carboxyl-terminal hydrolase L1 by 1,2-naphthoquinone through Cys152, but not Lys4

Covalent modification of cellular proteins by electrophiles affects electrophilic signal transduction and the dysfunction of enzymes that is involved in cytotoxicity. We have recently found a unique reaction which restores glyceraldehyde-3-phosphate dehydrogenase (GAPDH) that has been modified by 1,2-naphthoquinone (1,2-NQ) through a glutathione (GSH)-dependent S-transarylation reaction. We report here that ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) undergoes the same reaction. Exposure of human neuroblastoma SH-SY5Y cells to 1,2-NQ after pretreatment with buthionine sulfoximine (BSO) to deplete GSH resulted in an enhancement of covalent modification of UCH-L1 by 1,2-NQ. With recombinant human UCH-L1, we demonstrated that UCH-L1 underwent arylation by 1,2-NQ through Cys152 and Lys4, thereby decreasing its catalytic activity. Addition of GSH to an incubation mixture of 1,2-NQ-UCH-L1 adduct partially restored this decline in enzyme activity which was accompanied by decreased covalent attachment of 1,2-NQ, together with production of 1,2-NQ-GSH adduct. UCH-L1 in which Lys4 was mutated exhibited a lower level of covalent modification and enzyme inhibition, but completely recovered after addition of GSH. Taken together, these results suggest that Cys152 modification in UCH-L1 by 1,2-NQ is reversible via GSH-mediated S-transarylation reaction whereas Lys4 modification by 1,2-NQ is irreversible by GSH. Because UCH-L1 dysfunction has been associated with neurodegeneration, the electrophilic modification of Lys rather than Cys in UCH-L1 may be implicated in such neurodegenerative diseases.

β-Lapachone activity in synergy with conventional antimicrobials against methicillin resistant Staphylococcus aureus strains

The aim of this study was to evaluate the antimicrobial activity of lapachol, α-lapachone, β-lapachone and six antimicrobials (ampicillin, amoxicillin/clavulanic acid, cefoxitin, gentamicin, ciprofloxacin and meropenem) against twelve strains of Staphylococcus aureus from which resistance phenotypes were previously determined by the disk diffusion method. Five S. aureus strains (LFBM 01, LFBM 26, LFBM 28, LFBM 31 and LFBM 33) showed resistance to all antimicrobial agents tested and were selected for the study of the interaction between β-lapachone and antimicrobial agents, busing checkerboard method. The criteria used to evaluate the synergistic activity were defined by the Fractional Inhibitory Concentration Index (FICI). Among the naphthoquinones, β-lapachone was the most effective against S. aureus strains. FICI values ranged from 0.07 to 0.5, suggesting a synergistic interaction against multidrug resistant S. aureus (MRSA) strains. An additive effect was observed with the combination β-lapachone/ciprofloxacin against the LFBM 33 strain. The combination of β-lapachone with cefoxitin showed no added benefit against LFBM 31 and LFBM 33 strains. This study demonstrated that, in general, β-lapachone combined with beta lactams antimicrobials, fluoroquinolones and carbapenems acts synergistically inhibiting MRSA strains.

Synthetic cannabinoid quinones: preparation, in vitro antiproliferative effects and in vivo prostate antitumor activity

Chromenopyrazolediones have been designed and synthesized as anticancer agents using the multi-biological target concept that involves quinone cytotoxicity and cannabinoid antitumor properties. In cell cytotoxicity assays, these chromenopyrazolediones have antiproliferative activity against human prostate cancer and hepatocellular carcinoma. It has been shown that the most potent, derivative 4 (PM49), inhibits prostate LNCaP cell viability (IC₅₀ = 15 μM) through a mechanism involving oxidative stress, PPARγ receptor and partially CB₁ receptor. It acts on prostate cell growth by causing G₀/G₁ phase arrest and triggering apoptosis as assessed by flow cytometry measurements. In the in vivo treatment, compound 4 at 2 mg/kg, blocks the growth of LNCaP tumors and reduces the growth of PC-3 tumors generated in mice. These studies suggest that 4 is a good potential anticancer agent against hormone-sensitive prostate cancer.

Photoinduced reduction of divalent mercury by quinones in the presence of formic acid under anaerobic conditions

The laser flash photolysis technique (λ(exc)=355 nm) was used to investigate the mechanism of the HgCl(2) reduction mediated by CO(2)(-) radicals generated from quenching of the triplet states of 1,4-naphthoquinone (NQ) by formic acid. Kinetic simulations of the experimental signals support the proposed reaction mechanism. This system is of potential interest in the development of UV-A photoinduced photolytic procedures for the treatment of Hg(II) contaminated waters. The successful replacement of NQ with a commercial fulvic acid, as a model compound of dissolved organic matter, showed that the method is applicable to organic matter-containing waters without the addition of quinones.

Synthetic condensed 1,4-naphthoquinone derivative shifts neural stem cell differentiation by regulating redox state

Naphthoquinones are bioactive compounds widespread in nature that impact on several cellular pathways, including cell proliferation and survival, by acting as prooxidants and electrophiles. We have previously described the role of the synthetic isoxazole condensed 1,4-naphthoquinone derivative 1a in preventing apoptosis induced by distinct stimuli in several cell models. In addition, apoptosis regulators and executioners may control neural stem cell (NSC) fate, without involving cell death per se. Here, we hypothesize that 1a might also play a role in NSC fate decision. We found that exposure to 1a shifts NSC differentiation potential from neurogenic to gliogenic lineage and involves the generation of reactive oxygen species, without increasing cell death. Modulation of caspases and calpains, using cysteine protease inhibitors, failed to mimic 1a effects. In addition, incubation with the naphthoquinone derivative resulted in upregulation and nuclear translocation of antioxidant responsive proteins, Nrf2 and Sirt1, which in turn may mediate 1a-directed shift in NSC differentiation. In fact, antioxidants halted the shift in NSC differentiation potential from neurogenic to gliogenic lineage, while strongly reducing reactive oxygen species generation and Nrf2 and Sirt1 nuclear translocation in NSC exposed to 1a. Collectively, these data support a new role for a specific naphthoquinone derivative in NSC fate decision and underline the importance of redox environment control.

Redox cycling of 1,2-naphthoquinone by thioredoxin1 through Cys32 and Cys35 causes inhibition of its catalytic activity and activation of ASK1/p38 signaling

1,2-Naphthoquinone (1,2-NQ) is an atmospheric chemical capable of (1) redox cycling with electron donors and (2) covalent modification of nucleophilic groups on proteins. In the present study, we investigated its interaction with the redox protein, thioredoxin1 (Trx1), which led to oxidative stress-dependent cell damage. In experiments with purified wild-type Trx1 and its double mutant (32S/35S Trx1), we found that incubation of Trx1 with 1,2-NQ resulted in a redox cycling reaction, generating superoxide and hydrogen peroxide involving Cys32 and Cys35 and an arylation reaction resulting in covalent modification of Lys85 together with a loss of Trx activity. A significant fraction of the lost Trx1 activity following interaction with 1,2-NQ was restored by dithiothreitol. Exposure of RAW264.7 cells to 1,2-NQ generated reactive oxygen species (ROS) and caused a decrease in Trx activity. Trx is a negative regulator of apoptosis signal-regulating kinase 1 (ASK1), and under the conditions of the experiment, 1,2-NQ activated ASK1 and p38, leading to PARP cleavage and apoptotic cell death that were blocked by pretreatment with polyethylene glycol-catalase. These results suggest that Trx1 readily undergoes oxidative modification by 1,2-NQ through the proximal thiols Cys32 and Cys35. It seems likely that ROS production concomitant with decline in cellular Trx activity plays a role in the activation of ASK1/p38 signaling to promote apoptotic cell death cause by 1,2-NQ exposure.

Peroxiredoxin 6 is a molecular target for 1,2-naphthoquinone, an atmospheric electrophile, in human pulmonary epithelial A549 cells

1,2-Naphthoquinone (1,2-NQ) is an electrophile found in the atmosphere, which reacts readily with protein nucleophiles to form a stable protein adduct. Peroxiredoxin 6 (Prdx6) is predominantly expressed in lung tissue and functions in antioxidant defense by facilitating the repair of damaged cell membranes via reduction of peroxidized phospholipids. In the present study, human A549 pulmonary epithelial cells were exposed to 1,2-NQ to explore whether 1,2-NQ can bind covalently to Prdx6, thereby disrupting its catalytic activity. Two-dimensional SDS/PAGE followed by western blot analysis with a specific antibody against 1,2-NQ showed that Prdx6 was covalently modified by 1,2-NQ. Using purified human Prdx6, it was found that 1,2-NQ bound covalently to Prdx6 through Cys47, Lys144 and Cys91, resulting in a significant reduction in phospholipase A(2) activity. These results suggest that arylation of Prdx6 by 1,2-NQ may, at least in part, be involved in the cellular toxicity induced by 1,2-NQ.

Library synthesis and antibacterial investigation of cationic anthraquinone analogs

We report the parallel synthesis of a series of novel 4,9-dioxo-4,9-dihydro-1H-naphtho[2,3-d][1,2,3]triazol-3-ium chloride salts, which are analogs to cationic anthraquinones. Three synthetic protocols were examined leading to a convenient and facile library synthesis of the cationic anthraquinone analogs that contain double alkyl chains of various lengths (C(2)-C(12)) at N-1 and N-3 positions. The antibacterial activities of these compounds were evaluated against Gram-positive bacterium Staphylococcus aureus and Gram-negative bacterium Escherichia coli. The antibacterial activities of these compounds were expected to be associated with the structural features of naphthoquinone, cation and lypophilic alkyl chain and, interestingly, they showed much higher levels of antibacterial activities against G+ than G- bacteria. In addition, when the total number of carbon atoms of the alkyl groups at both N-1 and N-3 positions lies between 9 and 18, the bactericidal activity against S. aureus increased with increasing alkyl chain length at both N-atoms with MIC ≤ 1 μg/mL.

GSH-mediated S-transarylation of a quinone glyceraldehyde-3-phosphate dehydrogenase conjugate

Many cellular proteins with reactive thiols form covalent bonds with electrophiles, thereby modifying their structures and activities. Here, we describe the recovery of a glycolytic protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), from such an electrophilic attack by 1,2-napthoquinone (1,2-NQ). GAPDH readily formed a covalent bond with 1,2-NQ through Cys152 at a low concentration (0.2 μM) in a cell-free system, but when human epithelial A549 cells were exposed to this quinone at 20 μM, only minimal binding was observed although extensive binding to numerous other cellular proteins occurred. Depletion of cellular glutathione (GSH) with buthionine sulfoximine (BSO) resulted in some covalent modification of cellular GAPDH by 1,2-NQ and a significant reduction of GAPDH activity in the cells. Incubation of native, but not boiled, human GAPDH that had been modified by 1,2-NQ with GSH resulted in a concentration-dependent removal of 1,2-NQ from the GAPDH conjugate, accompanied by partial recovery of lost catalytic activity and formation of a 1,2-NQ-GSH adduct (1,2-NQ-SG). While GAPDH is recognized as a multifunctional protein, our results show that GAPDH also has a unique ability to recover from electrophilic modification by 1,2-NQ through a GSH-dependent S-transarylation reaction.