Somatic mutation, DNA damage and cytotoxicity induced by benzo[a]pyrenedione/benzo[a]pyrenediol redox couples in cultured mammalian cells

Detection of Carcinogen–Macromolecular Adducts in Humans

A major concern of molecular epidemiology is the identification of individuals at increased risk of cancer by obtaining evidence of high exposure to carcinogens that may lead to pathobiological lesions in target cells. DNA is considered to be a target for modification by mutagens and carcinogens; therefore, damage to DNA can be used as an internal, molecular dosimeter of carcinogen exposure. The reactive species of these carcinogens may bind either directly to DNA to form adducts or indirectly to cause secondary DNA lesions through free radicals and aldehydes. Highly sensitive and specific methods have been developed to measure DNA lesions and DNA repair products that are found in biological specimens from humans exposed to carcinogens in the environment. For example, DNA adducts have been measured in cells and tissues from people exposed environmentally to carcinogenic polycyclic aromatic hydrocarbons or alkylating agents. Antibodies recognizing carcinogen-DNA adducts have also been detected in human sera. Carcinogen-protein adducts are also being used as molecular dosimeters of carcinogen exposure. The advantages and limitations of the various methods used to measure carcinogen-macromolecular adducts are discussed here. The use of two or more complementary assays to obtain confirmatory results is recommended.

In vivo non-enzymatic repair of DNA oxidative damage by polyphenols

The non-enzymatic repair of DNA oxidative damage can occur in a purely chemical system, but data show that it might also occur in cells. Human hepatoma cells (SMMC-7721) and human hepatocyte cells (LO2) were treated with 200microM H(2)O(2) for 30min to induce oxidative DNA damage quantified by amount of 8-OHdG and degree of DNA strand breaks, without inducing enzymatic repair. The dynamics of enzymatic repair activity quantified by unscheduled DNA synthesis, within 30min after removal of H(2)O(2) enzymatic repair mechanism has not been initiated. However, pre-incubation with low micromolar level polyphenols, quercetin or rutin can significantly attenuate DNA damage in both cell lines, indicating that the polyphenols did not work through an enzymatic mechanism. Unscheduled DNA synthesis after removal of H(2)O(2) was also markedly decreased by quercetin and rutin. Combined with our previous studies of fast reaction chemistry, the inhibitory effect of polyphenols have to be assigned to non-enzymatic repair mechanism rather than to enzymatic repair mechanism or antioxidant mechanism.

Benzo[a]pyrene-induced elevation of GSH level protects against oxidative stress and enhances xenobiotic detoxification in human HepG2 cells

Glutathione (GSH) is one of the most important antioxidants in mammalian cells. It also plays an important role in chemical detoxification. Some evidence showed that polycyclic aromatic hydrocarbons, such as benzo[a]pyrene (B[a]P [50-32-8]), could increase GSH content as a defense mechanism against oxidative stress as well as to promote its detoxification. However, there has been very little study on clarifying the role GSH plays in antioxidation and detoxification actions. Therefore, the present study aims to analyze intracellular glutathione metabolism in the human hepatoma cells (HepG2) upon exposure to B[a]P. Exposure of the cells to B[a]P (1-100 microM) for 24 h did not cause significant cell death in this cell line. By selecting the sublethal concentration of 10 microM, B[a]P caused a significant increase in GSH and a small (13%) but significant decrease in glutathione reductase activity. However, there was no change in the activity of glutathione peroxidase, and no detectable increase in reactive oxygen species (ROS) production. Treatment with B[a]P caused up to 1.5 folds increase in gamma-glutamylcysteine synthatase (gamma-GCS) activity over control. Buthioneine sulfoximine (BSO), an inhibitor of gamma-GCS, could suppress GSH increase in a dose-dependent manner. Assessment of the oxidative state of the cells indicated that the increase in GSH caused the cells to become more reduced. Thus, the results concluded that cells were not suffering from oxidative stress at 24 h after treatment with 10 microM B[a]P. Upon analyzing the activities of detoxification enzymes, there was an increase in the activity of CYP1A subfamily monooxygenases and glutathione S-transferase. Both changes occurred prior to the changes in gamma-GCS activity and the increase in GSH. In summary, results of the present study demonstrate that B[a]P caused an activation of detoxification enzymes. The increase in intracellular GSH level was due to activation of gamma-GCS activities. Oxidative stress may not be an important risk factor for B[a]P (at 10 microM of up to 24 h) induced injury.

“In vitro” protection of DNA from Fenton reaction by plant polyphenol verbascoside

The protection effect of verbascoside (Ver) against Fenton reaction on plasmid pBR322 DNA was studied using agarose gel electrophoresis and UV-visible spectroscopy. The pBR322 plasmid DNA is damaged by hydroxyl radical (OH*) generated from the Fenton reaction with H2O2 and Fe(II) or Fe(III). This DNA damage is characterized by the diminution of supercoiled DNA forms or by the increase of relaxed or linear DNA forms after oxidative attack. The UV spectrum study showed that verbascoside can form complexes with Fe(II) or Fe(III), and the complexation can be reversed by the addition of EDTA. The formation constants of verbascoside-Fe complexes were estimated as 10(21.03) and 10(31.94) M(-2) for Fe(II) and Fe(III) respectively. The inhibition of Fenton reaction by verbascoside could be partially explained by the sequestration of Fe ions.

Mitochondrial damage prior to apoptosis in furanonaphthoquinone treated lung cancer cells

The mechanisms of the antitumor reactions of 2-methylnaphtho[2,3-b]furan-4,9-dione (FNQ3) to human lung adenocarcinoma A549 cells were investigated. A549 cells that received 1.25 microg/ml FNQ3 (IC(50) at 0.35 microg/ml) developed intensive mitochondrial H(2)O(2) production at 1 h. Selective structural mitochondrial swelling, alteration of mitochondrial membrane potential, and cytochrome c and caspase-9 release from the mitochondria occurred 18-24 h later. alpha-Tocopherol inhibited the alteration of both mitochondrial permeability and the leakage of procaspase-9. The caspase-9 was then activated in the cytosol. The expression of Bcl-2 oncoprotein was suppressed by FNQ3, and resulted in apoptosis. The higher dose of 5 microg/ml induced necrosis via severe mitochondrial breakage. These results showed that FNQ3 targets the mitochondria of A549 cells to produce a reactive oxygen species resulting in apoptosis and necrosis.

Theoretical study of fast repair of DNA damage by cistanoside C and analogs: mechanism and docking

Experiments show that the natural substances phenylpropanoid glycosides (PPGs) extracted from pelicularis spicata are capable of repairing DNA damaged by oxygen radicals. Based on kinetic measurements and experiments on tumor cells, a theoretical study of the interaction between PPG molecules and isolated DNA bases, as well as a DNA fragment has been performed. An interaction mechanism reported early has been refined. The docking calculations performed using junction minimization of nucleic acids (JUMNA) software showed that the PPG molecules can be docked into the minor groove of DNA and form complexes with the geometry suitable for an electron transfer between guanine radical and the ligand. Such complexes can be formed without major distortions of DNA structure and are further stabilized by the interaction with the rhamnosyl side-groups.

Differential protection by rat UDP-glucuronosyltransferase 1A7 against Benzo[a]pyrene-3,6-quinone- versus Benzo[a]pyrene-induced cytotoxic effects in human lymphoblastoid cells

UDP-glucuronosyltransferase 1A7 (UGT1A7) is a polyaromatic hydrocarbon (PAH)-inducible UGT with activity toward various benzo[a]pyrene (B[a]P) metabolites. To investigate the influence of rat UGT1A7 on B[a]P-induced cytotoxicity, human lymphoblastoid L3 cells were transfected with pMF6 (control expression vector), p167Dtk2 (microsomal epoxide hydrolase expression vector), or p167Dtk2-1A7 (epoxide hydrolase/UGT1A7 coexpression vector), and the cell populations were compared for sensitivity to B[a]P-induced effects. B[a]P inhibited cell proliferation and decreased relative cell survival of p167Dtk2 and p167Dtk2-1A7 cells to a similar extent. Metabolism studies using [(3)H]B[a]P revealed increased formation of glucuronide conjugates of B[a]P-4,5-diol, 3-OH-, or 9-OH-B[a]P and an unidentified metabolite by p167Dtk2-1A7 cells, but the presence of unconjugated metabolites suggested that glucuronidation capacity may be limited. No differences between p167Dtk2 and p167Dtk2-1A7 L3 cells were observed in the growth inhibitory effects of 3-OH-B[a]P or B[a]P-7,8-diol, but p167Dtk2-1A7-expressing cells were found to be less sensitive to B[a]P-3,6-quinone-induced effects on cell proliferation and relative cell survival. The effect was also observed in AHH-1 lymphoblastoid cells expressing UGT1A7 without epoxide hydrolase. The UGT1A7-expressing AHH-1 cells were also less sensitive to growth inhibition by B[a]P-1,6-quinone and B[a]P-6,12-quinone. Flow cytometric analysis of vehicle and B[a]P-3, 6-quinone-exposed cell populations showed an association between UGT1A7 expression and resistance to B[a]P-3,6-quinone-induced apoptosis and loss of cell viability. These data suggest that UGT1A7 may be preferentially active toward B[a]P-quinones and that UGT1A7 may represent the PAH-inducible UGT activity previously implicated in protection against toxic redox cycling by B[a]P-3,6-quinone.

Fast repair of hydroxy radical purine deoxynucleotide adducts by phenylpropanoid glycosides and their derivatives from Chinese herbs

DNA damaged by oxygen radicals has been implicated as a causative event in a number of degenerative diseases, including cancer and aging. So it is very significant to look for ways in which either oxygen radicals are scavenged prior to DNA damage or damaged DNA is repaired to supplement the cells' inadequate repair capacity. The repair activities and reaction mechanism of phenylpropanoid glycosides (PPGs) and their derivatives, isolated from Chinese folk medicinal herbs, towards both dGMP-OH* adducts and dAMP-OH* adducts were studied with the pulse radiolytic technique. On pulse irradiation of nitrous oxide saturated 2 mM dGMP or dAMP aqueous solution containing one of the PPGs or their derivatives, the transient absorption spectra of the hydroxyl adduct of dGMP or dAMP decayed with the formation of that of phenoxyl radicals of PPGs or their derivatives within several decades of microseconds after electron pulse irradiation. The result indicated that dGMP or dAMP hydroxyl adducts can be repaired by PPGs or their derivatives. The rate constants of the repair reactions were deduced to be 0.641-1.28 x 10(9) M(-1) s(-1) for dGMP-OH* and 0.2-0.491 x 10(9) M(-1) s(-1) for dAMP-OH*, which positively correlated to the number of phenolic hydroxyl groups in the glycoside structure. A deeper understanding of this new repair mechanism may help researchers to design strategies to prevent and/or intervene more effectively in free radical related diseases.

Repair effect of phenylpropanoid glycosides on thymine radical anion induced by pulse radiolysis

Repair effects on thymine radical anion by six phenylpropanoid glycosides (PPGs), isolated from Pedicularis species, were studied using pulse radiolysis method. The thymine radical anion was produced by the reaction of hydrated electron with thymine. PPGs were added into the thymine solution saturated with N(2). Kinetic analysis showed that transient absorption spectrum of thymine radical anion formed at first, and then after several microseconds of pulse radiolysis changed to that of PPG radical anion. The evidence indicated that thymine radical anion was repaired through one-electron transfer between the radical anion and PPG. Electrophilic phenyl-substituted unsaturated carboxylic group containing in PPGs' structure was able to capture electron from thymine radical anion before it undergo reversible protonation. The reaction rate constants of electron transfer from thymine radical anion to PPGs were within 1.16-2.29 x 10(9) dm(3) mol(-1) s(-1).

Repair of dGMP hydroxyl radical adducts by verbascoside via electron transfer: a pulse radiolysis study

The repair activity of verbascoside (VER), isolated from Pedicularis spicata, towards the oxidizing hydroxyl radical adduct of dGMP and its reaction mechanism were studied using pulse radiolysis. Upon pulse radiolysis of nitrous oxide saturated aqueous solution of 2'-deoxyguanosine-5'-monophosphate (dGMP) and VER, it was found that the transient absorption spectrum of the hydroxyl adduct of dGMP decays with the formation of that of the phenoxyl radical of VER, several tens of microseconds after the electron pulse. From the formation kinetics of the phenoxyl radical of VER, the rate constant of the repair reaction was determined to be 1.12 x 10(9) dm(3) mol(-1) s(-1).

The toxicity of menadione (2-methyl-1,4-naphthoquinone) and two thioether conjugates studied with isolated renal epithelial cells

Menadione (2-methyl-1,4-naphthoquinone) was used as a model compound to test the hypothesis that thioether conjugates of quinones can be toxic to tissues associated with their elimination through a mechanism involving oxidative stress. Unlike menadione, the glutathione (2-methyl-3-(glutathion-S-yl)-1,4-naphthoquinone; MGNQ) and N-acetyl-L-cysteine (2-methyl-3-(N-acetylcysteine-S-yl)-1,4-naphthoquinone; M(NAC)NQ) thioether conjugates were not able to arylate protein thiols but were still able to redox cycle with cytochrome c reductase/NADH and rat kidney microsomes and mitochondria. Interestingly, menadione and M(NAC)NQ were equally toxic to isolated rat renal epithelial cells (IREC) while MGNQ was nontoxic. The toxicity of both menadione and M(NAC)NQ was preceded by a rapid depletion of soluble thiols and was associated with a depletion of soluble thiols and was associated with a depletion of protein thiols. Treatment of IREC with the glutathione reductase inhibitor, 1,3-bis(2-chloroethyl)-1-nitrosourea, potentiated the thiol depletion and toxicity observed with menadione and M(NAC)NQ indicating the involvement of oxidative stress in this model of renal cell toxicity. The lack of MGNQ toxicity can be attributed to an intramolecular cyclization reaction which destroys the quinone nucleus and therefore eliminates its ability to redox cycle. These findings have important implications with regard to our understanding of the toxic potential of quinone thioether conjugates and of quinone toxicity in general.

Metabolism and cytotoxicity of benzo(a)pyrene in the human lung tumour cell line NCI-H322

1. The human lung tumour cells NCI-H322 metabolized benzo(a)pyrene (BP) at a rate of 160 pmol/10(6) cells/h for at least 8 h. About 30% of the total metabolites were water-soluble, 30% of which were conjugates with glutathione. The water-soluble fraction also contained BP sulphates but no BP glucuronides. 2. The cytotoxic potency of BP and its metabolites, 3-hydroxybenzo(a)pyrene (3-OH-BP) and (+/-) anti-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene (7,8-diol-BP), differed by an order of magnitude with the ranking 7,8-diol-BP greater than BP greater than 3-OH-BP. The cytotoxicity of BP was not detected at the end of a 24 h treatment period but became increasingly apparent at later times. In contrast, the cytotoxicity of 3-OH-BP was observed immediately after the treatment period and did not increase greatly thereafter. 7,8-Diol-BP caused both 'immediate' and 'late' effects. 3. The time course and concentration dependence suggested that the cytotoxicity of BP in NCI-H322 cells was not attributable to the formation of 3-OH-BP, but more likely resulted from the formation of other products such as 7,8-diol-BP.