Inhibitory effect of pentachlorophenol on gap junctional intercellular communication in rat liver epithelial cells in vitro

To understand the initiating/promoting actions of pentachlorophenol (PCP), a non-mutagenic hepatocarcinogen, and its metabolite, tetrachlorohydroquinone (TCHQ), we investigated the effects of each chemical on gap junctional intercellular communication (GJIC) in rat liver epithelial cells (WB cells) by the scrape-loading and dye transfer method. After treatment with PCP, the GJIC was initially inhibited at 4 h but was restored in 6-8 h, followed by a second phase of inhibition between 16 and 24 h. Both the first and second inhibitions were concentration-dependent and were restored by 2-4 h after removal of PCP. The phosphorylation state of connexin 43 (CX43) and its localization on the plasma membrane were unchanged up to 24 h after treatment; however, this was accompanied by a decrease in the CX43 protein level. No inhibitory effect was apparent on the GJIC of cells treated with TCHQ. These results suggest that PCP may play a critical role of promoting activity via non-mutagenic mechanisms.

Modulated gap junctional intercellular communication as a biomarker of PAH epigenetic toxicity: structure-function relationship

Cancer is a multistage multimechanism process involving gene and/or chromosomal mutations (genotoxic events), altered gene expression at the transcriptional, translational, and post-translational levels (epigenetic events), and altered cell survival (proliferation and apoptosis or necrosis), resulting in an imbalance of the organism's homeostasis. Maintenance of the organism's homeostasis depends on the intricate coordination of genetic and metabolic events between cells via extracellular and intercellular communication mechanisms. The release of a quiescent cell, whether normal or premalignant, from the suppressing effects of communicating neighbors requires the downregulation of intercellular communication via gap junctions, thereby allowing factors that control intracellular events to exceed a critical mass necessary for the cell to either proliferate or undergo apoptosis. Therefore, determining the role an environmental pollutant must play in the multistage carcinogenic process includes mechanisms of epigenetic toxicity such as the effects of a compound on gap junctional intercellular communication (GJIC). A classic example of a class of compounds in which determination of carcinogenicity focused on genotoxic events and ignored epigenetic events is polycyclic aromatic hydrocarbons (PAHs). The study of structure-activity relationships of PAHs has focused exclusively on the genotoxic and tumor-initiating properties of the compound. We report on the structure-activity relationships of two- to four-ringed PAHs on GJIC in a rat liver epithelial cell line. PAHs containing a bay or baylike region were more potent inhibitors of GJIC than the linear PAHs that do not contain these regions. These are some of the first studies of determine the epigenetic toxicity of PAHs at the epigenetic level.

Bay or baylike regions of polycyclic aromatic hydrocarbons were potent inhibitors of Gap junctional intercellular communication

Many polycyclic aromatic hydrocarbons (PAHs) are known carcinogens, and a considerable amount of research has been devoted to predicting the tumor-initiating potential of PAHs based on chemical structure. However, there has been little research into the effects of PAHs on the epigenetic events of tumor promotion and no structural correlation has been made thereof. Gap junctional intercellular communication (GJIC) activity was used in this study as an epigenetic biomarker to determine the structure-activity relationships of twelve different PAHs. The PAHs used were naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, anthracene, 1-methylanthracene, 2-methylanthracene, 9-methylanthracene, 9, 10-dimethylanthracene, phenanthrene, fluorene, 1-methylfluorene, and fluoranthene. Results showed that PAHs containing bay or baylike regions inhibited GJIC more than did the linear PAHs. The nonnaphthalene PAHs were not cytotoxic as determined by a vital dye uptake assay, but the naphthalene compounds were cytotoxic at the higher doses, indicating that the down regulation of GJIC by these naphthalenes could be a consequence of general membrane damage. Inhibition of GJIC by all the inhibitory PAHs was reversed when the cells were refreshed with PAH-free growth medium. Inhibition of GJIC occurred within 0.5-5 min and correlated with the aqueous solubility of the PAHs. The present study revealed that there are structural determinants of epigenetic toxicity as determined by GJIC activity.

Hydrogen peroxide inhibits gap junctional intercellular communication in glutathione sufficient but not glutathione deficient cells

Cell to cell communication via gap junctions is essential in the maintenance of the homeostatic balance of multicellular organisms. Aberrant intercellular gap junctional communication (GJIC) has been implicated in tumor promotion, neuropathy and teratogenesis. Oxidative stress has also been implicated in similar pathologies such as cancer. We report a potential link between oxidative stress and GJIC. Hydrogen peroxide, a known tumor promoter, inhibited GJIC in WB-F344 rat liver epithelial cells with an I50 value of 200 microM. Inhibition of GJIC by H2O2 was reversible as indicated by the complete recovery of GJIC with the removal of H2O2 via a change of fresh media. Free radical scavengers, such as t-butyl alcohol, propylgallate, and Trolox, did not prevent the inhibition of GJIC by H2O2, which indicated that the effects of H2O2 on GJIC was probably not a consequence of aqueous free radical damage. The depletion of intracellular GSH reversed the inhibitory effect of H2O2 on GJIC. The treatment of glutathione-sufficient cells with H2O2 resulted in the hyperphosphorylation of connexin43, which is the basic subunit of the hexameric gap junction protein, as determined by Western blot analysis. TPA, a well-known tumor promoter, also inhibits GJIC via hyperphosphorylation of GJIC, which is a result of protein kinase-C activation. However, H2O2 also induced hyperphosphorylation in GSH-deficient cells that had normal rates of GJIC. Therefore, the mechanism of GJIC inhibition must be different from the TPA-pathway and involves GSH.

The effects of anthracene and methylated anthracenes on gap junctional intercellular communication in rat liver epithelial cells

Polycyclic aromatic hydrocarbons (PAHs), many of which are known carcinogens, are derived from the pyrolysis of organic materials. A rich source of PAHs is cigarette smoke, which contains methylated anthracenes and phenanthrenes as the predominant PAHs. The tumor-promoting activity of cigarette smoke has been well documented. The down-regulation of gap junction intercellular communication (GJIC) by nongenotoxic chemicals and several oncogenes has been implicated in tumor promotion. Therefore, we determined the effects of the three isomers of methylanthracene on GJIC in WB-F344 rat liver epithelial cells. Anthracene and 2-methylanthracene did not significantly inhibit GJIC, whereas anthracene methylated in the 1 or 9 position reversibly inhibited GJIC with I50 values of 22 and 36 microM, respectively. Inhibition occurred within 15 min. In conclusion, the biological effect of methylanthracene depends on the ring position of the methyl group, and these inhibitory isomers could play a potential role in tumor promotion of methylated PAH-rich mixtures such as cigarette smoke and crude oil products.

Isolation of pl 4.6 extensin peroxidase from tomato cell suspension cultures and identification of Val-Tyr-Lys as putative intermolecular cross-link site

Extensins and kindred hydroxyproline-rich glycoproteins occur in dicot cell walls mainly as insoluble integral components that may form an intermolecularly cross-linked network interpenetrated by other polymers. Extensins also occur in muro as a small pool of soluble monomeric precursors to network extensin. These precursors were prepared in milligram quantities by salt elution from the surface of intact cells grown as tomato suspension cultures. Based on an FPLC (Superose-6) gel filtration assay of cross-linked extensin oligomers, a pl 4.6 extensin cross-linking peroxidase isozyme was partially purified from the culture growth medium. Purification involved: volume reduction, ultracentrifugation to remove pectin and co-adsorbed cationic peroxidase, followed by chromatography of anionic extensin peroxidase (pl 4.6) on DEAE-Trisacryl and TSK-gel DEAE-5PW columns. With tomato P1 extensin as substrate and 60 microM H2O2 as co-substrate, at 23 degrees pl 4.6 extensin peroxidase gave a Km of 0.22 mM P1 and a Vmax 0f 70 mumol P1 cross-linked min-1mg-1 enzyme, at the optimum pH 5.5. Assayed with 12 different extensins from representative monocots, dicots, and gymnosperms, the pl 4.6 isozyme cross-linked highly selectively, indicating two natural groups: cross-linking or CL-extensins and non-cross-linking or NCL-extensins. CL-extensins contained the X-Hyp-Val-Tyr-Lys motif and were also highly glycosylated. However, the simplest motif common to CL-extensins but absent from NCL-extensins was Val-Tyr-Lys. Thus, peroxidative coupling of extensin monomers and resistance of the resultant oligomers to depolymerization by anhydrous HF suggests that the intermolecular cross-link involves tyrosine or lysine.

Nongenotoxic effects of polycyclic aromatic hydrocarbons and their oxygenation by-products on the intercellular communication of rat liver epithelial cells

Since polycyclic aromatic hydrocarbons (PAHs) are known to have epigenetic effects, we evaluated the effect of the parent chemical and the ozonated products on in vitro cell to cell communication bioassays which measures a nongenotoxic event. The scrape loading/dye transfer (SL/DT) technique was used to determine the effect of the following PAHs on gap-junction intercellular communication (GJIC): fluorene, 1-methyl-fluorene, fluoranthene, anthracene, 9-methyl-anthracene, phenanthrene, pyrene, benzo(a)pyrene, and benzo(e)pyrene. The methylated PAHs were more inhibitory to GJIC than the unmethylated counterparts. Fluoranthene, which has an additional ring added to fluorene, was more effective in inhibiting GJIC than fluorene. The three-ringed PAHs were also more inhibitory than the four- and five-ringed PAHs. A time-course study of fluoranthene and of pyrene resulted in maximal inhibition occurring within 30 min of incubation with the cells. The cells recovered from the inhibition within 1 hr after fluoranthene and pyrene were removed from the cell culture medium. Pyene, vbenzo(a)pyrene, fluorene, and fluoranthene were ozonated until the parent compound was completely eliminated as determined by reverse-phase high-pressure liquid chromatography (RP-HPLC). An increased level of inhibition of GJIC was observed for the ozonated mixtures of by-products of pyrene, fluoranthene, and benzo(a)pyrene, but not for fluorene, as monitored with the SL/DT technique. The products of the ozonated pyrene mixture were fractionated and collected by RP-HPLC. Each fraction was found to be inhibitory to GJIC as monitored by fluorescence recovery after photobleaching. In conclusion, current treatment technologies, such as ozonation or biologically based oxidations and methylations, do not necessarily eliminate toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Counteraction of Paraquat Toxicity at the Chloroplast Level

Six pyridyl derivatives [benzylviologen, 2-anilinopyridine, 1,2-bis(4-pyridyl)ethane, 1,2-bis(4- pyridyl)ethylene, 2-benzoylpyridine, and 2-benzylaminopyridine] and five heme-iron derivatives [hemoglobin, hemin, hematin, ferritin, and ferrocene] were screened for their potential to coun- teract paraquat (1,1′-dimethyl-4.4′-bipyridinium ion) toxicity on pea (Pisum sativum L.) isolated chloroplasts. The H2O -> methylviologen(MV)/O2 and H2O → ferredoxin(Fd)/NADP+ were two Hill reactions assayed with these compounds. Antagonists of paraquat toxicity should inhibit the first Hill reaction but not the latter. All pyridyl derivatives examined did not inhibit the reaction H2O → MV/O2. Ferritin and ferrocene were also ineffective as inhibitors of this reaction. Hemoglobin inhibited the reaction H2O → MV/O2 without inhibiting the reaction H2O → Fd/NADP+, providing protection to pea chloroplasts against paraquat. Hemin and hematin inhibited both Hill reactions examined. They also inhibited H2O → diaminodurene(DAD)ox and durohydro-quinone → MV/O2 Hill reactions but not the dichlorophenol indophenolred → MV/O2 and DADred → MV/O2 Hill reactions. These results suggest that hemin and hematin are inhibiting the photosynthetic electron transport in the plastoquinone-pool region.

Counteraction of Paraquat Toxicity at the Chloroplast Level

Six pyridyl derivatives [benzylviologen, 2-anilinopyridine, 1,2-bis(4-pyridyl)ethane, 1,2-bis(4-pyridyl)ethylene, 2-benzoylpyridine, and 2-benzylaminopyridine] and five heme-iron derivatives [hemoglobin, hemin, hematin, ferritin, and ferrocene] were screened for their potential to counteract paraquat (1,1′-dimethyl-4.4′-bipyridinium ion) toxicity on pea (Pisum sativum L.) isolated chloroplasts. The H2O → methylviologen(MV)/O2 and H2O → ferredoxin(Fd)/NADP+ were two Hill reactions assayed with these compounds. Antagonists of paraquat toxicity should inhibit the first Hill reaction but not the latter. AU pyridyl derivatives examined did not inhibit the reaction H2O → MV/O2. Ferritin and ferrocene were also ineffective as inhibitors of this reaction. Hemoglobin inhibited the reaction H2O → MV/O2 without inhibiting the reaction H2O → Fd/NADP+, providing protection to pea chloroplasts against paraquat. Hemin and hematin inhibited both Hill reactions examined. They also inhibited H2O → diaminodurene(DAD)ox and durohydro- quinone → MV/O2 Hill reactions but not the dichlorophenol indophenolred → MV/O2 and DADred → MV/O2 Hill reactions. These results suggest that hemin and hematin are inhibiting the photo- synthetic electron transport in the plastoquinone-pool region.

Inhibition of the photosynthetic electron transport of isolated thylakoids by hemolyzed rabbit sera. Evidence for the potential involvement of parallel electron transport in photosystem I Mehler reactions

The inhibition patterns of rabbit sera (RS1 & RS2) from two different rabbits on the photosynthetic electron transport of isolated spinach thylakoids were studied. Fifty μl of RSI were required for 100% inhibition of a H2O → MV/O2 reaction, while only 10 μl of a 1:10 dilution of RS2 were needed for 100% inhibition. The RS2 serum was greatly hemolyzed. The γ-globulin fraction from purified rabbit serum (RS1) did not inhibit photosynthetic electron transport, indicating that the antibody fraction of the rabbit serum does not contain the inhibitor. It appears that the inhibitor is from the hemolyzed red blood cells. Rabbit sera added prior to chloroplast illumination caused no inhibition, while addition of rabbit sera during illumination inhibited a H2O → MV/O2 reaction within 1-3s. Aminotriazole, a catalase inhibitor, did not affect the efficacy of the rabbit sera indicating that the unknown rabbit serum inhibitor is not catalase. Various Hill reactions were employed to determine the site of inhibition. Rabbit sera inhibited the following reactions: DHQ/DCMU → MV/O2, DAD/Asc/DBMIB → MV/O2, and DCIP/Asc/DBMIB → MV/O2. Rabbit sera did not inhibit a H2O → DADox reaction indicating that inhibition is on the reducing side of PSI. However, a H2O → Fd/NADP(+) reaction was not inhibited by rabbit sera. NADP did not interfere with the ability of RS2 to inhibit a MV-mediated Mehler reaction. In simultaneously measured assays of Fd-mediated O2 and NADP(+) reductions, RS2 serum inhibited the reduction of O2 by ferredoxin without inhibiting the reduction of NADP(+). These results indicate the potential involvement of parallel (branched) electron transport of the reducing side of PSI in the reduction of oxygen.

Photooxidative reactions in chloroplast thylakoids. Evidence for a Fenton-type reaction promoted by superoxide or ascorbate

A methyl viologen (MV)(*) mediated Mehler reaction was studied using Type C and D chloroplasts (thylakoids) from spinach. The extent of photooxidative reactions were measured as (a) rate of ethylene formation from methional oxidation indicating the production of oxygen radicals, and (b) rate of malondialdehyde (MDA) formation as a measure of lipid peroxidation. Without added ascorbate, 1 μM FerricEDTA increased ethylene formation by greater than 4-fold, but had no effect on MDA production. Ascorbate (1 mM) produced a tripling of ethylene while it reduced MDA formation in the presence of iron. Radical scavengers diethyldithiocarbamate (DDTC), formate, 1,4-diazabicyclo (2.2.2octane) (DABCO), inhibited ethylene formation. Using 0,4 M mannitol to scavenge hydroxyl radicals, the rates of ethylene formation were reduced 40 to 60% with or without 1 μM Fe(III) EDTA. The strong oxidant(s) not scavenged by mannitol are hypothesized to be either alkoxyl radicals from lipid peroxidation, or 'site specific' formation of hydroxyl radicals in a lipophillic environment not exposed to mannitol. Singlet oxygen does not appear to be a significant factor in this system. Catalase strongly inhibited both ethylene and MDA synthesis under all conditions; 1 mM ascorbate did not reverse this inhibition. However, the strong superoxide dismutase (SOD) inhibition of ethylene and MDA formation was completely reversed by 1 mM ascorbate. This suggests that superoxide was functioning as an iron reducing agent and that in its absence, ascorbate was similarly promoting oxidations. Therefore, these oxidative processes were dependent on the presence of H2O2 and a reducing agent, suggesting the involvement of a Fenton-type reaction.