The acute effects of microcystin LR on the transcription of nine glutathione S-transferase genes in common carp Cyprinus carpio L

Hepatic and branchial glutathione S-transferases of two fish species: substrate specificity and biotransformation of microcystin-LR

Liver and gills of roach (Rutilus rutilus) and silver carp (Hypophthalmichthys molitrix) were examined for glutathione S-transferases (GSTs) contents and their substrate specificity and capacity to biotransform microcystin-LR (MC-LR). GSTs and other glutathione (GSH) affine proteins were purified using a GSH-agarose matrix and separated by anionic chromatography (AEC). Substrate specificities were determined photometrical for 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene (DCNB), 4-nitrobenzyl chloride (pNBC) and ethacrynic acid (ETHA). Biotransformation rate of MC-LR was determined by high performance liquid chromatography (HPLC). Roach exhibited different hepatic and branchial GST activities for used substrates (DNB, pNBC and DCNB) compared to silver carp but not for ethacrynic acid. It suggests that, both fish species have similar amount of pi and/or alpha class, which were the dominant GST classes in liver and gills. Gills of both fish species contained a higher number of GST isoenzymes, but with lower activities and ability of MC-LR biotransformation than livers. GST isoenzymes from roach had higher activity to biotransform MC-LR (conversion rate ranging up to 268 ng MC-LR min(-1) mL(-1) hepatic enzyme) than that isolated from silver carp. Without any prior contact to MC-LR or another GST inducer, roach seems to be better equipped for microcystin biotransformation than silver carp.

Molecular cloning, phylogenetic analysis and expression of a MAPEG superfamily gene from the pufferfish Takifugu obscurus

The microsomal glutathione S-transferases (MGSTs) of membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG) superfamily play an important role in xenobiotics detoxification. Compared to mammals, there is limited information on MAPEGS from fish. We cloned a full length of cDNA sequence of a MGST gene from the river pufferfish (Takifugu obscurus), studied its phylogenetic relationship, and measured its expression in different tissues and in liver of fish exposed to cadmium. Phylogenetic analysis revealed that the identified gene encoded for MGST3. Liver showed the highest expression of MGST3 transcripts. When MSGT expression was compared with the expression of other GSTs (GST-Alpha, GST-Mu and GST-Theta), a similar pattern of highest expression was observed in the liver. Upon Cd exposure (5 ppm) for 96 h, the highest expression of MGST was observed at 24 h. GST-Mu also showed highest expression at 24 h. These findings indicate that MGSTs may be playing a role in detoxification of xenobiotics or free radicals generated by Cd-induced oxidative stress in fish.

Microcystin dynamics in aquatic organisms

Eutrophication of surface water has increased significantly during the past decade, resulting in increased occurrences of toxic blooms. Cyanotoxins have become a global health threat to humans, wild animals, or domestic livestock. Hepatotoxic microcystins (MC) are the predominant cyanotoxins, which accumulate in aquatic organisms and are transferred to higher trophic levels. This is an issue of major concern in aquatic toxicology, as it involves the risk for human exposure through the consumption of contaminated fish and other aquatic organisms. The persistence and detoxification of MC in aquatic organisms are important issues for public health and fishery economics. Bioaccumulation of MC depends on the toxicity of the strains, mode of feeding, and detoxication mechanisms. Although mussels, as sessile filter feeders, seem to be organisms that ingest more MC, other molluscs like gastropods, as well as zooplankton and fish, may also retain average similar levels of toxins. Edible animals such as some species of molluscs, crustaceans, and fish present different risk because toxins accumulate in muscle at low levels. Carnivorous fish seem to accumulate high MC concentrations compared to phytophagous or omnivorous fish. This review summarizes the existing data on the distribution and dynamics of MC in contaminated aquatic organisms.

Glutathione transferases from Anguilla anguilla liver: identification, cloning and functional characterization

Glutathione transferases (GSTs) constitute a class of detoxifying enzymes involved in Phase II metabolism. Using GSH-affinity chromatografy followed by HPLC analysis, two GST isoforms were isolated from the Anguilla anguilla liver cytosol. The major GST belongs to the piscine-specific rho class and accounted for about 59% of total GST affinity eluted fraction, while the remaining 41% was represented by a Pi class GST. Both isoforms were cloned, heterologously expressed in Escherichia coli and their enzyme activities were characterized with respect to a broad spectrum of well-known GST substrates. Our data indicate that only a fraction of prototypical GST substrates are conjugated by these enzymes and that Pi class GST has higher specific activity than rho class GST against 1-chloro-2,4-dinitrobenzene (CDNB), ethracrynic acid, 4-nitroquinoline-1-oxide and p-nitrophenyl acetate while trans-2-nonenal is detoxified more efficiently by rho class GST. Analysis of the kinetics parameters of the conjugation against CDNB indicated that the utilization ratio K(cat)/K(m) is slightly higher for rho class GST with respect to pi class GSTs. Finally, to determine the potential for environmental inhibition of the GST isoforms, we examined the effect of the widely used herbicide atrazine as an inhibitor of catalytic activity. The inhibition studies revealed that atrazine was an effective inhibitor of GST-CDNB catalytic activities of both isoforms at micromolar concentrations, suggesting the sensitivity of these isoforms to pesticide inhibition at environmentally relevant concentrations.

The effect of cyanobacterial crude extract on the transcription of GST mu, GST kappa and GST rho in different organs of goldfish (Carassius auratus)

The glutathione S-transferases play important roles in the detoxification of microcystin. Core-sequences of three classes of GST (mu, kappa and rho) were cloned from goldfish (Carassius auratus L.) i.p. injected with cyanobacterial crude extract at two doses (50 and 200 microg MC-LReq kg(-1)BW). The relative changes of the mRNA abundance in liver, kidney and intestine were analyzed by real-time PCR. The transcription of GST mu was inhibited in intestine at both doses and the transcription of GST kappa was inhibited from 12 to 48h in kidney at both doses. The decreased transcription of GST rho was detected in all three organs at the high dose. It is suggested that transcription inhibition of GST rho might be significant in MCs toxicity at higher toxin concentration in omnivorous freshwater fish. Alteration in transcription of GSTs stimulated by MCs implicates an increased health risk to fish.

Detoxification and oxidative stress responses along with microcystins accumulation in Japanese quail exposed to cyanobacterial biomass

The cyanobacterial exposure has been implicated in mass mortalities of wild birds, but information on the actual effects of cyanobacteria on birds in controlled studies is missing. Effects on detoxification and antioxidant parameters as well as bioaccumulation of microcystins (MCs) were studied in birds after sub-lethal exposure to natural cyanobacterial biomass. Four treatment groups of model species Japanese quail (Coturnix coturnix japonica) were exposed to controlled doses of cyanobacterial bloom during acute (10 days) and sub-chronic (30 days) experiment. The daily doses of cyanobacterial biomass corresponded to 0.2-224.6 ng MCs/g body weight. Significant accumulation of MCs was observed in the liver for both test durations and slight accumulation also in the muscles of the highest treatment group from acute test. The greatest accumulation was observed in the liver of the highest treatment group in the acute test reaching average concentration of 43.7 ng MCs/g fresh weight. The parameters of detoxification metabolism and oxidative stress were studied in the liver, heart and brain. The cyanobacterial exposure caused an increase of activity of cytochrome P-450-dependent 7-ethoxyresorufin O-deethylase representing the activation phase of detoxification metabolism. Also the conjugation phase of detoxification, namely the activity of glutathione-S-transferase, was altered. Cyanobacterial exposure also modulated oxidative stress responses including the level of glutathione and activities of glutathione-related enzymes and caused increase in lipid peroxidation. The overall pattern of detoxification parameters and oxidative stress responses clearly separated the control and the lowest exposure group from all the higher exposed groups. This is the first controlled study documenting the induction of oxidative stress along with MCs accumulation in birds exposed to natural cyanobacterial biomass. The data also suggest that increased activities of detoxification enzymes could lead to greater biotransformation and elimination of the MCs at the longer exposure time.

Microcystin-induced variations in transcription of GSTs in an omnivorous freshwater fish, goldfish

The glutathione S-transferases are important enzymes in the microcystin-induced detoxication processes. In this experiment, we cloned the full-length cDNA of alpha, pi and theta-class-like glutathione S-transferase genes from goldfish (Carassius auratus L). Their derived amino acid sequences were clustered with other vertebrate alpha, pi and theta-class GSTs in a phylogenetic tree and the goldfish GST sequences have the highest similarity with those from common carp and zebrafish. Goldfish were i.p. injected with microcystins extract at two doses (50 and 200microgkg(-1)BW MC-LReq) and the relative changes of the mRNA abundance in liver, kidney and intestine were analyzed by real-time PCR. The transcription of GST alpha was suppressed in both liver and intestine, but induced in the kidney. Decreased transcription of GST theta was detected in liver, kidney and intestine in the low-dose group. The transcription of GST pi was suppressed in liver and intestine post-injection in both dose groups. These results suggested that the transcription of GST isoforms varied in different ways within an organ and among organs of goldfish exposed to MCs.

Acute effects of microcystins on the transcription of antioxidant enzyme genes in crucian carp Carassius auratus

Recent evidences suggested that oxidative stress may play a significant role in the pathogenesis of MCs toxicity. In the present study, the acute effects of microcystins on the transcription of antioxidant enzyme genes were investigated in liver of crucian carp i.p.-injected with 50 mug MC-LReq per kg body weight (BW). We reported the cDNA sequences for four kinds of antioxidant enzyme (GSH-PX, CAT, Cu/Zn SOD, and GR) genes, and evaluated the oxidant stress induced by MCs through analyzing the transcription abundance of antioxidant enzyme genes using real-time PCR method. The time-dependent change of relative transcription abundance and expression of the antioxidant enzyme genes were determined at 1, 3, 12, 24, and 48 h. The transcription abundance varied among antioxidant enzymes, with GSH-PX and GR down-regulation, and CAT and SOD significantly upregulation. Based on these data, we tentatively concluded that the oxidant stress was induced by MCs, and caused the different response of the antioxidant enzyme genes.

Hepatic expression profiling in smolting and adult coho salmon (Onchorhynchus kisutch)

Coho salmon are a critical Pacific salmon species that undergo complex physiological transformations as they migrate towards seawater and enter adult life stages. During these periods, coho may receive exposure to waterborne pollutants that coincide with outmigration through contaminated waterways and return to natal streams. However, little is known regarding the ontogenic modulation of gene expression during these critical life stages. Accordingly, the purpose of the present study was to characterize the hepatic transcriptome of smolting coho, adult males, and adult females by carrying out microarray analysis with a commercially available 16,000 cDNA element platform. Quantitative PCR (Q-PCR) analysis of genes involved in chemical biotransformation (cytochrome P450 isoforms 1A, and 2M1, glutathione S-transferase pi, microsomal GST), defense against metal exposure (metallothionein-A), and reproductive function (vitellogenin receptor) were developed for the purpose of analyzing specific genes of interest and to validate the microarray data. Microarray analysis identified 842 genes that were differentially expressed between smolts and adult males or females (p<0.001 and more than 2-fold difference). These 842 genes were not differentially expressed between adult males and females and, therefore, can be interpreted as a smolt-specific transcriptional profile. Of these 842 genes, 275 were well annotated and formed the basis for further bioinformatics analysis. Many of the differentially expressed genes were involved in basic cellular processes related to protein biosynthesis and degradation (24%), ion transport (12%), transcription (8%), cell structure (8%) and cellular energetics (6%). The majority of differentially expressed genes involved in signal transduction and energy metabolism were expressed at higher levels in adult coho relative to smolts. However, genes associated with cellular protection against chemical injury (i.e. biotransformation, DNA damage repair, and protection against oxidative stress) did not generally differ among the groups. Q-PCR studies revealed extensive interindividual variation in mRNA expression, but were consistent with the microarray results (R(2)=0.74). Collectively, our results indicate differences in liver gene expression in young smolting coho salmon relative to adults and extensive interindividual variation in biotransformation gene expression. However, we did not find a global lack of hepatic biotransformation capacity or poor cellular detoxification response capacity in smolting cohos based on mRNA profiles.

Transcriptional responses of alpha- and rho-class glutathione S-transferase genes in the liver of three freshwater fishes intraperitoneally injected with microcystin-LR: relationship of inducible expression and tolerance

Rho-class glutathione S-transferase (GST) is found only in teleost fish with no homologues in mammals. Silver carp (Hypophthalmichthys molitrix), grass carp (Ctenopharyngodon idellus), and Nile tilapia (Oreochromis nilotica) are three warm freshwater fishes with differential tolerance to microcystin-LR (MC-LR): Nile tilapia has a little higher tolerance than silver carp, but both have much higher tolerance than grass carp. Full-length cDNAs encoding the rho-class GST were cloned and sequenced from the livers of the three fishes. The silver carp, grass carp, and Nile tilapia rho-class GST cDNAs were 1078, 1104, and 904 bp in length, respectively, and all contained an open-reading frame (ORF) of 681 bp (encoding a polypeptide of 226 amino acids). Using beta-actin as an external control, semiquantitative RT-PCR was conducted to determine the constitutive and inducible expression level of alpha- and rho-class GST genes among the three fishes (6-12 g) intraperitoneally injected with MC-LR (50 mug kg(-1) body weight). Liver mRNA expression levels of alpha-class GST gene were found to be higher than those of rho-class GST gene in both exposed and control fish of silver carp and grass carp, whereas liver mRNA expression levels of rho-class GST gene were higher than those of alpha -class GST gene in both exposed and control fish of Nile tilapia. Increased liver transcription of GST isoforms was detected at 8-h postinjection of MC-LR in silver carp (alpha- and rho-class GST) and Nile tilapia (rho-class GST), and at 24-h postinjection in silver carp (alpha-class GST) and Nile tilapia (alpha-class GST), but an increase in liver transcription neither of alpha-class GST nor of rho-class GST was detected in grass carp at either 8-h or 24-h postinjection. The inducible expression of the liver GST gene showed a close relationship with their tolerance to MC-LR: high-resistant fish (phytoplanktivorous silver carp and Nile tilapia) had inducible liver expression of either alpha- or rho-class GST, and the high-sensitive fish (herbivorous grass carp) had no inducible liver expression of either one. We suggest that inducible expression (instead of constitutive expression) of the liver GST gene should play an important role in the tolerance to microcystin exposure, and that in addition to alpha-class GST, high-liver expression of rho-class GST gene might have facilitated the evolutionary radiation of teleost fish.

Purification and partial characterization of glutathione transferase from the teleost Monopterus albus

Glutathione transferases (GSTs) catalyze the transfer of glutathione to a variety of xenobiotic and toxic endogenous compounds. GSTs are phase II biotransformation enzymes and are proposed as biomarkers of environmental pollution. In this study, a cytosolic glutathione transferase (maGST) was purified from liver of the freshwater fish Monopterus albus by affinity chromatography. The maGST appeared to be a homodimer composed of two subunits each with a molecular weight of 26 kDa. This maGST showed high activity towards the substrates 1-chloro-2,4-dinitrobenzene (CDNB) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl). Kinetic analysis with CDNB as substrate revealed a K(m) of 0.28 mM and V(max) of 15.68 micromol/min per mg of protein. It had maximum activity in the pH range 7.0-7.5, a broad optimum T(m) range of 30 degrees C-55 degrees C, and a high thermal stability with 77% of its initial activity at 45 degrees C. This high thermal stability of maGST could be related to the physiological adaptation of M. albus to high temperatures in tropical and subtropical environments.