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

Abstract

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.

Seasonal variation of gut Cyanophyta contents and liver GST expression of mud carp (Cirrhina molitorella) and Nile tilapia (Oreochromis niloticus) in the tropical Xiangang Reservoir (Huizhou, China)

“…The liver GSTs of freshwater fish are responsible for the detoxification of MCs [11,[32][33][34]. In our study, the mRNA expression of the liver GST in Nile tilapia was positively correlated with the abundance of toxic cyanbacteria in the fore-gut, particularly for GSTA and GSTR2 (Pearson correlation index = 0.99 and 0.97, respectively).…”

“…We hypothesize that differences in the pattern of GST expression between the two species are related to their specific resistance to MCs. Liang et al [34] demonstrated that inducible expression of the liver GST gene was closely related to tolerance to MC-LR. The highly-resistant fish had inducible liver expression of either alpha-or rho class GST whereas the highly-sensitive fish had no inducible liver expression.…”

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

“…Although, many novel classes of GST sequences have been identified and classified from non-mammalian organisms, information of fish GSTs is limited. The alpha, pi and theta GSTs are widely found in fishes (George, 1994;Riol et al, 2001) and Fu and Xie (2006) reported five cytosolic GSTs (rho, mu, theta, alpha and pi) from liver of common carp, in which the transcription of GST alpha, theta, and pi changed greatly, while mu isoform was relatively stable and Liang et al (2007) reported that cyprinid fishes may show an alpha-class predominant expression pattern of liver GST genes. Pérez-López et al (2002) found that only some of the rainbow trout GST isoenzymes were specifically induced by the polychlorinated biphenyls treatment, rendering these isoenzymes as valuable biomarkers for exposure to these environmental pollutants.…”

A Pharm-Ecological Perspective of Terrestrial and Aquatic Plant-Herbivore Interactions

“…The gap in knowledge related to mechanisms required to process nutrients in marine herbivorous fishes, for example, has only recently begun to close (Choat and Clements, 1998;Clements et al, 2009). Moreover, despite a long history of investigating how aquatic animals process chemical contaminants (Chambers and Yarbrough, 1976;Katagi, 2010;Rewitz et al, 2006;Smital et al, 2004), studies investigating how they process dietary secondary metabolites have only recently been initiated (Gross and Bakker, 2012;Liang et al, 2007;Richardson et al, 2009). An understanding of pharmacology can fill these gaps in both terrestrial and aquatic systems.…”