Mycobacteria, but not mercury, induces metallothionein (MT) protein in striped bass, Morone saxitilis, phagocytes, while both stimuli induce MT in channel catfish, Ictalurus punctatus, phagocytes

Recent advances in molecular immunology indicate that the expression of inducible pro-inflammatory proteins is increased in vertebrates in response to both infectious disease agents and various xenobiotics. For example, iNOS, COX-2, and CYP1A are induced by both inflammation and AhR ligands. Moreover, the expression of these proteins in response to stimuli varies among individuals within populations. Little is known of the differences among fish in the inducibility of proinflammatory proteins in response to both infectious agents and xenobiotics. Through random screening of a striped bass, Morone saxitilis, peritoneal macrophage cDNA library, a full length metallothionein (MT) gene was cloned and sequenced. MT is a low-molecular weight (6–8 kDa), cysteine-rich metal binding protein. Metals are required by pathogenic bacteria for growth, and by the host defense system by serving as a catalyst for the generation of reactive oxygen intermediates (ROIs) by phagocytes. A recombinant striped bass MT (rMT) was expressed and purified, then used to generate a specific mAb (MT-16). MT protein expression was followed in freshly isolated striped bass and channel catfish, Ictalurus punctatus, phagocytes after in vitro exposure to the naturally occurring intracellular pathogen Mycobacteria fortuitum or to 0.1 and 1 μM mercury (Hg), as HgCl₂. MT expression was increased by 24 h in both channel catfish and striped bass phagocytes as a result of exposure to M. fortuitum cells. On the other hand, MT was induced by Hg in channel catfish cells, but not those of striped bass. These results indicate that metal homeostasis in phagocytes is different between catfish and striped bass. In addition, these data suggest that care should be taken to distinguish between inflammation-induced vs. metal-induced MT when using MT expression as a biomarker of metal exposure.     

Bioaccumulation and trophic transfer of mercury in striped bass (Morone saxatilis) and tautog (Tautoga onitis) from the Narragansett Bay (Rhode Island,USA)

We examined the bioaccumulation and trophic transfer of mercury in two marine finfish species, striped bass (Morone saxatilis) and tautog (Tautoga onitis), collected from the Narragansett Bay (Rhode Island, USA). For each of these target fish, white muscle tissue was analyzed for total mercury (Hg) and results were evaluated relative to fish age, body size, and Hg content of preferred prey. Dietary and stable isotope analysis was also used to elucidate the effect of trophic processes on Hg concentrations in fish. The Hg content of muscle tissue was positively correlated with fish age and length for both species, although striped bass accumulated Hg faster than tautog. Accelerated Hg bioaccumulation in striped bass is consistent with its high trophic level (trophic level = 4.07) and Hg-enriched prey (forage fish and macrocrustaceans; mean Hg content = 0.03 mg Hg kg wet wt^?1). In contrast, tautog maintain a lower trophic status (trophic level = 3.51) and consume prey with lower Hg levels (mussels and crabs; mean Hg content = 0.02 mg Hg kg wet wt^?1). Despite differences in Hg bioaccumulation between target fish, the mean Hg concentration of tautog exceeded levels in striped bass (0.24 and 0.16 mg Hg kg wet wt^?1, respectively) due to a disparity in age-at-catch between sampled groups (mean age of tautog and bass = 11.3 and 4.3 yr, respectively). Taking into account legal minimum catch lengths further revealed that 75.0% of legal-size striped bass (>70.2 cm TL; n = 4) and 44.8% of tautog (>40.6 cm TL; n = 29) had Hg levels beyond the US EPA regulatory threshold of 0.3 mg Hg kg wet wt^?1. Moreover, Hg-length relationships suggest that each target fish meets this threshold near their minimum legal catch length. Our findings reiterate the value of species ecology to improve predictions of fish Hg and permit better management of human contamination by this important dietary source.     

The use of polyclonal antibodies raised against rat and trout cytochrome P450 CYP1A1 orthologues to monitor environmental induction in the channel catfish (Ictalurus punctatus)

Induction of hepatic cytochrome P450-dependent microsomal mono-oxygenase by xenobiotics is a well-established phenomenon in teleost fish. As in laboratory mammals, fish possess multiple forms of cytochrome P450 that display overlapping substrate specificity. One such isoform, CYP1A1, which has been cloned and sequenced from rainbow trout, has been shown to be orthologous to rat CYP1A1 and, as in mammals, is inducible up to several hundred-fold by planar aromatic hydrocarbons, PCBs and dioxins. It has been suggested that induction of CYP1A1 orthologues might provide a sensitive biomonitor for environmental pollution by mixtures of such compounds. In the current study, polyclonal antibodies directed against CYP1A1 purified from rat and trout liver were used to monitor induction of the CYP1A1 orthologue in hepatic microsomes from the fresh water species, the channel catfish (Ictalurus punctatus). Catfish from a local fish farm were induced in the laboratory by three daily injections of 50 mg/kg of the PCB mixture Aroclor 1254 and compared with fish taken from a site in central Arkansas—the Bayou Meto, known to be polluted with dioxin. Hepatic microsomal activities towards ethoxyresorufin (EROD) and pentoxyresorufin (PROD) were measured and Western blot analysis carried out with the two antibodies. EROD was elevated in both the Aroclor-treated fish and in the Bayou Meto fish compared with untreated fish farm controls; smaller but significant increases were observed in PROD. Spearman's rank correlations of 0·74 and 0·89 were observed between EROD and immunoquantified cross-reactivity towards the rat CYP1A1 and trout CYP1A1 antibodies.