Identification of a multixenobiotic resistance mechanism in primary cultured epidermal cells from Oncorhynchus mykiss and the effects of environmental complex mixtures on its activity

The impact of chemosensitisation on bioaccumulation and sediment toxicity

Cellular multixenobiotic resistance (MXR) transport proteins enhance the efflux of numerous organic pollutants. However, MXR proteins may be blocked or saturated by xenobiotic compounds, acting as inhibitors - also called chemosensitisers. Although effective on a cellular level, the environmental relevance of chemosensitisers has not been conclusively demonstrated. Since sediments are an important source of bioaccumulating compounds in aquatic ecosystems, sediments and sediment-associated hydrophobic pollutants were investigated for their potential to increase exposure and toxicity in the presence of chemosensitisation. In this study, we address this issue by (1) comparing the net uptake of 17 hydrophobic environmental pollutants by zebrafish (Danio rerio) embryos in the presence and absence of the model chemosensitiser verapamil and (2) investigating the impact of verapamil on the dose-dependent effect on zebrafish embryos exposed to polluted sediment extracts. None of the 17 pollutants showed a reproducible increase in bioaccumulation upon chemosensitisation with verapamil. Instead, internal concentrations were subject to intra-species variation by a factor of approximately two. However, a significant increase in toxicity was observed upon embryo co-exposure to verapamil for one of three sediment extracts. In contrast, another sediment extract exhibited less toxicity when combined with verapamil. In general, the results indicate only a minor impact of verapamil on the uptake of moderately hydrophobic chemicals in zebrafish embryos.

Characterization of MXR activity in the sea anemone Bunodosoma cangicum exposed to copper

Transmembrane proteins of the ABC family contribute to a multiple xenobiotic resistance (MXR) phenotype in cells, driving the extrusion of toxic substances. This phenotype promotes a high degree of protection against xenobiotics. The present study provides a better understanding of the MXR activity in the podal disk cells of Bunodosoma cangicum exposed to copper, and further establishes the relationship between protein activity (measured by accumulation of rhodamine-B) and bioaccumulation of copper in these cells. Sea anemone cells were exposed for 24h to copper (0, 7.8 and 15.6μg/L) in presence and absence of MXR blocker (verapamil 50μM). Results indicate that copper exposure increases intracellular metal content when ABC proteins were blocked, causing an increase in cellular death. The present study also verified the relationship between MXR activity, ATP depletion, and general metabolic activity (by MTT). MXR activity decreased in treatment groups exposed to copper concentrations of 15.6μg/L and 10mM energy depleting potassium cyanide. Metabolic activity increased in cells exposed to 7.8μgCu/L, but 15.6μgCu/L was similar to 0 and 7.8μg/L. The presence of copper decreased the ABC proteins expression. The present study improves the knowledge of MXR in anemone cells and shows that this activity is closely associated with copper extrusion. Also, the copper exposure is able to modify the metabolic state and to lead to cytotoxicity when cells cannot defend themselves.

Regulation of hepatic abcb4 and cyp3a65 gene expression and multidrug/multixenobiotic resistance (MDR/MXR) functional activity in the model teleost, Danio rerio (zebrafish)

Multidrug/multixenobiotic resistance (MDR/MXR) confers resistance to a diverse range of potentially toxic pharmaceuticals and environmental contaminants through a cellular response that involves the coordinated induction and activity of the ATP-binding cassette (ABC) transporter P-glycoprotein (P-gp) and the Phase I metabolizing enzyme cytochrome P450 3A (CYP3A). In mammals, ligand-mediated pregnane X receptor (PXR) transcriptional activity regulates the induction of P-gp and CYP3A; however, this mechanism has not been well-characterized in piscine species. Zebrafish (Danio rerio) treated with the Pxr agonist pregnenolone 16α-carbonitrile (PCN) showed decreased P-gp (zebrafish Abcb4) and CYP3A (zebrafish Cyp3a65) mRNA levels after 48h exposure; however, treatment with PCN also resulted in increased hepatic MDR/MXR functional activity (i.e. increased Rhodamine 123 efflux) in vivo. Consistent with mammalian-like MDR/MXR regulated by PXR, the PCN-mediated modulation of hepatic Abcb4 and Cyp3a65 mRNA levels and MDR/MXR functional activity was attenuated by co-treatment with PCN and the mammalian PXR antagonist, ketoconazole (KTC). These results provide evidence that zebrafish Pxr may play a role in MDR/MXR through transcriptional regulation of abcb4 and cyp3a65 gene expression.

When evolution is the solution to pollution: Key principles, and lessons from rapid repeated adaptation of killifish (Fundulus heteroclitus) populations

For most species, evolutionary adaptation is not expected to be sufficiently rapid to buffer the effects of human‐mediated environmental changes, including environmental pollution. Here we review how key features of populations, the characteristics of environmental pollution, and the genetic architecture underlying adaptive traits, may interact to shape the likelihood of evolutionary rescue from pollution. Large populations of Atlantic killifish (Fundulus heteroclitus) persist in some of the most contaminated estuaries of the United States, and killifish studies have provided some of the first insights into the types of genomic changes that enable rapid evolutionary rescue from complexly degraded environments. We describe how selection by industrial pollutants and other stressors has acted on multiple populations of killifish and posit that extreme nucleotide diversity uniquely positions this species for successful evolutionary adaptation. Mechanistic studies have identified some of the genetic underpinnings of adaptation to a well‐studied class of toxic pollutants; however, multiple genetic regions under selection in wild populations seem to reflect more complex responses to diverse native stressors and/or compensatory responses to primary adaptation. The discovery of these pollution‐adapted killifish populations suggests that the evolutionary influence of anthropogenic stressors as selective agents occurs widely. Yet adaptation to chemical pollution in terrestrial and aquatic vertebrate wildlife may rarely be a successful “solution to pollution” because potentially adaptive phenotypes may be complex and incur fitness costs, and therefore be unlikely to evolve quickly enough, especially in species with small population sizes.

Is chemosensitisation by environmental pollutants ecotoxicologically relevant?

The active cellular efflux of toxicants is an efficient biological defense mode present in all organisms. By blocking this so-called multixenobiotic resistance transport-a process also referred to as chemosensitisation-, cellular bioaccumulation and the sensitivity of organisms towards environmental pollutants can increase. So far, a wide range of compounds, including pesticides, pharmaceuticals, fragrances, and surfactants, have been identified as chemosensitisers. Although, significant on a cellular level, the environmental impact of chemosensitisation on the organism level is not yet understood. Critically evaluating existing data, this paper identifies research needs to support our tentative conclusion that chemosensitisation may well enhance the risks of chemical exposure to aquatic organisms. Our conclusion is based on studies investigating the impact of individual chemicals and complex environmental mixtures on aquatic wildlife and a chemosensitiser mixture toxicity model which, however, is subject to great uncertainty due to substantial knowledge gaps. Those uncertainties include the inconsistent reporting of effect data, the lack of representative environmental contaminants tested for chemosensitisation, and the publishing of highly unreliable nominal exposure concentrations. In order to confirm the tentative conclusion of this paper, we require the significant and systematic investigation of a broader set of chemicals and environmental samples with a harmonised set of bioassays and rigorously controlled freely dissolved effect concentrations.

Histopathological lesions, P-glycoprotein and PCNA expression in zebrafish (Danio rerio) liver after a single exposure to diethylnitrosamine

The presence of carcinogenic compounds in the aquatic environment is a recognized problem. ABC transporters are well known players in the multidrug-resistance (MDR) phenomenon in mammals associated with resistance to chemotherapy, however little is known in fish species. Thus, the aim of this study was to induce hepatic tumours and evaluate long-term effects on P-glycoprotein (P-gp) and proliferating cell nuclear antigen (PCNA) proteins in Danio rerio liver, after exposure to diethylnitrosamine (DEN). Several hepatic histopathological alterations were observed in zebrafish after exposure to DEN including pre-neoplastic lesions 6 and 9 months post-exposure. After 3, 6 and 9 months of exposure to DEN, P-gp and PCNA proteins expression were up-regulated. In conclusion, this study has shown that zebrafish ABC transporters can play a similar role as in human disease, hence zebrafish can be used also as a biological model to investigate in more deep mechanisms involved in disease processes.

Current advances on ABC drug transporters in fish

Most members of the large ATP-binding cassette (ABC) gene family are transporters involved in substrate translocation across biological membranes. In eukaryotes, ABC proteins functioning as drug transporters are located in the plasma membrane and mediate the cellular efflux of a wide range of organic chemicals, with some transporters also transporting certain metals. As the enhanced expression of ABC drug transporters can confer multidrug resistance (MDR) to cancers and multixenobiotic resistance (MXR) to organisms from polluted habitats, these ABC family members are also referred to as MDR or MXR proteins. In mammals, ABC drug transporters show predominant expression in tissues involved in excretion or constituting internal or external body boundaries, where they facilitate the excretion of chemicals and their metabolites, and limit chemical uptake and penetration into "sanctuary" sites of the body. Available knowledge about ABC proteins is still limited in teleost fish, a large vertebrate group of high ecological and economic importance. Using transport activity measurements and immunochemical approaches, early studies demonstrated similarities in the tissue distribution of ABC drug transporters between teleosts and mammals, suggesting conserved roles of the transporters in the biochemical defence against toxicants. Recently, the availability of teleost genome assemblies has stimulated studies of the ABC family in this taxon. This review summarises the current knowledge regarding the genetics, functional properties, physiological function, and ecotoxicological relevance of teleostean ABC transporters. The available literature is reviewed with emphasis on recent studies addressing the tissue distribution, substrate spectrum, regulation, physiological function and phylogenetic origin of teleostean ABC transporters.

Stress-related gene expression changes in rainbow trout hepatocytes exposed to various municipal wastewater treatment influents and effluents

The present study sought to examine the performance of six different wastewater treatment processes from 12 wastewater treatment plants using a toxicogenomic approach in rainbow trout hepatocytes. Freshly prepared rainbow trout hepatocytes were exposed to increasing concentrations of influent (untreated wastewaters) and effluent (C(18)) extracts for 48 h at 15 °C. A test battery of eight genes was selected to track changes in xenobiotic biotransformation, estrogenicity, heavy metal detoxification, and oxidative stress. The wastewaters were processed by six different treatment systems: facultative and aerated lagoons, activated sludge, biological aerated filter, biological nutrient removal, chemically assisted primary treated, and trickling filter/solids contact. Based on the chemical characteristics of the effluents, the treatment plants were generally effective in removing total suspended solids and chemical oxygen demand, but less so for ammonia and alkalinity. The 12 influents differed markedly with each other, which makes the comparison among treatment processes difficult. For the influents, both population size and flow rate influenced the increase in the following mRNA levels in exposed hepatocytes: metallothionein (MT), cytochrome P4503A4 (CYP3A4), and vitellogenin (VTG). Gene expression of glutathione S-transferase (GST) and the estrogen receptor (ER), were influenced only by population size in exposed cells to the influent extracts. The remaining genes-superoxide dismutase (SOD) and multidrug resistance transporter (MDR)-were not influenced by either population size or flow rate in exposed cells. It is noteworthy that the changes in MT, ER, and VTG in cells exposed to the effluents were significantly affected by the influents across the 12 cities examined. However, SOD, CYP1A1, CYP3A4, GST, and MDR gene expression were the least influenced by the incoming influents. The data also suggest that wastewater treatments involving biological or aeration processes had the best performance. We found that the effects of municipal effluents on gene expression depended on the population size, the initial properties of the incoming influent, and the wastewater treatment method applied. Considering that the long-term goals of wastewater treatment is to produce clean effluents for the aquatic biota and independent of the incoming influent, more research is needed in developing treatment processes to better protect aquatic life from anthropogenic contamination.

Differential changes in gene expression in rainbow trout hepatocytes exposed to extracts of oil sands process-affected water and the Athabasca River

The oil sands region of northern Alberta represents the world's largest reserves of bitumen, and the accelerated pace of industrial extraction activity has raised concern about the possible impacts on the Athabasca River and its tributaries. An ecotoxicogenomic study was undertaken on Oncorhynchus mykiss trout hepatocytes exposed to extracts of water samples near the oil sand development area, as well as to oil sands process-affected water (OSPW) extracts using the quantitative reverse transcriptase polymerase chain reaction technique. The expression of the following genes (mRNA) was monitored to track changes in xenobiotic biotransformation (CYP1A1, CYP3A4, glutathione S-transferase, multi-drug resistance transporter), estrogenicity (estrogen receptor and vitellogenin), oxidative stress (superoxide dismutase and metallothionein) and DNA repair activity (DNA ligase). The extent of DNA-aromatic hydrocarbon adducts was also determined in cells by immuno-staining. A comparative analysis of gene expression between the river/lake and OSPW samples revealed that CYP3A4, metallothioneins, DNA ligase and GST genes, were specifically expressed by OSPW. Cells exposed to OSPW, commercial naphthenic acids, and benzo(a)pyrene showed increased polyaromatic hydrocarbon DNA-adducts, as determined by cell immunofluorescence analysis. Other genes were induced by all types of water samples, although the induction potential was stronger in OSPW most of the time (e.g., VTG gene was expressed nearly 15-fold by surface waters from the lake and river samples but increased to a maximum of 31-fold in OSPW). A multivariate discriminant function analysis revealed that the lake and river water samples were well discriminated from the OSPW. The CYP3A4 gene was the most highly expressed gene in cells exposed to OSPW and responded less to the lake or river water in the Athabasca River area. This study identified a suite of gene targets that responded specifically to OSPW extracts, which could serve as toxicogenomic fingerprints of OSPW contamination.

Regulation of xenobiotic transporter genes in liver and brain of juvenile thicklip grey mullets (Chelon labrosus) after exposure to Prestige-like fuel oil and to perfluorooctane sulfonate

Xenobiotic transport proteins are involved in cellular defence against accumulation of xenobiotics participating in multixenobiotic resistance (MXR). In order to study the transcriptional regulation of MXR genes in fish exposed to common chemical pollutants we selected the thicklip grey mullet (Chelon labrosus), since mugilids are widespread in highly degraded estuarine environments where they have to survive through development and adulthood. Partial sequences belonging to genes coding for members of 3 different families of ATP binding cassette (ABC) transporter proteins (ABCB1; ABCB11; ABCC2; ABCC3; ABCG2) and a vault protein (major vault protein, MVP) were amplified and sequenced from mullet liver. Their liver and brain transcription levels were examined in juvenile mullets under exposure to perfluorooctane sulfonate (PFOS) and to fresh (F) and weathered (WF) Prestige-like heavy fuel oil for 2 and 16 days. In liver, PFOS significantly up-regulated transcription of abcb1, abcb11 and abcg2 while in brain only abcb11 was up-regulated. Both fuel treatments significantly down-regulated abcb11 in liver at day 2 while abcc2 was only down-regulated by WF. mvp was significantly up-regulated by F and down-regulated by WF at day 2 in the liver. At day 16 only a significant up-regulation of abcb1 in the F group was recorded. Brain abcc3 and abcg2 were down-regulated by both fuels at day 2, while abcb1 and abcc2 were only down-regulated by F exposure. After 16 days of exposure only abcb11 and abcg2 were regulated. In conclusion, exposure to organic xenobiotics significantly alters transcription levels of genes participating in xenobiotic efflux, especially after short periods of exposure. Efflux transporter gene transcription profiling could thus constitute a promising tool to assess exposure to common pollutants.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.