Expression of P-glycoprotein gene in marine sponges. Identification and characterization of the 125 kDa drug-binding glycoprotein

P-glycoprotein induction and its energetic costs in rainbow trout (Oncorhynchus mykiss)

Biological organisms are constantly challenged by xenobiotics and have evolved mechanisms to reduce, neutralize, or repair toxic outcomes. The various chemical defenses all utilize energy, but their specific costs and impacts on energy budgets are currently unknown. In this study, the energetic costs associated with the induction and substrate transport of the efflux transporter P-glycoprotein (P-gp [ABCB1, MDR1]) were examined in rainbow trout. An intraperitoneal injection of the P-gp inducer clotrimazole (0, 0.1, 1.0, and 10 mg/kg) increased P-gp activity (as measured by a competitive rhodamine 123 transport assay in hepatocytes) in a dose-dependent manner reaching a maximum induction of 2.8-fold. Maximum P-gp induction occurred at 50 h post-administration with the highest dose; significant induction of P-gp activity remained elevated over constitutive values until the last sampling time point (168 h). In vitro measurements of hepatocyte respiration indicated that basal P-gp activity transporting R123 as a substrate did not significantly increase respiration rates (range 18.0 to 23.2 ng O₂/min/10⁶ cells); however, following the induction of P-gp by clotrimazole and exposure to the P-gp substrate R123, respiration rates increased significantly (3.52-fold) over baseline values. Using whole animal respirometry, it was shown that respiration rates in fish exposed to R123 only or induced with clotrimazole were not different from controls (range 1.2 to 2.1 mg O₂/kg/min); however, respiration rates were significantly increased in fish with induced P-gp levels and also exposed to R123. This work indicates that basal and induced levels of P-gp activity do not incur significant energetic costs to fish; however, upon induction of P-gp and concomitant substrate exposures, energetic costs can increase and could pose challenges to organisms facing limited energy resources.

The molecular mechanism of AhR-ARNT-XREs signaling pathway in the detoxification response induced by polycyclic aromatic hydrocarbons (PAHs) in clam Ruditapes philippinarum

The aryl hydrocarbon receptor (AhR) has been known primarily for its role in the regulation of several drug and xenobiotic metabolizing enzymes to mitigate environmental stresses. In this study, we interfere the expression of AhR gene to investigate the mechanism of AhR signaling pathway in the detoxification and antioxidation defense system that induced by Polycyclic Aromatic Hydrocarbons (PAHs) exposure by RNA interference (RNAi). The gene expressions of aryl hydrocarbon receptor nuclear translocator (ARNT), heat shock protein 90 (Hsp90) were evaluated after being exposed to benzo(a)pyrene (BaP) (4 μg/L) for 5 days and the positive correlations between AhR, ARNT, HSP90 indirectly indicating that AhR may have the ability to bind to ligands such as PAHs in Ruditapes philippinarum (R. philippinarum). Besides, the activities of detoxification enzymes were determined to investigate the role of AhR signaling pathway played in the metabolic detoxification. What's more, the gene expressions of protein kinase C (PKC) signaling pathway, mitogen-activated protein kinase (MAPKs) signaling pathway, NF-E2-related factor 2 (Nrf2) signaling pathway and antioxidant defense system indicated that AhR may regulate the Nrf2-Keap1 signaling pathway through Kelch-like ECH-associated protein-1 (Keap1) and MAPKs, PKC signaling pathways. In conclusion, adoption of RNA interference technology to explore the role of RpAhR gene played in the detoxification and antioxidation defense system under the PAHs stress at different time points can informe molecular endpoints for application towards ecotoxicology monitoring of bivalves.

Multixenobiotic resistance in Mytilus edulis: Molecular and functional characterization of an ABCG2- type transporter in hemocytes and gills

Among the cellular protection arsenal, ABC transporters play an important role in xenobiotic efflux in marine organisms. Two pumps belonging to B and C subfamily has been identified in Mytilus edulis. In this study, we investigated the presence of the third major subtype ABCG2/BCRP protein in mussel tissues. Transcript was expressed in hemocytes and with higher level in gills. Molecular characterization revealed that mussel ABCG2 transporter shares the sequence and organizational structure with mammalian and molluscan orthologs. Overall identity of the predicted amino acid sequence with corresponding homologs from other organisms was between 49% and 98%. Moreover, protein efflux activity was demonstrated using a combination of fluorescent allocrites and specific inhibitors. The accumulation of bodipy prazosin and pheophorbide A was heterogeneous in gills and hemocytes. Most of the used blockers enhanced probe accumulation at different levels, most significantly for bodipy prazosin. Moreover, Mrp classical blocker MK571 showed a polyspecificity. In conclusion, our data demonstrate that several ABC transporters contribute to MXR phenotype in the blue mussel including ABCG2 that forms an active pump in hemocytes and gills. Efforts are needed to distinguish between the different members and to explore their single function and specificity towards allocrites and chemosensitizers.

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.

Response of detoxification gene mRNA expression and selection of molecular biomarkers in the clam Ruditapes philippinarum exposed to benzo[a]pyrene

Benzo[a]pyrene (B[a]P) has a high carcinogenic potential. B[a]P concentrations and molecular biomarkers (mRNA expressions of Pgp, AhR, CYP4, CYP414A1, GST-pi, GST-S2, Cu/Zn-SOD and Mn-SOD) were assayed in gills and digestive glands of the clam Ruditapes philippinarum exposed to 0.03, 0.3 and 3 μg/L B[a]P for 21 days and then exposed to natural seawater for 15 days. Results showed that B[a]P was rapidly accumulated in and then eliminated from tissues of the clams. All gene mRNA expressions in the treated groups were induced significantly with the exception of CYP414A1 and Cu/Zn-SOD in the 0.03 μg/L B[a]P group. According to correlation analysis, mRNA expressions of AhR, GST-pi and Mn-SOD in gills and GST-pi in digestive glands had good correlations with B[a]P concentrations and could be used as molecular biomarkers of B[a]P exposure. This study investigated the molecular response of the genes mentioned above and selected useful molecular biomarkers for B[a]P pollution monitoring.

Inhibition of P-glycoprotein in the blood-brain barrier alters avermectin neurotoxicity and swimming performance in rainbow trout

The importance of the blood brain barrier (BBB) and the contribution to its function by the efflux transporter P-glycoprotein (P-gp) in teleosts were examined using the P-gp substrates and central nervous system neurotoxins ivermectin (22,23-dihydroavermectin B1a+22,23-dihydroavermectin B1b) [IVM]) and emamectin benzoate (4″-deoxy-49″epimethylaminoavermectin B1 benzoate [EB]). Trout were injected intraperitoneally with 0.01-1.0 and 1-50mg/kg of IVM or EB, respectively either alone or in combination with cyclosporin A (CsA: a P-gp substrate) at 1mg/kg. IVM affected the swimming performance (critical swimming speed, burst swimming distance, and schooling) at significantly lower concentrations than EB. When fish were exposed to IVM or EB in the presence of CsA, alterations to swimming were increased, suggesting that competition for P-gp in the BBB by CsA increased IVM and EB penetration into the CNS and decreased swimming capabilities. The effect of co-administration of CsA on swimming-related toxicity was different between IVM and EB-treated fish; EB toxicity was increased to a greater extent than IVM toxicity. The greater chemosensitization effect of EB vs. IVM was examined using a P-gp competitive inhibition assay in isolated trout hepatocytes with rhodamine 123 as a substrate. At the cellular level, IVM was a more potent inhibitor of P-gp than EB, which allowed for a greater accumulation of R123 in hepatocytes. These results provide evidence for a role of P-gp in the BBB of fish, and suggest that this protein protects fish from environmental neurotoxins.

Isolation of acetylated bile acids from the sponge Siphonochalina fortis and DNA damage evaluation by the comet assay

From the organic extracts of the sponge Siphonochalina fortis, collected at Bahía Bustamante, Chubut, Argentina, three major compounds were isolated and identified as deoxycholic acid 3, 12-diacetate (1), cholic acid 3, 7, 12-triacetate (2) and cholic acid, 3, 7, 12-triacetate. (3). This is the first report of acetylated bile acids in sponges and the first isolation of compound 3 as a natural product. The potential induction of DNA lesions by the isolated compounds was investigated using the comet assay in lymphocytes of human peripheral blood as in vitro model. The results showed that the administration of the bile acid derivatives would not induce DNA damages, indicating that acetylated bile acids are nontoxic metabolites at the tested concentrations. Since the free bile acids were not detected, it is unlikely that the acetylated compounds may be part of the sponge cells detoxification mechanisms. These results may suggest a possible role of acetylated bile acids as a chemical defense mechanism, product of a symbiotic relationship with microorganisms, which would explain their seasonal and geographical variation, and their influence on the previously observed genotoxicity of the organic extract of S. fortis.

Principles of biofouling protection in marine sponges: a model for the design of novel biomimetic and bio-inspired coatings in the marine environment?

The process of biofouling of marine structures and substrates, such as platforms or ship hulls, proceeds in multiple steps. Soon after the formation of an initial conditioning film, formed via the adsorption of organic particles to natural or man-made substrates, a population of different bacterial taxa associates under the formation of a biofilm. These microorganisms communicate through a complex quorum sensing network. Macro-foulers, e.g., barnacles, then settle and form a fouling layer on the marine surfaces, a process that globally has severe impacts both on the economy and on the environment. Since the ban of tributyltin, an efficient replacement of this antifouling compound by next-generation antifouling coatings that are environmentally more acceptable and also showing longer half-lives has not yet been developed. The sponges, as sessile filter-feeder animals, have evolved antifouling strategies to protect themselves against micro- and subsequent macro-biofouling processes. Experimental data are summarized and suggest that coating of the sponge surface with bio-silica contributes to the inhibition of the formation of a conditioning film. A direct adsorption of the surfaces by microorganisms can be impaired through poisoning the organisms with direct-acting secondary metabolites or toxic peptides. In addition, first, compounds from sponges have been identified that interfere with the anti-quorum sensing network. Sponge secondary metabolites acting selectively on diatom colonization have not yet been identified. Finally, it is outlined that direct-acting secondary metabolites inhibiting the growth of macro-fouling animals and those that poison the multidrug resistance pump are available. It is concluded that rational screening programs for inhibitors of the complex and dynamic problem of biofilm production, based on multidisciplinary studies and using sponges as a model, are required in the future.

Potentiation of the cytotoxic activity of copper by polyphosphate on biofilm-producing bacteria: a bioinspired approach

Adhesion and accumulation of organic molecules represent an ecologically and economically massive problem. Adhesion of organic molecules is followed by microorganisms, unicellular organisms and plants together with their secreted soluble and structure-associated byproducts, which damage unprotected surfaces of submerged marine structures, including ship hulls and heat exchangers of power plants. This is termed biofouling. The search for less toxic anti-biofilm strategies has intensified since the ban of efficient and cost-effective anti-fouling paints, enriched with the organotin compound tributyltin, not least because of our finding of the ubiquitous toxic/pro-apoptotic effects displayed by this compound [1]. Our proposed bio-inspired approach for controlling, suppressing and interfluencing the dynamic biofouling complex uses copper as one component in an alternative anti-fouling system. In order to avoid and overcome the potential resistance against copper acquired by microorganisms we are using the biopolymer polyphosphate (polyP) as a further component. Prior to being functionally active, polyP has to be hydrolyzed to ortho-phosphate which in turn can bind to copper and export the toxic compound out of the cell. It is shown here that inhibition of the hydrolysis of polyP by the bisphosphonate DMDP strongly increases the toxic effect of copper towards the biofilm-producing Streptococcus mutans in a synergistic manner. This bisphosphonate not only increases the copper-caused inhibition of cell growth but also of biofilm production by the bacteria. The defensin-related ASABF, a marine toxin produced by the sponge Suberites domuncula, caused only an additive inhibitory effect in combination with copper. We conclude that the new strategy, described here, has a superior anti-biofilm potential and can be considered as a novel principle for developing bio-inspired antifouling compounds, or cocktails of different compounds, in the future.

P-glycoprotein (P-gp) in the monogonont rotifer, Brachionus koreanus: molecular characterization and expression in response to pharmaceuticals

P-glycoprotein is involved in the efflux of diverse chemicals, including hydrophobic compounds and pharmaceuticals as a first line of defense. Here, we firstly identified and characterized the P-gp (Bk-P-gp) gene in the rotifer, Brachionus koreanus. Bk-P-gp was highly conserved in genomic organization compared to the human P-gp gene. Messenger RNA expression of Bk-P-gp revealed that it would be regulated by temperature change via 14 heat shock response elements in its promoter region. Bk-P-gp showed a high similarity of motifs/domains compared to those of vertebrates in its amino acid sequences. To check whether Bk-P-gp would be inducible, we exposed B. koreanus to six pharmaceuticals including antibiotics for use in aquaculture and observed dose- and time-dependency on transcripts of Bk-P-gp for 24h over a wide range of concentration. Efflux assay and membrane topology supported its conserved function for transportation of a number of chemicals upon cellular damage. To reveal the effect of pharmaceuticals on the rotifer, we measured survival rate and population growth rate after exposure to six pharmaceuticals. In an acute toxicity test, both NOEC and LC₅₀ values for all the pharmaceuticals were high for 24 h. ATP, CBZ, SMX, and TMP markedly inhibited the population growth of B. koreanus after exposure up to 100 mg/L for 10 days. In this paper, we demonstrated that various pharmaceuticals can retard growth rate with up-regulation of the P-gp gene as a cellular defense system. This finding provides a better understanding of molecular mechanisms involved in pharmaceutical-mediated cellular damage in B. koreanus.

The Multixenobiotic Resistance Mechanism in Aquatic Organisms

Many aquatic organisms thrive and reproduce in polluted waters. This fact indicates that they are well equipped with a defense system(s) against several toxic xenobiotics simultaneously because water pollution is typically caused by a mixture of a number of pollutants. We have found that the biochemical mechanism underlying such "multixenobiotic" resistance in freshwater and marine mussel, in several marine sponges, and in freshwater fish is similar to the mechanism of multidrug resistance (MDR) found in tumor cells that became refractory to treatment with a variety of chemotherapeutic agents. All these organisms possess a verapamil-sensitive potential to bind 2-acetylaminofluorene and vincristine onto membrane vesicles. They all express mRNA for mdr1 gene, and mdr1 protein product, the glycoprotein P170. Finally, in in vivo experiments, the accumulation of xenobiotics is enhanced in all investigated organisms in the presence of verapamil, the inhibitor of the P170 extrusion pump. The knowledge that the presence of one xenobiotic may block the pumping out, and hence accelerating accumulation, of others, may help us to understand and interpret our present and past data on different environmental parameters obtained using indicator organisms.

ABCB- and ABCC-type transporters confer multixenobiotic resistance and form an environment-tissue barrier in bivalve gills

Aquatic organisms and, in particular, filter feeders, such as mussels, are continuously exposed to toxicants dissolved in the water and, presumably, require adaptations to avoid the detrimental effects from such chemicals. Previous work indicates that activity of ATP-binding cassette (ABC) transporters protects mussels against toxicants, but the nature of these transporters and the structural basis of protection are not known. Here we meld studies on transporter function, gene expression, and localization of transporter protein in mussel gill tissue and show activity and expression of two xenobiotic transporter types in the gills, where they provide an effective structural barrier against chemicals. Activity of ABCB/MDR/P-glycoprotein and ABCC/MRP-type transporters was indicated by sensitivity of efflux of the test substrate calcein-AM to the ABCB inhibitor PSC-833 and the ABCC inhibitor MK-571. This activity profile is supported by our cloning of the complete sequence of two ABC transporter types from RNA in mussel tissue with a high degree of identity to transporters from the ABCB and ABCC subfamilies. Overall identity of the amino acid sequences with corresponding homologs from other organisms was 38-50% (ABCB) and 27-44% (ABCC). C219 antibody staining specific for ABCB revealed that this transporter was restricted to cells in the gill filaments with direct exposure to water flow. Taken together, our data demonstrate that ABC transporters form an active, physiological barrier at the tissue-environment interface in mussel gills, providing protection against environmental xenotoxicants.

Interaction of insecticides with mammalian P-glycoprotein and their effect on its transport function

We studied the effects of four commonly used insecticides (methylparathion, endosulfan, cypermethrin and fenvalerate) on P-glycoprotein isolated from multidrug-resistant cells. All the pesticides stimulated P-glycoprotein ATPase activity, with maximum stimulation of up to 213% in a detergent-solubilized preparation, and up to 227% in reconstituted liposomes. The ATPase stimulation profiles were biphasic, displaying lower stimulation, and in the case of methylparathion, inhibition of activity, at higher insecticide concentrations. Quenching of the intrinsic Trp fluorescence of purified P-glycoprotein was used to quantitate insecticide binding; the estimated K(d) values fell in the range 4-6 microM. Transport of the fluorescent substrate tetramethylrosamine (TMR) into proteoliposomes containing P-glycoprotein was monitored in real time. The TMR concentration gradient generated by the transporter was collapsed by the addition of insecticides, and prior addition of these compounds prevented its formation. The rate of TMR transport was inhibited in a saturable fashion by all the compounds, indicating that they compete with the substrate for membrane translocation. Taken together, these data suggest that the insecticides bind to Pgp with high affinity and effectively block drug transport. Inhibition of Pgp by pesticides may compromise its ability to clear xenobiotics from the body, leading to a higher risk of toxicity.

New perspectives on perfluorochemical ecotoxicology: inhibition and induction of an efflux transporter in the marine mussel, Mytilus californianus

The toxicological effects of perfluoroalkyl acids on the p-glycoprotein (p-gp) cellular efflux transporter were investigated using the marine mussel Mytilus californianus as a model system. Four of the perfluoroalkyl acids studied exhibit chemosensitizing behavior, significantly inhibiting p-gp transporter activity. The inhibitory potency is maximal for the longer chain acids perfluorononanoate (PFNA) and perfluorodecanoate (PFDA), with average IC50 values of 4.8 and 7.1 microM, respectively. Results indicate that PFNA inhibits p-gp by an indirect mechanism, and this inhibition is reversible and accompanied by a rapid loss of PFNA from the tissue. In addition, PFNA induces expression of the p-gp transporter after a 2-h exposure, a stress response that may result in a metabolic cost to the organism. Given that most organisms, including humans, share efflux transporters as a first line of defense against toxicants, the results of this study may have broader implications for the ecotoxicology of perfluoroalkyl acids.

Multiple drug resistance, antimutagenesis and anticarcinogenesis

Many cells are protected from excess levels of exogenous chemicals, including mutagens and carcinogens as well as pharmaceutical agents, by being actively extruded through the action of one or more of a series of ATP-binding cassette drug transporter proteins. Those known to be important in humans are the multidrug resistance proteins (P-glycoproteins, encoded by the mdr1 and 3 genes), multidrug-resistance-associated proteins (MRP1-7) and the breast cancer resistance protein (BCRP). These proteins have overlapping but distinct cellular locations and substrate specificities, and jointly govern the likelihood of penetration or distribution of a given mutagen or carcinogen into various tissues including the brain, testis, ovaries and fetus. Thus, they can affect the absorption, distribution and excretion of mutagens and carcinogens, as well as of their metabolites and conjugates, in most cases acting to prevent or reduce mutagenesis or carcinogenesis. However, because ABC transporters may limit the success of chemotherapy, there has been a considerable effort by the pharmaceutical industry to develop inhibitors of this transport process, and these are increasing in use. In general, the mutagenicity of many chemicals may be increased at the cellular levels by the action of these inhibitors, while the altered absorption characteristics favour greater uptake into the body. Thus, in many cases, such inhibitors may counter the antimutagenic and anticarcinogenic effect of the multidrug resistance mechanisms. There are exceptions, however. An increasing number of single nucleotide polymorphisms in multidrug resistance genes are being identified in humans, and may account for many of the significant differences in inter-individual susceptibility to exogenous and endogenous mutagenic and carcinogenic insults.

Emerging contaminants--pesticides, PPCPs, microbial degradation products and natural substances as inhibitors of multixenobiotic defense in aquatic organisms

The environmental presence of chemosensitizers or inhibitors of the multixenobiotic resistance (MXR) defense system in aquatic organisms could cause increase in intracellular accumulation and toxic effects of other xenobiotics normally effluxed by MXR transport proteins (P-glycoprotein (P-gps), MRPs). MXR inhibition with concomitant detrimental effects has been shown in several studies with aquatic organisms exposed to both model MXR inhibitors and environmental pollutants. The presence of MXR inhibitors has been demonstrated in environmental samples from polluted locations at concentrations that could abolish P-gp transport activity. However, it is not clear whether the inhibition observed after exposure to environmental samples is a result of saturation of MXR transport proteins by numerous substrates present in polluted waters or results from the presence of powerful MXR inhibitors. And are potent environmental MXR inhibitors natural or man-made chemicals? As a consequence of these uncertainties, no official action has been taken to monitor and control the release and presence of MXR inhibitors into aquatic environments. In this paper we present our new results addressing these critical questions. Ecotoxicological significance of MXR inhibition was supported in in vivo studies that demonstrated an increase in the production of mutagenic metabolites by mussels and an increase in the number of sea urchin embryos with apoptotic cells after exposure to model MXR inhibitors. We also demonstrated that MXR inhibitors are present among both conventional and emerging man-made pollutants: some pesticides and synthetic musk fragrances show extremely high MXR inhibitory potential at environmentally relevant concentrations. In addition, we emphasized the biological transformation of crude oil hydrocarbons into MXR inhibitors by oil-degrading bacteria, and the risk potentially caused by powerful natural MXR inhibitors produced by invasive species.

Accumulation of ivermectin in the brain of sea bream, Sparus aurata after intraperitoneal administration

Ivermectin, which is widely used in veterinary and human, has been considered safe due to its inability to penetrate into the central nervous system of higher vertebrates. This paper presents data on the ability of the drug to cross the blood-brain barrier of the marine teleost sea bream, Sparus aurata and accumulate in the brain. The concentration of the drug in the brain and the serum of the fish was assessed by the use of a direct competitive ELISA commercial kit. Our results showed a rapid uptake of the substance by the brain of the fish reaching a maximum concentration of 98.9ngg(-1) 8h post treatment. The trend of the absorption of the drug in brain followed that of the blood. Concentration of the drug in the brain remained high at each sampling point over the 24h duration of the experiment. In view of these findings, the need of study of the role of the blood-brain barrier and particularly the multidrug resistance mechanism in sea bream is outlined.

Response of multixenobiotic defence mechanism in Dreissena polymorpha exposed to environmental stress

Multixenobiotic defence mechanism (MXDM) has been recently described as a first line defence system in many aquatic organisms and it has raised attention for its potential as an early biomarker of exposure to environmental stress. In order to investigate the relevance of this biomarker in the freshwater zebra mussel Dreissena polymorpha, we examined its response to exposure and to cessation of exposure. For this purpose, depurated zebra mussels were transplanted to natural rivers or exposed to medium experimentally contaminated with fuel oil 2 in the laboratory. After the exposure period, mussels were submitted to depuration in clean water in the laboratory. The level of MXDM activity was assessed by the efflux method that allows a simple and fast measure of the rate of rhodamine B efflux. MXDM activity was induced following exposure and induction appears earlier after laboratory exposure (3 days) than after in the field exposure (3-14 days). The level of MXDM response could not be directly correlated to exposure dose in the laboratory and to isolated physicochemical parameters or AOX levels in the field. The behaviour of MXDM activity seems to reflect the occurrence of exposure to a global stress rather than to specific compounds. Our results highlighted the possibility to monitor MXDM response in the freshwater zebra mussel. In spite of the difficulty to interpret responses resulting from the lack of specificity of response, multixenobiotic defence mechanism displays qualities to represent a biomarker of general stress in freshwater ecosystems.

Le mécanisme de défense multixénobiotique (MDMX) chez les bivalves

Multixenobiotic defence mechanism (MXDM) consists in a cellular system that functions as membrane extrusion pumps effluxing organic compounds out of the cells. In bivalves, it represents a primordial protection against toxic effects of organic xenobiotics in preventing their cellular accumulation. It has raised attention during the last decade for its potential to be used as a biomarker of pollution. This article reviews the fundamental knowledge on the MXDM system in bivalves and the methods proposed to assess its activity. Finally, it reviews the major results of laboratory and field studies that enabled to hypothesise that MXDM could be used as a biomarker of environmental stress rather than of pollutant exposure.

Differential modulation of cytochrome P-450 1A and P-glycoprotein expression by aryl hydrocarbon receptor agonists and thyroid hormone in Xenopus laevis liver and intestine

Several defence mechanisms, such as cytochrome P450 1A (CYP1A) enzymes and P-glycoprotein (Pgp), may influence the intracellular concentration and consequently the toxicity of xenobiotics. The parallel expression of CYP1A and Pgp has been investigated in mammals and, to a lesser extent in fish, in search for evidence of co-ordinated responses to xenobiotic exposure. The aryl hydrocarbon receptor (AHR) agonists are well known CYP1A inducers but some of them resulted not to have a uniquely defined action on Pgp levels in mammalian and fish species. To the best of our knowledge, no detailed studies have been carried out so far on amphibians Xenopus laevis. For this reason, in this work, the time dependent responses of the hepatic CYP1A and Pgp, to the prototypical CYP1A inducers, benzo(a)pyrene (B(a)P), 3-methylcholanthrene (3MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in X. laevis have been assessed at the protein level and compared. The responsiveness of Xenopus intestinal Pgp to these compounds has also been analysed, as the epithelial cells lining the lumen of intestine represent another preferential site of Pgp expression. In addition, since the thyroid hormone has been demonstrated to down regulate the mdr gene during Xenopus development and in primary culture of Xenopus intestinal epithelial cells, the effects of 3,3',5-triiodo-L-thyronine (T(3)) on CYP1A and Pgp protein levels have been investigated in adult organisms. Western blot evidenced that a single injection of B(a)P (100 mg/kg), 3MC (20 mg/kg), and TCDD (3 microg/kg) elicited a statistically significant induction of hepatic CYP1A at all time points considered (72, 120 and 168 h) which decreased in time. The same trend of liver CYP1A induction was observed in T(3) treated Xenopus (15 microg/kg). Unlike CYP1A induction, the modulation of hepatic and intestinal Pgp expression exhibits an heterogeneous pattern. The basal levels of hepatic and intestinal Pgp were not statistically significant affected by treatments. In particular, the hepatic Pgp levels seem not to be induced by TCDD and T(3) at all times considered in comparison to control. For the first time the modulation of CYP1A and Pgp levels by B(a)P, 3MC and in particular by TCDD and T(3) in Xenopus has been demonstrated and the results herewith indicate that the two target defence mechanisms respond to AHR agonists in a dissimilar way in terms of proteins induction in Xenopus. Moreover, these data suggest additional experiments in order to clarify the complex mechanism, which adjusts the parallel expression of CYP1A and Pgp in Xenopus.

Cross-reactivity of some antibodies to human epitopes with shrimp Pandalus borealis proteins: a possible aid in validation and characterization of crustacean cells in vitro

Cell characterization of primary cultures in vertebrates is well established but not in marine invertebrates. This fact is hampering advances in the development of tissue cultures from this species. In the present study, a panel of antibodies to structural proteins, stress proteins, oncogenes and proliferation antigens, developed against mammalian antigens, were tested in paraffin sections of the crustacean Pandalus borealis tissues. Several tissues were analysed: hepatopancreas, gills, ovaries, epithelium under the cuticle and abdominal muscle. Specific antibodies to crustacean proteins are not commercially available. The immunocytochemical results show that antibodies to human epitopes cross-react with antigens in the crustacean Pandalus borealis indicating that some cellular proteins are highly conserved in evolution. Cytokeratin, proliferating cell nuclear antigen, ras and p-glycoprotein were detected by immunocytochemistry in Pandalus borealis. No immunoreactivity for Ki-67 and metallothionein was observed. This system can help in validation and characterization of invertebrate cultures.

Genetic and immunological characterisation of a multixenobiotic resistance system in the turbot (Scophthalmus maximus)

Pleiotropic resistance driven by transport proteins constitutes a very ubiquitous protection mechanism against natural or synthetic toxic compounds. The multidrug (MDR) or multixenobiotic (MXR) system has been identified in many different species, and may be used as a biomarker for pollution assessment. Here we report the existence of a gene encoding a MXR-related protein in a benthic fish species, the turbot Scophthalmus maximus, and its constitutive expression in several tissues. A 433bp cDNA fragment has been cloned by RT-PCR. The deduced amino-acid sequence shares close to 80% homology with class I or class II mammalian MDR proteins. This cDNA corresponds to a major mRNA of 5.6 kb and encodes a protein having an apparent molecular weight of 83 kDa. Constitutive expression levels assessed by semi-quantitative RT-PCR and Western blot, revealed that the kidney and the brain, and to a lesser extent, the heart, gills and intestine, are the organs which contain the highest amount of both MXR mRNAs or proteins. This tissue specific expression suggests a role for the identified mechanism in protection against endogenous or exogenous toxic compounds.

Enhanced xenobiotic transporter expression in normal teleost hepatocytes: response to environmental and chemotherapeutic toxins

Many aquatic organisms are resistant to environmental pollutants, probably because their inherent multi-drug-resistant protein extrusion pump (pgp) can be co-opted to handle man-made pollutants. This mechanism of multixenobiotic resistance is similar to the mechanism of multidrug resistance exhibited in chemotherapy-resistant human tumor cells. In the present study, a variety of techniques were used to characterize this toxin defense system in killifish (Fundulus heteroclitus) hepatocytes. The cellular localization and activity of the putative drug efflux system were evaluated. In addition, in vitro and in vivo studies were used to examine the range of expression of this putative drug transporter in the presence of environmental and chemotherapeutic toxins. The broad range of pgp expression generally observed in transformed mammalian cells was found in normal cells of our teleost model. Our findings suggest that the expression of the pgp gene in the killifish could be an excellent indicator of toxin levels or stressors in the environment.

Induction of the multixenobiotic defense mechanism (MXR), P-glycoprotein, in the mussel Mytilus californianus as a general cellular response to environmental stresses

A multixenobiotic resistance mechanism (MXR) related to the P-glycoprotein multidrug transporter protein (p-gp) has been identified and characterized in several marine invertebrates. p-gp activity and protein titer is induced by exposure to toxins, supporting the suggestion that the role for this transporter is protection from xenobiotics by reducing accumulation of toxins in cells. In this study, we report on the specificity of the induction of the transporter by various chemical and physical stressors. p-gp substrates (including the pesticides pentachlorophenol and chlorthal) as well as non-substrates (including DDE and sodium arsenite) induced p-gp activity and protein titer in the gill tissues of the mussel Mytilus californianus. Similarly, mussels exposed to heat shock of 20 degrees C or 25 degrees C exhibited increased p-gp titer and activity compared to mussels held at ambient (12 degrees C) temperature seawater. Some of the same treatments that induced an increase in p-gp caused a concomitant increase in hsp70, but hsp induction was not always associated with induction of the p-gp protein. These findings suggest that p-gp induction in mussels may be part of a general cellular stress response. This response, however, does not appear to be always coupled with the hsp70 response in mussels.

Multixenobiotic resistance as a cellular defense mechanism in aquatic organisms

Multixenobiotic resistance in aquatic organisms exposed to natural toxins or anthropogenic contaminants is a phenomenon analogous to multidrug resistance in mammalian tumor cell lines tolerant of anti-cancer drugs. Multidrug resistance is commonly due to the elevated expression of transmembrane P-glycoproteins (P-gp) which actively transport a wide variety of structurally and functionally diverse compounds. The purpose of this review is to place aquatic ecotoxicological data in context of the larger multidrug resistance field of study. Information on P-glycoproteins structure, mechanism of transport, and substrate specificity gained through traditional mammalian and cell culture models is examined in conjunction with recent work on aquatic species exposed to xenobiotics both in the field and in the laboratory. The physiological function of P-glycoproteins is explored through studies of gene knockout models and expression patterns in normal tissues and tumors. The effect of xenobiotic exposures on P-gp activity and protein titer is examined in wild and captive populations of aquatic invertebrates and vertebrates. Substrate overlap and evidence of co-expression of phase I detoxification enzymes (e.g. cytochromes P450) and P-gp are presented. The role of P-gp chemosensitizers as environmental pollutants and the ecotoxicological consequences of P-gp inhibition are highlighted. The overwhelming evidence suggests that P-glycoproteins provide aquatic organisms with resistance to a wide range of natural and anthropogenic toxins.

Identification of a multixenobiotic resistance mechanism in Xenopus laevis embryos

In the '90's a membrane-associated transport protein, discovered in aquatic organisms, was considered to be expressed in response to environmental xenobiotics. Like the multidrug resistance protein found in mammalian tumor cell lines, this protein confers resistance in organisms in polluted areas by binding xenobiotics and transporting them out of the cells in an energy-dependent manner. This study investigates the expression and the activity of a P-glycoprotein (Pgp) involved in a multixenobiotic resistance mechanism (MXRM) during the early developmental stages and in tissues of adult Xenopus laevis.

Chemosensitizers of the multixenobiotic resistance in amorphous aggregates (marine snow): etiology of mass killing on the benthos in the Northern Adriatic?

Periodically appearing amorphous aggregates, `marine snow', are formed in the sea and if settled as mats on the sea bottom cause death of benthic metazoans. Especially those animals are killed which are sessile filter feeders, e.g. sponges, mussels, or Anthozoa. The etiology of the toxic principle(s) is not yet well understood. Gel-like marine snow aggregates occurred in the Northern Adriatic during summer 1997. Samples of these aggregates were collected during the period July to September and the outer as well as the inner zones were analyzed for (i) cell toxicity, and (ii) chemosensitizing activity of the multixenobiotic resistance (MXR) mechanism. Organic extracts were prepared and cell toxicity was determined using mouse lymphoma cells. The experiments revealed that the major activity is seen in the center of the mats of the gel-like aggregates; a growth inhibitory activity of up to 54% (correlated to 5 ml of snow sample) was determined. The same extracts were used to determine the inhibition of the P-glycoprotein (Pgp) extrusion pump which confers the multixenobiotic resistance. The analyses were performed with cells from the sponge Suberites domuncula and with gills from the clam Corbicula fluminea in situ. Both systems have been shown to express the Pgp extrusion pump. The data show that extracts from the outer zone of the gel-like aggregate samples display pronounced inhibitory activity on the MXR extrusion pump and hence act as chemosensitizers by reversing the MXP property. These findings indicate that gel-like aggregates contain compounds in the outer zone, chemosensitizer of the Pgp extrusion pump, which lower the level of protection of metazoan animals towards dissolved compounds in their surrounding milieu, and in the center toxic compounds which are-very likely-even in the absence of chemosensitizers hazardous for the invertebrates.

The chemosensitizers of multixenobiotic resistance mechanism in aquatic invertebrates: a new class of pollutants

Mechanism of multixenobiotic resistance (MXR), identical to multidrug resistance (MDR) in tumor cells, has been found in aquatic invertebrates. The presence of this ATP-dependent membrane P-glycoprotein (Pgp) pump was confirmed by biochemical ('binding'), molecular (immunohistochemical, Western, Northern), physiological (verapamil-sensitivity) and toxicological (modulation of toxicity) methods. The inducibility of MXR in the presence of xenobiotics and its wide taxonomic distribution suggests its role as a general biological defense mechanism that rescues organisms by pumping potentially toxic xenobiotics out of the cells. Some xenobiotics, the chemosensitizers, can inhibit this defense mechanism. The presence of these MXR-inhibitors has important implications on environmental parameters like exposure, uptake, internal dose, bioaccumulation, response, synergism and toxicity. Such MXR-inhibitors, for example, enhance the accumulation of carcinogenic aromatic amines in mussel, with subsequent enhancement in production of their mutagenic metabolites, in induction of single strand breaks in DNA, and in induction of DNA-adducts. The property to inhibit defense mechanism of organisms classifies MXR-inhibitors among top-hazardous environmental chemicals. Therefore, we measured the concentration of chemosensitizers in water concentrates or sediment extracts as their potential to modulate the accumulation of fluorescent dyes in a cell-culture of NIH 3T3 mouse fibroblasts stable transfected with human MDR1 gene, or as the potential of native waters to decrease the efflux-rate of Rhodamine B from gills of mussels. We found significantly higher concentrations of MXR-inhibitors in samples from polluted marine sites or from polluted rivers than in samples from corresponding unpolluted sites. These concentrations were able to enhance the accumulation of fluorescent dyes or carcinogenic aromatic amines in clams, mussels, snails and sponges exposed to these xenobiotics, demonstrating the ecotoxicological relevance of MXR-inhibitors present in polluted waters.

Inhibitory effects of extracts from the marine alga Caulerpa taxifolia and of toxin from Caulerpa racemosa on multixenobiotic resistance in the marine sponge Geodia cydonium

The invasive growth of the introduced green alga Caulerpa taxifolia, already affecting the richness and diversity of the littoral ecosystems, has become a major ecological problem in the Mediterranean Sea. Previously, we demonstrated that the water pollutant tributyltin induces apoptosis in tissue of the marine sponge Geodia cydonium at concentrations of 3 μM and higher. Here we show that exposure of G. cydonium to low (non-toxic) concentrations of Caulerpa extract or purified caulerpin (10 μg/ml) together with low doses of tributyltin (1 μM; non-toxic), results in a strong apoptotic effect. Evidence is presented that the enhancement of toxicity of tributyltin by Caulerpa extract is at least partially caused by inhibition of the multixenobiotic resistance (MXR) pump by the algal toxin. Caulerpa extract, as well as caulerpin, strongly enhance the accumulation of the test substrate of MXR, rhodamine B, in the gills of the mussel Dreissena polymorpha, used as a model system for testing MXR-inhibiting potential.

Increased genotoxicity of acetylaminofluorene by modulators of multixenobiotic resistance mechanism: studies with the fresh water clam Corbicula fluminea

The presence of a 'multixenobiotic resistance' [MXR] mechanism in gills of the freshwater clam Corbicula fluminea was investigated. Western blot analyses of membrane vesicles from gills, applying antibodies to vertebrate P170 multidrug resistance (MDR) protein, revealed a 135 kDa immunoreactive protein. Verapamil caused a reduction of 3H-vincristine (3H-VCR) binding onto vesicles from clam. Exposure of clams to 3H-VCR in the presence of verapamil or staurosporine (STP) enhanced the accumulation of 3H-VCR over control values. Furthermore, clams were exposed instead to VCR, to a model carcinogen, 2-acetylaminofluorene (AAF), to determine the verapamil- and STP-dependent increase of single-strand breaks (SSBs) in DNA from gills of this organism. Verapamil caused no or little increase of SSBs induced by exposure to 0.01 or 0.10 microM AAF, respectively, as measured by the alkaline elution technique. In contrast, in the presence of STP a highly significant and dose-dependent enhancement of AAF-mediated SSBs was measured already at exposure to 0.01 microM AAF. These data indicate (i) that the clam C. fluminea is provided with a P-glycoprotein-like element of the MDR-mechanism, (ii) that this system can be poisoned by chemosensitizers such as verapamil and STP, (iii) the role of protein kinase C in the regulation of MXR function and (iv) the importance of the MXR modulators for the assessment of ecotoxicological effects of pollutants.

How do insect herbivores cope with the extreme oxidative stress of phototoxic host plants?

Plants of the Asteraceae and Hypericaceae possess secondary compounds that induce photooxidation in insect herbivores that consume them. One of the well-established modes of action of these substances is peroxidation of membrane lipids. Some herbivores counteract these defences by avoidance of light and tissues rich in phototoxins or the ability to detoxify these secondary substances. The cytochrome P-450 polysubstrate monooxygenase systems involved, the metabolic products, and a new putative toxin pump have been described. Dietary antioxidants (β-carotene, vitamin E, ascorbate) are additional defences against phototoxicity. They reduce mortality in herbivores exposed to phototoxins and some specialist herbivores have high constitutive levels. Adapted specialist insects also have higher constitutive levels of superoxide dismutase (SOD) and respond to phototoxins in their diet by the induction of catalase (CAT), glutathione reductase (GR), and increased levels of reduced glutathione (GSH). Artificial inhibition of the enzymes SOD and CAT had little effect on phototoxicity but inhibition of GSH synthesis in herbivores enhanced photooxidative effects of administered phototoxins on lipid peroxidation. While insects have many mechanisms to overcome plant photooxidants, the Asteraceae appear to have adopted a strategy of counterattack. We suggest and provide preliminary evidence that a second group of secondary substances, the sesquiterpene lactones, occurring in the Asteraceae can attack key antioxidant defences to synergise phototoxins. © 1995 Wiley-Liss, Inc.

A putative nicotine pump at the metabolic blood-brain barrier of the tobacco hornworm

In mammals, P-glycoprotein immunostaining at the blood-brain barrier has implicated the multidrug pump in the restricted movement of many cytotoxic agents into the central nervous system (CNS). Since many insects require a sophisticated blood-brain barrier system to protect their CNS from plant-derived neurotoxins, we have investigated the possibility that a P-glycoprotein homolog constitutes a component of the insect blood-brain barrier. We have used the nicotine-resistant tobacco hornworm (Manduca sexta) to address this issue. Manduca has been previously shown, in physiological studies, to have an alkaloid (nicotine/morphine/atropine) pump at its excretory malpighian tubules. We show (1) that the tubules are P-glycoprotein immunopositive, (2) that Manduca has a metabolic blood-brain barrier for nicotine, (3) that the barrier co-localizes with P-glycoprotein immunostaining, and (4) that detoxifying enzymes as well as the nicotine pump are likely to account for the metabolic blood-brain barrier to nicotine. These findings may provide insights on two major fronts, the troublesome problem of multi-insecticide resistance, a phenomenon that parallels multidrug resistance in tumor cells, and the problem of tolerance to addictive neuroactive drugs like nicotine or morphine.

Multidrug resistance and mutagenesis

Multidrug resistance, the phenomenon whereby the development of resistance to one drug is sometimes accompanied by the simultaneous development of resistance to a variety of other, often structurally unrelated, drugs, is frequently associated with the presence of an energy-dependent membrane-transport system which reduces the concentration of a drug or other chemical in the cytoplasm. The latter process (termed here MDR) occurs naturally in a number of normal mammalian tissues, including colon, jejunum, liver, kidney and bone marrow, as well as in other species including bacteria. The presence of MDR can reduce the mutagenic potential of a variety of compounds in mammalian and microbiological assays. MDR can be reversed by a diverse collection of compounds, many of which are hydrophobic cations with other physiological effects. An important consequence of these considerations is that MDR-reversing agents are potentially dangerous because, while having no intrinsic mutagenicity, they may significantly increase the mutagenicity of other compounds by poisoning protective MDR mechanisms in the body.