Measurements of the cytosolic Ah receptor among four strains of Drosophila melanogaster

Dose- and time-dependent expression of aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (ARNT) in PCB-, B[a]P-, and TBT-exposed intertidal copepod Tigriopus japonicus

The aryl hydrocarbon receptor (AhR) and aryl hydrocarbon nuclear translocator (ARNT) genes from the copepod Tigriopus japonicus (Tj) were cloned to examine their potential functions in the invertebrate putative AhR-CYP signaling pathway. The amino acid sequences encoded by the Tj-AhR and Tj-ARNT genes showed high similarity to homologs of Daphnia and Drosophila, ranging from 68% and 70% similarity for the AhR genes to 56% for the ARNT genes. To determine whether Tj-AhR and Tj-ARNT are modulated by environmental pollutants, transcriptional expression of Tj-AhR and Tj-ARNT was analyzed in response to exposure to five concentrations of polychlorinated biphenyl (PCB 126) (control, 10, 50, 100, 500 μg L(-1)), benzo[a]pyrene (B[a]P) (control, 5, 10, 50, 100 μg L(-1)), and tributyltin (TBT) (control, 1, 5, 10, 20 μg L(-1)) 24h after exposure. A time-course experiment (0, 3, 6, 12, 24h) was performed to analyze mRNA expression patterns after exposure to PCB, B[a]P, and TBT. T. japonicus exhibited dose-dependent and time-dependent upregulation of Tj-AhR and Tj-ARNT in response to pollutant exposure, and the degree of expression was dependent on the pollutant, suggesting that pollutants such as PCB, B[a]P, and TBT modulate expression of Tj-AhR and Tj-ARNT genes in the putative AhR-CYP signaling pathway.

Mining gene expression data for pollutants (dioxin, toluene, formaldehyde) and low dose of gamma-irradiation

General and specific effects of molecular genetic responses to adverse environmental factors are not well understood. This study examines genome-wide gene expression profiles of Drosophila melanogaster in response to ionizing radiation, formaldehyde, toluene, and 2,3,7,8-tetrachlorodibenzo-p-dioxin. We performed RNA-seq analysis on 25,415 transcripts to measure the change in gene expression in males and females separately. An analysis of the genes unique to each treatment yielded a list of genes as a gene expression signature. In the case of radiation exposure, both sexes exhibited a reproducible increase in their expression of the transcription factors sugarbabe and tramtrack. The influence of dioxin up-regulated metabolic genes, such as anachronism, CG16727, and several genes with unknown function. Toluene activated a gene involved in the response to the toxins, Cyp12d1-p; the transcription factor Fer3's gene; the metabolic genes CG2065, CG30427, and CG34447; and the genes Spn28Da and Spn3, which are responsible for reproduction and immunity. All significantly differentially expressed genes, including those shared among the stressors, can be divided into gene groups using Gene Ontology Biological Process identifiers. These gene groups are related to defense response, biological regulation, the cell cycle, metabolic process, and circadian rhythms. KEGG molecular pathway analysis revealed alteration of the Notch signaling pathway, TGF-beta signaling pathway, proteasome, basal transcription factors, nucleotide excision repair, Jak-STAT signaling pathway, circadian rhythm, Hippo signaling pathway, mTOR signaling pathway, ribosome, mismatch repair, RNA polymerase, mRNA surveillance pathway, Hedgehog signaling pathway, and DNA replication genes. Females and, to a lesser extent, males actively metabolize xenobiotics by the action of cytochrome P450 when under the influence of dioxin and toluene. Finally, in this work we obtained gene expression signatures pollutants (dioxin, toluene), low dose of gamma-irradiation and common molecular pathways for different kind of stressors.

Cytochrome P450 enzymes in aquatic invertebrates: recent advances and future directions

A variety of enzymes and other proteins are produced by organisms in response to xenobiotic exposures. Cytochrome P450s (CYP) are one of the major phase I-type classes of detoxification enzymes found in terrestrial and aquatic organisms ranging from bacteria to vertebrates. These enzymes metabolize a wide variety of substrates including endogenous molecules (e.g. fatty acids, eicosenoids, steroids) and xenobiotics (e.g. hydrocarbons, pesticides, drugs). Aquatic invertebrates, especially those in marine habitats, occupy every aspect of the environment, from above the surface (intertidal) to below the sediments. In turn, they have extremely diverse physiologies and are exposed to a vast array of potential toxicants. Aspects of aquatic invertebrate cytochrome P450 enzymes have been studied for the last 25 years. In a few phyla, P450 activities have been measured and are responsive to xenobiotic exposures. Until the last several years, little progress had occurred in the identification of P450 gene diversity in aquatic invertebrates. Molecular biology tools have greatly aided this search, and are likely to identify as much diversity for this protein superfamily as is present in higher marine and terrestrial organisms. Recent work has expanded our knowledge of the CYP superfamily, and new developments will rapidly advance the usefulness of these genes into such fields as biomarker research. Advances of the last decade are reviewed and insights are presented from related insect studies.

The aryl hydrocarbon receptor: a comparative perspective

The aryl hydrocarbon receptor (Ah receptor or AHR) is a ligand-activated transcription factor involved in the regulation of several genes, including those for xenobiotic-metabolizing enzymes such as cytochrome P450 1A and 1B forms. Ligands for the AHR include a variety of aromatic hydrocarbons, including the chlorinated dioxins and related halogenated aromatic hydrocarbons whose toxicity occurs through activation of the AHR. The AHR and its dimerization partner ARNT are members of the emerging bHLH-PAS family of transcriptional regulatory proteins. In this review, our current understanding of the AHR signal transduction pathway in non-mammalian and other non-traditional species is summarized, with an emphasis on similarities and differences in comparison to the AHR pathway in rodents and humans. Evidence and prospects for the presence of a functional AHR in early vertebrates and invertebrates are also examined. An overview of the bHLH-PAS family is presented in relation to the diversity of bHLH-PAS proteins and the functional and evolutionary relationships of the AHR and ARNT to the other members of this family. Finally, some of the most promising directions for future research on the comparative biochemistry and molecular biology of the AHR and ARNT are discussed.

Interaction of the regulatory domains of the murine Cyp1a1 gene with two DNA-binding proteins in addition to the Ah receptor and the Ah receptor nuclear translocator (ARNT)

The aromatic hydrocarbon (Ah) receptor complex is a ligand-activated transcriptional activator consisting of at least two protein components. The ligand-binding component is the AhR protein, a cytosolic receptor encoded by the Ahr gene, which, upon ligand binding, translocates to the nucleus in a heterodimeric complex with the ARNT (Ah receptor nuclear translocator) component. The complex binds to several discrete DNA domains containing aromatic hydrocarbon responsive elements (AhRE) present in the regulatory region of the murine cytochrome P(1)450 Cyp1a1 gene and of the other genes in the [Ah] gene battery. As a consequence of binding, a transcriptional complex is formed that activates the expression of these genes by as yet unidentified mechanisms. We have analyzed DNA-protein interactions in four of these domains, specifically, the AhREs located between -1085 and -482 (sites A, C, E, and D) of the upstream regulatory region of the murine Cyp1a1 gene. We found that two DNA-binding proteins, present in cytosolic and nuclear extracts of mouse Hepa-1 cells, showed overlapping DNA-binding specificities to those of the Ah receptor. One of these proteins had an apparent molecular mass of 35-40 kDa, bound only to AhRE3 (site D), and has been identified tentatively as a member of the C/EBP family of transcription factors. The second protein, purified by DNA-affinity chromatography, had an apparent molecular mass of 95 kDa and bound to a larger DNA motif that included the AhRE sequence, in AhRE3 and AhRE5 (sites D and A), but not in AhRE1 or AhRE2 (sites C and E). This protein was not AhR nor was it ARNT, since it was found in receptorless (Ahr-) and in nuclear translocation-defective (Arnt-) cells, as well as in cells that had not been exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin), a potent inducer of Cyp1a1 expression. Evidence from in vivo methylation protection indicated that two G residues flanking AhRE3, one of which is required for binding of the 95-kDa protein, may be protected from methylation in uninduced cells and become exposed upon dioxin treatment, suggesting that the 95-kDa protein may be constitutively bound to AhRE3, and be displaced by binding of the Ah receptor complex. These results lend support to the concept that the transcriptional regulation of the [Ah] battery genes could be modulated by combinatorial interactions of the Ah receptor complex with other transcription factors.

Genetic variation in cytochrome P-450-dependent demethylation in Drosophila melanogaster

The genetic variation in the basal capacity to N-demethylate aminopyrine, d-benzphetamine and ethylmorphine was studied in microsomes from adult Drosophila of 9 different strains. Ethylmorphine and d-benzphetamine N-demethylase activity varied about fourfold between the strains, with the highest capacity for both reactions in the Aflatoxin B1-sensitive Florida 9 and the lowest in the insecticide-resistant Hikone R. The two activities were closely correlated with each other but not with aminopyrine demethylation or any previously studied cytochrome P-450-dependent reaction, indicating a common determination by a separate cytochrome P-450 form(s). Aminopyrine N-demethylase activity was more than fourfold higher in the DDT-resistant Oregon R than in Berlin K. A genetic analysis of aminopyrine N-demethylation revealed that the high activity in the Oregon R(R) strain was inherited as an apparently semidominant second chromosome trait. The similar mode of inheritance as well as the close correlation between aminopyrine demethylase and the previously analysed biphenyl 4-hydroxylase activity suggests that these activities are under the same genetic control.

Characterization of the in vitro stability of the rat hepatic receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The in vitro stability of the Ah receptor from rat hepatic cytosol was evaluated by [3H]TCDD binding studies, gel filtration, and sucrose density gradient ultracentrifugation. Thermal inactivation of unoccupied receptor followed first-order kinetics between 5 and 40 degrees C, with an estimated Ea for inactivation of approximately 35 kcal/mol. Protease inhibitors did not reduce and dilution slightly increased the inactivation rate at 20 degrees C. Recovery and 20 degrees C stability decreased with increasing ionic strength. The TCDD-receptor complex was less susceptible to degradation at 20 degrees C, even in the presence of 0.4 M KCl. Specific binding was markedly pH dependent, with maximum recovery at 7.6. Analysis of the pH curve suggested that cysteine sulfhydryl groups may be involved in TCDD binding. Dithiothreitol (1 mM) maximized recovery and 20 degrees C stability, and addition of the thiol largely reactivated binding sites lost from cytosol prepared without it. Removal of low molecular weight components of cytosol by gel filtration resulted in a rapid 20 degrees C inactivation rate that could not be lessened by dithiothreitol. Glycerol (10% v/v) and EDTA (1.5 mM) maximized recovery of specific binding, but both decreased 20 degrees C stability in a concentration-dependent manner. Calcium chloride (4 mM) increased stability at 20 degrees C by approximately 20%, and retarded the characteristic shift in sedimentation coefficient from approximately 9 to approximately 6 S in high-salt sucrose gradients. The fact that sodium molybdate (20 mM) decreased recovery and 20 degrees C stability when dithiothreitol was present but slightly increased stability in its absence suggested an antagonism between the two compounds. Molybdate mitigated the inactivation induced by 0.4 M KCl, an effect which may be related to the observation of dual peaks in molybdate-containing high-salt sucrose gradients. These data indicate that thermal inactivation of the unoccupied rat hepatic Ah receptor primarily may be due to physical rather than enzymatic processes; (ii) sulfhydryl oxidation, removal of low molecular weight cytosolic components, and high ionic strength result in rapid rates of inactivation at 20 degrees C; and (iii) the large degree of protection conferred by TCDD binding implies a very tight ligand-receptor interaction, and as such accords with TCDDs extraordinary potency and persistence in producing its toxic and biochemical effects.

Species and tissue distribution of the 4S carcinogen binding protein and its possible role in cytochrome P-450 induction

A carcinogen binding protein (CBP) that is implicated in controlling the expression of rat cytochrome P-450c which is closely associated with aryl hydrocarbon hydroxylase (AHH) was examined in hepatic and extrahepatic tissues of the neonatal and adult New Zealand White (NZW) rabbits, the hepatic tissue of the BALB/cJ and DBA/2J mice, Brown Norway rat, Golden Syrian hamster and Hartley guinea pig. These animals and tissues were examined in order to determine whether there was a correlation of CBP levels and the reported presence or absence of inducibility of AHH in these tissues. The CBP was found in hepatic and extrahepatic tissue of the NZW rabbit and the hepatic tissues of all animals except the Hartley guinea pig. The Hartley guinea pig may provide a useful animal with which to further examine the role of CBP in cytochrome induction. Since the CBP is not a tissue specific protein and because it is found in both neonatal and adult NZW rabbit tissue, the data suggests that the CBP is not the limiting factor in the tissue specific induction of cytochromes nor in developmentally controlled induction of cytochromes previously reported in the rabbit.

Effects of pretreatment with 2,3,7,8-tetrachlorodibenzofuran on microsomal monooxygenase activity in Drosophila melanogaster

The effects of tetrachlorodibenzofuran pretreatment was studied in microsomes from adult Drosophila of the Karsnäs 60w strain. Pretreatment for 18 h with 0.2 or 1 mg TCDBF did not increase benzo[alpha]pyrene monooxygenase activity, 7-ethyoxyresorufin deethylase activity or the formation of 2-, 3- or 4-hydroxylated metabolites from biphenyl, in accordance with previous results with alpha-naphthoflavone pretreatment. The results suggest that adult Drosophila is non-responsive to induction by polycyclic aromatic hydrocarbons. The role of the TCDD receptor, which has been reported to be present in Drosophila, is obscure against this background. The implications for mutagenicity testing in Drosophila is discussed.