Kinetic and immunochemical studies of a receptor-like protein that binds aromatic hydrocarbons

Reduced cytochrome P4501A activity and recovery from oxidative stress during subchronic benzo[a]pyrene and benzo[e]pyrene treatment of rainbow trout

This study assessed the role of aryl hydrocarbon receptor (AHR) affinity, and cytochrome P4501A (CYP1A) protein and activity in polyaromatic hydrocarbon (PAH)-induced oxidative stress. In the 1-100nM concentration range benzo[a]pyrene (BaP) but not benzo[e]pyrene (BeP) competitively displaced 2nM [(3)H]2, 3, 7, 8-tetrachloro-dibenzo-p-dioxin from rainbow trout AHR2α. Based on appearance of fluorescent aromatic compounds in bile over 3, 7, 14, 28 or 50days of feeding 3μg of BaP or BeP/g fish/day, rainbow trout liver readily excreted these polyaromatic hydrocarbons (PAHs) and their metabolites at near steady state rates. CYP1A proteins catalyzed more than 98% of ethoxyresorufin-O-deethylase (EROD) activity in rainbow trout hepatic microsomes. EROD activity of hepatic microsomes initially increased and then decreased to control activities after 50days of feeding both PAHs. Immunohistochemistry of liver confirmed CYP1A protein increased in fish fed both PAHs after 3days and remained elevated for up to 28days. Neither BaP nor BeP increased hepatic DNA adduct concentrations at any time up to 50days of feeding these PAHs. Comet assays of blood cells demonstrated marked DNA damage after 14days of feeding both PAHs that was not significant after 50days. There was a strong positive correlation between hepatic EROD activity and DNA damage in blood cells over time for both PAHs. Neither CYP1A protein nor 3-nitrotyrosine (a biomarker for oxidative stress) immunostaining in trunk kidney were significantly altered by BaP or BeP after 3, 7, 14, or 28days. There was no clear association between AHR2α affinity and BaP and BeP-induced oxidative stress.

Preliminary array analysis reveals novel genes regulated by ovarian steroids in the monkey raphe region

We hypothesize that ovarian hormones may improve serotonin neuron survival. We sought the effect of estradiol (E) and progesterone (P) on novel gene expression in the macaque dorsal raphe region with Affymetrix array analysis. Nine spayed rhesus macaques were treated with either placebo, E or E+P via Silastic implant for 1 month prior to euthanasia (n=3 per treatment). RNA was extracted from a small block of midbrain containing the dorsal raphe and examined on an Agilent Bioanalyzer. The RNA from each monkey was labeled and hybridized to an Affymetrix HG_U95AV Human GeneChip Array. After filtering and sorting, 25 named genes remained that were regulated by E, and 24 named genes remained that were regulated by supplemental P. These genes further sorted into functional categories that would promote neuronal plasticity, transmitter synthesis, and trafficking, as well as reduce apoptosis. The relative abundance of four pivotal genes was examined in all nine animals with quantitative RT-PCR and normalized by glyceraldehyde 3-phosphate dehydrogenase (GAPDH). E+/-P caused a significant threefold reduction in JNK-1 (a pro-apoptosis gene, p<0.007); and a significant sixfold decrease in kynurenine mono-oxygenase (produces neurotoxic quinolones, p<0.05). GABA-A receptor (alpha3 subunit; benzodiazepine site) and E2F1 (interferes with cytokine signaling) were unaffected by E, but increased sevenfold (p<0.02) and fourfold (p<0.009), respectively, upon treatment with P. In summary, subsets of genes related to tissue remodeling or apoptosis were up- or down-regulated by E and P in a tissue block containing the dorsal raphe. These changes could promote cellular resilience in the region where serotonin neurons originate.

Acquired resistance to Ah receptor agonists in a population of Atlantic killifish (Fundulus heteroclitus) inhabiting a marine superfund site: in vivo and in vitro studies on the inducibility of xenobiotic metabolizing enzymes

New Bedford Harbor (NBH), MA, is a federal Superfund site that is heavily contaminated with polychlorinated biphenyls (PCBs) and other halogenated aromatic hydrocarbons (HAHs), including some potent aryl hydrocarbon receptor (AhR) agonists. A population of Atlantic killifish (Fundulus heteroclitus) continues to inhabit this site, despite accumulating extraordinarily high concentrations of PCBs (272 microg/g dry weight). To determine if NBH killifish have developed resistance to HAHs that act through the AhR, we examined the inducibility of cytochrome P4501A1 (CYP1A1), UDP glucuronosyl transferase (UGT), and glutathione S-transferase (GST) in fish from NBH and a reference site, Scorton Creek (SC, Cape Cod, MA; PCB concentrations 0.177 microg/g dry weight). 2,3,7,8-Tetrachlorodibenzofuran (TCDF) induced CYP1A1 mRNA, protein, and activity in SC fish in all tissues examined (liver, heart, gut, gill, kidney, spleen, and gonad). In contrast, NBH fish expressed low levels of CYP1A1 and showed no induction of CYP1A1 mRNA, protein, or activity by TCDF, or induction that was lower in magnitude or required higher doses of inducer. p-Nitrophenol UGT activity was not induced by TCDF in either population, while GST activity with 1-chloro-2,4-dinitrobenzene as substrate was induced only in NBH fish in one experiment. Inducibility of CYP1A1 by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or beta-naphthoflavone (BNF) was measured in primary hepatocyte cultures prepared from SC and NBH fish. TCDD induced CYP1A1 activity (ethoxyresorufin O-deethylase) to the same degree in hepatocytes from both populations, demonstrating the functionality of the AhR signaling pathway in NBH fish. However, hepatocytes from NBH fish were 14-fold less sensitive to TCDD than were those from SC fish. The nonhalogenated AhR agonist BNF also induced CYP1A1 in cells from both populations, although with only a 3-fold difference in sensitivity (NBH < SC). These results indicate that chronic exposure to high levels of HAHs has led to a reduction in the sensitivity of NBH killifish to AhR agonists. The resistance is systemic and pretranslational, and exhibits compound-specific differences in magnitude. These findings suggest an alteration in the AhR signal transduction pathway in NBH fish.

Arachidonic acid metabolism in the marine fish Stenotomus chrysops (Scup) and the effects of cytochrome P450 1A inducers

Cytochrome P450-mediated arachidonic acid (AA) metabolism was investigated in the marine fish scup, Stenotomus chrysops. Liver microsomes incubated with AA and NADPH produced epoxyeicosatrienoic acids (EETs) and their hydration products (dihydroxyeicosatrienoic acids, DHETs), midchain conjugated dienols (midchain HETEs), and C16-through C20-alcohols of AA (omega-terminal HETEs), all identified by HPLC and GC/MS. Gravid females had 4-fold lower AA metabolism rates than males but identical metabolite profiles. The 5,6-EET (inferred from stable metabolites) was most abundant (47% of total EETs) followed by 14,15-, 11,12-, and 8,9-EET (27, 13, and 13%, respectively). The 12-HETE represented 25% of total HETEs followed in abundance by 16-, 15-, 11-, 19-, 20-, 8-, and 9-HETE. Antibodies against scup CYP1A and a scup CYP2B-like protein inhibited liver microsomal AA metabolism by 30 and 46%, respectively. GC/MS analysis revealed EETs and DHETs as endogenous constituents in scup liver; the predominant EETs were 8,9- and 14,15-EET, followed by a lesser amount of 11,12-EET. Chiral analysis showed a preference for the S,R-enantiomers of endogenous 8,9-, 11,12-, and 14,15-EET (optical purities 80, 64, and 64%, respectively). Treatment of scup with the CYP1A inducer benzo(a)pyrene (BP) increased liver microsomal formation of EETs and HETEs by 2.7-fold in spring and 1.7-fold in summer. BP treatment did not affect microsomal EET regioselectivity, but shifted hydroxylation in favor of 19-HETE and induced 17-HETE formation. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) treatment in summer did not induce liver microsomal AA metabolism rates, yet BP and TCDD both increased endogenous EET content of liver (5- and 3-fold, respectively), with a shift to 14,15-EET. BP treatment increased the selectivity for the S,R-enantiomers of endogenous 8,9-, 11,12-, and 14,15-EET (optical purities 91, 84, and 83%, respectively). Kidney, gill, and heart microsomes all metabolized AA, at rates 10- to 30-fold less than liver microsomes. Similar amounts of endogenous 8,9- and 14,15-EET and less 11,12-EET were detected in heart and kidney, and there was a strong enantioselectivity for 8(R),9(S)-EET in heart (optical purity 78%) but not in kidney. BP treatment did not alter the total EET content in these organs but did shift the regiochemical profile in heart to favor 14,15-EET. Thus, scup liver and extrahepatic organs metabolize AA via multiple cytochrome P450 (CYP) forms to eicosanoids in vitro and in vivo. BP or TCDD induced endogenous AA metabolism in liver, altering EET regioselectivity and, with BP, stereoselectivity. While AhR agonists alter metabolism of AA in early diverging vertebrates expressing both CYP1A and AhR, the magnitude of effects may depend upon the type of inducer.

Rat liver 4S-benzo[a]pyrene-binding protein is distinct from glycine N-methyltransferase

Rat liver 4S-benzo[a]pyrene-binding protein (BAP) was reported to be identical to the subunit (Mr 32,500) of the tetrameric glycine N-methyltransferase (GNMT). We have reevaluated this study. When a liver cytosol was subjected to Sephacryl S-200 gel permeation chromatography, enzyme activity as well as cross-reactivity with anti-GNMT antibody were found only at the elution position of the purified GNMT. Chromatograph of the cytosol pretreated with [3H]benzo[a]pyrene showed two peaks in the void volume and at the position of an approximate Mr of 40,000. The latter, corresponding to the 4 S BAP, did not cross-react with the anti-GNMT antibody. An extract of nuclei in which GNMT was proposed to act as a mediator of cytochrome P4501A1 gene expression contained the tetrameric GNMT but no binding activity. The lung, in which no GNMT mRNA occurred, had the 4 S BAP. Extracts of Escherichia coli and COS-7 cells expressing large amounts of GNMT showed no 4 S BAP. These findings suggest that the 4 S BAP is distinct from the subunit of GNMT.

Characterization of the 4 S polycyclic aromatic hydrocarbon-binding protein in human liver and cells

The 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein (PBP) is a soluble protein that binds PAHs with high affinity in mouse, rat, and rabbit. Until now, this protein had not been detected in human placenta or human cells in culture by cytosol labeling and gradient centrifugation assay. Thanks to a preliminary fractionation of cytosol by sedimentation on sucrose gradient or/and gel permeation chromatography, we found that PBP was present in liver, MCF-7 cell line, and hepatocytes of human. To accurately quantitate PBP binding and determine specific binding parameters, a reduction in the amount of charcoal used to adsorb nonspecifically bound benzo[a]pyrene was required. By saturation analysis, the concentration of specific binding sites for [3H]BP in PBP fraction from human liver was 4.6 pmol/mg of protein compared with 14.7 +/- 1.4 pmol/mg in the same fraction from DBA/2J mouse liver. Kinetic studies analyzed by Scatchard and Woolf plots indicate that human liver and MCF-7 cells contain a low-affinity PBP form: the Kd derived from Woolf plot analysis were 14.2 +/- 1.4 and 26.2 +/- 1.8 nM, respectively. DBA/2J mouse possesses a higher-affinity PBP form, the same analysis indicating a Kd of 6.1 +/- 0.3 nM. These data demonstrate that, by comparison to the mouse liver, a lower-affinity form of PBP is present in reduced concentration in human liver, explaining the impossibility of detecting this protein by sedimentation of human cytosol in sucrose gradient.

Benzo[e]pyrene elicits changes in the biochemical activities and chromatographic behavior of murine hepatic cytochromes P-450 that are distinct from those induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The objective of this study was to examine the potential for a specific ligand of carcinogen binding protein (CBP) to induce changes in the overall character of hepatic microsomal cytochromes P-450 (P450) and to compare potential changes with those induced by an Ah receptor ligand. Benzo[e]pyrene (BeP) was previously shown to bind CBP with high affinity and Ah receptor with low affinity. In contrast, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) binds Ah receptor avidly and CBP weakly. Hepatic microsomes were prepared from C57BL/6J (B6) and DBA/2J (D2) mice treated with corn oil, BeP or TCDD. Relative to corn oil controls, pretreatment of B6 mice with BeP or TCDD increased the nmol P450/mg microsomal protein content 26 and 28%, respectively. In D2 mice, nmol P450/mg microsomal protein was increased 23% in the BeP pretreatment, while TCDD pretreatment had no effect relative to the corn oil controls. For the O-alkyl ethers of resorufin, rates of metabolism (per nmol P450) were affected differently in B6 and D2 by BeP pretreatment. Pentoxyresorufin O-dealkylase activity was reduced to 44% of control activity in B6 mice and increased 39% relative to controls in D2 mice. BeP pretreatment had no effect on ethoxyresorufin O-dealkylase activity in B6 mice, while this activity was decreased to 58% of controls in D2 mice. Additionally, benzyloxyresorufin O-dealkylase activity was reduced to 65% of control levels in B6 mice and not affected in D2 mice. Methoxyresorufin O-dealkylase activity was reduced in both strains to an average of 55% of control values. As expected, TCDD pretreatment resulted in increases of all O-dealkylations measured in both strains of mouse. For both inbred strains of mouse, anion exchange chromatography revealed a P450 peak associated with BeP pretreatment that was not present in chromatograms generated with corn oil or TCDD pretreatments. Results of enzyme linked immunosorbant assays also indicated that the pattern of P450 isoenzyme expression associated with BeP pretreatment was distinct from that associated with TCDD pretreatment. Overall, these data show that treatment with a specific ligand of CBP induces changes the biochemical activities and chromatographic behavior of P450 isozymes in murine hepatic microsomes. Moreover, they indicate that changes in P450 occurring after treatment with a CBP ligand are distinct from those changes that are associated with treatment with an Ah receptor ligand (TCDD). Differences between B6 and D2 strains suggest that the hepatic P450 changes occurring in response to pretreatment with a CBP ligand may be influenced by the presence of Ah receptor.

Presence of the 4 S polycyclic hydrocarbon-binding protein in H4-II-E cells

The 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein had been implicated in regulating the expression of rat cytochrome P450IA1 which is most closely associated with aryl hydrocarbon hydroxylase (AHH). We have now investigated the presence of both the 4 S PAH-binding protein and the 8 S Ah receptor in rat hepatoma H4-II-E cells as well as the induction of P450IA1 upon their exposure to PAH's such as benzo[a]pyrene (BP) and 3-methylcholanthrene (3MC), and halogenated dioxins such as 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDBF). Sucrose density gradient analyses and hydroxylapatite assays indicate that, in addition to the 8 S protein, the 4 S PAH binding protein is present in these cells. This protein interacts in a saturable and high affinity manner with BP and 3MC, but not with TCDD or TCDBF. Using a P450IA1 probe, the induction of gene expression was observed by Northern blot analysis of total cellular RNA after exposure of the H4-II-E cells to BP, 3MC, or TCDBF. Since the 4 S protein was observed to interact only with BP and 3MC, these results suggest that this protein may also play a role in the PAH-induced expression of cytochrome P450IA1 gene expression in H4-II-E.

Binding characteristics of 4S PAH-binding protein and Ah receptor from rats and mice

Cytochrome P-450IA1 (Cyto-P450IA1) is the isozyme most closely associated with aryl hydrocarbon hydroxylase (AHH). At least two distinct high-affinity binding proteins may regulate its expression, the 4S protein that primarily binds polycyclic aromatic hydrocarbons (PAHs), and the 8S Ah receptor that binds 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and like congeners. The present study was conducted to investigate ligand binding characteristics of the 4S and 8S binding proteins before and after separation from liver cytosol in the presence and absence of sodium molybdate. Liver cytosol and 4S and 8S receptor-enriched fractions from livers of male Sprague-Dawley rats (AHH-responsive), and from C57BL/6N (AHH-responsive) and DBA/2N and AKR/N mice (AHH-nonresponsive) served as sources of these proteins. Competitive binding studies were performed using 10 nM [3H]benzo[a]pyrene (BaP) or [3H]-TCDD in the presence and absence of a 200-fold excess of BaP, 3-methylcholanthrene (3-MC), and tetrachlorodibenzofuran (TCDBF). Specific PAH-binding activity was assayed by using either sucrose density gradient analysis or a hydroxylapatite assay. Our results indicate that before and after the separation of liver cytosol into 4S and 8S fractions, ligand binding characteristics were relatively unaltered for the 4S protein in comparison to that for the Ah receptor, particularly in the presence of molybdate. The 4S protein had high affinity for BaP and 3-MC but very low affinity for TCDBF; the 8S protein had high affinity for TCDBF, lesser affinity for 3-MC, and low affinity for BaP. In the presence of sodium molybdate, the Ah receptor fractions were significantly stabilized, whereas the 4S protein was relatively unaffected. After the separation of Ah receptor fraction from liver cytosol in the presence of molybdate, 3-MC consistently bound to a greater extent. These results affirm the existence of two distinct PAH-binding proteins.

The effect of polycyclic aromatic hydrocarbons on choline kinase activity in mouse hepatoma cells

Choline kinase catalyzes the first rate-limiting step in the pathway of biosynthesis of phosphatidylcholine. This enzyme was shown previously to be induced in liver by treatment of rats with polycyclic aromatic hydrocarbons (Ishidate et al. (1980) Biochem. Biophys. Res. Commun. 96, 946-952). The present study was undertaken to determine whether choline kinase in the murine hepatoma cell line, Hepa 1c1c7, is inducible by aromatic hydrocarbons and, if so, whether this induction is mediated by the aromatic hydrocarbon receptor. Treatment of Hepa 1c1c7 cells with 10 microM beta-naphthoflavone resulted in a 1.6-fold increase of choline kinase activity, but no response was seen when the cells were exposed to either 5.0 microM benzo[a]pyrene or 1.0 nM 2.3,7,8-tetrachlorodibenzo-p-doxin, both potent inducers of aryl hydrocarbon hydroxylase. Cell line variants with either deficient or elevated aromatic hydrocarbon receptors showed no increase in choline kinase activity following treatment with any of the polycyclic aromatic hydrocarbons. These results are not consistent with a role for the aromatic hydrocarbon receptor in increased choline kinase activity in Hepa 1c1c7 cells.

Purification to Homogeneity of the Major «4S» PAH Binding Protein from «Non Responsive » DBA/2N Mouse Liver by Affinity Chromatography

DBA/2N is a genetically non responsive inbred strain of mice in which administration of polycyclic aromatic hydrocarbons (PAHs) does not induce microsomal monooxygenase activity. DBA/2N mouse liver cytosol contains a polycyclic aromatic hydrocarbon-binding protein that sediments, in a sucrose gradient, at 4S (« 4S » PAH-BP). Its binding kinetic and physicochemical properties indicate that this protein is practically indistinguishable from the « 4S » PAH-BP identified and characterized in liver cytosol of rats and other PAH responsive rodents including C57 B1/6J mice. « 4S » PAH-BP was purified to homogeneity from DBA/2N mouse liver by ammonium sulfate fractionation of the cytosol, followed by Sephadex G-200 chromatography and, finally, affinity chromatography using 1-aminopyrene-Sepharose 6B. This procedure yielded about 50 μg of protein from 50–60 g of mouse liver, with a recovery of 18%. «4S » PAH-BP as a complex with3H-(benzo-a-pyrene) was more than 99% pure. A single band was seen on polyacrylamide gel electrophoresis under non denaturing conditions. H-BaP comigrated with the protein band.3H-BaP bound to the protein was displaced by PAHs with a specificity identical to that obtained using crude cytosol. On electrophoresis in SDS gels, the purified protein migrated as a single protein band with an apparent molecular weight of 40,000.