Implication of the "4S" polycyclic aromatic hydrocarbon binding protein in the transregulation of rat cytochrome P-450c expression

The multi-functional roles of GNMT in toxicology and cancer

Although glycine N-methyltransferase (GNMT) has been discovered for five decades, its function was not elucidated until recently. In this review, we discuss the multiple roles of GNMT in toxicology and cancer. Besides catalyzing the production of methylglycine (sarcosine) in one carbon metabolism pathway, GNMT was found to be able to bind a number of polycyclic aromatic hydrocarbons and inhibit DNA adducts formation. Moreover, GNMT exerts protective effects against the cytotoxicity and carcinogenicity of benzo(a)pyrene and aflatoxin B(1) in vitro and in vivo. Occupational study showed that workers who had genotypes with higher GNMT promoter activity may have lower content of oxidative damaged DNA products in their urine. In terms of cancer, recent studies using GNMT knockout mouse models demonstrated that GNMT deficiency has high penetrance in inducing the development of steatohepatitis and hepatocellular carcinoma. In terms of the mechanism, besides dysregulation of epigenetic modification, insights have been provided by recent identification of two novel proteins interacting with GNMT-DEPTOR and NPC2. These studies suggest that GNMT not only is involved in mTOR signaling pathway, but also plays an important role in the intracellular trafficking of cholesterol. The implication of these findings to the preventive medicine and translational research will be discussed.

Characterization of reduced expression of glycine N-methyltransferase in cancerous hepatic tissues using two newly developed monoclonal antibodies

Glycine N-methyltransferase (GNMT) is a protein with multiple functions. Recently, two Italian siblings who had hepatomegaly and chronic elevation of serum transaminases were diagnosed to have GNMT deficiency caused by inherited compound heterozygosity of the GNMT gene with missence mutations. To evaluate the expression of GNMT in cell lines and tissues from hepatocellular carcinoma (HCC) patients, we produced two monoclonal antibodies (mAbs) 4-17 and 14-1 using two recombinant GNMT fusion proteins. M13 phage peptide display showed that the reactive epitopes of mAbs 4-17 and 14-1 were amino acid residues 11-15 and 272-276 of human GNMT, respectively. The dissociation constants of the binding between GNMT and mAbs were 1.7 x 10(-8) M for mAb 4-17 and 1.8 x 10(-9) M for mAb 14-1. Both mAbs can identify GNMT present in normal human and mouse liver tissues using Western blotting (WB) and immunohistochemical staining assay (IHC). In addition, WB with both mAbs showed that none of 2 hepatoblastoma and 5 HCC cell lines expressed GNMT. IHC demonstrated that 50% (13/26) of nontumorous liver tissues and 96% (24/25) of HCC tissues did not express GNMT. Therefore, the expression of GNMT was downregulated in human HCC.

The 4S benzo(a)pyrene-binding protein is not a transcriptional activator of Cyp1a1 gene in Ah receptor-deficient (AHR -/-) transgenic mice

In an effort to better understand the role of the 4S benzo(a)pyrene-binding protein in the induction of CYP1A1 by PAHs, we used a genetically engineered mouse line deficient in Ah receptor (AHR -/-). First, we demonstrated through binding experiments analyzed by sucrose gradient sedimentation and gel permeation chromatography that AHR -/- mice have no detectable AHR protein. In contrast, this AHR-deficient line expressed a 4S protein which efficiently binds BP as it does in hepatic cytosol from C57BL/6 mice. In vivo BP exposure in AHR-deficient mice proved the inability to sustain any CYP1A1 mRNA or CYP1A1 protein induction. These findings demonstrate the requirement of an active AHR to sustain the transactivation pathway leading to CYP1A1 induction. Surprisingly, the 4S BP-binding protein, which was previously characterized as the glycine N-methyltransferase, was completely devoid of such an enzymatic activity after purification by Sephacryl gel permeation chromatography. Moreover, sedimentation and chromatographic experiments, under nondenaturing conditions, do not support the assumption of 4S protein as a subunit of a multimeric protein (GNMT) displaying a molecular mass of 150 kDa.

Glycine N-methyltransferase is an example of functional diversity. Role as a polycyclic aromatic hydrocarbon-binding receptor

The cytochrome P-4501A1 (CYP1A1) gene is regulated by several trans-acting factors including the 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein, which has recently been identified as glycine N-methyltransferase (GNMT) (Raha, A., Wagner, C., Macdonald, R. G., and Bresnick, E. (1994) J. Biol. Chem. 269, 5750-5756). The role of GNMT as a 4 S PAH-binding protein in mediating the induction of cytochrome P-4501A1 has been investigated further. GNMT cDNA, which was cloned into a pMAMneo vector containing the Rous sarcoma virus promoter and the neomycin resistance gene, was stably transfected into D422 Chinese hamster ovary (CHO) cells. Several positive clones were selected by reverse transcription-polymerase chain reaction and assayed for the expression of recombinant protein. Western blot analysis indicated the expression of significant levels of the 4 S protein in the stably transfected CHO cells (CHO-GNMT). Cytosolic preparations from the CHO-GNMT showed high benzo[a]pyrene (B[a]P) binding but no 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) binding activity when compared with clones transfected with the pMAMneo vector alone (CHO-neo) or the parental CHO cells. Challanging the CHO-GNMT cells with 4 microM B[a]P resulted in elevated levels of CYP1A1 mRNA. Equally effective in inducing CYP1A1 mRNA were benzo[e]pyrene and 3-methylcholanthrene. On the other hand, TCDD did not induce CYP1A1 gene expression in these cells. B[a]P-treated CHO-GNMT, expressing the 4 S protein, also showed CYP1A1 protein by Western blotting and exhibited ethoxyresorufin-O-deethylase activity; neither the CHO-neo or parental CHO cells were positive for any of these measures. No Ah receptor message or protein was detectable in the parental CHO, CHO-neo, or CHO-GNMT cells. Furthermore, no XRE binding activity was observed in TCDD-treated cytosolic preparations or nuclear extracts from CHO-GNMT cells that were treated with TCDD. These studies unequivocally establish that GNMT is a PAH-binding protein that can mediate the induction of CYP1A1 by PAHs such as B[a]P through an Ah receptor-independent pathway.

Role of the aromatic hydrocarbon receptor in the suppression of cytochrome P-450 2C11 by polycyclic aromatic hydrocarbons

The aromatic hydrocarbon (AH) receptor mediates the induction of cytochromes P-450 (CYP) of the CYP1A subfamily caused by polycyclic aromatic hydrocarbons (PAHs). CYP1A induction by PAHs is accompanied by down-regulation of CYP2C11, the predominant CYP expressed constitutively in the liver of male rats. We performed a structure-activity relationship study with a series of PAHs of the anthracene class in order to determine if the AH receptor is involved in CYP2C11 down-regulation. Anthracene, benz[a]anthracene, dibenz[a,c]anthracene, dibenz[a,h]anthracene, 7,12-dimethylbenz[a]anthracene, as well as 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3-methylcholanthrene decreased CYP2C11 immunoreactive protein levels to varying degrees in primary rat hepatocytes cultured on a laminin-rich extracellular matrix. The binding affinity of the PAHs for the rat liver cytosolic AH receptor correlated with the potency for transforming the cytosolic AH receptor to its DNA-binding form. In addition, the ability of the PAHs to suppress CYP2C11 correlated with both the AH receptor binding affinity and the AH receptor transformation potency. These results suggest that the AH receptor plays a role in the down-regulation of CYP2C11 caused by PAHs.

ATP depletion affects the phosphorylation state, ligand binding, and nuclear transport of the 4 S polycyclic aromatic hydrocarbon-binding protein in rat hepatoma cells

In the rat, cytochrome P4501A1 gene expression is thought to be regulated by several trans-acting factors including the 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein. Phosphorylation and dephosphorylation have been suggested to influence the function of many cytosolic receptors and transcription factors. The ATP level within H4IIE rat hepatoma cells could be depleted by treatment with sodium azide or 2,4-dinitrophenol; restoration of the original ATP levels occurred with addition of glucose to the cell culture. ATP depletion reduced the phosphate content of the 4 S protein by approximately 25-30%, which lowered the binding of benzo[a]pyrene (B[a]P) to the 4 S protein by >60%. This effect could not be reversed by the addition of ATP to the binding reaction mixtures. Alkaline phosphatase treatment of the purified 4 S protein in a cell-free system also reduced the B[a]P binding to the protein. Cells treated with a protein phosphatase inhibitor, okadaic acid, and a protein kinase inhibitor, staurosporin, affected the B[a]P binding of the 4 S protein positively and negatively, respectively. These data suggested that phosphorylation is involved in the interaction of the 4 S protein with the PAH. The nuclear translocation of the predominantly cytosolic binding protein has been investigated after ligand binding. Western blots with the immunopurified 4 S PAH-binding protein from cytosolic and nuclear lysates showed significant differences in the distribution of the 4 S receptor between cytosolic and nuclear compartments in control and ATP-depleted cells. Ligand binding stimulated the movement of the receptor into the nucleus, which was completely blocked by reducing the intracellular ATP concentration. These findings provide new information on the role of ATP and phosphorylation on the interaction of B[a]P with 4 S PAH-binding protein and its nuclear translocation.

Partial purification and characterization of a rat liver polychlorinated biphenyl (PCB) binding protein

A protein capable of specifically binding polychlorinated biphenyls (PCB) was partially purified from rat liver cytosol. After labeling with [3H]2,2',4,4',5,5'-hexachlorobiphenyl (6-CB), protein enrichment was guided by monitoring the protein-bound radioactivity through a sequence of purification steps including ion exchange chromatography and preparative gel electrophoresis. In addition, specific binding tests of individual fractions were carried out. An average 100-fold enrichment of the 40 kDa protein was achieved. A variety of ligands were tested in competitive binding studies with 6-CB. Whereas penta- and hexachloro-PCB congeners are high affinity competitors, the 3,3',4,4'-tetrachlorobiphenyl congener does not compete for 6-CB binding. Studies on the species and tissue distribution suggest a prevalence of the binding protein in tissues of the rat. Since the natural physiological ligand of the protein has not yet been identified, the function of the protein can only be speculated on.

Induction of cytochrome P450IA1 and its recombinant construct in H4IIE rat hepatoma cells

1. Our previous studies have shown that benzo(a)pyrene (BP), 3-methylcholanthrene (3MC) and tetrachlorodibenzofuran (TCDBF) can induce the expression of the cytochrome P450IA1 mRNA in the rat hepatoma cell line, H4IIE, although the kinetics of induction differed. 2. In the present study, by using biochemical, immunochemical and recombinant DNA approaches, the effects of these inducers have been examined on the steady state level of endogenous cytochrome P450IA1 protein and on induction of chloramphenicol acetyltransferase activity (CAT) in the H4IIE cells transfected with pMC1CAT (a recombinant construct consisting of CAT linked to 5' upstream DNA sequence of the rat cytochrome P450IA1 gene). 3. From 7-ethoxyresorufin O-deethylase activity (EROD) and immunochemical analysis of cytochrome P450IA1, the optimal concentrations of BP, 3MC and TCDBF for induction in the H4IIE cells were determined as 1, 0.1-1 and 0.1 microM, respectively. 4. The elevated expression of the protein was more sustained in the TCDBF-exposed cells than in the BP or 3MC-treated cells. 5. After 1.5 hr of treatment, little if any detectable P450IA1 protein was observed in the H4IIE cells although a considerable amount of mRNA was present. 6. In addition, no cytochrome P450IA2 protein was detected in the control or induced H4IIE cells. 7. H4IIE cells were transfected by pMC1CAT, and the induction ratio of CAT expression in the transfected H4IIE cells after BP, 3MC or TCDBF treatment was 10-, 17- and 40-fold, respectively. 8. These results indicate that the rat H4IIE cell line offers a valid homologous system for studies of the regulation of the rat cytochrome P450IA1 gene.

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.

Induction of CYP1A1 gene expression in H4-II-E rat hepatoma cells by benzo[e]pyrene

In the rat, expression of the CYP1A1 gene is closely associated with arylhydrocarbon hydroxylase (AHH) enzyme activity. AHH is an inducile enzyme activity known to play an important role in the bioactivation of polycyclic aromatic hydrocarbons (PAHs) to mutagenic and carcinogenic metabolites. PAH-induced expression of the CYP1A1 gene appears to be regulated by several trans-acting factors, including the Ah receptor and the 4S PAH-binding protein. In this study, we used the PAH isomers benzo[a]pyrene (BaP) and benzo[e]pyrene (BeP) to further evaluate the role of the 4S PAH-binding protein in induction of the CYP1A1 gene in H4-II-E rat hepatoma cells. Although BaP is believed to bind to both the Ah receptor and the 4S protein, BeP has been reported to bind exclusively to the 4S protein. The results of the study presented here indicate that BaP and BeP induce the expression of the CYP1A1 gene, as measured by ethoxyresorufin O-deethylase (EROD) activity, in a concentration-dependent manner. However, BaP is about 25 times as potent as BeP in inducing EROD activity in these cells. Slot-blot analysis of total RNA isolated from these cells indicated that BeP, BaP, and 3-methylcholanthrene increased the level of CYP1A1 mRNA expression. Sucrose-gradient analysis of BeP binding activity indicated that BeP bound with high affinity to the 4S PAH-binding protein, but not to the Ah receptor. These results suggest that the 4S protein may play a role in the PAH-induced expression of the CYP1A1 gene in rat H4-II-E cells.

Mechanism of benzo[a]pyrene-induced Cyp1a-1 gene expression in mouse Hepa 1c1c7 cells: role of the nuclear 6 s and 4 s proteins

Treatment of wild-type (wt) aryl hydrocarbon (Ah)-responsive mouse Hepa 1c1c7 cells with benzo[a]pyrene (B[a]P) caused a concentration-dependent induction of ethoxyresorufin O-deethylase (EROD) activity. In contrast, B[a]P was inactive as an inducer in Ah nonresponsive class 1 and class 2 mutant cell lines. In parallel experiments, the nuclear fractions from wt cells treated with 10(-7) M [3H]B[a]P contained both the 4 s carcinogen binding protein and the 6 s (Ah receptor) complexes, whereas only the 4 s complex was present in the nuclear fraction of the class 2 mutant cells. The results obtained from cotreatment of wt Hepa 1c1c7 cells with 10(-6) or 10(-7) M B[a]P and 5 x 10(-7) or 10(-7) M 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) showed that MCDF inhibited the induction of EROD activity and Cyp1a-1 mRNA levels by B[a]P. Moreover, using 10(-7) M [3H]B[a]P and unlabeled MCDF, it was shown that MCDF not only inhibited the induction response but also caused a concentration-dependent decrease in levels of the nuclear 6 s complex but not the 4 s complex. In contrast, in situ competition studies with unlabeled 10(-6) M benzo[ghi]-perylene (B[ghi]P) resulted in the elimination of the nuclear [3H]B[a]P 4 s complex (but not the 6 s complex); however, the EROD activity and Cyp1a-1 mRNA levels in cells treated with 10(-7) M B[a]P in the presence or absence of 10(-6) M B[ghi]P were not significantly different. These results indicate that the 4 s binding protein is not required for the induction of Cyp1a-1 gene expression in Hepa 1c1c7 cells and suggest that B[a]P and 2,3,7,8-tetrachlorodibenzo-p-dioxin induce Cyp1a-1 gene expression via a common mechanism which involves binding to the Ah receptor.

Ah receptor mediating induction of cytochrome P450IA1 in a novel continuous human liver cell line (Mz-Hep-1). Detection by binding with [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin and relationship to the activity of aryl hydrocarbon hydroxylase

The Ah receptor regulates induction of cytochrome P450IA1 and mediates certain toxicities of polyhalogenated aromatics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). It has been characterized previously in continuous cell lines, notably the mouse hepatoma line Hepa 1, the human squamous cell carcinoma line A431, and the human liver cell line Hep G2. The present work extends our knowledge of the Ah receptor in continuous human liver cell lines. Ah receptor can be detected in Mz-Hep-1, a hepatitis B virus-negative cell line derived from a Thorotrast-induced hepatocellular carcinoma. The mean concentration of Ah receptor in Mz-Hep-1 cells was 341 +/- 22 fmol/mg cytosol protein (mean +/- SEM, nine separate determinations). This is equivalent to approximately 30,000 sites per cell. The concentration of Ah receptor in Mz-Hep-1 cells is similar to that in Hepa 1 cells and approximately three times higher than that in Hep G2 cells. The Mz-Hep-1 Ah receptor sedimented in continuous sucrose gradients at approximately 9 S. Specificity of binding by [3H]TCDD was demonstrated by competitive binding of non-radiolabeled 2,3,7,8-tetrachlorodibenzofuran, 3-methylcholanthrene (MC), and dibenz[a,h]anthracene in 50-fold molar excess. Phenobarbital, which is not a substrate for P450IA1, did not compete with [3H]TCDD for binding to Mz-Hep-1 Ah receptor. Dexamethasone and estradiol also did not compete with [3H]TCDD for binding, suggesting non-identity of Ah receptor with glucocorticoid or estrogen receptor. In separate experiments, glucocorticoid receptor was identified in Mz-Hep-1 cells. By Scatchard plot analysis, the apparent equilibrium dissociation constant (Kd) for binding of [3H]TCDD to Mz-Hep-1 Ah receptor was estimated to be 4.4 nM, compared to 0.8 nM in Hepa 1 cells. By Woolf plot analysis the Kd was 5.4 nM, compared to 1.2 nM in Hepa 1 cells. The [3H]TCDD.Ah receptor complex extracted from nuclei of Mz-Hep-1 cells incubated with [3H]TCDD in culture at 37 degrees sedimented at approximately 6 S under conditions of high ionic strength. Aryl hydrocarbon hydroxylase (AHH) activity was detectable in Mz-Hep-1 cells after pretreatment with inducing chemicals. Mz-Hep-1 cells have the highest concentrations of Ah receptor in any continuous human liver cell line thus far investigated. The Mz-Hep-1 Ah receptor is similar physicochemically to that described in murine systems. AHH activity is inducible in Mz-Hep-1 cells.

Induction of cytochrome P450IA1 in rat hepatoma cell by polycyclic hydrocarbons and a dioxin

In this study, the effects of benzo[a]pyrene (BP), 3-methylcholanthrene (3MC) and tetrachlorodibenzofuran (TCDBF) on the expression of the cytochrome P450IA1 gene in the rat hepatoma cell line H4IIE were examined. The initial rate of increase in the steady-state concentration of the mRNA for this gene was similar with each of these inducers; however, the elevated level of this mRNA was more sustained after TCDBF treatment. Nuclear run-off assays suggested that the elevated level of the mRNA was caused principally by an effect upon transcription.

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.

Characterization of the Ah receptor mediating aryl hydrocarbon hydroxylase induction in the human liver cell line Hep G2

The Ah receptor, a soluble cytoplasmic receptor that regulates induction of cytochrome P450IA1 and mediates toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was detected and characterized in the continuous human liver cell line Hep G2. The mean concentration of specific binding sites for TCDD was 112 +/- 26 (SEM) fmol/mg cytosol protein as determined in eight separate cytosol preparations in the presence of sodium molybdate. This is equivalent to 14,000 binding sites per cell, approximately 40% of the sites per cell found in the mouse hepatoma line Hepa-1. The cytosolic Ah receptor from Hep G2 cells sedimented at 9 S and was specific for those halogenated and nonhalogenated aromatic compounds known to be agonists for the Ah receptor in rodent tissues and cells. Specific binding in the 9 S region was detected with both [3H]TCDD and 3-[3H]methylcholanthrene. 3-[3H]Methylcholanthrene did not bind to any component besides that at approximately 9 S. Phenobarbital, dexamethasone, and estradiol did not compete with [3H]TCDD for binding to the Hep G2 Ah receptor. Specific binding of [3H]triamcinolone acetonide to glucocorticoid receptor could also be demonstrated in Hep G2 cytosol. The apparent equilibrium dissociation constant (Kd) for binding of [3H]TCDD to Hep G2 Ah receptor was 9 nM by Woolf plot analysis, about an order of magnitude weaker than the affinity of [3H]TCDD for the mouse Hepa-1 Ah receptor or for the C57BL/6 murine hepatic Ah receptor. [3H]TCDD.Ah receptor complex, which was extracted from nuclei of Hep G2 cells incubated with [3H]TCDD at 37 degrees C in culture, sedimented at approximately 6 S under conditions of high ionic strength. Aryl hydrocarbon hydroxylase (AHH) activity was significantly induced after 24 h of incubation with polycyclic aromatic hydrocarbons: the EC50 for AHH induction was 5.3 microM for benz(a)anthracene and 1.3 microM for 3-methylcholanthrene. Modification of the preparative technique for cell cytosol, especially inclusion of 20 mM sodium molybdate in homogenizing and other buffers, was necessary to detect cytosolic Hep G2 Ah receptor. Hep G2 cells appear to conserve drug-metabolizing activity associated with cytochrome P450IA1 as well as the receptor mechanism which regulates its induction.

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.

«4S» Polycyclic Aromatic Hydrocarbon Binding Protein. Its Role as a Benzo(a) Pyrene Cytosolic Carrier to the Microsomes of DBA/2N Mouse Liver

Rodent liver cytosol and other biological systems contain two proteins that bind polycyclic aromatic hydrocarbons (PAH) in a non covalent manner and that sediment at a different rate when centrifuged on sucrose gradient. The role of the smaller protein (« 4S » PAH-BP) was studied. When DBA/2N mouse liver homogenate was incubated with3H-BaP, most of the radioactivity was found in the microsomal subcellular fraction. The cytosol binding activity apparently decreased but reincubation of the cytosol with the radioactive ligand completely restored « 4S » PAH-BP activity. The microsomal uptake of3H-BaP can be studied in a reconstituted system in which microsomes are incubated with radioactive benzo(a)pyrene in the presence of crude cytosol. In these conditions the microsomal uptake rate of3H-BaP increased with the temperature and at 37 °C ten minutes were required to reach the plateau. When cytosol was substituted by HEDG buffer, the amount of radioactivity found in the microsomes decreased drastically. 0.2 μM was the benzo(a)pyrene concentration required to saturate the microsomes. When microsomes were incubated with ammonium sulfate cytosolic fractions or with homogeneously purified « 4S » PAH-BP, the3H-BaP uptake was restored and reached the maximum with 3 μg/ml of purified protein. The radioactive benzo(a)pyrene bound to microsomes was oxidated in the presence of NADPH regenerating system. The oxidated products were discharged from microsomes only when « 4S » PAH-BP was either present during the incubation or added at its end. Thus, this protein is able to transfer benzo(a)pyrene to the microsomal metabolization sites and to facilitate the release of oxidized products and, presumably, bind them.

Radioligand-dependent differences in the molecular properties of the mouse and rat hepatic aryl hydrocarbon receptor complexes

A comparison of the molecular properties of the male Long-Evans rat and male C57BL/6 mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor complex was determined using 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-[3H]tetrachlorodibenzofuran (TCDF) as radioligands. In low salt buffer, the sedimentation coefficients, Stokes radii, relative molecular masses, frictional ratios, axial ratios and gel permeation chromatographic properties of the rat receptor complexes were ligand independent. In contrast, there were several ligand-dependent differences in the mouse Ah receptor complexes formed after incubation in low salt buffer and these include: sucrose density gradient analysis of the 2,3,7,8-[3H]TCDF receptor complex gave a 9.5 S specifically bound peak and a 2.6 S nonspecifically bound peak whereas the corresponding 2,3,7,8-[3H]TCDD receptor complex gave a single 9.6 S specifically bound peak; sucrose density gradient analysis of the two major peaks eluted from a Sephacryl S-300 column chromatographic separation of the 2,3,7,8-[3H]TCDF receptor complex gave two specifically bound peaks at 9.2 and 5.1 S. The molecular properties of the rat hepatic cytosolic receptor complexes incubated in high salt (0.4 M KCl) buffer were ligand independent with one exception, namely the significant difference in the sedimentation coefficient of the specifically bound disaggregated 2,3,7,8-[3H]TCDD receptor complex (6.8 S) and the corresponding 2,3,7,8-[3H]TCDF receptor complex (5.0 S). The major ligand-dependent differences in the mouse receptor complexes incubated in high salt (0.4 M KCl) were associated with the sedimentation coefficients of the complexes derived after direct incubation and after gel permeation chromatography. For example, both ligands gave two specifically bound complexes after chromatography on Sephacryl S-300 column and centrifugation of these fractions gave both the approximately 9 and approximately 5 S peaks; this suggested that there was some equilibration between the aggregated and disaggregated receptor complexes. The behavior of the 2,3,7,8-[3H]TCDF mouse receptor complex was similar after incubation in low or high salt buffer except that sucrose density gradient analysis of the gel permeation chromatographic fractions gave an additional specifically bound peak which sedimented at 7.2 S. These studies demonstrate that the molecular properties of the Ah receptor were dependent on the source of the cytosolic receptor preparation, the ionic strength of the incubation media, and the structure of the radioligand.

The 4S binding protein acts as a trans-regulator of the polycyclic hydrocarbon-inducible cytochrome P450

A model has been proposed for the induction of cytochrome P450c in liver by polycyclic hydrocarbons such as benzo(a)pyrene (BaP) and 3-methylcholanthrene (3MC). The polycyclic hydrocarbon interacts in specific, saturable, and high-affinity fashion with a rat liver cytosolic 4s binding protein. The latter enters the nucleus, complexes to 5' upstream regions of the cytochrome P450c gene, and stimulates the transcription. The 4s binding protein has been purified from rat liver and its substrate specificity has been determined. The affinity for 3MC or BaP is 1-2 mM. The binding protein has been demonstrated to complex with specific 5'-upstream regions of the P450c gene by using a filtration assay as well as exonuclease footprinting. In addition, the binding protein stimulates in vitro transcription with upstream regions of the P450c gene as template; these data confirm the hypotheses.

Binding characteristics of Ah receptors from rats and mice before and after separation from hepatic cytosols. 7-Hydroxyellipticine as a competitive antagonist of cytochrome P-450 induction

The Ah receptor, a soluble protein implicated in the mechanism of action of the toxic halogenated aryl hydrocarbons has been examined in rodent livers. Due to the difficulty of making reliable quantitative determinations on binding parameters for hydrophobic compounds in cytosols that contain several components, Ah receptors from livers of Sprague-Dawley rats and C57BL/6 mice have been separated, in a preparative manner, using sucrose density gradient centrifugation in a vertical rotor. The binding characteristics of Ah receptors, before and after separation, were assessed by competition of various chemicals as 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzofuran, 3-methylcholanthrene, benzo[a]pyrene, beta-naphthoflavone and ellipticines with [3H]3-methylcholanthrene and 2,3,7,8-tetrachloro[3H]dibenzo-p-dioxin as radioligands. The rationale of this approach is supported by the results obtained and the major conclusions are as follows. 1. The intrinsic binding characteristics of Ah receptors were dependent on the presence or absence of other cytosolic binding components (light-density component and 4-S carcinogen-binding protein). 2. In contrast with many previous unsuccessful attempts, the separation of the C57BL/6 Ah receptor allowed the unambiguous detection of a 9-S binding peak with [3H]benzo[a]pyrene as a radioligand. 3. The intrinsic binding characteristics of the separated Ah receptors of Sprague-Dawley rats and C57BL/6 mice were similar if not identical. 4. A good correlation exists between the competitive potency (IC50) of chemicals and their ability to induce aryl hydrocarbon hydroxylase activity, except for 7-hydroxyellipticine which binds to the Ah receptors of rat and mouse liver (IC50 approximately 5-10 microM) without inducing aryl hydrocarbon hydroxylase. 5. When coadministered with various inducers, 7-hydroxyellipticine antagonizes (from about 20% to 65%) the inducing ability of chemicals displaying similar (ellipticines) or weaker (chlorpromazine, phenothiazine) binding affinities for the Ah receptor.

Binding characteristics of 4-S proteins from rat and mouse liver. High affinity of ellipticines

The binding characteristics of 4-S components (carcinogen-binding protein) from livers of Sprague-Dawley rats, C57BL/6 and DBA/2 mice have been examined before and after separation from other binding components presents in the cytosol. Competitive potency of 3-methylcholanthrene, benzo[alpha]pyrene, beta-naphthoflavone and 20 ellipticines, a series of compounds differently substituted on the dimethyl-pyrido-carbazole nucleus and deprived of carcinogenic activity, has been determined with [3H]3-methylcholanthrene and/or [3H]benzo[a]pyrene as radioligands. The inducing ability of the same compounds for aryl hydrocarbon hydroxylase and for ethoxyresorufin-O-deethylase has been compared to their affinity for the 4-S protein and the Ah receptor respectively. The main results of this study are as follows. 1. The intrinsic binding characteristics of 4-S proteins were dependent on both the nature of the radioligand used and the presence or absence of other cytosolic binding components. 2. The heterocyclic ellipticines were revealed as strong ligands for the carcinogen-binding protein (stronger than benzo[alpha]pyrene for five derivatives substituted in the A ring of ellipticine), with IC50 values ranging from 0.047 microM (8-hydroxyellipticine) to 5.8 microM (N2-ethyl-9-hydroxyellipticinium). 3. When the affinity of ellipticines was plotted versus their inducing ability of aryl hydrocarbon hydroxylase and ethoxyresorufin-O-deethylase, it appears that a good correlation exists for the Ah receptor but not for the 4-S protein. It is concluded that these data, as well as the lack of enzymatic induction after benzo[alpha]pyrene treatment of DBA/2 mice, which display a high level of 4-S protein, do not support the implication of this binding component in the positive control of cytochrome P-450 induction.

The 4-5S carcinogen binding protein binds 1-aminopyrene, 1-nitropyrene and (+)-benzo[a]pyrenediolepoxide and is found in 12 tissues of the DBA/2J mouse

Twelve tissues from the BDA/2J mouse were examined in order to assess the tissue distribution of a soluble 4-5S carcinogen binding protein (CBP). The CBP binds numerous polycyclic aromatic hydrocarbons, regulates the expression of cytochrome P-450c and is a carrier protein of carcinogen metabolites. The highest specific binding was found in hepatic tissue (600 fmol per mg protein). The amount of CBP in the kidney and testis was about 60% of the value observed for the liver. Lower levels of CBP were observed in the reproductive tract (uterus and vagina), spleen, stomach, heart, urinary bladder, skin, large and small intestine and lung. It was shown that the CBP bound the environmental carcinogen 1-nitropyrene, its reduced metabolite 1-aminopyrene and the ultimate carcinogenic metabolite of benzo[a]pyrene or (+)-r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene but not the smaller coplanar PCB molecule, 3,3',4,4'-tetrachlorobiphenyl. The results suggest that (a) the CBP is not a tissue-specific protein in the mouse, (b) the ligands for the CBP are not limited to the unmetabolized procarcinogens but include both positively and negatively charged ligands, and (c) in vitro the CBP covalently binds the ultimate carcinogenic metabolite of benzo[a]pyrene.

Immunosuppression following exposure to 7,12-dimethylbenz[a]anthracene (DMBA) in Ah-responsive and Ah-nonresponsive mice

Recent reports suggest that the immunotoxicity of certain polycyclic aromatic hydrocarbons is associated with the Ah locus in mice. To test whether immunosuppression mediated by 7,12-dimethylbenz[a]anthracene (DMBA) is regulated by the Ah locus, several endpoints of immune function were measured in Ah-responsive B6C3F1 and Ah-nonresponsive DBA/2N and in Ah-congenic C57BL/6J (responsive B6-AhbAhd and nonresponsive B6-AhdAhd) mice dosed sc with up to 100 micrograms/g DMBA in corn oil. Some groups of B6C3F1 and DBA/2N mice were exposed to 100 micrograms/g benzo[a]pyrene (B(a)P) or 1 nmol 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for determination of hepatic microsomal monooxygenase activity. The body weights of all mice were unaffected by DMBA exposure, but thymus weights and spleen cellularity were decreased. Antibody plaque-forming cells (PFC) measured 4 days after iv sheep erythrocyte (SRBC) immunization were suppressed 99% in B6C3F1 and 96% in DBA/2 mice. Antibody PFC after in vitro immunization to SRBC were similarly suppressed 98% in both B6-AhbAhd and B6-AhdAhd Ah-congenic mice exposed to 100 micrograms/g DMBA. Responses to the T-cell mitogens concanavalin A and phytohemagglutinin were significantly suppressed in both B6C3F1 and DBA/2N strains, as was mitogenesis to bacterial lipopolysaccharide. The unidirectional mixed lymphocyte responses of the congenic strains were suppressed 76% in B6-AhbAhd and 85% in B6-AhdAhd, cytotoxic lymphocyte generation was suppressed 68% in B6-AhbAhd and 78% in B6-AhdAhd. The overall differences between immunosuppressive responses in splenocytes from B6-AhbAhd and B6-AhdAhd congenics were not significant. Induction of cytochrome P1-450, a marker of Ah responsiveness, was determined by 7-ethoxycoumarin O-deethylase monooxygenase activity in hepatic microsomes or splenocytes. This monooxygenase activity was not significantly increased in either B6C3F1 or DBA/2 mice exposed to DMBA, whereas B(a)P and TCDD exposure significantly induced enzyme activity in B6C3F1 hepatocytes. These data suggest that DMBA has an immunosuppressive action on murine splenocytes which is independent of the Ah locus and associated induction of cytochrome P1-450 xenobiotic-metabolizing enzymes.

Interaction of the 4 S polycyclic aromatic hydrocarbon-binding protein with the cytochrome P-450c gene

The induction of cytochrome P-450c, the isozyme most closely associated with aryl hydrocarbon hydroxylase activity in the rat, is mediated through a cytosolic polycyclic aromatic hydrocarbon (PAH)-binding protein(s). We have reported on the purification and characterization of a 4 S protein that interacts in a specific and saturable manner with [3H]benzo[a]pyrene and other PAHs. (W. H. Houser et al. (1985) Biochemistry 24, 7839-7845). We have also reported on the specific and saturable interaction of the 4 S protein with a plasmid containing 1.9 kbp of cloned rat P-450c sequence including exon 1, the 5' half of intron 1, and approximately 882 bp upstream information. Our investigations now show that incubation of this protein with a portion of the rat P-450c gene, followed by digestion with either lambda exonuclease or exonuclease III, tentatively identified two protected regions at -225 and -455 bp 5' from exon 1. To further study the significance of these protected regions, a 3.4-kbp fragment containing cytochrome P-450c promoter and 5'-upstream sequences (-882 to +2545) was fused to the chloramphenicol acetyl transferase (CAT) reporter gene and transfected into either rat epithelial RL-PR-C cells or rat hepatoma H-4-II-E cells. Both cell lines expressed CAT activity in response to induction by 3-methylcholanthrene (3MC), indicating that important regulatory regions responsive to 3MC are present in these constructs. However, neither cell line expressed CAT activity in response to 3MC when transfected with plasmids containing deletions (-95 to -724 or -240 to -720) in the regions shown to be protected by our footprinting studies. These results corroborate previous studies which indicated that the 4 S PAH-binding protein interacts in a specific manner with regions of the rat cytochrome P-450c gene. We conclude that the 4 S protein may play an important role in the regulation of expression of cytochrome P-450c in the rat.

Synergy of phenobarbital and 3-methylcholanthrene in "superinduction" of cytochrome P-450c mRNA but not enzyme activity

The combination of phenobarbital and 3-methylcholanthrene in the inductive process of the rat hepatic cytochrome P-450c gene was evaluated. Daily injections of phenobarbital (80 mg/kg, i.p.) had little or no effect on the amount of poly (A)+ RNA encoding cytochrome P-450c, whereas a single injection of 3-methylcholanthrene (25 mg/kg, i.p.) produced a significant accumulation at 15 hr in cytosolic mRNA coding for cytochrome P-450c. Four daily injections of phenobarbital followed by a single dose of 3-methylcholanthrene produced 5-24 times more poly (A)+ RNA coding for P-450c than 3-methylcholanthrene treatment alone. This superinduction of RNA transcripts was also observed for a species coding for cytochrome P-450d, which was increased 3-6 times over 3-methylcholanthrene treatment alone. However, the elevated concentration of transcripts for both the P-450d and P-450c RNA species did not result in an increase in the marker enzyme activity for cytochrome P-450c, 7-ethoxyresorufin O-deethylase. These data implicate a regulatory step in the induction of cytochrome P-450c enzyme activity which must occur at a level beyond transcription.

Multiple constitutive but phenobarbital-inducible rat hepatic nuclear RNA species homologous to a novel cytochrome P-450 cDNA

A cDNA clone, pPB8, representing partial information for a phenobarbital-inducible rat hepatic cytochrome P-450, immunochemically related to cytochrome P-450b and/or P-450e, hybridized to multiple hepatic nuclear RNA species. In addition to the 3.7 +/- 0.2 kb mRNA encoding this novel cytochrome P-450 isozyme, pPB8 hybridized to nuclear RNAs of 4.9 +/- 0.3, 5.4, 5.7 +/- 0.2, and 6.3 +/- 0.1 kb. These nuclear RNAs were constitutively expressed and were inducible to various extents by phenobarbital administration. The time course of induction of these nuclear RNA components suggested product-precursor relationships. A "full-length" cDNA clone, pPB8/7, synthesized from poly(A)+ RNA homologous to pPB8, detected two mRNA species of 4.6 and 1.8 kb. The 4.6 kb nuclear RNA was inducible by 3-methylcholanthrene, Aroclor 1254, and phenobarbital, while the 1.8 kb nuclear RNA was not appreciably affected. It is suggested that pPB8 and pPB8/7 were synthesized from distinct mRNAs that share homology in their 3' regions.

Structural details of the human cytochrome P-450c gene

Aryl hydrocarbon hydroxylase activity is most closely associated with cytochrome P-450c in the rat and cytochrome P1-450 in the mouse. The sequence for the orthologous human gene coding for this enzymatic activity has been determined from several sources: cytochrome P-450c isolated from human embryonic DNA [K. Kawajiri, J. Watanabe, O. Gotoh, Y. Tagashiri, K. Sogawa, and Y. Fujii-Kuriyama (1986) Eur. J. Biochem. 139, 219-225], human lymphocytes in our own laboratory, and cytochrome P1-450 isolated from the established human breast carcinoma cell line, MCF-7 [A.K. Jaiswal, F. J. Gonzalez, and D. W. Nebert (1985) Nucleic Acids Res. 13, 4503-4520]. The data from our laboratory agree well with the sequence derived from human embryonic DNA, but differs significantly from that reported for the gene isolated from MCF-7 cells. Among these differences are a 320-bp insert and a 650-bp deletion in intron 1 relative to the sequence derived from the established cell line. We observe two mRNA species that hybridize to cytochrome P-450c probes, one expected at 2.7 kb and an additional 2.0-kb species. Finally, we note additional hybridization bands in 11% of the population examined by Southern blot analysis, representing either a second rare allele or, more likely, a duplication of at least a portion of the cytochrome P-450c gene.

«4 S» Polycyclic Aromatic Hydrocarbon Binding Protein: Further Characterization and Kinetic Properties

A protein that binds polycyclic aromatic hydrocarbons (PAHs) with high affinity and sediments in a sucrose gradient at 4 S has been described in rat liver cytosol. This « 4 S » PAH binding protein precipitates at a 40–60% ammonium sulfate saturation. This partial purification procedure allows assay of this protein by using purified3H-benzo(a)pyrene (3H-BaP) as radioactive ligand and dextran-coated charcoal as adsorbent for unreacted3H-BaP. The3H-BaP binding activity measured as a function of pH shows its maximum activity between pH 7.3 and 10.5. The « 4 S » PAH binding protein is stable up to 42 °C even in the absence of the ligand. At 65 °C the binding sites for3H-BaP are destroyed. The binding activity assayed as a function of protein concentration is linear between 0.4 and 2 mg/ml at 0 °C, whereas at 37 °C higher protein concentrations (4 mg/ml) can be reached. Exposure to guanidine-HCl (3 M) and urea (5 M) for 20 min at 4 °C inhibits the PAH binding completely to the « 4 S » protein. Quick dilution or dialysis does not restore the binding activity. The dissociation rate of the « 4 S » PAH binding protein measured in the presence of an excess of unlabeled ligand at 0 °C is biphasic and shows a two-step, first-order kinetic pattern. At 37 °C the dissociation rate is linear and faster, and is complete after 5 min of incubation. The association rate shows the same behavior: the binding is complete after 10 min at 0 °C, whereas at 37 °C the reaction is 10 times as fast. The dissociation equilibrium constants at 0 °C and 37 °C are respectively 2.45 × 10−9M and 1.09×10−9M. The high rates of association and dissociation of BaP to « 4 S » PAH binding protein were used to set up an assay to exchange radioactive3H-BaP with cold BaP.

Binding of polynuclear aromatic hydrocarbons to the rat 4S cytosolic binding protein: structure-activity relationships

The relative competitive binding affinities of benzo[a]pyrene (B[a]P), benzo[e]pyrene, benzo[g, h, i]perylene, picene, 7,12-dimethylbenz [a]anthracene, 1,2,3,4-dibenz[a]anthracene, 1,2,5,6-dibenz[a]anthracene, perylene, 4H-cyclopenta[d,e,f]-phenanthrene, benz[a] anthracene, triphenylethylene and triptycene for the rat hepatic cytosolic 4S binding protein were determined using [3H]benzo[a]pyrene as the radioligand. With the exception of triphenlethylene, triptycene and 4H-cyclopenta[d,e,f]phenanthrene, the EC50 values for the remainder of these compounds were between 1.25 X 10(-7) and 2.5 X 10(-8) M with 1,2,5,6-dibenz[a]anthracene being the most active ligand. A comparison of the relative cytosolic Ah (9S) receptor binding affinities and aryl hydrocarbon hydroxylase (AHH) induction potencies of these hydrocarbons with their 4S protein binding affinities demonstrated the following: five compounds, namely 1,2,5,6-dibenz[a]-anthracene, 1,2,3,4-dibenz[a]anthracene, picene, benzo[a]pyrene and 3-methylcholanthrene exhibited high to moderate binding affinities for the 4S and 9S cytosolic proteins (EC50 values less than 10(-6) M) and induced AHH in rat hepatoma cells; three compounds, namely perylene, benzo[e]pyrene and benzo[g,h,i]perylene exhibited high affinities for the 4S binding protein (1.25 X 10(-7), 4.4 X 10(-8) and 2.9 X 10(-8) M, respectively) and low affinities (EC50 values greater than 10(-5) M) for the Ah receptor protein; moreover these three compounds did not induce AHH in rat hepatoma H-4-II E cells in culture. These data suggest that the 4S binding protein may not play a significant role in AHH induction although the results do not rule out a function for this protein in the transregulation of AHH and its associated cytochromes P-450.

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.

A regulatory polymorphism for rat hepatic cytochrome P-450g

Rat hepatic cytochrome P-450g is a male-specific hemoprotein found at significant levels only in adult animals. In the present study, two-dimensional gel electrophoretic and immunochemical methods were used to study a polymorphism of this isozyme and its ontogenetic regulation. Inbred ACI/Hsd and WF/Hsd rats were found to express high and low levels of cytochrome P-450g, respectively. F1 hybrids of these strains showed additive inheritance for this trait and the responsible gene was found to be autosomal. Cytochrome P-450g and another male-specific form of the enzyme, cytochrome P-450h, were characterized by a similar time-course for their ontogenetic expressions. However, unlike cytochrome P-450g, the level of cytochrome P-450h was indistinguishable in hepatic microsomes from mature ACI/Hsd and WF/Hsd rats. Considering these results, we tentatively conclude that the gene regulating the level of cytochrome P-450g is Cis-acting.

Further characterization of the polycyclic aromatic hydrocarbon binding properties of the 4S protein

A 4-S protein which specifically binds [3H]benzo(a)pyrene and other polycyclic aromatic hydrocarbons has been investigated in the rat using a hydroxylapatite assay and sucrose gradient analysis. Although there was significant interanimal variation, the specific polycyclic aromatic hydrocarbon binding activity appeared to be highest in 4-week-old male rats and declined with age. The specific [3H]benzo(a)pyrene binding activity was induced after pretreatment with either phenobarbital or isosafrole as evidenced by a 72 and 61% increase, respectively, over untreated controls. No apparent increase in specific binding activity was observed after pretreatment of animals with 3-methylcholanthrene. Pretreatment with either phenobarbital or isosafrole also resulted in the appearance of a small, nonspecific, benzo(a)pyrene binding peak at the 8- to 9-S region in the sucrose density gradients. This 8-S peak was not seen in untreated control animals and represented low affinity, high capacity binding sites. In contrast to the 8-S protein, the 4-S binding protein had low affinity for polychlorinated aromatic compounds such as tetrachlorodibenzodioxin and tetrachlorodibenzofuran. The addition of a 200-fold excess of tetrachlorodibenzofuran to incubations did not displace [3H]benzo(a)pyrene from the 4-S protein. The addition of sodium molybdate to isolation buffers, known to stabilize certain hormone receptors, did not alter the sedimentation coefficient or the specific binding activity of the 4-S protein. These experiments indicate that the 4-S protein does not appear to be a subunit of the 8-S protein. We conclude that in the rat the 4-S protein is distinct from the 8-S protein and the 4-S species may regulate the polycyclic aromatic hydrocarbon-induced expression of aryl hydrocarbon hydroxylase activity.

Regulation of cytochrome P-450c by glucocorticoids and polycyclic aromatic hydrocarbons in cultured fetal rat hepatocytes

The actions of polycyclic aromatic hydrocarbons and glucocorticoids to regulate the synthesis of cytochrome P-450c (the major isozyme induced by polycyclic aromatic hydrocarbons) were investigated in fetal rat hepatocytes maintained in primary monolayer culture. Treatment of hepatocytes in culture with 1,2-benzanthracene resulted in a 50-fold increase in 7-ethoxycoumarin O-deethylase activity. The level of P-450c increased in the cells in a time-dependent fashion as determined by immunoelectrophoretic analysis. The inductive effect of BA was potentiated approximately 1.6- to 2.3-fold when 1 microM dexamethasone was included in the culture medium. However, dexamethasone alone had little or no effect on the induction of P-450c. The rate of synthesis of P-450c was examined by immunoisolation of the specific isozyme from total cellular proteins radiolabeled with [35S]methionine and from the protein products formed during in vitro translation of the isolated mRNA. In addition, the amount of mRNA specific for cytochrome P-450c was determined by Northern blot analysis of RNA extracted from cultured cells. The changes in the rates of synthesis and mRNA levels were found to parallel the changes in enzyme activity. The concentration of dexamethasone required to cause a half-maximal increase in P-450c content in the presence of 1,2-benzanthracene was between 10(-8) and 10(-7) M. It is concluded that glucocorticoids act synergistically with polycyclic aromatic hydrocarbons to increase the levels of P-450c expressed in the fetal rat liver, and that this action is likely mediated by the classical type II glucocorticoid receptor.