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

The existence of the 4S polycyclic aromatic hydrocarbon-protein binding in 14-day-old chick embryo liver

Cytochrome P-450IA1, the isozyme most closely associated with aryl hydrocarbon hydroxylase (AHH), is regulated by two high-affinity binding proteins, the 4S polycyclic aromatic hydrocarbon (PAH)-binding protein which primarily binds PAHs and the 8S Ah (dioxin) receptor which binds 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and like congeners. The present study was conducted to determine whether the 4S protein existed in 14-day-old chick embryo liver when AHH activity is maximal to determine if they are linked as is the 8S Ah receptor and to confirm the existence of the dioxin receptor by investigating their ligand binding characteristics in the presence and absence of sodium molybdate, an agent that stabilizes steroid hormone receptors and partially stabilizes the dioxin receptor. Competitive ligand binding studies were performed with liver cytosol from livers of male 14-day-old chick embryos using [3H]-benzo[a]pyrene (B[a]P) or [3H]-TCDD in the presence and absence of a 200-fold excess of B[a]P, benzo[e]pyrene (B[e]P), 3-methylcholanthrene (3-MC), and tetrachlorodibenzofuran (TCDBF). Specific PAH-binding activity was assayed using sucrose gradient analysis. In the absence of molybdate, the 4S PAH-binding protein had high affinity for B[a]P, B[e]P, 3-MC, but very low affinity for TCDBF; the Ah receptor exhibited high affinity for TCDBF. In the presence of sodium molybdate, the Ah receptor was stabilized while the 4S PAH-binding protein was relatively unaffected. These results affirm the existence of two distinct PAH-binding proteins in 14-day-old chick embryo liver cytosol and suggest a linkage of the 4S protein to AHH.

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.

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.