Flavin-containing monooxygenase-3: induction by 3-methylcholanthrene and complex regulation by xenobiotic chemicals in hepatoma cells and mouse liver

Abstract

Flavin-containing monooxygenases often are thought not to be inducible but we recently demonstrated aryl hydrocarbon receptor (AHR)-dependent induction of FMO mRNAs in mouse liver by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Celius et al., Drug Metab Dispos 36:2499, 2008). We now evaluated FMO induction by other AHR ligands and xenobiotic chemicals in vivo and in mouse Hepa1c1c7 hepatoma cells (Hepa-1). In mouse liver, 3-methylcholanthrene (3MC) induced FMO3 mRNA 8-fold. In Hepa-1 cells, 3MC and benzo[a]pyrene (BaP) induced FMO3 mRNA >30-fold. Induction by 3MC and BaP was AHR dependent but, surprisingly, the potent AHR agonist, TCDD, did not induce FMO3 mRNA in Hepa-1 cells nor did chromatin immunoprecipitation assays detect recruitment of AHR or ARNT to Fmo3 regulatory elements after exposure to 3MC in liver or in Hepa-1 cells. However, in Hepa-1, 3MC and BaP (but not TCDD) caused recruitment of p53 protein to a p53 response element in the 5'-flanking region of the Fmo3 gene. We tested the possibility that FMO3 induction in Hepa-1 cells might be mediated by Nrf2/anti-oxidant response pathways, but agents known to activate Nrf2 or to induce oxidative stress did not affect FMO3 mRNA levels. The protein synthesis inhibitor, cycloheximide (which causes "superinduction" of CYP1A1 mRNA in TCDD-treated cells), by itself caused dramatic upregulation (>300-fold) of FMO3 mRNA in Hepa-1 suggesting that cycloheximide prevents synthesis of a labile protein that suppresses FMO3 expression. Although FMO3 mRNA is highly induced by 3MC or TCDD in mouse liver and in Hepa-1 cells, FMO protein levels and FMO catalytic function showed only modest elevation.

Fig. 1

Upregulation of FMO2 and FMO3 mRNA levels by 3MC in livers of adult male mice. Mice received injections of 80 mg/kg 3MC. Livers were removed after 6 h and mRNA levels were measured by real-time qPCR as described in Materials and Methods. mRNA levels are expressed relative to the level of that particular FMO mRNA in the control animals set at 1.0. Plotted bars in all figures represent mean ± S.D.; n=3 per group. **p<0.01 compared to vehicle control.

Fig. 2

In vivo recruitment of AHR and ARNT to the Cyp1a1 enhancer region after 3MC exposure. Adult male mice were injected with 80 mg/kg 3MC or the corn oil vehicle (control). Livers were removed 6 h later. ChIP assays were performed as described under Materials and Methods using antibodies to AHR and ARNT as well as to IgG (negative control). Plotted bars represent mean ± S.D.; n=3 per group. **p<0.01 compared to vehicle control.

Fig. 3

Chemical selectivity of FMO3 mRNA induction in Hepa1c1c7 mouse hepatoma cells. Cells were cultured as described under Materials and Methods and exposed to 1 μM 3MC, BaP, BNF or PCB126, or to 10 nM TCDD, 50 μM BHA, 10 μM menadione, 20 μM SUL or t-BHQ for 24 h. Control cells were exposed to DMSO (solvent control) at a concentration in the medium of less than 1%. Cells were harvested and total RNA was analyzed by real-time qPCR as described in Materials and Methods. Cells were treated in duplicate and the entire experiment was performed twice. Plotted bars represent mean ± S.D. for data combined from the two experiments (n=4). *p <0.05; **p<0.01 compared to solvent control.

Fig. 4

Time-dependent induction of FMO3 and CYP1A1 mRNA. Cells were exposed to 1 μM 3MC, BaP, BNF, PCB126, or 10 nM TCDD for 6, 12 or 24 h. Cells were harvested and mRNA levels were measured by real-time qPCR as described in Materials and Methods. Cells were treated in duplicate and the entire experiment was performed twice. Plotted bars represent mean ± S.D. for data combined from the two experiments. *p <0.05; **p<0.01 compared to solvent control.

Fig. 5

Dose-dependent induction of mRNAs for FMO3 and CYP1A1. Cells were exposed to varied concentrations of 3MC or to a single concentration of TCDD (10 nM) and harvested 24 h after exposure. Cells were treated in duplicate and the entire experiment was performed twice. Plotted bars represent mean ± S.D. for data combined from the two experiments. Plotted bars show mean ± S.D. *p <0.05; **p<0.01 compared to solvent control.

Fig. 6

Induction of FMO3mRNA by cycloheximide and superinduction of CYP1A1 mRNA by combined exposure to cycloheximide plus 3MC. Cells were pre-exposed to 10 μg/ml of CHX for 1 h, then 1 μM 3MC was added and cells were incubated for a further 23 hours in the continued presence of CHX and 3MC. Cells were harvested and mRNA levels were measured by real-time qPCR as described in Materials and Methods. *p<0.05; **p<0.01 compared to solvent control.

Fig. 7

Effect of 3MC treatment on recruitment of AHR or ARNT to Cyp1a1 and Fmo3 enhancer regions in Hepa1c1c7 cells. Cells were preincubated for 1 h with or without 10 μg/ml cycloheximide (CHX), then incubated for 4 h with 1 μM 3MC or the DMSO vehicle. ChIP assays were preformed in triplicate as described in the Materials and Methods. Significant differences compared to matched DMSO samples are represented by an asterisk (p< 0.01), whereas significant difference between 3MC-treated matched samples in the presence or absence of CHX are shown by the number sign (p<0.01).

Fig. 8

Treatment with 3MC or BaP recruits p53 to a p53 response element (p53RE) in the 5'-flanking region of the Fmo3 gene. Hepa1c1c7 cells were exposed to TCDD (10 nM), 3MC (1 μM), BP (1 μM), or the vehicle (DMSO) for 15h. ChIP assays were performed as described in Material and Methods using antibodies to IgG (2 μg per reaction) and p53 (3 μg per reaction). Primers were designed for the p53RE (−4885 to −4982) shown in the diagram. Results are reported as percentage enrichment of p53 binding to the p53RE of the total input and compared to the DMSO IgG control. Plotted bars represent mean ± S.D. for data combined from three experiments (n=3). *p <0.05; **p<0.01 compared to DMSO control. Panel A is a positive control which demonstrates binding of p53 to a p53 response element (p53RE) in the Cdnk1a gene, a gene that is a known target for regulation by p53. Panel B illustrates recruitment of p53 to a p53RE in the 5'-flanking region of the Fmo3 gene in Hepa-1 cells treated with either 3MC or BaP.

Fig. 8

Treatment with 3MC or BaP recruits p53 to a p53 response element (p53RE) in the 5'-flanking region of the Fmo3 gene. Hepa1c1c7 cells were exposed to TCDD (10 nM), 3MC (1 μM), BP (1 μM), or the vehicle (DMSO) for 15h. ChIP assays were performed as described in Material and Methods using antibodies to IgG (2 μg per reaction) and p53 (3 μg per reaction). Primers were designed for the p53RE (−4885 to −4982) shown in the diagram. Results are reported as percentage enrichment of p53 binding to the p53RE of the total input and compared to the DMSO IgG control. Plotted bars represent mean ± S.D. for data combined from three experiments (n=3). *p <0.05; **p<0.01 compared to DMSO control. Panel A is a positive control which demonstrates binding of p53 to a p53 response element (p53RE) in the Cdnk1a gene, a gene that is a known target for regulation by p53. Panel B illustrates recruitment of p53 to a p53RE in the 5'-flanking region of the Fmo3 gene in Hepa-1 cells treated with either 3MC or BaP.

Fig. 9

Postulated mechanism of FMO3 mRNA induction by 3MC and BaP. (Adapted from Mathieu et al. (Mathieu et al., 2001).)

Fig. 10

Catalytic activity of mouse liver microsomes for FMO substrates after treatment with TCDD in vivo. Mice were treated with TCDD (30 μg/kg) and liver was harvested 24 h later. Enyzme activity was measured with ethionamide (ETA) and sulindac sulfide (SS) as substrates as described in section 2.7. Plotted bars represent mean ± S.D.; n=3 for control males, TCDD-treated males and TCDD-treated females; only one female control was available. *p<0.05 compared to vehicle control.

Fig. 11

FMO3 protein content in mouse liver after treatment with TCDD in vivo. The ACQA/mass spectrometry method described in section 2.8 was used to quantitate the FMO3 protein content in microsomes from the same mice shown in Fig. 10.