Effect of 3-Methylcholanthrene Administration on Expression of Cytochrome P-450 Isoforms Induced by Phenobarbital in Rat Hepatocytes

Abstract

The effects of an inducer on expression of cytochrome P-450 (P-450) isoforms induced antecedently by another inducer are unknown. Thus, we examined the amount of phenobarbital (PB)-inducible P-450 isoforms (P-450 2B1/2B2) in hepatocytes from rats injected first with PB and then with 3-methylcholanthrene (MC) (PB+MC-treated animals) by quantitative immunohistochemistry. In addition, expression of P-450 2B2 mRNA was examined by in situ hybridization. In PB-treated animals, P-450 2B1/2B2 content increased in perivenular and midzonal hepatocytes. In PB+MC-treated animals, however, the PB-induced increase in 2B1/2B2 content was suppressed in perivenular hepatocytes but promoted in midzonal hepatocytes. The hybridization signal for P-450 2B2 mRNA appeared almost exclusively in perivenular hepatocytes after 24 hr of PB injection and disappeared after 48 hr of injection. In PB+MC-treated animals, however, strong hybridization signal was observed in midzonal and perivenular hepatocytes after 48 hr of PB injection. The promotion of the increase in P-450 2B1/2B2 content in midzonal hepatocytes in PB+MC-treated animals probably corresponds to the strong hybridization signal, whereas there appeared to be a divergence between the intensity of the signal and the content in perivenular hepatocytes. The results indicate that MC administration drastically influences the pattern of expression of P-450 isoforms induced by PB in perivenular and midzonal hepatocytes.

Keywords

cytochrome P-450 2B phenobarbital 3-methylcholanthrene quantitative immunohistochemistry in situ hybridization liver rat

Steroids, xenobiotics, and carcinogens induce synthesis of inducer-specific cytochrome P-450 (P-450) isoforms in the liver (Lu and West 1980; Murray and Reidy 1990). Many P-450 isoforms thus induced are predominantly distributed in perivenular hepatocytes around the central venule (Baron et al. 1981; Wolf et al. 1984; Bühler et al. 1992). Increase in dosage of a single inducer sometimes causes enlargement of the area in which induction of the P-450 isoform occurs, from perivenular to periportal hepatocytes (van Sliedregt and van Bezooijen 1990; Bars and Elcombe 1991).

Because animal systems are usually subject to the actions of various inducers, it is probable that inductions of various types of the P-450 isoform occur one after another in the liver. However, effects of an inducer on expression of the P-450 isoform induced antecedently by another inducer are unknown.

We therefore injected first phenobarbital (PB) and then 3-methylcholanthrene (MC) in rats and examined the amount of P-450 2B1/2B2, major PB-inducible P-450 isoforms, in periportal, midzonal, and perivenular hepatocytes by quantitative immunohistochemistry. In addition, expression of P-450 2B2 mRNA was examined by in situ hybridization. Furthermore, expression of P-450 2B1/2B2 proteins and their mRNA were examined by Western blotting and Northern hybridization.

Materials and Methods

A total of 115 male Sprague-Dawley rats, 8-12 weeks old (300-350 g), were used. The animals were fed laboratory chow and water ad libitum. They were divided into four groups. Animals of one group (PB+MC-treated) received an ip injection of PB dissolved in saline at a dose of 80 mg/kg body weight and then an ip injection of MC dissolved in corn oil at a dose of 25 mg/kg body weight 24 hr after PB injection. Those of the second group (PB-treated animals) received an ip injection of PB (80 mg/kg) and then an ip injection of corn oil instead of MC 24 hr after PB injection. Those of the third group (MC-treated animals) received an ip injection of saline instead of PB and then an ip injection of MC (25 mg/kg) 24 hr after saline injection. Those of the fourth group (control animals) received an ip injection of saline and then an ip injection of corn oil 24 hr after saline injection. The animals were sacrificed under sodium pentobarbital anesthesia 24 hr after the last injection. For in situ hybridization and Northern hybridization, animals injected ip with PB and sacrificed 24 hr after the injection were also used. The animal experiments were performed according to the guidelines of Kansai Medical University.

Immunohistochemical Methods

Under anesthesia, livers of animals (6 animals/group) were briefly perfused with saline, then perfused with cold 4% formaldehyde in 0.1 M phosphate buffer (pH 7.4) for 5 min. Slices cut from the left lobe of the liver were washed at 4C for 6 hr with 0.1 M phosphate buffer (pH 7.4) containing 8% sucrose. Serial frozen sections 6 μm thick were cut and placed on poly-l-lysine-coated glass slides. The thickness of the slides and sections was checked as described previously (Watanabe et al. 1996a,b).

A pair of the serial sections (about 2 × 3 mm) were immersed in 70% (v/v) methanol including 0.3% hydrogen peroxide at room temperature (RT) for 30 min, washed with PBS (three times for 5 min), and soaked for 30 min in normal goat serum (diluted 1:10 with PBS) at RT. Then one section was incubated in PBS containing anti-rat P-450 2B1/ 2B2 goat IgG (diluted 1:200 with PBS; Daiichi Kagaku, Tokyo, Japan) and the other was incubated with normal goat serum (diluted 1:50 with PBS) at 4C for 12 hr (80μl/section). After washing with PBS, the sections were incubated in PBS including horseradish peroxidase (HRP)-labeled rabbit anti-goat IgG (Cappel, West Chester, PA; diluted 1:200 with PBS) at 20C for 30 min. The sections were washed again with PBS and stained with PBS containing 3 mg diaminobenzidine and 0.03% (v/v) hydrogen peroxide at 20C for 20 min. Then the sections were dehydrated and mounted.

Some sections were stained with anti-rat P-450 1A1/1A2 antibody (Daiichi Kagaku) instead of anti-rat P-450 2B1/ 2B2 antibody by the indirect immunoperoxidase method under saturation conditions as described previously (Tanaka et al. 1997).

Microphotometry of P-450 2B1/2B2

Immunostained and mounted sections on the slides were set in a microphotometer (KWSP-1) (Watanabe and Kanamura 1991). Readings were made at 466 nm with a spot size of 4 μm, and the resulting percent transmissions were converted into absorbances. The number of measurements per group was 864 (4 portions/hepatocyte, 6 periportal, midzonal or perivenular hepatocytes/section, 1 section/block, 2 blocks/liver, and 6 animals/group for total or nonspecific absorbance). Then the specific absorbance (total minus nonspecific) was calculated.

Image Analysis of P-450 2B1/2B2

Sections used for microphotometry were subjected to image analysis with a microscope (Axiovert; Carl Zeiss, Oberkochen, Germany) linked to a video image processor (ARGUS 100-VEC; Hamamatsu Photonics, Hamamatsu, Japan) (Watanabe et al. 1994,1996a,b). Microscopic images through an IF-470 interference filter (Nikon; Tokyo, Japan) were transferred to the processor. Average absorbances in widely defined areas of the sections (0.998 mm × 0.998 mm; average staining intensity, antibody-reacted; ASIab) and absorbances in small portions (3.9 × 3.9 μm; staining intensity in portion, antibody-reacted; SIPab) in the cytoplasm of periportal, midzonal, and perivenular hepatocytes were measured. The average absorbance in a widely defined area of the adjacent section incubated with normal goat serum (average staining intensity, normal serum-reacted; ASIns), and the absorbances in corresponding portions in the section (staining intensity in portion, normal serum-reacted; SIPns) were also measured.

Microphotometry of Total Cytochrome P-450

The content of total P-450 in liver sections was measured microphotometrically as described previously (Watanabe and Kanamura 1991; Watanabe et al. 1993a,b). In short, absorbances at 450 nm and 490 nm in reduced and carbon monoxide-bound spectra of P-450 were measured in the cytoplasm of nine periportal, midzonal, or perivenular hepatocytes with a spot size of 5 μm. The extinction of total P-450 calculated from the absorbances was converted to the amount of total P-450 per unit cytoplasmic volume. The number of measurements per group was 864 (4 spots/cell, 27 cells/slice, 2 slices/liver, and 4 animals/group).

In Situ Hybridization of CYP2B2 mRNA

Distilled water used for in situ hybridization was pretreated with diethyl pyrocarbonate (DEPC). Under anesthesia, livers of animals (6 animals/group) were perfused with saline. Serial fresh frozen sections 6 μm thick were cut and placed on aminopropyltriethoxysilane-coated glass slides. The sections were immersed in cold 0.1 M phosphate buffer containing 4% formaldehyde (pH 7.4) for 15 min. After treatment with proteinase K (2μg/ml in 10 mM Tris-HCl containing 1 mM EDTA, pH 8.0) for 20 min at 37C, the sections were fixed again with buffered 4% paraformaldehyde as above, washed with PBS, immersed in 0.2 M HCl at 20C for 10 min, and washed again with PBS. The sections were acetylated with 0.1 M triethanolamine-HCl (pH 8.0) containing 0.25% acetic anhydride for 10 min, washed with PBS, and hybridized with diluted sodium citrate-sodium chloride solution (5 × SSC, 0.75M NaCl and 75 mM sodium citrate) containing 2% (w/v) blocking reagent attached to a DIG Detection System Kit (Boehringer Mannheim; Mannheim, Germany), 50 mM sodium phosphate, 7% (w/v) sodium dodecylsulfate (SDS), 0.1% (v/v) N-laurylsarcosine, 200μg/ml tRNA, 50% (v/v) formamide, and 100 pmol/ml of anti-sense or sense oligonucleotide probe labeled with digoxigenin (DIG) (pH 7.0).

The anti-sense sequence of the probe was 3′-CACTAACCGAGAGTGTCCGGTGGTAGGGAA-5′for CYP2B2 corresponding to amino acids 331-340 (Biggin et al. 1983; Omiecinsky et al. 1990). Hybridization was done at 38C for 16 hr in a moist chamber. After hybridization the sections were washed and stained with a DIG Detection System Kit (Boehringer Mannheim).

Western Blotting

Under anesthesia, homogenates and microsomes were prepared from livers of animals (6 animals/group) as described previously (Amatsu et al. 1995; Watanabe et al. 1993a), and subjected to SDS-PAGE followed by Western blotting according to the method of Burnette (1981) or subjected to Western dot-blot analysis (Watanabe et al. 1996b). Partially purified P-450 2B, obtained by DEAE cellulose column chromatography followed by hydroxyapatite column chromatography from liver microsomes from PB-treated rats, was used as a standard for Western blotting.

Northern Hybridization of CYP2B2

Under anesthesia, total RNA was extracted from livers of animals (6 animals/group) with ISOGEN (Nippon Gene; Tokyo, Japan), and mRNA was purified from the total RNA by Poly (A) column chromatography with a Poly (A) Quik mRNA Purification Kit (Stratagene; La Jolla, CA). After denaturation at 65C for 10 min, mRNA (5μg RNA/lane) was applied to a 1% agarose gel containing 2.2 M formaldehyde, subjected to electrophoresis, and transferred to a nylon membrane (Boehringer Mannheim). The membrane was then hybridized with a DIG-labeled oligonucleotide probe, washed, and stained with the DIG Detection System Kit (Boehringer Mannheim). For dot-blot Northern analysis, denatured mRNA immobilized on a nylon membrane was hybridized with the DIGlabeled oligonucleotide probe, washed, and stained as above. The stained spots on the filters were scanned with a ScanJet IIcx/T scanner (Hewlett Packard Japan; Tokyo, Japan) and analyzed with a personal computer (Macintosh 6100/60av; Apple Japan, Tokyo, Japan) and suitable software (NIH Image version 1.60; Bethesda, MD).

Other Biochemical Methods

Total P-450 content and P-450 2B1/2B2 content in liver homogenates or microsomes were measured by difference spectrophotometry (Watanabe et al. 1993a,b) and single radial immunodiffusion (Thomas et al. 1981), respectively. Protein content was measured by the method of Dulley and Grieve (1975).

Statistical Analysis

Data were subjected to one-way analysis of variance followed by Duncan's multiple-range test. All statistical comparisons were made above a 95% level of confidence.

Results

Effect of Anti-P-450 2B1/2B2 Antibody Dilution on Immunostaining Intensity

When specific absorbance (total absorbance minus absorbance caused by background) resulting from P-450 2B1/2B2 in immunostained spots on antigen-immobilized NC membranes was plotted as a function of the concentration of anti-P-450 2B1/2B2 antibody, the absorbance increased with increasing concentration of the antibody up to a dilution of 1:200 and then leveled off (Figure 1A). When the absorbance in the cytoplasm of perivenular hepatocytes in serial sections cut from a PB-treated rat was plotted as the antibody concentration, the absorbance increased with increasing concentration up to 1:200 and then saturated (Figure 1B). Thus, the antibody dilution of 1:200 used in the present study followed saturation kinetics.

Relationship Between Immunostaining Intensity and Amount of P-450 2B1/2B2 Immobilized on Nitrocellulose Membrane or Between Immunostaining Intensity and Section Thickness

To examine the relationship between immunostaining intensity and the amount of P-450 2B1/2B2, serially diluted liver microsomal preparations containing a known amount of P-450 2B1/2B2 prepared from a PB-treated rat were immobilized as spots on an NC membrane and immunostained under saturation conditions with anti-P-450 2B1/2B2 antibody. When specific absorbance measured in stained spots on the membrane was plotted as a function of the amount of P-450 2B1/2B2, there was a linear relationship between the absorbance and the amount (Figure 2).

Figure 1

Effect of anti-cytochrome P-450 2B1/2B2 antibody dilution on immunostaining intensity with the indirect immunoperoxidase reaction. A constant amount of microsomes immobilized on nitrocellulose (NC) membrane (1 μg protein/spot; A) or serial sections (6 μm thick; B) was prepared from livers of phenobarbital (PB)-treated animals. The membranes or sections were incubated with various concentrations of the antibody, processed for the indirect immunoperoxidase reaction and subjected to microphotometry with a spot size of 10 (for membranes) or 4 μm (for sections). The resulting staining intensity is expressed as specific (total - nonspecific) absorbance. Values are means ± SE for six experiments.

When corresponding portions in the cytoplasm of perivenular hepatocytes in serial sections cut at 4-, 6-, 8-, and 10- μm thickness from a PB-treated animal were analyzed by microphotometry after immunostaining, a linear relationship was also observed between specific absorbance and section thickness (Figure 3). Assuming that the serial sections contain a constant amount of P-450 2B1/2B2, Beer-Lambert's law holds for the present quantitative immunohistochemical methods. Thus, the absorbance in sections was proportional to the amount of P-450 2B1/2B2.

Immunohistochemistry of P-450 2B1/2B2

In control and MC-treated animals, immunostaining resulting from P-450 2B1/2B2 was moderate or weak in perivenular hepatocytes and was very weak or negligible in midzonal and periportal hepatocytes (Figure 4). In PB-treated animals, the staining was strong in perivenular hepatocytes, moderate in midzonal hepatocytes and weak or negligible in periportal hepatocytes, and weak or negligible in periportal hepatocytes. In PB+MC-treated animals, the staining was strong or moderate in perivenular and midzonal hepatocytes No staining was seen in sinusoidal cells, such as endothelial and Kupffer cells, and in the nuclei of hepatocytes from animals of the four groups. When adjacent sections were incubated with normal serum instead of anti-P-450 2B1/2B2 antibody, no staining was observed in hepatocytes and sinusoidal cells.

Figure 2

Relationship between immunostaining intensity of cytochrome P-450 2B1/2B2 (P-450 2B1/2B2) and amount of P-450 2B1/ 2B2 immobilized on an NC membrane. Content of P-450 2B1/2B2 in liver microsome from a PB-treated animal was measured by single radial immunodiffusion. Then the microsome was serially diluted, immobilized on an NC membrane, immunostained by the indirect immunoperoxidase method under saturation conditions, and subjected to microphotometry. The resulting staining intensity is expressed as specific absorbance. Values are means ± SE for six measurements.

Figure 3

Relationship between immunostaining intensity of cytochrome P-450 2B1/2B2 and section thickness. Corresponding fields in immunostained serial liver sections cut from a PB-treated animal were examined microphotometrically. The resulting staining intensity is expressed as specific absorbance. Values are means ± SE for six measurements.

Absorbance Resulting from P-450 2B1/2B2 in Sections

In control and MC-treated animals, the absorbance caused by P-450 2B1/2B2 was low in perivenular hepatocytes and very low in midzonal and periportal hepatocytes (Figure 5). In PB-treated animals, the absorbance increased remarkably in perivenular hepatocytes, moderately in midzonal hepatocytes, and did not change in periportal hepatocytes. In PB+MC-treated animals, the absorbance increased moderately in perivenular and midzonal hepatocytes but did not change in periportal hepatocytes. The value in perivenular hepatocytes was about half of that in PB-treated animals, whereas the value in midzonal hepatocytes was twice as much as that in PB-treated animals.

Conversion of Absorbance to Molar Content

As described above, immunostaining intensity resulting from P-450 2B1/2B2 was proportional to the amount of P-450 2B1/2B2. Thus, the intensity can be converted to an absolute value (molar content) as follows (Watanabe et al. 1994):

Antigen content = (SIPspec/ASIspec).

BC/D …. (1),

Figure 4

Immunostaining of cytochrome P-450 2B1/2B2 detected by the indirect immunoperoxidase method under saturation conditions in frozen sections cut from livers perfused with buffered 4% formaldehyde. Portions of sections from control (A), methylcholanthrene (MC)-treated (B; 24 hr after MC administration), PB-treated (C; 48 hr after PB injection), and PB+MC-treated animals (D; 48 hr after PB administration and 24 hr after MC administration). A portion of section cut from a PB-treated animal and incubated with normal serum instead of anti-P-450 2B1/2B2 antibody is superimposed in C. P, periportal area; C, central venule. Bar = 100 μm.

where SIPspec = specific immunostaining intensity in portions of sections (= SIPab-SIPns), ASIspec = average specific staining intensity in whole section (= ASIab-ASIns), BC = biochemically measured content (nmol/g liver), and D = specific gravity of the liver (1.07) (Watanabe et al. 1994). SIPspec measured by image analysis (data not shown) was similar to that measured by microphotometry. ASIspec was obtained by image analysis (control 0.006 ± 0.0018; PB-treated animals 0.067 ±0.0145; PB+MC-treated 0.059 ± 0.0078; MC-treated 0.005 ± 0.0016; means ± SE for six experiments). In the present study, the value “BC/(ASIspec .D)” = 307 is constant in all the groups, because ASIspec was proportional to content of P-450 2B1/2B2 measured biochemically in liver tissues. When this value is substituted into Equation 1, antigen content (nmol/cm³ cytoplasm) is:

Antigen content (nmol/cm³ cytoplasm) = 307 × SIPspec …. (2).

The immunostaining intensity shown in Figure 5 (left axis) could thus be converted to molar content of P-450 2B1/2B2 in sections (right axis). For example, if SIPspec is 0.208, P-450 2B1/2B2 content in the portion (nmol/cm³ cytoplasm) is:

P-450 2B1/2B2 content = (307 × 0.208) = 64 nmol/cm³ cytoplasm …… (3).

In Situ Hybridization for P-450 2B2 mRNA

In control and MC-treated animals, hybridization signal for P-450 2B1/2B2 mRNA was sparse in perivenular hepatocytes and negligible in midzonal and periportal hepatocytes (Figure 6). In PB-treated animals, the signal was strong in perivenular hepatocytes and weak or negligible in midzonal and periportal hepatocytes after 24 hr. After 48 hr of PB administration, however, the signal almost disappeared; only a weak reaction was occasionally seen in perivenular hepatocytes. In PB+MC-treated animals, however, the signal was strong in perivenular and midzonal hepatocytes and weak or negligible in periportal hepatocytes.

In animals of the four groups, hybridization signal for P-450 2B1/2B2 mRNA was negligible in sinusoidal cells. When adjacent sections were incubated with sense probe or unlabeled anti-sense probe, the signal was negligible or very weak in hepatocytes and sinusoidal cells.

Figure 5

Staining intensity of P-450 2B1/2B2 in the cytoplasm of perivenular (PV), midzonal (MZ), and periportal (PP) hepatocytes from control (C), PB-treated (PB), PB+MC-treated (P1M), and MCtreated (MC) animals. The intensity is expressed as specific absorbance (left axis) or molar content converted as described in the text (right axis). Values in control or MC-treated animals are in the order PV < MZ = PP, those in PB-treated animals are PV < MZ < PP, and those in PB+MC-treated animals are MZ = PV < PP (<, p > 0.05; =, p < 0.05; Duncan's multiple range test). Values in PV hepatocytes are in the order PB < P1M < C = MC and those in MZ hepatocytes are P1M < PB < C = MC. There are no significant differences between values in PP hepatocytes. Values are means ± SE for six animals.

Total P-450 Content in Hepatocyte Cytoplasm

In control animals, total P-450 content measured by difference microphotometry was greater in perivenular or midzonal than periportal hepatocytes (Figure 7). In PB-treated animals, the content of P-450 increased about 1.2-fold in periportal hepatocytes, 1.3-fold in midzonal hepatocytes, and 2.2-fold in perivenular hepatocytes. In PB+MC-treated animals, the content increased about 1.3-fold in periportal hepatocytes, 1.6-fold in midzonal hepatocytes, and 2.1-fold in perivenular hepatocytes. In MC-treated animals, the content increased about 1.2-fold in periportal hepatocytes, 1.3-fold in midzonal hepatocytes, and 1.5-fold in perivenular hepatocytes.

Proportion of P-450 2B1/2B2 to Total P-450 in Hepatocyte Cytoplasm

The proportion of P-450 2B1/2B2 to total P-450, calculated by dividing P-450 2B1/2B2 content by total P-450 content, increased in perivenular (from 12 to 58%) or midzonal hepatocytes (from 5 to 29%) in PBtreated animals. The value also increased in perivenular (to 28%) or midzonal hepatocytes (to 45%) in PB+MC-treated animals. The content of P-450 forms other than 2B1/2B2, calculated by subtracting 2B1/2B2 content from total P-450 content (45, 44, or 33 nmol/cm³ liver tissue for perivenular, midzonal, or periportal hepatocytes in control animals), remained unchanged in PB-treated animals and increased in PB+MC (77 or 44 nmol/cm³ for perivenular or periportal hepatocytes) or MC-treated animals (71, 59, or 41 nmol/cm³ for perivenular, midzonal, or periportal hepatocytes) except for midzonal hepatocytes in PB+MCtreated animals (43 nmol/cm³).

Figure 6

In situ hybridization for P-450 2B2 mRNA. Frozen sections cut from perfusion-fixed livers from control (A), MC-treated (B), PB-treated (24 hr after PB administration; (C-E); 48 hr after PB administration (F), or PB+MC-treated animals (G) were incubated with digoxigenin (DIG)-labeled anti-sense probe (A-C, F,G), sense probe (D), or unlabeled anti-sense probe (E). P, periportal area; C, central venule. Bar = 100 μm.

Figure 7

Total cytochrome P-450 amount per unit cytoplasmic volume in perivenular (PV), midzonal (MZ), and periportal (PP) hepatocytes from control (C), PB-treated (PB), PB+MC-treated (P1M) and MC-treated (MC) animals. Values in control animals are in the order PV = MZ < PP and those in PB-treated, PB+MC-treated or MC-treated animals are in the order PV < MZ < PP (<, p, 0.05; =, p < 0.05; Duncan's multiple range test). Values in PV hepatocytes are in the order PB = P1M < MC < C, those in MZ hepatocytes are P1M < PB = MC < C, and those in PP hepatocytes are P+M = PB = MC < C. Values are means ± SE for six animals.

Immunohistochemistry of P-450 1A1/1A2

In control animals and PB-treated animals, immunostaining caused by P-450 1A1/1A2 was weak in perivenular hepatocytes and very weak in midzonal and periportal hepatocytes (Figure 8). In MC-treated animals, the staining was strong in perivenular hepatocytes and very weak in midzonal and periportal hepatocytes. In PB+MC-treated animals, the immunostaining was moderate in perivenular and midzonal hepatocytes and weak or very weak in periportal hepatocytes. No staining was seen in sinusoidal cells and in the nuclei of hepatocytes in animals of the four groups. When adjacent sections were incubated with normal serum instead of anti-P-450 1A1/1A2 antibody, no immunostaining was observed in hepatocytes and sinusoidal cells in animals of the four groups.

Biochemical Results

When homogenates or microsomes were applied to SDS-PAGE followed by Western blotting with anti-P-450 2B1/2B2 antibody, two stained bands (2B1 and 2B2) were identified (data not shown). This is in accord with the result of Thomas et al. (1983). When homogenates or microsomes were immobilized on NC membrane and subjected to Western dot-blotting with anti-P-450 2B1/2B2 antibody, the immunostaining intensity was strong in PB-treated and PB+MC-treated animals and was weak in control and MC-treated animals (data not shown).

Figure 8

Immunostaining of cytochrome P-450 1A1/1A2 stained by the indirect immunoperoxidase method under saturation conditions in frozen fixed sections. Portions of sections from control (A), methylcholanthrene (MC)-treated (B), PB-treated (C), and PB+MCtreated animals (D). A portion of section cut from a PB-treated animal and incubated with normal rabbit serum instead of anti-P-450 1A1/1A2 antibody is superimposed in (B). P, periportal area; C, central venule. Bar = 100 μm.

Northern blot analysis (Figure 9) revealed a weak hybridization signal for CYP2B2 mRNA in control (OD = 0.07-0.13) and MC-treated animals (0.05-0.09). Although the signal was strong after 24 hr of PB injection (0.57-0.93), it became weak after 48 hr of injection (0.10-0.19). A strong signal for the message was found in PB+MC-treated animals (0.45-0.84).

Figure 9

Northern hybridization for P-450 2B2 mRNA. Extracted and purified mRNA from livers of control (Cont), PB-treated (24 hr after PB administration, PB (24); 48 hr after PB administration, PB), PB+MC-treated (PB+MC), and MC-treated animals (MC) was subjected to Northern (upper, 2 mg mRNA/lane) or Northern dot-blotting (lower, 2 mg mRNA/spot).

Total P-450 content in homogenates and microsomes (37 ± 3.1 nmol/g liver, 1.3 ± 0.14 nmol/mg protein in control animals; means ± SE for four animals) increased in PB- (61 ± 5.8 nmol/g liver, 2.2 ± 0.33 nmol/mg protein), PB+MC- (63 ± 9.9 nmol/g liver, 2.3 ± 0.31 nmol/mg protein), or MC-treated animals (51 ± 4.4 nmol/g liver, 1.8 ± 0.15 nmol/mg protein).

P-450 2B1/2B2 content in homogenates or microsomes in control animals was low (2 nmol/g liver or 0.1 nmol/mg microsomal protein, means for four animals). The content in homogenates or microsomes increased in PB- (22 nmol/g liver or 0.9 nmol/mg microsomal protein) or PB+MC-treated animals (19 nmol/g liver or 0.8 nmol/mg microsomal protein) but was unchanged in MC-treated animals. Content of P-450 forms other than 2B1/2B2 in homogenates or microsomes, calculated by subtracting 2B1/2B2 content from total P-450 content (35 nmol/g liver or 1.2 nmol/mg microsomal protein in control animals), remained unchanged in PB-treated animals and increased in PB+MC and MC-treated animals (44-49 nmol/g liver or 1.5-1.7 nmol/mg microsomal protein).

Discussion

As revealed in the present study, the PB-induced increase in P-450 2B1/2B2 content was suppressed in perivenular hepatocytes but promoted in midzonal hepatocytes in PB+MC-treated animals (48 hr after PB injection and 24 hr after MC injection). The suppression observed in perivenular hepatocytes could be due to an increase in turnover of the isoforms caused by an increase in the rate of degradation induced by MC administration, or to inhibition of the synthesis of the isoforms by the action of MC. The promotion found in midzonal hepatocytes probably relates to an increase in the synthesis of the isoforms. Thus, the pattern of intralobular expression of PB-inducible P-450 isoforms induced antecedently by PB is drastically changed by the action of MC. The promotion of increase in the isoforms in midzonal hepatocytes appears to compensate for the suppression of induction in perivenular hepatocytes.

The hybridization signal for P-450 2B2 mRNA increased exclusively in perivenular hepatocytes after 24 hr of PB injection and almost disappeared after 48 hr of injection in PB-treated animals. In PB+MC-treated animals, however, a strong hybridization signal for the mRNA was observed in midzonal and perivenular hepatocytes after 48 hr of PB injection. This suggests that transcription of the P-450 2B2 gene, induced by PB injection, continues to be activated by MC, or that degradation of mRNA is suppressed by MC. However, it is unlikely that the retention of P-450 2B2 mRNA in perivenular hepatocytes in PB+MC-treated animals is due to an activation of transcription of P-450 2B2 gene, because P-450 2B2 mRNA in perivenular hepatocytes did not increase in MC-treated animals. Therefore, the retention of P-450 2B2 mRNA in perivenular hepatocytes in PB+MC-treated animals is probably due to suppression of mRNA degradation.

In MC-treated animals, the intensity of immunostaining for P-450 1A1/1A2 increased markedly in perivenular hepatocytes, although the intensity for P-450 2B1/2B2 did not increase. In PB+MC-treated animals, however, the intensity for P-450 1A1/1A2 increased moderately in both midzonal and perivenular hepatocytes. This suggests that induction of P-450 1A1/1A2 is suppressed in perivemular hepatocytes but activated in midzonal hepatocytes. This is similar to the change seen in the immunostaining for P-450 2B1/2B2 in PB+MCtreated animals. The induction mechanism for P-450 1A1/1A2 has been considered as follows (Pollenz et al. 1993; Hord and Perdew 1994; Asaka et al. in press). After stimulation of MC or dioxin, arylhydrocarbon receptor in the hepatocyte cytoplasm translocates to the nucleus, associates with its translocator protein, and then binds to xenobiotic responsive element (XRE), an enhancer that drives expression of P-450 1A1/1A2 genes in response to the inducer. PB might act as an inhibitor at a certain step in the above-mentioned mechanism, although it is unknown which step is inhibited by PB.

The increase in P-450 2B1/2B2 content seen in PBtreated animals was suppressed despite the retention of a large amount of P-450 2B2 mRNA in perivenular hepatocytes in PB+MC-treated animals. There appears to be a divergence between the intensity of hybridization signal for P-450 2B2 mRNA and P-450 2B1/2B2 content. The divergence can be explained by suppression of translation in P-450 2B2 mRNA in perivenular hepatocytes from PB+MC-treated animals. It is possible that degradation of mRNA by ribonuclease after translation is repressed and that untranslated mRNA is retained in the cytoplasm (Graves et al. 1987). As the cause of repression of translation, competition between P-450 2B1/2B2 mRNA and P-450 1A1/1A2 mRNA for ribosomes (Graves et al. 1987; Mowry and Steitz 1987), phosphorylation of transcriptional regulatory factors (Hinnebusch 1990; Rhoads 1993), or binding of a repressor protein to ribosomes (Nomura et al. 1984; Aziz and Munro 1987) is considered.

As alternative reason for the divergence, suppression of an increase in P-450 2B1/2B2 by promotion of degradation of P-450 2B1 can be considered. However, a decrease in the staining intensity for the P-450 2B1 band in microsomes from PB+MC-treated animals was not observed in Western blots. This indicates that promotion of degradation of P-450 2B1 is negligible.

The relationship between immunostaining intensity resulting from P-450 1A1/1A2 and section thickness was curvilinear (Tanaka et al. 1997). The curvilinear relationship is attributed to steric hindrance, antibody trapping, loss of antigen from sections, or reduction in antibody binding owing to a high antibody concentration (Raivich et al. 1993; Watanabe et al. 1993b, 1996b). In the present results, however, the relationship between the intensity resulting from P-450 2B1/2B2 and section thickness was linear. Therefore, the staining intensity in sections measured in the present study was proportional to the content of P-450 2B1/2B2 and could therefore be converted to molar content in hepatocyte cytoplasm by the image processing method.

There appear to be two types of increase in amount of P-450 isoforms after drug administration: an additive increase without reduction in other constitutive forms and a substitutional increase with reduction in other constitutive forms (Tanaka et al. 1997). The increase in P-450 2B1/2B2 after PB or PB+MC administration appeared to be additive in hepatocytes of all three zones. The pattern of increase in P-450 isoforms in heptocytes is believed to differ according to inducers, isoforms, or location of hepatocytes in the lobule.

Footnotes

Acknowledgements

Supported by grants-in-aid from the Japanese Ministry of Education, Science and Culture (grants 06670041 and 09470005).

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