Triiodothyronine downregulates the periportal expression of alpha class glutathione S-transferase in rat liver

Most drug-metabolizing phase I and phase II enzymes, including the glutathione S-transferases (GST), exhibit a zonated expression in the liver, with lower expression in the upstream, periportal region. To elucidate the involvement of pituitary-dependent hormones in this zonation, the effect of hypophysectomy and 3,3',5-triiodo-L-thyronine (T3) on the distribution of GST was studied in rats. Hypophysectomy increased total GST activity both in the periportal and perivenous liver region. Subsequent T3 treatment counteracted this effect in the perivenous zone. However, analysis for either mu class M1/M2-specific (1,2-dichloro-4-nitrobenzene) or alpha class A1/A2-specific (7-chloro-4-nitrobenzo-2-oxa-1,3-diazole) GST activity revealed that T3 treatment did not significantly affect the perivenous activity of these GST classes. In contrast, T3 was found to significantly counteract the increase of alpha class GST activity caused by hypophysectomy in the periportal zone. To establish whether this effect was T3-specific, hepatocytes were isolated from either the periportal and perivenous zone by digitonin/collagenase perfusion and cultured either as pyruvate-supplemented monolayer or as co-culture with rat liver epithelial cells. Only in the latter it was found that T3 suppressed the A1/A2-specific GST activity and alpha class proteins predominantly in periportal cells. The data demonstrate that T3 is an important factor responsible for the low expression of alpha GST in the periportal region. T3 may be involved in the periportal downregulation of other phase I and II enzymes as well.

Effect of long-term ethanol exposure on the acinar distribution of hepatic glutathione S-transferase

The ability of ethanol to affect the regional distribution of individual glutathione S-transferase (GST) isoenzymes in rat liver was investigated by analyzing the expression levels in cell lysates obtained from the periportal or perivenous liver region after in situ digitonin perfusion. In control rats, a significant perivenous dominance of GST proteins and activities measured by the substrates 1-chloro-2,4-dinitrobenzene (broad spectrum), 1, 2-dichloro-4-nitrobenzene (M1/M2-specific), and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (A1/A2-specific) was found. In pair-fed rats exposed to ethanol (36% of total calories) for 2 weeks, all GST activities measured were significantly increased in both acinar zones. However, the relative increase was greater in the perivenous region. The induction of the A1/A2-specific activity was the most pronounced. HPLC analysis revealed for both regions that this increase was largely confined to the A2 subunit, with only minor effects observed on the A1 subunit. At the mRNA level, the constitutive perivenous dominance of both GST A1 and GST A2 expression became more pronounced after ethanol administration. The results demonstrate that long-term ethanol exposure induces individual GST isoenzymes differently and might have a profound effect on xenobiotic-induced regional liver damage.

Effect of ethanol on the expression of hepatic glutathione S-transferase: an in vivo/in vitro study

Ethanol, a human toxicant and a solvent in pharmacological research, is known to interfere with biotransformation of xenobiotics. We compared the in vivo and in vitro long-term effects of ethanol exposure on the expression of glutathione S-transferases (GST, EC 2. 5.1.18) in rat liver. Long-term in vivo ethanol treatment to achieve blood ethanol levels ranging between 10-50 mM was by liquid diet feeding. For in vitro experiments, rat hepatocytes co-cultured with rat liver epithelial cells were exposed to 17 and 68 mM ethanol for up to 10 days. Two weeks of liquid diet ethanol treatment increased total GST activity. Both Mu and Alpha classes and in particular the A1 and A2 subunits and the amount of their corresponding mRNAs were increased. Total GST activity was also increased in co-cultures after exposure to 68 mM ethanol for 10 days. However, the Mu class subunits M1 and M2 and the corresponding mRNAs were increased, rather than the Alpha class subunits. Thus, long-term exposure to ethanol induces hepatic GST both in vivo and in vitro, but different isoenzymes are affected. Consequently, extrapolation of in vitro data on GST expression and regulation to the in vivo situation must be judicious. During xenobiotic metabolism in cell culture, a shift in relative expression and induction of different GST forms may occur, resulting in either an under- or overestimation of effects.

Growth hormone-regulated periportal expression of CYP2C7 in rat liver

Most drug- and steroid-metabolizing cytochrome P450 (CYP) enzymes are expressed in the mammalian liver in a characteristic zonated pattern, with high expression in the downstream perivenous (centrilobular) region. Here, we report that CYP2C7, a member of the rat CYP2 family, is expressed preferentially in the opposite, periportal region. CYP2C7 mRNA, as detected by reverse transcription-polymerase chain reaction, was detected almost exclusively in cell lysates obtained from the periportal region, indicating a very steep acinar gradient. The amount of immunoreactive CYP2C7 protein in periportal cell lysates was also higher than in samples from the perivenous region. This gradient was reversed by hypophysectomy, which markedly and selectively reduced the periportal CYP2C7 protein content. Subsequent growth hormone infusion by osmotic minipumps restored the zonation by selectively increasing the amount of periportal CYP2C7 protein. Although hypophysectomy suppressed CYP2C7 mRNA and growth hormone counteracted it, regulation at this level did not appear to occur in a zone-specific fashion. This indicates that growth hormone-mediated zonal regulation of CYP2C7 protein has additional translational or posttranslational components. Ethanol treatment, which has been shown to affect growth hormone levels, significantly induced CYP2C7 mRNA, but not zone specifically. Our results demonstrate that growth hormone up-regulates the CYP2C7 gene by enhancing the expression of the protein specifically in the periportal liver region. Growth hormone may up-regulate other periportally expressed liver genes in a similar fashion.

Short-term ethanol exposure increases the expression of Kupffer cell CD14 receptor and lipopolysaccharide binding protein in rat liver

Gut-derived endotoxins (lipopolysaccharide, LPS) complexed to LPS-binding protein (LBP) activate liver Kupffer cells via their CD14 receptor. Pro-inflammatory cytokines are released and this is postulated to promote liver injury. We previously demonstrated enhanced expression of CD14 endotoxin receptor after 2 weeks of alcohol administration. A similar result, based on 6 weeks of ethanol treatment, was recently reported and suggested to correlate with alcohol-induced liver injury. To establish whether this occurs prior to or after the initiation of damage, we investigated the temporal effect of continuous ethanol exposure on the expression of CD14 and the associated LBP. In addition, we studied the effect of treatment with gadolinium chloride (GdCl3) that inactivates Kupffer cells and alleviates alcohol-induced liver damage. The amount of CD14 and LBP mRNA, as determined by reverse transcriptase-polymerase chain reaction (RT-PCR), was unchanged 4-8 h after intragastric ethanol administration. However, after 24-48 h of repeated ethanol administration, CD14 and LBP mRNA both increased significantly and reached a level similar to that observed after 6 weeks of ethanol exposure by liquid diet. Immunostaining experiments with ED2 antibody demonstrated that GdCl3 efficiently inactivated Kupffer cells. However, there was no concomitant reduction in the expression of CD14 mRNA, suggesting that compensatory infiltration by ED2-negative, but CD14-positive, macrophages had occurred. Our results demonstrate that soon after the initiation of ethanol exposure, i.e. within 24-48 h, the hepatic expression of both the CD14 receptor and LBP is increased. This suggests that these increases could contribute to the initiation of alcoholic damage rather than being a consequence of the injury.

No significant expression of CYP2E1 in rat liver stellate cells

The putative role of the ethanol-inducible cytochrome P450(CYP)2E1 in stimulating collagen synthesis by rat liver stellate cells was studied. Analysis of carefully isolated stellate cells revealed that their content of immunoreactive CYP2E1 protein and of CYP2E1 mRNA, as determined by reverse transcription, polymerase chain reaction (RT-PCR), was very low, i.e. only 0-4% of that in hepatocytes. We conclude that it is improbable that such low expression of CYP2E1 in stellate cells would have functional importance.

Induction of UDP-glycosyltransferase family 1 genes in rat liver: different patterns of mRNA expression with two inducers, 3-methylcholanthrene and beta-naphthoflavone

Uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), presently called UDP-glycosyltransferases, catalyse the detoxification of many toxic and carcinogenic compounds. Glucuronidation is also a major metabolic pathway for numerous drugs. The UGT1A6 gene (formerly known as UGT1*06 and UGT1A1) has been suggested to belong to the aryl hydrocarbon (Ah) gene battery, which consists of several genes encoding for drug-metabolising enzymes regulated by dioxin and other ligands of the Ah receptor. In this study, we analysed the localisation of UGT1A6 expression in rat liver by in situ hybridisation to mRNA. Two different RNA probes were used, one which was specific to UGT1A6 and the other against the C terminal sequence shared by all UGT1 genes. In this study, no UGT1A6 mRNA was detected in the control animals. However, other gene(s) of the UGT1 family were expressed in the perivenous region surrounding the central veins as detected by hybridisation with the probe against the common region of the UGT1 genes. Treatment with the lower dose (5 mg/kg) of 3-methylcholanthrene (3MC) induced expression of UGT1A6 perivenously. Treatment with the higher dose (25 mg/kg) of 3-Methylcholanthrene resulted in a more panacinar expression pattern. In contrast to the perivenous induction observed with 3-methylcholanthrene, treatment with 15 mg/kg of beta-naphthoflavone (BNF) resulted in strong induction in the periportal region. The results reveal an inducer-specific pattern of UGT1A6 expression similar to that demonstrated earlier for other Ah battery genes, namely CYP1A1, CYP1A2, GSTYalpha and ALDH3. The finding further supports the notion that common factors regulate the regional hepatic expression of Ah battery genes.

Aryl hydrocarbon receptor-associated genes in rat liver: regional coinduction of aldehyde dehydrogenase 3 and glutathione transferase Ya

The tumor-associated aldehyde dehydrogenase 3 (ALDH3) and the glutathione transferase (GST)Ya form are coded by members of the Ah (aryl hydrocarbon) battery group of genes activated in the liver by polycyclic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The physiological role of the Ah receptor (AHR), its gene-activating mechanism and its endogenous ligands are still poorly clarified. We had previously observed that 3-methylcholanthrene (3MC) and beta-naphthoflavone (betaNF) induced the AHR-associated CYP1A1/1A2 pair in different liver regions, an effect not explained by the acinar distribution of the AHR protein. Here, we investigated AHR-associated regional induction by comparing the expression patterns of ALDH3 and GSTYa. Analysis of samples from periportal and perivenous cell lysates from 3MC-treated animals revealed that ALDH3 mRNA, protein and benzaldehyde-NADP associated activity were all confined to the perivenous region. In contrast, such regio-specific induction was not seen after beta-NF induction. Immunohistochemically, a peculiar mono- or oligocellular induction pattern of ALDH3 was seen, consistently surrounding terminal hepatic veins after 3MC but mainly in the midzonal region after betaNF. A ligand-specific difference in regional induction of GSTYa1 mRNA was also observed: The constitutive perivenous dominance was preserved after 3MC while induction by betaNF was mainly periportal. A 3MC-betaNF difference was also seen by immunohistochemistry and at the GSTYa protein level, in contrast to that of the AHR-unassociated GSTYb protein. However, experiments with hepatocytes isolated from the periportal or perivenous region to replicate these inducer-specific induction responses in vitro were unsuccessful. These data demonstrate that the different acinar induction patterns by 3MC and betaNF previously observed for CYP1A1 and CYP1A2 are seen also for two other Ah battery genes, GSTYa1 and ALDH3, but in a modified, gene-specific form. We hypothesize that unknown protein(s) operating in vivo and modifying the Ah-mediated response at the common XRE element located upstream of these genes is affected zonespecifically by 3MC and betaNF.

Alcohol-induced expression of the CD14 endotoxin receptor protein in rat Kupffer cells

Gut-derived endotoxins (lipopolysaccharide, LPS) are believed to contribute to alcohol-induced liver disease (ALD) by stimulating Kupffer cells, the resident liver macrophages, to release proinflammatory cytokines. This activation is largely mediated by CD14, a high-affinity membrane-anchored receptor for LPS. We observed, by chemiluminescence-enhanced detection, an increase in immunoreactive CD14 protein in Kupffer cells isolated from rats treated with ethanol for 2 weeks. Immunocytofluorescence experiments confirmed that this increase was confined to the membranes of Kupffer cells from the alcohol-treated rats. The increase was regulated pretranslationally: a 3-fold elevation (p < 0.01) in the hepatic level of CD14 mRNA was observed. The marked increase in CD14 expression suggests a new mechanism by which alcohol increases the LPS-mediated cytokine signaling by the liver macrophages, thus promoting the interaction between alcohol and endotoxins in the development of liver damage.

Zonation of hepatic cytochrome P-450 expression and regulation

The CYP genes encode enzymes of the cytochrome P-450 superfamily. Cytochrome P-450 (CYP) enzymes are expressed mainly in the liver and are active in mono-oxygenation and hydroxylation of various xenobiotics, including drugs and alcohols, as well as that of endogenous compounds such as steroids, bile acids, prostaglandins, leukotrienes and biogenic amines. In the liver the CYP enzymes are constitutively expressed and commonly also induced by chemicals in a characteristic zonated pattern with high expression prevailing in the downstream perivenous region. In the present review we summarize recent studies, mainly based on rat liver, on the factors regulating this position-dependent expression and induction. Pituitary-dependent signals mediated by growth hormone and thyroid hormone seem to selectively down-regulate the upstream periportal expression of certain CYP forms. It is at present unknown to what extent other hormones that also affect total hepatic CYP activities, i.e. insulin, glucagon, glucocorticoids and gonadal hormones, act zone-specifically. The expression and induction of CYP enzymes in the perivenous region probably have important toxicological implications, since many CYP-activated chemicals cause cell injury primarily in this region of the liver.

Zonal distribution of transcripts of four hepatic transcription factors in the mature rat liver

Within the liver acinus the majority of genes are expressed in an ascending or descending gradient from the portal to the central vein. The regulatory factors involved in this spatial pattern of gene expression are still poorly understood. Many liver genes are regulated by liver-enriched transcription factors. Here we report on mRNA distribution of four transcription factors in cell lysates obtained from either the periportal or the perivenous region after zone-specific digitonin infusion and by in-situ hybridization. Northern blot analysis revealed that there were slightly more transcripts of C/EBP, HNF1 beta (n.s.) and HNF4 (p < 0.05), but fewer of HNF3 gamma (n.s.), in perivenous than in periportal lysates. A somewhat stronger staining in the perivenous region of HNF4 was also seen by in-situ hybridization. The moderate acinar zonation of the mRNAs of these transcription factors suggests that at best they could modulate but not govern the zonated expression of liver genes in the mature liver.

Selective centrilobular expression of the aryl hydrocarbon receptor in rat liver

The aryl hydrocarbon receptor (AHR) is a transcriptional activator of genes encoding a group of drug-metabolizing enzymes, including cytochrome P450 1A1 (CYP1A1), glutathione S-transferase, tumor-associated aldehyde dehydrogenase and quinone reductase. Both the constitutive and inducible expression of these genes in the liver is zonated, i.e., dominant in hepatocytes of the centrilobular region, a poorly understood position-dependent phenomenon. By comparing cell lysates obtained from opposite acinar regions we observed that immunoreactive AHR protein was almost exclusively confined to centrilobular cells. The AHR mRNA, as analyzed from cell lysates by reverse transcriptase polymerase chain reaction, exhibited a similar, although somewhat less pronounced zonation. By contrast, only slight zonation of the AHR nuclear translocator mRNA was observed. Treatment of rats with omeprazole, an atypical nonligand activator of the AHR, caused a zone-specific induction of CYP1A1 in the centrilobular region similar to that seen after pretreatment with the AHR ligand 3-methylcholanthrene. Our results suggest that the zone-restricted expression of AHR protein will allow the constitutive and inducible expression of AHR-regulated genes in the centrilobular region, but will limit their expression in the periportal region.

Distribution of glutathione S-transferase isoforms in rat liver after induction by beta-naphthoflavone or 3-methylcholanthrene

Regional differences in vulnerability to xenobiotic liver damage may relate to the distribution of the detoxication capacity of the glutathione S-transferases (GST). HPLC analysis of cell lysates obtained by digitonin infusion from either the periportal or the perivenous region revealed that the content of all the GST subunits investigated (1, 2, 3, 4 and 8) was higher in the perivenous region. The strongest perivenous dominance was observed for subunit 1 (Ya) and the alpha class appeared to be more zonated that the mu class. A similar perivenous dominance was observed by analysis of GST activity with either 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloronitrobenzene (DCNB) or trans-4-phenyl-3-buten-2-one (PBO) as substrate. In contrast, with cumene hydroperoxide (CuOOH) or tert-butyl hydroperoxide (tBOOH) as substrate a reciprocal twofold periportal dominance was observed. Induction by pretreatment with beta-naphthoflavone reduced or abolished the perivenous dominance of the alpha-subunits 1, 2 and 8. In contrast, after pretreatment with 3-methylcholanthrene, only the acinar gradient of subunits 2 (Yc) was abolished, while the strong perivenous gradient subunit 1 (Ya) was maintained and that of subunit 8 (Yk) increased. CDNB based assays demonstrated that beta-naphtoflavone treatment reduced (from 2.1 to 1.4) while 3-methyl cholanthrene enhanced (to 2.6) the perivenous/periportal GST activity ratio. Assays based on CuOOH or tBOOH indicated that neither the Se-dependent nor the Se-independent glutathione peroxidase activity nor its acinar distribution was affected by the inducers. These results demonstrated that although the expression of all investigated members of the alpha and mu classes is higher in the perivenous region, there are marked isozyme differences, the acinar gradient being particularly prominent for subunit 1 (Ya). The distinct difference in the acinar induction pattern of GST Ya between beta-naphthoflavone and 3-methylcholanthrene resembles that reported for cytochrome P450 (CYP1A1 and CYP1A2), also members of the aryl hydrocarbon (Ah) receptor genes, suggesting common regionally acting regulatory elements in the expression of these genes in the liver.

Zonation of cytochrome P450 enzyme expression in rat liver. Isozyme-specific regulation by pituitary dependent hormones

The effect of hypophysectomy and subsequent infusion of growth hormone (GH) or injections of triiodothyronine (T3) on the acinar expression pattern of four homonally regulated P450 isozymes was studied to elucidate the involvement of pituitary dependent hormones in regulating the characteristic centrilobular expression pattern of most members of the cytochrome P450 (CYP) gene family in rat liver. Hypophysectomy was previously observed to allow high expression of CYP2B1/2 and 3A1/2 in the normally silent periportal region. In the present study, it had much less effect on the zonation of the ethanol-inducible P450 2E1 form: only a moderate shift of 2E1 staining towards the periportal region was observed by immunohistochemistry. Subsequent injections with T3 moderately decreased CYP2E1 expression in the periportal region and no significant countereffect of GH was discerned. T3 treatment, previously observed to block only the periportal expression of CYP3A1/2, counteracted the increased CYP2B1/2 expression caused by hypophysectomy equally in the periportal and perivenous region. This was true both at the protein and mRNA level, as analysed from cell lysates obtained by in situ perfusion of livers by zone-restricted digitonin treatment. Thus, although hypophysectomy and subsequent GH and T3 treatment affect the total expression of CYP2B1/2, 2E1, and 3A1/2 similarly, the zonal effects were isozyme-specific. In contrast, the perivenous zonation normally seen for the dioxin-inducible P450 1A2 form was steepened rather than diminished by hypophysectomy, both in male and female rats. Administration of GH by the female-type continuous infusion had no effect in male rats, but partially counteracted the effect of hypophysectomy in females, suggesting an involvement of GH. In contrast to other CYP genes investigated, the female-characteristic expression of CYP2C12 was found to be completely non-zonated. Hypophysectomy and continuous GH administration dramatically affected the amount of mRNA of both P450 2C12 and the male-specific 2C11 form, but analysis of periportal and perivenous cell lysates indicated that these effects were not zone-specific. The distribution of the GH receptor was investigated to explain the zonal effects of GH. Immunohistochemically, a moderate perivenous dominance was observed, whereas the mRNA abundance of both GH receptor and GH binding protein was slightly higher in the periportal region. Thus, zonal regulation by GH does not appear to result from a GH receptor zonation; rather, a sinusoidal GH gradient may be involved. These data, combined with our previous results, indicate that pituitary-dependent hormones regulate the zone-specific expression of some P450 forms strongly (i.e. 2B1/2 and 3A1/2), and other forms are moderately regulated (i.e. 1A2 and 2E1), or are affected across the whole acinus (i.e. 2C11, 2C12).

Hormonal regulation of the zonated expression of cytochrome P-450 3A in rat liver

Most cytochrome P-450 enzymes are expressed characteristically in a zonated pattern in the liver. The factors responsible for this heterogenous expression are largely unknown. Here we report how growth hormone and tri-iodothyronine regulate the steroid-hydroxylating cytochrome P-450 (CYP) 3A forms, which are constitutively expressed mainly in the perivenous (downstream) liver region. By comparing cell lysates obtained from the periportal and perivenous acinar regions we observed that the elevated CYP3A expression observed after hypophysectomy was due mainly to a dramatic increase in the normally silent periportal region. This effect was particularly strong in females. Treatment with growth hormone re-established the perivenous expression pattern, a finding corroborated by immunohistochemical analysis of liver sections. Analysis of periportal and perivenous mRNA by reverse-transcriptase PCR demonstrated that in males the changes in CYP3A2 mRNA paralleled the changes at the protein level. In females, CYP3A2 mRNA was detected only after hypophysectomy, and the zonal protein changes seemed to be governed by changes in CYP3A1 mRNA levels. Treatment of hypophysectomized animals with tri-iodothyronine also suppressed the expression of CYP3A, both in males and females. However, this occurred almost exclusively in the periportal region. This was observed both at the protein level, as determined by immunoblotting and immunohistochemically, and at the CYP3A1 and 3A2 mRNA level. These results indicate that growth hormone and thyroid hormone regulate the expression of CYP3A genes zone-specifically by suppressing their transcription in the periportal (upstream) region of the liver.

Pretranslational induction of cytochrome P4501A enzymes by beta-naphthoflavone and 3-methylcholanthrene occurs in different liver zones

Most of the cytochrome P450 (CYP) genes are expressed in an uneven, zonated pattern in the liver. Factors regulating this regionally restricted expression are not well known. In this study we have analysed cell lysates obtained from opposite zones of rat liver by infusing digitonin to the perfused liver to study the zonation of CYP1A1 and CYP1A2 induction. 3-Methylcholanthrene induced CYP1A1 protein in perivenous cells, while a low dose of beta-naphthoflavone caused periportal induction. Analysis of CYP1A1 mRNA from cell lysates by reverse transcriptase-coupled polymerase chain reaction (RT-PCR) and in situ hybridization experiments both demonstrated that this inducer-specific differently localized effect occurred at the pretranslational level. A corresponding difference in the regional pattern of CYP1A2 induction was seen: induction by beta-naphthoflavone reversed the constitutive perivenous pattern into a periportal CYP1A2 mRNA pattern while induction after 3-methylcholanthrene treatment was more panacinar. Attempts to identify the regiospecific factors involved were made by comparing the in vitro induction of CYP1A1 by beta-naphthoflavone and 3-methylcholanthrene in hepatocytes isolated from the periportal and perivenous region. However, after isolation, induction seemed to be independent of the source of the cells. Our results demonstrate the existence in the liver of regionally acting factors that mediate the induction of CYP1A1 and 1A2 in a local and inducer-specific fashion. These factors could be Ah receptor associated binding proteins operating in vivo, but no longer in isolated cells.

Growth hormone mediates zone-specific gene expression in liver

The CYP2B1/2 gene pair is an example of many liver genes that exhibit a characteristic zonated expression pattern in the liver. The factors governing this zonation are poorly understood. We observed that after hypophysectomy the expression of CYP2B1/2 protein and mRNA in the normally silent periportal (upstream) region was high, in both male and female rats. Treatment with growth hormone counteracted the effect of hypophysectomy, limiting expression to the perivenous (downstream) region, completely in females and partially in males. This shows that a hormone zone-specifically regulates gene expression in the liver.

Zone-specific inducibility of cytochrome P450 2B1/2 is retained in isolated perivenous hepatocytes

The expression and induction of the cytochrome P450 2B1/2 isoenzyme is heterogeneous, exhibiting a regional pattern in the intact liver and a varied response to phenobarbital in isolated cultured hepatocytes. We report that P450 2B1/2 immunostaining of hepatocytes isolated from the perivenous liver region and cultured in the presence of phenobarbital is much stronger than that of cells identically treated but isolated from the periportal region. P450 2B1 mRNA, quantified by a sensitive and specific RNAase protection assay, is also preferentially induced in perivenous hepatocytes, demonstrating that the difference in induced expression is at the pretranslational level. Our results suggest that perivenous and periportal hepatocytes are differentially imprinted to retain regiospecific factors governing their inducibility after isolation.