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45
Review
The Induction of CYP2B Proteins in Rodents
by Phenobarbital-Like Inducers:
What Has Been Discovered and
What Remains to be Learned
Alan Anderson
Centre de recherche, L’Hτtel-Dieu de Quιbec, Quιbec G1R 2J6 Canada and Dιpartement de biologie,
Universitι Laval, Quιbec G1K 7P4 Canada
* Corresponding author: E-mail: Alan.Anderson@bio.ulaval.ca; Tel.: +1 418 6915281, Fax: +1 418 6915439
Received: 17-09-2007
Abstract
Cytochrome P450s (CYPs) are key enzymes of Phase I metabolism of xenobiotics and endobiotics in the liver, and many hepatic CYPs are inducible by xenobiotics. A striking example of inducible CYPs is furnished by the CYP2B proteins in rat and mouse liver, which are strongly induced by phenobarbital (PB). A 163-bp Sau3AI fragment in the CYP2B2 5’ flank confers PB inducibility on reporter genes in primary rat hepatocytes and has the properties of a trans-criptional enhancer. This fragment is referred to as the PB response unit (PBRU). The purpose of this review is to summarize present understanding of the molecular mechanism whereby the PBRU confers PB responsiveness, and to examine what remains to be learned concerning the induction of CYP2B proteins (chiefly in rodents, but also in humans) by PB and PB-like inducers.
Keywords: Phenobarbital, induction, CYP2B, constitutive androstane receptor, rats, mice
1. Introduction
In this review the standard cytochrome P450 no-menclature1 is used, where proteins are identified as CYP1A2, CYP2B1 etc, and the corresponding genes as CYP1A2, CYP2B1 etc. The names CYP2B1 and CYP2B2 apply only to the rat forms inducible by phenobarbital (PB), although very similar homologues exist in other mammals.1 A mouse homologue of rat CYP2B1 and CYP2B2 is termed CYP2B10, and, following the standard convention for mouse gene names, its gene is designated Cyp2b10. Nelson2 has concluded that CYP2B1 and Cyp2b10 are orthologues. Herein, the term CYP2B includes the closely related rat CYP2B1 and CYP2B2 and mouse Cyp2b10 genes. Occasional reference will also be made to genes for other CYP2B forms such as the consti-tutively expressed rat CYP2B3 gene,3–5 and the mouse Cyp2b9 gene,6–8 considered to be orthologues by Nel-son,2 as well as the PB-inducible human homologue CYP2B6,9 for which no identifiable rodent orthologue exists.10 The CYP2B1, CYP2B2 and CYP2B3 genes (Fig. 1A) form part of the CYP2ABFGST cluster on rat chromosome 1,11 and the syntenic Cyp2abfgst cluster is on mouse chromosome 7.10,12
The molecular mechanism whereby PB and PB-like inducers induce hepatic enzymes and, in particular, CYP2B proteins in the rat and mouse, has long been of in-terest.13,14 Much progress has been made over the past 10 to 15 years since the discovery of a 163-bp Sau3AI fragment in the CYP2B2 5’ flank conferring PB inducibility on a reporter gene in primary rat hepatocytes and having the properties of a transcriptional enhancer.15 We originally referred to the rat 163-bp Sau3AI fragment as a PB response element (PBRE), but further analysis led us to conclude that it contains a PB response unit (PBRU)16,17 (Fig. 1B). The purpose of this review is to summarize the progress that has been made since the discovery of the PBRU, and to cast a critical eye on what remains to be learned concerning the induction of CYP2B proteins by PB and PB-like inducers. A number of other reviews related to this question have appeared in the course of the past several years.18–24
Mice lacking the constitutive androstane receptor (CAR) have greatly reduced or undetectable levels of hepatic or small intestinal CYP2B10 mRNA after treatment with PB or PB-like inducers.25–28 The mechanism whereby CAR mediates transcriptional activation of rodent CYP2B genes is the paradigm for understanding the biolo-
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Figure 1. (A) Schematic representation of the locus of the CYP2B1, CYP2B2 and CYP2B3 genes on rat chromosome 1. The localizations of CYP2B1 and CYP2B2 PBRUs are illustrated. The other thin vertical lines represent the nine CYP2B exons, not all of which are resolved in this image. (B) Sequence of the rat CYP2B2 PBRU showing the positions of nuclear receptor recognition sites NR1, NR2 and NR3, as well as NF1, a GRE-like element, and the AF1 sequence (shaded). The position of the PBREM within the PBRU is also indicated. Not shown here are the binding sites for other transcription factors, the role of which in CYP2B expression, like that of NF1, is not clear. They include thyroid hormone receptor ί (TRί), liver X receptor (LXR), hepatocyte nuclear factor 4 (HNF-4) and heterodimers of PBX-PREP1.50
gical role of CAR.28–30 Over the past several years there have been numerous reports implicating CAR and the pregnane X receptor (PXR), both as sensors of xenochem-icals (“xenosensors”)31,32 and as regulators of genes coding for enzymes involved in metabolism and transport of endogenous compounds, notably bile acids,33 bilirubin34,35 and steroid hormones.36
2. Regulation of CYP2B Expression: Overview
The classical P450s inducible in rat liver by PB, CYP2B1 and CYP2B2, display remarkable inducibility by PB.14 In PB-treated Sprague-Dawley rats, hepatic mi-crosomal levels of CYP2B1 and CYP2B2 are, respectively, >200-fold and ˜27-fold greater than those of untreated rats.37,38 CYP2B1 and CYP2B2 mRNAs are virtually undetectable by hybridization in hepatic RNA prepared from untreated rats, but are among the most abundant
messages after PB or Aroclor 1254 administration.39–41 Most of the PB-dependent increase of CYP2B1 and CYP2B2 message level is due to increased transcription.42 Although PB has a multitude of effects on the liver,43 including a low-level induction of many genes,14,44 the rapid and high-level induction of CYP2B1 and CYP2B2 mRNA synthesis by PB39,42 suggests that the induction mechanism in this case is in some way unique or is accompanied by events specific to CYP2B1 and CYP2B2. In the rodent system generally, CYP2B genes are not PB-inducible in permanent cell lines21 and the only cultured cells in which CYP2B genes respond normally to PB treatment are primary hepatocytes.45,46 More specifically, in human HepG2 cells neither rodent CYP2B reporter constructs29 nor the endogenous CYP2B6 gene47 are responsive to PB-type in-ducers. A PB-responsive, differentiated HepG2 derivative, called WGA, has been described, but the increase after PB treatment of CYP2B6 mRNA in those cells was modest (4-fold),48 as compared to that of CYP2B1 or CYP2B2 proteins in rat liver37 or of CYP2B10 in mouse liver (>50-
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fold).49 Depending on the vector and the conditions of cell culture, the PB responsiveness of a CYP2B2 reporter construct in primary rat hepatocytes (25 to 75-fold)17,50 can be comparable to that of the CYP2B2 protein in rat liver. The PB responsiveness in mouse liver of a Cyp2b10 reporter construct assessed by tail vein injection (at least 100-fold)51 is also comparable to that of the CYP2B10 protein in mouse liver.
3. Regulation of CYP2B Expression
in Rats and Mice: the PBRU
and the PBREM
The sequences of the CYP2B1 and CYP2B2 PBRUs are identical except for a single base pair difference17 and the localization of the CYP2B1 PBRU has been confirmed by deletional analysis and reporter gene assays in primary rat hepatocytes.52 Nevertheless, there are striking differences in the tissue-specific expression of CYP2B1 and CYP2B2, most notably that in lung microsomes of Sprague -Dawley rats CYP2B2 is virtually undetectable whereas CYP2B1 is present constitutively but is only marginally (1.5-fold) PB-inducible.37 The regulatory sequences responsible for CYP2B1 expression in the lungs have yet to be characterized.
Although promoter-proximal sequences are doubtless required for basal transcriptional activity of CYP2B genes,15,43 there is at present no convincing evidence for their involvement in conferring PB responsiveness. Furthermore, Ramsden et al.53 found that in transgenic mice a rat CYP2B2 transgene including only the first 800 bp of 5’-flanking sequence exhibited constitutive expression in liver and kidney and was not PB-inducible, whereas a CYP2B2 transgene carrying 19 kb of 5’-flanking sequence exhibited liver-selective PB-inducible expression. These results provided the first indication that distal regulatory sequences were required for conferring PB responsiveness on CYP2B genes. Later the critical portion of the CYP2B2 5’ flank conferring PB responsiveness in transge-nic mice was limited to the region between –2500 to –1700.54 In accord with that conclusion, the PBRU that we localized by transfection of primary rat hepatocytes with reporter constructs is situated between –2155 and –2317 in the CYP2B2 5’ flank,17 near a liver-specific DNaseI hypersensitive site.55
The capacity of the rat CYP2B2 PBRU to confer PB responsiveness on heterologous promoters was confirmed in a quite different assay system involving in situ DNA injection into rat liver.56,57 A 177-bp fragment of the homologous region of the 5’-flank of the PB-inducible mouse Cyp2b10 gene also confers PB inducibility on heterolo-gous promoters and possesses the properties of a trans-criptional enhancer.58 It contains a 162-bp segment that is 92% identical to the rat CYP2B2 PBRU.16 Negishi and co-
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workers defined a 51-bp PB responsive enhancer module (PBREM) within the mouse Cyp2bl0 PBRU sequence. The PBREM confers PB responsiveness comparable to that of the full length PBRU in primary mouse hepatocy-tes when placed directly adjacent to the heterologous tk promoter.59 Hence, generally similar results have been obtained by transfection analysis in different laboratories using different experimental approaches and three PB-inducible CYP2B genes of two different rodent species. Furthermore, a number of PB-like inducers representing a variety of chemical classes have been shown to activate the mouse Cyp2bl059 and rat CYP2B260–62 distal enhancer elements.
4. Nuclear Receptors and CYP2B Induction
The rat CYP2B2 PBRU contains, among other putative transcription factor recognition sites, three DR4 elements (Fig. 1B). Two of the DR4 sites, NR1 and NR2, flank an NF1 site and were recognized as nuclear receptor binding sites by Negishi and coworkers in the homologous mouse Cyp2bl0 fragment.59 This fragment constitutes the PBREM.59 Except for a single base pair difference in the NR2 spacer, the CYP2B1, CYP2B2 and Cyp2bl0 PBREM sequences are identical.17 The third DR4 site, NR3, is upstream of NR1 and NR2.63,64 The human CYP2B6 5’-flank also contains a PBREM element consisting of two DR4 sites, NR1 and NR2, flanking an NF1 site.65
CAR, in the form of a heterodimer with the retinoic X receptor (RXR), binds to the retinoic acid ί2 response element (ίRARE)66,67 and to the NR1, NR2 and NR3 sites of the PBRU.64,67 CAR-RXR heterodimers also bind to the mouse mammary tumor virus (MMTV) liver X receptor DR4/5 response element (LXRE), the rat CYP3A1 (CYP3A23) DR3 response element, and the human CYP3A4 ER6 response element.67 Although HepG2 or other non PB-responsive cell lines are not appropriate models to study PB induction of CYP2B genes, such cell lines can and have been widely and successfully used to assess the potential of CAR, in the absence of added ligand, to activate transcription driven by numerous enhancers or enhancer elements. CAR activates transcription driven by the PBREM,29,50,67 the PBRU,50 and oligo-merized NR1,68 as well as oligomerized ίRARE, MMTV LXRE, CYP3A1 DR3, and CYP3A4 elements.67
Negishi and coworkers29,68 have shown that there is a PB-dependent (or PB-type inducer-dependent) nuclear accumulation of CAR in mouse liver. The synthetic compound 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) is a potent PB-like inducer in the mouse69,70 and is an agonist ligand for mouse CAR.67,71 The agonist activity of TCPOBOP on CAR may be unrelated to PB-type induction of rat or mouse CYP2B genes, however, because PB itself, at a concentration that induces both nuc-
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lear accumulation of CAR and Cyp2b10 expression in primary mouse hepatocytes,68 is not an agonist ligand of CAR.28,67,71 Rather, the ligand-independent transcriptional activation capacity of CAR-RXR heterodimers is thought to account for the PB-induced expression of CYP2B ge-nes.29,68 According to this widely held view,35,72–74 it is the PB-dependent displacement of CAR from the cytoplasm to the nucleus that explains PB-dependent transcriptional activation of CYP2B genes (Fig. 2).
Figure 2. Simplified schematic representation of the current model to account for the role of CAR in activating hepatic transcription of rodent CYP2B genes after treatment with PB. The drawing is not to scale. NR, one or more of the DR4 nuclear receptor binding sites of the CYP2B PBRU.
5. What is a PB-type Inducer?
In their recent review, Yamada et al.22 list some 100 different chemicals that induce CYPY2B mRNAs or proteins in rats or mice. Although about a dozen are barbiturates, most of the others have no evident structural relationship either with PB or with each other. Furthermore, even this daunting list is incomplete or still growing. For example, dieldrin, nonylphenol, and ligands of the GABAA/central benzodiazepine receptor and the peripheral benzodiazepine receptor (now called “translocator protein (18 kDA)”75), all known CYP2B inducers,61,76,77 are not included. The general question may be raised as to whether these CYP2B inducers all act by the mechanisms described in sections 1 to 4. In other words, are they all “PB-type inducers”? Implicit in the posing of this question is the notion that PB-type inducers not only induce the same genes as PB, but that they act by a similar mechanism. In any case, it is striking that a great many CYP2B-inducing chemicals do act via the PBRU or the PBREM,59–62 stimulate translocation of CAR from the cytoplasm to the nucleus in rodent hepatocytes,68 and fail to cause detectable Cyp2b10 induction in CAR-negative mice.25,28 Thus, to a large extent, whenever PB-type indu-cers have been examined, they have been found to exhibit these three properties.
The case of dexamethasone (Dex), which induces CYP2B78 as well as CYP3A genes,14 is clearly different from other CYP2B inducers, however. In the first place, in
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mice in which the glucocorticoid receptor gene had been subjected to targeted inactivation, hepatic CYP3A and CYP2B proteins were induced by PB, whereas only CYP3A but not CYP2B proteins were induced by Dex.79 These results indicate that the glucocorticoid receptor is required for Cyp2b induction by Dex. In CAR-negative mice on the other hand, whereas treatment with numerous PB-type inducers fails to lead to the normal increase in CYP2B mRNA,25–28 treatment with Dex does lead to induction of this messenger.28 Hence, it seems clear that there are at least two different molecular mechanisms leading to normal induction of CYP2B genes, one dependent on CAR and the other mediated by the glucocorticoid receptor. The sequence elements conferring responsiveness to Dex have yet to be fully characterized. In the rat CYP2B2 gene there are two candidates, the functional glucocorti-coid response element (GRE) identified by Jaiswal et al.80 and located approximately 1.3 kb upstream of the transcription start site, and the putative GRE located within the PBRU.16 Further study will be required to determine which if either is required to confer Dex responsiveness in hepatocytes, but the GRE at –1.3 kb appears to be the better candidate given the results of Jaiswal et al.80 indicating that it confers Dex responsiveness on a CAT reporter construct in H4II rat hepatoma cells.
There is also a case where CYP2B inducers act independently of the PBRU. The alkyloids strychnine and brucine are potent inducers of CYP2B1 and CYP2B2 in rat liver, although the profile of induced proteins and enzyme activities differs from that characteristic of PB.81 For example, treatment with PB leads to a greater increase of CYP2B1 than CYP2B2, whereas for strychnine and brucine the situation is reversed.81 These alkaloids are also potent inducers of CYP2B3,81,82 which, however, is not in-ducible by PB.3–5 The 5’ flank of the CYP2B3 gene does not contain a typical PBRU (Fig. 1A), although it does contain a mutated and presumably inactive variant ( Y. Paquet and A. Anderson, unpublished observations). In this respect CYP2B3 is similar to the mouse Cyp2b9 gene.58 Thus, strychnine and brucine induce CYP2B3 by a mechanism that does not require a functional PBRU and may induce CYP2B1 and CYP2B2 by the same mechanism.
6. AMP-activated Protein Kinase (AMPK) and CYP2B Induction
A major recent development concerning the regulation of CYP2B expression by PB-type inducers has been the conclusion that AMPK plays a role in the pro-cess.48,83–85 The AMPK cascade is a sensor of the cellular energy charge.86 The energy charge hypothesis to the effect that AMP levels mediate energy metabolism was first proposed by Daniel Atkinson.87,88 In its present form it entails the notion that elevated AMP levels activate AMPK, which then acts to inhibit ATP-consuming anabolic pro-
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cesses and to stimulate ATP-generating catabolic pathways, both acutely, by phosphorylating metabolic enzymes, and chronically, by effects on gene expression.86 The adenosine analog 5-aminoimidazole-4-carboxamide ribo-side (AICAR) can enter cells where it is phosphorylated to the corresponding nucleotide, referred to as ZMP, which mimics the effects of AMP on AMPK.89 Hence, AICAR is a pharmacological AMPK activator.
Rencurel et al.48 observed among other things that AICAR induced CYP2B proteins and mRNA in primary rat hepatocytes and CYP2B6 mRNA in WGA cells; that a constitutively active form of AMPK mimicked the effect of PB on CYP2B6 mRNA levels in WGA cells, whereas a dominant negative form of AMPK inhibited PB induction of CYP2B mRNA in primary rat hepatocytes; and finally that PB at very high concentrations (up to 10 mM) led to an increase of AMPK activity in WGA cells and in H4IIE cells. Shindo et al.85 observed that PB-like inducers activated AMPK in rat liver, as shown by increased phosp-horylation of its Thr-172 residue. They were, however, unable to detect induction by AICAR of hepatic CYP2B proteins or associated enzymatic activity in rats or mice. Another extensive series of experimental results83 supported a role for AMPK in the induction of the CYP2B6 gene in primary human hepatocytes and Cyp2b10 in primary mouse hepatocytes and in mouse liver. Here, the key observations were made using mice in which the genes for both catalytic subunits of AMPK had been subjected to liver-specific targeted inactivation. Primary hepatocytes of such mice were refractory to PB or TCPOBOP induction of CYP2B10 mRNA; in the liver, however, the levels of CYP2B10 mRNA after inducer treatment were similar to those of the wild type animals, but the basal level was increased by 100-fold. It may be, as the authors suggest,83 that circulating factors account for the increased basal level of Cyp2b10 in the mutant mice. But it is very curious that the absence of AMPK activity should be associated with high constitutive Cyp2b10 expression in vivo, whereas AMPK activators induce CYP2B gene expression in cell culture. In any case, it seems clear that although AMPK is not essential for expression of the Cyp2b10 gene it may well play a role in its induction by PB-type agents. The experiments of Blδttler et al.,84 performed with the chicken hepatoma cell line LMH in which the CYP2H1 gene is PB-inducible, provide further support for a role of AMPK in the induction process. The results suggest that PB-like inducers may activate a signaling cascade via the mitochondria leading to formation of reactive oxygen species and phosphorylation of the AMPK kinase LKB1, which in turn leads to phosphorylation and activation of AMPK and finally transcriptional activation of CYP2B genes. This provocative model will require further experimental testing. In any case it has the merit of opening a new approach to the resolution of the vexing problem noted by Rencurel et al.83 : despite extensive study of the molecular mechanism whereby PB induces CYP2B
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gene expression, an intracellular protein target of PB has yet to be identified.
7. Induction of CYP2B Genes
in Rodents by PB-like Inducers:
What Remains to be Learned?
Remarkable progress has been made over the past ten to fifteen years in understanding the mechanism of action of PB-like inducers on the expression of rodent (and human) CYP2B genes. We have seen a brief overview of that progress in this review. Progress has also been remarkable in understanding the effect of PB-like inducers on many other genes that respond to them or that depend for their expression on CAR or PXR activation. These latter issues have not been dealt with here, but have been treated extensively in several recent reviews.23,90–94
But what issues remain to be resolved concerning the major thrust of this review, that is the induction of CYP2B genes in rodents by PB-like inducers?
We have seen one such issue in section 6 above. What is the protein target of PB, if indeed there is one? This fundamental question still awaits a response. A related issue concerns the role of CAR, which of course is essential for normal expression of Cyp2b10. PB is not an agonist ligand of CAR, yet PB treatment causes nuclear accumulation of CAR in rodent hepatocytes. The process of PB-induced nuclear accumulation is as yet poorly characterized, although some elements are known.93 Another issue is the fate of CAR once it reaches the nucleus. It seems that the mere presence of CAR in the nucleus is not sufficient to activate CYP2B transcription. For example, bilirubin is as effective as PB in causing nuclear accumulation of CAR in primary mouse hepatocytes,95 but it is not a PB-type inducer and indeed it may suppress responsiveness to PB-type inducers.96 Another result leading to a similar conclusion is that the Ca2+/calmodulin-dependent ki-nase inhibitor KN-62 inhibited induction of CYP2B10 mRNA in primary mouse hepatocytes without concomitant inhibition of CAR nuclear accumulation.97 This led to the proposal that CAR undergoes an activation process in the nucleus.35,97,98 Hence, it appears that CAR can be nuclear and not stimulate CYP2B transcription. The nature of the putative nuclear activation of CAR is not characterized.
The properties of CAR-negative mice clearly show that a product of the mouse Car gene is required to confer normal levels of response to PB-like inducers on the Cyp2b10 gene.25,28 On the other hand, adding exogenous mouse CAR to primary rat hepatocytes has an inhibitory effect on PB responsiveness as assayed using a reporter construct driven by the rat CYP2B2 PBRU in its natural sequence context.17 Recent results from Auerbach et al.,99 showing that splice variants of human CAR behave differently from the reference form in the presence of certain
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compounds, may be pertinent in this regard. The case of hCAR3 is particularly relevant to the present discussion. Clotrimazole is an agonist ligand for this variant of human CAR,99 whereas it acts as an inverse agonist of the reference form of human CAR.71 This raises the intriguing possibility that the inhibitory effects of the reference form of mouse CAR on PB responsiveness of the PBRU-driven reporter construct in rat hepatocytes17 may result from interference with the activity of an endogenous splice variant required for CYP2B2 expression. Further investigation will be required to determine if the rat splice variants of CAR described by Kanno et al.100 respond differently than the reference form to PB and other PB-like inducers as the human hCAR3 does with respect to clotrimazole.
There is a final issue relating to CYPB2B induction by PB-like inducers that is yet to be resolved. Curiously, given the time that has elapsed and the effort invested, that issue is the precise definition of the PBRU sequence elements required to confer PB responsiveness. As we have seen, the 51-bp PBREM confers a PB response on a reporter construct in primary hepatocytes that is comparable to that of the PBRU when fused to a tk promoter.59 But when the PBRU in the natural sequence context of the CYP2B2 5’ flank was replaced with the PBREM, PB response was reduced by at least 4-fold.17 Furthermore, mutational analysis, once again in the natural sequence context, demonstrated that PBRU sequence elements outside the rat CYP2B2 PBREM are essential for maximal PB responsiveness and that one such element is likely to be NR3.17 In the mouse system, River-Rivera et al.51 undertook a series of experiments using reporter constructs in which the mouse Cyp2b10 PBRU, or fragments thereof, or chimeric PBRU sequences consisting of portions of the active Cyp2b10 PBRU and the inactive Cyp2b9 PBRU, were placed directly upstream of the rabbit CYP2C1 basal promoter and injected into the mouse tail vein to assess PB responsivess in the liver. Among the results was the observation that a 62-bp fragment of the Cyp2b10 containing the 51-bp PBRU was inactive in conferring PB responsiveness in the tail vein assay. The addition to the 62-bp fragment of a portion 3’ of the PBREM, which included the previously identified16 AF-1 sequence (Fig. 1B), partially restored PB responsiveness. The 62-bp fragment containing the PBREM also failed to confer PB responsiveness in the tail vein assay when it was placed directly upstream of the tk promoter.
In summary then, in transfected primary rat hepa-tocytes the rat CYP2B2 PBREM placed in the natural sequence context is only partially active in conferring PB responsiveness on a reporter construct; furthermore, a sequence on the 5’ side contributes to maximal responsive-ness.17 With the mouse tail vein system, the mouse Cyp2b10 PBREM placed directly adjacent to a basal promoter (the CYP2C1 promoter or the tk promoter) is inactive in conferring PB responsiveness on a reporter construct; in this case addition of a sequence on the 3’ side
partially restored PB responsiveness. The PBREM, however, is essentially as active as the PBRU when placed directly adjacent to a basal promoter and tested in a reporter assay in primary mouse59 or rat17 hepatocytes. Doubtless these different results depend in part at least on differences in experimental protocols. One reason why it is important to resolve the issue of the precise sequences required to confer PB responsiveness is related to the nature of the PBRU. Is it a unit in which several sites interact to confer full PB responsiveness? Or, is it fundamentally a single repeated element, that does not require interaction with other elements to confer full PB responsiveness? That issue is not yet resolved.
8. Acknowledgments
Work from our laboratory was supported by the Canadian Institutes of Health Research. I thank Yanick Paquet for his suggestions and Amaury Lachaud for his comments on the manuscript.
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Povzetek
Citokromi P450 (CYP) so klju~ni encimi faze I presnove ksenobiotikov in endobiotikov v jetrih. Mnogi od jetrnih CYP se s ksenobiotiki inducirajo. Tipi~ni primer inducibilnega CYP predstavlja CYP2B protein v jetrih podgan in mi{i, kjer je mo~no induciran s fenobabitalom (PB). Za 163-bp dolg Sau3AI fragment v 5’-neprevedeni regiji CYP2B2 je bila s poro~evalskim sistemom v primarnih podganjih hepatocitih dokazana inducibilnost s PB, fragment pa ima tudi lastnosti oja~evalca. Ta odsek DNA se imenuje enota odzivna na fenobarbital (PBRU). Namen tega ~lanka je povzeti napredek, ki je bil narejen od odkritja PBRU dalje, in kriti~no oceniti, kaj se moramo {e nau~iti o indukciji CYP2B proteinov (ne le pri glodalcih, ampak tudi pri ~loveku) s PB in njemu podobnimi induktorji.
Anderson:  The Induction of CYP2B Proteins in Rodents by Phenobarbital-Like Inducers: ...