Phenobarbital-induced activation of CYP2H1 and 5-aminolevulinate synthase genes in chick embryo hepatocytes is blocked by an inhibitor of protein phosphorylation

Haem repression of the housekeeping 5-aminolaevulinic acid synthase gene in the hepatoma cell line LMH

Haem is essential for the health and function of nearly all cells. 5-Aminolaevulinic acid synthase-1 (ALAS-1) catalyses the first and rate-controlling step of haem biosynthesis. ALAS-1 is repressed by haem and is induced strongly by lipophilic drugs that also induce CYP (cytochrome P450) proteins. We investigated the effects on the avian ALAS-1 gene promoter of a phenobarbital-like chemical, Glut (glutethimide), and a haem synthesis inhibitor, DHA (4,6-dioxoheptanoic acid), using a reporter gene assay in transiently transfected LMH (Leghorn male hepatoma) hepatoma cells. A 9.1 kb cALAS-1 (chicken ALAS-1) promoter-luciferase-reporter construct, was poorly induced by Glut and not by DHA alone, but was synergistically induced by the combination. In contrast, a 3.5 kb promoter ALAS-1 construct was induced by Glut alone, without any further effect of DHA. In addition, exogenous haem (20 microM) repressed the basal and Glut- and DHA-induced activity of luciferase reporter constructs containing 9.1 and 6.3 kb of ALAS-1 5'-flanking region but not the construct containing the first 3.5 kb of promoter sequence. This effect of haem was subsequently shown to be dependent on the -6.3 to -3.5 kb region of the 5'-flanking region of cALAS-1 and requires the native orientation of the region. Two deletion constructs of this approx. 2.8 kb haem-repressive region (1.7 and 1.1 kb constructs) retained haem-dependent repression of basal and drug inductions, suggesting that more than one cis-acting elements are responsible for this haem-dependent repression of ALAS-1. These results demonstrate that there are regulatory regions in the 5'-flanking region of the cALAS-1 gene that respond to haem and provide a basis for further investigations of the molecular mechanisms by which haem down-regulates expression of the ALAS-1 gene.

Induction of Drug Metabolism: The Role of Nuclear Receptors

Induction of drug metabolism was described more than 40 years ago. Progress in understanding the molecular mechanism of induction of drug-metabolizing enzymes was made recently when the important roles of the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), two members of the nuclear receptor superfamily of transcription factors, were discovered to act as sensors for lipophilic xenobiotics, including drugs. CAR and PXR bind as heterodimeric complexes with the retinoid X receptor to response elements in the regulatory regions of the induced genes. PXR is directly activated by xenobiotic ligands, whereas CAR is involved in a more complex and less well understood mechanism of signal transduction triggered by drugs. Most recently, analysis of these xenobiotic-sensing nuclear receptors and their nonmammalian precursors such as the chicken xenobiotic receptor suggests an important role of PXR and CAR also in endogenous pathways, such as cholesterol and bile acid biosynthesis and metabolism. In this review, recent findings regarding xenosensors and their target genes are summarized and are put into an evolutionary perspective in regard to how a living organism has derived a system that is able to deal with potentially toxic compounds it has not encountered before.

Evidence that the coactivator CBP/p300 is important for phenobarbital-induced but not basal expression of the CYP2H1 gene

We have previously identified an upstream 556-bp enhancer domain for the chicken CYP2H1 gene that responds to phenobarbital and binds several transcription factors, including the orphan chicken xenobiotic receptor (CXR). By contrast, the promoter lacks a CXR site and is not inducible by phenobarbital. Although it has been established that CXR can interact with the coactivator SRC-1, there are no reports as to whether other coactivators may be important for phenobarbital-mediated inducibility. Our studies using the adenovirus E1A wild-type protein, which inhibits the coactivators cAMP response element binding protein (CBP) and CBP associated factor (p/CAF), provide evidence for the involvement of one or both of these coactivators at the enhancer but not at the promoter of the CYP2H1 gene. The observations that mutant E1A proteins did not affect the enhancer activity and that inhibition by wild-type E1A was reversed by CBP and p/CAF confirmed the involvement of these coactivators in the induction process. We propose that the intrinsic histone acetyl transferase activity of one or both of these coactivators participates in chromatin remodeling thereby stimulating drug induction of the promoter. This proposal was supported by experiments with the histone deacetylase inhibitor, trichostatin A, which resulted in the superinduction of the drug response but had little effect on basal expression of the CYP2H1 gene. The work provides evidence for the first time for the involvement of the coactivators CBP and p/CAF in the phenobarbital-mediated induction of the CYP2H1 gene.

A duplicated HNF-3 binding site in the CYP2H2 promoter underlies the weak phenobarbital induction response

We are investigating induction of chicken cytochrome P450 genes by the sedative phenobarbital in chick embryo hepatocytes. The steady-state level of induced mRNA for the gene CYP2H1 is about 10-fold higher than that of a second gene, CYP2H2. Here, we show that a difference in drug-responsive enhancer activity does not underlie the differential response of these genes to phenobarbital since upstream enhancer regions are identical in these genes. The first 198 bp of CYP2H2 promoter sequence is identical to the CYP2H1 gene promoter, except that the functional HNF-3 binding site in the CYP2H1 promoter is replaced with a duplicated HNF-3 sequence in the CYP2H2 promoter. Transient expression analysis established that the promoter activity of the CYP2H2 gene was about ninefold lower than the CYP2H1 gene. Mutagenesis of either of the partially overlapping HNF-3 sites in the CYP2H2 gene substantially induced drug induction. Gel-shift analysis established that each of these HNF-3 sites bound HNF-3, most likely HNF-3beta. In-vitro footprint analysis demonstrated that all the identified sites in the CYP2H2 promoter bound protein except the duplicated HNF-3 region. However, protein binding was observed by in-vitro footprint analysis if either of the HNF-3 sites was mutated in the CYP2H2 promoter. Hence, duplication of the HNF-3 site in the CYP2H2 promoter does not allow binding of HNF-3 in the promoter context and may be predominantly, if not exclusively, responsible for the poor response of the CYP2H2 gene to phenobarbital.

The antiglucocorticoid RU486 inhibits phenobarbital induction of the chicken CYP2H1 gene in primary hepatocytes

The cytochrome P450 gene CYP2H1 is highly induced by phenobarbital in chick embryo hepatocytes. Recent studies have established that the orphan nuclear receptor CAR plays a critical role in the induction mechanism. Here, we show that a high concentration of the potent glucocorticoid and progesterone receptor antagonist RU486 almost completely blocks phenobarbital-induced accumulation of CYP2H1 mRNA in hepatocytes yet has no effect on basal expression. In marked contrast, CYP2H1 mRNA induced by the phenobarbital-type inducers glutethimide and 2-allylisopropylacetamide is not affected by RU486. RU486 inhibition is not mediated through the glucocorticoid or progesterone receptors. Transient transfection studies showed that RU486 does not repress through activation of the orphan receptor PXR and subsequent competition with CAR for binding to the upstream drug-responsive 556-base-pair enhancer. Additionally, none of the known functional transcription factor binding sites found in the enhancer region was a target of RU486 inhibition. Using an artificial construct containing multiple CAR binding sites, we also established that RU486 has no direct effect on the activity of exogenously expressed CAR. There is no evidence that phenobarbital binds to CAR; we propose that RU486 inhibits phenobarbital induction, either by interfering with a phenobarbital-dependent mechanism responsible for nuclear import of CAR or with the metabolism of phenobarbital to the true inducer. Whether a novel nuclear receptor that binds RU486 at high concentrations plays a role in the inhibitory action of RU486 is an interesting possibility.

Regulation of phenobarbital induction of the cytochrome P450 2b9/10 genes in primary mouse hepatocyte culture. Involvement of calcium- and cAMP-dependent pathways

Phenobarbital (PB) has long been known as an inducer of drug-metabolizing enzymes in liver, but the molecular mechanism underlying this induction is still poorly understood. Using primary mouse hepatocyte culture, we have investigated the possible involvement of different regulatory pathways in PB action, by exposing PB-treated cells to various protein kinase/phosphatase modulators. Our results showed a negative role of the cAMP-dependent pathway, as treatment with cAMP-dependent protein kinase (PKA) activators (10 microM dibutyryl-cAMP and 50 microM forskolin) dramatically inhibited PB-induced Cyp2b9/10 mRNA accumulation, whereas PKA inhibitor potentiated the PB responsiveness of this gene. The cGMP-dependent protein kinase (PKG) seems to play a positive role as PKG inhibitor reduced the PB-induced level of Cyp2b9/10 mRNA. We also obtained two lines of evidence for the involvement of Ca2+ in modulating PB action. Firstly, measurements of intracellular Fura-2 fluorescence ratio in murine hepatocytes showed that long-term PB incubation (24 and 48 h) led to a significant increase of [Ca2+]i. Secondly, treatment with an intracellular Ca2+ chelator (BAPTA-AM) nearly completely abolished PB-induced Cyp2b9/10 expression. Ca2+ thus appeared to mediate PB action likely via Ca2+/calmodulin-dependent protein kinase II, as KN62, a specific inhibitor of this enzyme, also dramatically inhibited PB induction of the Cyp2b9/10 genes.

Transcriptional regulation of 5-aminolevulinate synthase by phenobarbital and cAMP-dependent protein kinase

5-Aminolevulinate synthase (ALA-S) is a mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of the heme biosynthesis. There are two ALA-S isozymes encoded by distinct genes. One gene encodes an isozyme that is expressed exclusively in erythroid cells, and the other gene encodes a housekeeping isozyme that is apparently expressed in all tissues. In this report we examine the mechanisms by which phenobarbital and cAMP regulate housekeeping ALA-S expression. We have determined that cAMP and phenobarbital effects are additive and the combined action is necessary to observe the cAMP effect on ALA-S mRNA in rat hepatocytes. The role of the cAMP-dependent protein kinase (PKA) has been examined. A synergism effect on ALA-S mRNA induction is observed in rat hepatocytes treated with pairs of selective analogs by each PKA cAMP binding sites. A 870-bp fragment of ALA-S 5'-flanking region is able to provide cAMP and phenobarbital stimulation to chloramphenicol O-acetyltranferase fusion vectors in transiently transfected HepG2 cells. ALA-S promoter activity is induced by cotransfection with an expression vector containing the catalytic subunit of PKA. Furthermore, cotransfection with a dominant negative mutant of the PKA regulatory subunit impairs the cAMP analog-mediated increase, but the phenobarbital-mediated induction is not modified. Our data suggest that the transcription factor cAMP-response element binding protein (CREB) is probably involved in PKA induction of ALA-S gene expression. Finally, heme addition greatly decreases the basal and phenobarbital or cAMP analog-mediated induction of ALA-S promoter activity. The present work provides evidence that cAMP, through PKA-mediated CREB phosphorylation, and phenobarbital induce ALA-S expression at the transcriptional level, while heme represses it.

Phosphorylation/Dephosphorylation steps are crucial for the induction of CYP2B1 and CYP2B2 gene expression by phenobarbital

The effects of several protein kinase activators and protein phosphatase inhibitors on the phenobarbital (PB)-induced gene expression of CYP2B1 and CYP2B2 (CYP2B1/2B2) in adult rat hepatocytes were investigated. Insulin, epidermal growth factor, interleukin 6, cAMP, phorbol 12-myristate 13-acetate, tumor necrosis factor alpha, vanadate, and okadaic acid were found to suppress the induction of CYP2B1/2B2 at mRNA and protein levels in hepatocytes. cAMP and vanadate completely suppressed the induction of CYP2B1/2B2 gene expression in both rat hepatocytes and liver. The addition of genistein to vanadate-treated hepatocytes partially recovered the induction of CYP2B1/2B1 gene expression by PB. These results of the present study demonstrate that phosphorylation/dephosphorylation steps are crucial for the induction of CYP2B1/2B2 gene expression by PB.

Lack of modulation by phenobarbital of cyclic AMP levels or protein kinase A activity in rat primary hepatocytes

Results of previous studies have substantiated a negative modulatory role for cyclic AMP (cAMP) and protein kinase A (PKA) dependent processes on the phenobarbital (PB) induction response in hepatocytes. The current study was conducted to further examine the potential role of second messenger pathways in the initial phases of induction, specifically addressing the effects of PB on the expression of intracellular cAMP levels and associated PKA activity. Using a highly differentiated primary rat hepatocyte system, cells were exposed to PB for various intervals (30 sec to 48 hr), and levels of intracellular cAMP and subsequent PKA activity were determined. Although PB markedly induced CYP2B expression, exposure to this agent produced no detectable increases in cAMP levels and PKA activity at any of the times examined. These results demonstrated that the initial events stimulated by PB in rat hepatocytes do not include alterations of cAMP levels or associated PKA activities.

Modulation of murine phenobarbital-inducible CYP2A5, CYP2B10 and CYP1A enzymes by inhibitors of protein kinases and phosphatases

Phenobarbital causes a multitude of effects in hepatocytes, including increased cell proliferation, inhibition of apoptosis and upregulation of xenobiotic and endobiotic metabolizing enzymes. In this study, the involvement of several protein kinase and phosphatase pathways on constitutive and phenobarbital-induced activities of CYP2A5, CYP2B10 and CYP1A1/2 in primary mouse hepatocytes was determined using well-defined chemical modulators of intracellular protein phosphorylation and desphosphorylation events. A 48-h treatment of the hepatocytes with 2-aminopurine, a nonspecific serine/threonine kinase inhibitor, elicited dose-dependent increases in both basal and phenobarbital-induced CYP2A5 catalytic activity (assayed as coumarin 7-hydroxylation), the maximal induction being 60-fold greater than the control value upon cotreatment with 1.5 mM phenobarbital and 10 mM 2-aminopurine. In contrast, phenobarbital induction of CYP2B10 (pentoxyresorufin O-deethylase) and CYP1A1/2 (ethoxyresorufin O-deethylase) activities were blocked by 2-aminopurine. Increases in CYP2A5 activity were also observed after exposure of the hepatocytes to other protein kinase inhibitors affecting the cell cycle, i.e. roscovitine, K-252a and rapamycin. Inhibitors of protein kinases A and C, as well as tyrosine kinases, did not appreciably affect CYP2A5 activity levels. The serine/threonine phosphatase inhibitors tautomycin, calyculin A and okadaic acid all reduced both basal and phenobarbital-induced CYP2A5, CYP2B10 and CYP1A1/2 activities. These results further strengthen the concept that hepatic CYP2A5 is regulated in a unique way compared with CYP2B10 and CYP1A.

Evidence for involvement of cAMP-dependent pathway in the phenobarbital-induced expression of a novel hamster cytochrome P450, CYP3A31

Recently, we isolated a novel Syrian hamster cDNA clone that encodes a protein which has been named CYP3A31. In primary hepatocyte cultures, CYP3A31 is dramatically induced by phenobarbital. To elucidate the mechanism of this induction, we first studied the effects of cAMP on phenobarbital-induced CYP3A31 expression using forskolin and N6,O2'-dibutyryl cAMP in hepatocyte cultures. At 100 microM, forskolin significantly inhibited both the phenobarbital-induced CYP3A31 mRNAs expression and the testosterone 6beta-hydroxylation activity related to the CYP3A subfamily in rats, whereas 0.1 microM forskolin potentiated the phenobarbital induction of CYP3A31 mRNA and the testosterone 6beta-hydroxylation activity. Treatment with N6,O2'-dibutyryl cAMP resulted in an inhibition of phenobarbital-induced CYP3A31 gene expression and testosterone 6beta-hydroxylation activity. Increasing amounts of transfected cAMP-response element binding proteins (CREB) or CREB-binding proteins in hamster hepatocytes reduced the phenobarbital-induction of CYP3A31 mRNAs expression. These results suggest that in vitro induction of CYP3A31 by phenobarbital in Syrian hamster hepatocytes is regulated by a cAMP-dependent pathway.

Liver-enriched transcription factors, HNF-1, HNF-3, and C/EBP, are major contributors to the strong activity of the chicken CYP2H1 promoter in chick embryo hepatocytes

Chicken CYP2H1 promoter constructs express strongly in chick embryo hepatocytes at a level comparable with that of Rous sarcoma viral promoter. We have identified the transcription factors responsible for the active CYP2H1 promoter. Binding sites for transcription factors were located within the first 160 bp of promoter sequence using promoter deletion experiments and DNase I footprint analysis. Sequence analysis revealed characteristic sites for the liver-enriched transcription factors of the HNF-1, HNF-3, and C/EBP families and for the ubiquitous factor, USF. Protein binding to these sites was established by gel mobility shift assays. Mutagenesis and transient transfection experiments demonstrated that these sites, in combination, were responsible for the strong promoter activity with a substantial contribution from HNF-1 and HNF-3. The promoter was also active in mammalian HepG2 and COS-1 cell lines where expression was dependent on the identified transcription factor binding sites but promoter activity in the HeLa cells was low. Transactivation experiments revealed that promoter expression could be activated through the appropriate binding sites by exogenously expressed rat HNF-1alpha or HNF-1beta, rat HNF-3alpha or HNF-3beta and chicken C/EBP alpha. Transcriptional synergism between HNF-1 and C/EBP was observed in these transactivation experiments. A Barbie box-like sequence overlapped the USF element but was not functional. The results demonstrate that liver-enriched transcription factors and USF direct strong expression of the CYP2H1 promoter in transiently transfected cells. By comparison, in vivo expression of this gene in uninduced chick embryo hepatocytes is low but markedly increased by phenobarbital. Drug induction may therefore substantially reflect derepression of this inherently active promoter.

Characterization of a pretranscriptional pathway for induction by phenobarbital of cytochrome P450 3A23 in primary cultures of adult rat hepatocytes

Our laboratory has proposed that phenobarbital (PB), a typical lipophilic agent that induces some members of the supergene family of liver microsomal cytochromes P450 (e.g., CYP2B1/2 and CYP3A23), acts through a complex process inhibitable by the presence of growth hormone (GH), the absence of some components of the extracellular matrix, or a disrupted cytoskeleton. To verify that these manipulations of the culture environment block specific steps in the PB induction pathway rather than simply exerting nonspecific or toxic effects on CYP2B1/2 gene transcription, we have now examined PB induction of CYP3A23, a gene known to also be transcriptionally activated by dexamethasone (DEX) through a "nonclassical" pathway apparently involving the glucocorticoid receptor. We found that in primary cultures of adult rat hepatocytes treated with PB, induction of CYP3A23 mRNA, just as we reported for induction of CYP2B1/2 mRNA, required the use of Matrigel (a reconstituted basement membrane) and was blocked by the presence of cytoskeletal inhibitors (colchicine or cytochalasins) or of physiologic concentrations of GH in the culture medium. Moreover, PB induction of CYP3A23 and of CYP2B1/2 mRNAs was greatly diminished by inhibitors of cAMP-dependent protein kinase (PKA). In striking contrast, induction of CYP3A23 mRNA by DEX was unaffected by any of these alterations of the culture conditions that block its induction by PB. We conclude that the effects of extracellular matrix, GH, disruption of the cytoskeleton, and activation of cAMP-dependent protein kinase, pharmacologically define multiple, pretranscriptional steps in the pathway(s) for PB induction of liver cytochromes P450.