Paradoxical transcriptional activation of rat liver cytochrome P-450 3A1 by dexamethasone and the antiglucocorticoid pregnenolone 16 alpha-carbonitrile: analysis by transient transfection into primary monolayer cultures of adult rat hepatocytes

Scaffold-Based microRNA Therapies in Regenerative Medicine and Cancer

microRNA-based therapies are an advantageous strategy with applications in both regenerative medicine (RM) and cancer treatments. microRNAs (miRNAs) are an evolutionary conserved class of small RNA molecules that modulate up to one third of the human nonprotein coding genome. Thus, synthetic miRNA activators and inhibitors hold immense potential to finely balance gene expression and reestablish tissue health. Ongoing industry-sponsored clinical trials inspire a new miRNA therapeutics era, but progress largely relies on the development of safe and efficient delivery systems. The emerging application of biomaterial scaffolds for this purpose offers spatiotemporal control and circumvents biological and mechanical barriers that impede successful miRNA delivery. The nascent research in scaffold-mediated miRNA therapies translates know-how learnt from studies in antitumoral and genetic disorders as well as work on plasmid (p)DNA/siRNA delivery to expand the miRNA therapies arena. In this progress report, the state of the art methods of regulating miRNAs are reviewed. Relevant miRNA delivery vectors and scaffold systems applied to-date for RM and cancer treatment applications are discussed, as well as the challenges involved in their design. Overall, this progress report demonstrates the opportunity that exists for the application of miRNA-activated scaffolds in the future of RM and cancer treatments.

Arsenic decreases RXRα-dependent transcription of CYP3A and suppresses immune regulators in hepatocytes

Arsenite is critical pharmacologically as a treatment for advanced stage blood cancer. However, environmental exposure to arsenic results in multiple diseases. Previous studies have shown that arsenic decreases expression of CYP3A, a critical drug metabolizing enzyme in human and rat liver. In addition, acute and chronic arsenic exposure in liver stimulates an inflammatory response. Our work has shown that arsenite decreases nuclear levels of RXRα the nuclear receptor that, as a heterodimer partner with PXR, transactivates the CYP3A gene. These results suggest that arsenite decreases transcription of CYP3A by decreasing RXRα. The present report shows that exposure to 5 μM arsenite decreased the activity of a rat CYP3A promoter luciferase reporter in HepG2 cells. The activity of a RARE-luciferase reporter, that is transactivated by the retinoic acid receptor (RAR)/RXRα, was also decreased. Previous studies have shown that arsenic in the concentration range of 2-5 μM affects CYP3A mRNA. When rifampicin-treated primary human hepatocyte cultures were exposed to arsenite concentrations as low as 50 nM, CYP3A mRNA was decreased. Treatment of primary human hepatocytes with the proteasome inhibitor MG132 increased RXRα suggesting the involvement of the proteasome pathway in regulation of RXRα. Finally, arsenic induces a pro-inflammatory response in liver. Surprisingly, we show that in hepatocytes arsenite decreases expression of two inflammatory mediators, TNF and VEGF, an effect that is not predicted from suppression of RXRα activity.

Membrane-bound and exosomal metastasis-associated C4.4A promotes migration by associating with the α(6)β(4) integrin and MT1-MMP

Metastasis-associated C4.4A, which becomes upregulated during wound healing and, in some tumors, during tumor progression, is known to be frequently associated with hypoxia. With the function of C4.4A still unknown, we explored the impact of hypoxia on C4.4A expression and functional activity. Metastatic rat and human tumor lines upregulate C4.4A expression when cultured in the presence of CoCl(2). Although hypoxia-inducible factor 1α (HIF-1α) becomes upregulated concomitantly, HIF-1α did not induce C4.4A transcription. Instead, hypoxia-induced C4.4A up-regulation promoted in vivo and in vitro wound healing, where increased migration on the C4.4A ligands laminin-111 and -332 was observed after a transient period of pronounced binding. Increased migration was accompanied by C4.4A associating with α(6)β(4), MT1-MMP1, and TACE and by laminin fragmentation. Hypoxia also promoted the release of C4.4A in exosomes and TACE-mediated C4.4A shedding. The association of C4.4A with α(6)β(4) and MT1-MMP1 was maintained in exosomes and exosomal α(6)β(4)- and MT1-MMP1-associated C4.4A but not shed C4.4A sufficient for laminin degradation. Hypoxia-induced recruitment of α(6)β(4) toward raft-located C4.4A, MT1-MMP, and TACE allows for a shift from adhesion to motility, which is supported by laminin degradation. These findings provide the first explanation for the C4.4A contribution to wound healing and metastasis.

Interplay between cholesterol and drug metabolism

Cholesterol biosynthetic and metabolic pathways contain several branching points towards physiologically active molecules, such as coenzyme Q, vitamin D, glucocorticoid and steroid hormones, oxysterols, or bile acids. Sophisticated regulatory mechanisms are involved in maintenance of the homeostasis of not only cholesterol but also other cholesterogenic molecules. In addition to endogenous cues, cholesterol homeostasis needs to accommodate also to exogenous cues that are imported into the body, such as chemicals and medications. Steroid and nuclear receptors together with sterol regulatory element-binding protein (SREBP) mediate the fine tuning of biosynthetic and metabolic routes as well as transports of cholesterol and its derivatives. Similarly, drug/xenobiotic metabolism is the subject to the feedback regulation of cytochrome P450 enzymes and transporters. The regulatory mechanisms that maintain the homeostasis of cholesterogenic molecules and are involved in drug metabolism share similarities. Cholesterol and cholesterogenic compounds (bile acids, glucocorticoids, vitamin D, etc.) regulate the xenosensor signaling in drug-mediated induction of the major drug-metabolizing cytochrome P450 enzymes. The key cellular receptors, pregnane X receptor (PXR), constitutive androstane receptor (CAR), vitamin D receptor (VDR), and glucocorticoid receptor (GR) provide a functional cross-talk between the pathways maintaining cholesterol homeostasis and controlling the expression of drug-metabolizing enzymes. These receptors serve as metabolic sensors, resulting in a coordinate regulation of cholesterogenic compounds metabolism and of the defense against xenobiotic and endobiotic toxicity. Herein we present a comprehensive review of functional interactions between cholesterol homeostasis and drug metabolism involving the main nuclear and steroid receptors.

CEBPbeta, JunD and c-Jun contribute to the transcriptional activation of the metastasis-associated C4.4A gene

The glycosylphosphatidylinositol-anchored molecule C4.4A, which shares structural features with uPAR, is frequently expressed on carcinomas with upregulated expression during tumor progression. Moreover, rare expression on nontransformed epithelial cells is strongly increased during tissue remodeling, e.g., during wound healing. This strictly regulated expression prompted us to define transcriptional activation of the C4.4A gene. C4.4A transcription was analyzed in 2 syngenic rat tumor cell lines with low or high metastatic potential, respectively. Though genomic C4.4A DNA was present in both lines, C4.4A mRNA and transcription of a reporter construct containing the C4.4A promoter was only observed in the metastasizing subline. Deletions and point mutations in the C4.4A promoter-driven reporter construct revealed that activation of the TATA-less, GC-rich core promoter (-1 to -50 bp) does not suffice to initiate transcription that requires coactivation of a proximal response element (-71 to -88 bp) and can be further increased by more distal response elements (-89 to -133 bp). Mobility-shift and cotransfection studies showed that Sp3 binding enhances C4.4A transcription, whereas potential Sp1 binding sites were ineffective. C4.4A transcription essentially requires C/EBPbeta binding to a TRE/CCAAT composite element (-71 to -88 bp) as measured by ChIP assay. C4.4A transcription is strikingly enhanced by cotransfection with JunD or c-Jun, such that C4.4A is most strongly transcribed even in the C4.4A-negative tumor cell line after cotransfection with C/EBPbeta plus JunD or c-Jun. Thus, upregulation of C/EBPbeta during tumor progression and wound repair may well provide a sufficient trigger for transcription of the C4.4A gene.

PXR and CAR: nuclear receptors which play a pivotal role in drug disposition and chemical toxicity

Xenobiotic metabolism and detoxification is regulated by receptors (e.g., PXR, CAR) whose characterization has contributed significantly to our understanding of drug responses in humans. Technologies facilitating the screening of compounds for receptor interactions provide valuable tools applicable in drug development. Most use in vitro systems or mice humanized for receptors in vivo. In vitro assays are limited by the reporter systems and cell lines chosen and are uninformative about effects in vivo. Humanized mouse models provide novel, exciting ways of understanding the functions of these genes. This article evaluates these technologies and current knowledge on PXR/CAR-mediated regulation of gene expression.

Dexamethasone protects primary cultured hepatocytes from death receptor-mediated apoptosis by upregulation of cFLIP

Dexamethasone (DEX) pretreatment protected hepatocytes from TNF-alpha plus actinomycin D (ActD)-induced apoptosis by suppressing caspase-8 activation and the mitochondria-dependent apoptosis pathway. DEX treatment upregulated cellular FLICE inhibitory protein (cFLIP) expression, but did not alter the protein levels of Bcl-2, Bcl-xL, Mcl-1, and cIAP as well as Akt activation. The increased cFLIP mRNA level by DEX was inhibited by ActD, indicating that DEX upregulates cFLIP expression at the transcriptional step. DEX also inhibited Jo2-mediated hepatocyte apoptosis by blocking the formation of the death-inducing signaling complex and caspase-8 activation. Specific downregulation of cFLIP expression using siRNA reversed the antiapoptotic effect of DEX by increasing caspase-8 activation. Moreover, DEX administration into mice increased cFLIP expression in the liver and prevented Jo2-induced hepatic injury by inhibiting caspase-8 and -3 activities. Our results indicate that DEX exerts a protective role in death receptor-induced in vitro and in vivo hepatocyte apoptosis by upregulating cFLIP expression.

Differential regulation of hepatic CYP2B6 and CYP3A4 genes by constitutive androstane receptor but not pregnane X receptor

Accumulated evidence suggests that cross-talk between the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) results in shared transcriptional activation of CYP2B and CYP3A genes. Although most data imply symmetrical cross-regulation of these genes by rodent PXR and CAR, the actual selectivities of the corresponding human receptors are unknown. The objective of this study was to evaluate the symmetry of human (h) PXR and hCAR cross-talk by comparing the selectivities of these receptors for CYP2B6 and CYP3A4. Human hepatocyte studies revealed nonselective induction of both CYP2B6 and CYP3A4 by hPXR activation but marked preferential induction of CYP2B6 by selective hCAR activation. Gel shift assays demonstrated that hPXR exhibited strong and relatively equal binding to all functional response elements in both CYP2B6 and CYP3A4 genes, whereas hCAR displayed significantly weak binding to the CYP3A4 proximal ER6 motif. In cell-based transfection assays, hCAR displayed greater activation of CYP2B6 reporter gene expression compared with CYP3A4 with constructs containing both proximal and distal regulatory elements. Furthermore, in agreement with binding observations, transfection assays using promoter constructs containing repeats of CYP2B6 DR4 and CYP3A4 ER6 motifs revealed an even greater difference in reporter activation by hCAR. In contrast, hPXR activation resulted in less discernible differences between CYP2B6 and CYP3A4 reporter gene expression. These results suggest asymmetrical cross-regulation of CYP2B6 and CYP3A4 by hCAR but not hPXR in that hCAR exhibits preferential induction of CYP2B6 relative to CYP3A4 because of its weak binding and functional activation of the CYP3A4 ER6.

Arsenite decreases CYP3A23 induction in cultured rat hepatocytes by transcriptional and translational mechanisms

Arsenic is a naturally occurring, worldwide contaminant implicated in numerous pathological conditions in humans, including cancer and several forms of liver disease. One of the contributing factors to these disorders may be the alteration of cytochrome P450 (CYP) levels by arsenic. In rat and human hepatocyte cultures, arsenic, in the form of arsenite, decreases the induction of several CYPs. The present study investigated whether arsenite utilizes transcriptional or post-transcriptional mechanisms to decrease CYP3A23 in primary cultures of rat hepatocytes. In these cultures, a 6-h treatment with 5 microM arsenite abolished dexamethasone (DEX)-mediated induction of CYP3A23 protein and activity, but did not inhibit general protein synthesis. However, arsenite treatment only reduced DEX-induced levels of CYP3A23 mRNA by 30%. The effects of arsenite on CYP3A23 transcription were examined using a luciferase reporter construct containing 1.4 kb of the CYP3A23 promoter. Arsenite caused a 30% decrease in DEX-induced luciferase expression of this reporter. Since arsenite abolished induction of CYP3A23 protein, but caused only a small decrease in CYP3A23 mRNA, the effects of arsenite on translation of CYP3A23 mRNA were investigated. Polysomal distribution analysis showed that arsenite decreased translation by decreasing the DEX-mediated increase in CYP3A23 mRNA association with polyribosomes. Arsenite did not decrease intracellular glutathione or increase lipid peroxidation, suggesting that the effect of arsenite on CYP3A23 does not involve oxidative stress. Overall, the results suggest that low-level arsenite decreases both transcription and translation of CYP3A23 in primary rat hepatocyte cultures.

Effects of Cell Cycle Status on the Efficiency of Liposome-mediated Gene Transfection in Mouse Fetal Fibroblasts

Methods for cell cycle synchronization of mouse fetal fibroblast cells (MFFCs) were first selected and optimized. When MFFCs were cooled at 5 C for different periods of time, the highest percentage of cells at the G0/G1 phase (75.4+/-2.9%), with 3.5+/-0.3% of apoptotic cells, was achieved after 5 h of treatment. Extended cooling increased the number of apoptotic cells significantly. When MFFCs were treated with different concentrations of roscovitine (ROS) for different periods of time, the highest percentage of G0/G1 cells (83.5+/-1.8%), with 9.2+/-0.6% apoptotic cells, was obtained after exposure to 10 microM ROS for 24 h. When the cells were cooled at 5 C for 5 h followed by incubation in 10 microM ROS for 12 h, 83.6+/-1.9% were synchronized at the G0/G1 stage, with 3.6% undergoing apoptosis. Cell cycle progression was then observed after release of the MFFCs from different synchronization blocks. The highest percentages of S and G2/M cells (81% and 75%) were achieved at 12 and 20 h, respectively, after release of the MFFCs from the cooling plus ROS treatment, and these percentages were significantly higher than those obtained after release from the cooling or ROS alone blocks. Finally, MFFCs were transfected with pEGFP-N1 plasmid at the peak of the G0/G1, S, and G2/M phases, respectively, after release from the different blocks and both the transient and stable transfection efficiencies were determined. The GFP gene expression was greatly enhanced when transfection was performed at the time when most cells were at the G2/M stage after release from cooling, ROS alone, and cooling plus ROS treatments. Statistical analysis revealed a close correlation between the rate of G2/M cells and the transient and stable GFP gene expression efficiencies. Together, the results indicated that (a) the best protocol for cell cycle synchronization of MFFCs was a 5-h cooling at 5 C followed by incubation in 10 microM ROS for 12 h which produced both a high rate of synchronization in the G0/G1 phase with acceptable apoptosis and a high rate of G2/M cells after release; and (b) that the cell cycle status had marked effects on the efficiency of liposome-mediated transfection in MFFCs, with the highest transfection efficiency obtained in cells at the G2/M stage.

Arsenite decreases CYP3A4 and RXRalpha in primary human hepatocytes

Arsenic is a naturally occurring, worldwide contaminant implicated in numerous pathological conditions in humans, including cancer and several forms of liver disease. One of the contributing factors to these disorders may be the alteration of cytochrome P450 (P450) levels by arsenic. P450s are involved in the oxidative metabolism and elimination of numerous toxic chemicals. CYP3A4, a major P450 in humans, is involved in the metabolism of half of all currently used drugs. Acute exposure to arsenite decreases the induction of CYP1A1/2 proteins and activities in cultured human hepatocytes, as well as CYP3A23 in cultured rat hepatocytes. Here, in primary cultures of human hepatocytes, we assessed the effects of acute arsenite exposure on CYP3A4 and several transcription factors involved in CYP3A4 expression. The concentrations of arsenite used in these studies were nontoxic to the hepatocytes and failed to elicit an oxidative response. Treatment with arsenite in the presence of CYP3A4 inducers, rifampicin (Rif) or phenobarbital, caused major decreases in CYP3A4 mRNA, protein, and activity. In addition, the levels of CYP3A4 in untreated cells were decreased following arsenite treatment. Transcription of the CYP3A4 gene is primarily regulated by heterodimers of the retinoid X receptor alpha (RXRalpha) and the pregnane X receptor (PXR). We found that arsenite failed to affect expression of PXR or the transcription factor Sp1, yet caused a significant decrease in PXR responsiveness to Rif. Arsenite caused a large decrease in nuclear RXRalpha protein and, to a lesser extent, RXRalpha mRNA. These results suggest that arsenite inhibits both untreated and induced CYP3A4 transcription in primary human hepatocytes by decreasing the activity of PXR, as well as expression of the nuclear receptor RXRalpha.

EFFECT OF DEXAMETHASONE TREATMENT ON THE EXPRESSION AND FUNCTION OF TRANSPORT PROTEINS IN SANDWICH-CULTURED RAT HEPATOCYTES

Dexamethasone (DEX) is a well established inducer of CYP3A. These studies examined the influence of DEX treatment on transport protein expression and function in sandwich-cultured (SC) rat hepatocytes. Freshly isolated hepatocytes were cultured between two layers of gelled collagen and maintained in Dulbecco's modified Eagle's medium supplemented with DEX (0.1 microM, 0-48 h and 0.1-100 microM, 48-96 h). The expression of sinusoidal [(organic anion transporting polypeptide 1a1 (Oatp1a1), Oatp1a4, multidrug resistance-associated protein 3 (Mrp3), and Na(+)-dependent taurocholate cotransporting polypeptide (Ntcp)] and canalicular [bile salt export pump (Bsep), multidrug resistance protein 1a/b (Mdr1a/b), and Mrp2] transport proteins was determined by Western blot analysis. The accumulation and biliary excretion index (BEI; percentage of accumulated substrate in canalicular networks) of the probe substrates taurocholate (TC; 1 microM, 10 min), rhodamine 123 (Rh123; 10 microM, 30 min), and carboxy-2',7'-dichlorofluorescein (CDF; 10 microM, 10 min) were employed as measures of canalicular transport protein function in SC rat hepatocytes. DEX treatment increased CYP3A1/2, Oatp1a4, and Mrp2 expression, decreased the expression of Ntcp, and did not seem to alter the expression of Oatp1a1, Mrp3, Mdr1a/b, or Bsep. The BEI of CDF, an Mrp2 substrate, increased from 18 to 37% after DEX treatment (100 microM). The accumulation of TC, an Ntcp substrate, was reduced (<50% of control), whereas the BEI of TC, also a Bsep substrate, was unchanged. Treatment of SC rat hepatocytes with DEX resulted in alterations in the expression of CYP3A1/2 and some hepatic transport proteins. Modest alterations in hepatic transport protein function were consistent with changes in protein expression.

Isolation and Characterization of a New Major Intestinal CYP3A Form, CYP3A62, in the Rat

Based on information of the nucleotide sequence obtained from rat genome clones, a new CYP3A (CYP3A62) cDNA was isolated from the cDNA library of a rat liver. The CYP3A62 cDNA was 1746 base pairs (bp) in length, which included 1491 bp of an open reading frame and 93 bp and 209 bp of the respective 5'- and 3'-noncoding regions. Amino acid sequence deduced from CYP3A62 cDNA shared the highest similarity with rat CYP3A9 (79.9%) among human and rat CYP3A forms previously reported. CYP3A62 mRNA and protein were consistently detected in small intestines as well as livers. CYP3A62 was a major form in small intestines of both sexes but was a female-predominant form in livers of adult rats. CYP3A62 in both tissues of male and female rats were clearly enhanced by the treatment with dexamethasone. These expression profiles resembled those of CYP3A9. Despite clear detection of CYP3A62, no detectable levels of CYP3A1 and CYP3A2 proteins, as well as those of mRNAs, were found in the intestinal tract. Therefore, CYP3A62 may play major roles together with CYP3A9 and CYP3A18 in endogenous or exogenous detoxification at the absorption site.

Genomic characterization and regulation of CYP3a13: role of xenobiotics and nuclear receptors

We report that CYP3a13 gene, located on mouse chromosome 5, spans 27.5 Kb and contains 13 exons. The transcription start site is 35 bp upstream of the coding region and results in a 109 bp 5' untranslated region. CYP3a13 promoter shows putative binding sites for retinoid X receptor, pregnane X receptor, and estrogen receptor. CYP3a13 shows a broad tissue distribution with predominant expression in liver. Although CYP3a13 shares 92% nucleotide identity with the female-specific rat CYP3A9, its expression does not exhibit sexual dimorphism. Ligand activation of peroxisomal proliferator-activated receptor-gamma and retinoid X receptor inhibit expression of CYP3a13 at the transcription level in a tissue-specific manner. Another novel finding is hepatic induction of CYP3a13 by dexamethasone occurring only in pregnane X receptor null mice. We also report that pregnane X receptor is essential to maintain robust in vivo basal levels of CYP3a13 in contrast to CYP3a11. CYP3a13 protein expressed in vitro can metabolize clinically active drugs ethylmorphine and erythromycin, as well as benzphetamine. We conclude that CYP3a13 is regulated differentially by various nuclear receptors. In humans this may lead to altered drug metabolism, as many of the newly synthesized ligands/drugs targeted toward these nuclear receptors could influence CYP3A gene expression.

The expression of CYP2B6, CYP2C9 and CYP3A4 genes: a tangle of networks of nuclear and steroid receptors

Numerous chemicals increase the metabolic capability of organisms by their ability to activate genes encoding various xenochemical-metabolizing enzymes, such as cytochromes P450 (CYPs), transferases and transporters. For example, natural and synthetic glucocorticoids (agonists and antagonists) as well as other clinically important drugs induce the hepatic CYP2B, CYP2C and CYP3A subfamilies in man, and these inductions might lead to clinically important drug-drug interactions. Only recently, the key cellular receptors that mediate such inductions have been identified. They include nuclear receptors, such as the constitutive androstane receptor (CAR, NR1I3), the retinoid X receptor (RXR, NR2B1), the pregnane X receptor (PXR, NR1I2), and the vitamin D receptor (VDR, NR1I1) and steroid receptors such as the glucocorticoid receptor (GR, NR3C1). There is a wide promiscuity of these receptors in the induction of CYPs in response to xenobiotics. Indeed, this adaptive system appears now as a tangle of networks, where receptors share partners, ligands, DNA response elements and target genes. Moreover, they influence mutually their relative expression. This review is focused on these different pathways controlling human CYP2B6, CYP2C9 and CYP3A4 gene expression, and the cross-talk between these pathways.

Expression and cyclic variability of CYP3A4 and CYP3A7 isoforms in human endometrium and cervix during the menstrual cycle

CYP3A4, a cytochrome p450 (p450) isoform metabolizes estrogens, whereas CYP3A7, a fetal liver p450 isoform, is involved in estriol biosynthesis. The goal of this study was to evaluate expression of these enzymes in human uterine tissue during the proliferative and secretory phases of the menstrual cycle. Endometrium and cervix specimens were collected from women undergoing hysterectomy (n = 36). Total mRNA was extracted, quantified, and reverse-transcription polymerase chain reaction (RT-PCR) was carried out using consensus primers for CYP3A. The 453 base pairs PCR product was hybridized with specific internal oligonucleotide probes for CYP3A4 or CYP3A7 end labeled with (32)P gamma-ATP. The relative intensity of hybridization was determined by autoradiography. Expression of CYP3A7 in endometrium was significantly greater (approximately 10-fold) in the proliferative phase compared with the secretory phase (p < 0.05). CYP3A4 expression was comparable between the two phases. Expression of both enzymes was minimal in the cervix. Fluorescence in situ hybridization of paraffinized sections indicated localized expression of CYP3A enzymes in the glandular epithelium as well as the stroma. Comparison of relative fluorescence intensity indicated differential expression of CYP3A7 in various phases of the menstrual cycle. These results suggest that CYP3A expression in the endometrium of premenopausal women may vary depending on the menstrual cycle phase.

The nuclear pregnane X receptor: a key regulator of xenobiotic metabolism

The nuclear pregnane X receptor (PXR; NR1I2) is an important component of the body's adaptive defense mechanism against toxic substances including foreign chemicals (xenobiotics). PXR is activated by a large number of endogenous and exogenous chemicals including steroids, antibiotics, antimycotics, bile acids, and the herbal antidepressant St. John's wort. Elucidation of the three-dimensional structure of the PXR ligand binding domain revealed that it has a large, spherical ligand binding cavity that allows it to interact with a wide range of hydrophobic chemicals. Thus, unlike other nuclear receptors that interact selectively with their physiological ligands, PXR serves as a generalized sensor of hydrophobic toxins. PXR binds as a heterodimer with the 9-cis retinoic acid receptor (NR2B) to DNA response elements in the regulatory regions of cytochrome P450 3A monooxygenase genes and a number of other genes involved in the metabolism and elimination of xenobiotics from the body. Although PXR evolved to protect the body, its activation by a variety of prescription drugs represents the molecular basis for an important class of harmful drug-drug interactions. Thus, assays that detect PXR activity will be useful in developing safer prescription drugs.

Ectopic expression of MHC class II genes (RT1.B(I) beta/alpha) in rat hepatocytes in vivo and in culture can be elicited by treatment with the pregnane X receptor agonists pregnenolone 16 alpha-carbonitrile and dexamethasone

The synthetic steroid, pregnenolone-16-alpha-carbonitrile (PCN), has served for decades as a probe for a postulated series of hepatic defenses activated under situations of environmental "stress". PCN, an antiglucocorticoid, and also such glucocorticoids as dexamethasone (Dex) appear to stimulate hepatic metabolism and elimination of xenobiotics by binding to the nuclear pregnane X receptor (PXR) which then interacts with a distinct DNA response element associated with induction of cytochrome P450 3A genes. To explore the full domain of genes controlled by PCN/PXR, we used differential display to detect rat liver mRNA species selectively induced by PCN or by Dex. Sequence analysis identified one of many PCN induced cDNA fragments as RT1.B(I)beta, a member of the major histocompatability class II (MHC) gene family usually found only in antigen presenting cells. Northern blot analysis of RNA from rat liver or from cultured hepatocytes confirmed that amounts of RT1.B(I)beta mRNA and also of its companion gene, RT1.B(I)alpha mRNA, became readily detectable within 3-6 hours following treatment with PCN or Dex, whereas no induction was observed in spleen RNA. Induction by PCN of RT1.B(I)beta immunoreactive protein was localized to the hepatocytes as judged by immunofluorescence. We conclude that ectopic expression of MHC II genes, an unprecedented effect of steroids or drugs, is rapidly evoked by PCN acting on the liver, directly. The concept of a set of genes coordinately controlled to maintain homeostasis in parenchymal tissues during toxic stress must now be extended to include the immune system.

Regulation of cyp3a gene transcription by the pregnane x receptor

The pregnane X receptor (PXR) is a promiscuous nuclear receptor that has evolved to protect the body from toxic chemicals. PXR is activated by a structurally diverse collection of xenobiotics, including several widely used prescription drugs. Various lipophilic compounds produced by the body, such as bile acids and steroids, also activate PXR. PXR stimulates the transcription of cytochrome P450 3A monooxygenases and other genes involved in the detoxification and elimination of these potentially harmful chemicals. Assays that detect PXR activation have important implications for the design of future drugs in two respects. On the one hand, PXR activation assays can be used to determine whether candidate drugs are likely to induce CYP3A gene expression and interact with other medicines. On the other hand, PXR agonists may prove useful in the treatment of diseases in which toxic metabolites accumulate, such as cholestatic liver disease.

Receptor-dependent transcriptional activation of cytochrome P4503A genes: induction mechanisms, species differences and interindividual variation in man

1. The importance of CYP3A enzymes in drug metabolism and toxicology has yielded a wealth of information on the structure, function and regulation of this subfamily and recent research emphasis has been placed on the human forms, namely CYP3A4, CYP3A5, CYP3A7 and CYP3A43. 2. The current review will focus on the receptor-dependency of CYP3A regulation and includes consideration of the regulatory roles of the glucocorticoid (GR), pregnane X (PXR) and constitutive androstane (CAR) receptors. 3. Emphasis has been placed on the topics of expression and substrate specificity, assessment of induction, species differences in induction, CYP3A promoter sequences and regulation of gene expression, structural and functional aspects of receptor-mediated, CYP3A gene activation, receptor variants and interindividual variation in human CYP3A expression, the latter encompassing environmental, physiological and genetic aspects. 4. An outline of future research needs will be discussed in the context of receptor-mediated molecular mechanisms of CYP3A gene regulation and the impact on interindividual variations in CYP3A expression. 5. Taken collectively, this review highlights the importance of understanding the molecular mechanisms of CYP3A induction as a means of rationalizing human responses to many clinically used drugs, in addition to providing a mechanistically coherent platform to understand and predict interindividual variations in response and drug-drug interactions.

Dual effect of dexamethasone on CYP3A4 gene expression in human hepatocytes. Sequential role of glucocorticoid receptor and pregnane X receptor

Although CYP3A induction by dexamethasone has been extensively documented, its mechanism is still unclear because both the role of the glucocorticoid receptor and the ability of dexamethasone to activate the human pregnane X receptor have been questioned. In an attempt to resolve this problem, we investigated the response of CYP3A4 to dexamethasone (10 nm-100 microm) in primary human hepatocytes and HepG2 cells, using a variety of methods: kinetic analysis of CYP3A4 and tyrosine aminotransferase expression, effects of RU486 and cycloheximide, ligand binding assay, cotransfection of HepG2 cells with CYP3A4 reporter gene constructs and vectors expressing the glucocorticoid receptor, pregnane X receptor or constitutively activated receptor. In contrast to rifampicin (monophasic induction), dexamethasone produces a biphasic induction of CYP3A4 mRNA consisting of a low-dexamethasone component (nmol concentrations) of low amplitude (factor of 3-4) followed by a high-dexamethasone component (supramicromolar concentrations) of high amplitude (factor of 15-30). We show that the low-dexamethasone component results from the glucocorticoid receptor-mediated expression of pregnane X receptor and/or constitutively activated receptor which, in turn, are able to transactivate CYP3A4 in a xenobiotic-independent manner. At supramicromolar concentrations (>10 microm), dexamethasone binds to and activates pregnane X receptor thus producing the high-dexamethasone component of CYP3A4 induction. We conclude that, in contrast to the other xenobiotic inducers of CYP3A4, glucocorticoids play a dual role in CYP3A4 expression, first by controlling the expression of PXR and CAR under physiological conditions (submicromolar concentrations) through the classical glucocorticoid receptor pathway, and second by activating the pregnane X receptor under bolus or stress conditions (supramicromolar concentrations).

Studies on transcriptional regulation of Cyp3a16 gene in mouse livers by application of direct DNA injection method

CYP3A16 is expressed in the mouse liver specifically during a fetal and puberty life. Functional regions responsible for the transcriptional regulation of the Cyp3a16 gene were identified by an in vivo direct DNA injection into mouse livers followed by a luciferase assay. The results of the deletion analysis of a 5'-flanking sequence suggested the existence of an adult-specific repressor(s) interacting with the Cyp3a16 gene. In addition, a positive regulatory element was assumed to be present in a region from -146 to -56 of the Cyp3a16 gene. A hepatocyte nuclear factor 4 (HNF-4)-binding consensus sequence was found in this region. In fact, HNF-4alpha bound to this sequence as evidenced by a gel mobility shift assay. The role of the HNF-4-binding sequence was further examined by an introduction of mutations in this sequence. The introduction of the mutations resulted in a reduced activity of a luciferase in the assay. These results obtained by an application of the direct DNA injection method suggest that the HNF-4alpha activates the transcription of the Cyp3a16 gene in vivo.

Effect of selenium and vitamin E deficiency on differential gene expression in rat liver

To examine the molecular events associated with selenium (Se) and vitamin E (VE) deficiency, we applied cDNA array technology to define the transcriptional response in the liver of Se- and VE-deficient rats. VE deficiency alone did not induce any significant changes in expression profile among the genes evaluated. Se deficiency lead to a down-regulation of Se-dependent cGPx and to an induction of genes, encoding for detoxifying enzymes in liver (cytochrome P450 4B1, UDP-glucuronosyltransferase 1). Combined VE and Se deficiency was characterized by alterations in the expression level of genes encoding for proteins involved in inflammation (multispecific organic anion exporter, SPI-3 serine protease inhibitor) and acute phase response (alpha-1 acid glycoprotein, metallothionein 1). Additionally, a significant down-regulation in the expression level of genes important in the inhibition of apoptosis (defender against cell death 1 protein, Bcl2-L1), cell cycle (G1/S-specific cyclin D1) and antioxidant defense (gamma-glutamylcysteine synthetase catalytic subunit) was demonstrated. The experimental strategy identified several novel Se and VE sensitive genes.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

Dexamethasone enhances constitutive androstane receptor expression in human hepatocytes: consequences on cytochrome P450 gene regulation

The barbiturate phenobarbital induces the transcription of cytochromes P450 (CYPs) 2B through the constitutive androstane receptor (CAR; NR1I3). CAR is a member of the nuclear receptor family (NR1) mostly expressed in the liver, which heterodimerizes with retinoid X receptor (RXR) and was shown to transactivate both the phenobarbital responsive element module of the human CYP2B6 gene and the CYP3A4 xenobiotic response element. Because previous studies in rodent hepatocyte cultures have shown that the phenobarbital-mediated induction of CYP2B genes is potentiated by glucocorticoids, we examined the role of activated glucocorticoid receptor in this process. We show that submicromolar concentrations of dexamethasone enhance phenobarbital-mediated induction of CYP3A4, CYP2B6, and CYP2C8 mRNA in cultured human hepatocytes. In parallel, we observed that glucocorticoid agonists, such as dexamethasone, prednisolone, or hydrocortisone, specifically increase human car (hCAR) mRNA expression. Accumulation of hCAR mRNA parallels that of tyrosine aminotransferase: both mRNAs reach a maximum at a concentration of 100 nM dexamethasone and are down-regulated by concomitant treatment with the glucocorticoid antagonist RU486. Moreover, the effect of dexamethasone on hCAR mRNA accumulation appears to be of transcriptional origin because the addition of protein synthesis inhibitor cycloheximide has no effect, and dexamethasone does not affect the degradation of hCAR mRNA. Furthermore, dexamethasone increases both basal and phenobarbital-mediated nuclear translocation of CAR immunoreactive protein in human hepatocytes. The up-regulation of CAR mRNA and protein in response to dexamethasone explains the synergistic effect of this glucocorticoid on phenobarbital-mediated induction of CYP2B genes and the controversial role of the glucocorticoid receptor on phenobarbital-mediated CYP gene inductions.

Use of the nuclear receptor PXR to predict drug interactions

We recently cloned the human, rabbit, rat, and mouse orthologs of a novel member of the steroid/retinoid/thyroid hormone receptor family, which we have named the Pregnane X Receptor (PXRs). The discovery and characterization of PXR has led to an increased understanding of the molecular basis of many drug-drug interactions as well as a better understanding of xenobiotic metabolism in general. The key insights into PXR action was the finding that this nuclear receptor is linked to regulation of the cytochrome P450 3A monooxygenase (CYP3A) genes. Several lines of evidence indicate that PXR mediates the induction of CYP3A gene transcription. First, PXR is selectively expressed in the liver and intestine, the same tissues in which CYP3A gene expression is induced. Second, PXR binds as a heterodimer with the retinoid X receptor (RXR) to xenobiotic response elements that have been identified in CYP3A gene promoters. Third, PXR is activated by the remarkable array of compounds that are known to induce CYP3A gene transcription. And finally, PXRs from different species are differentially activated by certain compounds such as rifampicin and pregnenolone 16alpha-carbonitrile (PCN) in a manner that correlates with species-specific induction of CYP3A gene expression. We are now employing high throughput PXR activation and binding assays to identify drug candidates that induce CYP3A gene expression so that these compounds can be removed from the drug development process.

Humanized xenobiotic response in mice expressing nuclear receptor SXR

The cytochrome CYP3A gene products, expressed in mammalian liver, are essential for the metabolism of lipophilic substrates, including endogenous steroid hormones and prescription drugs. CYP3A enzymes are extremely versatile and are inducible by many of their natural and xenobiotic substrates. Consequently, they form the molecular basis for many clinical drug-drug interactions. The induction of CYP3A enzymes is species-specific, and we have postulated that it involves one or more cellular factors, or receptor-like xeno-sensors. Here we identify one such factor unequivocally as the nuclear receptor pregnenolone X receptor (PXR) and its human homologue, steroid and xenobiotic receptor (SXR). We show that targeted disruption of the mouse PXR gene abolishes induction of CYP3A by prototypic inducers such as dexamethasone or pregnenolone-16alpha-carbonitrile. In transgenic mice, an activated form of SXR causes constitutive upregulation of CYP3A gene expression and enhanced protection against toxic xenobiotic compounds. Furthermore, we show that the species origin of the receptor, rather than the promoter structure of CYP3A genes, dictates the species-specific pattern of CYP3A inducibility. Thus, we can generate 'humanized' transgenic mice that are responsive to human-specific inducers such as the antibiotic rifampicin. We conclude that SXR/PXR genes encode the primary species-specific xeno-sensors that mediate the adaptive hepatic response, and may represent the critical biochemical mechanism of human xenoprotection.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

Glucocorticoid-dependent maintenance of CYP2C11-dependent oxidation in male rat liver in vivo

1. Hormonal factors participate in the regulation of xenobiotic metabolising enzymes in liver. Hepatic xenobiotic oxidation capacity is decreased in adrenalectomised rats, which directly implicates adrenal hormones in the control of cytochrome P450 (CYP) expression. In addition, recent studies in cultured hepatocytes have demonstrated that low concentrations of glucocorticoid upregulate the male-specific CYP2C11, which is a major enzyme that catalyses xenobiotic and steroid hydroxylations in rat liver. The present study evaluated whether glucocorticoid or mineralocorticoid may be the adrenal factor that contributes to the in vivo expression of CYP2C11 in liver. 2. Adrenalectomy of male rats selectively decreased CYP2C11-dependent 2alpha-/16alpha-hydroxylation of testosterone and other steroid substrates to 60-70% of control, whereas activities mediated by other constitutive CYPs were unaffected. The decrease in CYP2C11 activity was due to impaired protein expression in liver after adrenalectomy. Administration of dexamethasone (DEX; 0.2 mg/kg i.p. daily for 6 days) restored CYP2C11 activity and protein, whereas the mineralocorticoid deoxycorticosterone (DOC) and adrenocorticotropic hormone (ACTH) were ineffective. 3. These findings establish that glucocorticoids have a partial role in the maintenance of CYP2C11 expression and associated microsomal oxidation in liver and provide a physiological correlate for similar observations made in vitro in hepatocyte culture.

Differential glucocorticoid responses of CYP3A23 and CYP3A2 are mediated by selective binding of orphan nuclear receptors

CYP3A2 and CYP3A23 are two cytochrome P450 genes in rat that are differentially regulated in both their constitutive activities and their responsiveness to glucocorticoids, the prototypic CYP3A inducers. CYP3A2 displays 20-25% of the response to glucocorticoids as CYP3A23 despite extensive sequence homology in their 5'-regulatory regions. Promoter deletion analyses revealed that the CYP3A2 -57 to -168 region, homologous to the CYP3A23 dexamethasone-responsive region, mediated its low level activation. When this region was analyzed by DNase I footprinting, three binding sites were shown to correspond to the functional elements described for CYP3A23: DexRE-1, DexRE-2, and Site A (J. M. Huss and C. B. Kasper (1998) J. Biol. Chem. 273: 16155-16162). The CYP3A2 DexRE-2 and Site A elements bear two mismatches each from the CYP3A23 elements but displayed similar binding patterns in footprinting and gel-shift analyses as their CYP3A23 counterparts. The region containing 3A2DexRE-1 has six mismatches and displayed unique footprinting and gel-shift patterns compared to 3A23DexRE-1. Functional assays revealed that four mismatches within the DexRE-1 and DexRE-2 elements accounted for the differential inducibility of the two isoforms. We propose that the reduced responsiveness of CYP3A2 is the result of preferential binding of COUP-TF at the CYP3A2 DexRE-1 site. In contrast, CYP3A23 DexRE-1 associates with an accessory factor(s) that acts in concert with downstream sites to mediate the strong glucocorticoid induction response observed for CYP3A23. Site A mismatches did not influence induction magnitude but were responsible for basal activity differences. Higher CYP3A23 basal activity appears to be due to an E-box in 3A23SiteA that interacts with USF1, a ubiquitous bHLH/leucine zipper transcription factor. This site is disrupted in the corresponding 3A2SiteA. Hence, 4 nucleotide mismatches within two elements account for the difference in glucocorticoid induction, and a single mismatch is responsible for the fivefold difference in the basal activities of CYP3A2 and CYP3A23.

A reporter gene assay to assess the molecular mechanisms of xenobiotic-dependent induction of the human CYP3A4 gene in vitro

1. A plasmid containing 1 kb of the CYP3A4 regulatory (promoter) region coupled to a reporter gene for secretary placental alkaline phosphatase (SPAP) was transfected into HepG2 cells. Transfected cells were dosed with several known inducers of CYP3A4 and the levels of SPAP were measured. The effect of co-transfecting a plasmid encoding the human glucocorticoid receptor on reporter gene activity was also examined. 2. Dexamethasone induced CYP3A4-dependent reporter gene expression in a concentration-dependent manner and induction was approximately doubled in the presence of the glucocorticoid receptor. Dexamethasone-dependent induction was blocked by RU-486 (a glucocorticoid receptor antagonist), in the presence of the co-transfected glucocorticoid receptor. 3. Induction of CYP3A4-dependent reporter gene expression and enhancement of the induction by the glucocorticoid receptor was also observed with pregnenolone-16alpha-carbonitrile (PCN), rifampicin, phenytoin, carbamazepine, phenylbutazone and phenobarbitone, all known in vivo inducers of CYP3A4 in man. 4. Metyrapone and sulfinpyrazone induced CYP3A4-dependent reporter gene expression, but induction was not enhanced by the glucocorticoid receptor. 5. Clotrimazole, erythromycin and triacetyloleandomycin (TAO) did not induce CYP3A4-dependent reporter gene expression, consistent with the observation that these inducers act through post-transcriptional mechanisms. 6. These results highlight differences in the molecular mechanisms of induction of CYP3A4 by the xenobiotics studied and indicate that the glucocorticoid receptor is involved in the induction of the CYP3A4 gene by some, but not all, CYP3A4 inducers. 7. We propose that the approach described here provides a useful in vitro approach for the identification of transcriptional regulators of the CYP3A4 gene.

Structure and expression of the rat CYP3A1 gene: isolation of the gene (P450/6betaB) and characterization of the recombinant protein

A P450 gene (P450/6betaB) of the CYP3A subfamily was isolated from a rat genomic library. Nucleotide sequencing of the exons revealed a high similarity with P450PCN1 cDNA (Gonzalez et al. (1985), J. Biol. Chem. 260, 7345-7441), but differed in 41 nucleotides, resulting in 11 changes and 2 deletions of amino acid residues. The P450/6betaB spanned about 30 kbp and consisted of 13 exons, and was in exon number and size identical with CYP3A2 gene except in the 6th exon, which was shorter than that of CYP3A2. 6beta-B mRNA, which may be transcribed from P450/6betaB, was detected on Northern blotting and by reverse transcription-polymerase chain reaction (RT-PCR). Profiles of the developmental change and induction by a treatment with several chemicals were very similar to those of P450PCN1 mRNA reported previously. P450PCN1 mRNA and gene, however, were not detected by PCR in rats. To determine whether P450/6betaB encodes an active protein, a cDNA was isolated and expressed. Expression of 6beta-B cDNA in COS-1 cells was carried out and revealed that the recombinant protein comigrated with purified P4506beta-4 previously identified as CYP3A1. The recombinant 6beta-B protein showed similar turnover rate and regioselectivity for testosterone with purified P4506beta-4 by the simultaneous addition of NADPH-cytochrome P450 reductase and cytochrome b5. These data suggest that P450/6betaB encodes an active P450 form corresponding to CYP3A1 and P450PCN1 reported previously does not exist in rats.

Environmental xenobiotics and the antihormones cyproterone acetate and spironolactone use the nuclear hormone pregnenolone X receptor to activate the CYP3A23 hormone response element

The pregnenolone X receptor (PXR), a new member of the nuclear hormone receptor superfamily, was recently demonstrated to mediate glucocorticoid agonist and antagonist activation of a hormone response element spaced by three nucleotides (DR-3) within the rat CYP3A23 promoter. Because many other steroids and xenobiotics can up-regulate CYP3A23 expression, we determined whether some of these other regulators used PXR to activate the CYP3A23 DR-3. Transient co-transfection of LLC-PK1 cells with (CYP3A23)2-tk-CAT and mouse PXR demonstrated that the organochlorine pesticides transnonachlor and chlordane and the nonplanar polychlorinated biphenyls (PCBs) each induced the CYP3A23 DR-3 element, and this activation required PXR. Additionally, this study found that PXR is activated to induce (CYP3A23)2-tk-CAT by antihormones of several steroid classes including the antimineralocorticoid spironolactone and the antiandrogen cyproterone acetate. These studies reveal that PXR is involved in the induction of CYP3A23 by pharmacologically and structurally distinct steroids and xenobiotics. Moreover, PXR-mediated PCB activation of the (CYP3A23)2-tk-CAT may serve as a rapid assay for effects of nonplanar PCBs.

SXR, a novel steroid and xenobiotic-sensing nuclear receptor

An important requirement for physiologic homeostasis is the detoxification and removal of endogenous hormones and xenobiotic compounds with biological activity. Much of the detoxification is performed by cytochrome P-450 enzymes, many of which have broad substrate specificity and are inducible by hundreds of different compounds, including steroids. The ingestion of dietary steroids and lipids induces the same enzymes; therefore, they would appear to be integrated into a coordinated metabolic pathway. Instead of possessing hundreds of receptors, one for each inducing compound, we propose the existence of a few broad specificity, low-affinity sensing receptors that would monitor aggregate levels of inducers to trigger production of metabolizing enzymes. In support of this model, we have isolated a novel nuclear receptor, termed the steroid and xenobiotic receptor (SXR), which activates transcription in response to a diversity of natural and synthetic compounds. SXR forms a heterodimer with RXR that can bind to and induce transcription from response elements present in steroid-inducible cytochrome P-450 genes and is expressed in tissues in which these catabolic enzymes are expressed. These results strongly support the steroid sensor hypothesis and suggest that broad specificity sensing receptors may represent a novel branch of the nuclear receptor superfamily.

Differential inducibility of specific mRNA corresponding to five CYP3A isoforms in female rat liver by RU486 and food deprivation: comparison with protein abundance and enzymic activities

The induction of cytochrome P450 3A (CYP3A) protein and mRNA by RU486 [17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-1-pro pyl-estra-4,9-dien-3-one] treatment and food deprivation in female rat liver was studied using Western blotting and competitive reverse transcription-polymerase chain reaction (RT-PCR). CYP3A apoprotein levels increased in response to food deprivation and to RU486 treatment, and the combination of RU486 treatment plus food deprivation had an apparent additive effect. Food deprivation and RU486 treatment also caused increases in CYP3A1, CYP3A18, and CYP3A23 mRNA, and the combined effects of these treatments on each of these mRNA forms were synergistic. CYP3A2 mRNA was not detected in any of the treatment groups, and there was a lack of concordance between CYP3A9 mRNA levels and the specific messages corresponding to the other CYP3A isoforms. CYP3A9 mRNA levels were highest in food-deprived animals, whereas RU486 inhibited CYP3A9 mRNA expression and suppressed the induction effect of food deprivation. Food deprivation and RU486 treatment each separately caused increased microsomal diazepam C3-hydroxylase activity, and the combined effects of these treatments on this monooxygenase were additive. In contrast, the [N-methyl-14C]erythromycin demethylase activity of the fasted, RU486-treated group of rats did not differ from that of the untreated group, and kinetic analyses revealed that both groups of animals exhibited similar Km and Vmax values. These results suggest that CYP3A9 may be primarily responsible for erythromycin N-demethylation and that the isoforms induced by the combination of fasting and RU486 administration are CYP3A1, CYP3A23, and, to a lesser extent, CYP3A18.

An oligonucleotide decoy for transcription factor E2F inhibits mesangial cell proliferation in vitro

The transcription factor E2F controls expression of several genes involved in cell proliferation including c-myc, c-myb, proliferating cell nuclear antigen (PCNA), and cdk2 kinase. Having established that both PCNA and cdk2 kinase are induced in rat mesangial cells (MC) by serum stimulation, we attempted to inhibit MC proliferation in vitro by transfecting these cells with cationic liposomes containing a synthetic double-stranded oligodeoxynucleotide (ODN) with high affinity for E2F. Using a gel mobility shift assay, we detected increased specific binding of E2F in MC following serum stimulation. This binding was completely inhibited by preincubation of MC nuclear extracts with the double-stranded ODN with high affinity for E2F but not by preincubation with a missense ODN containing two point mutations. MC were also transfected with a luciferase reporter gene construct containing three E2F binding sites. Luciferase activity was enhanced by serum stimulation of MC, and this effect was specifically abolished by cotransfection of MC with E2F decoy ODN. Furthermore, RT-PCR analysis revealed that serum-induced upregulation of PCNA and cdk2 kinase gene expression was inhibited by E2F decoy ODN transfection but not by transfection of missense ODN. These changes in gene expression were paralleled by a reduction in PCNA and cdk2 kinase protein expression in E2F decoy ODN transfected cells. MC number increased following serum stimulation. This effect was blunted by transfection with E2F decoy ODN but not by transfection of missense ODN. These data suggest that the transcription factor E2F plays a crucial role in the regulation of MC proliferation and that this factor can be successfully targeted to inhibit MC cell cycle progression.

Nuclear receptor involvement in the regulation of rat cytochrome P450 3A23 expression

Many genes of the cytochrome P450 3A (CYP3A) subfamily, including several human and rat isoforms, are inducible by glucocorticoids. In the rat CYP3A23 gene, a 110-base pair segment of the proximal 5'-flanking region mediates dexamethasone activation. Three binding sites (DexRE-1, DexRE-2, and Site A), identified by DNase I footprinting analysis, were characterized for their relative contribution to both basal activity and dexamethasone inducibility. Site-directed mutagenesis of DexRE-1 (-144 to -169) and DexRE-2 (-118 to -136) demonstrated that each contained a core imperfect AGGTCA direct repeat, which comprised a consensus nuclear receptor binding site, and was essential for dexamethasone responsiveness but was not required for basal activity. Competition gel shift and supershift analyses revealed that both sites can bind the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor. Site A (-85 to -110) was shown to be important for both basal activity and dexamethasone responsiveness. Point mutants displayed a reduced (2-3-fold) induction response, compared with 15-fold for wild-type, which was accompanied by a 40-60% drop in basal activity. Site A was shown to bind the liver-enriched nuclear receptor hepatocyte nuclear factor 4. Our studies demonstrate that the mechanism mediating glucocorticoid-inducible transcriptional activity of CYP3A23 involves multiple binding sites for members of the nuclear receptor superfamily.

Posttranslational elevation of cytochrome P450 3A levels and activity by dimethyl sulfoxide

The molecular mechanisms by which dimethyl sulfoxide (DMSO) enhances CYP3A protein in phenobarbital-treated primary cultured rat hepatocytes were examined. DMSO treatment rapidly increased CYP3A protein levels in the absence of an increase in CYP3A mRNA levels or an increase in CYP2B protein or mRNA levels. CYP3A levels were increased approximately 3.7- and 9-fold following 0.1% DMSO treatment for 6 and 48 h, respectively. Analyses of the polysomal distribution of CYP3A mRNA suggested that DMSO treatment did not significantly alter the translational efficiency of the CYP3A mRNA. Comparative analyses of immunodetectable protein levels following treatment with cycloheximide showed that DMSO clearly decreased the rate of CYP3A protein turnover but not that of CYP2B. Examination of testosterone metabolism in hepatocyte cultures revealed that DMSO pretreatment increased CYP3A-catalyzed 2 beta- and 6 beta-testosterone hydroxylation. When DMSO was in the culture medium, no inhibitory affect on CYP3A-catalyzed testosterone metabolism was observed, although a slight (15-21%) inhibitory effect was noted for CYP2B-catalyzed 16 alpha- and 16 beta-testosterone hydroxylation. These data provide evidence that DMSO increased CYP3A protein levels as a result of decreased protein degradation. DMSO increased both immunodetectable CYP3A protein levels and catalytic activity, in contrast to compounds that have been reported to stabilize CYP3A protein and inhibit activity.

Characterization of DNA-binding proteins required for glucocorticoid induction of CYP3A23

Cytochrome P450 (CYP) 3A23 is transcriptionally regulated in rat liver by such glucocorticoids as dexamethasone (DEX) and by such antiglucocorticoids as pregnenolone 16 alpha-carbonitrile (PCN). Based on studies of CYP3A23 gene fragments expressed in primary cultures of adult rat hepatocytes and tested for DNA-protein interactions, we have proposed that the mechanism of CYP3A23 induction by these steroid hormones involves the glucocorticoid receptor or a protein induced by glucocorticoids indirectly interacting with proteins constitutively bound to an enhancer element consisting of a direct repeat of 7-bp separated by two nucleotides in the 5'-flanking region of the CYP3A23 gene (L. Quattrochi et al., J. Biol. Chem. 270, 28,917, 1995). In the present study, we prepared and transiently expressed in cultured rat hepatocytes 20-bp double-stranded (ds)-oligonucleotides containing this direct repeat or various mutations of this direct repeat inserted into a chloramphenicol acetyltransferase (CAT) reporter plasmid. We found that both repeats were necessary for induction of CAT by either DEX or PCN. Analysis of proteins bound to CYP3A23 enhancer through the use of uv cross-linking revealed two rat liver nuclear proteins with molecular masses of approximately 130 and 100 kDa, as well as several proteins of molecular masses between 45 and 60 kDa, that specifically bind to the 20-bp ds-oligonucleotide CYP3A23 enhancer. Methylation interference assays determined that all guanine residues within the direct repeats of this oligonucleotide are important for protein binding. Mutations of these guanine residues abolished binding of nuclear proteins and eliminated DEX or PCN inducibility of CAT. These data suggest that constitutively bound proteins, interacting with the CYP3A23 enhancer possibly as a heterodimeric complex, play a role in the glucocorticoid inducibility of CYP3A23.

Identification of a functional glucocorticoid response element in the CYP3A1/IGC2 gene

The rat CYP3A subfamily of cytochrome P450 consists of steroid- and drug-metabolizing enzymes inducible by pregnenolone 16alpha-carbonitrile and by supra-physiological doses of dexamethasone. The induction of CYP3A by dexamethasone has been proposed to be mediated by a mechanism distinct from the glucocorticoid receptor mediated response. However, a synergistic induction of CYP3A has been observed with physiological doses of glucocorticoids and other CYP3A inducers. We have identified the presence of a glucocorticoid-responsive element in the CYP3A1/IGC2 gene that mediates the induction with physiological doses of glucocorticoids. A 219-bp dexamethasone responsive fragment of the CYP3A1/IGC2 gene localized at -2100/-1882 bp upstream of the transcription initiation site was identified in transfection experiments with HepG2 cells. Maximum induction was achieved with 50-100 nM dexamethasone. DNase I footprinting analysis revealed two glucocorticoid receptor-protected sequences in the 5' flank of the CYP3A1/IGC2 gene. Point mutations in footprint I (-1982/-1960-bp) completely abolished binding and transcription activation whereas a mutation in footprint II (-2001/-1986-bp) only decreased the binding and had no effect on transcription activation. These results led to the conclusion that the glucocorticoid response element present in footprint I mediated the dexamethasone response in transfection experiments with HepG2 cells. Pregnenolone 16alpha-carbonitrile failed to induce any transcriptional effect mediated by this response element in the HepG2 cells.

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.

Transfection of human endothelial cells

OBJECTIVE

The introduction of recombinant genes into endothelial cells provides a method to study specific gene products and their effect on cell function. In addition, endothelial cells can be used for implantation into vessels or prosthetic vascular grafts. Because transfection efficiencies in human endothelial cells have been low, it is important to develop improved gene transfer techniques. Therefore, several transfection methods were optimized and transfection efficiencies were determined.

METHODS

Transfection by particle-mediated gene transfer (biolistics) or by cationic liposomes were optimized and compared to calcium phosphate and DEAE-dextran. Transfection efficiency was determined using either a beta-galactosidase or placental alkaline phosphatase reporter gene. The effect of promoter strength was analyzed by transfecting plasmids with either the Rous sarcoma virus (RSV) promoter or cytomegalovirus (CMV) promoter regions.

RESULTS

Optimal conditions for particle-mediated gene transfer utilized gold particles of 1.6 microns diameter, a target distance of 3 cm, helium pressures of 8.96 MPa (1300 psi) and cell confluence of 75%. Transfection with different cationic liposomes demonstrated that one compound, N-(3-aminopropyl)-N,N-dimethyl-2,3-(bis-dodecyloxy)-1-propanimi nium bromide/dioleoyl phosphatidylethanolamine (gamma AP-DLRIE/DOPE), was optimal for gene transfer when 5 micrograms of DNA and 10 to 20 micrograms of lipid was used. With both gold particles and gamma AP-DLRIE/DOPE, the alkaline phosphatase reporter was more efficient than beta-galactosidase using comparable promoters and polyadenylation sites. CMV regulatory elements were more efficient than the RSV promoter in optimizing gene expression. Optimal gene transfer efficiency was 20.28% of cells with gamma AP-DLRIE/DOPE, 3.96% with biolistics, 2.09% with calcium phosphate and 0.88% with DEAE-dextran.

CONCLUSIONS

Gene expression is detectable in a high percentage of human endothelial cells after liposome-mediated transfection when expression is controlled by a strong promoter. Particle-mediated transfection is less efficient under these conditions, but more effective than liposomes when expression is driven by a relatively weak promoter. Calcium phosphate and DEAE-dextran are less useful.