Characterization of two nuclear proteins that interact with cytochrome P-450 1A2 mRNA. Regulation of RNA binding and possible role in the expression of the Cyp1a2 gene

Regulation of drug-metabolizing cytochrome P450 enzymes by glucocorticoids

The regulation of drug-metabolizing cytochrome P450 enzymes (CYP) is a complex process involving multiple mechanisms. Among them, transcriptional regulation through ligand-activated nuclear receptors is the crucial mechanism involved in hormone-controlled and xenobiotic-induced expression of drug-metabolizing CYPs. In this article, we focus, in detail, on the role of the glucocorticoid receptor (GR) in the transcriptional regulation of human drug-metabolizing CYP enzymes and the mechanisms of the regulation. There are at least three distinct transcriptional mechanisms by which GR controls the expression of CYPs: 1) direct binding of GR to a specific gene-promoter sequence called the glucocorticoid responsive element (GRE); 2) indirect binding of GR in the form of a multiprotein complex to gene promoters without a direct contact between GR and promoter DNA; and 3) up- or downregulation of other CYP transcriptional regulators or nuclear receptors (i.e., transcriptional regulatory cross-talk). However, due to the general effect of glucocorticoids on numerous cellular pathways and functions, the net transcriptional effect of glucocorticoids on drug-metabolizing enzymes is usually a combination of several mechanisms. Since synthetic glucocorticoids are widely prescribed in human pharmacotherapy for the treatment of many diseases, comprehensive understanding of the transcriptional regulation of drug-metabolizing CYPs via GR with respect to glucocorticoid therapy or glucocorticoid hormonal status will aid in the development of efficient individualized pharmacotherapy without drug-drug interactions.

Molecular mechanisms of cold-induced CYP1A activation in rat liver microsomes

Cytochrome P4501A (the CYP1A1 and CYP1A2 enzymes) is known to metabolize anthropogenic xenobiotics to carcinogenic and mutagenic compounds. CYP1A1 transcriptional activation is regulated via the aryl hydrocarbon receptor (AhR)-dependent signal transduction pathway. CYP1A2 activation may occur through the AhR-dependent or AhR-independent signal transduction pathways. We used male Wistar rats to explore possible mechanisms of CYP1A activation induced by exposure to cold and the effects of the protein-tyrosine kinase inhibitors genistein, herbimycin A, and geldanamycin on the properties of hepatic CYP1A1 and CYP1A2 proteins following exposure to cold and to classic CYP1A inducers. The molecular mechanisms of cold-induced CYP1A1 and CYP1A2 activation are different. The CYP1A2 activation apparently occurs at the post-transcriptional level. The CYP1A1 activation, whether caused by exposure to cold or by classic CYP1A inducers, is AhR-dependent and occurs at the transcriptional level. Protein tyrosine kinase inhibitors have no effect on benzo(a)pyrene-induced CYP1A expression but alter cold-induced CYP1A1 activity and the CYP1A1 mRNA level. Thus, treatment with herbimycin A or geldanamycin leads to an increase in CYP1A1 activity, while treatment with genistein increases CYP1A1 mRNA expression and decreases CYP1A2 activity. These data elucidate the molecular mechanisms of cold-induced CYP1A activation and the role of protein kinases in the regulation of CYP1A during exposure to cold. Our results can also help identify the differences between the molecular mechanisms underlying the effects of the classic CYP1A inducers and the effects of cooling.

Regulatory sequence responsible for insulin destabilization of cytochrome P450 2B1 (CYP2B1) mRNA

Diabetes has been reported to increase CYP2E1 (cytochrome P450) and CYP2B1 expression at both the mRNA and protein levels in rat livers. This increase has been attributed to mRNA stabilization and can be reversed by daily insulin treatment. In a previous study, we showed that this hormone directly down-regulates CYP2E1 and 2B1 expression through a post-transcriptional mechanism in rat hepatoma cell lines. We then aimed to identify the molecular mechanisms involved in this regulation. We first identified a 16-mer sequence that we later showed to be the actual functional target of insulin on the rat CYP2E1 mRNA. Similar work was performed with CYP2B1. We first investigated the presence of mRNA-protein interactions. Using cytoplasmic proteins of Fao cells treated or not with insulin (0.1 microM) and the full-length CYP2B1 mRNA as a probe, a major CYP2B1 RNA-protein complex was observed with RNase T1 protection experiments. With the use of different CYP2B1 mRNA probes and by means of competition experiments with antisense oligonucleotides, a protein fixation site was located on a 16-nt sequence in the 5' part of the coding region. This sequence has a hairpin loop structure, shows 80% sequence identity with a structure previously identified on CYP2E1 and is also responsible for the post-transcriptional effects of insulin on this mRNA. Protein(s) bound to both CYP2B1 and CYP2E1 sequences are cytosolic and have an apparent molecular mass of 60 kDa. The protein(s) that bind(s) to both these sequences and the insulin transduction signal involved in this regulation remain(s) to identified.

Focus on the intermediate state: immature mRNA of cytochromes P450--methods and insights

The scattered and limited data on hnRNAs (pre-mRNAs) of cytochromes P450 (CYP) are compiled and discussed for the first time. The methods for determination and quantification of hnRNAs are compared. In most cases, CYP hnRNA levels were determined as a parameter of transcriptional activity. It is known, however, that some CYPs, in particular CYP2E1, are in addition specifically and extensively regulated by post-transcriptional processes. Obviously, these processes also influence the processing of CYP hnRNAs so that their levels cannot be considered a mere result of transcription. The underlying mechanisms of post-transcriptional CYP hnRNA and mRNA regulation are not well understood. It is our aim therefore to bring together available data on CYP hnRNA and to discuss them in the light of recent advances in knowledge concerning pre-mRNA processing and interactions between RNA and low molecular weight interfering RNAs. By doing this, we hope to drive research in a direction which appears promising in providing some long-awaited answers with respect to mechanisms of post-transcriptional CYP regulation.

Effect of cold stress on expression of genes for the AhR-dependent pathway of CYP1 regulation in rat liver

The level of mRNA for cytochromes P450 (CYP1A1, CYP1A2, and CYP1B1) and CYP1 regulatory proteins (heat shock protein, aryl hydrocarbon receptor, aryl hydrocarbon receptor repressor, and aryl hydrocarbon receptor nuclear translocator) was measured in the liver of rats after cold stress (4 degrees C). The CYP1A1 mRNA level increased and remained high for 10 days after 5-day cold exposure. The level of mRNA for CYP1A2, heat shock protein, and aryl hydrocarbon receptor nuclear translocator decreased by the 10th day. The level of mRNA for CYP1B1, aryl hydrocarbon receptor, and aryl hydrocarbon receptor nuclear translocator remained unchanged over this period.

In vivo and mechanistic evidence of nuclear receptor CAR induction by artemisinin

Artemisinin (a sesquiterpene lactone endoperoxide) has become important in multi-drug treatment of malaria. There is evidence that artemisinin induces drug metabolism which could result in drug-drug interactions. The objective of this study was to characterize the inductive properties of artemisinin on drug-metabolizing cytochrome P450 (CYP450) enzymes. The possibility of artemisinin to induce CYP450 was studied in artemisinin-treated (orally for four days) and vehicle-treated rats using reverse transcriptase polymerase chain reaction (RT-PCR). The effect on enzymatic activities in mouse microsomes from multiple artemisinin administration (intraperitonally) to mice were also studied as well as the effect on the expression in mouse primary hepatocytes and HEK293 cells. Increased CYP2B1 mRNA levels in rats could be seen after artemisinin treatment as well as a weak but reproducible increase in the intensity of CYP1A2. Administration of artemisinin to mice up-regulated hepatic CYP2B10-dependent, and to a lesser extent, CYP2A5-dependent enzyme activities. In primary hepatocyte culture, artemisinin significantly increased the CYP2B10 mRNA levels whereas the CYP2A5 mRNA levels were increased to a lesser extent. No significant changes were seen in the levels of other CYP enzymes. Artemisinin was an activator of constitutive androstane receptor (CAR) but not pregnane X receptor (PXR) in HEK293 cells. The results demonstrate that the drug exerts its effects on drug metabolism via the CAR receptor that results in up-regulation of genes such as the Cyp2b. The weaker up-regulation of CYP2A5 might also be CAR-dependent or alternatively, a consequence of artemisinin toxicity. The results of this study are of importance when predicting potential drug-drug interactions in multi-drug therapies with artemisinin.

Identification of a 16-Nucleotide Sequence That Mediates Post-transcriptional Regulation of Rat CYP2E1 by Insulin

Insulin directly down-regulates the gene expression of the rat CYP2E1 by altering its mRNA stability (De Waziers, I., Garlatti, M., Bouguet, J., Beaune, P. H., and Barouki, R. (1995) Mol. Pharmacol. 47, 474-479). Because the regulation of CYP mRNA stability was poorly understood, the molecular mechanisms involved in this regulation in the rat hepatoma H4IIEC3 cell line were studied. By using RNase T1 protection methods, the formation of a major CYP2E1 RNA-protein complex was observed. By competition experiments, the binding site of this complex was located on a 16-nucleotide sequence in the 5'-proximal region of the CYP2E1-coding sequence. Insulin did not modify the binding pattern of proteins to this sequence. and transfections of expression vectors or antisense oligonucleotides were undertaken to demonstrate the actual functionality of the 16-mer sequence. The insertion of this sequence in a luciferase gene was sufficient to render the chimeric mRNA sensitive to insulin. Furthermore, transfection of H4IIEC3 cells with antisense oligonucleotide complementary to this sequence blocked the insulin effect on the CYP2E1 mRNA expression, i.e. its rapid degradation. All these results demonstrate that this 16-nucleotide sequence is implicated in the CYP2E1 post-transcriptional regulation by insulin.

Identification of a 43-kDa protein in human liver cytosol that binds to the 3'-untranslated region of CYP2A6 mRNA

Hepatic expression of cytochrome P450 2A6 (CYP2A6) varies widely in humans and is induced during hepatitis; however, the mechanism regulating CYP2A6 has not been established. The murine orthologue Cyp2a5 is regulated post-transcriptionally by mRNA stabilization. A 43-kDa protein that binds to the 3'-untranslated region (3'-UTR) of Cyp2a5 mRNA has been identified, but its role in mRNA stabilization is unclear. We hypothesized that similar interactions occur between cytosolic proteins in human liver and CYP2A6 3'-UTR mRNA. We identified, by RNA electrophoretic mobility shift assay, an hepatic cytosolic protein that binds specifically to sequences in the 3'-UTR of CYP2A6. Complexes did not form with denatured proteins and were eliminated with proteinase K digestion. Complex formation was inhibited with a molar excess of unlabeled CYP2A6 RNA but not by non-specific competitor RNA. Protein-mRNA interactions were not affected by probe denaturation, suggesting that RNA secondary structure is not essential for binding. UV cross-linking of complexes revealed RNA-binding proteins in both human and mouse liver cytosols with molecular masses of approximately 43 kDa. Using truncated RNA probes corresponding to various lengths of CYP2A6 mRNA, the protein-binding site was localized to a 50-nucleotide region between bases 1478 and 1527 of the 3'-UTR. Complex formation with hepatic cytosolic protein from four human subjects correlated with levels of hepatic CYP2A6 microsomal protein, suggesting a possible regulatory role. Further characterization of the RNA-binding protein, the primary binding site, and the influence of this interaction on CYP2A6 mRNA stability will help to elucidate the relevance of these findings to the post-transcriptional control of CYP2A6.

Molecular cloning and expression studies of a prolactin receptor in goldfish (Carassius auratus)

A full-length cDNA clone, of a size of 4.6 kb, for the goldfish prolactin receptor has been isolated. This cDNA clone encodes a protein of 600 amino acids homologous to prolactin receptors of other species. A Kyte-Doolittle hydropathy analysis of the receptor indicates that the translated protein consists of a signal peptide of 22 amino acids, an extracellular domain of 228 amino acids, a single transmembrane domain of 24 amino acids, and an intracellular domain of 346 amino acids. Several characteristic landmarks of prolactin receptor could be identified in this clone. These include the four conserved cysteine residues and the WS motif within the extracellular domain, and the box 1 and box 2 regions of the intracellular domain. Among all the prolactin receptor sequences known to date, this clone bears the closest resemblance to the tilapia prolactin receptor, although homology between these two fish prolactin receptors is rather low. There are only 57.4% of nucleotide and 48.3% of amino acid sequence identities between these two fish receptors. This receptor cDNA was transfected into CHO-K1 cells for functional analysis. RT-PCR analysis with a pair of gene specific primers indicate that the receptor was transcribed in the transfected cells. Using a cell proliferation assay based on the reduction of the tetrazolium salt 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, the receptor transfected CHO-K1 cells can be stimulated to proliferate upon the addition of ovine prolactin in the culture medium. The tissue distribution of the prolactin receptor in goldfish was studied by RT-PCR/Southern analysis and by Northern analysis. The results indicated that the receptor is expressed mostly in the kidney, the gill and the intestine of goldfish, corroborating with the osmoregulatory role of prolactin in fish. In addition, an appreciable level of the receptor is also found in the brain and gonads of goldfish. Northern analysis showed that there are two transcript sizes, a major 4.6 kb and a minor 3.5 kb mRNAs, in the kidney, gill and intestine.

Differential lobular induction in rat liver of glutathione S-transferase A1/A2 by phenobarbital

Phenobarbital and other xenobiotics induce drug-metabolizing enzymes, including glutathione S-transferase A1/A2 (rGSTA1/A2). We examined the mechanism of induction of rGSTA1/A2 in rat livers after phenobarbital treatment. The induction of rGSTA1/A2 was not uniform across the hepatic lobule; steady-state transcript levels were threefold higher in perivenous hepatocytes relative to periportal hepatocytes when examined by in situ hybridization 12 h after a single dose of phenobarbital. Administration of a second dose of phenobarbital 12 or 24 h after the first dose did not equalize the induction of rGSTA1/A2 across the lobule. The transcriptional activity of the rGSTA1/A2 gene was increased 3.5- to 5.5-fold in whole liver by phenobarbital, but activities were the same in enriched periportal and perivenous subpopulations of hepatocytes from phenobarbital-treated animals. The half-life of rGSTA1/A2 mRNA in control animals was 3.6 h, whereas it was 10.2 h in phenobarbital-treated animals. We conclude that phenobarbital induces rGSTA1/A2 expression by increasing transcriptional activity across the lobule but induction of rGSTA1/A2 is greater in perivenous hepatocytes due to localized stabilization of mRNA transcripts.

Phenotypic polymorphism and gender-related differences of CYP1A2 activity in a Chinese population

AIMS

To investigate the distribution characteristics of CYP1A2 in a Chinese population, and to examine gender-related differences in CYP1A2 activity.

METHODS

Two hundred and twenty-nine healthy subjects, 120 men and 109 women, were enrolled in this study. CYP1A2 activity was measured by plasma paraxanthine/caffeine (1,7X/1,3,7X) ratio 6 h after administration of 300 mg caffeine. The concentrations of paraxanthine and caffeine in plasma were detected by h.p.l.c.

RESULTS

A 16-fold variation of CYP1A2 activity (range 0. 09 to 1.46) was shown in this study. The coefficient of variation (CV %) of CYP1A2 activity was 62.9%. Non-normal distribution of CYP1A2 activity was indicated by the Shapiro-Wilk test (P<0.001). Probit plots of CYP1A2 activity revealed a bimodal distribution with breakpoint of 1,7X/1,3,7X ratio of 0.12. The percentage of poor metabolizers (PMs) was 5.24% (95% CI: 2.35% approximately 8.13%) in this Chinese population. Residual analysis of the data also supported bimodality (P<0.01). The CYP1A2 activity of men was higher than that of women (median: 0.33 vs 0.23, P<0.001). A probit plot of CYP1A2 activity in men was shifted to the left compared with that in women. Based on phenotype, the gender-related difference was observed in extensive metabolizers (EMs) (P<0.001), but not in PMs (P >0.1). In addition, there was no sex-related difference in the incidence of PMs (P >0.1).

CONCLUSIONS

There is a phenotypic polymorphism in CYP1A2 activity in this Chinese population, and CYP1A2 activity is higher in men than that in women.

Effects of a chargrilled meat diet on expression of CYP3A, CYP1A, and P-glycoprotein levels in healthy volunteers

BACKGROUND & AIMS

Carcinogenic heterocyclic amines and polycyclic aromatic hydrocarbons present in chargrilled meat are substrates for inducible CYP1A and CYP3A enzymes and for P-glycoprotein. We examined whether consumption of a chargrilled meat diet results in induction of these proteins.

METHODS

Ten healthy adults were fed a diet enriched with chargrilled meat for 7 days. Duodenal biopsy specimens were obtained on days 1, 5, and 12 and analyzed for CYP1A, CYP3A, and P-glycoprotein messenger RNA (mRNA) and protein. On days 5 and 12, hepatic CYP3A4 and CYP1A2 activities were measured and colon biopsies were performed. The levels of polycyclic aromatic hydrocarbon DNA adducts in peripheral blood mononuclear cells were measured on days 1, 4, 11, and 26.

RESULTS

There was no detectable induction of CYP3A4, CYP3A5, or P-glycoprotein mRNAs or protein in small intestine or colon and no induction of hepatic CYP3A4 enzyme activity. In contrast, the chargrilled meat diet resulted in unequivocal induction of CYP1A enzymes in the liver and small intestine of each subject. There was an inverse correlation between the level of peripheral polycyclic aromatic hydrocarbon DNA adducts measured on day 11 and both liver CYP1A2 activity (P = 0.027) and enterocyte CYP1A1 protein concentration (P = 0.046).

CONCLUSIONS

Ingestion of chargrilled meat results in induction of CYP1A enzymes but not CYP3A4 or P-glycoprotein. This observation, combined with the correlation between adduct levels and CYP1A expression, supports an adaptive role for CYP1A but not CYP3A4 or P-glycoprotein.