Induction of rat hepatic glucocorticoid-inducible cytochrome P450 3A by metyrapone

Metabolic changes in serum steroids for diagnosing and subtyping Cushing's syndrome

To evaluate the diagnostic value of serum levels of adrenal steroids for diagnosing and subtyping Cushing's syndrome. Patients diagnosed with endogenous Cushing's syndrome (34 and 19 patients with adrenal and pituitary Cushing's syndrome, respectively) and healthy controls (n = 34) were consecutively enrolled at Seoul National University from 2016 to 2020. Morning serum samples were collected before and 3 months after treatment. Serum steroids were profiled using liquid chromatography-mass spectrometry. The diagnostic value of each and the combination of steroids were assessed using the area under the curve of receiver operating characteristic (AUROC) and decision tree analysis. Tetrahydrocortisone and 6β-hydroxycortisol showed the highest AUROC (0.893 and 0.890, respectively) for the diagnosis of endogenous Cushing's syndrome. The decision tree composed of tetrahydrocortisone and 6β-hydroxycortisol correctly classified 79/87 (90.8 %) subjects. For subtyping into adrenal or pituitary Cushing's syndrome, dehydroepiandrosterone sulfate (DHEA-S) showed the highest AUROC (0.988), which was similar to that of plasma ACTH (0.994, P = 0.458). The decision tree composed of only DHEA-S correctly classified 51/53 (96.2 %) of the Cushing's syndrome subtype. DHEA-S showed a significant linear correlation with the plasma ACTH level, but not with the 24 -h urine free cortisol or dexamethasone suppression test results. All steroids, except allo-tetrahydrocortisol and tetrahydrocortisone, decreased significantly at 3 months post-treatment with similar patterns in both adrenal and pituitary Cushing's syndrome. Serum steroid profiling using a single morning serum sample provides valuable information for diagnosing and subtyping Cushing's syndrome.

Risk of adrenal crisis in dental patients: results of a systematic search of the literature

BACKGROUND

The authors performed a systematic search of the literature to identify the frequency of, risk of experiencing and factors associated with adrenal crises in dental patients.

METHODS

The authors searched PubMed and Ovid MEDLINE (1947-June 20, 2012) and Embase (1974-2012) for English-language articles related to cases of adrenal crisis in dentistry and extracted and analyzed data from the articles. The six authors determined whether the cases identified met a consensus definition of adrenal crisis.

RESULTS

Of 148 articles identified in the initial screening, 34 articles were included in the final review, from which six cases met the criteria of adrenal crisis. The authors categorized four cases as "suggestive of adrenal crisis" and two cases as "consistent with adrenal crisis." Risk factors were significant adrenal insufficiency, pain, infection, having undergone an invasive procedure, having received a barbiturate general anesthetic, and poor health status and stability at the time of presentation. The authors estimated risk to be less than one in 650,000 in patients with adrenal insufficiency.

CONCLUSIONS

Adrenal crisis is rare in dental patients, with only six reports of it having been published in the past 66 years. Risk is associated with unrecognized adrenal insufficiency, poor health status and stability at the time of treatment, pain, infection, having undergone an invasive procedure and having received a barbiturate general anesthetic.

CLINICAL IMPLICATIONS

Risk of adrenal crisis is reduced through proper evaluation of the patient, identification of risk factors and following appropriate preventive measures.

Drug interactions with mitotane by induction of CYP3A4 metabolism in the clinical management of adrenocortical carcinoma

Mitotane [1-(2-chlorophenyl)-1-(4-chlorophenyl)-2,2-dichloroethane, (o,p'-DDD)] is the only drug approved for the treatment for adrenocortical carcinoma (ACC) and has also been used for various forms of glucocorticoid excess. Through still largely unknown mechanisms, mitotane inhibits adrenal steroid synthesis and adrenocortical cell proliferation. Mitotane increases hepatic metabolism of cortisol, and an increased replacement dose of glucocorticoids is standard of care during mitotane treatment. Recently, sunitinib, a multityrosine kinase inhibitor (TKI), has been found to be rapidly metabolized by CYP3A4 during mitotane treatment, indicating clinically relevant drug interactions with mitotane. We here summarize the current evidence concerning mitotane-induced changes in hepatic monooxygenase expression, list drugs potentially affected by mitotane-related CYP3A4 induction and suggest alternatives. For example, using standard doses of macrolide antibiotics is unlikely to reach sufficient plasma levels, making fluoroquinolones in many cases a superior choice. Similarly, statins such as simvastatin are metabolized by CYP3A4, whereas others like pravastatin are not. Importantly, in the past, several clinical trials using cytotoxic drugs but also targeted therapies in ACC yielded disappointing results. This lack of antineoplastic activity may be explained in part by insufficient drug exposure owing to enhanced drug metabolism induced by mitotane. Thus, induction of CYP3A4 by mitotane needs to be considered in the design of future clinical trials in ACC.

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.

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).

Implications and consequences of enzyme induction on preclinical and clinical drug development

1. Enzyme induction has traditionally been studied during drug development to assess the potential of drug entities to interact with concomitant medications and alter their pharmacological effects, and clearly it is an unwanted phenomenon. However, another hurdle caused by induction occurs during preclinical development via the attainment of safety data, obtained by dosing high quantities of compound to species used in toxicology assessment. This review considers the techniques that can now be utilized in drug discovery, their relevance, the pharmacokinetic aspects of this phenomenon, and it discusses the consequences and implications of induction during preclinical and clinical development. 2. It is becoming increasingly routine to employ hepatocyte cultures and novel techniques such as quantitative real-time reverse transcriptase PCR to identify enzyme inducers in vitro. The major challenge is to utilize these in vitro data to predict the consequences of induction in vivo. From an understanding of pharmacokinetic principles and low clinical doses relative to preclinical studies, there is limited potential for induction by a development candidate to significantly alter the pharmacological efficacy of a co-administered drug. 3. The most comprehensive approach when considering induction involves integrating quantitative in vitro data, information on the pharmacokinetic behaviour of the compound and the PK/PD) relationship in order to predict its consequences. The generation of this holistic strategy would enable more detailed and informed decision-making about both the suitability of molecules for development and the development strategy itself.

Rapid determination of rat hepatocyte mRNA induction potential using oligonucleotide probes for CYP1A1, 1A2, 3A and 4A1

1. A new assay to quantify mRNA levels in small numbers of rat hepatocytes has been developed for cytochrome P450 (CYP) isoforms 1A1, 1A2, 3A and 4A1. The assay uses sets of oligonucleotide probes end-labelled with [35S]-dATP to hybridize to mRNA in control- or drug-treated rat hepatocytes cultured on Cytostar-T 96-well scintillating microplates. 2. The rat hepatocyte induction potential (RHIP) assays for CYP3A, 1A1, 1A2 and 4A1 are sensitive and selective and have an excellent qualitative relationship with CYP induction data ex vivo. The robustness of the CYP3A assay was determined following a run of > 40 plates. The variation of the dexamethasone (DEX) response on each plate, calculated as %coefficient of variation, showed that there was no significant difference between the variability of the response to DEX. 3. Assay specificity for each CYP isoform was achieved by designing probes (four per isoform) antisense to coding regions of each CYP gene sequence. In the CYP3A RHIP assay, pregnenalone 16alpha-carbonitrile (PCN), DEX, clotrimazole (CLOT) and miconazole (MIC) were all good inducers of CYP3A mRNA; beta-napthoflavone (BNF) and methylclofenapate (MCP), however, did not induce CYP3A mRNA, further defining the specificity of this methodology. Specificity was similarly confirmed for the other CYP isoforms. 4. Ind50, the concentration of inducer required to elicit a 50% induction of CYP-specific mRNA, was derived for prototypical CYP inducers: BNF 0.54 and 0.17 microM (CYP1A1 and 1A2 respectively), 3-methylcholanthrene (3MC) 0.11 and 0.04 microM (CYP1A1 and 1A2 respectively), PCN 0.03 microM, DEX 0.17 microM, CLOT 0.48 microM, MIC 3 microM, TAO 3 microM (CYP3A), MCP 1.8 microM, clofibrate (CLOF) 65 microM and ciprofibrate (CIP) 1.9 microM (CYP4A1). Ind50 for BNF and 3MC at CYP1A2 was 3-fold lower than that at CYP1A1 indicating a subfamily difference in inducer potency. 5. Reducing the numbers of animals and the amount of compound required to study CYP induction is an important advantage of the RHIP assays over conventional evaluations in vivo. Typically four rats are dosed for 4 days using oral doses in the range 50-500 mg kg(-1) day(-1). In comparison, the amount of hepatocytes required to carry out all the studies reported herein may be obtained from a single animal (< 2 x 10(8) viable cells) and CYP induction investigated using microg rather than g quantities of drug substance. 6. With appropriately designed oligonucleotide probes, the RHIP technology can assess CYP induction in human hepatocytes, which together with preclinical data can contribute to improving the quality of compounds progressing into the expensive process of drug development.

High throughput ribonuclease protection assay for the determination of CYP3A mRNA induction in cultured rat hepatocytes

1. A rapid 96-well plate based method for the determination of CYP3A mRNA induction in primary rat hepatocytes has been developed which has substantial advantages over current technologies including the ability to test the effect of relatively large numbers of new chemical entities on the expression of CYP3A mRNA in hepatocytes. 2. The ribonuclease protection assay detects changes in mRNA levels in small numbers of hepatocytes by the utilization of a radiolabelled antisense riboprobe that will hybridize CYP3A1 and CYP3A23. Using in situ hybridization techniques in conjunction with Amersham 96-well Cytostar-T scintillating microplates, there is no need for isolation of mRNA. A simple ribonuclease digestion step allows quantitative data to be generated easily within 1 week of hepatocyte isolation. 3. Rat hepatocytes were cultured for 48 h post-isolation on the Cytostar plates coated with a basal matrix of Matrigel. Prototypical CYP3A inducers (dexamethasone and pregnenolone 16alpha-carbonitrile) have been studied using various treatment periods from 0.5 to 24 h. Methylclofenapate and beta-naphthoflavone, prototypical inducers of CYP4A and CYP1A respectively, have been used as controls to show specificity of the [33P]-labelled riboprobe for the CYP3A family. 4. Time-dependent increases in CYP3A mRNA were demonstrated following exposure of hepatocytes to prototypical CYP3A inducers, but not for methylclofenapate or beta-naphthoflavone, so demonstrating specificity for CYP3A mRNA over CYP1A and CYP4A. Analysis of the 24-h induction data demonstrates that significant differences from controls can be determined and that induction potential can be assessed. The system has the potential to screen for overall CYP3A mRNA induction in response to compounds at an early stage in drug research.

Methapyrilene hepatotoxicity is associated with oxidative stress, mitochondrial disfunction and is prevented by the Ca2+ channel blocker verapamil

Methapyrilene (MP) is an unusual hepatotoxin in that it causes periportal necrosis in rats. The mechanism of acute methapyrilene hepatotoxicity has, therefore, been investigated in cultured male rat hepatocytes. Addition of methapyrilene to rat hepatocytes resulted in a time- and dose-dependent loss in cell viability between 4 and 8 h of incubation as judged by cellular enzyme leakage. The cytochrome P450 (CYP) inhibitor metyrapone protected against methapyrilene-mediated toxicity suggesting that MP is metabolised by CYP for toxicity. The concentration-dependent protection from methapyrilene toxicity afforded by metyrapone correlated with an inhibition of microsomal CYP2C11-associated androstenedione 16alpha hydroxylase activity, and hepatocytes prepared from hypophysectomised rats (containing reduced levels of microsomal immunodetectable CYP2C11 and associated androstenedione 16alpha hydroxylase activity) showed resistance to the toxic effects of methapyrilene. These data suggest that the toxicity of methapyrilene is predominantly dependent on the CYP2C11 isoform. Treatment of hepatocytes with a toxic concentration of MP caused oxidative stress as indicated by increases in NADP+ levels within 2 h and cellular thiol oxidation as evidenced by a reduction--but not complete loss--in glutathione levels. Methapyrilene hepatotoxicity was associated with an early loss in mitochondrial function, as indicated by mitochondrial swelling and significant losses in cellular ATP within 2 h. Co-incubation of methapyrilene-treated hepatocytes with inhibitors of inner mitochondrial transition permeability pore opening--cyclosporin A or the thiol reductant dithiothreitol--abrogated cell death suggesting that pore opening and loss of mitochondrial Ca2+ homeostasis play a significant role in methapyrilene-mediated cell death. Co-incubation of methapyrilene-treated hepatocytes with the phenylalkylamine calcium channel blocker verapamil--but not by treating cells in a nominally calcium-free medium--also abrogated cell death, suggesting that if Ca2+ is involved in cell killing then it is dependent on an intracellular Ca2+ pool. Pre-treatment of hepatocytes for 1 h with verapamil--to inhibit intracellular Ca2+ pool filling--increased the potency of verapamil protection against methapyrilene toxicity by approximately 100-fold. Taken together, these data indicate that methapyrilene intoxication leads to mitochondrial disfunction and suggest a critical role for a loss of mitochondrial Ca2+ homeostasis in this model of hepatocyte death.

Comparative induction of CYP3A and CYP2B in rat liver by 3-benzoylpyridine and metyrapone

3-Benzoylpyridine (3BP) is a major metabolite of HGG-12, and oxime that has been synthesized as a potential antidote to the toxic effects of soman and other anticholinesterases. Structural similarities exist between 3BP, the cytochrome P450 (CYP)-inducer metyrapone (MET) and other 3-substituted pyridines that interact with CYPs. The present study evaluated the regulatory effects of 3BP on CYP expression in rat liver. Both 3BP and MET (100 mg/kg) increased total hepatic microsomal holo-CYP content significantly 24 h after administration to male rats. Pronounced increases in activities mediated by CYP2B (androstenedione 16 beta-hydroxylation and 7-pentylresorufin O-depentylation) were produced by 3BP and MET, which correlated with respective 9- and 14-fold increases in CYP2B immunoreactive protein. In addition, both agents slightly increased rates of microsomal CYP3A-dependent steroid 6 beta-hydroxylation, troleandomycin metabolite complex formation and total CYP3A immunoreactive protein. Induction of the dexamethasone-inducible CYP3A23 mRNA to 4.5- and 2.5-fold of control was detected in liver of MET- and 3BP-induced rats; CYP3A2 mRNA levels were unchanged. Analogous in vitro studies revealed that MET was a preferential inhibitor of CYP3A-mediated steroid 6 beta-hydroxylation activity, but 3BP was inactive against constitutive steroid hydroxylase CYPs. These findings indicate that the structurally related 3BP and MET elicit similar induction effects on CYPs 2B and 3A23 in rat liver after in vivo administration, but differential inhibitory effects of the chemicals on CYP activity in vitro. Recent reports have implicated a microsomal binding site in the induction of CYP3A1/3A23 in rat liver. In light of the present findings, substituted pyridines like 3BP may be useful tools in structure-activity studies to evaluate the physicochemical requirements for binding to this protein.

Disruption of endogenous regulator homeostasis underlies the mechanism of rat CYP1A1 mRNA induction by metyrapone

The transcriptional induction of the cytochrome P-450 1A1 (CYP1A1) gene by xenobiotics such as polyaromatic hydrocarbons is dependent on their interaction with the aryl hydrocarbon receptor. Administration of the structurally unrelated compounds metyrapone (a cytochrome P-450 inhibitor) or dexamethasone (a glucocorticoid) to male rats does not induce hepatic CYP1A1 mRNA. However, administration of both metyrapone and dexamethasone to male rats results in the induction of hepatic CYP1A1 mRNA expression. The induction response is mimicked in vitro in cultured rat hepatocytes by the addition of metyrapone and dexamethasone to a serum-free culture medium, suggesting that these compounds act directly on the liver in vivo to effect hepatic CYP1A1 mRNA induction. An examination of the characteristics of CYP1A1 induction by metyrapone and dexamethasone in combination in vitro indicate that at least 6 h of treatment is required for detectable levels of CYP1A1 mRNA to accumulate in hepatocytes. In contrast, beta-naphthoflavone, which is known to bind to the aryl hydrocarbon receptor to effect CYP1A1 gene expression, induces detectable levels of CYP1A1 mRNA within 2 h of treatment. CYP1A1 mRNA is also induced when hepatocytes are treated with metyrapone in combination with the protein synthesis inhibitor cycloheximide but not with dexamethasone in combination with cycloheximide, indicating that CYP1A1 mRNA induction is strictly dependent on the presence of metyrapone and suggesting that the metyrapone-associated induction of CYP1A1 mRNA is dependent on a loss of a constitutively expressed protein that functions to suppress CYP1A1 gene expression. The role of dexamethasone in metyrapone-associated induction of CYP1A1 is probably mediated through the glucocorticoid receptor since the glucocorticoid receptor antagonist RU486 reduces the levels of CYP1A1 mRNA induced by metyrapone and dexamethasone in combination. Increasing the levels of the photosensitizer riboflavin present in the culture medium 10-fold and exposure to light increases the levels of CYP1A1 mRNA induced by metyrapone and dexamethasone in combination in vitro, suggesting that photoactivation of inducing medium constituent(s) might be required for induction. Failure to induce CYP1A1 mRNA by co-administration of metyrapone and dexamethasone in hepatocytes cultured in a balanced salt solution with or without photoactivation indicates that induction is dependent on a photoactivated component of the culture medium and not on metyrapone or dexamethasone alone. The addition of tryptophan in the presence of riboflavin to the balanced salt solution restores CYP1A1 mRNA induction by metyrapone alone and induction is increased when medium is exposed to light, indicating that induction is dependent on tryptophan photoactivation in vitro. Metyrapone failed to compete with 2,3,7,8-tetrachlorodibenzo-p-dioxin for specific binding to the aryl hydrocarbon receptor in rat liver cytosolic fractions. These results suggest that CYP1A1 might be induced in rats by metyrapone through an indirect mechanism associated with an elevation in the level of an endogenously generated inducer such as photoactivated product(s) of tryptophan and not because of metyrapone's interacting with the aryl hydrocarbon receptor. The dependence of CYP1A1 induction on dexamethasone or cycloheximide suggests that derepression by a glucocorticoid receptor-modulated negative-acting factor of CYP1A1 gene expression might be critical to induction by metyrapone.

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.

Developmental changes in the constitutive and inducible expression of cytochrome P450 3A2

Using a CYP3A2-specific oligonucleotide and an antipeptide antibody raised against the C terminus of CYP3A2 (VINGA) it is demonstrated that metyrapone administration to adult (12 weeks old) but not immature (3 weeks old) male Sprague Dawley rats induces the hepatic expression of CYP3A2 mRNA and protein. The constitutively expressed level of CYP3A2 protein in adult male rats is markedly lower than the levels expressed in immature rats as determined using the anti-VINGA antibody, in contrast to previous reports using antibodies that do not discriminate between CYP3A forms. Hepatic microsomal CYP3A2 protein expression, examined between 3 and 15 weeks of age, is extinguished between 9 and 12 weeks of age in contrast to immunoreactive CYP3A protein (determined using a nonselective antibody) and CYP3A-dependent androstenedione 6beta-hydroxylase activity. These data suggest that the regulation of the induction of CYP3A2 is developmentally controlled and that the major expressed adult form(s) of constitutively expressed CYP3A is not CYP3A2.

Effect of vitamin A deficiency on the expression of low affinity glucocorticoid binding site activity and glucocorticoid-dependent induction of CYP3A2 in rat liver

Maintenance of rats on a vitamin A-deficient diet resulting in undetectable levels of plasma retinol and significant reductions in relative testes weight compared to age-matched controls leads to the loss of liver membrane-bound low affinity glucocorticoid binding site (LAGS) activity without any effects on the levels of constitutively expressed CYP3A2 protein. Subsequent daily administration of retinol acetate to vitamin A-deficient rats results in the re-expression of LAGS activity to control levels by 7 days. To determine any role for the LAGS in the modulation of CYP3A2 expression by glucocorticoids, a single dose of dexamethasone 21-phosphate was administered to vitamin A-deficient rats and vitamin A-deficient rats induced to re-express LAGS by daily retinol acetate treatment. Retinol acetate administration alone induces CYP3A2 protein to apparent maximal levels since dexamethasone 21-phosphate does not further increase the induction response. However, CYP3A2 remains inducible to dexamethasone 21-phosphate in vitamin A-deficient rats. These data suggest that vitamin A status affects the expression of LAGS and CYP3A2 but that glucocorticoids regulate the induction of CYP3A2 by a mechanism(s) independent of their interaction with the LAGS.

Analysis of rat cytochrome P450 isoenzyme expression using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR)

A method was developed using reverse transcriptase-polymerase chain reaction (RT-PCR) to selectively detect and qualitatively determine the levels of mRNA expression of the major isoenzymes of cytochrome P450 (P450 1A1, 1A2, 2B1/2, 2C11, 2E1, 3A1, 3A2, and 4A1) and fatty acyl-CoA oxidase (FACO) in the rat. Total liver RNA was isolated from male Sprague-Dawley rats treated with various inducers of cytochrome P450 (P450) and analyzed for the presence and relative quantities of each P450 isoenzyme mRNA using this technique. The specificity of the oligonucleotide primers used in the detection of each P450 mRNA was tested and confirmed through the simultaneous analysis of liver microsomal protein preparations for the presence of constitutive or inducible P450 apoprotein and enzyme activities using western immunoblotting and specific enzyme activity measures, respectively. This method of P450 expression analysis is proven to be highly specific and readily applicable for the assessment of P450 enzyme induction and down-regulation in the rat during routine toxicology studies when expression of the gene product is regulated by transcriptional activation and/or mRNA stabilization.

Effects of metyrapone on expression of CYPs 2C11, 3A2, and other 3A genes in rat hepatocytes cultured on matrigel

Hepatocytes cultured on matrigel express many liver-specific functions, but the levels and activities of the predominant male-specific rat hepatic CYPs, 3A2 and 2C11, decline rapidly in culture. Metyrapone maintains the level of total cytochrome P450 of rat hepatocytes in primary culture, but the mechanism underlying this effect has not been completely elucidated. The present study sought to determine whether metyrapone acts solely to stabilise CYP proteins in rat hepatocytes cultured on matrigel, or whether it also influences mRNA levels of the encoding genes. Metyrapone maintained the level of total cytochrome P450 in cultured hepatocytes so that values were > 200% of those found in untreated control cells 24 hr after isolation. At this time, CYP3A2-mediated testosterone 6 beta-hydroxylation was approximately 7-fold higher in hepatocytes cultured in the presence of metyrapone than in control cells, and CYP2C11-dependent testosterone 2 alpha- and 16 alpha-hydroxylation activities were between 2 and 3-fold greater. The results inferred from catalytic activities were supported by immunoquantitation of CYP3A and 2C11 proteins. The trend of increased CYP protein levels in metyrapone-treated cells continued throughout the 48-hr culture period. In control cells, CYP3A2 and 2C11 mRNA levels fell abruptly in culture to reach values at 24 hr that were < 30% of those in freshly isolated cells; addition of metyrapone failed to arrest this fall. However, treatment of cells with metyrapone considerably elevated levels of one or more CYP3A subfamily mRNA species, as detected by a riboprobe based on the cDNA for CYP3A1 ("CYP3A1-like mRNA') that were demonstrated, by another riboprobe, not to be CYP3A2 or RNCYP3AM. RT-PCR of mRNA prepared from cultured hepatocytes, followed by restriction mapping of the cloned cDNAs was used to characterise the CYP3A induced by metyrapone. This revealed that elevated levels of the CYP3A1-like mRNA were attributable to induction of RL33/cDEX mRNA; there were no CYP3A1 cDNAs isolated from these cells. These data are interpreted as indicating that metyrapone stabilises the expression of cytochrome P450 in culture by both pre- and posttranslational mechanisms. The particular mechanism employed is gene-specific, whereby even the highly homologous genes CYP3A2, RL33/cDEX and, possibly, RNCYP3AM are subject to different types of regulation in the presence of metyrapone.

Characteristics of a membrane-associated steroid binding site in rat liver

The glucocorticoid dexamethasone binds a site in microsomes in a saturable manner which by competition studies also binds other classes of steroids. The characteristics of dexamethasone binding to microsomes is distinct from the cytosolic glucocorticoid receptor by virtue of a slower rate of association; a differential competition by the glucocorticoid receptor agonist triamcinolone acetonide and antagonist RU38486; and a lack of sensitivity to the reversible thiol reactive agent arsenite. However, both binding sites have a similar rate constant for complex dissociation; are sensitive to covalent thiol modification by N ethylmaleimide and iodoacetamide; and have a similar concentration-dependent sensitivity to the reversible thiol reactive agent methyl methanethiosulfonate. The binding of dexamethasone by microsomes therefore exhibits distinct properties from the soluble glucocorticoid receptor.