Induction of CYP1A1 gene by benzimidazole derivatives during Caco-2 cell differentiation. Evidence for an aryl-hydrocarbon receptor-mediated mechanism

The Caco-2 cell line, derived from a human colon adenocarcinoma, is unique in its property of spontaneously differentiating into a mature enterocyte cell type during its growth in culture. In this work, we compared the response of the CYP1A1 gene with the benzimidazole derivatives omeprazole and lansoprazole, and with the classical inducer beta-naphthoflavone in the Caco-2 cells at various culture stages. In addition, we characterized the Caco-2 aryl-hydrocarbon receptor. The protein-synthesis inhibitor cycloheximide led to a derepression of the CYP1A1 gene transcription, and to a superinduction when combined with either beta-naphthoflavone or benzimidazoles. Taking advantage of the spontaneous differentiation of Caco-2 cells in long-term cultures, we observed a difference in behavior between the classical inducer beta-naphthoflavone and the atypical inducer omeprazole. In the poorly differentiated cells, both compounds elicited comparable dose/response and rate of induction of CYP1A1 gene expression. In the fully differentiated cells, in contrast, the induction by omeprazole was only transient, whereas the response to beta-naphthoflavone was long lasting. The Caco-2 aryl-hydrocarbon receptor exhibited binding characteristics similar to those determined for human liver and other tissues. The induction of CYP1A1 transcription by benzimidazole derivatives in Caco-2 cells occurred with no direct binding of benzimidazole derivatives to the aryl-hydrocarbon receptor, as in human hepatocytes. However, transient transfection experiments clearly showed that the xenobiotic-responsive element enhancer, with which the activated aryl-hydrocarbon receptor interacts, could drive the induction of a heterologous promoter in the presence of benzimidazoles. Finally the presence of the activated aryl-hydrocarbon receptor in the nuclei of the Caco-2 cells exposed to these molecules was clearly demonstrated by gel-retardation experiments. These results question about the mechanism of ligand-independent activation of the aryl-hydrocarbon receptor and intracellular signaling, initiated by benzimidazole derivatives.

Detection of CYP3A5 allelic variant: a candidate for the polymorphic expression of the protein?

In liver samples from 19 Caucasian subjects, the CYP3A5 protein was detected in 74% of individuals (14/19), while the messenger was shown to be expressed in 100% of individuals, assessed by the RT-PCR method. In order to characterise the putative mutation(s) in the messenger, accounting for the absence of protein accumulation, the full coding region of the CYP3A5 cDNA was sequenced for two unrelated individuals, one expressing the protein at a high level and one being defective. A point mutation in exon 11 at position 1280 (C-->A) was found to cosegregate with the absence of protein accumulation in 2 of the 5 defective individuals. This mutation produces a change in the amino acid at position 398 (Thr-->Asn). Whether this mutation affects the stability of the protein or whether it is in linkage desequilibrium with other mutation(s) in the non-coding region of the messenger is not known. The C-->A change at 1280 generates a Tsp509 I site which can be used for routine evaluation of the frequency of this mutation in population studies.

Metabolism of the new immunosuppressor cyclosporin G by human liver cytochromes P450

Cyclosporin G is a new immunosuppressor structurally similar to cyclosporin A. Although this drug is pharmacologically as active as cyclosporin A, it is less toxic, in particular at the kidney level. The aim of this work was to identify the enzyme system(s) involved in the oxidative metabolism of cyclosporin G in man: (1) in a bank of human liver microsomes (n = 22), cyclosporin G oxidase activity correlated significantly with cyclosporin A oxidase activity (P < 0.0001) and with the level of CYP3A4 (P < 0.002), determined by immunoblot; (2) specific inhibitors of CYP3A4, troleandomycin, and ketoconazole, inhibited cyclosporin G oxidase activity by more than 80%; (3) antiCYP3A4 antibodies specifically inhibited this activity by nearly 90%; (4) cyclosporin A was a competitive inhibitor of cyclosporin G oxidase and vice versa; (5). Among a battery of cDNA-expressed CYPs, only CYP3A4 was able to generate detectable amounts of metabolites of cyclosporin G and cyclosporin A with a turnover number close to that calculated from experiments with liver microsomes; (6) in human hepatocytes in culture, pretreatment of cells with rifampicin and phenobarbital, 2 inducers of CYP3A4, produced a great increase in cyclosporin G oxidase activity, while beta-naphthoflavone, an inducer of CYP1As, did not. We conclude that CYP3A4 is the major enzyme involved in the oxidative metabolism of cyclosporin G in human liver.

Chondroitin sulfate proteoglycans in the developing central nervous system. I. cellular sites of synthesis of neurocan and phosphacan

We have used in situ hybridization histochemistry to examine the cellular sites of synthesis of two major nervous tissue proteoglycans, neurocan and phosphacan, in embryonic and postnatal rat brain and spinal cord. Both proteoglycans were detected only in nervous tissue. Neurocan mRNA was evident in neurons, including cerebellar granule cells and Purkinje cells, and in neurons of the hippocampal formation and cerebellar nuclei. In contrast, phosphacan message was detected only in astroglia, such as the Golgi epithelial cells of the cerebellum. At embryonic day 13-16, phosphacan mRNA is largely confined to areas of active cell proliferation (e.g., the ventricular zone of the ganglionic eminence and septal area of the brain and the ependymal layer surrounding the central canal of the spinal cord) as well as being present in the roof plate. The distribution of neurocan message is more widespread, extending to the cortex, hippocampal formation, caudate putamen, and basal telencephalic neuroepithelium, and neurocan mRNA is present in both the ependymal and mantle layers of the spinal cord but not in the roof plate. The presence of neurocan mRNA in areas where the proteoglycan is not expressed suggests that the short open reading frame in the 5'-leader of neurocan may function as a cis-acting regulatory signal for the modulation of neurocan expression in the developing central nervous system.

Neurocan and phosphacan: two major nervous tissue-specific chondroitin sulfate proteoglycans

Neurocan is a multidomain hyaluronan-binding chondroitin sulfate proteoglycan that is synthesized by neurons, whereas the astroglial proteoglycan phosphacan is an mRNA splice variant representing the entire extracellular portion of a receptor-type protein tyrosine phosphatase. A glycoform of phosphocan (phosphocan-KS) that contains both chondroitin sulfate and keratan sulfate is present in the postnatal rat central nervous system (CNS). The concentration of neurocan in brain increases during late embryonic development but then declines steeply during the early postnatal period together with hyaluronan, and neurocan also undergoes extensive proteolytic processing during the course of brain development. In contrast, the concentrations of both phosphocan and phosphocan-KS rise steadily after embryonic day 20 to reach a plateau at about 2 weeks postnatally. In the embryonic CNS the distribution of neurocan mRNA is more widespread than that of phosphocan, which is primarily present in regions of active cell proliferation. Neurocan mRNA is also present in areas where the proteoglycan is not expressed, and there is evidence that the short open reading frame in its 5'-leader may function as a cis-acting regulatory signal for the modulation of neurocan expression in the developing CNS. Neurocan and phosphocan bind saturably, reversibly, and with high affinity to neural cell adhesion molecules (Ng-CAM/L1, NCAM, TAG-1/axonin-1) and to tenascin-C. The proteoglycans and their ligands have overlapping localizations in the CNS, and binding of phosphocan to Ng-CAM/L1, NCAM, and tenascin-C is mediated by complex-type N-linked oligosaccharides on the proteoglycan. Neurocan and phosphocan also bind to neurons and are potent inhibitors of neuronal and glial adhesion and neurite outgrowth. Through their interactions with neural cell adhesion and extracellular matrix molecules, these proteoglycans may play a major role in modulating cell adhesion, neurite growth, and signal transduction across the plasma membrane during the development of the CNS.

Oxidative metabolism of zolpidem by human liver cytochrome P450S

The aim of this study was to identify the form(s) of cytochrome P450 (CYP) responsible for the biotransformation of zolpidem to its alcohol derivatives which, after rapid conversion to carboxylic acids, represents the main way of metabolism in humans. In human liver microsomes, zolpidem was converted to alcohol derivatives. Production of these correlated with the level of CYP3A4 and with cyclosporin oxidation and erythromycin N-demethylation activities, but not with the level of CYP1A2 nor with ethoxyresorufin O-deethylation or S-mephenytoin 4'-hydroxylation activities. Liver microsomes from CYP2D6-deficient patients exhibited normal activity. Production of alcohol derivatives was significantly inhibited by anti-CYP3A antibodies and by ketoconazole. Antibodies directed against other CYP forms (including CYP1A1, CYP1A2, CYP2A6, CYP2B4, and CYP2C8), and CYP-specific substrates or inhibitors (including propranolol, coumarin, mephenytoin, sulfaphenazole, quinidine, aniline, and lauric acid) produced a moderate or no inhibitory effect. cDNA-expressed CYP3A4 and CYP1A2 generated significant amounts of one of the alcohol derivatives, whereas CYP2D6 generated both of them in similar amounts. In human hepatocytes in primary culture, zolpidem was extensively and almost exclusively converted to one of the carboxylic acid derivatives, the main species identified in vivo. Treatment of cells with inducers of CYP1A (beta-naphthoflavone) and CYP3A (rifampicin and phenobarbital) greatly increased the rate of production of this metabolite. We conclude that the formation of alcohol derivatives of zolpidem is rate-limiting and principally mediated by CYP3A4. Both CYP1A2 and CYP2D6 participate in alcohol formation; but, because of their low relative level of expression in the human liver, their contribution is minor.

Differential effects of cytokines on the inducible expression of CYP1A1, CYP1A2, and CYP3A4 in human hepatocytes in primary culture

We have investigated the effect of cytokines, including interleukin-6 (Il-6), interleukin-1 alpha (Il-1 alpha), and tumor necrosis factor-alpha (TNF-alpha), on the inducible expression of cytochrome P450s (CYP) CYP1A1, CYP1A2, and CYP3A4 in human hepatocytes in primary culture. The ability of these cultures to mimic the acute phase response when stimulated with cytokines was evaluated using immunoblotting to measure the production of albumin, ferritin, fibrinogen, and ceruloplasmin. The cytokines exhibited specific patterns of action on the production of these proteins. Albumin was depressed by all the cytokines. In contrast to Il-6 and Il-1 alpha, TNF-alpha reduced the production of fibrinogen and ceruloplasmin but stimulated the production of ferritin. When cells were treated with the CYP inducer alone, large increases in the expression of CYP1A1 and CYP1A2 by beta-naphthoflavone and of CYP3A4 by rifampicin were observed at messenger RNA (mRNA) and protein levels, by ribonuclease protection and immunoblotting, respectively. When the cells were treated with the inducer plus cytokines, the induction of mRNA was greatly reduced. Again, specific patterns of action were revealed: Il-6 had the most potent effect on CYP3A4, whereas TNF-alpha was the most potent with CYP1A genes. In all cases, changes at the protein levels paralleled changes at the mRNA levels. In cells preinduced with beta-naphthoflavone or rifampicin, the decay with time of the levels of the CYP1A2 or CYP3A4 proteins, after the removal of the inducer, was not affected by cytokines. We conclude that cytokines strongly repress the inducibility of CYP1As and CYP3A4 genes at a transcriptional or a posttranscriptional level, but affect neither the rate of translation of CYP mRNAs nor the rate of degradation of the CYP proteins in these cultures.

Effect of anti-ulcer drugs on DNA synthesis in adult normal human hepatocytes in culture

The aim of this work was to investigate the effect of four H2 receptor antagonist, cimetidine, ranitidine, famotidine, nizatidine, and of two proton pump inhibitors, omeprazole and lansoprazole, on the mitotic response of human hepatocytes in primary culture. After plating at subconfluent density, cells were exposed to 0.2 to 20 mumol/L of these drugs for 48 hours, either in the absence or in the presence of epidermal growth factor (EGF). The rate of DNA synthesis was evaluated by [3H]-thymidine incorporation into genomic DNA. Both the basal rate of DNA synthesis and the extent of stimulation by EGF exhibited a wide interindividual variability, and were not correlated with the viability of freshly prepared cells. In contrast, the effects of anti-ulcer drugs on the rate of DNA synthesis were clearly reproducible from one culture to another. H2 receptor antagonists had no significant effect (P > .2) over the entire range of concentration tested, whereas omeprazole and lansoprazole significantly inhibited the rate of DNA synthesis by 60% to 90% at 30 mumol/L (P = .016). This effect was concentration dependent between 2 and 20 mumol/L. Neither of the drugs tested was cytotoxic under the conditions used in this work, as assessed by measurements of the do nov protein synthesis. We conclude that, in contrast to H2 receptor antagonists, omeprazole and lansoprazole are able to interfere with the replicative synthesis of DNA in human hepatocytes in culture, at suprapharmacological concentrations. Whether or not this effect is clinically significant remains to be established.

Evidence for the ligand-independent activation of the AH receptor

Benzimidazole derivatives are potent inducers of CYP1A1 in rabbit and human hepatocytes, but apparently do not bind the AH receptor. To resolve this paradoxical behaviour, studies have been concerned with the question of whether an alternative ligand-independent mechanism could explain the activation of the AH receptor. From experiments in cultured rabbit hepatocytes we show that benzimidazoles bind early and transiently to an unknown protein. Moreover, they are able to deplete the AHR in a time- and dose-dependent manner. In contrast, benzimidazoles are unable to induce CYP1A1 mRNA in mouse hepa-1 cells and to deplete the high-affinity AHR form from these cells. Taken together these data suggest that a signal transduction pathway, similar to that involved in the ligand-independent activation of steroid receptors, could only activate the low-affinity forms of AHR as those existing in rabbit and human cells.

Oxidative metabolism of lansoprazole by human liver cytochromes P450

The aim of this work was to identify the form(s) of human cytochrome P450 (P450) involved in the hepatic biotransformation of lansoprazole to its two main metabolites, i.e., the sulfone and the hydroxy derivative. In liver microsomes, the production of the sulfone of lansoprazole correlated with the level of P450 3A4, cyclosporin oxidase, and the production of the hydroxy derivative, as well as of omeprazole sulfone. The production of hydroxylansoprazole moderately correlated with the level of P450 3A4, cyclosporin oxidase, and (S)-mephenytoin 4'-hydroxylase. The production of the sulfone and of the hydroxy derivative of lansoprazole was significantly inhibited by anti-P450 3A4 antibodies, by cyclosporin and ketoconazole, and by tolbutamide. Anti-P450 2C8 and 2C3 antibodies moderately inhibited the biotransformation of lansoprazole, whereas they completely inhibited (S)-mephenytoin 4'-hydroxylase activity under the same conditions. In primary cultures of human hepatocytes, the biotransformation of lansoprazole and the oxidation of cyclosporin were strongly increased by rifampicin and phenobarbital, whereas (S)-mephenytoin 4'-hydroxylation was not. beta-Naphthoflavone did not induce the formation of the sulfones but stimulated the production of hydroxylansoprazole. Among several forms of cDNA-expressed human P450s, 3A4 generated significant amounts of the sulfones of lansoprazole and omeprazole and 2C18 was active for the production of hydroxylansoprazole but inactive in the 4'-hydroxylation of (S)-mephenytoin. We conclude that P450 3A4 is the major enzyme involved in the production of the sulfone of lansoprazole and that this P450, as well as P450 2C18 and/or another 2C-related form, could contribute to the production of hydroxylansoprazole.

Nucleotide sequence and molecular variants of rat receptor-type protein tyrosine phosphatase-zeta/beta

We have previously described the cloning of phosphacan, a chondroitin sulfate proteoglycan of nervous tissue which interacts with neurons, glia, neural cell adhesion molecules, and tenascin, and represents the extracellular domain of a receptor-type protein tyrosine phosphatase. We now report the complete cDNA and deduced amino acid sequences of the rat transmembrane phosphatase, and demonstrate that the phosphatase and the extracellular proteoglycan have different 3'-untranslated regions. Northern analysis showed three probable splice variants, comprising the extracellular proteoglycan (phosphacan) and long and short forms of the transmembrane phosphatase. PCR studies of rat genomic DNA indicated that there are no introns at the putative 5' and 3' splice sites or in the 2.6 kb segment which is deleted in the short transmembrane protein. Using variant-specific riboprobes corresponding to sequences in the 3'-untranslated region of phosphacan and in the first or second phosphatase domains of the transmembrane protein, in situ hybridization histochemistry of embryonic rat brain and spinal cord and early postnatal cerebellum demonstrated identical localizations of phosphacan and phosphatase mRNAs.

Sequence of the 5'-flanking region of CYP3A5: comparative analysis with CYP3A4 and CYP3A7

Two genomic clones containing the 3'-end of CYP3A7, the whole 5'-flanking region plus the first exon of CYP3A5 were isolated from two different human genomic banks and sequenced. The 5'-flanking region of CYP3A5, from nucleotide -1 to -1434, was 60% and 59% similar with the corresponding regions of CYP3A4 and CYP3A7, respectively. However, the similarity increased to 74% when regions from nucleotide -1 to -700 were compared and dropped to less than 42% for regions upstream from -700. The CYP3A5 promoter contains a CATA box instead of the typical TATA box and a basic transcription element (BTE). Other consensus sequences previously found in CYP3A4 and CYP3A7 were also identified in the 5'-flanking of CYP3A5 from -1 to -700. Both CYP3A5 and CYP3A7 genes appear to be tandemly associated and transcribed in the same direction on chromosome 7.

FK 506 renal toxicity and lack of detectable cytochrome P-450 3A in the liver graft of a patient undergoing liver transplantation

Many commonly used drugs are substrates for hepatic cytochrome P-450 3A in human beings, and its role in the metabolism of potentially toxic immunosuppressants has been highlighted (cyclosporine, FK 506). One hundred fifty human liver grafts were biopsied before and after liver transplantation, and levels of cytochromes P-450 3A, 1A2, 2D6 and 2C were estimated by means of the Western-blot technique and correlated with histological appearance, glycogen content and clinical course. In 15 of the grafts, cyclosporine oxidase was also measured, and in 12 of 15 recipients, urinary 6 beta-hydroxycortisol excretion was assayed. A wide range of cytochrome P-450 3A values were observed (25 to 366 arbitrary units/mg; mean, 105 +/- 63 arbitrary units/mg). In one graft (no. 730) no cytochrome P-450 3A was detectable on immunoblotting, despite increasing homogenate concentrations. In this sample, cytochromes P-450 1A2, 2D6, and 2C were present in normal ranges. Levels of cyclosporine oxidase and 6 beta-hydroxycortisol in the urine specimens of the recipient were found to be low. The recipient of graft 730 experienced reversible complications of FK 506 therapy despite adherence to the administration protocol and drug plasma level in the normal range. The subsequent appearance of the cytochrome P-450 3A was associated with the consequent tolerance of oral FK 506. The absence of detectable amounts of P-450 3A in one biopsy from a donated human liver graft dramatically emphasizes the extreme range of this enzyme levels and has important clinical implications.

The pig as a model for studying AH receptor and other PAH-binding proteins in man

In this report we compare the three major binding proteins existing in hepatic cytosols of pig and human. Many data suggest that for the AH receptor, which mediates the biological effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, a structural as well as a functional variability exists across species. Similar conclusions can be drawn from the interspecies characterization of benzo(a)pyrene binding proteins, namely the 4S protein and the 8S protein. Using fractionation procedures such as sucrose gradient sedimentation and gel permeation chromatography we obtained enriched fractions containing each of these binding proteins. By saturation experiments and analysis of data by Scatchard and Woolf plots we determined binding characteristics and we conclude that pig and human AH receptors were closely related proteins since their Kd (18 +/- 0.4nM) were found quite similar. On the other hand, 4S proteins from pig (Kd = 16.7 nM; Bmax = 5.5 pmol/mg) and from human (Kd = 14 nM; Bmax = 4.5 pmol/mg) as well as 8S proteins (Kd approximately 300 nM) also exhibit remarkable similarities.

Phosphacan, a chondroitin sulfate proteoglycan of brain that interacts with neurons and neural cell-adhesion molecules, is an extracellular variant of a receptor-type protein tyrosine phosphatase

We have identified cDNA clones encoding a chondroitin sulfate proteoglycan of rat brain (previously designated 3F8 and now named phosphacan) that binds to neurons and neural cell-adhesion molecules. A sequence of 1616 amino acids deduced from a 4.8-kb open reading frame contains the N-terminal amino acid sequence of the 3F8 core glycoprotein as well as four internal CNBr, tryptic, and endoproteinase Lys-C peptide sequences from the proteoglycan. The deduced amino acid sequence, beginning with a 24-amino acid signal peptide, reveals an N-terminal domain of 255 amino acids homologous to carbonic anhydrases. The entire amino acid sequence deduced from our cDNA clones corresponds to the extracellular portion of a human receptor-type protein tyrosine phosphatase (RPTP zeta/beta) with which it has 76% identity, and the proteoglycan may represent an mRNA splicing variant of the larger transmembrane protein. RNA analysis demonstrated that a probe to the N-terminal carbonic anhydrase domain of the proteoglycan hybridizes with rat brain mRNA of 9.5, 8.4, and 6.4 kb, whereas probes to the phosphatase domains hybridize with only the 9.5-kb message and with the 6.4-kb message (which corresponds to a previously identified variant of the transmembrane protein in which half of the extracellular domain is deleted). The 30 N-terminal amino acids of the 3H1 chondroitin/keratan sulfate proteoglycan of brain are identical to those of the 3F8 proteoglycan, and six internal tryptic peptide sequences also matched those found in sequenced peptides of the 3F8 proteoglycan and/or amino acid sequences deduced from the cDNA clones. We therefore conclude that the 3H1 chondroitin/keratan sulfate proteoglycan and the 3F8 chondroitin sulfate proteoglycan represent glycosylation and possible extracellular splicing variants of a receptor-type protein tyrosine phosphatase. These proteoglycans may modulate cell interactions and other developmental processes in nervous tissue through heterophilic binding to cell-surface and extracellular matrix molecules, and by competition with ligands of the transmembrane phosphatase.

What changes drug metabolism in critically ill patients? Two preliminary studies in isolated human hepatocytes

The metabolism of many drugs is abnormal in the critically ill patient. The causes of this are unknown, and investigation in patients is difficult. We therefore used isolated, cultured human hepatocytes to study the effects of hypoxia and induction on cytochrome P450 3A, the cytochrome responsible for the metabolism of many drugs. When hepatocytes were exposed to 5% oxygen, the amount of 3A produced, after induction with rifampicin, was five to 10 times less than the amount produced in 21% oxygen. In another study, we exposed isolated hepatocytes for 4 days to serum from five critically ill patients or from volunteers. At the end of this time, the functional ability of the hepatocytes to glucuronidate 14C progesterone was measured. Four of the five patients had a substance in their plasma that reduced the ability of the hepatocytes to glucuronidate progesterone. The nature of this substance and the reason that the serum from the fifth patient did not affect metabolism are unknown. This model is able to simulate the abnormalities which occur in critically ill patients. Further studies are needed to explain our observations and to identify the substances in the serum from critically ill patients that alter drug metabolism.

Involvement of human liver cytochrome P450 3A in vinblastine metabolism: drug interactions

Vinblastine biotransformation was investigated by using a human liver microsomes library. The drug was converted into one major metabolite (M) upon incubation with the microsomes. A large interindividual variation in vinblastine metabolism was observed among the samples tested, with a 4.4 ratio between the lowest and the highest metabolic rates. The biotransformation of vinblastine followed Michaelis-Menten kinetics (Km = 6.82 +/- 0.27 microM and Vmax = 0.64 +/- 0.06 nmol/min/mg protein). The involvement of the cytochrome P450 3A subfamily in vinblastine metabolism was demonstrated by the following body of evidence: (a) the competitive inhibition of vinblastine biotransformation by cytochrome P450 3A specific probes with Ki values of 0.17, 22.5, 14.8, and 35.3 microM for ketoconazole, erythromycin, troleandomycin, and vindesine, respectively; (b) the immunoinhibition of vinblastine metabolism by polyclonal anti-cytochrome P450 3A antibodies; (c) the highly significant correlation between the level of cytochrome P450 3A determined by Western blots and vinblastine metabolism (r = 0.759, P < 0.001); (d) the highly significant correlation between erythromycin N-demethylase activity (mediated by cytochrome P450 3A) and vinblastine metabolism (r = 0.83, P < 0.001); (e) the significant correlation between the CYP3A4 mRNA level and vinblastine metabolism (r = 0.60, P < 0.1). Although vincristine and navelbine (members of the Vinca alkaloid family) also inhibit the metabolism of vinblastine, suggesting the involvement of the cytochrome subfamily in their respective metabolisms, other anticancer drugs currently associated with vinblastine in chemotherapy (etoposide, Adriamycin, lomustine, and teniposide) also interfere with vinblastine biotransformation. These metabolic drug interactions may alter the antitumor activity and/or toxicity of the drug during anticancer chemotherapy.

Metabolism of cyclosporine after orthotopic liver transplantation

The aim of this work was to determine whether the extensive metabolism of cyclosporine, acquired in a donor by treatment with an inducer of cytochrome P450 3A (P450 3A) (cyclosporine oxidase), was transmissible to the recipient by orthotopic liver transplantation. For this purpose, male Wistar rats were divided into five groups including: control animals (group C), animals treated with dexamethasone (an inducer of P450 3A, 50 or 300 mg/kg/day, for 4 days, group D), animals transplanted with the livers of control rats (group G) or with the livers of dexamethasone-induced rats (group GD), and animals treated with beta-naphthoflavone (an inducer of P450 1A, group B). All animals received a single i.v. dose of 10 mg/kg cyclosporine 24 hr after either the last dose of inducer or the transplantation. For each group of animals, the area under the curve (AUC) of cyclosporine was calculated from the curves of blood cyclosporine levels (by radioimmunoassay) against time; liver microsomes were assayed for cyclosporine oxidase activity by HPLC, erythromycin demethylase and P450 3A level by western blot with specific anti-P450 3A antibodies. The decrease in the AUC in groups D and GD with respect to C and G was correlated with increased level of P450 3A (4-5-fold with respect to control) as well as of microsomal cyclosporine oxidase. In addition, cyclosporine oxidase activity of liver microsomes was specifically inhibited by anti-P450 3A antibodies and troleandomycin. The animals in group B did not exhibit increased metabolism of cyclosporine either in vivo or in vitro. We conclude that: (1) cyclosporine is predominantly oxidized in the rat liver by a form of P450 from the 3A subfamily; (2) the extensive metabolism of cyclosporine acquired by donor rats after treatment with dexamethasone is transmissible to the recipients through orthotopic liver transplantation.

Detection and characterization of a novel hepatic 8 S binding protein for benzo[a]pyrene distinct from the Ah receptor

Using fractionation procedures such as sucrose gradient sedimentation and gel permeation chromatography, a novel cytosolic binding protein for benzo[a]pyrene has been detected in liver of nonmammalian and mammalian species including human. This protein, called 8 S protein, cosediments with the Ah receptor after centrifugation in sucrose density gradient but can be separated from the Ah receptor and 4 S protein by gel permeation chromatography. Owing to its binding characteristics, the 8 S protein is clearly distinct from the Ah receptor. The [3H]BP binding parameters have been determined by saturation experiments. According to Scatchard and Woolf plots the Kd are 268 nM and 138 nM for DBA/2 mouse and guinea pig 8 S proteins, respectively. Bmax are 248 and 840 pmol/mg for DBA/2 mouse and guinea pig 8 S proteins, respectively. Apparent molecular mass of 8 S protein, according to Stokes radius (5 +/- 0.2 nm) and sedimentation coefficient, was estimated approximately 170,000 +/- 6000. After in vitro incubation of 8 S proteins with [3H]BP the charcoal, as well as the 4 S protein, exerts potent stripping effects on the [3H]BP binding. In contrast, after administration of [3H]BP to DBA/2 mice the 8 S protein-[3H]BP complex formed in vivo is more resistant to the stripping effects of charcoal and 4 S protein. Competition studies demonstrate that polycyclic aromatic hydrocarbons (PAHs) (BP > 1-aminopyrene > pyrene > 7,12-dimethylbenz[a]anthracene > 3-methylcholanthrene > benzo[e]pyrene > benzo[g, h, i,]perylene) are the best ligands of the 8 S protein. In contrast, the 2,3,7,8-tetrachlorodibenzo-p-dioxin is a poor ligand of this protein. Phenobarbital, steroid hormones, and omeprazole don't bind the 8 S protein. The at present unknown function of the 8 S protein and its role in the toxicology of PAHs are currently under investigation.

Major pathway of imipramine metabolism is catalyzed by cytochromes P-450 1A2 and P-450 3A4 in human liver

The metabolism of imipramine by human liver microsomes was examined using a combination of five strategies. Human hepatic microsomes produced N-desmethylimipramine (84%), 2-hydroxyimipramine (10%), and 10-hydroxyimipramine (6%). Preincubation of human hepatocytes in culture with beta-naphthoflavone and macrolides exclusively induced the formation of desmethylimpramine (552%, p < 0.05, and 234%, p < 0.003, respectively). Correlations were obtained between rates of imipramine demethylation and cytochrome P-450 (P-450) 1A2 (r = 0.88, p < 0.001) and P-450 3A (r = 0.80, p < 0.02) concentrations in human liver microsomal preparations from 13 different subjects. Anti-P-450 1A2 and anti-P-450 3A antibodies selectively inhibited N-demethylation (80% and 60%, respectively). N-Demethylation was completely inhibited when anti-1A2 and anti-3A were added simultaneously. Kinetic studies with human microsomes confirm the contribution of two different enzymes in the N-demethylation. The Km of 1A2 was similar to the high affinity Km in human liver microsomes, whereas the Km of 3A was similar to the low affinity Km in human liver microsomes. P-450 1A2 was apparently more efficient than 3A4 (lower Km and higher Vmax) but was expressed in much lower concentration. Human P-450s 1A2 and 3A4 expressed in yeast efficiently produced desmethylimipramine. These results suggest that P-450 1A2 and P-450 3A4 are the major enzymes involved in imipramine N-demethylation in human hepatic microsomes. Similar experiments were conducted using P-450 2D6, and they confirmed that P-450 2D6 catalyzes imipramine 2-hydroxylation. Interindividual variations in 3A4 hepatic content may explain the large variations in imipramine blood levels observed after uniform dosages and thus may explain the variations in clinical efficacy. Caution might be advised in the clinical use of tricyclic antidepressants when drugs are also administered that induce or inhibit P-450s 3A4 and 1A2.

Human liver microsomal cytochrome P450 3A isozymes mediated vindesine biotransformation. Metabolic drug interactions

Vindesine biotransformation was investigated using a bank of human liver microsomes. The drug was converted into one major metabolite (M) upon incubation with the microsomes. Large interindividual variations were observed: vindesine biotransformation rates ranged from 1.2 to 12.9 pmol/min/mg protein. Vindesine metabolic processes followed Michaelis-Menten kinetics: Km = 24.7 +/- 9.4 microM, Vmax = 1.5 +/- 0.8 nmol/min/mg protein. The involvement of human cytochrome P450 3A isozymes in vindesine metabolism was demonstrated by: (1) competitive inhibition of vindesine biotransformation by compounds known to be specifically metabolized by human cytochrome P450 3A. Apparent Ki values were 3.6, 17.9 and 19.8 microM for quinidine, troleandomycin and erythromycin, respectively; (2) immunoinhibition of vindesine metabolism by polyclonal anti-P450 3A antibody; (3) significant correlation between immunoquantified P450 3A and vindesine biotransformation (r = 0.800, P < 0.001); and (4) significant correlation between erythromycin N-demethylase activity, which was supported by P450 3A in humans, and vindesine biotransformation (r = 0.853, P < 0.001). Other vinca alkaloids also exerted an inhibitory effect on vindesine biotransformation with apparent Ki values of 3.8, 10.6 and 19.2 microM for vinblastine, vincristine and navelbine, respectively, suggesting a possible involvement of the same cytochrome subfamily in their hepatic metabolism. Moreover, a number of anticancer drugs currently associated with the vinca alkaloids, such as teniposide, etoposide, doxorubicin, lomustine, folinic acid and mitoxantrone, significantly inhibited vindesine biotransformation.