Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0006142 (breast cancer)
 160,383 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Exposure of MCF-7 breast cancer cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes an elevated cytochrome P450 content and a marked increase in the microsomal hydroxylation of 17 beta-estradiol (E2) at the C-2, C-4, C-15 alpha, and C-6 alpha positions. In this study we investigated the involvement of cytochromes P450 of the 1A gene subfamily in this metabolism of E2. Hydroxylation at each of these four positions of E2 was inhibited by P450 1A-subfamily inhibitors, alpha-naphthoflavone, benzo[a]pyrene, and 7-ethoxyresorufin. Northern blots showed that treatment of MCF-7 cells with TCDD resulted in production of the 2.6-kb CYP1A1 mRNA, but not the 3.0-kb CYP1A2 mRNA. Immunoblot analyses with anti-P450 1A antibodies confirmed the production of P450 1A1 protein in TCDD-treated MCF-7 cells. Anti-rat P450 1A IgG inhibited the hydroxylation of E2 at C-2, C-15 alpha, and C-6 alpha, but not hydroxylation at C-4. E2 hydroxylation by human cytochromes P450 1A1 and P450 1A2 was assessed in experiments with microsomes from Saccharomyces cerevisiae after transformation with cDNAs encoding the two cytochromes. The major hydroxylase activities of expressed human P450 1A1 were at the C-2, C-15 alpha, and C-6 alpha positions of E2; expressed human P450 1A2 catalyzed hydroxylation predominately at C-2. While both expressed P450s 1A1 and 1A2 had minor hydroxylase activities at the C-4 position, neither catalyzed a low-Km hydroxylation at C-4 similar to that observed with microsomes from TCDD-treated MCF-7 cells. These results provide strong evidence that P450 1A1 catalyzes the hydroxylations of E2 at the C-2, C-15 alpha, and C-6 alpha in incubations with microsomes from TCDD-treated MCF-7 cells, but suggest TCDD may also induce a cytochrome P450 E2 4-hydroxylase that is distinct from P450 1A1 or P450 1A2.
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PMID:17 beta-estradiol hydroxylation catalyzed by human cytochrome P450 1A1: a comparison of the activities induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in MCF-7 cells with those from heterologous expression of the cDNA. 153 70

Tamoxifen (TXF), a triphenylethylene antiestrogen, is the major therapeutic agent for breast cancer. In rare cases, TXF treatment appears to increase incidence of endometrial cancer. Also in rats, TXF was found to induce hepatocellular carcinoma. Previous studies suggested that metabolism of TXF may contribute to its antiestrogenic and anticancer activity. The current study demonstrates a novel route of TXF metabolism. TXF is metabolized by rat and human liver microsomes into a reactive intermediate (txf*) which binds irreversibly to microsomal proteins. The binding requires NADPH and O2 and is inhibited by CO, inhibitors of P-450, and antibodies to rat NADPH-P450 reductase, indicating catalysis by P450. Phenobarbital treatment of rats markedly increases binding, suggesting the involvement of induced P450s. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins from incubation of [14C] TXF with phenobarbital-treated microsomes exhibits a major radiolabeled zone which corresponds to a molecular weight of approximately 54,000, suggesting binding to a P-450. Cysteine and glutathione inhibited the binding of TXF without significantly affecting P-450-mediated metabolism of TXF, possibly by reacting with txf* or by competing for the same binding sites. Exposure of phenobarbital-treated microsomes and control-microsomes to 50 degrees C for 90 s, which inactivates the flavin-containing monooxygenase (FMO), diminished binding and pH 8.6 enhanced binding. Also, alternate FMO substrates inhibited binding. These findings indicate that P-450 and possibly FMO catalyze the reactions leading to the formation of txf*. However, incubations with single-labeled and dual-radiolabeled tamoxifen or with [14C]TXF-N-oxide demonstrated that monodesmethyl-TXF and TXF-N-oxide, the principal P-450 and FMO-mediated metabolites, respectively, are not on the major route of txf* formation, indicating that txf* could not be an aldehyde derived from tamoxifen nitrone. Thus, though the structure of txf* was not characterized, certain possibilities were excluded. Speculations on the structure of txf* and on its possible pharmacological and toxicological activity are presented.
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PMID:Cytochrome P-450-mediated activation and irreversible binding of the antiestrogen tamoxifen to proteins in rat and human liver: possible involvement of flavin-containing monooxygenases in tamoxifen activation. 193 68

The synthesis of 3-(cyclohexymethyl)-1-(4-aminophenyl)-3-azabicyclo[3.1.0]hexane-2, 4-dione (1h), with its optical enantiomers, and a series of novel achiral 1-(4-aminophenyl)-3-azabicyclo[3.1.1]haptane-2,4-diones (2a-i,k) is described. These compounds were tested in vitro for inhibition of human placental aromatase, a cytochrome-P450-dependent enzyme responsible for the conversion of androgens to estrogens. All of them displayed enzyme-inhibiting activity, and 3-cyclohexyl derivative 2g and 3-cyclohexylmethyl derivative 1h both proved more potent (greater than 140-fold) than the clinically effective agent aminoglutethimide [3-(4-aminophenyl)-3-ethylpiperidine-2,6-dione, AG]. As with AG and its derivatives, the 1R-(+)-enantiomer of 1h was responsible for the enzyme inhibitory activity. These novel compounds are of interest as potential drugs for endocrine therapy of hormone-dependent tumors, e.g. breast cancer.
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PMID:Synthesis and aromatase inhibitory activity of novel 1-(4-aminophenyl)-3-azabicyclo[3.1.0]hexane- and -[3.1.1]heptane-2,4- diones. 201 6

The antiestrogen tamoxifen (Tam or Nolvadex, ICI)-Z-1-[4-[2-(dimethylamino) ethoxy]phenyl]-1,2-diphenyl-1-butene is widely used in treatment of hormone-dependent breast cancer. The drug is extensively metabolized by cytochrome P450 dependent hepatic mixed function oxidase in man, yielding mainly the N-desmethyl metabolite (DMT). This study has been carried out to determine the P450 enzyme involved in the N-oxidative demethylation of Tam in microsomal samples from 25 human livers (23 adults, two children). This metabolic step was inhibited by carbon monoxide up to 75%. Tam was demethylated into DMT with an apparent Km of 98 +/- 10 microM; rates varied between 37 and 446 pmol/min/mg microsomal protein. These metabolic rates were strongly correlated with 6 beta-hydroxylation of testosterone (r = 0.83) and erythromycin N-demethylase (r = 0.75), both activities known to be associated with P450 IIIA enzyme. To further assess whether or not the Tam demethylation pathway is catalysed by the same P450, the inhibitory effect of TST on this reaction was determined. The competitive inhibition had an apparent Ki of 100 +/- 10 microM. Drugs such as erythromycin, cyclosporin, nifedipine and diltiazem were shown to inhibit in vitro the metabolism of tamoxifen. Furthermore the P450 IIIA content of liver microsomal samples, measured by Western blot technique using a monoclonal P450NF (nifedipine) antibody, was strongly correlated with DMT formation (r = 0.87). Tam N-demethylase activity was inhibited by more than 65% with polyclonal anti-human anti-P450NF. All these in vitro observations establish that a P450 enzyme of the IIIA sub-family is involved in the oxidative demethylation of tamoxifen in human liver.
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PMID:Identification of the cytochrome P450 IIIA family as the enzymes involved in the N-demethylation of tamoxifen in human liver microsomes. 203 44

The emergence of drug resistance is a major obstacle to effective cancer chemotherapy. The identification of novel agents that serve as selective, potent and nontoxic modulators of drug resistance is thus an important goal for improving the success of cancer treatment. Thaliblastine (TBL), a plant alkaloid and P-glycoprotein (P-gp) inhibitor, is presently shown to fully reverse 490-fold resistance to Adriamycin (AdR) in a multidrug-resistant (MDR) human breast cancer cell line (MCF/AdR) that overexpresses P-gp, whereas the same treatment had no effect on AdR cytotoxicity in the drug-sensitive parental MCF-7 cells. Mechanistic studies showed that this striking resistance reversal was achieved without alteration of cellular levels of glutathione and without inhibition of glutathione S-transferase, glutathione peroxidase or P450 reductase by TBL, each of which is significantly altered in MCF/AdR cells, and each of which has been proposed to contribute to AdR resistance in this MDR line. Rather, resistance reversal by TBL can be entirely explained by this drug's capacity to restore the intracellular accumulation of AdR in the resistant cells. These results establish that MDR associated with P-gp overexpression can be fully reversed by the potent P-gp inhibitor TBL. They further indicate that although changes in multiple drug-metabolizing enzymes may accompany the development of MDR, these multiple biochemical alterations need not correspond to multiple functional determinants for drug resistance.
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PMID:Complete reversal by thaliblastine of 490-fold adriamycin resistance in multidrug-resistant (MDR) human breast cancer cells. Evidence that multiple biochemical changes in MDR cells need not correspond to multiple functional determinants for drug resistance. 756 98

In a previous study (Vanden Bossche et al., Breast Cancer Res. Treat. 30 (1994) 43) the interaction between (+)-S-vorozole and the I-helix of cytochrome P450 19 (P450 aromatase) has been reported. In the present study we extended the "I-helix model" by incorporating the C-terminus of P450 aromatase. The crystal structures of P450 101 (P450 cam), 102 (P450 BM-3) and 108 (P450 terp) reveal that the C-terminus is structurally conserved and forms part of their respective substrate binding pocket. Furthermore, the present study is extended to the interaction between P450 aromatase and its natural substrate androstenedione and the non-steroidal inhibitors (-)-R-vorozole, (-)-S-fadrozole, R-liarozole and (-)-R-aminoglutethimide. It is found that (+)-S-vorozole, (-)-S-fadrozole and R-liarozole bind in a comparable way to P450 aromatase and interact with both the I-helix (Glu302 and Asp309) and C-terminus (Ser478 and His480). The weak activity of (-)-R-aminoglutethimide might be attributed to a lack of interaction with the C-terminus.
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PMID:A molecular model for the interaction between vorozole and other non-steroidal inhibitors and human cytochrome P450 19 (P450 aromatase). 762 53

Vorozole, the (+)-(S)-isomer of a new triazole compound, is a potent and selective aromatase inhibitor. In vitro, the compound is over a thousandfold more active than aminoglutethimide. In vivo, the compound very potently inhibits ovarian, peripheral, and tumoral aromatase. Vorozole shows an in vitro selectivity margin of 10,000-fold for aromatase inhibition as compared to inhibition of other P450- and non-P450-dependent reactions. This selectivity was confirmed in the rat in vivo. Vorozole, like ovariectomy, almost completely reduces tumor growth in the DMBA-induced mammary carcinoma model in the rat. In postmenopausal women, vorozole very potently inhibits peripheral conversion of androstenedione to estrone. After chronic administration, plasma estradiol levels are reduced while the levels of adrenal gluco- and mineralo-corticoids remain unchanged. Vorozole has excellent oral bioavailability and exerts linear, dose-proportional pharmacokinetics.
Breast Cancer Res Treat 1994
PMID:Vorozole, a specific non-steroidal aromatase inhibitor. 772 94

The conversion of androgens to estrogens occurs in a variety of cells and tissues, such as ovarian granulosa and testicular cells, placenta, adipose tissue, and various sites of the brain. The extragonadal synthesis of estrogens has great pathophysiological importance. Estrogens produced by, for example, adipose tissue have a role in the pathogenesis of certain forms of breast cancer and endometrial adenocarcinoma. The biosynthesis of estrogens is catalyzed by the aromatase, an enzyme localized in the endoplasmic reticulum that consists of two components: a cytochrome P450 (P450 Arom, P450 19 product of the CYP19 gene) and the NADPH cytochrome P450 reductase. The alignment of the amino acid sequences of human P450 19 with other mammalian P450s shows little sequence similarity, which indicates not only that P450 19 is a unique form of the P450 superfamily but also that the aromatase may be a good target for the development of selective P450 inhibitors. Aminoglutethimide (AG) is the pioneer drug of the reversible competitive nonsteroidal aromatase inhibitors. Since AG is a nonspecific aromatase inhibitor and presents some problems with tolerability, a number of structural analogues have been synthesized. For example, rogletimide is slightly less potent than AG but has the advantage of not inhibiting the cholesterol side-chain cleavage and is devoid of sedative action. Elongation of the ethyl substituent of AG and rogletimide leads to an increase in aromatase inhibition. Further studies led to the discovery of a new generation of much more potent aromatase inhibitors. An example is fadrozole. However, although fadrozole is a poor inhibitor of the cholesterol side-chain cleavage, it suppresses aldosterone release by ACTH-stimulated human adrenocortical cells. More selective aromatase inhibitors are the triazole derivatives. Examples are CGS 20267, CGS 47645, R 76 713, and ICI D1033. R 76 713's aromatase inhibitory effect is largely due to its (+)-S-enantiomer, vorozole. Computer modeling studies of the interaction of vorozole with part of the "I-helix" of P450 19 suggest that the chlorine-substituted phenyl ring of vorozole interacts with the gamma-carbonyl group of Glu-302. Thr-310, which corresponds to the highly conserved Thr-252 in P450 101, interacts with vorozole's triazole ring, and the 1-methyl-benzotriazole moiety binds near Asp-309.
Breast Cancer Res Treat 1994
PMID:Aromatase inhibitors--mechanisms for non-steroidal inhibitors. 794 4

Tamoxifen is the major therapeutic agent for the treatment of hormone-dependent breast cancer. Tamoxifen treatment appears to be associated with an increased incidence of endometrial carcinoma in humans and hepatocellular carcinoma in rats. These carcinogenic effects of tamoxifen might be induced by the formation of a tamoxifen reactive intermediate that binds covalently to macromolecules. Liver microsomal cytochrome P450s (CYPs) catalyze the metabolism of tamoxifen, forming a reactive intermediate that binds irreversibly to microsomal proteins, primarily to a 54 kDa protein (Mani, C. and Kupfer, D., Cancer Res., 51, 6052-6058, 1991). The current study identifies the P450 enzymes that catalyze the activation of tamoxifen to a reactive intermediate in rats and humans. Among the species examined, rats, chickens and humans demonstrate low tamoxifen binding activity, ranging from 0.1 to 0.4 nmol bound/mg protein/h. In contrast, hamsters and mice exhibit high binding, 1.2 and 1.6 nmol/mg protein/h respectively. Treatment of male rats with phenobarbital or pregnenolone-16 alpha-carbonitrile (PCN) markedly elevated the binding of tamoxifen to liver microsomal proteins. Methylcholanthrene treatment had no effect on binding. These findings suggested the involvement of CYP3A in catalysis of the covalent binding. Alternate substrates of CYP3A, cortisol and erythromycin, inhibited tamoxifen binding in liver microsomes from PCN- and phenobarbital-treated rats. Treatment of rats with troleandomycin (TAO), an inducer of CYP3A, followed by the dissociation of the TAO-CYP3A complex, elevated the covalent binding to liver microsomes approximately 3-fold. Antibodies against rat CYP3A1 strongly inhibited tamoxifen binding to liver microsomes from PCN- and phenobarbital-treated rats, whereas the antibodies anti-CYP2B1/2B2 did not inhibit binding. In humans, tamoxifen binding was inhibited by the anti-rat CYP3A1 IgG and also by alternate substrates of CYP3A. These results indicate that the activation of tamoxifen to a reactive intermediate by rat and human liver microsomes is principally catalyzed by CYP3A enzymes.
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PMID:Involvement of cytochrome P4503A in catalysis of tamoxifen activation and covalent binding to rat and human liver microsomes. 800 Dec 26

Tamoxifen and its metabolite 4-hydroxytamoxifen can both exist as geometrical isomers. Trans-tamoxifen is an oestrogen receptor antagonist and is used for the treatment of breast cancer. Trans-4-hydroxytamoxifen is 100 times more anti-oestrogenic than trans-tamoxifen. The cis isomers of tamoxifen and 4-hydroxytamoxifen are oestrogenic and weakly anti-oestrogenic or oestrogenic respectively. Both isomers of 4-hydroxytamoxifen have been detected in breast tumours of patients treated with trans-tamoxifen and it has been proposed that enzymatic isomerization of 4-hydroxytamoxifen occurs in vivo, resulting in resistance to tamoxifen therapy. We have investigated the isomerization of 4-hydroxytamoxifen by human liver microsomes and whether it is mediated by cytochromes P450. Microsomes from five of the 12 livers examined catalysed the interconversion of trans- and cis-4-hydroxytamoxifen (0.52 microM) when incubated for 40 min with an NADPH-generating system. Between 51 and 64% conversion of trans-4-hydroxytamoxifen was observed. Cis-4-hydroxytamoxifen was also converted to trans-4-hydroxytamoxifen (range 22-27%). Incubations with control, heat-treated microsomes resulted in approximately 1% isomerization of trans-4-hydroxytamoxifen. The extent of isomerization of trans- to cis-4-hydroxytamoxifen observed in microsomes from the other seven livers (range 2-8%) did not greatly exceed that seen in heat-inactivated microsomes. Enzymatic isomerization required NADPH and was inhibited by SKF 525A and ketoconazole, indicating the involvement of cytochromes P450. Enzymatic isomerization of trans-tamoxifen and trans-droloxifene (the 3-hydroxy synthetic analogue of tamoxifen) was not observed. These findings may have implications for the safe and effective use of tamoxifen.
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PMID:Interindividual variation in the isomerization of 4-hydroxytamoxifen by human liver microsomes: involvement of cytochromes P450. 800 Dec 29


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