Aromatase and its inhibitors--an overview

Quantum mechanical/molecular mechanical and docking study of the novel analogues based on hybridization of common pharmacophores as potential anti-breast cancer agents

In an attempt to identify some new potential leads as anti-breast cancer agents, novel hybrid compounds were designed by molecular hybridization approach. These derivatives were structurally derived from hybrid benzofuran-imidazole and quinazolinone derivatives, which had shown good cytotoxicity against the breast cancer cell line (MCF-7). Since aromatase enzyme (CYP19) is highly expressed in the MCF-7 cell line, the binding of these novel hybrid compounds to aromatase was investigated using the docking method. In this study, due to the positive charge on the imidazole ring of the designed ligands and also, the presence of heme iron in the active site of the enzyme, it was decided to optimize the ligand inside the protein to obtain more realistic atomic charges for it. Quantum mechanical/molecular mechanical (QM/MM) method was used to obtain more accurate atomic charges of ligand for docking calculations by considering the polarization effects of CYP19 on ligands. It was observed that the refitted charge improved the binding energy of the docked compounds. Also, the results showed that these novel hybrid compounds were adopted properly within the aromatase binding site, thereby suggesting that they could be potential inhibitors of aromatase. The main binding modes in these complexes were through hydrophobic and H bond interactions showing agreement with the basic physicochemical features of known anti aromatase compounds. Finally, the complex structures obtained from the docking study were used for single point QM/MM calculations to obtain more accurate electronic interaction energy, considering the electronic polarization of the ligand by its protein environment.

The Pattern of Glucocorticoid and Estrogen Receptors May Explain Differences in Steroid Dependency of Intrauterine Prostaglandin Production at Parturition in Sheep

BACKGROUND

We have recently described two distinct pathways of intrauterine prostaglandin (PG) synthesis: a cortisol-dependent/estradiol-independent mechanism within trophoblast tissue leading to elevations in fetal plasma PGE2, and an estradiol-dependent mechanism within maternal endometrium that leads to increased maternal plasma PGF2(2alpha). We hypothesized that the differential effects of cortisol and estradiol on intrauterine PGH synthase-II (PGHS-II) expression and PG production may be because of the tissue specific expression of the glucocorticoid and estradiol receptors (GR and ER, respectively) within the intrauterine tissues. In addition, we suggest that these two pathways of PG production are linked through the expression of P450(C17hydroxylase) (P450(C17)) and subsequent increase in placental estradiol synthesis.

METHODS

To test the hypotheses, we infused singleton, chronically catheterized fetal sheep beginning at day 125 of gestation (term 147 to 150 days) with (1) cortisol (0.45 mg/mL; n = 5); (2) cortisol and 4-hydroxyandrostenedione, a P450(aromatase) inhibitor (4-OHA: 1.44 mg/h; n = 5); (3) saline (n = 5); or (4) saline and 4-OHA (n = 5). PGHS-II, ER alpha, ER beta, and GR alpha were localized using immunohistochemistry. ER alpha, ER beta, P450(C17), and GR alpha protein expressions were determined by Western blot analysis. Data were analyzed by analysis of variance (ANOVA) (P < or =.05).

RESULTS

Fetal cortisol infusion in the presence or absence of a rise in placental estrogen synthesis increased placental expression of GR alpha; both PGHS-II and GR alpha localized to the uninucleate trophoblast cells of the placentome and were excluded from the maternal stroma and binucleate cells. Both forms of ER were excluded from the trophoblast tissue of the placentome. ER alpha, ER beta, and PGHS-II showed a similar pattern of distribution within the luminal epithelium of the endometrium; there were no alterations in the level of the ER in the presence of cortisol +/- 4-OHA. Placental P450(C17) protein expression was increased in the presence of a rise in fetal cortisol independent of changes in placental estrogen synthesis.

CONCLUSIONS

We concluded that the differential effects of cortisol and estradiol on intrauterine PGHS-II expression and PG production may be due to the tissue-specific expression of the GR and ER within the intrauterine tissues. Glucocorticoid effects on trophoblast PG production may be mediated in a positive feed-forward manner. We further suggest that either cortisol or a cortisol-stimulated intermediate, like PGE2, increased P450(C17) expression, leading to a rise in placental estradiol synthesis and triggering maternal intrauterine tissue PG production.

Models and Mechanisms of Acquired Antihormone Resistance in Breast Cancer: Significant Clinical Progress Despite Limitations

Translational research for the treatment and prevention of breast cancer depends upon the four Ms: models, molecules, and mechanisms in order to create medicines. The process, to target the estrogen receptor (ER) in estrogen-dependent breast cancer, has yielded significant advances in patient survivorship and the first approved medicines (tamoxifen and raloxifene) to reduce the incidence of any cancer in high- or low-risk women. This review focuses on the critical role of the few ER-positive cell lines (MCF-7, T47D, BT474, ZR-75) that continue to advance our understanding of the estrogen-regulated biology of breast cancer. More importantly, the model cell lines have provided an opportunity to document the development and evolution of acquired antihormone resistance. The description of this evolutionary process that occurs in micrometastatic disease during up to a decade of adjuvant therapy would not be possible in the patient. The use of the MCF-7 breast cancer cell line in particular has been instrumental in discovering a vulnerability of ER-positive breast cancer exhaustively treated with antihormone therapy. Physiologic estradiol acts as an apoptotic trigger to cause tumor regression. These unanticipated findings in the laboratory have translated to clinical advances in our knowledge of the paradoxical role of estrogen in the life and death of breast cancer.

Expression of the two cytochrome P450 aromatase genes in the male and female blue gourami (Trichogaster trichopterus) during the reproductive cycle

In this study, the involvement of the cytochrome P450 aromatase gene (CYP19) in the gametogenesis of the teleost blue gourami (Trichogaster trichopterus) is described. The blue gourami brain CYP19 (bgCYP19b) and gonadal CYP19 (bgCYP19a) aromatase genes were cloned and their expression analyzed during the different reproductive stages. The cloned cDNAs of the bgCYP19b and bgCYP19a were found to contain segments of 1518 bp (an open reading frame encoding a deduced protein of 506 residues) and 489 bp (encoding a peptide of 163 residues), respectively. Although the mRNA levels of bgCYP19b were very low in females until the vitellogenic phase, they were significantly higher in the final oocyte maturation stage. The aromatase gene mRNA levels in the gonads were significantly lower in females in the high vitellogenic stage, as compared to females during early vitellogenesis or maturation. In males, the mRNA levels of bgCYP19b were significantly lower in juveniles than in mature individuals. However, no significant differences were observed between mature non-reproductive and reproductive males. In addition, there was no significant difference between the expression of bgCYP19a in juvenile and non-nest building mature males, although a significant increase was detected in mature reproductive males. Although CYP19b expression was similar in both sexes, the expression of CYP19a was significantly different between males and females.

Effects of adlay (Coix lachryma-jobi L. var. ma-yuen Stapf.) hull extracts on the secretion of progesterone and estradiol in vivo and in vitro

Adlay (Coix lachryma-jobi L. var. ma-yuen Stapf.) has been used as a traditional Chinese medicine for dysfunction of the endocrine system. However, there have been few studies on the effects of adlay seed on the endocrine system. In the present study, both the in vivo and in vitro effects of methanolic extracts of adlay hull (AHM) on progesterone synthesis were studied. AHM was partitioned with four different solvents: water, 1-butanol, ethyl acetate, and n-hexane. Four fractions, namely, AHM-Wa (water fraction), AHM-Bu (1-butanol fraction), AHM-EA (ethyl acetate fraction), and AHM-Hex (n-hexane fraction), were respectively obtained. Granulosa cells (GCs) were prepared from pregnant mare serum gonadotropin-primed immature female rats and were challenged with different reagents, including human chorionic gonadotropin (hCG; 0.5 IU/ml), 8-bromo-adenosine-3',5'-cyclic monophosphate (8-Br-cAMP; 0.1 mM), forskolin (10 microM), 25-OH-cholesterol (10 microM), and pregnenolone (10 microM), in the presence or absence of AHM (100 microg/ml). The functions of steroidogenic enzymes, including protein expression of the steroidogenic acute regulatory protein (StAR), cytochrome P450 side chain cleavage enzyme (P450scc), protein kinase A (PKA), and aromatase activity, were investigated. The expression of StAR mRNA was also explored by using real-time reverse transcription-polymerase chain reaction. In the in vivo study, AHM decreased plasma progesterone and estradiol levels after an intravenous injection of AHM (2 mg/ ml/kg). In the in vitro studies, AHM decreased progesterone and estradiol via inhibition of (i) the cAMP-PKA signal transduction pathway, (ii) cAMP accumulation, (iii) P450scc and 3beta-HSD enzyme activities, (iv) PKA, P450scc and StAR protein expressions and StAR mRNA expression, and (v) aromatase activity in rat GCs. These results suggest that AHM decreased the production of progesterone via mechanisms involving the inhibition of the cAMP pathway, enzyme activities, and the protein expressions of P450scc and StAR in rat GCs.

Assessing the mammary gland of nonhuman primates: effects of endogenous hormones and exogenous hormonal agents and growth factors

This review provides a summary of the normal biology, development, and morphology of the breast in nonhuman primates (macaques), and of the major published work addressing hormonally-induced changes in the breast of these animals. The mammary glands of macaques are anatomically, developmentally, and physiologically similar to the human breast, with similar expression of sex steroid receptors (estrogen receptors alpha and beta, progesterone receptor A and B, androgen receptors), estrogen dependent markers, and steroid metabolizing enzymes. Genetic similarity between human beings and macaques is high, varying from 95-99% depending on the sequence evaluated. Macaques develop hyperplastic and cancerous lesions of the breast spontaneously, which are similar in type and prevalence to those of human beings. They have a reproductive physiology typical of anthropoid primates, including a distinct menarche and menopause, and a 28-day menstrual cycle. These similarities give unique value to the macaque model for evaluation of the effectiveness and safety of hormonal agents. Such agents considered in this review include estrogens and progestogens, combined therapies such as oral contraceptives and post-menopausal hormone therapies, androgens, selective estrogen receptor modulators, phytoestrogens, prolactin, somatotropin, epidermal growth factor, and other novel agents with hormonal or growth factor-like activity. This review also includes a consideration of selected background changes and typical strategies and markers used for evaluation of experimentally-induced changes, including biopsy-based strategies designed to control for inter-individual variability and minimize numbers of animals used.

Cloning and developmental expression of the cytochrome P450 aromatase gene (CYP19) in the European eel (Anguilla anguilla)

To characterize the involvement of the aromatase gene during the process of sex determination in the European eel (Anguilla anguilla), the expression of its gonadal form was determined during various developmental stages. The cloned cDNA from the European eel gonad (EeCYP19) contains an open reading frame of 1539 bp, encoding a deduced protein of 513 residues. The predicted amino acid sequence shows 97% identity with that of the Japanese eel, and 59-69% of identity with those of the CYP19 gonadal and brain forms of other teleost fish. Two potential initiation sites (ATG) were found downstream of the first ATG codon. A fluorescent-based method of real-time PCR was developed to quantify EeCYP19 expression. The expression levels of EeCYP19 in the gonads of adult males were approximately 12- and 30-fold lower than the levels in adult females and juvenile eels previously treated with E2, respectively. Expression of aromatase was found only in a single specimen in the control group. In contrast, no difference was found among sexes in the aromatase expression in the brain. Treatment with aromatase inhibitor (AI) of juvenile eel resulted in the total loss of aromatase expression in the gonads and brains. The results of this work revealed that AI treatment not only reduces the synthesis of estradiol, but reduces the expression levels of EeCYP19 as well. No evidence for the presence of a distinct extra-gonadal (brain) form of aromatase in the European eel could be provided.

Study of substrate specificity of human aromatase by site directed mutagenesis

Human aromatase is responsible for estrogen biosynthesis and is implicated, in particular, in reproduction and estrogen-dependent tumor proliferation. The molecular structure model is largely derived from the X-ray structure of bacterial cytochromes sharing only 15-20% identities with hP-450arom. In the present study, site directed mutagenesis experiments were performed to examine the role of K119, C124, I125, K130, E302, F320, D309, H475, D476, S470, I471 and I474 of aromatase in catalysis and for substrate binding. The catalytic properties of mutants, transfected in 293 cells, were evaluated using androstenedione, testosterone or nor-testosterone as substrates. In addition, inhibition profiles for these mutants with indane or indolizinone derivatives were obtained. Our results, together with computer modeling, show that catalytic properties of mutants vary in accordance with the substrate used, suggesting possible differences in substrates positioning within the active site. In this respect, importance of residues H475, D476 and K130 was discussed. These results allow us to hypothesize that E302 could be involved in the aromatization mechanism with nor-androgens, whereas D309 remains involved in androgen aromatization. This study highlights the flexibility of the substrate-enzyme complex conformation, and thus sheds new light on residues that may be responsible for substrate specificity between species or aromatase isoforms.

Effects of 4-hydroxyandrostenedione and hyperstimulation with pregnant mare serum gonadotrophin on early embryonic development in rats

A possible role of high oestradiol levels in mediating the adverse effects of hyperstimulation with pregnant mare serum gonadotrophin (PMSG) on early embryonic development in the rat was investigated using an aromatase inhibitor, 4-hydroxyandrostenedione (4-OHA), to inhibit endogenous oestradiol production. Three experiments were conducted in this study. In the first, varying doses of 4-OHA were administered either concurrently with human chorionic gonadotropin (hCG) to pro-oestrus female rats hyperstimulated at early di-oestrus stage with 20 IU PMSG or alone into nonhyperstimulated pro-oestrus females. At high doses of 1000, 2000, or 5000 µg/rat, 4-OHA substantially improved the survival of embryos in hyperstimulated females, while low doses of 100 and 500 µg/rat were ineffective. The protective effect of 4-OHA on embryo count was optimum at 2000 µg. When administered alone, only the highest dose of 5000 µg/rat 4-OHA increased embryo count. In the second experiment, higher doses of PMSG were studied (30 or 40 IU), with or without 5000 µg/rat 4-OHA given at the time of hCG injection. PMSG proved to be more detrimental with increasing dose, and 5000 µg/rat 4-OHA was able to rescue embryos from death in the 30, but not 40, PMSG group. In the third experiment, the influence of the timing of 4-OHA treatment on its ability to improve the embryo count in hyperstimulated females was examined by introducing 4-OHA 24 h earlier, rather than at the time of hCG treatment. The results showed the importance of timing of 4-OHA administration, as 5000 µg/rat 4-OHA was able to restore embryo survival in the 40 PMSG hyperstimulated group only when it was administered 24 h before hCG injection. Together, these results highlighted that 4-OHA, when administered at the appropriate time and dose, could reverse the negative effects of hyperstimulation from PMSG on early embryonic development. This may be due to its potent aromatase inhibiting properties that lead to the suppression of oestrogen production, thereby alleviating the supraphysiological level of oestradiol, which is typically present in PMSG-treated females. Interestingly, 4-OHA treatment on its own was able to positively influence embryo count when given at a high dose of 5000 µg/rat, and this may be associated with its weak androgenic properties. In conclusion, this study supports the hypothesis that excessive oestradiol is responsible for the negative effects of hyperstimulation with PMSG on early embryonic development.Key words: 4-hydroxyandrostenedione, embryonic development, PMSG, rat.

A comparison of ovariectomy models for estrogen studies

Many estrogen-replacement studies use ovariectomized animals as controls. However, ovariectomy greatly increases body weight and can enhance the peripheral synthesis of estrogen. Tamoxifen is commonly used as an antiestrogen, but it may elicit mixed agonist or antagonist actions. The aim of our study was to compare vascular function in mesenteric arteries among groups of rats with low estradiol levels. The groups (n = 5, each) of Sprague-Dawley rats were cycling (diestrus), ovariectomized (OVX), OVX + tamoxifen (OVX-T), OVX + 4-hydroxyandrostene-3,17-dione, an aromatase inhibitor (OVX-A) to prevent peripheral synthesis of estrogen, and control-fed OVX to prevent excess weight gain. Body weight was significantly elevated in only the non-control-fed OVX group. Estrogen levels were significantly greater in the cycling rats compared with the other groups, whereas uterine weights were significantly reduced in only the OVX-A and control-fed OVX groups. Methacholine relaxation was blunted only in the OVX-A and control-fed OVX groups, suggesting a possible estrogenic influence in the non-control-fed OVX and OVX-T groups. These data indicate the potential for confounding factors to decrease the efficacy of OVX controls.

Effect of the aromatase inhibitor, MEN 11066, on growth of two different MCF-7 sublines

The racemate compound MEN 11066 (1-[(benzofuran-2-yl)(4'-cyanophenyl)methyl]-1H-1,2,4-triazole) and its enantiomers, (+)-MEN 11623 and (-)-MEN 11622, showed potent and selective aromatase activity on human placental microsomes. In addition, to better evaluate their potency as anticancer drugs, the compounds were assayed on testosterone-induced cell proliferation to measure their ability in inhibiting oestrogen-dependent tumour growth. Two different sublines originated from the human breast carcinoma MCF-7 were used. One, named MCF-7(tumour aromatase) (TA), that had maintained its intrinsic aromatase activity, was more sensitive to estradiol or testosterone-induced growth than the second subline named MCF-7(human placental aromatase) (hPA). The latter had been transfected with the human placental aromatase cDNA, after recognizing that the parental cells had aromatase activity reduced to undetectable levels. The MEN compounds completely reverted the testosterone-induced proliferation in both MCF-7(TA) and MCF-7(hPA) cells, while they did not affect the estradiol-triggered proliferation as a proof of their specificity for aromatase enzyme. Interestingly, MCF-7(TA) cells were more susceptible to the effects of aromatase inhibitors than the MCF-7(hPA) cell. These data suggest the efficacy of aromatase inhibitors in breast cancer when the growth dependency from oestrogen is high and a relatively low aromatase activity may be extremely important for tumour development.

The biological activity of nonsteroidal vitamin D hormone analogs lacking both the C- and D-rings

1alpha,25-dihydroxyvitamin D is a key calcium-regulating hormone but also displays potent differentiating and antiproliferative activities on many cell types. The structural requirements of this secosteroid hormone have been extensively studied for the A-ring and side chain, whereas relatively little is known about the requirements of the natural CD-ring structure for the vitamin D-like biological activity. We have embarked on a vast program in which derivatives were synthesized and evaluated characterized by profound structural changes in the central C/D-region. This first series of nonsteroidal analogs consists of (1R,3S)-5-((Z,2E)-4-((1S,3S)-3-(4-hydroxy-4-methylpentyl)-1,2,2-++ +trimethylcyclopentyl)-2-butenylidene)-4-methylenecyclohexan e-1,3-diol (KS 176) and derivatives thereof. These analogs are characterized by the absence of normal C- and D-rings and by the presence of an unnatural five-membered ring which we call the E-ring. KS 176 with the otherwise natural side chain structure of 1alpha,25(OH)2D3 has between 10 and 30% of the biological activity of 1alpha,25(OH)2D3 when tested in vitro (prodifferentiating effects on HL-60 and MG-63; antiproliferating activity on MCF-7 and keratinocytes) but has minimal in vivo calcemic effects. Introduction of several side chain modifications created analogs with increased intrinsic noncalcemic biological properties, whereas their calcemic potency remains very low. These data demonstrate that the full CD-rings are not mandatory for the biological activity of 1alpha,25(OH)2D3 since they can be replaced by a new ring structure which generates an appropriate spacing of the A-seco B-rings in relation to the side chain. The biological activity of these nonsteroidal analogs probably involves a classical genomic activation since they are also active in transfection assays using an osteocalcin vitamin D responsive element coupled to a human growth hormone reporter gene.

Inhibition and inactivation of equine aromatase by steroidal and non-steroidal compounds. A comparison with human aromatase inhibition

In order to approach the detailed structure-function relationships of aromatase, we studied the inhibitory and inactivatory potencies of several steroidal androstenedione analogues (1: 4-hydroxyandrostenedione, 2: 4-acetoxyandrostenedione and 3: 7 alpha-(4'-amino)phenylthio-4-androstene-3, 17-dione) and non-steroidal imidazole derivatives (4: ketoconazole, 5: miconazole and 6: fadrozole) on equine aromatase in placental microsomes, a well established mammalian model. Human placental microsomes and the purified enzyme from equine testis were also used to compare inhibition by 1 and 2. In equine microsomes, all compounds tested exhibited a competitive inhibition, with Ki values of 4.1, 26 and 1.8 nM for 1, 2 and 3, and of 2400, 1.4 and 4 nM for 4, 5, and 6, respectively. The Km for androstenedione, the substrate mainly used in these studies, was 1.8 +/- 0.13 nM. The three non-steroidal derivatives did not inactivate equine aromatase, but 1 and 2 acted as comparable inactivators to a much higher degree than 3. Compound 1 inhibited in a similar manner (89-94%) purified or equine and human microsomal aromatases, whereas 2 inhibited microsomal aromatase more efficiently in the horse than in man (92% and 33% inhibition, respectively). There was only a 40% inhibition with 2 on the purified equine enzyme, which is no more in the natural membrane environment. The comparisons between equine and human microsomal aromatases allow precise functional and structural differences to be observed with these enzymes.

Pre- and post-translational regulation of aromatase by steroidal and non-steroidal aromatase inhibitors

Treatment of castrated quail with testosterone (T) reliably activates male copulatory behavior and, at the same time, increases the aromatase activity (AA), the number of aromatase-immunoreactive (ARO-ir) cells and the concentration of aromatase mRNA as measured by RT-PCR in the brain. All these effects can be mimicked by estrogens. The behavioral effects of T can be blocked by a variety of aromatase inhibitors and, in parallel, the AA is strongly inhibited in the preoptic area (POA). We showed recently that the steroidal inhibitor, 4-OH-androstenedione (OHA) markedly decreases the immunostaining density of brain ARO-ir cells while the non-steroidal inhibitor, R76713 (racemic Vorozole; VOR) unexpectedly increased the density of this staining, despite the fact that the enzyme activity was completely inhibited. To generalize these findings and try to identify the underlying mechanism, we compared here the effects of two steroidal (OHA and androstatrienedione [ATD]) and two non-steroidal (VOR and Fadrozole [FAD]) aromatase inhibitors on the aromatase immunostaining and aromatase mRNA concentration in the brain of castrated quail concurrently treated with T. The 4 inhibitors significantly blocked the activation by T of male copulation. The two steroidal inhibitors decreased the immunostaining of brain ARO-ir cells but both VOR and FAD markedly enhanced the density of this staining. In parallel, OHA and ATD completely blocked the T-induced increase in aromatase mRNA concentration, while VOR and FAD had no effect on these RNA concentrations in the POA-anterior hypothalamus and they decreased them only slightly in the posterior hypothalamus. Taken together these results suggest that the inhibition of AA by ATD or OHA and the subsequent removal of locally produced estrogens blocks the synthesis of aromatase presumably at the transcriptional level. By contrast, the two non-steroidal inhibitors tested here block AA but in parallel increase the aromatase immunostaining. This effect does not result from an enhanced transcription and it is therefore speculated that these compounds increase either the translation of the aromatase mRNA or the half-life of the protein itself.

Role of P-450 aromatase in sex determination of the diamondback terrapin, Malaclemys terrapin

Sex determination in the diamondback terrapin, Malaclemys terrapin, is temperature-dependent. Eggs incubated at 31 degrees C, and above, hatch in approximately 45 days as females. Eggs incubated below 27 degrees C hatch in about 60 days as males. Sex is not reversible after hatching. Nest temperatures in the wild can be as low as 20 degrees C and as high as 37 degrees C with as much as a 10 degrees C diel cycle. The shortest incubation time measured in nature was 56 days and the longest approaching 120 days. Nests in our study site produced predominantly (> 95%) male hatchlings. Treatment of developing embryos with estrogen produces females at male producing temperatures while treatment with fadrozole (a nonsteroidal aromatase inhibitor) induces partial male-like gonads. Treatment with a steroidal aromatase inhibitor (4-hydroxyandrostenedione, 4-OHA) had no effect on sex determination. Both fadrozole and 4-OHA are potent competitive inhibitors (Ki approximately 40-50 nM) for terrapin in vitro aromatase activity. These findings are consistent with aromatase expression being a key step in sex determination of terrapins. We have cloned a partial single copy P-450 aromatase from the terrapin using a cDNA library constructed from ovarian mRNA. This partial clone is highly homologous to other vertebrate aromatases.

Lignans and flavonoids inhibit aromatase enzyme in human preadipocytes

Lignans and flavonoids are naturally-occurring diphenolic compounds found in high concentrations in whole grains, legumes, fruits and vegetables. Seven lignans and six flavonoids were evaluated for their abilities to inhibit aromatase enzyme activity in a human preadipose cell culture system. The lignan, enterolactone (Enl) and its theoretical precursors, 3'-demethoxy-3O-demethylmatairesinol (DMDM) and didemethoxymatairesinol (DDMM) decreased aromatase enzyme activity, with Ki values of 14.4, 5.0 and 7.3 microM, respectively. The flavonoids, coumestrol, luteolin and kaempferol also decreased aromatase enzyme activity, with Ki values of 1.3, 4.8 and 27.2 microM, respectively. Aminoglutethimide, a pharmaceutical aromatase inhibitor, showed a Ki value of 0.5 microM. Kinetic studies showed the inhibition by all compounds to be competitive. Smaller decreases in aromatase activity were observed with the lignan, enterodiol (End) and its theoretical precursors, O-demethylsecoisolariciresinol (ODSI), demethoxysecoisolariciresinol (DMSI) and didemethylsecoisolariciresinol (DDSI). The flavonoids, O-demethylangolensin (O-Dma), fisetin and morin showed no inhibitory effects. The inhibition of human preadipocyte aromatase activity by lignans and flavonoids suggests a mechanism by which consumption of lignan- and flavonoid-rich plant foods may contribute to reduction of estrogen-dependent disease, such as breast cancer.

Adrenal effects of low-dose aminoglutethimide when used alone in postmenopausal women with advanced breast cancer

Aminoglutethimide (Ag) is a potent aromatase-enzyme inhibitor used in the treatment of patients with breast cancer. In the past, it has been administered in doses of 1,000 mg/d (usually with 40 mg hydrocortisone). At these dose levels, the drug also affects multiple cytochrome P-450 enzymes, including enzymes for adrenal steroid biosynthesis. Recently, lower-dose regimens (500 mg/d) of Ag have been found to be just as effective for breast cancer therapy, but less toxic than the higher conventional dose. There is limited information on the adrenal effects at the lower dosages, and it is not known whether these effects are clinically significant. We measured basal and synthetic corticotropin (Cortrosyn)-stimulated levels of adrenal steroids in postmenopausal breast cancer patients before and during treatment with low-dose Ag (500 mg/d) administered without a glucocorticoid preparation. Basal levels of progesterone, 17-OH progesterone, and 11-deoxycortisol were higher after 2 months' treatment (P < .01). After ACTH injection, peak levels of progesterone and 17-OH progesterone were higher (P < .01), but in contrast, peak levels of 18-OH corticosterone were lower during treatment (P < .02). Basal and peak levels of cortisol, aldosterone, and all other adrenal steroids were unchanged during treatment. We conclude that low-dose Ag treatment leads to partial inhibition of the 21-hydroxylase, 11-hydroxylase, and 18-hydroxylase adrenal enzymes. Since cortisol and aldosterone secretion remained normal with minimal shunting to or accumulation of adrenal androgen compounds, we believe that the mild inhibition was compensated for by further endogenous ACTH stimulation.

Breast cancer and expression of aromatase in breast adipose tissue

Extraglandular conversion of C19 steroids to estrogens takes place primarily in the stromal cell component of adipose tissue and is catalyzed by an enzyme complex comprising aromatase cytochrome P450 (P450arom; the product of the CYP19 gene) together with the flavoprotein NADPH-cytochrome P450 reductase. It has been proposed that increased local aromatase activity in breast adipose tissue may influence the growth o f breast carcinomas. Using competitive polymerase chain reaction following reverse transcription (RT-PCR), we quantified P450arom transcripts in breast adipose tissue from mastectomy specimens. In 10 of 15 patients, the highest transcript levels were found in the quadrant where the tumor was located. We also found the highest proportions of adipose stromal cells versus adipocytes in these quadrants. These findings suggest that regional differences in the relative proportions o f these histologic components give rise to local elevated concentrations of estrogens. Although the initiating events are not known, once a neoplastic change has occurred, tumor growth may be promoted by these locally increased estrogen levels. Our data regarding hormonally regulated use of tissue-specific promoters for CYP19 gene transcription suggest preferential utilization of certain upstream regulatory regions of the gene in breast adipose tissue, resulting in increased expression of aromatase.

Estradiol control of ornithine decarboxylase mRNA, enzyme activity, and polyamine levels in MCF-7 breast cancer cells: therapeutic implications

Previous studies have shown that natural polyamines-putrescine, spermidine, and spermine-play a key role in the mechanism of action of estrogens in breast cancer. Ornithine decarboxylase (ODC) is the first enzyme of the polyamine biosynthetic pathway. To examine estrogenic regulation of polyamine biosynthesis in breast cancer, we measured ODC mRNA, ODC activity, and polyamine levels in G1 synchronized MCF-7 cells. ODC mRNA and activity increased four-fold over that of cells in G1 phase between 8 to 16 h after the addition of estradiol. Polyamine levels showed a sharp increase by 8 h after the addition of estradiol and decreased by 12 h. We further examined whether synthetic homologs of putrescine or spermidine could replace natural polyamines in supporting MCF-7 cell growth. Treatment of MCF-7 cells with 1 mM difluoromethylornithine (DFMO), an inhibitor of ODC, suppressed putrescine, spermidine, and spermine levels by 74, 78, and 10%, respectively, within 48 h. Cells treated with DFMO for 48 h were supplemented with either putrescine or its homologs or spermidine or its homologs. Diaminopropane, diaminobutane (putrescine), and diaminopentane were capable of fully or partially reversing the growth inhibitory effects of DFMO, whereas diaminoethane had no significant effect. Among a series of triamines, H2N(CH2)nNH(CH2)3NH2 (where n = 2 to 8; abbreviated as APn n = 4 for spermidine, or AP4), spermidine was most effective in reversing the effects of DFMO, whereas compounds with shorter or longer methylene bridging regions were less effective. AP8 was ineffective in reversing the growth inhibitory effects of DFMO. At 10 microM concentration, AP8 also inhibited DNA synthesis by 66%, as measured by [3H]-thymidine incorporation. These data show that MCF-7 cells have a strong requirement for polyamines for their growth and that estradiol stimulates the polyamine cascade by inducing the ODC mRNA level. Our results also suggest that polyamine homologs such as AP8 might be potentially useful in breast cancer therapy.

Human carcinogenic risk assessment based on hormonal effects in a carcinogenicity study in rats with the antifungal agent, fluconazole

Fluconazole is an orally active bis-triazole antifungal agent that acts by selective inhibition of lanosterol 14 alpha-demethylase, a key enzyme for maintenance of the fungal cell wall. It is not genotoxic. In a 2 year carcinogenicity study in Sprague-Dawley rats, fluconazole decreased mammary fibroadenomas in females and adrenal pheochromocytomas in males, and increased hepatic adenomas in males. The pattern of these changes is explicable in terms of a hormonal imbalance, corroborated in other studies with fluconazole in rats by changes in the weights of hormone-sensitive organs and circulating levels of 17 beta-estradiol. The decreases in mammary tumors are probably a consequence of aromatase inhibition by fluconazole at high dose levels. The tumor effects observed in this study are extremely unlikely to be of relevance to humans, since the hormone effects observed in this study do not occur in humans treated with therapeutic dose levels of fluconazole. This study illustrates the importance of seeking a mechanistic interpretation of rodent tumor findings, which may then be assessed for its relevance to the clinical use of a drug.

Regulation of aromatase expression in human tissues

Extraglandular conversion of C19 steroids to estrogens takes place primarily in the stromal cell compartments of adipose tissue and is catalyzed by aromatase cytochrome P450 (P450arom, the product of the CYP19 gene). CYP19 gene expression and aromatase activity in breast adipose stromal cells in culture are subject to complex hormonal regulation, which was recently found to be mediated in part by alternative use of tissue-specific promoters of the CYP19 gene. It has been proposed that increased local aromatase activity in breast adipose tissue may influence the growth of breast carcinomas. Using competitive RT-PCR, we quantified P450arom transcripts in breast adipose tissue from mastectomy specimens. In 10 out of 15 patients, the highest transcript levels were found in the quadrant where the tumor was located. We also found the highest proportions of adipose stromal cells vs. adipocytes in these quadrants. These findings suggest that regional differences in the relative proportions of the histologic components give rise to local elevated concentrations of estrogens. Although the initiating events are not known, once a neoplastic change has occurred, tumor growth may be promoted by these locally increased estrogen levels. We are currently investigating alternative promoter use for CYP19 gene transcription to explain this association. Our results underscore the importance of aromatase inhibitors as effective agents in treatment of hormone-responsive breast cancer, since aromatase inhibitors reduce local aromatase activity as well as blood estradiol levels.

Aromatase inhibitors--mechanisms for non-steroidal inhibitors

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.

Tobacco alkaloid derivatives as inhibitors of breast cancer aromatase

The inhibition of estrogen biosynthesis by the use of aromatase inhibitors is emerging as a valuable approach to breast cancer therapy. Because smoking has a profound effect on estrogen-related processes we examined the ability of tobacco constituents to suppress estrogen production by breast cancer aromatase. N-n-octanoylnornicotine and N-(4-hydroxyundecanoyl) anabasine suppressed aromatase activity in culture of two human breast cancer cell lines, MDA-MB-231 (IC50 of 310 and 20 microM, respectively) and SK-BR-3 (IC50 of 450 and approximately 2 microM, respectively). MDA-MB-231 cells induced by 250 nM dexamethasone or 1 mM (Bt)2cAMP were slightly more sensitive to both inhibitors. Kinetic analyses showed that inhibition by N-(4-hydroxyundecanoyl)anabasine is competitive with respect to androstenedione as substrate, with apparent Ki values of 0.2 microM against microsomal aromatase activity derived from both (Bt)2cAMP-induced MDA-MB-231 cells and human breast tumor tissue. The corresponding apparent Ki against human placental microsomal aromatase activity was 0.4 microM. These results indicate that acyl derivatives of nornicotine and anabasine block estrogen formation in breast tumor cells and tissue and could contribute to the decreased intra-tissue estrogen levels in women who smoke.

Role of androgens in breast cancer

There is a vast amount of information on the concentration of different androgens in human breast cancer in pre- and post-menopausal women. Many years ago it was suggested that some androgens, in particular dehydroepiandrosterone (DHA), could be one of the parameters to establish whether these androgens can predict breast cancer. However, the enormous data available on the plasma and tissue concentrations, particularly DHA and DHA sulfate (DHA-S) allow confirmation that the quantitative values of these androgens are not significantly different in normal women from those with breast cancer. Another important aspect of androgens in breast cancer is their function as precursors of estrogens, hormones which play an important role in breast cancerization. However, it is not clear at present what the quantitative contribution of androgens "via aromatase" is to the formation of estrogens, because more recently it was found that estradiol in breast cancer tissues originates mainly "via sulfatase" using estrone sulfate as precursor. A point of further interest is that a series of authors have demonstrated that the administration of DHA to experimental animals with breast cancer significantly decreased the evolution of the disease. This part of the data is also contradictory because other experimental information has shown that administration of DHA can increase the incidence of granulosa cell tumors. Finally, it has been suggested that androgens, in their capacity as anti-estrogens, can be used to substitute anti-estrogens in cases where treatment with classical anti-estrogens has no response. In conclusion, more information concerning the plasma and tissular concentrations of androgens, their contribution as estrogen precursors and their biological response(s), is needed in order to have a clearer idea of the role of these steroids in breast cancer.

Growth suppression of MCF-7 human breast cancer cells by aromatase inhibitors: a new system for aromatase inhibitor screening

In our previous study we found that MCF-7 cells possess aromatase activity and stimulate estrogen receptor-mediated growth. The pathways through which androgens are converted to estrogens by aromatase and estrogens interact with estrogen receptors contribute significantly to growth stimulation. The administration of aromatase inhibitor results in suppression of growth stimulation by androgens. This system enabled us to assess directly the biological activities of aromatase inhibitors. Aromatase activity was inhibited in a dose-dependent manner by the addition of aminoglutethimide and CGS 16949A, competitive inhibitors, and of 14 alpha-hydroxy-4-androstene-3,6,17-trione and 4-hydroxy-androstenedione, mechanism-based inhibitors. After preincubation with mechanism-based inhibitors, aromatase activity was significantly suppressed, whereas after preincubation with competitive inhibitors, it was adversely increased. These effects were concentration- and time-dependent. Preincubation with competitive inhibitors resulted in augmentation of subsequent androgen stimulation of thymidine incorporation, while preincubation with mechanism-based inhibitors resulted in diminished stimulation by subsequent androgen administration. These results suggest that in MCF-7 cells competitive inhibitors adversely induce aromatase and accelerate the subsequent androgen stimulation of DNA synthesis. Suicide inhibitors are more effective than competitive inhibitors. This system will be useful for aromatase inhibitor screening.

A detailed molecular model for human aromatase

Using a variety of techniques, including sequence alignment, secondary structure prediction, molecular mechanics and molecular dynamics, we have constructed a model for the three-dimensional structure of P-450arom (human aromatase) based on that of P-450cam, the only cytochrome P-450 enzyme for which the crystal structure is known. The predicted structure is found to be in good agreement with current experimental data; both direct, from site-directed mutagenesis studies, and indirect, from the consideration of the structures and activities of known substrates and inhibitors.

Inhibition study of human placental aromatase with 3-deoxy- and 6-oxo-steroids using [1 beta-3H]16 alpha-hydroxyandrostenedione as a substrate: a comparison to that using [1 beta-3H]androstenedione

Inhibition of aromatase activity in human placental microsomes with 3-deoxy- (1, 2, 4, 6 and 8), 6-oxo- (3, 5 and 9), and 16 alpha-bromo- (7) steroids was studied using [1 beta-3H]16 alpha-hydroxyandrostenedione (16 alpha-OHAD) as a substrate and compared to that with the conventional substrate, [1 beta-3H]androstenedione. All the steroids inhibited both 16 alpha-OHAD and [1 beta-3H]androstenedione aromatization in a competitive manner. Based on Ki/Km ratios obtained in both series of experiments, their relative inhibitory activities in the series with 16 alpha-OHAD were not necessarily identical with those in the other series. The results would be important to understand the relationship of structure to function of aromatase in human placental microsomes.

Inhibitors of P450-dependent steroid biosynthesis: from research to medical treatment

A number of cytochrome P450-dependent enzymes are major targets for both steroidal and nonsteroidal compounds that may be of use in the treatment of a number of androgen-independent, androgen-, estrogen- and other steroid-dependent diseases. Compounds of interest are for example aminoglutethimide and derivatives; esters of 4-pyridineacetic acid; imidazole derivatives, such as ketoconazole, liarozole, fadrozole, CGS 18320 B; bis-chlorophenyl-pyrimidine analogues; triazole derivatives vorozole and CGS 20267, and steroidal aromatase inhibitors such as 4-hydroxyandrostenedione. Some of them (e.g. ketoconazole) triggered studies to find new possibilities in medical treatment. Others are of use clinically or under clinical evaluation to provide a palliative treatment and/or cure to patients with for example prostatic carcinoma, breast cancer, hypercortisolism and benign prostatic hyperplasia.