Inhibition of Leydig tumor cell steroidogenesis by 10-propargylestr-4-ene-3,17-dione, an irreversible aromatase inhibitor

The murine Leydig cell tumor (M5480A) was assayed for the presence of aromatase activity and for the effects of 10-propargylestr-4-ene-3,17-dione (PED), an aromatase inhibitor, on steroidogenesis. Microsomal preparations from these tumors contained low levels of aromatase activity which was PED sensitive. In addition, these Leydig tumor cells were maintained in primary culture and incubated under basal and gonadotropin-stimulated conditions with various doses of PED. Medium levels of progesterone, a major product of these cells, were found to decrease in a dose- and time-dependent manner upon addition of PED. To determine whether the observed effect was due to reduced synthesis or to increased metabolism of progesterone, tritiated progesterone was added to these cell cultures. Analysis of culture medium by high-performance liquid chromatography suggested that PED dramatically reduced the conversion of labeled progesterone to testosterone. Furthermore, examination of medium pregnenolone levels revealed diminished amounts of this steroid as well. Taken together, these results suggest that PED or its metabolites inhibit Leydig tumor cell steroidogenesis at several sites. Thus, in addition to its role as an aromatase inhibitor, this agent also has effects prior to pregnenolone production, as well as other effects in the pathway between progesterone and testosterone.

The 5-hydroxyl of myo-inositol is essential for uptake into HSDM1C1 mouse fibrosarcoma cells

We attempted to replace the myo-inositol in cellular inositol phosphatides with 5-deoxy-myo-inositol to evaluate the role of inositol 1,4,5-trisphosphate as a second messenger. This analog, lacking a 5-hydroxyl, might be incorporated into 5-deoxyphosphatidylinositol and converted to the corresponding phosphatidylcyclitol 4-phosphate but could not be converted to phosphatidylinositol 4,5-diphosphate, the precursor of the second messenger molecule inositol 1,4,5-trisphosphate. We synthesized 5-deoxy-myo-inositol and found that this analog does not replace myo-inositol as an essential growth factor for essential fatty acid deficient HSDM1C1 mouse fibrosarcoma cells. Furthermore, [5-3H]-5-deoxy-myo-inositol was neither incorporated into the phospholipids nor accumulated in the cytoplasm of these cells. It appears that this cell line has a specific myo-inositol uptake system that excludes a potentially harmful analog of inositol.

Hydroperoxides as inactivators of aromatase: 10 beta-hydroperoxy-4-estrene-3,17-dione, crystal structure and inactivation characteristics

The crystal structure of 10 beta-hydroperoxy-4-estrene-3,17-dione (10 beta-OOH) was determined, and its inhibition of human placental aromatase was investigated. In the absence of added NADPH, 10 beta-OOH caused a time-dependent loss of aromatase activity (e.g., 50% loss after 90 s with 2.16 microM 10 beta-OOH). Protection against this loss of activity was provided when a substrate, androstenedione, was included in the incubation. Centrifugation and resuspension of the 10 beta-OOH-treated microsomes in fresh buffer failed to restore the activity, but partial recovery could be effected by dithiothreitol. Experiments to detect destruction of aromatase protoheme were done but were inconclusive. In the presence of NADPH, 10 beta-OOH did not cause a time-dependent loss of activity but was instead a competitive inhibitor (Ki = 330 nM) of androstenedione (Km = 21 nM) aromatization. The added NADPH was not utilized for the aromatization of 10 beta-OOH to estrogens, and enhanced reduction of 10 beta-OOH to 10 beta-hydroxy-4-estrene-3,17-dione could not be detected. In addition, microsomes alone were incapable of using 10 beta-OOH to support the aromatization of androstenedione. Cumene hydroperoxide and H2O2 were also investigated as inactivators of aromatase. Losses of activity comparable to those found for 10 beta-OOH could only be observed at 500-1000-fold higher concentrations of these agents, and no protection was provided by either androstenedione or NADPH. Extensive destruction of microsomal protoheme was found with these nonsteroidal agents.

Study of the role of Schiff base formation in the aromatization of 3-[18O]testosterone and 3,17-di-[18O]androstenedione by human placental aromatase

Testosterone, prepared with 18O at the 3 position, and androstenedione, prepared with 18O at both the 3 and 17 positions, were incubated with human placental microsomes. The resulting estrogen metabolites (estradiol and estrone, respectively) as well as unconverted starting material were isolated and analyzed for their 18O content by gas chromatography-mass spectrometry. In each case, greater than 90% of the 18O present in the original substrate was retained in the products. This result argues against a role for Schiff base formation in the aromatase reaction.

Differential effect of alpha-difluoromethylornithine on the in vivo uptake of 14C-labeled polyamines and methylglyoxal bis(guanylhydrazone) by a rat prostate-derived tumor

The uptake of exogenously administered radiolabeled polyamines by a rat prostate-derived tumor line, the Dunning R3327 MAT-Lu, and various normal tissues was studied. Pretreatment of tumor cells in vitro with alpha-difluoromethylornithine (DFMO), a polyamine synthesis inhibitor, resulted in a markedly enhanced uptake of both [14C]putrescine and [14 C]spermidine. The in vitro uptake of [14C]putrescine by these cells was effectively inhibited by unlabeled spermine, spermidine, 1,8-diaminooctane, 1,7-diaminoheptane, 1,6-diaminohexane, 1,5-diaminopentane, 1,4-diaminopentane, and 1,4-diaminobutane, but less effectively by 1,4-diamino-2,3-butene and 1,4-diamino-2,3-butyne. The diamines, 1,3-diaminopropane and 1,2-diaminoethane, were ineffective in inhibiting [14C]putrescine uptake in vitro into the R3327 MAT-Lu cell line. When tumor-bearing animals were pretreated with DFMO or with DFMO and 5-alpha-dihydrotestosterone propionate, the tumor and prostate uptake of [14C]putrescine and [14C]-cadaverine was enhanced but not substantially increased in other tissues. In contrast to the in vitro results, spermidine and spermine were not enhanced substantially by DFMO pretreatment into any tissue, and their uptake into the tumor actually decreased. Ethylenediamine, which does not utilize the polyamine transport system, did not have its uptake increased into any tissue following DFMO pretreatment. The chemotherapeutic agent, methylglyoxal bis(guanylhydrazone), which utilizes the polyamine transport system for uptake into cells, exhibited uptake behavior different from that of the polyamines. Thus, methylglyoxal bis(guanylhydrazone) uptake into the tumor was not significantly increased or decreased by DFMO or by DFMO + 5-alpha-dihydrotestosterone propionate pretreatment, and only the ventral, but not the dorsal-lateral, lobe of the prostate showed increased uptake of methylglyoxal bis(guanylhydrazone) following DFMO + 5-alpha-dihydrotestosterone propionate pretreatment.

Inactivation of human placental 17 beta,20 alpha-hydroxysteroid dehydrogenase by 16-methylene estrone, an affinity alkylator enzymatically generated from 16-methylene estradiol-17 beta

The substrate 16-methylene estra-1,3,5(10)-triene-3,17 beta-diol (16-methylene estradiol-17 beta) and its enzyme-generated alkylating product, 3-hydroxy-16-methylene estra-1,3,5(10)-triene-17-one (16-methylene estrone), were synthesized to study the 17 beta- and 20 alpha-hydroxysteroid dehydrogenase activities which coexist in homogeneous enzyme purified from human placental cytosol. 16-Methylene estradiol, an excellent substrate (Km = 8.0 microM; Vmax = 2.8 mumol/mg/min) when enzymatically oxidized to 16-methylene estrone in the presence of NAD+ (256 microM), inactivates simultaneously the 17 beta- and 20 alpha-activities in a time-dependent and irreversible manner following pseudo-first order kinetics (t1/2 = 1.0 h, 100 microM, pH 9.2). 16-Methylene estradiol does not inactivate the enzyme in the absence of NAD+. 16-Methylene estrone (Km = 2.7 microM; Vmax = 2.9 mumol/mg/min) is an affinity alkylator (biomolecular rate constant k'3 = 63.3 liters/mol-s, pH 9.2; KI = 261 microM; k3 = 8.0 X 10(-4) S-1, pH 7.0) which also simultaneously inhibits both activities in an irreversible time-dependent manner (at 25 microM; t1/2 = 7.2 min, pH 9.2; t1/2 = 2.7 h, pH 7.0). Substrates (estradiol-17 beta, estrone, and progesterone) protect against inhibition of enzyme activity by 16-methylene estrone and 16-methylene estradiol. Affinity radioalkylation studies using 16-methylene [6,7-3H]estrone demonstrate that 1 mol of alkylator binds per mol of inactivated enzyme dimer. Thus, 16-methylene estradiol functions as a unique substrate for the enzymatic generation of a powerful affinity alkylator of 17 beta,20 alpha-hydroxysteroid dehydrogenase and should be a useful pharmacological tool.

Computer-assisted modeling of the picrotoxinin and gamma-butyrolactone receptor site

Three-dimensional models of the picrotoxinin and alkyl-substituted gamma-butyrolactone (GBL) receptor sites were constructed with the aid of a molecular graphics computer system (MMS-X). These two independently derived models proved to be very compatible, which suggested that both types of compounds share a common site of action. Since picrotoxinin is known to act at gamma-aminobutyric acid-regulated chloride channels, a hypothesis was made and tested that the convulsant GBL and picrotoxinin analogues physically impede the passage of chloride ions through the channel. It was also shown that it was theoretically possible for the anticonvulsant GBLs to act at this same site without blocking chloride flux. Finally, the model was applied to several convulsant and anticonvulsant compounds of different chemical classes and was found to be of somewhat general applicability.

Alpha-substituted gamma-butyrolactones: new class of anticonvulsant drugs

Alkyl-Substituted gamma-butyrolactones were synthesized and tested for their convulsant and anticonvulsant actions in mice and guinea pigs. The alpha-substituted compounds, alpha, alpha-dimethyl-, and alpha-ethyl-alpha-methyl-gamma-butyrolactone were anticonvulsant compounds with a spectrum of activity similar to that of ethosuximide. In contrast, beta-substituted compounds were convulsant agents similar to picrotoxinin. The alpha-substituted-gama-butyrolactones represent a new class of anticonvulsant drug with experimental and clinical potential.

Anticonvulsant properties of alpha, gamma, and alpha, gamma-substituted gamma-butyrolactones

Derivatives of gamma-butyrolactone (GBL) substituted on the alpha- and/or gamma-positions were synthesized and tested for their effects on behavior in mice, on the electroencephalographs and blood pressure of paralyzed-ventilated guinea pigs, and on electrical activity of incubated hippocampal slices. Several compounds, including alpha-ethyl-alpha-methyl GBL (alpha-EMGBL), alpha, alpha-dimethyl GBL, alpha, gamma-diethyl-alpha, gamma-dimethyl GBL, and gamma-ethyl-gamma-methyl GBL, prevented seizures induced by pentylenetetrazol, beta-ethyl-beta-methyl-gamma-butyrolactone (beta-EMGBL), picrotoxin, or all three compounds in mice and guinea pigs but had no effect on seizures induced by maximal electroshock or bicuculline. Neither gamma-hydroxybutyrate (GHB) nor alpha-isopropylidine GBL had any anticonvulsant activity. The anticonvulsant alpha-substituted compounds had a potent hypotensive effect and antagonized the hypertensive effect of beta-EMGBL, alpha-EMGBL was tested in incubated hippocampal slices and was found to depress basal activity and antagonize excitation induced by beta-EMGBL. These results demonstrate that alpha-alkyl-substituted GBL and, to a lesser extent, gamma-substituted derivatives are anticonvulsant agents and that their effects are strikingly different from those of GHB or beta-alkyl-substituted GBLs, which are epileptogenic. Possibly beta- and alpha-substituted GBLs act at the same site as agonists and antagonists, respectively.

Comparative studies of aromatase inhibitors in cultured human breast cancer cells

The presence of aromatase activity, estrogen receptors, and estrogenic responsiveness in MCF-7 human breast cancer cells has allowed this cell line to be used as a unique in vitro system for investigating the biological activities of potentially therapeutic aromatase inhibitors. We now report the results of studies which have examined the cytotoxicity, antiaromatase, and intrinsic estrogenic activities of aminoglutethimide, 1,2-dehydrotestolactone (testolactone), dihydrotestosterone, 4-hydroxy-4-androstene-3,17-dione, and 10-propargylestr-4-ene-3,17-dione within MCF-7 monolayer cultures. Cell viability was determined by trypan blue exclusion, and aromatase activity was assessed by quantifying the amounts of [3H]estradiol formed from [3H]testosterone. Estrogenic activity was assessed by examining the ability of each inhibitor to increase cytoplasmic progesterone receptor and deplete cytoplasmic estrogen receptor concentrations in these cells during a 5-day incubation period. Cytoplasmic progesterone and estrogen receptors were measured by the single-saturating-dose technique using [17 alpha-methyl-3H]-17 alpha, 21-dimethyl-19-norpregna-4,9-diene-3,20-dione and [3H]estradiol as the labeled ligands for each assay, respectively. The results showed that all of these compounds were noncytotoxic aromatase inhibitors in MCF-7 cells but that these agents demonstrated marked differences in inhibitory potency (10-propargylestr-4-ene-3,17-dione greater than 4-hydroxy-4-androstene-3,17-dione much greater than dihydrotestosterone much greater than testolactone = aminoglutethimide). The incubation of cells with 4-hydroxy-4-androstene-3,17-dione resulted in cytoplasmic progesterone and estrogen receptor responses that were similar in magnitude to those observed in other cultures incubated with equimolar concentrations of estradiol. None of the other four agents demonstrated estrogenic activity in this system. However, we have previously observed that dihydrotestosterone has substantial antiestrogenic action in this system. Taken together, these results indicate that some aromatase inhibitors may influence the hormonal regulation of human breast cancer cells by more than one mechanism.

A new hypothesis based on suicide substrate inhibitor studies for the mechanism of action of aromatase

Recently, it was discovered that 4-hydroxy-4-androstene-3,17-dione, 4-androstene-3,6,17-trione, and 1,4,6-androstatriene-3,17-dione, compounds previously reported to be competitive inhibitors of aromatase, cause a time-dependent loss of aromatase activity in human placental microsomes. We report here that 1,4-androstadiene 3,17-dione (Ki 0.32 microM; kinact 0.91 X 10(-3)/sec) and testolactone (Ki 35 microM; kinact 0.36 X 10(-3)/sec) also cause a similar loss of aromatase activity. The mechanism which explains the unexpected loss of activity caused by these five inhibitors is neither established nor apparent from current theories of the enzyme mechanism of action of aromatase. We propose an inactivation mechanism based on a new hypothesis for estrogen biosynthesis in which the third enzyme oxidation carried out by aromatase results in the formation of an enzyme-bound intermediate. This intermediate is released as an aromatized product via a facile elimination reaction which simultaneously regenerates the unaltered active enzyme. Various structural modifications made in these five inhibitors are hypothesized to redirect this elimination reaction so that the steroid intermediate remains covalently attached to the enzyme instead of being released as an aromatized product.

Effects of 4-hydroxy-4-androstene-3, 17-dione and 10-propargylestr-4-ene-3, 17-dione on the metabolism of androstenedione in human breast carcinoma and breast adipose tissues

The effects of 4-hydroxy-4-androstene-3, 17-dione (4-OH-A) and 10-propargylestr-4-ene-3, 17-dione (PED) on the aromatization of androstenedione (A) and the conversion of A to testosterone (T) were studied in incubations with breast carcinoma and breast adipose tissues. Parallel studies were carried out to determine the effects of 4-OH-A and PED on A metabolism in tissue from 5 patients with breast carcinoma. At 11 micro M, both compounds fully inhibited aromatization, whereas the conversion of A to T was decreased in only 2 incubations. Studies with varying concentrations of 4-OH-A and PED demonstrated that both compounds inhibited estrone (E1) formation by 80% at a concentration of 0.085 micro M, with maximum effect at 0.34 micro M. 90% inhibition of estradiol (E2) formation was observed at inhibitor concentrations of 0.17 micro M or greater. T formation was slightly affected at 0.67 microM, but was progressively inhibited with increasing 4-OH-A or PED concentrations, reaching 70% at 11 micro M. Similar experiments with 4-OH-A in breast adipose tissue homogenates showed that a concentration of 0.1 micro M was sufficient to inhibit aromatization while T inhibition required 11 micro M. 4-OH-A and PED are selective inhibitors of aromatization in human breast tissues and may provide a mechanism for controlling estrogen responsive processes.

Enzyme-generated intermediates derived from 4-androstene-3,6,17-trione and 1,4,6-androstatriene-3,17-dione cause a time-dependent decrease in human placental aromatase activity

Kinetic evidence is presented for a time-dependent decrease in human placental aromatase activity by enzyme-generated intermediates derived from two widely used steroids previously described as competitive inhibitors of estrogen biosynthesis. Thus, 4-androstene-3,6,17-trione binds to the enzyme with an apparent Ki of 0.43 microM and has a pseudo-first order overall rate constant for decrease in activity of 4.03x10(-3)sec-1, while 1,4,6-androstatriene-3,17-dione has an apparent Ki of 0.18 microM and a pseudo-first order overall rate constant for decrease in activity of 1.10x10(-3)sec-1. These findings imply that the potent inhibition of estrogen biosynthesis caused by these steroids results primarily from a decrease in enzyme activity caused by enzyme-generated intermediates from the parent steroids.

10 beta-propynyl-substituted steroids. Mechanism-based enzyme-activated irreversible inhibitors of estrogen biosynthesis

A series of 10 beta-propynyl-substituted steroids are reported as mechanism-based enzyme-activated irreversible inhibitors of the human placental aromatase which converts 4-androstene-3,17-dione to 1,3,5(10-estratrien-3-ol-17-one. Thus 10-propargylestr-4-ene-3,17-dione binds to the enzyme with an apparent Ki of 23 nM and causes time-dependent inactivation (pseudo-first order kinact = 1.11 X 10(-3) s-1). The 10-[(1S)-1-hydroxy-3-propynyl]estr-4-ene-3,17-dione analog of the known enzyme-generated intermediate, 4-androsten-19-ol-3,17-dione, behaved similarly and had an apparent Ki of 27 microM and a psuedo-first order kinact of 2.91 X 10(-3) s-1. The stereoisomeric 10-[(1R)-1-hydroxy-2-propynyl]estr-4-ene-3,17-dione did not cause time-dependent inactivation, but bound in a competitive manner with an apparent Ki of 2.5 microM. Finally, 10-(1-oxo-2-propynyl)-estr-4-ene-3,17-dione, the proposed enzyme-generated affinity label and analog of the second intermediate (3,17-dioxoandrost-4-en-19-al) in the enzymatic reaction, bound to the enzyme with an apparent Ki of 12 microM and caused time-dependent inactivation with a pseudo-first order kinact of 5.35 X 10(-4) s-1.

Human placental 17 beta-estradiol dehydrogenase and 20 alpha-hydroxysteroid dehydrogenase. Two activities at a single enzyme active site

Two soluble enzyme activities, 17 beta-estradiol dehydrogenase and 20 alpha-hydroxysteroid dehydrogenase, present in the cytosol fraction of term human placenta, were co-purified with a constant ratio of specific activities, approximating 100:1, respectively. The "pure enzyme" is a single band on sodium dodecyl sulfate disc gel electrophoresis. To evaluate whether catalysis of the estrogen and progestin substrates occurs at a single active site, alkylation studies using 16 alpha-bromoacetoxyprogesterone were designed. This affinity alkylating steroid binds at the enzyme-active site (km 256 microM; Vmax = 0.012 mumol/min/mg), inactivates the enzyme in an irreversible and time-dependent manner which follows pseudo-first order kinetics, and causes coincident loss of both the 17 beta- and 20 alpha-activities. Affinity radioalkylation studies using 16 alpha-[2'-3H]bromoacetoxyprogesterone indicate that 2 mol of steroid bind per mol of inactivated enzyme dimer (Mr = 68,000). Amino acid analyses of the acid hydrolysate of radioalkylated enzyme show that 16 alpha-bromoacetoxyprogesterone dicarboxymethylates a histidyl residue in the active site. These results are identical with those reported for 16 alpha-[2'-3H]bromoacetoxyestradiol 3-methyl ether inactivation and radioalkylation of identically purified "17 beta-estradiol dehydrogenase." Computer graphics were used to construct a model in which: 1) binding of estrogen and progestin substrates at one active site permits stereospecific catalysis; 2) the estrogen and progestin analogs' alkylating side arms have access to a common histidine residue. These observations clearly demonstrate that the catalysis of estrogen and progestin substrates can occur at a single active site of one enzyme.