Mechanism-based inactivation of 17 beta,20 alpha-hydroxysteroid dehydrogenase by an acetylenic secoestradiol

Human hydroxysteroid dehydrogenases and pre-receptor regulation: insights into inhibitor design and evaluation

Hydroxysteroid dehydrogenases (HSDs) represent a major class of NAD(P)(H) dependent steroid hormone oxidoreductases involved in the pre-receptor regulation of hormone action. This is achieved by HSDs working in pairs so that they can interconvert ketosteroids with hydroxysteroids resulting in a change in ligand potency for nuclear receptors. HSDs belong to two protein superfamilies the aldo-keto reductases and the short-chain dehydrogenase/reductases. In humans, many of the important enzymes have been thoroughly characterized including the elucidation of their three-dimensional structures. Because these enzymes play fundamental roles in steroid hormone action they can be considered to be drug targets for a variety of steroid driven diseases, e.g. metabolic syndrome and obesity, inflammation, and hormone dependent malignancies of the endometrium, prostate and breast. This article will review how fundamental knowledge of these enzymes can be exploited in the development of isoform specific HSD inhibitors from both protein superfamilies. Article from the Special issue on Targeted Inhibitors.

Biochemical factors governing the steady-state estrone/estradiol ratios catalyzed by human 17beta-hydroxysteroid dehydrogenases types 1 and 2 in HEK-293 cells

Human 17beta-hydroxysteroid dehydrogenase types 1 and 2 (17betaHSD1 and 17betaHSD2) regulate estrogen potency by catalyzing the interconversion of estrone (E1) and estradiol (E2) using nicotinamide adenine dinucleotide (phosphate) cofactors NAD(P)(H). In intact cells, 17betaHSD1 and 17betaHSD2 establish pseudo-equilibria favoring E1 reduction or E2 oxidation, respectively. The vulnerability of these equilibrium steroid distributions to mutations and to altered intracellular cofactor abundance and redox state, however, is not known. We demonstrate that the equilibrium E2/E1 ratio achieved by 17betaHSD1 in intact HEK-293 cell lines is progressively reduced from 94:6 to 10:90 after mutagenesis of R38, which interacts with the 2'-phosphate of NADP(H), and by glucose deprivation, which lowers the NADPH/NADP(+) ratio. The shift to E2 oxidation parallels changes in apparent K(m) values for purified 17betaHSD1 proteins to favor NAD(H) over NADP(H). In contrast, mutagenesis of E116 (corresponding to R38 in 17betaHSD1) and changes in intracellular cofactor ratios do not alter the greater than 90:10 E1/E2 ratio catalyzed by 17betaHSD2, and these mutations lower the apparent K(m) of recombinant 17betaHSD2 for NADP(H) only less than 3-fold. We conclude that the equilibrium E1/E2 ratio maintained by human 17betaHSD1 in intact cells is governed by NADPH saturation, which is strongly dependent on both R38 and high intracellular NADPH/NADP(+) ratios. In contrast, the preference of 17betaHSD2 for E2 oxidation strongly resists alteration by genetic and metabolic manipulations. These findings suggest that additional structural features, beyond the lack of a specific arginine residue, disfavor NADPH binding and thus support E2 oxidation by 17betaHSD2 in intact cells.

Estradiol−Adenosine Hybrid Compounds Designed to Inhibit Type 1 17β-Hydroxysteroid Dehydrogenase

The steroidogenic enzyme type 1 17beta-hydroxysteroid dehydrogenase (17beta-HSD) is involved in the synthesis of estradiol (E(2)), a hormone well-known to stimulate the growth of estrogen-sensitive tumors. To obtain compounds able to control E(2) formation, two moieties were linked with a methylene side chain: an adenosine moiety for interacting with the cofactor-binding site and an E(2) moiety for interacting with the substrate-binding site. When tested as inhibitors of type 1 17beta-HSD, the hybrid compounds inhibited the reductive activity (E(1) into E(2)) with IC(50) values ranging from 52 to 1,000 nM. The optimal side-chain length was determined to be eight methylene groups for a 16 beta-orientation. The presence of two components (E(2) and adenosine) is essential for good inhibition, since 16 beta-nonyl-E(2) and 5-nonanoyl-O-adenosine, two compounds having only one of the components, did not inhibit the enzyme. Moreover, the 3D-structure analysis of EM-1,745 complexed with type 1 17beta-HSD showed key interactions with both substrate- and cofactor-binding sites.

E-ring modified steroids as novel potent inhibitors of 17beta-hydroxysteroid dehydrogenase type 1

17beta-Hydroxysteroid dehydrogenases (17beta-HSDs) are an important class of steroidogenic enzymes that regulate the bioavailability of active estrogens and androgens and are as yet a relatively unexploited therapeutic target. Based on our investigations and those of others, E-ring modified steroids were identified as a useful template for the design of inhibitors of 17beta-HSD type 1, an enzyme involved in the conversion of estrone into estradiol. The synthesis and biological evaluation of a new series of N- and C-substituted 1,3,5(10)-estratrien-[17,16-c]-pyrazoles and the corresponding SAR are discussed. Among the N-alkylated analogues, the most potent inhibitor was the 1'-methoxyethyl derivative, 41, with an IC(50) of 530 nM in T47-D human breast cancer cells. The X-ray crystal structure of the 1'-isobutyl derivative, was determined. Further optimization of the template using parallel synthesis resulted in a library of C5'-linked amides from which 73 emerged. This pyridylethyl amide had an IC(50) of 300 nM and its activity, with that of 41, suggests the importance of hydrogen bond acceptor groups in the pyrazole side chain. Both 41 and 73 displayed selectivity over 17beta-HSD type 2, and preliminary investigations showed 41 to be nonestrogenic in vitro in a luciferase reporter gene assay in contrast to the parent pyrazole 25. Molecular modeling studies, which support these findings, and a QSAR, the predictive power of which was demonstrated, are also presented.

Minireview: cellular redox state regulates hydroxysteroid dehydrogenase activity and intracellular hormone potency

Hydroxysteroid dehydrogenases (HSDs) interconvert potent and relatively inactive forms of individual steroid hormones using nicotinamide cofactors NADPH/NADP(+) and NADH/NAD(+) [nicotinamide adenine dinucleotide (phosphate), reduced/oxidized forms]. Although reactions with purified enzymes in vitro may be driven in either direction depending on the assay conditions, HSD enzymes appear to function in one direction or the other in intact cells. At least for some of these enzymes, however, the apparent unidirectional metabolism actually reflects bidirectional catalysis that reaches a pseudoequilibrium state with a strong directional preference. This directional preference, in turn, derives from intracellular concentration gradients for the nicotinamide cofactors and the relative affinities of each HSD for these cofactors. Because the concentrations of free cofactor exceed those of steroids by many orders of magnitude, the activities of these enzymes are predominantly driven by cofactor abundance, which is linked to intermediary metabolism. Consequently, the amount of active steroids in cells containing HSDs may be modulated by cofactor abundance and, hence, intracellular redox state. We will review the evidence linking cofactor handling and HSD activity, speculate on additional ways that intracellular metabolism can alter HSD activity and, thus, hormone potency, and discuss fruitful avenues of further investigation.

Human 17beta-hydroxysteroid dehydrogenases types 1, 2, and 3 catalyze bi-directional equilibrium reactions, rather than unidirectional metabolism, in HEK-293 cells

Human 17beta-hydroxysteroid dehydrogenases (17betaHSDs) catalyze the interconversion of weak and potent androgen and estrogen pairs. Although the reactions using purified enzymes can be driven in either direction, these enzymes appear to function unidirectionally in intact cells: only reductive reactions for 17betaHSD1 and 17beta HSD3 and only oxidative reactions for 17betaHSD2. We show that, after exhaustive incubations with either 17beta-hydroxy- or 17-ketosteroid, the medium for HEK-293 cells expressing 17betaHSD1 or 17betaHSD3 contains a 92:8 ratio of reduced:oxidized steroid. Similarly, 17betaHSD2 yields a >95:5 ratio of oxidized:reduced steroids for both androgens and estrogens. Dual-isotope kinetic measurements show that the rates of the forward and reverse reactions are identical at these functional equilibrium states in intact cells for all three 17betaHSD isoforms, and these rates are much faster than those estimated from single-isotope flux studies. Mutation L36D converts 17betaHSD1 to an oxidative enzyme in intact cells, reversing the equilibrium distribution of estradiol:estrone to 5:95; however, the rates of the forward and reverse reactions at equilibrium are equal and comparable to those of the wild-type enzymes. The co-expression of 17betaHSD2 paradoxically increases the potency of estrone in transactivation assays, demonstrating the physiological relevance of "backwards" metabolism to estradiol. We conclude that 17betaHSD types 1, 2, and 3 catalyze both oxidative and reductive reactions in HEK-293 cells at intrinsic rates that are much faster than those estimated from single-isotope studies. These 17betaHSD isoforms do not drive steroid flux in one direction but rather may achieve functional equilibria in intact cells, reflecting thermodynamically driven steroid distributions.

A concerted, rational design of type 1 17beta-hydroxysteroid dehydrogenase inhibitors: estradiol-adenosine hybrids with high affinity

Human estrogenic 17beta-hydroxysteroid dehydrogenase (17beta-HSD type 1) catalyzes the final step in the synthesis of active estrogens that stimulate the proliferation of breast cancer cells. Based on the initial premise to make use of the binding energies of both the substrate and cofactor sites, and molecular modeling starting from the enzyme structure, several estradiol-adenosine hybrids were designed and synthesized. Among these hybrids, EM-1745 with a linker of 8-CH2 groups is proved to be the best competitive inhibitor with a Ki of 3.0 +/- 0.8 nM. The crystal structure of the EM-1745 enzyme complex at 1.6 A provides evidence at atomic resolution of strong interactions between both the steroid and cofactor moieties and the enzyme molecule, as illustrated by a deltaA-weighted 2Fo-Fc electron density map contoured at 3.0 delta. The substrate entry loop is further stabilized in this complex compared with previous complexes of the enzyme. These results confirm our initial strategy of combining studies of structural biology and enzyme mechanism in the inhibitor design, which may be applied to other steroidogenic enzymes involved in human diseases.

Overview of a rational approach to design type I 17beta-hydroxysteroid dehydrogenase inhibitors without estrogenic activity: chemical synthesis and biological evaluation

Hormone-sensitive diseases such as breast cancer are health problems of major importance in North America and Europe. Endocrine therapies using antiestrogens for the treatment and the prevention of breast cancer are presently under clinical trials. Antiestrogens are drugs that compete with estrogens for the estrogen receptor without activating the transcription of estrogen-sensitive genes. However, an optimal blockade of estrogen action could ideally be achieved by a dual-action compound that would antagonize the estrogen receptor and inhibit the biosynthesis of estradiol. Type I 17beta-hydroxysteroid dehydrogenase (17beta-HSD) was chosen as a key steroidogenic target enzyme to inhibit the formation of estradiol, which is the most potent estrogen. This article describes a rational approach that could lead to the development of compounds that exhibit both actions. The chemical syntheses of estradiol derivatives bearing a bromoalkyl and a bromoalkylamide side chain at the 16alpha-position are summarized. Two parameters were studied for biological evaluation of our synthetic inhibitors: (1) the inhibition of estrone reduction into estradiol by type I 17beta-HSD, and (2) the proliferative/antiproliferative cell assays performed on the estrogen-sensitive ZR-75-1 breast tumor cell line. First, the substitution of the 16alpha-position of estradiol by bromoalkyl side chain led to potent inhibitors of type I 17beta-HSD, but the estrogenic activity remained. Secondly, an alkylamide functionality at the 16alpha- or 7alpha-position of estradiol cannot abolish the estrogenic activity without affecting considerably the inhibitory potency on type I 17beta-HSD. In conclusion, the best dual-action inhibitor synthesized showed an IC50 of 13 +/- 1 microM for type I 17beta-HSD, while displaying antiestrogenic activity at 1.0 microM. Despite the fact that we did not obtain an ideal dual-action blocker, we have optimized several structural parameters providing important structure-activity relationship.

A 6beta-(thiaheptanamide) derivative of estradiol as inhibitor of 17beta-hydroxysteroid dehydrogenase type 1

In an effort to develop potent agents for reducing the levels of the active estrogen, estradiol, we developed a new category of 17beta-hydroxysteroid dehydrogenase (17beta-HSD) type 1 inhibitors. The compounds described possess a butyl methyl alkylamide side chain linked to the C6 position of estradiol by a thioether. With a series of epimeric mixtures, an optimal side-chain length of five methylene groups (between the amide group and steroid part) was first determined. Thereafter, both C6 epimers of optimized mixture were obtained after high-pressure liquid chromatography separation. 1H and 13C NMR experiments were performed to confirm the stereochemistry of each epimer. The 6beta-orientation of the side-chain was found to be crucial for enzymatic inhibition. Indeed, for the optimized side-chain length, the compound with a beta-orientation (5: N-butyl,N-methyl 7-(3',17'beta-dihydroxy-1',3',5'( 10')-estratriene-6'beta-yl)-7-thiaheptanamide) was 70-fold more potent than the 6alpha-analog. Compound 5 did not inactivate 17beta-HSD type 1, suggesting a reversible inhibitor. In addition, it was found to be a more potent inhibitor than the substrate estrone itself or a panel of three known inhibitors.

Synthesis of 16-(bromoalkyl)-estradiols having inhibitory effect on human placental estradiol 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD type 1)

The activity of 17 beta-HSD type 1, the enzyme that converts estrone into its more potent metabolite estradiol, has been demonstrated in all classical steroidogenic tissues and almost all peripheral tissues from both rat and human. Since 17 beta-HSD is one of the most important enzymes involved in active steroid hormone formation, its inactivation could be a clinical approach to the treatment of hormono-dependent diseases like breast cancer. Herein we report the synthesis of 16-(bromoalkyl)-estradiols and their potency to inhibit the human placenta cytosolic estradiol 17 beta-HSD (type 1). Synthetic analogues possess various side chain lengths and orientation (alpha or beta) at position 16 of the steroidal D ring. The most potent inhibitory effect was observed when the length of the side chain was 3 or 4 carbons. However, the 16 beta-(bromopropyl)-estradiol easily undergoes cyclization and its effect on 17 beta-HSD is lost. Consequently, 16 alpha-(bromopropyl)-E2, 16 alpha-(bromobutyl)-E2, and 16 beta-(bromobutyl)-E2 were the best inhibitors discussed in this paper.

Secosteroid mechanism-based inactivators and site-directed mutagenesis as probes for steroid hormone recognition by 3 alpha-hydroxysteroid dehydrogenase

Rat liver 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD, EC 1.1.1.50) inactivates circulating androgens, progestins, and glucocorticoids. 3 alpha-HSD is a member of the aldo-keto reductase superfamily, and the X-ray structure of the apoenzyme shows the presence of an (alpha/beta)8 barrel [Hoog, S. S., Pawlowski, J. E., Alzari, P. M., Penning, T. M., & Lewis, M. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 2517-2521]. As yet, a three-dimensional structure of the ternary complex E.NADPH.steroid is unavailable. To identify regions of the enzyme involved in steroid hormone recognition, we have employed mechanism-based inactivators and site-directed mutagenesis. (3 RS)-1,10-Seco-5 alpha-estr-1-yne-3,17 beta-diol (1) and (17 RS)- 17-hydroxy-14,15-secoandrost-4-en-15-yn-3-one (3) are secosteroids which contain latent Michael acceptors (alpha,beta-unsaturated alcohols) at opposite ends of the steroid nucleus (at the C-3 and C-17 positions, respectively). It was found that compounds 1 and 3 inactivated 3 alpha-HSD only in the presence of NAD+. The requirement for cofactor implies that 1 and 3 are oxidized to the corresponding alpha,beta-unsaturated ketones for inactivation to occur. Chemically prepared 17 beta-hydroxy-1,10-seco-5 alpha-estr-1-yn-3-one (2) and 14,15-secoandrost-4-en-15-yne-3,17-dione (4), the presumed products of 1 and 3 oxidation, behaved as stoichiometric inactivators of 3 alpha-HSD. In the presence and absence of NAD+, 2 and 4 inactivated > 50% of the enzyme in 10 s or less.(ABSTRACT TRUNCATED AT 250 WORDS)

Clues to the development of mechanism-based inactivators of 3 alpha-hydroxysteroid dehydrogenase: comparison of steroidal and nonsteroidal Michael acceptors and epoxides

A series of steroidal and nonsteroidal Michael acceptors that represent reaction products for 3 alpha-hydroxysteroid dehydrogenase were synthesized and evaluated as potential enzyme-generated inactivators. Introduction of exocyclic olefins either at C-2 or C-6 produced inhibitors with high affinity for the enzyme (0.05 to 5.0 microM). However, despite this affinity, none of these compounds produced time-dependent inactivation of the enzyme. By contrast, analogs based on 1-phenyl-2-propen-1-one were stoichiometric inactivators of the enzyme and ease of turnover of the parent latent Michael acceptor depended on the presence of an electron-withdrawing substituent at the para position. A series of steroidal and nonsteroidal epoxides in which the oxiranyl oxygen could be substituted for the 3-ketone (the acceptor carbonyl of a steroid substrate) were also synthesized and evaluated as potential mechanism-based inactivators. Steroidal 2 alpha,3 alpha-, and 3 alpha,4 alpha-epoxides as well as 3 alpha- and 3 beta-spiroepoxides did not bind to the enzyme and were unable to cause enzyme inactivation in either the presence or absence of pyridine nucleotide. In contrast, nitrostyrene oxides produced time-dependent inactivation, the rate of which was governed by the presence of an electron withdrawing group at the para position. These data indicate that the design of mechanism-based inactivators for 3 alpha-hydroxysteroid dehydrogenase requires the incorporation of electron-withdrawing groups adjacent to the latent enzyme-activated group and, as a result, the turnover and/or reactivity of these compounds is increased. Moreover, these compounds can be modeled on nonsteroids.

Evidence that enzyme-generated aromatic Michael acceptors covalently modify the nucleotide-binding site of 3 alpha-hydroxysteroid dehydrogenase

The non-steroidal allylic and acetylenic alcohols 1-(4'-nitrophenyl)prop-2-en-1-ol (I) and 1-(4'-nitrophenyl)prop-2-yn-1-ol (II) are oxidized by homogeneous 3 alpha-hydroxysteroid dehydrogenase to the corresponding alpha beta-unsaturated ketones 1-(4'-nitrophenyl)prop-2-en-1-one (III) and 1-(4'-nitrophenyl)prop-2-yn-1-one (IV), which then inactivate the enzyme selectively with high affinity; low effective partition ratios are observed for the parent alcohols [Ricigliano & Penning (1989) Biochem. J. 262, 139-149]. Inactivation of 3 alpha-hydroxysteroid dehydrogenase by compound (I) displays an NAD+ concentration optimum. Scavenging experiments indicate that the enzyme-generated inactivators (III) and (IV) alkylate the enzyme via a release-and-return mechanism. Several lines of evidence suggest that compounds (III) and (IV) covalently modify the NAD(P)(+)-binding site. First, micromolar concentrations of NAD(P)H offer substantial protection against enzyme inactivation mediated by Michael acceptors (III) and (IV). In these protection studies Kd measurements for NAD(P)H approached those measured by fluorescence titration of free enzyme. Secondly, under initial-velocity conditions compounds (III) and (IV) act essentially as competitive inhibitors of NAD+ binding, and as mixed competitive or non-competitive inhibitors against androsterone binding. Thirdly, enzyme inactivated with either compound (III) or compound (IV) fails to bind to NAD+ affinity columns (e.g. Affi-gel Blue). Under the same conditions of chromatography native enzyme and enzyme affinity-labelled at the steroid-binding site with 17 beta-bromoacetoxy-5 alpha-dihydrotestosterone is retained on the affinity column. A kinetic scheme that represents the inactivation of the homogeneous dehydrogenase by the enzyme-generated alkylators (III) and (IV) is presented.

Synthesis and evaluation of non-steroidal mechanism-based inactivators of 3 alpha-hydroxysteroid dehydrogenase

Two non-steroidal mechanism-based inactivators for 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) of rat liver have been synthesized: 1-(4'-nitrophenyl)-2-propen-1-ol (I), and 1-(4'-nitrophenyl)-2-propyn-1-ol (II). Both of these compounds inactivate homogeneous 3 alpha-HSD in a time- and concentration-dependent manner only in the presence of NAD+. Analysis of the pseudo-first-order inactivation data gave a Kd of 1.2 mM for the allylic alcohol and a t1/2 (time required to promote a 50% loss of enzyme activity) for the enzyme of less than 10 s at saturation. Similar inactivation studies with the acetylenic alcohol gave a Kd of 1.5 mM and a t1/2 for the enzyme of 9.9 min at saturation. The allylic alcohol and acetylenic alcohol are oxidized stereoselectively by the enzyme, yielding a Km of 2.0 mM and a Vmax. of 0.58 mumol/min per mg for the allylic alcohol and a Km of 0.75 mM and a Vmax. of 0.29 mumol/min per mg for the acetylenic alcohol. Effective partition ratios (kcat./kinact.) are low for both alcohols: for the allylic alcohol, 5.3; and for the acetylenic alcohol, 141. H.p.l.c. indicates that the Michael acceptors 1-(4'-nitrophenyl)-2-propen-1-one (III) and 1-(4'-nitrophenyl-2-propyn-1-one (IV) are the products of the enzymic oxidation of the corresponding alcohols. The latter compound (IV) was trapped as its monothioether adducts before h.p.l.c. analysis. The Michael acceptors III and IV inactivate the 3 alpha-HSD in the absence of NAD+ at a rate too high to accurately measure and titrate the enzyme in a stoichiometric manner. Enzyme inactivated by I and NAD+, II and NAD+, III or IV is not re-activated by gel filtration or dialysis, implying a stable covalent bond has been formed between the enzyme and the inactivators. A screen of five other HSDs, and two aliphatic alcohol dehydrogenases, indicates that alcohol I is a selective inactivator of rat liver 3 alpha-HSD. It is concluded that 3 alpha-HSD generates non-steroidal alkylating agents (III and IV) that potently inactivate the enzyme with low effective partition coefficients. This report of non-steroidal mechanism-based inactivators of 3 alpha-HSD may provide a precedent for the development of related compounds to act as suicide substrates of other HSDs.

Preparation of 14,15-secoestra-1,3,5(10)-trien-15-ynes, inhibitors of estradiol dehydrogenase

The conversion of estrone to 14,15-secoestratrien-15-ynes, inactivators of estradiol dehydrogenase from human term placenta, is described. The optically pure precursor 7-acetoxy-octahydro-2-phenanthrenecarboxylic acid methyl ester is prepared from estrone in five steps and 40% yield. The unsubstituted propargylic secoestratriene diol, a mechanism-based inactivator of estradiol dehydrogenase, and the corresponding acetylenic ketone, an affinity label inactivator of the same enzyme, arise from the phenanthrene ester in three and four steps. The propargylic secoestratriene diol also competes with [3H]estradiol for binding to calf uterus estrogen receptor and possesses weak uterotrophic activity.