Characterization of homogeneous recombinant rat ovarian 20alpha-hydroxysteroid dehydrogenase: fluorescent properties and inhibition profile.
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In rat ovary, 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD), a member of the aldo-keto reductase (AKR) superfamily, converts progesterone into the inactive progestin 20alpha-hydroxyprogesterone and has been implicated in the termination of pregnancy. Here we report a convenient overexpression system that permits the purification of milligram quantities of homogeneous recombinant 20alpha-HSD with wild-type enzyme activity. The availability of this enzyme has permitted detailed kinetic, inhibition and fluorescence analyses. The enzyme exhibited narrow steroid specificity, catalysing reactions only at C-20; it reduced progesterone and 17alpha-hydroxyprogesterone and oxidized 20alpha-hydroxypregnanes. It also turned over common AKR substrates, such as 9, 10-phenanthrenequinone and 4-nitrobenzaldehyde. The intrinsic fluorescence spectrum of 20alpha-HSD was characterized and was quenched on the binding of NADP(H), yielding a KNADPd of 0.36 microM and a KNADPHd of 0.64 microM. NADP(H) binding generated an energy transfer band that could not be quenched by steroids. Inhibition studies conducted with non-steroidal and steroidal anti-inflammatory drugs and synthetic oestrogens indicated that even though rat ovarian 20alpha-HSD and rat liver 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) share more than 67% amino acid identity, their inhibition profiles are markedly different. Unlike 3alpha-HSD, most of these compounds did not inhibit 20alpha-HSD. Only meclofenamic acid and hexoestrol were potent competitive inhibitors for 20alpha-HSD, yielding K(i) values of 18.9 and 14.3 microM respectively. These studies suggest that selective non-steroidal AKR inhibitors could be developed for 20alpha-HSD that might be useful in maintaining pregnancy and that specific inhibitors might be developed from either N-phenylanthranilates or biphenols. (+info)
26-cholesterol hydroxylase in rat corpora lutea: A negative regulator of progesterone secretion.
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From a subtracted cDNA library of rat luteal tissue, where cDNA fragments in functional luteal tissue were subtracted from those in regressing luteal tissue, a cDNA clone corresponding to 26-cholesterol hydroxylase (P450(C26)) was obtained. It is known that P450(C26) catalyzes the conversion of cholesterol to 26-hydroxycholesterol, which blocks cholesterol utilization in the cell, and that 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) catalyzes the conversion of progesterone to an inactive steroid, 20alpha-dihydroprogesterone (20alpha-OHP). Thus, using pseudopregnant rats as a model, physiological cooperation of P450(C26) and 20alpha-HSD in the reduction of progesterone release toward the end of the luteal phase was evaluated. Levels of P450(C26) and 20alpha-HSD mRNA were examined in corpora lutea from pseudopregnant rats by Northern blot or reverse transcription-polymerase chain reaction or both. P450(C26) mRNA was ubiquitously expressed in corpora lutea, and its expression increased toward the end of pseudopregnancy, while 20alpha-HSD was expressed in all corpora lutea on Day 16 (Day 0 = the day of after cervical stimulation) but not detected before Day 10. An inhibitor of 20alpha-HSD, STZ26 (D-homo-16-oxa-4-androstene-3,16alpha-dione), was administered at various doses to rats from Day 12 to 20, effectively suppressing the elevation of 20alpha-OHP in a dose-dependent manner but not the depletion of progesterone completely. The expression of P450(C26) mRNA was increased as STZ26 dose increased, which negatively correlated with the progesterone levels. These results strongly suggest that P450(C26) cooperated with 20alpha-HSD in the reduction of progesterone release from the rat luteal tissue at the end of the functional luteal phase. (+info)
Conversion of mammalian 3alpha-hydroxysteroid dehydrogenase to 20alpha-hydroxysteroid dehydrogenase using loop chimeras: changing specificity from androgens to progestins.
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Hydroxysteroid dehydrogenases (HSDs) regulate the occupancy and activation of steroid hormone receptors by converting potent steroid hormones into their cognate inactive metabolites. 3alpha-HSD catalyzes the inactivation of androgens in the prostate by converting 5alpha-dihydrotestosterone to 3alpha-androstanediol, where excess 5alpha-dihydrotestosterone is implicated in prostate disease. By contrast, 20alpha-HSD catalyzes the inactivation of progestins in the ovary and placenta by converting progesterone to 20alpha-hydroxyprogesterone, where progesterone is essential for maintaining pregnancy. Mammalian 3alpha-HSDs and 20alpha-HSDs belong to the aldo-keto reductase superfamily and share 67% amino acid sequence identity yet show positional and stereospecificity for the formation of secondary alcohols on opposite ends of steroid hormone substrates. The crystal structure of 3alpha-HSD indicates that the mature steroid binding pocket consists of 10 residues located on five loops, including loop A and the mobile loops B and C. 3alpha-HSD was converted to 20alpha-HSD by replacing these loops with those found in 20alpha-HSD. However, when pocket residues in 3alpha-HSD were mutated to those found in 20alpha-HSD altered specificity was not achieved. Replacement of loop A created a 17beta-HSD activity that was absent in either 3alpha- or 20alpha-HSD. Once loops A and C were replaced, the chimera had both 3alpha- and 20alpha-HSD activity. When loops A, B, and C were substituted, 3alpha-HSD was converted to a stereospecific 20alpha-HSD with a resultant shift in k(cat)/K(m) for the desired reaction of 2 x 10(11). This study represents an example where sex hormone specificity can be changed at the enzyme level. (+info)
Prostaglandin F2alpha-induced expression of 20alpha-hydroxysteroid dehydrogenase involves the transcription factor NUR77.
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Prostaglandin F(2)alpha (PGF(2)alpha) binding to its receptor on the rat corpus luteum triggers various signal transduction pathways that lead to the activation of a steroidogenic enzyme, 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD), which in turn catabolizes progesterone. The molecular mechanism underlying PGF(2)alpha-induced 20alpha-HSD enzyme activity has not yet been explored. In this report we show, using mice lacking PGF(2)alpha receptor and pregnant rats, that PGF(2)alpha is responsible for the rapid and massive expression of the 20alpha-HSD gene at the end of pregnancy leading to a decrease in progesterone secretion. We also present evidence that PGF(2)alpha enhances 20alpha-HSD promoter activity. We have determined a region upstream of the -1590 position in the 20alpha-HSD promoter that confers regulation by PGF(2)alpha in ovarian primary cells. This region encompasses a unique transcription factor-binding site with a sequence of a NUR77 response element. Deletion of this motif or overexpression of a NUR77 dominant negative protein caused a complete loss of 20alpha-HSD promoter activation by PGF(2)alpha. NUR77 also transactivated the 20alpha-HSD promoter in transient transfection experiments in corpus luteum-derived cells (GG-CL). This induction required the NUR77-transactivating domain. We also show that PGF(2)alpha induces a very rapid expression of NUR77 that binds to a distal response element located at -1599/-1606 but does not interact with another proximal putative NUR77 response element located downstream in the promoter. A rapid increase in NUR77 mRNA was observed in mice corpora lutea just before parturition at a time when 20alpha-HSD becomes expressed. This increase in the expression of both genes was not seen in PGF(2)alpha receptor knockout mice. By using cyclosporin A and PGF(2)alpha treatment, we established that inhibition of NUR77 DNA binding in vivo prevents PGF(2)alpha induction of the 20alpha-HSD gene in the corpus luteum. Taken together, our results demonstrate, for the first time, that PGF(2)alpha induces in the corpus luteum the expression of the nuclear orphan receptor and transcription factor, NUR77, which in turn leads to the transcriptional stimulation of 20alpha-HSD, triggering the decrease in serum progesterone essential for parturition. (+info)
Characterization of a human 20alpha-hydroxysteroid dehydrogenase.
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It has been suggested that 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) is a T-cell differentiation marker in mice. In the human, this enzyme has generally been associated with types 1 and 2 17beta-HSDs, which belong to the short-chain alcohol dehydrogenase family, whereas the rat, rabbit, pig and bovine 20alpha-HSDs are members of the aldoketo reductase superfamily, which also includes the 3alpha-HSD family. In this study, we report the cloning, from a human skin cDNA library, of a cDNA that shows, after transfection into human embryonic kidney (HEK-293) cells, high 20alpha-HSD activity but negligible 3alpha- and 17beta-hydroxysteroid dehydrogenase activities. A comparison of the amino acid sequence of the human 20alpha-HSD with those of other related 20alpha- and 3alpha-HSDs indicates that the human 20alpha-HSD shares 79.9, 68.7 and 52.3% identity with rabbit, rat and bovine 20alpha-HSDs, whereas it shows 97, 84 and 65% identity with human type 3, type 1 and rat 3alpha-HSDs. In contrast, the enzyme shares only 15.2 and 15.0% identity with type 1 and type 2 human 17beta-HSDs. DNA analysis predicts a protein of 323 amino acids, with a calculated molecular weight of 36 767 Da. In intact transfected cells, the human 20alpha-HSD preferentially catalyzes the reduction of progesterone to 20alpha-hydroxyprogesterone with a K(m) value of 0.6 microM, the reverse reaction (oxidation) being negligible. In a cell cytosolic preparation, the enzyme could use both NADPH and NADH as cofactors, but NADPH, which gave 4-fold lower K(m) values, was preferred. We detected the expression of 20alpha-HSD mRNA in liver, prostate, testis, adrenal, brain, uterus and mammary-gland tissues and in human keratinocyte (HaCaT) cells. The present study clearly indicates that the genuine human 20alpha-HSD belongs to the aldoketo reductase family, like the 20alpha-HSDs from other species. (+info)
The reactive oxygen species--and Michael acceptor-inducible human aldo-keto reductase AKR1C1 reduces the alpha,beta-unsaturated aldehyde 4-hydroxy-2-nonenal to 1,4-dihydroxy-2-nonene.
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The human aldo-keto reductase AKR1C1 (20alpha(3alpha)-hydroxysteroid dehydrogenase) is induced by electrophilic Michael acceptors and reactive oxygen species (ROS) via a presumptive antioxidant response element (Burczynski, M. E., Lin, H. K., and Penning, T. M. (1999) Cancer Res. 59, 607-614). Physiologically, AKR1C1 regulates progesterone action by converting the hormone into its inactive metabolite 20alpha-hydroxyprogesterone, and toxicologically this enzyme activates polycyclic aromatic hydrocarbon trans-dihydrodiols to redox-cycling o-quinones. However, the significance of its potent induction by Michael acceptors and oxidative stress is unknown. 4-Hydroxy-2-nonenal (HNE) and other alpha,beta-unsaturated aldehydes produced during lipid peroxidation were reduced by AKR1C1 with high catalytic efficiency. Kinetic studies revealed that AKR1C1 reduced HNE (K(m) = 34 microm, k(cat) = 8.8 min(-1)) with a k(cat)/K(m) similar to that for 20alpha-hydroxysteroids. Six other homogeneous recombinant AKRs were examined for their ability to reduce HNE. Of these, AKR1C1 possessed one of the highest specific activities and was the only isoform induced by oxidative stress and by agents that deplete glutathione (ethacrynic acid). Several hydroxysteroid dehydrogenases of the AKR1C subfamily catalyzed the reduction of HNE with higher activity than aldehyde reductase (AKR1A1). NMR spectroscopy identified the product of the NADPH-dependent reduction of HNE as 1,4-dihydroxy-2-nonene. The K(m) of recombinant AKR1C1 for nicotinamide cofactors (K(m) NADPH approximately 6 microm, K(m)(app) NADH >6 mm) suggested that it is primed for reductive metabolism of HNE. Isoform-specific reverse transcription-polymerase chain reaction showed that exposure of HepG2 cells to HNE resulted in elevated levels of AKR1C1 mRNA. Thus, HNE induces its own metabolism via AKR1C1, and this enzyme may play a hitherto unrecognized role in a response mounted to counter oxidative stress. AKRs represent alternative GSH-independent/NADPH-dependent routes for the reductive elimination of HNE. Of these, AKR1C1 provides an inducible cytosolic barrier to HNE following ROS exposure. (+info)
Characterization of the oxidative 3alpha-hydroxysteroid dehydrogenase activity of human recombinant 11-cis-retinol dehydrogenase.
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11-cis-Retinol dehydrogenase catalyzes the oxidation of cis-retinols, a rate-limiting step in the biosynthesis of 9-cis-retinoic acid. It is also active toward 3alpha-hydroxysteroids, and thus might be involved in steroid metabolism. To better understand the role of this enzyme, we produced stable transfectants expressing 11-cis-retinol dehydrogenase in human embryonic kidney 293 cells. In vitro enzymatic assays have demonstrated that, with an appropriate exogenous cofactor, the enzyme catalyzes the interconversion of 5alpha-androstane-3alpha,17beta-diol and dihydrotestosterone and that of androsterone and androstanedione. However, using intact transfected cells, we found that the enzyme catalyzes reactions only in the oxidative direction. Thus, it is possible that 5alpha-androstane-3alpha,17beta-diol (an inactive androgen) can be converted into dihydrotestosterone, the most potent androgen, by the action of 11-cis-retinol dehydrogenase. This reaction could constitute a non-classical pathway of production of active androgens in the peripheral tissues. We also showed that all-trans-, 9-cis- and 13-cis-retinol inhibit the oxidative 3alpha-hydroxysteroid steroid activity of 11-cis-retinol dehydrogenase with similar K(i) values. Since all-trans-retinol is a precursor of cis-retinols, its inhibitory effect on the activity suggests that it could play an important role in modulating the formation of 9-cis-retinoic acid. In addition, we examined the effect of several known enzyme modulators, namely carbenoxolone, phenylarsine oxide and phosphatidylcholine, on 11-cis-retinol dehydrogenase activity. Taken together, our results suggest that, in humans, this enzyme might play a role in the biosynthesis of both 9-cis-retinoic acid and dihydrotestosterone. (+info)
Dietary indoles and isothiocyanates that are generated from cruciferous vegetables can both stimulate apoptosis and confer protection against DNA damage in human colon cell lines.
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The natural indoles 3,3'-diindolylmethane (DIM), ascorbigen (ASG), indole-3-carbinol (I3C), and indolo[3,2-b]carbazole (ICZ), as well as the natural isothiocyanates sulforaphane (SUL), benzyl isothiocyanate (BITC) and phenethyl isothiocyanate (PEITC), all possess cancer chemopreventive properties. It is now shown that DIM, ICZ, SUL, and BITC can each stimulate apoptosis in human colon adenocarcinoma LS-174 and Caco-2 cells. Treatment of LS-174 cells with nontoxic doses of DIM, ASG, I3C, or ICZ affected an increase of up to 21-fold in cytochrome P450 1A1 (CYP1A1). None of these indoles caused an elevation in either aldo-keto reductase 1C1 (AKR1C1) or the gamma-glutamylcysteine synthetase heavy subunit (GCS(h)), but DIM, I3C, and ICZ produced a very modest increase in NAD(P)H:quinone oxidoreductase 1 (NQO1). By contrast, nontoxic doses of SUL, BITC, or PEITC failed to induce expression of CYP1A1 in LS-174 cells, but caused an increase of between 11- and 17-fold in the protein levels of AKR1C1, NQO1, and GCS(h). Treatment of the colon cell line with ICZ or SUL caused increases in the levels of mRNA for CYP1A1, AKR1C1, and NQO1 that were consistent with the enzyme data. Exposure of Caco-2 cells to media containing indoles or isothiocyanates gave similar results to those obtained using LS-174 cells. Evidence is presented that the ability of indoles and isothiocyanates to stimulate either xenobiotic response element- or antioxidant response element-driven gene expression accounts for the two groups of phytochemicals inducing different gene batteries. Pretreatment of LS-174 cells for 24 h with ICZ and SUL before exposure for 24 h to benzo(a)pyrene (BaP) reduced to <20% the number of single-strand DNA breaks produced by the carcinogen. Neither ICZ alone nor SUL alone were able to confer the same degree of protection against DNA damage produced by BaP as they achieved in combination. Similar results were obtained with H(2)O(2) as the genotoxic agent. Together, these phytochemicals may prevent colon tumorigenesis by both stimulating apoptosis and enhancing intracellular defenses against genotoxic agents. (+info)