Characterisation of an associate 17-beta-hydroxysteroid dehydrogenase activity and affinity labelling of the 3-alpha-hydroxysteroid dehydrogenase of Pseudomonas testosteroni

The 3-alpha-hydroxysteroid dehydrogenase and the 3-beta-hydroxysteroid dehydrogenase of Pseudomonas testosteroni were purified to homogeneity by polyaerylamide gel electrophoresis using the following stages: DEAE cellulose chromatography, affinity chromatography on oestrone-aminocaproate sepharose and Sephadex gel filtration. The pure 3-alpha-hydroxysteroid dehydrogenase was completely devoid of 3-beta-hydroxysteroid dehydrogenase activity but could oxidize estradiol 17-beta at an appreciable rate. This activity accounts for about 40 per cent of the total 17-beta-estradiol dehydrogenase of the crude bacterial extract. Affinity labelling of pure 3-alpha-hydroxysteroid dehydrogenase was carried out using 5-beta-pregnane 3,20-dione-12-alpha-iodoacetate and 5-alpha-androstane 3-one-17-beta-bromoacetate. With both reagents, inactivation was obtained only in the presence of coenzyme, the substrate protected against inactivation and the enzyme was fully inhibited with covalent binding of 1 mole of reagent per mole of subunit suggesting an active site directed inhibition. Histidine and methionine were identified as the labelled aminoacid residues.     

Differential estrogenic 17β-hydroxysteroid dehydrogenase activity and type 12 17β-hydroxysteroid dehydrogenase expression levels in preadipocytes and differentiated adipocytes

Estradiol (E2) is produced locally in adipose tissue and could play an important role in fat distribution and accumulation, especially in women. It is well recognized that aromatase is expressed in adipose tissue; however the identity of its estrogenic 17β-hydroxysteroid dehydrogenase (17β-HSD) partner is not identified. To gain a better knowledge about the enzyme responsible for the conversion of estrone into estradiol, we determined the activity and expression levels of known estrogenic 17β-HSDs, namely types 1, 7 and 12 17β-HSD in preadipocytes before and after differentiation into mature adipocytes using an adipogenic media. Estrogenic 17β-HSD activity was assessed using [¹⁴C]-labelled estrone, while mRNA expression levels of types 1, 7 and 12 17β-HSD were quantified using real-time PCR and protein expression levels of type 12 17β-HSD was determined using immunoblot analysis. The data indicate that there is a low conversion of E1 into E2 in preadipocytes; however this activity is increased 5-fold (p < 0.0001) in differentiated adipocytes. The increased estrogenic 17β-HSD activity is consistent with the increase in protein expression levels of 17β-HSD12.     

Luteolytic action of RU486: Modulation of luteal 3β-hydroxysteroid dehydrogenase and 20α-hydroxysteroid dehydrogenase activities in late pregnant rats

The effect of the synthetic antiprogestin RU486 on luteal function in late pregnant rats was studied by evaluating the activities of the enzymes 3β-hydroxysteroid dehydrogenase (3β-HSD) and 20α-hydroxysteroid dehydrogenase (20α-HSD). RU486 (2 mg/kg) administered to rats on day 18 of pregnancy at 10.00 h induced preterm delivery 26.4 ± 0.35 h (n = 8) after treatment. Luteal 3β-HSD activity increased 24 and 34 h after RU486 injection, but a significant and progressive decrease started at 48 h with the maximal reduction 72 h after RU486 treatment, when compared with controls. Serum progesterone concentration decreased at the time of 3β-HSD activity reduction. Interestingly, 20α-HSD activity started to increase 58 h after RU486 injection. The administration of the cyclooxygenase inhibitor, diclofenac (1.3 mg/kg), on days 17–19 of pregnancy to RU486-treated rats, delayed abortion and the duration of delivery, and prevented the decrease in 3β-HSD and the increase in 20α-HSD activities observed 58 h after antiprogesterone treatment. RU486 administered intrabursally (1 μg per ovary) on day 20 (14.00–15.00 h) increased 3β-HSD and decreased 20α-HSD luteal activities at 18.00 h on day 21 of pregnancy, without modifying serum progesterone concentration, when compared with normal pregnant rats. In conclusion, the luteolytic process after preterm delivery induced by RU486 administration in late pregnant rats is characterized by a decrease in luteal 3β-HSD activity and circulating progesterone, which may trigger the increase in luteal 20α-HSD activity. Prostaglandins seems to be involved in the increase of 20α-HSD activity and therefore, in the demise of corpora lutea.     

The subcellular localization of 17β-hydroxysteroid dehydrogenase type 4 and its interaction with actin

The porcine 17β-hydroxysteroid dehydrogenase type 4 is the key enzyme for the inactivation of estradiol. Its localization in peroxisomes was proven by immunogold electron microscopy. Interactions of the 17β-hydroxysteroid dehydrogenase with cytoskeletal proteins might be mandatory for a topical assignment of enzymatic activity to defined subcellular compartments.     

Distribution of 3α-hydroxysteroid dehydrogenase in rat brain and molecular cloning of multiple cDNAs encoding structurally related proteins in humans

3α-Hydroxysteroid dehydrogenase in the brain is responsible for production of neuroactive tetrahydrosteroids that interact with the major inhibitory gamma-aminobutyric acid receptor complexes. Distribution of 3α-hydroxysteroid dehydrogenase in different regions of the brain in rats was evaluated by activity assay and by Western immunoblotting using a monoclonal antibody against liver 3α-hydroxysteroid dehydrogenase as the probe. The olfactory bulb was found to contain the highest level of 3α-hydroxysteroid dehydrogenase activity, while moderate levels of the enzyme activity were found in other regions such as cerebellum, cerebral cortex, hypothalamus and pituitary. Some activity was found in the rest of the brain such as amygdala, brain stem, caudate putamen, cingulate cortex, hippocampus, midbrain, and thalamus. The protein levels of 3α-hydroxysteroid dehydrogenase in different regions of the brain as detected by Western immunoblotting are comparable to those of the enzyme activity. We used the rat cDNA as the probe to screen a human liver λ gt11 cDNA library. A total of four different cDNAs were identified and sequenced. One of the cDNAs is identical to that of the human chlordecone reductase cDNA except that our clone contains a much longer 5′-coding sequence than previously reported. The other three cDNAs display high degrees of sequence homology to those of both rat 3α-hydroxysteroid dehydrogenase and human chlordecone reductase. We are currently investigating the functional relationship between the enzymes encoded by these human cDNAs and 3α-hydroxysteroid dehydrogenase.     

Unusual evolution of 11β- and 17β-hydroxysteroid and retinol dehydrogenases

11β-hydroxysteroid dehydrogenases regulate glucocorticoid concentrations and 17β-hydroxysteroid dehydrogenases regulate estrogen and androgen concentrations in mammals. Phylogenetic analysis of the sequences from two 11β-hydroxysteroid dehydrogenases and four mammalian 17β-hydroxysteroid dehydrogenases indicates unusual evolution in these enzymes. Type 1 11β- and 17β-hydroxysteroid dehydrogenases are on the same branch; Type 2 enzymes cluster on another branch with β-hydroxybutyrate dehydrogenase, 11-cis retinol dehydrogenase and retinol dehydrogenase; Type 3 17β-hydroxysteroid dehydrogenase is on a third branch; while the pig dehydrogenase clusters with a yeast multifunctional enzyme on a fourth branch. Pig 17β-hydroxysteroid dehydrogenase appears to have evolved independently from the other three 17β-hydroxysteroid dehydrogenase dehydrogenases; in which case, the evolution of 17β-hydroxysteroid dehydrogenase activity is an example of functional convergence. The phylogeny also suggests that independent evolution of specificity toward C11 substituents on glucocorticoids and C17 substituents on androgens and estrogens has occurred in Types 1 and 2 11β- and 17β-hydroxysteroid dehydrogenases.     

Expression of human 17β-hydroxysteroid dehydrogenase in mammalian cells

The insert of 1278 bp containing the entire coding region of cDNA encoding human 17β-hydroxysteroid dehydrogenase (17β-HSD) was inserted into a pHS1 vector and expressed in HeLA human cervical carcinoma cells and COS-1 monkey kidney tumor cells. Western blot analysis indicated that the expressed protein migrates at the same position as the purified enzyme and is recognized by the antibody raised against purified human placental 17β-HSD. The expressed enzyme efficiently catalyzes the interconversion of estrone and estradiol while dehydroepiandrosterone and 5-androstene-3β,17β-diol are interconverted at a lower rate. The present data suggest the existence of two 17β-HSDs.     

Parallel solid-phase synthesis of 3β-peptido-3α-hydroxy-5α-androstan-17-one derivatives for inhibition of type 3 17β-hydroxysteroid dehydrogenase

Type 3 17β-hydroxysteroid dehydrogenase (17β-HSD), a key steroidogenic enzyme, transforms 4-androstene-3,17-dione (Δ⁴-dione) into testosterone. In order to produce potential inhibitors, we performed solid-phase synthesis of model libraries of 3β-peptido-3α-hydroxy-5α-androstan-17-ones with 1, 2, or 3 levels of molecular diversity, obtaining good overall yields (23–58%) and a high average purity (86%, without any purification steps) using the Leznoff's acetal linker. The libraries were rapidly synthesized in a parallel format and the generated compounds were tested as inhibitors of type 3 17β-HSD. Potent inhibitors were identified from these model libraries, especially six members of the level 3 library having at least one phenyl group. One of them, the 3β-(N-heptanoyl- -phenylalanine- -leucine-aminomethyl)-3α-hydroxy-5α-androstan-17-one (42) inhibited the enzyme with an IC₅₀ value of 227 nM, which is twice as potent as the natural substrate Δ⁴-dione when used itself as an inhibitor. Using the proliferation of androgen-sensitive (AR⁺) Shionogi cells as model of androgenicity, the compound 42 induced only a slight proliferation at 1 μM (less than previously reported type 3 17β-HSD inhibitors) and, interestingly, no proliferation at 0.1 μM.     

Purification of 5β-reductase from hepatic cytosol fraction of chicken

From the cytosol fraction (supernatant fluid at 105,000 g) of chicken liver, 4-en-3-oxosteroid 5β-reductase (EC 1.3.1.23) was purified by ammonium sulfate precipitation, followed by Butyl Toyopearl, DEAE-Sepharose, Sephadex G-75 and hydroxylapatite column chromatographies. The enzyme activity was quantitated from amount of the 5β-reduced metabolites derived from [4-¹⁴C]testosterone. During the purification procedures, 17β-hydroxysteroid dehydrogenase which was present in the cytosol fraction was separated from 5β-reductase fraction by the Butyl Toyopearl column chromatography. By the DEAE-Sepharose column chromatography, 3α- and 3β-hydroxysteroid dehydrogenases were able to be removed from 5β-reductase fraction. The final enzyme preparation was apparently homogenous on SDS-polyacrylamide gel electrophoresis. Purification was about 13,600-fold from the hepatic cytosol. The molecular weight of this enzyme was estimated as 37,000 Da by SDS-polyacrylamide gel electrophoresis and also by Sephadex G-75 gel filtration. For 5β-reduction of 4-en-3-oxosteroids, such as testosterone, androstenedione and progesterone, NADPH was specifically required as cofactor. K_m of 5β-reductase for NADPH was estimated as 4.22 × 10⁻⁶M and for testosterone, 4.60 × 10⁻⁶M. The optimum pH of this enzyme ranged from pH 5.0 to 6.5 and other enzymic properties of the 5β-reductase were examined.     

Differential inhibition of dehydrogenase and 5-ene→4-ene isomerase activities of purified 3β-hydroxysteroid dehydrogenase. Evidence for two distinct sites

The success in synthesis of [³H]5-androstene-3,17-dione, the intermediate product in the transformation of DHEA to 4-androstenedione by 3β-hydroxysteroid dehydrogenase/ 5-ene→4-ene isomerase (3β-HSD) offers the opportunity to determine whether or not the two activities reside in one active site or in two closely related active sites. The finding that N,N-dimethyl-4-methyl-3-oxo-4-aza-5-androstane-17β-carboxamide (4-MA) inhibits competitively and specifically the dehydrogenase activity whereas a non-competitive inhibition type with a K_i value 1000 fold higher was observed for the isomerase activity, indicated that dehydrogenase and isomerase activities belong to separate sites. Using 5-dihydro-testosterone and 5-androstane-3β,17β-diol, exclusive substrates for dehydrogenase activity, it was shown that dehydrogenase is reversible and strongly inhibited by 4-MA and that thus the irreversible step in the transformation of DHEA to 4-androstenedione is due to the isomerase activity.     

Structures important in mammalian 11β- and 17β-hydroxysteroid dehydrogenases

We have used the X-ray crystallographic structures of rat and human dihydropteridine reductase and Streptomyces hydrogenans 20β-hydroxysteroid dehydrogenase to model parts of the 3-dimensional structure of human 11β- and 17β-hydroxysteroid dehydrogenases. We use this information along with previous results from studies of Drosophila alcohol dehydrogenase mutants to analyze the structures in binding sites for NAD(H) and NADP(H) in 11β-hydroxysteroid dehydrogenase-types 1 and 2. We also examine the structure of an -helix at catalytic site of 17β-hydroxysteroid dehydrogenase-types 1, 2, 3, and 4. This -helix contains a highly conserved tyrosine and lysine. Adjacent to the carboxyl side of this lysine is a site proposed to be important in subunit association. We find that 11β- and 17β-hydroxysteroid dehydrogenases-type 1 have the same residues at the “anchor site” and conserve other stabilizing features, despite only 20% sequence identity between their entire sequences. Similar conservation of stabilizing structures is found in the 11β- and 17β-hydroxysteroid dehydrogenases-type 2. We suggest that interactions of the dimerization surface of -helix F with proteins or membranes may be important in regulating activity of hydroxysteroid dehydrogenases.     

Molecular cloning of multiple cDNAs encoding human enzymes structurally related to 3α-hydroxysteroid dehydrogenase

Rat liver 3α-hydroxysteroid dehydrogenase cDNA was previously cloned by us. In this study, we used the rat cDNA as the probe to screen a human liver lambda gt11 cDNA library. A total of four different cDNAs were identified and sequenced. The sequence of one of the cDNAs is identical to that of the human chlordecone reductase cDNA except that our clone contains a much longer 5′-coding sequence than previously reported. The other three cDNAs display high degrees of sequence homology to those of both rat 3α-hydroxysteroid dehydrogenase and human chlordecone reductase. Because 3α-hydroxysteroid dehydrogenase and human chlordecone reductase belong to the aldo-keto reductase superfamily, we named these human clones HAKRa to HAKRd. Northern blot analysis showed that the liver expresses the highest levels of all four clones. Expression of all four clones was also detected in the brain, kidney, lung, and testis, whereas the placenta expressed only the messenger RNA for HAKRb. Genomic blot analysis using HAKRb as the probe detected multiple DNA fragments hybridized to the probe and a high degree of restriction fragment length polymorphism, suggesting the complexity of this supergene family.     

Human 17β-hydroxysteroid dehydrogenase: overproduction using a baculovirus expression system and characterization

Estrogenic 17β-hydroxysteroid dehydrogenase (17β-HSD) plays a pivotal role in the synthesis of estrogens. We overproduced human placental estrogenic 17β-HSD using a baculovirus expression system for the study of the enzyme mechanism. A cDNA encoding the entire open reading frame of human 17β-HSD was inserted into the genome of Autographa californica nuclear polyhedrosis virus and expressed in Spodoptera frugiperda (Sf9) insect cells. Metabolic labeling and Western blot analysis using polyclonal antibodies raised against native human 17β-HSD indicated that a molecule with an apparent mass of 35 kDa was maximally expressed 60 h after infection. At that time interval, intracellular 17β-HSD activity reached 0.26 U/mg of protein in crude homogenate, about 70 times the level measured in human placenta. Purification of recombinant 17β-HSD was achieved by a single affinity fast liquid protein chromatography step yielding 24 mg of purified 17β-HSD protein per liter of suspension culture, with a specific activity of about 8 μmol/min/mg of protein for conversion of estradiol into estrone, at pH 9.2. In addition, the recombinant protein purified from infected Sf9 cells was assembled as a dimer with molecular mass and specific activity identical to those of the enzyme purified directly from placenta. The present data show that the baculovirus expression system can provide active 17β-HSD that is functionally identical to its natural counterpart and easy to purify in quantities suitable for its physico-chemical studies.     

5-Phosphorylribose 1-α-methylenebisphosphonate: Properties of a substrate analog of 5-phosphorylribose 1-α-diphosphate

The enzymatic synthesis of 5-phosphorylribose 1-α-methylenebisphosphonate (PRPCP), an analog of 5-phosphorylribose 1-α-diphosphate (PRPP), has been achieved by incubating Mg²⁺, β,γ-methylene ATP, and ribose 5-phosphate with pure Salmonella typhimurium PRPP synthetase (EC 2.7.6.1). The PRPCP was purified from the reaction mixture by ion-exchange chromatography, and was isolated as the ammonium salt. It was characterized by phosphate and ribose contents, and by ³¹P NMR spectroscopy. A study of the rates of hydrolysis of PRPP and PRPCP at 37°C shows that the methylene analog is more stable to chemical hydrolysis at pH's 4, 7, and 10. The products of base hydrolysis of PRPCP are methylenebisphophonate and ribose 5-phosphate. PRPCP serves as a good alternate substrate for mammalian orotate phosphoribosyltransferase (EC 2.4.2.10), but is a very poor substrate for this enzyme derived from yeast. PRPCP should be a useful analog in kinetic studies of phosphoribosyl transferases because its chemical decomposition product, methylene bisphosphonate, is identical to the nonnucleotide product produced by these enzymes.     

Rapid purification yielding highly active 17β-hydroxysteroid dehydrogenase: application of hydrophobic interaction and affinity fast protein liquid chromatography

Homogeneous human placental 17β-hydroxysteroid dehydrogenase was obtained by a procedure consisting of two fast protein liquid chromatographic (FPLC) steps using Phenyl-Sepharose hydrophobic interaction and Blue-Sepharose affinity columns. In the first chromatography, the enzyme eluted only when an additional decrease in ionic strength was inserted after the ammonium sulphate concentration had reached zero, thus enhancing the separation. In the affinity chromatography, separation of contaminating proteins occurred at different stages of loading and washing. The specific elution of the enzyme by the co-factor NADP⁺ is very efficient in obtaining a homogeneous preparation in high yield. The rapidity of FPLC was further increased by a maximum simplification of the intermediate steps, and the whole procedure lasted only two days. This preparation has a yield of more than 50% and a high specific activity, catalysing the formation of 7.9 μmol of estrone from estradiol per minute at pH 9.2 and 23°C. It has an apparent molecular mass of 35 000. This provides an efficient candidate for the purification of other membrane-associated proteins.     

Distribution of 17β-hydroxysteroid dehydrogenase gene expression and activity in rat and human tissues

The interconversion of estrone (E₁) and 17β-estradiol (E₂), androstenedione (4-ene-dione) and testosterone (T), as well as dehydroepiandrosterone and androst-5-ene-3β,17β-diol is catalyzed by 17β-hydroxysteroid dehydrogenase (17β-HSD). The enzyme 17β-HSD thus plays an essential role in the formation of all active androgens and estrogens in gonadal as well as extragonadal tissues. The present study investigates the tissue distribution of 17β-HSD activity in the male and female rat as well as in some human tissues and the distribution of 17β-HSD mRNA in some human tissues. Enzymatic activity was measured using ¹⁴C-labeled E₁, E₂, 4-ene-dione and T as substrates. Such enzymatic activity was demonstrated in all 17 rat tissues examined for both androgenic and estrogenic substrates. While the liver had the highestlevel of 17β-HSD activity, low but significant levels of E₂ as well as T formation were found in rat brain, heart, pancreas and thymus. The oxidative pathway (E₂→E₁, T→4-ene-dione) was favored over the reverse reaction in almost all rat tissues while in the human, almost equal rates were found in most of the 15 tissues examined. The widespread distribution of 17β-HSD in rat and human tissues clearly indicates the importance of this enzyme in peripheral sex steroid formation or intracrinology.     

Preparation of 12-ketochenodeoxycholic acid from cholic acid using coimmobilized 12α-hydroxysteroid dehydrogenase and glutamate dehydrogenase with NADP cycling at high efficiency

12-Ketochenodeoxycholic acid, an essential intermediate in the synthesis of chenodeoxycholic acid, has been enzymatically prepared from cholic acid. The specific oxidation of the 12α-hydroxyl group of cholic acid with NADP⁺ was catalysed by 12α-hydroxysteroid dehydrogenase (12α-hydroxysteroid: NAD⁺ oxidoreductase, EC 1.1.1.176), and the regeneration of NADP⁺ was obtained through the glutamate dehydrogenase (l-glutamate:NADP⁺ oxidoreductase, EC 1.4.1.4) catalysed reduction of α-ketoglutarate. The two enzymes were immobilized onto Sepharose CL-4B activated with tresyl chloride. The coimmobilized enzymes showed a cycling efficiency for the coenzyme similar to that of the free enzymes. High concentrations of cholic acid (up to 4%, w/v) were completely and specifically transformed into the 12-keto derivative using amounts of cofactor about 1600 times lower on a molar basis. The immobilized enzymes maintained 70% of the initial activity after 2 months of continuous use.     

3β-hydroxysteroid dehydrogenase activity in tissues of the human fetus determined with 5α-androstane-3β,17β-diol and dehydroepiandrosterone as substrates

3β-Hydroxysteroid dehydrogenase (3β-HSD)/Δ^5→4-isomerase activity in steroidogenic tissues is required for the synthesis of biologically active steroids. Previously, by use of dehydroepiandrosterone (3β-hydroxy-5-androsten-17-one, DHEA) as substrate, it was established that in addition to steroidogenic tissues 3β-HSD/Δ^5→4-isomerase activity also is expressed in extraglandular tissues of the human fetus. In the present study, we attempted to determine whether the C-5,C-6-double bond of DHEA serves to influence 3β-HSD activity. For this purpose, we compared the efficiencies of a 3β-hydroxy-5-ene steroid (DHEA) and a 3β-hydroxy-5α-reduced steroid (5α-androstane-3β,17β-diol, 5α-A-diol) as substrates for the enzyme. The apparent Michaelis constant (K_m) for 5α-A-diol in midtrimester placenta, fetal liver, and fetal skin tissues was at least one order of magnitude higher than that for DHEA, viz the apparent K_m of placental 3β-HSD for 5α-A-diol was in the range of 18 to 40 μmol/l (n = 3) vs 0.45 to 4 μmol/l for DHEA (n = 3); for the liver enzyme, 17 μmol/l for 5α-A-diol and 0.60 μmol/l for DHEA, and for the skin enzyme 14 and 0.18 μmol/l, respectively. Moreover, in 13 human fetal tissues evaluated the maximal velocities obtained with 5α-A-diol as substrate were higher than those obtained with DHEA. A similar finding in regard to K_ms and rates of product formation was obtained by use of purified placental 3β-HSD with DHEA, pregnenolone, and 3β-hydroxy-5α-androstan-17-one (epiandrosterone) as substrates: the K_m of 3β-HSD for DHEA was 2.8 μmol/l, for pregnenolone 1.9 μmol/l, and for epiandrosterone 25 μmol/l. The specific activity of the purified enzyme with pregnenolone as substrate was 27 nmol/mg protein·min and, with epiandrosterone, 127 nmol/mg protein·min. With placental homogenate as the source of 3β-HSD, DHEA at a constant level of 5 μmol/l behaved as a competitive inhibitor when the radiolabeled substrate, [³H]5α-A-diol, was present in concentrations of 20 to 60 μmol/l, but a lower substrate concentrations the inhibition was of the mixed type; similar results were obtained with [³H]DHEA as the substrate at variable concentrations in the presence of a fixed concentration of 5α-A-diol (40 μmol/l). These findings are indicative that both steroids bind to a common site on the enzyme, however, the binding affinity for these steroids appear to differ markedly as suggested by the respective K_ms. Studies of inactivation of purified placental 3β-HSD/Δ^5→4-isomerase by an irreversible inhibitor, viz 5,10-secoestr-4-yne-3,10,17-trione, were suggestive that the placental protein adopts different conformations depending on whether the steroidal substrate has a 5α-configuration, e.g. epiandrosterone, or a C-5,C-6-double bond e.g. DHEA or pregnenolone. The lower rates of product formation obtained with placenta and fetal tissues by use of 3β-hydroxy-5-ene steroids as substrates when compared with those obtained with 3β-hydroxy-5α-reduced steroids may be explained by a combination of factors, including: (i) inhibition of 3β-HSD activity by end products of metabolism of 3β-hydroxy-5-ene steroids, e.g. 4-androstene-3,17-dione formed with DHEA as substrate; (ii) higher binding affinity of the enzyme for 3β-hydroxy-5-ene steroids—and possibly for their 3-oxo-5-ene metabolites; (iii) lack of a requirement for the isomerization step with 5α-reduced steroids as substrates, and (iv) the possible presence in fetal tissues of an enzyme with 3β-HSD activity only (i.e. no Δ^5→4-isomerase).     

Intrinsic sterol- and phosphatidylcholine transfer activities of 17β-hydroxysteroid dehydrogenase type IV

Previous studies have shown that the 80 kDa 17β-hydroxysteroid dehydrogenase (17β-HSD) type IV comprises distinct domains, including an N-terminal region related to the short chain alcohol dehydrogenase multigene family and a C-terminal part related to the lipid transfer protein sterol carrier protein 2 (SCP2). In this study, we have investigated whether the SCP2-related part of the 80 kDa protein leads to an intrinsic sterol and phospholipid transfer activity, as shown earlier for the 60 kDa SCP2-related peroxisomal 3-ketoacyl CoA thiolase with intrinsic sterol and phospholipid transfer activity called sterol carrier protein x (SCPx). Our results indicate that a fraction rich in the 80 kDa form of 17β-HSD type IV exhibits high transfer activities for 7-dehydrocholesterol and phosphatidylcholine. In addition, a purified recombinant peptide derived from the SCP2-related domain of the 17β-HSD type IV has about 30% of the transfer activities for 7-dehydrocholesterol and phosphatidylcholine seen with purified recombinant human SCP2. We conclude that the 80 kDa type IV 17β-HSD represents a potentially multifunctional protein with intrinsic in vitro sterol and phospholipid transfer activity in addition to its enzymatic activity.     

Analysis of 17β-hydroxysteroid dehydrogenase types 5, 7, and 12 genetic sequence variants in breast cancer cases from French Canadian Families with high risk of breast and ovarian cancer

A family history and estrogen exposure are well-known risk factors for breast cancer. Members of the 17β-hydroxysteroid dehydrogenase family are responsible for important steps in the metabolism of androgens and estrogens in peripheral tissues, including the mammary gland. The crucial biological function of 17β-HSDs renders these genes good candidates for being involved in breast cancer etiology. This study screened for mutations in HSD17B7 and HSD17B12 genes, which encode enzymes involved in estradiol biosynthesis and in AKR1C3, which codes for 17β-HSD type 5 enzyme involved in androgen and progesterone metabolism, to assess whether high penetrance allelic variants in these genes could be involved in breast cancer susceptibility. Mutation screening of 50 breast cancer cases from non-BRCA1/2 high-risk French Canadian families failed to identify germline likely high-risk mutations in HSD17B7, HSD17B12 and AKR1C3 genes. However, 107 sequence variants were identified, including seven missense variants. Assessment of the impact of missense variants on enzymatic activity of the corresponding enzymes revealed no difference in catalytic properties between variants of 17β-HSD types 7 and 12 and wild-type enzymes, while variants p.Glu77Gly and p.Lys183Arg in 17β-HSD type 5 showed a slightly decreased activity. Finally, a haplotype-based approach was used to determine tagging SNPs providing valuable information for studies investigating associations of common variants in these genes with breast cancer risk.