Novel aspects of hormone action: intracellular ligand supply and its control by a series of tissue specific enzymes

Hydroxysteroid dehydrogenases (HSDs) in bacteria: a bioinformatic perspective

Steroidal compounds including cholesterol, bile acids and steroid hormones play a central role in various physiological processes such as cell signaling, growth, reproduction, and energy homeostasis. Hydroxysteroid dehydrogenases (HSDs), which belong to the superfamily of short-chain dehydrogenases/reductases (SDR) or aldo-keto reductases (AKR), are important enzymes involved in the steroid hormone metabolism. HSDs function as an enzymatic switch that controls the access of receptor-active steroids to nuclear hormone receptors and thereby mediate a fine-tuning of the steroid response. The aim of this study was the identification of classified functional HSDs and the bioinformatic annotation of these proteins in all complete sequenced bacterial genomes followed by a phylogenetic analysis. For the bioinformatic annotation we constructed specific hidden Markov models in an iterative approach to provide a reliable identification for the specific catalytic groups of HSDs. Here, we show a detailed phylogenetic analysis of 3α-, 7α-, 12α-HSDs and two further functional related enzymes (3-ketosteroid-Δ(1)-dehydrogenase, 3-ketosteroid-Δ(4)(5α)-dehydrogenase) from the superfamily of SDRs. For some bacteria that have been previously reported to posses a specific HSD activity, we could annotate the corresponding HSD protein. The dominating phyla that were identified to express HSDs were that of Actinobacteria, Proteobacteria, and Firmicutes. Moreover, some evolutionarily more ancient microorganisms (e.g., Cyanobacteria and Euryachaeota) were found as well. A large number of HSD-expressing bacteria constitute the normal human gastro-intestinal flora. Another group of bacteria were originally isolated from natural habitats like seawater, soil, marine and permafrost sediments. These bacteria include polycyclic aromatic hydrocarbons-degrading species such as Pseudomonas, Burkholderia and Rhodococcus. In conclusion, HSDs are found in a wide variety of microorganisms including bacteria and archaea, suggesting that steroid metabolism is an evolutionarily conserved mechanism that might serve different functions such as nutrient supply and signaling. Article from a special issue on steroids and microorganisms.

Vitamin D analogs: therapeutic applications and mechanisms for selectivity

The vitamin D endocrine system plays a central role in mineral ion homeostasis through the actions of the vitamin D hormone, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], on the intestine, bone, parathyroid gland, and kidney. The main function of 1,25(OH)(2)D(3) is to promote the dietary absorption of calcium and phosphate, but effects on bone, kidney and the parathyroids fine-tune the mineral levels. In addition to these classical actions, 1,25(OH)(2)D(3) exerts pleiotropic effects in a wide variety of target tissues and cell types, often in an autocrine/paracrine fashion. These biological activities of 1,25(OH)(2)D(3) have suggested a multitude of potential therapeutic applications of the vitamin D hormone for the treatment of hyperproliferative disorders (e.g. cancer and psoriasis), immune dysfunction (autoimmune diseases), and endocrine disorders (e.g. hyperparathyroidism). Unfortunately, the effective therapeutic doses required to treat these disorders can produce substantial hypercalcemia. This limitation of 1,25(OH)(2)D(3) therapy has spurred the development of vitamin D analogs that retain the therapeutically important properties of 1,25(OH)(2)D(3), but with reduced calcemic activity. Analogs with improved therapeutic indices are now available for treatment of psoriasis and secondary hyperparathyroidism in chronic kidney disease, and research on newer analogs for these indications continues. Other analogs are under development and in clinical trials for treatment of various types of cancer, autoimmune disorders, and many other diseases. Although many new analogs show tremendous promise in cell-based models, this article will limit it focus on the development of analogs currently in use and those that have demonstrated efficacy in animal models or in clinical trials.

Clinical, endocrine, and molecular findings in 17beta-hydroxysteroid dehydrogenase type 3 deficiency

The 17beta-hydroxysteroid dehydrogenases (17betaHSD) gene family comprises different enzymes involved in the biosynthesis of active steroid hormones. The 17betaHSD type 3 (17betaHSD3) isoenzyme catalyzes the reductive conversion of the inactive C19-steroid, Delta4-androstenedione (Delta4- A), into the biologically active androgen, testosterone (T), in the Leydig cells of the testis. It is encoded by the 17beta-hydroxysteroid dehydrogenase type 3 (HSD17B3) gene, which maps to chromosome 9q22. Mutations in the HSD17B3 gene are associated with a rare form of 46,XY disorder of sex development referred to as 17betaHSD3 deficiency (or as 17-ketosteroid reductase deficiency), due to impaired testicular conversion of Delta4-A into T. 46,XY patients with 17betaHSD3 deficiency are usually classified as female at birth, raised as such, but develop secondary male features at puberty. Diagnosis, and consequently early treatment, is difficult because clinical signs from birth until puberty may be mild or absent. Biochemical diagnosis of 17betaHSD3 deficiency requires measurement of serum T/Delta4-A ratio after hCG stimulation test in pre-pubertal subjects, while baseline values seem to be informative in early infancy and adolescence. However, low basal T/Delta4-A ratio is not specific for 17betaHSD3 deficiency, being sometimes also found in patients with other defects in T synthesis or with Leydig cells hypoplasia. Mutational analysis of the 17HSDB3 gene is useful in confirming the clinical diagnosis of 17betaHSD3 deficiency. This review describes clinical findings, diagnosis, and molecular basis of this rare disease.

Inhibition of 11beta-hydroxysteroid dehydrogenase type 1 in obesity

Excessive glucocorticoid exposure (Cushing's syndrome) results in increased adiposity associated with dysmetabolic features (including insulin resistance, hyperlipidaemia, and hypertension). Circulating cortisol levels are not elevated in idiopathic obesity, although cortisol production and clearance are increased. However, tissue glucocorticoid exposure may be altered independently of circulating levels by 11beta-hydroxysteroid dehydrogenase type 1 (11HSD1), an enzyme which generates active glucocorticoid within tissues, including in adipose tissue. Transgenic overexpression of 11HSD1 in mice causes obesity. In human obesity, 11HSD1 is altered in a tissue-specific manner with reduced levels in liver but elevated levels in adipose, which may lead to glucocorticoid receptor activation and contribute to the metabolic phenotype. The reasons for altered 11HSD1 in obesity are not fully understood. Although some polymorphisms have been demonstrated in intronic and upstream regions of the HSD11B1 gene, the functional significance of these is not clear. In addition, there is mounting evidence that 11HSD1 may be dysregulated secondarily to factors that are altered in obesity, including substrates for metabolism, hormones, and inflammatory mediators. 11HSD1 is a potential therapeutic target for the treatment of the metabolic syndrome. 11HSD1 knockout mice are protected from diet-induced obesity and associated metabolic dysfunction. Although many specific inhibitors of 11HSD1 have now been developed, and published data support their efficacy in the liver to reduce glucose production, their efficacy in enhancing insulin sensitivity in adipose tissue remains uncertain. The therapeutic potential of 11HSD1 in human obesity therefore remains highly promising but as yet unproven.

11β-Hydroxysteroid dehydrogenase Type 1 as a novel therapeutic target in metabolic and neurodegenerative disease

11beta-hydroxysteroid dehydrogenase Type 1 (11HSD1) catalyses regeneration of active 11-hydroxy glucocorticoids from inactive 11-keto metabolites within target tissues. Inhibition of 11HSD1 has been proposed as a novel strategy to lower intracellular glucocorticoid concentrations, without affecting circulating glucocorticoid levels and their responsiveness to stress. Increased 11HSD1 activity may be pathogenic, for example, in adipose tissue in obesity. Experiments in transgenic mice and using prototype inhibitors in humans show benefits of 11HSD1 inhibition in liver, adipose and brain tissue in treating features of the metabolic syndrome and cognitive dysfunction with ageing. The clinical development of potent selective 11HSD1 inhibitors is now a high priority.

11 beta-hydroxysteroid dehydrogenase type 1 in obesity and the metabolic syndrome

11 beta-Hydroxysteroid dehydrogenase type 1 (11HSD1) catalyses the in vivo conversion of inactive to active glucocorticoids. It is a widespread, highly regulated enzyme which amplifies the ligand available for intracellular glucocorticoid receptors. Excessive glucocorticoid exposure causes central obesity, hypertension, dyslipidaemia and insulin resistance, as seen with elevated plasma cortisol in Cushing's syndrome. Transgenic mice over-expressing 11HSD1 in their white adipose tissue are obese, hypertensive, dyslipidaemic and insulin resistant. Further, 11HSD1 knockout mice are protected from these metabolic abnormalities. In human idiopathic obesity, circulating cortisol levels are not elevated but 11HSD1 mRNA and activity is increased in subcutaneous adipose. The impact of increased adipose 11HSD1 on pathways leading to metabolic complications remains unclear in humans. Pharmacological inhibition of 11HSD1 has been achieved in liver with carbenoxolone, which enhances hepatic insulin sensitivity. Newer selective 11HSD1 inhibitors are in development, which may achieve reduced cortisol action in adipose tissue and confer therapeutic benefit in obese patients.

De la biologie à la clinique : le décès dû au cancer de la prostate peut-il maintenant être une exception ?

The most significant discovery of the second half of the XXth century in the field of prostate cancer therapy is probably the observation that the human prostate, as well as many other peripheral human tissues, synthesize locally an important amount of androgens from the inactive steroid precursors dehydroepiandrosterone (DHEA) and its sulfate DHEA-S. In parallel with these observations, two important discoveries also made by our group are applied in the clinic worldwide, namely the use of LHRH (luteininizing hormone-releasing hormone) agonists to completely block testicular androgens, while, simultaneously, the androgens made locally in the prostate from DHEA are blocked in their access to the androgen receptor by a pure antiandrogen of the class of flutamide. This treatment, called combined androgen blockade, has been the first treatment demonstrated to prolong life in prostate cancer. While the first studies were performed in patients with advanced and metastatic disease, our recent data indicate a remarkable level of efficacy of the same treatment applied to localized prostate cancer, namely a 90% possibility of cure. However, in order to be able to treat localized prostate cancer, early diagnosis must be achieved. In the first large-scale randomized study of prostate cancer screening, we have demonstrated that 99% of prostate cancers can be diagnosed at the localized or potentially curable stage, using simple annual measurement of PSA (prostatic specific antigen). Today's data show that with the simple application of the available diagnostic and therapeutic tools, death from prostate cancer should be an exception.

Intracrinology and the skin

The skin, the largest organ in the human body, is composed of a series of androgen-sensitive components that all express the steroidogenic enzymes required to transform dehydroepiandrosterone (DHEA) into dihydrotestosterone (DHT). In fact, in post-menopausal women, all sex steroids made in the skin are from adrenal steroid precursors, especially DHEA. Secretion of this precursor steroid by the adrenals decreases progressively from the age of 30 years to less than 50% of its maximal value at the age of 60 years. DHEA applied topically or by the oral route stimulates sebaceous gland activity, the changes observed being completely blocked in the rat by a pure antiandrogen while a pure antiestrogen has no significant effect, thus indicating a predominant or almost exclusive androgenic effect. In human skin, the enzyme that transforms DHEA into androstenedione is type 1 3beta-hydroxysteroid dehydrogenase (type 1 3beta-HSD) as revealed by RNase protection and immunocytochemistry. The conversion of androstenedione into testosterone is then catalyzed in the human skin by type 5 17beta-HSD. All the epidermal cells and cells of the sebaceous glands are labelled by type 5 17beta-HSD. This enzyme is also present at a high level in the hair follicles. Type 1 is the 5alpha-reductase isoform responsible in human skin for the conversion of testosterone into DHT. In the vagina, on the other hand, DHEA exerts mainly an estrogenic effect, this effect having been demonstrated in the rat as well as in post-menopausal women. On the other hand, in experimental animals as well as in post-menopausal women, DHEA, at physiological doses, does not affect the endometrial epithelium, thus indicating the absence of DHEA-converting enzymes in this tissue, and avoiding the need for progestins when DHEA is used as hormone replacement therapy.

Pathophysiology of modulation of local glucocorticoid levels by 11beta-hydroxysteroid dehydrogenases

11beta-Hydroxysteroid dehydrogenases (11beta HSDs) are enzymes that catalyse the interconversion of active glucocorticoids (cortisol and corticosterone) into their inactive 11-keto products (cortisone and 11-deoxycorticosterone). Two isozymes have been identified: 11beta HSD type 1 is a predominant reductase, reactivating glucocorticoids from inert metabolites, whereas 11beta HSD type 2 is a potent dehydrogenase, inactivating glucocorticoids. They play a major role in the modulation of local cortisol levels and hence access of active steroid to corticosteroid receptors. This review focuses on the clinical importance of 11beta HSDs. We describe recent research that has not only advanced our understanding of the physiological role of these enzymes, but also their role in common diseases, including primary obesity and essential hypertension. These data provide encouragement that novel therapies will arise from a fuller understanding of the 11beta HSD system.

17beta-hydroxysteroid dehydrogenase from the fungus Cochliobolus lunatus: structural and functional aspects

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) activity has been described in all filamentous fungi tested, but until now only one 17beta-HSD from Cochliobolus lunatus (17beta-HSDcl) was sequenced. We examined the evolutionary relationship among 17beta-HSDcl, fungal reductases, versicolorin reductase (Ver1), trihydroxynaphthalene reductase (THNR), and other homologous proteins. In the phylogenetic tree 17beta-HSDcl formed a separate branch with Ver1, while THNRs reside in another branch, indicating that 17beta-HSDcl could have similar function as Ver1. The structural relationship was investigated by comparing a model structure of 17beta-HSDcl to several known crystal structures of the short chain dehydrogenase/reductase (SDR) family. A similarity was observed to structures of bacterial 7alpha-HSD and plant tropinone reductase (TR). Additionally, substrate specificity revealed that among the substrates tested the 17beta-HSDcl preferentially catalyzed reductions of steroid substrates with a 3-keto group, Delta(4) or 5alpha, such as: 4-estrene-3,17-dione and 5alpha-androstane-3,17-dione.

Searching for the physiological function of 17beta-hydroxysteroid dehydrogenase from the fungus Cochliobolus lunatus: studies of substrate specificity and expression analysis

17beta-hydroxysteroid dehydrogenase from the filamentous fungus Cochliobolus lunatus (17beta-HSDcl) has recently been characterized. Since its function is still unclear, we performed substrate specificity studies to obtain some indications about its physiological function. Different steroids were studied as putative substrates of recombinant 17beta-HSDcl, androgens and estrogens, brassinosteroids, and the fungal steroid herbarulid. Among these androgens and estrogens were most efficiently converted. The following substrates in decreasing order were best reduced: 4-estrene-3,17-dione, 5alpha-androstane-3,17-dione, 4-androstene-3,17-dione and estrone. Two typical inhibitors were tested: carbenoxolone--a representative inhibitor of the SDR family and quercetin--a diagnostic inhibitor of carbonyl reductases. Among these two quercetin was more efficient. Expression studies revealed that 17beta-HSDcl is mainly expressed in the stationary phase of growth indicating its possible involvement in secondary metabolism.

Expression of 17beta-hydroxysteroid dehydrogenases in mesophilic and extremophilic yeast

17beta-hydroxysteroid dehydrogenases (17beta-HSDs) are enzymes responsible for reversible interconversions of biologically active 17-hydroxy and inactive 17-keto steroids. We have performed a survey of 17beta-HSD activity in yeast. Constitutive 17beta-HSD activity was found in three mesophilic yeast species: Candida tropicalis, Cryptococcus tsukubaensis, and Saccharomyces cerevisiae as well as in three extremophilic black yeast species: Hortaea werneckii, Trimmatostroma salinum, and Phaeotheca triangularis, indicating that 17beta-HSD activity is widely distributed among yeast. In extremophilic black yeast, NaCl modulated enzyme activity. Enzymes resembling 17beta-HSD from the filamentous fungus Cochliobolus lunatus were detected in Trimmatostroma salinum and Phaeotheca triangularis. Sequences with identity to the Saccharomyces cerevisiae YBR159w gene were not observed in other yeast species possessing a similar enzyme activity. The results suggest the existence of at least three different types of 17beta-HSD in yeast.

Role of 17 beta-hydroxysteroid dehydrogenases in sex steroid formation in peripheral intracrine tissues

In postmenopausal women, almost 100% of active sex steroids are synthesized in peripheral target tissues from inactive steroid precursors and, in adult men, approximately 50% of androgens are made locally in target tissues. This new field of endocrinology has been called intracrinology. The last and key step in the formation of all estrogens and androgens is catalyzed by a series of substrate-specific, cell-specific and unidirectional 17 beta-hydroxysteroid dehydrogenases (17 beta-HSDs). To date, seven human 17 beta-HSDs have been cloned, sequenced and characterized. The 17 beta-HSDs provide each cell with the means of precisely controlling the intracellular concentration of each sex steroid according to local needs.

Obesity and cortisol

Cortisol in obesity is a much-studied problem. Previous information indicates that cortisol secretion is elevated but that circulatory concentrations are normal or low, suggesting that peripheral disappearance rate is elevated. These studies have usually not taken into account the difference between central and peripheral types of obesity. Recent studies using saliva cortisol have indicated that the problem is complex with both high and low secretion of cortisol, perhaps depending on the status of the function of the hypothalamic-pituitary-adrenal gland axis. A significant background factor seems to be environmental stress. The results also suggest that the pattern of cortisol secretion may be important. Other neuroendocrine pathways are also involved, including the central sympathetic nervous system, the gonadal and growth hormone axes, and the leptin system. In concert, these abnormalities seem to be responsible for the abnormal metabolism often seen in central obesity. Several associated polymorphisms of candidate genes may provide a genetic background. Cortisol conversion to inactive metabolites may be a factor increasing central signals to secretion and may add to the increased secretion of cortisol induced by centrally acting factors. Perinatal factors have been found to be involved in the pathogenesis of obesity and its complications. The mechanism involved is not known, but available information suggests that programming of the hypothalamic-pituitary-adrenal axis may be responsible.

Characterization of fungal 17beta-hydroxysteroid dehydrogenases

To promote understanding of the evolution of the steroid hormone signalling and hydroxysteroid dehydrogenases (HSDs), comparative characterization of fungal 17beta-HSDs was performed. Constitutive 17beta-HSD activity was determined in cytosols of the fungi: Cochliobolus lunatus, Pleospora herbarum, Fusarium lini, Trichoderma viride, Mucor spinosus, Rhizopus nigricans and Pleurotus ostreatus. The reaction equilibrium in all species except P. ostreatus was shifted towards reduction. The preferential coenzyme for reduction of androstenedione was NADPH, while for oxidation of testosterone, NAD4 was preferred. The highest enzyme activities were found in the Ascomycete C. lunatus (152.4 nmol mg(-1) h(-1)) and in the Basidiomycete P. ostreatus (69.1 nmol mg(-1) h(-1)). No similarities on the protein and mRNA level between fungal 17beta-HSDs and the purified enzyme from C. lunatus were observed. To investigate the nature of these enzymes, 17beta-HSD was purified from P. ostreatus using ammonium sulphate precipitation, hydrophobic interaction chromatography, and affinity chromatography. The purified enzyme has an apparent molecular mass of approximately 35 kDa and is probably a dimer as determined by gel filtration. Chemical modifications exposed Lys, His and Tyr as important for enzyme activity. Additionally, no similarities of C. lunatus and P. ostreatus enzymes were found to bacterial 3alpha,20beta-HSD from Streptomyces hydrogenans, 3beta,17beta-HSD from Comamonas testosteroni and mammalian 17beta-HSD types 1 and 4. The results thus suggest that there are most probably different enzymes responsible for 17beta-HSD activity in filamentous fungi.

17Beta-hydroxysteroid dehydrogenases in normal human mammary epithelial cells and breast tissue

17Beta-hydroxysteroid dehydrogenase activity represents a group of several isoenzymes (17HSDs) that catalyze the interconversion between highly active 17beta-hydroxy- and low activity 17-ketosteroids and thereby regulate the biological activity of sex steroids. The present study was carried out to characterize the expression of 17HSD isoenzymes in human mammary epithelial cells and breast tissue. In normal breast tissues 17HSD types 1 and 2 mRNAs were both evenly expressed in glandular epithelium. In two human mammary epithelial cell lines, mRNAs for 17HSD types 1, 2 and 4 were detected. In enzyme activity measurements only oxidative 17HSD activity, corresponding to either type 2 or type 4 enzyme, was present. The role of 17HSD type 4 in estrogen metabolism was further investigated, using several cell lines originating from various tissues. No correlation between the presence of 17HSD type 4 mRNA and 17HSD activity in different cultured cell lines was detected. Instead, oxidative 17HSD activity appeared in cell lines where 17HSD type 2 was expressed and reductive 17HSD activity was present in cells expressing 17HSD type 1. These data strongly suggest that in mammary epithelial cell lines the oxidative activity is due to type 2 17HSD and that oxidation of 17beta-hydroxysteroids is not the primary activity of the 17HSD type 4 enzyme.

A novel 17beta-hydroxysteroid dehydrogenase in the fungus Cochliobolus lunatus: new insights into the evolution of steroid-hormone signalling

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) from the filamentous fungus Cochliobolus lunatus (17beta-HSDcl) catalyses the reduction of steroids and of several o- and p-quinones. After purification of the enzyme, its partial amino acid sequence was determined. A PCR fragment amplified with primers derived from peptide sequences was generated for screening the Coch. lunatus cDNA library. Three independent full-length cDNA clones were isolated and sequenced, revealing an 810-bp open reading frame encoding a 270-amino-acid protein. After expression in Escherichia coli and purification to homogeneity, the enzyme was found to be active towards androstenedione and menadione, and was able to form dimers of Mr 60000. The amino acid sequence of the novel 17beta-HSD demonstrated high homology with fungal carbonyl reductases, such as versicolorin reductase from Emericella nidulans (Aspergillus nidulans; VerA) and Asp. parasiticus (Ver1), polyhydroxynaphthalene reductase from Magnaporthe grisea, the product of the Brn1 gene from Coch. heterostrophus and a reductase from Colletotrichum lagenarium, which are all members of the short-chain dehydrogenase/reductase superfamily. 17beta-HSDcl is the first discovered fungal 17beta-hydroxysteroid dehydrogenase belonging to this family. The primary structure of this enzyme may therefore help to elucidate the evolutionary history of steroid dehydrogenases.

The key role of 17 beta-hydroxysteroid dehydrogenases in sex steroid biology

17 beta-Hydroxysteroid dehydrogenase (17 beta-HSD) controls the last step in the formation of all androgens and all estrogens. This crucial role of 17 beta-HSD is performed by at least five 17 beta-HSD isoenzymes having individual cell-specific expression, substrate specificity, regulation mechanisms, and reductive or oxidative catalytic activity. Both estrogenic and androgenic 17 beta-HSD activities were found in all 25 rhesus monkey and 15 human peripheral intracrine tissues examined. Type 1 17 beta-HSD is a protein of 327 amino acids catalyzing the formation of 17 beta-estradiol from estrone. Its x-ray structure was the first to be determined among mammalian steroidogenic enzymes. Initially crystallized with NAD, the crystal structure of type 1 17 beta-HSD has just been determined as a complex with 17 beta-estradiol, thereby illustrating the conformation of the substrate-binding site. Type 2 17 beta-HSD degrades 17 beta-estradiol into estrone and testosterone into androstenedione, and type 4 17 beta-HSD mainly degrades 17 beta-estradiol into estrone and androst-5-ene-3 beta, 17 beta-diol into dehydroepiandrosterone. Types 3 and 5 17 beta-HSD, on the other hand, catalyze the formation of testosterone from androstenedione in the testis and peripheral tissues, respectively. The various types of human 17 beta-HSD, because of their tissue-specific expression and substrate specificity, provide each peripheral cell with the necessary mechanisms to control the level of intracellular androgens and/or estrogens, a new area of hormonal control that we call intracrinology.

Characterization of estrogen-dependent growth of cultured MCF-7 human breast-cancer cells expressing 17beta-hydroxysteroid dehydrogenase type 1

17beta-hydroxysteroid dehydrogenase (17HSD) type I converts the weakly active estrogen, estrone, into highly active estradiol. In addition to being essential for gonadal estradiol biosynthesis, the enzyme is also expressed in a significant proportion of breast tumors. In order to study the role of the enzyme in estrogen-dependent growth of breast cancer, MCF-7 breast-cancer cells stably expressing human 17HSD type I were generated. In control MCF-7 cells a very low 17HSD activity was observed and, in line with its low estrogenic activity, estrone was devoid of the growth-enhancing effect of estradiol. The presence of the enzyme in the stably transfected MCF-7 cells resulted in a rapid conversion of estrone into estradiol but did not alter the estrogen-receptor concentration in the cells. However, in transfected cells, estrone had a growth-promoting effect practically identical to that of estradiol. The presence or absence of 17HSD type I in breast-cancer cells may therefore be decisive with regard to estrogen exposure and the estrogen-responsive growth of breast-cancer tissues.

Purification and characterization of 17beta-hydroxysteroid dehydrogenase from the filamentous fungus Cochliobolus lunatus

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) from the filamentous fungus Cochliobolus lunatus was purified in three steps, yielding a protein of an apparent molecular mass of 28 kDa. According to the obtained experimental data, the native form of the enzyme could be a dimer (60 kDa) and/or a tetramer (120 kDa). The enzyme was found to catalyse preferentially the reduction of steroid substrates using NADPH as an electron donor. Both androgens and estrogens are substrates for 17beta-HSD. Kinetic studies revealed the equilibrium ordered kinetic mechanism with NADPH as the first ligand to be bound to the enzyme followed by the addition of the substrate androstenedione. The purification and characterization of 17beta-HSD from Cochliobolus lunatus represents a step towards the elucidation of the role of this enzyme in fungal metabolism.

Human 17 beta-hydroxysteroid dehydrogenase type 1 and type 2 isoenzymes have opposite activities in cultured cells and characteristic cell- and tissue-specific expression

17 beta-Hydroxysteroid dehydrogenase (17HSD) isoenzymes catalyse the interconversion between highly active 17 beta-hydroxy- and low-activity 17-keto-steroids and thereby regulate the biological activity of sex steroids. The present study was carried out to characterize 17HSD activity and the expression of 17HSD type 1 and 2 isoenzymes in several human cell types and tissues. The data indicate that in cultured cells the direction of 17HSD activity is exclusively determined by the expression of these distinct isoenzymes. The intracellular environment could not modulate the direction of the enzyme activities in any of the cell types analysed. 17HSD type 1 acts as a reductase converting oestrone into oestradiol, whereas 17HSD type 2 possesses oxidative activity inactivating oestradiol by converting it into oestrone. The data, furthermore, suggest that of the two 17HSD type 1 mRNAs (1.3 and 2.3 kb), expression of the 1.3 kb mRNA is related to enzyme concentration in all the cell types studied. This mRNA is principally expressed in cells of placental and ovarian origin, but is also present in malignant breast epithelial cells. In contrast, 17HSD type 2 is more widely expressed. It is present in several oestradiol-metabolizing tissues as well as in some target cells of sex steroid action. The opposite reaction directions observed in the cultured cells, together with differences in the distribution of the isoenzymes, suggest that type 1 is involved in oestradiol production in females while type 2 plays a role in the inactivation of this sex steroid in peripheral tissues, both in females and in males. However, some examples exist of simultaneous expression of both enzymes in the same cell type or tissue.

Steroid hormone signalling system and fungi

Three components of the steroid hormone signalling system, 17 beta-hydroxysteroid dehydrogenase, androgen binding proteins and steroid hormone signalling molecule testosterone were determined in the filamentous fungus Cochliobolus lunatus for the first time in a fungus. Their possible role in C. lunatus is discussed in comparison with their role in mammalian steroid hormone signalling system. The results are in accordance with the hypothesis, that the elements of primordial signal transduction system should exist in present day eukaryotic microorganisms.

Identification and tissue distribution of a novel variant of 11 beta-hydroxysteroid dehydrogenase 1 transcript

A novel variant of 11 beta-hydroxysteroid dehydrogenase 1 (11 beta-HSD1) mRNA was identified from the ovine liver by reverse transcription-polymerase chain reaction (RT/PCR), and was named 11 beta-HSD1C mRNA. Sequence analysis of the RT-PCR product revealed that 11 beta-HSD1C mRNA was the product of an alternative exon-splicing within the 11 beta-HSD1 gene in which exon 5 was spliced out. Although it caused a deletion of 48 amino acids in the deduced 11 beta-HSD1 protein, this alternative splicing did not result in a shift within the predicted open reading frame of 11 beta-HSD1 cDNA. Thus, 11 beta-HSD1C mRNA was predicted to code for a protein of 244 amino acids. Using RT-PCR, we also examined the expression of 11 beta-HSD1C mRNA in ovine fetal organs and in maternal myometrium, endometrium, chorion, amnion and placenta. The 11 beta-HSD1C mRNA was expressed ubiquitously, similar to 11 beta-HSD1A mRNA, but at a lower abundance. Furthermore, since levels of 11 beta-HSD1C mRNA were directly related to those of 11 beta-HSD1A mRNA, there is no tissue-specificity for this shorter transcript and the only factor regulating its production appears to be 11 beta-HSD1A mRNA itself. To determine whether 11 beta-HSD1C mRNA encoded a functional enzyme, we inserted the cDNA into the expression vector pRc/CMV, and transfected the construct into Chinese hamster ovary cells. The transfected cells expressed a mRNA of expected size but contained no detectable 11 beta-HSD activity. When combined with cellular extracts of 11 beta-HSD1A cDNA transfected cells, they also did not alter either the dehydrogenase or reductase activity. The functional significance of the 11 beta-HSD1 transcript lacking exon 5 (11 beta-HSD1C mRNA) remains to be determined.

Co-expression of two distinct isoforms of 11 beta-hydroxysteroid dehydrogenase in the ovine placenta

We have previously described two distinct isoforms of 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) with respect to enzymatic activity in the ovine liver and kidney. To determine which isoform(s) is expressed in the ovine placenta, we studied the characteristics of 11 beta-HSD activity in placental tissues collected at days 140-143 of pregnancy. 11 beta-HSD activity was determined by a radiometric conversion assay using cortisol and cortisone as physiological substrates. At 100 nM cortisol, the placental 11 beta-HSD utilized NAD as cofactor, but displayed preference for NADP at 10 microM cortisol. Kinetic characteristics were examined in the presence of alternate cofactors, in order to determine whether this difference in the cofactor requirement represents distinct enzymes. With NAD as cofactor, the placental 11 beta-dehydrogenase had a Km (110 +/- 18 nM) compatible with the kidney enzyme, but displayed a Km (12 +/- 2 microM) similar/identical to the liver 11 beta-HSD when NADP was used. By contrast, the placental 11-oxoreductase showed preference for NADPH regardless of cortisone concentration. Kinetic analysis, using NADPH as cofactor, revealed a single species of 11-oxoreductase activity with a Km of 4 +/- 0.9 microM and a Vmax of 3.1 +/- 0.5 pmol/mg/min. Finally, since the NAD-dependent 11 beta-HSD in the ovine placenta displayed similar/identical kinetic characteristics to the enzyme described previously in the ovine kidney where a truncated 11 beta-HSD transcript was identified, we have also determined whether this transcript is expressed in the placenta by Northern blotting. It was found that the truncated 11 beta-HSD transcript was undetectable in the total RNA samples. These results demonstrate that both liver- and kidney-types of 11 beta-HSD activities are expressed in the ovine placenta, thus providing further evidence for the existence of a NAD-dependent 11 beta-HSD distinct from the well-characterized hepatic NADP-dependent enzyme. Furthermore, the lack of the truncated 11 beta-HSD transcript in the placenta suggests that the NAD-dependent enzyme identified in placenta and kidney is the product of a gene distinct from 11 beta-HSD.

Ovine 11 beta-hydroxysteroid dehydrogenase: from gene to function

Two distinct isoforms of 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) with respect to enzymatic activity were identified in the ovine liver and kidney. 11 beta-HSD1 (the hepatic isoform) was reversible and NADP(H)-dependent. By contrast, 11 beta-HSD2 (the renal isoform) was unidirectional and NAD-dependent. Ovine placenta contained both forms of 11 beta-HSD activities. The cDNA encoding ovine 11 beta-HSD1 was cloned, and used as a probe to study 11 beta-HSD1 gene expression in fetal sheep during development. It was found that fetal and adult liver was the major site of 11 beta-HSD1 biosynthesis, and that 11 beta-HSD1 gene expression was regulated in a tissue-specific and developmentally programmed manner. Two non-functional variants of 11 beta-HSD1 were also identified. In addition, sheep kidney was unique in that both 11 beta-HSD1 mRNA and activity were absent. Although the physiological significance of 11 beta-HSD in individual fetal organs during development remains largely speculative, 11 beta-HSD in the fetal pituitary may contribute, at least in part, to the proposed resetting of cortisol negative feedback on pituitary ACTH during the last few days of gestation. In the fetal liver, the action of 11 beta-HSD may lead to the formation of cortisol which could act locally as well as systematically to modulate developmental processes. Placental 11 beta-HSD may protect fetus from exposure to the growth-inhibiting effects of maternal glucocorticoids.

Evidence for distinct isoforms of 11 beta-hydroxysteroid dehydrogenase in the ovine liver and kidney

We have previously identified a unique 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) transcript in the ovine kidney. To examine whether this is indicative of a distinct isoform with respect to enzymatic activity, we studied and compared the characteristics of 11 beta-HSD activity in the ovine liver and kidney. 11 beta-HSD activity was determined by a radiometric conversion assay using cortisol and cortisone as physiological substrates. Although in both liver and kidney, the enzyme was localized by subcellular fractionation in the microsomes, the renal 11 beta-HSD displayed distinct characteristics in that it expressed only dehydrogenase activity and utilized almost exclusively NAD as cofactor (the respective activity in the presence of NAD and NADP was 190 +/- 26 and 12 +/- 2 pmol/min/mg protein). By contrast, the liver enzyme contained both dehydrogenase and reductase activities, and displayed preference for NADP and NADPH, respectively. Furthermore, with cortisol as substrate, the kidney 11 beta-HSD had a Km of 68 +/- 7 nM which was over 100 times lower than the hepatic enzyme (8 +/- 1 microM). In addition, the renal 11 beta-HSD activity was inhibited in a dose-dependent fashion by both carbenoxolone, a potent inhibitor of 11 beta-HSD, and the end product cortisone, whereas the liver enzyme showed little inhibition by either substance. In summary, these results provide strong evidence for the existence of distinct isoforms of 11 beta-HSD with respect to enzymatic activity in the ovine liver and kidney. In addition, the characteristics of the kidney enzyme closely resemble those of that described previously in the rabbit renal aldosterone target cells, and thus further demonstrating the presence of an isoform of 11 beta-HSD distinct from the NADP-dependent enzyme purified and cloned from the rat liver.

Physiology of the steroid-thyroid hormone nuclear receptor superfamily

Glucocorticoids, other steroid hormones, thyroid hormones and vitamin-derived hormones (including retinoids) all exert their effects by the regulation of hormone-responsive target genes within the cell nucleus. These hormones bind to a series of specific nuclear receptor proteins that function as hormone-inducible transcription factors. The receptors are structurally homologous, are related to the avian erythroblastosis oncogene v-erbA, and exhibit remarkable evolutionary conservation. Together they form the steroid-thyroid hormone nuclear receptor superfamily. This chapter describes the structure and functions of the various family members and highlights the differences and similarities that occur between individual receptor proteins. Type I receptors, which include glucocorticoid receptor and other steroid receptor proteins, interact as homodimers with target sequences of DNA containing two receptor binding sites arranged as a palindrome. Type II receptors, which include receptors for retinoids, thyroid hormone and vitamin D3, bind as heterodimers (or homodimers) to DNA sequences in which two or more receptor-binding sites are arranged as a direct repeat or as other more complex configurations. The complexity of both receptor-DNA and receptor-receptor interactions predicts the potential for considerable cross-talk between various hormone-activated pathways. Thus, the specificity of hormone action and its regulation is discussed in relation to the structural and functional characteristics of the receptors and their molecular mechanisms of action. Finally, potential sites of regulation of hormone action, from circulating hormone levels in the periphery to their delivery to the cell and final site of action in the nucleus, are highlighted to provide a perspective for the following chapters in this volume and to indicate their clinical significance.

Structure, regulation and role of 3 beta-hydroxysteroid dehydrogenase, 17 beta-hydroxysteroid dehydrogenase and aromatase enzymes in the formation of sex steroids in classical and peripheral intracrine tissues

In addition to the classical steroidogenic tissues, namely the ovaries, testes, adrenals and placenta, a large series of human peripheral tissues possess all the enzymatic systems required for the formation of active androgens and oestrogens from a relatively large supply of precursor steroids provided by the adrenals. This chapter describes the structure, function, tissue-specific expression and regulation of the 3 beta-HSD and 17 beta-HSD gene families as well as some information about the aromatase gene. While, so far, most therapeutic approaches have been aimed and limited at controlling steroid formation by the classical steroidogenic tissues, it is clear that major efforts should now be turned towards intracrinology in order to understand better the physiological mechanisms controlling local steroid formation in peripheral target tissues and thus be in a position to develop novel therapeutic approaches that take into account the high proportion of steroids that are made locally and are responsible for the growth and function of normal as well as cancerous tissue.

Lactogenic hormones and extracellular matrix regulate expression of IGF-1 linked to MMTV-LTR in mammary epithelial cells

The cell line MD-IGF-1, containing an ovine IGF-1 cDNA driven by the mouse mammary tumor virus-long terminal repeat (MMTV-LTR) promoter, was used to study expression of IGF-1 linked to the MMTV-LTR in bovine mammary epithelial cells in response to various hormonal and substratum stimuli. Acute sensitivity of the MMTV-LTR promoter to glucocorticoids and sex steroids was ascertained by transient transfection of parental MAC-T cells with an MMTV-CAT construct. Specifically, CAT activity was induced by glucocorticoids, but not by 17 beta-estradiol or progesterone. Induction of MD-IGF-1 cells with dexamethasone (DEX) alone triggered a 29.5-fold increase in secretion of recombinant IGF-1 (348.9 vs 11.8 pg/micrograms DNA), and stimulated a 1.7-fold increase in total DNA within 72 h. Growth of MD-IGF-1 cells was enhanced by exogenous IGF-1, insulin, and TGF-alpha. In contrast, TGF-beta inhibited cell proliferation, while epidermal growth factor, estrogen, progesterone, and testosterone had no effect. Extracellular matrix from the Engelbreth-Holm-Swarm (EHS) tumor, in the presence of DEX, prolactin (PRL), and insulin stimulated a 29.4-fold increase in secretion of IGF-1 (591.9 pg/microgram DNA), compared with cells in absence of hormones (20.1 pg/micrograms DNA). EHS and DEX plus PRL triggered a 63.2-fold increase in IGF-1 secretion (689.1 pg/micrograms DNA), compared with MD-IGF-1 cells cultured on plastic (10.9 pg/micrograms DNA), in the absence of hormones. These data indicate that the MMTV-LTR is regulated by both lactogenic hormones and extracellular matrix in MD-IGF-1 cells and that the MMTV-LTR may be a useful regulatory element for targeting expression of foreign proteins in bovine mammary epithelial cells.