Control of cell proliferation by steroids: the role of 17HSDs.

Sex steroid hormone signaling regulates the development, growth, and functioning of the breast and the prostate and plays a role in the development and progression of cancer in these organs. The intracellular concentration of active sex steroid hormones in target tissues is regulated by several enzymes, including 17beta-hydroxysteroid dehydrogenases (17HSDs). Changes in the expression patterns of these enzymes may play a pathophysiological role in malignant transformation. We recently analyzed the mRNA expressions of the 17HSD type 1, 2, and 5 enzymes in about 800 breast carcinoma specimens. Both 17HSD type 1 and 2 mRNAs were detected in normal breast tissue from premenopausal women but not in specimens from postmenopausal women. The patients with tumors expressing 17HSD type 1 mRNA or protein had significantly shorter overall and disease-free survival than the other patients. The expression of 17HSD type 5 was significantly higher in breast tumor specimens than in normal tissue. Cox multivariate analyses showed that 17HSD type 1, tumor size, and estrogen receptor alpha (ERalpha) had independent prognostic significance. We developed, using a LNCaP prostate cancer cell line, a model to study the malignant transformation of prostate cancer and showed that androgen-sensitive LNCaP cells are transformed into neuroendocrine-like cells when cultured without androgens and, eventually into highly proliferating androgen-independent cells. We conducted Northern hybridizations and microarrays to analyze the gene expression during these processes. Substantial changes in the expressions of steroid metabolizing enzymes occurred during the transformation process. The variations in steroid-metabolizing enzymes during cancer progression may be crucial in the regulation of the growth and function of organs.

Enzymes as modulators in malignant transformation.

Experimental data suggest that sex steroids have a role in the development of breast and prostate cancers. The biological activity of sex steroid hormones in target tissues is regulated by several enzymes, including 17beta-hydroxysteroid dehydrogenases (17HSD). Changes in the expression patterns of these enzymes may significantly modulate the intracellular steroid content and play a pathophysiological role in malignant transformation. To further clarify the role of 17HSDs in breast cancer, we analyzed the mRNA expressions of the 17HSD type 1, 2, and 5 enzymes in 794 breast carcinoma specimens. Both 17HSD type 1 and 2 mRNAs were detected in normal breast tissue from premenopausal women but not in specimens from postmenopausal women. Of the breast cancer specimens, 16% showed signals for 17HSD type 1 mRNA, 25% for type 2, and 65% for type 5. No association between the 17HSD type 1, 2, and 5 expressions was detected. The patients with tumors expressing 17HSD type 1 mRNA or protein had significantly shorter overall and disease-free survival than the other patients. The expression of 17HSD type 5 was significantly higher in breast tumor specimens than in normal tissue. The group with 17HSD type 5 overexpression had a worse prognosis than the other patients. Cox multivariate analyses showed that 17HSD type 1 mRNA, tumor size, and ERalpha had independent prognostic significance. Using an LNCaP prostate cancer cell line, we developed a cell model to study the progression of prostate cancer. In this model, androgen-sensitive LNCaP cells are transformed in culture conditions into more aggressive, androgen-independent cells. The model was used to study androgen and estrogen metabolism during the transformation process. Our results indicate that substantial changes in androgen and estrogen metabolism occur in the cells during the process. A remarkable decrease in oxidative 17HSD activity was seen, whereas reductive activity seemed to increase. Since local steroid metabolism controls the bioavailability of active steroid hormones of target tissues, the variations in steroid-metabolizing enzymes during cancer progression may be crucial in the regulation of the growth and function of organs.

Expression of 3beta-hydroxysteroid dehydrogenase type 1, P450 aromatase, and 17beta-hydroxysteroid dehydrogenase types 1, 2, 5 and 7 mRNAs in human early and mid-gestation placentas.

The placenta is responsible for the production of progesterone (P) and estrogens during human pregnancy. In this study, the expression of several key steroidogenic enzymes was investigated in different cell types of human placenta during early and mid-gestation by in situ hybridization. 3Beta-hydroxysteroid dehydrogenase type 1 (3beta-HSD1), P450 aromatase (P450arom) and 17beta-hydroxysteroid dehydrogenase type 1 (17HSD1) were expressed abundantly in syncytiotrophoblast (ST) cells. These three enzymes were also detected in some column cytotrophoblast (CCT) cells. 17HSD5 was found in intravillous stromal (IS) cells in low levels, suggesting that androgens may be synthesized and metabolized in the placenta. 17HSD7 was found in all types of placental cells. Moreover, 17HSD2 was localized in IS cells. The expression level of 17HSD2 gradually increased during pregnancy weeks 7-16, concurrently with the androgen production by the male fetus. The present study provides evidence that CCT and IS cells participate in P and estrogen biosynthesis, in addition to ST cells. 17HSD2 also converts 20alpha-dihydroprogesterone (20-OH-P) to P, whereas 17HSD5 and 17HSD7 inactivate P. Therefore, the action of 3beta-HSD1 and 17HSD2 on P biosynthesis in the placenta is countered by 17HSD5 and 17HSD7, which may provide an optimal level of P for the maintenance and progression of pregnancy.

17 beta-hydroxysteroid dehydrogenases--their role in pathophysiology.

17 beta-Hydroxysteroid dehydrogenases (17HSDs) regulate the biological activity of sex steroid hormones in a variety of tissues by catalyzing the interconversions between highly active steroid hormones, e.g. estradiol and testosterone, and corresponding less active hormones, estrone and androstenedione. Epidemiological and endocrine evidence indicates that estrogens play a role in the etiology of breast cancer, while androgens are involved in mechanisms controlling the growth of normal and malignant prostatic cells. Using LNCaP prostate cancer cell lines, we have developed a cell model to study the progression of prostate cancer. In the model LNCaP cells are transformed in culture condition into more aggressive cells. Our data suggest that substantial changes in androgen and estrogen metabolism occur in the cells, leading to increased production of active estrogens during the process. In breast cancer, the reductive 17HSD type 1 activity is predominant in malignant cells, while the oxidative 17HSD type 2 mainly seems to be present in non-malignant breast epithelial cells. Deprivation of an estrogen response by using specific 17HSD type 1 inhibitors is a tempting approach in treating estrogen-dependent breast cancer. Our recent studies demonstrate that in addition to sex hormone target tissues, estrogens may be important in the development of cancer in some other tissues previously not considered to be estrogen target tissues, such as the gastrointestinal tract.

Sex steroid hormone metabolism and prostate cancer.

The growth and function of the prostate is dependent on androgens. The two predominant androgens are testosterone, which is formed in the testis from androstenedione and 5alpha-dihydrotestosterone, which is formed from testosterone by 5alpha-reductases and is the most active androgen in the prostate. Prostate cancer is one of the most common cancers among men and androgens are involved in controlling the growth of androgen-sensitive malignant prostatic cells. The endocrine therapy used to treat prostate cancer aims to eliminate androgenic activity from the prostatic tissue. Most prostate cancers are initially responsive to androgen withdrawal but become later refractory to the therapy and begin to grow androgen-independently. Using LNCaP prostate cancer cell line we have developed a cell model to study the progression of prostate cancer. In the model androgen-sensitive LNCaP cells are transformed in culture conditions into more aggressive, androgen-independent cells. The model was used to study androgen and estrogen metabolism during the transformation process. Our results indicate that substantial changes in androgen and estrogen metabolism occur in the cells during the process. A remarkable decrease in the oxidative 17beta-hydroxysteroid dehydrogenase activity was seen whereas the reductive activity seemed to increase. The changes suggest that during transformation estrogen influence is increasing in the cells. This is supported by the cDNA microarray screening results which showed over-expression of several genes up-regulated by estrogens in the LNCaP cells line representing progressive prostate cancer. Since local steroid metabolism controls the bioavailability of active steroid hormones in the prostate, the variations in steroid-metabolizing enzymes during cancer progression may be crucial in the regulation of the growth and function of the organ.

17 beta-hydroxysteroid dehydrogenases and cancers.

17 beta-Hydroxysteroid dehydrogenases (17HSDs) catalyze the interconversions between active 17 beta-hydroxysteroids and less-active 17-ketosteroids thereby affecting the availability of biologically active estrogens and androgens in a variety of tissues. The enzymes have different enzymatic properties and characteristic cell-specific expression patterns, suggesting differential physiological functions for the enzymes. Epidemiological and endocrine evidence indicate that estrogens play a key role in the etiology of breast cancer while androgens are involved in mechanisms controlling the growth of prostatic cells, both normal and malignant. Recently, we have developed, using LNCaP prostate cancer cell lines, a cell model to study the progression of prostate cancer. In the model LNCaP cells are transformed in culture condition to more aggressive cells, able to grow in suspension cultures. Our results suggest that substantial changes in androgen and estrogen metabolism occur in the cells during the process. These changes lead to increased production of active estrogens during transformation of the cells. Data from studies of breast cell lines and tissues suggest that the oxidative 17HSD type 2 may predominate in human non-malignant breast epithelial cells, while the reductive 17HSD type 1 activity prevails in malignant cells. Deprivation of an estrogen response by using specific 17HSD type 1 inhibitors is a tempting approach to treat estrogen-dependent breast cancer. Our recent studies demonstrate that in addition to sex hormone target tissues, estrogens may be important in the development of cancer in some other tissues previously not considered as estrogen target tissues such as colon. Our data show that the abundant expression of 17HSD type 2 present in normal colonic mucosa is significantly decreased during colon cancer development.

Structure and function of 17beta-hydroxysteroid dehydrogenase type 1 and type 2.

17beta-Hydroxysteroid dehydrogenases (17HSDs) catalyze the interconversions between high-activity 17beta-hydroxysteroids and low-activity 17-ketosteroids. Several distinct 17HSD isoenzymes have been characterized. They have unique tissue distribution patterns suggesting a specific function for each of the isoenzymes in modifying sex steroid hormone activity. The activities of 17HSDs are essential for gonadal sex steroid biosynthesis and they are also involved in the modulation of steroid hormone action in peripheral tissues. 17HSD type 1 (17HSD1) is needed for estradiol biosynthesis in ovarian granulosa cells and it is also expressed in breast tissue, thus increasing locally estradiol concentration. 17HSD type 2 (17HSD2) is another 17HSD enzyme involved in estrogen metabolism. The type 2 enzyme has an opposite activity catalyzing estradiol to estrone, thereby reducing the exposure of tissues to estrogen action. Preliminary data suggest that 17HSD2 may predominate in human non-malignant breast epithelial cells, while 17HSD type 1 activity prevails in malignant cells. Determination of the three-dimensional structure of human 17HSD1 has led to an atomic level description of the estradiol binding pocket of the enzyme and an understanding of its mechanism of action, and the molecular basis for the estrogen-specificity of the enzyme. Deprivation of an estrogen response by using specific 17HSD1 inhibitors is a tempting approach to treat estrogen-dependent breast cancer.

Cytokine regulation of the expression of estrogenic biosynthetic enzymes in cultured rat granulosa cells.

Both P450 aromatase (P450arom) and 17beta-hydroxysteroid dehydrogenase (17HSD) type 1 are key enzymes in the ovarian E(2) biosynthesis. Cytokines have been suggested to be mediators between the immune and the reproductive systems, and they may play a role as paracrine or autocrine ovarian regulatory factors. Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNFalpha) have been shown to modulate the FSH-induced E(2) production in immature rat granulosa cells. The aim of the present study was to investigate the effects of these cytokines on the activity and expression of the 17HSD type 1 enzyme in cultured undifferentiated granulosa cells. Furthermore, the expression of P450arom was also analyzed. The granulosa cells obtained from the ovaries of immature DES-treated rats were initially cultured for 48 h with no other treatment and then incubated with or without the test reagents for an additional 48 h. The treatment of the granulosa cells with cytokines alone did not affect the activity of 17HSD type 1 as assessed by the conversion of tritiated substrate. However, both TNFalpha and IL-1beta caused a dose-dependent inhibition of the recombinant FSH-induced enzyme activity and the Forskoline-induced expression of 17HSD type 1 and P450arom mRNAs. The cytokines only slightly inhibited the 8-Br-cAMP-induced P450arom expression. In contrast, the inhibitory cytokine effects on 17HSD type 1 expression and activity were not abolished by the presence of 8-Br-cAMP. Despite the presence of inhibitors of protein kinase C (staurosporine) or tyrosine kinases (genistein), the inhibitory effects of TNFalpha and IL-1beta on the Forskoline-induced expression of 17HSD type 1 and P450arom and the Forskoline-induced 17HSD activity were not blocked. The data show a dose dependent inhibitory effect of TNFalpha and IL-1beta on gonadotropin action, opposite to the follicular development by down-regulating the expressions of estrogen biosynthetic enzymes. The cytokine effects on P450arom expression are mainly derived from a decrease in gonadotropin-induced cAMP production, while the inhibitory mechanisms on 17HSD type 1 expression involve distal sites from cAMP generation. The protein kinase C and tyrosine kinase pathways are likely not to be involved in the latter mechanisms.

Activin-A, but not inhibin, regulates 17beta-hydroxysteroid dehydrogenase type 1 activity and expression in cultured rat granulosa cells.

17beta-Hydroxysteroid dehydrogenase type 1 (17HSD type 1) catalyzes the reduction of estrone (E(1)) to biologically more active estradiol (E(2)). In the present study, the effect of activin, inhibin, and follistatin on 17HSD activity and 17HSD type 1 expression in cultured, unluteinized rat granulosa cells was examined. Furthermore, the effects of these hormones on 17HSD type 1 expression were compared with the expression of P450 aromatase (P450arom). Rat granulosa cells were pre-incubated in serum-free media for 3 days, followed by a 2-day treatment with activin, inhibin, follistatin and 8-Br-cAMP. Activin in increasing concentrations appeared to effect a dose-dependent increase in 17HSD activity. In addition, increasing concentrations of activin also increased 17HSD type 1 mRNA expression. Addition of 8-Br-cAMP at concentrations of 0.25 and 1.5 mmol/l together with activin significantly augmented the stimulatory effects of activin alone in the cultured cells. Neither inhibin, nor follistatin, either alone or in combination with 8-Br-cAMP, had any notable effects on 17HSD activity and 17HSD type 1 expression. Preincubation of activin with increasing concentrations of follistatin significantly diminished the stimulatory effect of activin. In the presence of follistatin, activin did not significantly increase the 8-Br-cAMP-induced 17HSD activity and 17HSD type 1 expression. The culturing of granulosa cells in the presence or the absence of inhibin or follistatin with or without 8-Br-cAMP did not alter the effect of these peptides on P450arom expression in rat granulosa cells as judged by Northern blot analysis of total RNA. However, cAMP-induced P450arom expression was enhanced by activin treatment, except when follistatin was present. This is in line with the suggested role of follistatin as an activin-binding protein, which limits the bioavailability of activin to its membrane receptors. Thus, the results support the notion of a paracrine/autocrine role of activin in follicular steroidogenesis of growing follicles.

Estrogen metabolism as a regulator of estrogen action in the mammary gland.

Estrogen action in the target cells is dependent on estrogen receptor activity and intracellular estrogen concentration, which, in turn, is affected by the serum concentration and local metabolism in these cells. During the reproductive years the main source of estrogens is the ovarian follicles, but in postmenopausal women most of the estrogens are formed in peripheral tissues. 17Beta-hydroxysteroid dehydrogenases (17HSDs) catalyze the reaction between 17beta-hydroxysteroids and 17-ketosteroids, and several distinct 17HSD isoenzymes have been characterized. 17HSD type 1 catalyzes the reaction from low-activity estrone to high-activity estradiol. The type 2 enzyme has an opposite activity, thereby reducing the exposure of tissues to estrogen action. 17HSD type 1 is expressed both in steroidogenic tissues and in the target tissues of steroid action, such as normal and malignant breast tissue, where it may be responsible for maintaining the high intracellular estradiol concentration seen in breast cancer specimens. Therefore, 17HSD type 1 inhibitors may be useful in the treatment and/or prevention of estrogen-dependent malignancies, such as breast cancer. This article deals mainly with 17HSD types 1 and 2 and their role in estrogen action in breast tissue.

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.

Human 17beta-hydroxysteroid dehydrogenase type 2 messenger ribonucleic acid expression and localization in term placenta and in endometrium during the menstrual cycle.

According to the current hypothesis, 17beta-hydroxysteroid dehydrogenases (17HSDs) regulate the extent of estrogen influence in the endometrium by converting estradiol (E2) locally into a biologically less active sex steroid, estrone (E1), and vice versa. Recently, we have shown that both 17HSD type 1 and type 2 are expressed in the human endometrium, and in the present work, using in situ hybridization, we show that 17HSD type 2 is localized in the glandular epithelial cells as previously shown for the type 1 enzyme, but in contrast to type 1, the expression of type 2 is highest at the end of the cycle. Hence, we hypothesize that the differential expression of the two 17HSD enzymes, with opposite activities in same cell types, could modulate intracellular E2 concentrations during the end of the luteal phase of the menstrual cycle. We further analyzed the expression of 17HSD type 1 and type 2 mRNAs in term human placenta. Expression of 17HSD type 1 mRNA was detected in the syncytiotrophoblasts, and signals for type 2 mRNA were found inside the villi, corresponding to cytotrophoblasts. The expression of 17HSD type 2 in the placenta may serve to maintain the presence of inactive sex steroids and attenuate the formation of biologically potent androgens and estrogens.

Mouse 17 beta-hydroxysteroid dehydrogenase type 2 mRNA is predominantly expressed in hepatocytes and in surface epithelial cells of the gastrointestinal and urinary tracts.

17 beta-Hydroxysteroid dehydrogenase (17HSD) type 2 efficiently catalyzes the conversion of the high activity 17 beta-hydroxy forms of sex steroids into less potent 17-ketosteroids. In the present study in situ hybridization was utilized to analyze the cellular localization of 17HSD type 2 expression in adult male and female mice. The data indicate that 17HSD type 2 mRNA is expressed in several epithelial cell layers, including both absorptive and secretory epithelia as well as protective epithelium. In both males and females, strong expression of 17HSD type 2 was particularly detected in epithelial cells of the gastrointestinal and urinary tracts. The mRNA was expressed in the stratified squamous epithelium of the esophagus, and surface epithelial cells of the stomach, small intestine and colon. The hepatocytes of the liver and the thick limbs of the loops of Henle in the kidneys, as well as the epithelium of the urinary bladder, also showed strong expression of 17HSD type 2 mRNA in both male and female mice. In the genital tracts, low 17HSD type 2 expression was detected in the seminiferous tubules, the uterine epithelial cells and the surface epithelium of the ovary. Expression of the mRNA was also detected in the sebaceous glands of the skin. The results indicate that in both male and female mice, 17HSD type 2 is expressed mainly in the various epithelial cell types of the gastrointestinal and urinary tracts, and therefore suggest a role for the enzyme in steroid inactivation in a range of tissues and cell types not considered as classical sex steroid target tissues.

Ontogeny of 17beta-hydroxysteroid dehydrogenase type 2 mRNA expression in the developing mouse placenta and fetus.

17beta-Hydroxysteroid dehydrogenase type 2 (17HSD type 2) catalyzes the inactivation of estradiol, testosterone and dihydrotestosterone into biologically less active 17-keto forms. Our recent Northern analysis indicated that the enzyme is expressed both in mouse placenta and fetus. The present data indicate that in the placenta the distribution of enzyme expression changes during pregnancy. In the choriovitelline placenta (day 8) 17HSD type 2 was expressed both in mural and polar giant cells. Later, on days 9-12.5, the mRNA was also detected in the junctional zone, and in late gestation (days 14.5-17.5), 17HSD type 2 mRNA was predominantly expressed only at the labyrinth region. In the fetus, 17HSD type 2 expression appears in the liver on day 11. At day 12 the expression was strongly increased in the liver, and at the same time moderate mRNA expression was also detected in the esophagus and intestine. In these tissues, high constitutive expression of 17HSD type 2 was then maintained throughout pregnancy. At later stages of development (days 15-16) the mRNA was, furthermore, detected in epithelial cells of the stomach, tongue, oropharynx and nasopharynx as well as in the kidney. We conclude that the expression pattern of 17HSD type 2 in the developing placenta and fetus suggests a role for the enzyme in maintaining a barrier to the transfer of active 17-hydroxy forms of sex steroids between the fetus and maternal circulation.

Cloning of mouse 17beta-hydroxysteroid dehydrogenase type 2, and analysing expression of the mRNAs for types 1, 2, 3, 4 and 5 in mouse embryos and adult tissues.

17beta-Hydroxysteroid dehydrogenases (17HSDs) are responsible for the conversion of low-activity sex steroids to more potent forms, and vice versa. 17HSD activity is essential for the biosynthesis of sex steroids in the gonads, and it is also one of the key factors regulating the availability of active ligands for sex-steroid receptors in various extragonadal tissues. In this study, we have characterized mouse 17HSD type 2 cDNA, and analysed the relative expression of 17HSD types 1, 2, 3, 4 and 5 mRNAs in mouse embryos and adult male and female tissues. The cDNA characterized has a open reading frame of 1146 bp, and encodes a protein of 381 amino acids with a predicted molecular mass of 41837 kDa. Northern-blot analysis of adult mouse tissues revealed that, of the different 17HSDs, the type 2 enzyme is most abundantly expressed. High expression of the enzyme, which oxidizes both testosterone and oestradiol, in several large organs of both sexes indicates that it is the isoform having the most substantial role in the metabolism of sex steroids. Interestingly, four of the five 17HSD enzymes were also detected by Northern blots of whole mouse embryos, and each of the enzymes showed a unique pattern of expression. The oestradiol-synthesizing type 1 enzyme predominates in early days of development embryonic day 7, but after that the oxidative type 2 enzyme becomes the predominant form of all 17HSDs. The data therefore suggest that there is transient oestradiol production in the early days of embryonic development, after which inactivation of sex steroids predominates in the fetus and placenta.

Expression and regulation of 17 beta-hydroxysteroid dehydrogenase type 1.

The current data indicate that during a woman's reproductive years, 17 beta-hydroxysteroid dehydrogenase type 1 is the major 17 beta-hydroxysteroid dehydrogenase (17HSD) involved in glandular oestradiol biosynthesis. The type 1 enzyme catalyses reduction from low-activity oestrone to high-activity oestradiol in ovarian granulosa cells and placental syncytiotrophoblasts, in which it is abundantly expressed. In addition to steroidogenic cells, 17HSD type 1 is present in certain peripheral tissues in which it reduces circulating oestrone, thus regulating the intracellular ligand supply for oestrogen receptors. Several factors and second messenger pathways are involved in the cell-specific expression of 17HSD type 1. In ovarian granulosa cells, 17HSD type 1 expression is strictly regulated by pituitary gonadotrophins, steroid hormones and growth factors, while in peripheral tissues progestins and retinoic acids, at least, affect 17HSD type 1 concentrations.

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.

Mutated human androgen receptor gene detected in a prostatic cancer patient is also activated by estradiol.

Androgens are necessary for the development of prostatic cancer. The mechanisms by which the originally androgen-dependent prostatic cancer cells are relieved of the requirement to use androgen for their growth are largely unknown. The human prostatic cancer cell line LNCaP has been shown to contain a point mutation in the human androgen receptor gene (hAR), suggesting that changes in the hAR may contribute to the abnormal hormone response of prostatic cells. To search for point mutations in the hAR, we used single strand conformation polymorphism analysis and a polymerase chain reaction direct sequencing method to screen 23 prostatic cancer specimens from untreated patients, 6 prostatic cancer specimens from treated patients, and 11 benign prostatic hyperplasia specimens. One mutation was identified in DNA isolated from prostatic cancer tissue, and the mutation was also detected in the leukocyte DNA of the patient and his offspring. The mutation changed codon 726 in exon E from arginine to leucine and was a germ line mutation. The mutation we found in exon E of the hAR gene does not alter the ligand binding specificity of the AR, but the mutated receptor was activated by estradiol to a significantly greater extent than the wild-type receptor. The AR gene mutation described in this study might be one explanation for the altered biological activity of prostatic cancer.

Regulation of oestrogen action: role of 17 beta-hydroxysteroid dehydrogenases.

The target cell responses to steroid hormones, such as oestrogens, are dependent on the expression of their receptors. Apart from receptor concentration, another key regulatory factor in steroid hormone action is the intracellular hormone concentration, which is affected by three main variables: the concentration of the steroid in plasma, local production and local conversion into metabolites. During the reproductive years the main source of oestrogens is the ovarian follicle, but in postmenopausal women most of the oestrogens are formed in peripheral tissues. The present overview deals with the formation of active oestrogens in steroidogenic tissues and in oestrogen target tissues, and the main focus is on 17 beta-hydroxysteroid dehydrogenases, which catalyse the interconversion between oestradiol and oestrone. It is evident that different 17 beta-hydroxysteroid dehydrogenase isoenzymes are responsible for the oxidation/reduction of oestradiol or oestrone in oestrogen target cells. Because these enzymes are involved in the biosynthesis and metabolism of oestrogens, they have an important physiological significance for the growth of oestrogen-dependent tissues and, hence, the growth and progression of hormone-dependent tumours.

Role of 17 beta-hydroxysteroid dehydrogenase type 1 in endocrine and intracrine estradiol biosynthesis.

Enzymes with 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) activity catalyse reactions between the low-active female sex steroid, estrone, and the more potent estradiol, for example. 17 beta-HSD activity is essential for glandular (endocrine) sex hormone biosynthesis, but it is also present in several extra-gonadal tissues. Hence, 17 beta-HSD enzymes also take part in local (intracrine) estradiol production in the target tissues of estrogen action. Four distinct 17 beta-HSD isozymes have been characterized so far, and the data strongly suggests that different 17 beta-HSD isozymes have distinct roles in endocrine and intracrine metabolism of sex steroids. Current data suggest that 17 beta-HSD type 1 is the principal isoenzyme involved in glandular estradiol production both in humans and rodents. During ovarian follicular development and luteinization, rat 17 beta-HSD type 1 is regulated by gonadotropins, and the effects of gonadotropins are modulated by steroid hormones and paracrine growth factors. Human 17 beta-HSD type 1 favors the reduction reaction, thereby converting estrone to estradiol both in vitro and in cultured cells. Hence, the enzymatic properties of the enzyme are also in line with its suggested role in estradiol biosynthesis. Interestingly, 17 beta-HSD type 1 is also expressed in certain target tissues of estrogen action such as normal and malignant human breast and endometrium. Hence, 17 beta-HSD type 1 could be one of the factors leading to a relatively high tissue/plasma ratio of estradiol in breast cancer tissues of postmenopausal women. We conclude that 17 beta-HSD type 1 has a central role in regulating the circulating estradiol concentration as well as its local production in estrogen target cells.