**12. 17β-HSD dehydrogenase and multifunstional izoforms: localization, function and relevance to clinical therapeutic strategies**

17βHydroxysteroid dehydrogenases (17β-HSDs, 17HSD/KSRs) are NAD(H)- and/or NADP(H)-dependent enzymes that catalyze the oxidation and reduction of active 17βhydroxy- and low active/inactive 17-ketosteroids, respectively. In the presence of substantial excess of a suitable cofactor and/or in the absence of a preferred cofactor, 17HSD/ KSRs can be compelled to catalyze both oxidative and reductive reactions. Depending on their reductive or oxidative activities, they modulate the intracellular concentration of inactive and active steroids. Acting as oxidoreductases at the 17-position of the steroid, they play a key role in estrogen/androgen steroid metabolism by catalyzing the final steps of steroid biosynthesis. Both estrogens and androgens have the highest afnity for their receptors in the 17β-hydroxy form and hence, 17HSD/KSR enzymes regulate the biological activity of the sex hormones.17KSR activities are essential for estradiol and testosterone biosynthesis in the gonads, but they are also present in certain extragonadal tissues and can convert lowactivity precursors to their more potent forms in peripheral tissues. Instead, 17HSD activities tend to decrease the potency of estrogens and androgens and consequently may protect tissues from excessive hormone action [10, 225].

Up to now, 14 different subtypes have been identified in mammals and they differ in tissue distribution, sub-cellular localization, function and catalytic preference (oxidation or reduction using the cofactor NAD(H) and NADP(H), respectively) (Table 1). In fact, 17β-HSDs have diverse substrate specificities in vivo as they also catalyze the conversions of other substrates than steroids as for example lipids or retinoids. Until recently, besides 17β-HSD3 and 17β-HSD14, 17β-HSD1 and 2 were thought to be exclusively converting sex steroids. However, the participation of the two latter enzymes (17β-HSD1 and 2) in retinoic acid metabolism recently was suggested. Other 17β-HSD types were already known to be multifunctional and some of them play important roles in different metabolic pathways.

17β-HSD7 is mainly involved in cholesterol synthesis, 17β-HSD4 is implicated in βoxidation of fatty acids, 17β-HSD5 participates in both prostaglandin and steroid metabolism, and 17β-HSD12 is required in fatty acid elongation. 17β-HSD10 catalyzes the oxidation of short chain fatty acids. 17β-HSD6 and 9 play a role in retinoid conversion. For some 17β-HSDs, the physiological function is not yet clear. For several types of 17β-HSDs participation in the pathophysiology of human diseases has been postulated [225]. The specificity of each 17β-HSD subtype for a preferred substrate together with distinct tissue localization, suggests that these proteins are promising therapeutic targets for diseases like breast cancer, endometriosis, osteoporosis, and prostate cancer. For some of them, their

138 Dehydrogenases

fetal programming [215, 216, 221].

of *HSD11B2* gene encoding 11β-HSD have low birth weight. Intrauterine growth retardation in human is associated with increased fetal cortisol levels and reduced placental 11β-HSD2 activity [217]. Studies on prenatal exposure to 11β-HSD inhibitors such as glycyrrhetinic acid and carbenoxolone have indicated that these agents cause fetal growth retardation and adult offspring changes that are very similar to those that are caused by prenatal exposure to glucocorticoids such as dexamethasone (readily crosses the placenta) [221]). Mice that are homozygous for disrupted alleles of *HSD11B2* (i.e. 11β-HSD2–/– mice) also have lower birth weight and the offspring display anxiety-related behaviors in adulthood. It seems that the conditions of increased fetal glucocorticoid levels, in response to different maternal restrictions, sometimes have persistent effects in the offspring - so-called concept of developmental physiological programming and that placental 11β-HSD2 is a key player in

**12. 17β-HSD dehydrogenase and multifunstional izoforms: localization,** 

17βHydroxysteroid dehydrogenases (17β-HSDs, 17HSD/KSRs) are NAD(H)- and/or NADP(H)-dependent enzymes that catalyze the oxidation and reduction of active 17βhydroxy- and low active/inactive 17-ketosteroids, respectively. In the presence of substantial excess of a suitable cofactor and/or in the absence of a preferred cofactor, 17HSD/ KSRs can be compelled to catalyze both oxidative and reductive reactions. Depending on their reductive or oxidative activities, they modulate the intracellular concentration of inactive and active steroids. Acting as oxidoreductases at the 17-position of the steroid, they play a key role in estrogen/androgen steroid metabolism by catalyzing the final steps of steroid biosynthesis. Both estrogens and androgens have the highest afnity for their receptors in the 17β-hydroxy form and hence, 17HSD/KSR enzymes regulate the biological activity of the sex hormones.17KSR activities are essential for estradiol and testosterone biosynthesis in the gonads, but they are also present in certain extragonadal tissues and can convert lowactivity precursors to their more potent forms in peripheral tissues. Instead, 17HSD activities tend to decrease the potency of estrogens and androgens and consequently may

Up to now, 14 different subtypes have been identified in mammals and they differ in tissue distribution, sub-cellular localization, function and catalytic preference (oxidation or reduction using the cofactor NAD(H) and NADP(H), respectively) (Table 1). In fact, 17β-HSDs have diverse substrate specificities in vivo as they also catalyze the conversions of other substrates than steroids as for example lipids or retinoids. Until recently, besides 17β-HSD3 and 17β-HSD14, 17β-HSD1 and 2 were thought to be exclusively converting sex steroids. However, the participation of the two latter enzymes (17β-HSD1 and 2) in retinoic acid metabolism recently was suggested. Other 17β-HSD types were already known to be multifunctional and some of them play important roles in different metabolic pathways.

17β-HSD7 is mainly involved in cholesterol synthesis, 17β-HSD4 is implicated in βoxidation of fatty acids, 17β-HSD5 participates in both prostaglandin and steroid

**function and relevance to clinical therapeutic strategies** 

protect tissues from excessive hormone action [10, 225].


**Table 1.** Human 17β-Hydroxysteroid dehydrogenases

expression level can be used as prognostic marker in breast or prostate cancer. The selective inhibition of the concerned enzymes might provide an effective treatment and a good alternative for treatment of steroid dependent diseases [246]. Having in mind multifunctionaloty 17β-HSD enzymes, the biological and clinical aspects of each isoform will be described separately.

Hydrohysteroid Dehydrogenases – Biological Role and Clinical Importance – Review 141

dependence of these phenotypes on androgens. Interestingly, the androgen exposure during pregnancy in the HSD17B1-TG mice resulted in benign ovarian serous cystadenomas in adulthood. As ovarian serous borderline tumours are positively associated with a history of PCOS, thus with a history of (foetal) hyperandrogenism, 17β-HSD1 may promote ovarian carcinogenesis via increased estrogen concentration, but also via enhanced androgen production. Endometrial hyperplasia in HSD17B1-TG mice closely resembled human disease and it was efficiently reversed by 17β-HSD1 inhibitor treatment. The data concerning the expression of 17β-HSD1 in normal and diseased human endometrium are not fully conclusive. However, in most of the studies, the 17β-HSD1 expression is detected in normal endometrium, endometriosis specimens and endometriotic cancer. Other 17β-HSD enzymes including 17β-HSD2, 17β-HSD5, 17β-HSD7 and 17β-HSD12 have also been detected in the endometrium under different pathological conditions like endometriosis and PCOS [10]. Collectively, the data suggest that 17β-HSD1 inhibition is one of the several possible approaches to reduce estrogen production both in eutopic and in ectopic

17β-HSD type-2: 17-HSD/KSR2 converts 17β-hydroxy forms of estrogens and androgens (estradiol, testosterone and 5α-dihydrotestosterone) to their less active 17-keto forms (estrone, androstenedione and 5α-androstanedione).The enzyme also possesses 20α-HSD activity, thereby activating 20α-hydroxyprogesterone to progesterone. The 17β-HSD2 enzyme is widely and abundantly expressed in both adult and fetal tissues such as placenta, uterus, liver, the gastrointestinal and urinary tracts. Due to its expression pattern and enzymatic characteristics, it has been suggested that the 17β-HSD2 enzyme protects tissues from excessive steroid action [251]. 17β-HSD2 is localised in the endoplasmic reticulum, and it is widely expressed in various estrogen and androgen target tissues both in human and in rodents including breast endometrium, placenta and prostate. Furthermore, the 17β-HSD2 expression in the placenta and in foetal liver and intestine, together with the observed oxidative 17β-HSD2activity, are the basis for the hypothesis, suggesting a role for the

Phylogenic analyses have indicated that 17β-HSD2 is a close homologue of retinoidconverting enzymes and has a high sequence similarity to retinol dehydrogenase type 1. In addition, studies have shown that retinoic acid (RA) induces expression of 17β-HSD2 in a dose- and time-dependent manner in human endometrial epithelial and placental cells [10]. Recent data from transgenic mice (HADS17B2-TG) provide evidence for importance of 17β-HSD2 for prenatal eye morphogenesis and eye development [10]. These TG mice overexpressing human 17β-HSD2 showed growth retardation, disrupted spermayogenesis, female masculinization, delayed eye opening, squint appearance of the eyes and some of these defects closely resembeled those identified in retinoid receptor mutant mice. The most notable changes in the HSD17B1TG mice are well explained by alterations in sex steroid action, whereas in the HSD17B2-TG mice the connection to sex steroids is weaker. The opposite mouse model of deficiency of 17β-HSD2 provide evidence for the essential role of 17β-HSD2. Embryonic death in the HSD17B-KO mice is reported, related to lack of action of 17β-HSD2 enzyme in placenta. Furthermore, the treatment of pregnant female mice with an

enzyme in lowering the sex steroid exposure of the foetus.

endometrial tissue.

**17β-HSD type1**: 17β-HSD1 catalyzes the activation of estrone (E1) to the most potent estrogen estradiol (E2), predominantly considered as an ezyme of estradiol biosynthesis. It is abundantly expressed in granulosa cells of developing follicles and variable amounts of the enzyme are also expressed in human breast epithelial cells. The enzyme is known to have a crucial role in the development of estrogen-dependent diseases. Based on the in vitro studies, human (h) 17β-HSD1 has been considered as highly estrogen specific, with markedly lower catalytic efficacy towards androgenic substrates. There is a clear difference in the substrate specificity between human and rodent 17β-HSD1 enzymes; the catalytic efficacy of rodent enzyme in vitro is similar for both androgens and estrogens. According a recent review by Saloniemi et al. [10], the h17β-HSD1 is not fully estrogenen-specific but it possesses significant androgenic sctivity. The enzyme catalyses both oxidative (17-hydroxy to 17-keto) and reductive (17-keto to 17-hydroxy) 17b-HSD activity with a proper cofactor added in vitro. However, in cultured cells, the h17β-HSD1 has been shown to catalyse predominantly the reductive reaction [247]. Although h17β-HSD1 expression in various peripheral tissues is low, its catalytic efficacy is markedly higher than those measured for 17β-HSD7 and 17β-HSD12 [248, 242], suggesting an important role for 17β-HSD1 in peripheral E2 formation. Data from animal models further demonstrated the ability of h17β-HSD1 to enhance estrogen action in target tissues and its decrease after treating the mice with 17β-HSD1 inhibitors [10]. These data suggest that 17β-HSD1 plays a major role in determining the gradient between the E2 concentrations in serum and peripheral tissues. An increased E2/E1 ratio by the 17β-HSD1 point out the pivotal role of 17β-HSD1 in breast cancer, ovarian tumor, endometriosis, endometrial hyperplasia and uterine leiomyoma [249, 250]. Consequently, inhibition of 17β-HSD1 is considered as a valuable therapeutic approach for treatment of these deseases. In vivo evaluation of 17β-HSD1 inhibitors is complicated by the fact that the rodent enzymes only show moderate homology/identity to the human one. Due to these species differences, there is a high probability that inhibitors optimized for activity toward rodent 17β-HSD1 do not inhibit the human enzyme. In addition, rodents and humans vary considerably in enzyme distribution in the different tissues. Attempts to overcome these problems include xenograft models using nude mice.

Recently generated mouse genetic model for overexpression of17β-HSD1 (HSD17B1-TG mice) by Saloniemi et al [10] provided valuable data about common female reproductive disorders like Polycystic Ovarian Syndrome (PCOS), ovarian carcinogenesis and endometiosis. Overexpression of hHSD17B1 leads to increased androgen exposure during embryonic development that caused androgen-dependent phenotypic alterations in female, such as increased anogenital distance, lack of vaginal opening and combination of vagina with urethra. These alterations observed in the HSD17B1-TG females were effectively rescued by prenatal anti-androgen (flutamide) treatment, further confirming the dependence of these phenotypes on androgens. Interestingly, the androgen exposure during pregnancy in the HSD17B1-TG mice resulted in benign ovarian serous cystadenomas in adulthood. As ovarian serous borderline tumours are positively associated with a history of PCOS, thus with a history of (foetal) hyperandrogenism, 17β-HSD1 may promote ovarian carcinogenesis via increased estrogen concentration, but also via enhanced androgen production. Endometrial hyperplasia in HSD17B1-TG mice closely resembled human disease and it was efficiently reversed by 17β-HSD1 inhibitor treatment. The data concerning the expression of 17β-HSD1 in normal and diseased human endometrium are not fully conclusive. However, in most of the studies, the 17β-HSD1 expression is detected in normal endometrium, endometriosis specimens and endometriotic cancer. Other 17β-HSD enzymes including 17β-HSD2, 17β-HSD5, 17β-HSD7 and 17β-HSD12 have also been detected in the endometrium under different pathological conditions like endometriosis and PCOS [10]. Collectively, the data suggest that 17β-HSD1 inhibition is one of the several possible approaches to reduce estrogen production both in eutopic and in ectopic endometrial tissue.

140 Dehydrogenases

will be described separately.

expression level can be used as prognostic marker in breast or prostate cancer. The selective inhibition of the concerned enzymes might provide an effective treatment and a good alternative for treatment of steroid dependent diseases [246]. Having in mind multifunctionaloty 17β-HSD enzymes, the biological and clinical aspects of each isoform

**17β-HSD type1**: 17β-HSD1 catalyzes the activation of estrone (E1) to the most potent estrogen estradiol (E2), predominantly considered as an ezyme of estradiol biosynthesis. It is abundantly expressed in granulosa cells of developing follicles and variable amounts of the enzyme are also expressed in human breast epithelial cells. The enzyme is known to have a crucial role in the development of estrogen-dependent diseases. Based on the in vitro studies, human (h) 17β-HSD1 has been considered as highly estrogen specific, with markedly lower catalytic efficacy towards androgenic substrates. There is a clear difference in the substrate specificity between human and rodent 17β-HSD1 enzymes; the catalytic efficacy of rodent enzyme in vitro is similar for both androgens and estrogens. According a recent review by Saloniemi et al. [10], the h17β-HSD1 is not fully estrogenen-specific but it possesses significant androgenic sctivity. The enzyme catalyses both oxidative (17-hydroxy to 17-keto) and reductive (17-keto to 17-hydroxy) 17b-HSD activity with a proper cofactor added in vitro. However, in cultured cells, the h17β-HSD1 has been shown to catalyse predominantly the reductive reaction [247]. Although h17β-HSD1 expression in various peripheral tissues is low, its catalytic efficacy is markedly higher than those measured for 17β-HSD7 and 17β-HSD12 [248, 242], suggesting an important role for 17β-HSD1 in peripheral E2 formation. Data from animal models further demonstrated the ability of h17β-HSD1 to enhance estrogen action in target tissues and its decrease after treating the mice with 17β-HSD1 inhibitors [10]. These data suggest that 17β-HSD1 plays a major role in determining the gradient between the E2 concentrations in serum and peripheral tissues. An increased E2/E1 ratio by the 17β-HSD1 point out the pivotal role of 17β-HSD1 in breast cancer, ovarian tumor, endometriosis, endometrial hyperplasia and uterine leiomyoma [249, 250]. Consequently, inhibition of 17β-HSD1 is considered as a valuable therapeutic approach for treatment of these deseases. In vivo evaluation of 17β-HSD1 inhibitors is complicated by the fact that the rodent enzymes only show moderate homology/identity to the human one. Due to these species differences, there is a high probability that inhibitors optimized for activity toward rodent 17β-HSD1 do not inhibit the human enzyme. In addition, rodents and humans vary considerably in enzyme distribution in the different tissues. Attempts to overcome these problems include xenograft models using nude mice.

Recently generated mouse genetic model for overexpression of17β-HSD1 (HSD17B1-TG mice) by Saloniemi et al [10] provided valuable data about common female reproductive disorders like Polycystic Ovarian Syndrome (PCOS), ovarian carcinogenesis and endometiosis. Overexpression of hHSD17B1 leads to increased androgen exposure during embryonic development that caused androgen-dependent phenotypic alterations in female, such as increased anogenital distance, lack of vaginal opening and combination of vagina with urethra. These alterations observed in the HSD17B1-TG females were effectively rescued by prenatal anti-androgen (flutamide) treatment, further confirming the 17β-HSD type-2: 17-HSD/KSR2 converts 17β-hydroxy forms of estrogens and androgens (estradiol, testosterone and 5α-dihydrotestosterone) to their less active 17-keto forms (estrone, androstenedione and 5α-androstanedione).The enzyme also possesses 20α-HSD activity, thereby activating 20α-hydroxyprogesterone to progesterone. The 17β-HSD2 enzyme is widely and abundantly expressed in both adult and fetal tissues such as placenta, uterus, liver, the gastrointestinal and urinary tracts. Due to its expression pattern and enzymatic characteristics, it has been suggested that the 17β-HSD2 enzyme protects tissues from excessive steroid action [251]. 17β-HSD2 is localised in the endoplasmic reticulum, and it is widely expressed in various estrogen and androgen target tissues both in human and in rodents including breast endometrium, placenta and prostate. Furthermore, the 17β-HSD2 expression in the placenta and in foetal liver and intestine, together with the observed oxidative 17β-HSD2activity, are the basis for the hypothesis, suggesting a role for the enzyme in lowering the sex steroid exposure of the foetus.

Phylogenic analyses have indicated that 17β-HSD2 is a close homologue of retinoidconverting enzymes and has a high sequence similarity to retinol dehydrogenase type 1. In addition, studies have shown that retinoic acid (RA) induces expression of 17β-HSD2 in a dose- and time-dependent manner in human endometrial epithelial and placental cells [10]. Recent data from transgenic mice (HADS17B2-TG) provide evidence for importance of 17β-HSD2 for prenatal eye morphogenesis and eye development [10]. These TG mice overexpressing human 17β-HSD2 showed growth retardation, disrupted spermayogenesis, female masculinization, delayed eye opening, squint appearance of the eyes and some of these defects closely resembeled those identified in retinoid receptor mutant mice. The most notable changes in the HSD17B1TG mice are well explained by alterations in sex steroid action, whereas in the HSD17B2-TG mice the connection to sex steroids is weaker. The opposite mouse model of deficiency of 17β-HSD2 provide evidence for the essential role of 17β-HSD2. Embryonic death in the HSD17B-KO mice is reported, related to lack of action of 17β-HSD2 enzyme in placenta. Furthermore, the treatment of pregnant female mice with an

anti-estrogen or with progesterone did not prevent the foetal loss of the HSD17B2-KO mice, thus indicating that embryonic deaths is likely not due to the lack of progesterone or due to an increased action of estrogens.

Hydrohysteroid Dehydrogenases – Biological Role and Clinical Importance – Review 143

tissues. Using specific probes and antibodies, human 17β-HSD5 has been localized in liver, adrenal, testis, basal cells of the prostate, and in prostatic carcinoma cell lines [251].

17β-HSD type-6: 17-HSD/KSR6 is part of the catabolic cascade of 5α-dihydrotestosterone (DHT). The 17β-HSD6 shows low dehydrogenase activity with DHT, testosterone and estradiol and possesses a weak oxidative 3α-HSD activity. The 17β-HSD6 enzyme shares 65% sequence identity with retinol dehydrogenase type 1 and it is most abundantly

17β-HSD type-7: 17β-HSD7 is expressed in the developing follicles and in luteinized cells, being the enzyme of ovarian estradiol biosynthesis. Both rodent and human 17β-HSD7 catalyze exclusively the conversion of estrone to estradiol. The 17β-HSD7 is abundantly expressed in corpus luteum during pregnancy and the enzyme is considered to be important in E2 production, especially during pregnancy. In addition, 17β-HSD7 mRNA has been detected in placental, mammary gland and kidney samples [251]. The 17β-HSD7 enzyme was first characterised as a prolactin receptor-associated protein in the rat corpus

A role for mouse 17β-HSD7 in cholesterol biosynthesis was also suggested by the studies, showing a similar expression pattern of 17β-HSD7 and cholesterogenic enzymes during mouse embryonic development. Data from HSD17B7-KO mouse embryos evidently showed the essential role of 17β-HSD7 for cholesterol biosynthesis in vivo. The lack of 17β-HSD7 resulted in a marked blockage in foetal de novo cholesterol synthesis. Histological analysis revealed that the 17β-HSD7 deficiency results in defects in the development of nerve system, vasculature, heart, associated with defect in cholesterol synthesis. HSD17B-KO deficient mice exhibit embryonic lethal phenotypes Tese data suggest a possible role of 17β-HSD7 in cholesterol biosynthesis in mice, while its role in E2 production in vivo needs

17β-HSD type-8: The *Ke 6* gene product has been characterized as a protein whose abnormal regulation is linked to the development of recessive polycystic kidney disease in mice and later it was discovered to be a 17βHSD8. In *in vitro* conditions, 17β-HSD8 converts most eficiently estradiol to estrone and, to some extent, it also catalyses oxidative reactions of androgens and the reduction from estrone to estradiol. The 17β-HSD8 is abundand in kidney, liver and gonads. Interestingly, in the ovary, 17β-HSD8 is present in cumulus cells

17β-HSD type-10: The 17β- -HSD10 has a very broad substrate profile. Interestingly, it has been proposed that this enzyme plays an important role in the pathological processes of Alzheimer's disease (AD), mainly because 17β-HSD10 binds to amyloid-β peptide and appears to be up-regulated in patients suffering from this disease [225]. The mechanism by which 17β-HSD10 contributes to the pathology of AD is still not completely understood. The protein-protein interaction of 17β-HSD10 with amyloid-β appears to inhibit the enzymatic activity of 17β-HSD10. In vitro studies with a potent 17β-HSD10 inhibitor [257] have shown that inhibition of this enzyme can prevent its interaction with the amyloid-β peptide,

Recently, up-regulation of 17β-HSD5 was found in breast and prostate cancer [256].

expressed in liver and prostate, at least in rodent tissues [251].

luteum, although its role in prolactin signalling has remained unknown.

and not in granulosa or luteal cells like 17βHSD1 and 7, respectively [251].

further clarification [10].

Osteoporosis is well known to occurs in elderly people when the level of active sex steroids decreases. Estrogen replacement therapy is beneficial for the treatment of osteoporosis but it is no longer recommended because of adverse effects (breast, endometrial and ovarian cancers, stroke, thromboembolism). Since 17β-HSD2 oxidizes E2 into E1, decreasing the amount of E2 in bone cells, inhibition of this enzyme is a promising approach for the treatment of this disease [225]. Ovariectomized cynomolgus monkeys were used as an osteoporosis model to evaluate the efficacy of 17β-HSD2 inhibitors. Decrease in bone resorption and maintenance of bone formation was achieved in this experimental model.

17β-HSD type-3: 17-HSD/KSR3 17β-HSD3 converts ∆4-androstenedione into testosterone and it is essential for testosterone biosynthesis. The enzyme is present exclusively in the testis and the deficiency of the active enzyme results in male pseudohermaphroditism [252]. In addition to the conversion of androstenedione to testosterone, the enzyme is capable of catalyzing conversion of 5α-androstanedione to 5α- dihydrotestosterone as well as estrone to estradiol [108]. Messender RNA for 17β-HSD3 are over-expressed in prostate cancer tissues. As T is known to be responsible for cell proliferation in androgen dependent diseases, 17β-HSD3 inhibitors (exerting effects equivalent of chemical castration) could be therapeutics for the treatment of such diseases [225]. Day et al. [253] developed the first xenograft model in castrated mice to evaluate 17β-HSD3 inhibitors and strong suppression of tumor growth by 81% was found, suggesting that 17β-HSD3 inhibition might be an efficient strategy for the treatment of hormone dependent prostate cancer.

There are only few observations in human male deficient in 17β-HSD as rare mutation associated with 46XY disorder of sexual development [254]. Patients with 17β-HSD deficiency are usually classified as female at birth (although abdominal testes) but developed secondary male features at pubery with diminished virilization [255].

17β-HSD type-4: Among 17-HSD/KSRs, type 4 is an unique multifunctional enzyme consisting of 17-HSD/KSR-, hydratase- and sterol carrier 2-like domains. 17β-HSD4 is ubiquitously expressed, but in some tissues it shows cell-specific expression. In the brain it is present only in Purkinje cells, in the lung only in bronchial epithelium and in the uterus in luminal and glandular epithelium. The deficiency of 17β-HSD4 leads to disease known as Zellweger syndrome [251].

17β-HSD type-5: 17-HSD/KSR5 is also known as type 2 3α-HSD, and diferently from other 17-HSD/KSRs it belongs to the AKR (aldo-keto reductase) family. With other members of the AKR family (type 1 3α-HSD, type 3 3α-HSD and 20α-HSD), 17β-HSD5 shares 84%, 86% and 88% identity, respectively. Both human and mouse 17β-HSD5 catalyze the conversion of androstenedione to testosterone, and additionally possess 3α-HSD activity. Human 17β-HSD5 has been previously identified predominantly as 3α-HSD. Human, but not mouse, 17β-HSD5 also converts progesterone to 20α-dihydroprogesterone effectively. 17β-HSD5 appears to be involved in the formation of androgens in the testis and several peripheral tissues. Using specific probes and antibodies, human 17β-HSD5 has been localized in liver, adrenal, testis, basal cells of the prostate, and in prostatic carcinoma cell lines [251]. Recently, up-regulation of 17β-HSD5 was found in breast and prostate cancer [256].

142 Dehydrogenases

an increased action of estrogens.

Zellweger syndrome [251].

anti-estrogen or with progesterone did not prevent the foetal loss of the HSD17B2-KO mice, thus indicating that embryonic deaths is likely not due to the lack of progesterone or due to

Osteoporosis is well known to occurs in elderly people when the level of active sex steroids decreases. Estrogen replacement therapy is beneficial for the treatment of osteoporosis but it is no longer recommended because of adverse effects (breast, endometrial and ovarian cancers, stroke, thromboembolism). Since 17β-HSD2 oxidizes E2 into E1, decreasing the amount of E2 in bone cells, inhibition of this enzyme is a promising approach for the treatment of this disease [225]. Ovariectomized cynomolgus monkeys were used as an osteoporosis model to evaluate the efficacy of 17β-HSD2 inhibitors. Decrease in bone resorption and maintenance of bone formation was achieved in this experimental model.

17β-HSD type-3: 17-HSD/KSR3 17β-HSD3 converts ∆4-androstenedione into testosterone and it is essential for testosterone biosynthesis. The enzyme is present exclusively in the testis and the deficiency of the active enzyme results in male pseudohermaphroditism [252]. In addition to the conversion of androstenedione to testosterone, the enzyme is capable of catalyzing conversion of 5α-androstanedione to 5α- dihydrotestosterone as well as estrone to estradiol [108]. Messender RNA for 17β-HSD3 are over-expressed in prostate cancer tissues. As T is known to be responsible for cell proliferation in androgen dependent diseases, 17β-HSD3 inhibitors (exerting effects equivalent of chemical castration) could be therapeutics for the treatment of such diseases [225]. Day et al. [253] developed the first xenograft model in castrated mice to evaluate 17β-HSD3 inhibitors and strong suppression of tumor growth by 81% was found, suggesting that 17β-HSD3 inhibition might be an

There are only few observations in human male deficient in 17β-HSD as rare mutation associated with 46XY disorder of sexual development [254]. Patients with 17β-HSD deficiency are usually classified as female at birth (although abdominal testes) but

17β-HSD type-4: Among 17-HSD/KSRs, type 4 is an unique multifunctional enzyme consisting of 17-HSD/KSR-, hydratase- and sterol carrier 2-like domains. 17β-HSD4 is ubiquitously expressed, but in some tissues it shows cell-specific expression. In the brain it is present only in Purkinje cells, in the lung only in bronchial epithelium and in the uterus in luminal and glandular epithelium. The deficiency of 17β-HSD4 leads to disease known as

17β-HSD type-5: 17-HSD/KSR5 is also known as type 2 3α-HSD, and diferently from other 17-HSD/KSRs it belongs to the AKR (aldo-keto reductase) family. With other members of the AKR family (type 1 3α-HSD, type 3 3α-HSD and 20α-HSD), 17β-HSD5 shares 84%, 86% and 88% identity, respectively. Both human and mouse 17β-HSD5 catalyze the conversion of androstenedione to testosterone, and additionally possess 3α-HSD activity. Human 17β-HSD5 has been previously identified predominantly as 3α-HSD. Human, but not mouse, 17β-HSD5 also converts progesterone to 20α-dihydroprogesterone effectively. 17β-HSD5 appears to be involved in the formation of androgens in the testis and several peripheral

efficient strategy for the treatment of hormone dependent prostate cancer.

developed secondary male features at pubery with diminished virilization [255].

17β-HSD type-6: 17-HSD/KSR6 is part of the catabolic cascade of 5α-dihydrotestosterone (DHT). The 17β-HSD6 shows low dehydrogenase activity with DHT, testosterone and estradiol and possesses a weak oxidative 3α-HSD activity. The 17β-HSD6 enzyme shares 65% sequence identity with retinol dehydrogenase type 1 and it is most abundantly expressed in liver and prostate, at least in rodent tissues [251].

17β-HSD type-7: 17β-HSD7 is expressed in the developing follicles and in luteinized cells, being the enzyme of ovarian estradiol biosynthesis. Both rodent and human 17β-HSD7 catalyze exclusively the conversion of estrone to estradiol. The 17β-HSD7 is abundantly expressed in corpus luteum during pregnancy and the enzyme is considered to be important in E2 production, especially during pregnancy. In addition, 17β-HSD7 mRNA has been detected in placental, mammary gland and kidney samples [251]. The 17β-HSD7 enzyme was first characterised as a prolactin receptor-associated protein in the rat corpus luteum, although its role in prolactin signalling has remained unknown.

A role for mouse 17β-HSD7 in cholesterol biosynthesis was also suggested by the studies, showing a similar expression pattern of 17β-HSD7 and cholesterogenic enzymes during mouse embryonic development. Data from HSD17B7-KO mouse embryos evidently showed the essential role of 17β-HSD7 for cholesterol biosynthesis in vivo. The lack of 17β-HSD7 resulted in a marked blockage in foetal de novo cholesterol synthesis. Histological analysis revealed that the 17β-HSD7 deficiency results in defects in the development of nerve system, vasculature, heart, associated with defect in cholesterol synthesis. HSD17B-KO deficient mice exhibit embryonic lethal phenotypes Tese data suggest a possible role of 17β-HSD7 in cholesterol biosynthesis in mice, while its role in E2 production in vivo needs further clarification [10].

17β-HSD type-8: The *Ke 6* gene product has been characterized as a protein whose abnormal regulation is linked to the development of recessive polycystic kidney disease in mice and later it was discovered to be a 17βHSD8. In *in vitro* conditions, 17β-HSD8 converts most eficiently estradiol to estrone and, to some extent, it also catalyses oxidative reactions of androgens and the reduction from estrone to estradiol. The 17β-HSD8 is abundand in kidney, liver and gonads. Interestingly, in the ovary, 17β-HSD8 is present in cumulus cells and not in granulosa or luteal cells like 17βHSD1 and 7, respectively [251].

17β-HSD type-10: The 17β- -HSD10 has a very broad substrate profile. Interestingly, it has been proposed that this enzyme plays an important role in the pathological processes of Alzheimer's disease (AD), mainly because 17β-HSD10 binds to amyloid-β peptide and appears to be up-regulated in patients suffering from this disease [225]. The mechanism by which 17β-HSD10 contributes to the pathology of AD is still not completely understood. The protein-protein interaction of 17β-HSD10 with amyloid-β appears to inhibit the enzymatic activity of 17β-HSD10. In vitro studies with a potent 17β-HSD10 inhibitor [257] have shown that inhibition of this enzyme can prevent its interaction with the amyloid-β peptide, suggesting 17β HSD10 as a potential target for the treatment of AD.Transgenic mice overexpressing human 17β-HSD10 suggesting that inhibition of 17β-HSD10 could protect from cerebral infarction and ischemia [258].

Hydrohysteroid Dehydrogenases – Biological Role and Clinical Importance – Review 145

studies for better understanding of regulatory mechanisms of the expression of HSD enzymes. Elucidation of molecular events involved in transcription control of HSD is of great importance for molecular desigh of new HSD inhibitors and development of new strategies for appropriate treatment of steroid-dependent deceases without use of invasive

*Inst. Experimental Morphology, Pathology and Anthropology with Museum, Bulgarian Academy of* 

The authors thank to Professor Richard Sharpe for providing samples from experimental models for hormonal manipulations, Chris McKinnel for technical expertise in immunohistochemistry. We are also grateful to Professor Michail Davidoff and Assoc. Professor Mariana Bakalska for studies on EDS experimental model. Authors' work was supported in part by Grant DEER # 212844 funded by FP7-ENV-CP and Grant # DO 02/113 funded by NF "Scientific Research" of Ministry of Education Youth and Science in Bulgaria.

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[5] Giudice L. Clinical practice. Endometriosis. New England Journal of Medicine 2010;

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action, and disease. Vitamines & Hormones 2000; 58: 121–148.

[4] Cancer Research:http://info.cancerresearchuk.org/cancerstats/

*Dept. Anatomy and Histology, Medical University, Plovdiv, Bulgaria* 

techniques.

**Author details** 

Nina Atanassova

Yvetta Koeva

*Sciences, Sofia, Bulgaria* 

**Acknowledgement** 

**14. References** 

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17β-HSD type-12: The mammalian 17β-HSD12 was initially characterised as a 3-ketoacyl-CoA reductase, involved in the long-chain fatty acid synthesis, particularly essential for brain arachidonic acid synthesis. Both the human and the mouse 17β-HSD12 share 40% sequence similarity with 17β-HSD3, and the data indicate that 17β-HSD12 is an ancestor of 17β-HSD3. In human and rodents, 17β-HSD12 is expressed universally and the highest expression of 17β-HSD12 is detected in tissues involved in the lipid metabolism, including the liver, kidney hearth, and skeletal muscle. In mice, the expression has also been detected in brown and white adipose tissue. 17β-HSD12 expression is also regulated by sterol regulatory element binding proteins, identically to that shown to be involved in fatty acid and cholesterol biosynthesis. Interestingly, a reduced expression of 17β-HSD12 in cultured breast cancer cells results in significant inhibition of cell proliferation that is fully recovered by supplementation of arachidonic acid. In addition to its putative role in fatty acid synthesis, human 17β-HSD12 has been shown to catalyse the conversion of E1 to E2 in cultured cells, and the enzyme was suggested to be a major enzyme converting E1 to E2 in postmenopausal women [10]. Analysis of the HSD17B12-KO embryos indicated that the embryos initiated gastrulation but further organogenesis was severely disrupted. The mutant embryos exhibited severe defects in the neuronal development (ectoderm-derives), they failed to grow several mesoderm-derived structures. Therefore, the embryos at the age of E8.5–E9.5 were avoid of all normal embryonic structures that caused their death.
