Both reductive forms of 17 beta-hydroxysteroid dehydrogenase (types 1 and 3) are expressed during development in the mouse testis

HSD17B1 Compensates for HSD17B3 Deficiency in Fetal Mouse Testis but Not in Adults

Hydroxysteroid (17β) dehydrogenase (HSD17B) enzymes convert 17-ketosteroids to 17beta-hydroxysteroids, an essential step in testosterone biosynthesis. Human XY individuals with inactivating HSD17B3 mutations are born with female-appearing external genitalia due to testosterone deficiency. However, at puberty their testosterone production reactivates, indicating HSD17B3-independent testosterone synthesis. We have recently shown that Hsd17b3 knockout (3-KO) male mice display a similar endocrine imbalance, with high serum androstenedione and testosterone in adulthood, but milder undermasculinization than humans. Here, we studied whether HSD17B1 is responsible for the remaining HSD17B activity in the 3-KO male mice by generating a Ser134Ala point mutation that disrupted the enzymatic activity of HSD17B1 (1-KO) followed by breeding Hsd17b1/Hsd17b3 double-KO (DKO) mice. In contrast to 3-KO, inactivation of both HSD17B3 and HSD17B1 in mice results in a dramatic drop in testosterone synthesis during the fetal period. This resulted in a female-like anogenital distance at birth, and adult DKO males displayed more severe undermasculinization than 3-KO, including more strongly reduced weight of seminal vesicles, levator ani, epididymis, and testis. However, qualitatively normal spermatogenesis was detected in adult DKO males. Furthermore, similar to 3-KO mice, high serum testosterone was still detected in adult DKO mice, accompanied by upregulation of various steroidogenic enzymes. The data show that HSD17B1 compensates for HSD17B3 deficiency in fetal mouse testis but is not the enzyme responsible for testosterone synthesis in adult mice with inactivated HSD17B3. Therefore, other enzymes are able to convert androstenedione to testosterone in the adult mouse testis and presumably also in the human testis.

Morphohistometric and steroidogenic parameters during testicular and epididymal differentiation in cavy (Galea spixii) fetuses

Sexual differentiation and steroidogenic mechanisms have an important impact on postnatal gonadal phenotypic development. Thus, establishing the activities that lead to male phenotypic development can provide a better understanding of this process. This study examined the prenatal development of cavies to establish morphological and histometric development patterns and protein and enzyme immunolocalization processes that are responsible for androgen synthesis in the testes and epididymis. Histological and histometric analyses of the diameter of the seminiferous cords and epididymal ducts of male fetuses on Days 25, 30, 40, and 50 were performed, as well as immunohistochemistry of the steroidogenic enzymes 5α-reductase and 17β-HSD, the androgen receptor, and the anti-Müllerian hormone (AMH). Our findings showed a cellular grouping of gonocytes from Day 30 onward that was characteristic of the seminiferous cord, which was not present in the lumen at any of the studied dates. From Day 50 onward, the differentiation of the three anatomical regions of the epididymis was evident, the head (caput), body (corpus), and tail (cauda), with tissue distinctions. Furthermore, the diameters of the seminiferous cords and epididymal ducts significantly increased with age. On Day 50, the tail showed the greatest diameter of the three regions. The Sertoli and Leydig cells exhibited AMH immunoreactivity at all dates. In addition, the Leydig cells and epididymal epithelial tissue were immunopositive for 5α-reductase, 17β-HSD, and the androgen receptor; therefore, these factors influenced the development and maintenance of the testis and epididymis during cavy prenatal development.

New Insights into Testosterone Biosynthesis: Novel Observations from HSD17B3 Deficient Mice

Androgens such as testosterone and dihydrotestosterone (DHT) are essential for male sexual development, masculinisation, and fertility. Testosterone is produced via the canonical androgen production pathway and is essential for normal masculinisation and testis function. Disruption to androgen production can result in disorders of sexual development (DSD). In the canonical pathway, 17β-hydroxysteroid dehydrogenase type 3 (HSD17B3) is viewed as a critical enzyme in the production of testosterone, performing the final conversion required. HSD17B3 deficiency in humans is associated with DSD due to low testosterone concentration during development. Individuals with HSD17B3 mutations have poorly masculinised external genitalia that can appear as ambiguous or female, whilst having internal Wolffian structures and testes. Recent studies in mice deficient in HSD17B3 have made the surprising finding that testosterone production is maintained, male mice are masculinised and remain fertile, suggesting differences between mice and human testosterone production exist. We discuss the phenotypic differences observed and the possible other pathways and enzymes that could be contributing to testosterone production and male development. The identification of alternative testosterone synthesising enzymes could inform the development of novel therapies to endogenously regulate testosterone production in individuals with testosterone deficiency.

Ablation of the canonical testosterone production pathway via knockout of the steroidogenic enzyme HSD17B3, reveals a novel mechanism of testicular testosterone production

Male development, fertility, and lifelong health are all androgen-dependent. Approximately 95% of circulating testosterone is synthesized by the testis and the final step in this canonical pathway is controlled by the activity of the hydroxysteroid-dehydrogenase-17-beta-3 (HSD17B3). To determine the role of HSD17B3 in testosterone production and androgenization during male development and function we have characterized a mouse model lacking HSD17B3. The data reveal that developmental masculinization and fertility are normal in mutant males. Ablation of HSD17B3 inhibits hyperstimulation of testosterone production by hCG, although basal testosterone levels are maintained despite the absence of HSD17B3. Reintroduction of HSD17B3 via gene-delivery to Sertoli cells in adulthood partially rescues the adult phenotype, showing that, as in development, different cell-types in the testis are able to work together to produce testosterone. Together, these data show that HS17B3 acts as a rate-limiting-step for the maximum level of testosterone production by the testis but does not control basal testosterone production. Measurement of other enzymes able to convert androstenedione to testosterone identifies HSD17B12 as a candidate enzyme capable of driving basal testosterone production in the testis. Together, these findings expand our understanding of testosterone production in males.

Spermatogonial Type 3 Deiodinase Regulates Thyroid Hormone Target Genes in Developing Testicular Somatic Cells

Premature overexposure to thyroid hormone causes profound effects on testis growth, spermatogenesis, and male fertility. We used genetic mouse models of type 3 deiodinase (DIO3) deficiency to determine the genetic programs affected by premature thyroid hormone action and to define the role of DIO3 in regulating thyroid hormone economy in testicular cells. Gene expression profiling in the neonatal testis of DIO3-deficient mice identified 5699 differentially expressed genes. Upregulated and downregulated genes were, respectively, involved according to DAVID analysis with cell differentiation and proliferation. They included anti-Müllerian hormone and genes involved in the formation of the blood-testis barrier, which are specific to Sertoli cells (SCs). They also included steroidogenic genes, which are specific to Leydig cells. Comparison with published data sets of genes enriched in SCs and spermatogonia, and responsive to retinoic acid (RA), identified a subset of genes that were regulated similarly by RA and thyroid hormone. This subset of genes showed an expression bias, as they were downregulated when enriched in spermatogonia and upregulated when enriched in SCs. Furthermore, using a genetic approach, we found that DIO3 is not expressed in SCs, but spermatogonia-specific inactivation of DIO3 led to impaired testis growth, reduced SC number, decreased cell proliferation and, especially during neonatal development, altered gene expression specific to somatic cells. These findings indicate that spermatogonial DIO3 protects testicular cells from untimely thyroid hormone signaling and demonstrate a mechanism of cross-talk between somatic and germ cells in the neonatal testis that involves the regulation of thyroid hormone availability and action.

Temporal expression pattern of genes during the period of sex differentiation in human embryonic gonads

The precise timing and sequence of changes in expression of key genes and proteins during human sex-differentiation and onset of steroidogenesis was evaluated by whole-genome expression in 67 first trimester human embryonic and fetal ovaries and testis and confirmed by qPCR and immunohistochemistry (IHC). SRY/SOX9 expression initiated in testis around day 40 pc, followed by initiation of AMH and steroidogenic genes required for androgen production at day 53 pc. In ovaries, gene expression of RSPO1, LIN28, FOXL2, WNT2B, and ETV5, were significantly higher than in testis, whereas GLI1 was significantly higher in testis than ovaries. Gene expression was confirmed by IHC for GAGE, SOX9, AMH, CYP17A1, LIN28, WNT2B, ETV5 and GLI1. Gene expression was not associated with the maternal smoking habits. Collectively, a precise temporal determination of changes in expression of key genes involved in human sex-differentiation is defined, with identification of new genes of potential importance.

Wt1 dictates the fate of fetal and adult Leydig cells during development in the mouse testis

Wilms' tumor 1 (Wt1) is a tumor suppressor gene encoding ∼24 zinc finger transcription factors. In the mammalian testis, Wt1 is expressed mostly by Sertoli cells (SCs) involved in testis development, spermatogenesis, and adult Leydig cell (ALC) steroidogenesis. Global knockout (KO) of Wt1 is lethal in mice due to defects in embryogenesis. Herein, we showed that Wt1 is involved in regulating fetal Leydig cell (FLC) degeneration and ALC differentiation during testicular development. Using Wt1(-/flox);Amh-Cre mice that specifically deleted Wt1 in the SC vs. age-matched wild-type (WT) controls, FLC-like-clusters were found in Wt1-deficient testes that remained mitotically active from postnatal day 1 (P1) to P56, and no ALC was detected at these ages. Leydig cells in mutant adult testes displayed morphological features of FLC. Also, FLC-like cells in adult mutant testes had reduced expression in ALC-associated genes Ptgds, Sult1e1, Vcam1, Hsd11b1, Hsd3b6, and Hsd17b3 but high expression of FLC-associated genes Thbs2 and Hsd3b1. Whereas serum LH and testosterone level in mutant mice were not different from controls, intratesticular testosterone level was significantly reduced. Deletion of Wt1 gene also perturbed the expression of steroidogenic enzymes Star, P450c17, Hsd3b6, Hsd3b1, Hsd17b1, and Hsd17b3. FLCs in adult mutant testes failed to convert androstenedione to testosterone due to a lack of Hsd17b3, and this defect was rescued by coculturing with fetal SCs. In summary, FLC-like cells in mutant testes are putative FLCs that remain mitotically active in adult mice, illustrating that Wt1 dictates the fate of FLC and ALC during postnatal testis development.

Down regulation of gene related sex hormone synthesis pathway in mouse testes by miroestrol and deoxymiroestrol

Miroestrol and deoxymiroestrol are phytoestrogens isolated from tuberous root of Pueraria candollei var. mirifica. Modulatory effects of miroestrol and deoxymiroestrol on enzymes involved in sex-hormone synthesis pathway in male C57BL/6 mice were investigated using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). Miroestrol and deoxymiroestrol suppressed the expressions of 3β-HSD, 17β-HSD1, and CYP17 while CYP19 mRNA expression was slightly decreased. In addition, the expression of 17β-HSD2 was induced in correlation with those did by estradiol. These observations supported that miroestrol and deoxymiroestrol could exhibit the same effect as estradiol regarding regulation of testicular gene related sex hormone synthesis pathway.

Role of 11β-OH-C(19) and C(21) steroids in the coupling of 11β-HSD1 and 17β-HSD3 in regulation of testosterone biosynthesis in rat Leydig cells

Here we describe further experiments to support our hypothesis that bidirectional 11β-HSD1-dehydrogenase in Leydig cells is a NADP(H) regenerating system. In the absence of androstenedione (AD), substrate for 17β-HSD3, incubation of Leydig cells with corticosterone (B) or several C(19)- and C(21)-11β-OH-steroids, in the presence of [(3)H]-11-dehydro-corticosterone (A), stimulated 11β-HSD1-reductase activity. However, in presence of 30 μM AD, testosterone (Teso) synthesis is stimulated from 4 to 197 picomole/25,000 cells/30 min and concomitantly inhibited 11β-HSD1-reductase activity, due to competition for the common cofactor NADPH needed for both reactions. Testo production was further significantly increased (p<0.05) to 224-267 picomole/25,000 cells/30 min when 10 μM 11β-OH-steroids (in addition to 30 μM AD) were also included. Similar results were obtained in experiments conducted with lower concentrations of AD (5 μM), and B or A (500 nM). Incubations of 0.3-6.0 μM of corticosterone (plus or minus 30 μM AD) were then performed to test the effectiveness of 17β-HSD3 as a possible NADP(+) regenerating system. In the absence of AD, increasing amounts (3-44 pmol/25,000 cells/30 min) of 11-dehydro-corticosterone were produced with increasing concentrations of corticosterone in the medium. When 30 μM AD was included, the rate of 11-dehydro-corticosterone formation dramatically increased 1.3-5-fold producing 4-210 pmol/25,000 cells/30 min of 11-dehydro-corticosterone. We conclude that 11β-HSD1 is enzymatically coupled to 17β-HSD3, utilizing NADPH and NADP in intermeshed regeneration systems.

Bisphenol A impairs follicle growth, inhibits steroidogenesis, and downregulates rate-limiting enzymes in the estradiol biosynthesis pathway

Bisphenol A (BPA) is used as the backbone for plastics and epoxy resins, including various food and beverage containers. BPA has also been detected in 95% of random urine samples and ovarian follicular fluid of adult women. Few studies have investigated the effects of BPA on antral follicles, the main producers of sex steroid hormones and the only follicles capable of ovulation. Thus, this study tested the hypothesis that postnatal BPA exposure inhibits antral follicle growth and steroidogenesis. To test this hypothesis, antral follicles isolated from 32-day-old FVB mice were cultured with vehicle control (dimethyl sulfoxide [DMSO]), BPA (4.4-440 μM), pregnenolone (10 μg/ml), pregnenolone + BPA 44 μM, and pregnenolone + BPA 440 μM. During the culture, follicles were measured for growth daily. After the culture, media was subjected to ELISA for hormones in the estradiol biosynthesis pathway, and follicles were processed for quantitative real-time PCR of steroidogenic enzymes. The results indicate that BPA (440 μM) inhibits follicle growth and that pregnenolone cotreatment was unable to restore/maintain growth. Furthermore, BPA 44 and 440 μM inhibit progesterone, dehydroepiandrosterone, androstenedione, estrone, testosterone, and estradiol production. Pregnenolone cotreatment was able to increase production of pregnenolone, progesterone, and dehydroepiandrosterone and maintain androstenedione and estrone levels in BPA-treated follicles compared with DMSO controls but was unable to protect testosterone or estradiol levels. Furthermore, pregnenolone was unable to protect follicles from BPA-(44-440 μM) induced inhibition of steroidogenic enzymes compared with the DMSO control. Collectively, these data show that BPA targets the estradiol biosynthesis pathway in the ovary.

Down-regulation of murine testicular 17β-HSD3 and hepatic CYP1A2 enzymes by a bovine testes extract

Purpose

We investigated the effects of a bovine testes extract (BTE), which was developed as an alternative product for andropausal men, on expression of testicular enzymes responsible for sex hormone synthesis genes and a carcinogen activation related gene.

Methods

Expression of testicular CYP1A2, CYP11A1, CYP17, 3β-HSD, and 17β-HSD3 mRNAs as well as hepatic CYP1A2 mRNA were semi-quantitatively determined by RT-PCR. In addition, expression of hepatic CYP1A2 protein and methoxyresorufin O-demethylase activity were carried out.

Results

Bovine testes extract did not alter the testicular expression of CYP11A1, CYP17, and 3β-HSD mRNAs, while that of CYP11A1 was significantly down-regulated by testosterone. Interestingly, administration of BTE for 3 weeks significantly suppressed testicular 17β-HSD3 and hepatic CYP1A2 mRNA. Correspondingly, methoxyresorufin O-demethylase (MROD) activity and expression of hepatic CYP1A2 protein were significantly decreased.

Conclusions

These findings strongly suggested considering risks versus benefits and raised concerns regarding the use of BTE as an alternative medication or health supplement in andropausal men due to its potential for suppressing expression of both 17β-HSD3 and CYP1A2 mRNAs, testicular enzymes responsible for sex hormone gene synthesis.

Leydig cell re-generation and expression of cell signaling molecules in the germ cell-free testis

Leydig cells in the rat testis can be specifically ablated with ethane dimethane sulfonate (EDS) and will subsequently re-generate. In this study, we have characterized Leydig cell re-generation and expression of selected cell-signaling molecules in a germ cell-free model of EDS action. This model offers the advantage that re-generation occurs on a stable background without confounding changes from the regressing and repopulating germ cell population. Adult rats were treated with busulfan to remove the germ cell population and Leydig cells were then ablated with EDS. Testicular testosterone levels declined markedly within 24 h of EDS treatment and started to recover after 8 days. After EDS treatment there were marked declines in levels of Leydig cell-specific mRNA transcripts coding for steroidogenic enzymes cytochrome P450 11a1 (Cyp11a1), cytochrome P450 17a1 (Cyp17a1), 3beta-hydroxysteroid dehydrogenase type 1 (Hsd3b1), 17beta-hydroxysteroid dehydrogenase type 3 (Hsd17b3) and the LH receptor. Levels of all transcripts recovered within 20 days of EDS treatment apart from Hsd17b3, which remained undetectable up to 20 days. Immunohistochemical localization of CYP11A1 during the phase of early Leydig cell re-generation showed that the Leydig cell precursors are spindle-shaped peritubular cells. Studies on factors which may be involved in Leydig cell re-generation showed there were significant but transient increases in platelet-derived growth factor A (Pdgfa), leukemia inhibitory factor (Lif), and neurofilament heavy polypeptide (Nefh) after EDS, while desert hedgehog (Dhh) levels declined sharply but recovered by 3 days. This study shows that the Leydig cell precursors are peritubular cells and that expression of Pdgfa and Lif is increased at the start of the re-generation process when precursor proliferation is likely to be taking place.

Formation of cystic ovarian follicles associated with elevated luteinizing hormone requires estrogen receptor-beta

Stringent regulation of LH secretion from the pituitary is vital to ovarian function in mammals. Two rodent models of LH hypersecretion are the transgenic LHbeta-C-terminal peptide (LHbetaCTP) and estrogen receptor-alpha (ERalpha)-null (alphaERKO) mice. Both exhibit ovarian phenotypes of chronic anovulation, cystic and hemorrhagic follicles, lack of corpora lutea, interstitial/stromal hyperplasia, and elevated plasma estradiol and testosterone. Because ERbeta is highly expressed in granulosa cells of the ovary, we hypothesized the intraovarian actions of ERbeta may be necessary for full manifestation of phenotypes associated with LH hyperstimulation. To address this question, we generated female mice that possess elevated LH, but lack ERbeta, by breeding the LHbetaCTP and ERbeta-null (betaERKO) mice. A comparison of LHbetaCTP, alphaERKO, and betaERKO(LHCTP) females has allowed us to elucidate the contribution of each ER form to the pathologies and endocrinopathies that occur during chronic LH stimulation of the ovary. alphaERKO ovaries respond to elevated LH by exhibiting an amplified steroidogenic pathway characteristic of the follicular stage of the ovarian cycle, whereas wild-type(LHCTP) and betaERKO(LHCTP) females exhibit a steroidogenic profile more characteristic of the luteal stage. In addition, the hemorrhagic and cystic follicles of the LHbetaCTP and alphaERKO ovaries require the intraovarian actions of ERbeta for manifestation, because they were lacking in the betaERKO(LHCTP) ovary. In turn, ectopic expression of the Leydig cell-specific enzyme, Hsd17b3, and male-like testosterone synthesis in the alphaERKO ovary are unique to this genotype and are therefore the culmination of elevated LH and the loss of functional ERalpha within the ovary.

Characterization of the hypothalamic-pituitary-gonadal axis in estrogen receptor (ER) Null mice reveals hypergonadism and endocrine sex reversal in females lacking ERalpha but not ERbeta

To determine the role of each estrogen receptor (ER) form (ERalpha, ERbeta) in mediating the estrogen actions necessary to maintain proper function of the hypothalamic-pituitary-gonadal axis, we have characterized the hypothalamic-pituitary-gonadal axis in female ER knockout (ERKO) mice. Evaluation of pituitary function included gene expression assays for Gnrhr, Cga, Lhb, Fshb, and Prl. Evaluation of ovarian steroidogenic capacity included gene expression assays for the components necessary for estradiol synthesis: i.e. Star, Cyp11a, Cyp17, Cyp19, Hsd3b1, and Hsd17b1. These data were corroborated by assessing plasma levels of the respective peptide and steroid hormones. alphaERKO and alphabetaERKO females exhibited increased pituitary Cga and Lhb expression and increased plasma LH levels, whereas both were normal in betaERKO. Pituitary Fshb expression and plasma FSH were normal in all three ERKOs. In the ovary, all three ERKOs exhibited normal expression of Star, Cyp11a, and Hsd3b1. In contrast, Cyp17 and Cyp19 expression were elevated in alphaERKO but normal in betaERKO and alphabetaERKO. Plasma steroid levels in each ERKO mirrored the steroidogenic enzyme expression, with only the alphaERKO exhibiting elevated androstenedione and estradiol. Elevated plasma testosterone in alphaERKO and alphabetaERKO females was attributable to aberrant expression of Hsd17b3 in the ovary, representing a form of endocrine sex reversal, as this enzyme is unique to the testes. Enhanced steroidogenic capacity in alphaERKO ovaries was erased by treatment with a GnRH antagonist, indicating these phenotypes to be the indirect result of excess LH stimulation that follows the loss of ERalpha in the hypothalamic-pituitary axis. Overall, these findings indicate that ERalpha, but not ERbeta, is indispensable to the negative-feedback effects of estradiol that maintain proper LH secretion from the pituitary. The subsequent hypergonadism is illustrated as increased Cyp17, Cyp19, Hsd17b1, and ectopic Hsd17b3 expression in the ovary.

DNA microarray analysis of genes involved in the process of differentiation in mouse Leydig cell line TTE1

A Leydig cell line, TTE1, was established from the temperature-sensitive simian virus 40 large T-antigen transgenic mice. The cells showed temperature-sensitive growth characteristics and a differentiated phenotype at a nonpermissive temperature. To identify differentially expressed genes in the process of Leydig cell differentiation, the authors carried out microarray analysis of TTE1 cells cultured at permissive and nonpermissive temperatures. The resulting fluorescence-labeled cDNAs synthesized from mRNAs were hybridized with Clontech's Atlas glass mouse 1.0 microarrays. Of the 1081 genes analyzed, the levels of 31 genes were changed, with 24 genes showing increased levels of expression and the remaining 7 genes showing decreased levels. Tie2 was the most changed transcript, with a 13.5-fold upregulation under the differentiated condition. The authors believe this to be the first report of broadscale gene expression in Leydig cell differentiation using the microarray technology. The ability to analyze broadscale gene expression in this manner provides a powerful tool for investigating the molecular mechanisms of Leydig cell functions.

Establishment of Leydig cell line, TTE1, from transgenic mice harboring temperature-sensitive simian virus 40 large T-antigen gene

A Leydig cell line, TTE1, has been established from transgenic mice harboring a temperature-sensitive simian virus 40 (tsSV40) large T-antigen gene. The cells grew at a permissive temperature (33 degrees C), but growth was markedly prevented at a nonpermissive temperature (39 degrees C). T-antigen was expressed in the nuclei at 33 degrees C but disappeared at 39 degrees C, indicating that the cells show a temperature-sensitive growth phenotype reflected by the tsSV40 large T-antigen. TTE1 cells did not show any colony-forming activity in soft agar and form tumors in subcutaneous tissue in nude mice, indicating that the cells were not transformed. Alkaline phosphatase and 3beta-hydroxysteroid dehydrogenase (HSD) activities or expression of cytokeratin and vimentin were observed. Reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that TTE1 cells expressed mRNAs encoding 17beta-HSD types 1 and 3, and inhibin-alpha. The cells with unique characteristics, therefore, should serve useful model study the function of Leydig cell.

Localization of 17beta-hydroxysteroid dehydrogenase/17-ketosteroid reductase isoform expression in the developing mouse testis--androstenedione is the major androgen secreted by fetal/neonatal leydig cells

The final step in the biosynthesis of testosterone is reduction of androstenedione by the enzyme 17beta-hydroxysteroid dehydrogenase/ 17-ketosteroid reductase (17betaHSD/17KSR). In this study, we have examined expression of the four known reductive isoforms of 17betaHSD/ 17KSR (types 1, 3, 5, and 7) in the developing mouse testis and have determined changes in the localization of isoform expression and testosterone secretion during development. Using RT-PCR isoforms 1, 3, and 7 were shown to be expressed in the seminiferous tubules of neonatal testis, whereas isoforms 3 and 7 were expressed in the interstitial tissue of the adult testis. The type 7 isoform is unlikely to be involved in androgen synthesis and further study concentrated on the type 3 isoform. Developmentally, isoform type 3 was expressed in the seminiferous tubules up to day 10, showed little or no expression on day 20 and from day 30 was confined to the interstitial tissue. In situ hybridization confirmed that the type 3 isoform was expressed only in the seminiferous tubules in fetal testes and in the interstitial tissue in adult testes. In accordance with the localization of enzyme messenger RNA expression 17-ketosteroid reductase enzyme activity was very low in isolated interstitial tissue from neonatal testes while interstitial tissue from adult testes showed high activity. Seminiferous tubules from both neonatal and adult testes showed high levels of enzyme activity. The major androgen secreted by the interstitial tissue of prepubertal animals was androstenedione up to day 20 while 5alpha-androstanediol and/or testosterone were the major androgens secreted from day 30 onwards. These results show that fetal Leydig cells do not express significant levels of a reductive isoform of 17betaHSD/ 17KSR and that androstenedione is the major androgen secreted by these cells. Production of testosterone up until puberty is dependent upon 17betaHSD/17KSR activity in the seminiferous tubules--a "two cell" requirement for testosterone synthesis. Expression of the 17betaHSD/17KSR type 3 isoform (the main reductive isoform in the testis) declines in the seminiferous tubules before puberty but then reappears in the developing adult Leydig cell population.

Expression of 3beta-hydroxysteroid dehydrogenase type I and type VI isoforms in the mouse testis during development

Six isoforms of the enzyme 3beta-hydroxysteroid dehydrogenase (3betaHSD) have been identified in the mouse, each the product of a distinct gene. Two of these isoforms (type I and type VI) are detectable in the adult testis but changes in their expression during development are unknown. In this study we have examined changes in testicular expression and localization of mRNA encoding the type I and type VI isoforms of 3betaHSD. Total 3betaHSD (type I plus type VI) mRNA was measured by reverse transcription-polymerase chain reaction and showed a peak of expression at day 5 after birth followed by a decline and then a further rise after day 10 that continued up to adulthood. When each isoform was measured individually it was clear that the type I isoform was expressed at all ages from embryonic day 13 to adulthood. In contrast, the type VI isoform was only expressed at significant levels during fetal life on embryonic day 13 and then not again until after day 10 postnatally. Expression of the type VI isoform mRNA increased markedly after day 10 so that by adulthood it was the predominant 3betaHSD isoform present in the testis. Closer examination of the timing of type VI expression showed that the isoform mRNA was first detectable at a significant level on day 11. In-situ hybridization confirmed that the type I isoform is the only one expressed in the fetal/neonatal animal and showed that expression was limited to the interstitial tissue. In the adult, both type I and type VI expression was within the interstitial tissue. The timing of 3betaHSD type VI mRNA expression suggests, strongly, that this isoform is expressed only by adult-type Leydig cells in the mouse testis and that this development starts shortly before day 11. The limited expression of the type VI isoform means that it will be a useful marker in studies of adult Leydig cell development.

Fetal development of Leydig cell activity in the mouse is independent of pituitary gonadotroph function

During fetal development the testes secrete anti-Mullerian hormone and testosterone to induce formation of the male phenotype. Adult Leydig cells secrete testosterone under the control of LH, but the role of the fetal pituitary in regulating fetal Leydig cell function is unclear. To study the early relationship between pituitary and Leydig cell function, we have examined the development of fetal pituitary LH levels and Leydig cell function in normal mice and in hypogonadal (hpg) mice that lack GnRH and, thus, circulating gonadotropins. In normal and hpg mice, pituitary LH content was barely detectable until embryonic day 17 (E17), when levels began to increase significantly in both groups. Pituitary levels of LH in hpg mice were, however, only about 10% of normal at all ages. Full-length LH receptor transcripts were first detectable in fetal testes on E16 in both normal and hpg mice. In normal mice, levels of testicular messenger RNA (mRNA) encoding cytochrome P450 side-chain cleavage and 17alpha-hydroxylase increased from E13 to reach a peak around birth. In hpg mice, levels of mRNA encoding these enzymes were normal until around birth, at which time there was a significant decline. Levels of testicular mRNA encoding 3beta-hydroxysteroid dehydrogenase type I were similar in normal and hpg mice and showed little change during development. Intratesticular testosterone reached a peak on E18 in normal animals before declining again after birth. In hpg mice, intratesticular testosterone levels were normal throughout fetal development and on the day of birth, but were barely detectable by postnatal day 5. Results show 1) that fetal Leydig cell function in the mouse is normal in the absence of endogenous circulating gonadotropins; 2) that Leydig cells become dependent on gonadotropins shortly after birth; and 3) that pituitary LH synthesis can start in the absence of GnRH but is dependent on LH for a normal level of synthesis and secretion.

Localisation and regulation of 17beta-hydroxysteroid dehydrogenase type 3 mRNA during development in the mouse testis

The final step in the biosynthesis of testosterone is the reduction of androstenedione to testosterone catalysed by the enzyme 17beta-hydroxysteroid dehydrogenase (17betaHSD). Five isoforms of the enzyme have been identified in the mouse and the type 3 isoform has been shown to be the predominant reductive form present in the adult human and mouse testis. In this study the regulation of 17betaHSD type 3 isoform mRNA levels and the cellular localisation of the enzyme mRNA have been studied in the mouse testis. To examine regulation of 17betaHSD type 3 mRNA expression in the testis, mRNA levels were measured during development in normal mice and in mice lacking circulating gonadotrophins (hpg) or functional androgen receptors (Tfm). In these mutants testicular descent does not occur at the normal time (25 days) and control animals were, therefore, rendered cryptorchid at 19 days. In neonatal mice, it has been shown a peak of type 3 expression occurs around day 5 and this was found to be normal in all groups in the current study. In normal animals there was a marked increase in type 3 isoform expression between 25 and 30 days and this continued into adulthood. In cryptorchid animals the increase in type 3 mRNA levels after 25 days was less marked than in untreated controls and by 90 days was about 15% of normal animals. In Tfm mice, levels of 17betaHSD type 3 mRNA failed to show any increase around puberty (25 days) and in adult Tfm mice, levels were less than 1% of cryptorchid controls. In hpg mice, levels of type 3 mRNA increased slowly after puberty and were about 30% of cryptorchid controls by 90 days. Studies using in situ hybridisation showed that the type 3 isoform was expressed only in the interstitial tissue of the adult normal mouse testis. No specific hybridisation could be determined in adult hpg or Tfm testes. Results show that 17betaHSD type 3 is an interstitial enzyme in the testis and is, probably, localised in the Leydig cells. During neonatal development expression of 17betaHSD type 3 is independent of gonadotrophin action while the increase in type 3 expression at puberty is primarily dependent upon androgen action although testicular descent and gonadotrophins are also required.

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.

Sequence of mouse 17beta-hydroxysteroid dehydrogenase type 3 cDNA and tissue distribution of the type 1 and type 3 isoform mRNAs

The enzyme 17beta-hydroxysteroid dehydrogenase (17betaHSD) interconverts 17-ketosteroids and 17beta-hydroxysteroids. Five isoforms of the enzyme have been identified in the human, two of which (types 1 and 3) have been shown to catalyse the reduction reaction preferentially. The cDNA encoding mouse 17betaHSD type 3 was isolated from testis cDNA using the RACE technique and primers based on the human sequence. The mouse protein is 305 amino acids in length which is five short of the human protein with four of these amino acids missing at the N-terminus. The predicted amino acid sequence is 72.5% identical and 94.8% similar to the human sequence. Tissue distribution of mRNA encoding both types 1 and 3 17betaHSD was studied using reverse transcription and the polymerase chain reaction (RT-PCR). Highest levels of type 1 mRNA were found in the ovary whereas highest levels of type 3 were in the testis. All other tissues tested contained mRNA encoding both isoforms of the enzyme although levels were markedly lower than in the gonads.