Gonadotropic regulation of aromatase activity in the adult rat testis

Estrogens in males: what have we learned in the last 10 years?

This review focuses on the role of estrogen in men, mainly in male reproduction. The continuing increase in data obtained, and recent discoveries in this area will enable a better understanding of male physiology; these, in turn, will have important clinical implications.

Estrogen in the adult male reproductive tract: a review

Testosterone and estrogen are no longer considered male only and female only hormones. Both hormones are important in both sexes. It was known as early as the 1930's that developmental exposure to a high dose of estrogen causes malformation of the male reproductive tract, but the early formative years of reproductive biology as a discipline did not recognize the importance of estrogen in regulating the normal function of the adult male reproductive tract. In the adult testis, estrogen is synthesized by Leydig cells and the germ cells, producing a relatively high concentration in rete testis fluid. Estrogen receptors are present in the testis, efferent ductules and epididymis of most species. However, estrogen receptor-alpha is reported absent in the testis of a few species, including man. Estrogen receptors are abundant in the efferent ductule epithelium, where their primary function is to regulate the expression of proteins involved in fluid reabsorption. Disruption of the alpha-receptor, either in the knockout (alphaERKO) or by treatment with a pure antiestrogen, results in dilution of cauda epididymal sperm, disruption of sperm morphology, inhibition of sodium transport and subsequent water reabsorption, increased secretion of Cl-, and eventual decreased fertility. In addition to this primary regulation of luminal fluid and ion transport, estrogen is also responsible for maintaining a differentiated epithelial morphology. Thus, we conclude that estrogen or its alpha-receptor is an absolute necessity for fertility in the male.

Aromatase expression in the human male

The role of estrogens produced by the testis may involve negative feedback regulation of androgen biosynthesis. Estrogens are also associated with contractile processes of seminiferous tubules, and might have mitogenic effects on Sertoli and Leydig cells. To investigate the location of aromatase (estrogen synthetase) in the testes, tissue from normal human subjects, aged 3 months to 72 years were studied using immunocytochemistry. In mature testes, aromatase immunostain was always associated with Leydig cells and was absent from Sertoli cells. Aromatase activity ranged from 0.014-0.55 pmol estrogen per mg/h and was significantly correlated with the immunostain intensity (P<0.02). Activity and immunostain intensity did not correlate with increasing age. Rather, the highest levels were measured in four of six testes of men aged 18-20 years, three of whom also had the strongest immunostain in larger and more prominent Leydig cell clusters than those in the other specimens. A low level of aromatase activity but no immunostain was detected in prepubertal testes. However, in several prepubertal patients with Peutz-Jegher's Syndrome (PJS) with bilateral multifocal sex cord tumors and enlarged seminiferous tubules and Sertoli cells, aromatase was expressed in these Sertoli cells, but absent from normal Sertoli and Leydig cells. Increased aromatase expression in these tissues involved activation of upstream regulatory elements of the gonadal P II promoter of P-450(arom). In a prepubertal boy with gynecomastia but without PJS, aromatase excess appeared to be due to increased aromatization in skin fibroblasts and lymphocytes. Several members of the patient's family including his sister also expressed high levels of aromatase. This condition appears to be inherited in an autosomal dominant manner.

Effects of human recombinant luteinizing hormone and follicle-stimulating hormone in patients with acquired hypogonadotropic hypogonadism: study of Sertoli and Leydig cell secretions and interactions

Experimental data suggest that FSH-stimulated Sertoli cells can enhance LH-induced Leydig cell testosterone (T) production. The function of Leydig and Sertoli cells can be selectively studied by using recombinant human LH (rhLH) and recombinant human FSH (rhFSH) in patients with complete gonadotropin deficiency. The aim of the present study was to assess the secretion of testicular T, estradiol (E2), and inhibin B and the physiological relevance of the Sertoli-Leydig cell interaction in man. For that purpose, six patients with acquired complete hypogonadotropic hypogonadism received the following treatments for three periods of 1 month in a random order: 1) rhLH, 900 IU/day sc; 2) rhFSH, 150 IU/day sc; and 3) combined rhLH/rhFSH treatments. Each treatment period was separated by a washout period of 15 days. Plasma LH, FSH, T, E2, and inhibin B were measured before and every 10 days during each treatment. During rhLH administration, mean plasma LH levels rose significantly from 0.4 +/- 0.2 IU/L to 11.7 +/- 1.2 IU/L (P < 0.01) and plasma FSH levels did not change. rhFSH administration induced a significant increase in plasma FSH levels (from 0.5 +/- 0.4 to 12.1 +/- 1.4 IU/L; P < 0.01), whereas mean plasma LH levels remained low. Mean plasma E2 levels were unchanged during rhFSH treatment, but they increased significantly during rhLH from 22 +/- 4 to 54 +/- 8 pmol/L (P < 0.01) and during rhLH plus rhFSH administration. rhFSH treatment induced a sustained elevation of mean plasma inhibin B levels from 58 +/- 13 to 175 +/- 25 pg/mL (P < 0.01), similar to the increase occurring during rhFSH plus rhLH administration. In contrast, mean plasma inhibin B levels did not increase during rhLH administration. Finally, a similar and significant increase in mean plasma T levels occurred during both rhLH and rhLH plus rhFSH treatment from 0.9 +/- 0.3 to 5.4 +/- 0.7 nmol/L (P < 0.01) and from 1.0 +/- 0.4 to 6.0 +/- 0.9 nmol/L (P < 0.01), respectively. In contrast, during rhFSH treatment mean plasma T levels remained unchanged when compared with baseline.

IN CONCLUSION

1) the increase of plasma E2 induced by rhLH and the absence of effect of rhFSH confirm that Leydig cells are the major site of testicular E2 production in man; 2) the secretion of inhibin B is increased by rhFSH and not by rhLH, and, thus, Sertoli cells seem to be the main source of inhibin B production; and 3) the increase of plasma T induced by rhLH is not enhanced by rhFSH. These results suggest that the stimulatory effect of FSH on Leydig cell steroidogenesis by a Sertoli cell paracrine factor does not seem to play a major physiologic role in man.

Characterization of a region upstream of exon I.1 of the human CYP19 (aromatase) gene that mediates regulation by retinoids in human choriocarcinoma cells

The biosynthesis of estrogens is catalyzed by aromatase P450 (P450arom), the product of the CYP19 gene. The tissue-specific expression of the CYP19 gene is regulated by means of tissue-specific promoters through the use of alternative splicing mechanisms. Thus, transcripts containing various 5'-untranslated termini are present in ovary, brain, adipose stromal cells, and placenta. Sequence corresponding to untranslated exon I.1 is present uniquely in 5'-termini of transcripts expressed in human placenta and choriocarcinoma cells, as a consequence of expression driven by a distal promoter, I.1. The goal of the present study was the identification of regulatory elements in this promoter region. Various deletion mutations of the upstream flanking region of exon I.1 were constructed using the PCR or restriction enzyme digestion. The genomic fragments were fused upstream of the luciferase reporter gene. These constructs were transfected into human choriocarcinoma (JEG3) cells. The longest construct employed, -924/+10 bp, expressed the highest luciferase reporter gene activity. The -64/+10 bp and -125/+10 bp constructs showed no reporter gene expression. Transfection of the -201/+10 bp construct resulted in reporter gene expression, but at a lower level than that of the -924/+10 bp construct, and this expression was induced by serum as well as by LG69 and TTNPB, ligands specific for RXR and RAR respectively, as well as by vitamin D. These results parallel the actions of the ligands on aromatase activity. Mutation or deletion of an imperfect palindromic sequence (AGGTCATGCCCC) located at -183 to -172 bp upstream of the transcriptional start site of exon I.1 resulted in loss of basal- and retinoid-induced reporter gene expression. Gel retardation analysis using nuclear extracts of JEG3 cells treated with retinoids and the imperfect palindromic sequence as probe, showed that proteins present in the nuclear extracts bound to this sequence in a specific fashion. The binding activities were elevated by incubation of the cells with LG69 and TTNPB, ligands specific for RXR and RAR respectively. Binding of nuclear proteins to the palindromic sequence was displaced either by anti-RXR alpha serum or by anti-VDR serum, suggesting the formation of a heterodimer of RXR alpha and VDR. These results suggest that the imperfect palindromic sequence upstream of exon I.1 plays an important but novel role in the regulated expression of the CYP19 gene in choriocarcinoma cells.

Exon-specific northern analysis and rapid amplification of cDNA ends (RACE) reveal that the proximal promoter II (PII) is responsible for aromatase cytochrome P450 (CYP19) expression in human ovary

Estrogens are synthesized from C19 steroids by a unique form of cytochrome P450, aromatase cytochrome P-450 (P-450AROM; the product of the CYP19 gene). We have shown that tissue-specific expression of human P-450AROM is determined, in part, by the use of alternative promoters. Previous methods of analysis for determining the specific 5'-termini of the different transcripts included S1 nuclease protection, primer extension, and Northern analysis. In the present study we have used the RACE procedure (rapid amplification of cDNA ends) to amplify and clone the 5' termini of P-450AROM transcripts expressed in human corpus luteum (CL). Sequencing of the resulting clones supports the results of the previously performed studies. Specifically, the proximal promoter, PII, is the predominant promoter utilized in CL, such that the start of transcription occurs 26 bp downstream of the putative TATA sequence. A minority of the clones possess an alternative 5'-end, namely I.3. Exon-specific Northern analysis confirms that the majority of the P-450AROM transcripts in CL tissue contain sequence specific for promoter II. Similarly, exon-specific Northern analysis indicates that transcripts in human follicles, as well as granulosa cells in culture, contain primarily sequence specific for promoter II.

Molecular cloning of a cDNA showing alternative splicing of the 5'-untranslated sequence of mRNA for human aromatase P-450

A new type of full-length cDNA clone encoding human aromatase P-450 was isolated from a human placental cDNA library. The clone, designated as pES-4, has a 3130-bp insert. The nucleotide sequences of the translated region and the 3'-untranslated region of the insert of pES-4 are exactly identical with those of the cDNA clone characterized previously. However, the sequence of the 5'-untranslated region of the insert has characteristic feature, i.e. an extra sequence of 109 bp is present at a junction between exon 1 and exon 2 on the processed human aromatase mRNA. Analysis of the genomic clones containing the region between exon 1 and exon 2 of the human aromatase P-450 gene reveals that the 109-bp genomic segment, encoding the same sequence as the extra sequence observed in pES-4, is located approximately 10-kbp downstream of exon 1 and that the nucleotide sequences of the 5'-flanking and the 3'-flanking regions of the segment conform to the GT-AG rule for RNA splicing. By means of reverse transcription and polymerase chain reaction, relative amounts of the pES-4-type mRNA are estimated to be approximately 4.8% and 2.3% of the processed aromatase P-450 mRNA in human placenta and human BeWo choriocarcinoma cells, respectively. These results indicate that the segment of 109 bp between exon 1 and exon 2 is a new exon hitherto unidentified and that heterogeneity observed in the 5'-untranslated sequence of human aromatase P-450 mRNA is, at least in part, caused by alternative splicing of this new exon.

Tissue-specific promoters regulate aromatase cytochrome P450 expression

In the human, estrogen biosynthesis occurs in several tissue sites, including ovary, placenta, adipose, and brain. Recent work from our laboratory has indicated that tissue-specific expression of aromatase cytochrome P450 (P450arom), the enzyme responsible for estrogen biosynthesis, is determined, in part, by the use of tissue-specific promoters. Thus the expression of P450arom in human ovary appears to utilize a promoter proximal to the translation start-site. This promoter is not utilized in placenta but instead, the promoter used to drive aromatase expression in placenta is at least 40 kb upstream from the translational start-site. In addition, there is a minor promoter used in the expression of a small proportion of placental transcripts which is 9 kb upstream from the start of translation. Transcripts from these promoters are also expressed in other fetal tissues including placenta-related cells such as JEG-3 choriocarcinoma cells, hydatidiform moles, and other fetal tissues such as fetal liver. On the other hand, in adipose tissue expression of P450arom may be achieved by yet another, adipose-specific promoter. The various 5'-untranslated exons unique for expression driven by each of these promoters are spliced into a common intron/exon boundary upstream from the translational start-site. This means that the protein expressed in each of the various tissue-specific sites of estrogen biosynthesis is identical.

Regulation of human aromatase cytochrome P450 gene expression

In the human, estrogen biosynthesis occurs in several tissue sites, including ovary, placenta, adipose, and brain. Recent work from our laboratory has indicated that tissue-specific expression of aromatase cytochrome P450 (P450arom), the enzyme responsible for estrogen biosynthesis, is determined, in part, by the use of tissue-specific promoters. Thus the expression of P450arom in human ovary appears to utilize a promoter proximal to the translation start-site. This promoter is not utilized in placenta but instead, the promoter used to drive aromatase expression in placenta is at least 40 kb upstream from the translational start-site. In addition, there is a minor promoter used in the expression of a small proportion of placental transcripts which is 9 kb upstream from the start of translation. Transcripts from these promoters are also expressed in other fetal tissues including placenta-related cells such as JEG-3 choricarcinoma cells, hydatidiform moles, and other fetal tissues such as fetal liver. On the other hand, in adipose tissue expression of P450arom may be achieved by yet another, adipose-specific promoter. The various 5'-untranslated exons unique for expression driven by each of these promoters are spliced into a common intron/exon boundary upstream from the translational start-site. This means that the protein expressed in each of the various tissue-specific sites of estrogen biosynthesis is identical.

Characterization of a cis-acting regulatory element involved in human-aromatase P-450 gene expression

The characteristics of a cis-acting regulatory region involved in the human-aromatase P-450 gene have been examined by transient expression analysis. The region spans from -242 - -166 relative to the cap site of the gene. A fragment containing the region excised from the gene enhances heterologous promoter activity as well as its own promoter activity in a position-independent and orientation-independent manner. The fragment exerts its enhancer activity in human BeWo choriocarcinoma cells in which the aromatase P-450 gene is expressed, but not in other cell lines tested. Deletion of 38 bp from the 3' end of the fragment results in a complete loss of enhancer activity. A gel-retardation assay with nuclear extracts from BeWo cells suggests the existence of a nuclear factor(s) which interacts with the fragment. These results suggest that the regulatory element in the fragment is involved in efficient transcription of the human-aromatase P-450 gene.

Alternative promotion of aromatase P-450 expression in the human placenta

We have previously reported the isolation and characterization of the human gene encoding aromatase cytochrome P-450 (P-450AROM). The gene had been demonstrated to span at least 52 kb and contain ten exons, the first of which, exon I.1, is untranslated. Here we report the isolation and characterization of a P-450AROM cDNA from a human placental primer-extended cDNA library which contains a unique 5' sequence. This cDNA has been isolated and sequences used to screen a human placental genomic library for the presence of a unique first exon. The exon (exon I.2) lies 9 kb 5' of the second, ATG-containing exon (exon II) and is spliced onto exon II at the same site as that reported for exon I.1. DNA sequence analysis indicates that exon I.2 has a putative TATA (TAAA) sequence 33 base pairs (bp) upstream from a putative transcription start site and putative CAAT (CATT) binding sequence beginning at 54 bp upstream from this start site. Polymerase chain reaction (PCR) amplification experiments indicate that mRNA containing exon I.2-specific sequences can be demonstrated in tissues of fetal, but not adult, origin. These data have been confirmed by Northern analysis in the placenta. Characterization of this genomic clone containing exons I.2 and II now establishes the P-450AROM gene to be at least 72 kb in length and raises new questions regarding tissue specific and developmental control of aromatase expression in the human.

Structural and functional characterization of human aromatase P-450 gene

The gene encoding aromatase P-450 (CYP XIX) has been isolated from two types of human genomic DNA libraries. It spans at least 70 kb and consists of 10 exons. The translational initiation site and the termination site are located in exon 2 and exon 10, respectively. The promoter region of the gene contains a TATA box, a CAAT box and two putative AP-1 binding sites beginning at -28, -83, -55 and -68 bp, respectively, from the transcriptional initiation site. In addition, a palindromic nucleotide sequence is observed between -209 and -196 and two types of repetitious hexanucleotide (consensus: AATGAA and CCATAAGG) are also present within the regions between -485 and -433 and between -358 and -331. Transient expression studies of chloramphenicol acetyltransferase constructs bearing various lengths of 5'-flanking region of the gene show that the region between -500 and -243 contains negative cis-acting element(s), whereas the region between -242 and -183 is required for efficient transcriptional activity. Northern blot analysis demonstrates that the expression of aromatose P-450 gene is remarkably stimulated by treatment of cells with 12-O-tetradecanoyl-phorbol 13-acetate. By chloramphenicol acetyltransferase assay the region up to nucleotide position -242 relative to the transcriptional initiation site is shown to participate in the transcriptional responsiveness to this phorbol ester.

Estrogen-dependent Leydig cell protein recognized by monoclonal antibody to MCF-7 cell line

A protein (27,000 molecular weight) was previously found in rat Leydig cells after treatment with estradiol (E2) and human chorionic gonadotropin (hCG) in vitro. The effect of hCG occurred through increased E2 production. This hormone-regulated rat testicular protein was compared to an estrogen-regulated protein of similar physical characteristics isolated from a human mammary cancer cell line (MCF-7) and present in normal human estrogen target organs. The Leydig cells from rat and human tissue showed specific immunofluorescence and immunoperoxidase staining in the cytoplasm upon incubation with a monoclonal antibody (C11) to the estrogen-regulated protein from MCF-7 cells. Leydig cells after exposure to E2 or hCG showed the highest fluorescence intensity; this intensity was reduced by treatment with Tamoxifen. No reaction was associated with other testicular cells. The estrogen-regulated protein from human cell lines is therefore immunologically similar to that from the rat Leydig cell. The monoclonal antibody should be useful for further characterization of the Leydig cell protein.