DNA sequences and their binding proteins required for Sertoli cell-specific transcription of the rat androgen-binding protein gene

Testosterone Retention Mechanism in Sertoli Cells: A Biochemical Perspective

Mechanism(s) involved in regulating Intratesticular Testosterone levels (iT) have assumed importance in recent years, from the point of view of hormonal contraception. Contraceptives using Testosterone (T) in combination with Progestins (P), for more effective suppression of pituitary gonadotropins thereby iT, are not 100% effective in suppressing spermatogenesis in human males, likely due to pesrsistence of Intratesticular Dihydrotestosterone (iD) in poor-responders. Several lacunae pertaining to the mechanism of action of principal male hormone T during spermatogenesis remain to be resolved. Notably, the mechanism through which T brings about the stage-specific differentiation of germ cells lacking Androgen Receptors (AR). Testosterone is a highly anabolic steroid with a rapid tissue clearance rate. T is intratesticular substrate for synthesis of Dihydrotestosterone (DHT) and Estradiol (E2) involved in spermtaogenesis. Therefore, it is important to delineate the mechanism(s) for retention of iT, in order to understand regulation of its bioavailability in testis. In depth studies, pertaining to the role of androgen-binding protein(s) in sequestration, retention and bioavailability of T/DHT are required to understand male fertility regulation. The appropriate approach to overcome this lacuna would be development of mice lacking functional testicular Androgen-Binding Protein (ABPKO), but not deficient T/DHT, Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH), in order to understand its physiological functions. Insights gained about androgen retention mechanism(s) from the ABPKO murine model will be of immense help in improving the efficacy of male hormonal contraceptives and infertility management.

Upstream stimulatory factor induces Nr5a1 and Shbg gene expression during the onset of rat Sertoli cell differentiation

Within the testis, each Sertoli cell can support a finite number of developing germ cells. During development, the cessation of Sertoli cell proliferation and the onset of differentiation establish the final number of Sertoli cells and, thus, the total number of sperm that can be produced. The upstream stimulatory factors 1 and 2 (USF1 and USF2, respectively) differentially regulate numerous Sertoli cell genes during differentiation. To identify genes that are activated by USF proteins during differentiation, studies were conducted in Sertoli cells isolated from 5- and 11-day-old rats, representing proliferating and differentiating cells, respectively. Usf1 mRNA and USF1 protein levels were increased between 5 and 11 days after birth. In vitro studies revealed that USF1 and USF2 DNA-binding activity also increased at 11 days for the promoters of four potential target genes, Fshr, Gata4, Nr5a1, and Shbg. Chromatin immunoprecipitation assays confirmed that USF recruitment increased in vivo between 5 and 11 days after birth at the Fshr, Gata4, and Nr5a1 promoters. Expression of Nr5a1 and Shbg, but not of Fshr or Gata4, mRNAs was elevated in 11-day-old Sertoli cells compared with 5-day-old Sertoli cells. Transient transfection of USF1 and USF2 expression vectors up-regulated Nr5a1 and Shbg promoter activity. RNA interference assays demonstrated that USF1 and USF2 contribute to Nr5a1 and Shbg expression in differentiating cells. Together, these data indicate that increased USF levels induce the expression of Nr5a1 and Shbg during the differentiation of Sertoli cells, whereas Fshr and Gata4 expression is not altered by USF proteins during differentiation.

Role of basic helix-loop-helix (bHLH) and CREB transcription factors in the regulation of Sertoli cell androgen-binding protein expression

Differentiation of Sertoli cells is marked by the presence of novel gene products such as transferrin and androgen-binding protein (ABP). Transcriptional regulation of Sertoli cell differentiation is, in part, controlled through the binding of specific transcription factors to response elements within these genes promoters. Transferrin gene expression has been shown to be regulated by the binding and interactions of basic helix-loop-helix (bHLH) and cAMP response element binding protein (CREB) to an E-box and cyclic AMP response element (CRE), respectively. Interaction between the bHLH and CREB is facilitated through subsequent binding of CREB-binding protein (CBP)/p300. The hypothesis tested in the current study is that ABP expression is regulated by a similar mechanism. The ABP promoter activation was analyzed through the use of transfection assays, site-directed mutagenesis, and electromobility shift assays (EMSA). Transient transfections of rat Sertoli cells used a reporter construct containing the proximal 619 bp of the ABP promoter. Observations suggest that cAMP and follicle stimulating hormone (FSH) upregulate the expression of ABP. Mutational studies of the three E-boxes and the CRE of the 619-bp ABP promoter indicate that all of these elements are critical for stimulation of promoter activity. EMSA revealed a weak interaction between an E-box-2 and the CRE that are overlapping in the promoter. An artificial promoter that contains only an E-box and CRE was created to further test this hypothesis. The artificial promoter was stimulated by both FSH and cAMP. Experiments with mutants of the artificial promoter demonstrate that both response elements contribute to the optimal activation of the promoter construct. The overexpression of the bHLH inhibitor Id (i.e., inhibitor of differentiation) that binds bHLH proteins and eliminates DNA binding was found to suppress hormone activation of the ABP promoter. Combined observations of the ABP promoter and artificial promoter provide insight into a common mechanism for gene regulation in differentiated Sertoli cells involving a role for both the bHLH and CREB family of transcription factors.

Molecular and functional characterization of sex hormone binding globulin in zebrafish

SHBG (sex hormone binding globulin) transports androgens and estrogens in the blood of vertebrates including fish. Orthologs of SHBG in fish are poorly defined, and we have now obtained a zebrafish SHBG cDNA and characterized the zebrafish SHBG gene and protein through molecular biological, biochemical, and informatics approaches. Amino-terminal analysis of zebrafish SHBG indicated that its deduced precursor sequence includes a 25-residue secretion polypeptide and exhibits 22-27% homology with mammalian SHBG sequences and 41% with a deduced fugufish SHBG sequence. The 356-residue mature zebrafish SHBG (39,243 Da) sequence comprises a tandem repeat of laminin G-like domains typical of SHBG sequences; contains three N-glycosylation sites; and exists as a 105,000 +/- 8700 Da homodimer. Zebrafish SHBG exhibits a high affinity and specificity for sex steroids. An RT-PCR indicated that SHBG mRNA first appears in zebrafish larva, and SHBG mRNA was localized within the liver and gut at this stage of development by whole-mount in situ hybridization. In adult fish, SHBG mRNA was found in liver, testis, and gut. In the liver, immunoreactive SHBG was present in hepatocytes and concentrated in intrahepatic bile duct cells, whereas in the testis it was confined to cells surrounding the seminiferous tubule cysts. In the intestine, immunoreactive SHBG was present in the stroma and epithelial cells of the villous projections and the surrounding muscle. The production and presence of SHBG in the gut of developing and adult zebrafish suggests a novel role for this protein in regulating sex steroid action at this site.