Diverse roles for sex hormone-binding globulin in reproduction

Abstract

Sex hormone-binding globulin (SHBG) transports androgens and estrogens in blood and regulates their access to target tissues. Hepatic production of SHBG fluctuates throughout the life cycle and is influenced primarily by metabolic and hormonal factors. Genetic differences also contribute to interindividual variations in plasma SHBG levels. In addition to controlling the plasma distribution, metabolic clearance, and bioavailability of sex steroids, SHBG accumulates in the extravascular compartments of some tissues and in the cytoplasm of specific epithelial cells, where it exerts novel effects on androgen and estrogen action. In mammals, the gene-encoding SHBG is expressed primarily in the liver but also at low levels in other tissues, including the testis. In subprimate species, Shbg expression in Sertoli cells is under the control of follicle-stimulating hormone and produces the androgen-binding protein that influences androgen actions in the seminiferous tubules and epididymis. In humans, the SHBG gene is not expressed in Sertoli cells, but its expression in germ cells produces an SHBG isoform that accumulates in the acrosome. In fish, Shbg is produced by the liver but has a unique function in the gill as a portal for natural steroids and xenobiotics, including synthetic steroids. However, salmon have retained a second, poorly conserved Shbg gene that is expressed only in ovary, muscle, and gill and that likely exerts specialized functions in these tissues. The present review compares the production and functions of SHBG in different species and its diverse effects on reproduction.

FIG. 1.

Organization of the human SHBG gene and tertiary structure of human SHBG showing the location and topography of the steroid-binding site. A) The 4.3-kb human SHBG transcription unit that is expressed in hepatocytes under the control of an approximately 800-bp promoter sequence (red). The positions and relative sizes of exons 1–8 that encode the signal polypeptide for secretion and the two LG domain structures that constitute the mature SHBG monomer are color-coded to match regions for which the tertiary structure has been resolved. Exon 1 includes a 60-bp, 5′ untranslated region and the translation start site for the SHBG precursor polypeptide sequence, which includes the signal polypeptide sequence that is removed during secretion of the mature polypeptide, as well as the 3 amino-terminal residues of the mature SHBG protein. B) Tertiary structure of the human SHBG monomer showing the amino-terminal LG domain that has been crystallized with steroid ligands in the single steroid-binding site joined by a linker region (red) to the carboxy-terminal LG domain (gray) that has been modeled from the amino-terminal LG domain structure. The position of the disordered sequence over the steroid-binding site that is influenced by occupancy of a zinc-binding site (zinc molecule shown as a purple ball) and steroid-ligand binding is shown (dashed green tracing). During synthesis and secretion, SHBG monomers dimerize spontaneously in a head-to-head manner to form a linear homodimer at a dimerization domain, the position of which is indicated, and this process is influenced by occupancy of a calcium-binding site (calcium molecule shown as a green ball). C) The topography of the SHBG steroid-binding site, showing how androgens (5α-androstane-3β,17β-diol) and estrogens (estradiol) are oriented differently in the steroid-binding site and how this induces subtle differences in the positioning of surface residues (e.g., Trp84 [arrow]).

FIG. 2.

The 200-bp human SHBG proximal promoter sequence, showing occupancy of cis-elements by transcription factors in response to different physiological states and the corresponding transcriptional activity of the promoter. A) Under conditions when HNF4A levels in hepatocytes are high, the cis-element (1), which resembles a nuclear hormone DR1 response element, is occupied by HNF4A, resulting in maximal transcriptional activity that may be enhanced by differences in growth hormone pulsatility. HNF4A also binds to a second DR1 sequence (3), but this has a more modest effect on transcriptional activity. A binding-site USF1/2 (4) has little effect on the transcriptional activity of SHBG in liver cells. The identity of the transcription factor that binds to the cis-element (2) and its effect on transcription in the liver is not known. B) PPARG-2 competes with HNF4A for binding at the DR1 sequence (3) and preferential binding of a PPARG-2 variant with superior transcriptional activity to this site will repress SHBG promoter activity. C) Under conditions where HNF4A levels are reduced, the DR1 site closest to the transcription start site is occupied by COUP-TF, and this results in a marked reduction of SHBG transcription.

FIG. 3.

Positions of key regulatory elements that control human SHBG transcription in different cell types in the testis. In Sertoli cells, the binding site of USF1/2 to the SHBG proximal promoter (see Fig. 2) blocks its transcription in Sertoli cells. Removal of this USF1/2-binding site from the human SHBG proximal promoter restores its activity in Sertoli cells in vivo in transgenic mice and in mouse Sertoli cells in culture. This modified human SHBG promoter also responds to hormones in the same way as the rat Shbg in Sertoli cells, producing a transcript encoding the SHBG precursor that can be secreted. A conserved SP1-binding site in the corresponding rat Shbg promoter plays a key role in its expression in Sertoli cells, and it probably acts in the same way in the human SHBG promoter that lacks the USF1/2-binding site. In round spermatids and spermatocytes, the SHBG promoter sequence that is active in hepatocytes is silent. Instead, a promoter sequence flanking an alternative exon 1 sequence located approximately 2 kbp upstream is active and under the control of at least two cis-elements for key transcription factors (SPZ1 and CREB/CREM) that influence the expression of genes in these cells.