Immunohistochemical detection of a sex hormone binding globulin like antigen in tissue sections of normal human prostate, benign prostatic hypertrophy and normal human endometrium

Sex hormone-binding globulin provides a novel entry pathway for estradiol and influences subsequent signaling in lymphocytes via membrane receptor

The complex effects of estradiol on non-reproductive tissues/cells, including lymphoid tissues and immunocytes, have increasingly been explored. However, the role of sex hormone binding globulin (SHBG) in the regulation of these genomic and non-genomic actions of estradiol is controversial. Moreover, the expression of SHBG and its internalization by potential receptors, as well as the influence of SHBG on estradiol uptake and signaling in lymphocytes has remained unexplored. Here, we found that human and mouse T cells expressed SHBG intrinsically. In addition, B lymphoid cell lines as well as both primary B and T lymphocytes bound and internalized external SHBG, and the amount of plasma membrane-bound SHBG decreased in B cells of pregnant compared to non-pregnant women. As potential mediators of this process, SHBG receptor candidates expressed by lymphocytes were identified in silico, including estrogen receptor (ER) alpha. Furthermore, cell surface-bound SHBG was detected in close proximity to membrane ERs while highly colocalizing with lipid rafts. The SHBG-membrane ER interaction was found functional since SHBG promoted estradiol uptake by lymphocytes and subsequently influenced Erk1/2 phosphorylation. In conclusion, the SHBG-SHBG receptor-membrane ER complex participates in the rapid estradiol signaling in lymphocytes, and this pathway may be altered in B cells in pregnant women.

Reduced levels of stromal sex hormone-binding globulin and androgen receptor dysfunction in the sperm storage region of the rat epididymis

The cauda epididymidis is the major sperm storage region whose androgenic supply, essential for the sperm viability, is provided by the vasculature and is dependent upon testosterone diffusion through the stromal tissue to reach the epithelial cells. We have focused our efforts on examining the regulation of this important epididymal region by evaluating the impact of the androgen disrupter cimetidine on the epithelial–stromal androgenic microenvironment. Male rats received 100 mg/kg cimetidine (CMTG) or saline (CG) for 50 days, serum testosterone levels were measured and the epididymal cauda region was processed for light and transmission electron microscopy. In the proximal cauda region, the duct diameter was measured and birefringent collagen in the stroma was quantified. TUNEL-labeled epithelial cells were quantified, and androgen receptor (AR), karyopherin alpha (KPNA) and sex hormone-binding globulin (SHBG) levels were analyzed by immunofluorescence and Western blot. CMTG showed reduced duct diameter and high number of apoptotic epithelial cells. In the epithelium, the total AR concentration and the KPNA immunoreactivity were reduced, and a weak/absent AR nuclear immunofluorescence was observed in contrast to the enhanced AR immunolabeling observed in the cytoplasm of the epithelial cells. A significant reduction of collagen and SHBG levels in the stroma was also observed. Cimetidine treatment impairs AR nuclear import in the epithelium, causing androgenic dysfunction and subsequent epithelial cell apoptosis and duct atrophy. The connective tissue atrophy and reduction of SHBG stromal levels associated with epithelial androgenic dysfunction indicate a possible role of stromal SHBG in the androgenic supply of the sperm storage region of the epididymis.

Interactions of sex hormone-binding globulin with target cells

Sex hormone-binding globulin (SHBG) was initially described as a plasma protein synthesized in, and secreted by, the liver. It was discovered by its ability to bind certain androgens and estrogens and, for many years, was believed to serve as a transporter/reservoir for the steroids which it bound. Subsequently, it became clear that the cell membranes of selected tissues contained a receptor for SHBG (R(SHBG)). This review deals with what is known of that receptor - its anatomy, physiology and biochemistry.

Human sex hormone-binding globulin gene expression- multiple promoters and complex alternative splicing

BACKGROUND

Human sex hormone-binding globulin (SHBG) regulates free sex steroid concentrations in plasma and modulates rapid, membrane based steroid signaling. SHBG is encoded by an eight exon-long transcript whose expression is regulated by a downstream promoter (P(L)). The SHBG gene was previously shown to express a second major transcript of unknown function, derived from an upstream promoter (P(T)), and two minor transcripts.

RESULTS

We report that transcriptional expression of the human SHBG gene is far more complex than previously described. P(L) and P(T) direct the expression of at least six independent transcripts each, resulting from alternative splicing of exons 4, 5, 6, and/or 7. We mapped two transcriptional start sites downstream of P(L) and P(T), and present evidence for a third SHBG gene promoter (P(N)) within the neighboring FXR2 gene; PN regulates the expression of at least seven independent SHBG gene transcripts, each possessing a novel, 164-nt first exon (1N). Transcriptional expression patterns were generated for human prostate, breast, testis, liver, and brain, and the LNCaP, MCF-7, and HepG2 cell lines. Each expresses the SHBG transcript, albeit in varying abundance. Alternative splicing was more pronounced in the cancer cell lines. P(L)- P(T)- and P(N)-derived transcripts were most abundant in liver, testis, and prostate, respectively. Initial findings reveal the existence of a smaller immunoreactive SHBG species in LNCaP, MCF-7, and HepG2 cells.

CONCLUSION

These results extend our understanding of human SHBG gene transcription, and raise new and important questions regarding the role of novel alternatively spliced transcripts, their function in hormonally responsive tissues including the breast and prostate, and the role that aberrant SHBG gene expression may play in cancer.

Evidence that fibulin family members contribute to the steroid-dependent extravascular sequestration of sex hormone-binding globulin

Sex hormone-binding globulin (SHBG) binds steroids in the blood but is also present in the extravascular compartments of some tissues. Mice expressing a human SHBG transgene in the liver have human SHBG in their blood. In these animals, human SHBG accumulates within the stromal matrix of the endometrium and epididymis. This is remarkable because these tissues do not express the transgene. Human SHBG administered intravenously to wild-type mice in the presence of estradiol is rapidly sequestered within the endometrial stroma, and this prompted us to search for SHBG interacting proteins. Yeast two-hybrid screens revealed that fibulin-1D and fibulin-2 interact with the amino-terminal laminin G domain of SHBG. These interactions were verified in GST-pull down assays in which human SHBG bound the carboxyl-terminal domains of fibulin-1D and fibulin-2 in a steroid-dependent manner, with estradiol being the most effective ligand, and were enhanced by reducing the N-glycosylation of human SHBG. Like human SHBG, fibulin-1 and fibulin-2 concentrate within the endometrial stroma. In addition, SHBG co-immunoprecipitates with these fibulins in a proestrus uterine extract. These matrix-associated proteins may therefore sequester plasma SHBG within uterine stroma where it can control sex-steroid access to target cells. Given the interplay between fibulins and numerous proteins within the basal lamina, interactions between SHBG and matrix proteins may exert novel biological effects.

Access of reproductive steroids to target tissues

The access of reproductive steroids to their target cells varies considerably between tissues, and is influenced to a great extent by their interactions with plasma steroid-binding proteins, and with SHBG and CBG in particular. An increased awareness of how SHBG and CBG function within the blood circulation, and within extravascular compartments of steroid-responsive target tissues, needs to be incorporated into the design and evaluation of therapies involving the administration of both natural and synthetic steroids, which influence female reproduction and healthy aging.

Sex hormone-binding globulin in the human prostate is locally synthesized and may act as an autocrine/paracrine effector

Sex hormone-binding globulin (SHBG) is a plasma protein synthesized and secreted by the liver. Its initial description stemmed from its ability to bind estrogens and androgens and its capacity to regulate the free concentration of the steroids that bind to it. Additionally, it participates in signal transduction for certain steroid hormones at the cell membrane. It binds with high affinity to a specific membrane receptor (R(SHBG)) in prostate stromal and epithelial cells, wherein the SHBG.R(SHBG) complex forms. An appropriate steroid binds to this complex and results in increases of intracellular cAMP. These two disparate functions of SHBG, regulation of the concentration of free steroids in plasma and signal transduction in selected tissues, raise the question of how its synthesis and secretion might be regulated so as to best perform these two disparate functions. In this paper we demonstrate that SHBG is produced in human prostate cancer cell lines (LNCaP, DU 145, and PC 3) as well as in cultured human prostate epithelial and stromal cells. In addition, in tissue sections of human prostate, we demonstrate the presence of SHBG (immunocytochemistry) and SHBG mRNA (in situ hybridization). These observations are consistent with the hypothesis that SHBG, destined to participate in signaling at the cell membrane, is locally regulated and produced.

Receptors for androgen-binding proteins: internalization and intracellular signalling

In plasma, most steroid hormones are bound and transported by the specific binding protein, testosterone-estradiol-binding globulin (TeBG). For years, it was believed that the only function of this protein was to regulate the concentration of free steroids in plasma. However, a number of reports have provided evidence for the presence of specific TeBG receptors on plasma membranes. Furthermore, the interaction of TeBG with its receptor was shown to be inhibited when steroids are bound to TeBG, suggesting that TeBG is an allosteric protein. The purpose of this manuscript is to review the evidence that androgen-binding proteins bind to membrane receptors, and, in some cells, this binding stimulates cAMP accumulation, and transfer TeBG/ABP into tissue as a consequence of receptor mediated endocytosis. Recent studies from our laboratories have demonstrated binding and uptake of TeBG by MCF-7 breast cancer cells. The interaction of unligated rabbit TeBG with membranes from MCF-7 cells resulted in a time and concentration-dependent increase in adenylate cyclase activity. The TeBG alone also had a reproducible effect on intact cells by increasing cAMP accumulation by 30-35%. The addition of DHT to cells, after TeBG has been allowed to bind, resulted in increases in cAMP of greater than 4-fold. This effect was not blocked by antiandrogens. These data support the hypothesis that extracellular SHBG is a regulator of cellular function through a membrane receptor that is coupled to adenylate cyclase.

Structure, function, and regulation of androgen-binding protein/sex hormone-binding globulin

Despite over 20 years of research, the functions of ABP and SHBG remain elusive. The major reason for this lack of knowledge has been the unavailability of natural mutants with clinical defects for study. There is strong evidence that these binding proteins do act to modulate the gene regulatory actions of nuclear sex steroid receptors by controlling the availability of androgens and estrogens. In plasma, SHBG controls the metabolic clearance rate of sex steroids. In addition there is strong evidence that they have a much broader function. The identification of plasma membrane receptors in target tissues and the finding of homologous domains in several developmental proteins support other functions. Moreover, other experiments suggest the proteins may actually be hormones or growth factors. These findings are not compatible with a model that has the proteins only regulating free steroid hormone levels. Obviously, much more experimentation will be necessary to reveal the functions of ABP and SHBG. The recent discoveries have offered several clues to their functions and open new routes for study. These experiments, coupled with newly developed techniques, such as gene knockout by homologous recombination, make one optimistic that the functions of these unique proteins will be deciphered in the near future.

Binding of sex-hormone-binding globulin (SHBG) to testicular membranes and solubilized receptors

Sex-hormone-binding globulin (SHBG) binds to a specific protein on the surface of prostate, epididymis, and a human breast cancer cell line (MCF-7), and is internalized by these cells. The present study demonstrated specific binding of SHBG to receptor on membranes prepared from rat testes. The binding was saturable, specific, and time and temperature dependent. Scatchard analysis of these binding studies suggested that SHBG binds to a single class of sites on testicular membranes with a Kd at 37 degrees C of 5 x 10(-8) M and a binding capacity of 30 +/- 0.6 pmol/mg protein. These binding characteristics are similar to the SHBG receptor on human prostate and MCF-7 cells. Solubilization of the receptor resulted in a 5-fold increase in its binding capacity (158 +/- 0.3 pmol/mg protein) and a 10-fold decrease in binding affinity (Kd at 37 degrees C = 6.5 x 10(-7) M). The apparent molecular weight of the testicular SHBG receptor, as estimated by gel filtration, was M(r) = 174,000.

CONCLUSION

a specific binding site for SHBG was identified on testicular membranes. This binding site has been tentatively identified as a SHBG receptor based on its physical properties in testicular membrane preparations and following solubilization.

Pharmacokinetics and pharmacodynamics of oral contraceptive steroids: factors influencing steroid metabolism

The time-dependent alterations in the serum concentrations of ethinyl estradiol, gestodene, and 3-keto-desogestrel during treatment with 30 micrograms of ethinyl estradiol + 75 micrograms of gestodene or 30 micrograms of ethinyl estradiol + 150 micrograms of desogestrel were investigated during 12 months. The levels of gestodene and 3-keto-desogestrel increased between days 1 and 21 of each cycle, reaching maximal levels during the third and sixth cycles. The serum concentrations of gestodene were fourfold to fivefold higher than those of 3-keto-desogestrel. The ethinyl estradiol levels increased significantly between days 1 and 10 during each cycle and were significantly higher by 70% during intake of ethinyl estradiol/gestodene compared with ethinyl estradiol/desogestrel, although the dose was identical. Intake of gestodene, in addition to 35 micrograms of ethinyl estradiol + 2 mg of cyproterone acetate, caused a rise in ethinyl estradiol levels. During treatment with ethinyl estradiol/gestodene and an additional 150 micrograms of levonorgestrel, there was a continuous increase in gestodene levels, although sex hormone-binding globulin level did not change. During treatment with 30 or 35 micrograms of ethinyl estradiol and 75 micrograms of gestodene, 150 micrograms of desogestrel, or 2 mg of cyproterone acetate, there were large intraindividual and interindividual variations in the steroid levels and ratios of estrogen: progestogen levels. There was no correlation with the occurrence of intermenstrual bleedings. It is concluded that ethinyl estradiol and nortestosterone derivatives may inhibit steroid-metabolizing enzymes in the liver, which results in a rise in the serum levels of contraceptive steroids. The cause of the large intraindividual variations is as yet unknown, but it is probably from changes in steroid metabolism.