Immunohistochemical detection of the sex steroid-binding plasma protein in human mammary carcinoma cells

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.

Sex Hormone-Binding Globulin (SHBG), estradiol and breast cancer

The human serum Sex Hormone-Binding Globulin (SHBG) plays an important role in breast cancer pathophysiology and risk definition, since it regulates the bioavailable fraction of circulating estradiol. We here summarize data reported over the years concerning the involvement of SHBG and SHBG polymorphisms in the definition of breast cancer risk. We also report what is known about the direct action of SHBG in breast cancer cells, illustrating its interaction with these cells and the subsequent initiation of a specific intracellular pathway leading to cross-talk with the estradiol-activated pathway and, finally, to the inhibition of several effects of estradiol in breast cancer cells. In conclusion, as a result of its unique property of regulating the estrogen free fraction and cross-talking with the estradiol pathways, by inhibiting estradiol-induced breast cancer cell growth and proliferation, SHBG is associated with a reduced risk of developing the neoplasm after estrogen exposure.

Phytoestrogens as modulators of steroid action in target cells

Although numerous reports exist on the potential beneficial role of nutritional phytoestrogens in human health, their molecular mechanism in target cells is still not completely understood. Phytoestrogens promote estrogen and antiestrogen effects by interacting with numerous molecules, carrier proteins, enzymes and membrane and nuclear receptors, directly or indirectly involved in the transfer of estrogen signals. The hypothesis that the ER beta subtype plays a key role in antiproliferative effect of phytoestrogens, especially in breast cancer, is examined here. This review focus on the effects of phytoestrogens in developmental processes such as those linked to reproductive function, tumorigenesis and angiogenesis.

The shbg Gene and Hormone Dependence of Breast Cancer: A Novel Mechanism of Hormone Dependence of MCF-7 Human Breast Cancer Cells Based upon SHBG

BACKGROUND: SHBG (sex hormone binding globulin) is a 45 kDa glycoprotein thatbinds sex steroid with high specificity and affinity. SHBG is produced in various tissues including breast, liver, endometrium, and prostate via activated ER alpha and is secreted into plasma. SHBG regulates the activity of bioavailable sex steroid in plasma and in cells and also modulates cell growth regulation. METHODS: The predictive value of SHBG on the efficacy of hormone therapy against human breast cancer was determined. To evaluate the role of shbg gene expression in estrogen-dependent cell growth of MCF-7 breast cancer, cDNA cloning and determination of the expression of the shbg gene of MCF-7 cells was performed using PCR, RT-PCR Southern blotting. RESULTS: The SHBG titer (17 beta -estradiol binding capacity of SHBG) showed high predictability for the hormone dependence of breast cancer. Tumors of patientswith high SHBG titers showed a 91.8% response rate (N = 49). In contrast, tumors of patients with low SHBG titers showed only an 8.2% response rate (N = 61). >From our experimental results using MCF-7 cells, it is suggested that the SHBG titer includes SHBG secreted from liver and breast cancer cells. MCF-7 cells showed high expression of the wild type shbg gene, hybridized with Hammond's SHBG probe, which represents the 3'portion of SHBG-cDNA cloned from hepatocytes. E2 (17 beta-estradiol) induced the expression of the wild type shbg gene. However, the exon VII splicing variant of the shbg gene did not respond to E2 induction. CONCLUSIONS: From our results and the reports of other investigators, it is suggested that loss of hormone dependence in breast cancer may be caused by the loss of wild type shbg gene and the appearance of the exon VII splicing variant.The shbg-E2 complex binds to SHBG receptor (SHBGR) in cell membrane and internalizes through SHBGR mediated endocytosis causing the production of intracellularcAMP and E2-responsive second messenger. SHBG functions as a nuclear protein. From these data, we prepared a model of a novel mechanism of hormone dependence of breast cancer based upon SHBG and the shbg gene.

Sex hormone binding globulin expression and colocalization with estrogen receptor in the human Fallopian tube

The detection of sex hormone binding globulin (SHBG) or SHBG mRNA in several sex steroid target tissues, has raised the possibility that SHBG modulates the action of sex steroids outside the vascular compartment. The presence of SHBG mRNA was investigated by RT-PCR in the poly (A+) RNA fraction of the human Fallopian tube. Human and rat liver were used as positive and negative control tissues, respectively. The electrophoretic analysis of the amplified PCR products showed bands at 219 bp, corresponding to the expected size of the SHBG cDNA, in the Fallopian tube and human liver but not in rat liver, indicating that SHBG might be synthesized by the Fallopian tube. The cellular localization of SHBG and of estrogen receptor (ER) was examined by immunohistochemistry in consecutive sections of Fallopian tube tissues for individual staining or double immunostaining in the same section. Specific immunostaining of SHBG was present in the epithelial, vascular and muscle cells of the ampullary and isthmic region. In epithelial cells, immunoreactive SHBG was present in the apical end with the highest concentration close to the luminal membrane. The ER was localized in the nuclei of epithelial, stromal and muscle cells of the ampulla and isthmus. Double immunostaining showed that SHBG and ER are colocalized principally in epithelial cells of the ampulla and in muscle cells of the isthmus. In conclusion, the detection of SHBG and SHBG mRNA and the localization of SHBG in estrogen target cells was shown. These findings support the hypothesis that SHBG might regulate sex steroid action at the tissue level.

Sex hormone binding globulin mRNA in human breast cancer: detection in cell lines and tumor samples

Sex hormone binding globulin (SHBG) is a high affinity binding protein for estrogens and androgens. SHBG has been found in breast tissue and cell lines through immunostaining. The goal of this series of experiments was to determine whether mRNA for SHBG is expressed in breast cancer cell lines and tumor tissue. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to detect SHBG and beta-2 microglobulin (control for tissue extractions). Three breast cancer cell lines, ZR-75-1, MCF-7, and MDA-MB-231 and 56 breast tissue samples were collected and analysed for SHBG mRNA expression. mRNA was successfully extracted from 30 of these breast tissue samples. SHBG mRNA was detected in ZR-75-1, MCF-7 and MDA-MB-231 cells, and in 11 of the breast tissue samples. Two PCR products were routinely amplified from the breast cancer cell line RNA, one at approximately 500 bp and another at approximately 300 bp. The DNA sequence of the 300 bp PCR produce was consistent with alternate splicing of the SHBG mRNA, where exon 7 is deleted, and is accompanied by a point deletion at the beginning of exon 8. SHBG protein production from the three breast cancer cell lines was detected by immunoprecipitation using an affinity purified SHBG antibody. SHBG mRNA was found in 11 of 30 samples of breast tissue. Some samples expressed only the 500 bp or the 300 bp PCR product, whereas others expressed both PCR products. The presence of SHBG mRNA in these samples was not associated with either the presence or absence of steroid receptors. SHBG mRNA is thus expressed in breast cancer cell lines, and in some breast tissue samples.

Interactions between phytoestrogens and human sex steroid binding protein

The interactions of human Sex steroid binding protein (SBP) and the lignans [Nordihydrogaiaretic acid (NDGA) enterolactone (Ent), enterodiol (End)] and isoflavonoid phytoestrogens [Equol (Eq), diazein Dad), genistein (Gen)] were studied. The phytoestrogens had different dose-dependent inhibitory effects on steroid binding by SBP. Their relative efficiencies were: Ent> or = NDGA = Eq > Gen for displacing E2 and Eq > Ent > NDGA > Gen for displacing T. End and Dad were much less active. Scatchard analysis suggested that NDGA had similar non- competitive effects on T and E2 binding by reducing the number of binding sites without changing the association constants. But Eq seemed to inhibit E2 binding non-competitively and T binding competitively. NDGA binding to SBP reduced the immunorecognition of SBP by monospecific anti-SBP antibodies, suggesting that NDGA changed SBP immunoreactivity. Unlike NDGA, Eq binding to SBP caused no immunological changes in SBP, indicating qualitative differences in the effects of the lignan and isoflavonoid. Our results indicate that phytoestrogens may modulate the SBP activity and so influence the role of this protein in the delivery of hormonal information to sex steroid-dependent cells.

Antioxidants and hormone-mediated health benefits of whole grains

Lignans and phytoestrogens have been associated with protective effect against hormone-related diseases, for example, cancer of the breast and prostate, and potential mechanisms for this effect have been reported. Antioxidants also appear to have some protective effect against diseases associated with reactive free radicals such as coronary heart disease and cancer. Whole grains contain some of these substances particularly the mammalian lignan precursors, vitamin E, other phenolic compounds, Se, and phytic acid. These substances may in part be responsible for the reduced risk of cancer and coronary heart disease associated with intake of high-fiber diets containing whole grains. Because they are more associated with the fiber in the outer layers of the grain, the intake of whole vs. refined grain is emphasized for optimum health benefits.

Endocytosis of human sex steroid-binding protein in monkey germ cells

We investigate in this study the hypothesis of human sex steroid-binding protein hSBP internalization into germ cells in a primate model. Human SBP was purified from late-pregnancy serum and labeled either with colloidal gold particles (18 nm) or with [3H]delta 6-testosterone by photoaffinity treatment. The germ cells were isolated from sexually mature monkey testis or caput epididymis (Macaca fascicularis) by mechanical means and cell suspensions (4 x 10(6) per 100 microliters culture medium) were incubated in presence of hSBP-gold complex (60 ng/100 microliters) or hSBP-[3H]delta 6-testosterone complex (66 ng/100 microliters, 20,000 cpm) for 2, 5, 15, 45, and 60 min. The samples were processed for electron microscopy followed by autoradiographic treatment for the radiolabeled samples. Localization of the label occurred over the whole germ cell lineage whichever tracer was used. Spermatogonia, spermatocytes, spermatids, testicular and epididymal spermatozoa exhibited specific binding sites over the plasma membrane associated with clathrin-like coated pits and vesicles. At 34 degrees C, intracellular localization of the labeled ligand was found within coated vesicles, in early and late endosomes. In addition, in early spermatogenic cells, labeled ligand was detected in the nuclei and/or associated with the nuclear envelope whereas in late spermatids and residual bodies, the labeling was accumulated in multivesicular, prelysosomal structures. Quantitative analysis of the "labeled cells/total cells" ratio exhibited a negative correlation to the maturation steps, epididymal spermatozoa being the least labeled. The cellular distribution is similar with one or the other protein in the same spermatogenic cells. Unlabeled hSBP treatment prior to labeled hSBP reduced significantly the internalization. Lowering the temperature to 4 degrees C prevented endocytosis and enhanced membrane binding. EDTA pretreatment strongly decreased hSBP internalization and modified the early endocytic steps, namely, the pinching off of the coated vesicles. It is concluded that monkey germ cells are able to internalize the human sex steroid-binding protein through specific endocytic organelles. This endocytosis leads to the labeling of the nuclei in the early spermatogenic cells and of the multivesicular bodies in the late germ cells. This strongly suggests that steroid-binding proteins may be required for spermatogenesis in acting at the germ cell lineage level either by themselves or by serving as steroid transmembrane carriers.

Effects of hormones on SBP mRNA levels in human cancer cells

The human plasma sex steroid binding protein (SBP) has been previously shown to be synthesized in liver cells. The hormonal regulation studies of hepatic SBP mRNA demonstrate that it is controlled by estradiol, antiestrogen tamoxifen, dihydrotestosterone, triiodothyronine and insulin in a similar way as secreted SBP. The metabolic inhibitor cycloheximide was unable to prevent the estrogen or thyroid hormone induced increase in SBP mRNA. The slight stimulation of SBP synthesis by estradiol suggests that non-steroidal factors may be involved in its regulation and that the estrogen regulatory mechanism could also be partly post-transcriptional. In endometrial (Ishikawa cells) and prostatic (LNCaP cells) carcinoma cells, SBP mRNA has been detected suggesting that SBP may play a role in the uptake and intracellular mechanism of action of sex steroid in target cells.

Effects of sex hormone binding globulin (SHBG) on human prostatic carcinoma

The purpose of this study was to determine what effects sex hormone binding globulin (SHBG) might have on the growth and steroid content of human prostate carcinoma. Two human prostate carcinoma cell lines were used for this study, ALVA-41 and ALVA-101. The first part of the study was to determine the effect of SHBG or albumin on the uptake of [3H]DHT in the cells. In this experiment both SHBG and albumin inhibits the uptake of [3H]DHT into each of the cell lines when studied in vitro. The degree of inhibition was dependent on the binding capacity of the protein. When [3H]thymidine uptake was measured in each of the cell lines following either the addition of SHBG or albumin to the culture media, an increase in uptake and presumably DNA synthesis was noted in the ALVA-41 and ALVA-101 cells for SHBG additions but not for albumin. Further, this stimulation was increased when testosterone was added to the media, however, [3H]thymidine uptake was decreased by high concentrations of dihydrotestosterone (DHT) or if the SHBG was saturated with DHT prior to being added to the media. The cells also demonstrate high affinity cell membrane receptors for SHBG. Finally, using a 3', 550 bp cDNA or SHBG, 1.9 and 2.8 kb mRNAs were detected on Northern analysis of the ALVA-101 and ALVA-41 cells. These data indicate SHBG can inhibit uptake of steroids into the prostate, but also it may act as a stimulus for growth through a SHBG cell surface receptor. In addition, the growth effect may be through an autocrine effect from SHBG or a SHBG-related peptide.

Sex steroid binding protein (SBP) receptors in estrogen sensitive tissues

Since the discovery of a specific membrane binding site for sex steroid binding protein (SBP) in human decidual endometrium and in hyperplastic prostate numerous speculations have been raised on the existence of an additional non-receptor-mediated system for steroid hormone action. In the present work SBP cell membrane binding was investigated in human estrogen target tissues other than those previously studied either in the absence of steroids or in the presence of varying amounts (10(-10)-10(-6) M) of estradiol, testosterone and dihydrotestosterone, respectively. Plasma membranes obtained by differential centrifugation from homogenized samples of pre-menopausal endometrium, endometrium adenocarcinoma, normal liver and post-menopausal breast showed a specific binding of highly purified [125I]SBP: a major displacement of labeled SBP was elicited by radioinert SBP, while no significant displacement occurred when other human plasma proteins were used as cold competitors (molar excess ranging 500-10,000-fold). A specific, time-dependent binding of [125I]SBP was also observed in MCF-7 and in Hep-G2 cell lines. The different patterns of specific binding, observed in membranes from different tissues when SBP was liganded with different sex steroid molecules, leads us to consider the tissue individuality of the receptor as a further entity in the membrane recognition system for SBP.

Sex hormone binding globulin: origin, function and clinical significance

Sex hormone binding globulin (SHBG) is a glycoprotein possessing high affinity binding for 17 beta-hydroxysteriod hormones such as testosterone and oestradiol. It is probably synthesized in the liver, plasma concentrations being regulated by, amongst other things, androgen/oestrogen balance, thyroid hormones, insulin and dietary factors, it is involved in transport of sex steroids in plasma and its concentration is a major factor regulating their distribution between the protein-bound and free states. Its detailed role in the delivery of hormones to target tissues is not yet clear. Plasma SHBG concentrations are affected by a number of different diseases, high values being found in hyperthyroidism, hypogonadism, androgen insensitivity and hepatic cirrhosis in men. Low concentrations are found in myxoedema, hyperprolactinaemia and syndromes of excessive androgen activity. Concentrations are also affected by drugs such as androgens, oestrogens, thyroid hormones and anti-convulsants. Measurement of SHBG is useful in the evaluation of mild disorders of androgen metabolism and enables identification of those women with hirsutism who are more likely to respond to oestrogen therapy. Testosterone:SHBG ratios correlate well with both measured and calculated values of free testosterone and help to discriminate subjects with excessive androgen activity from normal individuals.

Direct effect of plasma sex hormone binding globulin (SHBG) on the metabolic clearance rate of 17 beta-estradiol in the primate

Sex hormone binding globulin (SHBG) has been shown to be a major determinant of testosterone clearance in the primate. It has also been suggested that SHBG would also be a determinant of estradiol clearance (MCR-E2). However, published studies have suggested that the MCR-E2 do not always vary with changes in the level of SHBG. Therefore, the present study was undertaken to address this issue. The baseline MCR-E2 was determined in adult male pigtail macaques (Macaca nemestrina). Following the baseline determination of MCR-E2 the animals were infused with either purified human (h) SHBG or antibody against hSHBG, which also has a high degree of cross-reactivity with primate SHBG. Following the infusions of either hSHBG or anti-SHBG, MCR-E2 was again determined. In addition, luteinizing hormone (LH) was measured using a mouse Leydig cell bioassay. Following the infusion of hSHBG, a marked increase in serum SHBG was noted and the MCR-E2 decreased. Associated with the increase in SHBG, the SHBG bound T levels decreased and LH increased. Following the infusion of antibody, serum SHBG decreased, and the MCR-E2 also decreased. With the decrease in SHBG following the antibody infusion, non-SHBG bound T increased and serum LH fell. This study demonstrates that an increase in the serum SHBG levels is associated with a decrease in MCR-E2, however, an acute decrease in serum SHBG also decreases the MCR-E2. This later result demonstrates that factors in addition to serum SHBG binding may be important in determining the MCR-E2.

Sex steroid-binding protein: identification and comparison of the primary product following cell-free translation of human and monkey (Macaca fascicularis) liver RNA

A very close similarity in molecular, steroid-binding and immunological properties have been demonstrated for the sex steroid-binding proteins of plasma from human (hSBP) and monkey (mSBP): both are glycoproteins composed of two similar subunits able to bind one steroid molecule and to cross-react with the same antibodies. After translation of human and monkey (Macaca fascicularis) liver mRNAs by a wheat-germ embryo extract, in the presence of labelled amino-acids, we have characterized in both cases a single radioactive polypeptide immunologically related to SBP, migrating in SDS-PAGE as a single band and having a molecular weight of about 42,000. This protein could be displaced from the antibody by pure unlabelled SBP in excess. The difference in molecular weight between the in vitro translation product and the native SBP sub-unit is probably due to the absence of glycosylation in the neo-synthesized protein. The radioactivity incorporated into mSBP was 4 times higher than the radioactivity incorporated into hSBP, suggesting that the amount of mRNA for SBP is higher in monkey than in human liver. Our results show that the two sub-units of hSBP and mSBP derive from a common precursor, representing respectively 0.0050% and 0.0013% of the total neosynthesized proteins in monkey and in human liver.

Half-life of plasma sex steroid-binding protein (SBP) in the primate

We have recently shown that the metabolic clearance of testosterone in plasma is directly dependent on sex steroid-binding protein (SBP or SHBG) levels [J. steroid. Biochem. 22, 739 (1986)]. In order to further understand the relationship between these two parameters, we have measured the half-life of SBP in plasma of female rhesus monkeys. SBP was purified to homogeneity from pooled Macaca nemestrina serum, and iodinated with 125I. The labeled protein ([125I]nSBP) was purified by chromatography on DEAE-agarose and fractions identified as immunologically reactive against anti-human SBP were collected. Protein purity of [125I]nSBP was established by SDS gel electrophoresis using an unlabelled Macaca nemestrina SBP as standard. The labeled protein was infused intravenously into two different adult female Macaca mulatta (rhesus) monkeys. Plasma samples were collected at short intervals during the first 24 h after infusion, and then daily for 7-9 days. The clearance profile of labeled SBP in plasma was quantitated by radioactivity measurement and immunoprecipitation. Analysis of the results indicate that the rate of SBP clearance in plasma has two components, the t1/2 (app) of the first component is 7.5 h (r = 0.94), and the t1/2 (app) of the second component is 3.95 days (r = 0.95). Over 90% of the injected 125I-nSBP was removed from plasma within 24 h at a rate corresponding to the t1/2 (app) of the first component. The data indicate that most of the SBP rapidly distributes into extravascular spaces during the first 24 h following infusion, and are consistent with the hypothesis that SBP may be directly involved in sex steroid hormone transport into tissues.

Identification of the primary translation product of the sex steroid-binding protein from monkey liver mRNA in a cell-free system

The synthesis of monkey (Macaca fascicularis) Sex steroid-Binding Protein (mSBP) in a wheat germ cell-free system in response to liver RNA was demonstrated by use of a specific antiserum raised against purified native human SBP. Antibodies precipitate a single translation product behaving as a 42 kDa protein in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Western blots of monkey sera subjected to SDS-PAGE and immunorevelation show that the native mSBP migrates as 2 molecular species (50 and 53 kDa) present in the approximate ratio of 1:10, respectively. The difference in apparent molecular weights of the primary translation product and the reduced mature mSBP may represent glycosylation that occurs post translationally. We describe for the first time the biosynthesis of mSBP at the molecular level and suggest that both components of mSBP derive from a common differentially processed precursor. Its mRNA is poorly represented, since the neosynthesized mSBP represents about 0.005% of the total proteins encoded by liver mRNA.

Are corticosteroid-binding globulin and sex hormone-binding globulin hormones?

Because it no longer seemed reasonable to us that the sole function of the steroid-binding proteins in plasma was to serve as a buffer reservoir for steroid hormones, we conducted experiments which sought out other possibilities. Both CBG and SHBG bind to cell membranes, and this interaction partakes of the general characteristics of peptide hormone-membrane receptor systems. Additionally, human CBG has the ability to cause an increase in the activity of membrane-bound adenylate cyclase in MCF-7 cells, and this, in turn, results in an increase in cellular cAMP content. Thus, CBG appears to be a protein hormone. As a first consideration, one might presume that because CBG's half-life is measured in days, it would be counted among the hormones which, for the most part, are tonic in their effects, e.g., thyroid hormone. However, two important considerations tend to believe this presumption: (1) CBG which is unoccupied by steroid is not hormonally active (Figure 5): (2) Depending upon the time of day, circulating CBG is approximately 0-60% occupied in normal humans. These observations result in a circumstance in which a substantial portion of circulating CBG is available for activation by bursts of cortisol secretion. It seems prudent to speculate that, because steroids are essential for CBG's activity, the hormonal role of CBG may be entwined with, or complementary to the steroids which it binds. Finally, we should comment on the impact that our model of CBG as a hormone has on the view that only unbound steroid can be hormonally active. First, it should be stated that we have not addressed this question experimentally. Although there is evidence that CBG may be required for cortisol action, we feel that an obligate role for it is not documented adequately. At this time, we believe that CBG's hormonal role is compatible with a hypothesis that encompasses the view that unbound steroid hormones can diffuse into cells in some tissues and that both free and bound steroid can enter cells in others. Obviously, the final word on these important topics, as always, awaits the proper experiments.

Subcellular distribution and selectivity of the protein-binding component of the recognition system for sex-hormone-binding protein-estradiol complex in human decidual endometrium

In order to assess the subcellular distribution of the protein-binding component of the recognition system for sex-hormone-binding protein-estradiol complex (Strel'chyonok, O.A., Avvakumov, G.V. and Survilo, L.I. (1984) Biochim. Biophys. Acta 802, 459-466), plasma membranes and other subcellular fractions were prepared from homogenate of human decidual endometrium (8-12 weeks of pregnancy). Specific binding of 125I-labeled sex-hormone-binding protein complexed with estradiol was found preferentially in the plasma membrane fraction. The protein-binding component of the recognition system was found to specifically bind sex-hormone-binding protein complexed with estrogens (estradiol, estriol, estrone, with close affinities) but not with androgens (testosterone and 5 alpha-dihydrotestosterone). Specific membrane binding of 125I-labeled sex-hormone-binding protein complexed with 17 alpha-pregna-2,4-dien-20-yn-[2,3-d]isoxazol-17-ol (danazol) was also detected. This allowed us to suggest a putative mechanism for the danazol pharmacological action on endometrium which involves the interaction of the steroid-protein complex with the plasma membrane of endometrial cells. It is proposed that sex-hormone-binding protein takes part in the guided transport of steroids into endometrial cells.

Specific binding of human corticosteroid-binding globulin to cell membranes

Specific binding sites for corticosteroid-binding globulin were detected on membranes prepared from human prostates. The binding sites are typical of membrane receptors: they are saturable and specific and have high affinity. There was little specific binding at 4 degrees C and 23 degrees C. Maximal specific binding was obtained at 37 degrees C. Scatchard analysis revealed the presence of a single set of binding sites with an apparent dissociation constant of 8.7 X 10(-7) M and a binding capacity of 22 pmol/mg of membrane protein. The sites were specific for corticosteroid-binding globulin; binding was not inhibited by human testosterone/estradiol-binding globulin, by albumin, or by transferrin. The density of specific binding sites in membranes obtained from several organs from the rhesus monkey is consistent with the hypothesis that corticosteroid-binding globulin is involved in the transport of steroid hormones into target tissues.

Changing concepts of active androgens in blood

We have provided a brief historical review of developments in our understanding of the endocrine mechanisms underlying the expression of androgen action in women. An alternative to the free hormone concept is considered which proposes that, at least in some target cells, androgens bound to SHBG are the biologically relevant molecules. In nearly every instance, the changes in blood levels of SHBG that have been observed are consistent with this idea. At present there are only bits of direct evidence to support the hypothetical mechanism proposed. As already mentioned, control of androgen action at the level of cellular uptake would provide obvious advantages as well as a potential mechanism to explain the antagonism between androgens and oestrogens which is still a mystery. It is important to note that the proposed mechanism is not obligatory for androgen or other steroid hormone action. Synthetic steroids which do not bind to SHBG or CBG clearly can gain access to target cells by simple diffusion and bind to intracellular receptors. Compounds such as methyltestosterone and dexamethasone are metabolized much more slowly than their natural counterparts and therefore are cleared slowly from the circulation. It is possible that the well-known difficulties in selecting appropriate therapeutic regimens with such compounds is related to the fact that they bypass an important regulatory step in steroid hormone action-modulated entry into target cells. Hopefully, the recent development of powerful new tools of molecular endocrinology will hasten the answer to the question: What is the active androgen in blood?

Direct effect of sex steroid-binding protein (SBP) of plasma on the metabolic clearance rate of testosterone in the rhesus macaque

We report direct evidence for the effect of the sex steroid-binding protein (SBP) on the metabolic clearance rate of testosterone (MCRT). Pure rhesus SBP or human SBP was infused intravenously into three different cycling female rhesus monkeys. MCRT was measured before and after SBP had reached 150-300% of basal levels. A decrease in MCRT was observed in all cases. The effect of SBP on MCRT was tested further in four additional cycling females by infusing immunoaffinity-purified monospecific human SBP antibodies known to cross-react with rhesus SBP. SBP dropped to 54, 40, 4 and 2% of basal levels with a concomitant increase of 118, 190, 320 and 640% of basal MCRT. In one of these animals, pure rabbit SBP was administered after the anti-human SBP infusion resulting in a decrease in MCRT. The magnitude of the SBP effect on MCRT is related to the distribution of testosterone (T) bound to SBP and albumin in the plasma. Calculations show that as long as the percent of T bound to SBP is equal or higher than the percent of T bound to albumin, the influence on MCRT is small. However, if SBP is reduced to the extent that T is bound mostly to albumin, the redistribution of T is associated with a dramatic increase in MCRT. We conclude that under normal conditions each animal has an optimum concentration of plasma SBP which binds a maximum amount of T. If SBP increases above this level, little effect on MCRT will result. However, a drop below the optimum level, as is the case in certain physiological or clinical conditions, will produce a large increase in the clearance of T.

Testosterone-estradiol-binding globulin binds to human prostatic cell membranes

Specific binding sites for human testosterone-estradiol-binding globulin have been found on human prostatic cell membranes. Scatchard analysis reveals both a high and a low affinity binding site for [125I]testosterone-estradiol-binding globulin. The high affinity site is specific for testosterone-estradiol-binding globulin, whereas the low affinity site also binds human corticosteroid-binding globulin and human transferrin.