Rabbit sex hormone-binding globulin: expression in the liver and testis during postnatal development and structural characterization by truncated proteins

Although sex hormone binding globulin (SHBG) is found in the blood plasma of adult humans and rabbits and the gene is expressed in their livers, it is not detected in the plasma of adult rodents nor is it expressed in adult rodent livers. Thus the rabbit represents a good model to study the metabolism and function of SHBG in the blood. We have used a cloned rabbit SHBG cDNA to detect mRNA expression in rabbits during the postnatal period, and to construct truncated SHBG proteins for structure/function analysis. The SHBG mRNA appeared in the testis as early as 3 days after birth. The level increased gradually in abundance throughout postnatal development, and attained a maximum at 12 weeks of age when the gonads were fully matured. In contrast, SHBG mRNA in the livers of male and female animals increased to a maximum by 4 weeks of age, and were maintained at this level until 12 weeks before subsiding to the initial levels. The increase and decrease in SHBG mRNA levels in the liver were accompanied by similar changes in serum SHBG. This suggests that SHBG in the blood circulation comes from the liver and this might also provide a source of SHBG for the male reproductive tract before formation of the blood-testis barrier. To elucidate the minimal sequence of rabbit SHBG responsible for steroid-binding, a panel of 13 truncated SHBG proteins was constructed, expressed in Escherichia coli, and biochemically purified for study. It was shown that the complete protein sequence of rabbit SHBG was important for maintaining a stable steroid-protein complex. Unlike human SHBG for which a truncated protein of the first 206 residues of the 373 amino acid protein can still bind steroid, removal of 43 or more residues from the C-terminus of rabbit SHBG completely abolished steroid-binding.

Insulin-like growth factor-1 and breast cancer risk in postmenopausal African-American women

We explored the relationship between insulin-like growth factor-1 (IGF-1) concentrations and breast cancer risk. Also, we examined whether obesity, sex hormone-binding globulin (SHBG), and estradiol influenced IGF-1 concentrations. A pilot study of 60 postmenopausal African-American women (30 cases and 30 controls) was used. Plasma concentrations of IGF-1 were higher among the cases, as compared to the controls. A negative trend was seen for plasma concentrations of IGF-1 and TNM (tumor-node-metastasis) stage and IGF-1 and body mass index. IGF-1 was found to be associated negatively with SHBG. After adjustment, plasma concentrations of IGF-1 remained significantly and positively associated with breast cancer risk (odds ratio, 1.183; 95% confidence interval, 1.167-1.201). No significant associations for breast cancer risk were observed for estradiol, SHBG, and body mass index. Further research with a larger sample is needed to clarify the relationships between obesity and IGF-1 concentrations to breast cancer risk in this population.

Sex hormone-binding globulin in gestational diabetes

BACKGROUND

Insulin is an important regulator of serum sex hormone-binding globulin (SHBG) concentration which works by inhibiting its production in hepatocytes. Low SHBG level is associated with increased insulin resistance and hyperinsulinemia. Our purpose was to compare maternal serum SHBG level between normal and gestational diabetic pregnant women and to study the relationships between SHBG, SHBG/insulin and SHBG/glucose ratio and several endocrine, metabolic and clinical parameters.

METHODS

Serum SHBG concentrations were measured in 34 women with gestational diabetes and in 32 matched controls. Glucose, insulin, C-peptide, fructosamine, beta-HCG, cholesterol, triglycerides, HDL cholesterol, LDL cholesterol, apolipoprotein A, apolipoprotein B, total and free T4, total and free estriol, T3 and IGF-1 were measured. Insulin sensitivity was estimated using the short insulin tolerance test.

RESULTS

SHBG, SHBG/insulinemia ratio and SHBG/glucose ratio were significantly lower in the diabetic group (309.54 +/- 112.22 vs 460.54 +/- 144.54, p = 0.00001), (33.55 +/- 16.62 vs 72.56 +/- 66.50, p = 0.0006 using log-transformed values), (5.88 +/- 1.87 vs 3.39 +/- 1.23, p < 0.00001). SHBG was negatively correlated with insulinemia (r = -0.40, p = 0.001), C-peptide (r = -0.41, p = 0.001), glycemia (r = -0.27, p = 0.02), diastolic blood pressure (r = -0.41, p = 0.001) and beta-HCG (r = -0.41, p = 0.001) and positively correlated with LDL-c (r = 0.25, p = 0.04) and apolipoprotein B (r = 0.33, p = 0.007).

CONCLUSIONS

SHBG concentrations are lower in gestational diabetic pregnant women and are related to insulin levels but not to peripheral insulin sensitivity. Since insulinemia was similar in normal and gestational diabetic pregnant women, we speculate that gestational diabetes is characterized by a higher peripheral insulin resistance, a fasting normal insulinemia and a higher hepatic insulin sensitivity, at least in other actions than on carbohydrate metabolism. The role of sex steroids, T4 and IGF-1 in regulating SHBG appears to be limited during pregnancy.

Synthesis and regulation of sex hormone-binding globulin in obesity

Sex hormone-binding globulin (SHBG) is a plasma glycoprotein with high binding affinity for testosterone and dihydrotestosterone and lower affinity for estradiol. SHBG is synthesized in the liver, and its plasma level is important in the regulation of plasma free and albumin-bound androgens and estrogens. Obesity and particularly excess visceral fat, known risk factors for cardiovascular and metabolic diseases, are associated with decreased testosterone levels in males and SHBG levels in both sexes. SHBG is usually positively correlated with high-density lipoprotein cholesterol and negatively correlated with triglyceride and insulin concentrations. A positive association between SHBG and various measures of insulin sensitivity has been demonstrated in both sexes, suggesting that decreased SHBG levels may be one of the components of the metabolic syndrome. We have examined pituitary-adrenocortical function, glucose tolerance, and lipoprotein and hormone levels in a large cohort of Finnish males. Abdominal obesity appears to be associated with slight hypocortisolemia and increased sensitivity to exogenous adrenocorticotropin stimulation, which may contribute to the hyperinsulinemia and related metabolic changes including decreased SHBG levels in males.

Conjugated estrogens and breast cancer risk

Available epidemiologic data suggest the possibility that the use of oral conjugated equine estrogens (CEE) 0.625 mg/day as a first-choice dose could be associated with a very limited (if any) breast cancer risk increase. Some biological peculiarities of oral CEE back the possibility of a limited detrimental effect on breast tissue, due to either direct or indirect actions. Direct actions. Some experimental findings suggest that the 17 alpha-dihydroderivatives of equilenin and equilin (15% of the CEE components) have a non-estrogenic or even an anti-estrogenic effect on breast tissue. This could partially counterbalance the stimulatory action of the other CEE components. Indirect actions. Oral estrogens, through their metabolic and hepatocellular effects (emphasized by the first liver passage) cause a sharp increase in sex hormone binding globulin (SHBG) level which is followed by a lower quantity of both estrogen and androgen in the free, bioavailable, form. More importantly, they cause a decrease in circulating insulin-like growth factor I (IGF-I) activity, due to both a reduction in IGF-I synthesis by the liver and an increase in IGF-binding protein-1 level. A strong relationship between breast cancer risk and the concentration of circulating IGF-I in premenopausal women has been recently found. Actually, estrogens and IGF-I have a synergistic effect on cell proliferation, and IGF-I is necessary for maximum estrogen-receptor activation in breast cancer cell lines. The possibility does exist that the SHBG level increase and the IGF-I bioavailability decrease, caused by oral CEE, balance the increased estrogen stimulation on breast tissue.

Estradiol increases the production of sex hormone-binding globulin but not insulin-like growth factor binding protein-1 in cultured human hepatoma cells

OBJECTIVE

To study the effects of E2 on insulin-like growth factor binding protein-1 (IGFBP-1) and sex hormone-binding globulin (SHBG) production with cultured human HepG2 hepatoma cells.

DESIGN

Experimental cell culture.

SETTING

Academic medical center.

PATIENT(S)

None.

INTERVENTION(S)

Addition of E2 to cell culture medium.

MAIN OUTCOME MEASURE(S)

Intracellular and released concentrations of IGFBP-1 and SHBG.

RESULT(S)

Estradiol did not affect the intracellular or extracellular IGFBP-1 concentration, whereas the intracellular SHBG concentration increased significantly in response to 0.5-2.5 microM of E2.

CONCLUSION(S)

Whereas the two binding proteins share a number of regulatory factors, their regulation by E2 is dissimilar in human hepatoma cells. Estradiol does not affect the intracellular or secreted IGFBP-1 concentration, but it does increase the production of SHBG.

Heterologous expression of wild type and deglycosylated human sex steroid-binding protein (SBP or SHBG) in the yeast, Pichia pastoris. Characterization of the recombinant proteins

Wild type, partially and fully-deglycosylated human sex steroid-binding protein (SBP or SHBG) cDNAs lacking the native cucaryotic signal sequence were cloned into a yeast expression vector containing the Saccharomyces cerevisiae alpha-factor for extracellular secretion. Following transformation into Pichia pastoris, the wild type and all constructed mutants were successfully expressed. The levels were lower for the deglycosylated mutants indicating that oligosaccharide side chains may play a role in SBP secretion. Under fermentation conditions, the wild type protein was expressed at a level of 4 mg/l while the fully-deglycosylated mutant T7A/N351Q/N367Q was expressed at about 1.5 mg/l. The latter was purified from several fermentation runs and was found to be completely deglycosylated, electrophoretically homogeneous and fully active. The aminoterminus was found to have the sequence NH2QSAHDPPAV- indicating that cleavage of the alpha-factor occurred at the A(+7)-Q(+8) peptide bond. The molecular mass of the subunit was determined to be 39,717.8 Da, which is in complete agreement with the amino acid sequence of the T7A/N351Q/N367/Q mutant. The equilibrium constants for the dissociation of 5alpha-dihydrotestosterone and steroid binding specificity were found to be identical to that of the human plasma protein indicating that the missing N-terminal segment NH2-LRPVLPT and the removal of oligosaccharide side chains do not affect the stability and active conformation of the protein. In conclusion, the data presented reveal that the SBP mutant T7A/N351Q/N367/Q is the protein of choice for solving the three-dimensional structure.

Hepatocyte nuclear factor-4 controls transcription from a TATA-less human sex hormone-binding globulin gene promoter

Hepatocytes are the major source of sex hormone-binding globulin (SHBG), a glycoprotein that transports sex steroids in the blood and regulates their access to target tissues. The human SHBG proximal promoter was analyzed by DNase I footprinting, and the functional significance of 6 footprinted regions (FP1-FP6) within the proximal promoter was studied in human HepG2 hepatoblastoma cells. Two footprinted regions (FP1 and FP3) contain binding sites for the chicken ovalbumin upstream promoter-transcription factor (COUP-TF) and hepatocyte nuclear factor-4 (HNF-4). In experiments where SHBG promoter-luciferase reporter gene constructs were co-transfected into HepG2 cells with COUP-TF and/or HNF-4 expression vectors, HNF-4 markedly increased transcription, whereas COUP-TF suppressed this probably by displacing HNF-4 from their common FP1-binding site. This COUP-TF/HNF-4-binding site within FP1 includes a TTTAA sequence, located at nucleotides -30/-26 upstream of the transcription start site, which fails to interact with human TFIID, TATA-binding protein in vitro. When this sequence was replaced with an idealized HNF-4-binding site, the transcriptional activity of the promoter increased in HepG2 cells. Taken together, these data imply that an interplay between COUP-TF and HNF-4 at a site within FP1 regulates human SHBG expression and that HNF-4 controls transcription from this TATA-less promoter by somehow substituting for TATA-binding protein in the recruitment of a transcription preinitiation complex.

Human sex hormone-binding globulin gene expression in transgenic mice

The sex hormone-binding globulin gene (shbg) is expressed in the liver and testis as well as in several other tissues that play important roles in reproduction. Expression of shbg in the human liver results in the production of plasma sex hormone-binding globulin (SHBG), which regulates the bioavailability of sex steroids in the blood. Although shbg is not expressed in rodent livers postnatally, it gives rise to the androgen-binding protein in their testes upon sexual maturation. Human shbg is also expressed in the testis, but its products and their function are less well characterized. To study the expression of human shbg in different tissues and the consequences of overexpressing this gene in vivo, we have produced several lines of mice containing approximately 11-kilobase (kb; shbg11) or 4.3-kb (shbg4) human shbg genomic fragments that comprise all eight exons encoding SHBG as well as approximately 6 kb or approximately 0.9 kb of 5'-flanking DNA, respectively. Northern blots indicated that human shbg transcripts were most abundant in liver, kidney, and testis of the shbg11 mice. The 4.3-kb shbg transgenes were expressed at similar levels in liver and kidney, but the abundance of human shbg transcripts in their testes was much lower than that in shbg11 mice. Primer extension analysis indicated that transcription starts 60 bp from the translation initiation codon for SHBG in liver and kidney of shbg11 mice, and that the shbg transcripts in their testis are derived from a separate promoter flanking an alternative exon that replaces the exon containing the translation initiation codon for SHBG or androgen-binding protein. At the cellular level, the human shbg transgenes are expressed in clusters of hepatocytes located mainly within the periportal region of hepatic lobules and in the epithelial cells lining the proximal convoluted tubules of the kidney. This results in high levels of human SHBG in serum (1.45-1.72 nmol/ml) and urine (6-16 pmol/ml) of mature male shbg mice. The abundance and distribution of human shbg transcripts in the Sertoli cells of shbg11 mice vary throughout the spermatogenic cycle, with levels increasing in the Sertoli cell cytoplasm until stage VII of spermatogenesis and declining after stage IX. At stages X-XII of spermatogenesis, these transcripts concentrate at the adluminal compartment of the Sertoli cells, and this suggests that they have a role in the elongation phase of spermiogenesis. The presence of human SHBG in the blood of shbg transgenic mice may result in serum levels of testosterone that are 10-100 times higher than those in wild-type littermates. Despite this, their reproductive performance is normal, and there is no obvious phenotypic abnormalities even in animals homozygous for the transgenes.

Expression of sex hormone-binding globulin exon VII splicing variant messenger RNA in human uterine endometrial cancers

We have demonstrated the intracellular expression of sex hormone-binding globulin (SHBG) exon VII splicing variant mRNA in human uterine endometrial cancer using the reverse transcription-PCR-Southern blot and DNA sequencing analyses. Analysis of the missing base pairs proved that they corresponded to the entire exon VII, which is considered to encode a portion of the steroid-binding site, suggesting that the steroid-binding affinity of this variant might be different from that of the SHBG wild type. In uterine endometrial cancers, the wild-type mRNA levels significantly (P < 0.01) decreased, and the ratio of the SHBG variant to wild-type mRNA levels (P < 0.01) increased with the advance of histological dedifferentiation. These results suggest that dedifferentiation of endometrial cancers might induce a reduction in their estrogen-dependent properties via intracellular SHBG.

Expression of sex hormone-binding globulin and corticosteroid-binding globulin mRNAs in corpus luteum of human subjects

To understand the biology of sex steroids in human ovarian corpus luteum, the expression of intracellular sex hormone-binding globulin (SHBG) and corticosteroid-binding globulin (CBG) mRNAs as a manifestation of intracellular SHBG and CBG expression was determined. The expression of SHBG and CBG mRNAs was detected in all samples analyzed. Luteal SHBG mRNA level showed no significant change during the endometrial secretory phase of the menstrual cycle. On the other hand, luteal CBG mRNA level was significantly higher (p < 0.05) at the mid-secretory phase than that at the early and late secretory phases of the endometrium. These findings suggest that human ovarian corpus luteum synthesizes SHBG and CBG intracellularly, CBG being plausibly involved in the functional life span of corpus luteum.

Reduced testosterone, 17 beta-oestradiol and sexual hormone binding globulin, and increased insulin-like growth factor-1 concentrations, in healthy nulligravid women aged 19-25 years who were first and/or second degree relatives to breast cancer patients

Differences in hormonal and constitutional parameters between women with at least one first and/or second degree relative with breast cancer (RBC) and women without such affected relatives were studied in a group of healthy, nulligravid women aged 19-25 years. Present oral contraceptive (OC) users were analysed separately. In women not presently exposed to OCs we found significant correlations between RBC and reduced concentrations of testosterone during both the follicular (P < 0.001) and luteal menstrual cycle phases (P = 0.016). 17 beta-oestradiol was also significantly negatively correlated with RBC in the follicular (P = 0.044) and in the luteal phase (P = 0.027). RBC was significantly correlated with a lower waist/hip ratio (P = 0.044) compared with women without such a history. In multivariate analyses, the results for testosterone but not 17 beta-oestradiol remained significant. In these analyses high IGF-1 (P = 0.05) in the follicular phase and low sexual hormone-binding globulin (SHBG) (P = 0.04) in the luteal phase were also related to RBC. Including all 66 women in a multivariate model that analysed the specific effects from OCs and RBC on plasma testosterone showed that plasma testosterone was significantly lower among present OC users (P = 0.004) and in women with RBC (P = 0.005) during cycle days 5-10, with a significant positive two-way interaction between present OC use and RBC (P = 0.007). During cycle days 18-23 plasma testosterone showed a significant negative relationship with present OC use (P < 0.001) and RBC (P = 0.016) no significant interaction was seen during cycle days 18-23. Factors not significantly related to RBC were height, weight, breast size, age at menarche, p-progesterone and p-prolactin. It is concluded that a family history of breast cancer significantly lowered plasma testosterone concentrations in both cycle phases among healthy, nulligravid women compared with women without such history.

Characterization of the "estrogenicity" of tamoxifen and raloxifene in HepG2 cells: regulation of gene expression from an ERE controlled reporter vector versus regulation of the endogenous SHBG and PS2 genes

The estrogenic character of tamoxifen and raloxifene was studied on three different genes, an ERE-reporter construct and two endogenous genes, sex hormone binding globulin (SHBG) and pS2, in two variants of the human liver carcinoma cell line HepG2. On the ERE-reporter construct and the pS2 gene both tamoxifen and raloxifene acted as pure estrogen antagonists, whereas on the SHBG gene they functioned as partial estrogens/antiestrogens at concentrations below 1 microM and as full "agonists" at concentrations higher than 1 microM. The fold stimulatory effect of tamoxifen and raloxifene on SHBG protein expression was similar in the estrogen receptor (ER) expressing HepG2 cells (HepER3) and the parental non-ER expressing HepG2 cells at concentrations above 1 microM. In contrast, the 17beta-estradiol analogue moxestrol stimulated SHBG expression only in the HepER3 cells. Both tamoxifen and raloxifene had an additive effect to estrogen receptor-dependent SHBG gene expression in the HepER3 cells in the presence of saturating concentrations of moxestrol. However, a significant difference was observed in that a much higher concentration of moxestrol was required to see an additive effect of raloxifene compared to tamoxifen. The cytokine IL1-beta completely blocked the tamoxifen-dependent induction of SHBG gene expression in HepER3 cells, but only partly blocked the effect of moxestrol mediated by the ER. In conclusion, our results suggest that the mechanism for the liver-selective "estrogenic" character of tamoxifen and raloxifene is mediated by a non-ER dependent pathway.

Expression of sex hormone-binding globulin mRNA in uterine cervical cancers

To explore the role of sex hormone-binding globulin (SHBG) in the intracellular steroidal actions in human uterine cervical cancers, the expression of SHBG mRNA as a manifestation of intracellular SHBG expression was investigated using the competitive reverse transcription-polymerase chain reaction-Southern blot analysis. The expression of SHBG mRNA was detected in all cervical endometria and cancers analyzed. The levels of SHBG mRNA in cervical cancers were significantly lower (p < 0.01) than in the normal cervical endometrium. In cervical cancers, the levels of SHBG mRNA in cervical adenocarcinomas were significantly higher (p < 0.01) than in keratinizing and small cell nonkeratinizing squamous cell carcinomas, and tended to be higher than in large cell nonkeratinizing squamous cell carcinomas. There was no difference in expression among the clinical stages of cervical cancers. These data suggest that human uterine cervical cancers, especially adenocarcinomas, might synthesize SHBG intracellularly, and might conserve the activity of SHBG-related steroidal mechanisms to some extent.

A study of serum androgen and cortisol levels in female patients with rheumatoid arthritis. Correlation with disease activity

Androgen status and the role played by androgens in the pathogenesis of rheumatoid arthritis (RA) in female patients are a matter of debate. In the present study serum testosterone (T), DHEAS, sex hormone binding globulin (SHBG) and cortisol levels were determined in 55 RA women, both in pre- and post-menopausal (M) status, and in a group of healthy subjects. Patients were divided into two groups according to disease activity and a correlation analysis of hormonal levels against serum IL1beta levels was performed. No significant differences were found in serum T levels between RA patients and controls, both in preM (1.38 +/- 0.4 vs 1.35 +/- 0.3 nmol/l; p = ns) and in postM status (1.21 +/- 0.2 vs 1.10 +/- 0.2 nmol/l; p = ns). Serum SHBG levels were lower in RA patients than in control subjects, both in pre and in postM status. DHEAS levels were significantly lower in preM RA patients than in controls (2.34 +/- 1.2 vs 5.93 +/- 1.6 mu mol/l; p < 0.001) while cortisol levels were significantly higher in preM active RA patients than in controls (466.2 +/- 30.3 vs 411 +/- 66.2 nmol/l; p = 0.02). IL1beta levels were significantly higher in RA patients than in controls both in pre- and postM subjects (70 +/- 33.8 vs 23.1 +/- 2.9 and 92 +/- 27.4 vs 31.9 +/- 3.1 fmol/l, p < 0.001, respectively). Although androgen status could play a role in the pathogenesis of RA, at present it is not possible to exclude the influence of RA itself on sex hormone profile.

Roles of estrogen and SHBG in prostate physiology

Heretofore, the function of estrogen in the prostate, other than as an antiandrogen, has been unclear. In this review of a growing fund of knowledge about both estrogen and the plasma protein, sex hormone-binding globulin (SHBG), or testosterone-estradiol binding globulin (TeBG), the hypothesis is proposed that estrogen, mediated by SHBG, participates with androgen in setting the pace of prostatic growth and function. It is suggested that the estrogen not only directs stromal proliferation and secretion, but also, through IGF-I, conditions the response of the epithelium to androgen.

Hepatic expression of sex hormone-binding globulin associated with the postnatal surge of serum androgen-binding activity in the Djungarian hamster

Serum androgen-binding capacity in Djungarian hamsters, as in many other mammals, increases within days after birth and remains elevated until puberty. This increased activity has been attributed to a hepatic glycoprotein, sex hormone-binding globulin (SHBG), but expression of SHBG by the postnatal liver has not been demonstrated. Therefore, a full-length SHBG cDNA was cloned from the liver of neonatal hamsters and the expression of SHBG during development was examined. Hepatic SHBG RNA levels, as measured by both competitive RT-PCR and Northern analysis, were very low in fetal animals but increased significantly within 24 h of birth. Maximal values were maintained for 1 week after parturition, and then declined to basal adult levels. The developmental pattern in hepatic SHBG immunoactivity, as determined by Western analysis, mirrored that of hepatic SHBG mRNA. However, changes in serum SHBG immunoactivity and steroid-binding activity occurred approximately 1 week later. There were no sex differences in the levels of hepatic SHBG mRNA or protein during development, but serum immunoactivity tended to be higher in females at puberty. Sex- and age-related differences in the relative abundance of SHBG isoforms were also noted. Results of these studies demonstrate that Djungarian hamsters express an authentic SHBG and indicate that the postnatal surge in serum androgen-binding activity is due to perinatal up-regulation of SHBG expression.

Phytoestrogens: epidemiology and a possible role in cancer protection

Because many diseases of the Western Hemisphere are hormone-dependent cancers, we have postulated that the Western diet, compared to a vegetarian or semivegetarian diet, may alter hormone production, metabolism, or action at the cellular level by some biochemical mechanisms. Recently, our interest has been mainly focused on the cancer-protective role of some hormonelike diphenolic phytoestrogens of dietary origin, the lignans and the isoflavonoids. The precursors of the biologically active compounds originate in soybean products (mainly isoflavonoids), whole grain cereal food, seeds, and probably berries and nuts (mainly lignans). The plant lignan and isoflavonoid glycosides are converted by intestinal bacteria to hormonelike compounds with weak estrogenic but also antioxidative activity; they have now been shown to influence not only sex hormone metabolism and biological activity but also intracellular enzymes, protein synthesis, growth factor action, malignant cell proliferation, differentiation, and angiogenesis in a way that makes them strong candidates for a role as natural cancer-protective compounds. Epidemiologic investigations strongly support this hypothesis because the highest levels of these compounds in the diet are found in countries or regions with low cancer incidence. This report is a review on recent results suggesting that the diphenolic isoflavonoids and lignans are natural cancer-protective compounds.

Regulation of sex hormone-binding globulin production by isoflavonoids and patterns of isoflavonoid conjugation in HepG2 cell cultures

The effect of the isoflavonoid phytoestrogens daidzein, equol, and genistein on sex hormone-binding globulin (SHBG) levels, SHBG mRNA transcript levels, and SHBG gene methylation was studied in HepG2 cell cultures by fluoroimmunometric SHBG assay and Northern and Southern hybridizations, respectively. The effect of 17 beta-estradiol on these parameters was studied as a control. The metabolism of isoflavonoids in HepG2 cells was determined by isotope dilution gas chromatography-mass spectrometry, after ion-exchange chromatography. Daidzein and equol increased SHBG levels in parallel intracellularly and extracellularly, whereas genistein increased SHBG levels only within the cells, resembling thus the effect of 17 beta-estradiol. The difference may originate from the fact that genistein has more hydroxyl groups than daidzein and equol. The regulation of SHBG production by phytoestrogens appears to occur at the post-transcriptional level. Firstly, daidzein, equol, or genistein did not have a clear effect on the steady-state SHBG mRNA levels. Secondly, no effect on SHBG gene methylation was observed by genistein. The findings applied also to 17 beta-estradiol. However, as the SHBG gene was more methylated in SHBG-negative MCF-7 cells than in SHBG-positive HepG2 cells, DNA methylation may play a role in the tissue-specific activation of this gene. The metabolism of isoflavonoids in HepG2 cells yielded mainly unconjugated and sulfated compounds. Similar metabolism in hepatocytes in vivo might retain their biological activity in tissues responsive to estrogens.

Regulation of sex hormone-binding globulin secretion and gene expression by cycloheximide in vitro

The role of protein synthesis in sex hormone-binding globulin (SHBG) secretion and gene expression was studied in HepG2 cell cultures. Inhibition of protein synthesis by cycloheximide suppressed SHBG levels. Triiodothyronine and estradiol increased SHBG production, and cycloheximide reduced their effects to an extent which correlated with the degree of suppression obtained with the drug alone. Insulin decreased SHBG production, and the effect of the treatment with insulin and cycloheximide together did not differ from that with cycloheximide alone. Cycloheximide did not, alone or with the hormones, decrease SHBG levels more markedly extra- than intracellularly. Therefore, cycloheximide does not impair the secretion of SHBG which is synthesized in the presence of the drug. In contrast to SHBG protein levels, cycloheximide increased SHBG mRNA levels. When the effect of cycloheximide on the rate of SHBG mRNA decay was tested, the drug was found to extend the half-life of SHBG mRNA. Of the hormones, insulin decreased and triiodothyronine modestly increased SHBG mRNA levels, whereas estradiol had no clear effect. Treatment with cycloheximide together with any of the hormones resulted in an increase in SHBG mRNA levels. We conclude that protein synthesis inhibition does not impair the secretion of SHBG produced under such conditions, but stabilizes SHBG mRNA by removing some hepatic protein species involved in the regulation of its degradation.

Body mass and physical activity are important predictors of serum androgen concentrations in young healthy men

The relative importance of body mass index (BMI), demographic, and life-style factors in predicting androgen and sex hormone-binding globulin (SHBG) concentrations of young healthy men has not been clearly established. We evaluated the role of age, BMI, height, habitual physical activity, education, smoking, coffee and alcohol intake as predictors of serum testosterone, dihydrotestosterone, dehydroepiandrosterone sulfate, androstenedione, and SHBG concentrations in 100 healthy men 18-22 years old. BMI was the only independent predictor of dihydrotestosterone and dehydroepiandrosterone sulfate, and both BMI and physical activity were independent predictors of serum testosterone concentrations, whereas other variables did not appear to play a role of comparable importance.

Serum soluble interleukin-2 receptors as an index of the biological activity of thyroid hormones in hyperthyroidism

In order to examine whether serum soluble Interleukin-2 Receptors (sIL-2R) could be used as a marker of the biological effects of the thyroid hormones, we measured the sIL-2R, sex hormone binding globulin and beta-2 microglobulin levels in thirty-three hyperthyroid patients (14 with Graves' disease, 17 with Toxic Nodular Goiter and 2 with toxic adenoma) before and during treatment with antithyroid drugs. We found that serum sIL-2R concentrations of the patients, at diagnosis, were significantly higher compared with normal controls (2424 +/- 1447 vs 459 +/- 184 U/ml). All hyperthyroid patients had sIL-2R levels > mean + 2SD of normal controls, with 28 of the 33 patients having sIL-2R concentrations higher than 1011 U/ml (mean + 3SD of normal controls). Only 15 patients had SHBG levels higher than 3SD above the mean for the normal controls and 28 had SHBG levels 2SD above the mean for the normal controls. Three of the 5 hyperthyroid patients with normal SHBG levels at presentation had abnormally high sIL-2R levels. In all patients sIL-2R levels decreased gradually during therapy down to normal levels when euthyroidism was achieved. A strong positive correlation was found between sIL-2R, SHBG and T3 and T4 concentrations. Serum B2-microglobulin (B2-m) levels were higher than the upper normal limit only in 9 patients, but a significant decrement was observed in all patients when euthyroidism was achieved. The above results indicate that serum sIL-2R levels could be a useful marker of the in vivo biological effects of the thyroid hormones on lymphocytes in hyperthyroid patients.

Over-expression of human sex steroid-binding protein (hSBP/hABP or hSHBG) in insect cells infected with a recombinant baculovirus. Characterization of the recombinant protein and comparison to the plasma protein

Human sex steroid-binding protein (hSBP/hABP or hSHBG) was over-expressed in High Five and Sf9 cells adhered to plates and in suspension. The adherent cells expressed to levels of 2.3 mg/l and 1.4 mg/l after 4 and 6 days, respectively, while Sf9 cells grown in suspension yielded 4.67 mg/l after 6 days. Recombinant hSBP/hABP, purified to homogeneity by immunoadsorption, was found to fold similarly to native plasma hSBP/hABP and to display similar sequence epitopes after heat denaturation. The recombinant protein binds dihydrotestosterone, testosterone, and 17 beta-estradiol with KdS of 0.6, 2.4, and 14.2 nM, respectively, which are similar to plasma hSBP/hABP. The recombinant protein contains N-linked and O-linked oligosaccharide side-chains but the monomer exhibits a slightly lower molecular weight than plasma hSBP/hABP (40 kDa vs 44 kDa) which may be due to the absence of one N-linked side-chain or to shorter oligosaccharide side-chains. The partial N-terminal sequence LRPVLP(T)Q of recombinant hSBP/hABP is identical to plasma hSBP/hABP but appears to be less heterogeneous. These results indicate that recombinant baculovirus SBP represents a good model for investigating the structure of plasma hSBP/hABP. The expression system will allow the isolation of preparative amounts of SBP mutants generated by combinatorial site-directed mutagenesis to advance investigations on structure-function relationships and undertake crystallization trials for X-ray diffraction analyses.

Regulation of sex-hormone-binding globulin production by endogenous estrogens in vitro

The effect of endogenous estrogens on sex-hormone-binding globulin (SHBG) production was studied in HepG2 cells. 17 beta-estradiol, estrone, and estrogens from both 2- and 16 alpha-hydroxylative pathways stimulated SHBG production, but not in parallel with their binding affinities for the estrogen receptor. Thus, the underlying mechanism may be other than a pure interaction with the estrogen receptor.

Stimulation of sex hormone-binding globulin mRNA and attenuation of corticosteroid-binding globulin mRNA by triiodothyronine in human hepatoma cells

We examined the time course and dose response of the triiodothyronine (T3) effect on mRNAs for sex hormone-binding globulin (SHBG) and corticosteroid-binding globulin (CBG) in cells of the human hepatoma line HepG2. After 7 h of exposure to a saturating dose of T3, SHBG mRNA was unchanged but increased to 1.5 +/- 0.1 times the unstimulated control at 22 h. Maximal stimulation (2.3 +/- 0.6) was observed at 2-3 days. Corticosteroid-binding globulin mRNA was unchanged for 22 h after exposure to T3 but diminished thereafter to 64% by day 3. At 3-4 days of exposure, the changes in both SHBG mRNA and CBG mRNA were dose-responsive to the T3 concentration. For both mRNAs, half-maximal response occurred between 10 and 20 pmol/l bioavailable T3. Cortisol-binding proteins secreted by HepG2 cells after 3 days in culture also were T3 dose-responsive. No re-uptake of secreted CBG by the cells was observed, suggesting that the T3 effect on CBG secretion occurs during production of the mature protein. These data suggest that T3 stimulates the expression of the SHBG gene and attenuates the expression of the CBG gene. The effects of T3 on these genes are consistent with the increase in circulating SHBG and decrease in circulating CBG observed in hyperthyroidism. The HepG2 cells may be a useful human cell line in which to study the diversity of the molecular mechanisms of T3 action.