Mammalian expression of the human sex steroid-binding protein of plasma (SBP or SHBG) and testis (ABP). Characterization of the recombinant protein

The Roles of Androgens in Humans: Biology, Metabolic Regulation and Health

Androgens are an important and diverse group of steroid hormone molecular species. They play varied functional roles, such as the control of metabolic energy fate and partition, the maintenance of skeletal and body protein and integrity and the development of brain capabilities and behavioral setup (including those factors defining maleness). In addition, androgens are the precursors of estrogens, with which they share an extensive control of the reproductive mechanisms (in both sexes). In this review, the types of androgens, their functions and signaling are tabulated and described, including some less-known functions. The close interrelationship between corticosteroids and androgens is also analyzed, centered in the adrenal cortex, together with the main feedback control systems of the hypothalamic-hypophysis-gonads axis, and its modulation by the metabolic environment, sex, age and health. Testosterone (T) is singled out because of its high synthesis rate and turnover, but also because age-related hypogonadism is a key signal for the biologically planned early obsolescence of men, and the delayed onset of a faster rate of functional losses in women after menopause. The close collaboration of T with estradiol (E2) active in the maintenance of body metabolic systems is also presented Their parallel insufficiency has been directly related to the ravages of senescence and the metabolic syndrome constellation of disorders. The clinical use of T to correct hypoandrogenism helps maintain the functionality of core metabolism, limiting excess fat deposition, sarcopenia and cognoscitive frailty (part of these effects are due to the E2 generated from T). The effectiveness of using lipophilic T esters for T replacement treatments is analyzed in depth, and the main problems derived from their application are discussed.

Characterization and comparison of recombinant full-length ursine and human sex hormone-binding globulin

Sex hormone‐binding globulin (SHBG) regulates the bioavailability of sex steroid hormones in the blood. Levels of SHBG increase markedly in brown bears (Ursus arctos) during hibernation, suggesting that a key regulatory role of this protein is to quench sex steroid bioavailability in hibernation physiology. To enable characterization of ursine SHBG and a cross species comparison, we established an insect cell‐based expression system for recombinant full‐length ursine and human SHBG. Compared with human SHBG, we observed markedly lower secretion levels of ursine SHBG, resulting in a 10‐fold difference in purified protein yield. Both human and ursine recombinant SHBG appeared as dimeric proteins in solution, with a single unfolding temperature of ~ 58 °C. The thermal stability of ursine and human SHBG increased 5.4 and 9.5 °C, respectively, in the presence of dihydrotestosterone (DHT), suggesting a difference in affinity. The dissociation constants for [³H]DHT were determined to 0.21 ± 0.04 nm for human and 1.32 ± 0.10 nm for ursine SHBG, confirming a lower affinity of ursine SHBG. A similarly reduced affinity, determined from competitive steroid binding, was observed for most steroids. Overall, we found that ursine SHBG had similar characteristics to human SHBG, specifically, being a homodimeric glycoprotein capable of binding steroids with high affinity. Therefore, ursine SHBG likely has similar biological functions to those known for human SHBG. The determined properties of ursine SHBG will contribute to elucidating its potential regulatory role in hibernation physiology.

Androgen-Regulated Cardiac Metabolism in Aging Men

The prevalence of cardiovascular mortality is higher in men than in age-matched premenopausal women. Gender differences are linked to circulating sex-related steroid hormone levels and their cardio-specific actions, which are critical factors involved in the prevalence and features of age-associated cardiovascular disease. In women, estrogens have been described as cardioprotective agents, while in men, testosterone is the main sex steroid hormone. The effects of testosterone as a metabolic regulator and cardioprotective agent in aging men are poorly understood. With advancing age, testosterone levels gradually decrease in men, an effect associated with increasing fat mass, decrease in lean body mass, dyslipidemia, insulin resistance and adjustment in energy substrate metabolism. Aging is associated with a decline in metabolism, characterized by modifications in cardiac function, excitation-contraction coupling, and lower efficacy to generate energy. Testosterone deficiency -as found in elderly men- rapidly becomes an epidemic condition, associated with prominent cardiometabolic disorders. Therefore, it is highly probable that senior men showing low testosterone levels will display symptoms of androgen deficiency, presenting an unfavorable metabolic profile and increased cardiovascular risk. Moreover, recent reports establish that testosterone replacement improves cardiomyocyte bioenergetics, increases glucose metabolism and reduces insulin resistance in elderly men. Thus, testosterone-related metabolic signaling and gene expression may constitute relevant therapeutic target for preventing, or treating, age- and gender-related cardiometabolic diseases in men. Here, we will discuss the impact of current evidence showing how cardiac metabolism is regulated by androgen levels in aging men.

Modulation of SHBG binding to testosterone and estradiol by sex and morbid obesity

OBJECTIVE

Sex hormone-binding globulin (SHBG) binds and transports testosterone and estradiol in plasma. The possibility that SHBG is a mixture of transporting proteins has been postulated. We analyzed in parallel the effects of obesity status on the levels and binding capacity of circulating SHBG and their relationship with testosterone and estradiol.

DESIGN

Anthropometric measures and plasma were obtained from apparently healthy young (i.e. 35 ± 7 years) premenopausal women (n = 32) and men (n = 30), with normal weight and obesity (BMI >30 kg/m²).

METHODS

SHBG protein (Western blot), as well as the plasma levels of testosterone, estradiol, cortisol and insulin (ELISA) were measured. Specific binding of estradiol and testosterone to plasma SHBG was analyzed using tritium-labeled hormones.

RESULTS

Significant differences in SHBG were observed within the obesity status and gender, with discordant patterns of change in testosterone and estradiol. In men, testosterone occupied most of the binding sites. Estrogen binding was much lower in all subjects. Lower SHBG of morbidly obese (BMI >40 kg/m²) subjects affected testosterone but not estradiol. The ratio of binding sites to SHBG protein levels was constant for testosterone, but not for estradiol. The influence of gender was maximal in morbid obesity, with men showing the highest binding/SHBG ratios.

CONCLUSIONS

The results reported here are compatible with SHBG being a mixture of at least two functionally different hormone-binding globulins, being affected by obesity and gender and showing different structure, affinities for testosterone and estradiol and also different immunoreactivity.

Is Immunohistochemical Sex Hormone Binding Globulin Expression Important in the Differential Diagnosis of Adenocarcinomas?

Adenocarcinomas (AC) are the most frequently encountered carcinomas. It may be quite challenging to detect the primary origin when those carcinomas metastasize and the first finding is a metastatic tumor. This study evaluated the role of sex hormone binding globulin (SHBG) positivity in tumor cells in the subclassification and detection of the original organ of adenocarcinomas. Between 1994 and 2008, 64 sections of normal tissue belonging to ten organs, and 116 cases diagnosed as adenoid cystic carcinoma and mucoepidermoid carcinoma of the salivary gland, lung adenocarcinoma, invasive ductal carcinoma of the breast, adenocarcinoma of stomach, colon, gallbladder, pancreas and prostate, endometrial adenocarcinoma and serous adenocarcinoma and mucinous adenocarcinoma of the ovary, were sent to the laboratory at the Department of Pathology at the Yuzuncu Yil University School of Medicine, where they were stained immunohistochemically, using antibodies against SHBG. The SHBG immunoreactivity in both the tumor cells and normal cells, together with the type, diffuseness and intensity of the staining were then evaluated. In the differential diagnosis of the adenocarcinomas of the organs, including the glandular structures, impressively valuable results are encountered in the tumor cells, whether the SHBG immunopositivity is evaluated alone or together with other IHC markers. Further extensive research with a larger number of cases, including instances of cholangiocarcinoma and cervix uteri AC [which we could not include in the study for technical reasons] should be performed, in order to appropriately evaluate the role of SHBG in the differential diagnosis of AC.

Sex hormone-binding globulin (SHBG) expression in ovarian carcinomas and its clinicopathological associations

Sex hormone-binding globulin (SHBG) is known as a carrier protein. It is classically thought to be mainly synthesized in the liver and then secreted into the circulating system, where it binds to sex steroids with a high affinity and modulates the bio-availability of the hormones. Other organs known to produce SHBG include brain, uterus, testis, prostate, breast and ovary, and the local expressed SHBG may play an important role in tumor development. However, SHBG expression status and its clinicopathological significance in ovarian cancer cells are not reported yet. In our present study, we examined and found the variable SHBG expression in four ovarian cancer cell lines (OV-90, OVCAR-3, SKOV-3 and ES-2) by immunocytochemistry and Western blotting. We then extended our study to 248 ovarian carcinoma samples, which were collected at The Norwegian Radium Hospital, Oslo University Hospital with complete clinical information, and discovered that SHBG was variably expressed in these ovarian carcinomas. Higher level of SHBG expression was significantly associated with more aggressive histological subtype (p = 0.022), higher FIGO stage (p = 0.018) and higher histological grade (grade of differentiation, p = 0.020), although association between SHBG expression and OS/PFS was not observed. Our results demonstrate that ovarian cancer cells produce SHBG and higher SHBG expression in ovarian carcinoma is associated with unfavorable clinicopathological features.

Efficient eukaryotic expression system for authentic human sex hormone-binding globulin

Sex hormone-binding globulin (SHBG) is the main carrier for androgens and oestrogens in humans. It mediates the transport of steroid hormones in the circulation and testicular fluid, and regulates their bioavailability to steroid-responsive tissues. In addition, the protein interacts with membrane receptors expressed in target tissues. Binding to the receptors is suspected to facilitate the uptake of steroid hormones and/or elicit cellular signal transduction. The identity of the SHBG receptor has not yet been resolved, in part due to a lack of sufficient quantities of authentic SHBG for receptor purification and molecular characterization. We have successfully addressed this problem by establishing an episomal expression system in human embryonic kidney cells that produces 5 mg of fully active human SHBG per litre. The recombinant protein resembles native SHBG in terms of structure, glycosylation pattern and steroid-binding activity. Moreover, the protein interacts with plasma membranes in steroid target tissues, an activity not observed with SHBG from other recombinant expression systems. Thus our studies have removed an important obstacle to the further elucidation of the role SHBG plays in steroid hormone action.

Arginine-140 and isoleucine-141 determine the 17beta-estradiol-binding specificity of the sex-steroid-binding protein (SBP, or SHBG) of human plasma

Arginine-140 and isoleucine-141 were identified as key determinants of 17beta-estradiol (E(2)) binding affinity of the sex-steroid-binding protein (SBP, or SHBG) of human plasma. Amino acid residues that differ between human and rabbit SBP sequences were replaced in the human protein and the products tested for lowered E(2)binding activity as are seen in the rabbit protein. Only mutants containing either R140K or I141L replacements display an E(2) equilibrium dissociation constant (Kd) higher than the wild type, reaching a value of 30 nM when both were present. The 5alpha-dihydrotestosterone (DHT) equilibrium dissociation constant of these mutants was unaffected. The quadruple mutant M107I/I138V/R140K/I141L yielded an E(2) Kd of 65 nM, significantly closer to the 80 nM rabbit SBP E(2) Kd value. Although mutants containing the M107I and I138V replacements in the absence of R140K and I141L had normal E(2) Kds, the presence of the M107I replacement in the quadruple mutant was necessary to obtain an accurate E(2) Kd value by competitive Scatchard analysis. Molecular modeling using coordinates for the recently determined N-terminal domain of human SBP revealed a significant shift of the F56 phenyl ring away from ring A of E(2) in mutant models containing the R140K and I141L replacements. We conclude that R140 and I141 are required for sustaining the right proximity of the phenyl ring of F56 to ring A of 17beta-estradiol, thus optimizing the E(2)-binding affinity of human SBP.

The sex steroid binding protein (SBP or SHBG) of human plasma: identification of Tyr-57 and Met-107 in the steroid binding site

Tyrosine-57 (Y57) and methionine-107 (M107) have been identified in the binding site of the sex steroid binding protein (SBP) (or sex hormone binding globulin) of human plasma by replacing the two amino acids with a number of residues of varying structure. Replacement of Y57 with phenylalanine resulted in a fourfold increase in the K(d) of 5 alpha-dihydrotestosterone but left the K(d) of 17 beta-estradiol unchanged. Except in two cases, no further loss in binding took place when replacing Y57 with other residues, suggesting that the phenolic group of Y57 may form a hydrogen bond with the ligand. Replacement of M107 with isoleucine increased the 5 alpha-dihydrotestosterone K(d) fourfold to a value equal to that of rabbit SBP, which contains isoleucine at the corresponding position; however, the K(d) of 17 beta-estradiol remained unchanged. Replacement of M107 with threonine resulted in a tenfold decrease in 5 alpha-dihydrotestosterone binding affinity, whereas replacement with leucine left the K(d) unchanged. These data indicate that substitutions on the beta-carbon of the amino acid side-chain at position 107 causes significant loss of binding affinity but, as in the case of Y57, the activity was not totally eliminated. We conclude that Y57 and M107 form part of a structural motif within the steroid binding site and specifically contribute binding energy to ring A of 5 alpha-dihydrotestosterone but not to ring A of 17 beta-estradiol. We also propose that the integrated contribution of several side chains may be required to optimize the ligand affinity of the steroid binding site. This proposal may fit a 'lock and key' model where little movement of the side chains occurs during binding as might be expected for a rigid structure like the steroid nucleus.

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.

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.

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.

Localization of the steroid-binding site of the human sex steroid-binding protein of plasma (SBP or SHBG) by site-directed mutagenesis

The amino-terminal region of the human sex steroid-binding protein of plasma (SBP or SHBG) containing K134 and M139 was found to represent part of the steroid-binding site. This was accomplished by constructing and expressing site-directed mutants having the following replacements: M139L, M139K, M139S, K134A, H235S, and Y57F. The results indicated that M139L and H235S were fully-active, K134A and Y57F were 50 and 67% active, M139K was 7% active, and M139S was inactive. These results support affinity-labeling data indicating that both K134 and M139 are located in or near the site, and suggest that Y57 may play a role in steroid binding. The fully active H235S mutant reveals that H235 is not involved in the steroid-binding process.

Complete enzymatic deglycosylation of native sex steroid-binding protein (SBP or SHBG) of human and rabbit plasma: effect on the steroid-binding activity

An enzymatic procedure for the complete removal of the N-linked and O-linked oligosaccharide side chains of the sex steroid-binding proteins (SBP or SHBG) of human and rabbit plasma under native conditions is described. Deglycosylation was catalyzed by N-glycanase, neuraminidase, and O-glycanase and was monitored by SDS-PAGE, lectin blotting, and molecular weight analyses by electrospray mass spectrometry. Digestion of rabbit SBP with N-glycanase generated a major 39,777-Da protein and two minor ones of 39,389 and 39,545 Da. The molecular weight of the major protein agrees with the molecular weight calculated from the sequence of the sugar-free polypeptide monomer (39,769 Da: Griffin, P.R., Kumar, S., Shabanowitz, J., Charbonneau, H., Namkung, P.C., Walsh, K.A., Hunt, D.F., & Petra, P.H., 1989, J. Biol. Chem. 264, 19066-19075), whereas the other two are deglycosylated proteolytic cleavage products lacking the TQR and TQ sequences at the amino-terminus. The N- and O-linked side chains of human SBP were removed by sequential digestion with N-glycanase and neuraminidase/O-glycanase. A 38,771-Da protein was generated, which agrees well with the molecular weight of the sugar-free polypeptide monomer (Walsh, K.A., Titani, K., Kumar, S., Hayes, R., & Petra, P.H., 1986, Biochemistry 25, 7584-7590). N-deglycosylation of human and rabbit SBP has no effect on the steroid-binding activity, but removal of the O-linked side chains of N-deglycosylated human SBP results in an apparent 50% loss of steroid-binding activity and an increase in the Kd for the binding of 5 alpha-dihydrotestosterone from 0.3 mM to 0.9 nM.(ABSTRACT TRUNCATED AT 250 WORDS)