Radioligand assays for oestradiol and progesterone conjugated to protein reveal evidence for a common membrane binding site in the medial preoptic area-anterior hypothalamus and differential modulation by cholera toxin and GTPgammaS

Three steroid-binding globulins, their localization in the brain and nose, and what they might be doing there

Steroid-binding globulins (SBGs) such as sex hormone binding globulin, corticosteroid binding globulin, and vitamin-D binding protein are receiving increasing notice as being actively involved in steroid actions. This paper reviews data of all three of these SBGs, focusing on their presence and possible activity in the brain and nose. We have found all three proteins in the brain in limbic areas such as the paraventricular (PVN) and supraoptic nuclei (SON) as well as other areas of the hypothalamus, hippocampus, and medial preoptic area. There is also evidence that all three are made in the PVN and SON, in conjunction with the neuropeptides oxytocin and vasopressin. The localization of these three SBGs is more variable within areas of the main olfactory area and the vomeronasal organ. However, all three are found in the mucus of these areas, suggesting that one of their functions is to sequester aerosol steroids, such as pheromones, and deliver them to sensory cells and then to deeper sensory areas. In this manuscript, we present multiple models of SBG action including: A) SBG binding to a membrane receptor, B) this SBG receptor being associated with a larger protein complex including cytoplasmic steroid receptors, C) when the SBGs binds to their SBG receptors, second messengers within the cells respond, D) after SBG binding to its receptor, it releases its associated steroid into the membrane's lipid bilayer, from which it gains access into the cell only when bound by an internal protein, E) the SBG, possibly with its bound SBG receptor, is internalized into the cell from which it can gain access to numerous organelles and possibly the cell's nucleus or F) associate with intracellular steroid receptors, G) SBGs produced in target cells are released from those cells upon specific stimulation, and H) according to the Free Steroid Hypothesis steroids released from the extracellular SBG passively diffuse across the plasma membrane of the cell. These models move the area of steroid endocrinology forward by providing important paths of steroid activity within many steroid target cells.

Sex hormone binding globulin and corticosteroid binding globulin as major effectors of steroid action

Contrary to the long-held postulate of steroid-hormone binding globulin action, these protein carriers of steroids are major players in steroid actions in the body. This manuscript will focus on our work with sex hormone binding globulin (SHBG) and corticosteroid binding globulin (CBG) and demonstrate how they are actively involved in the uptake, intracellular transport, and possibly release of steroids from cells. This manuscript will also discuss our own findings that the steroid estradiol is taken up into the cell, as demonstrated by uptake of fluorescence labeled estradiol into Chinese hamster ovary (CHO) cells, and into the cytoplasm where it may have multiple actions that do not seem to involve the cell nucleus. This manuscript will focus mainly on events in two compartments of the cell, the plasma membrane and the cytoplasm.

Internalization of sex hormone-binding globulin into neurons and brain cells in vitro and in vivo

BACKGROUND

Sex hormone-binding globulin (SHBG) is a 94-kDa homodimer that binds steroids and is made in the hypothalamus. We have demonstrated that infusions of SHBG into the hypothalami of rats increase their female sexual receptivity except when SHBG is coupled to dihydrotestosterone (DHT) suggesting that SHBG has an active function in behavioral neuroendocrinology.

METHODS

This study examines the possibility that SHBG is internalized by neuronal and/or non-neuronal brain cells as one possible mode of action using in vitro and in vivo techniques.

RESULTS

First, analysis of the uptake of radiolabeled SHBG ((125)I-SHBG) found (125)I-SHBG uptake in HT22 hippocampal cells stably transfected with cDNA for ER beta (HT22-ER beta). The addition of DHT to (125)I-SHBG significantly inhibited (125)I-SHBG uptake in HT22-ER beta cells but not in HT22-ER alpha or HT22 wild-type cells. SHBG internalization was specific as it did not occur in either the human neuroblastoma cell line SK-N-SH or the glioma cell line C6. Second, SHBG was labeled with a fluor (Alexa-555), and infused into the lateral cerebroventricles of ovariectomized rats. Optimal SHBG uptake was seen 10 min after these infusions. SHBG uptake was seen in specific parts of the choroid plexus and periventricular cells as well as into cells in the paraventricular nucleus, the medial forebrain bundle, and the habenula.

CONCLUSIONS

These studies suggest that SHBG is internalized by brain cells, which may be affected by the presence of ER beta. The gonadal steroids have numerous effects in brain and the discovery that the steroid-binding protein SHBG is taken up into neurons and brain cells may demand a change in thinking about how steroids are delivered to brain cells to affect neurophysiology.

Nongenomic effects of progesterone on the contraction of muscle cells from the guinea pig colon

Progesterone (PG) affects muscle cells by genomic mechanisms through nuclear receptors and by nongenomic mechanisms through unidentified pathways. This study aimed to determine the pathways mediating its nongenomic actions. Experiments were performed in dissociated muscle cells from guinea pig colons. Nongenomic actions were defined as those occurring within 10 min of PG exposure. PG blocked the contraction to CCK-8 and NKA (10(-7) M) but did not impair ACh (10(-7) M) and KCl (2.5 x 10(-2) M)-induced contraction. Both CCK-8 and NKA contract muscle cells by releasing calcium from intracellular stores, whereas ACh and KCl can utilize extracellular calcium. PG also blocked the contraction induced by inositol 1,4,5-trisphosphate, thapsigargin, and caffeine, agents that contract muscle cells by releasing calcium from storage sites. The nongenomic actions of PG were transient because they were absent 1 h after the first PG dose, remaining unresponsive after a second PG dose was administered. Furthermore, PG had no effect on the contraction induced by CCK-8 and thapsigargin in muscle cells from animals pretreated with daily intramuscular PG for 4 days. Cytosolic incorporation experiments of [(3)H]PG showed that pretreatment with unlabeled PG significantly reduced the radiolabeled PG incorporation in the cytosol. We conclude that the nongenomic actions of PG on colonic muscle cells transiently blocked calcium release from storage sites, and this response became rapidly desensitized. This effect does not appear to be specific to PG because other steroid hormones such as aldosterone and testosterone can also induce it.

A Nongenomic Action of 17β-Estradiol as the Mechanism Underlying the Acute Suppression of Secretion of Luteinizing Hormone1

The objective of the present study was to determine the ability of 17beta-estradiol (E(2)) and conjugated forms of E(2) (E(2) conjugated to BSA [E(2)-BSA] and a novel conjugate, E(2) conjugated to a small peptide [E(2)-PEP]) to prevent the GnRH-induced secretion of LH and to determine the role of estradiol receptors (ERs) and ER subtypes (ERalpha, also known as ESR1, and ERbeta, also known as ESR2) in the mediation of the acute action of E(2) in primary cultures of ovine pituitary cells. Preincubation of cells for 15 min with E(2), E(2)-BSA, or E(2)-PEP prevented the GnRH-induced secretion of LH (P < 0.01). Treatment of cells with nonestrogenic steroid hormones did not affect secretion of LH when given alone, nor did these steroids impair the E(2)-induced inhibition of LH secretion (P > 0.1). Likewise, treatment of cells with the ER-antagonists tamoxifen, hydroxytamoxifen, or ICI 182 780 did not affect (P > 0.1) secretion of LH when given alone but did prevent (P < 0.01) the inhibition by E(2) and the E(2)-conjugates on GnRH-induced secretion of LH. When cells were treated with subtype-selective ER agonists, the ERalpha agonist (propylpyrazole-triol), but not the ERbeta agonist (diarylpropionitrile), decreased (P < 0.01) the GnRH-induced secretion of LH. In conclusion, the rapidity by which E(2) prevented GnRH-induced release of LH in ovine pituitary cells suggests that this inhibition is mediated via a nongenomic action of E(2). The inhibition of GnRH-induced secretion of LH proved to be steroid specific and mediated by ERs. It may occur specifically through ERalpha. The fact that E(2)-BSA or E(2)-PEP mimicked the action of E(2) suggests that this effect was mediated by an ER associated with the plasma membrane.

Estradiol control of expression and levels of estradiol-binding proteins in the medial preoptic area, medial hypothalamus and pituitary

The brains of mammals have at least three estradiol-binding proteins: estradiol receptor-alpha (ERalpha), ERbeta, and sex hormone-binding globulin (SHBG). In this study we compare the effects of estradiol treatment on the expression of mRNA for these three estradiol-binding proteins in two reproductively important brain areas, the medial preoptic area-anterior hypothalamus (MPOA-AH) and medial hypothalamus (MH) as well as in the hippocampus in ovariectomized rats, using the reverse transcriptase-polymerase chain reaction (RT-PCR). We also used surface-enhanced laser desorption ionization time of flight (SELDI-TOF) mass spectrometry (MS) to analyze the effects of estradiol in ovariectomized rats on SHBG levels in the MPOA-MH as well as the neurohypophysis. In vivo estradiol treatment in ovariectomized rats eliminated or significantly reduced expression of all three estradiol-binding proteins in both the MPOA-AH and MH. This change in ERalpha, ERbeta, and SHBG expression did not occur in the hippocampus. Both Northern blot and DNA sequence analysis confirmed the results of the RT-PCR for SHBG. SELDI-TOF MS analysis demonstrated that in vivo estradiol treatments resulted in dramatically decreased levels of SHBG in the hypothalamus and that a reduction in SHBG mRNA by estradiol treatment also resulted in a reduction in SHBG protein levels. Estradiol treatment also eliminated detectable SHBG from the neurohypophysis, suggesting that estradiol controls SHBG levels in this release site. That in vivo estradiol treatments had the same inhibitory effects on mRNA levels for SHBG and both ERs suggests similar translational control mechanisms for all three steroid-binding proteins in the brain. That estradiol treatments also reduced pituitary SHBG suggests that such treatment releases SHBG from the neurohypophysis.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

5alpha-Reduced androgens block estradiol-BSA-stimulated release of oxytocin

In this study we test the postulate that estradiol conjugated to bovine serum albumin (E-BSA) acts via receptors for the steroid-binding protein sex hormone binding globulin (SHBG) by attempting to block E-BSA-stimulated release of oxytocin with two antagonists of SHBG receptor actions: the 5alpha-reduced androgens dihydrotestosterone (DHT) and 3alpha-diol. Simultaneous superfusion with either DHT or 3alpha-diol significantly blocked E-BSA-stimulated release of oxytocin. We also found that a wide range of free 17beta-estradiol was unable to stimulate oxytocin release, suggesting that E-BSA stimulates receptors other than those for free estradiol to release oxytocin, perhaps SHBG receptors.

Uncoupling of 5-HT1A receptors in the brain by estrogens: regional variations in antagonism by ICI 182,780

Previously we have shown that 17beta-estradiol (in vivo and in vitro) rapidly decreases the function of serotonin(1A) (5-HT(1A)) receptors, allowing us to hypothesize that 17beta-estradiol accomplished this via activation of a membrane estrogen receptor. Hippocampus and frontal cortex obtained from ovariectomized rats were incubated with 17beta-estradiol or bovine serum albumin (BSA)-estradiol in the presence or absence of the estrogen receptor (ER) antagonist ICI 182,780. Membranes were prepared to measure R(+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding (a measure of 5-HT(1A) receptor coupling and function). In both hippocampus and frontal cortex, 17beta-estradiol and BSA-estradiol (50 nM) decreased R(+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding. ICI 182,780 blocked the effect of both the estrogens in hippocampus, but only the effect of 17beta-estradiol in frontal cortex. Due to the inability of ICI 182,780 to block the effects of BSA-estradiol in frontal cortex, similar experiments were performed using the selective estrogen receptor modulator tamoxifen as the agonist. Tamoxifen (100 nM and 1 microM) decreased R(+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding. ICI 182,780 (1 microM) blocked the ability of tamoxifen to decrease 5-HT(1A) receptor coupling in the hippocampus, but not in the frontal cortex. Taken together, these data support the existence of a pharmacologically distinct ER in hippocampus vs. frontal cortex that might be responsible for rapid uncoupling of 5-HT(1A) receptors.

Membrane-bound progesterone receptors coupled to G proteins in the fungus Rhizopus nigricans

Steroid binding sites with high affinity for progesterone (Kd=40+/-14 nM determined by binding, and Kd=71+/-22 nM determined by displacement studies) and lower affinity for 21-hydroxyprogesterone and for testosterone, but no affinity for estradiol-17beta, onapristone and alpha-naphthoflavone were detected in the enriched plasma membrane fraction of the fungus Rhizopus nigricans. The amount of steroid binding sites is in accordance with the value of B(max)=744+/-151 fmol (mg protein)(-1). In the membrane fraction, progesterone induced about 30% activation of G proteins over basal level, as determined by GTPase activity (EC50=32+/-8 nM) and by the guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding rate (EC50=61+/-21 nM). The affinity of receptors for progesterone was substantially decreased in the presence of GTPgammaS and of cholera toxin. Our results suggest the existence of progesterone receptors in the membrane of Rhizopus nigricans and their coupling to G proteins.

Rapid non-genomic and genomic responses to progestogens, estrogens, and glucocorticoids in the endocrine pancreatic B cell, the adipocyte and other cell types

Rapid biologic responses to injected steroids were described as early as 60 years ago. More recently, evidence has been presented that 17beta-estradiol given i.v. will double the uterine cAMP activity within 15 s (Proc Natl Acad Sci USA 1967;58:1711-8), and also that estrogens will bind to the outer surfaces of endometrial cells (Nature 1977;265:69-72), suggesting that these steroids can both engage and direct intracellular events. Unfortunately, studies of such rapid membrane effects of steroids have languished due to the accumulation of compelling data for the more slowly manifest actions of these compounds at the level of nuclear DNA. We report a number of observations in women, in experimental animals, and in isolated organ or cell systems using 17beta-estradiol, progesterone or glucocorticoids which provide ample evidence for rapid intracellular metabolic responses to these steroids, mediated by their actions at the cellular plasma membrane. Such rapid responses have been shown in various classic targets or not, such as the B cell of the endocrine pancreas and the fat cell. They involve plasma membrane binding, changes in membrane electrical activity, Ca2+ handling, G and Ras proteins, cAMP, cGMP, IP(3), DAG, phosphodiesterases, protein kinases, tyrosine kinases, ER kinases, and mitogen activated protein kinases (MAPks) and nitric oxide synthase. These recent findings are discussed in detail and should lead to a fuller understanding of the cellular effects of the steroid hormones.

A sexual arousability model involving steroid effects at the plasma membrane

This review will discuss the status of research related to sexual arousability. It will also present a model for sexual arousability based on current knowledge of steroids effects at the membranes of cells. Steroids have multiple rapid actions that are suggested to result from actions at membrane-associated receptors. When stimulated by steroids these receptors alter G-protein coupling in a manner unique to this complex. Initial stimulation of the receptors by steroids alters the coupling pattern of G-proteins and of other binding sites associated with the complex. This change in G-protein coupling is a stable alteration and thus may serve as a long-term change in the system, which is a requirement of sexual arousability. Stimulation of this receptor system by a surge of oxytocin at ejaculation or orgasm then decouples the G-protein and reduces arousability. Sex hormone binding globulin may be an important ligand at this complex. This model suggests completely new relationships among steroids and their receptors that may complement or diverge from actions at known intracellular receptors.

Sex hormone binding globulin stimulates female sexual receptivity

Sex hormone binding globulin (SHBG) is found in the brain and acts directly on plasma membrane-associated receptors in the prostate gland. Infusing SHBG into the medial preoptic area or medial basal hypothalamus of female rats increases their female sexual receptivity. SHBG, SHBG plus estradiol (SHBG-E), and SHBG-E plus oxytocin all significantly increased female sexual receptivity over vehicle or estradiol plus oxytocin infused controls, as measured by lordosis quotients and receptivity scores, at 40, and 90 min after their infusions into the medial preoptic area. When infused into the medial basal hypothalamus, SHBG-E plus oxytocin resulted in significantly increased sexual receptivity 20 and 40 min after infusion when compared to its estradiol plus oxytocin control group. SHBG produced in the brain may be released endogenously to have immediate effects on reproductive physiology and behavior.

Sex-steroid induction of endogenous opioid inhibition on oxytocin secretory responses to stress

In pregnancy, endogenous opioids inhibit enhanced basal and stressor-stimulated oxytocin neurone activity and secretion. By contrast, stress responses of the hypothalamo-pituitary-adrenal (HPA) axis are reduced in pregnancy. We investigated whether the high levels of oestradiol and progesterone of pregnancy could induce these changes. Silastic capsules containing oestradiol or progesterone (or control capsules) were implanted s.c. in virgin female rats for 16 or 17 days, with or without progesterone removal on day 15 to mimic the progesterone withdrawal seen at the end of pregnancy. Plasma concentrations of oxytocin, adrenocorticotrophic hormone (ACTH) and corticosterone were measured in jugular vein blood samples from conscious rats. Under basal conditions, naloxone (5 mg/kg) increased oxytocin secretion in all groups, but had no greater effect in sex-steroid treated rats, and did not induce Fos expression in the supraoptic nucleus. Forced swimming, a stressor, increased oxytocin secretion at 5 min in vehicle-injected controls, and this response was slightly attenuated in the sex-steroid treated groups. Pretreatment with naloxone greatly enhanced the response in the sex-steroid treated rats, and was less effective in the controls. In rats treated with oestradiol alone, naloxone prolonged the response. Thus, the combined sex-steroid treatment enhanced the responsiveness of oxytocin neurones to the stressor, while simultaneously restraining oxytocin secretion via endogenous opioid inhibition. In the same rats, ACTH and corticosterone secretion was also stimulated by the stressor, but the hypothalamo-pituitary-adrenal (HPA) axis response was not attenuated in sex-steroid treated rats. Naloxone weakly reduced the HPA axis response in controls and was ineffective in the sex-steroid treated rats. We conclude that oestradiol and progesterone may be responsible for inducing the opioid restraint and enhanced oxytocin neurone responsiveness in pregnancy.