Glucocorticoid-recognizing and -effector sites in rat liver plasma membrane. Kinetics of corticosterone uptake by isolated membrane vesicles--II. Comparative influx and efflux

The ATP-binding cassette proteins ABCB1 and ABCC1 as modulators of glucocorticoid action

Responses to hormones that act through nuclear receptors are controlled by modulating hormone concentrations not only in the circulation but also within target tissues. The role of enzymes that amplify or reduce local hormone concentrations is well established for glucocorticoid and other lipophilic hormones; moreover, transmembrane transporters have proven critical in determining tissue responses to thyroid hormones. However, there has been less consideration of the role of transmembrane transport for steroid hormones. ATP-binding cassette (ABC) proteins were first shown to influence the accumulation of glucocorticoids in cells almost three decades ago, but observations over the past 10 years suggest that differential transport propensities of both exogenous and endogenous glucocorticoids by ABCB1 and ABCC1 transporters provide a mechanism whereby different tissues are preferentially sensitive to different steroids. This Review summarizes this evidence and the new insights provided for the physiology and pharmacology of glucocorticoid action, including new approaches to glucocorticoid replacement.

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.

Rapid non-classical effects of steroids on the membrane receptor dynamics and downstream signaling in neurons

Contribution to Special Issue on Fast effects of steroids. Although rapid effects of steroid hormones on membrane receptors and intracellular signaling molecules have been extensively studied in neurons, we are only beginning to understand the molecular mechanisms behind these non-classical steroid actions. Single molecule tracking (SMT) studies on live cells demonstrated that surface trafficking of membrane receptors determines their ligand binding properties and downstream signaling events. Recent findings suggest that one of the underlying mechanisms of non-classical steroid actions is the alteration of receptor movements on the membrane surface. In order to highlight this novel aspect of steroid effects, we first address the types of receptor movements in the plasma membrane and the role of cortical actin dynamics in receptor movement. We then discuss how single molecules and the surface movements of receptors can be detected in live cells. Next, we review the fundamental processes, which determine the effect of steroids on the plasma membrane: steroid movement through the lipid bilayer and the role of steroid membrane receptors. Using glutamate and neurotrophin receptors (NTRs) as examples, we demonstrate the features of receptor dynamics in the membrane. In addition, we survey the available data of rapid steroid actions on membrane receptor trafficking: we discuss how glucocorticoids act on the surface diffusion of glutamate receptor molecules and how estradiol acts on NTRs and gamma-aminobutyric acid type A receptors (GABA_ARs) and their related signaling events as well as on cortical actin. Finally, we address the physiological relevance of rapid steroid action on membrane receptors dynamics.

Estradiol's interesting life at the cell's plasma membrane

Clearly, we have presented here evidence of a very complex set of mechanisms and proteins involved with various and intricate actions of steroids at the plasma membrane. Steroids do MUCH more at the plasma membrane than simply passing passively through it. They may sit in the membrane; they are bound by numerous proteins in the membrane, including ERs, SHBG, steroid-binding globulin receptors, and perhaps elements of cellular architecture such as tubulin. It also seems likely that the membrane itself responds graphically to the presence of steroids by actually changing its shape as well, perhaps, as accumulating steroids. Clara Szego suggested in the 1980s that actions of E2 at one level would act synergistically with its actions at another level (e.g. membrane actions would complement nuclear actions). Given the sheer number of proteins involved in steroid actions, just at the membrane level, it seems unlikely that every action of a steroid on every potential protein effector will act to the same end. It seems more likely that these multiple effects and sites of effect of steroids contribute to the confusion that exists as to what actions steroids always have. For example, there is confusion with regard to synthetic agents (SERMs etc.) that have different and often opposite actions depending on which organ they act upon. A better understanding of the basic actions of steroids should aid in understanding the variability of their clinical effects.

Estrogens, androgens and generalized behavioral arousal in gonadectomized female and male C57BL/6 mice

General arousal has been operationally defined as an enhanced motor activity and enhanced intensity of response to sensory stimuli. Even though the effects of gonadal hormones on mating behavior have been much studied, their potential effect on generalized arousal, as defined above, has never been evaluated. In the present study we employed a thoroughly validated assay of general arousal to determine the effects of estradiol (E) and testosterone (T) in gonadectomized female and male mice, respectively. The steroids were administered in three different ways: A fast-acting, water soluble preparation given intraperitoneally, an oil solution given subcutaneously, and an oil solution in a subcutaneous Silastic capsule. Motor activity and responses to sensory stimuli were recorded for 24h, 91h, and seven days following hormone administration, respectively. All measures of arousal varied according to the day/night cycle. The water soluble steroid preparation had no reliable effect. When the same doses of estradiol and testosterone were administered subcutaneously in an oil vehicle no effect of either treatment on arousal was observed. The subcutaneously implanted capsule containing estradiol or testosterone had a delayed effect on motor activity in females (four to seven days) but no effect in males. The long time required by the gonadal hormones for affecting arousal would be consistent with, but does not prove, a genomic action. The limited effects of E and T in our arousal assay suggest to us that the strongest actions of these hormones on arousal occur in the context of sequences of responses to sexually relevant stimuli.

Expression of corticosteroid-binding globulin in human astrocytoma cell line

Glial tumor cells are known to be sensitive to glucocorticoids (GC) in vivo and in vitro. Here we studied the expression of corticosteroid-binding globulin (CBG) in the low-grade malignant human astrocytoma cell line 1321N1. CBG was observed in cytoplasm of most of these cells with immunocytochemistry. RT-PCR revealed the presence of the respective mRNA. Only scattered cells contained nuclear immunoreactivity for glucocorticoid receptor as visualized by double immunostaining. Immunoreactive CBG could be recovered from the supernatant of cultures that had been exposed to 10(-5) M cortisol. Our observations indicate the endogenous expression of CBG in 1321N1 cells which may occur independently from classical glucocorticoid receptor pathways. Cortisol seems to facilitate liberation of CBG in a paracrine manner, perhaps through membrane action of the steroid. Effects of adrenal steroids on proliferation and apoptosis of certain glial tumors may in part depend on these mechanisms.

The biological roles of extracellular and intracytoplasmic glucocorticoids in skeletal cells

Osteoporosis is the most common metabolic disease characterized by loss of the normal density of bone, resulting in fragile bone and a higher risk of fractures. Patients under glucocorticoids treatment are susceptible to glucocorticoid-induced osteoporosis (GIO). The normal bone turnover depends on a balance between osteoblasts and osteoclasts. The skeletal cells including osteoblasts, osteoclasts, osteocytes and their precursors demonstrate altered features while they are cocultured with different extracellular glucocorticoids, or their intracytoplasmic glucocorticoids modified by genetic manipulation of 11beta-HSD isozyme. However, recent studies have also demonstrated different or even contradictive outcomes on whether the glucocorticoids inhibit or increase biological activity of these skeletal cells. Focusing on the roles of extracellular glucorticoids, intracytoplasmic glucocorticoids and the mechanism of transmembrane passage of the glucocorticoids, this review reveals that glucocorticoids may exert either inhibitive or enhancing influence on these skeletal cells, but relying on the difference in cell origins, methodology, and types of glucocorticoids. In addition, the effects of glucocorticoids may be dose- and time-dependent.

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.

Neurosteroids, neuroactive steroids, and symptoms of affective disorders

Neurosteroids (NS) are steroids synthesized by the brain. Neuroactive steroids (NAS) refers to steroids that, independent of their origin, are capable of modifying neural activities. NAS bind and modulate different types of membrane receptors. The gamma amino butyric acid (GABA) and sigma receptor complexes have been the most extensively studied. Oxidized ring A reduced pregnanes, tetrahydroprogesterone (THP), and tetrahydrodeoxycorticosterone (THDOC) bind to the progesterone intracellular receptor (PR), and in this way can also regulate gene expression. Animal experimentation showed that salient symptoms of depression, viz., anxiety, sleep disturbances, and memory and sexual dysfunctions, are modulated by NAS. In turn, psychotropic drugs modulate NS and NAS levels. NS levels as well as NAS plasma concentrations change in patients with depression syndromes, the levels return to normal baseline with recovery, but normalization is not necessary for successful therapy. Results from current studies on the evolution of nervous systems, including evolutionary developmental biology as well as anatomical and physiological findings, almost preclude a categorical classification of the psychiatric ailments the human brain succumbs to. The persistence in maintaining such essentialist classifications may help to explain why up to now the search for biological markers in psychiatry has been an unrewarding effort. It is proposed that it would be more fruitful to focus on relationships between NAS and symptoms of psychiatric disorders, rather than with typologically defined disorders.

Membrane initiated steroid signaling (MISS): computational, in vitro and in vivo evidence for a plasma membrane protein initially involved in genomic steroid hormone effects

Steroid hormones are indispensable for control of vital processes, development, reproduction and modulation of behavior. Lack or complete dysfunction of glucocorticoid (GC) signaling, in particular, have lethal consequences. Even a minor change in the level of circulating cortisol can be of physiological and clinical significance. Analysis of the action mechanisms of GC is therefore of major importance, especially since natural and synthetic GC are widely used in the therapy of GC-responsive diseases. According to the classical genomic concept of steroid hormone action, the effects of GC are mediated by specific nuclear receptors (GR). Recent findings provide evidence for the existence of additional, plasma membrane (PM) located steroid receptors, which are thought to be responsible for rapid, non-genomic responses. We present evidence for a PM-residing protein, termed "steroid hormone recognition and effector complex" (SHREC), which plays a pivotal role in the complex network of PM-related non-genomic responses leading to GR-mediated genomic effects. This conclusion was drawn from studies using different in vitro and in vivo models of a GC-target: (1) highly purified fractions of osmotically active PM-vesicles isolated from rat and human hepatocytes, (2) a computational model of SHREC generated by the "automated receptor modeling" (ARM) method, (3) rat liver cell lines transfected with a GC-responsive reporter gene construct and (4) young women orally administered with selected steroids. We conclude that SHREC and the GR participate in the same signaling chain, SHREC<-->steroid interaction is the initial step and an interdependent part of the complete GC signal propagation, thus called "membrane initiated steroid signaling" (MISS).

Steroids, neuroactive steroids and neurosteroids in psychopathology

The term "neurosteroid" (NS) was introduced by Baulieu in 1981 to name a steroid hormone, dehydroepiandrosterone sulfate (DHEAS), that was found at high levels in the brain long after gonadectomy and adrenalectomy, and shown later to be synthetized by the brain. Later, androstenedione, pregnenolone and their sulfates and lipid derivatives as well as tetrahydrometabolites of progesterone (P) and deoxycorticosterone (DOC) were identified as neurosteroids. The term "neuroactive steroid" (NAS) refers to steroids which, independent of their origin, are capable of modifying neural activities. NASs bind and modulate different types of membrane receptors. The GABA and sigma receptor complexes have been the most extensively studied, while glycine-activated chloride channels, nicotinic acetylcholine receptors, voltage-activated calcium channels, although less explored, are also modulated by NASs. Within the glutamate receptor family, N-methyl-d-aspartate (NMDA) receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and kainate receptors have also been demonstrated to be a target for steroid modulation. Besides their membrane effects, once inside the neuron oxidation of Ring A reduced pregnanes, THP and THDOC, bind to the progesterone intracellular receptor and regulate gene expression through this path. The involvement of NASs on depression syndromes, anxiety disorders, stress responses to different stress stimuli, memory processes and related phenomena such as long-term potentiation are reviewed and critically evaluated. The importance of context for the interpretation of behavioral effects of hormones as well as for hormonal levels in body fluids is emphasized. Some suggestions for further research are given.

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.

Membrane-initiated steroid signaling (MISS): genomic steroid action starts at the plasma membrane

Plasma membrane (PM) steroid recognition sites are thought to be responsible only for rapid, non-genomic responses without any link to the nuclear receptor-mediated genomic effects of steroids. We focused on a PM "glucocorticoid-importer" (GC-importer) that imports GC into rat liver cells. This site interacts also with particular gestagens (progesterone, P; medroxyprogesterone, MP; ethynodiol, Ethy) and estrogens (ethinylestradiol, EE(2); mestranol), which do not bind to the nuclear GC receptor (GR). To elucidate the role of the GC-importer, we transfected a rat wild-type hepatocyte (CC-1) and a hepatoma cell line, unable to import GC (MH 3924), with a GC<-->GR-responsive luciferase (luc)-reporter gene. Selected steroids were tested for their ability to induce or inhibit luc expression. Corticosterone (B) and dexamethasone (Dex), but also the GC-antagonists cortexolone (Cortex), P and MP, induced luc. Even the PM-impermeable BSA-derivatives of B, Dex and Cortex did so to almost the same extent as the free steroids. MH 3924 cells respond stronger than CC-1 to luc inducing steroids. Luc expression was inhibited by RU 38 486, but also by EE(2) and Ethy. The thiol reactive mesylate-derivatives of B, Dex and Cortex induced to a considerably lesser extent than the free or BSA-steroids. The thiol reagent mersalyl blocks cellular entry of GC and inhibits luc induction in CC-1 cells. Incubation with EE(2) and B of PM-vesicles, isolated from liver cells, resulted in a decrease of the density of two 75 and 52kDa G-proteins reflecting a diminished exchange of GDP by GTP.

CONCLUSION

the PM-residing GC-importer, now renamed "Steroid Hormone Recognition and Effector Complex" (SHREC) is an interdependent part of the complete GC signal propagation in which G-proteins are involved. Free SH-groups of SHREC are a prerequisite for genomic GC activity. Specific interactions between SHREC and GC-agonist/-antagonist trigger steroid-dependent signaling. However, import of the ligand into the cell terminates it. Thus, the PM-related non-genomic steroid responses are clearly linked to the GR-related genomic effects.

Amphibians as a model for the study of endocrine disruptors

Evidence shows that environmental compounds can interfere with the endocrine systems of wildlife and humans. The main sink of such substances, called endocrine disruptors (EDs), which are mainly of anthropogenic origin, is surface water; thus, aquatic vertebrates such as fishes and amphibians are most endangered. Despite numerous reports on EDs in fishes, information about EDs in amphibians is scarce, and this paucity of information is of particular concern in view of the worldwide decline of amphibians. EDs could contribute to changes of amphibian populations via adverse effects on reproduction and the thyroid system. In amphibians, EDs can affect reproduction by (anti)estrogenic and (anti)androgenic modes of action that produce severe effects including abnormal sexual differentiation. ED actions on the thyroid system cause acceleration or retardation of metamorphosis, which may also affect population levels. Our broad knowledge of amphibian biology and endocrinology indicates that amphibians are very suitable models for the study of EDs. In particular, effects of EDs on the thyroid system triggering metamorphosis can be determined easily and most sensitively in amphibians compared to other vertebrates. A new classification of EDs according to their biological modes of action is proposed because EDs have quite heterogeneous chemical structures, which do not allow prediction of their biological effects. Methods and strategies are proposed for identification and risk assessment of EDs, whether as pure test substances or as mixtures from environmental samples. Effects of EDs on the thyroid system of amphibians can be assessed by a single animal model (Xenopus laevis), whereas the various types of reproduction need comparative studies to investigate whether general endocrine principles do exist among several species of anurans and urodeles. Thus, at least one anuran and one urodelean model are needed to determine ED interference with reproduction.

Seasonal regulation of membrane and intracellular corticosteroid receptors in the house sparrow brain

A number of studies have demonstrated seasonal regulation of the adrenocortical response to stress, or of corticosteroid binding globulins, but very few studies have examined seasonal regulation of corticosteroid receptor levels. As a result, there have been few attempts to produce an integrated picture of seasonal plasticity of the stress response. We measured baseline and stress-induced corticosterone (CORT), corticosteroid binding globulin and neuronal cytosolic and membrane corticosteroid receptor levels in male and female, wild-caught house sparrows (Passer domesticus) during three different seasons over the annual cycle (nesting, molting and winter). We identified three neuronal corticosteroid receptors in the house sparrow brain: two intracellular receptors and one membrane-associated receptor. Little is known about corticosteroid receptors in neuronal membranes of avian and mammalian species, but we found that the levels of membrane corticosteroid receptors varied seasonally, being lowest during the nesting season. Cytosolic corticosteroid receptor numbers (both low and high affinity receptors) also varied seasonally. In contrast to the membrane bound receptors, however, the numbers of low and high affinity cytosolic receptors were lowest during winter. In addition, mean levels of total basal and stress-induced CORT in the plasma varied seasonally. Both basal and stress-induced levels of total CORT were significantly higher during nesting than during winter or molt. Finally, corticosteroid binding globulin levels in plasma were also seasonally regulated, in a pattern similar to total CORT, so that estimated free CORT levels did not vary between seasons. These data indicate that multiple components of the stress response are seasonally regulated in birds obtained from wild populations. Interactions between these regulated components provide a basis for seasonal differences in behavioural and physiological responses to stress.

Noninvasive corticosterone treatment rapidly increases activity in Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii)

Recent evidence supports the hypothesis that corticosteroids influence behavioral changes associated with stressful events. Most investigations into this relationship focus on the long-term behavioral effects of corticosterone. Because many behavioral responses to environmental perturbations occur within minutes, we determined what rapid effects corticosterone may have on behavior. With this goal in mind, we devised and evaluated a method of corticosterone delivery which allowed us to examine immediate effects of corticosterone on behavior in a noninvasive manner. White-crowned sparrows (Zonotrichia leucophrys gambelii) were allowed access to mealworms (Tenebrio molitor) injected with corticosterone. Once ingested, the corticosterone moves across the digestive epithelium into the circulation. This method was evaluated using two vehicles: dimethyl sulfoxide and peanut oil. We tested the efficiency and consistency of corticosterone transfer into the circulation for both vehicles. Dimethyl sulfoxide gave a more efficient transfer of corticosterone. Injecting mealworms with corticosterone (carried in dimethyl sulfoxide) and feeding those worms to white-crowned sparrows increased circulating corticosterone to a discrete, repeatable level which peaked within 7 min and was cleared within 60 min. Using this method, we demonstrated that intermediate levels of corticosterone caused an increase in perch hopping in white-crowned sparrows within 15 min of hormone administration. An increase in perch hopping indicated elevated locomotor activity that is consistent with behavioral responses to natural perturbations. High levels of corticosterone did not induce this behavioral change. In light of the rapid effect of corticosterone on behavior, we propose that corticosterone was acting through a nongenomic mechanism.

Glucocorticoid-recognizing and -effector sites in rat liver plasma membrane. Kinetics of corticosterone uptake by isolated membrane vesicles. III. Specificity and stereospecificity

In previous papers we provided evidence for a glucocorticoid (GC) responsive site in a highly purified rat liver plasma membrane (PM) fraction, which has proved to be osmotically active, 'right side-out' vesicles, free of CBG, glucocorticoid receptors (GR) and ATP (J. Steroid Biochem. Molec. Biol. 42 (1992) 737-756 and 757-771). This site, now called 'GC importer', mediates active transmembrane transport of corticosterone (B). Pronounced specificity, including stereo- and enantiomeric specificity, of ligand-GC importer interaction was demonstrated by competition assays using 54 different steroidal hormones and molecules. Important structural prerequisites for ligands with high specificity for the GC importer are plane C21-steroid hormones with 1-ene and/or 4-ene or 5alpha-reduced configuration, and/or OH-group(s) at C11beta>C17alpha>C21. Unexpectedly, other preferred ligands are C17alpha-ethynyl steroids like estrogens with an OH- or OCH3-group at C3 (EE2, mestranol) as well as progestins with C3-OH and 4-ene configuration (ethynodiol). C21-steroids with 11alpha-OH, 11-keto, 16alpha-CH3, 16beta-CH3, 16alpha-OH or 5beta-reduced configuration are low specificity ligands. The importer even displays different specificity for enantiomers (levonorgestrel>L-norgestrel). Altogether, the GC importer preferentially recognizes active GC and natural progestins which act as GC-antagonist (e.g. prednisolone>11beta-cortisol = B > or = progestins). Synthetic GC-agonists (e.g. dexamethasone, betamethasone, triamcinolone), most synthetic progestins, biologically inactive GC (e.g. 11alpha-cortisol, prednisone, cortisone, 11-dehydro-B), mineralocorticoids (aldosterone), natural estrogens (e.g. E1, E2, E3), DES and vitamin D3 derivatives do not interact with the GC importer. Osmotic shrinkage experiments revealed that interaction of high as well as low specificity ligands with the GC importer comprises reversible binding and transport through the PM. The ligand specificity profile of the GC importer and the GR exhibit pronounced differences, suggesting that both GC recognizing sites are different proteins. Performing immunoblotting, using specific mono- and polyclonal antibodies directed against the intracellular rat GR, of the PM pretreated with the membrane protein solubilizing detergent CHAPSO, we found that specific steroid binding to the PM site is not due to contamination with GR. Colchicine, daunorubicine, quinine, reserpine, verapamil and vinblastine, representatives of lipophilic xenobiotics which are known to be transported out of cells by the glycoprotein P170, did not compete with B for uptake into PM-vesicles, indicating that the GC importer is not a member of the ABC/mdr superfamily. The GC importer seems to be an additional link in the chain of steroid signal transduction and may be functionally involved in the action of natural GC-agonists and GC-antagonists.

Glucocorticoid-recognizing and -effector sites in rat liver plasma membrane. Kinetics of corticosterone uptake by isolated membrane vesicles--I. Binding and transport

To gain insight into the mechanisms governing cellular uptake of glucocorticoids, we studied the binding and membrane transport of corticosterone (B) on a highly purified plasma membrane fraction from rat liver that was homogenized using a gentle, isotonic procedure. The fraction was mostly in the form of right-side out and osmotically active vesicles that were free of intracellular glucocorticoid receptors (GCR), transcortin (CBG) and ATP. Our uptake and binding studies carried out at 22 degrees C with [3H]B in physiological concentrations resulted in the following findings: (1) unlabeled B competed with [3H]B for uptake by the membrane vesicles; half-maximal competition of specific uptake was achieved with a 10- to 11-fold molar excess of unlabeled B. (2) [3H]B uptake was a saturable process of unusual kinetics (multiple sigmoidity); modified Scatchard plots revealed three significantly different apparent Kd-values of 1.3, 4.7 and 17.3 nM, corresponding to free B in the blood of non-stressed rats (4-16 nM). (3) Osmotic shrinkage of the vesicles led to a linear decrease in specific uptake, while non-specific uptake was independent of vesicle volume. Passive diffusion of [3H]B took place in leaky, but not in intact, vesicles. Reversible binding to, and mediated transport through, the membrane were interdependent parts of a strongly linked process. B was accumulated inside the vesicle up a concentration gradient by an active transport that followed first-order kinetics (Kt:3.9 nM); for its statistically reliable mathematical formulation and kinetic analysis, a replot was developed that revealed that relative accumulation increased with decreasing external hormone concentration. (4) Comparative binding studies disclosed that the apparent Kd-values (86.5 +/- 7.3 and 77.0 +/- 14.3 nM, respectively) of the [3H]B interactions with CBG and GCR did not differ (P greater than 0.3). These findings permit the conclusion that a plasma membrane-inserted carrier for B, effectively operating at physiological concentrations in the blood, is involved in a functional and regulatory manner in the biological action of glucocorticoids.