Studies on the arrangement of glucocorticoid receptors in the plasma membrane of S-49 lymphoma cells

Dexamethasone mediates pancreatic cancer progression by glucocorticoid receptor, TGFβ and JNK/AP-1

Glucocorticoids such as dexamethasone are widely co-prescribed with cytotoxic therapy because of their proapoptotic effects in lymphoid cancer, reduction of inflammation and edema and additional benefits. Concerns about glucocorticoid-induced therapy resistance, enhanced metastasis and reduced survival of patients are largely not considered. We analyzed dexamethasone-induced tumor progression in three established and one primary human pancreatic ductal adenocarcinoma (PDA) cell lines and in PDA tissue from patients and xenografts by FACS and western blot analysis, immunohistochemistry, MTT and wound assay, colony and spheroid formation, EMSA and in vivo tumor growth and metastasis of tumor xenografts on chicken eggs and mice. Dexamethasone in concentrations observed in plasma of patients favored epithelial–mesenchymal transition, self-renewal potential and cancer progression. Ras/JNK signaling, enhanced expression of TGFβ, vimentin, Notch-1 and SOX-2 and the inhibition of E-cadherin occurred. This was confirmed in patient and xenograft tissue, where dexamethasone induced tumor proliferation, gemcitabine resistance and metastasis. Inhibition of each TGFβ receptor-I, glucocorticoid receptor or JNK signaling partially reversed the dexamethasone-mediated effects, suggesting a complex signaling network. These data reveal that dexamethasone mediates progression by membrane effects and binding to glucocorticoid receptor.

Further evidence for a membrane receptor that binds glucocorticoids in the rodent hypothalamus

In parallel with their well-characterized delayed genomic effects, steroid hormones exhibit rapid, non-genomic effects at molecular, cellular and behavioral levels. We have proposed a model of rapid, non-genomic glucocorticoid inhibition of hypothalamic neuroendocrine cells through a putative membrane-associated glucocorticoid receptor (GR). Here we tested for plasma membrane GR immunoreactivity and binding in the hypothalamic supraoptic and paraventricular nuclei. Selective cross-linking of membrane proteins with membrane-impermeant BS³ and subsequent Western blot analysis with a monoclonal GR antibody revealed a reduction in the intensities of a ∼98kDa immunoreactive band and a ∼64kDa band in the rat paraventricular and supraoptic nuclei, and of a 64kDa band in hippocampal tissue, which suggested that these proteins are associated with the membrane. Saturation binding of [³H]-corticosterone and [³H]-dexamethasone in rat and mouse hypothalamic tissue revealed a K_d 4-24-fold lower and a B_max 4-7-fold lower for the membrane-associated GR compared to the intracellular GR, suggesting a lower affinity and abundance of the glucocorticoid binding sites in the membrane than in the cytosol. Together, these findings suggest the presence of a low-affinity, low-abundance membrane-associated GR in the hypothalamus that shares homology with the intracellular GR, and are consistent with physiological evidence of rapid, non-genomic glucocorticoid actions in hypothalamic neuroendocrine cells that are GR dependent.

Effects of Glucocorticoids in the Immune System

Glucocorticoids (GCs) are steroid hormones with widespread effects. They control intermediate metabolism by stimulating gluconeogenesis in the liver, mobilize amino acids from extra hepatic tissues, inhibit glucose uptake in muscle and adipose tissue, and stimulate fat breakdown in adipose tissue. They also mediate stress response. They exert potent immune-suppressive and anti-inflammatory effects particularly when administered pharmacologically. Understanding these diverse effects of glucocorticoids requires a detailed knowledge of their mode of action. Research over the years has uncovered several details on the molecular action of this hormone, especially in immune cells. In this chapter, we have summarized the latest findings on the action of glucocorticoids in immune cells with a view of identifying important control points that may be relevant in glucocorticoid therapy.

Membrane glucocorticoid receptor activation induces proteomic changes aligning with classical glucocorticoid effects

Glucocorticoids exert rapid nongenomic effects by several mechanisms including the activation of a membrane-bound glucocorticoid receptor (mGR). Here, we report the first proteomic study on the effects of mGR activation by BSA-conjugated cortisol (Cort-BSA). A subset of target proteins in the proteomic data set was validated by Western blot and we found them responding to mGR activation by BSA-conjugated cortisol in three additional cell lines, indicating a conserved effect in cells originating from different tissues. Changes in the proteome of BSA-conjugated cortisol treated CCRF-CEM leukemia cells were associated with early and rapid pro-apoptotic, immune-modulatory and metabolic effects aligning with and possibly "priming" classical activities of the cytosolic glucocorticoid receptor (cGR). PCR arrays investigating target genes of the major signaling pathways indicated that the mGR does not exert its effects through the transcriptional activity of any of the most common kinases in these leukemic cells, but RhoA signaling emerged from our pathway analysis. All cell lines tested displayed very low levels of mGR on their surface. Highly sensitive and specific in situ proximity ligation assay visualized low numbers of mGR even in cells previously thought to be mGR negative. We obtained similar results when using three distinct anti-GR monoclonal antibodies directed against the N-terminal half of the cGR. This strongly suggests that the mGR and the cGR have a high sequence homology and most probably originate from the same gene. Furthermore, the mGR appears to reside in caveolae and its association with caveolin-1 (Cav-1) was clearly detected in two of the four cell lines investigated using double recognition proximity ligation assay. Our results indicate however that Cav-1 is not necessary for membrane localization of the GR since CCRF-CEM and Jurkat cells have a functional mGR, but did not express this caveolar protein. However, if expressed, this membrane protein dimerizes with the mGR modulating its function.

Anabolic androgenic steroids effects on the immune system: a review

Androgenic anabolic steroids (AAS) are synthetic derivatives of the male hormone testosterone. AAS are used by athletes and recreational users of all ages to enhance their athletic performance and/or physical appearance. While several adverse effects of AAS abuse have been described, their effect on the immune system has not been clearly elucidated. The literature generally indicates that supraphysiologic doses of AAS with an intact steroid nucleus are immunosuppressive, that is they reduce immune cell number and function. While those with alterations to the steroid nucleus are immunostimulatory as they induce the proliferation of T cells and other immune cells. Specifically, several common AAS have been shown to adversely influence lymphocyte differentiation and proliferation, antibody production, Natural Killer Cytotoxic activity and the production of certain cytokines, thereby altering the immune reaction. These effects may be profound and long lasting depending on the dosing regime, types or combinations of AAS used and the extent and duration of AAS abuse. Nevertheless, the effects of long term use of supraphysiologic doses of AAS on the immune system remain uncertain.

Glucocorticoid signaling: a nongenomic mechanism for T-cell immunosuppression

Glucocorticoids were long believed to exert their effects through transcriptional regulation of glucocorticoid-receptor target genes. However, there is accumulating evidence for nongenomic glucocorticoid-receptor-dependent modulation of signal transduction pathways. Here, we review rapid glucocorticoid activities and focus on a novel mechanism that underlies nongenomic glucocorticoid-induced immunosuppression in T cells. The findings demonstrate a physical and functional interaction between the glucocorticoid receptor and the T-cell receptor (TCR) complex. In its unligated state, the glucocorticoid receptor has an important role in TCR signaling but, after glucocorticoid-receptor-ligand binding (caused by short-term treatment with the synthetic glucocorticoid dexamethasone), the TCR complex is disrupted, leading to impaired TCR signaling. These data reveal a dichotomal functional role for glucocorticoid receptors: one in the cytosol as part of the TCR complex and the other as a nuclear regulator of gene transcription. Drugs that selectively target membrane-bound glucocorticoid receptors might represent a novel immunosuppressive approach.

Xenoestrogens are potent activators of nongenomic estrogenic responses

Studies of the nuclear transcriptional regulatory activities of non-physiological estrogens have not explained their actions in mediating endocrine disruption in animals and humans at the low concentrations widespread in the environment. However, xenoestrogens have rarely been tested for their ability to participate in the plethora of nongenomic steroid signaling pathways elucidated over the last several years. Here we review what is known about such responses in comparison to our recent evidence that xenoestrogens can rapidly and potently elicit signaling through nongenomic pathways culminating in functional endpoints. Both estradiol (E(2)) and compounds representing various classes of xenoestrogens (diethylstilbestrol, coumestrol, bisphenol A, DDE, nonylphenol, endosulfan, and dieldrin) act via a membrane version of the estrogen receptor-alpha on pituitary cells, and can provoke Ca(2+) influx via L-type channels, leading to prolactin (PRL) secretion. These hormones and mimetics can also cause the oscillating activation of extracellular regulated kinases (ERKs). However, individual estrogen mimetics differ in their potency and temporal phasing of these activations compared to each other and to E(2). It is perhaps in these ways that they disrupt some endocrine functions when acting in combination with physiological estrogens. Our quantitative assays allow comparison of these outcomes for each mimetic, and let us build a detailed picture of alternative signaling pathway usage. Such an understanding should allow us to determine the estrogenic or antiestrogenic potential of different types of xenoestrogens, and help us to develop strategies for preventing xenoestrogenic disruption of estrogen action in many tissues.

Hormone multifunctionalities: a theory of endocrine signaling, command and control

A theory is presented outlining how organisms can function and benefit from multifunctionality of hormones in order to enhance greatly the information-carrying potential of endocrine signaling. Hormones are produced continuously as micropulses, and intermittently as larger pulses. It is generally believed that micropulses generate fluctuating basal hormone concentrations, which may consistently elicit particular responses among diverse variables. Evidence is discussed suggesting that in contrast to the hormone micropulses, the larger endogenous hormone pulses may elicit responses which may differ from one pulse to another and may therefore serve different physiological functions. In this paper we postulate that an endogenous hormone pulse is a specific form of a multisignal message that serves a certain physiological function. Different pulses of a hormone may be signals of diverse multisignal messages that serve different functions. A multisignal message may elicit congruous responses by selectively enhancing some actions and suppressing other actions of the component signals. Various roles of signals of multisignal messages are discussed, as well as processes that may be involved in the diversity and selectivity of actions of different pulses of a hormone. Hormones also are converted into other hormones; we analyze how precursor and derived hormones may function independently of each other, and how precursor hormones may give rise to permissive effects. Mechanisms involved in therapeutic and adverse effects of hormone administrations are analyzed, and a strategy is suggested for developing more selective hormonal therapies.

Membrane estrogen receptor-alpha levels predict estrogen-induced ERK1/2 activation in MCF-7 cells

INTRODUCTION

We examined the participation of a membrane form of estrogen receptor (mER)-alpha in the activation of mitogen-activated protein kinases (extracellular signal-regulated kinase [ERK]1 and ERK2) related to cell growth responses in MCF-7 cells.

METHODS

We immunopanned and subsequently separated MCF-7 cells (using fluorescence-activated cell sorting) into mER-alpha-enriched (mERhigh) and mER-alpha-depleted (mERlow) populations. We then measured the expression levels of mER-alpha on the surface of these separated cell populations by immunocytochemical analysis and by a quantitative 96-well plate immunoassay that distinguished between mER-alpha and intracellular ER-alpha. Western analysis was used to determine colocalized estrogen receptor (ER)-alpha and caveolins in membrane subfractions. The levels of activated ERK1 and ERK2 were determined using a fixed cell-based enzyme-linked immunosorbent assay developed in our laboratory.

RESULTS

Immunocytochemical studies revealed punctate ER-alpha antibody staining of the surface of nonpermeabilized mERhigh cells, whereas the majority of mERlow cells exhibited little or no staining. Western analysis demonstrated that mERhigh cells expressed caveolin-1 and caveolin-2, and that ER-alpha was contained in the same gradient-separated membrane fractions. The quantitative immunoassay for ER-alpha detected a significant difference in mER-alpha levels between mERhigh and mERlow cells when cells were grown at a sufficiently low cell density, but equivalent levels of total ER-alpha (membrane plus intracellular receptors). These two separated cell subpopulations also exhibited different kinetics of ERK1/2 activation with 1 pmol/l 17beta-estradiol (E2), as well as different patterns of E2 dose-dependent responsiveness. The maximal kinase activation was achieved after 10 min versus 6 min in mERhigh versus mERlow cells, respectively. After a decline in the level of phosphorylated ERKs, a reactivation was seen at 60 min in mERhigh cells but not in mERlow cells. Both 1A and 2B protein phosphatases participated in dephosphorylation of ERKs, as demonstrated by efficient reversal of ERK1/2 inactivation with okadaic acid and cyclosporin A.

CONCLUSION

Our results suggest that the levels of mER-alpha play a role in the temporal coordination of phosphorylation/dephosphorylation events for the ERKs in breast cancer cells, and that these signaling differences can be correlated to previously demonstrated differences in E2-induced cell proliferation outcomes in these cell types.

Membrane estrogen receptor-alpha levels in MCF-7 breast cancer cells predict cAMP and proliferation responses

Introduction

17β-estradiol (E₂) can rapidly induce cAMP production, but the conditions under which these cAMP levels are best measured and the signaling pathways responsible for the consequent proliferative effects on breast cancer cells are not fully understood. To help resolve these issues, we compared cAMP mechanistic responses in MCF-7 cell lines selected for low (mER^low) and high (mER^high) expression of the membrane form of estrogen receptor (mER)-α, and thus addressed the receptor subform involved in cAMP signaling.

Methods

MCF-7 cells were immunopanned and subsequently separated by fluorescence activated cell sorting into mER^high (mER-α-enriched) and mER^low (mER-α-depleted) populations. Unique (compared with previously reported) incubation conditions at 4°C were found to be optimal for demonstrating E₂-induced cAMP production. Time-dependent and dose-dependent effects of E₂ on cAMP production were determined for both cell subpopulations. The effects of forskolin, 8-CPT cAMP, protein kinase A inhibitor (H-89), and adenylyl cyclase inhibitor (SQ 22,536) on E₂-induced cell proliferation were assessed using the crystal violet assay.

Results

We demonstrated a rapid and transient cAMP increase after 1 pmol/l E₂ stimulation in mER^high cells; at 4°C these responses were much more reliable and robust than at 37°C (the condition most often used). The loss of cAMP at 37°C was not due to export. 3-Isobutyl-1-methylxanthine (IBMX; 1 mmol/l) only partially preserved cAMP, suggesting that multiple phosphodiesterases modulate its level. The accumulated cAMP was consistently much higher in mER^high cells than in mER^low cells, implicating mER-α levels in the process. ICI172,780 blocked the E₂-induced response and 17α-estradiol did not elicit the response, also suggesting activity through an estrogen receptor. E₂ dose-dependent cAMP production, although biphasic in both cell types, was responsive to 50-fold higher E₂ concentrations in mER^high cells. Proliferation of mER^low cells was stimulated over the whole range of E₂concentrations, whereas the number of mER^high cells was greatly decreased at concentrations above 1 nmol/l, suggesting that estrogen over-stimulation can lead to cell death, as has previously been reported, and that mER-α participates. E₂-mediated activation of adenylyl cyclase and downstream participation of protein kinase A were shown to be involved in these responses.

Conclusion

Rapid mER-α-mediated nongenomic signaling cascades generate cAMP and downstream signaling events, which contribute to the regulation of breast cancer cell number.

The interplay between the glucocorticoid receptor and nuclear factor-kappaB or activator protein-1: molecular mechanisms for gene repression

The inflammatory response is a highly regulated physiological process that is critically important for homeostasis. A precise physiological control of inflammation allows a timely reaction to invading pathogens or to other insults without causing overreaction liable to damage the host. The cellular signaling pathways identified as important regulators of inflammation are the signal transduction cascades mediated by the nuclear factor-kappaB and the activator protein-1, which can both be modulated by glucocorticoids. Their use in the clinic includes treatment of rheumatoid arthritis, asthma, allograft rejection, and allergic skin diseases. Although glucocorticoids have been widely used since the late 1940s, the molecular mechanisms responsible for their antiinflammatory activity are still under investigation. The various molecular pathways proposed so far are discussed in more detail.

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.

Correlation of membrane glucocorticoid receptor levels with glucocorticoid-induced apoptotic competence using mutant leukemic and lymphoma cells lines

We have studied the presence and functional implications of membrane glucocorticoid receptor (mGR) in several wild type (WT) and mutant mouse lymphoid cell lines (nuclear transfer decrease, NT(-); nuclear transfer increase, NT(i); and receptorless, R(-)). Direct fluorescent antibody staining revealed large aggregates of mGR-specific fluorescing antigens in the plasma membrane of the WT and mGR-enriched (mGR(++)) S-49 cells. While R(-) cells totally lacked mGR, this receptor level was low in NT(-) and NT(i) groups. FACS analysis corroborated these results, showing a approximately 4-10-fold difference between the highest mGR levels (mGR(++)) and the R(-) and NT(i) cells. Membrane extracts were analyzed for mGR by immunoblotting. Multiple receptor forms, ranging in M(r) from 94,000 to > 200,000, were observed in the WT cells, while only smaller peptides (85,000-94,000) were found in NT(-) cells. No detectable immunoreactive bands were identified in either membrane or cytosol immunoprecipitates of NT(i) and R(-) cell groups. Within 48 h post dexamethasone exposure > 98% of WT and mGR(++) S-49 cells underwent apoptosis, compared to 0-30% in the mutant cells, albeit the total receptor number is two to three times higher in NT(i) compared to WT. These results suggest a better correlation between the quantity and quality of mGRs (rather than total cellular GRs) and the ability of glucocorticoids (GCs) to lyse lymphoid cells.

Rapid actions of steroid receptors in cellular signaling pathways

Steroid hormones regulate cellular processes by binding to intracellular receptors that, in turn, interact with discrete nucleotide sequences to alter gene expression. Because most steroid receptors in target cells are located in the cytoplasm, they need to get into the nucleus to alter gene expression. This process typically takes at least 30 to 60 minutes. In contrast, other regulatory actions of steroid hormones are manifested within seconds to a few minutes. These time periods are far too rapid to be due to changes at the genomic level and are therefore termed nongenomic or rapid actions, to distinguish them from the classical steroid hormone action of regulation of gene expression. The rapid effects of steroid hormones are manifold, ranging from activation of mitogen-activated protein kinases (MAPKs), adenylyl cyclase (AC), protein kinase C (PKC), and heterotrimeric guanosine triphosphate-binding proteins (G proteins). In some cases, these rapid actions of steroids are mediated through the classical steroid receptor that can also function as a ligand-activated transcription factor, whereas in other instances the evidence suggests that these rapid actions do not involve the classical steroid receptors. One candidate target for the nonclassical receptor-mediated effects are G protein-coupled receptors (GPCRs), which activate several signal transduction pathways. One characteristic of responses that are not mediated by the classical steroid receptors is insensitivity to steroid antagonists, which has contributed to the notion that a new class of steroid receptors may be responsible for part of the rapid action of steroids. Evidence suggests that the classical steroid receptors can be localized at the plasma membrane, where they may trigger a chain of reactions previously attributed only to growth factors. Identification of interaction domains on the classical steroid receptors involved in the rapid effects, and separation of this function from the genomic action of these receptors, should pave the way to a better understanding of the rapid action of steroid hormones.

Membrane estrogen and glucocorticoid receptors--implications for hormonal control of immune function and autoimmunity

Membrane steroid receptors (mSRs) have recently re-emerged as candidates for mediating steroid effects which do not fit the paradigm of nuclear transcription factor mechanisms. We have studied two steroid-binding classes of mSRs, and have noted striking similarities in their characteristics (immunocytochemical appearance, biochemical properties, proteolytic sensitivity, signaling pathways, regulation, and molecular origins). These observations strengthen the conclusion that mSRs can be modified versions of intracellular steroid receptors. The membrane estrogen receptors (mERs) we studied are involved in estrogen-induced release of prolactin. Membrane glucocorticoid receptors (mGRs) in both mouse and human lymphoma cells are necessary for the initiation of glucocorticoid-induced therapeutic apoptosis which is related to the developmental phenomenon of thymic involution. Diseases of autoimmunity such as systemic lupus erythematosus and arthritis are related to estrogen status. Since both of these mSRs have recently been found in both normal and cancerous lymphoid cells, actions of these mSRs may have important consequences for functions and diseases of the immune system. Therefore, the study of these forms of steroid receptors may present novel therapeutic opportunities for the use of steroids and steroid analogs.

Identification and characterization of membrane estrogen receptor from MCF7 estrogen-target cells

Estrogens control the proliferation of estrogen-target cells through a receptor mediated pathway. We have recently presented evidence that estradiol cancels the proliferative inhibition exerted by albumin on estrogen-target cells (indirect-negative hypothesis). We postulate that this mechanism requires the presence of a membrane estrogen receptor (mER)-membrane albumin receptor complex. Confirmation for mERalpha in MCF7 cells is now made using both the C542 monoclonal and ER-21 polyclonal antibodies (Ab)s specific for ERalpha. Western blot analysis of purified membrane proteins with ERalpha Abs revealed multiple high M(r) mERs (92 k, 110 k, and 130 k M(r)), as well as a 67 k M(r) mER; immunoreactive proteins were competed by inclusion of 500-fold molar excess C542 peptide. Ligand blot analysis of similar extracts with estradiol-peroxidase identified several potential mERs as well; two of these proteins were also recognized by C542 and ER-21 Abs (110 and 67 k M(r)). Fluorescence, confocal and electron microscopy of MCF7 cells fixed in 2.0% paraformaldehyde/0.1% glutaraldehyde identified specific mERalpha sites by immunocytochemistry. Specific binding of 3H-17beta-estradiol was reduced by a 200-fold molar excess of unlabeled 17beta-estradiol, but not by testosterone and progesterone. These results suggest that the ER on the plasma membrane of MCF7 cells is similar, but not identical to its intracellular counterpart. We propose that the observed mER actively participates in the estrogen-mediated proliferation of MCF7 cells.

Rapid androgen actions on calcium signaling in rat sertoli cells and two human prostatic cell lines: similar biphasic responses between 1 picomolar and 100 nanomolar concentrations

Androgen-induced calcium fluxes and gap junctional intercellular communication (GJIC) were studied in three different cell types. A transient (2-3 min duration) increase in intracellular calcium levels was observed within 20-30 sec of androgen addition, which was followed by a plateau phase with steroid concentrations higher than 1 nM. The kinetics of the calcium responses were similar in immature rat Sertoli cells, which contain normal nuclear receptors; the human prostatic tumor cell line, LNCaP, which contains a mutated nuclear receptor; and the human prostatic cell line, PC3, which does not contain a nuclear receptor. The human A431 tumor cell line did not respond to androgens. Concentrations of testosterone and the synthetic androgen, R1881, between 1-1000 pM induced transient calcium increases with ED(50) values near 1 pM and 1 nM, whereas dihydrotestosterone (DHT) was not active at these concentrations. At concentrations higher than 1 nM, testosterone, R1881, and DHT were equipotent in stimulating an increase in calcium that lasted for more than 10 min, with ED(50) values between 5 and 20 nM. Testosterone covalently bound to albumin was also active, whereas 11 related androstane compounds as well as progesterone and estradiol-17beta were inactive at 1000 nM. The calcium response induced by the three androgens (10 nM) was abolished in all cell types by hydroxyflutamide (1000 nM) and finasteride (1000 nM), but not by cyproterone acetate (1000 nM). The calcium response was also abolished in the absence of extracellular calcium and strongly inhibited by the presence of verapamil. Exposure of the responsive cells to brief (150-sec) pulses of androgens generated calcium responses that were similar to those after continuous exposure. After exposure of Sertoli cells for only 30 sec to 100 nM testosterone, the calcium response lasted for at least 50 min. Although nuclear binding of androgens could be demonstrated, there was no evidence for tight binding to the plasma membrane under similar conditions. When protein synthesis was inhibited, an enhancement of GJIC between rat Sertoli cells, but not between LNCaP cells or PC3 cells, was observed within 15 min of the addition of 10 nM testosterone. Because nuclear androgens are not present in PC3 cells and many functional properties of the responsive system are different from the nuclear receptor in all three cell types, we postulate the existence of an alternative cell surface receptor system with biphasic response characteristics (high and low affinity). The calcium signals are probably coupled to the regulation of gap junctional efficiency between Sertoli cells. The low-affinity receptors may convey complementary androgen signals at elevated local levels such as in the testis, when nuclear receptors are (over)saturated.

Partial purification and biochemical characterization of a membrane glucocorticoid receptor from an amphibian brain

A membrane receptor for corticosterone (mGR) in the brain of the roughskin newt (Taricha granulosa) has been previously identified. This manuscript reports the evaluation of several chromatographic resins for enrichment of the newt mGR solubilized from neuronal membranes. A protein with an apparent molecular weight of 63 kDa was purified to near homogeneity following sequential purification using ammonium sulfate fractionation, wheat germ agglutinin (WGA)-agarose chromatography, hydroxylapatite chromatography, and an immobilized ligand affinity resin (Corticosterone-Sepharose). Other studies employed a novel protein differential display strategy and a photoaffinity labeling strategy to visualize candidate receptor proteins following SDS-PAGE. Both of these techniques also identified a 63 kDa protein, agreeing with the estimation of molecular weight from the purification data. Furthermore, the use of 2D SDS-PAGE following the photolabeling procedure showed the candidate 63 kDa protein to have a pI of approximately 5.0. Taken together these data suggest that the newt mGR is an acidic glycoprotein with an apparent molecular weight of 63 kDa. Because these characteristics of newt mGR are inconsistent with the characteristics of intracellular glucocorticoid receptors, these two receptor proteins are apparently distinct.

Glucocorticoids-potent modulators of astrocytic calcium signaling

Glucocorticoids are the first line of choice in the treatment of cerebral edema associated with brain tumors. High-dose glucocorticoids reduce the extent of edema within hours, often relieving critical increases in intracranial pressure, but the mechanisms by which glucocorticoids modulate brain water content are not well-understood. A possible target of action may be glucocorticoid receptor-expressing astrocytes, which are the primary regulators of interstitial ion homeostasis in brain. In this study, we demonstrate that two glucocorticoids, methylprednisolone and dexamethasone, potentiate astrocytic signaling, via long-range calcium waves. Glucocorticoid treatment increased both resting cytosolic calcium (Ca2+i) level and the extent and amplitude of Ca2+ wave propagation two-fold, compared to matched controls. RU-486, a potent steroid receptor antagonist, inhibited the effects of methylprednisolone. The glucocorticoid-associated potentiation of Ca2+ signaling may result from upregulation of the cellular ability to mobilize Ca2+ and release ATP, because both agonist-induced Ca2+i increments (via ATP and bradykinin) and ATP release were proportionally enhanced by glucocorticoids. In contrast, neither gap junction expression (as manifested connexin 43 immunoreactivity) nor functional coupling was significantly affected by methylprednisolone. Confocal microscopy revealed both the expression of glucocorticoid receptors and nuclear translocation of these receptors when exposed to methylprednisolone. We postulate that the edemolytic effects of glucocorticoids may result from enhanced astrocytic calcium signaling.

Immunoaffinity isolation of native membrane glucocorticoid receptor from S-49++ lymphoma cells: biochemical characterization and interaction with Hsp 70 and Hsp 90

The membrane glucocorticoid receptor (mGR), previously correlated with glucocorticoid-induced lymphocytolytic competency, was purified under nondenaturing conditions from mGR-enriched mouse S-49 T lymphoma cells. Proteins were immunoaffinity batch adsorbed to BUGR-2 monoclonal antibody-coupled protein A Sepharose 4B beads, and elution by epitope competition was compared with standard denaturation procedures. Elution with BUGR-2 epitope peptides released multiple mGRs (42-150 kDa) and heat shock proteins 70 and 90, suggesting that mGR interacts with these protein chaperones under physiological conditions. The mGR-heat shock protein 90 interaction was inhibited by 1 microM geldanamycin. Several other mGR binding partners were captured and most were dissociated from mGR by 0.6 M salt. Peptide maps of purified mGR displayed immunoreactive bands unique to mGR. Scatchard analysis estimated a k(d) value of 239 nM and a Bmax of 384 fmol/mg protein for mGR, compared to a k(d) of 19.5 nM and a Bmax of 90.3 fmol/mg protein for the intracellular GR (iGR). The rank order of affinities for mGR were RU-486 > dexamethasone > triamcinolone acetonide = aldosterone. Other steroids had no significant binding affinity. These results show that epitope-purified mGR on the plasma membrane of mouse lymphoma cells is similar but not identical to iGR.

Functional testosterone receptors in plasma membranes of T cells

T cells are considered to be unresponsive to testosterone due to the absence of androgen receptors (AR). Here, we demonstrate the testosterone responsiveness of murine splenic T cells in vitro as well as the presence of unconventional cell surface receptors for testosterone and classical intracellular AR. Binding sites for testosterone on the surface of both CD4(+) and CD8(+) subsets of T cells are directly revealed with the impeded ligand testosterone-BSA-FITC by confocal laser scanning microscopy (CLSM) and flow cytometry, respectively. Binding of the plasma membrane impermeable testosterone-BSA conjugate induces a rapid rise (<5 s) in [Ca2+]i of Fura-2-loaded T cells. This rise reflects influx of extracellular Ca2+ through non-voltage-gated and Ni2+-blockable Ca2+ channels of the plasma membrane. The testosterone-BSA-induced Ca2+ import is not affected by cyproterone, a blocker of the AR. In addition, AR are not detectable on the surface of intact T cells when using anti-AR antibodies directed against the amino and carboxy terminus of the AR, although T cells contain AR, as revealed by reverse transcription-polymerase chain reactions and Western blotting. AR can be visualized with the anti-AR antibodies in the cytoplasm of permeabilized T cells by using CLSM, though AR are not detectable in cytosol fractions when using the charcoal binding assay with 3H-R1881 as ligand. Cytoplasmic AR do not translocate to the nucleus of T cells in the presence of testosterone, in contrast to cytoplasmic AR in human cancer LNCaP cells. These findings suggest that the classical AR present in splenic T cells are not active in the genomic pathway. By contrast, the cell surface receptors for testosterone are in a functionally active state, enabling T cells a nongenomic response to testosterone.

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.

Cell cycle regulation of membrane glucocorticoid receptor in CCRF-CEM human ALL cells: correlation to apoptosis

The human leukemic cell line (CCRF-CEM) and a subline enriched for the plasma membrane-resident glucocorticoid receptor (mGR) were studied for the influence of the cell cycle on the expression and function of mGR. Three synchronization procedures (double thymidine, colcemid, and combined thymidine-colcemid blocks) were used. Fluorescent microscopy and flow cytometry simultaneously assessed antibody-tagged mGR and DNA. In addition, mGR was quantitated and characterized by immunoprecipitation and immunoblotting. Apoptosis was assayed by DNA fragmentation (TUNEL assay) and by cell survival (trypan blue exclusion). All synchronization procedures demonstrated that progression from DNA replication (S) to the second growth phase and mitosis (G2/M) leads to cells having the highest levels of mGR expression and being highly glucocorticoid sensitive in the apoptosis assays: 32 and 80% sensitivity of wild type and mGR-enriched cells, respectively, compared with 12 and 30% sensitivity in asynchronous cells. Therefore, mGR expression appears to be cell cycle regulated, with its highest expression at late S-G2/M, when the cells are most sensitive to the lymphocytolytic effects of glucocorticoids.

Nongenomic effects of an anti-idiotypic antibody as an estrogen mimetic in female human and rat osteoblasts

We investigated the early effects of the anti-idiotypic antibody (clone 1D5), which recognized the estrogen receptor (ER), on cytosolic free calcium concentration ([Ca2+]i) and its long term effects on creatine kinase (CK) specific activity in female human and rat osteoblasts. These actions were compared to the known membrane and genomic effects of 17 beta estradiol (E2). Like E2, clone 1D5 increased within 5 s [Ca2+]i in both cell types by two mechanisms: 1) Ca2+ influx through voltage-gated Ca2+ channels as shown by using EGTA a chelator of extracellular Ca2+, and nifedipine, a Ca2+ channel blocker; 2) Ca2+ mobilization from the endoplasmic reticulum as shown by using phospholipase C inhibitors, such as neomycin and U-73122, which involved a Pertussis toxin-sensitive G-protein. Clone 1D5 and E2 stimulated CK specific activity in human and rat osteoblasts with ten fold higher concentrations than those needed for the membrane effects (0.1 microgram/ml and 10 pM, respectively). Both effects were gender-specific since testosterone and 5 alpha-dihydotesterone were uneffective. Tamoxifen and Raloxifene, two estrogen nuclear antagonists, inhibited CK response to 1D5 and E2 and Ca2+ response to 1D5, but not Ca2+ response to E2. By contrast, (Fab')2 dimer, a proteolytic fragment of 1D5 with antagonist properties, inhibited both membrane and genomic effects of 1D5 and E2. In conclusion, these results imply that clone 1D5 has an estrogen like activity both at the membrane and nuclear levels in female human and rat osteoblasts. 1D5 must therefore interact with membrane binding sites, penetrate the cells, and reach the nuclear receptors by an as yet uncharacterized mechanism.

Glucocorticoid-induced long-term remission in primary cerebral lymphoma: case report and review of the literature

We report a 25-year old immunocompetent woman with a high grade primary non-Hodgkin's lymphoma of the central nervous system (PNHL-CNS) in whom the administration of dexamethasone alone during three months produced a complete clinical and radiological response lasting over four years. If complete remission of PNHL-CNS induced by glucocorticoids are well known, the opportunity to observe glucocorticoid-induced remission for a long period of time without radio- and chemotherapy is rare. Only nine other cases of PNHL-CNS with complete remission induced by glucocorticoids lasting from 6 to 60 months, were found in the literature and are summarized here. Duration of glucocorticoids therapeutic effect in PNHL-CNS is probably underestimated. Glucocorticoids cannot be recommended as sole initial treatment for PNHL-CNS. However, we suggest standard therapies to be delayed in those patients responding completely to glucocorticoids where radio- and chemotherapy should be contraindicated (kidney, liver, bone marrow failure, pregnancy).

Membrane estrogen receptor-enriched GH(3)/B6 cells have an enhanced non-genomic response to estrogen

We immunoselected GH(3)/B6 cells for a membrane estrogen receptor (mER) using antibodies generated against the rat intracellular ER (iER). Immunocytochemistry with anti-ER antibodies revealed bright fluorescence distributed in patches over the surface of mER-enriched cells, while cells immuno-depleted for mER showed only low-level membrane immunofluorescence. Quantitation via digital image analysis confirmed that immunoenriched populations show increases in both stained cell numbers and intensity of staining. Short-term culturing with serum reversibly decreased the intensity of immunostaining in mER-enriched cells to immuno-depleted cell levels. The mER-enriched populations initially contained ∼85% immunopositive cells in defined medium, but when cultured continuously with serum gradually decline to ∼22% immunopositive cells by 10 weeks. Cells enriched for mER showed a significant increase in rapid (after 2 or 5 min) prolactin release when treated with 17β-estradiol, while mER-depleted cells lacked this response. Immunoprecipitabie membrane proteins isolated from mER-enriched cells were 60,000, 74,000 and ∼ 200,000 MW, compared to an iER size of 67,000. Therefore, the presence and level of an mER that is antigenically related to iER is correlated with the ability of GH(3)/B6 cells to mediate a rapid action of estrogen.

Identification and localization of steroid-binding and nonsteroid-binding forms of the glucocorticoid receptor in the mouse P1798 lymphosarcoma

Glucocorticoid receptors (GCRs) were characterized in sublines of the mouse P1798 lymphosarcoma that are sensitive (S) or resistant (R) to glucocorticoid-mediated apoptosis. Previous work had identified two steroid-binding GCRs in S and R cells: a 97 kDa wild-type GCR in S cells (WT-GCR), and a 45 kDa truncated GCR in R cells (TR-GCR). A third GCR, a 97 kDa nonsteroid-binding GCR (NSB-GCR), was also identified in R cells. Using subcellular fractionation and Western blotting, we now show that in contrast to the WT-GCR which is localized in both the cytoplasm and nucleus of S cells, the NSB-GCR is localized predominantly in R cell nuclei. Moreover, gel filtration chromatography revealed that treatment with 400 mM NaCl and heat did not significantly alter the Stokes radius of the NSB-GCR suggesting that this receptor is not present in a heterooligomeric complex with other proteins. The TR-GCR was localized predominantly in the soluble cytoplasmic fraction but also in the crude membrane fractions of R cell nuclei, suggesting that this receptor is tightly associated with nuclear structures. It was not detected in the soluble nuclear fraction. Unexpectedly, a 45 kDa nonsteroid-binding immunoreactive protein was detected in crude membrane fractions of S cells. These studies describe a complex GCR system in the P1798 lymphosarcoma that necessitates a further consideration of glucocorticoid signaling in S and R cells.

Identification of progesterone binding sites in the plasma membrane of the filamentous fungus Cochliobolus lunatus

Plasma membrane associated binding sites for progesterone have been identified in the filamentous fungus Cochliobolus lunatus (C. lunatus). The Kd for progesterone determined by Scatchard analysis was 13.9 +/- 5.7 nM and the Bmax was 250-360 fmol/mg protein. A broad ligand specificity of these binding sites is suggested by the observation that all tested steroids, regardless of their capability to act as inducers of the 11 beta-steroid hydroxylase, competed at 250-fold excess with [3H]progesterone binding. A biological role of these plasma membrane associated steroid binding sites is nevertheless suggested since in protoplasts which were devoid of them, 11 beta-steroid hydroxylase could not be induced. Progesterone binding sites were present in the plasma membrane as well as in the cytosol and were detected in this fraction, in contrast to the plasma membrane fraction, only under special experimental conditions in respect to redox state. Kd and Bmax of cytosol binding sites were of the same order of magnitude compared to the plasma membrane progesterone binding sites. Ethisterone and 4-cholesten-3-one which cannot induce 11 beta-hydroxylase competed efficiently for plasma membrane binding sites; ethisterone, however also competed for cytosol binding sites and acted, in contrast with 4-cholesten-3-one, as antagonist in the induction of 11 beta-steroid hydroxylase in C. lunatus. On the basis of presented evidence we concluded that C. lunatus contains binding sites for steroids in the plasma membrane and in the cytosol and that both types of binding site are involved in the process of induction of enzymes which transform steroids in this fungus.