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

The Potential of Pharmaceutical Hydrogels in the Formulation of Topical Administration Hormone Drugs

Hormones have attracted considerable interest in recent years due to their potential use in treatment of many diseases. Their ability to have a multidirectional effect leads to searching for new and increasingly effective drugs and therapies. Limitations in formulating drug forms containing hormones are mainly due to their low enzymatic stability, short half-life and limited bioavailability. One of the solutions may be to develop a hydrogel as a potential hormone carrier, for epidermal and transdermal application. This review discusses the main research directions in developing this drug formulation. The factors determining the action of hormones as drugs are presented. An analysis of hydrogel substrates and permeation enhancers that have the potential to enhance the efficacy of hormones applied to the skin is reviewed.

Genomic variants reducing expression of two endocytic receptors in 46,XY differences of sex development

Transporter-dependent steroid hormone uptake into target cells was demonstrated in genetically engineered mice and fruit flies. We hypothesized that mutations in such transporters may cause differences in sex development (DSD) in humans. Exome sequencing was performed in 16 genetically unsolved cases of 46,XY DSD selected from an anonymized collection of 708 lines of genital fibroblasts (GF) that were taken from individuals with incomplete virilization. Selection criteria were based on available biochemical characterization of GF compatible with reduced androgen uptake. Two unrelated individuals were identified with mutations in LDL receptor-related protein 2 (LRP2), a gene previously associated with partial sex steroid insensitivity in mice. Like Lrp2^-/- mice, affected individuals had non-descended testes. Western blots on GF confirmed reduced LRP2 expression, and endocytosis of sex hormone-binding globulin was reduced. In three unrelated individuals, two with undescended testes, mutations in another endocytic receptor gene, limb development membrane protein 1 like (LMBR1L), were detected. Two of these individuals had mutations affecting the same codon. In a transfected cell model, mutated LMBR1L showed reduced cell surface expression. Our findings suggest that endocytic androgen uptake in complex with sex hormone-binding globulin is relevant in human. LMBR1L may play a similar role in androgen uptake.

Physiologically Based Pharmacokinetic Modeling Involving Nonlinear Plasma and Tissue Binding: Application to Prednisolone and Prednisone in Rats

The pharmacokinetics (PK) of prednisolone (PNL) exhibit nonlinearity related to plasma protein binding, tissue binding, metabolic interconversion with prednisone (PN), and renal elimination. Blood and 11 tissues were collected from male Wistar rats after steady-state (SS) infusion and after subcutaneous boluses of 50 mg/kg of PNL. Concentrations of PNL and PN were measured by liquid chromatography-tandem mass spectrometry. Plasma and tissue profiles were described using a complex physiologically based pharmacokinetics (PBPK) model. Concentrations of PN and PNL were in rapid equilibrium in plasma and tissues. The tissue partition coefficients (K _p ) of PNL calculated from most subcutaneously dosed tissue and plasma areas were similar to SS infusion and in silico values. The blood-to-plasma ratio of PNL was 0.71 with similar red blood cell and unbound-plasma concentrations. Plasma protein binding (60%-90%) was related to corticosteroid-binding globulin (CBG) saturation. Tissue distribution was nonlinear. The equilibrium dissociation constant (K _d ) of PNL shared by all tissues was 3.01 ng/ml, with the highest binding in muscle, followed by liver, heart, intestine, and bone and the lowest binding in skin, spleen, fat, kidney, lung, and brain. Fat and bone distribution assumed access only to interstitial space. Brain PNL concentrations (K _p = 0.05) were low owing to presumed P-glycoprotein-mediated efflux. Clearances of CBG-free PNL were 1789 from liver and 191.2 ml/h from kidney. The PN/PNL ratio was nonlinear for plasma, spleen, heart, intestine, bone, fat, and linear for the remaining tissues. Our PBPK model with multiple complexities well described the PK profiles of PNL and PN in blood, plasma, and diverse tissues. SIGNIFICANCE STATEMENT: Because steroids, such as prednisolone and prednisone, have similar and complex pharmacokinetics properties in various species, receptors in most tissues, and multiple therapeutic and adverse actions, this physiologically based pharmacokinetics (PBPK) model may provide greater insights into the pharmacodynamic complexities of corticosteroids. The complex properties of these compounds require innovative PBPK modeling approaches that may be instructive for other therapeutic agents.

Transitioning from Basic toward Systems Pharmacodynamic Models: Lessons from Corticosteroids

Technology in bioanalysis, -omics, and computation have evolved over the past half century to allow for comprehensive assessments of the molecular to whole body pharmacology of diverse corticosteroids. Such studies have advanced pharmacokinetic and pharmacodynamic (PK/PD) concepts and models that often generalize across various classes of drugs. These models encompass the "pillars" of pharmacology, namely PK and target drug exposure, the mass-law interactions of drugs with receptors/targets, and the consequent turnover and homeostatic control of genes, biomarkers, physiologic responses, and disease symptoms. Pharmacokinetic methodology utilizes noncompartmental, compartmental, reversible, physiologic [full physiologically based pharmacokinetic (PBPK) and minimal PBPK], and target-mediated drug disposition models using a growing array of pharmacometric considerations and software. Basic PK/PD models have emerged (simple direct, biophase, slow receptor binding, indirect response, irreversible, turnover with inactivation, and transduction models) that place emphasis on parsimony, are mechanistic in nature, and serve as highly useful "top-down" methods of quantitating the actions of diverse drugs. These are often components of more complex quantitative systems pharmacology (QSP) models that explain the array of responses to various drugs, including corticosteroids. Progressively deeper mechanistic appreciation of PBPK, drug-target interactions, and systems physiology from the molecular (genomic, proteomic, metabolomic) to cellular to whole body levels provides the foundation for enhanced PK/PD to comprehensive QSP models. Our research based on cell, animal, clinical, and theoretical studies with corticosteroids have provided ideas and quantitative methods that have broadly advanced the fields of PK/PD and QSP modeling and illustrates the transition toward a global, systems understanding of actions of diverse drugs. SIGNIFICANCE STATEMENT: Over the past half century, pharmacokinetics (PK) and pharmacokinetics/pharmacodynamics (PK/PD) have evolved to provide an array of mechanism-based models that help quantitate the disposition and actions of most drugs. We describe how many basic PK and PK/PD model components were identified and often applied to the diverse properties of corticosteroids (CS). The CS have complications in disposition and a wide array of simple receptor-to complex gene-mediated actions in multiple organs. Continued assessments of such complexities have offered opportunities to develop models ranging from simple PK to enhanced PK/PD to quantitative systems pharmacology (QSP) that help explain therapeutic and adverse CS effects. Concurrent development of state-of-the-art PK, PK/PD, and QSP models are described alongside experimental studies that revealed diverse CS actions.

Physiologically Based Pharmacokinetics of Dexamethasone in Rats

Blood and multitissue concentration-time profiles for dexamethasone (DEX), a synthetic corticosteroid, were measured in male rats after subcutaneous bolus and infusion dosing. A physiologically based pharmacokinetics (PBPK) model was applied for 12 measured tissues. Tissue partition coefficients (K _p ) and metabolic clearance were assessed from infusion studies. Blood cell to plasma partitioning (0.664) and plasma free fraction (0.175) for DEX were found to be moderate. DEX was extensively partitioned into liver (K _p = 6.76), whereas the calculated K _p values of most tissues ranged between 0.1 and 1.5. Despite the moderate lipophilicity of DEX (log P = 1.8), adipose exhibited very limited distribution (K _p = 0.17). Presumably due to P-glycoprotein-mediated efflux, DEX concentrations were very low in brain compared with its expected high permeability. Infusion studies yielded K _p values from male and female rats at steady state that were similar. In silico K _p values calculated for different tissues by using GastroPlus software were similar to in vivo values except for adipose and liver. Glucocorticoid receptors are found in diverse tissues, and these PBPK modeling results may help provide exposure profiles driving pharmacodynamic effects of DEX. SIGNIFICANCE STATEMENT: Our physiologically based pharmacokinetics model describes the experimentally determined tissue and plasma dexamethasone (DEX) pharmacokinetics (PK) profiles in rats reasonably well. This model can serve for further investigation of DEX tissue distribution in rats as the PK driving force for PD effects in different tissues. No major sex differences were found for DEX tissue distribution. Knowledge gained in this study may be translatable to higher-order species including humans.

Rapid and structure-specific cellular uptake of selected steroids

Steroid hormones and their respective nuclear receptors are essential mediators in numerous physiologic and pathophysiologic processes, ranging from regulation of metabolism, immune function, and reproductive processes to the development of hormone-dependent cancers such as those of the breast and prostate. Because steroids must enter cells before activating nuclear receptors, understanding the mechanisms by which cellular uptake occurs is critical, yet a clear understanding of these mechanisms has been elusive. It is generally assumed that diffusion-driven uptake is similar across various steroids whereas an elevated cellular concentration is thought to reflect active uptake, but these assumptions have not been directly tested. Here we show that intact cells rapidly accumulate free steroids to markedly elevated concentrations. This effect varies widely depending on steroid structure; more lipophilic steroids reach more elevated concentrations. Strong preferences exist for 3β-OH, Δ5-steroids vs. 3-keto, Δ4-structural features and for progestogens vs. androgens. Surprisingly, steroid-structure-specific preferences do not require cell viability, implying a passive mechanism, and occur across cells derived from multiple tissue types. Physiologic relevance is suggested by structure-specific preferences in human prostate tissue compared with serum. On the other hand, the presence of serum proteins in vitro blocks much, but not all, of the passive accumulation, while still permitting a substantial amount of active accumulation for certain steroids. Our findings suggest that both passive and active uptake mechanisms make important contributions to the cellular steroid uptake process. The role of passive, lipophilicity-driven accumulation has previously been largely unappreciated, and its existence provides important context to studies on steroid transport and action both in vitro and in vivo.

Modeling Corticosteroid Pharmacokinetics and Pharmacodynamics, Part III: Estrous Cycle and Estrogen Receptor-Dependent Antagonism of Glucocorticoid-Induced Leucine Zipper (GILZ) Enhancement by Corticosteroids

Our previous report examined the pharmacokinetics (PK) of methylprednisolone (MPL) and adrenal suppression after a 50 mg/kg IM bolus in male and female rats, and we described in detail the development of a minimal physiologically based pharmacokinetic/pharmacodynamic (mPBPK/PD) model. In continuation of such assessments, we investigated sex differences in genomic MPL responses (PD). Message expression of the glucocorticoid-induced leucine zipper (GILZ) was chosen as a multitissue biomarker of glucocorticoid receptor (GR)-mediated drug response. Potential time-dependent interplay between sex hormone and glucocorticoid signaling in vivo was assessed by comparing the enhancement of GILZ by MPL in the uterus [high estrogen receptor (ER) density] and in liver (lower ER density) from male and female rats dosed within the proestrus (high estradiol/progesterone) and estrus (low estradiol/progesterone) phases of the rodent estrous cycle. An expanded-systems PD model of MPL considering circadian rhythms, multireceptor (ER and GR) control, and estrous variations delineated the determinants controlling receptor/gene-mediated steroid responses. Hepatic GILZ response was ∼3-fold greater in females, regardless of estrous stage, compared with males, driven predominantly by increased MPL exposure in females and a negligible influence of estrogen interaction. In contrast, GILZ response in the uterus during proestrus in females was 60% of that observed in estrus-phased females, despite no PK or receptor differences, providing in vivo support to the hypothesis of estrogen-mediated antagonism of glucocorticoid signaling. The developed model offers a mechanistic platform to assess the determinants of sex and tissue specificity in corticosteroid actions and, in turn, reveals a unique PD drug-hormone interaction occurring in vivo. SIGNIFICANCE STATEMENT: Mechanisms relating to sex-based pharmacodynamic variability in genomic responses to corticosteroids have been unclear. Using combined experimental and systems pharmacology modeling approaches, sex differences in both pharmacokinetic and pharmacodynamic mechanisms controlling the enhancement of a sensitive corticosteroid-regulated biomarker, the glucocorticoid-induced leucine zipper (GILZ), were clarified in vivo. The multiscale minimal physiologically based pharmacokinetics/pharmacodynamic model successfully captured the experimental observations and quantitatively discerned the roles of the rodent estrous cycle (hormonal variation) and tissue specificity in mediating the antagonistic coregulation of GILZ gene synthesis. These findings collectively support the hypothesis that estrogens antagonize pharmacodynamic signaling of genomic corticosteroid actions in vivo in a time- and estrogen receptor-dependent manner.

Modeling Corticosteroid Pharmacokinetics and Pharmacodynamics, Part I: Determination and Prediction of Dexamethasone and Methylprednisolone Tissue Binding in the Rat

The plasma and tissue binding properties of two corticosteroids, dexamethasone (DEX) and methylprednisolone (MPL), were assessed in the rat in anticipation of developing physiologically based pharmacokinetic and pharmacokinetic/pharmacodynamic models. The tissue-to-plasma partition coefficients (K _P) of DEX and MPL were measured in liver, muscle, and lung in vivo after steady-state infusion and bolus injection in rats. Since K _P is often governed by reversible binding to macromolecules in blood and tissue, an attempt was made to assess K _P values of DEX and MPL by in vitro binding studies using rat tissue homogenates and to compare these estimates to those obtained from in vivo kinetics after dosing. The K _P values of both steroids were also calculated in rat tissues using mechanistic tissue composition-based equations. The plasma binding of DEX and MPL was linear with moderate binding (60.5% and 82.5%) in male and female rats. In vivo estimates of steroid uptake appeared linear across the tested concentrations and K _P was highest in liver and lowest in muscle for both steroids. Assessment of hepatic binding of MPL in vitro was severely affected by drug loss at 37°C in male liver homogenates, whereas DEX was stable in both male and female liver homogenates. With the exception of MPL in liver, in vitro-derived K _P estimates reasonably agreed with in vivo values. The mechanistic equations modestly underpredicted K _P for both drugs. Tissue metabolism, saturable tissue binding, and active uptake are possible factors that can complicate assessments of in vivo tissue binding of steroids when using tissue homogenates. SIGNIFICANCE STATEMENT: Assuming the free hormone hypothesis, the ratio of the unbound drug fraction in plasma and in tissues defines the tissue-to-plasma partition coefficient (K _P), an important parameter in physiologically based pharmacokinetic modeling that determines total drug concentrations within tissues and the steady-state volume of distribution. This study assessed the plasma and tissue binding properties of the synthetic corticosteroids, dexamethasone and methylprednisolone, in rats using ultrafiltration and tissue homogenate techniques. In vitro-in vivo and in silico-in vivo extrapolation of K _P was assessed for both drugs in liver, muscle, and lung. Although the extrapolation was fairly successful across the tissues, in vitro homogenate studies severely underpredicted the K _P of methylprednisolone in liver, partly attributable to the extensive hepatic metabolism.

Characterization of membrane receptor binding activity for cortisol in the liver and kidney of the euryhaline teleost, Mozambique tilapia (Oreochromis mossambicus)

Glucocorticoids (GCs) regulate an array of physiological responses in vertebrates. Genomic GC actions mediated by nuclear steroid receptors require a lag time on the order of hours to days to generate an appreciable physiological response. Experimental evidence has accumulated that GCs, can also act rapidly through a nongenomic mechanism to modulate cellular physiology in vertebrates. Causal evidence in the Mozambique tilapia (Oreochromis mossambicus) suggests that the GC cortisol exerts rapid, nongenomic actions in the gills, liver, and pituitary of this euryhaline teleost, but the membrane receptor mediating these actions has not been characterized. Radioreceptor binding assays were conducted to identify a putative GC membrane receptor site in O. mossambicus. The tissue distribution, binding kinetics, and pharmacological signature of the GC membrane-binding activity were characterized. High affinity (Kd=9.527±0.001 nM), low-capacity (Bmax=1.008±0.116 fmol/mg protein) [(3)H] cortisol binding was identified on plasma membranes prepared from the livers and a lower affinity (Kd=30.08±2.373 nM), low capacity (Bmax=4.690±2.373 fmol/mg protein) binding was found in kidney membrane preparations. Competitors with high binding affinity for nuclear GC receptors, mifepristone (RU486), dexamethasone, and 11-deoxycorticosterone, displayed no affinity for the membrane GC receptor. The association and dissociation kinetics of [(3)H] cortisol binding to membranes were orders of magnitude faster (t1/2=1.7-2.6 min) than those for the intracellular (nuclear) GC receptor (t1/2=10.2h). Specific [(3)H] cortisol membrane binding was also detected in the gill and pituitary but not in brain tissue. This study represents the first characterization of a membrane GC receptor in fishes and one of only a few characterized in vertebrates.

Non-genomic effect of glucocorticoids on cardiovascular system

Glucocorticoids (GCs) are essential steroid hormones for homeostasis, development, metabolism, and cognition and possess anti-inflammatory and immunosuppressive actions. Since glucocorticoid receptor II (GR) is nearly ubiquitous, chronic activation or depletion of GCs leads to dysfunction of diverse organs, including the heart and blood vessels, resulting predominantly from changes in gene expression. Most studies, therefore, have focused on the genomic effects of GC to understand its related pathophysiological manifestations. The nongenomic effects of GCs clearly differ from well-known genomic effects, with the former responding within several minutes without the need for protein synthesis. There is increasing evidence that the nongenomic actions of GCs influence various physiological functions. To develop a GC-mediated therapeutic target for the treatment of cardiovascular disease, understanding the genomic and nongenomic effects of GC on the cardiovascular system is needed. This article reviews our current understanding of the underlying mechanisms of GCs on cardiovascular diseases and stress, as well as how nongenomic GC signaling contributes to these conditions. We suggest that manipulation of GC action based on both GC and GR metabolism, mitochondrial impact, and the action of serum- and glucocorticoid-dependent kinase 1 may provide new information with which to treat cardiovascular diseases.

Glucocorticoids turn over slowly in human adipose tissue in vivo

CONTEXT

Lipophilic plasma glucocorticoids are thought to gain rapid access to intracellular compartments in adipose tissue. In other organs, transport can be regulated in a steroid- and tissue-specific manner. Moreover, 11β-hydroxysteroid dehydrogenase type 1 (11βHSD1) generates additional cortisol within adipose.

AIM

The aim was to measure the rate of exchange of cortisol between plasma and adipose for comparison with rates of intracellular cortisol generation by 11βHSD1.

PARTICIPANTS AND INTERVENTIONS

With ethical approval, otherwise healthy females (n = 6) undergoing hysterectomy for benign indications were infused with tracer 9,11,12,12-[(2)H](4)cortisol (d4-cortisol). Adipose biopsies and peripheral venous samples were obtained during surgery after 3.9-5.5 h of infusion. Glucocorticoids were quantified using liquid chromatography tandem mass spectrometry.

RESULTS

In plasma, d4-cortisol concentrations and appearance rates of cortisol and d3-cortisol (reflecting 11βHSD1 activity) did not change during surgery. In both omental and sc adipose, cortisol concentrations were lower than in plasma, consistent with differences in corticosteroid binding globulin, and enrichment with d4-cortisol was low (sc, 7.2 ± 0.6%; omental, 7.4 ± 0.7%; vs. plasma, 15.5 ± 1.0%). The rate of accumulation of d4-cortisol in adipose depots was 0.5 ± 0.1 (sc) and 0.4 ± 0.1 (omental) nmol/kg · h, and the proportion of intraadipose cortisol replaced each hour only 10.7 ± 1.0 and 10.4 ± 0.7%, respectively. The contribution of 11βHSD1 to this turnover could not be quantified because very little substrate d3-cortisone accumulated in adipose during infusion.

CONCLUSIONS

Slow turnover of the adipose glucocorticoid pool suggests that rapid acute fluctuations in circulating cortisol are not reflected in adipose, so that 11βHSD1 activity (previously estimated to generate 9 nmol cortisol/kg · h in sc adipose) may play a relatively important role in modulating activation of glucocorticoid receptors.

Dexamethasone and corticosterone induce similar, but not identical, muscle wasting responses in cultured L6 and C2C12 myotubes

Dexamethasone-treated L6 (a rat cell line) and C2C12 (a mouse cell line) myotubes are frequently used as in vitro models of muscle wasting. We compared the effects of different concentrations of dexamethasone and corticosterone (the naturally occurring glucocorticoid in rodents) on protein breakdown rates, myotube size, and atrogin-1 and MuRF1 mRNA levels in the two cell lines. In addition, the expression of the glucocorticoid receptor (GR) and its role in glucocorticoid-induced metabolic changes were determined. Treatment with dexamethasone or corticosterone resulted in dose-dependent increases in protein degradation rates in both L6 and C2C12 myotubes accompanied by 25-30% reduction of myotube diameter. The same treatments increased atrogin-1 mRNA levels in L6 and C2C12 myotubes but, surprisingly, upregulated the expression of MuRF1 in L6 myotubes only. Both cell types expressed the GR and treatment with dexamethasone or corticosterone downregulated total cellular GR levels while increasing nuclear translocation of the GR in both L6 and C2C12 myotubes. The GR antagonist RU38486 inhibited the dexamethasone- and corticosterone-induced increases in atrogin-1 and MuRF1 expression in L6 myotubes but not in C2C12 myotubes. Interestingly, RU38486 exerted agonist effects in the C2C12, but not in the L6 myotubes. The present results suggest that muscle wasting-related responses to dexamethasone and corticosterone are similar, but not identical, in L6 and C2C12 myotubes. Most notably, the regulation by glucocorticoids of MuRF1 and the role of the GR may be different in the two cell lines. These differences need to be taken into account when cultured myotubes are used in future studies to further explore mechanisms of muscle wasting.

The effects of non-genomic glucocorticoid mechanisms on bodily functions and the central neural system. A critical evaluation of findings

Mounting evidence suggests that--beyond the well-known genomic effects--glucocorticoids affect cell function via non-genomic mechanisms. Such mechanisms operate in many major systems and organs including the cardiovascular, immune, endocrine and nervous systems, smooth and skeletal muscles, liver, and fat cells. Non-genomic effects are exerted by direct actions on membrane lipids (affecting membrane fluidity), membrane proteins (e.g. ion channels and neurotransmitter receptors), and cytoplasmic proteins (e.g. MAPKs, phospholipases, protein kinases, etc.). These actions are mediated by the glucocorticoids per se or by the proteins dissociated from the liganded glucocorticoid receptor complex. The MR and GR also activate non-genomic mechanisms in certain cases. Some effects of glucocorticoids are shared by a variety of steroids, whereas others are more selective. Moreover, "ultra-selective" effects-mediated by certain glucocorticoids only-were also shown. Disparate findings suggest that non-genomic mechanisms also show "demand-specificity", i.e. require the coincidence of two or more processes. Some of the non-genomic mechanisms activated by glucocorticoids are therapeutically relevant; moreover, the "non-genomic specificity" of certain glucocorticoids raises the possibility of therapeutic applications. Despite the large body of evidence, however, the non-genomic mechanisms of glucocorticoids are still poorly understood. Criteria for differentiating genomic and non-genomic mechanisms are often loosely applied; interactions between various mechanisms are unknown, and non-genomic mechanism-specific pharmacological (potentially therapeutic) agents are lacking. Nevertheless, the discovery of non-genomic mechanisms is a major breakthrough in stress research, and further insights into these mechanisms may open novel approaches for the therapy of various diseases.

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).

Dexamethasone upregulates the expression of the nuclear cofactor p300 and its interaction with C/EBPbeta in cultured myotubes

Muscle wasting during sepsis and other catabolic conditions is, at least in part, mediated by glucocorticoids and is associated with upregulated transcription of multiple genes in the ubiquitin-proteasome proteolytic pathway. In addition to transcription factors, nuclear cofactors, including p300, regulate gene transcription. We tested the hypothesis that glucocorticoids upregulate the expression of p300 in muscle cells. Treatment of cultured L6 myotubes, a rat skeletal muscle cell line, with dexamethasone resulted in a dose- and time-dependent increase in p300 protein and mRNA levels. Surprisingly, the effect of dexamethasone on p300 levels was not inhibited by the glucocorticoid receptor (GR) antagonist RU38486 and RU38486 exerted an agonist effect on p300, increasing its expression. Co-immunoprecipitation showed that treatment of the myotubes with dexamethasone resulted in protein-protein interaction between p300 and C/EBPbeta, but not C/EBPdelta. The present results suggest that glucocorticoids upregulate the expression of p300 and its interaction with C/EBPbeta in skeletal muscle. Increased expression and activity of p300 may be involved in the regulation of gene transcription in glucocorticoid-dependent muscle wasting.

Do antidepressants regulate how cortisol affects the brain?

Although the effects of antidepressants on glucocorticoid hormones and their receptors are relevant for the therapeutic action of these drugs, the molecular mechanisms underlying these effects are unclear. Studies in depressed patients, animals and cellular models have demonstrated that antidepressants increase glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) expression and function; this, in turn, is associated with enhanced negative feedback by endogenous glucocorticoids, and thus with reduced resting and stimulated hypothalamic-pituitary-adrenal (HPA) axis activity. In a series of studies conducted over the last few years, we have shown that antidepressants modulate GR function in vitro by inhibiting membrane steroid transporters that regulate the intracellular concentration of glucocorticoids. In this paper, we will review the effects of membrane steroid transporters and antidepressants on corticosteroid receptors. We will then present our unpublished data on GR live microscopy in vitro, showing that ligand-induced translocation of the GR starts within 30 seconds and is completed within minutes. Furthermore, we will present our new data using an in situ brain perfusion model in anaesthetised guinea-pigs, showing that entry of cortisol to the brain of these animals is limited at the blood-brain barrier (BBB). Finally, we will present a comprehensive discussion of our published findings on the effects of chemically unrelated antidepressants on membrane steroid transporters, in mouse fibroblasts and rat cortical neurones. We propose that antidepressants in humans could inhibit steroid transporters localised on the BBB and in neurones, like the multidrug resistance p-glycoprotein, and thus increase the access of cortisol to the brain and the glucocorticoid-mediated negative feedback on the HPA axis. Enhanced cortisol action in the brain might prove to be a successful approach to maximise therapeutic antidepressant effects.

Cortisol rapidly suppresses intracellular calcium and voltage-gated calcium channel activity in prolactin cells of the tilapia (Oreochromis mossambicus)

Cortisol was previously shown to rapidly (10-20 min) reduce the release of prolactin (PRL) from pituitary glands of tilapia (Oreochromis mossambicus). This inhibition of PRL release by cortisol is accompanied by rapid reductions in (45)Ca(2+) and cAMP accumulation. Cortisol's early actions occur through a protein synthesis-independent pathway and are mimicked by a membrane-impermeable analog. The signaling pathway that mediates rapid, nongenomic membrane effects of glucocorticoids is poorly understood. Using the advantageous characteristics of the teleost pituitary gland from which a nearly pure population of PRL cells can be isolated and incubated in defined medium, we examined whether cortisol rapidly reduces intracellular free calcium (Ca(i)(2+)) and suppresses L-type voltage-gated ion channel activity in events that lead to reduced PRL release. Microspectrofluorometry, used in combination with the Ca(2+)-sensitive dye fura 2 revealed that cortisol reversibly reduces basal and hyposmotically induced Ca(i)(2+) within seconds (P < 0.001) in dispersed pituitary cells. Somatostatin, a peptide known to inhibit PRL release through a membrane receptor-coupled mechanism, similarly reduces Ca(i)(2+). Under depolarizing [K(+)], the L-type calcium channel agonist BAY K 8644, a factor known to delay the closing of L-type Ca(2+) channels, stimulates PRL release in a concentration-dependent fashion (P < 0.01). Cortisol (and somatostatin) blocks BAY K 8644-induced PRL release (P < 0.01; 30 min), well within the time course over which its actions occur, independent of protein synthesis and at the level of the plasma membrane. Results indicate that cortisol inhibits tilapia PRL release through rapid reductions in Ca(i)(2+) that likely involve an attenuation of Ca(2+) entry through L-type voltage-gated Ca(2+) channels. These results provide further evidence that glucocorticoids rapidly modulate hormone secretion via a membrane-associated mechanism similar to that observed with the fast effects of peptides and neurotransmitters.

Proteins of multiple classes may participate in nongenomic steroid actions

Responses to steroids initiated from non-nuclear receptors impinge on a wide variety of cellular responses and utilize nearly all known signal transduction webs. While the mechanisms by which steroid receptors localize in the membrane are still unclear, it is apparent that this alternative localization allows steroid receptors to participate in a wide range of complex functions influencing cell proliferation, death, and differentiation. The central debate still remains the identity of the protein class or classes that mediate membrane-initiated (nongenomic) responses. The data thus far have supported several possibilities, including: nuclear steroid receptor-like forms in non-nuclear locations; other known (nonsteroid) membrane receptors or channels with additional steroid-binding sites; enzymes; transporters; receptors for serum steroid-binding proteins; unique and previously undescribed proteins; or chimeras of typical steroid receptor domains with other unique or known protein domains. Categorizing membrane steroid receptor proteins based exclusively on the actions of antagonists and agonists, without considering cell context and protein partnering issues, may mislead us into predicting more receptor subtypes than really exist. However, the plethora of signaling and functional outcomes may indicate the participation of more than one kind of steroid-binding protein. Resolving such unanswered questions will require future investigative focus on this alternative arm of steroid action, which is likely to yield as many therapeutic opportunities as have nuclear steroid mechanisms.

Photoaffinity labeling identification of thyroid hormone-regulated glucocorticoid-binding peptides in rat liver endoplasmic reticulum: an oligomeric protein with high affinity for 16beta-hydroxylated stanozolol

Steroid-binding proteins unrelated to the classical nuclear receptors have been proposed to play a role in non-genomic actions of the17alpha-alkylated testosterone derivative (17alpha-AA) stanozolol (ST). We have previously reported that male rat liver endoplasmic reticulum contains two steroid-binding sites associated with high molecular mass oligomeric proteins: (1) the ST-binding protein (STBP); and (2) the low-affinity glucocorticoid-binding protein (LAGS). To further explore the role of LAGS on the mechanism of action of ST, we have now studied: (1) the interaction of ST and its hydroxylated metabolites with solubilized LAGS and the cytosolic glucocorticoid receptor (GR); and (2) the effects of hormones on the capability of STBP to bind ST. We found that, unlike 17alpha-methyltestosterone, neither ST nor its hydroxylated metabolites bind to GR. However, the 16beta-hydroxylation of ST significantly increases the capability of LAGS to bind ST. Interestingly, 3'-hydroxylation of ST abrogates the capability of LAGS to bind ST. ST (k(i)=30 nM) and 16beta-hydroxystanozolol (k(i)=13 nM) bind with high affinity to LAGS, and are capable of accelerating the rate of dissociation of previously bound dexamethasone from the LAGS. STBP and LAGS are strongly induced by ethinylestradiol. However, unlike STBP, LAGS is regulated by thyroid hormones and growth hormone, which proves that these steroid-binding activities are associated with different binding sites. These findings seem to suggest a novel mechanism for ST whereby membrane-associated glucocorticoid-binding activity is targeted by the 16beta-hydroxylated metabolite of ST. ST and its 16beta-hydroxylated metabolite modulate glucocorticoid activity in the liver through negative allosteric modulation of LAGS, with the result of this interaction an effective increase in classical GR-signaling by increasing glucocorticoid availability to the cytosolic GR.

Antidepressant fluoxetine enhances glucocorticoid receptor function in vitro by modulating membrane steroid transporters

1. Incubation of LMCAT fibroblast cells with antidepressants potentiates glucocorticoid receptor (GR)-mediated gene transcription in the presence of dexamethasone and cortisol, but not of corticosterone. We have shown that antidepressants do so by inhibiting the LMCAT cell membrane steroid transporter (which is virtually identical to the multidrug resistance P-glycoprotein) and thus by increasing dexamethasone or cortisol intracellular concentrations. However, previous experiments with the antidepressant fluoxetine in the presence of dexamethasone have produced negative results (Pariante et al. (2001). Br. J. Pharmacol., 134, 1335-1343). 2. We have since re-examined the effects of fluoxetine on GR-mediated gene transcription in the presence of dexamethasone. Moreover, we have examined the effects of fluoxetine on GR-mediated gene transcription in the presence of cortisol and corticosterone, and on the intracellular accumulation of radioactive cortisol and corticosterone. Finally, we have examined the effects of fluoxetine on inhibition of P-glycoprotein activity in Caco-2 cells. 3. We now find that fluoxetine (1-10 micro M) enhances GR-mediated gene transcription in the presence of dexamethasone and cortisol (+140-170%), but not of corticosterone, and increases the intracellular accumulation of (3)H-cortisol (+5-15%), but not of (3)H-corticosterone. Moreover, fluoxetine (10 micro M) induces approximately 30% inhibition of PGP activity in Caco-2 cells. 4. Our results show that fluoxetine, like other antidepressants, inhibits membrane steroid transporters.

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.

Nongenomic membrane actions of glucocorticoids in vertebrates

For decades, it was widely assumed that glucocorticoids (GCs) work solely through changes in gene expression to exert their physiological actions, a process that normally takes several hours to occur. However, recent evidence indicates that GCs might also act at the membrane through specific receptors to exert multiple rapid effects on various tissues and cells. GCs modulate hormone secretion, neuronal excitability, behavior, cell morphology, carbohydrate metabolism and other processes within seconds or minutes. These early actions occur independent of the genome and are transduced by the same biochemical effector pathways responsible for mediating rapid responses to neurotransmitters. The biological significance of most rapid GC effects are not well understood, but many might be related to the important functions that this hormone plays in modulating stress responses.

Membrane oestrogen receptors on rat pituitary tumour cells: immuno-identification and responses to oestradiol and xenoestrogens

Our laboratory has identified plasma membrane oestrogen receptors on a GH3/B6 rat pituitary tumour cell line and several sublines which produce rapid (within minutes), non-genomic responses to oestrogens. Oestrogen receptors have been identified by their binding to nine different antibodies (Abs) which together recognize at least seven epitopes on the oestrogen receptor-alpha. GH3/B6/F10 cells, a membrane oestrogen receptor-enriched subline, elevate intracellular calcium levels in response to 10 nM oestradiol. Prolactin release in these cells is triggered by both 1 pM and 1 nM oestradiol and diethylstilbestrol (DES). A membrane oestrogen receptor-alpha immunocytochemical signal rapidly disappears (at 3 min) and reappears (at 12-15 min) when 1 nM oestradiol, 10 nM diethylstilbestrol, or 10 nM nonylphenol is applied to the cells. This suggests that both oestrogens and xenoestrogens can utilize this alternative pathway for oestrogenic action. Xenoestrogens, which have so far shown weak effects in genomic assay systems, should now be retested for activity in eliciting membrane-initiated oestrogenic responses.