The mechanism of [3H]dexamethasone uptake into prolactin producing rat pituitary cells (GH3 cells) in culture

Cartilage-targeting dexamethasone prodrugs increase the efficacy of dexamethasone

Intra-articular administration of glucocorticoids such as dexamethasone is a common treatment for osteoarthritic inflammation and pain. Despite its potent anti-inflammatory properties, multiple barriers hinder the drug's effectiveness in the articular space. In particular, the high turnover rate of the synovial fluid and the dense cartilage extracellular matrix (ECM) lead to poor drug penetration into cartilage. In order to increase the infiltration and retention time, two dexamethasone prodrugs were developed. Firstly, dexamethasone was conjugated to polycationic chitosan, which led to deep and sustained infiltration of the drug into full thickness cartilage, due to its strong electrostatic interactions with the high negative fixed charges of the cartilage ECM. Secondly, dexamethasone was conjugated to a collagen type II-binding peptide, WYRGRL, and this prodrug was shown to be retained in the deep zones of cartilage through specific interactions with cartilage-specific collagen type II bundles. In both cases, active dexamethasone was released from the carrier by ester linkage hydrolysis. Complexing dexamethasone with either chitosan or collagen type II-affinity carriers increased its binding and therapeutic efficacy inside cartilage, compared to the free drug. Both dexamethasone conjugates significantly reduced levels of inflammatory markers and slowed the loss of glycosaminoglycans in an ex vivo model. A single dose of a cartilage-targeting dexamethasone prodrug represents a promising alternative to the repetitive glucocorticoid injections needed to compensate for its rapid clearance from the joint cavity.

The activity of organic anion transporter-3: Role of dexamethasone

Human organic anion transporter-3 (hOAT3) is richly expressed in the kidney, where it plays critical roles in the secretion, from the blood to urine, of clinically important drugs, such as anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. In the current study, we examined the role of dexamethasone in hOAT3 transport activity in the kidney HEK293 cells. Cis-inhibition study showed that dexamethasone exhibited a concentration-dependent inhibition of hOAT3-mediated uptake of estrone sulfate, a prototypical substrate for the transporter, with IC₅₀ value of 49.91 μM. Dixon plot analysis revealed that inhibition by dexamethasone was competitive with a Ki = 47.08 μM. In contrast to the cis-inhibition effect of dexamethasone, prolonged incubation (6 h) of hOAT3-expressing cells with dexamethasone resulted in an upregulation of hOAT3 expression and transport activity, kinetically revealed as an increase in the maximum transport velocity V_max without meaningful alteration in substrate-binding affinity K_m. Such upregulation was abrogated by GSK650394, a specific inhibitor for serum- and glucocorticoid-inducible kinases (sgk). Dexamethasone also enhanced sgk1 phosphorylation. Our study demonstrated that dexamethasone exhibits dual effects on hOAT3: it is a competitive inhibitor for hOAT3-mediated transport, and interestingly, when entering the cells, it stimulates hOAT3 expression and transport activity through sgk1.

A convenient, high-throughput assay for measuring the relative cell permeability of synthetic compounds

We describe here a convenient procedure for assessing the relative cell permeability of chemical compounds. The assay can be used in a high-throughput mode and is particularly applicable for the evaluation of the relative permeability of compounds in a combinatorial library. The compound of interest is conjugated to a dexamethasone derivative. The entry of the conjugate into living mammalian cells triggers the nuclear transport of a Gal4 DNA binding domain-glucocorticoid receptor ligand binding domain-VP16 activation domain fusion protein and, consequently, the activation of a Gal4-responsive luciferase reporter gene. The relative cell permeability is thus described quantitatively by the level of luciferase expression. The experiments take only a few days once chemical synthesis and conjugation are finished.

Corticosteroids: the mainstay in asthma therapy

Inflammation is now marked as a central feature of asthma pathophysiology and aims of current asthma management are not only to treat acute symptoms of wheezing, breathlessness, chest tightness, cough but also to suppress the underlying inflammatory component. Despite the availability of a number of drugs, corticosteroids remain the mainstay in the management of all types of asthma as these are the most potent and effective antiinflammatory agents available so far. Corticosteroids suppress virtually every step in inflammation. However therapeutic doses of oral glucocorticoids are associated with a range of adverse reactions. To overcome these side effects, inhalations have been developed to deliver glucocorticoids directly to the lungs and in the process a number of aerosol preparations have become available, which have advantage of significantly lower toxicity due to low systemic absorption from the respiratory tract and rapid inactivation. Despite considerable efforts by pharmaceutical industry, it has been difficult to develop novel therapeutic agents for asthma management, which could surpass inhaled corticosteroids. Currently the data favours using inhaled corticosteroids as monotherapy in the majority of patients in all kinds of asthma. If combination therapy is recommended to achieve additional control in severe asthma cases, other drugs such as beta-agonists, antileukotrienes, theophylline, etc. are considered as adjunct therapies to corticosteroids. This review discusses the importance of corticosteroids as first line therapy for asthma treatment with the availability of inhaled corticosteroids for chronic treatment and oral formulations for treating acute exacerbations of moderate to severe asthma.

Simple reporter gene-based assays for hairpin poly(amide) conjugate permeability and DNA-binding activity in living cells

Hairpin poly(amide)s (HPs) are sequence specific DNA-binding compounds that have engendered considerable interest as potential pharmacological agents to manipulate the expression of specific genes. However, recent reports have indicated that the ability of HP conjugates to pass through cell membranes is sensitive to the cell type employed and the nature of the conjugate. Furthermore, while binding of HPs to DNA sequences in vitro is relatively well understood, packing of DNA into chromatin in living cells makes predicting the efficiency with which a given poly(amide) will bind its cognate site less certain. Previous methods to evaluate HP permeability and binding in vivo, while effective, are somewhat tedious and qualitative. We report here two related reporter gene-based assays that provide a more convenient and quantitative measure of poly(amide) permeability and DNA binding activity in living cells. We anticipate that these methods will complement existing tools and facilitate the development of HP conjugates with the desired biological activity.

Identification of novel glucocorticoid-response elements in human elastin promoter and demonstration of nucleotide sequence specificity of the receptor binding

Glucocorticoids exert their action on gene expression through activation of cytoplasmic glucocorticoid receptors (GRs) that bind to glucocorticoid response elements (GREs). The consensus GRE consists of two half sites (underlined), AGAACANNNTGTTCT. We have recently cloned the entire human elastin gene. Nucleotide sequencing of the promoter region disclosed the presence of three putative GREs with the downstream half-site sequence TGTTCC that has homology with the consensus GRE, although the upstream half site showed no homology. To examine the functionality of these putative GREs in binding to the GRs, we performed gel mobility shift and supershift assays with synthetic oligomers containing the putative GREs and a recombinant GR protein, expressed in a baculovirus system. All three GREs identified in the elastin promoter bound the receptor. A chimeric oligonucleotide containing the upstream consensus GRE half site and the downstream elastin promoter GRE half site was capable of binding the receptor, and this binding could be competed with the elastin promoter GRE. Nonconservative substitution of single nucleotides (positions 1-6) in the elastin GRE indicated that mutations in the positions 1-3 and 6 had relatively little effect, but substitutions in positions 4 and 5 rendered the oligomer less effective in competing for the binding. These observations suggest that the downstream half site of GREs in the human elastin promoter is sufficient for receptor binding and certain nucleotides are critical for the efficient binding. The results also imply that the three GREs within the human elastin promoter are active and mediate the glucocorticoid-induced up-regulation of human elastin promoter activity.

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

To elucidate the initial step in the interaction between glucocorticoids (GC) and the hepatocyte, we examined at 22 degrees C further kinetic properties of active corticosterone (B) transport mediated by a putative, plasma membrane-inserted carrier for GC (GCC) as previously reported [Alléra and Wildt, J. Steroid Biochem. Molec. Biol. 42 (1992) 737-756]. We used a purified, well-characterized, osmotically active vesicle fraction of plasma membrane (PM), free of ATP, isolated from rat liver and a method developed by us to describe transport processes mathematically: (1) uptake (U) of 7 nM B into the vesicles (influx, I) occurred very rapidly whereby T1/2 = 8.3 s, the time (S) required for half maximum transport; the influx velocity (dU/dS = V) decreased degressively with time following second-order kinetics characterized by an initial transport V (VT0) of 177.7 fmol/mg membrane protein/s. (2) VToI of B-influx rose with temperature biphasically (P less than 0.025): activation energy above and below 15 degrees C (at PM phase transition) amounted to 9.5 and 26.5 kJ/mol. Neither at 45 nor at 60 degrees C did transport take place, revealing the high thermolability of GCC. (3) Efflux (E) of 6.5 nM B, i.e. transport out of the vesicles preincubated with the steroid, showed that influx had resulted in a 19.6-fold intravesicular hormone accumulation, indicating active ("uphill") transport. (4) The efflux velocity (dE/dS = V) exhibited almost the same kinetic quality as that of influx: it decreased following mainly second-order kinetics whereby T1/2 = 8.0 s. However, its whole time-course was much slower and the VT0 of efflux (VToE) was 6.3 lower than VToI. (5) Using physics and thermodynamics, we deduced that the affinity (AF) between B and GCC is proportional to the square of VT0. (6) Thus, because AF approximately (1/6.3)2, AF of the B-GCC interaction after completion of influx was calculated to be 40 times lower (Kd = 708 nM; delta G degrees = -34.9 kJ/mol) than at outset of influx, whereby delta G degrees = -44.0 kJ/mol. Concluding from these and previous findings, we present a new hypothesis on B transport into the hepatocyte: There is no difference (P greater than 0.3) in free enthalpy between transcortin (CBG) and the intracellular GC receptor interacting with B (delta G degrees = -40.1 and -40.4 kJ/mol). The GCC, however, is characterized by its ability to switch from a high- to lower-affinity when interacting with B (and vice versa due to metabolic energy input).(ABSTRACT TRUNCATED AT 400 WORDS)

DNA-binding by the glucocorticoid receptor: a structural and functional analysis

The glucocorticoid receptor belongs to a family of ligand activated nuclear receptors. This family includes, in addition to the receptors for steroid hormones, receptors for thyroid hormone, retinoic acid and 1,25-dihydroxy vitamin D3 as well as some receptors with as yet unknown ligands. The glucocorticoid receptor DNA-binding domain has been expressed in E. coli. The purified protein binds to the same DNA sequences as the native receptor and is therefore suitable for biochemical and structural studies of the DNA-binding function of the receptor protein. This protein has been shown to bind as a dimer to its DNA-binding site. Protein-protein interactions facilitate DNA-binding and a segment responsible for these interactions has been identified close to the C-terminal zinc-binding site. The family of nuclear receptors, with their related DNA-binding sites, provides an opportunity to study determinants for DNA sequence recognition. A segment close to the N-terminal zinc ion has been shown to be responsible for the target specificity of glucocorticoid and estrogen receptors. DNA-binding domains of nuclear receptors include nine conserved cysteine residues which have been shown to coordinate two zinc ions and zinc has been shown to be required for the structural integrity and DNA-binding ability of the glucocorticoid receptor DNA-binding domain. A motif for DNA recognition, based around zinc ions, was first described for transcription factor IIIA and nuclear receptors were believed to recognize DNA via a similar motif. However, the three-dimensional structure determination of the glucocorticoid receptor DNA-binding domain shows that its structure is clearly different from that of the TFIIIA type zinc-binding domains.

Dexamethasone inhibition of rat hepatoma cell growth and cell cycle traverse is reversed by insulin

(1) The growth of 7800 C1 Morris hepatoma cells was inhibited by dexamethasone. The inhibition was detectable at 1 nM and half-maximal effect was obtained with approx. 13 nM dexamethasone. About 80% growth inhibition was obtained with 250 nM of the hormone and the growth rate was normalized on cessation of treatment. (2) These hepatoma cells contain dexamethasone receptors with equilibrium dissociation constant of 0.24 nM and a capacity of 24 fmol/mg cell protein. Treatment of the cells with insulin did not change these dexamethasone binding properties. Binding experiments showed that 2, 10 and 100% of the receptors were occupied when the cells were incubated with 1 nM, 7 nM and 250 nM dexamethasone, respectively. (3) Insulin completely counteracted the growth inhibition by dexamethasone and antagonized the induction of peroxisomal acyl-CoA oxidase and tyrosine aminotransferase caused by the glucocorticoid. (4) Micro-flow fluorometry showed that the cultures had a major diploid DNA stem line and a minor tetraploid stem line. Changes in diploid, tetraploid and S phase cells of the diploid stem line were scored. Dexamethasone reduced the proportion of cells in S phase and of tetraploid cells. Insulin partly reversed the action of dexamethasone in S phase, but prevented the reduction in tetraploid cells caused by dexamethasone. (5) The mitotic rate was significantly reduced by dexamethasone and this effect was reversed by insulin. (6) Continuous [3H]methyl-thymidine labelling showed a growth fraction of unity in all treatment groups. (7) It is concluded that dexamethasone induces growth inhibition by reducing the G1-S transition. Insulin is able to counteract this effect and increase the rate of DNA synthesis.

Uptake and receptor binding of dexamethasone in cultured 7800 C1 hepatoma cells in relation to regulation of cell growth and peroxisomal beta-oxidation

1. Uptake and binding of dexamethasone to glucocorticoid receptor has been studied in Morris hepatoma 7800 C1 cells in relation to its effect on cell growth and peroxisomal beta-oxidation. 2. Intact cells showed saturable, specific dexamethasone binding of limited capacity and Scatchard analysis revealed one single class of binding sites with equilibrium dissociation constant (Kd) of 0.24 nM similar to other glucocorticoid receptors. However, the binding capacity of 24 fmol/mg cell protein is less than 5% of previously reported values. 3. Uptake of [3H]dexamethasone by intact cells was temperature dependent giving a linear Arrhenius plot with a calculated energy of activation of 58.5 kJ mol-1 x degree-1. 4. Cytosol fractions had specific binding proteins for glucocorticoid hormones with sedimentation coefficient of ca 7S. No specific binding sites for [3H]dexamethasone was demonstrated in purified membrane fractions. 5. Dexamethasone and the synthetic fatty acid analogue tetradecylthio acetic acid (TTA) both inhibited the growth of the 7800 C1 cells and induced the peroxisomal acyl-CoA oxidase activity. A combination of the two compounds gave additive effects. Both these effects of dexamethasone and TTA were counteracted by insulin. 6. We conclude that dexamethasone induces growth inhibition and enzyme induction by binding to functional intracellular glucocorticoid receptors. The action of dexamethasone is consistent with a dissolution in the membrane from where it diffuses passively into the cell and binds to specific receptors in an energy dependent step. 6. The synergistic action of dexamethasone and TTA and the counteraction exerted by insulin are not due to changes in the dexamethasone receptor affinity or binding capacity.