Jan Carlstedt-Duke

Molecular interactions of steroid hormone receptor with its enhancer element: Evidence for receptor dimer formation

A steroid hormone responsive element (GRE/PRE), sufficient to confer glucocorticoid and progesterone inducibility when linked to a reporter gene, was used in band-shift assays to examine its molecular interactions with steroid hormone receptors. Both progesterone and glucocorticoid receptors bound directly and specifically to the GRE/PRE. The purine contact sites for both form A and form B chicken progesterone receptor, as well as those for rat glucocorticoid receptor, are identical. A peptide fragment produced in bacteria that primarily contain the DNA binding domain of the glucocorticoid receptor binds first to the TGTTCT half-site of the GRE/PRE, and a second molecule binds subsequently to the TGTACA (half-site) of the GRE/PRE in a cooperative manner. Utilizing the peptide fragment and the protein A-linked fragment, we demonstrated that the receptor interacts with its cognate enhancer as a dimer.

The glucocorticoid receptor binds to a sequence overlapping the TATA box of the human osteocalcin promoter: a potential mechanism for negative regulation.

Expression of the human osteocalcin promoter is negatively regulated by glucocorticoids in vivo. In vitro DNase I and exonuclease III footprinting analysis showed binding of purified glucocorticoid receptor in close proximity to and overlapping with the TATA box of the osteocalcin gene. These results imply competition or interference with binding of the TATA box-binding transcription factor IID as a mechanism of repression of this gene by glucocorticoids. In support of this notion, point mutation analysis of the receptor binding site indicated that flanking nucleotides and not the TATA box motif per se were important for receptor interaction. Moreover, DNA binding competition assays showed specific binding of the receptor only to the TATA box region of the osteocalcin gene and not to the corresponding region of an immunoglobulin heavy-chain promoter.

EVIDENCE THAT THE BETA -ISOFORM OF THE HUMAN GLUCOCORTICOID RECEPTOR DOES NOT ACT AS A PHYSIOLOGICALLY SIGNIFICANT REPRESSOR

Alternative splicing of the human glucocorticoid receptor (hGR) primary transcript generates two receptor isoforms, hGRα and hGRβ, with different carboxyl termini diverging at amino acid 727. By reverse transcriptase-polymerase chain reactions it was previously demonstrated that the hGRβ message had a widespread tissue distribution. To demonstrate the presence of hGRβ as protein we produced specific rabbit antisera to hGRβ, as well as a hGRβ-specific mouse monoclonal IgM antibody, by peptide immunizations. By SDS-polyacrylamide gel electrophoresis and Western immunoblotting we showed that hGRβ is endogenously expressed at the protein level in HeLa cells and human lymphatic leukemia cells. Using an antibody directed against an epitope shared by both isoforms we showed a relatively lower expression of the hGRβ form. We also showed that hGRβ bound to hsp90 by immunoprecipitation of in vitro translated hGRβ in reticulocyte lysate with hsp90-specific antibodies, a coprecipitation occurring also in the presence of dexamethasone. We could not demonstrate that hGRβ inhibited the effects of dexamethasone-activated hGRα on a glucocorticoid-responsive reporter gene. In conclusion, low hGRβ expression levels and hGRβ-hsp90 interaction maintained in the presence of ligand and lack of inhibition of hormone-activated hGRα effects challenge the concept of the hGRβ isoform as a proposed dominant negative inhibitor of hGRα activity.

Characterization of two novel mutations in the glucocorticoid receptor gene in patients with primary cortisol resistance

OBJECTIVE Primary glucocorticoid resistance is characterized by decreased sensitivity to cortisol signalling. We have performed genetic analysis of the glucocorticoid receptor (GR) gene in 12 unrelated patients with primary cortisol resistance as defined by a pathological dexamethasone suppression test.

Structure and function of the glucocorticoid receptor

Glucocorticoids cause changes in the expression of target genes via interaction with an intracellular receptor protein, the glucocorticoid receptor. This signal transduction process can be divided into a number of steps, each of which represents a functional facet of the receptor protein. These steps include (i) receptor transformation to an active form resulting from specific interaction with glucocorticoid steroid hormones, (ii) homo-dimerization, (iii) DNA-binding to specific hormone response elements in the genome and (iv) modulation of the expression levels of linked genes. These aspects of glucocorticoid receptor function have been studied using a combination of tertiary structure determination, biochemical assays and a genetic approach using a yeast system to screen for mutant receptors that are altered in function. The results show that contacts involving both the DNA and steroid binding domains are involved in dimerization and high affinity DNA binding. Genetic experiments have illuminated the role of amino acids within the recognition helix of the DNA-binding domain in discriminating between cognate DNA response elements for the glucocorticoid receptor and closely related binding sites for other nuclear receptors. Squelching experiments suggest that the N-terminal transactivation domain of the receptor contacts components of the general transcriptional machinery that appear to be distinct from the TATA binding protein, TFIID, during transactivation of gene expression by the DNA-bound receptor.

Detection of the 2,3,7,8-tetpachlorodibenzo-p-dioxin (TCDD) receptor in rat liver by isoelectric focusing in polyacrylamide gels

Abstract A stereospecific, high-affinity binding protein (receptor protein) for 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD) in rat liver cytosol can be demonstrated using isoelectric focusing in polyacrylamide gels. In its native form, the receptor protein focuses at a varying pI possibly due to aggregation. However, if limited proteolysis of the receptor is carried out, the trypsinized receptor focuses as a single sharp peak at pI 5.15–5.25. Using this method the receptor can be separated from a serum TCDD-binding protein (pI 5.7–5.8) that is resistant to dextran-coated charcoal (DCC) treatment. The binding of [ 3 H] TCDD to the receptor is competed for by 2,3,7,8-tetrachlorodibenzofuran (TCDBF), 3-methylcholanthrene (MC), and ⨿-naphthoflavone (⨿NF) but not by phenobarbital (PhB) or pregnenolone-16α-carbonitrile (PCN). The method described can be used for detection and quantitation of the TCDD receptor.

Characterization of an antiserum against the glucocorticoid receptor

An immunoglobulin (IgG) fraction from serum of a rabbit immunized with a highly purified preparation of glucocorticoid receptor from rat liver cytosol contained specific antibodies to glucocorticoid receptor. This was shown following incubation of the [3H]triamcinolone acetonide-glucocorticoid receptor (TA-GR) complex with the IgG fraction by (I) adsorption of the [3H]triamcinolone acetonide-glucocorticoid receptor (TA-GR) complex with the IgG fraction by (I) adsorption of the [3H]TA-GR-antibody complex to protein A linked to Sepharose, (II) an increased sedimentation rate of the [3H]TA-GR-antibody complex compared to that of the [3H]TA-GR complex, and (III) an increased molecular size of the [3H]TA-GR-antibody complex when compared to that of the [3H]TA-GR complex as judged from gel filtration. The antibody fraction was characterized with regard to titer, cross-reactivity and specificity. The antibodies cross-reacted with the glucocorticoid receptor from various rat tissues (liver, thymus and hippocampus), as well as with the glucocorticoid receptor from human normal lymphocytes, chronic lymphatic leukemia cells and human hippocampus. In the rat liver, the antibody bound to both the nuclear and the cytosolic glucocorticoid receptor (stokes radius 6.1 nm). It did not cross-react with the proteolytic fragments of the glucocorticoid receptors, the 3.6 nm complex or the 1.9 nm complex. Binding of the antibodies was not seen to the androgen, estrogen or progestin receptors in rat or to rat serum transcortin. With an indirect competitive ELISA (enzyme-linked immunosorbent assay) combined with various separation techniques, based on different physicochemical principles, it was shown that the glucocorticoid receptor was the only detectable antibody binding protein from rat liver cytosol using this assay system. These findings also indicate an immunochemical similarity between glucocorticoid receptors in different tissues as well as in different species, but not between glucocorticoid receptors and other steroid hormone receptor proteins. The cytosolic and nuclear glucocorticoid receptors in rat liver were shown to be immunochemically similar.

Involvement of multidrug resistance proteins (MDR) in the modulation of glucocorticoid response

Glucocorticoid resistance is a problem in the treatment of many diseases. One possible factor involved in the modulation of a glucocorticoid response is the export of glucocorticoids out of the cell. It has been shown that multidrug resistance protein 1 (MDR1, ABCB1), a member of the ABC family, is capable of transporting some glucocorticoids. This paper uses a mouse cell line, LMCAT in which the glucocorticoid response can be modulated by inhibitors of multidrug resistance proteins. Glucocorticoids fall into three categories. Firstly, those that are transported by an Abcb1a/Abcb1b transporter and whose transport can be inhibited by inhibitors of ABCB1 activity. Functional Abcb1a/Abcb1b was detected by inhibition of rhodamine efflux by these drugs and mRNA for Abcb1a and Abcb1b were detected in these cells. Secondly, those that are not transported. Finally, those that are transported by an Abcc1a transporter. Calcein transport out of these cells was blocked by treatment with probenecid indicating a functional Abcc1a transporter. Abcc1a mRNA was also detected in these cells. Thus, this paper provides insight into the mechanisms of glucocorticoid transport in cells and demonstrates a diversity of two independent mechanisms of transport of glucocorticoids by Abcb1a/Abcb1b and Abcc1a with individual patterns of steroid specificity.

Interleukin-1 beta induces expression of cyclooxygenase-2 mRNA in human gingival fibroblasts.

The effect of interleukin-1β (IL-1β) on the expression of cyclooxygenase-1 and −2 (COX-1 and COX-2) mRNA and its relation to prostaglandin E2 (PGE2) biosynthesis in human gingival fibroblasts was studied. IL-1β increased levels of mRNA for COX-2 whereas the COX-1 mRNA level was unaffected. The increased COX-2 mRNA levels were accompanied by enhanced PGE2 formation. The phorbol, 12-myristate 13-acetate (PMA), known to stimulate protein kinase C (PKC), also induced expression of COX-2 mRNA. When gingival fibroblasts were treated simultaneously with IL-1β and PMA, the cytokine IL-1β synergistically increased levels of COX-2 mRNA, accompanied by a corresponding increase in PGE2 biosynthesis. The anti-inflammatory steroid, dexamethasone (DEX) abolished the enhanced expression of COX-2 mRNA as well as PGE2 formation induced by IL-1β, PMA or the combination of IL-1β and PMA. The study indicates that the IL-1β induced PGE2 formation is mediated by an enhanced gene expression of COX-2 in gingival fibroblasts suggesting that the enzyme COX-2 may play an important role in the regulation of prostanoid formation at inflammatory lesions in gingival tissue.

Glucocorticoid Receptor β: View II

There is a clear role for mechanisms that modulate glucocorticoid receptor (GR) function. The non-steroid-binding GRβ isoform has been proposed to play a role in this modulation but the published data are contradictory. The relative levels of this isoform appear to be low. Alternative mechanisms for the modulation of glucocorticoid action are described and contrasted with the proposed role for GRβ.