Identification by microarray analysis of aspartate aminotransferase and glutamine synthetase as glucocorticoid target genes in a mouse Schwann cell line

Schwann cells have been identified as targets for glucocorticoids. Besides genes implicated in the myelination process, the target genes of glucocorticoids have not been identified in these cells. For that purpose, we performed microarray analysis on MSC80 (mouse Schwann cells) treated with a synthetic glucocorticoid, dexamethasone. These cells express a functional glucocorticoid receptor (GR), but none of the other steroid receptors. This allowed us to identify genes specifically regulated by GR in the absence of the mineralocorticoid receptor. Among the 5000 genes analyzed, 12 were at least two-fold upregulated and 91 genes were at least two-fold down-regulated upon treatment with dexamethasone. Because of their potential role in Schwann cell homeostasis, we selected, for further analysis, the upregulated genes encoding glutamine synthetase (GS) and cytosolic aspartate aminotransferase (cAspAT). These genes play a crucial role in the glutamate cycle which was shown to be vital in neuron-astrocyte cross-talk in the central nervous system. Their activation was confirmed by semi-quantitative and real-time PCR. A detailed analysis of cAspAT promoter activity revealed that the mechanism of regulation by GR in Schwann cells differs from that in hepatoma cells, suggesting a cell-specific regulation. The transactivation potency of the two Glucocorticoid Responsive Units (GRU) present in the cAspAT promoter seems to be dependent on the levels of the GR in MSC80 cells. Furthermore, we show that an increase in GR levels under certain circumstances could considerably potentiate the effects of glucocorticoids on the cAspAT promoter via synergistic activation of both GRU, To the opposite, an enhancement in GR levels did not further potentiate Dex-activation of the GS promoter, showing a differential mechanism of action of GR in the context of both promoters.     

A functional glucocorticoid-responsive unit composed of two overlapping inactive receptor-binding sites: evidence for formation of a receptor tetramer.

An unusual glucocorticoid-responsive element (called GRE A) was found to mediate the induction of the cytosolic aspartate aminotransferase gene by glucocorticoids and was bound by the glucocorticoid receptor in a DNase I footprinting assay. GRE A consists of two overlapping GREs, each comprising a conserved half-site and an imperfect half-site. The complete unit was able to confer glucocorticoid inducibility to a heterologous promoter (delta MTV-CAT). Mutation of any of the half-sites, including the imperfect ones, abolished inducibility by the hormone, demonstrating that each of the isolated GREs was inactive. In electrophoretic mobility shift assays, purified rat liver glucocorticoid receptor (GR) formed a low-mobility complex with GRE A, presumably containing a GR tetramer. When purified bacterially expressed DBD was used, low-mobility complexes as well as dimer and monomer complexes were formed. In inactive mutated oligonucleotides, no GR tetramer formation was detected. Modification of the imperfect half-sites in order to increase their affinity for GR gave a DNA sequence that bound a GR tetramer in a highly cooperative manner. This activated unit consisting of two overlapping consensus GREs mediated glucocorticoid induction with a higher efficiency than consensus GRE.     

UV cross-linking/immunoprecipitation assay: glucocorticoid receptor-adrenergic receptor gene sequence interaction

The rat beta 1-adrenergic receptor (beta 1-AR) gene contains glucocorticoid response element (GRE) half-sites at positions -2767 and -945. In electrophoretic mobility shift assay (EMSA) experiments, neither beta 1-AR GRE half-site recognized glucocorticoid receptors (GRs) obtained from baculovirus high-level expression systems or from mammalian cells. We have developed a sensitive UV cross-linking/immunoprecipitation assay, using a 524-bp fragment containing the prototypical GRE obtained from the rat tyrosine aminotransferase promoter sequence and using antibodies recognizing mammalian GR. Using this assay, we provide evidence that rat beta 1-AR gene sequences recognize mammalian GRs expressed in mouse 3T3 cells and that the site of GR interaction does not appear to specifically contain the beta 1-AR GRE half-sites. This represents one of the first reports demonstrating the utility of a UV cross-linking/immunoprecipitation assay in the detection of mammalian GR interaction with beta 1-AR sequences, is consistent with the lack of specific DNA-GR protein complexes observed in EMSA experiments using oligonucleotide probes containing the beta 1-AR GRE half-sites, and provides evidence that mammalian GR interaction occurs at complex rate beta 1-AR gene sequences.     

Dynamics of gene targeting and chromatin remodelling by nuclear receptors

Activation of the murine-mammary-tumour virus (MMTV) promoter by the glucocorticoid receptor (GR) is associated with a chromatin structural transition in the B nucleosome region of the viral long terminal repeat (LTR). We have reconstituted this nucleoprotein transition with chromatin assembled on MMTV LTR DNA with Drosophila embryo extracts, purified GR, and HeLa nuclear extract. Chromatin remodelling in vitro is ATP-dependent and maps to a region identical with that found in vivo. We demonstrate specific, glucocorticoid response element dependent, binding of purified GR to a large, multi-nucleosome MMTV chromatin array and show that GR-dependent chromatin remodelling is a multistep process. In the absence of ATP, GR binds to multiple sites on the chromatin array and inhibits nuclease access to GR recognition sites. On the addition of ATP, GR induces remodelling resulting in a large increase in access of enzymes to their sites within the transition region. These findings are complemented by studies in living cells; using a tandem array of MMTV-Ras reporter elements and a form of GR labelled with the green fluorescent protein, we have observed direct targeting of the receptor to response elements in live mouse cells. Whereas the ligand-activated receptor is associated with the MMTV promoter for observable periods, photobleaching experiments provide direct evidence that the hormone-occupied receptor undergoes rapid exchange between chromatin and the nucleoplasmic compartment. The results both in vitro and in vivo are consistent with a dynamic model ('hit and run') in which GR first binds to chromatin after ligand activation, recruits a remodelling activity and is then lost from the template.