Dexamethasone increases the number of RNA polymerase II molecules transcribing integrated mouse mammary tumor virus DNA and flanking mouse sequences

Dynamic regulation of glucocorticoid signalling in health and disease

Activation of the glucocorticoid receptor (GR) by endogenous and synthetic glucocorticoids regulates hundreds of genes to control regulatory networks in development, metabolism, cognition and inflammation. Elucidation of the mechanisms that regulate glucocorticoid action has highlighted the dynamic nature of hormone signalling and provides novel insights into genomic glucocorticoid actions. The major factors that regulate GR function include chromatin structure, epigenetics, genetic variation and the pattern of glucocorticoid hormone secretion. We review our current understanding of the mechanisms that contribute to GR signalling and how these contribute to glucocorticoid sensitivity, resistance and side effects.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.

Chromatin structure of recombinant Moloney murine leukemia virus proviral DNAs that contain tax-responsive sequences from human T-cell lymphotropic virus type II in the presence and absence of tax

Human T-cell lymphotropic virus types I and II (HTLV-I and HTLV-II) are replication-competent retroviruses which contain two additional regulatory proteins, tax and rex. tax is a transcriptional transactivator of the HTLV-I or HTLV-II long terminal repeat (LTR) and also of some heterologous promoters. To investigate the mechanism of tax transactivation, we used chimeric Moloney murine leukemia viruses (M-MuLVs) with LTRs containing tax-responsive sequences from the HTLV-II LTR (nucleotides -273 to -32). Mo+HTLV-II+ M-MuLV contained the HTLV II sequences inserted into the wild-type M-MuLV LTR at nucleotide -150, whereas delta Mo+HTLV-II+ M-MuLV contained the same sequences inserted into an M-MuLV LTR lacking its own enhancer region. HTLV-II tax (tax II)-positive mouse cells (15S-5a) infected with Mo+HTLV-II+ M-MuLV or delta Mo+HTLV-II+ M-MuLV showed higher rates of viral transcription in nuclear run-on assays than did infected tax-negative NIH 3T3 cells. The chromatin structure of these viruses was investigated by high-resolution mapping of DNase I-hypersensitive (HS) sites. Three prominent HS sites were associated with HTLV-II sequences in proviral chromatin both in tax-positive and in tax-negative cells. The spacing resembled that of the 21-base-pair (bp) repeats, but the HS sites were displaced approximately 50 bp upstream of the 21-bp repeats. This suggested that cellular proteins bound to the HTLV-II sequences in the presence or absence of tax. No direct effect of tax on chromatin structure was found. These in vivo results were consistent with results of in vitro DNase footprinting studies performed by other investigators.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.

Dual function of a nuclear factor I binding site in MMTV transcription regulation

Using linker-scanning mutagenesis we had previously identified four elements within the MMTV LTR which are necessary for transcriptional stimulation by glucocorticoid hormones. Two of them overlapped with regions to which the glucocorticoid receptor binds in vitro. The third element contained a NF-I binding site, and the fourth the TATA box. Here we show that mutations that abolish in vitro binding of NF-I had a negative effect also on the basal activity of the MMTV promoter of LTR-containing plasmids stably integrated in Ltk- fibroblasts. The analysis of double mutants altered in the NF-I plus either one of the receptor binding elements further demonstrated that the NF-I site functionally cooperated with the proximal (-120) element, which alone was extremely inefficient in stimulation. The stronger distal (-181/-172) element was independent of NF-I and showed functional cooperativity with the proximal hormone-binding element.

Chromatin structure of hormone-responsive Moloney murine leukemia virus proviruses that contain sequences from mouse mammary tumor virus

The chromatin structure of chimeric Moloney murine leukemia viruses (M-MuLVs) containing a glucocorticoid response element (GRE) from mouse mammary tumor virus (MMTV) inserted into the long terminal repeat (LTR) was investigated. Nuclear run-on assays indicated that transcription from the chimeric proviruses was induced 2- to 4-fold by dexamethasone. The wild-type M-MuLV 5' LTR contained a DNase I hypersensitive (HS) site at the TATA sequences, as well as four sites in the enhancer region. The chimeric LTRs contained these sites, as well as three additional sites in the MMTV sequences. Two of the MMTV sites were present in the absence of hormone, while one was hormone-induced. In addition, internal MMTV sequences appeared protected from DNase I digestion in the absence of hormone, suggesting bound protein. Hormone treatment resulted in loss of the DNase I protection.

Regulatory elements within the murine leukemia virus enhancer regions mediate glucocorticoid responsiveness

Enhancer elements within nonleukemogenic (Akv) and T-cell leukemogenic (SL3-3) murine leukemia viruses demonstrate strong cell type preference in transcriptional activity. These transcription elements are additionally regulated by the synthetic glucocorticoid dexamethasone, and this pattern of regulation varies according to cell type. The sequences required for dexamethasone regulation for both Akv and SL3-3 are shown to include a 17-nucleotide consensus sequence previously termed the glucocorticoid response element (GRE). Although the GREs are identical for both viral enhancers, the sequences surrounding these elements differ, as does the spatial arrangement of the GRE sequences with respect to one another. It is proposed that the spatial arrangement of the GREs, as well as their precise sequence context, determines the difference in the response to dexamethasone of the enhancers in different cell types.

Regulation by aldosterone of Na+,K+-ATPase mRNAs, protein synthesis, and sodium transport in cultured kidney cells

Transepithelial Na+ reabsorption across tight epithelia is regulated by aldosterone. Mineralocorticoids modulate the expression of a number of proteins. Na+,K+-ATPase has been identified as an aldosterone-induced protein (Geering, K., M. Girardet, C. Bron, J. P. Kraehenbuhl, and B. C. Rossier, 1982, J. Biol. Chem., 257:10338-10343). Using A6 cells (kidney of Xenopus laevis) grown on filters we demonstrated by Northern blot analysis that the induction of Na+,K+-ATPase was mainly mediated by a two- to fourfold accumulation of both alpha- and beta-subunit mRNAs. The specific competitor spironolactone decreased basal Na+ transport, Na+,K+-ATPase mRNA, and the relative rate of protein biosynthesis, and it blocked the response to aldosterone. Cycloheximide inhibited the aldosterone-dependent sodium transport but did not significantly affect the cytoplasmic accumulation of Na+,K+-ATPase mRNA induced by aldosterone.

Functional dissection of the hormone and DNA binding activities of the glucocorticoid receptor

We have identified two separate regions of the 795 amino acid rat glucocorticoid receptor that interact with hormonal ligands and DNA respectively. The functional regions were defined by direct assays of segments of the receptor coding sequence translated in vitro. Hormone affinity measurements suggested that residues near the receptor C-terminus are the primary determinants of ligand binding, whereas sequence-specific DNA binding activity resides between amino acids 440 and 546. DNA binding efficiency was stimulated only modestly by prior hormone binding. The receptor regions identified in these in vitro studies correspond to those that mediate ligand-dependent transcriptional enhancement in vivo.