Interleukin-6 repression is associated with a distinctive chromatin structure of the gene

Phenylmethimazole and a thiazole derivative of phenylmethimazole inhibit IL-6 expression by triple negative breast cancer cells

Inhibition of interleukin-6 (IL-6) holds significant promise as a therapeutic approach for triple negative breast cancer (TNBC). We previously reported that phenylmethimazole (C10) reduces IL-6 expression in several cancer cell lines. We have identified a more potent derivative of C10 termed COB-141. In the present work, we tested the hypothesis that C10 and COB-141 inhibit TNBC cell expressed IL-6 and investigated the potential for classical IL-6 pathway induced signaling within TNBC cells. A panel of TNBC cell lines (MDA-MB-231, Hs578T, MDA-MB-468) was used. Enzyme linked immunosorbent assays (ELISA) revealed that C10 and COB-141 inhibit MDA-MB-231 cell IL-6 secretion, with COB-141 being ~6.5 times more potent than C10. Therefore, the remainder of the study focused on COB-141 which inhibited IL-6 secretion, and was found, via quantitative real time polymerase chain reaction (QRT-PCR), to inhibit IL-6 mRNA in the TNBC panel. COB-141 had little, if any, effect on metabolic activity indicating that the IL-6 inhibition is not via a toxic effect. Flow cytometric analysis and QRT-PCR revealed that the TNBC cell lines do not express the IL-6 receptor (IL-6Rα). Trans-AM assays suggested that COB-141 exerts its inhibitory effect, at least in part, by reducing NF-κB (p65/p50) DNA binding. In summary, COB-141 is a potent inhibitor of TNBC cell expressed IL-6 and the inhibition does not appear to be due to non-specific toxicity. The TNBC cell lines do not have an intact classical IL-6 signaling pathway. COB-141's inhibitory effect may be due, at least in part, to reducing NF-κB (p65/p50) DNA binding.

Characterization, promoter analysis and expression of the interleukin-6 gene in blunt snout bream, Megalobrama amblycephala

Interleukin-6 (IL-6) is one of the most important multifunctional cytokines, playing essential roles in mediating the innate and adaptive immune responses. In this study, il-6 gene and its promoter from blunt snout bream, Megalobrama amblycephala, were characterized, and its expression at the transcript level in healthy fish and after bacterial infection was determined by quantitative real-time PCR. The results showed that the M. amblycephala il-6 (Mamil-6) cDNA had an ORF of 699 bp, encoding 232 amino acids, and contained 9 instable motifs in the 3' UTR. The deduced MamIL-6 possessed a 24-amino acid signal peptide and was located in the cytoplasm. Although sequence alignment and phylogenetic analysis revealed that IL-6 is poorly conserved in vertebrates, the protein and genomic structure of il-6 gene was well conserved. Analysis of the Mamil-6 promoter revealed the presence of a conserved TATA box and six major cis-regulatory elements, including C/EBPβ (NF-IL6), AP-1, CRE, GRE, GATA and NF-κB binding sites. In healthy fish, Mamil-6 was the most abundant in the spleen. After Aeromonas hydrophila infection, Mamil-6 was significantly up-regulated in all 6 immune-related tissues examined, suggesting that Mamil-6 plays an important role in the blunt snout bream immune system.

ARTD1 Suppresses Interleukin 6 Expression by Repressing MLL1-Dependent Histone H3 Trimethylation

ADP-ribosyltransferase diphtheria-toxin like 1/poly(ADP-ribose) polymerase 1 (ARTD1/PARP1) is a chromatin-associated protein in the nucleus and plays an important role in different cellular processes such as regulation of gene transcription. ARTD1 has been shown to coregulate the inflammatory response by modulating the activity of the transcription factor nuclear factor κB (NF-κB), the principal regulator of interleukin 6 (IL-6), an important inflammatory cytokine implicated in a variety of diseases such as cancer. However, to what extent and how ARTD1 regulates IL-6 transcription has not been clear. Here, we show that ARTD1 suppresses lipopolysaccharide (LPS)-induced IL-6 expression in macrophages, without affecting the recruitment of the NF-κB subunit RelA to the IL-6 promoter and independent of its enzymatic activity. Interestingly, knockdown of ARTD1 did not alter H3 occupancy but increased LPS-induced trimethylation of histone 3 at lysine 4 (H3K4me3), a hallmark of transcriptionally active genes. We found that ARTD1 mediates its effect through the methyltransferase MLL1, by catalyzing H3K4me3 at the IL-6 promoter and forming a complex with NF-κB. These results demonstrate that ARTD1 modulates IL-6 expression by regulating the function of an NF-κB enhanceosome complex, which involves MLL1 and does not require ADP-ribosylation.

Regulation of the Interleukin-6 gene expression during monocytic differentiation of HL-60 cells by chromatin remodeling and methylation

The pro-inflammatory cytokine Interleukin (IL)-6 is involved in the proliferation and differentiation of leukocytes and non-immune cells, but its overproduction is associated with inflammatory and autoimmune disorders. The main producers of IL-6 are mature monocytes, whereas progenitor cells and the promyeloid cell line HL-60 do not synthesize IL-6. In contrast, HL-60 cells differentiated into monocytic cells were able to express IL-6 after lipopolysaccharide (LPS) stimulation. This study investigated the chromatin structure of the IL-6 promoter and the effect of methylation on IL-6 gene regulation during monopoiesis. The results show that the proximal IL-6 promoter regions I to III (+13/-329) were inaccessible in undifferentiated HL-60 cells but became significantly accessible in differentiated HL-60 cells stimulated with LPS. Region IL-6 VI (-1099/-1142) remained closed, but the upstream region IL-6 VII (-2564/-2877) relaxed after differentiation and LPS treatment. The opening of IL-6 IV (-309/-521) and IL-6V (-500/-722), containing DNA and histone methylation sites, was differentiation-dependent only. Demethylation experiments using 5-aza-2'-deoxycytidine (AZA) followed by LPS stimulation revealed a significant enhanced IL-6 mRNA expression and protein release by HL-60 cells. AZA treatment resulted in significant increased IL-6 promoter accessibilities, identifying methylation as an important repressor of IL-6 gene regulation in promyeloid cells. The histone deacetylase (HDAC) inhibitor trichostatin A (TSA) had no effect on IL-6 promoter accessibility. Our data indicate that during monopoiesis the proximal IL-6 promoter is reorganized into an accessible conformation allowing transcription of IL-6 after LPS stimulation. DNA methylation appears to be the essential epigenetic mechanism in IL-6 gene expression of mature monocytes and their progenitors by controlling the chromatin structure.

Binding of NF-κB to nucleosomes: effect of translational positioning, nucleosome remodeling and linker histone H1

NF-κB is a key transcription factor regulating the expression of inflammatory responsive genes. How NF-κB binds to naked DNA templates is well documented, but how it interacts with chromatin is far from being clear. Here we used a combination of UV laser footprinting, hydroxyl footprinting and electrophoretic mobility shift assay to investigate the binding of NF-κB to nucleosomal templates. We show that NF-κB p50 homodimer is able to bind to its recognition sequence, when it is localized at the edge of the core particle, but not when the recognition sequence is at the interior of the nucleosome. Remodeling of the nucleosome by the chromatin remodeling machine RSC was not sufficient to allow binding of NF-κB to its recognition sequence located in the vicinity of the nucleosome dyad, but RSC-induced histone octamer sliding allowed clearly detectable binding of NF-κB with the slid particle. Importantly, nucleosome dilution-driven removal of H2A–H2B dimer led to complete accessibility of the site located close to the dyad to NF-κB. Finally, we found that NF-κB was able to displace histone H1 and prevent its binding to nucleosome. These data provide important insight on the role of chromatin structure in the regulation of transcription of NF-κB dependent genes.

In eukaryotes DNA is hierarchically packaged into chromatin by histones. The fundamental subunit of chromatin is the nucleosome. The packaging of DNA into nucleosomes not only restricts DNA accessibility for regulatory proteins but also provides opportunities to regulate DNA based processes. Accessibility of transcription factor NF-κB to their recognition sequences embedded in nucleosomes is highly controversial. On one hand in vivo studies have suggested that packaging of DNA into chromatin plays an important role in regulating the expression of NF-κB dependent genes, and on the other hand some in vitro studies reported that NF-κB can bind by itself to its recognition sequences embedded in the nucleosome. In this study, we show that NF-κB can specifically bind to its recognition sequences placed at the end of the nucleosome but not when placed inside the nucleosome core. We then demonstrate that disruption of nucleosome is necessary for the productive binding of NF-κB. Finally, we show that the presence of histone H1 does not affect the specific binding of NF-κB to its cognate sequence, when its binding region overlaps with the binding site of NF-κB. We propose that histone eviction is needed for NF-κB to bind specifically to its recognition sequence embedded in the nucleosome.

Heteroarylketones inhibit astroglial interleukin-6 expression via a STAT3/NF-κB signaling pathway

Background

Elevated brain levels of the pleiotropic cytokine interleukin-6, which is mainly secreted from activated local astrocytes, contribute to pathological events including neuroinflammation and neurodegeneration. Thus, inhibition of pathological IL-6 expression provides a rationale strategy for targeting the onset or further progression of neurological disorders including Alzheimer's disease, multiple sclerosis, Parkinson's disease and traumatic brain injury. The purpose of this study was to identify and to characterize new potent inhibitors of astrocytic IL-6 expression for further therapeutic development of novel anti-inflammatory and neuroprotective drugs.

Methods

Oncostatin M (OSM)-treated human glioma U343 cells were used as model for induction of astrocytic IL-6 expression. This model was characterized by immunoblotting, siRNA technique, ELISA and qRT-PCR and used to screen low molecular weight compound libraries for IL-6-lowering effects. To validate bioactive compounds identified from library screens, bacterial lipopolysaccharide was used to induce IL-6 expression in cultivated primary astrocytes and in mice in vivo. To dissect underlying molecular mechanisms, protein extracts from OSM-treated U343 cells were analyzed by phospho-specific immunoblotting and immunocytochemistry as well as by co-immunoprecipitation.

Results

OSM-treatment (100 ng/ml; 24 h) led to 30-fold increase of IL-6 secretion from U343 cells. The temporal profile of IL-6 mRNA induction displayed a biphasic induction pattern with peak synthesis at 1 h (6.5-fold) and 16 h (5.5-fold) post stimulation. IL-6 protein release did not show that biphasic pattern and was detected as early as 3 h post stimulation reaching a maximum at 24 h. The screen of compound libraries identified a set of heteroarylketones (HAKs) as potent inhibitors of IL-6 secretion. HAK compounds affected the second peak in IL-6 mRNA synthesis, whereas the first peak was insensitive to HAK treatment. HAK compounds also suppressed lipopolysaccharide-induced IL-6 expression in primary murine astrocytes as well as in brain and plasma samples from lipopolysaccharide-treated mice. Finally, HAK compounds were demonstrated to specifically suppress the OSM-induced phosphorylation of STAT3 at serine 727 and the physical interaction of pSTAT3^S727 with p65.

Conclusion

Heteroarylketone compounds are potent inhibitors of IL-6 expression in vitro and in vivo and may represent a new class of potent anti-inflammatory and neuroprotective drugs.

Mechanistic insights into the events that lead to synergistic induction of interleukin 6 transcription upon activation of the aryl hydrocarbon receptor and inflammatory signaling

The aryl hydrocarbon receptor (AHR) is the ligand-activated transcription factor responsible for mediating the toxicological effects of dioxin and xenobiotic metabolism. However, recent evidence has implicated the AHR in additional, nonmetabolic physiological processes, including immune regulation. Certain tumor cells are largely nonresponsive to cytokine-mediated induction of the pro-survival cytokine interleukin (IL) 6. We have demonstrated that multiple nonresponsive tumor lines are able to undergo synergistic induction of IL6 following combinatorial treatment with IL1beta and the AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin. Such data implicate the AHR in tumor expansion, although the mechanistic basis for the AHR-dependent synergistic induction of IL6 has not been determined. Here, we demonstrate that ligand-activated AHR is involved in priming the IL6 promoter through binding to nonconsensus dioxin response elements located upstream of the IL6 start site. Such binding appears to render the promoter more permissive to IL1beta-induced binding of NF-kappaB components. The nature of the AHR-dependent increases in IL6 promoter transcriptional potential has been shown to involve a reorganization of repressive complexes as exemplified by the presence of HDAC1 and HDAC3. Dismissal of these HDACs correlates with post-translational modifications of promoter-bound NF-kappaB components in a time-dependent manner. Thus the AHR plays a role in derepressing the IL6 promoter, leading to synergistic IL6 expression in the presence of inflammatory signals. These observations may explain the association between enhanced expression of AHR and tumor aggressiveness. It is likely that AHR-mediated priming is not restricted to the IL6 promoter and may contribute to the expression of a variety of genes, which do not have consensus dioxin response elements.

MeCP2/H3meK9 are involved in IL-6 gene silencing in pancreatic adenocarcinoma cell lines

The aim of the present study was to analyse the molecular mechanisms involved in the Interleukin-6 (IL-6) silencing in pancreatic adenocarcinoma cell lines. Our results demonstrate that TNF-alpha, a major IL-6 inducer, is able to induce IL-6 only in three out of six cell lines examined. 5-aza-2'-deoxycytidine (DAC), but not trichostatin A (TSA), activates the expression of IL-6 in all cell lines, indicating that DNA methylation, but not histone deacetylation, plays an essential role in IL-6 silencing. Indeed, the IL-6 upstream region shows a methylation status that correlates with IL-6 expression and binds MeCP2 and H3meK9 only in the non-expressing cell lines. Our results suggest that critical methylations located from positions -666 to -426 relative to the transcription start site of IL-6 may act as binding sites for MeCP2.

Hyperactivated NF-{kappa}B and AP-1 transcription factors promote highly accessible chromatin and constitutive transcription across the interleukin-6 gene promoter in metastatic breast cancer cells

Interleukin-6 (IL-6), involved in cancer-related inflammation, acts as an autocrine and paracrine growth factor, which promotes angiogenesis, metastasis, and subversion of immunity, and changes the response to hormones and to chemotherapeutics. We explored transcription mechanisms involved in differential IL-6 gene expression in breast cancer cells with different metastatic properties. In weakly metastatic MCF7 cells, histone H3 K9 methylation, HP1 binding, and weak recruitment of AP-1 Fra-1/c-Jun, NF-kappaB p65 transcription factors, and coactivators is indicative of low chromatin accessibility and gene transcription at the IL-6 gene promoter. In highly metastatic MDA-MB231 cells, strong DNase, MNase, and restriction enzyme accessibility, as well potent constitutive transcription of the IL-6 gene promoter, coincide with increased H3 S10 K14 phosphoacetylation and promoter enrichment of AP-1 Fra-1/c-Jun and NF-kappaB p65 transcription factors and MSK1, CBP/p300, Brg1, and Ezh2 cofactors. Complementation, silencing, and kinase inhibitor experiments further demonstrate involvement of AP-1 Fra-1/c-Jun and NF-kappaB p65/RelB members, but not of the alpha estrogen receptor in promoting chromatin accessibility and transcription across the IL-6 gene promoter in metastatic breast cancer cells. Finally, the natural withanolide Withaferin A was found to repress IL-6 gene transcription in metastatic breast cancer cells upon dual inhibition of NF-kappaB and AP-1 Fra-1 transcription factors and silencing of IL-6 promoter chromatin accessibility.

Inflammatory signaling and aryl hydrocarbon receptor mediate synergistic induction of interleukin 6 in MCF-7 cells

The pleiotropic cytokine interleukin 6 (IL-6) is involved in immune cell homeostasis. Additionally, IL-6 expression and signaling in tumor cells have been shown to elicit both protumor and antitumor properties. There is a plethora of mechanistic knowledge regarding how IL-6 signal transduction translates to biological responses. However, there is little understanding as to what factors control IL-6 expression within a tumor cell environment. The studies presented herein show that, in MCF-7 breast and ECC-1 endocervical cancer cells, the stimulation of aryl hydrocarbon receptor (AHR) activity, in combination with IL-1beta or phorbol 12-myristate 13-acetate (PMA) treatment, results in a marked synergistic induction of IL-6 levels over what is seen without AHR activation. Chromatin immunoprecipitation experiments suggest that the regulation of IL-6 mRNA expression occurs at the chromatin level, as AHR presence on the IL-6 promoter was observed in response to treatment with AHR ligand. Synergistic induction of IL-6 expression was sustained for 72 hours, with accumulation of IL-6 protein reaching levels 4.8-fold above IL-1beta treatment alone. In addition, transcriptional regulation of the prototypic AHR responsive gene Cyp1a1 was negatively regulated by PMA and IL-1beta treatment. Silencing of RELA expression alleviated IL-1beta-mediated repression of AHR transcriptional activity, whereas PMA-mediated repression was maintained. Additionally, small interfering RNA studies reveal that AHR and RELA are necessary for synergistic induction of IL-6. The findings presented here reveal the AHR as a potential therapeutic target for selective modulation of IL-6 expression in some tumor cell types. The data also suggest a possible previously unrecognized mechanism of AHR-mediated tumor promotion.

Identification of a novel regulatory region in the interleukin-6 gene promoter

Interleukin-6 (IL6) is an important pleiotropic cytokine that is regulated at the transcriptional level. To date, most work on its regulation has focused on a 1.2kb region 5' from the start of transcription, similar to published reports on other cytokine genes. This report demonstrates for the first time that a cytokine gene can be regulated by cis-acting regions much further upstream than previously examined. Comparative genomic analysis showed that a 120 kb region contains blocks of sequence conservation between human and rodent genomes, and that a 15 kb region proximal to the start of transcription contains 10 highly homologous sequence blocks of between 100 and 250 bp. By means of a reporter gene assay, a novel transcriptionally active region located between -5307 and -5202 bp upstream from the start of transcription was identified. Electrophoretic mobility shift assays showed nuclear protein(s) binding to this region, thus raising the possibility that the regulatory activity shown by the reporter gene constructs may be mediated by these proteins. These results suggest that the regulation of IL6 expression involves a much larger upstream region than previously examined and the control of IL6 transcription is likely to be regulated by a complex mechanism of modular cis-regulatory elements.

Potentiation of NF-kappaB-dependent transcription and inflammatory mediator release by histamine in human airway epithelial cells

BACKGROUND AND PURPOSE

In asthma, histamine contributes to bronchoconstriction, vasodilatation and oedema, and is associated with the late phase response. The current study investigates possible inflammatory effects of histamine acting on nuclear factor kappaB (NF-kappaB)-dependent transcription and cytokine release.

EXPERIMENTAL APPROACH

Using BEAS-2B bronchial epithelial cells, NF-kappaB-dependent transcription and both release and mRNA expression of IL-6 and IL-8 were examined by reporter assay, ELISA and quantitative RT-PCR. Histamine receptors were detected using qualitative RT-PCR and function examined using selective agonists and antagonists.

KEY RESULTS

Addition of histamine to TNFalpha-stimulated BEAS-2B cells maximally potentiated NF-kappaB-dependent transcription 1.8 fold, whereas IL-6 and IL-8 protein release were enhanced 7.3- and 2.7-fold respectively. These responses were, in part, NF-kappaB-dependent and were associated with 2.6- and 1.7-fold enhancements of IL-6 and IL-8 mRNA expression. The H(1) receptor antagonist, mepyramine, caused a rightward shift in the concentration-response curves of TNFalpha-induced NF-kappaB-dependent transcription (pA(2)=9.91) and release of IL-6 (pA(2)=8.78) and IL-8 (pA(2)=8.99). Antagonists of histamine H(2), H(3) and H(4) receptors were without effect. Similarly, H(3) and H(4) receptor agonists did not affect TNFalpha-induced NF-kappaB-dependent transcription, or IL-6 and IL-8 release at concentrations below 10 microM. The anti-inflammatory glucocorticoid, dexamethasone, inhibited the histamine enhanced NF-kappaB-dependent transcription and IL-6 and IL-8 release.

CONCLUSIONS AND IMPLICATIONS

Potentiation of NF-kappaB-dependent transcription and inflammatory cytokine release by histamine predominantly involves receptors of the H(1) receptor subtype. These data support an anti-inflammatory role for H(1) receptor antagonists by preventing the transcription and release of pro-inflammatory cytokines.

A critical role for p53 in the control of NF-kappaB-dependent gene expression in TLR4-stimulated dendritic cells exposed to Genistein

Considerable research has focused on the anti-inflammatory and antiproliferative activities exhibited by the soy isoflavone genistein. We previously demonstrated that genistein suppresses TNF-alpha-induced NF-kappaB-dependent IL-6 gene expression in cancer cells by interfering with the mitogen- and stress-activated protein kinase 1 activation pathway. However, effects of isoflavones on immune cells, such as dendritic cells, remain largely unknown. Here we show that genistein markedly reduces IL-6 cytokine production and transcription in LPS-stimulated human monocyte-derived dendritic cells. More particularly, we observe that genistein inhibits IL-6 gene expression by modulating the transcription factor NF-kappaB. Examination of NF-kappaB-related events downstream of TLR4 demonstrates that genistein affects NF-kappaB subcellular localization and DNA binding, although we observe only a minor inhibitory impact of genistein on the classical LPS-induced signaling steps. Interestingly, we find that genistein significantly increases p53 protein levels. We also show that overexpression of p53 in TLR4/MD2 HEK293T cells blocks LPS-induced NF-kappaB-dependent gene transcription, indicating the occurrence of functional cross-talk between p53 and NF-kappaB. Moreover, analysis of IL-6 mRNA levels in bone marrow-derived p53 null vs wild-type dendritic cells confirms a role for p53 in the reduction of NF-kappaB-dependent gene expression, mediated by genistein.

Keeping up NF-κB appearances: Epigenetic control of immunity or inflammation-triggered epigenetics

Controlled expression of cytokine genes is an essential component of an immune response and is crucial for homeostasis. In order to generate an appropriate response to an infectious condition, the type of cytokine, as well as the cell type, dose range and the kinetics of its expression are of critical importance. The nuclear factor-kappaB (NF-kappaB) family of transcription factors has a crucial role in rapid responses to stress and pathogens (innate immunity), as well as in development and differentiation of immune cells (acquired immunity). Although quite a number of genes contain NF-kappaB-responsive elements in their regulatory regions, their expression pattern can significantly vary from both a kinetic and quantitative point of view, reflecting the impact of environmental and differentiative cues. At the transcription level, selectivity is conferred by the expression of specific NF-kappaB subunits and their respective posttranslational modifications, and by combinatorial interactions between NF-kappaB and other transcription factors and coactivators, that form specific enhanceosome complexes in association with particular promoters. These enhanceosome complexes represent another level of signaling integration, whereby the activities of multiple upstream pathways converge to impress a distinct pattern of gene expression upon the NF-kappaB-dependent transcriptional network. Today, several pieces of evidence suggest that the chromatin structure and epigenetic settings are the ultimate integration sites of both environmental and differentiative inputs, determining proper expression of each NF-kappaB-dependent gene. We will therefore discuss in this review the multilayered interplay of NF-kappaB signaling and epigenome dynamics, in achieving appropriate gene expression responses and transcriptional activity.

Inhibition of inflammatory cytokine secretion from mouse microglia cells by DHMEQ, an NF-κB inhibitor

Activation of microglia has been implicated in various neurodegenerative disorders, and thus the inhibition of microglial activity may suppress these disorders. Earlier we designed and synthesized an NF-kappaB inhibitor, dehydroxymethylepoxyquinomicin (DHMEQ) that showed anti-inflammatory and anti-tumor activities in vivo. In the present research, we studied whether DHMEQ would inhibit the activation of mouse microglial cells. DHMEQ inhibited lipopolysaccharide (LPS)-induced activation of NF-kappaB in an electrophoresis mobility shift assay. It also inhibited LPS-induced secretions of TNF-alpha and IL-6 from mouse microglial cell line 6-1 cells.

Glucocorticoid receptor activation of the I kappa B alpha promoter within chromatin

The glucocorticoid receptor (GR) is a ligand-activated transcription factor that induces expression of many genes. The GR has been useful for understanding how chromatin structure regulates steroid-induced transcription in model systems. However, the effect of glucocorticoids on chromatin structure has been examined on few endogenous mammalian promoters. We investigated the effect of glucocorticoids on the in vivo chromatin structure of the glucocorticoid-responsive I kappa B alpha gene promoter, the inhibitor of the ubiquitous transcription factor, nuclear factor kappa B (NF kappa B). Glucocorticoids inhibit NF kappa B activity in some tissues by elevating the levels of I kappa B alpha. We found that glucocorticoids activated the I kappa B alpha promoter in human T47D/A1-2 cells containing the GR. We then investigated the chromatin structure of the I kappa B alpha promoter in the absence and presence of glucocorticoids with the use of micrococcal nuclease, restriction enzyme, and deoxyribonuclease (DNaseI) analyses. In untreated cells, the promoter assembles into regularly positioned nucleosomes, and glucocorticoid treatment did not alter nucleosomal position. Restriction enzyme accessiblity studies indicated that the I kappa B alpha promoter is assembled as phased nucleosomes that adopt an "open" chromatin architecture in the absence of hormone. However, glucocorticoids may be required for transcription factor binding, because DNaseI footprinting studies suggested that regulatory factors bind to the promoter upon glucocorticoid treatment.

The mouse mammary tumor virus promoter adopts distinct chromatin structures in human breast cancer cells with and without glucocorticoid receptor

Steroid receptors represent a class of transcription regulators that act in part by overcoming the often repressive nature of chromatin to modulate gene activity. The mouse mammary tumor virus (MMTV) promoter is a useful model for studying transcriptional regulation by steroid hormone receptors in the context of chromatin. The chromatin architecture of the promoter prevents the assembly of basal transcription machinery and binding of ubiquitous transcription factors. However, in human breast carcinoma T47D cells lacking the glucocorticoid receptor (GR), but expressing the progesterone receptor (PR), nucleosome B (nuc B) assumes a constitutively hypersensitive chromatin structure. This correlation led us to test the hypothesis that the chromatin structure of nuc B was dependent on GR expression in T47D cells. To examine this possibility, we stably co-transfected the MMTV promoter and the GR into T47D cells that lacked both the GR and the PR. We found that in T47D cells that lack both the GR and the PR or express only the GR, nuc B assumes a constitutively "open" chromatin structure, which allows hormone independent access by restriction endonucleases and transcription factors. These results suggest that in GR(+)/pr(-) T47D cells, the MMTV chromatin structure permits GR transcriptional activation, independent of chromatin remodeling.