Gabriel Pascual

A SUMOylation-dependent pathway mediates transrepression of inflammatory response genes by PPAR-gamma.

Peroxisome proliferator-activated receptor-γ (PPAR-γ) has essential roles in adipogenesis and glucose homeostasis, and is a molecular target of insulin-sensitizing drugs. Although the ability of PPAR-γ agonists to antagonize inflammatory responses by transrepression of nuclear factor kappa B (NF-κB) target genes is linked to antidiabetic and antiatherogenic actions, the mechanisms remain poorly understood. Here we report the identification of a molecular pathway by which PPAR-γ represses the transcriptional activation of inflammatory response genes in mouse macrophages. The initial step of this pathway involves ligand-dependent SUMOylation of the PPAR-γ ligand-binding domain, which targets PPAR-γ to nuclear receptor corepressor (NCoR)–histone deacetylase-3 (HDAC3) complexes on inflammatory gene promoters. This in turn prevents recruitment of the ubiquitylation/19S proteosome machinery that normally mediates the signal-dependent removal of corepressor complexes required for gene activation. As a result, NCoR complexes are not cleared from the promoter and target genes are maintained in a repressed state. This mechanism provides an explanation for how an agonist-bound nuclear receptor can be converted from an activator of transcription to a promoter-specific repressor of NF-κB target genes that regulate immunity and homeostasis.

Induced ncRNAs allosterically modify RNA-binding proteins in cis to inhibit transcription

With the recent recognition of non-coding RNAs (ncRNAs) flanking many genes, a central issue is to obtain a full understanding of their potential roles in regulated gene transcription programmes, possibly through different mechanisms. Here we show that an RNA-binding protein, TLS (for translocated in liposarcoma), serves as a key transcriptional regulatory sensor of DNA damage signals that, on the basis of its allosteric modulation by RNA, specifically binds to and inhibits CREB-binding protein (CBP) and p300 histone acetyltransferase activities on a repressed gene target, cyclin D1 (CCND1) in human cell lines. Recruitment of TLS to the CCND1 promoter to cause gene-specific repression is directed by single-stranded, low-copy-number ncRNA transcripts tethered to the 5′ regulatory regions of CCND1 that are induced in response to DNA damage signals. Our data suggest that signal-induced ncRNAs localized to regulatory regions of transcription units can act cooperatively as selective ligands, recruiting and modulating the activities of distinct classes of RNA-binding co-regulators in response to specific signals, providing an unexpected ncRNA/RNA-binding protein-based strategy to integrate transcriptional programmes.

Molecular determinants of crosstalk between nuclear receptors and toll-like receptors

Nuclear receptors (NRs) repress transcriptional responses to diverse signaling pathways as an essential aspect of their biological activities, but mechanisms determining the specificity and functional consequences of transrepression remain poorly understood. Here, we report signal- and gene-specific repression of transcriptional responses initiated by engagement of toll-like receptors (TLR) 3, 4, and 9 in macrophages. The glucocorticoid receptor (GR) represses a large set of functionally related inflammatory response genes by disrupting p65/interferon regulatory factor (IRF) complexes required for TLR4- or TLR9-dependent, but not TLR3-dependent, transcriptional activation. This mechanism requires signaling through MyD88 and enables the GR to differentially regulate pathogen-specific programs of gene expression. PPARgamma and LXRs repress overlapping transcriptional targets by p65/IRF3-independent mechanisms and cooperate with the GR to synergistically transrepress distinct subsets of TLR-responsive genes. These findings reveal combinatorial control of homeostasis and immune responses by nuclear receptors and suggest new approaches for treatment of inflammatory diseases.

Peroxisome proliferator-activated receptor gamma-dependent repression of the inducible nitric oxide synthase gene.

ABSTRACT The peroxisome proliferator-activated receptor γ (PPARγ) is a member of the nuclear receptor superfamily that activates target gene transcription in a ligand-dependent manner. In addition, liganded PPARγ can inhibit transcription of genes induced by gamma interferon (IFN-γ) and/or lipopolysaccharides (LPSs), including the inducible nitric oxide synthase (iNOS) gene. Inhibition of the iNOS promoter is achieved partially through antagonizing the activities of NF-κB, AP-1, and STAT1, which are known to mediate effects of LPS and IFN-γ. Previous studies have suggested that transrepression of these factors by nuclear receptors involves competition for limiting amounts of the general coactivators CREB-binding protein (CBP) and p300. CBP and p300 are thought to be recruited to nuclear receptors through bridging factors that include SRC-1, although CBP also interacts directly with PPARγ through its amino terminus. These observations have raised questions concerning the involvement of SRC-1-like factors in CBP recruitment and transrepression. We here provide evidence that PPARγs ability to repress iNOS transcription requires the ligand-dependent charge clamp that mediates interactions with CBP and SRC-1. Single amino acid mutations in PPARγ that abolished ligand-dependent interactions with SRC-1 and CBP not only resulted in complete loss of transactivation activity but also abolished transrepression. Conversely, a CBP deletion mutant containing the SRC-1 interaction domain but lacking the N-terminal PPARγ interaction domain was inactive as a PPARγ coactivator and failed to rescue transrepression. Together, these findings are consistent with a model in which transrepression by PPARγ is achieved by targeting CBP through direct interaction with its N-terminal domain and via SRC-1-like bridge factors.

Histone H2A Monoubiquitination Represses Transcription by Inhibiting RNA Polymerase II Transcriptional Elongation

Solving the biological roles of covalent histone modifications, including monoubiquitination of histone H2A, and the molecular mechanisms by which these modifications regulate specific transcriptional programs remains a central question for all eukaryotes. Here we report that the N-CoR/HDAC1/3 complex specifically recruits a specific histone H2A ubiquitin ligase, 2A-HUB/hRUL138, to a subset of regulated gene promoters. 2A-HUB catalyzes monoubiquitination of H2A at lysine 119, functioning as a combinatoric component of the repression machinery required for specific gene regulation programs. Thus, 2A-HUB mediates a selective repression of a specific set of chemokine genes in macrophages, critically modulating migratory responses to TLR activation. H2A monoubiquitination acts to prevent FACT recruitment at the transcriptional promoter region, blocking RNA polymerase II release at the early stage of elongation. We suggest that distinct H2A ubiquitinases, each recruited based on interactions with different corepressor complexes, contribute to distinct transcriptional repression programs.

Nuclear receptors versus inflammation: mechanisms of transrepression

Inflammation is a beneficial host response to external challenge or cellular injury that leads to the activation of a complex array of inflammatory mediators, finalizing the restoration of tissue structure and function. Although a beneficial response, prolonged inflammation can be detrimental to the host, contributing to the pathogenesis of many disease states. Considerable attention has been focused on the ability of several members of the nuclear receptor superfamily to inhibit transcriptional activation by signal-dependent transcription factors that include nuclear factor kappaB and activator protein 1, thereby, attenuating inflammatory responses to both acute and chronic challenge. An important general mechanism responsible for this activity is referred to as transrepression, in which nuclear receptors interfere with signal-dependent activation of inflammatory response genes through protein-protein interactions with coregulatory proteins and promoter-bound transcription factors, rather than direct, sequence-specific interactions with DNA.

Control of Proinflammatory Gene Programs by Regulated Trimethylation and Demethylation of Histone H4K20

Regulation of genes that initiate and amplify inflammatory programs of gene expression is achieved by signal-dependent exchange of coregulator complexes that function to read, write, and erase specific histone modifications linked to transcriptional activation or repression. Here, we provide evidence for the role of trimethylated histone H4 lysine 20 (H4K20me3) as a repression checkpoint that restricts expression of toll-like receptor 4 (TLR4) target genes in macrophages. H4K20me3 is deposited at the promoters of a subset of these genes by the SMYD5 histone methyltransferase through its association with NCoR corepressor complexes. Signal-dependent erasure of H4K20me3 is required for effective gene activation and is achieved by NF-κB-dependent delivery of the histone demethylase PHF2. Liver X receptors antagonize TLR4-dependent gene activation by maintaining NCoR/SMYD5-mediated repression. These findings reveal a histone H4K20 trimethylation/demethylation strategy that integrates positive and negative signaling inputs that control immunity and homeostasis.

Narrowing in on Cardiovascular Disease: The Atheroprotective Role of Peroxisome Proliferator-Activated Receptor γ

Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor that has been suggested to play protective roles in the pathogenesis of diseases that are characterized by chronic inflammation, such as atherosclerosis. The study of nuclear receptors, including PPAR gamma, has led to the discovery of anti-inflammatory processes that are collectively known as transrepression. In this review, we will highlight some of the mechanisms of PPAR gamma-mediated transrepression that have surfaced throughout the past decade. We will also discuss the existing evidence for an atheroprotective role of PPAR gamma as a repressor of inflammatory genes and as a key determinant of distinct monocyte-derived subpopulations that may serve an anti-inflammatory, homeostatic role in atherogenesis.

Macrophage peroxisome proliferator activated receptor γ as a therapeutic target to combat Type 2 diabetes

The peroxisome proliferator activated receptor γ is a member of the nuclear receptor superfamily of ligand-dependent transcription factors and is the molecular target of antidiabetic thiazolidinediones that exert insulin sensitizing effects in adipose tissue, skeletal muscle and the liver. In addition to the well described effects of peroxisome proliferator activated receptor γ in insulin target tissues, it is now apparent that its expression in macrophages is critical in the regulation of macrophage phenotype, whole body glucose metabolism and in mediating, in part, the antidiabetic actions of thiazolidinediones. As macrophages are major contributors to tissue inflammation and are resident in tissues responsible for maintaining glucose homeostasis, the therapeutic potential of these cells in the treatment of Type 2 diabetes is of significant clinical interest.