Metabolic labeling and LC-MS/MS-based identification of interleukin-1α-induced secreted proteomes from epithelial cells in the presence or absence of serum.

The unbiased identification of cytokine-induced, secreted proteins from cells cultured in serum-containing medium is challenging. Here, we describe an experimental and bioinformatics workflow to label interleukin-1α-regulated proteins in living cells with the methionine analogue L-homopropargylglycine. We detail their purification and identification by means of CLICK-chemistry-based biotinylation followed by nanoHPLC-MS/MS. A side-by-side comparison of enriched proteins and their ontologies to serum-free conditions demonstrates the sensitivity and specificity of this approach to study the inducible secreted proteomes of epithelial cells.

Thapsigargin: key to new host-directed coronavirus antivirals?

Despite the great success of vaccines that protect against RNA virus infections, and the development and clinical use of a limited number of RNA virus-specific drugs, there is still an urgent need for new classes of antiviral drugs against circulating or emerging RNA viruses. To date, it has proved difficult to efficiently suppress RNA virus replication by targeting host cell functions, and there are no approved drugs of this type. This opinion article discusses the recent discovery of a pronounced and sustained antiviral activity of the plant-derived natural compound thapsigargin against enveloped RNA viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), Middle East respiratory syndrome coronavirus (MERS-CoV), and influenza A virus. Based on its mechanisms of action, thapsigargin represents a new prototype of compounds with multimodal host-directed antiviral activity.

Monitoring the Levels of Cellular NF-κB Activation States.

The NF-κB signaling system plays an important regulatory role in the control of many biological processes. The activities of NF-κB signaling networks and the expression of their target genes are frequently elevated in pathophysiological situations including inflammation, infection, and cancer. In these conditions, the outcome of NF-κB activity can vary according to (i) differential activation states, (ii) the pattern of genomic recruitment of the NF-κB subunits, and (iii) cellular heterogeneity. Additionally, the cytosolic NF-κB activation steps leading to the liberation of DNA-binding dimers need to be distinguished from the less understood nuclear pathways that are ultimately responsible for NF-κB target gene specificity. This raises the need to more precisely determine the NF-κB activation status not only for the purpose of basic research, but also in (future) clinical applications. Here we review a compendium of different methods that have been developed to assess the NF-κB activation status in vitro and in vivo. We also discuss recent advances that allow the assessment of several NF-κB features simultaneously at the single cell level.

Multi-level inhibition of coronavirus replication by chemical ER stress.

Coronaviruses (CoVs) are important human pathogens for which no specific treatment is available. Here, we provide evidence that pharmacological reprogramming of ER stress pathways can be exploited to suppress CoV replication. The ER stress inducer thapsigargin efficiently inhibits coronavirus (HCoV-229E, MERS-CoV, SARS-CoV-2) replication in different cell types including primary differentiated human bronchial epithelial cells, (partially) reverses the virus-induced translational shut-down, improves viability of infected cells and counteracts the CoV-mediated downregulation of IRE1α and the ER chaperone BiP. Proteome-wide analyses revealed specific pathways, protein networks and components that likely mediate the thapsigargin-induced antiviral state, including essential (HERPUD1) or novel (UBA6 and ZNF622) factors of ER quality control, and ER-associated protein degradation complexes. Additionally, thapsigargin blocks the CoV-induced selective autophagic flux involving p62/SQSTM1. The data show that thapsigargin hits several central mechanisms required for CoV replication, suggesting that this compound (or derivatives thereof) may be developed into broad-spectrum anti-CoV drugs.

Regulation of Transcription Factor NF-κB in Its Natural Habitat: The Nucleus.

Activation of the transcription factor NF-κB elicits an individually tailored transcriptional response in order to meet the particular requirements of specific cell types, tissues, or organs. Control of the induction kinetics, amplitude, and termination of gene expression involves multiple layers of NF-κB regulation in the nucleus. Here we discuss some recent advances in our understanding of the mutual relations between NF-κB and chromatin regulators also in the context of different levels of genome organization. Changes in the 3D folding of the genome, as they occur during senescence or in cancer cells, can causally contribute to sustained increases in NF-κB activity. We also highlight the participation of NF-κB in the formation of hierarchically organized super enhancers, which enable the coordinated expression of co-regulated sets of NF-κB target genes. The identification of mechanisms allowing the specific regulation of NF-κB target gene clusters could potentially enable targeted therapeutic interventions, allowing selective interference with subsets of the NF-κB response without a complete inactivation of this key signaling system.

HDAC3 functions as a positive regulator in Notch signal transduction.

Aberrant Notch signaling plays a pivotal role in T-cell acute lymphoblastic leukemia (T-ALL) and chronic lymphocytic leukemia (CLL). Amplitude and duration of the Notch response is controlled by ubiquitin-dependent proteasomal degradation of the Notch1 intracellular domain (NICD1), a hallmark of the leukemogenic process. Here, we show that HDAC3 controls NICD1 acetylation levels directly affecting NICD1 protein stability. Either genetic loss-of-function of HDAC3 or nanomolar concentrations of HDAC inhibitor apicidin lead to downregulation of Notch target genes accompanied by a local reduction of histone acetylation. Importantly, an HDAC3-insensitive NICD1 mutant is more stable but biologically less active. Collectively, these data show a new HDAC3- and acetylation-dependent mechanism that may be exploited to treat Notch1-dependent leukemias.

Distinct IL-1α-responsive enhancers promote acute and coordinated changes in chromatin topology in a hierarchical manner.

How cytokine-driven changes in chromatin topology are converted into gene regulatory circuits during inflammation still remains unclear. Here, we show that interleukin (IL)-1α induces acute and widespread changes in chromatin accessibility via the TAK1 kinase and NF-κB at regions that are highly enriched for inflammatory disease-relevant SNPs. Two enhancers in the extended chemokine locus on human chromosome 4 regulate the IL-1α-inducible IL8 and CXCL1-3 genes. Both enhancers engage in dynamic spatial interactions with gene promoters in an IL-1α/TAK1-inducible manner. Microdeletions of p65-binding sites in either of the two enhancers impair NF-κB recruitment, suppress activation and biallelic transcription of the IL8/CXCL2 genes, and reshuffle higher-order chromatin interactions as judged by i4C interactome profiles. Notably, these findings support a dominant role of the IL8 "master" enhancer in the regulation of sustained IL-1α signaling, as well as for IL-8 and IL-6 secretion. CRISPR-guided transactivation of the IL8 locus or cross-TAD regulation by TNFα-responsive enhancers in a different model locus supports the existence of complex enhancer hierarchies in response to cytokine stimulation that prime and orchestrate proinflammatory chromatin responses downstream of NF-κB.

Single-Cell Analysis of Multiple Steps of Dynamic NF-κB Regulation in Interleukin-1α-Triggered Tumor Cells Using Proximity Ligation Assays.

The frequently occurring heterogeneity of cancer cells and their functional interaction with immune cells in the tumor microenvironment raises the need to study signaling pathways at the single cell level with high precision, sensitivity, and spatial resolution. As aberrant NF-κB activity has been implicated in almost all steps of cancer development, we analyzed the dynamic regulation and activation status of the canonical NF-κB pathway in control and IL-1α-stimulated individual cells using proximity ligation assays (PLAs). These systematic experiments allowed the visualization of the dynamic dissociation and re-formation of endogenous p65/IκBα complexes and the nuclear translocation of NF-κB p50/p65 dimers. PLA combined with immunostaining for p65 or with NFKBIA single molecule mRNA-FISH facilitated the analysis of (i) further levels of the NF-κB pathway, (i) its functionality for downstream gene expression, and (iii) the heterogeneity of the NF-κB response in individual cells. PLA also revealed the interaction between NF-κB p65 and the P-body component DCP1a, a new p65 interactor that contributes to efficient p65 NF-κB nuclear translocation. In summary, these data show that PLA technology faithfully mirrored all aspects of dynamic NF-κB regulation, thus allowing molecular diagnostics of this key pathway at the single cell level which will be required for future precision medicine.

NF-κB p65 dimerization and DNA-binding is important for inflammatory gene expression.

Increasing evidence shows that many transcription factors execute important biologic functions independent from their DNA-binding capacity. The NF-κB p65 (RELA) subunit is a central regulator of innate immunity. Here, we investigated the relative functional contribution of p65 DNA-binding and dimerization in p65-deficient human and murine cells reconstituted with single amino acid mutants preventing either DNA-binding (p65 E/I) or dimerization (p65 FL/DD). DNA-binding of p65 was required for RelB-dependent stabilization of the NF-κB p100 protein. The antiapoptotic function of p65 and expression of the majority of TNF-α-induced genes were dependent on p65's ability to bind DNA and to dimerize. Chromatin immunoprecipitation with massively parallel DNA sequencing experiments revealed that impaired DNA-binding and dimerization strongly diminish the chromatin association of p65. However, there were also p65-independent TNF-α-inducible genes and a subgroup of p65 binding sites still allowed some residual chromatin association of the mutants. These sites were enriched in activator protein 1 (AP-1) binding motifs and showed increased chromatin accessibility and basal transcription. This suggests a mechanism of assisted p65 chromatin association that can be in part facilitated by chromatin priming and cooperativity with other transcription factors such as AP-1.-Riedlinger, T., Liefke, R., Meier-Soelch, J., Jurida, L., Nist, A., Stiewe, T., Kracht, M., Schmitz, M. L. NF-κB p65 dimerization and DNA-binding is important for inflammatory gene expression.

RNAi-Based Identification of Gene-Specific Nuclear Cofactor Networks Regulating Interleukin-1 Target Genes.

The potent proinflammatory cytokine interleukin (IL)-1 triggers gene expression through the NF-κB signaling pathway. Here, we investigated the cofactor requirements of strongly regulated IL-1 target genes whose expression is impaired in p65 NF-κB-deficient murine embryonic fibroblasts. By two independent small-hairpin (sh)RNA screens, we examined 170 genes annotated to encode nuclear cofactors for their role in Cxcl2 mRNA expression and identified 22 factors that modulated basal or IL-1-inducible Cxcl2 levels. The functions of 16 of these factors were validated for Cxcl2 and further analyzed for their role in regulation of 10 additional IL-1 target genes by RT-qPCR. These data reveal that each inducible gene has its own (quantitative) requirement of cofactors to maintain basal levels and to respond to IL-1. Twelve factors (Epc1, H2afz, Kdm2b, Kdm6a, Mbd3, Mta2, Phf21a, Ruvbl1, Sin3b, Suv420h1, Taf1, and Ube3a) have not been previously implicated in inflammatory cytokine functions. Bioinformatics analysis indicates that they are components of complex nuclear protein networks that regulate chromatin functions and gene transcription. Collectively, these data suggest that downstream from the essential NF-κB signal each cytokine-inducible target gene has further subtle requirements for individual sets of nuclear cofactors that shape its transcriptional activation profile.

The Activation of IL-1-Induced Enhancers Depends on TAK1 Kinase Activity and NF-κB p65.

The inflammatory gene response requires activation of the protein kinase TAK1, but it is currently unknown how TAK1-derived signals coordinate transcriptional programs in the genome. We determined the genome-wide binding of the TAK1-controlled NF-κB subunit p65 in relation to active enhancers and promoters of transcribed genes by chromatin immunoprecipitation sequencing (ChIP-seq) experiments. Out of 35,000 active enhancer regions, 410 H3K4me1-positive enhancers show interleukin 1 (IL-1)-induced H3K27ac and p65 binding. Inhibition of TAK1 or IKK2 or depletion of p65 blocked inducible enhancer activation and gene expression. As exemplified by the CXC chemokine cluster located on chromosome 4, the TAK1-p65 pathway also regulates the recruitment kinetics of the histone acetyltransferase CBP, of NF-κB p50, and of AP-1 transcription factors to both promoters and enhancers. This study provides a high-resolution view of epigenetic changes occurring during the IL-1 response and allows the genome-wide identification of a distinct class of inducible p65 NF-κB-dependent enhancers in epithelial cells.

Cyclin-dependent kinase 6 is a chromatin-bound cofactor for NF-κB-dependent gene expression.

Given the intimate link between inflammation and dysregulated cell proliferation in cancer, we investigated cytokine-triggered gene expression in different cell cycle stages. Transcriptome analysis revealed that G1 release through cyclin-dependent kinase 6 (CDK6) and CDK4 primes and cooperates with the cytokine-driven gene response. CDK6 physically and functionally interacts with the NF-κB subunit p65 in the nucleus and is found at promoters of many transcriptionally active NF-κB target genes. CDK6 recruitment to distinct chromatin regions of inflammatory genes was essential for proper loading of p65 to its cognate binding sites and for the function of p65 coactivators, such as TRIP6. Furthermore, cytokine-inducible nuclear translocation and chromatin association of CDK6 depends on the kinase activity of TAK1 and p38. These results have widespread biological implications, as aberrant CDK6 expression or activation that is frequently observed in human tumors modulates NF-κB to shape the cytokine and chemokine repertoires in chronic inflammation and cancer.

The coactivator role of histone deacetylase 3 in IL-1-signaling involves deacetylation of p65 NF-κB.

Histone deacetylase (HDAC) 3, as a cofactor in co-repressor complexes containing silencing mediator for retinoid or thyroid-hormone receptors (SMRT) and nuclear receptor co-repressor (N-CoR), has been shown to repress gene transcription in a variety of contexts. Here, we reveal a novel role for HDAC3 as a positive regulator of IL-1-induced gene expression. Various experimental approaches involving RNAi-mediated knockdown, conditional gene deletion or small molecule inhibitors indicate a positive role of HDAC3 for transcription of the majority of IL-1-induced human or murine genes. This effect was independent from the gene regulatory effects mediated by the broad-spectrum HDAC inhibitor trichostatin A (TSA) and thus suggests IL-1-specific functions for HDAC3. The stimulatory function of HDAC3 for inflammatory gene expression involves a mechanism that uses binding to NF-κB p65 and its deacetylation at various lysines. NF-κB p65-deficient cells stably reconstituted to express acetylation mimicking forms of p65 (p65 K/Q) had largely lost their potential to stimulate IL-1-triggered gene expression, implying that the co-activating property of HDAC3 involves the removal of inhibitory NF-κB p65 acetylations at K122, 123, 314 and 315. These data describe a novel function for HDAC3 as a co-activator in inflammatory signaling pathways and help to explain the anti-inflammatory effects frequently observed for HDAC inhibitors in (pre)clinical use.