The expanding regulatory network of STING-mediated signaling

Protein-Ligand Interactions in the STING Binding Site Probed by Rationally Designed Single-Point Mutations: Experiment and Theory

STING protein (stimulator of interferon genes) plays an important role in the innate immune system. A number of potent compounds regulating its activity have been reported, mostly derivatives of cyclic dinucleotides (CDNs), natural STING agonists. Here, we aim to provide complementary information to large-scale "ligand-profiling" studies by probing the importance of STING-CDN protein-ligand interactions on the protein side. We examined in detail six typical CDNs each in complex with 13 rationally devised mutations in STING: S162A, S162T, Y167F, G230A, R232K, R232H, A233L, A233I, R238K, T263A, T263S, R293Q, and G230A/R293Q. The mutations switch on and off various types of protein-ligand interactions: π-π stacking, hydrogen bonding, ionic pairing, and nonpolar contacts. We correlated experimental data obtained by differential scanning fluorimetry, X-ray crystallography, and isothermal titration calorimetry with theoretical calculations. This enabled us to provide a mechanistic interpretation of the differences in the binding of representative CDNs to STING. We observed that the G230A mutation increased the thermal stability of the protein-ligand complex, indicating an increased level of ligand binding, whereas R238K and Y167F led to a complete loss of stabilization (ligand binding). The effects of the other mutations depended on the type of ligand (CDN) and varied, to some extent. A very good correlation (R² = 0.6) between the experimental binding affinities and interaction energies computed by quantum chemical methods enabled us to explain the effect of the studied mutations in detail and evaluate specific interactions quantitatively. Our work may inspire development of high-affinity ligands against the common STING haplotypes by targeting the key (sometimes non-intuitive) protein-ligand interactions.

Vascular disrupting agents in cancer therapy

Tumor blood vessel formation is a key process for tumor expansion. Tumor vessels are abnormal and differ from normal vessels in architecture and components. Besides oxygen and nutrients supply, the tumor vessels system, due to its abnormality, is responsible for: hypoxia formation, and metastatic routes. Tumor blood vessels can be a target of anti-cancer therapies. There are two types of therapies that target tumor vessels. The first one is the inhibition of the angiogenesis process. However, the inhibition is often ineffective because of alternative angiogenesis mechanism activation. The second type is a specific targeting of existing tumor blood vessels by vascular disruptive agents (VDAs). There are three groups of VDAs: microtubule destabilizing drugs, flavonoids with anti-vascular functions, and tumor vascular targeted drugs based on endothelial cell receptors. However, VDAs possess some limitations. They may be cardiotoxic and their application in therapy may leave viable residual, so called, rim cells on the edge of the tumor. However, it seems that a well-designed combination of VDAs with other anti-cancer drugs may bring a significant therapeutic effect. In this article, we describe three groups of vascular disruptive agents with their advantages and disadvantages. We mention VDAs clinical trials. Finally, we present the current possibilities of VDAs combination with other anti-cancer drugs.

A STING to inflammation and autoimmunity

Various intracellular pattern recognition receptors (PRRs) recognize cytosolic pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Cyclic GMP-AMP synthase (cGAS), a cytosolic PRR, recognizes cytosolic nucleic acids including dsDNAs. The recognition of dsDNA by cGAS generates cyclic GMP-AMP (GAMP). The cGAMP is then recognized by STING generating type 1 IFNs and NF-κB-mediated generation of pro-inflammatory cytokines and molecules. Thus, cGAS-STING signaling mediated recognition of cytosolic dsDNA causing the induction of type 1 IFNs plays a crucial role in innate immunity against cytosolic pathogens, PAMPs, and DAMPs. The overactivation of this system may lead to the development of autoinflammation and autoimmune diseases. The article opens with the introduction of different PRRs involved in the intracellular recognition of dsDNA and gives a brief introduction of cGAS-STING signaling. The second section briefly describes cGAS as intracellular PRR required to recognize intracellular nucleic acids (dsDNA and CDNs) and the formation of cGAMP. The cGAMP acts as a second messenger to activate STING- and TANK-binding kinase 1-mediated generation of type 1 IFNs and the activation of NF-κB. The third section of the article describes the role of cGAS-STING signaling in the induction of autoinflammation and various autoimmune diseases. The subsequent fourth section describes both chemical compounds developed and the endogenous negative regulators of cGAS-STING signaling required for its regulation. Therapeutic targeting of cGAS-STING signaling could offer new ways to treat inflammatory and autoimmune diseases.

Nucleic Acid Sensing in Mammals and Plants: Facts and Caveats

The accumulation of nucleic acids in aberrant compartments is a signal of danger: fragments of cytosolic or extracellular self-DNA indicate cellular dysfunctions or disruption, whereas cytosolic fragments of nonself-DNA or RNA indicate infections. Therefore, nucleic acids trigger immunity in mammals and plants. In mammals, endosomal Toll-like receptors (TLRs) sense single-stranded (ss) or double-stranded (ds) RNA or CpG-rich DNA, whereas various cytosolic receptors sense dsDNA. Although a self/nonself discrimination could favor targeted immune responses, no sequence-specific sensing of nucleic acids has been reported for mammals. Specific immune responses to extracellular self-DNA versus DNA from related species were recently reported for plants, but the underlying mechanism remains unknown. The subcellular localization of mammalian receptors can favor self/nonself discrimination based on the localization of DNA fragments. However, autoantibodies and diverse damage-associated molecular patterns (DAMPs) shuttle DNA through membranes, and most of the mammalian receptors share downstream signaling elements such as stimulator of interferon genes (STING) and the master transcription regulators, nuclear factor (NF)-κB, and interferon regulatory factor 3 (IRF3). The resulting type I interferon (IFN) response stimulates innate immunity against multiple threats-from infection to physical injury or endogenous DNA damage-all of which lead to the accumulation of eDNA or cytoplasmatic dsDNA. Therefore, no or only low selective pressures might have favored a strict self/nonself discrimination in nucleic acid sensing. We conclude that the discrimination between self- and nonself-DNA is likely to be less strict-and less important-than assumed originally.

Combination of anti-vascular agent - DMXAA and HIF-1α inhibitor - digoxin inhibits the growth of melanoma tumors

Vascular disrupting agents as DMXAA inhibit tumor growth only for a short period of time followed by rapid tumor regrowth. Among others, hypoxia and presence of transcription factor HIF-1α are responsible for tumors regrowth. The aim of our study was to investigate the inhibition of murine melanoma growth by combining two agents: anti-vascular - DMXAA and the HIF-1α inhibitor - digoxin and explaining the mechanism of action of this combination. After DMXAA treatment tumor size was reduced only for a limited time. After 7 days regrowth of tumors was observed and number of vessels was increased especially in tumor's peripheral areas. DMXAA also induced an influx of immune cells: macrophages, CD8+ cytotoxic lymphocytes, NK cells, CD4+ lymphocytes. Administration of digoxin alone inhibited the growth of tumors. Administration of both agents in the proper sequence significantly inhibited the regrowth of tumors better than either agents alone. Combination therapy reduced number of newly formed vessels. In tumors of mice treated with combination therapy, the number of macrophages M1, CD8+ cytotoxic lymphocytes, NK cells and to a lesser extent CD4+ cells was increased. The combination of anti-vascular agents with HIF-1α inhibitors appears to be an effective therapeutic option.

The in vivo evidence for regulated necrosis

Necrosis is a hallmark of several widespread diseases or their direct complications. In the past decade, we learned that necrosis can be a regulated process that is potentially druggable. RIPK3- and MLKL-mediated necroptosis represents by far the best studied pathway of regulated necrosis. During necroptosis, the release of damage-associated molecular patterns (DAMPs) drives a phenomenon referred to as necroinflammation, a common consequence of necrosis. However, most studies of regulated necrosis investigated cell lines in vitro in a cell autonomous manner, which represents a non-physiological situation. Conclusions based on such work might not necessarily be transferrable to disease states in which synchronized, non-cell autonomous effects occur. Here, we summarize the current knowledge of the pathophysiological relevance of necroptosis in vivo, and in light of this understanding, we reassess the morphological classification of necrosis that is generally used by pathologists. Along these lines, we discuss the paucity of data implicating necroptosis in human disease. Finally, the in vivo relevance of non-necroptotic forms of necrosis, such as ferroptosis, is addressed.

The Scaffolding Protein IQGAP1 Interacts with NLRC3 and Inhibits Type I IFN Production

Sensing of cytosolic nucleotides is a critical initial step in the elaboration of type I IFN. One of several upstream receptors, cyclic GMP-AMP synthase, binds to cytosolic DNA and generates dicyclic nucleotides that act as secondary messengers. These secondary messengers bind directly to stimulator of IFN genes (STING). STING recruits TNFR-associated NF-κB kinase-binding kinase 1 which acts as a critical node that allows for efficient activation of IFN regulatory factors to drive the antiviral transcriptome. NLRC3 is a recently characterized nucleotide-binding domain, leucine-rich repeat containing protein (NLR) that negatively regulates the type I IFN pathway by inhibiting subcellular redistribution and effective signaling of STING, thus blunting the transcription of type I IFNs. NLRC3 is predominantly expressed in lymphoid and myeloid cells. IQGAP1 was identified as a putative interacting partner of NLRC3 through yeast two-hybrid screening. In this article, we show that IQGAP1 associates with NLRC3 and can disrupt the NLRC3-STING interaction in the cytosol of human epithelial cells. Furthermore, knockdown of IQGAP1 in THP1 and HeLa cells causes significantly more IFN-β production in response to cytosolic nucleic acids. This result phenocopies NLRC3-deficient macrophages and fibroblasts and short hairpin RNA knockdown of NLRC3 in THP1 cells. Our findings suggest that IQGAP1 is a novel regulator of type I IFN production, possibly via interacting with NLRC3 in human monocytic and epithelial cells.

cGAS-cGAMP-STING: The three musketeers of cytosolic DNA sensing and signaling

Innate immunity is the first line of host defense against invading pathogens. The detection of aberrant nucleic acids which represent some conserved PAMPs triggers robust type I IFN-mediated innate immune responses. Host- or pathogen-derived cytosolic DNA binds and activates the DNA sensor cGAS, which synthesizes the second messenger 2'3'-cGAMP and triggers STING-dependent downstream signaling. Here, we highlight recent progress in cGAS-cGAMP-STING, the Three Musketeers of cytosolic DNA sensing and signaling, and their essential roles in infection, autoimmune diseases, and cancer. We also focus on the regulation of these critical signal components by variant host/pathogen proteins and update our understanding of this indispensable pathway to provide new insights for drug discovery. © 2016 IUBMB Life, 68(11):858-870, 2016.