Stimulator of interferon genes (STING) is a cytosolic endoplasmic reticulum anchored receptor protein involved in the propagation of innate immune sensing of cytosolic DNA through the production of Interferon- (IFN-). Mechanistic studies have shown IFN- production within a tumor microenvironment can result in activation of tumor antigen-specific CD8+ T-cell immunity that can lead to tumor regression. STING activation by STING agonists should result in innate T-cell mediated anti-tumor immunity in the tumor microenvironment and have significant potential as a cancer therapeutic. Conversely, we hypothesize that: inhibition of STING will lead to a decreased production of IFN- which will reduce the immune response to cytosolic DNA and RNA and thereby reduce life-threatening conditions in systemic lupus erythematosus (SLE). Molecular Dynamics (MD) equilibrated crystal structures for human HAQ, REF, and wild type (WT) STING alleles were clustered to find optimal conformations for computational chemical library screening via computational docking utilizing rigid receptor, induced fit, and quantum polarized ligand models. Models for both STING agonists and antagonists were developed. A novel low-molecular-weight organic molecule that is not based on a cyclic dinucleotide (such as STING?s normal ligand, 2?,3?-cyclic-GAMP) was found as a strong binder of STING. The compound was synthesized in our laboratory and its structure was confirmed using LC/MS and 1H and 13C NMR. Proteins representing both the HAQ and WT alleles (representing 78.3% of the human population) were tested against our compound with 2?,3?-cGAMP and DMSO as positive and negative controls, respectively in a luciferase reporter model. In short, pIRF-3 (the immediate downstream protein activated by STING) was measured by luminescence in THP-1 monocytic leukemic cells and gave a signal for the luminescence of luciferin that was approximately 100 fold weaker than 2?,3?-cGAMP. Moreover, in Surface Plasmon Resonance (SPR) experiments we determined that our compound possesses a KD of ~400nM. We hypothesize that our compound is a partial agonist that can be converted to a full antagonist using iterative rounds of computational modeling, synthesis, and experimental testing. We are designing analogs of this compound as potential antagonists of STING for SLE therapy. The strong binding of the compound will be preserved while exploring R group modifications that can suppress the STING pathway for SLE. It is noteworthy that antagonists of STING are anticipated to play a strong role in ameliorating life threatening conditions in SLE. What is needed are confirmatory experiments that our lead compound can be optimized as a STING antagonist that will be effective in SLE. We are employing our expertise in computational drug discovery, synthetic and medicinal chemistry, biophysical binding measurements, and in vitro cell-based assays, to perform ?lead exploration? studies that will result in compounds that can be tested in lupus-prone mice to determine their efficacy before engaging in a full blown lead optimization initiative.

Public Health Relevance

A key protein in the cell's process of alerting the immune system is STING (STimulator of INterferon Genes) that is activated by abnormal localization of DNA in the cell. We have recently identified a compound that binds to STING that we hypothesize will reduce the titer of autoantibodies that bind DNA and accumulate in the kidneys and heart, potentially causing life-threatening crises such as kidney or heart failure. We intend to develop a drug for lupus based on this promising compound to greatly improve the health and quality of life of SLE patients.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Exploratory/Developmental Grants (R21)
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Macromolecular Structure and Function A Study Section (MSFA)
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Johnson, David R
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University of South Florida
Schools of Arts and Sciences
United States
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