Supersulfide donors and their therapeutic targets in inflammatory diseases

Abstract

Inflammation is one defense mechanism of the body that has multiple origins, ranging from physical agents to infectious agents including viruses and bacteria. The resolution of inflammation has emerged as a critical endogenous process that protects host tissues from prolonged or excessive inflammation, which can become chronic. Failure of the inflammation resolution is a key pathological mechanism that drives the progression of numerous inflammatory diseases. Owing to the various side effects of currently available drugs to control inflammation, novel therapeutic agents that can prevent or suppress inflammation are needed. Supersulfides are highly reactive and biologically potent molecules that function as antioxidants, redox regulators, and modulators of cell signaling. The catenation state of individual sulfur atoms endows supersulfides with unique biological activities. Great strides have recently been made in achieving a molecular understanding of these sulfur species, which participate in various physiological and pathological pathways. This review mainly focuses on the anti-inflammatory effects of supersulfides. The review starts with an overview of supersulfide biology and highlights the roles of supersulfides in both immune and inflammatory responses. The various donors used to generate supersulfides are assessed as research tools and potential therapeutic agents. Deeper understanding of the molecular and cellular bases of supersulfide-driven biology can help guide the development of innovative therapeutic strategies to prevent and treat diseases associated with various immune and inflammatory responses.

Keywords: hydropersulfides; hydropolysulfides; inflammation; inflammatory responses; persulfides; polysulfides; supersulfide donors; supersulfides.

Figure 1

Overview of supersulfides and their biological functions. Supersulfides are classified into LMW supersulfides and protein-bound supersulfides, both of which play vital roles in cellular processes. These processes include regulating enzyme activity, managing oxidative stress, participating in energy metabolism, and modulating immune responses. Cys, cysteine; GSH, glutathione; H, hydrogen; LMW, low-molecular-weight.

Figure 2

Dual enzymatic functions of CARS. As a cysteinyl-tRNA synthetase, CARS performs dual enzymatic functions as it catalyzes the formation of tRNA-bound CysSSH adducts, which enables the incorporation of CysSSH into proteins and promotes the generation of protein-bound supersulfides. In addition, through a pyridoxal phosphate (PLP)-dependent reaction, CARS synthesizes CysSSH by utilizing a second cysteine molecule as the sulfur donor, independent of ATP and tRNA. This activity contributes to sulfur-oxygen hybrid respiration within mitochondria.

Figure 3

Components of the inflammatory process. A typical inflammatory pathway consists of inducers, sensors, mediators and effectors. Inflammation is initiated by infections and tissue damage, which are recognized by PRRs. This recognition triggers the secretion of immune mediators, such as cytokines, which subsequently affect target tissues.

Figure 4

Binding of ligand-TLRs and their signaling transduction. TLRs recognize their specific ligands, typically dimerize on activation, and recruit adaptor molecules containing the same TIR domain to transmit signals. This process ultimately leads to the production of inflammatory mediators.

Figure 5

Inhibitory effects of supersulfides on the TLR signaling pathway. Administration of NAC-S2 significantly improved the survival rate of mice subjected to lethal endotoxin shock. Mechanistically, TLR ligands, including zymosan A, polyinosinic-polycytidylic acid sodium salt (poly I:C), and LPS, are recognized by TLR2, TLR3, and TLR4, respectively. TLR activation induces the expression of pro-inflammatory mediators, including TNF-α, IFN-β, and iNOS, through MyD88-dependent or TRIF-dependent signaling pathways. Supersulfide donors, such as NAC-S2, suppress the production of these inflammatory mediators by inhibiting the phosphorylation of signaling proteins. NaHS, sodium hydrosulfide.

Figure 6

Structural features and signaling pathway of RLRs. The structure and functions of MDA5 are like those of RIG-I. However, MDA5 lacks the repressor domain, which means it does not have self-inhibitory functions. LGP2, however, lacks the CARD domain and therefore cannot transmit signals. The binding of viral RNA to the CTD induces conformational changes in RLRs. These conformational changes facilitate the interaction between MAVS and either RIG-I or MDA5, which leads to the transcription of IFN-I via IRF3-, IRF7-, and NF-κB mediated pathways. LGP2, acts as a modulator and promotes MDA5-mediated signal transduction while suppressing RIG-I-mediated signaling.

Figure 7

Suppression of IFN signaling by supersulfides. The binding of IFN-I to receptors induces the phosphorylation of JAK1 and TYK2, leading to the release of phosphorylated STAT1/2 heterodimers. These dimers bind to the transcription factor IRF9 and thus form the ISGF3 complex, which translocates to the nucleus and binds to the promoter region of ISRE, thereby activating the transcription of ISGs. Similarly, IFN-γ induces the phosphorylation of STAT1 through its binding to IFN-γ receptors, promoting ISG expression by binding to GAS. NAC-S2 inhibits JAK/STAT signaling by blocking their phosphorylation, thereby reducing ISG expression. TYK2, tyrosine kinase 2; IRF9, IFN regulatory factor 9; ISGF3, IFN-stimulated gene factor 3; ISRE, IFN-stimulated response element; GAS, γ-interferon-activated site; IFNGR1 and IFNGR2, interferon γ receptor 1 and 2.

Figure 8

The mechanisms of NLRP3 inflammasome activation. NLRP3 inflammasome activation occurs via two distinct steps. During the priming step, transcriptional upregulation of NLRP3 inflammasome components including NLRP3, pro-caspase-1, and the immature form of cytokines is induced by the recognition of PAPMs or cytokines, leading to NF-κB signaling activation. The activation step is triggered by diverse DAMPs, such as extracellular ATP, pore-forming toxins, ionophore, and crystalline particles. Alterations in cellular homeostasis during this step are believed to represent a common event upstream of NLRP3 inflammasome complex assembly. This perturbation recruits components proteins, resulting in the processing of NLRP3 inflammasome and the maturation of cytokines, including IL-1β and IL-18. IL-1R, interleukin-1 receptor; Nek7, NIMA-related kinase 7.

Figure 9

Typical supersulfide donors. The diagram illustrates various synthetic supersulfide donors, including both organic and inorganic compounds, some of which are endogenously produced. These compounds serve as valuable tools for investigating the biological functions of supersulfides and their therapeutic potential both in vitro and in vivo studies.

Figure 10

Negative regulation of the NLRP3 inflammasome by supersulfides. LPS priming induces the upregulation of slc7a11, which encodes the cystine transporter xCT. Increased xCT expression enhances cystine uptake, promoting the production of CysSSH via CARS and establishing a negative feedback loop to limit excessive inflammatory mediator production. ATP sensing by the P2X purinoceptor 7 (P2X7) receptor, however, triggers the efflux of GSH and GSSH, along with ROS accumulation, leading to redox imbalance. This imbalance activates the NLRP3 inflammasome. Suppression of GSSH efflux through exogenous GSSG administration significantly inhibits NLRP3 inflammasome activation, underscoring the regulatory role of supersulfides in inflammation.

Figure 11

Protective effects of supersulfides in pulmonary disease. Liquid chromatography-mass spectrometry (LC-MS/MS)-based breathomics analyses have identified supersulfides in EBCs from COVID-19 patients. Supersulfides manifest protective effects against viral infections by using three key mechanisms: suppression of pro-inflammatory cytokine production, inhibition of oxidative stress, and modification of viral proteases. These effects were validated by treatments with supersulfide donors in various animal models.