Li Ge,1 Ming Yang,2 Na-Na Yang,3 Xin-Xin Yin,2 and Wen-Gang Song4
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Abstract
INTRODUCTION
Oxidative stress in the cell results from the robust oxidizing potential of excess reactive oxygen species (ROS) [1]. Acute oxidative stress may result from various conditions, such as vigorous exercise, inflammation, ischemia and reperfusion (I/R) injury, surgical bleeding, and tissue transplantation [2–4]. Chronic/persistent oxidative stress is closely related to the pathogenesis of many lifestyle-related diseases, aging, and cancer [5–8]. However, many clinically tested antioxidants exhibit high toxicity levels that limit their usage to a narrow range of therapeutic dosages, and result in ineffective prevention of oxidative stress-related diseases [9]. Thus, identifying effective antioxidants with little-to-no side effects is very important for the treatment of multiple diseases.
H2 is a flammable, colorless, odorless gas that can act as a reducing agent under certain circumstances. It was previously considered physiologically inert in mammalian cells, and was not thought to react with active substrates in biological systems. Recently, H2 has emerged as a novel medical gas with potentially broad applications. Dole, et al. first reported the therapeutic effects of H2 in 1975 in a skin squamous carcinoma mouse model [10]. Thereafter, inhaling high pressure H2 was demonstrated as a treatment for liver parasite infection-induced hepatitis [11]. In 2007, Ohsawa and colleagues discovered that H2 has antioxidant properties that protect the brain against I/R injury and stroke by selectively neutralizing hydroxyl radicals (·OH) and peroxynitrite(ONOO-) [1].
To date, H2 preventive and therapeutic effects have been observed in various organs, including the brain, heart, pancreas, lung, and liver. H2 mediates oxidative stress and may exhibit anti-inflammatory and anti-apoptotic effects [12–14]. H2 not only provides a safe and effective disease treatment mechanism, but also prompts researchers to re-visit the significance and benefits of medicinal gas in the human body. This review summarizes recent progress toward potential preventive and therapeutic applications of H2 and addresses possible underlying molecular mechanisms.
POTENTIAL MECHANISMS OF H2 AS A THERAPEUTIC AGENT
The exact molecular mechanisms of the effects of low-dose H2 remain unclear. H2 can modulate signal transduction across multiple pathways, but its primary molecular targets have not been determined. Examining critical overlapping signaling molecules would help mapcrosstalk among critical pathways. To fully explain the biological functions of H2, its molecular mechanisms of action must be clarified. Potential mechanisms are proposed and summarized in Figure Figure11.
H2 biological effects and possible mechanisms of action
(A) H2 has selective anti-oxidative, anti-inflammatory and anti-apoptotic properties. Exogenous damage due to such factors as radiation induces excess cellular ROS production. H2 penetrates biomembranes and effectively reaches cell nuclei. H2 selectively scavenges ·OH and ONOO- and thus prevents DNA damage. H2 also downregulates the expression of pro-inflammatory and inflammatory cytokines, such as IL-1β, IL-6, TNF-α, ICAM-1, and HMGB-1, and of pro-apoptotic factors, such as caspase-3, caspase-12, caspase-8 and Bax. H2upregulates the expression of anti-apoptotic factors, such as Bcl-2 and Bcl-xL. (B) H2 modulates signal transduction within and between many pathways. ?¶The exact targets and molecular mechanisms of H2 are unknown. ?: Does cross-talk occur among various signaling pathways? If so, how is it triggered? Further studies should explore other signaling pathways that may take part in H2-related disease mitigation.
Selective anti-oxidation
The role of H2 as an antioxidant has garnered the most attention among many proposed biological activities. H2 is a specific scavenger of ·OH and ONOO-, which are very strong oxidants that react indiscriminately with nucleic acids, lipids, and proteins, resulting in DNA fragmentation, lipid peroxidation, and protein inactivation. Fortunately, H2 does not appear to react with other ROS that have normal physiological functions in vivo [1].
H2 administration decreases expression of various oxidative stress markers, such as myeloperoxidase, malondialdehyde, 8-hydroxy-desoxyguanosine8-OHdG, 8-iso-prostaglandin F2a, and thiobarbituric acid reactive substances in all human diseases and rodent models [15–19]. Recent reports also revealed that H2-selective anti-oxidation mitigates certain pathological processes in plants and retains freshness in fruits [20–23]. In 2016, researchers proposed that H2 could decrease ROS content in Ganoderma lucidumdepending on the presence of endogenous glutathione peroxidase [24].
Anti-inflammation
A 2001 study found that breathing high-pressure H2 could cure parasite-induced liver inflammation, and was the first demonstration of the anti-inflammatory properties of H2 [11]. H2 has exhibited anti-inflammatory activities in various injury models. Typically, H2 inhibits oxidative stress-induced inflammatory tissue injury via downregulation of pro-inflammatory and inflammatory cytokines, such as interleukin (IL)-1β, IL-6, tumor necrosis factor-α(TNF-α) [25, 26], intercellular cell adhesion molecule-1 [27], high-mobility group box 1(HMGB-1) [27], nuclear factor kappa B (NF-κB) [28], and prostaglandin E2 [29]. H2 improved survival rate and reduced organ damage inseptic mice by downregulating early and late pro-inflammatory cytokines in serum and tissues, suggesting the potential use of H2 as a therapeutic agent for conditions associated with inflammation-related sepsis/multiple organ dysfunction syndrome [30]. Additionally, H2 released from intestinal bacteria has been suggested to suppress inflammation [31].
Anti-apoptosis
H2 exerts anti-apoptotic effects by up- or downregulating apoptosis-related factors. For example, H2inhibits expression of the pro-apoptotic factors, B-cell lymphoma-2-associated X-protein [32], caspase-3 [33], caspase-8 [32], and caspase-12 [34], and upregulates the anti-apoptotic factors, B-cell lymphoma-2 and B-cell lymphoma-extra large [32, 35]. H2 further inhibits apoptosis by regulating signal transduction within and between specific pathways. Hong, et al. first confirmed in 2014 that the H2-triggered neuroprotective effect is at least partially associated with anti-apoptotic protein kinase B pathway (also known as the Akt/glycogen synthase kinase 3β(GSK3β) pathway)activation in neurons [35].
Gene expression alterations
H2 administration induces expression of diverse genes, including NF-κB [36], c-Jun N-terminal kinase (JNK) [37, 38], proliferation cell nuclear antigen [39], vascular endothelial growth factor (VEGF) [40], glial fibrillary acidic protein (GFAP) [41, 42], and creatine kinase [43]. Some of these molecules may be secondarily regulated by H2, and some may be direct H2 targets. In the normal rat liver, H2 was found to have little effect on the expression of individual genes, but gene ontology analysis demonstrated upregulation of oxidoreduction-related genes [44]. The anti-inflammatory and anti-apoptotic properties of H2 could be realized by modulating expression of pro-inflammatory and inflammatory cytokines, and apoptosis-related factors.
H2 as a gaseous signal modulator
Oxidative stress impacts multiple signaling pathways, including the extracellular signal-regulated protein kinase (ERK)1/2, NF-κB, JNK, andnuclear factor-erythroid 2p45-related factor 2 (Nrf2) pathways. Along with selectively scavenging ·OH, H2 may alleviate oxidative stress-induced injury by targeting these pathways [45–47]. Additional studies confirmed that H2 could exert anti-inflammatory effects by regulating Toll-like receptor 4 (TLR4) signaling [48], and anti-apoptotic effects through Ras-ERK1/2-MEK1/2 and Akt pathway inactivation [49]. H2 may also protect against allergic reactions by directly modulating FcεRI-related signaling, rather than through radical-scavenging activity [50].
Since H2 may influence multiple signaling pathways to exert broad effects, crosstalk between these pathways likely influences H2 therapeutic outcomes. The effects of H2 as a gaseous signal modulator in a therapeutic setting may involve a network of signaling molecules, and future research using various animal and cell models is needed to confirm the benefits of H2 in such settings.