4.2.1. Hydrogen as an Antioxidant Treatment
Controversy surrounds antioxidant therapies because ROS have essential functions in living organisms. Balancing oxidative stress is the key issue, and bioavailability and bioaccessibility are needed to elicit an effective response to drugs. Antioxidants have effective chemical activities
in vitro, however, many failures have been shown in proving
in vivo fair effects [
108]. Many cerebrovascular studies have investigated the effects of the representative antioxidant vitamin E. A meta-analysis of the effect of vitamin E on stroke revealed a 10% reduction in ischemic stroke accompanied by a 22% increase in hemorrhagic stroke. Antioxidants are likely to cause the progression of cancer [
109]. Stem cell like cancer cells have powerful antioxidative properties that protect them from oxidative stress and thus prevent their apoptosis [
110]. Oxidative stress upon normal cells might induce them to become cancer cells that are highly resistant to further oxidative stress. Inducing oxidative stress under these conditions is an approach to treat cancers that several trials are investigating [
111]. However, this approach is likely to kill normal cells and induce new cancer development from normal cells. Thus, oxidative stress must be controlled according to clinical circumstances. The clinical findings of antioxidant therapies have not always been favorable [
112]. This might be because mitochondria do not effectively take up antioxidants that would then interfere with the essential mechanisms of oxidative stress that protects cells from infection or other invasive cellular injury [
113]. Ohsawa
et al. [
18] have found that molecular hydrogen has powerful antioxidant effects with unique features. In cultured cells H
2 scavenges hydroxyl radicals but not superoxide or hydrogen peroxide and nitric oxide (NO). Levels of intracellular superoxide increased in cells cultured with the mitochondrial respiratory complex III inhibitor, antimycin A and then rapidly converted to hydrogen peroxide and hydroxyl radicals. Culture with H
2 decreased levels of hydroxyl radicals but not those of superoxide, hydrogen peroxide or steady-state levels of NO in cells. Even nuclear levels of hydroxyl radicals were notably decreased. As a result, H
2 prevented a decline in the mitochondrial membrane potential and the decrease in cellular ATP synthesis suggestive of effective antioxidants.
To defend cells against bacterial invasion, hydrogen peroxide is converted to hypochlorous acid by myeloperoxidase, indicating that oxidative stress is important for survival [
114]. Additionally, NO functions as a neurotransmitter that is essential for blood vessel dilation and it protects against endothelial cell activation, suggesting that oxidative stress is important for survival [
115]. Treatment with H
2 reduces levels of hydroxyl radicals but not those of superoxide or hydrogen peroxide, which have physiological roles in cell survival.
Most hydrophilic compounds are retained at membranes and never reach the cytoplasm, whereas hydrophobic compounds such as vitamin E cannot penetrate biomembranes in the absence of specific carriers or receptors. In contrast, H2 can diffuse into cytoplasm and intracellular organelles such as mitochondria, the ER and the nucleus.
High concentrations of H
2 are not cytotoxic. Breathing high concentrations of H
2 in gas has been used to treat decompression sickness and arterial gas thrombosis after deep diving [
116].
Several disease models have been created to determine the effects of molecular hydrogen (
Table 1) [
19,
70,
117–
129]. Hydrogen can be ingested mainly by gas inhalation or drinking hydrogen-rich water. Hydrogen can be inhaled via hydrogen gas delivered through a ventilator with a face mask. Arterial blood levels of H
2 increase depending upon the concentration of inhaled H
2 gas. The diffusion of H
2 gas has been monitored in the rat myocardium [
118], in which the H
2 concentration was increased by two thirds in the ischemic compared with the non-ischemic myocardium. Ischemic volume was decreased one day after middle cerebral artery occlusion in a rat model of cerebral infarction that had inhaled H
2, the wider volume difference one week after occlusion between these and control rats indicated improvements in chronic ischemic stress [
18]. The inhalation of H
2 reduces inflammatory responses associated with ventilator-induced lung injury at local and systemic levels via its antioxidant, anti-inflammatory and anti-apoptotic effects [
130].
Drinking hydrogen-rich water is a straightforward method of daily administration at outpatient clinics because up to 0.8 mM H
2 (1.6 ppm,
w/v) can be dissolved in water under atmospheric pressure. Glass or plastic containers are unsuitable for conserving H
2 since it can penetrate rapidly, whereas aluminum bags can retain H
2 for long periods [
113]. Hydrogen-rich water can be prepared by dissolving hydrogen gas in water under high pressure or by the reaction of magnesium metal with water. The detection of H
2 at μM levels in the liver and the fact that the H
2 concentration peaks at five minutes after hydrogen-rich water reaches the stomach suggests that the liver is a good H
2 target [
122].
4.2.2. Hydrogen as an Antioxidant Treatment Candidate for NASH
The effects of hydrogen on chemically induced liver damage have been studied in mouse models of liver damage induced by GalN/LPS, CCl
4 and diethylnitrosamine (DEN) [
125]. Hydrogen was given intraperitoneally every three hours after the administration of chemicals. Serum levels of TNF-α and IL-6 and transaminase levels decreased in mice with GalN/LPS-induced acute liver injury after H
2 administration. Hepatic fibrogenesis markers such as collagen-α1 or α-smooth muscle actin (SMA) were reduced in model mice with CCl
4-induced liver cirrhosis and proliferative activities of hepatocytes were reduced in mice with DEN induced hepatic tumorigenesis.
Kawai
et al. have reported that drinking hydrogen-rich water has favorable effects in NASH models [
70]. Plasma transaminase levels, histological NAS, hepatic TNF-α, IL-6 and fatty acid synthesis-related gene expression and the oxidative stress biomarker 8-OHdG were decreased in the livers of established MCD diet-induced NASH models administered with hydrogen-rich water or anti-oxidative pioglitazone. Although the decrease in hepatic cholesterol was smaller in the group given hydrogen-rich water, serum oxidative stress was reduced and antioxidant function was higher than that in the pioglitazone group. Another NASH mouse model was constructed to determine whether hydrogen affects hepatocarcinogenesis. The Stellic Research Institute created a mouse model of streptozotocin-induced NASH (STAM) that represents hepatocarcinogenesis within 16 weeks [
131]. Hydrogen-rich water reduced the number and size of hepatocellular carcinoma lesions in this model compared with controls.
The consumption of hydrogen-rich water might effectively treat NASH by reducing hepatic oxidative stress, inflammation, and hepatocarcinogenesis. We emphasize that because the results of many investigations into the effects of antioxidants upon diseases associated with oxidative stress have been disappointing, hydrogen might be the same as those of previously discouraged agents. More basic and clinical understanding of this novel potential treatment option is required.