**3. Vitamin E and influenza virus infection**

Vitamin E is the most active natural fat-soluble antioxidant capable of protecting unsaturated fatty acids in cellular membranes from peroxidation, thereby contributing to membrane stability [33]. Both human clinical trials and animal studies have shown a beneficial effect of

Studies in the last decade established that the nuclear factor (erythroid-derived 2)-like 2 (NRF2) encoded in humans by the NSF2 gene, is a protein regulating the expression of antioxidant proteins that protect against oxidative damage triggered by injury and inflammation. NRF2 controls the basal and induced expression of antioxidant response element-dependent genes to regulate the physiological and pathophysiological outcomes of oxidant exposure. NRF2 has a substantial impact on oxidative stress and toxicity, regulating the antioxidant defense [35]. At this point of view, the oxidative stress is caused by the imbalance between production of reactive oxygen species (ROS) and the body's ability to detoxify the reactive intermediates.

Recent studies described the role of NRF2 gene coded protein in the development of oxidative stress. The antioxidant pathway controlled by NRF2 is pivotal for protection of lungs against the development of influenza virus infection-induced pulmonary inflammation and injury under oxidative conditions. The NRF2-mediated antioxidant system is essential to protect the lungs from oxidative injury and inflammation induced by influenza virus infection [36, 37].

supplemental vitamin E on the immune system [34].

**Figure 1.** Respiratory tract damages, causes by influenza virus infection.

70 Vitamin E in Health and Disease

It has been proven that oxidative stress in the influenza virus-infected organism provokes free-radical oxidation of unsaturated lipid chains in the cell membranes (lipid peroxidation), which reduces their permeability as a whole. In the presence of antioxidant deficiency, as described below, when all cell membranes are exposed and/or damaged, influenza infection proceeds with severe pathology and results in serious damage at all levels in the body [38].

It was established that, during influenza infection in mice, the activity of antioxidant enzymes SOD and catalase were changed, along with a decrease of the amounts of endogenous lowmolecular-weight antioxidants such as α-tocopherol (**Table 1**), glutathione, and ascorbate [19, 24, 39–41],). Endogenic levels of vitamin E were significantly decreased in lung, liver, and blood plasma [19, 23, 42]. In addition, changes in cytochromes were recorded as well as decreases in the activities of liver cytochrome P-450-dependent monooxygenases [18, 43]. Together, these facts indicate that, in the course of the disease, the buffering capacity of the organism's antioxidant protection diminished [18, 19, 22, 23, 25].

These data demonstrate that, during influenza virus infection, a decrease in natural antioxidant vitamin E was established, accompanied by a significant increase in endogenous lipid peroxidation products.

Oxidative damage in the course of influenza virus infection is quite large, even when registered in experimental animals (mice) at low virus-inoculation doses. In conditions involving non-infected animals with suppressed antioxidant defense systems, the consequent inoculation of influenza virus resulted in an acceleration of oxidative stress and graduated tissue damage.

Different conditions can favor the host's susceptibility to influenza virus infection; among them are cold exposure and stressors of physical, chemical, and psychological origin. For example, immobilization and cold-restraint stress are widely used experimental models that are accompanied by a considerable decrease in the antioxidative capacity of the animal organism; they are also used for the indirect modulation of antioxidant deficiency in experimental animals [19, 44–47].

Because of the significant role of oxidative stress in the pathogenesis of influenza virus infection, a lot of work has been done to test the influence of antioxidants on the course of influenza. Drugs stimulating NRF2 pathway are tested for treatment of diseases causing oxidative


**Table 1.** Endogenous content of vitamin E [nmol/mg protein] in lung, liver, and blood plasma of mice experimentally infected with influenza virus A/Aichi/2/68 H3N2 (1.5 MLD50).

stress, influenza virus infection included [48]. Experiments on in vivo models, predominantly in mice, hold a significant place in such investigations.

Several different non-antioxidant functions of vitamin E may be essential for the maintenance of cell integrity and functions, such as its role as an anti-phospholipase A2 agent, that is, as a stabilizer of the lipid bilayer of membranes against hydrolyzed and oxidized lipids [65].

Vitamin E and Influenza Virus Infection http://dx.doi.org/10.5772/intechopen.80954 73

The *in vivo* investigations on influenza virus-infected laboratory animals and the clinical data on influenza patients revealed a negative correlation between pulmonary inflammations and endogenous levels of vitamin E in the body. Exogenous vitamin E supplementation has been

It is clear that influenza virus infection is a powerful prooxidant that causes a significant increase in lipid peroxidation products in lung, liver, and blood plasma as well as a decrease in natural antioxidants (vitamin E, glutathione) and cytochrome P-450 (CYP). Moreover, in the liver, cytochrome *c* reductase and liver monooxygenases (aniline hydroxylase, ethylmorphin-N-demethylase, analgin-N-demethylase, and amidopyrine-N-demethylase) are inhibited as

As mentioned above, investigations on mice experimentally infected with influenza virus found that endogenous vitamin E content was significantly decreased after influenza virus inoculation. In addition, the amount of cytochrome P-450 in the liver and the activity of cytochrome *c* reductase decreased by about two times on the 5th–7th days post virus inoculation. The decrease in cytochrome P-450 was found to correlate with increases in the concentration of lipid peroxidation products in liver, lung, and blood [18]. Influenza virus infection significantly inhibits liver monooxygenase activity. As a consequence, products from the decreased enzymatic function accumulate in the liver, resulting in the destruction of cytochrome P-450

The effects of influenza virus infection on liver monooxygenases and lipid peroxidation are different from the effects of the hydrophobic xenobiotic substrates of cytochrome P-450. Evidently, the oxidative stress induced in the liver by hydrophobic xenobiotics is a consequence of enhanced oxidation by cytochrome P-450-dependent monooxygenases. The decrease in liver monooxygenase activity resulting from influenza virus infection is accompanied by increases in lipid peroxidation products in the liver, which is not a result of activation of cytochrome P-450-dependent monooxygenases. It may be presumed that influenza virus induces free-radical processes outside the liver, thus producing free radicals and/or activated oxygen species. These reactive compounds must diffuse or be transported over the hepato-

The protective effect of vitamin E against lipid peroxidation was dose-dependent and was more pronounced on the 5th day as compared to the 7th day after virus inoculation [18, 22]. This agrees with data from Peterhans [5] and Jacoby and Choi [38]. Vitamin E supplementation led to stabilization of cytochrome P-450. Concentrations of the hepatic cytochrome P-450 in infected mice reached the values found in control (non-infected animals) after vitamin E supplementation (120 or 240 mg/kg b.w.), because the monooxygenase activities

tied to reducing severe symptoms of lung disease [16, 18, 24, 66].

compared to their activity in control (non-infected) animals.

and its transformation to the catalytically inactive P-420 form.

cyte barrier to initiate lipid peroxidation in the liver.

were restored.

**4. Effects of vitamin E supplementation**

Among the antioxidants tested against influenza virus infections in mice [17, 49–52] α-tocopherol (vitamin E) occupies the leading position. This is because of its efficacy in preventing oxidative damage through its free-radical scavenging activity [16, 18, 24, 42, 53–55].

Protein expression of NRF2 is found to be increased in both the cholesterol-fed and the vitamin E-supplemented rabbits via activation of NRF2 pathway, resulting in induction of several antioxidant genes. Vitamin E appeared to afford the protection effect of NRF2 [48, 55]. Besides, it was found that vitamin E prevents the NRF2 suppression by allergens in alveolar macrophages, proved for asthmatic model in vivo [56, 57].

These data clearly show the role of antioxidants, such as vitamin E, which can be manifested in several ways: (i) to capture free radicals in enzymatic or non-enzymatic mechanism(s), (ii) to suppress their generation, and (iii) to affect these processes in an indirect way, for example, by inhibiting viral replication.

As vitamin E is a lipid-soluble substance and possesses a hydrophobic tail, it tends to accumulate within the interior of lipid membranes. There, it acts as the most important chain-breaker, as it reacts with lipid peroxyl radicals about four times faster than they can react with adjacent fatty acid side chains. It is well known that vitamin E is able to prevent oxidative damage [58–61], because its lipophilic structure contributes to easy and passive diffusion through the cell membranes, allowing it to reach the mitochondria and the single-plated reticulum. In this way, vitamin E protects them against lipid peroxidation and damage (**Figure 2**). Especially important is its termination of free-radical chain reaction, which protects membrane polyunsaturated fatty acids from oxidation involving reactive oxygen species [61].

Vitamin E is known to affect inflammatory responses in different tissues, including the lung, not only via direct quenching of oxidative stress [42, 62], but also through modulation of oxidative eicosanoid pathways and prostaglandin synthesis [58, 63, 64], inhibition of inflammatory mediators [59], and control of apoptotic lipid signaling [60]. A stabilizing role of Vitamin E has a stabilizing role for membrane phospholipids [61] (**Figure 2**).

**Figure 2.** "Bermuda triangle" composed by the pathogenesis of influenza virus infection in the infected body. Vitamin E action is directed to the storm center.

Several different non-antioxidant functions of vitamin E may be essential for the maintenance of cell integrity and functions, such as its role as an anti-phospholipase A2 agent, that is, as a stabilizer of the lipid bilayer of membranes against hydrolyzed and oxidized lipids [65].

The *in vivo* investigations on influenza virus-infected laboratory animals and the clinical data on influenza patients revealed a negative correlation between pulmonary inflammations and endogenous levels of vitamin E in the body. Exogenous vitamin E supplementation has been tied to reducing severe symptoms of lung disease [16, 18, 24, 66].

It is clear that influenza virus infection is a powerful prooxidant that causes a significant increase in lipid peroxidation products in lung, liver, and blood plasma as well as a decrease in natural antioxidants (vitamin E, glutathione) and cytochrome P-450 (CYP). Moreover, in the liver, cytochrome *c* reductase and liver monooxygenases (aniline hydroxylase, ethylmorphin-N-demethylase, analgin-N-demethylase, and amidopyrine-N-demethylase) are inhibited as compared to their activity in control (non-infected) animals.
