Vitamin E: A potential therapy for gastric mucosal injury

Abstract

Context: Many scientific reports have shown the involvement of oxidative stress and inflammation as well as diminished gastroprotective substances in the pathogenesis of gastric lesions using various models. Therefore, treatment with antioxidants like tocopherol and tocotrienol may afford beneficial effects in attentuating the formation of the gastric lesions.

Objective: The aim of this work was to summarize documented reports on the effects of vitamin E on various models of gastric lesion.

Methods: A literature search was performed from databases in Medline (PubMed), Web of Science, ScienceDirect, and Googlescholar from June to December 2013.

Results and conclusion: The potential roles of tocopherol and tocotrienol in modifying the effects of ulcerogenic agents are discussed in this review. The protective effects of the vitamin E might involve ameliorating oxidative stress and inflammation as well as restoration of endogenous gastroprotective substances. This vitamin has the potential to be used as a therapy for gastric mucosal injury.

• Gastric lesion
• inflammation
• oxidative stress
• stress hormones
• tocopherol
• tocotrienol

Introduction

Vitamin E is a general term used that to refer two chemical compounds with a similar structure which are tocopherol and tocotrienol. Both have a chromanol ring and a long phytyl chain. The tocopherol has a saturated phytyl chain, whilst tocotrienol has three unsaturated bonds. There are four isomers in nature for each type, namely α-, β-, γ-, and δ-tocopherol or tocotrienol depending on the methyl group positions on the chromanol ring (Bardhan et al., 2011; Wong & Radhakrishnan, 2012).

Vitamin E effects on various disorders have been extensively studied either in humans or experimental animals. It was shown to be protective against many diseases like cardiovascular (Catalán et al., 2012; Norsidah et al., 2013), cerebrovascular (Park et al., 2011), liver diseases (D'Adamo et al., 2013; Kamisah et al., 2009), cancer (Behery et al., 2013; Hodul et al., 2013), and diabetes (Baburao Jain & Anand Jain 2012). Both α-tocopherol and tocotrienol reduced gastric lesion index in experimental animals that were exposed to ulcerogens (Cuevas et al., 2011; Fesharaki et al., 2006; Mohd Fahami et al., 2012).

Gastric lesion formation is influenced by many factors such as oxidative stress, endogenous gastroprotective substance, and inflammation. There are many models used to explore the protective effects of the vitamin E on gastric lesion development. The commonly used models are non-steroidal anti-inflammatory drugs (NSAIDs)-induced (Jiang et al., 2009), ethanol-induced (Cuevas et al., 2011), stress-induced (Ibrahim et al., 2012), Helicobacter pylori-induced (Kim et al., 2002), acid-induced (Ishihara et al., 2008), and pyloric ligation induced (Laloo et al., 2013). These models induce the development of gastric ulcer via different mechanisms.

Method

A literature search was conducted from databases in Medline (PubMed), Web of Science, ScienceDirect, and Googlescholar from June to December 2013. The key terms of search were “vitamin E and gastric lesion”, “vitamin E and gastric mucosal”, “tocopherol and gastric lesion”, “tocopherol and gastric mucosal”, “tocotrienol and gastric lesion”, and “tocotrienol and gastric mucosal”. Any gastric lesions that involved precancerous or cancerous gastric lesions were excluded.

Effects of vitamin E on different mechanisms involved in the pathogenesis of gastric lesions

Oxidative stress in the stomach

Involvement of free radicals which leads to an increase in oxidative stress has been postulated as one of the mechanisms in the pathogenesis of gastric mucosal injury. Superoxide anion, hydrogen peroxide, and hydroxyl radical are believed to be the culprits (Wada et al., 1997). Co-administration of superoxide dismutase and catalase enzymes which are responsible for detoxifying the radicals has been shown to reduce the incidence of gastric lesions in rats (Ohta & Nishida, 2003). α-Tocopherol was shown to be an effective and potent scavenger of these radicals in an in vitro experiment (Suzuki et al., 1998).

An increase in oxidative stress is usually measured by the formation of malondialdehyde (MDA) content, an end product of lipid peroxidation. Elevated MDA content in the rat stomach was consistently observed in various models of gastric lesion, such as stress- (Kamisah et al., 2011), aspirin-, ethanol- (Cuevas et al., 2011), and Helicobacter pylori-induced models (Oh et al., 2005) (Table 1). Administration of α-tocopherol or tocotrienol-rich fraction (75% tocotrienol and 25% tocopherol) was able to reduce the increase in oxidative stress in these models of gastric lesions (Cuevas et al., 2011; Ishihara et al., 2008; Kamisah et al., 2011; Oh et al., 2005). Being an antioxidant, the findings were very much expected. However, tocotrienol which was reported to possess better antioxidant properties than tocopherol (Bardhan et al., 2011), showed a similar effect in terms of protection against oxidative stress (Nur Azlina et al., 2005) but significantly lower gastric lesion index than α-tocopherol (Nur Azlina et al., 2009) in rats that were exposed to repetitive restraint stress.

Table 1. Effects of vitamin E oxidative stress parameters in various gastric lesion models.

		Gastric parameters							Reference
Vitamin E	Induction model	MDA	GSH	GPx	SOD	Catalase	XO	MPO
α-Tocopherol	Indomethacin	nd	+	nd	+	+	nd	+	Odabasoglu et al., 2008
α-Tocopherol + tocotrienol	Indomethacin	+	±	nd	nd	nd	nd	nd	Nafeeza & Kang, 2005
Tocotrienol-rich fraction	Aspirin	+	nd	nd	nd	nd	nd	nd	Jaarin et al., 2002
α-Tocopherol	Aspirin	+	+	+	+	nd	±	+	Fesharaki et al., 2006; Cuevas et al., 2011
α-Tocopherol	Ethanol	+	nd	nd	nd	nd	nd	nd	Cuevas et al., 2011; Senol et al., 2011
Tocotrienol-rich fraction	Ethanol	+	nd	nd	nd	nd	nd	nd	Jaarin et al., 1999, 2000
α-Tocopherol	HCl+ethanol	+	nd	nd	nd	nd	nd	nd	Suzuki et al., 1998
α-Tocopherol	Restraint stress	+	+	±	±	nd	nd	nd	Nur Azlina et al., 2009
Tocotrienol	Restraint stress	+	+	+	±	nd	nd	nd	Nur Azlina et al., 2009
α-Tocopherol	WIRS	+	nd	nd	nd	nd	+	nd	Kamisah et al., 2011
Tocotrienol-rich fraction	WIRS	+	nd	nd	nd	nd	+	nd	Kamisah et al., 2011
α-Tocopherol	H. pylori + WIRS	+	nd	nd	nd	nd	nd	nd	Oh et al., 2005
α-Tocopherol	H. pylori	nd	nd	nd	nd	nd	nd	+	Sugimoto et al., 2006
α-Tocopherol	C48/80	+	nd	+	nd	nd	+	+	Ohta et al., 2006

+, beneficial effect; ±, no effect; nd, not determined; MDA, malondialdehyde; GSH, glutathione; GPx, glutathione peroxidase, SOD, superoxide dismutase; XO, xanthine oxidase; MPO, myeloperoxidase; WIRS, water-immersion restraint stress.

The free radicals can also be endogenously detoxified by glutathione. In many models of gastric lesions, the gastric glutathione content was significantly decreased. Treatment with α-tocopherol and/or tocotrienol in these models afforded protection against the reduction in the glutathione content (Fesharaki et al., 2006; Nur Azlina et al., 2009; Odabasoglu et al., 2008; Qodriyah et al., 2002). The antioxidant vitamin might spare the glutathione by scavenging the free radicals before they could react with the glutathione.

Antioxidant enzyme activities of glutathione peroxidase, superoxide dismutase, and catalase in the stomach are usually measured to assess oxidative stress status. Superoxide dismutase converts superoxide anion to oxygen and hydrogen peroxide (H₂O₂), which later can be detoxified by catalase or glutathione peroxidase (Mates et al., 1999). In the presence of ferric ion, hydrogen peroxide is converted into hydroxyl radical (OH^•) (Cohn et al., 2004; Puppo & Halliwell, 1988). In animals exposed to ulcerogenic substances, the antioxidant enzyme activities in the stomach were greatly reduced, which could be prevented by the administrations of either tocopherol or tocotrienol (Fesharaki et al., 2006; Nur Azlina et al., 2009; Odabasoglu et al., 2008). The vitamin E might preserve the enzyme activities by itself acting to scavenge the free radicals.

A compromised blood flow in the gastric microenvironment following an exposure to inducers also contributes to gastric lesion formation (Brzozowski et al., 2008; Konturek et al., 2010). This leads to ischemia-reperfusion-like injury (Kwichien et al., 2002) in which the conversion of xanthine dehydrogenase into xanthine oxidase occurs (McCord, 1985). Xanthine oxidase is one of the primary sources of oxyradicals, especially in the gastrointestinal tract as it is present at high concentration in organs (Parks & Granger, 1986). It predominantly exists as xanthine dehydrogenase but converts into the oxidase form in the pathological conditions like ischemia-reperfusion, producing superoxide anion, hydrogen peroxide, and hydroxyl radical (McCord, 1985). Administration of allopurinol inhibited oxidative stress in the gastric mucosa, confirming the involvement of xanthine oxidase in the pathogenesis of gastric lesions (Kurose et al., 1993). It seemed that α-tocopherol had no obvious effect on enzyme activity in rats that were given aspirin (Fesharaki et al., 2006) but showed a protective effect in rats administered C48/80, a mast cell degranulator (Ohta et al., 2006) (Table 1). While in rats subjected to water-immersion restraint stress, the rise in xanthine oxidase activity was similarly inhibited in both α-tocopherol- and tocotrienol-rich fraction-pretreated groups (Kamisah et al., 2011; Mohd Fahami et al., 2012). It is believed that vitamin E improves gastric mucosal microcirculation, thus retards enzyme conversion and hence gastric lesion formation (Table 1).

Another source of oxyradicals is myeloperoxidase enzyme. It is a marker enzyme of neutrophil infiltration (Fesharaki et al., 2006). It produces hypochlorus acid from hydrogen peroxide and chloride ion. The acid is cytotoxic and plays an important role in killing pathogens (Klebanoff, 2005). Myeloperoxidase activity was reported to be elevated in aspirin-, indomethacin-, H. pylori-, and C48/80-induced gastric lesion models (Fesharaki et al., 2006; Odabasoglu et al., 2008; Ohta et al., 2006; Sugimoto et al., 2006). The increase in myeloperoxidase activity was attenuated by α-tocopherol administration, which also reduced gastric lesions and formation of leukocyte-generated oxygen radicals (Fesharaki et al., 2006; Kurose et al., 1993; Odabasoglu et al., 2008; Ohta et al., 2006; Sugimoto et al., 2006). These findings suggest that α-tocopherol reduced the severity of gastric lesion via inhibition of neutrophil infiltration. It might retard the earlier oxidative events that led to neutrophil infiltration. However, no study so far has reported the effect of tocotrienol on this enzyme in gastric lesion rat models.

Diminished endogenous gastroprotective factors

Gastric mucus is one of the important protective factors. It consists of viscous, adherent, elastic, and transparent gel which is formed by 95% water and 5% glycoprotein. The mucus covers the mucosa of the gastrointestinal tract (Repetto & Llesuy, 2002). The gastric adherent mucus content was unaltered in rats that were fed a diet containing various doses of tocotrienol-rich fraction (60, 100, 150, and 300 mg/kg) for 4–8 weeks and administered absolute ethanol, indomethacin, or aspirin even though there was a reduction in gastric lesion index compared with the untreated control (Jaarin et al., 1999, 2000, 2002; Nafeeza et al., 2002; Qodriyah et al., 2002). A similar finding was also observed in rats given a diet containing α-tocopherol (20, 30, 50, and 300 mg/kg) for 4–8 weeks and challenged with aspirin (Jaarin et al., 2002; Nafeeza et al., 2002). These findings suggest that neither tocotrienol nor tocopherol has any significant effect on gastric mucus secretion or production.

Inconsistent outcomes on gastric acidity are seen in stress-induced gastric lesion models. Some studies showed reduced gastric acidity (Ibrahim et al., 2008; Nur Azlina et al., 2005) while some other showed otherwise (al-Moutairy & Tariq, 1996; Nur Azlina et al., 2013) after exposure to stress. However, the gastric acidity was unaltered in the NSAIDs-induced model (Nafeeza et al., 2002; Qodriyah et al., 2002). α-Tocopherol and tocotrienol pretreatments were shown to reverse the negative effect of stress on gastric acidity in rats (Ibrahim et al., 2008; Nur Azlina et al., 2013), while other studies had shown no effect (Jaarin et al., 2002; Nafeeza et al., 2002; Nur Azlina et al., 2005; Qodriyah et al., 2002). Stress possibly increases the gastric acidity initially and causes ulcerative damage to the stomach, then followed by a reduction in the gastric acidity due to oxidative damage to the gastric acid-secreting cells, which results in the cell dysfunction. The beneficial effect of the vitamins might be attributable to their antioxidant property by blocking the oxidative attack to the secreting cells. Tocotrienol was demonstrated to inhibit gastric acidity in rats exposed to stress, comparable with that of the omeprazole, a therapeutic drug to treat gastric ulcers, which potently inhibits gastric acid secretion (Nur Azlina et al., 2013).

One of the important endogenous substances that maintains gastric mucosal integrity is prostaglandin, in particular prostaglandin E₂ (PGE₂). On one hand, α-tocopherol increased gastric PGE₂ level in many gastric lesion models (Fesharaki et al., 2006, Jiang et al., 2009; Tariq, 1988). Tocotrienol, on the other hand, was also not effective in increasing the PGE₂ level in rats given indomethacin (Qodriyah et al., 2002) or ethanol (Jaarin et al., 2000). However, when the two were combined, a protective effect of the combination was observed (Nafeeza & Kang, 2005).

Prostaglandin is produced from arachidonic acid by the action of cyclooxygenase (COX) enzyme. COX exists in two isoforms: constitutive COX-1 and inducible COX-2 (Takeuchi, 2012). NSAIDs significantly inhibited COX (Liu et al., 2012). Stress had no significant effect on gastric COX-1 but it upregulated gastric COX-2 mRNA (Nur Azlina et al., 2013). Upregulation of COX-2 indicates an increase in inflammation, which reduces protective gastric PGE₂ content. Tocotrienol pretreatment was shown to downregulate COX-2 expression after stress. It also upregulated gastric COX-1 mRNA expression, the enzyme which is involved in PGE₂ synthesis, hence increased PGE₂ content in the stomach. A similar effect on the COX-1 expression was not seen with omeprazole (Nur Azlina et al., 2013).

Inflammation in gastric mucosa

As previously mentioned, many other studies had also demonstrated the involvement of inflammation in the pathogenesis of gastric lesions. Many pro-inflammatory biomarkers have been identified in various models of gastric lesions (Oh et al., 2005; Senol et al., 2011). On one hand, interferon-γ (IFN-γ) was significantly elevated in rats subjected to water-immersion restraint with H. pylori infection but no elevation was seen in the group with H. pylori infection only. Tumor necrosis factor-α (TNF-α), on the other hand, was increased in both groups in the presence or absence of stress with H. pylori (Oh et al., 2005). It seems that IFN-γ has a more important role in stress than TNF-α. In the ethanol-induced model, other than the two cytokines (IFN-γ and TNF-α), interleukin-2 (IL2) and interleukin-4 (IL4) levels were also augmented (Senol et al., 2011). These cytokines have important roles in gastric inflammation (Oh et al., 2005). α-Tocopherol showed no inhibitory effects on the cytokines in rats given ethanol despite of a reduction in the gastric mucosal lipid peroxidation and gastric lesion index (Senol et al., 2011).

Other than the cytokines, heat-shock proteins (HSPs) are also known to play a role in the cytoprotective effect in many gastrointestinal injuries (Tsukimi & Okabe, 2001). HSPs are believed to contribute to gastric mucosal defense mechanism and gastric lesion healing by protecting key enzymes that are related to anti-inflammation. The protein expressions of the HSP60, HSP70, and HSP90 were reduced in rats exposed to water-immersion restraint stress plus H. pylori infection. Administration of α-tocopherol augmented the protein expression of HSP27, but not HSP70 in this model of gastric injury. The HSP27 expression was not prominent in the stress + H. pylori group without α-tocopherol (Oh et al., 2005). It prevented neutrophil accumulation in H. pylori-mediated gastric mucosal lesions by inhibiting the neutrophilic expression of CD11b/CD18 (Sugimoto et al., 2006). Anti-inflammatory effect of tocotrienol was also seen with its ability to decrease the gastric expression of inducible COX-2 in the rats exposed to stress (Nur Azlina et al., 2013).

Roles of catecholamines and hormones

The role of catecholamines and hormones in gastric lesion formation was primarily studied in the stress-induced model. Stress activates hypothalamic–pituitary–adrenal axis and sympatho-adrenal–medullary systems in the central nervous system, causing the releases of adrenocorticotropin hormone (ACTH), corticosterone, noradrenalin, and adrenalin (Ainsah et al., 2000; Charmandari et al., 2005). Increased secretions of ACTH and corticosterone were seen in rats following an exposure to water-immersion restraint stress (Mohd Fahami et al., 2012) and repetitive restraint stress (Nur Azlina & Nafeeza, 2008). These hormones regulate the release of noradrenalin and adrenalin (Szabo et al., 1987). Increased adrenalin and noradrenalin levels in the blood would promote vasoconstriction in the gastric mucosa. This would lead to ischemia–reperfusion injury upon conversion of the xanthine dehydrogenase into xanthine oxidase during ischemic phase with a burst of oxyradical formation (Siriussawakul et al., 2010) and then increased formation of the gastric lesion. Thus, the elevated levels of ACTH, corticosterone, adrenalin, and noradrenalin are related to the increased incidence of gastric lesions (Ibrahim et al., 2012; Mohd Fahami et al., 2012).

The effects of α-tocopherol and tocotrienol on various hormones and neurotransmitters are summarized in Table 2. The vitamins attenuated the increased plasma ACTH, corticosterone, adrenalin, and noradrenalin in rats exposed to water-immersion restraint stress (Ibrahim et al., 2012; Mohd Fahami et al., 2012). In a repetitive restraint stress model, tocotrienol pretreatment managed to prevent the increase in plasma noradrenalin level and completely inhibited gastric lesion formation, similar effects were not observed with α-tocopherol. However, both vitamin E pretreatments had prevented the increase in plasma corticosterone in the rats exposed to stress (Nur Azlina & Nafeeza, 2008). It seems that in gastric mucosal injury pathogenesis, catecholamine plays a more important role than corticosterone. While in another study, both types of vitamin E at the same dose (60 mg/kg) did not completely suppress the incidence of gastric lesions and the elevation of catecholamine (noradrenalin and adrenalin) back to their non-stress levels in rats exposed to acute stress (Mohd Fahami et al., 2012). The discrepancy between these studies could be due to the difference in the models used that is acute versus repetitive stress. The rats exposed to stress repeatedly may have developed an adaptive behaviour towards stress and this would reduce or prevent gastric lesion formation.

Table 2. Vitamin E effects on hormones and neurotransmitters.

		Parameter
Vitamin E	Induction model	Corticosterone	ACTH	Gastrin	NA	Adr	ACh	Histamine	References
α-Tocopherol	WIRS	+	+	+	+	+	+	nd	Mohd Fahami et al., 2012; Ibrahim et al., 2008, 2011, 2012
α-Tocopherol	WIRS	±	±	nd	nd	nd	nd	nd	Ohta et al., 2009
Tocotrienol-rich fraction	WIRS	+	+	+	+	+	+	nd	Mohd Fahami et al., 2012; Ibrahim et al., 2008, 2011, 2012
α-Tocopherol	Restraint stress	+	nd	±	±	nd	nd	nd	Nur Azlina et al., 2005; Nur Azlina & Nafeeza, 2008
Tocotrienol	Restraint stress	+	nd	+	+	nd	nd	nd	Nur Azlina et al., 2005; Nur Azlina & Nafeeza, 2008
α-Tocopherol	C48/80	nd	nd	nd	nd	nd	nd	±	Ohta et al., 2009

+, Beneficial effect; ±, no effect; nd, not determined; WIRS, water-immersion restraint stress; NA, noradrenalin; Adr, adrenalin; Ach, acetylcholine.

On one hand, Ibrahim et al. (2012) demonstrated that α-tocopherol had completely prevented the increase in plasma corticosterone in rats subjected to water-immersion restraint stress. Ohta et al. (2009), on the other hand, reported that α-tocopherol had no obvious effect on both plasma corticosterone and ACTH in rats exposed to the same model for 6 h. The disparity in these studies, even though they used a similar model, could be the duration of the stress exposure in which the former used a shorter duration of stress (3.5 h) and the timing of the vitamin dosing. Ibrahim et al. (2012) had administered the vitamin E for 28 d prior to stress, while Ohta et al. (2009), even though a higher dose of the α-tocopherol was used, the administration was at the onset of the stress.

Gastrin is a hormone that physiologically regulates gastric acid secretion from parietal cells. Its release by G cells in the stomach antrum is one of the stimuli for gastric acid secretion (Grabowska & Watson, 2007). The relationship between gastrin and gastric lesions is not consistent. Its plasma level increases in rats with ethanol-induced gastric lesions (Castro et al., 2010) and in rats with ischemia-reperfusion-induced gastric lesions (Brzozowski et al., 1999), but a few other studies had reported that the plasma gastrin level was decreased in rats with remarkable gastric lesions induced by water-immersion restraint stress (Ibrahim et al., 2008) and repetitive restraint stress (Nur Azlina et al., 2005).

The role of vitamin E on gastrin in gastric lesion pathogenesis is not fully explored. Only a few studies had investigated the vitamin E effects. α-Tocopherol and tocotrienol-rich fraction pretreatments were shown to decrease the reduction in plasma gastrin in rats subjected to water-immersion restraint and the same pattern was observed with the gastric acidity (Ibrahim et al., 2008). While another study showed that only tocotrienol but not α-tocopherol restored the reduction in the gastrin level after exposure to repetitive restraint stress. In this study, the gastric acid concentration in the tocotrienol-treated group and subjected to stress was comparable with its non-stress group, whilst α-tocopherol had no significant effect on the gastric acidity (Nur Azlina et al., 2005). It can be concluded that both α-tocopherol and tocotrienol may be protective in terms of gastrin level restoration in acute stress but in repetitive stress, tocotrienol exerts a better effect than α-tocopherol. The vitamins may reduce or restore the hormone level by acting as an antioxidant to combat the oxidative damage to the secreting cells.

Another factor that may be involved in the pathogenesis of gastric lesion is gastric motility. It was reported that gastric hypermotility accompanied by an increase in plasma acetylcholine product (measured as total choline) increased the incidence of gastric lesions in rats subjected to stress. The hypermotility was observed together with the increased amplitude and frequency of the gastric contractions (Ibrahim et al., 2011). The study regarding the effects of α-tocopherol and tocotrienol on gastric motility and/or acetylcholine is still lacking. To date, in a single work that has been conducted. Ibrahim et al. (2011) reported that both tocotrienol-rich fraction and α-tocopherol mitigated the increased frequency and amplitude of the gastric contractions induced by stress in rats, together with a reduction in the plasma acetylcholine product and gastric lesion index.

In a study that used C48/80, the compound stimulated serotonin and histamine release from the extragastric connective tissue mast cells (Takeuchi et al., 1986). In this model, α-tocopherol had no effect on the serum histamine and serotonin, but totally prevented gastric lesions (Ohta et al., 2006). Therefore, it is unlikely that vitamin E prevents gastric lesion formation by affecting the release of hormones from connective tissue mast cells.

Conclusion

The summary of possible protective mechanisms of vitamin E is shown in Figure 1. It can be concluded that vitamin E, namely tocopherol and tocotrienol, confers its protection against ulcerogenic factors/agents primarily via its antioxidant and anti-inflammatory mechanisms. Both tocopherol and tocotrienol have comparable gastroprotection against gastric mucosal injury. It may be recommended to be taken as a supplement to avoid gastric lesions precipitated by any stress or other aggressive agents or as a component of a therapy in critically ill patients. However, so far no clinical studies have been conducted on the use of vitamin E in patients with gastric lesions. This kind of study should be carried out in order to confirm its gastroprotective properties in humans.

Figure 1. Possible gastroprotective effects of vitamin E in the pathogenesis of gastric mucosal injury.

Declaration of interest

The authors declare no conflict of interest. This work was supported by grants FF-280-2011 and FF-032-2012 from the Faculty of Medicine, Universiti Kebangsaan Malaysia.