Effect of Gamma Tocopherol Enriched Supplementation on Response to Inhaled O3 Exposure

Allergic asthma (AA) is the most commonly encountered respiratory disease in children and adults in the United States and is a leading cause of morbidity worldwide. Among the most disruptive expressions of disease in AA is acute asthma exacerbation. The Center for Disease Control (CDC) lists ambient air pollutants and environmental tobacco smoke as among the most common triggers for acute asthma exacerbation. Ozone (O3) is the most commonly encountered ambient air pollutant in the US.

Increased oxidative stress and decreased antioxidant capability have been observed in asthmatics. These pollutants are pro-inflammatory and are associated with increased oxidative stress, which would exacerbate reactive oxygen and reactive nitrogen species (ROS and RNS)-induced injury in asthmatics. O3 injures epithelial cells, releasing secondary mediators which activate inflammatory cells, in part by ligation of Toll-like receptor 4 (TLR4), the primary receptor for lipopolysaccharide (LPS). TLR4 activation of inflammatory cells activates (nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) and induces oxidative stress. O3 and LPS has been associated with exacerbation of asthma, and the investigators have reported that O3 and LPS augments allergic airway inflammation in allergic asthmatics (AA). Development of interventions to mitigate these responses will greatly decrease disease morbidity. Given the role that oxidants play in the pathophysiology of asthma exacerbation, defects in antioxidant levels would increase risk for acute asthma exacerbation. Nutritional deficiencies in vitamin E, ascorbate and selenium have been linked to asthma severity, and asthmatics have decreased antioxidant levels in airway fluid. It has been shown that vitamins C and E are decreased in airway fluids of asthmatics. Additionally, genetic factors may increase risk for oxidant induced exacerbation of asthma. Many investigators have reported that persons who are homozygous for the null polymorphism of the Glutathione-S-Transferase Mu1 (GSTM1) gene and unable to produce GSTM1 protein (the GSTM1 null genotype) have increased risk of acute pollutant-induced exacerbation of asthma. This lab has shown in healthy volunteers that the GSTM1 null genotype is associated with increased inflammatory response to O3, with no impact on the nociceptive response to this pollutant.

Other researchers have shown that the GSTM1 null genotype is associated with increased response to secondhand tobacco smoke, diesel exhaust, and other particulate matter components. Romieu et al demonstrated that children with asthma in Mexico City had increased susceptibility to O3-induced exacerbations if they had the GSTM1 null genotype. This group also found that GSTM1 null AAs selectively benefited from antioxidant intervention. The GSTM1 null genotype is found in 20-40% of the population, and may be overrepresented in allergic populations. Taken together, these observations show that the sizable GSTM1 null population is at risk for pollutant-induced airway disease, and that antioxidant intervention targeting the action of ROS and RNS will benefit asthmatics and especially GSTM1 null asthmatics.

A non-exclusive list of proposed antioxidants includes radical scavengers such as ascorbate, a-tocopherol (aT), y-tocopherol (γT) or inducers of Nuclear factor erythroid-2-related factor 2 (NRF2), which activate NRF2 with subsequent broad upregulation of acute phase II and antioxidant proteins. These agents are available in naturally occurring foods and as nutritional supplements. A number of animal, cell culture and epidemiological studies support the idea that antioxidant supplementation is useful in allergic airway disease. However, despite these studies and widespread public and scientific enthusiasm regarding use of such agents in asthmatics, there are scant human data to support or refute the use of such interventions for either acute or chronic allergic airways disease.

Gamma tocopherol has both radical scavenging and anti-inflammatory actions which may play important roles in decreasing pollutant-induced and allergic injury to the airway. Like other isoforms of vitamin E, γT is a potent ROS scavenger and is also a powerful nucleophile that traps electrophiles such as peroxynitrite in lipophilic compartments. One mechanism by which γT is cytoprotective is scavenging of RNS at the un-substituted C-5 position on the hydroxy-chroman ring of γT to form 5-NO-γ-tocopherol (5-NO-γT). Overall, vitamin E provides general protection of DNA, lipids and proteins from radical stress. An example of such protection is shown in rodent studies of prostate cancer in which intake of γT is associated with decreased DNA methylation of CpG rich regions of the NRF2. NRF2 is a master regulator of numerous cytoprotective antioxidant enzymes.

Supplementation with γT also prevents protein nitration and attenuates loss of plasma vitamin C in plasma in a rodent peritonitis model of inflammation. The investigators have also shown that γT inhibits Cyclooxygenase (COX)-2 and 5-Lipoxygenase (LO) in LPS-stimulated macrophages and interleukin (IL)-1b stimulated epithelial cells. These actions are mediated primarily by the γT metabolite 2, 7, 8-trimethyl-2S-(.-carboxyethyl)-6-hydroxychroman (γ-CEHC) which requires hydroxylation of the γ-methyl group of γT by cytochrome P450 (CYP450). In carrageenan- induced inflammation in male Wistar rats, γT decreases prostaglandin (PGE2) and leukotriene (LTB4) production, suggesting that LO-mediated production of leukotrienes may also be inhibited by γT. Tocopherols, including γT, also modulate gene expression of a number of inflammatory genes. Thus, γT and other tocopherols decrease production of a number of pro-inflammatory cytokines at the transcriptional level.

In animals, it has also been shown that gamma tocopherol reduces baseline eosinophilia in the airway. In early phase I studies of gamma tocopherol-enriched mixed tocopherols (γT-mT) the investigators have shown that γT-mT inhibits monocyte induced cytokine production, decreases baseline nitrosative stress, and inhibits LPS eosinophil and neutrophil influx in healthy volunteers.

SUMMARY:

There is widespread opinion that antioxidant nutrients like gamma tocopherol ( γT, a form of Vitamin E) are an untapped and inexpensive intervention for environmentally triggered acute asthma. However, there is a crucial gap in evidence-based support of such interventions in asthma. A lack of coordinated research assessing specific antioxidant regimens from preclinical, phase I and phase II studies impedes development of phase III antioxidant trials in asthmatic populations. It is also unclear which physiological endpoints are most relevant in such studies.