The Epigenetic Effects of Diet and Radiation

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There are a variety of environmental factors that affect our epigenetic profile.

Two of these that trigger epigenetic responses are diet and radiation exposure. Obese individuals demonstrate metabolic disorders that cause oxidative stress, which has been linked genetic mutation and altered gene expression. Oxidative stress is an imbalance in reactive oxygen species and antioxidant defenses. Exposure to radiation has also been connected to changes in DNA methylation as well as increased oxidative damage in biogenic substances (Batra et al., 2009).

DNA methylation (DNAm) is an epigenetic mechanism that can turn off the expression of genes with the addition of a methyl group into a cytosine ring in the promoter region of the gene. DNA methylation can be used as a biomarker for epigenetic age, and obesity has been linked to epigenetic age with implications that it accelerates tissue aging (Horvath et al., 2014). Diet and obesity can also affect metabolic pathways that fight negative results from radiation induced oxidative stress (Vares et al., 2014).

Certain dietary supplements, called methyl donation agents (MDA), can alter the magnitude of the negative effect of certain forms of radiation (Batra et al., 2009).

One study that focused on the relationship between obesity, radiation, and epigenetic modification to determine if diet-induced obesity (DIO) influenced radiation- induced genotoxicity, or the amount of damage done to genetic information was conducted by Vares et al. (2014). In addition, they investigated how epigenetic regulation is related to diet and ionizing radiation. Two separate mouse strains were tested,

C57BL/6J DIO and C3H, as well as AML12 mouse liver cells. The first strain aforementioned were pre-diabetic for type 2 diabetes with elevated blood glucose and Printz 2 impaired glucose tolerance while the C3H mice are high-fat diet-induced obesity resistant, and the AML12 liver cells are simply a normal, unaltered mouse cell line. The researchers fed half of the C57BL/6J mice a 10% fat diet and the other half a 60% fat diet

(high-fat diet), doing the same for the C3H strain. Half of each group of the differently fed mice from each strain was irradiated four times a day with Gy X-rays.

The scientists analyzed the promoter methylation of 24 liver cancer-related genes in both irradiated and non-irradiated C57BL/6J mice (Figure 1). Gene expression levels of four tumor suppressor genes were significantly lower in obese and irradiated mice compared to control mice, as evident in figure 1 below. Microarray analysis of the

C57BL/6J mice miRNA showed that 97 genes, mainly linked to metabolism, stress defense mechanisms, and inflammatory processes related to obesity, were modulated significantly in the group fed a high fat diet. With their findings, the scientists in this study believe that the expression of several anti-tumor genes is affected by rapid promoter hypermethylation in obese mice. Promoter-specific hypermethylation is an increase in the epigenetic methylation of cytosine and adenosine residues in DNA and occurs often in liver carcinogenesis, affected by diet (Vares et al., 2014).

The scientists also introduced the AML12 cultured mouse cells to free fatty acids to mimic obesity related responses in the liver cells. With the liver cell and free fatty acid experiment, they determined that the addition of these acids caused the cells undergo more oxidative stress, determined by an increase in the presence of more reactive oxygen species (Figures 2, 3). Oxidative stress, which has been linked to the consequences of obesity through an increased production of reactive oxygen species from an overabundance of fatty acids, made mouse cells more sensitive to radiation exposure. Printz 3

Therefore, the combination of both obesity and irradiation are suggested to result in both a disruption of epigenetic mechanisms and deficient response systems against reactive oxygen species (Vares et al., 2014). Printz 4

As previously mentioned in the introduction, DNAm age has been thought to be a potential biomarker for epigenetic aging of such tissues. A study by Horvath et al. (2014) looked at 1215 human DNA samples from liver tissue, blood, adipose tissue, and muscle.

First, they analyzed the relationship between DNAm age, which is a measure of cell age, and chronological age in order to determine the correlation between the two (Figure 4, A-

D). They found that DNAm age and chronological age have a strong linear relationship, and defined the regression of the two together as age acceleration (Horvath et al., 2014). Printz 5

Once they determined that the two are in fact significantly related in liver tissues, adipose tissues, and blood, the researchers also looked at a second dataset of tissues in order to study the affect of BMI on age acceleration. With this analysis, shown in figure

4, G-H, they confirmed a significant correlation between age acceleration and a high BMI in liver tissues. However, there was no significant data proving that BMI affects age in non-liver tissues, which provides evidence that this biomarker for age acceleration is specific to the liver. Their findings suggest that obesity/high BMI-induced oxidative stress may directly point to acceleration of DNAm age in the liver, thus affecting the epigenome (Horvath et al., 2014).

A third study conducted relating epigenetic modification through radiation and the effect of diet on the modulation was conducted by Batra et al. (2009). Gama radiation (γ - radiation) is a demethylating agent that causes DNA damage, which could be reduced through the supplementation of certain dietary elements called methyl donating agents

(MDA) such as folate, choline, and methionine. These dietary supplements have been proven to alter DNAm by directly influencing C1 one-carbon flux. The C1 metabolic pathway regulates one-carbon flux in regards to DNA methylation and repair/synthesis.

Without a sufficient amount of dietary MDA, there is a competition between DNAm and nucleotide sequencing dependent on folate and choline metabolic pathways. This leads to an irreversible reaction in which the conversion of the substrate 5,10- methyelenetetrahydrofolate (5,10-methyleneTHF) to 5-methyltetrahydrofolate (5-methyl

THF) decreases and diminishes the amount of folate available for DNAm, altering its level. This affects the methylation of amino acids homocysteine to methionine, which is Printz 6 then converted to S-adenosylmethionine (SAM) (Batra et al., 2009). SAM is a major methyl donor that is found in almost every tissue in the body (Chiang et al., 1996)

It is suggested that gamma radiation induced stress may enhance MDA deficiency due to the need for methyl groups in DNA repair pathways formed to protect cells against the genotoxicity of the radiation. Irradiation produces oxidative stress on bio-molecules by production of reactive oxygen species, including the molecules of MDAs. MDA deficiency caused by radiation is suggested to negatively affect the C1 flux towards

DNAm, thus altering the epigenome. Therefore, the researchers in this experiment sought to compare the effect of γ -radiation on the DNAm of normal control diet fed mice to its effect on methyl-supplemented diet fed mouse livers, studying the cofactors, such as methionine and SAM, needed to mobilize C1 flux in the liver to determine the possible affect of irradiation and diet of C1 flux metabolism (Batra et al., 2009).

The researchers of this study found that the mice fed normal diets had significant decreases in metabolites needed for C1 flux after being exposed to γ -radiation, whereas the mice fed methyl-supplemented diets had folate levels above the baseline concentration observed in the control, non-irradiated mice. It is suggested based on these findings that these higher baseline concentrations can influence the C1 metabolic pathway towards DNAm. The supplementation of MDA in diet prevents γ -radiation-induced stress and can enhance certain cofactors of C1 flux, such as methionine synthase (Figure 5). The prevention of stress from radiation through added methyl donating supplements in turn prevented epigenetic modification to cells. Figure 6 below shows the change in DNAm levels in each group after irradiation. There was no significant change in the MSD fed Printz 7 mice 48 hours after irradiation, but there was a decrease in NCD fed mice (Batra et al.,

2009). Printz 8

These three studies show that everyday factors such as diet and radiation affect our epigenome more than most people are aware. Both dietary supplements and dietary obesity can affect epigenetic modification as well as methylation response to irradiation.

The first study, conducted by Vares et al. (2014), demonstrated how obesity can affect epigenetic profiles and how its adverse affects can enhance the negative epigenetic effects of radiation exposure. The second experiment by Horvath et al. (2014) discussed another way obesity can affect the epigenome based on DNAm as a biomarker that provides evidence of change in epigenetic age. It also can be concluded that dietary supplements can prevent the negative epigenetic effects of radiation exposure by adding

MDAs to certain metabolic pathways, such as C1 flux, exemplified by the third Printz 9 experiment aforementioned. In conclusion, adverse epigenetic modification can be caused through diet induced obesity and high BMI, and prevented through the regulation of certain methyl-donating dietary supplements.

Literature Cited:

Chiang PK, Gordon RK, Tal J, Zeng GC, Doctor BP, Pardhasaradhi K, McCann PP

(1996). S-adnosylmethionine and methylation. FASEB J 10:47180

Batra V, Sridhar S, Devasagayam TPA (2009). Enhanced one-carbon flux towards DNA

methylation: Effect of dietary methyl supplements against γ -radiation-induced

epigenetic modifications. doi:10.1016/j.cbi.2009.11.010

Horvath S, Erhart W, Brosch M, Ammerpohl O, von Schonfels W, Ahrens M, Heits N,

Bell J, Tsai P, Spector T, Deloukas P, Siebert R, Sipos B, Becker T, Rocken C,

Schafmayer C, Hampe J (2014) Obesity accelerates epigenetic aging of human liver.

www.pnas.org/cgi/doi:101073/pnas.1412759111

Vares G, Wang B, Ishii-Ohba H, Nenoi M, Nakajima T (2014) Diet-induced obesity

modulates epigenetic responses to ionizing radiation in mice. PLoS ONE 9(8):

e106277. doi:10.1371/journal.pone.0106277