Location: Jean Mayer Human Nutrition Research Center On Aging
2017 Annual Report
Objectives
LAB NAME: Vitamin Metabolism & Aging
1. Develop new and efficient methods for assessing nutritional status in individuals for B vitamins and their functional markers with internal standards obtained from plants grown on D2O or yeast grown on C-13 glucose.
2. Determine the biochemical, pathological and functional impact of nutritional status and genetic variations in B complex vitamin metabolism with special emphasis on the effect of age, in human, animal and cell culture models. Consider the nutritional status of other B vitamins while studying the effect of each B vitamin.
3. Determine the impact of mandatory folic acid fortification of cereals including effects of improved nutritional status as well as excess intake of folic acid on outcomes including disease risk, cognitive function, inflammation and immune response, with emphasis on understanding the mechanism behind the side-effects.
Approach
LAB NAME: Vitamin Metabolism & Aging
We will use biochemical, molecular biological and epidemiological approaches to study the role of B vitamins and the genes involved in their metabolism, in modulating processes associated with aging, disease development and increased risk for diseases. We will explore the association between pyridoxal 5’-phosphate, the active form of vitamin B6 and inflammation by measuring the immunomodulatory compounds in plasma that are produced from pyridoxal 5’-phosphate-dependent reactions in a cohort of elders. Decrease in vitamin B12 status can potentially result in decrease in muscle strength. We will explore the association between vitamin B12 and muscle strength and power, to determine the feasibility of an intervention trial to improve muscle strength. Fortification of cereals with folic acid was adopted by the US and many other countries to reduce neural tube defects. While natural folate from plants and meat can enter the metabolic pathway directly, folic acid, the synthetic form of the vitamin in supplements and fortified foods, has to be reduced by dihydrofolate reductase prior to entering the metabolic pathway. A 19bp deletion polymorphism in intron 1 of dihydrofolate reductase has been associated with increased risk for cancer in supplement users. We will determine the effect of this 19bp deletion on gene expression and enzyme activity of dihydrofolate reductase and the reactions of folate pathway. Intake of folic acid in excess of the ability of the body to metabolize it has been associated with negative health outcomes. Using a mouse model we will determine the effect of excess intake of folic acid on immune function and response to infections. We will explore the association between the polymorphism in dihydrofolate reductase gene and folic acid intake in modulating the risk for breast cancer using the samples and data from the PLCO cohort.
Progress Report
Pyridoxal 5’phosphate (PLP,) the bioactive form of vitamin B6, is a cofactor for enzymes in the kynurenine pathway of tryptophan degradation that produce immunomodulatory compounds. Increased plasma kynurenine to tryptophan ratio, a marker of kynurenine pathway activation, is seen in subjects with cardiovascular disease and has been shown to be indicative of inflammation severity. Using an Ossabaw pig model, we examined if an atherosclerotic diet causes kynurenine pathway activation by comparing it to a heart healthy diet and atorvastatin treatment. On average, pigs on atherosclerotic diet had hydroxykynurenine concentrations 5.8 times that of pigs on the same diet plus atorvastatin and 2.9 times that of pigs on a heart healthy diet only. There were no other statistically significant interactions observed for any other kynurenine metabolite, PLP, or neopterin, a marker of inflammation in plasma. Our results show that an atherogenic diet can cause an increase in tryptophan degradation. Lack of additional differences between diets could be due to inadequate treatment duration and variability in response due to use of wild-type animals.
While adequate folate nutrition is necessary for optimal immune function, high folate intake is associated with negative outcomes such as reduced immune function, metabolic syndrome and exacerbation of vitamin B12 deficiency-associated conditions. We investigated the potential mechanism by which high folic acid diet can bring about these negative outcomes using a mouse model. Young and old mice were fed a diet with adequate or excess (20x) folic acid for 3 months, and samples were collected. Small intestinal and colonic mucosal layers turn over frequently, and we used these tissues to determine damage to DNA and changes in gene expression due to high folic acid diet.
Telomeres are protective structures at the ends of chromosomes. DNA integrity and epigenetics regulate the length of telomeres. Short telomeres are associated with chronic diseases. High folic acid diet was associated with a non-significant trend for shorter telomere length in young mice, suggesting DNA damage. In old mice, telomere length was significantly longer in those on high folic acid diet when compared to those on control diet. Further research is needed to determine if this is due to loss of epigenetic control of telomere length.
Mitochondrial DNA (mtDNA) codes for proteins of oxidative phosphorylation system, and maintaining an adequate mtDNA copy number is necessary for cell survival. Studies in animal models have shown that folate deficiency can affect mtDNA content. Current data suggest that excess folic acid intake can result in conditions that resemble folate deficiency. In young mice, there was no difference in mtDNA content in control vs high folic acid diet. MtDNA content of the old mice fed a high folic acid diet was 7-fold higher than those fed a control diet. While higher mtDNA copy number is considered to have beneficial effects on survival, very high mtDNA copy number has been shown to be accompanied by nucleoid enlargement, defective transcription and mtDNA deletions in old mice. Thus, the dramatic increase in mtDNA content due to excess folic acid diet may be detrimental. Further research is required to establish if the high mtDNA content due a high folic acid diet can result in mitochondrial dysfunction in aged
mice.
To investigate how a high folic acid diet can exacerbate vitamin B12 deficiency, we also determined changes in expression of vitamin B12 transport genes (CUBN, TCN2, amnionless, gp330) of small intestinal mucosa in aged mice. There was no difference in the expression of the genes studied between the control and high folic acid diets. Our results suggest that a high folic acid diet may not affect vitamin B12 transport in small intestine.
Accomplishments
1. High folate intake is linked with nerve-damage risk in older adults with a common gene variant. An estimated one in six people in the U.S. carry two copies of a genetic variation in TCN2, the gene for a vitamin B12 transport protein transcobalamin. For some of these individuals, the TCN2 variation (referred to as GG) can lead to conditions related to vitamin B12 deficiency even if individuals consume normal amounts of B12. In an epidemiological study involving 171 adults aged 60 years and older, ARS-funded researchers in Boston, Massachusetts, in collaboration with scientists from Boston University and Pfizer Inc., found that individuals with the GG variant of TCN2 were three times more likely to have peripheral neuropathy—nerve damage commonly associated with vitamin B12 deficiency—when compared to individuals without the variant. Among study subjects who consumed more than twice the Recommended Dietary Allowance (RDA) of 800 micrograms per day of folate, individuals with the GG variant had seven-fold higher odds for peripheral neuropathy compared to those without the variation. Our data suggest that increased risk of peripheral neuropathy in older adults with the TCN2 variation may be avoided by limiting folate intake to the Recommended Dietary Allowance and exercising caution with regard to the consumption of folic acid supplements.
2. Folate from bacteria regulates cell growth of host nematode. Animal germ stem cells (GSCs) are adult stem cell populations that provide reproductive cells to allow species propagation. Like humans, the nematode C. elegans cannot produce folate and have to obtain it from diet. ARS-funded researchers in Boston, Massachusetts, in collaboration with scientists from University of Georgia, determined if folate compounds produced by bacteria can affect GSC growth of the host nematode C. elegans. They showed that a specific folate compound (10-formyl tetrahydrofolate) produced by bacteria can stimulate GSC growth, while other related folate compounds did not have this function. This property of folate is not related to its function as a vitamin in the folate metabolic pathway. A specific protein that transports folate across cell membrane (receptor FOLR1) is necessary for this phenomenon. Normally folate is transported to cells using a protein called reduced folate carrier. However, FOLR1 is present in cancer cells and immune cells during inflammation, where its function is not known. The results of this study suggest that folate receptors may be involved in cell growth in diseases like cancer, and folate produced by gut bacteria may provide signals for cell growth. This study sheds light on the mechanism behind the effect of gut microbiome in influencing the health and disease risk in humans.
