Abstract
Elevated plasma total homocysteine (tHcy) is associated with increased risk of cardiovascular disease, but the mechanisms underlying this association are not completely understood. Cellular hypomethylation has been suggested to be a key pathophysiologic mechanism, since S-adenosylhomocysteine (AdoHcy), the Hcy metabolic precursor and a potent inhibitor of methyltransferase activity, accumulates in the setting of hyperhomocysteinemia. In this study, the impact of folate and methionine on intracellular AdoHcy levels and protein arginine methylation status was studied. Human endothelial cells were incubated with increasing concentrations of folinic acid (FnA), a stable precursor of folate, with or without methionine restriction. The levels of intracellular AdoHcy and AdoMet, tHcy in the cell culture medium, and protein-incorporated methylarginines were evaluated by suitable liquid chromatography techniques. FnA supplementation, with or without methionine restriction, reduced the level of tHcy and did not affect intracellular AdoMet levels. Interestingly, FnA supplementation reduced intracellular AdoHcy levels only in cells grown under methionine restriction. Furthermore, these cells also displayed increased protein arginine methylation status. These observations suggest that folic acid supplementation may enhance cellular methylation capacity under a low methionine status. Our results lead us to hypothesize that the putative benefits of folic acid supplementation in restoring endothelial homeostasis, thus preventing atherothrombotic events, should be reevaluated in subjects under a methionine restriction diet.
Highlights
Folate intake reduces risk of neural tube defects [1] and may protect against several other clinical conditions, including certain cancers [2,3]
Incubation of human umbilical vein endothelial cells (HUVECs) with folinic acid (FnA), a stable precursor of folate, in the presence of either dose of Met tested elicited a dose-dependent decrease in the concentration of total homocysteine (tHcy) in the cell culture medium (Figure 1)
We observed a dose-dependent decrease in extracellular tHcy concentrations in HUVECs exposed to FnA, probably reflecting an increase of biochemical clearance of Hcy by remethylation
Summary
Folate intake reduces risk of neural tube defects [1] and may protect against several other clinical conditions, including certain cancers [2,3]. Elevated levels of homocysteine (Hcy) are accepted as a risk factor/biomarker/predictor for developing cardiovascular disease [4]. Nutrients 2018, 10, 404 folic acid supplementation on cardiovascular disease prevention via lowering plasma Hcy levels has been disputed. Recent reviews and meta-analyzes have shown that folic acid supplementation does not reduce cardiovascular disease risk [5,6]. It is not clear whether Hcy itself exerts an atherogenic effect and is a direct causal risk factor for vascular disease [7]. AdoHcy is further converted to Hcy through a reversible reaction in which AdoHcy synthesis, rather than its hydrolysis, is thermodynamically favored [8]. Several studies have shown that AdoHcy, a potent inhibitor of most methyltransferases, accumulates in the setting of hyperhomocysteinemia (HHcy) [7,9,10,11,12,13,14]
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