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Folic Acid Impairs the Uptake of 5-Methyltetrahydrofolate in Human Umbilical Vascular Endothelial Cells

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Background Adequate folate status supports endothelial structure and function. Folic acid (FA), an oxidized synthetic folate, which is present in the plasma of patients consuming fortified food or FA supplements, may impair cellular uptake of physiological, reduced folates. We studied the effect of FA on uptake of the dominant circulatory folate, 5-methyltetrahydrofolate (5MTHF) in endothelial cells. Methods and Results For short-term effects of FA, primary human umbilical vein endothelial cells (HUVECs) were maintained in growth medium containing 200 nM 5MTHF and preincubated with 20 nM FA 10 minutes before the 5MTHF uptake assessment. For long-term effects, HUVECs were cultured for 3 passages in growth medium containing either 200 nM 5MTHF, or a combination of 100 nM 5MTHF and 100 nM FA. 5MTHF uptake was assessed after exposing cells to 200 nM [¹³C5]-5MTHF, after which intracellular [¹³C5]-5MTHF was quantified using liquid chromatography/tandem mass spectrometry. Acute FA exposure caused a 57% reduction in 5MTHF uptake compared with control conditions (51 ± 12 vs. 22 ± 7 fmol·min⁻¹·mg⁻¹ protein; P = 0.01). Long-term exposure to FA reduced 5MTHF uptake by 41% (51 ± 12 vs. 30 ± 11 fmol·min⁻¹·mg⁻¹ protein; P = 0.05) and reduced total cellular 5MTHF levels by 47 ± 21% in HUVEC (P = 0.02). Conclusion Unmetabolized FA, which appears in the plasma after consumption of fortified food or FA supplements, may impair uptake of 5MTHF, the dominant bioactive form of folate, in HUVEC.
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... The extent of tHcy-lowering was not significantly different between the genotypes. But circulating tHcy concentration is predominantly determined by liver and kidney folate homeostasis, therefore not necessarily reflecting folate transport and intracellular folate metabolism (19). ...
... In addition, the subdued increase in serum 5-MTHF upon supplementation in subjects with deletion allele (especially del/ins genotype) implies that more FA might have remained unmetabolized, even though the extent of tHcy-lowering did not differ significantly between genotypes. There have been concerns about the detrimental effects of unmetabolized folic acid [UMFA; (16,19,22)]. In a study of the US National (1999)(2000)(2001)(2002) where there was a mandatory FA fortification policy, Morris et al. (22,23) found that the presence of detectable serum UMFA (in ∼33% of the participants) was related to poorer cognitive test performance, whereas serum 5-MTHF was associated with better performance. ...
... In a study of the US National (1999)(2000)(2001)(2002) where there was a mandatory FA fortification policy, Morris et al. (22,23) found that the presence of detectable serum UMFA (in ∼33% of the participants) was related to poorer cognitive test performance, whereas serum 5-MTHF was associated with better performance. There is evidence that UMFA could interfere with the endothelial cell uptake and cerebral transport (bloodto-brain) of circulating 5-MTHF in vivo (16,19). Although UMFA may only appear in the circulation for a short period [1-2 h; (24)], there is evidence that circulating UMFA may persist longer with sustained FA supplementation (25). ...
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... Further details are provided in the figure. High-dose folic acid supplementation has been linked to abnormal folate metabolism and issues related to folate deficiencies [75][76][77] although some caution is required in the interpretation of these studies and conclusions [78]. In our previous research on neonatal hydrocephalus, we also found a negative effect of folic acid in precipitating hydrocephalus in a susceptible rat model of this congenital condition [12]. ...
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... In support of this hypothesis, PSMA facilitates folic acid uptake by neoplastic prostate cells induced to express PSMA [77]. Folic acid also blocks the uptake of the major circulating folate, methyltetrahydrofolate, by PSMA [123]. Thus, cells expressing both high levels of PSMA and dihydrofolate reductase may preferentially metabolize folic acid to support DNA synthesis (Fig. 2, reactions 1-3 and 6) rather than other aspects of one-carbon metabolism, as would be expected with methyltetrahydrofolate (Fig. 2, reaction 6). ...
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... This generalized increase in FA intake has led to the observation that un-metabolized folic acid (UMFA) can now be found in umbilical cord blood, and in infants' circulation (Kalmbach et al., 2008;Plumptre et al., 2015;Sweeney et al., 2005). UMFA competes with 5-methylene tetrahydrofolate (5-MTHF) for folate receptor and transporter molecules; 5-MTHF is the natural folate and is immediately available for recycling of homocysteine by the one carbon cycle (1-CC) (Smith et al., 2017). This competition between natural folate and UMFA may have an impact on the resetting of methyl/epigenetic tags, which initially takes place very early in the male germline, and again during late embryogenesis (Blake et al., 2021;Ly et al., 2015). ...
... UMFA can be detected in mothers' milk, and in their infants [3]; since most of the folates share the same receptor and FA competes with 5-MTHF for uptake into the folate cycle [78], this raises an important issue surrounding a potential impact of UMFA on the regulation of epigenetic tags/methylation. ...
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Citation: Menezo, Y.; Elder, K.; Clement, A.; Clement, P. Folic Acid, Folinic Acid, 5 Methyl TetraHydroFolate Supplementation for Mutations That Affect Epigenesis through the Folate and One-Carbon Cycles.
... Similarly, dhfr-1 RNAi reduced the rate of 5MTHF uptake and assimilation over time (Fig. 2e). These findings suggest an inhibitory role of FA in the assimilation of folate cycle intermediates, supporting previous in vitro work 22,23 . ...
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... Внаслідок цього ФК має здатність інгібувати поглинання природного біоактивного фолату 5-MTГФ в ендотеліальні клітини через механізм, який, імовірно, передбачає взаємодію з PCFT. Оскільки споживання синтетичної ФК разом із метафоліном може призводити до збільшення неметаболізованої ФК в плазмі, це може вплинути на стан фолатів ендотеліальних клітин та бути тригером ятрогенної гестаційної ендотеліопатії [43]. ...
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