Intrauterine vitamin B2 uptake of preterm and full-term infants

Riboflavin deficiency alters cholesterol homeostasis partly by reducing apolipoprotein B100 synthesis in HepG2 cells

Riboflavin deficiency led to lower blood cholesterol level and higher content of hepatic cholesterol in rats and the mechanisms are not clarified yet. We hypothesized that riboflavin deficiency might alter cholesterol homeostasis via apolipoprotein B100, one of the important proteins in cholesterol transport. To test this hypothesis, HepG2 cells were cultured in riboflavin-deficient media for 4 days to develop riboflavin deficiency. Compared to riboflavin-sufficient cells, the mRNA (0. 37 ± 0.04 vs 1.03 ± 0.29 relative expression level, n = 3) and protein expressions of apolipoprotein B100 (intracellular: 173.7 ± 14.4 vs 254.8 ± 47.2 μg/mg protein; extracellular: 93.8 ± 31.1 vs 161.6 ± 23.9 μg/mg protein; n = 3) were significantly reduced in riboflavin-deficient cells (P < 0.05). Endoplasmic reticulum oxidoreductin 1 and protein disulfide isomerase, two enzymes involved in the oxidative folding of apolipoprotein B100, were also lower remarkably in expression at both mRNA and protein levels. Meanwhile, intracellular cholesterol was increased (256.3 ± 17.1 μM/g protein vs 181.4 ± 23.9 μM/g protein, n = 4) and extracellular cholesterol decreased (110.0 ± 23.2 μM/g protein vs 166.2 ± 34.6 μM/g protein, n = 4) significantly in riboflavin-deficient cells (P < 0.05). Very low-density lipoprotein was also diminished (29.0 ± 6.1 μM/g protein vs 67.0 ± 11.0 μM/g protein, n = 4) in the culture media (P < 0.05). These findings suggest that riboflavin deficiency alters cholesterol homeostasis partly by reducing apolipoprotein B100 synthesis in HepG2 cells.

Dietary Reference Values for riboflavin

Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) derives dietary reference values (DRVs) for riboflavin. The Panel considers that the inflection point in the urinary riboflavin excretion curve in relation to riboflavin intake reflects body saturation and can be used as a biomarker of adequate riboflavin status. The Panel also considers that erythrocyte glutathione reductase activation coefficient is a useful biomarker, but has limitations. For adults, the Panel considers that average requirements (ARs) and population reference intakes (PRIs) can be determined from the weighted mean of riboflavin intake associated with the inflection point in the urinary riboflavin excretion curve reported in four intervention studies. PRIs are derived for adults and children assuming a coefficient of variation of 10%, in the absence of information on the variability in the requirement and to account for the potential effect of physical activity and the methylenetetrahydrofolate reductase 677TT genotype. For adults, the AR and PRI are set at 1.3 and 1.6 mg/day. For infants aged 7–11 months, an adequate intake of 0.4 mg/day is set by upward extrapolation from the riboflavin intake of exclusively breastfed infants aged 0–6 months. For children, ARs are derived by downward extrapolation from the adult AR, applying allometric scaling and growth factors and considering differences in reference body weight. For children of both sexes aged 1–17 years, ARs range between 0.5 and 1.4 mg/day, and PRIs between 0.6 and 1.6 mg/day. For pregnant or lactating women, additional requirements are considered, to account for fetal uptake and riboflavin accretion in the placenta during pregnancy or the losses through breast milk, and PRIs of 1.9 and 2.0 mg/day, respectively, are derived.

This publication is linked to the following EFSA Supporting Publications article: http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2017.EN-1268/full

Riboflavin deficiency induces a significant change in proteomic profiles in HepG2 cells

Riboflavin deficiency is widespread in many regions over the world, especially in underdeveloped countries. In this study, we investigated the effects of riboflavin deficiency on protein expression profiles in HepG2 cells in order to provide molecular information for the abnormalities induced by riboflavin deficiency. HepG2 cells were cultured in media containing different concentrations of riboflavin. Changes of cell viability and apoptosis were assessed. A comparative proteomic analysis was performed using a label-free shotgun method with LC-MS/MS to investigate the global changes of proteomic profiles in response to riboflavin deficiency. Immunoblotting test was used to validate the results of proteomic approach. The cell viability and apoptosis tests showed that riboflavin was vital in maintaining the cytoactivity of HepG2 cells. The label-free proteomic analysis revealed that a total of 37 proteins showing differential expression (±2 fold, p < 0.05) were identified after riboflavin deficiency. Bioinformatics analysis indicated that the riboflavin deficiency caused an up-regulation of Parkinson's disease pathway, steroid catabolism, endoplasmic reticulum stress and apoptotic process, while the fatty acid metabolism, tricarboxylic citrate cycle, oxidative phosphorylation and iron metabolism were down-regulated. These findings provide a molecular basis for the elucidation of the effects caused by riboflavin deficiency.

Transcriptional regulation of the albumin gene depends on the removal of histone methylation marks by the FAD-dependent monoamine oxidase lysine-specific demethylase 1 in HepG2 human hepatocarcinoma cells

Lysine-specific demethylase (LSD) 1 is an FAD-dependent demethylase that catalyzes the removal of methyl groups from lysine-4 in histone H3, thereby mediating gene repression. Here we tested the hypothesis that riboflavin deficiency causes a loss of LSD1 activity in HepG2 human hepatocarcinoma cells, leading to an accumulation of lysine-4-dimethylated histone H3 (H3K4me2) marks in the albumin promoter and aberrant upregulation of albumin expression. Cells were cultured in riboflavin-defined media providing riboflavin at concentrations representing moderately deficient (3.1 nmol/L), sufficient (12.6 nmol/L), and supplemented (301 nmol/L) cells in humans for 7 d. The efficacy of treatment was confirmed by assessing glutathione reductase activity and concentrations of reduced glutathione as markers of riboflavin status. LSD activity was 21% greater in riboflavin-supplemented cells compared with riboflavin-deficient and -sufficient cells. The loss of LSD activity was associated with a gain in the abundance of H3K4me2 marks in the albumin promoter; the abundance of H3K4me2 marks was ∼170% higher in riboflavin-deficient cells compared with sufficient and supplemented cells. The abundance of the repression mark, K9-trimethylated histone H3, was 38% lower in the albumin promoter of riboflavin-deficient cells compared with the other treatment groups. The expression of albumin mRNA was aberrantly increased by 200% in riboflavin-deficient cells compared with sufficient and supplemented cells. In conclusion, riboflavin deficiency impairs gene regulation by epigenetic mechanisms, mediated by a loss of LSD1 activity.

Secondary coenzyme Q10 deficiency and oxidative stress in cultured fibroblasts from patients with riboflavin responsive multiple Acyl-CoA dehydrogenation deficiency

Coenzyme Q10 (CoQ10) is essential for the energy production of the cells and as an electron transporter in the mitochondrial respiratory chain. CoQ10 links the mitochondrial fatty acid β-oxidation to the respiratory chain by accepting electrons from electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO). Recently, it was shown that a group of patients with the riboflavin responsive form of multiple acyl-CoA dehydrogenation deficiency (RR-MADD) carrying inherited amino acid variations in ETF-QO also had secondary CoQ10 deficiency with beneficial effects of CoQ10 treatment, thus adding RR-MADD to an increasing number of diseases involving secondary CoQ10 deficiency. In this study, we show that moderately decreased CoQ10 levels in fibroblasts from six unrelated RR-MADD patients were associated with increased levels of mitochondrial reactive oxygen species (ROS). Treatment with CoQ10, but not with riboflavin, could normalize the CoQ10 level and decrease the level of ROS in the patient cells. Additionally, riboflavin-depleted control fibroblasts showed moderate CoQ10 deficiency, but not increased mitochondrial ROS, indicating that variant ETF-QO proteins and not CoQ10 deficiency are the causes of mitochondrial ROS production in the patient cells. Accordingly, the corresponding variant Rhodobacter sphaeroides ETF-QO proteins, when overexpressed in vitro, bind a CoQ10 pseudosubstrate, Q10Br, less tightly than the wild-type ETF-QO protein, suggesting that molecular oxygen can get access to the electrons in the misfolded ETF-QO protein, thereby generating superoxide and oxidative stress, which can be reversed by CoQ10 treatment.

Riboflavin deprivation inhibits macrophage viability and activity – a study on the RAW 264.7 cell line

Riboflavin, or vitamin B2, as a precursor of the coenzymes FAD and FMN, has an indirect influence on many metabolic processes and determines the proper functioning of several systems, including the immune system. In the human population, plasma riboflavin concentration varies from 3·1 nm(in a moderate deficiency, e.g. in pregnant women) to 10·4 nm(in healthy adults) and 300 nm(in cases of riboflavin supplementation). The purpose of the present study was to investigate the effects of riboflavin concentration on the activity and viability of macrophages, i.e. on one of the immunocompetent cell populations. The study was performed on the murine monocyte/macrophage RAW 264.7 cell line cultured in medium with various riboflavin concentrations (3·1, 10·4, 300 and 531 nm). The results show that riboflavin deprivation has negative effects on both the activity and viability of macrophages and reduces their ability to generate an immune response. Signs of riboflavin deficiency developed in RAW 264.7 cells within 4 d of culture in the medium with a low riboflavin concentration (3·1 nm). In particular, the low riboflavin content reduced the proliferation rate and enhanced apoptotic cell death connected with the release of lactate dehydrogenase. The riboflavin deprivation impaired cell adhesion, completely inhibited the respiratory burst and slightly impaired phagocytosis of the zymosan particles. In conclusion, macrophages are sensitive to riboflavin deficiency; thus, a low riboflavin intake in the diet may affect the immune system and may consequently decrease proper host immune defence.

Dynamin 2 Regulates Riboflavin Endocytosis in Human Placental Trophoblasts

Riboflavin is thoroughly established to be indispensable in a multitude of cellular oxidation-reduction reactions through its conversion to coenzyme forms flavin mononucleotide and flavin adenine dinucleotide. Despite its physiological importance, little is known about specific mechanisms or proteins involved in regulating its cellular entry in humans. Studies involving biochemical modulators and immunological inhibition assays have indirectly revealed that riboflavin internalization and trafficking occurs at least in part through a clathrin-dependent receptor-mediated endocytic process. Here, using a two-tiered strategy involving RNA interference and the overexpression of dominant-negative constructs, we directly show the involvement of this endocytic mechanism through the requirement of the pluripotent endocytic vesicle scission enzyme, dynamin 2 GTPase, in human placental trophoblasts. Similar to the endocytic control ligand, transferrin, riboflavin is shown to exhibit 50% dependence on the functional expression of dynamin 2 for its active cellular entry. Furthermore, this reduced vitamin uptake correlates with >2-fold higher riboflavin association at the cell surface. In addition, fluorescent ligand endocytosis assays showing colocalization between rhodamine-riboflavin and the immunostained caveolar coat protein, caveolin 1, suggest that the active absorption of this important nutrient involves multiple and distinct endocytosis pathways.

HepG2 cells develop signs of riboflavin deficiency within 4 days of culture in riboflavin-deficient medium

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are essential coenzymes in redox reactions. For example, FAD is a coenzyme for both glutathione reductase and enzymes that mediate the oxidative folding of secretory proteins. Here we investigated short-term effects of moderately riboflavin-deficient culture medium on flavin-related responses in HepG2 hepatocarcinoma cells. Cells were cultured in riboflavin-deficient (3.1 nmol/l) medium for up to 6 days; controls were cultured in riboflavin-sufficient (532 nmol/l) medium. The activity of glutathione reductase decreased by 98% within 4 days of riboflavin-deficient culture. Transport rates of riboflavin increased in response to riboflavin depletion, whereas expression of enzymes mediating flavocoenzyme synthesis (flavokinase and FAD synthetase) decreased in response to depletion. The oxidative folding and synthesis of plasminogen and apolipoprotein B-100 was impaired within 4 days of culture in riboflavin-deficient medium; this is consistent with impaired processing of secretory proteins in riboflavin-deficient cells. Riboflavin depletion was associated with increased DNA-binding activities of transcription factors with affinity for endoplasmic reticulum stress elements and nuclear factor kappaB (NF-kappaB) consensus elements, suggesting cell stress. Moreover, the abundance of the stress-induced protein GADD153 was greater in riboflavin-deficient cells compared with controls. Riboflavin deficiency was associated with decreased rates of cell proliferation caused by arrest in G1 phase of the cell cycle. These studies are consistent with the hypothesis that HepG2 cells have a great demand for riboflavin and that cell stress develops rapidly if riboflavin supply is marginally low.

Riboflavin deficiency impairs oxidative folding and secretion of apolipoprotein B-100 in HepG2 cells, triggering stress response systems

Secretory proteins such as apolipoprotein B-100 (apoB) undergo oxidative folding (formation of disulfide bonds) in the endoplasmic reticulum (ER) before secretion. Oxidative folding depends on flavoproteins in eukaryotes. Here, human liver (HepG2) cells were used to model effects of riboflavin concentrations in culture media on folding and secretion of apoB. Cells were cultured in media containing 3.1, 12.6, and 300 nmol/L of riboflavin, representing moderately deficient, physiological, and pharmacological plasma concentrations in humans, respectively. When cells were cultured in riboflavin-deficient medium, secretion of apoB decreased by >80% compared with controls cultured in physiological medium. The nuclear translocation of the transcription factor ATF-6 increased by >180% in riboflavin-deficient cells compared with physiological controls; this is consistent with ER stress. Nuclear translocation of ATF-6 was associated with activation of the unfolded protein response. Expression of stress-response genes coding for ubiquitin-activating enzyme 1, growth arrest and DNA damage inducible gene, and glucose regulated protein of 78 kDa was greater in riboflavin-deficient cells compared with other treatment groups. Finally, phosphorylation of the eukaryotic initiation factor (eukaryotic initiation factor 2alpha) increased in riboflavin-deficient cells, consistent with decreased translational activity. We conclude 1) that riboflavin deficiency causes ER stress and activation of unfolded protein response in HepG2 cells, and 2) that riboflavin deficiency decreases protein secretion in HepG2 cells. Decreased secretion of apoB in riboflavin-deficient cells might interfere with lipid homeostasis in vivo.

Riboflavin deficiency causes protein and DNA damage in HepG2 cells, triggering arrest in G1 phase of the cell cycle

Eukaryotes convert riboflavin to flavin adenine dinucleotide, which serves as a coenzyme for glutathione reductase and other enzymes. Glutathione reductase mediates the regeneration of reduced glutathione, which plays an important role in scavenging free radicals and reactive oxygen species. Here we tested the hypothesis that riboflavin deficiency decreases glutathione reductase activity in HepG2 liver cells, causing oxidative damage to proteins and DNA, and cell cycle arrest. As a secondary goal, we determined whether riboflavin deficiency is associated with gene expression patterns indicating cell stress. Cells were cultured in riboflavin-deficient and riboflavin-supplemented media for 4 days. Activity of glutathione reductase was not detectable in cells cultured in riboflavin-deficient medium. Riboflavin deficiency was associated with an increase in the abundance of damaged (carbonylated) proteins and with increased incidence of DNA strand breaks. Damage to proteins and DNA was paralleled by increased abundance of the stress-related transcription factor GADD153. Riboflavin-deficient cells arrested in G1 phase of the cell cycle. Moreover, oxidative stress caused by riboflavin deficiency was associated with increased expression of clusters of genes that play roles in cell stress and apoptosis. For example, the abundance of the pro-apoptotic pleiomorphic adenoma gene-like 1 gene was 183% greater in riboflavin-deficient cells compared with riboflavin-sufficient controls. We conclude that riboflavin deficiency is associated with oxidative damage to proteins and DNA in liver cells, leading to cell stress and G1 phase arrest.

Current perspectives on the cellular uptake and trafficking of riboflavin

The role of riboflavin in cell maintenance and growth, and the mechanism by which it is absorbed into various human tissues and cell lines has been extensively studied over the past decade. Evidence suggests two absorption mechanisms, a saturable-active component that dominates at near physiological vitamin concentrations and a passive component that is revealed at oversupplemented riboflavin conditions. Various transport modulator studies consistently suggest a highly riboflavin specific, temperature-dependent active transport mechanism that is regulated by the Ca2+/calmodulin pathway. The PKA and PKG pathways have also been implicated in absorption regulation. The long-standing model that riboflavin absorption involves a carrier-mediated transporter has recently been challenged through studies suggesting a receptor-mediated endocytic component. The presence of a soluble, human riboflavin binding protein in the transport stratagem has been shown to play an important role in fetal development. The relationship of this binding protein with the riboflavin specific membrane bound protein, though currently not well defined, may involve a protein-protein interaction that plays a primary role in this proposed receptor-mediated component.

Oxidative folding of interleukin-2 is impaired in flavin-deficient jurkat cells, causing intracellular accumulation of interleukin-2 and increased expression of stress response genes

Secretory proteins such as interleukin (IL)-2 undergo oxidative folding (disulfide formation) in the endoplasmic reticulum (ER) before secretion. Studies in yeast have suggested that oxidative folding depends on the flavoprotein Ero1p; unfolded proteins accumulate in the ER, triggering cellular stress response. Here, human lymphoid cells (Jurkat cells) were used to model effects of cellular flavin supply on secretion of IL-2 (containing one disulfide bond) and cellular stress response. Cells were cultured in media containing 0.85, 3.1, 12.6 or 300.6 nmol/L riboflavin for 5 wk, representing severely deficient, moderately deficient, physiologic and pharmacologic plasma concentrations in humans, respectively. Transport rates of riboflavin were increased in severely and moderately deficient cells compared with cells cultured in physiologic medium; this increase was not sufficient to prevent intracellular depletion of riboflavin, as judged by glutathione reductase activity and intracellular concentrations of glutathione. Intracellular accumulation of IL-2 was greater in severely deficient cells than in other groups. Nevertheless, severely deficient cells secreted normal amounts of IL-2 into the extracellular space, mediated by increased transcriptional activity of the IL-2 gene. Riboflavin-deficient cells responded to intracellular accumulation of IL-2 with increased expression of genes encoding ubiquitin-activating enzyme E1 and X box-binding protein, consistent with cellular stress. These findings are consistent with the hypothesis that flavin deficiency may cause cellular stress by accumulation of unfolded proteins in human cells.

The relationship between maternal and neonatal umbilical cord plasma homocyst(e)ine suggests a potential role for maternal homocyst(e)ine in fetal metabolism

OBJECTIVE

Data on fetal blood homocyst(e)ine concentrations are not available. We tested the hypothesis that homocyst(e)ine crosses the maternal/placental/fetal interphases and is sequestered by the fetus.

STUDY DESIGN

The concentration of homocyst(e)ine was determined at parturition in peripheral venous plasma from 35 nulliparous healthy pregnant women and umbilical arterial and venous plasma from their conceptus.

RESULTS

Findings demonstrated a descending concentration gradient of plasma homocyst(e)ine from maternal vein to umbilical vein and to umbilical artery; the decrease at each interphase approximated 1 micromol/L. The neonate weight and gestational age were inversely related to maternal homocyst(e)ine concentrations.

CONCLUSION

The umbilical vein to umbilical artery homocyst(e)ine decrement suggests that uptake of homocyst(e)ine occurs in the fetus. The likely incorporation of homocyst(e)ine into the fetal metabolic cycle may implicate maternal homocyst(e)ine as having a potential nutritional role in the fetus. Further studies are required to explain the role of homocyst(e)ine in fetal metabolism and development.