Regulatory mechanisms of intestinal folate uptake in a rat model of folate oversupplementation

Low-dose daily folic acid (400 μg) supplementation does not affect regulation of folate transporters found present throughout the terminal ileum and colon of humans: a randomized clinical trial

BACKGROUND

Folic acid supplementation during the periconceptional period reduces the risk of neural tube defects in infants, but concern over chronic folic acid exposure remains. An improved understanding of folate absorption may clarify potential risks. Folate transporters have been characterized in the small intestine, but less so in the colon of healthy, free-living humans. The impact of folic acid fortification or supplementation on regulation of these transporters along the intestinal tract is unknown.

OBJECTIVE

The objective was to characterize expression of folate transporters/receptor (FT/R) and folate hydrolase, glutamate carboxypeptidase II (GCPII), from the terminal ileum and throughout the colon of adults and assess the impact of supplemental folic acid.

METHODS

In this 16-wk open-labeled randomized clinical trial, adults consumed a low folic acid-containing diet, a folate-free multivitamin, and either a 400 μg folic acid supplement or no folic acid supplement. Dietary intakes and blood were assessed at baseline, 8 wk, and 16 wk (time of colonoscopy). Messenger RNA (mRNA) expression and protein expression of FT/R and GCPII were assessed in the terminal ileum, cecum, and ascending and descending colon.

RESULTS

Among 24 randomly assigned subjects, no differences in dietary folate intake or blood folate were observed at baseline. Mean ± SD red blood cell folate at 16 wk was 1765 ± 426 and 911 ± 242 nmol/L in the 400 and 0 μg folic acid group, respectively (P < 0.0001). Reduced folate carrier, proton-coupled folate transporter, and folate-receptor alpha expression were detected in the terminal ileum and colon, as were efflux transporters of breast cancer resistance protein and multidrug resistance protein-3. Other than a higher mRNA expression of FR-alpha and GCPII in the 400 μg supplement group in the ascending colon, no treatment differences were observed (P < 0.02).

CONCLUSIONS

Folate transporters are present throughout the terminal ileum and colon; there is little evidence that a low dose of folic acid supplementation affects colonic absorption. This trial was registered at clinicaltrials.gov as NCT03421483.

Folic Acid: Sources, Chemistry, Absorption, Metabolism, Beneficial Effects on Poultry Performance and Health

Recently, there has been an increasing interest in the study of the effects of folic acid (FA) on poultry because it was observed that FA could overcome problems in poultry health while improving its performance. FA, or folate, is a water-soluble B vitamin essential in poultry, so FA intake must be available in the feed. Sources of FA in feed come from plants or animals, and animal sources have relatively more stable FA. The ingested FA will be absorbed in the intestinal lumen and transported into the liver through the blood vessels. Therefore, FA has a positive effect on the performance and health status of poultry. The effect of FA on poultry performance is to increase reproductive tract development, FA content in eggs, hatchability, weight gain, average initial body weight, feed intake, relative growth rate, chick body weight, breast fillet percentage, and reduce FCR and white striping score. At the same time, the effect on poultry health influences antioxidant activities, thyroid hormones, blood biochemicals, anti-inflammatory gene expressions, and immune responses. The present review deals with FA sources, chemistry, absorption, metabolism, effects on performance, and poultry health, which are based on valid basic information.

Folic Acid Absorption Characteristics and Effect on Cecal Microbiota of Laying Hens

This experiment was conducted to investigate the characteristics of folic acid (FA) absorption in laying hens and the effect of FA supplementation on cecal microbiota. A total of 432 healthy hens (30-week-old) were randomly assigned to four diets supplemented with FA: 0, 1, 6, and 24 mg/kg of feed for 8 w. Blood, duodenum, jejunum, ileum, cecum, and cecal chyme samples (six samples per treatment) were collected from the hens at the end of the feeding trial. Expression profiles of folate transport and transformation genes in intestine and cecal microbiota were detected. Results showed that serum folate level significantly increased (P < 0.01) with an increase in dietary FA supplementation, reaching a plateau at 6 mg/kg FA supplementation. The expression of FA transport and transformation genes was not affected in the cecum (P > 0.05) by dietary FA supplementation; however, it was affected in the duodenum, jejunum, and ileum and mostly showed a downward trend in treatment groups (P < 0.05). The genes affected include duodenal folate receptor (Folr) and dihydrofolate reductase (Dhfr), jejunal proton-coupled folate transporter (Pcft) and reduced folate carrier (Rfc), and ileal ATP binding cassette subfamily C member (Abcc2), Abcc3, Rfc, Folr, and Dhfr. Furthermore, according to the operational taxonomic unit classification and taxonomic position identification, the cecal microbiota population of the hens was not affected by dietary FA supplementation at the phylum, class, order, family, genus, and species levels (P > 0.05). However, the relative abundance of some microbiota was affected by dietary FA supplementation (P < 0.05). In conclusion, FA transport from the intestinal lumen into enterocytes, and then into the bloodstream, is strictly regulated, which may be associated with the regulation of the expression profiles of genes involved in FA absorption. Pathogenic bacteria decreased in the cecum, especially at 24 mg/kg supplementation, but the beneficial bacteria (Bifidobacteriaceae) decreased at this level, too. Overall, FA supplementation at 6 mg/kg, which was selected for folate-enriched egg production, did not affect the health and metabolism of laying hens negatively.

Clinical Implications of Folate Transport in the Central Nervous System

Folates are essential for key biosynthetic processes in mammalian cells and play a crucial role in the maintenance of central nervous system homeostasis. Mammals lack the metabolic capacity for folate biosynthesis; hence, folate requirements are largely met through dietary sources. To date, three major folate transport pathways have been characterized: the folate receptors (FRs), reduced folate carrier (RFC), and proton-coupled folate transporter (PCFT). This article reviews current knowledge on the role of folate transport systems in mediating folate delivery to vital tissues, particularly the brain, and how these pathways are modulated by various regulatory mechanisms. We will also briefly highlight the clinical significance of cerebral folate transport in relation to neurodevelopmental disorders associated with folate deficiency.

Folate transporter expression in placenta from pregnancies complicated with birth defects

BACKGROUND

Folate plays a fundamental role for fetal development, participating in cell division, embryogenesis, and fetal growth. The fetus depends on maternal supply of folate across the placenta. The objective of this study was to compare the expression of Folate Receptor-α (FR-α), Reduced Folate Carrier (RFC), and Proton Coupled Folate Transporter (PCFT) in placentas from pregnancies complicated with birth defects (BD) and controls.

METHODS

Case-control study, including placentas of BD-complicated pregnancies (n = 25) and a control group (n = 25). We determined the placental expression of FR-α, RFC, and PCFT by immunohistochemistry. Optical density was measured to obtain a relative quantification of the expression.

RESULTS

The expression of PCFT was greater in placentas from pregnancies complicated with BD than in those from the control group (p < .01). The expression of FR-α and RFC was not different between groups.

CONCLUSION

The expression of PCFT in placentas from BD-complicated pregnancies is increased, possibly as an adaptive response to increase the folate flux at the maternal-fetal interface.

Mechanistic target of rapamycin (mTOR) regulates trophoblast folate uptake by modulating the cell surface expression of FR-α and the RFC

Folate deficiency in fetal life is strongly associated with structural malformations and linked to intrauterine growth restriction. In addition, limited availability of methyl donors, such as folate, during pregnancy may result in abnormal gene methylation patterns and contribute to developmental programming. The fetus is dependent on placental transfer of folate, however the molecular mechanisms regulating placental folate transport are unknown. We used cultured primary human trophoblast cells to test the hypothesis that mechanistic target of rapamycin complex 1 (mTORC1) and 2 (mTORC2) regulate folate transport by post-translational mechanisms. Silencing raptor (inhibits mTORC1) or rictor (inhibits mTORC2) markedly decreased basal folate uptake. Folate uptake stimulated by insulin + IGF-1 was mediated by mTORC2 but did not involve mTORC1. mTORC1 or mTORC2 silencing markedly decreased the plasma membrane expression of FR-α and RFC transporter isoforms without affecting global protein expression. Inhibition of the ubiquitin ligase Nedd4-2 had no effect on folate transport. In conclusion, we report for the first time that mTORC1/C2 are positive regulators of cellular folate uptake by modulating the cell surface abundance of specific transporter isoforms. We propose that regulation of placental folate transport by mTOR signaling provide a direct link between placental function, gene methylation and fetal programming.

Identification of regulatory mechanisms of intestinal folate transport in condition of folate deficiency

Folic acid is an essential micronutrient, deficiency of which can lead to disturbance in various metabolic processes of cell. Folate transport across intestine occurs via the involvement of specialized folate transporters viz. proton coupled folate transporter (PCFT) and reduced folate carrier (RFC), which express at the membrane surfaces. The current study was designed to identify the regulatory mechanisms underlying the effects of folate deficiency (FD) on folate transport in human intestinal cell line as well as in rats and to check the reversibility of such effects. Caco-2 cells were grown for five generations in control and FD medium. Following treatment, one subgroup of cells was shifted on folate sufficient medium and grown for three more generations. Similarly, rats were fed an FD diet for 3 and 5 months, and after 3 months of FD treatment, one group of rats were shifted on normal folate-containing diet. Increase in folate transport and expression of folate transporters were observed on FD treatment. However, when cells and rats were shifted to control conditions after treatment, transport and expression of these genes restored to the control level. FD was found to have no impact on promoter methylation of PCFT and RFC; however, messenger RNA stability of transporters was found to be decreased, suggesting some adaptive response. Overall, increased expression of transporters under FD conditions can be attributed to enhanced rate of transcription of folate transporters and also to the increased binding of specificity protein 1 transcription factor to the RFC promoter only.

Reduced SP1-mediated transcriptional activation decreases expression of intestinal folate transporters in response to ethanol exposure

SCOPE

The study was designed to identify the regulatory mechanisms underlying the effects of ethanol exposure on intestinal folate transport and to investigate the reversibility of such effects.

METHODS AND RESULTS

Caco-2 cells were grown in control and ethanol containing medium for 96 h. Thereafter, one subgroup of cells was shifted on ethanol free medium and grown for next 72 h. For in vivo studies, rats were given 1g ethanol/kg body weight/day either for 3 or 5 months and after 3 months of ethanol treatment, one group of rats received no ethanol for 2 months. A significant decrease in folic acid transport as well as expression of folate transporters was observed on ethanol treatment and the effects were reversible upon removal of ethanol. Ethanol exposure had no impact on CpG island methylation of the folate transporters however, an increase in their mRNA half-life was observed that seems to be a homeostatic mechanism. Chromatin immunoprecipitation assay revealed a decrease in binding of SP1 transcription factor to the promoter regions of folate transporters.

CONCLUSION

Reduced binding of SP1 to the promoter region of folate transporters may be a part of the regulatory mechanism resulting in decreased expression of folate transporters on ethanol exposure.

Regulation at multiple levels control the expression of folate transporters in liver cells in conditions of ethanol exposure and folate deficiency

Complex regulatory mechanisms control the expression of folate transporters within cells. Liver is the primary reserve of the folate stores within the body. As excessive alcohol consumption or inefficient dietary folate intake are known to create folate deficiency, so therefore the current study was designed to explore various regulatory mechanisms controlling the expression of folate transport in liver cells in conditions of ethanol exposure and folate deficiency. In order to see whether the effects mediated by the treatments are reversible or not, ethanol removal, and folate repletion was done after ethanol exposure and folate deficiency treatment respectively. Folate deficiency resulted an increase, whereas ethanol treatment decreased the folic acid uptake within the cells. The alterations in folic acid uptake were in agreement with the observed changes in the expression of folate transporters. Ethanol exposure resulted an increase in promoter methylation of reduced folate carrier; however, folate deficiency had no effect. The effects produced by ethanol exposure and folate deficiency were found to be reversible in nature as depicted in case of ethanol removal and folate repletion group. Rate of synthesis of folate transporters was found to be increased whereas half lives of mRNA of folate transporters was found to be decreased on folate deficiency treatment and reverse was the case on ethanol treatment. Overall, alteration in the expression of folate transporters under ethanol exposure and folate deficient conditions can be attributed to those regulatory mechanisms which work at the mRNA level.

The intestinal absorption of folates

The properties of intestinal folate absorption were documented decades ago. However, it was only recently that the proton-coupled folate transporter (PCFT) was identified and its critical role in folate transport across the apical brush-border membrane of the proximal small intestine established by the loss-of-function mutations identified in the PCFT gene in subjects with hereditary folate malabsorption and, more recently, by the Pcft-null mouse. This article reviews the current understanding of the properties of PCFT-mediated transport and how they differ from those of the reduced folate carrier. Other processes that contribute to the transport of folates across the enterocyte, along with the contribution of the enterohepatic circulation, are considered. Important unresolved issues are addressed, including the mechanism of intestinal folate absorption in the absence of PCFT and regulation of PCFT gene expression. The impact of a variety of ions, organic molecules, and drugs on PCFT-mediated folate transport is described.

Natural folates from biofortified tomato and synthetic 5-methyl-tetrahydrofolate display equivalent bioavailability in a murine model

Folate deficiency is a global health problem related to neural tube defects, cardiovascular disease, dementia, and cancer. Considering that folic acid (FA) supply through industrialized foods is the most successful intervention, limitations exist for its complete implementation worldwide. Biofortification of plant foods, on the other hand, could be implemented in poor areas as a complementary alternative. A biofortified tomato fruit that accumulates high levels of folates was previously developed. In this study, we evaluated short-term folate bioavailability in rats infused with this folate-biofortified fruit. Fruit from tomato segregants hyperaccumulated folates during an extended ripening period, ultimately containing 3.7-fold the recommended dietary allowance in a 100-g portion. Folate-depleted Wistar rats separated in three groups received a single dose of 1 nmol of folate/g body weight in the form of lyophilized biofortified tomato fruit, FA, or synthetic 5-CH3-THF. Folate bioavailability from the biofortified tomato was comparable to that of synthetic 5-CH3-THF, with areas under the curve (AUC(0-∞)) of 2,080 ± 420 and 2,700 ± 220 pmol · h/mL, respectively (P = 0.12). Whereas, FA was less bioavailable with an AUC(0-∞) of 750 ± 10 pmol · h/mL. Fruit-supplemented animals reached maximum levels of circulating folate in plasma at 2 h after administration with a subsequent steady decline, while animals treated with FA and synthetic 5-CH3-THF reached maximum levels at 1 h. Pharmacokinetic parameters revealed that biofortified tomato had slower intestinal absorption than synthetic folate forms. This is the first study that demonstrates the bioavailability of folates from a biofortified plant food, showing its potential to improve folate deficiency.

Reduced expression of folate transporters in kidney of a rat model of folate oversupplementation

Folic acid is the key one-carbon donor required for de novo nucleotide and methionine synthesis. Its deficiency is associated with megaloblastic anemia, cancer and various complications of pregnancy. However, its supplementation results in reduction of neural tube defects and prevention of several types of cancer. The intake of folic acid from fortified food together with the use of nutritional supplements creates a state of folate oversupplementation. Fortification of foods is occurring worldwide with little knowledge of the potential safety and physiologic consequences of intake of such high doses of folic acid. So, we planned to examine the effects of acute and chronic folate oversupplementation on the physiology of renal folate transport in rats. Male Wistar rats were procured and divided into two groups. Rats in group I were given semisynthetic diets containing 2 mg folic acid/kg diet (control) and those in group II were given folate-oversupplemented rat diet, i.e., 20 mg folic acid/kg diet (oversupplemented). Six animals from group I and group II received the treatment for 10 days (acute treatment) and remaining six for 60 days (chronic treatment). In acute folate-oversupplemented rats, 5-[(14)C]-methyltetrahydrofolate uptake was found to be significantly reduced, as compared to chronic folate-oversupplemented and control rats. This reduction in uptake was associated with a significant decrease in the mRNA and protein levels of the folate transporters. Results of the present investigation showed that acute oversupplementation led to a specific and significant down-regulation of renal folate uptake process mediated via transcriptional and translational regulatory mechanism(s).

Safety and fortification with folic acid in neonatal period

Folic acid, the essential vitamin, and its active forms are substantial parts of many biochemical processes in the human body. In the period of rapid growth of organism or in cell growth, body’s demands for folate increase. Its impact in neonatal period varies even in premature newborns. Fortification with folic acid and its substitution in the treatment of anaemia are the important parts in the comprehensive care in premature newborns. To determine optimal dose in this group of patients is difficult. The determination of red blood cell folate concentration levels is the most accurate indicator of long-term folate level status in the body. Unmetabolised folic acid in circulation of newborns could have potentially adverse effects. Toxicity of folic acid is not a concern as folate is water-soluble and easily excreted by kidneys when in excess but on the other side growing organism of preterm newborn and disruption of metabolic balance could be potential risks.

Decreased activity of folate transporters in lipid rafts resulted in reduced hepatic folate uptake in chronic alcoholism in rats

Folic acid is an essential nutrient that is required for one-carbon biosynthetic processes and for methylation of biomolecules. Deficiency of this micronutrient leads to disturbances in normal physiology of cell. Chronic alcoholism is well known to be associated with folate deficiency, which is due in part to folate malabsorption. The present study deals with the regulatory mechanisms of folate uptake in liver during chronic alcoholism. Male Wistar rats were fed 1 g/kg body weight/day ethanol (20 % solution) orally for 3 months, and the molecular mechanisms of folate uptake were studied in liver. The characterization of the folate transport system in liver basolateral membrane (BLM) suggested it to be a carrier mediated and acidic pH dependent, with the major involvement of proton coupled folate transporter and folate binding protein in the uptake. The folate transporters were found to be associated with lipid raft microdomain of liver BLM. Moreover, ethanol ingestion decreased the folate transport by altering the Vmax of folate transport process and downregulated the expression of folate transporters in lipid rafts. The decreased transporter levels were associated with reduced protein and mRNA levels of these transporters in liver. The deranged folate uptake together with reduced folate transporter levels in lipid rafts resulted in reduced folate levels in liver and thereby to its reduced levels in serum of ethanol-fed rats. The chronic ethanol ingestion led to decreased folate uptake in liver, which was associated with the decreased number of transporter molecules in the lipid rafts that can be ascribed to the reduced synthesis of these transporters.

Adaptive transport of folic acid across renal epithelia in folate-deficient rats

Folate (vitamin B(9)) is an essential vitamin for a wide spectrum of biochemical reactions; however, unlike bacteria and plants, mammals are devoid of folate biosynthesis and thus must obtain this cofactor from exogenous sources. The activities of folate transporters on the kidneys play an important role in conserving folate excretion and reabsorption across the apical membrane of the renal proximal tubules. The different transport system activities may become identifiable in response to external stimuli, such as folate availability and exposure to chemotherapeutic agents. We have explored the effect of folate deficiency on the activity and expression of folate transporters in rat kidneys. Wistar rats were fed a folate-containing diet (2 mg folic acid kg(-1) diet) or a folic acid-free diet over a 3-month period, and mechanisms of folate transport were studied in renal brush border membrane vesicles and basolateral membrane vesicles. The renal folate uptake process is saturable and pH dependent, and it involves the folate receptor and reduced folate carrier (RFC) systems and possibly the proton coupled folate transporter (PCFT) system. We found that folate deficiency increased the renal brush border membrane and basolateral folate uptake by increasing the number of transporter molecules. The observed up-regulation of mRNA expression was also associated with a significant increase in RFC and PCFT expression at the protein level.

Folate malabsorption is associated with down-regulation of folate transporter expression and function at colon basolateral membrane in rats

Folates, an essential component (important B vitamin) in the human diet, are involved in many metabolic pathways, mainly in carbon transfer reactions such as purine and pyrimidine biosynthesis and amino acid interconversions. Deficiency of this micronutrient leads to the disruption of folate-dependent metabolic pathways that lead to the development of clinical abnormalities ranging from anaemia to growth retardation. Folate deficiency due to alcohol ingestion is quite common, primarily due to malabsorption. The present study dealt with the mechanistic insights of folate malabsorption in colonic basolateral membrane (BLM). Wistar rats (n 12) were fed 1 g/kg body weight per d ethanol (20 %) solution orally for 3 months and folate transport was studied in the isolated colonic BLM. The folate exit across colon BLM shows characteristics of carrier-mediated process with the major involvement of reduced folate carrier (RFC). The chronic ethanol ingestion decreased the uptake by decreasing the affinity by 46 % (P < 0·01) and the number of transport molecules by 43 % (P < 0·001) at the colon BLM. The decreased uptake was associated with down-regulation of proton-coupled folate transporter (PCFT) and RFC expression at mRNA and protein levels. The extent of decrease was 44 % (P < 0·01) and 24 % (P < 0·05) for PCFT and 23 % (P < 0·01) and 57 % (P < 0·01) for RFC at mRNA and protein levels, respectively. Moreover, folate transporters were associated with lipid rafts (LR) of colon BLM, and chronic alcoholism decreased the association of these transporters with LR.

Biochemical and molecular mechanisms of folate transport in rat pancreas; interference with ethanol ingestion

Folic acid is an essential nutrient that is required for one-carbon biosynthetic processes and for methylation of biomolecules. Deficiency of this micronutrient leads to disturbances in normal physiology of cell. Chronic alcoholism is well known to be associated with folate deficiency which is due, in part to folate malabsorption. The present study deals with the mechanistic insights of reduced folate absorption in pancreas during chronic alcoholism. Male Wistar rats were fed 1 g/kg body weight/day ethanol (20% solution) orally for 3 months and the mechanisms of alcohol associated reduced folate uptake was studied in pancreas. The folate transport system in the pancreatic plasma membrane (PPM) was found to be acidic pH dependent one. The transporters proton coupled folate transporter (PCFT) and reduced folate carrier (RFC) are involved in folate uptake across PPM. The folate transporters were found to be associated with lipid raft microdomain of the PPM. Ethanol ingestion decreased the folate transport by reducing the levels of folate transporter molecules in lipid rafts at the PPM. The decreased transport efficiency of the PPM was reflected as reduced folate levels in pancreas. The chronic ethanol ingestion led to decreased pancreatic folate uptake. The decreased levels of PCFT and RFC expression in rat PPM were due to decreased association of these proteins with lipid rafts (LR) at the PPM.

Nutritional deficiencies and phospholipid metabolism

Phospholipids are important components of the cell membranes of all living species. They contribute to the physicochemical properties of the membrane and thus influence the conformation and function of membrane-bound proteins, such as receptors, ion channels, and transporters and also influence cell function by serving as precursors for prostaglandins and other signaling molecules and modulating gene expression through the transcription activation. The components of the diet are determinant for cell functionality. In this review, the effects of macro and micronutrients deficiency on the quality, quantity and metabolism of different phospholipids and their distribution in cells of different organs is presented. Alterations in the amount of both saturated and polyunsaturated fatty acids, vitamins A, E and folate, and other micronutrients, such as zinc and magnesium, are discussed. In all cases we observe alterations in the pattern of phospholipids, the more affected ones being phosphatidylcholine, phosphatidylethanolamine and sphingomyelin. The deficiency of certain nutrients, such as essential fatty acids, fat-soluble vitamins and some metals may contribute to a variety of diseases that can be irreversible even after replacement with normal amount of the nutrients. Usually, the sequelae are more important when the deficiency is present at an early age.