Human intestinal folate transport: cloning, expression, and distribution of complementary RNA

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.

Functional role of serine 318 of the proton-coupled folate transporter in methotrexate transport

This study revealed the importance of serine 318 (S318) residue for proton-coupled folate transporter (PCFT, SLC46A1) functioning. Substitution of S318 with arginine or lysine impaired transport of methotrexate (MTX), but substitution with alanine (has a simple side chain structure), or cysteine (structurally similar to serine), had no significant effect on MTX transport. The initial uptake rate of MTX by S318A and S318C mutant at pH 5.0, followed by Michaelis-Menten kinetics with a K_m value of approximately 2.3 μM (for S318A) and 2.9 μM (for S318C), was similar to that of the wild-type. The normalized V_max value of the S318A mutant, calculated by dividing the V_max value by the Western blot protein band's relative intensity, was approximately 2-fold greater than that of the wild-type. The normalized V_max value of the S318C mutant was approximately 0.8-fold smaller than the wild-type. Results obtained showed that the substitution of S318 with basic amino acid residues results in the loss of transport activity, even though PCFT mutants are expressed at the cell membrane. Alternatively, the substitution of S318 with neutral amino acids did not significantly affect the transport function of PCFT.

The nutritional requirements of Caenorhabditis elegans

Animals require sufficient intake of a variety of nutrients to support their development, somatic maintenance and reproduction. An adequate diet provides cell building blocks, chemical energy to drive cellular processes and essential nutrients that cannot be synthesised by the animal, or at least not in the required amounts. Dietary requirements of nematodes, including Caenorhabditis elegans have been extensively studied with the major aim to develop a chemically defined axenic medium that would support their growth and reproduction. At the same time, these studies helped elucidating important aspects of nutrition-related biochemistry and metabolism as well as the establishment of C. elegans as a powerful model in studying evolutionarily conserved pathways, and the influence of the diet on health.

Possible selection of host folate pathway gene polymorphisms in patients with malaria from a malaria endemic region in North East India

BACKGROUND

Recent studies in experimental mice have shown that mild deficiency of methylenetetrahydrofolate reductase (MTHFR) enzyme confers protection against malaria, thus providing an important basis for the hypothesis that MTHFR polymorphism, i.e. C677T, might have been subjected to selection pressure against malaria. The present study was undertaken in a malaria endemic region in North East India to assess whether a similar selection advantage exists for other genes in folate metabolism pathway.

METHODS

A total of 401 subjects including 131 symptomatic malaria, 97 asymptomatic malaria and 173 normal healthy controls were analysed for nine polymorphisms (single-nucleotide polymorphisms [SNPs] in eight genes and insertion/deletion in one gene): MTHFR C677T, methionine synthase reductase (MTRR) A66G, glutamate carboxypeptidase II (GCPII) C1561T, cystathionine beta-synthase (CBS) 844ins68, reduced folate carrier-1 (RFC-1) G80A, serine hydroxymethyltransferase (SHMT) C1420T, methionine synthase (MTR) A2756G, MTHFR G1793A (D 919G), glycine N-methyltransferase (GNMT) 1289 by PCR-RFLP technique. Differences in frequencies of genotype distribution of each polymorphic marker between these groups were evaluated.

RESULTS

MTRR A2756G, SHMT C1420T, GCPII C1561T, MTRR A2756G and GNMT C1289T and RFC1 G80A polymorphisms showed significantly different prevalence between different groups analyzed. No significant differences were seen in the distribution of other polymorphisms.

CONCLUSIONS

The study gives a clue for the possible selection of specific polymorphisms in the genes involved in the folate metabolism pathway by malaria parasite.

Stereoselective recognition of amethopterin enantiomers by the rat proton-coupled folate transporter

The stereoselective transport of methotrexate (L-amethopterin, L-MTX) and its enantiomer (D-amethopterin, D-MTX) by the rat proton-coupled folate transporter (rPCFT) were examined using rPCFT-expressing HEK293 cells. The initial rate of uptake of [3H]-L-MTX by the rPCFT followed Michaelis-Menten kinetics, with a Km value of 2.1 µM. Dixon plots revealed that the uptake of L-MTX by the rPCFT was inhibited in a competitive manner by unlabeled L-MTX and D-MTX, with Ki values of approximately 1.3 and 150 µM, respectively. The initial rate of uptake of D-MTX by the rPCFT also followed Michaelis-Menten kinetics, with a Km value of 190 µM. The results of the current study demonstrate that the different enantiomers of MTX are transported in a highly stereoselective manner by the rPCFT, with the uptake clearance of L-MTX being approximately 46-fold greater than that of D-MTX. The observed stereoselectivity of the rPCFT was found to be comparable with that of the human PCFT.

RFC1 and non-syndromic cleft lip with or without cleft palate: an association based study in Italy

The molecular basis of orofacial development is largely unknown and needs to be unravelled. Non-syndromic cleft lip with or without cleft palate (NSCL/P) is the most common craniofacial malformation, with an incidence of about 1/700 live births, although variable according to ethnicity. Being a multifactorial disease, it arises as a result of an interplay between genetic and environmental factors. Several approaches have been developed to identify susceptibility genes. Genes belonging to the folate/homocysteine pathway are attracting increasing interest because folate supplementation before and during early pregnancy can reduce the risk of NSCL/P. We performed a family based association study in order to assess if a genetic variant of RFC1 could be involved in NSCL/P onset. We genotyped 404 unrelated probands and their relatives for three biallelic polymorphic variants (rs1051266, rs4818789 and rs3788205), that were selected because they produced conflicting results on previous investigations. Evidence of association was found between the investigated polymorphisms and NSCL/P in our sample of the Italian population, albeit with weak significance levels. Results from this investigation provided a support of previous studies suggesting a role of RFC1 in NSCL/P aetiology, reinforcing the concept that genetic predisposition to NSCL/P varies enormously within different ethnic groups.

Relationship between reduced folate carrier gene polymorphism and non-syndromic cleft lip and palate in Indian population

OBJECTIVE

Folate metabolism involves absorption, transport, modifications and interconversions of folates. The reduced folate carrier does not participate directly in folate metabolism but plays a major role in intracellular transport of metabolically active 5-methyltetrahydrofolate and maintains the intracellular concentrations of folate. The purpose of this study was to identify the prevalence of reduced folate carrier 1 (RFC1) A80G polymorphism and to further delineate its association with non-syndromic cleft lip and palate (NSCLP) in a south Indian population.

METHODS

In the present case-control study, we studied RFC1 gene A80G polymorphism to evaluate its impact on NSCLP risk in south Indian population. Blood samples of 142 cases with NSCLP and 141 controls were collected and genotyped using PCR-RFLP.

RESULTS

The genotype distribution in the control group followed Hardy-Weinberg equilibrium (p = 0.633). The G allele frequency of cases was 64.8% (184/284) and was significantly lower than that found in the control group 56.4% (160/282). The genotype distributions between NSCLP cases and controls was not significantly different (p = 0.131). The allelic model significantly increased the risk of NSCLP (G versus A; OR = 1.40; 95% CI: 1.00-1.97; p = 0.050). In subgroup analysis, the A80G variant showed significant association for the CLP group in dominant and allelic models.

CONCLUSIONS

Altogether, our findings support the hypothesis that RFC1 A80G variant may contribute to NSCLP susceptibility in a south Indian population.

Effect of epigallocatechin gallate on drug transport mediated by the proton-coupled folate transporter

Folic acid (FA) is a water-soluble vitamin, and orally ingested FA is absorbed from the small intestine by the proton-coupled folate transporter (PCFT). In the present study, we investigated whether epigallocatechin gallate (EGCG), one of the tea catechins, affects the transport of FA by PCFT. EGCG inhibited the uptake of FA into Caco-2 cells and human PCFT-expressing HEK293 cells (PCFT-HEK293 cells). The initial rate of uptake of FA into PCFT-HEK293 cells followed Michaelis-Menten kinetics (K(m) = 1.9 µM). Dixon plots revealed that PCFT-mediated FA uptake was competitively inhibited by EGCG (K(i) ≒ 9 µM). The uptake of the PCFT substrate methotrexate (MTX) was competitively inhibited by EGCG as well (K(i) ≒ 15 µM). In conclusion, it is suggested that when FA or MTX is ingested with tea, it is likely that the intestinal absorption of these compounds by PCFT is inhibited, which could result in insufficient efficacy.

Stereoselective transport of human His27- and Arg27-reduced folate carrier

The stereoselective transport of methotrexate (L-amethopterin, L-MTX) and its antipode (D-amethopterin, D-MTX) by human reduced folate carrier (hRFC) has been examined in HEK293 cells expressing H27-hRFC and R27-hRFC. The uptake of both L-MTX and D-MTX increased as the extracellular pH increased from 6.0 to 7.4. The initial uptake rate of L-MTX into the H27- and R27-hRFCs of the HEK293 cells followed Michaelis-Menten kinetics with Km values of approximately 0.24 and 0.47 µM, respectively. Dixon plots revealed that the [(3)H]-L-MTX uptake mediated by the H27- and R27-hRFCs was inhibited competitively by unlabeled L-MTX with Ki values of approximately 0.1 and 0.5 µM, respectively. D-MTX also competitively inhibited the H27- and R27-hRFC mediated uptake of [(3)H]-L-MTX with Ki values of approximately 3.4 and 3.2 µM, respectively. The RFC-mediated uptake clearance of L-MTX was approximately 15-fold greater than that of D-MTX in the H27-hRFC-HEK293 cells, and was more than 30-fold greater than that of D-MTX in the R27-hRFC-HEK293 cells. The results of the current study have revealed that the enantiomers of MTX enantiomers can be transported in a stereoselective manner by the H27- and R27-hRFCs because of significant differences in the affinities of the enantiomers to the hRFC.

Delivery of therapeutic agents by nanoparticles made of grapefruit-derived lipids

Although the use of nanotechnology for the delivery of a wide range of medical treatments has potential to reduce adverse effects associated with drug therapy, tissue-specific delivery remains challenging. Here we show that nanoparticles made of grapefruit-derived lipids, which we call grapefruit-derived nanovectors (GNVs), can transport chemotherapeutic agents, siRNA, DNA expression vectors and proteins to different types of cells. We demonstrate the in vivo targeting specificity of GNVs by co-delivering therapeutic agents with folic acid, which in turn leads to significantly increasing targeting efficiency to cells expressing folate receptors. The therapeutic potential of GNVs was further demonstrated by enhancing the chemotherapeutic inhibition of tumor growth in two tumor animal models. GNVs are less toxic than nanoparticles made of synthetic lipids and, when injected intravenously into pregnant mice, do not pass the placental barrier, suggesting they may be a useful tool for drug delivery.

Folate malabsorption and its influence on DNA methylation during cancer development

The folate transport across the epithelial of the intestine, colon, kidney, and liver is essential for folate homeostasis. The relative localization of transporters in membranes is an important determinant for the vectorial flow of substrates across the epithelia. Folate deficiency is a highly prevalent vitamin deficiency in the world, and alcohol ingestion has been the major contributor. It can develop because of folate malabsorption in tissues, increased renal excretion dietary inadequacy, and altered hepatobiliary metabolism. Additionally, folate-mediated one-carbon metabolism is important for various cellular processes, including DNA synthesis and methylation. In this regard, the contribution of alcohol-associated and dietary folate deficiency to methylation patterns is under intense investigation, especially in cancer. The epigenetic events have increasing relevance in the development of strategies for early diagnosis, prevention, and treatment of cancer.

Alcohol intake and folate antagonism via CYP2E1 and ALDH1: effects on oral carcinogenesis

The interaction of folate and alcohol consumption has been shown to have an antagonistic effect on the risk of oral cancer. Studies have demonstrated that increased intake of folate decreases the risk of oral cancer, while greater alcohol consumption has an opposite effect. However, what is poorly understood is the biological interaction of these two dietary factors in relation to carcinogenesis. We hypothesize that cytochrome P450 2E1 (CYP2E1) and the family of aldehyde dehydrogenase 1 (ALDH1) enzymes may play a causal role in the occurrence of oral cancer. Chronic and high alcohol use has been implicated in the induction of CYP2E1, which oxidizes ethanol to acetaldehyde. Acetaldehyde is a known carcinogen. As the first metabolite of ethanol, it has been shown to interfere with DNA methylation, synthesis and repair, as well as bind to protein and DNA to form stable adducts, which lead to the eventual formation of damaged DNA and cell proliferation. Studies using liver cells have demonstrated that S-adenosyl methionine (SAM), which is a product of folate metabolism, regulates the expression and catalytic activity of CYP2E1. Our first hypothesis is that as increased levels of folate lead to higher concentrations of SAM, SAM antagonizes the expression of CYP2E1, which results in decreased conversion of ethanol into acetaldehyde. Thus, the lower levels of acetaldehyde may lower risk of oral cancer. There are also two enzymes within the ALDH1 family that play an important role both in ethanol metabolism and the folate one-carbon pathway. The first, ALDH1A1, converts acetaldehyde into its non-carcinogenic byproduct, acetate, as part of the second step in the ethanol metabolism pathway. The second, ALDH1L1, also known as FDH, is required for DNA nucleotide biosynthesis, and is upregulated at high concentrations of folate. ALDH1L1 appears to be a chief regulator of cellular metabolism as it is strongly downregulated at certain physiological and pathological conditions, while its upregulation can produce drastic antiproliferative effects. ALDH1 has three known response elements that regulate gene expression (NF-Y, C/EBPβ, and RARα). Our second hypothesis is that folate interacts with one of these response elements to upregulate ALDH1A1 and ALDH1L1 expression in order to decrease acetaldehyde concentrations and promote DNA stability, thereby decreasing cancer susceptibility. Conducting future metabolic and biochemical human studies in order to understand this biological mechanism will serve to support evidence from epidemiologic studies, and ultimately promote the intake of folate to at-risk populations.

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.

Intestinal absorption of water-soluble vitamins in health and disease

Our knowledge of the mechanisms and regulation of intestinal absorption of water-soluble vitamins under normal physiological conditions, and of the factors/conditions that affect and interfere with theses processes has been significantly expanded in recent years as a result of the availability of a host of valuable molecular/cellular tools. Although structurally and functionally unrelated, the water-soluble vitamins share the feature of being essential for normal cellular functions, growth and development, and that their deficiency leads to a variety of clinical abnormalities that range from anaemia to growth retardation and neurological disorders. Humans cannot synthesize water-soluble vitamins (with the exception of some endogenous synthesis of niacin) and must obtain these micronutrients from exogenous sources. Thus body homoeostasis of these micronutrients depends on their normal absorption in the intestine. Interference with absorption, which occurs in a variety of conditions (e.g. congenital defects in the digestive or absorptive system, intestinal disease/resection, drug interaction and chronic alcohol use), leads to the development of deficiency (and sub-optimal status) and results in clinical abnormalities. It is well established now that intestinal absorption of the water-soluble vitamins ascorbate, biotin, folate, niacin, pantothenic acid, pyridoxine, riboflavin and thiamin is via specific carrier-mediated processes. These processes are regulated by a variety of factors and conditions, and the regulation involves transcriptional and/or post-transcriptional mechanisms. Also well recognized now is the fact that the large intestine possesses specific and efficient uptake systems to absorb a number of water-soluble vitamins that are synthesized by the normal microflora. This source may contribute to total body vitamin nutrition, and especially towards the cellular nutrition and health of the local colonocytes. The present review aims to outline our current understanding of the mechanisms involved in intestinal absorption of water-soluble vitamins, their regulation, the cell biology of the carriers involved and the factors that negatively affect these absorptive events.

Effect of chronic kidney disease on the expression of thiamin and folic acid transporters

BACKGROUND

Chronic kidney disease (CKD) is associated with significant cardiovascular, neurological and metabolic complications. Thiamin and folate are essential for growth, development and normal cellular function, and their uptake is mediated by regulated transport systems. While plasma folate and thiamin levels are generally normal in patients with CKD, they commonly exhibit features resembling vitamin deficiency states. Earlier studies have documented impaired intestinal absorption of several B vitamins in experimental CKD. In this study, we explored the effect of CKD on expression of folate and thiamin transporters in the key organs and tissues.

METHODS

Sprague-Dawley rats were randomized to undergo 5/6 nephrectomy or sham operation and observed for 12 weeks. Plasma folate and thiamin concentrations and gene expression of folate (RFC, PCFT) and thiamin transporters (THTR-1 and THTR-2) were determined in the liver, brain, heart and intestinal tissues using real-time PCR. Hepatic protein abundance of these transporters was determined using western blot analysis.

RESULTS

Plasma folate and thiamin levels were similar between the CKD and the control groups. However, expressions of both folate (RFC and PCFT) and thiamin (THTR-1, THTR-2) transporters were markedly reduced in the small intestine, heart, liver and brain of the CKD animals. Liver protein abundance of folate and thiamin transporters was significantly reduced in the CKD animals when compared with the sham-operated controls. Furthermore, we found a significant reduction in mitochondrial folate and thiamin transporters in the CKD animals.

CONCLUSIONS

CKD results in marked down-regulation in the expression of folate and thiamin transporters in the intestine, heart, liver and brain. These events can lead to reduced intestinal absorption and impaired cellular homeostasis of these essential micronutrients despite their normal plasma levels.

Stereoselective transport of amethopterin enantiomers by the proton-coupled folate transporter

Stereoselective transport of methotrexate (L-amethopterin, L-MTX) and its antipode (D-amethopterin, D-MTX) by the proton-coupled folate transporter (PCFT) was examined using PCFT-expressing HEK293 cells (PCFT-HEK293 cells). Uptake of both L-MTX and D-MTX was pH-dependent and decreased with an increase in the extracellular pH from 5.0 to 7.4. The initial uptake rate of L-MTX into PCFT-HEK293 cells followed Michaelis-Menten kinetics with a K(m) value of approximately 5.0 microM. Dixon plots revealed that L-MTX uptake was inhibited competitively by unlabeled L-MTX, D-MTX, and folic acid (FA), with K(i) values of approximately 3.6, 180, and 2.1 microM, respectively. The initial uptake rate of D-MTX into PCFT-HEK293 cells also followed Michaelis-Menten kinetics with a K(m) value of 211 microM. The V(max) value of D-MTX was similar to that of L-MTX. The present study revealed that the transport of MTX enantiomers by PCFT is highly stereoselective with the uptake clearance of L-MTX being approximately 40-fold greater than that of D-MTX. It was also revealed that this high stereoselectivity results from the difference in K(m) values, and not V(max) values, between the enantiomers. The observed stereoselectivity was consistent with the differences in the intestinal absorption of MTX enantiomers in humans.

Plasma folate, related genetic variants, and colorectal cancer risk in EPIC

BACKGROUND

A potential dual role of folate in colorectal cancer (CRC) is currently subject to debate. We investigate the associations between plasma folate, several relevant folate-related polymorphisms, and CRC risk within the large European Prospective Investigation into Cancer and Nutrition cohort.

METHODS

In this nested case-control study, 1,367 incident CRC cases were matched to 2,325 controls for study center, age, and sex. Risk ratios (RR) were estimated with conditional logistic regression and adjusted for smoking, education, physical activity, and intake of alcohol and fiber.

RESULTS

Overall analyses did not reveal associations of plasma folate with CRC. The RR (95% confidence interval; Ptrend) for the fifth versus the first quintile of folate status was 0.94 (0.74-1.20; 0.44). The polymorphisms MTHFR677C-->T, MTHFR1298A-->C, MTR2756A-->G, MTRR66A-->G, and MTHFD11958G-->A were not associated with CRC risk. However, in individuals with the lowest plasma folate concentrations, the MTHFR 677TT genotype showed a statistically nonsignificant increased CRC risk [RR (95% CI; Ptrend) TT versus CC=1.39 (0.87-2.21); 0.12], whereas those with the highest folate concentrations showed a nonsignificant decreased CRC risk [RR TT versus CC=0.74 (0.39-1.37); 0.34]. The SLC19A180G-->A showed a positive association with CRC risk [RR AA versus GG 1.30 (1.06-1.59); <0.01].

CONCLUSIONS

This large European prospective multicenter study did not show an association of CRC risk with plasma folate status nor with MTHFR polymorphisms.

IMPACT

Findings of the present study tend to weaken the evidence that folate plays an important role in CRC carcinogenesis. However, larger sample sizes are needed to adequately address potential gene-environment interactions.

Low folate transport across intestinal basolateral surface is associated with down-regulation of reduced folate carrier in in vivo model of folate malabsorption

The process of folate transport regulation across biological membranes is of considerable interest because of its ultimate role in providing one-carbon moieties for key cellular metabolic reactions and exogenous requirement of the vitamin in mammals. Although, intestinal folate malabsorption is established phenomena in alcoholism; however, there is no knowledge regarding the mechanism of folate exit across intestinal basolateral membrane (BLM) to circulation during alcohol associated malabsorption. In the present study, male Wistar rats were fed 1 g/kg body weight/day ethanol (20% solution) orally for 3 months and regulatory characteristics of folate transport at BLM surface were evaluated. The folate transport was found to be carrier mediated, saturable, with pH optima at 7.0, besides exhibiting Na(+) independence. The chronic alcohol ingestion resulted in alteration of transport kinetics, shifting the process to K(+) dependent one besides affecting the status of S--S linkage of the transport system. Importantly, chronic ethanol ingestion reduced the folate exit across the BLM by decreasing the affinity of transporter (high K(m)) for substrate and by decreasing the number of transporter molecules (low V(max)) on the surface. The decreased basolateral transport activity was associated with down-regulation of the reduced folate carrier (RFC) which resulted in decreased RFC protein levels in BLM in rat model of alcoholism. The study suggests that during alcohol ingestion, RFC mediated deregulated folate transport across BLM also attributes to folate malabsorption.

Molecular and functional characteristics of proton-coupled folate transporter

Proton-coupled folate transporter (PCFT) has recently been identified as the molecular entity of the carrier-mediated intestinal folate transport system. PCFT has been demonstrated to be most abundantly expressed in the upper small intestine, localizing at the brush border membrane of epithelial cells, transport folate and its analogs more efficiently at lower (acidic) pH by a H(+)-coupled cotransport mechanism, and have a high affinity for folate with a Michaelis constant (K(m)) of a few microM at pH 5.5 and somewhat lower affinities for reduced folates and methotrexate (MTX). A loss of PCFT function due to a homozygous mutation in its gene has been indicated to be responsible for hereditary folate malabsorption. Thus, PCFT has all the characteristics of the brush border H(+)-coupled cotransporter for folate and analogs, which has long been suggested to be present in the intestine. Furthermore, sulfasalazine was found to be a potent inhibitor of PCFT, suggesting that it is a risk factor that would cause malabsorption of folate and also MTX, when coadministered in the treatment of rheumatoid arthritis. Understanding the molecular and functional characteristics of PCFT should be important and helpful in exploring therapeutic strategies for folate malabsorption and in optimizing therapies using antifolate drugs.

New perspectives on folate transport in relation to alcoholism-induced folate malabsorption--association with epigenome stability and cancer development

Folates are members of the B-class of vitamins, which are required for the synthesis of purines and pyrimidines, and for the methylation of essential biological substances, including phospholipids, DNA, and neurotransmitters. Folates cannot be synthesized de novo by mammals; hence, an efficient intestinal absorption process is required. Intestinal folate transport is carrier-mediated, pH-dependent and electroneutral, with similar affinity for oxidized and reduced folic acid derivatives. The various transporters, i.e. reduced folate carrier, proton-coupled folate transporter, folate-binding protein, and organic anion transporters, are involved in the folate transport process in various tissues. Any impairment in uptake of folate can lead to a state of folate deficiency, the most prevalent vitamin deficiency in world, affecting 10% of the population in the USA. Such impairments in folate transport occur in a variety of conditions, including chronic use of ethanol, some inborn hereditary disorders, and certain diseases. Among these, ethanol ingestion has been the major contributor to folate deficiency. Ethanol-associated folate deficiency can develop because of dietary inadequacy, intestinal malabsorption, altered hepatobiliary metabolism, enhanced colonic metabolism, and increased renal excretion. Ethanol reduces the intestinal and renal uptake of folate by altering the binding and transport kinetics of folate transport systems. Also, ethanol reduces the expression of folate transporters in both intestine and kidney, and this might be a contributing factor for folate malabsorption, leading to folate deficiency. The maintenance of intracellular folate homeostasis is essential for the one-carbon transfer reactions necessary for DNA synthesis and biological methylation reactions. DNA methylation is an important epigenetic determinant in gene expression, in the maintenance of DNA integrity and stability, in chromosomal modifications, and in the development of mutations. Ethanol, a toxin that is consumed regularly, has been found to affect the methylation of DNA. In addition to its effect on DNA methylation due to folate deficiency, ethanol could directly exert its effect through its interaction with one-carbon metabolism, impairment of methyl group synthesis, and affecting the enzymes regulating the synthesis of S-adenosylmethionine, the primary methyl group donor for most biological methylation reactions. Thus, ethanol plays an important role in the pathogenesis of several diseases through its potential ability to modulate the methylation of biological molecules. This review discusses the underlying mechanism of folate malabsorption in alcoholism, the mechanism of methylation-associated silencing of genes, and how the interaction between ethanol and folate deficiency affects the methylation of genes, thereby modulating epigenome stability and the risk of cancer.

Postnatal ontogeny of intestinal GCPII and the RFC in pig

In humans and pigs, hydrolysis of dietary polyglutamyl folates is carried out by intestinal brush border folate hydrolase [glutamate carboxypeptidase II (GCPII)], whereas the transport of the monoglutamyl folate derivatives occurs via the intestinal brush border reduced folate carrier (RFC). The study objective was to measure the expression of intestinal GCPII and RFC during postnatal development of pigs and their effects on plasma and liver folate concentrations. Duodenum, jejunum, ileum, liver, and plasma samples were collected from female Yorkshire pigs at birth, 24 h, 1 wk, 3 wk, and 6 mo (n=6 at each time point). GCPII mRNA transcripts and protein (normalized using beta-actin), and enzyme activity (normalized per mg mucosal protein) were highest in all segments of small intestine at birth and were undetectable in ileum after 1 wk, whereas jejunal protein and activity predominated at 6 mo. RFC mRNA transcripts were present in all segments of small intestine at birth and declined significantly throughout development to 6 mo. Conversely, RFC protein increased twofold during the first 24 h and remained constant throughout development in all segments of small intestine. Liver RFC mRNA transcripts were detected at birth but were reduced by 6 mo. Liver folate concentration increased throughout postnatal development, whereas plasma folate levels increased during the first 24 h but decreased over time, reflecting the pattern of RFC expression in small intestine. These findings show that intestinal GCPII and intestinal and hepatic RFC all exhibit ontogenic changes in the pig that are reflected in postnatal folate status.

Renal conservation of folates role of folate transport proteins

Folates play vital roles in one-carbon metabolism that produces the early substrates necessary for nucleotide synthesis and salvage. Folates are essential vitamins in that humans cannot synthesize them and are totally dependent on the diet to obtain them. As water-soluble vitamins, they would be easily filtered by the kidney and lost to the tubular fluid but for a highly efficient renal conservation mechanism. This renal "folate trap" is made up of alpha-folate receptors and reduced folate carriers. The locations of these transporters are such that they direct folate transport from the apical/luminal sides of kidney cells to the basolateral/plasma sides. In addition, other transporters such as organic anion transporters and multidrug resistance proteins are also found in kidney cells and play a role in renal elimination of folate analogues such as antifolate cancer chemotherapy drugs. This chapter discusses how these transporter activities manifest themselves in folate and antifolate pharmacokinetics. It also discusses effects of alcohol on renal reabsorption of folates.

A80G polymorphism of reduced folate carrier 1 (RFC1) and C776G polymorphism of transcobalamin 2 (TC2) genes in Down's syndrome etiology

CONTEXT AND OBJECTIVE

There is evidence that polymorphisms of genes involved in folate metabolism may be associated with higher risk that mothers may bear a Down's syndrome (DS) child. This study therefore had the objective of investigating the A80G polymorphism of the reduced folate carrier 1 (RFC1) gene and the C776G polymorphism of the transcobalamin 2 (TC2) gene as maternal risk factors for DS among Brazilian women.

DESIGN AND SETTING

Analytical cross-sectional study with control group, at Faculdade de Medicina de São José do Rio Preto (Famerp).

METHODS

Sixty-seven mothers of DS individuals with free trisomy 21, and 113 control mothers, were studied. Molecular analysis of the polymorphisms was performed by means of the polymerase chain reaction with restriction fragment length polymorphism (PCR-RFLP), followed by electrophoresis on 2% agarose gel.

RESULTS

The frequencies of the polymorphic alleles were 0.51 and 0.52 for RFC1 80G, and 0.34 and 0.34 for TC2 776G, in the case and control groups, respectively. Thus, there were no differences between the groups in relation to either the allele or the genotype frequency, for both polymorphisms (P = 0.696 for RFC1 A80G; P = 0.166 for TC2 C776G; P = 0.268 for combined genotypes).

CONCLUSION

There was no evidence of any association between the RFC1 A80G and TC2 C776G polymorphisms and the maternal risk of DS in the sample evaluated.

Differentiation-dependent regulation of the intestinal folate uptake process: studies with Caco-2 cells and native mouse intestine

Differentiation of intestinal epithelial cells is accompanied by alterations in levels of expression of many genes, including those involved in nutrient uptake. Effects of differentiation of intestinal epithelial cells on the physiological and molecular parameters of the intestinal folate uptake process are not well characterized. To address this issue, we used two models, Caco-2 cells and native mouse intestine. Studies with Caco-2 cells showed a significant increase in the initial rate of carrier-mediated folic acid uptake during differentiation (i.e., as the cells transitioned from preconfluent to confluent and then to postconfluent stages). This increase was associated with an increase in the level of expression of the human reduced folate carrier (hRFC) and the human proton-coupled folate transporter (hPCFT) both at the protein and mRNA levels with differentiation; it was also associated with a significant increase in activity of the hRFC and hPCFT promoters. Studies with native mouse intestine showed a significantly higher folate uptake in villus compared with crypt cells, which was again associated with a significantly higher level of expression of the mouse RFC and PCFT at the protein and mRNA levels. Together, these studies demonstrate that the intestinal folate uptake process undergoes differentiation-dependent regulation and that this regulation is mediated via changes in the level of expression of both the RFC and PCFT. In addition, the studies suggest the possible involvement (at least in part) of a transcriptional mechanism(s) in this type of regulation of the intestinal folate uptake process.

Correlation of polymorphism of MTHFRs and RFC-1 genes with neural tube defects in China

BACKGROUND

Maternal periconceptional supplementation of folate reduces the incidence of neonatal Neural Tube Defects, indicating that changes in folate metabolism play a role in formation of NTDs. The mutations on two genes involved in folate metabolism, the C677 of the MTHFR gene and the RFC-1(A80G) gene are potential risk factors of NTDs.

METHODS

In this study, we analyzed the genotypic distributions and allele frequencies of MTHFR C677T and RFC-1 A80G polymorphisms in DNA samples from mothers with at least one previous child with NTDs (the NTD group) and controls.

RESULTS

Our results indicated that there was a significant difference in the genotype and allele frequencies of RFC-1 80A-->G between the NTD group and controls (p = .008 and p = .017, respectively). There was, however, no significant difference in the genotype and allele frequencies of the MTHFR 677C-->T polymorphism between the NTD group and controls. The NTD group was further separated into the upper and lower types by location of abnormalities. The frequency of RFC-1 80A/G and 80G/G was significantly higher in the upper group than the control (p = .009 and p = .005, respectively). The frequency of G-alleles was also significantly higher in the upper group than the control (OR 2.42; p = .006; 95% CI: 1.28-4.58). For the MTHFR C677 gene, the frequency of T-alleles was significantly lower in the lower defect type than the control group (OR 0.32; p = .027; 95% CI: 0.11-0.9).

CONCLUSIONS

These results suggest that in the Shanxi population RFC-1 polymorphisms may play a role in NTD risk, whereas the impact of MTHFR C677T polymorphisms requires further clarification. Birth Defects Research (Part A) 2008.

Functional characterization of PCFT/HCP1 as the molecular entity of the carrier-mediated intestinal folate transport system in the rat model

Proton-coupled folate transporter/heme carrier protein 1 (PCFT/HCP1) has recently been identified as a transporter that mediates the translocation of folates across the cellular membrane by a proton-coupled mechanism and suggested to be the possible molecular entity of the carrier-mediated intestinal folate transport system. To further clarify its role in intestinal folate transport, we examined the functional characteristics of rat PCFT/HCP1 (rPCFT/HCP1) expressed in Xenopus laevis oocytes and compared with those of the carrier-mediated folate transport system in the rat small intestine evaluated by using the everted tissue sacs. rPCFT/HCP1 was demonstrated to transport folate and methotrexate more efficiently at lower acidic pH and, as evaluated at pH 5.5, with smaller Michaelis constant (K(m)) for the former (2.4 microM) than for the latter (5.7 microM), indicating its characteristic as a proton-coupled folate transporter that favors folate than methotrexate as substrate. rPCFT/HCP1-mediated folate transport was found to be inhibited by several but limited anionic compounds, such as sulfobromophthalein and sulfasalazine. All these characteristics of rPCFT/HCP1 were in agreement with those of carrier-mediated intestinal folate transport system, of which the K(m) values were 1.2 and 5.8 microM for folate and methotrexate, respectively, in the rat small intestine. Furthermore, the distribution profile of the folate transport system activity along the intestinal tract was in agreement with that of rPCFT/HCP1 mRNA. This study is the first to clone rPCFT/HCP1, and we successfully provided several lines of evidence that indicate its role as the molecular entity of the intestinal folate transport system.

Association of RFC1 A80G and MTHFR C677T polymorphisms with Alzheimer's disease

We examined polymorphisms in reduced folate carrier gene (RFC1) and methylenetetrahydrofolate reductase gene (MTHFR) for association with sporadic AD (SAD) in Chinese population. Significant associations of RFC1 A80G G allele and GG genotype with SAD (p=0.008, OR=1.312, 95%CI=1.072-1.605, and p=0.042, OR=1.383, 95%CI=1.012-1.890) were found. Further stratification of total samples by APOE epsilon4 carrier status, age/age at onset and gender revealed that RFC1 A80G G allele was an APOE epsilon4-independent risk factor for late-onset AD, and it might increase the risk of AD in females. No significant associations of MTHFR C677T allele and genotype with AD were observed in total samples, but significant associations of T allele and TT genotype with AD (p=0.031, OR=1.586, 95%CI=1.042-2.414, and p=0.028, OR=2.250, 95%CI=1.074-4.712) were identified in APOE epsilon4 carrier subgroup, suggesting that MTHFR 677 T allele and APOE epsilon4 allele may synergistically act to increase AD risk. No significant effect of RFC1 G80A and MTHFR C677T polymorphisms on plasma folate and homocysteine levels was detected.

Structure and function of the reduced folate carrier a paradigm of a major facilitator superfamily mammalian nutrient transporter

Folates are essential for life and folate deficiency contributes to a host of health problems including cardiovascular disease, fetal abnormalities, neurological disorders, and cancer. Antifolates, represented by methotrexate, continue to occupy a unique niche among the modern day pharmacopoeia for cancer along with other pathological conditions. This article focuses on the biology of the membrane transport system termed the "reduced folate carrier" or RFC with a particular emphasis on RFC structure and function. The ubiquitously expressed RFC is the major transporter for folates in mammalian cells and tissues. Loss of RFC expression or function portends potentially profound physiological or developmental consequences. For chemotherapeutic antifolates used for cancer, loss of RFC expression or synthesis of mutant RFC protein with impaired function results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and low levels of substrate for polyglutamate synthesis. The functional properties for RFC were first documented nearly 40 years ago in murine leukemia cells. Since 1994, when RFC was first cloned, tremendous advances in the molecular biology of RFC and biochemical approaches for studying the structure of polytopic membrane proteins have led to an increasingly detailed picture of the molecular structure of the carrier, including its membrane topology, its N-glycosylation, identification of functionally and structurally important domains and amino acids, and helix packing associations. Although no crystal structure for RFC is yet available, biochemical and molecular studies, combined with homology modeling, based on homologous bacterial major facilitator superfamily transporters such as LacY, now permit the development of experimentally testable hypotheses designed to establish RFC structure and mechanism.

The MTR A2756G polymorphism is associated with an increase of plasma homocysteine concentration in Brazilian individuals with Down syndrome

Individuals with Down syndrome (DS) present decreased homocysteine (Hcy) concentration, reflecting a functional folate deficiency secondary to overexpression of the cystathionine ss-synthase gene. Since plasma Hcy may be influenced by genetic polymorphisms, we evaluated the influence of C677T and A1298C polymorphisms in the methylenetetrahydrofolate reductase gene (MTHFR), of A2756G polymorphism in the methionine synthase gene (MTR), and of A80G polymorphism in the reduced folate carrier 1 gene on Hcy concentrations in Brazilian DS patients. Fifty-six individuals with free trisomy 21 were included in the study. Plasma Hcy concentrations were measured by liquid chromatography_tandem mass spectrometry with linear regression coefficient r(2) = 0.9996, average recovery between 92.3 to 108.3% and quantification limits of 1.0 micromol/L. Hcy concentrations >15 micromol/L were considered to characterize hyperhomocystinemia. Genotyping for the polymorphisms was carried out by polymerase chain reaction followed by enzyme digestion and allele-specific polymerase chain reaction. The mean Hcy concentration was 5.2 +/- 3.3 micromol/L. There was no correlation between Hcy concentrations and age, gender or MTHFR C677T, A1298C and reduced folate carrier 1 A80G genotype. However, Hcy concentrations were significantly increased in the MTR 2756AG heterozygous genotype compared to the MTR 2756AA wild-type genotype. The present results suggest that the heterozygous genotype MTR 2756AG is associated with the increase in plasma Hcy concentrations in this group of Brazilian patients with DS.

H+-coupled nutrient, micronutrient and drug transporters in the mammalian small intestine

The H(+)-electrochemical gradient was originally considered as a driving force for solute transport only across cellular membranes of bacteria, plants and yeast. However, in the mammalian small intestine, a H(+)-electrochemical gradient is present at the epithelial brush-border membrane in the form of an acid microclimate. Over recent years, a large number of H(+)-coupled cotransport mechanisms have been identified at the luminal membrane of the mammalian small intestine. These transporters are responsible for the initial stage in absorption of a remarkable variety of essential and non-essential nutrients and micronutrients, including protein digestion products (di/tripeptides and amino acids), vitamins, short-chain fatty acids and divalent metal ions. Proton-coupled cotransporters expressed at the mammalian small intestinal brush-border membrane include: the di/tripeptide transporter PepT1 (SLC15A1); the proton-coupled amino-acid transporter PAT1 (SLC36A1); the divalent metal transporter DMT1 (SLC11A2); the organic anion transporting polypeptide OATP2B1 (SLC02B1); the monocarboxylate transporter MCT1 (SLC16A1); the proton-coupled folate transporter PCFT (SLC46A1); the sodium-glucose linked cotransporter SGLT1 (SLC5A1); and the excitatory amino acid carrier EAAC1 (SLC1A1). Emerging research demonstrates that the optimal intestinal absorptive capacity of certain H(+)-coupled cotransporters (PepT1 and PAT1) is dependent upon function of the brush-border Na(+)-H(+) exchanger NHE3 (SLC9A3). The high oral bioavailability of a large number of pharmaceutical compounds results, in part, from absorptive transport via the same H(+)-coupled cotransporters. Drugs undergoing H(+)-coupled cotransport across the intestinal brush-border membrane include those used to treat bacterial infections, hypercholesterolaemia, hypertension, hyperglycaemia, viral infections, allergies, epilepsy, schizophrenia, rheumatoid arthritis and cancer.

The molecular identity and characterization of a Proton-coupled Folate Transporter--PCFT; biological ramifications and impact on the activity of pemetrexed

Membrane transport of folates is essential for the survival of all mammalian cells and transport of antifolates is an important determinant of antifolate activity. While a major focus of attention has been on transport mediated by the reduced folate carrier and folate receptors, a very prominent carrier-mediated folate transport activity has been recognized for decades with a low-pH optimum and substrate specificity distinct from that of the reduced folate carrier which operates most efficiently at neutral pH. This low-pH transporter represents the mechanism by which folates are absorbed in the small intestine and it is also widely expressed in other human tissues and solid tumors. Recently, this laboratory discovered the molecular identity of this transporter which is genetically unrelated to the reduced folate carrier. This transporter is proton-coupled, electrogenic, and manifests a substrate specificity that is similar to that of the low-pH transport activity previously described in mammalian cells. The key role this transporter plays in intestinal folate absorption has been confirmed by the demonstration of a mutation in this gene in the rare autosomal recessive disorder, hereditary folate malabsorption. This article reviews (1) the characteristics and prevalence of the low-pH folate transport activity, (2) its relationship to, and properties of, the recently identified Proton-Coupled Folate Transporter (PCFT), (3) the physiological and pharmacological roles of this transporter, particularly with respect to pemetrexed, and (4) the historical controversy, now resolved, on the mechanism of intestinal folate absorption.

Cloning and functional characterization of a folate transporter from the nematode Caenorhabditis elegans

Two putative orthologs to the human reduced folate carrier (hRFC), folt-1 and folt-2, which share a 40 and 31% identity, respectively, with the hRFC sequence, have been identified in the Caenorhabditis elegans genome. Functional characterization of the open reading frame of the putative folt-1 and folt-2 showed folt-1 to be a specific folate transporter. Transport of folate by folt-1 expressed in a heterologous expression system showed an acidic pH dependence, saturability (apparent K(m) of 1.23 +/- 0.18 microM), a similar degree of inhibition by reduced and substituted folate derivatives, sensitivity to the anti-inflammatory drug sulfasalazine (apparent K(i) of 0.13 mM), and inhibition by anion transport inhibitors, e.g., DIDS. Knocking down (silencing) or knocking out the folt-1 gene led to a significant inhibition of folate uptake by intact living C. elegans. We also cloned the 5'-regulatory region of the folt-1 gene and confirmed promoter activity of the construct in vivo in living C. elegans. With the use of the transcriptional fusion construct (i.e., folt-1::GFP), the expression pattern of folt-1 in different tissues of living animal was found to be highest in the pharynx and intestine. Furthermore, folt-1::GFP expression was developmentally and adaptively regulated in vivo. These studies demonstrate for the first time the existence of a specialized folate uptake system in C. elegans that has similar characteristics to the folate uptake process of the human intestine. Thus C. elegans provides a genetically tractable model that can be used to study integrative aspects of the folate uptake process in the context of the whole animal level.

Chiral recognition of amethopterin enantiomers by the reduced folate carrier in Caco-2 cells

Stereoselectivity of the human reduced folate carrier (RFC1) in Caco-2 cells was examined using methotrexate (L-amethopterin, L-MTX) and its antipode (D-amethopterin, D-MTX) as model substrates. The initial uptake rate of L-MTX into Caco-2 cells followed Michaelis-Menten kinetics with a Km value of approximately 1 microM. The Eadie-Hofstee plot of the RFC1-mediated L-MTX uptake showed that it was mediated by a single transport system, RFC1. Dixon plots revealed that L-MTX uptake was inhibited competitively by folic acid (FA), L-MTX and D-MTX, with Ki values of approximately 0.8, 1.5 and 180 microM, respectively. The results showed that the affinities of FA and L-MTX to RFC1 were approximately 120-fold greater than that of D-MTX. The uptake of L- and D-MTX into Caco-2 cells was also measured using LC-MS/MS analysis, which revealed that the L-MTX uptake was at least 7-fold greater than that of D-MTX. The present study revealed significant stereoselectivity of RFC1 toward amethopterin enantiomers with the L-isomer being much more favored.

Downregulation of the reduced folate carrier transport activity by phenobarbital-type cytochrome P450 inducers and protein kinase C activators

The sodium dependent reduced folate carrier (Rfc1; Slc19a1) provides the major route for cellular uptake of reduced folates and antifolate drugs such as methotrexate (MTX) into various tissues. Despite its essential role in folate homeostasis and cancer treatment, little is known about Rfc1 regulation. A barbiturate recognition box, which as yet has only been found in the promoter region of xenobiotic metabolizing enzymes, particularly those of the CYP450 enzyme family, was predicted in the 5' untranslated region of rat rfc1 cDNA. We have therefore investigated the regulation of Rfc1 by phenobarbital (PB)-type CYP450 inducers on the functional, transcriptional and translational level in a suitable in vitro model for rat liver. A decrease of >75% in substrate uptake was observed following treatment (48 h) with 1-10 times therapeutic plasma concentrations of PB-type CYP450 inducers like PB, carbamazepine, chlorpromazine, clotrimazole and with 0.1-1 ng/ml of the constitutive androstane receptor agonist TCPOBOP. This was not associated with reduced mRNA and protein levels. Further mechanistic investigations revealed that short-term treatment (2 h) of cells with protein phosphatase 1/2A inhibitor okadaic acid (80.5 ng/ml) and proteinkinase C inducer phorbol 12-myristate 13-acetate (PMA; 0.62 microg/ml) almost abolished Rfc1 mediated MTX uptake. Finally, the reduction in Rfc1 activity caused by PB, TCPOBOP and PMA was reversed by simultaneous incubation with the specific PKC inhibitor bisindolylmaleimide (BIM; 21 ng/ml). These results demonstrate that clinically relevant concentrations of PB-type CYP450 inducers cause a significant PKC-dependent reduction in Rfc1 uptake activity at the posttranscriptional level.

G80A reduced folate carrier SNP influences the absorption and cellular translocation of dietary folate and its association with blood pressure in an elderly population

The functional consequences of the G80A RFC SNP on the expressed reduced folate carrier protein were evaluated by looking at the relationship between intake of folate, plasma folate and cellular stores of the vitamin. The effect on homocysteine was also examined. Homocysteine is a thiol that is known to be inversely associated with folate, and which is considered to be both thrombo- and athrogenic. At high levels, homocysteine may also interfere with nitric oxide mediated vasodilation, cause oxidative injury to, and proliferation of the vascular endothelium, and alter the elastic properties of the vascular wall, contributing to increased blood pressure. Participants (119; 52 male, 67 female) from a NSW retirement village were assessed. Independent of gender, the assimilation of folate from dietary sources into red cells showed a significant association for GG (r=0.399; p=0.022) and GA (r=0.564; p<0.0001) subjects, but not homozygous recessive (AA) individuals (r=0.223; p=0.236). The same genotype based pattern of significance was shown for the association between dietary folate and plasma folate (GG: r=0.524; p=0.002, GA: r=0.408; p=0.002). No genotype-related pattern of significance was shown for the association between dietary folate and homocysteine. When examined by gender, some differences were apparent; one-way ANOVA showed that genotype influenced diastolic blood pressure in males (p=0.019), while only females showed a significant correlation between dietary folate and blood pressure within specific genotypes (Systolic pressure GA: r=-0.372; p=0.025, carriage of A: r=0.-0.357; p=0.011. Diastolic pressure GA: r=-0.355; p=0.034, carriage of A: r=0.-0.310; p=0.029). The G80A RFC SNP had an impact on the absorption and cellular translocation of dietary folate and its association with blood pressure in an elderly population.

Intestinal absorption of water-soluble vitamins: an update

PURPOSE OF REVIEW

The molecular biology revolution has led to a significant improvement in our understanding of biological and physiological processes. Such expansion of knowledge has also covered the field of intestinal absorption of water-soluble vitamins and is the subject of this review.

RECENT FINDINGS

Impressive progress has been made in the understanding of the mechanisms and regulation of transport of water-soluble vitamins at the cellular and molecular levels. In addition, the 5' regulatory regions of the genes that encode a number of the involved transporters have been cloned and characterized in vitro and in vivo in transgenic mice, thus providing important information about transcriptional regulation of these events. Furthermore, confocal imaging of live intestinal epithelial cells has led to significant progress in understanding the mechanisms involved in intracellular trafficking and membrane targeting of the carrier proteins and how clinical mutations lead to interference with transport. Finally, the identification in the large intestine of efficient and specialized carrier-mediated systems that are capable of absorbing a number of the bacterially synthesized vitamins (thiamin, folate, biotin, riboflavin, pantothenic acid) has raised the possibility that this source of vitamins may play a role in regulating (fine tuning) the normal body homeostasis of these vitamins, and especially the vitamin level in the local colonocytes.

SUMMARY

Water-soluble vitamin absorption involves regulated and specific mechanisms. Interference with the function of these mechanisms may lead to deficiency. The large intestine is capable of absorbing water-soluble vitamins that are synthesized by the normal microflora.

G80A reduced folate carrier SNP modulates cellular uptake of folate and affords protection against thrombosis via a non homocysteine related mechanism

Dietary folate is absorbed in the jejunum by the 'Reduced Folate Carrier' binding protein. This protein also sequesters extracellular folate for use by many cells in the body. As several biosynthetic pathways require folate for critical life processes, any change in the properties of this protein could lower folate bioavailability, cellular levels of the vitamin, and thus influence health. Since folate lowers thrombogenic homocysteine, we examined the prevalence of a common genetic polymorphism encoding the Reduced Folate Carrier (G80A RFC) to see if it acts as a risk factor for thrombotic vascular disease via an effect on homocysteine disposition in a cohort of 156 patients. The odds ratio indicates a significant protective effect of the mutant A allele against thrombosis: OR = 0.56(95% CI; 0.34-0.92). chi2; p = 0.022 (Yates corrected chi2; p = 0.031). The polymorphism had no impact on homocysteine, but did increase the level of extracellular to intracellular folate as might be predicted by the biological role of the expressed protein. This, and not homocysteine level, may be what affords protection against thrombosis.

Role of reduced folate carrier in intestinal folate uptake

Studies from our laboratory and others have characterized different aspects of the intestinal folate uptake process and have shown that the reduced folate carrier (RFC) is expressed in the gut and plays a role in the uptake process. Little, however, is known about the actual contribution of the RFC system toward total folate uptake by the enterocytes. Addressing this issue in RFC knockout mice is not possible due to the embryonic lethality of the model. In this study, we describe the use of the new approach of lentivirus-mediated short hairpin RNA (shRNA) to selectively silence the endogenous RFC of the rat-derived intestinal epithelial cells (IEC-6), an established in vitro model for folate uptake, and examined the effect of such silencing on folate uptake. First we confirmed that the initial rate of [(3)H]folic acid uptake by IEC-6 cells was pH dependent with a markedly higher uptake at acidic compared with alkaline pH. We also showed that the addition of unlabeled folic acid to the incubation buffer leads to a severe inhibition ( approximately 95%) in [(3)H]folic acid (16 nM) uptake at buffer pH 5.5 but not at buffer pH 7.4. We then examined the effect of treating (for 72 h) IEC-6 cells with RFC-specific shRNA on the levels of RFC protein and mRNA and observed substantial reduction in the levels of both parameters ( approximately 80 and 78%, respectively). Such a treatment was also found to lead to a severe inhibition ( approximately 90%) in initial rate of folate uptake at buffer pH 5.5 (but not at pH 7.4); uptake of the unrelated vitamin, biotin, on the other hand, was not affected by such a treatment. These results demonstrate that the RFC system is the major (if not the only) folate uptake system that is functional in intestinal epithelial cells.

Renal tubular reabsorption of folate mediated by folate binding protein 1

Renal tubular reabsorption of filtered folate is essential for the conservation and normal homeostasis of this important vitamin. Different molecular mechanisms have been implicated in epithelial folate transport, including folate receptors. Defective expression or antibody inactivation of these is associated with embryonic defects also correlated with low folate intake; however, their contribution to renal tubular folate reabsorption has not been established. With the use of targeted inactivation of the folate binding protein 1 (folbp1) and folate binding protein 2 (folbp2) genes in mice, the role of folate receptors in renal epithelial folate reabsorption was evaluated during low and normal folate intake. Inactivation of folbp1 was associated with (1) loss of (3)H-folic acid binding to crude kidney membranes, (2) increase in renal folate clearance, and (3) increase in urinary excretion and decrease in renal uptake of injected (3)H-methyltetrahydrofolate. No changes in renal folate handling were observed as a result of folbp2 inactivation. Thus, folbp1 is essential for normal renal tubular folate reabsorption, preventing excessive urinary folate loss. Folbp1 is heavily expressed in choroid plexus, yolk sac, and placenta, supporting a role of folbp1 in folate transport in other tissues. The greatest significance of folbp1 for renal folate uptake was observed at conditions of low folate intake, providing a possible explanation for the ability of folate supplementation to prevent developmental defects associated with folbp1 inactivation.

Stereoselectivity of the reduced folate carrier in Caco-2 cells

Stereoselectivity of the human reduced folate carrier (RFC1) was examined in Caco-2 cells using methotrexate (l-amethopterin or l-MTX) and its antipode (d-amethopterin or d-MTX) as model substrates. The initial uptake rate of folic acid (FA) was concentration dependent, with a K(m) value of approximately 0.6 microM. The Eadie-Hofstee plot of the RFC1-mediated FA uptake revealed a single component for FA uptake into Caco-2 cells, demonstrating that only RFC1 is involved in FA uptake. l-MTX inhibited FA uptake in a competitive manner with a K(i) value of approximately 2 microM, similar to the K(m) value of l-MTX. d-MTX also competitively inhibited FA uptake with a K(i) value being approximately 120 microM, indicating that the affinity of d-MTX is ca. 60-fold less than that of l-MTX. The stereoselectivity of human RFC1 observed in the present study was consistent not only with the stereoselectivity of rabbit RFC1 observed in rabbit intestinal brush border membrane vesicles but also with the reported differences in oral absorption of amethopterin enantiomers in humans.

Intestinal solute carriers: an overview of trends and strategies for improving oral drug absorption

A large amount of absorptive intestinal membrane transporters play an important part in absorption and distribution of several nutrients, drugs and prodrugs. The present paper gives a general overview on intestinal solute carriers as well as on trends and strategies for targeting drugs and/or prodrugs to these carriers in order to increasing oral bioavailability and distribution. A number of absorptive intestinal transporters are described in terms of gene and protein classification, driving forces, substrate specificities and cellular localization. When targeting absorptive large capacity membrane transporters in the small intestine in order to increase oral bioavailabilities of drug or prodrug, the major influence on in vivo pharmacokinetics is suggested to be dose-dependent increase in bioavailability as well as prolonged blood circulation due to large capacity facilitated absorption, and renal re-absorption, respectively. In contrast, when targeting low-capacity transporters such as vitamin transporters, dose independent saturable absorption kinetics are suggested. We thus believe that targeting drug substrates for absorptive intestinal membrane transporters could be a feasible strategy for optimizing drug bioavailability and distribution.

Recent advances in carrier-mediated intestinal absorption of water-soluble vitamins

Significant progress has been made in recent years toward understanding the mechanisms and regulation of intestinal absorption of water-soluble vitamins from the diet, especially those that are transported by a specialized carrier-mediated mechanism (i.e., ascorbic acid, biotin, folate, riboflavin, thiamin, and pyridoxine). The driving force involved in the uptake events and the molecular identity of the systems involved have been identified for a number of these vitamins. In addition, information about regulation of the uptake process of these micronutrients by intracellular and extracellular factors has been forthcoming. Furthermore, the 5' regulatory region of the genes that encode a number of these transporters has been characterized, thus providing information about transcriptional regulation of the transport events. Also of interest is the identification of existence of carrier-mediated mechanisms in human colonocytes that are capable of absorbing some of the vitamins that are synthesized by normal microflora of the large intestine. Although the contribution of the latter source of vitamins toward overall host nutrition is not clear and requires further investigations, it is highly likely that it does contribute toward the cellular homeostasis of these vitamins in the localized colonocytes.