Jejunal perfusion of simple and conjugated folates in celiac sprue

Gastrointestinal Handling of Water-Soluble Vitamins

Nine compounds are classified as water-soluble vitamins, eight B vitamins and one vitamin C. The vitamins are mandatory for the function of numerous enzymes and lack of one or more of the vitamins may lead to severe medical conditions. All the vitamins are supplied by food in microgram to milligram quantities and in addition some of the vitamins are synthesized by the intestinal microbiota. In the gastrointestinal tract, the vitamins are liberated from binding proteins and for some of the vitamins modified prior to absorption. Due to their solubility in water, they all require specific carriers to be absorbed. Our current knowledge concerning each of the vitamins differs in depth and focus and is influenced by the prevalence of conditions and diseases related to lack of the individual vitamin. Because of that we have chosen to cover slightly different aspects for the individual vitamins. For each of the vitamins, we summarize the physiological role, the steps involved in the absorption, and the factors influencing the absorption. In addition, for some of the vitamins, the molecular base for absorption is described in details, while for others new aspects of relevance for human deficiency are included. © 2018 American Physiological Society. Compr Physiol 8:1291-1311, 2018.

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.

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.

Apical membrane targeting and trafficking of the human proton-coupled transporter in polarized epithelia

The human proton-coupled folate transporter (hPCFT) is a recently discovered intestinal transporter involved in folate uptake in epithelia (and possibly other cells). Little is currently known about the structure-function relationship of the different domains of this transporter, particularly which regions are important for substrate transport as well as targeting of the transporter to the apical cell surface of polarized cells. Here we have investigated the role of the COOH-terminal domain and a well-conserved sequence separating transmembrane (TM) domains TM2 and TM3 (DXXGRR; amino acids 109-114) speculated by others to be important for transport function. Using live cell imaging approaches, we show that 1) an hPCFT-yellow fluorescent protein construct is functionally expressed at the apical membrane domain and is localized differentially to the human reduced folate carrier; 2) the predicted cytoplasmic COOH-terminal region of hPCFT is not essential for apical targeting or transporter functionality; 3) mutations that ablate a consensus beta-turn sequence separating predicted TM2 and TM3 abolished apical [(3)H]folic acid uptake as a consequence of endoplasmic reticulum retention of mutant, likely misfolded, transporters; and 4) cell surface delivery of hPCFT is disrupted by microtubule depolymerization or by overexpression of the dynactin complex dynamitin (p50). For the first time, our data present information regarding structure-function and membrane targeting of the hPCFT polypeptide, as well as the mechanisms that control its steady-state expression in polarized cells.

Folate uptake in the human intestine: promoter activity and effect of folate deficiency

The intestinal folate absorption process occurs via a specialized mechanism that involves the reduced folate carrier (RFC). In humans, multiple variants of the hRFC (driven by multiple promoters) have been identified with variant I being the prominent form expressed in the intestine. While it is known that promoter B (pB) of hRFC drives the expression of this variant, little is known about the minimal region required for basal activity of this promoter in human intestinal epithelial cells. Also not known is whether folate absorption in the human intestine is up-regulated during folate deficiency (as occur in animal studies), and if so, whether transcriptional mechanisms via activation of hRFC pB are involved in such regulation. To address these issues, we have used deletion constructs of the hRFC pB and determined their activity in two human intestinal epithelial cell lines: the colon-derived Caco-2 cells, and the duodenum-derived HuTu-80 cells. Our results showed that activity of hRFC pB to be significantly higher in Caco-2 cells compared to HuTu-80 cells, a finding that corresponds with a higher level of folate uptake and endogenous hRFC mRNA levels in the former compared to the latter cell type. The minimal region required for basal activity of hRFC pB in Caco-2 cells was found to be encoded in a sequence between -1088 and -1043, while in HuTu-80 cells it was encoded in a sequence between -1431 and -1088. Growing Caco-2 cells in a folate deficient medium led to a significant and specific up-regulation in folate uptake. This up-regulation was associated with a parallel increase in hRFC protein and mRNA levels, and in the activity of hRFC pB. The most responsive sequence of pB to the effect of folate deficiency was found to be encoded in a sequence between -2016 and -1431, i.e., outside the minimal region of the pB. These results show that different minimal regions for hRFC pB are utilized by different intestinal epithelial cells. In addition, folate-deficiency was found to up-regulate folate uptake by human intestinal epithelial cells and that this regulation involves activation of hRFC pB.

Absorption of water-soluble vitamins

Water-soluble vitamins are required as enzyme cofactors in a wide variety of metabolic reactions. Riboflavin, niacin, and vitamin C are essential in redox reactions; thiamine and biotin are involved in macronutrient metabolism; and folate, vitamin B12, pyridoxine, and riboflavin play important roles in the regulation of S-adenosylmethionine production and DNA synthesis. Each of the water-soluble vitamins appears to require its own membrane transport process for absorption across the enterocyte. The absorption of vitamin B12, or cobalamin (Cbl), is unique in requiring multiple processes from the stomach to the ileum that involve at least four different binding proteins. Whereas all water-soluble vitamins are absorbed from the small intestine, folate, biotin, and riboflavin can be transported across colonic epithelial cells, with uncertain clinical significance. This article reviews recent studies on the requirement, metabolism, and deficiency state of each water-soluble vitamin, followed by a discussion of current knowledge on the regulation of its intestinal absorption.

Expression of the reduced folate carrier SLC19A1 in IEC-6 cells results in two distinct transport activities

Intestinal absorption of folates has been characterized as a facilitative process with a low pH optimum. Studies with intestinal epithelial cells have suggested that this activity is mediated by the reduced folate carrier (RFC1). In this paper, we report on folate transport characteristics in an immortalized rat IEC-6 cell line that was found to exhibit the predominant influx activity for methotrexate (MTX) at pH 5.5 with a low level of activity at pH 7.4. Transfection of this cell line with an RFC1 construct resulted in clones exhibiting increased MTX uptake at both the pHs and high folic acid uptake only at the low pH. For the two clones with the highest level of transport activity, relative MTX influx at the two pHs was reversed. Moreover, the low pH MTX influx activity ([MTX](e) = 0.5 microM) was markedly inhibited by 20 microM folic acid while influx at neutral pH was not. Furthermore, in the presence and absence of glucose at low pH, MTX and folic acid influx activity was inhibited by azide, while MTX influx at pH 7.4 was stimulated by azide in the absence of glucose but was unchanged in the presence of glucose and azide. This was contrasted with the results of transfection of the same RFC1 construct into an L1210 murine leukemia cell line bearing a nonfunctional endogenous carrier. In this case, the activity expressed was only at pH 7.4. These data indicate that RFC1 can exhibit two distinct types of folate transport activities in intestinal cells that must depend on tissue-specific modulators.

Mechanism of folate transport across the human colonic basolateral membrane

Previous studies from our laboratory have demonstrated the existence of a folate transporter in the human colonic apical membranes. The current studies were undertaken to examine the possible presence and function of a folate carrier in the human colonic basolateral membrane vesicles (BLMV). BLMV were purified from mucosal scrapings of colons of organ donors by a Percoll-density gradient centrifugation technique, and uptake studies were performed using a rapid filtration technique. Our results on [(3)H]Pte-Glu uptake are summarized as follows: 1) uptake was sensitive to osmolarity of the incubation medium; 2) Na(+) removal from the incubation medium did not affect folate uptake into BLMV; 3) uptake was significantly increased with decreasing incubation buffer pH from 8 to 4; 4) uptake demonstrated saturation kinetics with an apparent Michaelis constant of 9.6 +/- 0.48 microM and a maximal velocity of 8.10 +/- 0.36 pmol x mg protein(-1) x 10 s(-1); 5) uptake was markedly inhibited by the structural analog methotrexate (inhibitory constant = 8.28 +/- 1.0 microM); 6) uptake into BLMV demonstrated a trans-stimulation phenomenon; 7) anion exchange inhibitors DIDS and SITS significantly inhibited folate uptake; and 8) uptake was potential-insensitive, as voltage clamping of vesicles or making them inside positive with K(+)/valinomycin failed to influence folate uptake. Western blot analysis using purified human colonic basolateral membrane preparations and specific polyclonal antibodies against the human reduced folate carrier (hRFC) has shown expression of the hRFC protein at this membrane domain. These data demonstrate the existence of a pH-dependent, DIDS-sensitive, electroneutral, carrier-mediated mechanism for folate transport across the human colonic basolateral membranes.

Adaptive regulation of intestinal folate uptake: effect of dietary folate deficiency

Folate is an essential micronutrient that, in mammals, must be obtained from exogenous sources via intestinal absorption. Previous studies have characterized different aspects of the mechanism of the intestinal folate uptake process. Much less, however, is known about regulation of this process. In this study, we examined the effect of dietary folate deficiency on intestinal folate uptake using the rat as an animal model. The results showed that dietary folate deficiency leads to a significant (P < 0.01) and specific upregulation in the transepithelial transport of folic acid. The upregulation in transepithelial folate transport 1) was found to be due to an induction in carrier-mediated folate uptake across the brush-border membrane (BBM) and was mediated via a significant (P < 0.01) increase in the maximal velocity but not the apparent Michaelis constant of the uptake process, 2) was associated with a marked increase in the steady-state mRNA level of reduced folate carrier-1 and in the level of the expressed protein at the intestinal BBM, and 3) was associated with a marked (>10-fold) increase in the activity of the intestinal BBM form of folate hydrolase. Results of this study demonstrate, for the first time, that dietary folate deficiency leads to a marked upregulation in intestinal folate uptake and in the activity of folate hydrolase. Furthermore, the upregulation in folate uptake is associated with an increase in mRNA and protein levels of folate carrier, suggesting possible involvement of a transcriptional regulatory mechanism(s) in the upregulation.

Intestinal folate transport: identification of a cDNA involved in folate transport and the functional expression and distribution of its mRNA

Although the mechanism of folate intestinal transport has been the subject of intensive studies, very little is known about the molecular identity of the transport system(s) involved. In this investigation, we screened a mouse intestinal cDNA library using as probe the cDNA clone of a reduced folate carrier (RFC1) of mouse leukemia L1210 cells, and identified a positive clone, IFC1(RFC1). The cloned cDNA consisted of 2274 base pairs with an open reading frame that encodes a putative polypeptide of 512 amino acids with a predicted molecular mass of 58,112 daltons and 12 putative transmembrane domains. The polypeptide appears to carry a net positive charge (pI = 8.6) which may be important for its interaction with the negatively charged substrate. Functional identity of the IFC1(RFC1) clone was established by expression in Xenopus oocytes. An 11-fold increase in 5-methyltetrahydrofolate (5-MTHF) uptake was observed in oocytes injected with 10 ng IFC1(RFC1) cRNA compared to water-injected controls. The expressed folate uptake in the cRNA injected oocyte was (1) 4,4'-diisothiocyanatosilbene-2,2'-disulfonic acid (DIDS)-sensitive; and (2) saturable with an apparent Km of 1.99 +/- 0.32 micrometers and a V(max) of 3782 +/- 188 fmol/oocyte per h. The distribution of mRNA species complementary to IFC1(RFC1) in different mouse tissues was examined by Northern blot analysis. In addition to the small intestine, expression of such mRNA species were also found in the kidney, large intestine, brain, heart and liver. Furthermore, mRNA species complementary to IFC1(RFC1) were also detected by Northern blot analysis in the small intestine of human and other animal species (rat and rabbit). Expression of mRNA complementary to IFC1(RFC1) was markedly higher in rat intestinal villus cells than in crypt cells. These results represent the first identification of a folate transporter in mammalian intestine.

Jejunal brush-border folate hydrolase. A novel enzyme

Dietary folate, a vitamin required for DNA synthesis and cell regeneration, occurs as pteroylpolyglutamates that are hydrolyzed to pteroylglutamate during the process of intestinal absorption. Studies from our laboratory over the past 15 years have shown that jejunal brush-border folate hydrolase is essential and rate-limiting in folate absorption. Brush-border folate hydrolase activity and pteroylpolyglutamate hydrolysis are inhibited in disease and conditions associated with folate deficiency, including celiac and tropical sprue, the use of sulfasalazine to treat inflammatory bowel disease, and chronic alcoholism. Brush-border folate hydrolase is an exopeptidase located on the jejunal brush-border surface that liberates hydrolytic products of pteroylpolyglutamates in a progressive fashion, with a final release of pteroylglutamate. Subsequent steps in folate absorption include uptake by a brush-border folate-binding-protein receptor and transport across the brush-border membrane into the enterocyte. These steps are probably followed by an intracellular synthesis of pteroylglutamates for folate-dependent reactions and intracellular hydrolysis to pteroylglutamate for transport across the basolateral membrane to the portal circulation. In pigs, the active form of jejunal brush-border folate hydrolase has a molecular weight of 240 kd and is probably a homodimer of the 120-kd protein found after immunoprecipitation with specific antibody. Regulating the synthesis and expression of brush-border folate hydrolase may be critical to the availability of dietary folate.

Folic acid transport in organ-cultured mucosa of human intestine. Evidence for distinct carriers

The transport of folic acid was investigated in organ-cultured endoscopic biopsy specimens of intestinal mucosa from normal subjects. In the proximal small intestine at pH 5.5 and 6.5, [3H]folic acid accumulated to concentrations 2.64- and 2.17-fold higher than those of the medium, respectively, but at pH 7.5 the concentration was same as that of the medium. Saturability of initial rates of uptake was demonstrable with respect to luminal concentrations of [H+] and folic acid. Two proton-dependent transport mechanisms were evident: (a) a carrier with a low affinity for H+ and a low capacity for folic acid, and (b) a high-affinity, high-capacity carrier (Km, 635 and 54.6 nmol/L [H+]; Vmax, 0.066 and 2.583 pmol folic acid/microL intracellular water per 10 minutes, respectively). Kinetic studies of folic acid uptake at pH 5.5 and 7.5 revealed a difference in Km for folic acid (15.76 and 34.38 mumol/L, respectively) with no change in Vmax. In the colon, folic acid did not accumulate against a concentration gradient, and the initial rate of uptake was not affected by luminal pH. The accumulation of folic acid in colonic mucosa was significantly higher at pH 5.5 than at 7.5 and showed significant regional variation, with optimal uptake in the sigmoid colon. Methotrexate inhibited folic acid uptake competitively both in the proximal small intestine and in the cecum (inhibition constant 6.9 and 54 mumol/L, respectively, at pH 5.5). These data indicate the presence of a proton-dependent, active transport of folic acid in the proximal small intestine at pH 5.0-6.5. At pH 7.5 and in the mucosa of the colon, uptake of folic acid proceeds by facilitated diffusion through a low-affinity carrier.

The intestinal absorption of dietary folates in health and disease

Dietary folates exist as pteroylpolyglutamates (PteGlun) that undergo hydrolysis to pteroylmonoglutamate (PteGlu) forms during the process of intestinal absorption. Using the technique of jejunal perfusion of separately labeled folates, our laboratory has demonstrated that hydrolysis of PteGlun occurs on the surface of the jejunum and is a prerequisite for folate absorption. An intestinal brush border pteroylpolyglutamate hydrolase (BB-PPH) has been identified in human and pig jejunum with characteristics that are distinct from those of an intracellular hydrolase (IC-PPH). Functional parallels of BB-PPH with in vivo hydrolysis of PteGlun in human and pig intestine and the clinical responsiveness of BB-PPH to different disease states indicate that this enzyme plays the major physiological role in folate absorption. Folate malabsorption is found in diseases which affect the jejunal mucosa and in response to various drugs. In most of these clinical conditions, folate malabsorption results from suppression of both of the processes of hydrolysis of PteGlun and jejunal uptake of PteGlu. Ongoing studies in miniature pigs are aimed at definition of the sequence of development of folate malabsorption in chronic alcoholism.

Depressed jejunal secretion of water and ions in response to prostaglandin E1 in adult celiac disease

In order to test in man the concept that intestinal fluid secretion originates from the crypts of Lieberkühn, we assessed the jejunal secretory effect of intraluminal prostaglandin E1 (PGE1) (0.9 microgram/kg/min) in untreated adult celiac disease (ACD, N = 7), treated ACD (N = 4) and normal subjects using the intestinal perfusion technique. In untreated ACD (1) water and solutes were malabsorbed (or secreted) in the basal period; (2) fluid and ion secretion seen during PGE1 infusion and net secretory effect of PGE1 (difference in transport between basal and PGE1 periods) were reduced (P less than 0.01); (3) the effects of PGE1 inhibition of sodium insorption, and stimulation of sodium exsorption, were decreased and abolished, respectively. In treated patients PGE1-induced secretion returned towards normal values. Our finding of a depressed secretory response to PGE1 in the face of marked crypt hypertrophy do not support the concept that crypt cells are the major site of intestinal fluid secretion.

Availability of monoglutamyl and polyglutamyl folates in normal subjects and in patients with coeliac sprue

Intestinal folate absorption was assessed in six normal subjects and in four patients with coeliac sprue who were studied before and after treatment by dietary gluten exclusion. Comparisons were made of the luminal disappearance from the perfused jejunum of 3H-pteroylmonoglutamate and pteroyl 14C-glutamylhexaglutamate, and of the 48-hour urinary recovery of each isotope after perfusion and a tissue saturating dose of folic acid. The labelled urinary folates consisted of folic acid, 10-formyltetrahydrofolate, and 5-methyltetrahydrofolate. In each group urinary recovery of 3H was significantly greater than that of 14C, confirming the evidence from jejunal perfusion that the availability of monoglutamyl folate is greater than that of polyglutamyl folate. According to the urinary recovery data, both folates were poorly absorbed in untreated coeliac sprue, but were normally absorbed after treatment. Assuming uniform displacement of the absorbed labelled folates by the parenteral flushing dose, the finding of greater urinary isotope recovery than of luminal folate disappearance from the perfused proximal jejunal segment suggests an adaptation of the distal small bowel for folate absorption in coeliac sprue.

Folate conjugase: two separate activities in human jejunum

Previous perfusion studies of the human jejunum suggested that conjugated folate is hydrolyzed on the mucosal surface. The techniques of cell fractionation and DEAE and gel chromatography led to the identification of two separate folate conjugase activities in human jejunal mucosa: one membrane-bound and concentrated in the brush border, the other soluble and intracellular. These enzyme activities exhibit different pH optima, molecular weights, and inhibition characteristics. Folate conjugase in the brush border may accomplish the initial digestion of dietary pteroylpolyglutamates.