Intestinal absorption of water-soluble vitamins in health and disease

Association of TM4SF4 with the human thiamine transporter-2 in intestinal epithelial cells

BACKGROUND

The human thiamine transporter-2 (hTHTR-2) is involved in the intestinal absorption of thiamine. Recent studies with membrane transporters of other nutrients/substrates have shown that they have associated proteins that affect different aspects of their physiology and cell biology. Nothing is known about protein(s) that interact with hTHTR-2 in intestinal epithelial cells and influence its physiological function and/or its cell biology.

AIMS

The aim of this study was to identify protein partner(s) that interact with hTHTR-2 in human intestinal cells and determine the physiological/biological consequence of that interaction.

METHODS

The yeast split-ubiquitin two-hybrid approach was used to screen a human intestinal cDNA library. GST-pull-down and cellular co-localization approaches were used to confirm the interaction between hTHTR-2 and the associated protein(s). The effect of such an interaction on hTHTR-2 function was examined by (3)H-thiamine uptake assays.

RESULTS

Our screening results identified the human TransMembrane 4 SuperFamily 4 (TM4SF4) as a potential interactor with hTHTR-2. This interaction was confirmed by an in vitro GST-pull-down assay, and by live-cell confocal imaging of HuTu-80 cells co-expressing hTHTR-2-GFP and mCherry-TM4SF4 (the latter displayed a significant overlap of these two proteins in intracellular vesicles and at the cell membrane). Co-expression of hTHTR-2 with TM4SF4 in HuTu-80 cells led to a significant induction in thiamine uptake. In contrast, silencing TM4SF4 with gene-specific siRNA led to a significant decrease in thiamine uptake.

CONCLUSIONS

These results show for the first time that the accessory protein TM4SF4 interacts with hTHTR-2 and influences the physiological function of the thiamine transporter.

Molecular identification and functional characterization of the human colonic thiamine pyrophosphate transporter

Colonic microbiota synthesize a considerable amount of thiamine in the form of thiamine pyrophosphate (TPP). Recent functional studies from our laboratory have shown the existence of a specific, high-affinity, and regulated carrier-mediated uptake system for TPP in human colonocytes. Nothing, however, is known about the molecular identity of this system. Here we report on the molecular identification of the colonic TPP uptake system as the product of the SLC44A4 gene. We cloned the cDNA of SLC44A4 from human colonic epithelial NCM460 cells, which, upon expression in ARPE19 cells, led to a significant (p < 0.01, >5-fold) induction in [(3)H]TPP uptake. Uptake by the induced system was also found to be temperature- and energy-dependent; Na(+)-independent, slightly higher at acidic buffer pH, and highly sensitive to protonophores; saturable as a function of TPP concentration, with an apparent Km of 0.17 ± 0.064 μM; and highly specific for TPP and not affected by free thiamine, thiamine monophosphate, or choline. Expression of the human TPP transporter was found to be high in the colon and negligible in the small intestine. A cell surface biotinylation assay and live cell confocal imaging studies showed the human TPP transporter protein to be expressed at the apical membrane domain of polarized epithelia. These results show, for the first time, the molecular identification and characterization of a specific and high-affinity TPP uptake system in human colonocytes. The findings further support the hypothesis that the microbiota-generated TPP is absorbable and could contribute toward host thiamine homeostasis, especially toward cellular nutrition of colonocytes.

Appropriate nutrient supplementation in celiac disease

Reduced levels of iron, folate, vitamin B12, vitamin D, zinc, and magnesium are common in untreated celiac disease (CD) patients probably due to loss of brush border proteins and enzymes needed for the absorption of these nutrients. In the majority of patients, removal of gluten from the diet leads to histological recovery and normalization of iron, vitamin, and mineral levels. Iron deficiency anemia is the most common extra-intestinal sign of CD and usually resolves with adherence to a gluten-free diet. However, deficiencies of both folate and vitamin B12 may persist in some patients on a gluten-free diet, thus requiring vitamin supplementation to improve subjective health status. Similarly, exclusion of gluten from the diet does not always normalize bone mineral density; in these cases, supplementation of vitamin D and calcium is recommended. Resolution of mucosal inflammation may not be sufficient to abrogate magnesium deficiency. Since gluten-free cereal products have a lower magnesium content as compared with gluten-containing counterparts, a magnesium-enriched diet should be encouraged in CD patients. In this article we discuss the frequency and clinical relevance of nutrient deficiency in CD and whether and when nutrient supplementation is needed.

Double-blind, randomized placebo-controlled clinical trial of benfotiamine for severe alcohol dependence

Alcohol dependence is associated with severe nutritional and vitamin deficiency. Vitamin B1 (thiamine) deficiency erodes neurological pathways that may influence the ability to drink in moderation. The present study examines tolerability of supplementation using the high-potency thiamine analog, benfotiamine (BF), and BF's effects on alcohol consumption in severely affected, self-identified, alcohol dependent subjects. A randomized, double-blind, placebo-controlled trial was conducted on 120 non-treatment seeking, actively drinking, alcohol dependent men and women volunteers (mean age=47 years) from the Kansas City area who met DSM-IV-TR criteria for current alcohol dependence. Subjects were randomized to receive 600 mg benfotiamine or placebo (PL) once daily by mouth for 24 weeks with 6 follow-up assessments scheduled at 4 week intervals. Side effects and daily alcohol consumption were recorded. Seventy (58%) subjects completed 24 weeks of study (N=21 women; N=49 men) with overall completion rates of 55% (N=33) for PL and 63% (N=37) for BF groups. No significant adverse events were noted and alcohol consumption decreased significantly for both treatment groups. Alcohol consumption decreased from baseline levels for 9 of 10 BF treated women after 1 month of treatment compared with 2 of 11 on PL. Reductions in total alcohol consumption over 6 months were significantly greater for BF treated women (BF: N=10, -611 ± 380 standard drinks; PL: N=11, -159 ± 562 standard drinks, p-value=0.02). BF supplementation of actively drinking alcohol dependent men and women was well-tolerated and may discourage alcohol consumption among women. The results do support expanded studies of BF treatment in alcoholism.

Big brains, meat, tuberculosis, and the nicotinamide switches: co-evolutionary relationships with modern repercussions?

Meat-eating was a game changer for human evolution. We suggest that the limiting factors for expanding brains earlier were scarcities of nicotinamide and tryptophan. In humans and some other omnivores, lack of meat causes these deficiencies. Nicotinamide adenine dinucleotide (NADH) is necessary to synthesize adenosine triphosphate (ATP) via either glycolysis or via the mitochondrial respiratory chain. NAD consumption is also necessary for developmental and repair circuits. Inadequate supplies result in "de-evolutionary" brain atrophy, as seen with pellagra. If trophic nicotinamide/tryptophan was a "prime mover" in building bigger brains, back-up mechanisms should have evolved. One strategy may be to recruit extra gut symbionts that produce NADH precursors or export nicotinamide (though this may cause diarrhea). We propose a novel supplier TB that co-evolved early, which did not originally and does not now inevitably cause disease. TB has highly paradoxical immunology for a pathogen, and secretes and is inhibited by nicotinamide and its analogue, isoniazid. Sharp declines in TB and diarrhea correlated with increased meat intake in the past, suggesting that dietary vitamin B3 and tryptophan deficiencies (also associated with poor cognition and decreased lifespans) are still common where meat is unaffordable.

Mammalian colonocytes possess a carrier-mediated mechanism for uptake of vitamin B3 (niacin): studies utilizing human and mouse colonic preparations

Niacin (vitamin B3; nicotinic acid) plays an important role in maintaining redox state of cells and is obtained from endogenous and exogenous sources. The latter source has generally been assumed to be the dietary niacin, but another exogenous source that has been ignored is the niacin that is produced by the normal microflora of the large intestine. For this source of niacin to be bioavailable, it needs to be absorbed, but little is known about the ability of the large intestine to absorb niacin and the mechanism involved. Here we addressed these issues using the nontransformed human colonic epithelial NCM460 cells, native human colonic apical membrane vesicles (AMV) isolated from organ donors, and mouse colonic loops in vivo as models. Uptake of ³H-nicotinic acid by NCM460 cells was: 1) acidic pH (but not Na⁺) dependent; 2) saturable (apparent Km = 2.5 ± 0.8 μM); 3) inhibited by unlabeled nicotinic acid, nicotinamide, and probenecid; 4) neither affected by other bacterially produced monocarboxylates, monocarboxylate transport inhibitor, or by substrates of the human organic anion transporter-10; 5) affected by modulators of the intracellular protein tyrosine kinase- and Ca²⁺-calmodulin-regulatory pathways; and 6) adaptively regulated by extracellular nicotinate level. Uptake of nicotinic acid by human colonic AMV in vitro and by mouse colonic loops in vivo was also carrier mediated. These findings report, for the first time, that mammalian colonocytes possess a high-affinity carrier-mediated mechanism for nicotinate uptake and show that the process is affected by intracellular and extracellular factors.

Characterization of the human mitochondrial thiamine pyrophosphate transporter SLC25A19 minimal promoter: a role for NF-Y in regulating basal transcription

Transcriptional regulation of expression of the human mitochondrial thiamine pyrophosphate transporter (the product of the SLC25A19 gene) is unknown. To understand this regulation, we cloned and characterized the 5'-regulatory region of the SLC25A19 gene (1,080 bp). The cloned fragment was found to possess promoter activity in transiently transfected human-derived liver HepG2 cells. 5'- and 3'-deletion analysis has identified the minimal region required for basal SLC25A19 promoter activity to be between -131 and +20 (using the distal transcriptional start site as +1). The minimal promoter lacks typical TATA motif and contains two inverted CCAAT boxes (binding sites for NF-Y transcriptional factor). By means of mutational analysis, the critical role of both the upstream and downstream CCAAT boxes in basal SLC25A19 promoter activity was established; however, each of these boxes alone was found to be unable to support promoter activity. EMSA and supershift EMSA (with the use of specific antibodies against NF-Y subunits) studies, as well as chromatin immunoprecipitation assay, demonstrated the binding of NF-Y to both CCAAT boxes in vitro and in vivo, respectively. The requirement for NF-Y in SLC25A19 promoter activity in vivo was directly confirmed by the use of a dominant negative NF-YA mutant in transiently transfected HepG2 cells. These studies report for the first time the characterization of the SLC25A19 promoter and demonstrate an essential role for NF-Y in its basal activity.

Differentiation-dependent regulation of intestinal vitamin B(2) uptake: studies utilizing human-derived intestinal epithelial Caco-2 cells and native rat intestine

Intestinal epithelial cells undergo differentiation as they move from the crypt to the villi, a process that is associated with up- and downregulation in expression of a variety of genes, including those involved in nutrient absorption. Whether the intestinal uptake process of vitamin B(2) [riboflavin (RF)] also undergoes differentiation-dependent regulation and the mechanism through which this occurs are not known. We used human-derived intestinal epithelial Caco-2 cells and native rat intestine as models to address these issues. Caco-2 cells showed a significantly higher carrier-mediated RF uptake in post- than preconfluent cells. This upregulation was associated with a significantly higher level of protein and mRNA expression of the RF transporters hRFVT-1 and hRFVT-3 in the post- than preconfluent cells; it was also accompanied with a significantly higher rate of transcription of the respective genes (SLC52A1 and SLC52A3), as indicated by the higher level of expression of heterogeneous nuclear RNA and higher promoter activity in post- than preconfluent cells. Studies with native rat intestine also showed a significantly higher RF uptake by epithelial cells of the villus tip than epithelial cells of the crypt; this again was accompanied by a significantly higher level of expression of the rat RFVT-1 and RFVT-3 at the protein, mRNA, and heterogeneous nuclear RNA levels. These findings show, for the first time, that the intestinal RF uptake process undergoes differentiation-dependent upregulation and suggest that this is mediated (at least in part) via transcriptional mechanisms.

Main issues in micronutrient supplementation in phenylketonuria

For almost all patients with PKU, a low phenylalanine diet is the basis of the treatment despite a widely varying natural protein tolerance. A vitamin and mineral supplement is essential and it is commonly added to a phenylalanine-free (phe-free) source of L-amino acids. In PKU, many phe-free L-amino acid supplements have age-specific vitamin and mineral profiles to meet individual requirements. The main micronutrient sources are chemically derived and their delivery dosage is usually advised in three or more doses throughout the day. Within the EU, the composition of VM (vitamin and mineral) phe-free L-amino acid supplements is governed by the Foods for Special Medical Purposes (FSMP) directive (European Commission Directive number 1999/21/EC and amended by Directive 2006/141/EC). However the micronutrient composition of the majority fails to remain within FSMP micronutrient maximum limits per 100 kcal due to their low energy content and so compositional exceptions to the FSMP directive have to be granted for each supplement. All patients with PKU require an annual nutritional follow-up, until it has been proven that they are not at risk of any vitamin and mineral imbalances. When non-dietary treatments are used to either replace or act as an adjunct to diet therapy, the quality of micronutrient intake should still be considered important and monitored systematically. European guidelines are required about which micronutrients should be measured and the conditions (fasting status) for monitoring.

Conditional knockout of the Slc5a6 gene in mouse intestine impairs biotin absorption

The Slc5a6 gene expresses a plasma membrane protein involved in the transport of the water-soluble vitamin biotin; the transporter is commonly referred to as the sodium-dependent multivitamin transporter (SMVT) because it also transports pantothenic acid and lipoic acid. The relative contribution of the SMVT system toward carrier-mediated biotin uptake in the native intestine in vivo has not been established. We used a Cre/lox technology to generate an intestine-specific (conditional) SMVT knockout (KO) mouse model to address this issue. The KO mice exhibited absence of expression of SMVT in the intestine compared with sex-matched littermates as well as the expected normal SMVT expression in other tissues. About two-thirds of the KO mice died prematurely between the age of 6 and 10 wk. Growth retardation, decreased bone density, decreased bone length, and decreased biotin status were observed in the KO mice. Microscopic analysis showed histological abnormalities in the small bowel (shortened villi, dysplasia) and cecum (chronic active inflammation, dysplasia) of the KO mice. In vivo (and in vitro) transport studies showed complete inhibition in carrier-mediated biotin uptake in the intestine of the KO mice compared with their control littermates. These studies provide the first in vivo confirmation in native intestine that SMVT is solely responsible for intestinal biotin uptake. These studies also provide evidence for a casual association between SMVT function and normal intestinal health.

Oral delivery of glucagon-like peptide-1 and analogs: alternatives for diabetes control?

Type 2 diabetes mellitus (T2DM) is one of the most prevalent diseases worldwide. Current treatments are often associated with off-target effects and do not significantly impact disease progression. New therapies are therefore urgently needed to overcome this social burden. Glucagon-like peptide-1 (GLP-1), an incretin hormone, has been used to control T2DM symptomatology. However, the administration of peptide or proteins drugs is still a huge challenge in the pharmaceutical field, requiring administration by parenteral routes. This article reviews the main hurdles in oral administration of GLP-1 and focuses on the strategies utilized to overcome them.

A high-affinity and specific carrier-mediated mechanism for uptake of thiamine pyrophosphate by human colonic epithelial cells

All mammals require exogenous sources of thiamine (vitamin B1), as they lack the ability to synthesize the vitamin. These sources are dietary and bacterial (the latter is in reference to the vitamin, which is synthesized by the normal microflora of the large intestine). Bacterially generated thiamine exists in the free, as well as the pyrophosphorylated [thiamine pyrophosphate (TPP)], form. With no (or very little) phosphatase activity in the colon, we hypothesized that the bacterially generated TPP can also be taken up by colonocytes. To test this hypothesis, we examined [(3)H]TPP uptake in the human-derived, nontransformed colonic epithelial NCM460 cells and purified apical membrane vesicles isolated from the colon of human organ donors. Uptake of TPP by NCM460 cells occurred without metabolic alterations in the transported substrate and 1) was pH- and Na(+)-independent, but energy-dependent, 2) was saturable as a function of concentration (apparent K(m) = 0.157 ± 0.028 μM), 3) was highly specific for TPP and not affected by free thiamine (or its analogs) or by thiamine monophosphate and unrelated folate derivatives, 4) was adaptively regulated by extracellular substrate (TPP) level via what appears to be a transcriptionally mediated mechanism(s), and 5) appeared to be influenced by an intracellular Ca(2+)/calmodulin-mediated regulatory pathway. These findings suggest the involvement of a carrier-mediated mechanism for TPP uptake by colonic NCM460 cells, which was further confirmed by results from studies of native human colonic apical membrane vesicles. The results also suggest that the bacterially synthesized TPP in the large intestine is bioavailable and may contribute to overall body homeostasis of vitamin B1 and, especially, to the cellular nutrition of the local colonocytes.

Methyl donor deficiency affects small-intestinal differentiation and barrier function in rats

Dietary methyl donors and their genetic determinants are associated with Crohn's disease risk. We investigated whether a methyl-deficient diet (MDD) may affect development and functions of the small intestine in rat pups from dams subjected to the MDD during gestation and lactation. At 1 month before pregnancy, adult females were fed with either a standard food or a diet without vitamin B12, folate and choline. A global wall hypotrophy was observed in the distal small bowel (MDD animals 0·30 mm v. controls 0·58 mm; P< 0·001) with increased crypt apoptosis (3·37 v. 0·4 %; P< 0·001), loss of enterocyte differentiation in the villus and a reduction in intestinal alkaline phosphatase production. Cleaved caspase-3 immunostaining (MDD animals 3·37 % v. controls 0·4 %, P< 0·001) and the Apostain labelling index showed increased crypt apoptosis (3·5 v. 1·4 %; P= 0·018). Decreased proliferation was observed in crypts of the proximal small bowel with a reduced number of minichromosome maintenance 6 (MDD animals 52·83 % v. controls 83·17 %; P= 0·048) and proliferating cell nuclear antigen-positive cells (46·25 v. 59 %; P= 0·05). This lack of enterocyte differentiation in the distal small bowel was associated with an impaired expression of β-catenin and a decreased β-catenin–E-cadherin interaction. The MDD affected the intestinal barrier in the proximal small bowel by decreasing Paneth cell number after immunostaining for lysosyme (MDD animals 8·66 % v. controls 21·66 %) and by reducing goblet cell number and mucus production after immunostaining for mucin-2 (crypts 8·66 v. 15·33 %; villus 7 v. 17 %). The MDD has dual effects on the small intestine by producing dramatic effects on enterocyte differentiation and barrier function in rats.

Biological mechanisms for nutritional regulation of maternal health and fetal development

This review paper highlights mechanisms for nutritional regulation of maternal health and fetal development. Malnutrition (nutrient deficiencies or obesity) in pregnant women adversely affects their health by causing or exacerbating a plethora of problems, such as anaemia, maternal haemorrhage, insulin resistance, and hypertensive disorders (e.g. pre-eclampsia/eclampsia). Maternal malnutrition during gestation also impairs embryonic and fetal growth and development, resulting in deleterious outcomes, including intrauterine growth restriction (IUGR), low birthweight, preterm birth, and birth defects (e.g. neural tube defects and iodine deficiency disorders). IUGR and preterm birth contribute to high rates of neonatal morbidity and mortality. Major common mechanisms responsible for malnutrition-induced IUGR and preterm birth include: (i) abnormal growth and development of the placenta; (ii) impaired placental transfer of nutrients from mother to fetus; (iii) endocrine disorders; and (iv) disturbances in normal metabolic processes. Activation of a series of physiological responses leading to premature and sustained contraction of the uterine myometrium also results in preterm birth. Recent epidemiologic studies have suggested a link between IUGR and chronic metabolic disease in children and adults, and the effects of IUGR may be carried forward to subsequent generations through epigenetics. While advanced medical therapies, which are generally unavailable in low-income countries, are required to support preterm and IUGR infants, optimal nutrition during pregnancy may help ameliorate many of these problems. Future studies are necessary to develop effective nutritional interventions to enhance fetal growth and development and alleviate the burden of maternal morbidity and mortality in low- and middle-income countries.

Host-gut microbiota metabolic interactions

The composition and activity of the gut microbiota codevelop with the host from birth and is subject to a complex interplay that depends on the host genome, nutrition, and life-style. The gut microbiota is involved in the regulation of multiple host metabolic pathways, giving rise to interactive host-microbiota metabolic, signaling, and immune-inflammatory axes that physiologically connect the gut, liver, muscle, and brain. A deeper understanding of these axes is a prerequisite for optimizing therapeutic strategies to manipulate the gut microbiota to combat disease and improve health.

A pivotal role of vitamin B9 in the maintenance of regulatory T cells in vitro and in vivo

Dietary factors regulate immunological function, but the underlying mechanisms remain elusive. Here we show that vitamin B9 is a survival factor for regulatory T (Treg) cells expressing high levels of vitamin B9 receptor (folate receptor 4). In vitamin B9-reduced condition in vitro, Treg cells could be differentiated from naïve T cells but failed to survive. The impaired survival of Treg cells was associated with decreased expression of anti-apoptotic Bcl2 and independent of IL-2. In vivo depletion of dietary vitamin B9 resulted in the reduction of Treg cells in the small intestine, a site for the absorption of dietary vitamin B9. These findings provide a new link between diet and the immune system, which could maintain the immunological homeostasis in the intestine.

Thiamine in nutrition therapy

Clinicians involved with nutrition therapy traditionally concentrated on macronutrients and have generally neglected the importance of micronutrients, both vitamins and trace elements. Micronutrients, which work in unison, are important for fundamental biological processes and enzymatic reactions, and deficiencies may lead to disastrous consequences. This review concentrates on vitamin B(1), or thiamine. Alcoholism is not the only risk factor for thiamine deficiency, and thiamine deficiency is often not suspected in seemingly well-nourished or even overnourished patients. Deficiency of thiamine has historically been described as beriberi but may often be seen in current-day practice, manifesting as neurologic abnormalities, mental changes, congestive heart failure, unexplained metabolic acidosis, and so on. This review explains the importance of thiamine in nutrition therapy and offers practical tips on prevention and management of deficiency states.

Tspan-1 interacts with the thiamine transporter-1 in human intestinal epithelial cells and modulates its stability

The human thiamine transporter-1 (hTHTR-1) contributes to intestinal thiamine uptake, and its function is regulated at both the transcriptional and posttranscriptional levels. Nothing, however, is known about the protein(s) that may interact with hTHTR-1 and affects its cell biology and physiology. We addressed this issue in the present investigation using a bacterial two-hybrid system to screen a human intestinal cDNA library with the complete coding sequence of hTHTR-1 as a bait. Our results showed that a member of the tetraspanin family of proteins, Tspan-1, interacts with hTHTR-1. Coimmunoprecipitation and glutathione S-transferase (GST)-pulldown assays confirmed the existence of such an interaction between hTspan-1 and hTHTR-1 in human intestinal epithelial Caco-2 cells. Furthermore, live cell confocal imaging demonstrated that hTspan-1 and hTHTR-1 colocalize in human intestinal epithelial HuTu-80 cells. The importance of the interaction between hTspan-1 and hTHTR-1 for cell biology of the thiamine transporter was examined in HuTu-80 cells stably expressing hTHTR-1. Coexpression of hTspan-1 in these cells led to a significant decrease in the rate of degradation of hTHTR-1 compared with cells expressing the hTHTR-1 alone; in fact the half-life of the hTHTR-1 protein was twice longer in the former cell type compared with the latter cell type (12 h vs. 6 h, respectively). This finding was also confirmed at the functional level when a significantly higher thiamine uptake was observed in cycloheximide-treated (6 h) cells expressing hTHTR-1 together with hTspan-1 compared with those expressing hTHTR-1 alone. These studies demonstrate for the first time that Tspan-1 is an interacting partner with hTHTR-1 and that this interaction affects hTHTR-1 stability.

Cys(294) is essential for the function of the human sodium-dependent multivitamin transporter

The sodium-dependent multivitamin transporter (SMVT) plays an important role in biotin uptake in the intestine and other cell types. While significant knowledge has been gained with regard to regulation and cell biology of the SMVT system, there is little known about its structure-function relationships. Here we examined the role of each of the ten conserved (among species) cysteine residues in the function of the human SMVT (hSMVT) using site-directed mutagenesis. Our results showed a significant impairment in biotin uptake only in cells transfected with hSMVT mutated at Cys(294), but not at the other conserved cysteine residues; the impairment in biotin uptake caused by mutating Cys(294) was not related to the polar status of substituting amino acid. The inhibition in hSMVT function upon mutating Cys(294) was mediated via a significant reduction in the V(max), but not the apparent K(m), of the biotin uptake process, suggesting a decrease in the number (and/or activity) of hSMVT but not affinity. Biotinylation assay confirmed this suggestion by showing a marked reduction in the level of expression of the mutated protein at the cell membrane, without affecting total cellular level of induced hSMVT. These results show an important role for Cys(294) in the function and cell biology of hSMVT.