In vitro and in vivo characterization of the minimal promoter region of the human thiamin transporter SLC19A2

Tumor necrosis factor α impedes colonic thiamin pyrophosphate and free thiamin uptake: involvement of JNK/ERK1/2-mediated pathways

The aim of this study was to examine the effect of TNFα (i.e., a predominant proinflammatory cytokine produced during chronic gut inflammation) on colonic uptake of thiamin pyrophosphate (TPP) and free thiamin, forms of vitamin B1 that are produced by the gut microbiota and are absorbed via distinct carrier-mediated systems. We utilized human-derived colonic epithelial CCD841 and NCM460 cells, human differentiated colonoid monolayers, and mouse intact colonic tissue preparations together with an array of cellular/molecular approaches in our investigation. The results showed that exposure of colonic epithelial cells to TNFα leads to a significant inhibition in TPP and free thiamin uptake. This inhibition was associated with: 1) a significant suppression in the level of expression of the colonic TPP transporter (cTPPT; encoded by SLC44A4), as well as thiamin transporters-1 & 2 (THTR-1 & -2; encoded by SLC19A2 & SLC19A3, respectively); 2) marked inhibition in activity of the SLC44A4, SLC19A2, and SLC19A3 promoters; and 3) significant suppression in level of expression of nuclear factors that are needed for activity of these promoters (i.e., CREB-1, Elf-3, NF-1A, SP-1). Furthermore, the inhibitory effects were found to be mediated via JNK and ERK1/2 signaling pathways. We also examined the level of expression of cTPPT and THTR-1 & -2 in colonic tissues of patients with active ulcerative colitis and found the levels to be significantly lower than in healthy controls. These findings demonstrate that exposure of colonocytes to TNFα suppresses TPP and free thiamin uptake at the transcriptional level via JNK- and Erk1/2-mediated pathways.

Hypoxia inhibits colonic uptake of the microbiota-generated forms of vitamin B1 via HIF-1α-mediated transcriptional regulation of their transporters

Hypoxia exerts profound effects on cell physiology, but its effect on colonic uptake of the microbiota-generated forms of vitamin B1 (i.e., thiamin pyrophosphate [TPP] and free thiamine) has not been described. Here, we used human colonic epithelial NCM460 cells and human differentiated colonoid monolayers as in vitro and ex vivo models, respectively, and were subjected to either chamber (1% O2, 5% CO2, and 94% N2) or chemically (desferrioxamine; 250 μM)-induced hypoxia followed by determination of different physiological-molecular parameters. We showed that hypoxia causes significant inhibition in TPP and free thiamin uptake by colonic NCM460 cells and colonoid monolayers; it also caused a significant reduction in the expression of TPP (SLC44A4) and free thiamin (SLC19A2 and SLC19A3) transporters and in activity of their gene promoters. Furthermore, hypoxia caused a significant induction in levels of hypoxia-inducible transcription factor (HIF)-1α but not HIF-2α. Knocking down HIF-1α using gene-specific siRNAs in NCM460 cells maintained under hypoxic conditions, on the other hand, led to a significant reversal in the inhibitory effect of hypoxia on TPP and free thiamin uptake as well as on the expression of their transporters. Finally, a marked reduction in level of expression of the nuclear factors cAMP responsive element-binding protein 1 and gut-enriched Krüppel-like factor 4 (required for activity of SLC44A4 and SLC19A2 promoters, respectively) was observed under hypoxic conditions. In summary, hypoxia causes severe inhibition in colonic TPP and free thiamin uptake that is mediated at least in part via HIF-1α-mediated transcriptional mechanisms affecting their respective transporters.

Reduced Thiamine Availability and Hyperglycemia Impair Thiamine Transport in Renal Glomerular Cells through Modulation of Thiamine Transporter 2

Thiamine helps transketolase in removing toxic metabolites, counteracting high glucose-induced damage in microvascular cells, and progression of diabetic retinopathy/nephropathy in diabetic animals. Diabetic subjects show reduced thiamine levels. Hyperglycemia and reduced thiamine availability concur in impairing thiamine transport inside the blood-retinal barrier, with thiamine transporter-2 (THTR2) primarily involved. Here, we examined the behavior of thiamine transporter-1 (THTR1), THTR2, and their transcription factor Sp1 in response to high glucose and altered thiamine availability in renal cells involved in diabetic nephropathy. Human proximal tubule epithelial cells, podocytes, glomerular endothelial, and mesangial cells were exposed to high glucose and/or thiamine deficiency/oversupplementation. Localization and modulation of THTR1, THTR2, and Sp1; intracellular thiamine; transketolase activity; and permeability to thiamine were examined. Reduced thiamine availability and hyperglycemia impaired thiamine transport and THTR2/Sp1 expression. Intracellular thiamine, transketolase activity, and permeability were strongly dependent on thiamine concentrations and, partly, excess glucose. Glomerular endothelial cells were the most affected by the microenvironmental conditions. Our results confirmed the primary role of THTR2 in altered thiamine transport in cells involved in diabetic microvascular complications. Lack of thiamine concurs with hyperglycemia in impairing thiamine transport. Thiamine supplementation could represent a therapeutic option to prevent or slow the progression of these complications.

Proinflammatory cytokines inhibit thiamin uptake by human and mouse pancreatic acinar cells: involvement of transcriptional mechanism(s)

Thiamin (vitamin B1) plays critical roles in normal metabolism and function of all mammalian cells. Pancreatic acinar cells (PACs) import thiamin from circulation via specific carrier-mediated uptake that involves thiamin transporter-1 and -2 (THTR-1 and -2; products of SLC19A2 and SLC19A3, respectively). Our aim in this study was to investigate the effect(s) of proinflammatory cytokines on thiamin uptake by PACs. We used human primary (h)PACs, PAC 266-6 cells, and mice in vivo as models in the investigations. First, we examined the level of expression of THTR-1 and -2 mRNA in pancreatic tissues of patients with chronic pancreatitis and observed severe reduction in their expression compared with normal control subjects. Exposing hPACs and PAC 266-6 to proinflammatory cytokines (hyper IL-6, TNF-α, and IL-1β) was found to lead to a significant inhibition in thiamin uptake. Focusing on hyper-IL-6 (which also inhibited thiamin uptake by primary mouse PACs), the inhibition in thiamin uptake was found to be associated with significant reduction in THTR-1 and -2 proteins and mRNA expression as well as in activity of the SLC19A2 and SLC19A3 promoters; it was also associated with reduction in level of expression of the transcription factor Sp1 (which is required for activity of these promoters). Finally, blocking the intracellular Stat3 signaling pathway was found to lead to a significant reversal in the inhibitory effect of hyper IL-6 on thiamin uptake by PAC 266-6. These results show that exposure of PACs to proinflammatory cytokines negatively impacts thiamin uptake via (at least in part) transcriptional mechanism(s).NEW & NOTEWORTHY Findings of the current study demonstrate, for the first time, that exposure of pancreatic acinar cells to proinflammatory cytokines (including hyper IL-6) cause significant inhibition in vitamin B1 (thiamin; a micronutrient that is essential for normal cellular energy metabolism) and that this effect is mediated at the level of transcription of the thiamin transporter genes SLC19A2 and SLC19A3.

Thiamine transporter 2 is involved in high glucose-induced damage and altered thiamine availability in cell models of diabetic retinopathy

Thiamine prevents high glucose-induced damage in microvasculature, and progression of retinopathy and nephropathy in diabetic animals. Impaired thiamine availability causes renal damage in diabetic patients. Two single-nucleotide polymorphisms in SLC19A3 locus encoding for thiamine transporter 2 are associated with absent/minimal diabetic retinopathy and nephropathy despite long-term type 1 diabetes. We investigated the involvement of thiamine transporter 1 and thiamine transporter 2, and their transcription factor specificity protein 1, in high glucose-induced damage and altered thiamine availability in cells of the inner blood-retinal barrier. Human endothelial cells, pericytes and Müller cells were exposed to hyperglycaemic-like conditions and/or thiamine deficiency/over-supplementation in single/co-cultures. Expression and localization of thiamine transporter 1, thiamine transporter 2 and transcription factor specificity protein 1 were evaluated together with intracellular thiamine concentration, transketolase activity and permeability to thiamine. The effects of thiamine depletion on cell function (viability, apoptosis and migration) were also addressed. Thiamine transporter 2 and transcription factor specificity protein 1 expression were modulated by hyperglycaemic-like conditions. Transketolase activity, intracellular thiamine and permeability to thiamine were decreased in cells cultured in thiamine deficiency, and in pericytes in hyperglycaemic-like conditions. Thiamine depletion reduced cell viability and proliferation, while thiamine over-supplementation compensated for thiamine transporter 2 reduction by restoring thiamine uptake and transketolase activity. High glucose and reduced thiamine determine impairment in thiamine transport inside retinal cells and through the inner blood-retinal barrier. Thiamine transporter 2 modulation in our cell models suggests its major role in thiamine transport in retinal cells and its involvement in high glucose-induced damage and impaired thiamine availability.

Wernicke’s Encephalopathy after Pancreaticoduodenectomy: A Case Report

Background: Wernicke’s encephalopathy (WE) is caused by thiamine deficiency and classically appears as changes in mental status, oculomotor manifestations, and gait ataxia. WE is mostly found in patients with chronic alcoholism and malnutrition, and has rarely been reported following pancreaticoduodenectomy. Case Presentation: A 77-year-old woman was admitted to our hospital complaining of loss of appetite and weakness of the lower extremities. No abnormalities were found on blood examination. One year earlier, she had undergone pancreaticoduodenectomy for bile duct cancer, and pancreatic fistula developed as a postoperative complication. On hospital day 8, her level of consciousness deteriorated, and she experienced difficulty bending the lower limbs and walking. Computed tomography and upper gastrointestinal series showed dilatation of the residual stomach and stenosis of the gastrojejunostomy. Laboratory test results were normal except for blood gas analysis, which revealed severe lactic acidosis. A diagnosis of WE was suspected due to lactic acidosis secondary to thiamine deficiency. Definitive diagnosis was confirmed by magnetic resonance imaging of the brain. Intravenous infusion of high-dose thiamine (1,200 mg/day) was commenced, leading to improvement of her neurological condition. Conclusions: Physicians should recognize that pancreaticoduodenectomy, when complicated by gastrojejunostomy stenosis resulting in pancreatic fistula, can lead to WE. In this case, blood gas analysis was helpful in arriving at the correct diagnosis. A high degree of suspicion should be maintained if any of the classical features of WE are observed following pancreaticoduodenectomy.

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.

Compound heterozygous SLC19A3 mutations further refine the critical promoter region for biotin-thiamine-responsive basal ganglia disease

Mutations in the gene SLC19A3 result in thiamine metabolism dysfunction syndrome 2, also known as biotin-thiamine-responsive basal ganglia disease (BTBGD). This neurometabolic disease typically presents in early childhood with progressive neurodegeneration, including confusion, seizures, and dysphagia, advancing to coma and death. Treatment is possible via supplement of biotin and/or thiamine, with early treatment resulting in significant lifelong improvements. Here we report two siblings who received a refined diagnosis of BTBGD following whole-genome sequencing. Both children inherited compound heterozygous mutations from unaffected parents; a missense single-nucleotide variant (p.G23V) in the first transmembrane domain of the protein, and a 4808-bp deletion in exon 1 encompassing the 5′ UTR and minimal promoter region. This deletion is the smallest promoter deletion reported to date, further defining the minimal promoter region of SLC19A3. Unfortunately, one of the siblings died prior to diagnosis, but the other is showing significant improvement after commencement of therapy. This case demonstrates the power of whole-genome sequencing for the identification of structural variants and subsequent diagnosis of rare neurodevelopmental disorders.

Role of HIF-1α in the hypoxia inducible expression of the thiamine transporter, SLC19A3

Ensuring continuous intracellular supply of thiamine is essential to maintain metabolism. Cellular homeostasis requires the function of the membrane bound thiamine transporters THTR1 and THTR2. In the absence of increased dietary intake of thiamine, varying intracellular levels to meet metabolic demands during pathophysiological stressors, such as hypoxia, requires adaptive regulatory mechanisms to increase thiamine transport capacity. Previous work has established the up-regulation of SLC19A3 (THTR2) gene expression and activity during hypoxic stress through the activity of the hypoxia inducible transcription factor 1 alpha (HIF-1α). However, it is unknown whether HIF-1α acts directly or indirectly to trans-activate expression of SLC19A3. This work utilized the breast cancer cell line BT-474 treated with 1% O2 or a hypoxia chemical mimetic deferoxamine to determine the minimal promoter region of SLC19A3 responsible for hypoxia responsiveness. In silico sequence analysis determined two contiguous hypoxia responsive elements in close proximity to the transcriptional start site of the SLC19A3 gene. Using a HIF-1α transcriptional factor ELISA assay, HIF-1α was capable of binding to a dsDNA construct of the SLC19A3 minimal promoter. Chromatin immunoprecipitation assay established that SP1 was bound to the SLC19A3 minimal promoter region under normoxic conditions. However, HIF-1α binding to the minimal promoter region occurred during hypoxic treatments, while no SP1 binding was observed under these conditions. This work demonstrates the direct binding and activation of SLC19A3 expression by HIF-1α during hypoxic stress, suggesting an important adaptive regulatory role for HIF-1α in maintaining thiamine homeostasis.

Identification and characterization of the minimal 5'-regulatory region of the human riboflavin transporter-3 (SLC52A3) in intestinal epithelial cells

The human riboflavin (RF) transporter-3 (product of the SLC52A3 gene) plays an important role in intestinal RF absorption. Our aims in this study were to identify the minimal 5'-regulatory region of the SLC52A3 gene and the regulatory element(s) involved in its activity in intestinal epithelial cells, as well as to confirm promoter activity and establish physiological relevance in vivo in transgenic mice. With the use of transiently transfected human intestinal epithelial HuTu 80 cells and 5'-deletion analysis, the minimal SLC52A3 promoter was found to be encoded between -199 and +8 bp (using the start of the transcription start site as position 1). Although several putative cis-regulatory elements were predicted in this region, only the stimulating protein-1 (Sp1) binding site (at position -74/-71 bp) was found to play a role in promoter activity, as indicated by mutational analysis. Binding of Sp1 to the minimal SLC52A3 promoter was demonstrated by means of EMSA and supershift assays and by chromatin immunoprecipitation analysis. Studies with Drosophila SL2 cells (which lack Sp activity) confirmed the importance of Sp1 in driving the activity of the SLC52A3 minimal promoter; they further showed that Sp3 can also do the activation. Finally, with the use of luciferase gene fusions, the activity of the cloned SLC52A3 promoter was confirmed in vivo in transgenic mice. These studies report, for the first time, on the identification and characterization of the SLC52A3 promoter and also demonstrate the importance of Sp1 in regulating its activity in intestinal epithelial cells.

Identification and characterization of 5'-flanking region of the human riboflavin transporter 1 gene (SLC52A1)

The human SLC52A1 gene encodes the riboflavin transporter-1 (RFVT-1), a plasma membrane protein that transports vitamin B2 (riboflavin, RF) into cells, and thus, plays a role in controlling cellular homeostasis of RF in those tissues that express the carrier protein (e.g. placenta and intestine). Currently, there is nothing known about transcriptional regulation of the SLC52A1 gene, therefore, we aimed to clone and characterize its 5'-flanking region. Using rapid amplification of the cDNA ends (5'-RACE), we identified one transcription start site (TSS). A 579 bp segment of the 5'-flanking region of this gene was cloned which exhibited robust promoter activity upon transfection in human intestinal epithelial cells. Deletion analysis revealed that the core promoter activity to be embedded in a region between -234 and -23 that lacked TATA element, was GC-rich, and harbored several putative cis-regulatory sites including KLFs, AP-2, EGRF and Sp-1. Mutating each of these sites led to a significant decrease in promoter activity (which was highest for the Sp-1 site), suggesting their possible involvement in regulating SLC52A1 transcription. Focusing on the Sp-1 site, EMSA, super-shift and ChIP analysis was performed that established the interaction of the Sp-1 transcription factor with the SLC52A1 promoter; also, co-transfection of the minimal SLC52A1 promoter with an Sp-1 containing vector in Drosophila SL-2 cells led to significant promoter activation. These results are the first to reveal the identity of the minimal SLC52A1 promoter and to establish an important role for Sp-1 in its activity.

Mechanisms involved in the inhibitory effect of chronic alcohol exposure on pancreatic acinar thiamin uptake

Pancreatic acinar cells (PAC) obtain thiamin from the circulation via a carrier-mediated process that involves thiamin transporters 1 and 2 (THTR-1 and THTR-2; products of SLC19A2 and SLC19A3, respectively). Chronic alcohol exposure of PAC inhibits thiamin uptake, and, on the basis of in vitro studies, this inhibition appears to be transcriptionally mediated. The aim of this study was to confirm the involvement of a transcriptional mechanism in mediating the chronic alcohol effect in in vivo settings and to delineate the molecular mechanisms involved. Using transgenic mice carrying full-length SLC19A2 and SLC19A3 promoters, we found that chronic alcohol feeding led to a significant reduction in the activity of SLC19A2 and SLC19A3 promoters (as well as in thiamin uptake and expression of THTR-1 and -2). Similar findings were seen in 266-6 cells chronically exposed to alcohol in vitro. In the latter studies, the alcohol inhibitory effect was found to be mediated via the minimal SLC19A2 and SLC19A3 promoters and involved the cis-regulatory elements stimulating protein 1 (SP1)/gut-enriched Kruppel-like factor and SP1-GG-box and SP1/GC, respectively. Chronic alcohol exposure of PAC also led to a significant reduction in the expression of the SP1 transcription factor, which upon correction (via expression) led to the prevention of alcohol inhibitory effects on not only the activity of SLC19A2 and SLC19A3 promoters but also on the expression of THTR-1 and -2 mRNA and thiamin uptake. These results demonstrate that the inhibitory effect of chronic alcohol exposure on physiological/molecular parameters of thiamin uptake by PAC is mediated via specific cis-regulatory elements in SLC19A2 and SLC19A3 minimal promoters.

Adaptive regulation of human intestinal thiamine uptake by extracellular substrate level: a role for THTR-2 transcriptional regulation

The intestinal thiamine uptake process is adaptively regulated by the level of vitamin in the diet, but the molecular mechanism involved is not fully understood. Here we used the human intestinal epithelial Caco-2 cells exposed to different levels of extracellular thiamine to delineate the molecular mechanism involved. Our results showed that maintaining Caco-2 cells in a thiamine-deficient medium resulted in a specific and significant increase of [3H]thiamine uptake compared with cell exposure to a high level of thiamine (1 mM). This adaptive regulation was also associated with a higher level of mRNA expression of thiamine transporter-2 (THTR-2), but not thiamine transporter-1 (THTR-1), in the deficient condition and a higher level of promoter activity of gene encoding THTR-2 (SLC19A3). Using 5'-truncated promoter-luciferase constructs, we identified the thiamine level-responsive region in the SLC19A3 promoter to be between -77 and -29 (using transcriptional start site as +1). By means of mutational analysis, a key role for a stimulating protein-1 (SP1)/guanosine cytidine box in mediating the effect of extracellular thiamine level on SLC19A3 promoter was established. Furthermore, extracellular level of thiamine was found to affect SP1 protein expression and binding pattern to the thiamine level-responsive region of SLC19A3 promoter in Caco-2 cells as shown by Western blotting and electrophoretic mobility shift assay analysis, respectively. These studies demonstrate that the human intestinal thiamine uptake is adaptively regulated by the extracellular substrate level via transcriptional regulation of the THTR-2 system, and report that SP1 transcriptional factor is involved in this regulation.

Glucose-induced down regulation of thiamine transporters in the kidney proximal tubular epithelium produces thiamine insufficiency in diabetes

Increased renal clearance of thiamine (vitamin B(1)) occurs in experimental and clinical diabetes producing thiamine insufficiency mediated by impaired tubular re-uptake and linked to the development of diabetic nephropathy. We studied the mechanism of impaired renal re-uptake of thiamine in diabetes. Expression of thiamine transporter proteins THTR-1 and THTR-2 in normal human kidney sections examined by immunohistochemistry showed intense polarised staining of the apical, luminal membranes in proximal tubules for THTR-1 and THTR-2 of the cortex and uniform, diffuse staining throughout cells of the collecting duct for THTR-1 and THTR-2 of the medulla. Human primary proximal tubule epithelial cells were incubated with low and high glucose concentration, 5 and 26 mmol/l, respectively. In high glucose concentration there was decreased expression of THTR-1 and THTR-2 (transporter mRNA: -76% and -53% respectively, p<0.001; transporter protein -77% and -83% respectively, p<0.05), concomitant with decreased expression of transcription factor specificity protein-1. High glucose concentration also produced a 37% decrease in apical to basolateral transport of thiamine transport across cell monolayers. Intensification of glycemic control corrected increased fractional excretion of thiamine in experimental diabetes. We conclude that glucose-induced decreased expression of thiamine transporters in the tubular epithelium may mediate renal mishandling of thiamine in diabetes. This is a novel mechanism of thiamine insufficiency linked to diabetic nephropathy.

Relative contribution of THTR-1 and THTR-2 in thiamin uptake by pancreatic acinar cells: studies utilizing Slc19a2 and Slc19a3 knockout mouse models

Thiamin is essential for normal function of pancreatic acinar cells, and its deficiency leads to a reduction in pancreatic digestive enzymes. We have recently shown that thiamin uptake by rat pancreatic acinar cells is carrier-mediated and that both thiamin transporter (THTR)-1 and THTR-2 are expressed in these cells; little, however, is known about the relative contribution of these transporters toward total carrier-mediated thiamin uptake by these cells. We addressed this issue using a gene-specific silencing approach (siRNA) in mouse-derived pancreatic acinar 266-6 cells and Slc19a2 and Slc19a3 knockout mouse models. First we established that thiamin uptake by mouse pancreatic acinar cells is via a carrier-mediated process. We also established that these cells as well as native human pancreas express THTR-1 and THTR-2, with expression of the former (and activity of its promoter) being significantly higher than that of the latter. Using gene-specific siRNA against mouse THTR-1 and THTR-2, we observed a significant inhibition in carrier-mediated thiamin uptake by 266-6 cells in both cases. Similarly, thiamin uptake by freshly isolated primary pancreatic acinar cells of the Slc19a2 and Slc19a3 knockout mice was significantly lower than uptake by acinar cells of the respective littermates; the degree of inhibition observed in the former knockout model was greater than that of the latter. These findings demonstrate, for the first time, that both mTHTR-1 and mTHTR-2 are involved in carrier-mediated thiamin uptake by pancreatic acinar cells.

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.

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.

Pancreatic beta cells and islets take up thiamin by a regulated carrier-mediated process: studies using mice and human pancreatic preparations

Thiamin is essential for the normal function of the endocrine pancreas, but very little is known about uptake mechanism(s) and regulation by beta cells. We addressed these issues using mouse-derived pancreatic beta-TC-6 cells, and freshly isolated primary mouse and human pancreatic islets. Results showed that thiamin uptake by beta-TC-6 cells involves a pH (but not Na+)-dependent carrier-mediated process that is saturable at both the nanomolar (apparent K(m) = 37.17 +/- 9.9 nM) and micromolar (apparent K(m) = 3.26 +/- 0.86 microM) ranges, cis-inhibited by thiamin structural analogs, and trans-stimulated by unlabeled thiamin. Involvement of carrier-mediated process was also confirmed in primary mouse and human pancreatic islets. Both THTR-1 and THTR-2 were found to be expressed in these mouse and human pancreatic preparations. Maintaining beta-TC-6 cells in the presence of a high level of thiamin led to a significant (P < 0.01) decrease in thiamin uptake, which was associated with a significant downregulation in level of expression of THTR-1 and THTR-2 at the protein and mRNA levels and a decrease in transcriptional (promoter) activity. Modulators of intracellular Ca2+/calmodulin- and protein-tyrosine kinase-mediated pathways also altered thiamin uptake. Finally, confocal imaging of live beta-TC-6 cells showed that clinical mutants of THTR-1 have mixed expression phenotypes and all led to impairment in thiamin uptake. These studies demonstrate for the first time that thiamin uptake by the endocrine pancreas is carrier mediated and is adaptively regulated by the prevailing vitamin level via transcriptional mechanisms. Furthermore, clinical mutants of THTR-1 impair thiamin uptake via different mechanisms.

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.

High prevalence of low plasma thiamine concentration in diabetes linked to a marker of vascular disease

AIMS/HYPOTHESIS

To assess thiamine status by analysis of plasma, erythrocytes and urine in type 1 and type 2 diabetic patients and links to markers of vascular dysfunction.

METHODS

Diabetic patients (26 type 1 and 48 type 2) with and without microalbuminuria and 20 normal healthy control volunteers were recruited. Erythrocyte activity of transketolase, the concentrations of thiamine and related phosphorylated metabolites in plasma, erythrocytes and urine, and markers of metabolic control and vascular dysfunction were determined.

RESULTS

Plasma thiamine concentration was decreased 76% in type 1 diabetic patients and 75% in type 2 diabetic patients: normal volunteers 64.1 (95% CI 58.5-69.7) nmol/l, type 1 diabetes 15.3 (95% CI 11.5-19.1) nmol/l, p < 0.001, and type 2 diabetes 16.3 (95% CI 13.0-9.6) nmol/l, p < 0.001. Renal clearance of thiamine was increased 24-fold in type 1 diabetic patients and 16-fold in type 2 diabetic patients. Plasma thiamine concentration correlated negatively with renal clearance of thiamine (r = -0.531, p < 0.001) and fractional excretion of thiamine (r = -0.616, p < 0.001). Erythrocyte transketolase activity correlated negatively with urinary albumin excretion (r = -0.232, p < 0.05). Thiamine transporter protein contents of erythrocyte membranes of type 1 and type 2 diabetic patients were increased. Plasma thiamine concentration and urinary excretion of thiamine correlated negatively with soluble vascular adhesion molecule-1 (r = -0.246, p < 0.05, and -0.311, p < 0.01, respectively).

CONCLUSIONS/INTERPRETATION

Low plasma thiamine concentration is prevalent in patients with type 1 and type 2 diabetes, associated with increased thiamine clearance. The conventional assessment of thiamine status was masked by increased thiamine transporter content of erythrocytes.

Vitamin B1 (thiamine) uptake by human retinal pigment epithelial (ARPE-19) cells: mechanism and regulation

Retinal abnormality and visual disturbances occur in thiamine-responsive megaloblastic anaemia (TRMA), an autosomal recessive disorder caused by mutations in the human thiamine transporter-1 (hTHTR-1). Human retinal pigment epithelial cells play a pivotal role in supplying thiamine to the highly metabolically active retina but nothing is known about the mechanism, regulation or biological processes involved in thiamine transport in these cells. To address these issues, we used human-derived retinal pigment epithelial ARPE-19 cells to characterize the thiamine uptake process. Thiamine uptake is energy- and temperature-dependent, pH-sensitive, Na+-independent, saturable at both the nanomolar (apparent Km, 30 +/- 5 nM) and the micromolar (apparent Km, 1.72 +/- 0.3 microM) concentration ranges, specific for thiamine and sensitive to sulfhydryl group inhibition. The diuretic amiloride caused a concentration-dependent inhibition in thiamine uptake, whereas the anti-trypanosomal drug, melarsoprol, failed to affect the uptake process. Both hTHTR-1 and hTHTR-2 are expressed in ARPE-19 cells as well as in native human retinal tissue with expression of the former being significantly higher than that of the latter. Uptake of thiamine was adaptively regulated by extracellular substrate level via transcriptionally mediated mechanisms that involve both hTHTR-1 and hTHTR-2; it was also regulated by an intracellular Ca2+-calmodulin-mediated pathway. Confocal imaging of living ARPE-19 cells expressing TRMA-associated hTHTR-1 mutants (D93H, S143F and G172D) showed various expression phenotypes. These results demonstrate for the first time the existence of a specialized and regulated uptake process for thiamine in a cellular model of human retinal pigment epithelia that involves hTHTR-1 and hTHTR-2. Further, clinically relevant mutations in hTHTR-1 lead to impaired cell surface expression or function of the transporter in retinal epithelial ARPE-19 cells.

Tough beginnings: alterations in the transcriptome of cloned embryos during the first two cell cycles

Cloned embryos produced by somatic cell nuclear transfer (SCNT) display a plethora of phenotypic characteristics that make them different from fertilized embryos, indicating defects in the process of nuclear reprogramming by the recipient ooplasm. To elucidate the extent and timing of nuclear reprogramming, we used microarrays to analyze the transcriptome of mouse SCNT embryos during the first two cell cycles. We identified a large number of genes mis-expressed in SCNT embryos. We found that genes involved in transcription and regulation of transcription are prominent among affected genes, and thus may be particularly difficult to reprogram, and these likely cause a ripple effect that alters the transcriptome of many other functions, including oxidative phosphorylation, transport across membrane, and mRNA transport and processing. Interestingly, we also uncovered widespread alterations in the maternal (i.e., non-transcribed) mRNA population of SCNT embryos. We conclude that gene expression in early SCNT embryos is grossly abnormal, and that this is at least in part the result of incomplete reprogramming of transcription factor genes.

Regulation of the human biotin transporter hSMVT promoter by KLF-4 and AP-2: confirmation of promoter activity in vivo

The mechanism of biotin uptake in human intestine has been well characterized and involves the human sodium-dependent multivitamin transporter (hSMVT), yet little is known about the molecular/transcriptional regulation of the system. Previous investigations cloned the 5' regulatory region of the hSMVT gene and identified the minimal promoter. To expand these investigations, we compared activity of the hSMVT promoter in three human intestinal epithelial cell lines (NCM460, Caco-2, and HuTu-80) and contrasted a renal epithelial cell line (HEK-293). We analyzed the role of putative cis-elements in regulating promoter activity and confirmed activity of the cloned hSMVT promoter in vivo. In vitro studies demonstrated that all cell lines utilized the same minimal promoter region, and mutation of specific cis-regulatory elements [Kruppel-like factor 4 (KLF-4) and activator protein-2 (AP-2)] led to a decrease in promoter activity in all intestinal cell types but not in renal cells. Using electrophoretic mobility shift assays, we identified two specific DNA/protein complexes. Using oligonucleotide competition and antibody supershift analysis, we determined that KLF-4 and AP-2 were involved in forming the complexes. In HEK-293 cells, overexpressing KLF-4 increased the endogenous hSMVT message levels threefold and activated a cotransfected hSMVT promoter-reporter construct. In vivo studies using hSMVT promoter-luciferase transgenic mice established physiological relevance and showed the pattern of hSMVT promoter expression to be similar to endogenous mouse SMVT mRNA expression. The results demonstrate, for the first time, the importance of KLF-4 and AP-2 in regulating the activity of the hSMVT promoter in the intestine and provide direct in vivo confirmation of hSMVT promoter activity.

Developmental maturation of intestinal and renal thiamin uptake: studies in wild-type and transgenic mice carrying human THTR-1 and 2 promoters

Thiamin (B1) is an essential micronutrient for normal growth and development. Mammals obtain thiamin through intestinal absorption, while in the kidney thiamin is reabsorbed to prevent its loss in the urine, both processes are specialized, carrier-mediated and involve thiamin transporters-1 and 2 (THTR-1 and THTR-2, respectively; products of the SLC19A2 and SLC19A3 genes). Although thiamin appears to play an important role in neonatal growth, little is currently known about the possible regulation of intestinal and renal thiamin uptake during developmental maturation. We addressed these issues by examining intestinal and renal thiamin uptake and expression of THTR-1 and THTR-2 during early stages of life. We utilized wild-type mice (mice express orthologues of both thiamin transporters) and transgenic mice expressing human SLC19A2 or SLC19A3 promoter-reporter transgenes as a model system and examined carrier-mediated thiamin uptake, mTHTR-1 and 2 protein and mRNA levels and luciferase activity in suckling (13 days), weanling (25-27 days), and adult (60-65 days) mice. Carrier-mediated thiamin uptake by jejunal and renal brush border membrane vesicles (BBMV) both decreased with maturation (suckling>weanling>adult) and were associated with a reduction in mTHTR-1 and mTHTR-2 protein, mRNA levels, and the activity of human SLC19A2 and SLC19A3 promoter-reporter constructs in the intestines and kidneys of transgenic mice. These results are the first to demonstrate that intestinal and renal thiamin uptake are developmentally regulated during early stages of life, mediated through mTHTR-1 and mTHTR-2, and suggest the possible involvement of transcriptional regulatory mechanism(s) in this regulation.

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.

Functional analysis of two regulatory regions of the human Na+-dependent vitamin C transporter 2, SLC23A2, in human vascular smooth muscle cells

Uptake of vitamin C occurs through the Na+ -dependent vitamin C transporters (SVCT1 and 2), the products of two separate genes. In cultured human vascular smooth muscle cells (hVSMC), we found expression of only the hSVCT2 transcript and identified an additional 5'-UTR transcript variant that we termed exon 1b, in addition to the previously described exon 1a. We cloned and tested the promoter functionality of the two genomic regions of the hSVCT2 upstream of these alternative first exons in hVSMC. Both demonstrated activity, and deletion constructs demonstrated that the minimal promoter regions were within approximately 100 bp relative to their adjacent exons.

Differentiation-dependent up-regulation of intestinal thiamin uptake: cellular and molecular mechanisms

Differentiation of intestinal epithelial cells is associated with up-and-down regulation of expression of a variety of genes including those involved in nutrient uptake. Nothing is known about possible differentiation-dependent regulation of the intestinal thiamin uptake process and the cellular and molecular mechanisms involved in such regulation. Using as models human-derived intestinal epithelial Caco-2 cells and crypt/villus epithelial cells isolated from wild-type and transgenic mice carrying promoters for human thiamin transporter-1 and -2 (hTHTR-1 and hTHTR-2), we addressed this issue. Our results showed that differentiation of Caco-2 cells is associated with a significant up-regulation in carrier-mediated thiamin uptake. Up-regulation was associated with a significant increase in the level of expression of hTHTR-1 and hTHTR-2 protein and mRNA as well as in activity of the corresponding transfected human thiamin transporter-1 (SLC19A2) and -2 (SLC19A3) promoters. Deletion analysis identified the differentiation-responsive region to be at position -356 to -275 bp for the SLC19A2 promoter and at position -77 to -13 bp for the SLC19A3 promoter. In addition, a critical and specific role in the differentiation-mediated effects for an NF1 binding site (-348 to -345 bp) in the SLC19A2 promoter and a SP1/GC-box binding site (-48 to -45 bp) in the SLC19A3 promoter were established using mutational analysis. The physiological relevance of in vitro findings with Caco-2 cells was confirmed in wild-type and transgenic mice by demonstrating that thiamin uptake and mRNA levels of the mouse THTR-1 and THTR-2, as well as activity of human SLC19A2 and SLC19A3 promoters expressed in transgenic mice, were all significantly higher in intestinal villus compared with crypt epithelial cells. These studies demonstrate for the first time that differentiation of intestinal epithelial cells is associated with an up-regulation in thiamin uptake process and that this up-regulation appears to be mediated via transcriptional regulatory mechanisms that involve the SLC19A2 and SLC19A3 genes.

Adaptive regulation of intestinal thiamin uptake: molecular mechanism using wild-type and transgenic mice carrying hTHTR-1 and -2 promoters

Thiamin participates in metabolic pathways contributing to normal cellular functions, growth, and development. The molecular mechanism of the human intestinal thiamin absorption process involves the thiamin transporters-1 (hTHTR-1) and -2 (hTHTR-2), products of the SLC19A2 and SLC19A3 genes. Little is known about adaptive regulation of the intestinal thiamin uptake process or the molecular mechanism(s) involved during thiamin deficiency. In these studies, we addressed these issues using wild-type mice and transgenic animals carrying the promoters of the hTHTR-1 and -2. We show that, in thiamin deficiency, a significant and specific upregulation in intestinal carrier-mediated thiamin uptake occurs and that this increase is associated with an induction in protein and mRNA levels of mTHTR-2 but not mTHTR-1; in addition, an increase in the activity of the SLC19A3, but not the SLC19A2, promoter was observed in the intestine of transgenic mice. Similar findings were detected in the kidney; however, expression of both thiamin transporters and activity of both human promoters were upregulated in this organ in thiamin deficiency. We also examined the effect of thiamin deficiency on the level of expression of mTHTR-1 and mTHTR-2 messages and activity of the human promoters in the heart and brain of transgenic mice and found an increase in mTHTR-1 mRNA and a rise in activity of the SLC19A2 promoter in thiamin-deficient mice. These results show that the intestinal and renal thiamin uptake processes are adaptively upregulated during dietary thiamin deficiency, that expression of mTHTR-1 and mTHTR-2 is regulated in a tissue-specific manner, and that this upregulation is mediated via transcriptional regulatory mechanism(s).

Characterization of the 5'-regulatory region of the human thiamin transporter SLC19A3: in vitro and in vivo studies

Transcriptional regulation of expression of the human thiamin transporter-2 (the product of the SLC19A3 gene) is unknown. In this study, we cloned the 5'-regulatory region of the human SLC19A3 gene (2,016 bp), identified the minimal promoter region required for basal activity, demonstrated a critical role for specific cis-regulatory elements in determining the promoter activity, and confirmed activity and physiological relevance of the cloned SLC19A3 promoter in vivo. With the use of transiently transfected human intestinal epithelial Caco-2 cells and 5'-deletion analysis, the minimal promoter region required for basal activity of the SLC19A3 promoter was found to be encoded in a sequence between -77 and +59 by using the start of transcription initiation as position 1. This minimal region was found to contain a number of putative cis-regulatory elements, with a critical role for a stimulating protein-1 (SP1)/GC-box binding site (at position -48/-45 bp) established by means of mutational analysis. With the use of EMSA and supershift assays, the binding of SP1 and SP3 to the minimal promoter region was also demonstrated. In transiently transfected Drosophila SL2 cells, both SP1 and SP3 transactivated the SLC19A3 minimal promoter in a dose-dependent manner and in combination demonstrated an additive stimulatory effect. Functionality of the full-length SLC19A3 promoter was confirmed in vivo in transgenic mice expressing the promoter-luciferase reporter gene. These studies report the first characterization of the SLC19A3 promoter in vitro and in vivo and demonstrate the importance of an SP1 cis-regulatory element in regulating promoter activity of this important human gene.

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.