Regulation of basal promoter activity of the human thiamine pyrophosphate transporter SLC44A4 in human intestinal epithelial cells

IQGAP-2: a novel interacting partner with the human colonic thiamin pyrophosphate transporter

The human colonic thiamin pyrophosphate transporter (hcTPPT) mediates the uptake of the microbiota-generated and phosphorylated form of vitamin B1 (i.e., thiamin pyrophosphate) in the large intestine. Expression of hcTPPT along the absorptive tract is restricted to the large intestine, and the transporter is exclusively localized at the apical membrane domain of the polarized epithelial cells/colonocytes. Previous studies have characterized different physiological/pathophysiological aspects of the hcTPPT system, but nothing is currently known on whether the transporter has interacting partner(s) that affect its physiology/biology. We addressed this issue using a Y2H to screen a human colonic cDNA library and have identified three putative interactors, namely IQGAP-2, SNX-6, and DMXL-1. Focusing on IQGAP-2 (whose expression in human colonocytes is the highest), we found (using fluorescent microscopy imaging and coimmunoprecipitation approaches) the putative interactor to colocalize with hcTPPT and to directly interact with the transporter. Also, overexpressing IQGAP-2 in NCM460 cells and in human primary differentiated colonoid monolayers was found to lead to significant (P < 0.01) induction in TPP uptake, while knocking down (using gene-specific siRNAs) caused significant (P < 0.01 and P < 0.05) decrease in uptake. Furthermore, overexpressing IQGAP-2 in NCM460 cells was found to lead to a significant enhancement in hcTPPT protein stability. Finally, we found the expression of IQGAP-2 to be markedly suppressed in conditions/factors that negatively impact colonic TPP uptake. These results identify the IQGAP-2 as an interacting partner with the hcTPPT in human colonocytes and show that this interaction has physiological and biological consequences.NEW & NOTEWORTHY This study reports on the identification of IQGAP-2 as an interacting partner with the hcTPPT in human colonocytes and how that impacts the transporter's physiology and cell biology.

Transcriptional regulation of genetic variants in the SLC40A1 promoter

Solute carrier 40A1 (SLC40A1) encodes ferroportin, which is the only known transmembrane protein that exports elemental iron from mammalian cells and is essential for iron homeostasis. Mutations in SLC40A1 are associated with iron-overload disorders. In addition to ferroportin diseases, SLC40A1 expression is downregulated in various cancer types. Despite the clinical significance of the SLC40A1 transporter, only a few studies have investigated genetic variants in SLC40A1. The present study was performed to identify genetic variations in the SLC40A1 promoter and functionally characterize each variant using in vitro assays. We investigated four haplotypes and five variants in the SLC40A1 promoter. We observed that haplotype 3 (H3) had significantly lower promoter activity than H1, whereas the activity of H4 was significantly higher than that of H1. Luciferase activity of H2 was comparable to that of H1. In addition, four variants of SLC40A1, c.-1355G>C, c.-662C>T, c.-98G>C, and c.-8C>G, showed significantly increased luciferase activity compared to the wild type (WT), whereas c.-750G>A showed significantly decreased luciferase activity compared to the WT. Three transcription factors, cAMP response element-binding protein-1 (CREB-1), chicken ovalbumin upstream promoter transcription factor 1, and hepatic leukemia factor (HLF), were predicted to bind to the promoter regions of SLC40A1 near c.-662C>T, c.-98G>C, and c.-8C>G, respectively. Among these, CREB-1 and HLF bound more strongly to the variant sequences than to the WT and functioned as activators of SLC40A1 transcription. Collectively, our findings indicate that the two SLC40A1 promoter haplotypes affect SLC40A1 transcription, which is regulated by CREB-1 and HLF.

Bacterial lipopolysaccharide inhibits colonic carrier-mediated uptake of thiamin pyrophosphate: roles for TLR4 receptor and NF-κB/P38/JNK signaling pathway

This study investigated the effect of the bacterial endotoxin lipopolysaccharide (LPS) on colonic uptake of thiamin pyrophosphate (TPP), the biologically active form of vitamin B1 that is generated by gut microbiota. We used three complementary models in our study: in vitro (human-derived colonic epithelial NCM460), ex vivo (human differentiated colonoid monolayers), and in vivo (mouse colonic tissue). The results showed that exposure of NCM460 cells to LPS leads to a significant inhibition of carrier-mediated TPP uptake as well as in decreased expression of the colonic TPP transporter (cTPPT) protein, mRNA, and heterologous nuclear RNA (hnRNA) compared with untreated controls. Similarly, exposure of human differentiated colonoid monolayers and mice to LPS caused significant inhibition in colonic carrier-mediated TPP uptake and in cTPPT protein, mRNA, and hnRNA expression. The effect of LPS on colonic TPP uptake and cTTPT expression was also found to be associated with a significant reduction in activity of the SLC44A4 promoter as well as in decreased expression of the nuclear factor Elf-3 (E74-like ETS transcription factor 3), which is needed for promoter activity. Finally, we found that knocking down the Toll-like receptor 4 (TLR4) and blocking the nuclear factor kappa B (NF-κB), JNK, and p38 signaling pathways with the use of pharmacological inhibitors lead to significant abrogation in the degree of LPS-mediated inhibition in TPP uptake and cTPPT expression. These results demonstrated that exposure of colonic epithelia to LPS inhibits colonic TPP uptake via transcriptional mechanism(s) and that the effect is mediated via TLR4 receptor and NF-κB/p38/JNK signaling pathways.NEW & NOTEWORTHY This study examined the effect of the bacterial lipopolysaccharide (LPS) on the colonic uptake of thiamin pyrophosphate (TPP), the biologically active form of vitamin B1. Three complementary models were used: in vitro (human NCM460 cells), ex vivo (human colonoids), and in vivo (mice). The results showed LPS to significantly suppress TPP uptake and the expression of its transporter, and that these effects are mediated via the membrane TLR4 receptor, and involve the NF-κB/p38/JNK signaling pathways.

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.

Direct transcriptomic comparison of xenobiotic metabolism and toxicity pathway induction of airway epithelium models at an air-liquid interface generated from induced pluripotent stem cells and primary bronchial epithelial cells

The airway epithelium represents the main barrier between inhaled air and the tissues of the respiratory tract and is therefore an important point of contact with xenobiotic substances into the human body. Several studies have recently shown that in vitro models of the airway grown at an air-liquid interface (ALI) can be particularly useful to obtain mechanistic information about the toxicity of chemical compounds. However, such methods are not very amenable to high throughput since the primary cells cannot be expanded indefinitely in culture to obtain a sustainable number of cells. Induced pluripotent stem cells (iPSCs) have become a popular option in the recent years for modelling the airways of the lung, but despite progress in the field, such models have so far not been assessed for their ability to metabolise xenobiotic compounds and how they compare to the primary bronchial airway model (pBAE). Here, we report a comparative analysis by TempoSeq (oligo-directed sequencing) of an iPSC-derived airway model (iBAE) with a primary bronchial airway model (pBAE). The iBAE and pBAE were differentiated at an ALI and then evaluated in a 5-compound screen with exposure to a sub-lethal concentration of each compound for 24 h. We found that despite lower expression of xenobiotic metabolism genes, the iBAE similarly predicted the toxic pathways when compared to the pBAE model. Our results show that iPSC airway models at ALI show promise for inhalation toxicity assessments with further development.

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.

Pathophysiology, prevention, and treatment of beriberi after gastric surgery

Beriberi is a nutritional complication of gastric surgery, caused by deficiency of vitamin B₁, or thiamine. Thiamine deficiency leads to impaired glucose metabolism, decreased delivery of oxygen by red blood cells, cardiac dysfunction, failure of neurotransmission, and neuronal death. This review describes the history and pathophysiology of beriberi as well as the relationship between beriberi and nutritional deficiencies after gastric surgery. A literature review of the history and pathophysiology of beriberi and the risk factors for thiamine deficiency, particularly after gastric resection or bariatric surgery, was performed. Recommendations for nutritional follow-up post gastric surgery are based on current national guidelines. Patients may have subclinical thiamine deficiency after upper gastrointestinal surgery, and thus beriberi may be precipitated by acute illness such as sepsis or poor dietary intake. This may occur very soon or many years after gastrectomy or bariatric surgery, even in apparently well-nourished patients. Prompt recognition and administration of supplemental thiamine can decrease morbidity and mortality in patients with beriberi. Dietary education post surgery and long-term follow-up to determine nutritional status, including vitamin and mineral assessment, is recommended for patients who undergo gastric surgery.

Enterohemorrhagic Escherichia coli infection inhibits colonic thiamin pyrophosphate uptake via transcriptional mechanism

Colonocytes possess a specific carrier-mediated uptake process for the microbiota-generated thiamin (vitamin B1) pyrophosphate (TPP) that involves the TPP transporter (TPPT; product of the SLC44A4 gene). Little is known about the effect of exogenous factors (including enteric pathogens) on the colonic TPP uptake process. Our aim in this study was to investigate the effect of Enterohemorrhagic Escherichia coli (EHEC) infection on colonic uptake of TPP. We used human-derived colonic epithelial NCM460 cells and mice in our investigation. The results showed that infecting NCM460 cells with live EHEC (but not with heat-killed EHEC, EHEC culture supernatant, or with non-pathogenic E. Coli) to lead to a significant inhibition in carrier-mediated TPP uptake, as well as in level of expression of the TPPT protein and mRNA. Similarly, infecting mice with EHEC led to a significant inhibition in colonic TPP uptake and in level of expression of TPPT protein and mRNA. The inhibitory effect of EHEC on TPP uptake by NCM460 was found to be associated with reduction in the rate of transcription of the SLC44A4 gene as indicated by the significant reduction in the activity of the SLC44A4 promoter transfected into EHEC infected cells. The latter was also associated with a marked reduction in the level of expression of the transcription factors CREB-1 and ELF3, which are known to drive the activity of the SLC44A4 promoter. Finally, blocking the ERK1/2 and NF-kB signaling pathways in NCM460 cells significantly reversed the level of EHEC inhibition in TPP uptake and TPPT expression. Collectively, these findings show, for the first time, that EHEC infection significantly inhibit colonic uptake of TPP, and that this effect appears to be exerted at the level of SLC44A4 transcription and involves the ERK1/2 and NF-kB signaling pathways.

Functional Expression of Choline Transporters in the Blood-Brain Barrier

Cholinergic neurons in the central nervous system play a vital role in higher brain functions, such as learning and memory. Choline is essential for the synthesis of the neurotransmitter acetylcholine by cholinergic neurons. The synthesis and metabolism of acetylcholine are important mechanisms for regulating neuronal activity. Choline is a positively charged quaternary ammonium compound that requires transporters to pass through the plasma membrane. Currently, there are three groups of choline transporters with different characteristics, such as affinity for choline, tissue distribution, and sodium dependence. They include (I) polyspecific organic cation transporters (OCT1-3: SLC22A1-3) with a low affinity for choline, (II) high-affinity choline transporter 1 (CHT1: SLC5A7), and (III) choline transporter-like proteins (CTL1-5: SLC44A1-5). Brain microvascular endothelial cells, which comprise part of the blood-brain barrier, take up extracellular choline via intermediate-affinity choline transporter-like protein 1 (CTL1) and low-affinity CTL2 transporters. CTL2 is responsible for excreting a high concentration of choline taken up by the brain microvascular endothelial cells on the brain side of the blood-brain barrier. CTL2 is also highly expressed in mitochondria and may be involved in the oxidative pathway of choline metabolism. Therefore, CTL1- and CTL2-mediated choline transport to the brain through the blood-brain barrier plays an essential role in various functions of the central nervous system by acting as the rate-limiting step of cholinergic neuronal activity.

Genetic defects of thiamine transport and metabolism: A review of clinical phenotypes, genetics, and functional studies

Thiamine is a crucial cofactor involved in the maintenance of carbohydrate metabolism and participates in multiple cellular metabolic processes within the cytosol, mitochondria, and peroxisomes. Currently, four genetic defects have been described causing impairment of thiamine transport and metabolism: SLC19A2 dysfunction leads to diabetes mellitus, megaloblastic anemia and sensory-neural hearing loss, whereas SLC19A3, SLC25A19, and TPK1-related disorders result in recurrent encephalopathy, basal ganglia necrosis, generalized dystonia, severe disability, and early death. In order to achieve early diagnosis and treatment, biomarkers play an important role. SLC19A3 patients present a profound decrease of free-thiamine in cerebrospinal fluid (CSF) and fibroblasts. TPK1 patients show decreased concentrations of thiamine pyrophosphate in blood and muscle. Thiamine supplementation has been shown to improve diabetes and anemia control in Rogers' syndrome patients due to SLC19A2 deficiency. In a significant number of patients with SLC19A3, thiamine improves clinical outcome and survival, and prevents further metabolic crisis. In SLC25A19 and TPK1 defects, thiamine has also led to clinical stabilization in single cases. Moreover, thiamine supplementation leads to normal concentrations of free-thiamine in the CSF of SLC19A3 patients. Herein, we present a literature review of the current knowledge of the disease including related clinical phenotypes, treatment approaches, update of pathogenic variants, as well as in vitro and in vivo functional models that provide pathogenic evidence and propose mechanisms for thiamine deficiency in humans.

Metabolic functions of the human gut microbiota: the role of metalloenzymes

Covering: up to the end of 2017 The human body is composed of an equal number of human and microbial cells. While the microbial community inhabiting the human gastrointestinal tract plays an essential role in host health, these organisms have also been connected to various diseases. Yet, the gut microbial functions that modulate host biology are not well established. In this review, we describe metabolic functions of the human gut microbiota that involve metalloenzymes. These activities enable gut microbial colonization, mediate interactions with the host, and impact human health and disease. We highlight cases in which enzyme characterization has advanced our understanding of the gut microbiota and examples that illustrate the diverse ways in which metalloenzymes facilitate both essential and unique functions of this community. Finally, we analyze Human Microbiome Project sequencing datasets to assess the distribution of a prominent family of metalloenzymes in human-associated microbial communities, guiding future enzyme characterization efforts.

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.

Molecular mechanisms involved in the adaptive regulation of the colonic thiamin pyrophosphate uptake process

A considerable amount of the thiamin generated by gut microbiota exists in the form of thiamin pyrophosphate (TPP). We have previously shown that human colonocytes possess an efficient carrier-mediated uptake process for TPP that involves the SLC44A4 system and this uptake process is adaptively regulated by prevailing extracellular TPP level. Little is known about the molecular mechanisms that mediate this adaptive regulation. We addressed this issue using human-derived colonic epithelial NCM460 cells and mouse colonoids as models. Maintaining NCM460 cells in the presence of a high level of TPP (1 mM) for short (2 days)- and long-term (9 days) periods was found to lead to a significant reduction in [³H] TPP uptake compared with cells maintained in its absence. Short-term exposure showed no changes in level of expression of SLC44A4 protein in total cell homogenate (although there was a decreased expression in the membrane fraction), mRNA, and promoter activity. However, a significant reduction in the level of expression of the SLC44A4 protein, mRNA, and promoter activity was observed upon long-term maintenance with the substrate. Similar changes in Slc44a4 mRNA expression were observed when mouse colonoids were maintained with TPP for short- and long-term periods. Expression of the transcription factors ELF3 and CREB-1 (which drive the SLC44A4 promoter) following long-term exposure was unchanged, but their binding affinity to the promoter was decreased and specific histone modifications were also observed. These studies demonstrate that, depending on the period of exposure, different mechanisms are involved in the adaptive regulation of colonic TPP uptake by extracellular substrate level.

Evaluating the Association of Common Variants of the SLC44A4 Gene with Ulcerative Colitis Susceptibility in the Han Chinese Population

OBJECTIVE

The SLC44A4 gene was recently reported to be associated with ulcerative colitis (UC) susceptibility in the Indian and Japanese populations. The aim of our study was to investigate the association of common variants within the SLC44A4 gene and the susceptibility to UC among the Han Chinese.

METHODS

We examined 16 tag single nucleotide polymorphisms (SNPs) within the SLC44A4 gene in a Han Chinese population that consisted of 311 UC patients and 675 healthy controls; both SNP and haplotypic association analyses were performed.

RESULTS

We found that rs2736428 was significantly associated with UC risk (allelic p = 0.0004), and the CT and TT genotypes of rs2736428 had a higher distribution compared with the CC genotypes (genotypic p = 0.001), suggesting that the T allele was a risk allele (odds ratio = 1.45, 95% confidence interval = 1.18-1.78). Moreover, one haplotype block that included rs2736428 was found to be strongly associated with UC risk as well (global p < 0.001).

CONCLUSION

Our results provide further supportive evidence for an important role of the SLC44A4 gene in the pathogenesis of UC.

Mechanism(S) Involved in the Colon-Specific Expression of the Thiamine Pyrophosphate (Tpp) Transporter

Microbiota of the large intestine synthesizes considerable amount of vitamin B1 (thiamine) in the form of thiamine pyrophosphate (TPP). We have recently demonstrated the existence of an efficient and specific carrier-mediated uptake process for TPP in human colonocytes, identified the TPP transporter (TPPT) involved (product of the SLC44A4 gene), and shown that expression of TPPT along the gastrointestinal (GI) tract is restricted to the colon. Our aim in this study was to determine the molecular basis of the colon-specific expression of TPPT focusing on a possible epigenetic mechanism. Our results showed that the CpG island predicted in the SLC44A4 promoter is non-methylated in the human colonic epithelial NCM460 cells, but is hyper-methylated in the human duodenal epithelial HuTu80 cells (as well as in the human retinal pigment epithelial ARPE19 cells). In the mouse (where TPPT expression in the GI tract is also restricted to the colon), the CpG island predicted in the Slc44a4 promoter is non-methylated in both the jejunum and colon, thus arguing against possible contribution of DNA methylation in the colon-specific expression of TPPT. A role for histone modifications in the tissue-specific pattern of Slc44a4 expression, however, was suggested by the findings that in mouse colon, histone H3 in the 5’-regulatory region of Slc44a4 is tri-methylated at lysine 4 and acetylated at lysine 9, whereas the tri-methylation at lysine 27 modification was negligible. In contrast, in the mouse jejunum, histone H3 is hyper-trimethylated at lysine 27 (repressor mark). Similarly, possible involvement of miRNA(s) in the tissue-specific expression of TPPT was also suggested by the findings that the 3’-UTR of SLC44A4 is targeted by specific miRNAs/RNA binding proteins in non-colonic, but not in colonic, epithelial cells. These studies show, for the first time, epigenetic mechanisms (histone modifications) play a role in determining the tissue-specific pattern of expression of TPPT in the GI tract.