The human colonic thiamine pyrophosphate transporter (hTPPT) is a glycoprotein and N-linked glycosylation is important for its function

Hiding in Plain Sight: Modern Thiamine Deficiency

Thiamine or vitamin B1 is an essential, water-soluble vitamin required for mitochondrial energetics—the production of adenosine triphosphate (ATP). It is a critical and rate-limiting cofactor to multiple enzymes involved in this process, including those at the entry points and at critical junctures for the glucose, fatty acid, and amino acid pathways. It has a very short half-life, limited storage capacity, and is susceptible to degradation and depletion by a number of products that epitomize modern life, including environmental and pharmaceutical chemicals. The RDA for thiamine is 1.1–1.2 mg for adult females and males, respectively. With an average diet, even a poor one, it is not difficult to meet that daily requirement, and yet, measurable thiamine deficiency has been observed across multiple patient populations with incidence rates ranging from 20% to over 90% depending upon the study. This suggests that the RDA requirement may be insufficient to meet the demands of modern living. Inasmuch as thiamine deficiency syndromes pose great risk of chronic morbidity, and if left untreated, mortality, a more comprehensive understanding thiamine chemistry, relative to energy production, modern living, and disease, may prove useful.

Developmental maturation of the colonic uptake process of the microbiota-generated thiamin pyrophosphate

The water-soluble vitamin B1 is essential for normal human health and physiology. In its main biologically active form, i.e., thiamin pyrophosphate (TPP), the vitamin plays many critical roles in cell metabolism; thus, its deficiency leads to a variety of adverse effects. Humans/mammals obtain vitamin B1 from two exogenous sources: diet and gut microbiota. Considerable amount of the microbiota-generated vitamin B1 exists in the form of TPP, and colonocytes can efficiently absorb this TPP via a high-affinity and specific carrier-mediated mechanism that involves the recently cloned colonic TPP transporter (cTPPT; product of SLC44A4 gene). There is nothing currently known about colonic uptake of TPP during early stages of life and whether the process undergoes developmental regulation. We addressed this issue using the mouse as animal model. Our results showed that colonic uptake of TPP undergoes developmental upregulation as the animal moves from the suckling period to weanling and adulthood. This upregulation in uptake was found to be associated with a parallel induction in level of expression of the cTPPT protein, mRNA, and heterogeneous nuclear RNA, suggesting possible involvement of transcriptional mechanism(s). We also found a parallel upregulation in the level of expression of the two nuclear factors that drive activity of the SLC44A4 promoter (i.e., CREB-1 and Elf-3) with maturation. These results demonstrate, for the first time, to our knowledge, that colonic TPP uptake process and cTPPT expression are developmentally upregulated and that this upregulation is likely driven via transcriptional mechanism(s).NEW & NOTEWORTHY The colonic carrier-mediated uptake process of the microbiota-generated and phosphorylated form of vitamin B1, i.e., thiamin pyrophosphate, undergoes ontogenic changes that parallel the development of the gut microbiota (and their ability to generate vitamins) during early stages of life.

Wernicke's encephalopathy - from basic science to clinical practice. Part 1: Understanding the role of thiamine

Wernicke's encephalopathy (WE) is an acute neuropsychiatric state. Untreated, WE can lead to coma or death, or progress to Korsakoff syndrome (KS) - a dementia characterized by irreversible loss of anterograde memory. Thiamine (vitamin B1) deficiency lies at the heart of this condition. Yet, our understanding of thiamine regarding prophylaxis and treatment of WE remains limited. This may contribute to the current undertreatment of WE in clinical practice. The overall aim of this review is to identify the best strategies for prophylaxis and treatment of WE in regard to (a) dose of thiamine, (b) mode of administration, (c) timing of switch from one mode of administration to another, (d) duration of administration, and (e) use of magnesium along thiamine as an essential cofactor. Evidence from randomized controlled trials and other intervention studies is virtually absent. Therefore, we have to resort to basic science for proof of principle instead. Here, we present the first part of our clinical review, in which we explore the physiology of thiamine and the pathophysiology of thiamine deficiency. We first explore both of these in their historical context. We then review the pharmacodynamics and pharmacokinetics of thiamine, exploring the roles of the six currently known thiamine compounds, their transporters, and target enzymes. We also explore the significance of magnesium as a cofactor in thiamine-facilitated enzymatic reactions and thiamine transport. In the second (forthcoming) part of this review, we will use the findings of the current review to make evidence-based inferences about strategies for prophylaxis and treatment of WE.

Thiamine mimetics sulbutiamine and benfotiamine as a nutraceutical approach to anticancer therapy

Malignant cells frequently demonstrate an oncogenic-driven reliance on glycolytic metabolism to support their highly proliferative nature. Overexpression of pyruvate dehydrogenase kinase (PDK) may promote this unique metabolic signature of tumor cells by inhibiting mitochondrial function. PDKs function to phosphorylate and inhibit pyruvate dehydrogenase (PDH) activity. Silencing of PDK expression has previously been shown to restore mitochondrial function and reduce tumor cell proliferation. High dose Vitamin B1, or thiamine, possesses antitumor properties related to its capacity to reduce PDH phosphorylation and promote its enzymatic activity, presumably through PDK inhibition. Though a promising nutraceutical approach for cancer therapy, thiamine's low bioavailability may limit clinical effectiveness. Here, we have demonstrated exploiting the commercially available lipophilic thiamine analogs sulbutiamine and benfotiamine increases thiamine's anti-cancer effect in vitro. Determined by crystal violet proliferation assays, both sulbutiamine and benfotiamine reduced thiamine's millimolar IC50 value to micromolar equivalents. HPLC analysis revealed that sulbutiamine and benfotiamine significantly increased intracellular thiamine and TPP concentrations in vitro, corresponding with reduced levels of PDH phosphorylation. Through an ex vitro kinase screen, thiamine's activated cofactor form thiamine pyrophosphate (TPP) was found to inhibit the function of multiple PDK isoforms. Attempts to maximize intracellular TPP by exploiting thiamine homeostasis gene expression resulted in enhanced apoptosis in tumor cells. Based on our in vitro evaluations, we conclude that TPP serves as the active species mediating thiamine's inhibitory effect on tumor cell proliferation. Pharmacologic administration of benfotiamine, but not sulbutiamine, reduced tumor growth in a subcutaneous xenograft mouse model. It remains unclear if benfotiamine's effects in vivo are associated with PDK inhibition or through an alternative mechanism of action. Future work will aim to define the action of lipophilic thiamine mimetics in vivo in order to translate their clinical usefulness as anticancer strategies.

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.

Genetics of ulcerative colitis: putting into perspective the incremental gains from Indian studies

Ulcerative colitis (UC), one of the two clinical subtypes of inflammatory bowel disease is perceived as a potential 'sleeping giant' in the Indian subcontinent. Clinical manifestation is overall believed to be the same across ethnic groups but overwhelming genetics from large European and fewer non-European studies have revealed shared as well as unique disease susceptibly signaturesbetween them, pointing to population specific differences at genomic and environmental levels. A systematic recount of the four major eras in UC genetics spanning earliest linkage analysis, cherry picked candidate gene association studies, unbiased genomewide association studies, their logical extension in trans-ethnic setting (Immunochip study), lastly whole exome sequencing efforts forrare variant burden; and lessons learnt thereof in context of genetically distinct Indian population was attempted in this review. Genetic heterogeneity manifesting at allelic/locus level across these approaches has been the consistent finding through the range of pan India studies. On the other hand, these salient findings also highlight the limitations of even the best of these genetic leadsfor prognostic/clinical application. The imminent need, therefore, for the UC research community to adopt newer approaches/tools with improved study design to (i) gain better insight into genetic/mechanistic basis of disease; (ii) identify biomarkers of immediate translational value; and (iii) develop new/alternate therapeutic options is emphasized at the end.

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.

Membrane topology of rat sodium-coupled neutral amino acid transporter 2 (SNAT2)

Sodium-coupled neutral amino acid transporter 2 (SNAT2) is a subtype of the amino acid transport system A that is widely expressed in mammalian tissues. It plays critical roles in glutamic acid-glutamine circulation, liver gluconeogenesis and other biological pathway. However, the topology of the SNAT2 amino acid transporter is unknown. Here we identified the topological structure of SNAT2 using bioinformatics analysis, Methoxy-polyethylene glycol maleimide (mPEG-Mal) chemical modification, protease cleavage assays, immunofluorescence and examination of glycosylation. Our results show that SNAT2 contains 11 transmembrane domains (TMDs) with an intracellular N terminus and an extracellular C terminus. Three N-glycosylation sites were verified at the largest extracellular loop. This model is consistent with the previous model of SNAT2 with the exception of a difference in number of glycosylation sites. This is the first time to confirm the SNAT2 membrane topology using experimental methods. Our study on SNAT2 topology provides valuable structural information of one of the solute carrier family 38 (SLC38) members.

Structure/functional aspects of the human riboflavin transporter-3 (SLC52A3): role of the predicted glycosylation and substrate-interacting sites

The human riboflavin (RF) transporter-3 (hRFVT-3; product of the SLC52A3 gene) plays an essential role in the intestinal RF absorption process and is expressed exclusively at the apical membrane domain of polarized enterocytes. Previous studies have characterized different physiological/biological aspects of this transporter, but nothing is known about the glycosylation status of the hRFVT-3 protein and role of this modification in its physiology/biology. Additionally, little is known about the residues in the hRFVT-3 protein that interact with the ligand, RF. We addressed these issues using appropriate biochemical/molecular approaches, a protein-docking model, and established intestinal/renal epithelial cells. Our results showed that the hRFVT-3 protein is glycosylated and that glycosylation is important for its function. Mutating the predicted N-glycosylation sites at Asn94 and Asn168 led to a significant decrease in RF uptake; it also led to a marked intracellular (in the endoplasmic reticulum, ER) retention of the mutated proteins as shown by live-cell confocal imaging studies. The protein-docking model used in this study has identified a number of putative substrate-interacting sites: Ser16, Ile20, Trp24, Phe142, Thr314, and Asn315 Mutating these potential interacting sites was indeed found to lead to a significant inhibition in RF uptake and to intracellular (ER) retention of the mutated proteins (except for the Phe142 mutant). These results demonstrate that the hRFVT-3 protein is glycosylated and this glycosylation is important for its function and cell surface expression. This study also identified a number of residues in the hRFVT-3 polypeptide that are important for its function/cell surface expression.

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.