Cellular uptake of biotin: mechanisms and regulation

Characterisation of Indian gut microbiome for B-vitamin production and its comparison with Chinese cohort

The human gut microbiota can biosynthesize essential micronutrients such as B-vitamins and is also known for its metabolic cooperative behaviour. The present study characterises such B-vitamin biosynthesizers, their biosynthetic pathways, explores their prevalence and abundance, examines how lifestyle or diet affects them in multiple Indian cohorts and compares it with the Chinese cohort. To achieve this, publicly available faecal metagenome data of healthy individuals from multiple Indian (two urban and three tribal populations) and a Chinese cohort were analysed. The distribution of prevalence and abundance of B-vitamin biosynthesizers showed similar profiles to that of the entire gut community of the Indian cohort, and there were 28 B-vitamin biosynthesizers that had modest or higher prevalence and abundance. The omnivorous diet affected only the prevalence of a few B-vitamin biosynthesizers; however, lifestyle and/or location affected both prevalence and abundance. A comparison with the Chinese cohort showed that fourteen B-vitamin biosynthesizers were significantly more prevalent and abundant in Chinese as compared with Indian samples (False Discovery Rate (FDR) <= 0·05). The metabolic potential of the entire gut community for B-vitamin production showed that within India, the tribal cohort has a higher abundance of B-vitamin biosynthesis pathways as compared with two urban cohorts namely, Bhopal and Kasargod, and comparison with the Chinese cohort revealed a higher abundance in the latter group. Potential metabolic cooperative behaviour of the Indian gut microbiome for biosynthesis of the B-vitamins showed multiple pairs of species showed theoretical complementarity for complete biosynthetic pathways genes of thiamine, riboflavin, niacin and pantothenate.

Comparative bioavailability of vitamins in human foods sourced from animals and plants

Vitamins are essential components of enzyme systems involved in normal growth and function. The quantitative estimation of the proportion of dietary vitamins, that is in a form available for utilization by the human body, is limited and fragmentary. This review provides the current state of knowledge on the bioavailability of thirteen vitamins and choline, to evaluate whether there are differences in vitamin bioavailability when human foods are sourced from animals or plants. The bioavailability of naturally occurring choline, vitamin D, vitamin E, and vitamin K in food awaits further studies. Animal-sourced foods are the almost exclusive natural sources of dietary vitamin B-12 (65% bioavailable) and preformed vitamin A retinol (74% bioavailable), and contain highly bioavailable biotin (89%), folate (67%), niacin (67%), pantothenic acid (80%), riboflavin (61%), thiamin (82%), and vitamin B-6 (83%). Plant-based foods are the main natural sources of vitamin C (76% bioavailable), provitamin A carotenoid β-carotene (15.6% bioavailable), riboflavin (65% bioavailable), thiamin (81% bioavailable), and vitamin K (16.5% bioavailable). The overview of studies showed that in general, vitamins in foods originating from animals are more bioavailable than vitamins in foods sourced from plants.

Intermediate role of gut microbiota in vitamin B nutrition and its influences on human health

Vitamin B consists of a group of water-soluble micronutrients that are mainly derived from the daily diet. They serve as cofactors, mediating multiple metabolic pathways in humans. As an integrated part of human health, gut microbiota could produce, consume, and even compete for vitamin B with the host. The interplay between gut microbiota and the host might be a crucial factor affecting the absorbing processes of vitamin B. On the other hand, vitamin B supplementation or deficiency might impact the growth of specific bacteria, resulting in changes in the composition and function of gut microbiota. Together, the interplay between vitamin B and gut microbiota might systemically contribute to human health. In this review, we summarized the interactions between vitamin B and gut microbiota and tried to reveal the underlying mechanism so that we can have a better understanding of its role in human health.

A new-type HOCl-activatable fluorescent probe and its applications in water environment and biosystems

The outbreak and spread of Corona Virus Disease 2019 (COVID-19) has led to a significant increase in the consumption of sodium hypochlorite (NaOCl) disinfectants. NaOCl hydrolyzes to produce hypochlorous acid (HOCl) to kill viruses, which is a relatively efficient chlorine-based disinfectant commonly used in public disinfection. While people enjoy the convenience of NaOCl disinfection, excessive and indiscriminate use of it will affect the water environment and threaten human health. Importantly, HOCl is an indispensable reactive oxygen species (ROS) in human body. Whether its concentration is normal or not is closely related to human health. Excessive production of HOCl in the body contributes to some inflammatory diseases and even cancer. Also, we noticed that the concentration of ROS in cancer cells is about 10 times higher than that in normal cells. Herein, we developed a HOCl-activatable biotinylated dual-function fluorescent probe BTH. For this probe, we introduced biotin on the naphthalimide fluorophore, which increased the water solubility and enabled the probe to aggregate in cancer cells by targeting specific receptor overexpressed on the surface of cancer cell membrane. After reacting to HOCl, the p-aminophenylether moiety of this probe was oxidatively removed and the fluorescence of the probe was recovered. As expected, in the PBS solution with pH of 7.4, BTH could give full play to the performance of detecting HOCl, and it has made achievements in detecting the concentration of HOCl in actual water samples. Besides that, BTH had effectively distinguished between cancer cells and normal cells through a dual-function discrimination strategy, which used biotin to enrich the probe in cancer cells and reacted with overexpressed HOCl in cancer cells. Importantly, this dual-function discrimination strategy could obtain the precision detection of cancer cells, thereby offering assistance for improving the accuracy of early cancer diagnosis.

Cell type-specific biotin labeling in vivo resolves regional neuronal and astrocyte proteomic differences in mouse brain

Proteomic profiling of brain cell types using isolation-based strategies pose limitations in resolving cellular phenotypes representative of their native state. We describe a mouse line for cell type-specific expression of biotin ligase TurboID, for in vivo biotinylation of proteins. Using adenoviral and transgenic approaches to label neurons, we show robust protein biotinylation in neuronal soma and axons throughout the brain, allowing quantitation of over 2000 neuron-derived proteins spanning synaptic proteins, transporters, ion channels and disease-relevant druggable targets. Next, we contrast Camk2a-neuron and Aldh1l1-astrocyte proteomes and identify brain region-specific proteomic differences within both cell types, some of which might potentially underlie the selective vulnerability to neurological diseases. Leveraging the cellular specificity of proteomic labeling, we apply an antibody-based approach to uncover differences in neuron and astrocyte-derived signaling phospho-proteins and cytokines. This approach will facilitate the characterization of cell-type specific proteomes in a diverse number of tissues under both physiological and pathological states.

Microbiome distinctions between the CRC carcinogenic pathways

Colorectal cancer (CRC) is the third most commonly diagnosed cancer, the third leading cause of cancer-related deaths, and has been on the rise among young adults in the United States. Research has established that the colonic microbiome is different in patients with CRC compared to healthy controls, but few studies have investigated if and how the microbiome may relate to CRC progression through the serrated pathway versus the adenoma-carcinoma sequence.Our view is that progress in CRC microbiome research requires consideration of how the microbiome may contribute to CRC carcinogenesis through the distinct pathways that lead to CRC, which could enable the creation of novel and tailored prevention, screening, and therapeutic interventions. We first highlight the limitations in existing CRC microbiome research and offer corresponding solutions for investigating the microbiome's role in the adenoma-carcinoma sequence and serrated pathway. We then summarize the findings in the select human studies that included data points related to the two major carcinogenic pathways. These studies investigate the microbiome in CRC carcinogenesis and 1) utilize mucosal samples and 2) compare polyps or tumors by histopathologic type, molecular/genetic type, or location in the colon.Key findings from these studies include: 1) Fusobacterium is associated with right-sided, more advanced, and serrated lesions; 2) the colons of people with CRC have bacteria typically associated with normal oral flora; and 3) colons from people with CRC have more biofilms, and these biofilms are predominantly located in the proximal colon (single study).

Photocytotoxic cancer cell-targeting platinum(ii) complexes of glucose-appended curcumin and biotinylated 1,10-phenanthroline

Mixed-ligand platinum(ii) complexes, [Pt(phen)(pacac)](NO3) (1), [Pt(phen)(cur)](NO3) (2), [Pt(bt-phen)(cur)](NO3) (3) and [Pt(phen)(scur)](NO3) (4), where phen is 1,10-phenanthroline, bt-phen is 5-biotin-1,10-phenanthroline, pacac is 1,3-diphenyl-1,3-propanedioate anion, Hcur is curcumin and Hscur is diglucosylcurcumin, were prepared, characterized and their anticancer activity studied. Complexes 2-4 showed absorption bands within 410-430 nm (ε, 2.1 × 104 to 2.8 × 104 M-1 cm-1) in 10% DMSO-DPBS (Dulbecco's phosphate-buffered saline) and emission bands near 530 nm (λex = 410-430 nm) with a fluorescence quantum yield (ΦF) value of ∼0.02. The curcumin complexes showed stability over a study period of 48 h. The photocytotoxicity was studied using human cervical HeLa, human liver HepG2, human breast cancer MDA-MB 231 and human lung adenocarcinoma A549 cancer cells along with human immortalized lung epithelial HPL1D as normal cells. Complexes 2-4 showed apoptotic photo-induced cell death in light of wavelength 400-700 nm (IC50, half maximal inhibitory concentration: 6-28 μM) by reactive oxygen species (ROS), while remaining inactive in the dark (IC50: 43-95 μM). The selectivity of the complexes 3 and 4 was enhanced significantly towards the cancer cells than towards the normal cells, thus making them targeted photochemotherapeutic agents. The ROS formation and mode of cell death were studied from 2',7'-dichlorofluorescein diacetate (DCFDA) and annexin-V/FITC (fluorescein isothiocyanate)-PI assays, respectively. Preferential nuclear and mitochondrial localization was evidenced from inductively coupled plasma mass spectrometry (ICP-MS) studies.

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.

Acute oxygen sensing-Role of metabolic specifications in peripheral chemoreceptor cells

Acute oxygen sensing is essential for humans under hypoxic environments or pathologic conditions. This is achieved by the carotid body (CB), the key arterial chemoreceptor, along with other peripheral chemoreceptor organs, such as the adrenal medulla (AM). Although it is widely accepted that inhibition of K⁺ channels in the plasma membrane of CB cells during acute hypoxia results in the activation of cardiorespiratory reflexes, the molecular mechanisms by which the hypoxic signal is detected to modulate ion channel activity are not fully understood. Using conditional knockout mice lacking mitochondrial complex I (MCI) subunit NDUFS2, we have found that MCI generates reactive oxygen species and pyridine nucleotides, which signal K⁺ channels during acute hypoxia. Comparing the transcriptomes from CB and AM, which are O₂-sensitive, with superior cervical ganglion, which is practically O₂-insensitive, we have found that CB and AM contain unique metabolic gene expression profiles. The "signature metabolic profile" and their biophysical characteristics could be essential for acute O₂ sensing by chemoreceptor cells.

Crossing the border - Solute entry into the chlamydial inclusion

Chlamydiales comprise important human and animal pathogens as well as endosymbionts of amoebae. Generally, these obligate intracellular living bacteria are characterized by a biphasic developmental cycle, a reduced genome and a restricted metabolic capacity. Because of their metabolic impairment, Chlamydiales essentially rely on the uptake of diverse metabolites from their hosts. Chlamydiales thrive in a special compartment, the inclusion, and hence are surrounded by an additional membrane. Solutes might enter the inclusion through pores and open channels or by redirection of host vesicles, which fuse with the inclusion membrane and release their internal cargo. Recent investigations shed new light on the chlamydia-host interaction and identified an additional way for nutrient uptake into the inclusion. Proteome studies and targeting analyses identified chlamydial and host solute carriers in inclusions of Chlamydia trachomatis infected cells. These transporters are involved in the provision of UDP-glucose and biotin, and probably deliver further metabolites to the inclusion. By the controlled recruitment of specific solute carriers to the inclusion, the chlamydial resident thus can actively manipulate the metabolite availability and composition in the inclusion. This review summarizes recent findings and new ideas on carrier mediated solute uptake into the chlamydial inclusion in the context of the bacterial and host metabolism.

Targeted tumor detection: guidelines for developing biotinylated diagnostics

We highlight the prominent role of hydrophilicity in the preferential cellular uptake process of biotinylated fluorescent probes.

Selective accumulation of biotin in arterial chemoreceptors: requirement for carotid body exocytotic dopamine secretion

KEY POINTS

Biotin, a vitamin whose main role is as a coenzyme for carboxylases, accumulates at unusually large amounts within cells of the carotid body (CB). In biotin-deficient rats biotin rapidly disappears from the blood; however, it remains at relatively high levels in CB glomus cells. The CB contains high levels of mRNA for SLC5a6, a biotin transporter, and SLC19a3, a thiamine transporter regulated by biotin. Animals with biotin deficiency exhibit pronounced metabolic lactic acidosis. Remarkably, glomus cells from these animals have normal electrical and neurochemical properties. However, they show a marked decrease in the size of quantal dopaminergic secretory events. Inhibitors of the vesicular monoamine transporter 2 (VMAT2) mimic the effect of biotin deficiency. In biotin-deficient animals, VMAT2 protein expression decreases in parallel with biotin depletion in CB cells. These data suggest that dopamine transport and/or storage in small secretory granules in glomus cells depend on biotin.

ABSTRACT

Biotin is a water-soluble vitamin required for the function of carboxylases as well as for the regulation of gene expression. Here, we report that biotin accumulates in unusually large amounts in cells of arterial chemoreceptors, carotid body (CB) and adrenal medulla (AM). We show in a biotin-deficient rat model that the vitamin rapidly disappears from the blood and other tissues (including the AM), while remaining at relatively high levels in the CB. We have also observed that, in comparison with other peripheral neural tissues, CB cells contain high levels of SLC5a6, a biotin transporter, and SLC19a3, a thiamine transporter regulated by biotin. Biotin-deficient rats show a syndrome characterized by marked weight loss, metabolic lactic acidosis, aciduria and accelerated breathing with normal responsiveness to hypoxia. Remarkably, CB cells from biotin-deficient animals have normal electrophysiological and neurochemical (ATP levels and catecholamine synthesis) properties; however, they exhibit a marked decrease in the size of quantal catecholaminergic secretory events, which is not seen in AM cells. A similar differential secretory dysfunction is observed in CB cells treated with tetrabenazine, a selective inhibitor of the vesicular monoamine transporter 2 (VMAT2). VMAT2 is highly expressed in glomus cells (in comparison with VMAT1), and in biotin-deficient animals VMAT2 protein expression decreases in parallel with the decrease of biotin accumulated in CB cells. These data suggest that biotin has an essential role in the homeostasis of dopaminergic transmission modulating the transport and/or storage of transmitters within small secretory granules in glomus cells.

Commensal 'trail of bread crumbs' provide pathogens with a map to the intestinal landscape

Growth of a microorganism in a host is essential for infection, and bacterial pathogens have evolved to utilize specific metabolites to enhance replication in vivo. Now, emerging data demonstrate that pathogens rely on microbiota-derived metabolites as a form of bacterial-bacterial communication to gain information about location within a host and modify virulence gene expression accordingly. Thus, metabolite-sensing is critical for pathogens to establish infection. Here, we highlight recent examples of how the foodborne pathogen enterohemorrhagic Escherichia coli O157:H7 (EHEC) exploits microbiota-derived metabolites to recognize the host intestinal environment and control gene expression that results in controlled expression of virulence traits.

Effects of Fatty Liver Induced by Excess Orotic Acid on B-Group Vitamin Concentrations of Liver, Blood, and Urine in Rats

Fatty liver is caused when rats are given orotic acid of the pyrimidine base in large quantities. The lack of B-group vitamins suppresses the biosynthesis of fatty acids. We investigated how orotic acid-induced fatty liver affects the concentrations of liver, blood, and urine B-group vitamins in rats. The vitamin B6 and B12 concentrations of liver, blood, and urine were not affected by orotic acid-induced fatty liver. Vitamin B2 was measured only in the urine, but was unchanged. The liver, blood, and urine concentrations of niacin and its metabolites fell dramatically. Niacin and its metabolites in the liver, blood, and urine were affected as expected. Although the concentrations of vitamin B1, pantothenic acid, folate, and biotin in liver and blood were decreased by orotic acid-induced fatty liver, these urinary excretion amounts showed a specific pattern toward increase. Generally, as for the typical urinary excretion of B-group vitamins, these are excreted when the body is saturated. However, the ability to sustain vitamin B1, pantothenic acid, folate, and biotin decreased in fatty liver, which is hypothesized as a specific phenomenon. This metabolic response might occur to prevent an abnormally increased biosynthesis of fatty acids by orotic acid.

The sodium/multivitamin transporter: a multipotent system with therapeutic implications

The Na(+)/multivitamin transporter (SMVT) is a member of the solute:sodium symporter family that catalyzes the Na(+)-dependent uptake of the structurally diverse water-soluble vitamins pantothenic acid (vitamin B5) and biotin (vitamin H), α-lipoic acid-a vitamin-like substance with strong antioxidant properties-and iodide. The organic substrates of SMVT play central roles in the cellular metabolism and are, therefore, essential for normal human health and development. For example, biotin deficiency leads to growth retardation, dermatological disorders, and neurological disorders. Animal studies have shown that biotin deficiency during pregnancy is directly correlated to embryonic growth retardation, congenital malformation, and death of the embryo. This chapter focuses on the structural and functional features of the human isoform of SMVT (hSMVT); the discovery of which was greatly facilitated by the cloning and expression of hSMVT in tractable expression systems. Special emphasis will be given to mechanistic implications of the transport process of hSMVT that will inform our understanding of the molecular determinants of hSMVT-mediated transport in dynamic context to alleviate the development and optimization of hSMVT as a multipotent platform for drug delivery.

Chronic alcohol exposure inhibits biotin uptake by pancreatic acinar cells: possible involvement of epigenetic mechanisms

Chronic exposure to alcohol affects different physiological aspects of pancreatic acinar cells (PAC), but its effect on the uptake process of biotin is not known. We addressed this issue using mouse-derived pancreatic acinar 266-6 cells chronically exposed to alcohol and wild-type and transgenic mice (carrying the human SLC5A6 5'-promoter) fed alcohol chronically. First we established that biotin uptake by PAC is Na(+) dependent and carrier mediated and involves sodium-dependent multivitamin transporter (SMVT). Chronic exposure of 266-6 cells to alcohol led to a significant inhibition in biotin uptake, expression of SMVT protein, and mRNA as well as in the activity of the SLC5A6 promoter. Similarly, chronic alcohol feeding of wild-type and transgenic mice carrying the SLC5A6 promoter led to a significant inhibition in biotin uptake by PAC, as well as in the expression of SMVT protein and mRNA and the activity of the SLC5A6 promoters expressed in the transgenic mice. We also found that chronic alcohol feeding of mice is associated with a significant increase in the methylation status of CpG islands predicted to be in the mouse Slc5a6 promoters and a decrease in the level of expression of transcription factor KLF-4, which plays an important role in regulating SLC5A6 promoter activity. These results demonstrate, for the first time, that chronic alcohol exposure negatively impacts biotin uptake in PAC and that this effect is exerted (at least in part) at the level of transcription of the SLC5A6 gene and may involve epigenetic/molecular mechanisms.

Biotin uptake by mouse and human pancreatic beta cells/islets: a regulated, lipopolysaccharide-sensitive carrier-mediated process

Biotin is essential for the normal function of pancreatic beta cells. These cells obtain biotin from their surroundings via transport across their cell membrane. Little is known about the uptake mechanism involved, how it is regulated, and how it is affected by internal and external factors. We addressed these issues using the mouse-derived pancreatic beta-TC-6 cells and freshly isolated mouse and human primary pancreatic beta cells as models. The results showed biotin uptake by pancreatic beta-TC-6 cells occurs via a Na(+)-dependent, carrier-mediated process, that is sensitive to desthiobiotin, as well as to pantothenic acid and lipoate; the process is also saturable as a function of concentration (apparent Km = 22.24 ± 5.5 μM). These cells express the sodium-dependent multivitamin transporter (SMVT), whose knockdown (with doxycycline-inducible shRNA) led to a sever inhibition in biotin uptake. Similarly, uptake of biotin by mouse and human primary pancreatic islets is Na(+)-dependent and carrier-mediated, and both cell types express SMVT. Biotin uptake by pancreatic beta-TC-6 cells is also adaptively regulated (via transcriptional mechanism) by extracellular substrate level. Chronic treatment of pancreatic beta-TC-6 cells with bacterial lipopolysaccharides (LPS) leads to inhibition in biotin uptake. This inhibition is mediated via a Toll-Like receptor 4-mediated process and involves a decrease in membrane expression of SMVT. These findings show, for the first time, that pancreatic beta cells/islets take up biotin via a specific and regulated carrier-mediated process, and that the process is sensitive to the effect of LPS.

Membrane transporters in a human genome-scale metabolic knowledgebase and their implications for disease

Membrane transporters enable efficient cellular metabolism, aid in nutrient sensing, and have been associated with various diseases, such as obesity and cancer. Genome-scale metabolic network reconstructions capture genomic, physiological, and biochemical knowledge of a target organism, along with a detailed representation of the cellular metabolite transport mechanisms. Since the first reconstruction of human metabolism, Recon 1, published in 2007, progress has been made in the field of metabolite transport. Recently, we published an updated reconstruction, Recon 2, which significantly improved the metabolic coverage and functionality. Human metabolic reconstructions have been used to investigate the role of metabolism in disease and to predict biomarkers and drug targets. Given the importance of cellular transport systems in understanding human metabolism in health and disease, we analyzed the coverage of transport systems for various metabolite classes in Recon 2. We will review the current knowledge on transporters (i.e., their preferred substrates, transport mechanisms, metabolic relevance, and disease association for each metabolite class). We will assess missing coverage and propose modifications and additions through a transport module that is functional when combined with Recon 2. This information will be valuable for further refinements. These data will also provide starting points for further experiments by highlighting areas of incomplete knowledge. This review represents the first comprehensive overview of the transporters involved in central metabolism and their transport mechanisms, thus serving as a compendium of metabolite transporters specific for human metabolic reconstructions.

Utility of transporter/receptor(s) in drug delivery to the eye

The eye is a highly protected organ, and designing an effective therapy is often considered a challenging task. The anatomical and physiological barriers result in low ocular bioavailability of drugs. Due to these constraints, less than 5% of the administered dose is absorbed from the conventional ophthalmic dosage forms. Further, physicochemical properties such as lipophilicity, molecular weight and charge modulate the permeability of drug molecules. Vision-threatening diseases such as glaucoma, diabetic macular edema, cataract, wet and dry age-related macular degeneration, proliferative vitreoretinopathy, uveitis, and cytomegalovirus retinitis alter the pathophysiological and molecular mechanisms. Understanding these mechanisms may result in the development of novel treatment modalities. Recently, transporter/receptor targeted prodrug approach has generated significant interest in ocular drug delivery. These transporters and receptors are involved in the transport of essential nutrients, vitamins, and xenobiotics across biological membranes. Several influx transporters (peptides, amino acids, glucose, lactate and nucleosides/nucleobases) and receptors (folate and biotin) have been identified on conjunctiva, cornea, and retina. Structural and functional delineation of these transporters will enable more drugs targeting the posterior segment to be successfully delivered topically. Prodrug derivatization targeting transporters and receptors expressed on ocular tissues has been the subject of intense research. Several prodrugs have been designed to target these transporters and enhance the absorption of poorly permeating parent drug. Moreover, this approach might be used in gene delivery to modify cellular function and membrane receptors. This review provides comprehensive information on ocular drug delivery, with special emphasis on the use of transporters and receptors to improve drug bioavailability.

Functional and molecular aspects of biotin uptake via SMVT in human corneal epithelial (HCEC) and retinal pigment epithelial (D407) cells

Sodium-dependent multivitamin transporter (SMVT) is a vital transmembrane protein responsible for translocating biotin and other essential cofactors such as pantothenate and lipoate. Unlike primary cultures of corneal and retinal pigment epithelial (RPE) cells, immortalized cells can be subcultured many times, yet maintain their physiological properties. Hence, the purpose of this study was to delineate the functional and molecular aspects of biotin uptake via SMVT on immortalized human corneal epithelial (HCEC) and RPE (D407) cells. Functional aspects of [(3)H] biotin uptake were studied in the presence of different concentrations of unlabeled biotin, pH, temperature, metabolic inhibitors, ions, substrates, structural analogs and biotinylated prodrug (Biotin-Acyclovir (B-ACV)). Molecular identity of SMVT was examined with reverse transcription-polymerase chain reaction. Biotin uptake was found to be saturable in HCEC and D407 cells with K (m) of 296.2 ± 25.9 and 863.8 ± 66.9 μM and V (max) of 77.2 ± 2.2 and 308.3 ± 10.7 pmol/mg protein/min, respectively. Uptake was found to be pH, temperature, energy, and sodium-dependent. Inhibition of biotin uptake was observed in the presence of structural analogs and specific substrates. Further, uptake was lowered in the presence of B-ACV indicating the translocation of biotinylated prodrug by SMVT. A distinct band at 774 bp confirmed the molecular existence of SMVT in both the cells. This study shows for the first time the functional and molecular presence of SMVT in HCEC and D407 cells. Therefore, these cell lines may be utilized as in vitro models to study the cellular translocation of biotin-conjugated prodrugs.

Targeted lipid based drug conjugates: a novel strategy for drug delivery

A majority of studies involving prodrugs are directed to overcome low bioavailability of the parent drug. The aim of this study is to increase the bioavailability of acyclovir (ACV) by designing a novel prodrug delivery system which is more lipophilic, and at the same time site specific. In this study, a lipid raft has been conjugated to the parent drug molecule to impart lipophilicity. Simultaneously a targeting moiety that can be recognized by a specific transporter/receptor in the cell membrane has also been tethered to the other terminal of lipid raft. Targeted lipid prodrugs i.e., biotin-ricinoleicacid-acyclovir (B-R-ACV) and biotin-12hydroxystearicacid-acyclovir (B-12HS-ACV) were synthesized with ricinoleicacid and 12hydroxystearicacid as the lipophilic rafts and biotin as the targeting moiety. Biotin-ACV (B-ACV), ricinoleicacid-ACV (R-ACV) and 12hydroxystearicacid-ACV (12HS-ACV) were also synthesized to delineate the individual effects of the targeting and the lipid moieties. Cellular accumulation studies were performed in confluent MDCK-MDR1 and Caco-2 cells. The targeted lipid prodrugs B-R-ACV and B-12HS-ACV exhibited much higher cellular accumulation than B-ACV, R-ACV and 12HS-ACV in both cell lines. This result indicates that both the targeting and the lipid moiety act synergistically toward cellular uptake. The biotin conjugated prodrugs caused a decrease in the uptake of [(3)H] biotin suggesting the role of sodium dependent multivitamin transporter (SMVT) in uptake. The affinity of these targeted lipid prodrugs toward SMVT was studied in MDCK-MDR1 cells. Both the targeted lipid prodrugs B-R-ACV (20.25 ± 1.74 μM) and B-12HS-ACV (23.99 ± 3.20 μM) demonstrated higher affinity towards SMVT than B-ACV (30.90 ± 4.19 μM). Further, dose dependent studies revealed a concentration dependent inhibitory effect on [(3)H] biotin uptake in the presence of biotinylated prodrugs. Transepithelial transport studies showed lowering of [(3)H] biotin permeability in the presence of biotin and biotinylated prodrugs, further indicating a carrier mediated translocation by SMVT. Overall, results from these studies clearly suggest that these biotinylated lipid prodrugs of ACV possess enhanced affinity towards SMVT. These prodrugs appear to be potential candidates for the treatment of oral and ocular herpes virus infections, because of higher expression of SMVT on intestinal and corneal epithelial cells. In conclusion we hypothesize that our novel prodrug design strategy may help in higher absorption of hydrophilic parent drug. Moreover, this novel prodrug design can result in higher cell permeability of hydrophilic therapeutics such as genes, siRNA, antisense RNA, DNA, oligonucleotides, peptides and proteins.

Biotin: biochemical, physiological and clinical aspects

Significant progress has been made in our understanding of the biochemical, physiological and nutritional aspects of the water-soluble vitamin biotin (vitamin H). It is well know now that biotin plays important roles in a variety of critical metabolic reactions in the cell, and thus, is essential for normal human health, growth and development. This is underscored by the serious clinical abnormalities that occur in conditions of biotin deficiency, which include, among other things, growth retardation, neurological disorders, and dermatological abnormalities (reviewed in 1). Studies in animals have also shown that biotin deficiency during pregnancy leads to embryonic growth retardation, congenital malformation and death (Watanabe 1983; Cooper and Brown 1958; Mock et al. 2003; Zempleni and Mock 2000). The aim of this chapter is to provide coverage of current knowledge of the biochemical, physiological, and clinical aspects of biotin nutrition. Many sections of this chapter have been the subject of excellent recent reviews by others (Wolf 2001; McMahon 2002; Mock 2004; Rodriguez-Melendez and Zempleni 2003; Said 2004; Said et al. 2000; Said and Seetheram 2006), and thus, for more information the reader is advised to consider these additional sources.

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.

Role of the putative N-glycosylation and PKC-phosphorylation sites of the human sodium-dependent multivitamin transporter (hSMVT) in function and regulation

The sodium-dependent multivitamin transporter (SMVT) is a major biotin transporter in a variety of tissues including the small intestine. The human SMVT (hSMVT) polypeptide is predicted to have four N-glycosylation sites and two putative PKC phosphorylation sites but their role in the function and regulation of the protein is not known and was examined in this investigation. Our results showed that the hSMVT protein is glycosylated and that this glycosylation is important for its function. Studies utilizing site-directed mutagenesis revealed that the N-glycosylation sites at positions Asn(138) and Asn(489) are important for the function of hSMVT and that mutating these sites significantly reduces the V(max) of the biotin uptake process. Mutating the putative PKC phosphorylation site Thr(286) of hSMVT led to a significant decrease in the PMA-induced inhibition in biotin uptake. The latter effect was not mediated via changes in the level of expression of the hSMVT protein and mRNA or in its level of expression at the cell membrane. These findings demonstrate that the hSMVT protein is glycosylated, and that glycosylation is important for its function. Furthermore, the study shows a role for the putative PKC-phosphorylation site Thr(286) of hSMVT in the PKC-mediated regulation of biotin uptake.

Surprising substrate versatility in SLC5A6: Na+-coupled I- transport by the human Na+/multivitamin transporter (hSMVT)

Iodide (I(-)) is an essential constituent of the thyroid hormones triiodothyronine and thyroxine, which are required for the development of the central nervous system in the fetus and newborn. I(-) uptake in the thyroid is mediated by the Na(+)/I(-) symporter (NIS). NIS has gained particular medical interest due to its sensitivity to the environmental pollutant perchlorate (ClO(4)(-)) and its implication in radioiodide cancer treatment. Recently, others have shown that I(-) absorption in the intestine is mediated by NIS (Nicola, J. P., Basquin, C., Portulano, C., Reyna-Neyra, A., Paroder, M., and Carrasco, N. (2009) Am. J. Physiol. Cell Physiol. 296, C654-662). However, their data suggest the participation of other systems in the homeostasis of I(-), in particular because in vivo uptake studies revealed a ClO(4)(-)-insensitive transport component. Here, we describe Na(+)-coupled I(-) uptake by the human Na(+)/multivitamin transporter (hSMVT), a related protein isolated from the placenta, where it was suggested to supply the fetus with the water-soluble vitamins biotin and pantothenic acid, and α-lipoic acid. hSMVT-mediated Na(+)/I(-) symport is inhibited by the other three organic hSMVT substrates but not by NIS substrates; notably, hSMVT is insensitive to ClO(4)(-). Because hSMVT is found in the intestine and in many other tissues, we propose that hSMVT may play an important role in the homeostasis of I(-) in the body.

Vitreal pharmacokinetics of biotinylated ganciclovir: role of sodium-dependent multivitamin transporter expressed on retina

PURPOSE

The objective of this study was to investigate the role of sodium-dependent multiple vitamin transporter (SMVT) on Biotin-Ganciclovir (biotin-GCV) uptake on both human retinal pigmented epithelium cell line (ARPE-19) and rabbit retina. Study also aims to delineate the vitreal pharmacokinetics of biotin-GCV.

METHOD

ARPE-19 was employed to study the in vitro uptake experiments. New Zealand white albino rabbits were used to study in vivo retinal uptake and vitreal pharmacokinetics following intravitreal administration of biotin-GCV. In vitro uptake kinetics of [3H] biotin was determined at various initial concentrations. Competitive inhibition studies were conducted in the presence of unlabelled biotin, desthiobiotin, pantothenic acid, and lipoic acid. Various other uptake studies were performed to functionally characterize the transporter. To provide the molecular evidence of this transporter, Reverse Transcription-Polymerase Chain Reaction (RT-PCR) studies were also conducted. In vivo retinal/choroidal uptake studies were carried out with New Zealand albino rabbits. Unconscious animal ocular microdialysis studies were performed in order to evaluate intravitreal pharmacokinetics of GCV and Biotin-GCV.

RESULTS

Uptake of [3H] biotin into ARPE-19 was linear over 7 min, and found to be saturable with K(m) of 138.25 muM and Vmax of 38.85 pmol/min/mg protein. Both pantothenic acid and lipoic acid decreased significantly in uptake of biotin in the concentration-dependent manner. Uptake of biotin into ARPE-19 was found to be temperature, energy, and Na+ dependent but Cl(-)independent. Further, RT-PCR studies identified a band exhibiting presence of hSMVT on ARPE-19. Biotin-GCV is recognized by SMVT system present on the ARPE-19 and rabbit retina. Vitreal Pharmacokinetics profile reveals that most of the parameters were not significantly different for GCV and Biotin-GCV. However, use of Biotin-GCV may result in sustain levels of regenerated GCV in vitreous.

CONCLUSIONS

SMVT was identified and functionally characterized on ARPE-19 cells. Further, Biotin-GCV shares this transport system. Vitreal pharmacokinetics of the conjugate was determined in unconscious rabbit model.

Biotin

Biotin is a water-soluble vitamin and serves as a coenzyme for five carboxylases in humans. Biotin is also covalently attached to distinct lysine residues in histones, affecting chromatin structure and mediating gene regulation. This review describes mammalian biotin metabolism, biotin analysis, markers of biotin status, and biological functions of biotin. Proteins such as holocarboxylase synthetase, biotinidase, and the biotin transporters SMVT and MCT1 play crucial roles in biotin homeostasis, and these roles are reviewed here. Possible effects of inadequate biotin intake, drug interactions, and inborn errors of metabolism are discussed, including putative effects on birth defects.

Cell and molecular aspects of human intestinal biotin absorption

Humans cannot synthesize biotin and thus must obtain this vitamin from exogenous sources. The intestine is exposed to 2 sources of biotin: a dietary source and a bacterial source, which is normal microflora of the large intestine. Dietary protein-bound biotin is converted to free biotin prior to absorption. Absorption of free biotin in the small and large intestine involves a saturable and Na(+)-dependent carrier-mediated process that is shared with pantothenic acid and lipoate. For this reason, the involved transport system is referred to as the sodium-dependent multivitamin transporter (SMVT); in humans, it is designated as hSMVT. The hSMVT system has been cloned, demonstrated to be exclusively expressed at the apical membrane of enterocytes, and shown, by means of gene-specific short interfering RNA, to be the main biotin uptake system that operates in human intestinal epithelial cells. The 5'-regulatory region of the hSMVT gene has also been cloned and characterized both in vitro and in vivo. Further, the human intestinal biotin uptake process was adaptively up-regulated in biotin deficiency via a transcriptionally mediated mechanism(s) that involves Kruppel-like factor 4 sites. Studies on cell biology of hSMVT have shown a region in the cytoplasmic C-terminal domain of the polypeptide to be essential for its targeting to the apical membrane domain of epithelial cells. Intracellular trafficking of the hSMVT protein appears to involve distinct trafficking vesicles that require an intact microtubules network and the motor protein dynein for their mobility.

Biotin accounts for less than half of all biotin and biotin metabolites in the cerebrospinal fluid of children

BACKGROUND

Biotin is likely transported into cerebral spinal fluid (CSF) via one or more specific transporters. Concentrations of biotin in CSF measured by using modern analytic techniques that are specific for biotin and biotin metabolites have not previously been reported.

OBJECTIVES

We aimed to accurately measure the concentration of biotin and major biotin metabolites, biotin sulfoxide (BSO) and bisnorbiotin (BNB), in the CSF of children.

DESIGN

Concentrations of biotin were determined initially as total avidin-binding substances (TABS) in CSF obtained by lumbar puncture from 55 children. Biotin, BSO, and BNB were quantitated by HPLC and an avidin-binding assay in CSF samples from a subset of 11 children.

RESULTS

Concentrations of TABS in CSF averaged 1.6 nmol/L with substantial variability (SD = 1.3 nmol/L). CSF concentrations of biotin and biotin analogs varied widely, but substantial amounts of BSO were detected in every sample. Biotin accounted for 42 +/- 16%, BSO for 41 +/- 12%, and BNB for 8 +/- 14% of the total. It was surprising that the molar sum of biotin, BSO, and BNB on average was >200-fold the TABS concentrations from the same CSF sample. Using several analytic approaches, we found no masking of detection, nor did we find degradation of biotin or BSO. Gel electrophoresis and streptavidin Western blot detected several biotinylated proteins in CSF.

CONCLUSIONS

Biotin appears to be bound to protein covalently, reversibly, or both, and this binding likely accounts for the increase in detectable biotin after HPLC. Protein-bound biotin may play an important role in biotin nutriture of the brain.

Biotin and biotinidase deficiency

Biotin is a water-soluble vitamin that serves as an essential coenzyme for five carboxylases in mammals. Biotin-dependent carboxylases catalyze the fixation of bicarbonate in organic acids and play crucial roles in the metabolism of fatty acids, amino acids and glucose. Carboxylase activities decrease substantially in response to biotin deficiency. Biotin is also covalently attached to histones; biotinylated histones are enriched in repeat regions in the human genome and appear to play a role in transcriptional repression of genes and genome stability. Biotin deficiency may be caused by insufficient dietary uptake of biotin, drug-vitamin interactions and, perhaps, by increased biotin catabolism during pregnancy and in smokers. Biotin deficiency can also be precipitated by decreased activities of the following proteins that play critical roles in biotin homeostasis: the vitamin transporters sodium-dependent multivitamin transporter and monocarboxylate transporter 1, which mediate biotin transport in the intestine, liver and peripheral tissues, and renal reabsorption; holocarboxylase synthetase, which mediates the binding of biotin to carboxylases and histones; and biotinidase, which plays a central role in the intestinal absorption of biotin, the transport of biotin in plasma and the regulation of histone biotinylation. Symptoms of biotin deficiency include seizures, hypotonia, ataxia, dermatitis, hair loss, mental retardation, ketolactic acidosis, organic aciduria and also fetal malformations. This review focuses on the deficiencies of both biotin and biotinidase, and the medical management of such cases.

Holocarboxylase synthetase regulates expression of biotin transporters by chromatin remodeling events at the SMVT locus

The sodium-dependent multivitamin transporter (SMVT) is essential for mediating and regulating biotin entry into mammalian cells. In cells, biotin is covalently linked to histones in a reaction catalyzed by holocarboxylase synthetase (HCS); biotinylation of lysine 12-biotinylated histone H4 (K12Bio H4) causes gene silencing. Here, we propose a novel role for HCS in sensing and regulating levels of biotin in eukaryotic cells. We hypothesized that nuclear translocation of HCS increases in response to biotin supplementation; HCS then biotinylates histone H4 at SMVT promoters, silencing biotin transporter genes. Jurkat lymphoma cells were cultured in media containing 0.025, 0.25, or 10 nmol/l biotin. The nuclear translocation of HCS correlated with biotin concentrations in media; the relative enrichment of both HCS and K12Bio H4 at SMVT promoter 1 (but not promoter 2) increased by 91% in cells cultured in medium containing 10 nmol/l biotin compared with 0.25 nmol/l biotin. This increase of K12Bio H4 at the SMVT promoter decreased SMVT expression by up to 86%. Biotin homeostasis by HCS-dependent chromatin remodeling at the SMVT promoter 1 locus was disrupted in HCS knockdown cells, as evidenced by abnormal chromatin structure (K12Bio H4 abundance) and increased SMVT expression. The findings from this study are consistent with the theory that HCS senses biotin, and that biotin regulates its own cellular uptake by participating in HCS-dependent chromatin remodeling events at the SMVT promoter 1 locus in Jurkat cells.

Molecular mechanisms involved in the adaptive regulation of human intestinal biotin uptake: A study of the hSMVT system

Biotin, a water-soluble micronutrient, is vital for cellular functions, including growth and development. The human intestine utilizes the human sodium-dependent multivitamin transporter (hSMVT) for biotin uptake. Evidence exists showing that the intestinal biotin uptake process is adaptively regulated during biotin deficiency. Nothing, however, is known about molecular mechanism(s) involved during this adaptive regulation. This study compared two human-derived intestinal epithelial cell lines (HuTu-80 and Caco-2) during biotin-deficient or biotin-sufficient states and with an approach that assessed carrier-mediated biotin uptake, hSMVT protein and RNA levels, RNA stability, and hSMVT promoter activity. The results showed that during biotin deficiency, a significant and specific upregulation in carrier-mediated biotin uptake occurred in both human intestinal epithelial cell lines and that this increase was associated with an induction in protein and mRNA levels of hSMVT. The increase in mRNA levels was not due to an increase in RNA stability but was associated with an increase in activity of the hSMVT promoter in transfected human intestinal cells. Using promoter deletion constructs and mutational analysis in transiently transfected HuTu-80 and Caco-2 cells, a biotin deficiency-responsive region was mapped to a 103-bp area within the hSMVT promoter that contains gut-enriched Kruppel-like factor (GKLF) sites that confer the response to biotin deficiency. These results confirm that human intestinal biotin uptake is adaptively regulated and provide novel evidence demonstrating that the upregulation is not mediated via changes in hSMVT RNA stability but rather is due to transcriptional regulatory mechanism(s) that likely involve GKLF sites in the hSMVT promoter.

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.

Biotin uptake by rabbit corneal epithelial cells: role of sodium-dependent multivitamin transporter (SMVT)

PURPOSE

The objective of this research was to investigate the presence of sodium-dependent multivitamin transporter (SMVT) on rabbit corneal epithelial cells.

METHODS

Primary cultured rabbit corneal epithelial cells (rPCECs)and freshly excised rabbit corneas were used for characterization of biotin uptake and transport, respectively. Reverse transcription-polymerase chain reaction (RT-PCR) was performed to confirm the molecular identity of SMVT. Liquid chromatography/tandem mass spectrometry (LC-MS/MS) analysis was performed to examine the presence of biotin in rabbit tears.

RESULTS

Uptake of biotin by rPCECs was found to be time and concentration dependent with Km of 32.52 microM and Vmax of 10.43 pmol min- 1 mg protein- 1. Biotin was significantly inhibited in the presence of pantothenic acid and lipoic acid. Biotin uptake was found to be energy and Na+ dependent but H+ and Cl- independent. The uptake was inhibited by valeric acid in a concentration-dependent manner but not much affected in the presence of biotin methyl ester and biocytin with no free carboxyl group. Modulators of both PKC- and PKA-mediated pathways had no effect on biotin uptake, but calcium-calmodulin inhibitor significantly inhibited its uptake. Sodium-dependent multivitamin transporter was identified by RT-PCR in rPCECs. Transport experiments across the rabbit corneas revealed the functional localization of SMVT on the apical side of the cornea, and thereby corroborating with in vitro results with cultured corneal cells. Finally, LC-MS/MS analysis showed the presence of biotin in rabbit tears.

CONCLUSIONS

Results obtained from both in vitro and exvivo studies suggest the possible role of SMVT expressed on corneal epithelial cells for the uptake of biotin, which co-transports pantothenic acid and lipoic acid. Further, the presence of biotin in tears suggests the physiological significance of this transporter in rabbit corneal epithelium.

Functional characterization of sodium-dependent multivitamin transporter in MDCK-MDR1 cells and its utilization as a target for drug delivery

The objective of this research is to characterize a sodium-dependent multivitamin transporter (SMVT) in MDCK-MDR1 cells (Madin-Darby canine kidney cells transfected with the human MDR1 gene) and to investigate the feasibility of utilizing the MDCK-MDR1 cell line as an in vitro model to study the permeability of biotin-conjugated prodrugs of anti-HIV protease inhibitors. Mechanism of [3H]biotin uptake and transport was delineated. Transepithelial permeability of the biotin-conjugated prodrug, i.e., biotin-saquinavir, was also studied. Reverse transcription polymerase chain reaction (RT-PCR) was carried out to confirm the existence of SMVT in MDCK-MDR1 cells. Biotin uptake was Na+, pH, and temperature dependent, but energy independent. Uptake of biotin was found to be saturable with a Km of 13.0 microM, Vmax 21.5 of pmol min-1 (mg of protein)-1, and Kd of 0.12 microL min-1 (mg of protein)-1. Both apical and basal uptake and transepithelial transport of [3H]biotin showed that SMVT localized predominantly on the apical membrane of MDCK-MDR1 cells. [3H]Biotin uptake was inhibited by excess unlabeled biotin and its structural analogues, i.e., desthiolbiotin and valeric acid, and other vitamins such as lipoic acid and pantothenic acid, but not by acetic acid, benzoic acid, biotin methyl ester, and biocytin. Biotin-saquinavir caused lowering of [3H]biotin uptake, which indicates that it is recognized by SMVT. Apical to basal transport of [3H]biotin was also significantly inhibited in the presence of excess biotin or biotin-saquinavir. Transepithelial transport studies of biotin-saquinavir in MDCK-MDR1, wild type MDCK, and Caco-2 cells revealed that permeability of biotin-saquinavir was similar in all three cell lines. A band of SMVT mRNA at 862 bp was identified by RT-PCR. A sodium-dependent multivitamin transporter, SMVT, responsible for biotin uptake and transport, was identified and functionally characterized in MDCK-MDR1 cells. Therefore, the MDCK-MDR1 cell line may be utilized as an in vitro model to study the permeability of biotin-conjugated prodrugs such as HIV protease inhibitors.

Biotin uptake and cellular translocation in human derived retinoblastoma cell line (Y-79): A role of hSMVT system

The objective of this research was to investigate the presence of a specialized carrier-mediated system for biotin and delineate uptake mechanism and intracellular trafficking of biotin in the human derived retinoblastoma cell line (Y-79). Human derived retinoblastoma cell line, Y-79, was used for uptake studies. Uptake of [3H]Biotin was determined at various concentrations, pH, temperatures, in the absence of sodium and in the presence of other vitamins and metabolic inhibitors to delineate the mechanism of uptake. Uptake was determined in the presence of various intracellular regulatory pathways (protein kinase A & C, protein tyrosine kinase and calcium-calmodulin) modulators. Reverse transcription polymerase chain reaction (RT-PCR) was performed to confirm the molecular identity of human sodium-dependent multivitamin transporter (hSMVT). Uptake of [3H]Biotin in Y-79 cells were found to be saturable at micromolar concentration range, with apparent Km of 8.53 microM and Vmax of 14.12 pmol/min/mg protein, but linear at nanomolar concentration range. Uptake was sodium, pH, temperature and energy-dependent, but chloride independent; inhibited by the structural analogue desthiobiotin, pantothenic acid and lipoic acid at milimolar concentrations and not at nanomolar concentrations. Uptake of [3H]Biotin was trans-stimulated by the intracellular biotin. Ca2+/calmodulin pathways appeared to play important roles in the regulation of riboflavin uptake in Y-79 cells via significant reduction in Vmax (66%) and Km (28%) of the uptake process. A human sodium-dependant multivitamin transporter, hSMVT, was identified by RT-PCR in Y-79. These studies demonstrated for the first time the existence of a human sodium dependant multivitamin transporter (hSMVT), a specialized carrier-mediated system for biotin uptake, in human derived retinoblastoma cells.

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.

The acute phase response alters cationic amino acid transporter expression in growing chickens (Gallus gallus domesticus)

The effect of an acute phase response (APR) on cationic amino acid transporter (CAT1-3) mRNA expression in liver, muscle, bursa and thymus was determined in broiler strain chickens. The APR was initiated by injecting Salmonella typhimurium lipopolysaccharide subcutaneously (LPS; 1 mg/kg bw). In Experiment 1, CAT1-3 mRNA expression was determined at multiple time points following LPS administration. LPS increased bursa and liver total and high affinity CAT mRNA expression (P<0.05) and transiently increased pectoralis total CAT mRNA expression (P<0.05). Total CAT mRNA expression in the thymus decreased 7.7-fold from 0 to 8 h after LPS injection (P<0.05). In Experiment 2, fasted chicks were uninjected or LPS-injected. LPS increased total and high affinity CAT mRNA 2-fold in both the bursa and liver (P<0.05) and did not change thymus total and high affinity CAT mRNA expression (P>0.05). LPS increased liver weight only (P<0.05) and did not alter the plasma lysine and arginine concentration (P>0.05). In Experiments 3 and 4, thymocyte proliferation and total protein content were dependent upon the media lysine concentration (P<0.001). The inability of the thymus to compete for lysine and arginine during the APR may limit the ability of thymocytes to develop during infections.

Uptake, localization, and noncarboxylase roles of biotin

Evidence is emerging that biotin participates in processes other than classical carboxylation reactions. Specifically, novel roles for biotin in cell signaling, gene expression, and chromatin structure have been identified in recent years. Human cells accumulate biotin by using both the sodium-dependent multivitamin transporter and monocarboxylate transporter 1. These transporters and other biotin-binding proteins partition biotin to compartments involved in biotin signaling: cytoplasm, mitochondria, and nuclei. The activity of cell signals such as biotinyl-AMP, Sp1 and Sp3, nuclear factor (NF)-kappaB, and receptor tyrosine kinases depends on biotin supply. Consistent with a role for biotin and its catabolites in modulating these cell signals, greater than 2000 biotin-dependent genes have been identified in various human tissues. Many biotin-dependent gene products play roles in signal transduction and localize to the cell nucleus, consistent with a role for biotin in cell signaling. Posttranscriptional events related to ribosomal activity and protein folding may further contribute to effects of biotin on gene expression. Finally, research has shown that biotinidase and holocarboxylase synthetase mediate covalent binding of biotin to histones (DNA-binding proteins), affecting chromatin structure; at least seven biotinylation sites have been identified in human histones. Biotinylation of histones appears to play a role in cell proliferation, gene silencing, and the cellular response to DNA repair. Roles for biotin in cell signaling and chromatin structure are consistent with the notion that biotin has a unique significance in cell biology.

Biotin uptake by human proximal tubular epithelial cells: cellular and molecular aspects

Cellular and molecular regulation of renal biotin uptake in humans is not well defined. The contribution of the human Na(+)-dependent multivitamin transporter (hSMVT) to carrier-mediated biotin uptake by human proximal tubular epithelial cells is not clear. The aim of this study was to address these issues, with the human-derived proximal tubular epithelial HK-2 cells used as a model. First, we characterized the mechanism of biotin uptake by these cells and obtained evidence for involvement of an Na(+)-, temperature-, and energy-dependent carrier-mediated uptake system. This system was inhibited by the biotin structural analog desthiobiotin, pantothenic acid, and lipoate. These findings suggest involvement of the hSMVT system in the uptake process. This was confirmed by demonstrating that the hSMVT system is expressed in HK-2 cells at the protein and mRNA levels and by selective silencing of the hSMVT gene with the use of gene-specific small interfering RNAs, which led to specific and significant inhibition of carrier-mediated biotin uptake. Of the two recently cloned promoters of the hSMVT gene, promoter 1 was more active than promoter 2 in these cells. Pretreatment of HK-2 cells with modulators of PKC- and Ca(2+)/calmodulin-mediated pathways (but not those that modulate PKA-, protein tyrosine kinase-, or nitric oxide-mediated pathways) led to significant alterations in biotin uptake. Maintaining the HK-2 cells in a biotin-deficient growth medium led to a marked upregulation in biotin transport, which was associated with an increase in hSMVT protein and RNA levels and an increase in activity of the hSMVT promoters. These results demonstrate that biotin uptake by human renal epithelial cells occurs via the hSMVT system and that the process is regulated by intracellular PKC- and Ca(2+)/calmodulin-mediated pathways. The uptake process appears to be adaptively regulated by extracellular biotin level, which involves transcriptional regulatory mechanism(s).

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.

Biotin uptake by human intestinal and liver epithelial cells: role of the SMVT system

It has been well established that human intestinal and liver epithelial cells transport biotin via an Na+-dependent carrier-mediated mechanism. The sodium-dependent multivitamin transport (SMVT), a biotin transporter, is expressed in both cell types. However, the relative contribution of SMVT toward total carrier-mediated uptake of physiological (nanomolar) concentrations of biotin by these cells is not clear. Addressing this issue is important, especially in light of the recent identification of a second human high-affinity biotin uptake mechanism that operates at the nanomolar range. Hence, we employed a physiological approach of characterizing biotin uptake by human-derived intestinal Caco-2 and HepG2 cells at the nanomolar concentration range. We also employed a molecular biology approach of selectively silencing the endogenous SMVT of these cells with specific small interfering RNAs (siRNAs), then examining carrier-mediated biotin uptake. The results showed that in both Caco-2 and HepG2 cells, the initial rate of biotin uptake as a function of concentration over the range of 0.1 to 50 nM to be linear. Furthermore, we found that the addition of 100 nM unlabeled biotin, desthiobiotin, or pantothenic acid to the incubation medium had no effect on the uptake of 2.6 nM [3H]biotin. Pretreatment of Caco-2 and HepG2 cells with SMVT specific siRNAs substantially reduced SMVT mRNA and protein levels. In addition, carrier-mediated [3H]biotin (2.6 nM) uptake by Caco-2 and HepG2 cells was severely (P 0.01) inhibited by the siRNAs pretreatment. These results demonstrate that the recently described human high-affinity biotin uptake system is not functional in intestinal and liver epithelial cells. In addition, the results provide strong evidence that SMVT is the major (if not the only) biotin uptake system that operates in these cells.