Characterization of a sodium-dependent vitamin transporter mediating the uptake of pantothenate, biotin and lipoate in human placental choriocarcinoma cells

Biotin Enhances Testosterone Production in Mice and Their Testis-Derived Cells

Late-onset hypogonadism, a male age-related syndrome characterized by a decline in testosterone production in the testes, is commonly treated with testosterone replacement therapy, which has adverse side effects. Therefore, an alternative treatment is highly sought. Supplementation of a high dosage of biotin, a water-soluble vitamin that functions as a coenzyme for carboxylases involved in carbohydrate, lipid, and amino acid metabolism, has been shown to influence testis functions. However, the involvement of biotin in testis steroidogenesis has not been well clarified. In this study, we examined the effect of biotin on testosterone levels in mice and testis-derived cells. In mice, intraperitoneal treatment with biotin (1.5 mg/kg body weight) enhanced testosterone levels in the serum and testes, without elevating serum levels of pituitary luteinizing hormone. To investigate the mechanism in which biotin increased the testosterone level, mice testis-derived I-10 cells were used. The cells treated with biotin increased testosterone production in a dose- and time-dependent manner. Biotin treatment elevated intracellular cyclic adenosine monophosphate levels via adenylate cyclase activation, followed by the activation of protein kinase A and testosterone production. These results suggest that biotin may have the potential to improve age-related male syndromes associated with declining testosterone production.

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.

Thyroglobulin in smoking mothers and their newborns at delivery suggests autoregulation of placental iodide transport overcoming thiocyanate inhibition

BACKGROUND

Placental transport of iodide is required for fetal thyroid hormone production. The sodium iodide symporter (NIS) mediates active iodide transport into the thyroid and the lactating mammary gland and is also present in placenta. NIS is competitively inhibited by thiocyanate from maternal smoking, but compensatory autoregulation of iodide transport differs between organs. The extent of autoregulation of placental iodide transport remains to be clarified.

OBJECTIVE

To compare the impact of maternal smoking on thyroglobulin (Tg) levels in maternal serum at delivery and in cord serum as markers of maternal and fetal iodine deficiency.

METHODS

One hundred and forty healthy, pregnant women admitted for delivery and their newborns were studied before the iodine fortification of salt in Denmark. Cotinine in urine and serum classified mothers as smokers (n=50) or nonsmokers (n=90). The pregnant women reported on intake of iodine-containing supplements during pregnancy and Tg in maternal serum at delivery and in cord serum were analyzed.

RESULTS

In a context of mild-to-moderate iodine deficiency, smoking mothers had significantly higher serum Tg than nonsmoking mothers (mean Tg smokers 40.2 vs nonsmokers 24.4 μg/l, P=0.004) and so had their respective newborns (cord Tg 80.2 vs 52.4 μg/l, P=0.006), but the ratio between Tg in cord serum and maternal serum was not significantly different in smokers compared with nonsmokers (smoking 2.06 vs nonsmoking 2.22, P=0.69).

CONCLUSION

Maternal smoking increased the degree of iodine deficiency in parallel in the mother and the fetus, as reflected by increased Tg levels. However, placental iodide transport seemed unaffected despite high thiocyanate levels, suggesting that thiocyanate-insensitive iodide transporters alternative to NIS are active or that NIS in the placenta is autoregulated to keep iodide transport unaltered.

Alpha-lipoic acid supplementation and diabetes

Diabetes is a common metabolic disorder that is usually accompanied by increased production of reactive oxygen species or by impaired antioxidant defenses. Importantly, oxidative stress is particularly relevant to the risk of cardiovascular disease. Alpha-lipoic acid (LA), a naturally occurring dithiol compound, has long been known as an essential cofactor for mitochondrial bioenergetic enzymes. LA is a very important micronutrient with diverse pharmacologic and antioxidant properties. Pharmacologically, LA improves glycemic control and polyneuropathies associated with diabetes mellitus; it also effectively mitigates toxicities associated with heavy metal poisoning. As an antioxidant, LA directly terminates free radicals, chelates transition metal ions, increases cytosolic glutathione and vitamin C levels, and prevents toxicities associated with their loss. These diverse actions suggest that LA acts by multiple mechanisms both physiologically and pharmacologically. Its biosynthesis decreases as people age and is reduced in people with compromised health, thus suggesting a possible therapeutic role for LA in such cases. Reviewed here is the known efficacy of LA with particular reference to types 1 and 2 diabetes. Particular attention is paid to the potential benefits of LA with respect to glycemic control, improved insulin sensitivity, oxidative stress, and neuropathy in diabetic patients. It appears that the major benefit of LA supplementation is in patients with diabetic neuropathy.

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.

Sodium-dependent multivitamin transporter gene is regulated at the chromatin level by histone biotinylation in human Jurkat lymphoblastoma cells

The sodium-dependent multivitamin transporter (SMVT) is essential for mediating and regulating biotin entry into mammalian cells. In cells, holocarboxylase synthetase (HCS) mediates covalent binding of biotin to histones; biotinylation of lysine-12 in histone H4 (K12BioH4) causes gene repression. Here we propose a novel role for HCS in sensing and regulating levels of biotin in eukaryotic cells. We hypothesize that nuclear translocation of HCS increases in response to biotin supplementation; HCS then biotinylates histone H4 at SMVT promoters, silencing biotin transporter genes. We show that nuclear translocation of HCS is a biotin-dependent process that might involve tyrosine kinases, histone deacetylases, and histone methyltransferases in human lymphoid (Jurkat) cells. The nuclear translocation of HCS correlated with biotin concentrations in cell culture media; the relative enrichment of both HCS and K12BioH4 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 K12BioH4 at the SMVT promoter was inversely linked to SMVT expression. 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 (K12BioH4 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.

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.

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.

The blood-brain barrier sodium-dependent multivitamin transporter: a molecular functional in vitro-in situ correlation

The molecular mechanism of biotin brain uptake was investigated using an in vitro bovine blood-brain barrier (BBB) cell model and an in situ mouse brain perfusion technique. A functional uptake/transport correlation of the in vitro and in situ characteristics of biotin uptake was investigated. Morphological and immunochemical characteristics (e.g., factor VIII expression) of the primary culture of brain microvessel endothelial cells (BMECs) were confirmed. Gene expression of the multidrug resistance (Mdr1) and sodium-dependent multivitamin (SMVT) transporters was also determined in BMECs. Biotin transport was saturable and Na(+)-dependent at the luminal side of the BBB. The estimated half-saturation concentrations (K(m)) of biotin uptake in vitro and in situ were 49.1 and 35.5 microM, respectively, supporting the presence of a carrier-mediated biotin transport system. Inhibition studies using various biotin derivatives and structural analogs demonstrated the structural requirements for biotin-SMVT interaction. Desthiobiotin and pantothenic acid significantly inhibited the uptake of biotin, whereas 2-iminobiotin and diaminobiotin were very weak inhibitors. Based on our results, there was a good correlation between the in vitro and in situ BBB models, suggesting that when a single membrane transporter is involved in substrate uptake, flexibility in choosing the experimental model can be afforded. The current results are also consistent with the suggestion that the properties of the BBB are likely to be organ-specific rather than species-specific. Further mechanistic and comparative studies are needed to validate these results. In conclusion, the in vitro transporter-based mechanism studies produced valuable molecular functional transport results that correlated well with in situ results.

Biotin biochemistry and human requirements

Human biotin turnover and requirements can be estimated on the basis of (1) concentrations of biotin and metabolites in body fluids, (2) activities of biotin-dependent carboxylases, and (3) the urinary excretion of organic acids that are formed at increased rates if carboxylase activities are reduced. Recent studies suggest that the urinary excretions of biotin and its metabolite bisnorbiotin, activities of propionyl-CoA carboxylase and beta-methylcrotonyl-CoA carboxylase in lymphocytes, and urinary excretion of 3-hydroxyisovaleric acid are good indicators of marginal biotin deficiency. On the basis of studies using these indicators of biotin deficiency, an adequate intake of 30 microg (123 nmoles) of biotin per day is currently recommended for adults. The dietary biotin intake in Western populations has been estimated to be 35 to 70 microg/d (143-287 nmol/d). Recent studies suggest that humans absorb biotin nearly completely. Conditions that may increase biotin requirements in humans include pregnancy, lactation, and therapy with anticonvulsants or lipoic acid.

Regulation of gene expression by biotin (review)

In mammals, biotin serves as coenzyme for four carboxylases, which play essential roles in the metabolism of glucose, amino acids, and fatty acids. Biotin deficiency causes decreased rates of cell proliferation, impaired immune function, and abnormal fetal development. Evidence is accumulating that biotin also plays an important role in regulating gene expression, mediating some of the effects of biotin in cell biology and fetal development. DNA microarray studies and other gene expression studies have suggested that biotin affects transcription of genes encoding cytokines and their receptors, oncogenes, genes involved in glucose metabolism, and genes that play a role in cellular biotin homeostasis. In addition, evidence has been provided that biotin affects expression of the asialoglycoprotein receptor and propionyl-CoA carboxylase at the post-transcriptional level. Various pathways have been identified by which biotin might affect gene expression: activation of soluble guanylate cyclase by biotinyl-AMP, nuclear translocation of NF-kappaB (in response to biotin deficiency), and remodeling of chromatin by biotinylation of histones. Some biotin metabolites that cannot serve as coenzymes for carboxylases can mimic biotin with regard to its effects on gene expression. This observation suggests that biotin metabolites that have been considered "metabolic waste" in previous studies might have biotin-like activities. These new insights into biotin-dependent gene expression are likely to lead to a better understanding of roles for biotin in cell biology and fetal development.

Monocarboxylate transporter 1 mediates biotin uptake in human peripheral blood mononuclear cells

Here the hypothesis was tested that monocarboxylate transporters (MCT) mediate biotin transport in human lymphoid cells. Uptake of [(3)H]biotin was measured in human lymphoid cells [peripheral blood mononuclear cells (PBMC) and Jurkat cells] under conditions known to affect MCT-mediated transport. When biotin uptake into PBMC was measured in the presence of excess concentrations of competitors (MCT substrates) and MCT inhibitors, transport rates decreased significantly to <75 and <67%, respectively, of controls. Biotin uptake correlated with the concentration of protons in culture media, consistent with cotransport of protons and the carboxylate biotin by MCT. Efflux of biotin from PBMC was stimulated by extracellular lactate (a known substrate for MCT), consistent with countertransport of the two substrates by the same transporter. PBMC responded to proliferation with parallel increases of transport rates for both biotin and lactate, providing circumstantial evidence that the same transporter mediates uptake of the two substrates in PBMC. Transfection of Jurkat cells with an expression vector encoding MCT1 caused a 50% increase in biotin uptake; in contrast, overexpression of MCT1 did not affect biotin uptake in various nonlymphoid cell lines. These findings are consistent with the hypothesis that MCT mediate biotin uptake in human lymphoid cells.

Localization of biotinidase in the brain: implications for its role in hearing loss in biotinidase deficiency

Biotinidase deficiency is an autosomal recessively inherited disorder characterized by neurological and cutaneous features, including sensorineural hearing loss. Although many of the features of the disorder are reversible following treatment with biotin, the hearing loss appears to be irreversible. To better characterize the nature of the hearing loss in this disorder, location of the expression and presence of biotinidase within the brain was examined using Northern blot analysis, in vitro hybridization of a cDNA panel, and immunohistochemical staining. Results indicate low, but detectable expression of biotinidase throughout the brain, but increased concentrations of biotinidase within the dorsal cochlear nucleus, ventral cochlear nucleus, and superior olivary complex of the brainstem, as well as, in the hair cells and spiral ganglion of the cochlea. These findings suggest that biotinidase and possibly biotin plays an important role in hearing.

Biotin dependency due to a defect in biotin transport

We describe a 3-year-old boy with biotin dependency not caused by biotinidase, holocarboxylase synthetase, or nutritional biotin deficiency. We sought to define the mechanism of his biotin dependency. The child became acutely encephalopathic at age 18 months. Urinary organic acids indicated deficiency of several biotin-dependent carboxylases. Symptoms improved rapidly following biotin supplementation. Serum biotinidase activity and Biotinidase gene sequence were normal. Activities of biotin-dependent carboxylases in PBMCs and cultured skin fibroblasts were normal, excluding biotin holocarboxylase synthetase deficiency. Despite extracellular biotin sufficiency, biotin withdrawal caused recurrent abnormal organic aciduria, indicating intracellular biotin deficiency. Biotin uptake rates into fresh PBMCs from the child and into his PBMCs transformed with Epstein Barr virus were about 10% of normal fresh and transformed control cells, respectively. For fresh and transformed PBMCs from his parents, biotin uptake rates were consistent with heterozygosity for an autosomal recessive genetic defect. Increased biotin breakdown was ruled out, as were artifacts of biotin supplementation and generalized defects in membrane permeability for biotin. These results provide evidence for a novel genetic defect in biotin transport. This child is the first known with this defect, which should now be included in the identified causes of biotin dependency.

Targeted PEG-based bioconjugates enhance the cellular uptake and transport of a HIV-1 TAT nonapeptide

We previously described the enhanced cell uptake and transport of R.I-K(biotin)-Tat9, a large ( approximately 1500 Da) peptidic inhibitor of HIV-1 Tat protein, via SMVT, the intestinal biotin transporter. The aim of the present study was to investigate the feasibility of targeting biotinylated PEG-based conjugates to SMVT in order to enhance cell uptake and transport of Tat9. The 29 kDa peptide-loaded bioconjugate (PEG:(R.I-Cys-K(biotin)-Tat9)8) used in these studies contained eight copies of R.I-K(biotin)-Tat9 appended to PEG by means of a cysteine linkage. The absorptive transport of biotin-PEG-3400 (0.6-100 microM) and the bioconjugate (0.1-30 microM) was studied using Caco-2 cell monolayers. Inhibition of biotin-PEG-3400 by positive controls (biotin, biocytin, and desthiobiotin) was also determined. Uptake of these two compounds was also determined in CHO cells transfected with human SMVT (CHO/hSMVT) and control cells (CHO/pSPORT) over the concentration ranges of 0.05-12.5 microM and 0.003-30 microM, respectively. Nonbiotinylated forms of these two compounds, PEG-3350 and PEG:(R.I-Cys-K-Tat9)8, were used in the control studies. Biotin-PEG-3400 transport was found to be concentration-dependent and saturable in Caco-2 cells (K(m)=6.61 microM) and CHO/hSMVT cells (K(m)=1.26 microM). Transport/uptake was significantly inhibited by positive control substrates of SMVT. PEG:(R.I-Cys-K(biotin)Tat9)8 also showed saturable transport kinetics in Caco-2 cells (K(m)=6.13 microM) and CHO/hSMVT cells (K(m)=8.19 microM). Maximal uptake in molar equivalents of R.I-Cys-K(biotin)Tat9 was 5.7 times greater using the conjugate versus the biotinylated peptide alone. Transport of the nonbiotinylated forms was significantly lower (P<0.001) in all cases. The present results demonstrate that biotin-PEG-3400 and PEG:(R.I-Cys-K(biotin)Tat9)8 interact with human SMVT to enhance the cellular uptake and transport of these larger molecules and that targeted bioconjugates may have potential for enhancing the cellular uptake and transport of small peptide therapeutic agents.

Proliferation of peripheral blood mononuclear cells causes increased expression of the sodium-dependent multivitamin transporter gene and increased uptake of pantothenic acidopen star

Antigenic or mitogenic stimulation of peripheral blood mononuclear cells (PBMC) causes rapid cell proliferation. PBMC proliferation is associated with increased activities of pantothenic acid-dependent metabolic pathways, suggesting increased demand for pantothenic acid. We sought to determine whether PBMC respond to proliferation by increased cellular uptake of pantothenic acid and, if so, by what mechanism(s) the increased uptake is mediated. Uptake of pantothenic acid into PBMC was mediated by the sodium-dependent multivitamin transporter, SMVT, as judged by sodium dependency of uptake, substrate affinity and specificity, and RT-PCR of PBMC RNA. Proliferating PBMC accumulated two times more [3H]pantothenic acid than quiescent PBMC. Rates of [3H]pantothenic acid uptake paralleled rates of PBMC proliferation, as judged by uptake of [3H]thymidine. The increased uptake of [3H]pantothenic acid into proliferating PBMC was mediated by increased expression of SMVT (as judged by RT-PCR using total RNA from PBMC), leading to an increased number of transporters on the cell surface (as judged by maximal transport rates for pantothenic acid). We conclude that proliferating PBMC increase expression of the gene encoding SMVT to increase uptake of pantothenic acid.

Targeting the sodium-dependent multivitamin transporter (SMVT) for improving the oral absorption properties of a retro-inverso Tat nonapeptide

PURPOSE

To investigate the potential for delivering large peptides orally by altering their absorptive transport pathways and improving intestinal permeability. The absorptive transport of retro-inverso (R.I.-) K-Tat9 and R.I.-K(biotin)-Tat9, novel peptidic inhibitors of the Tat protein of HIV-1, and their interactions with human SMVT (hSMVT), a high affinity, low capacity transporter, were investigated using Caco-2 and transfected CHO cells.

METHODS

Following synthesis on a PAL resin using Fmoc chemistry, the transport of R.I.-K-Tat9 (0.01-25 microM) and R.I.-K(biotin)-Tat9 (0.1-25 microM) was evaluated across Caco-2 cells. The transport and kinetics of biotin, biocytin and desthiobiotin (positive controls for SMVT) were also determined. Uptake of R.I.-K-Tat9 and R.I.K(biotin)-Tat9 (both 0.1-10 microM) was determined in CHO/hSMVT and CHO/pSPORT (control) cells.

RESULTS

The absorptive transport of R.I.-K-Tat9 was passive, low (Pm approximately 1 x 10(-6) cm/sec) and not concentration dependent. R.I.K(biotin)-Tat9 permeability was 3.2-fold higher than R.I.-K-Tat9 demonstrating active (Ea = 9.1 kcal/mole), concentration dependent and saturable transport (Km = 3.3 microM). R.I.-K(biotin)-Tat9 uptake in CHO/hSMVT cells (Km = 1.0 microM) was - 500-fold greater than R.I.-K-Tat9 (at 10 microM). R.I.-K(biotin)-Tat9 transport in Caco-2 and CHO/hSMVT cells was significantly inhibited by known substrates of SMVT including biotin, biocytin, and desthiobiotin. Passive uptake of R.I.-K(biotin)-Tat9 was significantly greater than R.I.-K-Tat9 uptake in CHO/pSPORT cells.

CONCLUSIONS

These results demonstrate that the structural modification of R.I.-K-Tat9 to R.I.-K(biotin)-Tat9 altered its intestinal transport pathway resulting in a significant improvement in its absorptive permeability by enhancing nonspecific passive and carrier-mediated uptake by means of SMVT. The specific interactions between R.I.-K(biotin)-Tat9 and SMVT suggest that targeting approaches utilizing transporters such as SMVT may substantially improve the oral delivery of large peptides.

In vivo biotin supplementation at a pharmacologic dose decreases proliferation rates of human peripheral blood mononuclear cells and cytokine release

Theoretically, vitamin supplements may either enhance or reduce protein synthesis and proliferation in peripheral blood mononuclear cells (PBMC). In the present study, we determined whether administration of a pharmacologic dose of biotin affects proliferation rates of PBMC and cytokine release. Healthy adults (n = 5) ingested 3.1 micromol biotin/d for 14 d; blood and urine were collected pre- and postsupplementation. PBMC were isolated by density gradient and incubated with the mitogen concanavalin A for up to 3 d. At timed intervals during mitogen stimulation, we measured the following: 1) cellular uptake of [(3)H]thymidine to determine proliferation rates; 2) concentrations of various cytokines released into the medium; and 3) the percentages of PBMC subsets as judged by CD surface markers. Biotin supplementation caused a significant decrease of PBMC proliferation. At 2 d after mitogen stimulation, [(3)H]thymidine uptake by postsupplementation PBMC was 66 +/- 21% of the uptake by presupplementation PBMC (P < 0.05). Similarly, concentrations of interleukin-1beta (2 d after mitogen) and interleukin-2 (1 d after mitogen) in media from postsupplementation PBMC were 65 +/- 28% and 44 +/- 23%, respectively, of those for presupplementation PBMC (P < 0.01). Percentages of PBMC subsets were not affected by 14 d of biotin supplementation. Overall, this study provides evidence that administration of pharmacologic doses of biotin for 14 d decreases PBMC proliferation and synthesis of interleukin-1beta and interleukin-2.

Mechanisms for the transport of unconjugated bilirubin in human trophoblastic BeWo cells

To evaluate mechanisms that mediate passage of unconjugated bilirubin (UCB) across placenta, the transport of [3H]UCB was studied in the human trophoblastic, BeWo cell line. When plotted against the unbound UCB concentration [Bf], uptake exhibited saturative kinetics with a similar apparent Km ( approximately 30 nM) for BeWo cells grown either in polarized (Transwell) or non-polarized fashion (dish). UCB release from cells, but not uptake, was inhibited by sulfobromophthalein but not by taurocholate, and almost abolished by MK571, a specific inhibitor of the activity of multidrug resistance-associated proteins (MRPs). MRP1 and MRP5 were both present in BeWo cells and the expression of MRP1, but not MRP5, was markedly higher in polarized cells. These data indicate that UCB is taken up from the fetal circulation by a still undefined, saturative process not shared by other organic anions and is then excreted to maternal circulation by proteins of the MRP family.

The clearance and metabolism of biotin administered intravenously to pigs in tracer and physiologic amounts is much more rapid than previously appreciated

Understanding of biotin pharmacokinetics and regulation of metabolism is essential for the determination of the biotin requirement for humans. Using Landrace-Cambrough pigs as a model, we initially demonstrated that biotin binding to protein accounts for only a small percentage of the total biotin in plasma. A physiologic amount of [14C]biotin was administered intravenously to three pigs; nine blood samples were collected over 48 h. Plasma concentrations of 14C-labeled metabolites were negligible for the first 2 h after biotin infusion. Disappearance curves of total 14C and of [14C]biotin were similar; both fit a triexponential function consistent with a three-compartment, open model. To characterize the rapid early phase of disappearance more precisely, a physiologic amount of [14C]biotin was administered intravenously to five pigs; eight blood samples were collected over the first hour and 16 total samples over 48 h. Again a triexponential function provided an excellent fit. The mean half-life values (+/- 1 SD) for the three phases were 0.11 +/- 0.07, 1.43 +/- 0.42 and 22 +/- 4 h. The [14C]biotin accumulated primarily in the liver, kidney and muscle. When administered intravenously at tracer doses to three pigs, [3H]biotin exhibited similar early pharmacokinetics; however, substantial quantities of a 3H-labeled metabolite appeared after 1 h. These studies provide evidence that egress of biotin from plasma is more rapid than previously appreciated. The slower second and third phases may represent transport into the cytosol, biotransformation into intermediates and covalent binding to intracellular proteins. Similar pharmacokinetics are likely to be seen in humans.

Structure and function of mammalian sodium-dependent multivitamin transporter

This review focuses on the advances made in recent years in the understanding of the multivitamin transporter, a unique and important transporter that transports not one but three different unrelated water-soluble vitamins namely, pantothenate, biotin and lipoate. The transport process mediated by the transporter is active and is energized by the transmembrane sodium ion gradient as well as the membrane potential. The transporter belongs to the sodium-coupled glucose transporter family. The ubiquitous expression of this transporter in mammalian tissues and the fact that it is highly conserved across the species indicate the nutritional relevance and importance of this transporter.

Electrogenic nature of rat sodium-dependent multivitamin transport

We report on the electrogenic nature of the transport process mediated by the rat sodium-dependent multivitamin transporter. In Cos-7 cells, the relationship of Na(+) concentration versus biotin and pantothenate uptake rate was sigmoidal with a Na(+):substrate stoichiometry of 2:1. In Cos-7 cells expressing rat SMVT biotin transport was significantly higher when the membrane was hyperpolarized and considerably reduced when the membrane was depolarized. Similarly, biotin uptake in X. laevis oocytes expressing rat SMVT was inhibited with depolarized oocyte membrane by altering the K(+) permeability across the membrane. It is concluded that the transport of biotin and pantothenate mediated by rat SMVT is electrogenic with a Na(+):substrate coupling ratio of 2:1 and that the transport process is associated with the transfer of one net positive charge across the membrane per transport cycle.

Molecular mechanism of the intestinal biotin transport process

Previous studies have characterized different aspects of the cellular/membrane mechanism and regulation of the intestinal uptake process of the water-soluble vitamin biotin. Little, however, is known about the molecular mechanisms of the uptake process. In this study, we have identified a cDNA from rat small intestine that appears to be involved in biotin transport. The open reading frame of this cloned cDNA consisted of 1,905 bases and was identical to that identified for the vitamin transporter in placental tissue. Significant heterogeneity, however, was found in the 5' untranslated region of this clone, with three distinct variants (II, III, IV) being identified in the small intestine; the placental variant (variant I), however, was not present in the small gut. Variant II was found to be the predominant form expressed in the rat small and large intestines. Functional identity of the cloned intestinal cDNA was confirmed by stable expression in COS-7 cells, which showed a four- to fivefold increase in biotin uptake in transfected COS-7 cells compared with controls. The induced biotin uptake in transfected COS-7 cells was found to be 1) Na(+) dependent, 2) saturable as a function of concentration with an apparent K(m) of 8. 77 microM and a V(max) of 779.7 pmol. mg protein(-1). 3 min(-1), and 3) inhibited by unlabeled biotin and pantothenic acid and their structural analogs. The distribution of complementary mRNA transcripts of the cloned cDNA along the vertical and longitudinal axes of the intestinal tract was also determined. Results of this study describe the molecular characteristics of the intestinal biotin absorption process and report the identification of a cDNA that encodes a Na(+)-dependent biotin uptake carrier that appears to exist in the form of multiple variants.

Identification of the plasma membrane H+-biotin symporter of Saccharomyces cerevisiae by rescue of a fatty acid-auxotrophic mutant

Bakers' yeast is auxotrophic for biotin (vitamin H) and depends on the efficient uptake of this compound from the environment. A mutant strain with strongly reduced biotin uptake and with reduced levels of protein biotinylation was identified. The strain was auxotrophic for long-chain fatty acids, and this auxotrophy could be suppressed with high levels of biotin in the medium. After transformation of this mutant with a yeast genomic library, the unassigned open reading frame YGR065C was identified to complement this mutation. This gene codes for a protein with 593 amino acids and 12 putative transmembrane helices. Northern blot analysis revealed that, in wild-type cells, the corresponding mRNA levels were increased at low biotin concentrations. Likewise, cellular biotin uptake was increased with decreasing biotin availability. Expression of YGR065C under the control of the constitutive ADH1 promoter resulted in very high biotin transport rates across the plasma membrane that were no longer regulated by the biotin concentration in the growth medium. We conclude that YGR065C encodes the first biotin transporter identified for a non-mammalian organism and designate this gene VHT1 for vitamin H transporter 1.

Molecular and functional characterization of the intestinal Na+-dependent multivitamin transporter

We have cloned a Na+-dependent multivitamin transporter from rabbit intestine (riSMVT). The cDNA codes for a protein of 636 amino acids with 12 putative transmembrane domains. When expressed in mammalian cells, the cDNA induces Na+-dependent uptake of the vitamins pantothenate and biotin. Lipoate is also a substrate for the cDNA-induced uptake process. The affinity constant for the cDNA-specific transport of pantothenate and biotin is approximately 2 and approximately 8 microM, respectively. The Na+:vitamin stoichiometry is greater than 1, indicating that the transport process is electrogenic. The SMVT-specific transcripts of 3.2 kbp are equally distributed throughout the small intestine. We have also cloned SMVT from the human intestinal cell line Caco-2. The Caco-2 SMVT cDNA codes for a protein of 635 amino acids which is homologous to riSMVT and is identical to the SMVT expressed in the human choriocarcinoma cell line JAR. Caco-2 SMVT also catalyzes Na+-dependent uptake of pantothenate, biotin, and lipoate. In oocytes expressing Caco-2 SMVT, all three vitamins evoke inward currents, confirming the electrogenicity of the transport process.

Human placental Na+-dependent multivitamin transporter. Cloning, functional expression, gene structure, and chromosomal localization

We have cloned the human Na+-dependent multivitamin transporter (SMVT), which transports the water-soluble vitamins pantothenate, biotin, and lipoate, from a placental choriocarcinoma cell line (JAR). The cDNA codes for a protein of 635 amino acids with 12 transmembrane domains and 4 putative sites for N-linked glycosylation. The human SMVT exhibits a high degree of homology (84% identity and 89% similarity) to the rat counterpart. When expressed in HRPE cells, the cDNA-induced transport process is obligatorily dependent on Na+ and accepts pantothenate, biotin, and lipoate as substrates. The relationship between the cDNA-specific uptake rate of pantothenate or biotin and Na+ concentration is sigmoidal with a Na+:vitamin stoichiometry of 2:1. The human SMVT, when expressed in Xenopus laevis oocytes, induces inward currents in the presence of pantothenate, biotin, and lipoate in a Na+-, concentration-, and potential-dependent manner. We also report here on the structural organization and chromosomal localization of the human SMVT gene. The SMVT gene is approximately 14 kilobase pairs in length and consists of 17 exons. The SMVT gene is located on chromosome 2p23 as evidenced by somatic cell hybrid analysis and fluorescence in situ hybridization.

Biotin uptake by human colonic epithelial NCM460 cells: a carrier-mediated process shared with pantothenic acid

Previous studies showed that the normal microflora of the large intestine synthesizes biotin and that the colon is capable of absorbing intraluminally introduced free biotin. Nothing, however, is known about the mechanism of biotin absorption in the large intestine and its regulation. To address these issues, we used the human-derived, nontransformed colonic epithelial cell line NCM460. The initial rate of biotin uptake was found to be 1) temperature and energy dependent, 2) Na+ dependent (coupling ratio of 1:1), 3) saturable as a function of concentration [apparent Michaelis constant (Km) of 19.7 microM], 4) inhibited by structural analogs with a free carboxyl group at the valeric acid moiety, and 5) competitively inhibited by the vitamin pantothenic acid (inhibition constant of 14.4 microM). Pretreatment with the protein kinase C (PKC) activators phorbol 12-myristate 13-acetate (PMA) and 1, 2-dioctanoyl-sn-glycerol significantly inhibited biotin uptake. In contrast, pretreatment with the PKC inhibitors staurosporine and chelerythrine led to a slight, but significant, increase in biotin uptake. The effect of PMA was mediated via a marked decrease in maximal uptake velocity and a slight increase in apparent Km. Pretreatment of cells with modulators of the protein kinase A-mediated pathway, on the other hand, showed no significant effect on biotin uptake. These results demonstrate, for the first time, the functional existence of a Na+-dependent, specialized carrier-mediated system for biotin uptake in colonic epithelial cells. This system is shared with pantothenic acid and appears to be under the regulation of an intracellular PKC-mediated pathway.

Cloning and functional expression of a cDNA encoding a mammalian sodium-dependent vitamin transporter mediating the uptake of pantothenate, biotin, and lipoate

Previous studies have shown that a Na+-dependent transport system is responsible for the transplacental transfer of the vitamins pantothenate and biotin and the essential metabolite lipoate. We now report the isolation of a rat placental cDNA encoding a transport protein responsible for this function. The cloned cDNA, when expressed in HeLa cells, induces Na+-dependent pantothenate and biotin transport activities. The transporter is specific for pantothenate, biotin, and lipoate. The Michaelis-Menten constant (Kt) for the transport of pantothenate and biotin in cDNA-transfected cells is 4.9 +/- 1.1 and 15.1 +/- 1.2 microM, respectively. The transport of both vitamins in cDNA-transfected cells is inhibited by lipoate with an inhibition constant (Ki) of approximately 5 microM. The nucleotide sequence of the cDNA (sodium-dependent multivitamin transporter (SMVT)) predicts a protein of 68.6 kDa with 634 amino acids and 12 potential transmembrane domains. Protein data base search indicates significant sequence similarity between SMVT and known members of the Na+-dependent glucose transporter family. Northern blot analysis shows that SMVT transcripts are present in all of the tissues that were tested. The size of the principal transcript is 3.2 kilobases. SMVT represents the first Na+-dependent vitamin transporter to be cloned from a mammalian tissue.