Recent advances in carrier-mediated intestinal absorption of water-soluble vitamins

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.

Renal tubular reabsorption of folate mediated by folate binding protein 1

Renal tubular reabsorption of filtered folate is essential for the conservation and normal homeostasis of this important vitamin. Different molecular mechanisms have been implicated in epithelial folate transport, including folate receptors. Defective expression or antibody inactivation of these is associated with embryonic defects also correlated with low folate intake; however, their contribution to renal tubular folate reabsorption has not been established. With the use of targeted inactivation of the folate binding protein 1 (folbp1) and folate binding protein 2 (folbp2) genes in mice, the role of folate receptors in renal epithelial folate reabsorption was evaluated during low and normal folate intake. Inactivation of folbp1 was associated with (1) loss of (3)H-folic acid binding to crude kidney membranes, (2) increase in renal folate clearance, and (3) increase in urinary excretion and decrease in renal uptake of injected (3)H-methyltetrahydrofolate. No changes in renal folate handling were observed as a result of folbp2 inactivation. Thus, folbp1 is essential for normal renal tubular folate reabsorption, preventing excessive urinary folate loss. Folbp1 is heavily expressed in choroid plexus, yolk sac, and placenta, supporting a role of folbp1 in folate transport in other tissues. The greatest significance of folbp1 for renal folate uptake was observed at conditions of low folate intake, providing a possible explanation for the ability of folate supplementation to prevent developmental defects associated with folbp1 inactivation.

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).

Oral rehydration therapy: new explanations for an old remedy

Diarrheal diseases are among the most devastating illnesses globally, but the introduction of oral rehydration therapy has reduced mortality due to diarrhea from >5 million children, under the age of 5, in 1978 to 1.3 million in 2002. Variations of this simple therapy of salts and sugars are prevalent in traditional remedies in cultures world-wide, but only in the past four decades have the scientific bases for these remedies begun to be elucidated. This review aims to provide a broad understanding of the cellular basis of oral rehydration therapy. The features integral to the success of oral rehydration therapy are active glucose transport in the small intestine, commensal bacteria, and short-chain fatty acid transport in the colon. The review examines these processes and their regulation and considers new approaches that might supplement oral rehydration therapy in controlling diarrheal diseases.

Mechanism and regulation of folate uptake by human pancreatic epithelial MIA PaCa-2 cells

After the liver, the pancreas contains the second highest level of folate among human tissues, and folate deficiency adversely affects its physiological function. Despite that, nothing is currently known about the cellular mechanisms involved in folate uptake by cells of this important exocrine organ or about folate uptake regulation. We have begun to address these issues, and in this report we present the results of our findings on the mechanism of folate uptake by the human-derived pancreatic MIA PaCa-2 cells. Our results show folic acid uptake to be 1). temperature and energy dependent; 2). pH dependent, with a markedly higher uptake at acidic pH compared with neutral or alkaline pH; 3). Na(+) independent; 4). saturable as a function of substrate concentration (apparent K(m) = 0.762 +/- 0.10 microM); 5). inhibited (with similar affinity) by reduced, substituted, and oxidized folate derivatives; and 6). sensitive to the inhibitory effect of anion transport inhibitors. RT-PCR and Western blot analysis showed expression of the human reduced folate carrier (hRFC) at the RNA and protein levels, respectively. The functional contribution of hRFC in carrier-mediated folate uptake was confirmed by gene silencing using gene-specific small interfering RNA. Evidence also was found suggesting that the folate uptake process by MIA PaCa-2 cells is regulated by cAMP- and protein tyrosine kinase (PTK)-mediated pathways. These studies demonstrate for the first time the involvement of a specialized, acidic pH-dependent, carrier-mediated mechanism for folate uptake by human pancreatic MIA PaCa-2 cells. The results also show the involvement of hRFC in the uptake process and suggest the possible involvement of intracellular cAMP- and PTK-mediated pathways in the regulation of folate uptake.