Cloning of the Human Thiamine Transporter, a Member of the Folate Transporter Family

Abstract

We have isolated a cDNA from human placenta, which, when expressed heterologously in mammalian cells, mediates the transport of the water-soluble vitamin thiamine. The cDNA codes for a protein of 497 amino acids containing 12 putative transmembrane domains. Northern blot analysis indicates that this transporter is widely expressed in human tissues. When expressed in HeLa cells, the cDNA induces the transport of thiamine (K t ‫؍‬ 2.5 ؎ 0.6 M) in a Na ؉ -independent manner. The cDNA-mediated transport of thiamine is stimulated by an outwardly directed H ؉ gradient. Substrate specificity assays indicate that the transporter is specific to thiamine. Even though thiamine is an organic cation, the cDNA-induced thiamine transport is not inhibited by other organic cations. Similarly, thiamine is not a substrate for the known members of mammalian organic cation transporter family. The thiamine transporter gene, located on human chromosome 1q24, consists of 6 exons and is most likely the gene defective in the metabolic disorder, thiamine-responsive megaloblastic anemia. At the level of amino acid sequence, the thiamine transporter is most closely related to the reduced-folate transporter and thus represents the second member of the folate transporter family.

Cell Biology of the Human Thiamine Transporter-1 (hTHTR1)

“…3 H]thiamine uptake in cells expressing hTHTR1-EGFP (ϳ1.5-fold uptake) were similar to those reported elsewhere on expression of hTHTR1 cDNA (2,14). Thus, the functionality of the hTHTR1-EGFP fusion protein was not impaired by COOHterminal fusion to GFP, and the fusion protein correctly targeted to the cell surface.…”

“…For example, thiamine pyrophosphate (the coenzyme form) is important for normal carbohydrate metabolism and energy production (1,2). Thiamine is not synthesized by humans and other mammals and is obtained from dietary sources via absorption into intestinal epithelia.…”

“…12 and 13). The full-length hTHTR1 protein encodes a 497-amino acid polypeptide with 12 predicted transmembrane-spanning segments and cytoplasmic NH 2 -and COOH-terminal regions (2,8). Several clinically identified mutations have been characterized in hTHTR1 (2,(7)(8)(9)(10)14), including ten prematurely truncated mutants, resulting from either point mutation, deletion, or insertion of nucleotides (reviewed in Refs.…”

“…3) as the site of mutations that link with TRMA inheritance (2,(7)(8)(9)(10). SLC19A2 encodes a saturable, high affinity human thiamine transporter known as hTHTR1 (2,(7)(8)(9)(10), which is expressed in many tissues, including duodenal epithelium (11). The functional properties of hTHTR1 mimic the biochemical characteristics of thiamine uptake in intestinal preparations (reviewed in Refs.…”

High-dose thiamine therapy counters dyslipidaemia in streptozotocin-induced diabetic rats

“…In the latter study, thiamine metabolites were analysed and a selective decline in TPP concentration found in the liver and heart. Thiamine enters hepatocytes mainly by the high-affinity thiamine transporter [39,40] and TMP enters cells by the reduced folate transporter [41]. In the cytosol, TMP is hydrolysed rapidly to thiamine, and thiamine is phosphorylated to TPP by thiamine pyrophosphokinase (TPPK) [41].…”