THTR1 and THTR2 (Thiamine transporters 1 and 2)
Aliases: TC1 (THTR1); None (THTR2)
Gene name: Solute carrier family 19 member 2/3 (SLC19A2/3)
The human thiamine transporters 1 and 2 (THTR1 and THTR2) are high affinity transporters involved in the cellular accumulation of thiamine (vitamin B1). The concentrative transport mediated by these transporters is energized by a proton gradient. Both THTR1 and THTR2 are widely distributed and are ubiquitously expressed both intracellularly and on the cell surface of various human tissues, such as small and large intestine, kidney, liver, muscle, brain and placenta. THTR2 is the main absorptive transporter for thiamine in the human intestine. Impaired functionality of THTR1 and THTR2 transporters results in various thiamine deficiency disorders. Mutations in the human SLC19A2 gene have been reported to be responsible for the thiamine-responsive megaloblastic anemia (TRMA or Rogers syndrome), whereas defects in THTR2 may cause biotin-responsive basal ganglia disease (BBGD). THTR2 was implicated in the withdrawal of the Janus Kinase (JAK) 2 inhibitor fedratinib from a late-stage clinical trial. Fedratinib inhibited THTR2 activity which led to the development of Wernicke’s encephalopathy. Although THTR1 and THTR2 are both considered as targets for drug–drug and vitamin–drug interactions, their investigation is not specifically recommended in the current FDA or EMA guidelines.
Both THTR1 and THTR2 have 12 transmembrane domains and two N-glycosylation sites. They are expressed in many organs and important physiologic barriers including the intestine, kidney proximal tubule and blood–brain barrier (BBB) [1, 2]. THTR2 is primarily expressed in the kidney, with highest expression on the apical membrane of renal proximal tubule epithelial cells, and on the apical face of the intestinal epithelium, with the highest levels in the proximal half of the human small intestine. In comparison, THTR1 is predominantly located on the basolateral (blood-side) membrane of proximal tubule and intestinal cells. THTR2 is present on the apical (blood) side of the blood-brain barrier (BBB) whereas THTR1 is found on the basolateral side .
Function, physiology, and clinically significant polymorphisms
THTR1 and THTR2 are high-affinity/low-capacity thiamine transporters that depend on a transcellular proton gradient to transport thiamine from the intestine and urine into the cell. Once inside the cells, thiamine can be phosphorylated and transported into the blood for distribution to various tissues . The main phosphorylated thiamine derivative is thiamine diphosphate (ThDP), an indispensable cofactor for enzymes involved in oxidative energy metabolism. Other, less abundant thiamine forms include thiamine monophosphate (ThMP), thiamine triphosphate (ThTP) and adenosine thiamine triphosphate (AThTP) .
THTR1 and THTR2, in conjunction with FOLT (Reduced folate transporter 1), belong to the solute carrier family 19 (SLC19) of vitamin transporters. The SLC19 family members transport the B group vitamins, such as folic acid (FOLT/SLC19A1) and thiamine (THTR1/SLC19A2 and THTR2/SLC19A3), across the cell membrane using differences in pH as a driving force [4, 6]. The uptake of thiamine into the cell, as tested in rat intestinal brush border membrane vesicles in the absence of a H+ gradient, is Na+ and biotransformation-independent, completely inhibited by thiamine analogues, and reduced by alcohol consumption and aging . Although FOLT is also capable of transporting thiamine monophosphate, THTR1 and THTR2 are only capable of transporting thiamine and have no folate transport activity .
THTR1/2 have been considered as highly selective for thiamine until recent studies showed interactions with commonly prescribed drugs, such as the biguanide metformin, a first-line therapeutic drug used to treat type 2 diabetes. THTR1 and THTR2 share common substrates and inhibitors. The classical clinical inhibitor of THTR2 is the Janus kinase (JAK2) inhibitor fedratinib . Other inhibitors include trimethoprim (THTR1/2), amprolium (THTR1/THTR2) and metformin (THTR2 only; also a substrate) [10, 11]. A transfected cell-based in vitro assessment of thiamine transport by THTR2 showed that it can also be inhibited by quinapril, amsacrine, sertraline, amoxapine, chlorhexidine, centrimonium, phenformin, chloroquine, verapamil, famotidine and amitriptyline [10, 12].
Studies in mice have revealed that thiamine uptake by mouse pancreatic acinar cells is a carrier-mediated process that is significantly inhibited in Slc19a2 and Slc19a3 knockout mouse models . Slc19a2(-/-) mice developed diabetes mellitus, associated with thiamine-responsive megaloblastic anemia syndrome (TRMA), with an enhanced response to insulin and reduced insulin secretion. In the glucose tolerance test, fasting blood glucose level was significantly increased in Slc19a2(−/−) mice compared to the wild-type. When insulin secretion test and insulin tolerance test were performed on mice fed a thiamine-deficient diet, insulin secretion was significantly impaired and hypoglycemic response was prolonged in Slc19a2(−/−) mice compared to the wild type. Problems with insulin secretion and tolerance are known to lead to diabetes mellitus . Mutations in the SLC19A2 gene were also shown to cause TRMA in humans . A loss-of-function SLC19A2 mutation in a family with early-onset diabetes and mild TRMA traits was found to be transmitted in an autosomal dominant fashion. SLC19A2 deficiency causes reduced insulin secretion in combination with mitochondrial dysfunction, a cell cycle arrest and a loss of protection against oxidative stress . Mutations in the SLC19A3 gene, on the other hand, result in a recessive disorder called biotin-responsive basal ganglia disease (BBGD) .
Thiamine deficiency has been classically associated with malnutrition, alcoholism and various diseases such as HIV infection . It is common in chronic alcoholism and may lead to Wernicke-Korsakoff syndrome. A study in rats found that chronic alcohol feeding causes a significant inhibition in thiamine transport across renal epithelial cells. This inhibition was associated with a marked decrease in THTR1 and THTR2 expression at the protein and mRNA levels .
While many drug-induced thiamine deficiency mechanisms are related to thiamine metabolism, cases that involve the inhibition of THTR2 resulting in disruption of thiamine uptake are quite rare. Thiamine transporters 1 and 2 interact with drugs like fedratinib, trimethoprim and metformin . Inhibition of THTR2 has a significant impact on thiamine intestinal absorption and renal reabsorption, resulting in thiamine deficiency. Many reported cases of drug-induced thiamine deficiency were due to inhibition of the thiamine disposition pathway. The clinical development of the Janus kinase (JAK2) inhibitor fedratinib was terminated due to inhibition of the THTR2 transporter that resulted in thiamine deficiency and subsequently caused Wernicke’s encephalopathy in myelofibrosis patients . It was concluded that the toxicity observed in this clinical trial was a result of a transporter-mediated drug-nutrient interaction in patients with myelofibrosis, a population susceptible to thiamine deficiency .
Although vitamin transporters are currently not on the list of transporters that commonly mediate clinically relevant drug–drug interactions, the failure of fedratinib suggests that THTR2 should be considered a potential new target for drug–vitamin interactions. Nevertheless, currently there is no recommendation to investigate THTR1 or THTR2 in the latest FDA and EMA regulatory guidances.
In vitro substrates used experimentally
Intestine, kidney proximal tubule, blood–brain barrier (BBB), muscle, placenta, liver.
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