Abstract
SLC19A2 and SLC19A3, also known as thiamine transporters (THTR) 1 and 2, respectively, transport the positively charged thiamine (vitamin B1) into cells to enable its efficient utilization. SLC19A2 and SLC19A3 are also known to transport structurally unrelated cationic drugs, such as metformin, but whether this charge selectivity extends to other molecules, such as pyridoxine (vitamin B6), is unknown. We tested this possibility using Madin-Darby canine kidney II (MDCKII) cells and human embryonic kidney 293 (HEK293) cells for transfection experiments, and also using Caco-2 cells as human intestinal epithelial model cells. The stable expression of SLC19A2 and SLC19A3 in MDCKII cells (as well as their transient expression in HEK293 cells) led to a significant induction in pyridoxine uptake at pH 5.5 compared with control cells. The induced uptake was pH-dependent, favoring acidic conditions over neutral to basic conditions, and protonophore-sensitive. It was saturable as a function of pyridoxine concentration, with an apparent Km of 37.8 and 18.5 μm, for SLC19A2 and SLC19A3, respectively, and inhibited by the pyridoxine analogs pyridoxal and pyridoxamine as well as thiamine. We also found that silencing the endogenous SLC19A3, but not SLC19A2, of Caco-2 cells with gene-specific siRNAs lead to a significant reduction in carrier-mediated pyridoxine uptake. These results show that SLC19A2 and SLC19A3 are capable of recognizing/transporting pyridoxine, favoring acidic conditions for operation, and suggest a possible role for these transporters in pyridoxine transport mainly in tissues with an acidic environment like the small intestine, which has an acidic surface microclimate.
Highlights
(THTR1) and THTR2 [6, 7], respectively, can transport pyridoxine, based upon the fact that both pyridoxine and thiamine have cationic charge and that other structurally unrelated cationic drugs, such as metformin, fedratinib, and trimethoprim, are substrates for these transporters [8, 9]
Pyridoxine uptake was not altered by transient introduction of SLC19A1, suggesting that this transporter, which is another member of SLC19A family and known as reduced folate carrier 1, does not have such a capability for pyridoxine transport
We explored whether SLC19A2 and SLC19A3, known thiamine transporter 1
Summary
(THTR1) and THTR2 [6, 7], respectively, can transport pyridoxine, based upon the fact that both pyridoxine and thiamine (vitamin B1) have cationic charge and that other structurally unrelated cationic drugs, such as metformin, fedratinib, and trimethoprim, are substrates for these transporters [8, 9]. For pyridoxine transport, they were both found to operate in an opposite manner of pH dependence, favoring acidic conditions over neutral to basic conditions, as indicated by an increase in the specific uptake of pyridoxine (5 nM) by each transporter with a decrease in pH in stable transfectant MDCKII cells (Fig. 3).
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