Rb. Zhao et al., Impact of the reduced folate carrier on the accumulation of active thiaminmetabolites in murine leukemia cells, J BIOL CHEM, 276(2), 2001, pp. 1114-1118
The thiamin transporter encoded by SLC19A2 and the reduced folate carrier (
RFC1) share 40% homology at the protein level, but the thiamin transporter
does not mediate transport of folates, By using murine leukemia cell lines
that express no, normal, or high levels of RFC1, we demonstrate that RFC1 d
oes not mediate thiamin influx. However, high level RFC1 expression substan
tially reduced accumulation of the active thiamin coenzyme, thiamin pyropho
sphate (TPP), This decreased level of TPP, synthesized intracellularly from
imported thiamin, resulted from RFC1-mediated efflux of TPP, This conclusi
on was supported by the following observations. (i) Efflux of intracellular
TPP was increased in cells with high expression of RFC1. (ii) Methotrexate
inhibits TPP influx. (iii) TPP competitively inhibits methotrexate influx.
(iv) Loading cells, which overexpress RFC1 to high levels of methotrexate
to inhibit competitively RFC1-mediated TPP efflux, augment TPP accumulation
. (v) There was an inverse correlation between thiamin accumulation and RFC
1 activity in cells grown at a physiological concentration of thiamin. The
modulation of thiamin accumulation by RFC1 in murine leukemia cells suggest
s that this carrier may play a role in thiamin homeostasis and could serve
as a modifying factor in thiamin nutritional deficiency as well as when the
high affinity thiamin transporter is mutated.