Human intestinal folate transport: cloning, expression, and distribution of complementary RNA

Despite intensive investigations, very little is known about the molecular identity(ies) of the intestinal folate transport system(s), especially in humans. The aim of this study was to isolate a functional human intestinal folate carrier complementary DNA (cDNA) clone and determine the distribution of complementary RNA at the tissue and cellular levels.

Hybridization screening, modified Marathon cDNA amplification, expression in Xenopus oocytes, Northern analysis, and in situ hybridization were used.

The hIFC-1 cDNA contains an open reading frame for 591 amino acids (relative molecular mass = 64,826, pI = 9.4, 12 transmembrane domains, three protein kinase C phosphorylation sites, and one N-glycosylation site) with 74% DNA and 66% amino acid sequence homologies with the mouse cDNA counterpart. Xenopus oocytes injected with hIFC-1 cRNA show induced folate uptake that was (1) saturable with substrate concentration (apparent Michaelis constant = 0.71 +/- 0.06 micromol/L; maximum velocity = 128 +/- 3 fmol x h(-1) x oocyte(-1)), (2) inhibited by methotrexate, folinic acid, and folic acid (Ki = 0.84 micromol/L, 0.71 micromol/L, and 10 micromol/L, respectively), and (3) sensitive to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (Ki = 0.29 mmol/L). Northern analysis showed wide distribution of hIFC1-complementary messenger RNA species in various human tissues. In situ hybridization on sections of human jejunum showed preferential hIFC-1 expression in epithelial cells, especially in the upper half of the villi.

These results represent the first molecular characterization of a human small intestinal folate carrier.