Mediated internalization of folates is required for cellular macromolecular biosynthesis. Multiple carrier-mediated mechanisms have been identified that can fulfill this role in a variety of mammalian cell types, including neoplastic cells, with and without proliferative potential. The absorption of dietary folates also relies on the function of a carrier-mediated system in mature luminal epithelium of small intestine. The various carrier-mediated systems can be distinguished by their preferences for various folate compounds as permeants as well as by differences in temperature and pH dependence. The widely studied one-carbon, reduced-folate transport system is mediated by a transporter encoded by the newly discovered RFC-1 (reduced-folate carrier) gene. The characteristics of this gene in rodent and human cells are similar, consistent with the close similarity between these species of folate transport mediated by this transporter. However, differences occur in the form of tissue-specific expression, alternate splicing, and 5' end mRNA heterogeneity, as well as in promoter utilization regulating transcription. RFC-1 gene expression also appears to regulate luminal epithelial cell folate absorption in small intestine. However, the properties of RFC-1-mediated folate transport in these cells is anomalous when compared with that seen in nonabsorptive cell types. Detailed mechanisms as to the regulation of RFC-1 transcription are now emerging along with other information on structure and function of the transporter and its alteration following mutation.

The differential polarized distribution of the reducedfolate transporter (RFT-1) and folate receptor ␣ (FR␣), the two proteins involved in the transport of folate, has been characterized in normal mouse retinal pigment epithelium (RPE) and in cultured human RPE cells. RPE cells mediate the vectorial transfer of nutrients from choroidal blood to neural retina. Whereas FR␣ is known to be present in many cell types of the neural retina, in situ hybridization analysis in the present study demonstrated that RFT-1 is present only in RPE. Laser-scanning confocal microscopy using antibodies specific for RFT-1 demonstrated an apical distribution of this protein in cultured human and intact mouse RPE, which contrasts with the basolateral distribution of FR␣ in these cells. The expression of RFT-1 in the RPE cell apical membrane was confirmed by functional studies with purified apical membrane vesicles from bovine RPE. These studies, done with N 5 -methyltetrahydrofolate (the predominant folate derivative in blood) and folate as substrates, have shown that RFT-1 functions in a Na ؉ -and C1؊ -independent manner. The transporter is specific for folate and its analogs. A transmembrane H ؉ gradient influences the transport function of this protein markedly; the transport mechanism is likely to be either folate/H ؉ co-transport or folate/OH ؊ exchange. Based on the differential polarization of FR␣ and RFT-1 in RPE, we suggest that these two proteins work in a concerted manner to bring about the vectorial transfer of folate across the RPE cell layer from the choroidal blood to the neural retina. This constitutes the first report of the differential polarization of the two folate transport proteins in any polarized epithelium.The one-carbon derivatives of the water-soluble vitamin folic acid are essential for intermediary metabolism. These derivatives are required for the synthesis of purine and pyrimidine nucleotide precursors of RNA and DNA and also for metabolism of several amino acids. Since folate and its one-carbon

Folate is an essential micronutrient that, in mammals, must be obtained from exogenous sources via intestinal absorption. Previous studies have characterized different aspects of the mechanism of the intestinal folate uptake process. Much less, however, is known about regulation of this process. In this study, we examined the effect of dietary folate deficiency on intestinal folate uptake using the rat as an animal model. The results showed that dietary folate deficiency leads to a significant (P < 0.01) and specific upregulation in the transepithelial transport of folic acid. The upregulation in transepithelial folate transport 1) was found to be due to an induction in carrier-mediated folate uptake across the brush-border membrane (BBM) and was mediated via a significant (P < 0.01) increase in the maximal velocity but not the apparent Michaelis constant of the uptake process, 2) was associated with a marked increase in the steady-state mRNA level of reduced folate carrier-1 and in the level of the expressed protein at the intestinal BBM, and 3) was associated with a marked (>10-fold) increase in the activity of the intestinal BBM form of folate hydrolase. Results of this study demonstrate, for the first time, that dietary folate deficiency leads to a marked upregulation in intestinal folate uptake and in the activity of folate hydrolase. Furthermore, the upregulation in folate uptake is associated with an increase in mRNA and protein levels of folate carrier, suggesting possible involvement of a transcriptional regulatory mechanism(s) in the upregulation.