Previous studies have defined a novel route of internalization for the essential vitamin 5-methyltetrahydrofolate in MA104 cells that begins with binding of the vitamin to the membrane receptor for folate. One of the critical steps in the pathway is the passage of 5-methyltetrahydrofolate through the membrane into the cytoplasm. Utilizing both probenecid and low temperature as selective inhibitors, we have successfully blocked transmembrane movement of the vitamin into the cytoplasm without affecting binding to the receptor or the internalization of the vitamin-receptor complex, which suggests that passage is through an anion carrier. This anion carrier, which mediates inward movement of 5-methyltetrahydrofolate after it dissociates from the receptor, also appears to mediate the efflux of folylmonoglutamate, but not folylpolyglutamate, when the concentration of the former in the cytoplasm is sufficiently high. Since we also found that the synthesis of folylpolyglutamates is regulated in these cells, most likely the intracellular concentration of the vitamin is controlled by regulating the flux of folylmonoglutamate through this carrier. (J. Clin. Invest.
J. Neurochem. (2008) 104, 1494–1503.
Abstract
Reduced derivatives of folic acid (folates) play a critical role in the development, function and repair of the CNS. However, the molecular systems regulating folate uptake and homeostasis in the central nervous system remain incompletely defined. Choroid plexus epithelial cells express high levels of folate receptor α (FRα) suggesting that the choroid plays an important role in CNS folate trafficking and maintenance of CSF folate levels. We have characterized 5‐methyltetrahydrofolate (5‐MTHF) uptake and metabolism by primary rat choroid plexus epithelial cells in vitro. Two distinct processes are apparent; one that is FRα dependent and one that is independent of the receptor. FRα binds 5‐MTHF with high affinity and facilitates efficient uptake of 5‐MTHF at low extracellular folate concentrations; a lower affinity FRα independent system accounts for increased folate uptake at higher concentrations. Cellular metabolism of 5‐MTHF depends on the route of folate entry into the cell. 5‐MTHF taken up via a non‐FRα ‐mediated process is rapidly metabolized to folylpolyglutamates, whereas 5‐MTHF that accumulates via FRα remains non‐metabolized, supporting the hypothesis that FRα may be part of a pathway for transcellular movement of the vitamin. The proton‐coupled folate transporter, proton‐coupled folate transporter (PCFT), mRNA was also shown to be expressed in choroid plexus epithelial cells. This is consistent with the role we have proposed for proton‐coupled folate transporter in FRα‐mediated transport as the mechanism of export of folates from the endocytic compartment containing FRα.
The uptake of [3H]MTX in relation to 5CH3[3H]THF by leukemic lymphoblasts in vitro may correlate positively with treatment outcome in children with B-lineage ALL. A larger study of homogeneously treated patients is necessary to confirm these results.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.