Clustered folate receptors deliver 5-methyltetrahydrofolate to cytoplasm of MA104 cells

Previously, a high affinity, glycosylphosphatidylinositol-anchored receptor for folate and a caveolae internalization cycle have been found necessary for potocytosis of 5-methyltetrahydrofolate in MA104. We now show by cell fractionation that folate receptors also must be clustered in caveolae for potocytosis. An enriched fraction of caveolae from control cells retained 65-70% of the [³H]folic acid bound to cells in culture. Exposure of cells to the cholesterol-binding drug, filipin, which is known to uncluster receptors, shifted ~50% of the bound [³H]folic acid from the caveolac fraction to the noncaveolac membrane fraction and markedly inhibited internalization of [³H]folic acid. An mAb directed against the folate receptor also shifted ~50% of the caveolae-associated [³H]folic acid to noncaveolac membrane, indicating the antibody perturbs the normal receptor distribution. Concordantly, the mAb inhibited the delivery of 5- methyl[³H]tetrahydrofolate to the cytoplasm. Receptor bound 5- methyl[³H]tetrahydrofolate moved directly from caveolae to the cytoplasm and was not blocked by phenylarsine oxide, an inhibitor of receptor-mediated endocytosis. These results suggest cell fractionation can be used to study the uptake of molecules by caveolae.