Folate receptor allows cells to grow in low concentrations of 5-methyltetrahydrofolate.

Abstract

The folate receptor mediates the uptake of 5-methyltetrahydrofolate in certain cultured cells by a process called potocytosis. When these cells are grown in physiological concentrations of folate, the receptor ineases the icy of vitamin uptake by 30-fold. We now show that PAM 212 cells, a mouse keratinocyte cell line, are unable to grow in 1 nM 5-methyltetrahydrofolate unless they express a functional folate receptor. These results suggest that under certain conditions, tissue cells in the body may depend on the folate receptor to obtain enough 5-methyltetrahydrofolate for growth.Potocytosis is a high-affinity uptake process that cells use to obtain essential, low molecular weight molecules (1) from their environment. Membrane-bound proteins at the cell surface concentrate the molecules within closed caveolae. Once the molecules are released into the caveolar space, they diffuse into the cytoplasm through carrier proteins in the membrane.Caveolae are the membrane-bound transport vehicles for potocytosis (1). This membrane specialization is present on the surface of a wide variety of cells (2, 3) but has been studied most thoroughly in endothelial cells (4-6). In these cells, caveolae form vesicles or open channels that appear to transport molecules from the blood to the tissue spaces. The invaginated morphology of caveolae makes them easy to recognize in thin-section electron micrographs (7). Rapidfreeze, deep-etch images have shown that the cytoplasmic surface of invaginated and flat caveolae membranes is decorated with a characteristic striated coat. This coat is resistant to removal with high salt (7). A protein component ofthe coat has recently been identified and named caveolin (7). The membrane coat may play a role in controlling the closing or pinching off of caveolae to form the transport compartment for potocytosis.Potocytosis was discovered by studying the receptormediated uptake of 5-methyltetrahydrofolate in folatedepleted MA104 cells (8)(9)(10)(11)(12). The folate receptor is a glycosyl-phosphatidylinositol-anchored membrane protein that cycles in and out of the cell by caveolae. Each cycle delivers a quantity of the vitamin to the interior of closed caveolae, where it dissociates from the receptor and diffuses through anion carriers in the membrane into the cytoplasm. When cells are grown in physiological concentrations of 5-methyltetrahydrofolate, the receptor increases the efficiency of vitamin uptake by 30-fold (9). It is not known whether this increased efficiency confers any survival advantage on cells growing under physiological conditions. If it does, then cells cultured in the presence of 1-10 nM 5-methyltetrahydrofolate should grow more rapidly if they express the folate receptor. The availability ofthe cDNA for the folate receptor (13) Transfection. PAM 212 cells (16) were plated onto 100-mm dishes (1.5 x 105 cells per dish) and cultured in 10 ml of cDMEM containing Polybrene at 3 mg/ml (15). After 18 hr, the medium was replaced with 3 ml of cDMEM containing Polybrene at 3 mg/ml....

Relationship between folate-binding protein expression and cisplatin sensitivity in ovarian carcinoma cell lines

“…FBP expression levels have been reported to be inversely regulated by the extra-and intracellular folate concentration (Kamen et al, 1986;Kane et al, 1988) but, in our hands, only one (L-SKOV3) out of five ovarian carcinoma cell lines tested showed FBP up-modulation when adapted at physiological folate concentrations (20 nM) . In the same study, all the cell lines showed a marked decrease in endogenous folates compared with cells grown in standard medium (2.3 ,UM), and the amount of total folate in the cells was in general directly proportional to the level of membrane FBP expression, consistent with the concept that intracellular folate content is maintained primarily by FBP in low-folate cultured cells (Matsue et al, 1992 (Table). These data support the notion that it is not FBP expression levels per se that underlie the cell response to cisplatin.…”

Folate receptor overexpression is associated with poor outcome in breast cancer

“…Physiological folates are transported into cells by either a low affinity reduced folate carrier (K m 5 10 25 M) or a high affinity folate receptor (K d 5 10 210 M). [1][2][3][4][5] The reduced folate carrier is ubiquitously expressed and constitutes the sole folate uptake pathway for most normal cells.The high affinity receptor is a glycosylphosphatidylinositolanchored membrane protein originally identified as a mAb-defined antigen in placenta and trophoblastic cells. It is rarely expressed or inaccessible in most normal cells.…”

“…Physiological folates are transported into cells by either a low affinity reduced folate carrier (K m 5 10 25 M) or a high affinity folate receptor (K d 5 10 210 M). [1][2][3][4][5] The reduced folate carrier is ubiquitously expressed and constitutes the sole folate uptake pathway for most normal cells.…”

Loss of heterozygosity at the 5,10-methylenetetrahydrofolate reductase locus in human ovarian carcinomas

“…Moreover, in conditions of shortage of 5-CH3-H4 folate, the cell can physiologically up-modulate FBP expression in order to increase coenzyme uptake from the extracellular fluids (Kamen and Capdevila, 1986;Kane et al, 1988;Matsue et al, 1992;Miotti et al, 1995). Therefore, at least for the subset of ovarian carcinomas displaying LOH at the MTHFR locus, our working hypothesis seems to be confirmed, even if not directly proved.…”

“…It is well known that in several cases FBP expression is physiologically regulated by the intracellular folate content (Kamen and Capdevila, 1986;Kane et al, 1988;Matsue et al, 1992;Miotti et al, 1995), most probably by the 5-methyltetrahydrofolate (5-CH3-H4 folate) content, since this coenzyme is the natural ligand of FBP and represents the predominant circulatory form (>90% of the overall seric folate, under physiological conditions) (Antony, 1992). Intracellular 5-CH3-H4 folate shortage may be consequent on limited bioavailability of extracellular folates or on biochemical defects in the intracellular regeneration or cellular retention of this folate form.…”