Influence on immunoreactive folate-binding proteins of extracellular folate concentration in cultured human cells.

Abstract

The influence of extracellular folate concentration on cellular levels of the folate transport protein and its soluble product was studied directly in cultured human nasopharyngeal carcinoma (KB) cells. As determined by radioimmunoassay, levels of the folate transport protein and the soluble folate-binding protein were 58±17 (mean±SD) and 5±2 pmol/mg cell protein, respectively, in KB cells maintained in standard medium (containing 2,300 nM folic acid). These levels significantly increased to 182±34 and 26±6 pmol/mg cell protein, respectively, in KB cells serially passaged in low folate medium (containing 2-10 nM 5-methyltetrahydrofolate). Increases in folate-binding protein levels occurred more rapidly in KB cells serially passaged in very low folate medium containing < 2 nM folate and were prevented by the addition of 100 nM 5-methyltetrahydrofolate or 0.1-1 ;&M 5-formyltetrahydrofolate to this medium. When KB cells which had been passaged in low folate medium were passaged back into either standard medium or low folate medium supplemented with reduced folates, the levels of both folate-binding proteins fell linearly towards the levels in KB cells continuously maintained in standard medium. The folate transport protein was identified in and underwent similar changes in human and mouse mammary tumor cells. These studies indicate that the folate transport system is probably regulated by the extracellular folate concentration through changes in intracellular metabolite levels.

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

“…Because these culture conditions ensured adequate delivery of folate to all cell lines used, the differential cisplatin responsiveness cannot be attributed to different availability of endogenous folates but instead might reflect differences in the capacity to use them. 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).…”

Resistance to antifolates

“…The impact of FR expression on the activity of antifolates is influenced by the folate present and its concentration in vitro or in vivo and the relative affinities of the folate and antifolate for this transporter. Hence, in standard media with folic acid levels of 2.3 mm, transport of MTX via FRs is markedly suppressed, with a comparable decrease in growth inhibition by this agent due to the 300-fold higher affinity of folic acid for the receptor (Kane et al, 1986;Chung et al, 1993). Likewise, activity of the nonpolyglutamatable TS inhibitor, CB300638 which has a very high affinity for folate receptor but a low affinity for RFC, was marked suppressed when 1 mm folic acid was added to medium containing either 1 or 20 nm 5-CHO-THF (Theti et al, 2003).…”

Mechanism of cytotoxicity of 5,10-dideazatetrahydrofolic acid in human ovarian carcinoma cells in vitro and modulation of the drug activity by folic or folinic acid

“…Many aspects of the cellular pharmacology of DDATHF have already been investigated in detail. DDATHF appears to be a good substrate for membrane folate-binding proteins (mFBP) (Kane et al, 1988;Westerhof et al, 1991), which probably act as a relevant carrier for its intracellular transport. The intracellular transport can also be mediated by the reduced folate carrier (Pizzorno et al, 1993).…”