title: pH dependence of methotrexate transport by the reduced folate carrier and the folate receptor in L1210 leukemia cells. Further evidence for a third route mediated at low pH.
abstract: F2-MTX'A is an L1210 leukemia cell line with a functional detect in the reduced folate carrier and high level expression of folate receptor beta. The pH dependence of methotrexate (MTX) influx by folate receptor beta in F2-MTX'A cells was characterized and compared with that of the reduced folate carrier in parental L1210 cells. MTX influx by folate receptor beta had a pH optimum of 6.5, whereas influx mediated by the reduced folate carrier showed a pH optimum of 7.5. Increased folate receptor beta-mediated MTX influx at pH 6.5 relative to pH 7.5 was accompanied by a 5-fold increase in binding affinity of the receptor for MTX without a change in the number of binding sites. At pH 6.2, approximately 24% of MTX influx in F2-MTX'A cells proceeded by another mechanism. This transport route became active at pH < 7.5, operated optimally at pH 6.0 to 6.5, and, unlike folate receptor beta-mediated MTX influx, was insensitive to the presence of low levels of folic acid (100 nM). MTX influx by the low pH system showed saturability, with a Ki of 5.3 microM and a Vmax of 1.53 nmol/g dry wt/min, was energy dependent, was inhibited by sulfobromophthalein with a Ki of 148 microM, and had similar relative affinities for folic acid, leucovorin, and 5 methyltetrahydrofolate. Influx of 5-methyltetrahydrofolate was also mediated by this route. The data provide further confirmatory evidence for an MTX influx route in F2-MTX'A cells, optimal at low pH and distinct from the reduced folate carrier or the folate receptor.
authors.affiliation: Department of Medicine, Einstein Cancer Center, Bronx, NY 10461, USA.
authors.forename: E E
authors.initials: EE
authors.lastname: Sierra
authors.forename: K E
authors.initials: KE
authors.lastname: Brigle
authors.forename: M J
authors.initials: MJ
authors.lastname: Spinella
authors.forename: I D
authors.initials: ID
authors.lastname: Goldman
mesh_terms: D000818:Animals; D001692:Biological Transport; D002352:Carrier Proteins / Q000502:physiology*; D002455:Cell Division / Q000187:drug effects; D058973:Folate Receptors, GPI-Anchored; D005493:Folic Acid Antagonists / Q000493:pharmacokinetics*; D006863:Hydrogen-Ion Concentration; D007939:Leukemia L1210 / Q000378:metabolism*; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D008727:Methotrexate / Q000493:pharmacokinetics* / Q000494:pharmacology; D051379:Mice; D011956:Receptors, Cell Surface; D058978:Reduced Folate Carrier Protein; D013448:Sulfobromophthalein / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D005493:Folic Acid Antagonists; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D011956:Receptors, Cell Surface; D058978:Reduced Folate Carrier Protein; C548700:Slc19a1 protein, mouse; C492292:Slc19a2 protein, mouse; D013448:Sulfobromophthalein; D008727:Methotrexate