IMPACT OF OVEREXPRESSION OF THE REDUCED FOLATE CARRIER (RFC1), AN ANION-EXCHANGER, ON CONCENTRATIVE TRANSPORT IN MURINE L1210 LEUKEMIA-CELLS

Transport of reduced folates in murine leukemia cells is mediated by t he bidirectional reduced folate carrier (RFC1) and independent unidire ctional exit pumps, RFC1 has been proposed to be intrinsically equilib rating, generating transmembrane gradients by exchange with inorganic and organic anions. his paper defines the role of high level carrier e xpression, through transfection with RFC1 cDNA, on concentrative trans port of the folate analog, methotrexate (MTX) in murine L1210 leukemia cells, RFC1 was expressed in the MTX(r)A line, which lacks a function al endogenous carrier to obtain the MTX(r)A-R16 clonal derivative, Inf lux was increased similar to 9-fold in MTX(r)A-R16 cells without a cha nge in K-m. The efflux rate constant was increased by a factor of 5.1 relative to L1210 cells, and this resulted in only a a,l-fold increase in the steady-state level of free intracellular MTX, [MTX](i), when [ MTX](e) was 1 mu M. The concentrative advantage for RFC1 (the ratio of [MTX](i) in MTX(r)A-R16 to L1210 cells) increased from 1.8 at 0.1 mu MTX to 3.8 at an [MTX](e) level of 30 mu M. Augmented transport in MTX (r)A-R16 cells was accompanied by a 2-fold increase in accumulation of MTX polyglutamate derivatives and a similar to 50% decrease in the EC 50 for 5-formyltetrahydrofolate and folic acid and the MTX IC50 relati ve to L1210 cells. These alterations paralleled changes in [MTX], and not the much larger change in influx at low [MTX](e) levels, consisten t with the critical role that free intracellular folates and drug play in meeting cellular needs for folates and as a determinant of antifol ate activity, respectively, The data indicate that RFC1 produces a lar ge and near symmetrical increase in the bidirectional fluxes of MTX re sulting in only a small increase in the transmembrane chemical gradien t at low extracellular folate levels, Hence, increased expression of R FC1, alone, may not be an efficient adaptive response to folate depriv ation, and other factors may come into play to account for the marked increases in concentrative folate transport which occur when cells are subjected to low folate-selective pressure.