Functional activity of the reduced folate carrier in KB, MA104, and IGROV-I cells expressing folate-binding protein

The role of a membrane-associated folate binding protein (mFBP) in transport of folate analogues was investigated in three epithelial cell lines that were grown in high folate medium and folate-conditioned medium and express different levels of mFBP: human nasopharyngeal KB cells, monkey kidney MA104 cells, and IGROV-I ovarian carcinoma cells. Folate analogues were selected for which mFBP exhibits a low affinity, i.e., methotrexate (MTX) and 10-ethyl-10-deazaaminopterin (10-EdAM) or a (moderately) high affinity as compared to folic acid, i.e., N-(5[N-(3,4-dihydro-2-methyl-4-oxoquinazolin-6-ylmethyl(-N-m ethylamino]-2-theonyl)-L-glutamic acid (ZD1694), N10-propargyl-5,8-dideazafolic acid (CB3717), and 5,10-dideazatetrahydrofolic acid. Regardless of the medium folate status, growth inhibition studies with IGROV-I and MA104 cells demonstrated a lack of correlation between the affinity of mFBP for the antifolate drugs and their sensitivity profile; both cell lines were highly sensitive to growth inhibition by MTX, 10-EdAM, ZD1694 and 5,10-dideazatetrahydrofolic acid, but were insensitive for CB3717. The same drug sensitivity profile was observed for KB cells, with the exception that these cells were also sensitive to growth inhibition by CB3717 but only in folate-conditioned medium. This overall drug sensitivity profile appeared to correlate with the differential efficiency of drug transport via the "classical" reduced folate/MTX carrier (RFC), rather than by mFBP. Characteristics that further supported functional RFC activity in KB, IGROV-I, and MA104 cells included: (a) the growth inhibitory effects of the drugs could be prevented by the reduced folate leucovorin rather than by folic acid; (b) rates for uptake of [3H]10-EdAM were 2-4-fold higher than for [3H]MTX at 1 microM extracellular concentrations and coincided with the affinity of the RFC for these drugs, rather than those of the mFBP; (c) uptake of [3H]10-EdAM and [3H]leucovorin was markedly inhibited by leucovorin and 10-EdAM, respectively, or by an N-hydroxysuccinimide ester of MTX (irreversibly labeling RFC) but only to a minor extent by folic acid or an N-hydroxysuccinimide ester of folic acid (irreversibly labeling mFBP); and, finally, (d) labeling with an N-hydroxysuccinimide ester of [3H]MTX identified a protein with a molecular weight within the range of that reported for the RFC in human leukemic cells. Altogether, these results indicate that both RFC and mFBP are coexpressed in three epithelial cell lines and that RFC is the preferential route of entry for antifolate compounds, even when mFBP is expressed to very high levels.