ROLE OF MEMBRANE-ASSOCIATED FOLATE BINDING-PROTEIN IN THE CYTOTOXICITY OF ANTIFOLATES IN KB, IGROV1, AND L1210A CELLS

Based on differential levels of membrane-associated folate binding pro tein (mFBP) expression, murine L1210 leukemia, human KB epidermoid car cinoma, and human IGROV1 ovarian carcinoma cells maintained under low (physiological) folate conditions (2 nM folinic acid) were used as mod el systems to investigate the potential role of mFBP in antifolate tra nsport. In addition, L1210 parental cells were compared to a subline, L1210A, expressing high levels of mFBP and defective reduced folate ca rrier. Antifolates for which KB-derived mFBP has high affinity (5, 10- dideazatetrahydrofolic acid [DDATHF] and homo-DDATHF [0.24 and 0.78 re spectively relative to folic acid]) and low affinity (methotrexate [0. 002]) were chosen for this study. Protection against DDATHF/homo-DDATH F induced cytotoxicity was achieved preferentially by folic acid compa red to folinic acid in IGROV1 and L1210A cells. In IGROV1 cells, cytot oxicity IC(50)s were increased 18- and 5.5-fold for DDATHF and homo-DD ATHF respectively by 20 nM folic acid. Moreover, greater protection wa s observed in L1210A cells, where IC(50)s were increased 354- and 80-f old for these same compounds by 20 nM folic acid. Similar protection w as not observed in KB cells, suggesting that KB mFBP was not functiona l in DDATHF transport. Although mFBP expression may be an important de terminant in the cytotoxicity of antifolates for certain tumor cells, our data demonstrate a lack of correlation between levels of mFBP and function of mFBP for DDATHF transport in the models studied.