Human cells fail to proliferate in Cbl/folic acid–deficient medium. (A) K562 cells were seeded at 1.2 × 105 cells/well (in 1-mL volumes) and cultured in RPMI (○), RPMI lacking Cbl and folic acid (▵), RPMI lacking Cbl supplemented with 10 μmol/L folic acid (□), RPMI lacking folic acid supplemented with 3.7 nmol/L CN-Cbl (▴). All media were supplemented with 10% FCS. QUSO treatment of FCS does not affect cell proliferation. (B) K562 cells were seeded at 1 × 104 cells/well and cultured in RPMI supplemented with 10% FCS (○) or 10% QUSO-treated FCS (•). At various times of culture, an aliquot (10 μL) of cells was removed for determination of cell numbers. Results are expressed as the mean ± SEM of a single cell number determination from three individual cultures.
