The effects of folate deficiency on thymidylate synthetase activity, deoxyuridine suppression, cell size and doubling time in a cultured human myeloid cell line.

To help understand the pathogenesis of megaloblastic anaemia, we have studied folate-deprived HL60 cells. Cells grown with no added folic acid developed macrocytosis, a prolonged doubling time, a grossly increased deoxyuridine-suppressed value, and markedly reduced thymidylate synthetase activity. Cells in medium containing 50 nmol/l added folic acid became macrocytic with similar biochemical changes, but their doubling time was only marginally prolonged. In 100 nmol/l added folic acid, there was slight macrocytosis and a normal doubling time, but marked biochemical alterations were still present. The findings demonstrate that folate deficiency causes diminished thymidylate synthetase activity and macrocytosis in human myeloid cells, but that such cells may nevertheless demonstrate no prolongation of their doubling time, indicating either that their supply of thymidine triphosphate (TTP) is sufficient, or that misincorporation of deoxyuridine triphosphate (dUTP) is occurring. This suggests that the ineffectiveness of haemopoiesis in folate deficiency may result from damage to bone marrow cells in some way other than arrest in S-phase by a reduced delivery of TTP to the DNA replication fork.