Enhancement of Cytosine Arabinoside Cytotoxicity by Granulocyte/Macrophage Colony-stimulating Factor and Granulocyte Colony-stimulating Factor in a Human Myeloblastic Leukemia Cell Line

Enhancement of the cytotoxicity of cytosine arabinoside (ara-C) by granulocyte/macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF), and the mechanisms involved, were studied in the AML-193 human leukemia cell line. AML-193 cells require GM-CSF and G-CSF(CSFs) for optimum growth, and 24 h deprivation of CSFs decreased DNA synthesis measured in terms of ³H-thymidine incorporation. The DNA synthesis gradually recovered upon addition of CSFs. To examine the sensitivity to ara-C under different growth conditions, two groups of cell suspensions, one pretreated with CSFs after 24 h deprivation (CSFs(+) cells), and the other held continuously under CSFs-free conditions (CSFs(-) cells), were exposed to 1.0 μg/ml of ara-C for 16 h. In clonogenic assays, CSFs(+) cells showed higher sensitivity to ara-C than CSFs(-) cells. These cell groups showed no significant difference in ara-C triphosphate accumulation or retention, though the amount of ara-C incorporated into the acid-insoluble fraction was two times greater in CSFs(+) cells than CSFs(-) cells, and that difference became even clearer in the retention pools. These data suggest that the enhancement of cytotoxicity by CSFs was due to the promotion of ara-C incorporation into DNA as a result of an increase of the cell fraction in the S phase.