Effects of ConditionedMediumuponProliferation,Deoxynucleotide Metabolism,and AntimetaboliteSensitivityof Promyelocytic LeukemicCells in Vitro'1

In order to investigate the interaction of factors from leukocyteconditioned medium with leukemic cells, effects of an ammonium sulfate-precipitated conditioned medium concentrate were tested upon HL-60 cells. This preparation increased tritiated thymidine incorporation into DNA of HL-60 cells markedly, an effect which was found to be attributable in large part to greater thymidine accumulation in the intracellular nucleotide triphosphate pool. A modestly expanded population of DNA synthetic phase cells was also demonstrated by flow cytometry. Similar effects were noted of the K562 and KG-1 cell lines and were also demonstrable with giant cell tumor-conditioned medium. These effects were not demonstrable with a purified preparation of granulocyte-monocyte colony-stimulated factor. Because of the altered pattern of nucleotide metabolism noted, the effect of the conditioned medium concentrate upon 5-fluorouracil sensitiv ity was tested. Following continuous 24-h exposure to 5-fluo rouracil at 5 x 10~6 M, tritiated thymidine incorporation of HL-60 cells increased in parallel with depletion of endogenous thymidylate. Conditioned medium concentrate markedly sensitized cells to this effect of 5-fluorouracil and also increased growth retardation, cytotoxicity, and cell cycle arrest as assessed by flow cytometry. These studies thus demonstrated marked effects of a factor in conditioned medium on deoxynucleotide uptake and metabolism of the HL-60 line. These effects occurred in conjunction with, but were relatively more marked than, effects upon cell cycle distribution and were found to influence chemo therapy sensitivity.