In order to investigate the interaction of factors from leukocyte-conditioned 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 sensitivity was tested. Following continuous 24-h exposure to 5-fluorouracil at 5 × 10-6m, 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 chemotherapy sensitivity.

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This work was supported in part by Grant CH224 from the American Cancer Society, Inc., and was performed in part under the auspices of the United States Department of Energy and the Los Alamos National Flow Cytometry and Sorting Research Resource funded by the Division of Research Resources of NIH (Grant P41-RR01315-01A1) and the Department of Energy. Presented in part at the 76th Annual Meeting of the American Association for Cancer Research (1).

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